Effect of Metal Active Sites on the Product ... - ACS Publications

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Effect of Metal Active Sites on the Product Distribution over Composite Catalysts in the Direct Synthesis of Aromatics from Syngas Tianhui Yang, Like Cheng, Na Li, and Dianhua Liu* State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China S Supporting Information *

ABSTRACT: The conversion of syngas to aromatics over composite catalysts, comprising a mixture of Fe2O3-SiO2 and Nb-/Ni- modified HZSM-5 (HZ), CuO-ZnO-Al2O3 (CZA) and Nb-/Ni- modified HZ, and a mixture of above two kinds of metal oxides and Ni-HZ, were investigated at 330 °C, 4 MPa and H2:CO = 2:1. Catalysts were characterized to analyze the properties. Fe 2O3-SiO2/modified-HZ had the high selectivity of BTX, while CuO-ZnO-Al2O3/modified-HZ had the high selectivity of durene. Fe-CZA/Ni-HZ exhibited higher selectivity of trimethylbenzene (triMB) than both Fe/Ni-HZ and CZA/Ni-HZ, lower selectivity of BTX and tetramethylbenzene (tetraMB), compared with Fe/Ni-HZ and CZA/Ni-HZ, respectively. Reaction pathways were speculated to illustrate the diverse distribution of aromatics. The incorporation of Nb and Ni on the parent HZSM-5 increased the selectivity of aromatics due to their various acidity. The highest selectivity of BTX was obtained over Fe/Ni-HZ and the highest selectivity of durene was achieved over CZA/Nb-Ni-HZ. aromatics at a higher temperature above 300 °C, they show a high selectivity of methane and low selectivity of liquid hydrocarbons, which are not suitable for the reaction of syngas to aromatics.15 Precipitated iron-based metal oxides can produce liquid hydrocarbons in a wide temperature range. Hence, in comparison with supported cobalt-based metal oxides, precipitated iron-based metal oxides are more suitable for the reaction of syngas to aromatics in spite of their higher water−gas shift (WGS) activity. Fe-MnO/ZSM-5 has been used to convert syngas into aromatics.2,8 Composite catalyst comprising a physical mixture of KFeCo and ZSM-5 zeolite has been studied to produce gasoline-range isoparaffins and aromatics.16 In addition to that, Fe2O3-SiO2, with a small amount addition of silicon to decentralize active center and increase the mechanical strength of catalyst, was widely studied in converting syngas to hydrocarbons.17,18 There is no research studied on the reaction of syngas to aromatics over Fe2O3-SiO2 catalyst mixed with ZSM-5 zeolites. Besides, copper-based and palladium-based metal oxides have great catalytic performance to produce methanol. Syngas can convert to aromatics through the intermediate step of syngas to methanol or dimethyl ether.4,6 The copper-based multimetal oxides (such as CuO-ZnO-Al2O3) are widely used to produce methanol at a temperature of 250−270 °C.19,20 In the 1970’s,

1. INTRODUCTION Aromatic hydrocarbons, as important petrochemical raw materials, are usually produced by the method of naphtha reforming. Except for that, the approach to produce aromatic hydrocarbons from syngas was put forward by Chang1 for the first time, and it has been one of the promising technologies to relieve the dependence of petroleum feedstock nowadays. The aromatic hydrocarbons can be obtained from syngas over composite catalyst systems composed of metal oxides and ZSM-5 zeolites.1−4 The reaction can be conducted in dualreactor with optimized condition for each kinds of catalyst;5,6 however, it was researched that the composite catalysts in single reactor had a remarkable catalytic performance for their synergistic effect.7,8 The iron-based and cobalt-based metal oxides can transform syngas into liquid fuel through Fischer−Tropsch synthesis route, providing a wide range of hydrocarbons (C1 ∼ C60).9,10 ZSM-5 zeolites have great catalytic performance in the aspect of cracking, isomerization, and aromatization of hydrocarbons for their unique shape selectivity and acidity. Therefore, the combination of ZSM-5 zeolites with iron- or cobalt-based metal oxides, as composite catalysts, can increase the selectivity of gasoline-range hydrocarbons (C5 ∼ C11) and aromatic hydrocarbons.11,12 However, aromatization requires a higher reaction temperature compared with the production of gasoline-range hydrocarbons. Supported cobalt-based metal oxides, with low water−gas shift (WGS) activity, have a high selectivity of long-chain paraffin in low temperature.13,14 Nevertheless, when mixed with ZSM-5 to convert syngas into © XXXX American Chemical Society

Received: August 21, 2017 Revised: September 26, 2017 Accepted: September 28, 2017

