(652f) Experimental Study on Catalytic Pyrolysis Mechanism of n-Pentane over HZSM-22 Zeolite

Since 1950s, scientists have begun the catalytic pyrolysis research and paid attention to the application of catalytic pyrolysis in the heavy oil process. Some relative patents were sequentially published in many countries. Up to now, lots of reports about catalysts and technologies on catalytic pyrolysis have been studied and developed. Catalytic pyrolysis, which combines a catalytic cracking on catalyst and thermal pyrolysis reactions in catalyst void, has the advantages of both catalytic cracking and thermal pyrolysis. It can raise the yields of light olefins, expand the flexibility of products distribution, and simultaneously lower reaction temperature and decrease energy consumption for the whole system, thus it has broad application prospect. Zeolite-based catalyst have been introduced into the catalytic pyrolysis of hydrocarbons and it shows excellent selectivity to light olefins. Catalytic pyrolysis starts a new director to explore the cracking of hydrocarbons, however, the research about catalytic pyrolysis is still not enough, especially the lack of mechanism research.

In this research,the catalytic studies were performed on an homemade reactor with an internal diameter (ID) of 6 mm, packed with 200-400 mg of HZSM-22 (particle size in the 20-30 mesh range and diluted with SiC) at 600-900 ^oC and 1 atm. The catalyst bed length was fixed to be 200 mm long by adjusting the SiC loading. The reactor feed gas was a blend of n-pentane and nitrogen and the mole ratio was about 1:50. An Agilent Micro 3000GC equipped with three channels was used to collect chromatogram every 4 min.

This has permitted, thanks to design of experiments, to show the importance of temperature on n-paraffin catalytic pyrolysis mechanism. Through a series of n-pentane pyrolysis on HZSM-22 over a comprehensive temperature range, we find that the steady-state rate of n-pentane conversion has been related to catalytic activity of HZSM-22 and the reaction temperature. Low-temperature domain (below 700 ^oC) that thermal pyrolysis does not occur, HZSM-22 shows excellent catalytic activity for n-pentane. It is shown that the apparent activation energy of the overall n-pentane transformation decreases to about 115 kJ·mol^-1 mol less than half of pure thermal pyrolysis. As temperature rising above 700 ^oC, emergence of thermal pyrolysis can accelerate the conversion of n-pentane and the rate is higher than the sum of pure catalytic cracking and thermal cracking at the same temperature. The catalytic/thermal cracking synergy in HZSM-22 has been studied. The results have revealed, for the first time, the detailed difference between pure catalytic cracking and the synergy effect of catalytic/thermal cracking which reduces above 1/3 the apparent activation energy. The activity of HZSM-22 nearly vanishes when the temperature exceeds 800 ^oC.

*Corresponding author. Email:gliu@tju.edu.cn

This work was supported by the National Natural Science Fund of China (U1232134).