Rationally Designing Bifunctional Catalysts as an Efficient Strategy To

Dec 18, 2018 - Rationally Designing Bifunctional Catalysts as an Efficient Strategy To Boost CO2 Hydrogenation Producing Value-Added Aromatics...
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Rationally Designing Bifunctional Catalysts as an Efficient Strategy to Boost CO2 Hydrogenation Producing Value-Added Aromatics Yang Wang, Li Tan, Minghui Tan, Peipei Zhang, Yuan Fang, Yoshiharu Yoneyama, Guohui Yang, and Noritatsu Tsubaki ACS Catal., Just Accepted Manuscript • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 18, 2018

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ACS Catalysis

Rationally Designing Bifunctional Catalysts as an Efficient Strategy to Boost CO2 Hydrogenation Producing Value-Added Aromatics Yang Wanga, Li Tana, Minghui Tanb, Peipei Zhanga, Yuan Fanga, Yoshiharu Yoneyamaa, Guohui Yanga,b,*, Noritatsu Tsubakia,* aDepartment

of Applied Chemistry, Graduate School of Engineering, University of Toyama, Gofuku 3190, Toyama 9308555, Japan bState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China ABSTRACT: The efficient conversion of CO2 to useful chemicals is a promising way to reduce the atmospheric CO2 concentration, and also reduce reliance on the fossil-based resources. Although lots of progress have been made on the production of basic chemicals, like methanol, through CO2 hydrogenation, the direct conversion of CO2 to value-added aromatics, especially para-xylene (PX), is still in a great challenge due to the inertness nature of CO2 and high barrier for C-C coupling. Herein a bifunctional catalyst composed of Cr2O3 and H-ZSM-5 zeolite (Cr2O3/H-ZSM-5) was designed for the direct conversion of CO2 to aromatics. Due to the concertedly synergistic effect between the two components in this bifunctional catalyst, aromatics selectivity of ~76% at CO2 conversion of 34.5% was achieved, and there was no catalyst deactivation after 100 h of long-term stability test. Moreover, a modified bifunctional catalyst Cr2O3/H-ZSM-5@S-1 (silicalite-1) consisting of a core-shell structured H-ZSM-5@S-1 zeolite capsule component was proposed to realize the target synthesis of BTX (benzene, toluene, and xylene), especially PX (paraxylene). The precise suppression on undesired side reactions was accomplished on the Cr2O3/H-ZSM-5@S-1, because neutralizing acidic sites at outer surface of H-ZSM-5 by S-1 stopped isomerization of PX to ortho- or meta-xylene, as well as other side reactions. Consequently, the fractions of BTX and PX in aromatics products were increased from 13.2% and 7.6% to 43.6% and 25.3%, respectively, with almost unchanged catalyst activity and total aromatics selectivity. KEYWORDS: CO2 conversion, Aromatics synthesis, Bifunctional catalysts, Tandem process, Single-pass INTRODUCTION Over the past centuries, the excessive reliance on fossil fuels has resulted in huge amounts of CO2 emissions. And CO2 is the most major cause to some serious environmental concerns, such as global warming, ocean acidification, climate change, etc.1 Therefore, the effective conversion of CO2 into value-added chemicals not only controls the CO2 concentration in the atmosphere, but also offers an alternative way instead of the fossil resources, realizing an sustainable human society.2,3 Accordingly, various CO2 conversion strategies, including electrochemical, photochemical, and thermochemical methods have been proposed in the past decades.4–6 Among all of the methods, thermochemical CO2 conversion has appealed great attention due to its high efficiency, controllable selectivity, and tremendous potential for industrial application.7,8 CO2 hydrogenation is the easiest way for the direct CO2 conversion into valuable chemicals. Wide attention has been paid to CO2 hydrogenation to basic chemicals, such as CO, CH3OH, HCOOH, and CH4.9–13 However, the inertness of CO2 and high kinetic barriers for C-C bond formation hamper the opportunity for long-carbon-chain growth. It is still a challenge to synthesize long-chain hydrocarbons directly from CO2.14–16 Recently, a direct CO2 conversion route was developed to synthesize gasoline-range hydrocarbons, in which CO2 was firstly reduced to CO via reverse water-gas shift (RWGS) reaction and then the formed CO was hydrogenated into hydrocarbons via Fischer-Tropsch synthesis (FTS) process.17 Furthermore, bifunctional catalysts being composed of methanol synthesis catalyst and acidic zeolite were

