Furfuryl Alcohol a Versatile, Eco-Sustainable Compound in Perspective
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Renewable agricultural biomass derived chemicals, their modifications and uses have seen multiplicity in numerous applications and important processes with major impacts on the pursuit for eco-sustainability. Such applications range include the energy sector, chemistry, pharmacy, the textile industry, paints and coatings, plastic industry, to name but a few. This field of lignocellulosic derived chemicals interconnects several scientific disciplines ranging from agriculture, biochemistry, engineering, environmental sciences, forestry, pharmacy, medicine, etc. hence making it difficult to have a single expert view on these complicated interactions. Therefore, the idea to create a focused review, specifically, on FA (an important furanic compound) is the main objective of this article. FA and its resultant derivatives exhibits an array of capabilities and fascinating properties in various fields of applications. As a compound or with co-reactants, it finds interesting applications as base and/or intermediate chemical compound, hypergolic rocket fuels, in flame resistant composites and coatings used in aerospace, auto, and the built environment; it also finds application as mortars, cementitious grouts, impregnating materials, and sealants due to its exceptional resistance to common corrosive chemicals such as acids, alkalis and other solvents when it is cross-linked. Coupled with its environmental and economic benefits FA has proved to be a remarkable eco-sustainable bio-derived compound.
KeywordsLignocellulosic Agricultural biomass Hemicellulose Furfural Furfuryl alcohol Eco-sustainability Furan polymers
Chemicals obtained from inedible lignocellulosic agricultural biomass, has been noted to be one of the most promising environmentally benign, sustainable and industrially applicable alternatives to petroleum feedstock [1, 2, 3, 4, 5]. Hence, lignocellulosic biomass offers an enormous assortment of derivable chemical compounds capable of producing materials analogous to and even exceeding those derived from fossil chemicals [6, 7, 8, 9, 10, 11]. Available data indicate that with commensurate policies and investments to promote the use of agricultural waste residues, there are associated benefits such as considerably reduction in the dependence on fossil derived chemicals [3, 4, 10, 11], increased job opportunities in the agricultural and allied sector , and consequent impact on energy security [2, 13]. Moreover, with the increasing concerns over the climatic impact of greenhouse effect coupled with the volatility in oil prices and attendant undesirable environmental issues of fossil hydrocarbons, many scientists agree that it is exigent and timely to consider the vast opportunities offered by non-edible agricultural lignocellulosic biomass [14, 15, 16].
1.1 Physical Structure and Properties of FA
2 FA Production
2.1 Vapour Phase Process
The reaction being slightly exothermic, liberates about 60.7 kJ/mol, hence the flowing oil acts as a cooling system. The gaseous mixture of reaction products enters a condensation system comprising a packed column 7, a pump 8, and a cooler 9. The pump circulates unrefined FA through the cooler 9 and unto the packing of column 7 where it meets a countercurrent of the gaseous products. From the latter stream, most of the condensables are liquefied. The remaining portion, consisting of unreacted hydrogen and the saturation quantities of the condensables at the column temperature, is recompressed by a ROOTS pump 10 and added to the hydrogen feed to check losses. A small bleed stream prevents a build-up of impurities. The condensed portion is fed into a reboiler system consisting of tank 11, a circulation pump 12, and a heater 13 energised by steam. The vapour produced by this system enters a packed vacuum distillation column 14. The head vapour of this column is liquefied by a condenser 15 maintained at reduced pressure by a vacuum pump 16. Most of the condensate is returned to the column as reflux, while the rest represents a small head fraction consisting of 2-methyl furan, unreacted furfural, and reaction water from the 2-methyl furan formation and polymerisation effects. The sump fraction is the purified FA [23, 30].
2.2 Liquid Phase Process
The head vapours of the column are liquefied in condenser 15, the resulting distillate being partly returned to the column to effect rectification and partly collected in tank 16. This distillate is pure FA. Vacuum pump 17 maintains a reduced pressure to permit distillation at moderate temperatures [23, 30].
FA conversion proceeds at much lower temperatures and pressures compared to the liquid phase
The lower temperatures give the added advantage of reducing the quantity of other by-products formed; hence yielding a higher crude grade of FA.
