Research Article Cite This: ACS Catal. 2019, 9, 5147−5156
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Selective Conversion of Syngas to Aromatics over Fe3O4@MnO2 and Hollow HZSM‑5 Bifunctional Catalysts Yanfei Xu,†,# Jingge Liu,‡,§,# Jie Wang,† Guangyuan Ma,† Jianghui Lin,† Yong Yang,∥ Yongwang Li,∥ Chenghua Zhang,*,∥ and Mingyue Ding*,†,⊥
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†
School of Power and Mechanical Engineering, Hubei International Scientific and Technological Cooperation Base of Sustainable Resource and Energy, Wuhan University, Wuhan 430072, China ‡ State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China § University of Chinese Academy of Sciences, Beijing 100049, China ∥ Synfuels China Co., Ltd., Beijing 101407, China ⊥ Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China S Supporting Information *
ABSTRACT: Although considerable efforts have been made toward converting syngas to liquid fuels and value-added chemicals, selectively converting syngas to aromatics remains a big challenge because of severe deactivation and low selectivity. Here, we reported a bifunctional catalyst composed of Fe3O4@ MnO2 and hollow HZSM-5, which could synthesize aromatics from syngas with a high selectivity of 57% at CO conversion >90%. The catalyst retained good stability for 180 h under industrially relevant conditions. The electron transfer from the Mn to Fe species in the core−shell Fe3O4@MnO2 catalyst promoted the formation of olefins intermediates, which were subsequently diffused onto the acid sites of HZSM-5, further converting to aromatics. Shortened channels and cavity structures of hollow HZSM-5 strengthened the diffusion of reactants and products, enhancing the catalyst stability via the suppression of carbon deposition. The present research provides insight into developing a potential bifunctional catalyst candidate for selectively converting syngas to aromatics. KEYWORDS: syngas to aromatics, Fe3O4@MnO2, hollow zeolite, stability, Fischer−Tropsch synthesis
1. INTRODUCTION Increasing energy demands and fast economy development have made the quest for alternative energy sources more urgent, especially clean liquid fuels. Syngas (CO/H2) can be derived from various carbon resources, such as coal, biomass, natural gas, and even waste. Fischer−Tropsch synthesis (FTS) is a promising technology for the conversion of syngas to various high value-added chemicals and liquid fuels and has been commercialized for decades. However, selectivity control is still challenging because the hydrocarbon products of FTS follow the wide and unselective Anderson−Schulz−Flory (ASF) distribution. The design of novel catalysts to optimize the hydrocarbons distribution will improve the FTS technology. Aromatics, as one of the most important basic chemicals (including over 30% petrochemicals), is widely applied for fuel additive and the production of polymers, resins, and films. Conventionally, aromatics are produced from petroleum cracking, which becomes more and more scarce with the growing shortage of petroleum resources and fast development of the economy.1−3 In recent years, new strategies for © 2019 American Chemical Society
production of aromatics have been developed from hydrogenation of carbon oxides (CO and/or CO2).4,5 Syngas can be transformed to aromatics via the intermediate methanol, known as syngas−methanol−aromatics (SMA).6,7 Alternatively, syngas can also be converted to olefins via the Fischer−Tropsch (F−T) synthesis route and then continually to aromatics (syngas−olefins−aromatics (SOA)).8 Cu−Znbased catalyst is generally chosen for syngas to methanol (220−300 °C),9−12 while a zeolite catalyst is selected for methanol to aromatics (>400 °C).1,13,14 The mismatch of reaction temperature in the two-step process restrains the yield of aromatics through SMA route. Due to the match of operating conditions between syngas to olefins (300−400 °C)15−17 and olefins to aromatics (300−500 °C),18−20 SOA technology has rapidly developed. In particular, the coupling of the Fe-based FTS catalyst with zeolite has drawn increasing attention in optimizing the aromatization Received: March 12, 2019 Revised: April 23, 2019 Published: April 29, 2019 5147
DOI: 10.1021/acscatal.9b01045 ACS Catal. 2019, 9, 5147−5156
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ACS Catalysis
Figure 1. Catalytic performance for syngas to aromatics. (A) CO conversion and hydrocarbon distribution over different F−T/zeolite catalysts. Reaction conditions: 320 °C, 2.0 MPa, 4000 mL·h−1·g−1, H2/CO = 1. The mass ratio of F−T/zeolite = 1:1 unless otherwise noted. (B, C) Comparison of product distribution over Fe3O4@MnO2 (B) and Fe3O4@MnO2−Hol HZSM-5 (27) (C). ASF plot and the chain growth probability (α value) comparison of the two catalysts are also given. Wn is the weight fraction of a product with n carbon atoms.(a) C5+ hydrocarbons except for aromatics. (b) Under optimum reaction conditions: 320 °C, 4.0 MPa, 4000 mL·h−1·g−1, H2/CO = 1.
