Catalytic Ethylene Oligomerization over Ni/Al-HMS: A Key Step in Conversion of Bio-Ethanol to Higher Olefins

Yanyong Liu

doi:10.3390/catal8110537

Abstract

Al-modified hexagonal mesoporous silica (HMS) materials were synthesized using dodecylamine as a template according to the methods reported in the literature. FT-IR spectra proved that Al3+ ions entered in the HMS framework in Al-HMS (prepared by sol-gel reaction) but Al3+ ions existed in the extra-framework in Al/HMS (prepared by post-modification). NH3-TPD indicated that either Al-HMS or Al/HMS had solid acid sites on the surface, and the acidic strength of Al/HMS was stronger than that of Al-HMS. For ethylene oligomerization at 200 °C under 1 MPa, Ni/Al-HMS showed an ethylene conversion of 96.3%, which was much higher than that over Ni/Al/HMS (45.6%). The selectivity for C4H8, C6H12, C8H16, and C8+ was 37.7%, 24.5%, 24.0%, and 9.1% for ethylene oligomerization over Ni/Al-HMS, respectively. Ni/Al-MCM-41, which has been reported as an effective catalyst for ethylene oligomerization in the literature, showed a high ethylene conversion (95.2%) similar to that of Ni/Al-HMS in this study. However, the selectivity for C8H16 over Ni/Al-MCM-41 (16.3%) was lower than that over Ni/Al-HMS (24.0%) in the ethylene oligomerization. For ethanol dehydration at 300 °C under 1 MPa, a commercial H-ZSM-5 catalyst showed a high ethylene yield (91.2%) after reaction for 24 h using a feed containing 90 wt.% ethanol and 10 wt.% water. In this study, a two-step process containing two fixed-bed reactors and one cold trap was designed to achieve the direct synthesis of higher olefins from bio-ethanol. The cold trap was used to collect the water formed from ethanol dehydration. By using H-ZSM-5 as a catalyst for ethanol dehydration in the first reactor and using Ni/Al-HMS as a catalyst for ethylene oligomerization in the second reactor, higher olefins were continuously formed by feeding a mixture containing 90 wt.% ethanol and 10 wt.% water. The yields of higher olefins did not decrease after reaction for 8 h in the two-step reaction system.

Highlights

The conversion of biomass to liquid fuels has become an important research field because biomass utilization has an effect in reducing greenhouse gas emission [1]
Ni/Al-hexagonal mesoporous silica (HMS) was an effective catalyst for the oligomerization of ethylene to higher olefins
The existence of Al3+ ion in the neighborhood of Si4+ ion was important to improve the catalytic activity of Ni/Al-HMS in the oligomerization of ethylene

Summary

Introduction

The conversion of biomass to liquid fuels has become an important research field because biomass utilization has an effect in reducing greenhouse gas emission [1]. Bio-ethanol is produced in a large scale from sugary, starchy, and lignocellulosic biomasses by yeast fermentation [2]. Bio-ethanol accounts for above 90% of worldwide biofuel production. Bio-ethanol is mainly used as fuel for automobiles by blending with gasoline. Bio-ethanol is an attractive fuel; the needs of transportation fuel for automobiles will be reduced due to the popularization of electric vehicle and hydrogen-fueled car. Bio-ethanol is expected to be in surplus in future years. It is necessary to develop new technologies to utilize the surplus bio-ethanol. Production of hydrogen by the steam reforming of bio-ethanol is an available technology [3].

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