Journal of Analytical and Applied Pyrolysis

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1 J. Anal. Appl. Pyrolysis 83 (2008) Contents lists available at ScienceDirect Journal of Analytical and Applied Pyrolysis journal homepage: Uncatalysed and potassium-catalysed pyrolysis of the cell-wall constituents of biomass and their model compounds Daniel J. Nowakowski, Jenny M. Jones * Energy and Resources Research Institute, School of Process, Environmental and Materials Engineering (SPEME), University of Leeds, Leeds, LS2 9JT, UK ARTICLE INFO ABSTRACT Article history: Received 16 July 2007 Accepted 20 May 2008 Available online 28 May 2008 Keywords: Biomass Lignocellulose Pyrolysis Catalysis Potassium Mechanism Cell-wall components (cellulose, hemicellulose (oat spelt xylan), lignin (Organosolv)), and model compounds (levoglucosan (an intermediate product of cellulose decomposition) and chlorogenic acid (structurally similar to lignin polymer units)) have been investigated to probe in detail the influence of potassium on their pyrolysis behaviours as well as their uncatalysed decomposition reaction. Cellulose and lignin were pretreated to remove salts and metals by hydrochloric acid, and this dematerialized sample was impregnated with 1% of potassium as potassium acetate. Levoglucosan, xylan and chlorogenic acid were mixed with CH 3 COOK to introduce 1% K. Characterisation was performed using thermogravimetric analysis (TGA) and differential thermal analysis (DTA). In addition to the TGA pyrolysis, pyrolysis gas chromatography mass spectrometry (PY GC MS) analysis was introduced to examine reaction products. Potassium-catalysed pyrolysis has a huge influence on the char formation stage and increases the char yields considerably (from 7.7% for raw cellulose to 27.7% for potassium impregnated cellulose; from 5.7% for raw levoglucosan to 20.8% for levoglucosan with CH 3 COOK added). Major changes in the pyrolytic decomposition pathways were observed for cellulose, levoglucosan and chlorogenic acid. The results for cellulose and levoglucosan are consistent with a base catalysed route in the presence of the potassium salt which promotes complete decomposition of glucosidic units by a heterolytic mechanism and favours its direct depolymerization and fragmentation to low molecular weight components (e.g. acetic acid, formic acid, glyoxal, hydroxyacetaldehyde and acetol). Base catalysed polymerization reactions increase the char yield. Potassium-catalysed lignin pyrolysis is very significant: the temperature of maximum conversion in pyrolysis shifts to lower temperature by 70 K and catalysed polymerization reactions increase the char yield from 37% to 51%. A similar trend is observed for the model compound, chlorogenic acid. The addition of potassium does not produce a dramatic change in the tar product distribution, although its addition to chlorogenic acid promoted the generation of cyclohexane and phenol derivatives. Postulated thermal decomposition schemes for chlorogenic acid are presented. ß 2008 Elsevier B.V. All rights reserved. 1. Introduction The pyrolysis of cellulose and cellulosic materials has been investigated under a variety of conditions and from several points of view, such as chemical utilization [1], fire and fire retardation [2], and other technical subjects. Interest in the thermochemical conversion of biomass into fuels and chemical feedstocks also prompts research into the thermal decomposition (depolymerization) of the cell-wall lignocellulosic material. The chemical composition of biomass varies among species, but biomass consists of about 70% carbohydrates or sugars, 30% of lignin, * Corresponding author. Tel.: ; fax: address: j.m.jones@leeds.ac.uk (J.M. Jones). and most species also contain about 5% of a third portion of smaller molecular fragments, called extractives, and inorganic components. Cellulose is the chief constituent of wood and makes up about 50% of the dry wood [3]. Lignin exists as one of the essential wood and biomass components, ranging amount from 10% to 30% [4]. Hemicellulose is the other major cell-wall component, and can comprise up to 35%. Potassium is a key plant nutrient (N P K) whose concentration varies over the growing season. Of all the metals present in biomass, it is potassium that has the greatest influence on its thermal conversion properties, catalysing both the pyrolysis and the combustion/gasification stage [5 8]. This influence is still not completely understood for lignocellulosic material, although, as discussed below, there have been some excellent mechanistic studies in uncatalysed and catalysed cellulose pyrolysis [9 19] and /$ see front matter ß 2008 Elsevier B.V. All rights reserved. doi: /j.jaap

2 D.J. Nowakowski, J.M. Jones / J. Anal. Appl. Pyrolysis 83 (2008) a more limited number concerning hemicellulose [20,21] and lignin [22 27]. The cellulose decomposition mechanism has been studied in some detail [9 16]. From thermal analytical studies, Shafizadeh [9] has proposed a simple model, in which the primary decomposition step is the formation of an active cellulose. The subsequent step is the formation of char and gases by one decomposition pathway or the production of primary volatiles by another route. The primary volatiles resemble cellulose monomeric units, and include levoglucosan [28], levoglucosenone (1,6-anhydro-3,4-dideoxy-b- D-glycero-hex-1-enopyranos-2-ulose) and 1,4:3,6-dianhydro-a-Dglucopyranose have also been isolated. The levoglucosan yields obtained by pyrolysis of a number of different celluloses varied from 20% to 60% [29]. Except for levoglucosan, these compounds have only been detected in minor amounts (in the range of 1%) [30,31]. Low molecular weight products include carbon dioxide, methane, water, acetone and acetic acid, formaldehyde, glycolaldehyde, butanone, 2-furaldehyde, 5-hydroxymethyl-2-furaldehyde and furan derivatives [32]. It was also noted [12,14,16] that the presence of certain ions (K +, Ca 2+,NH 4 + ) in cellulose promotes char and gas formation at the expense of the tar yield. The effect of removing metals by acid washing has been studied. Julien et al. [12] showed that a mild pretreatment could reduced the ash content from 0.22% to 0.02% and that about 90% of the original inorganic ions, potassium, calcium and silicon were removed. The total organic liquid yield was increased substantially when the inorganic cations are reduced and fast heating rates applied. The very low quantities of acids and aldehydes obtained when acid-washed treated cellulose was pyrolysed indicate that decomposition of the glucose unit from cellulose has been greatly reduced. The pretreated cellulose (low metal content) gave a much higher yield of levoglucosan (33.1% compared with 3.7% for the untreated cellulose) and the amount of hydroxyacetaldehyde was drastically reduced from 11.1% to 0%. Levoglucosan is thus formed at the expense of hydroxyacetaldehyde by removal of inorganic ions from original cellulose. Julien et al. [12] suggest that