performance. Guan et al.21 prepared an Fe−MnO/GaZSM-5 tandem catalyst, which presented 40% aromatics selectivity. Under the optimized reaction conditions, the Pd-modified Fe/ HZSM-5 catalyst exhibited 45% aromatics selectivity in liquid hydrocarbons.22 Recently, Zhao et al.8 coupled Na−Zn−Fe5C2 with the hierarchical HZSM-5, which presented 51% aromatics selectivity at a high CO conversion (above 85%). The combination of the Fe-based FTS catalyst with zeolite can exhibit high catalytic activity and aromatics selectivity, whereas zeolite deactivates rapidly in the SOA route, leading to a short lifetime. Yang et al.23 reported 48% aromatics selectivity over a Fe/Ni-HZSM-5 catalyst, but the reaction was only conducted for 12 h. A higher aromatics selectivity of 53% was obtained over a FeK-HZSM-5 catalyst, which remained stable only for 4 h.24 In the syngas to aromatics reaction, intracrystalline diffusion limitations inside micropores of zeolite restrain the diffusion of reactant and product molecules, which causes the inevitable carbon deposition covering on the acid sites of zeolite, resulting in the rapid deactivation of the catalyst.25 Numerous researchers design hierarchical zeolites to enlarge micropore structures, promoting the diffusion of reactant and product molecules as well as enhancing the catalyst stability.26−28 However, both the difficulties of controlling the desilication and/or dealumination process in forming hierarchical structures and the easy destruction of enlarged pore structures during reaction seriously restrict further industrial applications. Thus, designing a novel bifunctional catalyst with superior stability at high catalytic activity and aromatics selectivity is very challenging. Hollow materials (HM), which possess hollow cores and microporous/mesoporous shells, have gained increasing attention due to their excellent features of low density, large hollow interiors, high surface areas, and tunable shell thickness.29−33 The hollow cores within HM can provide large space for the storage of drugs and loading of active species, acting as a nanocontainer and nanoreactor. Fur-
thermore, microporous/mesoporous shells with tunable thickness and pore sizes enhance the mass transfer of reactants and products inside and outside of HM. The excellent physicochemical properties of HM provide us a new method to solve the obstacle in SOA. Herein, we have developed a novel bifunctional catalyst by coupling Fe3O4@MnO2 with hollow HZSM-5 zeolite, which presented an outstanding aromatics selectivity of 57% at CO conversion over 90%. No deactivation was observed within 180 h reaction over the bifunctional catalyst, demonstrating the superior catalytic stability reported in the SOA route. This work provides good potential in practical industrial applications for the selective conversion of syngas to aromatics.
2. RESULTS AND DISCUSSION 2.1. Catalytic Performance for SOA. The catalytic activity and product distribution in the SOA reaction are shown in Figure 1. As shown in Figure 1A, the traditional FeMnKSi catalyst exhibits 17.2% selectivity to CH4, 30.0% selectivity to C2−4 hydrocarbons (the ratio of olefins to paraffins (o/p) = 1.92), and 52.8% selectivity to C 5 + hydrocarbons (o/p = 1.23) at a CO conversion of 79.7% (Figure 1A, order 1), obeying an ideal ASF distribution. No aromatics are formed in the reaction. A combination of FeMnKSi with traditional HZSM-5 results in the formation of aromatics, accompanied by obvious decrease of olefins (Figure 1A, order 2−3). In addition, the species in the water phase are analyzed (Figure S1). The mass ratio of oxygenates/(oxygenates + hydrocarbons) is less than 2%, and large amounts of hydrocarbons are olefins over FeMnKSi. After combination of FeMnKSi with HZSM-5, the olefin selectivity decreases obviously and the aromatics are formed, suggesting that aromatics are mainly produced via the consumption of olefins on the acid sites of HZSM-5.8 An increase in aromatics selectivity from 19.5 to 29.5% is exhibited, accompanied by the decrease of the SiO2/Al2O3 ratio in traditional HZSM-5 from 80 to 27, implying that increasing acid sites via the decrease of 5148
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Figure 2. Characterization of Fe3O4@MnO2 catalyst. (A) SEM image of Fe3O4@MnO2. The scale bar represents 500 nm. (B) TEM image of Fe3O4@MnO2. The scale bar represents 100 nm (inner patterns, 2 nm). (C) EDX elemental mapping of Fe and Mn on Fe3O4@MnO2. The scale bar represents 30 nm. (D) XRD patterns of Fe3O4@MnO2. (E) XPS spectra of Fe 2p on Fe3O4 and Fe3O4@MnO2.
