Review of Catalytic Pyrolysis of Biomass for Bio-oil

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1 Review of Catalytic Pyrolysis of Biomass for Bio-oil Kaiqi Shi, Shuangxi Shao*, Qiang Huang, Xuwen Liang, Lan Jiang, Ya Li Key Laboratory of Biomass Green Transformation Ningbo University of Technology Ningbo, China Abstract - Pyrolysis utilizes biomass to produce a liquid product bio-oil which is widely used as a potential energy source and a low-cost feedstock for chemical production. Considerable efforts have been made to convert biomass to bio-oil since the oil crisis in mid-i970s. This review focuses on the recent developments in catalysts used in pyrlysis of biomass for bio-oil production. In this review, 5 types of catalysts such as metal oxide, molecular sieve, mineral, transition metal and others, have been given. Molecular sieves, such as HZSM-5, ZSM-5, MCM-41, SBA-15, HUSV, REV and their modified catalysts are most widely used in biomass catalytic pyrolysis process. The effects of catalysts on bio-oil yields, quality, composition, stability etc. are also discussed. Although catalysts in pyrolysis of biomass produce markedly influences on bio-oil products, the variety of catalyst types and properties make generalizations difficult to define, in regard to trying to critically analyze the literature. Keywords - bio-oil; catalytic pyrolysis; biomass I. INTRODUCTION The demand for energy and its resources is increasing day by day due to the rapid outgrowth of population and urbanization. As the major conventional fossil energy resources like coal, petroleum going to be extinct, renewable sources of energy are constantly examined as alternatives for fossil fuels[l]. Biomass is one of the main available renewable energy[2]. Biomass conversion methods can be divided into two broad pathways: biological and thermochemical, among which pyrolysis is considered to be an emerging technology for bio-oil production[3]. Currently, catalytic pyrolysis of different biomasses have been greatly studied to increase biooil yield or improve its quality[4]. This review focuses on the recent developments in the catalysts used in biomass pyrolysis process. There are many types of catalysts, such as metal oxide, molecular sieve, mineral, transition metals and so on. II. A. Metal oxide catalyst PYROLYSIS CATALYSTS Demiral et al. applied activated alumina in pyrolysis of olive and hazelnut bagasse biomass samples. The maximum bio-oil yields for the bio-oils obtained were found as 37.07% [1]. The oxygen contents of the bio-oils were also markedly reduced while the yield of bio-oil was reduced by the use of catalysts. Pullin et al. investigated the same catalyst in pyrolysis of perennial shrub with the oil yield as 3S.6% m steam atmosphere[5]. Wan et al. induced microwave to enhance the catalytic pyrolysis with Ab03, which may be microwave absorbent[ 6]. Yorgun et al. found activated alumina catalyst had a strong influence on the miscanthus giganteus pyrolysis product and conversion yield [7]. The yield of bio-oil product is between 47% -51% due to different pyrolysis parameters. However, it cannot contribute to any considerable improvement of the bio-oil yield and composition compared to non-catalytic pyrolysis. Wang et al. compared catalysts Ab03 with AI-SBA-15 and ZSM-5. The obtained bio-oil had the lowest oxygen content (26.71%) and the highest calorific value (25.94 Mlkg- I ). Alumina catalyst loaded could also clearly enhance the formation of aliphatics and aromatics[s]. Nokkosmaki et al. studied conversion of pyrolysis vapours of pine sawdust in micro and bench scales with zinc oxide catalyst [9]. The pyrolysis liquids produced were homogeneous stable one-phase oils with lower viscosity. In Piltlin et al.work[lo], cotton seed, as a biomass source, was pyrolysed in fixed-bed reactor under various sweeping gas (N2) flow rates at different pyrolysis temperatures. MgO catalyst addition decreased the quantity of bio-oil yet, comparing with non-catalytic work, but the quality of bio-oil in terms of calorific value, hydrocarbon distribution and removal of oxygenated groups increased. The fuel obtained via catalytic pyrolysis mainly consisted of lower weight hydrocarbons in the diesel range. Nokkosmaki et al. developed a novel microscale test method was for testing pine sawdust pyrolysis with ZnO, MgO catalysts[ii]. They connected a pyrolyser