碱氧化松木制取单酚化学品*

朱妤婷，刘 竞，吕 微，

REUBROYCHAROEN Prasert4，王晨光 1,2,3†

（1.中国科学院广州能源研究所，广州 510640；2.中国科学院可再生能源重点实验室，广州 510640；3.广东省新能源和可再生能源研究开发与应用重点实验室，广州 510640；4.泰国朱拉隆功大学科学学院化学技术系，曼谷 10330；5.中国科学院大学，北京 100049）

0 Introduction

Currently, scientists are making unremitting efforts to valorize biomass, the world’s richest renewable energy sources, into chemicals and materials to meet the challenge of shortage of non-renewable energy.[1-2]Lignin is one of the three major components of biomass with a content of about 30%.[3]Cellulose and hemicellulose can be efficiently converted to high value-added chemicals by suitable chemical or biological processes.[4]Lignin has the highest aromatic substance among biomass in the world, while currently burned as a low-grade fuel because of its resistant structure that hard to be converted.[5-6]Therefore, the conversion of lignin to high value products is gaining more and more attention by scientists all over the world.Hydrogenolysis,[5,7-8]oxidation,[9-12]bioengineering were developed for the valorization of lignin into valuable chemicals and materials.[4,13]

Vanillin (3-methoxy-4-hydroxy benzaldehyde) is one of the most famous fragrances and the highest-yielding aromatic compound in the world.[14]It is used in a wide variety of applications such as flavoring agents for chocolate and ice cream, ripeners, sunscreens, catalyst,intermediate of the drugs like aldomet, L-dopa, and trimethaprim.[14-16]Nowadays, annual produced vanillin is about 20 000 tons by three routes, petroleum-based guaiacol route (85%), lignin route (about 15%) and vanilla beans route (less than 1%).[14,17]Vanillin is the main product of alkaline oxidation of lignin, especially type-G lignin.Thus，given its renewable and abundant,lignin can be the alternative of petroleum-based guaiacol and become the primary source of vanillin in the future.

Nitrobenzene gave high yield of vanillin due to its strong oxidizability in the alkaline oxidation of lignin.[18]However, the nitrobenzene method has been deprecated due to reduction product of nitrobenzene is toxicity and hard to be separated from aromatics.[14,19]Currently,environmental friendly O2and air are used as oxidant.[11,14,20-21]Both homogenous and heterogenous catalysts are widely used for promote the selective oxidation of lignin to monophenols.On the one hand,homogenous catalysts like CuSO4·H2O achieved vanillin yield of 18wt.% from pine wood in the presence of NaOH.[22]The addition of FeCl3into the CuSO4catalytic system exhibited an enhanced monomer yield when compared to the NaOH only alkaline system.[23]On the other hand, heterogeneous catalysts such as Co-, Pd-,Mn-, Fe-based solid catalyst,[24]Pd/CeO2,[25]LaCoO3,[9]Pt-based catalysts[26]were widely applied to the oxidation of lignin or lignin models.Although heterogeneous catalyst has an advantage of easily separated from the alkali solution, their insufficient mass transfer with solid lignin might weaken their efficiency.[6]Furthermore,some catalysts were only exhibited activity on model compounds rather than on real lignin.[26-27]

In this work, pine wood is reported to produce monophenols by using cheap NaOH as homogeneous catalyst, and O2as green oxidant, under a mild reaction conditions.Effects of NaOH concentration, agitation speed and oxygen pressure on monophenols yields were studied.Alkaline oxidation of vanillin under the same condition as pine wood system was carried out to investigate the degradation of vanillin.GC, GC-MS and HPLC were used to analyze the contents of monophenols and carboxylic acids, while GPC was applied for the characterization of molecular weight distribution of products.This work provides some valuable information for the future research of valorization of lignin, even the conversion of lignocellulose into chemicals and materials.

