Heterogeneous Catalyst For Oxidation Research Articles

Depolymerization of lignin: Recent progress towards value-added chemicals and biohydrogen production

The need for alternative sources of energy became increasingly urgent as demand for energy and the use of fossil fuels both soared. When processed into aromatic compounds, lignin can be utilized as an alternative to fossil fuels, however, lignin's complex structure and recalcitrance make depolymerization impractical. This article presented an overview of the most recent advances in lignin conversion, including process technology, catalyst advancement, and case study-based end products. In addition to the three established methods (thermochemical, biochemical, and catalytic depolymerization), a lignin-first strategy was presented. Depolymerizing different forms of lignin into smaller phenolic molecules has been suggested using homogeneous and heterogeneous catalysts for oxidation or reduction. Limitations and future prospects of lignin depolymerization have been discussed which suggests that solar-driven catalytic depolymerization through photocatalysts including quantum dots offers a unique pathway to obtain the highly catalytic conversion of lignin.

Metal-Support Interaction in Gold Zeolitic Octahedral Metal Oxide and the Catalytic Activity for Low-Temperature Alcohol Oxidation.

Au nanoparticles are efficient catalysts for selective oxidations. The interaction between Au nanoparticles and supports is critical for achieving high catalytic activity. Herein, Au nanoparticles are supported on a zeolitic octahedral metal oxide based on Mo and V. The charge of Au is controlled by the surface oxygen vacancies of the supports, and the redox property of the zeolitic vanadomolybdate is highly dependent on Au loading. The Au-supported zeolitic vanadomolybdate is used as a heterogeneous catalyst for alcohol oxidation under mild conditions with molecular oxygen as an oxidant. The supported Au catalyst can be recovered and reused without the loss of activity.

Direct Valorization of Recycled Palladium as Heterogeneous Catalysts for Total Oxidation of Methane

Electronic waste as an alternative source of palladium is used for the preparation of heterogeneous catalysts for methane oxidation. The strategy developed in this paper involves two steps: optimization of the leaching protocol to recover the Pd from the waste, and development of a suitable impregnation protocol directly from the leachate, which is a strongly acidic solution. The precipitation-deposition technique is particularly suitable for the preparation of catalysts from acidic media. The impact of Ba, Bi, and Ag impurities on the catalytic performance is studied on model catalysts. The presence of these elements and their impact on the catalytic properties are analyzed and discussed using X-ray diffraction, nitrogen physisorption, scanning and transmission electron microscopy. Through variation of the pH, the impurities co-deposited with Pd can be limited and the catalytic performance of catalysts derived from real waste can be greatly enhanced.

Kinetic analysis of aerobic oxidation catalyzed by a hybrid heterogeneous-homogeneous system containing supported Mn and V oxides and N-hydroxyphthalimide

The activities of transition metal oxide (TMO) heterogeneous catalysts based on individual components such as VOX, and MnOX supported on TiO2 and TiO2-SiO2 as cocatalysts in catalytic systems complemented with N-hydroxyphthalimide (NHPI) in the processes of aerobic liquid-phase radical chain oxidation of alkylarenes (RH) was investigated. The mechanisms of catalysis by NHPI in a liquid phase and participation of TMO in the chain initiation stage were discussed. Catalytic activities of Mn- and V-oxide catalysts were compared with the results of molecular hydrogen temperature programmed reduction profiles (H2-TPR) of the catalysts, revealing correlation between them. Reaction enthalpies calculated via a semi-empirical method for model structures of catalytic centers explained the obtained experimental results. Finally, catalytic cumene oxidation was presented using a reactions scheme identifying the concomitant role of heterogeneous initiation and homogeneous propagation and termination steps for the oxidation process proceeding via a free-radical mechanism.

Distinguishing homogeneous advanced oxidation processes in bulk water from heterogeneous surface reactions in organic oxidation

Clarifying the reaction pathways at the solid-water interface and in bulk water solution is of great significance for the design of heterogeneous catalysts for selective oxidation of organic pollutants. However, achieving this goal is daunting because of the intricate interfacial reactions at the catalyst surface. Herein, we unravel the origin of the organic oxidation reactions with metal oxide catalysts, revealing that the radical-based advanced oxidation processes (AOPs) prevail in bulk water but not on the solid catalyst surfaces. We show that such differing reaction pathways widely exist in various chemical oxidation (e.g., high-valent Mn3+ and MnOX) and Fenton and Fenton-like catalytic oxidation (e.g., Fe2+ and FeOCl catalyzing H2O2, Co2+ and Co3O4 catalyzing persulfate) systems. Compared with the radical-based degradation and polymerization pathways of one-electron indirect AOP in homogeneous reactions, the heterogeneous catalysts provide unique surface properties to trigger surface-dependent coupling and polymerization pathways of a two-electron direct oxidative transfer process. These findings provide a fundamental understanding of catalytic organic oxidation processes at the solid-water interface, which could guide the design of heterogeneous nanocatalysts.

