A Study of the Hydrogen Exchange Reactions Occurring during Loy

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A Study of the Hydrogen Exchange Reactions Occurring during Loy Yang Lignite Pyrolysis Using Deuterium-Labeled Water and Gas Chromatography
Mass Spectrometry Analysis Alfons V. Larcher*,† and Shiro Kajitani†,‡ † ‡

Fuels and Energy Technology Institute, Curtin University of Technology, GPO Box U1987, Perth, Western Australia, Australia Energy Engineering Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), Energy Engineering Research Laboratory, 2-6-1 Nagasaka, Yokosuka, Kanagwa 240-0196, Japan ABSTRACT: Hydrogen exchange has been studied during lignite pyrolysis by heating mixtures of an oven-dried (105 °C) Loy Yang lignite and deuterium-labeled water (D2O) in a quartz U-tube reactor. A typical experiment consisted of heating the lignite from ambient temperature to 500 ( 2 °C at 20 °C/min under a flow of nitrogen. A higher-heating rate experiment (6 °C/s) was also performed. The collected tar fractions were analyzed by gas chromatography and mass spectrometry, which allowed detailed analysis of individual components. Inspection of the mass spectra of some of the components showed changes that reflected deuterium had been incorporated into their molecular structure during pyrolysis. The relative extent of deuterium incorporation was quantitated by mass fragmentography of selected ions, with the molecular ion in most cases being used. Four compounds were chosen for detailed analysis: 1-methylnaphthalene, 2-methylphenol, 1-octadecene, and n-octadecane. The pyrolysis experiments showed that a significant amount of deuterium was incorporated into the aromatic and phenolic components of the tar, while the amount of deuterium incorporated into the 1-alkene and n-alkane components was much smaller. A series of experiments was completed to determine the influence of experimental parameters such as the heating rate, purge gas flow, and amount of deuterium-labeled water present. Additional experiments using in-line mass spectrometry monitored the fate of the deuterium-labeled water during pyrolysis and the generation of chemically produced pyrolytic water. Reaction mechanisms for the observed hydrogen exchange reactions are proposed, and the significance of these to the overall lignite pyrolysis process is discussed.

1. INTRODUCTION The pyrolysis of coal by heating in an inert atmosphere generates liquid and gaseous products as well as a carbon-rich, hydrogen-depleted char residue.1,2 The liquid and gaseous products comprise a variety of organic and inorganic compounds, with the organic compounds that are liquid or semisolid at room temperature being termed the “tar” fraction. The chemical reactions occurring during the coal pyrolysis process and their mechanism have been the subject of appreciable study because any research in this area can be applied directly in the more efficient utilization of coal and coal tars as sources for fuels and petrochemical feedstocks.3,4 A significant body of coal pyrolysis research has been completed in which molecules labeled with the deuterium or tritium isotopes of hydrogen have been used to elucidate the reactions occurring. Typically, isotopically labeled molecules such as water, hydrogen, or hydrogen-donor molecules have been used, with tritiated species being detected by liquid scintillation and deuterium-containing species by nuclear magnetic resonance (NMR). The work in this area prior to 2004 has been reviewed in a detailed monograph,5 with a more recent paper continuing the tritiated hydrogen work with a pulsed addition technique being used to monitor the uptake of the gas by the coal.6 The effect of mineral matter on this process has also been studied,7 while earlier research used tritiated water to study the hydrogen exchange characteristics of a coal.8 The exchange reactions of deuterium-labeled tetralin and naphthalene with a coal have been r 2011 American Chemical Society

studied using a variety of techniques,9 while a recent study used deuterated methane to study coal pyrolysis with CO2 reforming of methane.10 The research completed with isotopically labeled water has identified that “labile” hydrogen, i.e., that attached to nitrogen, sulfur, and oxygen (NSO), heteroatoms in coals readily exchange with the hydrogen present in water. A number of investigations have been performed using model compounds and isotopically labeled water in closed reactor systems, making analysis of the products more straightforward than analysis of those from the complex coal pyrolysis process. Extensive exchange of heteroatom hydrogen such as the hydroxyl hydrogen in the OH group in phenols was observed at 100 °C over 6 h, while aromatic hydrogen exchanged to a lesser extent.8 Similar experiments were completed with coals, with the results obtained from the model experiments being used to rationalize the observed trends in deuterium incorporation. Researchers determining the isotopic composition of coals and sedimentary organic matter have observed the exchange of hydrogen between these materials and water vapor at 115 °C, which they attributed once again to labile NSO hydrogen.11,12 This work extends the studies described above by investigating the exchange of hydrogen between water and coal throughout the coal pyrolysis process, including preheating and tar formation. Received: February 7, 2011 Revised: May 10, 2011 Published: May 20, 2011 3029

