• FEATURE ARTICLES • February 2010 Vol.53 No.2: 337–350
doi: 10.1007/s11426-010-0045-8
Catalytic selective oxidation or oxidative functionalization of methane and ethane to organic oxygenates
WANG Ye*, AN DongLi & ZHANG QingHong
State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
Received October 11, 2009; accepted November 23, 2009
Selective oxidation or oxidative functionalization of methane and ethane by both homogeneous and heterogeneous catalysis is presented concerning: (1) selective oxidation of methane and ethane to organic oxygenates by hydrogen peroxide in a water medium in the presence of homogeneous osmium catalysts, (2) selective oxidation of methane to formaldehyde over highly dispersed iron and copper heterogeneous catalysts, (3) selective oxidation of ethane to acetaldehyde and formaldehyde over supported molybdenum catalysts, and (4) oxidative carbonylation of methane to methyl acetate over heterogeneous catalysts containing dual sites of rhodium and iron.
Methane, ethane and other lower alkanes usually constitute the components of natural gas [1]. Methane is also con-tained in coal-bed gas, landfill gas and methane hydrate resources. These abundant resources may be used more ef-ficiently as chemical feedstocks or as clean fuels provided that the lower alkanes are transformed into chemicals or liquid fuels with high efficiency. The technology for chemical utilization of methane is indirect, involving steam reforming of methane to synthesis gas (H2 + CO) and the subsequent transformation of synthesis gas to methanol or hydrocarbon fuels via methanol synthesis or Fischer- Tropsch synthesis. However, the steam reforming of meth-ane is not only an energy-intensive but also a high-cost process. 65%–75% of the capital cost of the indirect ap-proach is associated with the methane reforming process. Therefore, to develop a more efficient and economical di-rect route for methane transformation is highly desirable.
Methane and ethane possess saturated C-H or C-C bonds and represent two of the most stable organic molecules. The activation of the saturated C-H bond is an active research field in organometallic chemistry, with systems capable of activating C-H bonds of saturated hydrocarbons having been developed [1–6]. However, the selective functionaliza-tion of C-H bonds in methane or ethane to a target product, especially organic oxygenates, remains unattained. This is because methane and ethane are very stable molecules, therefore the activation of these molecules often requires severe conditions. Additionally, the target product (such organic oxygenates as methanol and formaldehyde) is typi-cally far more reactive than methane or ethane under these severe conditions and may undergo consecutive oxidations to CO and CO2 (Figure 1). The bond energies of the weak-est C-H or C-C bond in many target products are lower than those in the lower alkanes. For example, the bond energies of the C-H bonds in methanol and formaldehyde are respec-tively 388.7 and 364 kJ mol-1, which are significantly lower than that in methane (434.7 kJ mol-1). Therefore, how to suppress the consecutive conversions of the target products
338 Wang Ye, et al. Sci China Chem February (2010) Vol.53 No.2
Figure 1 Schematic plot illustrating the difficulty in obtaining a target product with high selectivity during the oxidations of lower alkanes.
at a reasonably higher conversion of alkane is a critical challenge.
Many studies have concerned the development of het-erogeneous and homogeneous catalysts for selective oxida-tion or oxidative functionalization of methane to organic oxygenates [7–12]. This review highlights advances in the selective oxidation or oxidative functionalization of meth-ane and ethane with an emphasis on the results obtained in our laboratory.
2 Homogeneous oxidative functionalization of methane
Concerning progress in homogeneous catalysis for selective conversion of methane, Periana et al. [13–15] demonstrated that HgII, PtII and PdII complexes or compounds catalyzed the oxidation of methane to methyl bisulfate (CH3OSO3H) or acetic acid in concentrated sulfuric acid. Figure 2 shows the proposed reaction mechanism for the oxidative func-tionalization of methane in the presence of a PtII bipyrimidine complex [14]. Methyl bisulfate was obtained with a selectivity of 81% at a methane conversion of 90% at 220°C in oleum in the presence of the PtII catalyst. The rate constant of the oxidation of methane to methyl sulfate is ~100 times higher than that of further oxidation of methyl bisulfate in the presence of the PtII catalyst [14,16], and thus, high selectivity to methyl bisulfate is achieved. Because methanol may be obtained from the hydrolysis of methyl bisulfate, this system affords an efficient route for the oxi-dation of methane to methanol by sulfuric acid. The protec-tion of methanol as methyl bisulfate reduces the reactivity of the methyl group of the product, contributing to the high selectivity of the target product at high single-pass methane conversion [16]. Cationic gold also functions as a catalyst for the oxidation of methane in sulfuric acid, but selenic acid, which is a stronger oxidant than sulfuric acid, must be added as an oxidant to keep cationic gold from being re-duced to metallic gold [17]. The separation and recovery of methanol from concentrated sulfuric acid seem quite diffi-
Figure 2 The proposed reaction mechanism for the oxidation of methane to methyl bisulfate with concentrated sulfuric acid catalyzed by the Pt(II) bipyrimidine complex.
cult and the turnover frequency (TOF) of these systems re-mains low (<5 h1). The recovery and reoxidation of the produced SO2 must also be considered to complete the catalytic cycle. Furthermore, concentrated sulfuric acid is not a green reagent. All these hamper the commercial ap-plication of the Periana system, i.e. the sulfuric acid-based methane selective oxidation system. A platinum bipyrimidine complex-immobilized solid polymer has been utilized as a heterogeneous catalyst for the oxidation of methane in sul-furic acid [18].
In addition to the sulfuric acid medium, the oxidation of methane in trifluoroacetic acid or in a trifluoroacetic anhy- dride medium to methyl trifluoroacetate (CF3COOCH3) in the presence of an oxidant such as potassium peroxodisul- fate (K2S2O8) or hydrogen peroxide and an electrophilic catalyst such as the PdII complex has also been reported, but the TOF is low [19,20]. With the combination of three re- dox couples, i.e. PdII/Pd0, p-benzoquinone/hydrobenzo- quinone, and NO2/NO, molecular oxygen is also utilized for the oxidation of methane in trifluoroacetic acid (Figure 3). A TOF of 0.7 h1 for the CF3COOCH3 formation was ob-tained at 80°C [21]. PtII bipyridyl complex grafted onto SiO2 was also used in the same system to replace the ho-mogeneous PdII complex. A TOF of 0.5 h1 was reported for CF3COOCH3 formation [22]. In the trifluoroacetic acid me-dium, Fujiwara and co-workers [23,24] reported the oxida-tive carbonylation of methane or other alkanes to acetic acid or other carboxylic acids in the presence of CO and K2S2O8 (oxidant) catalyzed by Pd(OAc)2-Cu(OAc)2, VO(acac)2 or CaCl2. However, a subsequent study showed that the carbon source of the carboxylic acid group is from CF3COOH but not from CO in a similar system, indicating the participation of CF3COOH, the solvent, in the reaction [25]. Because the
Wang Ye, et al. Sci China Chem February (2010) Vol.53 No.2 339
Figure 3 One-pot conversion of methane to methyl trifluoroacetate by the combination of PdII/Pd0−quinone/hydroquinone−NO2/NO redox cou-ples in CF3COOH at 80°C.
direct reaction between methane and CF3COOH to produce CH3COOH and CHF3 may occur [26,27], the use of CF3COOH as a reaction medium should be very careful. Bell and co-workers have studied the homogeneous cata-lytic sulfonation of methane into methanesulfonic acid by SO3 or by SO2 in the presence of an oxidant such as H2O2 and O2 in a strong acid (H2SO4 or CF3SO3H) medium [28,29].
Green oxidation requires not only an environmentally benign oxidant, e.g. H2O2 or O2, but also such a non-toxic solvent as water. Several studies have questioned whether or not organic solvents as CF3COOH and CH3CN may take part in the reaction [26,27,30,31]. There are ample incen-tives to develop efficient homogeneous catalysts for the selective oxidation of methane and ethane in a water me-dium. It has been reported that a di-iron-substituted silico-tungstate catalyzed the oxidation of methane by H2O2 in water, and methyl formate and CO2 were obtained as the main products [32]. At 80°C, the turnover number (TON) for CH4 conversion was 21.6 in 48 h, and the selectivity to methyl formate and CO2 were respectively 54% and 44%, with a homogeneous γ-SiW10[Fe(OH2)]2O38
6 catalyst. Thus, the TOF for methyl formate formation was ~0.24 h1.
We have investigated the catalytic behaviors of a series of transition metal chlorides for the oxidation of methane and ethane with H2O2 in a water medium [33]. OsCl3 exhib-its the highest activity among these transition metal chloride catalysts for selective oxidations of both methane and eth-ane as shown in Figure 4. TOFs for the formations of C1 oxygenate (CH3OH, HCHO and CH3OOH) and C2 oxygen-ates (CH3CH2OH and CH3CHO) respectively reached 12 and 41 h1 at 90°C. CO2 was also formed from methane and ethane in the presence of OsCl3, and the selectivities to C1 and C2 oxygenates were respectively 61% and 85%. HAuCl4 and FeCl3 also showed satisfactory catalytic effects for the selective oxidations of methane and ethane with H2O2 in a water medium. HAuCl4 provided a TOF of ~10 h1 and a selectivity of 57% for the oxidation of methane to C1 oxy-genates at 90°C. However, precipitation was observed after the reaction, possibly due to the formation of Au0 powder, indicating that AuIII was unstable under the reaction condi-tions.
