Ethanol is mainly used as a substitute for imported oil in order to reduce their dependence on imported energy supplies. The substantial gains made in fermen­tation technologies now make the production of ethanol for use as a petroleum substitute and fuel enhancer, both economically competitive (given certain assumptions) and environmentally beneficial. The most commonly used feedstock in developing countries is sugarcane, due to its high productivity when supplied with sufficient water. Where water availability is limited, sweet sorghum or cassava may become the preferred feedstocks. Other advantages of sugarcane feedstock include the high residue energy potential and modern management prac­tices which make sustainable and environmentally benign production possible while at the same time allowing continued production of sugar. Other feedstocks include saccharide-rich sugarbeet, and carbohydrate-rich potatoes, wheat and maize.

Ethanol fermentation, also referred to as alcoholic fermentation, is a biological process in which sugars such as glucose, fructose and sucrose are converted into cellular energy and thereby produce ethanol and carbon dioxide as metabolic waste products. Because yeasts perform this process in the absence of oxygen, ethanol fermentation is classified as anaerobic. Ethanol fermentation occurs in the production of alcoholic beverages and ethanol fuel, and in the rising of bread dough.

Typically, sugars are extracted from the biomass feedstock by crushing and washing (or in the case of starchy feedstocks like corn, by breakdown of starch to sugars). The sugar syrup is then mixed with yeast and kept warm, so that the yeast breaks down the sugars into ethanol. However, the fermented product is only about 10% ethanol, so a further stage of distillation is required to concentrate the ethanol to 95%. If the ethanol is intended for blending with gasoline, a “dehydration” phase may be required to make 100% pure ethanol. In the near future, ethanol may be made from cellulose, again by breakdown into sugars for fermentation. Cellulose is widely and cheaply available from many other biomass feedstocks, energy crops, agricultural and forestry residues [11].

One of the most promising fermentation technologies to be identified recently is the ‘‘Biostil’’ process which uses centrifugal yeast reclamation, and continuous evaporative removal of the ethanol. This allows the fermentation medium to be continuously sterilized and minimizes water use. The Biostil process markedly lowers the production of stillage, while the non-stop nature of the fermentation process allows substrate concentrations to be constantly kept at optimal levels and therefore fermentation efficiency is maximized. Improved varieties of yeast, pro­duced through clonal selection techniques have also raised the tolerance levels of the yeast to alcohol concentrations, again improving efficiency.

Ethanol or ethyl alcohol, CH3CH2OH, has been described as one of the most exotic synthetic oxygen-containing organic chemicals because of its unique combination of properties as a solvent, a germicide, a beverage, an antifreeze, a fuel, a depressant and especially because of its versatility as a chemical inter­mediate for other organic chemicals.

A great number of bacteria are capable of ethanol formation. Many of these microorganisms, however, generate multiple end products in addition to ethyl alcohol. These include other alcohols (butanol, isopropylalcohol, 2, 3-butanediol), organic acid (acetic acid, formic acid, and lactic acids), polyols (arabitol, glycerol and xylitol), ketones (acetone) or various gases (methane, carbon dioxide, hydrogen). Many bacteria (i. e. Enterobacteriaceas, Spirochaeta, Bacteroides, etc.) metabolize glucose by the Embden-Meyerhof pathway. Briefly, this path utilizes 1 mol of glucose to yield 2 mol of pyruvate which are then decarboxylated to acetaldehyde and reduced to ethanol. Besides that the Entner-Doudoroff pathway is an additional means of glucose consumption in many bacteria.

The organisms of primary interest to industrial operations in fermentation of ethanol include Saccharomyces cerevisiae, S. uvarum, Schizosaccharomyces pombe and Kluyueromyces sp. Yeast, under anaerobic conditions, metabolize glucose to ethanol primarily by way of the Embden-Meyerhof pathway. The overall net reaction involves the production of 2 mol each of ethanol, but the yield attained in practical fermentations however does not usually exceed 90-95% in theory. This is partly due to the requirement for some nutrient to be utilized in the synthesis of new biomass and other cell maintenance related reactions.

