Chromophores from hexeneuronic acids: identification of HexA-derived chromophores
Hexeneuronic acids (HexA) have long been known as triggers for discoloration processes in glucuronoxylan-containing cellulosic pulps. They are formed under the conditions of pulping from 4-O-methylglucuronic acid residues, and are removed in an “A stage” along the bleaching sequences, which mainly comprises acidic washing treatments. The chemical structures of HexA-derived chromophoric compounds 4–8, which make up 90% of the HexA-derived chromophores, are reported here for the first time. The compounds are ladder-type, mixed quinoid-aromatic oligomers of the bis(furano)-[1,4]benzoquinone and bis(benzofurano)-[1,4]benzoquinone type. The same chromophoric compounds are generated independently of the starting material, which can be either a) HexA in pulp, b) the HexA model compound methyl 1-13C-4-deoxy-β-L-threo-hex-4-enopyranosiduronic acid (1) or c) a mixture of the primary degradation intermediates of 1, namely 5-formyl-furancarboxylic acid (2) and 2-furancarboxylic acid (3). Isotopic labeling (13C) in combination with NMR spectroscopy and mass spectrometry served for structure elucidation, and final confirmation was provided by X-ray structure analysis. 13C-Isotopic labeling was also used to establish the formation mechanisms, showing all the compounds to be composed of condensed, but otherwise largely intact, 2-carbonylfuran and 2-carbonylfuran-5-carboxylic acid moieties. These results disprove the frequent assumption that HexA-derived or furfural-derived chromophores are linear furanoid polymers, and might have a direct bearing on structure elucidation studies of “humins”, which are formed as dark-colored byproducts in depolymerization of pentosans and hexosans in different biorefinery scenarios.
KeywordsCellulose Pulp Chromophores Hexeneuronic acids Bleaching A-stage Furan Furancarboxylic acid Ladder-type oligomers
The pulp and paper industry is a major economic factor in many countries. Today, with biorefinery topics becoming more and more prominent, paper mills are in transition from classical pulp producers to more diverse biorefineries. But still, the major product of these industries is cellulose, with pulping and bleaching being the two main operations in its manufacture. A major part of chemical, energy and technology costs is devoted to removing chromophores from pulp to reach target brightness. The fewer chromophores are left in pulp, the more difficult it gets to remove the remaining ones. For glucuronoxylan-containing pulps, the role of hexeneuronic acids (HexA) has been recognized as critical with regard to color formation (“brightness reversion”). The origin of this moiety in pulp has been briefly repeated in the previous part of this series (Rosenau et al. 2017).
By applying the well-established technique of CRI (chromophore release and identification method, see also the previous parts of this series) (Rosenau et al. 2004, 2005, 2007; Korntner et al. 2015), identification of the chromophores formed from HexA or from its furanoid primary degradation products was possible, and their structures—all of them being of the bis(furano)-[1,4]benzoquinone or bis(benzofurano)-[1,4]benzoquinone type—are hereby reported.
Materials and methods
Commercial chemicals were of the highest grade available and were used without further purification.1 Reagent-grade solvents were used for all extractions and workup procedures. Distilled water was used for all aqueous extractions and for all aqueous solutions. n-Hexane, diethyl ether, ethyl acetate, and petroleum ether for chromatography were distilled before use. All reactions involving non-aqueous conditions were conducted in oven-dried (140 °C, overnight) or flame-dried glassware under an inert argon or nitrogen atmosphere. TLC was performed using Merck silica gel 60 F254 pre-coated plates. Flash column chromatography was performed using Baker silica gel (40 µm particle size). All products were purified to homogeneity by TLC/GCMS analysis. The use of brine refers to saturated aqueous NaCl solution. All given yields refer to isolated, pure products.
1H NMR spectra were recorded at 400.13 MHz for 1H and at 100.62 MHz for 13C NMR in CDCl3 at room temperature, if not stated otherwise. Chemical shifts, relative to tetramethylsilane (TMS) as the internal standard, are given in δ ppm values, and coupling constants are given in Hz. 13C peaks were assigned by means of high-resolved 1D-13C, APT, HSQC and HMBC spectra.
