One Step Alkylation of Benzene with Syngas over Non-noble Catalysts

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Applied Chemistry

One Step Alkylation of Benzene with Syngas over Non-noble Catalysts Mixed with Modified HZSM-5 Fan Yang, Yuehua Fang, xiangyu liu, Xiaohui Liu, David I. Muir, Aimee Maclennan, and Xuedong Zhu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02156 • Publication Date (Web): 12 Jul 2019 Downloaded from pubs.acs.org on July 22, 2019

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Industrial & Engineering Chemistry Research

One Step Alkylation of Benzene with Syngas over Non-noble Catalysts Mixed with Modified HZSM-5 Fan Yang a, Yuehua Fang a, Xiangyu Liu a, Xiaohui Liu b*, David Muir c, Aimee Maclennan c, and Xuedong Zhu a*

a

Engineering Research Center of Large Scale Reactor Engineering and Technology,

Ministry of Education, East China University of Science & Technology, Shanghai 200237, P. R. China b

Shanghai Key Laboratory of Functional Materials Chemistry, Research Institute of

Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science & Technology, Shanghai 200237, P. R. China c

Canadian Light Source Inc., 44 Innovation Boulevard, Saskatoon, SK S7N 2V3,

Canada

Author Information Corresponding Author *E-mail: [email protected]; [email protected].

Abstract: Para-xylene is an important chemical for industry. In this work, alkylation of benzene with syngas (ABS) is carried out to prepare toluene and xylene over zinc/chromium (Zn/Cr) oxide and modified HZSM-5. The catalyst with Zn/Cr ratio of 1.6 and 20% (wt) silicate-1 zeolite (S1) coated HZSM-5 has the best catalytic performance, exhibiting 34.4% benzene conversion, and 94.7% total selectivity of toluene and xylene, with 73.2% of xylene as para-xylene. The catalyst is carefully characterized, results showed that the activity was enhanced by excessive zinc atoms, which would replace the surface chromium atoms and this substitution led to more oxygen vacancies. Besides, by coating a layer of S1 on HZSM-5, the production of trimethylbenzene was suppressed and the selectivity of para-xylene among all xylene was increased, due to the coverage of zeolite surface acid sites. These findings are helpful to understanding the one-pot syngas conversion and benzene alkylation 1

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

processes.

Keywords: Zn/Cr oxide, HZSM-5@S1, one-pot syngas conversion, alkylation of benzene

1. Introduction Para-xylene is one of the most significant chemicals owing to its extensive applications in synthetic resin, pharmaceutical, chemical fiber, and pesticide industries. 1,2

It is mainly produced by catalytic reforming of naphtha, toluene disproportionation

and C8 aromatic isomerization.3-5 Compared to these methods, alkylation of toluene/benzene with methanol is carried on at 400-500 °C/0.3-0.4 MPa and could result in high utilization of toluene/benzene.6-11 Instead of methanol, syngas has been used directly as alkylation agent for toluene/benzene at similar temperatures,12,13 such processes are much more efficient in both energy and cost.14-20 In our previous work, alkylation of benzene with syngas (ABS) was successfully developed by using a catalyst containing platinum (Pt), cerium (Ce) and H-ZSM5 zeolite, in which benzene conversion is 34.2% and total selectivity of toluene and xylene is 96.7% with 23.0% of xylene as para-xylene.12 However, Pt is a noble metal and it would be beneficial to find an inexpensive catalyst as substitute. For Pt catalyst, ABS was carried out under relatively high temperature as benzene conversion decreased remarkably at lower temperature, so the desired catalyst to replace should have good thermal stability in order to achieve a high benzene conversion. Zinc/chromium (Zn/Cr) oxide is very stable under high temperatures therefore it has been used in similar reactions for one-step conversion of syngas into other hydrocarbons.21-24 The operating temperature (350-450 oC) for Zn/Cr oxide matches well with that (400-500 °C) required for benzene alkylation,25-30 so it would be a good candidate for ABS. Additionally, developing catalysts with higher para-xylene selectivity would be significant, as para-xylene is more valuable. The fractions of xylene isomers are effected by the xylene isomerization happened on the 2


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external surface acid sites of HZSM-5 and the equilibrium ratio of three xylene isomers is about 1:2:1 for para-xylene, meta-xylene and ortho-xylene, respectively.31-33 By reducing the surface acidity, the para-xylene selectivity would increase. Accordingly, catalysts by coupling Zn/Cr oxide and modified HZSM-5 could be very promising in alkylation of benzene with syngas. In this work, a bifunctional catalyst containing Zn/Cr oxide and modified HZSM-5 zeolite was developed for the alkylation of benzene with syngas. To obtain an optimal catalyst, a series of Zn/Cr oxides with different Zn/Cr ratios and HZSM-5 modified by silicate-1 zeolite (S1) were studied. Characterization methods like XRD, SEM, NH3TPD, XPS and XAS were employed. The reaction of one-pot alkylation of benzene with syngas will be especially practical in plants with installations for toluene disproportionation and xylene isomerization.

