In chemical hydrolysis, pretreatment and hydrolysis may be carried out in a single step. There are two basic types of acid hydrolysis processes commonly used: dilute acid and concentrated acid, each with variations.

Acid Hydrolysis

Acid-catalyzed process can be divided into two general approaches, based on concentrate acid/low temperature and dilute-acid/high temperature hydrolysis. Sulfuric acid is the common acid employed although, however, hydrochloric, nitric and trifluoracetic acids, phosphoric acid, weak organic acids have also been used.

Concentrated-Acid Hydrolysis

Concentrate acid processes enable the hydrolysis of both hemicelluloses and cellulose. The solubilization of polysaccharides is reached using different acid concentrations, like 72% H2SO4, 41% HCl or 100% TFA [45]. Concentrate-acid — based processes have the advantage to allow operating at low/medium tempera­tures leading to the reduction in the operational costs. Hydrolysis of cellulosic materials by concentrated sulphuric or hydrochloric acids is a relatively old pro­cess. The concentrated acid process uses relatively mild temperatures, and the only pressures involved are those created by pumping materials from vessel to vessel. Reaction times are typically much longer than for dilute acid. This method gen­erally uses concentrated sulphuric acid followed by a dilution with water to dis­solve and hydrolyze or convert the substrate into sugar and provides a complete and rapid conversion of cellulose into glucose and hemicelluloses into 5-carbon sugars with little degradation. The critical factors needed to make this process economically viable are to optimize sugar recovery and cost-effectively recovery of the acid for recycling. The solid residue from the first stage is dewatered and soaked in a 30-40% concentration of sulphuric acid for 1-4 h as a pre-cellulose hydrolysis step. The solution is again dewatered and dried, increasing the acid concentration to about 70%. After reacting in another vessel for 1-4 h at low temperatures, the contents are separated to recover the sugar and acid. The sugar/ acid solution from the second stage is recycled to the first stage to provide the acid for the first-stage hydrolysis. The primary advantage of the concentrated acid process is the potential for high sugar recovery efficiency. The acid and sugar are separated via ion exchange and then, acid is re-concentrated via multiple effect evaporators. The low temperatures and pressures employed allow the use of rel­atively low cost materials such as fiberglass tanks and piping. The low tempera­tures and pressures also minimize the degradation of sugars. Unfortunately, it is a relatively slow process and cost — effective acid recovery systems have been dif­ficult to develop. Without acid recovery, large quantities of lime must be used to neutralize the acid in the sugar solution. This neutralization forms large quantities of calcium sulfate, which requires disposal and creates additional expense. Moreover, the equipment corrosion is an additional disadvantage. Nevertheless, there seems to be a renewed interest in these processes [209] owing to the mod­erate operation temperatures and because no enzymes are required.

Formic and acetic acids are good solvents for lignin, and they can hydrolyze lignin in lignocellulosic material under elevated temperature thus resulting in delignifi — cation. At present, organic acids-based fractionations are preformed in formic or acetic acid solutions without and with the addition of catalysts (Table 11.2) [86-103]. As can be seen, the catalysts mainly used are inorganic acid such as hydrochloric acid, sulfuric acid and hydrogen peroxide.

11.4.1 Effect of Treatment on the Structure of Lignocellulosic Material

11.4.1.1 Reactions of Lignin

The chemical modifications of lignin during organic acid fractionation are mainly b-O-4 cleavage, lignin condensation, hydrolysis of LCC structures and the native ester structures, and esterification of the hydroxyl groups. By analyzing the dissolved lignin in a Milox pulping process with three stages, it was found that b-aryl ether bonds were mainly cleaved in the second stage. The precipitated lignin dissolved in the first stage contained a high amount of sugars, and its molecular weight increased with increased stages [104]. Lignin model compounds were investigated by reflux in 85% formic acid, and it was found that the compounds were completely consumed in 1 h. Primary, secondary and phenolic hydroxyl groups of the model compounds were partially converted into corresponding formates [105]. Compared to MWL, the dissolved lignin from acetic acid frac­tionation contained more acetyl groups in Ca and Cy, indicating that hydrolysis of native esters and acetylation (esterification) occurred simultaneously during the fractionation process. However, no acetylation was involved in the formic acid fractionation [106]. In addition, the cleavage of LCC structures was confirmed by the low contaminated sugars in the lignin fractionation precipitated from water [106].

