Energy & Fuels 1995,9, 1104-1105
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L'ommunzcatzons Release of Sulfur Dioxide from Sulfur-Containing Phenolic Resins during Temperature-Programmed Oxidation S. D. Brown, K. Ismail, and C. E. Snape* Department of Pure and Applied Chemistry, University of Strathclyde, Thomas Graham Building, 295 Cathedral Street, Glasgow G1 lZ,Scotland, U.K.
A. W. Harding, J. M. Jones, and K. M. Thomas Northern Carbon Research Laboratories, University of Newcastle-upon-Tyne, Bedson Building, Newcastle-upon-Tyne NE1 7RU, U.K. Received July 10, 1995. Revised Manuscript Received August 11, 1995 The significant progress achieved in the speciation of the thiophenic and non-thiophenic (sulfidic) forms in coals and other fossil fuels using X-ray techniques (XPS and XANES),temperature-programmed reduction (TPR) and pyrolysis, temperature-programmed oxidation (WO) and selective chemical modification has been docuFor mented in a number of recent review high-vacuum techniques and the well-swept reactor vessels used in the pyrolysis techniques, solids which do not soften upon heating are required for calibration purposes. Silica-immobilized sulfur compounds were shown previously to be ideal calibrants for TPR,4,5with the SiO-C linkage stable at temperatures up to ca. 500 "C. Phenol-formaldehyde resins which enable different moieties to be incorporated into a highly cross-linked matrix are an alternative class of materials just as suitable. A series of sulfur-containing coresins have been prepared using phenol with, as the second component, dibenzothiophene, diphenyl sulfide, phenyl benzyl sulfide, and thioanisole, and a preliminary report of their use in TPR has been published.6 The principle of TPO is the same as that of TPR except the sulfur forms are being oxidized to sulfur dioxide (SOz, small amounts of COS are also released) as opposed to being reduced to hydrogen sulfide at characteristic oxidation temperatures. The technique has been developed by LaCount and c o - w ~ r k e r s ~ over -~ a number of years and is now referrred t o as "controlledatmosphere programmed temperature oxidation" (CAP-
* Corresponding author. (1)Davidson, R. M. Organic sulphur in coal; IEA Coal Research CW 60; August 1993; Fuel 1994,73, 988-1005. (2) Calkins, W. H. Fuel 1994,73,475-484. (3) Snape, C. E.; Mitchell, S. C.; Ismail, K.; Garcia, R. Speciation of organic sulphur forms in heavy oils, petroleum source rocks and coals. Euroanalysis VI1I : Reviews on Analytical Chemistry; Littlejohn, D.; Thorburn Burns, D., Eds.; Royal Society of Chemistry: London, 1994; pp 103-120. (4) Ismail, K.; Garcia, R.; Mitchell, S. C.; Snape, C. E.; Buchanan 111, A. C.; Britt, P. F.; Franco, D.; Yperman, J. Proc. 1993 Int. Conf. on Coal Science, Banff, Canada 1993,2,3-6. (5) Ismail, K.; Garcia, R.; Mitchell, S. C.; Snape, C. E.; Buchanan 111, A. C.; Britt, P. F.; Franco. D.; Maes, I.; Yperman, J. Energy Fuels, in press. (6) Ismail, K.; Love, G. D.; Mitchell, S. C.; Brown, S. D.; Snape, C. E. Prepr. Pap.-Am. Chem. SOC.,Diu.Fuel Chem. 1994,39(2), 551557. (7) LaCount, R. B.; Anderson, R. R.; Friedman, S.; Blaustein, S. Fuel 1987,66, 909. 0887-0624/95/2509-1104$09.00/0
TO). Early work was conducted using a mixture of 10% vlv of oxygen in helium7 where pyrite produced SO2 in the temperature range 250-380 "C, the same as that found for aliphatic sulfide polymers. This problem was solved using pure oxygen which curtails pyrite oxidation until above 470 0C.8,9 Typical CAPTO traces for coals obtained with tungsten oxide catalyst comprise two distinct peaks at 290 and 420 "C that have been assigned to aliphatic and aromatic-bound sulfur, respe~tively.'-~In this Communication,TPO with on-line mass spectrometry (MS) has been conducted on the series of four phenolic resites (cured resoles) containing different sulfur functionalities in a TGA apparatus to ascertain whether the release of SO2 is dependent primarily on sulfur functionality or whether other factors, particularly the concentration of volatile matter and chemistry involving the interconversion of sulfur forms, are also important for macromolecular systems. The results obtained for the resites have been compared with those of a typical high-volatile UK bituminous coal (Daw Mill; IS0 711, 81.3% dmmf C, 35.7% daf VM) containing a relatively low concentration of pyrite (0.28% db, 1.4% total sulfur). The preparation and general characteristics of the sulfur-containing resites are being reported fully elsewhere.6J0 A mole ratio of 3:l (phenol to hydroxy sulfurcontaining component) was adopted to ensure that a reasonably high degree of cross-linking was achieved in the initial preparation of the resoles via base catalysis. XANES has shown that the sulfidic moieties were neither oxidized nor converted to thiophenes to a significant extent during the preparation of the resites.1° The procedure for the TGA-MS apparatus has been described elsewhere.11-13 The samples were heated from ambient to 1000 "C at a rate of 15 "C min-I in a (8) LaCount, R. B.; Kern, D. G.; King, W. P.; Trulli, T. K.; Walker, D. K. Prepr. Pap.-Am. Chem. Soc., Diu.Fuel Chem. 1992,37(3), 10831086. (9) LaCount, R. B.; Kern, D. G.; King, W. P.; LaCount Jr., R. B.; Miltz Jr., D. J.; Stewart, A. L.; Trulli, T. K.; Walker, D. K.; Wicker, R. K. Fuel 1993,72, 1203-1209. (10) Ismail, K.; Sirkecioglu, 0.;Andresen, Jm.; Brown, S. D.; Hall, P. J.;Steedman, W.; Snape, C. E. Polymer submitted for publication. (11)Brown, S. D.; Thomas, K. M. Fuel 1993,72,359-363. (12)Hindmarsh, C. J.; Wang, W.; Thomas, K. M.; Crelling, J. C. Fuel 1994,73, 1229-1234.
