Characterization of aquaporin-driven hydrogen peroxide transport

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FeK-on-3D Graphene-Zeolite Tandem Catalyst with High Efficiency and Versatility in Direct CO2 Conversion to Aromatics Shunwu Wang, Tijun Wu, Jun Lin, Jing Tian, Yushan Ji, Yan Pei, Shirun Yan, Minghua Qiao, Hualong Xu, and Baoning Zong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b04328 • Publication Date (Web): 11 Oct 2019 Downloaded from pubs.acs.org on October 21, 2019

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FeK-on-3D Graphene-Zeolite Tandem Catalyst with High Efficiency and Versatility in Direct CO2 Conversion to Aromatics Shunwu Wang†, Tijun Wu†, Jun Lin‡, Jing Tian†, Yushan Ji†, Yan Pei†, Shirun Yan†, Minghua Qiao*,†, Hualong Xu†, Baoning Zong*,§ †Collaborative

Innovation Center of Chemistry for Energy Materials, Department of Chemistry

and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, 2205 Songhu Road, Shanghai 200438, P. R. China ‡Key

Laboratory of Nuclear Analysis Techniques, Shanghai Institute of Applied Physics, Chinese

Academy of Sciences, 2019 Jialuo Road, Shanghai 201800, P. R. China §State

Key Laboratory of Catalytic Materials and Chemical Engineering, Research Institute of

Petroleum Processing, SINOPEC, 18 Xueyuan Road, Beijing 100083, P. R. China * Email: [email protected] (M. Q.) * Email: [email protected] (B. Z.) KEYWORDS: CO2, aromatics, 3D porous graphene, iron catalyst, zeolite ABSTRACT: Direct converting CO2 with renewable H2 to aromatics can transform green-house gas and intermittent reproducible energies into valuable organic building blocks. However, the catalytic efficiency for this purpose remains low on existing catalysts containing either metal

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oxides (the methanol route) or iron (the olefin route) as the CO2 hydrogenation component(s). In this contribution, benefited from the exceptional activity of the honeycomb-structured graphene (HSG)-supported, potassium-promoted iron (FeK1.5/HSG) in hydrogenating CO2 to light olefins, and with the help of the tandem HZSM-5, CO2 was converted to aromatics with high selectivity of 41% among all the carbon-containing products (inclusive of CO) or 68% among all the hydrocarbons at CO2 single-pass conversion of 35% and high space velocity of 26,000 mL h–1 gcat– 1,

which results in an unprecedentedly high space time yield of aromatics of 11.8 μmolCO2 gcat–1

s–1. Furthermore, the dual-layer packing configuration of the FeK1.5/HSG-zeolite catalyst enables flexible adjustment of the aromatics spectrum simply by changing the type of the tandem zeolite. This work for the first time shows promise for the realization of a high-efficiency and versatile CO2-to-aromatics technology.

INTRODUCTION Aromatics occupy about one-third share of the market for commodity petrochemicals.1 Nowadays, aromatics are produced from fossil fuels, mainly petroleum, and to a much lesser extent, coal. Benzene is the third most important organic building blocks, with ethylbenzene being the fourth. Toluene can be demethylated to benzene or disproportionated to benzene and xylenes. The major chemical use for individual xylenes is oxidation of p-xylene to terephthalic acid, mxylene to isophthalic acid, and o-xylene to phthalic anhydride. Heavy aromatics with high solvency and controlled evaporation characteristics make them versatile performers in many industrial and agricultural applications.2 Considering the huge demand and broad applications of aromatics, their production from sustainable carbon resources is of significant economic and social implications.

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Some recent works successfully demonstrated the feasibility of converting the green-house gas, CO2, with the help of renewable H2 from sunlight, wind, tide, or wave, to aromatics in high selectivity. In direct conversion of CO2 to gasoline-range hydrocarbons over the In2O3/HZSM-5 catalyst, Gao et al. identified aromatics with low selectivity of 14.6%.3 Liu and co-workers reported that the ZnAlOx&HZSM-5 catalyst prepared by grinding ZnAlOx with HZSM-5 showed high aromatics selectivity of 73.9% at CO2 conversion of 9.1%. Operando DRIFTS suggested that methanol and dimethyl ether (DME, by dehydration of methanol) were formed by hydrogenating the formate species on ZnAlOx and then transmitted to HZSM-5, where they were converted to the olefin intermediates and finally, aromatics.4 Over a ZnZrO/HZSM-5 catalyst also prepared by grinding ZnZrO with HZSM-5, CO2 was converted to aromatics with selectivity up to 73% at CO2 conversion of 14%. Characterizations including in situ DRIFTS and chemical trapping-MS indicated that CO2 hydrogenation on ZnZrO generated the CHxO intermediates that transferred to the micropores of HZSM-5, in which they produced aromatics through the olefin intermediates.5 Tan and co-workers employed the ZnCrOx-ZnZSM-5 composite catalyst for CO2 hydrogenation, on which the aromatics selectivity of 56.5% was obtained at CO2 conversion of 19.9%.6 On the Cr2O3/HZSM-5 catalyst combined by powder mixing, Tsubaki and co-workers obtained the aromatics selectivity of 75.6% at CO2 conversion of 34.5%. In that case, 5.42 vol.% of CO was added in the H2/CO2 mixture gas to replenish the oxygen vacancies on Cr2O3.7 The design concept of the above metal oxide/ZSM-5 catalysts for converting CO2 to aromatics is based on methanol synthesis and aromatization of methanol. Because of the chemical inertness of the CO2 molecule, the low hydrogenation activity of the metal oxides, and the severe thermodynamic restriction of CO2 hydrogenation to methanol,8 the space time yields of the aromatics (STYaro, moles of CO2 converted to aromatics per gram catalyst per second) on these

