- Email: [email protected]
Direct Transformation of Syngas to Aromatics over Na-Zn-Fe5C2 and Hierarchical HZSM-5 Tandem Catalysts
Article
Direct Transformation of Syngas to Aromatics over Na-Zn-Fe5C2 and Hierarchical HZSM-5 Tandem Catalysts Bo Zhao, Peng Zhai, Pengfei Wang, ..., Qianwen Zhang, Weibin Fan, Ding Ma [email protected] (W.F.) [email protected] (D.M.)
HIGHLIGHTS An unprecedented aromatic yield via direct syngas conversion is obtained The aromatic selectivity is dependent on the acidity and porous structure of HZSM-5 The pivotal role of the density and strength of acid sites is discussed
Aromatics, the basic chemicals, could be produced from syngas (CO/H2) directly over a physical mixture of a highly effective FTS catalyst, Na-Zn-Fe5C2, and HZSM-5 zeolite. The Na-Zn-Fe5C2 component was highly selective toward olefins; the appropriate density and strength of the Brønsted acid sites and the hierarchical pore structure of the HZSM-5 component converted the formed olefins and endowed the tandem catalyst with an unprecedented aromatic yield.
Zhao et al., Chem 3, 1–11 August 10, 2017 ª 2017 Published by Elsevier Inc. http://dx.doi.org/10.1016/j.chempr.2017.06.017
Please cite this article in press as: Zhao et al., Direct Transformation of Syngas to Aromatics over Na-Zn-Fe5C2 and Hierarchical HZSM-5 Tandem Catalysts, Chem (2017), http://dx.doi.org/10.1016/j.chempr.2017.06.017
Article
Direct Transformation of Syngas to Aromatics over Na-Zn-Fe5C2 and Hierarchical HZSM-5 Tandem Catalysts Bo Zhao,1,6 Peng Zhai,1,6 Pengfei Wang,2,3 Jiaqi Li,1 Teng Li,1 Mi Peng,1 Ming Zhao,4 Gang Hu,5 Yong Yang,2,3 Yong-Wang Li,2,3 Qianwen Zhang,4 Weibin Fan,2,* and Ding Ma1,7,*
SUMMARY
The Bigger Picture
Direct synthesis of aromatics from syngas is a great challenge because of severe operating conditions and low yield of aromatics. Making this process more competitive than the MTA (methanol to aromatics) process will require high energy efficiency and low CO2 emission. A combination of Na-Zn-Fe5C2 and hierarchical HZSM-5 with uniform mesopores dramatically changed the product distribution of Fischer-Tropsch synthesis, leading to 51% aromatic selectivity under the stable stage with CO conversion >85%. C12+ heavy hydrocarbons almost disappeared, and the catalyst showed good stability. The hierarchical zeolitic structure and Brønsted acidity of HZSM-5 could be precisely tuned by controlling the alkali treatment conditions and the degree of ion exchange. The appropriate density and strength of the Brønsted acid sites and the hierarchical pore structure of HZSM-5 endowed the catalyst with an unprecedented aromatic yield. This work shows a broad area for development for syngas conversion.
The depletion of petroleum reserves threatens economic development and regional stability nowadays. Building an alternative route for fuel and chemical production has become a focused research topic in recent years. Although conversion of syngas to fuels by the FischerTropsch synthesis (FTS) process has been industrialized in various countries, expanding its products to value-added chemicals is still a great challenge. Aromatics are very important feedstock; they are used as blending mixtures for gasoline with a high octane number in some countries and are the most important molecular platform for the polymer industry. With the fast development of the polymer industry, the gap between supply and demand calls for a new strategy for the synthesis of aromatics. We have established an intriguing process that directly converts syngas to aromatics on the basis of a physical mixture of a highly effective FTS catalyst and zeolite, which shows enormous potential in comparison with the traditional multiple-step pathway.
INTRODUCTION Transformation of syngas (CO/H2) derived from shale gas, biomass, and coal has been developed as a promising alternative to oil for the preparation of liquid fuels and commodity chemicals.1–5 The Fischer-Tropsch synthesis (FTS) process is well known. The product selectivity of FTS is limited by the law of polymerization, i.e., Anderson-Schulz-Flory distribution, which restricts industrial application as the result of a very wide product distribution. Recently, different novel catalyst design strategies have been developed to confine the products into a narrow range by altering the electronic property of metal active sites and the steric structure of the catalyst.6–12 With S and Na as promoters, high selectivity to low olefins was obtained over a supported Fe catalyst.7 In addition, the introduction of acidic zeolite in an FTS catalyst initiates consecutive reactions, such as cracking and isomerization of primary products, leading to excellent selectivity toward high-quality gasoline or diesel.8–10 Very recently, it was reported that coupling of high-temperature methanol synthesis catalyst with SAPO (silico-alumino-phosphate) molecular sieves produced more than 80% of C2–C4 olefins in hydrocarbons, although high temperature (673 K) facilitated the water gas shift reaction and generated a large amount of CO2.11,12 Aromatics, mainly produced from petroleum refinement processes, are used as a fuel additive in some countries and are the most important platform molecules for the polymer industry. With the fast development of the economy, the gap between
Chem 3, 1–11, August 10, 2017 ª 2017 Published by Elsevier Inc.