Efficient abatement of recalcitrant p-phenol sulfonic acid by CoFe2-xMoxO4 catalyzed peroxymonosulfate

P-phenol sulfonic acid (PSA) is a very noxious and refractory emerging pollutant owing its chemical stability. In this study, CoFe2-xMoxO4 as heterogeneous catalysts were fabricated via a hydrothermal route and investigated as peroxymonosulfate (PMS) activator for PSA removal. X-ray diffraction (XRD), Transmission electron microscopy (TEM), and Brunauer-Emmett-Teller (BET) analyses indicate a well-maintained structure, suitable particle size, and larger surface area nanoparticles which offer a further active surface for the catalytic reaction. X-Ray photoelectron spectroscopies (XPS) confirm the valence and chemical structure of CoFe2-xMoxO4 with different ratios of Mo ion. CoFe2-xMoxO4/PMS degraded 88% with rate constant of 0.109 min−1 in 60 min under the reaction conditions ([PSA] = 185 mg L−1, [CoFe2-xMoxO4] = 0.3 g L−1, [PMS] = 5 g L−1, T = 20 °C, pH = 3.5). The increasing content of Mo (i.e. 0.1–1.0) in CoFe2-xMoxO4 enhanced the degradation rate, signifying that the catalytic performance was positively correlated with Mo content in the catalyst composition. The results of electron paramagnetic resonance (EPR) suggested that SO4·− and HO· radicals predominated in PMS activation by CoFe2-xMoxO4. Overall, the findings offer the applicability of such metal oxide heterogeneous catalysts for the remediation of highly recalcitrant pollutants in ambient conditions.

Mononuclear ruthenium (II) complex covalently anchored on melem and g-C3N4 as efficient heterogeneous catalysts for chemical water oxidation

Ru-melem and Ru-C3N4 were synthesized by a simple and facile strategy to construct a novel covalently anchoring by introducing easily synthesized amide bond as a bridge connecting the Ru-terpy and melem or g-C3N4, respectively. The covalent anchoring of Ru complex on melem or C3N4 not only makes these materials exhibit water oxidation activity under CeIV-driven (CeIV = Ce(NH4)2(NO3)6) reaction condition, but also makes the obtained heterogeneous catalysts show higher catalytic activity than the corresponding homogeneous catalysts, which reveals that the covalent anchoring strategy of Ru complex is beneficial to improve the catalytic activity of homogeneous Ru catalysts. The synthetic method of hybrid catalysts offers an insightful strategy for enhancing water oxidation activity of molecular catalysts.

Mn(III) Schiff base complexes containing crown ether rings immobilized onto MCM-41 matrix as heterogeneous catalysts for oxidation of alkenes

Six catalysts MnL1–MnL6, containing two crown ether rings and their analogs supported on the MCM-41 heterogeneous substrate (MnL1@MCM41-MnL5@MCM41) were synthesized and characterized. A mixture of molecular oxygen, as an oxidant, and these catalysts were used for the epoxidation of styrene. As a general result, the supported catalysts showed better performance compared with the unsupported analogs. On the other hand, the supported species, in addition to recyclability, did not require an axial base and reducing agent.

Selective Styrene Oxidation Catalyzed by Phosphate Modified Mesoporous Titanium Silicate

Selective oxidation of organics over an efficient heterogeneous catalyst under mild liquid phase conditions is a very demanding chemical reaction. Herein, we first report the modification of the surface of mesoporous silica MCM-41 material by phosphate for the efficient incorporation of Ti(IV) in the silica framework to obtain highly ordered 2D hexagonal mesoporous material STP-1. STP-1 has been synthesized by using tetraethyl orthosilicate, triethyl phosphate, and titanium isopropoxide as Si, P, and Ti precursors, respectively, in the presence of cationic surfactant cetyltrimethylammonium bromide (CTAB) under hydrothermal conditions. The observed specific surface area and pore volume of STP-1 were 878 m2g−1 and 0.75 ccg−1, respectively. Mesoporous STP-1 has been thoroughly characterized by XRD, FT-IR, Raman spectroscopy, SEM, and TEM analyses. Titanium incorporation (Ti/Si = 0.006) was confirmed from the EDX analysis. This mesoporous STP-1 was used as a heterogeneous catalyst for the selective oxidation of styrene into benzaldehye in the presence of dilute aqueous H2O2 as an oxidizing agent. Various reaction parameters such as the reaction time, the reaction temperature, and the styrene/H2O2 molar ratio were systematically studied in this article. Under optimized reaction conditions, the selectivity of benzaldehyde could reach up to 93.8% from styrene over STP-1. Further, the importance of both titanium and phosphate in the synthesis of STP-1 for selective styrene oxidation was examined by comparing the catalytic result with only a phosphate-modified mesoporous silica material, and it suggests that both titanium and phosphate synergistically play an important role in the high selectivity of benzaldehyde in the liquid phase oxidation of styrene.

