Sulfation of Wheat Straw Soda Lignin with Sulfamic Acid over Solid Catalysts

Soda lignin is a by-product of the soda process for producing cellulose from grassy raw materials. Since a method for the industrial processing of lignin of this type is still lacking, several research teams have been working on solving this problem. We first propose a modification of soda lignin with sulfamic acid over solid catalysts. As solid catalysts for lignin sulfation, modified carbon catalysts (with acid sites) and titanium and aluminum oxides have been used. In the elemental analysis, it is shown that the maximum sulfur content (16.5 wt%) was obtained with the Sibunit-4® catalyst oxidized at 400 °C. The incorporation of a sulfate group has been proven by the elemental analysis and Fourier-transform infrared spectroscopy. The molecular weight distribution has been examined by gel permeation chromatography. It has been demonstrated that the solid catalysts used in the sulfation process causes hydrolysis reactions and reduces the molecular weight and polydispersity index. It has been established by the thermal analysis that sulfated lignin is thermally stabile at temperatures of up to 200 °C. According to the atomic force microscopy data, the surface of the investigated film consists of particles with an average size of 50 nm. The characteristics of the initial and sulfated β-O-4 lignin model compounds have been calculated and recorded using the density functional theory.


Introduction
Lignocellulosic biomass is known to be the most promising renewable carbon source on Earth [1,2]. The lignin fraction in lignocellulosic biomass ranges from 10-35%, depending on the biomass' origin and type. In particular, hardwoods contain 20-30% lignin, conifers can contain 25-38%, agro-industrial wastes can contain 11-20%, and herbaceous plants can contain 8-15% [1]. The lignin from woody biomasses is an aromatic polymer consisting of phenylpropane units built from organic substituents, including ether, carboxyl, and alcohol groups, and the composition and amount of which depend on the initial biomass and the isolation method used [3].
The unique structure of lignin makes it a good candidate for chemical processing into valuable products, for example, in the production of phenolic and aromatic chemicals and
The oxidized carbon samples were prepared from a Sibunit-4 (S4) commercial mesoporous carbon material by oxidization in a mixture of 20 vol% of oxygen in N 2 in the presence of water vapors at temperatures of 400 and 500 • C for 2 h [32]. The catalyst fractions were 1.0-1.6 mm (granular) and 0.056-0.094 mm (nongranular).
Sulfated Sibunit ® was obtained by the treatment of the graphite-like Sibunit ® material with sulfuric acid in the presence of nitric acid as described in [33,34].
The catalyst's acidity was estimated using the point of zero charge (PZC) by the Sørensen-de Bruijn method [35,36], i.e., by measuring the acidity of a solid suspension upon reaching the isoelectric point (non-measurable electrode values) corresponding to pH of the solid suspension.
Sulfation of WSSL with sulfamic acid was carried out in a dioxane medium according to the modified procedure proposed in [27]. A total of 2 g of WSSL, 5 g of sulfamic acid (or ammonium sulfamate), and 50 mL of 1,4-dioxane were loaded into a heat-resistant three-necked flask with a capacity of 250 mL and placed in a thermostat with a mechanical stirrer; then, the mixture was heated to 90 • C under continuous stirring. Upon reaching the desired temperature, the reaction mixture was added with 0.5 g of a catalyst (see Table 1) and this temperature was held for 3 h. After that, the reaction mixture was cooled down to room temperature, the solvent was decanted, the residue was neutralized with a 5-7% aqueous ammonia solution to pH~8-9 (50 mL), and a solid catalyst was filtered off. The WSSL aqueous solution was purified from low molecular weight compounds by dialysis carried out in an MF-503-46 MFPI cellophane dialysis bag (US) with a pore size of 3.5 kDa for 8-10 h, changing the water every hour. Next, the aqueous solution of sulfated lignin was evaporated to dryness under vacuum with a rotary evaporator and sulfated lignin in the form of an ammonium salt was obtained as a solid residue. The elemental analysis of the sulfated samples was carried out on a Thermo Quest Flash EA-1112 elemental analyzer (Rome, Italy).
The Fourier-transform infrared (FTIR) spectra were recorded on a Shimadzu IRTracer-100 FTIR spectrophotometer (Kyoto, Japan) in the range of 4000-400 cm −1 . The spectral data were processed using the LabSolution IR software. Solid samples for the FTIR spectroscopy study were tablets of the substance in the potassium bromide matrix prepared under the same conditions (i.e., the same time of mixing with potassium bromide, pressure, and evacuation time). The substance concentration was 3 mg per 1000 mg of KBr.
The ultraviolet-visible (UV-Vis) spectra were recorded on a Leki Instruments SS2109-UV scanning spectrophotometer (Lempäälä, Finland) using 1-cm quartz cells. The cell thermostating (±0.1 K) was performed by a Haake K15 thermostat connected to a Haake DC10 controller. The measured absorbance of the process solutions was 220-400 nm. All the measurements were performed at 298 K. The weight average molecular weight M w , number average molecular weight M n , and polydispersity index PDI of the WSSL samples were determined by gel permeation chromatography (GPC) on an Agilent 1260 Infinity II multidetector GPC/SEC system with a refractive detector, viscometer, and light scattering detectors. The water-soluble samples were separated on two Agilent PL aquagel-OH Mixed-M columns using the aqueous solution of 0.1 M LiNO 3 as a mobile phase. The column was calibrated using Agilent polyethylene glycol standards (US). The tetrahydrofuran-soluble samples were separated on an Agilent PLgel Mixed-B column with tetrahydrofuran stabilized with 250 ppm of butylhydroxytoluene (BHT) as a mobile phase. The column was calibrated using Agilent polystyrene standards (US). The eluent flow rate was 1 mL/min and the sample volume was 100 µL. Before the analysis, the samples were dissolved in the mobile phase (~5 mg/mL) and filtered through a 0.22-µm Agilent PTFE membrane filter. The data collection and processing were performed with the Agilent GPC/SEC MDS software.
The thermal analysis included the thermogravimetry (TG) and differential thermogravimetry (DTG) investigations and was carried out on a NETZSCH STA 449 F1 Jupiter synchronous thermal analysis instrument. Thermograms were taken in a corundum crucible in the argon atmosphere at temperatures of 30-800 • C and a sample heating rate of 10 • C/min.

