Propane Dehydrogenation on Chromia/Silica and Chromia/Alumina Catalysts

The catalytic activity of CrOx/SiO2 (SC, 0.08 to 0.99% chromium by weight), CrOx/Al2O3 (AC, 0.16 to 1.55%), and alpha-Cr2O3 for the dehydrogenation of propane has been investigated at 723 to 823 K in a flow apparatus. Before catalytic experiments, SC and AC samples were activated with oxygen at 773 K (s.o. treatment) followed or not followed by reduction with CO or H2 at 623 K. Occasionally, SC samples were dehydrated under vacuum at 1023 K before the s.o. treatment. Assuming all chromium to be in the +6 state after s.o., the average oxidation number of chromium (nBAR) is nBAR congruent-to 2 in SC samples reduced with CO and nBAR congruent-to 3 with H2. In AC catalysts, nBAR values progressively increase in consecutive reduction experiments, from nBAR = 2.2 to nBAR = 4.2. All catalysts are highly selective to propene and undergo a deactivation process during time on stream. The s.o. treatment completely restores the activity. The reduction mode does not influence the catalytic activity. Surface species formed upon adsorption at room temperature (RT) of CO or NO were studied by IR spectroscopy on CrOx/SiO2 submitted to treatments analogous to those used before catalytic experiments. In SC catalysts reduced with CO, the formation of carbonyls and nitrosyls of three different Cr(II) species was confirmed. Interestingly, all these species were absent when the sample was (i) reduced with H2 at 773 K, or (ii) heated with H2O after reduction with CO, or (iii) exposed to propane after reduction with either CO or H2. After treatments (i) through (iii), a carbonyl and a dinitrosyl of Cr(III) were the only species formed on CO or NO adsorption at RT. Relying on (i) the dependence of the activity on chromium content, (ii) IR results. and (iii) ESR data, it is suggested that mononuclear Cr(III) with two coordinative vacancies is the active site for the dehydrogenation of propane. The catalytic activity of CrOx/SiO2, CrOx/Al2O3, and alpha-Cr2O3 is compared with that of CrOx/ZrO2, measured previously. Possible reasons for the substantially higher activity of CrOx/ZrO2 are discussed.