Topics in Catalysis Vol. 27, Nos. 1–4, February 2004 (# 2004)
`
`11
`
`Recent catalytic advances in the chemistry of substituted furans
`from carbohydrates and in the ensuing polymers
`Claude Moreaua, , Mohamed Naceur Belgacemb, and Alessandro Gandinib
`aLaboratoire de Mate´riaux Catalytiques et Catalyse en Chimie Organique UMR 5618, CNRSÿENSCM Ecole Nationale Supe´rieure
`de Chimie de Montpellier 8, Rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France
`bLaboratoire de Ge´nie des Proce´de´s Papetiers, UMR 5518, CNRSÿCTPÿEFPGÿINPG Ecole Franc¸ aise de Papeterie
`et des Industries Graphiques Institut National Polytechnique de Grenoble BP 65, 38402 Saint-Martin d’He´res, France
`
`In this review, an overview is given on the last development of catalytic methods for the preparation of substituted furans from
`carbohydrates and ensuing polymers. The review starts with the recent aspects in the synthesis of some key furan monomers in the
`presence of solid catalysts. In the second part, selected examples are given of polymerization systems leading to furan-based
`materials with promising properties, thus constituting a serious alternative to petroleum-based counterparts. Finally, a short
`examination is given on what could be the future of furan chemistry with the recent development of ionic liquids as solvents.
`
`KEY WORDS: carbohydrates; catalysis; furanic monomers; furanic polymers.
`
`1. Introduction
`
`The growing interest for the preparation of non-
`petroleum chemicals has naturally led to the development
`of the nonfood transformation of carbohydrates, the
`most abundant source of renewable materials on earth,
`into valuable chemicals [1]. This renewed interest was
`quite obvious with the development of the chemistry of
`furanic compounds, particularly for the preparation of
`nonpetroleum-derived polymeric materials
`such as
`polyesters, polyamides and polyurethanes [2,3].
`Two basic nonpetroleum chemicals readily accessible
`from renewable resources, furfural (2-furancarboxalde-
`hyde, (1)) arising from the acid-catalyzed dehydration of
`pentoses, and 5-hydroxymethylfurfural
`(5-hydroxy-
`methyl-2-furancarboxaldehyde (2)) arising from the
`acid-catalyzed dehydration of hexoses, are suitable
`starting materials for the preparation of further mono-
`mers required for polymer applications. Whereas the
`former is industrially available (200 000 tons/year), the
`latter was only produced on a pilot plant scale [4].
`However,
`in spite of the high potential of furanic
`intermediates derived from (2), this basic disubstituted
`furan is not yet produced on industrial scale, as the
`present politico-economical situation still favors the
`petroleum route. The general strategy representing the
`approach of furanic first-generation monomers (1 and 2)
`is sketched in scheme 1.
`5-Hydroxymethylfurfural (2) has been used for the
`manufacture of phenolic resins by reaction of
`the
`aldehyde group as well as of the hydroxymethyl group
`[1]. However, high potential organic intermediate
`
` To whom correspondence should be addressed.
`
`E-mail: cmoreau@cit.enscm.fr
`
`chemicals are the various ensuing products of (2) as
`partially illustrated in scheme 2. 2,5-Disubstituted furan
`derivatives are particularly suitable as they can replace
`their corresponding aromatic counterparts [5,6]. For
`example, as it will be seen in the second part of this
`is capable of
`review, 2,5-furandicarboxylic acid (7)
`replacing terephthalic or
`isophthalic acids
`for
`the
`preparation of polyesters, polyamides and polyur-
`ethanes. 2,5-Furandicarboxaldehyde (6) is a starting
`material for the preparation of 2,5-bis(aminomethyl)-
`furan and also for the preparation of Schiff bases. 2,5-
`Bis(hydroxymethyl)-furan (4) is already used in the
`manufacture of polyurethane foams [7]. The fully
`saturated 2,5-bis(hydroxymethyl)-tetrahydrofuran (5)
`can be used as a diol in the preparation of polyesters.
`Finally, the new compound 2-hydroxymethyl-5-vinyl-
`furan (8) can be a starting material for the preparation
`of epoxy resins.
`5-Hydroxymethylfurfural has also been identified as
`a novel scaffold for the generation of disubstituted furan
`derivatives, an important component of pharmacologi-
`cally active compounds, which are associated with a
`wide spectrum of biological activities [8].
