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Rinnakkaistallenteet Luonnontieteiden ja metsätieteiden tiedekunta
2019
Kraft lignin reaction with paraformaldehyde
Paananen, Hanna
Walter de Gruyter GmbH
Tieteelliset aikakauslehtiartikkelit
© 2019 Walter de Gruyter GmbH, Berlin/Boston.
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http://dx.doi.org/10.1515/hf-2019-0147
https://erepo.uef.fi/handle/123456789/24168
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KRAFT LIGNIN REACTION WITH PARAFORMALDEHYDE
Hanna Paananen and Tuula T. Pakkanen
Department of Chemistry, University of Eastern Finland, P.O. Box 111, FI-80101, Joensuu, Finland Corresponding Author: tuula.pakkanen@uef.fi, tel. +358504354379
ABSTRACT
Lignin is the second most abundant biopolymer and will be an important source for carbon-containing compounds in the future. Based on their similar phenolic structures, lignin has great potential to become a valuable substitute for phenol in phenol-formaldehyde resin adhesives. To meet this aim, the NaOH-catalyzed reaction of kraft lignin with formaldehyde was studied by using paraformaldehyde as a formaldehyde source. The advantage of using paraformaldehyde, the solid polymer of formaldehyde, is the simple composition of the depolymerized solution. According to the results of differential scanning calorimetry (DSC), the lignin reaction was found to require a high NaOH concentration in order for the reaction with paraformaldehyde to proceed at reasonably low temperatures compared to the curing temperature of phenol-formaldehyde resins (approximately 150
C). On the other hand, high alkalinity conditions are known to favor the disproportionation of
formaldehyde to formic acid and methanol. Due to the moderate reactivity of lignin, the Cannizzaro reaction can compete with the methylolation reaction of lignin. Based on the results of 13C, 31P and
1H-13C HSQC NMR, methylolation was found to be the main reaction occurring in the lignin- formaldehyde reaction.
Keywords: differential scanning calorimetry; 1H-13C HSQC NMR; metylolation of lignin;
paraformaldehyde; softwood kraft lignin
* Corresponding author: Tuula T. Pakkanen, Phone: +358 50 435 4379; e-mail: tuula.pakkanen@uef.fi
2 INTRODUCTION
Lignin is the second most abundant biopolymer and is one of the few sources of aromatic compounds in nature. The large amounts of waste lignin available from the pulping industry have initiated large- scale research on the utilization of lignin in various applications. (Zakzeski et al., 2010) Based on similar phenolic structures, one of the main future applications of lignin can be the partial or full replacement of phenol in phenol-formaldehyde resins. (Zhao et al. 1994, Moubarik et al., 2013) Lignin-phenol-formaldehyde resins have already attracted attention because of their promising results (Khan et al., 2004) in usability and mechanical tests. (Zhao et al. 1994, Danielson & Simonson, 1998a,b)
The aromatic structure of lignin is based on three different types of monolignol units, the ratio of which has an influence on the reactivity of lignin. (Shimizu et al., 2012; Wang et al., 2016; Pang et al., 2017) Often, lignin has to be preactivated by depolymerization to incorporate lignin into e.g., the phenol-formaldehyde resin structure. Evaluation of the lignin reactivity is important for optimization of the depolymerization degree. (Du et al., 2014) The reactivity of lignin has been previously studied with the Mannich reaction, which involves the condensation reaction of a carbonyl compound with dimethylamine and formaldehyde. (Wang et al., 2016) Equation 1 presents the Mannich reaction for a guaiacyl unit of lignin. The main product of the reaction is the condensation product of lignin but also hydroxymethylation (methylolation) of lignin by formaldehyde is possible in this reaction. The reactivity value is determined after the reaction on the basis of the nitrogen content of the lignin. In this study, a reaction of lignin with formaldehyde derived from paraformaldehyde was chosen and suitable conditions for the reactivity assessment were examined using differential scanning calorimetry (DSC).
3 (1)
Paraformaldehyde (PFA), the solid polymer of formaldehyde, is easy to transport, store and handle.
(Thavarajah et al., 2012) The number of monomers in paraformaldehyde can range up to 100.