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catalysts were generally below 1 μmolCO2 gcat–1 s–1 (Table S1). Even with the addition of 5.42 vol.% of CO to H2/CO2, the STYaro was still only 0.9 μmolCO2 gcat–1 s–1 on the Cr2O3/HZSM-5 catalyst (Table S1). Provided that a more efficient hydrogenation component was used for methanol synthesis, the hydrogenation of the precursors to aromatics would also be accelerated due to the close proximity between the catalyst components for methanol synthesis and aromatization demanded by such catalysts, which is an inherent dilemma for catalysts converting CO2 to aromatics via the methanol synthesis–aromatization route. Intriguingly, there is a consensus in above works that the olefins are the more direct intermediates than methanol/DME to aromatics. Moreover, thermodynamic calculations manifest that, as compared to CO2 hydrogenation to methanol, CO2 hydrogenation to the olefins is more favorable, taking ethylene as an example (Figure 1A). Also, the aromatization of ethylene to typical aromatics such as benzene, toluene, ethylbenzene, and p-xylene is thermodynamically favored at least in the temperature range of 273–773 K (Figure 1B). Hence, converting CO2 to aromatics via the olefination–aromatization route is thermodynamically viable. Although in the synthesis of iso-paraffins from CO2 hydrogenation over the Na-Fe3O4/zeolite catalysts aromatics were produced simultaneously, the aromatics fraction in the hydrocarbons was limited.9,10 In an early work targeting at aromatics production from CO2 hydrogenation via the olefination– aromatization route, Kuel and Lee obtained the aromatics selectivity of 21.2% at CO2 conversion of 37.4% with the STYaro of only 0.02 μmolCO2 gcat–1 s–1 over the fused iron-HZSM-5 catalyst (Table S1).11 Liu and co-workers developed a Na/Fe-HZSM-5 catalyst, on which the aromatics selectivity was 54.7% at CO2 conversion of 21.8%, and the STYaro was increased to 1.1 μmolCO2 gcat–1 s–1 (Table S1).12 Gascon and co-workers reported the aromatics selectivity of 27.1% at CO2 conversion of 47.4% with enhanced STYaro of 1.7 μmolCO2 gcat–1 s–1 over the Fe2O3@KO2/ZSM-5

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catalyst (Table S1).13 By replacing one half of CO2 with CO, the STYaro was additionally increased to 2.6 μmol gcat–1 s–1.13 On the ZnFeOx-nNa/HZSM-5 catalyst, Cui et al. further improved the aromatics selectivity and the STYaro to 76.5% and 3.6 μmolCO2 gcat–1 s–1, respectively (Table S1),14 indicating that a properly designed iron-based component can facilitate efficient production of aromatics from CO2 via the olefination–aromatization route. Recently, we developed a honeycomb-structured graphene (HSG)-supported FeK1.5 catalyst (FeK1.5/HSG) that displayed the highest STY of light olefins from CO2,15 which shows promise as an olefination component to boost the STYaro from CO2 if coupling with the acidic HZSM-5 zeolite. Herein, we show that FeK1.5/HSG and HZSM-5(50) (SiO2/Al2O3 molar ratio = 50) combined in a dual-layer mode (labelled as FeK1.5/HSG|HZSM-5(50)), instead of being homogeneously mixed (labelled as FeK1.5/HSG + HZSM-5(50)), is highly active and selective in converting CO2 to aromatics. The FeK/HSG catalysts with lower or higher potassium contents are inferior to the FeK1.5/HSG catalyst in terms of the aromatics selectivity and STYaro when combined with HZSM-5(50) in the dual-layer mode (Table S2). On the FeK1.5/HSG|HZSM-5(50) catalyst, the aromatics selectivity is 41% among all the carbon-containing products (inclusive of CO) or 68% among all the hydrocarbons at CO2 conversion of 35% and space velocity (SV) of 26,000 mL h–1 gcat–1. The resulting STYaro amounts to 11.8 μmolCO2 gcat–1 s–1, which represents the highest value reported in the open literature as far as we are aware of. Furthermore, the dual-layer packing configuration of the FeK1.5/HSG|zeolite catalyst enables flexible adjustment of the aromatics spectrum simply by changing the type of the tandem zeolite.

