Steering the reaction pathway of syngas

Nature Communications volume 13, Article number: 2742 (2022) Cite this article

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Significant progress has been demonstrated in the development of bifunctional oxide-zeolite catalyst concept to tackle the selectivity challenge in syngas chemistry. Despite general recognition on the importance of defect sites of metal oxides for CO/H2 activation, the actual structure and catalytic roles are far from being well understood. We demonstrate here that syngas conversion can be steered along a highly active and selective pathway towards light olefins via ketene-acetate (acetyl) intermediates by the surface with coordination unsaturated metal species, oxygen vacancies and zinc vacancies over ZnGaOx spinel−SAPO-34 composites. It gives 75.6% light-olefins selectivity and 49.5% CO conversion. By contrast, spinel−SAPO-34 containing only a small amount of oxygen vacancies and zinc vacancies gives only 14.9% light olefins selectivity at 6.6% CO conversion under the same condition. These findings reveal the importance to tailor the structure of metal oxides with coordination unsaturated metal sites/oxygen vacancies in selectivity control within the oxide-zeolite framework for syngas conversion and being anticipated also for CO2 hydrogenation.

Syngas is an important intermediate platform for the utilization of carbon resources such as coal, natural gas and biomass, which can be converted to a variety of high-value chemicals and fuels. The selectivity control in syngas chemistry remains a challenge although significant progress has been made in fundamental studies and industrial applications of Fischer-Tropsch synthesis (FTS) technology since its invention almost a century ago1,2. It was demonstrated that composite catalysts by coupling partially reducible metal oxides and zeolites or zeotypes (OXZEO) enabled syngas direct conversion to a variety of chemicals, e.g., light olefins, gasoline range isoparaffins, benzene-toluene-xylene (BTX), and even oxygenates, with their selectivities all surpassing the Anderson-Schultz-Flory (ASF) distribution limit3,4,5,6,7. For example, the selectivity of light olefins among hydrocarbons reached 80% at 17% CO conversion over ZnCrOx-SAPO-34 at 400 °C, 2.5 MPa3 while 49% CO conversion and 83% selectivity of light olefins over ZnCrOx-AIPO-18 at 390 °C, 10 MPa8. ZnCrOx-mordenite gave 83% ethylene selectivity and 7% CO conversion at 360 °C, 2.5 MPa6. Furthermore, similar metal oxide-zeolite systems were also developed for CO2 hydrogenation to a variety of chemicals and fuels. For instance, ZnZrOx-SAPO-34 for light olefins synthesis9, ZnZrOx-ZSM-510, and ZnAlOx-H-ZSM-511 for aromatics synthesis, and In2O3-H-ZSM-5 for gasoline-range hydrocarbon synthesis12.

It is generally recognized that within the framework of OXZEO catalyst concept, CO/H2 activation takes place over metal oxides and C–C coupling over zeolites3,4. The partial reducibility of metal oxides was essential in controlling the overall activity of CO conversion. For example, partially reduced MnOx enabled CO dissociation and conversion to surface carbonate and carbon species, which were converted to CO2 and hydrocarbons upon H2 introduction. In comparison, no carbonate species were detected on unreduced MnOx, revealing the pivotal role of surface oxygen vacancies in syngas conversion13. Similarly, the reducibility phenomenon was reported for Zn-based catalysts13,14,15,16. ZnAl2O4 with a Zn/Al ratio of 1/2 achieved the highest CO or CO2 conversion, which was attributed to the largest amount of oxygen vacancies thus promoting CO and CO2 activation and conversion14. Zn/Cr ratios also affected H2 reduction and thus the formation ability of oxygen vacancies, which significantly influenced the activity and selectivity of syngas conversion15,17. ZnO-ZrO2 aerogel catalyst provided high surface area and large amount of oxygen vacancies, which also played important roles in bifunctional ZnO-ZrO2−ZSM-5 catalyst for CO2 hydrogenation to aromatics16. Despite significant progress, the actual structure of oxygen vacancies, and their catalytic roles are far from being well understood in OXZEO catalysis.

Here, we show that the reaction pathways of syngas conversion strongly depend on the defect sites of metal oxides and hence the final product distribution. Partially reducible ZnGaOx spinel containing coordination unsaturated Ga3+ sites and oxygen vacancies steers syngas conversion pathway towards light olefins, whereas spinel with a similar composition but only little oxygen vacancies and zinc vacancies is remarkably less active and non-selective for light olefins.