GCMS analyses were carried out on an Agilent 6890 N/5975B instrument in the ESI (70 eV) ionization mode. Melting points, determined on a Kofler-type micro hot stage with Reichert-Biovar microscope, are uncorrected. Elemental analyses were performed at the Microanalytical Laboratory at the University of Vienna. All compounds showed satisfactory microanalytical data within the limits of 0.24%.
Analytical data of the two cellulosic pulps used
Molar massa (g/mol)
Carbonyl group contenta (µmol/g)
Carboxyl group contentb (µmol/g)
HexA contentc (µmol/g)
Brightnessd (% ISO)
Carbohydrate compositione (%0)
Gal 0.2%, Glu 91.8%, Xyl 6.5%
Gal 0.3%, Glu 79.3%, Xyl 19.1%
GPC and CCOA method
The gel permeation chromatography of celluloses with concomitant Mw-related profiling of carbonyl groups (Potthast et al. 2005) (“CCOA method”) (Röhrling et al. 2002a, b; Potthast et al. 2003) and carboxyl groups (“FDAM method”) (Bohrn et al. 2005, 2006) was based on multiple detection by refractive index (RI), multi-angle laser light scattering (MALLS) and fluorescence, using the setup of instruments and protocol of the selective fluorescence labeling (“CCOA method”) as previously reported. The method was applied to characterization of the different pulp samples. Especially the carboxyl-selective FDAM technique is useful in the present case to reflect changes in HexA content through the inherent carboxylic moieties.
Isolation of HexA-derived chromophores from pulp according to the CRI method
The CRI method was used as previously published (Rosenau et al. 2004, 2007). In summary, the pulp was suspended in a highly pure (bidistilled) solvent in the presence of BF3 acetate complex/sodium sulfite as catalysts. CRI was applied to 2.8 kg of the pulp samples to obtain chromophores in sufficiently “large” amounts. This treatment releases all electron-rich aromatic and/or quinoid compounds, which are subsequently chromatographically separated under careful exclusion of oxygen and in the presence of tocopherol antioxidants in the eluent.
The amount of the individual components was between 0.3 and 1.2 mg, which normally would have been sufficient to derive the structure of the compounds by a combination of NMR, IR and MS. Due to the highly symmetric structure of the compounds, the spectra (especially in 1H and 13C NMR) were not highly characteristic and revealing. Fortunately, the high tendency of the compounds to crystallize could be used to establish their identity by single crystal X-ray structure as compounds 4–8, which was then correlated with the results of the spectroscopic techniques.
Comparison of the analytical data showed the compounds to be identical to those obtained from HexA model compound 1 and a mixture of its primary degradation products 5-formylfuran-2-carboxylic acid (2) and furan-2-carboxylic acid (3), see below.
Isolation of HexA-derived chromophores from model compound 1
Methyl 4-deoxy-β-L-threo-hex-4-enopyranosiduronic acid (1, 190 mg, 1 mmol), or its isotopomers 1a-1f, were dissolved in 10 ml of sulfuric acid (pH 0) which had been deaerated by flushing with argon for 5 min. The mixture was stirred under argon and under exclusion of light (vessel wrapped in aluminum foil) at 50 °C. The solution turned yellow immediately and the color changed gradually into dark brown within 10 min. After about 30 min a brownish-black precipitate formed while at the same time the liquid phase turned nearly colorless again. TLC analysis (MeOH/CH2Cl2/H2O, v/v/v = 5:10:1) of the precipitate showed the presence of 5 compounds which did not change over prolonged stirring. After 2 h the stirring was discontinued and the solution cooled to r.t. TLC control showed the liquid phase to be free from organic solutes, indicating complete conversion of initially soluble organics into the insoluble black precipitate. The precipitate was washed with distilled water until the washings were neutral, and subsequently with 5 ml of diethylether to remove adhering water. The precipitate was dissolved in 2 ml of N,N-dimethylformamide, diluted with 5 ml of dry toluene (HPLC grade), transferred to a chromatographic column and chromatographed on silica gel using toluene/ethanol (v/v = 95:5) as the eluent, with the ethanol component in the binary solvent system significantly improving the separation of the carboxyl-containing compounds as compared to pure toluene. Five well-separable compounds were eluted, corresponding to 92% of the starting mass.