`Several attempts have since been made to develop
`new catalytic low-cost processes, more performant,
`more selective and environmentally safe,
`for
`the
`transformation of fructose and fructose precursors into
`5-hydroxymethylfurfural and 2,5-disubstituted furans.
`This review will summarize the last developments in this
`field with emphasis on the preparation of some key
`intermediates, as illustrated in scheme 2, and their use in
`the preparation of polymeric materials.
`As reported in a previous paper [9], a considerable
`interest has emerged these last thirty years for the
`application of solid heterogeneous catalysts in fields
`
`1022-5528/04/0200–0011/0 # 2004 Plenum Publishing Corporation
`
`Petitioners' Exhibit 1020, Page 1 of 20
`
`

`
`C. Moreau et al./Chemistry of substituted furans
`
`VEGETAL BIOMASS
`
`Wood, Jerusalem artichoke, Wheat, Maize, Sugar Beet, Sugar Cane, Rice
`
`SOURCE OF
`
`
`Hemicelluloses, Cellulose, Starch, Inulin, Sucrose
`
`
`
`SOURCE OF
`
`
`PENTOSES
`
`
`
`
`
`
`HEXOSES
`
`First generation furanic monomers
`
`H
`
`O
`
`
`
`O
`
`HO
`
`5-Hydroxymethylfurfural
`(2)
`
`O
`
`H
`
`O
`
`
`
`Furfural
`(1)
`
`12
`
`
`
`
`
`
`
`other than oil refining and petrochemistry. Indeed,
`zeolites and related materials can be used as highly
`selective and active catalysts in the synthesis of bulk and
`fine chemicals [10ÿ17]. Furthermore, their acidic and
`basic properties, as well as their hydrophilic and
`hydrophobic properties, can be combined with the
`structural properties of these materials in order to take
`advantage of
`their
`sorption and shape selectivity
`properties, the latter being an advantageous feature of
`zeolites compared to other heterogeneous catalysts.
`
`2. Part I: Furan-based monomers
`
`2.1. 5-Hydroxymethylfurfural
`
`The most convenient method for the preparation of
`5-hydroxymethylfurfural is the acid-catalyzed dehydra-
`tion of fructose (scheme 3).
`A review by Lewkowski on the chemistry of 5-
`hydroxymethylfurfural
`(2) and its derivatives has
`recently appeared [18], but is unfortunately far from
`being updated. Apart from the reviews by Gaset [19,20],
`the two more recent and exhaustive reviews are those by
`Kuster [21] and Descotes [22].
`Two basic articles by Kuster [23] and van Bekkum
`[24] report an exhaustive study of all the parameters
`capable of influencing the course of the homogeneously
`catalyzed dehydration of fructose in water as the most
`convenient solvent. Whatever the operating conditions,
`the highest selectivity in (2) (75%) was obtained in the
`presence of methyl
`isobutyl ketone as simultaneous
`extraction solvent. In another work, the introduction of
`vapor extraction of (2) yields a product with a high
`purity, but to the detriment of the yield [25].
`When acidic ion-exchange resins are used in water in
`place of mineral acids, some improvements are obtained
`in terms of process due to the presence of solid catalysts,
`but the selectivity in 5-hydroxymethylfurfural
`is not
`significantly improved [26ÿ28].
`However, in the literature, the highest selectivities in
`5-hydroxymethylfurfural
`(2) have been obtained in
`DMSO as
`the solvent under moderate operating
`conditions. Fructose is selectively and quantitatively
`converted into (2) in the absence of catalyst as well as in
`[29ÿ32]. The
`the presence of
`ion-exchange resins
`advantage of using DMSO as solvent is that this dipolar
`
`H3O+
`
`HO
`
`O
`
`H3O+
`
`H
`
`O
`
`O
`5-Hydroxymethyl furfural
`
`(2)
`
`O
`
`OH
`
`Levulinic acid
` +
` Formic acid
` +
` Humins
`
`OH
`
`O
`
`H
`
`OH
`OH
`
`HO
`
`H
`
`H
`
`HO
`
`Fructose
`
`Scheme 1. From vegetal biomass to furfural (1) and hydroxymethyl-
`furfural (2), the two first-generation furanic monomers.
