Copper Active Site Research Articles

Achieving strong thermal stability in catalytic reforming of methanol over in-situ self-activated nano Cu2O/ZnO catalyst with dual-sites of Cu species

Copper based catalysts with excellent catalytic activity and hydrogen selectivity are widely used in methanol steam reforming (MSR). Yet, the hydrogen pre-reduction step results in hydrogen consumption and energy loss. In addition, the copper nanoparticles are extremely sintered (≥ 300 ℃) to render deactivation caused by the low Taman temperature of 407 ℃. To solve these specific problems, a nano Cu2O/ZnO catalyst with double copper active sites (Cu+ and Cu0) was designed. Cu+ with high reducibility could lessen the catalyst's self-activation time using methanol as the hydrogen carrier. Nano ZnO could support and scatter the active components, effectively prevent the sintering and aggregation of copper nanoclusters. Furthermore, the synergy between the carrier and the active components existed according to H2-TPR. In order to maximize the hydrogen yield and reduce undesirable CO, the quadratic polynomial functions were established to investigate the effects of temperature (T = 450–550 °C), molar ratio of steam to carbon (SCMR=1.5–2.5 mol/mol), and weight-hour space velocity (WHSV=4–6 h−1) on H2 yield (HY) and potential H2 yield (PHY) based on the response surface methodology (RSM) in conjunction with the Box-Behnken design (BBD). The HY (92.1%) and PHY (97.2%) were obtained under T = 550 °C, SCMR= 2.0 mol/mol, and WHSV= 4.7 h−1. Within 36 h, stable 99% hydrogen selectivity was attained at 550 °C. The activity is induced by a new double copper active reaction path substituting the conventional Cu0-determined path. These discoveries provide a basis for the development of efficient catalysts with commercial potential.

Thermocatalytic Hydrogenation of CO2 to Methanol Using Cu-ZnO Bimetallic Catalysts Supported on Metal–Organic Frameworks

The thermocatalytic hydrogenation of carbon dioxide (CO2) to methanol is considered as a potential route for green hydrogen storage as well as a mean for utilizing captured CO2, owing to the many established applications of methanol. Copper–zinc bimetallic catalysts supported on a zirconium-based UiO-66 metal–organic framework (MOF) were prepared via slurry phase impregnation and benchmarked against the promoted, co-precipitated, conventional ternary CuO/ZnO/Al2O3 (CZA) catalyst for the thermocatalytic hydrogenation of CO2 to methanol. A decrease in crystallinity and specific surface area of the UiO-66 support was observed using X-ray diffraction and N2-sorption isotherms, whereas hydrogen-temperature-programmed reduction and X-ray photoelectron spectroscopy revealed the presence of copper active sites after impregnation and thermal activation. Other characterisation techniques such as scanning electron microscopy, transmission electron microscopy, and thermogravimetric analysis were employed to assess the physicochemical properties of the resulting catalysts. The UiO-66 (Zr) MOF-supported catalyst exhibited a good CO2 conversion of 27 and 16% selectivity towards methanol, whereas the magnesium-promoted CZA catalyst had a CO2 conversion of 31% and methanol selectivity of 24%. The prepared catalysts performed similarly to a CZA commercial catalyst which exhibited a CO2 conversion and methanol selectivity of 30 and 15%. The study demonstrates the prospective use of Cu-Zn bimetallic catalysts supported on MOFs for direct CO2 hydrogenation to produce green methanol.

Cover Feature: Molecular Mechanism of the Mononuclear Copper Complex‐Catalyzed Water Oxidation from Cluster‐Continuum Model Calculations (ChemSusChem 7/2022)

The Cover Feature shows the mononuclear copper complex-catalyzed water oxidation proceeds via an H2O2 intermediate, which is further oxidized to O2. This process is analogous to the reverse process of O2 activation and reduction occurring in the natural copper-dependent monooxygenase. The present finding could have far-reaching implications on the reactivity and catalysis of mononuclear copper active site in both synthetic and biological systems. More information can be found in the Research Article by P. Wu et al.

