Observation and Characterization of Cellulose Pyrolysis Intermediates

VOLUME 18, NUMBER 1

JANUARY/FEBRUARY 2004

© Copyright 2004 American Chemical Society

Articles Observation and Characterization of Cellulose Pyrolysis Intermediates by 13C CPMAS NMR. A New Mechanistic Model Jan B. Wooten,*,† Jeffrey I. Seeman,‡ and Mohammad R. Hajaligol† Philip Morris USA Research Center, P.O. Box 26583, Richmond, Virginia 23261, and SaddlePoint Frontiers, 12001 Bollingbrook Place, Richmond, Virginia 23236-3218 Received March 16, 2003. Revised Manuscript Received July 22, 2003

The composition of char from heated Avicel cellulose was monitored as a function of heating time and temperature, using 13C cross-polarization magic-angle spinning (CPMAS) NMR. Complex NMR line shapes observed in the carbohydrate region of the spectra are indicative of the presence of multiple carbohydrate forms. By successive spectral subtractions of the 300 °C pyrolysis char, the complex line shapes were separated into three distinct carbohydrate components that correspond to the crystalline cellulose starting material (SM), an intermediate cellulose (IC) that resembles a low degree-of-polymerization (low-DP) amorphous cellulose, and a disordered final carbohydrate (FC) that is characterized by a very broad 13C line width. Curve fitting was used to monitor the changes in the approximate abundance of these different carbohydrate forms relative to the aliphatic, aromatic, carboxyl, and ketone clusters of compounds of the char. The time evolution of the IC, together with its spectral line shapes, associate this component with the “active cellulose” intermediate that has long been postulated in many kinetic mechanisms for cellulose pyrolysis. After a heating period of 30 min, FC was the only remaining carbohydrate component. When subjected to prolonged heating, FC converted to aromatic carbons but not to aliphatic carbons, with little or no loss in char mass. This property distinguishes the FC as a char component that has not previously been recognized. Pyrolyses of cellulose with 1% K+ as KCl, and of pectin at 300 °C and cellulose at 350 °C were also performed. Evaluation of the combined data led to a new model for low-temperature cellulose pyrolysis. In this model, all char products are formed from IC, with FC being capable of forming aromatic carbon.

1. Introduction The thermal decomposition of cellulose (1), which is a polymer composed of repeating D-glucose units (shown in its β-glucopyranose form in structure 2), has been studied intensely for many years.1-5 Fundamental and * Author to whom correspondence should be addressed. E-mail: [email protected]. † Philip Morris USA, Inc. ‡ SaddlePoint Frontiers. (1) Gaur, S.; Reed, T. B. Thermal Data for Natural and Synthetic Fuels; Marcel Dekker: New York, 1998.

applied studies of cellulose pyrolysis have attempted to elucidate the decomposition pathways, product distributions, and chemical kinetics in the presence and absence of oxygen. When heated at moderate temperatures (2) Moldoveanu, S. C. Analytical Pyrolysis of Natural Organic Polymers; Elsevier: Amsterdam, 1998. (3) Antal, M. J., Jr.; Va´rhegyi, G. Ind. Eng. Chem. Res. 1995, 34, 703-717. (4) Sanders, E. B.; Goldsmith, A.; Seeman, J. I. J. Anal. Appl. Pyrolysis 2003, 66, 29-50. (5) Hajaligol, M.; Waymack, B.; Kellogg, D. Fuel 2001, 80, 17991807.

10.1021/ef0300601 CCC: $27.50 © 2004 American Chemical Society Published on Web 12/16/2003

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Energy & Fuels, Vol. 18, No. 1, 2004

(300-450 °C) under an inert atmosphere, cellulose undergoes various dehydration, fragmentation, elimination, and condensation reactions to give a plethora of gaseous products, a complex semivolatile liquid tar, and a residual carbonaceous char. Char is the material that is left in the pyrolysis vessel and can be composed of aromatic, aliphatic, carboxyl, and ketone molecular groups, as well as residual carbohydrate forms. The relative proportion of these molecular substructures varies with both the heating time and temperature.6-10 Although the gaseous and liquid products are relatively easy to characterize using a combination of gas chromatography (GC), high-performance liquid chromatography (HPLC), infrared (IR) spectroscopy and mass spectrometry (MS) methods, the char is composed of high-molecular-weight, nonvolatile, and insoluble structures and, thus, is difficult to analyze. Moreover, the char becomes progressively more refractory as the heating temperature increases.