Calculation Details
In the calculation, the Gaussian 09W [37] and GaussView 0.5 [38] program packages were used. The molecular structure of the initial and sulfated β-O-4 lignin were optimized using the density functional theory (DFT) with the 6-31G(d, p) basis set via the Becke-3-parameter-Lee-Yang-Parr (B3LYP) hybrid functional levels [39,40]. The vibrational frequencies were calculated at the same level of theory; the determined frequency values were corrected with a scaling factor of 0.9608 taking into account a systematic error of the basis set [41]. In addition, the HOMO and LUMO frontier orbital surfaces and energies, molecular electrostatic potential (MEP) surface, and Mulliken atomic charges were calculated for the optimized structures at the same level of theory.

Synthesis of the WSSL Sulfonic Derivatives
The main drawbacks of the sulfation of hydroxyl-containing organic compounds with sulfamic acid in the presence of urea are the impossibility of isolating urea from the reaction mass and the competing reaction of the carbamatization of hydroxyl groups [42][43][44][45].
In this work, we examined the effect of the type of a solid mesoporous acid catalyst on the degree of sulfation and the composition and structure of the products of WSSL sulfation with sulfamic acid. The catalysts used were Sibunit oxidized at 500 • C (granules, Sibunit-ox-500g), Sibunit oxidized at 400 • C (powder, Sibunit-ox-400), sulfonated Sibunit (Sulfated Sibunit), TiO 2 , and γ-Al 2 O 3 . The acidity of the catalysts was estimated using the PZC.
It is evident from the data in Table 1 that lignin sulfation of the aluminum γ-Al 2 O 3 and titanium TiO 2 oxides yields the WSSL derivatives with the lowest sulfur content (8.1 and 5.5 wt%, respectively); as the catalyst acidity increases, the degree of lignin sulfation decreases due to the competing polymer degradation processes. In addition, the aluminum and titanium oxides have fairly large sets of Lewis acid and base centers. The concentration and strength of these centers depends on the method used to obtain the corresponding hydroxides, the calcination temperature determining the crystal structure formed, and the nature of the chemical modifying agents [46].
It should be noted that the data obtained in this work are slightly different from the results reported in the studies on preparing the sulfo-derivatives of betulin [17,35], which may be due to the nature of the initial compounds and their different stabilities in the acid hydrolytic destruction processes [1][2][3].
In addition, as reported in [24], the Lewis acids used as the catalysts for WSSL sulfation lead to the production of WSSL sulfates with a sulfur content similar to that of the products of sulfation with sulfamic acid in the presence of some organic bases (e.g., thiourea, DMF, and pyridine).
It is noteworthy that the maximum sulfur content (16.5 wt%) in the synthesized WSSL derivatives noticeably exceeds the values reported earlier for the sulfation of lignins over base catalysts. In particular, when sulfating WSSL with sulfamic acid in the presence of urea in 1,4-dioxane, lignin sulfates with a maximum sulfur content of 10.1 wt% were obtained [24].
The use of Sibunit-4 modified under different conditions leads to the production of sulfated products with a higher degree of lignin sulfation compared to its value in the sulfated products obtained with the aluminum and titanium oxides. The maximum sulfur content (16.5 wt.%) in the WSSL-sulfated derivative was obtained from the Sib-unit-ox-400 graphite-like catalysts. When the Sibunit-ox-500 g granular graphite-like catalyst was used, the sulfur content in the products was lower (10.7 wt%) than in the case of the Sibunit-ox-400 powder catalyst, which can be explained by the diffusion limitations imposed on the delivery of reagents to the catalyst reaction sites. A similar effect of the difference between the catalyst surfaces on the sulfation process was described in [47], where granular catalysts were shown to be less active in the hydrogenation of flax shives.
The high degree of the sulfation of the catalysts based on the Sibunit-4 graphite-like material can be attributed to the fact that the reaction occurs both at the aliphatic and aromatic hydroxyl groups of the lignin macromolecule or to the growth in the number of hydroxyl groups available for sulfation, which appear during the depolymerization of lignin (Table 1). On the contrary, as was proven previously [26,28], the sulfation of lignin with sulfamic acid in the presence of urea is only accompanied by the selective sulfation of aliphatic hydroxyls of lignin without affecting the aromatic ones.
In the Brønsted acids (Table 1), we observed the following dependence of the sulfur content in the lignin sulfate on the catalyst acidity: the lower the pH pzc value, the lower the sulfur content in the WSSL sulfo derivatives. Such a catalytic effect of the acid catalysts on the sulfation process may be due to the presence of both the acid and base centers on their surface. The carbonyl and ether groups and their combinations forming the pyrone structures in the Sibunit carbon material-based catalyst are known to be basic. Aromatic π-electrons in the graphite layers can also be responsible for the basic properties of carbon [48]. As the acidity decreases, the number of base centers on the catalyst surface obviously increases; this, in turn, leads to an increase in its activity in the sulfation process. According to the correlated dependences of the degree of lignin sulfation on the acid nature of the catalysts, the mechanism of their catalytic action is similar to the catalytic effect of some organic bases on the sulfation of natural compounds with sulfamic acid [24,26,28,[48][49][50][51].
The proposed mechanism of the WSSL sulfation with sulfamic acid on the Sulfated Sibunit catalyst allows us to describe the process as where R is the catalyst matrix, R is the WSSL molecule, and X − H + is the catalyst functional group. According to our reaction scheme proposed above, during the catalytic sulfation of WSSL, the zwitterionic form of sulfamic acid is first sorbed on the catalyst matrix, which is followed by its decomposition into sulfur trioxide and ammonia (the limiting stage) [52]. After that, sulfur trioxide reacts with the formation of a reactive sulfating complex with 1,4-dioxane, which in turn sulfates the WSSL molecule. Next, the catalyst matrix and the acidic form of WSSL sulfate exchange via ammonium cations. This scheme is similar to our scheme for the catalytic sulfation of betulin [17,35].
Thus, the acid-base properties of the solid catalyst surface govern the lignin sulfation process. Brønsted acids catalyze the modification of WSSL with sulfamic acid better than Lewis acids. The high degree of sulfation with the catalysts based on the Sibunit-4 graphitelike material can be related to the occurrence of the reaction both at the aliphatic and aromatic hydroxyl groups of the lignin macromolecule or to the growth in the number of hydroxyl groups available for sulfation as a result of the depolymerization of lignin (Table 1). When using the Sibunit-based catalysts, along with the sulfation process, similarly to the growth of the catalyst acidity, the lignin depolymerization processes are intensified, which causes the formation of sulfated lignin with the lowest weight average molecular weight (Table 1).