Degradation of Methylene Blue Using Fe-ZSM-5 Zeolite as a Heterogeneous Fenton Catalyst

A Fe2O3 modified ZSM-5 hybrid was synthesized by solid phase exchange method in this work. The prepared Fe-ZSM-5 material was characterized by XRD and SEM and was used as a heterogeneous Fenton catalyst in oxidation of methylene blue. The influences of different parameters such as the initial pH value, the amount of catalyst, amount of H2O2, contact time and reaction temperature on the removal rate of methylene blue were evaluated. Results showed that under the optimum conditions of catalyst dosage of 1.1 g/L, initial pH value of 3.0, reaction temperature of 30°Cand H2O2 30 mmol/L, 98.2% of decolorization rate could be achieved in 60 minutes. This work provides a promising catalyst for the decolorization of methylene blue in aqueous solution.

Manganese-sulfonato porphyrin adsorbed on amino kaolinite as heterogeneous catalyst for oxidation and polymerization reactions

Due to its medical applications, poly-caprolactone (PCL) is one of the most studied polymers. Herein, adsorption was used as a route to synthesize a new kaolinite/metalloporphyrin-based material for application in caprolactone (ε-CL) polymerization. Kaolinite (Kaol) was modified with tris(hydroxymethyl)aminomethane (TRIS), and the resulting Kaol-TRIS was used as adsorbent for 5,10,15,20-tetrakis(4-sulfonatophenyl)manganese(III) porphyrinate (MnTSPP). Kaol-MnTSPP was also prepared for comparison purposes. Kaolinite grafting with TRIS molecules increased the Kaol basal value from 7.2 to 12.7 Å. MnTSPP immobilization on Kaol-TRIS decreased the intensity and broadened the Kaol-TRIS reflection peaks, evidencing Kaol-TRIS exfoliation. Mathematical equation models of Sips and Langmuir were used to fit the adsorption data, indicating a monolayer adsorption model (maximal adsorption = 8.78 mg·g−1). Kaol-TRIS-MnTSPP stability and efficiency as a catalyst of cis-cyclooctene (C8) and cyclohexane oxidations were tested. Kaol-TRIS-MnTSPP obtained from 600 mg·L−1 MnTSPP aqueous solution provided the highest C8 conversion to cyclooctenoxide (81%). MnTSPP did not leach from Kaol-TRIS-MnTSPP, evidencing that TRIS acted as a binder for MnTSPP and enhanced its attachment to the matrix. In contrast, purified Kaol interacted with MnTSPP weakly: MnTSPP completely leached from Kaol-MnTSPP after the first catalytic cycle. Finally, Kaol-TRIS-MnTSPP was used to initiate ε-CL polymerization. Monitoring of the reaction by infrared absorption spectroscopy showed that the bands around 3438 cm−1, attributed to free hydroxyls in the ε-CL monomer aliphatic structure, became less intense. This indicated that the ε-CL cyclic monomer structure was lost, with subsequent polymerization. This work has proven that the chemical modification of Kaol with subsequent adsorption of a metalloporphyrin is an important route for synthesizing initiator catalysts of polymerization reactions in the synthesis of new materials.

Advances in limitations and opportunities of clean biofuel production to promote decarbonization

Factors such as higher fuel prices, limited and geologically posited fossil fuel resources along with the impact of fossil fuels on global warming are highly unsustainable for growing economies around the globe. For this reason, an unconventional sustainable energy root is desired. In recent years, biodiesel has gained much consideration because of its similar combustion properties like fossil fuels and its synthesis from edible and non-edible oils via a catalytic process. Various catalysts (homogeneous and heterogeneous in nature) have been investigated for sustainable and green production of biodiesel. The heterogeneous catalyst calcium oxide is an abundant, cheap and has a high activity for transesterification reaction. The review provides that there is strong need to focus on exploring the continuous heterogeneous nanoscale calcium oxide catalyst to improve the efficiency and cost-effectiveness biodiesel production technology. Calcium oxide catalytic activity can be improved by nanotechnology approach. This approach helps in the production of suitable catalyst at a low cost, minimizes the waste, and results in higher activity and selectivity for desired reaction. This study provides an overview on the literature existing on the preparation of nanosized-Calcium oxide (CaO), utilization for conversion of ester into biodiesel along, advantages over other heterogeneous catalysts and importance as a future catalyst for the production of biodiesel.

