G. Cerrato, C. Morterra, M. Delgado
Sep 8, 2006
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Microporous and Mesoporous Materials
Abstract
Abstract Sulfated zirconia samples having a variable Ga 2 O 3 content (in the 1–15% molar range) were synthesized. The promoting effect of gallium was studied in the catalytic isomerization of n -butane at 523 K, by feeding n -butane and H 2 (with a 1:4 ratio). Catalytic activity was found to be greatly dependent on gallium loading. Catalysts containing 3–5 mol% Ga 2 O 3 doubled the activity of sulfated zirconia, whereas for a sample with 15 mol% Ga 2 O 3 the catalytic activity was completely lost. Surface chemistry of these materials was studied by means of FTIR spectroscopy and adsorption microcalorimetry, using selected probe molecules (CO and 2,6-dimethylpyridine). IR spectroscopy showed that gallium-containing sulfated zirconia samples exhibit both Lewis and Bronsted acidity. Lewis acidity is attributed to coordinatively unsaturated Zr 4+ ions located in defective surface sites, whereas Bronsted acidity is associated to surface sulfate groups. Samples with a Ga 2 O 3 content between 1 and 9 mol% show a combination of Lewis and Bronsted acidity that quantitatively decreases with increasing gallium oxide content. Samples having a Ga 2 O 3 content equal to 9 mol% or greater show the following specific features: (i) these samples are much more difficult to dehydrate than those with smaller gallium content, and the hydroxy groups interact by hydrogen bonding, (ii) the sulfate groups progressively lose their covalent character, and (iii) both Lewis and Bronsted acidity of the samples decreases drastically. Couples of Lewis and Bronsted acid sites appear to be needed for catalytic activity in n -butane isomerization, and they present an optimum ratio when the catalyst is brought to a medium-high dehydration degree and when its Ga 2 O 3 content is of about 3–5 mol%.