2. Experimental Section 2.1. Source of Reagents. Zn(NO3)2·6H2O, Cr(NO3)3·9H2O, (NH4)2CO3, tetraethyl-orthosilicate (TEOS), tetrapropylammonium hydroxide (TPAOH, 40%wt aqueous solution) and benzene were all purchased from Shanghai Titan Scientific Co. Ltd, HZSM-5 zeolite(Si/Al=12.5) was purchased from Nankai University Catalyst Co. Syngas (N2:H2:CO=1:2:1), hydrogen and other gases were purchased from Shanghai Jiajie Specialty Gas Co. 2.2. Catalyst Preparation. Zn/Cr oxides with different Zn/Cr ratios were prepared by co-precipitation as reported before.21-24 Typically, a metal salt solution containing specific amount of zinc nitrate and chromium nitrate and a solution of (NH4)2CO3 were pumped into a beaker by two peristaltic pumps. In the beaker, the temperature was kept at 60 ± 0.5 oC and pH was kept at 7.5 ± 0.1 to obtain suspensions, thereafter the suspensions were aged at 60 oC

for 12 h. Subsequently the samples were filtered, washed thoroughly by deionized

water and dried at 100 oC overnight. Finally the samples were calcined under 550 oC for 6h in air to obtain the corresponding Zn/Cr oxides. These Zn/Cr oxides are named 3



as ZnxCr in the following, wherein the ‘x’ represents the Zn/Cr molar ratio. For comparison, sample Zn1.6Cr was also synthesized by two other methods, one is through physical mixing of specific amount of ZnO and Zn0.5Cr which will be named as mixZn1.6Cr and the other is by impregnation method using Zn0.5Cr and a certain amount of Zn(NO3)·6H2O, which will be named as impZn1.6Cr.12 Silicate-1 (S1) coated HZSM-5 was synthesized by hydrothermal method. Firstly a solution containing TEOS, TPAOH and deionized water (1SiO2: 0.25TPAOH: 60H2O: 4EtOH) was stirred for 2 h under 40 oC to yield clear colloid. HZSM-5 was added into this colloid by different SiO2/HZSM-5 mass ratios. Then the mixture was sealed into a Teflon autoclave for crystallization under 175 oC lasting 3 days, and the autoclave was kept rotating with a rate of 30 r/min. The S1 coated HZSM-5 zeolites are denoted as Z5@yS1, the ‘y’ here is the mass percentage of S1/HZSM-5. For example, Z5@10S1 means 1 g HZSM-5 was coated by 10 wt% S1 (0. 1g). 2.3. Catalytic Tests. In the catalytic evaluation of ABS, ZnxCr and HZSM-5 were mixed by a mass ratio of 1:1, the mixture was pelleted into 20-40 mesh particles. The constituted catalyst containing Zn/Cr oxides and HZSM-5 will be named as ZnxCr/Z5. Before the reaction, 1g ZnxCr/Z5 was first loaded into a stainless steel tube (i.d. 10 mm, customized by Swagelok Company), then the catalyst was reduced by a hydrogen flow with a rate of 50 ml (S. T. P.)/min at 480 oC, 1 atm for 2 h, after reduction the reactor was cooled to 450 oC, thereafter a co-feed of syngas and benzene was introduced into the reactor, and the pressure was adjusted to 3 MPa by a back pressure valve (KBP1J0A4-A5A20000, Swagelok Company). The flow of syngas was set at 200 ml (S. T. P.)/min by a mass flow controller (D08-1D/ZM, Beijing Sevenstar Electronics Company) and the flow of benzene was set at 0.05 ml/min by a high pressure constant flow pump (P100, Shanghai Wufeng Technology Company). The molar ratio of H2: CO: C6H6 in the feed was calculated to be 8:4:1 approximately, the gas hourly space velocity (GHSV) was 12000 h-1 (S T. P.) and the liquid hourly space velocity (LHSV) was 3 h-1. The products from the tube were first cooled by a condenser with running water as coolant, after which the gas and liquid phases were separated by a gas-liquid separator at 0 oC. Finally, the gas 4


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phase was introduced into an online chromatography produced by Shimadzu using a TDX-02 packed column (2 m) connected to a TCD detector and a TDX-01 packed column (30m) connected to an FID detector. The liquid phase was collected, weighted and analyzed by an offline chromatograph produced by Agilent using a DB-WAX column (30 m) connected to an FID detector. In other evaluation experiments, all conditions were the same as mentioned above except that the feeds and catalysts were adjusted, which will be described below. 2.4. Characterization. The specific Zn/Cr ratios of the ZnxCr catalysts were determined by inductively coupled plasma atomic emission spectrometer (ICP-AES) using Agilent 167nm785nm/725. X-ray diffraction (XRD) patterns were acquired through Bruker D8-Advance X-ray polycrystalline diffractometer with Cu Kα radiation at 40 kV, 40 mA. Measurements were performed from 5o to 80o at a scanning speed of 4 o/min. Powder diffraction files from the International Center of Diffraction Data (ICDD) and the software Jade 6.5 were applied for the XRD pattern analysis. Specific surface and pore structures were acquired by N2 physical adsorption at 196 oC on Micromeritics ASAP2460 using liquid nitrogen as coolant. All samples were heated to 300 oC in vacuum for 12 h to remove gases before N2 adsorption. Specific area was calculated by Brunauer-Emmett-Teller (BET) method, pore volume and average pore diameter were obtained by Barrett-Joyner-Halenda (BJH) method. X-ray photoelectron spectroscopy (XPS) was measured over Thermo ESCALAB 250, monochromatic Al Kα was used as the X-ray source (1486.6 eV, anode operating at 14 kV and 30 mA). C1s peak at 284.8 eV was used for energy calibration. The software XPSpeak41 was used for data analysis. X-ray absorption spectroscopy (XAS) measurements were conducted at the Canadian Light Source for the Zn K-edge on the IDEAS beamline. Zn foil was used for energy calibration and both fluorescence and transmission yields were recorded. Ammonia temperature programed desorption (NH3-TPD) was carried out on Micromeritics ASAP2720. The zeolites were firstly degassed in a U-tube at 550 oC for 5



1 h, subsequently the sample was cooled to room temperature and the tube was purged with ammonia for 30 min. Before recording, each sample was swept under helium at 120 oC, and the TCD signal was recorded under a helium flow of 25 ml/min. Scanning electron microscopy (SEM) was taken on FEI Nova NanoSEM 450 with an accelerating voltage of 2.0 kV.

