Selective conversion of CO₂ and H₂ into aromatics

Abstract

Transformation of greenhouse gas CO₂ and renewable H₂ into fuels and commodity chemicals is recognized as a promising route to store fluctuating renewable energy. Although several C₁ chemicals, olefins, and gasoline have been successfully synthesized by CO₂ hydrogenation, selective conversion of CO₂ and H₂ into aromatics is still challenging due to the high unsaturation degree and complex structures of aromatics. Here we report a composite catalyst of ZnAlO_x and H-ZSM-5 which yields high aromatics selectivity (73.9%) with extremely low CH₄ selectivity (0.4%) among the carbon products without CO. Methanol and dimethyl ether, which are synthesized by hydrogenation of formate species formed on ZnAlO_x surface, are transmitted to H-ZSM-5 and subsequently converted into olefins and finally aromatics. Furthermore, 58.1% p-xylene in xylenes is achieved over the composite catalyst containing Si-H-ZSM-5. ZnAlO_x&H-ZSM-5 suggests a promising application in manufacturing aromatics from CO₂ and H₂.

Selective conversion of CO₂ and H₂ into aromatics remains challenging due to the high unsaturation degree and complex structure of aromatics. Here the authors report a composite catalyst of ZnAlOx and H-ZSM-5 which promotes the formation of aromatics with high selectivity while inhibiting CO and CH₄ formation in CO₂ hydrogenation reactions.

Introduction

The utilization of fossil resources such as coal, oil, and natural gas has brought us unprecedented economy and social development in the past two centuries¹, however, continuously increasing the emissions of greenhouse gas CO₂ is threatening our living environment. The renewable energy resources (for example, solar, tidal, wind and biomass) can generate abundant power, but low-efficiency and fluctuating nature limits their widespread applications². These problems above could be effectively overcome via CO₂ hydrogenation to fuels (e.g., gasoline) and commodity chemicals (e.g., methanol, olefins and aromatics) because hydrogen can be acquired from the clean electricity^3–6.

Considering that CO₂ (Δ_fG^o = −396 kJ mol⁻¹) is a chemical inert molecular³, CO₂ hydrogenation reactions are generally operated under high pressure and hydrogen content over reductive metal catalysts so as to improve the conversion efficiency. Nowadays, C₁ chemicals such as methanol (MeOH), dimethyl ether (DME), formic acid (HCOOH), methane (CH₄), and carbon monoxide (CO) have been selectively synthesized under above conditions^3,4,6,7. However, production of C₂₊ hydrocarbons such as olefins and liquid fuels from CO₂ are not easy because of high kinetic barriers for C–C coupling^6,8. In the earlier studies, combination of reverse-water-gas-shift (RWGS, CO₂ + H₂ → CO + H₂O) and Fischer–Tropsch (FT, CO + H₂ → C_nH_m) synthesis reactions was considered as a promising method to generate long-chain hydrocarbons from CO₂^2,9. Nevertheless, due to the restriction of Anderson–Schulz–Flory (ASF) distribution^10,11, the selectivity of C₂–C₄ and gasoline fraction hydrocarbons does not exceed 58% and 48%^12,13, respectively. More recently, high selective olefins or gasoline has been achieved from CO₂ hydrogenation via utilization of oxide/zeolite bifunctional catalysts which have been successfully applied in syngas-to-olefins (STO) or aromatics (STA) reactions^14–20. Some typical results are as follows: In−Zr oxide, ZnGa₂O₄ or ZnO-ZrO₂ combined with SAPO-34 zeolites achieved more than 80% C₂–C₄ olefins^21–23, meanwhile, In₂O₃ or Na-Fe₃O₄ coupled with H-ZSM-5 zeolites reached to approximately 80% gasoline-range hydrocarbons^24,25. Because metal catalysts or the Brønsted acid sites of the zeolites can catalyze hydrogenation reactions^26,27, it is challenging to synthesize aromatics with high unsaturation degree and complex structures under conditions of high H₂ content. Up to now, there are no reports on the highly selective conversion of CO₂ and H₂ into aromatics.

