Studies on Mo/HZSM-5 Complex catalyst for Methane Aromatization

Transcription

1 Journal of Natural Gas Chemistry 13(2004)36 40 Studies on Mo/HZSM-5 Complex catalyst for Methane Aromatization Qun Dong 1, Xiaofei Zhao 1, Jian Wang 1, M Ichikawa 2 1. Department of Petrochemical Engineering, Daqing Petroleum Institute, Daqing , China 2. Catalysis Research Center, Hokkaido University, Kita-Ku, N-11, W-10, Sapporo 060, Japan [Manuscript received November 05, 2003; revised February 23, 2004] Abstract: The influence of adding Fe, Cr, Co, and Ga into 3%Mo/HZSM-5 catalyst on methane aromatization, and the influence of additives ratio on methane conversion, selectivity to hydrocarbons and coke, as well as distribution of aromatics were investigated. The experimental results showed that the addition of Fe, Cr, Co and Ga promoted the dehydrogenation and dissociation of methane. The results of NH 3-TPD indicated that the acidity of HZSM-5 was changed by adding Fe and Co components, consequently the catalytic properties of Mo/HZSM-5 were changed. It was also revealed that strong acid sites were the center of methane aromatization. The results of XRD characterization showed that the crystallinity of Mo on ZSM-5 zeolite was increased after adding Fe, Co additives. Key words: methane, aromatization, benzene, HZSM-5 zeolite, Fe, Ga, Cr, Co 1. Introduction 2. Experimental Since the eighties, natural gas plays a more and more important role both in energy industry and as a raw material. The interests are focused on the process of oxidative coupling and carbureting to form lowgrade olefins. Recently, it was found that methane dehydrogenates and condensates into aromatics such as benzene and naphthalene selectively over Mo/HZSM- 5 catalyst under the atmospheric pressure, 973 K, and this process has become a hot subject for research [1]. The catalytic performance of 3%Mo/HZSM-5 with Fe, Cr and Ga additives in different ratios has been studied, and it was reported that carbon accumulated heavily on Mo/HZSM-5 resulting in notable decrease in reactivity [2]. In this work, the addition of Fe, Cr, Co and Ga to Mo/HZSM-5 was found to increase the methane conversion, reduce the carbon formation and increase the yield of benzene appreciably Catalyst preparation The Mo/HZSM-5 catalyst was prepared by incipient wetness impregnation of NH 4 /HZSM-5 zeolite (n(si)/n(al)=39.5 supplied by Japan- EAST Corporation Nanyang research institute) with an aqueous solution of ammonium molybdate. The impregnated sample was dried at room temperature for 24 h, then dried at 393 K for another 2 h and calcined in air at 773 K for 4 h. After that the Mo- Fe/HZSM-5, Mo-Cr/HZSM-5, Mo-Ga/HZSM-5 and Mo-Co/HZSM-5 catalysts were prepared by impregnating Mo/HZSM-5 with Fe, Cr, Ga or Co nitrate, and then the Mo- second metal /HZSM-5 was dried at room temperature for 24 h, at 393 K for 2 h, then calcined in air at 773 K for 4 h. The Mo- second metal/hzsm-5 compound catalysts were also prepared by co-impregnating HZSM-5 with an aqueous Corresponding author.

