Characterization and Comparison of Fast PyrolysisBio-oils from Pinewood, Rapeseed Cake, and Wheat Straw Using 13C NMR and Comprehensive GC × GC
Abstract
Fastpyrolysis bio-oils are feasible energy carriers and a potential sourceof chemicals. Detailed characterization of bio-oils is essential tofurther develop its potential use. In this study, quantitative 13C nuclear magnetic resonance (13C NMR) combinedwith comprehensive two-dimensional gas chromatography (GC × GC)was used to characterize fast pyrolysis bio-oils originated from pinewood,wheat straw, and rapeseed cake. The combination of both techniquesprovided new information on the chemical composition of bio-oils forfurther upgrading. 13C NMR analysis indicated that pinewood-basedbio-oil contained mostly methoxy/hydroxyl (≈30%) and carbohydrate(≈27%) carbons; wheat straw bio-oil showed to have high amountof alkyl (≈35%) and aromatic (≈30%) carbons, while rapeseedcake-based bio-oil had great portions of alkyl carbons (≈82%).More than 200 compounds were identified and quantified using GC ×GC coupled to a flame ionization detector (FID) and a time of flightmass spectrometer (TOF-MS). Nonaromatics were the most abundant andcomprised about 50% of the total mass of compounds identified andquantified via GC × GC. In addition, this analytical approachallowed the quantification of high value-added phenolic compounds,as well as of low molecular weight carboxylic acids and aldehydes,which exacerbate the unstable and corrosive character of the bio-oil.
Detailed characterization of fast pyrolysis bio-oils originatedfrom pinewood, wheat straw, and rapeseed cake was studied using acombination of quantitative 13C NMR and GC × GC-FID/TOF-MS.
Introduction
Lignocellulosic biomass has emerged asa potential alternative source of specialty chemicals, gaseous andliquid fuels, and thermal energy. Among different processing routes,the transformation of lignocellulosic biomass into liquid bio-oilthrough the fast pyrolysis process is receiving increased attention.1−4 Fast pyrolysis bio-oil exhibits key advantages: it is produced athigh yields (up to 70 wt %), suitable for decentralized production,and practical for handling, transport, and storage and has a muchhigher energy density as compared to the parent biomass.5,6 However, fast pyrolysis bio-oils are complex mixtures of water andoxygenated compounds, including carbohydrates, heterocyclics, phenolics,carboxylic acids, aldehydes, ketones, esters, and alcohols.7,8 To fully realize the potential of the fast pyrolysis process, detailedknowledge on the chemical composition of the bio-oil is essential.7,9,10
Several analytical techniqueshave been developed to characterize pyrolysis bio-oil, and recentreview articles summarize the different analytical strategies developedso far.7,11 A powerful technique for bio-oil characterizationis nuclear magnetic resonance (NMR). NMR allows quantitative analysisof the complete bio-oil sample, rather than a fraction, and givesinformation on the type of chemical functional groups.12−14 In this way, NMR characterization of pyrolysis bio-oil can helpto determine optimum operational conditions for the pyrolysis processand to find the suitable (alternative) feedstocks for producing thedesired bio-oils, which are chemically stable over an extended timeperiod.15 However, the majority of compoundspresent in bio-oils have very low concentrations (<0.2 wt %), anda detailed compositional analysis requires the combination of severaltechniques.7 Comprehensive two-dimensionalgas chromatography (GC × GC) has proven to be very powerful forthe quantitative analysis of different types of pyrolysis oils16−19 and complex hydrocarbon matrices,20,21 as it providesdetailed information on the molecular composition of their volatilefraction. Overall high performance liquid chromatography (HPLC) andGC allow a primary qualitative and quantitative classification ofthe detectable components.22 Only a portionof the sample is identified and the higher MW components can be determined,e.g., by gel permeation chromatography (GPC) to be up 1000–2000Da without any further information regarding their structure. ComprehensiveLC also shows promise as demonstrated for the aqueous phase of a bio-oilby Tamasini et al.23 Nevertheless, NMRprovides insight into the different structures and functionalitiesof the various components and, hence, assists the interpretation ofthe results of the other analytical methods.
In this work, threedifferent bio-oils originating from pinewood, wheat straw and rapeseedcake are characterized by means of 13C NMR spectroscopyand GC × GC coupled to a flame ionization detector (FID) anda time-of-flight mass spectrometer (TOF-MS), and their compositionis compared. 13C NMR is used to characterize the majorfunctional groups, while GC × GC-FID/TOF-MS is applied to identifyand quantify individual volatile compounds.
