Review articlePorous nanomaterials as green catalyst for the conversion of biomass to bioenergy
Introduction
In past few decades technological advancement of the mankind is mostly dependent on oil as it is the primary source of energy. With the exponential economical growth of the developed and developing countries like India and China, the demand is likely to be increased more and more in the forthcoming years. Rapid utilization of natural fossil fuels as a greater part of energy sources at the advent of massive civilization and industrialization resulted depletion of these reserves [1], [2]. A new recyclable and renewable resource needs to be focused to balance this crisis. Increased CO2 emission from the natural resource also found to be a major contributing factor of global warming and this has a devastating effect on earth’s eco-system. Owing to these serious concerns a constant effort has been devoted by the researchers to seek for an alternative source of energy in the near future [3]. In recent times, there is an increased interest for the biobased chemicals as non-conventional energy resources such as carbohydrates, non-food biomass, ligonocellulosic compounds, and bioethanol. High oxygen content in the molecular structure of carbohydrates is a limitation in this context [4]. Oxygen content can be lowered by using three main pathways. Firstly, by the removal of the highly oxidized carbon molecules such as CO2, formic acid or formaldehyde. Formation of ethanol, butanol and CO2 is an example of fermentative conversion of carbohydrates. Removal of oxygen from the molecule by hydrogenolysis is another method which typically removes oxygen to form water combining with one molecule of hydrogen. Typical example of hydrogenolysis is one-pot conversion of cellulose into polyols, which are very important intermediates for the production of perfumes, beer, polyesters, polyethers, polyurethanes, pharmaceuticals, etc. [5]. The third option is the dehydration of carbohydrates to furans and levulinic acid (LA). Although bioethanol serves as a fuel supplement when mixed with gasoline, it can act as a long term renewable fossil fuel alternative. In large quantities it is currently produced from grains such as corn and this is a major concern as this directly competes with the food supply [6]. Although bioethanol can be produced from biomass sources like straw and cob in good yields but its relatively low energy density (18.2 MJ/L) and high CO2 release on ignition, which make its ecologically less attractive. To produce more economical and sustainable alternative with lesser drawbacks lignocellulosic compounds have shown promising results for future perspectives [7]. In 1951, Newth et al. first published an article on furan production from carbohydrate [8]. Since then researcher’s interest for the production of bioenergy from biomass through catalytic processes has grown gradually [9]. In 1980s HMF production from carbohydrates were mainly based on aqueous mineral acid catalyzed systems. The history of HMF synthesis and its real field application have been reported by Lewkowski’s furan chemistry review in 2001 [10]. Ionic liquids are used as eco-friendly solvents by Lima [11] and Stark [12], and immobilized on silica supports [13] for selective sugar dehydration. Many researchers experimented HMF as an introductory compound to produce highly demanding chemicals such as promising next generation polyester building block monomers (2,5-furandicarboxylic acid (FDCA) [14], [15], 2,5-bis(hydroxymethyl)furan (BHMF) [16], [17], 2,5-bis(hydroxymethyl)tetrahydrofuran (BHMTF)) [18], [19] and potential biofuel candidates (2,5 dimethylfuran (DMF) [20], [21], 5-ethoxymethylfurfural (EMF) [22], ethyl levulinate (EL) [23] and γ-valerolactone (GVL)) [24] (Fig. 1) directly from biomass via green catalytic processes.
Lignocellulosic compounds have versatile uses and they have abundant supply mainly from agricultural industry and paper producing plants. The major constituents of these compounds are 40–50% cellulose, 16–33% hemicelluloses and 15–30% lignin, and these are available in several industrial waste streams [25], [26], [27]. As, cellulose is the primary component, it has gained more attraction for the biomass conversion processes. Several methods are implicated for the hydrolysis of cellulose using various types of catalysts and solvents such as mesoporous carbon functionalized with metal [28], [29], [30] or acid groups [31], ionic liquids [32], [33], supercritical water [34], [35], and sulfonated ion exchange resins [36]. Degradation of cellulose breaking β-1,4-glycosidic bonds is a complicated procedure. To overcome this complicacy ionic liquids have been introduced to obtain a homogeneous solution prior to hydrolysis. Cellulose hydrolysis under some simplified condition can also yield high amount of glucose though it has several limitation like large portion of unreacted cellulose and separation of glucose from the homogeneous solution. Due to complex hydrogen bonded chemical structure present in the lignocellulosic compound requires various pre-treatment before enzymatic hydrolysis, has very narrow cost effectiveness. Using bifunctional solid catalyst Pt/γ-Al2O3 direct conversion of cellulose to sugar alcohols is possible up to a certain extent [37]. Further, conversion of amorphous cellulose into glucose using sulphonated activated carbon has been demonstrated by Onda et al. with considerably high product yield [38]. Moreover, several nanocomposite materials with variable density are designed to hydrolyse the cellulose, fails to show promising results due to leaking of polycyclic aromatic hydrocarbon containing 
SO3H groups [39], [40]. As study progressed, it is possible to produce a high yield of glucose from biomass employing sulphonated mesoporous carbon [41]. Hence a detailed research has been devoted to design solid acid catalyst with substantial surface modification utilizing acidic functionalized groups to increase the product selectivity as well as efficiency for biomass degradation.
