Characterization of pyrolytic lignins with different activities obtained from bio-oil☆

Furong Leng,Yurong Wang,Junhao Chen,Shurong Wang*,Jinsong Zhou,Zhongyang Luo

State Key Laboratory of Clean Energy Utilization,Zhejiang University,Hangzhou 310027,China

1.Introduction

As a renewable liquid fuel from biomass pyrolysis,bio-oil is expected to replace fossil fuels to deal with energy and environmental crisis[1].However,the crude bio-oil can only be used as a low-grade fuel due to its inferior properties including high oxygen content,low heating value,high acidity,and thermal sensitivity[2,3].Therefore,bio-oil must be upgraded before achieving its high-grade utilization.The common upgrading technologies include catalytic hydrogenation,catalytic cracking,catalytic reforming,catalytic esterification,and supercritical extraction[4,5].As the composition of bio-oilis complicated,single upgrading technology cannot realize the efficient conversion of overall components in bio-oil.In addition,it may reduce economic efficiency when bio-oil is directly upgraded since it has some valuable compounds.The pre-separation of bio-oil can provide appropriate fractions for subsequent extraction of valuable chemicals and high-efficiency upgrading.Researchers have found that acids and ketones can be converted to polyalcohols,hydrocarbons and alkene under mild conditions with high catalyst stability,whereas sugars and phenols are easy to cause catalyst deactivation due to carbon deposition[6,7].Molecular distillation of bio-oil can enrich different chemical families with distinct properties into various bio-oil fractions,which obtains the distilled fraction with high reactivity and the heavy fraction rich in sugars and phenolic compounds[8,9].The distilled fraction can be used to produce hydrogen or bio-gasoline with appropriate upgrading technology,while the heavy fraction is used to produce emulsion fuel or to extract valuable chemicals[10–13].In addition,the separation of large-molecular compounds from bio-oil can be realized based on their distinct properties.Sugars can be dissolved and separated directly using water owing to their strong hydrophilicity[12,14].Pyrolytic lignin(PL),the fragment produced from lignin pyrolysis,is mainly composed of waterinsoluble phenolic polymers,with a content up to 30 wt%based on crude bio-oil[15].It can be converted to monophenols through depolymerization for further upgrading,and can also be used in the synthesis of chemicals like phenolic resin,adhesive,and carbon fiber[16,17].It's worth noting that condensation and polymerization reactions between PL and carbohydrates and aldehydes in bio-oil are easy to occur,leading to the aging of bio-oil and the increasing of viscosity[18].In consequence,the isolation and structure characterization of PL are important to study the aging of bio-oil and the efficient upgrading of bio-oil aqueous phase.

The common water extraction method always introduces a lot of water to obtain PL with high purity in order to remove the water soluble compounds from bio-oil[15].The residual aqueous phase can be used to further extract water-soluble phenols,sugars,etc.[19]The side-chain functional groups and the interunit linkages in PL extracted by water are kept relatively complete,but adding too much water causes high energy consumption in the subsequent separation and upgrading process of aqueous phase.If the dosage of water is reduced,other separation methods need to be introduced for the further purification of PL.Wanget al.[20]combined acid and alkali solutions with solvent extraction to sequentially extract monophenols and PLs from the water-insoluble phase of lauan bio-oil,and found that some water soluble sugars still remained in the water-insoluble phase.In order to solve this problem,this paper introduced methanol–water extraction method to further separate the water-insoluble phase of bio-oil and extract PLs with different activities.Several characterization technologies were used to analyze the physical properties,structural units,sidechain functional groups,and interunit linkages of PLs.The final char yields of PLs pyrolysis were also studied by thermogravimetric analysis.

2.Materials and Methods

2.1.Separation of PLs from bio-oil

The bio-oil was prepared in a fluidized bed reactor with a feeding rate of 5 kg·h−1designed by Zhejiang university.The reaction temperature was 500–550 °C,and the residence time was about 1 s.The feedstock was lauan sawdust of 0.25-0.42 mm(40–60 mesh),which belongs to hardwood biomass.The detailed operation procedures refer to the literature[21].Appropriate bio-oil after filtration pretreatment was added dropwise into the same mass of water under ultrasonication.Afterwards,the solution was centrifuged for 15 min at the speed of 8000 r·min−1.The top layer was then decanted.The residual waterinsoluble phase was mixed with methanolin a massratio of1:2,producing a clear solution with the aid of ultrasonication.Water was added into the solution in a mass ratio of 3:1.After then,this mixture was centrifuged at a high speed of 8000 r·min−1again to obtain the precipitate,which was named as high-molecular-weight pyrolytic lignin(HPL).The residual solution was evaporated with vacuum distillation at a low temperature of 25°C to remove the methanol.The suspension was then centrifuged again at the same speed of 8000 r·min−1to obtain the low-molecular-weight pyrolytic lignin(LPL).To remove the water in LPL,it was dissolved in tetrahydrofuran(THF) first,and then evaporated under vacuum and 25°C.