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supply and demand calls for a new strategy for the synthesis of aromatics. Starting from syngas, typically, a two-step process is needed, i.e., we need to transform syngas to methanol on a CuZn catalyst (493–573 K) first, and then convert methanol to aromatics over a zeolite catalyst (MTA, >673 K).13 However, this two-step process is highly cost and energy effective. If we could combine the two processes into one, i.e., direct conversion of syngas into aromatics (STA), this would not only greatly decrease the equipment and operation costs but would also have an advantage in product separation and recycling because the reactants are in the gas phase and the main products are liquid. (n + 6)CO + (2n + 9)H2 / Cn+6H2n+6 + (n+6)H2O
(n R 0)
Because of the mismatch between the operating temperature of the CuZn catalyst and that of aromatization catalysts,14 many efforts have been devoted to coupling a high-temperature methanol synthesis catalyst with an MTA catalyst.15,16 Pioneering work by Chang et al.17 indicated that 70% aromatics selectivity could be reached over a Zn-Cr/HZSM-5 catalyst, demonstrating the effectiveness of this concept. However, the consecutive reactions required severe operating conditions and showed low reactivity. On the other hand, some researchers combined an Fe-based FTS catalyst with zeolite to directly synthesize aromatics from syngas. Guan et al.18 reported 40.4% aromatics selectivity in C5+ products on an Fe/MnO-GaZSM-5 catalyst, but rapid deactivation occurred with time on stream. Recently, Yan et al.19 investigated the influence of reaction temperature, pressure, and space velocity on the production of aromatics on an Fe-Pd/HZSM-5 catalyst, yet it is difficult to obtain high aromatic selectivity as well as high CO conversion on these catalysts.20,21 As the combination of high CO conversion and high aromatic selectivity would confer an immediate advantage in industrial applications of the STA process, the development of more efficient catalyst is highly desired. Here, we have constructed a tandem catalyst system by combining Na-Zn-Fe5C2 catalysts (with high CO conversion and high selectivity toward alkenes)22 with hierarchical HZSM-5 for the direct conversion of syngas to aromatics. By physically mixing the highly selective Na-Zn-Fe5C2 catalyst with the mesoporous HZSM-5 zeolite, we successfully obtained high aromatic selectivity with high CO conversion. The total selectivity of undesired C1 products, including CH4 and CO2, has been successfully suppressed to less than 35%, and the yield of aromatics reached as high as 32.6% in a single run. 1Beijing
RESULTS Preliminary Catalytic Performance for STA Table 1 shows the catalytic activity and product distribution of syngas conversion at 613 K and 20 bar. Fe-based FTS processes generally operate between 523 and 623 K. The main products are alkanes containing 30.8% CH4 over a simple SiO2supported iron catalyst (Table 1, entry 1). At such a temperature range, aromatization of alkanes through dehydrogenation/oligomerization/cyclization over a solid acid catalyst is not possible because it needs a high reaction temperature, e.g., 823 K.23 Therefore, a combination of Fe and HZSM-5 (SiO2/Al2O3 = 30) (Table 1, entry 2) generated C2+ paraffin as the main product, with aromatic selectivity of only 10.9% at 613 K. By comparing with the catalytic results of Fe/SiO2, it can be deduced that aromatics are formed at the expense of olefins on acidic HZSM-5. Recently, a high-performance catalyst, Na-Zn-Fe5C2 (termed as FeZnNa), was developed by the co-precipitation method (Figures S1–S3).22 The molar weight ratio of
2
Chem 3, 1–11, August 10, 2017
National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
2State
Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, 27 South Taoyuan Road, Taiyuan, Shanxi 030001, China
3Synfuels
China, Beijing 100195, China
4Faculty
of Materials and Chemical Engineering, Beijing Institute of Petrochemical Technology, Beijing 100029, China
5Israel
Chemicals Limited, Shanghai 200021,
China 6These 7Lead
authors contributed equally
Contact
*Correspondence: [email protected] (W.F.), [email protected] (D.M.) http://dx.doi.org/10.1016/j.chempr.2017.06.017
Please cite this article in press as: Zhao et al., Direct Transformation of Syngas to Aromatics over Na-Zn-Fe5C2 and Hierarchical HZSM-5 Tandem Catalysts, Chem (2017), http://dx.doi.org/10.1016/j.chempr.2017.06.017
Table 1. Comparison of the CO Conversion and Product Distribution of Different Catalysts for Syngas Conversion Entry
Catalyst
Al (mmol/g)a
–
B Site (mmol/g)b
XCO (%)
–
38.6
CO2 Selectivity (%)
Selectivity in Hydrocarbons (%) CH4
C2–C4 Olefin
C2–C4 Paraffin
C5+c
Aromatics
11.3
30.8
13.6
17.0
38.6
–
1
Fe/SiO2
2
Fe/HZSM-5
1.06
1.02
28.1
13.7
29.5
1.0
29.1
29.5
10.9
3
FeZnNa
–
–
74.9
27.3
10.9
25.5
4.4
58.1
1.1
4
FeZnNa@HZSM-5
1.06
1.02
88.3
27.7
11.3
1.4
38.6
28.9
19.8
5
[email protected]
1.04
0.46
84.2
26.5
9.9
0.7
35.6
22.7
30.9
6
[email protected]
1.36
0.35
89.2
26.9
9.8
1.0
26.8
21.8
40.5
7
[email protected]
1.36
0
86.6
27.2
10.1
20.4
15.8
52.6
2.1
8
[email protected]
1.69
0.36
88.5
27.8
9.8
2.9
21.9
38.2
26.9
9
[email protected]
1.36
0.20
88.8
27.5
9.6
1.0
25.6
13.2
50.6
Reaction conditions: 120 mg catalyst (20 mg FeZnNa + 100 mg ZSM-5, physical mixture), CO/H2/CO2/Ar = 24:64:8:4, 613 K, 20 bar, total flow rate = 20 mL/min, time on stream = 40 hr. Catalyst was treated in 5%H2/N2 at 623 K for 2 hr before the reaction. a Aluminum content per gram of zeolite, determined by ICP. b Brønsted sites per gram of zeolite, determined by 1H MAS NMR. c C5+ products except for aromatics.
Zn and Fe was 1:1, with 0.8 wt % sodium content caused by residual precipitate. Seventy-eight percent of the hydrocarbon product was olefins, and undesired C1 (CO2, CH4) products were successfully suppressed with high CO conversion (Table 1, entry 3). It was found that Fe5C2 was the active phase in this catalyst, and the sodium acted as an electronic promoter that affected the elementary reaction on the surface to produce more alkenes. The thermodynamic calculation shows that, different from alkanes, aromatization of alkenes is feasible below 623 K and long-chain alkenes are apt to cyclization without oligomerization (Figure S4).24 Thus, the alkene-favorable Na-Zn-Fe5C2 catalyst would be an ideal platform for successive aromatization reactions. After physically mixing Na-Zn-Fe5C2 with HZSM-5, the aromatic selectivity increased sharply from 1.1% to 19.8% (Table 1, entry 4), whereas C2–C4 olefin selectivity decreased dramatically from 25.5% to 1.4%, and the olefin to paraffin ratio of hydrocarbons decreased greatly from 5.8 to 0.036. Simultaneously, CO conversion increased from 74.9% to 88.3% in comparison with pure Na-Zn-Fe5C2 catalyst. This can be attributed to the tension force by consecutive consumption of intermediates, which in turn favors the conversion of the initial reactants. The sharp increase in C2–C4 paraffin selectivity from 4.4% to 38.6% is due to the occurrence of dealkylation of polyalkylbenzene and the hydrogen transfer reaction within the reaction network.25 Hydrogen generated during the aromatization process is transferred into unsaturated hydrocarbons, leading to the formation of additional paraffin. Modified HZSM-5 by Base Treatment Although HZSM-5 can catalyze different types of acid-based reactions, such as dehydration of alcohols, cracking of hydrocarbons, oligomerization and cyclization of olefins, and dehydrogenation of paraffin,26 different types of reactions require different acidity, porosity, and reaction conditions. As the acid properties of zeolites depend primarily on the location, the geometry, and the coordination of aluminum in its framework, zeolite acidity can be engineered to improve its catalytic performance in specific reactions. In the current study, removal of excessive acid sites and creation of additional pore structures seem to be a promising way to further increase the selectivity toward aromatics, because aromatization reactions usually require mild
