Biomass to fuels: The role of zeolite and mesoporous materials

Microporous and Mesoporous Materials 144 (2011) 28–39

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Biomass to fuels: The role of zeolite and mesoporous materials q Carlo Perego, Aldo Bosetti ⇑ Eni S.p.A., Research Center for Non-Conventional Energies, Istituto Eni Donegani, Via G. Fauser 4, 28100 Novara, Italy

a r t i c l e

i n f o

Article history: Received 16 September 2010 Received in revised form 19 November 2010 Accepted 26 November 2010 Available online 16 December 2010 Keywords: Biomass Biofuels Catalysis Zeolite

a b s t r a c t Biomass is an abundant and carbon–neutral renewable energy resource for the production of biofuels, moving the market dependence away from fossil-based energy sources. The main problem is how to efficiently remove the abundant oxygen content from biomass-derived products and convert it into a hydrophobic molecule with the appropriate combustion or chemical properties. Many efforts have been devoted to the search of heterogeneous catalytic systems, more selective, safe and environmentally friendly. In this scenario zeolites and mesoporous compounds may help chemists to develop new biofuel generation processes. The development of new catalysts in the field of conversion of biomass to biofuels requires knowledge of the complex nature of the substrates to be converted. Starting from the main chemical aspects of the different biomass platforms, an overview of some of the zeolite and mesoporous materials technologies currently used commercially or tested at pilot and laboratory scale, is presented in this paper. Ó 2010 Elsevier Inc. All rights reserved.

1. Introduction Expansion and progress, particularly in emerging countries, will boost the need for energy in all the end-use sectors, in particular in the transportation utilization [1]. As reported by 2009 Energy Outlook of Energy Information Administration of DOE, the world liquid fuel supply forecast is still increasing [2]. In 2030 106.6 million barrels per day is the expected demand of liquid fuel (Fig. 1). The transportation sector accounts for the largest increment in total liquids demand, at nearly 80% of the total world increase. Unconventional liquids play an increasingly important role in meeting demand for liquid fuels over the course of the IEO2009 projections. Fuels derived from shale oils, oils sands, extra heavy oils, coal to liquids and gas to liquids processes, and biofuels are considered unconventional liquids. In the reference case, 12.6% of world liquids supply in 2030 comes from unconventional sources, including 1.5 million barrels per day from OPEC and 11.9 million from non-OPEC sources. As illustrated in Fig. 2, biofuels represent a significant part of unconventional supplies, i.e. increasing as absolute value and as percentage with respect to the other unconventional sources. Biofuels are liquids or gases for transport purposes that are produced from biomass.

q Keynote lecture presented at the 16th International Zeolite Conference (16th IZC), jointly organised with the 7th International Mesostructured Materials Symposium (7th IMMS), held in Sorrento, Italy, July 4–9, 2010. ⇑ Corresponding author. Tel.: +39 0321 447348; fax: +39 0321 447241. E-mail address: [email protected] (A. Bosetti).

1387-1811/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2010.11.034

The increasing demand of biofuels is due to several key issues, the first one is that bio-based resources are renewable and CO2 neutral in contrast with fossil fuels. In fact, while electricity and heat can be generated from a wide spectrum of alternatives (sun, wind, hydro, geothermal heat, etc.) the production of transportation fuels can just rely on carbon-biomass, as only alternative carbon source to fossils ones. However, the main driver is the strong political focus on renewable biofuel alternatives together with the increasing severity of regulations everywhere in the World. The Renewable Energy Directive (RED) of European Union requires biofuels to reach 10% of total automotive fuel consumptions by the year 2020. Biofuels must contribute to CO2 reduction of 35% at the introduction of the new Directive, till to reach 50% CO2 reduction by 2017 (60% for the new production plants). US Renewable Fuel Standard (RFS) requires around 2.2 MBPD biofuels by 2022 (30% of the transport pool), with corn ethanol capped at 1 MBPD. First-generation biofuels, produced primarily from agricultural crops, traditionally grown for food and animal feed purposes, are the initial step in this direction. The main first-generation biofuels are bioethanol, used as a gasoline substitute, produced from sugar containing plants or cereals crops, and biodiesel, produced from vegetable oils after conversion into the corresponding fatty acid methyl esters. However, most of first-generation biofuels have several drawbacks, including the competition with food crops, the competition for water, the potentially negative impact on biodiversity, the limited greenhouse gas emission reduction (with the exception of sugarcane ethanol) and the high production cost [3]. Many of these problems could be addressed by the production of the second generation biofuels, manufactured from agricultural

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compounds may help chemists to develop new biofuel generation processes, due to their important properties, namely, high concentration of active sites, high thermal/hydrothermal stability and enhanced shape selectivity. The aim of this contribution is to summarise some challenges facing zeolite and mesoporous catalysts in new industrial processes.

Million bpd 120 100 80 60

2. The role of zeolite catalyst in biomass conversion

40 20 0

2006 Non-OPEC Conventional

2030 (reference) OPEC Conventional

Total Unconventional

Total

(EIA 2009) Fig. 1. World liquid fuel supply 2006–2030 by EIA.

and forest residues and from ligno-cellulosic non-food energy crops. Second generation biofuels are expected to be superior to many of the first-generation biofuels in terms of energy balances, greenhouse gas emission reduction, land requirement and competition for land, food, fiber and water. The main reason they have not yet been taken up commercialization, despite their potential advantages, is that the involved production technologies are not technically proven at a commercial scale and their costs are at the moment estimated to be significantly higher than that of most first-generation biofuels. Therefore, there is still much work to be done for the improvement of the existing processes and for the development of new efficient technologies. Acid and base catalysis plays a crucial role in most of the processes currently used for the production of biofuels, such as the transesterification of vegetable oils with methanol in the biodiesel process (catalysed by alkali, i.e. NaOH, MeONa, KOH) and the hydrolysis of cellulose to fermentable sugars for bioethanol production (catalysed by H2SO4). The homogeneous catalysts, used in both these processes, have some drawback, mainly due to the neutralization step needed at the end of the reaction. In order to avoid these problems, many efforts have been devoted to the search of solid catalysts, more selective, safe and environmentally friendly. In this scenario zeolites and mesoporous

Zeolites have found wide application as solid acid catalysts or catalyst carriers in the oil refining and petrochemical industries, where they have been gradually replacing conventional homogeneous and heterogeneous catalysts (e.g. free liquid acids; amorphous mixed oxides). In most of the processes, zeolite and mesoporous materials are involved in acid-catalyzed reactions that proceed through the formation of carbocation-like intermediates. Therefore the chemistry of the catalytic transformations is closely related to the chemistry of the carbocations in the restricted microporous environment. Normally, the reactions are performed in apolar gas or liquid phases where the substrate to be converted is an hydrocarbon oil or an hydrocarbon-like molecule. To produce biofuels, we should take in account that the composition and the structure of biomass raw materials are totally different from petroleum. Lignocellulose (wood and its derivatives) is the cheapest and most abundant source of biomass and is essentially composed of cellulose (38–50%), hemicelluloses (23–32%) and lignin (15–25%) [4]. All these compounds are polymeric molecule insoluble in most of conventional organic solvents. The transport of heavy biomass molecules into catalyst pores is very cumbersome and severe mass transfer limitations should be expected. To be transformed the biomass require a dispersant as reaction medium and the so obtained

Table 1 Chemical composition of various biomass feedstock. Biomass substrate (dry base)

Sawdust Switchgrass Arundo donax Algae Organic urban waste Fossil heavy oil

Fig. 2. World production unconventional liquid fuels 2006–2030 by EIA.