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Table 1. Proximate and Ultimate Analysis (weight %) of the Loy Yang Lignite Used in This Studya proximate analysis

ultimate analysis (daf)

M

Ad

Vdaf

C

H

N

S

O

Cl

36

1.1

52.2

70.4

5.4

0.62

0.28

23.2

0.10

Abbreviations: daf, dry and ash-free basis; M, moisture (105 °C ovendried basis); Ad, ash yield (dry basis); Vdaf, volatile matter (dry and ashfree basis). a

Table 2. Details of the Prepared Lignite/Water Mixtures D2O or H2O (wt %)

abbreviation used in text

14.6 (H2O)

LYL/H2O

14.6 (D2O)

LYL/D2O

23.9 (D2O)

LYL/D2Oþ

This has been achieved by pyrolyzing mixtures of a dried (105 °C) Loy Yang lignite and deuterium-labeled water under a variety of conditions. The isolated tar fractions were analyzed by gas chromatography and mass spectrometry (GC
MS), a technique that couples the high resolving power of capillary gas chromatography and the structural elucidation ability of mass spectrometry. Incorporation of deuterium into the components of the tar fraction was detected by changes in the mass spectra of each compound, with the extent of deuterium incorporation being determined by mass fragmentography. This allowed the direct determination of the incorporation of deuterium into the different compound classes of the lignite’s tar fraction, rather than extrapolating from model system studies. By using an in-line mass spectrometer (MS), the fate of the added deuterium-labeled water and the generation of pyrolytic water and its deuterated analogues were additionally studied. Chemical and physical processes have been proposed to rationalize the observed deuterium incorporation trends, and the significance of these reactions to the overall coal pyrolysis process is discussed. Similar techniques have been used to elucidate the reaction mechanisms occurring during the maturation of sedimentary organic matter.13
15

2. EXPERIMENTAL SECTION 2.1. Materials. The coal matrix used in the experiments was a lignite (Loy Yang) from the Latrobe Valley, Victoria, Australia, which was used in a previous study16 and whose properties are listed in Table 1. The asmined lignite, containing more than 60 wt % water, was air-dried, crushed and sieved, with the fraction between 63 and 150 μm used for experimentation. Deuterium-labeled water (deuterium oxide, D2O) was purchased from ACROS Chemicals (99.8% atom grade), while deionized water was prepared by passing normal tap water through a MilliPore purification system. The dichloromethane solvent used for the experiments was analytical reagent (AR) grade, while AR grade sodium sulfate was dried at 105 °C overnight before being used. We prepared lignite/deuterium-labeled water mixtures first by drying portions of the prepared sieved lignite at 105 °C to a constant weight and storing them in a desiccator. The moisture content of the lignite determined by this method was 36 wt %. A portion of the dried lignite was then weighed into a glass screw-capped vial after which an amount of deuterium oxide was added and the mixture reweighed. The mixture was agitated thoroughly with a spatula for approximately 20 s before the vial was capped. Further mixing was effected by approximately 20 end-overend tilts of the vial, and after being kept at ambient temperatures

