Probing cobalt localization on HZSM-5 for efficient methane dehydroaromatization catalysts

Abstract

A new catalyst family of Co-exchanged HZSM-5 zeolite catalyst without additional active metals was for the first time modeled for the non-oxidative conversion of methane toward aromatics, which provides an alternative and promising way to produce aromatics from huge methane resources compared with the traditional routes from coal and crude oil. The prepared catalyst contains a well-defined Co site in the HZSM-5 zeolite anchored at Brønsted acid sites and is much appropriate for activating CH bond in CH4 for controllable formation of CHx species rather than coke. Over the Co-exchanged HZSM-5 catalyst, the maximum selectivity to total aromatics (mainly benzene and naphthalene) over 80% could be obtained. Compared to almost all non-Mo-based catalysts including V, Cr, Mn, Fe, Zn, Cu, Ga and W investigated in literature, the Co-exchanged catalysts exhibited several times higher TOF values (s−1). To corroborate the distinct Co speciation associated with the ion-exchanged catalyst and illustrate the structure–activity relationship of the well-modeled catalyst, a series of comparative activity experiments, various spectroscopic and microscopic techniques (ICP-AES, XRD, FT-IR, XPS, UV–Vis DR, and TEM) and chemical transient experiments (H2-TPR, CO-TPR, NH3-TPD) and DFT calculations were performed. The exchanged Co(II) species connected with zeolite framework are much stable in reductive gases and could not be reduced and carburized, which should be the active sites for appropriately activating CH4 into CHx species available for subsequent aromatic formation. DFT calculations confirmed that these Co-exchanged sites required much higher energy for the dissociation of CH bond in CH4 than supported Co clusters, suggesting that the formed CHx species have a low possibility of further cleavage of CH bonds and are stable for the subsequent aromatization reactions. The distinct Co locations and chemical states in HZSM-5 showing different catalytic behaviors has actually provided us a new insight into how to construct an active metal-zeolite catalyst for selective conversion of CH4 to value-added chemicals.