Hydrogenolysis process for lignosulfonate depolymerization using synergistic catalysts of noble metal and metal chloride

Abstract

A novel hydrogenolysis process for lignosulfonate depolymerization was proposed using a noble metal catalyst cooperated with metal chloride in methanol. Hydrogenolysis performance was significantly affected by the catalysts, via a synergistic catalytic effect between the Lewis acid (metal chloride) and the noble metal. Reaction conditions were also optimized, with 8.6% aliphatic alcohols and 9.2% monomers obtained at 280 °C for 5 h over Pt/C cooperated with CrCl₃. Analysis of the depolymerized products indicated that CrCl₃ had a catalytic promoting effect on cleavage of β-O-4 bonds with the synergistic catalytic effect of Pt/C. Meanwhile, the noble metal catalysed saturation of the depolymerized products and suppressed them from condensing into residues. The sulfonic groups of lignosulfonate were cleaved and they did not cause Pt/C poisoning. The catalyst showed good recyclability, with no significant loss of catalytic activity after three runs.

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1. Introduction

Lignin is a principal component of the renewable biomass, accounting for 15–30% and 40% of the weight and energy content, respectively.¹ As an abundant byproduct of biomass refineries, it has attracted more and more attention, particularly with respect to falling reserves of fossil fuels and increasing environmental problems. Lignin is a three-dimensional amorphous polymer composed of methoxylated phenylpropane units interconnected by a variety of carbon–carbon and ether linkages. However, the exact structure of lignin is difficult to determine because of its varying composition, particularly in terms of type and quantity of linkages. The numbers of methoxy groups on aromatic rings also differ from plant to plant.²

Every year a large amount of specific lignin known as lignosulfonate is generated by the pulp and paper industry. The lignosulfonate not only contains hydrophobic groups such as aromatic and aliphatic groups, but also contains many hydrophilic groups, such as sulfonic, carboxyl and phenolic hydroxyl groups. Thus, it has a high sulfur content and large molecular weight. As shown in Fig. 1, the structure of lignosulfonate is complex, with typical bonds including β-O-4, β-β, 4-O-5, 5-5, and so on.³ Traditional use of lignosulfonate has mainly been based on its dispersive, stabilizing and binding nature, making it useful in dispersing agents and binders.⁴ However, in general, only a small percentage of lignosulfonates is used, with most disposed as waste, likely causing environmental hazards. Therefore, making full use of lignosulfonates is important both economically and environmentally.⁵ Its aromatic units mean that lignosulfonate has great potential in depolymerization of highly added value aromatic monomers and high-grade biofuels by catalytic methods. There are few related studies on this, however.

Fig. 1 Principal linkages and functional groups in lignosulfonate.

Shin et al.⁶ reported the depolymerization of lignosulfonate by peroxidase. Oxidation products of 2,6-dimethoxy-1,4-benzoquinone, benzoic acid, butyl phthalate and bis(2-ethylhexyl)phthalate were achieved, but the efficiency was very low. Horacek et al.⁷ conducted the depolymerization of sodium lignosulfonate in a tailor-made stainless steel tubular flow reactor over the catalyst of NiW/Al₂O₃ at 320 °C. Many methoxyphenols were achieved, with guaiacol being the main product at 1.8% yield. But the catalyst was easy to deactivate, and the yield of phenolic monomers was also low. Zhao et al.⁸ also studied the catalytic oxidation of sodium lignosulfonate over the H₅PMo₁₀V₂O₄₀ catalyst at 190 °C. They obtained about 15% yield of the oil liquid product, but the residual solid was the main product, with yield up to 65%. Results indicated serious condensation, making coking of the catalyst inevitable. Overall, the low yields of value-added products and the serious condensation have limited the development of lignosulfonate depolymerization. Another challenge is the strong interaction between sulfur and the active sites of the catalyst, which can lead to the deactivation of catalysts.³

In this study, a novel hydrogenolysis process was proposed for lignosulfonate depolymerization using noble metal cooperated with metal chloride in methanol. The value-added products, such as aliphatic alcohols and aromatic monomers, were achieved at high yields. The effects of reaction conditions on the lignosulfonate depolymerization and product distribution were carefully studied. Results showed that hydrogenolysis and condensation reactions took place during the process with intensive competition. Detailed characterizations of the original lignosulfonate, the oligomers and the residues were also carried out, and analysis showed that the synergetic catalysts had a promoting effect on cleavage of β-O-4 bonds and sulfonic groups. The lignosulfonate degradation performed well in this hydrogenolysis process.

