High-temperature reaction of SiO2 with methanol: Nucleophilic assistance of some N-unsubstituted benzazoles

• Different forms of SiO2 react slowly with methanol at 350 °C to give mainly TMOS.
• Some N-unsubstituted benzazoles increase considerably the amount of reacted SiO2.
• The key role of heterocycles is nucleophilic assistance to the reaction.
• Azole ring of heterocycles is methylated by MeOH during the reaction.
• Both SiO2-surface and TMOS catalyze the methylation of benzazole.

SiO2-containing materials (quartz, Pyrex glass, silica gel, and H-Y zeolite) react slowly with methanol at 350 °C under both supercritical and gas-phase conditions. The amount of SiO2 reacted with supercritical methanol depends on the kind of the material and is varied from 0.05 wt% for quartz sand to 4.0 wt% for wide-porous silica gel for 5 h of the reaction. The main products of the reaction are methyl orthosilicates, mainly tetramethyl orthosilicate. N-Unsubstituted 1H-indole, 1H-benzimidazole, and 1H-indazole additives considerably increase the amount of reacted SiO2-containing material. Noteworthy, quartz sand “solubility” is increased ca 14-fold when indole is used at the same reaction conditions. These benzazoles provide a nucleophilic assistance to the reaction between SiO2 and methanol. During the assistance, the azole ring is methylated by methanol, and the participation of SiO2-surface or tetramethyl orthosilicate molecule facilitates the alkylation reaction of benzazole used.

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