ZnGaOx oxides prepared by coprecipitation method were denoted as ZnGaOx_NP and those by hydrothermal method were named as ZnGaOx_F. The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images in Fig. 1a–d, and Supplementary Figs. 1, 2a, b, and 3a–d show that the hydrothermal ZnGaOx_F sample exhibits a hydrangea shape formed by thin flakes of ~21 nm thickness and micrometer size on the plane direction. In comparison, all coprecipitation ZnGaOx_NP samples exhibit nanoparticle morphologies (Fig. 1e–g, and Supplementary Figs. 2c, d, 3e–j, and 4), which do not appear to preferentially expose certain crystal faces. Various faces are observed including {111} and {010} in the [101] orientation over ZnGaOx_NP sample (Fig. 1g and Supplementary Fig. 3e–j).

a–d ZnGaOx_F sample. a, b Scanning electron microscopy (SEM) images. c, d High-resolution transmission electron microscopy (HRTEM) side view images. d The enlarged image of a selected area with orange frame in c viewed along the [\(1\bar{1}0\)] orientation. e–g ZnGaOx_NP sample. TEM images with g viewed along the [101] orientation. Note that [uvw] indexed a crystal axis, (hkl) a crystal plane, and {hkl} a group of crystal planes with the same atomic configuration70. h XRD pattern of ZnGaOx samples.

X-ray diffraction (XRD) patterns confirm that all ZnGaOx samples exhibit the same spinel crystal phase (JCPDS number of 38-1240, Fig. 1h, and Supplementary Figs. 5 and 6a) with the Zn/Ga ratio ranging from 0.2 to 3.1 (Supplementary Table 1). Therefore, they are non-stoichiometric spinels18,19. We first compared a nanoparticle spinel ZnGaOx_NP-A with a nanoflake spinel ZnGaOx_F with similar surface and bulk Zn/Ga molar ratios, as analyzed by inductively coupled plasma optical emission spectrometer (ICP-OES), scanning electron microscopy with energy dispersive X-ray detector (SEM-EDX), and X-ray photoelectron spectroscopy (XPS) (Fig. 2a, Supplementary Table 1). Interestingly, these two oxides give significantly different catalytic performance in syngas conversion upon being physically mixed with SAPO-34, respectively (Fig. 2b, Supplementary Table 2). CO conversion over ZnGaOx_NP-A−SAPO-34 is 32.3%, almost 5 times higher than 6.6% over ZnGaOx_F−SAPO-34, while the light olefins selectivity over the former is also 5 times higher than that over the latter (77.6% versus 14.9%). Note that hydrocarbons selectivity in this study is reported excluding CO2 to simplify the discussion since CO2 selectivity is similar for all catalysts (Supplementary Table 2). Even being normalized by the specific surface area of oxides, ZnGaOx_NP-A still exhibits a yield of light olefins 7 times higher than ZnGaOx_F (0.047 versus 0.007 mmol m−2 h−1) (Fig. 2c).

a Zn/Ga molar ratio of ZnGaOx samples corresponding to the data in Supplementary Table 1. The inset is morphology diagram, with purple, dark pink, and red balls referring to Zn, Ga, and O atoms, respectively. b Reaction performance of syngas conversion over ZnGaOx−SAPO-34. c Hydrocarbon formation rate normalized by specific surface area of oxides. Reaction conditions: OX/ZEO = 1 (mass ratio, 20–40 mesh), H2/CO = 2.5 (v/v), 400 °C, 4 MPa, 1600 mL g−1 h−1.

The catalytic activity can be further optimized by varying the composition of ZnGaOx, as shown in Supplementary Tables 1 and 2. Notably, all nanoparticle ZnGaOx_NP samples demonstrate much superior performance than ZnGaOx_F upon being physically mixed with SAPO-34 (Fig. 2b, c and Supplementary Table 2). ZnGaOx_NP with a surface Zn/Ga molar ratio of 1.2 (Fig. 2a) gives a highest CO conversion, i.e., 49.3% with 75.6% selectivity of light olefins (Fig. 2b and Supplementary Table 2). This CO conversion is higher than most results reported for Cr-free oxides under similar reaction conditions20. The formation activity of light olefins is 0.14 mmol m−2 h−1, which is 20 times higher than that over ZnGaOx_F (Fig. 2c). Moreover, ZnGaOx_NP−SAPO-34 delivers a rather good stability. CO conversion remains at 46% and selectivity of light olefins at 70% after 120 h on stream (Supplementary Fig. 7).

