Characterization of Char from Pyrolysis of Chlorogenic Acid

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Characterization of Char from Pyrolysis of Chlorogenic Acid Ramesh K.Sharma, Mohammad R. Hajaligol,* Pamela A. Martoglio Smith, Jan B. Wooten, and Vicki Baliga Research Center, Philip Morris U.S.A., P.O. Box 26583, Richmond, Virginia 23261 Received March 21, 2000. Revised Manuscript Received July 19, 2000

Pyrolysis of chlorogenic acid was studied under varying conditions of temperature and reaction environment. The objective was to study the effect of pyrolysis conditions on the composition of the solid residue, i.e., char. Runs were made at atmospheric pressure under oxidative and nonoxidative (inert) atmospheres and at temperatures ranging from 250 to 750 °C. The characterization of char was done in terms of its elemental composition and surface area, and by Fourier transform infrared (FTIR) and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. The surface morphology of char was studied by scanning electron microscopy (SEM). The char yield in non-oxidative runs decreased from 80% at 250 °C to 20% above 550 °C. In oxidative runs, the char was completely oxidized at 550 °C. The surface area of char increased with temperature to a maximum of 196 m2/g at 650 °C. SEM analysis indicated that the pyrolysis of chlorogenic acid first formed a melt followed by formation of varying structures that decomposed rapidly at high temperatures. The H/C and O/C ratios of the char decreased as the temperature increased. NMR analysis showed that the resonance bands corresponding to carbonyl groups mostly disappeared above 350 °C and the phenolic groups became almost totally absent in 650 °C char. The aromatic character of char was enhanced with increasing temperature. FTIR studies indicated a gradual decrease in the intensities of OH and CdO stretches at high temperatures. At 750 °C, most bands disappeared, resulting in a char that was mainly an aromatic polymer of carbon atoms. The oxidative pyrolysis enhanced mainly the surface area at the expense of char yield. The results are consistent with the analysis of the evolved gases.

Introduction Chlorogenic acid, common name for 3-O-caffeoylquinic acid, is a plant material that contains both aliphatic and aromatic groups and may be a good representative model compound for other similar biomasses such as rutin, etc. The structure of chlorogenic acid (Figure 1) shows that it is an ester formed from caffeic and quinic acids and has the following a formula: C16(H2O)9, representative of carbohydrates. Chlorogenic acid is one of the components of coffee seeds and tobacco leaves and is extracted commercially from coffee seeds. Pyrolysis of chlorogenic acid may be helpful in understanding the pyrolysis of similar biomasses. A number of studies are reported in the literature on the pyrolysis of chlorogenic acid. Upon pyrolysis, chlorogenic acid is reported to undergo a rapid decomposition to form volatile products and a solid residue, i.e., char. The char may undergo further reactions to form additional products. Zane and Wender,1 Sakuma et al.,2 and Schlotzhauer et al.3 observed phenol, catechol, substituted catechols, benzoic acid, and quinide to be the major volatile components of the pyrolysis of chlorogenic * Author to whom correspondence should be addressed. (1) Zane, A.; Wender, S. H. Tob. Sci. 1963, 7, 21-24. (2) Sakuma, H.; Matsushima, S.; Munakata, S.; Sugawara, S. Agric. Biol. Chem. 1982, 46 (5), 1311-1317. (3) Schlotzhauer, W. S.; Snook, M. E.; Chortyk, O. T.; Wilson, R. L. J. Anal. Appl. Pyrol. 1992, 22, 231-238.

Figure 1. Structure of chlorogenic acid.

acid. Despite the above studies there is no information in the literature on the composition of char. The nature of char may affect the secondary products from pyrolysis of chlorogenic acid. In this work, the effect of pyrolysis conditions on the composition of char was studied. The pyrolysis was done at atmospheric pressure and at temperatures ranging from 250 to 750 °C under approximately isothermal conditions. Both oxidative and non-oxidative atmospheres were used. The char represented the solid residue that remained after pyrolysis and consisted of organic material with a composition varying from barely pyrolyzed chlorogenic acid to a highly carbonized material. The characterization was done by scanning electron microscopy (SEM), environmental scanning electron microscopy (ESEM), Fourier transform infrared (FTIR)

10.1021/ef000058z CCC: $19.00 © 2000 American Chemical Society Published on Web 08/19/2000

