# Direct conversion of methane to formaldehyde and CO on B2O3 catalysts

## Abstract

Direct oxidation of methane to value-added C1 chemicals (e.g. HCHO and CO) provides a promising way to utilize natural gas sources under relatively mild conditions. Such conversions remain, however, a key selectivity challenge, resulting from the facile formation of undesired fully-oxidized CO2. Here we show that B2O3-based catalysts are selective in the direct conversion of methane to HCHO and CO (~94% selectivity with a HCHO/CO ratio of ~1 at 6% conversion) and highly stable (over 100 hour time-on-stream operation) conducted in a fixed-bed reactor (550 °C, 100 kPa, space velocity 4650 mL gcat−1 h−1). Combined catalyst characterization, kinetic studies, and isotopic labeling experiments unveil that molecular O2 bonded to tri-coordinated BO3 centers on B2O3 surfaces acts as a judicious oxidant for methane activation with mitigated CO2 formation, even at high O2/CH4 ratios of the feed. These findings shed light on the great potential of designing innovative catalytic processes for the direct conversion of alkanes to fuels/chemicals.

## Introduction

The catalytic transformation of methane to value-added chemicals is of significant interest for the efficient utilization of natural gas sources1,2, especially due to the recent shale-gas revolution3. In particular, the direct conversion of methane via selective oxidation to C1 chemicals (i.e. CH3OH, HCHO, and CO) is most attractive, because all of these products are important platform molecules/intermediates for the production of fuels and chemicals4,5. Selective oxidation of methane involves the cleavage of C–H bonds and formation of C–O bonds, leading to CH3OH, HCHO, and CO as the desired partial oxidation products but also to CO2 as an undesired complete oxidation product. Such oxidation processes are induced by nucleophilic attack of active O species at the H or C atom in CH4 (or reactive intermediates) before electron transfer6. Because the desired C1 partial oxidation products (i.e. CH3OH, HCHO, and CO) have much higher electrophilicity than CH4, they are kinetically more favored in oxidation, leading to dominant formation of undesired CO2 at methane conversions even less than 5% on conventional catalysts7,8,9,10,11,12 (e.g., V2O513, MoO314, and Fe2O315). Lattice O anions exposed on these oxide surfaces act as the nucleophilic and oxidative centers that can be regenerated fast via dissociative adsorption of gaseous O2 during catalytic cycles (also described as the Mars van Krevelen mechanism16). To our best knowledge, high selectivities to the desired C1 partial oxidation products on these traditional metal oxide catalysts are able be obtained merely at low methane conversions (<2%) or low O2/CH4 ratios (<0.5)7,8,9,10,11,12,13,14,15, making such oxidation processes impractical.

Directly using molecular O2 or derived O· radicals to oxidize CH4, instead of the more nucleophilic and reactive lattice O anions, would render facile control over the extent of CH4 oxidation. Recent studies17,18 have shown that nonmetallic B-based materials (e.g. BN19,20,21, B4C22, SiB623, and B2O324,25) can catalyze oxidative dehydrogenation of C2–C4 alkanes to alkenes with extraordinarily low selectivities to CO2, reflecting, in turn, that these catalysts probably use the moderate O2 or O· oxidants to activate the alkane reactants. The BN catalyst has also been attempted for methane oxidation at temperatures above 690 °C, which yielded CO, CO2, and methane coupling products (i.e. C2H4 and C2H6) with respective selectivities of 76.6, 4.3, and 19.3 at 20.5% methane conversion26. We expect that the selectivity of valuable C1 products for methane oxidation on the B-based catalysts can be significantly improved at milder temperatures under which HCHO and CH3OH can be better stabilized kinetically. Moreover, experiments of nuclear magnetic resonance spectroscopy27 and X-ray photoelectron spectroscopy22 have revealed that B(OH)xO3-x (where x = 0–3) layers formed on the B-based catalysts act as the true active phase, irrespective of their bulk contents.

According to the above considerations, we here study supported B2O3 catalysts for selective methane oxidation at relatively mild temperatures. Our results show that B2O3-based catalysts are highly selective in the direct conversion of methane to HCHO and CO, and these selectivities are unexpectedly insensitive to the O2/CH4 ratios. Structural characterization, kinetic measurements, and isotopic labeling experiments are combined to discern that molecular O2 bonded to coordinately unsaturated BO3 centers on the B2O3 surfaces is the crucial oxidant that accounts for the selective methane oxidation. The mechanistic understanding of methane oxidation on the unique B2O3 surfaces would inspire the design of the next generation of heterogeneous catalysts for selective oxidation of hydrocarbons.