`
`H
`
`O
`
`O
`
`Furfural
`(1)
`
`HO
`
`O
`
`H
`
`O
`
`5-Hydroxymethylfurfural
`(2)
`
`OH
`
`
`
`HO
`
`O
`
`H
`
`O
`
`
`
`
`
`O
`
`2-hydroxymethyl-5-
`
`vinylfuran
`(8)
`
`
`OH
`
`Furfuryl alcohol
`(3)
`
`
`
`HO
`
`O
`
`
`2,5-bis(hydroxymethyl)-furan
`(4)
`
`
`
`O
`
`HO
`
`OH
`
`
`
`2,5-bis(hydroxymethyl)-
`
`terahydrofuran
`(5)
`
`H
`
`O
`
`
`
`O
`
`
`
`2,5-furandicarboxaldehyde
`(6)
`
`OH
`
`O
`
`
`
`O
`
`
`
`HO
`
`O
`
`2,5-furandicarboxylic acid
`(7)
`
`Scheme 2. Some of key furan derivatives.
`
`Scheme 3. Simplified reaction scheme for the dehydration of fructose.
`
`Petitioners' Exhibit 1020, Page 2 of 20
`
`

`
`C. Moreau et al./Chemistry of substituted furans
`
`13
`
`aprotic solvent prevents the formation of levulinic acid
`and humins as well, but the corresponding disadvantage
`is concerned with the separation of DMSO, (2) and
`water formed, and also with some possible toxic S-
`containing by-products arising from the decomposition
`of DMSO.
`Taking into account the results reported in the recent
`literature on the dehydration of fructose into (2), in
`particular, those dealing with the use of strongly acidic
`ion-exchange resins,
`it was expected that
`the low
`selectivity to 5-hydroxymethylfurfural (2) observed in
`water as solvent could result from the presence of
`hydronium species within the macropores of the resins,
`leading to its further degradation [9]. It was then
`postulated that microporous zeolitic materials as cata-
`lysts could lead to improvements in the dehydration
`reaction, mainly due to the possible tuning of their
`acidic and basic properties,
`their hydrophilic and
`hydrophobic properties, and their adsorption and shape
`selectivity properties, the latter being an advantageous
`feature of zeolites compared to other heterogeneous
`catalysts [9].
`Fructose dehydration was then performed in the
`presence of microporous catalysts, namely, dealumi-
`nated H-form zeolites in a water/methyl isobutyl ketone
`(1/5 by volume) mixture [33ÿ35]. A mordenite in the
`protonic form, with a Si/Al ratio of 11 and a low
`mesoporous volume, was found to offer the best balance
`between activity, selectivity and by-product amounts at
`165 C. A selectivity in 5-hydroxymethylfurfural (2) of
`91ÿ92% was obtained up to a fructose conversion of
`76%. However, in most other cases, the selectivity tends
`to decrease by increasing the Si/Al ratio,
`i.e., by
`increasing the acidity of the catalytic sites, thus allowing
`secondary reactions to take place, such as the formation
`of formic and levulinic acid or polymeric materials
`referred to as humins. Compared to other catalytic
`systems, the structure of mordenite with only parallel
`large elliptical channels will allow the accessibility of
`fructose to the catalytic sites and the rapid diffusion of
`(2) once formed, while avoiding its rearrangement into
`higher molecular weight compounds.
`A pilot plant using a new solidÿliquidÿliquid reactor,
`resulting from the modification of a liquidÿliquid
`extraction pulsed column, was then developed [36]. 5-
`Hydroxymethylfurfural
`is extracted continuously
`(2)
`with methyl isobutyl ketone in a countercurrent manner
`with respect to the aqueous fructose phase and catalyst
`feed. In such a process, the residence time of the
`intermediate in the aqueous phase is shortened, and the
`consequence was a gain in selectivity of about 10%.
`From an economical point of view,
`it was more
`interesting to start from raw fructose-containing pre-
`cursors such as sucrose and inulin hydrolysates instead
`of fructose itself. In the presence of the H-mordenite
`ðSi=Al ¼ 11Þ, selectivities to 5-hydroxymethylfurfural
`between 92 and 97% for fructose conversions up to 54%
`
`at 165 C have been obtained [34]. Under the operating
`conditions used, glucose does not react significantly and
`is only acting as a spectator in the dehydration step. The
`production of (2) from fructose-containing precursors is
`then a more interesting and economical route, as the
`unreacted glucose can be easily separated from the
`reaction medium in a liquidÿliquid extractor working in
`a countercurrent mode.