Since the lower temperature impacts reduction of other by-products, it also has the advantage of consuming less furfural feedstock per approximately 0.5-kg of FA produced.
Increased lifetime of catalyst employed.
Notwithstanding, the choice of FA synthesis method from furfural is largely dependent on the economics and environmental concerns. However, both gas-phase and liquid-phase catalytic processes have been successfully shown to have their respective advantages, inclusive of better yields and ease of obtaining refined FA in a single-continuous process [5, 30, 39, 40, 41, 42, 43]. Furthermore, a life cycle assessment (LCA) and life cycle costing (LCC) on the environmental and economic impact of furfuryl alcohol production using corncobs as raw material showed that increasing electricity consumption efficiency and furfural product yield, decreasing transportation distance from corncob buyers and suppliers, choosing the suitable corncob compression technique, and optimising the wastewater reuse system were the key contributing factors that resulted in reducing the overall environmental and economic impacts of this process .
3 Nature and Chemistry of FA
FA is a predominant member of the heterocyclic furan family [45, 46]. It is classified as a primary (1°) alcohol due to the typical characteristics of having one carbon-atom bonded to a carbon atom carrying the hydroxyl group. Although FA exhibits the chemical behaviour of primary alcohols however it exhibits an atypical chemical characteristics by readily reacting with strong acids to form a complex resinous material [46, 47, 48, 49]. This peculiar ability of a supposedly primary alcohol has intrigued chemists, technologists, and scientists for decades and subsequently various attempts have been made to explain this phenomena [50, 51, 52, 53].
3.1 Polycondensation Reactions of FA
3.1.1 Complex Chemistry of FA Polycondensates
The complexity in the mechanism and products of FA polycondensates is well-known [46, 59]. The isolation of varied polyfurfuryl alcohol resins confirmed to consist of combination species such as 2-oxymethyl-5-furfuryl furan, 2-oxymethyl-5-(5ˊ-furfuryl)-furfuryl furan, di-furfuryl ether, di-2-furylmethane, formaldehyde, and levulinic acid, under acidic systems, are well documented [60, 61, 62]. Furthermore, it has been shown that polymers of FA catalysed by acids, non-acids (such as γ-alumina) or heat alone differ uniquely in chemical properties and compositions [63, 64]. Over the years, works by Krishnan et al. , Dunlop et al. , Gandini et al. , Choura et al. , and others have employed both mechanistic, theoretical, computer simulations, chemo-rheological, and kinetic studies in attempting to explain this puzzle, by either clear-cut evidences and/or tentatively. Equations 1–6 summarises the schemes and structures hitherto proposed by various studies for the polycondensates of FA.
184.108.40.206 Equation 1: Intermolecular Water Loss
220.127.116.11 Equation 2: Furan Chain Linkage
18.104.22.168 Equation 3: Ether Formation
22.214.171.124 Equation 4: Hydrolytic Cleavage of Furan Ring
126.96.36.199 Equation 5: Possible Crosslinking
It has been suggested that the crosslinking of FA polycondensates resins consists of a variant repeating-structural units and not a homogenous system as supposed. Furthermore, it was postulated that, possibly, formaldehyde is formed at certain stage which condenses with the intermediate products to form a complex polymer network [67, 71, 72].
188.8.131.52 Equation 6: Possible Crosslinking
4 Selected Applications of FA
4.1 Rocket Fuels
Furfuryl Alcohol releases about 26 MJ/kg heat combustion when it burns, hence its use as an alternative hypergolic propellant for rocket engines [23, 29, 52, 75]. Kulkarni et al. demonstrated that rocket fuel blends consisting of 3-carene, norbornadiene, FA, ethylidene norbornene, and kerosene in different weight proportions exhibited good synergistic hypergolic ignition with red fuming nitric acid as oxidiser with almost no ignition delays. They concluded that these fuel blends exhibited high combustion efficiency of over 95% with very good performance comparable to, and even exceeding existing rocket fuels; coupled with the advantages of nontoxicity, eco-friendliness, safe handling and transporting . Furthermore, Bhosale et al. showed that FA used in hypergolic ionic biofuel blend presented a low-cost, technologically promising, affordable, benign and high performance hypergolic fuel for applications in missile propulsions and satellite launch vehicles .