Paraformaldehyde can be depolymerized to the formaldehyde monomer by heating above 100°C (Grajales et al., 2015) or by using warm water (approximately 60°C) under slightly basic conditions.
(Helander, 2000; Kiernan, 2000) An aqueous solution of depolymerized paraformaldehyde does not need methanol as a stabilizer, which is the important component in the commonly used 37%
formaldehyde solution to reduce oligomerization of different compounds derived from formaldehyde and its reaction with methanol.
Paraformaldehyde has been used in the preparation of phenolic resins. (Rojas & Williams, 1979;
Cook & Hess, 1991; Park et al., 2002; Kouisni et al., 2012; Taverna et al., 2019) The advantages of paraformaldehyde over the 37% formaldehyde solution have been found to be a higher reaction rate and better reaction yields. (Rojas & Williams, 1979) Both water and methanol had a notable decelerating influence on the polymerization rate. Recently, paraformaldehyde has become an important one-carbon reagent source for synthetic organic chemistry. (Li & Wu, 2015) Paraformaldehyde serves as a practical source of HCHO for organic reactions in which anhydrous reaction conditions are required. (Grajales et al., 2015) Further, aqueous solutions of paraformaldehyde are very well known among biochemists (Fox et al., 1985; Nogueira et al., 1997;
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Kiernan, 2000; Thavarajah et al., 2012) and are widely used for chemical fixation because methanol, present in the commercial formaldehyde solution, hinders the fixation process.
In the present study, we present a differential scanning calorimetry method for determination of the kraft lignin reactivity with formaldehyde by using solid paraformaldehyde (PFA) as the formaldehyde source. The reaction temperatures and enthalpies obtained from the method can be utilized for assessment of the lignin reactivity. The lignin–formaldehyde reaction using depolymerized paraformaldehyde is studied in a sodium hydroxide-catalyzed reaction. NaOH has an essential role in this reaction as the reaction catalyst, as a reagent to enhance the solubility of lignin as well as to promote the depolymerization of paraformaldehyde. The first two roles can be strongly interdependent in the case of lignin. (Chakar & Ragauskas, 2004; Holopainen et al., 2004) The reactivity of lignin with depolymerized paraformaldehyde is assessed as a function of the increasing NaOH concentration by using differential scanning calorimetry (DSC). Changes occurring in the lignin structure during the reaction with formaldehyde are examined with NMR methods (13C, 31P and 1H-13C HSQC) at increasing temperatures. Kraft lignin from spruce and pine is used in this study.
Softwood lignin contains predominantly guaiacyl-type structures and therefore its reactivity is expected to be high. (Gellerstedt, 2015)
EXPERIMENTAL Materials
A commercial formaldehyde solution (37 wt% solution containing 10-15% of methanol), sodium acetate (p.a. anhydrous, ≥ 99.0%), 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (95%), pyridine (anhydrous, 99.8%), cyclohexanol (99%) and chromium(lll) acetylacetonate (97%) were obtained from Sigma-Aldrich. Paraformaldehyde (97%) was from Alfa-Aesar. Sodium hydroxide (98.8%), dimethylsulfoxide D6 and deuterium oxide (99.96%) were purchased from VWR
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Chemicals. Chloroform D (99.80% D) was purchased from Euriso-top. Kraft lignin isolated from pine and spruce was selected as a representative of softwood lignin for this study. The elemental composition was 63.4% carbon, 6.0% hydrogen, 2.4% nitrogen and the calculated oxygen content 28.2%. Reference samples are available upon request from the authors. Lignin is freeze-dried and stored in a desiccator before use.
Preparation of paraformaldehyde solution
A paraformaldehyde solution was prepared using the method of Nogueira et al. (Nogueira et al., 1997) with minor modifications. Solid paraformaldehyde (PFA) was added to warm water (60°C) in a H2O:
PFA weight ratio of 5:1. After 10 minutes of stirring, the pH was adjusted to 10 using 1 M NaOH solution and after 15 minutes the solution became clear. The solution was cooled down to room temperature and filtered. The solution was kept in a refrigerator and protected from light. The composition and stability of the paraformaldehyde solution were determined using 13C NMR.