Bioinspired laccase-mimicking catalyst for on-site monitoring of thiram in paper-based colorimetric platform

A long-standing goal has been to create artificial enzymes with natural enzyme-like catalytic activity. Herein, a laccase-mimicking catalyst (GSH-Cu) is designed by simulating the copper active sites and spatial amino acid microenvironment of natural enzymes. In particular, the engineered GSH-Cu shows a catalytic function that conforms to Michaelis-Menten kinetics of natural laccase. The high catalytic activity of GSH-Cu can be easily inhibited by thiram through surface passivation to produce copper nanoparticles. We demonstrate that the developed GSH-Cu with high stability and recyclability can be used to fabricate effective colorimetric sensor for sensitive detection of thiram. The resulting absorption intensity can be employed to quantify thiram in the range of 2.5–250 ng mL−1, which meets the detection requirement in fruit. Bestowed with the feasibility analysis of colorimetric output, a portable platform is designed by integrating GSH-Cu based test paper with a conventional smartphone for conveniently on-site quantified thiram. The proposed strategy about engineering enzyme-mimicking catalysts with excellent catalytic performance will open avenues for boosting the sensing application.

Theoretical perspective on mononuclear copper-oxygen mediated C–H and O–H activations: A comparison between biological and synthetic systems

Dioxygen activations constitute one of core issues in copper-dependent metalloenzymes. Upon O 2 activation, copper-dependent metalloenzymes such as particulate methane monooxygenases (pMMOs), lytic polysaccharide monooxygenases (LPMOs) and binuclear copper enzymes PHM and DβM, are able to perform various challenging C–H bond activations. Meanwhile, various copper-oxygen core containing complexes have been synthetized to mimic the active species of metalloenzymes. Dioxygen activation by mononuclear copper active site may generate various copper-oxygen intermediates, including Cu(II)-superoxo, Cu(II)-hydroperoxo, Cu(II)-oxyl as well as the Cu(III)-hydroxide species. Intriguingly, all these species have been invoked as the potential active intermediates for C–H/O–H activations in either biological or synthetic systems. Due to the poor understanding on reactivities of copper-oxygen complex, the nature of active species in both biological and synthetic systems are highly controversial. In this account, we will compare the reactivities of various mononuclear copper-oxygen species between biological systems and the synthetic systems. The present study is expected to provide the consistent understanding on reactivities of various copper-oxygen active species in both biological and synthetic systems. Reactivities of various mononuclear copper-oxygen have been compared between the synthetic and biological systems, showing that all species are reactive toward O–H activations but only [CuO] + species could be responsible for C–H activations in monooxygenases.

Second-Sphere Lattice Effects in Copper and Iron Zeolite Catalysis.

Transition-metal-exchanged zeolites perform remarkable chemical reactions from low-temperature methane to methanol oxidation to selective reduction of NOx pollutants. As with metalloenzymes, metallozeolites have impressive reactivities that are controlled in part by interactions outside the immediate coordination sphere. These second-sphere effects include activating a metal site through enforcing an "entatic" state, controlling binding and access to the metal site with pockets and channels, and directing radical rebound vs cage escape. This review explores these effects with emphasis placed on but not limited to the selective oxidation of methane to methanol with a focus on copper and iron active sites, although other transition-metal-ion zeolite reactions are also explored. While the actual active-site geometric and electronic structures are different in the copper and iron metallozeolites compared to the metalloenzymes, their second-sphere interactions with the lattice or the protein environments are found to have strong parallels that contribute to their high activity and selectivity.

Atomically Defined Undercoordinated Copper Active Sites over Nitrogen-Doped Carbon for Aerobic Oxidation of Alcohols.