2.2.Analysis of bio-oil and its fractions

The chemical components in crude bio-oil and its derived fractions were detected through Trace DSQ II gas chromatography-mass spectrometer(GC–MS,Thermo Fisher Scientific Company)equipped with a DB-WAX column(Agilent Company).In the process of analysis,the column temperature was kept at 40°C for 1 min firstly,and then it was heated up to 240 °C at a heating rate of 8 °C·min−1and kept at this temperature for 10 min.

The contents of C,H and N in PLs were analyzed through a Vario MICRO Elemental analyzer(Elementar Analysensysteme GmbH,Germany),whereas the content of O was estimated by difference.Heating values of PLs were estimated according to the formula:3.55[C]2−232[C]−2230[H]+51.2[C]×[H]+131[N]+20600[22].The distribution of molecular weight for PLs was detected through PL GPC 50 plus gel permeation chromatography equipped with PL gel chromatographic columns(103,105,500 Å,300 mm × 7.5 mm).Each sample was dissolved in THF(～5 mg·ml−1),then centrifuged for 15 min at the speed of 8000 r·min−1.The top layer was filtered using an organic membrane of 0.45 μm.The column temperature was 50 °C,and the mobile phase was THF with a flow rate of 1 ml·min−1.

Nicolet 5700 Fourier transform infrared spectroscopy(FTIR)was used to detect the functional groups of PLs.Each PL sample was dissolved in THF and loaded on the solid KBr slide,and then dried under an infrared lamp.Each spectrum of the sample was detected at the range of 4000–400 cm−1.The 2D13C–1H heteronuclear single quantum coherence correlation nuclear magnetic resonance(HSQC-NMR)spectroscopy of PLs was carried out on an Agilent 600 MHz DD2 nuclear magnetic resonance apparatus.Appropriate amount of each sample was dissolved in 0.5 ml DMSO-d6with tetramethylsilane as internal standard for chemical shift.The temperature was 25°C.The relaxation delay of1H dimension was 1.5 s,while that of13C dimension was 2 s.The total running time lasted 10 h.The solvent signal(δC/δH=39.5/2.49)was used as a reference point,and the1JC–Hvalue was 146 Hz.

STA 449 F5 Jupiter thermogravimetric analyzer(TG)was used to analyze the charyield of PL pyrolysis.About5 mg of each sample was heated from ambient temperature to 800°C with a heating rate of 20 °C·min−1.The nitrogen purging flow was 40 ml·min−1.

3.Results and Discussion

3.1.Separation of PLs

The water content of original bio-oil was 37.67 wt%,and 46.69 wt%of water-soluble organic compounds could be obtained from bio-oil through water extraction.Based on the results from GC–MS,crude bio-oil mainly included phenols(37.24%),ketones(26.18%),and acids(18.52%),while the relative content of sugars was only 0.84%.The aqueous phase from water extraction mainly consisted of small molecular compounds with strong water hydrophilicity,such as phenols(26.15%),acids(24.50%),ketones(20.3%),and furans(11.52%).The relative content of acetic acid reached 18.02%,which was the highest in the aqueous phase,whereas that of sugars increased slightly to 1.60%.These results illustrate that simple water extraction could separate most small molecular compounds with high water solubility from bio-oil.The water-insoluble phase was a viscous and semi-solid mixture.It was dissolved and dispersed in the methanol to break the intermolecular interactions.After then,the addition of water and evaporation of methanol were carried out in sequence to stepwise precipitate PLs.During this process the residual water solution after the removal of LPL still contained 6.03 wt%of organic compounds based on crude bio-oil,which were mainly monophenols(60.57%)and sugars(10.59%).This might be ascribed to the fact that complex chemical interactions existed in bio-oil[23].It was difficult to completely break the intermolecular interactions between PLs and other chemical compounds just using a little water.Therefore,some small molecular compounds were also be separated out from water together with PLs.Using methanol to disperse water-insoluble phase followed by water addition could further remove the residual water-soluble compounds in the water-insoluble phase.