also proposed for the efficient conversion of CO2 to C5+ hydrocarbons.18,19 Aromatics, as the most important platform molecules for polymer industry, are mainly produced from petroleum refinement process. With the diminishing of the crude oil reserves, exploration on an alternative strategy for aromatics synthesis is required to meet the growing demand.20,21 So far, there has been a lot of studies on the direct conversion of syngas (CO + H2) to aromatics, employing a methanol mediated pathway or via a Fischer-Tropsch synthesis process.22–26 The direct and selective conversion of syngas to para-xylene (PX) was even realized over a hybrid catalyst comprised of ZnCr2O4 spinel and core-shell structured zeolite.27 Furthermore, CO2 was also employed as alternative carbon sources for aromatics synthesis via an indirect route. For example, our group had realized CO2 conversion to aromatics in a combined catalysis system, in which CO2 methanation and the methane aromatization were performed in two consecutive reactors with different reaction conditions.28 However, the low efficiency of this process encourages us to develop a novel strategy to realize the direct conversion of CO2 to aromatics with excellent catalytic performance. In this work, we present a novel tandem catalysis model for the direct conversion of CO2 to aromatics, during which a methanolmediated pathway was realized over a bifunctional catalyst composed of Cr2O3 and H-ZSM-5 zeolite (Cr2O3/H-ZSM-5, Scheme 1). Surprisingly, high aromatics selectivity (~76%) with a single-pass CO2 conversion of 34.5% was achieved. Meanwhile, the undesired C1 products such as CH4 and CO were successfully suppressed. In addition, no obvious catalyst deactivation was

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detected during 100 h long-term stability test. The synergistic effect between Cr2O3 and H-ZSM-5 endowed excellent performance for aromatics synthesis in this tandem process by using CO2 as feedstock. In order to realize more commercial merits in the aromatics products, we further designed an improved CO2 conversion route, where a core-shell structured zeolite capsule catalyst was employed and enhanced the selectivity of more valuable aromatics such as BTX (benzene, toluene, and xylene) remarkably. By using the new capsule zeolite component, the outer surface acid sites of zeolite, where the undesirable alkylation and isomerization of para-xylene (PX) reaction usually occurred, were successfully passivated by a non-acidic silicalite-1 zeolite shell (H-ZSM-5@S-1). As a result, higher selectivity of BTX (benzene, toluene, and xylene, 43.6%) and PX (25.3%) were achieved over the bifunctional catalyst Cr2O3/H-ZSM-5@S-1. To the best of our knowledge, this is the first study to report the direct conversion of CO2 to aromatics in a single-pass. This work not only provides a promising strategy for the direct conversion of CO2 to aromatics, but also enriches the horizon of the value-added utilization of CO2.

RESULTS AND DISCUSSION

Figure 1. The catalytic performance of varied catalysts for CO2 conversion to aromatics. CH4, black; CH3OH+DME, magenta; C2-C4, blue; aromatics, red; non-aromatics C5+, green. The number in the parenthesis of the catalyst name correspond to the Si/Al ratio of H-ZSM-5. Reaction conditions: 350 oC, 3 MPa, H2/CO2 = 3 (5.42vol% CO), 10 ml·min-1, TOS = 6 h. Catalyst weight, 0.5 g, oxide/zeolite mass ratio = 1, 0.25 g Cr2O3 for methanol synthesis. *CO2 hydrogenation reaction over Cr2O3 or Cr2O3/H-ZSM-5 without CO contained in the feed gas.

