Effect of Metal Doping on the Initial Pyrolysis ... - ACS Publications

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Energy & Fuels 2008, 22, 2816–2825

Effect of Metal Doping on the Initial Pyrolysis Chemistry of Cellulose Chars Joshua G. Lee, Eun-Jae Shin, Ryan A. Pavelka, Mathew S. Kirchner, Dimitri Dounas-Frazer, Bryan D. McCloskey, David E. Petrick, J. Thomas McKinnon, and Andrew M. Herring* Department of Chemical Engineering, Colorado School of Mines, Golden, Colorado 80401-1887 ReceiVed October 26, 2007. ReVised Manuscript ReceiVed April 17, 2008

Laser pyrolysis (LP) using a CO2 laser was used to rapidly heat Avicel cellulose char with and without doping by Na, K, Ca, Mg, Co, Ni, Cu, Pd, and Zn as their respective acetates in the uncharred cellulose. LP-molecular beam mass spectrometry (LP-MBMS) was used to probe the chemistry of the pyrolysis of the chars and to characterize the initial volatilized reacting plume of pyrolyzate. The mass spectra were deconvoluted by multivariant factor analysis (MVA). MVA revealed that, at the 1 wt % doping level, Cu did little to change the pyrolysis chemistry of cellulose char, among the other metals, Na, K, Ca, Mg, Co, Ni, and Zn did effect the pyrolysis chemistry, and Co and to a lesser extent Pd acted by a differing mechanism. A liquid N2 cold trap coated with diphenyldisulfide was used to understand the chemistry of the evolving pyrolyzate plume. The washings from this trap were analyzed by gas chromatography-mass spectrometry (GC-MS) allowing for speciation of both the chemically trapped radicals and the nonradical volatiles. It was clear that the species trapped had a much greater degree of methyl substitution than observed in the LP-MBMS. Radicals trapped included hydrogen, methyl, and phenyl but did not include cyclopentadienyl or benzyl. Zn, which was volatilzed by the LP, had the highest yield of radicals. In general, the cold-trapped 1 and 2 aromatics had a higher level of methyl substitution. Cu and Pd increase the formation of aromatic species during the pyrolysis of cellulose char and have a variable effect on radical production. Na, K, Mg, Ca, Co, and Ni suppress the formation of aromatic species and have a variable effect on radical production.

Introduction There is increasing interest in the production of renewable fuels and chemicals from biomass. The thermochemical conversion of biomass is one promising route that is currently being investigated. Biomass thermochemical conversion includes both gasification and pyrolysis. Additionally, biomass combustion processes are often imperfect and actually involve pyrolysis of biomass and biomass char. Pyrolysis processes yield a mixture of solid char, liquid oil, and gaseous products, whose relative ratios may be varied by the residence time of the pyrolysis event. Traditional slow pyrolysis yields predominantly large amounts of char, whereas the more recently studied fast pyrolysis processes yield predominantly pyrolysis oil.1–5 However, the widespread use of biomass pyrolysis to produce, for example, pyrolysis oil, a dispatchable energy source that may be reformed to syngas and, therefore, other usable fuels is hampered by the fact that such oils include entrained ash and polycyclic aromatic hydrocarbons (PAHs) and other molecules predisposed to the * To whom correspondence should be addressed. Telephone: (303) 3842082. Fax: (303) 273-3730. E-mail: [email protected]. (1) Oasmaa, A.; Kuoppala, E.; Solantausta, Y. Fast pyrolysis of forestry residue. 2. Physicochemical composition of product liquid. Energy Fuels 2003, 17 (2), 433–443. (2) Oasmaa, A.; Kuoppala, E.; Gust, S.; Solantausta, Y. Fast pyrolysis of forestry residue. 1. Effect of extractives on phase separation of pyrolysis liquids. Energy Fuels 2003, 17 (1), 1–12. (3) Oasmaa, A.; Kuoppala, E. Fast pyrolysis of forestry residue. 3. Storage stability of liquid fuel. Energy Fuels 2003, 17 (4), 1075–1084. (4) Bridgwater, A. V. Fast Pyrolysis of Biomass: A Handbook; CPL Press: Newbury, U.K., 2002; Vol. 2, p 426. (5) Shafizadeh, F. Introduction to pyrolysis of biomass. J. Anal. Appl. Pyrolysis 1982, 3 (4), 283–305.

formation of intractable tars.6,7 Because the particle size of the biomass is never infinitely small, it follows that some portion of the biomass will char significantly before being pyrolzed, and this is a possible mechanism for the formation of PAH and tars.8,9 The pyrolysis of biomass char is also important to such diverse and important applications from cigarette smoking to using biomass in conventional heating systems to forest fires. The major constituents of biomass are cellulose, hemicellulose, and lignin, of which cellulose is often the most abundant and has the most defined structure.10 In addition, it is thought that, of all of the constituents of biomass, cellulose pyrolysis gives rise to the highest yields of PAHs5,11 and the lignin may simply (6) Britt, P. F.; Buchanan, I. A. C.; Kidder, M. K.; Owens, J.; Clyde, V. Influence of steroid structure on the pyrolytic formation of polycyclic aromatic hydrocarbons. J. Anal. Appl. Pyrolysis 2003, 66 (1-2), 71–95. (7) Marsh, N. D.; Ledesma, E. B.; Sandrowitz, A. K.; Wornat, M. J. Yields of polycyclic aromatic hydrocarbons from the pyrolysis of catechol [ortho-dihydroxybenzene]: Temperature and residence time effects. Energy Fuels 2004, 18 (1), 209–217. (8) McGrath, T. E.; Chan, W. G.; Hajaligol, M. R. Low temperature mechanism for the formation of polycyclic aromatic hydrocarbons from the pyrolysis of cellulose. J. Anal. Appl. Pyrolysis 2003, 66 (1-2), 51–70. (9) Herring, A. M.; McKinnon, J. T.; Petrick, D. E.; Gneshin, K. W.; Filley, J.; McCloskey, B. D. Detection of reactive intermediates during laser pyrolysis of cellulose char by molecular beam mass spectroscopy, implications for the formation of polycyclic aromatic hydrocarbons. J. Anal. Appl. Pyrolysis 2003, 66 (1-2), 165–182. (10) Langan, P.; Nishiyama, Y.; Chanzy, H. A revised structure and hydrogen-bonding system in cellulose II from a neutron fiber diffraction analysis. J. Am. Chem. Soc. 1999, 121 (43), 9940–9946. (11) McGrath, T. E.; Wooten, J. B.; Chan, W. G.; Hajaligol, M. R. Formation of polycyclic aromatic hydrocarbons from tobacco: The link between low temperature residual solid (char) and PAH formation. Food Chem. Toxicol. 2007, 45 (6), 1039–1050.