One early approach to characterize cellulose char involved chemical degradation of the insoluble complex into soluble, analyzable fragments. Thus, an estimate of the concentration of aromatic groups and the crosslinking of these structures was obtained using permanganate oxidation.11 However, most recent studies have relied on spectroscopic methods to obtain information about the composition of char. IR spectroscopy is useful for identifying the oxygenated functional groups that form on the char surface, whereas 13C cross-polarization magic-angle spinning (CPMAS) NMR is useful for making quantitative comparisons between the various molecular substructures in the char.6-10 Both techniques are useful, because they analyze the solid, bulk material directly. A particularly fruitful approach is to combine these methods with pyrolysis-mass spectrometry (Py-MS), so that the volatile pyrolysis products can be correlated with the composition of the char.7,8 Many kinetic models have been proposed for the thermal decomposition of cellulose.3,12-21 The various (6) Julien, S.; Chornet, E.; Tiwri, P. K.; Overend, R. P. J. Anal. Appl. Pyrolysis 1991, 19, 81-104. (7) Boon, J. J.; Pastorova, I.; Botto, R. E.; Arisz, P. W. Biomass Bioenergy 1994, 7, 25-32. (8) Pastorova, I.; Botto, R. E.; Arisz, P. W.; Boon, J. J. Carbohydr. Res. 1994, 262, 27-47. (9) Sekiguchi, Y.; Frye, J. S.; Shafizadeh, F. J. Appl. Polym. Sci. 1983, 28, 3513-3525. (10) Morterra, C.; Low, M. J. D. Carbon 1983, 21, 283-288. (11) Shafizadeh, F.; Sekiguchi, Y. Carbon 1983, 21, 511-516. (12) Broido, A.; Nelson, M. A. Combust. Flame 1975, 24, 263-268. (13) Bradbury, A. G. W.; Sakai, Y.; Shafizadeh, F. J. Appl. Polym. Sci. 1979, 23, 3271-3280. (14) Mok, W. S.-L.; Antal, M. J., Jr. Thermochim. Acta 1983, 68, 165-186. (15) Piskorz, J.; Radlein, D.; Scott, D. S. J. Anal. Appl. Pyrolysis 1986, 10, 121-137. (16) Piskorz, J.; Radlein, D.; Scott, D. S.; Czernik, S. J. Anal. Appl. Pyrolysis 1989, 16, 127-142. (17) Diebold, J. P. Biomass Bioenergy 1994, 7, 75-85. (18) Milosavljevic, I.; Suuberg, E. Ind. Eng. Chem. Res. 1995, 34, 1081-1091. (19) DiBlasi, C. D. Ind. Eng. Chem. Res. 1996, 35, 37-46. (20) Va´rhegyi, G.; Antal, M. J., Jr.; Jakab, E.; Szabo, P. J. Anal. Appl. Pyrolysis 1997, 42, 73-87.

Wooten et al. Scheme 1. The Broido-Shafizadeh Model12,13

models have increased in complexity over time, as researchers have sought to explain the distribution of decomposition products under a variety of heating conditions and to predict the rates of evolution of the products. In numerous studies, thermogravimetric analysis (TGA)3,18,21,22 has been used to calculate global reaction kinetics and distinguish between various hypothetical models. Weight loss of the initial starting material is considered, but char structure and composition typically are not. One of the earliest proposed kinetic models, known as the “Broido-Shafizadeh model,”12,13 is shown in Scheme 1. In this model, the char is a product that is derived from an “active cellulose” intermediate. Active cellulose was introduced into the model as a reaction intermediate because, at low heating temperatures (259-312 °C), an initiation period occurs during which a weight loss occurs that accelerates with heating time.13 Moreover, the overall global kinetics for cellulose pyrolysis has been observed to be first order, with a high activation energy of ca. 200-240 kJ/mol,3,22 which implies that a high-energy initiation step exists. The model in Scheme 1 ultimately was determined to be inadequate, because it does not account for experimentally observed changes in the distribution of the volatile products when the heating rate and temperature increase.6,15-17,21,23 To account for the divergent product pathways, moreelaborate models were developed. For example, the socalled “Waterloo” multipath model shown in Scheme 2 was proposed by Piskorz et al.16 and later adopted by Julien et al.6 The model shown in Scheme 2 postulates two types of reactions that compete for the starting material: (i) cleavage reactions to form a “low degreeof-polymerization (low-DP) cellulose” and (ii) dehydration, elimination, and other reactions that form char, water, and carbon dioxide directly from the cellulose. The different decomposition pathways are influenced by the presence of alkali-metal ions, the temperature, and the heating rate. In the absence of metal ions, higher temperatures and heating rates favor the formation of hydroxyacetaldehyde (glycolaldehyde, 3) and less char, whereas lower temperatures and heating rates favor the formation of levoglucosan (4) and more char.16,21 The presence of even trace amounts of Na+ or K+ ions significantly increases the formation of hydroxyacetaldehyde, with a concomitant decrease in levoglucosan.24

Partitioning between char formation, transglycosylation and levoglucosan formation, and the formation of (21) Banyasz, J. L.; Li, S.; Lyons-Hart, J. L.; Shafer, K. H. J. Anal. Appl. Pyrolysis 2001, 57, 223-248. (22) Antal, M. J., Jr.; Va´rhegyi, G.; Jakab, E. Ind. Eng. Chem. Res. 1998, 37, 1267-1275. (23) Richards, G. N. J. Anal. Appl. Pyrolysis 1987, 11, 251-255.

Cellulose Pyrolysis Intermediates by

13C

CPMAS NMR

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Scheme 2. The Waterloo Model16

Scheme 3. The Diebold Model17,30,31

low-molecular-weight gases is dependent on the temperature and the heating rate.1 Evidence for both cationic25 and free-radical intermediates26 in cellulose thermal depolymerization has been presented. Any comprehensive mechanism must explain the dramatic changes in product distribution that result from the addition of small amounts of alkali metals.24 The first step in defining a mechanism is identifying as many of the reaction intermediates and products as possible. It is inconceivable that any one set of reactions can explain the diversity and plethora of pyrolysis products observed to date, from monomers2 to oligosaccharides.27,28 Indeed, the futility of using a simple rate law to model cellulose pyrolysis was first reported more than 30 years ago.29 One approach to address this complexity is to formulate even more complex global models, as shown by Diebold’s more recent (1994) model (Scheme 3).17,30,31 These models, and our new model presented below, do not explicitly consider the complexities that are due to the intermolecular hydrogen bonding32 between the different chains within the cellulose microfibrils. In numerous studies, TGA18,22 and evolved gas analysis by Fourier transform infrared (FTIR) spectroscopy21 have been used to calculate global reaction kinetics and to distinguish between various hypothetical models. As can be seen in Schemes 1-3, many of the kinetic models incorporate an active cellulose, or low-DP cellulose, as a putative intermediate. In fact, the carbohydrate intermediates in cellulose pyrolysis remain ill-defined, and their existence has even been questioned.3,20,22,24 The uncertainty persists because numerical simulations of some kinetic models at primarily higher temperatures (24) Essig, M. G.; Richards, G. N.; Schenck, E. M. In Cellulose and Wood Chemistry and Technology; Schuerch, C., ed.; Wiley: New York, 1989; pp 841-862. (25) Byrne, G. A.; Gardiner, D.; Holmes, F. H. J. Appl. Chem. 1966, 16, 81-88. (26) Kislitsyn, A. N.; Rodionova, Z. M.; Savinykh, V. I.; Guseva, A. V. Zh. Prikl. Khim. (S.-Peterburg) 1971, 44, 2518-2524. (27) Helleur, R. J.; Budgell, D. R.; Hayes, E. R. Anal. Chim. Acta 1987, 192, 367-372. (28) Piskorz, J.; Majerski, P.; Radlein, D.; Vladars-Usas, A.; Scott, D. S. J. Anal. Appl. Pyrolysis 2000, 56, 145-166. (29) Halperin, Y.; Patai, S. Isr. J. Chem. 1969, 7, 673-683. (30) Diebold, J. P. Biomass Bioenergy 1994, 7, 1-6. (31) Brown, A. L.; Dayton, D. C.; Daily, J. W. Energy Fuels 2001, 15, 1286-1294. (32) Attalla, R. H.; VanderHart, D. L. Solid State Nucl. Magn. Reson. 1999, 15, 1-19.