The yield and physical properties of PLs are presented in Table 1.9.61 wt%of PLs was separated from bio-oil.HPL was prior to precipitate from the solution in the presence of methanol due to its complex structure and high degree of polymerization.Subsequently,the methanol was distilled out,and 1.23 wt%of LPL precipitated out from the aqueous phase.The carbon content of HPL was higher than that of LPL,whereas the oxygen content was 32.52 wt%in LPL which was higher than that in HPL.Consequently,the heating value of HPL was slightly higher than that of LPL.

Table 1Yield and ultimate analysis of PLs

Table 2Approximate molecular weight distribution of PLs(%)

3.2.Molecular weight distribution

The determination of molecular weight provides comprehensive information of PLs about the degree of polymerization and dispersion.According to the literatures[24,25],the classification of molecular weight ranges is shown in Table 2.The weight-average(Mw)and number-average(Mn)molecular weight of HPL were 833 and 513,respectively,while those ofLPL were relatively lower,511 and 348,respectively.The degree of polymerization shows that HPL included plenty of dimers to pentamers,whose contents were all in the range of 12%–18%,whereas LPL mainly consisted of trimers,with a content of 75.38%.It is worth noting that the contents of polymers with molecular weight higher than 1526 had significant difference in two PLs.It was up to 10.75%in HPL,but just 0.71%in LPL.Moreover,the contents of hexamers to decamers in HPL were apparently higher than those in LPL.These results were on account of that macromolecular polymers first precipitated from the solution due to their hydrophobicity and complex structures.The high dispersity of HPL indicates it had wide molecular weight distribution,which might be because a few small molecular compounds were separated out during the separation process of HPL,and the degree of polymerization for high polymers was also scattered.

Fig.1.Relative intensities of typical functional groups for PLs.

3.3.Structure analysis of PLs

3.3.1.FTIR analysis of PLs

The absorbance intensity of aromatic skeletal vibration at 1515 cm−1(Iar)in FTIR spectra of PLs was set as a standard reference value,and the relative absorbance intensity(Iw/Iar)of each functional group was calculated based on the ratio of its vibration intensity(Iw)relative to the standard,as shown in Fig.1.The main characteristic peaks of HPL and LPL exhibit good consistency.The peak at 3374 cm−1corresponds to O–H stretching vibrations in hydroxyl groups[26],and LPL had more hydroxyl groups due to the high relative intensity of corresponding signals.The peaks at 1714 cm−1,1672 cm−1and 1608 cm−1are attributed to the conjugated and unconjugated C=O stretching vibration,which are mainly generated by the linkage cleavage in the lignin pyrolysis process[27].The higher relative intensities of these signals for LPL illustrate that more carbonyl groups existed in LPL compared to HPL.The bands at 1331 cm−1and 1116 cm−1indicate the presence of the syringyl units,while those at 1274 cm−1and 1155 cm−1represent the presence of guaiacyl units in the two PLs.In addition,the relative absorbance intensities of these two basic structures in LPL were higher than those in HPL,suggesting that LPL included more syringyl and guaiacyl units.Hardwood lignin has complex structures,of which the basic structural units mainly include syringyl and guaiacyl units[28].The FTIR analysis of PLs shows that a large number of syringyl and guaiacyl units were retained in the process of lignin pyrolysis.Carbonyl and hydroxyl groups in PLs provided more active sites which could facilitate their subsequent utilization.

Fig.2.HSQC-NMR spectrum of HPL.

3.3.2.NMR analysis of PLs

2D NMR can provide more accurate structure information through the synergistic effect of13C and1H signals,and solves the problem of overlapping signals in 1D NMR.For HPL and LPL,the 2D HSQC-NMR spectra(δC/δH135–50/8.0–2.6)of the main side-chain and aromatic regions are presented in Figs.2 and 3.The main identified structures in PLs are presented in Fig.4.The HSQC spectra show that the side chains and the main structural units of HPL and LPL had similarity.The hemiquantitative analysis of some interunit linkages and functional groups is mainly calculated according to13C NMR.The integral of the region(102–162)was set as the internal reference,assuming that this region included six aromatic carbons plus 0.12 ethylene carbon,namely a complete aromatic ring containing 6.12 carbons[29].The relative contents of functional groups and interunit linkages of PLs,which were calculated based on the reference[30,31],are presented in Table 3.