To understand the distinctively different activity of nanoparticle and nanoflake ZnGaOx spinel, we first looked into the activity of ZnGaOx_NP and ZnGaOx_F alone in syngas conversion. The results in Supplementary Table 3 show that both oxides give similar CO conversion around 5.5%. However, they behave significantly different when they combined with SAPO-34 respectively as composites. ZnGaOx_NP−SAPO-34 provides CO conversion > 37% and light olefins selectivity > 71% at different OX/ZEO mass ratios (Supplementary Fig. 8a). It indicates that the reaction equilibrium is successfully shifted and the reaction channel from intermediates to light olefins is opened up in the presence of SAPO-34, consistent with a recent theoretical study21. Thus, it forms a tandem catalytic process. Furthermore, the composite with OX/ZEO = 1 (mass ratio) gives optimal performance. By contrast, introducing SAPO-34 to ZnGaOx_F hardly affects the overall conversion (<8%) and light olefins selectivity (<16%) with the OX/ZEO ratio changing from 1/2 to 2/1 (Supplementary Fig. 8b, c). The main products are methane (45%) and light paraffins (40%). It implies that ZnGaOx_F is intrinsically of low activity or the intermediates generated over ZnGaOx_F cannot be effectively converted to desired products by SAPO-34 catalyst in contrast to ZnGaOx_NP. The reaction most likely has gone through different pathways over the two types of oxides.

The in-situ Fourier Transform Infrared (FT-IR) spectra in Fig. 3a confirm the distinct intermediates generated over the two ZnGaOx oxides. Upon exposing ZnGaOx_NP to syngas at 400 °C, two strong absorption bands appear at ~1368 and ~1589 cm−1, which are generally ascribed to formate species22. Such formate species is also observed over ZnGaOx_F, although the intensity is weaker. Formate species has been widely observed over methanol synthesis catalysts and is generally considered to be the precursor of methanol4,9,23,24. It was also reported for the metal oxides of the OXZEO composites which go via methanol-olefins pathway4,8,25,26. However, neither the intensity of IR formate signal correlates well with CO conversion of the corresponding OXZEO catalysts (Supplementary Fig. 9) nor the methanol concentration produced by ZnGaOx correlates with the hydrocarbons produced by OXZEO catalysts (Supplementary Fig. 10).

a In-situ FT-IR differential spectra of syngas conversion over H2-reduced ZnGaOx_NP (navy line) and ZnGaOx_F (brown line) at 400 °C. b CO conversion as a function of acetate intensity at 1525 cm−1 of FT-IR spectra of different ZnGaOx samples in Supplementary Fig. 9a.

In comparison, over ZnGaOx_NP surface, additional signals at ~1400 (δCH3), ~1450 (υC-O) and ~1525 cm−1 (υC=O) are obviously observed (Fig. 3a and Supplementary Fig. 9a), which are characteristic acetate species27,28,29,30. The band at 1671 cm−1 is assigned to acetyl group27,28,29,30. The acetate species likely originates from ketene being chemisorbed on the surface hydroxyl group, and acetyl group could be the product of hydrogenated ketene in the absence of zeotypes. Acetate species, a product of C–O breaking and C–C coupling, was not reported over typical methanol synthesis catalysts previously4,9,23,24. Formation of acetate species over ZnGaOx-NP was also validated by solid-state Nuclear Magnetic Resonance study31. Figure 3b displays that CO conversion of ZnGaOx−SAPO-34 bifunctional catalysts correlates well with the intensity of the representative IR signal for acetate species (1525 cm−1). Therefore, the reaction likely proceeds through ketene/acetyl/acetate pathway over ZnGaOx-SAPO-34 although methanol contribution cannot be completely excluded because SAPO-34 is a classical catalyst for methanol-to-olefins.

To understand the origin of different reaction pathways over the two types of ZnGaOx spinel, we set out to investigate their structures and active sites. Although metal oxides are less studied as catalysts directly, the increasing number of studies have proposed the important role of oxygen vacancies3,25,26 in OXZEO catalyzed syngas conversion. Therefore, we turned to CO-temperature programmed reduction (TPR) first. The profiles in Fig. 4a demonstrate a much more facile reduction of ZnGaOx_NP than ZnGaOx_F. A strong signal of CO2 centers around 300 °C over ZnGaOx_NP. This signal appears to overlap with another one starting from ~ 400 °C, which is likely CO disproportionation. Nevertheless, the integrated area of CO2 signals below 400 °C in CO-TPR (Fig. 4a, Supplementary Fig. 11, and Supplementary Tables 4 and 5) can still reflect the reducibility. In contrast, the reduction peak is significantly weaker over ZnGaOx_F indicating very few reducible defect sites below 400 °C. H2-TPR shows a similar trend (Supplementary Fig. 12). Figure 4b displays that the specific formation rates of hydrocarbons and light olefins correlate monotonically with the reducibility of ZnGaOx oxides regardless of nanoparticles or nanoflakes, which is consistent with previous studies3,13,25,26, revealing again the essential role of reducibility.