A

DOI: 10.1021/acs.iecr.7b03450 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Mobile Company first put forward that ZSM-5 zeolites could convert methanol to hydrocarbons (MTH).21,22 Since that, MTH reaction including the methanol to aromatics (MTA) process has been extensively investigated. MTA reactions are usually carried out at high temperature above 400 °C in which the catalytic performance of copper-based multimetal oxides is greatly suppressed. However, when applied to the reaction of syngas to aromatics, the composite catalyst, combined of ZSM5 zeolites and CuO-ZnO-Al2O3 or other metal oxides, shows higher conversion rate due to the break of thermodynamic equilibrium.4 For MTA, continuing research efforts have been focused on the modification of ZSM-5 with various metals in order to enhance the selectivity of aromatics, such as Zn, Ni, Cu, Ag, and Ga.23−26 For instance, Ni-modified ZSM-5 are active in the methylation of toluene with methanol.25 Liu et al.27 considered that Nb-modified γ-Al2O3 efficiently converted methanol to dimethyl ether, which is the intermediate of MTA. MTA is considered to produce light aromatic hydrocarbons (BTX). However, compared with MTA, significant difference in the distribution of aromatic product was found in the reaction of syngas to aromatics over the PdO-SiO2/HZSM-5 catalyst system. Fujimoto et al.4 found that aromatic hydrocarbons obtained from syngas over PdO-SiO2/HZSM-5 were mostly tetramethylbenzene and pentamethylbenzene. The reaction of syngas to aromatic rich gasoline was investigated by many researchers. Therefore, a further study is needed to investigate the reaction of syngas to aromatics of high selectivity at lower temperature regions of 320−360 °C. The lower temperature region, which approaches the active temperature of metal oxides, can match with the aromatization temperature of ZSM-5 zeolites. In addition, the low temperature provides the advantage of reducing the amount of carbon deposit and prolonging the catalyst life. In this study, Nb- and Ni-modified HZSM-5 were prepared. The metal-oxide/modified-HZ composite catalysts, consisting of iron-based metal oxide (Fe2O3-SiO2) or copper-based multimetal oxides (CuO-ZnO-Al2O3) and the modified HZSM-5, were applied to the reaction of syngas to aromatics. All composite catalysts presented a high selectivity of aromatics. Different metal oxides had a significant effect on the distribution of aromatic hydrocarbons, and metal modified on HZSM-5 were conducive to improve the selectivity of aromatics. The properties of catalysts were characterized by X-ray powder diffraction (XRD), N2 adsorption−desorption (BET), scanning electron microscopy (SEM), and ammonia temperature-programmed desorption (NH3-TPD).

the method reported by Rocha et al.29 The catalyst was then dried in air overnight, dried at 110 °C for 12 h and calcined at 500 °C for 4 h. The Ni-HZSM-5 and Nb-HZSM-5 were prepared in the same procedure using the method of one-step incipient wetness impregnation. The designated loading of Ni or Nb was 3 wt % of HZSM-5. The method of physical mixing with silica sol (PMS) was adopted to prepare composite catalysts, which was similar to the method described in the literature.30 Powder of metal oxide and modified HZSM-5 zeolites was mixed through silica sol as a bonding agent. The Fe2O3-SiO2 or CuO-ZnO-Al2O3 powder was immersed in the diluted silica sol (12 wt %) for a while, and then they were mixed with the as-prepared HZSM-5 zeolite powder in a mass ratio of 1:1 in a round-bottomed flask under the condition of vigorous stirring. The samples were dried in the air and calcined at 500 °C for 2 h after being filtered and washed to remove the redundant silica. Then these Fe2O3SiO2/modified-HZ catalysts were denoted as Fe/HZ, Fe/NbHZ, Fe/Ni-HZ, and Fe/Nb-Ni-HZ. While, these CuO-ZnOAl2O3/modified-HZ catalysts were denoted as CZA/HZ, CZA/ Nb-HZ, CZA/Ni-HZ, and CZA/Nb-Ni-HZ. The Fe/Ni-HZ and CZA/Nb-Ni-HZ catalysts were also prepared by the method of powder mixing (PM) without silica sol. At last, the multimetal oxides consisted of Fe2O3-SiO2 and CuO-ZnOAl2O3 powder in a mass ratio of 1:1, and then they were mixed with Ni-HZSM-5 in a mass ratio of 1:1 by the method of PMS. This composite catalyst was designed as Fe-CZA/Ni-HZ. All of the composite catalysts were compressed into tablets and then crushed and sieved into 20−40 mesh for catalytic experiments. 2.2. Catalytic Activity Tests. The reactions were performed in a fixed bed tubular reactor with 14 mm internal diameter and 450 mm length. The composite catalysts, mixed with 2 g of quartz (20−40 mesh), were packed in the middle of the reactor which was the isothermal zone. The rest of the tubular reactor was filled with quartz. Prior to the reaction, the Fe2O3-SiO2/modified-HZ composite catalysts were first dried in the flowing N2 for 1.5 h and then reduced in the flowing syngas (H2:CO = 2:1, GHSV = 500 h−1) at 300 °C for 10 h in an atmospheric pressure. While, the CuO-ZnO-Al2O 3/ modified-HZ composite catalysts were reduced in the flowing H2/N2 (H2:N2 = 5:95, GHSV = 1500 h−1) at 170 °C for 3 h and 170−300 °C for 7 h in an atmospheric pressure. Moreover, the Fe-CZA/Ni-HZ catalyst was reduced in the flowing syngas (H2:CO:N2 = 2:1:3, GHSV = 1000 h−1) at 170 °C for 3 h, 170−300 °C for 4 h, and 300 °C for 3 h in an atmospheric pressure. After reduction, the reactions were conducted for 12 h at 4 MPa and 330 °C with flowing syngas (H2:CO = 2:1, GHSV = 1813 h−1) and N2 as an internal standard. The liquid products were collected through compensation and gas−liquid separation. The liquid hydrocarbons were determined by PE 580 gas chromatograph using Elite Wax (30 m * 250 μm * 6 μm) capillary column with a flame ionization detector (FID). The effluent gas were analyzed online using an Agilent 7890B gas chromatograph equipped with a FID and two thermal conductivity detectors (TCD). A HP-AL/S (25 m × 320 μm × 8 μm) capillary column with a FID was used to detect C1 ∼ C4 hydrocarbons. Porapak-Q and MolSieve 5A packed column with two TCD were used to detect H2, CO, N2, and CO2. The product selectivity was calculated according to carbon balance based on inlet/outlet flows of gas composition analysis, and mass balance based on carbon basis was performed by analyzing liquid product weight and gas composition of inlet/outlet flows.

