Introduction

The formation of furans from sugars has been known since the early nineteenth century (Dias et al., 2010; Van Putten et al., 2013a, b). Furfural was discovered in 1821 by Dobereiner, by the distillation of bran with dilute sul­furic acid (Kamm et al., 2006; Van Putten et al., 2013b). The resulting compound was first named furfurol (the name comes from the Latin word furfur that means bran cereal, while finishing ol means oil). The furfural molecule has an aldehyde group and a furan ring with aromatic character, and a characteristic smell of almonds. In the presence of oxygen, a colorless solution of furfural tends to become initially yellow, then brown, and finally black. This color is due to the formation of oligomers/ polymers with conjugated double bonds formed by radical mechanisms and can be observed even at concen­trations as low as 10~5 M (Zeitsch, 2000a). Despite the fact that furfural has an LD50 between 50 and 2330 mg/ kg for mice, rats, guinea pigs and dogs, man tolerates its presence in a wide variety of fruit juices, wine, coffee and tea (Zeitsch, 2000a; Hoydonckx et al., 2007). The highest concentrations of furfural are present in cocoa and coffee (55—255 ppm), in alcoholic beverages (1—33 ppm) and in brown bread (26 ppm) (Zeitsch, 2000a). There is no commercially attractive route for the production of furfural from petrochemical resources (Mamman et al., 2008). The synthesis of HMF from biomass was already described in 1895 by Dull (1895) and Kiermayer (1895). Due to its high potential as a plat­form chemical for a variety of applications, furfural and HMF were mentioned by Bozell in the "top 10 + 4" list ofbiobasedchemicals (Bozell and Petersen, 2010), along with 2,5-furandicarboxylic acid (FDCA), which is formed by oxidation of HMF (Van Putten et al., 2013a).

The formation of furans from sugars takes place through an acid-catalyzed dehydration of sugar mole­cules at elevated temperature. In general furfural is formed from C-5 sugars and HMF is formed from C-6 sugars. It is therefore not surprising that furans, especially HMF, can be found in essentially all carbohy­drate containing heat-treated food. Furfural is known to have some toxic effects, whereas for HMF it is still un­clear (Van Putten et al., 2013a). The hydrolysis of poly­saccharides and subsequent dehydration into furfural and HMF may be promoted by Bransted or Lewis acid catalysts (Dias et al., 2010; Van Putten et al., 2013a). Furfural production through traditional processes is accompanied by acidic waste stream production and high energy consumption. Marcotullio and de Jong state that modern furfural production process concepts will have to consider environmental concerns and energy requirements besides economics moreover will have to be integrated within widened biorefinery concepts (Marcotullio and de Jong, 2010). The industrial use of aqueous mineral acids as the catalysts, such as sulfuric acid for furfural production, poses serious operational (corrosion), safety and environmental problems (large amounts of toxic waste). Hence, it is seen desirable to replace conventional aqueous mineral acids by "green" nontoxic catalysts for converting sugars into furfural and HMF. The use of solid acids as catalysts may have several advantages over liquid acids, such as easier separation and reuse of the solid catalyst, longer catalyst lifetimes, toleration of a wide range of temperatures and pressures, and easier/safer catalyst handling, storage and disposal. A road map to furfural, HMF and levulinic acid has recently been presented by the group of Dumesic (Wettstein et al., 2012).

Furfural Production and Applications

The industrial production of furfural was driven by the need of the United States to become self-sufficient during the First World War. Between 1914 and 1918, intensive exploration for converting agricultural wastes into industrially more valuable products was initiated. In 1921, the Quaker Oats company in Iowa initiated the production of furfural from oat hulls using "left over" re­actors (Zeitsch, 2000a). Over time, there was an increased industrial production of furfural and the discovery of new applications. Nowadays, the annual world produc­tion of furfural is about 300,000 tons and, although there is industrial production in several countries, the main

production units are located in China, the Dominican Republic and South Africa (Kamm et al., 2006; Zeitsch, 2000a; Hoydonckx et al., 2007; Mamman et al., 2008).