When hydrogen peroxide was added into the organic acid system, peroxyformic or peracetic acid was generated in situ through an equilibrium reaction between organic acid and hydrogen peroxide, and electrophilic HO+ ions were formed [107]. The HO+ ions reacted with lignin through ring hydroxylation, oxidative ring opening, substitution of side chains, cleavage of b-aryl ether bonds and epoxidation [108].

In the conventional method to isolate vanillin from the oxidized solution, the remaining lignin is precipitated by acidification adding carbon dioxide or a mineral acid like sulfuric acid. A liquid-liquid extraction with organic solvents, such as benzene, toluene, or ethyl ether enables the recovery of vanillin from the acidified liquid fraction [98]. The vanillin is co-extracted for a sodium bisulfite aqueous solution in the form of vanillin-bisulfite complex insoluble in the organic solvent. Finally, the vanillin complex in aqueous fraction must be acidified to recover free vanillin [162]. The neutralization and isolation of the vanillin from the lignin precipitated are remarkable cost factor and can present technical problems. A large amount of acidic solution is required and, eventually, the precipitation of the high molecular weight compounds causes losses of vanillin. A direct liquid-liquid extraction to obtain sodium vanillate from the oxidized solution was suggested by Sandborn and Howard [162] and Bryan [163], applying solvents, or a mixture, immiscible with water (alcohols as n-and iso-butanol [162] and iso-propanol [163]). In this case, besides sodium bisulfite method, the vanillin can also be recovered from organic phase by carrier-steam distillation [164].

Although the sodium bisulfite method provides high selectivity, the bisulfite derivative of vanillin is not sufficiently stable to carry out one-stage stripping requiring, therefore, the use of multiple extraction steps [165]. Kaygorodov et al. [165] have reported data on possible extractants for vanillin recovery and discussed on related disadvantages: difficulties in vanillin stripping or solvent recovery, toxicity, price, and solubility in water of some solvents. The aliphatic alcohols of the series C6-C8 were evaluated to extract vanillin from weakly alkaline media, which could eliminate problems related to the emulsification of the extraction system and vanillin sorption by the precipitated when acidification is applied.

Hydrogen production by mixed (non-sterile) cultures would probably have lower production costs and in addition has many other advantages including continuous hydrogen production without light input, a large variety of carbon substrates, including organic wastewaters can be used as carbon sources, hydrogen can be produced at ambient temperatures, and sterile conditions are not required. Innocula of mixed anaerobic bacteria can be derived from a variety of sources including;

sewage sludge, cattle dung compost, river sediment [19], anaerobically digested sludge, acclimated sludge and animal manure [7].

The presence of non-hydrogen producing organisms can be a disadvantage for a mixed culture since they can consume a proportion of the substrate and can also use H2 as an electron donor. As well, the product of dissimulatory sulfate reduction, H2S, can be a potential catalyst poison. Thus, the selection of hydrogen producing organisms can be very important for establishing efficient and clean hydrogen production. The ability of some hydrogen producing bacteria to form spores can be used as an advantage to eliminate non-spore forming methanogens using pre-treatment methods based on this ability. Using chemical inhibitors such as acid and base, or operating the continuous culture under low HRT (hydraulic retention time) and pH conditions can also eliminate methanogens. Nevertheless, heat treatment between 75 and 121 °C is the mostly used method to select spore forming Clostridia. However, heat treatment can also eliminate non-spore forming H2 producers such as Enterobacter species, and can select some spore forming hydrogen consumers like acetogens. As an alternative, hydrogen consumption can be reduced by sparging with N2 or releasing the produced H2 from the headspace [3].

