Effect of probe molecules with different proton affinities on the coordination of boron atoms in dehydrated zeolite H-[B]ZSM-5

The influence of probe molecules with proton affinities of PA = 812–930 kJ mol−1 adsorbed at Brønsted acid sites in boron-substituted zeolite ZSM-5 on the coordination of boron atoms in this material has been studied by 1H and 11B MAS NMR spectroscopy. The probe molecules are, in the sequence of their PA values, acetone (PA = 812 kJ mol−1), tetrahydrothiophene (PA = 849 kJ mol−1), ammonia (PA = 854 kJ mol−1), acetamide (PA = 864 kJ mol−1), dimethyl sulfoxide (PA = 884 kJ mol−1) and pyridine (PA = 930 kJ mol−1). By 11B MAS NMR spectroscopy, it has been found that loading of zeolite H-[B]ZSM-5 with probe molecules characterized by proton affinities of PA ⩾ 854 kJ mol−1 leads to a transformation of trigonally coordinated boron species (B[3]) into tetrahedrally coordinated boron species (B[4]). This coordination change is accompanied by a decrease of the 11B quadrupole coupling constant from CQCC = 2.7 ± 0.1 MHz for B[3] species to CQCC ⩽ 0.85 MHz for B[4] species. In addition, a proton transfer from the SiOH[B] groups in the vicinity of framework boron atoms in zeolite H-[B]ZSM-5 to probe molecules with a proton affinity of PA ⩾ 854 kJ mol−1 occurs. This PA value is ca. 30 kJ mol−1 higher in comparison with the proton affinity required for a protonation of probe molecules adsorbed at Brønsted acid sites in aluminosilicate-type zeolites (PA = 821 kJ mol−1).