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 materials.
Nitrobenzene 을 hydrogenation 시켜 aniline 을 제조하는 반응에서 생성되는 coke 가 $Ni/SiO_2$ 촉매를 deactivation 시키는 현상에 관하여 조사 하였다. 본 실험에서는 electrobalance 로 coke 의 무게를 연속적으로 측정하였으며 gas chromatography 로 product 를 분석하여 conversion 을 구하였다.
Coke content 가 시간에 따라 증가하고, conversion 이 시간에 따라 감소하는 것으로 촉매가 deactivation 되고 있음을 알 수 있었다. 여러 가지 deactivation function 을 regression method 로 test 해 본 결과 exponential 과 inverse quadratic function 이 가장 적합한 것으로 나타났으며, conversion 과 coke content 의 관계는 exponential 과 quadratic equation 이 가장 적합한 것으로 나타났다. 또한 coke content 가 6-7% 까지는 monolayer coke 가 생성되는 것으로 나타났으며 그 이상에서는 multilayer coke 가 생성되는 경향을 보여 주었다.
Voorhies correlation 은 두 개의 직선으로 나타났는데, 30-50분 후에는 coking 이 diffusion limited process 가 됨을 알 수 있었다. 또한 온도가 증가할수록 coking 과 conversion 이 증가하였고, $H_2/NB$ ratio 와 W/F 가 증가할수록 이들이 감소하는 경향을 보여주었다. W/F 가 감소할수록 coke content 가 감소하고 nitrobenzene 이 coke 로 잘 변하는 성질로 부터 coke 가 parallel mechanism 으로 형성됨을 알 수 있었으며 11.25 - 20.52% coke content 에 대하여 12.8 - 19.2%의 촉매표면적이 감소되는 것으로부터 coke 가 uniform 및 pore blocking mechanism 으로 생성됨을 알 수 있었다. 또한 흡착된 무게를 제거한 순수 coke 의 무게를 구하기 위하여 coke content correction factor 를 이용하였다.
