Roles of Phosphoric Acid and H3PW12O40/K3PW12O40 Catalyst in the Conversion of Corn Stalk Cellulose into 5-Hydroxymethylfurfural: A New Insight into Mechanism by Experimental and Theoretical Investigation
The conversion of cellulose to 5-hydroxymethylfurfural (5-HMF) relies on synergistic catalysis by Brønsted and Lewis acids and is governed by cellulose accessibility in heterogeneous systems. Herein, we demonstrate that phosphoric acid (H3PO4) markedly facilitates the conversion of corn stalk cellulose into 5-HMF catalyzed by spherical particles of phosphotungstic acid (PTA) supported on its potassium salt (K3PW12O40). The H3PO4 solution effectively dissolves corn stalk cellulose. Mechanistic investigation reveals that H3PO4 and H2O molecules are initially inserted into the cellulose molecular chains. Subsequent ionization of the H3PO4 molecules generates charged cellulose fragments, creating a strong thermodynamic driving force. The resulting electrostatic repulsion among these charged species ultimately leads to dissociation of the cellulose framework. Concurrently, our results demonstrate that the H3PW12O40/K3PW12O40 particles function as dual Lewis and Brønsted acids, efficiently catalyzing the conversion of the dissolved cellulose to 5-HMF with a yield of 54%. This yield surpasses that obtained from untreated cellulose (38.7%) under the identical reaction conditions. The reaction pathway was investigated through a combination of experimental and theoretical approaches. Results indicate that the dehydration reaction between ring-opening glucose and protonated PTA leads to the formation of a new intermediate featuring an unsaturated tungsten center. This species acts as a Lewis acid site capable of accepting electron pairs and thereby facilitating the isomerization of ring-opening glucose to fructose. Over six consecutive catalytic cycles, the yield of 5-HMF remained nearly constant at 40.8%, demonstrating the high stability and recyclability of the H3PW12O40/K3PW12O40 catalyst. Such recyclability not only improves the process sustainability but also underscores the potential of this heterogeneous catalyst for cost-effective industrial applications.
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