Scheme 4. The Va´ rhegyi-Antal Model20,33

and heating rates have been more successful at predicting weight-loss data by omitting the active cellulose intermediate. In 1998, Antal et al. concluded that the pyrolysis behavior of several cellulose samples is well represented by a simple, single-step, irreversible, firstorder rate law with a single high activation energy.22 A year earlier, Va´rhegyi et al. found a better fit to the thermoanalytical curve when the active cellulose was omitted from the reaction model; they proposed a new model, which is shown in Scheme 4.20,33 Nevertheless, other researchers have retained the two-step pathway with the active cellulose intermediate, even under flash pyrolysis conditions.17,19,34,35 Several reports have presented evidence for the depolymerization of cellulose and the formation of cellulose with a lower DP at low pyrolysis temperatures.29,36-38 Limited data suggest that amorphous cellulose will form less char than crystalline cellulose.39,40 Recently, Le´de´ et al.35 directly observed a transient “intermediate liquid compound” (ILC) in small pellets of cellulose that had been heated by radiant flash pyrolysis in an imaging furnace. The water-soluble ILC was analyzed by HPLC and found to be composed predominantly of anhydro-oligosaccharides. Because the char seems to emanate from the ILC, Le´de´ et al. associated the ILC with the active cellulose in the Broido-Shafizadeh model. Chars formed during biomass pyrolysis are critical to the total product formation for many reasons. First, additional heating can form and release volatile components from the char. Second, reactions between the volatilized gases and the char are possible.3,22 To make a full accounting of the decomposition products, char and especially the appearance and disappearance of the carbohydrate intermediates must be included in the kinetic model, along with the volatile products. (33) Va´rhegyi, G.; Jakab, E.; Antal, M. J., Jr. Energy Fuels 1994, 8, 1345-1352. (34) Boutin, O.; Ferrer, M.; Le´de´, J. Fuel 1998, 81, 13-31. (35) Le´de´, J.; Blanchard, F.; Boutin, O. Fuel 2002, 81, 1269-1279. (36) Golova, O. P. Russ. Chem. Rev. 1975, 44, 687-697. (37) Pacault, A.; Sauret, G. C. R. Acad. Sci. 1958, 246, 608-611. (38) Chatterjee, P. K.; Conrad, C. M. Text. Res. 1966, 36, 487-494. (39) Pastorova, I.; Arisz, P. W.; Boon, J. J. Carbohydr. Res. 1993, 248, 151-165. (40) Broido, A.; Weinstein, M. Combust. Sci. Technol. 1970, 1, 279285.

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Intermediate products from cellulose pyrolysis are most readily measured under slow heating conditions. The yield and distribution of the pyrolysis products have been measured under various slow heating protocols. The heating times typically were kept constant (ranging from 5 min to several hours) and the heating temperature varied from 190 °C to 800 °C. Sample heating was performed either under nitrogen, helium, or vacuum. Both FTIR and NMR analyses have been used to characterize the composition of cellulose char under the slow heating protocols; however, the char has never been examined as a function of heating time while keeping the temperature constant.6-10,41-43 In this work, our goals were (i) to observe, characterize, and quantify the chars formed during low-temperature biomass pyrolysis;6 (ii) to distinguish between char reaction intermediates and final products; and (iii) to utilize this information to elucidate further the pyrolysis mechanisms of cellulose pyrolysis. To achieve these goals, the evolution of the char, both as a function of heating temperature and residence time, was examined. 13C CPMAS NMR was used to characterize the char composition that resulted from heating Avicel cellulose at incremental periods in the range of 5-210 min at temperatures similar to the initial decomposition temperature. A temperature that was too low would not result in significant pyrolysis; a temperature that was too high would lead to too rapid thermal degradation and a loss of information about thermally unstable intermediates. Analysis by 13C CPMAS NMR is particularly useful in these experiments, because the incipient char intermediates can be readily distinguished from the unreacted cellulose on a semiquantitative basis. In addition, the low-temperature pyrolyses of cellulose with 1% added potassium chloride (KCl) and citrus pectin were investigated to assist in the understanding of the pyrolysis of pure cellulose. In this study, the following key results are reported: (1) For the first time, analytical spectroscopic data is presented for a low-DP amorphous cellulosic structure (observed after 5 min of heating, with ca. 15% weight loss) that corresponds to and supports the so-called active cellulose intermediate that has been hypothesized in the Bradbury-Shafizadeh model,13 the Piskorz model,16 the Diebold model,1,17 and other models.21 (2) The 13C CPMAS NMR data allow the observation of a series of pyrolysis products and quantification of their time dependencies. (3) A new, major carbohydrate component of the char, distinct from the active cellulose intermediate carbohydrate, is observed. (4) Finally, a new model of cellulose pyrolysis is proposed, on the basis of the derivation of underlying precursor-product relationships. 2. Experimental Section 2.1. Samples. Avicel PH-102 microcrystalline cellulose powder (obtained from FCI) was used in this work. This purified cellulose is obtained from fibrous plants. The ash (41) Sekiguchi, Y.; Shafizadeh, F. J. Appl. Polym. Sci. 1984, 29, 1267-1286. (42) Wind, R. A.; Li, L.; Maciel, G. E.; Wooten, J. B. Appl. Magn. Reson. 1993, 5, 161-176. (43) Soares, S.; Ricardo, N. M. P. S.; Jones, S.; Heatley, F. Eur. Polym. J. 2001, 37, 737-745.

