Study on furfural conversion into aromatics over Zn/HZSM-5 and Fe/HZSM-5 catalysts

ZSM-5 zeolite material (Si/Al ratio = 25) was synthesized with silica source of TEOS and TPAOH template. The zeolite is modified into proton form (HZSM-5) with 343 m2/g of BET surface area, 324 m2/g of micropore area, 0.1491 cm3/g of micropore volume and 5.77 nm of BJH adsorption average pore width. Zinc oxide and iron oxide are dispersed onto HZSM-5 catalyst surface with different contents by wet impregnation method. The results of HZSM-5, Zn/HZSM- 5, Fe/HZSM-5 catalyst materials still retain the micropore structure of ZSM-5 zeolite. These materials are used as catalysts for furfural pyrolysis in the inert atmosphere (N2) with the temperatures ranged from 400 to 700 °C. The conversion of furfural to aromatic hydrocarbons on catalysts is evaluated by furfural conversion, conversion into aromatics and aromatic hydrocarbons selectivity. Result shows that 3 %Zn/HZSM-5 and 2 %Fe/HZSM-5 catalyst favor for furfural pyrolysis at 600 oC. The furfural conversion, the conversion into BTXN and the BTXN selectivity are respectively 48.36 %, 21.18 %, 16.18 % with 3 %Zn/HZSM-5 catalyst and 64.41 %, 16.47 %, 26.81 % with 2%Fe/HZSM-5 catalyst. These results are the basic research for the upgrade of pyrolysis oil into fuels.

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Cite this paper: Vietnam J. Chem., 2020, 58(5), 602-609 Article DOI: 10.1002/vjch.202000025 602 Wiley Online Library © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH Study on furfural conversion into aromatics over Zn/HZSM-5 and Fe/HZSM-5 catalysts Huynh Van Nam 1,2 , Truong Thanh Tam 2 , Van Dinh Son Tho 1* 1 School of Chemical Engineering, Hanoi University of Science and Technology, 1 Dai Co Viet, Hai Ba Trung district, Hanoi 10000, Viet Nam 2 Faculty of Natural Sciences, Quy Nhon University, 170 An Duong Vuong, Quy Nhon City, Binh Dinh province 55000, Viet Nam Received February 26, 2020; Accepted July 28, 2020 Abstract ZSM-5 zeolite material (Si/Al ratio = 25) was synthesized with silica source of TEOS and TPAOH template. The zeolite is modified into proton form (HZSM-5) with 343 m 2 /g of BET surface area, 324 m 2 /g of micropore area, 0.1491 cm 3 /g of micropore volume and 5.77 nm of BJH adsorption average pore width. Zinc oxide and iron oxide are dispersed onto HZSM-5 catalyst surface with different contents by wet impregnation method. The results of HZSM-5, Zn/HZSM- 5, Fe/HZSM-5 catalyst materials still retain the micropore structure of ZSM-5 zeolite. These materials are used as catalysts for furfural pyrolysis in the inert atmosphere (N2) with the temperatures ranged from 400 to 700 °C. The conversion of furfural to aromatic hydrocarbons on catalysts is evaluated by furfural conversion, conversion into aromatics and aromatic hydrocarbons selectivity. Result shows that 3 %Zn/HZSM-5 and 2 %Fe/HZSM-5 catalyst favor for furfural pyrolysis at 600 o C. The furfural conversion, the conversion into BTXN and the BTXN selectivity are respectively 48.36 %, 21.18 %, 16.18 % with 3 %Zn/HZSM-5 catalyst and 64.41 %, 16.47 %, 26.81 % with 2%Fe/HZSM-5 catalyst. These results are the basic research for the upgrade of pyrolysis oil into fuels. Keywords. Furanic, aromatic, ZSM-5 zeolite, pyrolysis, biomass. 1. INTRODUCTION Fossil fuel resources are dwindling along with environmental concerns that have spurred various studies to produce alternative fuels from renewable carbon neutral sources (agricultural and forestry by- products such as wood, sawdust, bagasse, rice husks, straw, etc.). The process of converting biomass on catalysts into biofuels is expected to replace part of fossil fuels (oil, coal) and solve existing problems, [1] which has attracted a lot of interested in research of scientists in the world. Furanic compounds (furan, furfural, 5-methyl furan, etc.) are one of the main components of pyrolysis oil, they are formed from the decomposition process of hemicellulose and cellulose in biomass. [2,3] Among