10.1021/ef700637s CCC: $40.75  2008 American Chemical Society Published on Web 06/10/2008

Initial Pyrolysis Chemistry of Cellulose Chars

depolymerize to give single-ring aromatics. For this reason, the fundamental study of the pyrolysis of cellulose chars is important for obtaining a mechanistic understanding of the formation of large molecules, such as PAHs, for which a mitigation strategy would be desirable. Pure cellulose is readily purified and obtainable, although we note that it is not representative of cellulose in a native form, which would be in intimate contact with hemicellulose, lignin, metal ions, proteins, and other minor constituents. However, we study pure cellulose char here because it represents a model material for the study of the pyrolysis of biomass char in general. Thermal decomposition of cellulose generally proceeds by one of two pathways either dehydration to anhydrocellulose, which further decomposes to gases and char, or depolymerization of cellulose at somewhat high temperatures, producing tar (mainly levoglucosan and aromatic species containing up to five rings) and combustible gases.5 We have shown previously that the charring chemistry of cellulose is unique among the major biomass components comprising of lignin, cellulose, hemicellulose, and pectin; in that, it is responsible for the production of PAH via a low-temperature route.9,12 High-temperature growth of PAH is thought to occur either by the hydrogen abstraction, acetylene addition (HACA) mechanism or by other routes, such as the addition of methyl radical.13 When cellulose is charred, it gradually forms PAH moieties that grow in size with charring time, which are then liberated during subsequent pyrolysis events,9,14 the so-called low-temperature route to PAH from biomass.8 This does not occur with hemicellulose or pectin or even lignin, despite the fact that lignin has a major aromatic component.12 These PAH fragments are liberated during pyrolysis of the charred cellulose and may react further with smallmolecule fragments to give the final product slate. Simple cations, such as K, have already been shown to effect the thermal pathway of cellulose decomposition.15–20 The pyrolysis chemistry of cellulose has been studied when the cellulose has been doped with Na, K, Ca, Mg, Fe, Co, and Zn.21–23 The presence (12) Herring, A. M.; McKinnon, J. T.; Gneshin, K. W.; Pavelka, R.; Petrick, D. E.; McCloskey, B. D.; Filley, J. Detection of reactive intermediates from and characterization of biomass char by laser pyrolysis molecular beam mass spectroscopy. Fuel 2004, 83 (11-12), 1483–1494. (13) Richter, H.; Howard, J. B. Formation of polycyclic aromatic hydrocarbons and their growth to sootsA review of chemical reaction pathways. Prog. Energy Combust. Sci. 2000, 26 (4-6), 565–608. (14) Hajaligol, M.; Waymack, B.; Kellogg, D. Low temperature formation of aromatic hydrocarbon from pyrolysis of cellulosic materials. Fuel 2001, 80 (12), 1799–1807. (15) Evans, R. J.; Milne, T. A. Molecular characterization of the pyrolysis of biomass. 2. Applications. Energy Fuels 1987, 1 (4), 311–319. (16) Evans, R. J.; Milne, T. A. Molecular characterization of the pyrolysis of biomass. 1. Fundamentals. Energy Fuels 1987, 1 (2), 123–137. (17) Kellogg, D. S.; Waymack, B. E.; McRae, D. D.; Peishi, C.; Dwyer, R. W. The initiation of smolderin combustion in cellulosic fabrics. J. Fire Sci. 1998, 16 (2), 90–104. (18) Kellogg, D. S.; Waymack, B. E.; McRae, D. D.; Dwyer, R. W. Smolder rates of thin cellulosic materials. J. Fire Sci. 1997, 15 (5), 390– 403. (19) Sekiguchi, Y.; Shafizadeh, F. The effect of inorganic additives on the formation, composition, and combustion of cellulosic char. Combust. Flame 1984, 29, 1267–1286. (20) Shafizadeh, F.; Bradbury, A. G. W.; DeGroot, W. F.; Aanerud, T. W. Role of inorganic additives in the smoldering combustion of cotton cellulose. Ind. Eng. Chem. Prod. Res. DeV. 1982, 21 (1), 97–101. (21) Klampfl, C. W.; Breuer, G.; Schwarzinger, C.; Ko¨ll, B. Investigations on the effect of metal ions on the products obtained from the pyrolysis of cellulose. Acta Chim. SloV. 2006, 53 (4), 437–443. (22) McGrath, T. E.; Hoffman, J. A.; Wooten, J. B.; Hajaligol, M. R. The effect of inorganics on the formation of PAH during low temperature pyrolysis of cellulose. Prepr. Pap.sAm. Chem. Soc., DiV. Fuel Chem. 2002, 47 (1), 418–419. (23) Soares, S.; Camino, G.; Levchik, S. Effect of metal carboxylates on the thermal decomposition of cellulose. Polym. Degrad. Stab. 1998, 62 (1), 25–31.