Figure 1. Schematic of the pyrolysis reactor used in this study. overnight to equilibrate, the vial was stored in a laboratory freezer. The mixtures were removed from the freezer prior to use, and the contents were allowed to equilibrate to ambient temperature before being used in pyrolysis experiments. Details of the lignite/D2O and lignite/H2O mixtures prepared are listed in Table 2. The lignite/H2O mixture was prepared similarly but by using deionized instead of deuterium-labeled water. 2.2. Apparatus and Pyrolysis Procedure. Pyrolysis experiments were performed using a ChemBET 3000 (Quantachrome Instruments) apparatus that had been modified so that gas flowed directly into one arm of the quartz U-tube reactor cell, while the exit arm gas flowed into a Teflon tube [60 cm  6 mm (inside diameter)] and then into a small gas impinger filled with approximately 10 mL of dichloromethane. The end of the impinger bubbler tube was fitted with a porous frit. The internal diameter of the quartz U-tube was 4 mm, while the total gas path length along the tube was 46 cm. In the water monitoring experiments, a tee-piece fitting containing an airtight septum was placed before the impinger with one end of a 1 m deactivated fused silica capillary column (0.22 mm outside diameter, 0.15 mm inside diameter) being inserted into the septum, using a syringe needle as a guide, and positioned so that the end of the capillary was in the U-tube reactor flow. The capillary tube led a small flow (1
2 mL/min) of gas into a ThermoStar mass spectrometer (Pfeiffer Vacuum, Asslar, Germany) allowing continuous monitoring of selected ions. The delay time from generation of a pyrolysis component in the U-tube to its analysis in the MS was determined by injection of an aliquot of air into the entry port of the U-tube and monitoring the time delay to an oxygen (m/z 32) signal in the MS. This delay time was used to align the furnace temperature readings with the MS readings. A schematic of the pyrolysis reactor is shown in Figure 1. Portions (approximately 200 mg) of the prepared lignite/water mixture were weighed into the bottom of the U-tube, and with the aid of a thin wire, small plugs of quartz wool were placed at either side of the section of the lignite. For the experiments in which the ThermoStar MS was used to monitor the evolved water, a smaller amount of the lignite/ water mixture (approximately 10 mg) was used to prevent the condensation of liquid water on cooler parts of the apparatus (see section 3.8). The U-tube with associated ChemBET 3000 gas entry and exit fittings was assembled, and the bottom two-thirds of the quartz U-tube was placed into the furnace of the reactor. The Teflon tube leading to the impinger was connected to the exit port of the U-tube using a small length of silicone tubing. For the normal flow experiments, a nitrogen flow of 25 mL/min was begun and the system was allowed to purge free of air for at least 15 min before being heated. After this time, the furnace was heated from ambient temperature to 500 ( 2 °C at a rate of 20 °C/min, after which it was held at the maximal temperature for 20 min. During this process, the water and tar generated were seen to condense on the cooler 3030

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Energy & Fuels parts of the exit branch of the U-tube, while the dichloromethane in the impinger developed a light yellow color. The faster heating rate experiments were performed by preparing the U-tube and contents outside the furnace and then quickly inserting them into the preheated (500 °C) furnace. After pyrolysis, the furnace was cooled, the U-tube removed, and the gas flow stopped. The Teflon tube was disconnected and, along with associated fittings, was rinsed with dichloromethane from the impinger. The U-tube and its contents were rinsed into the body of the impinger, once again with dichloromethane from the impinger. A thin wire was used to dislodge the quartz wool plugs and lignite in the U-tube. At the end of this process, all the components involved were further rinsed with a minimal amount of AR dichloromethane. The dichloromethane solution containing the isolated tar, pyrolyzed lignite (char), and quartz wool plugs was then transferred to a 40 mL vial. The extract from the experiment was slowly reduced to a volume of approximately 5 mL when the open vial was placed in a 30
35 °C water bath, after which it was removed from the water bath and allowed to settle at ambient temperature for a few minutes. It was essential that the extract was not allowed to dry completely as that would result in a significant loss of lower-molecular weight volatile material. The extract’s supernatant was eluted through a small bed of sodium sulfate packed in a Pasteur pipet (bed dimensions of 0.3 cm  1 cm), with the sodium sulfate being held in the Pasteur pipet with a small plug of cotton wool. The residue in the vial (char and quartz wool) was rinsed with small amounts of dichloromethane and treated likewise. The dry, particle-free extract eluted from this column could then be analyzed by GC
MS. For the preheated lignite experiment, the U-tube and lignite matrix was prepared and heated as described above, but to a final temperature of 250 °C. After this initial heating step, the U-tube was cooled and contents were extracted and treated as in the normal experiment, excluding the volume reduction and sodium sulfate column cleanup steps. The contents of the 40 mL vial that included the lignite matrix were filtered through Whatman 1 filter paper, including a final rinse step with dichloromethane. The filter paper and its contents were then airdried and further dried to a constant weight at 105 °C. After cooling, the lignite was isolated from the filter paper contents via removal of the quartz wool plugs with tweezers, before it was again pyrolyzed using the procedure described above to a final temperature of 500 °C. The overall material balance of a typical experiment was determined by pyrolyzing a dried (105 °C) sample of the Loy Yang lignite from ambient temperature to 500 °C at a rate of 20 °C/min and maintaining the final temperature for 20 min before cooling. During the experiment, condensed water and tar were seen to form on the exit arm of the U-tube. The exit purge flow gas was monitered by MS (using the m/z 18 ion) for the presence of water by the procedures outlined in section 3.8. Initially, water vapor was detected in the purge gas flow as the condensed water/ tar mixture on the exit arm of the U-tube dried in the gas stream. When the condensed water had completely dried, corresponding to a sharp decrease in the m/z 18 ion intensity, the U-tube and its contents were reweighed. The weight difference, 36.4% (dry weight basis), was attributed to the generation of volatiles such as gases and water during pyrolysis. The tar on the exit arm of the U-tube was carefully extracted with dichloromethane ensuring that no quartz wool or char was incorporated into the extract. This was essentially completed by pushing an aliquot of solvent through the system (from the gas entry end) using a syringe and appropriate tubing. In this way, the quartz wool plugs at the exit end of the U-tube acted as a filter to ensure that only the solubilized tar contents were removed. After the U-tube had been extracted, it was dried under a nitrogen flow and reweighed. The weight loss, 6.5% (dry weight basis), was attributed to the amount of tar generated during pyrolysis, while the remaining weight, 57.1% (dry weight basis), was assigned as the char yield for the pyrolysis. Replicate analyses showed that the reproducibility of the results was within 10% relative. The