3. Results and Discussion

3.1. Characterizations of Zn/Cr Oxides XRD The XRD patterns of Zn/Cr oxides, ZnO, and Cr2O3 are shown in Figure 1a. Zn0.5Cr and Zn0.8Cr possess the typical spinel structure, presenting XRD peaks at about 2θ = 30.2o, 35.7o, 43.3o, 54.0o, 57.5o and 63.1o. However, some zincite peaks at 2θ = 31.8o, 34.4o, 36.4o, 47.6o, 56.6o, 68.1o and 69.1o will also appear when the Zn/Cr molar ratio is 1.6, denoting to the existence of zincite phase, and the zincite will gradually become the main phase when Zn/Cr molar ratios are higher than 2.4. Moreover, the most intense peaks located at 28~38o are enlarged in Figure 1b. It can be seen that the peaks of Zn0.8Cr shift towards higher angles compared with those of Zn0.5Cr (ZnCr2O4 spinel). For example, the peak attributed to the (311) face of ZnCr2O4 spinel changes from 35.7o to 35.9o. As reported, in a nonstoichiometric spinel structure, the excessive zinc can occupy the positions of chromium.34-36 The radius of Zn atom (134 pm) is smaller than that of Cr atom (185 pm), so the peaks which are characterized as ZnCr2O4 spinel shift to higher angles according to Bragg’s law.37,38 For comparison, mixZn1.6Cr was prepared by physical mixing of specific amount of ZnO and Zn0.5Cr, and impZn1.6Cr was synthesized through impregnation method by Zn0.5Cr and a certain amount of Zn(NO3)·6H2O, respectively. As shown in Figure1a, among mixZn1.6Cr, impZn1.6Cr and Zn1.6Cr, the shoulder at 36.4o in Zn1.6Cr is obviously less sharp and lower. It demonstrates that there is less zincite in Zn1.6Cr than those in the other two samples, in spite of the fact that they have the same compositions. 6


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From these XRD results, it can be concluded the atoms of Zn/Cr oxides are arranged strictly in the way of spinel when the Zn/Cr ratio is 0.5, if more zinc is added into the spinel structure, the excess zinc will substitute some chromium in the ZnCr2O4 spinel. By further increasing Zn content, excess zinc will mainly exist in the form of zincite.

Texture Properties Table 1 lists the textural properties of various Zn/Cr oxides. Pure Cr2O3 and ZnO have rather small specific surface, 18 m2g-1 and 15 m2g-1 respectively. Zn/Cr oxides have larger specific surfaces because zinc and chromium oxides can form ZnCr2O4 spinel which is a mesoporous material.26,27 Due to which, Zn0.5Cr owns the highest specific surface of 80 m2g-1. With the increment of zinc addition, the value drops gradually from 80 m2g-1 to 39 m2g-1. It is interesting that, from sample Zn0.8Cr to Zn3.2Cr, the changes of specific surface are not very significant, varying in a range from 68 m2g-1 to 59 m2g-1. In contrast, impZn1.6Cr and mixZn1.6Cr exhibit very low surface area and small pore size, which is possibly caused by the blockage of channels and pores inside the spinel structure by zinc species.

X-ray Absorption Spectroscopy X-ray absorption spectroscopy is employed to study the chemical states. Figure 2a shows the Zn K-edge near edge spectra (XANES) of Zn foil, ZnO and Zn/Cr oxides. Three peaks at 9665 eV, 9668 eV and 9673 eV along with one shoulder at 9680 eV are observed in the spectra of Zn0.5Cr and Zn0.8Cr. These features are attributed to those of ZnCr2O4 spinel.34-36 In Zn0.8Cr, the peak at 9668 eV is higher and the shoulder at 9680 eV is lower compared to Zn0.5Cr. As reported previously, this phenomenon is caused by the replacement of Cr atoms at octahedral sites by Zn atoms, this finding is in agreement with the XRD results.34-36 The XANES spectra of samples with Zn/Cr ratios higher than 0.8 are very similar to that of ZnO, but much different from those of Zn0.8Cr or Zn0.5Cr. Figure 4b shows the Zn K-edge extended region results (EXAFS) in R-space. In Zn foil, one dominate peak (2.27 Å) corresponding to Zn-Zn interatomic distance can be 7



found. There are two peaks in EXAFS results of zincite and Zn0.5Cr, in which the first peak (1.53 Å) denotes to the Zn-O distance in both structures, and the second peak denotes to Zn-O-Zn distance (2.88 Å) in zincite or Zn-O-Cr distance (2.96 Å) in ZnCr2O4 spinel.39,40 Notably, only the peak at 1.53 Å attributed to the first shell is resolved for Zn1.6Cr, indicating the amorphous nature of Zn1.6Cr,41 in agreement with the XRD results that the peaks of zincite in Zn1.6Cr is less obvious compared with mixZn1.6Cr and impZn1.6Cr.

