Textural Mesoporosity and the Catalytic Activity of Mesoporous Molecular Sieves with Wormhole Framework Structures

Abstract

Three different water−alcohol cosolvent systems were used to assemble mesoporous molecular sieve silicas with wormhole framework structures (previously denoted HMS silicas) from an electrically neutral amine surfactant (S°) and a silicon alkoxide precursor (I°). The fundamental particle size and associated textural (interparticle) porosity of the disordered structures were correlated with the solubility of the surfactant in the water−alcohol cosolvents used for the S°I° assembly process. Polar cosolvents containing relatively low volume fractions of CnH2n+1OH alcohols (n = 1−3) gave heterogeneous surfactant emulsions that assembled intergrown aggregates of small primary particles with high textural pore volumes (designated HMS−HTx). Conversely, three-dimensional, monolithic particles with little or no textural porosity (designated HMS−LTx) were formed from homogeneous surfactant solutions in lower polarity cosolvents. Aluminum substituted Al-HMS−HTx analogues with high textural porosity and improved framework accessibility also were shown to be much more efficient catalysts than Al-HMS−LTx or monolithic forms of hexagonal Al-MCM-41 for the sterically demanding condensed phase alkylation of 2,4-di-tert-butylphenol with cinnamyl alcohol. Transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) studies verified the textural differences between wormhole HMS and electrostatically assembled hexagonal MCM-41 and SBA-3 molecular sieves. Power law fits to the scattering data indicated a surface fractal (Ds = 2.76) for HMS−HTx, consistent with rough surfaces. A second power law at lower-q indicated the formation of a mass fractal (Dm = 1.83) consistent with branching of small fundamental particles. Hexagonal MCM-41 and SBA-3 silicas, on the other hand, exhibited scattering properties consistent with moderately rough surfaces (Ds = 2.35 and 2.22, respectively) and large particle diameters (≫1 μm). HMS−LTx silicas showed little or no mass fractal character (Dm = 2.87), and no surface fractal scattering.