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Brønsted acidity27,28 and a hierarchical pore structure, which ensures fast product diffusion and desorption.29,30 We treated pristine HZSM-5 with NaOH solutions of different concentrations, which not only adjusts its acidity but also creates mesostructured porosity.31,32 The treated zeolites were termed x-HZSM-5, where x represents the NaOH concentration. For example, 0.1-HZSM-5 means HZSM-5 treated with 0.1 M NaOH solution at 353 K for 1 hr and then exchanged with 1 M NH4NO3 solution three times (all the samples were exchanged three times unless specified otherwise). Figure 1A shows that all the treated HZSM-5 samples show diffraction patterns characteristic of MFI structure, but the diffraction lines of the samples treated with R0.6 M NaOH solution were obviously broadened and decreased in intensity. This phenomenon suggests that NaOH treatment partially destroyed HZSM-5 structure by desilication,33 whereas the main MFI architecture was preserved. The N2 adsorption/desorption isotherms clearly show that HZSM-5 has a typical microporous structure. In contrast, mesoporous structures were formed in NaOHtreated HZSM-5, as revealed by its H4-type N2 sorption isotherms (Figures 1B and 1C). Upon NaOH treatment, an obvious increase in the mesopore volume was observed despite the fact that the surface area and micropore volume remained similar (except for 2.0-HZSM-5; its surface area and micropore volume declined significantly as a result of severe destruction of the ZSM-5 structure) (Table S1). The mesopore volume and diameter determined from the desorption branch by the Barrett-Joyner-Halenda (BJH) method increased with increasing NaOH concentration up to 1.0 M, indicating the presence of large numbers of mesopores in the sample. Transmission electron microscopy shows that 2–3 mm pristine HZSM-5 crystals with smooth surfaces became sponge-like because plenty of mesopores ranging from 4 to 10 nm were created as a result of the NaOH treatment (Figures 1D and 1E). These results indicate that the hierarchical pore structure in ZSM-5 crystals was formed through a desilication process (Table S2). Given that acid site and aluminum in HZSM-5 play a critical role in the aromatization process, the distribution and reconstruction of aluminum species and acid sites in parent HZSM-5 and those treated with NaOH were tracked with NH3-temperature-programmed desorption (TPD), pyridine-infrared, 27Al magic angle spinning (MAS) and multiple quantum (MQ)-MAS nuclear magnetic resonance (NMR), and 1 H MAS NMR techniques (Figures 2 and S5–S7). Treatment with 0.1 M NaOH solution partially removed framework aluminum (AlF, at 55 ppm, the main origin of Brønsted acid sites) and led to the formation of species with distorted framework Al (AlDF, with relatively weak Brønsted acidity), and six-coordinated (Al6NF, at about 0 ppm) Al species.34 With the increase of NaOH concentration (0.6 M), more AlF species were expelled from the zeolite framework; five-coordinated (Al5NF, around 30 ppm) and ex-framework or tri-coordinated Al species (a very broad hump, socalled invisible Al) were clearly resolved on 27Al MAS and 27Al MQ MAS NMR spectra (Figures 2A, 2D, and 2E).35,36 As strong Brønsted acid sites (BAS) are associated with AlF, the removal of AlF from the zeolitic lattice will decrease the amount of BAS in the catalyst. The NH3-TPD profile indicates that the ammonia desorption associated with strong acid sites (>573 K) decreased with the degree of NaOH treatment (Figure 2C). Direct evidence about BAS is provided by the 1H MAS NMR profile (Figure 2B). It shows that the peaks characteristic of Brønsted acid sites (4.0 and 6.1 ppm) decreased sharply after NaOH treatment (0.1 M), suggesting the removal of large amounts of BAS in comparison with parent HZSM-5 (0.46 versus 1.02 mmol/g). When the concentration of NaOH solution increased to 0.6 M, the concentration
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Figure 1. The Structure of the Modified HZSM-5 (A) X-ray diffraction patterns of modified HZSM-5. (B) N 2 adsorption-desorption isotherms for modified HZSM-5. (C) Pore-size distributions of HZSM-5 derived from the N2 adsorption-desorption isotherms by the BJH method. The number before HZSM-5 represents the NaOH concentration of the solution that treated HZSM-5. (D) TEM micrograph of the HZSM-5. The scale bar represents 50 nm. (E) TEM micrograph of 0.6-HZSM-5. The scale bar represents 50 nm.
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Figure 2. The Acidity and Framework Structure of Modified ZSM-5 (A) 27 Al MAS NMR spectra. (B) 1 H MAS NMR spectra. (C) NH 3 -TPD profiles for HZSM-5 prepared with different concentrations of NaOH or different ionexchange times. (D) 27 Al MQ MAS NMR spectra of HZSM-5. (E) 27 Al MQ MAS NMR spectra of 0.6-HZSM-5.
of BAS decreased further to 0.35 mmol/g, and its strength also weakened (Figure 2C). STA on FeZnNa and Modified ZSM-5 Tandem Catalysts After Brønsted acidity decreased with 0.1 M NaOH treatment, the aromatic selectivity increased greatly to 30.9% on [email protected] with a decrease in selectivity toward methane and C2–C4 paraffins (Table 1, entry 5). With the further reduction of strong Brønsted acidity sites, the selectivity toward aromatics reached 40.5% on [email protected] (Table 1, entry 6; Figures S8–S10), which is twice that of FeZnNa@HZSM-5 catalyst. Although BAS is the active site for aromatization, as supported by the very low aromatic selectivity obtained over the [email protected] catalyst (which has not been exchanged with NH4NO3 solution and does not have BAS; Table 1, entry 7), an excessive amount of strong BAS could facilitate the cracking of long-chain hydrocarbons and generation of coking species,37,38 which decrease the catalytic stability and selectivity toward aromatics. The acidity of BAS increased with 2.0 M NaOH-treated HZSM-5 by desilication, leading to the decline in aromatic selectivity (Table 1, entry 8). Alongside acidity, the hierarchical pore structure created during NaOH treatment can also promote the catalytic performance of the FeZnNa@zeolite catalyst. Such a pore structure can serve as highly effective micro-reactors for aromatization of
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alkene produced over FeZnNa catalyst by greatly facilitating the diffusion of reactants and products, resulting in improved accessibility and transformation efficiency. The reaction behavior of both C2H4 and C3H6 was investigated to study the aromatization properties of different ZSM-5 zeolites (Table S3; Figures S11 and S12). Na-type ZSM-5 cannot catalyze the transformation of light olefins, which confirmed the deduction above. Although oligomerization seems to occur on both HZSM-5 and 0.6-HZSM-5, cyclization and dehydrogenation occur much more easily on the latter catalyst. More importantly, the mesoporous zeolite shows high catalytic stability in a diluted C2H4 atmosphere, whereas HZSM-5 almost died after 35 hr on stream. Rapid deactivation was also observed on untreated HZSM-5 when n-C6H12 was used as feed (Table S4; Figures S13 and S14). Besides desilication and the creation of the hierarchical pore structure by NaOH solution treatment, the catalytic performance of treated HZSM-5 can be finely tuned by control of the type and concentration of cations attached to AlF of ZSM-5. If ZSM-5 is fully exchanged with proton, e.g., ion-exchanged with NH4NO3 three times, one AlF site has one associated proton, thus, equivalent to one BAS. We can finely tune the zeolitic acidity of zeolite by controlling the degree of proton-sodium exchange. For example, after treating HZSM-5 with 0.6 M NaOH, we can exchange it with 1 M NH4NO3 solution just once to get 0.6-ZSM-5-a. Compared with 0.6-HZSM-5 (exchanged with NH4NO3 three times), it has the same pore structure and almost identical aluminum species distribution but different amounts of protons associated with AlF species (0.20 versus 0.35 mmol BAS/g zeolite) (Figure 2B; Table S5). In addition, the strength of BAS was also concomitantly modulated, with more strong BAS removed (Figure 2C). Such a sample exhibited promising aromatics selectivity, being as high as 50.6% when coupling with FeZnNa (Table 1, entry 9; Figures S15 and S16), whereas the conversion of CO remains almost intact, consequently resulting in an aromatic yield of more than 32% in a single run. This is the highest aromatic yield reported in a single-run reaction in the FTS process. A comparison of the hydrocarbon selectivity of FeZnNa and [email protected] indicates that almost all olefins participate in the aromatization process (Figure 3).