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Figure 2. A typical temperature profile for sample.

spectroscopy, and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, as well as in terms of the surface area and elemental composition of the char. The results of characterization are discussed in relation to the evolved gases. Experimental Section Chlorogenic acid was obtained from Fisher Scientific and was predominantly the trans isomer with 99% purity. It was an extract of coffee beans and had a melting point of 208 °C. The elemental analysis of chlorogenic acid showed that it contained no ash, nitrogen, or sulfur. The carbon, hydrogen, and oxygen contents exactly matched those calculated from the formula of chlorogenic acid, suggesting that the sample contained no impurities. This observation was supported further by the NMR and FTIR analyses of the sample. The NMR spectrum was consistent with the structure of chlorogenic acid and the FTIR spectrum matched the reference spectrum for trans chlorogenic acid given in the Aldrich spectra library. Pyrolysis Runs. The chars were produced in a tubular reactor by pyrolyzing chlorogenic acid. The reactor consisted of a 0.5 in. diameter quartz tube heated by a sliding 6 in. long stainless steel block furnace. The furnace provided about 4 in. length of uniform temperature profile. A chromel/alumel thermocouple was placed inside the tube embedded in the sample to measure the temperature. The carrier gas was helium in non-oxidative runs and a mixture of 2% oxygen in helium in oxidative runs. The concentration of oxygen in the oxidative runs was kept low in order to prevent the complete oxidation of chlorogenic acid. A porcelain boat containing up to 300 mg of chlorogenic acid, spread as a thin layer, was placed about 3 in. from the downstream end of the tube. The pyrolyzing gas was introduced at a flow rate of 220 N cm3/min that corresponded to a residence time of 550 ms at 200 °C and 250 ms at 750 °C. The furnace initially rested over an empty portion of the tube and was equilibrated at the desired temperature. After the temperature was reached, the furnace was moved over the sample to initiate pyrolysis. The runs were made at atmospheric pressure and at temperatures ranging from 250 to 750 °C under approximately isothermal conditions (except for the initial heat-up time). The typical temperature profiles for the sample at two reactor temperatures are shown in Figure 2. The sample reached within 5 °C of the desired temperature in ca. 3 min at 750 °C and in 4.5 min at 200 °C. The product gases were passed through a Cambridge pad before being vented into a hood. A sample of product gases was also analyzed on-line by a Balzer QMG 511 quadrupole mass spectrometer. The spectrometer was operated at 24 eV energy to minimize molecular fragmentation. The run duration was set based on the total ion current in the mass spectrometer.

Sharma et al. The total ion current passed through a maximum and returned to the baseline approximately 10 min after the sample reached the desired temperature. This indicated that the pyrolysis and evolution of most components were complete mostly in 10 min. Under constant analysis conditions the ion intensity of a species was taken to represent its concentration in the gas phase. At the end of the run the product char was allowed to cool to the ambient temperature before being recovered. It was stored over dry silica gel under vacuum until analyzed. The char yield was calculated from the amount of char based on the initial mass of chlorogenic acid. The experimental error in the yield measurements was less than (1%. Char Characterization. The elemental analysis of char was performed at Galbraith Laboratories, Inc. The surface area was measured in an automated volumetric gas adsorption apparatus (Autosorb 1 from Quantachrome Co.) using nitrogen as an adsorbate at 77 K. Prior to adsorption measurements, the sample was outgassed at 120 °C for 2 h. Typically, 400 mg of sample was used in each area measurement. The surface morphology of char was studied by scanning electron microscopy (SEM). For this, the samples were analyzed mostly without being taken out of the boat to maintain the integrity of the surface. A coating of 8 nm Au/Pd film was applied to the sample using a Cressington 208 HR sputter coater. The coated samples were then examined and imaged by a Topcon SM720 field emission scanning electron microscope. The chlorogenic acid was also analyzed by environmental scanning electron microscopy (ESEM). The ESEM was a Philips Electron Optics model XL30 scanning electron microscope capable of operating both in the conventional backscattering mode as well as in secondary electron imaging modes. Compared to the conventional SEM, the sample in the ESEM can be analyzed in both dry and wet modes and at pressures of up to 50 Torr and temperatures of up to 1500 °C. In this work, the ESEM was operated at 30 kV. A small amount of chlorogenic acid (1-2 mg) was spread on the hot stage crucible of the ESEM. The crucible was then heated to 900 °C at 20 °C/min and the physical transformations occurring in the sample were recorded. The pressure in the chamber containing the sample was maintained at 2-4 Torr using either water vapors or a mixture of hydrogen (30%) and argon. The behavior of chlorogenic acid was similar in the two modes wet or dry. Solid-state 13C CPMAS NMR spectra were obtained on a Varian Unity 200 spectrometer at a carbon resonance frequency of 50.3 MHz. The NMR probe was a Doty Scientific (Columbia, SC) high-speed magic-angle spinning probe. The MAS spinning speed was ∼8100 Hz, fast enough to move the spinning sidebands away from the region of interest. Infrared spectra were recorded on a Spectra-Tech IR-Plan microscope interfaced to a Nicolet Magna 560 FTIR spectrometer. The samples were held between two KBr salt plates, held in a micro-compression cell. In some cases, pre-flattening in a diamond cell was necessary prior to mounting. A spectral resolution of 4 cm-1 was used, and the apodization function was of the Happ-Genzel type. Since the intensity of the FTIR signal from various samples was not the same, the amount of sample in each analysis was different. This led to variations in the concentration and the path length with different samples.