hydroxyacetaldehyde is formed by a primary ring fragmentation of cellulose. The degree of polymerization of cellulose may also favour levoglucosan formation, and Basch and Lewin [19] note that the number of free chain ends in the polymer is also an important factor. Richards [17] has proposed a heterolytic mechanism for the formation of hydroxyacetaldehyde from cellulose pyrolysis (catalysed route), while the formation of levoglucosan results from a homolytic mechanism (uncatalysed route) [18], and it is assumed that inorganic cations (K + and Ca 2+ ) can act as catalysts for complete decomposition of glycosidic units by a heterolytic mechanism [12]. Isotopic labelling studies of glycerine (a model compound for carbohydrate monomers) [33] highlight the complexity of this decomposition step. In this case, up to four competing reactions were deduced for the fragmentation of glycerine to acetaldehyde and formaldehyde, depending upon the presence of acid (phosphoric) or basic (e.g. potassium salts) additives. Hemicelluloses (consisting of various saccharides i.e. xylose, mannose, galactose, glucose, etc.), under low heating rates at atmospheric pressure, undergo thermal decomposition at temperatures below 473 K [20]. The overall decomposition mechanism of hemicellulose pyrolysis under these conditions also involves competing reactions. The first is the low-temperature polymerization process (<523 K) to form components such as polysaccharides, which result in the formation of char, carbon monoxide, carbon dioxide and water. At somewhat higher temperatures ( K), these polysaccharides undergo decomposition to generate volatile anhydrosugars and related monomeric compounds. Shafizadeh et al. [21] studied pyrolysis of D-xylose and methyl-b-d-xylopyranoside (selected as a model compounds) as well as 4-O-methylglucuroxylan and O-acetyl-4-O-methylglucuroxylan extracted from cotton wood, before and after addition of 10 wt.% zinc chloride and sodium hydroxide. The main pyrolysis products were char, carbon monoxide, carbon dioxide, water and tar fraction. Addition of sodium hydroxide produced increased amounts of low molecular weight products from carbonyl fragmentation and disproportionation reactions. Zinc chloride promoted the formation of 2-furalaldehyde, water and char. The study indicated that the pyrolysis of xylan polysaccharides and related model compounds involves thermal cleavage of the glycosidic groups and the glycosyl units can then condense randomly to form tarry products or can be degraded to a variety of compounds through a combination of competitive dehydration, fragmentation and disproportionation reactions. The dehydration reactions were catalysed by acid conditions (ZnCl 2 ), and alkali (NaOH) promoted the fragmentation and rearrangement reactions. A numbers of studies have been reported on the thermal degradation of lignin under pyrolytic conditions [22 27]. These studies show that the volatile products from lignin pyrolysis are liberated over the wide range of temperature ( K). The distribution of products is very dependent on pyrolysis conditions and type of the lignin as well as on its method of isolation [22 24]. The major gas products were carbon monoxide, carbon dioxide, water, light hydrocarbons, acetic acid, acetaldehyde, methanol and formaldehyde [22,24] and the aliphatic OH group content has been correlated with the yields of formaldehyde and water [23]. Acetylation of lignins has shown that the free OH groups also promote the scission of COOH groups [25]. The influence of ZnCl 2 and NaCl on the pyrolysis mechanism of lignin has also been studied by thermogravimetric analysis (TGA) [25]. In the presence of ZnCl 2, water and formaldehyde formation shifted to lower temperature. NaCl promoted dehydration, demethoxylation and recombination of the primary free radicals, but in this case no shift of the maximum peak temperature was observed. A free-radical mechanism of pyrolytic lignin decomposition was proposed by Britt et al. [26] from a series of pyrolysis experiments of model compounds. The char, an intermediate solid residue from the pyrolysis of lignin, is believed to contribute in formation of polycyclic aromatic hydrocarbons (PAHs), particularly at low temperatures. Sharma and Hajaligol [27,34,35] studied the effect of pyrolysis conditions (temperature, presence of inorganics: Na and K) on the formation of selected PAHs from chlorogenic acid (a natural product) and alkali lignin. The removal of sodium and potassium from the lignin sample decreases the formation of char and PAHs. Co-pyrolysis of chlorogenic acid with lignin also affected the PAH distribution. Chlorogenic acid (esters formed between trans-cinnamic acid and ( )quinic acid) is a relatively simple and widespread natural product, and as a polyphenol, it contains similar functional groups to those present in lignin [35,36]. In this work chlorogenic acid was chosen as a model compound to study and understand the impact of potassium on the pyrolytic decomposition mechanism of polyphenols. This influence of potassium on the cellulose pyrolysis mechanism is now quite well established [12,20] but it is not clear if the other cell-wall components in biomass are catalysed to the same extent, and whether they can be considered to behave independently in catalytic thermal conversion. The present work compares the influence of potassium on the pyrolysis behaviour of the cell-wall constituents of biomass components (cellulose, hemicellulose and lignin) and model compounds (levoglucosan, chlorogenic acid) and examines in some detail the pyrolysis product distribution and components. This enables an extended

3 14 D.J. Nowakowski, J.M. Jones / J. Anal. Appl. Pyrolysis 83 (2008) decomposition reaction scheme to be presented for cellulose (and levoglucosan) and, from the study of chlorogenic acid, gives further insight into lignin decomposition. 2. Experimental 2.1. Materials The following cell-wall components were investigated: cellulose, hemicellulose (oat spelt xylan) and lignin (Organosolv), as well as model compounds, levoglucosan (an intermediate product of cellulose decomposition) and chlorogenic acid (structurally similar to the lignin). Their structures are shown in Fig. 1. All compounds were purchased from the Sigma Aldrich Company Ltd Sample preparation Demineralization Hydrochloric acid treatment of cellulose and lignin samples was performed by heating 10 g of sample in 50 cm 3 of 2.0 M HCl for 6 h at 333 K. After 48 h the sample, left in the HCl solution, was again heated at 333 K for 6 h. The sample was filtered, then washed using de-ionized water until the filtrate was Cl free (checked by 0.1 M silver nitrate solution). The sample was then oven dried at 333 K to constant weight. Fig. 1. Structures of model compounds used in this study: (a) cellulose, (b) xylan (hemicellulose; representative structure of O-acetyl-galactoglucmannan), (c) representative structure of lignin Organosolv and (d) chlorogenic acid, a natural product, and model compound for lignin.