5 (27) further increases the aromatics selectivity to 40.2% (Figure 1A, order 5), matched to FeMnKSi-Hol HZSM-5 (27). In addition, the aromatics selectivity increases continually to 48.7% by tuning the ratio of F−T catalyst/Hol HZSM-5 zeolite from 1:1 to 1:4 (Figure 1A, order 5−7). To illustrate the function of hollow HZSM-5 zeolite, the detailed product distributions are shown in parts B and C, respectively, of Figure 1. The product distribution of Fe3O4@ MnO2 is mainly composed of olefins and paraffins (Figure 1B), in which the olefins formed are completely converted to aromatics and C2−4 paraffins in the role of Hol HZSM-5 (27) as the second component (Figure 1C), further confirming the excellent aromatization ability of the Hol HZSM-5 (27) catalyst. The liquid hydrocarbons (C5+) are mainly composed of aromatics, which are identified mostly to be toluene, xylene, ethyltoluene, trimethylbenzene, and dimethylethylbenzene (Table S2). A α value (chain growth probability) of 0.72 is exhibited for the Fe3O4@MnO2−Hol HZSM-5 (27) bifunctional catalyst, lower than that of 0.79 for Fe3O4@MnO2, confirming the restrained formation of long-chain hydrocarbons by combining Fe3O4@MnO2 with Hol HZSM-5 (27). The product distribution of the bifunctional catalyst obviously deviates from the ASF distribution, which may be attributed to
SiO2/Al2O3 ratio facilitates the formation of aromatics. More interestingly, in our quest for obtaining more aromatics products, a designed hollow HZSM-5 zeolite (Hol HZSM-5 (27), the SiO2/Al2O3 ratio is 27) is coupled with FeMnKSi, which further increases the aromatics selectivity to 33.8% (Figure 1A, order 4). On the other hand, syngas is first converted to olefins, and then to aromatics via the tandem catalyst in the SOA reaction. More olefins obtained in the first process can provide the driving force to produce more aromatics by consecutive consumption of olefins intermediates. It is generally accepted that Mn promoter is used expectedly to modify electronic characteristics of Fe-based FTS catalysts, promoting the formation of olefins.34,35 The traditional FeMnKSi catalyst with higher Mn loading increases the aromatics selectivity after physically mixing with Hol HZSM-5 (27) (Table S1), confirming the enhanced formation of aromatics by Mn promoter. In particular, a Fe3O4@MnO2 catalyst is prepared by a one-pot hydrothermal method, which produces 7.7% CH4, 22.4% C2−4 hydrocarbons (o/p = 4.93), and 69.9% C5+ hydrocarbons (o/p = 4.10) (Figure 1A, order 9), obviously promoting the formation of olefins while successfully suppressing the undesired CH4 product compared to FeMnKSi (Figure S2). The coupling of Fe3O4@MnO2 with Hol HZSM5149
DOI: 10.1021/acscatal.9b01045 ACS Catal. 2019, 9, 5147−5156
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Figure 3. Structural characterization of hollow HZSM-5 zeolite. (A) SEM image of hollow HZSM-5. The scale bar represents 500 nm. (B) TEM image of hollow HZSM-5. The scale bar represents 50 nm (inner patterns, 20 nm). (C) XRD patterns of hollow HZSM-5. (D) N2 adsorption− desorption isotherm of hollow HZSM-5 and traditional HZSM-5.
aromatics selectivity is still obtained with a higher CO conversion over 94% as the H2/CO ratio is in the range of 1:1−2:1, suggesting that the coupling of Fe3O4@MnO2 with Hol HZSM-5 is suitable for the utilization of syngas derived from coal and biomass resources because syngas with the corresponding H2/CO ratio was produced mainly from those carbon resources. Under the optimum conditions of 320 °C, 4.0 MPa, 4000 mL·h−1·g−1, and H2/CO = 1, an optimal aromatics selectivity of 56.6% at a CO conversion of 90.3% is obtained (Figure 1A, order 8). As far as we know, such an excellent aromatics selectivity with a high catalytic activity has not been achieved in the SOA route. Subsequently, we investigated the influence of the presence of CO2 in syngas on the catalytic performance of the bifunctional catalyst (Figure S5). The addition of CO2 suppresses the conversion of CO. It is surprising that the presence of even 38 vol % CO2 in syngas does not exert a significant effect on the hydrocarbon distribution. The selectivity of aromatics remains stable as the CO2 percentage increases, indicating that the bifunctional catalyst is CO2 tolerant. This observation suggests that our catalyst is applicable to the utilization of CO2-rich syngas. In particular, the CO2-rich tail gases are also recycled used in the F−T synthesis industry, further decreasing energy consumption stemming from the CO2 byproduct. 2.3. Structural Characterization. To reveal the nature of the superior aromatics performance, we resorted to multiple characterization techniques to investigate the structure of the Fe3O4@MnO2-Hol HZSM-5 (27) bifunctional catalyst. The prepared Fe3O4@MnO2 catalyst is presented in the form of a nanoplate-shaped particle with crystal size of about 80−130