DISCUSSION The hierarchical pore structure and suitable BAS density and strength of [email protected] are pivotal for its high selectivity toward aromatics in FTS. From the results of the olefin conversion experiments, a simplified scheme for this process is shown as Scheme S1, where olefins play an irreplaceable role as the central hub. The dramatic decrease in the selectivity of heavy hydrocarbons (C12+) suggests that these molecules have been cracked into olefins and subsequently transformed into aromatics and light paraffins on acid sites. Significantly, the selectivity toward CH4 and CO2 is less than 35%, ensuring that most carbon resources transformed into aromatics (dominated by C6–9 aromatics) and light hydrocarbons. The high CO conversion and high product selectivity are pivotal for the separation and practical application of this direct STA process.
EXPERIMENTAL PROCEDURES Synthesis of FeZnNa Catalyst FeZnNa catalyst was prepared by a co-precipitation method. Equal molar weights of ferric nitrate and zinc nitrate were dissolved in water together for a mixed solution. An aqueous solution of sodium carbonate and mixed solution was added to a flask dropwise under stirring, and the pH value was controlled at 8.5–9.0 for the dropping process. After aging for 5 hr, the precipitate was washed with deionized
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Figure 3. Comparison of Product Distribution over FeZnNa and [email protected] with All the Carbon-Containing Products Counted Reaction conditions: 20 mg FeZnNa, 20 mg FeZnNa + 100 mg 0.6-ZSM-5-a, CO/H 2 /CO 2 / Ar = 24:64:8:4, 613 K, 20 bar, total flow rate = 20 mL/min, time on stream = 40 hr. Catalyst was treated in 5% H2 /N 2 at 623 K for 2 hr before the reaction.
water (5 3 100 mL) and dried at 393 K for 10 hr and then calcined in a muffle furnace at 623 K for 4 hr. The content of sodium was 0.8 wt % derived from inductively coupled plasma (ICP) atomic emission spectrometry analysis (conducted with a PROFILE SPEC). Synthesis of Modified ZSM-5 H-ZSM-5 with a SiO2/Al2O3 ratio of 30 was purchased from Nankai University Catalyst. HZSM-5 (4.0 g) was treated in 100 mL of aqueous solutions of NaOH at different concentrations for 1 hr at 353 K. The filter cake was washed with deionized water, dried at 393 K for 10 hr, and then calcined at 623 K for 5 hr. The powder obtained was denoted as NaZSM-5. NaZSM-5 (1.0 g) was exchanged in 100 mL of an aqueous solution of 1.0 mol/L NH4NO3. The filter cake was dried at 393 K for 10 hr and then calcined at 623 K for 5 hr to get proton-formed zeolite. Unless otherwise indicated, the zeolite used in the reaction was exchanged three times. Synthesis of Reference Catalysts Fe/SiO2 and Fe/HZSM-5 were prepared by the incipient impregnation method using ferric nitrate as the precursor. After impregnation, the sample was dried at 393 K for 10 hr, followed by calcination in air at 773 K for 5 hr. The loading of Fe in each catalyst was fixed at 10.0 wt %.
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MAS NMR and MQ MAS 1 H and 27Al MAS NMR experiments were performed on a Bruker Avance III 600 spectrometer at resonance frequencies of 600.5, 156.5, and 119.3 MHz. Before each 1H MAS NMR experiment, about 0.3 g of the sample was packed into a glass tube, which was later connected to a vacuum line. The temperature was gradually increased at a rate of 2 K/min to 693 K and kept at a pressure below 10 2 Pa for 20 hr to dehydrate the sample. After the samples were cooled to room temperature, the glass tubes were flame sealed. Then the samples were transferred into a 4 mm ZrO2 rotor under a dry nitrogen atmosphere in a glove box. 1H MAS NMR spectra were acquired with a Hahn-echo sequence for removal of the background signal arising from the probe; the p/2 and p pulse lengths were 4.2 and 8.4 ms, respectively, at a spinning rate of 13 kHz and a recycle delay of 5 s. The 27 Al MAS NMR spectra were acquired on a 4 mm probe with a spinning rate of 13 kHz, a p/18 pulse length of 0.3 ms, a recycle delay of 1 s, and with 1 M Al(NO3)3 solution as a reference. The chemical shifts of 1H were referenced to trimethylsilane. MQ MAS spectra were recorded with a sequence of three pulses. The pulse lengths were p1 = 6.8 ms, p2 = 2 ms, and p3 = 29 ms. The data were processed and sheared after Fourier transformation with Bruker Topspin. Catalytic Tests The reaction was carried out in a three-channel fixed-bed reactor. All the catalyst was sieved through a 40–60 mesh. FeZnNa and ZSM-5 were physically mixed and put into a stainless steel pipe with a quartz tube. Before the reaction, all catalyst was pretreated in a 5% H2 stream at 623 K for 5 hr and cooled down to 553 K. A syngas (24% CO, 8% CO2, 64% H2, and 4% Ar) stream was pumped at a flow rate of 20 mL/min. The pressure in the reactor was tuned to 20 bar and the temperature of the catalyst bed was maintained at 613 K. Olefin conversion experiment conditions: zeolite 100 mg, 613 K, 20 bar, C2H4/Ar/N2 = 20:5:75, C3H6/Ar/N2 = 7.5:2.5:90, C3H8/Ar/ N2 = 10:2:88, total flow rate 20 mL/min. Feed of n-C6H12 0.015 mL/min by Lab Alliance Series II pump, flow rate of N2 20 mL/min, 1.0 bar. The products were separated by a condensing collector. Gaseous products were detected online by an Agilent GC 7890A system. A Hayesep-Q plot and a MolSieve 5A plot were packed on a thermal conductivity detector for the separation of CO2, Ar, CH4, and CO. An HP PLOT Al2O3 was packed on a flame ionization detector (FID) for the separation of hydrocarbons (C1–C7). CO conversion was calculated by the internal standard method with Ar. Liquid products were detected on an Agilent GC 7820 system with an HP-5 capillary column and an FID. The detailed aromatic product distribution was determined by Agilent GC-MS (7890A GC+ 5975 MS).
SUPPLEMENTAL INFORMATION Supplemental Information includes Supplemental Experimental Procedures, 16 figures, 5 tables, and 1 scheme and can be found with this article online at http://dx. doi.org/10.1016/j.chempr.2017.06.017.
AUTHOR CONTRIBUTIONS D.M. and W.F. conceived and designed this work; B.Z. and P.Z. designed the experiments, catalyst preparation, and catalytic reactions; P.W. did the NMR measurements; J.L., T.L., M.P., and M.Z. did part of the catalytic evaluation as well as other
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characterization measurements; D.M. and P.Z. wrote the paper; and all authors participated in the analysis of the experimental data and discussions of the results, as well as preparation of the paper.
ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of China (91645115, 21473003, 21222306, 21473229, and 91545121) and 973 Project (2017YFB0602200 and 2013CB933100). Received: February 28, 2017 Revised: March 27, 2017 Accepted: June 26, 2017 Published: August 3, 2017
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Direct Transformation of Syngas to Aromatics over Na-Zn-Fe5C2 and Hierarchical HZSM-5 Tandem Catalysts Bo Zhao, Peng Zhai, Pengfei Wang, ..., Qianwen Zhang, Weibin Fan, Ding Ma [email protected] (W.F.) [email protected] (D.M.)
