February 2011, Volume 2, No.1 International Journal of Chemical and Environmental Engineering
The Study of Hydroxymethylfurfural as a Basic Reagent for Liquid Alkanes Fuel Manufacture from Agricultural Wastes Kambiz Tahvildari, a Saeed Taghvaei ,a Maryam Nozari *a a Department of Chemistry, Islamic Azad University, North Tehran branch, Tehran, Iran *Corresponding author: [email protected]
Abstract Renewable biomass resources have the potential to serve as a sustainable supply of fuels and chemical intermediates. The spentbiomass residues will contain fragments from lignin, residual carbohydrates and other organic matter. Two of these biomass residues are the molasses and bagasse of sugar cane which have several carbohydrates compounds in them. This paper discusses the relative advantages of different process options to convert carbohydrates to 5-hydroxymethyl-2-furaldehyde (HMF), a valuable intermediate for fine chemicals, pharmaceuticals and furan-based polymers. A great variety of acid catalysts has been used in this process, such as mineral acids, Lewis acids, strong acid cation-exchange resins, H-form zeolites and supported heteropolyacids. Several reaction media have been adopted, such as the more convenient water, but also anhydrous organic solvents, ionic liquids and recently biphasic water/organic co-solvent systems. Keywords: 5-hydroxymethyl-2-furaldehyde, Carbohydrates, Acid catalysis, Fuels
1. Introduction We are entering an era of diminishing the availability of petrochemical resources to produce energy and chemical materials which represent the basis for the synthesis of many useful products. Being concerned about the green house effect and future re-arrangement of the economy from petroleum to biological raw materials have resulted in using fuels from renewable resources. Abundant biomass resources can provide alternative routes for a sustainable supply of both transportation fuels and valuable intermediates (such as alcohols, aldehydes, ketones and carboxylic acids) for production of drugs and polymer materials [1, 19]. From the estimated annual production of biomass by biosynthesis about 75% are carbohydrates (mainly in the form of cellulose, starch and saccharose), 20% are lignin and only 5% are compounds of completely different structure, for example, fats, oils, protein and various other substances . Therefore, attention should be focused on efficient access to carbohydrates and their conversion to chemical materials. Carbohydrates are readily available, relatively inexpensive and renewable, and they are precursor chemicals for the synthesis of a large number of substances such as furfural and 5-hydroxymethyl-2furaldehyde (HMF), levulinic acid, etc. via established chemical methods. The high content of oxygenated functional groups in carbohydrates is an advantage, in contrast to the drawback of such functionality, for the conversion of carbohydrates to fuels [3-19]. Indeed, carbohydrates in addition to their
use in food chemistry, find new applications as source of green chemistry for the production of chemicals . One of the interesting reactions of carbohydrates is acid catalyzed triple dehydration of C6-sugar monomers to produce HMF [3-9]. HMF (1) is a versatile intermediate between biomassbased carbohydrates chemistry and petroleum-based industrial organic chemistry, (2) and its derivatives could potentially replace voluminously consumed petroleumbased building blocks, (3) is currently used to make plastics and fine chemicals .
2. The History of HMF In 1985, Dul and Kiermeyer while working independently, published a method for synthesis and chemical reaction of the compounds which they called ''Oxymethyl Furfural" . Several years later, other authors published their results. They worked out the modern method of HMF synthesis and studied the mechanism of its formation. Till now, over 1000 papers have been published, which is a proof for the great importance of this kind of compounds .
3. Aspect of Synthesis of HMF The synthesis of HMF is based on the triple dehydration of C6-sugar monomers in acidic moieties. By looking at the scheme of producing HMF, one could have an impression that the synthesis of HMF is simple (Scheme 1). But after dehydration, complications can arise, such as the rehydrating of HMF, which often yield
the by-products levulinic acid and formic acid. Another competing side-reaction is the polymerization of HMF and/or Fructose to form humin polymers .
and lignocellulosic material (such as cellulose, xylan, hemi-cellulose, etc.) . Other mixed carbohydrate sources include crude fructose, purified fructose, high fructose corn syrup intermediates and by-products but are not limited to them . Sugarcane bagasse, as a byproduct of the sugar industry, is an abundant source of hemicelluloses that can be hydrolyzed to yield fermentation feed stock for the production of chemicals . The production of carbohydrates from lignocellulosic biomass is technically performed using diluted acids at high temperature [31, 32], concentrated acids at low temperatures  or using biotechnological methods such as enzyme or micro organism . So the monosaccharides which are released during this process can be used as carbohydrate sources. The other feed stock which is from the converted industrial wastes is the molasses of sugar cane or sugar beet which has approximately 30%-50% sucrose. The usual sources for sucrose are the juice of sugar cane, sugar beet and other sucrose-containing materials. After the readily recoverable sucrose has been extracted from these sources, the mother liquors which are generally termed "molasses" will still contain a relatively large amount of sucrose along with other sugars such as glucose, fructose, raffinose, etc. So it is highly desirable to use sucrose for producing HMF .