Selective aerobic oxidation of alcohols offers an attractive means to address challenges in the modern chemical industry, but the development of non-noble metal catalysts with superior efficacy for this reaction remains a grand challenge. Here, this study reports on such a catalyst based on atomically defined undercoordinated copper atoms over nitrogen-doped carbon support as an efficient, durable, and scalable heterogeneous catalyst for selective aerobic oxidation of alcohols. This catalyst exhibits extremely high intrinsic catalytic activity (TOF of 7692 h-1 ) in the oxidation of cinnamyl alcohol to afford cinnamaldehyde, along with exceptional recyclability (at least eight cycles), scalability, and broad substrate scope. DFT calculations suggest that the high activity derives from the low oxidation state and the unique coordination environment of the copper sites in the catalyst. These findings pave the way for the design of highly active and stable single atom catalysts to potentially address challenges in synthetic chemistry.

Simultaneous synthesis of aniline and γ-butyrolactone from nitrobenzene and 1,4-butanediol over Cu-CoOx-MgO catalyst via catalytic hydrogen transfer process: Effect of calcination temperature

This study examined a hydrogen transfer reaction from 1,4-butanediol to nitrobenzene for γ-butyrolactone and aniline simultaneously over Cu-CoOx-MgO catalysts. The catalyst was developed by co-precipitation followed by metal hydroxycarbonate mixing and calcination at three different temperatures (500, 700, and 900 °C). The hydrogenation of nitrobenzene to aniline and the dehydrogenation of 1,4-butanediol to γ-butyrolactone was accomplished in a gas-phase fixed-bed continuous reactor system at 250 °C. The catalyst Cu-CoOx-MgO-500 calcined at 500 °C showed outstanding performance because of the higher copper dispersion and negligible spinel species (CuCo2O4 /MgCo2O4) compared to the other two catalysts. The order of activity decreased with increasing calcination temperature from 500° to 900°C. The activity trend followed the order, Cu-CoOx-MgO-500 > Cu-CoOx-MgO-700 > Cu-CoOx-MgO-900. The effect of calcination on the catalytic properties was analyzed by atomic absorption spectroscopy elemental analysis, Brunauer-Emmett-Teller surface area, N2O pulse chemisorption, temperature-programmed reduction-H2, X-ray diffraction, and X-ray photoelectron spectroscopy. The results showed that Cu-CoO-MgO-500 exhibited more active copper sites (Cu0/Cu+1) and optimal metal-support interactions that decrease the reduction temperature of copper. On the other hand, Cu-Co and Mg-Co spinel (CuCo2O4/MgCo2O4) content, which adversely affects the catalyst activity, increased with increasing calcination temperature. In summary, simultaneous hydrogenation and dehydrogenation reactions via hydrogen transfer reactions have potential commercial applications.

Capture of activated dioxygen intermediates at the copper-active site of a lytic polysaccharide monooxygenase.

Metalloproteins perform a diverse array of redox-related reactions facilitated by the increased chemical functionality afforded by their metallocofactors. Lytic polysaccharide monooxygenases (LPMOs) are a class of copper-dependent enzymes that are responsible for the breakdown of recalcitrant polysaccharides via oxidative cleavage at the glycosidic bond. The activated copper-oxygen intermediates and their mechanism of formation remains to be established. Neutron protein crystallography which permits direct visualization of protonation states was used to investigate the initial steps of oxygen activation directly following active site copper reduction in Neurospora crassa LPMO9D. Herein, we cryo-trap an activated dioxygen intermediate in a mixture of superoxo and hydroperoxo states, and we identify the conserved second coordination shell residue His157 as the proton donor. Density functional theory calculations indicate that both superoxo and hydroperoxo active site states are stable. The hydroperoxo formed is potentially an early LPMO catalytic reaction intermediate or the first step in the mechanism of hydrogen peroxide formation in the absence of substrate. We observe that the N-terminal amino group of the copper coordinating His1 remains doubly protonated directly following molecular oxygen reduction by copper. Aided by molecular dynamics and mining minima free energy calculations we establish that the conserved second-shell His161 in MtPMO3* displays conformational flexibility in solution and that this flexibility is also observed, though to a lesser extent, in His157 of NcLPMO9D. The imidazolate form of His157 observed in our structure following oxygen intermediate protonation can be attributed to abolished His157 flexibility due steric hindrance in the crystal as well as the solvent-occluded active site environment due to crystal packing. A neutron crystal structure of NcLPMO9D at low pH further supports occlusion of the active site since His157 remains singly protonated even at acidic conditions.