to a gas chromatograph (GC) with atomic emission detector. It is found that the carbon yields in liquid fraction decreased and product distribution in pyrolysis vapours was rather similar with ZnO and Mgo. But with MgO, the compounds of pyrolysis vapours comprised mainly gases, water and degradation products of polysaccharides as well as some aromatic hydrocarbons. B. Molecular sieves Molecular sieves are widely used as catalyst in biomass pyrolysis process, due to their porous properties. Sharma et al. introduced acidic catalyst HZSM-5 as a catalyst in fast pyrolysis for upgrading bio-oil to hydrocarbon fuels [12].The maximum amount of organic distillate in the effluent was 22 wt % of non-phenolic fraction and it contained up to 78 wt % /111$ IEEE Supported by Zhejiang Science Research Programme (20 I OC35050) and Ningbo Scicncc & Tcchnology Plan (2009AI0031). 317

2 of aromatic hydrocarbons. Stephanidis et al. [13] tested three catalysts, the strongly acidic zeolites H-ZSM-S and silicalite (with very low number of acid sites) and the mildly acidic mesoporous aluminosilicate AI-MCM-41, in the upgrading of the biomass pyrolysis vapor. The use of zeolite H-ZSM-S decreased the total liquid yield (bio-oil) via decreasing the organic phase of bio-oil and increasing its water content, accompanied by increase of gases and moderate formation of coke on the catalyst. While the zeolite silicalite and the AI MCM-41 induced similar effects, but a less extent. Cao et al.[14] employed HZSM-S, SBA-IS and MCM-41 in copyrolysis of wood biomass and waste tire. Co-pyrolysis is in favor of inhibiting the formation of polycyclic aromatic hydrocarbons (P AHs) produced from tire. There exist a hydrogen transfer and a synthetic effect during co-pyrolysis of the biomass and tire. They improve the quality of the oil with SBA-IS, which is more significant than MCM-41 or HZSM-S for reducing the density and viscosity of the oil and it can effectively decompose some large molecular compounds into small ones. Lu et al. studied catalytic cracking and reforming to upgrade the stability and quality of bio-oil. HUSY/y-Ab03 and REY /y Alz03 catalysts exhibited better deoxygenating activities while HZSM-S/y-Alz03 catalyst exhibited preferred selectivity for production of iso-alkanes and aromatics[is]. Aho et al. tested Beta, Y, ZSM-S, and mordenite in the pyrolysis of pine [16]. The yield of the pyrolysis product phases was only slightly influenced by the structures, at the same time the chemical composition of the bio-oil was dependent on the structure of acidic zeolite catalysts. The formation of ketones was higher over ZSM-S and the amount of acids and alcohols lower than over the other bed materials tested. It was possible to successfully regenerate the spent zeolites without changing the structure of the zeolite. Wang et al. investigated pyrolysis of herb residue with ZSM-S, AI SBA-lS and alumina catalysts [8]. The results indicated that the maximum bio-oil yield of 34.26% was obtained at 4S0 C with 10 wt% alumina catalyst loaded. The presence of all catalysts decreased the oxygen content of bio-oils and increased the calorific values. The order of the catalytic effect for upgrading the pyrolytic oil was Ab03 > AI-SBA- IS > ZSM-S. The bio-oil with the lowest oxygen content (26.71%) and the highest calorific value (2S.94 MJkg- 1 ) was obtained with 20 wt% alumina catalyst loaded, which would also enhance the formation of aliphatics and aromatics. Aho et al.prepared hybrid catalysts consisting of a zeolite (ZSM-S or Beta) and bentonite [17].The hybrid catalysts exhibited similar properties as the combined starting materials. Catalytic pyrolysis over pure ZSM-S and Beta as well as hybrid catalysts has been successfully carried out in a dual-fluidized bed reactor. De-oxygenation of the produced bio-oil over the different zeolitic materials was increased compared to noncatalytic pyrolysis over quartz sand. MCM-41 is one of the latest members of the mesoporous family of materials. They possess a hexagonal array of uniform mesopores (1.4-lO nm), high surface areas (> looo m 2 /g) and moderate acidity. Due to porous properties, the MCM-41 materials are currently under study in a variety of processes as catalysts or catalyst supports. Nilsen investigated the effect of metal sites in Me-AI-MCM-41 (Me = Fe, Cu or Zn) on the catalytic behavior during the pyrolysis of