Figure 17.4 gives an overview of some of the main out­lets of furfural. Most of the furfural produced worldwide is converted through a hydrogenation process into fur — furyl alcohol, which is primarily used as foundry resin but also increasingly applied as resin to improve wood durability and for the manufacturing of polymers and plastics (Dias et al., 2010). The aldehyde group and furan ring furnish the furfural molecule with outstanding prop­erties for use as a selective solvent (Zeitsch, 2000a; Hoydonckx et al., 2007; Sain et al., 1982). Furfural has the ability to form a conjugated double bond complex with molecules containing double bonds, and therefore is used industrially for the extraction of aromatics from lubricating oils and diesel fuels, or unsaturated com­pounds from vegetable oils. Furfural is used as a fungicide and nematocide in relatively low concentrations (Zeitsch, 2000a). Additional advantages of furfural as an agrochem­ical are its low cost, safe and easy application, and rela­tively low toxicity to humans. Nakagawa and Tomishige (Nakagawa and Tomishige, 2012) have recently reviewed

the catalyst system used to produce 1,5-pentanediol from tetrahydrofurfuryl alcohol. Other furan compounds ob­tained from furfural include levulinic acid (Gurbuz et al., 2012) and tetrahydrofuran. Furfural and many of its derivatives can be used for the synthesis of new poly­mers based on the chemistry of the furan ring (Hoydonckx et al., 2007; Sain et al., 1982; Win, 2005; Gandini and Belga — cem, 1997; Moreau et al., 2004). Furfural derivatives are also excellent starting points for fuel applications (Lange et al., 2012; Gruter and de Jong, 2009; de Jong et al., 2012a, b). Commercially, the pentosans (mainly xylan) pre­sent in the hemicellulose fraction of agricultural streams such as corn cobs and sugarcane bagasse are hydrolyzed, using homogeneous acid catalysts in water, giving rise to pentose (xylose), which, by dehydration and cyclization reactions, leads to furfural with a theoretical mass yield of approximately 73% (Scheme 17.1). Nowadays also other feedstocks are considered. Huber and his group developed a new process to produce furfural from waste aqueous hemicellulose solutions from the pulp and paper and cellulosic ethanol industries using a continuous two- zone biphasic reactor (Xing et al., 2011). A two-stage hybrid fractionation process was investigated to produce

cellulosic ethanol and furfural from corn stover. In the first stage, zinc chloride (ZnCl2) was used to selectively solubi­lize hemicellulose. During the second stage, the remaining solids were converted into ethanol using commercial cellulase and fermentative microorganisms. Yoo et al. found that the furfural yield from the hemicellulose hy­drolysates could be up to 58% based on carbon (Yoo et al., 2012). Yemis and Mazza researched the potential of a microwave-assisted process that provided a highly efficient conversion of wheat straw, triticale straw, and flax shives: obtained furfural yields based on carbon were 48%, 46%, and 72%, respectively (Yemis and Mazza, 2011, 2012). Sahu and Dhepe also presented a solid acid — catalyzed one-pot method for the selective conversion of solid hemicellulose without its separation from other lignocellulosic components, such as cellulose and lignin resulting in 56% furfural yields in biphasic systems (Sahu and Dhepe, 2012). An interesting approach was dis­closed by vom Stein and coworkers (vom Stein et al., 2011) by working with "real samples". They prepared aqueous solutions of FeCl3—NaCl (or seawater) to evaluate the dehydration of xylose into furfural, which can be extracted in situ into 2-methyltetrahydrofuran (2-MTHF) as second phase. Furfural was also successfully obtained when aqueous nonpurified xylose effluents directly from lignocellulose fractionation are tested (vom Stein et al., 2011). Also Marcotullio and De Jong observed good results with FeCl3 (Marcotullio and De Jong, 2010).