OsO4 is an oxidant which functions as a catalyst for the selective oxidation of alkenes to corresponding diols [34,35]. Relatively few studies have concerned the selective oxida-
Figure 4 Selective oxidation of methane (a) and ethane (b) to C1 and C2 oxygenates with H2O2 in a water medium catalyzed by various metal ox-ides. Reaction conditions: metal chlorides, 1.0 mmol dm3; CH4 or C2H6, 3 MPa; H2O2, 0.5 mol dm3; H2O solvent, 10 cm3; temperature, 90°C; reac-tion time, 1 h.
tion of alkanes using Os-based catalysts [36,37]. Shul’pin and co-workers [36] reported that OsCl3 catalyzed the oxi-dation of alkanes to alcohols and aldehydes with H2O2 in acetonitrile. Mayer and co-workers [37] demonstrated that the aqueous solution of OsO4 and NaIO4 oxidized methane to methanol at 50°C and at a methane pressure of 9.5 atm. In their system, the final methanol concentration reached 0.6% of the starting OsO4 and NaIO4 concentrations and 2.7% of the starting methane concentration after 120 h of reaction [37]. However, it was clarified that OsO4 did not function for CH4 oxidation with H2O2 instead of NaIO4 be-cause of the rapid decomposition of H2O2 by OsO4 [38]. In our system, we found that no C1 or C2 oxygenates were formed when OsO4 was used instead of OsCl3, but the con-sumption of H2O2 was 100% in 1 h at 90°C, excluding the possibility of OsO4 as the active species. Thus, the present OsCl3-H2O2 system is a unique Os-based homogeneous catalytic system for selective oxidation of lower alkanes.
We have examined the possibility of using other oxidants for the oxidation of methane and ethane in the presence of OsCl3. The result revealed that NaIO4, NaClO4 and NaClO could not oxidize either methane or ethane into the corre-sponding oxygenates. C1 or C2 oxygenates were formed in the oxidation of methane or ethane when tert-butyl hydrop-eroxide (TBHP) was used as the oxidant to replace H2O2. However, the activity with TBHP was significantly lower. TOFs for methane and ethane oxidations with TBHP in the presence of OsCl3 were respectively 5.4 and 11 h1.
The working state of osmium during the oxidation of methane with H2O2 was characterized by UV-Vis spectros-copy. The result suggested that OsIII was oxidized to OsIV in the presence of H2O2 in a water medium. This may be be-cause the standard electrode potential for the H2O2/H2O redox pair (E0 = 1.78 V) is much higher than that for the OsIV/OsIII redox pair (E0 = 0.45 eV for OsCl6
2/OsCl63) in a
340 Wang Ye, et al. Sci China Chem February (2010) Vol.53 No.2
water medium. After the oxidation of methane, OsIV was observable, indicating that osmium existed in the +IV state during the reaction. Thus, we speculate that OsIV species may be involved in the selective oxidations of methane and ethane with H2O2. This speculation was confirmed by the experimental fact that an OsIV compound, i.e. Na2OsCl6 also functioned for the oxidations of methane and ethane with H2O2. Further investigations clarified that NaIO4 also oxi-dized OsIII to OsIV. However, NaIO4 could not function as an efficient oxidant for the oxidation of methane or ethane using OsCl3 as a catalyst. Thus the possibility that an OsIV oxo species functions for the oxidation of methane or ethane, and H2O2 oxidizes the reduced osmium (OsII or OsIII) spe-cies to regenerate the OsIV oxo species is excluded. We propose that the role of osmium catalyst is to activate H2O2 to generate active oxygen species, which are responsible for the oxidation of methane and ethane. The oxidation of methane and ethane is suppressed by a radical scavenger, hydroquinone. Thus, the redox of OsIV/OsIII may take part in the activation of H2O2 to generate active oxygen species, which may be the hydroxyl (·OH) or the hydroperoxide (•OOH) radical. These radical species may then activate methane or ethane by abstracting a hydrogen atom, forming methyl or ethyl radicals. The subsequent reactions between the radicals and/or between the radical with methane or ethane may provide the products. The redox of FeIII/FeII may carry out the same reactions [31]. It was found that FeCl3 was also a better catalyst for the oxidations of meth-ane and ethane than other transition metal chlorides (Figure 4). However, the unproductive decomposition of H2O2 was more serious in the presence of FeCl3, and this resulted in lower concentrations of oxygenate products as compared to OsCl3.
3 Selective oxidation of methane to formalde-hyde over heterogeneous catalysts
Many heterogeneous catalysts are effective for the oxidation of methane to formaldehyde at 500–700°C, and among a number of heterogeneous catalysts reported to date, sil-ica-supported molybdenum and vanadium oxides exhibit relatively better catalytic performances (reproducible HCHO single-pass yield <5%) [8-12]. As far as the reaction mechanism is concerned, many early studies considered that the lattice oxygen species, particularly the terminal oxygen species (i.e. Mo=O and V=O), were responsible for the ac-tivation and oxidation of methane to HCHO [39-44]. Such a mechanism may be represented by the following reactions:
CH4 + 2MoVI=O → HCHO + H2O + 2MoIV
2MoIV + O2 → 2MoVI = O
This type of redox mechanism is also known as a Mars-van Krevelen mechanism, which has generally been
proposed for the selective oxidation of alkenes to oxygen-ates [45–47]. Wan et al. [48] investigated the activation of the C-H bond of methane on a model Mo3O9 cluster using the DFT method, and demonstrated that the terminal oxygen species (Mo=O) abstracted H atoms from methane.
Recently, Bell et al. reported mechanistic insights for the MoOx-catalyzed oxidation of methane to HCHO [49–53]. Their studies were based on isolated MoOx species dis-persed on SiO2 (<1 Mo/nm2), which were believed to be active for the oxidation of methane to HCHO [49]. Bell et al. questioned the possibility that multiple isolated molybdate centers could participate in the oxidation of methane and the reduction of molecular oxygen [50,51]. They found that the isolated MoVIOx species are readily reduced by H2 to MoIVOx species, but methane was not an effective reductant for this reduction, with ~50–500 ppm of MoVI in the catalyst reduced to MoIV during the steady-state oxidation of meth-ane to HCHO. Thus, they ruled out the possibility of the Mar-van Krevelen mechanism for the isolated MoOx-cata-lyzed selective oxidation of methane. Instead, they proposed that the active species for methane oxidation was a peroxide species generated by the reaction of oxygen with the low concentration of reduced MoIVOx species (Figure 5) [51]. Further spectroscopic studies and DFT calculations sug-gested that the isolated MoVI molybdate species on SiO2 existed as di-oxo species, while the reduced MoIV species were mono-oxo species [52,53].
Research has sought to improve the catalytic perform-ances for MoOx and VOx-based catalysts. We found that the supporting of MoOx or VOx into SBA-15, a typical mesoporous silica with relatively larger pores (> 5 nm), significantly enhanced the catalytic performances for HCHO formation. For example, the HCHO yield over the 10 wt% MoOx/SBA-15 was 3.7% at 650°C, while it was
Figure 5 The proposed reaction mechanism for the selective oxidation of methane to formaldehyde by oxygen over silica-supported isolated MoOx sites.
Wang Ye, et al. Sci China Chem February (2010) Vol.53 No.2 341
only 1.0% over the 10 wt% MoOx/Cab-O-Sil under the same reaction conditions [54]. The 3 wt% VOx/SBA-15 exhibited significantly higher HCHO selectivity than the VOx/Cab-O-Sil especially at higher CH4 conversions (Fig-ure 6) [55,56]. A HCHO yield of 3.7% was obtained over the 3 wt% VOx/SBA-15 catalyst at a HCHO selectivity of 81%. In addition to the higher dispersion of VOx species over SBA-15 owing to its higher surface area, we speculate that the larger porous channel of SBA-15 may favor the rapid desorption of HCHO and thus enhance HCHO selec-tivity. Another significant feature of the mesoporous mate-rial-supported VOx catalysts is their higher space-time yield (STY) for HCHO formation. Berndt et al. [57] once re-ported a STY for HCHO formation of 2.2 kg kgcat
1 h1 over a VOx/MCM-41 prepared by impregnation, which was sig-nificantly better than the best one reported over the VOx/SiO2 catalyst (~0.82 kg kgca
1 h1) [58]. We achieved a better HCHO STY of 2.79 kg kgcat
1 h1 over our 3 wt% VOx/SBA-15 catalyst. Fornés et al. [59] also reported the superior performances of the VOx/SBA-15 catalysts for the selective oxidation of methane, and they obtained a HCHO STY of 2.38 kg kgcat
1 h1. By using a thermolytic molecular precursor (TMP) method, site-isolated VOx/SBA-15 catalyst was prepared via the grafting of organic vanadium precur-sors, OV[OSi(OtBu)3]3 and OV(OtBu)3. The catalyst dem-onstrated a very high space-time yield (5.84 kg kgcat
1 h1) and TOF (0.48 s1) at 625°C although the single-pass HCHO yield was not high (1.25% at HCHO selectivity of 24%) [60]. By using a direct hydrothermal synthesis via co-condensation of monomeric vanadium species and tetra-ethoxysilicate species in aqueous solutions containing a surfactant (cetyltrimethylammonium bromide), Nguyen et al. [61] succeeded in preparing VOx/MCM-41 catalysts with monomeric VOx species, which contained one V=O bond and three bridging V-O-Si bonds or one hydroxyl group and two bridging V-O-Si bonds. This VOx/MCM-41 catalyst was more selective than that prepared by the impregnation method and afforded a much higher HCHO STY (6.46 kg
Figure 6 Comparison of catalytic performances of the 3 wt% VOx/SBA- 15 and the 3 wt% VOx/Cab-O-Sil for the selective oxidation of methane by oxygen. Reaction conditions: W = 0.10 g, T = 550-650°C; P(CH4) = P(O2) = 16.9 kPa; F = 120 cm3 min1.
kgcat1 h1 at 600°C).