Scheme 1. Illustration on the direct conversion of CO2 to aromatics over the bifunctional catalyst. The combination of methanol synthesis with the sequential methanol to aromatics (MTA) reaction in a tandem catalysis process is a significant challenge because of the different operating conditions. Typically, MTA reaction usually proceeds at high temperature (≥ 400 oC) due to the kinetic requirement, while exothermal methanol synthesis is unfavorable at such high temperature. Theoretically, our analysis based on Le Chatelier’s principle manifests that the CO2 direct conversion to aromatics is feasible due to the thermodynamically driving force in the tandem process (Figure S1). The methanol synthesis process, providing intermediates for the following MTA reaction, is the limited step in the tandem catalysis of CO2 to aromatics. Hence, the first challenge is to find an appropriate high-temperature methanol synthesis catalyst that matches well with the subsequent MTA reaction. In our research, we found that Cr2O3 catalyst achieved methanol and DME selectivity up to 97% with 29.1% of CO under 350 oC and 3.0 MPa, suggesting it a promising catalyst candidate for the tandem catalysis reaction to efficiently convert CO2 to aromatics (Figure 1).

Figure 2. TEM images of Cr2O3 (a) and H-ZSM-5 (b), SEM (c) and TEM (d) images of Cr2O3/H-ZSM-5. Cr2O3 has been widely studied as a high-temperature methanol synthesis catalyst from syngas (CO + H2). The oxygen vacancy (Ovacancy) on the surface of Cr2O3 catalyst are beneficial to C-O bond activation, meanwhile making Cr2O3 catalyst possible for CO2 activation and hydrogenation for methanol synthesis.29,30 The oxygen vacancies in Cr2O3 were characterized by X-ray photoelectron spectrum (XPS, Figure S2), the high resolution peak at 531.0 eV can be attributed to the oxygen atoms adjacent to vacancies (Ovacancy).31–33 The chemisorbed CO2* species at Ovacancy can be hydrogenated to methanol through a formate route, as proved by former researchers.19,34,35 Cr2O3 prepared by precipitation method is composed of uniformly dispersed nanoparticles, as shown in the TEM and SEM images (Figure 2a and Figure S3b). The XRD diffraction peaks of Cr2O3 correspond well with the eskolaite that has hexagonal structure (Figure S4). The lattice fringes with 0.36 nm interspace, as shown in the high resolution TEM (HRTEM, Figure S5), belong to (012) lattice plane of Cr2O3. Since the expectable performance of Cr2O3 catalyst in methanol synthesis, H-ZSM-5 as a typical MTA reaction catalyst was introduced for aromatics direct synthesis from CO2 via a tandem