Wooten et al. content of this sample was 500 °C).42 Thus, the influence of the unpaired electrons on the quantitative comparisons of our samples is minimal. Despite all these considerations, the emphasis here is on the change in the relative amounts of the various molecular groups with regard to heating time and not their absolute abundance. To monitor the temporal changes in the composition of the cellulose chars, curve fitting of the NMR spectra was used. The fitting routine is a component of the Varian 6.1c software. A simple system macro was written that copies the set-fitted parameters back into the input file to facilitate iterative improvements.

3. Results and Discussion 3.1. Pyrolysis of Cellulose at 300 °C. Figure 1 shows a series of 13C CPMAS NMR spectra of the Avicel cellulose samples that were heated at 300 °C, as a function of time. As discussed previously, the term “char” refers to all the material that remains in the pyrolysis boat. At a heating temperature of 300 °C, cellulose decomposes at a sufficiently slow rate so that the appearance of the various pyrolysis products in the char can be readily detected by NMR, especially in the early stages of heating. Curve fitting and spectral subtraction techniques were used to discriminate between the carbohydrate products. Details for the use of these two techniques will be described in the following two sections. 3.2. Deconvolution by Spectral Subtraction. A close inspection of the spectra in Figure 1 reveals significant details about the decomposition of cellulose. After the sample was heated for 10 min, the cellulose carbon resonances do not merely broaden; rather, the


relative intensity of some of the cellulose resonance peaks changes noticeably. In particular, the ratio of the peak at 89 ppm to the peak at 84 ppm diminishes and inverts for the sample that has been heated for 20 min. The ratio of these peaks is a measure of the relative crystallinity of the cellulose. A higher ratio indicates a more ordered, regular polymer, whereas a lower ratio indicates a more disordered polymer.32,47 After the char was heated for 30 min, the carbohydrate resonances exhibit only telltale evidence of the sharp peaks of the starting material, whereas the overall line shape begins to appear similar to the broad line observed in the 60min sample. Only this broad resonance (a width of 842 Hz at half-height) at 75 ppm remains at longer heating times. Heating the sample for longer than 60 min causes the 75-ppm peak to decrease; however, its line shape and width do not change any further. It is important to distinguish between the various mechanisms that may cause resonance line broadening in solids. They include (i) dispersion of chemical shifts, which is due to the presence of numerous but similar molecular species; (ii) a nonuniform chemical environment, which is due to molecular disorder; and (iii) the effects which are due to paramagnetic centers, e.g., unpaired electrons. In this work, resonance line broadening is attributed to the first aforementioned factor (i). We discuss these issues more fully in Section 4.3. No changes in the resonances in the carbohydrate region (Figure 1) were observed for samples that were heated for g60 min. Hence, the spectrum of the 60-min sample was used as a suitable model spectrum of the carbohydrate end products. To separate out the carbohydrate spectral components in the spectra of the samples that were heated at shorter times, the following procedure was used. The broad carbohydrate resonance at 75 ppm in Figure 1, visible by inspection at times >30 min, can be accurately reproduced by apodizing the time-domain spectrum (free induction decay, FID) of a ball-milled cellulose spectrum with a Gaussian decay function that has a time constant of 800 µs. The Gaussian weighted and transformed spectrum of ballmilled (amorphous) cellulose (Figure 2B) exactly matches the carbohydrate region of the 60-min char spectrum (Figure 2C). When the Gaussian-broadened spectrum of amorphous cellulose is subtracted from the 60-min char spectrum, the result gives a flat baseline between 60 and 100 ppm (Figure 2D). A similar operation that uses the FID of the crystalline starting material does not give a flat baseline. We call the molecular component that is represented by the 75-ppm peak and a second resonance at ca. 105 ppm (see Figure 2B) the “final carbohydrate” (FC), because the broad line widths do not permit an assignment to a particular carbohydrate structure. With a good representative spectrum for the FC component, and with the spectrum of unheated Avicel cellulose starting material, a means to deconvolute the complex spectra of char from other times into their component parts is available. The series of spectra in Figure 1 shows that some unaltered cellulose (SM) remains in the char after heating for 30 min. FC is the only remaining carbohydrate component in the char (47) Earl, W. L.; VanderHart, D. L. Macromolecules 1981, 14, 570574.

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Wooten et al.

