Comparison of thermal behavior of natural and hot-washed sisal fibers based on their main components: Cellulose, xylan and lignin. TG-FTIR analysis of volatile products

1 Comparison of thermal behavior of natural and hot-washed sisal fibers based on their main components: cellulose, xylan and lignin. TG-FTIR analysis of volatile products Mónica Benítez-Guerrero 1*, Jorge López-Beceiro 2, Pedro E. Sánchez-Jiménez3 and José Pascual-Cosp 1. 1 Departamento de Ingeniería Civil, Materiales y Fabricación, Universidad de Málaga, Escuela de Ingenierías. C/ Dr. Ortiz Ramos s/n. Campus Teatinos. 29071 Málaga. Spain. 2 Departamento de Ingeniería Industrial II. Escola Politécnica Superior. Universidade da Coruña, Avda. Mendizábal, 15403 Ferrol, Spain. 3 Instituto de Ciencia de Materiales de Sevilla, CSIC-Universidad de Sevilla, C. Américo Vespucio 49, 41092 Sevilla, Spain *Corresponding author. Email: [email protected] Tel: +34 951952594 Fax: +34 951952601 Abstract: This paper presents in a comprehensive way the thermal behavior of natural and hot-.washed sisal fibers, based on the fundamental components of lignocellulosic materials: cellulose, xylan and lignin. The research highlights the influence exerted on the thermal stability of sisal fibers by other constituents such as non-cellulosic polysaccharides (NCP) and mineral matter. Thermal changes were investigated by thermal X-ray diffraction (TXRD), analyzing the crystallinity index (%Ic) of cellulosic samples, and by simultaneous thermogravimetric and differential thermal analysis coupled with Fourier-transformed infrared spectrometry (TG/DTA-FTIR), which allowed to examine the evolution of the main volatile compounds evolved during the degradation under inert and oxidizing atmospheres. The work demonstrates the potential of this technique to elucidate different steps during the thermal decomposition of sisal, providing extensible results to other lignocellulosic fibers, through the analysis of the evolution of CO2, CO, H2O, CH4, acetic acid, formic acid, methanol, formaldehyde and 2-butanone, and comparing it with the volatile products from pyrolysis of the biomass components. The hydroxyacetaldehyde detected during pyrolysis of sisal is indicative of an alternative route to that of levoglucosan, generated during cellulose pyrolysis. Hot-washing at 75 ºC mostly extracts non-cellulosic components of low decomposition temperature, and reduces the range of temperature in which sisal decomposition occurs, causing a retard in the pyrolysis stage and increasing TbNCP and TbCEL, temperatures at the maximum mass loss rate of non-cellulosic polysaccharides and cellulose decompositions, respectively. However, enriching sisal fibers in cellulose produces a decrease of TbCEL under an oxidizing atmosphere, and furthermore, a delay of the combustion process, displacing TbCOM to higher temperatures. The results and findings of the paper would help further understanding of thermal processes where agave fibers are involved, as the decomposition of their composites. Keywords: Sisal fiber; Hot-water treatment; Biomass components; TGA-FTIR; Gas evolution; Pyrolysis; Combustion. 2 1. Introduction The large number and diversity of lignocellulosic products as well as plurality of their applications in many industrial sectors, implies the need to comprehensively know their thermal behavior, stability and degradation processes at elevated temperatures. Pyrolysis studies of synthetic biomass, obtained by mixing its fundamental constituents, revealed that, in general, the overall degradation of biomass follows the principle of additivity [1,2]. However, different authors [3,4] show that interaction among the constituents exits, whereby the proportion of the pyrolysis products differs from that of natural biomass. It has been demonstrated that the addition of extractives and mineral compounds to the fundamental biomass components [5-7], and the degree of crystallinity of cellulose [8], affect the pyrolysis process. The influence of extractives and mineral matter on thermal decomposition of different lignocellulosic materials has been studied extensively [9-11]. Water washing is a simple and economical pretreatment, which has been used to remove alkali metals from biomass, to avoid the toxicity of mists generated by the combustion of biomass, so preventing accumulation problems (deposition, corrosion, erosion, etc.) in combustion and gasification plants, and also to improve process yields [12-14]. Hot washing process reduces the proportion of substances adhered to the cell walls, removing organic compounds, as a hemicellulose fraction and other low molecular weight carbohydrates, and minerals such as salts of alkali, alkaline earth and transition metal cations. In addition to the traditional purposes, the use of Agave species fibers has spread to a variety of processes of interest in many industrial sectors, due to their ease of cultivation, with high productivity and low cost associated, as well as their renewable and biodegradable character. Furthermore, their biomass component proportion makes them suitable for processability and biofuel production [15,16]. In particular, Agave sisalana fiber or sisal fiber is one of the most