2. Experimental

2.1. Materials

Sodium lignosulfonate was purchased from Aladdin Reagent Co., Ltd (Shanghai, China). 5 wt% Ru/C, 5 wt% Pd/C, 5 wt% Pt/C and CrCl₃ were also purchased from Aladdin. Tetrahydrofuran (THF) and methanol were provided by Da Mao chemical reagent factory (Tianjin, China). KCl, CaCl₂, ZnCl₂, AlCl₃, and MgCl₂ were purchased from Tianjin Fu Yu Fine Chemical Co., Ltd. (Tianjin China). Other reagents were supplied by Jinhuada Chemical Reagent Co., Ltd (Guangzhou, China). All commercial reagents in this study were analytic grade and were used without further purification. 5 wt% Ni/C catalyst was prepared by an incipient wetness impregnation method following a previously described procedure.⁹

2.2. Typical process for lignosulfonate hydrogenolysis

In a typical run, 0.5 g sodium lignosulfonate, 1 mmol metal chloride, 0.1 g 5 wt% noble metal and 40 mL CH₃OH were charged into a 100 mL stainless autoclave (316L stainless, Weihai Chemical Machinery Co., Ltd. Shandong, China) equipped with mechanical agitation. The stirring rate was 400 rpm. After being displaced by H₂ three times and charged to 3 MPa, the reactor was heated to 280 °C for 5 h. When the reaction was finished, the mixture was cooled down to room temperature during 30 min using flowing water.

2.3. Product separation and analysis

The typical product separation procedures are shown in Fig. S1.† An Agilent 6890 gas chromatogram (GC) with a thermal conductivity detector (TCD) and a flame ionization detector (FID) was used to measure the gas fraction, and the gaseous products from this process were less than 1 wt% of the feed lignin. Therefore, the yield of gas products was negligible and not taken into account in the mass balance. Filtration of the product mixture was first carried out. The solid fraction (include catalysts and residues) was washed three times with methanol, 1 M HCl solution and water, respectively. After that, the solid fraction was dried at 60 °C until it achieved a constant weight. The filtrate was diluted by CH₃OH to a given volume for qualitative and quantitative analysis. Qualitative analysis of the volatile products was carried out by gas chromatography/mass spectrometry (GC-MS) using a HP-INNOWAX column (30 m × 0.25 mm × 0.25 μm). The oven temperature was programmed at 60 °C for 2 min, and then ramped up to 260 °C with 10 °C min⁻¹ and then held for another 10 min. The injector was kept at 280 °C in spit mode (5 : 1) with helium as the carrier gas. Quantitative analysis of the volatile products was determined by SHIMADZU GC 2014C with a FID and a HP-INNOWAX column. The oven temperature program was the same as the GC-MS analysis. Acetophenone was used as the internal standard chemical.

The degree of lignosulfonate liquefaction represented for the yield of liquid products, was calculated by the lignosulfonate weight loss ratio (eqn (1)). The yields of aliphatic alcohols, monomers and other volatile products were evaluated according to eqn (2)–(5), based on the GC results. The yield of residues was calculated based on eqn (6) and the yield of oligomers (namely the nonvolatile products) was measured by the mass balance, as shown in eqn (7):

Degree of lignosulfonate liquefaction (D_L) = (W_F − W_R)/W_F × 100% (1)

Yield of volatile products (Y_V) = W_V/W_F × 100% (2)

Yield of aliphatic alcohols (Y_A) = W_A/W_F × 100% (3)

Yield of monomers (Y_M) = W_M/W_F × 100% (4)

Yield of other volatile products (Y_Other) = Y_V − Y_A − Y_M (5)

Yield of the residues (Y_R) = W_R/W_F × 100% (6)

Yield of the oligomers (Y_O) = D_L − Y_V (7)

W_F: the weight of feed lignosulfonate; W_R: the weight of residues; W_V: the weight of volatile products; W_A: the weight of aliphatic alcohols; W_M: the weight of monomers.

