Study of Silica-Immobilized Sulfur Model Compounds as Calibrants for

This study deals with a series of model compounds for oxidized sulfur functionalities. It is proven by AP-TPR and AP-TPR-MS that the sulfur groups of ...
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Energy & Fuels 2000, 14, 1002-1008

Study of Silica-Immobilized Sulfur Model Compounds as Calibrants for the AP-TPR Study of Oxidized Coal Samples J. Van Aelst, J. Yperman,* D. V. Franco, and L. C. Van Poucke Material Chemistry Division, IMO, Limburgs Universitair Centrum, Universiteitslaan, B-3590 Diepenbeek, Belgium

A. C. Buchanan, III and P. F. Britt Chemical & Analytical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, M.S. 6197 Oak Ridge, Tennessee 37831-6197 Received February 8, 2000. Revised Manuscript Received June 26, 2000

Desulfurization of coal prior to combustion has developed the need for an accurate technique to determine which sulfur groups are present before and after chemical treatment. Atmospheric pressure-temperature programmed reduction (AP-TPR) is a technique that can determine the sulfur distribution by using model compounds as calibrants. This study deals with a series of model compounds for oxidized sulfur functionalities. It is proven by AP-TPR and AP-TPR-MS that the sulfur groups of these model compounds are decomposed and that decomposition products (SO/SO2) are only partially reduced. Diaryl sulfoxides and sulfones are found to be more stable than their aryl alkyl-substituted counterparts, and sulfones are more stable than the structurally related sulfoxides. Sulfates are generally found to be the most stable oxidized forms of sulfur.

Introduction Fossil fuels such as coal contain an amount of sulfur ranging from less than 0.5 wt % to more than 10 wt %. During combustion this sulfur is evolved into the atmosphere mainly as sulfur oxides. Besides the negative effect on human health the evolution of these gases is a threat to our environment. The ultimate result is the acidification of soils, forests, and surface water.1 To deal with this problem governments have been tightening legislation worldwide during the past decades. This has created the need for efficient methods to limit harmful emissions. One possible solution is to desulfurize coal prior to combustion by means of chemical treatment.2-7 To determine the precise effect of chemical treatments on coal an accurate technique that can compare sulfur distribution before and after treatment is necessary. Atmospheric pressure-temperature programmed reduction, on-line coupled with a potentiometric total sulfur detection setup, (AP-TPR) has been * Corresponding author: E-mail: [email protected] (1) Record, F. A.; Bubenick, D. V.; Kindya, R. J. Acid Rain Information Book; Noyes Data Corp., Park Ridge, NJ, 1982. (2) Soto, L. M.; Ledo, H.; Caldero´n, Y.; Marı´n, J.; Galarraga, F. J. Chromatogr., A 1998, 824, 45-52. (3) Durusoy, T.; Bozdemir, O.; Erincin, E.; Yu¨ru¨m, Y. Fuel 1977, 74 (1). (4) Juszczak, A.; Domka, F.; Kozlowski, M.; Wachowska, H. Fuel 1995, 74, 725. (5) Rojas-Chapana, J. A.; Giersig, M.; Tributsch, H. Fuel 1996, 75, 923. (6) Stambaugh, E. Desulphurization. In Encyclopedia of Chemical Technology, 32nd ed.; 1980. (7) Onursal, B. New cleaning technologies advance coal; part 1: flotation and separation. Coal Mining 1984, 21 (5), 38.

