Hydroxylation of Benzene to Phenol by Molecular Oxygen over an Organic–Inorganic Hybrid Catalyst: Schiff Base Manganese Complex Attached to Molybdovanadophosphoric Heteropolyacid

Abstract

The organic-inorganic hybrid catalyst L-MnPMoV was prepared simply by combining a schiff base Mn complex (L-Mn, L: N,N 0 -disalicylidene-1, 6-hexanediamine) with the Keggin-structured molybdovanadophosphoric heteropolyacid (PMoV). The proposed composition and structure of the catalyst were evidenced by TG, elemental analysis, FT-IR, and UV-Vis characterizations. Its catalytic performance was evaluated in the direct hydroxylation of benzene to phenol by molecular oxygen with ascorbic acid as the reducing agent. Various reaction parameters were changed to attain the optimal conditions. The hybrid catalyst has a formula [{Mn(C 20 H 22 N 2 O 2 )(Cl)} 2 (H 4 PMo 11 VO 40 )], with the two terminal oxygen atoms in the PMoV Keggin structure coordinately linked to the two Mn(III) ions in two L-Mn units, respectively. It exhibits a remarkably enhanced yield to phenol compared to the pure PMoV due to the synergy effect between the Schiff-base manganese complex and PMoV.

Liquid phase hydroxylation of benzene to phenol over vanadyl acetylacetonate supported on amine functionalized SBA-15

“…The phenol yield first increases obviously up to 9.5 % at an acetic acid concentration of 80 %, and then sharply decreases to 1.6 % as the acetic acid concentration is increased to 100 %. The results confirm that a proportion of acetic acid and water is helpful for the dissolution of reactants and products because benzene tends to be dissolved in the organic phase and phenol is transferred to aqueous phase [23]. So the optimum acetic acid concentration is 80 %.…”

“…For example, in the work of Kharat et al [20], SBA-15 was first modified by APTES, and the Keggin type vanadium substituted molybdophosphoric acid was then supported on the amine functionalized SBA-15, which showed good activity and stability in the direct hydroxylation of benzene to phenol, owing to strong acidbase interaction between heteropoly anions and amine group. For improving the stability of the POM catalysts, organic units such as ionic liquids and organic polymers have been used to prepare POM based organic-inorganic hybrid heterogeneous catalysts, such as L-Mn-PMoV [23], PMoV n -b-CyDs [24] and Py n PMo 11 V [25]. Although high activity and convenient recoverability were achieved, complex steps and a serial of solvents were required [26].…”

Hybrid compounds of Schiff base Cu, Fe, Co complexes with molybdovanadophoric heteropolyacids: synthesis, characterization and their catalytic performance to hydroxylation of benzene with H2O2

“…Thus the factors facilitate the stabilization of low oxidation state metal or V 4+ species, and formation of high oxidation state metal or V 5+ oxo‐peroxo intermediate species are favorable for accelerating oxidation reactions. In the prepared hybrid compounds, the Schiff base metal complex acts as active center as well as a modifier to create an electron donor–acceptor system between transition metal center and vanadium‐substituted heteropolyacid, as depicted in previous reports . As a result, this electron transfer between the Schiff base metal complex and heteropolyacid facilitates the stabilization of intermediate species, leading to the improvement of oxidative activity.…”

“…In the present study, the preparation of hybrid compounds of Schiff base complex and heteropolyacid is via electrostatic interactions. As reported in the typical preparation of hybrid polyoxometalate complex, in a suitable solvent the simple direct mixing of polyanion and cationic metallorganic species leads to insoluble hybrid compound formation . In the presence of strong heteropolyacid H 3+ n PMo 12− n V n O 40 , the 2,3‐ diaminopyridine Schiff base ligand derivatives saldmp or fpdmp with 2‐pyridyl on the diimine bridge and corresponding metal complexes accept the proton at the pyridyl residues associated with the remaining heteropolyacid anion to form hybrid compounds.…”

“…For example, Neumann and co‐workers studied a series of organometallic–polyoxometalate hybrid compounds (M‐salen–, Pt(II)pyridinium–, Pd(II) phenanthroline–crown ether–polyoxometalate), and demonstrated that a stabilized charge transfer relationship (organometallic donor–polyoxometalate acceptor) is formed to connect the two functional centers . Wang Jun's group investigated the hydroxylation of benzene in the presence of various organic–inorganic hybrid polyoxometalate‐based compounds, including L‐Mn‐PMoV (L: N , N ′‐disalicylidine‐1,6‐ hexanediamine), pyridine‐PMoV and Dmim] 2.5 PMoV, demonstrating that the synergy between the organic and inorganic unit in hybrid compounds has a promoting effect on the improvement of the reactivity of polyoxometalate.…”