Synthesis–Structure–Activity Relationship in Cu-MOR for Partial Methane Oxidation: Al Siting via Inorganic Structure-Directing Agents

Abstract

In the pursuit of controlling the propensity of Cumordenite (MOR) for the selective oxidation of CH 4 , we take a closer look at intrinsic zeolite parameters. Via synthesis design, we vary the relative proportion of Al situated near the 8-rings and 12rings of MOR zeolite. This is accomplished using different Al sources impacting the local degree of silica dissolution and zeolite formation as evidenced by crystallization times and morphological differences. Interrogating the crystalline system with steric probe molecules in conjunction with spectroscopic techniques such as 1 H magic angle spinning (MAS) NMR, infrared spectroscopy, as well as temperature-programmed desorption confirms discrete changes of the Al within the unit cell. The subsequent copper exchange allows for the generation of Cu-MOR materials of different inclinations for the activation of methane in the stepwise formation of MeOH. Here, an increasing degree of acid sites in more easily accessible locations (e.g., 12-ring) correlates with increasing maximum productivity toward MeOH at moderate exchange degrees. X-ray absorption spectroscopy supports this notion, finding a higher concentration of self-reduction-resistant framework-associated Cu 2+ species, previously established as the active sites in the selective oxidation of CH 4 .

Copper-zeolites Prepared by Solid-state Ion Exchange - Characterization and Evaluation for the Direct Conversion of Methane to Methanol

“…This ratio is based on the active site being a dicopper active site (Cu-O-Cu or Cu-O-O-Cu), or the combination of two monomeric sites (Cu-OH) [11,15]. It has been found that both the zeolite framework and placement of Cu have large impacts on the formation of the active sites and as a consequence, the activity towards methanol formation [12]. Therefore, finding new materials and synthesis methods is still very important in the attempts to optimize the MTM process.…”

“…Among the most investigated are MOR [4][5][6], CHA [7,8], and 10]. MOR has been found to be one of the most active zeolites with methanol productivities close to 0.5 mol MeOH /mol Cu [11,12]. This is often viewed to be the highest obtainable productivity, given a Cu…”

Influence of ion mobility on the redox and catalytic properties of Cu ions in zeolites

“…2). 62,71,76,[78][79][80] Interestingly, formation of Cu x O y moieties by activation in O 2 or other oxidants, and subsequent reaction with methane with breaking of the multimeric cores and formation of framework interacting Cu + ions, imply that Cu ions are reversibly moving in the zeolite cages during the redox cycle. 71…”

“…In brief, the asymmetry of the n(O-H) band has been described as due to the overlap of two distinct components, representative for Brønsted acid sites located in the 8-and 12-membered rings of the MOR framework (giving rise to the low and high frequency shoulders of the band, respectively). 80,82,83 The case of ZSM-5 is even more complex, due to the higher number of symmetry independent tetrahedral sites (12 vs. 4 in MOR). Nonetheless, the distribution of monovalent cations (including Cu + , Zn + , Cd + , Cs + ) has been investigated by several techniques in order to infer the existence of preferential substitutional sites for Al.…”

Direct observation of a new aluminum Lewis acid site in a zeolite

“…The abundance of proximal Al atoms is directed by site occupation probabilities influenced by Al content and poorly understood energetic preferences for Al distributions which depend on numerous factors during zeolite synthesis and post-synthetic treatment. Synthetic protocols to direct an increased abundance of adjacent Al sites have been reported [19][20][21], but either for Al-rich zeolites or directed to for pairs specifically. Statistically, higher Si/Al ratios in siliceous zeolites yield lower probabilities of adjacent Al pairs, let alone Al triplets.…”