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of inorganic salts can lower the primary pyrolysis temperature, increase the char yield by accelerating decomposition reactions, increase or decrease the rate of oxidation of the aromatic component of the char, and increase or decrease the heat released from the oxidation of char to CO2.22 To understand how cellulosic biomass may be cleanly converted to useful fuels and chemicals, it is important to understand the fundamental reaction steps that occur during the pyrolysis of the cellulose chars that are formed in all thermochemical conversion processes involving cellulosic biomass. We were motivated to see the effect of metal dopants on this process because many metals are present in biomass. We extended the study to a number of transition metals that could be imagined to have beneficial catalytic effects on the thermo-chemical conversion of biomass during pyrolysis. It has been proposed that the impregnation of wood with catalytically active transition metals with low volatility could lead to a practical reduction of tar in wood pyrolysis.24 Furthermore, because small amounts of metal ions are known to profoundly affect cellulose pyrolysis, it is also important to determine how, whether by catalytic or stoichiometric reaction, these metals ions influence the initial pyrolysis product slate of cellulose char. Here, we use laser heating with a CO2 laser to rapidly heat the cellulose char samples to >800 °C and measure the laser pyrolysis products. Laser pyrolysis has been used to successfully study wood pyrolysis under conditions that simulate fire heat fluxes.25 The motivation for using laser heating with large cellulose char pellets is not the precise control of temperature but the generation of detectable quantities of reactive intermediates under a well-controlled heat flux. Because so little is currently known about the reactive intermediates in the pyrolysis of cellulose chars, it seemed reasonable to first experimentally confirm, for the first time, the identity of the reactive intermediates and infer that these would be produced in the temperature regime likely in pyrolysis processes. Our attempts to detect these species while using smaller samples (i.e., of the size of the laser beam) in which the temperature is more precisely known and there is less likelihood of interparticle interactions will be discussed in a later publication from our research group. The reaction products are detected either immediately with a laser pyrolysis-molecular beam mass spectrometer (LP-MBMS) or after a longer but still short evolution time by a N2(l) cold trap coated with the radical-trapping agent diphenyldisulfide. Metal doping is achieved by incipient wetness using water-soluble metal acetates that decompose during charring. The charring temperature of 375 °C was chosen to ensure that the metal acetate would decompose and because it is know to be the temperature at which the maximum yield of char is obtained from cellulose. We have shown previously that no secondary cracking products are detected by LP-MBMS.9 The species detected by LP-MBMS, therefore, represent not only the initial reacting product slate liberated to the gas phase but are also representative of the char structure before the pyrolysis event. LP-MBMS produces complex mass spectra that are not easily interpreted. Using multivariate analysis, it is possible to show both the similarities and differences between samples and also reveal correlated behavior among the mass variables. The correlation may be due to several reasons: (1) products of the same reaction (24) Bru, K.; Blin, J.; Julbe, A.; Volle, G. Pyrolysis of metal impregnated biomass: An innovative catalytic way to produce gas fuel. J. Anal. Appl. Pyrolysis 2007, 78 (2), 291–300. (25) Lee, C. K.; Chaiken, R. F.; Singer, J. M. Charring pyrolysis of wood in fires by laser simulation. Int. Symp. Combust. 1977, 16 (1), 1459– 1470.

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pathways, (2) products of independent reaction pathways that have similar kinetics that can not be resolved by the experimental conditions used, and (3) multiple ions from the fragmentation of a compound in the ionizing process. The use of multivariate statistics allows for the extraction of the chemical trends to the extent that the experiment allows them to be expressed. In the cold-trap experiment, the reaction is arrested further along the reaction coordinate but still not at completion. This allows us to study how the evolved reactive pyrolyzate characterized by LP-MBMS reacts with time. All of the volatilzed and ablated materials from the pyrolysis event is quenched on the cold surface of the collection cup, and any radicals are neutralized and trapped as phenylsulfide adducts. Traditionally, dimethyldisulfide has been used as a radical trap in flame work,26,27 but its high volatility precluded its use in solid-state trapping. The result is a snapshot of the evolving gas plume allowing us to separate solid-phase charring chemistry from the gas-phase reactivity of the pyrolyzate plume. Experimental Section Materials. The metal acetates were obtained from commercial sources as follows: sodium acetate 99.99% (Fisher Scientific); potassium acetate, 99% (Baker); magnesium acetate, reagent-grade (Baker); calcium acetate, reagent-grade (Fisher Scientific); cobalt acetate, reagent-grade (Baker); nickel acetate, 99% (Fisher Scientific); palladium acetate, 99% (Pressure Chemical); copper acetate, 99% (Alpha); or zinc acetate, 99.1%, (Sigma-Aldrich). Avicel cellulose was also obtained from a commercial source (Analtech). Diphenyl disulfide (DPDS, 99%) was obtained from Aldrich. The Avicel was doped with the metal acetates by incipient wetness and dried at 75 °C overnight to produce cellulose pellets doped from 1 to 5 wt %. Pellets were pressed from 3 g of cellulose or doped cellulose at 138 MPa in a 3.175 cm L stainless-steel pellet press to give a pellet of 4 mm thickness and then charred under Ar (10.7 cm3/s) at 375 °C for 30 min unless otherwise stated. The pellets were then cooled under Ar and transferred to the sample chamber of the pyrolysis apparatus under N2. The pyrolysis experiments were then performed under He after evacuating and backfilling the pyrolysis chamber 3 times. LP-MBMS. The LP-MBMS system using a 65 W CO2 laser and LP experiments employed have been described previously.9 Multivariate Factor Analysis (MVA). The techniques of Windig et al. are used in this work.28–31 This approach is based on factor analysis followed by factor rotation methods that are a combination of (i) the KEY SET method to look for mass variables with loadings that are extreme in the matrix and (ii) the variance diagram (VARDIA) to look for a clustering behavior of the loadings.28–31 Data reduction is accomplished by finding correlated masses that can be expressed by a new single variable or factor. The first factor is the linear combination of masses that describes (26) Hausmann, M.; Homann, K. H. Scavenging of hydrocarbon radicals from flames with dimethyl disulfide. 1. Characterization and discussion of the method and the scavenging process. Phys. Chem. Chem. Phys. 1995, 99 (6), 853–862. (27) Hausmann, M.; Homann, K. H. Analysis of radicals from flames by scavenging with dimethyl disulfide. Phys. Chem. Chem. Phys. 1990, 94 (11), 1308–1312. (28) Windig, W.; Lippert, J. L.; Robbins, M. J.; Kresinske, K. R.; Twist, J. P.; Snyder, A. P. Interactive self-modeling multivariate analysis. Chemom. Intell. Lab. Syst. 1990, 9 (1), 7–30. (29) Windig, W.; Meuzelaar, H. L. C. Nonsupervised numerical component extraction from pyrolysis mass spectra of complex mixtures. Anal. Chem. 1984, 56 (13), 2297–2303. (30) Windig, W.; Kistemaker, P. G.; Haverkamp, J. Chemical interpretation of differences in pyrolysissMass spectra of simulated mixtures of biopolymers by factor analysis with graphical rotation. J. Anal. Appl. Pyrolysis 1982, 3 (3), 199–212. (31) Windig, W.; Haverkamp, J.; Kistenaker, P. G. Anal. Chem. 1983, 55, 81.

