Corresponding Author: Kelly Ross, Pacific Agri-Food Research Center, Agriculture and Agri-Food Canada, 4200 Highway 97,Summerland, BC, V0H 1Z0, Canada. Tel: +1-250-494-6411.
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Comparative Analysis of Pyrolysis Products from a Variety ofHerbaceous Canadian Crop Residues
Kelly Ross and Giuseppe Mazza
Pacific Agri-Food Research Center, Agriculture and Agri-Food Canada,4200 Highway 97, Summerland, BC, V0H 1Z0, Canada
Abstract: Interest in recovering and valorizing agricultural biomass residues has increased in recent years inresponse to emerging economic opportunities and the potential for more sustainable use of renewable and non-renewable resources. Agricultural crop residues are a major source of lignocellulose, with considerable potentialfor use as a renewable resource and lignin’s polyphenolic structure makes it a potential source of high valueproducts such as fine chemicals. To achieve this requires characterization of the chemical composition ofbiomass and in this study we determined the lignin and structural carbohydrate content of various sources ofagricultural biomass such as flax shives, wheat straw, wheat bran, triticale straw, triticale bran, barley straw, oatstraw and mustard straw along with selected lignin samples that were isolated using an environmentally benignionic liquid (1-ethyl-3-methylimidazolium acetate ([emim]Ac)). Pyrolysis-GC/MS was used to determine thethermal degradation products of the select biomass and lignin samples. The main phenolic compounds obtainedfrom pyrolysis included catechol, guaiacol, 4-vinyl guaiacol, eugenol, vanillin and isoeugenol. We quantifiedthe lignin content of the select biomass samples directly from pyrolysis-GC/MS measurements. Lignin valuesobtained from pyrolysis-GC/MS (Py-lignin) were compared against conventional acid insoluble lignin valuesand good correlation was found between lignin values obtained from pyrolysis-GC/MS and acid insoluble lignincontent within species (R =0.98). Also a good correlation was established between species (R =0.97). This work2 2
provides information that demonstrates that pyrolysis is a depolymerization technology capable of producinghigh value phenolic molecules from crop residues. This work also provides information demonstrating thatanalytical pyrolysis can be used to assess the lignin content in monocotyledonous and dicotyledonousherbaceous crop residues.
INTRODUCTION raised ethical concerns with the food for fuel debate.
The use of non-renewable resources as a source of harvesting is one of the largest sources of annuallyenergy and chemicals and their impact on the environment renewable biomass [4]. Herbaceous agricultural residueshas provided the impetus for the development of “green” do not compete directly with food production [5]. Canadaprocesses and the use of renewable resources as has 27 million ha of cropland and produces 75 millionreplacements for non-renewable resources in order to metric tons of agricultural crops [6]. Therefore a largecontribute to increased environmental, social and amount of agricultural residues are produced every yeareconomic sustainability. Forest products, agricultural in Canada. Consequently, herbaceous crop residues as ancrops and residues and animal and municipal wastes are alternative source to food crop biorefinerey feedstocksall sources of renewable biomass [1]. The use of have attracted considerable interest. agricultural food and feed crops such as maize and sugar As the utilization of herbaceous crop residues hascane to replace fossil fuels as an energy source attracted increasing attention, their thermochemicalin the United States and Brazil [2, 3] has affected conversion has received considerable study. Pyrolysisagricultural commodity markets and has subsequently of biomass can be described as the direct thermal
Herbaceous agricultural residues obtained after food crop
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decomposition of the organic matrix in the absence of with multivariate principal component analysis and partialoxygen to obtain an array of solid (char), liquid (tar) and least squares analysis has been used to assess changesgas products [4]. The plant cell walls comprising the in the lignin and carbohydrate composition of softwoodorganic matrix of biomass contain three major polymers: pulp fibres induced by kraft pulping [27]. The work ofcellulose (30-50%), hemicellulose (15-35%) and lignin Fahmi et al. [30] coupled Py-GC/MS with multivariate(10-30%); the proportions of these three polymers are principle component analysis and partial least squaresdependent on botanical source [6, 7]. The total complex of analysis to develop and validate a rapid screeningthese polymers is often referred to as lignocellulose and methodology for determining lignin content of ryegrassthe breakdown products of three biopolymers can be (Lolium) and fescue (Festuca) species. Absoluteutilized for several applications [8-11]. Cellulose can yield quantification (i.e. mass yields) of pyrolysis productssolvents (e.g. ethanol, acetone, butanol), 5-hydroxymethyl using internal standards has also been studied [21, 28]furfural and lubricants [1, 9, 12] and hemicellulose can however, accurate quantification of all GC detectableyield furfural, xylose and xylooligimers [1, 3, 9]. Lignin compounds is limited due to the large number of productsis a highly branched and substituted polyphenolic