Elemental analysis (%) C

H

O

N

S

45.8 47.8 47.1 46.1 44.3 86.1

6.1 5.8 5.8 7.4 6.7 11.8

42.7 45.0 46.4 41.3 44.5 <0.1

<0.1 1.1 0.6 4.8 3.5 0.1

<0.1 0.1 0.1 0.4 0.9 2.0

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sluggish medium is itself heterogeneous. The recovery and reuse of the solid catalyst at the end of the reaction is another critical issue. As illustrated in Table 1 in which several typical bio-starting material are compared to fossil heavy oil, biomass composition is not often homogeneous and elemental composition includes oxygen (the most abundant), nitrogen and sulphur [5]. The presence of heteroatoms reduce heat content of molecules and hinder the blending with existing fossil fuels [1]. As for heavy oil, in general to produce a fuel we have to break down large molecules, but in case of biomass we have to remove larger amount of heteroatoms too. Actually it is necessary to remove the abundant oxygen content from biomass-derived products and convert it into a hydrophobic molecule with the appropriate combustion or chemical properties. By this treatment, the hydrophilic substrates start to become more and more hydrophobic and the interactions with the active centre of the catalysts are probably changing during the course of the reaction. The reduction of the oxygen content in biomass by dehydration and hydrogenolysis is associated with the formation of the water in the medium. The water, sometime used as convenient dispersant solvent for biomass or naturally present in the starting material, can poison active sites of the catalyst or modify their activities. There is also the possibility that the polymeric molecules react on the external active sites and the reaction continues in the internal active pores, maybe with smaller compounds and with a different mechanism. Therefore, in biotransformation the interactions between biomass components with solid catalysts and zeolites are different with respect to hydrocarbon ones and are not based only on apolar carbocation-like relationships in the active centers. Zeolites and zeolite like materials seem to have not the same importance for biomass conversion as they have in petroleum industry so far. However, as soon as the ligno-cellulosic biomass is deconstructed to a sufficient extent, the further transformations of the obtained platform chemicals can take advantage of the unique properties of zeolites and related materials [6]. In the following sections the main chemical aspects of the different biomass platforms, vegetable oils, sugars, lignin and bio-oils, will be presented together with some examples of recent lab and pilot scale or industrial processes based on the aforementioned catalysts. 3. Vegetable oils Vegetable oils are triglyceride esters of fatty acids. In principle they could be used directly as diesel fuel, however, due to several drawbacks (e.g. high viscosity, encrustations in the internal combustion chambers, blockage of the injectors, and dilution of the lubricant) it is advisable to convert them into biodiesel [7]. Biodiesel is a mixture of fatty acid methyl esters (FAME) produced by transesterification of triglycerides with methanol in presence of a proper catalyst (Fig. 3). Glycerol is the main by-product, whose amount is equivalent to approximately 10% of the total FAME production. While various biodiesel feedstock are renewable, its competition with food source is a major concern. Non-edible oil should be preferred (castor oil, waste vegetable oils, algae based oil, etc.). Any vegetable (or animal) fat can be used to prepare biodiesel, but the source of feedstock should fulfil two requirements: a O CH2 O C R O CH O C R O CH2 O C R

CH2 OH

+ 3 CH3OH

CH

OH

CH2 OH

O + 3 R C OCH3 FAME (Fatty Acid Methyl Ester)

Fig. 3. Vegetable oils transesterification to biodiesel.

cheap price (more than 80% production cost of biodiesel is the feedstock price) and local availability (in terms of large and constant volume production or waste reuse). The process and properties of the final produced biodiesel are strongly influenced by the length of the fatty acid chains, the degree of un-saturation of the starting triglyceride, the presence of other chemical functional groups in the chain, the free fatty acids (FFA) contents, the melting point of fat and the degree of moisture and impurities [8]. Vegetable oil is composed from over 100 substances, and different oils have different compositions that can vary even for the same oil during the year. As a consequence, a vegetable oil process should be flexible versus feed. In principle, both acid and base catalysts could be used in the transesterification process, however, base catalysts are generally preferred for their superior activity (approximately 4000 times higher that that of acid catalysts) [9]. Today the commercial biodiesel production plants are utilizing homogeneous alkaline catalysts (usually sodium hydroxide or sodium methylate) [10]. The reaction is usually carried out with an excess of methanol (6/1 w/w) at 60 °C. The resulting reaction mixture is a biphasic system consisting in a polar phase, containing most of the glycerol, the catalyst and a part of co-produced soaps, and an apolar phase containing the FAMEs, a fraction of methanol, traces of the catalyst and most of the soaps. Actually, the current technology for biodiesel production has two main shortcomings. Firstly, the presence of free fatty acids and water in the feedstock causes the lost of the catalyst and the formation of soap. Secondly, the alkaline catalyst must be neutralised at the end of the reaction, and the resulting salt is difficult to remove from the glycerol to get a high purity grade product. To avoid these problems, many heterogeneous catalysts have been proposed and reported in the literature. Different heterogeneous catalysts have been developed to catalyze the transesterification of vegetable oils, including zeolite and mesoporous materials [11]. A comprehensive list of zeolite, micro-mesoporous material considered as transesterification catalyst has been recently reported in a review by Miertus et al. [8]. López et al. reported an interesting comparison among different solid catalyst and liquid homogeneous catalyst (NaOH and H2SO4) in the transesterification of triacetin as model compound for larger triglycerides as found in vegetable oils and fats [12]. The reaction were performed at 60 °C using a 6:1 (methanol:triacetin) initial molar ratio and 2 wt.% of solid catalyst in comparison with 0025% NaOH and 0.25% H2SO4 (final reaction time 500 min). In these conditions the catalytic activity decreases as follows: NaOH > H2SO4 > ETS-10 (Na, K) > Amberlyst-15 > Sulfated Zirconia > Nafion NR50 > MgO  Tungstated Zirconia > Supported Phosphoric Acid > H-Beta > ETS-10 (H). The maximum conversion of triacetin reached with the different catalysts at 500 min of reaction time are summarized in Fig. 4. The data indicate that homogeneous catalysts were more active than heterogeneous ones on a weight basis. Furthermore microporous acid zeolite is not a valid alternative to homogeneous

Triacetin conversion (%)

30

100 80 60 40 20 0 NaOH

H2SO4

Amberlyst15

Sulfated zirconia

MgO

Zeolite HBeta

ETS-10 (H) ETS-10 (Na,K)

Fig. 4. Triacetin conversion with different basic and acid catalysts (T = 60 °C; methanol/triacetin = 6/1 mol/mol; reaction time = 500 min).