4 D.J. Nowakowski, J.M. Jones / J. Anal. Appl. Pyrolysis 83 (2008) Impregnation 3 g of sample (cellulose and lignin), analysed before impregnation for moisture content, was impregnated by potassium (as potassium acetate) to yield a 1 wt.% K-impregnated sample. Levoglucosan, xylan and chlorogenic acid were mixed with CH 3 COOK to introduce 1% K. After addition of potassium acetate the sample was moistened by 5 cm 3 of de-ionized water, mixed and then oven dried at 333 K to constant weight Thermogravimetric analysis (TGA) Pyrolysis tests were performed using a TGA analyser (Station Redcroft Simultaneous Analyser STA-780 Series). A typical sample mass of 10 mg was heated at 25 K/min in a purge of nitrogen with the final temperature of 1173 K, and then the sample was held at 1173 K for 15 min Differential thermal analysis (DTA) DTA characterisation tests were performed using the Station Redcroft Simultaneous DTA Analyser STA-780 Series. A typical sample mass of 10 mg was heated at 20 K/min in a purge of nitrogen with the final temperature of 1173 K, and then the sample was held at 1173 K for 15 min Analytical pyrolysis tests (PY GC MS) PY GC MS tests were performed on each sample using a CDS 1000 pyrolyser coupled to a Hewlett-Packard GC-MSD gas chromatograph. The column was a RTX 1701 (14% cyanopropylphenyl, 86% dimethylpolysiloxane; 60 m, 0.25 mm i.d., 0.25 mm d.f.). The gas chromatograph oven was held at 313 K for 2 min and then programmed at 4 K/min to 523 K, held for 30 min. Approximately 1.5 mg of sample was placed in 20 mm quartz tube in between quartz wool. The sample was pyrolysed at a set point temperature of 873 K at a ramp rate of 20 K/ms with the final dwell time of 20 s. 3. Results and discussion 3.1. TGA pyrolysis and DTA analysis The deferential thermogravimetric (DTG) results comparing the influence of potassium in the TGA pyrolysis experiments of cellulose, levoglucosan and xylan are shown in Fig. 2a c. The DTA profile of cellulose shown in Fig. 3 indicated that the pyrolysis of untreated (raw) cellulose is an endothermic process and begins at approximately at 525 K and, as the temperature is increased, proceeds very rapidly, reaching a maximum rate of the weight loss at 641 K, and leaving the residue (char) of 7.7%. The same pyrolytic and thermal behaviour was observed for the HCl treated cellulose sample with the maximum weight loss rate at 643 K (shifted 2 K to higher temperature in comparison to untreated sample) with the char yield of 9.2%. Thermogravimetric analysis of potassiumimpregnated cellulose revealed that this sample is thermally degraded in two steps (Fig. 2a). Upon addition of potassium two clearly resolved peaks for cellulose are present. The DTG profile showed the first maximum at 517 K and the second maximum at 598 K. 72.3% of the sample was volatilized and 27.7% was converted to char. The yields for TGA pyrolysis of biomass component samples and model compounds are given in Table 1. DTA analysis for K-impregnated cellulose sample (DTA profile shown in Fig. 3) indicated that the first stage of the decomposition is endothermic, while the second one is moderately exothermic. Fig. 2. DTG profiles for (a) cellulose, (b) levoglucosan and (c) xylan (hemicellulose) samples. The DTG profiles for levoglucosan and xylan are shown in Fig. 2b and 2c respectively. The DTG curve for raw levoglucosan showed the peak maximum at 599 K, and for xylan at 588 K. The presence of potassium shifts the maximum peak temperature for levoglucosan to 587 K and for xylan the maximum peak temperature remains invariable. Potassium promotes the char formation stage

5 16 D.J. Nowakowski, J.M. Jones / J. Anal. Appl. Pyrolysis 83 (2008) Fig. 3. DTA curves for pyrolysis of cellulose samples. in the case of levoglucosan. The addition of potassium into the xylan (as described in Section 2.2) had no influence on the main pyrolysis product distribution char yields of both raw and potassium acetate added samples are comparable. The DTA results for the xylan samples indicate an endothermic nature of the pyrolysis process (not shown). The DTG results comparing the influence of potassium on the pyrolysis experiments of lignin and chlorogenic acid are shown in Fig. 4a and b respectively. The major weight loss for untreated and HCl dematerialized lignin samples occurs approximately at between 500 and 750 K with the maximum peak temperatures at 670 and 672 K. The presence of potassium shifts the main peak temperature (by approximately 90 K) to 603 K. Chlorogenic acid, a model phenolic compound, decomposes between 480 and 720 K with the main peak temperature at 636 K. A noticeable shoulder on the DTG profile is present at 530 K. When potassium is added to the chlorogenic acid sample the initial point of the pyrolysis process is lowered (400 K), and the decomposition occurs in two stages with the maximum peak temperatures at 501 and 587 K. Potassium significantly increases the char yield of lignin from 36.8 for the raw sample to 50.7% for the K- impregnated sample. A smaller increase is seen for chlorogenic acid from 31.2% up to 36.0%. DTA results for lignin Organosolv samples (Fig. 5) indicate an endothermic nature for the pyrolytic process in all cases. Fig. 4. DTG profiles for (a) lignin Organosolv and (b) chlorogenic acid samples Analytical pyrolysis tests Pyrolysis gas chromatography mass spectrometry (PY GC MS) analysis has been introduced to study the generation of Table 1 Pyrolysis yields for TGA pyrolysis of biomass components and model compounds samples Sample Char (%) Volatiles (%) Cellulose raw Cellulose HCl treated Cellulose K-impregnated Levoglucosan Levoglucosan + potassium acetate Xylan Xylan + potassium acetate Lignin Organosolv raw Lignin Organosolv HCl treated Lignin Organosolv K-impregnated Chlorogenic acid Chlorogenic acid + potassium acetate Fig. 5. DTA curves for pyrolysis of lignin (Organosolv) samples.