XPS The Zn/Cr oxides are analyzed by XPS, and the Zn/Cr molar ratios on the surface as determined by XPS fitting are listed in Table 1. Compared with the Zn/Cr molar ratios of the bulk obtained by ICP-AES, it can be seen that the values of the surface are generally higher. In agreement with the results from N2 adsorption, it is certain that the excess zinc tends to appear at the surface of the Zn/Cr oxides. Figure 3a shows the Cr 2p spectra with two broad peaks centered around 577 eV and 586 eV, representing the Cr 2p spin-orbit split peaks.42 Five components were fit into the Cr 2p3/2 profile between 576.5-578.9 eV, due to the discrete multiplet structure of Cr3+ in Zn0.5Cr or Cr2O3.42 Another peak at 579.6 eV was fit into the Cr 2p3/2 profile of the Zn/Cr oxides, this peak can be assigned to Cr6+.43,44 In Zn0.5Cr and Cr2O3, high degrees of fitting can be obtained by the five components. However, one more peak at 574.8 eV denoting to less charged Cr3+ is required for the higher goodness of fit in Zn1.6Cr and Zn3.2Cr, since their XPS spectra shift towards lower binding energies (BEs) compared to Zn0.5Cr and Cr2O3. By inspecting the fitting parameters in Table S1, it is obvious that Zn1.6Cr contains as much as 20.5% of less charged Cr3+, and the content drops to 17.5% in Zn3.2Cr. Generally, it was thought that Cr only plays a role as a promoter to acquire a larger specific surface in Zn/Cr catalysts.26,27 Nevertheless, the peak at 574.8 eV in Cr 2p3/2 spectrum implies that there exists electron interactions between Cr and other elements. Figure 3b shows the Zn 2p XPS spectra, in all samples the spin-orbit split 2p1/2 and 2p3/2 peaks are observed at about 1045 eV and 1021 eV, respectively. The Zn 2p peak 8


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of Zn0.5Cr is much narrower, similar to that of ZnO, and both of the two samples have similar Zn 2p BEs.45 For Zn1.6Cr and Zn3.2Cr, two components are fit into each of the Zn 2p spin-orbit split peaks, with one set of peaks at 1044.7 eV and 1021.6 eV, the same as those found in Zn0.5Cr and ZnO. While two other peaks are resolved at higher BEs, for instance, at 1045.4 eV and 1022.3 eV for Zn1.6Cr. According to previous reports, in Zn/Cr spinel the binding energies of Zn will increase when it occupies the position of chromium.46-49 This suggests that the substitution degree of Cr by Zn is higher for Zn1.6Cr than that for Zn3.2Cr as evidenced by higher peak intensities at 1045.4 eV and 1022.3 eV. But there is excess zincite in Zn3.2Cr as revealed by XRD, thus its higher BE peaks are not as intense as that of Zn1.6Cr. The O 1s XPS spectra are also shown in Figure 3c. After curve fitting, there are three peaks at 530.2 eV, 531.7 eV and 532.6 eV, which are denoted as O1, O2 and O3. O1 species at 530.2 eV is the lattice oxygen of ZnO, Zn/Cr spinel or Cr2O3. O2 at 531.7 eV and O3 at 532.6 eV are assigned to less charged oxygen anions and dissociatively chemisorbed oxygen respectively, which are thought to come from oxygen vacancies or surface defects.50-53 The contents of lattice oxygen (O1) are 70.4%, 78.9% and 78.2% in ZnO, Zn0.5Cr, and Cr2O3 respectively and the other two oxygen species only make up less than 30%. For Zn1.6Cr and Zn3.2Cr, total O2 and O3 increases to 59.2% and 38.1% respectively while O1 deceases to 40.8% and 61.9%. The increment of O2 is especially obvious in Zn1.6Cr and Zn3.2Cr, O2 in the two samples are almost twice as much as those of the other three samples. This indicates there are more oxygen vacancies in Zn1.6Cr and the interaction between Cr and Zn is more intense. In summary, it has been found that zinc atoms will substitute some chromium atoms when Zn/Cr ratio is higher than 0.5, and this part of zinc has a higher binding energy. However, this replacement will cause disorder in atom distribution because of the different valences and coordination environment between zinc and chromium.49 By further increasing Zn/Cr ratio, more chromium atoms are replaced by the excess zinc and structure like ‘-Zn-O-Zn-’ will appear in the original spinel. As a result, the ZnO phase is more amorphous in Zn1.6Cr, which could lead to more defects or oxygen vacancies. Besides, this substitution would result in a surface enrichment of zinc, so the 9



surface Zn/Cr molar ratio, as determined by XPS, is always larger than that of the bulk. When there is too much zinc like in Zn3.2Cr, it will gradually agglomerate into zincite and its crystallinity is restored, causing diminutions of the zinc with higher binding energy and the defects or oxygen vacancies.

3.2. Characterizations of Zeolites XRD Figure 4 shows the XRD results of zeolite samples. All zeolites exhibit typical MFI structures, suggesting that the products are all ZSM-5 or ZSM-5/S1 composite. For the zeolites, there is no remarkable change in XRD can be found, which means the MFI topologies of HZSM-5 before or after S1 coating are remained.