Results and Discussion Char Yield. Figure 3 shows the effect of temperature on char yield. As already mentioned, the char represented the solid residue that remained after pyrolysis and consisted of organic material with a composition varying from barely pyrolyzed chlorogenic acid at low temperatures to a highly carbonized material at high

Char from Pyrolysis of Chlorogenic Acid

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Figure 4. Relative peak areas of major volatilized components from non-oxidative pyrolysis of chlorogenic acid. Figure 3. Effect of temperature on char yield in oxidative and non-oxidative pyrolysis of chlorogenic acid.

temperatures. The char also included any coke that might have been formed by the reactions among the volatile components. In non-oxidative runs, the yield of char decreased with increase in temperature from 80% at 250 °C to 20% above 550 °C. The oxidative atmosphere led to char yields that were identical to those in the non-oxidative runs at low temperatures but lower above 450 °C. Above 550 °C, the char yield became negligibly small due to rapid oxidation of chlorogenic acid and the char in the presence of oxygen. The char yield was also virtually independent of the mass pyrolyzed indicating that any condensation reactions among the volatile products leading to the formation of secondary char or soot were absent under the above pyrolysis conditions. Such reactions, which occur mainly due to the inability of the volatile products to escape rapidly from the thick substrate sample, are generally expected to increase the char yield.4 Preliminary calculations showed that the apparent activation energy for pyrolysis was low at 79 kJ/mol suggesting that the pyrolysis might be affected by transport limitations. These limitations could be due to chlorogenic acid forming a melt during pyrolysis, as shown by the SEM results. However, it should be realized that the pyrolysis of chlorogenic acid is complex and the calculated activation energy values may represent the activation energy for the rate-controlling step. The major components of the volatile product were water, CO, and CO2. Their concentrations increased with temperature. At low temperatures, quinic acid was also a main component that condensed on the pad. Above 650 °C, the volatile product contained some hydrogen. This showed that the major reactions during pyrolysis were dehydration, decarboxylation, and decarbonylation of chlorogenic acid. Above 650 °C, there might be direct dehydrogenation of chlorogenic acid. Of particular interest were the relative concentrations of the organic components in the volatile product. Among these, the major components along with their relative peak areas are shown in Figure 4. The components are given in terms of the masses. The masses may correspond to phenol (m/z ) 94), catechol (110), benzoic acid (122), vinyl catechol (136). Methyl catechol (124), (4) Mok, W. S.; Antal, M. J.; Szabo, P.; Varhegyi, G.; Zelei, B. Ind. Eng. Chem. Res. 1992, 31, 1162-1166.

