貴金屬催化劑上兩步法選擇性降解有機(jī)溶劑型木質(zhì)素

劉凌濤 張 斌 李 晶 馬 丁 寇 元

(北京大學(xué)化學(xué)與分子工程學(xué)院,北京大學(xué)綠色化學(xué)研究中心,北京分子科學(xué)國(guó)家實(shí)驗(yàn)室,北京100871)

1 Introduction

Biomass is considered to be the only sustainable organic carbon footprint in the nature.1So far,a great deal of effort has been addressed on converting cellulose,the most abundant component in biomass,to liquid fuels2-10or platform chemicals.11-20Cellulose can be hydrogenolyzed to polyols on Ru/C in subcritical water,12or glycol using supported WC catalyst.13It could also be converted in ionic liquids to hydroxymethylfurfural.15,17-18Lignin is one of the three main components of biomass,which makes up 15%-30%dry weight of lignocellulose. However,direct hydrogenating isolated lignin to alternative fuels has few reports.21-25Previously,we had developed a twostep method for the conversion of lignin to liquid fuels using wood sawdust as raw material,26including(1)selective degradation of lignin into fragment molecules and(2)hydrodeoxygenation of these fragment molecules(mainly monomer and dimers)into alkanes and methanol.The reaction pathway for the second step was studied by Lercher et al.27-29and Dyson et al.30On the other hand,the detailed chemical transformation of lignin in the first step remains unclear.This is largely because in our original system wood sawdust was used as the starting material.As such,the cellulose and hemicellulose,which account for about 80%in wood,were also converted upon hydrotreating and therefore impeding a clear understanding of the chemical transformation of lignin in the process.In this paper,we used isolated lignin as the starting material to convert it to liquid fuels via the two-step process developed in this group and studied the degradation reactions of lignin in the first step. Combined Fourier transform infrared(FTIR)spectroscopy, X-ray photoelectron spectroscopy(XPS),element analysis (EA),and gel permeation chromatography(GPC)analysis provide a clearer overall picture of the fate of lignin upon hydrotreating over noble metal catalyst.

2 Experimental

2.1 Chemicals

Dioxane,toluene,ethanol,chloroform,diethyl ether,n-dodecane,sodium bicarbonate(NaHCO3),anhydrous sodium sulphate(Na2SO4),and barium bitrate(Ba(NO3)2)were all of analytical reagent grade and were used as received.Pd/C and Rh/ C were purchased from Shaanxi Kaida Chemical Engineering Co.,Ltd.

2.2 Dioxane lignin preparation

Dioxane lignin was prepared according to a procedure described in the literature(Fig.S1(see Supporting Information)).31Birch(Betula platyphylla Suk)sawdust(10 g)was extracted with a toluene and ethanol mixture(volume ratio 2:1) for 24 h in a Soxhlet extractor.After drying in the air,the sawdust was placed in a flask with a dioxane and deionized water mixture(volume ratio 9:1)containing HCl(0.7%(w)).After extracting at 90-95°C for 6 h,the extractor was removed and the solid residue was washed with fresh dioxane.NaHCO3was added to neutralize the combined solution.The solution was concentrated under reduced pressure to get a viscous black liquid,into which dioxane(10 mL)was added.This mixture was added drop by drop to a Na2SO4aqueous solution(1000 mL, 1%(w)).The solution was slowly stirred for 1 h,and then filtrated.The solid was washed with water until no SO2-4could be detected by Ba(NO3)2,and then dried under vacuum at 60°C overnight to obtain the dioxane lignin(～1.0 g).

2.3 Reaction procedure

A two-step method was used.All the reactions were carried out in an autoclave(60 mL).For step 1,dioxane lignin(0.5 g), Rh/C(0.1 g,5%(w)),phosphoric acid,and solvent(40 mL,dioxane and water(volume ratio 1:1),were mixed in the autoclave.The autoclave was pressurized with H2(4 MPa)at room temperature and then heated to set temperature.After reacting for 10 h,the autoclave was put into cold water to quench the reaction and then the reaction mixture was filtered and the filtrate was analyzed.

