Selective defunctionalization by TiO 2 of monomeric phenolics from lignin pyrolysis into simple phenols Ofei D. Mante a,⇑ , Jose A. Rodriguez b , Suresh P. Babu c a Sustainable Energy Technologies Department, Brookhaven National Laboratory, Upton, NY 11973, USA b Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA c Global and Regional Solutions Directorate, Brookhaven National Laboratory, Upton, NY 11973, USA highlights Highly selective process for the production of simple phenols. TiO 2 defunctionalized monomeric phenolics from lignin. Over 95% conversion of monomeric phenolics is attainable. Catechols were the most reactive monomeric phenolic. Anatase TiO 2 performed much better than rutile TiO 2 . graphical abstract article info Article history: Received 18 July 2013 Received in revised form 26 August 2013 Accepted 1 September 2013 Available online 11 September 2013 Keywords: Lignin Pyrolysis Aromatic chemicals Phenols Monomeric phenolics Titanium dioxide abstract This study is focused on defunctionalizing monomeric phenolics from lignin into simple phenols for applications such as phenol/formaldehyde resins, epoxidized novolacs, adhesives and binders. Towards this goal, Titanium dioxide (TiO 2 ) was used to selectively remove hydroxyl, methoxy, carbonyl and car- boxyl functionalities from the monomeric phenolic compounds from lignin to produce mainly phenol, cresols and xylenols. The results showed that anatase TiO 2 was more selective and active compared to rutile TiO 2 . Catechols were found to be the most reactive phenolics and 4-ethylguaiacol the least reactive with anatase TiO 2 . An overall conversion of about 87% of the phenolics was achieved at 550 °C with a cat- alyst-to-feed ratio of 5 w/w. Over 97% conversion of phenolics is achievable at moderate temperatures (550 °C or 6600 °C) and a moderate catalyst-to-feed ratio of 6.5:1. The reactivity of catechols on TiO 2 sug- gests that titania is a promising catalyst in the removal of hydroxyl moiety. Published by Elsevier Ltd. 1. Introduction The development of technologies for lignin utilization has be- come important with the advent of integrated lignocellulosic eth- anol industries. This is because lignin conversion into fuel additives, chemicals and materials would enhance the economic viability of producing biofuels from biomass in a biorefinery. To- day, lignin residue, a byproduct of the lignocellulosic ethanol pro- cess and the pulp and paper industry is commonly used as fuel for the generation of heat and power. However, other uses and appli- cations include production of carbon fibers, polymer fillers, resins, adhesives, binders and aromatics chemicals (benzene, toluene, xy- lene, phenol and terephthalic acid) (Holladay et al., 2007). As re- ported by Holladay et al. (2007), utilization of lignin residue for high value products and chemicals other than for heat and power could result in revenue improvement in the range of $12–35 billion in an integrated lignocellulosic ethanol plant. Lignin depolymerization and conversion into aromatic chemi- cals is an attractive route and various approaches have been exten- sively reviewed in the literature by Zakzeski et al. (2010) and 0960-8524/$ - see front matter Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.biortech.2013.09.003 ⇑ Corresponding author. Tel.: +1 5404491980. E-mail address: [email protected](O.D. Mante). Bioresource Technology 148 (2013) 508–516 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Ofei D. Mante a,⇑, Jose A. Rodriguez b, Suresh P. Babu c
a Sustainable Energy Technologies Department, Brookhaven National Laboratory, Upton, NY 11973, USAb Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USAc Global and Regional Solutions Directorate, Brookhaven National Laboratory, Upton, NY 11973, USA
h i g h l i g h t s
� Highly selective process for theproduction of simple phenols.� TiO2 defunctionalized monomeric
phenolics from lignin.� Over 95% conversion of monomeric
phenolics is attainable.� Catechols were the most reactive
monomeric phenolic.� Anatase TiO2 performed much better
than rutile TiO2.
g r a p h i c a l a b s t r a c t
a r t i c l e i n f o
Article history:Received 18 July 2013Received in revised form 26 August 2013Accepted 1 September 2013Available online 11 September 2013
This study is focused on defunctionalizing monomeric phenolics from lignin into simple phenols forapplications such as phenol/formaldehyde resins, epoxidized novolacs, adhesives and binders. Towardsthis goal, Titanium dioxide (TiO2) was used to selectively remove hydroxyl, methoxy, carbonyl and car-boxyl functionalities from the monomeric phenolic compounds from lignin to produce mainly phenol,cresols and xylenols. The results showed that anatase TiO2 was more selective and active compared torutile TiO2. Catechols were found to be the most reactive phenolics and 4-ethylguaiacol the least reactivewith anatase TiO2. An overall conversion of about 87% of the phenolics was achieved at 550 �C with a cat-alyst-to-feed ratio of 5 w/w. Over 97% conversion of phenolics is achievable at moderate temperatures(550 �C or 6600 �C) and a moderate catalyst-to-feed ratio of 6.5:1. The reactivity of catechols on TiO2 sug-gests that titania is a promising catalyst in the removal of hydroxyl moiety.
Published by Elsevier Ltd.
1. Introduction
The development of technologies for lignin utilization has be-come important with the advent of integrated lignocellulosic eth-anol industries. This is because lignin conversion into fueladditives, chemicals and materials would enhance the economicviability of producing biofuels from biomass in a biorefinery. To-day, lignin residue, a byproduct of the lignocellulosic ethanol pro-
cess and the pulp and paper industry is commonly used as fuel forthe generation of heat and power. However, other uses and appli-cations include production of carbon fibers, polymer fillers, resins,adhesives, binders and aromatics chemicals (benzene, toluene, xy-lene, phenol and terephthalic acid) (Holladay et al., 2007). As re-ported by Holladay et al. (2007), utilization of lignin residue forhigh value products and chemicals other than for heat and powercould result in revenue improvement in the range of $12–35 billionin an integrated lignocellulosic ethanol plant.
