J. Poźniczek

Oxide supports for 12-tungstosilicic acid catalysts in gas phase synthesis of MTBE

Abstract A series of simple and mixed oxides: SiO 2 , AlPO 4 , SiO 2 -Al 2 O 3 , TiO 2 , kaolin and γ-Al 2 O 3 differing by their basicities as characterised by effective negative charge on oxygen calculated according to Sanderson’s theorem was used for the preparation of supported H 4 SiW 12 O 40 catalysts. Using gas phase synthesis of MTBE as the catalytic test reaction it has been shown that on the supports of highest basicity, i.e. γ-Al 2 O 3 and kaolin the heteropolyacid was decomposed resulting in the catalyst which exhibited only low activity. On the other hand, the least basic oxide SiO 2 gave the most active catalysts. However, the sequence of activities differed from that of basicities by the fact that the activity of TiO 2 -supported catalyst was at the coverage with H 4 SiW 12 O 40 Θ =0.25 nearly as high as SiO 2 supported one and at Θ =1.0 TiO 2 -supported catalyst was the best. This indicates that the basicity of the support is only one of the factors determining the activity of supported heteropolyacid catalysts in acid–base type reactions. On the other hand good correlation was obtained between the catalytic activity and the neutralisation heat of the acid sites on the surface of catalyst which was determined using thermometric titration with n -butylamine solution in toluene.

Catalytic conversion of ethyl alcohol on polyaniline protonated with 12-tungstosilicic acid

Abstract It has been demonstrated that protonation of polyemeraldine base with 12-tungstosilicic acid leads to the preparation of a new, polymer supported catalyst which shows significantly higher activity and altered selectivity in ethyl alcohol conversion as compared to crystalline 12-tungstosilicic acid studied in the same experimental conditions. The improved activity is associated with higher dispersion of Keggin units bonded to the polymer surface via protonation reaction which in turn leads to their improved thermal stability as shown by DTA, FTIR and other studies.

Gas phase synthesis of MTBE on dodecatungstosilicic acid as the catalyst

Abstract Catalytic synthesis of methyl-tert-butyl ether (MTBE) in gas phase on solid H4SiW12O40 was studied at 40 and 80°C using differential constant flow reactor. Independently sorption of substrates, methanol and isobutene, as well as the product MTBE was studied using sorption balance. From the fact that methanol was easily sorbed by the whole volume of the solid (depending on pressure its uptake at 40°C reached up to 12 CH3OH molecules per 1 Keggin unit (KU)) while isobutene remained only adsorbed at the surface of heteropolyacid both pristine and saturated with methanol it was concluded that catalytic reaction occurs at the surface of H4SiW12O40 and reaction scheme has been proposed including the formation of tert-butyl carbenium ion with the participation of protons supplied by the catalyst. The assumption that reaction of this carbocation with methanol supplied also from the bulk or next-to-surface layer is the rate determining step led to the kinetic equations enabling to interpret the observed negative reaction order with respect to methanol at the steady state at 40°C but about one at initial period of the run. Reaction was characterised by the apparent activation energy as low as 25 kJ mol−1. At 80°C reaction order with respect to isobutene was one but that with respect to methanol decreased to about 0.5 indicating that reaction was diffusion controlled.

Polyaniline doped with heteropolyanions: Spectroscopic and catalytic properties

Abstract Acid-base properties of heteropolyacids make it possible to introduce them quite easily into a polyaniline matrix via classical acid-base doping. The present work shows that a new type of polymer-supported catalyst can be obtained in this way. It has been discovered that such catalysts are active in ethyl alcohol conversion. Of the two methods of synthesis studied, i.e. preparation of polyaniline and its protonation in two separate steps and the one-step procedure, the former results in more catalytically active products. This is presumably caused by the hindered diffusion of heteropolyanions between polymer chains in the former case. Thus, they stay mainly at the surface of the polymer matrix and are more easily accessible for the alcohol.

Catalytic reactions of ethanol on H3+xPMo12−xVxO40 catalysts

The catalytic decomposition of ethanol on heteropolyacids (HPA) of the series H3+xPMo12−xVxO40 (x=0,1,2,3) was investigated at 240°C, using a pulse method. It has been shown that besides ethyl ether, ethylene, acetaldehyde and unreacted alcohol, a certain amount of alcohol was irreversibly sorbed by HPA. The amount of the latter decreased with x, which is considered to be due to the tighter bonding of Keggin units (KU) with an increasing number of intramolecular hydrogen bonds. The yield of ethyl ether forming at the surface was almost constant but that of ethylene and acetaldehyde decreased in the series x=0,1,2, which was connected with the increasingly difficult penetration of HPA molecules into the bulk. Some differences in the behavior of the sample with x=3 were the result of partial decomposition of this thermally stable catalyst.AbstractС помощью импульсного метода изучали каталитическое разложение этанола на гетерополикислотах серии H3+xPMo12−xVxO40 (x=0, 1, 2, 3) при 240°C. Показано, что наряду с этиловым эфиром, этиленом, ацетальдегидом и непрореагировавшим спиртом, определенное количество спирта необратимо сорбируется гетерополикислотой. Количество последнего уменьшается с уменьшением х, что полагают следствием более сильного связывания единиц Кеггина с увеличивающимся количеством внутримолекулярных водородных связеи. Выход этилового эфира, образующегося на поверхности, был почти постоянным, а выход этилена и ацетальдегида уменьшается в серии x=0, 1, 2, чтя связывают с увеличивающимися затруднениями внедрения молекул гетерополикислот в массу. Некоторые расхождения в поведении образцов с x=3 являлись результатом парциального разложения этих термически стабильных катализаторов.

