Boron removal and reinsertion studies in 10B–11B exchanged HAMS-1B (H-[B]-ZSM-5) borosilicate molecular sieves using solid-state NMR

Abstract

Novel atomic-level insight in boron removal and reinsertion into the framework of a HAMS-1B (H-[B]-ZSM-5) borosilicate molecular sieve was obtained by a combination of wet chemistry and one-/two-dimensional 11B solid-state NMR (SSNMR) spectroscopy. Uncalcined HAMS-1B shows only tetrahedral boron. However, three boron species are observed in 11B SSNMR spectra of as-prepared and then calcined HAMS-1B: tetrahedral framework boron ([4]Bfr), trigonal framework boron ([3]Bfr), and non-framework trigonal boron ([3]BNF). A picture has emerged as to the origins of these three species. Trigonal boron species are formed via hydrolysis by reaction with the water formed from water release and water formed by oxidation and removal of the template during calcination. The trigonal boron species are readily removed from the framework by slurrying in water or mild acid solutions. Tetrahedral boron remains at a concentration about equal to that in the calcined sieve not slurried, indicating that it is more difficult to remove. The extent of boron removal and reinsertion is pH dependent. We demonstrate that boron is removed to a greater extent at low pH and can be reinserted when pH is increased. Boron reinsertion into the framework is proven by 11B SSNMR on a series of 10B–11B exchanged borosilicate zeolites. We found that when boron is reinserted it enters at higher concentrations (∼40% more) as tetrahedral boron, not trigonal boron, thus reversing partial hydrolysis and removal during calcination.