them, furfural is of highest interest due to its high specificity for furanic compounds as well as its high reactivity and versatility. [4-6] In addition to the need to remove the element oxygen to improve the quality of bio-oil, the excess functions present in furanic compounds (such as high toxicity, high oxygen content, low calorific value, incomplete combustion and deposit formation, etc.) are also detrimental when it is used directly as a fuel. Therefore, study on furanic compounds conversion is needed to upgrade pyrolysis oil. The different types of catalysts for furfural conversion to biofuels, fuel additives and chemicals are seriously studied. [5] Furanic compounds are converted into aromatic hydrocarbons and other hydrocarbons by process of pyrolysis on ZSM-5 zeolite. The reaction medium can be inert gas (N2, He), hydrogen, methane, propylene, etc., the reaction temperature usually ranges from 400 to 700 o C. [7-9] ZSM-5 zeolite catalyst is a reasonable choice because its pore size and structure that is suitable for a higher selectivity of aromatics. The metals such as Zn, Ga, Ag, Pt, Pd, Ir, etc. are chosen to be doped into zeolite because these metals are reported to be beneficial for the formation of aromatic compounds such as deoxygenation and hydrogenation reactions. [7-14] In this paper, furfural, a specific compound of the furan family, will be conducted pyrolysis on HZSM-5 catalyst and HZSM-5 catalyst is modified by oxides of zinc (Zn) and iron (Fe). Results are valuated according to conversion into aromatics and aromatic hydrocarbons selectivity such as benzene, toluene, Vietnam Journal of Chemistry Van Dinh Son Tho et al. © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 603 xylene and naphthalene (BTXN). According to Cheng et al. (2011), [7] conversion of furan on HZSM-5 catalyst occurs by the mechanism as shown in figure 1. The furan molecules are adsorbed onto the capillary of HZSM- 5 to form intermediate compounds such as 2,2- methylenebisfuran, benzofuran at temperatures of 400-600 o C. The products are composed of CO, CO2, olefin (C2H4, C3H6) and aromatic compounds (benzene, toluene, xylene and naphtalene). The selectivity of aromatic hydrocarbons and olefins decreases with increasing WHSV, whereas the selectivity of unsaturated hydrocarbon compounds (ethylene, cyclopentadien) increases. The appropriate temperature for the formation of aromatic hydrocarbons is ranges from 450-600 o C. The selectivity of CO, CO2 and olefin increases when the reaction temperature is over 600 o C. In addition, the process also causes Diels-Alder reaction (benzofuran and water are formed by condensation of two furans), decarbonylation (CO and allene are formed from furans), oligomerization (olefins, aromatics and hydrogen are formed from allen), alkylation (forms furan and olefins) and condensation create coke on the catalyst surface, reducing catalytic activity after about 30 minutes of reaction. Figure 1: Mechanism of furan conversion into aromatics on HZSM-5 catalyst [7] Figure 2: Mechanism of furan pyrolysis on HZSM-5 catalysis at 600 °C [8] Similarly, according to the research results of Vaitheeswaran et al. (2013), [8] furan metabolism on HZSM-5 catalyst occurred by Diels-Alder reaction mechanism and Ring-Opening. As well as the research results of Cheng et al. (2011), [7] benzofuran is an intermediate of metabolism, the product includes CO, CO2, olefins, alkadienes, alkynes and aromatics (benzene, toluene, xylene, naphtalene, etc.). If the reaction medium is present with gases such as methane, propylene or methanol, the efficiency of forming hydrocarbons increases, because then the Diels-Alder reaction prevails. [9,13,14] 2. MATERIAL AND METHODS 2.1. Experimental methods ZSM-5 zeolite with SiO2/Al2O3 ratio = 50 (Si/Al = 25) was synthesized by hydrothermal method according to Liu et al. (2018). [15] 26.16 g of tetra-n- propylammoniumhydroxide (TPAOH, C12H29NO 1M, Aldrich) and 3.16 g of urea (CO(NH2)2 99 wt.