Here, we report a composite catalyst made by nano-scaled spinel structural ZnAlO_x oxide and H-ZSM-5 zeolite (ZnAlO_x&H-ZSM-5), which exhibits 73.9% aromatics selectivity with only 0.4% CH₄ selectivity among the carbon products without CO in CO₂ hydrogenation reaction. RWGS reaction is largely suppressed by increasing H₂/CO₂ ratio or introducing CO. 58.1% p-xylene in xylenes is achieved over the composite catalyst containing Si-H-ZSM-5. Reaction mechanism and the causes of excellent aromatization performance are also explored.

Results and Discussion

Catalytic results

CO₂ hydrogenation reactions were conducted over ZnAlO_x&H-ZSM-5 under reaction conditions of H₂/CO₂/Ar = 3/1/0.2, pressure 3.0 MPa, and 593 K. The effects of space velocity on CO₂ conversion and product selectivity are shown in Fig. 1a. It is surprising to find that the selectivity of aromatics among the carbon products without CO reaches as high as 73.9% with 9.1% CO₂ conversion and 57.4% CO selectivity at space velocity = 2000 ml g⁻¹ h⁻¹. This aromatics selectivity is much higher than the about 40% obtained over Na-Fe₃O₄/H-ZSM-5²⁵ or 14.6% acquired over In₂O₃/H-ZSM-5²⁴, respectively. Besides that, the liquid (or gasoline-range) hydrocarbons including C₅₊ and aromatics (excluding CO) run up to 80.3% along with merely 0.4% CH₄. As the space velocity substantially rises to 10,000 ml g⁻¹ h⁻¹, the aromatics selectivity only slightly decreases to 56.9%. This result is quite different from STA reactions, which were often operated at the GHSV <1500 ml g⁻¹ h⁻¹ so as to peruse high aromatics selectivity^19,20. It also can be seen from Fig. 1a that MeOH, DME and C_2-4 olefins are progressively climbing with the space velocity growing, which suggests that these byproducts might be acted as the intermediates for aromatization. The CO₂ hydrogenation behaviors of various catalysts are compared in Fig.1b. ZnO exhibits 98.2% MeOH selectivity (excluding CO) with 2.6% CO₂ conversion and 63.5% CO selectivity. After mixed with H-ZSM-5, only 29.7% aromatics selectivity was achieved. Compared to ZnO, ZnAlO_x shows a higher CO₂ conversion (5.1%) but lower CO selectivity (54.9%). The DME and MeOH selectivity is up to 42.8% and 56.2% over ZnAlO_x, respectively. It implies that AlO_x component is acted as the active site for MeOH dehydration. In contrast with the composite catalyst ZnO&H-ZSM-5, the composite catalyst ZnAlO_x&H-ZSM-5 shows a much better aromatization performance, over which 66.2% aromatic selectivity is obtained under the same reaction conditions. As shown in Fig. 1b, although the CO₂ conversion and liquid hydrocarbons selectivity are approximate, the aromatics selectivity for ZnAlO_x&H-ZSM-5 prepared by two components grinding in an agate mortar is 12.4% higher than ZnAlO_x + H-ZSM-5 prepared by the granules of two components mixing. It is apparent that the proximity of oxides and zeolites for the former catalyst is better than the latter one. It means that the closer proximity favors the selective generation of aromatics. As a comparison, the dual-bed configuration catalyst ZnAlO_x/H-ZSM-5 with H-ZSM-5 downstream from the ZnAlO_x provided the poorest behavior with 17.9% aromatics selectivity. The effect of reaction temperature on the CO₂ hydrogenation over ZnAlO_x&H-ZSM-5 is investigated. Although raising temperature can increase the CO₂ conversion, the ability of aromatization is considerably weakened (Supplementary Fig. 1). The optimized weight ratio of oxide/zeolite is 1:1 (Supplementary Fig. 2). We also studied the CO₂ hydrogenation over the mixture of conventional methanol synthesis catalyst CuZnAlO_x and H-ZSM-5. As shown in Supplementary Fig. 3, the RWGS reaction is predominant, which leads to more than 95% CO selectivity. According to the conditions for CO₂ hydrogenation, CO hydrogenation over ZnAlO_x&H-ZSM-5 was investigated at H₂/CO/Ar = 3/1/0.2, space velocity = 6000 ml g⁻¹ h⁻¹, pressure 3.0 MPa, and 593 K. As depicted in Supplementary Fig. 4, the aromatics selectivity (excluding CO₂) is only 12.8% with 4.7% CO conversion. It implies that the excellent aromatization behavior of ZnAlO_x&H-ZSM-5 in CO₂ hydrogenation is not related to self-promotion mechanism of CO reported by Cheng et al.¹⁹.