Figure 2. Comparison of 13C NMR line widths for different cellulosic forms: (A) ball-milled cellulose, (B) ball-milled (amorphous) celluose (the FID was apodized with a 800 µs Gaussian function), (C) Avicel cellulose heated at 300 °C for 60 min (from Figure 1), and (D) difference spectrum between spectra C and B (spectrum C - spectrum B).

after prolonged heating. At intermediate heating times (10-30 min), the carbohydrate region of the spectra exhibits complex line shapes, which is representative of a mixture of components. The SM and FC components were directly identified from the NMR spectra. To visualize the remaining carbohydrate component, spectral subtraction was used, as shown in Figure 3. By successively subtracting the appropriate intensities of SM and the model spectrum of FC from the spectra of samples that were heated at 10-30 min, the spectra of other cellulose pyrolysis products were revealed. The separation of the SM and FC resonance components from the remaining carbohydrate intermediates is possible for two complimentary and fortuitous reasons: the SM resonances are significantly narrower and the FC resonances are significantly broader than the resonances of the intermediates. Moreover, the line widths of the SM and FC components do not change throughout the entire range of heating periods. In the subtraction process, either dips in the spectra or a bowed baseline give a clear indication of the fractional amount to be subtracted. The resulting difference spectra shown in Figure 4 reveal the spectral signatures of a species that we call intermediate cellulose (“IC”). More discussion about IC is found in the following sections. 3.3. Deconvolution by Curve Fitting. To estimate the relative abundance of each cluster of components in each sample as a function of the heating time, spectral curve fitting was used. There is significant overlap between some of the char resonance lines; consequently, the curve-fitting routine does not necessarily give a unique fit for any particular spectrum. Accordingly, the following strategy was used to obtain

Figure 3. Deconvolution procedure for isolating the 13C NMR spectrum of “intermediate cellulose” (IC): (A) Avicel cellulose heated at 300 °C for 20 min (from Figure 1), (B) unheated Avicel cellulose, (C) difference spectrum (spectrum A spectrum B), and (D) ball-milled cellulose. The FID of the spectrum in Figure 3D was apodized with a Gaussian function so that the Fourier transformed (FTIR) spectrum precisely matched the line shape and width of the FC portion of the spectrum in Figure 3C. Spectrum E is the difference spectrum between spectra C and D (spectrum C - spectrum D), showing IC.

consistent results. For each molecular substructure, the spectrum that displayed the best-resolved resonance was used to make the initial fit. For example, the sample that was heated for 60 min gave the best representation of the peak at 75 ppm (FC). In the case of the Avicel cellulose starting material, the spectrum of unheated cellulose was used. For components that had two or more resonances, the line widths and the ratio of the intensities of the resonances were kept constant; the intensities were varied only as a group. To obtain the best fit for all the remaining resonances, after the best line widths were determined, either the line widths were kept constant or small incremental changes were made from spectrum to spectrum. The relative concentrations of eight components were followed throughout the thermolysis, using a careful, consistent series of curve fittings. Five types of noncarbohydrate molecular substructures and three types of carbohydrate molecular substructures were detected in the char during the decomposition of the cellulose polymer, as listed in Table 1. The chemical-shift assignments for cellulose and noncarbohydrate resonances in


13C

CPMAS NMR

Figure 4. 13C CPMAS NMR difference spectra of IC obtained according to the spectra subtraction procedure shown in Figure 3. Heating times are indicated on the left-hand side of each spectrum.

the NMR spectra of the chars are well established and are presented in Table 1. Our use of the term “functional substructure” denotes a possible cluster of compounds, each of which has the structural functional group described by its name; for example, “Me” represents a cluster of compounds that have a methyl group, and IC and FC are carbohydrate-type compounds. It is also important to note that the NMR spectra were conducted at ambient temperature, whereas the char components were generated and were reacting at a temperature of 300 °C. Hence, the spectra represent the cooled materials, not the forms of these compounds at the thermolysis temperature. 3.4. Pyrolysis Profile. The time evolution of the various char substructures obtained from the curvefitting procedure for the series of samples that have been heated at 300 °C is shown in Figures 5 and 6. For clarity, the resonance intensities of the carbohydrate components are plotted separately from the noncarbohydrate char components. The total combined peak integrals for Figures 5 and 6 were summed and normalized to 100% total carbon. For molecular substructures that were fitted with multiple resonance lines, the peak integrals were summed together. At the shortest heating time that was used (5 min), the carbohydrate region of the spectrum of the heated cellulose remained virtually unchanged from the starting material (see Figure 1). Nonetheless, the weightTable 1.

13C


Figure 5. Carbohydrate forms present in Avicel cellulose heated at 300 °C, as a function of heating time, as measured by 13C CPMAS NMR. As indicated in Figure 1, after 60 min, the char mass remains constant within the precision of the measurement. Hence, in this figure and in Figure 6, the data points at g60 min reflect a change in the percentage composition of the char that is not due to any loss of material. Heating times do not include the sample warm-up period (see Experimental Section). Legend is as follows: (0) starting material, SM; (2) IC, intermediate cellulose, IC; and (O) final carbohydrate, FC.

Figure 6. Noncarbohydrate forms present in Avicel cellulose heated at 300 °C, as a function of heating time, as measured by 13C CPMAS NMR. See Figure 5 caption for additional details. Legend is as follows: (b) aromatic, Ar; (*) aliphatic, Al; (O) ketone; (2) carboxyl, CO2; and (×) methyl, Me.

loss data shown in Figure 1 reveals the loss of almost 23% of the starting mass to volatile components. Integration of the weak 13C resonances, which correspond to the noncarbohydrate components in the char of the 5-min sample, indicates that only ca. 6% of the total carbon in the heated residue are noncarbohydrate components. Multiplying this percentage by the fractional mass loss of the original sample (0.774) reveals that the noncarbohydrate components in the char

Chemical-Shift Assignments for Avicel Cellulose and Cellulose Char

substance

symbol

Avicel microcrystalline cellulose intermediate carbohydrate final carbohydrate methyl aliphatic aromatic carboxyl ketone

SM IC FC Me Al Ar CO2 K

13C

CPMAS NMR resonances (ppm)

63.16, 65.59, 72.41, 75.16, 84.53, 89.07, 105.51 63.25, 73.45, 102.67 74.67, 104.99 14.09 35.81 128.27, 131.08, 152.8 173 205.59

