Influence of the structure of alumina–aluminium phosphate (AAP) catalysts containing Fe3+ and Cr3+ on their catalytic activity for isopropyl alcohol decomposition

Abstract

Phosphate catalysts of the general formula Al₁₀₀P_xM₂₀, where M = Al, Cr, Fe and x= 0, 4.5, 9, 18, 36, 72 and 144, have been prepared by coprecipitation of the corresponding nitrate salts and phosphoric acid with ammonia at pH 9.5. The resulting solids, after characterization by surface area and XRD measurements, were checked for their total surface acidity (Brönsted and Lewis) by pyridine titration and also tested for their catalytic activity towards the isopropanol dehydration. It was found that for the Al₁₀₀P_xAl₂₀ solids the surface density of the acid sites increases on addition of phosphorus in a fairly regular manner. Substitution of some Al by Cr or Fe blurrs the picture of increasing acidity described above. The catalytic activity for isopropyl alcohol dehydration, calculated as the number of molecules decomposed per acid site per second, shows a minimum at x= 18–36 for Al₁₀₀P_xAl₂₀ and Al₁₀₀P_xFe₂₀, while for Al₁₀₀P_xCr₂₀ a continuous increment with x is observed. The activity minima of the first two sets of solids coincide with the total absence of crystallinity as detected by XRD measurements. On the contrary, the continuous increment of activity for the Cr-containing catalysts is related to the presence of crystalline α-Cr₂O₃. This behaviour is attributed to competition between the diffusional and chemical steps controlling the reaction's process. On the amorphous solids, where high values of activation energy and decreased activity are observed, the chemical reaction seems to be the slow and rate-determining step, while diffusion is fast because of the large porosity of the solids. The other samples show activation energies that are half those of the amorphous solids and larger activities. The controlling step in these cases seems to be diffusion, a fact related either to smaller pores or larger rate constants.