Surface enhanced dynamic nuclear polarization solid-state NMR spectroscopy sheds light on Brønsted–Lewis acid synergy during the zeolite catalyzed methanol-to-hydrocarbon process

Abstract

Surface-enhanced dynamic nuclear polarization solid-state NMR spectroscopy has been applied to identify the role of surface-carbene species and elucidating Brønsted–Lewis acid synergy during the zeolite-catalyzed methanol-to-hydrocarbon process.

Initial Carbon−Carbon Bond Formation during the Early Stages of Methane Dehydroaromatization

“…The slight downfield nature of signals could be attributed to the bi‐functional nature of the zeolite environment, which is also paramagnetic in nature . In addition, two more important oxygenate‐based intermediates have been identified particularly in the Mo/ZSM‐5_50 sample: (i) acetal (≈104 ( 13 C) and ≈6.5 ( 1 H) ppm in Figure c and (ii) surface‐formate (≈174 ( 13 C) and ≈10 ( 1 H) ppm in Figure S13 a) species …”

“…Simultaneously, it is important to recognize that methane activation upon molybdenum is also associated with deoxygenation (see XPS spectrum of spent catalyst Figure S5 b, and additional discussion in Section S2.3), which eventually leads to the formation of CO (Figure b) during the process. The Brønsted acid sites of the zeolite then undergo CO insertion to form the corresponding surface‐formate species, which then lead to the formation of acetal upon nucleophilic attack from the in situ formed surface‐methoxy species (pathway b in Scheme ) . All three key intermediates, i.e., surface‐formate, surface‐methoxy, and acetal, were experimentally verified in this work (Figure , Figure ).…”

“…This same observation was also observed for unreacted methane (Figure b and b), as its multiple adsorbed forms were also identified and hence, further advocates for the local heterogeneity within the material. The existence of surface‐formate species, i.e., the 13 C correlations observed at 170–173 ppm correlating with a 1 H‐signal at ≈8 ppm (Figure d), is indicative of the occurrence of CO insertion, which is known to generate hydrocarbon pool species during the zeolite‐catalyzed MTH process (see below) . However, it should also be mentioned that these species can only be formed in the very early stages of the reaction making use of O atoms present in the zeolite and metal oxide Mo precursor (cf.…”

Initial Carbon−Carbon Bond Formation during the Early Stages of Methane Dehydroaromatization

“…[39] Thes light downfield nature of signals could be attributed to the bi-functional nature of the zeolite environment, which is also paramagnetic in nature. [40] In addition, two more important oxygenate-based intermediates have been identified particularly in the Mo/ZSM-5_50 sample:(i) acetal ( % 104 ( 13 C) and % 6.5 ( 1 H) ppm in Figure 2c [28,41] and (ii)surface-formate ( % 174 ( 13 C) and % 10 ( 1 H) ppm in Figure S13 a) species. [28,42] To probe rigid/immobilized molecules,b oth 2D 13 C-1 H and 13 C- 13 Cd ipolar-based correlation spectra were acquired (Figure 3, see also Figure S12, S14-S16, S18-S19).…”

“…Theexistence of surface-formate species,i.e., the 13 C correlations observed at 170-173 ppm correlating with a 1 Hsignal at % 8ppm (Figure 3d), is indicative of the occurrence of CO insertion, [46][47][48][49][50][51] which is known to generate hydrocarbon pool species during the zeolite-catalyzed MTH process (see below). [28,41,42] However,i ts hould also be mentioned that these species can only be formed in the very early stages of the reaction making use of Oa toms present in the zeolite and metal oxide Mo precursor (cf.F igure 1b). [11,12,24] On the basis of the above-mentioned results,d ifferent reaction pathways for the MDAp rocess are proposed in Scheme 2.…”