According to theFood and Agriculture Organization of the United Nations (FAO), theproduction of wood pellets, which are used as fuel, reached 26 milliontons in 2014.24 This production was mainlydriven by increasing consumption in Europe to meet renewable energygoals and represented a growth of 16% over the previous year. Europeand North America accounted for almost all global pellet productionwith respective shares of 60 and 33%. Pine is one of the most importantsoftwoods for the production of pellets, contains low amounts of ashand nitrogen (≈1.0 and 0.2 wt % on a dry basis, respectively),and is adaptable to varied environmental conditions.25 The valorization of pinewood pellets, but also its industrialwaste (e.g., sawdust, shavings, waste ships, etc.), beyond renewableheat and electric power requires advanced technologies like fast pyrolysissupported by detailed compositional characterization of the bio-oil.As a distinctive feature, pinewood contains less hemicellulose andextractives and more cellulose, lignin, and nitrogen as compared withfast-growing agricultural residues.26
FAO’s statistics rank wheat as the sixth among the world agriculturalcommodities with a harvested area of 218 million hectares and a totalyield of 713 million tons of wheat flour in 2013.24 Wheat is one of the most important crops in Europe andAsia with respective shares of 8% and 14% of the global harvestedarea. Although the yield of wheat straw depends on specific varietiesand is widely affected by agronomic and climatic factors, an averageratio of 1.3 kg of straw per kg of grain is found for the most commonvarieties.27 Assuming a grain-to-flourmass ratio of 1.3 results in an annual yield of ≈1200 milliontons of wheat straw in 2013.27 These figuresdemonstrate the potential of wheat straw as a renewable source offuels and chemicals via fast pyrolysis. In addition, wheat straw isalso representative of residues from other fast-growing agriculturalresidues with high ash content (≈8 wt %) like barley, rice,and oats.26
According to the latestfigures of the EU Vegetable Oil and Protein Meal Industry FEDIOL,the total annual production of rapeseed in 2014 was 69 million metrictons worldwide resulting in a yield of vegetable oil of 27 millionmetric tonnes.28 Around 90% of the totalproduction of rapeseed is harvested in Europe. Rapeseed cake is themajor byproduct in the production of vegetable oil by cold pressingof rapeseed. Together, cellulose, hemicellulose, and lignin accountfor less than one-fourth of the dry mass of rapeseed cake. The remainingportion consists of crude protein, triglycerides, other extractives,and ashes.29,30 Rapeseed cake is valorized asanimal feed, because of the high content of proteins and triglycerides.However, the valorization of rapeseed cake via fast pyrolysis hasbeen also considered,30 and detailed informationon the chemical composition of the rapeseed cake bio-oil is thus essential.Among the starting materials of the studied bio-oils rapeseed cakecontains the lowest amount of oxygen (≈35 wt %) and the highestamount of nitrogen and sulfur (≈5.0 and 1.0 wt %, respectively).29,30
In this work, we present a detailed characterization studyof these three distinct bio-oils using a combination of quantitative 13C NMR and GC × GC-FID/TOF-MS. 13C NMR spectroscopyis used to characterize the major functional groups. The 13C NMR spectra are integrated over spectral regions to determine thepercentages of carbons in functional groups based on chemical shift.The identified components from GC × GC analysis are grouped accordingto their organic carbon number and organic functionality, leadingto remarkable differences in chemical composition and stability betweenthe three types of bio-oils. This knowledge provides practical guidelinesfor improved bio-oil upgrading strategies.
Materialsand Methods
Feedstocks
The bio-oils characterized in this studyoriginated from pinewood (PW), wheat straw (WS), and rapeseed cake(RC). Bio-oils were produced in a rotating cone reactor (RCR) fastpyrolysis plant at different temperatures. PW bio-oil was producedat 500 °C, while WS and RC bio-oils were produced at 480 and550 °C, respectively. Detailed descriptions of the RCR fast pyrolysissetup and the applied experimental conditions have been previouslyreported.31
Elemental Analysis
Elemental analysis was carried out with a Flash EA2000 elementalanalyzer (Interscience, Belgium) equipped with a thermal conductivitydetector (TCD). The elemental composition of each bio-oil was derivedbased on three repeat analyses. Uncertainties on the amount of carbon,hydrogen, oxygen, and nitrogen detected with this method were withinthe vendor specifications.
Nuclear Magnetic Resonance (NMR) Analysis
All bio-oil samples were freeze-dried for 12 h and kept under vacuumovernight. Then 30–50 mg of dried bio-oil was completely dissolvedin 600 μL of deuterated DMSO-d6 (dimethylsulfoxide-d6). The 13C NMRspectra were recorded on a Varian 400-MR spectrometer at a resonancefrequency of 100.614 MHz using a 5 mm broadband probe. The solventsignal of 39.52 ppm was used as the internal reference. The 13C NMR spectra were acquired with 45° pulse angle, proton decoupling,sweep width of 25000 Hz, and corresponding acquisition time of 1.311s. Acquisition of 10 000 transients using a 5 s pulse delayresulted in a good signal-to-noise ratio after 17.5 h of total timeof measurement per sample. All experiments were performed at 25 °C.The spectra were processed using MestReNova to perform baseline correctionsand integrations.
The accuracy of the NMR analysis depends uponseveral factors, such as signal-to-noise ratio, decoupling, relaxationdelay, line shape consideration, and baseline correction. Measurementsas accurate as ±5% can be achieved when the above-mentioned factorsare optimized.12,32 For this purpose, reasonablesignal-to-noise ratios were achieved to provide adequate recoveryof the signal. In addition, proton decoupling was used to avoid nuclearOverhauser enhancement (NOE) of the 13C signal from attachedprotons.
Two-Dimensional Gas Chromatography (GC × GC) Analysis
SamplePreparation
Bio-oil samples were dissolved in tetrahydrofuran(THF). Dibutyl ether and fluoranthene were used as internal standards.Samples were stored in a refrigerator at temperatures in the rangeof 3–5 °C. The prepared samples were subsequently analyzedby both GC × GC-FID and GC × GC-TOF-MS.
GC ×GC-FID/TOF-MS Setup
GC × GC analysis were carried outwith a Thermo Scientific TRACE GC × GC (Interscience, Belgium).The columns (a nonpolar/medium polar column set) and the modulator(a two stage cryogenic modulator) were positioned together in a singleoven. Both columns were connected to a piece of deactivated fusedsilica column (Rxi Guard, 0.1 m × 0.25 mm, Restek) by means ofa SilTite metal ferrule from SGE. The columns combinations placedin the same oven are described in Table 1.
| combination | first column | second column |
|---|---|---|
| 1 | MXT-1a 60 m long ×0.25 mm I.D. × 0.25 μm | BPX-50b 2 m long × 0.15 mm I.D.× 0.15 μm df |
| 2 | RTX-1 PONAa 50m long × 0.25 mm I.D. × 0.5 μm df | BPX-50 2 m long × 0.15 mm I.D. × 0.15 μm df |
aDimethyl polysiloxane(Restek).
b50% phenyl polysilphenylene-siloxane(SGE).