Lignin depolymerization and conversion into aromatic chemi-cals is an attractive route and various approaches have been exten-sively reviewed in the literature by Zakzeski et al. (2010) and
Amen-Chen et al. (2001). The following include some of the majorpathways to transform lignin into aromatic chemicals: hydropro-cessing (hydrodeoxygenation, hydrogenolysis, hydrogenation andhydrocracking), solvolysis, base-catalyzed depolymerization(BCD), acid-catalyzed depolymerization (ACD), oxidative depoly-merization (OD), pyrolysis, gasification, electrocatalysis and acidol-ysis (Amen-Chen et al., 2001; Bu et al., 2012; Elliott, 2007;Gluckstein et al., 2010; Kleinert and Barth, 2008a,b; Zakzeskiet al., 2010). Among the bulk chemicals that could be producedfrom lignin by the above methods, the conversion to aromatichydrocarbons (BTX) has been extensively explored compared tophenol. Nonetheless, phenol is of equal industrial importance andserves as the chemical building block for making products suchas Bisphenol-A, phenolic resins and caprolactam (Holladay et al.,2007).
For phenol production from lignin, the Noguchi process (Goh-een David, 1966) has been reported as one of the technologies thatproduces high yields. Recently, Kleinert and Barth (2008a,b) alsoreported lignin conversion by solvolysis process (Lignin-to-Liquid)using mixtures of formic acid and alcohols to produce alkylatedphenols and aliphatic hydrocarbons. The problems that exist withthese potential approaches are the requirement of hydrogen orhydrogen donor solvents, high pressure reactors, long reactiontimes, separation/recovery of chemicals and wide distribution ofchemicals in the product stream. For commercial applications,these obstacles would have to be addressed. Pyrolysis of lignin isalso an alternative process but produces a wide distribution ofmulti-functional phenolics in lower concentrations. As a result,pyrolysis of lignin over different catalysts has been explored inan effort to produce high yields of valuable chemicals. Most ofthe catalytic pyrolysis studies have so far focused on the use of zeo-lites (Choi and Meier, 2013; de Wild et al., 2009; Jackson et al.,2009; Ma et al., 2012; Thring et al., 2000; Zhao et al., 2010) withZSM-5 as the primary catalyst for deoxygenating lignin pyrolysisvapors into mainly hydrocarbons (BTX, naphthalenes and PAH).
Titanium dioxide (TiO2) on the other hand has been shown to bea promising photocatalyst for indirect photocatalytic degradationof lignin wastewater. (Kamwilaisak and Wright, 2012; Ma et al.,2008; Machado et al., 2000; Perez et al., 1998). The TiO2-photoas-sisted process in the presence of H2O2 is able to fragment the ligninstructure to yield quinones and other organic chemicals (Ma et al.,2008). In this study, TiO2 was explored in removing the differentoxygenated functionalities of the monomeric phenolics that is pro-duced from lignin pyrolysis. From a chemical industry perspective,it would be attractive to develop a technology that would producea narrow stream of high concentration of products rather thanwide mixtures of low concentration of chemicals. This investiga-tion reports of a highly selective defunctionalization process forconverting monomeric phenolics to simple phenols at atmosphericpressure without the use of hydrogen. The study is believed to bethe first published account of anatase TiO2 in selectively promotingdemethoxylation (DMO), dehydroxylation (DHO), decarboxylation(DCB) and decarbonylation (DCO) of monomeric phenolics intosimple phenols.
2. Methods
2.1. Materials
In this study, kraft lignin (alkali), titanium dioxide (anatase andrutile TiO2) and all the pure compounds used in the calibration ofthe GCMS were obtained from Sigma–Aldrich (Sigma–Aldrich, St.Louis, MO, USA). The moisture content of the lignin was deter-mined using MJ33 Compact Infrared Moisture Analyzer (MettlerToledo, Greifensee, Switzerland) before each experimental run.
The received TiO2 powders from Sigma–Aldrich were first calcinedat 550 �C under dry air flow for 3 h before use.
2.2. Methods
The kraft lignin used was subjected to thermogravimetric anal-ysis in a TA Instruments Q50 TGA. About 5 mg of kraft lignin sam-ple was heated at 20 �C/min from 25 �C to 750 �C with 60 mL/minof N2 as the carrier gas. The pyrolysis of the kraft lignin was con-ducted using a CDS Analytical Pyroprobe 5200 HPR with a microreactor (CDS Analytical, PA, USA) interfaced with Agilent 7890Agas chromatograph and 5975C mass spectrometer detector(Fig. 1). The pyrolysis experiments were performed in a directPy–GC/MS mode at atmospheric pressure. In the directPy–GC/MS mode, the volatiles and gases are immediately and di-rectly transferred to the GC/MS for analysis without trapping. Forthe neat pyrolysis, 0.3–0.6 mg of kraft lignin sample was loadedinto a quartz tube of 25 mm in length and 1.9 mm in diameter.In the experiment with TiO2, the primary vapors from lignin pyro-lysis were swept through a micro reactor packed with titaniumdioxide. The loaded sample in the quartz tube and TiO2 in thestainless steel tube reactor were held in place at the center withsmall plugs of quartz wool. The quartz tubes were weighed beforeand after pyrolysis using a Mettler Toledo MS105 semi-micro bal-ance with sensitivity of 0.01 mg (Mettler Toledo, Greifensee, Swit-zerland). Model compounds; catechol, 3-methycatechol, 2-methoxy-4-vinylphenol, vanillin and guaiacol were also reactedwith TiO2 at similar conditions. The Pyroprobe control softwarewas programmed to rapidly heat the sample to 550 �C and holdfor 20 s. Parametric studies were performed to investigate the ef-fect of process temperature (450, 550 and 650 �C) and catalyst-to-feed ratio of 2, 5, and 10 (w/w). Each experiment was conductedat least five times to ensure reproducibility and accuracy of results.
The vapors and gases after reaction were swept by ultra-highpurity helium carrier gas into the GC/MS for separation and detec-tion. The products were separated with a 60 m � 0.25 mm DB-1701 capillary column of 0.25 lm film thickness. The GC inletwas set to 275 �C and a split ratio of 75:1 was used. The column fol-lowed a temperature program of 40 �C for 1 min, 6 �C/min to130 �C, 10 �C/min to 275 �C held for 20 min. Column flow was con-trolled to maintain a constant linear velocity of 1 mL/min. The ionsource and the interface of the mass spectrometer detector wereheld at 230 �C and 250 �C, respectively. Electron ionization of thecompounds was performed at 69.9 eV and the ions were separatedby their mass-to-charge (m/z) ratios in the range of 28–350.