proven to be a reliable technique to determine the sulfur distribution in solid materials.8 In this technique a sample is heated in a hydrogen atmosphere. Sulfur groups are hydrogenated and are evolved as H2S. Each different type of sulfur functionality is hydrogenated/ reduced in a different temperature interval. The potentiometric detection of H2S as a function of temperature results in a kinetogram with peaks and shoulders that represent different sulfur groups. To allocate the peaks and shoulders of an AP-TPR kinetogram, a series of model compounds for different sulfur functionalities is needed. Ideally, these solid calibrants would neither melt nor evaporate before the sulfur groups are hydrogenated. In the past, sulfur compounds immobilized on resins, resoles, and silica matrixes have been used.9,10 Silica-immobilized compounds proved to be especially useful because the Si-O-Car linkage employed in these calibrants is known to be stable up to 500 °C, even in hydrogenating atmospheres.11 So far, little effort was made to study calibrants for oxidized sulfur groups in fossil fuels. Nevertheless, many chemical desulfurization treatments result in the formation of oxidized sulfur groups in the residue. In this work model compounds for sulfones, sulfoxides, and inorganic sulfates are (8) Yperman, J.; Maes, I. I.; Van den Rul, H.; Mullens, S.; Van Aelst, J.; Franco, D. V.; Mullens, J.; Van Poucke, L. C. Anal. Chim. Acta 1999, 395, 143. (9) Bar, H.; Aizenshtat, Z. J. Anal. Appl. Pyrol. 1991, 19, 265. (10) Ismail, K.; Sirkecioglu, O.; Andresen, J. M.; Brown, S. D.; Hall, P. J.; Steedman, W. Polymer 1996, 37 (18), 4041. (11) Lafferty, C. J.; Mitchell, S. C.; Garcia, R.; Snape, C. E.; Buchanan, C. A., III; Britt, P. F.; Klavetter, E. Energy Fuels 1993, 7, 331.

10.1021/ef000019n CCC: $19.00 © 2000 American Chemical Society Published on Web 08/31/2000

Calibrants for the AP-TPR Study of Oxidized Coal

studied. Results from AP-TPR experiments will be supplemented with results from AP-TPR coupled to a mass spectrometer. Experimental Section As described below, silica-immobilized model compounds are synthesized through reaction of Cabosil fumed silica with a specific sulfur-containing phenol.12 As a result of this reaction Si-O-C linkages are formed. The AP-TPR results of these sulfoxide and sulfone model compounds are used to resolve and interpret AP-TPR kinetograms from real (coal) samples. Silica-immobilized sulfoxides could not be synthesized in a pure form, because the sulfoxides show some oxidation and decomposition during the immobilization process. To determine whether non immobilized sulfoxides could be used instead of immobilized sulfoxides, a comparison is made between the immobilized and non immobilized sulfones. Finally some experiments are performed on several inorganic sulfates that might occur in coal. Model Compounds. p-Hydroxythioanisole and 4,4′-thiodiphenol (Aldrich) were used without further purification. 4,4′Sulfonyldiphenol (Aldrich) was recrystallized from ethanol/ benzene (purity of 99.9% by GC analysis). Iodometric titration of the moist m-chloroperbenzoic acid (m-CPBA, Aldrich, 5786%) determined the m-CPBA content to be 69.7 wt %.13 Although the m-CPBA oxidation of p-hydroxythioanisole to the sulfoxide and the sulfone could be accomplished in good yields, isolation of the product from the m-chlorobenzoic acid was difficult. Therefore, the phenols were protected as the acetate by standard methods (KOH/acetic anhydride),14 and the mchlorobenzoic acid was washed away by base extraction. The sulfoxide and sulfone of p-hydroxyphenyl benzyl sulfide15 were found to be thermally unstable and decompose at temperatures of ca. 230 °C. These compounds were unsuitable for the surface-attachment procedure (200 °C for 60 min) and were not investigated further. Analytical GC analysis was performed on a Hewlett-Packard 5890 Series II gas chromatograph equipped with a HP 7673 autosampler and a J & W Scientific 30 m × 0.25 mm DB-1 methylsilicone capillary column (0.25 µm film thickness). Mass spectra were obtained at 70 eV on a Hewlett-Packard 5792A/ 5890 Series II GC/MS system equipped with a capillary column identical to that used for GC analysis. The phenols were typically silylated to the trimethylsilyl ethers with bis(trimethylsilyl)trifluoroacetamide in pyridine (1:1) before analysis. Detailed procedures for the synthesis of surface-attached model compounds have been previously described and only the highlights will be presented here.16 Surface-attached substrates were prepared by adsorption of the phenol (0.5 equiv to surface OH) onto a dried (200 °C, 4 h under vacuum) surface of a fumed silica (Cabosil M-5, Cabot Corp., 200 ( 25 m2 g-1, ca. 4.5 OH nm-2) by solvent evaporation of a benzene slurry. Surface attachment was performed in a sealed, evacuated tube at 200 °C for 60 min in a fluidized sand bath. Excess phenol was removed by Soxhlet extraction (16 h) with benzene. The final product, which is a free-flowing white powder, was dried under vacuum and analyzed by a base hydrolysis procedure. (12) Ismail, K.; Mitchell, S. C.; Brown, S. D.; Snape, C. E.; Buchanan, A. C., III; Britt, P. F.; Franco, D. V.; Maes, I. I.; Yperman, J. Energy Fuels 1995, 9 (4), 707. (13) Organic Syntheses; Nolan, W. E., Ed.; Wiley & Sons: New York, 1988; Collect. Vol. 6, p 276-277. (14) Bordwell, F. G.; Boutan, P. J. J. Am. Chem. Soc. 1957, 79, 717722. (15) Oae, S. Organosulfur Chemistry: Structure and Mechanisms; CRC Press: Boca Raton, FL, 1991; pp 253-281. (16) (a) Buchanan, A. C., III; Britt, P. F.; Skeen, J. T.; Struss, J. A.; Elam, C. L. J. Org. Chem. 1998, 63, 9895. (b) Buchanan, A. C., III; Britt, P. F.; Thomas, K. B. Energy Fuels 1998, 12, 649. (c) Buchanan, A. C., III; Britt, P. F.; Thomas, K. B.; Biggs, C. A. J. Am. Chem. Soc. 1996, 118, 2182.

