Decarbonylation of Carboxylic Acids over Supported Metal Catalysts

Removal of oxygen from biomass-derived feedstocks such as carbohydrates and vegetable oil is often needed to produce chemicals and fuels. In this study, oxygen was removed from the model compounds heptanoic acid and propanoic acid by either decarboxylation, which yields carbon dioxide (CO2) and an alkane, and/or decarbonylation, which forms carbon monoxide (CO), water (H2O), and a linear alkene. Although decarbonylation produces an α-olefin as a primary product, the double bond can be isomerized to form an internal olefin or be hydrogenated to form an alkane. Recent work on decarboxylation/decarbonylation of carboxylic acids over transition metal catalysts is often performed in the presence of dihydrogen to inhibit catalyst deactivation, however, paraffins are the major product in those systems.

The effects of metal type, support composition, metal loading, reaction phase, acid concentration, and conversion on activity, selectivity, and stability are presented. We studied the liquid- and gas-phase decarbonylation of carboxylic acids on Pt, Pd, and Rh nanoparticles supported on carbon and silica supports in a continuous-flow fixed-bed reactor at temperatures ranging from 533 to 573 K. The liquid-phase turnover frequency (TOF) of heptanoic acid conversion over Pt at 573 K was fairly constant, 0.0050 s-1, as the catalyst dispersion and metal loading was varied. The liquid-phase TOF of Pd at 573 K was 0.00070 s-1 and was independent of support composition, weight loading, and acid concentration. A shift in product selectivity from decarboxylation products, paraffin and CO2, to decarbonylation products, olefins and CO, as previously discussed in the literature was most likely a result of changes in conversion. However, the decarboxylation products observed in the current study were likely formed in secondary side reactions such as water-gas shift (WGS) and hydrogenation.

Low conversion and high acid concentration experiments in liquid-phase and gas-phase operation suggest that the main reaction path for heptanoic acid and propanoic acid conversion is the decarbonylation reaction. Some direct decarboxylation was observed when operating in the gas-phase at very low concentrations of acid. The reaction was zero order in acid during the liquid-phase operation and high partial pressures during gas-phase operation, but was observed to be negative order in acid at very low partial pressures.

Characterization of catalysts after reaction revealed metal sintering, loss of surface area and loss of exposed metal during the liquid-phase operation. X-ray diffraction and electron microscopy revealed Pd sintering on a carbon support when operating in the liquid-phase at high acid concentration, but negligible Pd sintering when acid concentration was below 0.10 M. Palladium nanoparticles were more stable on the silica support during the liquid-phase operation. Furthermore, Pd sintering was negligible during the gas-phase experiments regardless of the support composition and acid concentration. Nevertheless, N2 physisorption and H2 chemisorption revealed a loss of surface area and metal exposed during the liquid-and gas-phase operation, even when metal sintering was not observed. Evidently, there was blocking of active sites and/or the porous structure by strong absorption of side products or carbonaceous species.

A Pt catalyst recovered from the liquid-phase reaction could not be regenerated after air calcination and H2 reduction at mild temperatures, and/or hot wash in acid. However, catalyst regeneration after gas-phase operation was successful. In situ treatment at 623 K with N2 and H2 of the spent catalyst recovered the initial catalytic activity. This suggested that the main cause for catalyst deactivation in the gas-phase was deposition of organic molecules onto the active sites.