A. Bielański

Physicochemical and catalytic properties of ployaniline protonated with 12-molybdophosphoric acid

Using acid–base-type doping of polyaniline with H3PMo12O40, two series of catalyst samples were prepared. In series Sl the doping and polymerization were carried out simultaneously, leading to a uniform distribution of the dopant over the whole volume of the polymer. In series SII the doping was achieved by protonation of already formed polyemeraldine base with H3PMo12O40. Since this process must involve diffusion in the solid matrix, the surface becomes enriched in heteropolyanions (Keggin units) due to their limited diffusivity. The examination of physicochemical properties of the samples by XP, EPR and FTIR spectroscopies indicated that the Keggin units incorporated into the polymer matrix retain their identity and represent catalytically active centres. The insertion of the dopant species increases the electrical conductivity of the polymer by several orders of magnitude. The catalytic activity was tested in ethyl alcohol conversion and both the products of acid–base [C2H4 and (C2H5)2O] and redox (CH3CHO) type reactions were observed. Owing to the surface enrichment of the SII series of catalysts their catalytic activity was much higher than that of samples of the Sl series nominally doped to the same level. In both types of catalysts the selectivity to the redox reaction product (CH3CHO) was greatly enhanced in comparison with unsupported H3PMo12O40.

Infrared study of the thermal decomposition of heteropolyacids of the series H3+xPMo12–xVxO40

F.t.i.r. spectroscopy has been used for the study of dehydration of heteropolyacids (HPA) of the series H3+xPMo12–xVxO40 carried out in vacuo at 298–673 K and also for the study of ammonia sorption on the same samples. Pumping at room temperature results in the departure of water of crystallization; the protons, which form H5O+2 ions in the hydrated samples, are involved in the formation of hydrogen bonds between the HPA anions (Keggin units) characterized by a band at 3250 cm–1. The intensity of this band progressively decreases with increasing temperature of outgassing, indicating the departure of the ‘water of constitution’. No bands were present for OH groups that were not involved in the formation of hydrogen bonds. Within the range 400–1100 cm–1 the bands characteristic of Keggin unit vibrations were observed. The substitution of Mo by V atoms results in the shift of most of these to lower frequencies. The formation of hydrogen bonds influences most strongly the vibrations of MoO groups, hence these terminal oxygen atoms are involved in the formation of such bonds. Ammonia is sorbed in the form of NH+4 ions, thus confirming the presence of Bronsted-acid sites. No coordinatively bound ammonia was observed. Formation of NH+4 ions influences most strongly the vibrations of MoO and Mo—Ob—Mo groups, but the Mo—Oc—Mo groups (Ob is an oxygen atom at the common apex of two [MoO6] octahedra, Oc is an oxygen atom at the common edge of two [MoO6] octahedra) indicating the position of NH+4 ion in the immediate vicinity of three terminal and three Ob atoms.

FTIR study of hydration of dodecatungstosilicic acid

The dehydration of H4SiW12 O40·15.6 H2O was studied in situ in the IR chamber. On evacuation at room temperature the departure of most loosely bonded water characterized by bands at 3550 and 1616 cm−1 was observed. In the remaining hexahydrate the band at 3445 cm−1 was ascribed to the hydrogen bond between the Od oxygen atom of the Keggin unit and dioxonium H5O2+ ion, the presence of which is manifested by the 1710 and 1100 cm−1 vibrations. All these bands vanish in the case of anhydrous H4SiW12O40, in which the band at 3106 cm−1 ascribed to the hydrogen bond between neighbouring HPA anions Od−H+−Oc is still present. The dehydration of hexahydrate is accompanied by splitting of the W=Od band into 987 and 1010 cm−1 reflecting the change of the kind of hydrogen bond in which the Od oxygen atom is involved. Based on the above results it was concluded that protons forming oxonium ions in hydrated solid heteropoly acid are more strongly bonded than those in anhydrous one which are forming hydrogen bonds between neighbouring Keggin units.

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.

XPS study of polyaniline supported dodecatungstosilicic acid catalyst

Abstract The presence of hydrogenoheteropolyanions at the polyaniline surface supported H 4 SiW 12 O 40 catalyst playing most probably the role of catalytically active Bronsted acid centers in cumene cracking was confirmed by the XPS measurements. These centers are partially removed by heating at 573 K in the atmosphere of dry air but easily regenerated by the water vapor introduced into the reactor in the stream of helium carrier gas.

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 hydrogenation of C60 fullerene

Catalytic hydrogenation of C60 with H2 or by hydrogen transfer reactions using Pd/SiO2, Rh/Al2O3 and Ru/Al2O3 has been studied. The final products containing partially hydrogenated C60 fullerence C60H42−C60H46 were characterized by FTIR, UV and NMR methods.

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.