Energy & Fuels, Vol. 14, No. 5, 2000 1003 The surface-immobilized compounds (ca. 150 mg) were stirred in 1 N NaOH (30 mL) overnight, p-HOC6H4CH2CH2Ph in 1 N NaOH was added as an internal standard, the solution was acidified with HCl to pH < 5, extracted with CH2Cl2 or diethyl ether (3 × 7 mL), the combined organic layers washed with H2O (1 × 10 mL), dried over MgSO4, and filtered. The solvent was removed under reduced pressure, and the material was silylated with N,O-bis(trimethylsilyl)trifluoroacetamide in pyridine (2.5 M, 0.3 mL). Multiple analysis provided surface coverages, reported as mmol of organic per gram of derivatized silica, that were reproducible to at least (5%. The samples of •PhS(O)2CH3 and •PhS(O)2C6H4OH had surface coverages (purities) of 0.123 mmol g-1 (96.3%) and 0.0745 mmol g-1 (99.3%), respectively. The surface-attached materials, denoted by •, are linked by a p-silyloxy linkage unless otherwise noted. p-Hydroxyphenyl Methyl Sulfoxide. A stirred solution of p-methylthiophenyl acetate14 (9.82 g, 53.9 mmol) in CH2Cl2 (50 mL) was cooled to -40 °C in a CO2/CH3CN bath under argon. A solution of m-CPBA (13.89 g, 53.9 mmol) in CH2Cl2 (150 mL) was added dropwise over 60 min to the cooled solution. The reaction mixture was stirred at -40 °C for 60 min and an aliquot was removed for analysis by GC and GC-MS. After 3 h, a solution of m-CPBA (0.93 g, 3.6 mmol) in CH2Cl2 (20 mL) was added to consume the remaining starting material, and the mixture was stirred for an additional 60 min at -40 °C. The reaction mixture was poured into a separatory funnel and washed with saturated NaHCO3 (2 × 100 mL), water (1 × 100 mL), and brine (1 × 100 mL). The organic layer was dried over Na2SO4, filtered, and the solvent removed under reduced pressure to yield a colorless oil (10.90 g, 102% yield). The reaction was quantitative by GC analysis and contained 96% sulfoxide and 4% sulfone. The phenol was deprotected in 71% yield by a standard method (CH3OH/H2O/K2CO3).17 The product was recrystallized two times from ethanol/hexanes to yield a white powder (4.38 g, 52% overall yield) containing 98.6% sulfoxide and 0.6% sulfone by GC analysis. p-Hydroxyphenyl Methyl Sulfone. A stirred solution of p-methylthiophenyl acetate14 (9.95 g, 54.7 mmol) in CH2Cl2 (300 mL) was cooled to 0 °C under an argon atmosphere. A solution of m-CPBA (28.12 g, 113.6 mmol) in CH2Cl2 (300 mL) was added over 2 h. The reaction mixture was warmed to room temperature and stirred for 15 h. The m-chlorobenzoic acid was removed by filtration, and the solids were washed with CH2Cl2 (40 mL). The filtrate was poured into a separatory funnel and washed with saturated NaHCO3 (3 × 150 mL) and brine (1 × 50 mL). The organic layer was dried over Na2SO4, filtered, and solvent removed under reduced pressure to yield a white solid (10.95 g, 93.6% yield). The phenol was deprotected in 86% yield by a standard method (CH3OH/H2O/ K2CO3).17 The final product (6.92 g, 73.5% overall yield) had a purity of 99.6% by GC analysis and was used without additional purification for surface attachment. Di-(p-hydroxyphenyl) Sulfoxide. A stirred solution of di(p-acetoxyphenyl) sulfide (19.295 g, 63.89 mmol) in CH2Cl2 (150 mL) was cooled to -10 °C in an ice/salt bath under an argon atmosphere. A solution of m-CPBA (16.46 g, 63.89 mmol) in CH2Cl2 (150 mL) was added over 1 h. The reaction mixture was stirred for 1 h and an aliquot was removed for analysis by GC and GC-MS. The reaction was not complete, so a solution of m-CPBA (6.53 g, 25.35 mmol) in CH2Cl2 (50 mL) was added over 30 min. The reaction mixture was stirred for 1 h at -10 °C. The m-chlorobenzoic acid was removed by filtration, and the solids were washed with CH2Cl2 (50 mL). The filtrate was poured into a separatory funnel and washed with saturated NaHCO3 (3 × 200 mL), water (1 × 100 mL), and brine (1 × 100 mL). The organic layer was dried over Na2SO4, filtered, and solvent removed under reduced pressure to yield a white solid (20.08 g, 98.8% yield). The crude product contained a mixture of sulfide (15%), sulfoxide (48%), and (17) Bu¨chi, G.; Weinreb, S. M. J. Am. Chem. Soc. 1971, 93, 746.

