Pyrolysis of Mesoporous Silica-Immobilized 1,3-Diphenylpropane. Impact of Pore Confinement and Size

Abstract

Mesoporous silicas such as SBA-15 and MCM-41 are being actively investigated for potential applications in catalysis, separations, and synthesis of nanostructured materials. A new method for functionalizing these mesoporous silicas with aromatic phenols is described. The resulting novel hybrid materials possess silyl aryl ether linkages to the silica surface that are thermally stable to ca. 550 degrees C, but can be easily cleaved at room temperature with aqueous base for quantitative recovery of the organic moieties. The materials have been characterized by nitrogen physisorption, FTIR, NMR, and quantitative analysis of surface coverages. The maximum densities of 1,3-diphenylpropane (DPP) molecules that could be grafted to the surface were less than those measured on a nonporous, fumed silica (Cabosil) and were also found to decrease as a function of decreasing pore size (5.6-1.7 nm). This is a consequence of steric congestion in the pores that is magnified at the smaller pore sizes, consistent with parallel studies conducted using a conventional silylating reagent, 1,1,3,3-tetramethyldisilazane. Pyrolysis of the silica-immobilized DPP revealed that pore confinement leads to enhanced rates and altered product selectivity for this free-radical reaction compared with the nonporous silica, and the rates and selectivities also depended on pore size. The influence of confinement is discussed in terms of enhanced encounter frequencies for bimolecular reaction steps and pore surface curvature that alters the accessibility and resultant selectivity for hydrogen transfer steps.

Reaction-dependent heteroatom modification of acid–base catalytic cooperativity in aminosilica materials

“…1 for the adsorption isotherms) in all cases. It is critical to maintain the same pore diameters among all catalysts because previous studies demonstrated that the pore diameter affects the cooperativity between amines and surface silanols [3,[59][60][61][62][63]. The amine functionalization procedure did not alter the pore diameter, indicating that the procedure was sufficiently mild.…”

Selectivity Enhancement in Dynamic Kinetic Resolution of Secondary Alcohols through Adjusting the Micro-Environment of Metal Complex Confined in Nanochannels: A Promising Strategy for Tandem Reactions

“…On the other hand, for all SBA-15-supported ruthenium species, Ru/Si-9, Ru/Si-8, and Ru/Si-7, the racemization efficiency decreased with increasing pore size, 15 whereas the Ru-catalyzed acylation was totally suppressed (entries 2−4). Catalysts supported on SBA-15 exhibited selectivity over racemization and acylation.…”

Silica-Based Mesoporous Nanospheres

“…[182][183][184] Grafting is commonly carried out by silylation on free ( Si-OH) and geminal silanol (¼ Si(OH) 2 ) groups. If a high surface coverage with functional groups is desired, it is important to maintain a large amount of surface silanol groups after removal of the surfactant.…”