How smoking causes cancer Cancer Research UK

Smoking is by far the biggest preventable cause of cancer. Thanks to years of research, the links between smoking and cancer are now very clear. Smoking accounts for more than 1 in 4 UK cancer deaths, and nearly a fifth of all cancer cases.

Tobacco was responsible for more than 100 million deaths worldwide in the 20th Century. The World Health Organisation has estimated that, if current trends continue, tobacco could cause a billion deaths in the 21st Century.

The good news is that most of these deaths are preventable, by giving up smoking. Speak to your GP or pharmacist, or visit NHS Smokefree for free advice and support.

Which cancers are caused by smoking?

Smoking causes more than 4 in 5 cases of lung cancer. Lung cancer has one of the lowest survival rates of all cancers, and is the most common cause of cancer death in the UK.

Smoking also increases the risk of at least 13 other cancers including cancers of the larynx (voice box), oesophagus (gullet), mouth and pharynx (throat), bladder, pancreas, kidney, liver, stomach, bowel, cervix, ovary, nose and sinuses and some types of leukaemia. There is also some evidence that smoking could increase the risk of breast cancer.

More information about different cancer types.

Why is smoking so addictive?

Smoking is very addictive because tobacco contains a powerful drug called nicotine. Cigarettes are deliberately designed to give you a fast nicotine hit. It takes less than 20 seconds for the drug to reach your brain from inhaled cigarette smoke.

Nicotine causes addiction in much the same way as heroin or cocaine. It is just as addictive as these harder drugs.

This is the reason why most smokers say they want to quit but find it so difficult. If you start smoking, you may find it very hard to stop later on.

What parts of your body does smoking affect?

Chemicals in cigarette smoke enter our blood stream and can then affect the entire body. This is why smoking causes so many diseases, including at least 14 types of cancer, heart disease and various lung diseases.

How does smoking cause cancer?

The main way that smoking causes cancer is by damaging our DNA, including key genes that protect us against cancer. Many of the chemicals found in cigarettes have been shown to cause DNA damage, including benzene, polonium-210, benzo(a)pyrene and nitrosamines.

This is already bad news, but its made worse by other chemicals in cigarettes. For example chromium makes poisons like benzo(a)pyrene stick more strongly to DNA, increasing the chances of serious damage. And chemicals like arsenic and nickel interfere with pathways for repairing damaged DNA. This makes it even more likely that damaged cells will eventually turn cancerous.

How long does it take for smoking to cause cancer?

It usually takes many years, or decades, for the DNA damage from smoking to cause cancer. Our bodies are designed to deal with a bit of damage but its hard for the body to cope with the number of harmful chemicals in tobacco smoke.

Each cigarette can damage DNA in many lung cells, but it is the build up of damage in the same cell that can lead to cancer. However research has shown that for every 15 cigarettes smoked there is a DNA change which could cause a cell to become cancerous. This is why its better to give up smoking sooner rather than later.

How does smoking weaken the bodys defences?

Smokers are also less able to handle toxic chemicals than those with healthy lungs and blood.

We all have special cleaner proteins called detoxification enzymes that mop up harmful chemicals and convert them into harmless ones. But the chemicals in smoke, such as cadmium, can overwhelm these cleaners.

Other chemicals like formaldehyde and acrolein kill cilia, the small hairs that clean toxins from your airways.

Cigarette smoke also impacts the immune system increasing cells which can encourage tumour growth in the lungs and suppressing the ones which kill cancer cells.

Thank you for reading article How smoking causes cancer Cancer Research UK

Butadieno - Wikipedia, la enciclopedia libre

Butadieno

De Wikipedia, la enciclopedia libre

Saltar a:navegación, búsqueda

Nombre IUPAC
1,3-butadieno
General
Otros nombres	Butadieno
Fórmula semidesarrollada	CH2=CH-CH=CH2
Fórmula molecular	C4H6
Identificadores
Número CAS	106-99-0[1]
ChEBI	39478
PubChem	7845
Propiedades físicas
Apariencia	Coloro
Masa molar	54.09 g/mol
Punto de fusión	164 K (-109 °C)
Punto de ebullición	269 K (-4 °C)
Viscosidad	0.29
Propiedades químicas
Solubilidad en agua	735
Peligrosidad
Punto de inflamabilidad	358 K (85 °C)
Valores en el SI y en condiciones estándar (25 °C y 1 atm), salvo que se indique lo contrario.
[editar datos en Wikidata]

El 1,3-butadieno es un alqueno que se produce en la destilación del petróleo. El 1,3-butadieno ocupa el lugar número 36 entre las sustancias químicas de más alta producción en los EE. UU. Es un gas incoloro de olor levemente parecido a la gasolina.

El nombre Butadieno también puede hacer referencia a su isomero, 1,2-butadieno. Aunque su importancia es casi nula al lado del 1,3-butadieno.

Cerca del 75% del 1,3-butadieno que se manufactura se usa para fabricar caucho sintético. El caucho sintético es usado extensamente en neumáticos para automóviles y camiones. También se usa para fabricar plásticos, entre los que se incluyen los acrílicos. La gasolina contiene pequeñas cantidades de 1,3-butadieno.

Historia

En 1863, un químico francés aisló un hidrocarburo previamente desconocido de la pirólisis del Alcohol amílico. Este hidrocarburo se identificó como butadieno en 1886, después de que Henry Edward Armstrong lo aislara de entre los productos de pirólisis del petróleo. En 1910, el químico ruso Serguéi Lébedev trabajó con el butadieno polimerizado, y obtuvo un material con propiedades similares al caucho. Este polímero es, sin embargo, demasiado suave para sustituir el caucho natural en muchas funciones, sobre todo los neumáticos del automóvil.

La industria de butadieno se originó en los años previos a la Segunda Guerra Mundial. Muchas de las naciones beligerantes se dieron cuenta de que, en caso de guerra, podrían ser separados de las plantaciones de caucho controlado por el Imperio Británico, y trataron de eliminar su dependencia del caucho natural. En 1929, Eduard Tschunker y Bock Walter, trabajando para IG Farben en Alemania, hizo un copolímero de estireno y butadieno, que podría ser utilizado en los neumáticos del automóvil. La producción mundial se disparó rápidamente, con butadieno que se produce a partir de alcohol de grano en la Unión Soviética y los Estados Unidos y de acetileno derivado del carbón en Alemania.

Usos

La mayoría del butadieno se polimeriza para producir caucho sintético. Mientras que el polibutadieno es en sí muy suave, casi líquido, los copolímeros preparados a partir de mezclas de butadieno con estireno y/o de acrilonitrilo, tales como acrilonitrilo butadieno estireno (ABS), acrilonitrilo-butadieno (NBR) y estireno-butadieno (SBR) son resistentes y elásticos . SBR es el material más utilizado para la fabricación de neumáticos de automóviles. Cantidades más pequeñas de butadieno se utiliza para hacer el intermediario del nylon, adiponitrilo, mediante la adición de una molécula de cianuro de hidrógeno a cada uno de los dobles enlaces en un proceso llamado hidrocianación desarrollado por DuPont. Otros materiales de caucho sintético como el cloropreno, y el sulfolano solvente también son fabricados a partir de butadieno. El butadieno se utiliza en la producción industrial de 4-vinilciclohexeno a través de una reacción de dimerización de Diels Alder. El vinilciclohexeno es una impureza común que se encuentra en el almacenamiento de butadieno. El Ciclooctadieno y el ciclododecatrieno se producen a través de reacciones dimerizadas y trimerizadas, catalizadas por níquel y titanio, respectivamente. El Butadieno también es útil en la síntesis de cicloalcanos y cicloalquenos, ya que reacciona con los dobles y los triples enlaces través de la reacción de Diels-Alder.

Referencias

1. Número CAS

• ATSDR en Español - ToxFAQs: 1,3-butadieno: Departamento de Salud y Servicios Humanos de EE.UU. (dominio público)

Obtenido de «http://es.wikipedia.org/w/index.php?title=Butadieno&oldid=73391910»

Categorías:

• Química industrial
• Dienos

Thank you for reading article Butadieno - Wikipedia, la enciclopedia libre

Butadiene - Wikipedia

Butadiene

Da Wikipedia, l'enciclopedia libera.

Vai a:navigazione, ricerca

Questa voce o sezione sull'argomento sostanze chimiche non cita le fonti necessarie o quelle presenti sono insufficienti. Puoi migliorare questa voce aggiungendo citazioni da fonti attendibili secondo le linee guida sull'uso delle fonti.

Butadiene
Nome IUPAC
1,3-butadiene
Nomi alternativi
butadiene bietilene divinile viniletilene
Caratteristiche generali
Formula bruta o molecolare	C4H6
Massa molecolare (u)	54,09
Aspetto	gas incolore di odore caratteristico
Numero CAS	106-99-0
Proprietà chimico-fisiche
Densità (g/cm3, in c.s.)	0,62 (liquefatto in c.n.)
Solubilità in acqua	735 ppm w (20 °C)
Temperatura di fusione	108,9 °C (164,25 K)
Temperatura di ebollizione	4,4 °C (268,75 K)
Tensione di vapore (Pa) a 293,15 K	245 kPa
Indicazioni di sicurezza
Flash point	76 °C (197,15 K)
Limiti di esplosione	1,1 - 16,3 %
Temperatura di autoignizione	414 °C (687,15 K)
TLV (ppm)	2
Simboli di rischio chimico
pericolo
Frasi H	220 - 350 - 340 - 280
Consigli P	210 - 202 - 308+313 - 377 - 381 - 405 - 403 [1]

Il butadiene è un alchene composto da quattro atomi di carbonio ("buta" da butano) che presenta due doppi legami ("diene" da di- che indica due ed -ene che indica un doppio legame).

Struttura

Nei dieni, a seconda dei numeri che precedono il nome della molecola, si possono avere diverse strutture che differiscono tra loro per la posizione dei due doppi legami. Ad esempio nell'1,3-butadiene, abbiamo una molecola organica composta da 4 atomi di carbonio e un doppio legame tra il carbonio 1 e il carbonio 2 e un altro doppio legame tra il carbonio 3 e il carbonio 4: CH₂=CHCH=CH₂.

L'1,2-butadiene rappresenta il composto isomero di struttura.

Reattività

La reattività rispetta quella relativa agli alcheni, in più, quale diene coniugato, può subire addizione elettrofila 1,2 e 1,4: si ottengono delle miscele nelle quali a bassa temperatura predomina l'addotto 1,2 (cineticamente favorito) mentre a temperature maggiore prevale l'addotto 1,4 (termodinamicamente più stabile).

