A Zr-Al-Beta zeolite with open Zr(iv) sites: an efficient bifunctional Lewis–Brønsted acid catalyst for a cascade reaction

Abstract

Herein, a Zr-Al-Beta zeolite was successfully prepared via a one-step strategy, that is, direct grafting of Zr species on commercial Al-Beta in ethanol. The presence of bifunctional Lewis acidic open Zr(IV) sites and Brønsted acidic framework Al sites was demonstrated by UV-vis spectroscopy, XPS, FTIR spectroscopy (CO and pyridine) and NH₃-TPD. Compared with Zr-De-Al-Beta derived from the traditional two-step strategy, Zr-Al-Beta exhibits lower silanol concentration because of the absence of the dealumination process (as observed via the FTIR spectra of the silanol region and ²⁹Si MAS NMR spectra). For the cascade Meerwein–Ponndorf–Verley (MPV) reductive etherification of cinnamaldehyde (CAL), the CAL conversion and 1-cinnamyl 2-propyl ether yield of Zr-Al-Beta (97.3% and 93.9%) were much higher than those of Zr-De-Al-Beta (41.0% and 26.9%) after reaction for 5 h. Moreover, Zr-Al-Beta showed higher activity than Zr-De-Al-Beta for etherification; this demonstrated higher Brønsted acid activity of Zr-Al-Beta; after the selective poisoning of Brønsted acid sites by 2,6-di tert butylpyridine, the turnover frequency (TOF, 30 min) of Zr-Al-Beta (27.9) was about 7.9 times that of Zr-De-Al-Beta (3.5), indicating higher Lewis acid activity of Zr-Al-Beta for the MPV reduction; based on the characterization and catalytic activity test results, the good catalytic performance of Zr-Al-Beta should be attributed to its bifunctional activities owing to the presence of external open Zr(IV) sites and framework Al sites. Finally, this one-step strategy can also be developed for the preparation of Zr-Al-Beta with elevated Zr contents (2Zr-Al-Beta) and Hf-Al-Beta with high activity for cascade reactions.