6 D.J. Nowakowski, J.M. Jones / J. Anal. Appl. Pyrolysis 83 (2008) Fig. 6. Pyrolysis GC MS chromatogram for cellulose (HCl treated) sample. The main peaks assigned from mass spectral detection are as follows: (1) propane; (2) hydroxyacetaldehyde; (3) propanoic acid; (4) cis-3-oxabicyclo[3.2.0]heptane-2,4- dione; (5) furfural; (6) 2-hydroxy-2-cyclopenten-1-one; (7) 2-furanmethanol; (8) 3,4-dihydroxy-3-cyclobutene-1,2-dione; (9) 3-methyl-1,2-cyclopentanedione; (10) 2,5-dimethyl-4-hydroxy-3(2H)-furanone; (11) 2H-pyran-2-one; (12) (Z)-2- butene-1,4-diol; (13) 2,3-anhydro-D-mannosan; (14) 2,3-anhydro-D-galactosan; (15) 3,4-anhydro-D-galactosan; (16) 1,4:3,6-dianhydro-a-D-galactosan; (17) 5- (hydroxymethyl)-2-furancarboxyaldehyde; (18) Unknown; (19) 1,6-anhydro-b-Dglucopyranose (levoglucosan). hydrocarbons produced during pyrolysis. Prospective assignments of the main peaks were made from mass spectral detection (NIST05A MS library) and from the literature [37 39] and are given under the each figure. The result of pyrolysis GC MS analyses are given in several figures, described below. Please note that the prospective assignments of the peaks in these figures are listed in the figure captions. Selected cellulose, levoglucosan, xylan and lignin key markers were identified for the most abundant thermal degradation compounds, and these are listed in Appendix A, together with main fragments and their relative abundances by the NIST05A library. Chromatograms from PY GC MS analyses of HCl treated and potassium impregnated cellulose samples are given in Figs. 6 and 7 respectively. There was no difference between the product distributions from raw and HCl treated cellulose and thus, the comparison has been made for HCl treated sample (which was a precursor for impregnation) and the K-impregnated sample. The main cellulose thermal degradation products for uncatalysed and potassium-catalysed pyrolysis are summarized in Scheme 1. Five anhydrosugar derivatives have been identified in the degradation products of raw and HCl treated cellulose (Fig. 6): 2,3- anhydro-d-mannosan; 2,3-anhydro-D-galactosan; 3,4-anhydro-Dgalactosan; 1,4:3,6-dianhydro-a-D-galactosan and 1,6-anhydro-b- D-glucopyranose. 1,6-anhydro-b-D-glucopyranose is normally referred to as levoglucosan. Levoglucosan is the major primary pyrolysis product of glycans (cellulose) under neutral [18] or acid conditions [30]. It is generally accepted that the generation of levoglucosan (with other anhydrosugars) is the first step in the formation of volatile compounds during the pyrolysis of cellulose. The other major volatile products formed during pyrolysis both of raw and HCl treated samples are: cis-3-oxabicyclo[3.2.0]heptane- 2,4-dione; 2-hydroxy-2-cyclopenten-1-one; 3,4-dihydroxy-3- cyclobutene-1,2-dione; 2H-pyran-2-one; 2-butene-1,4-diol and 5-(hydroxymethyl)-2-furancarboxyaldehyde. In addition there were minor peaks identified as propane, hydroxyacetaldehyde, propanoic acid, 2-furanmethanol, 3-methyl-1,2-cyclopentanedione and 2,5-dimethyl-4-hydroxy-3(2H)-furanone. The presence of potassium promotes the decomposition of levoglucosan, as well as the other anhydrosugars, and leads the decomposition process of cellulose to different (low molecular weight) products such as acetic acid, propanoic acid and 1- hydroxy-2-butanone. The major decomposition product of potassium impregnated cellulose are furfural, 2-cyclopenten-1-one, dihydroxyfuran-2(3h)-one and 2-hydroxy-4-methyl-2-cyclopenten-1-one. Other products detected were 2-furanmethanol, phenol and phenol derivatives. 2-hydroxy-3-propenyl-2-cyclopenten-1- one, 5-hydroxymethyl-dihydrofuran-2-one and levoglucosan were also detected but with very low abundance. The PY GC MS chromatograms for raw levoglucosan and levoglucosan with potassium acetate added are shown in Figs. 8 and 9 respectively. The main levoglucosan thermal degradation products for uncatalysed and potassium-catalysed pyrolysis are summarized in Scheme 2. The main products of the uncatalysed levoglucosan pyrolytic decomposition are the anhydrosugars: Fig. 7. Pyrolysis GC MS chromatogram for potassium impregnated cellulose sample. The main peaks assigned from mass spectral detection as follows: (1) acetic acid; (2) acetic acid, 2-propenyl ester; (3) propanoic acid; (4) 1-hydroxy- 2-butanone; (5) furfural; (6) 2-cyclopenten-1-one; (7) 2-furanmethanol; (8) 2- methyl-2-cyclopenten-1-one; (9) 1-(2-furanyl)-ethanone; (10) 2,3-dimethyl-2- cyclopenten-1-one; (11) dihydroxyfuran-2(3h)-one; (12) 2(5H)-furanone; (13) 2-hydroxy-4-methyl-2-cyclopenten-1-one; (14) 2-hydroxy-3-methyl-2- cyclopenten-1-one; (15) phenol; (16) 2- and 4-methylphenol; (17) 3-ethyl-2- hydroxy-2-cyclopenten-1-one; (18) 4,4-diethyl-3-methyleneoxetan-2-one; (19) 2,4-dimethylphenol; (20) 2-hydroxy-3-propenyl-2-cyclopenten-1-one; (21) 5- (hydroxymethyl)-2-furancarboxyaldehyde; (22) 1,6-anhydro-b-D-glucopyranose (levoglucosan). Fig. 8. Pyrolysis GC MS chromatogram for levoglucosan sample. The main peaks assigned from mass spectral detection as follows: (1) acetone; (2) acetic acid; (3) propanal; (4) furfural; (5) 2-hydroxy-2-cyclopenten-1-one; (6) 2(5H)-furanone; (7) 2-hydroxy-2-methyl-2-cyclopenten-1-one; (8) 2H-pyran-2-one; (9) maltol; (10) formic acid, 2-propenyl ester; (11) 3,4-anhydro-D-galactosan; (12) 2,6-anhydro-Dmannosan; (13) anhydro-d-mannosan; (14) 2,3-anhydro-D-galactosan; (15) n/d (column bleed); (16) D-mannose; (17) 1,6-anhydro-b-D-glucopyranose (levoglucosan).

