Selective Production of Aromatics Directly from Carbon Dioxide

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Selective production of aromatics directly from carbon dioxide hydrogenation Xu Cui, Peng Gao, Shenggang Li, Chengguang Yang, Ziyu Liu, Hui Wang, Liangshu Zhong, and Yuhan Sun ACS Catal., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019

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Selective production of aromatics directly from carbon dioxide hydrogenation Xu Cui1,2,4, Peng Gao1,4*, Shenggang Li1,3, Chengguang Yang1, Ziyu Liu1, Hui Wang1, Liangshu Zhong1,3, and Yuhan Sun1,3* 1

CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai

Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201203, China 2 3

University of the Chinese Academy of Sciences, Beijing 100049, China School of Physical Science and Technology, ShanghaiTech University, Shanghai

201203, China 4

These authors contributed equally: Xu Cui and Peng Gao

*Corresponding author. Tel: +86–021–20608002, Fax: +86–021–20608066. E-mail: [email protected] (P. Gao); [email protected] (Y. Sun) Abstract: Conversion of carbon dioxide (CO2) to fuels and chemicals with the help of renewable hydrogen (H2) is a very attractive approach to reduce CO2 emissions and replace dwindling fossil fuels. However, it is still a great challenge to synthesize aromatics directly from CO2 hydrogenation, because CO2 is thermodynamically very stable, and the aromatics are highly unsaturated products with complex structures. Here, we demonstrate that the combination of a sodium-modified spinel oxide ZnFeOx, which alone shows excellent performance for CO2 hydrogenation to olefins, and hierarchical nanocrystalline HZSM-5 aggregates can realize a highly efficient synthesis of aromatics directly from CO2 and H2. The maximum of aromatics selectivity was up to 75.6% among all hydrocarbons at 41.2% CO2 conversion. Additionally, the selectivity toward CO and CH4 is usually less than 20% over this catalyst system. The suitable amount of the residual sodium, hierarchical pore structure and appropriate density of Brønsted acid sites endow the composite catalyst with an outstanding aromatics yield and high catalytic stability. Keywords: CO2 hydrogenation; Aromatics; Fischer−Tropsch synthesis; Spinel oxides; Hierarchical zeolites; Brønsted acid sites 1

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1. INTRODUCTION In view of the dramatic impact of carbon dioxide (CO2) emission on global climate change and the inevitable depletion of fossil fuels, the valorization of CO2 is gaining interest in the scientific and industrial communities. With the assistance of hydrogen (H2) originated directly from renewable energy (solar, wind, biomass, etc), the conversion of CO2 to value-added products not only facilitates greenhouse gas reduction but also produces commodity chemicals that can be used either as fuels or as precursors in many industrial processes.1-3 Recently, the efficient hydrogenation of CO2 to chemicals and energy carriers, for example, methanol, formic acid, carbon monoxide and hydrocarbons, has been extensively investigated.4-8 Aromatics are some of the most important platform chemicals to produce various polymers, petrochemicals, and medicine. However, with increasing petroleum consumption and mounting demand for aromatics, it is imperative to develop an alternative route to replace the traditional petroleum processes for the production of aromatics, for example, naphtha reforming and oil cracking. In this context, direct CO2 hydrogenation to aromatics is particularly attractive. It is still a great challenge to synthesize hydrocarbons with two or more carbons directly from CO2 hydrogenation, because CO2 is a very stable molecule. Very recently, researchers designed bifunctional catalysts by combining components for methanol synthesis with zeolites for the methanol-to-hydrocarbon process to realize the direct one-step production of hydrocarbons from CO2 hydrogenation, which resulted in a significant breakthrough in the synthesis of isoparaffins, olefins and aromatics, although low catalytic activity and high CO selectivity remain the challenges.9-19 The CO2 conversion was usually 40%. The C–C coupling from methanol is thermodynamically more favorable at high temperature (>300 oC), while methanol synthesis is thermodynamically favored at much lower temperature.5, 20,21

Consequently, the mismatch of their reaction temperatures results in the production

of a large amount of CO via the endothermic reverse water gas shift (RWGS) reaction. It is also possible to use a modified Fischer−Tropsch synthesis (FTS) route to directly 2


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convert CO2 to hydrocarbons, where CO2 was first converted to CO via the RWGS reaction and then CO was transformed into hydrocarbons via FTS. As a slightly endothermic reaction, the conversion of CO2 by RWGS is favored at the high temperatures (>300 oC) applied for the traditional iron-based FTS reaction,5, 22 which would overcome the above equilibrium constraints. However, normal paraffins and olefins are the main products in FTS and the hydrocarbon distribution was limited by the Anderson–Schulz–Flory model.23-26 In addition, the low CO2 adsorption rate on the surface leads to a high H/C ratio on the catalyst surface, which benefits the hydrogenation of surface adsorbed intermediates and results in the facile formation of methane (CH4). As CH4 is thermodynamically the most stable compound among all hydrocarbons, it is almost the only hydrocarbon formed from CO2 hydrogenation when equilibrium is reached.22, 27 Although some studies on the synthesis of isoparaffins from CO2 suggested that aromatic hydrocarbons could form simultaneously over the Na– Fe3O4/zeolite multifunctional catalyst system, the selectivity to aromatics among all hydrocarbons remained limited.28-30 Therefore, more efficient catalysts both with high CO2 conversion and high aromatics selectivity are highly desirable for commercial applications of the direct CO2 hydrogenation to aromatics process. In this work, we fabricated a highly effective composite catalyst containing a Namodified spinel oxide ZnFeOx, which provides two types of active sites (Fe3O4 and Fe5C2) and shows high CO2 conversion and considerable olefins selectivity, and a hierarchical nanocrystalline HZSM-5 zeolite for selective conversion of CO2 into aromatics. This catalyst displays high aromatics selectivity with high CO2 conversion (~40%), and the combined selectivity of CO and CH4 is less than 20%. The outstanding yield of aromatics (up to 29%) is achieved over the composite catalyst containing ZnFeOx modified with a suitable amount of Na and micro/mesoporous nanocrystalline HZSM-5. Moreover, the fraction of p-xylene in xylenes is up to 75% over the composite catalyst containing silylated HZSM-5. 2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. ZnFeOx-nNa nanocatalysts (Fe:Zn = 3:1) with 3