with different response factors [21]. Alves et al. [29]biopolymer formed from the polymerization of three types quantified the lignin content of pine and spruce woodof methoxylated phenylpropane molecules, specifically using Py-GC/MS by relating the ratio of the lignin derivedconiferyl, sinapyl and p-coumaryl alcohol [11, 13]. products over the total products (called Py-lignin). ThisDepolymerization of lignin into base chemicals for treatment was able to successfully estimate the acidindustrial production of food and industrial products, insoluble lignin content from Py-lignin measurements fromsuch as vanillin, sryingol, syringaldehyde, ferulic acid, various softwood species using a common model. Thevinyl-guaiacol, is one of the key areas for valorization of authors suggested that the model might be used for otherbiomass [4, 11, 14, 15] in a biorefinery setting where the softwoods and, in principle since it was not demonstrated,production of value-added chemicals in addition to that Py-lignin measurements might also be useful forbioenergy is essential for achieving economic viability quantifying lignin content in hardwoods and grasses.[16, 17]. Pyrolysis is considered to be a promising Small sample size, ease of sample preparation and rapidtechnology that can be used to convert biomass into analysis times are the main advantages of analyticalclean energy and valuable chemicals [4, 18]. pyrolysis over using conventional wet chemistry as a
Biomass pyrolysis is affected by many factors such method to determine lignin content. as heating rate, temperature and composition of biomass Lignin is an important constituent of herbaceous[18, 19]. Analytical pyrolysis (Py) combined with on-line biomass and it is of interest not only to determine itsgas chromatography (GC) separation and mass quantity but it is also important to characterize thespectrometry (MS) using electron ionization provides a pyrolysis products of lignin [31]. Due to the complexity ofsimple and reliable micro-scale model of the fast pyrolysis lignin and the difficulty of extraction, literature related toprocess and enables molecular characterization of the the pyrolysis behavior of lignin is limited. Howevercompounds derived from biomass and also provides recently there have been a studies performedinformation on the relative amount of the molecules characterizing the pyrolysis products of lignin extractedpresent in the biomass [20, 21]. Py-GC/MS has been used from various sources, such as wheat straw, moso bambooto study the effect of different factors on pyrolysis and rice straw with mild enzymatic acid hydrolysis [4, 19,products such as: 1) particle size [22]; physiological 31-33]. Nevertheless there is still limited knowledge of thematurity [23]; genetic modifications [24] and chemical pre- pyrolysis of lignin for production of fine chemicals. Totreatments [25]. Although a significant data set on the utilize lignin to produce fine chemicals, such as vanillin,chemical composition of the resulting products has been guaiacol, 4-vinyl guaiacol, syringol, etc, it is necessary toobtained, an efficient data comparison to evaluate the investigate the pyrolysis products of lignin. Recently,effect of different biomass types separately from ionic liquids (ILs) have received attention as promisingpyrolyzing conditions can be difficult and a comparison green solvents for selectively extracting lignin fromof the pyrolysis products from a wide selection of lignocellulosic biomass [34]. Briefly, ILs are organic saltsbiomass at common pyrolysis conditions is required [21]. that usually melt below 100 °C. They are non-flammable
Py-GC/MS has been used as an analytical tool to and recyclable solvents with extremely low volatility andassess content of carbohydrates and lignin in a variety of high thermal stability [35]. The work of Fu et al. [34]biomass types [26-30]. Analytical Py-GC/MS combined successfully used the ionic liquid (1-ethyl-3-
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methylimidazolium acetate ([emim]Ac)) to extract relatively MATERIALS AND METHODShigh purity lignin from wheat straw, triticale straw and flaxshives. Samples: Triticale straw and bran (×Triticosecale
Since analysis of components in various agricultural Wittm. ex A. Camus cv, AC Ultima) samples wereresidues is essential to understand their utilization provided by Agriculture and Agri-Food Canada,potential due to variations in compositions that are Lethbridge, Alberta. CPS wheat (Triticum aestivum L.),dependent on plant species, plant tissues and growth durum wheat (Triticum durum L.), barleyconditions [6, 36] the objectives of this work were to: (Hordeum vulgare L.), oat (Avena sativa L.) andcharacterize the pyrolysis products of various kinds of mustard (Brassica compestris L.) straws along withcrop residues; characterize the pyrolysis products of CPS wheat and rye (Secale Cereale L.) bran werelignin extracted with ionic liquid (1-ethyl-3- provided by Agriculture and Agri-Food Canada,methylimidazolium acetate ([emim]Ac)) from wheat straw, Saskatoon Saskatchewan. Flax shives (Linumtriticale straw and flax shives; and develop a common usitatissimum L.; cvs, CDC Bethune and Flanders) weremodel that is capable of estimating lignin content of a provided by Biolin Research Inc. Saskatoon,variety of herbaceous crop residues using Py-lignin Saskatchewan. The compositions of these samples aremeasurements. presented in Table 1.