Two different detectorsmounted on different GC × GC setups were used. The detectorswere a FID and a TOF-MS. Table 2 gives a summary of the GC × GC settings. Parameterschanged depending on the used column combination as well as the bio-oilsample. The concentration of the internal standards was optimizedto give peak highs comparable to those of the quantified compounds.Columns combination, split flow, and temperature program were tunedto obtain a compromise between enhanced resolution in the first dimensionand minimization of wrap around in the second dimension.
GC × GC Settings for FID Analysisa
| detector | FID | TOF-MS, 25–500 amu | ||||
|---|---|---|---|---|---|---|
| origin of the bio-oil | pine wood | wheat straw | rapeseed cake | pine wood | wheat straw | rapeseed cake |
| column combination | 1 | 1 | 2 | 1 | 2 | 2 |
| injector, temperature [°C] | PTV, 350 | PTV, 350 | PTV, 300 | Split/Splitless,300 | Split/Splitless, 300 | Split/Splitless,300 |
| split flow [mL min–1] | 30 | 10 | 50 | 150 | 10 | 12 |
| carriergas [mL min–1] | 2.1 | 2.1 | 2.1 | 3.5 | 2.9 | 2.9 |
| oven temperature program [°C] | –25→350 | –25→350 | –40 →300 | –40→300 | –25→350 | –40→300 |
| heating rate [°C min–1] | 3 °C min–1 | |||||
| modulationperiod [s] | 7 | |||||
| detector acquisition rate [Hz] | 100 | 100 | 100 | 30 | 30 | 30 |
aColumn combination 1 = MXT-1/BPX-50,column combination 2 = RTX-1 PONA/BPX-50.
GC × GC–FID/TOF-MS Data Acquisitionand Quantification
Data acquisition and processing were carriedout using Thermo Scientific’s Chrom-Card data system for theFID and Thermo Scientific’s XCalibur software for the TOF-MS.The raw GC × GC-FID data files were exported as CDF files andimported into GC Image software (Zoex Corporation, USA). With theaid of the GC Image software the contour plotting, retention timemeasurement, peak fitting and blob integration were performed. Eachblob was tentatively identified based on both their chemical groupand number of carbon atoms. The combined information from the patternin the chromatogram obtained by the orthogonal separation of GC ×GC-FID and the National Institute of Standards and Technology (NIST)library MS confirmation was used for the tentative identificationof the peaks. The mass fraction wt %i ofeach compound was calculated using the mass fraction of the internalstandard (3-chlorothiophene) wt %st, peak volumes obtainedand the response factor relative to methane:
1where fi is the relative responsefactor for compound i, Vi is the peak volume of compound i, fst is the relative response factorfor the internal standard, and Vst isthe peak volume of the internal standard. The response factor of eachcompound is calculated by means of eq 2 which is based on the effective carbon number approach:33,34
2where Mi is the molar mass of compound i, MCH4 is the molar mass ofmethane (the chosen reference compound), and Ci,eff is the effective carbon number of compound i. The effective carbon number is approximately equal tothe carbon number of the compound in the case of hydrocarbons. Foroxygen containing compounds a correction method is applied. The effectivecarbon number is calculated as a function of both the carbon numberof the compound and the type of oxygen containing functional groups.The effective carbon number is calculated by means of eq 3:35
3where Ci is the carbon number of compound i and n is the correction value for functional groups(n = 1 for aldehydes, n = 1 formonoethers, n = 0.5 for primary alcohols, etc.).It has been demonstrated that the experimental response factors ofoxygen compounds agree well with the ones calculated based on theeffective carbon number approach.35 Theagreement between the calculated and the experimental effective carbonnumber for oxygen containing compounds has also been experimentallyvalidated elsewhere.16
Resultsand Discussion
Elemental Composition
As can beseen from Table 3 thecontents of carbon and hydrogen, and consequently the molar H/C ratiosof PW and WS bio-oils were comparable. However, the oxygen and nitrogencontents of PW and WS bio-oils differed notably. The higher nitrogencontent of WS bio-oil is consistent with the typical nitrogen contentof such a feedstock.36 RC bio-oil stoodout because of its high carbon and hydrogen contents, and its remarkablylow oxygen content which was less than half of the corresponding oxygencontents of the other investigated bio-oils. The relatively low molarO/C ratio for the RC bio-oil is consistent with the presence of significantamounts of triglycerides and high molecular weight triglyceride-derivedpyrolysis products, e.g., fatty acids with molar O/C ratios of ≈0.1.In addition, RC bio-oil showed a high nitrogen content which was expectedbecause crude protein is the major constituent of the starting material.30
Elemental Compositionof the Bio-Oilsa
| origin ofbio-oil | C (wt %) | H (wt %) | O (wt %) | N (wt %) | O/C molar ratio | H/C molar ratio |
|---|---|---|---|---|---|---|
| pinewood | 44.0 ± 0.45 | 7.46 ± 0.11 | 48.0 ± 0.17 | 0.092 ± 0.002 | 0.82 | 2.0 |
| wheat straw | 45.9 ± 2.48 | 7.69 ± 0.11 | 41.3 ± 2.37 | 2.19 ± 0.26 | 0.68 | 2.0 |
| rapeseedcake | 65.5 ± 0.21 | 9.79 ± 0.02 | 18.7 ± 0.01 | 5.62 ± 0.01 | 0.21 | 1.8 |
aEach reported value along with itscorresponding standard deviation corresponds to three repeated analyses.