Texture Properties The pore and surface date of zeolites are displayed in Table 2. All HZSM-5 or S1 coated HZSM-5 exhibit specific surfaces about 310 m2g-1. The corresponding pore diameters and pore volumes are very close, which fluctuate around 2.5 nm and 0.16 cm3g-1. The results also support the conclusion of XRD, confirming that the micropore MFI structure is well retained after modification.

Acidity and Morphology of Zeolites To study the change caused by S1 coating, Figure 5 shows the SEM pictures of zeolites. It can be seen that the original HZSM-5 was hexagon shape, while for the sample with 10% wt S1 coating, some dentations can be observed. With the increase of the S1 coating, more S1 is on the original surface. The HZSM-5 can be wrapped completely when the amount of S1 is over 20% wt and the new HZSM-5 surface composed of S1 is a little rougher than before. Figure 6 shows their NH3-TPD profiles. The original HZSM-5 has two distinctive peaks, the peak at about 220 oC stands for weak acid sites and the peak at 450 oC stands for strong acid sites. After the S1 coating, both peaks became lower especially for the peak at 450 oC. The total amount of acidity decreases from 0.69 mmol/g to 0.31 mmol/g 10


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when S1 content increases from 10% wt to 25% wt. The differences in acidities and morphologies imply that the formation of S1 was an epitaxial growth outside of HZSM-5, in consequence, total acidities decreased due to the cover of surface acidities. During the experiment, it has been found that effects of mechanical mixing on the properties of zeolites or Zn/Cr oxides are not obvious, so the characterization of mixed catalysts are not present.

3. 3. Catalytic Performance Activities and Selectivities in Alkylation of Benzene with Syngas Zn/Cr oxide was coupled with HZSM-5 zeolite to achieve the whole reaction. In the gaseous phase of ABS, the products are CO2, methane and C2~C4 hydrocarbons. No methanol/ketene can be found (Table 3) because they could be rapidly converted over acid sites in HZSM-5.14, 22 The liquid phase of ABS are mainly composed of toluene and xylene as the alkylation products. And the side products, ethylbenzene and C9+ species, are considered to come from alkylation between benzene and C2/C3. Table 3 and Table 4 present the catalytic performances of ZnO/Z5, Cr2O3/Z5 and all ZnxCr/Z5 catalysts. In Table 4, the benzene conversions of ZnO/Z5 and Cr2O3/Z5 are both only about 10%, while those of ZnxCr/Z5 show significant improvements. Benzene conversion of Zn0.5Cr/Z5 is 27.6%, and with excess zinc it increases to 39.8% in Zn1.6Cr, then reduces to 23.9% in Zn6.4Cr when there is too much zinc. The activities of mixZn1.6Cr/Z5 and impZn1.6Cr/Z5 are lower, exhibiting benzene conversions of 21.4% and 23.1% respectively due to the blockage of channels and pores as shown in Table 1. For all experiments, the total selectivity to toluene and xylene is 91-95%. Different from the thermal equilibrium composition as shown in Figure S1, toluene selectivities (~70%) are much higher than xylene selectivities (~20%), but toluene can further be converted into xylene in disproportionation setups so this reaction is still of great value to the industry. For the gaseous phase, CO conversions vary in the same way as those of benzene, reaching a maximum of 29.8% in Zn1.6Cr. The CO2 and CH4 selectivities are about 40% and 30%, respectively for each catalyst, 11



making them the main components of gaseous phase.

Reaction Routes Study Considering the coupling process consists of the synthesis of intermediates from syngas and an in-situ benzene alkylation reaction, syngas was fed into Zn/Cr oxides separately to study the possible reaction route. As shown in Table S2, the products are mainly C1-C4 hydrocarbons and methanol, wherein methanol is probably the intermediate since it is most likely to react with benzene to produce toluene/xylene. The CO conversions of pure ZnO and Cr2O3 are very low, only 5.4% and 6.1% respectively. For ZnxCr, conversions range from 8.9% to 12.2%, showing noticeable promotions. The selectivity towards methanol increases from 8.4% in Zn0.5Cr to 12.3% in Zn1.6Cr, then drops to 9.7% in Zn3.2Cr. Besides, Zn1.6Cr also shows the highest CO conversion, which is 12.2% (Table S2). It can be seen that Zn/Cr oxides with high activity in ABS also exhibit good performance in high temperature methanol synthesis (HMS). It should be noticed that most of the C6+ aromatics come from alkylation of benzene, because as shown in Table S3, the selectivities towards aromatics are very limited by directly feeding syngas into Zn1.6Cr/Z5. By analyzing the results in Table S2 and Table S3, it is clear that ABS contains two steps, the first step is involved with syngas converting over Zn/Cr oxides and the second step is the alkylation of benzene over zeolites. To reveal the structure-performance connections, surface O2 contents were also quantified through multiplying the percentage of fitted O2 species curve area in whole O1s curve area by the normalized surface oxygen fractions (proportion of surface oxygen atoms in all oxygen, zinc and chromium atoms) determined by XPS results. Relationship between conversions and quantified surface O2 contents is shown in Figure 7, it can be seen that there is a positive correlation between benzene/CO conversions and surface O2 contents in both HMS and ABS. Overall, more oxygen vacancies (O2) will lead to higher benzene/CO conversions, Zn1.6Cr possesses the largest amount of oxygen vacancies and its performance in the two reactions are the best. So it is reasonable to attribute the conversion promotions to the increase of oxygen 12


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vacancies.