the secondary aromatization reaction, occurring on the acid sites of HZSM-5.36 2.2. Modification of Reaction Conditions. Besides the intrinsic structure of the tandem catalyst, aromatization performance is significantly affected by reaction conditions (Figure S3). It is found that, accompanied by the elevated reaction temperature from 280 to 360 °C, CO conversion increases gradually from 34.5 to 93.8%, whereas the aromatics selectivity increases first and then begins to decrease (Figure S3A). A maximum value of aromatics selectivity is presented at 320 °C. In addition, the catalytic activity increases from 51.1 to 90.3% accompanied by the increase of reaction pressure from 1.0 to 4.0 MPa, with an increasing selectivity of aromatics via the consumption of CH4 and C2−4 paraffins (Figure S3B), which may be attributed to the increasing retention time of olefins intermediates produced with higher reaction pressure, promoting the aromatization reaction of olefins. In addition, the increasing GHSV results in the decrease of CO conversion, while it facilitates the formation of C5+ hydrocarbons (Figure S3C). The aromatics selectivity gradually increases with the increase of GHSV and reaches the maximum value of 52.0% at 12000 mL·h−1·g−1. Furthermore, the effect of the H2/CO ratio on the catalytic performance is investigated (Figure S4). Upon increasing the H2/CO ratio from 1:1 to 4:1, CO conversion increases from 90.3 to 96.2%, and the CO2 selectivity decreases from 45.0 to 28.8%, demonstrating that a higher H2/CO ratio might suppress the formation of CO2 byproduct. In particular, a maximum value of aromatics selectivity (56.6%) is displayed in the H2/CO ratio of 1:1, which presents a slowly decreasing trend with the increase of H2/CO ratio. More than 40% 5150
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gradient is indispensable for the synthesis of hollow structure. In contrast, the irregular Al distribution in commercial HZSM5 may result in the random dissolution in the interior of zeolite, leading to the formation of mesoporous structure rather than hollow structure (Figure S6D). The acidity of HZSM-5 zeolites is analyzed by NH3-TPD and Py-IR. As shown in Figure S7A, both traditional HZSM-5 and hollow HZSM-5 exhibit two desorption peaks, in which the high-temperature peak is ascribed to the strong acid sites and the low-temperature peak corresponds to the weak acid sites. Both the strong and weak acid sites present an increasing trend with a decrease in SiO2/Al2O3 ratio from 80 to 27. The calculated value listed in Table S3 shows that the total acid sites of HZSM-5 (80) is 71 μmol·g−1, which increases distinctly to 313 μmol·g−1 for HZSM-5 (27). Synchronously, the aromatics selectivity increases from 19.5 to 29.5%, confirming that the increase of acid sites facilitates the formation of aromatics. As traditional HZSM-5 (27) is pretreated by TPAOH, the formed Hol HZSM-5 (27) has lower total acid sites, which may be ascribed to the immigration of Si species from inside to outside of HZSM-5 during the process of forming a hollow cavity, causing the decrease of acid sites via the increasing amount of Si species on the surface of zeolite.39 However, the coupling of FeMnKSi with Hol HZSM-5 (27) leads to the increase of aromatics selectivity to 33.8% compared to FeMnKSi-HZSM-5 (27), indicating that suitable acid sites are crucial for obtaining aromatics. Brönsted and Lewis acid sites of zeolites are further analyzed by Py-IR. As shown in Figure S7B, two bands at approximately 1545 and 1455 cm−1 are ascribed to the Brönsted acid sites and Lewis acid sites, respectively. The amounts of both the Brönsted and Lewis acid sites have a similar changing trend with that of strong and weak acid sites based on the tuning of SiO2/Al2O3 ratio of traditional zeolite (Table S3). Especially, the amount of Brönsted acid sites is presented in the order of HZSM-5 (27) > Hol HZSM-5 (27) > HZSM-5 (80). It is generally considered that the Brönsted acid sites of zeolite have a positive effect on the aromatization reaction. In the present study, the aromatics selectivity exhibits the order of Hol HZSM-5 (27) > HZSM-5 (27) > HZSM-5 (80) (Figure S8), which is not consistent with the changing trend of Brönsted acid sites. This may be attributed to that the weaker acidity of HZSM-5 (80) is not sufficient to the aromatization reaction, while the stronger acidity of HZSM-5 (27) leads to the hydrogenation of olefins and the overcracking of heavy hydrocarbons on the excessive acid sites.7,8,36 The suitable Brönsted acid sites and peculiar morphology of Hol HZSM-5 play an important role in promoting the formation of aromatics. 2.4. Stability of the Bifunctional Catalyst. The stability of zeolite is a crucial index to the potential industrial application for the selective conversion of syngas to aromatics. Synthesizing aromatics via general routes such as methanol aromatization, methane aromatization, and propane aromatization processes has commonly suffered serious deactivation due to the carbon deposition.40−42 Looking for a novel zeolite catalyst with superior stability has drawn more and more attention. As shown in Figure 4, CO conversion remains a stable state with time on stream for the FeMnKSi-HZSM-5 (27) catalyst, implying that the FeMnKSi catalyst in the first stage has excellent stability for converting syngas. However, the aromatics selectivity presents a distinct decrease, revealing the easy deactivation of traditional HZSM-5 in the aromatization