2 Journal of Natural Gas Chemistry Vol. 13 No solution of the second metal nitrate and ammonium molybdate at the same time Evaluation of catalysts The evaluation was carried out in a continuous flow fixed-bed micro-reactor unit, with a tubular quartz reactor (inner diameter: 8 mm). 0.2 g catalyst (20 40 mesh) was loaded. The reactions were conducted at 973 K under a total pressure of 101 kpa. The catalyst samples were pretreated at 973 K for 40 min in helium stream at gas hourly space velocity (GHSV) of 1500 ml/(g h). Then, the stream was switched to the feed gas mixture of 98%CH 4 (7.5 ml/min) and 2%Ar. The hydrocarbon products such as H 4, C 2 H 4, C 3 H 6, C 3 H 8, C 6 H 6, and C 10 H 8 etc were analyzed with an on-line Shmadzu GC14B gas chromatograph with a one meter long Porapak-P column connected to FID, and a GC8A gas chromatograph with a two meter long activated carbon column connected to TCD for the separation of H 2, Ar, CO, CH 4 and CO 2. The six-way sampling valve was heated to 533 K. The regulation of flow rate at the inlet and outlet of reactor were achieved by monitoring the concentrations of Ar in the stream and the conversion of methane. The selectivity to hydrocarbon products was calculated by the balance of carbon atoms Catalyst characterization The structure and surface acidity of the catalysts were studied using NH 3 -TPD and XRD techniques. NH 3 -TPD experiments were carried out in an automatic analysis apparatus (AMI Company, U.S.A) equipped with a thermal conductive detector (TCD). The tests were carried out with 0.3 g 80 mesh samples placed in a quartz tube (6 mm id). It was first flushed with He at 773 K for 2 h. Then, it was cooled down to 318 K and ammonia was adsorbed until saturation, then it was flushed with He till the TCD baseline was kept stable. The samples were heated to 873 K at a rate of 15 K/min, at the same time the NH 3 desorption curve was recorded. The XRD spectrum was determined with an X ray diffraction apparatus (Ricoh Company, Japan), using Cu, K α radiation, 40 kv, 45 ma, 4 o /min, from 5 o to 60 o. 3. Results and discussion 3.1. Results of aromatization on multi-metal catalyst The different second metals on the catalytic performance of methane aromatization were investigated for the Mo/HZSM-5 catalysts. Table 1 shows the aromatization results on the catalysts 3%Mo/HZSM- 5 modified with Fe, Cr, Ga and other elements respectively. From Table 1 significant changes of the catalytic performance can be found after adding of Fe, Cr and Ga additives. Conversion of methane, selectivity of hydrocarbons and yield of benzene and naphthalene are increased remarkably, meanwhile the selectivity to coke is decreased considerably except for Ag and Cu. The methane conversion follows the order: Ti>Cr>Co>Zn>Ni=Rh>Re>Ga>Au>Fe>Mo>Ag Table 1. The catalytic performances of methane on different complex catalysts a Catalsts Conversion (%) Selectivity (%) HC distribution (%) Yield (%) HC Coke CO C 2 C 6 H 6 C 7 H 8 C 10 H 8 C 6 H 6 C 10 H 8 Mo/HZSM Co-Mo/HZSM Fe-Mo/HZSM Ga-Mo/HZSM Cr-Mo/HZSM Ni-Mo/HZSM Zn-Mo/HZSM Ti-Mo/HZSM Rh-Mo/HZSM Re-Mo/HZSM Au-Mo/HZSM Ag-Mo/HZSM Cu-Mo/HZSM Fe-Mo/HZSM-5 b Co-Mo/HZSM-5 b a: sampling at 80 th minute of reaction, b: co-impregnated catalyst

3 38 Qun Dong et al./ Journal of Natural Gas Chemistry Vol. 13 No >Cu, and the increase of benzene yield on the catalysts modified with different elements follows the order Co>Cr>Ga>Ni>Zn>Fe>Rh>Re>Ga>Au>Mo. The complex catalysts which have better selectivity to hydrocarbons are: Co-Mo, Fe-Mo, Ga-Mo, Cr-Mo, Ni-Mo, and Au-Mo/HZSM-5 catalysts. The highest methane conversion was obtained by adding Ti, but the selectivity for aromatics declined sharply to 35.9%, and the selectivity to coke rose to 63%. The selectivity to hydrocarbons on Ga-Mo/HZSM-5 reached 84%, the selectivity to coke was less than that on the unmodified Mo/HZSM-5 catalyst, and C = 2 components in the products were decreased. These results demonstrated that Co-Mo, Fe-Mo, Cr-Mo, Ni-Mo, Zn-Mo/HZSM-5 catalysts are favorable for promoting the