Highly Enhanced Catalytic Stability of Copper by the Synergistic Effect of Porous Hierarchy and Alloying for Selective Hydrogenation Reaction

Supported copper has a great potential for replacing the commercial palladium-based catalysts in the field of selective alkynes/alkadienes hydrogenation due to its excellent alkene selectivity and relatively high activity. However, fatally, it has a low catalytic stability owing to the rapid oligomerization of alkenes on the copper surface. In this study, 2.5 wt% Cu catalysts with various Cu:Zn ratios and supported on hierarchically porous alumina (HA) were designed and synthesized by deposition–precipitation with urea. Macropores (with diameters of 1 μm) and mesopores (with diameters of 3.5 nm) were introduced by the hydrolysis of metal alkoxides. After in situ activation at 350 °C, the catalytic stability of Cu was highly enhanced, with a limited effect on the catalytic activity and alkene selectivity. The time needed for losing 10% butadiene conversion for Cu1Zn3/HA was ~40 h, which is 20 times higher than that found for Cu/HA (~2 h), and 160 times higher than that found for Cu/bulky alumina (0.25 h). It was found that this type of enhancement in catalytic stability was mainly due to the rapid mass transportation in hierarchically porous structure (i.e., four times higher than that in bulky commercial alumina) and the well-dispersed copper active site modified by Zn, with identification by STEM–HAADF coupled with EDX. This study offers a universal way to optimize the catalytic stability of selective hydrogenation reactions.

The impact of reductants on the catalytic efficiency of a lytic polysaccharide monooxygenase and the special role of dehydroascorbic acid.

Monocopper lytic polysaccharide monooxygenases (LPMOs) catalyse oxidative cleavage of glycosidic bonds in a reductant-dependent reaction. Recent studies indicate that LPMOs, rather than being O2 -dependent monooxygenases, are H2 O2 -dependent peroxygenases. Here, we describe SscLPMO10B, a novel LPMO from the phytopathogenic bacterium Streptomyces scabies and address links between this enzyme's catalytic rate and in situ hydrogen peroxide production in the presence of ascorbic acid, gallic acid and l-cysteine. Studies of Avicel degradation showed a clear correlation between the catalytic rate of SscLPMO10B and the rate of H2 O2 generation in the reaction mixture. We also assessed the impact of oxidised ascorbic acid, dehydroascorbic acid (DHA), on LPMO activity, since DHA, which is not considered a reductant, was recently reported to drive LPMO reactions. Kinetic studies, combined with NMR analysis, showed that DHA is unstable and converts into multiple derivatives, some of which are redox active and can fuel the LPMO reaction by reducing the active site copper and promoting H2 O2 production. These results show that the apparent monooxygenase activity observed in SscLPMO10B reactions without exogenously added H2 O2 reflects a peroxygenase reaction.

Optimization of Nanostructured Copper Sulfide to Achieve Enhanced Enzyme-Mimic Activities for Improving Anti-Infection Performance

