165 Part III Chemical Reactions, Sustainable Processes, and Environment Ideas in Chemistry and Molecular Sciences Advances in Synthetic Chemistry Edited by Bruno Pignataro Copyright 2010 WILEY VCH V[.]
Trang 1Part III Chemical Reactions, Sustainable Processes, and Environment
Ideas in Chemistry and Molecular Sciences: Advances in Synthetic Chemistry Edited by Bruno Pignataro
Copyright 2010 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim
Trang 28 Furfural and Furfural-Based Industrial Chemicals
Ana S Dias, S´ergio Lima, Martyn Pillinger, and Anabela A Valente
8.1 Carbohydrates for Life
In recent years we have been confronted with the reduction of fossil oil reserves, fluctuations of fossil fuel prices and the increase of CO2 emissions, and the consequent problem of the greenhouse effect These environmental, social, and economic problems have created the need for sustainable alternatives to fossil fuels and chemicals [1] The use of plant biomass as starting material is one
of the alternatives to decrease the dependency on fossil oil The biomass can
be transformed into energy, transportation fuels, various chemical compounds, and materials such as natural fibers by biochemical, chemical, physical, and thermal processes (Figure 8.1) [2–6] However, when choosing the raw material,
it is important to avoid the competition with food and feed applications and the consequent rise in prices
Carbohydrates are among the most abundant organic compounds on earth and represent the major portion of the world’s annually renewable biomass Sources of carbohydrates include conventional forestry, by-products of wood processing (e.g., wood chips, pulp, and paper industrial residue), agricultural crops and surplus (e.g., corn stover, wheat, and rice straw), and plants (e.g., switchgrass) grown
on degraded soils and algae The bulk of the carbohydrate biomass comprises poly/oligosaccharides, such as hemicelluloses, cellulose, starch, inulin, and sucrose
In particular, lignocellulose plant matter is available in large quantities and is relatively cheap
Cellulose and hemicellulose can be found in the cell wall of all plants cells Cel-lulose is a linear polymer composed ofβ-d-glucopyranose (glucose) units forming
microfibrils that give strength and resistance to the cell wall The hemicellulose consists of a wide variety of polysaccharides (composed of pentoses, hexoses, hex-uronic acids), which are interspersed with the microfibrils of cellulose, conferring consistency and flexibility to the structure of the cell wall [8]
The fermentation and the chemical conversion of carbohydrates into value-added compounds have received increasing interest in the last decade, and in a biorefinery different advantages may be taken from both processes [9–16] Some of the most
Ideas in Chemistry and Molecular Sciences: Advances in Synthetic Chemistry Edited by Bruno Pignataro
Copyright 2010 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim
Trang 3Carbohydrates Starch Hemicellulose Cellulose
Lignin
Lipids, oil
SynGas C1 C2 C3 C4 C5 C6
Aromatics
Direct polymers
Methanol Ethanol Glycerol Lactic acid Propionic acid
Levulinic acid Furfural Lysine 5-Hydroxymethylfurfural
Malic acid Succinic acid
SynGas
Sugar
- Glucose
- Fructose
- Xylose
Lignin
Lipids/oil
Figure 8.1 Bio-based products from the different biomass feedstocks (Adapted from [7]).
