Catalyst Species Research Articles

TiO2 supported pallidum-bipyridyl complex as an efficient catalyst for Suzuki–Miyaura reaction in aqueous-ethanol

Owing to their improved catalytic stability and ability to undergo repeated cycles, solid-supported catalysts show great potential for various catalytic reactions. In this study, we synthesized a catalyst comprising a palladium-2,2-bipyridine complex supported on TiO2 nanoparticles (TiO2@BDP-PdCl2) fully characterised and investigated its efficacy in Suzuki–Miyaura cross coupling reactions involving phenyl boronic acid with various aryl halides under mild reaction conditions. The 2,2′- bipyridine (bp) has shown excellent complexation properties for Pd (II) and it could be easily anchored onto functionalized TiO2 support by the bridging carboxylate ions. The composition and structure of the as-prepared catalyst was characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), Transmission Electron Microscope (TEM), X-ray photoelectron spectroscopy (XPS), and UV–Vis spectroscopy. The catalyst easily demonstrated separability, enhancing its practicality in catalytic processes. Subsequent utilization showed a consistent activity level, suggesting the stabilization of the aggregated catalyst species. This research sheds light on the importance of catalyst stability and maintenance during consecutive reaction cycles.

A method to synthesize specific active Cu sites of single-atom catalysts with high stability for acetylene hydration

Single-atom catalysts (SACs) have received much attention in the realm of energy and catalytic conversion due to their maximum atomic efficiency. Herein, we report a cascade anchoring strategy for the preparation of a Cu-S1O2 species of single-atom catalyst attached to a carbon carrier containing P and S (Cu-S1O2 SA/CPS) with a content of 12.4 wt%. Over the Cu-S1O2 SA/CPS catalyst, the conversion of 95.8% and selectivity of 87.2% for acetylene hydration could still be achieved at 70 h (T = 200°C, GHSV(C2H2) = 90 h−1 and VH2O/VC2H2 = 4). X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) tests reveal that the Cu atoms of Cu-S1O2 SA/CPS are predominantly coordinated in a trinary manner (Cu-S1O2). Based on high-resolution aberration-corrected high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM), it is demonstrated that the Cu single-atom sites are highly dispersed in Cu-S1O2 SA/CPS. It is evident from the scanning electron microscopy (SEM) that Cu-S1O2 SA/CPS has a two-dimensional layered structure. The specific structure of the active site Cu is primarily attributed to the coordination of S and O atoms, resulting in its superior stability for acetylene hydration towards the synthesis of acetaldehyde. Density functional theory (DFT) calculations confirm that the formation of the Cu-S1O2 site facilitates the activation of acetylene, which is a pivotal step in the acetylene hydration process and considered as the rate-determining step. This article not only introduces an innovative strategy in the synthesis of Cu SACs but also represents a significant breakthrough in the stability of Cu SACs in acetylene hydration.

Unraveling the Prominent Existence of Trace Metals in Photocatalysis: Exploring Iron Impurity Effects.

Metal impurities can complicate the identification of active catalyst species in transition metal catalysis and electrocatalysis, potentially leading to misleading findings. This study investigates the influence of metal impurities on photocatalysis. Specifically, the photocatalytic reaction of inert alkanes using chlorides without the use of an external photocatalyst was studied, achieving successful C(sp3)-H functionalization. The observations reveal that Fe and Cu impurities are challenging to avoid in a typical laboratory environment and are prominently present in normal reaction systems, and iron impurities play a dominant role in the aforementioned apparent 'metal-free' reaction. Additionally, iron exhibits significantly higher catalytic activity compared to Cu, Ce, and Ni at low metal concentrations in the photocatalytic C(sp3)-H functionalization using chlorides. Considering the widespread presence of Fe and Cu impurities in typical laboratory environments, this study serves as a reminder of their involvement in reaction processes.

Resolving the Active Role of Isolated Transition Metal Species in Ni-Based Catalysts for Dry Reforming of Methane

Resolving the Active Role of Isolated Transition Metal Species in Ni-Based Catalysts for Dry Reforming of Methane

Unveiling Mechanistic Complexity in Manganese-Catalyzed C-H Bond Functionalization Using IR Spectroscopy Over 16 Orders of Magnitude in Time.