a CO-TPR profiles with m/z = 44 (CO2) signals in the effluents monitored by an online mass spectrometer. b Mass specific hydrocarbon formation rate as a function of integral area of CO2 signals below 400 °C in CO-TPR profiles of different ZnGaOx. Reaction conditions: OX/ZEO = 1/4 (mass ratio), H2/CO = 2.5 (v/v), 400 °C, 4 MPa, and 20,000 mL g−1 h−1. c, d Quasi-in-situ EPR spectra of ZnGaOx before and after H2 reduction, and the inset showing the surface Zn/Ga and Zn/O ratios determined by AP-XPS results. The purple and green colors refer to treatment conditions of UHV-O2 (degassed in ultra-high vacuum, and then exposed to O2) and H2, respectively. c ZnGaOx_NP. d ZnGaOx_F.

The reduction process can remove surface oxygen atoms, thereby leaving oxygen vacancies and coordinatively unsaturated metal sites, which are further investigated by quasi-in-situ electron paramagnetic resonance (EPR)32,33,34,35. Figure 4c shows that fresh ZnGaOx_NP exhibits a very weak EPR signal at g = 2.004. However, it intensifies significantly together with a new signal showing up at g = 1.97 upon H2 reduction (Supplementary Fig. 13a). In comparison, only a weak signal attributed to manganese impurity36 is detected for fresh ZnGaOx_F (Fig. 4d, and Supplementary Fig. 13b), which may has been brought in during catalyst synthesis (Supplementary Table 1). Upon H2 reduction, a signal at g = 2.004 appears and no other signals are observed over nanoflakes. g = 2.004 signal was frequently reported to be related to the presence of a free electron in the conduction band37,38, or defect sites over singly ionized zinc vacancy of zinc-based oxides32,39,40,41,42. However, it was also assigned to unpaired electron trapped at an oxygen vacancy site of oxides such as ZnO35,41,43, ZnxGayOz37,44,45, or TiO238,46. The assignment of g = 1.97 also remains controversial in different studies, e.g., singly ionized oxygen vacancies with one trapped electron33,47, zinc vacancies42,48,49, or donor centers such as ionized impurity atoms in the crystal lattice of ZnO oxide50,51. Volatility of zinc species has been frequently reported for ZnO and Zn-based oxides, which is facilitated in vacuum and hydrogen atmosphere52,53,54,55. Thus formation of zinc vacancies over both reduced ZnGaOx can be expected upon H2 reduction due to removal of oxygen atoms. This is confirmed by in-situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) experiments (the insets of Fig. 4c, d and Supplementary Table 6). Therefore, it is reasonable to attribute the g = 2.004 signal in both H2-reduced oxides to singly ionized zinc vacancies. Since the reducibility of ZnGaOx_F at 400 °C is relatively low (Fig. 4a and Supplementary Fig. 12), the signal intensity of oxygen vacancies over ZnGaOx_F should also be low, in contrast to significant reduction signal over ZnGaOx_NP. Therefore, the g = 1.97 signal of H2-reduced ZnGaOx_NP can be attributed to singly ionized oxygen vacancies.

Photoluminescence (PL) spectroscopy was conducted to further elucidate the defect structures of ZnGaOx oxides. Figure 5a shows emission peaks around 700 nm over both the fresh and reduced ZnGaOx_NP, which are generally attributed to oxygen vacancies56,57, in agreement with CO-TPR and EPR results. In addition, upon H2 reduction, another emission signal near 350 nm in the ultraviolet (UV) region becomes significantly intensified, consistent with the previous observation for ZnGa2O458. This was attributed to the formation of the distorted Ga-O octahedral structure, due to the removal of O atoms and thus forming coordination unsaturated Ga3+ sites, as displayed in the structure model in Fig. 5a56,59,60,61. This is further validated by a decreased O/Ga ratio of reduced ZnGaOx-NP by AP-XPS (the inset of Fig. 5a), whereas ZnGaOx_F does not show much change of the surface O/Ga ratio (the inset of Fig. 5b). In comparison, the fresh ZnGaOx_F exhibits an emission spectrum significantly different from that of ZnGaOx_NP, with a strong and wide signal at 400~650 nm and a weak signal around 700 nm (Fig. 5b). The former signal generally corresponds to the characteristic coordination saturated Ga3+ in the Ga-O octahedral structure (model of Fig. 5b)61,62 due to the charge transfer between Ga3+ ions located at the center of octahedral sites and its six first-neighbor O2- ions56,61. Furthermore, the quasi-in-situ H2-treated ZnGaOx_F at 400 °C gives almost the identical spectrum as the fresh sample, indicating no obvious electronic structure change (Fig. 5b).