Figure 1. Schematic of the chemical cold-trap apparatus.

the maximum variance in the data set. The subsequent factors are similarly extracted on the basis of the residual variance after the effect of the previous factors has been removed from the data. Significant data reduction is possible because the masses are highly correlated. This data reduction makes possible a graphical display of the data that not only shows trends but can also provide chemical insight. Each mass has a correlation coefficient, “the loading”, Rij, with each of the factors. The masses with high loadings on a particular factor are correlated, and that factor represents that group of masses and is quantified for each sample by the factor score, Fj. The factor score for a sample is a linear combination of the masses based on the loading of a mass for a factor and the intensity of that mass, Z, for each sample as shown in the equation

Fj ) R1jZ1 + R2jZ2 + ... + RnjZn Using the factor loadings for the masses and factor scores for the samples, factor analysis can reflect the presence of chemical components or trends in the data set. After the preparation of the correlation matrix between masses and the extraction of the initial factors, a third operation is performed, the rotation of the mathematically derived factors to coincide with real chemical components. Chemical Cold Trapping. The cold trap consisted of a hollow stainless-steel pipe filled with liquid nitrogen (at a maximum of 78 K) attached to a brass cup. A central hole was drilled into the brass cup to allow the laser beam to pass through (Figure 1). A small quantity of DPDS (0.5 g) was heated until it just melted and was poured into the brass cup to coat the inside surface. The pellet was then mounted inside the brass cup so that the laser beam would hit the pellet and the ejected debris and volatiles would be trapped on the coated brass cup and react with the DPDS. The apparatus was placed inside the Ar-flushed combustion chamber used for the MBMS experiments but with the MBMS isolated. Laser pyrolysis was performed at 13 W, and the pellet was lased while continually spun at 1.2 rpm on a stepper motor for two full revolutions. The process was repeated for four pellets before the cold-trapped volatiles and reactive intermediates were analyzed by gas chromatography-mass spectrometry (GC-MS). The sample cup was washed with a mixture of 5 µL of chlorobenzene (as a reference standard) in 8 mL of acetone. Air was passed over the collected washings, until they were reduced to 50 µL. A total of 0.8 µL of the mixture was injected into a HP 5890 series II GC


Figure 2. Char yield for cellulose and metal acetate-doped cellulose charred at 375 °C for 1/2 h: 1 wt %, ]; 5 wt %, [.

containing a 30 m, 0.25 mm inner diameter, 0.25 µm film column (DB-35MS stationary phase) coupled to a HP 5971 series mass selective detector. The temperature profile used was 55 °C for 8 min and then a 10 °C/min ramp to 205 °C for 5 min followed by a 10 °C/min ramp to 250 °C for 12 min. The injectors were set to 250 °C. The mass spectra of the various peaks detected by the GC were compared to National Institute of Standards and Technology (NIST)32 spectra of known compounds. Various controls were performed to investigate any interfering pyrolsyis chemistry of DPDS. A pure sample of DPDS only showed DPDS in its chromatogram under the GC conditions described above. To verify that the DPDS does not interact with brass, acetone, or chlorobenzene and to see if the temperature extremes present in these experiments (from the GC inlet, the liquid nitrogen, and possibly radiation reflected from the char surface) causes reactions within the DPDS itself, a control experiment was designed. A pellet of solid aluminum was made approximately the same size as a charred cellulose pellet. The outside of the aluminum pellet was coated with EMS graphite conductive adhesive 112. A hole was drilled in the center, and the pellet was placed on the pellet mount. The remainder of the experiment was performed as previously described for cellulose pellets.

