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 U mol(-1) adsorbed at Bronsted acid sites in boron-substituted zeolite ZSM-5 on the coordination of boron atoms in this material has been studied by H-1 and B-11 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 B 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 U 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 B-11 quadrupole coupling constant from C-QCC = 2.7 0.1 MHz for B-[3] species to C-QCC <= 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 U mol(-1) occurs. This PA >= value is ca. 30 U mol-1 higher in comparison with the proton affinity required for a protonation of probe molecules adsorbed at Bronsted acid sites in alummosilicate-type zeolites (PA = 821 kJ mol(-1)). (c) 2006 Elsevier Inc. All rights reserved.