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(OtBu)3)3(THF)2 and Cr­(Al­(OiPr)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 °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 D2 showed that the rate law is in all cases first order in H2 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 H2 addition to propene proceeds, at least in
part, via a pairwise mechanism. The experimental data for propene
hydrogenation suggests adsorption/desorption pre-equilibria of H2 (or D2) and propene followed by H2 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.