Acta Chim. Slov. 2006, 53, 437–443 437 Scientific Paper Investigations on the Effect of Metal Ions on the Products Obtained From the Pyrolysis of Cellulose Christian W. Klampfl1*, Gerold Breuer1, Clemens Schwarzinger2 and Berndt Köll3 1 Institute of Analytical Chemistry, Johannes Kepler-University, Altenbergerstr. 69, A-4040 Linz, Austria. 2 Institute for Chemical Technology of Organic Materials, Johannes Kepler-University, Altenbergerstr. 69, A-4040 Linz, Austria. 3 T1 Papier Group, Fabrikstr. 20, A-4050 Traun, Austria. Received 22-12-2005 Abstract Investigations on the influence of different metal ions, employed in the form of their chlorides, on the pyrolyzate of microcrystalline cellulose and pulp were performed. Cellulose/metal ion mixtures were pyrolyzed and the volatile reaction products enriched on a cartridge filled with Tenax, followed by GC-MS analysis after thermodesorption of the trapped pyrolysis products. Different metal ions, namely Na+, Ca2+, Mg2+ and Fe3+ were tested at levels ranging from 0.18 - 2.2 mM g-1. Evaluation of the GC-MS pyrograms revealed that the distribution of the selected 37 pyrolysis products was significantly influenced by both, the type and the concentration of the metal ion added. Keywords: pyrolysis, thermodesorption, cellulose, metal ions. 1. Introduction The characterization of products resulting from the thermolysis or pyrolysis of cellulose and related materials has been the subject of a series of investigations published during a time span covering several decades1-14. Analytical methods used for the characterization of these reaction products comprise a wide range of different instrumental technologies. Those include methodologies monitoring only a single parameter (e.g. change in weight) like thermogravimetry but also spectroscopic methods, like infrared spectrometry8,9, nuclear magnetic resonance spectroscopy11, chromatographic methods, mainly gas chromatography (GC) or high performance liquid chromatography (HPLC)5 and finally hyphenated techniques like GC or HPLC coupled to mass spectrometric [MS] detection.3,4,11,15 Particularly the latter methods can provide comprehensive information on the nature and quantity of the products originating from the pyrolysis of the investigated samples. The addition of metal ions to influence the pyrolysis of cellulose and with it the distribution of volatile and non-volatile products generated by this process has been reported in several publications.16-34 Major driving forces for these investigations come from quite different fields of interest, comprising works dedicated to the general interest in mechanistical and kinetical aspects of this pyrolysis reaction16,17,19,23,25-30, experiments focusing on the selective increase in yield for a specific pyrolysis product after the addition of salts and also phosphoric acid18,32,33 but also investigations coming from the fields of fire prevention22. To investigate the effect of these additives on the pyrolysis process, different types of analytical techniques have been employed. Principally two different approaches can be distinguished. First, the direct analysis of the pyrolysis mixture using e.g. thermogravimetry17,24, infrared spectroscopy16,17,30 electron spin resonance spectrometry32,33 and MS.30 Second, if a more detailed picture is required, the pyrolysis products have to be separated first using chromatographic methods, commonly GC. Consequently the resolved pyrolysis products can be either detected using a simple GC-detector18,23,24,32 or further characterized employing a more sophisticated spectroscopic detection method particularly MS.22,28,29 For the presented work we chose an approach using off-line pyrolysis with subsequent trapping of the pyrolysis products on an appropriate sorbent, followed by thermodesorption onto a GC-MS system. By this we wanted to demonstrate that the selected task can be accomplished without dedicated pyrolysis-GC instruments. In this work off-line pyrolysis GC-MS of cellulose and cellulose doped with different metal ions has been performed. The data obtained from these experiments can provide useful information with respect to the products originating from smoldering cellulose and pulp. Thereby valuable findings regarding the manipulation of base-materials used in the production of cigarette paper may