nm in length and 10−20 nm in thickness (Figure 2A,B). The corresponding HRTEM image (inner patterns in the Figure 2B) displays the Fe3O4 phase and amorphous MnO2 adhered on the surface of Fe3O4. The EDX elemental mapping (Figure 2C) confirms the formation of the Fe3O4@MnO2 core−shell structure. The appearance of Fe3O4 diffraction peaks whereas there are no Fe−Mn solid solution diffraction peaks in the XRD patterns (Figure 2D) implies no formation of Fe−Mn solid solution. The Fe/Mn molar ratio of the overall catalyst is 1.04 (determined by inductively coupled plasma (ICP)), while that on the surface is only 0.21 (determined by XPS), certifying the location of amorphous MnO2 on the external surface of Fe3O4. The formation mechanism of Fe3O4@MnO2 is shown in Scheme S1. First, FeSO4·7H2O is converted to Fe(OH)2 suspension after addition of NaOH. Subsequently, KMnO4 is added into the Fe(OH)2 suspension and reduced to MnO2 accompanied by the formation of Fe3O4. The addition of polyvinylpyrrolidone (PVP) in this process strengthens the stability of Fe3O4 particles and restrains their aggregation via strong adsorption of PVP on the surface of Fe3O4. MnO2 formed here is adhered easily onto the surface of Fe3O4 nanoparticles via the enhanced hydroxyl radical affinity produced on the surface of MnO2, resulting in the formation of an Fe3O4@MnO2 core−shell structure. In addition, XPS spectra (Figure 2E) show that the Fe 2p peaks of Fe3O4 nanoparticles shift toward lower binding energy with the addition of Mn into Fe3O4, which may be ascribed to the enhanced electron transfer from MnO2 to Fe3O4 in Fe3O4@ MnO2. The electron transfer from Mn to Fe species facilitates the dissociated adsorption of CO and weakens the hydrogenation ability, promoting the formation of olefins.34,35 As a result, more olefins are produced over Fe3O4@MnO2 in comparison to FeMnKSi, which diffuse onto the acid sites of zeolite for conversion to aromatics. The traditional HZSM-5 zeolite as the second component is solid crystal, which is presented in the form of block-shaped particles with a crystal size of about 1 μm in length (Figure S6A,B). Typical diffraction peaks of the MFI topology structure are exhibited for the HZSM-5 zeolite (Figure S6C). In contrast, the as-prepared Hol HZSM-5 (27) has regular morphology and uniform size with dimensions of about 170 nm × 140 nm (Figure 3A). A large regular hollow cavity with about 130 nm length and 100 nm width is created in the interior of zeolite and the thickness of the shell is only about 20 nm (Figure 3B), implying that post-treatment of TPAOH shortens obviously the length of HZSM-5 channels and creates a new cavity structure with the diameter of about 100 nm inside HZSM-5. The XRD pattern indicates that the modified treatment of TPAOH does not destroy the MFI topology structure of zeolite (Figure 3C). The N2 adsorption− desorption isotherms of Hol HZSM-5 (27) shows the presence of a H2 hysteresis loop with an abrupt step around P/P0 = 0.45 in the desorption branch (Figure 3D), confirming the large void and relatively intact zeolite shells.37 It is widely accepted that an Al distribution gradient exists in the HZSM-5 zeolite synthesized using TPA+ as the template.38 The interior of zeolite possesses higher Si content, while the exterior possesses higher Al content. Because Al species can protect Si species from extraction, dissolution is easier to occur in the interior when treating zeolite with alkali solution. The continued dissolution process of interior Si and Al species toward the exterior of zeolite gradually leads to the formation of hollow structure (Scheme S2). The zeolite with Al distribution 5151
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stream. Both CO conversion and the aromatics selectivity (about 34%) remain stable within 70 h on stream, indicating that the replacement of traditional HZSM-5 (27) by hollow HZSM-5 (27) can convert effectively olefin intermediates produced by FeMnKSi to aromatics, enhancing obviously the catalytic stability in the SOA reaction. On the basis of the above results (Figure 3), a shorter channel length and larger hollow cavity are created for Hol HZSM-5 (27) by the pretreatment of TPAOH on HZSM-5 (27), revealing that the zeolite channel length and hollow cavity created play a critical role in improving the catalytic stability of the bifunctional catalyst. Nanosized HZSM-5 (27) zeolites with different crystal sizes from 160 to 1000 nm are further synthesized to investigate the effect of zeolite channel length on the aromatization performance. The morphology and catalytic performance of these zeolites are shown in Figure 5. CO conversion of FeMnKSi shows no obvious change after mixing with these different zeolites (Figure S9), whereas the aromatics selectivity decreases to some extent (Figure 5D). The aromatics selectivity decreases rapidly (about 14%) within 30 h for the HZSM-5 (27) with a crystal size of 1000 nm. With the
Figure 4. Stability of bifunctional catalysts with different HZSM-5 zeolites. Reaction conditions: 320 °C, 2.0 MPa, 4000 mL·h−1·g−1, H2/ CO = 1.