Fig. 1.

Catalytic performance for CO₂ hydrogenation. a The effect of space velocity over ZnAlO_x&H-ZSM-5. Reaction conditions: 593 K, 3.0 MPa, H₂/CO₂/Ar = 3/1/0.2. b Comparisons of the CO₂ conversion and product selectivity over various catalysts. Reaction conditions: Space velocity = 12,000 (for ZnO and ZnAlO_x) or 6000 (for composite catalysts) ml g⁻¹ h⁻¹, 3.0 MPa, 593 K, H₂/CO₂/Ar = 3/1/0.2. Note that the C₅₊ excludes aromatics. C_2-4⁼ and C_2-4^o refer to C₂–C₄ olefins and paraffins, respectively; ZnAlO_x&H-ZSM-5 prepared by grinding; ZnAlO_x + H-ZSM-5 prepared by granules mixing; ZnAlO_x/H-ZSM-5 denoted as dual-bed catalysts

Catalytic stability

The catalytic stability of ZnAlO_x&H-ZSM-5 in CO₂ hydrogenation reaction was studied. It is apparent from Fig. 2 that the composite catalyst delivers a good stability in a 100 h test. The CO₂ conversion, liquid hydrocarbons, and CH₄ selectivity among the carbon products without CO levels off at around 5.4, 74.0, and 0.5% after 50 h on stream. Although the aromatics selectivity is slightly decreased from 72.0% in the initial state to 66.0% after 100 h on stream, the formation rate of aromatics can keep about 15 mg g⁻¹ h⁻¹ because the CO selectivity progressively declines.

Fig. 2.

The stability test for CO₂ hydrogenation over ZnAlO_x&H-ZSM-5. Reaction conditions: Space velocity = 6000 ml g⁻¹ h⁻¹, 3.0 MPa, 593 K, H₂/CO₂/Ar = 3/1/0.2. Note that the liquid hydrocarbons include aromatics

Selectivity optimization

In order to improve the efficiency for CO₂ hydrogenation to valuable hydrocarbons such as aromatics, light olefins and gasoline, the RWGS reaction should be suppressed as much as possible. It is very interesting to find from Fig. 3a that as the H₂/CO₂ ratio rises from 3/1 to 9/1, the CO₂ conversion is dramatically increased from 5.8 to 15.7% with the CO selectivity sharply decreasing from 56.9 to 28.7%. It means that the generated CO can be further converted by hydrogenation in the high H₂/CO₂ ratio. Although raising H₂/CO₂ ratio can slightly decrease aromatics selectivity, their formation rate keeps growing. Further increasing H₂/CO₂ ratio to 15/1 leads to a decline in aromatization rate. Moreover, the liquid hydrocarbons are not sensitive to H₂/CO₂ ratio. As shown in Fig. 3b, another effective method to weaken RWGS reaction is adding CO into the mixture gas of CO₂ and H₂. The CO selectivity is nearly monotonically decreasing with CO/CO₂ ratio increasing and as low as 12.6% at CO/CO₂ ratio = 1.8/1, nevertheless, the aromatics or liquid hydrocarbons selectivity is slightly influenced after CO introduction. As known, RWGS is a typical equilibrium reaction⁴. Therefore, introduction product CO does not benefit the formation of itself. These results above are vital to applications. On the one hand, the source of feed gas is more extensive because its composition can be adjusted in a wide range; on the other hand, the product CO can be recycled and accumulated so as to practically depress the RWGS reaction. As presented in Fig. 3c, aromatics with 9 to 10 carbons are predominant over ZnAlO_x&H-ZSM-5. We tried to optimize the distributions of aromatics by using Si-H-ZSM-5 made by tetraethoxysilane (TEOS) modification. Compared to ZnAlO_x&H-ZSM-5, ZnAlO_x&Si-H-ZSM-5 obviously produces more toluene (2.7%) and xylenes (16.8%) (Fig. 3c). Moreover, 58.1% p-xylene in xylenes, 25.3% ethylene and 11.9% propylene (excluding CO) along with little C_1-3 alkanes are obtained over ZnAlO_x&Si-H-ZSM-5 (Fig. 3c and Supplementary Fig. 5). The catalytic behavior for ZnAlO_x&Si-H-ZSM-5 would be interesting to industry, because ethylene, propylene and p-xylene are the most important commodity hydrocarbon chemicals.