8


comprise only ca. 4.6 wt % of the starting mass. This result shows that, although a significant amount of the starting mass is lost to volatile components at the earliest stages of heating, only a very small amount of noncarbohydrate char is formed. For more-prolonged heating, the carbon resonances in the carbohydrate region of the spectrum become progressively broader until a single featureless resonance at ca. 75 ppm remains after ca. 1 h of heating. Several reports have documented the formation of these molecular groups in cellulose char.7-9,41 Two carbohydrate components (IC and FC) were spectroscopically detected and distinguished for the first time in this work. The transient species IC, which is discussed in more detail later in this paper, forms and then disappears within a heating time of ca. 30 min. Five noncarbohydrate components also appear at the early stages of heating and persist at some concentration for the entire 3.5 h of heating. These substructures are methyl (Me), aliphatic (Al), aromatic (Ar), carboxyl (CO2), and ketone (K) groups, on the basis of the chemical shifts of the resonances observed (see Table 1). Figure 5 shows that the starting cellulose decreases linearly with time until its depletion after 30 min of heating. For IC, three linessat 63, 74, and 103 ppms were used in the curve fitting; these three lines correspond approximately to the three groups of peaks shown in Figure 4. The actual line shapes of the IC component are complex; however, the choice of these three lines was observed to be a good approximation. The so-called IC quickly reaches a maximum concentration after a heating period of ca. 5 min and then diminishes to zero at ca. 50-60 min. The resonance line widths and shapes of IC provide clues about its chemical composition. Several observations can be made about the difference spectra that we attribute to IC, shown in Figure 4. First, the resonance at 89 ppm (cellulose C-4, a crystalline component) is reduced relative to the resonance at 84 ppm. This observation indicates that IC has little or no crystalline regions that are characteristic of native cellulose.32,47 The spectrum of low-DP crystalline cellulose should not differ significantly from the spectrum of a higher-DP polymer. Thus, IC is shown not to be a low-DP form of crystalline cellulose but rather is a disordered carbohydrate that is derived from fragmented cellulose. Next, the samples exhibit complex line shapes. The 10-min sample, for example, exhibits line shapes that are composed of some broader and some narrower components. This shows that IC is a complex mixture that probably consists of a distribution of D-glucose oligomers, some of which contain structural modifications. Finally, all three spectra exhibit resonances that are broader than that of SM but, nonetheless, are narrower than the 75-ppm resonance associated with FC. Overall, the spectral features in Figure 4 are reminiscent of an irregular-length, amorphous, low-DP cellulose or glycosan oligomers. The third carbohydrate component, FC, reaches a maximum concentration after 30 min and diminishes slowly, persisting even after prolonged heating to 3.5 h. Previously, Pastorova et al. observed a broadening of the baseline in the 13C NMR of cellulose samples that were heated at 270 and 290 °C for 2.5 h,8 which likely is related to the resonance observed herein for FC.

Wooten et al.

However, Pastorova et al. attributed this broadening to amorphous cellulose or an amorphous component,8 contrary to the observation of resonances for FC in this work. Section 4.3 presents our evidence that the line width of the FC resonance is greater than what has been observed for authentic amorphous cellulose.48,49 Notably, the weight-loss data in Figure 1 shows that the weight of the sample residue becomes essentially constant after ca. 60 min. No measurable weight loss within the precision of the balance was recorded upon heating the residue for an additional 2.5 h. During this interval of prolonged heating, the FC component decreases by almost half (10 unit %), converting to aromatic carbons primarily (the so-called “Ar” component) without a simultaneous measurable weight loss from the char. Noteworthy, the rate of loss of FC is essentially the same as the rate of appearance of Ar groups from 60 to 220 min (compare Figures 5 and 6). Thus, chemical transformations continue to occur in the char, even after the char weight has become constant, within the experimental error of the measurements. Figure 6 charts the evolution of these noncarbohydrate carbons, relative to heating time at 300 °C. The rapid decrease in the cellulose starting material is accompanied by the appearance of several pyrolysis products, along with a significant weight loss. The most abundant char product contains aromatic carbons. Large amounts of aliphatic carbons are also formed, with smaller amounts of carboxyl and ketone carbons. At the point during heating when the weight of the char becomes constant (ca. 60 min), additional aromatic carbons (and possibly carboxyl and ketones) continue to form in the solid residue; however, no aliphatic carbons form. The aliphatic groups remain constant, even after the additional heating period of 2.5 h. FC is the only carbohydrate substructure that remains in the char after a heating period of 60 min, and it appears to convert to aromatic carbons but not aliphatic carbons. (By “appears”, we mean that FC is the component that loses the major mass and Ar is the component that gains the major mass in this time period.) The aliphatic carbons must originate from carbohydrate material as well. Scheme 5 shows our new model that explains the lowertemperature pyrolysis of cellulose. Additional evidence for this model will be presented later in this paper. In contrast to the model of Va´rhegyi et al. that is shown in Scheme 4,20 our new model incorporates an intermediate (IC), which is carbohydrate in structure. Scheme 5 differs from literature proposals3,20,22,33 in that some direct formation of char components other than IC originates directly from SM. To distinguish between these two models, additional pyrolyses of pure cellulose were performed at 300 °C at low heating times. Chars for heating times in the range of 0-8 min were obtained. Figure 7A shows the spectrum of the char after 8 min. After 8 min, 25% of the original mass was lost to volatile components. Figure 7B shows the difference spectrum of the 8-min char minus SM, i.e., the unheated pure cellulose; the residual is almost entirely IC (see Figures 3 and 4). An attempt was made to subtract the spectrum (48) VanderHart, D. L. In National Bureau of Standards Internal Report No. 82-2534; National Bureau of Standards, Gaithersburg, MD, 1982. (49) Maciel, G. E.; Kolodziejski, W. L.; Bertran, M. S.; Dale, B. E. Macromolecules 1982, 15, 686-687.