EXPERIMENTAL SECTION Materials. Li2O (97%) from Aladdin and HCl (36.5 wt%), Fe(NO3)3·9H2O, K2CO3, and

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anhydrous ethanol of analytical grade (A.R.) from Sinopharm were used as received. SAPO-34 (SiO2/Al2O3 = 0.5), HY (SiO2/Al2O3 = 5.4), Hβ (SiO2/Al2O3 = 25), HMCM-22 (SiO2/Al2O3 = 30), NaZSM-5 (SiO2/Al2O3 = 50, exchange degree of 80%), and HZSM-5 (SiO2/Al2O3 = 27, 50, 160) from Nankai University catalyst company were calcined at 773 K in air for 4 h before use. The gases were all from Shanghai Youjiali. Catalyst Preparation. The preparation details and basic physical characteristics of the FeK1.5/HSG catalyst have been described before.15 In brief, HSG was fabricated by reacting Li2O with CO. Iron and potassium with nominal loadings of 20 wt% and 1.5 wt%, respectively, were successively loaded on HSG. The as-prepared FeK1.5/HSG and the zeolite powders were respectively pressed into pellets at 25 MPa, crushed, and sieved to granules of 60–80 meshes (250–400 μm). If not specified, 150 mg of the FeK1.5/HSG granules were placed upstream of 150 mg of the zeolite granules, using 300 mg of quartz sand (60–80 meshes) as the partition. The as-combined catalyst is labelled as FeK1.5/HSG|zeolite. Catalytic Testing and Product Analysis. The reaction was carried out in a stainless steel fixedbed reactor with an inner diameter of 10 mm. The catalyst was reduced on site in flowing CO (99.9%, 30 mL min–1) at 623 K for 8 h with a heating rate of 2 K min–1. The reaction conditions were 613 K, 2.0 MPa, and H2/CO2/N2 ratio of 72/24/4 by volume. N2 was the internal standard during gaseous product analysis. If not specified, the SV was 26,000 mL h–1 gcat−1 relative to the total catalyst weight. During the reaction, H2, CO2, N2, CO, and CH4 were analyzed gas chromatographically online with a 2 m-long TDX-01 packed stainless steel column connected to a thermal conductivity detector (TCD). The hydrocarbons were analyzed online with a PONA capillary column (50 m ×

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0.25 mm × 0.50 μm) connected to a flame ionization detector (FID). The C1−C4 olefins and paraffins in the gas phase were additionally analyzed online with a PoraPlot Q capillary column (12.5 m × 0.53 mm × 20 μm) connected to a FID. The aromatics were analyzed offline with a FFAP capillary column (30 m × 0.32 mm × 1.0 μm) connected to a FID. The catalytic activity was expressed as iron time yield to hydrocarbons from CO2 (STYHC, moles of CO2 converted to hydrocarbons per gram catalyst per second). The hydrocarbon selectivities were calculated on carbon basis with the exception of CO. The productivity of aromatics (STYaro) was represented as moles of CO2 converted to the aromatics per gram catalyst per second. The carbon balance of the products in all catalytic runs were within 95−98%.

RESULTS AND DISCUSSION Effect of HZSM-5 on Product Distribution. Figure 2 compares the product distributions over the FeK1.5/HSG catalyst in the absence and presence of HZSM-5(50) under the reaction conditions of 613 K, H2/CO2 ratio of 3, 2.0 MPa, SV of 26,000 mL h−1 gcat−1 relative to the total catalyst weight, and time on stream (TOS) of 24 h. The textural properties of the FeK1.5/HSG catalyst have been reported previously.15 As shown in Figure 2A, the FeK1.5/HSG catalyst alone afforded the CH4 selectivity of 26%, C2–C4 selectivity of 70% (C2=–C4= of 59%), and C5+ selectivity of 4.1% (exclusive of CO) at CO2 conversion of 46%, as high reaction temperature favors the production of light olefins.16 No aromatics was identified. The hydrocarbon products obey the classical Anderson–Schulz–Flory (ASF) distribution with the chain-growth factor (α) of 0.41. When HZSM-5(50) was placed downstream of FeK1.5/HSG, the product spectrum changed dramatically (Figure 2B). The most prominent change is that the high carbon-number products,

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including aromatics and the C5+ n- and iso-paraffins, were boosted at the expense of light olefins, clearly evidencing the occurrence of the olefination–aromatization reaction. Aromatics occupy 68% of the hydrocarbons (carbon basis) at CO2 conversion of 35%, which were transformed from the olefins via oligomerization/cyclization/hydrogen transfer on the acid sites of HZSM-5.17 The aromatics selectivity is only 5.9% lower than the value reported on the ZnAlOx&HZSM-5 catalyst (Table S1).4 However, the CO2 conversion on the ZnAlOx&HZSM-5 catalyst was only 9.1%, which substantiates that the olefination–aromatization route is competitive to the methanol synthesis–aromatization route in selectivity. The increment in the C5+ paraffins also shown in Figure 2B is attributed to oligomerization/isomerization/cracking of light olefins also taking place on the acid sites.18 On the other hand, consistent with previous reports,13,19 the CO selectivity decreased from 44% to 39% in the presence of HZSM-5(50). According to Gascon and coworkers, in the presence of the zeolite, a fraction of unreacted CO formed via the reversed water– gas shift (RWGS) reaction on the iron-based catalyst could diffuse to the zeolite and produce mainly ethylene after several transformations, thus reducing the CO selectivity.13 It is somewhat unexpected that accompanying with the depletion of light olefins, the CH4 selectivity was noticeably suppressed to 3.5%. We assume that the coupling of olefination with aromatization may enhance olefination with respect to methanation. Considering that light olefins bind more tightly than alkanes on the iron catalysts, as a higher reaction temperature is demanded to obtain light olefins than alkanes,16 and the formation of light olefins from CO2 and H2 is thermodynamically less favorable than the formation of aromatics from light olefins, the coupling of olefination with aromatization can both kinetically and thermodynamically propel the formation and subsequent consumption of light olefins. The fast conversion of light olefins to aromatics empties the active sites on the iron catalyst, which become readily available for the formation of