On the other hand, the application of Lewis acidic Sn-β zeolite along with aqueous HCl can convert glucose to 5-HMF at 180 °C in a biphasic system with approximately 60% HMF selectivity, though corrosiveness of HCl is a limiting factor in the context of green chemical pathway [42]. Replacing the aqueous phase by N,N-dimethylformamide, solid acid resin and solid base hydrotalcites in a single reactor system is also an effective procedure for this biomass conversion process [43].
However, often the expensive ionic liquids (IL) are used as solvents and in those cases the recovery of HMF from high boiling ionic liquid is quite troublesome. Water, formed during dehydration reaction often deactivates the ILs and high boiling point solvents like DMSO and DMF are unable to resolve the drawbacks related to the separation issues over different bi-functional acid catalysts [44]. High concentration of oligomeric compounds generated as by-products in the organic solvent mediated dehydration. Therefore these methods are neither suitable nor cost effective for the large scale production of HMF. Due to the aforesaid drawbacks of monophasic solvent system using high boiling point organic system more systematic research efforts are concentrated to utilize biphasic solvent for HMF production [45]. In this biphasic solvent system alkylphenol solvents have been used, which can selectively partition furanic compounds from acidic aqueous solutions of mineral acids in the presence of homogeneous metal catalysts. Here the organic part acts as a separating unit and HMF is generated subsequently. This removes the separation related problem caused in the former method, efficiently recycle the aqueous phase and due to the use of heterogeneous catalysts it can be reutilized for the next reaction. A wide range of homogeneous and heterogeneous catalysts can work efficiently under biphasic reaction conditions using water/tetrahydrofuran (THF) solvent mixture to yield HMF like FePO4 [46], zirconium containing mesoporous MCM-41 silica [47], where surface acidity plays major role in this dehydration reaction. In the latter case the biphasic medium is composed of water/methylisobutylketone solvents and level of zirconium doping in the 2D-hexagonal mesoporous silica framework controlled the catalytic efficiency of the material. FePO4 can act as partly ‘soluble’ homogeneous acid catalyst at higher reaction temperature and it showed higher activity for the biphasic solvent systems than with water alone [46].
Another determining factor to measure the productiveness of biphasic solvents is partition coefficient, which is the ratio of HMF in organic phase to that of the aqueous phase. Higher the ratio denotes more effective extraction and increases HMF selectivity. The nature of the organic solvents along with the presence of inorganic salts is additional factor that determines the outcome of these processes. For an example sodium chloride (NaCl) in aqueous phase also acts to improve the partition coefficient of HMF [48]. A wide range of primary and secondary alcohols, ketones, and cyclic ethers in the C3-C6 range are employed as extracting solvents by Román-Leshkov and Dumesic in the biphasic systems saturated with inorganic salts like NaCl. They have observed that among these biphasic systems bearing C4 solvents like THF showed the highest HMF yields. However, sodium chloride is often harmful to plants on disposal. Thus, potassium, zinc and phosphate salts may be utilized in this context to enhance greenness in the process [49]. Using biphasic reaction it is therefore possible to receive high HMF yield without any unwanted by-products, which is also cost effective and this involves simple extraction method. So, current research efforts are directed to the biphasic solvent systems to provide highly selective HMF synthesis that can be used for commercial purpose [50]. Subsequently, numerous journals articles have been published in past few years, which documented these beneficial effects of biphasic solvent systems for the catalytic biomass conversion from carbohydrates and lignocellulosic compounds.
In this context, various modified solid acid catalysts have been developed with tunable pore architecture for the conversion of biomass to important platform chemical HMF. HMF is an important bio-sourced intermediate, formed from carbohydrates such as glucose or fructose and a potential feedstock for fuels and fine chemicals. Among the solid acids that are intensively studied in the recent times, the porous metal oxides, sulphonated porous organic polymers (POPs) [51], functionalized zeolites, alluminosillicates (ZSM-5), immobilized ionic-liquids and acid functionalized mesoporous silica materials recently opened a new window for biomass conversion with very little pitfalls. In the next few sections we will discuss various factors associated with this reaction descriptively and also with their limitation and future scopes as well.