ethyl catechol (138), and quinide (174) were also observed in small concentrations. The assignment of the masses is consistent with the MS/MS data, the work at the National Renewable Energy Laboratory (NREL),5 and the data from field ionization mass spectrometry (FIMS).6 The FIMS data were particularly helpful since, unlike in mass spectrometric analysis, there was a negligible fragmentation of the ions in the FIMS method. The m/z ) 78, in this study, may represent either benzene or a fragmented ion from methyl catechol. The largest peak was that of phenol followed in decreasing order by catechol and benzoic acid at low temperatures and by benzene at high temperatures. There was a maximum at 350 °C in the relative peak areas for phenol, catechol, and benzoic acid. On the other hand, the concentration of benzene increased continuously with temperature. The results suggested that catechol, benzoic acid, and vinyl catechol might be the primary products of pyrolysis, with benzene the secondary product. As mentioned before, all the above components, except benzene, were also reported earlier.1-3 The composition of the volatile product from oxidative runs was essentially similar to that from non-oxidative pyrolysis, except for some differences. Phenol and catechol were still the main components, but their relative areas were lower compared to those in the non-oxidative runs. This may be due to an enhanced oxidation of these components to water, CO, and CO2. It was interesting to note that although the presence of oxygen enhanced the oxidation of chlorogenic acid, the oxygen probably did not alter the relative rates of the major reactions except those leading to complete oxidation. Char Characterization. Surface Area. The BET surface areas of various char samples are presented in Figure 5. The chars from the non-oxidative pyrolysis below 550 °C had a negligible surface area but the area increased dramatically to 196 m2/g for the char prepared at 650 °C, before decreasing slightly at 750 °C. The decrease in the surface area at 750 °C suggested that the use of high temperatures was detrimental to the pore structure of the char. However, there was an optimum temperature for maximizing the surface area. (5) Evans, R. J.; Shin, E. J.; Nimlos, M. R. National Renewable Energy Laboratory (NREL), Golden, CO, personal communication, 1999. (6) Malhotra, R. SRI International, Menlo Park, CA, personal communication, 1999.

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Figure 5. Effect of temperature on surface area of char.

Since the SEM results (presented later) indicated the presence of pores in chars even below 550 °C, the negligible surface area values suggested that the pores might be virtually closed at low temperatures, thus preventing any access to the adsorbing gas. Further, the SEM reveals only large macropores that contribute only little to the surface area. The high surface areas of chars above 550 °C were probably due to the micropores that were an order of magnitude smaller than the smallest structures displayed in the SEM micrographs. The use of oxidative pyrolysis enhanced the surface area and lowered the temperature for the maximum area to 450 °C. The enhancement could be due to the oxidation of the organic components that may be present at the poremouth by the oxygen. This is consistent with the decrease in char yield in the oxidative runs at high temperatures. The effect of temperature on surface area may be represented better by the dashed curve in the figure. The use of oxygen enhances the surface area probably by promoting the opening up of the pores that would otherwise be inaccessible. Scanning Electron Microscopy Analysis. Analysis by scanning electron microscopy (SEM) showed that the individual particles of chlorogenic acid varied from equant-shaped to platelet-shaped particles (Figure 6a). The particles tended to form tight aggregates of up to 200 µm in diameter. Pyrolysis of chlorogenic acid led to the formation of a volcano-like cone of char. The external surface of the cone was smooth and “glassy” indicating that the chlorogenic acid formed a melt above 200 °C. This was confirmed by the observations made using ESEM that showed that the chlorogenic acid started softening around 180 °C and formed a continuous melt at ca. 200 °C. This was followed by reaction to form volatile products within the melt resulting in the formation of bubbles. Further increase in temperature led to the growth and breaking of the bubbles as well as to the formation of new bubbles. The ESEM work was done at 2-4 Torr. The above behavior of chlorogenic acid is different from that of many other biomasses such as cellulose which do not undergo melting and give volatile products with a composition different than that from the chlorogenic acid. This shows that the chlorogenic acid may not be the ideal model compound for such biomasses. The SEM results for the char at 250 °C showed that it consisted of solid regions of smooth texture and

