A Counterion-Catalyzed (S⁰H⁺)(X^-I⁺) Pathway toward Heat- and Steam-Stable Mesostructured Silica Assembled from Amines in Acidic Conditions

Abstract

An alternative pathway to assemble mesoporous molecular sieve silicas is developed using nonionic alkylamines and N,N-dimethylalkylamines (S0) as structure-directing agents in acidic conditions. The synthesized mesostructures possess wormhole-like frameworks with pore sizes and pore volumes in the range of 20−90 Å and 0.5−1.3 cm3/g, respectively. The formation of the mesophase is controlled by a counterion-mediated mechanism of the type (S0H+)(X-I+), where S0H+ are protonated water molecules that are hydrogen bonded to the lone electron pairs on the amine surfactant headgroups (S0H+), X- is the counteranion originating from the acid, and I+ are the positively charged (protonated) silicate species. We found that the stronger the ion X- is bonded to S0H+, the more it catalyzes the silica condensation into (S0H+)(X-I+). Br- is shown to be a strong binding anion and therefore a fast silica polymerization promoter compared to Cl- resulting in the formation of a higher quality mesophase for the Br- syntheses. We also showed that the polymerization rate of the silica, dictated by the counterion, controls the morphology of the mesostructures from nonuniform agglomerated blocks in the case of Br- syntheses to spherical particles for the Cl- syntheses. Next to many benefits such as low temperature, short synthesis time, and the use of inexpensive, nontoxic, and easily extractable amine templates, the developed materials have a remarkable higher thermal and hydrothermal stability compared to hexagonal mesoporous silica, which is also prepared with nonionic amines but formed through the S0I0 mechanism.