a, b Quasi-in-situ Photoluminescence emission spectroscopy. H2-reduced samples in comparison to the fresh ones at the excitation wavelength of 290 nm (Supplementary Fig. 14), with the inset showing the surface ratio of O/Ga measured by in-situ AP-XPS. The inset model showing the Ga-O octahedral structure after H2 treatment. a ZnGaOx_NP. b ZnGaOx_F. c NH3-TPD profiles of H2-reduced ZnGaOx oxides. d Mass specific hydrocarbon formation rate as a function of the amount of medium strength acid sites of ZnGaOx estimated by NH3-TPD. Reaction conditions: OX/ZEO = 1/4 (mass ratio), H2/CO = 2.5 (v/v), 400 °C, 4 MPa, 20,000 mL g−1 h−1.

The above results indicate that the coordination unsaturated Ga3+ sites, oxygen vacancies and zinc vacancies co-exist on reduced ZnGaOx_NP oxide, while only oxygen vacancies and zinc vacancies exist on reduced ZnGaOx_F. The coordinatively unsaturated Ga3+ sites generally exhibit Lewis acidity63,64, which is evidenced by in-situ FT-IR differential spectra of pyridine adsorption on the surface of ZnGaOx oxides (Supplementary Fig. 15). The amount of Lewis acid sites can be quantified by temperature-programmed desorption of ammonia (NH3-TPD)26,65. Figure 5c and Supplementary Fig. 16a show that all reduced ZnGaOx samples give broad and asymmetric NH3 desorption peaks in the range of 100~400 °C, but the concentration differs (Supplementary Table 7). Fitting of NH3-TPD profiles (Supplementary Fig. 16b–e and Supplementary Table 8) indicates the presence of weak (around 170 °C) and medium strength acid sites (around 250 °C)66. The NH3-desorption peak below 200 °C is generally related to hydrogen-bonded physisorption sites67, while the peak around 250 °C could be contributed by the defect sites of coordination unsaturated sites26,65. Therefore, the number of defect sites on ZnGaOx surface could be quantified by the amount of NH3 desorbing from the medium strength acid sites. Interestingly, as shown in Fig. 5d, the mass specific light olefins formation rate is positively correlated with the concentration of these coordination unsaturated metal sites, but not with that of weak strength acid sites (Supplementary Fig. 17). Thus, the light olefins formation rate per defect site can be estimated to be 502 h−1, assuming one defect site adsorbing one NH3 molecule. The above results demonstrate that the presence of coordination unsaturated Ga3+ together with oxygen vacancies and zinc vacancies lead to a much more active ZnGaOx spinel in generating ketene-acetate (acetyl) intermediates. Interestingly, Lai et al. recently also revealed the essential role of coordination unsaturated Cr3+ together with the oxygen vacancies in the cleavage of the C–O bond over the highly reduced ZnCr2O4 (110) surface21. Thus, incorporation of SAPO-34 would direct the reaction pathway towards light olefins (Fig. 6).

ZnGaOx oxides containing coordination unsaturated Ga3+, oxygen vacancy and zinc vacancy sites are much more active and selective in syngas conversion to light olefins.

ZnGaOx spinels with similar compositions but different morphologies were synthesized, which allows to elucidate the catalytic role of different defect sites over metal oxides in OXZEO catalyzed syngas conversion. Quasi-in-situ PL, EPR and in-situ FT-IR reveal that ZnGaOx_NP (nanoparticles) is much more reducible, which gives the coordination unsaturated Ga3+ species, oxygen vacancies and zinc vacancies. Such a surface facilitates the formation of ketene-acetate (acetyl) intermediates during CO/H2 activation, which allows subsequent conversion to light olefins by SAPO-34 and displacement of the reaction equilibrium. Consequently, the selectivity of light olefins reaches 75.6% at 49.3% CO conversion, which is 9 times higher than that obtained by ZnGaOx_NP alone. In comparison, ZnGaOx_F (nanoflakes) containing only oxygen and zinc vacancies catalyzes CO/H2 activation generating formate species, which are hardly converted to light olefins by SAPO-34 because CO conversion is only 6.6% and light olefins selectivity is only 14.9%. Instead, the products are mainly composed of CH4 and paraffins for both ZnGaOx_F alone and ZnGaOx_F−SAPO-34 composite. Although the detailed structure of the active sites and the elementary steps forming intermediates still need further investigation, the results here have already demonstrated that the structure of reducible metal oxides can be tailored to convert syngas selectively to value-added chemicals. These findings are also expected to be applicable to CO2 hydrogenation to value-added chemicals and fuels.