A small concentration of oxygen must be provided to the fermenting yeast as it is a necessary component in the biosynthesis of polyunsaturated fats and lipids. Typical amounts of O2 maintained in the broth are 0.05-0.10 mm Hg oxygen tension. Yeast is highly susceptible to ethanol inhibition. Concentration of 1-2% (w/v) is sufficient to retard microbial growth and at 10% (w/v) alcohol, the growth rate of the organism is nearly halted.

Based on a capital cost of $2.50-3.00 per U. S. gallon of annual capacity (for production plants of around 50 million gallons/year), the fixed costs are about 60 cents/gallon. Operating costs are expected to be about 35 cents/gallon and feedstock costs in the range of 30-50 cents/gallon. Assuming an electricity co­product credit equivalent to 10-15 cents/gallon, total costs could range from about $1.10 to 1.35/gallon. Currently, ethanol is produced from corn, and sells for around $1.20-1.50/gallon. Other options for producing ethanol, such as with thermal gasification instead of biological breakdown of cellulose, might reduce the cost further. Costs are also expected to decline over time with improvements in technology and operating experience.

The bioconversion of biomass to mixed alcohol fuels can be accomplished using the MixAlco process. Through bioconversion of biomass to a mixed alcohol, more energy from the biomass will end up as liquid fuels than in converting biomass into ethanol by yeast fermentation. The process involves a biological/ chemical method for converting any biodegradable material (e. g., urban wastes, such as municipal solid waste, biodegradable, and sewage sludge, agricultural residues such as corn stover, sugarcane bagasse, cotton gin trash, manure) into useful chemicals, such as carboxylic acids (e. g., acetic, propionic, butyric acid), ketones (e. g., acetone, methyl ethyl ketone, diethyl ketone) and biofuels, such as a mixture of primary alcohols (e. g., ethanol, propanol, n-butanol) and/or a mixture of secondary alcohols (e. g., isopropanol, 2-butanol, 3-pentanol). Because of the many products that can be economically produced, this process is a true biorefinery.

There have been attempts to describe the variety of solvation interactions, in which ILs are involved (for example: hydrogen bonding, dispersive, dipolar, ionic), by a set of empirical parameters that could be used to predict reaction products, yields, kinetics, and solubility [37,38,93-105]. The empirical parameters are determined by mixing the IL with a dye or a probe molecule. The interactions of the IL with the dye/ probe are then characterized by absorption spectroscopy [37, 38, 87, 97-101] or gas chromatography [93, 94, 96].

The set of solvent polarity parameters developed by Kamlet and Taft [102-105] has been widely used to predict cellulose and biomass solubility in IL [25, 37, 38, 87, 106-108]. The three parameters a, b, and p* characterize the IL hydrogen — bond acidity (ability to donate hydrogen bonds), hydrogen-bond basicity (abil­ity to accept), and polarity, respectively [102-104]. They are measured by absorption spectroscopy with mixtures of IL with three different solvatochromic dyes: (2,6-dichloro-4-(2,4,6-triphenyl-1-pyridinio)phenolate, 4-nitroaniline, and N, N-diethyl-4-nitroaniline [37, 87, 107].

The parameter a depends mostly on the IL cation. All three protons in the imidazolium cation can form hydrogen bonds, giving the cation a values between 0.45 and 0.63 [37, 49, 107]. Ammonium cations can have higher a values than the imidazolium cation (1.10) [107]. As for the parameter b, it depends mainly on the anion. The parameter b was measured for a series of ILs with the [BMIM] cation: it was highest for the acetate anion (1.20) [37], followed by the anions dimethylphosphate (1.12) [37], chloride (0.83) [37], dicyanamide (0.60) [37], trifluoromethanesulfonate (0.48) [37], tetrafluoroborate (0.38) [101], hexafluorophosphate (0.21) [101], and hexafluoroantimonate (0.15) [107].

In general, ILs with high hydrogen-bond basicity were best suited for cel­lulose dissolution. They usually have an anion with high basicity, such as chloride, acetate, formate, and diethylphosphate [25, 37, 38, 106, 108]. ILs with bromide, biscyanamide or thiocyanate anions have lower hydrogen-bond basi­city. Cellulose swells in these ILs but are only partially dissolved [37, 106]. ILs with tetrafluoroborate, hexafluorophosphate, and trifluoromethanesulfonate anions are generally poor solvents for cellulose [37, 96, 106]. It was confirmed that cellulose is soluble in [BMIM][Cl], but not in [BMIM][BF4] and [BMIM][PF6], which can be explained by the much larger hydrogen-bond basicity of [BMIM][Cl] [96].