Many studies have been conducted on dark fermentative hydrogen production by using mixed cultures. Generally these studies can be divided into three groups; batch, fed-batch and continuous cultures. Batch studies with a variety of mixed cultures have demonstrated reasonable hydrogen yields. A mixed culture carrying out fermentation of synthetic wastewaters gave a hydrogen yield of 2.48 mol H2/mol glucose [33]. A mixture of aerobic and anaerobic sludges derived from lake mud was reported to give 1.4 mol H2/mol glucose [34]. Sludges subjected to different pretreatments were effective in producing hydrogen; heat conditioned aerobic sludge, 2 mol H2/mol glucose [35]; heat treated anaerobic sludge, 1.75 mol H2/mol glucose [36]; acid treated anaerobic sludge, 1 mol H2/mol glu­cose [37]; treated microflora from cow dung, 2.27 mol H2/h TSS; heat shocked microflora from soil, 0.92 mol H2/mol glucose [38]. Using sucrose as a carbon source with heat treated anaerobic sludge resulted in 1.9 and 3.4 mol H2/mol sucrose in two different studies [39, 40]. Using glucose with heat treated anaerobic sludge resulted in hydrogen yields of 0.98 and 1 mol H2/mol glucose [41, 42].

Fed-batch cultures can be useful for industrial processes since this mode of operation can help prevent product inhibition. Therefore, fed-batch operation could show improved yields over batch cultures since it could prevent the pH drop associated with accumulation of volatile fatty acids. Fed-batch cultures have successfully been employed in a number of studies; olive mill wastewater was treated by a mixed culture and gave 14.7 mmol H2/gVSS degraded [43], it was also used for hydrogen production (17.82 mmol H2/l-reactor-h) using anaerobic POME sludge [44], and a mixed culture from windrow yard compost gave 7.44 mmol H2/L-reactor-h from glucose [45].

However, using continuous cultures may have important advantages for industrial applications. CSTR (continuously stirred tank reactor) and UASB (up- flow anaerobic sludge blanket) reactors are the two main types of continuous

Fig. 10.2 Dark fermentation system

reactors that have been used for hydrogen production. Glucose has been widely used as substrate for dark fermentative hydrogen production by mixed cultures in continuously operated CSTRs. Different studies have reported hydrogen yields between 1.1 and 1.9 mol H2/mol glucose [42, 46-48]. Similarly, using pure sucrose in CSTRs gave yields of 3.3 and 3.6 mol H2/mol sucrose [49-51]. However, the main disadvantage of these suspended culture systems is cell washout at high dilution rates. Therefore, immobilized systems have been devel­oped which not only prevent washout of cells, but also improve production rates since higher biomass concentrations are possible. Various techniques are possible, including the use of inert supports and self-immobilization. Cultures immobilized on activated carbon using sucrose as a sole carbon source gave 0.24-6.08 l H2/g VSS/h [52] or yields of 0.57 and 1.59 mol H2/mol sucrose [52, 53]. Granules formed by self-immobilization resulted in 0.86-2.2 mol H2/mol glucose in dif­ferent studies [54-58]. Other studies using mixed cultures in UASB reactors gave hydrogen yields between 0.84 and 2.47 mol H2/mol glucose [57, 59-61].

12.2.1.1 Origin and Isolation

Kraft pulping is the dominant process for production of pulp for paper [21]. The delignification is carried out in a strong alkaline solution composed mainly of OH — and HS — ions, removing around 90% of the initial lignin [22, 23]. In this process lignin undergoes reactions involving sulfidolytic cleavage of a — and b-aryl ether bonds in both phenolic and non-phenolic lignin units. Reactions of conjugate addition of carbanions to quinone methide intermediates lead to the increase of condensed structures in lignin [22]. Additionally, some other complex reactions between lignin and other wood components can also occur [8, 24].