HIGHLIGHTS An unprecedented aromatic yield via direct syngas conversion is obtained The aromatic selectivity is dependent on the acidity and porous structure of HZSM-5 The pivotal role of the density and strength of acid sites is discussed
Aromatics, the basic chemicals, could be produced from syngas (CO/H2) directly over a physical mixture of a highly effective FTS catalyst, Na-Zn-Fe5C2, and HZSM-5 zeolite. The Na-Zn-Fe5C2 component was highly selective toward olefins; the appropriate density and strength of the Brønsted acid sites and the hierarchical pore structure of the HZSM-5 component converted the formed olefins and endowed the tandem catalyst with an unprecedented aromatic yield.
Zhao et al., Chem 3, 1–11 August 10, 2017 ª 2017 Published by Elsevier Inc. http://dx.doi.org/10.1016/j.chempr.2017.06.017
Please cite this article in press as: Zhao et al., Direct Transformation of Syngas to Aromatics over Na-Zn-Fe5C2 and Hierarchical HZSM-5 Tandem Catalysts, Chem (2017), http://dx.doi.org/10.1016/j.chempr.2017.06.017
Article
Direct Transformation of Syngas to Aromatics over Na-Zn-Fe5C2 and Hierarchical HZSM-5 Tandem Catalysts Bo Zhao,1,6 Peng Zhai,1,6 Pengfei Wang,2,3 Jiaqi Li,1 Teng Li,1 Mi Peng,1 Ming Zhao,4 Gang Hu,5 Yong Yang,2,3 Yong-Wang Li,2,3 Qianwen Zhang,4 Weibin Fan,2,* and Ding Ma1,7,*
SUMMARY
The Bigger Picture
Direct synthesis of aromatics from syngas is a great challenge because of severe operating conditions and low yield of aromatics. Making this process more competitive than the MTA (methanol to aromatics) process will require high energy efficiency and low CO2 emission. A combination of Na-Zn-Fe5C2 and hierarchical HZSM-5 with uniform mesopores dramatically changed the product distribution of Fischer-Tropsch synthesis, leading to 51% aromatic selectivity under the stable stage with CO conversion >85%. C12+ heavy hydrocarbons almost disappeared, and the catalyst showed good stability. The hierarchical zeolitic structure and Brønsted acidity of HZSM-5 could be precisely tuned by controlling the alkali treatment conditions and the degree of ion exchange. The appropriate density and strength of the Brønsted acid sites and the hierarchical pore structure of HZSM-5 endowed the catalyst with an unprecedented aromatic yield. This work shows a broad area for development for syngas conversion.
The depletion of petroleum reserves threatens economic development and regional stability nowadays. Building an alternative route for fuel and chemical production has become a focused research topic in recent years. Although conversion of syngas to fuels by the FischerTropsch synthesis (FTS) process has been industrialized in various countries, expanding its products to value-added chemicals is still a great challenge. Aromatics are very important feedstock; they are used as blending mixtures for gasoline with a high octane number in some countries and are the most important molecular platform for the polymer industry. With the fast development of the polymer industry, the gap between supply and demand calls for a new strategy for the synthesis of aromatics. We have established an intriguing process that directly converts syngas to aromatics on the basis of a physical mixture of a highly effective FTS catalyst and zeolite, which shows enormous potential in comparison with the traditional multiple-step pathway.
INTRODUCTION Transformation of syngas (CO/H2) derived from shale gas, biomass, and coal has been developed as a promising alternative to oil for the preparation of liquid fuels and commodity chemicals.1–5 The Fischer-Tropsch synthesis (FTS) process is well known. The product selectivity of FTS is limited by the law of polymerization, i.e., Anderson-Schulz-Flory distribution, which restricts industrial application as the result of a very wide product distribution. Recently, different novel catalyst design strategies have been developed to confine the products into a narrow range by altering the electronic property of metal active sites and the steric structure of the catalyst.6–12 With S and Na as promoters, high selectivity to low olefins was obtained over a supported Fe catalyst.7 In addition, the introduction of acidic zeolite in an FTS catalyst initiates consecutive reactions, such as cracking and isomerization of primary products, leading to excellent selectivity toward high-quality gasoline or diesel.8–10 Very recently, it was reported that coupling of high-temperature methanol synthesis catalyst with SAPO (silico-alumino-phosphate) molecular sieves produced more than 80% of C2–C4 olefins in hydrocarbons, although high temperature (673 K) facilitated the water gas shift reaction and generated a large amount of CO2.11,12 Aromatics, mainly produced from petroleum refinement processes, are used as a fuel additive in some countries and are the most important platform molecules for the polymer industry. With the fast development of the economy, the gap between
Chem 3, 1–11, August 10, 2017 ª 2017 Published by Elsevier Inc.
1
Please cite this article in press as: Zhao et al., Direct Transformation of Syngas to Aromatics over Na-Zn-Fe5C2 and Hierarchical HZSM-5 Tandem Catalysts, Chem (2017), http://dx.doi.org/10.1016/j.chempr.2017.06.017
supply and demand calls for a new strategy for the synthesis of aromatics. Starting from syngas, typically, a two-step process is needed, i.e., we need to transform syngas to methanol on a CuZn catalyst (493–573 K) first, and then convert methanol to aromatics over a zeolite catalyst (MTA, >673 K).13 However, this two-step process is highly cost and energy effective. If we could combine the two processes into one, i.e., direct conversion of syngas into aromatics (STA), this would not only greatly decrease the equipment and operation costs but would also have an advantage in product separation and recycling because the reactants are in the gas phase and the main products are liquid. (n + 6)CO + (2n + 9)H2 / Cn+6H2n+6 + (n+6)H2O
(n R 0)
Because of the mismatch between the operating temperature of the CuZn catalyst and that of aromatization catalysts,14 many efforts have been devoted to coupling a high-temperature methanol synthesis catalyst with an MTA catalyst.15,16 Pioneering work by Chang et al.17 indicated that 70% aromatics selectivity could be reached over a Zn-Cr/HZSM-5 catalyst, demonstrating the effectiveness of this concept. However, the consecutive reactions required severe operating conditions and showed low reactivity. On the other hand, some researchers combined an Fe-based FTS catalyst with zeolite to directly synthesize aromatics from syngas. Guan et al.18 reported 40.4% aromatics selectivity in C5+ products on an Fe/MnO-GaZSM-5 catalyst, but rapid deactivation occurred with time on stream. Recently, Yan et al.19 investigated the influence of reaction temperature, pressure, and space velocity on the production of aromatics on an Fe-Pd/HZSM-5 catalyst, yet it is difficult to obtain high aromatic selectivity as well as high CO conversion on these catalysts.20,21 As the combination of high CO conversion and high aromatic selectivity would confer an immediate advantage in industrial applications of the STA process, the development of more efficient catalyst is highly desired. Here, we have constructed a tandem catalyst system by combining Na-Zn-Fe5C2 catalysts (with high CO conversion and high selectivity toward alkenes)22 with hierarchical HZSM-5 for the direct conversion of syngas to aromatics. By physically mixing the highly selective Na-Zn-Fe5C2 catalyst with the mesoporous HZSM-5 zeolite, we successfully obtained high aromatic selectivity with high CO conversion. The total selectivity of undesired C1 products, including CH4 and CO2, has been successfully suppressed to less than 35%, and the yield of aromatics reached as high as 32.6% in a single run. 1Beijing