Reactions in DSC
DSC measurements were carried out using a Mettler Toledo DSC 823e instrument (calibrated using an indium reference sample). The DSC samples of about 25 mg were placed in a 30 µl stainless-steel high-pressure capsule sealed with gold-plated copper seals. The samples were measured from 30 to 300°C at a heating rate of 10°C / min. The thermogram data were analyzed with STARe software.
The DSC results are the average of three parallel measurements to ensure the repeatability of the results.
In the preparation of the sample solutions, the total volume (400 L) of the samples was kept constant.
First, the 1 M NaOH solution and water were weighed in a vial, followed by the weighed amounts of solid paraformaldehyde and kraft lignin. The sample solution thus obtained was mixed using a magnetic stirrer for a few minutes to make the sample solution homogeneous. In the reactivity studies,
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lignin and paraformaldehyde were used in molar ratios of 1:1 and 1:2 based on a molar mass of 200 g/mol for C9 units of lignin. (Wang et al., 2016) The effect of NaOH on the reactivity of lignin with formaldehyde was studied by varying the molar ratio of lignin : NaOH from 10:1 to 2.6:1 using the four NaOH concentrations of 0.225 M, 0.45 M, 0.75 M, and 0.865 M. In the 1:1 case, the lignin and paraformaldehyde concentrations were 2.25 M, while in the 1:2 case the paraformaldehyde concentration was 4.5 M. The Cannizzaro reaction studies were performed in the absence of lignin using the same NaOH concentrations and using the paraformaldehyde concentrations of 2.25 M and 4.5 M.
Reactions in an autoclave
The kraft lignin reactions with paraformaldehyde were studied in a series of experiments in which weighed amounts of the paraformaldehyde and NaOH (1 M) solutions and lignin were packed into a Teflon-lined, stainless steel autoclave with a volume of 60 ml. The molar ratio of the reagents used in the reaction was lignin : paraformaldehyde : NaOH = 2.6 : 2.6 : 1. The reaction mixture was heated to the desired temperature (120, 140 and 150C) with a total heating time of around 45 minutes. After heating the autoclave was cooled in an ice bath for 45 minutes. The reaction mixture was quantitatively removed from the autoclave and freeze-dried for preparation of the NMR sample. The Cannizzaro reaction was carried out in the autoclave by using a paraformaldehyde: NaOH molar ratio of 2.6:1. The reaction mixture was heated to 120 C and the total heating time was 45 minutes. The reaction mixture was cooled in an ice bath to room temperature and an NMR sample was taken.
NMR characterization
The 100.62 MHz 13C NMR measurements were conducted using a Bruker UltrashieldTM AMX-400 spectrometer. The quantitative 13C NMR spectra were measured using an inverse gated–pulse program. The carbon spectra of paraformaldehyde samples were recorded with 700 scans and a
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relaxation time of 60 s. The composition of the paraformaldehyde solution was determined using CH3COONa as an internal reference.
The quantitative 13C NMR spectra of the pristine kraft lignin and lignin-paraformaldehyde reaction mixtures were measured in DMSO-d6 and the solvent signal was used as a chemical shift reference (13C δ = 39.50 ppm). The spectra were measured with 7000 scans and a relaxation delay of 30 s, using chromium acetylacetonate as a relaxation reagent with a Cr(acac)3 concentration of 17 mM. The 13C NMR spectrum of paraformaldehyde-NaOH mixtures in a Cannizzaro reaction study was recorded with 1000 scans and with a relaxation delay of 90 s. The 1H-13C HSQC NMR measurements were performed using a Bruker UltrashieldTM AVANCE-400 spectrometer. The 1H-13C HSQC spectra of lignin and the lignin-paraformaldehyde reaction product (150°C) were measured from the NMR samples used in the 13C measurement (for 1H DMSO δ = 2.5 ppm). The 1H-13C HSQC spectra were measured with 32 scans. Number of collected data points were 1024 for 1H and 256 for 13C with the relaxation delay of 2.5 s.