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light olefins in a next catalytic cycle. As a result, the aromatics selectivity on the FeK1.5/HSG|HZSM-5(50) catalyst exceeds the selectivity to light olefins on the FeK1.5/HSG catalyst alone.15 In contrast, such a promotion effect is not applicable to the formation of CH4. Therefore, the CH4 selectivity is apparently suppressed relative to the formation and subsequent consumption of light olefins to aromatics. Alternatively, Song and co-workers reported that CH4 can co-aromatize with the olefins on the Ag-Ga/ZSM-5 catalyst.20 The occurrence of such a reaction on the FeK1.5/HSG|HZSM-5(50) catalyst cannot be excluded, which awaits for further experimental elucidation. Benefited from the excellent hydrogenation activity of FeK1.5/HSG, the STYaro over the FeK1.5/HSG|HZSM-5(50) catalyst amounted to 11.8 μmolCO2 gcat−1 s−1. As demonstrated in Table S1, this value is above ten times of those on catalysts based on methanol synthesis–aromatization, and more than three times of the highest value reported so far on the ZnFeOx-nNa/HZSM-5 catalyst based on olefination–aromatization,14 which represents a significant progress in the production of useful chemicals from green-house gas CO2. The one-pass aromatics yield over the FeK1.5/HSG|HZSM-5(50) catalyst is only inferior to that over the ZnFeOx-nNa/HZSM-5 catalyst based on CO2 olefination–aromatization (Table S1),14 showing that the catalyst is highly promising in directly hydrogenating CO2 to aromatics. Effect of HZSM-5 on Phase Composition of FeK1.5/HSG. Since iron carbides such as Fe5C2, ε-Fe2C, and ε’-Fe2.2C have been acknowledged as the active phases for Fischer–Tropsch synthesis (FTS),21–23 the FeK1.5/HSG component in the FeK1.5/HSG|HZSM-5(50) catalyst after 24 h on stream was characterized by TEM, HRTEM, synchrotron-radiation XRD, and

57Fe

Mössbauer absorption spectroscopy. Figure 3A shows that the black nanoparticles (NPs) with average size of 13.5 nm were homogeneously dispersed on the pore walls of HSG. Figure 3B

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reveals that the NPs were mainly composed of χ-Fe5C2 with the (510) interplanar spacing of 2.05 Å, which is consistent with Figure 3C that the diffractions of χ-Fe5C2 (JCPDS 36-1248) dominate the XRD pattern. In Figure 3C, there are also some weak features arising from ε-Fe2C/ε’-Fe2.2C (JCPDS 36-1249) and γ-Fe2O3 (JCPDS 39-1346). In Figure 3D and Table S3, the doublet with isomer shift (IS) of 0.32 mm s–1 and quadrupole splitting (QS) of 1.06 mm s–1 is attributed to the Fe(III) species22 in nanosized γ-Fe2O3. The presence of χ-Fe5C2 is confirmed by three sextets with hyperfine magnetic fields (H) of 18.3, 21.5, and 11.1 T.24 The ε’-Fe2.2C phase is determined by the H of 16.8 T, which may be stabilized by HSG in the presence of the potassium promoter at 613 K.15 It should be reminded that the FeK1.5/HSG catalyst alone after reaction also owns identical iron phases and similar particle size distribution.15 Interestingly, more iron carbides were formed on the FeK1.5/HSG component in the tandem catalyst than on the FeK1.5/HSG catalyst alone (87.7% vs. 82.8%, inset in Figure 3D). The high content of iron carbides would also contribute to the high STYaro of the FeK1.5/HSG|HZSM-5(50) catalyst. In addition, it seems that the RWGS reaction does not limit the CO2 hydrogenation kinetics on the FeK1.5/HSG|HZSM-5(50) catalyst. Despite of its lower Fe(III) content for the RWGS reaction, the FeK1.5/HSG|HZSM-5(50) catalyst still exhibited high STYHC of 17.3 μmolCO2 gcat−1 s−1, which can be attributed to the coupling of the olefination–aromatization reaction and the increased content of iron carbides in the tandem catalyst. Ethylene as the Reactant. On the FeK1.5/HSG|HZSM-5(50) catalyst, ethylene was directly used as the reactant under otherwise the same reaction conditions described above. As expected, aromatics readily formed with the selectivity of 26% at ethylene conversion of 20% (Table S4), further proving the occurrence of the olefination–aromatization reaction pathway on this tandem catalyst. However, the aromatics selectivity is lower than that using CO2 and H2 as the reactants.