widely used due to its good mechanical properties, used in a great variety of traditional applications due to its hardness, coarseness and resistance to wear, which has gained great interest during the last decades in the manufacturing of composite materials, particularly as reinforcement of mortars, concrete and polymeric matrices [17-19], as polypropylene [20-22]. Sisal fibers are mainly composed of cellulose and non-cellulosic polysaccharides (NCP): hemicellulose O-acetyl-(4-O-methylglucurono)xylan [23] and pectins composed of rhamnose, arabinose and uronic acid at a 2.3:1:3.3 ratio [24], along with a smaller amount of lignin, whose molar ratio of constitutional units, p-hydroxyl phenol (H), guaiacyl (G) and syringyl (S), has been established in 2% H, 22% G and 76% S [25]. Furthermore, the fiber is constituted by a lower proportion of extractives, such as lipophilic components [26], simple phenolic derivatives and mineral matter, mainly represented by calcium oxalate deposits located between the elementary fibers comprising the fiber bundle [27]. The composition of sisal, as any natural fiber, varies depending on the origin, age and extraction process of the components, among other factors. The compositional variability in terms of constituents is very large as can be deduced from different reviews [15,28,29]. There is a lot of information scattered in the literature about the thermal behavior of sisal fibers, but few studies have analyzed it in depth [30-32]. The extensive work recently published by Martin et al. 3 [33] focuses on the thermal decomposition of sisal and the main biocomponents extracted from the fiber itself. However, given the complexity of thermal processes occurring in lignocellulosic materials, the comparison of thermogravimetric stages with their associated energy effects requires a detailed analysis, interpreted through the analysis of the decomposition of pure biomass components, adding a new perspective that would allow for a better definition of the phenomena occurring. Furthermore, the evolution of the thermal degradation products of sisal fibers is poorly described. Only the liquid fraction released during fast pyrolysis has been characterized [34], there being a lack of information about the gaseous products emitted. The thermogravimetry coupled with Fourier-transformed infrared spectroscopy (TG-FTIR) technique has been proved to be a valuable and simple tool, not only to analyze the gas phase during the thermolysis of lignocellulosic compounds [35-37], but also to clarify different steps during fiber decomposition. This paper aims to analyze and present in a comprehensive way the thermal behavior of natural and hot washed sisal fibers, discussing their thermogravimetric and differential thermal analysis (TGDTA) on the basis of the degradation of fundamental components, cellulose, lignin and xylan, and comparing the evolution of the gas phase decomposition products by TG-FTIR analysis. 2. Materials and Methods The sisal fibers used in this work were provided by Cayetano García del Moral S. L. (Cabra de Santo Cristo, Jaén, Spain). They were cut to 2 - 4 mm length and then subjected to a washing process with stirring in hot water at 75 °C for 2 hours. Hereinafter the samples will be referred to as Natural Sisal and Washed Sisal. Microcrystalline cellulose powder (Aldrich), alkali low sulfonate content lignin (Sigma Aldrich) and birch xylan (xylose ≥ 90%, Aldrich) were used as representatives of the main biomass components. X-ray diffraction (XRD) measurements were performed in a Philips X Pert PRO MPD, with a X Celerator real time multiple strip RTMS system. Thermodiffraction was performed in Helium atmosphere at temperatures up to 400 °C, using a TTK450N chamber for Aldrich cellulose and a one HTK1200N for Natural Sisal. Spectra were acquired every 20 °C, each one was recorded for 50 minutes in the 2θ range from 5 to 45°. Temperature was linearly changed between isotherms at 5 ºC/min, and the stabilization time at each isotherm was 5 minute. Data were analyzed employing X'Pert HighScore software. The crystallinity index, %Ic(XRD), was calculated at several temperatures using the equation proposed by Segal [38]: 1001)( max  I I =XRD%I am c where Imax is the maximum intensity of the (002) lattice reflection of cellulose I type, located at 2θ between 22 and 23º, and Iam is the intensity attributed to amorphous part, given at 2θ between 18º and 19º for I cellulose. 4 Thermogravimetric study was performed in a TA Instruments STD 2960 simultaneous thermal analyzer. The experimental setup consisted of a 10 °C/min linear heating program from 20 to 750 °C, with flow rates of 50 and 120 mL/min, both in air and in nitrogen. The gases released were immediately analyzed in a Bruker VECTOR 22 infrared spectrometer, coupled to the outlet port of the SDT. The transfer line and the cell were kept to a temperature of 200 °C. The spectrum of the evolved gas was recorded every 61 seconds, and consists of an average of 64 scans with a resolution of 4 cm-1 in the range from 4000 to 500 cm-1. Spectra were analyzed using Bruker OPUS 5.5 software, being baseline-corrected by a concave rubber band algorithm computed with 10 iterations on 64 points. Ashes from the combustion of natural and washed sisal fibers were semi quantitatively analyzed by X-ray fluorescence, using a PANalytical AXIOS spectrometer. 