Enhanced low-temperature behavior of selective catalytic reduction of NOx by CO on Fe-based catalyst with looping oxygen vacancy

The industrial applications of de-NOx require the high activity window of the catalysts to be shifted from high to low temperatures for energy saving. Selective catalytic reduction (SCR) of NOx by CO, as an alternative to NH3-SCR, is a promising de-NOx technology that facilitates the synergistic removal of pollutants. Herein, we report a new Fe2O3/amorphous SiO2 catalyst with high low-temperature activity for CO-SCR. A combination of experiments and density functional theory (DFT) calculations obtains direct evidence that an optimal content of oxygen vacancy enhances the low-temperature NO conversion ratio and N2 selectivity of Fe2O3-based catalyst to over 80% and maintain 100% CO conversion by promoting NN coupling via ONNO/N2O and NCO/NNCO2 intermediates. Compared with conventional defect engineering, such as doping other metals or artificially generating oxygen defects, the α-Fe2O3 (110) facet automatically enables an oxygen vacancy generation-filling cycle in the CO-SCR reaction, following Mars-van Krevelen (MvK) mechanism. This work advances a fundamental understanding of CO-SCR reaction mechanism over Fe2O3-based catalysts and provides insights for future studies to improve heterogeneous metal oxide catalysts.

Facile synthesis of M-CuFe2O4 (M= Al2O3, CeO2, La2O3, ZrO2, Mn2O3) catalysts using a one-pot method and their applications in high-temperature water gas shift reaction

Water gas shift reaction is an essential process of hydrogen production and carbon monoxide removal from syngas. In this study, the promotional effect of ZrO2, CeO2, La2O3, Al2O3, and Mn2O3 was investigated on the CO conversion and thermal stability of the copper ferrite in high-temperature water gas shift reaction (HTSR) and hydrogen purification. The powders were synthesized by a simple solid-state route and characterized by XRD, H2-TPR, SEM, FT-IR, TG-DTA, and BET analyses. Promoters (ZrO2, CeO2, La2O3, Al2O3, and Mn2O3) could affect the WGSR performance in activity and stability. In the M-CuFe2O4 catalyst, alumina acts as a texture promoter and aids in the fine dispersion of copper ferrite. The results indicated that the surface area of the Al2O3–CuFe2O4 (210 m2/g) catalyst was higher than the other samples. This catalyst presented higher CO conversion in HTSR and had higher stability at 1000 min on stream. It was found that the incorporation of different contents of alumina had a significant influence on the textural and catalytic properties of the CuFe2O4-based catalysts. The 30%Al2O3–70%CuFe2O4 catalyst exhibited the highest CO conversion of 65% at 350 °C, uniform pore size distribution, and intense interaction between copper ferrite and alumina, causing the effective stabilization of the active phase in the catalyst structure. The findings of this study represent that the solid-state method, due to its simplicity and creation of a mesoporous structure, can also be applied for the preparation of many heterogeneous metal oxide catalysts.

Synthesis of UiO-66-Sal-Cu(OH)2 by a Simple and Novel Method: MOF-Based Metal Thin Film as a Heterogeneous Catalyst for Olefin Oxidation

Metal-organic frameworks (MOFs), particularly UiO-66-NH2, are employed as a catalyst in many industrial catalyst applications. As converting catalysts into thin film significantly increases their catalytic properties for the epoxidation of olefins, we report a general approach to synthesizing MOF thin films (UiO-66-Sal-Cu(OH)2). Using the postsynthesis method (PSM), UiO-66-NH2 was functionalized with salicylaldehyde and entrapped on copper hydroxide nanoparticle surfaces using a modern strategy (MOF thin film). We used field-emission scanning electron microscopy (FE-SEM), EDX (energy-dispersive X-ray analysis), XRD (X-ray diffraction), FT-IR (Fourier transform infrared), BET (Brunauer-Emmett-Teller), TGA (thermogravimetric analysis), XPS (X-ray photoelectron spectroscopy), and ICP-MS (inductively coupled plasma mass spectrometry) to determine the structure and morphology of the synthesized UiO-66-Sal-Cu(OH)2. The oxidation of cyclooctene by the UiO-66-Sal-Cu(OH)2 thin film was studied. Due to its advantages, such as being environmentally friendly (base metal-loaded catalyst, room temperature, solvent-free reaction), reusability, and high yield, this compound can be an appropriate catalyst for the oxidation of olefins.

