Plug precursor assisted synthesis: A highly efficient method of tuning the acidic and structural properties of Al-SBA-15

Abstract

An acid-free strategy shows high flexibility for tuning the structural properties of Al-SBA-15 mesoporous materials. Full-open-, plug-containing- and inkbottle-type mesostructures have been directly hydrothermally synthesized by simply adjusting the P123/Si molar ratio of the initial reaction mixture from 0.015 to 0.0020. When attaching a plug precursor addition process (denoted as Plug Precursor Assisted Synthesis, PPAS), the plug structure can be controllably generated inside the straight mesopores of the resulting Al-SBA-15. The plug-containing Al-SBA-15 sample showed that more than 43% of the mesopores were blocked by the plug structures, even with a P123/Si molar ratio condition of as high as 0.0075. More importantly, it exhibited stronger acid strength than an Al-SBA-15 sample possessing full-open-type mesopores, and was characterized by the strongest acidity with a pH value of around 2. The strong acid strength was due to aluminum migration and rearrangement, as well as to structural reconstruction during the hydrothermal treatment to form plugs, which also accelerated the generation of more tetrahedral-coordinated Al species with high accessibility. In contrast, a plug-containing Al-SBA-15 sample synthesized from an HCl aqueous medium has a relatively lower aluminum content and a weaker acid strength, although the pH value was measured to be ca. 2. To account for this result, negatively charged silica fragments are suggested to have been previously neutralized by H+ so that further interaction with Al3+ was suppressed. Therefore, the inclusion of aluminum into framework reconstruction in the subsequent hydrothermal treatment under an H+-rich synthesis condition was inhibited.