Cooperative Catalysis Mechanism of Brønsted and Lewis Acids from Al(OTf)₃ with Methanol for β-Cellobiose-to-Fructose...

Al-containing catalysts, e.g., Al(OTf)₃, show good catalytic performance toward the conversion of cellulose to fructose in methanol solution. Here, we report the catalytic isomerization and alcoholysis mechanisms for the conversion of cellobiose to fructose at the PBE0/6-311++G(d,p), aug-cc-pVTZ theoretical level, combining the relevant experimental verifications of electrospray ionization mass spectrometry (ESI-MS), high-performance liquid chromatography (HPLC), and the attenuated total reflection-infrared (ATR-IR) spectra. From the alcoholysis of Al(OTf)₃ in methanol solution, the catalytically active species involves both the [CH₃OH₂]⁺ Brønsted acid and the [Al(CH₃O)(OTf)(CH₃OH)₄]⁺ Lewis acid. There are two reaction pathways, i.e., one through glucose (glycosidic bond cleavage followed by isomerization, w-G) and another through cellobiulose (isomerization followed by glycosidic bond cleavage, w-L). The Lewis acid ([Al(CH₃O)(OTf)(CH₃OH)₄]⁺) is responsible for the aldose-ketose tautomerization, while the Brønsted acid ([CH₃OH₂]⁺) is in charge of ring-opening, ring-closure, and glycosidic bond cleavage. For both w-G and w-L, the rate-determining steps are related to the intramolecular [1,2]-H shift between C1-C2 for the aldose-ketose tautomerization catalyzed by the [Al(CH₃O)(OTf)(CH₃OH)₄]⁺ species. The Lewis acid ([Al(CH₃O)(OTf)(CH₃OH)₄]⁺) exhibits higher catalytic activity toward the aldose-ketose tautomerization of glycosyl-chain-glucose to glycosyl-chain-fructose than that of chain-glucose to chain-fructose. Besides, the Brønsted acid ([CH₃OH₂]⁺) shows higher catalytic activity toward the glycosidic bond cleavage of cellobiulose than that of cellobiose. Kinetically, the w-L pathway is predominant, whereas the w-G pathway is minor. The theoretically proposed mechanism has been experimentally testified. These insights may advance on the novel design of the catalytic system toward the conversion of cellulose to fructose.