2.4. Characterization of the catalysts, the original lignosulfonate and hydrogenolysis products

The surface area and pore size distribution of the catalysts were determined using a Quantachrome chemical adsorption instrument. The catalysts were degassed at 200 °C for 12 h prior to measurements. Powder X-ray diffraction (XRD) patterns of the catalysts were measured using X-ray diffraction radiation (X Pert Pro MPD with Cu Ka (k = 0.154) radiation, Philip, the scanning angle (2θ) ranged from 10° to 80°). Scanning electron microscope (SEM) images and energy dispersive spectrometry (EDS) were obtained using a Hitach S-4800 instrument (10 kV). Transmission electron microscopy (TEM) investigations were measured on a Jeol TEM-100CX instrument at 200 kV accelerating voltage. The elemental analysis of the original lignosulfonate was carried out on a vario EL III element analyzer. The molecular weights of the original lignosulfonate and the oligomers were determined by gel permeation chromatography (GPC) on Waters e2695 with a 7.8 × 300 mm column (HR 4E THF). Tetrahydrofuran (THF) was used as the mobile phase and the average molecular weight of the sample was measured according to an external standard method with narrow polystyrene as the standard compound. Fourier translation infrared spectroscopy (FT-IR) spectra were measured on a Nicolet iS50 FT-IR spectrometer, using a KBr pelleting method. ¹H nuclear magnetic resonance (¹H-NMR) was conducted on a Bruker AVANCE III 300 WB spectrometer (7.05 T) with d₆-DMSO as the solvent.

3. Results and discussion

3.1. Catalyst characterization

Measurement of the main element composition was carried out. As listed in Table S1,† the sodium lignosulfonate exhibited notably the presence of Na and S, which was quite different from the other lignins.^10,11 The existence of high sulfur probably hampered lignosulfonate hydrogenolysis by poisoning the catalysts,^12–14 and the sulfur probably contaminated the down-stream products. These two factors are big challenges for clean utilization of lignosulfonate.

A typical run of lignosulfonate hydrogenolysis was carried out using synergistic catalysts of noble metal and metal chloride. First, the components of volatile products were qualitatively analyzed by GC-MS (Table S2†). Results highlighted some value-added products, such as aliphatic alcohols and monomers. The aliphatic alcohols mainly consisted of C4–C6 alcohols and the monomers contained alkylbenzenes, guaiacols and phenols. It should be noted that the volatile components did not contain any sulfur or chloride, indicating that the sulfonic groups had been eliminated from the lignosulfonate and that the dissolved metal chloride did not react with the feedstock. Therefore, the sulfur and chloride separated out and avoided contaminating the down-stream products.

Table 1 summarizes the lignosulfonate hydrogenolysis and product distribution with different catalysts. As detailed, only about 64.4% lignosulfonate was liquefied in the absence of any catalyst (Table 1, entry 1). Most was turned into residues (35.6%). Meanwhile, only 1.7% yield of aliphatic alcohols and 4.3% yield of monomers were produced. Addition of Pd/C showed only a small improvement in performance of the lignosulfonate hydrogenolysis. However, when lignosulfonate was reacted with Pd/C cooperated with metal chlorides, the liquefaction degree increased and the residues decreased significantly (Table 1, entries 3–8). When treated with Pd/C and CrCl₃, 83.9% lignosulfonate was liquefied and only 16.1% residues formed, giving 5.3% yield of aliphatic alcohols and 8.5% yield of monomers (Table 1, entry 8). It was considered that the ether bonds became more flexible for corruption when an acidic catalyst (metal chlorides) was used.^15–17 Therefore, higher liquefaction degree, higher monomer yield and lower residue yield were obtained in comparison with the acid-free process. Also, it is well known that Cl⁻ is a high electronegativity element and is widely used in biomass conversion as an excellent hydrogen bonding acceptor and nucleophilic reagent.^18,19 In this process, the Cl⁻ in the metal chloride could act as a hydrogen bond acceptor for lignosulfonate and as a polarization reagent for C–O bonding,²⁰ helping to weaken the ether bonds and promote the hydrogenolysis reaction. Moreover, comparing the performances with and without Pd/C, it can be seen clearly that the presence of Pd/C results in decrease of residues. Pd is an excellent hydrogenation catalyst, with outstanding adsorption capacity to H.^20,21 In this process, Pd/C was considered to promote stabilization of degraded products and prevent them from condensing into residues.

Table 1 Effect of catalysts on lignosulfonate hydrogenolysis and product distribution^a

Entry	Noble metal	Metal chloride	DL (%)	Yield of volatile products^b (wt%)			Residues (wt%)	Oligomers (wt%)
				Aliphatic alcohols	Monomers	Other volatile products
a Condition: 0.5 g sodium lignosulfonate, 0.1 g noble metal, 1 mmol metal chloride, 40 mL methanol, 3 MPa H2, 280 °C, 5 h.b Measured by GC 2014C, where acetophenone was used as internal standard chemical.
1	—	—	64.4	1.7	4.3	7.0	35.6	51.4
2	Pd/C	—	69.3	1.9	5.1	8.4	30.7	53.9
3	Pd/C	KCl	76.6	1.7	2.5	9.3	23.4	63.1
4	Pd/C	CaCl2	73.5	1.1	2.8	10.3	26.5	59.3
5	Pd/C	ZnCl2	77.7	1.5	5.6	9.5	22.3	61.1
6	Pd/C	MgCl2	81.8	3.1	7.5	17.1	18.2	53.6
7	Pd/C	AlCl3	81.7	2.6	7.0	14.6	18.3	57.5
8	Pd/C	CrCl3	83.9	5.3	8.5	17.8	16.1	48.5
9	Ni/C	CrCl3	76.2	3.1	7.9	12.1	23.8	52.9
10	Ru/C	CrCl3	75.2	2.2	5.6	6.7	24.8	60.7
11	Pt/C	CrCl3	84.6	8.6	9.2	18.8	15.4	48.0
12	—	CrCl3	72.5	3.1	7.7	12.8	27.5	48.9