Unmodified kraft lignin renders insoluble in aqueous solution at pH bellow the pKa values of the phenol groups of lignin, which are in the range 10.0-11.5 [25]. The pH currently used for precipitation and further filtration of lignin is near 7. Sulfuric acid or carbon dioxide have been used for this propose. Recently, the isolation of high purity kraft lignin (both softwood and hardwood lignins) has met a new advancement with Lignoboost process [26-28]. With this process, the washing at controlled conditions of precipitated lignin was improved leading to a final product with rather low content of ashes and carbohydrates, and then opening the perspectives to improve the existent applications or upgrading lignin in new valuable applications [29, 30]. Significant work concerning the recovery [31, 32] and fractionation [33, 34] of lignin from kraft liquors has been published in recent years, many of them via membrane processes.

The tremendous increase in the use of heavy metals over the past few decades has inevitably resulted in an increased flux of metallic substances in the environment. Industrial processes like petroleum refining, metal refining, coal combustion, tanning, metal extraction, electroplating, paints and pigments, the manufacture of batteries etc. discharge effluents in solid, liquid, and gaseous forms. They contain heavy metals such as lead, chromium, cadmium, nickel, arsenic, etc. But the major sources of heavy metals in the environment are traditional chemical processes for extraction of heavy metals. There are various disadvantages of these metal extraction processes like requirement of sufficient concentrations of elements in ores, environmental unfriendliness as huge amount of waste is generated, eco­nomically noncompetitive, nonrecovery of metals from low grade deposits i. e. minerals and inefficient use of energy.

Also, lot of metal containing effluent is produced during these processes, which is discharged as such without any treatment. This causes heavily loaded metal contaminated sites due to metal toxicity and non-biodegradability. The heavy metals are easily percolated through the soil and further trapped and biomagnified along the food chain via consumption of affected plants and animals. The increased concern about the environment and stringent national and international regulations on water pollution and the discharge of heavy metals makes it essential to develop efficient and cost effective technologies for their removal. Hence, it is the utmost need to extract metal ions (not only from the low grade ores but also from the contaminated sites) by the methods which are eco-friendly and greener in nature. The answer is provided by the nature itself: bioextraction.

Alcoholic fermentation involves the production of alcohols such as ethanol and butanol from biomass. Fermentation has been used since ages to produce alcohol from carbohydrate substrates. The feedstock for this fermentation has been agri­cultural commodities such as sugarcane, beet sugar and corn starch—the more common one being starch. The microorganism used for this fermentation has been mainly, yeast. The technology for such fermentations is very well established and has been commercially used for production of ethanol, with state-of-the art fer­mentation plants, the world over. However, the food versus fuel controversy has prompted the search for other feedstocks such as cellulosic/lignocellulosic biomass for use as substrate for fermentation to ethanol. The bioethanol so produced can be used as a fuel as such or, more appropriately, as an admixture in diesel, as gasohol. The use of such substrates, especially lignocellulosic substrates, for successful commercial fermentation, is still a challenging task mainly due to the highly refractory nature of lignocellulose. In addition to physicochemical methods for efficient hydrolysis of lignocellulose, enzymatic hydrolysis methods are also being researched the latter having an advantage of being carried out in situ in the fer­menter. Persistent research efforts in this area have resulted in the development of efficient and economic fermentation processes and technologies even for such a refractory material. The world’s first cellulosic ethanol demonstration plant has been set up at Yonroe-Tennesee and has begun operations since January 2010.

