Environmentally Benign Oxidation of Alkylphenols to p-Benzoquinones: A Comparative Study of Various Ti-Containing Catalysts

Catalytic properties of Ti-containing porous solids were compared in the oxidation of 2,3,6-trimethylphenol (TMP) with H₂O₂ to produce 2,3,5-trimethyl-1,4-benzoquinone (TMBQ, vitamin E key intermediate). Mesoporous titanium–silicates with di(oligo)nuclear Ti centers, metal–organic framework MIL-125 and amorphous TiO₂ demonstrated 100 % selectivity toward TMBQ. Titanium–silicates prepared by evaporation-induced self-assembly revealed superior performance in terms of product yield and catalyst reusability.

     

Titanium-monosubstituted polyoxometalates: relation between homogeneous and heterogeneous Ti-single-site-based catalysis

The similarity in the catalytic behaviour of titanium-monosubstituted Keggin type polyoxometalates [PTi(L)W₁₁O₃₉]^n- (Ti-POMs) and heterogeneous titanium single-site catalysts in selective oxidations with H₂O₂ is demonstrated. Recent achievements in the application of Ti-POMs as soluble molecular models for studying mechanisms of oxidation catalysis are reviewed.     

Full phenol peroxide oxidation over Fe-MMM-2 catalysts with enhanced hydrothermal stability

Iron-containing mesoporous mesophase materials Fe-MMM-2 have been synthesized by a sol–mesophase route under mild acidic conditions and characterized by DRS-UV–vis, XRD, and N₂ adsorption measurements. It was found that pH of the synthesis solution and iron content in the samples affect both the textural characteristics and the state of iron atoms. Isolated iron species predominate in silica framework under Fe < 2 wt% and pH  1.0 or Fe  1 wt% and pH < 2.0. These species are stable to leaching and highly active in full H₂O₂-based phenol oxidation. The increase in iron loading and pH of the synthesis solution lead to the agglomeration and formation of oligomeric iron species, which, in turn, results in the reduction of the catalytic activity of Fe-MMM-2 and the increase of iron leaching.     

How to reach 100% selectivity in H2O2-based oxidation of 2,3,6-trimethylphenol to trimethyl-p-benzoquinone over Ti,Si-catalysts

A comprehensive study of structure/activity/selectivity relationships and mechanistic aspects of the oxidation of 2,3,6-trimethylphenol (TMP) with aqueous H₂O₂ over a wide variety of titanium-silicate catalysts allowed us to infer requirements to an optimal catalyst and optimal reaction conditions for this reaction and to produce 2,3,5-trimethyl-1,4-benzoquinone (TMBQ, Vitamin E precursor) with nearly 100% selectivity at 100% substrate conversion. The main by-products in the TMP oxidation are C–C and C–O dimers, formed by coupling of intermediate phenoxyl radicals. The formation of TMBQ is favoured by (1) a poor coordinating solvent (MeCN), (2) elevated temperature (80 °C), (3) low TMP concentration (not higher than 0.1 M), (4) high H₂O₂/TMP molar ratio (ca. 3.5), and (5) low TMP/Ti ratio (<10–20). The crucial factors which determine the selectivity of Ti,Si-catalysts in TMP oxidation to TMBQ are mesoporosity and an optimal surface concentration (ca. 0.7–1.0 Ti atoms/nm²) of accessible highly dispersed, probably, dimeric Ti(IV) species. The catalysts prepared by a simple, affordable and cheap synthesis methodology via grafting titanium(IV) precursors onto the surface of commercial mesoporous silica completely fulfil these requirements and thus can be viewed as promising catalysts for environmentally benign TMBQ production.     

MIL-101 Supported Polyoxometalates: Synthesis,Characterization, and Catalytic Applications in Selective Liquid-Phase Oxidation

In this short review paper, we survey our recent achievements in the preparation of heterogeneous catalysts via incorporation of polyoxometalates (POMs) within cages of the mesoporous coordination polymer MIL-101, their physicochemical characterization, and application for liquid-phase selective oxidation of organic compounds with green oxidants — O₂ and aqueous H₂O₂. Special attention is paid to analyze a manifestation of confinement effects and to address the issues of catalytic activity and selectivity after immobilization and after recycling, catalyst resistance to POM leaching, and the nature of catalysis. The scope and limitations of POM/MIL-101 catalysts are discussed.     

Heterogeneous Selective Oxidation of Alkenes to α,β‐ Unsaturated Ketones over Coordination Polymer MIL‐101

The mesoporous metal‐organic framework MIL‐101 is an efficient heterogeneous catalyst for the selective allylic oxidation of alkenes with tert‐butyl hydroperoxide. The selectivity towards α,β‐unsaturated ketones reaches 86–93%. The temperature of the catalyst activation strongly affects the ketone yield. MIL‐101 is stable to chromium leaching, behaves as a true heterogeneous catalyst, can be easily recovered by filtration, and can be reused several times without loss of the catalytic performance.     

Alkene oxidation catalyzed by transition metal substituted Keggin-type heteropolyanions

Alkene oxidations with various oxidants (tert-butyl hydroperoxide, iodosylbenzene and molecular oxygen in the presence of isobutyraldehyde (IBA)) catalyzed by transition metal monosubstituted heteropolyanions, PW₁₁MO ₃₉ ^n– (PW₁₁M; M=Co^II, Mn^II, Cu^II, Ti^IV, Ru^IV, V^V and Nb^V), have been studied. Orders of catalytic activity of PW₁₁M are different for the oxidants studied. Radical chain mechanisms are proposed fort-BuOOH and O₂/IBA. Preliminary coordination of the oxidant to PW₁₁M is not a necessary step of its homolytic activation. Epoxidation with PhIO requires its coordination to the catalyst and most likely includes the formation of active metal-oxo species.     

Environmentally Benign Oxidation of Alkylphenols to p-Benzoquinones: A Comparative Study of Various Ti-Containing Catalysts

Catalytic properties of Ti-containing porous solids were compared in the oxidation of 2,3,6-trimethylphenol (TMP) with H₂O₂ to produce 2,3,5-trimethyl-1,4-benzoquinone (TMBQ, vitamin E key intermediate). Mesoporous titanium–silicates with di(oligo)nuclear Ti centers, metal–organic framework MIL-125 and amorphous TiO₂ demonstrated 100 % selectivity toward TMBQ. Titanium–silicates prepared by evaporation-induced self-assembly revealed superior performance in terms of product yield and catalyst reusability.