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sulfone (26%) by GC analysis. A portion of the reaction mixture (4.66 g) was purified by flash chromatography eluting with ethyl acetate/hexanes (1.5:1.0) to give di-(p-acetoxyphenyl) sulfoxide (3.86 g, 87% purity). The phenol was deprotected in 96% yield by a standard method (CH3OH/H2O/K2CO3).17 The product was purified by recrystallization in ethanol/hexanes (99.2% purity by GC analysis). Atmospheric Pressure-Temperature Programmed Reduction (AP-TPR). Before analysis, all samples were ground until the particle size was smaller than 88 µm. For the AP-TPR analysis of the silica-immobilized samples, 100 mg (instead of the usual 40 mg for coal samples) was used, because of the low sulfur loading on the silica matrix. For the analysis of the non immobilized products, an amount of sample was used and mixed with silica powder so that the wt % of sulfur agrees with that of the immobilized sample. The sample is placed in the reactor which is made of glass. The lower part consists of quartz and can stand temperatures up to 1200 °C, while the upper part (outside the oven) consists of borosilicate glass, including the gas inlet and outlet as well as the cooling system. This reactor (lower part) is heated in the oven at a rate of 5 °C/min from room temperature to 1000 °C. A hydrogen flow of 100 mL/min is sent through the reactor by means of the inside tube with a disperser at the end. Each different type of sulfur in the sample is hydrogenated/reduced in a specific temperature interval. The evolving H2S is detected potentiometrically. This potentiometric detection beaker holds an aqueous solution of sulfide antioxidant buffer, which converts all H2S into S2- and HS-. Because the pH of the detection solution is constant, the concentration ratio (HS-)/ (S2-) is also constant. In these circumstances, the S2- measured with a sulfide ion selective electrode, using a callibration curve, quantifies the total sulfur amount liberated from the coal or model compound sample. More detailed information about the technique can be found elsewhere.8 The differentation of this signal results in a kinetogram that represents the different types of sulfur that are present in the analyzed sample. The following reduction/hydrogenation order has been found: alkylthiols < arylthiols < disulfides < dialkyl sulfides < alkylaryl sulfides < pyrite ≈ iron sulfate < diaryl sulfides < simple thiophenes < troilite < some other inorganic sulfates and complex thiophenic structures. The kinetograms are presented with normalized intensities, meaning that raw AP-TPR data are divided by experimental AP-TPR sulfur recovery and multiplied by total sulfur content. This procedure presumes that all sulfur functionalities will be reduced to H2S to the same extent. Although some counter-indications (mainly from experiments with model compounds12) exist for this hypothesis, it still remains useful to normalize the profiles at this time in order to be able to make a quantitative comparison between different samples. Sometimes substantial differences in sulfur recovery (i.e., the ratio between the amount of sulfur detected by AP-TPR as H2S and the total sulfur content) of coals and model compound samples (especially oxidized) in a classical AP-TPR experiment are observed because of incomplete hydrogenation/reduction of some sulfur functionalities into H2S. To be able to investigate all other gases evolving during an AP-TPR experiment, a mass spectrometer (Fisons-VG Thermolab MS) is also connected after the watercooler of the APTPR reactor by a heated capillary. This coupling will be referred to as AP-TPR-MS. In this technique, on-line monitoring of the evolution of volatile organic compounds, up to a mass/charge ratio of 300, can help in clarifying the processes involved in the degradation of the coal matrix or model compound samples. In the current study, this detection enables the identification of possible other volatile sulfur-containing compounds, which are not fully reduced or hydrogenated to H2S, such as SO and SO2, that cannot be detected by the used ion-selective electrode. Since the signals from the mass spectrometer are difficult to interpret in a quantitative way, both detection modes are complementary. AP-TPR-MS experi-