important chemical transformations of carbohydrates are arguably the hydrolysis/ dehydration of polysaccharides into the furan platform products, furfural and 5-hydroxymethylfurfural (HMF) [16, 17] Furfural (Fur) has a wide industrial application profile and is considered as one of the top 30 building blocks that can be produced from biomass [7] HMF is promising as a versatile, renewable furan chemical for the production of chemicals, polymers, and biofuels, similar
to furfural [16, 18–20] While Fur has been produced on an industrial scale for decades, the production of HMF has not reached industrial scale, to the best of our knowledge
The hydrolysis/dehydration of polysaccharides into Fur and HMF may be promoted by Br¨onsted or Lewis acid catalysts The industrial use of aqueous mineral acids as catalysts, such as sulfuric acid for furfural production, poses serious operational (corrosion), safety, and environmental problems (large amounts
of toxic waste) Hence, it is desirable to replace conventional aqueous mineral acids
by ‘‘green’’ nontoxic catalysts for converting sugars into Fur 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 Several research groups have described approaches for converting hexoses (glucose and fructose) into HMF in the presence of solid acid catalysts with promising results [21–37] Recently, to prevent the nonselective HMF decomposition, Avantium Technologies, a company based in Amsterdam, has reported a new approach to obtain an HMF derivative In their work, the stable 5-(alkoxymethyl)furfural ether is formed from hexoses in the presence of an alcohol as solvent and an acid catalyst When the alcohol is ethanol, the resulting 5-(ethoxymethyl)furfural) has an energy density of 31.3 MJ l−1, which is as good as regular gasoline and diesel and significantly higher than ethanol The encouraging
Trang 48.3 Applications of Furfural 169
results of the engine tests performed with a Citroen Berlingo has boosted interest
in the development of furan products for application in transportation and aviation fuel/fuel additives and as bio-based polymers [16]
In this chapter, after an overview of the applications of Fur and the reaction mechanisms of dehydration/hydrolysis of polysaccharides into Fur, some of the most relevant results on the use of solid acid catalysts in the conversion of saccharides (in particular, xylose) into Fur are discussed
8.2 Fur – Evolution over Nearly Two Centuries
Furfural was discovered in 1821 by D¨obereiner, by the distillation of bran with dilute sulfuric acid [38, 39] The resulting compound was first named furfurol (the name
comes from the Latin word furfur that means bran cereal, while finishing ol means
oil) Between 1835 and 1840, Emmet noted that the fur could be obtained from the majority of vegetable substances The empirical formula of this product (C5H4O2) was discovered by Stenhouse in 1840 and, in the year 1845, with the discovery of the aldehyde function in the molecule, it was named furfural (al for aldehyde) The fur 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 Fur 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 [40]
The industrial production of fur was driven by the need of the United States
of America 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 Fur from oat hulls using ‘‘left over’’ reactors [40] Over time, there was an increased industrial production of Fur and the discovery of new applications [41] Currently, the annual world production of Fur is about 300 000 tons and, although there is industrial production in several countries, the main production units are located in China and in the Dominican Republic [42]
8.3 Applications of Furfural
The aldehyde group and furan ring furnish the Fur molecule with outstanding properties for use as a selective solvent [40, 41, 43] Fur 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 Fur is used
as a fungicide and nematocide in relatively low concentrations [40] Additional
Trang 5advantages of Fur as an agrochemical are its low cost, safe and easy application, and its relatively low toxicity to humans Despite the fact that Fur has an LD50
of 2330 mg kg−1 for dogs, man tolerates its presence in a wide variety of fruit juices, wine, coffee, and tea [40] The highest concentrations of Fur are present
in cocoa and coffee (55–255 ppm), in alcoholic beverages (1–33 ppm), and in brown bread (26 ppm) [44] Most of the fur produced worldwide is converted through a hydrogenation process into furfuryl alcohol (FA), which is used for manufacturing polymers and plastics Other furan compounds obtained from Fur include methylfuran and tetrahydrofuran Fur and many of its derivatives can be used for the synthesis of new polymers based on the chemistry of the furan ring [41, 43, 45, 46]
8.4 Mechanistic Considerations on the Conversion of Pentosans into Furfural
Commercially, the pentosans (mainly xylan) present in the hemicellulose fraction
of agricultural streams are hydrolyzed, using homogeneous acid catalysts in water, giving rise to pentose (xylose), which, by dehydration and cyclization reactions, leads to Fur with a theoretical mass yield of approximately 73% (Figure 8.2) The hydrolysis of pentosans into pentoses in the presence of H2SO4 is faster than the dehydration of the pentose monomers into Fur [40, 41] Hence, kinetic studies are generally focused on the rate limiting process, that is, the dehydration
of pentoses Xylose and arabinose are monomers found in pentosans, which can
be converted into Fur, and some studies have shown that the dehydration of arabinose is slower than that of xylose [40, 47] 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 [21, 40, 42, 43, 45, 46, 48, 49]
In the dehydration and cyclization of xylose into fur, three molecules of water are released per molecule of fur produced It is generally accepted that the xylose
to Fur conversion involves a complex reaction mechanism consisting of a series of elementary steps The two mechanisms presented have in common the fact that the furfural is formed from the xylopyranose ring and not from its open-chain aldehyde isomer (Figure 8.3 and 8.4) Considering the mechanism proposed by Zeitsch [40], the transformation of the pentose into Fur involves two eliminations
in the positions 1,2 and one elimination in the position 1,4 (Figure 8.3) The
O
O
O
H +
+
HO
Figure 8.2 Net reaction of conversion of pentosans into furfural.