Table 1: Acid Insoluble Lignin (AIL), Acid Soluble Lignin (ASL, Total Lignin (TL), Pyrolysis Lignin (Py-Lig) and Total Sugars Contents of Select Canadian Biomass Straws and Lignins
Sample Ash (%) AIL (%) ASL (%) Total Lignin (%) Py-Lignin (%) Glucans (%) Xylans (%) Galactans (%) Arabans (%) Mannans (%) Total Sugars (%)
The sample types are coded as follows: CPS wheat straw (WSC); durum wheat straw (WSD); barley straw (BS); oat straw (OS); mustard straw (MS); CPS wheat bran (WB); rye bran (RB);flax shives, cv. CDC Bethune (FSB), cv. Flanders (FSF); triticale straw, cv. AC Ultima (TSU), triticale bran, cv. AC Ultima (TB), lignin extracted from CPS wheat straw by ionic liquid 1-ethyl-3-methylimidazolium acetate, ([emim]Ac) (LW), lignin extracted from cv. AC Ultima triticale straw by ionic liquid [emim]Ac (LT) and lignin extracted from cv. CDC Bethune flax shives byionic liquid [emim]Ac (LF)Total sugars content (%) is the sum of glucans, xylans, galactans, arabans and mannans present in the biomass samples nd*= Ash content was not determined in these samples due to limited sample quantity
Table 2: Pyrolysis Products from Selected Biomass and Ionic Liquid Extracted Lignin Samples
Compound Peak # RT(min) MW WSC WSD BS OS MS WB RB FSB FSF TSU TB LW LT LF
Total Sum 35.61 35.76 34.33 27.58 38.63 31.12 23.52 42.24 35.97 36.98 24.56 54.22 56.92 48.93
The sample types are coded as follows: CPS wheat straw (WSC); durum wheat straw (WSD); barley straw (BS); oat straw (OS); mustard straw (MS); CPS wheat bran (WB); rye bran (RB);flax shives, cv. CDC Bethune (FSB) and cv. Flanders (FSF); triticale straw, cv. AC Ultima (TSU), triticale bran, cv. AC Ultima (TB), lignin extracted from CPS wheat straw by ionic liquid 1-ethyl-3-methylimidazolium acetate ([emim]Ac) (LW), lignin extracted from cv. AC Ultima triticale straw by ionic liquid [emim]Ac (LT) and lignin extracted from CDC Bethune flax shives by ionicliquid [emim]Ac (LF)C=carbohydrate, N=protein, Lg-H=p-hydroxyphenyl lignin, Lg-G=guaiacyl lignin, Lg-S=syringyl lignin, U-phenolic=undetermined phenolic source=Relative areas. The difference from 100 corresponds to non-diagnostic low molecular weight products and unidentified peaks a
Py-Lig (%) = Py-lignin was calculated as the ratio of the sum of the areas of the peaks from lignin products divided by the sum of the area of all used peaks (lignin, undetermined sourcephenolics, non-phenolics (i.e. carbohydrates and proteins) multiplied by 100%.
Milling of Samples: It has been reported that the particle pyrolysis products of the biomass presented in Table 2.size of a sample influence compositional analysis [6, 22]. For the experimental work correlating pryolysis ligninBiomass samples with larger particle sizes displayed (Py-Lignin) content with lignin content determined usinghigher cellulose, hemicellulose and lignin levels and a conventional wet chemistry, biomass samples with a widerlower ash level than the fraction with smaller particle sizes. range of particle sizes were tested. This allowed samplesBridgeman et al. [22] also found that the amounts of both possessing a wider range in lignin content to be tested.carbohydrate and lignin pyrolysis decomposition Wheat and triticale biomass samples with particle sizes >products were greater for biomass samples with larger 850µm and < 180µm were tested in addition to samplesparticle sizes. Therefore, the effect of particle size was with particle sizes between 180-850 µm. Flax biomasscontrolled in this work. Raw samples were milled using a samples with particle sizes > 850µm and < 150µm wereRetsch SM 2000 cutting mill (Retsch GmbH, Haan, tested in addition to samples with particle sizes betweenGermany) with a 2mm discharge screen. Milled samples 150-850 µm. (CPS wheat, durum wheat, barley, oat and triticale strawsand CPS wheat, rye and triticale brans) were separated Extractivesusing a Retsch AS 200 tap sieve shaker (Retsch GmbH, Removal and Determination: It has been reported thatHaan, Germany) with 20- and 80-mesh sieves shaken for water and ethanol extractives present in biomass samples5 min. The fraction with a particle size range of 180-850µm influence compositional analysis [6, 37]. With respect towas used for the experimental work characterizing the determination of acid insoluble lignin content, extractivespyrolysis products of the biomass presented in Table 2. are likely condensed or precipitated under the strongMilled flax shives were separated using 20 and 100 mesh acidic conditions used for the measurement of acidsieves; the fraction with the particle size range of 150-850 insoluble lignin, leading to a higher lignin level bias inµm was used for the experimental work characterizing the native biomass [6]. With regards to pyrolysis products
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characterization the work of Terron et al. [26] indicated content of the samples was determined as described bythat extractives affected the analysis of pyrolysis Sluiter et al. [40]. Briefly, the samples were ashed byproducts; extractive free material displayed higher complete combustion in a muffle furnace (Model F-A1730,amounts of carbohydrate product and lower amounts of Thermolyne Corporation, Dubuque, IA) equipped with alignin products. Therefore to ensure consistent data, temperature controller (Furnatrol II series 413, Thermolyneextractive-free biomass was used as the experimental Corporation, Dubuque, IA) running a temperature rampmaterial for the pyrolysis experiments. Removal of program as follows: 1) ramp from room temperature toextractives from the biomass was performed as indicated 105°C; 2) hold at 105°C for 12 min; 3) ramp to 250°C atby the NREL procedure [38]. As described