Synthesis, catalytic activity and the structural transformation of dimeric mono-Fe (Ⅲ)-substituted Keggin-type polyoxotungstates in the oxidation of cyclohexanol with H2O2

Abstract

Three dimeric mono-Fe(III)-substituted polyoxotungstates [C16H33(CH3)3N]10[(PW11FeO39)2O]·3.5H2O (1), [(C2H5)4N]10[(PW11FeO39)2O]·3H2O (2) and [(CH3)4N]10[(PW11FeO39)2O]·4H2O (3) were synthesized and characterized by elemental analysis, TGA-DSC, FT-IR, UV–vis and SEM. The catalytic activities of them were detected in the oxidation of cyclohexanol with H2O2 as oxidant. 1 with the longest carbon chain of organic countercations showed the best catalytic activity and exhibited a reaction-controlled phase transfer behavior. Many kinds of alcohols can be also efficiently oxidized into corresponding aldehydes or ketones based on 1. The fresh catalyst, the recovered one and the catalyst 1 treated with H2O2 were characterized by IR, UV and MALDI-TOF mass spectroscopy. The results manifested that the dimeric 1 decomposed into monomeric [C16H33(CH3)3N]4PW11Fe(H2O)O39·H2O (4) after the reaction. The temperature and H2O2 had an influence on the dimer-monomer transformation process. With the addition of H2O2, the solubility of 1 in the reaction solution was enhanced and 1 gradually decomposed into 4 with the temperature increasing, until decomposed completely at high temperature (70, 80 and 90 °C), eg., the dimer-monomer transformation completed at the first 3 h of the reaction at 70 °C. The mechanism of 1-catalyzed oxidation of cyclohexanol system with H2O2 was mainly a radical-chain mechanism. It was found that a faster reaction rate was observed during the transformation process based on 1 compared to 4 at the same amount of Fe in the catalyst. This suggested that there should exist the equilibrium of [(PW11FeO39)2O]10−, [PW11Fe(OH)O39]5- and [PW11Fe(H2O)O39]4- anions during the transformation process based on the radical decomposition of H2O2, which all played a catalytic role in the oxidation of cyclohexanol. But after the transformation, the [(PW11FeO39)2O]10- anion entirely decomposed into [PW11Fe(H2O)O39]4- anion until the reaction ended.