Boron Sites in Borosilicate Zeolites at Various Stages of Hydration Studied by Solid State NMR Spectroscopy

Borosilicate zeolites have recently been studied in many aspects because they are useful in substitution of framework metal ions (to Al or Ga) and in catalyzing chemical reactions that use relatively weak acid strength. Boron shows a great extent of flexibility in coordination conversion between trigonal boron, B[3], and tetrahedral boron, B[4]. Such conversion takes place in the framework of boron zeolites upon change of hydration level. Due to the character, boron can be deboronated from the proton form of dehydrated boron zeolites even under mild hydration treatment (see Reaction Scheme). The structures of framework boron atoms in boron zeolites, B-beta, B-SSZ-33, and B-SSZ-42 have been studied in the course of hydrationdehydration by employing solid state NMR methods. One and two dimensional 11B (I=3/2) NMR methods have been used for investigating such geometrical changes at a number of different hydrated situations. A new trigonal site has been identified using high resolution techniques such as 2D MQMAS and 1D MAS spectra of at a higher field (830 MHz at NHMFL). 1H-11B HETCOR NMR spectra showed improved resolution in correlating between boron sites and hydroxyl protons.