Geqian Fang,Jian Lin,Xiaodong Wang

1 CAS Key Laboratory of Science and Technology on Applied Catalysis,Dalian Institute of Chemical Physics,Chinese Academy of Sciences,Dalian 116023,China

2 University of Chinese Academy of Sciences,Beijing 100049,China

Keywords:Methane Oxygenates Supported metal catalysts Nanoparticle Single-atom catalysts

ABSTRACT Direct cost-effective conversion of abundant methane to high value-added oxygenates(methanol,formic acid,acetic acid,etc.) under mild conditions is prospective for optimizing the structure of energy resources.However,the C-H bond of products is more reactive than that of high thermodynamic stable methane.Exploring an appropriate approach to eliminate the“seesaw effect” between methane conversion and oxygenate selectivity is significant.In this review,we briefly summarize the research progress in the past decade on low-temperature direct conversion of methane to oxygenates in gas-solid-liquid phase over various transition metal(Fe,Cu,Rh,Pd,AuPd,etc.)based nanoparticle or single-atom catalyst.Furthermore,the prospects of catalyst design and catalysis process are also discussed.

1.Introduction

As an abundant and inexpensive resource,methane is widespread and constitutes the dominant component with a percentage of around 90% in natural gas,shale gas,methane hydrate and oilassociated [1].Compared with conventional combustion for heating,transforming methane to oxygenates (methanol,formic acid,acetic acid,etc.) has great economic potential and has attracted extensive attentions of both academic and industrial communities[2,3].However,the structure of methane molecule is similar to diamond with highly symmetrical spatial tetrahedral configuration,which makes it difficult to be captured by a catalyst.Moreover,the high HOMO-LUMO gap (9.91 eV) and ionization potential(12.61 eV) of methane make it hard to gain and loss of electron in the catalytic cycle [4,5].Meanwhile,the dissociation energy of the methane C-H bond is 440 kJ.mol-1,higher than most of the desired oxygenates products (methanol,formic acid,acetic acid,etc.),facilely resulting in the over oxidation for C-H species to form CO2and H2O finally [6–9].Thus,the valorization of methane by oriented transformation and the controllable activation of C-H bond have been regarded as a“Holy Grail of catalysis”in chemical reactions.

So far,traditional industrial approaches for the conversion of methane into liquid hydrocarbons mainly rely on an indirect route,generating syngas through methane-steam reforming first.The syngas was subsequently converted into target products by Fischer-Tropsch (FT) synthesis [10–12].Therein,both steam reforming and FT synthesis are highly energy-intensive with operation at high temperatures and even high pressures(Fig.1).Seeing the energy-intensive of indirect routes,the lower temperature and direct oxidation of methane without needing syngas medium has been explored in recent years.For the homogeneous gas–liquidphase oxidation,the soluble noble-metal-based catalysts were highly reactive for methane conversion,and oxygenate products could be protected by a strong acid medium under mild conditions[13,14].However,corrosion of strong acid solvent to the reactor,complex purification process of the products and nonrecoverable catalyst restricted the development and application of methane homogeneous catalysis system.Consequently,chemical looping,gas-solid-liquid phase ways,or other heterogeneous catalytic mode have been adopted for the valorization of methane[15–19].Although chemical looping is efficient for methane conversion,it needs to preform active oxygen species at high temperature and also the yield of products is limited by the usable lattice oxygen species of the solid substrate.Instead,the gas-solid-liquid phase system could be more reactive for methane oxidation at lower temperatures and with less energy-intensive [20,21].

Fig.1.State-of-the-art indirect and direct processes for converting methane to oxygenates.

Supported metal catalysts with nanoparticle dispersion were in the majority of methane conversion by gas-solid-liquid phase catalysis system in the early study stage [22–24].Compared to bulk metal oxides,the dispersed metal sites on these supports can promote the activation of metal oxides through the strong interaction between metal and support.Moreover,the metal species in a size of nanoparticles or single atoms can behave as active center.The catalytic performance could be improved by the synergistic effect between metal and support when compared with the pure support for the conversion of methane [22,23,25–27].With the progress of catalyst preparation technology,single-atom catalyst (SACs) with the atomically dispersed metal sites and unique electronic properties were first reported by Zhang and coworkers[28].This powerful new concept has so far spurred burgeoning interests and exhibited excellent catalytic performance in numerous reactions [29–33].Significantly,SACs have also achieved tremendous progress in the direct conversion of methane.Herein,we mainly highlight recent advances on low-temperature conversion of methane to oxygenates in gas-solid-liquid phase reactions via various supported metal nanoparticle catalysts and SACs in the past decade.The reported catalysts with their operation conditions and catalytic performance are summarized in Table 1.