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different residual Na concentrations denoted by n wt % were synthesized by a one-pot synthesis method. 96.96 g Fe(NO3)3·9H2O and 23.80 g Zn(NO3)2·6H2O were dissolved in 300 mL deionized water at 60 oC. Then, a 1.5 mol L–1 NaOH solution was added dropwise to the mixed metal nitrate solution under vigorous stirring until the pH value of the obtained suspension reached around 10.0. After the turbid liquid aging at 60 oC for 60 min, the formed suspension was centrifuged and washed with a certain amount of deionized water. The product was dried over night at 100 oC and calcined at 400 oC for 3 h. By controlling the washing times in the centrifugation step, the samples with various Na concentration were obtained. Fe-2.44Na was synthesized without zinc nitrite solution at otherwise the same conditions as for ZnFeOx-nNa samples. HZSM-5 zeolites with various SiO2/Al2O3 ratios were purchased from XinNian Petrochemical Additives Company. Typically, commercial HZSM-5 with a SiO2/Al2O3 ratio of 25 (C-HZSM-5) was used unless otherwise noted. Alkaline treatment (0.2 mol L–1 NaOH solution, 70 ºC, 1 h, and zeolite/liquid ratio = 100 g L–1) in two consecutive cycles was employed to prepare mesoporous C-HZSM-5-a. All samples were treated three times in a 1 mol L–1 NH4Cl solution at 90 ºC for 1 h, followed by being dried at 100 °C for 10 h and then calcined in air at 550 °C for 5 h before any catalytic experiments. In the synthesis of HZSM-5 aggregates (S-HZSM-5), a silica gel (30% SiO2 in water) solution and sodium aluminate (NaAlO2) were used as silica and aluminum sources, respectively. 47.50 g of silica gel and the calculated NaOH were well mixed together with stirring. In addition, 1.75 g of NaAlO2 dissolved into a certain amount of deionized water was dropped into the above mixture. After being stirred at ambient temperature for 4 h, 0.15 g of the prepared seeds (1.0 wt % of total SiO2 weight in the added gel) were then added under stirring. Furthermore, the gel was crystallized at 165 ºC for 3 days. Then, the product was filtered, washed, and dried at 100 ºC for 12 h. Finally, 10 g of zeolites were treated in 100 mL of NH4Cl solution (1 mol L–1) at 90 C for 1 h and calcined at 550 C for 4 h. This ion exchange process was repeated three times. 4


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2.2. Catalyst characterization. The phase and crystal structure of the samples were analyzed by powder X–ray diffraction (XRD) in the 2θ range 5o–90o using a Rigatku Ultima 4 X–ray diffractometer with Cu Ka radiation (40 kV, 40 mA) in the scanning speed of 2o min–1. For in situ XRD measurement, the sample was treated in pure H2 (30 mL min–1) during the process. The chemical composition of the samples was determined using an X–ray fluorescence (XRF) spectrometer (Rigaku ZSX Primus Ⅱ, Japan). The surface area and pore volume of catalysts were determined from the N2 adsorption/desorption isotherms at 77 K on a TriStar II 3020 instrument. Before the measurements, the samples were treated at 573 K for 10 h under vacuum for the dehydration. The morphology of the materials was characterized by using a Tecnai G220 high-resolution transmission electron microscope (HRTEM) and TEM operated at 200 kV and a SUPRRATM 55 scanning electron microscope (SEM) at 2.0 kV. Thermogravimetric (TG) analysis of the spent samples was performed on a STA449QMS thermal analyzer. The spent zeolites were heated in air (30 mL min–1) from 40 oC to 900 oC at a rate of 10 oC min–1.

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Al MAS NMR was recorded on a 156.4 MHz

Bruker AVANCE III 600 spectrometer with a 4 mm HX double-resonance MAS probe at a sample spinning rate of 14 kHz. The chemical shift of 27Al was referenced using 1 mol L–1 Al(NO3)3 solution. Hydrogen temperature-programmed reduction (H2–TPR) tests were performed on a Micromeritics ChemiSorb 2920. The signals were detected by a thermal conductivity detector (TCD). After pre-treatment, the temperature was increased to 800 oC with a temperature rate of 5 oC min–1 in the flow of 5%H2/95%Ar gas mixture (30 mL min–1). The temperature-programmed desorption of NH3 (NH3–TPD) experiment was carried out on a TP-5076 adsorption instrument. The zeolite was firstly treated in pure Ar (30 mL min–1) at 400 oC for 2 h. After cooling down to 100 oC, a flow of NH3 (30 mL min–1) was introduced into the sample; then the pure Ar flow was introduced at 100 o

C to remove any gas phase ammonia. Finally, the NH3–TPD was recorded in the

temperature range from 100 to 600 oC (10 oC min–1). The temperature-programmed desorption of CO2 (CO2–TPD) of the Fe-based 5


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catalyst was carried out by using an OmniStar GSD320 02 mass spectrometer to detect the CO2 desorption. The catalysts were saturated in CO2 at 50 oC and other experimental procedures are similar to those of NH3–TPD. Pyridine adsorbed infrared spectroscopy (Py–IR) was also performed to investigate the acidity of zeolites, which were performed by using a Thermo scientific (Nicolet 380) spectrometer with MCT detector. The sample was placed in a thermostatized cell and evacuated to 10–2 Pa at 450 oC for 2 h. Then, pyridine vapor was introduced until the zeolite was saturated. After vacuuming for 1 h, the IR desorption spectrum was recorded at 150 oC and 350 oC. For the characterization of the spent catalysts, the composite catalyst was cooled to room temperature in a pure flow of Ar (100 mL min–1) and then collected after passivated in 1%O2/99%Ar stream. The spent ZnFeOx-nNa samples were firstly recovered by magnet and then the sole HZSM-5 zeolites were separated from the quartz sand by handwork picking due to different colors between zeolites and quartz sand. 2.3. Catalytic evaluation. The CO2 hydrogenation reaction was conducted in a high-pressure, continuous-flow, fixed-bed reactor. 0.50 g of the catalyst (40-60 mesh) with ZnFeOx-nNa/HZSM-5 = 1/2 (mass ratio) (or 0.50 g ZnFeOx-nNa nanocatalyst) was used for test. Before reaction, the catalyst was firstly reduced in situ at 350 oC for 8 h in a pure H2 flow (100 mL min–1). The reaction condition was 320 oC, a pressure of 3.0 MPa, H2/CO2/N2 molar ratio of 73/24/3 and 4000 mL gcat–1 h–1. All the products were analyzed online with an Agilent GC 7890A gas chromatograph. Typically, N2, CO, CO2 and CH4 were detected using a gas chromatograph equipped with a thermal conductivity detector and a TDX-01 column. The hydrocarbon compounds and oxygenates were detected using another gas chromatograph equipped with a flame ionization detector. Calculations of the CO2 conversion were based on an internal standard method. CO selectivity and the hydrocarbons distribution were calculated on a molar carbon basis. The carbon balance was higher than 95%. The catalytic performances after 28 h on stream were typically used in our discussion. 3. RESULTS AND DISCUSSION 6