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ACS Catalysis catalysis model. H-ZSM-5 zeolites with varied Si/Al ratios were prepared by a hydrothermal method and the corresponding texture properties are listed in Table S1. Surprisingly, we found that the combination of Cr2O3 with H-ZSM-5 (Si/Al=40) as a bifunctional catalyst (Cr2O3/H-ZSM-5) can facilely realize aromatics direct synthesis with 75.9% selectivity at a single-pass CO2 conversion of 34.5% (Figure 1 and Table S2). As shown in Figure 2c and 2d, Cr2O3 was successfully mixed with H-ZSM-5 with fractions of Cr2O3 nanoparticles uniformly dispersed on the hexagonal HZSM-5 crystals (Figure S3a). The Si, Al, and Cr distributed uniformly in this bifunctional catalyst as shown in the corresponding elemental mapping images (Figure S3c). As we know, methanol synthesis from CO2 hydrogenation usually accompanied with the undesirable CO formation via the reverse water gas shift reaction (RWGS, CO2 + H2 = CO + H2O), which will be enhanced along with the increase of reaction temperature due to its endothermic property. But the formation of CO, on the bifunctional catalyst Cr2O3/H-ZSM-5, was obviously suppressed to a selectivity of only 11.4%. The lower CO selectivity can be attributed to the co-feeding of 5.42vol% CO in the feed gas. The catalytic performance of pure CO2 hydrogenation to aromatics was also performed at 350 oC and 3 MPa. A large amount of CO2 was converted to CO with a selectivity of 41.2% (Figure 1 and Table S3). The presence of CO in the feed gas can suppress the undesirable RWGS reaction due to the chemical equilibrium. Sun et al. verified the effect of CO co-feeding on the catalytic performance of the tandem CO2 conversion by varying the CO content in the feed gas. With the increase of CO content, the CO selectivity was significantly suppressed, suggesting that more CO2 was converted into valuable chemicals.19,33 On the other hand, the oxygen vacancy in Cr2O3 as characterized by XPS (Figure S2) has been recognized as active site for CO2 activation. Cr2O3, as a reducible metal oxide, can activate CO2 abiding by a mechanism: the cyclic creation and annihilation of oxygen vacancies. The cofeeding of CO can be employed as reducing agent to rescue the oxygen vacancies destroyed by CO2, which guarantee the efficient activation of the CO2 during the reaction.36 In order to clarify the state of oxygen vacancy of Cr2O3 with or without the presence of CO, the CO2 temperature-programmed desorption (CO2-TPD) of Cr2O3 pretreated in H2 or CO were performed. As shown in Figure S6, aside from a peak derived from the physisorbed CO2 at about 100 oC, two additional peaks between 200 and 500 oC assigned to the CO2 desorption from the oxygen vacancies were also detected. The CO2-TPD of Cr2O3 pretreated in CO exhibits higher CO2 desorption temperature and larger CO2 desorption amount compared with that of Cr2O3 pretreated in H2, which illustrated that the existence of CO during the reaction is favorable for the regeneration of oxygen vacancies. To further enhance its catalytic performance for aromatics synthesis, the optimal reaction conditions of this tandem process were further investigated. As listed in Table S2, the low temperature of 300 oC is unfavorable to aromatics synthesis due to the facts that the methanol dehydration forming olefins, oligomerization, cyclization, as well as aromatization of light hydrocarbons, require higher temperature.37 By increasing the reaction temperature to 350 oC, the selectivity of C2-4 olefins decreases, meanwhile the aromatics selectivity increases to ~76%. This result can be attributed to the higher temperature that efficiently facilitates the aromatics formation through the coupling of C2-4 olefins. Although higher reaction temperature of 400 oC further improves CO2 conversion up to 41.5%, the aromatics selectivity is lowered obviously. In fact, the over-higher reaction temperature led to the rise in the selectivity of methane and C2-4 paraffin. It is noteworthy that the selectivity of CO reaches 31.2% at 400 oC due to the enhanced RWGS reaction at high temperature. In addition, further increasing reaction pressure to 5 MPa has no obvious effect to improve the catalytic performance of Cr2O3/H-ZSM-5

for aromatics synthesis. We also combined other typical methanol synthesis catalysts, such as Cu-Zn-Al, In2O3, and Zn-ZrO2 with H-ZSM-5 zeolite for the same reaction, only the bifunctional catalyst Cr2O3/H-ZSM-5 exhibited significantly promising catalytic performance for aromatics synthesis directly from CO2 (Table S3). In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was performed to gain more insights into the mechanism of aromatics synthesis directly from CO2 (Figure S7). Under the reaction conditions, HCOO* species with asymmetric and symmetric OCO stretching vibrations at 1553 and 1354 cm-1, respectively, were observed for Cr2O3. The bands at 2924 and 2846 cm-1 can be attributed to the stretching vibrations of the C-H bonds.38,39 And HCOO* species have been widely recognized as the intermediates for methanol synthesis.35,40 For the bifunctional catalyst Cr2O3/H-ZSM-5, the peaks assigned to HCOO* almost disappeared, but the peaks of H3CO* at 1045 cm-1 and C-O-C at 1211 cm-1 appeared, indicating that the formation of ethers with the aid of H-ZSM-5 zeolite.41,42 The peaks at 1644 and 801 cm-1 can be assigned to the vibration of benzene ring skeleton and the substituents on benzene ring, respectively.43 The DRIFTS findings confirmed that the tandem process for CO2 hydrogenation to aromatics undergoes a methanol-mediated pathway. The catalytic performance of single H-ZSM-5 zeolite for methanol conversion has been tested. As shown in Table S4, 24.5% of aromatics was achieved at methanol conversion of 100%. The aromatics selectivity in MTA over H-ZSM-5 is lower than that of the bifunctional catalyst Cr2O3/H-ZSM-5 for direct conversion of CO2 to aromatics in a tandem process, which proves that the presented bifunctional catalyst is more beneficial for aromatics direct synthesis.