The predictions made from Kamlet-Taft parameters on biomass pretreatment efficiency were tested on maple wood flour (5 wt%) with the IL/wood mixture heated at 90°C. Two ILs, [BMIM][OAc] (b = 1.18) and [BMIM][MeSO4] (b = 0.60), were included in the study. The parameter b was tuned in the range 0.60-1.18 by the addition of water and the preparation of [BMIM] [OAc]/[BMIM][MeSO4] mixtures. Upon addition of 10 wt% water, b for [BMIM][OAc] decreased from 1.18 to 0.98, while for [BMIM][MeSO4], it only decreased from 0.60 to 0.57. After IL pretreatment and enzymatic hydrolysis, the glucose yield, xylose yield, crystallinity, and lignin extracted from wood was measured as a function of the IL parameter b. The yield of glucose and xylose released increased linearly with b. The lignin extraction efficiency and crystallinity of the pretreated wood were stable for b < 0.84. For b > 0.84, the lignin extraction efficiency increased with b and the crystallinity decreased dramatically with b [38].

For fair comparison of the catalytic deoxygenation methods of fatty acids, the criteria which reveal usefulness of the process have to be stated. The process was mainly developed for production of fuels from renewable feedstocks, which should in the nearest future replace, partially or totally, production of the diesel fuels from oil. Therefore, the product of the catalytic deoxygenation should be compatible with the specifications for diesel fuels. Moreover, costs of production should be considered.

The selectivity of the process is important for obtaining good grade fuel. The fuel from HDO and decarboxylation/decarbonylation processes has a high cetane number which makes them desirable product for implementation in diesel engines. The only problem with long-chain hydrocarbons can be poor low temperature properties which, however, can be diminished by, e. g., skeleton isomerization. Fuels from HDO and decarboxylation/decarbonylation processes have a minor amount of aromatics which are not desired in the diesel fuel.

Regarding catalytic cracking of fatty acids, the fuel obtained via this method has good low temperature properties. However, the cetane number of such fuel is relatively lower than the corresponding one for fuel obtained via HDO and decarboxylation/decarbonylation processes. The aromatics content is high using conventional zeolites and decrease with MgO — and Cs-based catalysts.

The carbon efficiency of HDO and decarboxylation/decarbonylation reaction is very high. Based on the stoichiometry of the reaction, for the HDO process, the carbon efficiency can reach 98% (with assumption of 70% selectivity toward hydrogenation of carboxylic group at 100% of conversion [20]) for fatty acids and 93% for triglycerides. For the decarboxylation/decarbonylation process, the carbon efficiency, as well based on stoichiometry of the reaction (at conversion of 100%), can reach 94% for fatty acids and 89% for triglycerides.

Carbon efficiency for catalytic cracking is low (assuming the product is used as a transportation diesel fuel). It is very difficult to state how much carbon from the feedstock can be used as a fuel because different cracking catalysts give different product distribution. Commonly used values of elemental carbon content of the product are given in some of the papers describing cracking of fatty acids; how­ever, those values are not representative because they do not take into account structure of the created compounds, from which some of them (e. g. aromatics) cannot be used as a diesel fuel. Similar misunderstanding comes with elemental oxygen content. It is a measure of deoxygenation of the feedstock, but it does not indicate in which form oxygen is present in the product mixture, which could influence fuel combustion properties. Therefore, in the case of catalytic cracking, when product distribution is broad and multiple side reactions occur, it is not wise to use carbon and oxygen content as a conversion and efficiency measure.

The process conditions from the point of implementation of the process on industrial scale have to be mild to decrease the cost of production, but still high enough to maintain good activity and selectivity. The optimum temperature ranges proposed in the deoxygenation of fatty acids and theirs derivatives are 250-350°C, 300-350°C, and 400-500°C for HDO, decarboxylation/decarbonylation process, and catalytic cracking, respectively. The milder temperatures of the HDO and decarboxylation/decarbonylation process show their superiority over catalytic cracking.

