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The influence of working pressure on the mechanisms of the CO 2 /H 2 reaction on a co-precipitated CuO/ZnO/Al 2 O 3 catalyst have been studied at 230°C and in the pressure range of 1-75 bar. In the CO 2 hydrogenation using CuO/ZnO/ Al 2 O 3 , the products were found to be CO, methanol and water almost exclusively. Only a trace of methane formation was observed. Methanol and carbon monoxide are competitively formed. The former is produced directly from CO 2 whatever the pressure whereas carbon monoxide stems either from CO 2 directly at high pressure or both methanol decomposition and CO 2 directly at low pressure.

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The catalytic dehydrocyclization of n-hexane was studied here for the first time using a number of compounds based on H₃PMo₁₂O₄₀. The described catalysts were prepared by either replacing the acidic proton with counter-ions such as ammonium or transition metal cations (NH₄⁺, Fe³⁺, K⁺), or by replacing Mo⁶⁺ with (Ni³⁺, Co³⁺, Mn³⁺) in the polyoxometalate framework, as reported earlier. For comparison purposes, the known (TBA)₇PW₁₁O₃₉ catalyst system was used. All reactions were conducted at different temperatures in the range 200 - 450. The Keggin structure of these heteropolycompounds was ascertained by XRD, UV and IR measurements. ³¹P NMR measurements and thermal behaviour of the prepared catalysts were also studied. These modified polyoxometalates exhibited heterogeneous superacidic catalytic activities in dehydrocyclization of n-hexane into benzene, cyclohexane, cyclohexene and cyclohexadiene. The catalysts obtained by substituting the acidic proton or coordination atom exhibited higher selectivity and stability than the parent compound H₃PMo₁₂O₄₀. Catalytic activity and selectivity were heavily dependent on the composition of the catalyst and on the reaction conditions. At higher temperatures, the catalyst exhibited higher conversion efficiency at the expense of selectivity. Using higher temperatures (>400) in the presence of hydrogen carrier gas, selectivity towards dehydrocyclization ceased and methane dominated. To explain the results, a plausible mechanism is presented, based on super-acidic nature of the catalyst systems.

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Heteropolyacids of Keggin structure, H 3 PMo 12 O 40 , H 3 PMo 11 WO 40 and the salts K 3 PMo 11 WO 40 and K 2.5 Fe 0.08 H 0.26 PMo 11 WO 40 were characterized by X-ray diffraction (XRD), UV-Vis spectroscopy, Fourier transform infrared (FTIR), low-temperature nitrogen adsorption and 31 P MAS NMR spectroscopy. The acid-base properties were evaluated using the isopropanol decomposition. They were tested in oxidative dehydrogenation (ODH) of ethylbenzene in the temperature range 300-400 • C at atmospheric pressure with the mild oxidants carbon dioxide and nitrous oxide, the major actors in the greenhouse effect. The results show that the best compromise between conversion and selectivity is obtained for the mixed K/Fe salt of PMo 11 WO 40 at a relatively low temperature, namely 350 • C.

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