0 1995 American Chemical Society
Communications
Energy h Fuels, Vol. 9, No. 6, 1995 1105
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Figure 2. Sulphur dioxide (m/z 64, x250 - -1 and carbon dioxide (m/z 44, -1 evolution profiles for Daw Mill coal. tion (overlap with carbon dioxide profiles, Figure 1).In
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Figure 1. Sulfur dioxide (mlz 64,- - -) and carbon dioxide (m/z44,-) evolution profiles for the sulfur-containing phenolic resites.
flow of heliudoxygen (80:20, 50 cm3 min-'). TGA experiments were also conducted on the resites in nitrogen to determine their volatile matter contents in the same regime as used for TPO. Figure 1 shows their TPO C02 and SO2 evolution profiles for the resites investigated. The corresponding TPO profiles for Daw Mill coal are shown in Figure 2. TGA indicated that the volatile yields of all the resites are close to 40%, but this yield is attained by ca. 700 "C for the thioanisole sample compared to 800-900 "C for diphenyl sulfide and phenyl benzyl sulfide and 1000 "C for dibenzothiophene. The C02 evolution profiles for the resites (Figure 1) peak in the range 500-550 "C compared to 480-530 "C for bituminous coals (Figure 2).11-13 For the coal and the thioanisole, diphenyl sulfide, and phenyl benzyl sulfide resites, the dominant peaks assigned to char all have lower temperature shoulders extending from ca. 300 to 480 "C attributable to volatile matter. The C02 profile for Daw Mill coal is typical of that obtained for many high-volatile bituminous and low-rank c0a1s.ll-l~ The dibenzothiophene resite is somewhat unusual in that two well-resolved peaks are observed in the C02 profile. However, the lower temperature peak at 440 "C and associated shoulder extending to below 300 "C clearly overlaps the temperature range for volatiles evolution from the other resites and the coal. Although the SO2 evolution profiles for the resites are complex, with the possible exception of thioanisole where virtually all the SO2 has evolved below ca. 500 "C, they comprise a significant contribution between 470-520 "C in the temperature range for char combus(13)Wang, W.; Brown, S. D.; Hindmarsh, C. J.;Thomas, K. M. Fuel 1994, 73,1381-1388.
the case of the non-thiophenic sulfur forms, this strongly suggests that, during devolatilization, the sulfides are converted to varying extents into more thermally stable thiophenes and their oxidised forms which remain in the char. In the TGA, the sweep gas does not actually pass through the sample bed so the phenomenon of SO2 evolving from char could well be more pronounced than in the well-swept CATPO reactor used by LaCount and co-worker~.~~~ SO2 evolves with the volatile matter between 300 and 480 "C for all the resites (Figure 1). With the exception of dibenzothiophene, low-temperature shoulders commencing a t 300 "C and preceding the evolution of the bulk of the volatile matter are evident in the SO2 profiles (Figure 1). The SO2 profile for the dibenzothiophene-containing resite overlaps those for the sulfidic resites investigated between 350 and 450 "C, corresponding to the major release of volatile species. This clearly demonstrates that the release of SO2 from thiophenes is governed primarily by the partition of the sulfur between volatile matter and char from the resins and not by the form of organic sulfur. The SO2 evolution profile for the coal is bimodal comprising two distinct peaks with maxima at ca. 430 and 500 "C (Figure 2). It is similar in appearance to those reported by LaCount and co-workers8t9but, since no catalyst was used here, the two peaks occur at higher temperatures (cf. 290 and 420 "C in CAFTO). The findings for the resites have confirmed that the higher temperature peak is attributable predominately to sulfur in the char while the lower temperature peak (this will also include a relatively small contribution from pyrite) that overlaps the broad shoulder on the C02 profile arises from the volatiles. Therefore, the lower temperature peak cannot be assigned to particular sulfur functionalities. If it is assigned incorrectly solely to nonthiophenic forms, their proportion of the total organic sulfur in the coal could be vastly overestimated by TPO.
Acknowledgment. The authors thank the Engineering and Physical Sciences Research Council (Grant Nos.GR/J/08997 and G W 7 9 4 2 6 ) and the Department of Trade and Industry for financial support. EF9501335