Table 1The reported catalysts with the operation conditions and catalytic performance in literatures

2.Catalytic Oxidation of Methane to Oxygenates Over Various Metal-based Catalysts

In nature,methane monooxygenase (MMO) can catalyze the oxidation of methane to methanol with ultrahigh activity and selectivity under aerobic conditions.The active site structure of soluble MMO (sMMO) has been demonstrated with a di-iron species in the hydroxylase subunit [61,62].The copper-oxo complex was identified as the active site motifs of particulate MMO(pMMO) [63].With that in mind,the biomimetic catalysis mode of Fe or Cu based for methane oxidation has been extensively studied.Significantly,with researches going deeper,other metal-based catalytic system is also reported and construction of active sites come into a more precisely atomic level.

2.1.Fe-based catalyst

In 2012,Hutchings’ group studied copper-promoted Fe-ZSM-5 catalyst for the oxidation of methane to methanol in aqueous conditions by using hydrogen peroxide as oxidant [34].The Fe4+=O species are reactive for the abstraction of H from methane,and the hydroperoxy ligand at the adjacent Fe center can quickly capture.CH3to form primary reaction product of CH3OOH.They found that the low selectivity of methanol was caused by the significant quantities of OH radicals that involved in the over-oxidation of methanol.Subsequently,the Cu2+addition was introduced to consume excessive hydroxyl radicals,which can improve the selectivity of methanol from 17% to 85%.The effect of hydroxyl radical scavenger (Na2SO3) on the activity of heterogeneous ZSM-5(30)and homogeneous Fenton’s reagent was also investigated [64].These experiments supported the hypothesis that lowtemperature selective oxidation of methane is centered on highly-reactive .CH3,?OH,and .O-2,rather the homolytic decomposition of H2O2and a subsequent free-radical chain mechanism.The activation barrier of 61 kJ.mol-1is much lower than that(95.7 kJ.mol-1) on sMMO reported [65].

Nevertheless,despite iron species are highly efficient for the oxidation of methane to methanol,the origin of the activity and the exact nature of the catalytic active sites are not yet fully understood.Consequently,Hutchings and coworkers used various spectroscopic and computational methods to demonstrate that extra framework oligomeric Fe species on ZSM-5 was the active component of the catalyst for methane oxidation [66].Dimitratos et al.further monitored the treatment of Fe-containing ZSM-5 with H2O2[67].It was revealed that the extra-framework binuclear core and an analogous Fe-OOH intermediates were present during the reaction by in situ Raman spectroscopy studies.The key FeIII-OOH intermediate in Fe-ZSM-5 was formed by heterolysis of the peroxide by the binuclear [Fe2(μ2-OH)2(OH)2-(H2O)2]2+species,which provided an spectroscopic evidence that the activation of methane contained potential similarities with the biomimetic oxidation property.The incorporation of other non-catalytic Al3+or Ga3+into the MFI-framework led to an increased migration of Fe3+to the extra-framework during initial heat pretreatment [68].These M3+trivalent cations were also able to stabilize the so-formed oligonuclear Fe sites during the catalytic cycle.

HCOOH and CO2,the main byproduct of methane oxidation to methanol,were believed to be formed by over-oxidation of methanol with excessive ?OH.However,Chadwick et al.claimed that HCHO was identified as an intermediate in the oxidation pathways to result in the conversion of CH3OOH to formic acid and ultimately to CO2over ZSM-5 catalysts with different Si/Al ratios[69].Meanwhile,hydrogen evolution was detected for the first time during the oxidation of methane via the following reaction(Reaction (1)):

Fig.2.(a) Initial [(H2O)2-Fe(III)-(μO)2-Fe(III)-(H2O)2]2+ model placed in the ZSM-5 zeolite pore.Color code:Si,yellow;Al,purple;Fe,blue;O,red;H,white.(b) Reaction energy diagram of heterolytic C-H bond cleavage of methane.(c) Schematic representation of the reaction network and most important steps underlying the oxidation of methane with H2O2 over Fe/ZSM-5 catalyst [71].

This reaction was demonstrated to arise from the oxidation of HCHO intermediate by lower concentrations of H2O2,which occurred at the beginning of the oxidation.The influence of Si/Al ratio in Cu-Fe/ZSM-5 on the productivity of methanol was also confirmed by Hellgardt’s group [70].

In view that most of theoretical work focused on the ratedetermining step of C-H bond activation,the calculations of the whole catalytic cycle were actually insufficient.According to previous work of experimental and theoretical,the mechanism of methane oxidation with H2O2over single-site Fe/ZSM 5 catalysts via periodic density functional theory(DFT) calculations were systematically investigated [71].Three different reaction networks were compared:(1)heterolytic,(2)homolytic,and(3)Fenton type activation.It was demonstrated that different Fe(III) and Fe(IV)clusters were possibly formed upon reaction with H2O2(Fig.2).These results revealed that methanol was facilely formed through heterolytic and Fenton-type reactions via combination of CH3and an OH group before or after oxidation of the Fe(III) species by the peroxo ligand.While the Fe(IV) clusters were shown to catalyze the homolytic reaction of methane C-H bond dissociation and H2O2decomposition to O2and [H+] ions,which can further transform the methyl group to CH3OOH.Meanwhile,the H2O molecule played a crucial role in the processes of active-site formation,H2O2adsorption,and the formation of methyl hydroperoxide via ligand exchange or a proton shuttling mechanism.Based on this proposal,introduction of H2O2could be a double-edged sword.The reaction temperature could be significantly decreased.However,the reaction network of methane to methanol also becomes unusual complexity due to the high reactivity of radical intermediates.

