(782a) Effect of Boron Promotion on the Performance of Platinum Catalysts for Propane Dehydrogenation

The activity, stability and selectivity of supported Pt catalysts in light alkane dehydrogenation reactions are often optimized by the addition of promotors such as Sn, Zn, Ga, In and Ge. Such promotors for example enhance the catalyst lifetime by reducing carbon deposition (coking) and/or particle sintering,¹ often at the expense of catalyst activity.

Recently, our group has reported that the addition of small amounts boron enhances the stability of Ni² and Co³ catalysts without affecting their activity and selectivity. Density Functional Theory (DFT) calculations show that boron binds strongly at sites where deleterious carbon deposition nucleates. By titrating these sites with boron, carbon deposition and growth becomes inhibited. Calculations furthermore suggest that this effect might be rather general.⁴ Indeed, the effect of boron promotion on the selectivity of Pd hydrogenation catalysts was recently reported by Tsang et al.⁵ and by Studt et al.⁶ Also in these studies, it is suggested that boron binds strongly at subsurface sites and hence modifies the surface electronic properties of the Pd catalyst.

Based on these insights, we evaluated the effect of boron promotion for Pt-based catalysts in propane dehydrogenation reactions. DFT calculations show that, although boron binds weaker to Pt than to Ni and Co, boron binds sufficiently strong at Pt subsurface octahedral sites to be stable against removal by hydrogen or carbon under dehydrogenation conditions (600Ë?C and 1 atm) with Î?G_rxn of 157 and 178 kJ/mol, respectively. Boron-promoted Pt/SiO₂ and Pt/Al₂O₃ catalysts were prepared with 3 wt. % Pt and with boron loadings ranging from 0.0 and 1.5 wt.%. To evaluate the effect of boron, propane dehydrogenation reactions were performed at 600 Ë?C and atmospheric pressure with C₃H₈/H₂/He (1/1/3). After reaction, the amount of deposited carbon was determined by oxidation.

For both series of catalysts, Pt/B-SiO₂ and Pt/B-Al₂O₃, the same trend was observed: the presence of small amounts of boron indeed improves the catalyst stability and reduces the amount of coke deposition. For example, coke formation is reduced threefold by the addition of 1 wt% boron to Pt/Al₂O₃. To elucidate the effect of boron promotion, several characterization techniques were used.

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