The energy contents of PW andWS bio-oils, assessed by their higher heating value (HHV) accordingto the correlation in eq 4,37 were 19 and 21 MJ kg–1, respectively. This correlation was validated for a wide range ofelemental compositions (C 0.00–92.3; H 0.43–25.2; S0.00–94.1; O 0.00–50.0; N 0.00–5.60; ash 0.00–71.4wt %).37 Additionally, this correlationhas been commonly used to assess the HHV of bio-oils as can be seenfrom recent publications.22,38−40Table 3 shows thatthe elemental compositions of the studied bio-oils are within thelimits of the correlation, except for the nitrogen content of rapeseedcake bio-oil (5.62 wt %) which slightly exceeds the upper limit of5.60 wt % by 0.36%. The typical HHV range for bio-oil is 14–19MJ kg–1.22 Based on theirenergy content, PW and WS bio-oils could replace heavy fuel oil. Ahigh energy content is a distinctive feature of the bio-oils fromthe RCR fast pyrolysis plant.31 This highenergy content is correlated to the moisture content in the feedstock,which is reduced to less than 10 wt %.
4where, C, H,S, O, N, and A represents carbon, hydrogen, sulfur, nitrogen, andash contents of bio-oils, respectively.
RC bio-oil showed acalculated HHV of 32 MJ kg–1. This remarkably highenergy content has also been reported for several bio-oils obtainedfrom the fast pyrolysis of rapeseed cake at temperatures exceeding500 °C.29,30,41,42 At these operation conditions, the raw bio-oilfrom rapeseed cake separates spontaneously into an organic fractionand an aqueous fraction. The organic fraction, which is the one characterizedhere, is expected to contain significant amounts of triglyceridesand fatty acids (cold pressing reduces the oil content of the rapeseedcake from ≈40–45 wt % to ≈14–15 wt %)28 along with products from the pyrolysis of otherethanol extractives (lipids, waxes, and resins). Based on its energycontent and elemental composition, RC bio-oil appears to be a goodcandidate for hydroprocessing into transportation fuels. However,this application requires removing a considerable part of the nitrogen.
13C NMR Analysis
NMR analysis of complex mixtures,such as pyrolysis bio-oil, offers a reasonable trade-off between functionalgroup identifications and analytical measurements. The advantage ofNMR analysis is that the whole bio-oil sample can be dissolved ina suitable solvent and a quantitative assessment of the chemical functionalgroups can be determined by integration of the defined regions ofspectra.12,15 The 13C NMR spectra of the bio-oilsare depicted in Figure 1. An overview of the carbon content as a percentage within a givenchemical shift range is summarized in Table 4. The assignments of the 13C NMRspectra are based on the works of Ingram et al.43 and Joseph et al.12 which provideinformation on the types of chemical functional groups in bio-oils.The 13C NMR spectra were divided into five chemical shiftregions. The region 1–54 ppm, excluding the DMSO solvent, correspondsto alkyl carbons. The alkyl hydrocarbon region accounts for energycontent, which is of primary interest when bio-oil is used as a fuel.This region can be further subdivided into primary carbons (6–24ppm) and secondary/tertiary carbons (24–34 ppm).
13C NMR spectra of the bio-oils obtained from(a) pinewood, (b) wheat straw, and (c) rapeseed cake.
Quantitative Analysis of the 13C NMR Spectra of the Bio-Oils Derived from Pinewood, WheatStraw, and Rapeseed Cake via Fast Pyrolysisa
| carbon content in each spectrum (%) | ||||
|---|---|---|---|---|
| type of carbon | chemical shifts (ppm) | pinewood bio-oil | wheat straw bio-oil | rapeseed cake bio-oil |
| alkyl carbons (total) | 1–54 | 19.78 | 35.73 | 81.83 |
| primary alkyl carbons | 6–24 | 13.78 | 20.44 | 16.64 |
| secondary/tertiary alkyl carbons | 24–34 | 5.55 | 15.18 | 64.81 |
| methoxy/hydroxy | 54–70 | 29.46 | 14.73 | 1.27 |
| carbohydrate | 70–103 | 26.64 | 5.56 | 0.17 |
| aromatic (total) | 103–163 | 11.03 | 30.14 | 12.09 |
| aromatic (syringyl) | 110–112 | 0.27 | 0.68 | 0.19 |
| aromatic (guaiacyl) | 112–125 | 6.32 | 16.03 | 1.37 |
| aromatic(general) | 125–163 | 4.44 | 13.43 | 10.54 |
| carbonyl | 163-215 | 13.10 | 12.37 | 4.64 |
aThe differenttype of carbon compounds present in the bio-oils are grouped accordingto their chemical shift range. Spectra were obtained at 25 °Cin DMSO-d6 at 100.614 MHz on Varian 400-MRspectrometer using inverse gated decoupling to avoid NOE effects.The strong DMSO solvent resonances at 39.5 ppm were excluded fromthis analysis.
Theregion between 54 and 70 ppm of the 13C NMR spectra correspondsto carbons adjacent to a heteroatom, mostly oxygen in ethers or alcoholsas well as carbon adjacent to nitrogen. This region provides informationon the oxygen or nitrogen functions present in bio-oils, e.g., lignin-derivedhydroxyl- and methoxy-phenols. Carbons adjacent to oxygen in carbohydratesresonate in the 70–103 ppm region of the 13C NMRspectra. The next integrated region is between 103 and 163 ppm andrepresents aromatic, including heteroaromatic, e.g. furans, and alkenecarbons in the bio-oils. The aromatics contents are important forsynthetic modification of bio-oils. This region was further subdividedbetween 110 and 112 ppm (syringyl carbon) and 112–125 (guaiacylcarbon). The final portion of 13C NMR spectra is the downfieldend between 163 and 215 ppm, which represents the carbonyl carbons.This spectral region is representative for acids, esters, ketones,and aldehydes.