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Unlike other light olefins, the oligomerization of ethylene is not efficiently catalyzed by acid catalysts, as the primary carbenium cation of ethylene is not as stable as the secondary carbenium cations.25 As illustrated in Figure 2A, aside from ethylene, the FeK1.5/HSG catalyst also generated substantial amounts of propylene and butylene during CO2 hydrogenation, which are more reactive than ethylene in oligomerization, thus justifying the higher aromatics selectivity using a mixture of CO2 and H2 on the FeK1.5/HSG|HZSM-5(50) catalyst. Similarly, Li and co-workers found that when methanol or ethylene was directly used as the reactant, the aromatics selectivity is less than one half of that from CO2 and H2.5 Liu and co-workers identified that the presence of H2O or CO2 can promote the conversion of ethylene and propylene into aromatics on HZSM-5.26 They proposed that H2O formed during the reaction may increase the strength of the acid sites on HZSM5,5,26 while CO2 may facilitate hydrogen transfer,26 which are essential for the occurrence of aromatization at relatively low reaction temperature. Combination Mode. On the metal oxide–HZSM-5 catalysts for converting CO2 to aromatics, the catalysts prepared by grinding both components together exhibited the highest aromatics selectivity. Other combination modes, including homogeneous mixing and dual-layer packing, resulted in more aliphatic hydrocarbons. It was concluded that the closer both components is, the higher the aromatics selectivity is.4,5,7 On the homogeneously mixed FeK1.5/HSG + HZSM-5(50) catalyst, however, we found that the aromatics selectivity (48%) is much lower than that on the FeK1.5/HSG|HZSM-5(50) duallayer catalyst with a longer distance between the two catalyst components (Figure 4). On the contrary, for the metal oxide–HZSM-5 catalysts, the dual-layer combination mode gave the poorest aromatics selectivity.4,5,7 These experimental facts indicate that the synergy between the active sites at the metal oxide–HZSM-5 interface, rather than the acid sites on HZSM-5 alone, is

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important for the immediate transformation of the methoxyl species into the olefin intermediates. Otherwise, the methoxyl species would be transformed into the aliphatic hydrocarbons on HZSM5, a reaction that competes for the formation of aromatics.27 The highest aromatics selectivity on the metal oxide/HZSM-5 catalysts prepared by grinding can thus be rationalized by the maximizing of the metal oxide–HZSM-5 interface for the generation of the olefin intermediates. On the FeK1.5/HSG + HZSM-5(50) catalyst, because FeK1.5/HSG is far more active than ZnAlOx and ZnZrO, the hydrogenation/hydrogenolysis of the oligomerization products transferred from HZSM-5(50) to the neighboring FeK1.5/HSG is inclined to occur, which produces more CH4, C2– C4, and C5+ products as manifested in Figure 4. According to the above catalytic results, the formation of aromatics from CO2 and H2 on the FeK1.5/HSG|HZSM-5(50) tandem catalyst can be divided into two consecutive steps (Scheme 1): (i) Light olefins are generated on the FeK1.5/HSG surface via CO2 hydrogenation, and (ii) light olefins diffuse into the micropores of HZSM-5, in which they are converted to aromatics. Since the formation of light olefins is thermodynamically more favorable than the formation of methanol from CO2 hydrogenation (Figure 1A), and FeK1.5/HSG is highly active in CO2 hydrogenation, the olefination–aromatization route is very efficient in producing aromatics from CO2. Because light olefins are direct precursors to aromatics, intimate contact between the two catalyst components is not necessary, which affords the opportunity to further improve the aromatics productivity on a temperature-gradient reactor to optimize the thermodynamics and kinetics of respective steps. Moreover, the aromatics distribution can be readily tuned by changing the type of the zeolite layer, as will be elaborated below. Zeolite Type: Effect of Pore Size. Aside from HZSM-5(50), we combined FeK1.5/HSG with SAPO-34, HY, Hβ, and HMCM-22 in the dual-layer mode to gain an insight into the zeolite type

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on the aromatics selectivity. The structural characters of the zeolites are compiled in Table S5. Figure 5 reveals that the aromatics selectivity was closely dependent on the zeolite type. Among these zeolites, SAPO-34 with the smallest pore size of 3.8 Å  3.8 Å gave the lowest aromatics selectivity, as the pore is too small even for the smallest ethylene molecule with the kinetic diameter of 3.9 Å to smoothly access the acid sites inside. Although both HY and Hβ have 12member large pores of 7.4 Å  7.4 Å and 6.6 Å  6.7 Å, respectively, Hβ has an additional 12member small pore of 5.6 Å  5.6 Å. Benzene has a kinetic diameter of 5.85 Å and a width of about 5 Å as calculated from bond lengths and bond angles. The kinetic diameters of toluene and p-xylene are about the same.28 Hence, Hβ displayed better shape-selectivity towards aromatics than HY. HMCM-22, similar to HZSM-5, has two sets of 10-member pores of 4.0 Å  5.5 Å and 4.1 Å  5.1 Å, but they are slightly smaller than those of HZSM-5 (5.1 Å  5.5 Å and 5.3 Å  5.6 Å), thus giving the second highest aromatics selectivity. It can be concluded that over the FeK1.5/HSG|zeolite tandem catalysts, the aromatics selectivity is mainly determined by the pore size of the zeolite, while the pore dimension (2- or 3-dimension) and the ring member (10 or 12) are not decisive, and the preferred pore diameter for aromatics formation is in the range of 5–6 Å. Effect of Acidity. Figure 5 also compares the effects of the cation (H+ and Na+) and the SiO2/Al2O3 ratio (27, 50, and 160) of ZSM-5 on the aromatics selectivity. Their NH3-TPD results are presented in Figure S1 and Table S6. With identical SiO2/Al2O3 ratio, NaZSM-5(50) with the exchange degree of 80% is less selective to aromatics than HZSM-5(50). Nevertheless, its aromatics selectivity is higher than that on HMCM-22, substantiating again the critical role of the pore size in aromatics formation. For H+-type ZSM-5, the aromatics selectivity is the highest with the SiO2/Al2O3 ratio of 50. Decreasing or increasing the ratio only lowered the aromatics selectivities. Thus, for ZSM-5 with identical pore structure, the effect of the acid property becomes