be acquired. Klampfl et al. Investigations on the Effect of Metal Ions 438 Acta Chim. Slov. 2006, 53, 437–443 2. Experimental 2.1. Instrumentation For pyrolysis a Pyroprobe 2000 (CDS Instruments, Oxford, PA, USA) was employed. Pyrolysis products were trapped on thermodesorption cartridges filled with Tenax TA (Supelco, Bellefonte, PA, USA). GC analysis was performed with an Agilent Model 6890N gas chromatograph (Agilent Technologies, Palo Alto, USA) equipped with a TDS 2 thermodesorption unit combined with a KAS 4 cold injection system (both Gerstel, Mülheim/Ruhr, Germany). For separation a DB-VRX GC column (J&W Scientific, Folsom, CA, USA) 60 m*0.25 mm I. D., film thickness 1.4 µm was used which was coupled to an Agilent MSD 5973 quadrupole mass spectrometer for detection. 2.2. Materials and Reagents Water was purified using a Milli-Q (Millipore, Bedford, MA, USA) system. The following metal salts were used: NaCl, CaCl2 x 2 H2O, MgCl2 x 6 H2O, FeCl3 (all Merck, Darmstadt, Germany). Microcrystalline cellulose (ALPHA CEL BH300®) was obtained from International Fiber Cooperation (New York, USA) and Lapponia pine pulp was from StoraEnso (Finland). He 4.6 (Linde, Stadl Paura, Austria) was used as carrier gas for GC experiments as well as for pyrolysis. 2.3. Procedures Pyrolysis experiments were performed by introducing 100 µg – 150 µg of the cellulose into a quartz tube which was subsequently plugged using glass wool. The quartz tube was introduced into the pyrolysis unit and heated to 500°C for 10 s employing a heating rate of 20 °C s-1. During the pyrolysis procedure a stream of helium gas was applied, transferring the volatile pyrolysis products onto a thermodesorption cartridge filled with Tenax TA, which was connected to the pyrolysis unit using a Swagelock union. The loaded cartridge was introduced into the thermodesorption unit, desorbed at 300 °C and the analytes trapped in the cold injection system (KAS) at -100°C. Finally they were transferred to the GC column employing a heating rate of 12°C s-1 and a final injector temperature of 260 °C that was maintained for 10 min. GC separation was performed using a start temperature of 35 °C and a ramp of 5 °C min-1 up to a final temperature of 260 °C that was maintained for 20 min. He 4.6 was used as carrier gas at flow rate of 1.4 mL min-1. The MS instrument was operated in the positive ion mode (EI ionization at 70 eV) and full scans from 15 – 420 mass units were acquired. Cellulose/metal ion mixtures were prepared by combining 200 – 250 mg of cellulose with 1 mL of the appropriate salt solution, containing 2 – 20 mg L-1 of the metal ion in Milli-Q purified water, in a vial. Subsequently, another 0.5 mL of water was added and the mixture placed into an ultrasonic bath for 2 min. Finally the vial was placed into a drying cabinet and maintained at 80 °C for 16 hours. After 12 hours the sample was removed temporarily and homogenized using an agate mortar. In the case of pulp a somewhat modified method had to be used. To 5 mg of pulp 100 µL of the aqueous salt solution were added whereby the whole solution was absorbed by the pulp. Drying was performed at 90 °C for 6 hours. 3. Results and Discussion 3.1 Pyrolysis of microcrystalline cellulose and pulp Firstly untreated microcrystalline cellulose was pyrolyzed and subsequently analyzed by GC-MS to optimize the experimental parameters and to identify the resulting pyrolysis products. To ensure the suitability of the selected procedure (in particular with respect to the trapping efficiency and capacity of the selected Tenax cartridges) a series of experiments was performed employing different amounts of cellulose for pyrolysis. These experiments were finally used to obtain the set of parameters given in section 2.3. Figure 1 shows a pyrogram obtained for such a sample, using these optimized conditions. The pyrogram obtained for Lapponia pulp was almost identical. As can be seen from this figure an acceptable separation of the different pyrolysis products could be achieved and the identification of 36 different pyrolysis products (listed in Table 1) was possible, using the NIST database of the GC-MS system and information taken from the 4e+07 V_______t> 8 910 11 12 \J I \11J18i. 21 \22 IwuaA I32l36l 10.00 15.00 20.00 25.00 30.00 35.00 40.00 [min] Figure 1. GC-MS pyrogram for microcrystalline cellulose. Experimental conditions: see section 2.3. Peaks: see Table 1. Klampfl et al. Investigations on the Effect of Metal Ions Acta Chim. Slov. 2006, 53, 437–443 439 literature.2 These 36 identified pyrolysis products were subsequently used as markers to judge the influence of different metal ions on the pyrolysis behavior of the investigated cellulose. For quantitative analysis and comparisons all peak areas obtained were normalized with respect to the amount of cellulose employed. 