3. Results and Discussion 3.1. Evolution of the crystallinity index of cellulose and sisal The diffraction patterns at different temperatures of microcrystalline cellulose and Natural Sisal are shown in Fig. 1. At room temperature, cellulose clearly presents reflections around 2θ = 15, 16.5, 20.5, 22.5 and 34.5º, corresponding respectively to the planes (101), (101), (021), (002) and (040) of I type allomorph, while sisal fiber only exhibits the main (002) peak, and a convolution of (101)-(10 1) reflections, which are superimposed to calcium oxalate hydrate Ca(COO)2·H2O ones (pattern 20-0231 of the Joint Committee on Powder Diffraction Standards - International Center for Diffraction Data, JCPDS – ICDD). For both, it is observed that the intensity of reflections decreases with increasing temperature, shifting the diffraction peaks towards a greater spacing. It can be explained due to the progressive deterioration of cellulose crystal lattice, until the structure becomes completely amorphous, observed before 310 ºC. Furthermore, the constitutive minerals of sisal fiber experiment transformations with increasing temperature, generating stable crystalline compounds under inert atmosphere. Figure 1. X-ray diffractograms at different temperatures, a of microcrystalline cellulose and b of Natural sisal, where can be observed reflexions from Ca(COO)2·H2O (■). 5 The evolution of %Ic(XRD) with temperature for both cellulose and sisal is displayed in Fig. 2, being observed a much more pronounced variation for the natural fiber. A rapid decrease occurs for sisal index at temperatures below than 100 °C, remaining stable at around 42% until 210 °C. In contrast, it diminishes slowly to 76% for cellulose. Near 230 ºC, the crystallinity index of both sisal and cellulose falls sharply, presenting almost the same slope in both cases. At 270 ºC it reaches the smallest value of 17% for sisal, and 65% for the studied cellulose, which is totally amorphized above 310 ºC. This behavior for sisal fiber is close to that reported by Saikia [32], which describes a reduction of the degree of crystallinity from 51 to 41% when sisal is heated in air at 177 °C, becoming completely amorphous at around 260 °C. Figure 2. Variation of the crystallinity index, %Ic(XRD), with temperature for microcrystalline cellulose (▲) and Natural Sisal (●). 3.2. TG-DTG/DTA analysis of the biomass components Previously to the analysis of the thermal decomposition of sisal fibers, it is important to understand the behavior of the main lignocellulosic components. Figure 3 exhibits TG, DTG and DTA plots for the studied cellulose, xylan, and lignin, under both air and nitrogen atmospheres. Temperature ranges, temperature of maximum mass loss rate (Tb) and the mass loss on a dry basis are summarized in Table 1. The weight on dry basis is calculated from each thermogram excluding the mass loss below 180ºC. 6 Figure 3. TG/DTG-DTA thermograms of the main biomass components, under nitrogen (a-b) and air purges (c-d) with flow rates of 120 mL/min. The inner inset in Fig. 3b notes three superimposed stages on DTG curve of lignin. Table 1: Thermoanalytical data TG/DTG-DTA of biomass components, under air and nitrogen atmospheres (120 mL/min flow rate). Tb a(ºC) 21 – 185 65 52 endo 185 – 329 325 61.9 325 endo 329 – 354 333 17.9 350 exo Cellulose Ash (737 ºC) 0.3 20 – 165 88 90 endo 165 –298 247 58.9 268 endo 298 –423 344 31.2 390 exo 423 – 502 435 1.3 438 exo 502 –653 562 0.7 565 exo Xylan 7.3 % wt bTpeak (ºC) DTA Air Ash (737ºC) Nitrogen 200 – 447 338 83.8 335 endo 10.8 23 – 161 85 80 endo 161 –242 228 7.2 242 –362 286 56.0 291 exo 362 – 548 460 8.8 458 exo Tb a(ºC) % wt bTpeak (ºC) DTA 23 – 200 66 56 endo a DTG peak temperature b mass loss percentage and ash/residue percentage in dry base 354 – 737 473 19.9 475, 600 exo 20.8 Lignin 26 – 180 86 98 endo 180 – 344 305 22.8 319 exo 344 – 426 380 6.6 403 exo Ash (737 ºC) 62.5 426 – 737 456 8.1 474 exo 26 – 176 88 90 endo 176 – 414 (278) 315 (358) 28.2 (292 exo) 312 endo (353 exo) 414 – 486 453 4.2 478 exo 486 – 737 678 9.9 647 exo 57.7 400 exo Residue (737 ºC) Residue (737ºC) Residue (737 ºC) 7 It can be distinguished fundamentally three stages, described underneath. i) In the low temperature range, below 180 °C, fundamentally physically adsorbed water evaporates. The mass losses account for about 4% for cellulose, 7% for xylan and 9% for lignin. The process implies a large endothermic effect with DTA peaks under nitrogen atmosphere at 56, 80 and 90 °C, respectively ii) Between 200 and 400 °C processes of depolymerization, decomposition and rearrangement occur, involving a considerable mass loss. Chemical bonds of the starting biomass break (primary reaction), generating a reactive residue (reactive char) plus liquid (tar) and gaseous compounds, taking place under inert and air atmosphere by different mechanisms, pyrolysis and oxidative pyrolysis respectively. The last one explains an accelerated decomposition in the presence of oxygen, related to oxidative type reactions described for cellulose [39,40] and