ConspectusAn understanding of the mechanistic processes that underpin reactions catalyzed by 3d transition metals is vital for their development as potential replacements for scarce platinum group metals. However, this is a significant challenge because of the tendency of 3d metals to undergo mechanistically diverse pathways when compared with their heavier congeners, often as a consequence of one-electron transfer reactions and/or intrinsically weaker metal-ligand bonds. We have developed and implemented a new methodology to illuminate the pathways that underpin C-H bond functionalization pathways in reactions catalyzed by Mn-carbonyl compounds. By integrating measurements performed on catalytic reactions with in situ reaction monitoring and state-of-the-art ultrafast spectroscopic methods, unique insight into the mode of action and fate of the catalyst have been obtained.Using a combination of time-resolved spectroscopy and in situ low-temperature NMR studies, we have shown that photolysis of manganese-carbonyl precatalysts results in rapid (<5 ps) CO dissociation─the same process that occurs under thermal catalytic conditions. This enabled the detection of the key states relevant to catalysis, including solvent and alkyne complexes and their resulting transformation into manganacycles, which results from a migratory insertion reaction into the Mn-C bond. By systematic variation of the substrates (many of which are real-world structurally diverse substrates and not simple benchmark systems) and quantification of the resulting rate constants for the insertion step, a universal model for this migratory insertion process has been developed. The time-resolved spectroscopic method gave insight into fundamental mechanistic pathways underpinning other aspects of modern synthetic chemistry. The most notable was the first direct experimental observation of the concerted metalation deprotonation (CMD) mechanism through which carboxylate groups are able to mediate C-H bond activation at a metal center. This step underpins a host of important synthetic applications. This study demonstrated how the time-resolved multiple probe spectroscopy (TRMPS) method enables the observation of mechanistic process occurring on time scales from several picoseconds through to μs in a single experiment, thereby allowing the sequential observation of solvation, ligand substitution, migratory insertion, and ultimate protonation of a Mn-C bond.These studies have been complemented by an investigation of the "in reaction flask" catalyst behavior, which has provided additional insight into new pathways for precatalyst activation, including evidence that alkyne C-H bond activation may occur before heterocycle activation. Crucial insight into the fate of the catalyst species showed that excess water played a key role in deactivation to give higher-order hydroxyl-bridged manganese carbonyl clusters, which were independently found to be inactive. Traditional in situ IR and NMR spectroscopic analysis on the second time scale bridges the gap to the analysis of real catalytic reaction systems. As a whole, this work has provided unprecedented insight into the processes underpinning manganese-catalyzed reactions spanning 16 orders of magnitude in time.

Catalysis using gold containing materials

This perspective article is based on a plenary lecture delivered by the author at Europacat 2023 in Prague on 29th August in recognition of the Boudart Award of EFCATS in 2021. The article charts the journey taken by the author in the discovery and application of gold catalysts over the preceding 40 years of research. Five topics are discussed. The first three parts describe three facets of monometallic gold catalysts. (1) CO oxidation and the hierarchy of activity of gold species supported on a reducible oxide Fe2O3; (2) acetylene hydrochlorination for the manufacture of vinyl chloride monomer using highly dispersed gold cations supported on carbon; (3) methane oxidation using molecular oxygen with gold nanoparticles dispersed on the zeolite H-ZSM-5. These first three parts emphasise that the active species of a gold catalyst is dependent on the reaction being catalysed and it is not a one size fits all for this catalysis. The final two parts concern gold palladium bimetallic catalysts. (4) alcohol oxidation and the new Cooperative Redox Enhancement effect (CORE); and (5) the use of in situ generation of hydrogen peroxide to enable new oxidative cascade processes. These examples demonstrate the rich tapestry that catalysis by gold can provide to the catalysis community globally, both from an academic and an industrial perspective as well as the societal impact that has been achieved by the commercialisation of gold catalysts.