Van Aelst et al.

Figure 1. AP-TPR kinetogram of the silica-immobilized ≈Ph-SO2-CH3. ments in pure H2 as well as in pure He gas stream help in the understanding of the processes occurring during these pyrolysis experiments, and in a better assignment of the different peak and shoulder signals to specific sulfur functionalities present in the original sample.

Results and Discussion Aryl Alkyl Sulfones. The model compound used as a model for the behavior of aryl alkyl sulfones is ≈PhSO2-CH3, with ≈ representing the link to the silica matrix. The AP-TPR kinetogram of this compound is shown in Figure 1. The signal shows one peak with a maximum at a temperature of 540 °C and a clear shoulder to the right of this peak. The peak for H2S evolution is the result of the hydrogenation/reduction of sulfur from the model compound. The shoulder may result from diffusion processes. Earlier particle size analysis indeed showed that silica can clot together upon heating.19 Below 500 °C this behavior is limited, but it becomes more prominent at higher temperatures. Because of the small loading of the sulfone on the silica matrix, the excess of silica can easily hinder both the entering and evolving gases. These diffusion processes are one of the reasons for the low AP-TPR sulfur recovery in this experiment (12%). The sulfur recovery of an AP-TPR experiment is the percentage of sulfur in the sample that is detected as H2S. An overview of each model compound with its AP-TPR temperature maximum and sulfur recovery is given in Table 1. Another reason for the low sulfur recovery could be the evolution of other volatile sulfur compounds, e.g., SO2. To check this possibility the sample is analyzed with AP-TPRMS. The results are shown in Figure 2. In this figure, the full line is the summation of the mass-to-charge ratios of 33 and 34 (i.e., HS+ and H2S+). This mass spectrum is the representation of the evolved H2S. The dotted line is the summation of the mass-to-charge ratios of 48 and 64 (i.e., SO+ and SO2+). This mass spectrum is the representation of the evolved SO2. (18) Mullens, S.; Yperman, J.; Bozdemir, T.; Durusoy, T.; Yurum, Y.; Franco, D. V.; Mullens, J.; Van Poucke, L. C. Proceeding of the Tenth International Conference on Coal Science 1999, Taiyuan, P. R. China, p 1135. (19) Van den Rul, H.; Yperman, J.; Buchanan, A. C., III; Britt, P. F.; Maes, I. I.; Franco, D. V.; Mullens, J.; Van Poucke, L. C. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. (San Francisco) 1997, 42 (1), 1031.

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Table 1: Overview of Model Compounds with AP-TPR Temperature Maximum and Sulfur Recovery model compound

temperature maximum (°C)

sulfur recovery as H2S (%)

ZnSO4‚7H2O BaSO4 CaSO4‚0.5H2O ≈Ph-SO2-CH3a ≈Ph-SO2-Ph-OHa HO-Ph-SO-CH3 HO-Ph-SO-Ph-OH

620 880 900 540 650 505 575

64 9 6 12 30 69 46

a

Note: ≈ representing the link to the silica matrix.

Figure 3. AP-TPR kinetogram of the non immobilized HOPh-SO2-CH3.

Figure 2. AP-TPR-MS mass spectra of the silica-immobilized ≈Ph-SO2-CH3 with evolution of H2S (full line) and SO2 (dotted line).

