Tr. Pauly et al., Textural mesoporosity and the catalytic activity of mesoporous molecular sieves with wormhole framework structures, J AM CHEM S, 121(38), 1999, pp. 8835-8842
Three different water-alcohol cosolvent systems were used to assemble mesop
orous molecular sieve silicas with wormhole framework structures (previousl
y denoted HMS silicas) from an electrically neutral amine surfactant (S deg
rees) and a silicon alkoxide precursor (I degrees). The fundamental particl
e size and associated textural (interparticle) porosity of the disordered s
tructures were correlated with the solubility of the surfactant in the wate
r-alcohol cosolvents used for the S degrees I degrees assembly process. Pol
ar cosolvents containing relatively low volume fractions of CnH2n+1OH alcoh
ols (n = 1-3) gave heterogeneous surfactant emulsions that assembled interg
rown aggregates of small primary particles with high textural pore volumes
(designated HMS-HTx). Conversely. three-dimensional, monolithic particles w
ith little or no textural porosity (designated HMS-LTx) were formed from ho
mogeneous surfactant solutions in lower polarity cosolvents. Aluminum subst
ituted Al-HMS-HTx analogues with high textural porosity and improved framew
ork 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 de
manding condensed phase alkylation of 2,4-di-tert-butylphenol with cinnamyl
alcohol. Transmission electron microscopy (TEM) and small-angle X-ray scat
tering (SAXS) studies verified the textural differences between wormhole HM
S and electrostatically assembled hexagonal MCM-41 and SBA-3 molecular siev
es. Power law fits to the scattering data indicated a surface fractal (D-s
= 2.76) for HMS-HTx, consistent with rough surfaces. A second power law at
lower-q indicated the formation of a mass fractal (D-m = 1.83) consistent w
ith branching of small fundamental particles. Hexagonal MCM-41 and SBA-3 si
licas, on the other hand, exhibited scattering properties consistent with m
oderately rough surfaces (D-s = 2.35 and 2.22, respectively) and large part
icle diameters (much greater than 1 mu m). HMS -LTx silicas showed little o
r no mass fractal character (D-m = 2.87), and no surface fractal scattering
.