The biochemistry of lignocellulose/cellulose conversion into easily fermentable sugars has been studied exhaustively and a multitude of reviews and treatises on the subject is available in the literature. This section will discuss the recent advances that have taken place in the area. Lignocellulose consists of cellulose which is a glucose polymer; hemicellulose, which consists of mixed hexoses and pentoses; and xylan which is a xylose polymer. All these substances need to be essentially converted into monosaccharides before they can be fermented by the usual fermentation process. This process of breaking down the complex poly­saccharides into simple soluble sugars is called saccharification. Saccharomyces cerevisiae was the predominantly used species for this conversion. However, this microorganism, which is conventionally used for conversion of starch into ethanol,

— First type of anaerobic digestion reactor developed; still in

use

— Good mass transfer

— Cannot be operated at high hydraulic loading rates

— Not suitable for biomass with low concentration of readily

biodegradable substances

— Can handle high-strength feedstocks with large amounts

of suspended solids

— High loading rates possible

— Active microbial mass retained in reactor

Applications

Batch CSTR produces 4,000 m3 of biogas per day in biomethanation plant at Karlsruhe, Germany Centralized biogas plant operating three thermophilic CSTRs with a total volume of 7,000 m3 in Lemvig, Denmark

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is not suitable for lignocellulosic-based ethanol production because Saccharomy — ces is capable of fermenting glucose alone to ethanol, whereas lignocellulosic biomass hydrolysate usually consists of a mixture of oligosaccharides. A number of novel enzymes and improved microbial strains, which have been specifically engineered to convert such a recalcitrant substance like lignocellulose into easily fermentable sugars, have been successfully developed. Some of the microbial strains developed are Scheffersomyces stiptis, Candida shehatae, Kluyveromyces marxianus, Escherichia coli and Zymonas mobilis [25]. The enzymes responsible for breaking down cellulose from lignocellulosic biomass comprise a multitude of enzymes which fall into the category of glycosyl hydrolases which exist in the form of a complex assembly of enzymes called “cellulosome”. These glycosyl hydrolases include both cellulases and non-cellulosic structural polysaccharidases. The details of the different modules of enzymes comprising the cellulosome are discussed in detail by Berg Miller et al. [26]. The true cellulases, present in the glycosyl hydrolases, cleave the p-1,4-glucosidic bonds of cellulose, resulting in the production of cellobiose. A number of other enzymes, having varying degrees of specificities, are responsible for hydrolyzing different forms of cellulose present in the cellulosic plant material. Non-cellulosic structural polysaccharidases are a diverse group of enzymes that are capable of cleaving the different types of bonds present in the main chain backbone (xylanases and mannanases) and the side chain constituents (arabinofuranosidases, glucuronidases, acetyl esterases, xylosidases, and mannosidases) of the substrate. Ladisch et al. [27] provide an elaborate description of the cellulose enzyme system and the mode of action of fungal cel — lulases. Current research efforts in the field of enzymatic hydrolysis of cellulosic plant material are going increasingly toward identifying newer glycosyl hydrolases. The currently followed approach for accomplishing this is by attempting to genetically modify the most efficient existing microbial systems such as those found in the grass eating ruminant animals, e. g., Fibronobacter succinogenes, Ruminococcus albus, and Ruminococcus flavefaciens into making newer and more efficient glycosyl hydrolases by using molecular engineering concepts. The research progress in the field is comprehensively outlined by Berg Miller et al. [26].

An alternative to the above-mentioned enzymatic hydrolysis of lignocellulosic biomass is acid hydrolysis, which has a few advantages over enzymatic hydrolysis in that, it is quick, no dedicated support of enzyme production system is required, and high temperatures can be used, allowing lower acid concentrations. However, the major drawback of acid hydrolysis process is the degradation of the hexoses and pentoses to acids such as hydroxymethyl furfural (HMF) from glucose and furfural from xylose, in addition to some other acids produced. These acids reduce the activity of the ethanol producing microorganisms. HMF further breaks down to formic acid which may lead to a total inhibition of ethanol formation. Figure 1.20 shows a schematic of the various acids produced during pretreatment/acid hydrolysis of lignocellulosic biomass.