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bubbles (Figure 6b). The film covering many of the bubbles had collapsed upon cooling, indicating that the matrix lacked rigidity needed to support the expanded shape of the bubbles. At 350 °C (Figure 6c), the bubbles became larger and grew up to 1 mm in diameter. There appeared to be closed vesicles within vesicles. The fracture surfaces were conchoidal in appearance. Within the film surface, localized eruptions resulted in the formation of packed clusters of bubbles and melt structures with diameters from less than 100 nm at some places to up to 1 µm at others. The surface texture of char became increasingly rough at 450 °C due to the formation of particles within the melt itself (Figure 6d). The char also contained open pores that might have been formed by the breaking and remelting of some of the bubbles. The film covering the bubbles was apparently very plastic and stretched to thickness as low as 20 nm. At 550 °C, the surface texture showed increased roughness as well as the appearance of precipitates in the form of globular particles and rectangular rodlike projections (Figure 7a). Evidence of surface degradation was apparent at this temperature. The morphology of char changed significantly at 650 °C and showed signs of increased degradation at the surface (Figure 7b). Precipitates in the form of equant-shaped globules, rectangular-shaped rods, and platelets covered much of the surface. As the temperature was increased to 750 °C, the external surface was found covered mostly with smooth open pores of different sizes (Figure 7c). The larger pores contained multiple smaller pores within forming annular spaces in the particle. The surface was covered with precipitates representing equant-, irregular-, or platelet-shaped particles (Figure 7d). The morphology of chars from the oxidative pyrolysis at low temperatures was essentially similar to that of the non-oxidative chars. However, the morphology changed much more rapidly with temperature in the oxidative atmosphere. At 450 °C, large bubbles that were formed in the melt appeared to break rapidly, probably due to a large increase in the rate of formation of the volatile products (Figure 8a). The exterior shell of the material was all that remained as the temperature was increased to 550 °C (Figure 8b). Manometersized globular structures as well as crystalline, angularshaped platelets were found to cover much of the surface. Interestingly, the oxidative chars also showed smooth as well as rough-textured cracks that were etched in the surface (indicated by arrows in Figure 8b). Such cracks were not observed in the case of nonoxidative chars. The difference in morphology was probably due to the difference in the rates of formation of volatile products which were much higher in oxidative pyrolysis and resulted in lower char yields compared to those from the non-oxidative pyrolysis. Elemental Composition. Figure 9 shows the relationship between the hydrogen/carbon (H/C) and oxygen/ carbon (O/C) ratios of the chars. The corresponding temperatures for some of the chars are also shown in the figure. The point labeled “25 °C” represents the unpyrolyzed chlorogenic acid. Both the H/C and O/C ratios decreased with increase in temperature indicating that the char became increasingly more carbonaceous in nature at high temperatures. Interestingly, there was



Figure 6. SEM micrographs of chlorogenic acid (a) and its non-oxidative chars prepared at 250 °C (b), 350 °C (c), and 450 °C (d).

almost a linear relationship between the decrease in the two ratios, at least until 650 °C. The major reactions at these temperatures appeared to be the dehydration, decarboxylation, and decarbonylation of chlorogenic acid into water, CO, and CO2. This is consistent with the composition of the volatile product that comprised mainly of CO, CO2, and water, with concentrations of the organic components being relatively low. Above 650 °C, the H/C ratio decreased dramatically compared to the O/C ratio suggesting a direct dehydrogenation of the char at the high temperatures. This is consistent with the evolution of hydrogen that was observed in the gas analysis. The H/C and O/C ratios and their relationship were not affected by the use of oxidative pyrolysis. Again, this observation is in agreement with the analysis of the volatile product that remained unaffected by the use of oxygen. NMR Analysis. Solid-state 13C CP/MAS NMR spectra of chlorogenic acid and its chars were obtained at the MAS spinning speed of ∼8100 Hz, fast enough to move the spinning sidebands away from the region of interest. The char at 750 °C could not be analyzed due to its high conductivity. The spectrum of chlorogenic acid (Figure 10) showed several sharp resonance peaks indicative of very ordered or crystalline states. The multiplicities of resonances in the individual groups of peaks were due to multiple crystalline modifications

and/or multiple molecules in the crystalline lattice. The assignment of various carbons in the structure was based on work with other bio-materials such as cellulose and pectin as well as on the results reported in the literature.7 The spectra for the non-oxidative chars are presented in Figure 11. The spectra changed progressively with temperature. The resonance bands tended to be broad relative to those from the un-pyrolyzed chlorogenic acid indicating increase in the chemical complexity and the amorphous nature of the chars with temperature. The structural changes already seem to be taking place at 250 °C. The changes appeared to be mostly due to melting and formation of an amorphous state since the resonance peaks only broadened and did not change appreciably in number or intensity. Any pyrolytic reactions were probably masked by the bulk of the sample that appeared to be essentially unchanged. Significant changes in spectra were observed above 250 °C. There was a progressive loss of oxygen functionality with temperature as indicated by the loss of carbonyl and carboxyl resonances. At 350 °C, the aliphatic resonances coalesced into a single-broad resonance at 37 ppm with an intensity that is lower than at 250 °C. The resonance bands corresponding to ester (7) Shafizadeh, F.; Sekiguchi, Y. Combust. Flame 1984, 55, 171179.