## Results

### Performances of B2O3-based catalysts in methane oxidation

Catalysts containing 20 wt% B2O3 supported on various oxides (i.e. Al2O3, SiO2, ZnO, TiO2, and ZrO2) were prepared using the wetness impregnation method with boric acid (H3BO3) as the boron source (see Methods section). This high B2O3 loading was chosen to ensure that multiple B2O3 layers were formed on each oxide support (Supplementary Table 1). Under the conditions studied (O2/CH4 ratio of 1.0 and 550 °C), these oxide supports themselves were inactive for methane activation, whereas all supported catalysts with 20 wt% B2O3 selectively converted methane to HCHO and CO (~94 % selectivity with a HCHO/CO ratio of ~1), together with a trace amount of desired CH3OH and C2 products (C2H4 and C2H6), which are irrespective of the nature of the support (Fig. 1a). It is noteworthy that these observed conversions are mainly attributed to the catalytic processes on the B2O3 surface, instead of gas-phase radical reactions, because the conversion and selectivity of methane oxidation on the supported B2O3 catalysts had negligible changes whether the empty space of the reactor was fully filled with inert SiC material or not (Supplementary Fig. 1) and their sum did not obey the empirical 100% rule28 that was observed in the previous gas phase chemistry (Supplementary Fig. 2).

In all cases, the sum of the selectivities to these desired C1 and C2 products were above 96%, while the selectivity to undesired CO2 was below 4% at ~6% methane conversion (controlled by the space velocity for rigorous selectivity comparison among the examined catalysts). Such high selectivities to the partial oxidation products reflect the superior control of the oxidation extent within the kinetic regime, brought forth by the unique property of the B2O3-based catalysts as described below, otherwise the fully oxidized CO2 would be predominant among the products if the thermodynamic equilibrium is established (e.g., 53.5% methane conversion and 72.8% CO2 selectivity at 550 °C and 100 kPa with an initial CH4/O2 molar ratio of 1/1; Supplementary Fig. 3). This highly selective methane oxidation process could be scaled up by combining efficient separation of the products from the effluent and recycling of the unconverted CH4 and O2 reactants29. Moreover, the fact that the molar ratio of HCHO to CO in the products is ~1.0 makes them potentially desired for the downstream acetic acid synthesis via hydrogenation of HCHO to methanol and its subsequent carbonylation30,31 and also for glycolic acid synthesis via direct carbonylation of HCHO32. The detected C2H6 and C2H4 products are likely ascribed to methane oxidative coupling as we reported previously21,26, reflecting the ability of B2O3-based catalysts to produce C2+ molecules from direct methane oxidation. Similar methane oxidation rates (mmol methane converted per surface area of the exposed B2O3 phase per hour) were obtained for the examined catalysts. These results suggest a negligible effect of the oxide supports on catalytic performance, a result that is consistent with formation of the multiple B2O3 layers on each oxide support at 20 wt% B2O3 loading (the loading threshold for forming a B2O3 monolayer for each of the oxide supports are shown in Supplementary Table 1).

We further studied the effects of O2/CH4 partial pressure ratios on the performances of the 20 wt% B2O3/Al2O3 catalyst at 550 °C, where methane conversions were kept ~6% via adjusting the space velocity for rigorous comparison of reactivity and selectivity. As shown in Fig. 1b, nearly constant selectivities (i.e., >95% selectivity to desired C1 and C2 products with <5% selectivity to undesired CO2) were obtained at ~6% methane conversion over a wide range of PO2/PCH4 ratios (0.67–2.00), reflective of the unique ability of B2O3 in preventing complete oxidation of the desired C1 and C2 products to CO2 and thus the potential of high-pressure operation for this catalytic process. Such remarkable selectivities to the desired C1 and C2 products and a negligible effect of the PO2/PCH4 ratio on activity makes this system superior to conventional oxide catalysts (e.g. supported V2O5 and MoO3 in Supplementary Table 2), which must operate at much lower PO2/PCH4 ratios (0.1–0.5) to minimize the over-oxidation of C1 and C2 products by the strongly nucleophilic lattice O anions on metal oxide surfaces16,33. Figure 1b also shows that methane oxidation rates increased with increasing the O2/CH4 ratio, indicating a positive reaction order with respect to O2. In addition, these B2O3-based catalysts exhibited exceptional stability at 550 °C as illustrated over 100 h time-on-stream stable methane oxidation for the B2O3/Al2O3 catalyst (Supplementary Fig. 4).