Fonti industriali

Industrialmente si ottiene principalmente in due modi:

• isolandolo dalla frazione C₄ di steamcracking; la separazione può essere fatta mediante distillazione estrattiva; in laboratorio si può fare la separazione mediante formazione di un complesso con acetato di cuproammonio;
• sintesi da n-butano: catalizzatore miscela Al₂O₃/CrO₃ in rapporto 4:1, pressione 0,2 atm, temperatura 650 °C;
• sintesi da etanolo, a temperatura 400-450 °C in presenza di ossidi metallici catalizzatori: 2 CH₃CH₂OH CH₂=CHCH=CH₂ + 2 H₂O + H₂.

Utilizzo industriale

Il butadiene viene utilizzato per preparare elastomeri quali il polibutadiene, il neoprene o copolimeri con stirene (l'SBS) o acrilonitrile.

Piccole quantità sono utilizzate nella sintesi del nylon via adiponitrile. È usato anche per preparare industrialmente il ciclododecatriene, tramite trimerizzazione.

Note tossicologiche

Esposizioni acute al butadiene causano sintomi vegetativi quali disturbi visivi, vertigini, stanchezza generale, abbassamento della pressione sanguigna, mal di testa, nausea, diminuzione del battito cardiaco e svenimento.

Diversi studi tossicologici indicano che un'esposizione cronica al butadiene può causare malattie cardiovascolari e cancro(categoria 1 delle liste IARC). Recenti studi stanno valutando l'effetto sulla sfera riproduttiva.

Note

1. ^ scheda del butadiene su IFA-GESTIS

Voci correlate

• Esaclorobutadiene

Altri progetti

• Wikimedia Commons contiene immagini o altri file su Butadiene

V · D · M Idrocarburi
Idrocarburi alifatici	Idrocarburi alifatici saturi Alcani CnH(2n + 2) Alcani lineari Metano · Etano · Propano · Butano · Pentano · Esano · Eptano · Ottano · Nonano · Decano · Undecano · Dodecano · Tridecano · Tetradecano · Pentadecano · Esadecano · Eptadecano · Ottadecano · Nonadecano · Eicosano · Eneicosano · Docosano · Tricosano · Triacontano · Entriacontano · Tetracontano · Pentacontano · Esacontano · Eptacontano · Ottacontano · Nonacontano · Ectano Alcani ramificati Isobutano · Isopentano · 3-metilpentano · Neopentano · Isoesano · Isoeptano · Isoottano · Isononano · Isodecano Cicloalcani Ciclopropano · Ciclobutano · Ciclopentano · Cicloesano · Cicloeptano · Cicloottano · Ciclononano · Ciclodecano · Ciclopentadecano Alchilcicloalcani Metilciclopropano · Metilciclobutano · Metilciclopentano · Metilcicloesano · Isopropilcicloesano · Bicicloalcani Casano (o biciclo[2.1.0]pentano) · Norbornano (o biciclo[2.2.1]eptano) · Biciclo[1.1.0]butano · Decalina (o biciclo[4.4.0]decano) Policicloalcani Adamantano · Diamantoidi · Peridrofenantrene · Peridrociclopentanofenantrene · Cubano · Prismano · Dodecaedrano · Basketano · Chiesano · Pagodano · Twistano Altri Spiroalcani Idrocarburi alifatici insaturi Alcheni CnH2n Alcheni lineari Etilene · Propilene · Butene · Pentene · Esene · Eptene · Ottene Alcheni ramificati Isobutene · Isopentene · Neopentene · Isoesene · Isoeptene · Isoottene Alchini CnH(2n - 2) Alchini lineari Acetilene · Metilacetilene · Butino · Pentino · Esino · Eptino · Ottino Alchini ramificati Isobutino · Isopentino · Neopentino · Isoesino · Isoeptino · Isoottino · Etilacetilene Cicloalcheni Ciclopropene · Ciclobutene · Ciclopentene · Cicloesene · Cicloeptene · Cicloottene Alchilcicloalcheni Metilciclopropene · Metilciclobutene · Metilciclopentene · Metilcicloesene · Isopropilcicloesene · Bicicloalcheni Norbornene Cicloalchini Ciclopropino · Ciclobutino · Ciclopentino · Cicloesino · Cicloeptino · Cicloottino Alcadieni Allene · Butadiene · Pentadiene · Esadiene · Isoprene · Cicloesadiene · 1,5-Cicloottadiene · Norbornadiene Altri Alcatriene · Alcadiino · Alchenino · Cumuleni · Cicloottatetraene · Ciclododecatriene
Idrocarburi aromatici	IPA Aceni Naftalene · Antracene · Naftacene · Pentacene · Esacene Altri Fluorene · Eliceni · Circuleni Alchilbenzeni Toluene · Xilene · Etilbenzene · Cumene · Stirene · Mesitilene · Pseudocumene Altri Benzene
Altri	Annuleni · Composti aliciclici

Portale Chimica: il portale della scienza della composizione, delle proprietà e delle trasformazioni della materia

Estratto da "http://it.wikipedia.org/w/index.php?title=Butadiene&oldid=71932672"

Categoria:

• Alcadieni

Categorie nascoste:

• Senza fonti - sostanze chimiche
• Senza fonti - ottobre 2011

Thank you for reading article Butadiene - Wikipedia

Catalytic oxidative dehydrogenation of 1-butene to 1,3

Highlights

Temperature Programmed Surface Reaction (TPSR) shows that oxidative dehydrogenation (ODH) takes place.

1-butene can be efficiently converted to 1,3-butadiene (BD) using CO₂ as a soft oxidant.

The main byproducts, tr/cis-2-butenes, can be converted to BD with similar yield.

A reaction mechanism is proposed based on the experiment results.

Abstract

1,3-Butadiene (BD) is an important raw material in petrochemical industry and its consumption is expected to increase dramatically. Catalytic oxidative dehydrogenation (ODH) of 1-butene is an alternative route to produce BD. In this study, a soft oxidant, CO₂ was used instead of O₂. Fe₂O₃ was selected from a preliminary screening using Temperature Programmed Surface Reaction (TPSR). The catalytic performance was greatly improved by supporting Fe₂O₃ on Al₂O₃. This contribution clearly shows that both Fe₂O₃ and CO₂ are involved in the reaction. The main byproducts, trans/cis-2-butenes, can also be converted to BD with similar BD yield. A preliminary reaction mechanism is proposed.

Keywords

• 1-Butene;
• 1,3-Butadiene;
• Oxidative dehydrogenation;
• Soft oxidant

Copyright © 2013 Elsevier B.V. All rights reserved.

Thank you for reading article Catalytic oxidative dehydrogenation of 1-butene to 1,3

N,N-dimesityl-1,4-diaza-1,3-butadiene C20H24N2 ChemSpider

(1E,2E)-N,N'-Dimesityl-1,2-ethandiimin [German] [ACD/IUPAC Name]

(1E,2E)-N,N'-Dimesityl-1,2-ethanediimine [ACD/IUPAC Name]

(1E,2E)-N,N'-Dimésityl-1,2-éthanediimine [French] [ACD/IUPAC Name]

Benzenamine, N,N'-(1E,2E)-1,2-ethanediylidenebis[2,4,6-trimethyl- [ACD/Index Name]

N,N-dimesityl-1,4-diaza-1,3-butadiene

(N,N'E,N,N'E)-N,N'-(ethane-1,2-diylidene)bis(2,4,6-trimethylaniline)

Benzenamine, N,N'-1,2-ethanediylidenebis[2,4,6-trimethyl-

Benzenamine,N,N'-1,2-ethanediylidenebis2,4,6-trimethyl-,N(E),N'(E)-

More...

Glyoxal bis(2,4,6-trimethylanil)

N,N'-1,2-Ethanediylidenebis[2,4,6-trimethylbenzenamine]

N,N'-Bis(2,4,6-trimethylphenyl)-1,4-diazabutadiene

N,N'-Bis(2,4,6-trimethylphenyl)ethanediimine

N,N'-Dimesitylethanediimine

Less...

Thank you for reading article N,N-dimesityl-1,4-diaza-1,3-butadiene C20H24N2 ChemSpider

Alkene Reactivity

Reactions of Alkenes Part II

3. Addition Reactions Involving Other Cyclic Onium Intermediates

Sulfenyl chloride additions are initiated by the attack of an electrophilic sulfur species on the pi-electrons of the double bond. The resulting cationic intermediate may be stabilized by the non-bonding valence shell electrons on the sulfur in exactly the same way the halogens exerted their influence. Indeed, a cyclic sulfonium ion intermediate analogous to the bromonium ion is believed to best represent this intermediate (see drawing on the left).
Two advantages of the oxymercuration method of adding water to a double bond are its high anti-stereoselectivity and the lack of rearrangement in sensitive cases. These characteristics are attributed to a mercurinium ion intermediate, analogous to the bromonium ion discussed above. In this case it must be d-orbital electrons that are involved in bonding to carbon. A drawing of this intermediate is shown on the right.

Hydroboration Stereoselectivity
The hydroboration reaction is among the few simple addition reactions that proceed cleanly in a syn fashion. As noted above, this is a single-step reaction. Since the bonding of the double bond carbons to boron and hydrogen is concerted, it follows that the geometry of this addition must be syn. Furthermore, rearrangements are unlikely inasmuch as a discrete carbocation intermediate is never formed. These features are illustrated for the hydroboration of -pinene in the following equation. Since the hydroboration procedure is most commonly used to hydrate alkenes in an anti-Markovnikov fashion, we also need to know the stereoselectivity of the second oxidation reaction, which substitutes a hydroxyl group for the boron atom. Independent study has shown this reaction takes place with retention of configuration so the overall addition of water is also syn.

The hydroboration of -pinene also provides a nice example of steric hindrance control in a chemical reaction. In the less complex alkenes used in earlier examples the plane of the double bond was often a plane of symmetry, and addition reagents could approach with equal ease from either side. In this case, one of the methyl groups bonded to C-6 (colored blue in the equation) covers one face of the double bond, blocking any approach from that side. All reagents that add to this double bond must therefore approach from the side opposite this methyl.

4. Hydrogenation

Addition of hydrogen to a carbon-carbon double bond is called hydrogenation. The overall effect of such an addition is the reductive removal of the double bond functional group. Regioselectivity is not an issue, since the same group (a hydrogen atom) is bonded to each of the double bond carbons. The simplest source of two hydrogen atoms is molecular hydrogen (H₂), but mixing alkenes with hydrogen does not result in any discernible reaction. Although the overall hydrogenation reaction is exothermic, a high activation energy prevents it from taking place under normal conditions. This restriction may be circumvented by the use of a catalyst, as shown in the following diagram.