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Figure 7. SEM micrographs of non-oxidative chars prepared at 550 °C (a), 650 °C (b), and 750 °C [(c) 100 µm, and (d) 2 µm].

Figure 8. SEM micrographs of oxidative chars at 450 °C (a) and 550 °C (b).

carbonyl groups mostly disappeared. A nonspecific aromatic resonance appeared at 129 ppm. Other aromatic resonances at 118.6, 142.9, and 165.4 ppm were not assigned, but appeared to be due to pyrolytic reactions. A weak peak at 209 ppm is characteristic of ketonic carbons. Features in this spectrum may provide insight into the mechanism of the homolytic pyrolysis of chlorogenic acid.8

The spectra appeared to change drastically above 350 °C suggesting extensive structural rearrangements. The char lost its aliphatic character and became more and more aromatic in nature. The resonances corresponding to phenolic groups also decreased progressively in (8) Lewis, I. C.; Singer, L. S. Electron Spin Resonance and the Mechanism of Carbonization. In Chemistry and Physics of Carbon, 17; Walker, P. L., Jr., Thrower, P. A., Eds.; Dekker: New York, 1981; p 1.


Figure 9. Relationship between H/C and O/C ratios of char.

intensity until they became almost totally absent in the 650 °C char. The resonance in the carbonyl region either became extremely weak or was absent. This suggested a rapid degradation of these groups. The decrease in the intensity of the resonance band at 155 ppm in the aromatic region, indicative of either furanyl or phenolic C-O functionalities, indicated that these compounds gradually lost their character until they became totally absent in the 650 °C char. At 450 °C, the weak carbonyl peak as well as most, but not all, of the aliphatic resonances had disappeared. The aromatic carbons that remained were typical of many carbonized organic materials. The resonances of the aromatic ring network appeared at 130 ppm with a shoulder due to aromatic oxygen-bound carbons at ca. 150 ppm. The phenolic, carboxyl, and carbonyl groups were still present in the char at 450 °C, although the concentrations of the latter two groups were very small. It is believed that the aliphatic and oxygen groups created links and loops between aromatic clusters of different sizes.9 Simultaneously with the loss of carbonyl and aliphatic carbons, the intensity for aromatic groups increased with temperature. The broadening of the base of the resonance for aromatic groups for 350 and 450 °C chars could be due to the enhancement of the amorphous nature of the char, as well as due to the presence of phenolic structures. Above 450 °C, the char seemed to be mainly aromatic in nature. The width of the aromatic resonances narrowed significantly probably due to a further loss of aromatic C-O resonances (150 ppm). At 550 °C, the aromatic carbons bonded to oxygen and the remaining aliphatic carbons were reduced in intensity. At 650 °C, the oxygen-bonded carbons were no longer distinguishable and only a very small aliphatic remained, indicating that the char was almost completely carbonized. The spectra of oxidative chars (Figure 12) are essentially similar to those of the non-oxidative chars indicating that many of the carbons that were oxidized were lost to the volatile product. These results appear to be at variance from those for cellulose chars prepared under inert and air-oxidation environments.7 For cellulose chars, with a heat-treatment temperature (HTT) of 400 °C, the aliphatic resonance was found to be significantly reduced while the resonance of aromatic (9) Fletcher, T. H.; Solum, M. S.; Grant, D. M.; Pugmire, R. J. Energy Fuels 1992, 6, 643-650.


