(101h) Studies on Ag-Modified ZSM-5 Zeolite for Alkanes Catalytic Cracking and Its Reaction Mechanism

The small molecular alkanes that are difficult to activate in crude oil bring challenges to the realization of direct cracking of crude oil to olefins. ZSM-5 zeolite with good propylene selectivity is considered to have a good application prospect. Transition metal-incorporation has the advantages of improving hydrothermal stability and enhancing cracking activity. In order to reveal influence of cracking reaction pathways of alkanes，parent ZSM-5 and Ag-ZSM-5 were prepared and employed in n-pentane catalytic cracking. It was found that the introduction of Ag can effectively improve the hydrothermal stability. After hydrothermal treatment of 0.01MAg modified zeolite, the conversion of n-pentane is about 30 wt% higher than that of unmodified zeolite at 923K. The cracking activity of n-pentane was improved, and the apparent activation energy decreased by 14.51 KJ/mol after Ag modification. The characterization results confirmed that the introduction of Ag covered some strong Brønsted acid sites and a reversible redox reaction occurred in the process of regeneration. Through on-line pulse reactor evaluation, it was found that dehydrogenation cracking at Ag site selectively promoted C-H bond breaking. At low conversion, the selectivity of C1-C3 alkanes decreases, and the selectivity of unstable intermediate 1-pentene increases. 1-pentene, which is easy to be protonated, tends to continue to cleave at the Brønsted acid site of zeolite. Increasing residence time can promote deep cracking and effectively improve the selectivity of low-carbon olefins. The catalytic cracking reaction network of n-pentane was constructed based on molecular reaction, which provided theoretical support for catalyst design and optimization of process conditions.