According to this figure, the model compound is decomposed with SO2 evolved at a maximum temperature of 490 °C. Reduction of the evolving SO2 to H2S reaches a maximum at 535 °C. Because the reduction seems to take place only above a certain temperature, a significant amount of SO2 is detected. This process clearly contributes to the low AP-TPR sulfur recovery reported as H2S above. Additional contributions to low H2S recoveries might be that other sulfur-containing volatiles are evolved, or that specific sulfur groups are formed and remain in the residue. These possibilities are indeed real because earlier work12 showed that secondary reactions via radical and dehydrogenation reactions can occur under AP-TPR conditions, leading to formation of altered sulfur groups. These mechanistic aspects will be investigated in the future. As mentioned above, the immobilized aryl alkyl sulfone was compared to the non immobilized compound (HO-Ph-SO2-CH3) dispersed in silica to examine the role of covalent immobilization. For this experiment, a certain amount of sample is used and mixed with silica powder so that the wt % of sulfur agrees with that of the immobilized sample. The AP-TPR kinetogram is shown in Figure 3 and exhibits no significant difference to the kinetogram of the immobilized compound in Figure 1. Both signals show the same temperature maximum around 540 °C and a shoulder to the right of this peak. The AP-TPR sulfur recoveries of the experiments are also comparable (12% for the immobilized and 18% for the non immobilized). The same reasons as mentioned above for the immobilized compound can be given to explain this low value: diffusion effects, evolution of other volatile sulfur-containing products, incomplete reduction of evolving SO2 and the remaining sulfur groups in the residue. It can be concluded that im-

Figure 4. AP-TPR kinetogram of the silica-immobilized ≈Ph-SO2-Ph-OH.

mobilizing this aryl alkyl sulfone on a silica matrix does not strongly affect its behavior in AP-TPR conditions. This indicates that, in this case, non immobilized compounds also provide satisfactory models for the APTPR experiments. Diaryl Sulfones. The model compound used to represent diaryl sulfones is ≈Ph-SO2-Ph-OH, with ≈ representing the link to the silica matrix. The AP-TPR kinetogram of this compound is shown in Figure 4. The signal shows one peak with a maximum at a higher temperature of 650 °C indicating that the diarylsulfone is more stable than the corresponding aryl alkyl sulfone. A less pronounced shoulder to the right of this peak is detected suggesting that the hydrogenation of this compound is less affected by diffusion or that secondary reactions are less prominent. This might be the reason the AP-TPR sulfur recovery is somewhat higher (30%) than for the aryl alkyl sulfone. But still this value is low, so an additional AP-TPR-MS experiment was performed to get better insight into this hydrogenation/ reduction process. The results are shown in Figure 5. Again the full line is the summation of the mass-tocharge ratios of 33 and 34, representing the evolved H2S. The dotted line is the summation of the mass-to-charge ratios of 48 and 64, representing the evolved SO2. From the mass spectral data, it can be concluded again that the compound seems to be rather stable, resulting in a higher temperature of decomposition than for the aryl

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Figure 5. AP-TPR-MS mass spectra of the silica immobilized Ph-SO2-Ph-OH with evolution of H2S (full line) and SO2 (dotted line). Figure 7. AP-TPR kinetogram of the non immobilized HOPh-SO-CH3.

Figure 6. AP-TPR kinetogram of the non immobilized HOPh-SO2-Ph-OH.

alkyl sulfone. As a consequence, the evolving SO2 is almost instantly reduced to H2S, and less SO2 is detected. This might be another reason for the slightly higher AP-TPR sulfur recovery of this compound, in comparison to the aryl alkyl sulfone. To compare with the immobilized diaryl sulfone, the non immobilized diaryl sulfone(HO-Ph-SO2-Ph-OH) was analyzed. As before, the sample was mixed with silica powder so that the wt % of sulfur agrees with that of the immobilized sample. The AP-TPR kinetogram is shown in Figure 6. This sample shows a peak with a temperature maximum at 615 °C. This is a significantly lower decomposition Tmax than for the immobilized diaryl sulfone. Immobilizing the diaryl sulfone on a silica matrix seems to have a stabilizing effect on the decomposition temperature of the diaryl sulfone. In addition, the AP-TPR sulfur recovery of the experiment of the non immobilized diaryl sulfone is lower (18%) than for the immobilized diaryl sulfone (30%). Aryl Alkyl Sulfoxide. As mentioned above, immobilization of the aryl alkyl sulfoxide on a silica matrix was not possible because the sulfoxide was partially oxidized and decomposed during the immobilization process. For this reason, only the non immobilized aryl alkyl sulfoxide (HO-Ph-SO-CH3) could be analyzed. For this compound too, the sample is mixed with silica powder to obtain a suitable wt % of sulfur. The AP-TPR result is shown in Figure 7. The kinetogram shows a peak with a temperature maximum at 505 °C. It should be remembered that the temperature maximum could be somewhat higher if the sulfoxide were immobilized. However, this lower Tmax for the sulfoxide compared