Methane monooxygenases (MMO) in methanotrophic bacteria catalyze the selective oxidation of methane to CH3OH by O2 within physiological temperature. Two dif-ferent forms of MMO, i.e. soluble MMO (sMMO) and par-ticulate MMO (pMMO), are known to exist, and the iron and copper centers dispersed in proteins are believed to be responsible for the respective selective oxidation of meth-ane by O2 in the sMMO and pMMO [62,63]. Therefore, it would be effective to design iron- and copper-based het-erogeneous catalysts with dispersed active sites for the se-lective oxidation of methane by O2. However, iron and copper are generally not employed as components in selec-tive oxidation catalysts because their oxides preferentially catalyze the complete oxidation of hydrocarbons to CO and CO2. Particularly, CuO is a well-known active component for the complete oxidation of methane [64]. Thus, the preparation of catalysts with highly dispersed iron or copper sites may be vital.
Two research groups have studied silica-supported iron oxides or cations for the selective oxidation of methane to HCHO by O2. Kobayashi and co-workers [65,66] reported that the doping of a small amount of Fe3+ (Fe/Si < 0.001) onto a kind of fumed silica by an impregnation method sig-nificantly enhanced the formation rate of HCHO. Arena and co-workers [10,67] disclosed that the FeOx/SiO2 catalyst prepared by an adsorption-precipitation method was more effective for HCHO formation than that prepared by the conventional impregnation method due to the enhanced dispersion of iron species.
Because the sol-gel method is known to be capable of producing supported catalysts with homogeneously distrib-uted active sites, we studied the catalytic performances of the FeOx-SiO2 catalysts prepared by the sol-gel method for methane selective oxidation [68]. We found that the 0.5 wt% FeOx-SiO2 catalyst prepared by the sol-gel method showed significantly higher methane conversion than the 0.5 wt% FeOx/SiO2 prepared by an impregnation method, while HCHO selectivities over the two catalysts were al-most the same under the same reaction conditions (Figure 7). The correlation between the catalytic performances and the characterizations with UV-Vis and H2-TPR suggested that the higher dispersion of iron species in the catalyst prepared by the sol-gel method accounted for its higher methane conversion. This is an unexpected result because it is gener-ally believed that the highly dispersed Fe3+ species is selec-tive but not active, while the FeOx clusters are more active and less selective for the oxidation of methane. Our studies suggest that the oligomeric FeOx clusters are less active than the isolated Fe3+ species for methane oxidation.
We have shown that the use of mesoporous silica as a support of FeOx leads to better catalysts for the selective oxidation of methane for HCHO [69–71]. We demonstrated that the Fe3+ sites incorporated in the framework of MCM-41 or SBA-15 were more selective than the FeOx
342 Wang Ye, et al. Sci China Chem February (2010) Vol.53 No.2
Figure 7 Comparison of catalytic performances of the 0.5 wt% FeOx- SiO2 and the 0.5 wt% FeOx/SiO2 catalysts respectively prepared by the sol-gel and the impregnation methods. Reaction conditions: W = 0.10 g, P(CH4) = P(O2) = 33.8 kPa, F = 120 mL min1.
clusters. We clarified that at a lower content of iron, the FeOx/SBA-15 catalysts prepared by the conventional im-pregnation method were also effective for the selective oxi-dation of methane by oxygen [72]. A HCHO yield of 1.9% was obtained at 625°C over the 0.05 wt% FeOx/SBA-15 catalyst prepared by impregnation. TOF of 2.0 mol (mol-Fe)1 s1 could be obtained over the catalyst with a much smaller content of iron (0.008 wt%). The increase in iron content from 0.05 wt% to 0.1–0.8 wt% increased the fraction of oligomeric FeOx clusters in the FeOx/SBA-15 catalysts. At the same time, both HCHO selectivity and methane conversion decreased. This further confirms our above conclusion that the FeOx clusters are not only less selective but also less active than the highly dispersed Fe3+ for the selective oxidation of methane.
In addition to the supported FeOx catalysts, such iron compounds as Fe2(MoO4)3 and FePO4 also exhibited satis-factory performance for selective oxidation of methane to HCHO by O2 [73,74]. We found that the FePO4 nano-clusters confined in the mesoporous channels of MCM-41 or SBA-15 exhibited significantly higher activity and selec-tivity for HCHO formation than the crystalline FePO4 [75,76]. For example, methane conversion rate per mole of iron increased more than 30 times and HCHO selectivity doubled by introducing FePO4 into SBA-15 (Figure 8). We clarified that the reducibility of FePO4 clusters was signifi-cantly enhanced as compared with that of crystalline FePO4. This could increase the activity of the FePO4 nanoclusters for the selective oxidation of methane because lattice oxy-gen may be responsible for the oxidation of methane over the FePO4-based catalyst. The dispersion of FePO4 species might be beneficial to the improvement of HCHO selectiv-ity. The modification of FeOx species introduced into SBA-15 or SiO2 by phosphorus also led to the formation of FePO4 nanoclusters and significantly enhanced HCHO se-lectivity [68,70].
Figure 8 Comparison of catalytic performances of FePO4 and the 5 wt% FePO4/SBA-15 for the selective oxidation of methane by oxygen. Reaction conditions: W = 0.20 g (0.50 g for FePO4), T = 450°C; P(CH4) = 33.8 kPa; P(O2) = 16.9 kPa; F = 60 cm3 min1.
In a communication, Groothaert et al. [77] reported that the chemisorbed oxygen on Cu-ZSM-5, which had been pretreated in O2 at ≥ 623 K, oxidized methane to CH3OH at ≥ 398 K. Spectroscopic characterizations suggested the formation of the bis(μ-oxo)dicopper core, [Cu2-(μ-O)2]
2+, and this species was proposed as being responsible for the oxidation of methane to CH3OH at low temperatures. However, the reaction did not proceed in a catalytic manner. The small amount of CH3OH formed by the stoichiometric reaction between the chemisorbed oxygen and methane molecules had to be extracted from the surface of the cata-lyst by a water/acetonitrile mixed solvent. Otherwise, the formed CH3OH would undergo further oxidation to CO2 over the Cu-ZSM-5.
We have systematically examined the catalytic perform-ances of various transition metal ions or oxide clusters loaded onto SBA-15. To ensure the high dispersion of the active species, we chose a very small content for each tran-sition metal (M/Si = 1/13200, where M = V, Cr, Mn, Fe, Co, Ni, Cu, Mo or W). We found that CuOx/SBA-15 exhibited the highest activity for the selective oxidation of methane to HCHO (Figure 9) [78]. Although SBA-15-supported MoOx and VOx clusters with higher loading amounts (> 1 wt%) catalyzed the selective oxidation of methane to HCHO with good yields [54–56,59–61], the Mo or V sites with such a low concentration were almost inactive for oxidation of methane. The FeOx/SBA-15 also showed a superior per-formance for HCHO formation (Figure 9). These observa-tions are significant because iron and copper are known to respectively be the active centers in the soluble MMO (sMMO) and the particulate MMO (pMMO) [62,63]. We further investigated the effect of copper content on catalytic performances, and clarified that a catalyst with a copper content of 0.008 wt% was best for HCHO formation [79]. EPR characterizations have indicated that this catalyst mainly contains isolated Cu2+ ions, and we propose that the selective oxidation of methane to HCHO requires high dis-persion of copper species [79]. By changing reaction condi-
Wang Ye, et al. Sci China Chem February (2010) Vol.53 No.2 343
Figure 9 Catalytic performances of various SBA-15-supported transition metal cations or metal oxide clusters for the selective oxidation of methane to HCHO by oxygen. Reaction conditions: catalyst, 0.10 g; T = 625°C; P(CH4) = P(O2) = 33.8 kPa; total flow rate, 120 mL min1.
tions, TOF for HCHO formation attains 5.6 s1, which is higher than that achieved over the “single-site” iron (~2.0 s1) [72] or the “single-site” vanadium (~0.5 s1) [60].