for lignin [41], which in turn results in the advancement of the maximum rate of decomposition. At this stage, endothermal volatilization and exothermal charring processes are responsible of the resulting energy balance [42,43]. Comparing biomass components, xylan decomposes at lower temperatures, followed by cellulose, while lignin does it over a wider range of temperatures, due to its heterogeneous structure, comprising aromatic rings with multiple branches and functional groups, whose bonds cleave along a wide range of temperature. Two sub-stages of exothermic nature can be observed in hemicellulose decomposition. The first one, very slight with Tb at around 228 ºC, can be associated to the decomposition of residual side chains, still present in the commercial xylan, assigned to the decomposition of 4-O-methyl glucuronic acid and acetyl groups of xylan by Shen et al. [44], while the second sub-stage, located at around 286 °C, is due to rupture of the xylan main chain. These sub-stages are hardly distinguishable in air, where the decomposition process occurs at much higher speed than in inert atmosphere. Cellulose is the biomass component which presents the maximum mass loss and maximum mass loss rate. Pyrolysis of cellulose can be described in a simplified form as a mechanism of chain depolymerization via transglycosylation reactions [45]. One important endothermic process is observed in nitrogen atmosphere at Tb = 338 °C, associated to the heat of evaporation of volatile products generated by the pyrolysis of the glycosyl units [40], and it matches with the collapse of the structure and the complete loss of the crystalline structure, as shown in Fig. 1A. In contrast, a sharper endotherm at Tb = 325 °C is observed in air. This shifting to lower temperature is due to the oxidative degradation of cellulose which starts at lower temperatures as a result of radical autoxidation mechanism that occurs in the presence of oxygen [39]. In air, immediately after the endothermic process, it can be distinguished a large exothermic one with Tb at 333 °C and maximum DTA peak at 350 °C, associated to the oxidation of volatiles generated in the preceding endothermic process. Lignin is the one of the biomass components which experiences the lower mass loss and at the lowest rate. Its thermal decomposition, in which at least three superimposed processes can be distinguished at around 278, 315 and 358 ºC in nitrogen atmosphere (as shown the inset of the Fig. 3B), present an overall exothermic process of greater intensity in air than in nitrogen. Furthermore, after pyrolysis stage, lignin is the component which presents the highest char content, about a 60%, generated 8 by condensation of the phenolic groups during its thermal degradation under inert atmosphere. The G- type lignin used in this work probably contributes to a higher char generation due to the own nature of the biomass polymer that, in one hand, allows a high degree of crosslinking facilitated by a reduced content in methoxyl groups, which in turn increases its thermal stability, and that, on the other hand, due to the own constituent guaiacyl units, is prone to experience thermal condensation reactions [46]. iii) At temperatures higher than 350-400 ºC, the thermal behavior depends on the atmosphere. Under inert gas, the structure of the products generated in the previous stage starts to change through cleavage of bonds and volatilization of heteroatoms, involved in the condensation and reorganization processes leading to the formation of products with high carbon content. This carbonization stage is accompanied by a small and gradual mass loss and by slight energy changes, as it can be respectively observed on the TG and DTA plots. In addition, the volatilized compounds undergo secondary thermolythic reactions in gas phase, which do not affect the mass of the remaining sample. The effect on the DTA signal is weak since the thermocouples of the instrument are located at the bottom of the sample and reference crucibles, relatively distant from where the evolved gases may react. Fisher et al. [47] maintain that secondary reactions can be thermally neutral, depending on the conditions under which the char is generated. In the case of cellulose, a small exothermic effect is observed next to the 338°C endotherm, at around 400 °C, which is associated to the charring process. Milosavljevic et al. [48] claim that the charring process of the cellulose can be exothermic (-2.6 kJ/g referred to residual carbon), endothermic (0.86 kJ/g) or combination of both, depending on the products generated. Similarly, exothermic effects at around 460 °C in the case of xylan, and at 480 and 650 °C in the case of lignin, are associated to carbonizing processes. In the presence of oxygen, the oxidation of volatile products (homogeneous reaction) and combustion of the char (heterogeneous reaction) take place in the range between 350 and 500 °C, depending on the nature of the lignocellulosic material. These exothermic phenomena appear at lower temperature and with higher intensity in xylan (390 ºC) than in cellulose (475 ºC). In addition, small exothermic effects are observed at higher temperatures, which can be associated to