As previously mentioned,the di-iron species in the hydroxylase subunit constitute the active structure of sMMO[61,62].To imitate the active site and local environment of sMMO,Gascon and coworkers built incorporation of isolated,antiferromagnetically coupled,high-spin Fe sites within the Al based-MOFs for catalyzing the direct conversion of methane to methanol[35].The use of nonredox nodes of the electrochemically synthesized MIL-53(Al,Fe)catalysts was critical to obtain a high recycling stability under aqueous H2O2condition.Despite of many competing reaction pathways that did not determine the rate of the methane oxidation to methanol,all of them possibly contributed to the decreased selectivity of methanol.Hence,the whole catalytic cycles in their previous work over MIL-53(Al,Fe) catalyst were further investigated by density functional theory calculations [72].It was indicated that both methane activation and H2O2conversion were proceeded on the same active sites.However,the reaction barrier of the latter reaction was over 60 kJ.mol-1lower than that of former.Although the activation of methane was limited by the decomposition of the second H2O2molecule,a lower reaction rate of methane conversion was necessary to enhance the selectivity of products.

Coincidentally,Tang et al.anchored Fe-O clusters on -OH and-OH2groups on nodes of another Zr-based MOFs modulated with trifluoroacetic acid(TFA)for effective oxidation of methane into C1 organic oxygenates [36].The electron-withdrawing effect of TFA was greater than acetic acid(AA),which led to the higher oxidation state of Fe-O clusters.Compared with the UiO-66(AA)-Fe catalysts,UiO-66(TFA)-Fe displayed a superior activity towards activating both methane and H2O2,thus leading to higher yield of C1 organic oxygenates.However,the contribution of MOFs support and Fe site to methane oxidation should be distinguished,which can better understand the structure–activity relationship between metal and support for further research.Given the flexible adjustability of MOFs,changing the effect of electronic and geometric structure via the modification of ligands or nodes for the conversion of methane to oxygenates is also worthy of study.

In addition to the properties of various supports,the dispersion state of metal sites (from nanoparticle to cluster or even single atoms) has also a great impact on the conversion of methane and oxidant.Single-atom catalysts can reach a maximum atomutilization efficiency.The unique electronic structure of the active centers in SACs has been proven importantly to obtain remarkable enhancements on catalytic activity in a variety of reactions compared with traditional metal nanoparticle catalysts [28–33].Recently,Bao et al.reported that graphene-confined single iron atoms could directly convert methane to C1 oxygenated products at room temperature[49].The unique FeN4/N4structure can easily absorb H2O2and decompose into H2O and two adsorbed O atoms to form O-FeN4-O active sites,which possessed the best activity for CH4conversion to the methyl radical than other O-MN4-O structures (Cr,Mn,Cu,and Co).Besides CH3OH and HCOOH,the HOCH2OOH was first identified as a product of methane conversion in mild conditions.They also found that ZSM-5 confined Fe singleatom catalyst could be highly efficient for the oxidation of methane to formic acid than Fe nanoparticles by H2O2under 80°C[50].Similarly,Luo et al.reported that the Fe single-atom catalysts exhibit 4 times higher turnover rate (TOR) compared to the dimercontaining Fe/ZSM-5 catalysts [51].These findings prove that Fe1SACs possess great potential for the conversion of methane to methanol in the H2O2-based aqueous system at low temperature.

2.2.Cu-based catalyst

Based on the previous literatures [73–77],copper-exchanged micropore topology materials also exhibited the mimics structure of the pMMO.However,the ascription of the active site of copper-based mimics pMMO structure in different pore microenvironment is still a controversial topic.Bokhoven at al.used braiding Cu K Edge X-ray absorption near-edge structure (XANES) to study the mechanism of the direct conversion of methane to methanol over copper hosted in zeolites.They found that a two-electron mechanism founded upon CuII/CuIredox couples complied with the studies of reactor based methanol from methane [74].Lercher and Yaghi group individually founded that the dinuclear copperoxo species were the probable active site of the Zr-based MOFs supported Cu catalysts for methane oxidation to methanol under stepwise conditions [76,77].The active oxygen species played a crucial role in tuning the catalytic performance for partial oxidation of alkane[7,78].The lattice oxygen sites as oxidant via chemical looping mode,are responsible for the abstraction of H atom from methane to form hydroxy and methoxy groups of oxygenates.