Catalytic ozonation of toluene and dichloromethane mixture at low temperatures over modified MnOx-based catalyst

Catalytic ozonation is an efficient method for VOCs abatement at low temperatures, in which the rational design of highly active heterogeneous catalysts constitutes the most significant step. In this study, Al2O3 was first optimized by loading MnOx. And then K was used for modifying MnOx/Al2O3 catalyst, which exhibited 2.4 × 10−3 mol∙g−1∙s−1 oxidation rate for dichloromethane and toluene oxidation with ca. 0.89 ppm ozone detected at 80 °C. That could be directly applied for removing toluene and dichloromethane mixture or coupled with other treatment technologies (such as biological purification). Besides, appropriate K doping could increase the amount of reactive surface oxygen species and the ratio of Mn2+, hence contributing to an enhanced oxidation ability. By contrast, an excess K doping inevitably lowered the activity of active surface oxygen species and interfered with the transformation of Mn3+ to Mn2+, which resulted in a weakened oxidative ability and thus a decreased catalytic activity. Finally, the reaction mechanisms over 0.1 K-MnOx/Al2O3 were tried to be demonstrated that the degradation of toluene mainly followed the pathway of toluene – phenylcarbinol – benzaldehyde – benzoic acid – phenyl group – small-molecule species and/or CO2 while the route of dichloromethane – methanol – formaldehyde – formic acid – small-molecule species and/or CO2 explained the catalytic oxidation of dichloromethane. All of these findings may facilitate the design of novel heterogeneous catalysts for VOCs oxidation and progress the development of catalytic ozonation for toluene and dichloromethane abatement.

PH-controlled assembling of POM-based metal–organic frameworks for use as supercapacitors and efficient oxidation catalysts for various sulfides

Two 3D POM-based metal–organic frameworks were prepared, which can be applied as electrode materials for supercapacitors, as well as efficient heterogeneous catalysts for the selective oxidation of sulfides to their corresponding sulfoxides.

Building N-hydroxyphthalimide organocatalytic sites into a covalent organic framework for metal-free and selective oxidation of silanes.

A novel crystalline covalent organic framework COF-NHPI was built by a bottom-up strategy to guarantee highly ordered embedment of the N-hydroxyphthalimide (NHPI) units as nitroxyl radical organocatalytic sites. The COF-NHPI was demonstrated to be a metal-free, highly selective and heterogeneous catalyst for the efficient oxidation of various silanes to the corresponding silanols. Mechanistic studies revealed that the critical phthalimido N-oxyl radical was generated in situ to govern the catalysis.

A robust strategy to boost the proton transfer of heterogeneous catalysts for efficient and sustainable water oxidation towards practical applications

A facile yet robust strategy was developed for accelerating the proton transfer of metal hydroxides via incorporating Lewis basic sites, thereby achieving sustainable water oxidation towards practical applications.

Preparation of N and Ce Co-doped MIL-101(Fe) Heterogeneous Catalysts for Efficient Electro-Fenton Oxidation

Design and synthesis of high-efficiency and stable heterogeneous catalysts for the electro-Fenton process remain challenging. In this study, a co-doped Ce and N metal–organic framework MIL-101(Fe) heterogeneous catalyst (Ce/N-MIL-101(Fe) was synthesized by a facile one-step solvothermal method. Polyvinyl pyrrolidone played dual roles during the fabrication process, which not only acted as a nitrogen source for doping but also prevented MIL-101(Fe) molecules from aggregating and reinforcing the formation crystal with a complete polyhedral crystal shape. The introduction of Ce significantly enhanced the density of active sites and promoted the Fe3+/Fe2+ cycle of the reaction system. Owing to the specific structure and properties, the rate of electron transfer in the catalytic process was significantly promoted. The synthesized Ce/N-MIL-101(Fe) catalyst exhibited a high degradation rate for dyes (>98%) with competitive stability and reusability. Moreover, compared with common electro-Fenton catalysts usually operated in a limited pH range of 3.0–6.0, the working pH range of Ce/N-MIL-101(Fe) was widened up to 10.0. The design concept for producing abundant active sites with co-doping into frameworks can be extended to more heterogeneous electro-Fenton catalysts constituted by other porous materials.