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catalysts in mild conditions. López et al. suggest internal mass transfer limitations as explanation of the poor performances. However, basic heterogeneous catalyst ETS-10 exhibited almost the same activities as H2SO4, but still lower than NaOH. Corma et al. evaluated glycerolysis of triglycerides using basic Cs-MCM-41, Cs-sepiolite and hydrotalcites [13]. The reaction was carried out at 240 °C and the best conversion was obtained with hydrotalcite (92%) without any significant loss of activity. Using soybean oil as substrate, a series of zeolite catalysts was tested by Suppes et al. [14], including NaX and ETS-10 zeolites exchanged with potassium and cesium. The ETS-10 catalysts provided higher conversions than the zeolite-X type catalysts. The increased conversion was attributed to the higher basicity of the ETS-10 material and larger pore structures that improved intra-particle diffusion. At 100 °C, the ETS-10 provided a conversion of 92% in 3 h. Unfortunately, the presence of FFA (oleic acid) quenched the reaction due to inhibition of basic sites. Currently, refined vegetable oils (containing less than 0.5% of free fatty acids, FFA) are the major feedstock for biodiesel production. However, waste greases, such as yellow grease from used cooking oils and animal fats, can also be employed because of their availability and low cost. The high concentration of FFAs (up to 15%) present in these inexpensive feedstocks, make them inappropriate for the conventional base-catalyzed transesterification route to biodiesel due to soap formation. Although the acid catalysts require a longer reaction time and a higher temperature than the alkali-catalysts, they are more efficient when the amount of free fatty acids in the oil exceeds 0.5%. In this case, a single-step, acid catalysed esterification/transesterification process is more economical than the alkali-catalyzed process, which requires an extra step to convert free fatty acids to methyl esters, thus avoiding soap formation. Homogeneous catalysts (H2SO4, HCl, BF3, H3PO4, and organic sulfonic acids) [15] although effective, lead to serious contamination problems, due to the formation of decomposition products that make essential the implementation of good separation and product purification protocols, which translate into higher production costs. Heterogeneous catalysts were tested. Solid sulfated oxides, such as SO4/ZrO2 and SO4/SnO2 proved to be active catalysts due to their high acid strength, but deactivation phenomena and sulphate leaching were detected under the transesterification conditions [16]. Esterification has been carried out using ion-exchange resins (Amberlyst-15) [17] and Nafion [18] as heterogeneous catalysts. However, most ion-exchange resins are not stable at temperatures above 140 °C, which prohibits their application to reactions that require higher temperatures. For this kind of application, inorganic acid catalysts, such as zeolites, are generally more suitable. Using microporous zeolites catalysts the mass-transfer resistance becomes a critical issue if large molecules, are used as substrates. In this case the reaction takes place mainly on the external surface of zeolite crystals. For this reason, only large-pore zeolites (faujasite, mordenite) have been successfully used in fatty acid esterifications [19]. Using waste frying oils, various zeolite catalysts with different acidity and pore structure were tested in the transesterification [20]. H+-exchanged MOR, MFI, FAU and BEA zeolites were employed. The yield increased linearly with enhancing of acid strength and increasing of amount of acid sites. The synthesis of biodiesel from soybean oil and methanol catalyzed by zeolite Beta modified with La3+ was described [21]. Results of the study showed that La-zeolite Beta shows higher conversion and stability than zeolite Beta for the production of biodiesel, which may be correlated to the higher quantity of external BrØnsted acid sites available for the reactants. It should be noted that the optimal reaction conditions were 4 h of reaction, 60 °C and a conversion of triglyceride equal to about 49%.

The catalytic performances reported so far by zeolites, both in acidic or basic form, are however lower than that of conventional catalyst (NaOH and Na methylate), and also that of other heterogeneous catalysts. This is probably the reasons why up to now, no zeolite catalysts have been considered for commercial biodiesel production processes. An additional improvement of biodiesel process deals with the utilization of the coproduced glycerol for the synthesis of oxygenated fuel components (e.g. glycerine etherification with olefins) [1]. At present, glycerol has already a great number of utilizations leading to a large number of products for non-fuel application, such as: propane diols (monomers for polyester or polyurethane materials), oligoglycerols (cosmetics, food additives and biodegradable lubricants), glycerol carbonate (solvent), glyceric acid (pharmaceuticals), epichlorohydrine, and glycidol (polymers and pharmaceuticals). In the future, the availability of glycerol could exceed the demand for traditional use and its price will decrease, making it a cost effective raw material for the preparation of fuel components and additives. This approach not only makes a valuable use of the by-product but could also increase the fuel yield in the overall biodiesel processes. In European Union, the leading worldwide producer of biodiesel, the glycerine price and production was stable at the beginning of the 2010 and followed an ongoing trend in the second part due to the imminent implementation of Renewable Energy Directive. Player in the market have predicted optimistic scenario for the second half of 2011. Glycerol cannot be added directly to fuels because of its low solubility and poor thermal stability that raise to engine problems at high temperatures. Therefore, glycerol must be transformed into derivatives that are compatible with diesel and biodiesel, prior to being added to the fuel [22–25]. Due to their high oxygen content, glycerol derivatives can be used as ignition accelerators, antiknock additives, viscosity and melting point enhancers and particle emission reducers. Among these products, glycerol ethers have been extensively studied for their promising physical, chemical and blending and properties. The reaction of glycerol with isobutene yields a mixture of the corresponding mono-, di- and tri-tertiary butyl ethers, as shown in Fig. 5. The mono-ethers are soluble in polar solvents [26], whereas the mixture of di- and tri-ethers are miscible with apolar media and can be used in the formulation of diesel fuels (as particulate matter emission reducers) or gasoline (as octane-booster in substitution of methyl-tertiarybutyl ether MTBE). The etherification reaction is efficiently promoted by both homogeneous and heterogeneous catalysts such as para-toluene sulfonic acid, acid ion exchangers resins like Amberlyst 15, and acid zeolites such as H–Y or H-Beta [27,28]. Typically, the reaction is carried out in liquid phase at 70–90 °C with a 3/1 isobutene/glycerol molar ratio. A process, in which glycerol and isobutene were reacted in presence of tertbutyl alcohol using zeolite Beta, was recently patented [29]. The zeolite Beta have a silicon to aluminum ratio greater than 150 and the presence of the alcohol reduces the formation of isobutylene oligomers (less than 5%) producing a more selective glycerol di-tert-butyl ethers mixture yield. An alternative reaction of glycerol in the biofuel production is the catalytic cracking pathway. Corma et al. [30] studied six FCC catalysts for the cracking of 50 wt.% glycerol-water solution at 500 °C, using glycerol as

OH OH OH

O

OH + 3

OH O

+

OH

O +

O

Fig. 5. Etherification of glycerol with isobutene.

O O

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representative of biomass derived oxygenates. Among the catalyst tested, a ZSM-5 based FCC additive and USY zeolites. The activity of the catalysts resulted different and USY was the highest active catalyst in the formation of a mixture of olefins C2–C4, aromatics and paraffins. Glycerol was also fed together with vacuum gasoil (VGO) without significantly altering in the product selectivity, suggesting that biomass-derived products could be co-fed with petroleum-derived streams in FCC process. In a similar way, triglyceride oils can be used to produce liquid fuels that contain linear and cyclic paraffin, olefins, aldehydes, ketones and carboxylic acids. Historically, pyrolysis products of vegetable oils were used as fuel during both World Wars. Vegetable oils are thermally unstable and a solid catalyst is compulsory to improve product yields. Many zeolites were tested in catalytic cracking, including ZSM-5, zeolite Beta and USY, using vegetable oils as only stream or as co-fed with fossil ones (VGO) [4]. Catalytic cracking of vegetable oils appears to be a process for the production of good RON gasoline and olefins, however, some problems should be resolved (coke formation, more stable catalytic system, etc.). A more radical innovation in bio-based diesel fuels considers the complete hydrogenation of the triglyceride feedstocks to hydrocarbon mixture, avoiding the side-production of glycerol and allowing a better integration of the process and the product in the exiting refinery infrastructure and fuel distribution system. Several companies have been developing such a kind of triglycerides hydroprocessing (e.g. Neste Oil, BP, Conoco-Phillips, Petrobras, Dynamic Fuels, Haldor Topsoe, Axens and UOP-Eni) [10,31]. The UOP/Eni Ecofining TM process is based on catalytic hydrodeoxygenation, decarboxylation and isomerization reactions (Fig. 6) to produce a diesel fuel rich in isoparaffins [32,33]. This alternative product is called greendiesel. As this kind of process is very flexible to the feedstocks, it can be considered also for inedible (e.g. jatropha and camelina) and unconventional (e.g. used and cooking oils, animal fats) triglycerides. In this concern greendiesel can be considered as a bridge between first and second generation biodiesel [34]. The main improvement of the Ecofining technology compared to the conventional FAME biodiesel, is that it allows refiners to obtain a synthetic fuel that has a similar chemical composition and similar chemical-physical properties compared to petroleum diesel. For this reason the product can be easily blended with conven-

O COR O COR O COR

RCH3 + 2H2O C3H8 +

RH + CO + H2O RH + CO2

Fig. 6. Vegetable oil transformation in Ecofining process.