It should be noted that the stoichiometric process of chemical looping was strictly not in a closed catalytic cycle.The role of copper species in the partial oxidative of methane with H2O2over Cu/ZSM-5 catalysts was studied under the gas-solid-liquid phase[37].Through myriad characterizations and reactivity experiments to study the structure–function relationships of Cu-species,it was found that 2.5% (mass) Cu supported on ZSM-5 (SiO2/Al2O3=23)can afford 42.8 mmol methane oxidation products and 97.4% high selectivity towards methanol.Infrared spectra with gas NO probe identified that Cu-I species were formed by coordination with square pyramidal AlO-4pair on Cu/ZSM-5 catalysts (Cu/Al <0.5).The Cu-I species not only generated ·OH but also acted as active sites for methane activation.In light of these results,ethane and propene oxidation reactions were further tested on Cu/ZSM-5 of varied SiO2/Al2O3and Cu/Al ratios in water using hydrogen peroxide as oxidant.It was confirmed that these catalytic cycles were also completed based on the role of Cu-I sites.

Mou et al.advocated that tricopper cluster complex was the active sites for the conversion of CH4into CH3OH under ambient conditions [38].They immobilized the CuICuICuItricopper complexes in negatively charged mesoporous silica nanoparticles(MSN) by ion exchange method.At the whole catalytic cycle,the tricopper clusters were separately activated and regenerated by O2and H2O2(Fig.3a).The much higher solubility of methane within the pores of the MSN can result in the conversion of CH4into CH3OH with H2O2with a catalytic efficiency approaching 100%.

Moreover,Guo et al.advocated that the M1-Oxentity in SACs could modify the chemical state of metal and benefit the enhancement of the activation of C-H bond in CH4.They used electrostatic adsorption methods to prepare Cu1/ZSM-5 catalyst with Cu1-O4sites for direct oxidation of methane to oxygenates at 50 °C(Fig.3b) [53].This catalyst can achieve about 74% CH3OH,15%HCOOH,and a total of 4800C1 oxygenates products within 30 min by using H2O2as the oxidant.EPR experiments showed that the concentration of hydroxyl radicals from Cu1/ZSM-5 SACs was 4 times less than 1.5% (mass) Cu/ZSM-5 in the aqueous solution.Thus,the high selectivity of C1 oxygenates was originated from the Cu1-O4entity that could block the overoxidation of reaction intermediates.

In addition to zeolites,porous carbon nitride material was also used for anchoring single-atom sites [52,79–81].More recently,Sun et al.found that Cu atoms embedded in a porous C3N4matrix can exhibit the ability for the selective oxidation of methane at ambient temperature [82].It was indicated that the highly dispersed Cu1-N4species were the underlying active sites for the methane conversion.However,this Cu1-N4site on graphene nanosheets has been ever regarded to be inactive for methane oxidation [49].These phenomena showed that the coordination environment between metal sites and support of SACs played a key role in low-temperature oxidation of methane.

Fig.3.(a) The turnover cycle driven by H2O2 in the catalytic oxidation of CH4 mediated by the immobilized [CuI CuI CuI(L)]1+ complex and O2 [38].(b) The model of methane to methanol over Cu1-ZSM-5 catalyst [53].

2.3.Rh-based catalyst

Compared with non-precious-metal centers,the noble-metalbased catalysts were usually considered to be more active for CH4conversion due to the lower activation barrier of the C-H bond of CH4[83].Stabilizing the CH3intermediate is a key step for the selective conversion of methane.Lee et al.used an Rh single-atom catalyst dispersed on tetragonal ZrO2support for the direct conversion of methane by using H2O2at 70 °C [55].Both the computational and the CH4-DRIFTs experimental data confirmed that *CH3intermediate can stably exist on Rh1/ZrO2single atom catalyst.Furthermore,ethane was also observed during the direct oxidation of methane by O2at 260 °C.These results further proved that*CH3can be stabilized on the surface of ZrO2supported single-atom catalyst for the further reaction.On the basis of this work,the Li group explored the atomistic mechanisms relating the activity and selectivity of methane to methanol on the local structure of Rh1sites of Rh1/ZrO2catalyst via DFT calculations[84].The H2O2dissociation on Rh1/ZrO2surface led to the formation of particular O2Rh/ZrO2-2H configuration for C-H bond adsorption and activation.Compare with the radical pathway,the *CH3had to overcome a large barrier to further react with the HOO-Rh site by non-radical pathway.The methane conversion to CH3OH or CH3OOH on the Rh1/ZrO2catalyst then proceeded via the radical mechanism under mild conditions.It was worth noting that the Rh sites were surrounded by the hydrogenation of the support ZrO2.It was then re-oxidized by H2O2to the ORh site rather than back to the O2Rh site of the initialization step.Thus,the pathway toward the production of CH3OOH and CO2was prohibited.