For step 2,the filtrate after reaction in step 1 was extracted with chloroform and then vaporized to remove the solvent.The residue was mixed with Pd/C(0.05 g,10%(w))and phosphoric acid aqueous solution(40 mL)in autoclave.The autoclave was pressurized with 4 MPa H2,heated to 250°C and reacted for 4 h.After reaction,the reaction mixture was extracted with diethyl ether and analyzed by gas chromatography(GC)and gas chromatography-mass spectrometry(GC-MS).

2.4 Product analysis

Dioxane lignin was characterized by FTIR(Vector 22,Bruker,United States)and Fourier transform ion cyclotron resonance mass spectrometry(FTICR MS,Bruker,United States) (Figs.S2,S3(see Supporting Information)).The monomer and dimer products from step 1 and their hydrodeoxygeantion products from step 2 were analyzed by an Agilent gas chromatography equipped with a HP-5 column and a flame ionization detector using n-dodecane as the internal standard.Molecular weight analysis was carried out over an Agilent gel permeation chromatography using chromatographic pure tetrahydrofuran as mobile phase and polystyrene as the standard.The XPS characterization was carried out on a Kratos Analytical Ltd.(Japan) instrument using an Al Kα(1486.7 eV)X-ray source,with the pressure of the measuring chamber set at 6.7×10-7Pa.C 1s binding energy was set to 284.8 eV as a reference.

3 Results and discussion

The separation method on lignin usually modifies its structure.25Lignin prepared by organic solvent extraction generally exhibits much smaller structural change compared to lignin obtained by chemical treating methods such as kraft lignin and hydrolyzed lignin and therefore is selected in the present study. Among various extracting solvents,dioxane was commonly used because of its good solubility towards lignin.Thus we prepared dioxane lignin as the starting material.

As dioxane lignin has a very low solubility in water which might prohibit its conversion,we used a mixture of dioxane and water as the reaction medium.Table 1 showed the results of hydrogenolysis of dioxane lignin using Rh/C and H3PO4at different conditions.Rh/C exhibited exceptional activity in CO bond cleavage when wood sawdust was used as the substrate26and therefore was selected as the metal catalyst in this study.After reacting at 200°C for 10 h,the yields of monomers and dimer were quite low,only 5.6%and 1.3%,respectively.Adding H3PO4(1%(w))increased the total yield of monomers and dimer to 10.9%.By increasing the H3PO4concentration to 3%(w),the total yield kept almost unchanged. Then we studied the temperature effect on the hydrogenolysis of dioxane lignin.The general trend is that the total yield of monomers and dimer increases with increasing temperature. For example,when the reaction temperature was increased to 220°C,the total yield of monomers and dimer increased slightly to 11.7%,whereas at 270°C,the highest yield(16.9%)was obtained.

The existence of these structurally well-defined monomers and dimer strongly suggests that the chemical process in step 1 involves C－O bond cleavages,and the fact that both Rh and an acid are required to obtain the maximized yield indicates that monomer and dimer were produced by a combined hydrogenolysis(catalyzed by Rh)and hydrolysis(catalyzed by H3PO4)of the C－O bond.Nevertheless,it has to be noted that these products only account for less than 20%of all the monomer units in lignin.A majority of lignin repeating units were converted to non-volatile compounds that were invisible by GC-MS analysis.To shed light on the structural changes of all lignin aromatic units during the reaction,we examined the crude products extracted by CHCl3from entries 2,5,and 6 in step 1 with combined FTIR,XPS,EA,and GPC analyses.

The FTIR spectra of lignin and the crude products at different temperatures were compiled in Fig.1.The intensities of the peaks at 1710 and 1032 cm-1,which were assigned to nonconjugated C＝O vibration32and the bending vibration of C－O in ether bond33respectively,decreased considerably with increasing the reaction temperature.The intensity of the vibration of C－O bond34in the guaiacyl structural unit at 1272 cm-1also decreased after reaction which was consistent with the GC results in Table 1.The intensities of the peaks from 1607 to 1423cm-1which were assigned to the vibration of the aromatic ring skeleton35kept almost unchanged,indicating that the aromatic structure was reserved during the reaction.The intensities of the peaks near 2931 cm-1appeared to increase after reaction indicating the increasing of C－H bond in the products.36From the FTIR result it is suggested that the major chemical transformations in step 1 include:(1)hydrogenation of the nonconjugated C＝O bond and(2)cleavage of the C－O bond,and consequently an increase in the C－H bonds.Meanwhile,the cleavage of C－O bond which was incomplete as the peak at 1272 cm-1did not disappear even after treating at 270°C.