Make-up hydrogen

CO2

tional refinery streams. In addition, all of the Ecofining by-products are already present during normal refinery operation and do not require any special handling. The greendiesel advantages, in comparison to mineral diesel fuels and FAME, are high cetane number (CN > 80); negligible O content, that means to have the same energy content as mineral diesel fuel and higher than FAME; better stability and blending properties, due to the absence of double bonds and oxygenated molecules. Besides, the hydrogenation is less sensitive to the quality of vegetable oil, in particular the fatty acid distribution and the degree of un-saturation that affect the properties of FAME biodiesel. A simplified flow diagram of Ecofining process is shown in Fig. 7. In the first stage, vegetable oil is combined with hydrogen and brought to reaction temperature, then it is sent to a reactor section where it is converted to greendiesel. The reactor section can consist of either a deoxygenation reactor or a combination of a hydroprocessing and an isomerization reactors, to achieve better cold flow properties in the green diesel product. The resulting mixture is separated from the recycle gas in the separator and the liquid stream sent to a fractionation section, producing propane, naphtha, and diesel products. In the hydroprocessing stage oxygen is removed from the triglyceride molecules via three competing reactions: hydrodeoxygenation, decarbonylation and decarboxylation (Fig. 6). The three carbon ‘‘backbone’’ yields propane that can be recovered easily when the process is integrated into a refinery. The oxygen contained in the feed is removed from the fatty acid chain either as CO/CO2 or water. In addition, all olefinic bonds are saturated, resulting in a product consisting of only n-paraffins. The hydroprocessing is carried out at moderate temperature (310 °C) using a bimetallic hydrotreating catalyst (e.g. Ni–Mo or Co–Mo catalyst), specifically tailored for the selected feedstock. Despite to the high cetane number, the high cloud point of the liquid stream coming out from the hydrotreating reactor has a great impact in limiting the volume that can be blended with mineral diesel. In order to overcome this restriction, this linear paraffinic stream is isomerized in a second stage. For such a purpose a proper hydroisomerization catalyst, based on a precious metal loaded on a mild acidic carrier has been developed. The scope of this second stage is to control the cold flow properties of the final green diesel. As well explained in the open literature [35–38], the diesel yield from the process will depend on the severity required in the isomerization reactor to meet cold flow specifications. Typical acidic supports for bifunctional catalysts used in the hydroisomerization reaction are: amorphous oxides or mixture of oxides (i.e. HF-treated Al2O3, SiO2–Al2O3,

Propane & light ends Water

Acid gas removal

Separator

HDO Reactor

Isomerization Reactor

Fractionation column

Vegetable oil Fig. 7. Simplified Ecofining process flowscheme.

Diesel

Naphtha or Jet

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ZrO2 =SO24 ), zeolites (Y, Beta, Mordenite, ZSM-5, ZSM-22), silicoaluminophosphates (SAPO-11, SAPO-31, SAPO-41) or mesoporous materials (MCM-41, AlMCM-41) [38]. Beside hydroisomerization, these catalysts also promote undesired cracking reactions and, in order to reduce the cracking extent, a proper combination of porosity and mild acidity is necessary. MSA, an amorphous silica–alumina with controlled porosity in the region of mesopores and a mild acidity, was found to be suitable for this purpose [39].

4. Sugar platform By far the most important class of organic compounds in terms of volume produced are carbohydrates as they represent roughly 75% of the annually renewable biomass production (Fig. 8) [40]. The rest part decays ad recycles along natural pathways. Therefore carbohydrates, expressed as Cn(H2O)n, are the major biofeedstocks from which to develop industrially and economically viable organic products to replace those derived from fossil sources. The non-food utilization of polysaccharides is confined to textile, paper and coating industries so far. Sugar starting platforms are non-edible cellulosic biomass and edible starch-sugar derived biomass. More in details, the constituent units of polysaccharides are glucose (cellulose, starch), fructose (inulin), xylose (xylan) or disaccharide units, most notably sucrose (glucose and fructose). These are the actual carbohydrate raw materials for industrial applications because they are quite cheap and ton-scale accessible. For example, the world annual production of sucrose is about 130 million tonnes [41]. Cellulose is a crystalline material with an extended, flat, helical conformation hydrophilic, insoluble in water and in most organic solvents [42]. It can be broken down chemically into its glucose units by treating it with concentrated acids at high temperature. Cellulose consists of a linear polysaccharide with b-1,4-linkages of D-glucopyranose monomers and its obvious primary sources are forests and cotton plantations. The average molecular mass equivalent is about 5000–7000 units according to its different origin (Fig. 9).

Renewable biomass (200 bill.tons/year)

0%

20%

40%

60%

80%

Carbohydrates 75% 20%

Lignin

Fats, proteins, terpenoids, alkaloids nucleic acids

5%

Fig. 8. Composition of renewable biomass.

O HO

OH

R

O HO O R

O O HO OH

OH O HO O R

R O OH

Cellulose R = OH Chitin R = NHAc Fig. 9. Structural representations of segment of cellulose and chitin.