Fig.3.HSQC-NMR spectrum of LPL.

Fig.4.Main determined substructures of PLs.

Table 3Estimation of functional groups and interunit linkages for PLs based on 13C NMR spectra

In the side-chain regions of PLs,the cross signals of methoxy groups(δC/δH55.6/3.73)could be obviously found in the HSQC spectra,and their contents in HPL and LPL reached 1.30/Ar and 1.38/Ar respectively.This phenomenon suggests that there were a large amount of methoxy groups in HPL and LPL,since PLs contained plenty of syringyl and guaiacyl units.In addition,there were more syringyl units in LPL.The signals at 90–77 in13C NMR correspond to Alkyl-O-Aryl and α-O-Aryl,with low contents for both PLs.However,the relative content of γ-OAlkyl and OHsecin the range of 77–65 for LPL was up to 0.56/Ar(solvent peak of THF were removed),which was far higher than that of HPL(0.01/Ar),indicating that the branches of LPL were more complex.The alkyl ether linkages mainly existed in LPL,which suggests that most of these linkages were derived from low molecular compounds.Although the ether and ester interunit linkages could be easily decomposed during lignin pyrolysis[28],some of them might be retained due to their high activation energies.Thus some ether linkages could be observed in PLs,such as β-β′resinol structures(B)which gave several signals at δC/δH84.8/4.65(Cα–Hα),53.5/3.06(Cβ–Hβ),as well as 71.0/3.82 and 4.18(Cγ–Hγ).The contents of resinol moieties in PLs were both 0.01/Ar.Some researchers[32]reported that the resonance signals of γ-ethers and sugars might be overlapped in the region of δC/δH65–62/4.0–3.5,which was also be observed in this paper.CH2-γ in γ-esters of lignin–carbohydrate complexes gave signals in the region of δC/δH65–62/4.5–4.0,but these signals always overlapped with the γ-esters in lignin.The results from HSQC-NMR show that the signals of esters in LPL were stronger than those in HPL.These results imply that the signals at δC/δH61.7/4.14 might be assigned to ester groups attached to a benzene ring,and these esters had low molecular weights.The amount of primary hydroxyl groups in LPL(0.13/Ar)was obviously higher than that in HPL(0.01/Ar),indicating there were more primary alcohols or sugars in the branches of LPL.The chemical shifts in the HSQC-NMR spectrum of LPL at δC/δH102.4/5.16,71.9/3.17,73.8/3.38,72.1/3.30,76.5/4.35,and 64.9/3.50 and 3.87 are attributed to C1,C2,C3,C4,C5and C6of levoglucosan(LG)respectively[33],proving the existence of LG in LPL,while there were almost no sugars in HPL.Most pyrolytic sugars could be removed in the separation of bio-oil with water extraction.However,due to the intermolecular interactions between sugars and PLs,a small amount of sugars still retained in the water-insoluble phase extracted by using a little water[20].In the subsequent methanol–water extraction process,high-molecular-weight polymers were prior to precipitate from the water,but sugars were still found in the residual water solution after the removal of methanol.As a result,LPL precipitated with a small amount of pyrolytic sugars.