Circumvention of Thermodynamic Limit Comparing the reaction of ABS and HMS, the CO conversions in HMS over ZnxCr oxides (Table S2) are only about half of those in ABS over ZnxCr/Z5 (Table 3). As shown in Figure S1, CO conversion in ABS is more thermodynamically favored than that in HMS, so it is believed that the difference is caused by the thermodynamic limit. To confirm that the thermodynamic limit has been overcome, it is presumed that all methyl groups of xylene and toluene are from methanol, and the yield of methanol in the two reactions are displayed in Table S4. It can be seen that methanol generated over ZnxCr/Z5 should be 7~9 times higher than that over ZnxCr. In order to evaluate the utilization of CO in ABS, CO efficiency is introduced as shown in Table 3. This value is defined as the proportion of C atoms which are converted into methyl groups of toluene and xylene in all converted CO. In Table 3, CO efficiency is improved with the increase of CO conversion, and 30-40% of reacted CO is utilized to produce toluene and xylene. As the counterpart of methanol selectivities in HMS, this value is also much higher ， further confirming that the thermodynamic limit has been overcome.

Modification of HZSM-5 It has been elucidated that Zn1.6Cr/Z5 is an effective catalyst for ABS. Zn1.6Cr/Z5 shows the best activity but the proportion of para-xylene, the most valuable product in all xylene, is only about 25% percent as shown in Table 4. As mentioned, this is caused by the isomerization of three xylene isomers over the external acid sites. Besides, the external surface acid sites of HZSM-5 will also lead to overalkylation,54,55 so the more C9+ products can be found in Zn1.6Cr/Z5. However, too much meta-xylene, orthoxylene and C9+ is not desirable for this system as they are less valuable. To achieve a higher para-xylene yield, a layer of silicate-1 (S1) was coated outside of HZSM-5 as mentioned above, in order to reduce the external surface acid sites of HZSM-5 then avoid the xylene isomerization and overalkylation. 13



Table 5 lists the catalytic performances of Zn1.6Cr coupled with S1 coated HZSM5. It can be seen that the conversion of benzene drops from 39.8% to 30.5% when S1 content increases from 0% wt to 25% wt, which is caused by the decrease in the total acidity amount of HZSM-5. Even though the total selectivity to toluene and xylene is still within 91-95% with toluene as the main product, the percentage of para-xylene in all xylenes increases from 25.9% to 74.4% and the selectivity to C9+ drops from 4.4% to 0.9%.

It is obvious to see from Table 4 and Figure 2 that the thicker layer of S1

will depress the conversions of benzene while be helpful for the yield of para-xylene. Taken every aspects into consideration, Zn1.6Cr/Z5@20S1 is regarded as the best catalyst as it opposes a higher para-xylene selectivity (73.2%) and benzene conversion (34.4%).

Stability Tests The activity is just one aspect of catalyst, and the stability is another important target. Here, the stability tests of Zn1.6Cr/Z5 and Zn1.6Cr/Z5@20S1 are shown in Figure 8. It can be seen, the changes of conversions and selectivities are not obvious within 100 hours for the two catalysts, indicating both Zn1.6Cr/Z5 and Zn1.6Cr/Z5@20S1 have excellent stability.

4. Conclusion A low-cost bifunctional ZnxCr/Z5 catalyst is developed for the alkylation of benzene with syngas (ABS). It could in-situ consume the intermediate generated from syngas to produce toluene and xylene, thus significantly improve the syngas utilization. Oxygen vacancies on ZnxCr oxides surface is important for the conversions of benzene and CO, which can be enhanced by the substitution of Cr atoms by excess Zn atoms in ZnCr2O4 spinel. Among the ZnxCr oxides, Zn1.6Cr has the best performance for HMS and it is also the best candidate to be coupled with HZSM-5 for ABS. The external surface acid sites of the HZSM-5 zeolite can be covered by coating a layer of S1, and it results in improvement of the para-xylene selectivity, meanwhile suppressing the formation of C9+ species. Benzene conversion of 34.4%, total selectivity to toluene and xylene of 14


Page 14 of 36

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94.7% (in which 73.2% of xylene is para-xylene) are achieved over Zn1.6Cr/Z5@20S1. Compared with the traditional process in which methanol synthesis and benzene alkylation are separated, this new route to produce para-xylene by one step ABS can make better use of syngas and save the equipment investment. Acknowledgements This project was sponsored financially by Shanghai Postdoctoral Scientific Program (No. 14R21410400) and the National Natural Science Foundation of China (No. 2177061270).

Supporting Information Fitting parameters of Cr 2p3/2 spectra, catalytic performance of HMS, catalytic performance of ABS using different packing methods of two components and feeds, methanol yield in HMS and ABS, thermodynamic analysis, and optimization of reaction conditions.

15



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21



Page 22 of 36

Table 1. Textural Properties and Surface Compositions of the Catalysts.

Sample

A (m2g-1) BET

D

(nm)

pore

V (cm3g-1) pore

Zn/Cr molar ratio a bulk

surface

Cr2O3

18

3.5

0.11

-

-

Zn0.5Cr

80

13.6

0.23

0.46

0.54

Zn0.8Cr

65

12.2

0.38

0.84

1.44

Zn1.6Cr

68

13.8

0.39

1.65

2.70

Zn2.4Cr

63

10.6

0.41

2.28

4.61

Zn3.2Cr

59

10.4

0.43

3.12

5.58

Zn6.4Cr

39

9.7

0.26

6.24

8.49

ZnO

15

3.8

0.08

-

-

mixZn1.6Cr

17

5.6

0.08

1.68

-

impZn1.6Cr

18

6.5

0.06

1.66

-

a. Zn/Cr molar ratios in the bulk were determined by ICP-AES, Zn/Cr molar ratios on the surface were determined by XPS.