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Besides Pd/C, Ni/C, Ru/C and Pt/C metal catalysts were also tested (Table 1, entry 9–11). The properties of these catalysts were compared using BET, XRD and SEM technologies. The N₂ adsorption–desorption isotherms are shown in Fig. S2 and Table S3† summarizes the BET surface area, total pore volume and pore diameter. Results show that the surface properties of these carbon-supported catalysts are similar. The Pt/C catalyst had the largest surface area (1239.76 m² g⁻¹), with total pore volume 0.65 cm³ g⁻¹ and pore diameter 3.82 nm. The XRD patterns of these catalysts (Fig. S2†) all exhibited obvious carbon peaks and no metal phase peaks, because of the low metal load and high metal dispersion. The SEM images (Fig. S3†) indicated that these carbon-supported catalysts possessed alike bulk shapes and small metal particles covering the surface. Overall, the properties of these catalysts were analogous. Differences in hydrogenolysis performances probably resulted from the distinctions between metal active components. Performances of lignosulfonate hydrogenolysis in the presence of Ni/C and Ru/C were no match for that in the presence of Pd/C. High residue yield was obtained because of condensation, while the presence of Pt/C was good for performance in production of volatile products. The yields of aliphatic alcohols and monomers were higher than those in the presence of Pd/C, especially the aliphatic alcohols. In terms of their formation, it was considered that aliphatic alcohols came from cleavage of long alkyl branch-chains on aromatic rings, which was conducive to realize the structure stability. Afterwards, derivatization occurred and various aliphatic alcohols were formed. Pt/C had higher catalytic activity on derivatization than Pd.^22,23 Under the effect of Pt/C, a large amount of aliphatic alcohols were eventually produced in the presence of hydrogen and methanol.^10,24 As it has better resistance capability to sulfur poisoning than does Pd,¹² Pt was able to keep the high catalytic activity, which probably contributed to the higher production of volatile products. Furthermore, in the presence of Pt/C, only 15.4% residues were obtained. Compared with the control experiment (Table 1, entry 12), this demonstrates that Pt/C was also able to promote stabilization of degraded products and prevent them from condensing into residues, even better than does Pd/C.

3.2. Effects of reaction condition on lignosulfonate hydrogenolysis

The effects of reaction temperature and time on lignosulfonate hydrogenolysis were investigated in the presence of Pt/C and CrCl₃. As shown in Fig. 2a, the hydrogenolysis process was highly temperature-dependent. The degree of lignosulfonate liquefaction and the yields of volatile products sharply increased with elevation of reaction temperature. As the temperature approached 280 °C, these reached their maximum, with 84.6% liquefaction degree, 8.6% aliphatic alcohols and 9.2% monomers. However, beyond that temperature, for example 300 °C, a drop was seen in liquefaction degree. The yields of volatile products also showed a similar downtrend. By way of explanation, the hydrogenolysis resulted in formation of aliphatic alcohols and monomers,²¹ with condensation occurring simultaneously. The monomers were condensed into oligomers, and the oligomers were further condensed into residues,²⁵ with the reactions competing intensely. A higher temperature promoted the depolymerization, hydrogenolysis and condensation reactions equally. But the excessive high temperature favored condensation and promoted formation of residues.¹⁰ Therefore, the yields of aliphatic alcohols and monomers decreased. A similar trend was exhibited for reaction time, although this was smaller in terms of effects than was temperature (Fig. 2b). An increase of residues was exhibited over prolonged time, probably attributed to intensified condensation.^10,20,21 The residues aggregated over the catalyst surface, which caused the catalytic activity to decline, which, eventually, resulted in slight decrease of volatile product yields.^26,27 Generally, 5 h was the most suitable time for lignosulfonate hydrogenolysis in this process.

Fig. 2 Effects of (a) reaction temperature and (b) time on the lignosulfonate hydrogenolysis and products distribution. Conditions: (a) 0.5 g lignosulfonate, 0.1 g Pt/C, 1 mmol CrCl₃, 3 MPa H₂, 5 h; (b) 0.5 g lignosulfonate, 0.1 g Pt/C, 1 mmol CrCl₃, 3 MPa H₂, 280 °C.