The reactivity for the supported metal catalysts relies on the synergistic role of support and metal sites in many catalytic reactions [22,23,30,85–87].The different chemical compositions and catalytic performances could derive from the same metal site by cooperation with various supports.Huang et al.adopted a hydrothermal method to prepare cerium dioxide(CeO2)nanowires supported rhodium (Rh) single-atom (SACs Rh-CeO2NWs) for the direct methane conversion to oxygenates[56].In situ DRIFTs measurements and DFT calculations showed that CeO2NWs played a vital role in the formation of ·OOH and ·OH radicals.The proportion of Ce3+in CeO2NWs was much higher than that in commercial CeO2,which favored the decomposition of H2O2.As a result,the CeO2NWs were used as support for loading Rh single-atom.The total selectivity and yield of CH3OH and CH3OOH was 93.9% and 1231.7 mmol?(g Rh)-1?h-1on Rh1-CeO2NWs,which was 1.66 and 6.5 times higher than those on Rh/CeO2NWs at 50 °C.It was suggested that the activation of the C-H bond in CH4faced stepped energy barriers toward*CH2on Rh clusters,indicating that further oxidation into CO2was preferred when compared with CeO2NWs supported Rh single atom catalysts.

In addition,Rh based catalyst not only favored the cleavage of methane C-H bond but also had good performance to carbonylation reaction [88,89].Compared to ZSM-5 supported other transition and platinum group metal (Fe,Co,Ni,Cu,Ru,Pd,Ir and Pt)catalysts,the Rh-based catalyst showed the best yield of CH3COOH in DCM with CO assistance [90].Flytzani-Stephanopoulos group and Tao group individually reported single-atom Rh1/ZSM-5 catalyst for the transformation of CH4to acetic acid with CO and O2as a carbonyl source and oxygen donor at 150 °C (Fig.4(a) and(b))[57,58].An isotope-labeling measurement also confirmed that the carbon of the carboxyl group and methyl-group in CH3COOH were derived from CO and CH4molecules,respectively.The activity was much higher than the stoichiometric process or free Rh cations.It was indicated that the Br?nsted acid sites (BAS) of the zeolite played a key role for the insertion of CO in the formation of acetic acid.This phenomenon was in accordance with Roma′n-Leshkov’s work[91].Rh cations were anchored on the internal surface of the micropore by replacement of BAS of H-ZSM-5 and form Rh1O5site.The possible mechanism was proposed in Fig.5(c).The Rh-CH3species can undergo the oxygen atom insertion to produce Rh-OCH3or CO molecules insertion to form Rh-COCH3,which contrasted to the well-accepted route through the carbonylation of intermediate methanol by CO.After the hydrolysis step,the methanol and acetic acid products released into the solution and the single atom dispersed Rh site was recovered.The adsorption and activation of O2,CO,and CH4at the Rh1sites was necessary for the production of CH3COOH.However,over-high pressure of O2or CO can increase the yield of formic acid or even lead to the poison of Rh single-atom site,thus deactivating this catalyst.The high activity of Rh single-atom catalyst for methane carbonylation gives a sound example for methane C-H bond activation by single-atom catalysts [92].

2.4.Pd-based catalyst

Inspired by the distinctly different catalytic activity or/and selectivity of single atom catalysts,Tao’s group prepared singly dispersed Pd1O4sites anchored on the“internal surface”in the micropores of ZSM-5,which exhibited excellent performance in partial oxidation of CH4to CH3OH at 50–95 °C (Fig.5(a)–(c)) [54].When 0.01%(mass) Pd anchored on H-ZSM-5 internal surface,the total yield of methanol can reach as high as 111.91 μmol,whereas only a few amounts of oxygenates products when Pd cations were supported on “open surfaces” of Al2O3or SiO2.However,the experimental results show that methanol could be oxidized to formic acid on 0.01%(mass) Pd/ZSM-5 by extra H2O2even at 50 °C.Thus,as the similar strategy adopted by study of CuFe/ZSM [28],the CuO center was impregnated in Pd1/ZSM-5 as a co-catalyst for dissociating extra H2O2to promote the methanol selectivity from 2.73% to 86.35% at 95 °C.Meanwhile,the Pd sites on the external surface of ZSM-5 showed poor activity for activation of methane,which gave a further proof that porous materials’“internal surface”provided an advantageous local environment for the stable atomic dispersed metal sites.

Fig.4.(a) AC-HAADF-STEM images of Rh1/ZSM-5 catalyst.(b) Catalytic performance of Rh based catalysts in the conversion of methane to oxygenates [57].(c) Reaction network for conversion of methane to CH3COOH on Rh1/ZSM-5 catalyst [58].

Fig.5.(a)Yields of products during the transformation of CH4 catalyzed by 0.01%,0.10%,and 2%(mass)Pd/ZSM-5 at 50°C.Catalytic performance of(b)0.01%Pd/ZSM-5 at 50,70,and 95 °C,and (c) 0.01 % Pd/ZSM-5 loaded with 2% (mass) CuO at 50,70,and 95 °C [54].(d) The partial oxidation of methane over Fe-ZSM-5 and Pd catalyst with H2O2 in situ generated from H2 and O2 [45].