Catalysts are substances that changes the rate (velocity) of a chemical reaction without being consumed or appearing as part of the product. Catalysts act by lowering the activation energy of reactions, but they do not change the relative potential energy of the reactants and products. Finely divided metals, such as platinum, palladium and nickel, are among the most widely used hydrogenation catalysts. Catalytic hydrogenation takes place in at least two stages, as depicted in the diagram. First, the alkene must be adsorbed on the surface of the catalyst along with some of the hydrogen. Next, two hydrogens shift from the metal surface to the carbons of the double bond, and the resulting saturated hydrocarbon, which is more weakly adsorbed, leaves the catalyst surface. The exact nature and timing of the last events is not well understood.
As shown in the energy diagram, the hydrogenation of alkenes is exothermic, and heat is released corresponding to the E (colored green) in the diagram. This heat of reaction can be used to evaluate the thermodynamic stability of alkenes having different numbers of alkyl substituents on the double bond. For example, the following table lists the heats of hydrogenation for three C₅H₁₀ alkenes which give the same alkane product (2-methylbutane). Since a large heat of reaction indicates a high energy reactant, these heats are inversely proportional to the stabilities of the alkene isomers. To a rough approximation, we see that each alkyl substituent on a double bond stabilizes this functional group by a bit more than 1 kcal/mole.

Alkene Isomer	(CH3)2CHCH=CH2 3-methyl-1-butene	CH2=C(CH3)CH2CH3 2-methyl-1-butene	(CH3)2C=CHCH3 2-methyl-2-butene
Heat of Reaction ( Hº )	30.3 kcal/mole	28.5 kcal/mole	26.9 kcal/mole

From the mechanism shown here we would expect the addition of hydrogen to occur with syn-stereoselectivity. This is often true, but the hydrogenation catalysts may also cause isomerization of the double bond prior to hydrogen addition, in which case stereoselectivity may be uncertain.
The formation of transition metal complexes with alkenes has been convincingly demonstrated by the isolation of stable platinum complexes such as Zeise's salt, K[PtCl₃(C₂H₄)].H₂O, and ethylenebis(triphenylphosphine)platinum, [(C₆H₅)₃P]₂Pt(H₂C=CH₂). In the latter, platinum is three-coordinate and zero-valent, whereas Zeise's salt is a derivative of platinum(II). A model of Zeise's salt and a discussion of the unusual bonding in such complexes may be viewed by clicking here. Similar complexes have been reported for nickel and palladium, metals which also function as catalysts for alkene hydrogenation.

A non-catalytic procedure for the syn-addition of hydrogen makes use of the unstable compound diimide, N₂H₂. This reagent must be freshly generated in the reaction system, usually by oxidation of hydrazine, and the strongly exothermic reaction is favored by the elimination of nitrogen gas (a very stable compound). Diimide may exist as cis-trans isomers; only the cis-isomer serves as a reducing agent. Examples of alkene reductions by both procedures are shown on the right.

5. Oxidations

(i) Hydroxylation

Dihydroxylated products (glycols) are obtained by reaction with aqueous potassium permanganate (pH > 8) or osmium tetroxide in pyridine solution. Both reactions appear to proceed by the same mechanism (shown below); the metallocyclic intermediate may be isolated in the osmium reaction. In basic solution the purple permanganate anion is reduced to the green manganate ion, providing a nice color test for the double bond functional group. From the mechanism shown here we would expect syn-stereoselectivity in the bonding to oxygen, and regioselectivity is not an issue.
When viewed in context with the previously discussed addition reactions, the hydroxylation reaction might seem implausible. Permanganate and osmium tetroxide have similar configurations, in which the metal atom occupies the center of a tetrahedral grouping of negatively charged oxygen atoms. How, then, would such a species interact with the nucleophilic pi-electrons of a double bond? A possible explanation is that an empty d-orbital of the electrophilic metal atom extends well beyond the surrounding oxygen atoms and initiates electron transfer from the double bond to the metal, in much the same fashion noted above for platinum. Back-bonding of the nucleophilic oxygens to the antibonding *-orbital completes this interaction. The result is formation of a metallocyclic intermediate, as shown below.

(ii) Epoxidation

Some oxidation reactions of alkenes give cyclic ethers in which both carbons of a double bond become bonded to the same oxygen atom. These products are called epoxides or oxiranes. An important method for preparing epoxides is by reaction with peracids, RCO₃H. The oxygen-oxygen bond of such peroxide derivatives is not only weak (ca. 35 kcal/mole), but in this case is polarized so that the acyloxy group is negative and the hydroxyl group is positive (recall that the acidity of water is about ten powers of ten weaker than that of a carboxylic acid). If we assume electrophilic character for the OH moiety, the following equation may be written.

It is unlikely that a dipolar intermediate, as shown above, is actually formed. The epoxidation reaction is believed to occur in a single step with a transition state incorporating all of the bonding events shown in the equation. Consequently, epoxidations by peracids always have syn-stereoselectivity, and seldom give structural rearrangement. You may see the transition state by clicking the Change Equation button. Presumably the electron shifts indicated by the blue arrows induce a charge separation that is immediately neutralized by the green arrow electron shifts.

---

The previous few reactions have been classified as reductions or oxidations, depending on the change in oxidation state of the functional carbons. It is important to remember that whenever an atom or group is reduced, some other atom or group is oxidized, and a balanced equation must balance the electron gain in the reduced species with the electron loss in the oxidized moiety, as well as numbers and kinds of atoms. Starting from an alkene (drawn in the box), the following diagram shows a hydrogenation reaction on the left (the catalyst is not shown) and an epoxidation reaction on the right. Examine these reactions, and for each identify which atoms are reduced and which are oxidized.

---

Epoxides may be cleaved by aqueous acid to give glycols that are often diastereomeric with those prepared by the syn-hydroxylation reaction described above. Proton transfer from the acid catalyst generates the conjugate acid of the epoxide, which is attacked by nucleophiles such as water in the same way that the cyclic bromonium ion described above undergoes reaction. The result is anti-hydroxylation of the double bond, in contrast to the syn-stereoselectivity of the earlier method. In the following equation this procedure is illustrated for a cis-disubstituted epoxide, which, of course, could be prepared from the corresponding cis-alkene. This hydration of an epoxide does not change the oxidation state of any atoms or groups.

(iii) Oxidative Cleavage of Double Bonds

Ozonolysis
In determining the structural formula of an alkene, it is often necessary to find the location of the double bond within a given carbon framework. One way of accomplishing this would be to selectively break the double bond and mark the carbon atoms that originally formed that bond. For example, there are three isomeric alkenes that all give 2-methylbutane on catalytic hydrogenation. These are 2-methyl-2-butene (compound A), 3-methyl-1-butene (compound B) and 2-methyl-1-butene (compound C), shown in the following diagram. If the double bond is cleaved and the fragments marked at the cleavage sites, the location of the double bond is clearly determined for each case. A reaction that accomplishes this useful transformation is known. It is called ozonolysis, and its application to each of these examples may be seen by clicking the "Show Reaction" button.

Ozone, O₃, is an allotrope of oxygen that adds rapidly to carbon-carbon double bonds. Since the overall change in ozonolysis is more complex than a simple addition reaction, its mechanism has been extensively studied. Reactive intermediates called ozonides have been isolated from the interaction of ozone with alkenes, and these unstable compounds may be converted to stable products by either a reductive workup (Zn dust in water or alcohol) or an oxidative workup (hydrogen peroxide). The results of an oxidative workup may be seen by clicking the "Show Reaction" button a second time. Continued clicking of this button repeats the cycle. The chief difference in these conditions is that reductive workup gives an aldehyde product when hydrogen is present on a double bond carbon atom, whereas oxidative workup gives a carboxylic acid or carbon dioxide in such cases. The following equations illustrate ozonide formation, a process that is believed to involve initial syn-addition of ozone, followed by rearrangement of the extremely unstable molozonide addition product. They also show the decomposition of the final ozonide to carbonyl products by either a reductive or oxidative workup.

Ozonolysis To learn more about the mechanism of this fascinating reaction Click Here.

---

The relatively stable ozonide, arterolane, is a component of a new antimalerial drug, Synriam. Combined with piperaquine, it provides effective treatment of Plasmodium falciparum, the most lethal form of malaria. The ozonide ring is similar to the bridged trioxide unit found in the Chinese herb artemisinin and its derivative artemether, which, combined with lumafantrine have also been used as the antimalerial drug, Coartem. Traditional medications such as chloroquine are effective in only 50 percent of patients where the parasite is drug-resistant.

Glycol Cleavage
The vicinal glycols prepared by alkene hydroxylation (reaction with osmium tetroxide or permanganate) are cleaved to aldehydes and ketones in high yield by the action of lead tetraacetate (Pb(OAc)₄) or periodic acid (HIO₄). This oxidative cleavage of a carbon-carbon single bond provides a two-step, high-yield alternative to ozonolysis, that is often preferred for small scale work involving precious compounds. A general equation for these oxidations is shown below. As a rule, cis-glycols react more rapidly than trans-glycols, and there is evidence for the intermediacy of heterocyclic intermediates (as shown), although their formation is not necessary for reaction to occur.

The following problems review many aspects of alkene chemistry. The first question allows you to choose the form of the question (i.e. starting alkene, reagent or product). The second question requires you to draw the product of a reaction selected from 54 possible combinations of alkene and reagent. The third question asks you to choose reagents which will convert propene into a designated product (more than one step may be needed). The fourth question is another reagent selection problem (the ketone can be ignored), and the fifth is similar but also requires selecting the starting alkene for the synthesis. The next two problems concern the ozonolysis cleavage of double bonds. The eighth question concerns four reactions in which stereospecific addition reactions have taken place. Finally, a random quiz may be examined.

This page is the property of William Reusch.   Comments, questions and errors should be sent to whreusch@msu.edu.
These pages are provided to the IOCD to assist in capacity building in chemical education. 05/05/2013

Free Radical Reactions of Alkenes

1. Addition of Radicals to Alkenes

Protons and other electrophiles are not the only reactive species that initiate addition reactions to carbon-carbon double bonds. Curiously, this first became evident as a result of conflicting reports concerning the regioselectivity of HBr additions. As noted earlier, the acid-induced addition of HBr to 1-butene gave predominantly 2-bromobutane, the Markovnikov Rule product. However, in some early experiments in which peroxide contaminated reactants were used, 1-bromobutane was the chief product. Further study showed that an alternative radical chain-reaction, initiated by peroxides, was responsible for the anti-Markovnikov product. This is shown by the following equations.