Section snippets
Biomass conversions over various acid catalysts
In the recent years, considerable progress has been made to obtain biofuels [52] and polyester building block chemicals [53], [54] from HMF. HMF is an important building block because it contains two different functionality, e.g. aldehyde and hydroxyl groups which permits the various kind of chemical transformation like hydrogenolysis [55], [56], oxidation [57], [58] and condensation [59] reactions. The synthesis of HMF from cellulose and different sugar derivatives are catalyzed using several
Mesoporous silica
Mesoporous materials having large BET surface area and pore volume with tunable pore diameters are extensively used in various application areas like gas adsorption [78], catalysis [79], optical densities [80], and sensing [81]. Materials with functionalized pores bearing organic groups, e.g. SO3H, COOH and uniformly distributed pore size are important contenders within the mesoporous family of materials. Among the mesoporous silica material SBA-15 materials are more acceptable in comparison to
Bifunctional catalyst
For initiating serial cascade reactions in the biomass conversion process it is necessary for a catalyst to have bifunctional properties like acidic and basic characters. Materials containing functionalized carbon, zirconia, titania found to have high thermal stability indeed essential for application like biomass conversion. HMF production by combining both acid and base functionalized mesoporous silica demonstrated by Peng et al. in an ionic liquid resulted approximately 0.54 mol HMF per mol
Porous metal oxides
Using soft-templates like aspartic acid [96] and salicylic acid [97] mesopores are introduced at the surface of the metal oxide materials, and resulting porous metal oxides can efficiently converts glucose and fructose to HMF. As because the surfaces of the metal oxides bearing hydroxyl group have a very little Brønsted acidity, the Brønsted acid strength of the resulting material is increased by sulphonation in which sulphuric acid groups with S
O bond are avidly bonded to metal atoms
Mixed oxides
Today 2,5-furandicarboxylic acid (FDCA) is regarded as an essential biomass derived chemical building block agent with high economic potential as it can replace terephthalic acid and PET (polyethylene terephthalate) manufacture. Neatu et al. have demonstrated an eco-friendly and environmentally sound method to synthesis FDCA [102]. Here a synergetic cooperation of Mn(III) and Mn(IV) along with a hematite phase allowed the efficient oxidation of FFCA to FDCA. Yang et al. illustrated a process
Porous carbon
Sulfonated carbons are much more efficacious in comparison to resin and oxide type solid acid catalysts due to preferable texture. To enhance hydrolysis of cellulose surface acidity and hydrophobicity/hydrophilicity is tuned by sulphonation under suitable conditions. Hence nanoporous carbon supported acidic sites or metal nanoparticles are efficiently utilized as catalyst for the biomass conversion utilizing this principle. Very recently Zhang et al. [105] have utilized sulphonated mesoporous
Zeolite
The petrochemical and fine industrial plants utilize zeolite crystals as a principle heterogeneous catalyst for more than four decades. Zeolites have both strong Brønsted and Lewis acid sites with properly arranged micropores, extraordinary thermal stability due to crystalline inorganic framework. However, zeolites are sensitive to hot water, which leads to limit its utilization in aqueous phase process like upgrading reactions and biomass conversion. The active acidic sites of zeolite (H-ZSM)
Sulphonated resins
Now-a-days there is a growing interest for using commercially synthesized sulphonated resin such as Amberlyst [121], EBD resins [122], and Nafion [123], [124] for the conversion of biomass to bioenergy. Polymerization of styrene followed by its sulphonation resulted the sulphonated Amberlyst-15 resin and it is the most well-known resins with rich macropores and mesopores with a very high acidity (4.0 mmol g−1). Sulphonated amberlyst-15 displayed 100% HMF yield for fructose conversion to HMF in a
Metal-organic frameworks (MOFs)
Owing to high BET surface area, modifiable micro-architecture and tunable pore diameter have made metal-organic frameworks (MOFs) very demanding in the context of heterogeneous catalyst. Hence, researchers have tried MOF derived solid acids as a heterogeneous catalyst for bio-transformation of carbohydrate to HMF. Through post-synthetic modification the functional group in the MOF containing organic ligand component adds a beneficial effect than conventional carbon materials and inorganic
Metal phosphate
Metal phosphates are a specific class of catalyst, showing very promising results for various dehydration reactions [134], [135]. Lewis acidity of the phosphate-immobilized anatase TiO2 (phosphate/TiO2) has been employed by Hara et al. for the selective conversion of glucose to HMF [136]. When NbPO, AlPO, TiPO and ZrPO were applied in glucose dehydration to HMF in aqueous phase, they showed high catalytic activity and their activity largely depends on the amount of strong acid sites present in
Future perspectives and conclusion
Although in recent times extensive fundamental research and experiments are carried out to optimize the HMF yield, commercial HMF production is still a challenge on economic point of view. The major limitation in this context is the absence of an effective and scalable system for dehydration of carbohydrates from glucose and glucose based polysaccharides. Despite high yields of HMF obtained by using ionic liquid or high boiling point polar aprotic medium with chromium catalyst, these systems
Acknowledgements
PB thanks CSIR, New Delhi for a senior research fellowship. AB wishes to thank DST, New Delhi for instrumental facilities through DST Unit on Nanoscience, DST-SERB and DST-UKIERI project grants.
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