Title Review of Reactions and Molecular Mechanisms in Cellulose

Review of Reactions and Molecular Mechanisms in Cellulose Title Pyrolysis Author(s) Kawamoto, Haruo Citation Current Organic Chemistry (2016), 20(23): 2444-2457 Issue Date 2016-05-25 URL http://hdl.handle.net/2433/240620 This is the ACCEPTED VERSION of the peer-reviewed article. The published manuscript is available at EurekaSelect via http://www.eurekaselect.com/142480; The full-text file will be made open to the public on 25 May 2017 in accordance with Right publisher's 'Terms and Conditions for Self-Archiving'.; This is not the published version. Please cite only the published version.; この論文は出版社版でありません。引用の際には出版社版をご確認ご利用ください。 Type Journal Article Textversion author Kyoto University Send Orders for Reprints to [email protected] Journal Name, Year, Volume 1 Review of Reactions and Molecular Mechanisms in Cellulose Pyrolysis Haruo Kawamoto Department of Socio-Environmental Energy Science, Graduate School of Energy Science, Kyoto University, Yoshida-Honmachi, Sakyo-Ku, Kyoto 606-8501, Japan Abstract: Cellulose, which is a polymer of β-1→4 linked D-glucose units, comprises about half the components of lignocellulosic biomasses. A better understanding of the chemistry involved in cellulose pyrolysis provides valuable insights into the development of efficient pyrolysis-based conversion technologies of biomass into biofuels, biochemicals and biomaterials. This review focuses on the reactions and molecular mechanisms that determine the reactivity and product selectivity in pyrolysis and the related conversion technologies. This information is useful for understanding the processing technologies conducted at high temperatures, such as wood drying and the production of cellulose-plastic composite materials. Because the cellulose pyrolysis behavior changes drastically in the temperature range of 300–350 °C, this review is divided into low- and high-temperature regimes. The intermediates produced from cellulose pyrolysis are further converted into various gas, liquid or solid products. Hence, these processes are addressed as primary pyrolysis and secondary reactions in this review. The roles of the crystalline cellulose in pyrolysis are noted. Keywords: Cellulose, pyrolysis, gasification, carbonization, molecular mechanism, controlled pyrolysis, hydrogen bond theory. 1. INTRODUCTION Cellulose is a crystalline polymer consisting of β-1→4 biofuels/biochemicals. The reactions proceeding in hot- linked D-glucose units and comprises about half of the components of lignocellulosic biomasses. Together with the compressed water including supercritical conditions are excluded, because these are closely related to hydrolysis massive amount of lignocellulosic biomass produced and reactions. The studies of the theoretical calculations for accumulated on the earth, cellulose is the most important bioresource, and can be utilized as renewable materials and specific pyrolysis reactions of cellulose and related model compounds are also excluded in this review. for the production of biofuels and biochemicals. The pyrolysis-based technologies, which include gasification, Cellulose pyrolysis is believed to proceed in two stages, fast pyrolysis, carbonization and torrefaction, are promising the volatile and non-volatile intermediates formed by the for this purpose. thermal degradation of cellulose, and the degradation into the final products including gas, liquid (tar and wood vinegar or Understanding the molecular mechanisms involved in the pyrolysis of lignocellulosic biomass is helpful to improve pyroligneous acid) and solid (char and coke) products. Thus, these processes are treated in this review as primary pyrolysis-based technologies, although these are prone to be pyrolysis and secondary reactions, although a strict treated as “black boxes”. Cellulose is the most widely studied lignocellulosic in the pyrolysis field. Hence, there are distinction between them is not always easy. many excellent reviews concerning cellulose pyrolysis [1-6]. The primary pyrolysis stage can be divided into two However, the reactions and molecular mechanisms involved regimes depending on the pyrolysis temperature, as in cellulose pyrolysis have received little focus. They are the summarized in the previous reviews authored by Shafizadeh main context of the present review, which was previously [1] and Antal [2]. Two competitive pathways are believed to published in Japanese [7]. be involved in cellulose pyrolysis [1,2,8,9]. One is the The present review does not include the following topics pathway to form char along with water and gaseous products at relatively low temperatures < 300 °C, and the other regarding cellulose pyrolysis, kinetic treatment on the mass- pathway produces volatile products selectively at higher loss behavior and