cyclization and dehydrogenation of C 2 olefins to form aromatics. Meanwhile, it was observed that the addition of Ga, Fe, Cr, Ni and Zn can inhibit the overdehydrogenation of methane. No obvious effects on Au-Mo, Ag-Mo, and Cu-Mo/HZSM-5 catalysts were observed. The preparation method also affects the catalyst s performance such as products distribution. The yields of benzene and naphthalene over Fe- Mo/HZSM-5 and Co-Mo/HZSM-5 catalyst prepared by co-impregnating are lower than those by sequence impregnating. Figure 1 shows the relationship of H 2 formation rates over the Mo/HZSM-5 catalysts modified with different additives Fe, Cr, Ga and Co. We can find easily that the rate of hydrogen formation increased notably on the catalysts with the addition of a second metal except Cu, Ag and Co. It is concluded that metal additives Fe, Cr, Ga and Co promoted the dehydrogenation and dissociation of methane over the Mo active site in the dehydrogenation and aromatization process of methane described in Ref [3]. Figure 1. H 2 formation rates of different catalysts Reaction conditions: 973 K, normal pressure, space velocity 1500 ml/(g h) 3.2. Ef fect of additives ratio on the catalytic activity of Mo/HZSM-5 Table 2 shows the effect of Fe/Mo atom ratio on the aromatization of methane. The results indicate that adding a small amount of Fe improved the catalysts activity, selectivity of hydrocarbons, increased the yield of benzene and naphthalene noticeably and reduced the coke formation to a minimum. It is interesting to note that the catalytic behavior of Fe modified Mo/HZSM-5 catalysts was sensitive to the additive amount. There was an optimum point. Further increase in Fe/Mo ratio caused the conversion of methane, selectivity of hydrocarbon and yield of benzene plus naphthalene decreased and the selectivity of coke increased. This was also true for Cr, Co and Ga modified Mo/HZSM-5 catalysts. At a content of 0.5% the Ga, Fe, Cr and Ga catalysts exhibit higher selectivity of benzene, leading to an increase about 1% in selectivity of benzene. Methane aromatization is related to the pore structure and acidity of HZSM- 5 [4]. The acidity of HZSM-5 catalysts changed with the amount of metal oxide added, while the Mo content was kept constant. It reached a maximum when the added amount was an optimal value. Beyond that point, the acidity decreased, and thus the conversion of methane and the yield of benzene both decrease.

4 Journal of Natural Gas Chemistry Vol. 13 No Table 2. Effect of Fe additive on aromatization results n(fe)/n(fe+mo) Conversion of Selectivity (%) Product distribution r(bz+np) a CH 4 (%) Hydrocarbons Coke of C 6 H 6 (%) (nmol/(g s)) a: r(bz+np) means the benzene and naphthalene formation rate (nmol/(g s)) Characterization of acidity of Mo/HZSM- 5 containing Fe and Co NH 3 -TPD technique was used to reveal the surface acidity of the Mo/HZSM-5, Fe-Mo/HZSM-5 and Co-Mo/HZSM-5 catalysts (Figure 2). It is seen that three peaks of the NH 3 -TPD curve correspond to the weak, moderately-strong and strong acid sites of Mo/HZSM-5, respectively. When Mo was introduced into HZSM-5, the strongly acidic site decreased its intensity and shifted to a higher temperature. The influence of Mo species located on the HZSM-5 is similar to that reported in Ref [5]. Adding Fe and Co additives to the Mo/HZSM-5 catalysts changed the acidities of catalysts remarkably, particularly in the strongly acidic site. For Co-Mo/HZSM-5 catalysts, the intensity of the peak for strongly acidic site became narrow and decreased, so did the peak for the moderately-strong acidic site. Figure 2. NH 3 -TPD characterization results of four catalysts (1) Mo/HZSM-5, (2) Co-Mo/HZSM-5, (3) Fe-Mo/HZSM-5 Introduction of Fe species into the Mo/HZSM-5 catalyst caused the peak for strongly acidic site become smooth, and the curve shape for the moderatelystrong acidic site was just like that of Co-Mo/HZSM-5 catalyst. The curve shapes of strong and moderatelystrong acidic site of Cr-Mo/HZSM-5 catalyst were similar to those for the Co-Mo/HZSM-5 