In addition to these acids, metals leached out from the hydrolysis equipments and other SO2 inhibitors released from additives also retard microbial growth and other metabolic activity. More than a 100 such inhibitors have been detected. Liu

et al. [28] have classified such inhibitory compounds on the basis of the functional group present on the inhibitor agent. These degradation products have an inhibi­tory effect on the ethanol producing organisms, reducing the yield of ethanol from the process. An obvious solution to this problem lies in the removal of the alde­hyde and/or other inhibitory agents at regular intervals. This can be carried out by physicochemical processes such as vacuum evaporation to reduce the volatile inhibitors; alkali treatment, using Ca(OH)2 or NaOH to precipitate out substances having aldehyde and ketone functional groups; and adding activated charcoal or diatomaceous earth to physically adsorb the inhibitory agents, thus improving the yield of ethanol. Use of anion — and cation — exchange resins has also been inves­tigated with results more favorable than all the other methods mentioned above. A combination of two or more methods is preferred depending on the nature of inhibitors present. Enzymatic treatment using peroxidases and laccases obtained from the lignolytic fungus Trametes versicolor has been found to improve the yield of ethanol by removing the phenolic inhibitors from the substrate. Alterna­tively, in situ ‘detoxification’ solutions to this problem are being explored. One such solution comprises development of inhibitor-tolerant strains of yeast or bacteria that can withstand the presence of the inhibitors. The inhibitor conversion pathways and mechanisms of in situ detoxification have been reviewed by Liu et al. [28].

Using recombinant yeast for improved ethanol production is yet another area in which ongoing research efforts are likely give good returns. The improvement of

cellulase expression in S. cerevisiae has also been exploited. The contribution of a number of researchers in developing strains with improved cellulose expression has been compiled by Liu et al. [28]. Similarly, hemicellulase expression in S. cerevisiae has also been studied.

The above developments, along with simultaneous advances in fermentation technologies, are certainly expected to take ethanol production from the more economical lignocellulosic biomass to new heights.

The fermentation technologies used for fermentations have also advanced rapidly, with many progressive modifications in the conventional batch and con­tinuous fermentation processes. Combination of both, the batch and continuous process, called the fed-batch processes have also been developed. Fermentations using novel immobilized cell systems have been used to enhance the efficiency and productivity of the fermentation processes. Methods such as ‘‘growth arrested process’’ have been developed, where a high productivity of intermediate meta­bolic products such as lactate and succinate, as well as other organic acids, has been achieved by arresting the growth of the microorganisms at the particular stage at which the target products are produced, by maintaining the conditions in the reactor which stop further growth of the organisms [29].

Another type of briquetting machine is the hydraulic piston press. This is different from the mechanical piston press in that the energy to the piston is transmitted from an electric motor via a high pressure hydraulic oil system. This machine is compact and light. Because of the slower press cylinder compared to that of the mechanical machine, it results in lower outputs. The briquettes produced have a bulk density lower than 1,000 kg/m3 due to the fact that pressure is limited to 40-135 kg/h. This machine can tolerate higher moisture content than the usually accepted 15% moisture content for mechanical piston presses. Pelletizing is closely related to briquetting except that it uses smaller dies (approximately 30 mm) so that the smaller products produced are called pellets. The pelletizer has a number of dies arranged as holes bored on a thick steel disk or ring and the material is forced into the dies by means of two or three rollers. The two main types of pellet presses are: flat and ring types.

Large capacity pelletizers are available in the range of 200 kg/h-8 ton/h. Thus, pellet press capacity is not restricted by the density of the raw material as in the case of piston or screw presses. Power consumption falls within the range of 15-40 kWh/ton.

Several acids served as catalysts with [BMIM][Cl] for the hydrolysis of corn stalk: hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, and maleic acid. Overall, hydrochloric acid was the most efficient catalyst. Sulfuric and nitric acids were also efficient, but required a higher loading to achieve the same yield in reducing sugars. At the same temperature (100°C), reactions with phosphoric and maleic acids were much slower than with the other acids, even at high loadings. The combination of hydrochloric acid (7 wt%) and [BMIM][Cl] was efficient in the hydrolysis of corn stalk, rice straw, pine wood, and bagasse [6]. Faster degradation of cellulose and hemicellulose was also observed at higher tem­peratures and for longer pretreatment times for Eucalyptus grandis [32]. The weight loss increased with the amount of hemicellulose, which was higher in softwoods (spruce and pine). More carbohydrates (polysaccharides and lignin) were hydrolyzed as the acid concentration increased [32]. Trifluoroacetic acid (0.2 wt%) also served as an acid catalyst in the dissolution of loblolly pine in [BMIM][Cl] at 120°C. Its effect was similar to sulfuric acid H2SO4 at the same molar concentration. After a 2-h treatment, 62 wt% of the loblolly pine was converted to soluble products. No further increase in the yield was seen after a