Results and Discussion Char Characterization. For comparative purposes, we first report the effect of metal doping on the char yields from cellulose and doped cellulose. This is because most doping studies for the charring studies of cellulose have been performed with metal chlorides and not all of these studies are complimentary.33 The char yields after heating in Ar(g) at ambient pressure for 1/2 h at 375 °C for the control Avicel cellulose and the 1 and 5 wt % metal acetate-doped cellulose are shown in Figure 2. 375 °C was chosen as the charring temperature because it falls just below the point at which char yields rapidly fall off and is well-above the decomposition temperature of all of the metal acetates studied. In general, the char yields decrease with an increasing atomic weight of the dopant metal. The highest yields occur when the cellulose is doped with 5 wt % Mg, Ca, or Zn acetate, and the lowest yields occur when doped with 5 wt % Pd acetate. The TGA under He for these samples is shown in Figure S1 in the Supporting Information. All of the samples show higher mass losses than the undoped pellet. Interestingly, the Pd doped samples may not have reached steady state because (32) Linstrom, P. J.; Mallard, W. G. NIST Chemistry WebBook; NIST Standard Reference Database Number 69. National Institute of Standards and Technology, Gaithersburg MD, 2003. (33) Shimada, N.; Kawamoto, H.; Saka, S. Different action of alkali/ alkaline earth metal chlorides on cellulose pyrolysis. J. Anal. Appl. Pyrolysis 2008, 81 (1), 80–87.


Figure 3. Typical LP-MBMS spectrum of the reacting pyrolysate plume in the first 0.25 s of a 6.5 W CO2 laser shot of cellulose charred at 375 °C for 1/2 h.

the onset of weight loss in a subsequent TGA experiment using 1 wt % Pd acetate-doped cellulose in nitrogen (Figure S1 in the Supporting Information) had an onset temperature for mass loss significantly lower than the charring temperature of 375 °C. Na and K have the least effect on char yield, and the addition of Cu appears to decrease it. With the exception of Cu, all of the other transition metals tested show the highest mass losses. We deduce from this that Pd clearly enhances the thermal decomposition of cellulose and that Mg, Ca, and Zn stabilize cellulose to thermal decomposition. In previous work, we have investigated the morphology of a Ni-doped cellulose char prepared under similar conditions. The scanning electron microscopy (SEM) of this char shows that the Ni is dispersed as irregular particles with a nominal diameter of 40 nm.34 We believe it is reasonable to assume that the metals in this study are similarly dispersed. Elemental analysis was not performed, because to be meaningful, it must be rigorously performed in the absence of oxygen. The chars in this study were protected from oxygen. When the pellets where exposed to oxygen, the yield of carbon dioxide rapidly increased with exposure tine, to the point that after a few days no hydrocarbons where observed.9 We, therefore, concluded that oxygen adsorption and reaction with the char was rapid and substantial and any elemental analysis would not truly reflect the composition of the char under study. LP-MBMS. LP-MBMS was used to probe the reacting pyrolyzate plume as close to the origin of the pyrolysis event as possible. In a typical 6.5 W LP-MBMS spectrum for Avicel cellulose charred for 1/2 h at 375 °C, the largest peaks are at m/z 28 and 44, CO and CO2, respectively. The rest of the mass spectrum shows mostly peaks typical of aromatic species and PAHs (Figure 3). At this laser power (6.5 W), modeling suggests an average temperature of ca. 400 °C and a maximum temperature of ca. 1202 °C for the 2 mm2 at 0.5 s of the laser irradiation.35 This result was confirmed using fine-wire thermocouple measurements recorded at a maximum temperature of 1250 °C. Additional confirmation came from SEM examination of cellulose char pellets containing finely divided Cu (mp of 993 °C) and Ni (mp of 1400 °C), which clearly showed that (34) Herring, A. M.; McKinnon, J. T.; McCloskey, B. D.; Filley, J.; Gneshin, K. W.; Pavelka, R. A.; Kleebe, H. J.; Aldrich, D. J. A novel method for the templated synthesis of homogeneous samples of hollow carbon nanospheres from cellulose chars. J. Am. Chem. Soc. 2003, 125 (33), 9916– 9917. (35) Petrick, D. E.; Gneshin, K. W.; McCloskey, B. D.; Herring, A. M.; McKinnon, J. T. Experiments and modeling of the temperature evolution in the pyrolysis of biomass char from CO2 laser heating. J. Anal. Appl. Pyrolysis 2007, manuscript in preparation.


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Figure 4. Major oxidation products relative to m/z 128 at 0.25 s of a 6.5 W CO2 laser shot for 1 wt % metal acetate-doped Avicel cellulose charred at 375 °C for 1/2 h. (White bars) Water, (gray bars) carbon monoxide, and (black bars) carbon dioxide.