The content of alkyl carbons of the studied bio-oilsshows the following order: PW bio-oil < WS bio-oil < RC bio-oil.More than 80% of the total carbon in RC bio-oil was found in the alkylregion. The proportion of alkyl carbons in RC bio-oil was about twicethat of alkyl carbons in PW bio-oil and approximately four times thatof alkyl carbons in WS bio-oil. Primary, secondary, and tertiary carbonsaccount for more than 98% of the alkyl carbons, thus the secondary/tertiary-to-primaryalkyl carbon ratio provides an indication of the average size of thealkyl chain. The secondary/tertiary-to-primary alkyl carbon ratiofor PW and WS bio-oils were respectively 0.43 and 0.74, while a ratioas high as 3.9 was found for RC bio-oil. These results indicate thatalkyl carbons in PW and WS are mostly in methyl groups, while thealkyl carbons in RC are alkyl chains with C ≥ 5. The 13C NMR and the elemental analyses (see Table 3) can be combined to provide further evidenceregarding the size of the alkyl chain in RC bio-oil. A hydrogen balanceshows that an alkyl chain size of C = 4.4 would be required if allof the hydrogen quantified by elemental analysis is attached to alkylcarbons.
Approximately 30% of the total carbon in PW bio-oiland 15% of the total carbon in WS bio-oil were found to be in themethoxy/hydroxy region. In contrast, only around 1% of the total carbonin RC bio-oil was found in that region. These results are consistentwith the higher lignin content of PW (≈25 wt % on a dry basis)when compared with the lignin content of WS (≈18 wt % on adry basis) and that of RC (≈4 wt %, on a dry basis).26 However, the lignin content is most likely relatedto the methoxy groups attached to the aromatics. The difference betweenPW and WS in terms of cellulose and hemicellulose contents is lesspronounced. Both, cellulose and hemicellulose pyrolysis products couldsubstantially contribute to the presence of hydroxy carbons.
A pattern similar to the one for methoxy/hydroxy carbons was observedfor carbohydrate carbons. Approximately 27% of the total carbon inPW bio-oil and 6% of the total carbon in WS bio-oil were found inthe carbohydrate region. Notably, the content of carbohydrate carbonsin RC bio-oil was found to be negligible. However, the mineral contentof the feedstock has a considerable effect on the yield and compositionof bio-oil.44 As soon as the pyrolysisproducts are formed, they can interact with catalytic minerals inthe residual solid. The presence of metal cations leads to the increasedformation of light oxygenates and pyrolytic water at the expense oflevoglucosan formation.44 While PW typicallycontains less than 1.0 wt % of ashes, WS contains approximately 8wt % (on a dry basis) which supports the consistency of the current 13C NMR analysis for carbohydrate carbons.26
Carbons corresponding to the aromatic/alkene regionwere more abundant in WS bio-oil, i.e., about 30% of the total carbon.Similar contents of aromatic/alkene carbons, i.e., around 10% of thetotal carbon, were found for the PW and RC derived bio-oils. The amountof syringyl carbons in PW bio-oil was low compared with its guaiacylcarbons content. This result is consistent with the respective proportionsof syringyl and guaiacyl units in pinewood lignin.45 Similar amounts of syringyl and guaiacyl carbons wouldbe expected in the WS bio-oil;46 however,results showed the same trend as for PW bio-oil. A plausible explanationfor this result is the high reactivity of the methoxyl groups47 associated with syringyl carbons, which is furtherpromoted by the minerals in WS. Yet, a significant amount of the carbonin this region was not identified either as syringyl or guaiacyl carbons(≈5% of the total carbon in PW bio-oil and 13% and 10% of thetotal carbon in WS bio-oil and RC bio-oil, respectively). The remainingpart in PW bio-oil and WS bio-oil is most probably composed by carbonsin heterocyclic aromatics such as furans. On the other hand, the contentof both syringyl and guaiacyl carbons in RC bio-oil is low which isin line with the low lignin content of the originated feedstock. Mostof the carbon corresponding to the aromatic/alkene region in RC bio-oilis probably composed by alkene carbons in the unsaturated fatty acidschains and their pyrolysis products. Rapeseed cake triglycerides arecharacterized by high contents of polyunsaturated fatty acids (mostlyoleic, linoleic, and linolenic).48
One of the most important results of the 13C NMR analysisof bio-oils is the carbonyl carbon content. Aldehydes and ketonesderived primarily from cellulose and hemicellulose, as well as aceticacids derived from the acetyl groups of the hemicellulose are expectedin PW and WS bio-oils. On the other hand, esters from triglyceridesand carboxylic acids are expected in RC bio-oil.42 The combination of phenolics and carboxylic acids leadsto condensation reactions which are detrimental for the quality ofthe bio-oil upon storage.49 PW bio-oiland WS bio-oil showed comparable amounts of carbonyl carbons correspondingto ≈10% of their total carbon. Carbonyl carbons accounted for≈5% of the total carbon in RC bio-oil. Again for this bio-oil 13C NMR and elemental analysis (see Table 3) results provide information on the watercontent of bio-oil. An oxygen balance showed that a maximum watercontent of 12 wt % would be present in this bio-oil if the remainingoxygen, i.e., oxygen not attached to the carbonyl group, is formingwater. The actual water content is expected to be <12 wt % as hasbeen experimentally found for bio-oil from the fast pyrolysis of rapeseedcake at 550 °C.30
Summarizing, 13C NMR revealed that PW bio-oil contains mostly methoxy/hydroxy,carbohydrate, and alkyl carbons. Aromatic carbon, mostly guaiacyland heteroatomic, along with short-chain alkyl carbons are the majorcarbon types in WS bio-oil. RC bio-oil is very rich in long chainalkyl carbons.
GC × GC-FID/TOF-MSAnalysis
The employed GC × GC-FID/TOF-MS analysis providescomplementary information to the elemental and 13C NMRanalysis of the three investigated bio-oils. The obtained GC ×GC-FID chromatograms are shown in Figure 2. It can be noted that different groups ofchemical compounds were identified by using the orthogonal separationof the GC × GC method, while the internal standards, i.e., 3-chlorothiopheneand fluoranthene, were adequately separated from the other compounds.The quality of the separation between peaks was assessed by the two-dimensionalresolution (RS2D) which has been defined for a pair ofcompounds A and B in terms of the peak width along each dimension(ωA1, ωA2, and ωB1, ωB2) and the difference in retention times foreach dimension (Δtr1 and Δtr2) as follows:50
The separation between peaks is considered acceptable if the two-dimensionalresolution is higher than 1 and good if its value is higher than 1.5.For the three studied bio-oils, two-dimensional resolutions higherthan 1.5 were calculated from the GC × GC chromatograms.