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3.1. Characterization of prepared ZnFeOx-nNa samples. ZnFeOx-nNa nanocatalysts (Fe:Zn = 3:1) with different residual Na contents were prepared by a simple one-pot synthesis method (Table S1). The samples with various Na concentrations, which are denoted by n wt % with n = 20.00, 5.79, 4.25, 2.68, 0.50 wt %, were obtained by controlling the amount of water in the filtration step. As shown in the XRD patterns of calcined samples (Figure 1a), ZnFeOx-nNa catalysts mainly consist of cubic ZnFe2O4 phase, whose crystallinity increases with decreasing Na contents, suggesting strong interaction between Fe and Zn oxides, though ZnFeOx-0.50Na presents relatively strong Fe2O3 diffractions. A catalyst with 2.44 wt % Na but without Zn was also prepared (Fe-2.44Na). Compared with Fe-2.44Na, the oxides with Zn exhibit smaller crystal sizes as calculated by XRD patterns (Table S1), which suggests Zn decreases the particle size of iron species. The phases of NaNO3 were detected at about 29.4o, 31.9°, 38.9° and 47.9° in the XRD patterns of ZnFeOx-20.00Na and ZnFeOx-5.79Na. For other samples, no diffraction peak associated with Na was observed due to its low concentration and good dispersion. Typically, ZnFeOx-4.25Na is composed of ZnFe2O4 nanoparticles with a small particle size of about 12 nm and the residual Na (4.25 wt %, determined by XRF) is well distributed on the surface of ZnFeOx-4.25Na nanoparticles (Figures 2a, S1a and S2). In addition, compared with ZnFeOx-5.79Na and ZnFeOx-20.00Na samples, the BET surface areas of ZnFeOx4.25Na, ZnFeOx-2.68Na and ZnFeOx-0.50Na are much larger due to the higher dispersion of the Na component on the surface, although this value decreases significantly with increasing Na content (Table S1 and Figure S3a,b). The CO2 adsorption properties of ZnFeOx-nNa with different Na contents were further investigated by CO2 temperature-programmed desorption (TPD). As shown in Figure 1c, three distinct peaks were observed for all the samples. The lower temperature peaks around 130 oC (α peak) are ascribed to the desorption of CO2 weakly adsorbed in the bulk phase, and the peaks in the temperature range of 300–400 oC (β peak) and 500–700 oC (γ peak) correspond to the desorption of CO2 interacting strongly with the surface basic sites, which shift gradually to higher temperature with increasing Na 7


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content. The increased Na loading also enhances the amount of CO2 uptake at strongly basic sites (Table S2). The electronic character in the near-surface region of ZnFeOxnNa was further investigated by XPS. Both ZnFeOx-0.50Na and ZnFeOx-4.25Na consist of Fe3+ species as indicated by the Fe 2p3/2 peak at 760.2 and 709.9 eV and associated shake-up structure (Figures 1d and S3d). These peaks are lower than the standard ferric oxide sample (710.5 eV),31,32 especially for ZnFeOx-4.25Na with the higher Na content, indicating charge transfer from sodium ions to the surface iron species, which enhances surface basicity. After pretreatment of ZnFeOx-nNa in H2 at 350 oC for 8 h, the XRD diffraction peaks of ZnFe2O4 disappears, while the peaks (2θ = 44.7°, 65.0°, and 82.3°) attributed to metallic Fe emerge in these catalysts (Figure S3c). Additionally, the diffraction peaks of FeO and ZnO are also clearly observed after reduction. To further explore the phase transformation of the representative ZnFeOx-4.25Na and ZnFeOx-0.50Na samples, the in situ temperature-resolved XRD technique was employed in H2 flow (Figure S4). The reductions of ZnFe2O4 to Fe3O4, Fe3O4 to FeO and FeO to α-Fe occur at 400 oC, 450 o

C and 500 oC for ZnFeOx-4.25Na, respectively. For ZnFeOx-0.50Na, the related phase

evolution was observed at 350 oC, 400 oC, 400 oC, respectively. The reduction behaviors of the Na promoted catalysts are also investigated by H2–TPR. As shown in Figure S5, ZnFeOx-0.50Na and ZnFeOx-2.68Na exhibit a broad reduction and the three-step reduction processes were clearly observed over high sodium-containing samples. Typically, the reduction peaks at around 300 °C and 450 °C are attributed to the reduction of ZnFe2O4 to Fe3O4 and Fe3O4 to FeO, respectively, and the peak above 600 o

C is ascribed to the reduction of FeO to α-Fe. According to in situ XRD analysis, the

formed FeO phase can be rapidly converted to α-Fe for ZnFeOx-0.50Na (Figure S4a), and thus the catalyst with much lower Na content shows a broad peak in the TPR profile. In addition, the reduction peak shifts to higher temperature with increasing Na content, indicating that the high sodium content restrains the reduction of iron oxide. This can be attributed to the inhibiting effect of the sodium on the adsorption of H2.33,34 Moreover, the addition of Zn significantly increases the reducibility of iron oxide (Figure S5), 8


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which benefits Fe3O4 and metallic Fe formation. Therefore, the iron oxide with Zn has more exposed active sites than that without Zn. 3.2. Structure and acidity of various HZSM-5 zeolites. Nanocrystalline HZSM5 (S-HZSM-5, SiO2/Al2O3 = 25) was synthesized by using a seed-assisted method with no organic template. SEM and TEM images show that the prepared zeolite has a cubic shape and can be considered as a “standard” nanocrystalline HZSM-5 in the range of 50–100 nm (Figures 3 and S6a). These nanocrystals tend to agglomerate into large particles with crystal sizes of 1–1.5 μm due to their high surface Gibbs energies. Commercial HZSM-5 zeolites with different SiO2/Al2O3 ratios possess an elongated prismatic shape with smooth surfaces, and the average particle size of the commercial zeolite with the same SiO2/Al2O3 ratio (C-HZSM-5) is approximately 6 μm in length and 1 μm in thickness (Table S3, Figures S6b and S7). The N2 adsorption/desorption isotherms clearly show that commercial HZSM-5 has a typical microporous structure (Figure S8a). In contrast, mesoporous structures are formed in S-HZSM-5 and NaOH treated C-HZSM-5-a zeolites (Figures 3, S6a,c and S7b). For comparison, C-HZSM-5 treated twice with 0.1 mol L–1 NaOH solution at 70 oC for 1 hour is termed as C-HZSM5-a. The pore size distributions of the samples determined using the Barrett-JoynerHalenda (BJH) from the adsorption branch of the nitrogen isotherms indicate the presence of 4 nm mesopores in the S-HZSM-5 and C-HZSM-5-a samples (Figure 4a). As for S-HZSM-5, a large amount of mesopores ranging from 7 to 15 nm are also formed. The SEM and TEM images of S-HZSM-5 indicate that these mesopores are derived from the stacking and agglomeration of intracrystalline zeolite nanocrystals. The mesopores (7–15 nm) are formed as some nanosized zeolites stick loosely together. These results suggest the direct creation of hierarchical pore structure in S-HZSM-5 during the synthesis procedure. Apart from the pore structure, the acid site and aluminum of zeolite also play a crucial role in the aromatization process. Therefore, various zeolites were characterized by NH3–TPD, 27Al MAS NMR and pyridine-adsorbed FT-IR. As shown in the NH3– TPD profiles, all samples show peaks centered around 200 oC, corresponding to NH3 9


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eluted from the weakly acidic sites, and higher temperature peaks around 475 oC are attributed to NH3 desorption associated with strongly acidic sites (Figure 4b). Upon NaOH treatment, the density of strong acid sites identified by NH3–TPD decreases and its strength also weakens (Figure 4b and Table S3). It is noteworthy that S-HZSM-5 exhibits much fewer strong acid sites compared with C-HZSM-5-a. As shown in Figure S8b, the