%, Fisher Acros) were dissolved in 43.32g of distilled water. The mixture was stirred at room temperature for about 1 hour to dissolve completely, then added with 21.16 g of tetraethoxysilane (TEOS, (C2H5O)4Si 99 wt.%, Merck) and 0.61g of aluminum isopropoxide (C9H21AlO3 98 wt.%, Merck). The mixture continued to be stirred for 24 hours at room temperature then put into autoclave to crystallize for 48 hours at 175 o C. The synthetic product was centrifuged and washed with distilled water until the neutral environment is reached (pH = 7). The sample was dried for 12 hours at 100 °C and put into the furnace at 500 °C for 12 hours to remove organic matter and stabilize the structure. HZSM-5 catalyst (proton form of ZSM-5) was synthesized by ZSM-5 exchanging twice with 1M NH4NO3 solution (Fisher Acros, 99 wt.%) for 24 hours at room temperature. [16] After ion exchange, the product was centrifuged and washed with distilled water to deion at 80 ºC, then was dried for 12 hours at 100 ºC and was heated in static air for 3 hours at 550 ºC. HZSM-5 sample is symboled as HZ. The metal-assisted HZSM-5 catalyst (Zn, Fe) was synthesized by wet impregnation method according to the literatures. [17,18] Zn/HZSM-5 catalysts with 1 wt.%, 3 wt.% and 5 wt.% of Zn were synthesized by stirring a solution of HZSM-5 and Zn(NO3)2.6H2O (Fisher Acros, 98 wt.%) for 24 hours at 80 °C, was dried for 12 hours at 105 °C and was heated for 6 hours at 550 °C. The catalysts has 1 wt.%, 3 wt.% and 5 wt.% of Zn, they are symboled as 1ZnHZ, 3ZnHZ and 5ZnHZ, respectively. The process is similar to the Fe/HZSM-5 catalyst, using Fe(NO3)3.9H2O (Fisher Acros, 98 wt.%) with 1 wt.%, 2 wt.% and 3 wt.% of Fe. The samples are Vietnam Journal of Chemistry Study on furfural conversion into aromatics © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 604 symboled as 1FeHZ, 2FeHZ and 3FeHZ, respectively. 2.2. Material Furfural 99 wt.% of Fisher Acros is pyrolysis in the condition of Weight Hourly Space Velocity (WHSV) of 5 h -1 (10 g of material/2 g of catalyst/h); carrier gas flow rate N2 of 50 mL/min and heating rate of 20-25 o C/min. [13] The furfural conversion, conversion into aromatics and the aromatics selectivity are determined by the formula: ( ) (1) ( ) (2) ( ) (3) The catalytic characteristics were assessed through the results obtained from X-ray diffraction (XRD, Rigaku HyPix-3000), Brunauer-Emmett- Teller theory (BET, ASAP 2010 Mircomeritics), Energy-dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM, JEOL 5410 LV) measurements. Gas chromatography-mass spectrometry (GC-MS) was used to determine the chemical composition products and it was performed on a GC-MS 7000D at laboratory of Electrical - Chemical - Physics, the Directorate for Standards, Metrology and Quality of Vietnam (STAMEQ). Analytical specifications were 0.8 mL/min of helium (He) flow rate, 50:1 of flow split ratio, 9 °C/min of heating rate, m/z = 40-600 of MSD scanning range, and 70 eV of ion source EI. 3. RESULTS AND DISCUSSION 3.1. Characterization of catalytic material Phase composition of catalyst material samples is analyzed by the XRD method with emission of CuKα, .5406 Å of wavelength (λ), 4 KV of voltage, scanning angle (2θ) from 5 to 8 o, 0.02o of scanning step, 0.6s of scanning time and at 25 o C. The degree of crystallization is calculated using major peaks with 2θ in the range of 22 to 25o.[19] Figure 3 shows a very high level of crystalline of the catalyst material. The main peaks of HZSM-5 at 2θ are 8. 4o, 9.02o, 23.28o, 24.1o and 24.58o within the range of typical peaks of ZSM-5 (2θ = 7-9o and 22 - 25 o ). [19] XRD patterns of the 3ZnHZ and 2FeHZ catalytic samples are similar to that of HZSM-5. This shows that the addition of Zn and Fe by wet impregnation method don’t change the structure of HZSM-5. Metal oxides are also highly dispersed on the surface of HZSM-5 catalyst. However, because the used Zn and Fe contents on HZSM-5 are quite low (from 1 wt.% to 5 wt.%), typical peaks for the metal oxide crystals were not found in XRD pattern. 