Synthesis and characterization of the Fe-substituted ZSM-22 zeolite catalyst with high n-dodecane isomerization performance

Abstract

A series of Fe-substituted ZSM-22 zeolites were synthesized via an organic structure-directing agent-free route with a small amount of precursor suspension. Characterizations by XRD, SEM, TEM, N2 adsorption, UV–vis, FT-IR, and XPS confirm the high crystallinity, uniform morphology, and smaller crystal size of the as-synthesized zeolites and the presence of Fe3+ in the framework of the zeolites. The acidity, especially the Brønsted acidity, decreases significantly due to the low acidity of Si–OH–Fe bridge hydroxyls compared with that of Si–OH–Al. Moreover, in the Fe-containing catalysts, Pt can easily be reduced and its dispersion slightly increases, as confirmed by H2 TPR and CO chemisorption. As a result, the Pt/[Al,Fe]-ZSM-22 catalysts exhibit superior selectivity in n-dodecane isomerization compared with the Pt/ZSM-22 catalyst. The higher monobranched isomer selectivity of the catalyst using Fe-substituted ZSM-22 zeolites may be due to more balanced bifunctionality between acid sites and metal sites and decreased crystallite size of the Fe-containing zeolites, which improves the diffusion of intermediates and prevents the products from further cracking. It is also observed that more dibranched isomers were formed with more Fe replacement even at higher temperatures due to reduced strong acidity by Fe replacement, which significantly suppressed the cracking and improved the selectivity to deeper isomerization.