`It must also be mentioned that fructose can be
`obtained from fructose precursors such as sucrose and
`inulin by catalytic heterogeneous processes using zeolitic
`materials instead of processes using mineral acids, ion-
`exchange resins or enzymes. It was shown that hydro-
`lysis of those precursors into glucose and fructose was
`selectively achieved over zeolites in their acidic form
`[37]. Glucose and fructose can be separated over zeolites
`in their cationic form [38,39]. In a similar manner,
`hydrolysis of glucose precursors such as starch, maltose
`and cellobiose has also been easily achieved over acidic
`zeolites. It was also shown that the selective isomeriza-
`tion of glucose into fructose was working over cation-
`exchange zeolites [40,41] and hydrotalcites [42]. In the
`latter case, a very high selectivity in fructose is obtained,
`but at a glucose conversion close to 20% only. It should
`also be added that hydrotalcites are easily regenerated,
`whereas, in the case of cation-exchange zeolites, some
`lixiviation phenomenon can occur. Such an approach
`may also be advantageously used in food applications
`for the preparation of
`invert sugars since zeolitic
`materials are known for their catalytic as well as for
`their sorption properties and are more easily recycled
`than ion-exchange resins [43,44].
`As far as the fructose route to 5-hydroxymethylfur-
`fural (2) is not yet developed on an industrial scale,
`another route to obtain 2 was to start from a cheap and
`readily available starting material such as 1a (scheme 4).
`This route involves hydroxymethylation of furfural (1a)
`with formaldehyde.
`Hydroxyalkylation of aromatics and heteroaromatics
`was recently shown to fail in the presence of aldehydes
`or epoxides as hydroxyalkylating agents and H-form
`zeolites as catalysts [45ÿ47]. However, one reference
`mentioned traces of 5-hydroxymethylfurfural (2) when
`furfural (1a) is reacted together with aqueous formalde-
`hyde in the presence of sulfonic ion-exchange resins
`[48]. After several attempts to determine the most
`
`O
`
`O
`
`O
`
`Furfural
`
`(1a)
`
`H
`
`H
`
`H
`
`HO
`
`H
`
`O
`
`H3O+
`
`O
`5-Hydroxymethyl furfural
`
`(2)
`
`Scheme 4. Simplified reaction scheme for hydroxymethylation of
`furfural.
`
`Petitioners' Exhibit 1020, Page 3 of 20
`
`

`
`14
`
`C. Moreau et al./Chemistry of substituted furans
`
`appropriate operating conditions to be used, solvent,
`source of formaldehyde and catalyst, it was found that
`the reaction could work with a large excess of aqueous
`formaldehyde in the presence of dealuminated H-
`mordenites at
`low temperatures [49]. However,
`the
`selectivity in (2) was relatively poor, around 30%,
`whatever be the acidic and hydrophobic properties of
`the catalyst. A slight increase in the selectivity to 5-
`hydroxymethylfurfural (2) was observed with toluene as
`the solvent, up to 50%, but at a low furfural conversion.
`In fact, the low selectivity results from the deactivation
`of the electron-withdrawing aldehyde group on the C-5
`carbon position. Owing to this deactivation effect, the
`principal reaction is the addition of formaldehyde to the
`aldehyde group of furfural leading to aldolized, croto-
`nized and heavier molecular weight compounds referred
`to as
`resins. When the electron demand of
`the
`substituent is reversed as, for example, by protecting
`the electron-withdrawing aldehyde function with an
`electron-donating 1.3-dithiolane function (scheme 5),
`the selectivity to the corresponding hydroxymethylated
`derivative easily reaches 90% [50].
`It is also worth mentioning that zeolitic materials
`have been successfully used in the dehydration of xylose
`and precursors to furfural
`[51]. Furfural
`is obtained
`through dehydration of pentoses, xylose in particular, or
`hemicelluloses, at high temperatures ð200–250 CÞ, and
`in the presence of mineral acids as catalysts, mainly
`sulfuric acid [52]. Under these conditions, the selectivity
`in furfural does not exceed 70%, except in the case of its
`continuous extraction with supercritical CO2, where it
`reaches 80% [53].