2. EXPERIMENT 2.1. Catalyst Preparation. The copper-based multimetal oxides (CuO-ZnO-Al2O3) were prepared by the optimum coprecipitation method for which details were analogous to the procedure described by Baltes et al.28 The iron-based metal oxides, Fe2O3-SiO2, were prepared by coprecipitation method using Si(OC2H5)4 as a silica source for which details were similar to the procedure described by Suo et al.17 The Nb-Ni-HZSM-5 was prepared using the method of twostep incipient wetness impregnation. The commercial powdered HZSM-5 (SiO2/Al2O3 = 50) was impregnated with the solution of Ni(NO3)2 for 5 h and then dried in air overnight, dried at 110 °C for 12 h, and calcined at 500 °C for 4 h. This Ni-HZSM-5 was then impregnated with the solution of ammonium diperoxodioxalo niobate complex (NH4)3[Nb(O2)2(C2O4)2], which was prepared in two steps based on B


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SiO2/modified-HZ and CuO-ZnO-Al2O3/modified-HZ composite catalysts was comparatively evaluated. As shown in Figure 1, all catalysts exhibited high CO conversion. While, the Fe2O3-SiO2/modified-HZ catalysts

Eight percent N2 was added into the syngas as an internal standard for the calculation of CO conversion and hydrocarbon selectivity. The CO conversion and hydrocarbon selectivity were calculated based on eq 1 and eq 2, respectively.12 In the equations, F0 and F are the flow rates of the syngas and effluent gas after the reaction, respectively, C0 are the concentrations of component in the syngas, Ci are the concentrations of component i in the effluent gas, and n is the carbon number in a product i. conversion of CO (%) = =

0 F 0CCO − FCCO 0 F 0CCO 0 CCO − C N02CCO/C N2 0 CCO

× 100 (1)

selectivity of product i (%) = =

FC in 0 F 0CCO − FCCO

C N02C in 0 C N2CCO − C N02CCO

× 100

Figure 1. CO conversion over the Fe2O3-SiO2/modified-HZ and CuO-ZnO-Al2O3/modified-HZ catalysts (T = 330 °C, P = 4 MPa, GHSV = 1813 h−1, H2/CO = 2:1, and time on stream = 12 h).

(2)

2.3. Characterization of the Catalyst. X-ray powder diffraction (XRD) patterns were obtained by a Rigaku D/max2550 X-ray Diffractometer, using a Cu Kα radiation (λ = 0.154 nm) operating at 40 kV and 100 mA to analyze the crystal phases of catalysts. The scanning was within a range of 5°−75° at a step size and scanning rate of 0.02° and 8°/min, respectively. The N2 adsorption−desorption isotherm were determined by ASAP2020 Adsorption Analyzer (Micrometeritics Corp.) in liquid nitrogen at −196 °C after evacuating at 200 °C for 10 h. The specific surface area of catalyst was calculated according to the BET equation, the pore size was evaluated by BJH equation, and the total pore volume was evaluated at a nitrogen relative pressure of 0.99. The micropore surface and volume were calculated through the t-plot method. The surface morphology of the catalyst was determined by HITACHI S3400N Vacuum Scanning Electron Microscopy (SEM) equipment at an acceleration voltage of 15 kV. Ammonia temperature-programmed desorption (NH3-TPD) were carried out by AutoChem II 2920 Adsorption Analyzer (Micrometeritics Corp.) to obtain the acidity of the catalysts. Each sample was pretreated at 550 °C for 1 h in flowing He (25 mL/min). After the pretreatment, the sample was cooled down to atmospheric temperature and saturated with 5% NH3-He gas. Then, desorption was carried out in flowing He (25 mL/ min) from 100 to 600 °C at a heating rate of 15 °C/min. A thermal conductivity detector (TCD) was applied to the detection of chemisorption ammonia concentration in the exit gas.