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CPMAS NMR


Figure 7. (A) Pyrolysis of pure cellulose at 300 °C after 8 min of heating (see Experimental Section) results in 25% weight loss. (B) Difference spectrum obtained by subtraction of the unheated SM from spectrum A. Subtraction of more than ca. 3% of FC from spectrum B results in a negative baseline, indicating that no more than 3% FC could be present in the 8-min char. Scheme 5. The New Model for Low-Temperature Cellulose Pyrolysis

of FC from the spectrum of Figure 7B. However, even the subtraction of more than ca. 3% of FC resulted in a negative baseline, which indicated that no more than 3% FC could be present in the 8-min char. Additional experiments, e.g., at slightly lower temperatures, would likely be useful in making further distinctions. Figure 1 demonstrates significant weight loss occurs at early heating times (e.g., 5 min), with some formation of IC and little conversion to other products, notably Ar groups. This indicates (see Scheme 5) that cellulose itself leads to volatile materials directly and simultaneously with the formation of IC. This is consistent with the identification of anhydro-oligosaccharides with levoglucosan-type groups, as well as the formation of levoglucosan itself from cellulose.27,50 3.5. Pyrolysis of Cellulose at 325 °C. A second series of cellulose char samples was prepared by heating Avicel cellulose at 325 °C for times in the range of 0-60 min. Figure 8 shows the evolution profiles for the (50) Sanders, E. B.; Seeman, J. I., unpublished results.

different molecular groups in the residue, as a function of heating time, as determined by curve-fitting analysis. In this series, neither SM nor IC was detected by 13C CPMAS NMR in samples that were heated for g5 min. Moreover, after 5 min, the char weight was reduced to ca. 18% of the initial cellulose weight. After 20 min, the char weight was reduced to ca. 16% and remained constant with further heating at this temperature (325 °C), up to 60 min. Thus, the vast majority of weight loss at 325 °C occurs almost immediately. The only significant carbohydrate component observed in the NMR of char in this entire series was FC. As in the case of the samples that were heated at 300 °C, the FC component exhibits a single resonance line at 75 ppm with a width of ca. 18 ppm. The mass loss during the initial heating period was ca. 80%, stabilizing at 84% after heating for only ca. 30 min. The only components in the residue that significantly change during the heating period between 5 and 60 min at 325 °C are the aromatic carbons and FC. The

10


Figure 8. Relative portions of char carbons for various molecular groups found in cellulose char heated at 325 °C, as a function of heating time. Heating times do not include the sample warm-up period (see Experimental Section). No weight loss of the char was measured after 5 min. As in the case of Figures 5 and 6 (300 °C), the data points at 5 min or longer reflect a change in the percentage composition of the char that is not due to any loss of material. Legend is as follows: (b) aromatic, Ar; (9) aliphatic, Al; (2) intermediate cellulose, IC; (4) final carbohydrate, FC; (0) ketone, K; (*) carboxyl, CO2; and ([) methyl, Me.

carbohydrate component comprises ca. 19.6% of the char carbons after 5 min and diminishes to ca. 8.3% after 60 min. The decrease in the relative amount of FC carbons (11.3%) almost exactly matches the increase in the amount of Ar carbons (11.2%), and the rate of change is very similar (see slopes in Figure 8). The other char components (apart from the aromatic groups)smost notably, the aliphatic carbonssremain remarkably constant with prolonged heating. These data reinforce the earlier conclusion that FC does not contribute to the aliphatic component of the char, as shown in Scheme 5. During the same heating interval, the char loses ca. 21% of its mass through volatilization. On the basis of the C/H/O analysis of the samples, most of this mass loss can be attributed to the loss of oxygen. The efficient conversion of the FC component to Ar at 325 °C through a dehydration process without forming aliphatic carbons is a result that is completely congruent with the samples that were heated at 300 °C. Relative to the mass of the original starting material, the amount of FC carbons that converted to aromatic carbons during the heating interval between 5 and 60 min is very low, amounting to 25%) IC before the observation of Ar, Me, Al, CO2, K, and 3.5 h). This component was tentatively described as “amorphous cellulose”,8 because broad 13C NMR line widths in solids are usually indicative of molecular disorder. However, the very large line width of this resonance (ca. 16-18 ppm) suggests that this interpretation is an oversimplification. Figure 2 illustrates the problem of the interpretation of the 13C line widths. The carbohydrate resonance of the cellulose sample that has been heated for 60 min (Figure 2C) exhibits a much broader line width than the individual resonances of an authentic amorphous