carbons bonded to oxygen increased. This difference in the characteristics of cellulose and chlorogenic acid chars may result as much from the use of different experimental conditions in the two cases as from any fundamental differences in the chlorogenic acid and cellulose chars. FTIR Analysis. The FTIR spectrum of chlorogenic acid (not shown) was complex and showed bands corresponding to OH, aromatic CH, aliphatic CH, CdO, and aromatic ring. The spectra for the select char samples are presented in Figure 13. As the temperature increased, there was an increasingly upward drift in baseline at high wavenumbers. This may be an indication of the increase in the carbon black content of the char, i.e., the char being relatively more carbonized at high temperatures. Various bands in the spectra corresponding to OH-stretch (at wavenumber of ∼3400 cm-1, broad), aromatic CH (3066-3051), aliphatic CH3 (2963-2968), aliphatic CH2 (2936-2932), aromatic CH3, i.e., CH3 in an aromatic ring (2925-2917), aromatic ring (1608-1578), and aromatic CH wag, i.e., an aromatic ring (various bands between 1000 and 700 cm-1) were identified. The stretches due to the OH and carbonyl groups showed a continuous decrease in intensity with increase in temperature due to increased loss of these functionalities. The CdO stretching region contained at least three overlapping bands making the analysis difficult. Some of the bands covered a range of wavenumbers since the bands showed a shift in the wavenumbers for different chars. Interestingly, there was a small band at 3637 cm-1 that may correspond to a free OH stretch, whereas the band at 3400 cm-1 represents a bonded OH stretch. The band for the free OH stretch became apparent only for the char prepared at 300 °C and grew to a maximum at 600 °C. The spectrum of the 750 °C char showed the absence of most bands and suggested that the char was mainly an aromatic polymer of carbon atoms. Mok et al.4 made essentially similar observations from the FTIR analysis of cellulose chars prepared at temperatures of up to 450 °C. The major chemical changes were reported to be dehydration, carbonyl group formation and elimination, and the decomposition of aliphatic and formation of aromatic char units. This indicates that at least some steps in the chemical transformations occurring during pyrolysis may be similar for different biomasses. The relative proportions of a functional group in different chars may be compared by comparing the corresponding peak areas. However, the peak area also depends on the path length and the concentration of the sample in the analysis. Due to the varying intensity of the FTIR signal, the amount of sample in different analyses could not be kept constant. This led to differences in the concentration and path length with different samples. As a result, the peak areas for a functional group in different samples could not be compared directly. The problem was overcome by comparing the functional group ratios. However, the significance of the ratios has to be considered in conjunction with the FTIR spectra that showed which part of the ratio contributed more to the variations in the ratios for different samples. Of particular interest were the bands corresponding to the OH group and their variations relative to the other bands such as aromatic CH or aromatic ring

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Figure 10.

13C

CPMAS NMR spectrum of chlorogenic acid.

Figure 11.

13C

CPMAS NMR spectra of non-oxidative chars.

stretches as well as the aliphatic CH2 plus aromatic CH3 stretches. Figure 14 shows the variations in the ratios

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of OH stretch to the aromatic ring stretch and aromatic ring wag. Both the ratios were high initially but


Figure 12.

13C


CPMAS NMR spectra of oxidative chars.

Figure 13. FTIR spectra of chars.

decreased as the temperature was increased. This indicated a loss of the OH functionality and an enhancement of the aromatic nature with increase in temperature. The band due to OH stretch was missing in the 750 °C char, indicating that the char had completely lost the hydroxyl group. At high temperatures the

bonded OH stretch was lost more rapidly relative to stretches for the aliphatic CH2 or aromatic CH3. Although the bulk of the OH stretch was mostly due to the bonded OH group, the absorbance due to the free OH stretch increased gradually with temperature. As shown in the figure, the ratio of the peak areas for the free and bonded OH stretches was at maximum at 600 °C. Interestingly, the surface area of char was also high in this temperature region. Although several bands may be used to monitor the aromatic nature of the char, the most suitable band may be the aromatic CH stretch. Figure 15 shows the ratio of the peak areas for the aromatic CH and the two OH stretches. The ratio increased with temperature to a maximum at 600 °C. At higher temperatures the ratio decreased and became negligible for the 750 °C char. Thus, the aromaticity of the char appeared to be at maximum at 600 °C. The reason for the decrease in aromaticity at 650 °C is not clear but the decrease could be due to pronounced graphitization and an increased loss of hydrogen from the char. The increased aromatization results in a poly-aromatic structure of the char and a decrease in the intensity of the aromatic CH stretch as the concentration of the carbon atoms at the surface decreases. Boon et al.10 observed a similar increase in the aromatic character and a loss of oxygen functionality for the cellulose chars at high temperatures. Figure 15 also shows the ratios of peak areas for the aliphatic CH2 plus aromatic CH3 stretch to the two OH stretches. The ratio with free OH stretch decreased with increase in temperature due to the increase in the intensity of the free OH stretch at high temperatures (10) Boon, J. P.; Pastorova, I.; Botto, R. E.; Arisz, P. W. Biomass Bioenergy 1994, 7, 25-32.

