Influence of Sn Promoter on Pd and Pt Catalysts for the Conversion of Heptanoic Acid and Propane

Silica-supported Pd catalysts can be used to selectively remove oxygen from biomass-derived carboxylic acids through decarbonylation. This work aims to improve Pd catalyst stability and selectivity towards desirable products by varying metal particle size as well as by introducing Sn to the catalyst. Catalysts were prepared by ion exchange or incipient wetness impregnation of metal precursors on Davisil 636 silica gel supports, followed by calcination in air and reduction in dihydrogen. Monometallic Pd and Sn catalysts, in addition to chemically-mixed and physically-mixed PdSn catalysts, were tested for activity in heptanoic acid conversion. Two-stage reactor beds were used to probe the effect of Sn proximity on Pd. X-Ray diffraction, hydrogen chemisorption, transmission electron microscopy, and X-ray absorption near edge structure-temperature-programmed reduction were used to characterize the catalysts.

The stability of the monometallic Pd catalyst during decarbonylation of heptanoic acid was influenced by the metal particle size, with smaller particles deactivating slower than larger particles. Whereas catalyst deactivation was reduced by adding Sn to Pd by impregnation, improved stability was also observed when Sn/SiO2 was physically mixed with Pd/SiO2. The physically mixed PdSn initially produced products characteristic of monometallic Pd, however, the two bimetallic catalysts showed similar product selectivities after 10 h on stream. The PdSn catalysts also expanded the reaction network to include hydrogenation and decarboxylative ketonization, with the primary product shifting from hexene to heptanal. Scanning transmission electron microscopy-energy dispersive spectroscopy of physically-mixed PdSn catalyst indicated that Sn migrated to the silica support particles containing Pd. In situ XANES analysis during butyric acid deoxygenation indicated that SnOx is the primary Sn phase associated with Pd in both the physically-mixed and chemically-mixed catalysts.

The promotional effect of Sn on Pt catalysts for propane dehydrogenation was also explored. The high temperatures and reducing conditions of the alkane dehydrogenation reaction are substantially different from the carboxylic acid reactions and likely result in a more reduced state of the Sn promoter. Bimetallic PtSn catalysts showed improved stability compared to monometallic Pt catalysts, which deactivated rapidly with time on stream. X-ray diffraction indicated the presence of PtxSny alloys in the bimetallic catalysts, suggesting that alloy phases play a critical role in improving catalyst stability. The PtSn particles could be regenerated by an oxidative treatment when supported on alumina but not on silica. Both Sn and trace amounts of carbon on Pt decreased propane hydrogenolysis activity in the presence of co-fed dihydrogen.