Catalytic synthesis of diphenylmethane from benzene and formalin with water-tolerant solid acids

The synthesis of diphenylmethane (DPM) from benzene and formalin (a mixture of formaldehyde and water) or paraformaldehyde has been studied using vari ous solid acids. The reaction between benzene and paraformaldehyde (oligome rs of formaldehyde) proceeded readily at 140 degreesC on typical solid acid s. The activity of a silica-composite of polymer resin, Aciplex-SiO2, was F ound to be superior to that of other solid acids such as zeolites (HY,H-ZSM -5 and beta -zeolite), heteropoly compounds (H3PW12O40 and Cs2.5H0.5PW12O40 ), and the other polymer resins (Nafion-H, Nafion-SiO2, and Amberlyst). The addition of water to the reactant mixture decreased greatly the activity o f Aciplex-SiO2. While the reaction between benzene and formalin (27.3 mol% HCHO aqueous solution) was indeed very slow over Aciplex-SiO2 at 140 degree sC, the increase in the temperature to 180 degreesC brought about 42.1% con version of HCHO with 88.2% of selectivity to DPM at 6 h, when a mixture of 40 cm(3) benzene (450 mmol) and 3.36 cm(3) formalin (HCHO 45 mmol and H2O 1 11 mmol) and 1 g of Aciplex-SiO2 were used. The turnover number (TON), whic h was defined as the number of product molecule divided by the number of ac id sites in the whole bulk, reached about 36 over Aciplex-SiO2 under the ab ove reaction conditions, indicating a catalytic reaction. This is the first example of catalytic synthesis of diphenylmethane from formalin. The high catalytic performance of Aciplex-SiO2 can probably be attributed to the lar ge number of acid sites (0.46 mmol g(-1)) available for the surface reactio n, together with the strong acidity and the high hydrophobicity. (C) 2001 E lsevier Science B.V. All rights reserved.