7 18 D.J. Nowakowski, J.M. Jones / J. Anal. Appl. Pyrolysis 83 (2008) Scheme 1. 2,3-anhydro-D-mannosan; 2,3-anhydro-D-galactosan; 3,4-anhydro-D-galactosan and levoglucosan. The other major decomposition products are propanal, 2-hydroxy-2-cyclopenten-1-one, furfural and 2-hydroxy-4methyl-2-cyclopenten-1-one. The GC MS integration results revealed that the presence of potassium promotes the generation of cyclopentene derivatives. Seven different derivatives were detected i.e.: 2-cyclopenten-1-one; 2- cyclopenten-1,4-dione; 2-methyl-2-cyclopenten-1-one; 2-ethyl- 2-cyclopenten-1-one; 2,3-dimethyl-2-cyclopenten-1-one; 2- hydroxy-3-methyl-2-cyclopenten-1-one and 2-hydroxy-3,5- dimethylcyclopenten-2-en-1-one. Potassium promotes further decomposition (dehydration) of levoglucosan to dianhydrosugars. 1,4:2,3-dianhydro-a-D-galactosan and 1,4:3,6-dianhydro-a-Dgalactosan were detected beside the 2,3-anhydro-D-galactosan and 3,4-anhydro-D-galactosan but in much lower abundance. Phenol and methyl- and dimethyl-phenol derivatives were detected only in the decomposition products of the levoglucosan sample with added CH 3 COOK.

8 D.J. Nowakowski, J.M. Jones / J. Anal. Appl. Pyrolysis 83 (2008) The main decomposition products of xylan (hemicellulose) are acetic acid, 1-hydroxy-2-butanone, ethyl 2-propenylester, 3,4- dihydroxy-3-cyclobutene-1,2-dione, furfural, cyclohexanone, phenol and its methoxy- and dimethoxy derivatives, 2-hydroxy-3- methyl-2-cyclopenten-1-one, carbonic acid, methyl 2-propenylester (tentative assignment), 3-methyl-2,4-(3H,5H)-furandione and 3,4-dihydroxycoumarin-6-ol. Components such 3,4-dihydroxy-3-cyclobutene-1,2-dione, 1-hydroxy-2-butanone, ethyl 2- propenylester, carbonic acid, methyl 2-propenylester, 3-methyl- 2,4-(3H,5H)-furandione and 3,4-dihydroxycoumarin-6-ol, for which formulas are given on the PY GC MS chromatogram of raw xylan sample and marked with a # (Fig. 10) were not detected when potassium was present in the sample during pyrolysis. Other thermal degradation products remain the same (with similar abundance) in both raw and K-impregnated xylan samples. Thus the influence of added potassium on the nature of the xylan pyrolysis products is small in comparison with cellulose. However, the xylan originated from oat spelt, a high K-content precursor, and also NaOH is commonly used in the extraction process [40]. Hence, the alkali content of the raw xylan sample is expected to be significant, and the impact of further addition of potassium small. The chromatogram for the HCl treated lignin (Organosolv) is shown in Fig. 11. No difference between the product distributions from raw and HCl treated lignin samples were observed. Thus, the comparison has been made between the HCl treated sample and the K-impregnated sample. For the HCl treated (and raw) lignin, the key markers for Guaiacol (G) and Syringol (S) lignin were detected by PY GC MS: 2-methoxy-4-methyl-phenol (G); 4-ethyl-2-methoxy-phenol (G); 2-methoxy-4-vinyl-phenol (G); 2-methoxy-4-(2-propenyl)-phenol (eugenol) (G); 2-methoxy-4-propyl-phenol (G); 2,6-dimethoxyphenol (S); 3,4-dimethoxyphenol (S); 2-methoxy-4-(1-propenyl)- phenol (G); 4-methoxy-3-(methoxymethyl)-phenol (G); 4- hydroxy-3-methoxy-benzaldehyde (vanillin) (G); 2,6-dimethoxy- 4-(2-propenyl)-phenol (S) and 4-hydroxy-3,5-dimethoxy-benzaldehyde (S). The components 4-ethyl-2-methoxy-phenol, eugenol, Scheme 2.