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Figure 2. Total ion chromatogram (TIC) of the tar fraction obtained from the pyrolysis of the LYL/D2O mixture (see Table 2) in both full scale deflection and magnified formats. material balance results are understandably different but on a similar order of magnitude for wire mesh reactor experiments in which, in some cases, heating rates of 60 °C/min are used.2 The isolated pyrolysis extracts were analyzed using an Agilent 5973 (Hewlett-Packard) instrument fitted with a 30 m  0.25 mm capillary column (0.25 μm HP-5MS phase), using split/splitless injection mode. The oven was programmed to heat from 40 to 300 °C at a rate of 4 °C/min, after which it was held at a constant temperature for 10 min. The mass spectrometer was used in scan mode with an ion range from 50 to 500 amu. Components corresponding to peaks in the GC
MS total ion trace (TIC) were assigned by computer matching their spectra against the instrument’s NIST mass spectral database. Authentic standards of some of the compounds of interest were purchased from Chem Service Inc. Peak integrations were performed electronically using the software’s manual integration function. The estimated detection limit of the integration method was estimated by assessing the minimum peak size that could be reliably integrated above the background signal noise. The reproducibility of the calculated extents of deuterium incorporation based on integrated data was found to be within 5% relative.

3. RESULTS 3.1. Initial Experiments. A sample of a Loy Yang lignite/ deuterium-labeled water mixture [sample LYL/D2O (Table 2)] was pyrolyzed in a quartz U-tube reactor under a flow of nitrogen from ambient temperature to 500 °C at a rate of 20 °C/min and was maintained at 500 °C for 20 min before cooling. The condensed pyrolysate (tar fraction) generated was collected by dissolution in dichloromethane, and after the solution was dried and concentrated, it was analyzed by GC
MS. The total ion chromatogram (TIC) obtained from this analysis is shown in Figure 2, in both full scale deflection (FSD) and magnified format. The magnified trace shows the presence of a number of lower-molecular weight components eluting before a retention time of 25 min and a regular series of n-alkane/n-alkene peaks. It can additionally be seen that an unresolved complex mixture (UCM) is present at higher retention times. A similar trace was obtained when an equivalent mixture was prepared with “normal” water (H2O). The dichloromethane extracts obtained as described above were further analyzed by mass fragmentography to assess whether deuterium had been incorporated into the organic 3031

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Figure 3. Structures and mass spectral fragmentation patterns of the compounds mentioned in the text.

