1. Introduction
The quest of seeking alternative energy sources to replace fossil fuel began in the beginning 21st century. Economic factor is major driven, followed by environmental lately : (1) their finite supply, (2) increasing price and unexpected fluctuations, and (3) greenhouse gases emission and global warming. All these weaknesses were synthesized into huge massive interests on searching for alternative, renewable, sustainable, and economically viable fuel such as biofuel. 1st generation of biofuel (bioethanol) was produced from sugars, starches, and vegetables oil. Bioethanol can be either mixed with gasoline or used as a sole fuel using dedicated engines; moreover, it has higher heat of vaporization and provide octane number compared to gasoline. Ethanol is already blended with gasoline and supported by vehicle manufacturers have resulted in vehicles that can use up an 85% ethanol – 15% gasoline mixture. Gasoline can use bioethanol as an oxygenated fuel to increase its oxygen content, causing better hydrocarbon oxidation and diminishing greenhouse gases.[1]
High cost of raw materials, startch and sugar derived from sugar cane and maize, limited stock, and endanger food supply chain, couldn’t make this type of biofuels couldn’t sustain much longer. Second generation of biofuel replaced it soon after that, which used lignocellulosic-based materials as feedstock. This type of materials are cheap, abundant, and renewable which counter the first generation’s drawbacks.
Lignocellulosic are composed of cellulose, hemicelluloses, and lignin in an intricate structure, which is recalcitrant to decomposition.
One of the best strategies to convert such biomass into sugars is enzymatic saccharification due to its low energy requirement and less pollution caused; but, the major problem is the low accessibility of cellulose because of rigid association of cellulose with lignin. This leads to difficulties within the conversion process; therefore,
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breaking down lignin seal in order to make cellulose more accessible to enzymatic hydrolysis for conversion is one main aim of pretreatment. In other words, pretreatment is the crucial and costly unit process in converting lignocellulosic materials into fuels.
A suitable pretreatment procedures involve (1) disrupting hydrogen bonds in crystalline cellulose, (2) breaking down cross-linked matrix of hemicelluloses and lignin, and finally, (3) raising the porosity and surface area of cellulose for subsequent enzymatic hydrolysis. There are several pretreatment methods including, physical pretreatment (grinding and milling, microwave and extrusion), chemical pretreatment (alkali, acid, organosolvent, ozonolysis, and ionic liquid), physicochemical pretreatment (steam explosion, liquid hot water, ammonia fiber explosion, wet oxidation and CO2
explosion) and biological pretreatment.[1]
Figure 1 Schematic pretreatment of lignocellulosic material[1]
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1.1. Cellulose
Cellulose, the main constituent of lignocellulosic biomass, is a polysaccharide that consists of a linear chain of D-glucose linked by β-(1,4)-glycosidic bonds to each other. The cellulose strains are associated together to make cellulose fibrils. Cellulose fibers are linked by a number of intra- and intermolecular hydrogen bonds. Cellulose is insoluble in water and most organic solvents.
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Figure 2 Structure of cellulose[2]
1.2. Hemicelluloses
Hemicellulose, located in secondary cell walls, are heterogenous branched biopolymers containing pentoses (β-D-xylose, α-L-arabinose), hexoses (β-D-mannose, β-D-glucose, α-D-galactose) and/or uronic acids (α-D-glucuronic, α-D-4-O-methyl-galacturonic and α-D-α-D-4-O-methyl-galacturonic acids). They are relatively easy to hydrolyze because of their amorphous, and branched structure (with short lateral chain) as well as their lower molecular weight. In order to increase the digestability of cellulose, large amounts of hemicelluloses must be removed as they cover cellulose fibrils limiting their availability for the enzymatic hydrolysis. Hemicellulose are relatively sensitive to operation condition, therefore, parameters such as temperature and retention time must be controlled to avoid the formation of unwanted products such as furfurals and hydroxymethyl furfurals which later inhibit the fermentation process.[1]
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Figure 3 Structure of hemicelluloses[3]
1.3. Lignin
Lignin is an aromatic polymer synthesized from phenylpropanoid precursors.