The pressure of the catalytic cracking can be as low as 0.1 MPa, whereas HDO and decarboxylation/decarbonylation processes are operating at elevated pressures. The HDO process was applied at different pressure, from 3.5 to 8 MPa. However, even at commonly used hydrogen pressure of 3.5 MPa the conversion and selectivity is high [20]. For decarboxylation/decarbonylation process the operating pressure was tested in between 1.5 and 2.7 MPa, with commonly used 1.5-2 MPa of argon or 5% hydrogen in argon mixture. Moreover, in HDO process increase of hydrogen pressure as well as decrease of fatty acid partial pressure results in conversion increase, whereas in decarboxylation/decarbonylation processes con­version was not affected by hydrogen pressure and passes through a maximum as a function of fatty acid partial pressure [44].

When comparing closer the most selective processes, i. e., HDO and decar — boxylation/decarbonylation, hydrogen consumption is an important issue. HDO reaction occurs in the presence of hydrogen high pressure which is a disadvantage because of higher process costs. The decarboxylation/decarbonylation process does not need hydrogen to occur. However, low amounts of hydrogen help in maintaining stability of the catalyst. Still, even with assumption that whole fatty acids in decarboxylation/decarbonylation process transform through decarbony — lation, the molar ratio (based on stoichiometry of stearic acid deoxygenation) of hydrogen consumption to fatty acid converted is 1, whereas in HDO process is 1.5-2.1.

The second disadvantage of HDO process is a need to use sulfur for maintaining activity of sulfided Ni-Mo/Al2O3 catalyst. Addition of the sulfur compounds could deteriorate fuel quality as well as generate costs for the additives, whereas in

decarboxylation/decarbonylation process no addition of sulfur is needed for cat­alyst activity.

Different deoxygenation methods are summarized and compared in Table 6.3.

6.3 Conclusions

This chapter shows possibility of deoxygenation of fatty acids and their derivatives from different feedstocks. Three different processes are described with in-depth analysis of their advantages and disadvantages for industrial application. Two of those processes, HDO and decarboxylation/decarbonylation, can be pointed out as promising for production of high grade diesel fuel from renewable sources. However, decarboxylation/decarbonylation over Pd/C catalyst does not need either utilization of hydrogen or addition of sulfur-containing compounds, which makes it more suitable for production of diesel fuels.

The catalytic deoxygenation process is already applied industrially (NExBTL). However, conventional diesel cannot be totally replaced by renewable diesel fuel, because natural oils feedstock is not sufficient enough to produce comparable amount of diesel fuel. The only plausible feedstock capable for production of oils in this quantity, without threatening food production, can be algae. With the

current extensive research on harvesting of algae and development of deoxygen­ation process, there is a strong possibility that after 20 years or even earlier transportation fuel based on algae and produced by deoxygenation will be widely applied.

It is the most abundant material on earth. About 50% of the CO2 fixed photo­synthetically is stored in the form of cellulose [43] as a result of the total photosynthetic activity [165]. The cereal straw contains 30-40% cellulose while in cotton, flex, etc. the contents are as high as 98%. This form of carbon if recycled can meet the future needs of food energy. Being highly resistant to acid hydrolysis, the recycling process is not without problems. Microorganisms play a pivotal role in recycling of cellulosic carbon. The higher eukaryotes are unable to hydrolyze this polymer. However, the ruminants do so with the help of intestinal microbes.

Cellulose is a homopolysaccharide of D-glucose units joined in a linear fashion through b-1,4-glycosidic linkage (chain length 1.5 x 104 glucose units). The cellulose molecules are joined to each other through hydrogen bonds and van der wall forces. The cellulose is insoluble in water and does not give characteristic color with iodine. There is a large number of microorganisms including bacteria and fungi which are capable of breaking down cellulose into monosaccharides either aerobically or anaerobically. The anaerobic bacteria include Bacteroids cellulosolvents, Bacillus sp., C. cellulolyticum, C. cellulovorans, Cellvibrio gilvus, Candida lusitance, etc. The fermentation of cellulose yields a variety of products, e. g., ethanol, lactate, acetate, butyrate, H2, CO2 , etc. Due to its water insoluble nature and impermeability to cell wall, the hydrolytic degradation of cellulose occurs through extracellular secretion of enzymes. A single enzyme cannot accomplish the task of cellulose hydrolysis and requires multiple enzymes.