showed higher CO conversion of about 99% than CuO-ZnOAl2O3/modified-HZ catalysts of about 91−95%. For the Fe2O3SiO2/modified-HZ catalysts, metal loading had little effect on the CO conversion. While for the CuO-ZnO-Al2O3/modifiedHZ catalysts, CZA/Nb-Ni-HZ significantly improved the CO conversion. This was due to the fact that the CO conversion over single iron-based metal oxide was much higher than that over single copper-based metal oxide at 330 °C.20,31 So the effect of metal loading on the CO conversion over Fe2O3-SiO2/ modified-HZ was not manifest compared with CuO-ZnOAl2O3/modified-HZ. The high CO conversion may attribute to the synergistic effect of the composite catalysts. For the composite catalysts, metal oxides and modified HZSM-5 zeolites were mixed with a small distance. Therefore, after intermediates were formed on the surface of metal oxides, they immediately entered the channel or adhered to the external surface of the zeolites to proceed the subsequent reaction of cracking, dehydrogenation, isomerization and aromatization.32 As a result, the thermodynamic equilibrium can be broken during the reaction process, leading to high CO conversion. The product distribution is shown in Figure 2. According to Figure 2, the products of aromatics, C5+ aliphatic hydrocarbons, C1 ∼ C4 gas hydrocarbons, and CO2 were detected. High aromatic selectivity was obtained suggesting that both ironbased metal oxides and copper-based multimetal oxides mixed with modified HZSM-5 showed excellent catalytic performance at low temperature of 330 °C. The result indicated that the composite catalysts consisting of metal oxides and zeolites showed great property in temperature matching. However, a little amount of methanol was only found over CuO-ZnOAl2O3/modified-HZ catalysts. Such a result demonstrated the different reaction process to produce aromatic hydrocarbons over different metal-oxide/modified-HZ catalyst systems. Fe2O3-SiO2/modified-HZ catalysts showed lower CO2 selectivity than CuO-ZnO-Al2O3/modified-HZ catalysts, indicating that water gas shift was inactive over Fe2O3-SiO2/modified-HZ catalysts compared with that of CuO-ZnO-Al2O3/modified-HZ catalysts. For Fe2O3-SiO2/modified-HZ catalysts, metal species loading increased the selectivity of carbon dioxide. This may

3. RESULT AND DISCUSSION 3.1. Catalytic Performance. The metal-oxide/modifiedHZ composite catalysts were applied to investigate the performance of syngas to aromatics and to compare the different distribution of aromatic hydrocarbons. Metal-modified HZSM-5 were applied to optimize the catalytic performance. 3.1.1. Comparison of the Catalytic Performance over Metal-Oxide/Modified-HZ Composite Catalysts. To investigate the impact of different metal oxides on the reaction of syngas to aromatics, the catalytic performance of the Fe2O3C


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the selectivity of benzene, toluene, and xylene increased on the order of Ni-HZ > Nb-Ni-HZ > Nb-HZ, in comparison with the parent HZ. The increase of xylene selectivity was more prominent than that of benzene and toluene. The BTX selectivity increased from 23.02% over Fe/HZ to 29.49% over Fe/Ni-HZ. The BTX selectivity over Fe/Nb-Ni-HZ (about 27.33%) was between that of Fe/Nb-HZ (about 25.09%) and Fe/Ni-HZ. The result indicated that bimetallic modified HZ was adverse to the generation of BTX. As for CuO-ZnO-Al2O3/ modified-HZ catalysts, the aromatic products of triMB, tetraMB, and A9+ aromatics showed higher selectivity than that of BTX. TetraMB is the aromatic product with high addedvalue, and its downstream products play a significant role in sophisticated technique of aerospace, defense, and other industries. TetraMB showed the highest selectivity in aromatics. In addition, metal loading on parent HZ increased the selectivity of tetraMB on the order of CZA/Nb-Ni-HZ > CZA/Ni-HZ > CZA/Nb-HZ > CZA/HZ. The selectivity of tetraMB increased from 19.41% over CZA/HZ to 31.34% over CZA/Nb-Ni-HZ. Figure 4 shows the distribution of tetraMB over the various CuO-ZnO-Al2O3/modified-HZ composite catalysts. As shown

Figure 2. Product distribution over the Fe2O3-SiO2/modified-HZ and CuO-ZnO-Al2O3/modified-HZ catalysts (T = 330 °C, P = 4 MPa, GHSV = 1813 h−1, H2/CO = 2:1, and time on stream = 12 h; C1 ∼ C4 hydrocarbons = alkane and alkene in the effluent gas; and C5+ = aliphatic hydrocarbons with five and more than five carbons in the liquid phase).

attribute to the synergistic effect of Fe2O3-SiO2 and HZSM-5 after metal modification. For Fe2O3-SiO2/modified-HZ catalysts, aromatics formed through consecutive oligomerization, cyclization, and dehydrogenation of short-chain olefins formed on the Fe2O3-SiO2.16 For CuO-ZnO-Al2O3/modified-HZ catalysts, aromatics generated through aromatization of methanol intermediates. Fe2O3-SiO2 and CuO-ZnO-Al2O3 composite with parent HZ exhibited only 40.3% and 33.4% aromatic selectivity, respectively. It was observed that after introducing metal species to parent HZ, aromatic selectivity over Fe2O3-SiO2/modified-HZ and CuO-ZnO-Al2O3/modified-HZ was markedly enhanced and the formation of C5+ aliphatic hydrocarbons was remarkably suppressed. The highest total aromatic selectivity for the Fe2O3-SiO2/modified-HZ and CuO-ZnO-Al2O3/modified-HZ catalysts were 47.43% and 46.55% over Fe/Ni-HZ and CZA/Nb-Ni-HZ, respectively. Figure 3 shows the distribution of aromatic hydrocarbons. As for Fe2O3-SiO2/modified-HZ catalysts, the distribution of