Fig. 3.

The results of selectivity optimization for CO₂ hydrogenation. a The effect of H₂/CO₂ ratio over ZnAlO_x&H-ZSM-5. Reaction conditions: Space velocity = 6000 ml g⁻¹ h⁻¹, 3.0 MPa, 593 K. b The effect of CO introduction over ZnAlO_x&H-ZSM-5. Reaction conditions: catalyst weight = 667 mg, 3.0 MPa, 593 K, inlet gas containing 66.7 ml min⁻¹ mixed gas (H₂/CO₂/Ar = 3/1/0.2) and 0–26.6 ml min⁻¹ CO. c Comparisons of catalytic behaviors over ZnAlO_x&H-ZSM-5 and ZnAlO_x&Si-H-ZSM-5. Reaction conditions: Space velocity = 6000 ml g⁻¹ h⁻¹, 3.0 MPa, 593 K, H₂/CO₂/Ar = 3/1/0.2. A9 and A10 are representative of aromatics containing 9 and 10 carbon atoms, respectively

Structural characterization

Figure 4a shows the X-ray diffraction (XRD) patterns of ZnAlO_x oxide. The reflection peaks of ZnAlO_x can be approximately assigned to cubic ZnAl₂O₄ gahnite²⁸ (JCPDS 05-0669, Supplementary Fig. 6). The reflection peaks of ZnO (Supplementary Fig. 7) are hardly detectable for ZnAlO_x. The Field-emission scanning electron microscopy (SEM) image (Supplementary Fig. 8) shows that the morphology of ZnAlO_x is porous and made by small particles. The BET surface area of ZnAlO_x (Supplementary Table 1) is up to 151.0 m² g⁻¹, which is much higher than ZnO (11.9 m² g⁻¹). High-resolution transmission electron microscopy (TEM) image in Fig. 4b gives us the fact that the ZnAlO_x sample consists of a lot of tiny nanoparticles with the sizes of <10 nm. Moreover, the lattice spaces of 0.28 and 0.45 nm which respectively correspond to the (220) and (111) planes of cubic ZnAl₂O₄ further confirm the spinel structure of ZnAlO_x²⁸. The element distribution analysis (Supplementary Fig. 9) shows that the Zn, Al, and O are uniformly dispersed. As presented in Supplementary Fig. 10, the ZnO sample in this study is consisted of about 100 nm nanoparticles with a clear lattice space. The H-ZSM-5 zeolite embraces a typical nano-sized MFI structure (Supplementary Fig. 11). The acidcontent of H-ZSM-5 is 1.3 times as high as Si-H-ZSM-5 (Supplementary Table 2). The temperature-programmed reduction (TPR) results in Fig. 4c and Supplementary Table 3 show that both of ZnAlO_x and ZnO can be hardly reduced by H₂ below 700 K and <7% zinc oxides might be reduced to their metal phase above 1000 K. As known, Zn²⁺ cannot be reduced by H₂ due to the positive free Gibbs energy²⁹. However, the reduction may proceed if H₂ is split into hydrogen atoms or alloy phase with other metals like Cu is formed³⁰. Furthermore, the spinel structure of ZnAlO_x has been proved to be more durable under H₂ reduction³¹. As a result, we consider that it is not zinc metal but Zn²⁺ in ZnAlO_x activates CO₂ hydrogenation reactions in this study. For a long time, the debate about the role of zinc species in industrial Cu/ZnO/Al₂O₃ catalysts for methanol synthesis has always existed. ZnO in Cu/ZnO/Al₂O₃ was not reckoned as catalytic centers but structural promoter or others^32,33. However, our results prove that in the absence of Cu species, ZnAlO_x can also convert CO₂ into MeOH and derived DME. The FT-IR spectra of catalysts after 2,6-di-tert-butyl-pyridine absorption (DTBPy-FTIR) are exhibited in Fig. 4d. DTBPy-FTIR was usually utilized to investigate the external surface Brønsted acid sites because the kinetic diameter (10.5 Angstrom) of DTBPy is larger than the pore opening (5.5 Angstrom) of H-ZSM-5. The characteristic bands at 3365, 1614 and 1531 cm⁻¹ are attributed to the DTBPy adsorbed on the external surface Brønsted acid sites of H-ZSM-5 zeolite. The negative band at 3616 cm⁻¹ is assigned to the decrease of bridging hydroxyls on the external surface³⁴. Surprisingly, these typical bands almost disappeared for the composite catalyst ZnAlO_x&H-ZSM-5. It indicates that external Brønsted acid of H-ZSM-5 can be shielded by ZnAlO_x after mixing, grinding and pressing under high pressure. The external Brønsted acid has no ability to aromatization but can promote the hydrogenation of unsaturated hydrocarbons to paraffins^15,26,27, which is detrimental to the aromatics selectivity. The aromatics or C_2-4 olefins selectivity follows the order ZnAlO_x&H-ZSM-5˃   ZnAlO_x + H-ZSM-5>ZnAlO_x/H-ZSM-5 (Fig. 1b), whereas the order of C_2-4 paraffins selectivity completely reverses, which verifies this point of view.