22


Page 23 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Texture Properties of Silicate-1 (S1) Coated HZSM-5 Sample

A

2 -1

D

(m g )

BET

(nm)

pore

V

3 -1

(cm g )

pore

HZSM-5

310

2.60

0.16

Z5@10S1

305

2.74

0.19

Z5@15S1

311

2.31

0.17

Z5@20S1

320

2.45

0.16

Z5@25S1

314

2.78

0.15

23



Page 24 of 36

Table 3. Conversion of CO and Composition of the Gaseous Phase. Selectivities of Gaseous Products (%)

CO Conv

Sample

(%)

CO2

CH4 MeOH C2H4 C2H6 C3H6 C3H8

CO C4+

efficiency a (%)

Cr2O3/Z5

15.5

39.3

36.0

-

3.3

9.5

3.0

8.9

0.0

16.6

Zn0.5Cr/Z5

24.1

40.5

31.6

-

3.7

8.5

2.1

9.3

4.2

32.8

Zn0.8Cr/Z5

28.2

40.4

30.6

-

4.0

7.6

2.7

10.1

4.6

34.0

Zn1.6Cr/Z5

29.8

38.9

34.8

-

4.3

8.5

2.0

8.9

2.7

39.0

Zn2.4Cr/Z5

27.8

40.2

32.4

-

3.9

7.6

2.6

10.9

2.3

35.7

Zn3.2Cr/Z5

26.5

42.4

29.8

-

3.3

7.1

2.2

10.8

4.3

31.5

Zn6.4Cr/Z5

24.1

39.6

30.9

-

3.3

7.1

3.3

11.4

4.3

27.2

ZnO/Z5

13.4

37.8

32.2

-

3.2

6.8

3.2

9.5

7.2

26.4

mixZn1.6Cr/Z5

17.7

37.6

34.4

-

3.7

9.0

2.0

8.0

5.1

32.6

impZn1.6Cr/Z5

18.6

41.1

31.2

-

3.8

9.1

2.7

7.4

4.7

33.9

Reaction conditions: temperature=450 oC, pressure=3 MPa, GHSV=12000 h-1, LHSV=3 h-1. a. CO efficiency = (CO transferred into toluene and xylene) / (converted CO) =

Feedbenz  Convbenz (1  Selecttol  2  Select xyl ) Feed CO  ConvCO

24


Page 25 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 4. Conversion of Benzene and Composition of the Liquid Phase. Benzene

Sample

Selectivity of Alkylation Products (%)

Phenyl Ring

Conversion (%)

toluene

xylene

ethylbenzene

C9+

Yield (%) a

Cr2O3/Z5

10.2

86.4

7.4

5.3

0.9

95.0

Zn0.5Cr/Z5

27.6

72.1

21.3

4.7

1.9

96.1

Zn0.8Cr/Z5

33.9

71.0

21.1

4.3

3.6

97.9

Zn1.6Cr/Z5

39.8

66.4

25.2

4.0

4.4

96.6

Zn2.4Cr/Z5

35.1

72.2

20.5

4.2

3.1

95.8

Zn3.2Cr/Z5

29.5

73.5

19.9

3.0

3.6

97.3

Zn6.4Cr/Z5

23.9

75.7

17.1

4.8

2.4

96.9

ZnO/Z5

13.8

81.2

10.6

4.5

3.7

95.4

mixZn1.6Cr/Z5

21.4

77.7

15.0

4.4

2.9

97.7

impZn1.6Cr/Z5

23.1

76.9

16.1

3.7

3.3

95.1

Reaction conditions: temperature=450 oC, pressure=3 MPa, GHSV=12000 h-1, LHSV=3 h-1. a. phenyl ring yield = (phenyl ring in the product) / (phenyl ring in the reactant) =[

m effulent

(1  Convbenz ) M benz   Convbenz  Selecti M i

]/

25


m feed M benz

Industrial & Engineering Chemistry Research 1 2 3 Table 5. Catalytic Performance of Siliate-1 Coated HZSM-5 Coupled with Zn1.6Cr 4 5 Selectivities of Alkylation Products (%) 6 Benzene 7 8 Conversion Sample ethylbe xylene a 9 toluene 10 (%) nzene para meta ortho 11 12 Zn1.6Cr/ Z5 39.8 66.4 6.5(25.9%) 13.2(52.2%) 5.5(21.9%) 4.0 13 14 Zn1.6Cr/ Z5@ 10S1 37.2 69.8 8.4(37.2%) 10.4(46.1%) 3.8(16.7%) 3.8 15 16 Zn1.6Cr/ Z5@ 15S1 36.5 71.3 11.0(50.4%) 8.5(39.0%) 2.3(10.6%) 3.9 17 18 Zn1.6Cr/ Z5@ 20S1 34.4 73.8 15.3(73.2%) 3.6(17.0%) 2.0(9.8%) 4.1 19 20Zn1.6Cr/ Z5@25S1 30.5 75.4 14.8(74.4%) 3.6(18.1%) 1.5(7.5%) 3.8 21 22 a. The number in the brackets is the proportion of each xylene in all xylene. 23 24 b. phenyl ring yield = (phenyl ring in the product) / (phenyl ring in the reactant) 25 26 m effulent m feed ]/ =[ 27 (1  Convbenz ) M benz  Convbenz  Selecti M i M benz 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



26


Page 26 of 36

Phenyl Ring Yield C9+

(%) b

4.4

96.6

3.9

95.7

3.0

96.8

1.2

95.4

0.9

95.3

Page 27 of 36

a ZnO Intensity (a.u.)