The weak OO bond of a peroxide initiator is broken homolytically by thermal or hight energy. The resulting alkoxy radical then abstracts a hydrogen atom from HBr in a strongly exothermic reaction. Once a bromine atom is formed it adds to the -bond of the alkene in the first step of a chain reaction. This addition is regioselective, giving the more stable carbon radical as an intermediate. The second step is carbon radical abstraction of another hydrogen from HBr, generating the anti-Markovnikov alkyl bromide and a new bromine atom. Each of the steps in this chain reaction is exothermic, so once started the process continues until radicals are lost to termination events.
This free radical chain addition competes very favorably with the slower ionic addition of HBr described earlier, especially in non-polar solvents. It is important to note, however, that HBr is unique in this respect. The radical addition process is unfavorable for HCl and HI because one of the chain steps becomes endothermic (the second for HCl & the first for HI).
Other radical addition reactions to alkenes have been observed, one example being the peroxide induced addition of carbon tetrachloride shown in the following equation

RCH=CH2   +   CCl4 (peroxide initiator)   >   RCHClCH2CCl3

The best known and most important use of free radical addition to alkenes is probably polymerization. Since the addition of carbon radicals to double bonds is energetically favorable, concentrated solutions of alkenes are prone to radical-initiated polymerization, as illustrated for propene by the following equation. The blue colored R-group represents an initiating radical species or a growing polymer chain; the propene monomers are colored maroon. The addition always occurs so that the more stable radical intermediate is formed.

RCH2(CH3)CH· + CH3CH=CH2 > RCH2(CH3)CH-CH2(CH3)CH· + CH3CH=CH2 > RCH2(CH3)CHCH2(CH3)CH-CH2(CH3)CH· > etc.

2. Allylic Substitution

We noted earlier that benzylic and allylic sites are exceptionally reactive in free radical halogenation reactions. Since carbon-carbon double bonds add chlorine and bromine in liquid phase solutions, radical substitution reactions by these halogens are often carried out at elevated temperature in the gas phase (first equation below). Formation of the ionic -complexes that are intermediates in halogen addition is unfavorable in the absence of polar solvents, and entropy generally favors substitution over addition.
The brominating reagent, N-bromosuccinimide (NBS), has proven useful for achieving allylic or benzylic substitution in CCl₄ solution at temperatures below its boiling point (77 ºC). One such application is shown in the second equation.

The predominance of allylic substitution over addition in the NBS reaction is interesting. The NBr bond is undoubtedly weak (probably less than 50 kcal/mol) so bromine atom abstraction by radicals should be very favorable. The resulting succinimyl radical might then establish a chain reaction by removing an allylic hydrogen from the alkene. One problem with this mechanism is that NBS is very insoluble in CCl₄, about 0.006 mole / liter at reflux. Although it is possible that the allylic bromination occurs at a solid-liquid interface, evidence for another pathway has been obtained. In the non-polar solvent used for these reactions, very low concentrations of bromine may be generated from NBS. This would serve as a source of bromine atoms, which would abstract allylic hydrogens irreversibly (an exothermic reaction) in competition with reversible addition to the double bond. The HBr produced in this way is known to react with NBS, giving a new bromine molecule and succinimide, as shown here. Ionic addition of bromine to the double bond would be very slow in these circumstances (CCl₄ is a nonpolar solvent).

HBr   +   (CH2CO)2NBr   >   Br2   +   (CH2CO)2NH

This mechanism is essentially the same as that for the free radical halogenation of alkanes, with NBS serving as a source of very low concentrations of bromine. Unsymmetrical allylic radicals will react to give two regioisomers. Thus, 1-octene on bromination with NBS yields a mixture of 3-bromo-1-octene (ca. 18%) and 1-bromo-2-octene (82%) - both cis and trans isomers.

RCH2CH=CH2   +   (CH2CO)2NBr   >   RCHBrCH=CH2   +   RCH=CHCH2Br   +   (CH2CO)2NH

Dienes

1. Properties of Dienes

When considering compounds having two or more double bonds in a molecule, it is useful to identify three distinct ways in which these functions may be oriented with respect to each other. First, the double bonds may be separated by one or more sp³-hybridized carbon atoms, as in 1,5-hexadiene. In this circumstance each double bond behaves independently of the other, and we refer to them as isolated. A second relationship has the double bonds connected to each other by a single bond, as in 1,3-hexadiene, and we refer to this arrangement as conjugated. Finally, two double bonds might share a carbon atom, as in 1,2-hexadiene. The central carbon atom in such a system is sp-hybridized, and we call such double bonds cumulated. These three isomers are shown in the following diagram, and three other similar isomers will be displayed on clicking the Change Examples button. In cases where stereoisomers are possible only the E-isomer is shown.
Another stereoisomeric factor associated with conjugated dienes will be demonstrated by clicking the Change Examples button a second time. Rotation about the single bond joining the two double bonds (colored blue) converts a trans-like s-trans conformation to its s-cis form. The energy barrier to this conformational isomerization is normally low, and the s-trans conformer is often more stable than the s-cis conformer, as shown in the diagram.

These categories are based on more than obvious structural variations. We find significant differences in the chemical properties of dienes depending on their structural type. For example, catalytic hydrogenation converts all the dienes shown here to the alkane hexane, but the heats of reaction (heat of hydrogenation) reflect characteristic differences in their thermodynamic stability. This is illustrated in the diagram on the right. Taking the heat of hydrogenation of 1-hexene (30.1 kcal/mole) as a reference, we find that the isolated diene, 1,5-hexadiene, as expected, generates double this heat of reaction on conversion to hexane. The cumulated diene, 1,2-hexadiene, has a 6 kcal/mole higher heat of reaction, indicating it is less stable than the isolated diene by this magnitude. On the other hand, conjugation of double bonds seems to stabilize a diene by about 5 kcal/mole. The increase in stability of 2,4-hexadiene over 1,3-hexadiene (both are conjugated) is due to the increased double bond substitution of the former, a factor noted earlier for simple alkenes.
The stabilization of dienes by conjugation is less dramatic than the aromatic stabilization of benzene. Nevertheless, similar resonance and molecular orbital descriptions of conjugation may be written. A resonance description, such as the one shown here, involves charge separation, implying a relatively small degree of stabilization.

CH2=CH-CH=CH2     (+)CH2-CH=CH-CH2:()

A molecular orbital model for 1,3-butadiene is shown below. Note that the lobes of the four p-orbital components in each pi-orbital are colored differently and carry a plus or minus sign. This distinction refers to different phases, defined by the mathematical wave equations for such orbitals. Regions in which adjacent orbital lobes undergo a phase change are called nodes. Orbital electron density is zero in such regions. Thus a single p-orbital has a node at the nucleus, and all the pi-orbitals shown here have a nodal plane that is defined by the atoms of the diene. This is the only nodal surface in the lowest energy pi-orbital, ₁. Higher energy pi-orbitals have an increasing number of nodes.

To examine a model of the p-orbital components for the s-cis conformer of a 1,3-diene.  

To examine the molecular orbitals of both s-trans and s-cis-1,3-butadiene  

2. Addition Reactions of Dienes

Addition reactions of isolated dienes proceed more or less as expected from the behavior of simple alkenes. Thus, if one molar equivalent of 1,5-hexadiene is treated with one equivalent of bromine a mixture of 5,6-dibromo-1-hexene, 1,2,5,6-tetrabromohexane and unreacted diene is obtained, with the dibromo compound being the major product (about 50%).

Similar reactions of conjugated dienes, on the other hand, often give unexpected products. The addition of bromine to 1,3-butadiene is an example. As shown below, a roughly 50:50 mixture of 3,4-dibromo-1-butene (the expected product) and 1,4-dibromo-2-butene (chiefly the E-isomer) is obtained. The latter compound is remarkable in that the remaining double bond is found in a location where there was no double bond in the reactant. This interesting relocation requires an explanation.

The expected addition product from reactions of this kind is the result of 1,2-addition, i.e. bonding to the adjacent carbons of a double bond. The unexpected product comes from 1,4-addition, i.e. bonding at the terminal carbon atoms of a conjugated diene with a shift of the remaining double bond to the 2,3-location. These numbers refer to the four carbons of the conjugated diene and are not IUPAC nomenclature numbers. Product compositions are often temperature dependent, as the addition of HBr to 1,3-butadiene demonstrates.

Bonding of an electrophilic atom or group to one of the end carbon atoms of a conjugated diene (#1) generates an allyl cation intermediate. Such cations are stabilized by charge delocalization, and it is this delocalization that accounts for the 1,4-addition product produced in such addition reactions. As shown in the diagram, the positive charge is distributed over carbons #2 and #4 so it is at these sites that the nucleophilic component bonds. Note that resonance stabilization of the allyl cation is greater than comparable stabilization of 1,3-butadiene, because charge is delocalized in the former, but created and separated in the latter.

An explanation for the temperature influence is shown in the following energy diagram for the addition of HBr to 1,3-butadiene. The initial step in which a proton bonds to carbon #1 is the rate determining step, as indicated by the large activation energy (light gray arrow). The second faster step is the product determining step, and there are two reaction paths (colored blue for 1,2-addition and magenta for 1,4-addition). The 1,2-addition has a smaller activation energy than 1,4-addition, but the 1,4-product is more stable than the 1,2-product. At low temperatures, the products are formed irreversibly and reflect the relative rates of the two competing reactions. This is termed kinetic control. At higher temperatures, equilibrium is established between the products, and the thermodynamically favored 1,4-product dominates.

3. Diels-Alder Cycloaddition

The unique character of conjugated dienes manifests itself dramatically in the Diels-Alder Cycloaddition Reaction. A cycloaddition reaction is the concerted bonding together of two independent pi-electron systems to form a new ring of atoms. When this occurs, two pi-bonds are converted to two sigma-bonds, the simplest example being the hypothetical combination of two ethene molecules to give cyclobutane. This does not occur under normal conditions, but the cycloaddition of 1,3-butadiene to cyanoethene (acrylonitrile) does, and this is an example of the Diels-Alder reaction. The following diagram illustrates two cycloadditions, and introduces several terms that are useful in discussing reactions of this kind.

In the hypothetical ethylene dimerization on the left, each reactant molecule has a pi-bond (colored orange) occupied by two electrons. The cycloaddition converts these pi-bonds into new sigma-bonds (colored green), and this transformation is then designated a [2+2] cycloaddition, to enumerate the reactant pi-electrons that change their bonding location.
The Diels-Alder reaction is an important and widely used method for making six-membered rings, as shown on the right. The reactants used in such reactions are a conjugated diene, simply referred to as the diene, and a double or triple bond coreactant called the dienophile, because it combines with (has an affinity for) the diene. The Diels-Alder cycloaddition is classified as a [4+2] process because the diene has four pi-electrons that shift position in the reaction and the dienophile has two.
The Diels-Alder reaction is a single step process, so the diene component must adopt a cis-like conformation in order for the end carbon atoms (#1 & #4) to bond simultaneously to the dienophile. Such conformations are called s-cis, the s referring to the single bond connecting the two double bonds. The s-cis and s-trans conformers of 1,3-butadiene are shown in the preceding diagram. For many acyclic dienes the s-trans conformer is more stable than the s-cis conformer (due to steric crowding of the end groups), but the two are generally in rapid equilibrium, permitting the use of all but the most hindered dienes as reactants in Diels-Alder reactions. In its usual form, the diene component is electron rich, and the best dienophiles are electron poor due to electron withdrawing substituents such as CN, C=O & NO₂. The initial bonding interaction reflects this electron imbalance, with the two new sigma-bonds being formed simultaneously, but not necessarily at equal rates.