As shown in Fig. 9.7, the saccharification of cellulosic material to glucose involves three types of enzymes: (i) endo-b-1, 4 glucosidase, (ii) exo-cellobio — hydrolase, and (iii) b-glucosidase. The activities of both endo-glucanase and exo — cellobiohydrolase are regulated by cellulose through feedback inhibition. The action of b-glucosidase removes cellobiose by hydrolyzing it to glucose that allows the cellulolytic enzymes to function more efficiently. However, b-gluco — sidase is sensitive to inhibition by its substrate as well as product. A high glucose tolerant b-glucosidase from Candida sp. [158] has been purified as efforts to tap cellulosic biomass to form glucose and its subsequent fermentation to ethanol.

Carbonization is a slow pyrolysis process where biomass is heated slowly to temperatures of around 400°C in the absence of oxygen and maintained for several days. The long heating duration allows adequate time for condensable vapor to be converted into char and non-condensable gases. It is the oldest form of pyrolysis and is generally used as a cheap and inexpensive alternative for making charcoal. Tar, pyroligenous acid, and combustible gases are the by-products of this process. As in torrefaction, here also, dehydration and depolymerization of hemicellulose takes place. The breaking and reforming of intermolecular and intramolecular bonds result in formation of high molecular weight and low molecular weight compounds, where the low molecular weight fragments are cracked into liquid and gaseous products and the high molecular weight fragments formed as a result of re­bonding, char together to form the solid char product. The product distribution depends on the size of feed, heating rate, and temperature at which the process is maintained. The yield of liquid (tar) increases with decrease in size of the feed and a simultaneous increase in the rate of heating. The general scheme of the car­bonization process (the Modified Broide-Shafizadeh scheme), and an example of a continuous carbonization process of pine bark and sawdust (the Tech-Air pyrolysis system) is described in the Asian Biomass Handbook [8]. The predominant product of the carbonization process, i. e., charcoal, besides being used as a solid fuel for cooking, has recently acquired new uses such as use of charcoal for soil improvement. The by-products of the process, i. e., the pyroligenous acid has applications in agriculture and as a deodorant. The gas fraction can be used as a supplementary fuel for the process.

Biomass can be an important energy source to create a more sustainable society. However, nature has created a large diversity of biomass with varying specifica­tions. In order to create highly efficient biomass-to-energy chains, torrefaction of biomass in combination with densification (pelletization/briquetting), is a prom­ising step to overcome logistic economics in large scale green energy solutions. Torrefaction of biomass can be described as a mild form of pyrolysis at temper­atures typically ranging between 200 and 320°C. During torrefaction the biomass properties are changed to obtain a much better fuel quality for combustion and gasification applications. Torrefaction combined with densification leads to a very energy dense fuel carrier of 20-25 GJ/ton [4].

Torrefaction is a thermochemical treatment of biomass at 200-320°C. It is carried out under atmospheric conditions and in the absence of oxygen. During the process, the water contained in the biomass as well as superfluous volatiles are removed, and the biopolymers (cellulose, hemicellulose and lignin) partly decompose giving off various types of volatiles. The final product is the remaining solid, dry, blackened material which is referred to as ‘‘torrefied biomass’’ or ‘‘bio-coal’’.

During the process, the biomass loses typically 20% of its mass (dry bone basis), while only 10% of the energy content in the biomass is lost. This energy (the volatiles) can be used as a heating fuel for the torrefaction process. After the biomass is torrefied it can be densified, usually into briquettes or pellets using conventional densification equipment, to further increase the density of the material and to improve its hydrophobic properties. With regard to brewing and food products, torrefication occurs when a cereal (barley, maize, oats, wheat, etc.) is cooked at high temperature to gelatinize the starch endosperm creating the expansion of the grain and creating a puffed appearance. The cereal can then be used whole or flaked. In brewing, the use of small quantities of torrefied wheat or barley in the mashing process aids in head retention and clings to the glass. Additionally, torrefied cereals are generally less expensive than equal amounts of malted products.