Zn6.4Cr Zn3.2Cr Zn2.4Cr Zn1.6Cr Zn0.8Cr Zn0.5Cr Cr2O3 10

20

30

40 50o 2 Theta ( )

60

70

80

green: Cr2O3 PDF#38-1479 red: ZnO (zincite) PDF#36-1451 black: ZnCr2O4 PDF#22-1107

b Zn6.4Cr Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Zn3.2Cr Zn2.4Cr mixZn1.6Cr impZn1.6Cr Zn1.6Cr Zn0.8Cr Zn0.5Cr

28

30

32 34 o 2 Theta ( )

36

38

Figure 1. XRD Patterns of Zn/Cr Oxides (a, b).

27



a

Intensity (a.u.)

9665 eV

ZnO Zn3.2Cr Zn1.6Cr

9668 eV

9664

9672

9680

9680 eV

Zn0.8Cr Zn0.5Cr

9673 eV

Zn foil

9650

9660

9670 9680 Energy (eV)

9690

o

1.53 A Zn-O

o

2.88 A Zn-O-Zn

b ZnO

FT(k3c(k))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 36

Zn3.2Cr Zn1.6Cr

o

2.96 A Zn-O-Cr Zn0.8Cr Zn0.5Cr

o

2.27 A Zn-Zn

Zn foil 0

1

2

3 4o Radial Distance (A)

5

6

Figure 2. XANES (a) and EXAFS (b) Spectra of Zn K-edge.

28


Page 29 of 36

6+

3+

Cr Cr 2p1/2

Cr Cr 2p3/2 579.6 eV

586.3 eV

(3- )+

Cr

a

Cr 2p3/2 576.5 - 578.9 eV

Intensity (a.u.)

Zn3.2Cr Zn1.6Cr

Zn0.5Cr Cr2O3 595

590

585 580 Binding Energy (eV)

Zn 2p1/2

575

Zn 2p3/2

1044.7eV

1021.6eV

570

b

ZnO 1045.4eV

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1022.3eV

Zn3.2Cr

Zn1.6Cr Zn0.5Cr 1050

1045

1040 1035 1030 1025 Binding Energy (eV)

29


1020


531.7 eV 532.6 eV O2 O3 11.5%

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

18.1%

530.2 eV O1 70.4%

30.0%

c ZnO

61.9%

Zn3.2Cr

8.1% 39.7% 19.5%

Page 30 of 36

40.8%

Zn1.6Cr

16.2% 78.9% 4.9%

5.6%

536

534

Zn0.5Cr 16.3%

78.2%

532 530 528 Binding Energy (eV)

Cr2O3 526

Figure 3. XPS Spectra of Cr 2p (a), Zn 2p (b) and O 1s (c) Orbitals.

30


Page 31 of 36

HZSM-5

Z5@10S1

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Z5@15S1

Z5@20S1 Z5@25S1 10

20

30 o 2Theta ( )

40

Figure 4. XRD Patterns of Zeolites.

31


50


Figure 5. SEM Images of Silicon-1 (S1) Coated HZSM-5 a) HZSM-5 b). Z5@10S1 c). Z5@15S1 d). Z5@20S1 e). Z5@25S1 red circle: uncoated HZSM-5

32


Page 32 of 36

Page 33 of 36

Acidity Amount

HZSM-5 0.69 mmol/g

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Z5@ 10S1 Z5@ 15S1 Z5@ 20S1 Z5@ 25S1

100

200

300 400 500 o Temperature / C

600

0.44 mmol/g 0.41 mmol/g 0.37 mmol/g 0.31 mmol/g

700

Figure 6. NH3-TPD Curve of Silicate-1 (S1) Coated HZSM-5

33



50 Zn1.6Cr O2/23.3%

Benzene-ABS 40

Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 36

CO-ABS Zn3.2Cr O2/18.3%

Zn0.5Cr O2/10.5%

30 Cr2O 3

O2/9.0% ZnO O2/6.6%

20 10

CO-HMS 0

5

10 15 20 Surface O2 Content (%)

25

Figure 7. Correlation between Surface O2 Content and Activities Surface O2 content = (O2 ratio in total oxygen)*(oxygen ratio in O, Zn and Cr) = SO2 SOtotal



PO PO  PZn  PCr

Wherein, Si is the fitted XPS curve area of i and Pi is the surface atomic ratio of i.

34


Page 35 of 36

80

Zn1.6Cr/Z5

select. toluene

60

Percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


conv. benzene

40

conv. CO

20

select. xylene 9+ select. ethylbenzene select. C

Zn1.6Cr/Z5@20S1

80

select. toluene

60 conv. CO

40 20

conv. benzene

select. xylene

0 0

20

px/xylene

select. ethylbenzene select. C

40 60 Time on Stream (hr)

80

9+

100

Figure 8. Stability of Zn1.6Cr/Z5 and Zn1.6Cr/Z5@20S1 in Alkylation of Benzene with Syngas.

35



Graphical Abstract

36


Page 36 of 36

One Step Alkylation of Benzene with Syngas over Non-noble Catalysts

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