Abstract
HZSM-5 was prepared by the hydrothermal method using TPAOH as a structure directing agent without Na+ addition. The calcination process for removing the organic templates was studied and particularly the influence of calcination temperature on zeolite structure and acid property was investigated. It was found that dehydrogenation of Brønsted acid sites in zeolite framework occurred with the elevation of calcination temperature, leading to a decrease of catalytic activity for n-decane cracking. To recover the Brønsted acid sites in zeolite lost during calcination, the calcinated zeolite was treated with various reagents, including H2O, H2O2, NH3 and NH4Cl, to turn Lewis acid sites back to Brønsted acid sites. Among them, the NH4Cl treatment was found to be the most effective way, which increased conversion of n-decane from 6% to 77% and increased the B/L acid sites ratio from 0.503 to 3.95. This enhancement of catalytic cracking ability is probably related to the formation of an intermediate active component ([AlO4]0) during high temperature calcination, and these intermediate sites promoted the protonation of Lewis acid sites in the NH4Cl treatment process.
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