The GC column was calibrated with pure forms of selectedcompounds to allow for quantification and confirmation of peakspresent in pyrolysis products from kraft lignin. The National Insti-tute of Standards and Technology (NIST) mass spectral library anda custom library created for the mass spectra of the known com-pounds were both used in the identification of compounds. The fol-lowing standards were purchased from Sigma–Aldrich; methanol,phenol, 2-methylphenol, 4-methylphenol, 2,4-dimethylphenol,3,5-dimethylphenol, 4-ethylphenol, benzene, toluene, ethylben-zene, xylene, guaiacol, 4-methylguaiacol, 2-methoxy-4-vinylphe-nol, catechol, eugenol, 4-methylcatechol, trans-isoeugenol,syringaldehyde, cis-isoeugenol, vanillin, coniferyl alcohol, syringol,3-methylcatechol, 1,2,3-trimethoxybenzene, 2,3-benzofuran, 4-allyl-2,6-dimethoxyphenol, 3-methoxycatechol, trans-3,5-dimethoxy-4-hydroxycinnamaldehyde, 3,5-dimethoxy-4-hydroxyacetophenone,2,5-dimethoxybenzyl alcohol and 3,5-dimethoxy- acetophenone.For each compound, a linear calibration curve was establishedfor concentrations of 250, 750, 1250, 2500 and 5000 lg/mL. Thecalibration curves had R2 value greater than 0.95. For othercompounds that had no calibration curve, the response factor ofa closely related compound was used to estimate their yields.
The response factor of vanillin was used in estimating the yields ofapocynin, 4-hydroxy-3-methoxyphenyl acetone (guaiacylacetone)and homovanillic acid. The response factor of 4-methylguaiacolwas used for 4-ethylguaiacol and 4-propylguaiacol and that ofmethanol was used for methanethiol. The non-condensable gasesand reaction water produced during pyrolysis were not measuredin the study. The focus was solely to measure the yields of GCMSdetectable organics from lignin pyrolysis and their transformationover titanium dioxide.
3. Results and discussion
3.1. Production of monomeric phenolics from kraft lignin
The TGA in Fig. S1a shows that approximately 49 wt.% of thekraft lignin can be volatilized with maximum decompositionoccurring between 200 �C and 500 �C and 51 wt.% of residue beingformed at 550 �C at a heating rate of 20 �C/s. The high formation ofchar residue is partly because of mineral impurities which are usu-ally present in kraft lignin. Both the TG and DTG plots show onemajor degradation step, which suggests that the kraft lignin mostlikely contained negligible amounts of cellulose and hemicellulose.The relatively small peak below 100 �C is due to vaporization ofwater and the other peak at 680 �C was attributed to char devola-tilization. The maximum rate of decomposition of the kraft ligninwas found to be 0.25%/�C at 322 �C. This rate is lower when com-pared to the maximum rate of decomposition (0.40%/�C) of anorganosolv lignin reported in a study by Shen et al. (2013). Thelower decomposition rate found for the kraft lignin suggests there-fore that it is less reactive.
The bench-scale pyrolysis of kraft lignin at 550 �C produced sev-eral high molecular weight compounds amounting to approxi-mately 130 individual chemical species. The total ionchromatogram illustrating the main product distribution fromkraft lignin pyrolysis is shown in Fig. S1b. The compounds with rel-atively high concentration were identified and quantified usingcalibration curves obtained from prepared standards of pure com-pounds. The average total yield of the GC/MS quantified
compounds obtained from 8 runs performed at the same conditionwas 11 wt.% of dry lignin feed. A detailed product distribution isshown in Fig. S2. The gravimetrically measured char yield averaged35.6 wt.% with a standard deviation of 7.16. The reason for the low-er char yield measured compared to the solid residue from the TGAexperiment was because small quartz wool fibers from the quartztube escaped during the removal of the tube from the coiled fila-ment in some of the experiments, thus the final weight was lowerthan what it should have been. As a result, wide variation in thechar yield was recorded and their use in further discussion wouldbe misleading and not relevant to the primary focus of the study.
The yields for the major GC/MS detectable organics reported inthis study are comparable to those reported in the literature forfast pyrolysis of isolated lignin. Typically, isolated lignin by itscomplex nature produces lower yields of organics during pyrolysiswhen compared with cellulose and hemicellulose. Jegers and Klein(1985) performed pyrolysis of kraft lignin at 400 �C for 7.5 min andproduced a total phenolics yield of 9.3%. Nowakowski and Bridg-water (2010) studied analytical pyrolysis of Alcell lignin at varyingtemperatures and the GC/MS analysis showed that maximum yieldof 10.2 wt.% for the identified phenolic compounds was achievableat 600 �C. A recent team study by Nowakowski et al. (2010) detailsresults on fast pyrolysis of two different types of lignin from four-teen laboratories. In that work, one laboratory reported 11.0 wt.%of total organics from fast pyrolysis of Asian lignin based on aGC/FID analysis. de Wild et al. (2009) conducted pyrolysis in a flu-idized bed at 400 �C and found that GRANIT and Alcell lignins pro-duced, respectively 9.0 wt.% and 7.0 wt.% of GC detectable phenoliccompounds. Patwardhan et al. (2011) also performed pyrolysis ofcornstover lignin isolated by the organosolv process at 500 �Cand the phenolic compounds quantified by GC–MS/FID analysiswas 17.1 wt.%. It can be inferred from the above reported resultsthat the differences in some of the organic yields is due to thesource of lignin and the pyrolysis conditions such as process tem-perature, reaction time and reactor configuration.
The phenolics identified and quantified from the pyrolysis ofkraft lignin in this study were mainly methoxylated phenols. Gua-iacols (guaiacol, 4-methyl guaiacol, 4-vinyl guaiacol, 4-ethylguaia-col and 4-propylguaiacol) constituted the highest fraction (35%) of
the total yield. The second highest class of phenolics was catecholsand accounted for 17% of the yield. Eugenols and syringols were,respectively 7.5% and 1% and the sum of other phenolics (vanillin,guaiacylacetone, homovanillic acid, and apocynin) constituted 19%of the yield. However, phenol, cresols, and xylenols constituted avery small fraction and corresponded to 1.3%, 3.1% and 1.8%,respectively of the total yield. Methanol and methanethiol werealso produced during the thermal depolymerization of the ligninand their yields amounted to 13.6% of the quantified products. Ithas to be pointed out that the type of methoxylated phenolics gen-erated during pyrolysis is dependent on the source of lignin (i.e.,hardwood, softwood or herbaceous feedstock) and the process con-ditions. This implies that, the high presence of guaiacyl phenolicsand the very low yields of syringyl phenolics suggest that the kraftlignin was isolated from softwood. The formation of sulfur derivedproducts such as methanethiol and dimethyl sulfide as identifiedby the GC/MS library are produced due to the white liquor(NaOH–Na2S) used in the pulping process delignification.