The 191.98 MHz 31P NMR measurements were conducted using a Bruker UltrashieldTM AMX-400 spectrometer. The quantitative 31P spectra were measured using an inverse gated–pulse program. The NMR samples were prepared just prior to measurement because of the limited stability of the formed phospholane derivative.(Granata & Argyropoulos, 1995)The samples (60 mg) of the pristine kraft lignin, HCl-treated lignin, and a lignin-paraformaldehyde reaction mixture were dissolved in a pyridine-DMF mixture (200 µl) (1:1 v/v). Then, 2-chloro-4,4,5,5-tetramethyl-1,3,2- dioxaphospholane (100 µl) was added followed by a pyridine solution (100 µl) containing pyridine (1 ml), cyclohexanol (30 µl), and Cr(acac)3 (5 mg). CDCl3 (500 µl) was used as the deuterated solvent.
The pristine lignin was freeze-dried before the sample preparation, while the sample from the lignin-
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paraformaldehyde reaction mixture was neutralized with HCl and then freeze-dried. The spectra were recorded using a total of 450 scans and a relaxation delay of 120 s.
RESULTS AND DISCUSSION
Composition of depolymerized paraformaldehyde solution
The NaOH-catalyzed reactions of kraft lignin with paraformaldehyde (PFA) were carried out using two different approaches. In the differential scanning calorimetry (DSC) studies, paraformaldehyde was used as a solid compound and depolymerized by heat during the DSC experiment. For the reactions, characterized with NMR, paraformaldehyde was first depolymerized in a warm dilute NaOH solution (0.007 M). The composition of an aqueous solution of freshly depolymerized paraformaldehyde is depicted in the 13C NMR spectrum (Figure 1) together with that of a commercial 37% formaldehyde solution. In the two solutions, the main components are methylene glycol (HO- CH2-OH), with the CH2 signal at 82.39 ppm and its dimeric form (HO-CH2-O-CH2-OH), which has the corresponding signal at 86.15 ppm. (Rivlin et al., 2015) The commercial formaldehyde solution also contains methanol and various hemiformals from the reactions of methanol with the methylene glycol species. (Maiwald et al., 2003) Both solutions were found to maintain their composition after one month of storage in a refrigerator (see Figure 1 for the 13C NMR spectrum of the aged PFA solution).
The NMR spectra of Figure 1 clearly indicated the simple composition and stability of the formaldehyde solution derived from paraformaldehyde containing the two methylene glycol components expected to be active in the NaOH-catalyzed methylolation reaction with lignin.
Furthermore, the depolymerized paraformaldehyde solution does not need a presence of methanol as a stabilizer, which would disturb and even decelerate the lignin-formaldehyde reaction. (Rojas &
Williams, 1979) Other advantages of the use of paraformaldehyde are the ease of preparation of
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repeatable samples for DSC studies and the narrow number of compounds present in the depolymerized paraformaldehyde solution, which will simplify NMR-based characterization of the lignin-formaldehyde reactions.
Figure 1
Differential scanning calorimetry (DSC) study on the kraft lignin reactivity with formaldehyde derived from paraformaldehyde
The influence of the alkalinity and the molar ratio of the starting compounds on the progress of the kraft lignin-paraformaldehyde reaction was examined using differential scanning calorimetry. At a very low NaOH concentration (0.007 M) only one high-temperature exotherm at approximately 252C was observed (Figure 2A). A similar exotherm at 275C was also observed for a pure formaldehyde sample prepared from paraformaldehyde under the same alkaline conditions (Figure 2A). The probable reactions giving rise to these exotherms could be a lignin-formaldehyde reaction and/or a high-temperature decomposition of formaldehyde formed from paraformaldehyde through depolymerization. In an alkaline solution, disproportionation of formaldehyde to formic acid and methanol (the Cannizzaro reaction) is also possible. Equation 2 depicts the Cannizzaro reaction in the case of formaldehyde, which is the most common aldehyde used in this reaction. Self-oxidation and reduction of two formaldehyde molecules produce formic acid and methanol as the reaction products.