In the aromatic regions,the basic structures of guaiacyl(G type)and syringyl(S type)units for PLs could be obviously found in their HSQCNMR spectra,which were consistent with the aforementioned results of FTIR analysis.C2,6–H2,6correlation signals in the syringyl units display at δC/δH103.8/6.71,meanwhile the signals at106.2/7.23 and 7.07 correspond to Cα-oxidized syringyl units(S′).Three different signals,C2–H2(δC/δH110.9/6.98),C5–H5(δC/δH114.9/6.77 and 6.94),and C6–H6(δC/δH119.0/6.80),are all assigned to guaiacyl units.The double cross signals of C5–H5in the guaiacyl units for LPL and HPL indicate the heterogeneity of guaiacyl units,which might be ascribed to the effect from the different branches of C4[34].Since the chemical shift of C2in the guaiacyl units is hardly affected by the substituents on other aromatic carbons[35,36],the content of C2can represent the amount of guaiacyl units,which could be calculated according to the signals at 113–108 in13C NMR.The number of C2,6in the syringyl units was measured based on the range of108–102 in13C NMR.Since the C2,6signals in the syringyl units are symmetric vibration,S/G=0.5S2,6/G2.The S/G values for HPL and LPL were 0.89 and 1.62,respectively,indicating that LPL contained more syringyl units.This is also in agreement with the amounts of methoxy groups.In addition,the signal at δC/δH126.1/6.76 arises from Cβ–Hβof cinnamaldehyde end groups.The signals at 196–191 in13C dimension arise from the side-chain carbonyl groups,which further indicates the existence of benzaldehyde and cinnamaldehyde structure in HPL(0.03/Ar)and LPL(0.09/Ar).The content of unconjugated carbonyl groups(210–200)in HPL was negligible,while that was 0.05/Arin LPL at the same range.The amounts of aliphatic carboxyls(175–168)and conjugated carboxyls(168–166)in LPL reached 0.09/Ar and 0.01/Ar respectively,while those in HPL were too small to be detected.The total content of carbonyls in LPL was 0.24/Ar,which was much higher than that in HPL(0.03/Ar).This was also con firmed by FTIR.Consequently,LPL was more suitable for the synthesis of phenolic resin with substituting for phenol due to the high reactivity of carbonyl groups.

For a general HSQC spectrum of lignin,the structure of lignin was complex,and more β-O-4,α-O-4,and aliphatic hydroxyl groups could be detected[37].However,the carbonyls,double bonds and phenolic hydroxyl groups of side chains in PLs indicate that the breakage of plenty of ether linkages and the dehydroxylation occurred during lignin pyrolysis.In general,the basic structural units and the interunit linkages of two PLs were similar.Nevertheless,compared to HPL,the side chains of LPL contained more active functional groups,more complicated interunit linkages,and a small amount of pyrolytic sugars.

3.4.TG analysis of PLs

Fig.5 reveals the pyrolysis behaviors of HPL and LPL under the heating rate of 20 °C·min−1.As can be seen,the pyrolysis behaviors of PLs had similarity.The mass loss could be mainly divided into three stages,which was similar to other reports[24,38].The mass changes of PLs were small in the range of 60–120 °C,which was ascribed to the release of free water and some volatile components.Most of the small molecular compounds were dissolved in the water during the water extraction process of PLs,therefore the average molecular weights of PLs were high,illustrating the first stage was mainly attributed to water evaporation.In the major stage of 120–500 °C,the mass losses of HPL and LPL increased significantly due to the release of a large quantity of volatile components,showing double peaks in the DTG curves.The pyrolysis of LPL exhibited a higher mass loss than HPL before 311 °C,and its maximum mass loss rate appeared at 215 °C.As mentioned in the analysis of molecular weight distribution,LPL included more small molecular compounds and oligomers,leading to its larger mass loss at relatively lower temperature.And some monomer compounds might directly escaped from LPL.HPL had broad molecular weight distribution,and also included some small molecular compounds.The first mass loss peak of HPL appeared at 194°C,but it achieved its maximum mass loss rate at 349.5°C,con firming that HPL contained more structures with high activation energies.The multiple mass loss peaks indicate the pyrolysis behaviors of PLs were still very complex.The residues of PLs were slowly decomposed over 500°C,and the final char residue yield of LPL was 31.14 wt%,lower than that of HPL(37.03 wt%).

Fig.5.TG and DTG curves of PLs.

4.Conclusions

In this work,methanol–water extraction method was adopted to separate the water-insoluble phase obtained by water extraction of crude bio-oil.A total yield of 9.61 wt%of PLs with different activities was acquired.The characterization of PLs shows that LPL had lower dispersity and was mainly composed of trimers(75.38%),while the contents of dimers to pentamers for HPL were all in the range of 12%–18%.HPL and LPL had similar elemental composition,and both contained guaiacyl and syringyl units.The S/G values for HPL and LPL reached 0.89 and 1.62,respectively.The side-chain structures in PLs both included hydroxyls,benzaldehyde,and cinnamaldehyde,and the main interunit linkages were β-β′resinol moieties,alkyl and aryl ether linkages.HPL had relatively simple structures,while LPL involved more abundant syringyl units,carbonyl groups(0.24/Ar)and hydroxyl groups,indicating that LPL had higher activity.Additionally,LPL was more sensitive to the temperature,and it achieved severer mass loss at relatively low temperature.The final char residue yield of LPL was lower than that of HPL.

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