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Al MAS NMR peaks at 55 ppm are ascribed to tetrahedrally coordinated

framework aluminum (AlF), which is the main source of strong Brønsted acid sites, and its intensity for C-HZSM-5 decreases sharply after alkali treatment. Additional evidence about Brønsted acid sites is provided by pyridine-adsorbed FT-IR spectroscopic studies (Figures 4c, S8c and Table S3). It was found that the peaks characteristic of Brønsted acid sites decreased significantly after NaOH treatment, indicating the removal of a large amount of Brønsted acid sites compared with parent C-HZSM-5. Compared with C-HZSM-5-a, S-HZSM-5 possesses a lower concentration of Brønsted acid sites. The quantity of framework aluminum species, including AlF and distorted framework aluminum (AlD), was also determined through the 27Al MAS NMR analysis by taking into account the intensities and/or areas of the corresponding signals. The peak areas at 40–70 ppm of C-HZSM-5 are nearly the same as that of S-HZSM-5 (Figure S8b) and the same weight of the two samples was used for the 27Al MAS NMR characterization, indicating that they have the similar amount of framework aluminum species. Furthermore, according to NH3–TPD, 27Al MAS NMR and FT-IR results, the density of strong Brønsted acid sites also decreases as expected at increasing SiO2/Al2O3 ratio. 3.3. CO2 hydrogenation over ZnFeOx-nNa catalysts. The CO2 conversion via RWGS is limited at low reaction temperature due to its endothermic nature. The equilibrium conversion of CO2 is 10.5–28.6% in the RWGS reaction at temperatures between 200 and 340 oC at an H2/CO2 ratio of 3 (Figure S9). As in FTS, formations of hydrocarbons from CO2 and H2 are all exothermic processes, which are favored at low temperature (Figure S9). Therefore, it is possible to increase the equilibrium conversion of CO2 in the above temperature regime. The CO2 hydrogenation performance of sole 10


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ZnFeOx-nNa was investigated first at the standard reaction condition of 320 oC, 3.0 MPa and 4000 mL gcat−1 h−1. For ZnFeOx-0.50Na with low sodium content, the CO2 conversion is the lowest and selectivities towards C2–4 and C5+ are 46.4% and 17.6%, respectively (Figure 5 and Table S4). The olefin/paraffin (o/p) molar ratio in the C2–4 branch is only 0.1, indicating that C2–C4 alkanes are the main products when the Na content is low. CO2 conversion increases remarkably at increasing Na concentration probably due to the increasing amounts of CO2 uptake at strongly basic sites, and a maximum of ~38.5% is reached at the Na contents of 2.68 and 4.25 wt %, while it decreases gradually with the further increase of the Na content (Figure 5). The o/p ratio increases drastically with increasing Na content, and o/p in the C2–4 hydrocarbons reaches the maximum of 7.7 when Na loading is 4.25 wt %. In addition, the RWGS reaction is suppressed over all the ZnFeOx-nNa catalysts and CO selectivity is below 15%. As shown in Figure S10, the stable activity of ZnFeOx-2.68Na is around 33 mmolCO2 gFe–1 h–1, which is much higher than that of Fe-2.44Na (22 mmolCO2 gFe–1 h–1). However, the two catalysts exhibit similar CO selectivity and hydrocarbon distribution (Table S4). In combination with the XRD and H2–TPR results, we conclude that the addition of Zn leads to smaller crystals of iron species and more exposed active sites. Therefore, the mass-specific activity increases significantly with the introduction of Zn. XRD and HRTEM characterizations show that ZnO, Fe3O4 and Fe5C2 are clearly presented in the ZnFeOx-4.25Na catalyst after reaction for 50 h (Figures 1b and 2b and Figure S3e). After the reduction of ZnFeOx-4.25Na in H2 prior to reaction, metallic Fe and FeO are formed, which are then transformed into Fe3O4 and Fe5C2 as a result of the interaction with oxygen and carbon species in the reaction atmosphere.28 We found that iron carbide phases still formed in the presence of the zeolite during the CO2 hydrogenation reaction (Figure S3e). Iron carbides are formed during the reaction, where their carbon atoms come from the sequential hydrogenation of CO2 via the RWGS reaction and CO + H2 → C + H2O, as well as the disproportion reaction of CO (2CO → C + CO2). Therefore, there is an equilibrium for carbon atoms for bulk carbide formation and reaction into hydrocarbons. The XRD patterns of spent ZnFeOx-nNa also 11


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show a clear trend that the iron phase transformation depends on the Na loading. With increasing Na concentration, the proportion of Fe5C2 increases and the XRD peak intensity of Fe3O4 decreases gradually (Figure 1b). As for ZnFeOx-0.50Na, nearly no peaks of the Fe5C2 phase can be detected due to the insufficient surface basic sites from CO2–TPD results,35 while almost all the characteristic peaks of iron were attributed to Fe5C2 phase for ZnFeOx-20.00Na. An increased Na content leads to a higher coverage of dissociatively adsorbed CO, which promotes the catalyst.36,37 During CO2 hydrogenation, CO is the main product over the sole Fe3O4 oxides (Figure S3f and Table S4). The bare Fe5C2 displays no activity for CO2 conversion (Figure S3f and Table S4), which is the active site for FTS.28, 32 Consequently, CO2 is first reduced to CO by H2 via the RWGS over Fe3O4, followed by the subsequent CO hydrogenation to olefins via FTS on Fe5C2 sites. Therefore, we speculate that an appropriate ratio of Fe3O4 and Fe5C2 sites endows the ZnFeOx-2.68Na and ZnFeOx-4.25Na catalysts to achieve high CO2 conversion (~38%) and low CO selectivity (~11%). Moreover, the dramatic change of the olefins selectivity with different Na contents can be attributed to the electronic modulation of Na on ZnFeOx surfaces. After catalyzing the CO2 hydrogenation reaction at 320 oC, the Fe 2p3/2 XPS peak shifts toward lower binding energies, and new peaks at 705.7 and 704.9 eV associated with iron carbides appear for the ZnFeOx-0.50Na and ZnFeOx-4.25Na samples, respectively (Figures 1d and S3d), which are much lower than standard Fe5C2 peak at 706.5 eV,38 suggesting the formation of electron-rich Fe5C2 species. This interaction between Na and Fe5C2 can inhibit the further hydrogenation of the olefins products on the catalyst surface leading to increased o/p ratio.32, 39 3.4. Catalytic results over ZnFeOx-nNa/HZSM-5. When ZnFeOx-4.25Na is physically mixed with C-HZSM-5, the aromatics selectivity increases remarkably from 1.1% to 29.6% and C5+ selectivity increases slightly from 49.7% to 62.5%, while C2– C4 olefin selectivity decreases greatly from 34.8% to 7.7%, and o/p ratio decreases sharply from 7.7 to 0.4 (Figure 6a). This may be ascribed to the aromatization of olefins (dehydrogenation, oligomerization and cyclization) over the zeolite acid sites. Simultaneously, the catalytic activity greatly increases from 3.35 to 9.47 10–2 molCO2 12