9 20 D.J. Nowakowski, J.M. Jones / J. Anal. Appl. Pyrolysis 83 (2008) Fig. 9. Pyrolysis GC MS chromatogram for levoglucosan with CH 3 COOK added sample. The main peaks assigned from mass spectral detection as follows: (1) acetic acid; (2) acetic acid, 2-propenyl ester; (3) 1-hydroxy-2-propanone; (4) 2- cyclopenten-1-one; (5) 2-cyclopenten-1,4-dione; (6) 2-methyl-2-cyclopenten-1- one; (7) 1-(2-furanyl)-ethanone; (8) 1,2-cyclohexanedione; (9) 2-ethyl-2- cyclopenten-1-one; (10) 2,3-dimethyl-2-cyclopenten-1-one; (11) 2-hydroxy-3- methyl-2-cyclopenten-1-one; (12) mequinol; (13) 2-hydroxy-3,5- dimethylcyclopenten-2-en-1-one; (14) phenol and phenol derivatives; (15) 3,4- anhydro-d-galactosan; (16) 2,6-anhydro-D-mannosan; (17) anhydro-d-mannosan; (18) 2,3-anhydro-D-galactosan; (19) D-mannosan; (20) 1,4:2,3-dianhydro-2-Oacetyl-b-D-glucopyranose; (21) 1,6-anhydro-b-D-glucopyranose (levoglucosan); (22) 1,6:3,4-dianhydro-2-O-acetyl-b-D-glucopyranose. 3,4-dimethoxyphenol, 4-hydroxy-3,5-dimethoxy-benzaldehyde and aspidinol (labelled on the chromatogram by *) were not detected in the potassium impregnated sample, whereas the abundances of compounds 2-methoxy-4-methyl-phenol, 4-methoxy-3-(methoxymethyl)-phenol, 1-(4-hydroxy-3-methoxyphenyl)- ethanone, 3-(2-pentenyl)-1,2,4-cyclopentatrione, 1-(4-hydroxy- 3,5-dimethoxyphenyl)-ethanone and desaspidinol (labelled on the chromatogram by #) were smaller in comparison to raw and HCl treated lignin samples. Fig. 11. Pyrolysis GC MS chromatogram for HCl treated lignin sample. The main peaks assigned from mass spectral detection as follows: (1): phenol; (2) mequinol; (3) 2-methyl-phenol; (4) 4-methyl-phenol; (5) 2-methoxy-4-methyl-phenol; (6) 2,3- and/or 2,4- and/or 2,5-dimethyl-phenol; (7): 1,4-dimethoxy-2-methylbenzene; (8) 4-ethyl-2-methoxy-phenol; (9) 2-methoxy-4-vinylphenol; (10) eugenol; (11) 2-methoxy-4-propylphenol; (12) 2,6-dimethoxyphenol; (13) 3- allyl-6-methoxyphenol; (14) 3,4-dimethoxyphenol, (15) 2-methoxy-4-(1- propenyl)-phenol; (16) 4-methoxy-3-(methoxymethyl)-phenol; (17) vanillin; (18) 1-(2,3,4-trihydroxyphenyl)-ethanone; (19) 1-(2,6-dimethoxy-4- methoxyphenyl)-ethanone; (20) 1-(4-hydroxy-3-methoxyphenyl)-ethanone; (21) 3-(2-pentenyl)-1,2,4-cyclopentatrione; (22) 2,6-dimethoxy-4-(2-propenyl)- phenol; (23) 1-(3,4-dimethoxyphenyl)ethanone; (24) 4-hydroxy-3- methoxybenzenoacetic acid; (25) 4-hydroxy-3,5-dimethoxy-benzaldehyde; (26) 1-(4-hydroxy-3,5-dimethoxyphenyl)-ethanone; (27) desaspidinol; (28) aspidinol; (29) 4-(4-ethylcyclohexyl)-1-pentyl-cyclohexene. The pyrolysis GC MS chromatogram for chlorogenic acid and chlorogenic acid with CH 3 COOK added are given in Figs. 12 and 13 respectively. The main decomposition products for raw chlorogenic acid were caffeic acid derivatives: 1,2-benzenediol; Fig. 10. Pyrolysis GC MS chromatogram for xylan (raw) sample. The main peaks assigned from mass spectral detection as follows: (1) acetic acid; (2) 1-hydroxy-2- propanone; (3) 1-hydroxy-2-butanone; (4#) carbonic acid, ethyl 2-propenylester; (5) furfural; (6) cyclohexanone; (7) 2-butanone; (8) 2-oxo-butanoic acid; (9) 2- hydroxy-2-cyclopenten-1-one; (10) 3-methyl-2-cyclopenten-1-one; (11) butylolacetone; (12) 2(5H)-furanone; (13#) 3,4-dihydroxy-3-cyclobutene-1,2- dione; (14) 2-hydroxy-3-methyl-2-cyclopenten-1-one; (15) 2-hydroxy-3,5- dimethylcyclopenten-2-en-1-one; (16) phenol; (17) 2-methoxyphenol; (18) 2- or/and 3-methylphenol; (19) 3-ethyl-2-hydroxy-2-cyclopenten-1-one; (20#) carbonic acid, methyl 2-propenylester; (21) 2-propen-1-ol; (22) 4- methylphenol; (23) Unknown; (24#) 3-methyl-2,4(3H, 5H)-furandione; (25) 2- methoxy-vinylphenol; (26) 2,6-dimethoxy-phenol; (27) 3- or/and 4- hydroxybenzaldehyde; (28) 2-methylbenzene-1,4-diol; (29#) 3,4- dihydroxycoumarin-6-ol; (30) 2,6-dimethoxy-4-(2-propenyl)-phenol. #: Not detected in xylan sample with CH 3 COOK; other thermal degradation products remain the same in all xylan samples. Fig. 12. Pyrolysis GC MS chromatogram for chlorogenic acid sample (uncatalysed). The main peaks assigned from mass spectral detection as follows: (1) 1,3- cyclopentadiene; (2) benzene; (3) acetic acid or/and acetic acid anhydride with formic acid; 4: 1,2-cyclohexanedione; 5: 2-cyclopenten-1,4-dione; (6) 2-methyl-2- cyclopenten-1-one; (7) 2-cyclohexen-1-one; (8) 1,2-cyclohexenedione; (9) 2- pentanol; (10) 1-hydroxy-2-butanone; (11) 3-methyl-1,2-cyclopentanedione; (12) phenol; (13) 2-methyl-phenol; (14) 4-methyl-phenol; (15) 3-methyl-phenol; (16) 2,3-dihydroxybenzaldehyde; (17) benzoic acid; (18) 2-coumaranone; (19) 2,3- dihydroxybenzofuran; (20) 3-methyl benzoic acid; (21) 1,2-benzenediol; (22) 1-(5- methyl-2-furanyl)-1-propanone; (23) 4-acetylcyclohexanone; (24) 3-methyl-1,2- benzenediol; (25) 4-methyl-1,2-benzenediol; (26) hydroquinone; (27) 4-ethyl-1,2- benzenediol; (28) 2-hydroxy-5-methylisophthalaldehyde; (29) 2-hydroxy-6- methylbenzaldehyde; (30) 3-hydroxy benzoic acid; (31) (R)-( )-quinic acid.

