High yield synthesis of nanoscale high-silica ZSM-5 zeolites via interzeolite transformation with a new strategy†
A novel interzeolite transformation strategy using internally confined organic structure directing agents (OSDAs), called the “OSDA-confined” strategy, is proposed for highly crystalline nanosized high-silica ZSM-5 zeolite synthesis, by which high yield and fast synthesis of ZSM-5 is achieved using BEA zeolite as a raw material. The transformation process was investigated by XRD, SEM, ICP, FTIR and UV Raman spectroscopy, which revealed that the decomposition of a parent zeolite and the crystallization process of products were well modulated by the confined OSDA. The transformation process could be divided into three stages consisting of raw material deconstruction, small size product formation and product growth. The strong etching power of NaOH contributes to the fast decomposition of BEA zeolites. In addition, tetrapropylammonium hydroxide (TPAOH) as an OSDA was confined in the parent zeolite instead of being added into the mother liquid, achieving a high local concentration of TPAOH, which could reduce the decomposition rate of the parent BEA zeolite and promote the efficient organization of the building units generated by the decomposition of the BEA zeolite. Thereof, a high yield was achieved by the synergistic effect of NaOH and occluded TPAOH. During the interzeolite transformation process, the six-membered rings (6Rs), five-membered rings (5Rs) and four-membered rings (4Rs) in the BEA zeolite framework were disassembled and combined with the help of 5Rs to produce the ZSM-5 zeolite structure. Moreover, due to the nanoscale and large surface area, the obtained ZSM-5 zeolite catalyst showed extremely high catalytic activity in the self-etherification reaction of benzyl alcohol (BA) to dibenzyl ether (DE). This study provides insights into the interzeolite transformation process, which would be helpful for achieving size control and high-efficiency synthesis.