cellulose (ball-milled cellulose; see Figure 2A). Maciel et al.49 and VanderHart48 have shown that the inherent order in the crystalline regions of the cellulose microfibrils can be destroyed by prolonged beating in a ball mill. In the spectrum of the ball-milled cellulose (Figure 2A), the characteristic resonances of crystalline cellulose at 65.4 ppm (C-6) and 89 ppm (C-4) (as seen in Figure 3B) are absent, whereas the corresponding resonances of disordered cellulose at 63.4 and 84.4 ppm, respectively, are enhanced. At the same time, there is an overall increase in the line widths of the resonances. For example, the line width of the cellulose C-1 resonance of crystalline Avicel cellulose is ca. 2.7 ppm, whereas the corresponding resonance in the ball-milled cellulose is ca. 4.1 ppm. The spectral changes in the ballmilled cellulose were attributed to the loss of crystallinity, which results from the disruption of the hydrogen bonds that organize the anhydroglucose chains into ordered microfibrils.48,49 Therefore, the 16.7-ppm line width of the carbohydrate resonance at 75 ppm in the heated cellulose (Figure 2C) is much too large to be attributed to these effects alone; other resonance broadening mechanisms must be active. One potential line-broadening mechanism that should always be considered in carbonaceous materials is the shortening of the spin-spin relaxation time (T2) of the 13C nuclei via interaction with the unpaired electrons in the sample. In the absence of nonrelaxation effects that might contribute to the line width, the line width that is due to spin-spin relaxation alone is equal to 1/πT2 (for Lorentzian line shapes). It is well-known that large numbers of unpaired electrons are created in char during the thermal decomposition of carbohydrates and that these electrons are stable radicals that are delocalized over large aromatic ring systems.62 In principle, the unpaired electrons can shorten the T2 value of any carbohydrate or other carbons that are in close proximity to the electrons.42 This broadening effect, which is due to relaxation, is referred to as “homogeneous” broadening, because all the nuclei are affected equally. However, the resonances of the carbons located closest to the unpaired electrons are so severely broadened by the large local dipolar fields that they cannot be detected at all.44,46 Therefore, only the carbon nuclei that are further removed from the electron spins are observable. We found that the number of unpaired electrons increases as the heating time increases, in direct proportion to the number of aromatic carbons in the char. However, at the longest heating time at 300 °C, based on electron paramagnetic resonance (EPR) and C/H/O analysis, we found only one unpaired electron for ca. 10 000 13C nuclei.42,63 There is no concomitant increase in the line width of the 75-ppm resonance with electron spins, even after heating for 3.5 h. Moreover, using a modified Hahn-echo, cross-polarization pulse sequence, we measured the T2 value of the resonances of both the ball-milled cellulose and the 75-ppm peak of a cellulose sample that had been heated at 300 °C for 2 h. The T2 values for both samples fall in a comparable range (4-16 ms). Thus, the width of the 75-ppm peak cannot be attributed to relaxation effects alone.64

(60) Zawadzki, J.; Wisniewski, M. J. Anal. Appl. Pyrolysis 2002, 62, 111-121. (61) Muller, M.; Czihak, C.; Schober, H.; Nishiyama, Y.; Vogl, G. Macromolecules 2000, 33, 1834-1840.

(62) Singer, L. S.; Lewis, I. C. Appl. Spectrosc. 1982, 36, 52-57. (63) Feng, J.; Wooten, J. B., unpublished results. (64) Chuang, I.-S.; Wooten, J. B., unpublished results.


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CPMAS NMR

On the basis of all the aforementioned considerations, we conclude that the 75-ppm resonance line is “inhomogeneously” broadened by a distribution of the diamagnetic shieldings from the multiplicity of chemical species present and is not due to relaxation effects. One example of a contribution to the chemical-shift dispersion might be the anisotropic shielding from the randomly oriented aromatic groups that do not contain unpaired electrons (i.e., ring-current shifts). 5. Summary and Conclusions The results reported herein afford us the opportunity to make some definitive statements about the mechanisms of cellulose pyrolysis and precursor-product relationships. The major findings and conclusions of this work are as follows: (1) On the basis of experiments that have been conducted at 300 and 325 °C, a new model for lowtemperature cellulose pyrolysis is proposed (see Scheme 5). This model incorporates several novel observations and is consistent with the vast amount of literature data. (2) Two carbohydrate and five noncarbohydrate types of products were observed and quantified. On the basis of its NMR line-width analysis, each of these is likely a mixture of structurally similar components. (3) A large percentage of the initial weight (>25%) is lost before the appearance of significant amounts of pyrolysis products other than IC. This observation implies pyrolysis pathways that lead directly to volatile products without the required involvement of a reaction intermediate. (4) For samples that have been heated for 5-30 min, the NMR spectra show the appearance of a transient carbohydrate component (“intermediate cellulose”, IC) that is likely to be the long-postulated “active cellulose” intermediate that appears in various kinetic models (see Schemes 1-3). (5) The intermediate (IC) is a product of cellulose pyrolysis, not a product of volatile components that react with the nascent char. IC disappears rapidly from the samples that have been heated at temperatures of >300 °C. IC is likely to be composed of oligosaccharides, which


include anhydroglucose oligomers. IC appears to be the “intermediate liquid compound” that was observed by Le´de´ et al. in flash pyrolysis experiments.34,35 (6) IC is a precursor to both aliphatic and aromatic groups in the char. (7) The disappearance of the sum of the starting cellulose and the intermediate cellulose follows firstorder reaction kinetics. (8) A new carbohydrate intermediatesthe “final carbohydrate” (FC)swas identified in the pyrolysis of pure cellulose but not in the pyrolysis of pectin or cellulose that had been treated with potassium salts. (9) FC is an intermediate carbohydrate that is distinct from IC. Under constant heating, the appearance and disappearance of FC is slower than that of IC. With prolonged heating, FC follows a different productprecursor relationship than the starting cellulose. (10) FC converts, with high efficiency, to aromatic groups, with little or no weight loss. FC apparently does not form aliphatic groups. (11) FC may be a reaction product of volatile pyrolysis products with the char. (12) FC is not formed from the 300 °C pyrolysis of KCl-treated cellulose or the 250 °C pyrolysis of Sigma citrus pectin. Acknowledgment. The work performed by one of us (J.I.S.) was funded by Philip Morris USA, Inc. We express our gratitude to Brad Crosby, Brian Miller, and Chris Allmond for preparing the samples and obtaining NMR spectra. We thank Dr. Jiwen Feng and Dr. I-Ssuer Chuang (Colorado State University) for making the EPR and 13C T2 measurements, respectively, and Professor Gary Maciel for providing the facilities at Colorado State University and helpful discussions. We acknowledge many technical discussions with Bruce Waymack, Dr. Geoffrey Chan, Dr. Tom McGrath, and Dr. Ramesh Sharma. We also thank one reviewer for very helpful comments (in particular, for a discussion of the temperature dependence of reaction rates) and Dr. Mark Solum (University of Utah) for his helpful comments regarding the manuscript. EF0300601

Observation and Characterization of Cellulose Pyrolysis Intermediates

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