Ga-promoted sulfated zirconia systems. II. Surface features and catalytic activity

Sulfated zirconia samples having a variable Ga2O3 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 H2 (with a 1:4 ratio). Catalytic activity was found to be greatly dependent on gallium loading. Catalysts containing 3–5 mol% Ga2O3 doubled the activity of sulfated zirconia, whereas for a sample with 15 mol% Ga2O3 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 Brønsted acidity. Lewis acidity is attributed to coordinatively unsaturated Zr4+ ions located in defective surface sites, whereas Brønsted acidity is associated to surface sulfate groups. Samples with a Ga2O3 content between 1 and 9 mol% show a combination of Lewis and Brønsted acidity that quantitatively decreases with increasing gallium oxide content. Samples having a Ga2O3 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 Brønsted acidity of the samples decreases drastically. Couples of Lewis and Brønsted 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 Ga2O3 content is of about 3–5 mol%.