reaction. In contrast, the FeMnKSi-Hol HZSM-5 (27) bifunctional catalyst displays excellent stability with time on
Figure 5. Stability of HZSM-5 zeolites. (A−C) SEM images of the synthesized HZSM-5 zeolite with different sizes: 160 nm (A), 350 nm (B), and 550 nm (C). The scale bar represents 500 nm. (D) Stability of HZSM-5 zeolites mixed with FeMnKSi. Reaction conditions: 320 °C, 2.0 MPa, 4000 mL·h−1·g−1, H2/CO = 1, F−T/zeolite catalyst mass ratio = 1:1. (E) TG curves of zeolites after reaction. (F) Coke deposition rate of zeolites. 5152
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Figure 6. Stability of the bifunctional catalyst. (A) Stability of Fe3O4@MnO2−Hol HZSM-5 (27) catalyst. Reaction conditions: 320 °C, 2.0 MPa, 4000 mL·h−1·g−1, H2/CO = 1, F−T/zeolite catalyst mass ratio = 1:4. (B) TEM image of spent Hol HZSM-5 (27) after 180 h reaction. The scale bar represents 500 nm. (C) TEM image of spent Fe3O4@MnO2 after 180 h reaction. The scale bar represents 50 nm (inner patterns, 10 nm).
(27)160nm (Figure S10 and Table S4). The decrease of surface area and pore volume for hollow HZSM-5 zeolite, caused by the accumulation of coke, is much slighter than that of traditional HZSM-5 zeolite (Table S5 and Figure S11), demonstrating an enhanced stability of hollow structures of hollow HZSM-5 compared to traditional zeolite. Furthermore, the carbon deposition rate on Hol HZSM-5 (27)170nm is the lowest (Figure 5F), which is only 0.6 times of that on HZSM-5 (27)160nm, indicating the better diffusion property of hollow zeolite. The diffusion limitation can be reduced drastically by hollow cavity of hollow materials, and increasing interest has been received in the field of drug delivery, membranes, batteries, electro-catalysis and photocatalysis for their excellent stability.43 To the best of our knowledge, this is the first application of hollow zeolite in the syngas thermochemistry. In particular, after FeMnKSi is replaced by Fe3O4@MnO2, the bifunctional catalyst exhibits superior stability with enhanced aromatics selectivity. As shown in Figure 6A, the aromatic selectivity of Fe3O4@MnO2-Hol HZSM-5 (27) bifunctional catalyst gradually increases to about 49% and then remains unchanged within 180 h on stream. The undesired CH4 and olefins are successfully suppressed to a very low level. CO conversion increases to about 95%, and then remains stable for 180 h. A comparison of the aromatics selectivity and catalytic stability with the previous reports is shown in Table S6, which indicates that this is the highest catalyst stability with high aromatics selectivity (about 49%) and high activity (CO conversion over 92%) reported for the conversion of syngas to aromatics in this study. TEM preformed on the spent Hol ZSM-5 catalyst shows that the hollow structure is not destroyed during 180 h reaction (Figure
shortening of the crystal size of HZSM-5, the catalytic stability is strengthened gradually. Compared to HZSM-5 (27)1000 nm, the aromatics selectivity of HZSM-5 (27)160 nm is enhanced apparently, which decreases only about 4% within 160 h, illustrating that the shortening of zeolite channel length can improve the catalytic stability. TG technology is used to quantify the amount of coke formed over zeolites during reaction (Figure 5E). The weight loss below 280 °C and that above 280 °C are attributed to the adsorbed water and deposited carbon, respectively. The weight loss of HZSM-5 (27)1000 nm is 11.3%, which decreases to 9.0% for HZSM-5 (27)160 nm, indicating that the zeolite with smaller crystal size suppresses the carbon deposition. In particular, the carbon deposition rate of HZSM-5 (27)1000 nm is 1.0 times higher than that of HZSM-5 (27)160 nm (Figure 5F), implying an enhanced diffusion property of the zeolite with smaller crystal size. Thus, it is reasonable to conclude that the decrease of crystal size of HZSM-5 (27) shortens the zeolite channel length, which facilitates the diffusion of reactants and products, promoting the transfer of internal coke precursors to the external surface of zeolite. As a result, the accumulation of coke is suppressed and catalytic stability of HZSM-5 zeolite is obviously enhanced. More interestingly, the catalytic stability of Hol HZSM-5 (27)170nm with a comparative crystal size is further enhanced in contrast with HZSM-5 (27)160 m, which decreases slightly (about 3%) within 200 h of reaction (Figure 5D), implying that hollow cavity created in the interior of Hol HZSM-5 (27) has a positive effect on the catalytic stability. After reaction, the MFI morphology is exhibited and the crystallinity of Hol HZSM-5 (27)170nm is higher than that of traditional HZSM-5 5153