1 Materials and methods

1.1 Materials

Pine wood (lignin, 23%; cellulose, 44%; hemicellulose,23%; others, 10%) was smashed and screened to <400 μm.High purity O2(99.99%) was purchased from Yigas Gases Co.ltd (Guangdong, China).NaOH, tetrahydrofuran (THF),hydrochloric acid (HCl) and all standards (vanillin,syringaldehyde, p-hydroxybenzaldehyde, vanillic acid,acetic acid, formic acid, methanol and ethanol) were purchased from Macklin Biochemical Technology Co.,Ltd.(Shanghai, China).All chemicals were analytical reagents and used without purification.

1.2 Alkaline oxidation experiment

Batch experiments were performed by using a 100 mL stainless-steel autoclave reactor equipped with a magnetic coupling mechanical stirring rod and a thermocouple.In a typical experiment, 0.2 g vanillin or 1 g pine wood was added into a reactor preloaded with 50 mL NaOH aqueous solution.Then, the reactor was sealed and purged three times and pressurized to 1 MPa with gas mixture of O2and N2.During the reaction, the mixture was stirred at a predefined speed (50 ～ 100 r/min).The heating procedure was preceded as following: room temperature(about 30°C) to target temperature, heating rate, 2.5°C/min;target temperature, 1 h; target temperature to room temperature, about 3 h.Finally, the reactor was opened to collect the reaction solution.

Fig.1 Schematic diagram of autoclave reactor

1.3 Analysis

1.3.1 Gas chromatography (GC) and gas chromatographymass spectroscopy (GC-MS)

GC and GC-MS were employed for qualitative and quantitative analysis of monophenols.Before GC and GC-MS analysis, an extraction process is needed to isolate aromatics.The slurry was acidified to pH ≈ 2.0 with HCl(36wt.%) to precipitate aromatic aldehydes and lignin fragment.[9]These precipitated products were extracted with THF until the THF layer appeared colorless.A small amount of NaHCO3solid and anhydrous Na2SO4was added into the THF phase to neutralize the residual acid from acidification and absorb the residual water in the solution, respectively.The resulted THF phase was analyzed on a GC-2014C (Shimadzu, Japan) equipped with and flame ionization detector (FID) and a HP-innowax capillary column (30 m × 0.25 mm, 0.25 µm).The temperature program was listed as following: injector temperatures, 240°C; detector temperature, 280°C; 60°C,2 min; 60 ～ 240°C, 10°C/min; 240°C, 10 min.Products and standards were identified by TRACE 1300ISQ GC/MS (Thermo Fisher Scientific, America) with the same column and condition as GC.An external standard method was used and every sample was tested for three times.

1.3.2 High Performance Liquid Chromatography (HPLC)

HPLC was used to quantitative analyze small carboxylic acids.The HPLC sample preparation method is as following.To detect volatile acids including formic acid and acetic acid, 0.5 mL acidified liquid was diluted with 1.5 mL deionized water, then filtered over a 0.45 µm Teflon filtrate pad.To analyze oxalic acid, acidified liquid was distilled to remove water, volatile small acids and HCl under reduced pressure at 35°C, and then 5 mL of acetonitrile was added to dissolve the aromatics.Then the acetonitrile phase was separated from solid crystal (mainly NaCl).The isolated acetonitrile phase was dried again to remove solvent and resolved in 5 mL H2O.A HPLC instrument (Waters e2695) equipped with a 2414 RI Detector was used for HPLC analysis of oxalic acid, using a Shodex SUGAR SH1011 column (300 mm × 8 mm, 6 μm).The eluent was 5 mmol/L H2SO4aqueous solution; flow rate was 0.5 mL/min; temperature was 30ºC; injection volume was 10 μL; retention time was 35 min.

