Influence of the Structure of Alumina Aluminum Phosphate (Aap) Catalysts Containing Fe-3(+) and Cr-3(+) on Their Catalytic Activity for Isopropyl-Alcohol Decomposition

 
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Influence of the Structure of Alumina Aluminum Phosphate (Aap) Catalysts Containing Fe-3(+) and Cr-3(+) on Their Catalytic Activity for Isopropyl-Alcohol Decomposition (EN)

Petrakis, D. E. (EN)

Πανεπιστήμιο Ιωαννίνων. Σχολή Θετικών Επιστημών. Τμήμα Χημείας (EL)
Petrakis, D. E. (EN)

Phosphate catalysts of the general formula Al100P(x)M20, 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 (Bronsted and Lewis) by pyridine titration and also tested for their catalytic activity towards the isopropanol dehydration. It was found that for the Al100P(x)Al20 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(x)Al20 and Al100P(x)Fe20, while for Al100P(x)Cr20 a continuous increment with x is observed. The activity minima of the first 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 alpha-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. (EN)

surface-chemistry (EN)

Πανεπιστήμιο Ιωαννίνων (EL)
University of Ioannina (EN)

Journal of the Chemical Society-Faraday Transactions (EN)

Αγγλική γλώσσα

1991

<Go to ISI>://A1991FL39300024

Royal Society of Chemistry (EN)



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