Figure 3. Catalytic performance of CO2 hydrogenation over the bifunctional catalyst with different proximity. a. physical mixing; b. granule mixing; c. dual bed. Reaction conditions: 350 oC, 3 MPa, H2/CO2 = 3 (5.42vol% CO), GHSV = 1200 ml·h-1·gcat-1, TOS = 6 h, and oxide/zeolite mass ratio = 1 (weight of bifunctional catalyst, 0.5 g).

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In order to clarify the roles of two components in the tandem process, the catalytic performances of Cr2O3 and Cr2O3/H-ZSM-5 were respectively performed under 3 MPa and 350 oC as a function of time factor (GHSV), as shown in Figure S8a and S8b. As the major products over Cr2O3, the selectivity of methanol and DME increased with shortening the contact time between pristine reactant gas with catalyst. For the tandem process over the bifunctional catalyst Cr2O3/H-ZSM-5, a volcano curve on the aromatics selectivity was observed with the increase of GHSV. The most suitable GHSV was about 1200 ml·h-1·gcat-1 considering CO selectivity. Lower reactant gas flow rate was not beneficial to the formation of aromatics. The increased GHSV could enhance the mass transfer during the tandem process, accelerating the transportation of the produced methanol over Cr2O3 to the channels of H-ZSM-5 for the sequential MTA reaction. This phenomenon is consistent with the catalytic performance over different catalyst intimacy modes, as compared in Figure 3 and Table S5. The closest proximity between Cr2O3 and ZSM-5 (mode a) led to the most excellent catalytic performance for aromatics production. Further increasing the intimacy of the two components by ball milling of Cr2O3 and ZSM-5 did not increase the selectivity of aromatics. With the prolonged distance between two components (mode b and c), the intermediate species produced on the Cr2O3 surface were difficult to reach the zeolite channels to initiate the sequential MTA step. What’s more, large amount of CH3OH was generated in the dual-bed integration manner (mode c) with the Cr2O3 catalyst being loaded above the H-ZSM-5 and isolated by quartz wool. It is obvious that spatial arrangement modes of the two active sites play a vital role in this tandem reaction.44,45 Zeolite supported catalyst was also prepared by a wetness impregnation method (denoted as Cr2O3/H-ZSM-5-I). Cr2O3/H-ZSM-5-I exhibits low CO2 conversion (14.9%, Table S5) and completely different products distribution, with light hydrocarbons (CH4 and C2-4 alkanes) as major products, compared with the bifunctional catalyst. So the efficient conversion of CO2 to aromatics in a tandem process should meet the following points: (a) sufficient amount of methanol synthesis catalyst for the following MTA process; (b) appropriate distance between the two active sites; (c) appropriate amount of exposed acidic sites of the zeolite. MTA reaction is an acid-catalyzed process, and the relatively strong acid strength is beneficial to the aromatics formation. It is no doubt that the variation of acidic property of H-ZSM-5 controls the product distribution in the tandem catalysis for CO2 hydrogenation. The NH3 temperature-programmed desorption (NH3-TPD) measurements on the acidic H-ZSM-5 zeolite with different Si/Al ratios (20, 40, 80, 120) exhibit two NH3 desorption peaks in each sample, and the peaks at high temperature are corresponded to the strong acid sites (Figure S9). The decreased NH3 desorption peak area and the lowered desorption temperature, along with the increased Si/Al ratios from 20 to 120, suggesting the weakening of the acid strength on zeolite samples. In the tandem reaction, as in Table S6, the aromatics selectivity obtained over the bifunctional catalyst Cr2O3/H-ZSM-5 with varied Si/Al ratios is increased almost linearly with the enhancement of the acidic sites density on the H-ZSM-5 zeolite component. This result confirms that strong acid sites facilitate aromatization reaction.46,47 However, Cr2O3/H-ZSM-5 (20) with too much higher density of acid sites exhibits lower selectivity to aromatics. Possibly, this can be explained by the overhydrogenation of C2-C4 olefins intermediates, on the excessive amount of strong acid sites, generating saturated C2-C4 hydrocarbons (32.6%).44,45 The catalytic performance can be tailored by varying the weight ratio of Cr2O3 and zeolite (Table S7). It clearly discloses that the bifunctional catalyst with the mass ratio of 1 : 1 exhibits the