33

Cotton wool is almost pure cellulose. In wood cellulose is part of a constructed fiber-reinforced composite in which cellulose chain molecule organized in fibrils constitute the plant reticulum material held together and protected by lignin acting as binder and encasement. To separate cellulose from wood for industrial applications, the wood must be treated to separate lignin and hemicellulose (pulping process). In contrast to cellulose, hemicellulose is derived from several sugars in addition to glucose, especially xylose but also including mannose, galactose, rhamnose, and arabinose. Hemicellulose consists of shorter chains – around 200 sugar units. Furthermore, hemicellulose is branched, whereas cellulose is unbranched, and this is the reason why it is amorphous. Hemicellulose surrounds the cellulose fibers and is a linkage between cellulose and lignin. It is relatively easy to hydrolyze hemicellulose to its monomer sugars compared to cellulose. Chitin is a polysaccharide composed of b-(1 ? 4)-linked 2-acetamido-2-deoxy-D-glucopyranosyl residues (Fig. 9). Chitin is the major organic constituent of the exoskeleton of insects, crabs, lobsters, etc. It is the most abundant byproduct of the fishing industries [41]. Starches are the principal food-reserve polysaccharides in the plant kingdom. The two components, amylose and amylopectin, vary in relative amount among the different sources. The majority of starches contain between 15% and 35% amylose [41]. Amylose is linear formed by long unbranched chains of glucopyranosyl units, while amylopectin is an-(1 ? 4)-glucan with a branch point via O-6 about every 25 units. Starches are insoluble in water and white potato, rice, wheat and corn are the major source of starch in the human diet. The main problem in the sugar platform is related to the sluggish solubility of polymeric carbohydrates. This fact makes the reaction heterogeneous concerning the substrate itself: i.e. no solid acid catalyst can be easily used. Currently automotive fuel production from sugars is limited in practice to the fermentative processing of starch hydrolysates or sucrose to ethanol, waiting for the development of cellulosic bio-alcohol production [43,44] as represented in Fig. 10. Alternatively, the fermentation process can produce lipids, useful basis for the automotive pool [45]. However, once the pentose and hexose are obtained, zeolites can offer unique achievements in biofuel transformations of hydrolysates. Actually many transformations in carbohydrate chemistry require acid catalysis and zeolites can serve as an environmental friendly alternative to the traditional chemical synthetic routes. In the last few years major advances have been described addressing the use of zeolites as promoters in classical and key steps for the derivatisation of simple sugars, e.g. O-glycosylation, sugar protection and deprotection, acetylation and oxidation [46]. As stated before, few zeolite systems appeared working on polymeric carbohydrates [47]. Using starch in water (25% weight) and zeolite H-USY with 3% ruthenium, the polysaccharide was transformed in polyhydric alcohols through a simultaneous hydrolysis and hydrogenation. The reaction was performed in 1 h at 180 °C in presence of hydrogen, transforming the starch prevalently in D-sorbitol. In the presence of the highly dispersed metal on the zeolite support, obtained by ion exchange, the starch (or cellulose) is hydrolyzed to its basic monosaccharide whose aldehyde or ketone group is then hydrogenated to hydroxyl group. H-mordenite and H-ZSM-5 or a different metal (copper, nickel) were also employed. In order to obtain glucose or fructose as raw materials, a different catalytic system has been proposed, modifying a mesoporous silica with sulfonic acid groups [48]. The sulfonated mesoporous silica was applied as hydrolyses catalyst of sucrose and starch, working as water tolerant recyclable solid acid, and showing higher conversion and turnover frequency than conventional Amberlyst and H-ZSM-5 catalysts. Yields of monosaccharides up

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Biofuels Lignin

Zeolite catalysis

Glucose

Cellulose

Biomass

Xylose Pentose Arabinose Hemicellulose

Fermentation

Mannose Hexose

Galactose EtOH, ButOH, Lipids

Glucose

Fig. 10. Conversion routes of polysaccharides to biofuels.

to 90% were obtained at 80 °C, thanks to the mesoporous channels able to adapt bulky sucrose and starch molecules. In a different approach, cellulose was dispersed in water (about 9% in weight) and transformed to glucose and other water soluble products by means of several zeolites (H-mordenite, H-Beta and H-ZSM-5). The reactions were performed at 150 °C with a catalyst/cellulose ratio 2/1 in 24 h [49]. The zeolite with high Si/Al showed a relatively higher activity for glucose formation. For example, H-Beta with Si/Al = 75 transformed cellulose in glucose with 11% yield, while using the same zeolite with Si/Al = 12 only 5% of glucose yield was found. Zeolite with higher Si/Al ratio possess a higher hydrophobic character preferring organic compounds to water interactions, which may be a cause of the different glucose yield. Once produced from polymeric feedstock, monosaccharides may be transformed into hydrocarbons and oxygenated additives that can find application in the automotive pool. One of the pioneering patent on simple sugar transformation to hydrocarbon with zeolites appeared in 1985 [50]. Using a steamed 40% HZSM-5 catalyst dispersed in a silica–alumina matrix and a 50 wt.% aqueous solution of glucose in a fluid bed reactor at 510 °C, mainly C8–C10 aromatics were produced. On the basis of carbon fed, about 20% of the monosaccharide carbon was converted to hydrocarbons. Unfortunately, coke was the main product (about 60% on carbon balance). Levulinic acid can be selectively produced from glucose, fructose or from cellulose and hemicellulose. Levulinic esters and methyl-tetrahydrofuran (MTHF) can be then produced from levulinic acid by esterification and hydrogenation. These products can be used as oxygenated diesel and fuel additives [1]. MTHF has a octane number of 87 and can be blended with gasoline up to 70%. Furthermore, MTHF reduces low reid vapour pressure of ethanol blending and has been approved by the US-DOE as a component of P series fuel. More recently, the dehydration of glucose to organic acids in both microporous and mesoporous aluminosilicates cat-

OH O

alyst powders was studied [51]. HY, montmorillonite, MCM-20 and MCM-41 were used in the production of levulinic acid from glucose or fructose passing through 5-hydroxymethyl furfural (HMF) formation as reported in Fig. 11. Mesoporous catalysts with pore diameters in the range 10–30 Å exhibited higher yields in levulinic and formic acids, e.g. MCM-20 and MCM-41. The formic acid was the major product. Probably, in this range the pores were large enough to accommodate both the glucose molecule and the furfural. Furthermore, coke formation was less significant with the mesoporous MCM catalysts. Furfural can be produced from hemicellulose by mineral acid catalysis and then can be transformed to levulinic acid using a two step synthesis, e.g. hydrogenation to furfuryl alcohol and subsequent zeolite catalyzed esterification with ethanol [52]. The acidic ZSM-5 with Si/Al = 30 showed the highest yield in ethyl levulinate at 125 °C. Other medium (ZSM-23) and large-pore zeolites (ZSM-12, Beta and mordenite) were also tested. In a different route, sugars obtained from the hydrolysis of ligno-cellulosic biomass can be converted into hydrocarbons or fuel oxygenated components by low temperature liquid phase catalytic processing. The advantage of these process, with respect to other treatments, are the high selectivities and yields of desired products. Unfortunately, biomass should be pretreated to prepare a convenient feed solution for subsequent liquid phase reactions. Dumesic and co-workers developed different synthetic routes involving the dehydration of sugars (fructose) over acid catalysts (mineral acid or acidic ion-exchange resins) to form furan derivatives (HMF), that can be subsequently undergo aldol condensation with acetone to form C9–C15 alkanes for use in diesel and jet fuels [53,54]. An alternative strategy begins the oxygen-removal process by converting sugars and polyols (sorbitol) in water over a Pt–Re catalyst to form alcohols, ketones, carboxylic acids and heterocyclic compounds [55]. The organic liquid effluent from the Pt–Re reaction can be transformed into transportation fuel components via ZSM-5 catalyzed reactions. Aromatic compounds can be

O OH

Glucose O

+

HCOOH

O

HMF (5-hydroxymethyl furfural)

Levulinic acid (4-Oxopentanoic acid)

Fig. 11. Conversion of glucose to levulinic acid.