(2)
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The NMR spectrum in Figure 1 indicates that the mild conditions, the low 0.007 M NaOH concentration (with pH around 10), and the moderate heating temperature of 60C used in the preparation of a depolymerized paraformaldehyde solution, are not sufficient to initiate the disproportionation of formaldehyde. In the absence of NaOH, the uncatalyzed Cannizzaro-type disproportionation of formaldehyde to formic acid and methanol has been detected in supercritical water (SCW) at 400C as well as under the high-temperature (250C) and high-pressure (4 MPa) conditions. (Bröll et al., 1999; Tsujino et al., 1999; Osada et al., 2004) The classical Cannizzaro reaction of formaldehyde is promoted by strongly alkaline conditions. The reaction then occurs at room temperature giving the same disproportionation products as the noncatalytic reaction. (Hu et al., 2011)
Figure 2
The Cannizzaro reaction of formaldehyde as well as hydroxymethylation of lignin have been found (Taverna et al. 2019) to depend strongly on the alkalinity and hence on pH. A pH value of 10.5 has been presented to be optimal for methylolation of plant lignin and for a negligible disproportionation of formaldehyde (Malutan et al. 2008) However, Taverna et al. have noticed in their study that in order to attain high conversions in hydroxymethylation of Kraft-type lignin, pH has to be adjusted above 11.
The influence of the NaOH catalyst concentration on the progress of the Cannizzaro reaction of formaldehyde derived from solid paraformaldehyde is shown in Figure 2B. The NaOH concentration was varied from 0.225 M to 0.865 M at two different paraformaldehyde concentration levels of 2.25 and 4.5 M. Depolymerization of paraformaldehyde to formaldehyde was expected to be complete at
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these high NaOH concentrations. The DSC profiles contain two exotherms with the lowest originating from the disproportionation reaction of formaldehyde monomers to methanol and formic acid. The size of this exotherm increases with increasing NaOH concentration, indicating a higher extent of the Cannizzaro reaction. The exotherm above 200C is probably caused by decomposition of the thermally unstable formic acid to CO and H2O. (Bröll et al., 1999; Helander, 2000) The assignment of the lowest exotherm is supported by the 13C NMR spectrum (Fig. 3) of a Cannizzaro reaction mixture from an autoclave reaction carried out at 120oC by using a PFA: NaOH mole ratio of 2.6:1.
Methanol and formic acid are observed as the main products as well as formation of hemiformal HOCH2OCH3 from a reaction between methyleneglycol and methanol.
Figure 3
The kraft lignin reaction with formaldehyde derived from paraformaldehyde was found to give one broad exothermic signal in the temperature range of 145 – 230C depending on the NaOH concentration and the lignin : paraformaldehyde mole ratio (Figure 4). The exotherm is most probably due to the formation of methylol groups on the phenolic units of lignin. The low solubility of lignin even in polar solvents can affect its reactivity with formaldehyde and therefore the lignin reactions were studied at four different NaOH concentrations using lignin : NaOH mole ratios from 10:1 to 2.6:1. An increase in the amount of NaOH clearly shifts the exothermic methylolation reaction to lower temperatures with the peak temperature of the exotherm showing a linear dependence on the NaOH concentration. At the high NaOH concentration (0.865 M) the exotherm exists at 145C, which is in the same range as the curing temperatures (152 – 157C) observed for lignin-phenol- formaldehyde resins. (Turunen et al., 2003) When the paraformaldehyde amount is increased relative to that of lignin (a lignin: PFA mole ratio of 1:2), the reaction occurs at higher temperatures compared to the lignin : PFA ratio of the 1:1 case. The shift of the exotherm to the higher temperatures with an
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increase in the amount of paraformaldehyde used may have been caused by involvement of the Cannizzaro reaction, competing with the methylolation. A similar exotherm shift was observed in the absence of lignin, when the amount of paraformaldehyde was doubled (Figure 2B).
Figure 4
The reaction enthalpies of the lignin-paraformaldehyde reactions (Figure 4) varied from -61 J/g to - 125 J/g depending on the amount of reagents used. The higher the NaOH alkalinity, the higher the reaction enthalpy values observed. NaOH promotes both the formaldehyde-lignin reaction and the disproportionation of formaldehyde. The higher enthalpy values obtained in the case of the lignin/PFA ratio of 1:2 are due to the fast Cannizzaro reaction competing with the methylolation reaction under strongly alkaline conditions. (Liang et al., 2018)
NMR characterization of kraft lignin reactions with formaldehyde derived from paraformaldehyde
The DSC results indicated that depolymerized paraformaldehyde reacts with kraft lignin. To find the structural changes occurring in lignin during heating in the presence of paraformaldehyde, the progress of the reactions was examined with autoclave reactions. The composition, which gave the lowest reaction temperature for the lignin-formaldehyde reaction in the DSC measurement, was chosen for the reaction mixtures to be studied. Lignin, paraformaldehyde, and NaOH were used in a mole ratio of 2.6 : 2.6 : 1. Three reaction mixtures were heated gradually to the desired temperature (120, 140, and 150°C). The highest temperature used was selected so that the obtained reaction mixture was still soluble in DMSO. After heating, the reaction mixtures were characterized with 31P,
13C and 1H-13C HSQC NMR methods.
