Characterization of large-pore MCM-41 molecular sieves obtained via hydrothermal restructuring

MCM-41 periodic mesoporous silicates were synthesized using cetyltrimethylammonium bromide, and their pore sizes were tailored by postsynthesis hydrothermal treatment method of Khushalani et al., allowing us to obtain large-pore MCM-41 samples with a high degree of structural ordering. It was shown that the pore size enlargement was accompanied by a significant improvement of pore size uniformity, a gradual decrease of the specific surface area, and the pore wall thickening. After a certain upper limit of pore size (i.e., about 6.5 nm) was reached, a further hydrothermal treatment led to samples of diminished quality. The structural uniformity decreases dramatically, i.e., the pore size distribution becomes broader and a small but noticeable amount of micropores develops. Moreover, the appearance of pore blocking effects (as inferred from the irreversibility of nitrogen adsorption-desorption isotherms) indicates that the pore geometry deviates significantly from its initial cylindrical shape characteristic for MCM-41 materials. The current study confirmed previous findings that the hydrothermal restructuring method is a convenient way to synthesize high quality MCM-41 silicates with tailored pore sizes, but only up to 6.5 nm.