the Cu had melted and that the Ni had not. However, Zn was the only metal observed in the LP-MBMS of any of the doped cellulose. Zn metal is vaporized at a slightly higher temperature (bp of 907 °C), but Na and K actually have lower boiling points, implying that they are not in their metallic state in the char. Before an in depth analysis of the hydrocarbon components was performed, we first looked at the peaks attributable to oxygenated species, m/z, 18, 28, and 44, H2O, CO, and CO2, respectively, which are compared to the intensity to m/z 128, C10H8, in Figure 4. In our experience, even the relative intensity of these peaks can be somewhat variable, but one important observation can be made that, in the 1 wt % doped samples of Cu and Pd, the relative amounts of the oxidization product are comparable to the control cellulose. This implies that the other metals studied, K, Mg, and Zn, catalyze the oxidative decomposition of the hydrocarbon in the char. MVA was used to analyze the LP-MBMS of the 1 wt % metal acetate-doped charred cellulose samples, excluding the peaks at m/z 18, 28, and 44 and the peaks of elemental Zn, m/z 64, 66, 67, and 68, because Zn is volatilized under the conditions of LP. The factors for 1 wt % metal acetate-doped cellulose are shown in Figure 5. The factor score plot for 1 wt % metaldoped cellulose (Figure 6) clearly groups the main group metals, K, Mg, Zn, and Ca, in the upper right quadrant. The transition metals, Cu and Pd, are on the LHS of the plot, with Cu close to the control and Co clearly showing anomalous behavior. The component fractions and component mass spectra are shown in Figures 7 and 8, respectively. Component 1 in Figure 8 is dominated by m/z 26, acetylene. This component also has a peak at m/z 29 that is commonly associated with carbohydrate and peaks such as m/z 63 and 95, protonated phenol, that could be associated with other oxygenated species. The rest of the masses in component 1 appear to be aromatic, m/z 77, 115, 128, 139, 152, 165, 178, 189, 202, 215, and 226. Component 2 in Figure 8, although appearing noisier than component 1, looks very similar to the LP-MBMS (without the contributions from CO2, CO, and H2O) of undoped cellulose (Figure 3). We see in component 2 the peaks assigned to single-ring aromatic and PAHs, m/z 77, 115, 128, 139, 152, 165, 178, 189, 202, 215, 226, and 239, which we have previously assigned9 and many of which are shown in Scheme 1. Additionally, we see peaks at m/z 63 and 94, phenol, but not m/z 91, toluene. Component 3 in Figure 8 is dominated by m/z 39 with smaller peaks at m/z 78 and 91, benzene and toluene, respectively. We have suggested previously that the propargyl radical could be assigned to the anomalously high m/z 39 peaks observed in LP-MBMS,

Figure 5. Factors for 0.25 s of a 6.5 W CO2 laser shot for 1 wt % metal acetate-doped Avicel cellulose charred at 375 °C for 1/2 h.

Figure 6. Factor score plot for Avicel cellulose charred at 375 °C for 1 /2 h undoped and doped with 1 wt % of various metal acetates pyrolyzed at 0.25 s of a 6.5 W CO2 laser shot.

but this can still only be taken tentatively because m/z 39 is a common fragment ion. The component distribution of cellulose and 1 wt % Cu-doped cellulose appear almost identical; both contain almost equal amounts of components 2 and 3 (Figure 7). This shows that Cu has very little effect on the charring chemistry of cellulose. The 1 wt % cellulose-doped char samples with K, Mg, Ca, and Zn are all dominated by component 3 (Figure 8). It could possibly be that these metals promote the formation of propargyl during pyrolysis. For the 1 wt % doped Co and Pd cellulose chars, significant amounts of component 1 are observed (especially for Co; Figure 7). This implies that acetylene production is enhanced by Pd and Co. Note that any species with a methyl group is either absent, m/z 91 and 239, or suppressed, m/z 139, 165, 189, and 215. Large peaks at low m/z 29, 53, 63, 82 and 86 would seem to imply that there is more residual carbohydrate in these char samples. Therefore, it would appear that Co and



Figure 7. Component fractions for Avicel cellulose charred at 375 °C for 1/2 h undoped and doped with 1 wt % of various metal acetates pyrolyzed at 0.25 s of a 6.5 W CO2 laser shot. (Black area) Component 1, (gray area) component 2, and (white area) component 3.

Pd enhance the production of acetylene, suppress charring chemistry, and reduce the initial amount of methyl-substituted aromatic hydrocarbons formed. Chemical Cold Trapping. We wished to confirm the reactive intermediates detected in the LP-MBMS experiments independently. Unfortunately, no chemically trapped radicals could be detected at 6.5 W, and thus, the laser power was increased to 13 W. Therefore, to further investigate the pyrolysis chemistry of the char we used a N2(l) cold trap coated with diphenyl disulfide to quench the products of LP. It was not possible to position the cold trap adjacent to the cellulose char, and thus, the species detected represent freezing the pyrolyzate plume a little further along the reaction coordinate than the LP-MBMS experiment that was essentially adjacent to the pyrolyzed sample. At this higher laser power (13 W), the maximum temperature in the middle of the irradiated area is calculated to be >1000 °C after 0.5 s.35 The products detected and identified by GC-MS and normalized to the chlorobenzene standard added after pyrolysis are shown in Figures 9 and 10. By GC injection, it was determined that the remaining 1% of the 99% pure DPDS was largely diphenylsulfide. Thus, an amount equivalent to 1% of the DPDS peak was subtracted from the diphenylsulfide peak in the data to follow. Only the expected DPDS, diphenylsulfide, acetone, and chlorobenzene were detected from the graphitecoated aluminum pellet control experiment. The structures as determined by a comparison of the mass spectra to library spectra are shown in Scheme 1. Only aromatic species and trapped radicals were detected, from the single-ring benezene, m/z 78, to the five-ringed species, benzo[ghi]fluoranthene, m/z 226. Our experiment is not strictly quantitative because it was impossible to add an internal standard to the char before pyrolysis. However, data from the average of a large number of single-shot experiments from the same char at the same laser power, 13 W, show that we loose 1 mg of mass and collect 0.12 mg of pyrolysis oil for each 2 mm diameter laser shot. Because our temperature model of the char predicts a 0.75 mg volatilization, then at least 0.25 mg of the mass are lost by ablation of char materials and therefore we are recovering 16 wt % of the char as pyrolysis oil.35 Constructing a calibration curve for the toluene peak showed that it typically represented a concentration of 800 ppm of the pyrolysis oil.36 This seems reasonable considering that a large amount of fixed gases are observed in the MBMS. We attempted to correct for species (36) Petrick, D. E. Detection and Thermal Analysis of ReactiVe Intermediates from Biomass Char Pyrolysis; Colorado School of Mines: Golden, CO, 2001.