residual combustion processes, accompanied by a very small mass loss, which will be discussed afterwards. However, in the case of lignin, the combustion stage does not constitute a noticeable phenomenon. It appears to pass through several consecutive mild stages, with small mass loss at Tb = 380 and 456 °C. Moreover, unlike what may be expected, the percentage of the final residue of lignin is slightly greater in air than in nitrogen (63% > 58%). This can also be explained by several reasons: a) that a partial oxidation has occurred, generating compounds of higher molecular weight; b) that a stable char has been formed due to the elevated content in aromatics [49]; c) that the metals present in lignin oxidize, thus increasing their mass. Di Blasi [50] describes many factors that affect the reactivity of the char which could explain the elevated percentage that remain after thermal decomposition of lignin under air. The data presented here confirm that the thermal stability of the biomass components decreases in the following order: lignin, cellulose and hemicellulose. These different behaviors are due to their different chemical composition and structure [51]. Hemicellulose has an amorphous structure, formed by 9 various polysaccharides with branches of different compositions, while cellulose is a polymer composed of linear chains of glucose, with a longer range order and a degree of polymerization of ten to one hundred times higher than that of hemicellulose. However, lignin is a highly cross-linked polymer consisting of highly branched units of phenylpropane, which confers a great thermal stability. 3.3. TG-DTG/DTA analysis of natural and hot-washed sisal fibers Thermal degradation of sisal and washed sisal has been studied by means of simultaneous TGDTA using purges of air and nitrogen, both at 50 and 120 mL/min. Figure 4 shows a comparison of their thermogravimetric curves in air and nitrogen. Table 2 shows the mass loss on a dry basis, ranges of temperature and maximum temperature of the mass loss rate (Tb) of each stage. Figure 4. Comparison of TG/DTG-DTA thermograms of Natural and Washed Sisal, under 50 mL/min flow rate. 16 polysaccharides and cellulose. As in the case of microcrystalline cellulose, but to a lesser extent, it is observed a broad band at above 2500 cm-1, which after correction of the spectrum remains at around 3600-3000 cm-1, and a characteristic band at around 1047 cm-1, as indicated by the arrows in Fig. 6. These bands are attributed to a glycol type condensed phase, which was swept by the purge gas, as it will be discussed in the section of gases evolved from cellulose. Furthermore, differently than in the case of the pure biopolymers, a triplet band centered at 858 cm-1 is observed during the decomposition of sisal, denoted by an asterisk in Fig. 6, which is assigned to hydroxyacetaldehyde, (OH)CH2C(O)H) [60]. Figure 6. FTIR spectra of gases released out during decomposition of Washed Sisal under nitrogen atmosphere, at TbPNC (295 ºC) and TbCEL (350 ºC). Arrows denote characteristic bands of the emitted glycol phase, while the asterisk shows the vibration assigned to hydroxyacetaldehyde. It is known that the absorbance at a specific wavenumber is linearly dependent on the concentration of gas, so the changes in absorbance reflect the tendency of concentration variation of the different gas species, provided that the FTIR procedures are the same [35,42]. Thermograms of Fig. 7, 8, 9 and 10 show the evolution of the main infrared bands associated with gaseous products for pure biomassic components and for washed sisal, which will be discussed in more detail in the following sections. Figure 11 shows the comparison of the evolution of these bands for the different components and the hot-washed sisal, under two flow rates of nitrogen, 50 and 120 mL/min. Data derived from the mentioned thermograms are collected in Tables 4 and 5, which show the relative intensities of different vibrations with respect to the maximum absorbance of CO2 emitted in the pyrolysis stage for each biocomponent and washed sisal. 17 Table 4 Maximum intensities of the main infrared bands of volatiles emitted by biomass components, under 120 mL/min of nitrogen or air purges, expressed as relative percentage respect to the maximum absorbance of CO2 evolved in the respective pyrolysis stages. Table 5 Maximun intensities of the main infrared bands of volatiles emitted by Washed Sisal, under different atmosphere and flow rate, expressed as relative percentage respect to the maximum absorbance of CO2 evolved in the respective pyrolysis stages. 