`In the presence of zeolites as catalysts, a close
`parallelism is observed with the results obtained for
`the dehydration of fructose over the same catalysts,
`except that toluene is the cosolvent instead of methyl
`isobutyl ketone. The transformation of xylose into
`furfural is easily achieved at 170 C with a selectivity
`as high as 90–95%, as long as the conversion is kept low,
`i.e., 30–40%. At those high temperatures, ion-exchange
`resins cannot compete with zeolites. Nevertheless,
`it
`should be mentioned in the case of xylose dehydration
`that the formation of heavy compounds is less probable
`compared to what occurs in fructose dehydration to 5-
`hydroxymethylfurfural (2), which can react through its
`hydroxymethyl function, thus yielding higher molecular
`weight compounds.
`Although one zeolite of the faujasite family was
`already checked several years ago in the dehydration of
`fructose, but with the formation of traces only of (2)
`[54], it is now clear that zeolitic materials with adequate
`structures and well-controlled properties could replace
`the catalytic systems used up to now.
`However, new catalytic systems have also been
`developed by other research groups during the last
`decade. Nb-based catalysts were used in the dehydration
`of fructose with high catalytic efficiency, at only 100 C,
`
`Scheme 5. Selective route to 5-hydroxymethylfurfural after protection
`and deprotection steps.
`
`
`
`in water and without any extraction solvent. A high
`selectivity in 5-hydroxymethylfurfural (2), 70–80%, at a
`fructose conversion of 30–50%, has been reported [55].
`A more complete work over niobium oxide, H3PO4-
`treated niobic acid and niobium phosphates treated at
`different temperatures, was published later [56] and has
`confirmed the previous results, i.e., the reactions work at
`low temperature ð100 CÞ, and high selectivities in 5-
`hydroxymethylfurfural, up to 100% as determined by
`GC-MS analysis, are obtained, but at low fructose
`conversion, 25–30%, and relatively short reaction times,
`about 0.5–1 h. A nearly similar approach was also
`carried out in the presence of Zr- and Ti-based catalysts
`with different
`structural
`forms
`[57]. Once again,
`excellent results are reported. Selectivities in (2) as high
`as 100% are reported over cubic zirconium pyropho-
`sphate and -titanium phosphate catalysts, at reaction
`times and fructose conversion of the same order of
`magnitude as those reported for Nb-based catalysts.
`Other recent articles by Ishida report the catalytic
`dehydration of fructose and glucose in the presence of
`lanthanide(III) salts. At 100 C in organic solvents,
`namely DMSO, dimethylformamide (DMF) and DMA,
`the yields in 5-hydroxymethylfurfural (2) are over 90%
`after a reaction time of 4 h in the presence of LaCl3 [58].
`As already reported in the literature, DMSO was found
`to be the best solvent that prevents the formation of
`levulinic acid and other by-products. In water at 140 C,
`both fructose and glucose are smoothly dehydrated into
`(2) with selectivities higher than 95%, but only around
`10% of conversion at 15 min is obtained in the initial
`stages of the reaction [59]. As reported by the authors,
`the material balance gradually decreases after 20 min,
`and the ultimate product yield of ca. 40% is obtained
`after 120 min, whatever be the starting quantity of
`hexose, fructose or glucose.
`Two very recent papers are also worth noting: (i) The
`first one [60] takes into account the fact that high
`selectivities
`in 5-hydroxymethylfurfural
`are often
`obtained in high boiling polar solvents,
`leading to
`expensive separation procedures, and examines the
`dehydration of fructose, glucose, sucrose and inulin in a
`sub- and supercritical mixture of acetone and water.
`Contrary to the reaction performed in supercritical water
`[61], no solids (humic acids) were produced. Under
`optimized operating conditions in acetone/water mix-
`tures, 10 g/L of fructose, 10 mmol/L of H2SO4, 180 C
`and 20 MPa, both selectivity in 5-hydroxymethylfurfural
`
`Petitioners' Exhibit 1020, Page 4 of 20
`
`

`
`C. Moreau et al./Chemistry of substituted furans
`
`15
`
`and fructose conversion increase with decreasing water
`content. A nearly complete fructose conversion is
`attained within 2 min in the solvent mixture containing
`90 v/v% of acetone. Also remarkable are the selectivities
`in 5-hydroxymethylfurfural
`(yields at 100% starting
`material conversion) obtained—77% from fructose,
`78% from inulin, 48% from glucose and 56% from
`sucrose—which led the authors to propose, from their
`new technical process, an acceptable price of about 2 €/kg
`for 5-hydroxymethylfurfural if fructose, or fructose-rich
`precursors, are available at a price of 0.5 €/kg; (ii) The
`second one, by Ribeiro and Schuchardt [62], who have
`investigated the
`catalytic one-pot
`cyclization and
`oxidation of
`fructose over bifunctional and redox
`catalysts, namely, cobalt acetylacetonate encapsulated
`in sol–gel silica. Although the clear objective was the
`preparation of 2,5-furandicarboxylic acid, it was shown
`that, at 165 C and under a 2 MPa air pressure, fructose is
`selectively
`converted into 5-hydroxymethylfurfural
`within 1 h and a fructose conversion close to 50% over a
`SiO2 gel catalyst.
