From the outside to the inside: Elucidation of the mechanism of pseudomorphic transformation of SBA-15 into MCM-41 by following its time-resolved conversion

Abstract

Pseudomorphic transformation of silica materials is a versatile approach for the fabrication of porous phases in which the particle morphology and pore structure can be controlled independently from each other. It has been applied already for a wide variety of different parent silica materials, however, there is still a controversy about the details of the mechanism, about the manner of how the transformation proceeds through the body of the particles. The pure homogeneous distribution model on the one hand, and the core-shell model, on the other hand, are the so-to-speak extreme poles in this controversy, not excluding that a mixed-mode model can be valid for a particular parent silica phase. Herein, the detailed mechanism for the pseudomorphic transformation of ordered mesoporous SBA-15 into MCM-41 is elucidated by following the time-resolved conversion of the parent silica material. Upon pseudomorphic transformation, the characteristic morphology of the parent SBA-15 particles remains unchanged, whereas the original pore structure undergoes a successive restructuring in the presence of hexadecyl-trimethylammonium ions as a structure-directing agent. The inflow of the alkaline transformation solution within the pore system of SBA-15 initiates undirected dissolution and re-condensation processes of the pristine pore walls, resulting in an undulation, a slight shrinkage and a partial obstruction of the original mesopores. Simultaneously small domains of MCM-41 with various spatial orientations are formed on the external particle surface, restricting the access to the undulated mesopores within the particle core. The resulting core-shell structure of the partially transformed materials provides interesting gas physisorption hysteresis phenomena, caused by a cavitation mechanism during the evaporation of adsorbate from partially blocked SBA-15 pores. PXRD reflections that can be assigned to the characteristically large pore spacing of SBA-15 were found within patterns even for materials with a supposedly complete conversion. The occurrence of these reflections can be attributed to inaccessible pore domains of SBA-15. With proceeding transformation, a reduction of the amount of inaccessible SBA-15 pores was observed, finally leading to a material with typical MCM-41 characteristics but with a slightly less pore ordering due to the abrasively conditions during the long hydrothermal treatment. • Time-resolved pseudomorphic transformation of ordered mesoporous SBA-15 into MCM-41. • Original particle morphology of SBA-15 remained unchanged after transformation. • Bimodal mesoporous core-shell structures obtained by partial transformation. • Hysteresis phenomena in partially transformed samples caused by cavitation. • Inaccessibility of SBA-15 pore domains at partially transformed samples proven by nitrogen physisorption and PXRD.