10 D.J. Nowakowski, J.M. Jones / J. Anal. Appl. Pyrolysis 83 (2008) Fig. 13. Pyrolysis GC MS chromatogram for chlorogenic acid sample (potassium catalysed). The main peaks assigned from mass spectral detection as follows: (1) 1,3-cyclopentadiene; (2) acetone; (3) 1,2-butanedione; (4) benzene; (5) acetic acid; (6) 1,2-cyclohexanedione; (A (secondary products)) 2-cyclopenten-1-one, 2- methyl-2-cyclopenten-1-one, cyclopentanone, 1-hydroxy-2-butanone; (7) 2- cyclohexen-1-one; (8) 1,2-cyclohexenedione; (9) 2-hydroxy-cyclohexanone; (10) 3-methyl-1,2-cyclopentanedione; (11) phenol; (12) 2-methyl-phenol; (13) 4- methyl-phenol; (14) 3-methyl-phenol; (B) 1,2-dihydroxy-benzaldehyde + 1,4- cyclohexanedione + 2-ethyl-phenol + 2,5-dimethyl-phenol; (C) ethylphenol + benzoic acid; (15) 3,5-diethyl-phenol; (16) 1,2-benzenediol; (17) 3- methyl-1,2-benzenediol; (18) 4-methyl-1,2-benzenediol; (19) 5-ethyl-1,2- benzenediol; (20) hydroquinone; (21) 4-ethyl-1,2-benzenediol; (22) 4-ethyl-1,3- benzenediol; (23) 2-hydroxy-6-methyl-benzaldehyde; (24) 2,3-dihydro-2,2- dimethyl-7-benzofuranol; (25) (R)-( )-quinic acid. 4-methyl-1,2-benzenediol; 4-ethyl-1,2-benzenediol and ( )- quinic acid. The postulated thermal decomposition scheme for chlorogenic acid is presented in Scheme 3; this is an extension to that proposed by Hajagol and co-workers [34]. The first step in the thermal decomposition of chlorogenic acid is breaking of the side chain at various positions from the aromatic (benzene) ring, as shown by the arrows labelled 1 4. These products were also detected at low heating rate pyrolysis [34], and are assumed to be primary pyrolysis products. These primary decomposition products can crack and rearrange further to produce secondary products, benzene, benzoic acid, 2-cumaranone, 2-hydroxy-5- methylisophthalaldehyde and 2-hydroxy-6-methylbenzaldehyde. Other secondary decomposition products are: phenol; hydroquinone; 1,3-cyclopentadiene and cyclohexa-3,5-diene- 1,2-dione. The presence of potassium in chlorogenic acid produces a similar PY GC MS fingerprint to that seen for the raw sample, and a similar decomposition pathway is proposed in Scheme 4. There are a few areas of the chromatogram that show changes, in particular small peaks due to the appearance of additional secondary reaction products including methyl derivative phenols, and increased hydroquinone, 1,2-cyclohexenedione, 2-hydroxycyclohexanone, and 1,2-cyclohexanedione. There is a decrease in the amount of benzene, benzoic acid, 2-cumeranone and 2- hydroxy-5-methylisophthalaldehyde. Obviously, polymerization reactions are also promoted to some extent as seen by the increased production of char. Scheme 3. Proposed pyrolytic decomposition pathway of chlorogenic acid (uncatalysed). Assignments: (a) ( )-quinic acid; (b) 2-hydroxy-6-methylbenzaldehyde; (c) 2- cumaranone; (d) benzoic acid; (e) 2-hydroxy-5-methylisophthalaldehyde; (f) hydroquinone; (g) benzene; (h) 4-ethyl-1,2-benzenediol; (i) 4-methyl-1,2-benzenediol; (j) 1,2- benzenediol; (k) cyclohexa-3,5-diene-1,2-dione; (l) phenol; (m) 1,3-cyclopentadiene.

11 22 D.J. Nowakowski, J.M. Jones / J. Anal. Appl. Pyrolysis 83 (2008) Scheme 4. Proposed pyrolytic decomposition pathway of chlorogenic acid (potassium catalysed). Assignments: (a)- ( )-quinic acid; (b) 2-hydroxycyclohexanone; (c) 1,2- cyclohexanedione; (d) o-cresol; (e) m-cresol; (f) p-cresol; (g) 4-ethyl-1,2-benzenediol; (h) 3-ethyl-1,2-benzenediol; (i) 4-methyl-1,2-benzenediol; (j) 2-hydroxy-6- methylbenzaldehyde; (k) 1,2-benzenediol; (l) cyclohexa-3,5-diene-1,2-dione; (m) hydroquinone; (n) phenol; (o) 1,3-cyclopentadiene. 4. Conclusions The uncatalysed route for cellulose decomposition produces one endothermic weight loss region, and relatively high yields of anhydrosugars, and low yields of char. This work has identified five different anhydrosugar derivatives in the tar together with rearrangement products. The potassium-catalysed pyrolysis route becomes exothermic due to the change in the pyrolysis product distribution. In this study, the char yield is seen to triple, and there is lower tar yield comprising mainly phenol and cyclopentene derivatives and lighter ring-open cracked species such as organic acids. These products are consistent with a base catalysed route in the presence of the potassium salt, and base catalysed polymerization reactions increasing the char yield. A very similar trend is seen for levoglucosan pyrolysis, a primary product of cellulose pyrolysis. The influence of potassium on the pyrolysis tar species of other cell-wall components and model compounds is less clear-cut. The PY GC MS fingerprints do not show the dramatic change seen for cellulose. Somewhat surprisingly there was little effect of potassium on the pyrolysis of oat spelt xylan. The temperature of maximum pyrolysis rate, measured by DTG, and the char yields, were unchanged upon addition of potassium. The degradation products contain, among other compounds, light organic acids and esters, furfural, and phenol and derivatives. A few of these components are not detected in the K-impregnated sample, but the main pyrolysis fingerprint looks very similar to that of the raw xylan. It is proposed that this is because the alkali metal content of the raw xylan is already significant, and so further addition of potassium produces little effect. The effect of potassium on lignin pyrolysis is very significant. The temperature of maximum conversion in pyrolysis shifts to lower temperature by 70 K, and catalysed polymerization reactions increase the char yield from 37% to 51%, although the overall reaction processes remain endothermic. A similar trend is observed for the model compound, chlorogenic acid. The pyrolysis reaction occurs in two distinct stages, as seen by DTG and these are both shifted to lower temperature in the catalysed reaction. For lignin, it is noticeable that the PY GC MS fingerprints of the raw and K-impregnated samples are similar, although some products present from the uncatalysed reaction were not detected for the catalysed reaction, and some were reduced in intensity. For chlorogenic acid pyrolysis, PY GC MS fingerprints of raw and K- doped samples are also similar, although certain secondary products are promoted by the presence of potassium, and new species such as methylphenols are detected in small amounts. Polymerization to form char is also promoted, although to a much smaller extent than for lignin. Acknowledgements J.M.J. is an Engineering and Physical Sciences Research Council (EPSRC, United Kingdom) Advanced Research Fellow and the authors are grateful to the EPSRC for financial support under Grant numbers GR/S49018 and GR/S49025.