The formation of these monomeric phenolics from lignin pyro-lysis is explained by free-radical reactions. It is believed that pyro-lysis of lignin is accompanied by a pool of free radicals generatedfrom the cleavages of the different linkages (mainly a- or b-O-4aryl ether bonds) between the phenylpropane units, cracking of al-kyl chains, removal of carbonyl groups, and homolytic cleavage ofmolecular intermediates (Amen-Chen et al., 2001; Dorrestijn andMulder, 1999; Hosoya et al., 2008; Shen et al., 2010; Vuori et al.,1987). These radicals, hydroxyl (HO⁄), methoxy (CH3O⁄), carbonyl(CO⁄), carboxyl (COOH⁄) and methyl CH�3
� �are among some of
the reactive species that dictate the product distribution from lig-nin pyrolysis. Depending on the process condition, these free rad-icals and the aromatic ring undergo several secondary reactions toform various condensable compounds such as methoxylated phen-olics, hydroxyl-phenols, methanol, H2O and non-condensablegases (H2, CH4, CO and CO2). It is worth pointing out that eventhough these radicals (methoxy, hydroxyl, methyl, carbonyl andcarboxyl) are produced during lignin fragmentation they are stillretained in considerable amounts in the final products. Hence, itsuggests that these groups are thermally stable to some extentand may explain why the formation of simple phenols such as phe-nol, cresols and xylenols which are of industrial importance, areusually produced in significantly less quantity during pyrolysis oflignin. It also indicates that the thermal reactions in lignin pyroly-sis are uncontrolled and non-selective. In the following sections ofthis publication, the use of titanium dioxide in lignin pyrolysis toassist further demethoxylation (DMO), dehydroxylation (DOH),decarbonylation (DCO), decarboxylation (DCB), and demethylation(DME) of the monomeric phenolics to produce a narrow slate ofsimple phenols will be discussed.
3.2. Defunctionalization of monomeric phenolics by anatase TiO2
The monomeric phenolics produced from the neat pyrolysis ofkraft lignin were passed over a bed of anatase TiO2 to primarilyinvestigate their transformation into a narrow distribution of sim-ple phenols. As can be seen notably from Table 1, the anatase TiO2
was found to be active and selective in converting most of themonomeric phenolics at 550 �C into mainly lower molecularweight phenols. Consequently, formation of CO2, methanethiol,methanol, and dimethyl sulfide was found to increase. Further-more, increases in intensity for trace amounts of 1-naphthalenol,1H-Indenol, and aromatic hydrocarbons were also observed. Theyield of the quantified GCMS detectable condensables from the lig-nin pyrolysis with TiO2 was 7.45 wt.%, lower than the yield fromthe neat pyrolysis (Table 1). The decrease in the yield after reactingthe vapors with TiO2 was attributed to removal of primarily meth-oxy, hydroxyl and carbonyl groups present in the monomeric
phenolics as non-condensable gases (CO2, CO, and CH4) andH2O(g). It is evident from the results that the monomeric phenolicsinitially produced from lignin pyrolysis underwent selective DMO,DOH, DCO, DME, methylation (MT) and possibly hydrogenationreactions in the presence of anatase TiO2 at 550 �C. The yield ofmethanol increased by 60% from 0.7 wt.% to 1.16 wt.%, while thatof methanethiol increased by 37% from 0.76 wt.% to 1.04 wt.%.The increase in methanol and methanethiol shows additional re-moval of –OCH3 and indicates DMO as one of the reactions beingpromoted by anatase TiO2. Also, the formation of dimethyl sulfide(DMS) further indicates the cleavage of C–OCH3 bond. Other non-quantified products such as H2O, CO, CO2 and CH4 would poten-tially show increase due to the cleavage of hydroxyl and carbonylfunctional groups as well as probable decomposition of methanoland methanethiol.
The average conversion for most of the monomeric phenolics at550 �C was between 70% and 95% (Table 1) with the exception of4-hydroxy-3-methoxyphenyl acetone (guaiacylacetone) with aconversion of about 54%. It is worth mentioning that catecholand 4-methylcatechol were found to be reactive phenolics andthey were hardly found in the product distribution from ligninpyrolysis with anatase TiO2. Overall, anatase TiO2 was found tobe very reactive towards catechols, phenolics with aldehyde(CHO) and carboxyl functional groups, guaiacols, and eugenols.The reactivity of the anatase TiO2 towards phenolics with ketonefunctionality is unclear since the conversion obtained for apocyninwas high (81%) and that of guaiacylacetone was relatively low(54%). The selectivity of the phenolics in the product stream as shownin Fig. 2 indicates that the total percentage for the simple phenolsincreased from 7% to 80% upon reaction with anatase TiO2. Cresols(31%) formed the highest fraction of the phenols produced, followedby phenol (21%), xylenol (20%) and ethylphenol (8.5%).
To gain insight into the production of the reported simple phe-nols, reactions of selected model compounds such as catechol, 3-methycatechol, 2-methoxy-4-vinylphenol, vanillin and guaiacolwith TiO2 were studied at the same conditions. The reactivities ofthe model compounds with anatase TiO2 as shown in Fig. S3a–e,were in good agreement with the results from the lignin pyrolysiswith TiO2 to a larger extent. It should be pointed out that somereaction products which were not present in the lignin pyrolysiswith TiO2 were however found in the case of some model com-pound experiments. This may be due to the absence of some radi-cal species that are present during actual lignin pyrolysis tests.Hence, reaction products of interest to the actual lignin experi-ments were the focus of discussion.