compounds present. The following compounds were chosen for detailed analysis (structures shown in Figure 3): 1-methylnaphthalene (C11H10), 2-methylphenol (C7H8O), 1-octadecene (C18H36), and n-octadecane (C18H38). These compounds were chosen to represent the aromatic, phenolic, and 1-alkene/nalkane hydrocarbon components of the tar fraction. The particular tar components chosen for study were at relatively high concentrations and well-resolved from other components in the tar, allowing for their detailed analysis by mass fragmentography. Confirmation of the identity of the compounds of interest was obtained by comparison of their mass spectra with library mass spectra, reference to standard publications, and the use of analytical standard materials. 3.2. Analysis of Aromatic Hydrocarbons. An example of the techniques used to analyze the components of interest is shown in Figure 4a for the 1-methylnaphthalene in the tar extract isolated from the LYL/D2O pyrolysis experiment. Partial mass fragmentograms are shown in a merged format for ions at m/z (mass to charge ratio) 142 (corresponding to the molecular ion of 1-methylnaphthalene, M), 143 (M þ 1), and 144 (M þ 2) for the extract of the LYL/D2O pyrolysis experiment described above. The inset in this figure shows the partial mass spectrum (from 138 to 145 amu) obtained at the apex of the m/z 142 mass fragmentogram as indicated. The partial rather than the full mass spectrum is shown so the changes in the distribution of ions around the parent ion can be seen. Figure 4b shows the same partial mass fragmentograms and mass spectrum obtained from the equivalent normal water LYL/H2O pyrolysis experiment. The peak in the m/z 142 mass fragmentogram from the LYL/ H2O experiment was identified as 1-methylnaphthalene by a NIST mass spectral library search of the full mass spectrum obtained at its apex, with the elution order of this peak with respect to other aromatic hydrocarbons also agreeing with published data.17 It is evident that the mass fragmentograms from the deuteriumlabeled water experiment (Figure 4a) show an enhanced signal for the m/z 143 ion compared to the same mass fragmentograms obtained from the normal water experiment (Figure 4b). As with most polyaromatic hydrocarbons, the base peak in 1-methylnaphthalene’s mass spectrum is the molecular ion (M), with a

Figure 4. Partial mass fragmentograms and mass spectra (insets) obtained from GC
MS analysis of 1-methylnaphthalene isolated from the pyrolysis of (a) the LYL/D2O mixture and (b) the LYL/H2O mixture (see Table 2).

minor amount of the m/z 143 (M þ 1) ion also present.17,18 Inspection of the partial mass spectra from the two experiments shows an expected enhanced m/z 143 ion (M þ 1) signal for the 1-methylnaphthalene obtained from the LYL/D2O experiment. This is attributed to a deuterium instead of a hydrogen atom being incorporated into the structure of some of the 1-methylnaphthalene formed during the pyrolysis process, thus resulting in a net increase of one in its molecular weight, and an enhanced m/z 143 signal. From additional inspection of the mass fragmentograms in panels a and b of Figure 4, it can be seen that the m/z 144 (M þ 2) mass fragmentogram from the LYL/D2O experiment is also enhanced relative to that of the LYL/H2O experiment, indicating that a significant amount of 1-methylnaphthalene containing two deuterium atoms has formed. This is also reflected in the relative amounts of the m/z 144 ion in the partial mass spectra shown from the two experiments. To determine the extent of incorporation of deuterium into 1-methylnaphthalene from the LYL/D2O experiment, the peaks in the m/z 142, 143, and 144 mass fragmentograms shown in panels a and b of Figure 4 were integrated and all areas were normalized to the parent ion (M) in each case (parent ion area assigned a value of 1.00). The results showed that, as expected, an enhanced level of M þ 1 ion was present relative to the M ion in the 1-methylnaphthalene from the LYL/D2O experiment. The actual amount of monodeuterated (D1) 1-methylnaphthalene formed in the LYL/D2O experiment was determined as the difference between the M þ 1 normalized areas from the LYL/ D2O and LYL/H2O experiments. This value (0.28) is listed in Table 3. Similarly, the integrated areas of the m/z 142 and 144 mass fragmentograms from the two experiments were used to determine the relative amount of dideuterated 3032

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Table 3. Deuterium Incorporation Data for the Compounds Studied relative amount of the deuterated species matrix LYL/D2O

LYL/D2O, preheated

LYL/D2O, faster heating rate

LYL/D2O. low flow

LYL/D2Oþ

compound

D1

D2

1-methylnaphthalene

0.28

0.10

2-methylphenol

0.25

0.06

1-octadecene