Stereospecificity

We noted earlier that addition reactions of alkenes often exhibited stereoselectivity, in that the reagent elements in some cases added syn and in other cases anti to the the plane of the double bond. Both reactants in the Diels-Alder reaction may demonstrate stereoisomerism, and when they do it is found that the relative configurations of the reactants are preserved in the product (the adduct). The following drawing illustrates this fact for the reaction of 1,3-butadiene with (E)-dicyanoethene. The trans relationship of the cyano groups in the dienophile is preserved in the six-membered ring of the adduct. Likewise, if the terminal carbons of the diene bear substituents, their relative configuration will be retained in the adduct. Using the earlier terminology, we could say that bonding to both the diene and the dienophile is syn. An alternative description, however, refers to the planar nature of both reactants and terms the bonding in each case to be suprafacial (i.e. to or from the same face of each plane). This stereospecificity also confirms the synchronous nature of the 1,4-bonding that takes place.

The essential characteristics of the Diels-Alder cycloaddition reaction may be summarized as follows:

(i) The reaction always creates a new six-membered ring. When intramolecular, another ring may also be formed.
(ii) The diene component must be able to assume a s-cis conformation.
(iii) Electron withdrawing groups on the dienophile facilitate reaction.
(iv) Electron donating groups on the diene facilitate reaction.
(v) Steric hindrance at the bonding sites may inhibit or prevent reaction.
(vi) The reaction is stereospecific with respect to substituent configuration in both the dienophile and the diene.

These features are illustrated by the following eight examples, one of which does not give a Diels-Alder cycloaddition. Try to predict the course of each reaction before disclosing the answers by pressing the "Show Products" button. The formation of a new six-membered ring should be apparent in every case where reaction occurs.

There is no reaction in example D because this diene cannot adopt a s-cis orientation. In examples B, C, F, G & H at least one of the reactants is cyclic so that the product has more than one ring, but the newly formed ring is always six-membered. In example B the the same cyclic compound acts as both the diene colored blue) and the dienophile (colored red). The adduct has three rings, two of which are the five-membered rings present in the reactant, and the third is the new six-membered ring (shaded light yellow). Example C has an alkyne as a dienophile (colored red), so the adduct retains a double bond at that location. This double bond could still serve as a dienophile, but in the present case the diene is sufficiently hindered to retard a second cycloaddition. The quinone dienophile in reaction F has two dienophilic double bonds. However, the double bond with two methyl substituents is less reactive than the unsubstituted dienophile due in part to the electron donating properties of the methyl groups and in part to steric hindrance. The stereospecificity of the Diels-Alder reaction is demonstrated by examples A, E & H. In A & H the stereogenic centers lie on the dienophile, whereas in E these centers are on the diene. In all cases the configuration of the reactant is preserved in the adduct.

Cyclic dienes, such as those in examples B, C & G, give bridged bicyclic adducts for which an additional configurational feature must be designated. As shown in the following diagram, there are two possible configurations for compounds of this kind. If a substituent (colored magenta here) is oriented cis to the longest or more unsaturated bridge (colored blue here), it is said to be endo. When directed trans to the bridge it is exo. When the Diels-Alder reaction forms bridged bicyclic adducts and an unsaturated substituent is located on this bicyclic structure (as in B & G), the chief product is normally the endo isomer "Alder's Endo Rule". Example C does not merit such a nomenclature, since stereoisomeric orientations of the substituent are not possible.

Additional information about the Diels-Alder reaction may be reached by clicking here.

---

Allenes The chemistry of cumulated dienes (allenes) is somewhat specialized and will not be described here. The interested reader may pursue this subject a bit further by clicking here.

The following problems concern reactions of dienes. The first question requires you to draw the products of 1,2- & 1,4-addition to some conjugated dienes. The second and third questions asks you to evaluate potential dienes and dienophile reactants in Diels-Alder reactions. For the fourth question you are asked to draw the products expected from some Diels-Alder reactions. The fifth question is similar, but asks you to draw the reactants that will give a specified Diels-Alder adduct. Finally, the sixth question provides a comprehensive test of Diels-Alder reactions.

This page is the property of William Reusch.   Comments, questions and errors should be sent to whreusch@msu.edu.
These pages are provided to the IOCD to assist in capacity building in chemical education. 05/05/2013

Thank you for reading article Alkene Reactivity

Catalyst Characterization with FESEM/EDX by the Example of

1. Introduction

Ag catalysts are of outstanding importance in the field of heterogeneous catalysis. Optimum distribution and morphology of the Ag particles must be ensured by controlled, tailored catalyst synthesis. Hence, there is a growing demand for the characterization of Ag-dispersed fine particle systems requiring high-resolution surface observation of particles down to a few tens of nanometers and elemental analysis by field emission scanning electron microscopy and energy-dispersive X-ray spectrometry (FESEM/EDX). It is beneficial to characterize the particle morphology by comparison of different imaging methods like secondary electron (SE)-, backscattered electron (BSE)- and transmitted electron (TE) detection. In scanning electron microscopy surface topography becomes visible due to the dependency of the SE yield on the angle of electron incidence. Together with the large depth of field informative images of irregularly shaped particle structures are obtained. The increased BSE yield of high atomic numbers (Z) such as Ag catalysts and promoters (e.g. Cs) compared to a low-density matrix and the high penetration depth of 20-30 keV electrons also allows imaging and analysis of inclusions that would be obscured at low beam energies. Both SE and BSE detectors, in particular at low beam voltages, can additionally reveal interesting surface features of fine Ag particles. A well-known example for a Ag catalyzed reaction is the -Al₂O₃ supported Ag-catalyzed epoxidation of 1,3-butadiene to 3,4-epoxybutene. The electrophilic addition of oxygen across the carbon-carbon double bond of 1,3-butadiene, resulting in a three-member ring structure that can undergo further chemical transformations to oxygenated products, such as ketones, alcohols, and ethers. Supported silver catalysts have been shown to epoxidize olefins with nonallylic hydrogen when an alkali promotor is doped on the surface. Thus, the direct kinetically controlled oxidation to the corresponding epoxide is preferred. The guiding hypothesis for this partially oxidation is that surface oxametallacycles are key intermediates for epoxidation on promoted Ag catalysts. Therefore, the preparative application of Ag and promoters (Cs, Ba) on the catalyst support material is of great importance. Another important aspect is sintering of Ag particles which may reduce the catalytically active surface and decreases the overall reaction performance. For this research, catalysts are produced by sequential impregnation of two mineralogically differing support materials (SC13, SLA2) with an aqueous active component solution (AgNO₃, CsNO₃). The two selected Ag catalyst systems are examined using mainly FESEM/EDX. The determination of metal amount were carried out with EDX area analysis and, if necessary, supplemented by EDX-spot analysis. In some cases, characterization of Ag-distributions by EDX mappings were made. Furthermore, for the Monte Carlo (MC)- simulations x-ray line scans of two different SEM preparation techniques (bulk specimen, thin-film supported specimen) were performed to underpin the relationships impressively. Temperature-programmed O₂ desorption (O₂-TPD) as well as N₂ sorption (BET) measurements are important analysis methods in catalysis chemistry, too. Both methods are used to support the FESEM/EDX investigation to provide complementary contributions with regard to the Ag distribution and the properties of the carrier surfaces.

2. Characterization and measurement methods

2.1. Ag catalyst systems

Huge varieties of materials are used in the preparation of heterogeneous catalysts, especially industrial catalysts. Catalysts can be divided into three groups of constituents, namely active catalytic agents, promoters, and supports [1]. Catalysts are manufactured by various methods, such as wet impregnation, leaching, drying and calcination. The major components of the catalyst system are the catalyst support (bulk material, e.g. Al₂O₃, TiO₂, SiO₂), which might influence the catalytic activity of the active components (metal-support interactions, MSI) [2] and the active metal e.g. Ag, Pd, Pt is the active agent. Increasing the surface area of the active agent is one function of the support. Maintaining a high dispersion of the active components is the other function. -Al₂O₃ with its small specific surface area has proved to be a wear-resistant carrier material for Ag and to be highly suited for the selective oxidation of 1,3-butadiene [3].

2.2. Catalyst preparation

The Ag catalysts for this research are produced by sequential incipient wetness impregnation of two mineralogically different carrier materials with an aqueous solution of AgNO₃ and CsNO₃ as active components. The following catalyst supports are applied:

• SC13 (Almatis) mineralogical: 80 % alpha-Al₂O₃, 20 % -Al₂O₃, Chemical composition: 99 % Al₂O₃, 0.05 % CaO, 0.03 % Fe₂O₃, 0.02% SiO_2, 0.4 % Na₂O.

• SLA92 (Almatis) mineralogical: Main phase Ca hexa aluminate (CA6) hibonite, secondary phase alpha-Al₂O₃, chemical composition: 91% Al₂O₃, 8.5 % CaO, 0.04 % Fe₂O₃, 0.07 % SiO₂, 0.4 % Na₂O.