Table 1 Conversion of dioxane lignin using H2with Rh/C and H3PO4(step 1)

Fig.1 FTIR spectra of lignin and its products from step 1 at different temperatures

FTIR analysis provided insightful information regarding the chemical reactions that occurred to lignin.To obtain quantitative data concerning the composition in lignin,e.g.,how many oxygen atoms were removed,XPS measurement and element analysis were applied.The XPS spectrum revealed the change of different types of carbon-oxygen bonds,and the relative ratio of C－H and C－O bonds.Fig.2 showed the XPS spectrum of dioxane lignin.The spectrum could be fitted into four peaks. The peak at 284.8 eV was the signal of 1s electron of carbons which bonded with C or H.The peaks at 286.4 and 287.7 eV were assigned to C which connected with O through single bond and double bond,respectively.The peak at 291.8 eV corresponded to the C in the carboxyl unit.33,37From Fig.3 it could be found that after reacting at different temperatures,the XPS spectra of the products changed dramatically compared to that of the dioxane lignin.These spectra,together with integration results of different types of C were provided in Fig.3.The carbon bonded with C or H increased from 35.5%in the starting material to 86.0%in the product after reacting at 270°C.The carbon bonded with O through single bond decreased from 54.3%to 14.0%,suggesting that the hydrotreating broke ca 75%total C－O linkages in lignin,corroborating with FTIR analysis that a majority of C－O bonds were broken.The percentage of the signals of the carbon bonded with O through double bond and in the carboxyl unit remained largely unchanged when the reaction temperature was below 250°C. However,a complete disappearance of these signals was observed for the product obtained after reaction at 270°C,highlighting that these C＝O and O－C＝O bonds were removed by energy intensive transformations that high temperature is necessary,probably by decarbonylation and decarboxylations.

Fig.2 XPS spectrum of dioxane lignin

Fig.3 XPS spectra of fresh lignin and lignin after degradation at different temperatures(step 1)

These products were further analyzed by element analysis with results provided in Table 2.The carbon,hydrogen,and oxygen contents in dioxane lignin were 58.96%,5.75%,and 34.56%,respectively,accounting for over 99%(w)of the lignin indicating that there were few other atoms,such as S and N,existing in the starting material.From the composition empirical formulas of C9H10.5O4.0for the starting material,C9H10.8O3.2, C9H11.1O2.5,and C9H11.3O2.0for the extracted products from reaction at 200,250,and 270°C,respectively,could be calculated. A clear trend was that the C and H contents in the product kept increasing at the compensation of a continuous decrease in the O content as the reaction temperature increased.Meanwhile, the relative ratio between H and C only slightly increased from 1.17 to 1.25,ruling out the possibility of significant hydrogenation towards unsaturated functionalities such as aromatic rings in lignin.The O/C molar ratio decreased from 0.44 to 0.22 after reacting at 270°C,meaning that half of the O was removed from the lignin.Hydrolysis of the C－O bond would not result in a notable decrease in O content,as the O atoms remained in the product.Therefore,the decreasing of O content was plausibly due to the following reactions:(1)dehydration of hydroxyl group,catalyzed by H3PO4;(2)hydrogenolysis of methoxyl group in the lignin;and(3)removing of the carboxyl group at high temperature.The contribution of each reaction to the removal of oxygen in lignin remains unknown and is a subject of future research.

Fig.4 Molecular weight analyses of lignin and products from step 1 by GPC

Fig.5 Proposed reaction scheme for the degradation of dioxane lignin1:schematic representation of lignin;2,3:schematic representation of lignin fragments;4:monomers and dimer of lignin

Table 3 Yields and selectivities in step 2 of hydrogenation of dioxane lignin using Pd/C and H3PO4as catalysts

We also analyzed the molecular weight(MW)of dioxane lignin and its products after the hydrotreating.As the dioxane lignin has a relatively small molecular weight(about 1300 Da, see Fig.S3(Supporting Information)),and that of the products is even smaller,it was not possible to determine the exact MW of the products by GPC.Nevertheless,we could use the retention time to qualitatively analyze the change of the MW.The result was shown in Fig.4.The dioxane lignin?s retention time was 27.2 min,and the monomer?s was 32.4 min.After the first step reaction,there were two main peaks on the spectra with retention time of 29.8 and 32.4 min.The extension of the retention time on the GPC indicated that the molecular weight of the products was smaller than the reagent.The peak at 32.4 min indicated that there was small molecule which might be monomer or dimer in the products.The results from GPC showed that the dioxane lignin was completely transformed, partly degraded to lower molecular weight products and others to monomer and dimer.