wooden based biomass[18]. The catalysts are all shown to improve the product distribution and the quality of the bio-oil with respect to the yield of phenols. The presence of Zn-AI-MCM-41 as the catalyst leads to the lowest yield of phenols, but this catalyst gives the best results with respect to reduction of coke produced. The investigations have shown that these mesoporous materials are promising catalysts for the pyrolysis of biomass, and that the type of metal is of more importance than the metal siting. Iliopoulou et al. [19] compared AI-MCM-41 materials with to a siliceous MCM-41 sample and to non-catalytic for in situ upgrading biomass pyrolysis vapours. The product yields and the quality of the produced bio-oil were significantly affected by the use of all MCM-41 catalytic materials. The major improvement in the quality of bio-oil with the use of AI-MCM-41 catalytic materials was the increase of phenols concentration (useful chemicals) and the reduction of corrosive acids (undesirable in fuel bio-oils). Higher Si/AI ratios of the AI-MCM-41 samples enhanced the production of the organic phase of the bio-oil, while lower Si/ Al ratios favored the conversion of the hydrocarbons of the organic phase towards gases and coke. Adam et al. [20] used AI-MCM-41 type mesoporous catalysts for converting the pyrolysis vapours of spruce wood in order to obtain better bio-oil properties. Four AI-MCM-41 type catalysts with a Si/ Al ratio of 20 were tested. The catalytic properties of AI-MCM-41 catalyst were modified by pore enlargement that allowed the processing of larger molecules and by introduction of Cu cations into the structure. It was shown that levoglucosan was completely eliminated, while acetic acid, furfural and furanes became quite important among cellulose pyrolysis products over the unmodified AI MCM-41 catalyst. The dominance of phenolic compounds of higher molecular mass was strongly cut back among the lignin products. Both the increase of the yield of acetic acid and furan and the decrease of large methoxyphenols were repressed to some extent over catalysts with enlarged pores. Adam et al. [16] compared four above AI-MCM-41 type catalysts with a Si/ Al ratio of 20, with a commercial FCC catalyst, a pure siliceous SBA-lS and an aluminum incorporated SBA-lS materials for improved pyrolysis bio-oil. The gas yield increased in each catalytic case, the coke yield remained the same or slightly decreased compared to the noncatalytic experiments. The aqueous part in the liquid phase increased in the catalytic runs. In the catalytic experiments the hydrocarbon and acid yields increased, while the carbonyl and the acid yields decreased. All catalysts tested reduced the undesirable product yield, while the desirable product yield remained the same or increased. With spruce the FCC, with Miscanthus the unmodified AI-MCM-41 are the best performing catalysts. Antonakou et al. [21] also evaluated three different samples of AI-MCM-41 materials (with different Si/ Al ratio) and three metal containing mesoporous samples (Cu-AI-MCM-41, Fe-AI-MCM-41 and Zn-AI-MCM- 41) in biomass pyrolysis for the production of bio-oil. It was 318

3 found that the presence of the MCM-4l material alters significantly the quality of the pyrolysis products. All catalysts were found to increase the amount of phenolic compounds, which are very important in the chemical (adhesives) industry. A low Sil Al ratio was found to have a positive effect on product yields and composition. Fe-Al-MCM-4l and Cu-Al MCM-4l are the best metal-containing catalysts in terms of phenols production. The presence of the Al-MCM-4l material was also found to decrease the fraction of undesirable oxygenated compounds in the bio-oil produced, which is an indication that the bio-oil produced is more stable. In Aho's study [4], catalytic pyrolysis of pine biomass was carried out in a fluidized bed reactor at 450 C. The acidic catalysts used as bed material in the reactor were [beta] zeolites with varying silica to alumina. The yield of the different product phases was clearly influenced by the zeolites acidity. The yield of the pyrolysis product phases was only slightly influenced by the structures, at the same time the chemical composition of the bio-oil was dependent on the structure of acidic zeolite catalysts[ 16]. Zeolites with stronger acidity formed less organic oil, and respectively