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The NaOH-catalyzed reaction of lignin with the methylene glycol species from depolymerized paraformaldehyde is expected to involve, besides the desired methylolation of the phenolic unit (Holopainen et al., 1997), a possible methylolation of the propane chain of the phenolic ring (Tollens reaction). The low number of reactive sites (vacant ortho positions) on the phenolic rings in lignin and large functional groups of the phenolic units can probably hinder further reactions of the methylol groups. The rigid structure of lignin owing to the π-π interactions between the aromatic rings probably complicates condensation of the methylol groups to methylene bridges. (Ma et al., 2018) Moreover, the Cannizzaro reaction, a disproportionation reaction of formaldehyde, is expected to compete with methylolation. (Marton et al., 1966; Malutan et al., 2008; Hu et al., 2011)
The kraft lignin reaction with depolymerized paraformaldehyde, carried out at 150°C, was first examined using 31P NMR to find which phenolic units of lignin are affected in the reaction with formaldehyde (Figure 5). For 31P NMR measurements, the hydroxyl groups of lignin were phosphitylated with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (Li et al., 2018) and thus the method provides information about the chemical surroundings of the hydroxyl groups. For the NMR sample preparation, a sample from the lignin-formaldehyde reaction mixture was first neutralized using 1 M HCl and then rinsed with water and finally freeze-dried. The effect of the HCl treatment on the pristine lignin structure was also examined.
Figure 5
In the 31P NMR spectrum (Figure 5) the major changes occurred in the aromatic OH region between 145-137 ppm resulting from a formaldehyde reaction with lignin. After the reaction, the sharp 31P signal of non-condensed guaiacyl units at 141 ppm disappeared almost totally and a new broad signal appeared in the range of 141-145 ppm, overlapping the 31P signal of the syringyl OH group at 143.5
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ppm. The disappearance of the guaiacyl signal and the shift of the p-hydroxyphenyl signal to 139 ppm can indicate a formaldehyde reaction at the C5 position of these phenolic structures. The new 31P signal around 143 ppm is in the range assigned in the literature to a 5-substituted phenolic OH, including the guaiacyl and syringyl units having the phenolic ortho positions substituted. (Pu et al., 2011; Balakshin & Capanema, 2015)
In the 31P NMR spectrum of the lignin-depolymerized paraformaldehyde reaction product (Figure 5), there are no major changes in the aliphatic OH region (145-149 ppm). The two sharp signals at 149- 150 ppm belong to the two main components of the paraformaldehyde solution, unreacted methylene glycol (HOCH2OH) and its dimeric form (HOCH2OCH2OH). (Li et al., 2018) The weak signals observed between 151-150 ppm are caused by the HCl treatment (the middle spectrum in Figure 5).
The quantitative 13C NMR spectra of samples from the three kraft lignin-depolymerized paraformaldehyde reaction mixtures obtained at the reaction temperatures of 120, 140, and 150C are presented in Figure 6. Major changes are observed in the chemical shift ranges of 60, 80-90, and 165- 170 ppm. The formaldehyde reaction with the C5 position of the phenolic ring of lignin is supported by the disappearance of the C5 signals around 115-116 ppm in the 13C NMR spectra of the reaction mixtures. A new signal appearing at 88 ppm can be due to a -CH2OCH2OH methylol group at the C5
position of the lignin phenolic ring. (Peng et al., 1992) The weak sharp signals of unreacted methylene glycols at approximately 82 and 84 ppm (see Table 1) are visible after the reaction, even though a lignin : PFA molar ratio of 1:1 was used in the reaction. (Rahimi et al., 2013; Aminzadeh et al., 2018)
Figure 6
Table 1.
