Variable anchoring of boron in zeolite beta

Abstract

The transformation of boron species in H–B-beta and H,Na–B-beta zeolites has been studied by 11B and 1H MAS NMR as well as 1H{ 11B} Rotational Echo Double Resonance in combination with infrared spectroscopy. Tetrahedral and trigonal boron are found in the hydrated zeolites. Only trigonal boron is observed in the pure proton form for the dehydrated zeolite. The degree of boron anchoring in the framework (number of B–O–Si bridges) increases with higher dehydration temperature. The 11B isotropic chemical shift of trigonal boron correlates with the number of B–O–Si connectivities, and can be utilized to identify B(OSi) x (OH) 3− x species with different degrees of framework anchoring, x: 10.5 ± 0.7 ppm ( x = 3), 15.4 ± 0.5 ppm ( x = 1), and 18.5 ± 0.6 ppm ( x = 0). The transformation of boron anchoring and coordination numbers is reversible, depending on the conditions (temperature, humidity). Five 1H NMR and four IR lines are resolved for H–B-beta and assigned to three different classes of OH groups: class A are defect silanol groups (1.7 ppm/3745 cm −1), class B are Si–OH⋯B [3] sites and weakly hydrogen bonded SiOH and BOH groups (2.0, 2.3 ppm/3735 cm −1), and class C groups are due to hydrogen bonded SiOH, BOH and possibly water (3.0, 4.3 ppm/∼3680, 3530 cm −1).