by Tamaki and 10°C/min; 4) hold at 250 °C for 30 min; 5) ramp to 575 °C atMazza [6], a water extraction was performed for 24 h using 20°C/min; 6) hold at 575 °C for 180 min; 7) and drop to anda conventional Soxhlet apparatus, which included: hold at 105 °C until removed. The remaining residue in theextraction tube (85 mL); boiling flask (500 mL); heating crucible was taken as the ash content.mantle (Glas-Col, Terre Haute, IN). The reflux rate of thewater was adjusted to provide 4 to 5 siphon cycles per Lignin Isolation: Lignin extraction from triticale straw (cv.hour. After the water extraction was complete, an ethanol AC Ultima), CPS wheat straw and flax shive (cv. Bethune)extraction was performed for an additional 7 h. The reflux was carried out as described by Fu et al. [34]. Samplesrate of the ethanol was adjusted to provide 6-10 siphon (500 mg) of straw were incubated in 10 g of ionic liquid (1-cycles per hour. The extractive-free biomass was dried in ethyl-3-methylimidazolium acetate ([emim]Ac) undera vacuum oven at 35°C for 24 h and kept at -20°C until nitrogen gas with magnetic stirring at a constanttested. The extractives obtained via evaporation of the temperature (150°C) for 90 min. After incubation, thesolvent at 40°C using a rotary evaporator were dried in a suspension was diluted with 100 mL of 0.1 M NaOH andvacuum oven at 35°C for 24 h. The mass of the dried centrifuged at 11,600 X g for 20 min. The supernatant wasextractives was measured. decanted to a plastic vial (120 mL) and the residue was
Determination of Lignin, Structural Carbohydrates and funnel. An aliquot of 1.5 mL of the supernatant wasAsh Content: Acid insoluble and acid soluble transferred into a 2 mL centrifuge tube and the pH waslignin, structural carbohydrates and ash content of the adjusted to 2 with sulfuric acid. The centrifuge tube wassamples were determined according to the NREL stored in a refrigerator at 4°C overnight to allow forprocedure for determination of structural carbohydrates complete precipitation of lignin. The suspension was thenand lignin in biomass [39]. The samples were hydrolyzed centrifuged at 8030 X g for 5 min. The supernatant waswith a two-step acid hydrolysis procedure. The samples discarded and the lignin precipitate was obtained. In orderwere subjected to an initial hydrolysis step using 72% to assess the purity of the isolated lignin samples, theH SO at 30 °C for 1 h, followed by hydrolysis using 4% samples were subjected to acid insoluble lignin and acid2 4
H SO at 121 °C for 1 h. Monosaccharides in the soluble lignin content determination along with structural2 4
hydrolyzate were quantitatively measured with HPLC carbohydrate content analysis according to the NRELequipped with a refractive index detector (Agilent 1100, procedure [39].Agilent Technologies Inc. Palo Alto, CA). The HPLCanalysis was carried out using a Biorad Aminex HPX-87P Pyrolysis-GC-MS: Prior to the pyrolysis-GC-MS analysis,column (300×7.8mm, Bio-Rad Laboratories, Hercules, CA) the milled samples with various particle size ranges werewith a Cation H Refill Cartridge guard column (30×4.6mm, all ball-milled for 3min at 30 Hz (Mix Mill MM301, RetschBio-Rad Laboratories, Hercules, CA). The column GmbH, Hann, Germany) to ensure a homogeneous sampletemperature was 75°C and the mobile phase was and remove particle size as a variable as it affects heatMilliQ water operating at a flow rate of 0.5 mL/min. Acid transfer during pyrolysis. Analytical pyrolysis (Py-insoluble lignin was determined gravimetrically as the ash- GC/MS) was performed following the method of Alvesfree acid insoluble residue resulting from the hydrolysis. et al. [29] with some modification. Using a CDS pyroprobeAcid soluble lignin, the low molecular fraction of lignin 5150 (CDS analytical Inc. Oxford, PA) pyrolyzer equippedpresent the filtrate, was calculated from the measuring UV with a coil filament, the samples (100-200 µg), were heatedabsorbance at 320 nm of the liquid phase resulting from in a quartz tube at 650°C for 10s with a temperature risethe hydrolysis. An absorptivity of 30 Lg cm was used to time of 10 °C/ms. The interface temperature was 280°C. A-1 -1
convert absorbance readings to mass values [39]. Ash gas chromatograph (6890N Agilent Technologies) with a
washed with 500 mL of distilled water using a Buchner
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split ratio of 100:1 was used for compound separation. Tamaki and Mazza [6]. In comparison, the wheat, rye andThe injector and detector temperatures were both 280°C.The sample was carried onto a 30m X 0.32mm i.d. X 0.25µmDB-5 (J&W Scientific) column with helium gas. Thecolumn conditions were as follows: 1) the oventemperature was held at 50°C for 10 min; 2) ramped at4°C/min to 290°C; and held for 10 min at 290 °C. Theeluting compounds were detected with an Agilent5973N mass selective detector controlled by MSDChemstation (Agilent Technologies, Palo Alto, CA).Ionization was carried out at a 70eV electron impactvoltage in an ion chamber heated at 250°C. The massrange scanned was m/z 35-650. Peak identifications werecarried out on the basis of mass fragmentation patternsand by comparing the MS data with NIST and WileyLibraries. In accordance with Lou et al. [4] a peaksimilarity of 80% was required to integrate on the area ofthe peaks. Py-lignin was calculated with Chemstationsoftware (Agilent Technologies, Palo Alto, CA) as theratio of the sum of the areas of the peaks from ligninproducts divided by the sum of all used peaks (lignin,undetermined source phenolics, non-phenolics (i.e.carbohydrates and proteins) multiplied by 100%. Thistreatment of the data was in agreement with the work ofAlves et al. [29].