As hydrogen is consumed during hydrogenolysis, the effect of hydrogen pressure was studied on lignosulfonate hydrogenolysis (Fig. 3a). Initially, the liquefaction degree increased gradually with increasing H₂ pressure, with the summit reached when 3 MPa H₂ was inflated. After that, the liquefaction degree showed a little drop with elevated H₂ pressure, with residues increasing. The aliphatic alcohols and monomers exhibited a slight decrease when the H₂ pressure exceeded 3 MPa. This was probably because the side reactions and the condensation were enhanced at higher H₂ pressure,²⁰ with suppressed volatility of volatile products by physical effects.²⁸ The influence of CrCl₃ catalyst dosages was also investigated. As shown in Fig. 3b, the lignosulfonate liquefaction degree and the aliphatic alcohol yield were increased with added catalyst dosage at less than 1.0 mmol. Beyond this, continuous increase in catalyst dosage caused slight drops in liquefaction and yields. The catalyst had a promoting catalytic effect not only on the lignosulfonate depolymerization, but also on the condensation,²¹ hence the increase of residues exhibited with excess catalyst dosage in this process. The side reactions of the volatile products occurred likewise. Fig. 4 further details the effect of Pt/C catalyst dosages. Gradual increasing trends in degree of liquefaction and the volatile product yield were exhibited with augmentation of Pt/C dosages. 84.6% of lignosulfonate was liquefied when 0.1 g Pt/C was used, with 8.6% aliphatic alcohols and 9.2% monomers. As the Pt/C dosage continued to be augmented, the aliphatic alcohols and monomers, as well as the lignosulfonate liquefaction increased only a little, indicating that 0.1 g Pt/C was sufficient for catalytic hydrogenolysis in this process.

Fig. 3 Effects of (a) hydrogen pressure and (b) CrCl₃ dosage on the lignosulfonate hydrogenolysis and products distribution. Conditions: (a) 0.5 g lignosulfonate, 0.1 g Pt/C, 1 mmol CrCl₃, 5 h, 280 °C; (b) 0.5 g lignosulfonate, 0.1 g Pt/C, 3 MPa H₂, 5 h, 280 °C.

Fig. 4 Effect of Pt/C dosage on the lignosulfonate hydrogenolysis and products distribution. Conditions: 0.5 g lignosulfonate, 1 mmol CrCl₃, 3 MPa H₂, 5 h, 280 °C.

3.3. Analysis of volatile products

Gaseous product generated from this process was measured by GC-TCD-FID. Besides the inflated H₂, the other gas fraction included CH₄, CO₂ and CO, which mainly came from the hydrogenolysis, as well as the demethoxylation and dealkylation reactions. Moreover, the lignosulfonate had a high content of sulfur (Table S1†). Pb(Ac)₂ solution was used to find out whether there was any H₂S in the gas fraction. Unexpectedly, the result showed no obvious PbS precipitation formed, which indicated that the sulfur in lignosulfonate did not evolve into the gas fraction. Therefore, the sulfur was probably turned into liquid products or residues. BaCl₂ solution was used to test the liquid products, and BaSO₃ precipitation was observed (as soon as HCl was added, the precipitation dissolved), indicating that some sulfur was indeed incorporated into the liquid products.

As observed in the volatile components (Table S2†), a large amount of aliphatic alcohols with branch chain were obtained. Their structures were stable and able to suppress further derivatization.^29,30 Among the monomers, many alkylbenzenes, guaiacols and phenols were formed through the hydrogenolysis process. The derivatization came about inevitably in methanol, producing compounds with methyl substituents on aromatic rings. The methylation being conducive to stabilizing the compound activity was considered to be responsible.^31–33 Quantitative analysis was also carried out by GC-FID according to an internal standard method, and the results are displayed in Table 2. 8.6% aliphatic alcohols were obtained, mainly including 2-methyl-1-propanol, 2-methyl-1-butanol and 2-methyl-1-pentanol. Moreover, 9.2% monomers were produced, with 3.4% 2-methyl-5-(1-methylethyl)-phenol and 0.6% 3,4,5-trimethyl-phenol. The phenols accounted for the main part of the monomers. Demethylation and demethoxylation¹⁵ were considered to result in the formation of phenols.