We have elucidated that the oxidation of methane pro-duces HCHO as a major primary product together with a small amount of CO2, while CO is formed mainly via the consecutive oxidation of HCHO over a copper-based cata-lyst. Pulse reaction studies have indicated that methane molecules react with the lattice oxygen of the catalyst, pro-ducing CO and CO2, and CuII in the catalyst is reduced at the same time. Therefore, we excluded the possibility that the lattice oxygen species may function for the selective oxidation of methane. In other words, the present cop-per-based catalytic system does not follow the Mars-van Krevelen mechanism. Detailed pulse reaction studies with (CH4 + O2) pulses combined with EPR characterizations further suggest that the reduced copper (CuI) sites generated by methane molecules during the reaction account for the activation of molecular oxygen, forming active oxygen spe-cies for the selective oxidation of methane to HCHO [79]. Figure 10 shows the proposed reaction mechanism for the selective oxidation of methane to HCHO over our CuOx/ SBA-15 catalysts with highly dispersed copper sites.
The reaction mechanism in Figure 10 has some similari-ties to those proposed for the pMMO, which catalyzes the selective oxidation of methane on the active copper centers
Figure 10 The proposed reaction mechanism for the selective oxidation of methane to HCHO by oxygen over the CuOx/SBA-15 catalyst.
in the presence of a biological reductant (NADH) as fol-lows,
CH4 + NADH + H+ + O2 → CH3OH + NAD+ + H2O
For the reaction mechanism of pMMO, Chan et al. pro-posed that the reduced trinuclear CuI sites activated mo-lecular oxygen to form a bis(μ3-oxo)CuIICuIICuIII species, capable of inserting an oxygen atom into the C-H bonds [80]. Yoshizawa and Shiota [81] suggested through a theo-retical study that the reduced mononuclear and dinuclear CuI sites might both be able to activate the molecular oxy-gen to generate active oxygen species. In the case of the mononuclear copper site, CuI reacted with O2, forming CuII-superoxo species (CuII-OO−), which may undergo fur-ther conversions to CuII-hydroperoxo and then to CuIII-oxo species. For the dinuclear copper site, the incorporation of oxygen molecules may form a (μ-η2:η2-peroxo) dicopper species, which is then transformed into a bis(μ-oxo) CuIICuIII species. In our system, we have demonstrated that the CuI site is also responsible for the activation of molecu-lar oxygen to form an active oxygen species for the selec-tive conversion of methane [79]. However, in our system, it is not the NADH but the substrate (methane) itself that works as the reductant to generate the CuI site. The concept of reductive activation of the dioxygen molecule concerns both homogeneous and heterogeneous selective oxidation of hydrocarbons, but generally, a sacrificial reductant such as metallic powder, carboxylic acid, CO or H2 is required [8]. Therefore, our system using the methane molecule as the reductant to activate oxygen is also unique.
We have not obtained enough information about the ac-tive oxygen species for our system although in situ Raman spectroscopic studies have been attempted. Yoshizawa and Shiota[81] proposed that the CuII-superoxo species formed by the interaction of the mononuclear CuI with oxygen would not involve the active oxygen under physiological condi-tions because the calculated activation energy for the H-atom abstraction from methane by this oxygen species was 155 kJ mol1. However, this species may be workable under the reaction conditions we have employed (T = 500–625°C). The activation energy for the 0.008 wt% CuOx/SBA-15 catalyst (141 kJ mol1) [79] is close to this value. Further studies concerning the nature of the active oxygen species and the HCHO formation mechanism are underway.
4 Selective oxidation of ethane to acetaldehyde and formaldehyde over heterogeneous catalysts
The C-H bond energies in methane and ethane are respec-tively 434.7 and 409.6 kJ mol1. Thus, similar to methane, ethane is also one of the most difficult organic molecules to be activated. Because C2 oxygenates, i.e. ethanol, acetalde-
344 Wang Ye, et al. Sci China Chem February (2010) Vol.53 No.2
hyde and acetic acid, are more reactive than methanol or formaldehyde, the selective conversion of ethane to oxy-genates such as aldehydes is under investigation [82]. Ad-vances have been made for the oxidative dehydrogenation of ethane to ethylene and the oxidation of ethane to acetic acid [83,84], such that this article will not involve these two reactions.
We have focused our studies on supported molybdenum oxide catalysts for the selective oxidation of ethane to alde-hydes. As described above, a number of studies have con-cerned the selective oxidation of methane by oxygen over the MoOx/SiO2 catalysts [39,41,44,48,49–53]. However, it remains questionable whether or not the supported MoOx catalysts are effective for the selective oxidation of ethane by oxygen to such oxygenates as CH3CHO. Reports con-cerning the selective oxidation of ethane by oxygen over the supported MoOx catalysts are scarce although there are pub-lications regarding MoOx/SiO2-catalyzed selective oxidation of ethane by N2O [85,86]. For the selective oxidation of methane to HCHO, it is generally believed that the isolated MoOx species are the active and selective species [49–53]. However, the structure-reactivity relationships for the oxi-dative dehydrogenation of propane by oxygen over sup-ported MoOx catalysts indicated that, in a Mo surface den-sity range of 0.4–4.5 Mo/nm2, the propane conversion rate per Mo atom increases with a rise in the domain size of the MoOx species [87,88]. These studies imply that different structures of MoOx species may be required for the conver-sions of different alkanes and the formations of different products. It is essential to determine what kind of MoOx structure is required for the oxidation of ethane. Different products, e.g. ethylene and acetaldehyde, may both be pro-duced, and the site requirements may be different for these two products. We have investigated the structure-perform- ance relationships for the MoOx/SBA-15-catalyzed selective oxidation of ethane by oxygen [89].
We found that the conversion of ethane increased with Mo content and reached a maximum at a Mo content of 20.1 wt% (Figure 11(a)) [89]. Not only CH3CHO but also HCHO is formed over our MoOx/SBA-15 catalysts, and the best Mo contents for the formations of these two kinds of oxygenates are different, indicating that the formations of CH3CHO and HCHO require different structures of Mo sites. We further calculated the rates per Mo atom for ethane conversion and for CH3CHO and HCHO formations (Figure 11(b)). The catalysts with Mo contents of 9.5–20.1 wt% exhibited higher rates of ethane conversion per Mo atom than those with Mo contents less than 6.6 wt%.
The catalysts with different Mo contents have been characterized in detail by various spectroscopic techniques, and the structures of Mo species are summarized in Table 1. At Mo contents lower than 9.5 wt%, the ordered mesopor-ous structure of SBA-15 is well sustained, and most of the MoOx species are highly dispersed in the mesoporous channels of SBA-15. The catalysts with Mo contents of 2.8
Figure 11 Effect of Mo content on the catalytic behavior of MoOx/SBA- 15 for the selective oxidation of ethane by oxygen. Reaction conditions: catalyst, 0.20 g; T = 600°C; total flow rate, 150 cm3 min1; P(C2H6) = P(O2) = 10.1 kPa.
Table 1 Structures of Mo species over the MoOx/SBA-15 catalysts
Mo content Structure of Mo species Mesoporous structure
≤ 4.9 wt% monomeric MoOx ordered
6.6–9.5 wt% oligomeric MoOx clusters ordered
15.4–20.1 wt% polymeric MoOx clusters and
moOx nanoparticles (10–20 nm) destroyed
≥ 23.5 wt% moO3 crystallites destroyed
and 4.9 wt% mainly contain monomeric MoOx species. The increase in Mo content to 9.5 wt% causes the formation of oligomeric MoOx species. Further increases of the Mo con-tent to higher than 15.4 wt% causes the collapse of the or-dered mesoporous structure and the appearance of crystal-line MoO3. However, the catalysts with Mo contents of 15.4 and 20.1 wt% comprise only a small amount of plate-like or belt-like MoO3 crystallites. Instead, MoOx nanoparticles of 10–20 nm and polymeric MoOx clusters are mainly ob-served. The growth of the MoOx species in the mesoporous channels to MoOx nanoparticles results in the collapse of the silica wall of SBA-15.
The structure-performance correlations strongly suggest that polymeric MoOx clusters or MoOx nanoparticles are more active toward C2H6 activation than the monomeric MoOx species. The highly dispersed (monomeric and small oligomeric) MoOx species favor the selective formation of CH3CHO, whereas the formation of HCHO requires the presence of polymeric MoOx clusters or MoOx nanoparticles. By analyzing the redox behavior of different Mo species, we propose that the higher reducibility of the polymeric MoOx clusters or MoOx nanoparticles accounts for the higher rate of ethane conversion.
To gain insights into the formation of HCHO, we have
Wang Ye, et al. Sci China Chem February (2010) Vol.53 No.2 345
performed kinetic measurements for the oxidation of ethane over the 20.1 wt% MoOx/SBA-15. The results suggest that C2H4 and CH3CHO are two primary products, whereas HCHO is mainly formed via the catalytic oxidation of C2H4 by O2 (Figure 12). The oxidations of C2H4 and CH3CHO over the MoOx/SBA-15 further support this reaction scheme [90]. We have further studied the site requirements for the formation of HCHO in the oxidation of C2H4 over the MoOx/SBA-15. Our results indicate that the polymeric MoOx clusters or MoOx nanoparticles are the active species for the selective oxidation of C2H4 to HCHO by O2 [90]. Further in situ FT-IR studies suggest an ethylene dialkoxide (−OCH2CH2O−) intermediate for the selective oxidation of C2H4 to HCHO (Figure 13).
