Adam Bielański

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.

ESR investigation of the redox processes occurring upon oxygen and propylene interaction with V2O5

Abstract ESR and chemical analysis were used to investigate the changes in reduction degree of vanadium pentoxide upon interaction with oxygen and propylene. It may be derived from our measurements that the redox processes accompanying interaction of oxygen at temperatures not exceeding 200 °C and propylene at temperatures up to 100 °C with partially reduced V 2 O 5 are restricted to the surface layers only. Assuming a model of two consecutive reactions of the first order the rate constants of the two-step oxidation processes, V 3+ → O 2 V 4+ → O 2 +V 5+ , and two-step reduction processes, → C 3 H 6 +V 4+ → C 3 H 6 V 3+ , were calculated by means of computer optimization. The kinetic data indicate that the activation energies of these reactions are 6.3 and 21 kJ/mol for the first and second steps of oxidation whereas for the first and second steps of reduction they are 33 and 59 kJ/mol, respectively. At temperatures exceeding 200 °C the rate of vanadium oxidation is determined by diffusion, the apparent activation energy of this process being 126 kJ/mol.

Kinetics of reduction of iron catalysts for ammonia synthesis

Abstract The reduction of industrial iron synthetic ammonia catalysts by a stoichiometric mixture of hydrogen and nitrogen has been investigated. The reduction was studied in the temperature range 450–550 °C and for several grain sizes using a spring microbalance in a flow system. It was found that the results satisfy the Seth and Ross kinetic equation for a mixed-control mechanism in which neither the surface reaction nor the diffusion determines the reaction rate. The parameters of the Seth and Ross equation were calculated by a least squares method.

The formation of MTBE on supported and unsupported H4SiW12O40

Abstract Sulphonated resins used in the industry as the catalysts in the synthesis of methyl tert -butyl ether (MTBE) until now, the most important anti-knocking additive to the gasoline, are not satisfactory because of environmental reasons and catalyst’s not adequate stability. This stimulates the search for new catalysts for this process. Among others heteropoly acids (HPA) are considered to be prospective ones. The present paper is the review of the authors’ investigations on physical chemistry of the MTBE synthesis in gas phase on dodecatungstosilicic acid H 4 SiW 12 O 40 (HSiW). It has been preceded by the study of the interaction of both substrates, methanol and iso -butylene and the product MTBE, with crystalline HSiW comprising classical sorption experiments, FTIR and calorimetric measurements. Together with reaction kinetic measurements it enabled to propose the mechanism of the catalytic reaction according to which iso -butylene not penetrating the bulk of HSiW crystallites gets protonated at the surface with the loosely bonded protons (protons forming hydrogen bond between HSiW anions) and subsequently reacts with methanol from next-to-surface layer or from the gas phase. Methanol penetrating into the bulk is forming protonated clusters thus diminishing the concentration of loosely bonded protons. At high enough methanol concentration this leads to the diminishing of the formation rate of carbocation. In the result reaction order positive at the initial state of catalytic process (low bulk concentration of methanol and high concentration of loose protons) turns into negative at the reaction steady state. A set of kinetic equations has been proposed describing all these processes. A series of oxide supports: SiO 2 , TiO 2 , SiO 2 -Al 2 O 3 , γ-Al 2 O 3 and AlPO 4 with increasing basicity (characterised by the net oxygen negative charge calculated according to Sanderson’s theorem) was used for the preparation of HSiW supported catalysts. It has been shown that the activity decreases with the increasing basicity as a result of strong bonding protons by the surface of the support.

Segregation in vanadia-molybdena catalysts in the course of oxidation and reduction processes

Segregation of vanadia-molybdena catalysts in the course of oxidation and reduction processes was determined by X-ray microprobe analyses of 10 3< and 30 3< MoO/sub 3//V/sub 2/O/sub 5/ samples prepared by fusion at 700/sup 0/C. The same areas of the crystallites were analyzed as prepared, after air-oxidation at 450/sup 0/C, and after reduction with benzene at 450/sup 0/C. Homogeneity remained unchanged during treatment, but the molybdenum/vanadium ratio of the outer crystallite layers decreased during oxidation and increased during reduction. If the concentration of molybdenum trioxide in the crystallite interior during oxidation exceeds its solubility limit in vanadium pentoxide, it would form V/sub 9/Mo/sub 6/O/sub 40/, which would first be redissolved on reduction. As reduction continues, molybdenum would migrate to the surface faster than vanadium and would form V/sub 9/Mo/sub 6/O/sub 40/ when its surface concentration exceeds the solubility limit. Such processes lead to the phase segregration of vanadia-molybdena catalysts observed during benzene oxidation to maleic anhydride.

Thermal stability of giant wheel type polyoxomolybdates and their derivatives

The paper compares the thermal behaviour of hydrated giant ring polyoxomolybdates: sodium and ammonium salts as well as sodium polyoxomolybdates containing methanol. Two stages of dehydration are observed: release of water of crystallization reaching its maximum rate at about 70 °C and release of water of constitution above 280 °C leading to a collapse of the ring structure and formation of a new solid phase.