Based on these analyses above,we proposed a reaction scheme for the first step of selective degradation of lignin (Fig.5).Lignin was degraded to low molecular weight lignin derived polymer and some monomer and dimer.The C－O－C bond in the lignin was much more favored to be broken through hydrolysis or hydrogenolysis than the C－C bond concluded from the model compound reaction(Table S1(see Supporting Information)).The cleavage of C－O bond,which might be due to the dehydration of hydroxyl group,the hydrogenolysis of methoxyl group etc.,resulted in the decreasing of O content in the products.

After the step 1 reaction,the solution was extracted and the products were placed in the autoclave for the step 2 reacting, under the condition that was optimized in our previous studies.27From Table 3 we could notice that the monomer and dimer were hydrodeoxygenated to alkanes using Pd/C and H3PO4as catalysts at 250°C for 4 h.The highest yield of alkanes was 17.4%in entry 6.Besides,the selectivity of alkanes was all over 100%.We proposed that in the second step,the dioxane lignin with low molecular weight was further partly degraded to its monomer and dimer,and these products were also hydrodeoxygenated to alkanes.

4 Conclusions

Dioxane lignin was degraded by a two-step method using supported noble metal catalysts and phosphoric acid.The monomer and dimer could be hydrodeoxygenated to alkanes which has carbon number in the range of gasoline and diesel. The characterization of the products in the first step showed that the lignin was degraded to low molecular weight lignin polymers and lignin monomer and dimer.The O content decreased dramatically.Metal catalyst played a key role in the C－O bond cleavage whereas the phosphoric acid probably promoted dehydration reaction that further decreased the oxygen content.These results provided us a clearer scheme of the lignin?s selective degradation to liquid fuels.

Supporting Information: available free of charge via the internet at http://www.whxb.pku.edu.cn.

(1) Huber,G.W.;Iborra,S.;Corma,A.Chem.Rev.2006,106,4044. doi:10.1021/cr068360d

(2) Huber,G.W.;Chheda,J.N.;Barrett,C.J.;Dumesic,J.A. Science 2005,308,1446.

(3) Rinaldi,R.;Palkovits,R.;Schueth,F.Angew.Chem.Int.Edit. 2008,47,8047.doi:10.1002/anie.200802879

(4) Deng,L.;Li,J.;Lai,D.M.;Fu,Y.;Guo,Q.X.Angew.Chem. Int.Edit.2009,48,6529.doi:10.1002/anie.200902281

(5) Bond,J.Q.;Alonso,D.M.;Wang,D.;West,R.M.;Dumesic,J. A.Science 2010,327,1110.doi:10.1126/science.1184362

(6) Bozell,J.J.Science 2010,329,522.doi:10.1126/ science.1191662

(7) Geilen,F.M.A.;Engendahl,B.;Harwardt,A.;Marquardt,W.; Klankermayer,J.;Leitner,W.Angew.Chem.Int.Edit.2010,49, 5510.doi:10.1002/anie.201002060

(8) Corma,A.;de la Torre,O.;Renz,M.;Villandier,N.Angew. Chem.Int.Edit.2011,50,2375.

(9) Du,X.L.;He,L.;Zhao,S.;Liu,Y.M.;Cao,Y.;He,H.Y.;Fan, K.N.Angew.Chem.Int.Edit.2011,50,7815.doi:10.1002/ anie.201100102

(10) Rackemann,D.W.;Doherty,W.O.S.Biofuel.Bioprod.Bior. 2011,5,198.doi:10.1002/bbb.267

(11) Fukuoka,A.;Dhepe,P.L.Angew.Chem.Int.Edit.2006,45, 5161.doi:10.1002/anie.200601921

(12) Luo,C.;Wang,S.;Liu,H.Angew.Chem.Int.Edit.2007,46, 7636.doi:10.1002/anie.200702661