FTIR Study
The IR spectroscopy data on the initial and modified WSSL are presented in Figure 1. It is evident that the modified WSSL profile differs significantly from the profile of the initial WSSL. In particular, the spectrum of the modified WSSL contains absorption bands at 1255-1210, 840-790, and 590-690 cm −1 , which are indicative of the presence of a sulfate group after the modification. In the region of 3500-2500 cm −1 , the absorption bands of the hydroxyl, alkyl, and ammonium groups are superimposed, which causes the significant broadening of the peaks.
According to our reaction scheme proposed above, during the catalytic sulfation of WSSL, the zwitterionic form of sulfamic acid is first sorbed on the catalyst matrix, which is followed by its decomposition into sulfur trioxide and ammonia (the limiting stage) [52]. After that, sulfur trioxide reacts with the formation of a reactive sulfating complex with 1,4-dioxane, which in turn sulfates the WSSL molecule. Next, the catalyst matrix and the acidic form of WSSL sulfate exchange via ammonium cations. This scheme is similar to our scheme for the catalytic sulfation of betulin [17,35].
Thus, the acid-base properties of the solid catalyst surface govern the lignin sulfation process. Brønsted acids catalyze the modification of WSSL with sulfamic acid better than Lewis acids. The high degree of sulfation with the catalysts based on the Sibunit-4 graphite-like material can be related to the occurrence of the reaction both at the aliphatic and aromatic hydroxyl groups of the lignin macromolecule or to the growth in the number of hydroxyl groups available for sulfation as a result of the depolymerization of lignin (Table  1). When using the Sibunit-based catalysts, along with the sulfation process, similarly to the growth of the catalyst acidity, the lignin depolymerization processes are intensified, which causes the formation of sulfated lignin with the lowest weight average molecular weight (Table 1).

FTIR Study
The IR spectroscopy data on the initial and modified WSSL are presented in Figure  1. It is evident that the modified WSSL profile differs significantly from the profile of the initial WSSL. In particular, the spectrum of the modified WSSL contains absorption bands at 1255-1210, 840-790, and 590-690 cm −1 , which are indicative of the presence of a sulfate group after the modification. In the region of 3500-2500 cm −1 , the absorption bands of the hydroxyl, alkyl, and ammonium groups are superimposed, which causes the significant broadening of the peaks.

