Effect of boron promotion on the performance of transition metal catalysts

SHIWEN LI; MARK SAEYS; VLADIMIR GALVITA; ESTEBAN L. FORNERO

Blankenberge

Simposio; The first symposium Chemical Research in Flanders - CRF; 2016

FWO, KU Leuven, Universiteit Gent, Vrije Universiteit Bussel, Universiteit Antwerp

The activity, stability and selectivity of industrial catalysts are often optimized by the addition of small amounts of promoters. Recently, our group has shown that the addition of small amounts of boron enhances the stability of Co1 and Ni2 catalysts without affecting their activity and selectivity. The stability of 20 wt% Co/Al2O3 Fischer-Tropsch catalysts was found to increase 3-fold by the addition of 0.5 wt% B (Figure 1-left). Computational studies indicate that the addition of small amounts of boron blocks sites where deleterious carbon deposits nucleate and grow, thereby preventing deactivation by carbon deposition. In this study we extend this concept to Pt-based catalysts. Boron-promoted Pt catalysts were prepared by a successive impregnation of the support with aqueous solutions of H3BO3 and of H2PtCl6. To evaluate the effect of boron, propane dehydrogenation was studied over 3wt.%Pt/SiO2 and 0.4wt.%B-3wt.%Pt/SiO2 catalysts (Figure 1-center). While the initial activity of both catalysts is similar, the activity decreases much slower for the promoted catalyst, and the activity of the promoted catalyst is twice the activity of the unprompted catalyst after 50 min on stream. To evaluate the stability and the location of the boron promoter on Pt, we computed the binding energy of boron at several sites on Pt (Figure 1-right). The calculations suggest that boron also binds strongly to Pt, and removal of boron by hydrogenation or by carbon is highly unfavorable under reaction conditions (600˚C and 1 atm) with Gibbs free reaction energies of 157 and 178 kJ/mol, respectively. This work hence provides initial experimental evidence that the promotional effect of boron is quite general, indicating the versatility of this promoter to enhance the stability for transition metal catalysts.3