4- h treatment [53]. The addition of AlCl3 led to a decrease in pH in a mixture of wood (Metasequoia glyptostroboides) and [BAIM][Cl] and [MAIM][Cl], which accelerated the dissolution of wood at a lower temperature. The amount of insoluble residues in the IL and pH decreased with increasing AlCl3 amount. The selection of the metal chloride affected the pH and the liquefaction efficiency: AlCl3 led to lower pH than SnCl2 and FeCl3. The stronger acidity led to higher liquefaction efficiency [16]. These results were consistent with a previous study in which the initial acid hydrolysis rates of cellobiose increased with increasing acid strength. The conversion of cellobiose to glucose was much faster for acids with negative pKa values, such as methanesulfonic acid (pKa = -1.9) and sulfuric acid (pKa = -3) [26].

From these results, it was argued that biomass does not dissolve in ILs directly, but that it needs to be hydrolyzed first before the dissolution of the hydrolysis products. Pine wood and wheat straw (mesh size smaller than 1 mm) were dissolved in [EMIM][OAc] with acetic acid as catalyst. After dissolution, a drop in pH was observed with formation and accumulation of acetic acid in the IL/biomass solution. The addition of acetic acid to [EMIM][OAc] accel­erated the dissolution of wheat straw. After dissolution and addition of water, the precipitate contained an amount of lignin that increased with the amount of acetic acid added, suggesting that acetic acid also acted as a co-solvent for lignin [47].

Indeed, IL pretreatments with acid may increase the yield of reducing sugars following enzymatic hydrolysis, but they also promote the degradation of cellulose and hemicellulose when conducted at higher temperatures and for longer times [6, 32, 47, 53]. Faster degradation of cellulose and hemicellulose was observed at higher temperatures and for longer pretreatment times for Eucalyptus grandis [32]. For the acid hydrolysis of loblolly pine in [BMIM][Cl], the yield of monosac­charides reached a maximum after 2 and 0.5 h of pretreatment at 120 and 150°C, respectively [53]. Similarly, the yield of reducing sugars after hydrolysis of corn stalk in [BMIM][Cl] with HCl reached a maximum for an incubation time of 30 min at 100°C [6]. High performance liquid chromatography (HPLC) of resi­dues from the acid-catalyzed pretreatment of loblolly pine in [BMIM][Cl] showed that the monosaccharides from biomass reacted by dehydration to form other compounds, such as 5-hydroxymethylfurfural and furfural [53]. 31P NMR spectra of the recycled IL after pretreatment of Eucalyptus grandis exhibited signatures from 5-hydroxymethylfurfural, acetol, 2-methoxy-4-methylphenol, catechol, and acetic acid [32]. Fourier-transform infrared (FTIR) spectroscopy of corn stalk after pretreatment in [BMIM][Cl] with sulfuric acid showed the functionalization of lignin with sulfonic groups [6]. The generation of these by-products reduces the total reducing sugar yield, can affect the enzymatic hydrolysis of the remaining cellulose and complicate the recycling of the IL.

The field of nanocatalysis (the use of nanoparticles to catalyze reactions) has undergone an explosive growth during the past decade, both in homogeneous and heterogeneous catalysts. Since nanoparticles have a large surface-to-volume ratio compared to bulk materials, they are attractive candidates for use as catalysts. Nanoparticles of metals, semiconductors, oxides and other compounds have been widely used for important chemical reactions.