UV-Vis
It is evident in the normalized UV absorption spectra ( Figure 2) that the main peak characteristic of the alcoholic solutions of lignins (270-290 nm) change nonmonotonically with the change in the catalyst used. The maximum absorption at 270-290 nm is caused by the nonconjugated phenolic groups in lignin [30,31]. The most pronounced peak in

UV-Vis
It is evident in the normalized UV absorption spectra ( Figure 2) that the main peak characteristic of the alcoholic solutions of lignins (270-290 nm) change nonmonotonically with the change in the catalyst used. The maximum absorption at 270-290 nm is caused by the nonconjugated phenolic groups in lignin [30,31]. The most pronounced peak in this region corresponds to the Sulfated-Sibunit catalyst and can be explained by a larger amount of the nonconjugated phenolic groups in the macromolecule as a result of the more intense depolymerization of the initial lignin (Table 1). For the cat-3 catalyst, which ensures the formation of a sulfated product with the highest sulfur content, this peak is suppressed almost completely. This can be explained by a decrease in the absorption intensity of the phenolic groups due to their transition to the sulfated structure or by a significant change in the polymer structure, after which the free OH groups can no longer effectively absorb UV radiation.
this region corresponds to the Sulfated-Sibunit catalyst and can be explained by a larger amount of the nonconjugated phenolic groups in the macromolecule as a result of the more intense depolymerization of the initial lignin (Table 1). For the cat-3 catalyst, which ensures the formation of a sulfated product with the highest sulfur content, this peak is suppressed almost completely. This can be explained by a decrease in the absorption intensity of the phenolic groups due to their transition to the sulfated structure or by a significant change in the polymer structure, after which the free OH groups can no longer effectively absorb UV radiation.

GPC
The data shown in Figure 3 show that the initial WSSL is characterized by a monomodal distribution represented by polymers with molecular weights from ~200 to 20,000 Da [24]. The modification of lignin by the catalysts strongly changes its structure. We note that, in all the cases, the modification of WSSL reduces the polymer molecular weights (see Table 1), which is reflected in the shift of the molecular weight distribution (MWD) curves to the low molecular weight region.
The TiO2 catalyst has the weakest effect on the molecular weight of lignin; the MWD curve shifts slightly to the right due to an increase in the molecular weights of the individual polymer molecules caused by the incorporation of sulfate groups into their structure. The PDI of the sample decreases to 2.06 due to the removal of the low molecular weight fraction during dialysis.
In the case of the Al2O3 catalyst used, the MWD curve of the sulfated lignin derivative changes: the pronounced peaks corresponding to molecular weights of ~900 and ~2000 Da appear, which are probably related to the products of the partial depolymerization of the initial lignin macromolecules. At the same time, the weak peak around ~4000 Da corresponds to the structures that degraded the least during sulfation. At the same time, due to a decrease in the fraction of high molecular weight structures, the PDI of the sample decreased to 1.47.
When the Sibunit ® -based catalysts are used, the fraction with Mw ~900 Da grows due to the intensified degradation of the lignin polymers. Moreover, the use of sulfated Sibunit ® and oxidized Sibunit ® yields a product with the MWD profiles identical in this region. The difference is in the almost complete absence of the polymer particles with an Mw

GPC
The data shown in Figure 3 show that the initial WSSL is characterized by a monomodal distribution represented by polymers with molecular weights from~200 to 20,000 Da [24]. The modification of lignin by the catalysts strongly changes its structure. We note that, in all the cases, the modification of WSSL reduces the polymer molecular weights (see Table 1), which is reflected in the shift of the molecular weight distribution (MWD) curves to the low molecular weight region. value of more than 3000 Da in the sample. Therefore, this sample has the highest uniformity and polydispersity (1.34). In the presence of granular Sibunit ® , probably due to the diffusion limitations, the depolymerization of lignin proceeds less intensively, the MWD curve contains a minor peak corresponding to the high molecular weight (3000 Da) structures, and the low molecular weight (900 Da) fraction is slightly larger than in the sample obtained using Al2O3. The TiO 2 catalyst has the weakest effect on the molecular weight of lignin; the MWD curve shifts slightly to the right due to an increase in the molecular weights of the individual polymer molecules caused by the incorporation of sulfate groups into their structure. The PDI of the sample decreases to 2.06 due to the removal of the low molecular weight fraction during dialysis.
In the case of the Al 2 O 3 catalyst used, the MWD curve of the sulfated lignin derivative changes: the pronounced peaks corresponding to molecular weights of~900 and~2000 Da appear, which are probably related to the products of the partial depolymerization of the initial lignin macromolecules. At the same time, the weak peak around~4000 Da corresponds to the structures that degraded the least during sulfation. At the same time, due to a decrease in the fraction of high molecular weight structures, the PDI of the sample decreased to 1.47.
When the Sibunit ® -based catalysts are used, the fraction with M w~9 00 Da grows due to the intensified degradation of the lignin polymers. Moreover, the use of sulfated Sibunit ® and oxidized Sibunit ® yields a product with the MWD profiles identical in this region. The difference is in the almost complete absence of the polymer particles with an M w value of more than 3000 Da in the sample. Therefore, this sample has the highest uniformity and polydispersity (1.34). In the presence of granular Sibunit ® , probably due to the diffusion limitations, the depolymerization of lignin proceeds less intensively, the MWD curve contains a minor peak corresponding to the high molecular weight (3000 Da) structures, and the low molecular weight (900 Da) fraction is slightly larger than in the sample obtained using Al 2 O 3 .