+

H2O + Coke

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C. Perego, A. Bosetti / Microporous and Mesoporous Materials 144 (2011) 28–39

Biomass

Fractionation and pretreatment

Hydrogenolysis or hydrogenation

OH

OH

OH H2

Aqueous Phase Reforming

OMe OH Acid Zeolite condensation

Base catalyzed condensation

Gasoline

HDO

Alkene Oligomerization

Kerosene, jet fuel

Alkene saturation

Dehydration

Diesel Fig. 12. The simplified scheme of Virent’s Bioforming Process™.

produced heating the effluent at atmospheric pressure and 400 °C over H-ZSM-5. The carbon in the sorbitol-derived organic phase is converted to paraffins (25%) and olefins C3–C4 (29%), whereas 38% of it is transformed into aromatic species (benzene, toluene, xylenes, C3–C6 substituted benzenes). In addition, pentanols and hexanols, coming out from the hydrogenation of the effluent, could be dehydrated over an acidic niobia catalyst to form branched C4 to C6 olefins. These olefins were oligomerized in combination with cracking reactions over H-ZSM-5 to form a distribution of branched olefins centered at C12. The overall process converts 50% of the carbon present in alcohols to fuel-grade components. More recently, Virent has developed the conversion of readily available biomass-generated sugar feedstocks to carbon–neutral hydrocarbon fuels or hydrogen [56]. The Virent’s catalytic Bio forming™ process combines its proprietary aqueous-phase reforming technology with conventional catalytic processing technologies used in petroleum refining (Fig. 12). The aqueous-phase reforming step produces hydrogen, carbon dioxide, alcohols, ketones, aldehydes and by-products alkanes, organic acids and furans by reaction of oxygenated hydrocarbons with water over a proprietary catalyst. The processes include catalytic hydrotreating and catalytic condensation processes, including ZSM-5 acid condensation, base catalyzed condensation, acid catalyzed dehydration, and alkylation. In the case of condensation route, the process use a reactor system with four different catalyst beds at same pressure, different temperature and no intermediate separation: hydrogenation catalyst (Ru/C), APR catalyst, tungstate zirconia catalyst and H-ZSM-5. Starting from sucrose and xylose, aromatic and isoalkanes were produced to be used as gasoline. 59% of the lower heating value of the sugars was recovered in hydrocarbons having greater than 5 carbons. On March 2010, Virent Energy System and Shell announced the start up of the demonstration plant (Madison, USA), converting plant sugars into gasoline with an expected capacity of 38,000 l/year biogasoline [57].

5. Lignin To enhance the prospects of complete usage of the biomass resources (wood, agricultural residues, etc.), commercially viable applications for lignin must be expanded. After polysaccharides, lignin is the most abundant organic polymer in the world [58]. Actually, 10–25 wt.% of biomass is typically composed of lignin

Guaiacyl (G)

OMe

MeO OH

Syringyl (S)

OH

p-hydroxyphenyl propane ( p-H)

Fig. 13. Schematic representation of the structural units of lignin.

which is a polymer of coniferyl alcohol, sinapyl alcohol and coumaryl alcohol. Lignin is created by enzymatic polymerization of these monomers that lead, respectively to guaiacyl (G), syringyl (S) and p-hydroxyphenyl propane (p–H)-type units (Fig. 13) [59]. The resulting structure is a complex macromolecule with a great variety of functional groups and over 10 different types of linkages. For example, the bonding in the polymer can occur at many different sites between oxygens, aromatic rings, doublebonds in the side chains and so on. The chemical reactivity of lignin is largely determined by the presence of phenol hydroxyl, benzylic hydroxyl and carbonyl groups, even if their frequency may vary according to the source of lignin and the wood species. Because it is not possible to isolate native lignin from wood without degradation, the true molecular mass of lignin is still unknown. The weight-average molecular mass for softwood milled wood lignin is estimated to be 20,000 [60]. The manner in which it is produced from ligninocellulose materials affects its colour, the structure and the reactivity (degree of degradation, contaminants). Two types of lignin are commercially available, lignosulfonate and kraft lignins. The lignosulfonates, also denominated lignin sulfonates, are byproduct of sulphite pulping and are water soluble polymers heterogeneous in terms of their polydispersity and structures. They are insoluble in common organic solvents. Vanillin and dimethyl sulphide are prepared from lignin sulfonates or, in alternative, they are used as dispersants or for binder and adhesive applications [60]. Kraft lignin are obtained from kraft pulping liquor by precipitation. Kraft lignins are soluble in alkaline water, acetone, dioxane. The World potential for lignin production in the existing pulp and paper industry is more than 50 million tons/year [61]. Currently residual lignin from paper pulping is burned off to generate heat and electricity. Due to its C9-aromatic precursors, lignin appears as suitable substrate to be converted in biofuels if it can be broken down into smaller compounds [62]. Furthermore, lignin has a lower contents of heteroatoms than raw biomass. Normally, oxygen is present in a percentage ranging from 29 to 33 on the basis of elemental analysis and nitrogen is almost absent [60]. Few main strategies can be used to convert lignin in fuels with zeolites: the direct upgrading, the catalytic pyrolysis and the hydrotreating. For instance, the upgrading of the lignin was performed with HZSM-5 catalyst (Si/A = 56) at 500–650 °C and 2,5–7,5 h 1 WHSV [63]. Starting from AlcellR lignin, e.g. the lignin derived from the pulping process in which ethanol was used as solvent, the reaction produced a mixture of gas, organic liquid and coke. Conversion was high and ranged between 50% and 85%. Using a WHSV of 5 h 1, the liquid product yield was 39% at 500 °C, but decreased to 34% at 600 °C. The highest yield of liquid product was 43%, obtained at 550 °C. In all experiments, the liquid fraction mainly consisted of aromatic hydrocarbons, benzene, toluene, xylenes, ethyl benzene, propyl benzene and other C9 aromatic compounds. Toluene was the predominant component. The gas phase was a mixture of propane, ethylene, propylene, carbon dioxide and carbon monoxide.

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As the temperature increased, the gas yield increased dramatically (68% at 650 °C). Char and coke yield decreased with increasing the temperature. The pyrolitic conversion of pure lignin at 600 °C was described in a recent article [64]. The catalysts examined, and compared to pure sand, were H-ZSM-5, K-ZSM-5, Al-MCM-41, supported phosphoric acid and a hydrotreating Co–Mo catalyst on alumina. Although outcomes over these five catalysts are all different, no trend or predictable result appears. H-ZSM-5 was the best catalyst for producing a deoxygenated liquid fraction yielded almost equal amounts of simple aromatics (benzene, toluene and xylenes, 46.7% yield) and naphthalenic ring compounds (46.2%). This mixture is attractive in gasoline blend utilization. The gas phase includes methane, hydrogen and CO/CO2. Forty heterogeneous catalysts, among them ZSM-5 exchanged by nickel, cobalt, iron and gallium, were tested in the catalytic pyrolysis of several feedstock including lignin [65]. Using one of the best catalysts Ni-exchanged-ZSM-5, lignin was transformed in lignin-derived aromatic monomer syringol, isoeugenol, coniferyl alcohol, sinapyl alcohol and heavier fragment corresponding to dimers and trimers. The amount of coke left on the catalyst in the performed test on lignin ranged from 5% to 15%. Lignin can be dehydroxygenated using sulfided NiMo and Co–Mo catalysts supported also on zeolites at 250–450 °C [1]. The main products from dehydroxygenation are phenols, cyclohexane and other aromatic compounds with liquid oil yields of about 60% of the starting lignin. UOP LLC patented a process for producing high yields of naphtha and diesel related products from lignin and cellulosic waste by one step mild hydrocracking/hydrotreating at 300–450 °C [66]. An advantage of the process is that there is no need to pyrolyze the biomass before hydrotreating with an increase in the yield of naphtha boiling range products. The lignin is mixed with a fluid to form a slurry in order to facilitate the contact between lignin and the catalyst in presence of hydrogen (6–10 Mpa). The fluid is a naphtha boiling range stream (for example pyrolysis oil) and the catalyst included Ni-Mo metals onto a large pore zeolite for allowing large molecoles into the pores for cracking to smaller molecular constituents (Beta, Y, mordenite). Using 40/60 weight ratio lignin/pyrolysis oil, 73% of the lignin on a dry basis was converted to light liquids in the naphtha boiling range. The removal of oxygen from lignin was greater than 90%. Recently, a process was patented in which lignin was converted to fuels with hydrogen generated from lignin depolymerised products [67]. The lignin in aqueous mixture was depolymerised using an acidic or basic catalysts in hydrogen atmosphere. The depolymerization generates an intermediate mixture of depolymerised lignin and light oxygenates, namely light alcohols having from 1 to 3 carbon atoms, with methanol being the predominant alcohol. These alcohols were reformed in order to generate a hydrogen stream for use in the depolymerization and hydrogenation step. The hydrogenation of the depolymerised product stream produces saturated and partially saturated ring compounds. The hydrogenation step was performed using a noble metal on zeolitic support. The University of Utah has developed a different multi-stage process for the production of high octane blending components from kraft lignin [68,69]. In the first step, the lignin was suspended in water (or methanol) and depolymerised using a base-catalyst at moderate temperature (300 °C). The products are small, low molecular weight phenols and Cs-exchanged X-type zeolites can be used. These products are subsequently hydrotreated in presence of sulfided Co–Mo or Ni–Mo catalysts. More in details, the depolymerised lignin product from the first stage is subjected to a two sequential hydroprocessing treatments. In the first treatment, the feed is subjected to exhaustive hydrodeoxygenation which yields hydrodeoxygenated products. In the second treatment, the hydro-