### Active sites of B2O3-based catalysts for methane oxidation

To provide insight into the nature of active sites on these supported B2O3 catalysts, 11B solid state nuclear magnetic resonance (NMR) was used. Two boron oxide species with chemical shifts centered at 67.9 and 57.1 ppm were observed for these samples (using NaBH4 as the reference compound, NMR spectra shown in Supplementary Fig. 5). These are ascribed to tri-coordinated BO3 and tetra-coordinated BO4 units, respectively34. Previous studies35,36 suggest that the BO4 units are derived from the additional bonding of lattice O anions of the oxide support to the BO3 units in B2O3. The BO4/BO3 molar ratio of the supported B2O3 catalysts increased with increasing basic strength of the oxide support (e.g. 0.46 for B2O3/SiO2, 1.45 for B2O3/Al2O3, and 1.65 for B2O3/ZrO2 in Supplementary Fig. 5) that can be explained by increased coordination of the B center to more basic lattice O anions of the oxide support. On the other hand, these BO4/BO3 ratios did not affect the methane oxidation rates of the supported B2O3 catalysts that were almost identical when normalized by the exposed B2O3 surface area (Fig. 1a). These results might indicate that the BO3 and BO4 units have similar catalytic activities.

To further confirm or disapprove our hypothesis, B-substituted ZSM-5 zeolite (B-ZSM-5) was prepared, where the B atoms embedded in the silicate framework were all tetra-coordinated by lattice O anions as confirmed by the single chemical shift of B at 54.1 ppm (Supplementary Fig. S5)37,38. Compared with the supported B2O3 catalysts (e.g., 20 wt% B2O3/Al2O3), the B-ZSM-5 sample showed negligible activity in catalyzing methane oxidation with near 45% selectivity to CO2 (Supplementary Fig. 6). It is thus concluded that the BO4 units with full coordination of B centers are catalytically inactive. The fact that the methane oxidation rates of the supported B2O3 catalysts are independent of the BO4/BO3 ratio measured by 11B NMR is likely due to that NMR is a bulk technique and that the top layer B2O3 is not bonded to the oxygen anions of oxide supports on these catalysts with multilayered B2O3, leading to exclusively active surface BO3 units for methane oxidation.

### Mechanism of methane oxidation on B2O3-based catalysts

Kinetic studies were carried out to provide molecular-level insights into the mechanism of methane activation on supported B2O3 catalysts. The conversion-selectivity relationship for methane oxidation on 20 wt% B2O3/Al2O3 (Supplementary Fig. 7) showed that the selectivities to HCHO and CH3OH both monotonically decreased as the conversion of CH4 increased from 2 to 10%, concomitant with increased selectivities to CO and CO2. These selectivity trends indicate that both HCHO and CH3OH are primary products formed from methane conversion. It has been proposed that the initial activation of methane by active O species on oxide surfaces forms bonded methoxy intermediates (CH3O) via cleaving a C–H bond in methane, which undergo further dehydrogenation to form HCHO or recombine the cleaved H atom to form CH3OH39,40. On B2O3 surfaces, methane selective oxidation to HCHO is the predominant reaction channel for methane activation, as evidenced by the high HCHO/CH3OH selectivity ratios obtained at low methane conversions (e.g. ~17 at 2% conversion, Supplementary Fig. 7). The secondary dehydrogenation of HCHO leads to the formation of CO, whereas the CO2 product comes from CO oxidation.

Figure 2a depicts that the methane oxidation rate increased with O2 pressure (0–80 kPa; 550 °C), but the dependence became weaker at higher O2 pressure that is indicative of higher O2 coverages on the B2O3 surface. In contrast, the methane oxidation rate increased linearly with the CH4 pressure in the same pressure range, suggesting that CH4 barely adsorbs on the BO3 sites during catalysis (Fig. 2b), which is consistent with the unfavorable adsorption of CH4 on solid surfaces brought forth by its highly symmetrical geometry and weakly polarized C–H bonds11. These effects of reactant concentrations on the oxidation rates are indicative of the Eley-Rideal mechanism41 on catalytic surfaces, in which one molecule adsorbed on the active site directly reacts with another one from the gas phase. Because of this, we propose that methane oxidation occurs between a gaseous CH4 molecule and an O2 molecule bonded to the BO3 sites. Similar Eley-Rideal-type pathways have been found for methane oxidation on Pd and Pt surfaces at high temperatures42,43, which unveils that two adsorbed O atoms formed from the dissociation of a O2 molecule on the catalyst surface are required to act concertedly in order to cleave the strong C–H bond of CH4.