ZnGaOx_NP, where NP denoted as nanoparticles, was synthesized by a coprecipitation method with the temperature of water bath kept at 60~65 °C and pH kept at 9~10. Aqueous solutions of Zn(NO3)2·6H2O and Ga(NO3)3 ∙ nH2O were prepared as the precursors and their molar ratios were 1:10, 1:6, 1:4, and 1:2, respectively. An aqueous solution of NaOH and Na2CO3 with a molar ratio of 7.1:1 was used as the precipitant. After precipitation, the suspensions were aged for 2 h under continuous stirring. The precipitates were washed with water, dried at 60 °C and then 110 °C overnight, followed by calcination at 500 °C for 1 h in air, respectively. The resulting samples were named as ZnGaOx_NP-A, ZnGaOx_NP-B, ZnGaOx_NP-C, and ZnGaOx_NP, respectively. The effect of calcination temperature, T, was also studied in the range of 600~800 °C for ZnGaOx_NP (named as ZnGaOx_NPT).

ZnGaOx_F, where F denoted as nanoflakes, was prepared by a hydrothermal method by adapting the previously reported method68. In detail, 1.21 g 2H2O ∙ Zn(CH3COO)2 and 2.81 g nH2O ∙ Ga(NO3)3 were dissolved in a mixed solution of 110 mL water and 55 mL ethylenediamine. After continuous stirring at room temperature for 1 h, the mixed solution was transferred into a 200 mL hydrothermal kettle, and heated at 180 °C for 24 h. The product was separated by centrifugation, washed several times with water, and then dried overnight at 60 °C, followed by calcination at 500 °C for 1 h in air.

SAPO-34 was synthesized following a hydrothermal method similar to a previous report3. Typically, 30% silica sol, AlOOH, 85% phosphoric acid and triethylamine (TEA) were well dispersed in distilled water with a mass ratio of SiO2:Al2O3:H3PO4:TEA:H2O = 0.11:1:1.8:3.4:10. Then the mixture was placed in a Teflon-lined autoclave, and kept at 200 °C for 24 h. The resulting solid product was collected by centrifuging and washed with water till the pH of the supernate was 7.0–7.5. After drying for over 12 h at 110 °C, the white powder was calcined at 550 °C for 4 h in air with a heating rate of 1 °C/min.

X-ray diffraction (XRD) was measured on a PANalytical Empyrean-100 equipped with a Cu Kα radiation source (λ = 1.5418 Å), operated at 40 mA and 40 kV. XRD patterns were recorded in the range of 2 theta = 10~90o. The crystal size was estimated using the Scherrer equation. Nitrogen adsorption−desorption experiments were carried out on a Quantachrome NOVA 4200e instrument. Before analysis, samples were degassed under vacuum at 300 °C for 5 h. Isotherms were recorded at 77 K. A non-local density function theory (NLDFT) pore size method was used. High-resolution transmission electron microscopy (HRTEM) images were obtained using a JEOL JEM-2100 electron microscope operated at an accelerating voltage of 200 kV. Before tests, the samples were ultrasonically dispersed in ethanol and a drop of the solution was placed onto a copper grid coated with a thin microgrids support film. High-resolution scanning electron microscopy (HRSEM) images were obtained using a Carl Zeiss Orion NanoFab Helium ion microscope. The low-resolution scanning electron microscopy (SEM) tests were performed on a Phenom proX apparatus with energy dispersive X-ray detector (EDX) elemental analysis. The accelerating voltage was 15 kV. The elemental content was measured using Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). Samples were dissolved in aqua regia solution and then sealed in an autoclave with Teflon lining. The autoclave was then placed in a microwave reactor for 0.5 h. The samples were then measured on a PerkinElmer ICP-OES 7300DV apparatus. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a SPECS PHOIBOS-100 spectrometer using an Al Kα (hν = 1486.6 eV, 1 eV = 1.603 × 10−19 J) X-ray source. The Ga 2p3/2 binding energy at 1118.7 eV was used for calibration. Typically, only the Ga 2p3/2 component of the Ga 2p and Zn 2p3/2 of the Zn 2p regions are fitted and quantified. The atomic ratio of elements i to j (\({n}_{i}/{n}_{j}\)) on the oxide surface was calculated based on