more water and polyaromatic hydrocarbons than less acidic zeolites. Kumar[22] pointed out that the components of biomass pyrolysis oils are temperature-dependent, but the pyrolysis reaction kinetics and the quality of bio-oil produced are independent of the presence of mordenite catalysts. C. Mineral catalysts A novel micro scale test method, a pyrolyser connected to a gas chromatograph, was developed for testing catalysts by N okkosmaki et at. [11]. The test method was applied to treating pyrolysis vapours of Scots pine sawdust with dolomite, limestone, ZnO and MgO. The carbon yields in liquid fraction decreased with all the catalysts studied. Product distribution in pyrolysis vapours was rather similar with ZnO or without any catalyst. With MgO, dolomite and limestone, the compounds of pyrolysis vapours comprised mainly gases, water and degradation products of polysaccharides as well as some aromatic hydrocarbons. Sharypov et al.carried out the pyrolysis in a hydrogen atmosphere of pine wood and synthetic polymers (polyethylene and polypropylene) mixtures [23]. The used catalysts were pyrrhotite and haematite materials activated by mechanochemical treatment. Iron ore materials were found catalytically active in the process of hydro pyrolysis of wood/polymers mixtures. By using these catalysts a significant increase of the distillable liquids amounts (by wt. %) and a sharp decrease of olefins and cycloparaffins content (by approximately two to three times) were observed. Dilcio et at. [24] applied colloidal FeS catalyst in generating bio-oils process. The addition of a dispersed iron sulphide catalyst gave conversions close to 100% for the biomass samples investigated at 10 MPa under conditions in the fixedbed reactor where significant diffusional resistances existed and reduced the oxygen content of the bio-oil by a further 10%. D. Transition metals In two-stage tests for cellulose, Dilcio et at. [24] used a commercial sulphided Ni/Mo Iy -A1203 catalyst at 400 C, increasing the hydrogen pressure from 2.5 to 10 MPa decreased the oxygen content of the oil by over 20% to 10% w/w. The H/C ratios were higher and OIC ratios smaller for the two-stage bio-oils compared to their single stage counterparts. However, the differences in the OIC ratios between the single and two-stage bio-oils increase with pressure. Xu[25] improved the bio-oil produced by vacuum pyrolysis of pine sawdust with a series of nickel-based catalysts. Results showed that the reduced Mo-l ONil y- A1203 catalyst had the highest activity with the acetic acid conversion of 33.2%. Upgrading of the raw bio-oil was investigated over reduced Mo-IONi/y-Ab03 catalyst. After the upgrading process, the ph value of the bio-oil increased from 2.16 to The water content increased from 46.2 wt. % to wt. %. The H element content in the bio-oil increased from 6.61 wt. % to 6.93 wt. %. They also investigated over MoNil y -Ab03 catalyst under mild conditions (373 K, 3 MPa hydrogen pressure) [26]. The ph value of the bio-oil increased from 2.33 to The water content increased from wt. % to 4l.55 wt. % and the gross calorific value increased from MJ/kg to MJ/kg. The hydrogen content in the biooil increased from 6.25 wt. % to 6.95 wt. %. The product properties of the upgraded bio-oil, particularly the hydrogen content and the acidity were considerably improved. The elimination of specific oxygenated groups of biomassderived pyrolysis oils (bio-oils) is necessary for improving their stability. Laurent et al. carried out the study of hydro deoxygenation of carbonyl, carboxylic and guaiacyl groups over sulfided CoMol y -Ab03 and NiMol y -Ab03 catalysts [27]. Model oxygenated compounds were used, namely 4-methylacetophenone, diethyldecanedioate and guaiacol, in the presence of sulfided cobalt-molybdenum and nickel-molybdenum supported on y alumina catalysts in a batch system. CoMo and NiMo catalysts have comparable activities and selectivities. However, the NiMo catalyst has a higher decarboxylating activity than CoMo and also leads to a higher proportion of heavy products during the conversion of guaiacol. Centeno and Laurent et al. investigated the influence of the support of CoMo sulfide catalysts and of the addition of potassium and platinum on the catalytic performances for the hydro deoxygenation of the model oxygenated compounds[28]. It was shown that no important role was played by any acidbase mechanism; dispersion determines the activity. The appropriate modifications of the hydro treating catalysts can lead to a more effective process for stabilization of the bio-oils by reaction with hydrogen. In Ate's[29]study, the perennial shrub Euphorbia rigida, was used as biomass sample for catalytic pyrolysis using Co Mo commercial catalyst (Criterion-534) in the water vapor atmosphere, which was mixed with feedstock in different percentages. Yield of bio-oil did not increase much with more use in catalyst but yields of 42.6% was routinely obtained using catalyst ratio of 20% and water vapor flow rate of 319