4.2 FA-Phenolic Binders
FA constitutes the sizeable portion in the widely used FURAN1 foundry binders, abounding mostly in patent literatures, consisting between 30–85% of total contents and generally used in three main variant combinations viz FA/UF (Urea formaldehyde), FA/PF (Phenol Formaldehyde) and FA/PF/UF system. With the added advantage of flexibility as FURAN foundry binders find applications in HOT-BOX, gas hardened processes and the traditional FURAN-NO-BAKE (FNB) system [29, 78].
4.2.1 FURAN NO-BAKE (FNB) Process
4.2.2 FURAN HOT BOX Process
Developed by the Quaker Oats company unlike the FBN system involves the application of heat (usually between 180 and 270 °C) and latent acid catalysts (such as the solutions of urea or ammonium salts of strong acids). It is usually employed in both light (such as Aluminium) and heavy (such as bronze) metal casting and is appropriate for mass production. Generally speaking, the resins employed in this process are UF resins modified with about 20–50% FA copolymers and PF resins modified with urea with the addition of small amounts of corn flour and paraffin wax to facilitate a thorough mixing of the resin with the sand (usually within the range of 1–2.5% based on sand quantity employed). The resins are properly mixed with the sand and proportionate catalyst, and then blown into a heated mould (core boxes) to initiate the curing reaction [78, 81].
4.2.3 FURAN Gas Hardened Process
Also referred to the Cold-Box process is well suited for mass moulding of small moulds and cores employing sulphur dioxide (SO2) as catalyst in a closed-air system, at room temperature, which rapidly sets the FA-phenolic resin sand mix .
4.3 Wood Preservation
“Furfurylation” of wood is a chemical process by which commercial wood properties are improved using FA as a low-viscosity modifying agent to change the wood structure and chemistry so that it becomes less susceptible to biodegradation and resistant to chemical attack. The insitu complex polymerisation process within the wood system has been known as an eco-efficient “green” alternative for the previously employed toxic and hazardous compounds such as salts of copper, chromium and arsenic [83, 84, 85, 86]. Furfurylated woods are known to be non-toxic materials suitable for internal and external applications where a high demand for performance and aesthetic characteristics are required . Lande et al. demonstrated that furfurylated wood was completely resistant to attack in areas of high termite activity . Similarly, Esteves et al. concluded that furfurylation of wood imparted hardness and improved the durability of the wood. They observed that the moisture behaviour of furfurylated wood decreased in relation to the wood equilibrium moisture content but had an increment in its dimensional stability; thereby enhancing reduction in anisotropy with no significant effect on the bending properties .
Dong et al.  in their work demonstrated a novel bio-based wood polymer nanocomposites successfully prepared from fast-growing poplar wood employing FA and nano-SiO2. They posited that SEM and FT-IR studies showed that the nano-SiO2 were incorporated in the wood and fixated on the wood cell via the effect of the polymerised FA this significantly improved the modulus of elasticity (MOR) of the wood. Furthermore, they showed that the thermal stabilisation and flame retardancy of the wood improved remarkably at 2.0% nano-SiO2 incorporation. In another study Hazarika et al.  investigated the properties of wood impregnated with melamine-formaldehyde-FA (MFFA) copolymer and montmorillionite (MMT) concluding that the wood exhibited improved higher dimensional stability, lower water uptake (%), enhanced resistance, and better mechanical properties such as flexural, tensile and hardness.
4.5 As an Industrial Solvent
Furfuryl Alcohol is a fine solvent which when used alone or in combination with other solvents finds application as a general cleaning solvent and paint softener. It also finds use as dispersant for dyes in the textile industry and finds application as solvents for many resinous materials [96, 97, 98]. Its solvent properties can easily be enhanced by slightly heating since its flashpoint is 75 °C .
4.6 Levulinic Acid
Although several attempts were reported for the production of LA from petroleum-based compounds, these approaches failed to be commercialised due to the high-cost and complex production processes involved . Hence, the industrially cost-effective method employed commercially remains the renewable lignocellulosic feedstock such as FA . It has been shown that when the conversion reaction of FA to LA is carried out in water, 80% yield was achieved; when performed in ketones (such as acetone and 2-butanone) a yield of 93% and above was achieved [101, 107].