Figure 8. Mass spectra for the three major components identified by MVA for Avicel cellulose charred at 375 °C for 1/2 h undoped and doped with 1 wt % of Na, K, Mg, Ca, Co, Cu, Pd, and Zn acetates pyrolyzed at 0.25 s of a 6.5 W CO2 laser shot.

loss as a result of volatilization by adding chlorobenzene to the pyrolyzate before processing and normalizing the data to this standard. We first consider the phenyl sulfide adducts that are presumably the result of the reaction between the radical species produced and diphenyl disulfide. These are labeled with an asterisk in Scheme 1, and possible routes to their formation are shown in Scheme 2. In the simplest case, a radical reacts with diphenyldisulfide to produce a phenylsulfide derivative and a phenylsulfide radical. In this way, the hydrogen radical gives benzenethiol, m/z 110 (1), methyl gives methyl sulfanyl benzene, m/z 124 (2), iso-propyl gives iso-propylsulfanyl benzene, m/z 152 (4), phenyl gives diphenylsulfide, m/z 186 (5), and methyl napthenyl gives 2-phenylsulfanylmethyl-napthalene, m/z 250 (7). Presumably, the phenylsulfide radicals are terminated by reaction with another radical (8) or with themselves to regenerate diphenyldisulfide (9).


Lee et al. Scheme 1

We also detect benzo[b]thiophene, m/z 134. This species could form from the reaction of ethynyl benzene with diphenyldisulfide followed by subsequent cyclyzation and elimination of phenyl (3). Finally, dibenzothiphene, m/z 184, was detected. This species could have been formed by trapping of phenyl diradical and subsequent elimination of hydrogen (6). The highest concentration of S adducts detected were benzenethiol and diphenylsulfide, but some of the contributions to these may have come from reactions 3 and 6 in Scheme 2. The less hydrogen radical is never implied as being a very abundant species in the reacting pyrolyzate plume. It was very interesting that we did not directly detect propragyl (although its presence might be implied), cyclopentadienyl, or the benzyl radicals. All of these radicals are invoked in common mechanisms of combustion, and their absence either indicates that they were not formed or that they are dramatically more reactive than the radicals detected. The hydrogen radical is by far the most abundant, and because acetylene is seen in the LP-MBMS, it is assumed that the HACA mechanism is operating in this system. It is curious that the only non-aromatic hydrocarbon radicals detected by this method are methyl and iso-propyl. The propargyl radical is an important reaction in the pyrolysis of hydrocarbons,13 and its formation has been implicated in our previous work.9 Because there seems to be ample supply of hydrogen radical and it is known that hydrogen is a predominant product of pyrolysis (which we unfortunately can neither detect in the LP-MBMS experiment or the coldtrap experiment described here), we speculate that we are observing the hydrogenated propargyl radical rather than simply the iso-propyl radical. A large number of benzene derivatives were detected substituted with methyl (1, 2, or 3), vinyl, ethynyl, and/or hydroxyl (phenol). Two-ring compounds were less numerous in variety but still abundant and included unsubstituted indene,

m/z 116, diphenyl, m/z 154, methyl diphenyl, m/z 182, benzylmethylbenzene, m/z 182, naphthalene and methyl and dimethyl naptahlene, m/z 128, 142, and 156, respectively, and the heterocycles methyl and ethyl benzofuran, m/z 132 and 146. There was little substitution of the higher PAHs, which included the three-ring acenapthalene, m/z 152, 9H-fluorene, m/z 166, anthracene and phenanthracene, m/z 178, and possibly methyl9H-fluorene, m/z 180. The largest PAHs detected were 4Hcyclopenta[def]phenanthrene, pyrene, and benzo[ghi]fluoranthene, m/z 190, 202, and 226, respectively. The oxygenated phenols and benzofuran species are interesting because we generally do not observe these oxygenated species prominently in the LP-MBMS of cellulose chars, although they do appear after MVA analysis (component 2 in Figure 8). Curiously, the cold trapped one- and two-ring aromatics contain a lot of methyl-substituted derivatives. Inspection of a typical LP-MBMS of cellulose char (Figure 3) shows very little methyl substitution of any of the prominent aromatic or PAHs detected. This is strong evidence that the methyl derivatives are formed by the gas-phase reaction of methyl radical with one- and two-ring aromatics. In other words, methyl PAHs do not form during solid-state charring chemistry of cellulose. However, there is no reason why this should be selective for the smaller ring aromatics. This implies that some of the benzene, toluene, indene, and naphthalene were formed in the evolving gas plume and reacted with methyl radicals. It also implies that, because they are not methylated, under these conditions, PAHs with three or more rings are formed during solid-phase charring chemistry. Three species were trapped but not categorically identified by GC-MS. Two of these m/z 118 and m/z 168 are m/z 2 higher than identified PAHs, m/z 116, indene, and m/z 166, flourene. This implies that these are the neutral versions of the nonring closed precursor molecules (Scheme 1). Indene is commonly



Figure 9. Compounds collected on the cold trap after lasing Avicel cellulose and 1 wt % metal-doped Avicel cellulose pellets charred for 1/2 h under argon at 375 °C and lased at 13 W, relative to the internal standard, chlorobenzene. AMU 78-142.