Trang 68.4 Mechanistic Considerations on the Conversion of Pentosans into Furfural 171
O OH
OH HO
HO
1
2 3 4
OH
OH2 HO
HO
O OH HO
HO
O OH
H2O
HO
O OH HO
O
OH
H2O
O
OH
O O
H +
− H +
− H 2 O
− H 2 O
− H 2 O
Figure 8.3 Mechanism of the dehydration of pentoses into furfural proposed by Zeitsch [40].
O OH
OH HO
+
− H 2 O
− H 2 O
O
HO
O O
OH2 HO
HO
CHO
Figure 8.4 Reaction mechanism proposed by Antal et al.
involving the protonation of the hydroxyl group in position C-2[48].
1,2-eliminations imply the involvement of two neighboring carbon atoms and the formation of a double bond between them, while the 1,4-elimination involves two carbon atoms separated by two carbon atoms and the formation of the furan ring Zeitsch summarizes the mechanism of conversion of the pentose into Fur in an acidic medium as a result of the transformation of hydroxyl groups of the pentose into H2O+groups, leading to the liberation of water molecules with the formation
of carbocations
According to Antal et al [48], there are two mechanistic alternatives to obtain
Fur from d-xylose, depending on the hydroxyl group that is protonated first – the hydroxyl group at position 1 or 2 (Figure 8.4, only the mechanism resulting from the protonation of the hydroxyl group at the C2 position is shown) Both mechanisms involve the xylopyranose isomers, which lead to the formation of Fur by the loss of three molecules of water A recent study of the xylose degradation using quantum mechanics modeling showed that the protonation of the hydroxyl group
at position 2 is more favorable (requires less energy) than that of position 1 [50]
Trang 7In acidic medium, the open-chain xylose undergoes isomerization into lyxose, which may be further dehydrated into Fur, albeit at a lower rate than that observed for the dehydration of xylose into Fur [48]
By-products formed in the xylose reaction may derive from the fragmentation of xylose, such as glyceraldehyde, glycolaldehyde, lactic acid, acetol [48] On the other hand, as Fur is formed it can be transformed into higher molecular weight products
by (i) condensation reactions between Fur and intermediates of conversion of xylose
to furfural (and not directly with xylose) and (ii) Fur polymerization [40] Aldol condensation between two molecules of Fur does not occur due to the absence
of a carbon atom in Hα position in relation to the carbonyl group [51] The side
reactions (i) and (ii) lead to oligomers and polymers and (i) is considered to be more relevant than (ii), although published characterization studies of the by-products formed are scarce [40] The extent of these side reactions can be minimized by reducing the residence time of Fur in the reaction mixture and by increasing the reaction temperature [40, 49, 52] If Fur is kept in the gas phase during the aqueous-phase reaction, it will not react with intermediates that are ‘‘nonvolatile’’
On the other hand, in a nonboiling system Fur yield increases with temperature possibly due to the entropy effect The formation of by-products of high molecular weight results in a decrease of entropy and the change in Gibbs free energy (G) becomes less negative: in the equation G = H − TS, the term (−TS)
becomes positive [52] Increasing the temperature will eventually lead to G ∼ 0,
reached at the ceiling temperature (Tc) For T > T c, fragmentation rather than the combination of molecules is favored Another strategy for minimizing Fur losses
is using a cosolvent immiscible with water to extract Fur from the aqueous phase (where the dehydration reaction of xylose takes place) as it is formed [21] Extraction using supercritical CO2also enhances Fur yields [53–55]
The above mechanistic considerations for the homogeneous-phase conversion
of xylose into Fur 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
8.5 Production of Furfural
Industrially, Fur is directly produced from the lignocellulosic biomass in the presence of mineral acids, mainly sulfuric acid, under batch or continuous mode operation (Table 8.1) Attempts to improve Fur yields have been made by process innovation, although the use of mineral acids remains a drawback [40, 52, 56] 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 Fur is therefore one of many industrial processes
Trang 88.5 Production of Furfural 173 Table 8.1 Industrial processes of furfural production.