Scheme 1. Conversion of hexoses to HMF
As HMF has many functional groups (It is simultaneously a primary aromatic alcohol, an aromatic aldehyde and has a furan ring system), it can be used as a precursor to diesel fuel, fuel additives, fine chemicals and plant protect agents [4, 23, 24]. For example, HMF can be converted to 2,5-furandicarboxylic acid (FDCA) by selective oxidation. FDCA can be used as a replacement of terephthalic acid in the production of polyesters such as polyethyleneterephthalate (PET) and polybutyleneterephthalate
(PBT). HMF is also currently under investigation as a treatment for sickle cell anomia . HMF is itself a rather unstable molecule. It can be found in natural products such as honey and a variety of heat processed food products formed in the thermal decomposition of carbohydrates . It is also important that HMF show a weak cytotoxicity and mutagenicity in human . The production of HMF has been studied for years, but an efficient method which is cost-effective and can produce HMF in high yields has yet to be found. Several extensive reviews are describing the chemistry of HMF and its derivatives. Moreau et al.  described the recent catalytic advances in substituted furans from biomass and focused especially on the ensuring polymers and their properties. A review by Lewkowski  on the chemistry of HMF and derivatives also appeared recently. The other relevant reviews are from Cottier and Descotes , and Kuster . The mechanism for the dehydration of fructose to HMF has been interpreted to proceed via two different routes, either via acyclic compounds or cyclic compounds . Antal proved experimentally that the mechanism of the HMF formation went through cyclic intermediates .
5. Catalysts The first synthesis of HMF was catalyzed by oxalic acid and till now 100 inorganic and organic compounds were used for producing HMF . Several mineral acids, such as HCl, H2SO4 and H3PO4 have been employed as homogeneous catalyzed dehydration of fructose to HMF [7, 26, 36-38]. Organic acids such as oxalic acid, levulinic acid and maleic acid can be used, too. These acid catalysts are utilized in dissolved form, so it causes difficulties in their regeneration, reuse and their disposal . Acid catalysts may also promote a subsequent rehydration reaction of HMF, thus causing carbon-carbon bond cleavage on the aldehyde to yield levulinic acid and formic acid . Moreover, in acidic media, oligomerization of fructose and/or HMF may occur, leading to both insoluble humins and soluble polymeric by-products. Indeed, in homogeneous acid catalyzed processes, too low selectivities (30%-50%) towards HMF were observed at a quite high substrate conversion (50%-70%) owing to the degradation to levulinic acid and formic acid . In order to avoid these problems, reusable or recyclable catalysts are preferred for use in the reaction as they provide for increased efficiency, economic and industrial feasibility. Examples of these kinds of catalysts include solid acid catalysts, ion exchange resins, zeolites, Lewis acids, clays and molecular sieves but are not limited to them . Solid acid catalysts often comprise a solid material which has been functionalized to influence acid groups that are catalytically active. Sulfated zirconia used as an
4. Feed Stocks The synthesis of HMF is based on the dehydration C6sugar monomers mainly glucose and fructose. This is partly because fructose has been shown to be more amenable to the dehydration reaction . Early processes for the production of HMF used crystalline fructose, but its usage is prevented because of its high cost. Depending on the method which is used, the feed stocks can be hexoses other than glucose and fructose (such as mannose, galactose), as well as other mono- di-, oligo- and poly-saccharides (such as sucrose, inulin, etc.)