4.7 Flavouring and Fragrances
Ethyl Maltol (hydroxyl ethyl pyrone) is a complex alcoholic heterocyclic compound which finds use as butterscotch, strawberry jammy, and brown sugar characteristic flavouring. It is another example of electrochemical reactions of FA that has been successfully converted into a commercial process for the production of flavours [111, 112].
FA resins (polymers and/or oligomers), alone or with co-reactants, find interesting applications in fields such as aerospace, scientific laboratories, and the auto industries. Due to their exceptional resistance to corrosive chemicals (e.g. acids, alkalis and other solvents), they are also used in built environment as cementitious grouts, mortars, coating, impregnating materials, and sealants [61, 72, 113, 114, 115]. These resins are also used in hospital operating floor coverings which demand low electrostatic resistivity to prevent electrostatic discharge from igniting flammable liquid substances often used in hospital environments . When reinforced with fibre-glass, a material that is resistant to corrosion and heat distortion (at elevated temperature) with low flame and smoke emission level is obtained. This material finds applications in reinforced tanks, pipes, reaction vessels, vats, and ducts [72, 117, 118, 119]. For example, Lecite® mortar, an FA resin, developed by Electro Chemical Engineering & Manufacturing, Emmaus, Pa, United States, was used in the construction of the scrubbing tower .
FA resins also find wide-industrial applications as binder matrix in various fibre-reinforced composites in the auto, aerospace and construction industries, which exhibit almost the same corresponding physical properties as those manufactured from the dominant phenol resins . Nu-Kast® pump, a product of Nukem Manufacturing United Sates, is an example of the outstanding versatility of FA resins. Cast entirely from FA monomer, this pump is light weight and compact, corrosion-proof inside and outside, great mechanical strength and resistant to severe shock with the ability to resist practically all commercial acid solutions, salts, alkalis, and organic solvents . FA resins form gap-filling glues when modified with urea yielding a material with exceptional strength. These adhesives exhibit good flexibility, resist cracking and deterioration upon aging. They also show good resistance to shrinkage under high pressure and temperature .
4.9 Polymer Concretes
FA is used to produce non-petroleum based high-quality polymer concrete with very good properties such as resistance to acid, and alkali, heat stability, faster curing time, improved strength and bonding factors when compared to Portland cement concretes . FA polymer concretes are usually employed in aggressive environments such as corrosion resistant baths, chemical resistant floorings, channel pipes and structural materials in nonferrous metallurgical plants . Muthukumar et al. demonstrated that low viscosity of FA resin used in polymer concretes resulted to low binder content with cost effective formulations hence a competitive advantage over other conventional binders such as epoxy and polyesters resins employed in production of polymer concretes .
A demonstrated water-compatible polymer concrete materials developed from FA used in rapid repaid repair systems for airport runways, in all-weather conditions, has been reported. The resulting surfaced runway was reported to exhibit commensurate durability and compressive strength of the original surface. The formulation was shown to polymerise and cure within 20-min exhibiting a compressive strength of 20 MPa. It proved to be stable even under adverse chemical conditions and withstood temperatures of up to 200 °C. Further tests on the rehabilitated road pothole slabs demonstrated that the concrete can resist high stresses under repeated loads successfully. These FA-polymer concrete could be installed in less than 30-min, under any weather condition, thereby reducing the cost of man-hours [125, 126].
4.10 Wood Adhesives
FA also finds application as a resin for wood adhesive. It has been shown that composite boards were prepared using wood powder as matrix and FA or prepolymers of FA (oligomeric systems) as binder with hydrogen peroxide/ferrous ion or nitric acid as an activator. The study demonstrated that the tensile strength and water resistance of the oligomeric systems were superior to that obtained with monomeric FA. Furthermore it was shown that the degree of polymerisation of the oligomeric FA influenced the properties of the wood composite and that the addition of the activator to the binder instead of the matrix system yielded better results, further suggesting that the activation proceeds primarily through the binder oxidation. The study further demonstrated that using of acetone-soluble fraction of pre-oxidised oligomeric FA as binder gave impressive results, and the boards exhibited a tensile strength over 50% above reference phenol/resorcinol/formaldehyde (PRF) boards .