TGA/DSC
The data of the thermal analysis are shown in Figures 4 and 5. value of more than 3000 Da in the sample. Therefore, this sample has the highest uniformity and polydispersity (1.34). In the presence of granular Sibunit ® , probably due to the diffusion limitations, the depolymerization of lignin proceeds less intensively, the MWD curve contains a minor peak corresponding to the high molecular weight (3000 Da) structures, and the low molecular weight (900 Da) fraction is slightly larger than in the sample obtained using Al2O3.     The fractions of the substance decomposed upon heating from 250 • C at a rate of 10 deg/min determined from the thermograms of the initial and sulfated lignin samples are listed in Table 2. It is evident that the investigated lignins were generally decomposed in a wide temperature range by 700 • C. A comparison with the decomposition of the samples during thermolysis shows that the investigated lignins have different weight loss indicators at the same temperatures. Sulphated lignin demonstrates the maximum weight loss, and this trend continues until the pyrolysis is completed. The fractions of the substance decomposed upon heating from 250 °C at a rate of 10 deg/min determined from the thermograms of the initial and sulfated lignin samples are listed in Table 2. It is evident that the investigated lignins were generally decomposed in a wide temperature range by 700 °C. A comparison with the decomposition of the samples during thermolysis shows that the investigated lignins have different weight loss indicators at the same temperatures. Sulphated lignin demonstrates the maximum weight loss, and this trend continues until the pyrolysis is completed.  Both lignins are decomposed mainly in the range from 250-500 °C, i.e., in the active pyrolysis zone. Above 400 °C, a coke residue starts forming due to the aromatization of the lignin structure [53,54].
The final stage of passive pyrolysis is accompanied by weight loss, which was 76.6 and 61.9% for the initial and sulfated lignin, respectively, by 800 °C. The coke residue of the initial lignin at any stage of its formation is smaller than that of sulfated lignin, which may indirectly indicate a smaller number of thermostable fractions in its structure.
It is evident from the thermograms presented in Figure 5 that the DTG decomposition curve for the initial straw lignin sample has a broad peak, characteristic of lignins, with a shoulder in the range from 200 to ~340 °C. According to [55], in the temperature range of 230-260 °C, the propane side chains of lignin degrade with the formation of the methyl, ethyl, and vinyl derivatives of guaiacol. In addition, at temperatures of ≤310 °C, the thermally unstable ether bonds are destructed [56]. The thermogram of the sulfated lignin also contains a pronounced shoulder in the range from 200 to ~340 °C, but unlike the case of the initial lignin, it includes a narrow intense peak with a maximum at 350 °C, which is probably related, among other things, to the removal of the sulfo groups.  Both lignins are decomposed mainly in the range from 250-500 • C, i.e., in the active pyrolysis zone. Above 400 • C, a coke residue starts forming due to the aromatization of the lignin structure [53,54].

AFM
The final stage of passive pyrolysis is accompanied by weight loss, which was 76.6 and 61.9% for the initial and sulfated lignin, respectively, by 800 • C. The coke residue of the initial lignin at any stage of its formation is smaller than that of sulfated lignin, which may indirectly indicate a smaller number of thermostable fractions in its structure.
It is evident from the thermograms presented in Figure 5 that the DTG decomposition curve for the initial straw lignin sample has a broad peak, characteristic of lignins, with a shoulder in the range from 200 to~340 • C. According to [55], in the temperature range of 230-260 • C, the propane side chains of lignin degrade with the formation of the methyl, ethyl, and vinyl derivatives of guaiacol. In addition, at temperatures of ≤310 • C, the thermally unstable ether bonds are destructed [56]. The thermogram of the sulfated lignin also contains a pronounced shoulder in the range from 200 to~340 • C, but unlike the case of the initial lignin, it includes a narrow intense peak with a maximum at 350 • C, which is probably related, among other things, to the removal of the sulfo groups.

AFM
The sulphated WSSL films were studied by atomic force microscopy (AFM). The AFM images ( Figure 6) show that the sulfated WSSL film surface consists of homogeneous particles about 50 nm in size that are uniformly distributed over the film's surface without forming aggregates. It should be noted that, upon the sulfation of WSSL with sulfamic acid in the presence of urea, the particle size in the film was 100-200 nm [24].
The sulphated WSSL films were studied by atomic force microscopy (AFM). The AFM images ( Figure 6) show that the sulfated WSSL film surface consists of homogeneous particles about 50 nm in size that are uniformly distributed over the film's surface without forming aggregates. It should be noted that, upon the sulfation of WSSL with sulfamic acid in the presence of urea, the particle size in the film was 100-200 nm [24]. The incorporation of functional (e.g., sulfate) groups into the polymer structure, which intensify the inter-and intramolecular interactions or polyelectrolyte effects [57][58][59], can enhance the aggregation and coagulation of a polymer. Since we obtained sulfated lignin with a higher (compared to the known [24]) sulfur content, and consequently a high content of sulfate functional groups, we can assume that in the investigated case, the coagulation of sulfated lignin occurs.
According to the phase-contrast image (Figure 6b), the film surface is homogeneous and contains no impurities.
The sulfated lignin particle size distribution (Figure 6d) is Gaussian. The incorporation of functional (e.g., sulfate) groups into the polymer structure, which intensify the inter-and intramolecular interactions or polyelectrolyte effects [57][58][59], can enhance the aggregation and coagulation of a polymer. Since we obtained sulfated lignin with a higher (compared to the known [24]) sulfur content, and consequently a high content of sulfate functional groups, we can assume that in the investigated case, the coagulation of sulfated lignin occurs.