Industrial process Catalyst Reaction type Temperature ( ◦ C)
Quaker Oats H 2 SO 4 Batch 153
Agrifurane H 2 SO 4 Batch 177 −161 Quaker Oats H 2 SO 4 Continuous 184 Escher Wyss H 2 SO 4 Continuous 170 Rosenlew Acids formed from
the raw material
Continuous 180
where the replacement of the ‘‘toxic liquid’’ acid catalysts by alternative ‘‘green’’ catalysts is of high priority Attempts have been made to develop heterogeneous catalytic processes for Fur production that offer environmental and economic benefits, but to the best of our knowledge none have been commercialized
The acid properties of solid acids may be negatively affected by the presence of water in the reaction medium Hence, one of the critical parameters in the choice of
a stable, active heterogeneous catalyst is its tolerance toward water [57–64] Several water-tolerant solid acids have been investigated in the conversion of saccharides into furan derivatives, including inorganic oxides and resins Inorganic oxides have led to important improvements with respect to catalyst stability, recyclability, activity, and selectivity in comparison to conventional mineral acids and commercial acid ion exchange resins
8.5.1
Crystalline Microporous Silicates
Conventional microporous zeolites, such as Faujasite HY and H-Modernite, seem quite promising, achieving selectivities to Fur in the range of 90–95% at xylose conversions between 30 and 40%, with water as solvent and in the presence of toluene as cosolvent, at 170◦C [21, 23] However, xylose conversion has to be kept low in order to avoid significant drops in the Fur selectivity Even then, the authors observed the formation of coke on the surface of the catalysts [21, 22, 46]
A novel microporous niobium silicate denoted as AM-11 was reported in 1998 and found to be a promising catalyst for gas-phase dehydration reactions, such as
the conversion of tert-butanol to isobutene [65–67] This solid contains octahedral
niobium(V) and tetrahedral silicon, and the charge associated with framework niobium is balanced by Na+and NH4+cations Calcined AM-11 possesses a sub-stantial amount of Br¨onsted and Lewis acidity [66] Microporous AM-11 crystalline niobium silicates were studied as solid acid catalysts in the dehydration of xylose
in water/toluene biphasic conditions (water and toluene (W/T)), at 140–180◦C After 6 hours at 160◦C, xylose conversions of up to 90% and furfural yields of
up to 50% were achieved, and the thermally regenerated catalysts could be reused
Trang 9without loss of activity or selectivity [68] The calcined AM-11 (prepared in the NH4+form) catalysts gave higher Fur yields at 6 hours (46% at 85% conversion) than the protonic form of commercial HY (Si/Al= 5; 39% yield at 94% conver-sion) and H-MOR (protonic form of zeolite mordenite, Si/Al= 6; 28% at 79% conversion), under identical reaction conditions Zeolites and AM-11 materials are sufficiently stable to be used at elevated temperatures and to be regenerated (quite easily) by thermal treatments under air This constitutes an important advantage
in comparison to ion exchange resins as solid acid catalysts The catalytic results obtained with the crystalline solid acids may be further optimized by, for example, using different solvent mixtures and compositions The extensive studies carried out by the group of Dumesic and coworkers on the use of different solvent mixtures for the dehydration of sugars into HMF and Fur using mineral acids as catalysts at high temperatures give valuable insights on the solvent effects [18] Preferably, the solvent(s) should have an excellent extracting capacity of the furan compound and should be used in minimal amounts, avoiding high dilution and long heating times The reactor design is another important issue For example, Moreau reported that
a significant increase in HMF selectivity is obtained by simultaneous extraction of HMF with methyl isobutyl ketone (MIBK) circulating in a countercurrent manner