The role of coordination-unsaturated Cu1-O3 species in the single-atom catalyst Cu/In2O3 for the preparation of acetic acid from methane and carbon dioxide

The co-conversion of CH4 and CO2 has emerged as a challenging issue in C1 chemistry. Converting these gases into a high-value product like acetic acid is an attractive solution. In this study, we employed density-functional theory calculations to design dual-active single-atom catalysts M/In2O3 (M=Co, Ni, Cu, Zn, Rh, Pd, Ag, Cd), which incorporated oxygen vacancies. These catalysts were created by introducing metal M into the surface of In2O3 (111) through doping. Cu/In2O3, Zn/In2O3, Ag/In2O3, and Cd/In2O3 were selected based on the stability of forming single-atom catalysts. Our findings indicated that the formation of the M/In2O3 surface activates the In surface activity and significantly lowers the activation energy for methane dehydrogenation. Among them, the Cu1-O3 species on the Cu/In2O3 surface demonstrates a lower activation energy for methane dehydrogenation and C-C coupling reactions. The "H-O-C-O" hydrogenation of CH3COO* species bonded with the "n"-type structure is more realistic. Microkinetic analysis indicated that the average turnover frequency for the forming of acetic acid on the Cu/In2O3 surface is three orders of magnitude greater than that of methyl formate. This research provides theoretical support for the development of copper single-atom catalysts for the efficient conversion of CO2 and CH4 into acetic acid.

Asymmetric Synthesis with Organoselenium Compounds - The Past Twelve Years.

The discovery and synthetic applications of novel organoselenium compounds and their reactions proceeded rapidly during the past fifty years and such processes are now carried out routinely in many laboratories. At the same time, the growing demand for new enantioselective processes provided new challenges. The convergence of selenium chemistry and asymmetric synthesis led to key developments in the 1970s, although the majority of early work was based on stoichiometric processes. More recently, greater emphasis has been placed on greener catalytic variations, along with the discovery of novel reactions and a deeper understanding of their mechanisms. The present review covers the literature in this field from 2010 to early 2023 and encompasses asymmetric reactions mediated by chiral selenium-based reagents, auxiliaries, and especially, catalysts. Protocols based on achiral selenium compounds in conjunction with other species of chiral catalysts, as well as reactions that are controlled by chiral substrates, are also included.

First-principles study of the electronic and catalytic properties of nickel-antimony (Ni-Sb) alloy catalyst for hydrogen evolution reaction

Hydrogen evolution reaction (HER) is a vital half-reaction for water splitting to produce H2. Exploring efficient, affordably priced, and environmentally sustainable HER catalysts is necessary for the development of renewable energy conversion and storage technologies. Here, the HER catalytic activity of hexagonal NiSb(010), hexagonal NiSb(001), hexagonal NiSb(101), cubic Ni3Sb(100), cubic Ni3Sb(010), cubic Ni3Sb(101), orthorhombic Ni3Sb(010), and orthorhombic Ni3Sb(101), is systematically investigated by density functional theory methods. Based on the calculated surface energy, all the studied surfaces are stable. Subsequently, we further evaluate their HER activity by calculating Gibbs free energy of hydrogen adsorption (ΔG*H) and exchange current density (i0). The results show that hexagonal NiSb and orthorhombic Ni3Sb are expected to be promising HER catalysts in Ni-Sb alloys because their ΔG*H values are respectively −0.03 and 0.03 eV, and they have larger i0 values. The synergistic interaction between the metal atoms of Ni and Sb can effectively promote electron transfer between the catalyst and reaction species, which is clearly demonstrated by the electronic structure analysis, which also illustrates that such high catalytic activity can be attributed to it. This work is conducive to guiding the design of this kind of alloy electrocatalysts for HER.

Cinchona-Based Hydrogen-Bond Donor Organocatalyst Metal Complexes: Asymmetric Catalysis and Structure Determination.

In this study, we describe the synthesis of cinchona (thio)squaramide and a novel cinchona thiourea organocatalyst. These catalysts were employed in pharmaceutically relevant catalytic asymmetric reactions, such as Michael, Friedel-Crafts, and A3 coupling reactions, in combination with Ag(I), Cu(II), and Ni(II) salts. We identified several organocatalyst-metal salt combinations that led to a significant increase in both yield and enantioselectivity. To gain insight into the active catalyst species, we prepared organocatalyst-metal complexes and characterized them using HRMS, NMR spectroscopy, and quantum chemical calculations (B3LYP-D4/def2-TZVP), which allowed us to establish a structure-activity relationship.

Modulation of active metal species in MOF-derived catalysts for efficient NO reduction by CO

MOF-derived porous catalysts achieve high catalytic activity for converting NO and CO to N2 and CO2 (100% selectivity).

The influence of H2O or/and O2 introduction during the low-temperature gas-phase sulfation of organic COS + CS2 on the conversion and deposition of sulfur-containing species in the sulfated CeO2-OS catalyst for NH3-SCR.