Figure 10. Compounds collected on the cold trap after lasing Avicel cellulose and 1 wt % metal-doped Avicel cellulose pellets charred for 1/2 h under argon at 375 °C and lased at 13 W, relative to the internal standard, chlorobenzene. AMU 146-250.

thought to be formed by the HACA mechanism: hydrogen abstraction from toluene followed by acetylene addition to form the precursor (radical of m/z 118) that then ring-closes.37 However, the methyl radical is strongly implied here as an additional mechanism for PAH gowth. We do not detect the benzyl radical, but we do see both ethynyl benzene and the (37) Bittner, J. D.; Howard, J. B. Composition profiles and reaction mechanisms in a near-sooting premixed benzene/oxygen/argon flame. Int. Symp. Combust. 1981, 18 (1), 1105–1116.

methyl radical. Therefore, we propose that indene is formed via the addition of the methyl radical to ethenyl benzene. Analogously, we propose that flourene is formed via methyl radical attack to biphenyl followed by ring closure. A small amount of a peak at m/z 188 was also detected, which we did not identify and does not offer an easy logical assignment. A casual glance of the data in Figures 9 and 10, particularly toward the higher molecular weight compounds, would suggest grouping the metals in three groups relative to the control: Na,


Lee et al. Scheme 2

K, Mg, Ca, Co, and Ni, which generally suppress the formation of aromatic species and have a variable effect on radical production; Cu and Pd, which generally increase the formation of aromatic species and have a variable effect on radical production; and Zn, which generally decreases the formation of aromatic species and increases the formation of radicals. This is curious because the LP-MBMS data placed Co with unique chemistry. Closer inspection of the data reveals many additional correlations. Zn is the only metal in this study that is volatilized during the pyrolysis event. It is to be expected, therefore, that because finely divided metals catalyze radical chemistry and Zn is present throughout the pyrolysis plume that Zn should give rise to an anomalously large increase in radical production. Indeed, Zn doping produces a very large amount of hydrogen radical; the peak for benzenethiol, relative intensity of 6.8, is almost 4 times that produced from undoped cellulose and larger than for any other dopant. This is also true for phenyl and phenyl diradical. It is not surprising that most of the stable species are suppressed by Zn because the hot Zn vapor presumably catalyzes the

decomposition of these products in the reacting pyrolyzate plume. The chemistry clearly occurs in the gas phase because Zn shows solid-state chemistry as recorded by MVA LP-MBMS grouped with K, Mg, and Ca (Figure 6). Cu and Pd also produce increased yields of hydrogen radical compared to the control but produce lower amounts of all other radicals detected and, in some cases, ethynyl phenyl, iso-propyl, and methylnapthenyl, none at all. Cu and Pd produce more of almost every neutral species compared to the control. The large amounts of hydrogen radical would of course have a profound effect on the eventual product slate had we allowed this reacting plume to react to completion. The effect of the remaining six metals studied, the alkali metals, Na and K, the alkali earth metals, Mg and Ca, and the first row transition metals, Co and Ni, is surprisingly uniform. They all mostly suppress the formation of the stable aromatic species detected. The effect in the production of radicals is not as uniform for these diverse metals. The hydrogen radical Mg produces more than the control; K, Ca, and Co produce about the same; and Na and Ni produce less. Ca produces significantly


more methyl than the control, with the other not affecting the yield. The ethynylphenyl radical is produced in greater abundance by Na, K, Ca, and Mg but is completely suppressed by Ni and Co. None of the iso-propyl radical is detected for the control charred cellulose, but it is seen for Na, K, Mg, Ca, and Co. Interestingly, K, Mg, and Ca had LP-MBMS spectra that after MVA showed a component dominated by m/z 39. We again speculate that the iso-propyl radical may in fact be an hydrogenated propargyl radical. The phenyl diradical is enhanced by Na but suppressed by all of the other metals. For the phenyl radical, there is an enhancement in production by Na and Mg doping, but for Ca, all of the other metals, K, Co, and Ni, suppress its formation compared to the control cellulose. Conclusions We have shown that metal doping effects the pyrolysis chemistry of cellulose char. LP-MBMS detects the pyrolyzate of cellulose char before any subsequent gas-phase chemistry occurs, and therefore, this data allows us to examine the char chemistry and observe the pyrolyzate before self-reaction. MVA analysis of the LP-MBMS grouped 1 wt % Cu-doped char and to some extent Pd with the control, showing little change in charring chemistry. K, Mg, Ca, and Zn gave a MVA of the LP-MBMS dominated by m/z 39. Co and to some extent Pd were dominated by m/z 26 and with a decrease in the methylsubstituted aromatic components seen for the control. Chemical trapping grouped the metal dopants as follows: Na, K, Mg, Ca, Co, and Ni, which generally suppress the formation of aromatic species and have a variable effect on radical production; Cu and Pd, which generally increase the formation of aromatic species and have a variable effect on radical production; and Zn, which generally decreases the formation of aromatic species and increases the formation of radicals.


Zn acts in the gas phase to catalyze the decomposition of aromatic species because it is volatilized in the LP. Cu and Pd stand out as doing little to change char chemistry during charring but to act as powerful solid-state catalysts during the pyrolytic decomposition of the char. The other metals all alter the char chemistry during charring. While not as catalytically active as Cu and Pd during the pyrolytic decomposition of the char, the other metals Na, K, Mg, Ca, Co, and Ni act to produce aliphatic and ethynylphenyl radicals with the exception of Co that appears to enhance the production of acetylene and suppress radical production. Interestingly, we never see any clear evidence for the cyclopentadienyl radical in these processes. The propargyl radical is implied but never proven, and the benzyl radical is only observed in the LP-MBMS experiment and never in the chemical cold trap of the reacting pyrolyzate plume. Acetylene and hydrogen radical are observed, implying that the HACA path of PAH growth is active. Additionally, the generation of the methyl radical in the pyrolysis of cellulose char and its reactivity in the gas phase to produce methyl-substituted and mediated aromatics was observed and appears to be an equally important path to PAH formation in these systems. Acknowledgment. The authors gratefully acknowledge the financial support of Philip Morris, Richmond, VA. We are also indebted to Dr. Robert J. Evans of the National Renewable Energy Laboratory (NREL) for many useful discussions concerning this work. Supporting Information Available: TGA data under He for h at 375 °C of charred cellulose and 1 wt % metal acetatedoped cellulose chars. This information is available free of charge via the Internet at http://pubs.acs.org. 1/

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