Effect of the Si/Al ratio of HZSM-5 zeolite on the production of dimethyl ether before and after ultrasonication
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Abstract: A series of as-synthesized HZSM-5 zeolites with different Si/Al ratios (25, 90, 120, 240 and 400) were post-treated by ultrasonication for an optimum time of 60 min. The morphology, acidity and textural properties of HZSM-5 were characterized with XRD, SEM, N2 adsorption and NH3-TPD techniques. The catalytic performance was evaluated by dehydration of methanol to dimethyl ether (DME), which is a promising gaseous automotive fuel in future. It was found that the Si/Al ratio of HZSM-5 had considerable impacts on its catalytic performance for dehydration of methanol to DME. Its activity increased with decreasing Si/Al ratio from 400 to 25. Ultrasonication of HZSM-5 could significantly improve its catalytic performance.
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Key words:
- Ultrasonication-HZSM-5 /
- -dimethyl /
- ether-Si/Al /
- ratio
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Figure 6 DME yield obtained in the conversion of methanol over HZSM-5 zeolites with different Si/Al ratio
Figure 7 DME and hydrocarbons (HC) selectivity obtained at 300 ℃ over the HZSM-5 with different Si/Al ratios
Figure 8 DME yield obtained in the conversion methanol over the ultrasonicated HZSM-5 zeolites with different Si/Al ratios
Figure 9 Effect of reaction time on conversion of methanol over HZSM-5 with different Si/Al ratios
Figure 10 Methanol conversions obtained at different reaction time over the ultrasonicated and unsonicated HZSM-5(25)
Figure 11 Arrhenius plots for dehydration of methanol to DME over the HZSM-5 with different Si/Al ratios
Figure 12 Arrhenius plot for dehydration of methanol to DME over ultrasonicated HZSM-5 with different Si/Al ratios
Table 1 Textural properties of HZSM-5 with different Si/Al ratios
Catalyst Surface area
A/(m2·g-1)Total pore volume
v/(cm3·g-1)Crystal size
d/nmHZSM-5(25) 357 0.32 31.6 HZSM-5(90) 340 0.33 40.1 HZSM-5(120) 332 0.35 33.1 HZSM-5(240) 320 0.37 35.7 HZSM-5(400) 306 0.40 35.3 HZSM-5(25) U 402 0.39 29.7 HZSM-5(90) U - - 28.5 HZSM-5(120) U - - 29.2 HZSM-5(240) U - - 29.1 HZSM-5(400) U 356 0.45 26.7 
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Table 2 Ammonia desorption enthalpy of HZSM-5 with different Si/Al ratios
Catalyst With ultrasonication
ΔH/(J·g-1)Without sonication
ΔH/(J·g-1)HZSM-5(25) 103.2 100.0 HZSM-5(90) 92.5 87.5 HZSM-5(120) 88.6 78.1 HZSM-5(240) 78.5 64.1 HZSM-5(400) 66.8 54.7
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Table 3 Apparent activation energies for dehydration of methanol to DME over ultrasonicated and unsonicated HZSM-5 with different Si/Al ratios
Catalyst Ea/(kJ·mol-1) HZSM-5(25) 25.38 HZSM-5(90) 24.06 HZSM-5(120) 24.68 HZSM-5(240) 23.20 HZSM-5(400) 26.80 HZSM-5(25) U 26.42 HZSM-5(90) U 24.01 HZSM-5(120) U 21.76 HZSM-5(240) U 24.84 HZSM-5(400) U 27.30
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