DFT Analysis
According to the phase-contrast image (Figure 6b), the film surface is homogeneous and contains no impurities.
The sulfated lignin particle size distribution (Figure 6d) is Gaussian.
As a model compound for the quantum-chemical calculation of lignin and sulfated lignin, we chose β-O-4 lignin, specifically, 4-(1-hydroxy-2-(4-methoxyphenoxy)ethyl)phenol ( Figure 7a). To study the effect of the sulfation mechanism on the physicochemical properties, the molar compounds of β-O-4 lignin with the sulfate group localized at the alkyl, aromatic, and alkyl and aromatic hydroxo groups were also calculated (Figure 7b-d).
In [64], we investigated the quantum-chemical characteristics of sulfated monolignols and we continue our study herein. The theoretically investigated molecules of β-O-4 lignin and its sulfate are shown in Figure 7.
As a model compound for the quantum-chemical calculation of lignin and sulfated lignin, we chose β-O-4 lignin, specifically, 4-(1-hydroxy-2-(4-methoxyphenoxy)ethyl)phenol (Figure 7(a)). To study the effect of the sulfation mechanism on the physicochemical properties, the molar compounds of β-O-4 lignin with the sulfate group localized at the alkyl, aromatic, and alkyl and aromatic hydroxo groups were also calculated (Figure 7(bd)). In [64], we investigated the quantum-chemical characteristics of sulfated monolignols and we continue our study herein. The theoretically investigated molecules of β-O-4 lignin and its sulfate are shown in Figure 7. First, the structures under study were optimized to find the most energetically favorable conformations.
The molecular geometry of the initial and sulfated β-O-4 lignin were optimized in the B3LYP/6-31G (d, p) basis set ( Figure 8).
It is evident in Figure 8 that a sulfate group incorporated into the β-O-4 lignin molecule changes the conformation and configuration of the latter. First, the structures under study were optimized to find the most energetically favorable conformations.
The molecular geometry of the initial and sulfated β-O-4 lignin were optimized in the B3LYP/6-31G (d, p) basis set ( Figure 8).
It is evident in Figure 8 that a sulfate group incorporated into the β-O-4 lignin molecule changes the conformation and configuration of the latter.
In the analysis of the charge transfer interactions, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and the corresponding energies are of great importance [65]. The HOMO and LUMO frontier molecular orbitals of a molecular system are conventionally used to estimate its chemical reactivity [66,67]. The HOMO can donate an electron, while the LUMO is considered to be an electron acceptor due to its ability to gain an electron [65]. At the occurrence of a chemical reaction in a molecular system, electrons pass through the HOMO-LUMO gap due to the energy change [68]. The HOMO-LUMO energy gap is used to predict the chemical reactivity of molecular systems [69]. Generally, a molecular system with a large HOMO-LUMO gap is less reactive than a system with a smaller gap. The HOMO and LUMO of the compounds under study and their energy plots are presented in Figure 9. In the analysis of the charge transfer interactions, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and the corresponding energies are of great importance [65]. The HOMO and LUMO frontier molecular orbitals of a molecular system are conventionally used to estimate its chemical reactivity [66,67]. The HOMO can donate an electron, while the LUMO is considered to be an electron acceptor due to its ability to gain an electron [65]. At the occurrence of a chemical reaction in a molecular system, electrons pass through the HOMO-LUMO gap due to the energy change [68]. The HOMO-LUMO energy gap is used to predict the chemical reactivity of molecular systems [69]. Generally, a molecular system with a large HOMO-LUMO gap is less reactive than a system with a smaller gap. The HOMO and LUMO of the compounds under study and their energy plots are presented in Figure 9.
The data of the HOMO-LUMO analysis ( Figure 9) show that the initial β-O-4 lignin has the highest ΔE value (5.39 eV). The sulfation of the alkyl hydroxide reduces it to 4.92 eV and the sulfation of the aromatic hydroxyl group to ΔE = 4.63 eV. The lowest ΔE value (4.54 eV) is obtained upon substitution of both the aromatic and aliphatic hydroxyl groups into the β-O-4 lignin molecule.
Thus, the HOMO-LUMO analysis allowed us to assume that β-O-4 lignin sulfated to the aliphatic position will be more stable than that sulfated to the aromatic position and the β-O-4 lignin with the sulfate groups in the aliphatic and aromatic positions has the highest reactivity. The HOMO and LUMO energies were used to the calculate electron affinity A, electronegativity χ, energy band gap ΔE, chemical potential μ, chemical hardness η, ionization energy I, softness ς, maximum charge transfer index ΔNmax, nucleophilic index N, and electrophilicity index ω for the initial and sulfated β-O-4 lignin by the techniques described in [70]. The data obtained are given in Table 3.  The data of the HOMO-LUMO analysis (Figure 9) show that the initial β-O-4 lignin has the highest ∆E value (5.39 eV). The sulfation of the alkyl hydroxide reduces it to 4.92 eV and the sulfation of the aromatic hydroxyl group to ∆E = 4.63 eV. The lowest ∆E value (4.54 eV) is obtained upon substitution of both the aromatic and aliphatic hydroxyl groups into the β-O-4 lignin molecule.
Thus, the HOMO-LUMO analysis allowed us to assume that β-O-4 lignin sulfated to the aliphatic position will be more stable than that sulfated to the aromatic position and the β-O-4 lignin with the sulfate groups in the aliphatic and aromatic positions has the highest reactivity.
The HOMO and LUMO energies were used to the calculate electron affinity A, electronegativity χ, energy band gap ∆E, chemical potential µ, chemical hardness η, ionization energy I, softness ς, maximum charge transfer index ∆N max , nucleophilic index N, and electrophilicity index ω for the initial and sulfated β-O-4 lignin by the techniques described in [70]. The data obtained are given in Table 3.  According to the data listed in Table 3, the chemical potentials of the initial and sulfated β-O-4 lignin are negative, i.e., these molecules are stable. The chemical hardness refers to the resistance of a chemical system to the deformation of the electron cloud during chemical treatment [71]. Chemically hard systems with a large HOMO-LUMO energy gap are less polarizable and relatively small, while soft systems with a small HOMO-LUMO energy gap are more polarizable and large [72].
It is evident from Table 3