GC × GC-FIDchromatograms for (a) pinewood-based bio-oil, (b) wheat straw-basedbio-oil, and (c) rapeseed cake-based bio-oil. Some representativecompounds are highlighted. The complete list of plausibly identifiedcompounds, classified according to their organic functionality, ispresented in Table S1 of the Supporting Information.
The total number of identifiedpeaks that were subsequently quantified in the three different bio-oilsamples amounted to 256. Those 256 peaks corresponded to 209 compoundsas some of these compounds were present either in two or the threestudied bio-oils. From those peaks, 74 corresponded to PW bio-oil,112 to WS bio-oil, and 70 to RC bio-oil. The identified compoundsaccounted for 22.6 wt % of PW bio-oil, 17.2 wt % of the WS bio-oil,and 12.3 wt % of the RC bio-oil. As can be seen from Table 5, most of the identified compoundsin PW bio-oil exhibited carbon numbers from C2 to C9; 32% of thatfraction was C2, while 38% was C6. The identified fraction in WS bio-oilhad carbon numbers from C2 to C20. C2, C3, and C6 compounds were themost abundant in WS bio-oil with 30%, 21%, and 18% of the identifiedfraction, respectively. The identified fraction in RC bio-oils presentedcarbon numbers from C4 to C19, with C4 and C18 as the most abundantones, i.e., 17% and 11% of the identified fraction, respectively.
Detailed Compositionof the Bio-Oils (wt %) by Group Type and Carbon Number Obtained viaGC × GC-FID/TOF-MS for Pinewood-Derived Bio-Oil (PW), Wheat Straw-DerivedBio-Oil (WS), and Rapeseed Cake-Derived Bio-Oil (RS)a
| carbon number | ||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| group type | bio-oil | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 |
| NA OX | PW | 7.18 | 2.14 | 0.36 | 0.29 | 0.43 | 0.13 | 0.03 | 0.03 | |||||||||||
| WS | 5.09 | 3.58 | 1.16 | 0.39 | 0.82 | 0.33 | 0.04 | 0.003 | 0.01 | 0.03 | 0.07 | 0.05 | 0.04 | 0.02 | 0.004 | |||||
| RS | 0.88 | 0.05 | 0.30 | 0.14 | 0.07 | 1.34 | ||||||||||||||
| HTC OX | PW | 0.62 | 0.64 | 0.54 | 0.15 | |||||||||||||||
| WS | 0.40 | 0.39 | 0.03 | 0.01 | ||||||||||||||||
| RS | 0.46 | I | ||||||||||||||||||
| CBHD | PW | 7.05 | ||||||||||||||||||
| WS | 1.24 | 0.03 | ||||||||||||||||||
| RS | ||||||||||||||||||||
| NA HC | PW | |||||||||||||||||||
| WS | 0.003 | 0.02 | 0.005 | 0.003 | 0.01 | |||||||||||||||
| RS | 0.12 | 0.31 | 0.23 | 0.24 | 0.46 | 0.43 | 0.16 | 0.31 | 0.15 | 0.55 | 0.13 | 0.39 | 0.95 | |||||||
| NANIT | PW | |||||||||||||||||||
| WS | ||||||||||||||||||||
| RS | 0.09 | 1.65 | ||||||||||||||||||
| A OX | PW | 0.50 | 0.76 | 1.19 | 0.57 | |||||||||||||||
| WS | 0.58 | 1.23 | 0.71 | 0.34 | 0.44 | 0.12 | 0.01 | 0.03 | ||||||||||||
| RS | 0.50 | 0.66 | 0.22 | - | 0.31 | |||||||||||||||
| A HC | PW | |||||||||||||||||||
| WS | ||||||||||||||||||||
| RS | 0.39 | 0.26 | 0.11 | |||||||||||||||||
| A NIT | PW | |||||||||||||||||||
| WS | ||||||||||||||||||||
| RS | 0.17 | 0.21 | 0.13 | |||||||||||||||||
| total | PW | 7.18 | 2.14 | 0.98 | 0.93 | 8.53 | 0.89 | 1.36 | 0.57 | 0.03 | ||||||||||
| WS | 5.09 | 3.58 | 1.16 | 0.79 | 3.03 | 1.57 | 0.78 | 0.36 | 0.45 | 0.12 | 0.03 | 0.01 | 0.05 | 0.07 | 0.07 | 0.04 | 0.02 | 0.034 | ||
| RS | 1.34 | 0.55 | 1.16 | 0.97 | 0.43 | 0.68 | 0.57 | 0.73 | 0.16 | 0.53 | 0.15 | 0.62 | 0.13 | 2.04 | 2.29 | |||||
aNA OX nonaromatic oxygenates (i.e., alcohols, carboxylicacids, aldehydes, and ketones); HTC OX heterocyclic oxygenates (i.e.,furans, pyrans, and others); CBHD carbohydrates; NA HC nonaromatichydrocarbons; NA NIT nonaromatic nitrogenates; A OX aromatic oxygenates;A HC aromatic hydrocarbons; A NIT aromatic nitrogenates.