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ACS Catalysis

gFe–1 h–1 because of the greater driving force for consecutive consumption of the reaction intermediates (Figure 6a). In addition, the remarkable increase in the selectivity of C2–4 paraffins from 4.5% to 21.4% is attributed to hydrogen transfer reactions.40,41 Furthermore, oxygenates formation is suppressed in the presence of HZSM-5 zeolite (Tables S4 and S5). The catalytic performance for C2H4 and C3H6 conversions over HZSM-5 zeolite was also investigated (Table S6). The lower olefins can be transformed into aromatics and C5+* hydrocarbons (C5+ products except for aromatics) with C5+ selectivity of >75% and high conversion (>90%), indicating aromatization and oligomerization reactions occur easily on HZSM-5 zeolite. To further investigate the role of HZSM-5, the detailed product distribution for ZnFeOx-4.25Na/S-HZSM-5 was analyzed as shown in Figure 6c. The linear α-olefins are the main olefins products over the sole ZnFeOx-4.25Na (Figure 6b). Compared with ZnFeOx-4.25Na, the presence of S-HZSM-5 zeolite significantly decreases the selectivity of olefins and alters the product distribution towards C5+ isoparaffins and aromatics (Figure 6b and 6c), which suggests isomerization of hydrocarbons also occur on HZSM-5. With the decreasing density of Brønsted acid sites upon NaOH treatment, the aromatic selectivity increases markedly to 42.2% over the ZnFeOx-4.25Na/C-HZSM5-a catalyst with a decrease in the CH4 and C2–4 paraffins selectivities. When ZnFeOx4.25Na is combined with S-HZSM-5, the selectivity toward the aromatics is further increased to 60%, which is twice more than that of ZnFeOx-4.25Na/C-HZSM-5. However, further decreasing the density of Brønsted acid sites by increasing the SiO2/Al2O3 ratio leads to reduced aromatics selectivity (Figure 6a). Figure S11 shows that the increase in the density of Brønsted acid sites from 9 to 294 μmol g–1 significantly increases the selectivity of aromatics and decreases the selectivities of C2– 4

olefins and C5+* hydrocarbons. However, a larger density of Brønsted acid sites (>294

μmol g–1) decreases the selectivity of aromatics and increases that of C2–4 olefins and paraffins as well as C5+* hydrocarbons. The CO2 conversion changes slightly with an increase in the density of Brønsted acid sites. It is widely accepted that the Brønsted acid site is responsible for the aromatization, and an excessive amount of strong 13


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Brønsted acid sites may decrease aromatics selectivity and result in the deactivation of the zeolite due to coke formation on acid sites and the blocking of zeolite channels.4244

In addition, the hydrogenation of olefins also occurs on strong Brønsted acid sites.45,46

Therefore, the composite catalyst must have an appropriate density of Brønsted acid sites. Moreover, the catalyst composed of ZnFeOx-4.25Na and the mesoporous nanocrystalline S-HZSM-5 zeolites exhibits the highest selectivity toward the aromatics, which is also attributed to the superior accessibility of the olefins intermediates to the acid sites. A comparison of the detailed product distributions over ZnFeOx-4.25Na and ZnFeOx-4.25Na/S-HZSM-5 reveals that most of the olefins participate in the aromatization reaction (Figure 6b,c). For those catalysts combined with different zeolites, CO selectivity is only 11% and the formation of undesired CH4 is successfully suppressed to below 10% among all hydrocarbons at CO2 conversion of ~36%. Consequently, the aromatics yield over the ZnFeOx-4.25Na/S-HZSM-5 catalyst is more than 19.3% in a single run, which is the highest for direct CO2 hydrogenation from the open literature. It has been shown that the proximity of the different active components in multifunctional catalysts had a significant influence on catalytic performance.9,28,47,48 By increasing the proximity of ZnFeOx-nNa and HZSM-5 components, the aromatics selectivity increases greatly and CO2 conversion is also enhanced (Figure S12a,b). However, the aromatics selectivity and the CO2 conversion decrease when further shortening the distance between these two components by mixing of ZnFeOx-4.25Na and S-HZSM-5 powders of the smaller sizes of 75–150 μm (Figure S12c). When ZnFeOx-4.25Na and S-HZSM-5 were integrated by grinding their powder mixture in an agate mortar (Figure S12d), the resulting composite catalyst possesses even closer proximity between iron-based sites and zeolite acid sites, exhibits a very low aromatics selectivity (2.5%), a high undesired CH4 selectivity up to 33.8%, and a much lower CO2 conversion (19.5%). As a comparison, 1.36%Na-7.41%Zn-20.26%Fe/S-HZSM-5 catalyst with a close intimacy was synthesized by an incipient wetness impregnation method and also shows a poor performance for CO2 hydrogenation (Table S5). In 14


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ACS Catalysis

addition, a large amount of olefins are formed, which suggests a significant deactivation of HZSM-5. The reason, we speculate, is that the sodium ions on the ZnFeOx-nNa surface poison the zeolite acid sites, which sharply decreased the number of strong acid sites, leading to severe deactivation with very low aromatic selectivity. Therefore, an appropriate distance between the two components is critical for achieving the best performance. In addition, the Na content on the ZnFeOx-nNa surface also significantly affects the formation of aromatics from CO2 hydrogenation. With increasing Na content, the aromatics selectivity increases and the selectivities toward CH4 and C2–4 paraffins decrease, so a maximal aromatics selectivity was obtained at the Na content of about 4.25 wt % with CH4 selectivity of 8.2% (Figure 7a). However, the aromatics selectivity decreases markedly when the Na concentration is above 4.25 wt % due to excessive poisoning of zeolite acid sites by the basic Na ions. Moreover, the effects of space velocity and the H2/CO2 ratio of fed gas on the performance were studied. The decrease of space velocity and H2/CO2 ratio are beneficial for aromatics formation from CO2 hydrogenation. We found that the aromatics selectivity reached as high as 75.6% with 41.2% CO2 conversion and 6.9% CO selectivity at the lower space velocity of 1000 mL gcat–1 h–1, so the aromatics yield reaches up to 29% (Figure 7b). This aromatics selectivity is much higher than that of about 55% acquired over Na-Fe3O4/HZSM-549 or that of 21% obtained over K-Fe3O4/H-ZSM-527, respectively. In addition, the selectivity of total C5+ (gasoline-range) hydrocarbons increases to 83.7%. With H2/CO2 ratio increasing from 2 to 6, the aromatics selectivity decreases moderately from 67.2% to 47.4%, and CO2 conversion increases substantially to 59.8% (Figure 7c). 3.5. Catalytic stability. To investigate the prospect of the composite catalyst in industrial applications, the stability of the ZnFeOx-4.25Na/S-HZSM-5 catalyst with granule stacking was tested at industrially relevant conditions (320 °C, 3.0 MPa and 4000 mL gcat–1 h–1). The CO2 conversion and aromatics selectivity increase significantly, while the C2–4 paraffins selectivity decreases during the initial 12 hours (Figure 8). After a time-on-stream of 100 hours, CO2 conversion and CO selectivity remain stable at around 36% and 11%. Although the aromatics selectivity decreases slightly from 60% 15


