Aldehyde-Alcohol Reactions\nCatalyzed under Mild Conditions\nby Chromium(III) Terephthalate Metal Organic Framework (MIL-101) and\nPhosphotungstic Acid Composites

Abstract

Porous materials based on chromium­(III) terephthalate\nmetal organic\nframeworks (MIL-101) and their composites with phosphotungstic acid\n(PTA) were studied as heterogeneous acid catalysts in aldehyde-alcohol\nreactions exemplified by acetaldehyde-phenol (A-P) condensation and\ndimethylacetal formation from benzaldehyde and methanol (B-M reaction).\nThe MIL-101 was synthesized solvothermically in water, and the MIL101/PTA\ncomposite materials were obtained by either impregnation of the already\nprepared MIL-101 porous matrix with phosphotungstic acid solution\nor by solvothermic treatment of aqueous mixtures of Cr­(NO₃)₃, and terephthalic and phosphotungstic acids. The MIL101/PTA\nmaterials appeared to be effective catalysts for both A-P and B-M\nreactions occurring at room temperature, with half-lives ranging from\n0.5 h (A-P) to 1.5–2 h (B-M) and turnover numbers over 600\nfor A-P and over 2900 for the B-M reaction, respectively. A synergistic\neffect of the strong acidic moieties (PTA) addition to mildly acidic\nBrønsted and Lewis acid cites of the MIL-101 was observed with\nthe MIL101/PTA composites. The ability of the PTA and MIL101/PTA materials\nto strongly absorb and condense acetaldehyde vapors was discovered,\nwith the MIL101/PTA absorbing over 10-fold its dry weight of acetaldehyde\ncondensate at room temperature. The acetaldehyde was converted rapidly\nto crotonaldehyde and higher-molecular-weight compounds while in contact\nwith MIL-101 and MIL101/PTA materials. The stability of the MIL-101\nand MIL101/PTA catalysts was assessed within four cycles of the 1-day\nalcohol-aldehyde reactions in terms of the overall catalyst recovery,\nPTA or Cr content, and reaction rate constants in each cycle. The\nloss of the catalyst over 4 cycles was approximately 10 wt % for all\ntested catalysts due to the incomplete recovery and minute dissolution\nof the components. The reaction rates in all cycles remained unchanged\nand the catalyst losses stopped after the third cycle. The developed\nMIL101/PTA composites appear to be feasible for industrial catalytic\napplications.