Toward efficient nanoporous catalysts: Controlling site-isolation and concentration of grafted catalytic sites on nanoporous materials with solvents and colorimetric elucidation of their site-isolation

We report that the polarity and dielectric constants of solvents used for grafting organosilanes on mesoporous materials strongly affect the concentration of grafted organic groups, the degree of their site-isolation, and the catalytic properties of the resulting materials. Polar and nonpolar organosilanes as well as polar-protic, dipolar-aprotic, and nonpolar solvents were investigated. Polar-protic solvents, which have high dielectric constants, resulted in smaller concentrations (similar to 1 -2 mmol/g) of polar organic groups such as 3-aminopropyl groups, higher surface area materials, site-isolated organic groups, and more efficient catalytic properties toward the Henry reaction of p-hydroxybenzaidehyde with nitromethane. On the other hand, dipolar-aprotic and nonpolar solvents resulted in larger concentrations (similar to 2-3 mmol/g) of grafted polar functional groups, lower-to-higher surface area materials, more densely populated catalytic groups, and poor-to-efficient catalytic properties toward the Henry reaction. Both the polar-protic and dipolar-aprotic solvents resulted in significantly lower concentration of grafted groups for nonpolar organosilanes such as (3-mercaptopropyl)trimethoxysilane compared to corresponding grafting of the polar amino-organosilanes. The relationship between the solvent properties and the percentage and degree of site-isolation of the grafted functional groups was attributed to differences in solvation of the organosilanes and silanols in various solvents and possible hydrogen-bonding between the organsilanes and the solvents. The degree of site-isolation of the amine groups, which affect the material's catalytic properties, was elucidated by a new colorimetric method involving probing of the absorption maxima (lambda max) on the d-d electronic spectrum of Cu2+ complexes with the amine-functionalized materials and the colors of the samples. The absorption lambda(max) and the colors of the materials were found to be uniquely dependent on the type of solvents used for grafting the organoamines. For instance, the monoamine- and diamine-functionalized samples grafted in methanol resulted in pale blue and light purple colors with lambda(max) at similar to 720 and 650 nm, respectively. These correspond to CuNO5 and CuN2O4 structures, respectively, which are indicative of the presence of site-isolated organcamines in samples grafted in methanol. The monoamine and diamine samples grafted in toluene resulted in purple and deep purple colors with lambda(max) at similar to 590 and 630 nm, respectively. These correspond to CuN2O4 and CuN4O2, which are indicative of the presence of closely spaced organoamines in samples grafted in toluene. The samples grafted in isopropanol gave colors and lambda(max) intermediate between those of samples grafted in toluene and methanol.