A mixture of trace compounds, less colored than the main products and corresponding to 3% of the starting mass, was eluted with toluene/ethanol (v/v = 85:15) at 45 °C. This mixture proved to be inseparable; it formed a brownish film upon drying that proved to be completely insoluble in all solvents tried, also under prolonged stirring and warming. Due to its mass-wise insignificance, this side product mixture was not further studied and discarded.
The five main compounds eluted were identical to compounds 4-8 as identified above. Elution order under the condition applied was 4 – 6 – 8 – 7 – 5.
Isolation of HexA-derived chromophores from the primary degradation products of HexA, 5-formylfuran-2-carboxylic acid (2) and furan-2-carboxylic acid (3)
A mixture of 5-formylfuran-2-carboxylic acid (2) and furan-2-carboxylic acid (3) was prepared by careful, mildly acidic treatment (pH 4.5 in aqueous phosphate buffer, r.t.) of methyl 4-deoxy-β-L-threo-hex-4-enopyranosiduronic acid (1, 190 mg, 1 mmol), according to the procedure given in the preceding part of this series. The reaction mixture was adjusted to pH 0 with 1 N sulfuric acid and further treated as given above for the direct degradation of 1.
Alternatively, a 2:1 mixture (mol/mol) of 5-formylfuran-2-carboxylic acid (2) and furan-2-carboxylic acid (3) or the respective isotopomers (2a–f and 3b–f) was dissolved in sulfuric acid (pH 0) and was further treated as above. A changed ratio between 2 and 3 changed the ratio among the five chromatographically isolated compounds, but did not afford any other different compounds than those five identified above.
Analytical data of the HexA-derived chromophores (4–8)
The following section lists the analytical data of the five chromophores isolated from hexeneuronic acid moieties, both in pulp and model compounds. The X-ray diffraction data have been deposited at the Cambridge Structural Database (CSD) and can be obtained for each of the five compounds free of charge.2
Furo[2,3-f]benzofuran-4,8-dione (4). M = 188.14 g mol−1. Calcd. for C10H4O4: C 63.84, H 2.14; found C 63.94, H 2.19. TLC: toluene/ethanol = 90:10, Rf = 0.42. m.p. = 201–204 °C (literature: m.p. >185 °C) (Takahashi and Kobayashi 2000). UV/Vis (CHCl3): λmax [nm] (log ε) 315 (3.8), 255 (4.0), 245 (4.0), 230 (4.3). MS m/z 188.2 (M+). 1H NMR: δ 7.73 (d, J = 1.8 Hz, 2H, H-3/H-7), 6.94 (d, J = 1.8 Hz, 2H, H-2/H-6). 13C NMR: δ 170.9 (C-4/C-8), 152.0 (C-4a/C-8a), 148.2 (C-2/C-6), 128.4 (C-3a/C-7a), 108.3 (C-3/C-7). The NMR data agree with those given in the literature within limits of less than 0.03 ppm for 1H and less than 0.4 ppm for 13C (Takahashi and Kobayashi 2000).
4,8-Dioxofuro[2,3-f]benzofuran-2,6-dicarboxylic acid (5). M = 276.16 g mol−1. Calcd. for C12H4O8: C 52.19, H 1.46; found C 52.25, H 1.42. TLC: toluene/ethanol = 90:10, Rf = 0.94; toluene/ethanol = 75:25, Rf = 0.71. m.p. >230 °C (decomp.). MS m/z 276.3 (M+). 1H NMR: δ 7.24 (s, 2H, H-3/H-7), 6.30 (s, br, 2H, COOH). 13C NMR: δ 170.9 (C-4/C-8), 163.9 (2-COOH/6-COOH), 151.9 (C-2/C-6), 151.4 (C-4a/C-8a), 129.1 (C-3a/C-7a), 114.0 (C-3/C-7).