Figure 8. AP-TPR-MS mass spectra of the non immobilized HO-Ph-SO-CH3 with evolution of H2S (full line), SO(dashed line), and SO2 (dotted line).

with the corresponding sulfone is consistent with its expected higher reactivity. To the right of this peak an important shoulder is seen, and the AP-TPR sulfur recovery is significantly higher (69%) than for both sulfones. This could mean that the reduction efficiency of a sulfoxide is higher than that of a sulfone. To check this assumption, an AP-TPR-MS analysis of the sample was performed and the results are shown in Figure 8. The full line is the summation of the mass-to-charge ratios of 33 and 34 and represents the evolution of H2S. The dashed line is the mass-to-charge ratio of 48 and represents the mass spectrum of the evolved SO. The dotted line is the mass-to-charge ratio of 64 and represents the mass spectrum of the evolved SO2. The decomposition of this model compound takes place in two steps and the signals of the mass-to-charge ratios of 48 and 64 do not exhibit the same course as they did in the case of the sulfone model compounds. In the first step, only SO is detected, while in the second step SO2 as well as SO is detected. The source of oxygen for formation of SO2 is unclear, but could possibly arise from residual oxygen or possibly from the presence of the OH group. After the first decomposition step, the H2S signal shows a peak, while after the second step, the H2S signal exhibits only a shoulder. This confirms the higher reduction efficiency of the sulfoxides. For this model compound it can thus be concluded that it starts to decompose around 510 °C and SO is released. Part of the SO is reduced to H2S. At higher temperatures

Calibrants for the AP-TPR Study of Oxidized Coal

Figure 9. AP-TPR kinetogram of the non immobilized HOPh-SO-Ph-OH.

Figure 10. AP-TPR-MS mass spectra of the non immobilized HO-Ph-SO-Ph-OH with evolution of H2S (full line), SO(dashed line), and SO2 (dotted line).

SO2 is produced using oxygen of the OH groups. Part of SO2 is reduced to H2S. Diaryl Sulfoxide. As a model compound for diaryl sulfoxides, HO-Ph-SO-Ph-OH was studied as a mixture with silica powder. The AP-TPR kinetogram is shown in Figure 9, and it shows a peak with a temperature maximum at 575 °C. Again it must be noted that the temperature maximum of the immobilized diaryl sulfoxide might be slightly higher than this. Again we find that the sulfoxide is more reactive than the corresponding sulfone, but that diaryl-substitution results in a more stable compound than aryl alkyl-substitution. The sulfur recovery of this experiment is relatively high (46%). This is higher than the sulfur recovery of the corresponding sulfone, which is consistent with the assumption that the reduction efficiency is higher for sulfoxides than for sulfones. The AP-TPR-MS results are shown in Figure 10. The full line represents, as in all above cases, the evolution of H2S, while the dashed and the dotted line show the evolution of SO and SO2, respectively. Again two steps can be noticed. In the first step SO is detected, while in the second step SO2 is also detected. For this model compound, more SO2 than SO is observed. After the first step, the H2S signal shows a peak, while after the second step, the H2S signal exhibits only a shoulder. This confirms again the higher reduction efficiency of the sulfoxides. The same conclusion as for the aryl alkyl sulfoxide can thus be made: this model compound starts to decompose around 580 °C and SO is released. Part

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Figure 11. AP-TPR kinetogram of CaSO4‚0.5H2O (full line) and BaSO4 (dotted line).