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prominent. HZSM-5(160) with the lowest acid amount (Table S6) is not advantageous to the aromatization of the olefins. When the acid sites are in excess, the formation of the C5+ paraffins and the cracking of the heavy hydrocarbons to the C1–C4 products are enhanced, as clearly evidenced by the product distribution on HZSM-5(27) (Figure 5). Zeolite-Dependent Aromatics Distribution. Figure 6 compares the aromatics distribution on NaZSM-5(50), HZSM-5(50), and HMCM-22 in combination with FeK1.5/HSG with comparatively high aromatics selectivities. Interestingly, on NaZSM-5(50), benzene and toluene are the main aromatics, while on HZSM-5(50) and HMCM-22, the dominant aromatics are ethylbenzene and propylbenzene, respectively. HZSM-5(50) also gives the highest selectivity to the xylenes among these zeolites. Therefore, taking advantage of the dual-layer combination mode of FeK1.5/HSG and zeolite, one can tune the aromatics products simply by switching the zeolite component, thus endowing the CO2 olefination–aromatization process unprecedented flexibility in producing various aromatics. Effect of Reaction Conditions: FeK1.5/HSG to HZSM-5(50) Weight Ratio. Figure 7 shows the effect of the FeK1.5/HSG to HZSM-5(50) weight ratio on converting CO2 to aromatics. By keeping the weight of FeK1.5/HSG and the SV relative to FeK1.5/HSG constant, increasing the amount of HZSM-5(50) to attain the ratio from 5: 1 to 1: 1 increased aromatics while reduced the CH4 and C2–C4 products. Further increasing the amount of HZSM-5(50) to the ratios of 1: 3 and 1: 5 reduced aromatics. On the other hand, the selectivity to the C5+ products was the highest at the ratio of 5: 1, minimized at the ratio of 1: 1, and then increased at higher amounts of HZSM5(50). These results suggest that on HZSM-5(50) the aromatization of the olefins is kinetically slower than the oligomerization of the olefins.17 At the ratio of 5: 1, the short contact time of the olefins with HZSM-5(50) does not suffice all the olefins to undergo aromatization. When the

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amount of HZSM-5(50) is in excess, the cracking of aromatics becomes significant due to the prolonged contact time, thus deteriorating the aromatics selectivity. Reaction Temperature. Figure 8A shows the evolutions of the aromatics selectivity, STYHC, and STYaro against the reaction temperature over the FeK1.5/HSG|HZSM-5(50) catalyst at 2.0 MPa. The aromatics selectivity increased obviously from 573 K to 613 K, and then decreased reluctantly at higher temperatures. High temperature kinetically favors the aromatization of light olefins over HZSM-5.29,30 However, due to the simultaneous rise of the cracking rates,31 more light hydrocarbons (C1–C4) were formed (Table S7). On the other hand, both the olefination and aromatization reactions are in general thermodynamically favored at low temperature (Figure 1). These kinetic and thermodynamic factors result in an optimal temperature for the production of aromatics. The STYaro also maximized at 613 K, but decreased more significantly than the aromatics selectivity at higher temperatures, which is mainly caused by the faster decrease in the STYHC. Reaction Pressure. The effect of the reaction pressure on the catalytic performance of the FeK1.5/HSG|HZSM-5(50) catalyst at 613 K is illustrated in Figure 8B. The aromatics selectivity and STYaro increased significantly with the pressure from 1.0 to 2.0 MPa, and then decreased moderately at higher pressures. Thermodynamically, the oligomerization of light olefins is favored at high pressure,25 which is consistent with the monotonic increase in the selectivity to high molecular-weight hydrocarbons (C5+ and aromatics) with the pressure (Table S8). On the other hand, the aromatization of the olefins on HZSM-5 involves not only oligomerization, but also cyclization and dehydrogenation,17 with dehydrogenation being believed as the rate-limiting step.5 Hence, the formation of aromatics is suppressed at higher reaction pressure due to the restricted dehydrogenation at increased H2 pressure.

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Stability. The FeK1.5/HSG|HZSM-5(50) catalyst with high aromatics selectivity and exceptional activity was subjected to a 120 h stability testing; the results are illustrated in Figure 9. The STYHC increased to 17.3 μmolCO2 gcat–1 s–1 at 6 h on stream, and remained above 17.2 μmolCO2 gcat–1 s–1 within 24 h on stream. The STYHC then decreased slightly and reached the steady state with the value of ca. 16.2 μmolCO2 gcat–1 s–1 after 50 h on stream. The change in the aromatics selectivity is not significant, which retained at above 66% throughout the reaction. The selectivity to the C5+ products remained constant at around 24%. The CH4 selectivity increased slightly, and the selectivity to the C2–C4 products decreased slightly during the reaction. Similarly, a slight increase in CH4 selectivity with time on stream was observed on the ZnFeOx-nNa/HZSM-5 catalyst with relatively high STYaro,14 while the reason remains to be explored. Nevertheless, these results manifest the robustness of both the FeK1.5/HSG and HZSM-5(50) components combined in the dual-layer mode in converting CO2 to aromatics. For this reaction, sintering and coking are main factors that cause catalyst deactivation. We have confirmed that the porous HSG can effectively restrict the sintering of the iron carbide NPs during CO2 hydrogenation under identical reaction conditions.15 On the other hand, CO2 and H2O have been used to regenerate the zeolite catalysts deactivated by coking,32,33 while they are just one of the reactants and products in the present case. Moreover, Li and co-workers discovered that the formation of polycyclic aromatics on HZSM-5 is significantly suppressed in CO2 hydrogenation to aromatics as compared to that directly using methanol as the reactant over the ZnZrO/HZSM-5 catalyst. The polycyclic aromatics have been regarded as the precursors for coke formation, which causes the deactivation of most zeolite catalysts in hydrocarbon conversion.5