5-Hydroxy-4,10-dioxo-benzofuro[5,6-b]benzofuran-8-carboxylic acid (6). M = 298.20 g mol−1. Calcd. for C15H6O7: C 60.42, H 2.03; found C 60.31, H 2.18. TLC: toluene/ethanol = 90:10, Rf = 0.60. m.p. > 255 °C (decomp.). MS m/z 298.4 (M+). 1H NMR: δ 10.3 (s, 1H, COOH), 7.73 (d, J = 1.8 Hz, 1H, H-1), 7.64 (d, J = 8.4 Hz, 1H, H-7), 6.92 (d, J = 1.8 Hz, 1-H, H-2), 6.55 (d, J = 8.4 Hz, 1H, H-6). 13C NMR: δ 175.0 (C-4), 172.2 (C-10), 167.3 (COOH), 157.0 (C-5), 151.4 (C-2), 150.9 (C-3a), 149.4 (C-8a), 149.2 (C-9a), 131.3 (C-7), 130.9 (C-10a), 124.8 (C-4a), 116.8 (C-4b), 114.6 (C-8), 114.3 (C-6), 108.9 (C-1).
1,7-Dihydroxy-6,12-dioxo-6,12-dihydro-benzo[1,2-b:4,5-b’]bisbenzofuran-4,10-dicar-boxylic acid (7). M = 408.27 g mol−1. Calcd. for C20H8O10: C 58.84, H 1.98; found C 58.72, H 2.08. TLC: toluene/ethanol = 90:10, Rf = 0.75. m.p. > 280 °C (decomp.). MS m/z 408.1 (M+). 1H NMR: δ 7.75 (d, J = 8.6 Hz, 2H, H-3/H-9), 6.95 (d, J = 8.6 Hz, 2H, H-2, H-8). 13C NMR: δ 175.0 (C-6/C-12), 166.3 (COOH/COOH), 156.7 (C-1/C-7), 150.2 (C-4a/C-10a), 148.1 (C-5a/C-11a), 133.9 (C-3/C-9), 127.8 (C-6a/C-12a), 115.0 (C-6b/C-12b), 114.8 (C-4/C-10), 114.3 (C-2/C-8).
Benzo[1,2-b:4,5-b’][3a,14a:7a,10a]difuran-bisbenzofuran-4,6,7,11,13,14-hexaone (8). M = 428.26 g mol−1. Calcd. for C22H4O10: C 61.70, H 0.94; found C 61.93, H 0.94. TLC: toluene/ethanol = 90:10, Rf = 0.68. m.p. > 230 °C (decomp.). MS m/z 428.5 (M+). 1H NMR: δ 7.75 (d, J = 1.8 Hz, 2H, H-3/H-10), 6.95 (d, J = 1.8 Hz, 2H, H-2/H-9). 13C NMR: δ 172.6 (C-4/C-11), 172.0 (C-6/C-13), 170.2 (C-7/C-14), 154.9 (C-4a/C-11a), 154.4 (C-5a/C-12a), 153.4 (C-7a/C-14a), 148.2 (C-2/C-9), 126.8 (C-3a/C-10a), 123.2 (C-6b/C-13b), 123.0 (C-6a/C-13a), 108.3 (C-3/C-10).
Results and discussion
Structure of the HexA-derived chromophores
In a next set of experiments, the HexA model compound methyl 4-deoxy-β-L-threo-hex-4-enopyranosiduronic acid (1)3 was used as the starting material and was subject to thermal and acidic stress—under conditions similar to industrial acidic washing stages. The black precipitate obtained was separated by column chromatography and provided a mixture of five well-defined compounds that, most notably, were identical to those obtained from the pulp. In the model compound case, the combined mass of the five chromophoric compounds amounted almost to the whole mass of aging products, nearly 95%. The similarity of the compounds to those isolated from pulp was readily proven by identical retention times in thin layer chromatography (TLC), high-performance chromatography (HPLC), using toluene with 10% ethanol as the eluent in both cases, and by HR-MS. We were thus able to conclude that, first, model compound 1 was indeed a quite fitting model for the HexA unit in pulp and, second, it produced the same chromophoric compounds as the real-world pulp.