MEP Analysis
The MEP data obtained by a robust quantum-chemical method are used to interpret and predict various aspects of the chemical reactivity [73,74]. The MEP is a real physical property, which can be determined by X-ray diffractometry [75] or rigorous calculations [76].
The MEP is conventionally visualized by building a surface that reflects the boundaries of molecules. This can be made by overlapping the van der Waals radii of all atoms in a molecule using the algorithms for calculating the surface of a molecule accessible to the solvent or using a constant electron density value [77].
The MEP maps (the electrostatic potential energy maps) illustrate the 3D charge distribution in the molecules and are used to determine the nature of intermolecular interactions [73]. Such maps help visualize the charge distribution and charge-related properties of molecules, as well as their size and shape. The analysis of the electrostatic potential is related to the electron density and used to establish the positions of the electrophilic and nucleophilic attacks and the interactions of hydrogen bonds [21,78,79].
Different colors in the MEP surface plot presented in Figure 10 correspond to different electrostatic potentials. The potential descending order is blue > green > yellow > orange > red.
The negative values (colored red) correspond to the electrophilic attack region and are located mainly on an oxygen atom in the sulfate group. The nucleophilic attack (positive) region is colored blue and related mainly to hydrogen atoms in the ammonium cation.
The MEP is conventionally visualized by building a surface that reflects the boundaries of molecules. This can be made by overlapping the van der Waals radii of all atoms in a molecule using the algorithms for calculating the surface of a molecule accessible to the solvent or using a constant electron density value [77].
The MEP maps (the electrostatic potential energy maps) illustrate the 3D charge distribution in the molecules and are used to determine the nature of intermolecular interactions [73]. Such maps help visualize the charge distribution and charge-related properties of molecules, as well as their size and shape. The analysis of the electrostatic potential is related to the electron density and used to establish the positions of the electrophilic and nucleophilic attacks and the interactions of hydrogen bonds [21,78,79].
Different colors in the MEP surface plot presented in Figure 10 correspond to different electrostatic potentials. The potential descending order is blue > green > yellow > orange > red. The negative values (colored red) correspond to the electrophilic attack region and are located mainly on an oxygen atom in the sulfate group. The nucleophilic attack (positive) region is colored blue and related mainly to hydrogen atoms in the ammonium cation.

Mulliken Atomic Charge Analysis
Atomic charges and charge transfer are frequently used in the chemical determination of molecular behavior and reactivity. Atomic charges still play an important role in quantum chemistry and the concept of an atomic charge has been refined in many studies. Since the Mulliken's description of atomic populations [80][81][82][83], many alternative definitions of atomic charges and populations have been presented [84]. The Mulliken charges are the total charges of atoms in a molecule, while the electrostatic potentials are generated by an electric field of the internal charge distribution [85,86]. The Mulliken atomic charges of the initial and sulfated β-O-4 lignin were calculated in the B3LYP/6-31 G(d, p) basis set.
The atomic charges of the initial and sulfated β-O-4 lignin obtained using the Mulliken population analysis are listed in Table 4.
The Mulliken atomic charges are related to the vibrational properties of a molecule and also affect the molecular polarizability, atomic charge effect, electronic structure, and many other properties of molecular systems [80][81][82][83].