The major chemical phenylpropane units of lignin consisting primarily of syringyl, guaiacyl and p-hydroxy phenol are linked together by a set of linkages to make a complicated matrix.[1]
Figure 4 Structure of lignin[4]
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1.4 Furfural
Lignocellulose biomass contains cellulose, hemicellulose, and lignin. For the past decade, attention has been focused on utilization cellulose for biofuels compared to biofuels-derived hemicellulose. Glucose can be converted to hydroxymethylfurfural (HMF), and subsequently upgraded to dimethylfuran (DMeF). Furfural (FFR) was identified as one of promising chemicals for sustainable fuels and chemicals in 21st century proposed by Bozell et al.[5]Furfural is produced by the hydrolysis and dehydration of xylan contained lignocelluloses. Large amount of furfural production began at the beginning of 1922 in USA by Quaker Oats Company. As the consequence development of furfural industry, the price reached ~$1700 per ton in 2002,[6] and
~$2000 per ton by June 2011.[7] Its properties make this heteroaromatic aldehyde as selective extractant,[8] effective fungicide,[9] effective inhibiting the growth of wheat smut through killing the fungus.[8]
Lignocellulose
Figure 5 Schematic of reductive upgrading pathway for biomass derived xylose[10]
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1.5 Hydrogenation
Hydrogenation is chemical reaction that typically encountered in fuel processing. In order to compete with commercial fuel (e.g. gasoline), furanic biofuels is needed to go through this process to increase energy density and miscibility in hydrocarbon fuels. Hydrogenation chemistry for furfural, includes hydrogenation of – CHO side chain to –CH2OH or –CH3, hydrogenation of furan ring, and its opening to pentanols, pentane-diols, and alkanes.
Figure 6 Furfural platform for biofuels[11]
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1.6 Hydrogen storage / Hydrogen-generating agent
[12]H2 gas is commonly used in hydrogenation, but in some occasions high pressure need to be avoided for safety issue. There are classes of compound that can generate hydrogen gas. The urgency of finding hydrogen generator in fuel cell research area boosted the popularity of these compounds on new application as hydrogen source.
1.6.1. Hydrogen-generator in complex metal
Complex metal hydrides (e.g. NaAlH4, LiAlH4, LiBH4) generally have the formula AxByHn, where A is an alkali metal cation and B is metal or metalloid to which the hydrogen atoms are covalently bonded. Certain binary metal hydrides such as MgH2
and AlH3 also have covalently bonded hydrogen atoms and are thus more similar to complex metal hydrides than intermetallic hydrides in hydrides are particularly promising due to their high theoretical gravimetric and volumetric hydrogen storage densities. However, they suffer from slow uptake and release kinetics, meaning that much of the stored hydrogen is not practically accessible due to the time it would take to release it.
1.6.2. Hydrogen-generator in chemical hydrides
Chemical hydrides (e.g. NaBH4, LiAlH4, NH3BH3) have high gravimetric hydrogen storage densities and release hydrogen by reaction by water. In effect, the hydrogen is stored both in the chemical hydride itself and the water. These reactions tend not to be easily reversible and the by-products must be extracted from the spent fuel mixture to be regenerated. However as the reactions can be controlled of parameters such as rate of water addition, pH, and the use of catalysts, chemical hydrides are particularly attractive for use in portable applications, where easy ―on-off‖
control is crucial.
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1.7. Furfuryl Alcohol (FFA)
Furfuryl alcohol is product hydrogenation of –CHO side chain to –CH2OH from furfural. Furfuryl alcohol is known as intermediate to various hydrogenated chemicals.
Several last decade, researches focused on production of furfuryl alcohol from furfural using numerous metals as catalysts, such as copper, nickel, platinum, palladium, and platinum oxides.[13] Since the research quite advancing in catalyst field, furfuryl alcohol could be produced not only from furfural but also from molecule xylose. Marco A.
Fraga group have successfully did the production of furfuryl alcohol from xylose using dual catalysts composed Pt/SiO2 and sulfated ZrO2.[14]
Table 1 Representative works from literature in the hydrogenation of furfural to furfuryl alcohol[15]
YFFA(%)
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