Figure 4. TetraMB distribution over CuO-ZnO-Al2O3/modified-HZ catalysts (T = 330 °C, P = 4 MPa, GHSV = 1813 h−1, H2/CO = 2:1, and time on stream = 12 h).

in Figure 4, durene had the highest selectivity in tetraMB, while other tetraMB had low selectivity. This attributed to that other tetraMB were retained in the pore of HZSM-5. They only formed on the active sites at external surface of HZSM-5 due to their larger molecular diameters. It was observed that the selectivity of tetraMB was increased after metal modification. In accordance with the result, metal loading simultaneously enhanced the selectivity of durene and other tetraMB on the order of CZA/Nb-Ni-HZ > CZA/Ni-HZ > CZA/Nb-HZ > CZA/HZ. CZA/Nb-Ni-HZ had the highest selectivity of durene. CZA/Nb-Ni-HZ greatly increased the durene selectivity of 26.04% that was 9.53% higher than CZA/HZ of 16.51%. It can be summarized that Fe2O3-SiO2/modified-HZ catalysts exhibited much higher selectivity of light aromatics such as BTX, while CuO-ZnO-Al2O3/modified-HZ catalysts displayed the superior selectivity of polymethylbenzene (polyMB, most likely with four or more methyl groups) such as durene. The distribution of A6 to A10 aromatics varied tremendously due to the different transformation pathway of composite catalytic systems.

Figure 3. Aromatic distribution over the Fe2O3-SiO2/modified-HZ and CuO-ZnO-Al2O3/modified-HZ catalysts (T = 330 °C, P = 4 MPa, GHSV = 1813 h−1, H2/CO = 2:1, and time on stream = 12 h; other A9+ = aromatic hydrocarbons with nine and more than nine carbons except the triMB and tetraMB).

aromatic products ranged from A6 to A9+. The selectivity of toluene, xylene, and triMB were higher than other aromatics. Light aromatics (benzene, toluene, and xylene, known as BTX) were significant aromatic products. Benzene had the lowest selectivity in BTX. The selectivity of xylene was higher than that of toluene. After incorporating metal species to HZSM-5, D


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HZ and CZA/Nb-Ni-HZ, respectively. The metal oxide and HZSM-5 zeolites were better mixed through the PMS method than the PM method leading to the enhancement of the synergistic effect and aromatization efficiency. It was found above that the distribution of aromatic hydrocarbons was diverse over the different metal-oxide/ modified-HZ composite catalyst systems. Then a catalyst evaluation was designed in order to investigate the regulation of aromatic distribution and analyze the reaction process. CuOZnO-Al2O3 was mixed with Fe2O3-SiO2 to form another multimetal oxide. Then Fe-CZA/Ni-HZ was applied to the reaction of syngas to aromatics. The CO conversion is given in Table 2. As shown in Table 2, the CO conversion of the Fe-CZA/Ni-HZ was as high as that

In addition, Ni oxide species enhanced the aromatization activity of the zeolite at low temperature of 330 °C, resulting in the highest selectivity of BTX over Fe/Ni-HZ and the obvious increase of durene selectivity over CZA/Ni-HZ. However, NbNi loaded exhibited the highest durene selectivity over CZA/ Nb-Ni-HZ, which may attribute to the acidity change after the incorporation of Nb oxide species. Figure 5 displays the distribution of xylene over Fe2O3-SiO2/ HZ composite catalysts. Nb or Ni incorporating to HZ

Table 2. CO Conversion over the Metal-Oxide/Ni-HZ Composite Catalystsa catalysts

Fe/Ni-HZ

CZA/Ni-HZ

Fe-CZA/Ni-HZ

CO conversion (%)

99.20

91.29

99.25

Reaction conditions: T = 330 °C, P = 4 MPa, GHSV = 1813 h−1, H2/ CO = 2:1, and time on stream = 12 h.

a

of Fe/Ni-HZ at about 99% and was much higher than that of CZA/Ni-HZ, which illustrated that Fe2O3 and Ni-HZ made more contribution to the CO conversion. Figure 6 contrasts the product distribution of different metaloxide/Ni-HZ catalyst systems. For product of effluent gas, the

Figure 5. Xylene distribution over the Fe2O3-SiO2/modified-HZ catalysts (T = 330 °C, P = 4 MPa, GHSV = 1813 h−1, H2/CO = 2:1, and time on stream = 12 h).

increased the selectivity of p, o, m-xylene simultaneously on the order of Fe/Ni-HZ > Fe/Nb-Ni-HZ > Fe/Nb-HZ > Fe/HZ. The selectivity of p-xylene and m-xylene (about 3−4%) were similar, which were lower than half the selectivity of o-xylene (about 7−9%). Table 1 compares the reaction result of different composite methods over Fe/Ni-HZ and CZA/Nb-Ni-HZ. The composite catalysts obtained through PM method exhibited relatively lower CO conversion and selectivity of aromatics than that of the PMS method. In addition, in comparison with the PM method, composite catalysts prepared by PMS method exhibited higher selectivity of BTX and tetraMB over Fe/NiTable 1. Reaction Result of Fe/Ni-HZ and CZA/Nb-Ni-HZ with Different Composite Methodsa Fe/Ni-HZ

Figure 6. Product distribution over the metal-oxide/Ni-HZ catalysts (T = 330 °C, P = 4 MPa, GHSV = 1813h−1, H2/CO = 2:1, and time on stream = 12 h; C1 ∼ C4 hydrocarbons = alkane and alkene in effluent gas; C5+ = aliphatic hydrocarbons with five and more than five carbons in liquid phase).