`
`2.2. 2,5-Bis(hydroxymethyl)-furan
`
`As already mentioned, 2,5-bis(hydroxymethyl)-furan
`(4) is used in the manufacture of polyurethane foams [7],
`and its fully saturated form (5) can be used as a diol in
`the preparation of polyesters. The market for this latter
`compound would be 8000 tons per year at a price of
`around 7 €/kg [63].
`The two catalytic routes to (4) developed up to now
`are the hexose route through hydrogenation of 5-
`hydroxymethylfurfural and the pentose route through
`hydroxymethylation of furfuryl alcohol (3) (scheme 6).
`
`2.2.1. Hydrogenation of 5-hydroxymethylfurfural
`Hydrogenation of 5-hydroxymethylfurfural (2) has
`been studied in an exhaustive manner by Descotes et al.
`[64]. The initial strategy was based on the assumption
`that the hydrogenation of (2) would be more rapid than
`the dehydration of fructose, thus avoiding the formation
`of secondary products by trapping (2) once formed.
`Unfortunately, the experimental results were not those
`that were expected.
`Hydrogenation of 5-hydroxymethylfurfural was then
`performed under conventional operating conditions of
`
`
`
`
`
`
`
`
`
`
`Scheme 6. Routes to 2,5-bis(hydroxymethyl)-furan.
`
`temperature and pressure and over conventional hydro-
`genation catalysts, Raney nickel, copper chromites and
`C-supported metals. Nearly quantitative yields in (4)
`and/or 2,5-bis(hydroxymethyl)-tetrahydrofuran (5) have
`been obtained in water as the solvent, in relatively short
`reaction times, at 140 C and 70 bar of hydrogen.
`Hydrogenation of
`the aldehyde group of
`(2) and
`saturation of the furan ring may occur in the presence
`of most of the catalysts used, but a careful control of the
`experimental conditions also allows to stop selectively at
`the 2,5-bis(hydroxymethyl)-furan stage.
`Most of the recent works on hydrogenation of furan
`derivatives were mainly concerned with the improve-
`ment of catalytic systems to perform hydrogenation of
`furfural into (3), which is still industrially using copper
`chromites as catalysts. Copper chromites are effectively
`known to selectively hydrogenate the carbonyl function
`while leaving the C–C double bond unchanged. They are
`also known to deactivate relatively rapidly in the
`absence of stabilizing species [65]. Moreover, new
`environmental constraints prevent Cr-containing cata-
`lysts from being used in the future, and this led to the
`development of new C-based catalytic systems. Copper
`dispersed on activated carbon catalyst was shown to
`display a higher activity when reduced at 300 C than at
`400 C and a selectivity to furfuryl alcohol comparable
`to that obtained with copper chromite catalysts in the
`vapor-phase hydrogenation of furfural [66]. Also worth
`noting is the absence of deactivation for 10 h onstream.
`Raney nickel catalysts are known for their absence of
`selectivity in the hydrogenation of furfural into furfuryl
`alcohol. Hydrogenation of the carbonyl bond competes
`with ring saturation into tetrahydrofurfural and further
`hydrogenation of the aldehyde group to yield tetra-
`hydrofurfuryl alcohol. However,
`in the liquid-phase
`hydrogenation of furfural, it was recently reported that
`Raney nickel catalysts modified with heteropolyacid
`salts achieve hydrogenation of furfural
`into furfuryl
`alcohol with a selectivity in alcohol as high as 98.1% at a
`furfural conversion of 98.5%, at 80 C in ethanol as
`solvent and 20 bar of hydrogen [67].
`
`2.2.2. Hydroxymethylation of furfuryl alcohol
`In the preparation of 2,5-bis(hydroxymethyl)-furan
`resins (4) used as active and fast-curing foundry binders
`[7], the first step is the addition of formaldehyde to
`furfuryl alcohol (3) in the presence of acetic acid as
`solvent and catalyst, followed by further polymerization
`with the starting alcoholto produce resins with the
`desired properties (scheme 6).