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Research Article
ACS Catalysis 6B). In addition, no carbon filament is observed on the surface of zeolite, revealing that no obvious carbon deposition is formed on the hollow HZSM-5 zeolite. Furthermore, there is no obvious change in shape and size of the spent Fe3O4@ MnO2 catalyst (Figure 6C). All of these results indicate that the Fe3O4@MnO2-Hol HZSM-5 (27) bifunctional catalyst displays excellent activity and aromatics selectivity as well as superior stability. Moreover, hollow zeolites with different inner sizes and wall thicknesses are further synthesized by varying the time of alkali treatment for obtaining more information about the formation of aromatics (Figure S12). Accompanied by the increase of alkali treatment time, the inner size presents an increasing trend, while the wall thickness of hollow zeolite decreases gradually. The wall thickness is about 65 nm, and no cavity is exhibited without the alkali treatment (Figure S12A). After 12 h of alkali treatment, the cavity inside the zeolite appears (about 40 nm of inner size), whereas the wall thickness decreases to 50 nm. As the alkali treatment time increases to 72 h, the inner size increases continually to 100 nm accompanied by the decrease of wall thickness to 20 nm. A contrary correlation is displayed between the inner size and wall thickness. All of these results indicate that both the inner size and wall thickness of hollow zeolite can be tuned effectively by changing the time of alkali treatment. The catalytic performance of hollow zeolites with different inner sizes and wall thicknesses is shown in Figure S12B. It can be seen that CO conversion is about 93% and remains unchanged with the tuning of inner size and wall thickness. The aromatics selectivity presents a slightly increasing trend with the increase of inner size and decrease of wall thickness. In particular, the aromatics distribution changes with the changing of inner size and wall thickness. A larger inner size and smaller wall thickness facilitate the formation of light aromatics (benzene, toluene, ethylbenzene, and xylene (BTEX)), whereas they suppress the formation of heavy aromatics (C9+ aromatics) (Figure S12C). The BTEX selectivity increases from 42.5 to 47.3% with the decrease of wall thickness from 50 to 20 nm, suggesting that the decreasing wall thickness of hollow zeolite suppresses the further alkylation of BTEX. 2.5. Reaction Scheme for SOA. On the basis of the above results, a reaction scheme of selectively converting syngas to aromatics over the Fe3O4@MnO2-Hol HZSM-5 (27) bifunctional catalyst is illustrated in Scheme 1. The bifunctional catalyst with two types of active sites displays excellent synergistic effect in SOA reaction. Syngas is first converted to olefins on iron active sites over Fe3O4@MnO2. The designed core−shell structure facilitates the electron transfer from Mn to
Fe species, which enhances the dissociation adsorption of CO and inhibits the hydrogenation ability, promoting the formation of olefins intermediates. The olefins intermediates produced over Fe3O4@MnO2 then diffuse to the acid sites of the hollow HZSM-5 zeolite, taking place the oligomerization and aromatization reactions. As a result, aromatics are selectively produced and subsequently diffuse out of pore structures of zeolite. The short lifetime of zeolite is a major problem in the industrial application. In SOA reaction, heavy hydrocarbons, especially aromatics, can polymerize within the long microporous channels of zeolite and form coke. Subsequently, a rapid zeolite deactivation occurs due to the coverage of acid sites and blocking of micropores by coke.25 The decrease of zeolite crystal size could shorten the diffusion path of products and facilitate the transfer of internal coke precursors to the external surface of zeolite, leading to an enhanced catalytic stability.44 In particular, in our work, the creation of a hollow cavity in the interior of zeolite is another novel strategy to reduce the diffusion length and enhance the mass transport. The large hollow cavity and short channel length of hollow zeolite provide an effective microreactor for the olefin aromatization reaction, playing a critical role in optimizing the aromatization performance and prolonging the lifetime of zeolite.