RESULTS AND DISCUSSION
Compositional Analysis of Crop Residues and IsolatedLignins: Table 1 presents the acid insoluble lignin (AIL),acid soluble lignin (ASL), total lignin (TL) (sum of AILand ASL), pyrolysis lignin (Py-Lig), total sugars and ashcontent of the CPS wheat straw (WSC); durum wheatstraw (WSD); barley straw (BS); oat straw (OS); mustardstraw (MS); CPS wheat bran (WB); rye bran (RB); flaxshives from cv. CDC Bethune (FSB) and cv. Flanders(FSF); triticale straw from cv, AC Ultima (TSU), triticalebran from cv. AC Ultima (TB) biomass samples along withlignin samples extracted from CPS wheat straw, cv. ACUltima triticale straw and cv. CDC Bethune flax shive with(1-ethyl-3-methylimidazolium acetate ([emim]Ac). Allresults are expressed on a dry basis. It can be seen thatthe CPS and durum wheat straw samples, barley straw,oat straw and mustard straw samples all exhibitedcomparable AIL, ASL and TL contents, ~ 19, 1.2 and 20%,respectively. The triticale straw sample displayed lowerASL and TL content, ~17 and 18%, respectively butcomparable ASL content (~1.2%). The total sugarscontent of all these straw samples was comparable, ~ 68%,except for the mustard straw which was considerablylower at ~57%. These results agree with the work of
triticale bran samples displayed lower AIL, ASL and TLcontents. Within the bran samples, the wheat bransamples exhibited higher AIL and TL content values, ~11and 13%, while the rye and triticale bran samplespossessed lower AIL and TL content values, ~8 and 10%.All bran samples displayed comparable ASL contentvalues ~1.7-1.8%. Also, the bran samples showed lowercarbohydrate contents, ~54-57 and contained the highestash contents, ~4-6%. The flax shives from cv. CDCBethune presented the highest AIL content ~27% whilethe flax shive cv. Flanders had an AIL content of ~21%,which are in agreement with values presented in theliterature [6, 11, 41]. The total sugars content for flax shivevarieties analyzed ranged from 56 - 60%, which is slightlyhigher than the values presented by Tamaki and Mazza[6]. The lignin samples extracted from CPS wheat straw,cv. AC Ultima triticale straw and cv. CDC Bethune flaxshives with IL [emim] Ac all showed increased AILcontents (~73-78%) and decreased total sugars content(~8-13%), which was expected. The ASL contents of thelignin samples extracted from CPS wheat straw and cv. ACUltima triticale straw showed slightly elevated ASL lignincontents.