`In the presence of solid catalysts such as ion-exchange
`resins, it was shown that it was possible to stop at the
`monomer stage, but with a 28% yield after 120 h of
`reaction [68].
`However, it has been shown that protonic zeolites
`with controlled structure and acidity were capable
`of performing hydroxymethylation of
`furfural
`into
`
`Petitioners' Exhibit 1020, Page 5 of 20
`
`

`
`16
`
`C. Moreau et al./Chemistry of substituted furans
`
`5-hydroxymethylfurfural, whereas ion-exchange resins
`were shown to be inactive [69–71]. The low selectivity
`was shown to result
`from the deactivation of
`the
`electron-withdrawing aldehyde group on the C-5 carbon
`position, but, when the electron demand of the sub-
`stituent was reversed, for example, by protecting the
`aldehyde function with an electron-donating 1,3-dithio-
`lane function, a high selectivity to the corresponding
`hydroxymethylated derivative was found. In view of the
`electron-donating properties of
`the hydroxymethyl
`group in furfuryl alcohol, it was then expected that the
`addition of formaldehyde should take place relatively
`selectively. Indeed, under appropriate operating condi-
`tions, in particular reaction kinetics, and catalyst choice
`avoiding formation of condensation products or poly-
`mers, high selectivities to the diol (4) have been obtained
`at a temperature close to 40–50 C, and with a large
`excess of formaldehyde in order to account for its weak
`adsorption onto the hydrophobic mordenite in protonic
`form with a Si/Al ratio of 100. Under these conditions,
`the selectivity to (4) is equal to or higher than 95%, even
`when the conversion of furfuryl alcohol is high, i.e., 80–
`90% corresponding to reaction times between 15 and
`30 min [69–71]. However, if a balance is required between
`conversion and selectivity, both parameters generally
`varying in opposite directions, the results obtained are
`particularly interesting since different sets of operating
`conditions can be found in order to be in line with
`industrial,
`energetic
`and economical
`aspects by
`controlling, at any time, the composition of the (3)/(4)
`mixture.
`
`2.2.3. Hydroxyethylation of furfuryl alcohol
`Similarly, hydroxyethylation of furfuryl alcohol was
`also investigated in order to obtain a new disubstituted
`alcohol that could be dehydrated to the corresponding
`vinyl derivative (scheme 7). 2-Alkenylfurans constitute
`an interesting class of potential monomers, since the
`
`
`
`
`
`Scheme 7. Reaction scheme for hydroxyethylation of furfuryl alcohol
`with aqueous acetaldehyde.
`
`reactivity of the olefinic bond in polyaddition systems
`should reflect the effect of the furan ring conjugated to
`the double bond, which has so far received relatively
`little attention [5].
`Hydroxyethylation of furfuryl alcohol (3) was then
`performed under experimental conditions close to those
`reported for the hydroxymethylation reaction, i.e., with
`a large excess of acetaldehyde, a low temperature, water
`as the solvent and a highly dealuminated H-mordenite
`as catalyst. The reaction scheme is shown in scheme 7
`and involves formation of 1-(5-hydroxymethyl-2-furyl)-
`ethanol (9) and its dehydration to the corresponding
`vinyl derivative, 5-hydroxymethyl-2-vinylfuran (8). The
`major problem was to avoid the aldolization and/or
`crotonization of acetaldehyde, resinification of (3) and
`condensation of the intermediate diol with another (3)
`molecule to give the dimer 1,1-bis-(5-hydroxymethyl-2-
`furyl)-ethane (10).
`In the presence of large-pore zeolites such as Y-
`Faujasites, the maximum selectivity to the carbinol (9) is
`reached relatively rapidly for the most reactive catalysts
`with Si/Al ratios of 15 and 21 respectively. The
`selectivity in diol
`is nearly constant with time for
`HY(15), while furfuryl alcohol conversion is always
`increasing without significant appearance of the other
`products (8) and (10). This means that a secondary
`reaction takes place through a parallel reaction network.