catalyst. The addition of Fe and Co to the Mo/HZSM-5 catalysts caused the peak for the weakly acidic site to rise slightly. The above results implied that the addition of Fe and Co to the Mo/HZSM-5 zeolite changed the acid center distribution of the catalyst and consequently changed the catalytic performance Change of acidity for Mo/HZSM-5 and Co-Mo/HZSM-5 before and after reaction NH 3 -TPD technique was used to study the surface acidity of Mo/HZSM-5 and Co-Mo/HZSM-5 catalysts before and after reaction. It is seen from Figure 3 that the peaks corresponding to the moderatelystrong and strong acid sites for Mo/HZSM-5 were stronger than those for Co-Mo/HZSM-5 before reaction, while the peak corresponding to the weakly acidic site was weaker than that for Co-Mo/HZSM-5. After reaction the peak of the strong acid site became smooth and that of the moderately-strong acidic sites decreased. With the addition of Co, the moderatelystrong peak became smaller and the weak peak lowered after the reaction. Inspection of the 4 curves in Figure 3 reveals that the peaks for the strongly acidic site and the moderately- strong acidic site all became smoother after the reactions. This demonstrates that the formation of coke on Mo/HZSM-5 and Co-Mo/HZSM-5 catalysts took place on acidity centers after the catalytic reactions. The strongly acidic site appears to be the center of methane aromatization and the moderately-strong acid sites only participate the reactions partly. This change of acidity in the strongly acidic and moderately-strong acidic sites

5 40 Qun Dong et al./ Journal of Natural Gas Chemistry Vol. 13 No caused the catalytic performance to vary in the reactions Effect of Fe, Co additives on the structure of the catalysts as studied by XRD XRD techniques were used to study the changes in structure of Mo/HZSM-5 and Fe-Mo/HZSM-5 catalysts before and after reaction (Figure 4). The three characterization diffraction peaks (2θ=22 25 o ) of HZSM-5 (curve 2, 2%Fe-3%Mo/HZSM-5 after reaction) are different from those for 2%Fe3%Mo/HZSM- 5 (before the reaction, curve 1). It demonstrates that Fe introduced into HZSM-5 reacted with the HZSM-5 catalyst and changed the crystalline structure at 973 K. Comparison was also made between Mo/HZSM- 5 after reaction (curve 3) and 2%Fe-3%Mo/HZSM-5 before reaction (curve 1) catalysts. The result shows that the curve 3 characterization diffraction peaks of Mo/HZSM-5 compared with curve 1 are not significantly changed. This indicates that no new compound was found when Fe was introduced to the catalyst, this is different from the catalyst containing Co additive which formed the Mo 2 C [6]. Figure 3. NH 3 -TPD characterization of Co- Mo/HZSM-5 and Mo/HZSM-5 catalysts before and after the reaction (1) Mo/HZSM-5, (2) Mo/HZSM-5 reaction, (3) Co-Mo/HZSM- 5, (4) Co-Mo/HZSM-5 reaction 4. Conclusions The influence of Fe, Cr, Co and Ga additives on methane aromatization reaction for 3%Mo/HZSM-5 catalyst was studied. The performance of the aromatization of methane over Mo/HZSM-5 catalysts was promoted by introducing Cr, Fe, Ga and Co additives into the Mo/HZSM-5 catalysts. Cr, Fe, Ga and Co additives are favorable to the dehydrogenation and dissociation of methane over the Mo active sites. It also shown that strong acid site is the center of the methane aromatization The results of XRD shows that the crystallinity of Mo on ZSM-5 zeolite was increased after adding Fe, Co additives. References Figure 4. XRD patterns of Mo/HZSM-5 and Fe- Mo/HZSM-5 before and after reaction (1) 2%Fe3%Mo/HZSM-5, (2) 2%Fe3%Mo/HZSM-5 after reaction, (3) 3%Mo/HZSM-5 after reaction, (4) 3%Mo/HZSM-5 [1] Wang L S, Huang T S, Tao L X et al. Catal Lett, 1993, 21: 35 [2] Szoke A, Solymosi F. Appl Catal, A, 1996, 142: 361 [3] Liu S, Dong Q, Ohnishi R et al. Chem Commun, 1997, 1455 [4] Liu Sh T, Xu Y D, Guo X X. Cuihua Xuebao (Chin J Catal), 1995, 16: 102 [5] Shu Y Y, Xu Y D, Wang L Sh. Cuihua Xuebao (Chin J Catal), 1997, 18: 392 [6] Dong Q, Liu Sh T, Wang L Sh. Acta Petrolei Sinca (Petroleum Processing Section), 1999, 5: 20