Where I represented the area of the characteristic peak and S represented the atomic sensitivity factor in Eq. (1), which was referred to the previous literature69. Ambient Pressure X-ray Photoelectron Spectroscopy (AP-XPS) experiments were carried out on SPECS PHOIBOS-150 ambient pressure XPS with Al Kα as the X-ray source. ZnGaOx oxides were first degassed in ultra-high vacuum (UHV) at 400 °C, and then heated in 0.5 mbar O2. After pretreatment, 0.5 mbar H2 was introduced into the analysis chamber. The Ga 2p3/2 binding energy at 1118.7 eV was used for calibration. Temperature-programmed desorption of NH3 (NH3‒TPD) was performed on a Micromeritics AutoChem 2910 instrument equipped with a thermal conductivity detector (TCD). Typically, 100 mg sample was loaded into a U-shape reactor. Before NH3‒TPD experiment, sample was pretreated at 400 °C for 1 h under flowing H2 and then heated under flowing Ar at 500 °C for 1.5 h. After cooling down to 100 °C under flowing Ar, the sample was exposed to 5 vol.% NH3/He. Then, the sample was swept by Ar at the same temperature until a stable baseline was obtained. Subsequently, the signal was recorded while the temperature increased from 100 to 600 °C at a heating rate of 10 °C/min. Temperature-programmed reduction (TPR) was performed on another Micromeritics AutoChem 2910 instrument equipped with a TCD. Typically, 100 mg sample was loaded into a U-shape reactor. Before CO-TPR or H2-TPR experiment, sample was pretreated at 500 °C for 1 h in flowing Ar. After cooling down to room temperature under flowing Ar, the TPR profile was recorded in 5 vol.% CO/He or 1 vol.% H2/Ar at a heating rate of 10 °C/min with the effluents monitored by an online quadrupole mass spectrometer (MS). Electron paramagnetic resonance (EPR) spectra were collected at 7 K on a Bruker A200 EPR spectrometer operated at the X-band frequency using power 1.0 mW, modulation amplitude 4.00 G and receiver gain 10000. The photoluminescence (PL) spectra were measured using QM400 with a Xe-lamp as the excitation source at room temperature. The excitation wavelength was fixed at 290 nm. H2 reduction was conducted at 400 °C for 1 h, and then sealed for quasi-in-situ study of EPR and PL. In-situ Fourier Transform Infrared (FT-IR) transmission spectra were recorded on a BRUKER INVENIO S spectrometer equipped with a quatz cell. Before tests, sample was pretreated in H2 atmosphere at 450 °C for 1 h. After cooling down to room temperature, the background spectrum was recorded. Then, sample was exposed to syngas atmosphere at 1 atm and heated to 400 °C for 1 h. After cooling down to room tempetature, sample spectrum was recorded. Each spectrum was obtained by averaging 32 scans collected at 4 cm−1 resolution. The sample spectrum was subtracted by the background spectrum. Pyridine adsorption test was conducted using the same facilities. The sample was degassed in vacuum at 450 °C for 1 h. Then pyridine was introduced at room temperature and subsequently degassed by evacuation. All spectra were collected under room temperature.

Catalytic reaction was performed in a continuous flow, fixed-bed stainless steel reactor equipped with a quartz lining. Typically, 300 mg composite catalyst (20–40 meshes) with oxide/zeolite = 1/1 (mass ratio) was used. 5 vol.% Ar was added to syngas as the internal standard for online gas chromatography (GC) analysis. Reaction was carried out under conditions: H2/CO = 2.5 (v/v), 400 °C, 4.0 MPa, gas hourly space velocity (GHSV) = 1600 mL g−1 h−1 unless otherwise stated. Products were analyzed by an online GC (Agilent 7890B), equipped with a TCD and a flame ionization detector (FID). Hayesep Q and 5 A molecular sieves packed columns were connected to TCD whereas HP-FFAP and HP-AL/S capillary columns were connected to FID. Oxygenates and hydrocarbons up to C12 were analyzed by FID, while CO, CO2, CH4, C2H4, and C2H6 were analyzed by TCD. CH4, C2H4, and C2H6 were taken as a reference bridge between FID and TCD.

