Supercritical Fluid-Mediated Alumination of Mesoporous Silica and Its Beneficial Effect on Hydrothermal Stability

Abstract

We have investigated the use of supercritical fluids (SCFs) as carriers/solvents during the postsynthesis alumination of mesoporous silica. SCFs were found to be ideally suited for transport of Al into mesoporous silica and to lead to Al-grafted aluminosilicate materials that exhibit exceptional hydrothermal (steam) stability even for highly aluminated materials. The improvements in steam stability arising from the use of SCFs as grafting media (as compared to aqueous or organic solvents) are remarkable, especially for Al-grafted MCM-41 materials with high (Si/Al < or = 10) Al contents. It is proposed that under supercritical fluid conditions Al is sorbed on the surface of the pore walls of the host Si-MCM-41 with little penetration into the pore wall region, that is, the low solvating power of SCFs ensures the deposition of Al onto rather than into the silica framework. This is because the host silica framework cannot undergo any significant hydrolysis (to allow penetration of Al into the pore wall region) during the SCF-mediated alumination. Removal of the Al (i.e., dealumination) which occurs during steaming is therefore less detrimental to the structural integrity of SCF-grafted Al-MCM-41 materials since any dealumination that occurs will not involve removal of Al from deep within the pore walls.

Porous Materials and Supercritical Fluids

“…To investigate whether or not there was any specific advantage of using the solvents in the supercritical state, an Al-grafted material was synthesized under the equivalent conditions but using liquefied propane instead of SCF propane. [109] This material was found to have lower hydrothermal stability, suggesting that the SCF route led to better dispersion of the Al, a change in the way that the Al interacts with the host silica surface, or a combination of both.…”

“…For example, MCM-41 silica materials have been aluminized by reaction with aluminium isopropoxide using either scCO 2 or supercritical propane as the solvent (reaction temperature = 40 C for CO 2 , 110 C for propane). [109] The samples were then calcined at 600 C for 4 h to obtain the Algrafted materials. [27] Al magic-angle spinning (MAS) NMR spectroscopy indicated that ca.…”

Advances in Microporous and Mesoporous Solids—Highlights of Recent Progress

“…[22,23] However, the reac- tions have not been adopted commercially because the limited thermal, hydrothermal, and mechanical stabilities of conventional mesoporous sieves, such as the M41S materials, make these supports impractical under typical conditions employed in large-scale industrial applications. In the last few years, improvements in hydrothermal stability of mesoporous silicates have been achieved by the addition of salts during the hydrothermal synthesis (which can improve the degree of condensation of the silicate framework, [24] but sometimes catalyze the collapse of mesopores [25] ), by modification of the sieve surface (to eliminate reactive surface silanol groups) by post-synthesis grafting of aluminum onto such mesostructures, [26] and by increasing the wall thickness. These methods have been reviewed by Liu and Pinnavaia.…”

Synthesis and Characterization of Bifunctionalized Ordered Mesoporous Materials

“…[33,34] Because of the low density, low viscosity, low surface tension, and high diffusivity inherent to supercritical fluids, [35] uniform dispersion of functional groups grafted onto the pore walls can be expected. [34,36] It has been reported that a large number of silanol groups are present in hybrid OMMs synthesized via direct synthesis, [37,38] meaning that a grafting reaction could be performed, at least in principle, on these materials to result in bifunctionalized mesoporous materials.…”