The total mass fractions of bio-oil quantified in the present studycompare favorably with those reported in literature (5–25 wt%).16,51 Most of the published studies on GC ×GC analysis of bio-oils report concentrations in terms of relativepeak area percentage and a direct comparison with our results is notpossible.52,53 Additionally, the total mass fractions reportedin our study correspond to peaks annotated with names of plausiblecompounds. A reasonable water content of 25 wt % can be assigned tothe bio-oil from PW based on published studies in which bio-oil producedfrom the same feedstock and by the same company has been used.54,55 Noticeably, the elemental composition reported for those PW bio-oilsclosely matches the elemental composition of the PW bio-oil from ourstudy with differences in mass percentages for C, H, and O of lessthan 5%. Water and the quantified fraction account for around 50 wt% of the bio-oil from PW. A water content of 28.4 ± 0.45 wt %was measured using Karl Fischer titration for the bio-oil from WS.A mass fraction of 46 wt % was then quantified for this bio-oil. Finally,hydrogen and oxygen balances based on the elemental composition combinedwith results from the 13C NMR analysis, show that the watercontent of RC bio-oil is around 10 wt %. This water content is consistentwith that reported for bio-oil from the same feedstock and producedat similar pyrolysis temperature.30 Whenwater and the quantified mass fraction are combined, around 20 wt% of this bio-oil was quantified, i.e., less than half in comparisonto the other studied bio-oils.
PW bio-oil and WS bio-oil containedrelatively high and comparable amounts of acetic acid. In addition,these bio-oils also contained significant amounts of glycolaldehydeand 1-hydroxy-2-propanone. PW bio-oil stood out due to its high contentof levoglucosan (≈6 wt %), which was the most abundantly presentcompound in the current investigation. This high content of levoglucosanis in agreement with the 13C NMR results for this bio-oil.The nitrogenated compound (Z)-9-octadecenamide wasthe most abundant one in RC bio-oil.
Of the identified compounds,nonaromatic ones were present in the highest concentrations. In PWbio-oil, these nonaromatic compounds were followed in abundance bythe carbohydrates, aromatics and heterocyclic compounds. A differentorder was observed for WS bio-oil in which aromatics were detectedin an amount exceeding that of the carbohydrates. In RC bio-oil nocarbohydrates were detected, while the aromatics were found to bepresent in a higher amount than the detected heterocyclic compounds.Despite the limited fraction that was detected and quantified by GC ×GC-FID/TOF-MS, the general trends observed in the main groups of compoundsare in line with the results from the 13C NMR analysis.
Our results for the main groups in PW bio-oil are in agreementwith those for the PW bio-oil from Cheng et al.56 who reported carbohydrates as the major class of the identifiedcompounds. Cheng et al.56 also reportedsignificant amounts of nonaromatics, ketones, and carboxylic acidsin PW bio-oil, which is also in agreement with the current results.Compounds that were found in PW bio-oil at concentrations exceeding1.0 wt % in our analysis, i.e., acetic acid, levoglucosan, and glycolaldehyde,were also found in PW bio-oil at concentrations higher than 1.0 wt% by Djokic et al.16 Our results for themain groups in WS bio-oil match those obtained by Charon et al.10 The current results are also in agreement withthose of Charon et al.10 for individualnonaromatic compounds that were found at high concentrations in WSbio-oil: i.e., acetic acid, glycolaldehyde, and 1-hydroxy-2-propanone.The significant amount of these light oxygenates in WS bio-oil isconsistent with the high amounts of alkali metals in wheat straw,which can act as catalyst for secondary cracking reactions.10,57
Smets et al. reported the presence of free fatty acids inRC bio-oil, which are expected to be degradation products of triglycerides.30 Additionally, in the same study, gel permeationchromatography (GPC) revealed the presence of fatty acids and triglyceridesin RC bio-oil. Finally, the peak of triglycerides increased with thepyrolysis temperature. The fatty acids identified by Smets et al.in RC bio-oil via GC-MS analysis were 9-octadecenoic acid, 9,12-octadecadienoicacid and hexadecanoic acid.30 Fatty acidswere also found in RC bio-oil in the present study, as specificallytetradecanoic acid and hexadecanoic acid have been identified. Inaddition, significant amounts of 11-octadecenoic acid, methyl esterand trans-13-octadecenoic acid, methyl ester were also found in RCbio-oil. Comparable amounts (≈3 wt %) of aromatic compoundswere identified in PW bio-oil and RC bio-oil, while the identifiedphenols were found in higher concentrations in WS bio-oil and RC bio-oil(i.e., ≈1 wt %). Finally, methoxy- and dimethoxyphenol derivativeswere detected in relatively high amounts in PW bio-oil (1.5 wt %).
To have a more detailed overview of the chemical composition ofthe different bio-oils under study Van Krevelen (VK) diagrams forthe aromatic and nonaromatic fractions (hydrocarbons and nitrogenatedcompounds are excluded) have been constructed; see Figure 3. VK diagrams for the nonaromaticoxygenates show that ketones and carboxylic acids, which are the mostabundant among the identified compounds, are broadly distributed interms of O/C and H/C ratios. Carbohydrates are well separated at theright-hand side of the VK diagram due to its high O/C molar ratios,which vary from 0.67 to 1. Heterocyclic compounds exhibit the lowestH/C ratios with a maximum value of 1.6.
The group of compounds at the top of the left-hand side ofthe VK diagrams for nonaromatic oxygenates (i.e., molar O/C ratiosof 0.05–0.15; molar H/C ratios of 1.8–2.0) exhibitscarbon numbers from C15 to C20 and probablyoriginated from extractives for the case of PW and WS bio-oil, andfrom extractives and triglycerides for the case of RC bio-oil. Onthe other hand, aromatic compounds are concentrated between O/C ratiosfrom 0.1 to 0.43 and H/C ratios from 0.86 to 1.45. Besides, phenolsappeared well separated from benzenediols and methoxy-dimethoxy phenolderivatives. Other aromatic oxygenates were more distributed and compriseindenes, substituted benzenediols, aromatic carboxylic acids, andaromatic ketones among others.