The hydrolysis of pentosans into pentoses in the presence of H2SO4 is faster than the dehydration of the pentose monomers into furfural (Zeitsch, 2000a; Hoy — donckx et al., 2007). Hence, kinetic studies are generally focused on the rate-limiting process, i. e. the dehydration of pentoses. Xylose and arabinose are monomers found in pentosans, which can be converted into furfural, and some studies have shown that the dehydration of arabi — nose is slower than that of xylose (Zeitsch, 2000a; Kootstra et al., 2009). The concentration of xylose in the various raw materials is almost always much higher than that of arabinose. Considering these factors, it seems reason­able to investigate the kinetics of the dehydration process using xylose as substrate (Zeitsch, 2000a; Sain et al., 1982; Win, 2005; Gandini and Belgacem, 1997; Moreau et al., 2004,1998; Antal et al., 1991; Root et al., 1959). In the dehy­dration and cyclization of xylose into furfural, three mol­ecules of water are released per molecule of furfural produced. Huber and coworkers developed a kinetic model for the dehydration of xylose to furfural in a biphasic batch reactor with microwave heating (62). There are four key steps in their kinetic model: (1) xylose dehydration to form furfural, (2) furfural reaction to form degradation products, (3) furfural reaction with xylose to form degradation products, and (4) mass transfer of furfural from the aqueous phase into the organic phase (methyl isobutyl ketone (MIBK)). It was estimated that furfural yields in a biphasic system can reach 85%, whereas at these same conditions in a monophase system furfural yields of only 30% are obtained (Weingarten et al., 2010). Also a kinetic model for the homogeneous conversion of D-xylose in high-temperature water was developed (Kim et al., 2011). Experimental testing evalu­ated the effects of operating conditions on xylose conver­sion and furfural selectivity, with furfural yields of up to 60% observed. Also the kinetics of formic acid-catalyzed xylose dehydration into furfural and furfural decomposi­tion was investigated using batch experiments within a temperature range of 130—200 °C (Lamminpaa et al.,

2012) . The study showed that the modeling must account for other reactions from xylose besides dehydration into furfural. Moreover, the reactions between xylose interme­diate and furfural play only a minor role and that furfural decomposition reactions must take the uncatalyzed reac­tion in water as solvent into account (Lamminpaa et al., 2012). By-products formed in the xylose reaction may also derive from the fragmentation of xylose, such as glyc — eraldehyde, glycolaldehyde, formic acid, lactic acid, ace — tol (Antal et al., 1991; Ahmad et al., 1995).

As furfural is formed it can be transformed into higher molecular weight products by (1) condensation reactions between furfural and intermediates of conversion of xylose to furfural (and not directly with xylose) and (2) furfural polymerization (Zeitsch, 2000a). Aldol condensation between two molecules of furfural does not occur due to the absence of a carbon atom in Ha position in relation to the carbonyl group (Chheda and Dumesic, 2007). The side reactions (1) and (2) lead to olig­omers and polymers with (1) are considered to be more relevant than (2), although published characterization studies of the by-products formed are scarce (Zeitsch, 2000a). The extent of these side reactions can be mini­mized by reducing the residence time of furfural in the reaction mixture and by increasing the reaction tempera­ture (Zeitsch, 2000a, b; Root et al., 1959; Zeitsch, 2000b). If furfural is kept in the gas phase during the aqueous phase reaction it will not react with intermediates, which are "nonvolatile". Agirrezabal-Telleria et al. (Agirrezabal- Telleria et al., 2011) developed new approaches for the pro­duction of furfural from xylose. They propose to combine relatively cheap heterogeneous catalysts (Amberlyst 70) with simultaneous furfural stripping using nitrogen un­der semibatch conditions. Nitrogen, compared to steam, does not dilute the vapor phase stream when condensed. This system allowed stripping 65% of the furfural con­verted from xylose and almost 100% of selectivity in the condensate. Moreover, high initial xylose loadings led to the formation of two water—furfural phases, which could further reduce purification costs. Constant liquid—vapor equilibrium during stripping could be maintained for different xylose loadings. The modeling of the experi­mental data was carried out in order to obtain a liquid—va­por mass transfer coefficient. This value could be used for future studies under steady-state continuous conditions