Based on the product distribution from the model compoundstudy, conceptual reaction pathways for the production of simplephenols from monomeric phenolics using anatase TiO2 have beenproposed as follows. For the formation of phenol, the model studyresults (Fig. S3a) show clearly catechol as the predominant precur-sor. Guaiacol and the other studied model compounds also pro-duced phenol to some extent. As shown in Fig. S3a–e, it isconceivable that DOH of catechol and DMO of guaiacol are proba-bly the direct pathways in the production of phenol. In the case ofguaiacol, studies on hydrodeoxygenation of guaiacol have showndemethylation followed by dehydroxylation as an alternative routeto direct demethoxylation in the formation of phenol (Bu et al.,2012; Bui et al., 2009). However, the absence of catechol in theproduct distribution from the model study of guaiacol over TiO2
(Fig. S3e) in this study suggests that the formation of phenol fromguaiacol probably did not proceed through demethylation. Othercontributing consecutive routes may include (1) DMO of 2-meth-oxy-4-vinylphenol into 4-vinylphenol followed by complete crack-ing of the vinyl side-chain into phenol, (2) DOH of methylcatecholfollowed by DME and (3) DCO of vanillin to yield guaiacol followedby DMO to phenol. Cresols on the other hand appear to be
Table 1GC/MS quantified yields of selected compounds from pyrolysis of kraft lignin without and with anatase TiO2 at 550 �C and a catalyst to feed ratio of 5 (w/w).
Major peak numbers Retention time [min] Major compounds Pyrolysis Pyrolysis with TiO2 ConversionAverage yielda (wt.%) Average yielda (wt.%)
BDL = below quantifiable detection limit. NQ = not quantified.a Average yields based on at least 5 runs.
Fig. 2. Comparison of the selectivity of the phenolic products: blue-pyrolysis of kraft lignin at 550 �C, red-pyrolysis of kraft lignin with anatase TiO2 at 550 �C and C/F of 5(w/w). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
produced significantly from both methylcatechol and 2-methoxy-4-vinylphenol but to a lesser extent from guaiacol and vanillin.No cresols were produced from catechol and this is because ofthe absence of methyl radicals. The results from the model studysuggest that cresols were generated from direct dehydroxylationof 4-methylcatechol and DMO of 2-methoxy-4-vinylphenol into4-vinylphenol followed by partial cracking of the side-chain. Directdemethoxylation of 4-methylguaiacol is also a potential path forthe production of cresols. In addition, DCO of vanillin into a guaia-col followed by simultaneous DMO and MT is a possible pathwayfor cresol formation. The remaining group of simple phenols (i.e.,xylenol and ethylphenol) could be produced mainly from vinyl/al-
lyl chain-substituted guaiacols such as methoxy-4-vinylphenoland eugenols. Guaiacols with 2 or 3 carbon alkyl substituent couldbe precursors as well. As shown in Fig. S3b, the reaction of 2-meth-oxy-4-vinylphenol with TiO2 resulted in significant formation ofethylphenol and minimal generation of xylenols. This suggeststhat, at least one of the consecutive reactions in forming an alkyl-phenol would have to include a hydrogenation step to saturate theunsaturated side chains (vinyl/allyl) attached to the aromatic ring.The reactions involving hydrogen addition could proceed withhydrogen atom abstraction or hydrogen atom transfer fromother intermediate species. For xylenol formation, the side chainswould have to be catalytically cracked to yield methyl radicals
for methylation reactions. Hence, it has been proposed that2-methoxy-4-vinylphenol in the presence of TiO2 undergo DMOto form a vinylphenol which may either proceed through sidechain hydrogenation to yield ethylphenol or side chain crackingand methylation to produce xylenol.
Further fundamental studies are underway as part of the pro-cess development to elucidate the surface chemistry of TiO2 in pro-moting these postulated reactions. The partial deoxygenationreactions promoted in the lignin and model compound study isattributed to the nature of adsorption sites on the bulk anataseTiO2 and the interaction with or between the adsorbed species.The electronic effect of the hydroxyl and methoxy moieties ofthe monomeric phenolics is believed to induce absorption (disso-ciatively or molecularly) on the TiO2 surface. Consequently, astrong interaction with the surface of the anatase and the absorbedspecies could break bonds like C–OH (dehydroxylation), C–OCH3
(demethoxylation) and O–CH3 (demethylation), leading to the for-mation of hydroxyl (OHads), methoxyl (OCH3ad) and methyl (CH3ad)surface adsorbates. The surface hydroxyls could react to produceH2O and the methoxyls could react to form CH3OH, CH4S, CH3SCH3
or even decompose to CO. Other adsorbates such as formate coulddecompose into CO2 with production of surface hydrogen. Like-wise, free radicals from pyrolytic rupture of the lignin and furtherhomolytic cleavage of molecular intermediates provide a reactiveenvironment for reactions leading to the removal of methoxy, hy-droxyl, methyl, carbonyl and carboxyl groups to occur. It has to bepointed out that intermolecular interaction of the adsorbed mole-cules and transferable adsorbed hydrogen may be playing a bigrole in the reduction of oxygen functionalities in the phenolics.
Reported studies in the literature on the reaction of monomericphenolics on TiO2 surfaces have been conducted mainly underhydroprocessing (Alonso et al., 2012; Bu et al., 2012) and photocat-alytic conditions (Zakzeski et al., 2010). For hydroprocessing, thephenolics are reduced under hydrogen pressures into phenol, cre-sols, catechol and hydrocarbons through hydrogenolysis, hydro-deoxygenation and hydrogenation reactions. Underphotocatalytic conditions, the phenolics are rather oxidized byTiO2 and products such as quinones, aldehydes and acids are pro-duced (Zakzeski et al., 2010). The TiO2 assisted photocatalytic deg-radation of lignin wastewater is believed to be promoted bytransfer of electrons from the oxidized substrates to the catalystsurface adsorbed species such as hydroxyl radicals or electronholes. (Kamwilaisak and Wright, 2012; Ma et al., 2008; Machadoet al., 2000).
3.3. Comparison between anatase and rutile TiO2
The surface property of TiO2 that is promoting the apparent par-tial deoxygenation reactions is unclear at this point in the study.However, since titania occurs in three main different polymorphs(rutile, anatase and brookite) and they exhibit different properties,the pyrolysis of lignin was also performed with rutile TiO2 to atleast have an insight into the potential properties that maybe con-tributing to the selective removal of –OH, –OCH3, –CO and –COOHgroups from the monomeric phenolics. The total ion chromato-grams in Fig. S4 shows a comparison between the effect of both ru-tile TiO2 and anatase TiO2 on the monomeric phenolics from thepyrolysis of kraft lignin at 550 �C using a catalyst-to-feed ratio of10 (w/w).