We have developed a double-bed catalytic reaction route for the production of HCHO directly from ethane by com-bining the 20.1 wt% MoOx/SBA-15 catalyst with an ethane oxidative dehydrogenation catalyst, i.e. Dy2O3-Li+-MgO- Cl−, which has been developed by Lunsford et al. [91]. With a single Dy2O3-Li+-MgO-Cl− catalyst bed, C2H4 was pro-duced with a yield of 54% at a selectivity of 83% at 590°C, but no HCHO was formed. The 20.1 wt% MoOx/SBA-15 catalyst alone catalyzed the oxidation of ethane to HCHO
Figure 12 Reaction pathways for the oxidation of ethane over the 20.1 wt% MoOx/SBA-15 catalyst.
Figure 13 The proposed reaction mechanism for the selective oxidation of ethylene to formaldehyde by oxygen over the MoOx/SBA-15 catalysts.
by O2, but the HCHO yield was only 3.1% at the same temperature. The combination of the Dy2O3-Li+-MgO-Cl− (upstream) and the 20.1 wt% MoOx/SBA-15 (downstream) in the same reactor provided a HCHO yield of 14% [90]. To our knowledge, this is the highest single-pass HCHO yield achieved for the catalytic oxidation of ethane by O2.
5 Other novel routes for the oxidative function- alization of methane or ethane to oxygenates
5.1 Oxidation of methane or ethane using halogen (bromine) as a mediator
Olah et al. [92] reported a two-step conversion of methane to methanol and dimethyl ether. In the first step, methane was selectively oxidized to methyl chloride (CH3Cl) by chlorine over a supported acid catalyst such as TaOF3/Al2O3 or Pt/Al2O3 at 180–250°C, and the hydrolysis of methyl chloride in the presence of excess water. An Al2O3-sup-ported metal oxide/metal hydroxide catalyst such as ZnO/ Al(OH)3/Al2O3 yielded methanol and dimethyl ether in the second step. The produced HCl in the second step had to be re-oxidized to Cl2 to complete the catalyst cycle. The fundamental concept here is that the use of halogen as a mediator may afford a route with high single-pass yield to organic oxygenates provided that the halogenation of methane provides functionalized products (CH3Cl or CH3Br) with a significantly higher selectivity than the oxidation of methane by oxygen.
Due to the lower redox potential of the Br2/HBr pair, Br2 may be a more suitable mediator than Cl2. Recently, Zhou et al. [93] succeeded in converting ethane to ethanol, diethyl ether or ethylene by using Br2 as a mediator. In their work, after bromination of ethane, the product mixture containing ethyl bromide and HBr directly reacted with metal oxides, giving metal bromides and alcohol, ether or alkene depend-ing on the metal oxides employed. Metal bromides are con-verted with O2 to recover the metal oxides and Br2. In their subsequent studies, Zhou et al. utilized HBr as a mediator, and demonstrated that methane was converted to CH3Br and CO through the reaction with O2 and HBr (40 wt% in water) over Ru/SiO2 or Rh/SiO2. In the second step, the reaction between CH3Br and CO may produce acetic acid [94], or CH3Br may be transformed to higher hydrocarbons over the MgO/H-ZSM-5 or MgO/SiO2 catalyst [95]. The oxidative bromination of methane also proceeded over such sil-ica-supported non-noble metal oxide catalysts as BaO/SiO2 at 650°C, yielding CH3OH, CH3Br and CO as the main products [96].
By considering that dibromodioxomolybdenum (MoO2Br2) may release Br2 when reacting with O2, Li and Yuan [97] have developed a catalytic system for the direct transformation of methane and oxygen into methanol and dimethyl ether at 220°C over a MoO2Br2/Zn-MCM-48. It is proposed that MoO2Br2 readily reacts with O2 to release Br2
346 Wang Ye, et al. Sci China Chem February (2010) Vol.53 No.2
in the first step, which oxidizes C-H bonds to form HBr and a C-Br bond. The formed MoO3 then scavenges the bromine atoms from HBr and C-Br to yield the oxidized products and regenerate MoO2Br2. The deactivation was observed because of the loss of bromine from the catalyst bed, and the increase in Zn content could enhance catalyst stability because of the participation of ZnO/ZnBr2 in the catalyst cycles (Figure 14). CH3OH and CH3OCH3 selectivity of 55% and 35% is respectively obtained, at a CH4 conversion of ~12% at 220°C over a 8 wt% MoO2Br2/Zn-MCM-48 catalyst with a Si/Zn molar ratio of 20. Further studies showed that the injection of Br2 with an amount of 2.0 mmol into the reactant mixture of methane and oxygen caused a significant increase in methane conversion from ~2% to 13% and C1 oxygenate (CH3OH, HCHO and CH3OCH3) selectivity from ~20% to ~88% over a HgO/Zn- MCM-41 catalyst [98]. With prolonging the time onstream to 2.5 h, the conversion of methane and selectivity to C1 oxygenates gradually decreased and respectively fluctuate around 10% and 55%.
5.2 Heterogeneous catalysis for oxidative carbonyla-tion of methane to methyl acetate
Among the homogeneous catalytic transformations of methane, the oxidative carbonylation (or carboxylation) of methane with CO in the presence of an oxidant (e.g. K2S2O8) to form acetic acid in a single step has been reported [23–27,99,100]. Acetic acid is mainly produced by the car-bonylation of methanol catalyzed by a homogeneous Rh carbonyl complex (the Monsanto process). The activity of the homogenous catalysts reported for the oxidative carbon-ylation of methane is quite low, and the TOF for acetic acid formation was less than 5 h1. Most of these systems are complex because of the use of K2S2O8 as an oxidant or CF3COOH as a solvent, which may be consumed during the reaction. As compared to the homogeneous system, a rela-tively simple heterogeneous catalytic system might be sat-isfactory. However, only few studies have been published concerning heterogeneous catalysis for the oxidative car-bonylation of methane.
Wang et al. [101] reported that methane was directly converted to methyl acetate in a single step via the reaction
Figure 14 Proposed reaction cycles for the selective conversion of meth-ane to methanol and dimethyl ether by oxygen over the MoO2Br2/Zn- MCM-48.
of methane with N2O and CO over Rh-doped FePO4 catalyst as follows,
2CH4 + N2O + CO → CH3COOCH3 + N2 + H2O
Other transition metal (e.g. Co, Ir, Ni, Pd, Ru or Re)-doped FePO4 samples and Rh-doped metal oxides (e.g. SiO2, Al2O3, MoO3 and V2O5) or molecular sieves (e.g. HZSM-5) were also tested for this reaction, but none exhib-ited significant activity for methyl acetate formation. Thus, the Rh-doped FePO4 is a unique catalyst for this reaction.
We have studied the site requirements for this novel catalytic reaction by comparing the structures and catalytic performances of two series of Rh-doped FePO4 samples prepared by two different methods [102]. Our results show that the Rh-FePO4 series of samples prepared from the mixed aqueous solution exhibit a higher rate and TOF for methyl acetate formation than the Rh/FePO4 series of sam-ples prepared by the impregnation method. It has been clari-fied through various characterizations that the Rh/FePO4 samples contain RhIII species or RhOx clusters mainly on the surface of FePO4, whereas RhIII cations are dispersed in the lattice of FePO4, forming a RhxFe1-xPO4 solid solution in the Rh-FePO4 samples. The structure-performance correlation suggests that the dual sites containing both rhodium and iron cations connected by phosphate groups (−FeIII−O−P−O−RhIII−O−) are the active centers for the oxidative carbonylation of methane to methyl acetate in the presence of CO and N2O. The RhxFe1-xPO4 solid solution formed in the Rh-FePO4 samples contains a higher concen-tration of the dual sites, and thus exhibits a better catalytic performance for methyl acetate formation.
We have succeeded in preparing RhxFe1-xPO4 nano-clusters in mesoporous channels of MCM-41 by a simple co-impregnation method [102]. This led to catalysts with higher concentrations of the dual sites, which accelerated the conversion of methane to methyl acetate. We obtained a methyl acetate formation rate of 0.696 mmol kgcat
1 h1 and a TOF for methyl acetate formation of 65.2 h1 at 723 K over the 0.11 wt% Rh-9.1 wt% FePO4/MCM-41 catalyst.
Because N2O is a weak oxidant, the presence of CO in the reactant mixture would mainly determine the chemical state of the active sites. Therefore, we have performed comprehensive CO-TPR studies combined with XPS meas-urements to elucidate the states of rhodium and iron in a CO atmosphere [103]. FT-IR spectra of CO chemisorbed over the supported RhxFe1-xPO4 nanocluster catalyst at different temperatures have also been recorded to obtain information about the possible reaction intermediate. Our results indi-cate that, for the Rh-FePO4 series of samples containing RhxFe1-xPO4 solid solutions, simultaneous reductions of RhIII and FeIII species to RhI and FeII occur in CO, whereas for the Rh/FePO4 samples prepared by the impregnation method, the reduction of RhIII species to Rh0 alone has been observed at lower temperatures. Simultaneous reductions of RhIII and FeIII have also been observed for the MCM-41-
Wang Ye, et al. Sci China Chem February (2010) Vol.53 No.2 347
supported RhxFe1-xPO4 nanocluster catalyst. A RhI geminal dicarbonyl species, RhI(CO)2, has been observed in the FT-IR studies of chemisorbed CO over the MCM-41-sup-ported RhxFe1-xPO4 nanocluster sample at temperatures effective for methyl acetate formation, suggesting that this species is a reaction intermediate. The FeII site formed after the interaction of the dual site with CO would activate N2O to generate an active oxygen species for the conversion of methane. Based on these results, we propose a reaction mechanism for the oxidative carbonylation of methane to methyl acetate in Figure 15.