Dry SC13 and SLA92 are pre-sieved and subjected to wet sieving (Retsch laboratory sieves manufactured according to DIN 3310, mesh width 0.045 mm 0.063 mm). The sieve fractions are dried at 120°C in a circulating air oven for 5 h. The desired active component solutions AgNO₃(aq) and CsNO₃(aq) are applied sequentially to the carrier materials and subjected to an ultrasound bath for 0.5 min (Sonorex RK 100H) [4]. The precursors are dried in a circulating air oven at 40°C for 5 h and then oxidized with 100 % O₂ (4.8) at 250°C for 10 min (4000 ml_STP h^-1). A reduction with 100 % H₂ (6.0) follows at 200°C for 10 min (1000 ml_STP h^-1). The concentrations of the active metal (oxidation number ± 0) components after conditioning are given by:

D1, SC13, 45/63 µm, 5 % Ag D2, SC13, 45/63 µm, 5 % Ag, 1500 ppm Cs D3, SC13, 45/63 µm, 10 % Ag D4, SC13, 45/63 µm, 10 % Ag, 1500 ppm Cs D5, SC13, 45/63 µm, 20 % Ag D6 SC13, 45/63 µm, 20 % Ag, 1500 ppm Cs

MZ06, SLA 92, 45/63 µm, 5 % Ag, 1500 ppm Cs MZ09, SLA 92, 45/63 µm, 10 % Ag, 1500 ppm Cs

2.3. Electron interactions with the specimen

Scanning electron microscopy in combination with energy-dispersive X-ray spectrometry (SEM/EDX) is a well-established and versatile method for the characterization of heterogeneous catalysts, especially Ag catalysts. They are predominantly composed of small metal particles dispersed onto a supporting material, generally a chemically inert oxide. Information about particle size distribution, deposition on the substrate surface, and elemental compositions can be obtained easily. In FESEM, a high brightness Schottky-type field emission (FE) cathode with its small beam diameter (spot size) enables imaging of features with high resolution and high contrast down to the nanoscale even on bulk substrates. Coincidently, the element composition of the specimen is available by excitation of inner shell electrons to collect characteristic X-rays with sufficient intensity for analytical information. Due to the high depth of field (D d² / , _{20 keV} = 0.0086 nm), where D is the depth of field, d is the apparent resolution, and is the electron wavelength, impressive images of differently shaped catalyst particles can be generated [5]. In the SEM, the primary electron beam creates different types of electron interactions, while penetrating the specimen. For this research, however, we utilize the following 4 important electron interactions with the specimen, namely the generation of

Secondary and backscattered electrons are essentially for the topographical imaging of the specimen surface and therefore are described in more detail in chapter 2.4. Transmitted electrons (TE) interact less with the specimen. The specimen appears more or less "transparent for electrons, depending on the thickness and density of the specimen. In energy-dispersive X-ray Spectroscopy (EDX), the X-rays are produced by inelastic scattering of primary beam electrons with bound inner shell electrons during their penetration into the matter. Subsequent deexcitation by transition of outer shell electrons to the inner shell vacancy results in emission of an element specific X-ray quantum [6]. These characteristic X-rays are essential for the determination of the elemental composition of a specimen. The electron penetration depth depends mainly on the primary beam energy and the target composition. The X-ray production range also depends on the critical excitation energy of the specific X-ray line (e.g. K or L) and is always smaller than the electron range. The electron range is the travelling distance from the primary beam electron incident at the target surface to the point where the electrons lost their energy by multiple interaction processes within the material. Figure 1 illustrates the different kinds of electron interactions with specimen and the according electron detectors.

Figure 1.

Different types of electron interactions with specimen and related detection modes.

2.4. Imaging modes

Secondary electrons (SE), backscattered electrons (BSE) and transmitted electrons (TE) are responsible for three different imaging modes in SEM. The fourth type of electron interaction is the generation of X-rays and, hence, not a typical imaging mode.

The 3 imaging modes are explained as follows:

• Backscattered electrons provide contrast based on atomic number (Z-contrast) and density. The BSE-detector will be used as a quick response to visualize materials heterogeneously composed and distributed. It will be applied often in combination with the EDX- unit to capture images for subsequent microanalysis. Depending on the primary beam energy, backscattered electrons are created inside the specimen by elastic scattering. They possess approximately the energy of the primary electrons. As a rule of thumb, the exit depth of the BSE is half the primary electron range. As a consequence, the imaging resolution achieved by backscattered electrons is worse than those achievable by secondary electrons.

• Low energetic (< 50 eV) secondary electrons are used for true surface imaging, because they are created in the vicinity of the primary beam impact on the target surface. They are responsible for high-resolution imaging (in-lens SE detection).

• Transmission-type images (also called T-SEM mode) are obtained by a special mounting device for TEM grids with a diode-type detector beneath. The detector is mounted like a specimen stub instead and also adjusted to the electron beam. The specimen must be sufficiently thin (< 200 nm) to permit penetration of beam electrons at 20-30 kV, which is the typical operation voltage in T-SEM mode. Imaging of mass-thickness contrast is enabled. The application of the TE- detector can provide additional information on the sub-surface structure of many particles which cannot be resolved clearly in the corresponding SE and BSE images. The thin-film-supported specimen permits imaging with enhanced contrast due to an improved signal-to-noise ratio and X-ray analysis with reduced scattering background [7], [8].

2.5. Specimen preparation for SEM / T-SEM

The specimens are fixed with conductive tabs onto Al-stubs as bulk specimen for conventional SEM imaging. For T-SEM imaging, the samples are suspended in ethanol, and a droplet is poured onto a carbon-filmed TEM-grid (400 meshes Cu, ca. 8 nm C) for transmitted electron measurements.

2.6. SEM equipment

The electron microscopic investigations are carried out with a DSM 982 Gemini, Zeiss corp., Germany, equipped with a 4-quadrant solid-state BSE detector, a high brightness in-lens SE detector and a lateral SE detector. The DSM 982 GEMINI is applied with a thermal (Schottky-type) field-emission electron source (SFE). Element-specific quantification is performed by the dedicated EDX unit, equipped with a 30 mm² Si(Li) detector INCA Pentafet, FWHM 129 eV @ MnK (Oxford corp., England).

2.7. O₂-TPD

O₂-TPD analysis is performed using a BELCAT-B (BEL INC. Japan) system. It is coupled with a GAM 400 quadrupole mass spectrometer (In Process Instruments, Germany) as a mass-selective detector. The samples are subjected to a preliminary in-situ treatment in an O₂ / He test gas mixture. For this purpose, the sample is heated to 250°C in steps of 5 K/min and then kept at 250°C for 1 h. After this, it is cooled down to room temperature (RT). At RT, the non-adsorbed O₂ is removed by rinsing with He. In the subsequent O₂-TPD experiment the pre-treated samples are heated linearly in He. A thermal conductivity detector (WLD) is applied. The quadrupole mass spectrometer determines the composition of the desorbed products.

2.8. N₂-BET

The BET surface area (S. Brunauer, P. H. Emmett, and E. Teller) is determined with N₂ (77 K) (BELSORP-mini II, BEL INC. Japan). All samples are subjected to a preliminary treatment in a vacuum at 200°C for 5 h. To determine the sorption isotherms, the amount of molecules adsorbed on the samples are measured as a function of the relative pressure p/p₀. The specific surfaces are calculated from the adsorption parts of the isotherms at 77 K in the relative pressure range from 0.01 to 0.35 p/p₀ with N₂.

3. Discussion

3.1. Electron microscopy affecting parameters and MC simulation

FESEM and the dedicated EDX unit are important aids in the determination of the relationship among catalyst particle (cluster) sizes, dispersion onto support, support morphology and the influence of promoters. For a better understanding it makes sense to calculate the practical electron range by the Monte Carlo simulation program MOCASIM [9]. Additionally, the depth of the X-ray generating region (X-ray range) is estimated for the materials under consideration. It is based on an analytical expression useful for most elements and is calculated by the equation 1 of Andersen-Hasler:

R=0.064(E01.68Ec1.68)

(1)

Whereby R (µm) is the X-ray range, E₀ (keV) is the primary electron (beam) energy, E_c (keV) is the critical excitation energy for the characteristic (analytical) X-ray line and is the density of the elements. The dimension of the primary X-ray generation volume is important for the information depth obtained. It depends on the beam energy and the X-ray line chosen for the measurement [10]. The dependency of the practical electron range (penetration depth) and the X-ray range on the material density and beam voltage (beam energy) of the catalyst materials Ag, Al, and Al₂O₃ is depicted in Tab. 1. The higher the density of the element or specimen, the less is the penetration depth of the electrons (practical electron range) and the X-ray range (information depth). As a consequence, the material should be ideally homogeneous over the electron range to minimize errors caused by specimen heterogeneity. The critical excitation energy is determined by the specific absorption edge for the electron shell of an element from which the analytical X-ray line will be emitted. Therefore, E_c is always slightly higher than the corresponding X-ray line.

Specimen	/ g cm-3	X-ray line / keV	Ec / keV	HV / kV	pract. Electron range / nm	X-ray range / nm
Al	2.7	1.487 (K)	1.56	5	361	304
10	1146	1085
15	2252	2192
20	3639	3585
Al2O3	3.5	-	-	5	278	235
10	884	837
15	1738	1691
20	2807	2766
Ag	10.5	2.984 (L1,2)	3.35	5	93	45
10	295	245
15	579	530
20	936	888

Table 1.

Parameters affecting the electron range and X-ray range. E_c is the critical excitation energy for the corresponding analytical X-ray line. The practical (pract.) electron range is calculated by MOCASIM.

The different penetration depths for heterogeneously composed materials will be overcome by using the T-SEM technique as it will be discussed later. As depicted in Figure 2 and Figure 3 the MC simulations elucidate the different excitation volumes for 5 keV and 20 keV beam energies. They interact with an Alox (Al₂O₃) support covered with an Ag cluster that is assumed to be 100 nm in thickness. It demonstrates the differences in the electron ranges as a function of beam energies and material density whereas the relative atomic number Ag (107.8) and the relative molecular weight Al₂O₃ (101.9) are close together. Remarkable is the beam spreading effect caused by multiple scattering in the bulk specimen, which leads to a pear-like shape of the electron trajectories (interaction volume). This will be drastically reduced by using thin supporting foils and thinning of the specimen or using sufficiently small particles, depending on the beam voltage and density of the specimen (see Figure 10).

Figure 2.

MC simulation of electron trajectories (interaction volumes) created by primary beam energy of 5 keV. Note that N is equal to the number of used trajectories (100) for MC simulation. NR corresponds to the number of BS-electrons in case of Ag (38) and Alox (Al₂O₃, 13).

Obviously, specimens that are heterogeneously composed of materials strongly different in density and atomic number require a careful X-ray analysis. Nevertheless, due to the irregular shape of the analyzed particles, the systematic error of EDX measurements is increased compared to that of a homogeneous specimen with flat and smooth surfaces. Commonly, the commercially available "standard less analysis software" are adjusted to ideally smooth samples. Usually, the EDX analysis results are expressed in weight-percent (wt.-%), in which the collected X-ray counts (intensity = f(element conc.)) will be converted into concentrations by means of an evaluation program taking into account fundamental X-ray parameters and the detector efficiency. The procedure is known as "standard less analysis" and is already established by the suppliers. To reduce errors resulting through different X-ray exit and scattering angles due to the different particle sizes, irregular dispersion, or agglomerated catalyst deposits, a huge population of particles should be chosen for elemental analysis of bulk specimen. To obtain e.g. the amount of metal covering the support, data collection at low magnification as demonstrated in Fig.4. should preferably be performed.

Figure 3.

MC simulation of electron trajectories created by a primary beam energy of 20 keV. Note the progress in the interaction volumes compared to Figure 2. N is equal to the number of used trajectories (100) for MC simulation. NR corresponds to the number of BS-electrons in case of Ag (32) and Alox (Al₂O₃, 13).

