Influence of Catalyst Acidity on the Hydrogenation and Dimerization of Conjugated Olefins over Supported NiMoS Catalysts

The hydrogenation and dimerization of isoprene and 4-methylstyrene over supported NiMoS catalysts have been studied under conditions relevant to the hydrogenation of Canadian oil sands naphtha, during which the undesired dimerization of conjugated olefins, present in naphtha, is known to occur. NiMoS was dispersed on SiO2-Al2O3 supports with varying Si/Al contents to determine the impact of the catalyst acidity on the dimerization reactions. The reactions were carried out in a microscale trickle-bed reactor operated at 200-250 degrees C, 3.4 MPa H-2, and liquid hourly space velocity (LHSV) of 2-4 h(-1) for reaction periods of 3 days of continuous operation. The acidity of the NiMoS/Al2O3 catalyst, measured by NH3 temperature-programmed desorption and pyridine adsorption diffuse reflectance infrared Fourier transform spectroscopy, was lower than that of the NiMoS/SiO2-Al2O3 catalysts, which increased with increased SiO2 content up to 20 wt % SiO2. The results showed that dimer formation correlated with the acidity of the catalysts. The dimer yield on the sulfided supports without NiMoS was about 40% of the yield observed on the NiMoS/SiO2-Al2O3 catalysts. The NiMoS catalysts showed stable activity over 3 days of continuous operation, whereas the sulfided supports deactivated rapidly because of low hydrogenation activity. Addition of 3 wt % NaOH to the NiMoS/S10 catalyst neutralized 60% of the acid sites, resulting in a 50% drop in dimerization yield.