18 3.4.1. Analysis of gases evolved from the pure lignocellulosic components The analysis of gases evolved in the decomposition of pure components in air and nitrogen is described, previously to study the more complex case of sisal fiber. i) Xylan: In nitrogen atmosphere, the following species are detected at temperatures below 350°C: CO2, CO, H2O, acetic acid, formic acid, methanol, formaldehyde and a ketone, with vibrations near 1178 and 1734 cm-1 (Fig. 7). Figure 7. Evolution of the main infrared bands of the volatiles emitted during xylan decomposition, under 120 mL/min nitrogen (a-c) and air (d-f) purges, with the corresponding DTG curve. Among the three biomass components considered, xylan is the one which presents the highest absorbance of CO2 and the highest CO/CO2 ratio (Table 4). CO2 comes from the breakdown of C=O and COOH groups [42]. It would mainly come from decarboxylation reactions of O-acetyl units according to 19 Shen et al. [61]. In the present case that contribution must be less important, since the starting material is a hemicellulose mostly composed of xylose units. The CO formation is attributed to decarbonylation reactions, specifically to the breakdown of COC and C=O [42], due to the decomposition of the ringopened intermediate products [61]. The H2O emission during the pyrolysis stage is mainly derived from the hydroxyl groups of xylopyranose units. The emission of acetic acid and methanol is attributed to the breakdown of the O-acetyl and O-methyl groups of the residual side chains, which are even present after the extraction and purification of commercial xylan. The presence of various ketones in the products resulting from xylan pyrolysis was identified using Py-GC/MS by Y. Wu et al. [62], who found at 260 °C mainly 1-hydroxy-2-butanone, 1-hydroxy-propanone, and, to a lesser extent, 4-hydroxy-3-hexanone. In nitrogen at temperatures above 400 °C certain gases generated by secondary reactions of decomposition of anhydrous sugars are distinguished. Thus, the CO2 emission continues, but slightly, and CH4 is detected with a maximum near 470 °C, close to the exothermic effect observed in the DTA of xylan (Table 1). CH4 is formed by breaking R-CH3 bonds, which requires elevated temperatures due to its high stability [61]. The CO2 emission starts to increase at 550 °C, reaching a maximum rate at about 700 °C, temperature at which a new CO emission begins. During the pyrolytic decomposition stage in air the same gases are detected than when using inert atmosphere. Their proportions with respect to the CO2 emitted are quite similar to those obtained under nitrogen atmosphere, with the exception of CO principally, whose emission diminishes in air (Table 4). Furthermore, mass losses in air and nitrogen are similar, 59 and 63% respectively (Table 1). It can be explained through the phenomena that affect the volatiles emitted in the decomposition step. In this case, the decomposition temperature of xylan (about 290 °C) is sufficiently low so that, firstly, most of the volatiles generated are not oxidized to CO or CO2 in air, and on the other hand, the self-generated CO would oxidize to CO2 in contact with air through the thermodynamically favored reaction: 2CO + O2  2CO2, which explains the lower proportion of CO. The CO2 emission during the combustion process, between 300 – 420 °C, is intense and presents an asymmetrical shape which indicates that the process is triggered by the temperature once the energy barrier of the combustion process is reached. Then, the combustion rate decreases as the remaining amount of sample decreases. The corresponding DTG plot reflects the same process, but the DTG peak is more resolved due to the delay and mixing of components that may occur during the gas flow from the TGA furnace to the FTIR detector [63]. Additionally, small emissions of CO2, H2O and CO, associated with mild exothermic effects are observed at around 440 ºC and at 570 °C (Fig. 7D and Table 4). The exotherm at 440 ºC corresponds to the completion of the 300 – 420 ºC combustion process, which produced an overheating on the programmed ramp. Then, the process was halted as the temperature decreased to recover the programmed values, as it can be observed on the DTG plot. The exotherm at 570 ºC corresponds to combustion of a more stable carbon residue, which resulted from degradation of the most stable structures. ii) Cellulose: In nitrogen, at temperatures below 400 °C, CO2, CO, H2O, formaldehyde, formic acid, and 2- butanone are detected, besides a glycol condensate (Fig. 8). This condensate is assigned to 1,2- 20 propanediol, intermediate reaction product, and to ethylene glycol, which was observed in pyrolytic decomposition of cellulose [64,65]. It should be noted that the signal of formic acid (1105 cm-1) is increased due to the glycol (1047 cm-1, Fig. 8B and 11C), which also hides methanol (1032 cm-1). Thus, it is not possible to determine the proportion of each component (Fig. 8A and 11F, and Table 4). Figure 8. Evolution of the main infrared bands of the volatiles emitted during microcrystalline cellulose decomposition, under 120mL/min nitrogen (a-c) and air (d-f) purges, with the corresponding DTG curve. In contrast to xylan most of the main gaseous products of cellulose are evolved together, in a narrower range of temperatures, with maximum close to 330 - 340 ºC, except for CH4, which is emitted at higher temperature. Cellulose is the component that produces the highest emissions of H2O and organic products with respect to the CO2 emitted (Table 4). The breaking of the glycosidic linkages and ring opening generate liquid and gaseous intermediates, whose identity and proportion are dependent on the pyrolysis conditions. Shafizadeh [66] and subsequently Shen et al. [67] collected and proposed different routes of formation of