Hierarchical hollow Al-rich nano ZSM-5 crystals for highly selective production of light olefins from naphthenes

Abstract

Rational design and fabrication of hierarchical zeolites with desired structures has long been of essential and practical interest to engineers and scientists. Featuring a hierarchical nano-shell and enclosed interior space, hollow nano ZSM-5 crystals exhibit excellent capability as the key catalyst in various important reactions. However, it has been an ever-increasing but challenging task to directly fabricate hierarchical hollow Al-rich nano ZSM-5 crystals since only limited mesoporosity in ZSM-5 zeolites with Si/Al < 15 can be generated by desilication. Here, a one-step Al-distribution regulated desilication approach combining the synthesis of an Al-rich parent ZSM-5 (Si/Al = 12.5) with locally concentrated Al (LCA) domains and a simple alkali-treatment step is presented. The existence of LCA domains and their impacts on the selective desilication process were validated by diffuse reflectance UV spectroscopy and filtrate analysis, and approximately 49% of Al atoms emerged in Z5-LCA samples as Al species in close contact. Compared with its parent zeolite, the hierarchical hollow nano ZSM-5 crystal exhibited increased catalytic activities as well as ∼2-fold improved selectivities for high-value light olefins (C₂H₄ and C₃H₆) in the catalytic cracking of decalin (a model bulky naphthene molecule) and a real naphtha feedstock. These findings expand our knowledge on achieving desirable zeolite structures by regulating framework Al distribution, and suggest a promising new catalyst toward highly selective production of light olefins from naphthene feedstocks.