Solid-state NMR studies of the borosilicate mesoporous molecular sieve MCM-41

Abstract

A series of boron-substituted mesoporous MCM-41 sieves has been synthesized with a wide ranging content of boron (Si/B ratio from 40 to 5) and has been studied by XRD, ¹¹B and ²⁹Si solid-state NMR. Boron enters the silicate framework and there are two different 4-coordinate boron sites in the as-prepared products. The interplanar d₁₀₀ spacing varies from 36 to 40 Å depending on the Si/B ratio. There is no significant contraction of the unit cell upon calcination, but some 4-coordinate boron is converted into the less stable 3-coordination. The 3-coordinate boron can be fully reconverted to 4-coordination by equilibrating the sample with the water vapour over a saturated aqueous solution of NH₄Cl at room temperature. When the sample is immersed in water for 3 days at 50 °C, some 3-coordinate boron is removed from the framework. The 4-coordinate B is removed upon further treatment of the hydrated calcined sample with water vapour at 150 °C.