Fig. 4.

Structural characterization of various catalysts. a X-ray diffraction patterns (XRD) of ZnAlO_x. b High-resolution transmission electron microscopy (TEM) image of ZnAlO_x. Scale bar: 10 nm. c H₂-TPR profiles. The relative intensity was normalized by weight. The relative intensity of the standard sample Ag₂O was divided by 50. d FTIR subtraction spectra relative to adsorption of DTBPy

Operando DRIFT study

The operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of CO₂ hydrogenation over ZnAlO_x at 593 K and 0.1 MPa are explored. It is obvious from Supplementary Fig. 12 that absorbed surface formate species were first formed, then, absorbed surface methoxy species were generated by hydrogenation of formate species. As presented in Fig. 5, compared to DRIFTS of CO₂ hydrogenation over ZnAlO_x, the bands for surface formate (1620, 1375 and 2910 cm⁻¹) and methoxy (2940 and 2840 cm⁻¹) species^35,36 over ZnAlO_x&H-ZSM-5 are sharply weakened, which suggests that these active surface species over ZnAlO_x were converted by H-ZSM-5. Combined the catalytic results in Fig. 1a with DRIFTS results discussed above, we propose a mechanism for CO₂ hydrogenation to aromatics over ZnAlO_x&H-ZSM-5. First, surface formate species on ZnAlO_x was hydrogenated to form surface methoxy species; then, intermediates including MeOH and DME produced from the dissociation of methoxy species were spread to H-ZSM-5 to synthesize olefins intermediates; finally, transformation of olefins to aromatics consecutively take place in the micropores of H-ZSM-5. Furthermore, compared to CO₂ hydrogenation over ZnAlO_x, CO hydrogenation generates much less surface formate species. That is to say, more active metal species sites of ZnAlO_x are utilized to activate H₂ during CO hydrogenation, so that higher C_2-4 paraffins but lower aromatics are obtained in Supplementary Figure 5 because hydrogenation reactions of intermediates like olefins are enhanced. From Supplementary Fig. 13, we believe that CO₂ hydrogenation cannot proceed over H-ZSM-5. Some evidences were put forward by Jiao et al.^14,17,20 to support the ketene intermediate mechanism for STO or STA reactions over oxides/zeolites composite catalysts, however, we consider that this mechanism does not work in CO₂ hydrogenation over ZnAlO_x&H-ZSM-5 due to very low concentration of CO in the reaction process.

