Derivation of a nickel-silica catalyst by coke formation

Abstract

Coking which causes deactivation of catalysts was reported to occur in the hydrogenation of nitrobenzene to aniline. In the present study, deactivation of nickel on silica catalyst by coking was investigated for the same reaction. The weight of coke varying with time was continuously measured by a Cahn electrobalance and the conversion of nitrobenzene was obtained from the analysis of products by gas chromatography. The coke content on catalyst increased with time and conversion decreased with time: furthermore, conversion linearly decreased in the conversion vs. coke content plot until coke content reached 6-7\%, which showed that the catalyst was deactivated by coke formation. There was a trend of forming monolayer coke until coke content reached 6-7\%, and thereafter it seemed that multilayer coke was formed. When the several types of deactivation functions were tested by regression method, exponential and inverse quadratic functions appeared to be the most suitable, whereas, for the conversion vs. coke content, exponential and quadratic functions were found to be the most suitable deactivation equations. In the log-log plot of coke content vs. time, two straight lines were obtained and the Voorhies`` constants showed that coking reaction shifted to diffusion limited process after 30-50 minutes. Coking and conversion increased with increasing temperature, but diminished with increasing hydrogen to nitrobenzene ratio (H$_2$/NB) or catalyst to feed ratio (W/F). Increase in coke content with decreasing W/F and coke formation in the nitrobenzene adsorption experiment showed that coke was formed from nitrobenzene by parallel mechanism. Coking decreased the total surface area by 12.8-19.2\% when the coke content was 11.25-20.52\%. The trend of medium loss in surface area showed that coke was formed by uniform poisoning mechanism as well as pore mouth poisoning mechanism. A coke content correction factor was employed to calculate real coke content without adsorbed ...