CZA/Nb-Ni-HZ

catalysts

PMS

PM

PMS

PM

CO conversion (%) products selectivity (%) CO2 aliphatics (%) C1∼C4 C5+ aromatics (%) BTX ethylbenzene TriMB TetraMB other A9+ total aromatics methanol

99.20

98.79

95.69

94.79

3.21

4.5

11.61

12.73

9.21 39.57

10.45 40.39

9.71 30.59

10.98 33.52

29.49 2.98 7.49 0.97 7.08 48.01 0

26.5 3.05 6.63 0.92 7.56 44.66 0

2.02 0.03 3.35 31.34 9.81 46.55 1.54

3.12 0.04 2.31 27.08 8.53 41.08 1.69

CO2 selectivity over Fe-CZA/Ni-HZ was slightly lower than that of CZA/Ni-HZ and was much higher than that of Fe/NiHZ, while the selectivity of C1 ∼ C4 hydrocarbons over FeCZA/Ni-HZ was higher than that of Fe/Ni-HZ and CZA/NiHZ. For the distribution of products in the liquid phase, after mixing the CuO-ZnO-Al2O3 with Fe2O3, a sharp decline of C5+ selectivity emerged. The Fe-CZA/Ni-HZ catalysts showed the lowest C5+ selectivity of merely 20.89%, which was 10.07% lower than that of CZA/Ni-HZ and 19.29% lower than that of Fe/Ni-HZ. The great increase in the selectivity of total aromatics was shown over the Fe-CZA/Ni-HZ catalyst. Fe/ Ni-HZ and CZA/Ni-HZ catalyst had the similar total aromatic selectivity, while that of Fe-CZA/Ni-HZ was nearly 9% higher.

Reaction conditions: T = 330 °C, P = 4 MPa, GHSV = 1813 h−1, H2/ CO = 2:1, and time on stream = 12 h.

a

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3.1.2. Transformation Pathway of Syngas to Aromatic over Metal-Oxide/Modified-HZ Composite Catalysts. Different reaction pathway of the metal-oxide/modified-HZ composite catalytic systems were investigated to explain the different distribution of aromatic hydrocarbons. The reaction networks for the formation of aromatic are described in Scheme 1 and Scheme 2. For Fe2O3-SiO2/modified-HZ catalysts, as shown in Scheme 1, C2= ∼ C4= as well as C5+ alkene were produced on Fe2O3SiO2 through Fischer−Tropsch synthesis. Then BTX generated from unsaturated aliphatic hydrocarbons through the reaction of cracking, cyclization, hydrogen transfer, and dehydrogenation. PolyMB hydrocarbons such as triMB and tetraMB, and other aromatics, were generated from the alkylation of light aromatics with olefins,16 which was difficult to proceed. Thus, BTX were preferentially produced over Fe2O3-SiO2/modifiedHZ catalysts. In addition, C1 ∼ C4 and C5+ aliphatic hydrocarbons were produced through isomerization, oligomerization, cracking, cyclization, and hydrogen transfer of light alkene and alkane. Hydrocarbon-Pool mechanism has been widely recognized to account for MTA,33−35 and the generation of aromatic hydrocarbons over CuO-ZnO-Al2O3/modified-HZ catalysts can be explained based on the Hydrocarbon-Pool mechanism, as shown in Scheme 2. Methanol was produced on CuO-ZnOAl2O3. After entering the channel of zeolites, it can reversibly convert to dimethyl ether under the reaction conditions and then transformed into light olefins through dehydration.36 A large number of Hydrocarbon-Pool species, usually polyMB, generated inside the zeolite channels from light olefins through oligomerization, cyclization, hydrogen transfer, and dehydrogenation. PolyMB favored undergoing the reaction of dealkylation to form xylene, triMB, and tetraMB as well as light hydrocarbons before diffusing out of the zeolite pores.35,37,38 At the high pressure of 4 MPa in the reaction, it was conducive to the methylation of xylene and triMB rather than dealkylation of polyMB. As it was known that durene in tetraMB and pseudocumene in triMB can just diffuse out of the pore for the

The formation of methanol was found over both Fe-CZA/NiHZ and CZA/Ni-HZ catalysts, which attributed to the catalytic performance of CuO-ZnO-Al2O3. Figure 7 displays the distribution of aromatic hydrocarbons. Compared with Fe/Ni-HZ, Fe-CZA/Ni-HZ greatly decreased

Figure 7. Aromatic distribution over the metal-oxide/Ni-HZ catalysts (T = 330 °C, P = 4 MPa, GHSV = 1813 h−1, H2/CO = 2:1, and time on stream = 12 h; other A9+ = aromatic hydrocarbons with nine and more than nine carbons except the triMB and tetraMB).