Advanced antibacterial methods are urgently needed to deal with possible infectious diseases. As promising alternatives to antibiotics, enzyme-mimic nanocatalysts face bottlenecks of low activities and indistinct catalytic mechanisms, which seriously restrict their development for anti-infection treatment. Herein, metastable copper sulfide (Cu2-xS) nanozymes with diversiform sizes and compositions were selected to adjust the electronic structure for enhancing enzyme-mimic activities. The as-synthesized large and thin nanoplates (L/TN nanoplates), with the stoichiometric ratio of Cu1.25S, were proven to possess the optimal peroxidase (POD)-mimic activity. Using quantum mechanics, it was theoretically revealed that the sulfur vacancies could alter the electronic structure of copper active sites and thus reduce the reaction energy barrier of H2O2 to·OH to promote the POD-mimic performance. Moreover, through enhanced enzyme-mimic activities, L/TN nanoplates achieved efficient depletion of glutathione and ascorbic acid for improving antibacterial performances. Further, synergizing with the NIR irradiation, the satisfactory destruction capability for bacteria and biofilm was achieved for L/TN nanoplates under an inflammatory level of hydrogen peroxide (50 μM). Altogether, this work provides a deeper understanding of geometrical and electronic properties-dependent antibacterial performance, and paves the way toward precise compositions and structures engineering of nanozymes.

In situ synthesis of copper metal-organic framework on paper-based device for dual-mode detection of volatile sulfur compounds in exhaled breath

Volatile sulfur compounds (VSCs) are considered as indicators of halitosis and precursors of periodontal diseases. Trace VSCs determination in-breath can provide noninvasive data for clinical diagnostics. In this work, a dual-mode colorimetric/ chemiluminescent (CL) paper-based device with a copper metal-organic framework (Cu-MOF) was developed for trace VSCs detection in exhaled breath. The Cu-MOF with 2,4,6-tris(4-carboxyphenyl)− 1,3,5-triazine (TATB) as a ligand was in situ synthesized on a paper surface by the solvothermal method. The porous Cu-TATB served as a gaseous confinement cavity to selectively capture and adsorb VSCs. When Cu-TATB reacted with VSCs, significant and sensitive color changes can be observed from aqua blue to greenish-brown. Also, the Cu-TATB had an excellent catalytic effect on the luminol-H2O2 CL system. The CL intensity was inhibited by VSCs because of sulfur in VSCs reacting with copper active sites in Cu-TATB to form CuS. In this way, dual-mode detection methods for VSCs in breath were established with limits of detection (LOD) of 0.2 µM for colorimetry and 8 nM for CL. This portable, low-cost and stable paper-based sensor realizes accurate and reproducible signals read-out and shows potential feasibility as a prospect sensor platform for exhaled breath analysis.

DFT Analysis of Methane C-H Activation and Over-Oxidation by [Cu2 O]2+ and [Cu2 O2 ]2+ Sites in Zeolite Mordenite: Intra- versus Inter-site Over-Oxidation.

Methane over-oxidation by copper-exchanged zeolites prevents realization of high-yield catalytic conversion. However, there has been little description of the mechanism for methane over-oxidation at the copper active sites of these zeolites. Using density functional theory (DFT) computations, we reported that tricopper [Cu3 O3 ]2+ active sites can over-oxidize methane. However, the role of [Cu3 O3 ]2+ sites in methane-to-methanol conversion remains under debate. Here, we examine methane over-oxidation by dicopper [Cu2 O]2+ and [Cu2 O2 ]2+ sites using DFT in zeolite mordenite (MOR). For [Cu2 O2 ]2+ , we considered the μ-(η2 :η2 ) peroxo-, and bis(μ-oxo) motifs. These sites were considered in the eight-membered (8MR) ring of MOR. μ-(η2 :η2 ) peroxo sites are unstable relative to the bis(μ-oxo) motif with a small interconversion barrier. Unlike [Cu2 O]2+ which is active for methane C-H activation, [Cu2 O2 ]2+ has a very large methane C-H activation barrier in the 8MR. Stabilization of methanol and methyl at unreacted dicopper sites however leads to over-oxidation via sequential hydrogen atom abstraction steps. For methanol, these are initiated by abstraction of the CH3 group, followed by OH and can proceed near 200 °C. Thus, for [Cu2 O]2+ and [Cu2 O2 ]2+ species, over-oxidation is an inter-site process. We discuss the implications of these findings for methanol selectivity, especially in comparison to the intra-site process for [Cu3 O3 ]2+ sites and the role of Brønsted acid sites.

