Pore Size Control of Highly Ordered Mesoporous Silica MCM-48

The synthesis of highly ordered cubic mesoporous silica MCM-48 with tunable pore sizes in the range 1.6−3.8 nm is reported. Pore size control has been achieved by using (mixtures of) gemini surfactants, [CH 3 (CH 2 ) n NMe 2 (CH 2 ) m NMe 2 (CH 2 ) n CH 3 ] 2+ 2Br -, of different average chain length ( m = 12 and n = 15, 21) as a structure-directing agent and by hydrothermal restructuring at pH > 9 and temperatures >100 °C. A mesoporosity index, M, is introduced to describe porosity features of periodic mesoporous silica (PMS) materials. M is shown to provide distinct values for high quality MCM-48 ( M 0.34−0.35) and MCM-41 materials ( M 0.29−0.31) and to sensitively indicate microporosity in mesoporous and macroporous solids. Surface silylation based on tetramethyldisilazane, HN(SiHMe 2 ) 2, reveals different surface silanol populations in dependency of the pore size, and hence, of the surface curvature.