Chemical Application ofBio-Oils
Phenolic compounds in bio-oils have attracted attentionbecause of its potential uses as fuel additives and chemical precursors.7,58 Some phenols and phenol derivatives can be used after separationas food antioxidants, transportation fuel additives, precursors forchemical products (pesticides, dyes, pharmaceutical products), andin the resin industry.58 In addition, formaldehydecould be suitable for the production of phenol-formaldehyde resinsin the polymer industry. Thirty-nine different phenolic monomers,e.g., phenols, benzenediols, and methoxy- and dimethoxyphenol derivatives,etc., were identified and quantified in this work using GC ×GC-FID/TOF-MS. Phenolic compounds detected in amounts ≥0.5wt % were phenol in RC bio-oil, and 2-methoxy-phenol, also denotedas guaiacol, and 4-methylguaiacol in PW bio-oil. Feedstock screeningand optimization of fast pyrolysis operating conditions for the enrichmentof phenolics can be carried out based on the quantitative informationprovided by the current combined GC × GC-FID/TOF-MS approach.
Levoglucosan as major compound found in PW bio-oil could be usedfor the manufacturing of pharmaceuticals, surfactants, and biodegradablepolymers. Low molecular carbonyl compounds such as acetaldehyde, glycolaldehyde(hydroxyacetaldehyde), and 2-furaldehyde (furfural) and volatile carboxylicacids are very reactive at ambient conditions with negative impacton bio-oil quality and storage infrastructure. Within this context,GC × GC-FID/TOF-MS can provide quantitative results, which canassist in the selection of additives for stabilization.
Theacidity of fast pyrolysis bio-oils is caused mainly by volatile carboxylicacids.59 The carboxylic acids with molecularweight below 100 detected using GC × GC are acetic acid, propionicacid, 3-butenoic acid, butyric acid, and 2-methyl-propanoic (isobutyric)acid. Taking the concentrations of low molecular weight carboxylicacids as an indicator of acidity, the PW and WS bio-oils showed similarresults (4.9 and 4.3 wt %, respectively). In contrast, the RC bio-oilshowed a rather low value (0.14 wt %). A similar trend for acidityis also observed based on total nonaromatic carboxylic acids (4.9and 4.9 vs 1.0 wt %) and on total nonaromatic carbonyl (includingcarboxylic acids, aldehydes, ketones, and esters, 10 and 11 vs 2.4wt %). The typical pH range for bio-oils from lignocellulosic materialsis 2–3,22 while the reported pHfor RC bio-oil is notably much higher (≈7).30 These results confirm the application of GC × GC forassessing acidity and corrosivity of bio-oils.
In addition tothe presented chromatographic techniques, which are usually employedfor the identification of individual components, spectroscopic methodsassist in the chemical group analysis. NMR analyses give strategicinformation on the type and relative percentage of chemical functionalitiespresent in bio-oils. As already mentioned, alkyl hydrocarbon groupscontribute significantly to the energy content of bio-oils. About66% of gasoline and 80% of diesel fuels have alkyl hydrocarbons.60 The content of alkyl hydrocarbons and consequentlythe overall energy value of the studied bio-oils follows the trendof rapeseed cake > wheat straw > pinewood. The aromatic contentsare important for synthetic modifications when considering the solubilityof a feedstock for downstream processing or end products derivatives.The aromatic contents of bio-oils follow the following trend: wheatstraw > rapeseed cake ≈ pinewood. On the other hand, thecarbonyl contents, such as aldehyde or ketones, provide useful informationfor modification or further improvements of bio-oil.15 Finally, NMR analysis can also provide an insight intothe stability of bio-oil. The methoxy/hydroxy, carbohydrate, and carbonylcarbon contents of bio-oil can be seen as an stability indicator.61 In our case, as expected, the stability orderis RC bio-oil with 5% of unstable carbon followed by WS bio-oil with30% of unstable carbon and finally PW bio-oil with 70% of unstablecarbon. These results indicate PW bio-oil is prone to phase separationupon storage and separating its carbohydrate fraction could be desirable.
Conclusions
A detailed chemical analysis of bio-oils producedfrom the fast pyrolysis of pinewood, rapeseed, and wheat straw wasperformed using a combination of quantitative 13C NMR spectroscopyand GC × GC-FID/TOF-MS, providing new insights for the developmentof bio-oil upgrading strategies. 13C NMR analysis showedthat pinewood bio-oil was rich in methoxy/hydroxyl groups and carbohydrates,while rapeseed cake bio-oil was rich in alkyl hydrocarbons. On theother hand, wheat straw bio-oil contained high amount of aromaticsand alkyl hydrocarbons. The alkyl hydrocarbon content of the bio-oilsas energy value index showed the following trend: rapeseed cake >wheat straw > pinewood. Using a GC × GC-FID/TOF-MS analyticalapproach, more than 200 individual compounds have been identifiedand quantified. Nonaromatic oxygenates were the most abundant compoundsin all investigated bio-oils. Pinewood and wheat straw bio-oils containedsignificant amounts of volatile carboxylic acids. Pinewood in particularcontained significant amounts of low molecular weight aldehydes, whichrises concern regarding its stability and suitability for long-termstorage before being used in biorefinery operations. The results indicatethat rapeseed cake bio-oil is chemically more stable and relativelynoncorrosive when compared with the two other bio-oils. In addition,valuable phenolic compounds were identified and quantified in allbio-oils.
Acknowledgments
This work is financially supportedby the CAPITA-WAVES: WAste biofeedstocks hydro-Valorisation processES,CAPITA-13-6 Project and the European Research Council under the EuropeanUnion’s Seventh Framework Programme FP7/2007-2013/ERC grantagreement no. 290793. Dr. Pieter Bruijnincx and Dr. Sandra Constant,both from Utrecht University, are greatly acknowledged for the scientificdiscussions of the NMR data.
Supporting Information Available
The SupportingInformation is available free of charge on the ACS Publications website atDOI: 10.1021/acssuschemeng.6b01329.
List of identifiedcompounds and detailed composition of the bio-oils obtained via aGC × GC-FID/TOF-MS analytical approach (PDF)
Supplementary Material
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