Herein, the typical components of blast furnace gas, including H2O and O2, were introduced to improve the NH3-SCR activity of the sulfated CeO2-OS catalyst during the gas-phase sulfation of organic COS + CS2 at 50 °C. The characterization results demonstrate that the introduction of O2 or H2O during gas-phase sulfation enhances the conversion of organic COS + CS2 on a cubic fluorite CeO2 surface and reduces the formation of sulfur and sulfates in the catalyst, but decreases the BET surface area and pore volume of the sulfated CeO2-OS catalyst. However, the introduction of O2 or H2O during the gas-phase sulfation increases the molar ratios of Ce3+/(Ce3+ + Ce4+) and Oβ/(Oα + Oβ + Oγ) on the sulfated CeO2-OS catalyst surface, thus promoting the formation of surface oxygen vacancies and chemisorbed oxygen, and these properties of the catalyst are further enhanced by the co-existence of O2 and H2O. Furthermore, the reduction of sulfates formed under the action of O2 or H2O decreases the weak acid sites of the sulfated CeO2-OS catalyst, but the few and highly dispersive sulfates present stronger reducibility, and the proportion of medium-strong acid sites of the catalyst increases. These factors help to improve the NH3-SCR activity of the sulfated CeO2-OS catalyst. Thus, there exists a synergistic effect of H2O and O2 introduction during gas-phase sulfation on the physical-chemical properties and catalytic performance of the sulfated CeO2-OS catalyst by organic COS + CS2 at 50 °C.

Ultrathin two-dimensional (2D) manganese-based metal-organic framework nanosheets for selective photocatalytic oxidation of thioether.

The efficiency of photocatalysts depends largely on the accessibility of reaction species to the active centre, the electron transfer and geometric matching between the active surface of the catalyst and reaction species. In this work, we successfully synthesized and designed one two-dimensional Mn(II) MOF with [Mn2(H2L1)(H2O)2(DMF)2]n·(CH3CH2OH)n (HSTC 3) by using MnCl2·4H2O and 5,5'-(anthracene-9,10-diyl)diisophthalic acid (H4L1), in which the adjacent layers are stacked with weak interactions, and the huge gap leads to the interpenetration between layers to form a 2D + 2D → 3D interpenetration frame. Based on the particularity of the structure of HSTC 3, ultrasonic wall breaking methods were tried to successfully peel HSTC 3 into nanosheets (HSTC 3-NS), thus achieving a significant improvement in a series of optoelectronic properties due to exposure to more active centres for HSTC 3-NS. These results significantly enhance the photocatalytic selective oxidation of thioether. This study provides a new insight into the post-synthesis modification of MOF photocatalyst and their application in photocatalytic organic synthesis.

First principles investigation of manganese catalyst structure and coordination in the p-xylene oxidation process

The oxidation of p-xylene to terephtalic acid is important for the production of polyethylene terephthalate (PET). This work investigates the coordination of the Mn catalyst species under operating conditions.

Enhanced ruthenium selectivity for the conversion of FAMEs to diesel-range alkanes by surface decoration of FeOx species

Ruthenium-based catalysts have been widely used in the lignin depolymerization and polystyrene hydrogenolysis reactions due to its superior hydrogenolysis activity for C-O and C-C bonds. However, serious cleavage of C-C bonds at high temperatures greatly limits its application in the production of green biodiesel from the conversion of natural oils and bio-derived fatty esters. In this work, we found that introducing a suitable second less-reactive metal (e.g., Fe, Zn) can effectively suppress the hydrogenolysis activity of ruthenium (Ru) metal for C-C bonds and exhibit a high selectivity (>90 %) to diesel-range alkanes (C15-C18 alkanes) in the conversion of fatty acid methyl esters (FAMEs) even at high reaction temperature (250 °C) over the Ru1Fe0.5 catalyst, while an obvious cracking reaction was observed from 210 °C over the monometallic Ru catalyst. Detailed characterization and theoretical calculation results reveal that the introduction of Fe species in the RuFe catalysts weakens the interaction between catalyst and the resulting alkanes, which inhibits the cracking of alkanes. Specifically, adding Fe species breaks the ensemble of Ru atoms and decreases the binding affinity of metallic Ru for H2, which suppresses the activity of Ru metal for the hydrogenolysis of C-C bonds and exhibits a high selectivity to diesel-range alkanes. This research provides valuable information for improving the hydrodeoxygenation (HDO) selectivity of Ru-based catalysts while inhibiting its high hydrogenolysis activity for C-C bonds.

