Preparation of bio-oil derived from catalytic upgrading of biomass vacuum pyrolysis vapor over metal-loaded HZSM-5 zeolites

Highlights

• HZSM-5 zeolites were modified by Fe, Co, Cu via impregnation method.

• The metal loading led to the increasing of Brønsted and Lewis acid distribution.

• The yield of refined bio-oil decreased over metal-loaded ZSM-5 catalysts.

• Target hydrocarbon yields were boosted 2.5 times using Fe/HZSM-5 and Co/HZSM-5.

• Fe/HZSM-5 was a preferable catalyst for the preparation of bio-oil.

Abstract

The Fe-, Co-, Cu-loaded HZSM-5 zeolites were prepared via impregnation method. The upgrading by catalyst on biomass pyrolysis vapors was conducted over modified zeolites to investigate their catalytic upgrading performance and anti-coking performance. The Brønsted acid sites amount on Cu-,Co-loaded HZSM-5 decreased sharply, while that of Lewis both increased. The yield of liquid fraction and refined bio-oil over metal loaded ZSM-5 catalysts decreased, while that of char almost kept constant. The physical property of refined bio-oil was promoted in terms of pH value, dynamic viscosity and higher heating value (HHV). FT-IR analysis revealed that the chemical structure of refined bio-oil obtained over Fe-, Co-, Cu-loaded HZSM-5 zeolites was highly similar. The yield of monocyclic aromatic and aliphatic hydrocarbon over Fe-,Co-loaded HZSM-5 were boosted by around 2.5 times compared with original ZSM-5 zeolites. Data analysis revealed that Cu/HZSM-5 presented the worst deoxygenation ability. The anti-coking capability of Fe/HZSM-5 was obviously better, i.e., the coke content showed an approximate decrease of 38%. Thus, this study provided an efficient Fe/HZSM-5 catalysts for preparation of bio-oil derived from catalytic upgrading of biomass pyrolysis vapor.

Introduction

Biomass, as the only carbon source in all renewable energy, is attracting significant attention nowadays [1], [2], [3]. The main approaches for the conversion of biomass are combustion, gasification, pyrolysis, fermentation, and esterification, wherein the pyrolysis technology had been largely applied for its simplicity of operator, high efficiency as well as low costs [3]. However, because the oxygen content and viscosity were relatively high, but the heating value was lower than the fossil fuels, the bio-oil which was obtained from biomass by pyrolysis cannot be used directly as an alternative fuels [4]. Therefore, it's necessary to upgrade the crude bio-oil [5].

Much efforts such as catalytic pyrolysis had been devoted to the refinement of bio-oil [6], [7]. The catalysis of pyrolytic vapors with HZSM-5 was conducted in a reactor equipped with a two-stage fixed-bed, and HZSM-5 catalysis modified the bio-oil quality effectively [8]. Previous research of catalytic pyrolysis of corncob over HZSM-5 showed a decrease of the yield of bio-oil, an increase of moisture and the yield of coke. Meanwhile, the aromatic hydrocarbons proportion in bio-oil increased obviously, while the oxygen content decreased, which improved the quality of bio-oil [9]. Cheng et al. used furan compounds as raw materials in the process of producing aromatics by HZSM-5, and the aromatics in the product included benzene, toluene and naphthalene with a small amount of heterogeneous oligomer [10].

The HZSM-5 performed well when producing bio-oil by catalytic pyrolysis in some way, however, its low selectivity and frequent coke formation limited its application. Recently, in the field of petroleum industry and environmental chemistry, some studies in the applications of the modified HZSM-5 zeolite achieved preferable results [11]. The aromatization performance of HZSM-5 loaded with Zn or Co in the catalytic process of producing aromatics from C3–C9 saturated alkanes was researched [12]. It was found that HZSM-5 zeolite modified with transition metal exhibited high aromatics selectivity. W.B. Widayatno et al. investigated the upgrading of bio-oil over Cu-modified β-zeolite. When β-zeolite is modified by a small amount of Cu, the selectivity to the hydrocarbon can be obviously promoted [13]. E.F. Iliopoulou et al. studied transition metal-modified ZSM-5 zeolites for the in situ upgrading of biomass pyrolysis vapors, finding that NiO modified catalysts being more reactive towards decreasing the organic phase and increasing the gaseous products, compared to the Co₃O₄ supported catalysts [14]. A. Aho et al. investigated the influence of the proton forms of beta, Y and ferrierite zeolites and their iron modified counterparts during upgrading of pine wood pyrolysis vapours under nitrogen atmosphere. The increase of de-oxygenation reactions over the proton form and iron modified zeolites increased selectivity towards organic compounds compared to the non-catalytic pyrolysis [15]. Plenty of researches on catalytic effects of metal-modified zeolite on bio-oil indicated that the use of metal-loaded zeolite improved the quality of bio-oil significantly. We selected Fe、Co and Cu as loaded metal because the presence of transition metals affects the mode of oxygen deletion by producing increased amounts of CO and CO₂ and less water, which makes more hydrogen available to form hydrocarbons. When the loadings of iron and cerium were 1.97% and 6.36% respectively, the yields of ethylene and propylene were 32.79% and 18.05% respectively. All these demonstrated that metal-loaded ZSM-5 catalysts presented better catalytic performance.

On the basis of the above studies, attempts were furtherly made to prepare modified HZSM-5 zeolites with three metal elements (iron, cobalt, copper) by impregnation method [16]. They were applied in the process of in-situ catalytic pyrolysis of biomass to understand their catalytic and coking performance. This work aimed to obtain bio-oil from biomass efficiently using the theory obtained from our experiments.