Zr-Al-Beta zeolite with open Zr(IV) sites: an efficient bifunctional Lewis-Bronsted acidic catalyst of cascade reaction

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 Bronsted 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 Bronsted acid activity of Zr-Al-Beta; after the selective poisoning of Bronsted 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.