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Because of the complex structure and several reaction possibilities, only the main reaction products have been discussed. (Wang et al., 2009) The Cannizzarro reaction, which provides formic acid (HCOOH), can explain the signal observed in DMSO at approximately 168 ppm (Figure 6), and it can be assigned to formic acid or to its sodium salt (HCOONa). (Moret et al., 2013; Yu et al., 2013) The broad nature of the formic acid signal can indicate an interaction with the lignin structure that will reduce the mobility of formic acid and produce a broad carbon signal.
The assignment of the carbon signals in Figure 6 was confirmed with the 1H-13C HSQC NMR measurements of kraft lignin and the lignin-depolymerized paraformaldehyde reaction mixture (150°C) presented in Figure 7 with the appropriate lignin structures. According to the 13C NMR spectrum (Figure 6), a new signal appeared at 58 ppm between the methoxy signal (55 ppm) and the Aγ signal of lignin (60 ppm) in consequence of the reaction with formaldehyde. In the 1H-13C HSQC NMR spectrum of the reaction 150°C mixture (Figure 7), there are two new signals at (4.5, 58.2) and (4.5, 63.2) ppm with the proton and carbon chemical shifts in the range typical for a –CH2OH methylol group bound to a guaiacyl structure. (Singh & Prathap, 1997; Zhao et al., 1994) The two 2D signals belong to CH2OH methylol groups originating from the reaction of methylene glycol (HO- CH2-OH) with the C5 position of the phenolic units in the guaiacyl structures A, B, and C. Similarly, the signal at (4.7, 88.1) ppm in the 2D spectrum can be assigned to the -CH2OCH2OH methylol group formed through the reaction of a methylene glycol dimer (HO-CH2-O-CH2-OH) with the C5 position of the lignin phenolic group. (Peng et al., 1992)
The signal of the methoxy carbon of the lignin structure at 55 ppm remains unchanged in the reaction with depolymerized paraformaldehyde. In the 1H-13C HSQC NMR spectrum of the reaction mixture of 150C, there are two weak signals at (3.7, 70.8) and (3.6, 71.8) ppm, which can belong to Tollens
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reaction products. The signals are in the range reported for methylene carbons of a methylol group (CH2OH) bound to the aliphatic side chain of the phenolic unit. (Peng et al., 1992) The amount of these products is not very high since they were not detected in the normal 13C NMR spectrum (Figure 6).
Figure 7
The 31P and 13C NMR results indicated that formaldehyde derived from paraformaldehyde reacts with kraft lignin mainly at the C5 (ortho) position of the guaiacyl structures. Both of the two methylene glycols derived from paraformaldehyde were found to be active in the methylolation reaction, as expected. The 13C NMR results of the three reaction mixtures also showed the presence of formic acid, the product from the competing Cannizzaro reaction. The high alkalinity (0.865 M) used in the reaction mixtures inevitably favors the disproportionation of formaldehyde. Based on the signal intensities of the species originating from the starting paraformaldehyde (Figure 6 and Table 1), the conversion of formaldehyde in the reaction mixtures seems to be moderately high with the higher reaction temperatures favoring the methylolation reaction over the Cannizzaro reaction.
CONCLUSIONS
Keeping in mind the phenol-formaldehyde resin application of lignin, our aim was to find a way to assess the reactivity of lignin materials with formaldehyde. The method based on a reaction between depolymerized paraformaldehyde and kraft lignin was chosen. The advantages of the paraformaldehyde solution compared to a normally used 37% formaldehyde solution was the narrow distribution of formaldehyde oligomers and the absence of methanol. Thus, the paraformaldehyde solid or its depolymerized solutions served as straightforward formaldehyde sources for the lignin
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reactivity assessment using DSC or NMR methods. The lignin reaction was found to require a high NaOH concentration in order for the reaction with formaldehyde to proceed at reasonably low temperatures compared to the curing temperature of the phenol-formaldehyde resins (around 150C).
On the other hand, the conditions of high NaOH concentration are known to favor the disproportionation of formaldehyde to formic acid and methanol. This Cannizzaro reaction was found to compete greatly with the methylolation reaction due to kraft-type lignin being only moderately reactive.