deoxygenated lignin product is subjected to partial ring hydrogenation and mild hydrocracking to produce the final reformulated hydrocarbon gasoline products. So far, all of these studies on lignin were performed on a lab and pilot scale and no commercial plants based on zeolite catalysts were built.

6. Bio-oils from biomass The direct conversion of whole biomass in order to produce biofuels could be achieved using thermochemical processes, such as pyrolysis, liquefaction and gasification [70]. Apart gasification, the other processes produce a liquid hydrocarbon fraction generically called bio-oil. For each process, the products are dependent on the operating temperature, pressure, heating rate and residence time. Bio-oil has a greater energy density than raw biomass, and, in the case of wood pyrolysis, bio-oil increases the energy density by a factor of 6–7 times over the starting chips [71]. The conversion of a biomass in a bio-oil should simplify handling transportation and storage increasing the feasibility for large-scale bioenergy facilities [72]. In a techno-economic evaluation, therefore, the production of bio-oils may reduce biofuel production costs. It is not a goal of the present paper to review pyrolysis and liquefaction technologies, already described in several recent publications [1,73–79]. In a brief overlook, pyrolysis is the thermal degradation of dried biomass by heat in the absence of oxygen, which results in the production of solid (charcoal), liquid (bio-oil) and fuel gaseous products. Depending on the operating conditions, the pyrolysis process can be divided into three subclasses: conventional (slow heating rate, long residence time), fast (high temperature range, heating rate 10–200 K/s, 0.5–10 s residence time) and flash pyrolysis (heating rate > 1000 K/s, residence time <0.5 s, very fine particle substrate). In the case of liquefaction or hydrothermal liquefaction, biomass is converted to liquefied products by heating in presence of a solvent, generally the constituent water of the raw biomass. This approach is especially designed for wet materials, saving the energy costs of drying feedstock before transformation. Treated in water in a temperature range of 200–370 °C and a pressure range from 4 to 20 MPa, biomass is depolymerised to a hydrophobic bio-oil. Gases (consisting of CO2, H2, CO and light hydrocarbons), solid residues, water soluble organic compounds and water are coproduced in the reaction. Hydrothermal processing is carried out sub or near the critical point of water (374 °C and 22 MPa) and typically residence times of 5–90 min have been applied [80]. Water under liquefaction conditions has several different roles. It is a reaction medium, a quite polar organic solvent due to the strong decrease of its dielectric constant with temperature and water itself shows a catalytic role in various acid/base catalyzed processes due to its higher degree of ionization at the increased temperature [81]. Under liquefaction conditions a large portion of the oxygen present in the biomass is removed as carbon dioxide [82]. As first point, the chemical composition of bio-oils is determined by the nature of the biomass from which they originate. On the other hand, bio-oil characteristics are related to the synthetic routes. During pyrolysis and liquefaction pathways, a large number of reactions occur, including hydrolysis, dehydration, isomerization, dehydrogenation, aromatization, retro-condensation and coking, with different degrees of importance due to the different operating conditions [1]. Pyrolysis oils are a complex mixture typically water soluble and have an higher oxygen content than liquefaction oils. Pyrolysis biooils are composed of acids, alcohols, aldehydes, esters, ketones, sugars, phenols, guaiacols, syringols, furans, lignin derived phenols, and other not well defined components (terpenes, etc.). Bio-oils present a content of water as high as 15–30 wt.% derived from

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the original moisture and the product of dehydration during the pyrolysis reaction. The presence of water lowers the heating value and flame temperature, but water reduces viscosity and enhances the fluidity of the oils. The pyrolysis bio-oils are non-thermodynamically controlled products and are not stable, degrading with time. Therefore they cannot be directly used for diesel or gasoline fuel blending [83,84]. In general, bio-oils can be used as fuel directly in designed engine motors for electricity production [85]. Liquefaction oils are normally water insoluble with a lower oxygen content and, therefore, higher energy content than pyrolysis oils [1,86]. The liquefaction oils are even more complex mixture of compounds, such as hydrocarbons, aldehydes, ketones, esters, amides, aromatics [87]. Some typical properties of pyrolysis and liquefaction bio-oils are reported in Table 2. Several zeolite and mesoporous catalysts were applied in the pyrolysis route to bio-oils, representing an interesting area of opportunity. Conversely, no significant and robust examples appeared in the literature of zeolite catalyzed liquefaction, probably due to the severe operating conditions of the hydrothermal process. The pyrolysis of wood based biomass has been performed in the presence of H-ZSM-5 catalyst obtaining an increase of the aromatic fraction in the resulting bio-oil (up to 30%) [87,88]. The acid sites of the catalyst promote a series of dehydration, decarbonylation, decarboxylation, isomerization and dehydrogenation reactions, converting the oxygenated products generated in the pyrolysis process into more stable aromatic compounds, mainly naphthalene, ethylbenzene and xylenes. The shape selectivity and the acid properties of the catalysts are crucial parameters in the choice of the fast pyrolysis catalysts. For example silicalite and ZSM-5 had the same pore structure but different acid sites and the aromatic yield changed from about 26 wt.% to below 10 wt.% passing from ZSM-5 to silicalite catalyzed reaction. Indeed the use of zeolite Beta, Y and silica–alumina lead to the production of large amounts of coke as the main product. High heating rates and catalyst-tofeed ratios are needed to ensure that pyrolized biomass compounds enter the pores of the ZSM-5 catalyst and that thermal decomposition is avoided. As a consequence, product selectivity was a function of the active site and pore structure of the catalyst [87]. In 2009 the University of Massachusetts licensed the catalytic fast pyrolysis technology to startup Anellotech to produce renewable biogasoline. The Company hopes to open a first small-scale commercial biofuel production plant by 2014 [89]. Operating under different reaction conditions, the use of H-ZSM-5 catalyst could increase the aromatic fraction in the biooil from 7 (obtained in the absence of any catalyst) to 74% [90]. The catalytic pyrolysis of bamboo was performed using NaY zeolite as catalyst [91]. The NaY catalyst was not only effective in increasing the yield of liquid product but altered the final oil composition. The components of liquids in the absence of the catalyst were a mixture of carboxylic, carbonylic, lactonic, phenolic and furan compounds, while in presence of NaY the oil mainly consisted of Table 2 Main properties of bio-oils in comparison with heavy fossil oil. Oil from Elemental composition (%) C H O N Viscosity (cP) Water content (wt.%) Distillation residue (wt.%) Heating value (MJ/kg) a b

Pyrolysis of wooda

Liquefaction of woodb

Heavy fossil fuela

58 7 40 0.2 100 15–30 Up to 50 19

76.2 6.7 17.0 0.0 >10000 <1.0 10 32.3

86.1 11.8 – 0.1 180 <0.1 1 40

Adapted from: A. Corma et al. Angew Chem Int. Ed. 46 (2007) 7184–7201. C. Xu et al. Energy and Fuels 22 (2008) 635–642.