3.2. Pyrolysis of microcrystalline cellulose and pulp with metal ions To investigate the effect of different amounts of metal ions added to the cellulose on the qualitative and quantitative composition of the pyrolyzate a series of samples containing microcrystalline cellulose and metal salts were prepared. The salts employed included alkaline metals (Na+), alkaline earth metals (Ca2+ and Mg2+) as well as a trivalent species (Fe3+) all used in the form of their chlorides. Mixtures were prepared containing between 0.18 mM g-1 and 4.4 mM g-1 of the metal ion, employing the procedure described in section 2.3. In Figure 2 pyrograms of pure cellulose and cellulose/metal salt mixtures at metal ion levels of around 1 mM g-1 are compared. A discussion of the observed effects is given in the following paragraphs. Table 1. Analytes identified from the pyrolysis of cellulose Continued Nr. Analyte Retention Quantitation time (m/z) 1 2-Butanone 6.47 TIC 2 Formic acid 5.56 46 3 Hydroxyacetaldehyde + Acetic acid 8.40 31 4 2,3-Butandione 9.09 43 5 Crotonaldehyde 11.38 41 6 Hydroxyacetone 11.63 TIC 7 Propenic acid 12.16 72 8 Ethanediol 12.41 31 9 2,3-Pentandione 12.83 TIC 10 2,5-Dimethylfurane 13.41 TIC 11 Methylfurane 15.23 TIC 12 1-Hydroxybutanone 15.56 57 13 3-Furfurale 17.51 TIC 14 2-Furfurale 18.52 TIC 15 2-Cyclopentenone 18.56 82 16 2-Furanemethanol 18.87 98 17 1-Acetyloxy-2-propanone 19.43 TIC 18 Cyclopenten-l,3-dione 20.61 96 19 2-Furanone 21.60 55 20 2-Hydroxy-2-cyclopentene-l-one 21.83 98 21 3-Methyl-2,5-furanedione 22.51 TIC 22 5-Methyl-2-furanone 22.69 55 23 Phenole 23.37 94 24 5-Methylfurfurale 23.64 TIC 25 2-Hydroxy-3-methyl-2-cyclopentene-1-one 25.75 112 26 o-Cresol 26.29 108 27 m-,p-Cresol 28 Levoglucosenone 29 2-Hydroxy-3-methyl-4-pyranone 30 Dihydro-4-hydroxy-2-furanone 31 Benzoic acid 32 1,2-Dihydroxybenzene 33 1,4:3,6 Dianhydro-?-glucopyranose 34 5-Hydroxymethyl-2-furfurale 35 Hydrochinone 36 1,2-Dihydroxy-3-methylbenzene 1 Total Ion Current 26.95 108 28.86 68 29.02 126 29.32 TIC 29.84 105 28.84 105 31.85 TIC 31.92 TIC 32.75 110 32.93 124 a) M,. lU likk MM JbwJJku^A^jj-^ b), jy JLiv^iLJIkJL^ c) J, .k.i ,i.,i.. I..,..y.,.1.1.1.1.1..iU.!i,..I!.,. i d) 10.00 15.00 20.00 25.00 30.00 35.00 Figure 2. GC-MS pyrogram for microcrystal-line cellulose (a), containing 2.2 mM g-1 Na+ (b), 1.25 mM g-1 Ca2+ (c), 1.25 mM g-1 Mg2+ (d) and 0.9 mM g-1 Fe3+ (e). Experimental conditions: see section 2.3. Klampfl et al. Investigations on the Effect of Metal Ions 440 Acta Chim. Slov. 2006, 53, 437–443 3.2.1. Cellulose/pulp Na+ mixtures First mixtures of cellulose and Na+ were tested at different metal levels ranging from 0.44 mM g-1 – 4.4mM g-1 Na+. As can be seen from Figure 2b no significant changes between the patterns obtained for pure cellulose and the Na+ treated cellulose could be observed even at a level of 2.2 mM g-1 Na+. In depth analysis of the individual pyrolysis products revealed that the first impression, originating from the visual evaluation of the chromatographic pattern, was correct. Changes in concentration were insignificant for most analytes. Exceptions were hydroxyacetone, 2,3-pentandione, 1-acetyloxy-2-propanone, 2-furanemethanol and 5-methylfurfurale, where a slight increase in concentration could be observed and 2,3-butandione, 2,5-dimethylfurane, 2-hydroxy-3-methyl-2-cyclopentene-1-one, 1,2-dihydroxybenzene and the cresol isomers with a more pronounced trend in the same direction. Reduction in concentration, compared to the untreated cellulose was found for benzoic acid and dihydro-4-hydroxy-2-furanone. With respect to the amount of Na+ employed no significant influence of the metal ion concentration on the obtained signal areas could be observed. 