CO conversion (\({{{\mbox{Conv}}}}_{{{\mbox{CO}}}}\)) was calculated on a carbon atom basis, i.e.

where \({{{\mbox{CO}}}}_{{{\mbox{inlet}}}}\) and \({{{\mbox{CO}}}}_{{{\mbox{outlet}}}}\) in Eq. (2) represented moles of CO at the inlet and outlet, respectively.

CO2 selectivity (\({{{\mbox{Sel}}}}_{{{{\mbox{CO}}}}_{2}}\)) was calculated according to

where \({{{{\mbox{CO}}}}_{2}}_{{{\mbox{outlet}}}}\) in Eq. (3) denoted moles of CO2 at the outlet.

The selectivity of individual hydrocarbon CnHm (\({{{\mbox{Sel}}}}_{{{{\mbox{C}}}}_{{{\mbox{n}}}}{{{\mbox{H}}}}_{{{\mbox{m}}}}}\)) among hydrocarbons (free of CO2) in Eq. (4) was calculated according to

Little C12+ hydrocarbons were detected. The selectivity to oxygenates was below 1%C and therefore neglected. The carbon balance over the OXZEO catalysts was over 95%.

All data supporting the findings of this study are available within the paper and its supplementary information files.

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This work was supported by the Ministry of Science and Technology of China (No. 2018YFA0704503, F.J.), the Chinese Academy of Sciences (XDA21020400, X.P.), the National Natural Science Foundation of China (Grant Nos. 91945302, X.P.; 22002153, F.J.; 22008234, D.M.), the Youth Innovation Promotion Association of Chinese Academy of Sciences (2019184, F.J.), Dalian Science and Technology Innovation Fund (2020JJ26GX028, F.J.), the Natural Science Foundation of Liaoning (2020-BS-019, F.J.). The lab-based SPECS AP-XPS instrument was supported by ME2 project under contract no. 11227902 (Z.L.) from National Natural Science Foundation of China. We thank the High Magnetic Field Laboratory of the Chinese Academy of Sciences for the EPR measurement, and Professors Shengfa Ye and Jihu Su for discussion on EPR results, Professor Xueqing Gong and Fei Li for discussion on photoluminescence results.

These authors contributed equally: Na Li, Yifeng Zhu, Feng Jiao.

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian, 116023, PR China

Na Li, Yifeng Zhu, Feng Jiao, Xiulian Pan, Yifan Li, Bing Bai, Dengyun Miao & Xinhe Bao

Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian, 116023, PR China

Na Li, Feng Jiao, Xiulian Pan, Qike Jiang, Changqi Xu, Shengcheng Qu, Dengyun Miao & Xinhe Bao

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

Na Li, Feng Jiao, Xiulian Pan, Jun Cai, Bing Bai & Dengyun Miao

State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai, 200050, PR China

Jun Cai & Zhi Liu

School of Physical Science and Technology, ShanghaiTech University, Shanghai, 201210, PR China

Jun Cai & Zhi Liu

Department of Chemical Physics, University of Science and Technology of China, Jinzhai Road 96, Hefei, 230026, PR China

Yifan Li

High Magnetic Field Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, 230031, PR China

Wei Tong

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N.L., Y.Z., and F.J. contributed equally to this work. N.L. performed most material synthesis, characterization and catalytic activity tests. Y.Z. performed early experimental explorations. F.J. designed (quasi-) in-situ reactors and participated in most characterization as well as data analysis. X.P. and X.B. initiated the project. X.P., F.J. and Y.Z. designed the experiments. Q.J. carried out some HRTEM characterization and corresponding analysis. J.C. and Y.L. performed XPS measurements. W.T. participated in part EPR exploratory experiments and analysis. C.X. and S.Q. participated in samples synthesis, catalytic activity tests and characterization. B.B. assisted in IR test. D.M. participated in the discussion of the results. Z.L. participated in APXPS analysis. N.L., F.J., X.B. and X.P. wrote the manuscript. All authors discussed the manuscript and have given approval to the final version of the manuscript.

Correspondence to Xiulian Pan or Xinhe Bao.

The authors declare no competing interests.

Nature Communications thanks Noritatsu Tsubaki and the other, anonymous, reviewers for their contribution to the peer review of this work. Peer reviewer reports are available.

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Li, N., Zhu, Y., Jiao, F. et al. Steering the reaction pathway of syngas-to-light olefins with coordination unsaturated sites of ZnGaOx spinel. Nat Commun 13, 2742 (2022). https://doi.org/10.1038/s41467-022-30344-1

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Received: 07 August 2021

Accepted: 19 April 2022

Published: 18 May 2022

DOI: https://doi.org/10.1038/s41467-022-30344-1

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