In recent years, nanomaterials have attracted extensive interest for their unique properties in various fields (such as catalytic, electronic and magnetic properties) in comparison with their bulk counterparts. In view of biomass conversion, nanocatalysts come into view as one of the most promising additives to make fuel combustion complete and fast, decrease ignition time, and therefore produce little or non-toxic by-products. In fact, the large surface areas of nanoscale catalysts as well as reports on novel chemical reactivity of particles with nanometer dimen­sions make these materials highly interesting.

Only limited studies are available in the open literature for the application of nano metal oxides in biomass pyrolysis/gasification [34, 35]. Regarding increased relative surface area of the nanomaterials, it is highly expected that nanocatalysts would have a better catalytic activity in enhancing the performance of biomass gasification/pyrolysis. Gokdai et al. found that variation in pyrolysis temperature had a distinct effect on gas evolution in the presence of nano SnO2 particles [36]. The maximum gas yield in this study was obtained by nano SnO2—hazelnut shell interaction at 700°C, while the pyrolytic oil yield obtained by nano SnO2 at 700°C reached its minimum value compared to the other catalysts used. This behavior of nano SnO2 can be explained by accelerated primary and secondary decomposition reactions of hazelnut shell in the presence of nano SnO2 due to the size (3-4 nm) and larger external surface area of the nanoparticles as given by Li et al. [34]. This behavior of nano SnO2 can also be seen by the comparison of the yields obtained by bulk SnO2. In view of the gaseous products generated, nano SnO2 showed better performance at higher temperatures among the catalysts used.

Li et al. prepared nano NiO and tested its activity during biomass pyrolysis using a thermogravimetric analyzer [34]. Lu et al. investigated that nano TiO2 and its modified catalysts were used for experiments and confirmed to have some good catalytic activities [37]. In this study, six nano metal oxides were used as catalysts to test whether they had the capability to upgrade the fuel properties of bio-oil or maximize the formation of some valuable chemicals. The experiments were performed using an analytical Py-GC/MS instrument which allows direct analysis of the pyrolytic products. The catalytic and non-catalytic products were compared to reveal the catalytic capabilities of these catalysts.

Among the six nano metal oxides, CaO was the most effective catalyst in altering the pyrolytic products. It reduced most of the heavy products (anhyd — rosugars and phenols), and eliminated the acids, while it increased the formation of hydrocarbons and cyclopentanones. Moreover, it increased four light products (acetaldehyde, acetone, 2-butanone and methanol) greatly, which made the catalytic bio-oil a possible raw material for the recovery of these products. ZnO was a mild catalyst because it only slightly altered the distribution of the pyrolytic products. With regard to the other catalysts, they all reduced the linear aldehydes, while they increased the methanol, linear ketones, phenols and cyclopentanones levels. They also reduced the anhydrosugars remarkably, except for NiO. Moreover, the catalysis by Fe2O3 was capable of forming various hydrocarbons, but with several PAHs. These catalytic effects suggested a potential for bio-oil quality improvement, due to the enhanced stability promotion due to the reduced aldehyde levels and increased methanol, and the heating value increase by the formation of cyclopentanones and hydrocarbons. In addition, the increased phenol content after catalysis enabled the recovery of the valuable phenols from the catalytic bio-oils. However, none of these catalysts except CaO were able to greatly reduce the acids, which could be a problem for the use of catalytic bio-oils as liquid fuels.

5.2 Conclusion

The sharp increase in the worldwide oil prices will play an important role in the realization of alternative, renewable energy systems such as bio-oil production, syngas generation from biomass in which the types of catalysts play an important role. Although catalytic behaviors of catalysts differing in acid/base properties, metal (Ni, Pt, etc.) content and porous structure on thermal biomass conversion are widely known, it is needed to develop new types of catalysis for biomass conversion in order to improve the quality of products. Nanoparticles with increased surface area are attractive candidates for such applications.