The role of protons in acid–base type reactions on heteropolyacid catalysts: gas-phase MTBE synthesis on H4SiW12O40

Basing on the kinetic and sorption experiments a mechanism of catalytic formation of methyl-tert-butyl ether (MTBE) on H4SiW12O40 was proposed. A reaction intermediate, carbenium ion C4H9+, is forming at the catalyst surface from isobutene supplied from gas phase and protons supplied by the solid. Subsequently C4H9+ reacts with methanol absorbed by the solid heteropolyacid. The model satisfactorily explains the anomalous reaction order with respect to methanol which under the applied conditions is positive at the initial stage of the catalytic process and becomes negative at its steady state.

DTA and TG analysis of heteropolyacids of the series H(3+x)PMo(12−x)VxO40·nH2O

DTA, TG and DTG of molybdovanadophosphoric acids of the series H(3+x)Mo(12−x)VxO40· nH2O (x=0, 1, 2, 3) were carried out. The samples with the highest content of water of crystallization (n=19–34) fuse above 40 °C, giving solutions of heteropolyacids which boil at 110 to 130 °C. The total loss of crystallization water occurs below 170–200 °C. The anhydrous acids decompose (dehydroxylation) at temperatures decreasing from the average value of 433 °C forx=0 to about 293 °C forx=3. For these vanadium-containing samples, an exothermic peak not accompanied by weight change appears at about 450, 430 and 390 °C forx=1,2 and 3, respectively, indicating a transformation in the solid state. Ifx=0, a weaker endothermic effect of dehydroxylation overlaps with a stronger exothermic effect, and only one exothermic effect exists, at 435 °C.ZusammenfassungAn Molybdovanadophosphorsäuren der Formel H(3+x)Mo(12−x)VxO40· H2O (x=0, 1, 2, 3) wurden DTA-, TG- und DTG-Untersuchungen durchgeführt. Die Proben mit dem größten Kristallwassergehalt (n=19–34) schmelzen oberhalb 40 °C und liefern damit Lösungen von Heteropolysäuren mit einem Siedepunkt zwischen 110 und 130 °C. Der vollständige Verlust des Kristallwassers erfolgt unterhalb 170–200 °C. Die wasserfreien Säuren zersetzen sich (Dehydroxylierung) bei Temperaturen mit einem Durchschnittswert von 433 °C fürx=0 bis herab zu 293 °C fürx=3. Bei diesen vanadiumhaltigen Proben tritt fürx=1, 2 und 3 bei den Temperaturen 450, 430 und 390 °C ein exothermer Peak auf, der von keinem Gewichtsverlust begleitet wird und somit auf eine Strukturänderung des festen Zustandes hinweist. Beix=0 wird ein stärkerer exothermer Effekt von einem schwächeren endothermen überlagert und es existiert bei 435 °C nur ein exothermer Effekt.РезюмеПроведены ДТА, ТГ и ДТГ исследования молибденванадофосф орных кислот общей формулы H3+xMo12−xVxO40·nH2O, гдеx=О, 1, 2, 3. Образцы с наибольшим содержан ием кристаллизационной воды (n=19–34) плавятся выше 40°, давая растворы гетерополи кислот с температурами кипен ия 110–130°. Безводные кисл оты разлагаются (дегидро ксилирование) при среднем значении тем ператур от 433° для х=0 до 293° для х=3. Для этих ванадийсоде ржащих образцов экзотермич еский пик, наблюдаемы й, соответственно, при 340,430 и 390° дляx=1,2,3, не сопровождается по терей веса, что указыв ает на фазовое превращение в твердом состоянии. Приx=0 более слабый энд отермический эффект дегидроксилировани я перекрывается боле е сильным экзотермиче ским эффектом, вследс твии чего он только и наблюдается при 435°.

Synergetic effect of silver and molybdenum (VI) oxide additives in vanadia catalysts for the selective oxidation of benzene

It has been shown that the selectivity of V2O5−MoO3 catalysts for benzene oxidation to maleic anhydride can be enhanced by small amounts of silver introduced by impregnation with AgNO3 solution. The effect of Ag2O additive is synergetic and increases with increasing concentration of MoO3 up to 28.5 mol %. The best effect was obtained when about 7 Ag atoms were introduced for 1000 (V+Mo) atoms at the surface.AbstractПоказано, что селективность катализаторов V2O5−MoO3 в окислении бензола до малеинового ангидрида повышает-ся добавлением небольших количеств серебра пропиткой растворами AgNO3. Эффект добавки Ag2O синергичен и увеличивается с увеличением концентрации MoO3 вплоть до 28,5 мол. %. Наилучший эффект был достигнут, когда около 7 атомов Ag было введено на поверхность с 1000 атомами (V+Mo).

Effect of dopant concentration on the catalytic properties of H4SiW12O40-polyaniline system

Ethanol conversion on polyaniline supported H4SiW12O40 was investigated at 443 – 483 K. Catalytic activity of the samples expressed as conversion rate per 1 m2 exhibits a maximum at H4SiW12O40 content, equal to about 30 wt.%. The proposed interpretation of these effects is given.

Ethanol conversion and coke formation on TiO2 supported molybdovanadophosphoric acids

The catalytic conversion of ethanol on H3+xPMo12−xVxO40/TiO2 catalysts (x=0,1,2) yielding both the products of alcohol dehydration (ethylene, diethyl ether) and dehydrogenation (acetaldehyde) but also ethane as secondary product is accompanied by the formation of coke. The amount and composition of coke was estimated on the basis of mass balance. The fact that the hydrogen/carbon atom ratio was low (∼0.6) supports the conclusion that coke is involved in the hydrogenation of primary ethylene.