3.2.2.Cellulose/pulp Ca2+ and Mg2+ mixtures Three different mixtures of cellulose and Ca2+ were prepared at levels of 0.25, 0.75 and 1.25 mM g-1 of Ca2+. As can be seen from Figure 2c the intensities of a series of analytes were substantially decreased at the 1.25 mM g-1 level. Detailed evaluation of the individual pyrograms revealed several different trends. First, for most substances a significant influence of the metal ion concentration could be observed, with higher concentrations leading to decreased peak areas. Exceptions were benzoic acid, 2-hydroxy-3-methyl-4-pyranone, 2-butanone and 2,5-dimethylfurane where no impact of the Ca2+ concentration could be found. Second, the addition of Ca2+ led to an increase in concentration (compared to the untreated cellulose) for the following substances: propenic acid (only at the 0.25, 0.75 mM g-1 level), 2,5 dimethylfurane, all furfurals, 3-methyl-2,5-furanedione (only at the 0.25 mM g-1 level), 2-hydroxy-3-methyl-4-pyranone, 5-methyl-2-furanone, levoglucosenone and 1,4:3,6 dianhydro-?-glucopyranose, whereas the concentrations obtained for phenol and cresols remained almost unchanged. All other analytes showed a more or less pronounced decrease in peak areas upon the addition of Ca2+. The effects observed after the addition of Mg2+ (0.42, 1.25 and 2.01 mM g-1) were similar to those described for Ca2+. In general Mg2+ led to an even more pronounced reduction in signal area than Ca2+. The sample containing 2 mM g-1 Mg2+ provided the least signal areas for all samples investigated in this study. 3.2.3. Cellulose/pulp Fe3+ mixtures The cellulose/Fe3+ mixtures showed an interesting behavior with respect to the increase and decrease of signal intensities upon the addition of the metal salt. As can be seen from Figure 2e most signals were significantly suppressed when a cellulose sample containing 0.9 mM g-1 Fe3+ was pyrolyzed. Nevertheless the formation of some substances was greatly enhanced. In particular levoglucosenone, 1,4:3,6 dianhydro-?-glucopyranose and the furfurals were present in much higher concentrations compared to the untreated cellulose sample. Figure 3 depicts the changes in the chromatographic pattern induced by increasing the Fe3+ concentration. Detailed inspection of the investigated analytes shows that, similar as in the case of Ca2+, reduced signal intensities are obtained with increasing concentration of the metal ion. Pyrolysis products that are not affected by the Fe3+ concentration are formic acid, the furanone derivatives, 2-cyclopentenone, benzoic acid and hydrochinone. In a few cases higher concentrations of Fe 3+ led to increased signal intensities. Examples for this type of behavior are phenol and the cresols. 5e+07 4e+07 3e+07 2e+07 1e+07 5e+07 4e+07 3e+07 2e+07 1e+07 A«. L...ill J IJuJLA^bUl Ll^LJLLJ 10.00 15.00 20.00 25.00 b) Jy-JlLl, \,,. ,.,ll,ll,J.. *^XJii Jjj^jLLl 15.00 20.00 25.00 30.00 5e+07 4e+07 3e+07 2e+07 1e+07 -Ml ,li......J, II.I.J. L.,l.jUr^U-i, 10.00 15.00 20.00 25.00 30.00 Figure 3. GC-MS pyrogram for microcrystalline cellulose containing 0.18 mM g-1 of Fe3+ (a), 0.54 mM g-1 of Fe3+ (b) and 0.9 mM g-1 Fe3+ (c). Experimental conditions: see section 2.3. Klampfl et al. Investigations on the Effect of Metal Ions Acta Chim. Slov. 2006, 53, 437–443 441 350.0E+3 300.0E+3 250.0E+3 200.0E+3 150.0E+3 100.0E+3 50.0E+3 OOO.OE+0 c 0.44 4.4 CellBH300 Na+ 0.25 0.75 1.25 0.42 1.25 2.1 Ca2+ Mg2+ 0.18 0.54 0.09 mM g-1 Fe3+ Figure 4. Dependence of the levoglucosenone concentration (data are mean values from three determinations) upon the type and amount of metal ion added. 3.3. Comparison of the effects generated by different metal ions on the pyrolysis products of microcrystalline cellulose Moving from Na+ to Ca2+ and finally Mg2+ a general trend, namely an increase in influence leading to decreased peak areas for most of the selected analytes can be observed. This is also clearly apparent from Figure 2, where the addition of Mg2+ led to the most significant reduction in intensities. Additionally it can be seen that Ca2+ and Mg2+ induce a significant decrease in peak areas for most of the analytes included in this study. The trivalent Fe3+ ion behaves somewhat different. Although (comparing the lowest concentration level of each metal ion) Fe3+ provides the minimum signal intensities for the majority of substances, increasing the metal concentration had a less pronounced effect than in the case of Mg2+. For this reason, focusing on the highest metal ion level, Mg2+ provided the most significant reduction in signal intensities. Focusing on the increased formation of levoglucosenone and 1,4:3,6 dianhydro-?