Operando x-Ray Absorption Spectroscopy Investigation of Secondary Metal Doping into Iron-Nitrogen-Carbon Catalysts for Oxygen Electroreduction

Oxygen reduction reaction (ORR) plays a key role in the development of fuel cell technology, and great efforts have been made to reduce the amount of platinum, required to speed up this sluggish reaction, or, even more desirable, to use efficient electrocatalysts based on earth-abundant metals. To date, numerous single-atom catalysts in the form of metal-doped carbon-nitrogen materials (MNC) have proved to be a promising alternative to ORR Pt-based materials. [1-3]In this study we employed advanced spectroscopic techniques, namely Mössbauer spectroscopy and operando X-ray absorption (XAS) spectroscopy, to understand the structural and electronic factors underlying an increase in the catalytic turn over frequency (TOF) of bimetallic FeSnNC and FeCoNC catalysts, relative to the parent FeNC materials. In particular, 57Fe Mössbauer spectroscopy identified a larger ratio of D1/D2 species in both bimetallic catalysts, supporting a larger ratio of Fe(III)-Nx/Fe(II)-Nx sites. The combination of extended X-ray absorption fine structure (EXAFS) modeling, and the analysis of operando X-ray absorption near-edge spectroscopy (XANES) spectra revealed a disordered carbon structure around FeNx active sites, and variations in the iron oxidation state that can be linked to the enhanced intrinsic catalytic reactivity (TOF). This talk is therefore intended to draw attention to the potential of the XAS technique, especially when applied to in-situ/operando studies in the field of electrocatalysis. References F. Luo, A. Roy, L. Silvioli, D.A. Cullen, A. Zitolo, M.T. Sougrati, I.C. Oguz, T. Mineva, D. Teschner, S. Wagner, J. Wen, F. Dionigi, U.I. Kramm, J. Rossmeisl, F. Jaouen and P. Strasser. P-block single-metal-site tin/nitrogen-doped carbon fuel cell cathode catalyst for oxygen reduction reaction. Nature Materials 2020, 19, 1215-1223A. Zitolo, N. Ranjbar, T. Mineva, J. Li, Q. Jia, S. Stamatin, G.F. Harrington, S.M. Lyth, P. Krtil, S. Mukerjee, E. Fonda, F. Jaouen. Identification of catalytic sites in cobalt-nitrogen-carbon materials for the oxygen reduction reaction. Nature Communications 2017, 8(1), 957A. Zitolo, V. Goellner, V. Armel, M. T. Sougrati, T. Mineva, L. Stievano, E. Fonda, F. Jaouen. Identification of catalytic sites for oxygen reduction in iron and nitrogen doped graphene materials. Nature Materials 2015, 14, 937-942

OER Activity of Iridium Nanoparticle Catalysts By RDE and Comparison with MEAs

To accelerate the deployment of green hydrogen, lowering the cost of water electrolyzer is one of the important issues. Iridium anode catalyst is a key material for highly efficient and stable operation of proton exchange membrane (PEM) water electrolyzer, however, limited resources of iridium lead to higher cost and the limited availability. Therefore, there is a strong need to develop novel catalysts that can reduce iridium usage without compromising on activity. Rotating electrode half-cell is a useful technique for OER activity measurement of anode catalysts [1], however broader consensus about the activity measurement protocol is still missing. In addition, it is also unclear whether the measured activity values from the RDE are consistent with those of the MEA. The RDE activity is influenced by various measurement parameters; catalyst loading on the disk electrode, ionomer content, electrolyte conductivity, potential range etc. Here, we demonstrate the impact of these RDE parameters on the measured OER activity using various iridium catalysts [2]. The OER activity was evaluated by linear sweep voltammogram at a scan rate of 1 mV sec-1 and a disk rotation rate of 3,600 rpm to minimize the contribution of capacitive current and mass transfer loss due to O2 bubbles. Too much catalyst loading on the disk electrode leads to a decrease in apparent OER activity, so that catalyst loading should be in the range where the catalyst activity is independent of the loading amount. Electrolyte solution was also found to have a significant effect on OER activity; comparing to the activity in perchloric acid solution (0.1 M HClO4), sulfuric acid solution (0.05 M H2SO4) and phosphoric acid (0.1 M H3PO4) showed 60% and 40% activity, respectively. To verify the obtained values of OER activity by RDE at the optimized condition, the OER activities were also determined by MEAs at corresponding conditions. The OER activities of MEA and RDE were found to be approximately the same regardless of catalyst species. Detailed measurement conditions and results will be discussed at the Meeting. References S. M. Alia, G. C. Anderson, J. Electrochem. Soc., 166 (2019) F282. T. Ioroi, T. Nagai, Z. Siroma, K. Yasuda, Int. J. Hydrogen Energy, 47 (2022) 38506.