in similar reaction systems (Agirrezabal-Telleria, 2011). Formic acid, a by-product of furfural process (Root et al., 1959), can be an effective catalyst for dehydration of xylose into furfural. There is a growing interest in the use of for­mic acid as catalyst because it has low corrosiveness and can be easily separated and reused. Using response sur­face methodology the optimal process parameters (xylose concentration 40 g/l, formic concentration 10 g/l, and a reaction temperature 180 °C) were determined to obtain high furfural yield and selectivity. Under these conditions, a maximum furfural yield of 74% and selectivity of 78% were achieved (Yang et al., 2012). Extraction using super­critical CO2 (scCO2) also enhances furfural yields (Kim et al., 2011; Sako et al., 1991,1992). The above mechanistic considerations for the homogeneous conversion of xylose into furfural using H2SO4 as catalyst may also be consid­ered for solid acid catalysts. Nevertheless, differences in product selectivity between homogeneous and heteroge­neous catalytic processes are expected due to effects such as shape/size selectivity, competitive adsorption (related to hydrophilic/hydrophobic properties), and strength of the acid sites.

Industrially, furfural is directly produced from the lignocellulosic biomass in the presence of mineral acids, mainly sulfuric acid, under batch or continuous mode operation (Table 17.9). Attempts to improve furfural yields have been made by process innovation, although the use of mineral acids remains a drawback (Zeitsch, 2000a, 69. 70). The cost and inefficiency of separating these homogeneous catalysts from the products makes their recovery impractical, resulting in large volumes of acid waste, which must be neutralized and disposed off. Other drawbacks include corrosion and safety problems. The production of furfural is therefore one of many industrial processes where the reduction or replacement of the "toxic liquid" acid catalysts by alter­native "green" catalysts is of high priority. Recently Mar — cotullio and De Jong (Marcotullio and de Jong, 2010,

2011) shed new light on some particular aspects of the chemistry of D-xylose reaction to furfural. Their aim was to clarify the reaction mechanism leading to furfural

and to define new green catalytic pathways for its pro­duction. Specifically, their objective was to reduce the use of mineral acids by the introduction of alternative catalysts, e. g. halides, in dilute acidic solutions at tem­peratures between 170 and 200 °C (Schadel et al.,

2010) . Results indicate that the Cl — ions promote the for­mation of the 1,2-enediol from the acyclic form of xylose, and thus the subsequent acid-catalyzed dehydration to furfural. For this reason the presence of Cl- ions led to significant improvements for H2SO4 catalyzed reactions. The addition of NaCl to a 50 mM HCl aqueous solution gave 90% selectivity to furfural. Follow-up experimental results by the same group show the halides to influence at least two distinct steps in the reaction leading from D-xylose to furfural under acidic conditions, via different mechanisms. The nucleophilicity of the halides appears to be critical for the dehydration, but not for the initial enolization reaction. By combining different halides syn­ergic effects become evident resulting in very high selec — tivities and furfural yields (Marcotullio and de Jong,

2011) . Also Rong et al. (2012) found that the addition of inorganic salts (e. g. NaCl, FeCls) promoted the yield of furfural from xylose. Another approach to reduce the inorganic waste streams is to perform the reaction at high temperatures. It was shown that the reaction pathway for the xylose decomposition in high — temperature liquid water can be changed by manipu­lating the temperature and pressure without any catalyst with a maximum furfural yield of 50% (Jing and Lu, 2007). Many attempts have been made to develop heterogeneous catalytic processes for furfural production that offer environmental and economic benefits, but to the best of our knowledge none has been commercialized (Van Putten et al., 2013b).