Abdullah et al.  in their work developed an eco-friendly and formaldehyde-free wood adhesive from tannin-FA renewable materials. A more recent work on FA-aldehyde plywood adhesive resins showed that comparatively FA-glyoxal (FAG) resin showed satisfactory results for plywood composite boards. It was demonstrated that the dry strength, 24-h wet strength and 2-h boiled-water wet strength were 1.02 MPa, 1.36 MPa and 1.46 MPa respectively, which is significantly higher than the standard requirements (≥ 0.7 MPa). Furthermore since the glyoxal is non-toxic and non-volatile it demonstrates that FAG resin can be considered a more eco-friendly and sustainable alternative to the FA-formaldehyde adhesives .
4.11 Carbon-Carbon Materials
When FA resins are pyrolysed above 450 °C they yield glassy-porous carbons which has been used in mesoporous absorbent systems. At higher pyrolysis temperatures of up to 1000 °C, high-grade carbon materials are produced which are industrially employed in carbon-carbon composites materials such as brakes and clutches, rocket motors, heatshield, aero-engine components, high-grade military gears and hardware, as well as biomedical devices [70, 130, 131, 132]. FA also finds application in the production of nano-porous membranes for desalination of brackish and seawater [133, 134].
Basso et al. in their studies have shown the possibility of producing a cheap and eco-friendly formaldehyde-free rigid foams with outstanding thermal performance from FA and tannin . In a related work, Basso et al. successfully developed mixed phenolic-polyurethane-type rigid foams using tannin-furfuryl alcohol natural materials co-reacted with polymeric isocyanate which the method can be adapted for industrial continuous lines production, thus, opening up new possibilities for large-scale manufacture of these sustainable foams. The underlying technology for such tannin-based foams is on a self-blowing process with mild exothermic reaction due to the self-condensation reaction of the FA under acidic conditions thereby initiating rapid evaporation, at ambient temperature, of the organic volatile during hardening . Similarly, relative low cost furanic foams (consisting only of FA systems) exhibiting excellent thermal stability under high temperatures has been investigated. These FA foams finds interesting applications such as in foundries to bind the sand of moulds and/or cores for casting engine heads and other kind of steel tools . Tondi et al. has demonstrated the upscaling of eco-sustainable tannin foams. These bio-derived tannin systems have similar reactivity than phenol and when co-reacted with FA produces polymers suitable for a wide range of applications such as in waste water remedial . Similarly, carbon foams with improved thermal conductivity and mechanical properties were prepared from tannin-based resin and exfoliated graphite used as filler. These organic-carbon foams were first prepared by suspending exfoliated graphite in an aqueous solution of tannin, FA, formaldehyde, diethyl ether and para-toulene-4-sulphonic acid at room temperature. These carbon foams find varied applications ranging from templates for preparation of the metallic and ceramic foams currently used in industry to electrodes and insulating liners for high temperature applications up to 2500 °C .
Furthermore, Jinwoo et al. has shown that low-cost mesocellular carbon foams from FA can be used in catalysts supports, high performance adsorbent systems for bulky pollutants, and in highly efficient electrode materials . FA has also been employed in the production of environmentally benign polyols which have found applications as replacement for petroleum-based polyols in polyurethane foams .
FA was employed in the materials used by the United States space agency in their space shuttle thermal protection systems (TPS). Reinforced carbon-carbon was produced from cured graphite fabric that was impregnated with phenolic resin laid up in complex shaped moulds. After the parts were rough trimmed it was impregnated with FA and pyrolysed converting the resin polymer to carbon. The impregnation and pyrolysis is done multiple times to increase density which also resulted in improved, mechanical and flame retardant properties of these parts .
Wang et al.  reported a robust, environmental-friendly method to synthesise polymer/clay aerogel nanocomposites materials from low density FA oligomeric systems and clay. Polymer/clay aerogels find applications ranging from catalyst supports, packaging, thermal insulation, absorption and structural applications.