Isotopic labeling experiments were further used to confirm the above hypothesis (i.e. BO3-surface-bonded O2 directly activates CH4). Pulses of a small amount of 18O2 into flowing 16O2 on the supported B2O3 catalysts at 550 °C did not lead to the formation of isotope-exchanged 18O16O species, excluding the presence of dissociative adsorption of O2 molecules on the B2O3 surface (Fig. 3a). In contrast, when CH4 was co-fed with 16O2 and pulses of 18O2 over the surface of the same B2O3 catalysts, a significant amount of 18O16O was detected (Fig. 3b). We thus infer that the O–O bond in the adsorbed O2 molecule is cleaved concertedly when it activates the C–H bond of CH4.

The above kinetic and isotopic assessments are combined to give a plausible pathway for the formation of HCHO from CH4 oxidation on B2O3-based catalysts as described in Fig. 4a. First, O2 adsorbs on two vicinal BO3 units with each O atom in O2 bound to one of the electron-deficient B centers (Step 1, Fig. 4a). A gaseous CH4 molecule then attacks this adsorbed O2 reactant, resulting in concurrent formations of hydroxy and methoxy species (Step 2, Fig. 4a). Formaldehyde and H2O are further produced from hydrogen abstraction of the methoxy moiety by a hydroxyl (Step 3, Fig. 4a). Desorption of these products from the active BO3 sites completes a catalytic turnover for methane partial oxidation on B2O3 (Step 4, Fig. 3a). The measurable O-exchange between O2 molecules in the presence of CH4 (Fig. 3b) indicates the reversibility of Steps 1 and 2 on B2O3 under reaction conditions. This suggests, in turn, that Step 3 is kinetically relevant. This finding is consistent with a previous report6 that the electronegativity of oxide catalysts affects the selectivity to HCHO. These elementary steps, taken together with the pseudo-steady-state approximation for all bound species and the quasi-equilibrated nature of all steps except Step 3, lead to an equation for methane conversion rates (r):

$$r = \frac{{k_3K_1K_2P_{{\mathrm{O}}2}P_{{\mathrm{CH}}4}}}{{1 + K_1P_{{\mathrm{O}}2}}}$$
(1)

Here, K1 and K2 are the respective equilibrium constants for Steps 1 and 2, whereas k3 is the kinetic constant for Step 3. The functional form of Equation 1 accurately describes all methane oxidation rate data measured within a wide reactant pressure range (Fig. 4b; regression-fitted parameters shown in Supplementary Table 3), supporting the proposed methane activation mechanism on the B2O3 surfaces (Fig. 4a).

## Discussion

In summary, nonmetallic B2O3-based catalysts are selective and stable in the partial oxidation of methane to HCHO and CO. Surface tri-coordinated BO3 units are the active sites for methane oxidation. O2 molecules bound to the electron-deficient B centers of these BO3 units are moderate oxidants for methane activation, exhibiting strong suppression of the formation of thermodynamically favored CO2. Further exploitation of such nonmetallic oxide catalysts will bring innovative strategies and catalyst systems for efficient and selective oxidation of methane (and other alkanes) to valuable chemicals.

## Methods

### Preparation of B2O3-based catalysts

B2O3 catalysts supported on various oxides (including Al2O3, TiO2, ZnO, ZrO2, and SiO2) were prepared by wetness impregnation method using boric acid (Sinopharm chemical reagent co. LTD) as the boron source. The preparation method was described below using B2O3/Al2O3 as an illustrative example: a certain amount of H3BO3 was dissolved in deionized water (10 mL) and then the resulting aqueous solution was added dropwise dropped to pure Al2O3 (0.5 g). After stirring vigorously for 1 h, the impregnated samples were heated to 65 °C and subsequently vacuumed at 65 °C for 8 h. The as-prepared products were then calcined at 600 °C for 5 h in air.

### Preparation of B-ZSM-5 samples

B-ZSM-5 was synthesized according to a previously reported method37. Typically, NaOH, 1–6 hexanediamine (HMDA) and tetrapropylammonium bromide (TPABr) were dissolved in 18.9 mL of deionized water. The solution was stirred for 30 min, and then 1.8 g of porous SiO2 (Aladdin) and 0.3373 g H3BO3 were slowly added. The gel was stirred for 1 h and then transferred into a teflon-lined stainless-steel autoclave for crystallization at 180 °C for 48 h. The obtained samples were separated by filtration, washed with deionized water, dried at 90 °C for 12 h and finally calcined under static air at 550 °C for 5 h.