the toluene and xylene selectivity of about 9.28% from 10.73% to 1.45% and 7.21% from 17.69% to 10.48%, respectively. The triMB selectivity increased from 12.19% to 18.38%. The most obvious shift was that the tetraMB selectivity over Fe-CZA/NiHZ dramatically increased from 0.97% to 21.07%. In comparison with CZA/Ni-HZ, the selectivity of xylene and triMB over Fe-CZA/Ni-HZ were 9.44% and 13.91% higher, respectively. On the contrary, the tetraMB selectivity was 7.57% lower. The variation of aromatic distribution may be attributed to the methylation of light aromatics through methanol.

Scheme 1. Reactions Pathway over Fe2O3-SiO2/Modified-HZ Catalysts

F


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Industrial & Engineering Chemistry Research Scheme 2. Reactions Pathway over CuO-ZnO-Al2O3/Modified-HZ Catalysts

shape selectivity prosperity of ZSM-5 zeolites.24,37 Moreover, pseudocumene can convert to durene through the reaction of methylation with methanol at the pore mouth of zeolites. As a result, the CuO-ZnO-Al2O3/modified-HZ catalysts had a high selectivity of durene rather than the selectivity of BTX or triMB. Besides, aliphatic hydrocarbons formed through cracking, hydrogen transfer, and dehydrogenation of long chain alkenes. It was known that toluene and xylene were easier to methylate than triMB. In accordance with the results, it can be speculated that, for the Fe-CZA/Ni-HZ catalyst, there were two dominating reaction pathways for the methanol formed on CuO-ZnO-Al2O3. A proportion methanol underwent the methylation reaction of toluene and xylene to triMB. The remaining methanol produced tetraMB through HydrocarbonPool species. The C2=-C4= formed on Fe2O3-SiO2 suppressed the equilibrium of dimethyl ether to light olefin, leading to a smaller quantity of Hydrocarbon-Pool species. Therefore, the lower tetraMB selectivity over Fe-CZA/Ni-HZ resulted from the less Hydrocarbon-Pool species formation, in comparison with CZA/Ni-HZ. 3.2. Properties of the Catalysts. The crystal structure of modified HZSM-5 zeolites, metal oxide, and composite catalysts were determined by X-ray diffraction, and the results are shown in Figure 8. All these zeolites exhibited typical diffraction peaks (2θ = 8−9°, 23−25°, 30°, and 45°) of HZSM5 structure (Figure 8a), suggesting that the impregnation of Ni or Nb had little impact on the HZSM-5 framework. No distinct diffraction peaks for NiO or Nb2O5 species were observed, suggesting that the Ni or Nb species were highly dispersed in the HZSM-5 support.39 However, metal species loading led to the reduction of the zeolite crystallinity. Compared with the parent HZ, Nb-HZ and Ni-HZ, Nb-Ni-HZ displayed relatively weaker diffraction peaks, indicating that the crystallinity declined with the increase of the loading amount of metal species. The XRD pattern of Fe2O3-SiO2 only exhibited broad diffraction peaks of Fe2O3 crystalline phase (Figure 8b), which

indicated that the iron oxide samples were amorphous.17 For the XRD pattern of Fe/Ni-HZ, both weak diffraction peaks of Fe2O3 and typical diffraction peaks of HZSM-5 zeolite were found, which manifest that the crystal structure remained in the composite catalyst. However, compared with Ni-HZ, Fe/NiHZ displayed obvious decline in the strength of diffraction peaks of HZSM-5. The CuO-ZnO-Al2O3 exhibited XRD pattern typical for the oxides of copper and zinc (Figure 8c).40 The CZA/Nb-Ni-HZ displayed both diffraction peaks of CuO, ZnO, and HZSM-5 zeolite, which manifest that the composite of CZA and Nb-Ni-HZ had little effect on the crystal structure. The composite catalyst also showed the decrease in the strength of diffraction peaks compared with copper-based oxide and Ni-HZ. The textural properties of modified HZSM-5, metal oxides, and composite catalysts were characterized by N2 isothermal adsorption−desorption. The specific surface area (SBET), pore volume (Vpore), and average pore size are summarized in Table 3 and the isotherms are presented in Figure 9. All of the zeolites exhibited type-IV isotherms similar to the mesoporous material (Figure 9a), with a distinct increase of N2 adsorption in the region 0.6 < P/P0 < 0.95.21,41 Metal species loading had no difference on isotherms. Parent HZ exhibited higher SBET and Vpore than those of metal species modified HZ (Table 3). The Nb-Ni-HZ had the strongest reduction in the SBET and Vpore. The Ni-HZ exhibited lower SBET and Vpore compared with NbHZ, suggesting that Ni may more efficiently locate on HZ providing more active sites. The Ni-HZ and Nb-Ni-HZ exhibited lower Smic, and Vmic compared with parent HZ demonstrated that metal species were located on the external surface and in the micropore of zeolites.39 The Nb-HZ dramatically decreased the Smic and Vmic, indicating that Nb species were preferentially located in the micropore. The slight varieties of average pore size showed that metal species loading had not much effect on the pore structure and pore size distribution.42 The isotherms and the textural properties of Fe2O3-SiO2 and CuO-ZnO-Al2O3 suggested that the metal oxides had much mesopore (Figure 9b and Table 3).40 The G