Figure 4.

Magnified (100 x) BSE image of a large population of Al₂O₃ particles differently covered with Ag, prepared on a conventional specimen stub.

Figure 5.

High-magnification (30,000) image of a single Al₂O₃ particle covered with different sized Ag clusters, prepared on a conventional specimen stub.

Therefore a great number of particles should be included in the analyzed region to average the influence of different shaped and sized particles. As an example, a low-magnified, large number of supporting alumina (Alox = Al₂O₃) particles partially covered with Ag is shown in Figure 4 and produced under conditions mentioned in chapter 2.2. The magnification of 100 x is recommended for an EDX analysis to determine the amount of Ag deposited. The alumina particle diameter varies between 30 µm and 80 µm. For comparison, a highly magnified single alumina particle (sample D4) with differently sized and shaped Ag deposits is presented in Figure 5. It represents a high-magnification (30,000 x) image of a single substrate particle, covered with differently sized Ag clusters. The size range of the deposited Ag is between 20 nm and 850 nm.

3.2. EDX line scans

Figure 6 shows an EDX line scan which is simulated by MC calculation using a bulk specimen geometry with an Ag deposit of 100 nm in thickness onto an alumina (Alox) support. It reveals the course of Ag-L, Al-K and O-K X-rays across the specimen. The X-ray signals of Al and O dropped drastically down over the whole Ag cluster, contrarily the Ag signal becomes dominant. The lack in signal strength (intensity) is caused by absorption of the weak Al and O X- rays in the dense Ag particle. The maximum of the Ag signal, observed at the ends of the Ag deposit (edges) in the simulation is conspicuous. This is not observed in practice, only in the simulation.

Figure 7 is an example of an EDX line scan across the Ag agglomerate (approximately 620 nm) on a selected surface image of sample D6, which is generated by backscattered electrons. The line scan is shown in detail in Figure 8. One can see the pronounced maximum of the Ag signal and the minimum of the Al signal, whereas the minimum of the O signal is less distinct as expected. It is a hint for the presence of subsurface oxygen or Ag bulk-dissolved oxygen, which will discussed in chapter 3.6 in detail by means of O₂-TPD and BET. The comparison between the experimental EDX line scan of sample D6 (Figure 8) and the simulated line scan (see Figure 6) is feasible.

Figure 6.

MC simulation of an X-ray line scan (20 kV beam voltage) across a single particle deposited on an alumina (Alox) substrate.

Figure 7.

BSE image of sample D6 and EDX line scan across a selected Ag deposit.

Figure 8.

Line scan in detail of Figure 7.

To demonstrate the imaging power of the T-SEM mode an example of a very tiny single alumina particle (D4) partially covered with Ag deposits in comparison to a extended particle (Figure 5) is shown in Figure 9. The dense Ag clusters appear as dark spots on the nearly "electron-transparent" alumina matrix. The graphical representation of an MC calculation based on an Al₂O₃ (Alox) substrate of 92 nm in thickness and covered with a 20 nm thick Ag deposit on a TEM-grid support is depicted in Figure 10. It illustrates the transmitted electron trajectories with a strongly reduced excitation volume in contrary to Figure 3.

Figure 9.

Transmitted (TE) electron image (100,000 x) of an irregularly shaped single alumina particle (light grey) partly covered with Ag clusters (dark), prepared on a TEM-grid support.

The MC simulation of the electron beam trajectories in Figure 10 and the X-ray line scan in Figure 11 visualizes the interaction of primary electrons with matter of different densities and thickness on a thin-film support. The broadening of the beam (skirt) depends on the specimen thickness and density.

Figure 10.

MC simulation of the electron trajectories of an Ag deposit on an Alox (Al₂O₃) substrate, supported by an 8 nm carbon film (C) (TEM-grid). Note the broadening of the beam by passing the beam electrons through the matter. Backscattering events are drastically reduced (NR =1).

Figure 11.

X-ray line scan simulation of the thin film-supported (C) specimen. The course of the X-ray scan across the specimen is similar to that in Figure 6, but the intensity is decreased (indicated by the statistical fluctuations) due to the reduced beam interaction volume.

Thinner samples have better performance in resolution and contrast. In order to get highly magnified images even of tiny (sub nanometer) Ag particles, a TEM grid has to be prepared.

3.3. Ag distribution by EDX

In Figure 12 one can see the correlation of measured Ag concentration and the percentage of Ag area coverage visualized by image analysis (Figure 13). The image analysis is based on the intensity of the BSE signal.

Figure 12.

Ag area coverage derived from the BSE signal and the correlation to the Ag concentration measured by EDX.

Figure 13.

Color rendering of a BSE image. The yellow areas represent Ag deposits. The red colored areas show the matrix (Al₂O₃).

The yellow spots represent the Ag deposits, approximately 13.5% from the total area. It corresponds to approximately 11 wt.-% Ag measured by EDX. This procedure will not replace the numerical calculation by the evaluation program, but visualizes in a first approximation the dependence of the Ag concentration on the BSE contrast. The Ag distribution on the carrier material is discussed with regard to the penetration depth of primary beam electrons and lower limits of detection for X-ray analysis. It is crucial to catalytic activity that Ag particles cover the carrier material almost homogeneously. Sintering of Ag particles to Ag agglomerates has to be prevented [11]. Using sample D6 (20 % Ag / 1500 ppm Cs) as an example, the FESEM images (Figure 14) and EDX analysis results (Tab. 2) shall be discussed. The image reveals uniform grain sizes of the carrier material and variable coverage (bright areas) by Ag particles or Ag clusters. In the quadrants Q1, Q2, Q3, and Q4 of the SEM image of sample D6 the coverage of the carrier grains by Ag particles (Ag-clusters) varies (bright areas). The apparently uniform catalyst grains are assumed to result from the wet sieving of the carrier material (SLA92 and SC13). The EDX spectrum in Figure 15 reveals all significant elements Ag, Al, O, and Cs. Obviously, some of the grains are densely covered with Ag, others not. Figure 16 shows the shape of huge Ag agglomerates of the D6 sample in a higher magnification (30,000 x). There is no significant difference in the measured Ag surface content between the 4 quadrants in Figure 14, which indicates an "apparently homogeneous" distribution of Ag.

Figure 14.

SEM image of sample D6 (100 x, 20 kV) showing the 4 quadrants and the whole (integral) surface area analyzed by EDX (Tab.2).

Table 2.

Results (wt %) of the EDX analysis of sample D6 shown in the FESEM image Figure 14 (100 x, 20 kV).

Figure 15.

EDX analysis of sample D6 (Figure 14) revealing Ag, Al, O, and Cs as significant elements.

According to Tab. 2, the average Ag concentration is 15.1% (average of Int1, Q1 to Q4) and the Ag concentrations in the quadrants do not vary significantly (Q1 = 15.9 % Ag, Q2 =13.1 % Ag, Q3 = 15.2 % Ag, and Q4 = 16.4 % Ag). But the Ag concentrations doesn't correlate well with the absolute amounts of Ag (20 wt %) used for catalyst preparation. In general, the heterogeneous catalysts are influenced considerably by the penetration depth of the electron beam because of layer thickness, Ag cluster size, surface coverage, preparation performance and, under certain conditions, Ag bulk-dissolved oxygen (see chapter 3.6). Hence, the Ag values determined by EDX could differ from the absolute Ag concentrations of the prepared samples. The accuracy is strongly influenced by morphological effects and heterogeneity of the Ag coverage of the analyzed samples. Availability of reliable standards in this respect would be beneficial. In either case, for an absolute Ag determination a quantitative digestion should be carried out (4 wt% HF) and analyzed by ICP-AES (Inductive coupled plasma - atom emission spectroscopy) under appropriate conditions.

Figure 16.

Shape of huge Ag agglomerates of the D6 sample at 20 kV and 30,000 x.

3.4. EDX mapping

The dispersion of the Ag and Cs metal deposited on the catalyst support can also be characterized by the EDX mapping technique. Topographical and elemental imaging is possible at the same time. EDX mapping is a valuable tool to indicate the quality of the Ag dispersion. The electron beam is scanned pixel by pixel across a selected area of interest. In the following example, the element mappings for a selected specimen area (Figure 17, BSE image, bright spots indicating metal) revealed an inhomogeneous distribution of Ag (Figure 18) and Cs (Figure 19) of the catalyst support. Generally, the brighter the color appears, the higher is the concentration of the specific element. In both cases it correlates with the results in Tab. 3. Cs as a promoter facilitates Ag distribution on the -Al₂O₃ surface based on the high coverage and lack of crystallites with large contact angles [12]. The presence of Cs in the catalyst improves the distribution of Ag over the support and the Ag/Al interfacial area. Area P1 (see Figure 17 and Tab. 3), for instance, shows a remarkably high silver content of 66%, which is about twice as high as the average. The deviation from the average amount of Cs is not noticeable, because Cs is only present in small quantities (ppm). Nevertheless, the distribution of Ag and Cs can be shown very impressively by the time-consuming mapping. Spectra of the analyzed regions are shown in Figure 20. In summary it can be said that the inhomogeneity of metal distribution on the catalyst carrier material can be shown very clearly also for small amounts of metal. This example demonstrates a less quality of the preparation. One can see that the texture of the substrate SLA2 is very rough in comparison to SC31, which means that a homogeneous distribution of Ag and Cs is not so easy to achieve and requires an improvement in preparation. In case of an excellent preparation, this is a optimal Ag distribution for both catalyst systems, the question is which of the two catalyst systems is more favourable for the Ag-catalyzed epoxidation of 1,3-butadiene (1,3-BD) to 3,4-epoxy-1-butene (EpB) in the reactor.

Figure 17.

BSE image of a selected area of an Ag catalyst particle (MZ06). The marked (red) areas P1, P2, P3 and P4 represent analyzed regions.

Figure 18.

Map of Ag-L showing the inhomogeneous distribution of Ag. The Ag concentration correlates with the intensity of the green area (measuring time about 5 h).

Figure 19.

Map of Cs-L showing the inhomogeneous distribution of Cs. The Cs concentration is correlated with the intensity of the blue area(measuring time about 5 h).

Figure 22.

Spectra of the analyzed regions (see Figure 17).

The rate of a catalyzed reaction should be proportional to the surface area of the active agent. Therefore it is desirable to have the active phase in form of the smallest possible particle. But the most undesired contribution to the reduction of the active surface (deactivation) is sintering (welding together of particles by applying heat below the melting point). The function of the support is to increase the active surface and to reduce the rate of sintering of the metal particles. On the other hand, the interaction between the lattice oxygen of the carriers and the metal particles also influences the behaviour of the active metal agent. The support can modify the electronic character of the metal particle regarding to its adsorption and reactivity properties. Furthermore, the bond between the metal particle and the support can influence the shape of the metal particle (clusters). Both effects are so-called metal-support interactions (MSI). This effect decreases for supported Ag catalysts in the SiO₂ > Al₂O₃ > C sequence [2].