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at the beginning of the reaction to 53% after 100 hours of time-on-stream, the aromatics selectivity among all C5+ hydrocarbons remain stable at 78%. The high stability of ZnFeOx-4.25Na/S-HZSM-5 can be ascribed to the following factors. The strong interaction between Fe and Zn in the spinel structure of ZnFeOx suppresses the sintering of the iron species, and thus CO2 conversion is stable during 100 hours of reaction (Figure S1b,c). As mentioned above, a higher density of strongly acidic sites can cause severe coke formation. For ZnFeOx-4.25Na/C-HZSM-5, the selectivity toward aromatics decreases rapidly from 48% to 24% and C2–4 olefins increase greatly after only 40 hours, suggesting rapid deactivation on untreated microporous zeolite (Figure S13a). However, ZnFeOx-4.25Na/C-HZSM-5-a with fewer strong acid sites for NaOH treated zeolite is much more stable (Figure S13b). Aside from a suitable acidity, the hierarchical micro/mesoporous structure can also enhance the catalytic stability of the composite catalyst. It was found that the mesoporous zeolite showed less coke deposition, since such a pore structure greatly benefited the diffusion of olefin intermediates and aromatic products (Figure S14). This phenomenon is apparent over the ZnFeOx-4.25Na/S-HZSM-5 catalyst due to the much lower diffusion limitation as a result of the generation of multiple mesopores (4 and 7–15 nm) and the nanocrystal size of S-HZSM-5 zeolite.50-52 3.6. Tuning the fraction of para-xylene. Among the aromatic products, paraxylene (p-xylene) is a more important large-scale feedstock, which is used to produce terephthalic acid and other chemicals.53,54 As shown in Figure 9, aromatics with 9 carbons (A9) were formed as the major products with a fraction of 25% among all hydrocarbons together with 11% heavier aromatics (A≥10) over our ZnFeOx-4.25Na/SHZSM-5 catalyst. It was suggested that the formed BTX (benzene, toluene and xylene) inside the micropores of zeolites can be easily alkylated to form heavier aromatics during its diffusion to the external surface of zeolites.55 The mesoporous structure is beneficial for the diffusion of larger aromatic products, and thus decreases the fraction of heavier aromatics (Figure S15). To optimize the distribution of aromatics, we passivated the exterior acid sites to suppress the secondary reaction by silylation of H16


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ZSM-5 (S-HZSM-5-Si) through tetraethoxysilane (TEOS) modification.15,

45, 53

Compared with the parent S-HZSM-5, the crystallinity of the zeolite decreases after TEOS modification (Figure S8d). The surface elemental compositions of S-HZSM-5 and S-HZSM-5-Si from SEM and EDS analyses show that the Si/Al ratio increases significantly from 12.8 to 15.5, indicating that the external surface of the zeolite is successfully coated by SiO2 (Figure S7f and Table S7). In addition, the total amount of strong acid sites for S-HZSM-5 determined by NH3-TPD is 1.9 times higher than Si-SHZSM-5 (Table S3). Our catalytic results show that CO2 conversion and CO selectivity remain almost unchanged over the S-HZSM-5-Si zeolite (Table S5). Compared with ZnFeOx-4.25Na/S-HZSM-5, the selectivities of A9 and A≥10 are much lower over ZnFeOx-4.25Na/S-HZSM-5-Si, and the fraction of p-xylene in xylenes increases from 56% to 75% (Figure 9). Therefore, TEOS modification of zeolite can significantly enhance the selectivity of p-xylene from CO2 hydrogenation. 4. CONCLUSIONS In conclusion, we discovered that the integration of sodium-modified spinel oxide ZnFeOx, which enabled the hydrogenation of CO2 to CO over Fe3O4 sites and olefins formation via Fischer−Tropsch synthesis over Fe5C2 sites, and the hierarchical nanocrystalline HZSM-5 zeolites responsible for the aromatization of olefins can realize the direct synthesis of aromatics from CO2 hydrogenation with excellent catalytic performance. A suitable amount of residual sodium (around 4.25 wt %), hierarchical pore structure and appropriate density of Brønsted acid sites over the ZnFeOx-nNa/HZSM-5 composite catalyst are decisive factors for the high selectivity toward aromatics and high catalytic stability. The total aromatics selectivity reaches 75.6% with CO2 conversion of 41.2% at 320 oC and the proportion of p-xylene in xylenes reaches up to 75% by selective passivation of the exterior acid sites of HZSM5. It is also noteworthy that the combined selectivity toward CH4 and CO is less than 20%, ensuring that most of the CO2 is transformed into value-added products (aromatics and light hydrocarbons). Therefore, our results demonstrate a promising prospect for the industrial applications of this direct CO2 hydrogenation to aromatics process. 17


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:XXXXXX Detail material preparations and catalytic test as well as Figures S1–S15 and Tables S1– S7 as described in the text (PDF). AUTHOR INFORMATION Corresponding Author *E-mail for Peng Gao: [email protected]; *E-mail for Yuhan Sun: [email protected] Author Contributions P.G. and Y.S. conceived the project, analyzed the data and wrote the paper. X.C., P.G., H.W., L.Z. and S.L. drafted the manuscript. X.C., Z.L. and C.Y. prepared the samples. X.C, P.G. and C.Y. performed the catalytic evaluation. X.C, P.G., Z.L. and S.L. characterized the samples. All authors discussed the results and commented on the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21773286, U1832162, 91845105), the “Transformational Technologies for Clean Energy and Demonstration”, Strategic Priority Research Program of the Chinese Academy of Sciences (XDA21090204, XDA21090201), Youth Innovation Promotion Association CAS (2018330), Science and Technology Commission of Shanghai Municipality (19QA1409900), the Ministry of Science and Technology of China (2016YFA0202802, 2018YFB0604700), and Shanghai Functional Platform for Innovation Low Carbon Technology. REFERENCES 18