37

carboxylic and carbonylic compounds. Acetic acid was the main component and its content was two times higher than that from a non-catalytic reaction. Proton forms of Beta, Y, ZSM-5 and mordenite were tested as catalyst in the pyrolysis of pine in a fluidized bed reactor, while quartz sand was used as a reference material [92]. The formation of ketones was higher over ZSM-5 and the amount of acids and alcohols lower than over the other catalysts. Beta, Y, ZSM-5 produced more amount of polyaromatic compounds than mordenite and quartz sand. Furthermore, and in accordance with other findings, coke formation was higher with zeolites (17–30 wt.% coke deposited on material) than in absence of catalyst. Mesoporous catalysts, such as Al-MCM-41, significantly changed the composition of bio-oil, increasing the yields of phenols, hydrocarbons and polycyclic aromatic hydrocarbons (PAH), while decreasing the yields of oxygenated carbonyl and acid compounds [93]. In order to improve the Al-MCM-41 performances, the catalysts were modified by introduction of transition metals (Cu, Fe, Zn) [94]. Factors such as the large pore shape and size, the acidity and the redox properties of the metals could affect the catalytic pyrolysis product distribution. The production of liquids decreased in comparison with no-catalytic runs, but Fe-Al-MCM-41 and Cu-Al-MCM-41 provided the best results in terms of phenols production. The effect of feedstock was significant. Wood Lignocel produced higher levels of hydrocarbons and Miscanthus the higher levels of phenols for all the catalysts tested. Different catalytic upgrading of bio-oils to fuel has been proposed up to now. So far, four main tracks have been studied with respect to the upgrading of bio-oil with improved quality: cracking (e.g. FCC), decarboxylation, hydrodeoxygenation and hydrotreating [95]. Hydrotreating and catalytic cracking are the most studied pathways [96]. Acidic zeolites could be used in these reactions, taking advantage of the unique properties of these materials. The catalytic systems and the operating conditions need a fine tuning in order to avoid the formation of coke [97]. Different catalysts have been tested, including H-ZSM-5, H-Y zeolite, H-mordenite, silicalite, silica/alumina, SAPO 5 and SAPO 11 [97–101]. Among these catalysts, H-ZSM-5 yielded the highest amount of liquid products (up to 34 wt.% of feed). The product composition comprised mostly aromatics for ZSM5 and aliphatics for silica/alumina catalysts. The upgrading of wood pyrolysis oils was studied using H-ZSM-5 and H–Y zeolites [102]. While the upgraded liquid obtained by using the H-ZSM-5 consisted of easily separable organic and aqueous layers, the liquid obtained by using the H–Y consisted of a single phase in which the organic components were either dispersed or dissolved in the water. Gaseous products include CO2, CO, light alkanes, and light olefins. Large amounts of coke (6–29 wt.% of feed), char (12–37 wt.% of feed), and tar (12–37 wt.% of feed) formed during the upgrading over zeolites. Hydrotreating process are typically performed at 300–600 °C in presence of heterogeneous Co–Mo, Ni–Mo based catalysts under high pressure of hydrogen. In these conditions oxygen is removed as H2O and CO2. The hydrogenation of aromatics is not desired, since it would increase the hydrogen consumption [96]. The hydrotreatment of bio-oils was performed using typical hydrotreating catalyst, usually without zeolites in the catalyst formulation [103]. Honeywell/UOP has developed a two step process for the upgrading of bio-oils produced by fast pyrolysis to give high yield of naphtha, aviation and diesel fuels [104]. The first partial deoxygenation step is carried out at low temperature (315–340 °C) in order to remove the thermal unstable bio-oil components, such as the short chain carboxylic acids and the olefins, that would decompose to coke if heated under more severe conditions. The second hydrotreating stage, carried out at

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higher temperature (405 °C), involves the full hydrodeoxygenation of the stabilized bio-oil to produce a hydrocarbon oil with less than 2% oxygen. Both these reactions are promoted by sulfided Co–Mo/ Al2O3 and Ni–Mo/Al2O3 catalysts. The second stage can be carried out in presence of a combined zeolite and amorphous silica–alumina catalyst with metal loading (e.g. Ni, Mo, W). Zeolites, including Beta, Y, ZSM-5, mordenite, promote the cracking of heavy compounds that are outside the range of gasoline and diesel fuel. In a different approach the cracking stage can be carried out in a separate reactor, after separation of the stable oil into light and heavy fractions. The heavy fraction (which boils above 350 °C) is sent to the hydrocracker to completely convert the oil to gasoline and diesel blending components [105]. 7. Conclusions Biomass is an abundant and sustainable source of carbon for the production of fuels. To develop processes for the efficient utilization of biomass resources, it is important to understand biomass feedstock chemistry and how a catalyst can be used to depolymerize the biomass feedstock and to remove the heteroatoms present. The production routes of biofuels, both of 1st and 2nd generations, are characterized by a wide application of catalytic processes, including zeolites and mesoporous materials. Few processes, based on these last material catalysts, are at a pilot plant status, so far. No large scale industrial plants still exist and ZSM-5 will probably be the first zeolite catalyst applied in the small demonstration unit for the production of biogasoline in the Bioforming™ process, according to a recent announcement. The ‘‘next’’ generation of biofuels should be produced by more sustainable routes to ensure competitive energy supplies for the transportation sector. The ‘‘next’’ biofuels should respect the actual and future fuels normative in term of emission compliance, environmental impacts, carbon balance, standards requirements and new engine constrains. Furthermore, biofuels costs will be an important item in replacing fossil sources and low-cost renewable feedstocks will be preferred even if these possess intrinsic complexity and recalcitrance to be transformed in well-defined products. The catalytic technology will play also an important role in the ‘‘next’’ generation development. The design operation and control of reactor is one of the challenges in a developing catalytic renewable process. The reactions involving biomass are generally carried out in multiphase reactors (fluidized bed reactor, trickle bed reactor, fixed bed reactor, slurry phase reactor, etc.) to facilitate adequate contact between fluid phases and the catalytic system. The integration between process requirements and catalyst design is one of the difficult tasks to make the overall process to be successful [106]. Therefore, significant technology breakthroughs are required to overcome the technical barriers still existing. Zeolites and mesoporous materials could contribute in this task offering new efficient conversions routes of the whole biomass and multistep integrated processes. These materials might be used not only in the conversion of biomass to fuel or in the following upgrading step, but new applications might come up from the study of biomass pretreatments such as a selective deconstruction steps of the raw materials. Finding the breakthroughs will require both fundamental investigations and industrial oriented developments. References [1] G.W. Huber, S. Iborra, A. Corma, Chem. Rev. 106 (2006) 4044–4098. [2] Adapted from International Energy Outlook 2009, Report #:DOE/EIA0484(2009), http://www.eia.doe.gov/oiaf/ieo/liquid_fuels.html. [3] S.N. Naik, V.V. Goud, P.K. Rout, A.K. Dalai, Renewable Sustain. Energy Rev. 14 (2010) 578–597.

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[24]

[25] [26] [27] [28] [29]

[30] [31] [32] [33]

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