Fig. 5.

Operando DRIFT spectra over various catalysts. Conditions: 593 K, 0.1 MPa. CO₂&H₂ refers to the mixed gas of H₂/CO₂/Ar = 3/1/0.2; CO&H₂ refers to the mixed gas of H₂/CO/Ar = 3/1/0.2

In summary, composite catalyst of ZnAlO_x with a nano-scaled spinel structure and H-ZSM-5 exhibits an excellent performance for CO₂ hydrogenation to aromatics. The selectivity of aromatics (excluding CO) reaches as high as 73.9% with 9.1% CO₂ conversion and 0.4% CH₄ selectivity. Increasing H₂/CO₂ ratio or introducing CO can effectively suppress RWGS reaction without obviously weakening aromatization. The composite catalyst containing Si-H-ZSM-5 presents higher p-xylene, ethylene and propylene selectivity. It is not zinc metal but Zn²⁺ in ZnAlO_x activates CO₂ hydrogenation. The shield of the external Brønsted acid of H-ZSM-5 by ZnAlO_x is beneficial to aromatization. MeOH, DME and olefins are acted as reaction intermediates. CO₂ hydrogenation over ZnAlO_x&H-ZSM-5 shows much higher aromatics selectivity than CO hydrogenation because the former reaction generates more formate species on ZnAlO_x surface so that less metal species sites are left for hydrogenation of unsaturated intermediates like olefins. ZnAlO_x&H-ZSM-5 suggests a promising application in manufacturing aromatics from CO₂ and H₂.

Methods

Catalyst preparation

ZnAlO_x oxide was prepared by a conventional co-precipitation method. Typically, solution A was made by 59.5 g Zn(NO₃)₂ 6H₂O and 75.0 g Al(NO₃)₃ 9H₂O dissolved in 150 ml deionized water and solution B was prepared by 23.5 g (NH₄)₂CO₃ dissolved in 150 ml deionized water. Solutions A and B were dividedly added into one beaker by two peristaltic pumps with constant pH of 7.1–7.3 at 343 K under continuous stirring for 0.5 h, then followed by aging for 3 h at the same temperature. After filtered and washed by 200 ml deionized water for three times, the obtained mixture was dried at 373 K overnight and then calcined at 773 K for 4 h.

Commercial Na-ZSM-5 zeolite was supplied from Zhongke New Catalytic Technology Company, China. Na-ZSM-5 zeolites were transformed into NH₄-ZSM-5 by exchanging 50 g Na-ZSM-5 with 0.5 l NH₄NO₃ (1 mol l⁻¹) aqueous solution at 353 K for 2 h, followed by filtration and washing with deionized water. After repeating the exchanging process for three times, the resultant samples were dried overnight at 373 K, followed by calcination at 823 K for another 4 h in air to obtain the H-ZSM-5 zeolite.

Si-H-ZSM-5 zeolite was prepared by tetraethoxysilane (TEOS) modification. In brief, an organic solution was firstly made by 11.5 g TEOS mixed with 5 ml cyclohexane; then 10.0 g H-ZSM-5 was added into this solution, impregnated for 24 h at room temperature; after that, organic solvent was removed by evaporation at 353 K and the resultant powder was dried overnight at 373 K, followed by calcination at 823 K for another 4 h in air. After repeating the procedure above for twice, the Si-H-ZSM-5 was obtained.

The composite catalysts were typically prepared by physical mixing. Unless specially stated, the weight ratio of oxides and zeolites was 1:1. For preparation of the composite catalyst ZnAlO_x&H-ZSM-5, ZnAlO_x oxide and H-ZSM-5 zeolite were grinded in an agate mortar for 4 min, pressed under 40 MPa and granulated into the required size in the range of 0.4–0.8 mm.

