Boron vs. aluminum in ZSM-5 zeolites: Solid-state NMR, acidity, and C1/C2 reactant conversion

Three zeolite catalysts with comparable amounts of aluminum and/or boron ([Al]ZSM-5, [B,Al]ZSM-5, and [B]ZSM-5) are herein synthesized. Water (H 2 O), ammonia (NH 3 ), acetonitrile- d 3 (ACN), and trimethylphosphine oxide (TMPO) are applied as probe molecules to investigate the acidity of the respective materials in combination with 11 B, 27 Al, and 29 Si MAS NMR spectroscopy. Ammonia is not protonated to ammonium on Si(OH)B groups and only LAS-bound ammonia persists desorption. Thus, ammonia gives a realistic, quantitative picture of the present acid sites. ACN interacts only with Si(OH)Al as desired. The strong base TMPO results in a misleading, not quantifiable picture. Subsequent hydration is unsuited to distinguish BAS and LAS densities. The samples were catalytically tested in the conversion of methanol, ethanol, and ethylene. [B]ZSM-5 is unreactive in hydrocarbon formation due to absence of BAS, instead LAS are present. The mixed [B,Al]ZSM-5 shows a decreased lifetime in MTO conversion compared to the [Al]ZSM-5, due to LAS presence. A sometimes reported superior reactivity of [B,Al]ZSM-5 catalysts is thus explained primarily by an optimized BAS density. • Synthesis of [Al]ZSM-5, [B]ZSM-5, and [B,Al]ZSM-5 with similar physicochemical properties. • Ammonia is identified as quantitative probe for BAS and LAS, but is only adsorbed on Si(OH)B. • TMPO-loading and hydration is unsuited to characterize the acid site densities quantitatively. • No reactivity of Si(OH)B in methanol, ethanol, or ethylene conversion. • [B,Al] ZSM-5 outperformed by [Al]ZSM-5 in terms of MTO lifetime due to LAS presence.