X-H Bond Activation on Cr(III),O Sites (X = R, H): Key Steps in Dehydrogenation and Hydrogenation Processes

We synthesized isolated Cr(III) sites on SiO2-Al2O3 and Al2O3 by grafting and subsequent controlled thermal treatment of Cr(OSi((OBu)-Bu-t)(3))(3)(THF)(2) and Cr(Al((OPr)-Pr-t)(4))(3) on alumina. CrOx/Al2O3 was obtained from incipient wetness impregnation of Al2O3 with CrO3 in H2O followed by calcination, as carried out for the synthesis of industrial Cr-based dehydrogenation catalysts. These materials were characterized by IR, EPR, XAS, and by the adsorption of the probe molecules CO and pyridine, and the results were compared to previously reported isolated Cr(III)/SiO2. All of these materials were active in propane dehydrogenation at 550 degrees C, where higher TOFs were obtained for Cr(III)/SiO2-Al2O3 and Cr(III)/Al2O3 than for CrOx/Al2O3 and Cr(III)/SiO2. For mechanistic studies the reverse reaction, propene hydrogenation, was studied. Here, the order of reactivity was opposite that of dehydrogenation, with CrOx/Al2O3 and Cr(III)/SiO2 being more active in hydrogenation than Cr(III)/SiO2-Al2O3 and Cr(III)/Al2O3. Kinetic analysis and labeling studies with D-2 showed that the rate law is in all cases first order in H-2 partial pressure but had different dependence on propene partial pressure from catalyst to catalyst. We found small normal kinetic isotope effects of 1 <= KIE <= 2, activation enthalpies up to 40 kJ mol(-1), and large negative activation entropies between -100 and -194 J K-1 mol(-1) for the different Cr catalysts. We also performed parahydrogen-induced polarization (PHIP) experiments, which showed that H-2 addition to propene proceeds, at least in part, via a pairwise mechanism. The experimental data for propene hydrogenation suggests adsorption/desorption pre-equilibria of H-2 (or D-2) and propene followed by H-2 activation and insertion of propene. DFT computations for propane dehydrogenation and propene hydrogenation on Cr(III) on a periodic alumina model show that a mechanism involving X-H activation (X = R, H) is energetically feasible with activation enthalpies and entropies that are comparable to the experimentally determined values.