Roles of the free radical and carbenium ion mechanisms in pentane cracking to produce light olefins

Abstract

In order to explore the roles of the free radical and carbenium ion mechanisms in alkane cracking to produce light olefins, pentane cracking tests have been carried out over a zeolite catalyst at 600–800 °C, and a particular attention has been paid to the measurement of the product distribution. The cracking modes for pentane included “uncatalytic pyrolysis” following the free radical mechanism, “catalytic cracking” following the carbenium ion mechanism, and “catalytic pyrolysis” following the dual mechanisms (involving the concurrence of the free radical and carbenium ion mechanisms): (1) The free radical mechanism did not occur until 700 °C under the studied conditions, and it led to an ethylene-rich product distribution. (2) Zeolite catalyst initiated the carbenium ion mechanism that led to a propene-rich product distribution. (3) The dual mechanisms exhibited a wide distribution of ethylene and propene, which approximated to that of the free radical mechanism from that of the carbenium ion mechanism with increasing reaction temperature. The kc/kt and kcp/(kt+kc) parameters have been proposed to determine the relationship between the free radical and carbenium ion mechanisms. It was found that the carbenium ion mechanism greatly contributed to pentane cracking at the low temperatures, and the free radical mechanism took over the dominant position at the high temperatures. A proper proportion between the carbenium ion and free radical mechanisms promoted pentane cracking. In addition, yield of propene plus ethylene (P + E) and ratio of propene to ethylene (P/E) have been employed to evaluate the light olefins production. In the research scope, pentane cracking at 740 °C over 200 mg K-ZSM-22 zeolites seemed the optimal process, which has taken the consumption of energy and catalyst as well as the production and distribution of light olefins into consideration.