Torrefied and densified biomass has several advantages which makes it a competitive option compared to conventional biomass (wood) pellets:

• Higher energy density.

• Energy density of 18-20 GJ/m3 compared to 10-11 GJ/m3 driving a 40-50% reduction in transportation costs.

• More homogeneous composition.

Torrefied biomass can be produced from a wide variety of raw biomass feed­stocks while yielding similar product properties. The main reason for this is that about all biomass are built from the same polymers (lignocelluloses). In general (woody and herbaceous) biomass consists of three main polymeric structures: cellulose, hemicellulose and lignin. Together, these are called lignocelluloses. Torrefaction of biomass leads to improved grindability of biomass. This leads to more efficient co-firing in existing coal-fired power stations or entrained-flow gasification for the production of chemicals and transportation fuels.

Fischer-Tropsch process (or Fischer-Tropsch Synthesis) is a set of chemical reactions that convert a mixture of carbon monoxide and hydrogen into liquid hydrocarbons. The process, a key component of gas to liquids technology, pro­duces a petroleum substitute, typically from coal, natural gas or biomass for use as synthetic lubrication oil and as synthetic fuel. The F-T process has received intermittent attention as a source of low-sulfur diesel fuel and to address the supply or cost of petroleum-derived hydrocarbons.

Generally, the Fischer-Tropsch process is operated in the temperature range of 150-300°C (302-572°F). Higher temperatures lead to faster reactions and higher conversion rates but also tend to favor methane production. As a result, the temperature is usually maintained at the low to middle part of the range. Increasing the pressure leads to higher conversion rates and also favors formation of long- chained alkanes both of which are desirable. Typical pressures range from one to several tens of atmospheres. Even higher pressures would be favorable, but the benefits may not justify the additional costs of high-pressure equipment. A variety of catalysts can be used for the Fischer-Tropsch process, but the most common are the transition metals cobalt, iron and ruthenium. Nickel can also be used, but tends to favor methane formation.

Abundant lignocellulose biomass has the potential to become a sustainable source of fuels and chemicals. It needs to realize that this potential requires the eco­nomical conversion of recalcitrant lignocellulose into useful intermediates, such as sugars. With the development of biotechnology, the fermentation of sugar can lead to production of various bio-energy and value-added chemicals, such as bioethanol and biodiesel. Therefore, the development of an efficient pretreatment of biomass for monosugars production is the entry point of bio-based chemical industry. Ionic liquids have unique properties compared with conventional organic solvents. The full dissolution of cellulose and lignocellulose in ILs allows a full map of homogenous utilization of them in association with advanced catalytic and sepa­ration technologies. Bearing all of these significant progresses in our mind, from in-depth understanding of the dissolution mechanism, chemically catalytic and enzymatic hydrolysis, to in situ pretreatment-enzymatic hydrolysis, a clear path­way and potential to the production of bio-energy and chemicals from biomass in ILs has been illustrated. To take the full advantage of the opportunities afforded by ILs in biomass processing and conversion, there are still a number of challenges ahead on their potential industrial applications [77], for example:

1. The design and preparation of cheaper, non-toxic, enzyme-compatible ILs capable of dissolving cellulose, on the basis of in-depth understanding of dis­solution mechanism of cellulose in ILs;

2. Hydrolytic dynamic study of cellulose in ILs, which will provide in-depth information and knowledge for the design and development of high-efficient

catalysts;

3. Integration of sustainable energy methodologies, advanced catalytic technolo­gies, and separation technologies into the ILs platforms;

4. Development of efficient and facile separation technologies for recovery of ILs and separation of hydrolyzed sugars for downstream applications;

5. Metabolism of ILs by microorganism and gene modification of microorganism aiming to increase their tolerance to ILs.

Considering their abundance, recyclability, and biodegradability, purified cellulose substrates have a great potential in composite materials for biomedical applica­tions, tissue engineering, and sensing. The dissolution of cellulose in ILs increased their processability to facilitate their mixing with other composite components, chemical functionalization, and their extrusion to form materials with the desired shape.