6 Concluding remarks
Advances in the selective oxidation or oxidative function-alization of methane and ethane to organic oxygenates have been highlighted. Significant progress has been made in the oxidative functionalization of methane by homogeneous catalysis in recent years. In concentrated sulfuric acid, methane can be converted to methyl bisulfate, a precursor of methanol, with high single-pass yields in the presence of electrophilic catalysts such as the PtII or AuI complex. Methane can also be oxidized to methyl trifluoroacetate in the trifluoroacetic acid medium in the presence of the PdII or PtII complex. The oxidative carbonylation or sulfonation of methane to acetic acid or methanesulfonic acid has also received interests. Among various transition metal chlorides, osmium chloride provides the highest activity and selectiv-ity for the selective oxidation of methane and ethane by hydrogen peroxide in a water medium. Turnover frequen-cies of 12 and 41 h1 have been attained at 90°C for the conversions of methane and ethane to C1 and C2 oxygenates.
Figure 15 The proposed reaction mechanism for the oxidative carbon-ylation of methane to methyl acetate over the catalysts containing dual sites of RhIII and FeIII connected by phosphate groups.
It has been clarified that the redox of OsIV/OsIII may take part in the activation of hydrogen peroxide to generate ac-tive oxygen species for the activation of methane or ethane.
Concerning the heterogeneous oxidation of methane to formaldehyde at higher temperatures, although there is no significant breakthrough in the single-pass yield, strategies for the preparation of highly selective catalysts have been proposed. We and several other groups have demonstrated that the dispersion of active phases or species such as MoOx, VOx and FePO4 in mesoporous materials significantly en-hance catalytic performances for formaldehyde formation. Over a VOx/MCM-41 catalyst with monomeric VOx species, a formaldehyde space time yield of 6.46 kg kgcat
1 h1 has been attained at 600°C. The FePO4 nanoclusters confined in the mesoporous channels of SBA-15 provide a much higher methane conversion rate and higher formaldehyde selectiv-ity than the crystalline FePO4.
The “single-site” CuOx species loaded on SBA-15 are found to be highly active for the selective oxidation of methane to formaldehyde. A turnover frequency for for-maldehyde formation attains 5.6 s1 based on copper at 625 °C, which is the highest reported. Reaction mechanism stud-ies have elucidated that the selective oxidation of methane by oxygen does not follow the Mars-van Krevelen mechanism and lattice oxygen is not responsible for for-maldehyde formation over both isolated copper and mono-meric molybdenum sites. Instead, the CuI or MoIV generated during the reaction by methane accounts for the activation of oxygen, generating such active oxygen species as the CuII-superoxo species or the MoVI-peroxo species for methane selective oxidation. The reaction mechanism over CuI-based species has some similarities to that proposed for the particulate methane monooxygenase.
Structure-performance relationships have been investi-gated for the MoOx/SBA-15-catalyzed selective oxidation of ethane by oxygen. Polymeric MoOx clusters or MoOx nanoparticles are more active toward ethane activation than the monomeric MoOx species because of the higher reduci-bility of the former species. Acetaldehyde and formalde-hyde are both formed over the MoOx/SBA-15 catalysts but the formations of these two oxygenates require different structures of Mo species. The highly dispersed (monomeric and small oligomeric) MoOx species favor the selective formation of acetaldehyde, whereas the formation of for-maldehyde requires the presence of polymeric MoOx clus-ters or MoOx nanoparticles. Formaldehyde is formed by the selective oxidation of ethylene, and the polymeric MoOx clusters or MoOx nanoparticles are the active species for this catalytic reaction. We have further developed a double-bed catalytic reaction route for the production of formaldehyde directly from ethane by combining the MoOx/SBA-15 cata-lyst with an ethane oxidative dehydrogenation catalyst.
The use of bromine as a mediator affords an efficient route with a high single-pass yield to organic oxygenates because the bromination of methane may provide a func-
348 Wang Ye, et al. Sci China Chem February (2010) Vol.53 No.2
tionalized product (CH3Br) with a higher selectivity than the oxidation of methane by oxygen. The concept has been es-tablished, but efficient and stable catalysts for both the bro-mination of methane and the subsequent conversion of methyl bromide to methanol and dimethyl ether or other oxygenates remain undeveloped. Another novel heteroge-neous catalytic reaction of oxidative carbonylation of methane to methyl acetate in the presence of CO and N2O has been demonstrated. Dual sites of rhodium and iron cations connected by phosphate groups are proposed as the active centers for the direct formation of methyl acetate. However, this process cannot use oxygen as an oxidant, and the single-pass yield of methyl acetate is low (< 1%). The development of low-temperature and high-efficient hetero-geneous catalytic systems capable of using oxygen for the selective oxidation or oxidative functionalization of meth-ane is the focus of future investigation.
Financial support by the National Natural Science Foundation of China (Grant Nos. 20433030, 20625310, 20773099 and 20873110), the National Basic Program of China (Grant Nos. 2005CB221408 and 2010CB732303), the Key Scientific Project of Fujian Province (2009HZ0002-1), and the Program for New Century Excellent Talents in Fujian Province (to Q. Z.) is greatly acknowledged.
1 Shilov AE, Shul’pin GB. Activation and Catalytic Reactions of Satu-rated Hydrocarbons in the Presence of Metal Complexes. Dordrecht: Kluwer Academic Publishers, 2000
2 Goldberg KI, Goldman AS. Activation and Functionalization of C-H Bonds (ACS Symp Ser 885). Washinton D C: Am Chem Soc, 2004
3 Arndtsen BA, Bergman RG, Mobley TA, Petersen TH. Selective in-termolecular carbon-hydrogen bond activation by synthetic metal complexes in homogeneous solution. Acc Chem Res, 1995, 28: 154–162
6 Bergman RG. C-H activation. Nature, 2007, 446: 391–393 7 Lunsford JH. Catalytic conversion of methane to more useful chemi-
cals and fuels: a challenge for the 21st century. Catal Today, 2000, 63: 165–174
8 Otsuka K, Wang Y. Direct conversion of methane into oxygenates. Appl Catal A Gen, 2001, 222: 145–161
9 Tabata K, Teng Y, Takemoto T, Suzuki E, Bañares MA, Peña MA, Fierro J LG. Activation of methane by oxygen and nitrogen oxide. Catal Rev -Sci Eng, 2002, 44: 1–58
10 Arena A, Parmaliana A. Scientific basis for progress and catalytic de-sign in the selective oxidation of methane to formaldehyde. Acc Chem Res, 2003, 36: 867–875
11 Wang Y, Otsuka K. Recent developments in partial oxidation of methane (in Japanese). Shokubai, 2004, 46: 595–600
12 Wang Y. Advances in selective oxidation and oxidative functionali-zation of methane (in Chinese). Petrochem Tech, 2005, 34: 205–212
13 Periana RA, Taube DJ, Evitt, E R, Löffler D G, Wentrcek P R, Voss G, Masuda T. A mercury-catalyzed, high-yield system for the oxida-tion of methane to methanol. Science, 1993, 259: 340–343
14 Periana RA, Taube DJ, Gamble S, Taube H, Satoh T, Fujii H. Plati-num catalysts for the high-yield oxidation of methane to a methanol derivative. Science, 1998, 280: 560–564
15 Periana RA, Mironov O, Taube D, Bhalla G, Jones CJ. Catalytic oxidative condensation of CH4 to CH3COOH in one step via CH ac-tivation. Science, 2003, 301: 814–818