Symmetry Breaking in Solution-Phase [Cu(tsc)2(H2O)2]2+: Emergent Asymmetry in Cu-S Distances and in Covalence.

The structure of aqueous Cu(II)-bis-thiosemicarbazide, [Cu(tsc)2]2+, is reported following EXAFS and MXAN analyses of the copper K-edge X-ray absorption (XAS) spectrum. The rising K-edge feature at 8987.1 eV is higher energy than those of crystalline models, implying unique electronic and structural solution states. EXAFS analysis (k = 2-13 Å-1; 2 × Cu-N = 2.02 ± 0.01 Å; 2 × Cu-S = 2.27 ± 0.01 Å; Cu-Oax = 2.41 ± 0.04 Å) could not resolve 5- versus 6-coordinate models. However, MXAN fits converged to an asymmetric broken symmetry 6-coordinate model with cis-disposed TSC ligands (Cu-Oax = 2.07 and 2.54 Å; Cu-N = 1.94 Å, 1.98 Å; Cu-S = 2.20 Å, 2.41 Å). Transition dipole integral evaluation of the sulfur K-edge XAS 1s → 3p valence transition feature at 2470.7 eV yielded a Cu-S covalence of 0.66 e-, indicating Cu1.34+. The high Cu-S covalence and short Cu-S bond in aqueous [Cu(tsc)2(H2O)2]2+ again contradict the need for a protein rack to explain the unique structure of the blue copper active site. MXAN models of dissolved Cu(II) complex ions have invariably featured broken centrosymmetry. The potential energy ground state for dissolved Cu(II) evidently includes the extended solvation field, providing a target for improved physical theory. A revised solvation model for aqueous Cu(II), |[Cu(H2O)5]·14H2O|2+, is presented.

Enhanced Catalytic Performance of Quasi-HKUST-1 for the Tandem Imine Formation.

Copper-based metal-organic framework (Cu3 (BTC)2 (H2 O)3 ]n ⋅nH2 OMeOH (HKUST-1) has been subjected to thermolysis under air atmosphere at different temperatures ranging from 100 to 300 °C. This treatment produces the partial removal of ligands, the generation of structural defects and additional porosity in a controlled way. The resulting defective materials denoted according to the literature as quasi-MOFs, were subsequently employed as heterogeneous catalysts in the one pot synthesis of N-benzylideneaniline from aniline and benzyl alcohol in open air as terminal oxidant at 70 °C under base- and dehydrating agent-free conditions. The Q-HKUST catalysts calcined at 240 °C (QH-240) was the most efficient in the series, promoting imine synthesis. Data from Knoevenagel condensation of malononitrile shows that in QH-240 the distances of Cu ions in HKUST-1 cavities are preserved, increasing the Knoevenagel activity, but a strong rearrangement takes place at 300 °C or above. The unsaturated copper active sites with simultaneous presence of micro- and mesopores in QH-240 are responsible for this excellent catalytic performance. The effective parameters on catalytic activity of QH-240 including deligandation temperature, the amount of catalyst, the ratio of reactants, and reaction temperature as well as the stability and recyclability of the catalyst were also investigated. The possible mechanism used by QH-240 follows alcohol aerobic oxidation and subsequent anaerobic condensation of aldehyde intermediate with aniline.

In-situ synthesis of highly dispersed Cu-CuxO nanoparticles on porous carbon for the enhanced persulfate activation for phenol degradation