Catalytic tests

Catalytic reaction experiments in Fig.1 and Supplementary Figure 2 and 3 were performed in a 16-channel continuous flow fixed-bed stainless steel reactor (from Yashentech Corporation, Shanghai, China) with an 8.3 mm inner diameter for each channel. The inlet gas flow and composition along with the reaction temperature and pressure for each channel were identical. All the reaction products were kept in the gas phase and analyzed online by two gas chromatographs (Aglient 7890A) equipped with a HP-PLOT/Q capillary column connected to a flame ionization detector (FID) and a TDX-1 column (produced from DICP) connected to a thermal conductivity detector (TCD). CH₄ was taken as a reference bridge between FID and TCD. Ar was used as an inner standard. The CO₂ conversion and the CO selectivity, hydrocarbons (C_nH_m), MeOH and DME selectivity among the carbon products without CO (including C_nH_m, MeOH and DME) were calculated with the followed equations.

$${\mathrm{CO}}_2\mathrm{conversion}=\left({\mathrm{CO}}_{2\mathrm{in}}-{\mathrm{CO}}_{2\mathrm{out}}\right)∕\left({\mathrm{CO}}_{2\mathrm{in}}\right)\times 100\%$$ 1

CO_2in: moles of CO₂ at the inlet; CO_2out: moles of CO₂ at the outlet;

$$\mathrm{CO}\mathrm{selectivity}={\mathrm{CO}}_{\mathrm{out}}∕\left({\mathrm{CO}}_{2\mathrm{in}}-{\mathrm{CO}}_{2\mathrm{out}}\right)\times 100\%$$ 2

CO_out: moles of CO at the outlet;

$$\begin{array}{c}\hfill \hfill \\ \hfill \hfill & \hfill {\mathrm{C}}_n{\mathrm{H}}_m\mathrm{selectivity}={\mathrm{N}}_{\mathrm{C}n\mathrm{H}m}∕\left(\mathrm{total}\mathrm{carbon}\mathrm{atoms}\mathrm{of}\right.\hfill \\ \hfill \hfill \\ \hfill \hfill & \hfill \left.\mathrm{products}\mathrm{detected}\mathrm{by}\mathrm{FID}\right)\times 100\%\hfill \\ \hfill \hfill \\ \hfill \hfill & \hfill \mathrm{MeOH}\mathrm{selectivity}={\mathrm{N}}_{\mathrm{MeOH}}∕\left(\mathrm{total}\mathrm{carbon}\mathrm{atoms}\mathrm{of}\right.\hfill \\ \hfill \hfill \\ \hfill \hfill & \hfill \left.\mathrm{products}\mathrm{detected}\mathrm{by}\mathrm{FID}\right)\times 100\%\hfill \\ \hfill \hfill \\ \hfill \hfill & \hfill \mathrm{DME}\mathrm{selectivity}={\mathrm{N}}_{\mathrm{DME}}∕\left(\mathrm{total}\mathrm{carbon}\mathrm{atoms}\mathrm{of}\right.\hfill \\ \hfill \hfill \\ \hfill \hfill & \hfill \left.\mathrm{products}\mathrm{detected}\mathrm{by}\mathrm{FID}\right)\times 100\%\hfill \end{array}$$

N_CnHm: carbon atoms number of C_nH_m; N_Me6OH: carbon atoms number of MeOH; N_DME: carbon atoms number of DME.

Other catalytic reaction experiments were performed in a typical fixed-bed stainless steel reactor with 8 mm inner diameter. All products were analyzed online by two tandem gas chromatographs. One is Agilent 7890A equipped with a HP-PLOT/Q capillary column connected to FID and a TDX-1 column connected to TCD. The other one is Agilent 6890N equipped with a HP-FFAP capillary column connected to FID and a Porapak Q column connected to TCD. The detailed analysis for aromatics such as ethylbenzene, p-xylene, m-xylene and o-xylene is dependent on HP-FFAP capillary column.

Author contributions

Y.N., who made the initial discovery, designed and conducted the experiments and wrote the manuscript. Z.C. and Y.F. conducted the experiments. Y.L. discussed the results. W.Z. and Z.L. supervised the experiments and discussed the results.

Data availability

The data supporting the findings of this study are available within the article and its Supplementary Information files. All other relevant source data are available from the corresponding author upon reasonable request.

Competing interests

The authors declare no competing interests.

Electronic supplementary material

Supplementary Information accompanies this paper at 10.1038/s41467-018-05880-4.