However, determination of the chemical structures of the compounds was far from being trivial. While mass spectrometry allowed deriving a chemical formula that corresponded with that derived from microanalysis (see experimental), fragmentation patterns were largely inconclusive. Evidently, all compounds contained no other elements than carbon, hydrogen, and oxygen. The molecular formulae derived indicated structures containing 10 (4), 12 (5), 15 (6), 18 (7), and 22 (8) carbon atoms with rather low C/H ratios of 2.5 (4,6,7), 3 (5) and even 5.5 (8). This was clearly indicative of highly condensed structures which were in addition moderately oxygenated (C/O ratios between 2 and 2.5). While not much information was expected from the 1H NMR spectra because of the low H content, also most 13C spectra NMR spectra were disappointingly void of indicative resonances, although well resolved. Except for 6, which showed the expected number of 15 carbons, all other compounds displayed peaks only for half the number of contained carbons. Evidently, those compounds were symmetric with a two-fold symmetry element and exclusively contained pairs of magnetically equivalent carbons. Moreover, all peaks were rather sharp and well-resolved so that inclusion compounds or valence isomerism were excluded. The opposite case was observed for compounds contained as key chromophores in pulps, such as 2,5-dihydroxy-1,4-benzoquinones (DHBQ) (Hosoya et al. 2013a) or 5,8-dihydroxy-1,4-naphthoquinone (Hosoya et al. 2013b), which are highly resonance-stabilized moieties and hence give rather broad resonances in 13C NMR. To conclude, we failed to derive the chemical structure of the five isolated compounds by a combination of the conventional analytical techniques (NMR, MS, IR, microanalysis).
Properties, structure comparison, and reactivity of the chromophores
Compound 4 is a known compound—it has been reported as an intermediate in the synthesis of organic conductors (Takahashi and Kobayashi 2000). The other four compounds have not been described so far and represent novel structures. With the exception of 6, all compounds are symmetric with an inversion center I. This corresponds to the NMR data showing only “half the number” of carbon resonances for those four molecules, and to the mass spectra being poor in informative fragmentation mass peaks. The inversion center is evidently located in the geometric middle of the central 1,4-benzoquinone moiety in these molecules.
From the viewpoint of formal structure, compound 4 can be seen as a basic motif from which the other four compounds are derived: 5 has two carboxyl substituents in addition, 7 two anellated p-hydroxybenzoic acid moieties, and 8 two [2,3-f]-anellated benzofuran units. In all of these compounds the additional substituents (relative to 4) are introduced in a symmetric way, so that the inversion symmetry is maintained. Only in compound 6, just one p-hy-droxy-benzoic acid substituent is appended onto 4, breaking the symmetry; 6 is the only non-symmetric compound among the isolated chromophores, with each of its 15 carbons giving an individual resonance in 13C NMR since no magnetic equivalence due to symmetry reasons is prevailing.
The compounds are ladder-type oligomers of mixed furanoid/benzoid and mixed aromatic/quinoid nature. In all five chromophores, there are six-membered rings, either p-benzoquinone units or p-hydroxybenzoic acid units, anellated to (= fused with) furan units, i.e. the six-membered rings are amended with ethenyloxy units. By this anellation (lat. anellus = little ring), a C = C double bond is at the same time part of the furan unit and the p-benzoquinone unit (or p-hydroxybenzoic acid, respectively). The furan-fused benzoquinones can also be conceived as furan moieties linked through carbonyl bridges; alternatively, they can be seen as partially oxidized phenolic units that are at the same time linked through ortho-positioned aryl-O-aryl and aryl–aryl carbon bonds, or as 2,5-dihydroxy-1,4-benzoquinone units of which the hydroxyl groups are incorporated into fused heterocyclic aromatics (furan units) or etherified with phenol derivatives (2-hydroxy group of 2,4-dihy-droxy-benzoic acid). It is interesting that every second furan unit is flipped by 180° so that the two oxygens of the furans constitute at the same time two para-configured oxygen-substituents of the connecting six-membered structures. The compounds seem to have a pronounced Janus character with regard to contained structural elements, and these ambiguities in the structural description already reflect the fact that the electronic structure will be rather complex and cannot be described in simple terms of (anti)aromaticity or substituent effects. The UV/Vis and fluorescence properties of the chromophores are currently studied, along with a computational treatment of their electronic structures. Preliminary results indicate interesting semiconducting properties of the parent compound and predict the derived (radical)-ions to be very stable or even persistent, both being interesting features for organic conductors. This indeed would be a completely new and unexpected facet of the unwanted “black sludge” from HexA degradation. This matter will be discussed in an upcoming account.