highest aromatics selectivity. On the bifunctional catalyst with the mass ratio of 1 : 2, the products are mainly concentrated at liquid petroleum gas (LPG), since the hydrocracking effect occurred on the surface of acidic zeolites. The greater number of strong acid sites is responsible for the undesirable hydrocracking reaction as characterized by NH3-TPD (Figure S10). The bifunctional catalyst with fewer acidic zeolite, however, leads to higher C5+ selectivity (22.9%), which can be ascribed to the incomplete tandem process. As a vital factor for practical application, the long-term stability of the bifunctional catalyst Cr2O3/H-ZSM-5 was also investigated under 3 MPa and 350 oC, as in Figure 4. Both of CO2 conversion and aromatics selectivity remained stable at around 34% and 75% respectively during reaction of 100 h. The XRD pattern (Figure S4) and SEM image (Figure S11) of the spent catalyst revealed that the crystal size of Cr2O3 was unchanged after long-term stability test of 100 h. For other aromatics synthesis routes using methanol, propane, or methane as feedstocks over H-ZSM-5 zeolite, coke deposition is the key reason for catalyst deactivation.48–50 But the tandem process over bifunctional catalyst reported here overcomes this problem. The spent Cr2O3/H-ZSM-5 bifunctional catalyst realizes almost zero coke deposition even after 100 h long-term stability test, as demonstrated in the TGA result (Figure S12). The specific surface area, average pore diameter, and pore volume of the used Cr2O3/H-ZSM-5 have no obvious changes compared with the fresh Cr2O3/H-ZSM-5 (as shown in Table S1 and Figure S13), which demonstrates the excellent stability of the bifunctional catalyst Cr2O3/H-ZSM-5 in the tandem process. The high pressure H2 atmosphere plays a key role in suppressing the coke deposition on zeolite.25

Figure 4. The long-term stability of Cr2O3/H-ZSM-5 for aromatics synthesis directly from CO2. Reaction conditions: 350 oC, 3 MPa, H /CO = 3 (5.42vol% CO), GHSV = 1200 ml·h-1·g 2 2 cat 1, TOS = 6 h, and oxide/zeolite mass ratio = 1 (weight of bifunctional catalyst, 0.5 g). Although high aromatics selectivity was achieved over the bifunctional Cr2O3/H-ZSM-5 catalyst through a tandem catalysis process, the major products were trimethylbenzenes with a fraction of 40.1% and heavier aromatics (C≥10) of 22.6%. Among all kinds of aromatics, BTX (benzene, toluene, and xylene), especially para-xylene (PX), are recognized as the most valuable chemicals. It has been proved that the formed BTX inside the micropores of zeolites is inevitably alkylated to heavier aromatics while it is diffusing to the external surface of zeolites.51,52 PX is readily isomerized to ortho- and meta-xylene at outer surface acidic sites of H-ZSM-5. Hence, the passivation of the external acid sites of zeolite will be an effective strategy to increase the BTX and PX selectivity. Thus, a core-shell structured zeolite (H-