Exploring the influence of chemical state of Cu species on CO-SCR performance in spinel-type CuM2O4 (M = Co, Mn, Fe, Ni, and Cr): The synergy between Cu2+ and surface oxygen vacancy

The influence of the chemical state of Cu species in spinel-type catalysts on the selective catalytic reduction (SCR) of NO with CO has not been well understood. A series of CuM2O4 (M = Co, Mn, Fe, Ni, and Cr) were synthesized and studied in this work. Notably, CuCo2O4 demonstrated the best CO-SCR performance, exhibiting the de-NOx activity exceeding 90 % in a broad temperature window of 143–381 °C. Furthermore, CO conversion surpassed 90 % above 225 °C. Characterizations revealed that the synergy between Cu2+ and surface oxygen vacancy in the Cu2+–Ov–Mδ+ sites (δ represents the M cations in the lower valence states) played a vital role in enhancing the CO-SCR performance of the CuM2O4 catalysts. The superior CO-SCR performance of CuCo2O4 was also attributed to the optimal interplay between Cu and Co cations, resulting in superior Cu2+/Cu+ redox cycle and improved lattice oxygen mobility. In situ DRIFTS verified that Cu2+ was the primary site during the CO-SCR reaction, and two possible mechanisms (oxygen vacancy-assisted and lattice oxygen-assisted routes) over CuM2O4 were carefully proposed. This study unravels the nature of the CO-SCR activity of the CuM2O4 catalysts and provides novel perspectives on the development of Cu-based catalysts for CO-SCR applications.

Two‐Dimensional BiOBr Nanosheets Coupled with FeOOH Quantum Dots as Composite Photo‐Fenton Catalysts for Organic Pollutant Degradation under Visible Light

AbstractFeOOH quantum dots (QDs) were successfully anchored on 2D BiOBr nanosheets as composite photo‐Fenton catalysts by a simple precipitation and impregnation method. Using Rhodamine B (RhB) as an organic dye model, the photo‐Fenton catalytic activity of the composite catalyst was significantly higher than that of BiOBr and FeOOH. Among them, 1.8 % FeOOH/BiOBr has the highest photo‐Fenton catalytic activity. The main reason is that the interface effect between FeOOH and BiOBr can enable photogenerated electrons to rapidly transfer from the surface of BiOBr to FeOOH to participate in the Fe3+/Fe2+ cycle. Meanwhile, the quantum size of FeOOH increases the density of active sites on the surface of the composite catalyst. The influence of external factors on the catalytic activity of the catalyst was also verified. Finally, the electron transfer direction and main active species (h+, ⋅O2−) of the composite catalyst were discussed by electrochemical analysis and radical trapping experiments, and a reasonable catalytic degradation mechanism was proposed.

Selective oxidation of methacrolein to methacrylic acid over CsH3PMo11VO40 with structural defects

A novel synthetic strategy using microwave-assisted precipitation was proposed to prepare defective CsH3PMo11VO40 for the oxidation of methacrolein (MAL) to methacrylic acid (MAA). A systematic comparison of catalysts synthesized using different heating methods and preparation conditions was conducted. Microwave treatment was found to accelerate the crystallization rate of the catalyst and increase vanadyl species (VO2+) content on its surface, yielding the formation of defective Keggin structures. A moderate degree of defective Keggin units improves the oxidation capacity of active Mo species and facilitates the activation of oxygen into lattice oxygen, enhancing the oxidation of MAL. Therefore, the catalyst obtained through microwave heating demonstrates significantly improved catalytic performance compared to the one prepared via conventional heating. The defective Keggin structure plays a key role in catalytic enhancing. Under optimal conditions, the 70MH-40 catalyst exhibits the highest MAL conversion of 90.3% and MAA selectivity of 87.2%.