Characterization of Pyrolysis Products of Crop Residuesand Isolated Lignins: In this work GC/MS was used toanalyze the components of the pyrolysate obtained fromthe various biomass and lignin samples. Representativepyrograms for triticale straw (cv. AC Ultima) and ligninisolated from triticale straw (cv. AC Ultima) using ionicliquid (1-ethyl-3-methylimidazoliumacetate ([emim]Ac) areshown in Figure 1. Table 2 shows the molecules obtainedfrom pyrolysis of the various biomass samples. Theidentified compounds were classed as various sourcesincluding: carbohydrate (C), protein (N), p-hydroxyphenyllignin (Lg-H), guaiacyl lignin (Lg-G), syringyl lignin (Lg-S)and an undetermined phenolic source (U-phenolic)[26,42,43]. The values in Table 2 are the relative areas foreach compound. The non-phenolic sum represents thetotal of all identified carbohydrate and protein basedproducts; phenolic sum represents the total of allidentified phenolic products; the lignin sum representsthe total of all identified lignin products; the Lg-H, Lg-Gand Lg-S sums represent the total of all identified ligninproducts from p-hydrophenyl, guaiacyl and syringyllignin units respectively; the U-phenolic sum representsthe total of all identified phenolic products from anundetermined source; and the total sum represents thesum of all identified non-phenolic products and all
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Fig. 1: Representative pyrograms of triticale (AC Ultima) from hydroxyphenyl (Lg-H) were the least abundant. Thestraw (A) and lignin extracted from AC Ultima ratio of Lg-G to Lg-S products for the CPS wheat straw,triticale straw by ionic liquid 1-ethyl-3-methylim cv. AC Ultima triticale straw and cv. CDC Bethune flaxidazolium acetate ([emim]Ac) (B). Some key peaks shives were ~ 7:1, 5:1, 13:1, respectively. The values of theof phenolic origin are labeled with peak numbers ratios differ to those presented in the literature for
identified phenolic products. The difference from 100 trend does exist indicating that the flax shive lignincorresponds to the fact that a peak similarity of 80% was contains a higher ratio Lg-G to Lg-S units compared torequired to identify and integrate the area of the peaks [4]; wheat straw, which is in agreement with values from thethe area associated with the unidentified peaks make up literature. Pyrolysis data may be more reflective of thethe difference [26]. Py-Lig (%), given in Table 1, was monomer composition of the samples since the maincalculated as the ratio of the sum of the areas of the peaks factor affecting release of monomer products duringfrom lignin products divided by the sum of the area of all pyrolysis is the amount of energy required for the ruptureused peaks (lignin,undetermined source phenolics, non- of primary bonds (C-C or C-O-C) [31]. With nitrobenzenephenolics (i.e. carbohydrates and proteins) multiplied by oxidation monomer identification is affected by the degree100%. of condensation of the lignin as it only determines the
There were approximately 72 identifiable pyrolysis phenolic monomers present in uncondensed lignin. Also,products identified. Key pyrolysis products originating nitrobenzene oxidation results are dependent on thefrom carbohydrate origin include: acetic acid, furfural, 2- choice of external standards [18]. hydroxy, cyclopenten-1-one, 2-cyclopentene-1,4-dione, The lignin samples isolated from biomass using the ILlevoglucose and D-allose. The relative area of the ([emim]Ac) contained the largest amounts of Lg-G, Lg-Spyrolysis product originating from a carbohydrate source and Lg-H products. The ratio of Lg-G to Lg-S products forwas proportional to the total sugars content of the the lignin samples isolated from CPS wheat straw, cv. AC
samples. Similarly, the lignin samples isolated frombiomass using ionic liquid (1-ethyl-3-methylimidazoliumacetate ([emim]Ac) contained the smallest relative areascorresponding to carbohydrate products. Interestingly,the relative area assigned to furfural was markedly large inthe lignin sample, isolated from flax shive (cv. CDCBethune) using ionic liquid (1-ethyl-3-methylimidazoliumacetate ([emim]Ac); this sample also displayed a markedlylarge xylan content (Table 1).
Lignin composition depends upon genetic origin. Inmonocotyledonous Angiospermae grasses, such aswheat, triticale, rye, oats and barley, lignin is composed ofguaiacyl (Lg-G), syringyl (Lg-S) and p-hydroxyphenyl(Lg-H) units [13]. Chemical analysis of wheat straw withnitrobenzene oxidation has shown that the Lg-G to Lg-Sratio is ~1:1 while the Lg-H units are present in loweramounts [11]. In dicotyledonous Angiospermae, plantssuch as flax, lignin is composed primarily of guaiacyl (Lg-G) and syringyl (Lg-S) units [13]. Nitrobenzene oxidationof flax shives has shown the presence of Lg-G, Lg-S andLg-H units; the Lg-G to Lg-S ratio is ~2:1 while the Lg-Hunits are present in the low amounts [11, 44]. Thepyrolysis results displayed in Table 2 indicate thatgenerally, the most abundant lignin pyrolysis productsoriginated from guaiacyl (Lg-G) lignin units. Ligninproducts originating from syringyl (Lg-S) lignin units werethe second most abundant and lignin products originating
chemical analysis with nitrobenzene oxidation although a