RESULTS Preliminary Catalytic Performance for STA Table 1 shows the catalytic activity and product distribution of syngas conversion at 613 K and 20 bar. Fe-based FTS processes generally operate between 523 and 623 K. The main products are alkanes containing 30.8% CH4 over a simple SiO2supported iron catalyst (Table 1, entry 1). At such a temperature range, aromatization of alkanes through dehydrogenation/oligomerization/cyclization over a solid acid catalyst is not possible because it needs a high reaction temperature, e.g., 823 K.23 Therefore, a combination of Fe and HZSM-5 (SiO2/Al2O3 = 30) (Table 1, entry 2) generated C2+ paraffin as the main product, with aromatic selectivity of only 10.9% at 613 K. By comparing with the catalytic results of Fe/SiO2, it can be deduced that aromatics are formed at the expense of olefins on acidic HZSM-5. Recently, a high-performance catalyst, Na-Zn-Fe5C2 (termed as FeZnNa), was developed by the co-precipitation method (Figures S1–S3).22 The molar weight ratio of
2
Chem 3, 1–11, August 10, 2017
National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
2State
Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, 27 South Taoyuan Road, Taiyuan, Shanxi 030001, China
3Synfuels
China, Beijing 100195, China
4Faculty
of Materials and Chemical Engineering, Beijing Institute of Petrochemical Technology, Beijing 100029, China
5Israel
Chemicals Limited, Shanghai 200021,
China 6These 7Lead
authors contributed equally
Contact
*Correspondence: [email protected] (W.F.), [email protected] (D.M.) http://dx.doi.org/10.1016/j.chempr.2017.06.017
Please cite this article in press as: Zhao et al., Direct Transformation of Syngas to Aromatics over Na-Zn-Fe5C2 and Hierarchical HZSM-5 Tandem Catalysts, Chem (2017), http://dx.doi.org/10.1016/j.chempr.2017.06.017
Table 1. Comparison of the CO Conversion and Product Distribution of Different Catalysts for Syngas Conversion Entry
Catalyst
Al (mmol/g)a
–
B Site (mmol/g)b
XCO (%)
–
38.6
CO2 Selectivity (%)
Selectivity in Hydrocarbons (%) CH4
C2–C4 Olefin
C2–C4 Paraffin
C5+c
Aromatics
11.3
30.8
13.6
17.0
38.6
–
1
Fe/SiO2
2
Fe/HZSM-5
1.06
1.02
28.1
13.7
29.5
1.0
29.1
29.5
10.9
3
FeZnNa
–
–
74.9
27.3
10.9
25.5
4.4
58.1
1.1
4
FeZnNa@HZSM-5
1.06
1.02
88.3
27.7
11.3
1.4
38.6
28.9
19.8
5
[email protected]
1.04
0.46
84.2
26.5
9.9
0.7
35.6
22.7
30.9
6
[email protected]
1.36
0.35
89.2
26.9
9.8
1.0
26.8
21.8
40.5
7
[email protected]
1.36
0
86.6
27.2
10.1
20.4
15.8
52.6
2.1
8
[email protected]
1.69
0.36
88.5
27.8
9.8
2.9
21.9
38.2
26.9
9
[email protected]
1.36
0.20
88.8
27.5
9.6
1.0
25.6
13.2
50.6
Reaction conditions: 120 mg catalyst (20 mg FeZnNa + 100 mg ZSM-5, physical mixture), CO/H2/CO2/Ar = 24:64:8:4, 613 K, 20 bar, total flow rate = 20 mL/min, time on stream = 40 hr. Catalyst was treated in 5%H2/N2 at 623 K for 2 hr before the reaction. a Aluminum content per gram of zeolite, determined by ICP. b Brønsted sites per gram of zeolite, determined by 1H MAS NMR. c C5+ products except for aromatics.
Zn and Fe was 1:1, with 0.8 wt % sodium content caused by residual precipitate. Seventy-eight percent of the hydrocarbon product was olefins, and undesired C1 (CO2, CH4) products were successfully suppressed with high CO conversion (Table 1, entry 3). It was found that Fe5C2 was the active phase in this catalyst, and the sodium acted as an electronic promoter that affected the elementary reaction on the surface to produce more alkenes. The thermodynamic calculation shows that, different from alkanes, aromatization of alkenes is feasible below 623 K and long-chain alkenes are apt to cyclization without oligomerization (Figure S4).24 Thus, the alkene-favorable Na-Zn-Fe5C2 catalyst would be an ideal platform for successive aromatization reactions. After physically mixing Na-Zn-Fe5C2 with HZSM-5, the aromatic selectivity increased sharply from 1.1% to 19.8% (Table 1, entry 4), whereas C2–C4 olefin selectivity decreased dramatically from 25.5% to 1.4%, and the olefin to paraffin ratio of hydrocarbons decreased greatly from 5.8 to 0.036. Simultaneously, CO conversion increased from 74.9% to 88.3% in comparison with pure Na-Zn-Fe5C2 catalyst. This can be attributed to the tension force by consecutive consumption of intermediates, which in turn favors the conversion of the initial reactants. The sharp increase in C2–C4 paraffin selectivity from 4.4% to 38.6% is due to the occurrence of dealkylation of polyalkylbenzene and the hydrogen transfer reaction within the reaction network.25 Hydrogen generated during the aromatization process is transferred into unsaturated hydrocarbons, leading to the formation of additional paraffin. Modified HZSM-5 by Base Treatment Although HZSM-5 can catalyze different types of acid-based reactions, such as dehydration of alcohols, cracking of hydrocarbons, oligomerization and cyclization of olefins, and dehydrogenation of paraffin,26 different types of reactions require different acidity, porosity, and reaction conditions. As the acid properties of zeolites depend primarily on the location, the geometry, and the coordination of aluminum in its framework, zeolite acidity can be engineered to improve its catalytic performance in specific reactions. In the current study, removal of excessive acid sites and creation of additional pore structures seem to be a promising way to further increase the selectivity toward aromatics, because aromatization reactions usually require mild
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Brønsted acidity27,28 and a hierarchical pore structure, which ensures fast product diffusion and desorption.29,30 We treated pristine HZSM-5 with NaOH solutions of different concentrations, which not only adjusts its acidity but also creates mesostructured porosity.31,32 The treated zeolites were termed x-HZSM-5, where x represents the NaOH concentration. For example, 0.1-HZSM-5 means HZSM-5 treated with 0.1 M NaOH solution at 353 K for 1 hr and then exchanged with 1 M NH4NO3 solution three times (all the samples were exchanged three times unless specified otherwise). Figure 1A shows that all the treated HZSM-5 samples show diffraction patterns characteristic of MFI structure, but the diffraction lines of the samples treated with R0.6 M NaOH solution were obviously broadened and decreased in intensity. This phenomenon suggests that NaOH treatment partially destroyed HZSM-5 structure by desilication,33 whereas the main MFI architecture was preserved. The N2 adsorption/desorption isotherms clearly show that HZSM-5 has a typical microporous structure. In contrast, mesoporous structures were formed in NaOHtreated HZSM-5, as revealed by its H4-type N2 sorption isotherms (Figures 1B and 1C). Upon NaOH treatment, an obvious increase in the mesopore volume was observed despite the fact that the