The differential scanning calorimetry method gave numerical reactivity values, which can be utilized in evaluation of the relative reactivity within a series of lignin samples. A suitable composition for the DSC determination found in our study was the lignin : paraformaldehyde : NaOH mole ratio of 2.6 : 2.6 : 1 enabling the reaction to occur at approximately 150C. The NMR results indicated that methylolation is the principal reaction occurring during the DSC measurement of kraft lignin.
ACKNOWLEDGEMENTS
The Finnish Funding Agency for Technology and Innovation (Business Finland EVIM project) and the European Union/European Regional Development Fund are gratefully acknowledged for their financial support within the EVIM project. Furthermore, a scholarship (H.P.) from the University of Eastern Finland is acknowledged.
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Table 1. The 13C NMR integrals of lignin-paraformaldehyde reactions. The spectra are presented in Figure 5.
Signal / Sample
Aromatic C C5 Paraformaldehyde Aγ Methoxy
168 ppm
105-155 ppm
115 ppm
88 ppm
84 ppm 82 ppm 63 ppm
60 ppm
58 ppm
55 ppm
lignin - 8.30 0.55 - - - 0.17 0.21 - 1.00
120°C 0.26 8.58 0.22 0.31 0.04 0.08 0.23 0.23 0.25 1.00
140°C 0.22 8.55 0.25 0.35 0.06 0.12 0.28 0.20 0.23 1.00
150°C 0.18 8.53 0.24 0.51 0.07 0.09 0.39 0.19 0.20 1.00
24 Figure 1
25 Figure 2
26 Figure 3
27 Figure 4
28 Figure 5
29 Figure 6
30 Figure 7
31 Figure legends
Figure 1: 13C NMR spectra of a commercial 37% formaldehyde solution (black), an aqueous solution of paraformaldehyde (PFA) (red) and a PFA solution aged for one month (blue). The NaOH and PFA concentrations in the paraformaldehyde solution are 0.007 M and 5.4 M (13 wt%), respectively. The spectra were measured in D2O.
Figure 2: The section A represents DSC curve of the paraformaldehyde-NaOH reactions under slightly basic conditions (0.007 M) in the presence and the absence of kraft lignin. The profiles in the section B represent the DSC curves of the paraformaldehyde-NaOH reactions at two different PFA concentrations with the increasing NaOH content.
Figure 3: A Cannizzaro reaction carried out in an autoclave at 120°C. The PFA : NaOH molar ratio of 2.6:1 was used in the reaction. The 13C NMR spectrum was measured in D2O as the solvent. The signals and integrals are listed in the table. MG stands for methylene glycol (HO-CH2-OH) and HF for hemiformal (HO-CH2-OCH3).
Figure 4: DSC curves of kraft lignin-paraformaldehyde reactions. Influence of the NaOH concentration on the lignin-paraformaldehyde reaction at a lignin/PFA molar ratio of 1:1 (above) and 1:2 (below). The NaOH concentrations used are 0.225 (1), 0.45 (2), 0.75 (3) and 0.865 M (4).
Figure 5: 31P NMR spectra of untreated kraft lignin (upper), lignin treated with HCl (middle) and the kraft lignin-depolymerized paraformaldehyde reaction mixture (lower), which was heated to 150°C.
Cyclohexanol was used as an internal standard (IS, δ = 145 ppm).
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Figure 6: The 13C NMR spectra of kraft lignin-depolymerized paraformaldehyde autoclave reaction mixtures obtained at temperatures 120°C (blue), 140°C (brown) and 150°C (red) compared to the spectrum of the unmodified lignin (black) having a MeO/Ar mole ratio of 0.72. DMSO was used as the solvent (δ = 39.50 ppm). The lignin : PFA : NaOH molar ratio in the reaction mixtures was 2.6 : 2.6 : 1.
Figure 7: 1H-13C HSQC NMR spectra of the kraft lignin (left) and a sample from the kraft lignin- depolymerized paraformaldehyde reaction mixture heated to 150°C (right). The lignin : PFA : NaOH molar ratio in the reaction mixture was 2.6 : 2.6 : 1. DMSO-d6 was used as the solvent and the solvent signal was used as the reference (δ = 39.5, 2.5 ppm). A, B and C represent the lignin structures corresponding to the signals in the 1H-13C HSQC spectra. (Constant et al., 2016)