Clearly, it can be seen that the anatase TiO2 was highly activeand selective than rutile TiO2 in transforming the monomericphenolics from lignin pyrolysis into simple phenols. From thequantitative analysis of the major products shown in Table 2, theanatase was found to produce 51% more total yield of simple phe-nols than rutile TiO2. The results showed that the selected mono-meric phenolics decreased significantly from 8.8% wt.% to less
than 0.1 wt.% in presence of anatase powder and marginally re-duced to 5.15 wt.% for rutile powder. This translates to averageconversion of 40% of the phenolics on the rutile TiO2 and near com-plete conversion of the phenolics on the anatase TiO2. Furthermore,the total yield of methanol and methanethiol was 1.81 wt.% foranatase compared with 1.24 wt.% for the rutile powder. This indi-cates that demethoxylation of the phenolics was highly promotedwith the anatase TiO2. In contrast to rutile TiO2, the anatase pow-der produced in addition to simple phenols, BTX, ethylene benzeneand styrene, probably due to promotion of complete deoxygen-ation of some phenolics at a higher catalyst-to-feed ratio.
It is worth noting that phenolics with hydroxyl, carboxyl and car-bonyl moieties showed relatively high conversions on rutile surface.This observation compliments the fact that hydroxyl, carboxyl andcarbonyl groups attached to the aromatic ring appear to be reactivemoieties that are fairly removed from the monomeric phenolics onthe surface of both polymorphs by DOH, DCB and DCO, respectively.One other important information that became apparent with the useof rutile was the potential source of the phenol. In the earlier discus-sion, it was suggested generally that phenol is generated primarilyfrom catechol on TiO2. This hypothesis was cross checked since cat-echol is one of the two major phenolics that had a high conversionvalue (80%) when rutile TiO2 was used. The results showed that phe-nol yield increased from 0.14 wt.% to about 0.6 wt.%. The amount ofcatechol produced without a catalyst was 0.82 wt.%, meaning thatthe theoretical amount of phenol that can be produced from com-plete conversion of catechol is 0.69 wt.%, but according to the exper-iment, 78% of catechol was converted. This implies that the increasein the theoretical yield of phenol should be 0.54 wt.% under the pre-mise that DOH of catechol is the main pathway. In comparison, theexperimental increase of phenol produced with rutile was about85% of the theoretical increase. This finding indicates that dehydr-oxylation of catechol could be the primary pathway for phenol pro-duction and suggests that a small fraction of catechol could beconverted to 2-methyl phenol via DOH followed with methylation.It should be pointed out that DMO of guaiacol is another competingpathway for direct generation of phenol. However, DMO was notfavorable on rutile TiO2. Another analysis on the yield of 4-methyl-phenol supports the notion that dehydroxylation of 4-methylcatecolmay be the predominant pathway for its production. It was foundthat the increase in the yield of 4-methylphenol in the experimentwas 0.38 wt.% and it corresponds to about 49% of the theoreticalyield (0.77 wt.%) assuming dehydroxylation of 4-methyl catecholwas the sole reaction mechanism. This is interesting because theyield of 4-methyl catechol showed a decrease of 52% after undergo-ing catalysis with rutile TiO2.
The results indicate that the reactivities of the bulk structures,anatase and rutile are different with the monomeric phenolicsfrom lignin. The performance of the anatase compared with the ru-tile suggests that surface properties including surface area, phasestructure and grain size all affect the removal of hydroxyl, meth-oxy, carbonyl and carboxyl moieties attached to the aromatic ringof the phenolics. Studies on anatase to rutile transition have shownthat the rutile phase typically has lower specific surface area andthe crystallite sizes are larger (Ahmed et al., 2011; Hanaor and Sor-rell, 2011). This implies that the anatase TiO2 probably performedmuch better than the rutile because it has more active sites avail-able due to its higher surface area and also provided a larger sur-face area for adsorption of the species. Studies on photocatalysishave also reported that anatase exhibits higher density levels ofadsorbed radicals than rutile TiO2 (Hanaor and Sorrell, 2011).
3.4. Influence of process conditions on production of simple phenols
The effect of catalyst-to-feed (C/F) ratio and the pyrolysistemperature on the conversion of phenolics were investigated
Table 2GC/MS quantified yields of selected compounds from pyrolysis of kraft lignin with rutile and anatase TiO2 at 550 �C and a catalyst to feed ratio of 10 (w/w).
Major peak numbers Retention time [min] Major compounds Average yielda (wt.%) Conversion of phenolics (%)
No catalyst Anatase TiO2 Rutile TiO2 Anatase TiO2 Rutile TiO2
employing one-factor-at-a-time (OFAT) approach to study C/F ra-tios of 2, 5 and 10 w/w at fixed temperature of 550 �C and processtemperatures of 450, 550 and 650 �C at a C/F ratio of 5 (w/w). Theresults were obtained from at least five runs for each set of exper-imental parameter, making a minimum of 30 runs. Fig. S5 com-pares the total ion chromatograms for the product distributionsat C/F ratio of 0, 2, 5 and 10 (w/w). Visually, it can be seen thatthe conversion of the phenolics increased with an increase in thecatalyst-to-feed ratio. Qualitatively, the yields of the simple phe-nols showed that increase in C/F ratio increased the productionof phenol, cresols and trimethylphenols through a maximum andthen dropped with a further increase in C/F. From the graph inFig. 3a, the maximum yield of phenol, cresols and trimethylphenolswere produced at a C/F ratio of 5 (w/w). The yields of xylenols, eth-ylphenol and BTX on the other hand showed a steady increase withincrease in C/F ratios up to 10 (w/w). A similar trend was found forthe yield of styrene. The results indicate that at a higher C/F ratio,some of the phenol, cresols and trimethylphenols may have under-gone further deoxygenation to produce benzene, toluene, xylene,and styrene. This suggests that hydrogenation reactions were mostlikely promoted as a result of more active sites at a higher C/F ratio.The study also showed that the yield of methanol decreased signif-icantly at a C/F of 10 (w/w). The reason could be that at higher C/Fratio, methanol formation is hindered and the adsorbed methoxyspecies plausibly decompose into CO and hydrogen species. Thiscould explain the source of hydrogen species for the hydrogenationreactions that seem to have been promoted at a higher C/F ratio.