16 Wolf D. High yields of methanol from methane by C-H bond activa-tion at low temperatures. Angew Chem Int Ed, 1998, 37: 3351–3353
17 Jones CJ, Taube D, Ziatdinov VR, Periana RA, Nielsen RJ, Oxgaard J, Goddard III W A. Selective oxidation of methane to methanol catalyzed, with C-H activation, by homogeneous, cationic gold. Angew Chem Int Ed, 2004, 43: 4626–4629
18 Palkovits R, Antonietti M, Kuhn P, Thomas A, Schüth F. Solid cata-lysts for the selective low-temperature oxidation of methane to methanol. Angew Chem Int Ed, 2009, 48: 6909–6912
19 Kao LC, Huston AC, Sen A. Low-temperature, palladium(II)-cata- lyzed, solution-phase oxidation of methane to methanol derivative. J Am Chem Soc, 1991, 113: 700–701
20 Muehlhofer M, Strassner T, Herrmann WA. New catalyst systems for the catalytic conversion of methane into methanol. Angew Chem Int Ed, 2002, 41: 1745–1747
21 An Z, Pan X, Liu X, Han X, Bao X. Combined redox couples for catalytic oxidation of methane by dioxygen at low temperatures. J Am Chem Soc, 2006, 128: 16028–16029
22 An Z, Pan X, Liu X, Han X, Bao X. A silica-immobilized Pt2+ cata-lyst for the selective, aerobic oxidation of methane via an elec-tron-transfer chain. J Nat Gas Chem, 2008, 17: 120–124
23 Jia C, Kitamura T, Fujiwara Y. Catalytic functionalization of arenas and alkanes via C-H bond activation. Acc Chem Res, 2001, 34: 633–639
24 Asadullah M, Kitamura T, Fujiwara Y. Calcium-catalyzed selective and quantitative transformation of CH4 and CO into acetic acid. Angew Chem Int Ed, 2000, 39: 2475–2478
25 Reis PM, Silva J AL, Palavra AF, da Silva JJRF, Kitamura T, Fuji-wara Y, Pombeiro A J L. Single-pot conversion of methane into ace-tic acid in the absence of CO and with vanadium catalysts such as amavadine. Angew Chem Int Ed, 2003, 42: 821–823
26 Wilcox EM, Roberts GW, Spivey JJ. Thermodynamics of light al-kane carboxylation. Appl Catal A Gen, 2002, 226: 317–318
27 Zerella M, Mukhopadhyay S, Bell AT. Synthesis of mixed acid an-hydrides from methane and carbon dioxide in acid solvents. Org Lett, 2003, 5: 3193–3196
28 Mukhopadhyay S, Bell AT. A high-yield approach to the sulfonation of methane to methanesulfonic acid initiated by H2O2 and a metal chloride. Angew Chem Int Ed, 2003, 42: 2990–2993
29 Mukhopadhyay S, Bell AT. Direct catalytic sulfonation of methane with SO2 to methanesulfonic acid (MSA) in the presence of molecu-lar O2. Chem Commun, 2003, 1590–1591
30 Yamanaka I, Soma M, Otsuka K. Partial oxidation of light alkanes by reductively activated oxygen in Eu-catalytic system At 40°C. Res Chem Intermed, 2000, 26: 129–135
31 Shul’pin GB, Nizova GV, Kozlov YN, Cuervo LG, Süss-Fink G. Hydrogen peroxide oxygenation of alkanes including methane and ethane catalyzed by iron complexes in acetonitrile. Adv Synth Catal, 2004, 346: 317–332
32 Mizuno M, Seki Y, Nishiyama Y, Kiyoto I, Misono M. Aqueous phase oxidation of methane with hydrogen peroxide catalyzed by di-iron-substituted silicotungstate. J Catal, 1999, 184: 550–552
33 Yuan Q, Deng W, Zhang Q, Wang Y. Osmium-catalyzed selective oxidation of methane and ethane with hydrogen peroxide in aqueous medium. Adv Synth Catal, 2007, 349: 1199–1209
34 Schröder M. Osmium tetraoxide cis hydroxylation of unsaturated substrates. Chem Rev, 1980, 80: 187–213
35 Kolb HC, van Nieuwenhze MS, Sharpless KB. Catalytic asymmetric dihydroxylation. Chem Rev, 1994, 94: 2483–2547
36 Shul’pin GB, Süss-Fink G, Shul’pina LS. Oxygenation of alkanes with hydrogen peroxide catalysed by osmium complexes. Chem Commun, 2000, 1131–1132
37 Osako T, Watson EJ, Dehestani A, Bales BC, Mayer JM. Methane oxidation by aqueous osmium tetroxide and sodium periodate: Inhi-bition of methanol oxidation by methane. Angew Chem Int Ed, 2006, 45: 7433–7436
38 Bales BC, Brown P, Dehestani A, Mayer JM. Alkane oxidation by osmium tetroxide. J Am Chem Soc, 2005, 127:2832–2833
39 Spencer ND. Partial oxidation of methane to formaldehyde by means
Wang Ye, et al. Sci China Chem February (2010) Vol.53 No.2 349
dation with molecualr oxygen. J Catal, 1989, 116: 399–406. 41 Amridis MD, Rekoske JE, Dumesic JA, Rudd DF, Spencer ND,
Pereira C J. Simulation of methane partial oxidation over sil-ica-supported MoO3 and V2O5. AIChE J, 1991, 37: 87–97.
42 Smith MR, Ozkan US. The partial oxidation of methane to formal-dehyde: Role of different crystal planes of MoO3. J Catal, 1993, 141: 124–139
43 Irusta S, Cornaglia LM, Miró EE, Lombardo EA. The role of V=O sites on the oxidation of methane to formaldehyde over V/SiO2. J Catal, 1995, 156: 167–170
44 Parmaliana A, Arena F, Sokolovskii V, Frusteri F, Giordano NA. A comparative study of the partial oxidation of methane to formalde-hyde on bulk and silica supported MoO3 and V2O5 catalysts. Catal Today, 1996, 28: 363–371
45 Sokolovskii V. Principles of oxidative catalysis on solid oxides. Catal Rev-Sci Eng, 1990, 32:1–49
46 Grasselli RK. Fundamental principles of selective heterogeneous oxidation catalysis. Top Catal, 2002, 21: 79–88
48 Fu G, Xu X, Lu X, Wan H. Mechanisms of methane activation and transformation on molybdenum oxide based catalysts. J Am Chem Soc, 2005, 127: 3989–3996
49 Ohler N, Bell AT. Selective oxidation of methane over MoOx/SiO2: isolation of the kinetics of reactions occurring in the gas phase and on the surfaces of SiO2 and MoOx. J Catal, 2005, 231: 115–130
50 Ohler N, Bell AT. A study of the redox properties of MoOx/SiO2. J Phys Chem B, 2005, 110: 23419–23429
51 Ohler N, Bell AT. Study of the elementary processes involved in the selective oxidation of methane over MoOx/SiO2. J Phys Chem B, 2006, 110: 2700–2709
52 Chempath S, Zhang Y, Bell AT. DFT studies of the structure and vi-brational spectra of isolated molybdena species supported on silica. J Phys Chem C, 2007, 111: 1291–1298
53 Chempath S, Bell AT. A DFT study of the mechanism and kinetics of methane oxidation to formaldehyde occurring on silica-supported molybdena. J Catal, 2007, 247: 119–126
54 Yang W, Wang X, Guo Q, Zhang Q, Wang Y. Superior catalytic performance of phosphorus-modified molybdenum oxide clusters encapsulated inside SBA-15 in the partial oxidation of methane. New J Chem, 2003, 27: 1301–1303
55 Lin B, Wang X, Guo Q, Yang W, Zhang Q, Wang Y. Excellent cata-lytic performances of SBA-15-supported vanadium oxide for partial oxidation of methane to formaldehyde. Chem Lett, 2003, 32: 860–861
56 Wang X, Lin B, Yang W, Guo Q, Zhang Q, Wang Y. SBA-15 sup-ported vanadium oxide catalyst for selective oxidation of methane (in Chinese). Acta Chim Sinica, 2004, 62: 1738–1744
57 Berndt H, Martin A, Brückner A, Schreier E, Müller D, Kosslick H, Wolf G -U, Lücke B. Structure and catalytic properties of VOx/MCM materials for the partial oxidation of methane to formaldehyde. J Catal, 2000, 191: 384–400
58 Parmaliana A, Frusteri F, Mezzapica A, Scurrel MS, Giordano N. Novel high activity catalysts for partial oxidation of methane to for-maldehyde. J Chem Soc Chem Commun, 1993, 751–752
59 Fornés V, López C, López HH, Martínez A. Catalytic performance of mesoporous VOx/SBA-15 catalysts for the partial oxidation of meth-ane to formaldehyde. Appl Catal A Gen, 2003, 249: 345–354
60 Ruddy DA, Ohler NL, Bell AT, Tilley TD. Thermolytic molecular precursor route to site-isolated vanadia-silica materials and their catalytic performance in methane selective oxidation. J Catal. 2006, 238: 277–85
61 Nguyen LD, Loridant S, Launay H, Pigamo A, Dubois JL, Millet JM M. Study of new catalysts based on vanadium oxide supported on mesoporous silica for the partial oxidation of methane to formalde-hyde: Catalytic properties and reaction mechanism. J Catal, 2006, 237: 38–48