conversion technologies to temperatures. This idea was proposed by Kilzer and Broido *Address correspondence to this author at the Department of Socio- [8], and many kinetic investigations have been conducted Environmental Energy Science, Graduate School of Energy Science, Kyoto with these pathways. University, Yoshida-Honmachi, Sakyo-Ku, Kyoto 606-8501, Japan; Tel: +81-75-753-4737; Fax: +81-75-753-4737; E-mail: 2. TEMPERATURE OF CELLULOSE PYROLYSIS [email protected] XXX-XXX/15 $58.00+.00 © 2015 Bentham Science Publishers 2 Journal Name, 2014, Vol. 0, No. 0 Principal Author Last Name et al. The temperature ranges where cellulose pyrolysis occurs Furnace have been studied by thermogravimetric analysis (TGA). temperature These data provide the information for the slow pyrolysis 700 700°C 650°C conducted at relatively low heating rates. Cellulose starts to 600 600°C decrease in weight around 300 °C with a standard heating 550°C rate of 10 °C/min under nitrogen flow, and the mass loss 500 500°C C) attains a maximum around 350 °C, which can be easily ° 450°C observed in the derivative TG (DTG) curve. The DTG peak 400 430°C temperature shifts to higher temperatures when using higher heating rates. However, this shift can be explained by the 300 response lag in the temperature measurement and the heat of Temperature ( 200 evaporation for the volatile products [10-14]. Crystalline cellulose exhibits a DTG peak at temperatures 100 higher than that of ball-milled [15-17] or ammonia-swollen 0 [18] non-crystalline cellulose. Thus, the mass loss rate 0 50 100 increases directly with an increase in the content of the non- crystalline region [19]. The higher reactivity of the non- Heating time (s) crystalline cellulose is confirmed with the disappearance of Fig. (1). Temperature profiles from the in situ measurement hydroxyl groups in the non-crystalline region of cellulose as of cellulose (Avicel PH-101), when a small amount of indicated by the hydrogen–deuterium exchange method [20]. cellulose in a ceramic boat was plunged into the furnace Simple glycosides with relatively low melting points melt preheated at various temperatures under nitrogen flow [32]. and decompose in the temperature range lower than that of cellulose pyrolysis [21,22]. The size of the crystallites in Other than thermal reactions for cellulose cellulose, which is greater in the order of Halocynthia depolymerization, two processes, devolatilization and char (389 °C) > cotton (366 °C) > microcrystalline cellulose formation, which have opposite effects, affect the (341 °C), is reported to affect the DTG peak temperature decomposition temperature of cellulose during pyrolysis. (value in the parenthesis is the DTG peak temperature) [23]. Devolatilization is an endothermic process and tends to The stability of cellulose against heat relies on its crystalline decrease the cellulose temperature, which causes plateauing nature. of the sample temperature [10-12,31] as observed in Figure 1. Milosavljevic et al. [33] estimated the heat demand for the Fibrillation by a homogenizer reduces the thermal devolatilization process as 538 J/g (volatiles). However, the stability of cellulose. The DTG peak temperature decreases formation of solid carbonized materials (char) is a strong from around 360 to 300 °C along with the appearance of a exothermic process, which is estimated as 2 kJ/g (char) [33]. shoulder at 250 °C, where the major product, water, is Consequently, the decomposition temperature of cellulose in formed through dehydration reactions [24]. a steady state of pyrolysis varies depending on the balance of Although cellulose is converted into char that maintains these processes [33,34]. the original shape, but reduces in size under slow heating 3. PRIMARY PYROLYSIS REACTIONS conditions, cellulose is completely converted into molten intermediates under fast pyrolysis conditions with very high 3.1. Low-Temperature Regime (< 300 °C) heating rates [25-31]. These fast conditions, which can be At temperatures < 300 °C, cellulose decomposition is accomplished with a radiation flash pyrolysis or an ablative slow, but some structural alternations occur, which include pyrolysis, are promising for bio-oil production from biomass. The volatilization process via the molten intermediates can reducing and plateauing of the degree of polymerization be visualized using a high-speed camera [27,28]. (DP), crystallization and thermal decomposition of reducing end (RE) groups and molecules in the non-crystalline regions. The research group of Lédé extensively studied the formation and devolatilization of the molten intermediates 3.1.1. Reducing the Degree of Polymerization during the fast pyrolysis of cellulose and reported that the The DP of cellulose decreases to a plateau (around

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