Graphene/titanium carbide composites were synthesised employing sol-gel infiltration and spark plasma sintering (SPS). FA was used as the polymerisable carbon source. The graphene used was casted into a sponge-like shape consisting of three-dimensional network of graphene sheet whilst the sol-gel infiltration synthesis method allowed for the formation of nano-structured ceramics inside the porous structure of the graphene networks, hence forming the composites. Titanium-carbide (TiC) composites are ultra-high temperature ceramics (UHTC) with low thermal expansivity and density (4.93 g/cm3), high melting points (3067 °C), high Vickers hardness (28–35 GPa), high Young’s modulus (410–450 GPa) and high thermal and electrical conductivity. Their investigated applications includes usage as cutting tools, refractory components, super-computers, electronic elements, in aerospace engineering and so on [145, 146].
Ebrahimi et al. has reported the preparation of FA functionalised carbon nanotube (CNT) and epoxide novolac resin composites with high char yield. The epoxidised novolac resin (ENR) composites exhibited high thermal stability and char residue. The study demonstrated that modification of oxidised CNTs with FA resulted in improved dispersion in the resin matrix .
4.14 Sundry Applications
Nobuo et al. described a process for producing diamond powder by a shock compressing method using FA as a carbon precursor . FA has also been used in the production of bio-based nanocomposites, batteries and nuclear-grade graphitic rods for use in nuclear plants [71, 149, 150].
In their work Nanaji et al.  demonstrated that utilizing FA as an alternative source of carbon precursor (for the first time) a smart, efficient and cost-effective methodology employing a modified evaporation induced self-assembly (EISA), strategy was used to synthesise mesoporous carbon (MC), which exhibited excellent textural parameters, employed in super-capacitors. They showed that the resulting carbon synthesised with the modified EISA method exhibited a higher specific surface area with large pore volume and more ordered graphitic carbon. The wettability studies demonstrated that the functionalised mesoporous carbon surface had superior hydrophilic properties as compared against the non-functionalised mesoporous carbon film surface. In a related study Gao et al.  also has developed a boron-doped mesoporous carbons (BOMCs) for making of super-capacitors. Different boron contents were prepared by nano-casting using silica KIT-6, FA and boric acid as the template, carbon, and boron sources respectively.
Furfuryl Alcohol modified melamine sponge (MS) for high-efficient oil spill clean-up and recovery has been reported by Feng et al. The FA modified MS exhibited excellent hydrophobicity, improved thermal and mechanical properties, and showed excellent oil sorption capacities (75–160 g/g for various oils or organic solvents) and better recyclability capabilities; thus proposing such FA modified MS as potential candidates for high efficient absorbents for oil-water separation. The further demonstrated that FA modified commercial MS can be synthesised using a simple non-toxic and expensive modifying agents or solution .
FA functionalised water-soluble graphene dispersions, fabricated by the exfoliation of graphite by Diels–Alder cycloaddition reaction has been reported by Zhang et al. The study demonstrated that the high-quality graphene-FA so produced exhibited no significant structural defects less than a few layers. Furthermore, positing that facile procedure so reported could be used for the synthesis of versatile functional graphene with other organic group on the surface of graphene and the hydroxyl groups of FA for a variety of applications . Furthermore, FA has been reportedly used in the manufacture of esters, synthetic fibres and rubbers [97, 107, 155].
Owing to the ever increasing call for eco-sustainable chemicals and materials, FA derived from non-edible lignocellulosic biomass has continued to show increasing potential as a choice alternative to fossil-derived chemicals and materials in many industrial and materials applications as shown in this review article.
Non-edible lignocellulosic agricultural biomass offers us the bottomless opportunity for a cheaper, greener and eco-sustainable abundant resources. Coupled with advances in science and technology this eco-sustainable resources can be harnessed cheaply and effectively thereby reducing the overdependence on depleting and comparatively expensive fossil derived chemicals, mitigate greenhouse gas emissions, improve local economies and ensure energy security.
FA has been used to present the vast opportunities that chemicals from non-edible lignocellulosic agricultural residues can offer mankind; coupled with the intriguing chemistry and nature of FA reactions with co-reactants or alone, it avails scientists and technologists the capability to modify, tailor and transform it into materials and chemicals to meet specific end-use and applications with interesting properties not obtainable in fossil derived chemicals and materials.
FURAN is a common terminology used to refer to binders containing furfuryl alcohol and either urea or phenol formaldehyde or mixtures of both.
We appreciate the Polymer and Physical Chemistry Unit at the Centre for Rubber Science and Technology, CRST, Nelson Mandela University, South Africa, for their support.
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