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ACS Catalysis ZSM-5@S-1) with H-ZSM-5 as core and neutral silicalite-1 (S-1) as shell was fabricated to design more promising bifunctional catalyst, whereby to realize higher BTX and PX selectivity in the tandem process of CO2 hydrogenation. To the prepared core-shell H-ZSM-5@S-1 zeolite, the H-ZSM-5 core was successfully encapsulated by S-1 shell, which was confirmed by the changed zeolite morphology and size, as shown in SEM images (Figure S14). It is clear that the original H-ZSM-5 with rounded hexagonal shape evolved into a sharp hexagonal structure of HZSM-5@S-1 after S-1 encapsulating. The acidity of the core-shell structured zeolite capsule was evaluated by NH3-TPD, and the results are compared in Figure S15. The remarkably weakened acidic strength of H-ZSM-5@S-1 indicates that the S-1 shell neutralized the surface acidic sites of H-ZSM-5. For the new bifunctional catalyst of Cr2O3/H-ZSM-5@S-1, the CO2 conversion and the total aromatics selectivity are still stable, comparable to those of Cr2O3/H-ZSM-5 catalyst (Table S8). It is interesting that the fractions of BTX and PX in aromatics were tremendously enhanced from 13.2% and 7.6% to 43.6% and 25.3%, respectively, as compared in Figure 5 and Table S9. This result further demonstrates that more value-added aromatics, like BTX and PX, can be precisely synthesized directly through CO2 hydrogenation over the modified bifunctional catalyst of Cr2O3/HZSM-5@S-1.

Figure 5. (a) Fractions of products in aromatics, (b) scheme of highly selective production of BTX over Cr2O3/H-ZSM-5@S-1. Reaction conditions: 350 oC, 3 MPa, H2/CO2 = 3 (5.42vol% CO), GHSV = 1200 ml·h-1·gcat-1, TOS = 6 h, and oxide/zeolite mass ratio = 1 (weight of bifunctional catalyst, 0.5 g).

CONCLUSION A bifunctional catalyst composed of Cr2O3 and H-ZSM-5 was developed for CO2 hydrogenation to aromatics directly through a tandem catalysis process. High aromatics selectivity of 75.9% with only 1.5% CH4 and 11.4% CO selectivity was achieved at the CO2 conversion of 34.5%. The bifunctional catalyst Cr2O3/HZSM-5 also exhibited excellent stability, and no catalyst deactivation was observed in 100 h long-term stability test. The catalytic performance could be further manipulated by tuning the acid strength of zeolites and the mass ratio of the oxide/zeolite. In particular, the integration mode demonstrated that the proximity between two types of active sites in the bifunctional catalyst played a crucial role in promoting the direct conversion of CO2 to aromatics. Furthermore, a modified bifunctional catalyst Cr2O3/HZSM-5@S-1 consisting of a core-shell structured H-ZSM-5@S-1 zeolite capsule catalyst was found to further realize the direct synthesis of more value-added aromatics, by CO2 hydrogenation. With suppression of undesired side reactions on the external surface of acidic zeolite by passivation method, the BTX and PX selectivity of the bifunctional catalyst Cr2O3/H-ZSM-5@S-1 were strikingly promoted from 13.2% and 7.6% to 43.6% and 25.3%, respectively, keeping comparable aromatics selectivity to that of unmodified bifunctional catalyst Cr2O3/H-ZSM-5 simultaneously. This report provides a very promising strategy for the direct conversion of CO2 to aromatics, especially BTX and PX, by which we can effectively diminish the greenhouse effect caused

by CO2, at the same time realize more economical utilization of CO2 for value-added chemicals synthesis.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

ACKNOWLEDGMENT This work was financially supported by the JST-ACT-C (No. JPMJCR12YT) and JST-MIRAI (JPMJMI17E2) projects of Japan Science and Technology Agency, Japan. Dr. Yang thanks the financial support from the National Natural Science Foundation of China (91645113 and U1510110) and the Key Research Program of Frontier Sciences, CAS, Grant No. QYZDB-SSW-JSC043.

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