1.3.3 Gel permeation chromatography (GPC)

A 1260 LC system (Agilent technology, America)equipped with a refractive index detector (RID) was used for GPC analysis of the THF phase.The analysis condition is listed as following: column, PL gel mixed-C(300 mm × 7.5 mm, 5 µm); eluent, THF; flow rate, 1 mL/min;injection volume, 20 µL; temperature, 35°C.

1.3.4 Calculation formulas

For the alkaline oxidation of pine wood, yields of monopheol (Ymonophenol), acetic acid (Yaceticacid) and solid residue (Ysolidresidue), and selectivity of vanillin (Svanillin)were calculated by Equations (1) ～ (4), respectively.

WhereWmonophenol,Wpine wood,Wacetic acid,Wsolid residue,Wtotalmonophenolsare weights of monophenol, pine wood, acetic acid, solid residue and total monophenols, respectively.

In the case of the alkaline oxidation of vanillin, vanillin conversion (Cvanillin), yield of oxalic acid (Yoxalicacid), and carbon balance (CB) were calculated by Equations (5) ～(7), respectively.

WhereWremainedvanillinandWinitialvanillinare weights of remained vanillin and initial vanillin, respectively;Moxalicacidis mole of oxalic acid;Minitialvanillinis carbon mole of initial vanillin;Mtotalcarbonis total carbon moles of all known products;Noxalicacidis carbon number of oxalic acid molecule.Yields of other products were calculated by a similar equation as Equation (6).

2 Results and discussion

2.1 Alkaline oxidation of pine wood to vanillin

According to GC/MS analysis, aromatics and acetic acid are main volatile products of alkaline oxidation of pine wood.Vanillin was the main monophenol and its selectivity was about 60% ～ 75%.Acetovanillone gave secondly high yield among monophenols.Other monophenols were includingp-hydroxybenzaldehyde, feruilic acid, guaiacol,benzyl alcohol and phenol.A large amount of acetic acid was formed, which can be generated by alkaline oxidation of lignin in the presence of oxygen.[28]However, acetic acid can also be derived from the degradation of carbohydrates (both cellulose and hemicellulose).[29-30]According to Fig.2 ～ 4, reaction parameters such as NaOH concentration, O2pressure and agitation speed have significant effects on yields of these products.

2.1.1 Effects of NaOH concentration

After reaction, lignin was almost completely dissolved in the alkali solution with NaOH concentration of 5wt.% ～ 15wt.%, which implying that NaOH can accelerate the dissolution of lignin in the presence of oxygen.During the reaction, a cationic adduct was formed when Na+ions combined with lignin, which played a role of polarization of ether bonds between monolignol units and consequently reduced cleavage energy of these ether bonds.[31]At the same time, OH-ions abstract proton from theα-carbon atom ofβ-O-4 bond and thus leading to breaking ofβ-O-4 bond, that resulting in depolymerization of lignin.[32]As can be seen in Fig.2, yields of all monophenols had no obvious changes and kept at low level, while were all increased at NaOH concentration of 13.8wt.%.In addition, high yield of acetic acid was obtained probably due to degradation of cellulose and hemicellulose, because only a few solid residue left in the solution after reaction.At NaOH concentration of 13.8wt.%, vanillin yield was 8.9% and yield of total monophenols was about 15%, accompanied with lowest acetic acid yield.According to these results,we can learn that NaOH play important roles of depolymerization of lignin and promotion of lignin oxidized to monphenols.However, at high agitation speed of 1 100 r/min, too much oxygen dissolved in the alkali solution and mass transfer was sped up, thus leaded to fast degradation of both cellulose and hemicellulose and causing resource waste.Therefore, it is needed to study the effect of agitation speed on monophenols yield and solid residue reservation.