CO2, CO, CH4, formaldehyde, methanol and acetic acid, among other products of 21 decomposition of cellulose. In our study acetic acid has not been detected, so the bands located at 1178, 1772 and 1795 cm-1, shown in Fig. 8B, 8C, 11G, 11H and 11L are associated to acetone, formaldehyde and formic acid. Shen et al. [67] indicate that the formation of CO is favored by high temperatures and long residence times, which explain that CO formation is mainly affected by the secondary reactions, especially of aldehyde-type low molecular weight compounds. Nevertheless, CO2 is mainly formed in the early stages of the pyrolysis of cellulose. Li et al. [60] found that formaldehyde formation is favored by a lower purge flow and by longer residence times, which would explain its formation through secondary reactions. They explain that at high temperatures, near 550 °C, formaldehyde decomposes to generate CO and H2, which is consistent with the observation of Shen et al. Various ketones have been proposed and detected at the primary decomposition and pyrolysis of cellulose. Thus, 2,3-butanedione is among the products of pyrolysis of levoglucosan, a major decomposition product of cellulose [68]. 2-butanone, detected here at temperatures below 400 ºC, has also been detected by Py-GC/MS analysis [62]. According to other reports, it may come from decarboxylation of levulinic acid [66]. Radlein et al. [69] found formic acid in the pyrolysis products of different celluloses, which would be originated from the decomposition of levoglucosan according to Hosoya et al. [70]. In nitrogen, at temperatures above 450 °C, CO2, CO and CH4 are detected, resulting from side reactions. In this stage, maximum emission of CO2 and CH4 is observed at about 530 ºC, while the maxima of CO2 and CO appear at 650°C, as shown in Fig. 8A, 11A, 11E and 11I. The products detected during the decomposition in air of cellulose are the same than under inert atmosphere. However, compared to that of CO2, the emission of organic compounds and H2O has decreased significantly in air, while the proportion of CO is maintained in almost the same proportion as in nitrogen (Table 4). Since the mass loss after pyrolysis under air and nitrogen are similar, the explanation lies primarily in the changes suffered by the emitted gaseous compound. In an oxidizing atmosphere, when the temperature is high enough, it could happen that at the same instant in which the decomposition/pyrolysis of the cellulose occurs, the volatile organics generated oxidize, which would be favored by the presence of steam. Thus, a gasification reaction explains the lower proportion of H2O detected. On the other hand, the consumption of CO according to the reaction 2CO + O2  2CO2 is less thermodynamically favored, which explains its higher relative proportion under air atmosphere. The evolution of CO2 in the combustion process comprises a wide interval of temperatures, from 370 to 700 ºC and maximum at 470 ºC, where CO and H2O are detected. Exothermic effects, around 540 and 600 ºC, detected in the DTA of the cellulose, correspond mainly to the emission of CO2, as observed in Fig. 8B, associated to additional combustion phenomena. iii) Lignin: Unlike cellulose and hemicellulose, degradation of lignin under inert atmosphere covers a very wide range of temperature, from 130 to 500 °C (Fig. 9). The gases detected in this interval are CO2, CO, H2O, CH4 and CH3OH. No other organic compounds were detected. 22 Figure 9. Evolution of the main infrared bands of the volatiles emitted during lignin decomposition under 120 mL/min nitrogen purge, with the corresponding DTG curve. Of the three biomass components, lignin is the one which produces the highest proportion of CH4 and CH3OH and the lowest of CO with respect to the maximum absorbance of CO2 emitted at the pyrolysis stage (Table 4). At temperatures below 500 °C, the emission of H2O can be associated to aliphatic and aromatic hydroxyl groups, and the emission of CO2 and CO to cracking and reforming of the carboxyl, carbonyl and ester groups, present in the side chains of the phenylpropane units [41]. Their evolution above 500 °C is associated with the secondary pyrolysis of volatiles. In particular, several authors claim that CO is originated at the breaking of two different types of ether bonds: at low temperature the ether bonds between different lignin subunits, whilst at high temperature diaryl ether groups are involved [71,72]. Below 400°C, the evolution of CH3OH and CH4 is mainly associated with the cleavage and demethylation of methoxyl groups (-O-CH3) present in lignin [42,73]. Methanol 23 formation may also be contributed by the -CH2OH groups located on the carbon of the alkyl lateral chain of the phenylpropane units [41,71]. Above 400°C methane emission continues, reaching a second maximum at 525 °C, more intense than the first one. It is produced by side reactions, and also by primary reactions of condensation and reorganization of the char. At these high temperatures the rupture of the aromatic rings starts, generating methyl radicals and methylene groups, and finally forming methane [41] [71]. From 600 °C methane signal begins to disappear, because it breaks down into H2 and C [72]. The composition