It is important to note that the structure of the compounds as identified does not agree with linear furan polymers as postulated in the literature (Gopalakrishnan et al. 2014; Gandini 2008, 2011; Gandini et al. 2008, 2009). Hitherto it was assumed that furfural and other furan derivatives link in a way that furan units are bridged by C1 links and form relatively flexible systems with extended conjugation that accounts for their color.
The structure determination of the chromophores might have yet another important implication. In biorefinery scenarios, hexoses and pentoses are often converted into furan derivatives that are to be further used as chemical building blocks. Formation of furfural from xylose/xylan and 5-hydroxymethylfurfural (HMF) from glucose/cellulose are the most well-known examples. These conversions are always accompanied by formation of black condensation products that decrease the yield and cause problems in separation and purification. These dark by-products are usually denoted as “humins”—perhaps indicating similar color and complexity, but being structurally completely different from humic matter in soil. Such humins are formed by side reactions of mono-substituted or disubstituted furans, exactly as the HexA-derived chromophores were. Without having done in-depth structure determination of humins formed upon furfural or HMF production, we would like to point out the likelihood that those humins have very similar structures to the chromophores derived from HexA, thus being similar ladder-type oligomers of mixed aromatic and quinoid characters and mixed five-membered and six-membered structural building blocks.
The solution as to the formation mechanism was as simple as it was plausible. From a retrosynthetic point of view, the “trick” was to dissect the molecule not into p-benzoquinoid structures, but rather into furanoid fragments (Scheme 4). This way, compound 4 is composed of two 2-furanoyl (2-carbonylfuran) units that are linked in a symmetric way so that the carbonyl group of the first unit is linked to C-3 of the second, and the carbonyl group of the second linked to C-3 of the first furan. Compound 5 would be constructed in a similar way, with two 2-furanoyl-5-carboxylic acid units being symmetrically joined through carbonyl-C3 bonds. The two 2-furanoyl units making up 4 are two furan-2-carboxylic acid (3) units that are completely unchanged (apart from the two bonds linking them and a hydroxyl loss at the COOH group). The same applies to the 2-furanoyl-5-carboxylic acid units in 5: they represent almost unchanged 5-formylfuran-2-carboxylic acid (2) precursors.
One interesting aspect of compound 5 was that the carboxyl groups of the former HexA units (C6) are engaged in the bonds making up the central p-benzoquinone unit (Scheme 6). The two “exo”-carboxyl groups, in turn, actually represent the former C1-carbons in HexA, i.e. formyl (aldehyde) groups. However, in all isolated chromophores (and also in spectra of chromophore mixtures) we have never detected any aldehyde functions, which are readily discernible in NMR spectra by proton resonances >9 ppm and carbon resonances >180 ppm. Evidently, the aldehyde groups have been oxidized to the corresponding carboxylic acids quite rapidly. An experiment in an inert atmosphere could verify that this oxidation is—at least in part—an autoxidation by oxygen: the chromophore mixtures initially contained aldehydes which had disappeared after 1 h in contact with air. But neither was it examined to which chromophores the unstable aldehydes belonged nor whether an additional redox process with an oxidizing intermediate was involved. Since the labeling experiments were able to unambiguously establish the position and linkages of the 2-furanoyl building blocks the minor issue of the oxidant in the C1-oxidation was not further followed.
The oxygen-bridged intermediate (IDA in Scheme 9) would open up to eventually give a p-hydroxybenzoic acid motif, as indeed observed in 6 and 7. Detailed mechanistic investigations—which will be communicated in a separate account—have shown 5-formylfuran-2-carboxylic acid (2) to be the actual co-reactant of core chromophore 4 and thus precursor of 6 and 7. Furan-2-carboxylic acid (3) reacts in a similar Diels–Alder mode with 4, but the final products have anellated benzoic acid (not p-hydroxybenzoic acid) units and are thus missing the hydroxyl groups of 6 and 7 (they can be imagined as deoxy-6 and bisdeoxy-7). From the Diels–Alder intermediate IDA, opening of the oxygen bridge is the first step that proceeds regioselectively, governed by the higher stability of the intermediate tertiary carbenium ion in α-position to the carboxyl function, followed by loss of the neighboring proton. The formyl group is lost in an unconventional process similar to the deformylation described in the preceding paper. Why the formyl substituent is eliminated—which would be expected to be reasonably stable—and not the hydroxyl group—which on the other hand would be reckoned to be rather elimination-prone—will be discussed in the above-mentioned treatise, also in connection with computational results. The final step is an oxidation of the cyclohexenone (hydroxycyclohexadiene) product driven by rearomatization, to establish the p-hydroxybenzoic acid moieties found in chromophores 6 and 7.
