Significant Influence of Zn on Activation of the C‐H Bonds of Small Alkanes by Brønsted Acid Sites of Zeolite

Abstract

Herein, we analyze earlier obtained and new data about peculiarities of the H/D hydrogen exchange of small C(1)-n-C(4) alkanes on Zn-modified high-silica zeolites ZSM-5 and BEA in comparison with the exchange for corresponding purely acidic forms of these zeolites. This allows us to identify an evident promoting effect of Zn on the activation of C-H bonds of alkanes by zeolite Brønsted sites. The effect of Zn is demonstrated by observing the regioselectivity of the H/D exchange for propane and n-butane as well as by the increase in the rate and a decrease in the apparent activation energy of the exchange for all C(1)-n-C(4) alkanes upon modification of zeolites with Zn. The influence of Zn on alkane activation has been rationalized by dissociative adsorption of alkanes on Zn oxide species inside zeolite pores, which precedes the interaction of alkane with Brønsted acid sites.

ZnO Nanoclusters Supported on Dealuminated Zeolite β as a Novel Catalyst for Direct Dehydrogenation of Propane to Propylene

“…It is reported that ZnO supported on Al 2 O 3 could serve as Lewis acid sites (LAS), which can efficiently catalyze PDH. [14] In accordance with previous reports, small ZnO nanoclusters (SN ZnO) are capable of activating CÀ H bond, [18,19] suggesting its potentially catalytic ability for PDH. However, active ZnO nanoclusters are very small and unstable, especially under harsh operation conditions.…”

High Zn/Al ratios enhance dehydrogenation vs hydrogen transfer reactions of Zn-ZSM-5 catalytic systems in methanol conversion to aromatics

“…Frequently, the outstanding catalytic performance of bifunctional metal-zeolite systems is attributed to the synergic effect of different active sites, in this case, Zn sites and Brønsted sites. This active-site synergy, which is proposed to tune the properties of the active centers and considered to be caused by proximity and interaction, has been described in previous literature as the origin of the unique catalytic properties of metalcontaining zeolites [78][79][80][81]. In this section, however, it is important to highlight that it is not necessarily the synergy between the Zn sites and the zeolite protonic acid sites, but simply an optimum concentration of both sites controlling which of the two possible mechanistic pathways for the aromatization that is dominating in each case.…”

1H and 13C MAS NMR studies of light alkanes activation over MFI zeolite modified by Zn vapour

“…The results pointed to formation of n-propyl zinc surface species and ZnOH groups at the onset of propane reaction and accounted for propane activation via dissociative adsorption over ZnO fragments. This result was confirmed by in situ 1 H MAS NMR studies of H/D-exchange reaction between deuterated alkanes and residual acidic sites over ZnO-modified zeolites [8]. Later on, Stepanov et al [9] used in situ MAS NMR for the investigation of methane activation over ZnOmodified zeolite H-BEA and also concluded to the dissociative adsorption over ZnO fragments.…”