Table 2 Main components of the volatile products^a

Component	Yield^b (wt%)
a Condition: 0.5 g sodium lignosulfonate, 0.1 g Pt/C, 1 mmol CrCl3, 40 mL methanol, 3 MPa H2, 280 °C, 5 h.b Measured by GC 2014C, where acetophenone was used as internal standard chemical. Components listed are those represented by more than 0.1% of yield as determined by GC 2014C.
Monomers	9.2
Benzene, 1,2,3-trimethyl-	0.2
Benzene, 1,2-dimethoxy-	0.1
3,4-Dimethoxytoluene	0.1
Phenol, 2-methoxy-	0.2
1,4-Benzenediol, 2,3,5-trimethyl-	0.1
Phenol, 2-methoxy-4-methyl-	0.4
Phenol	0.2
Ethanone, 1-(4-hydroxy-3-methoxyphenyl)-	0.1
Phenol, 2,3,6-trimethyl-	0.1
Phenol, 2,3,5-trimethyl-	0.3
Phenol, 3,5-dimethyl-	0.1
3-tert-Butyl-4-hydroxyanisole	0.2
Phenol, 2-methoxy-4-propyl-	0.3
Phenol, 3,4-dimethyl-	0.2
Phenol, 2,3,4,6-tetramethyl-	0.1
Phenol, 3,4,5-trimethyl-	0.6
p-Isopropylphenetole	0.1
Benzene, 1-butyl-4-methoxy-	0.1
Phenol, 4-propyl-	0.2
Phenol, 2-methoxy-6-(1-propenyl)-	0.1
Phenol, 2-(1,1-dimethylethyl)-6-methyl-	0.1
Benzene, 2-methoxy-4-methyl-1-(1-methylethyl)-	0.2
Phenol, 2-methyl-5-(1-methylethyl)-	3.4
Ethanone, 1-(2,4,5-triethylphenyl)-	0.2
Benzene, 1,2-diethyl-3,4,5,6-tetramethyl-	0.1
Benzene, 1,2,3,4-tetramethyl-5-(1-methylethyl)-	0.8
Benzene, 1-methoxy-4-(1-methyl-2-propenyl)-	0.1
1,4-Benzenediol, 2,6-dimethyl-	0.4
1,3-Benzenedicarb-oxylicacid, dimethyl ester	0.1
Aliphatic alcohols	8.6
1-Propanol, 2-methyl-	6.7
1-Butanol	0.2
3-Heptanol	0.2
Ethanol, 2-methoxy-	0.2
1-Butanol, 2-methyl-	0.7
1-Pentanol	0.2
1-Pentanol, 2-methyl-	0.3
Propanoic acid, 2-hydroxy-, methyl ester	0.1
Other volatile products
Pentanoic acid, methyl ester	0.2
Butanoic acid, 2-methyl-, methyl ester	0.2
Cyclopentene, 1-methyl-	0.1
2-Cyclopenten-1-one, 2,3,4-trimethyl-	0.2
2-Cyclopenten-1-one, 2,3,4,5-tetramethyl-	0.6
Benzofuran, 2,3-dihydro-2,2,4,6-tetramethyl-	0.3
1H-Inden-1-one, 2,3-dihydro-3,4,7-trimethyl-	0.1

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3.4. Analysis of the oligomers

Characterizations of the original lignosulfonate and the oligomers were made carefully using various measurements. Elemental analysis was used to figure out the element components (Table S4†). Compared with the original lignosulfonate (Table S1†), the oligomers exhibited only small contents of sulfur and sodium. This indicates that the sulfonic groups in the lignosulfonate were cleaved significantly and incorporated into the liquid phase, in agreement with the above test results.

The average molecular weights of the original lignosulfonate and the oligomers were also measured. From Table 3, the sodium lignosulfonate possessed a weight average molecular weight of 3679 g mol⁻¹. After the hydrogenolysis treatment, the molecular weights of obtained oligomers had decreased significantly. In the absence of any catalyst, the original lignosulfonate was depolymerized to the oligomer with weight average molecular weight of 1078 g mol⁻¹. Addition of CrCl₃ promoted this degradation process, and both the average molecular weight and the dispersion degree of the oligomers decreased. The best degradation performance was achieved in the presence of Pt/C cooperated with CrCl₃. The number average molecular weight, the weight average molecular weight and the dispersion degree of oligomers were 654 g mol⁻¹, 846 g mol⁻¹ and 1.25, respectively. These results confirm that the lignosulfonate degradation performed well in this hydrogenolysis process.