`MFI zeolites appeared as good candidates for the
`hydroxyethylation reaction because of their reduced
`pore dimensions (5.5 A˚ ) compared to those of other
`catalysts and therefore to their ability to reduce
`secondary reactions provided that the reaction occurs
`within the pores of the zeolite. Indeed, a selectivity of
`about 55% in the carbinol 1-(5-hydroxymethyl-2- furyl)-
`ethanol (9) is achieved within 2 h. Finally, appropriate
`modifications of
`the experimental conditions allow
`obtaining a selectivity as high as 95% in the intermediate
`carbinol, but with a rapid deactivation of the catalyst
`after 30 min of reaction [72].
`Anyway, in spite of the rapid deactivation of the
`catalyst, hydroxyethylation of
`furfuryl alcohol over
`zeolites is the first example reported for the catalytic
`hydroxyalkylation of heteroaromatics in the presence of
`acetaldehyde, which corresponds, in an indirect manner,
`to a Wittig reaction.
`
`2.3. 2,5-Furandicarboxaldehyde
`
`Oxidation of 5-hydroxymethylfurfural (2) is a reaction
`of particular interest as far as complete oxidation yields
`2,5-furandicarboxylic acid (7), a material that has proper-
`ties and applications similar to those of both terephthalic
`and isophthalic acid [5,6]. Other partially oxidized
`compounds (scheme 8) are all involved as intermediates
`for the preparation of surfactants, synthetic materials or
`resins [73]. Among them, another symmetric compound of
`
`Petitioners' Exhibit 1020, Page 6 of 20
`
`

`
`C. Moreau et al./Chemistry of substituted furans
`
`17
`
`interest is 2,5-furandicarboxaldehyde (6), used as such
`or as a precursor to symmetrical diamines and Schiff’s
`bases and more difficult to reach in a selective manner.
`In terms of market,
`terephthalaldehyde would be
`available at around 6–7 €/kg on a basis of 500 tons/year
`[63].
`Up to now, high yields of 2,5-furandicarboxaldehyde
`were only obtained under noncatalytical conditions, in the
`presence of stoichiometric quantities of classical oxidants
`[74,75] or in the presence of electrophilic agents [76].
`Several papers were devoted to the catalytic oxidation
`of 5-hydroxymethylfurfural (2). In the presence of noble
`metal catalysts, oxidation of (2) does not stop selectively
`at the dialdehyde stage [73,77,78]. On the contrary,
`selective oxidation of (2) into 2,5-furandicarboxylic acid
`and 5-formyl 2-furancarboxylic acid was achieved.
`Recent results on in situ oxidation of 5-hydroxymethyl-
`furfural starting from fructose and using a membrane
`reactor or encapsulation of a PtPb/C oxidation catalyst
`in silicon beads do not lead to any selectivity improve-
`ment [79].
`Hydrogen peroxide was then used as oxidant and the
`synthetic titanium silicalite (TS1) as recyclable catalyst
`[80]. Unfortunately, under operating conditions close or
`identical
`to those described in the literature for
`oxidation reactions over the TS1 catalyst in the presence
`of 30% aqueous hydrogen peroxide, attempts to oxidize
`5-hydroxymethylfurfural
`in methanol or water were
`unsuccessful as the maximum yield obtained in 2,5-
`furandicarboxaldehyde (6) was only 25%. Although the
`above process appeared as a very elegant one using a
`recyclable catalyst and water as both solvent and
`reaction product, we have turned to more conventional
`oxidation catalysts based on vanadium oxide as the
`
`HO
`
`H
`
`O
`
`H
`
`O
`
`H
`
`O
`O
`O
` 2,5-Furandicarboxaldehyde
`5-Hydroxymethylfurfural
` (6)
`(2)
`
`HO
`
`OH
`
`O
`
`O
`
`H
`
`O
`
`O
`
`O
`
`OH
`
`5-Hydroxymethyl-2-furancarboxylic acid 5-Formyl-2-furancarboxylic acid
` (11)
`
`(12)
`
`HO
`
`O
`
`O
`
`O
`
`OH
`
`2,5-Furandicarboxylic acid
`(7)
`
`Scheme 8. Reaction scheme for the oxidation of 5-hydroxymethyl-
`furfural.
`
`active phase and titanium oxide or aluminum oxide as
`carriers.
`The oxidation of 5-hydroxymethylfurfural into 2,5-
`furandicarboxaldehyde was carried out
`in a batch
`reactor at 90 C in the presence of
`supported
`V2O5=TiO2 catalysts prepared with different vanadium
`loadings, and toluene