surface area and micropore volume remained similar (except for 2.0-HZSM-5; its surface area and micropore volume declined significantly as a result of severe destruction of the ZSM-5 structure) (Table S1). The mesopore volume and diameter determined from the desorption branch by the Barrett-Joyner-Halenda (BJH) method increased with increasing NaOH concentration up to 1.0 M, indicating the presence of large numbers of mesopores in the sample. Transmission electron microscopy shows that 2–3 mm pristine HZSM-5 crystals with smooth surfaces became sponge-like because plenty of mesopores ranging from 4 to 10 nm were created as a result of the NaOH treatment (Figures 1D and 1E). These results indicate that the hierarchical pore structure in ZSM-5 crystals was formed through a desilication process (Table S2). Given that acid site and aluminum in HZSM-5 play a critical role in the aromatization process, the distribution and reconstruction of aluminum species and acid sites in parent HZSM-5 and those treated with NaOH were tracked with NH3-temperature-programmed desorption (TPD), pyridine-infrared, 27Al magic angle spinning (MAS) and multiple quantum (MQ)-MAS nuclear magnetic resonance (NMR), and 1 H MAS NMR techniques (Figures 2 and S5–S7). Treatment with 0.1 M NaOH solution partially removed framework aluminum (AlF, at 55 ppm, the main origin of Brønsted acid sites) and led to the formation of species with distorted framework Al (AlDF, with relatively weak Brønsted acidity), and six-coordinated (Al6NF, at about 0 ppm) Al species.34 With the increase of NaOH concentration (0.6 M), more AlF species were expelled from the zeolite framework; five-coordinated (Al5NF, around 30 ppm) and ex-framework or tri-coordinated Al species (a very broad hump, socalled invisible Al) were clearly resolved on 27Al MAS and 27Al MQ MAS NMR spectra (Figures 2A, 2D, and 2E).35,36 As strong Brønsted acid sites (BAS) are associated with AlF, the removal of AlF from the zeolitic lattice will decrease the amount of BAS in the catalyst. The NH3-TPD profile indicates that the ammonia desorption associated with strong acid sites (>573 K) decreased with the degree of NaOH treatment (Figure 2C). Direct evidence about BAS is provided by the 1H MAS NMR profile (Figure 2B). It shows that the peaks characteristic of Brønsted acid sites (4.0 and 6.1 ppm) decreased sharply after NaOH treatment (0.1 M), suggesting the removal of large amounts of BAS in comparison with parent HZSM-5 (0.46 versus 1.02 mmol/g). When the concentration of NaOH solution increased to 0.6 M, the concentration
4
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Figure 1. The Structure of the Modified HZSM-5 (A) X-ray diffraction patterns of modified HZSM-5. (B) N 2 adsorption-desorption isotherms for modified HZSM-5. (C) Pore-size distributions of HZSM-5 derived from the N2 adsorption-desorption isotherms by the BJH method. The number before HZSM-5 represents the NaOH concentration of the solution that treated HZSM-5. (D) TEM micrograph of the HZSM-5. The scale bar represents 50 nm. (E) TEM micrograph of 0.6-HZSM-5. The scale bar represents 50 nm.
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Figure 2. The Acidity and Framework Structure of Modified ZSM-5 (A) 27 Al MAS NMR spectra. (B) 1 H MAS NMR spectra. (C) NH 3 -TPD profiles for HZSM-5 prepared with different concentrations of NaOH or different ionexchange times. (D) 27 Al MQ MAS NMR spectra of HZSM-5. (E) 27 Al MQ MAS NMR spectra of 0.6-HZSM-5.
of BAS decreased further to 0.35 mmol/g, and its strength also weakened (Figure 2C). STA on FeZnNa and Modified ZSM-5 Tandem Catalysts After Brønsted acidity decreased with 0.1 M NaOH treatment, the aromatic selectivity increased greatly to 30.9% on [email protected] with a decrease in selectivity toward methane and C2–C4 paraffins (Table 1, entry 5). With the further reduction of strong Brønsted acidity sites, the selectivity toward aromatics reached 40.5% on [email protected] (Table 1, entry 6; Figures S8–S10), which is twice that of FeZnNa@HZSM-5 catalyst. Although BAS is the active site for aromatization, as supported by the very low aromatic selectivity obtained over the [email protected] catalyst (which has not been exchanged with NH4NO3 solution and does not have BAS; Table 1, entry 7), an excessive amount of strong BAS could facilitate the cracking of long-chain hydrocarbons and generation of coking species,37,38 which decrease the catalytic stability and selectivity toward aromatics. The acidity of BAS increased with 2.0 M NaOH-treated HZSM-5 by desilication, leading to the decline in aromatic selectivity (Table 1, entry 8). Alongside acidity, the hierarchical pore structure created during NaOH treatment can also promote the catalytic performance of the FeZnNa@zeolite catalyst. Such a pore structure can serve as highly effective micro-reactors for aromatization of
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alkene produced over FeZnNa catalyst by greatly facilitating the diffusion of reactants and products, resulting in improved accessibility and transformation efficiency. The reaction behavior of both C2H4 and C3H6 was investigated to study the aromatization properties of different ZSM-5 zeolites (Table S3; Figures S11 and S12). Na-type ZSM-5 cannot catalyze the transformation of light olefins, which confirmed the deduction above. Although oligomerization seems to occur on both HZSM-5 and 0.6-HZSM-5, cyclization and dehydrogenation occur much more easily on the latter catalyst. More importantly, the mesoporous zeolite shows high catalytic stability in a diluted C2H4 atmosphere, whereas HZSM-5 almost died after 35 hr on stream. Rapid deactivation was also observed on untreated HZSM-5 when n-C6H12 was used as feed (Table S4; Figures S13 and S14). Besides desilication and the creation of the hierarchical pore structure by NaOH solution treatment, the catalytic performance of treated HZSM-5 can be finely tuned by control of the type and concentration of cations attached to AlF of ZSM-5. If ZSM-5 is fully exchanged with proton, e.g., ion-exchanged with NH4NO3 three times, one AlF site has one associated proton, thus, equivalent to one BAS. We can finely tune the zeolitic acidity of zeolite by controlling the degree of proton-sodium exchange. For example, after treating HZSM-5 with 0.6 M NaOH, we can exchange it with 1 M NH4NO3 solution just once to get 0.6-ZSM-5-a. Compared with 0.6-HZSM-5 (exchanged with NH4NO3 three times), it has the same pore structure and almost identical aluminum species distribution but different amounts of protons associated with AlF species (0.20 versus 0.35 mmol BAS/g zeolite) (Figure 2B; Table S5). In addition, the strength of BAS was also concomitantly modulated, with more strong BAS removed (Figure 2C). Such a sample exhibited promising aromatics selectivity, being as high as 50.6% when coupling with FeZnNa (Table 1, entry 9; Figures S15 and S16), whereas the conversion of CO remains almost intact, consequently resulting in an aromatic yield of more than 32% in a single run. This is the highest aromatic yield reported in a single-run reaction in the FTS process. A comparison of the hydrocarbon selectivity of FeZnNa and [email protected] indicates that almost all olefins participate in the aromatization process (Figure 3).