3. CONCLUSIONS In summary, we prepared a bifunctional catalyst composed of Fe3O4@MnO2 with hollow HZSM-5 zeolite which could convert syngas into aromatics with high selectivity and activity as well as good stability, suggesting a potential industrial application. The aromatics selectivity reached 57% at a CO conversion of over 90%. The catalyst remained in a stable state under industrial-relevant conditions, and no catalyst deactivation was exhibited with 180 h on stream. The enhancement of electron transfer between Fe and Mn elements over the Fe3O4@MnO2 core−shell catalyst promoted the formation of olefins intermediates, which further improved the production of target aromatics over HZSM-5 zeolite. The decrease in the length of zeolite channel can shorten the diffusion path of reactants and products, facilitating the transfer of internal coke precursors to the external surface of zeolite. As a result, the accumulation of coke is suppressed and catalytic stability of HZSM-5 zeolite is obviously enhanced. Furthermore, the creation of hollow structure in the interior of HZSM-5 zeolite is another strategy to enhance the mass transport and prolong the life of zeolite. 4. EXPERIMENTAL SECTION 4.1. Catalyst Preparation. The Fe3O4@MnO2 sample is prepared by a one-pot hydrothermal method. Typically, 12.5 mmol of FeSO4·7H2O (AR) and 5 g of PVP-K30 (GR) are added to 500 mL of deionized water with stirring to form a clear solution. Subsequently, the mixture is heated to 90 °C. Then 5 mL of 5.0 M NaOH (AR) is added dropwise under stirring, and 50 mL solution of KMnO4 (AR) is slowly added. After being aged for 10 h, the products are filtered and washed with ethanol (AR) and deionized water. The fresh-made catalyst is dried overnight at 50 °C and directly used for SOA reaction without further thermal treatment. The Fe/Mn molar ratio of the obtained catalyst is 1:1. The preparation of traditional FeMnKSi is shown in the Supporting Information.
Scheme 1. Reaction Scheme for Syngas to Aromatics over Fe3O4@MnO2−Hol HZSM-5 (27) Bifunctional Catalyst
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ACS Catalysis
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The traditional solid HZSM-5 zeolites are synthesized by a hydrothermal method. Tetrapropylammonium hydroxide (TPAOH, 25%) is used as the template. The silicon source is tetraethyl orthosilicate (TEOS, AR), and the aluminum source is aluminum isopropoxide (98%). A series of solid HZSM-5 zeolites with different particle sizes can be synthesized by adjusting the concentration of TPAOH. The molar composition of the synthesis mixture is 27TEOS/ 1Al2O3:(7.3−4.1)TPAOH/1000H2O. After being stirred for 10 h at room temperature, the mixture is heated at 85 °C under vigorous stirring to remove alcohol generated by hydrolysis of TEOS. Then a quantity of water is added to maintain the volume. The clear liquid is transferred in a Teflon-lined stainless-steel autoclave and crystallized at 170 °C for 3 days. The product is filtered, washed, and dried at 120 °C overnight. Finally, the traditional solid HZSM-5 zeolites are obtained after calcining in static air at 540 °C for 6 h. Commercial HZSM-5 zeolites (SiO2/Al2O3 = 27, 80) are purchased from Nankai University Catalyst Company, China. Hollow HZSM-5 zeolite is prepared by further treating the traditional zeolite with 0.2 M TPAOH aqueous solution (10 mL per gram zeolite) at 170 °C for 3 days. The product is filtered, washed,and dried at 120 °C overnight. Finally, it is then calcined in static air at 540 °C for 6 h. The obtained hollow zeolite is named Hol HZSM-5 (27). Hollow HZSM-5 zeolites with different inner sizes and wall thicknesses are synthesized by varying the time of alkali treatment. The FTS−zeolite bifunctional catalysts are typically prepared by granule mixing FTS catalyst with HZSM-5 zeolite at a mass ratio of 1:1 unless otherwise noted. 4.2. Catalyst Characterization and Catalytic Test. Catalyst characterization and catalytic tests are described in the Supporting Information.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.9b01045.
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Research Article
Tables S1−S6, Schemes S1 and S2, Figures S1−S12, and experimental details (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
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[email protected]. ORCID
Chenghua Zhang: 0000-0002-7012-9219 Mingyue Ding: 0000-0001-8769-4153 Author Contributions #
Y.X. and J.L. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the International Cooperation and Exchange Program of the National Natural Science Foundation of China (51861145102) and the Science and Technology Program of Shenzhen (JCYJ20180302153928437). This work is also supported by Synfuels China Co., Ltd. 5155
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