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Figure 15. Comparison of peak area ratios for the aromatic CH and aliphatic CH2 stretches with respect to the OH stretches. Figure 14. Comparison of peak area ratios for the OH stretches and aromatic ring stretches.

observed above. On the other hand, the ratio with bonded OH stretch was high in the intermediate temperature range of 350-550 °C. Notice that the ratios with bonded OH stretch were extremely small. However, the results indicated that the aliphatic CH2 plus aromatc CH3 groups maximized at the intermediate temperature. At high temperatures these groups were lost due to the aromatization reactions. This is in agreement with the observed hydrogen evolution at high temperatures. The spectra for oxidative chars are not presented since the spectra were virtually similar to those for the non-oxidative chars. Thus the role of oxygen on the appearance and disappearance of various functional groups was small. This is consistent with the NMR results. Conclusions The following conclusions may be drawn from this study. 1. The char yield decreases with increase in temperature to a minimum at 20% above 450 °C in nonoxidative pyrolysis. In oxidative runs, the yield becomes negligible above 550 °C. 2. When heated to temperatures of 250 °C or above, chlorogenic acid first forms a melt, followed by the formation, growth, and breaking of bubbles in the melt and the release of the volatile products.

3. At low temperatures, the char particles have irregular appearance with a few bubbles inside the particles. The melt consists of solid regions of smooth texture and bubbles; the underlying surfaces are filled with bubbles. At high temperatures, the particles become more rounded and the bubbles become larger. The surface of the melt becomes increasingly rough due to the growth of globular, rodlike, and platelet structures which degrade further to leave a carbonized frame of bubbles and pores. In the presence of oxygen, these carbonized structures are oxidized completely above 550 °C. 4. The maximum surface area of char in non-oxidative runs is obtained at 650 °C. The oxygen enhances the surface area and lowers the temperature for maximum surface area to 450 °C. 5. The composition of char is dependent on the pyrolysis conditions. Both the hydrogen and oxygen contents of char decrease as the temperature is increased. The H/C ratio decreases from ∼1 at 250 °C, to 0.2 at 750 °C, indicating an increase in the aromaticity and carbonaceous nature of char. 6. The NMR spectra of chars differ progressively with temperature from that of chlorogenic acid. At 250 °C, the resonance bands are broad, relative to those from un-pyrolyzed chlorogenic acid, indicating an increase in the amorphous nature of the sample. Above 250 °C, there is a steady loss of oxygen functionality and a loss of carbonyl resonances. The phenolic, carboxyl, and carbonyl groups are still present in the 350 °C char although the concentration of the latter two groups is


small. As the temperature increases, the char loses its aliphatic character completely and becomes more aromatic in nature. The resonances corresponding to phenolic groups also decrease progressively in intensity until they become totally absent in the 650 °C char. 7. The FTIR results suggest dramatic chemical changes in chlorogenic acid above 250 °C. The hydroxyl groups decrease. A free OH stretch appears at 300 °C and is at maximum at 600 °C. Aliphatic CH2 and CH3 stretches are at maximum at 350 °C and decrease until they disappear at 500 °C. The CH3 stretch on an aromatic ring appears to be at its maximum at 500-550 °C. On the other hand, the aromatic character (CH and aro-


matic ring stretches) increases and is highest at 600 °C. At 750 °C, all the bands due to OH, CH, CH2, and CH3 stretches vanish. The char is mainly an aromatic polymer of carbon atoms. The composition of the oxidative chars is virtually the same as that of the nonoxidative chars. Acknowledgment. The authors are grateful to Philip Morris Management for their support of this research, to Tony Penn for help in running the Autosorb, and to Marc Krauss for providing the ESEM data. EF000058Z

Characterization of Char from Pyrolysis of Chlorogenic Acid

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