Table 3 Average molecular weight of the original lignosulfonate and the oligomers^a

Materials	Mn	Mw	Mz	D
a Mn: number average molecular weight; Mw: weight average molecular weight; Mz: Z-average molecular weight; D: dispersion degree.
Original lignosulfonate (g mol^−1)
Sodium lignosulfonate	2968	3679	4592	1.28
Oligomers (g mol^−1)
No catalyst	795	1078	1527	1.36
CrCl3	725	871	1250	1.28
Pt/C + CrCl3	654	846	1120	1.25

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To follow the evolution of the lignosulfonate, FT-IR analysis was carried out (Fig. 5). According to the literature,^{2,10,11,34,35} 2923 and 2873 cm⁻¹ peaks were assigned to C–H stretch in methyl and methylene groups. A 1705 cm⁻¹ peak was regarded as the characteristic absorption of the carbonyl. The strong absorbencies at 1602, 1507 and 1459 cm⁻¹ suggested the existence of benzene structures in lignosulfonate. The peaks at 1216 cm⁻¹ and 1106 cm⁻¹ were considered to be, respectively, the characteristic vibrations of the guaiacyl and the C–O stretching in the alkoxy functional group. The peak attributed to the –SO₃H group was seen at 1043 cm⁻¹. Comparing the oligomers with the original lignosulfonate, it can be clearly seen that the characteristic peaks had significantly changed after the treatment, indicating significant lignosulfonate evolution. For example, the original lignosulfonate had a high intensity peak of –SO₃H group (1043 cm⁻¹). After treatment in the absence of catalyst, this group showed only a small decrease. Whereas, with addition of CrCl₃, the peak intensity of this group sharply decreased, even disappeared, indicating that the acid catalyst promoted cleavage of the –SO₃H group in the hydrogenolysis process. The guaiacyl group (1216 cm⁻¹) and alkoxy groups (1106 cm⁻¹) also showed a similar trend of change. After addition of CrCl₃, intensity of the peaks decreased significantly. It was considered that demethoxylation had occurred.³⁶ The volatile products, which showed a large amount of phenols and a small amount of guaiacols (Table 2), also provided evidence of this. Moreover, the peak intensities of the alkyl groups (2923 and 2873 cm⁻¹) increased in the presence of Pt/C, indicating the occurrence of methylation, correlating well with methyl as the major substituent in the volatile products' results.

Fig. 5 FT-IR spectra of the original lignosulfonate and the oligomers with different catalysts.

The original lignosulfonate and the oligomers were further characterized using ¹H-NMR (Fig. 6). According to the literature,^10,11,32 the peaks at 9.60–7.80 and 7.70–6.00 ppm were assigned to the phenolic OH and aromatic protons, respectively. The appearance of peaks at 3.73–3.59 ppm was attributed to the presence of protons in β-O-4 substructures. The peak at 3.43–3.36 ppm was assigned to the protons in the phenylcoumarane substructures. The chemical shifts at 3.10–2.90 and 2.29–1.81 ppm could be assigned, respectively, to the presence of protons in β-β substructure and alkyl group on aromatic rings. The profiles shown in Fig. 6 demonstrate that the lignosulfonate structure had significantly changed after hydrogenolysis. The original lignosulfonate and the obtained oligomers in the absence of catalyst both exhibited an obvious peak attributed to a β-O-4 group. With addition of CrCl₃, this group decreased significantly and even disappeared; meanwhile, the content of phenolic OH increased. This indicated that the ether bond had been cleaved through hydrogenolysis. Table 4 also lists the relative quantification of the functional groups based on the integral peak areas. As for the aromatic H, the content changed only a little in each sample, indicating that the aromatic ring remained intact, which correlates well with results of the volatile products' components (Table S2†). The increase in alkyl groups after the hydrogenolysis process suggested the occurrence of the methylation. These phenomena are in good agreement with the FT-IR result (Fig. 5). Moreover, all the nonvolatile products in the presence of CrCl₃ catalyst showed a significant increase in phenylcoumarane as well as the β-β groups.

Fig. 6 ¹H NMR of the original lignosulfonate and the oligomers with different catalysts.

Table 4 Relative quantification by ¹H NMR

Groups	Chemical shift (ppm)	Content (%)
		Original lignosulfonate	No catalyst	CrCl3	Pt/C + CrCl3
Phenolic OH	9.60–7.80	4.9	5.1	9.1	9.3
Aromatic H	7.70–6.00	6.5	6.6	6.8	6.8
β-O-4	3.73–3.59	2.7	2.2	0	0
Phenylcoumarane	3.43–3.36	5.1	5.3	8.0	7.8
β-β	3.10–2.90	2.0	2.1	3.5	3.5
Alkyl groups	2.29–1.81	12.2	15.0	17.1	17.5