(13)Ji,N.;Zhang,T.;Zheng,M.;Wang,A.;Wang,H.;Wang,X.; Chen,J.G.Angew.Chem.Int.Edit.2008,47,8510.doi: 10.1002/anie.200803233

(14) Zhao,H.;Holladay,J.E.;Brown,H.;Zhang,Z.C.Science 2007,316,1597.doi:10.1126/science.1141199

(15) Binder,J.B.;Raines,R.T.J.Am.Chem.Soc.2009,131,1979. doi:10.1021/ja808537j

(16) Zhang,Z.;Zhao,Z.K.Bioresource Technol.2010,101,1111. doi:10.1016/j.biortech.2009.09.010

(17) Mascal,M.;Nikitin,E.B.Angew.Chem.Int.Edit.2008,47, 7924.doi:10.1002/anie.200801594

(18) Hu,S.;Zhang,Z.;Song,J.;Zhou,Y.;Han,B.Green Chem. 2009,11,1746.doi:10.1039/b914601f

(19)Yan,N.;Zhao,C.;Gan,W.J.;Kou,Y.Chin.J.Catal.2006,27, 1159.[顏 寧,趙 晨,甘維佳,寇 元.催化學(xué)報(bào),2006,27, 1159.]

(20)Yan,N.;Zhao,C.;Luo,C.;Dyson,P.J.;Liu,H.;Kou,Y.J.Am. Chem.Soc.2006,128,8714.doi:10.1021/ja062468t

(21)Pepper,J.M.;Rahman,M.D.Cell.Chem.Technol.1987,21, 233.

(22) Thring,R.W.;Katikaneni,S.P.R.;Bakhshi,N.N.Fuel Process.Technol.2000,62,17.doi:10.1016/S0378-3820(99) 00061-2

(23)Jackson,M.A.;Compton,D.L.;Boateng,A.A.J.Anal.Appl. Pyrol.2009,85,226.doi:10.1016/j.jaap.2008.09.016

(24) Stark,K.;Taccardi,N.;Bosmann,A.;Wasserscheid,P. ChemSusChem 2010,3,719.doi:10.1002/cssc.200900242

(25) Zakzeski,J.;Bruijnincx,P.C.A.;Jongerius,A.L.;Weckhuysen, B.M.Chem.Rev.2010,110,3552.doi:10.1021/cr900354u

(26)Yan,N.;Zhao,C.;Dyson,P.J.;Wang,C.;Liu,L.T.;Kou,Y. ChemSusChem 2008,1,626.doi:10.1002/cssc.200800080

(27) Zhao,C.;Kou,Y.;Lemonidou,A.A.;Li,X.;Lercher,J.A. Angew.Chem.Int.Edit.2009,48,3987.doi:10.1002/ anie.200900404

(28) Zhao,C.;Kou,Y.;Lemonidou,A.A.;Li,X.;Lercher,J.A. Chem.Commun.2010,46,412.

(29) Zhao,C.;He,J.;Lemonidou,A.A.;Li,X.;Lercher,J.A. J.Catal.2011,280,8.doi:10.1016/j.jcat.2011.02.001

(30)Yan,N.;Yuan,Y.;Dykeman,R.;Kou,Y.;Dyson,P.J.Angew. Chem.Int.Edit.2010,49,5549.doi:10.1002/anie.201001531

(31) Pepper,J.M.;Siddiqueullah,M.Can.J.Chem.1961,39,1454. doi:10.1139/v61-185

(32)Derkacheva,O.;Sukhov,D.Macromol.Symp.2008,265,61. doi:10.1002/masy.200850507

(33) He,J.X.;Zhang,W.;Li,K.J.;Cui,S.Z.;Wang,S.Y.J.Text. Res.2009,30,13.[何建新,章 偉,李克兢,崔世忠,王善元.紡織學(xué)報(bào),2009,30,13.]

(34)Jung,H.J.G.;Himmelsbach,D.S.J.Agric.Food Chem.1989, 81.

(35) Zhang,A.P.;Liu,C.F.;Sun,R.C.Ind.Crop.Prod.2010,31, 357.doi:10.1016/j.indcrop.2009.12.003

(36) Faix,O.Holzforschung 1991,45(Suppl.),21.

(37) Dorris,G.M.;Gray,D.G.Cell.Chem.Technol.1978,12,9.