Fig.2 Effect of NaOH concentration on product yield and selectivity (conditions: 1 g pine wood, 1 MPa O2, 1 100 r/min,170°C, 1 h)

2.1.2 Effects of agitation speed

As showed in Fig.3a, solid residue yield increased when agitation speed decreased from 1 100 r/min to 50 r/min at NaOH concentration of 13.8wt.%.A highest solid residue yield of 52% was obtained at 50 r/min.Meanwhile, a decrease of acetic acid yield showed when agitation speed decreased, which might due to the reduction of degradation of carbohydrate.Vanillin yield increased from 8.9wt.% to 14.7wt.% as the agitation speed decreased from 1 100 r/min to 400 r/min, then decrease to 9.7wt.% when agitation speed decreased to 50 r/min.And the changes of acetovanillone yield and total yield of monophenols were similar to that of vanillin yield.

Fig.3 Effect of agitation speed of product yield and selectivity(conditions: (a) 1 g pine wood, 1 MPa O2, 13.8wt.% NaOH,170°C, 1 h; (b) 1 g pine wood, 1 MPa O2, 7.5wt.% NaOH,170°C, 1 h)

However, low solid residue yield (29%) was obtained at 400 r/min when monophenols yield at its highest level.To obtain more solid residue, NaOH concentration was decrease from 13.8wt.% to 7.5wt.% since NaOH can also dissolve carbohydrate.As Fig.3b shown, solid residue yield of 73% and monophenols yield of 19.5wt.% was achieved at 50 r/min.The increase of agitation speed causing degradation of pine wood that resulted in reduction of solid residue yield and increase of acetic acid yield.It is notable that a much higher monophenols yield of 25.8wt.% with vanillin selectivity of 74.5% was appeared when increased agitation speed from 50 r/min to 400 r/min, and 45% solids remained in the solution.From the results we have obtained, one can conclude that agitation can control both conversion of lignin to monophenols and degradation of carbohydrate.The main reason might be agitation can control the dissolved oxygen content thereby influenced the reaction in the alkali solution.Therefore, effect of oxygen on monophenols yield will be discussed in the next section.

2.1.3 Effects of oxygen pressure

As can be seen in Fig.4, solid residue yield decrease from 75% to 45% when oxygen particle pressure increase from 0.1 to 1 MPa, a further increase of oxygen pressure leaded to harder degradation of pine wood.In the case of yields of oxidation products, they showed a similar trend when the oxygen partial pressure changed: increased from 0.1 MPa to 1 MPa, followed by decreased from 1 MPa to 2 MPa.It is obvious that oxygen plays a crucial role in the oxidation of lignin to monophenols.Oxygen dissolved in the solution participated in the formation of phenoxyl radical and quinone methide hydroperoxide, the most important intermediate, thus accelerated the generation of vanillin.[9,33]

Fig.4 Effect of oxygen partial pressure on product yield and selectivity (conditions: 1 g pine wood, 7.5wt.% NaOH, 400 r/min,170°C, 1 h)

2.2 Degradation of vanillin during alkaline oxidation

According to the results above, a pretty high monophenols yield (25.8%) with high selectivity of vanillin (74.5%)was obtained and about half of the solids (mainly carbohydrate) was left as residue, by alkaline oxidation of pine wood under suitable condition.However, monophnols yield rapidly reduced under harsh condition such as strong agitation, higher NaOH concentration and oxygen pressure.The decrease of monophenols yield probably mainly caused by over oxidation of these aromatics formed in the system.Therefore, oxidation of the major monophenol, i.e.vanillin, under the conditions in pine wood system was conducted to verify the deduction.

2.2.1 Oxidation of vanillin to vanillic acid

As showed in Fig.5, a large amount of vanillin degraded under the same condition with alkaline oxidation of pine wood.The degradation of vanillin gave vanillic acid,carboxylic acids, etc.Vanillic acid is the only detected monophenol product of vanillin, which was resulted from oxidation of vanillin.Vanillic acid can also be found in the pine wood system, implying that conversion of vanillin to vanillic acid may cause the decrease of vanillin selectivity.An increase of O2pressure from 1 to 2 MPa resulted in vanillic acid yield increase by 60%.