10 20 30 40 50 60 70 80 0 50000 100000 150000 200000 1900 3800 5700 7600 1600 3200 4800 6400 2(deg) HZ In te n si ty ( c p s) 3ZnHZ 2FeHZ Figure 3: XRD patterns of HZ, 3ZnHZ and 2FeHZ catalyst Table 1 shows the EDX spectra of HZ, 3ZnHZ and 2FeHZ catalyst samples. The result from the EDX spectrum shows that the ratio of Si/Al is about 23. It is slightly lower than the initial target ratio (Si/Al = 25) but this is completely consistent with experimental conditions. On the other hand, the process of putting zinc and iron oxides on HZSM-5 catalyst was successful, with the content equivalent to the calculated content. Specifically, the results measure 2.83 wt.% of the Zn on HZSM-5 compared with 3 % of the theoretical value and 1.94 wt.% of the Fe on HZSM-5 compared with 2 wt.% of the theoretical value. The surface morphology of HZ, 3ZnHZ and 2FeHZ catalysts is shown by scanning electron microscopy (SEM) images in figure 4. Most of the crystals exhibited a hexagonal-like shape that is Vietnam Journal of Chemistry Van Dinh Son Tho et al. © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 605 typical for MFI. [15,20] However, it can be seen that the process of putting zinc oxide and iron oxide on the surface do not change the structure and surface shape of the catalyst. These two oxides also disperse evenly on the catalytic surface. Similar to the case of XRD results, zinc and iron oxide particles cannot be observed at this magnification because they are introduced in low mass. Table 1: EDX spectra of HZ, 3ZnHZ and 2FeHZ catalyst samples Samples Weight (wt.%) Si/Al O Al Si Zn Fe HZ 57.82 1.75 40.43 - - 23 3ZnHZ 54.21 1.70 41.26 2.83 - 24 2FeHZ 53.76 1.85 42.45 - 1.94 23 Figure 4: SEM images of HZ, 3ZnHZ and 2FeHZ catalyst Figure 5 shows nitrogen adsorption-desorption isotherms at 77 K (A) and Barrett-Joyner-Halenda (BJH) pore size distribution of the HZ, 3ZnHZ and 2FeHZ catalysts (B). The nitrogen adsorption curves of the catalyst samples show a type IV isothermal line. The adsorption-desorption loops of the three catalyst samples are in the range from p/p0 = 0.4 to p/p0 = 1, it is characterizing the microporeous material. This again proves that the structure of the catalyst material is preserved after the wet impregnation process of zinc and iron oxides. However, table 2 shows that the surface properties and pore structure of catalyst samples are different. HZ catalyst has a BET surface area of about 343 m 2 /g, while the BET surface area of 3ZnHZ catalyst is 302 m 2 /g and 2FeHZ is 313 m 2 /g. In particular, the BET surface area of 3ZnHZ and 2FeHZ catalysts decreases mainly due to the reduction of the surface area of the micropore. The pore volume is also reduced due to the accumulation of metal oxides on the surface and in the pore of the catalyst. [21] This result is similar to the research results of Schultz et al. (2017) [22] or Zheng et al. (2017). [23] 0.0 0.2 0.4 0.6 0.8 1.0 80 90 100 110 120 HZ 3ZnHZ Q u an ti ty A d so rb ed ( cm ³/ g S T P ) Relative Pressure (p/p°) 2FeHZ A 0 10 20 30 40 50 60 70 80 HZ 3ZnHZ P o re V o lu m e (c m ³/ g ) Pore Width (nm) 2FeHZ B Figure 5: Nitrogen adsorption-desorption isotherms at 77 K (A) and Barrett-Joyner-Halenda (BJH) pore size distribution (B) of the HZ, 3ZnHZ and 2FeHZ catalysts Vietnam Journal of Chemistry Study on furfural conversion into aromatics © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 606 Table 2: Surface properties of catalyst samples ID Catalyst samples SBET (m 2 /g) Smicro (m 2 /g) Sext (m 2 /g) Vp (cm 3 /g) dp (nm) 1 HZ 343 324 19 0.1494 5.77 2 3ZnHZ 302 280 22 0.1294 5.86 3 2FeHZ 313 271 41 0.1247 4.94 SBET: BET surface area; Smicro: Micropore area; Sext: External surface area; Vp: Micropore volume; dp: BJH Adsorption average pore width. 