and evolution of gases emitted by lignin in this study turn out to be virtually identical to those of alkaline lignin studied by Yang et al. [42]. Other studies describe the emission of other organic compounds, not detected in the present work, such as formaldehyde, formic acid and phenols originated by the rupture of phenylpropane units. The type of lignin and the extraction method would explain the differences in the composition of the emitted gases. The basic reason why a smaller variety of volatile organic compounds is detected is that char formation reactions are favored against the generation of volatile compounds. G type lignins, as the one used in this study are prone to undergo condensation and coupling reactions between phenylpropane units, causing them to generate more amount of char and emit less amount of phenolic derivatives [41], which would be characterized by intense vibrations around 1500, 1250 and 1100 cm-1 [73]. On the other hand, lignins containing little sulfur, as the one studied, are more affected by this type of condensation reactions. Fenner and Lephardt [73] indicate that the incorporation of sulfur facilitates the fragmentation of phenylpropane units and decreases its condensation. 3.4.2. Analysis of the gases emitted by hot-washed sisal fiber The evolution with temperature of the main infrared bands of the gases emitted during the decomposition under inert and oxidant atmosphere of washed sisal fibers is shown in Fig. 10. The gaseous components observed at different stages are summarized below: i) At temperatures below 150 °C, H2O emission is mainly from to the moisture in the fibers. ii) In the first sub-step, below 300 °C, the gases CO, CO2 and H2O come from the decomposition of sisal lignin and mainly from hemicelluloses and other NCP still present in the washed sisal. The emission of acetic acid, formic acid, methanol, formaldehyde, and some ketone, which may be tentatively assigned to 2-butanone, is mainly due to decomposition of hemicellulose. Sisal lignin should also contribute to the emission of methanol and acetic acid as suggested by other studies with HGS type lignins [41]. Nevertheless, in studies with wood, the emission of formic acid at this stage has been associated only with the hemicellulose and not with lignin [74]. Notably, the proportion of acetic acid emitted (Table 5) is proportionally much higher than that observed in the pure xylan (Table 4). This is because sisal hemicelluloses contain more side chains with acetyl groups than the xylan studied. 24 Figure 10. Evolution of the main infrared bands of the volatiles emitted during microcrystalline cellulose decomposition, under 120 mL/min nitrogen (a-c) and air (d-f) purges, with the corresponding DTG curve. In the second sub-stage, between 300 and 400 °C, the emitted compounds are mainly due to the pyrolysis of sisal cellulose: CO, CO2, H2O, glycol, formaldehyde, methanol, formic acid and ketone, assigned tentatively to 2-butanone, as well as hydroxyacetaldehyde which starts to be generated in the previous stage. The latter compound may appear as a product of decomposition of cellulose by a competitive route to the formation of levoglucosan, the main component of cellulose tar [60,67]. The low amount of hydroxyacetaldehyde emitted during the pyrolysis of sisal cellulose suggests a route of decomposition different from that of the microcrystalline cellulose studied in this work, in which this compound was not observed. The maximum emissions of all these compounds match with the maximum mass loss rate, as shown in Fig. 10. Acetic acid bands decrease very quickly at the end of the first substage. Thus, the vibrations of the carbonyl groups that remain in the second sub-step can be only assigned to formaldehyde, formic and 2-butanone (Fig. 10C). To a lesser extent sisal lignin contribute to the 25 emission of CO, CO2 and H2O, and probably also to methanol, formic acid and acetic acid. The emission of CH4 in nitrogen starts at around 360°C (Fig. 10A and 11I). It is mainly associated with the rupture of the methoxyl groups of lignin, but its signal is weak, being obscured by the C-H stretching vibrations of other organic compounds. Additionally, at the end of this sub-stage in air, the combustion of sisal hemicellulose and other NCP occurs, emitting CO2 and H2O (Fig. 10D). Figure 11. Comparison of the evolution of the main infrared bands of gaseous compounds evolved under nitrogen atmosphere, during decomposition of biomass components under 120 mL/min purges, and decomposition of Washed Sisal under purges of 50 and 120 mL/min. The asterisk of Fig. 11i pyrogram on the first stage of CH4 evolution corresponds to background signal and not to methane vibration. iii) Both in nitrogen and in air, emission of most organic compounds disappears at temperatures above 400 °C. Under inert atmosphere, the evolution of gases is similar to that of pure cellulose, as it is the majority component. Emission of CO2 and CO start again at 400 ºC due to side reactions, being the growth of CO intensity much slower. Emission of CH4 is also observed to increase at 400 ºC, whose signal remains at a nearly constant value up to 580 °C, when it began to decrease and a new increase of