12 D.J. Nowakowski, J.M. Jones / J. Anal. Appl. Pyrolysis 83 (2008) Appendix A Key markers assignment for PY GC MS of cellulose, levoglucosan, xylan and lignin samples Name of compound, chemical formula Structure, chemical formula Type of key marker Molecular weight m/z 2-Cyclopenten-1-one, C 5 H 6 O Levoglucosan (100.0%), (5.5%) Phenol, C 6 H 6 O Cellulose, Lignin (100.0%), (6.6%) 2-Cyclopenten-1,4-dione, C 5 H 4 O 2 Cellulose (100.0%), (5.4%) 2H-Pyran-2-one, C 5 H 4 O 2 Cellulose, Levoglucosan (100.0%), (5.4%) Furfural, C 5 H 4 O 2 Cellulose, xylan (100.0%), (5.4%) 2-Methyl-2-cyclopenten-1-one, C 6 H 8 O Cellulose (100.0%), (6.6%) 2-Hydroxy-2-cyclopenten-1-one, C5H 6 O 2 Cellulose (100.0%), (5.6%) 2-Furanmethanol, C 5 H 6 O 2 Cellulose (100.0%), (5.6%) Cyclohexanone, C 6 H 10 O Xylan (100.0%), (6.6%) 2-Ethyl-2-cyclopenten-1-one, C 7 H 10 O Levoglucosan (100.0%), (7.7%) 3-Methyl-1,2-cyclopentanedione, C 6 H 8 O 2 Cellulose (100.0%), (6.7%) 2-Hydroxy-3-methyl-2- Xylan (100.0%), (6.7%) cyclopenten-1-one, C 6 H 8 O 2 2-Hydroxy-4-methyl-2- Cellulose (100.0%), (6.7%) cyclopenten-1-one, C 6 H 8 O 2 3-Methyl-2,4-(3H,5H)- Xylan (100.0%), (5.6%) furandione, C 5 H 6 O 3 3,4-Dihydroxy-3-cyclobutene-1, Cellulose, xylan (100.0%), (4.5%) 2-dione, C 4 H 2 O 4 Dihydroxyfuran-2(3H)-one, C 4 H 4 O 4 Cellulose (100.0%), (4.3%)

13 24 D.J. Nowakowski, J.M. Jones / J. Anal. Appl. Pyrolysis 83 (2008) Appendix A (Continued ) Name of compound, chemical formula Structure, chemical formula Type of key marker Molecular weight m/z 2,4-Dimethylphenol, C 8 H 10 O Cellulose (100.0%), (8.8%) Mequinol, C 7 H 8 O 2 Lignin (100.0%), (7.7%) 5-(Hydroxymethyl)-2- furancarboxyaldehyde, C 6 H 6 O 3 Cellulose (100.0%), (6.7%) 2,5-Dimethyl-4-hydroxy-3(2H)- Cellulose (100.0%), (6.7%) furanone, C6H 8 O 3 2-Hydroxy-3-propenyl-2- Cellulose (100.0%), (8.8%) cyclopenten-1-one, C 8 H 10 O 2 2-Methoxy-4-vinyl-phenol, C 9 H 10 O 2 Xylan, Guaiacol lignin (100.0%), (9.9%) 4-Hydroxy-3-methoxy-benzaldehyde Guaiacol lignin (100.0%), (8.9%) (vanillin), C 8 H 8 O 3 4-Ethyl-2-methoxy-phenol, C 9 H 12 O 2 Guaiacol lignin (100.0%), (9.9%) 3,4-Dimethoxyphenol, C 8 H 10 O 3 Guaiacol lignin (100.0%), (8.9%) 2,6-Dimethoxyphenol, C8H 10 O 3 Syringol lignin (100.0%), (8.9%) 1,6-Anhydro-b-D-glucopyranose Cellulose (100.0%), (6.8%), (levoglucosan), C 6 H 10 O (1.2%) 2-Methoxy-4-(1-propenyl)-phenol, Guaiacol lignin (100.0%), (11.0%) C 10 H 12 O 2 2-Methoxy-4-(2-propenyl)-phenol Guaiacol lignin (100.0%), (11.0%) (eugenol), C 10 H 12 O 2 1-(3,4-Dimethoxyphenyl)ethanone, Lignin (100.0%), (11.1%) C 10 H 12 O 3 4-Hydroxy-3-methoxy-benzeneacetic Guaiacol lignin (100.0%), (10.0%), acid, C 9 H 10 O (1.2%)

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