DISCUSSION The hierarchical pore structure and suitable BAS density and strength of [email protected] are pivotal for its high selectivity toward aromatics in FTS. From the results of the olefin conversion experiments, a simplified scheme for this process is shown as Scheme S1, where olefins play an irreplaceable role as the central hub. The dramatic decrease in the selectivity of heavy hydrocarbons (C12+) suggests that these molecules have been cracked into olefins and subsequently transformed into aromatics and light paraffins on acid sites. Significantly, the selectivity toward CH4 and CO2 is less than 35%, ensuring that most carbon resources transformed into aromatics (dominated by C6–9 aromatics) and light hydrocarbons. The high CO conversion and high product selectivity are pivotal for the separation and practical application of this direct STA process.
EXPERIMENTAL PROCEDURES Synthesis of FeZnNa Catalyst FeZnNa catalyst was prepared by a co-precipitation method. Equal molar weights of ferric nitrate and zinc nitrate were dissolved in water together for a mixed solution. An aqueous solution of sodium carbonate and mixed solution was added to a flask dropwise under stirring, and the pH value was controlled at 8.5–9.0 for the dropping process. After aging for 5 hr, the precipitate was washed with deionized
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Figure 3. Comparison of Product Distribution over FeZnNa and [email protected] with All the Carbon-Containing Products Counted Reaction conditions: 20 mg FeZnNa, 20 mg FeZnNa + 100 mg 0.6-ZSM-5-a, CO/H 2 /CO 2 / Ar = 24:64:8:4, 613 K, 20 bar, total flow rate = 20 mL/min, time on stream = 40 hr. Catalyst was treated in 5% H2 /N 2 at 623 K for 2 hr before the reaction.
water (5 3 100 mL) and dried at 393 K for 10 hr and then calcined in a muffle furnace at 623 K for 4 hr. The content of sodium was 0.8 wt % derived from inductively coupled plasma (ICP) atomic emission spectrometry analysis (conducted with a PROFILE SPEC). Synthesis of Modified ZSM-5 H-ZSM-5 with a SiO2/Al2O3 ratio of 30 was purchased from Nankai University Catalyst. HZSM-5 (4.0 g) was treated in 100 mL of aqueous solutions of NaOH at different concentrations for 1 hr at 353 K. The filter cake was washed with deionized water, dried at 393 K for 10 hr, and then calcined at 623 K for 5 hr. The powder obtained was denoted as NaZSM-5. NaZSM-5 (1.0 g) was exchanged in 100 mL of an aqueous solution of 1.0 mol/L NH4NO3. The filter cake was dried at 393 K for 10 hr and then calcined at 623 K for 5 hr to get proton-formed zeolite. Unless otherwise indicated, the zeolite used in the reaction was exchanged three times. Synthesis of Reference Catalysts Fe/SiO2 and Fe/HZSM-5 were prepared by the incipient impregnation method using ferric nitrate as the precursor. After impregnation, the sample was dried at 393 K for 10 hr, followed by calcination in air at 773 K for 5 hr. The loading of Fe in each catalyst was fixed at 10.0 wt %.
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MAS NMR and MQ MAS 1 H and 27Al MAS NMR experiments were performed on a Bruker Avance III 600 spectrometer at resonance frequencies of 600.5, 156.5, and 119.3 MHz. Before each 1H MAS NMR experiment, about 0.3 g of the sample was packed into a glass tube, which was later connected to a vacuum line. The temperature was gradually increased at a rate of 2 K/min to 693 K and kept at a pressure below 10 2 Pa for 20 hr to dehydrate the sample. After the samples were cooled to room temperature, the glass tubes were flame sealed. Then the samples were transferred into a 4 mm ZrO2 rotor under a dry nitrogen atmosphere in a glove box. 1H MAS NMR spectra were acquired with a Hahn-echo sequence for removal of the background signal arising from the probe; the p/2 and p pulse lengths were 4.2 and 8.4 ms, respectively, at a spinning rate of 13 kHz and a recycle delay of 5 s. The 27 Al MAS NMR spectra were acquired on a 4 mm probe with a spinning rate of 13 kHz, a p/18 pulse length of 0.3 ms, a recycle delay of 1 s, and with 1 M Al(NO3)3 solution as a reference. The chemical shifts of 1H were referenced to trimethylsilane. MQ MAS spectra were recorded with a sequence of three pulses. The pulse lengths were p1 = 6.8 ms, p2 = 2 ms, and p3 = 29 ms. The data were processed and sheared after Fourier transformation with Bruker Topspin. Catalytic Tests The reaction was carried out in a three-channel fixed-bed reactor. All the catalyst was sieved through a 40–60 mesh. FeZnNa and ZSM-5 were physically mixed and put into a stainless steel pipe with a quartz tube. Before the reaction, all catalyst was pretreated in a 5% H2 stream at 623 K for 5 hr and cooled down to 553 K. A syngas (24% CO, 8% CO2, 64% H2, and 4% Ar) stream was pumped at a flow rate of 20 mL/min. The pressure in the reactor was tuned to 20 bar and the temperature of the catalyst bed was maintained at 613 K. Olefin conversion experiment conditions: zeolite 100 mg, 613 K, 20 bar, C2H4/Ar/N2 = 20:5:75, C3H6/Ar/N2 = 7.5:2.5:90, C3H8/Ar/ N2 = 10:2:88, total flow rate 20 mL/min. Feed of n-C6H12 0.015 mL/min by Lab Alliance Series II pump, flow rate of N2 20 mL/min, 1.0 bar. The products were separated by a condensing collector. Gaseous products were detected online by an Agilent GC 7890A system. A Hayesep-Q plot and a MolSieve 5A plot were packed on a thermal conductivity detector for the separation of CO2, Ar, CH4, and CO. An HP PLOT Al2O3 was packed on a flame ionization detector (FID) for the separation of hydrocarbons (C1–C7). CO conversion was calculated by the internal standard method with Ar. Liquid products were detected on an Agilent GC 7820 system with an HP-5 capillary column and an FID. The detailed aromatic product distribution was determined by Agilent GC-MS (7890A GC+ 5975 MS).
SUPPLEMENTAL INFORMATION Supplemental Information includes Supplemental Experimental Procedures, 16 figures, 5 tables, and 1 scheme and can be found with this article online at http://dx. doi.org/10.1016/j.chempr.2017.06.017.
AUTHOR CONTRIBUTIONS D.M. and W.F. conceived and designed this work; B.Z. and P.Z. designed the experiments, catalyst preparation, and catalytic reactions; P.W. did the NMR measurements; J.L., T.L., M.P., and M.Z. did part of the catalytic evaluation as well as other
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characterization measurements; D.M. and P.Z. wrote the paper; and all authors participated in the analysis of the experimental data and discussions of the results, as well as preparation of the paper.
ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of China (91645115, 21473003, 21222306, 21473229, and 91545121) and 973 Project (2017YFB0602200 and 2013CB933100). Received: February 28, 2017 Revised: March 27, 2017 Accepted: June 26, 2017 Published: August 3, 2017
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