RELEASE OF BORON DURING CONVERSION OF MFI BORALITE TO AMMONIUM AND HYDROGEN FORMS

Conversion of synthetic boralites with the MFI struture to ammonium and hydrogen forms is accompanied by release of skeletal boron - up to 50% of initial content - without changes in the crystallinity (determined by XRD). The deboronation level β of the hydrogen form is greater than that of the ammonium variety, and for both types it increases with increasing B/u.c., i.e., with decreasing SiO2/B2O3 molar ratio. As a result of hydrolysis, ammonolysis, and dehydroxylation reactions, boron vacancies occur that loosen the structure and cause the u.c. volume of the hydrogen forms to be greater than are the corresponding "as-made" boralites. Aluminium impurities (from the synthesis raw materials) are significant for the modified forms with B/u.c. ≤ 0.78 (SiO2/B2O3 ≥ 245) and can generate Brönsted acidity. The boron content (B/u.c.), the deboronation level (β), the content of vacancies per unit cell, and the unit cell content of the modified forms depends on a single synthesis parameter, viz. the SiO2/B2O3 of the reaction mixture. © 1990 Butterworth-Heinemann.