A plot of conversions of the various phenolics measured at C/Fratios of 2, 5, and 10 w/w is shown in Fig. 3b. It can be seen that
most of the phenolics were converted at a higher catalyst-to-feedratio. Even though the conversions increased with an increase inC/F ratio, the overall yield of the simple phenols and BTX was foundto decrease slightly from 4.66 wt.% at C/F of 5 (w/w) to 4.35 wt.% atC/F of 10 (w/w). The decrease in the overall yield was attributed topossible loss of aromatic hydroxyl groups as H2O due to the pro-motion of hydrogenation reaction at a C/F ratio of 10 (w/w). Fromthis study, it appears that maximum yield of simple phenols couldbe achieved with a C/F ratio of 6.5 (w/w). The conversion plot alsoindicates that catechols are by far the most reactive phenolic and4-ethylguaiacol is the least reactive with the surface of anataseTiO2. The order of the phenolics reactivity on the anatase is cate-chol > 4-methylcatechol > guaiacol > vanillin > isoeugenol > 2-meth-oxy-4-vinylphenol > 4-methylguaiacol > 4-ethylguaiacol (Fig. 3b). Itcan be inferred from the sequence that the type of side chainattached to the aromatic ring in guaiacols also affect its reactivity.The vinyl side chain appeared to be more reactive than methyl andethyl groups probably due to the presence of a p-bond.
The temperature was found to affect the production of simplephenols from the monomeric phenolics as shown in Fig. 4. Increas-ing the temperature from 450 �C to 650 �C at a constant catalyst-to-feed ratio of 5 (w/w) caused an increase in the yield through amaximum and then decreased slightly at 650 �C. The increase inphenols with temperature from 450 �C is primarily due to en-hanced devolatilization of the kraft lignin and the decrease in yieldof phenols at 650 �C could be because some phenols were trans-formed into aromatics (BTX) either thermally or catalytically. Onthe contrary, the total yields on phenols including BTX increasedlinearly from 1.8 wt.% at 450 �C to 4.86 wt.% at 650 �C. The
Fig. 4. The effect temperature on the yields of: (A) simple phenols and BTX, and (B) methanol and methanethiol produced in the pyrolysis of kraft lignin with anatase TiO2 at acatalyst-to-feed ratio of 5 (w/w).
Fig. 3b. The effect of catalyst-to-feed ratio on the conversion of some of the major monomeric phenolics identified in the pyrolysis of kraft lignin with anatase TiO2 at 550 �C.
Fig. 3a. The effect of catalyst-to-feed ratio on the yields of: (A) simple phenols and BTX, and (B) methanol and methanethiol produced in the pyrolysis of kraft lignin withanatase TiO2 at 550 �C.
formation of methanol was somehow unchanged from 450 �C to550 �C, but decreased at 650 �C. The lowest yield of methanethiolwas produced at 450 �C and the highest yield was at 550 �C. Itcould be that, at a high temperature, the methoxys adsorbed on
TiO2 were rather decomposed into CO than forming methanoland mathanethiol.
Other key minor products from catalytic pyrolysis of kraft ligninwere also analyzed based on their relative peak area percent in the
total ion chromatogram to ascertain the effect of temperature andcatalyst-to-feed ratio. The results revealed that the formation ofnaphthalene and PAH increased with increase in temperature.The amount of 1-naphthalenol also increased with increase in tem-perature from 450 �C to 550 �C, but decreased at 650 �C. Con-versely, the formation of 1H-indenol seem not to be affected bytemperature. The increase in temperature also appeared not to fa-vor CO2 and dimethyl sulfide formation. It can be inferred from thepeak area results that further increase in temperature beyond550 �C may have caused dehydroxylation of 1-naphthalenols intonaphthalenes and promoted condensation of aromatics into PAHs.In contrast to the temperature effect, the catalyst-to-feed ratio hada much stronger influence on the yields of 1H-indenol, 1-naphthal-enol, naphthalenes, PAHs and carbon dioxide according to theircorresponding peak area percent. For example, the catalytic pyro-lysis at 550 �C and a C/F ratio of 10 (w/w) generated more thantwice the yields of naphthalenes and PAHs from the catalyticpyrolysis at 650 �C and C/F of 5 (w/w). Consequently, the yieldsof 1H-indenol and 1-naphthalenol were lower in yields at a higherC/F ratio than at a higher temperature. Also, more CO2 was gener-ated at a high C/F ratio than at high temperature. Since pyrolysis at650 �C and a catalyst-to-feed ratio of 10 increased the formation ofundesirable aromatic hydrocarbons and PAHs, the catalytic pyroly-sis of lignin with anatase TiO2 would have to be conducted at mod-erate temperatures and a moderate catalyst ratio.
Considering the range of parameters investigated, increasingthe catalyst-to-feed ratio was found to produce a much narrowerproduct distribution than increasing the temperature and the cat-alytic effect was definitely stronger than the thermal effect and ex-plains why most of the multifunctional phenolics weredefunctionalized at a high C/F ratios. In a nut shell, the study sug-gests that pyrolysis of kraft lignin with anatase TiO2 at moderatetemperatures (<600 �C) and a catalyst ratio of 6.5 are suitable con-ditions for the production of simple phenols.
4. Conclusion
The results showed that anatase TiO2 is a very active catalyst forselective defunctionalization of monomeric phenolics from ligninpyrolysis. Catalyst-to-feed ratio, temperature, and the type of poly-morph of TiO2 were found to influence production of simple phe-nols. Defunctionalization of phenolics can be promoted by TiO2
without the use of hydrogen pressure. Since high conversions ofphenolics-to-phenols can be attained on anatase TiO2, a pyrolyticdepolymerization process that produces high yield of monomericphenolics from lignin could enhance the overall yield of simplephenols. Thus, high formation of char during lignin pyrolysis is achallenge that needs to be overcome.
Acknowledgement
The authors acknowledge funding support from the BNL Labo-ratory-Directed Research and Development Program, LDRD Project#12-024.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.biortech.2013.09.003.
References
Ahmed, A.Y., Kandiel, T.A., Oekermann, T., Bahnemann, D., 2011. Photocatalyticactivities of different well-defined single crystal TiO2 surfaces: anatase versusrutile. J. Phys. Chem. Lett. 2 (19), 2461–2465.
Alonso, D.M., Wettstein, S.G., Dumesic, J.A., 2012. Bimetallic catalysts for upgradingof biomass to fuels and chemicals. Chem. Soc. Rev. 41 (24), 8075–8098.
