Building Porous Ni(Salen)-Based Catalysts from Waste Styrofoam via Autocatalytic Coupling Chemistry for Heterogeneous Oxidation with Molecular Oxygen

The development of robust and industrially viable catalysts from plastic waste is of great significance, and the facile construction of high performance heterogeneous catalyst systems for phenol-quinone conversions remains a grand challenge. Herein, a feasible strategy is demonstrated to reclaim Styrofoam into hierarchically porous nickel-salen-loaded hypercrosslinked polystyrene (PS@Ni-salen) catalysts with high activities through an unusual autocatalytic coupling route. The salen is immobilized onto PS chain by Friedel-Crafts alkylation of benzyl chloride derivatives, and the generated hydrogen chloride coordinately promotes the simultaneous crosslinking and bridge formation between aromatic rings via a Scholl coupling route, leading to hierarchically porous networks. After the metallization with Ni, the resultant networks exhibit high catalytic activity for the oxidation of 2,3,6-trimethylphenol to 2,3,5-trimethyl-1,4-benzoquinone under mild conditions (303 K, 1 bar of O2 ). This catalyst also demonstrates attractive recycling performance without an obvious loss of catalytic efficiency over five consecutive cycles. This methodology might provide a potential sustainable alternative to construct environmentally benign and cost-effective catalysts for specific organic transformation.

Solvent-free benzylic oxidation of aromatics over Cu(II)-containing propylsalicylaldimine anchored on the surface of mesoporous silica catalysts

A sustainable method was used to produce aromatic ketones by the solvent-free benzylic oxidation of aromatics over mesoporous Cu(II)-containing propylsalicylaldimine anchored on the surface of Santa Barbara Amorphous type material-15 (CPSA-SBA-15) catalysts. For comparison, mesoporous Cu(II)-containing propylsalicylaldimine anchored with Mobil Composition of Matter-41 (CPSA-MCM-41) was assessed for these reactions under similar reaction conditions. The washed CPSA-SBA-15(0.2) (W-CPSA-SBA-15(0.2)) catalyst was prepared using an easy chemical method for the complete removal of non-framework CuO nanoparticle species on the surface of pristine CPSA-SBA-15(0.2) (p-CPSA-SBA-15(0.2) prepared with 0.2 wt% of Cu, and its catalytic activity was evaluated with different reaction parameters, oxidants and solvents. In order to confirm the catalytic stability, the recyclability was assessed, and the performance of hot-filtration experiments was also evaluated. All the catalysts used for these catalytic reactions were characterized using many instrumental techniques to pinpoint the mesoporous nature and active sites of the catalysts. The proposed reaction mechanism has been well documented on the basis of catalytic results obtained for solvent-free oxidation of aromatics. Based on the catalytic results, we found that W-CPSA-SBA-15(0.2) is a very ecofriendly catalyst with exceptional catalytic activity.

Highly Efficient Synthesis of p-Benzoquinones Catalyzed by Robust Two-Dimensional POM-Based Coordination Polymers

Selective oxidation of alkyl-substituted phenols offers efficient access to p-benzoquinones (BQs) that serve as key components for synthesizing biologically active compounds, but rational manufacture of efficient recyclable catalysts for such a reaction remains a severe challenge. Herein, two crystalline 2D polyoxometalate-based coordination polymers (POMCPs), formulated as H₃[Cu^I₃(L)₃]₂[PM₁₂O₄₀]·xH₂O (M = Mo, x = 4 for 1; M = W, x = 6 for 2; and HL = 4-(1H-tetazol-5-yl)pyridine), are prepared by a mineralizer-assisted one-step synthesis strategy and explored as heterogeneous catalysts for p-BQs synthesis. Both compounds have been characterized through elemental analysis, EDS analysis, infrared spectroscopy, UV-vis diffuse reflectance spectrum, EPR, XPS, BET, single-crystal, and powder X-ray diffraction. Single-crystal X-ray diffraction analysis indicates that both 1 and 2 exhibit an interesting 2D sheet structure composed of 2-connected Keggin type anions [PM₁₂O₄₀]^3- and hexa-nuclear {Cu^I₆(HL)₆} cluster-based metal-organic chains via Cu···O interactions. When used as catalysts, POMCPs 1 and 2 have excellent catalytic activities in the selective oxidation of substituted phenols to p-BQs with H₂O₂. Notedly, in the model reaction from 2,3,6-trimethylphenol (TMP) to the vitamin E key intermediate trimethyl-p-benzoquinone (TMBQ), the catalytic activities expressed by turnover frequency (TOF) of 1 and 2 can reach an unprecedented 2400 and 2000 h^-1, respectively, at close to 100% TMBQ yield. The truly heterogeneous nature, stability, and structural integrity of both catalysts were ascertained by FTIR, PXRD techniques, and the following cycles. Mechanism studies reveal that both catalysts can involve a dual reaction pathway through a heterolytic oxygen atom transfer mechanism and homolytic radical mechanism. Moreover, the 2D POMCPs with highly accessible bilateral active sites and efficient mass transfer efficiency possess superior catalytic performance to their analogous 3D species.

Multiunit Catalysts with Synergistic Reactivity: Three-Dimensional Polyoxometalate-Based Coordination Polymers for Highly Efficient Synthesis of Functionalized p-Benzoquinones

The rational design of highly efficient catalysts for the synthesis of functionalized p-benzoquinones (p-BQs) is of great significance for the manufacture of bioactive compounds. Herein, two 3D crystalline polyoxometalate-based coordination polymers (POMCPs) are used as heterogeneous catalysts for the synthesis of p-BQs, which are H[Cu^II(ttb)(H₂O)₃]₂[Cu^II(ttb)Cl]₂[PW₁₂O₄₀]·4H₂O (1) (Httb = 1-(tetrazol-5-yl)-4-(triazol-1-yl)benzene) and [ClCu₆^I(trz₎₄][ClCu₅^I(trz)₄]₂[Cu^II(H₂O)][PW₁₂O₄₀] (2) (trz = 1,2,4-triazole). Both compounds were characterized by elemental analysis, IR, XPS, solid diffuse reflective spectroscopy, TG analysis, and single-crystal X-ray diffraction. In 1, Keggin anions [PW₁₂O₄₀]^3- locate in 1D square channels constructed from wave-like Cu-ttb layers to form a 3D POMCP by coordinating to Cu ions, and in 2, [PW₁₂O₄₀]^3- anions situate in eight-membered Cu-trz channels via Cu···O interactions to yield a 3D POMCP structure. The catalytic activities of 1 and 2 have been evaluated in the selective oxidation of alkylphenols/alkoxybenzenes/methylnaphthalene, especially in the oxidation reaction of 2,3,6-trimethylphenol (TMP) to 2,3,5-trimethyl-p-benzoquinone (TMBQ, vitamin E key intermediate), with H₂O₂ as oxidant. By using catalysts 1 and 2 under optimal conditions, the yield of TMBQ can reach 99% and 96% within 10-20 min, respectively. Both catalysts demonstrated high turnover frequencies (300 h^-1 for 1 and 600 h^-1 for 2) and the truly heterogeneous nature. 1 and 2 catalyzed the synthesis of p-BQs on the basis of effective cooperative catalytic activities by POMs and metal nodes.