62 Merkx M, Kopp DA, Sazinsky MH, Blazyk JL, Müller J, Lippard SJ.
Dioxygen activation and methane hydroxylation by soluble methane monooxygenase: A tale of two irons and three proteins. Angew Chem Int Ed, 2001, 40: 2782–2807
63 Chan SI, Chen KH-C, Yu SS-F, Chen C-L, Kou S-J. Toward Deline-ating the structure and function of the particulate methane monooxy-genase from methanotrophic bacteria. Biochemistry, 2004, 43: 4421– 4430
64 Park PW, Ledford JS. The influence of surface structure on the cata-lytic activity of alumina supported copper oxide catalysts. Oxidation of carbon monoxide and methane. Appl Catal B Environ, 1998, 15: 221–231
65 Kobayashi T, Nakagawa K, Tabata K, Haruta M. Partial oxidation of methane over silica catalysts promoted by 3d transition metal ions. J Chem Soc Chem Commun, 1994, 1609–1610
66 Kobayashi T, Guihaume N, Miki J, Kitamura N, Haruta M. Oxidation of methane to formaldehyde over Fe/SiO2 and Sn-W mixed oxides. Catal Today, 1996, 32: 171–175
67 Arena F, Gatti G, Martra G, Coluccia S, Stievano L, Spadaro L, Fa-mulari P, Parmaliana A. Structure and reactivity in the selective oxi-dation of methane to formaldehyde of low-loaded FeOx/SiO2 catalysts. J Catal, 2005, 231: 365–380
68 He J, Li Y, An D, Zhang Q, Wang Y. Selective oxidation of methane to formaldehyde by oxygen over silica-supported iron catalysts. J Natural Gas Chem, 2009, 18: 288–294
69 Zhang Q, Yang W, Wang X, Wang Y, Shishido T, Takehira K. Co-ordination structures of vanadium and iron in MCM-41 and the cata-lytic properties in partial oxidation of methane. Microporous Mesoporous Mater, 2005, 77: 223–234
70 Wang Y, Yang W, Yang L, Wang X, Zhang Q. Iron-containing het-erogeneous catalysts for partial oxidation of methane and epoxidation of propylene. Catal Today, 2006, 117: 156–162
71 Wang Y. Selective oxidation of hydrocarbons catalyzed by iron- containing heterogeneous catalysts. Res Chem Intermed, 2006, 32: 235–251
72 Zhang Q, Li Yang, An D, Wang Y. Catalytic behavior and kinetic features of FeOx/SBA-15 catalyst for selective oxidation of methane by oxygen. Appl Catal A Gen, 2009, 356: 103–111
73 Otsuka K, Wang Y, Yamanaka I, Morikawa A. Kinetic study of the partial oxidation of methane over Fe2(MoO4)3 catalyst. J Chem Soc Faraday Trans, 1993, 89: 4225–4230
74 Wang Y, Otsuka K. Catalytic oxidation of methane to methanol with H2-O2 gas mixture at atmospheric pressure. J Catal, 1995, 155: 256–267
75 Wang X, Wang Y, Tang Q, Guo Q, Zhang Q, Wan H. MCM-41- supported iron phosphate catalyst for partial oxidation of methane to oxygenates with oxygen and nitrous oxide. J Catal, 2003, 217: 457– 467
76 Wang Y, Wang X, Su Z, Guo Q, Tang Q, Zhang Q, Wan H. SBA-15-supported iron phosphate catalyst for partial oxidation of methane to formaldehyde. Catal Today, 2004, 93-95: 155–161
77 Groothaert MH, Smeets PJ, Sels BF, Jacobs PA, Schoonheydt RA. Selective oxidation of methane by the bis(μ-oxo)dicopper core stabi-lized on ZSM-5 and mordenite zeolites. J Am Chem Soc, 2005 127: 1394–1395
78 Li Y, Chen S, Zhang Q, Wang Y. Copper-catalyzed selective oxida-tion of methane to formaldehyde by oxygen. Chem Lett, 2006, 35: 572–573
79 Li Y, An D, Zhang Q, Wang Y. Copper-catalyzed selective oxidation of methane by oxygen: Studies on catalytic behavior and functioning mechanism of CuOx
/SBA-15. J Phys Chem C, 2008, 112: 13700– 13708
80 Chen PPY, Yang RBG, Lee JCM, Chan SI. Facile O-atom insertion into COC and COH bonds by a trinuclear copper complex designed to harness a singlet oxene. Proc Natl Acad Sci USA, 2007, 104: 14570–14575
81 Yoshizawa K, Shiota Y. Conversion of methane to methanol at the mononuclear and dinuclear copper sites of particulate methane monooxygenase (pMMO): A DFT and QM/MM study. J Am Chem Soc, 2006, 128: 9873–9881
350 Wang Ye, et al. Sci China Chem February (2010) Vol.53 No.2
82 Zhang Z, Zhao Z, Xu C. Research advances in the catalysts for the selective oxidation of ethane to aldehydes (in Chinese). Chinese Sci Bull, 2005, 50: 833–840
83 Dai HX, Au CT. The oxidative dehydrogenation of ethane. Curr Top Catal, 2002, 3: 33–80
84 Li X, Iglesia E. Support and promoter effects in the selective oxida-tion of ethane to acetic acid catalyzed by Mo-V-Nb oxides. Appl Catal A Gen, 2008, 334: 339–347
85 Mendelovici L, Lunsford JH. Partial oxidation of ethane to ethylene and acetaldehyde over a supported molybdenum oxide. J Catal, 1985, 94: 37–50
86 Erdőhelyi A, Máté F, Solymosi F. Partial oxidation of ethane over silica-supported alkali metal molybdate catalysts. J Catal, 1992, 135: 563
87 Chen K, Xie S, Bell AT, Iglesia E. Structure and properties of oxida-tive dehydrogenation catalysts based on MoO3/Al2O3. J Catal, 2001, 198: 232–242
88 Chen K, Bell AT, Iglesia E. The relationship between the electronic and redox properties of dispersed metal oxides and their turnover rates in oxidative dehydrogenation reactions. J Catal, 2002, 209: 35–42
89 Lou Y, Wang H, Zhang Q, Wang Y. SBA-15-supported molybdenum oxides as efficient catalysts for selective oxidation of ethane to for-maldehyde and acetaldehyde by oxygen. J Catal, 2007, 247: 245–255
90 Lou Y, Zhang Q, Wang H, Wang Y. Catalytic oxidation of ethylene and ethane to formaldehyde by oxygen. J Catal, 2007, 250: 365–368
91 Conway SJ, Wang DJ, Lunsford JH. Selective oxidation of methane and ethane over Li+-MgO-Cl− catalysts promoted with metal oxides. Appl Catal A Gen, 1991, 79: L1–L5
92 Olah GA, Gupta B, Farina M, Felberg JD, Ip W M, Husain A, Kar-peles R, Lammertsma K, Melhotra AK, Trivedi NJ. Selective mono-halogenation of methane over supported acid or platinum metal cata-lysts and hydrolysis of methyl halides over γ-alumina-supported metal oxide/hydroxide catalysts. A feasible path for the oxidative conversion of methane into methyl alcohol/dimethyl ether. J Am Chem Soc, 1985, 107: 7097–7105
93 Zhou XP, Yilmaz A, Yilmaz GA, Lorkovic IM, Laverman LE, Weiss M, Sherman JH, McFarland EW, Stucky GD, Ford PC. An integrated process for partial oxidation of alkanes. Chem Commun, 2003, 2294–2295
94 Wang KX, Xu HF, Li WS, Zhou XP. Acetic acid synthesis from methane by non-synthesis gas process. J Mol Catal A Chem, 2005, 225: 65–69
95 Liu Z, Huang L, Li WS, Yang F, Au CT, Zhou XP. Higher hydrocarbons from methane condensation mediated by HBr. J Mol Catal A Chem, 2007, 273: 14–20
96 Lin R, Ding Y, Gong L, Li J, Chen W, Yan L, Lu Y. Oxidative bro-mination of methane on silica-supported non-noble metal oxide cata-lysts. Appl Catal A Gen, 2009, 353: 87–92
97 Li F, Yuan G. Hydrated dibromodioxomolybdenum (VI) supported on Zn-MCM-48 for facile oxidation of methane. Angew Chem Int Ed, 2006, 45: 6541–6544
98 Li F, Yuan G, Yan F, Yan F. Bromine-mediated conversion of meth-ane to C1 oxygenates over Zn-MCM-41 supported mercuric oxide. Appl Catal A Gen, 2008, 335: 82–87
99 Lin M, Sen A. Direct catalytic conversion of methane to acetic acid in an aqueous medium. Nature, 1994, 368: 613–615
100 Lin M, Hogan TE, Sen A. Catalytic carbon-carbon and carbon- hydrogen bond cleavage in lower alkanes. low-temperature hydroxy-lations and hydroxycarbonylations with dioxygen as the oxidant. J Am Chem Soc, 1996, 118: 4574–4580
101 Wang Y, Katagiri M, Otsuka K. Oxidative carbonylation of methane to methyl acetate on a rhodium-doped iron phosphate catalyst. Chem Commun, 1997, 1187–1188
102 Yuan Q, Zhang Q, Wang Y. Direct conversion of methane to methyl acetate with nitrous oxide and carbon monoxide over heterogeneous catalysts containing both rhodium and iron phosphate. J Catal, 2005, 233: 221–233
103 Wang Y, Yuan Q, Zhang Q, Deng W. Characterizations of unsup-ported and supported rhodium-iron phosphate catalysts effective for oxidative carbonylation of methane. J Phys Chem C 2007, 111: 2044–2053
![Titan 21 October 2011. Titan [2003] Second-largest moon in SS Density ~1900 kg/m 3 thick atmosphere! Sublimation N 2 (90%), methane, ethane ethane may.](https://static.documents.pub/doc/80x56/56649d395503460f94a13e02/titan-21-october-2011-titan-2003-second-largest-moon-in-ss-density-1900.jpg)