Amen-Chen, C., Pakdel, H., Roy, C., 2001. Production of monomeric phenols bythermochemical conversion of biomass: a review. Bioresour. Technol. 79 (3),277–299.
Bu, Q., Lei, H., Zacher, A.H., Wang, L., Ren, S., Liang, J., Wei, Y., Liu, Y., Tang, J., Zhang,Q., Ruan, R., 2012. A review of catalytic hydrodeoxygenation of lignin-derivedphenols from biomass pyrolysis. Bioresour. Technol. 124, 470–477.
Bui, V.N., Toussaint, G., Laurenti, D., Mirodatos, C., Geantet, C., 2009. Co-processing ofpyrolisis bio oils and gas oil for new generation of bio-fuels: hydrodeoxygenationof guaïacol and SRGO mixed feed. Catal. Today 143 (1–2), 172–178.
Choi, H.S., Meier, D., 2013. Fast pyrolysis of Kraft lignin—Vapor cracking overvarious fixed-bed catalysts. J. Anal. Appl. Pyrol. 100, 207–212.
de Wild, P., Van der Laan, R., Kloekhorst, A., Heeres, E., 2009. Lignin valorisation forchemicals and (transportation) fuels via (catalytic) pyrolysis andhydrodeoxygenation. Environ. Prog. Sustainable Energy 28 (3), 461–469.
Dorrestijn, E., Mulder, P., 1999. The radical-induced decomposition of 2-methoxyphenol. J. Chem. Soc. Perkin Trans. 2 (4), 777–780.
Gluckstein, J.A., Hu, M.Z., Kidder, M., McFarlane, J., Narula, C.K., Sturgeon, M.R., 2010.Final Report: Investigation of Catalytic Pathways for Lignin Breakdown intoMonomers and Fuels. ORNL/TM-2010/281; Other: ED1908000; RAED002; TRN:US201101%%298.
Goheen David, W., 1966. Hydrogenation of lignin by the noguchi process. LigninStructure and Reactions, vol. 59. American Chemical Society, pp. 205–225.
Hanaor, D.H., Sorrell, C., 2011. Review of the anatase to rutile phase transformation.J. Mater. Sci. 46 (4), 855–874.
Holladay, J.E., White, J.F., Bozell, J.J., Johnson, D., 2007. Top Value-Added Chemicalsfrom Biomass – Volume II—Results of Screening for Potential Candidates fromBiorefinery Lignin. PNNL-16983; Other: BM0102070; TRN: US200805%%262.
Hosoya, T., Kawamoto, H., Saka, S., 2008. Secondary reactions of lignin-derivedprimary tar components. J. Anal. Appl. Pyrol. 83 (1), 78–87.
Jackson, M.A., Compton, D.L., Boateng, A.A., 2009. Screening heterogeneous catalystsfor the pyrolysis of lignin. J. Anal. Appl. Pyrol. 85 (1–2), 226–230.
Jegers, H.E., Klein, M.T., 1985. Primary and secondary lignin pyrolysis reactionpathways. Ind. Eng. Chem. Process Des. Dev. 24 (1), 173–183.
Kamwilaisak, K., Wright, P.C., 2012. Investigating laccase and titanium dioxide forlignin degradation. Energy Fuels 26 (4), 2400–2406.
Kleinert, M., Barth, T., 2008a. Phenols from lignin. Chem. Eng. Technol. 31 (5), 736–745.
Kleinert, M., Barth, T., 2008b. Towards a lignincellulosic biorefinery: direct one-stepconversion of lignin to hydrogen-enriched biofuel. Energy Fuels 22 (2), 1371–1379.
Ma, Y.-S., Chang, C.-N., Chiang, Y.-P., Sung, H.-F., Chao, A.C., 2008. Photocatalyticdegradation of lignin using Pt/TiO2 as the catalyst. Chemosphere 71 (5), 998–1004.
Ma, Z., Troussard, E., van Bokhoven, J.A., 2012. Controlling the selectivity to chemicalsfrom lignin via catalytic fast pyrolysis. Appl. Catal. A 423–424, 130–136.
Machado, A.E.H., Furuyama, A.M., Falone, S.Z., Ruggiero, R., Perez, D.d.S., Castellan,A., 2000. Photocatalytic degradation of lignin and lignin models, using titaniumdioxide: the role of the hydroxyl radical. Chemosphere 40 (1), 115–124.
Nowakowski, D.J., Bridgwater, A.V., 2010. Analytical and fast pyrolysis studies oflignin. In: International Biomass Valorisation Congress, Amsterdam.
Nowakowski, D.J., Bridgwater, A.V., Elliott, D.C., Meier, D., de Wild, P., 2010. Ligninfast pyrolysis: results from an international collaboration. J. Anal. Appl. Pyrol. 88(1), 53–72.
Patwardhan, P.R., Brown, R.C., Shanks, B.H., 2011. Understanding the fast pyrolysisof lignin. ChemSusChem 4 (11), 1629–1636.
Perez, D.d.S., Castellan, A., Grelier, S., Terrones, M.G.H., Machado, A.E.H., Ruggiero, R.,Vilarinho, A.L., 1998. Photochemical bleaching of chemical pulps catalyzed bytitanium dioxide. J. Photochem. Photobiol. A 115 (1), 73–80.
Shen, D., Hu, J., Xiao, R., Zhang, H., Li, S., Gu, S., 2013. Online evolved gas analysis byThermogravimetric–Mass Spectroscopy for thermal decomposition of biomassand its components under different atmospheres: Part I. Lignin. Bioresour.Technol. 130, 449–456.
Thring, R.W., Katikaneni, S.P.R., Bakhshi, N.N., 2000. The production of gasolinerange hydrocarbons from Alcell� lignin using HZSM-5 catalyst. Fuel Process.Technol. 62 (1), 17–30.
Zakzeski, J., Bruijnincx, P.C.A., Jongerius, A.L., Weckhuysen, B.M., 2010. The catalyticvalorization of lignin for the production of renewable chemicals. Chem. Rev.110 (6), 3552–3599.
Zhao, Y., Deng, L., Liao, B., Fu, Y., Guo, Q.-X., 2010. Aromatics production via catalyticpyrolysis of pyrolytic lignins from bio-oil. Energy Fuels 24 (10), 5735–5740.