effective solid catalyst for conversion of fructose to HMF in non-aqueous solutions such as acetone-DMSO mixture. In this mixture, a high fructose conversion of 93.6% with HMF yield of 72.8% could be obtained . The use of solid phase catalysts in a chromatography column to synthesis and purity of HMF limited exposure time to heat and enabled synthesis at a lower temperature. Lower temperature resulted in reducing energy cost and time for heating and cooling the reaction. Another advantage is the ability of the reaction to proceed and separate the product from the unwanted side-products. Studies concerning the application of ion-exchanging resin for the synthesis of HMF are numerous. Nakamura obtained HMF in 80% yield using strongly acidic ionexchange resin . Other researchers used different ion exchange resin and Mercudier, Rigal, El-Hajj and Rapp could produce HMF in high yield [22, 40]. Highly acidic cation-exchange resins such as those derivatives with sulfonic acid groups are also effective catalysts, providing the acidity of mineral acids together with the advantages of the heterogeneous catalysts . Ion exchange catalysts limit the reaction temperature to under 130 0C; however, this temperature range seems to be sufficient to overcome the activation energy barriers . The use of inorganic salts for the synthesis of HMF was the subject of numerous studies. For example, ammonium phosphates (the yield 32%), triethylaminephosphate (36%), pyridiniumphosphate (44%), zirconiumphosphate and zirconyl chloride improve the yield up to 90%. Ammonium sulphate, chromium trichloride or zinc chloride were used, too . Fayet and Gelas utilized various pyridinium salts such as poly-4-vinylpyridinium hydrochloride, pyridinium trifluoroacetate, hydrochloride, hydrobromide, perbromate and p-toluene sulfonate. Starting from fructose, they obtained HMF in 70% average yield . Metals such as Zn, Al, Cr, Ti, Th, Zr and V can be used as ions, salts or complexes as catalysts. Such use has not brought about improved results and yields of HMF have continued to be low . Szmant and Chundry used BF3 ET2O catalyst with DMSO as a solvent in the conversion of high-fructose corn syrup (HFCS), but it is not economically practical since it cannot be recovered and reused . The procedure for obtaining HMF from carbohydrate resources in the presence of niobium-based catalysts and niobic acid (Nb2O5 .nH2O) has been reported to have an intermediate selectivity of about 30% for 80% conversion of fructose. Niobium pentoxide and niobium phosphate are known to display quite high Brønsted acid strength, while no levulinic and formic acids were obtained as well [42, 43]. The dehydration of fructose can be achieved using metal halides such as FeCl2, CrCl2, CrCl3, FeCl3, CuCl2, CuCl2, VCl3, MoCl3, PdCl2, PtCl2, PtCl4, RuCl3 or RhCl3 and HMF yields ranging from 63% to 83%. Not all of the metal halides were effective, for example the alkali
chloride, LaCl3 and MnCl2 did not work . Zhang and co-workers have reported a metal chloride/ionic liquid system that gives good HMF yields for both fructose (83% with Pt or Rh chloride, 65% with CrCl2) and glucose (68% with CrCl2) . NHC/metal (NHC=N-Heterocyclic Carbene) complexes as catalyst for sugar dehydration reaction was used . Glucose might be converted into fructose first and subsequent into HMF over the NHC/Cr catalyst. In this case the fructose concentration would be relatively low even when the glucose substrate loading was high since fructose would merely be an intermediate in the conversion of glucose into HMF . The NHC/Cr catalysts were also tested in dimethylsulfoxide (DMSO) as the solvent and lower HMF yields were obtained both from fructose (28%-52%) and glucose (25%-32%). Catalysts with bulky NHC ligands showed higher efficiency in the DMSO system. NHC/Cr ionic liquid system has been used for the selective conversion of sugar into HMF and HMF yields were as high as 96% and 81% for fructose and glucose respectively . The new system is tolerant towards high substrate loading and the catalyst and ionic liquid can be recycled and HMF is provided as the sole product isolated after simple extraction .
6. Types of Solvents for Converting Carbohydrates to HMF The types of solvents and their influence on the efficiency of the dehydration reaction are closely connected with temperature conditions. Cottier divided methods into 5 groups depending on the types of solvents and the temperature of the process : - Aqueous processes carried out at temperature below 200 0C - Aqueous processes carried out at temperature over 200 0C - Processes in non-aqueous medium - Processes in mixed solvents - Processes without solvent and microwave processes The dehydration of fructose to form HMF has been conducted in water [12, 14, 15], organic solvents , organic/water mixtures , ionic liquids [12, 21] and more recently, biphasic water /organic systems [19, 46]. The use of water as a solvent does increase the solubility of fructose in the solvent phase, but it also promotes sidereactions such as humin formation and HMF decomposition to acids (HMF yields