Materials based on purified cellulose or cellulose composites have been developed in ionic liquids using various techniques [14, 140], including mixing multiple components with dissolved cellulose in ionic liquids [140-146], grafting different monomer units onto cellulose to create copolymers [147, 148], and dis­solving cellulose into polymerizable ionic liquids [149]. The development of these composites enhanced the mechanical properties [141-144], thermal stability [141, 148], magnetic properties [143], and solubility in dimethyl sulfoxide and dimethylformamide [147], compared to pure cellulose. Films of carbon nanotubes coated with cellulose served as effective scaffolds for the growth of HeLa cells [145]. The biocompatibility of activated charcoal was significantly enhanced with a heparin-cellulose coating deposited in ionic liquids [146]. The regeneration of cellulose after dissolution in ILs using supercritical CO2 [148] or liquid nitrogen freeze drying [149] enabled the formation of micro — and nanoporous cellulose foams, that can be used for insulation, catalysis or as scaffolds for tissue engineering.

Cellulose dissolution in ILs also enabled the immobilization of chemical reagents [150, 151], drugs [152], and enzymes on solid substrates [153, 154]. Enzymes immobilized on a solid substrate still served as effective catalysts, while their stabilization and reuse were improved by immobilization [153, 154]. The immobilization of 1-(2-pyridylazo)-2-naphthol was exploited to detect Zn, Mn and Ni ions at concentrations as low as 10-6 mol/l [150]. The immobilization of calix [4] arenes on cellulose was used in nitrogen oxide NOx sensing [151].

Cellulose derivatives, including acetates, carboxymethylates, benzoylates, sul­fonates, phthalates, have been synthesized by the dissolution of cellulose in ILs followed by their chemical functionalization. These derivatives are widely used in coatings, films, membrane separation, textiles, and composites [14, 155-158]. Cellulose was directly converted to 5-hydroxymethylfurfural using CrCl2, CrCl3, or RuCl3 as a catalyst [95, 159, 160], and to hexitols using Ru nanoparticles [161]. The molecule 5-hydroxymethylfurfural is believed to be the building block for a wide variety of commodity chemicals. Its derivatives have potential applications as resins, polymers, herbicides, pharmaceuticals, plasticizers, and solvents. [95].

Hydrogen was successfully produced from glucose and cellulose in ILs using Ru as a catalyst. The use of 13C6-glucose in the reaction revealed that glucose decomposed into formic acid, which then decomposed into H2 and CO2 [162].

The second-generation bioethanol production facilities depend on lignocellulosic biomass, unlike the first-generation bioethanol plants that use corn starch or sugar. Demands on agricultural land for food production are expected to increase sig­nificantly in the coming decades and hence use of marginal land to grow and harvest the highest possible levels of biomass using plants such as switchgrass and Miscanthus will contribute significantly to ensure sustainable production of renewable fuel in the future. In order to enhance their productivity, these grasses have to be targeted for intensive research aimed at improving the biomass yield and other attempts to change the characteristics of the chemical contents (e. g., lignin, hemicellulose, cellulose).

Expanding the industry to use biomass feedstock from the agricultural and forestry waste materials and enhanced plant biomass from biofuel crops from marginal lands might be the best ways to get more bioethanol and reduce net emission of GHG. Hence, it is important to develop strategies to increase the yield of plant biomass in a unit area of marginal land, and save the arable land for food production. In this context, the following strategies can be employed to enhance biomass production and ensure a sustainable and constant supply of lignocellulosic biomass for bioethanol production.

Similar to recombinant E. coli, ethanologenic strains, K. oxytoca M5A1 was engineered with PDC/ADH from Z. mobilis for ethanol production from glucose and xylose [135]. The maximal volumetric productivity from xylose was comparable to glucose and almost twice as that previously obtained with E. coli KO11. Stabilization was achieved by chromosomal integration of the heterologous genes [40], allowing the strain to be used in hydrolysates and in simultaneous saccharification and fermentation (SSF) processes. This strain co-ferments glu­cose, arabinose, and xylose to ethanol, by this order of peference [19]; of notice is the fact that K. oxytoca is able to naturally metabolize XOS, as mentioned earlier [145].