Compared to O2or H2O,H2O2is a more efficient clean oxidant,which can decrease the reaction temperature of partial oxidation of methane(POM),but H2O2is expensive relative to gaseous oxygen.Thus,in situ generate H2O2from hydrogen and oxygen to transform methane to methanol is very attractive.The Pd-based catalyst was very effective for the direct synthesis of hydrogen peroxide from H2and O2[93,94].With that in mind,Park and coworkers adopted Pd supported on sulfonic acid-functionalized hyper-crosslinked porous polymer(cs-HCPP)for in situ generation of H2O2,and then cooperated this catalyst with Fe-ZSM-5 catalyst by physical mixing for methane conversion at 30°C(Fig.5(d))[45].The results of catalytic performance clearly implied that acid-functionalized hypercrosslinked porous polymer(HCPP)was beneficial for H2O2generation over Pd-based catalyst without adding acid and providing efficient oxidant for POM on Fe-ZSM-5 catalyst.

As a comparison,Huang et al.approved that precious-metalbased catalysts were more active for cleaving the C-H bond of CH4but suffered from poor methanol selectivity.Considering this,they constructed a nanointerface structure between CuPdO2and CuO for converting CH4to CH3OH using H2O2or O2as the oxidant[46].This heterojunction structure could form a high ratio of Pd4+species,which weighted towards C-H dissociation of CH4.Various characterizations revealed that the electron transfer from Cu to Pd led to longer Pd-O bond on Pd0.3Cu0.7O/C catalyst.These specific Pd coordination environments could suppress the dehydrogenation of CH3OH to *CHO,resulting in the improvement of CH3OH selectivity.

2.5.AuPd based catalyst

The previous work has confirmed that the supported AuPd nanoparticle presented significantly better performance than the pure Pd/TiO2materials for direct synthesis of H2O2[95].This idea prompted Hutchings et al.to use AuPd/TiO2for the oxidation of methane to methanol via in situ synthesis H2O2with O2and H2[39].Compared to the 5%(mass)AuPd/TiO2catalyst,physical mixtures of 2.5% (mass) Au/TiO2and 2.5% (mass) Pd/TiO2catalyst exhibited no synergistic effect,which gave inferior activity and selectivity.Actually,the nature of adopting an in situ approach to produce H2O2was based on the promotion of O2conversion into reactive oxyhydrogen species by AuPd alloy with H2under mild conditions.Besides in situ formation strategy for capturing O2,the radical process was also suitable for inducing molecular oxygen into the primary oxidation products.Given the support/metal interface can lead to the high H2O2degradation rates,the unsupported Au-Pd colloidal nanoparticles were used to catalyze CH4oxidation to CH3OH with O2under a little H2O2as the radical initiating agent [41].18O2isotopic labeling reactions proved that more than 70% O2was incorporated into primary oxygenate CH3-OOH.The initial activation of CH4to·CH3through a radical mechanism was the rate-determining step,while an excess of H2O2and TiO2support could limit product formation.Therefore,lower concentration of H2O2coupled with no presence of support were indispensable for higher selectivity to primary oxygenates.The utilization efficiency in the reactivity of H2O2can be also increased as compared to the reaction via using preformed H2O2.Although the ultra-low amount of hydrogen peroxide was used in this work,the usage of precious-metal also increased by many times with the formation of unsupported colloidal nanoparticles.

The ·OH for abstracting H atom of methane to form ·CH3originated from the decomposition of H2O2,whilst the adsorbed state·OH of the active sites was reactive than that in the solution.The key to maintain enough local concentration of H2O2in the vicinity of the metal nanoparticles was to prevent the diffusion away of H2O2from the active sites.Xiao et al.constructed a molecularfence catalyst for enhanced methanol productivity in methane oxidation by in situ formed peroxide[40].The model and TEM images of the AuPd@ZSM-5-R catalysts were shown in Fig.6(a)–(c).The AuPd alloy nanoparticles were embedded in ZSM-5 zeolites and further hydrophobic modification the external surface of the zeolite by appending organosilanes.As for the experiment results,there was a positive correlation between the conversion of methane and the hydrophobicityof AuPd@ZSM-5-Cx(x=0,3,6,16,carbon-chain numbers of organosilanes) catalysts.It can be found that the conversion of methane reached 17.3% on AuPd@ZSM-5-C16 catalyst,which was enhanced by 2.7-fold than that on AuPd@ZSM-5 catalyst with unmodified organosilanes.The results of H2O2titration and Fenton reaction tests indicated that～92% of the produced H2O2diffused into the solution on AuPd@ZSM-5,whereas 86% of H2O2was captured within the zeolite crystals over AuPd@ZSM-5-C16.After nine recycle tests,the AuPd@ZSM-5-C16 catalyst still exhibited methanol productivity of 87.6 mmol.(g AuPd)-1.h-1at 16.9% methane conversion,which proved that the molecular-fence catalyst was efficient and reliable.This work revealed that besides the construction of active sites,the design of the wettability on the catalyst surface was also necessary for the diffusion of reactants and products.Meanwhile,compared to a single component,the synergistic effect of multicomponent active centers could exhibit higher activity in catalysis reaction.