3.2. Effect of metal content on HZSM-5 To compare the effect of zinc and iron oxides content on the HZSM-5 catalyst activity, the study conducts many changes of zinc and iron oxides content on the surface of HZSM-5 catalyst. The results are evaluated by the furfural conversion and aromatics selectivity (BTXN). During the experimental process, the study selects three representative catalyst samples for comparison. Figure 6 shows the selectivity of aromatic hydrocarbons during furfural pyrolysis on different catalysts at 600 o C. When furfural pyrolysis is non- catalytic it doesn't produce aromatic hydrocarbons. The BTXN selectivity of HZSM-5 catalyst is quite low about 2.32 %. The main product is benzene accounting for 54.95 % of total BTXN products, the remaining is toluene, naphthalene and without xylene. When catalyst is added by zinc or iron oxides, the BTXN selectivity is significantly increased. Specifically, HZSM-5 catalyst contains 1 wt.% Zn, the BTXN selectivity is up to 15.52 %, 16.18 % for the catalyst containing 3 wt.% Zn and 36.34 % for the catalyst containing 5 wt.% Zn. Aromatic hydrocarbon products are BTX and only a small amount of naphthalene. NC HZ 1ZnHZ 3ZnHZ 5ZnHZ 1FeHZ 2FeHZ 3FeHZ 0 10 20 30 40 50 60 S e le c ti v it y ( % ) Benzene Toluene Xylene Naphthalene BTXN Figure 6: BTXN selectivity of furfural pyrolysis on HZ, ZnHZ and FeHZ catalysts The results are similar to furfural pyrolysis with Fe-containing catalysts. When Fe content is increased from 1 to 3 wt.%, the BTXN selectivity also is increased, respectively by 16.47 %, 26.81 % and 32.10 %. In addition, if the BTXN selectivity is almost the same as with the Zn-containing catalyst, the selectivity of benzene is the highest with the Fe- containing catalyst. Especially, the selectivity of naphthalene increases significantly compared to Zn- containing catalysts. This result is completely consistent with the studies of Li et al. (2016) [17] or the studies of Mullen et al. (2015). [24] The Fe/HZSM-5 catalyst is highly activity for formation of benzene and naphthalene. Figure 7 shows the furfural conversion and conversion into BTXN of catalysis pyrolysis at 600 o C. The furfural conversion is the weight of furfural transformed into products. Meanwhile, conversion into BTXN is the weight of carbon in furfural transformed into carbon in aromatic hydrocarbons (BTXN). Results show that furfural catalysis pyrolysis has a lower furfural conversion than non- catalytic pyrolysis. If the furfural conversion of non-catalytic pyrolysis (NC sample) is 71.3 %, the furfural conversion of HZSM-5 catalytic pyrolysis is only 14.14 %. The addition of zinc and iron oxides to HZSM-5 catalyst makes the increassing conversion of pyrolysis. The furfural conversion increases in proportion to the amount of metal on NC HZ 1ZnHZ 3ZnHZ 5ZnHZ NC HZ 1FeHZ 2FeHZ 3FeHZ 0 10 20 30 40 50 60 70 80 90 C o n v e rs io n ( % ) Furfural conversion Conversion into BTXN Figure 7: Furfural conversion and conversion into BTXN on HZ, ZnHZ and FeHZ catalysts at 600 o C Vietnam Journal of Chemistry Van Dinh Son Tho et al. © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 607 catalyst. Fe-containing catalysts have higher furfural conversion than Zn-containing catalysts. The furfural conversion is 21.35 %, 48.36 % and 55.09 % with 1, 3 and 5 wt.% Zn-containing catalyst, respectively. While the furfural conversion is 53.41 %, 64.41 % and 67.16 % with 1, 2 and 3 wt.% Fe- containing catalyst, respectively. On the other hand, adding metal oxide to surface of HZSM-5 catalyst significantly increases aromatic hydrocarbon conversion of furfural. The conversion into BTXN of furfural is only 3.22 % with HZSM-5 catalyst. But if adding 1 wt.% Zn to catalyst, the conversion into BTXN of furfural is 21.02 %. This value is 21.18 % with 3 wt.% Zn-containing catalyst and 34.39 % with 5 wt.% Zn-containing catalyst. The same phenomenal also observed with Fe- containing catalysts but it is lower than Zn- containing catalysts. The conversion into BTXN of furfural on Fe-containing catalysts are respectively 13.22 %, 16.47 % and 16.27 % for catalysts containing 1, 2 and 3 wt.% Fe. So, when the Fe content on the catalyst increases, the furfural conversion increases but the conversion into BTXN is ap