Fig.5 Degradation of vanillin under alkaline oxidation condition (conditions: (a) 0.2 g vanillin, 1 MPa O2, 7.5wt.%NaOH, 400 r/min, 170°C.1 h; (b) 0.2 g vanillin, 1 MPa O2,7.5wt.% NaOH, 1 100 r/min, 170°C 1 h; (c) 0.2 g vanillin, 2 MPa O2, 7.5wt.% NaOH, 400 r/min, 170°C 1 h; (d) 0.2 g vanillin, 1 MPa O2, 15wt.% NaOH, 400 r/min, 170°C 1 h)

2.2.2 Decomposition of vanillin to small carboxylic acids

Under alkaline condition in the presence of O2, vanillin would be decomposed into small molecule acids including formic acid, acetic acid, oxalic acid.Formic acid might be formed by oxidative cleavage of the methoxy group on vanillin, while the formation of acetic acid and oxalic acid might attributed to benzene ring-opening of vanillin.[34-35]46% of vanillin was converted to carboxylic acids under the same condition with pine wood alkaline oxidation (Fig.5, a).A similar total yield of carboxylic acids was achieved when O2pressure increase from 1 MPa to 2 MPa, while much lower vanillin left in the system (Fig.5, c) for vanillin suffered drastic degradation under higher oxygen pressure.Both vanillin conversion rate and yields of carboxylic acids decreased when alkali concentration increased due to high alkali concentration can protect the vanillin from degradation (Fig.5, d).[19,34,36-38]In addition,a decrease of vanillin conversion rate and yields of carboxylic acids was achieved at higher agitation speed of 1 100 r/min (Fig.5, b).The slow degradation of vanillin might due to short contact time between dissolved O2and vanillin at such high agitation speed.To understand the effect of agitation on contact between vanillin and dissolved O2, further research is needed in the future.

2.2.3 Repolymerization of vanillin

It must also be mentioned that the carbon balance of these vanillin oxidation reactions were poor, implying that vanillin degraded to other products.Vanillin would decompose into smaller molecules such as CO2and H2O,or probably repolymerized into high molecular weight products (HMWP) under alkaline condition.In order to obtained more information about alkaline oxidation of vanillin, GPC was used to analyze molecule weight distribution of HMWP (Fig.6).GPC results showed that HMWP generated in all reactions.Obviously, a mass of HMWP withMw of about 580 g/mol were obtained at high O2pressure of 2 MPa, which is consistent with the observation of unknown products.Our findings lead us to conclude that vanillin is unstable under alkaline oxidation condition and might condense to HMWP, thus leading to reduction of vanillin yield in the pine wood system.

Fig.6 Molecular weight distribution of nonvolatile products(condition: a, 0.2 g vanillin, 1 MPa O2, 7.5wt.% NaOH, 400 r/min,170°C.1 h; b, 0.2 g vanillin, 1 MPa O2, 7.5wt.% NaOH, 1 100 r/min,170°C 1 h.c, 0.2 g vanillin, 2 MPa O2, 7.5wt.% NaOH, 400 r/min,170°C 1 h.d, 0.2 g vanillin, 1 MPa O2, 15wt.% NaOH, 400 r/min,170°C 1 h)

3 Conclusion

In summary, momophenol can be efficiently extracted from pine wood, especially vanillin, and carbohydrates were left as solid residue.Under mild condition, yields vanillin, monophenols and solid residue were 19.9%, 25.8%and 45%, respectively.Agitation speed plays important roles of controlling the liquefaction of lignin and carbohydrates and oxidative conversion of lignin to monophenols.A decrease of agitation speed from 1 100 r/min to 400 r/min resulted in higher monophenols and solid residue yield.In addition, the decrease of monophenols yield was caused by over oxidation of monophenols, mainly vanillin.Vanillin would decompose to small carboxylic acids by ring-opening reaction, or condensed to high molecular weight products.