Highly Active W-h2so4/hzsm-5 Catalyst for Direct Conversion of Methane into Aromatic

Dehydroaromatization of methane (DHAM) under non oxidative condition was studied over tungsten loaded HZSM-5 catalysts to produce aromatic hydrocarbons. The catalysts were prepared by impregnation method using different conditions: in neutral and acidified solution. The activity of W/HZSM-5 prepared by neutral solution and W-H2SO4/HZSM-5 prepared in acidified condition were compared. The results showed that the optimum activity of W-H2SO4/HZSM-5 catalyst exceeded that of W/HZSM-5 catalyst. The effect of Si/Al ratio of WH2SO4/HZSM-5 catalyst was also studied. The WH2SO4/HZSM-5 catalyst with Si/Al ratio = 30 was found to be the most promising for the DHAM reaction. The remarkable activity of the catalyst is attributed to the presence of dual effects: suitable content of octahedral polymeric and tetrahedral monomeric tungstate species accompanied by proper amount and strength of acid sites in the catalyst. Kata kunci: methane; aromatization; W/HZSM-5 Introduction Methane, the main component of natural gas is an important source of energy. Currently, the main use of natural gas is combustion process for heating purposes. However, the usage of natural gas for liquid fuels and higher hydrocarbon is limited. Direct conversion of methane to more valuable chemicals is still a challenging field in catalysis (Hassan et. al., 2006). Previous studies have been conducted on dehydroaromatization of methane (DHAM). The studies reported that the DHAM reaction could take place in the presence of HZSM-5 catalysts containing transition metal. Most research work has mainly used Mo metal supported HZSM-5-catalyst operated at 973 K. At such low temperature, methane conversion (XCH4) was low due to thermodynamic limitation (Xiong et al., 2001). To reach a high methane conversion, higher reaction temperature is required. However, Mo-based catalysts used for DHAM suffers inevitably due to serious loss of Mo component by sublimation under such high temperature. Therefore, it is of interest to find the catalyst with high activity and stabil at higher operation temperatures suitable for DHAM reaction. The previous works reported that the activity and stability of catalyst can be improved by the addition of a second metal component on Mo/HZSM-5 (i.e Ru or Zn on Mo/HZSM-5). Currently, W/HZSM-5 catalyst was also reported to have highly active catalyst for DHAM reaction in the absence of oxygen. Xiong et al. (2001) reported that incorporation of Zn (or Mn, La, Zr) into the W/HZSM-5 catalyst could improve the performance of the catalyst for DHAM reaction operating under higher temperatures. The present work studies the DHAM reaction over W supported HZSM-5 catalysts. The effects of preparation conditions and Si/Al ratios of HZSM-5 are reported. Material and Methods Preparation and characterization of catalyst The 3%W/HZSM-5 catalyst was prepared by the conventional impregnation method. The HZSM-5 zeolite with a SiO2/Al2O3 ratio of 30 was supplied commercially (Zeolyst international Co. Ltd.). After impregnation with an aqueous solution of ammonium meta tungsten ((NH4)6W12O40.H2O), the zeolite was dried at 120 C for 2 h, and calcined in static air at 500C for 5 h. Another set of a series of 3% WH2SO4/HZSM-5 catalysts with different Si/Al ratios were prepared by impregnating HZSM-5 with ((NH4)6W12O40.H2O) and H2SO4 solution (pH = 2–3). The acidity of the catalysts was measured by means of TPD-ammonia using a Micromeritics TPD/TPR/O analyzer. The nature of W species on the catalysts was determined by means of UV diffuse reflectance spectra. UV DRS spectra were performed on a Perkin-Elmer Lamda-900 spectrometer. Catalytic testing Catalytic testing was carried out at atmospheric pressure in a fixed-bed continuous flow system with a quartz reactor of 9 mm internal diameter and 300 mm length. Before reaction, the catalyst was pretreated in a flow of nitrogen at Feed gas containing CH4 + 10% N2 was passed through over the catalyst bed at GHSV of 1800 ml/(g.h). Nitrogen was used as an internal standard for calculating the methane conversion and selectivity of the reaction products. The reaction products were analyzed by a Hewlett-Packard 5890 on-line GC *) Department of Chemical Engineering, Muhammadiyah University of Surakarta 61 Jl. Raya Pabelan, Surakarta, Indonesia, Telp/Fax: +62-271-717417 Email: [email protected] Highly Active W-H2SO4/HZSM-5 ... (Kusmiyati, et al.) equipped with TCD using Porapak Q, molecular sieve 5A, UCW 982, and DC 200 columns. Results and Discussion The result of catalysts activity can be seen in Table 1. The catalysts were prepared by impregnating HZSM-5 using different solution of ammonium metatungstate, 3% W/HZSM-5 prepared in neutral solution and 3%WH2SO4/HZSM-5 acidified solution catalysts prepared in acidified solution. The result shows that 3%W-H2SO4/HZSM-5 gives higher methane conversion (9.59%) than W/HZSM-5 (8.40%). The results show that 3%W-H2SO4/HZSM-5 displays the highest aromatics selectivity (99.5%). It appears that the acidified catalyst give benefit on the activity of catalyst for increasing methane conversion and selectivity to aromatic. Table 1. Catalytic activity of 3% W/HZSM-5 and 3% W-H2SO4/HZSM-5 prepared by neutral and acidified solution, respectively for dehydroaromatization of methane at 973 K, GHSV = 1800 ml/(g.h) , Feed Gas = CH4 + 10% N2, 1 atm at 973 K. Catalyst Methane conversion (%) Aromatic selectivity (%) 3%W/HZSM-5 (Si/Al=30) 8.40 98.23 3%WH2SO4/HZSM-5 (Si/Al=30) 9.59 99.35 The effect of Si/Al ratios on the catalytic performances of WH2SO4/HZSM-5 catalyst for methane dehydrogenation and aromatization at 800 C, 1 atm and GHSV of 1800 ml/(g.h) catalysts are shown in Figure 1. Feed gas contains a mixture of CH4 90% + N2 10%. As illustrated in Figures 1A and 1B, the conversion of methane, the selectivity to benzene decreased with increasing Si/Al ratio of HZSM-5. The selectivity to aromatic over the W-H2SO4/HZSM-5 (Si/Al =50) catalyst was close to that over the Si/Al =80 catalyst. A methane conversion over 3%wt WH2SO4/HZSM-5 catalyst with Si/Al = 30 approaches a maximum value of 22.08%, and further decreases with time. A maximum aromatic selectivity of 97.49% was achieved over the 3% WH2SO4/HZSM-5 (Si/Al ratio = 30) catalyst. On the other hand, the C2 selectivity of 3% W-H2SO4/HZSM-5 catalyst with various Si/Al ratios increase with an increase in the time on stream, as seen in Figure 1 (C). A gradual but significant increment in the on-stream C2 selectivity from 2.69% to 12.19% was observed over the 3% WH2SO4/HZSM-5 catalyst with Si/Al = 30. Figure. 1. Catalytic performances for methane dehydrogenation and aromatization at1073 K, GHSV = 1800 ml/(g.h) and a on the 3%WH2SO4/HZSM-5 (Si/Al=30) (Ο),(Si/Al=50) () and (Si/Al=80) (∆) catalysts (A) for methane conversion; (B) for benzene selectivity; (C) for C2 selectivity. Further investigation was carried out to study the effect of GHSV on the catalytic activity of WH2SO4/HZSM-5 with different Si/Al ratios. During