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Ultima triticale straw and cv. CDC Bethune flax shives factors were not used for quantitative evaluation,using the IL ([emim]Ac) were ~ 2:1, 2:1, 6:1, respectively. therefore in order to obtain absolute values a comparisonTerron et al. [26] indicated that the lignin rich fractions between Py-lignin and acid insoluble lignin content andobtained from alkaline extraction of wheat straw showed total lignin content was performed for all the cropa decreased ratio of Lg-G to Lg-S pyrolysis products, residues. The Py-lignin and acid insoluble lignin and totalwhich was attributed to enhanced removal of Lg-G units lignin contents for the samples determined by wetduring alkaline extraction. Further, the resulting chemistry are shown in Table 1. It should be noted herehemicellulose rich fraction obtained after alkaline that Table 1 does not contain all the experimental data thatextraction of lignin, showed increased levels of Lg-G units was used to generate the data presented in Figures 2-5.suggesting preferential association of hemicelluose with For the experimental work correlating Py-Lignin contentguaiacyl type lignin. This may offer an explanation for the with lignin content determined using conventional wetresults observed in the present work. chemistry biomass samples with a wider range of particle
Key lignin derived molecules obtained from pyrolysis sizes were tested. This allowed for samples possessing aof the crop residues include: guaiacol, 4-vinyl guaiacol, wider range in lignin content to be tested. CPS and durumeugenol, isoeugenol, vanillin, syringol and wheat straw and bran along with cv. AC Ultima triticalesyringaldehyde. 1,2 benzenediol (catechol) is phenolic straw and bran samples with particle sizes > 850µm and <molecule that was found in appreciable quantity in the 180µm were tested in addition to samples with particlepyrolysis product of all of the biomass samples, however sizes between 180-850 µm. Flax shives (cv CDC Bethune)its origin cannot be assigned to a lignin source. Wheat with particle sizes > 850µm and < 150µm were tested inand triticale bran samples only presented appreciable addition to samples with particle sizes between 150-850amounts of catechol and 4-vinyl guaiacol. In a biorefinery µm. Figures 2 shows the correlation betweenthe production of value-added chemicals other than conventionally determined lignin content and Py-ligninbiofuels and bioenergy derived from a generic feedstock content within species for the triticale straw and branis required for economic viability [16]. Herbaceous samples. Good correlation between Py-lignin results andbiomass is a potential source of a variety of bioactive those corresponding to both the acid insoluble lignin andphenolic molecules that can provide health benefits total lignin were obtained. The coefficient of correlationand/or be used as high value ingredients. Vanillin, obtained for the common triticale species was higheugenol, 4-vinyl guaiacol and syringaldehyde have been (R =0.98). Figure 3 shows the correlation betweenshown to possess anti-inflammatory activity [45-47]. conventional wet chemistry determined lignin content andCatechol has been shown to possess antimicrobial Py-lignin content between species within the same familyactivity against harmful intestinal bacteria such as C. (Poaceae) for the wheat, barely, oats and rye samples.difficile, C. perfringens and E. coli [48]. Guaiacol and Again good correlation between Py-lignin results andsyringol family compounds extracted from natural plant those corresponding to both the acid insoluble lignin andvinegar have been shown to regulate blood glucose level total lignin were obtained as the coefficient of correlationand improve blood flow [49]. Vinyl guaiacol and vanillin was high (R =0.92). In terms of botanical classification,are known flavoring agents in foods, beverages, or triticale, wheat, barley, oats and rye are classified asperfumes [15]. monocotyledonous angiosperms while flax and mustard
Correlating Py-Lignin with Conventional Wet Chemistry correlation between conventionally determined ligninDetermined Lignin Content: Investigation of content and Py-lignin content for the flax and mustardcomponents in various agricultural residues is essential to samples. Good correlation was obtained between the Py-understand their utilization potential. Lignin is an lignin results and those corresponding to both the acidimportant constituent of herbaceous crop residues and it insoluble lignin and total lignin (R =0.93). Table 3is of interest to determine the quantity of lignin in provides a summary of all of the regression equations. Forherbaceous crop residues. Direct quantification of the each sample group, the regression equations generatedlignin content of a variety of herbaceous crop residues describing the relationship between Py-lignin and acidwas obtained from Py-GC/MS pyrograms. Py-lignin values insoluble lignin along with Py-lignin and total lignin(sum of lignin pyrolysis products divided by the sum of exhibited slightly different slopes, ranging from 0.927-lignin and non-lignin pyrolysis products) is not a measure 1.061. Figure 5 shows the relationship betweenof the absolute lignin content as individual response conventionally determined lignin content and Py-lignin
2
2
are dicotyledonous angiosperms. Figure 4 shows the
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Table 3: Regression of pyrolysis (Py)-lignin versus acid insoluble lignin (AIL) and total lignin (TL) content of various crop residues Regression Equation: Regression Equation:
Biomass Type Py-Lignin vs. AIL Py-Lignin vs. TL nFlax and mustard straw Y=0.9696x + 1.52 Y=1.016x -0.506
R =0.92 R =0.93 242 2
Wheat, barley and oat straw Y= 0.9348x + 1.0349 Y= 0.9872x - 1.1775Wheat and rye bran R =0.92 R =0.92 212 2
Triticale straw and bran Y=1.0605x + 0.4939 Y=1.0839x - 1.3742R =0.98 R =0.98 152 2
All samples Y=0.9934x + 0.691 Y=1.0357x - 1.325R =0.97 R =0.97 602 2
n= sample size