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Table 3. Textural Properties of the Modified HZSM-5 Zeolites, Metal Oxides, and Composite Catalysts catalyst HZ Nb-HZ Ni-HZ Nb-Ni-HZ Fe2O3-SiO2 CuO-ZnOAl2O3 Fe/Ni-HZ CZA/Nb-NiHZ

SBET (m2/g)

Smic (m2/g)

Vtotal (cm3/g)

Vmic (cm3/g)

pore size (nm)

289 284 274 269 85 54

114 94 103 105 6 4

0.340 0.326 0.323 0.312 0.169 0.170

0.059 0.050 0.053 0.054 0.005 0.003

4.71 4.57 4.73 4.65 7.90 12.62

133 121

42 39

0.219 0.232

0.024 0.022

5.41 7.71

Figure 9. N2 adsorption−desorption isotherm of (a) modified HZSM5 zeolites and (b) metal oxides and composite catalysts. Figure 8. XRD patterns of (a) modified HZSM-5 zeolites, (b) Ni-HZ, Fe/Ni-HZ and Fe2O3-SiO2 zeolite, and (c) (1) Nb-Ni-HZ, (2) CZA/ Nb-Ni-HZ, and (3) CuO-ZnO-Al2O3 with the diffraction peaks of (◊) HZSM-5, (▼) ZnO, and (●) CuO.

composite catalysts exhibited different textural properties from both metal oxides and zeolites in the aspect of isotherm, surface properties, and pore properties. Figure 10 shows the morphologies of Fe/Ni-HZ and CZA/ Nb-Ni-HZ catalysts. For each catalyst, the distribution of metal oxides and zeolites was clear and uniform.43 In addition, no apparent catalyst crystal was found, presenting the aggregation of small particles in different size.

Figure 10. SEM images of (a) Fe/Ni-HZ and (b) CZA/Nb-Ni-HZ.

H


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were produced on copper-based metal oxides, resulting in different distribution of aromatics.

The acidity change was reflected by NH3-TPD, presented in Figure 11. The amount of ammonia desorption of zeolite

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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/acs.iecr.7b03450. Flow diagram for the reactions (Figure S1) and the GCMC analysis for production distribution (Figure S2) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tianhui Yang: 0000-0002-2708-7666 Like Cheng: 0000-0003-4656-8294 Notes

The authors declare no competing financial interest.

Figure 11. NH3-TPD profiles of modified HZSM-5 zeolites.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Key R&D Program of China (2017YFB0602204).

corresponded to the number of acid sites, and the temperature of ammonia peaks indicated the acid strength. The ammonia peaks at about 220 and 440 °C were assigned to weak and strong acid sites, respectively.44 The acid sites of parent HZSM5 zeolites were in general derived from framework aluminum.37,45 Compared with parent HZSM-5, metal loading slightly widened the peak of weak acid sites and reduced the peak of strong acid sites on the order of Nb-Ni > Ni > Nb. The temperature of peak for weak acid sites became slightly lower, suggesting that strength of weak acidity of catalysts were on the order of Nb-Ni < Ni < Nb. Nb loading slightly influenced the strong acid sites compared with parent HZSM-5, which was in accordance with the smallest improvement of BTX and durene selectivity over Fe/Nb-HZ and CZA/Nb-HZ, respectively. Ni loading moderately diminished the strong acid sites, which gave rise to the highest selectivity of BTX over Fe/Ni-HZ among Fe2O3-SiO2/modified-HZ catalysts. Nb-Ni loading eliminated a large amount of strong acid sites because more metal species impregnation covered on external surface and in the pore mouth of zeolites.23 In addition, Nb-Ni-HZ had the largest number of weak acid sites. The variation of acid sites led to the highest selectivity of durene over CZA/Nb-Ni-HZ among CuO-ZnO-Al2O3/modified-HZ catalysts.

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4. CONCLUSION Syngas was converted to aromatic hydrocarbons through a onestep reaction process over Fe2O3-SiO2/modified-HZ catalysts, CuO-ZnO-Al2O3/modified-HZ catalysts, and Fe-CZA/Ni-HZ. These metal-oxide/modified-HZ composite catalysts showed high efficiency at low temperature of 330 °C, 4 MPa with different distribution of aromatics. Fe2O3-SiO2/modified-HZ catalysts primarily produced light aromatic such as BTX, while CuO-ZnO-Al2O3/modified-HZ catalysts mainly produced polyMB such as durene. Fe-CZA/Ni-HZ displayed lower selectivity of BTX than Fe/Ni-HZ and of tetraMB than CZA/ Ni-HZ, and showed higher selectivity of triMB than both of them. After the incorporation of Nb and Ni, Fe/Ni-HZ and CZA/Nb-Ni-HZ displayed the highest selectivity of BTX and durene, respectively, due to the proper acidity of the modified zeolites. Two reaction pathways showed that unsaturated hydrocarbon formed on iron-based metal oxide and methanol I


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