Moreover,the beneficial effect of Cu2+to prohibit the overoxidation of formic acid was also found in AuPd/TiO2system[43].Trimetallic AuPd/Cu/TiO2catalysts presented 5 times higher productivity than AuPd/TiO2under the same test conditions.However,when replacing the preformed H2O2with H2and O2,the productivity of both H2O2and oxygenate was decreased.The authors conjectured this opposite result that a special interaction existed in Au/Pd and Cu sites with close proximity.Although the rate of H2O2decomposition could be decreased by the co-deposition of Cu with AuPd,the sites of H2and O2conversion to H2O2were also blocked.

Fig.6.(a–c) Models and TEM images of the AuPd@ZSM-5-R catalysts.(d) Reaction performance for the oxidation of methane with H2 and O2 over various catalysts [40].(e) Rate model from the H2O2 decomposition on AuPd nanocatalysts during POM [96].

There is no doubt that the generation of hydrogen peroxide into hydroxyl radicals plays a pivotal role in the activation of methane for decreasing reaction energy consumption.As a result,the catalyst properties (particle size,Au/Pd ratio,support,metal loading,Pd oxidation state) and reaction conditions (H2O2concentration,reaction temperature,methane pressure) have the non-negligible impact on H2O2decompositions or synthesis over AuPd alloy based catalyst for methane oxidation to methanol.When using preformed or in situ synthesis of H2O2for the selective methane oxidation,the influence of metal loading and Pd oxidation state of supported AuPd alloy catalyst have been separately investigated by Hutchings et al.and Tsubaki et al.[54,93,94].The metallic Pd likely resulted in the rapid H2O2decomposition than the Pd2+on AuPd/TiO2.Higher TOF(Turnover Frequency,TOF)and suppression of deleterious H2O2decomposition could be achieved by decreasing the metal loading [97,98].On the contrary,the higher Pd-Au loading catalyst have a better performance with the hydrogen and oxygen mixture in gas.The results of XPS,XRD measurements and catalytic performance suggested that these relatively larger nano-sized particles on PdAu/CNTs catalyst were more stable and active during the direct selective oxidation progress[42].The bivalent state Pd site was proved to play a vital role in achieving high methanol yield and selectivity,but the in-depth investigation of the related mechanism was not discussed.

Recently,Stach et al.used multivariate linear regression analysis of 143 H2O2decomposition kinetics on the AuPd-based catalyst samples reported in literatures to study the effect of physical properties and solution variables on the decomposition of H2O2during the methane conversion process (Fig.6(e)) [96].Inhibiting deleterious H2O2self-decomposition was considered to be crucial to increase the productivity of methane conversion and to achieve high utilization of H2O2.According to the result of the rate equation and fitted rate models,the H2O2decomposition efficiency could be enhanced by follow strategies:(1) using smaller AuPd nanoparticles,(2) decreasing the concentration of H2O2,(3) using colloidal nanocatalysts without support,(4) increasing the Au/Pd ratio.However,they found that the effect of methane pressure on the overall rate of H2O2decomposition was minimal.

2.6.Other typical metal-based catalysts

Apart from Fe and Cu as typical active sites of non-preciousmetal based catalysts for the CH4oxidation,Ni single atomic site has been reported to enhance the CH4oxidation performance.Wu and coworkers synthesized a single-atom Ni catalyst with 4 nitrogen coordination through the surface digging effect of Ni NPs on N-doped commercial carbon nanotubes[59].When evaluating their performance for methane to methanol,they found that compared to pure NiN4construction,the product of CH3OOH can be reduced on defective NiN4catalysts with higher CH3OOH formation energy.Subsequently,Song et al.prepared single chromium atoms supported on nano-TiO2[60].CH3OOH and HOCH2OOH were the main C1 oxygenated products on 1% (mass) Cr1/TiO2catalyst with H2O2oxidant at 50 °C.They inferred that Cr1site played for activating CH4to ·CH3and interacted with surface-bound -OOH of TiO2to form CH3OOH.The experiment of CH3OH as reactant further verified that HOCH2OOH came from the oxidation of CH3OH.

For noble-metal-based catalysts,except the excellent performance of Pd or Rh-based catalysts,the Ir-based catalysts were also confirmed highly active for activation of alkane C-H bond at lower temperatures[99,100].Recently,it was discovered that encapsulation of Ir NPs by Cu-based MOFs derived bicomponent IrO2/CuO catalyst with the bottom-up synthesis strategy can exhibit an inspired performance for methane to methanol by using air as oxidant [47].Combined with characterization and performance tests,they believed that C-H bond cleavage occurred on the surface between IrO2and CuO and then the methoxy generated on CuO sites.It is worth noting that,the side liquid products were identified as ethanol and aldehyde,but the generation mechanism of C2 products were not mentioned.Afterwards,they regulated the electronic structure of Ir with Ru [48].The optimum catalytic performance is achieved at top of the volcano-type relationship between XPS and CH4-TPR results,where the strongest electrophilic state of the Ru4+and Ir4+existed.These studies demonstrated low-cost air cooperating with hybrid oxides has an economic potential to further scale-up application for the direct oxidation of methane to methanol.