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3.5. Analysis of residues

To examine the reason for formation of the residues, the FT-IR analysis of this fraction was measured. Fig. S4† presents the spectra of the samples with or without catalysts, showing only small differences on some special characteristic peaks. For example, in the absence of any catalyst, the peaks of the –SO₃H group (1043 cm⁻¹) and benzene ring (1602, 1420 cm⁻¹) were exhibited obviously, indicating that condensation of lignosulfonate and aromatic products was the main reason for formation of residues.³⁷ However, when the catalysts were added, the peak intensities of these two groups decreased. In particular, addition of Pt/C and CrCl₃ caused the peak for the –SO₃H group almost to disappear and the peak for the benzene ring decreased significantly. This indicates that Pt/C cooperated with CrCl₃ promoted efficient cleavage of the –SO₃H group and suppressed the condensation, which correlated well with the fewest residues in the hydrogenolysis test (Table 1). The significant decrease of sulfur content in the residues (2.47% to 0.66%) also confirmed the promoting effect of Pt/C and CrCl₃ catalysts on the –SO₃H group cleavage (Table S5†).

3.6. Proposed catalytic mechanism

As discussed above, a synergistic catalytic effect was exhibited by CrCl₃ and Pt/C on lignosulfonate hydrogenolysis. Based on knowledge of the reported mechanism^20,38,39 and the results of this study, a possible reaction pathway for generation of monomers and aliphatic alcohols is proposed (Fig. S5†). The Cr cation was first conjuncted with an O atom and the electron-abundant benzene ring to form a stable complex (1). Simultaneously, the high electronegativity Cl was attached to the lignin molecule, resulting in weakening of the C–O bond. Cleavage of the –SO₃Na group also occurred during this stage. Then, the β-O-4 and –OCH₃ bonds were cleaved under the polarization effect of Cl⁻ and the synergic effect of Pt/C, and simple chemicals (4) and oligomers (5) were formed. Pt/C was able to capture hydrogen not only to promote the hydrogenolysis, but also to realize the saturation of depolymerized products and suppress them from condensing. Afterwards, the dealkylation of long carbon chains occurred in propyl-phenols (4) through hydrogenolysis.⁴⁰ And through a series of derivatizations (alkylation and so on), many alkylphenols and aliphatic alcohols were obtained eventually. The oligomers (5) were also able to further convert into monomers.

3.7. Catalyst recycle

The recyclability of the catalyst was investigated carefully. The results in Fig. S6† demonstrate that Pt/C exhibited good recyclability in this hydrogenolysis process. Only slight decreases were observed in lignosulfonate conversion and volatile product yields in the third run, with a slight increase in residues. This indicates that the evolved sulfur did not cause catalyst poisoning. ICP-AES analysis showed no Pt leaching in the solvent. SEM images of the fresh and recovered Pt/C (Fig. S7†) showed that there was no significant change in particle size and morphology, but that a slight tar formation was exhibited. The FT-IR spectrum (Fig. S8†) of the recovered catalyst also exhibited a characteristic infrared absorption of lignosulfonate and aromatic products, in which the characteristic absorptions of the benzene ring (1602, 1459 cm⁻¹) and the alkoxy group (1106 cm⁻¹) could be observed, thus revealing the source of the tar. TEM images of the catalysts (Fig. S9†) indicated that the size of Pt nanoparticles in Pt/C had increased after three runs. Therefore, the slight tar formation on the catalyst surface and the growth in size of Pt nanoparticles were considered to be responsible for the decrease in catalyst activity.

4. Conclusion

Depolymerization of sodium lignosulfonate can be realized through hydrogenolysis under the synergistic effect of Pt/C and CrCl₃ in methanol. Value-added products, such as aliphatic alcohols and monomers, were achieved at high yields. Reaction temperature, time, hydrogen pressure and catalyst dosage had significant influences on the lignosulfonate hydrogenolysis and product distribution. 84.6% lignosulfonate could be liquefied at 280 °C, 3 MPa H₂ for 5 h, producing 8.6% aliphatic alcohols and 9.2% monomers. The volatile products, oligomers and residues were measured by GC-MS, GC, elemental analysis, GPC, FT-IR, ¹H NMR, SEM and TEM. Analysis results showed that Pt/C and CrCl₃ catalyzed cleavage of β-O-4 bonds and suppressed them from condensing into residues. The sulfonic groups of lignosulfonate were also cracked and evolved into the liquid phase, while they did not cause catalyst poisoning. Moreover, the Pt/C catalyst showed a good recyclability. Hence this efficient hydrogenolysis process provides a beneficial reference for future clean utilization of lignosulfonate.

Acknowledgements

The authors gratefully acknowledge the financial support of the Natural Science Foundation of China (No. 51476178), and the National Key Technology R&D Program, (No. 2014BAD02B01).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra16725j

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