Abstract
Well-defined Ga(III) sites on SiO2 are highly active, selective, and stable catalysts in the propane dehydrogenation (PDH) reaction. In this contribution, we evaluate the catalytic activity toward PDH of tricoordinated and tetracoordinated Ga(III) sites on SiO2 by means of first-principles calculations using realistic amorphous periodic SiO2 models. We evaluated the three reaction steps in PDH, namely, the C–H activation of propane to form propyl, the β-hydride (β-H) transfer to form propene and a gallium hydride, and the H–H coupling to release H2, regenerating the initial Ga–O bond and closing the catalytic cycle. Our work shows how Brønsted–Evans–Polanyi relationships are followed to a certain extent for these three reaction steps on Ga(III) sites on SiO2 and highlights the role of the strain of the reactive Ga–O pairs on such sites of realistic amorphous SiO2 models. It also shows how transition-state scaling holds very well for the β-H transfer step. While highly strained sites are very reactive sites for the initial C–H activation, they are more difficult to regenerate. The corresponding less strained sites are not reactive enough, pointing to the need for the right balance in strain to be an effective site for PDH. Overall, our work provides an understanding of the intrinsic activity of acidic Ga single sites toward the PDH reaction and paves the way toward the design and prediction of better single-site catalysts on SiO2 for the PDH reaction.
Highlights
The high demand of light olefins[1] and the large abundance of shale gas,[2] mostly constituted of light alkanes, have stimulated interest in on-site propane dehydrogenation (PDH).[1,3] PDH involves activation of the C(sp3)−H bond of propane as a first step, which is still nowadays a very challenging reaction.[4]Because of the highly endothermic nature of alkane dehydrogenation, this reaction is generally carried out at 550 °C to obtain reasonable conversion to the alkene product
After construction of the Ga(III) sites on SiO2 amorphous periodic models, we have evaluated the reactivity of a variety of Ga−O pairs with different degrees of strain
We considered tri- and tetracoordinated Ga with one additional siloxane group coordinated to the Ga center because these are the proposed initial catalytic sites in the silica-supported well-defined Ga(III) PDH catalyst
Summary
The high demand of light olefins[1] and the large abundance of shale gas,[2] mostly constituted of light alkanes, have stimulated interest in on-site propane dehydrogenation (PDH).[1,3] PDH involves activation of the C(sp3)−H bond of propane as a first step, which is still nowadays a very challenging reaction.[4]Because of the highly endothermic nature of alkane dehydrogenation, this reaction is generally carried out at 550 °C to obtain reasonable conversion to the alkene product. The two historical heterogeneous catalysts that are used for this reaction in industry correspond to alumina-supported PtSn nanoparticles and the CrOx/Al2O3 system, known as the Houdry or Catofin catalysts.[5] Recent research developments have helped to launch a PtGa-based catalyst.[6] The Crbased catalyst is thought to have Cr(III) active sites dispersed on alumina. Gaexchanged zeolites can convert light alkanes such as propane directly into aromatics and H2 in a process a tandem dehydrogenation−aromatization proposed to process.[7−13] involve Ga2O3 promotes PDH reaction, but it suffers from fast deactivation, presumably because of reduction of the catalyst under reaction conditions.[14,15] More recently, silica-supported well-defined Ga(III) single-site catalysts have been developed[16] using a combined approach of surface organometallic chemistry[17−23] and a thermolytic precursor using [Ga(OSi-
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