Spectrum	O	Na	Al	Si	Ca	Ag	Cs	Sum
P1	21.8	0.45	10.08	0.15	0.67	66.33	0.45	100
P2	54.1	0.32	31.16	0.00	6.62	7.60	0.18	100
P3	52.5	0.18	34.09	0.08	3.59	9.53	0.00	100
P4	38.6	0.28	19.59	0.04	2.16	38.32	0.92	100
average	41.8	0.31	23.73	0.07	3.26	30.45	0.39	100
max.	54.1	0.45	34.09	0.15	6.62	66.33	0.92
min.	21.8	0.18	10.08	0.00	0.67	7.60	0.00

Table 3.

All results in wt.-%, derived from the BSE image in Figure 17.

Another tool to elucidate the distribution of elements is the so called CAMEO imaging. Figure 21 represents a CAMEO image from the examined specimen which is based on BSE detection. CAMEO^TM is a tool to convert X-ray energies into visible wavelengths. In comparison to the X-ray mapping technique, the CAMEO procedure is much faster, but it may lead in some cases to a false color rendering (color overlap) caused by adjacent X-ray energies. As depicted in Figure 21 one can see that the brownish areas obscured the small Cs-spots. Therefore, Cs and Ag are not distinguishable because they are located at the same area of surface.

Figure 21.

CAMEO rendering of the area shown in Figure 17. The green colored areas represent Ag, whereas the brownish areas represent Al, respectively.

3.5. Remarks

All analytical results have been obtained at 20 kV acceleration voltages, which turned out to be the best choice for excitation conditions. Because the in-lens detector is switched off above 20kV, therefore high resolution SE imaging is disabled. The EDX results are determined by the manufacturer's spectrum evaluation software (Oxford corp.). Originally, all quantitative results are calculated with two significant fractional decimal digits provided with 1 errors, which includes the errors resulting from spectra processing (background subtraction, filtered least squares fitting, peak overlap). Notably, this may not reflect uncertainties caused by surface topography and other systematic influences. According to experience, the actual uncertainties are considerably higher, especially for light elements. We suggest for C and O a relative uncertainty of 5 20%, for all other elements 1-5% [13]. Another important issue is the estimation of the limit of detection (LOD), which can be calculated from a synthetic spectrum by the equation 2.

Where B is the number of background counts, 3 B^1/2 represents 3 error of background measurement, C is the concentration of the element, P corresponds to the number of counts in the X-ray line after background subtraction. In practice, calculation will be performed by a special program tool named spectrum synthesis, which is provided with the INCA-Energy evaluation software [14].

LOD 3 / w t.- %		I / nA	tm / s	U / kV
Ag	Cs
0.21	-	0.23	60	20
0.18	0.27	0.23	100	20
0.09	0.15	0.71	100	20
0.06	0.09	0.71	300	20

Table 4.

Calculated 3 LODs regarding to equation 2 and [14]. The matrix composition, except for alumina, is assumed to be 1.8 wt.-% Ca, 0.3 wt.-% Na, 0.2 wt.-% P.

For the 3 LOD estimation, the composition of the sample matrix and the acquisition parameters like measuring time t_m, beam voltage U, current I, detector parameters, X-ray take-off angle are required. Tab. 4 shows 3 LODs which are calculated for 3 different measuring times and aperture adjustments (beam current). For Ag, this is of minor importance, since the amounts of Ag are sufficiently high in comparison to the promoters.The pore size of the alumina support is in a similar order of magnitude as the X-ray generating range (see Fig. 1b), which leads to an uncertainty in the measurements in these regions. As a consequence, the EDX analysis is considered to be semi-quantitative on such Al₂O₃ supports. A loss of X-rays (Al-K and O-K) emitted from mesoporous media compared to that of dense monocrystalline alumina is described in literature [15]. It may be caused by charge effects due to the specific surface of particles and distribution of pores. In the course of our various measurements, no significant charging was observed. Compared to common inductively coupled plasma atomic emission spectroscopy (ICP-AES) the values of metal deposition determined by EDX are frequently higher. One explanation is a sometimes observable incomplete chemical digestion prior to ICP measurements. In the case of Al₂O₃ we recommend a digestion with HF (4%). As already mentioned, EDX with an electron penetration depth in the order of several nanometers up to microns is a technique for surface analysis, while the ICP technique is applicable to the quantitative determination of the bulk composition with the detection limits in the µg/l (ppb) range. Due to the incipient wetness impregnation technique applied here for surface preparation, the values of metal deposition analyzed by EDX are expected to be higher than those by ICP-AES [16]. The differences in the amounts of Ag are plausible. The reason therefore is that the D-samples and MZ-samples are completely chemically digested, which is a physical homogenization. In contrast, the sample preparation for FESEM/EDX is non-destructive, which means that the sample is not physically homogenized. Tab. 5 gives a selected overview about catalysts characterization methods applied on sample D1.

Promoter CsNO3 / wt.-%	ICP Ag / wt.-%	EDX Ag / wt.-%	BET m2g-1
0.15	4.46	5.76	6.03

Table 5.

A listing of commonly applied laboratory methods (ICP, EDX, BET) for characterization of a catalyst with nominal 5 wt.-% Ag, grain fraction 45-63 µm (sample D1)

3.6. O₂-TPD and BET measurement

To get a deeper insight it makes sense to combine SEM with O₂-TPD experiments and BET measurements, which additionally were carried out. The TPD experiment measures the temperature-dependent desorption rate of a molecule from the catalyst surface [17]. Typically, O₂ / Ag interactions are studied on the Ag monocrystal surfaces (110) and (111) [18]. This means that silver may storage O₂, with the amount adsorbed being dependent on temperature and O₂ partial pressure. O₂ dissociatively adsorbs on the Ag catalyst and may assume the following characteristic forms:

• Surface oxygen

• Subsurface oxygen

• Ag bulk-dissolved oxygen

Ag bulk-dissolved oxygen may act as a storage of converted surface oxygen and, hence, be supplied later on. Subsurface O₂ increases the coordination number of Ag surface atoms, which results in a smaller binding strength of surface oxygen and favourably influences the epoxidation reaction of 1,3-BD. The surface area has been corrected by the subsurface oxygen value. Comparison of the measured amount of O₂ desorbed with the amount theoretically required for an O₂ monolayer shows that the measured amount of desorbed O₂ is higher by a factor of 2. This may be explained by the presence of subsurface O₂ [19].

O₂-TPD studies reveal significant differences between MZ samples with an SLA92 carrier and D samples with an SC13 carrier for the same Ag contents. In general it can be said that the desorbed O₂ amounts of the MZ samples are below those of the D samples by a factor of 5. Moreover, the temperatures of the desorption maximums of the MZ samples are lower. This difference (binding strength of oxygen) results in a strongly variable reactivity of the adsorbed oxygen species. Figure 22 shows O₂ desorption and temperature of the samples D1, D3, and D5 with different Ag concentration on the support that is not doped with Cs. With increasing Ag concentration, the desorptions maxima are shifted towards lower temperatures (red line). This indicates that epoxidation already may start at lower temperatures. Figure 23 shows that the sample D6 doped with Cs reaches a higher temperature at the desorption maximum than the non-doped sample D5.

Figure 22.

O₂ detection (WLD) by O₂-TPD of the samples D1 (5 % Ag), D3 (10 % Ag), and D5 (20 % Ag) versus temperature (red line).

This indicates that the adsorbate is stabilized by the presence and the grade of distribution of Cs and Ag on the surface, which correlates with TPD results, FESEM, EDX mapping and the reaction performance. The determination of the specific surface areas of the catalysts D3, D4, D5, D6, MZ06 and MZ09 gives an important hint regarding to metal distribution of MSI effects. Tab. 6 lists the specific surface areas for the mentioned catalysts determined with N₂ as adsorptive. All isotherms are of the IUPAC type II (s-shaped) and, hence, can be evaluated according to the BET theory [20], [21]. The Cs-doped samples (D4, D6) have a slightly higher surface than the non-doped samples (D3, D5) with the same Ag content, which correlates to a higher desorption rate for O₂-TPD. In case of the samples MZ06 and MZ09 one can see that the difference in BET surface is not remarkable, even in the presence of different Ag amounts. The total error for BET measurement is about 0.1 m²/g.

Figure 23.

O2 detection (WLD) by O2-TPD of samples D5 (20 % Ag) and D6 (20 % Ag + Cs) versus temperature (red line).

Table 6.

Specific surface areas from sample D3 D6 and MZ06 and MZ09.

The BET reveals that there is a significant difference in the catalyst surface, which is expressed by the topological images of FESEM of the D samples (SC31) and MZ (SLA92) samples. Note: Very small amounts (0,1g) of sample are used for the investigations (FESEM, TPD). In the case of BET-Measurement the sample weight is in the range of 0.1 g up to 10 g, depending on the specific surface. The carrier material SC13 (high BET Surface) was not found to be suited for the epoxidation of 1,3-butadiene (Figure 24) on Ag particles. Already at 180°C a total oxidation of 1,3-butadiene to CO₂ and H₂O does occur, also when Cs-doped catalysts are used. In contrast to this, epoxidation of the Ag catalyst with SLA29 (less BET surface) carrier material results in an EpB selectivity of 74 % (200°C, SV = 2590 h^-1) at a 1,3-BD conversion rate of 15 % [22]. The constitution of the oxametallacycle intermediate is depicted in Figure 25. The guiding hypothesis is that surface oxametallacycles are key intermediate for epoxidation on Ag / Cs catalysts. The intermediate EpB(ads), finally leading to molecular EpB, is probably strongly adsorbed on the catalyst surface indicated by theoretically calculations which also support its identity as an oxametallacycle. The oxametallacycle intermediate is more thermodynamically stable than EpB by approx. 24 kcal/mol. Moreover, the transition state for EpB formation from the oxametallacycle intermediate is actually lower in energy than the reactants, butadiene and oxygen [23].

Figure 24.

Epoxidation of 1,3-butadiene over Ag / Cs catalyst on Al₂O₃ to 3,4-epoxy-1-butene.

Figure 25.

The "intermediate EpB" finally leading to molecular EpB. The oxametallacycle intermediate is more thermodynamically stable than EpB. Moreover, the transition state for EpB formation from the oxametallacycle intermediate is actually lower in energy than the reactants, butadiene and oxygen

Thank you for reading article Catalyst Characterization with FESEM/EDX by the Example of