Mulliken Atomic Charge Analysis
Atomic charges and charge transfer are frequently used in the chemical determination of molecular behavior and reactivity. Atomic charges still play an important role in quantum chemistry and the concept of an atomic charge has been refined in many studies. Since the Mulliken's description of atomic populations [80][81][82][83], many alternative definitions of atomic charges and populations have been presented [84]. The Mulliken charges are the total charges of atoms in a molecule, while the electrostatic potentials are generated by an electric field of the internal charge distribution [85,86]. The Mulliken atomic charges of the initial and sulfated β-O-4 lignin were calculated in the B3LYP/6-31 G(d, p) basis set.
The atomic charges of the initial and sulfated β-O-4 lignin obtained using the Mulliken population analysis are listed in Table 4. The Mulliken atomic charges are related to the vibrational properties of a molecule and also affect the molecular polarizability, atomic charge effect, electronic structure, and many other properties of molecular systems [80][81][82][83].
The sulfate groups introduced into the β-O-4 lignin structure change the Mulliken charges of almost all the atoms (Table 4).
In particular, the charge observed for the 1C atom of the initial β-O-4 lignin is −0.1031, for lignin sulfated at the aliphatic hydroxyl it is −0.06686, for lignin sulfated at the aromatic hydroxyl it is −0.10693, and for lignin sulfated at both the aliphatic and aromatic hydroxyl groups the charges are both −0.10445. A somewhat different picture is observed for the 5C atom: for the initial sample, we have 0.10358; for sample 2, 0.08089; for sample 3, 0.06617; and for sample 4, 0.15057. The visible changes in the Mulliken atomic charges also affect the oxygen, hydrogen, nitrogen, and sulfur charges. Thus, the incorporated functional sulfate group influence the properties and charge of the entire β-O-4 lignin molecule.

Theoretical FTIR Analysis
The density functional theory (DTF) [87,88] is widespread in the theoretical examination of the physicochemical characteristics of natural substances. Here, the calculation of spectroscopic properties is of great importance for predicting the relevant physicochemical characteristics [89][90][91][92].
To refine the spectroscopic characteristics of the initial and sulfated β-O-4 lignins, a theoretical FTIR spectrum was calculated. The 6-31G (d, p) DFT FTIR spectra of the initial and sulfated β-O-4 lignins contain absorption bands with different relative intensities (see Figure 11 and Table 5). and sulfated β-O-4 lignins contain absorption bands with different relative intensities (see Figure 11 and Table 5).  Figure 11. Theoretical FTIR spectra of the investigated molecules. Table 5. Some important wavenumbers (cm −1 ) scaled by a factor of 0.9608 for the investigated molecules.  Figure 11. Theoretical FTIR spectra of the investigated molecules. O-H vibrations In the theoretical FTIR spectra of the initial β-O-4 lignin, stretching vibrations of OH groups were observed around 3673 and 3623 cm -1 , respectively. For sulfated β-O-4 lignin (samples 2 and 3), the OH-group stretching vibrations were observed at 3662 and 3670 cm −1 , respectively. For lignin sulfated to the aliphatic and aromatic positions, no OH stretching vibrations were observed.
C-H vibrations The aliphatic C-H group gives rise to several fundamental frequencies, including stretching (symmetric and asymmetric in-plane and out-of-plane bending vibrations). The asymmetric and symmetric tension modes appear, as a rule, at approximately 3100 and 3000 cm -1 [79,93] O-S vibrations In the theoretical FTIR spectra, stretching vibrations of the O-S group for lignin sulfated at the aliphatic hydroxyl group β-O-4 lignin were observed at 1302, 1101, 858 cm −1 at 1303, 1098, and 861 cm −1 , and for lignin sulfated at both the aliphatic and aromatic positions, the absorption bands were observed at 1276, 1272, 1095, 1087, 955, and 890 cm −1 . In the experimental FTIR spectra of the sulfated natural substances, the vibration of this group was observed at 1249-1259 and 815-850 cm -1 , which is consistent with the results reported in [94,95].
C=C vibrations In the theoretical FTIR spectra, C=C stretching vibrations for the initial and sulfated β-O-4 lignin were observed at 1611-1568 cm −1 .
N-H vibrations Vibrations of N-H groups were observed in the range of 3452-3336 cm -1 only in β-O-4 lignin sulfated at different positions due to the presence of an ammonium cation.

Conclusions
First, this research proposed to carry out the sulfation of wheat straw soda lignin with sulfamic acid over solid catalysts of different types.
Then, it was demonstrated that the maximum sulfur content (16.5 wt%) can be obtained with the Sibunit-4 ® catalyst oxidized at 400 • C, while the Lewis acids (aluminum and titanium oxides) appeared to be less efficient.
The incorporation of a sulfate group was proven by the elemental analysis and FTIR spectroscopy. For the WSSL sulfates, in contrast to the initial WSSL, the absorption bands in the region of 1255-1210 cm −1 were observed, which correspond to the sulfate group's vibration.
Using GPC, it was found that the solid catalysts used in the sulfation process cause hydrolysis reactions and reduce the molecular weight and polydispersity index.
The characteristics of the model compounds of the initial and sulfated β-O-4 lignin were calculated within the density functional theory using the MEP, HOMO-LUMO, and Mulliken atomic charge analysis.