Genesis of Strong Bronsted Acid Sites in WZr-KIT-6 Catalysts and Enhancement of Ethanol Dehydration Activity

Using a one-pot synthesis technique, tungsten and zirconium were simultaneously incorporated into an ordered mesoporous KIT-6 framework. A series of such materials, denoted WZr-KIT-6, were synthesized with W and Zr loadings ranging from 0 to 10 mol % each. At sufficiently high Zr loadings, ssNMR spectra of neat as well as pyridine-adsorbed catalyst confirm that the fresh WZr-KIT-6 materials exhibit both Lewis and Bronsted acid sites of high strength, resulting in correspondingly high ethylene yields during ethanol dehydration. Such yields surpass those observed on W-x-KIT-6 and Zr-y-KIT-6 materials with W and Zr loadings identical with those of WxZry-KIT-6. Interestingly, air regeneration of the spent WZr-KIT-6 catalyst further enhances ethanol dehydration activity, with ethylene yields approaching those reported with HZSM-5 and SAPO-34 catalysts under similar operating conditions. This enhancement correlates with ssNMR evidence of the formation of additional strong Bronsted acid sites following the regeneration step, presumably from the water produced during combustion of the coke deposits. On the basis of ssNMR characterization of acid strength, these acidic protons are assigned to the hydroxyl groups bound to metals in W-O-Zr structures and to the stronger acidic protons on heteropolytungstate structures. The formation of strong Bronsted acid sites, comparable to those observed in H-ZSM-5 and H-Beta, in mesoporous WZr-KIT-6 materials should be particularly attractive for reactions that are prone to rapid deactivation by coking in microporous catalysts.