of the SO is reduced to H2S. At higher temperatures SO2 is produced using oxygen of the OH groups. Part of SO2 is reduced to H2S. Inorganic Sulfates. In previous work, AP-TPR results from FeSO4‚7H2O were reported.8 The AP-TPR profile of this product is characterized by a peak with a temperature maximum at 570 °C. It was proven that this sulfate decomposed with SO2 evolution. This SO2 was then reduced to H2S. The AP-TPR sulfur recovery was very high (84%). This unusually high recovery was explained through the catalytic effect20-22 of metallic Fe on the reduction of SO2. In this current study, a series of other sulfates was investigated. Figure 11 shows the AP-TPR results of CaSO4‚0.5H2O and BaSO4. Both CaSO4‚0.5H2O and BaSO4 occur in coal as inclusions or separate particles. Both kinetograms show a peak with a temperature maximum at 900 °C and 880 °C, respectively, and very low AP-TPR sulfur recoveries of only 6% and 9%, respectively. The signal for both sulfates does not reach the baseline within the temperature interval of the experiment. This means that hydrogenation/reduction of the decomposition products is not finished at these temperatures and that, consequently, the AP-TPR sulfur recovery of both experiments must be very low. The other sulfate that has been investigated is ZnSO4‚ 7H2O. The reason this sulfate was chosen to be investigated is that, in coal, ZnS can be present which can possibly be oxidized to the corresponding sulfate. Furthermore, this sulfate has a lower decomposition temperature and catalytic effects from Zn might be expected. The AP-TPR result is shown in Figure 12. As expected, the kinetogram shows one peak with a temperature maximum at 620 °C. Decomposition and hydrogenation/reduction of the decomposition products seem to be complete within the AP-TPR temperature interval. The sulfur recovery is indeed higher (64%) than in the case of calcium and barium sulfate but lower than for iron sufate, probably because of its low decomposition temperature and probably the catalytic effects of Zn. (20) Mastral, A. M.; Izquierdo, M. T.; Mayoral, C.; Pardos, C. J. Coal Qual. 1992, 1, 11. (21) Futurama, S.; Koyonagi, S.; Kamiya, Y. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1991, 36, 5136. (22) Maldonado-Ho´dar, F. J.; Rivera-Utrilla, J.; Mastral-Lamarca, A. M.; Ferro-Garcia, M. A. Fuel 1995, 74, 818.

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Figure 12. AP-TPR kinetogram of ZnSO4‚7H2O.

Conclusions It was shown in this work that all model compounds for oxidized sulfur groups are decomposed during the AP-TPR experiment. Despite the excess of H2, sulfones are decomposed to SO2 which is then partially hydrogenated to H2S. Sulfoxides are decomposed to SO. Part of the evolved SO is hydrogenated to H2S and part is transformed to SO2 which is in turn partially hydrogenated to H2S. As can be expected hydrogenation of SO seems to be more efficient than hydrogenation of SO2, resulting in a higher AP-TPR sulfur recovery for the sulfoxides. Immobilization of the sulfur group on a silica matrix has an effect on the temperature of decomposition of some compounds. Inorganic sulfates are also decomposed before hydrogenation/reduction occurs. Because of the high temper-

Van Aelst et al.

ature of decomposition of CaSO4‚0.5H2O and BaSO4 hydrogenation/reduction is not finished at the end of the AP-TPR experiment. This results in a very low AP-TPR sulfur recovery. The AP-TPR sulfur recovery of ZnSO4‚ 7H2O is higher because the product has a lower temperature of decomposition and because Zn has a catalytic effect on the hydrogenation/reduction of the decomposition products. The results of this study can be used to interpret APTPR kinetograms of oxidized coal samples. When the AP-TPR signal shows an increase at temperatures above 800 °C the presence of inorganic sulfates can be inferred. When the signal shows an increase between 500 and 650 °C, the presence of primarily sulfones and sulfoxides (plus a few inorganic sulfates) can be expected. Diaryl sulfoxides and sulfones have been found to exhibit much higher Tmax (575-650 °C) than their corresponding aryl alkyl counterparts (505-540 °C). This study has demonstrated that for complex samples complementary APTPR-MS experiments are needed. Acknowledgment. The authors thank J. Kaelen and K. Van Vinckenroye for their technical assistance in the AP-TPR experiments. J. Van Aelst was financed with a specialization grant from “Het Vlaams Instituut voor de bevordering van het wetenschappelijk technologisch onderzoek in de industrie (IWT)”. A. C. Buchanan, III, and Phillip F. Britt acknowledge support by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy under Contract DE-AC05-96OR22464 with Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp. EF000019N