Huang and coworkers found that a higher amount of Ru4+species of RuCu NLs/SiO2-300 catalyst also exhibited remarkable activity towards CH4oxidation by using O2and a trace amount of H2O2[44].O2-TPO results confirmed that the Cu sites could affect the O2activation on the surface Ru species of RuxCu1-xNLs structure.Both the ratios of Ru4+/Ru0and Cu2+can be tuned by the calcination temperature,which then affected the activity of POM.It can be imagined that more and more noble-metal based catalysts such as Au,Ag or binary/ternary metal alloy might exhibit enhanced performance for the direct oxidation of methane to oxygenates in the future work.

3.Conclusion and Outlook

In this review,an up-to-date overview of direct conversion of methane to oxygenates (methanol,formic acid and acetic acid) in gas-solid-liquid phase reactions over typically reported metal based (Fe,Cu,Rh,Pd,AuPd,etc.) systems have been summarized and discussed,which mainly focused on the dispersion as nanoparticles or single atoms.The clean oxidant of H2O2plays a key role in the decrease of reaction temperature,but the whole pathway becomes complicated.The dissociation of H2O2,the formation of active sites,and optimal utilization of ·OH for minimum byproducts etc.,all of these processes should be considered.When using H2O2as the oxidant,the formed ·OH is more reactive in the abstraction of H from methane and tuning the selectivity of oxidation products when compared with other oxygen species(.O-2,·OOH,O2,etc.).Particularly,MFI type of micropore ZSM-5 supported metal catalyst became one of the classical catalytic system studied for direct activation of methane.The properties of support not only anchor the highly dispersed metal centers but also affect the reactivity of metal sites for the cleavage of methane C-H bond and insertion of O species or CO molecule.Thus,it is worth studying the discrepant performance between conversion of H2O2to ·OH and cleavage of methane C-H bond for the different geometric and electronic structures of hybrid support (MOFs,COFs,CN,etc.).By focusing on this target,the conversion of methane and the selectivity of oxygenates,that is,the “trade off”effect during the methane conversion,can be hoped to be well tacked in the near future.

It is believed that highly dispersed metal catalyst benefits for the improvement of methane activation.Single-atom catalysts have maximum atom utilization and improve the activity of methane conversion under mild conditions.However,the low metal loading (<1% (mass)) was not favorable for the further activation of methane under mild conditions.Therefore,follow-up studies should develop high metal loading SACs (especial nonnoble-metals-based) to advance the yield of oxygenates in one pot.More importantly,a highly dispersed metal cluster or atom pair could provide multiple active sites for catalysis C-C coupling tandem reaction for synthesis of C2+oxygenates.Compared to the use of expensive H2O2,in situ synthesis mode of this oxidant from H2and O2(Air) over AuPd alloy or Pd nanoparticles are moreefficient and economical.The forms of active oxygen species are important in tuning the catalytic selectivity and activity.While using O2as oxidant,it usually needs to be activated at higher temperature to participate in the reaction.With the introduction of additional H2,which is easily to produce active hydrogen species on transition metals surface,O2could be further activated to produce reactive oxyhydrogen species at low temperature.Based on this viewpoint,the further process could focus on subnanometer cluster or atom pair of Pd or AuPd catalyst to increase unit yield of H2O2and oxygenates.

As solvent and extractor,water can benefit the desorption of oxygenates from active sites.Although the H2O can be used as oxidant in methane oxidation reaction in some degree,the oxidative ability of ·OH produced by decomposition of hydrogen peroxide is much higher than that from H2O in low temperature system.Meanwhile,the diffusion effect of gas–liquid interface from low water-solubility of CH4results in a low local concentration around active sites.Chemical looping is an efficient way to convert CH4to CH3OH with a relatively small mass transfer limitation.Thus,a catalyst with lower temperature of activation,higher amount of methane adsorption and abundant reducible lattice oxygen is significantly demanded.From the view of practical application,the excellent cyclic stability is the important research of chemical looping for further study in direct conversion methane to methanol.Since there is potential possibility that the yield of methanol may be limited by stoichiometry via chemical looping mode,it is worth studying and challenging to improve methane conversion via co-feeding of CH4,O2(N2O,O2+H2),and H2O (dilute H2O2)by continuous flow mode of operation.

In the thermocatalysis systems,the energy required for the reaction can only be supplied by heat.The electro-,photo-,photothermal-or photoelectrocatalysis systems powered by light and/or electricity could effectively overcome the reaction barrier of C-H bond activation for POM at lower temperatures.And the plasma catalysis mode for methane coupling to multicarbon oxygenates without additional carbonyl is also a new opportunity for converting methane to high-value chemicals at more mild reaction conditions.These strategies are beneficial for fundamental research or future practical application of methane direct conversion to oxygenates under mild conditions.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was funded by National Natural Science Foundation of China (22022814,21878283),Youth Innovation Promotion Association CAS (2017223),“Strategic Priority Research Program”of the Chinese academy of Sciences (XDB17020100),the National Key projects for Fundamental Research and Development of China(2016YFA0202801).