Copper-mediated activation of persulfate (PS) has been widely investigated for the degradation of refractory organic pollutants. However, the dispersion of the active copper sites in the catalysts remains a great challenge. Herein, a series of well-dispersed Cu-CuxO embedded into carbon matrix composites (Cu-CuxO@C) were fabricated via the pyrolysis of HKUST-1 precursor. The Cu-CuxO@C catalysts were fully characterized by SEM, HRTEM, XRD, BET, Raman, FT-IR, and XPS. The results show that the Cu-CuxO@C calcinated at 700 °C (Cu-CuxO@C-700) contained multivalent Cu components. Their heterogeneous activation of PS for phenol degradation was also investigated. Cu-CuxO@C-700 removed completely phenol with a kobs value greater than 0.40 min−1. Such superior catalytic activity is mainly attributed to the synergistic effect of carbon matrix adsorption and multivalent Cu active site electron transfer. The reusability studies reveal that the performance of the used Cu-CuxO@C-700 could be reactivated through re-calcination. Moreover, quenching tests and EPR analyses revealed that OH and SO4− are involved in phenol degradation. The electron transfer process and resistance of Cu-CuxO@C-700 were described by electrochemical method. Finally, the catalytic mechanism was explored by XPS, FT-IR and Raman analyses. This study provides new insights into the fabrication of heterogeneous catalysts for sulfate radical activiation.

Insights of the Dynamic Copper Active Sites in Ethylene Oxychlorination Studied by the Multivariate UV–vis–NIR Resolution Kinetic Approach

Monitoring and simulating the events occurring on the catalysts under the real reaction conditions has significant meaning for elucidating the dynamic changes of the active sites during the reactio...

In Situ Hybridizing Cu3(BTC)2 and Titania to Attain a High‐Performance Copper Catalyst: Dual‐Functional Role of Metal‐Support Interaction on the Activity and Selectivity

AbstractProduction of value‐added chemicals sourced from biomass platform molecules has tremendous significance for sustainable chemical engineering. Herein, we reported a successful fabrication of a highly dispersed copper nanoparticles catalyst, CuO#TiO2, via a facile route of in situ homogeneously hybridizing the metal organic framework Cu3(BTC)2 with titania, and subsequent calcination. Distinctly from the copper catalysts prepared by the conventional routes (impregnation or co‐precipitation), this novel CuO#TiO2 catalyst has a small particle size (∼5 nm), easy‐reducible copper species, and weakened Lewis's acidity. As a result, it shows superior high activity (∼20.8 molFUR/molCu ⋅ h) as well as high furfural alcohol yield (>99 %) at 140 °C and 2 MPa for furfural hydrogenation. Moreover, the CuO#TiO2 can be conveniently regenerated by calcination at 200 °C in the air, with no appreciable changes in its structure or activity, a test that we repeated 8 times. Through comprehensive structure characterizations, it has revealed a significant interaction between the CuO and TiO2 originated from the in situ hybridization, which enables a bi‐functional effect on the resulted CuO#TiO2 catalyst: one is to generate more copper active sites by promoting the copper dispersion and reducibility; the other is to hinder the side reactions by decrease the support Lewis acidity. This work demonstrates a practical strategy to rationally design a highly efficient catalytic system in furfural hydrogenation catalysis.

Experimental and theoretical approaches for the development of 4H-Chromene derivatives as inhibitors of tyrosinase

ABSTRACT Tyrosinase (TYR) is a key enzyme that catalyzes the synthesis of melanin in plants, microorganisms and mammalian cells. Kojic acid (KA) is a well-known TYR inhibitor widely used as a popular cosmetic skin-lightening ingredient. However, KA is reported to have poor inhibitory activity against pigmentation within intact melanocytes or in clinical assays. In this study, a series of dihydropyrano[3,2-b] chromenedione (DHPC) (1a, 2a, 3a, 4a, 5a, 6a and 7a) and 1,8-dioxooctahydroxanthene (DOHX) (1b, 2b, 3b and 4b) derivatives were evaluated using a DPPH radical-scavenging assay. These results showed that 7a exhibited the most potent radical-scavenging activity. Compound 7a was characterised by X-ray crystallographic studies. Molecular docking was carried out to shed light on the mode of action and types of interaction between the compounds and the target. A metal-binding study suggested that these synthetic heterocyclic compounds may behave as competitive inhibitors for the L-DOPA binding site of the TYR. Finally, molecular modeling provided important insight into the mechanism of binding interactions with the TYR copper active site.