### Measurement of catalytic performance

Catalytic methane oxidation was conducted using a fixed-bed quartz tubular reactor (7 mm inner dimeter) with plug-flow hydrodynamics. The B2O3-based catalyst (0.15–0.18 mm sieved particles, ~200 mg, corresponding to a volume of 0.38 mL) was first pretreated in flowing O2/N2 (1/1 in volume) for 2 h under 580 °C and then cooled to the reaction temperature under N2. CH4/He (90/10%), O2 (99.99%), and N2 (99.99%) were individually controlled using three mass flow controllers (Sevenstar Technology Co., Ltd) to provide the reaction gas feed. The feed rate reported here is the weight hourly space velocity (WHSV), in which the gas volume refers to the standard ambient temperature and pressure. The concentrations of reaction products in the effluent were analyzed by an online gas chromatography (GC2060, Shanghai Ruimin GC Instruments, Inc). Samples in the quantitative ring were separated by Porapak column (6 m×3 mm) and then quantified using a thermal conductivity detector (TCD) for He, CH4, and CO244. The other gases are introduced into a flame ionization detector (FID) and subsequently analyzed including CO, CH4, CO2, C2H4, C2H6, HCHO, and CH3OH. Control experiments with SiC showed that there was negligible methane conversion without the catalyst. In all tests, carbon mass balances exceeded 98%. The CH4 conversion (XCH4) and the carbon selectivity of each product i (Si) were calculated using a standard normalization method (He as internal standard gas) based on the carbon balance, which were defined as

$$X_{{\mathrm{CH}}4} = \left( {{\mathrm{1}} - \frac{{P_{{\mathrm{CH}}4}^{{\mathrm{out}}}}}{{P_{{\mathrm{CH}}4}^{{\mathrm{in}}}}}} \right) \times 100\%$$
(2)
$$S_i = \frac{{n_iP_{\mathrm{i}}^{{\mathrm{out}}}}}{{{\sum} {n_i} P_{\mathrm{i}}^{{\mathrm{out}}}}} \times 100\%$$
(3)

Here, $$P_{{\mathrm{CH}}4}^{{\mathrm{out}}}$$ and $$P_{{\mathrm{CH}}4}^{{\mathrm{in}}}$$ are the corresponding partial pressures of methane at the outlet and inlet of the reactor, while $$P_{\mathrm{i}}^{{\mathrm{out}}}$$ and ni are the outlet partial pressure and the carbon number of each product i formed from methane oxidation, respectively.

### Structural characterization

Specific surface areas of catalysts were measured by the Brunauer–Emmett–Teller (BET) method, using a Micromeritics Tristar 3020 surface area and porosimetry analyzer. Prior to measurement, all samples were degassed at 150 °C for 6 h. 11B solid nuclear magnetic resonance (11B-NMR) analysis was recorded on a Bruker NMR 500 DRX spectrometer at 500 MHz and referenced to NaBH4 (3.2 ppm). Isotopic labeling experiments were performed in a fixed-bed single-pass flow micro-reactor. A mixture of N2 and 16O2 (research grade, 99.99%) was fed to the 20 wt% B2O3/Al2O3 catalyst bed at 550 °C until the baseline was stabilized and then an 18O2 (Cambridge Isotope Lab., 99%; 1 mL each time) pulse was injected into the flow using a syringe. The chemical and isotopic compositions of the reactor effluent were measured by online mass spectrometry (MS, Pfeiffer, OminStar TM) at intervals of 10 s with a m/z scanning from 1 to 50. The m/z signals of 32, 34, and 36 represent 16O16O, 16O18O, and 18O18O, respectively.

## Data availability

The datasets generated and analyzed during the current study are available from the corresponding authors upon a reasonable request.

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## Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21922201, 21872113, 91945301, 21673189, and 91545114) and the Fundamental Research Funds for the Central Universities (No. 20720190036 and 20720160032).

## Author information

Authors

### Contributions

Shu.W. and Y.W. conceived the idea for the project. J.Ti., J.Ta., and M.Y. conducted the material synthesis. P.H., M.Y., and J.Ta. performed the structural characterizations and catalytic test. Sha.W., Y.W., J.L., and Z.Z. discussed the catalytic results. J.Ti. drafted the manuscript under the guidance of Y.W. and Shu.W. All authors discussed and commented on the manuscript.

### Corresponding authors

Correspondence to Shuai Wang or Yong Wang.

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### Competing interests

The authors declare no competing interests.

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Tian, J., Tan, J., Zhang, Z. et al. Direct conversion of methane to formaldehyde and CO on B2O3 catalysts. Nat Commun 11, 5693 (2020). https://doi.org/10.1038/s41467-020-19517-y

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