CONCLUSIONS

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We have demonstrated that on the FeK1.5/HSG|HZSM-5(50) catalyst combined in the duallayer mode, high selectivity and unprecedentedly high productivity of aromatics can be realized simultaneously by means of CO2 hydrogenation. The working mechanism of this tandem catalyst is based on olefination of CO2 over the potassium-promoted iron carbide sites, followed by aromatization of light olefins over the acid sites of the zeolite. Unlike the existing metal oxide/HZSM-5 catalysts based on methanol synthesis and aromatization that call for close proximity of both catalyst components, the composition of aromatics on the dual-layer catalyst can be readily tuned by changing the zeolite layer, which endows this process unprecedented flexibility in producing desired aromatics molecules. The work shows promise for the development of a highefficiency and versatile CO2-to-aromatics technology, which propels the inversion of CO2 from a notorious greenhouse gas to a valuable carbon source.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.xxxxxxx. Characterization techniques, Figure S1, and Tables S1–S8 as described in the text (PDF)

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (M. H. Qiao). * E-mail: [email protected] (B. N. Zong). Notes

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The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Key R&D program of China (2017YFB0602204), the National Natural Science Foundation of China (21872035), the Science and Technology Commission of Shanghai Municipality (08DZ2270500), the International Joint Laboratory on Resource Chemistry (IJLRC), and the Shanghai Synchrotron Radiation Facility (SSRF). REFERENCES 1.

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10. Wei, J.; Ge, Q. J.; Yao, R. W.; Wen, Z. Y.; Fang, C. Y.; Guo, L. S.; Xu, H. Y.; Sun, J. Directly Converting CO2 into a Gasoline Fuel. Nat. Commun. 2017, 8, 15174, DOI 10.1038/ncomms15174. 11. Kuei, C. K.; Lee, M. D. Hydrogenation of Carbon Dioxide by Hybrid Catalysts, Direct Synthesis of Aromatics from Carbon Dioxide and Hydrogen. Can. J. Chem. Eng. 1991, 69, 347−354, DOI 10.1002/cjce.5450690142. 12. Xu, Y. B.; Shi, C. M.; Liu, B.; Wang, T.; Zheng, J.; Li, W. P.; Liu, D. P.; Liu, X. H. Selective Production of Aromatics from CO2. Catal. Sci. Technol. 2019, 9, 593−610, DOI 10.1039/C8CY02024H. 13. Ramirez, A.; Chowdhury, A. D.; Dokania, A.; Cnudde, P.; Caglayan, M.; Yarulina, I.; AbouHamad, E.; Gevers, L.; Ould-Chikh, S.; de Wispelaere, K.; Van Speybroeck, V.; Gascon, J.

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FIGURE CAPTIONS

Figure 1. Gibbs free energy changes. (A) CO2 hydrogenation to methanol or ethylene as a representative for light olefins, and (B) aromatization of ethylene to some typical aromatics molecules. Figure 2. Effect of HZSM-5 on product distribution. (A) Over the FeK1.5/HSG catalyst alone, and (B) over the FeK1.5/HSG|HZSM-5(50) catalyst. Reaction conditions: 150 mg of FeK1.5/HSG, 150 mg of HZSM-5(50), 613 K, H2/CO2 = 3, 2.0 MPa, SV = 26,000 mL h−1 gcat−1 (relative to the total catalyst weight), and TOS = 24 h. Carbon number 5 in (B) represents the C5+ products exclusive of aromatics. Figure 3. Structural characterization of the FeK1.5/HSG component in the FeK1.5/HSG|HZSM5(50) catalyst after 24 h on stream. (A) TEM image, (B) HRTEM image, (C) synchrotron-radiation XRD pattern, and (D) 57Fe Mössbauer spectrum and fitted sub-spectra. Inset compares the contents of the iron species in the absence (top) and presence (bottom) of HZSM-5(50) after reaction. Figure 4. Effect of combination mode of FeK1.5/HSG and HZSM-5(50) on CO2 conversion and product selectivity. Reaction conditions: 150 mg of FeK1.5/HSG, 150 mg of HZSM-5(50), 613 K, H2/CO2 = 3, 2.0 MPa, SV = 26,000 mL h−1 gcat−1 (relative to the total catalyst weight), and TOS = 24 h. The C5+ selectivity is exclusive of aromatics. Figure 5. Effect of zeolite type on CO2 conversion and product selectivity over the FeK1.5/HSG|zeolite catalysts. Reaction conditions: 150 mg of FeK1.5/HSG and 150 mg of zeolite, 613 K, H2/CO2 = 3, 2.0 MPa, SV = 26,000 mL h−1 gcat−1 (relative to the total catalyst weight), and TOS = 24 h. a Exclusive of aromatics.

