Selective synthesis of benzoquinones over Cu(ii)-containing propylsalicylaldimine functionalized mesoporous solid catalysts.

The major product, 2,3,5-trimethyl-1,4-benzophenone (TMB[double bond, length as m-dash]O), was synthesised by an eco-friendly liquid-phase oxidation of 2,3,6-trimethylphenol (TMP-OH) over Cu(ii)-containing propylsalicylaldimine (CSA) functionalized mesoporous solid catalysts, namely, CSASBA-15(0.2), CSASBA-15(0.1) and CSAMCM-41(0.2), synthesized using various amounts of copper in a simple post-grafting method using different mesoporous silica materials, e.g., thick-silica walled SBA-15 and thin-silica walled MCM-41. The benzoquinones, i.e., 2,6-disubstituted p-benzoquinones (DSBQs), were also synthesised by the slurry-phase oxidation of di/tri-substituted phenols using the prepared catalysts. A promising chemical treatment was used for the removal of extra-framework copper species from the active surface of CSASBA-15(0.2), and the catalytic activity of the recovered catalyst, i.e. green mesoporous CSASBA-15(0.2) or W-CSASBA-15(0.2), was evaluated. Various reaction parameters, oxidants and solvents were used in this catalytic oxidation. To confirm the catalytic stability, recyclability and hot-catalytic filtration experiments were performed. On the basis of all catalytic results, it is worth noting that the mesoporous CSASBA-15(0.2) is an outstanding and a promising heterogeneous catalyst for the selective synthesis of TMB[double bond, length as m-dash]O and DSBQs, and produces 98% TMB[double bond, length as m-dash]O selectivity with 100% TMP-OH conversion at 353 K for 40 min and 97-99% DSBQ selectivity with 98-100% di/tri-substituted phenols conversion at 330 K for 1-3 h. The green mesoporous catalyst has unprecedented catalytic activity compared with that of other CSA functionalized mesoporous solid catalysts.