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of Iron-based Fischer-Tropsch Synthesis Catalyst Promoted with Potassium or Sodium. Catal. Commun. 2007, 8, 1957-1962. (35) Wei, J.; Sun, J.; Wen, Z. Y.; Fang, C. Y.; Ge, Q. J.; Xu, H. Y. New Insights into the Effect of Sodium on Fe3O4-Based Nanocatalysts for CO2 Hydrogenation to Light Olefins. Catal. Sci. Technol. 2016, 6, 4786-4793. (36) Ribeiro, M. C.; Jacobs, G.; Davis, B. H.; Cronauer, D. C.; Kropf, A. J.; MarshaW, C. L., Fischer-Tropsch Synthesis: An In-Situ TPR-EXAFS/XANES Investigation of the Influence of Group I Alkali Promoters on the Local Atomic and Electronic Structure of Carburized Iron/Silica Catalysts. J. Phys. Chem. C 2010, 114, 7895-7903. (37) Galvis, H. M. T.; Koeken, A. C. J.; Bitter, J. H.; Davidian, T.; Ruitenbeek, M.; Dugulan, A. I.; de Jong, K. P., Effects of Sodium and Sulfur on Catalytic Performance of Supported Iron Catalysts for the Fischer-Tropsch Synthesis of Lower Olefins. J. Catal. 2013, 303, 22-30. (38) Yang, C.; Zhao, H. B.; Hou, Y. L.; Ma, D. Fe5C2 Nanoparticles: A Facile Bromide-Induced Synthesis and as an Active Phase for Fischer-Tropsch Synthesis. J. Am. Chem. Soc. 2012, 134, 15814-15821. (39) Govender, N. S.; Botes, F. G.; de Croon, M. H. J. M.; Schouten, J. C. Mechanistic Pathway for C2+ Hydrocarbons over an Fe/K Catalyst. J. Catal. 2014, 312, 98-107. (40) Olsbye, U.; Svelle, S.; Bjorgen, M.; Beato, P.; Janssens, T. V. W.; Joensen, F.; Bordiga, S.; Lillerud, K. P. Conversion of Methanol to Hydrocarbons: How Zeolite Cavity and Pore Size Controls Product Selectivity. Angew. Chem. Int. Ed. 2012, 51, 5810-5831. (41) Stocker, M. Methanol-to-Jydrocarbons: Catalytic Materials and Their Behavior. Micropor. Mesopor. Mat. 1999, 29, 3-48. (42) Zhao, B.; Zhai, P.; Wang, P.; Li, J.; Li, T.; Peng, M.; Zhao, M.; Hu, G.; Yang, Y.; Li, Y.-W.; Zhang, Q.; Fan, W.; Ma, D. Direct Transformation of Syngas to Aromatics over Na-Zn-Fe5C2 and Hierarchical HZSM-5 Tandem Catalysts. Chem 2017, 3, 323-333. (43) Sahoo, S. K.; Viswanadham, N.; Ray, N.; Gupta, J. K.; Singh, I. D. Studies on Acidity, Activity and Coke Deactivation of ZSM-5 during n-Heptane Aromatization. Appl. Catal. A-Gen. 2001, 205, 1-10. (44) Kim, J.; Choi, M.; Ryoo, R. Effect of Mesoporosity Against the Deactivation of MFI Zeolite Catalyst during the Methanol-to-Hydrocarbon Conversion Process. J. Catal. 2010, 269, 219228. 22


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ACS Catalysis

(45) Cheng, K.; Zhou, W.; Kang, J. C.; He, S.; Shi, S. L.; Zhang, Q. H.; Pan, Y.; Wen, W.; Wang, Y., Bifunctional Catalysts for One-Step Conversion of Syngas into Aromatics with Excellent Selectivity and Stability. Chem 2017, 3, 334-347. (46) Senger, S.; Radom, L., Zeolites as Transition-Metal-Free Hydrogenation Catalysts: A Theoretical Mechanistic Study. J. Am. Chem. Soc. 2000, 122, 2613-2620. (47) Jiao, F.; Li, J. J.; Pan, X. L.; Xiao, J. P.; Li, H. B.; Ma, H.; Wei, M. M.; Pan, Y.; Zhou, Z. Y.; Li, M. R.; Miao, S.; Li, J.; Zhu, Y. F.; Xiao, D.; He, T.; Yang, J. H.; Qi, F.; Fu, Q.; Bao, X. H. Selective Conversion of Syngas to Light Olefins. Science 2016, 351, 1065-1068. (48) Cheng, K.; Gu, B.; Liu, X. L.; Kang, J. C.; Zhang, Q. H.; Wang, Y. Direct and Highly Selective Conversion of Synthesis Gas into Lower Olefins: Design of a Bifunctional Catalyst Combining Methanol Synthesis and Carbon-Carbon Coupling. Angew. Chem. Int. Ed. 2016, 55, 4725-4728. (49) Xu, Y.; Shi, C.; Liu, B.; Wang, T.; Zheng, J.; Li, W.; Liu, D.; Liu, X., Selective Production of Aromatics from CO2. Catal. Sci. Technol. 2019, 9, 593-610. (50) Cheng, K.; Zhang, L.; Kang, J. C.; Peng, X. B.; Zhang, Q. H.; Wang, Y. Selective Transformation of Syngas into Gasoline-Range Hydrocarbons over Mesoporous H-ZSM-5Supported Cobalt Nanoparticles. Chem.-Eur. J. 2015, 21, 1928-1937. (51) Rownaghi, A. A.; Hedlund, J. Methanol to Gasoline-Range Hydrocarbons: Influence of Nanocrystal Size and Mesoporosity on Catalytic Performance and Product Distribution of ZSM-5. Ind. Eng. Chem. Res. 2011, 50, 11872-11878. (52) Chen, D.; Moljord, K.; Fuglerud, T.; Holmen, A. The Effect of Crystal Size of SAPO-34 on the Selectivity and Deactivation of the MTO reaction. Micropor. Mesopor. Mat. 1999, 29, 191-203. (53) Zhang, P.; Tan, L.; Yang, G.; Tsubaki, N. One-Pass Selective Conversion of Syngas to paraXylene. Chem. Sci. 2017, 8, 7941-7946. (54) Lyons, T. W.; Guironnet, D.; Findlater, M.; Brookhart, M. Synthesis of p-Xylene from Ethylene. J. Am. Chem. Soc. 2012, 134, 15708-15711. (55) Zhang, J. G.; Qian, W. Z.; Kong, C. Y.; Wei, F. Increasing para-Xylene Selectivity in Making Aromatics from Methanol with a Surface-Modified Zn/P/ZSM-5 Catalyst. ACS Catal. 2015, 5, 2982-2988.

23


ACS Catalysis

b

 ZnFe2O4 ◇ Fe2O3  NaNO3  

















ZnFeOx-20.00Na











20



30



 



 





◇  ◇

◇ 

 

10







★ Fe5C2  Fe3O4  ZnO

40

c



50

2θ (o)



 ZnFeO

-5.79Na x













ZnFeOx-4.25Na

ZnFeOx-20.00Na 

80



★★



★★ 

★



 



 



★★

★ ★



★★

★★ ★ ★



★★

★★ ★ ★

  

★★ ★ ★



ZnFeOx-5.79Na

 



ZnFeOx-4.25Na 

ZnFeOx-2.68Na





 

90 10

20





30

40

50

2θ (o)





ZnFeOx-0.50Na 

60

d

70

80

90

Fe3+ Fe0 o

R-320 C-50 h R-320 oC-8 h

 ZnFeOx-5.79Na

ZnFeOx-4.25Na

R-320 oC-4 h 25 oC

ZnFeOx-4.25Na

ZnFeOx-2.68Na ZnFeOx-0.50Na

100

ZnFeOx-20.00Na



★★



ZnFeOx-0.50Na

70

★★

  

ZnFeOx-2.68Na

60





Intensity (a.u.)

Intensity (a.u.)



Intensity (a.u.)

a

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

200

300

400

500 o

Temperature ( C)

600

711 706.5

700

735

730

725

720

715

710

705

700

Binding Energy (eV)

Figure 1. Characterizations of ZnFeOx-nNa catalysts with different Na content. XRD patterns for (a) calcined and (b) spent ZnFeOx-nNa catalysts. (c) CO2–TPD spectra. (d) Fe 2p XPS of ZnFeOxZnFeOx-4.25Na treated in reaction atmosphere (H2/CO2/N2 = 73/24/3) at 320 °C, 3.0 MPa and 4000 mL gcat−1 h−1. R-320 oC-50 h (blue line) suggests that the catalyst was firstly reduced in H2 at 350 °C for 8 h and then treated in reaction atmosphere for 50 h at 320 oC.