With the linkage analysis by means of isotopic labeling, all chromophores 4-8 were shown to consist of non-fragmented and largely unchanged furanoyl units (Schemes 5, 6, 7, 8, 9). It should be noted that this is in stark contrast to the chromophores from (over-)oxidized celluloses, such as 2,5-dihydroxy-1,4-benzoquinone or 5,6-dihydroxy-1,4-naphthoquinone, which are formed by complex degradation of oxidized units in cellulose to C2–C4 units and recondensation of such small fragments. In the HexA-derived chromophores, the furanoyl degradation intermediates 2-furancarboxylic acid (3) and 5-formylfuran-2-carboxylic acid (2) are linked together and undergo follow-up transformations, but they remain largely intact without further degradation. They build up the chromophores like whole bricks a wall.
In the present description, the chemical structures of chromophores derived from hexeneuronic acids in cellulosic pulps (HexA), compounds 4-8, have been elucidated for the first time, and comprehensive analytical characterization was performed. There are no differences between the chromophores isolated from real-world pulp and those derived from HexA model compound 1, which clearly argues in favor of the suitability of this model compound. The number of chromophores generated from HexA is rather limited, with five chromophores accounting for more than 90% of the total mass of chromophores generated. In all isolated chromophores, largely intact 2-furanoyl units and 2-furanoyl-5-carboxylic acid units are contained. The small number and the intact building blocks are two distinct differences to chromophores isolated from cellulosic pulps as a consequence of oxidation and aging: in that case many different chromophores are formed by a far-reaching degradation of oxidized cellulose units and recondensation of fragments to the actual chromophores.
The HexA-derived chromophores 4–8 are ladder-type, mixed furanoid/benzoid and mixed aromatic/quinoid oligomers. They are strictly planar, highly conjugated compounds and highly chromophoric. Even a 1 nM equilomar mixture of the five compounds still exhibits a yellow shade. The formation of the chromophores from the underlying 2-furanoyl units and 2-furanoyl-5-carboxylic acid units was clarified by means of 13C-isotopomers of model compound 1. By 13C NMR units where two 13C labels have become directly covalently linked in the product are easily identified by the homonuclear 1JC,C coupling causing the appearances of doublets instead of only singlets that are found otherwise (without direct carbon–carbon coupling between the labels). Similar to a puzzle where the arrangement of the individual pieces must fit to give the final picture, the number of furanoyl units per chromophore molecule and their linkage sites can be determined by this methodology, which was done for all five chromophores. The combination of isotopic labeling and NMR spectroscopy, as also used previously (Adelwöhrer et al. 2009; Krainz et al. 2010; Malz et al. 2007), once more proved to be a powerful tool in chromophore research.
In the follow-up papers in this series, we will explore the chemical behavior of the HexA-derived chromophores under bleaching conditions and their interrelation with the three key chromophore classes (Rosenau et al. 2005) found in cellulosic materials.
Note: this part on general analysis is equal to the corresponding description part in the preceeding part of this series as the work is closely related—no unlawful duplication is intended.
See www.ccdc.cam.ac.uk for more information.
See the preceding part of this series for synthesis of the compound and its 13C-isotopomers.
Open access funding provided by University of Natural Resources and Life Sciences Vienna (BOKU). The authors would like to thank the Austrian Forschungsförderungsgesellschaft (FFG) for financial support through Project 847169 (“Chromophores II”) and through the project FLIPPR2 (Future Lignin and Pulp Processing Research), along with its partner companies. We are indebted to the partner companies in this project for providing cellulosic pulps, valuable inputs to every-day pulping and bleaching practice as well as interesting discussions.
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