Role of the mesoporous diameters of hierarchical ZSM-22/MCM-41 zeolite for n-alkane isomerization

Abstract

Achieving highly dispersed Pt particles over ZSM-22 zeolite as bifunctional catalyst for n-alkane isomerization through simple synthesis methodology is critical for its practical applications. In this work, the mesoporous MCM-41 layers with different diameters are prepared rationally to reach this goal and in-depth understand the role of mesoporous sizes on acidity and Pt properties as well as the isomerization performance. Characterizations demonstrate the presence of microporous ZSM-22 zeolite as core and the well-established MCM-41 material with the diameters in the range of 2.2–4.5 nm as shell. Compared to parent ZSM-22 zeolite, partial deconstruction of the original structure by desilication causes the decreased acidity, while the effect of MCM-41 layers on acidity is negligible. However, the increase of mesopore diameters leads to obviously improved Pt dispersion by providing much broader mesoporous channels and higher surface areas. Therefore, the micro/mesoporous catalysts exhibit higher isomer selectivity than the parent Pt/ZSM-22 catalyst in n-dodecane isomerization, due to the balanced bifunctionality between the highly dispersed Pt sites and well-optimized Brønsted acid sites. Moreover, the enhanced selectivity of multi-branched isomers over the micro/mesoporous catalysts can be not only attributed to the presence of mesoporous layers by improving the secondary rearrangement of the iso-intermediates, but also related to the increment of Pt dispersion providing more sites for rapid hydrogenation of the multi-branched intermediates. Such an enhanced isomerization performance arising from the rational regulation of the mesoporous layer is noteworthy, providing new insight to design isomerization catalyst with more balanced bifunctionality and highly dispersed Pt particles.