Fig. 2: Pyrolysis (Py)-lignin content versus Fig. 4: Pyrolysis (Py)-lignin content versusconventionally determined acid insoluble lignin conventionally determined acid insoluble lignin(AIL) and total lignin (TL) content: triticale bran (AIL) and total lignin (TL) content: flax shive andand straw samples mustard straw samples
Fig. 3: Pyrolysis (Py)-lignin content versus Fig. 5: Pyrolysis (Py)-lignin content versusconventionally determined acid insoluble lignin conventionally determined acid insoluble(AIL) and total lignin (TL) content: wheat, lignin (AIL) and total lignin (TL) content: allbarley, oat, rye straw and bran samples samples
content for all the biomass samples. Despite the slight Py-lignin results and those corresponding to bothdifferences in slopes, the combination of the results of all the acid insoluble lignin and total lignin (R =0.97). This issamples (triticale, wheat, barley, oats, rye, flax and relevant because it shows that it is possible to estimatemustard) also provided a good correlation between both acid insoluble lignin content and total lignin content
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from the Py-lignin content for all the angiosperm large number of samples a common model may be able tospecies examined, both monocotyledonous and accurately describe the relationship between Py-lignindicotyledonous plants, using the same model. The work content and lignin content determined using conventionalof Alves et al. [29] speculated that the model they wet chemistry for grasses, softwoods and hardwoods. developed, which successfully correlated Py-lignincontent with acid insoluble lignin content, for two CONCLUSIONSsoftwood (gymnosperm) species might possibly be usedfor other softwoods too. They also speculated that Characterization of the chemical composition of cropdetermination of Py-lignin could also be used to quantify residues is essential to harnessing their utilizationthe lignin content of angiosperms such as, hardwoods potential. Also, effective use of crop residues depends onand grasses [29]. They questioned whether the slope of efficient isolation and depolymerization technologies. Inthe regression line would be similar or different from the this work, pyrolysis-GC/MS was used to determine theslopes generated for the spruce (0.86) and pine (0.71) thermal degradation products of various herbaceous cropsoftwood samples they tested. The present work on residues. This work provides information on the use ofangiosperms both monocotyldenous and dicotyledonous, pyrolysis as a depolymerization technology capable ofindicates that the slopes were comparable between the producing phenolic molecules from crop residues whichangiosperm plants modeled (~1.0). The slope values in the may serve as high value chemicals. The study showedcurrent work for angiosperms were slightly higher than for that analytical pyrolysis can be used to assess ligningymnosperms that have been examined. Also the content in monocotyledonous and dicotyledonousintercept of the regression equation obtained from herbaceous crop residues with a precision comparable tocombining all of the Py-lignin and AIL contents of the that of the conventional wet chemistry method forspruce and pine samples was 4.0132 while the intercept of determining acid insoluble lignin content. Lignin contentthe regression equation obtained from combining all of determined by conventional wet chemistry wasthe Py-lignin and AIL or TL contents of the angiosperm successfully estimated from Py-lignin measurements fromsamples of the present work was 0.691 and -1.325. This a variety of herbaceous crop residues using a commonresult may be due to the evolutionary history of the model.biosynthetic pathways of the plants. In dicotyledons,including plants such as flax and hemp and hardwoods, ACKNOWLEDGMENTSlignin is mainly derived from coniferyl and sinapylalcohols in various quantities. In monocotyledons, such The authors are grateful to the ABIP-NAFGEN andas cereals, lignin is composed of guaiacyl-syringyl-type ABIP-CBioNet networks for the financial support. Thelignin cores and 4-hydroxycinnamic acid groups [11, 13]. technical assistance provided by Lana Fukumoto andIn gymnosperms (i.e. softwoods), lignin structural Dongbao Fu along with the editorial input of Gordonelements are predominantly derived from more than 95% Neish is greatly appreciated.coniferyl alcohol [50]. It was noted by Torri et al. [21],whose work quantified the pyrolysis products of REFERENCESswitchgrass, sorghum, corn stover and poplar, thatpyrolysate composition can be affected by several factors 1. Reddy, N. and Y. Yang, 2005. Biofibers fromthat become crucial when analyses are performed with Agricultural Byproducts for Industrial Applications.a small sample amounts. Bridgeman et al. [22] indicated Trends in Biotechnol., 23(1): 22-27. that the amounts of both carbohydrate and lignin 2. Kim, S. and B.E. Dale, 2004. Global Potentialpyrolysis decomposition products were greater for Bioethanol Production from Wasted Crops and Cropbiomass samples with larger particle sizes. The particle Residues. Biomass Bioenergy, 26(4): 361-375. size of the samples used by Alves et al. [29] were of a 3. Orts, W.J., K.M. Holtman and J.N. Seiber, 2008.different size fraction (250-450 µm) than the samples Agricultural Chemistry and Bioenergy. J. Agriculturaltested in this work (150-850 µm); this may be responsible and Food Chemistry, 56(11): 3892-3899.for the differences in slope and intercept values. However, 4. Lou, R., S. Wu and G. Lv, 2010. Effect of Conditionsthe slopes and intercepts are within the same order of on Fast Pyrolysis of Bamboo Lignin. J. Analyticalmagnitude so it can be reasonably expected that for a and Applied Pyrolysis, 89: 191-196.
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