-glucopyranose upon the addition of metal ions, it can be seen that the most pronounced effect is generated by Fe3+ followed by Mg2+, Ca2+ and finally Na+. A graphical description of this behavior can be seen in Figure 4. 3.4. Pyrolysis of levoglucosane in the presence of Mg2+ Levoglucosane is an intermediate generated during the pyrolysis of cellulose.35 According to the literature, further reactions of this substance during the pyrolysis process lead to the formation of a substantial number of components detected in the final pyrolyzate.7,35 Therefore experiments were performed to investigate the behavior of this important pyrolysis intermediate upon the addition of metal ions in particular Mg2+. Results from these investigations revealed that for levoglucosane a similar trend was observed in the presence of Mg2+ (1.25 mM g-1) as in the case of cellulose and pulp. Figure 5 depicts a pyrogram of levoglucosane and one of a levoglucosane/Mg2+ (1.25 mM g-1) mixture. Figure 5. Pyrogram of levoglucosane (a) and a levoglucosane/Mg2+ (1.25 mM g-1) mixture (b). Peak assignment see Table 1 except for: 37 = 2,5-furandicarboxaldehyde; 38 = 1,6:3,4-dianhydro-ß-d-galactopyranose; 39 = 1,6:2,3-dianhydro-ß-d-mannopyranose. 10.0 20.0 30.0 40.0 Klampfl et al. Investigations on the Effect of Metal Ions 442 Acta Chim. Slov. 2006, 53, 437–443 Particularly furane-, furfurale- and pyranone-derivatives as well as levoglucosenone and 1,4:3,6 dianhydro-?-glucopyranose showed increased signal intensities when the levoglucosane was treated with Mg2+. These findings can be taken as an evidence that in the pyrolytic decomposition of cellulose the presence of Mg2+ also plays an important role with respect to the reactions following the intermediate production of levoglucosane. 4. Conclusions The presented investigations revealed that the addition of metal ions strongly influences the distribution of products resulting from the pyrolysis of cellulose and pulp. A strong decrease in concentration for the majority of the analyzed pyrolysis products was observed upon the addition of Ca2+, Mg2+ and Fe3+, whereas Na+ did not significantly influence the chromatographic pattern of the pyrolyzate. In contrast to this behavior the yield of some compounds in particular levoglucosenone, 1,4:3,6 dianhydro-?-glucopyranose, furfurals and some pyrane and furane derivatives was enhanced when metal ion doped cellulose was pyrolyzed. These findings can help to develop strategies for the selective manipulation of the amount of certain classes of compounds produced during the pyrolysis of cellulose and pulp, which can be seen as an important issue in the manufacturing of cigarette paper. 5. Acknowledgements The authors would like to thank Prof. Andreas Zemann and Dr. Thomas Haselgrübler (both WFT Research) for fruitful discussions and help in the experimental work. 6. References 1. J. Piskorz, D. Radlein, D.S. Scott, J. Anal. Appl. Pyrol. 1986, 9, 121-137. 2. 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Telysheva, D. Meier, S. Radtke, O. Faix, J. Anal. Appl. Pyrol. 2003, 68-69, 197-211. Klampfl et al. Investigations on the Effect of Metal Ions Acta Chim. Slov. 2006, 53, 437–443 443 33. G. Dobele, G. Rossinskaja, T. Dizhbite, G. Telysheva, D. 35. H. Kawamoto, M. Murayama, S. Saka, , J. Wood Sci. 2003, Meier, O. Faix, J. Anal. Appl. Pyrol. 2005, 74, 401-405. 49, 469-473. 34. B. Chatterjee, B. Reddy, P. Jena, S.N. Khanna, B.K. Rao, Preprints of Symposia-American Chemical Society, Division of Fuel Chemistry 2005, 50, 22-23. Povzetek Opravili smo raziskave vpliva različnih kovinskih ionov v obliki kloridov na nastajanje pirolizata mikrokristalinične celuloze in vlaknine. Mešanice celuloza/kovinski ioni smo pirolizirali in hlapne razgradne produkte koncentrirali z uporabo kolonic, napolnjenih s Tenaxom. Termodesorpciji je sledila analiza z uporabo GC-MS. Ione Na+, Ca2+, Mg2+ in Fe3+ smo nanesli v intervalu vsebnosti 0,18 - 2,2 mM mg-1. Analiza kromatogramov je pokazala, da je produkcija 37 produktov pirolize, ki smo jih opazovali, zelo odvisna od dodatka kovinskih ionov. Klampfl et al. Investigations on the Effect of Metal Ions