24


Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure 2. HRTEM micrograph of (a) calcined and (b) spent ZnFeOx-4.25Na catalyst.

Figure 3. Morphology of the synthesized S-HZSM-5 zeolite. (a) SEM and (b) HRTEM images of S-HZSM-5.

25


ACS Catalysis

a 4 nm

dV/dlog (D) (a.u.)

C-HZSM-5(355)

C-HZSM-5(132) C-HZSM-5(72) S-HZSM-5-Si S-HZSM-5 C-HZSM-5-a

C-HZSM-5

5

10

15

20

25

30

Pore diameter (nm)

b

c

C-HZSM-5(355)

150 oC

C-HZSM-5(355)

C-HZSM-5(132)

C-HZSM-5(132)

Absorbance (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

C-HZSM-5(72) S-HZSM-5-Si S-HZSM-5 C-HZSM-5-a

C-HZSM-5(72) S-HZSM-5-Si 1546 cm-1

1455 cm-1

S-HZSM-5 C-HZSM-5-a

C-HZSM-5

C-HZSM-5

100

200

300

400

Temperature (oC)

500

600 1575

1550

1525

1500

1475

Wavenumbers (cm-1)

1450

1425

Figure 4. Physiochemical properties of various HZSM-5 zeolites. (a) Pore distribution of the various HZSM-5 samples. (b) NH3–TPD profiles for various HZSM-5 zeolites. (c) FT-IR spectra of pyridine adsorption over HZSM-5 zeolites at 150 oC.

26


Page 27 of 31

Conversion and selectivity (%)

45 10

40 35 30

8

CO2 Conv.

25

o/p (C2-4) 6

20

4

15 CO Sel. 10

2

5

o/p molar ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

0

0

a a a a Na .50N -2.68N -4.25N -5.79N -20.00 0 eO x nFeO x nFeO x nFeO x nFeO x Z Z ZnF Z Z Figure 5. Catalytic results over ZnFeOx-nNa catalysts with different Na contents. Reaction conditions: T = 320 °C, P = 3.0 MPa, molar H2/CO2/N2 = 73/24/3 and WHSV = 4000 mL gcat–1 h–1.

27


ACS Catalysis

a

CH4

Conversion, selectivity or hydrocarbon distribution (%)

100

C2-4o

C2-4=

C5+*

Aromatics

10

80 60 40

8 6

CO2 Conv.

4 CO

20 Sel. 0

12

2

) -5 e 2) 72) -5-a -5 Non -HZSMHZSM-HZSMSM-5( M-5(13 -5(355 M C S -HZ HZS CS Z C CC-H

Activity (10-2 molCO2 gFe-1 h-1)

0

b 28

Aromatic Olefin Isoparaffin n-paraffin

24 20 16 12 8 4 0

c

28

Hydrocarbon distribution (%)

ZnFeOx-4.25Na or ZnFeOx-4.25Na/zeolite catalyst


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

24

Aromatic Olefin Isoparaffin n-paraffin

20 16 12 8 4 0

1 3 5 7 9 11 13 15 17 19

Carbon number

1 3 5 7 9 11 13 15 17 19

Carbon number

Figure 6. Catalytic performance for CO2 hydrogenation. (a) Comparisons of the CO2 conversion and product selectivity over sole ZnFeOx-4.25Na or composite catalysts containing ZnFeOx-4.25Na and various zeolites (mass ratio = 1/2). The detailed hydrocarbon product distribution obtained over (b) ZnFeOx-4.25Na and (c) ZnFeOx-4.25Na/S-HZSM-5 catalysts. Reaction conditions: T = 320 °C, P = 3.0 MPa, molar H2/CO2/N2 = 73/24/3 and WHSV = 4000 mL gcat–1 h–1. C5+* means C5+ products except for aromatics.

28


a

90 80

C2-4o

CH4

C2-4=

C5+*

Aromatics

40

70 CO2 Conv. 30

60 50

CO Sel.

40

20

30 10

20 10


0

0 n=0.05

n=2.68

n=4.25

n=5.79 n=20.00

ZnFeOx-nNa/S-HZSM-5 multifunctional catalyst

Hydrocarbon destribution (%)

b 100

C2-4o

CH4

C2-4=

C5+*

Aromatics

40

CO2

80

50

Conv.

30 60 20

40 CO Sel.

20 0

1000

10

2000

4000

6000 -1

8000


0

-1

Space velocity (mL g h )


c 100

CH4

C2-4o

C2-4=

C5+*

Aromatics

70 60 50

80 60 40 20

CO2

40

Conv.

30

CO Sel.

20 10

0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis


Page 29 of 31

0 2

3

4

5

6

Ratio of H2/CO2

Figure 7. CO2 hydrogenation performance over composite catalysts. (a) CO2 conversion, CO selectivity and hydrocarbon distribution over different ZnFeOx-nNa/S-HZSM-5 catalysts (n = 20.00, 5.79, 4.25, 2.68, 0.50 wt %). The effect of (b) space velocity and (c) H2/CO2 ratio on catalytic performance over ZnFeOx-4.25Na/S-HZSM-5. The catalysts with ZnFeOx-4.25Na/S-HZSM-5 mass ratio = 1/2 were evaluated under the reaction conditions shown in Figure 6. 29


ACS Catalysis

Hydrocarbon dostribution (%)

100

40

CO2 Conv.

90

Aromactics in C5+

80

35 30

70 60

25

Aromactics

50

20

40

15

CO Sel.

30

10

20

C2-4o CH4 5 C2-4=

10 0


CH4 C20-C40 C2=-C4= C20-C40 Aromatics Aro. in C5+

0

0

20

40

60

80

100

Reaction time (h) Figure 8. Stability test of ZnFeOx-4.25Na/S-HZSM-5 catalysts (mass ratio = 1/2) under reaction conditions given in Figure 6.


40 35

80 ZnFeOx-4.25Na/S-HZSM-5 ZnFeOx-4.25Na/S-HZSM-5-Si

70

30

60

25

50

20

40

15

30

10

20

5

10

0

= o * e e e e e CH 4C 2-4 C 2-4 C 5+nzen luennzen ylen A 9 ≥10 ylen o x A pBe T ylbe X h t E

p-xylene content in xylenes (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 31

0

Figure 9 Comparisons of hydrocarbons distribution over ZnFeOx-4.25/S-HZSM-5 and ZnFeOx4.25/S-HZSM-5-Si (ZnFeOx-4.25/HZSM-5 mass ratio = 1/2) under reaction conditions given in Figure 6. A9 and A≥10 are representative of aromatics containing 9 as well as 10 and more than 10 carbon atoms, respectively. C5+* means C5+ products except for aromatics.

30


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ACS Catalysis

Table of Contents artwork

31


Selective Production of Aromatics Directly from Carbon Dioxide

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