Bifunctional Behavior of Mo/HZSM-5 Catalysts in Methane Aromatization

Abstract

Catalytic functions of Mo/HZSM-5 toward methane aromatization are examined under various activation and reaction conditions. Molybdenum ions are necessary in the activation of methane. High benzene yields of around 7% are obtained with a selectivity of more than 96% at 973 K over a 2 wt % Mo/HZSM-5 catalyst in a tubular reactor. Pretreatment with hydrogen is found to be beneficial to the initialization of aromatization. The aromatization activity increases upon decreasing the Si/Al ratio in HZSM-5, indicating an acidity dependence. This is supported by the fact that a Mo catalyst supported on cesium-exchanged ZSM-5 losses its catalytic aromatization activity at all. The effective coordination between molybdenum ion sites and Brønsted acid sites in the framework of ZSM-5 is important in the activation of the stable C−H bonds in methane and the eventual aromatization to form benzene. The constant O1s binding energy in the XPS spectra of both fresh and used catalysts suggests that coke formation does not occur on the SiO4 tetrahedra but rather on the molybdenum sites supported on the external surface of ZSM-5.

Recent progress in methane dehydroaromatization: From laboratory curiosities to promising technology

“…The effect of zeolite type was described in Section 2. Researchers have found that catalysts such as unsupported Mo 2 C [93−95], unmodified HZSM-5 [64,69], and Mo supported on carriers without Brønsted acid sites, including Na-ZSM-5 [16,64], Cs-ZSM-5 [96], Ga-ZSM-5 [60], and TiO 2 [97], show low or no activity towards benzene formation. However, hybrid catalysts that are physical mixtures, such as Mo/SiO 2 +HZSM-5 or Mo 2 C+HZSM-5, exhibit remarkable activity, giving yields of benzene and naphthalene that are 100-300 times greater than either component alone [30].…”

“…Scheme 4. Possible route for aromatics and coke formation [114] Su et al [96] examined O 1s binding energy in XPS spectra of both fresh and used catalysts. Their results suggested that coke formation does not occur on SiO 4 tetrahedron, but on molybdenum sites present on the external surface of ZSM-5.…”

Dual effects of supported W catalysts for dehydroaromatization of methane in the absence of oxygen

“…Moreover, the effect of GHSV ranging from 1800-9000 ml/(g.h) on the activity of 3% W-H 2 SO 4 /HZSM-5 catalysts with different Si/Al ratios indicates that the maximum activity appears on the catalyst with Si/Al = 30 as shown in figure 3(a), (B) for methane conversion and aromatics selectivity, respectively. It seems that in addition to the pore structures being shape-selective, the strength of the acid sites in the HZSM-5 catalyst also contribute to achieving optimum catalyst activity in DHAM reaction as has been reported by several authors [1][2][3][4][5][6][7][8][9][10][11][12]16]. The decrease in aromatic selectivity after reaching a maximum value suggested the event of coke deposition in the catalyst.…”

“…Dehydroaromatization of methane (DHAM) to aromatics have received considerable attentions [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] in the study of catalytic reactions. The most common catalysts reported to be promising for DHAM are HZSM-5-supported Mo and also W catalysts [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18].…”

“…The most common catalysts reported to be promising for DHAM are HZSM-5-supported Mo and also W catalysts [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Some of the characteristics of an active DHAM catalyst include a highly dispersed active metal species on the surface and also a proper amount of acidity for the support [1][2][3][4][5][6][7][8][9][10][11][12]. Mo-based catalysts supported on HZSM-5 have been used for catalytic reaction of DHAM in the absence of oxygen.…”