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Figure 6. The aromatics spectra on NaZSM-5(50), HZSM-5(50), and HMCM-22 in combination with FeK1.5/HSG. Reaction conditions: 150 mg of FeK1.5/HSG and 150 mg of the zeolite, 613 K, H2/CO2 = 3, 2.0 MPa, SV = 26,000 mL h−1 gcat−1 (relative to the total catalyst weight), and TOS = 24 h. Figure 7. Effect of the FeK1.5/HSG to HZSM-5(50) weight ratio on the catalytic performance of the FeK1.5/HSG|HZSM-5(50) catalyst. Reaction conditions: 150 mg of FeK1.5/HSG and 30, 50, 150, 450, or 750 mg of HZSM-5(50), 613 K, H2/CO2 = 3, 2.0 MPa, SV = 26,000 mL h−1 g−1 for bare FeK1.5/HSG and SV = 52,000 mL h−1 g−1 relative to FeK1.5/HSG for tandem catalysts, and TOS = 24 h. a Exclusive of aromatics. Figure 8. Effects of reaction temperature and pressure on the aromatics selectivity, STYHC, and STYaro over the FeK1.5/HSG|HZSM-5(50) catalyst. (A) Effect of the reaction temperature. Other reaction conditions: 150 mg of FeK1.5/HSG and 150 mg of HZSM-5(50), H2/CO2 = 3, 2.0 MPa, and TOS = 24 h. Data are reported at comparable CO2 conversion levels between 34% and 36% by adjusting the SV. (B) Effect of the reaction pressure. Other reaction conditions: 150 mg of FeK1.5/HSG and 150 mg of HZSM-5(50), 613 K, H2/CO2 = 3, and TOS = 24 h. Data are reported at comparable CO2 conversion levels between 35% and 45% by adjusting the SV. Figure 9. Stability of the FeK1.5/HSG|HZSM-5(50) catalyst in converting CO2 to aromatics. Reaction conditions: 150 mg of FeK1.5/HSG and 150 mg of HZSM-5(50), 613 K, H2/CO2 = 3, 2.0 MPa, and SV = 26,000 mL h−1 gcat−1 (relative to the total catalyst weight). The hydrocarbon selectivities are normalized with the exception of CO.

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Figure 1.

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Figure 2.

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Figure 3. A

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30

20

20

10

10

0

No zeolite

Homogeneous mixing

Dual layer

CO2 conversion and CO selectivity (%)

80

Hydrocarbon distribution (%)

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

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0

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Figure 5.

Aromatics 100

90

90

80

80

70

70

60

60

50

50

CO

40

40

30

CO2

30

0 60

0)

(1

H

ZS

M

-5

-5

(5

7) M

-5

ZS

M ZS H

aZ

(2

(5 -5

M

SM

C M H

N

H

2 -2

β H

H

PO -3

SA

ze

)

0 0)

10

Y

10

4

20

ol ite

20

o

Hydrocarbon distribution (%)

C5+a

C2-C4

CO2 conversion and CO selectivity (%)

CH4

100

N

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

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Figure 6.

40 C8

C9

C10 NaZSM-5(50) HZSM-5(50) HMCM-22

30 25 20 15 10 5

en yl e be hy nz le m th en et yl e hy b en lp ro ze py ne lb en ze ne du re ne

e

lu

en

lto

di m

et

pr op

hy et

tr im et

hy

lb

en z

es

e

le n

xy

nz en

ne

lb e

ue

et hy

to l

en e

0

nz

Selectivity (%)

C7

C6

35

be

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

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Figure 7.

C2-C4

C5+a

Aromatics

100

90

90

80

80

70

70

60

60

50

50

CO

40

40 30

30

CO2

20

20

10

10

CO2 conversion and CO selectivity (%)

CH4

100

Hydrocarbon distribution (%)

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

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0

0 No zeolite

5:1

3:1

1:1

1:3

1:5

FeK1.5/HSG:HZSM-5 weight ratio

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Figure 8.

Aromarics STYHC

60

STYaro

20

A

Selectivity (%)

50 40

10

30 20

5

STY (μmolCO2 gcat1 s1)

70

15

10 0 613

573

0

693

653

Temperature (K)

60

20

Aromatics STYHC

B

STYaro

15

50 40

10 30 20

5

STY (μmolCO2 gcat1 s1)

70

Selectivity (%)

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

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10 0 1.0

1.5

2.0

2.5

3.0

0

Pressure (MPa)

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Figure 9.

20

90

15

1

60

1

70

STY (μmolCO2 gcat s )

80

Selectivity (%)

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

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CH4

50

C2-C4

Aromatics

40

C5+

10

STYHC

30 5

20 10

0

0 0

20

40

60

80

100

120

Time on stream (h)

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SCHEME 1. Illustration of the olefination–aromatization reaction pathway for converting CO2 to the aromatics over the FeK1.5/HSG|HZSM-5(50) tandem catalyst.

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Table of Contents

Coupling of honeycomb-structured graphene supported FeK with HZSM-5 affords unprecedentedly high productivity of aromatics from greenhouse gas CO2 hydrogenation.

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