4 1.3 cm/s. More polar fraction and less substituted aromatics and aliphatics were obtained when water vapor was used. Therefore, it is possible to produce petroleum-like liquid pyrolysis products by catalytic pyrolysis of E. rigida in the presence of water vapor. Zhang et al. [30] upgraded liquid fuel from the pyrolysis of biomass by sulfided Co-Mo-P catalyst in an autoclave. The significant difference between the raw pyrolytic oil and the upgraded oil was that the former was methanol-soluble while the latter was oil-soluble. E. Others There are also many other types of catalyst used in pyrolysis of biomass for high quality bio-oil. Busetto et al. upgraded biooil with ruthenium based Shvo homogeneous catalyst [31]. The Shvo catalyst maintains its performances under acidic "bio-oil conditions" leading to the almost quantitative conversion of the polar double bonds within I h. The activity of the Shvo catalyst was also investigated for the hydrogenation of a bio-oil from poplar in solvent free conditions, changing the chemical nature of the pyrolysis oil. Aldehydes, ketones and non-aromatic double bonds were almost totally hydrogenated. Du et al. studied fast pyrolysis of biomass for bio-oil with ionic liquid (ILs) l-butyl-3- methylimidazolium chloride and l-butyl-3-methylimidazolium tetrafluoroborate as catalysts and microwave irradiation[32]. The bio-oil yield from rice straw reached 38% and that from sawdust reached 34%. The main components in bio-oil are furfural, acetic acid, and l-hydroxy-2-butanone, and their contents mainly depend on the source of biomass and the type of IL used in pyrolysis. Wan et al. evaluated the effects of metal oxides, salts, and acids catalysts including K2Cr207, Alz03, KAc, H3B03, Na2HP04, Mg on product selectivity of microwave-assisted pyrolysis of com stover and aspen wood[6]. KAc, A1203, MgClz, H3B03, and NazHP04 were found to increase the bio-oil yield by either suppressing charcoal yield or gas yield or both. These catalysts may function as a microwave absorbent to speed up heating or participate in so-called "in situ upgrading" of pyrolytic vapors during the microwave-assisted pyrolysis of biomass. Chloride salts in particular simplify the chemical compositions of the resultant bio-oils and therefore improve the product selectivity of the pyrolysis process. Kumar et al. optimized of process for the production of bio-oil from eucalyptus wood with different catalysts such as kaoline clay, fly ash and so on[22]. The optimum conditions for the high yield of bio-oil are with fast heating rate, pyrolysis temperature of 450 C.The reaction kinetics and the quality of bio-oil produced are independent of the presence of the mentioned catalysts. Besides, there are also many commercial catalysts, namely DHC-32[33], HC-K l.3q[33], BP 3189[34], Criterion-424 [34], ReUSY [35] conducted in pyrolysis of biomass for bio-oil which belongs to some type of the catalysts mentioned above. III. CONCLUSIONS Catalysts are commonly used in pyrolysis of biomass for bio-oil production and its upgrading. Among the 5 types of catalysts mentioned, molecular sieves are the most popular one, which have also desired effect on bio-oil producing, such as higher yields, better stability and so on. Other catalysts are also studied to develop the pyrolysis process of biomass. However, there still have some problems. Some indications of catalyst deactivation were observed [9] and aromatics and polycyclic aromatic hydrocarbons (PAHs) were significantly increased by the use of some catalysts [l3], which needs to be further improved. ACKNOWLEDGMENT Thanks the Ningbo Science and Technology Bureau and Ningbo Municipal Government for this programme's grant. 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