in a continuous catalytic heterogeneous pulsed column reactor [26]
8.5.2
Functionalized Mesoporous Silicas
The application of mesoporous solid acids to convert sugars into furan derivatives may be advantageous in relation to microporous materials by avoiding diffusion limitations and fast catalyst deactivation Micelle-templated mesoporous silicas are especially promising supports for liquid-phase acid catalysis because they have high specific surface area and pore volume, together with a regular pore structure and tuneable pore size, which enables rapid diffusion of reactants and products through the pores, thus minimizing consecutive reactions
Heteropolyacids (HPAs) are promising candidates as green catalysts and are already used in several industrial processes, such as the hydration of olefins [57,
59, 69–72] The advantages of HPAs in homogeneous liquid-phase catalysis are their low volatility, low corrosiveness, high flexibility, safety in handling, and generally high activity and selectivity compared to conventional mineral acids Furthermore, side reactions such as sulfonation, chlorination, and nitration, which normally occur in the presence of mineral acids, are absent in the reactions catalyzed by HPAs The Keggin-type HPAs are typically represented by the formula H8−x[XM12O40], where X is the heteroatom, x is its oxidation state and M is the addenda atom (Mo6 +or W6 +
H4SiMo12O40(SiMo) were investigated in the liquid-phase dehydration of d-xylose
to Fur [73] The catalytic results depend on the reaction temperature, type of solvent, and HPA composition The most promising systems were the tungsten-containing HPAs used with either dimethyl sulfoxide (DMSO) or W/T as solvent: Fur yields
Trang 108.5 Production of Furfural 175
70
60
50
40
30
20
0
10
Time (h)
Figure 8.5 Dependence of furfural yield on reaction time using DMSO as the solvent and PW (◦), SiW () or PMo
(–) as the catalyst, or using W/T as the solvent and PW (•)
or SiW ( ) as the catalyst, at 140 ◦C [73] Copyright Elsevier
(2005) with kind permission.
achieved within 8 hours at 140◦C were below 70% (Figure 8.5) The catalytic performance of the heteropolytungstate PW was on a par with that for sulfuric acid for the cyclodehydration of xylose into furfural, in homogeneous phase, using DMSO as solvent, at 140◦C Kinetic studies showed that the initial reaction rate exhibits a first-order dependence on the initial concentration of xylose and a nonlinear dependence on the initial concentration of HPA
Heterogenization of HPAs can facilitate product separation, catalyst recovery, and recycling [70, 71] When supporting HPAs on ordered mesoporous silica, the immobilized species may interact more strongly with plain silica, due to the high dispersions achieved [74] The most important and common HPAs for catalysis are the Keggin acids since they are the most stable and readily available ones In particular, PW possesses the highest acid strength and thermal stability [71] Complexation of PW with the hydroxyl groups of the hexagonally ordered mesoporous silica MCM-41 (Mobil Composition of Matter), for example, is thought
to lead to SiOH2+groups that can act as counterions for the polyanion [75–77] Catalysts based on PW supported on silica have been used in the dehydration of xylose [78] A series of composites comprising PW immobilized in micelle-templated silicas (e.g., MCM-41) with large unidimensional mesopores were prepared by either incipient wetness impregnation or immobilization in amino-functionalized silicas These materials exhibited higher activity than the bulk HPA, and Fur yields after 4 hours were similar to those obtained with H2SO4(58%), using DMSO as solvent, at 140◦C Strong host–guest interactions and active site isolation for the materials with low HPA loadings (15 wt%)