Silica-Immobilized Sulfur Compounds as Solid Calibrants for

Energy & Fuels 1995,9, 707-716

707

Silica-Immobilized Sulfur Compounds as Solid Calibrants for Temperature-Programmed Reduction and Probes for the Thermal Behavior of Organic Sulfur Forms in Fossil Fuels K. Ismail, S. C. Mitchell, S. D. Brown, and C. E. Snape" Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow Gl lXL, U.K.

A. C. Buchanan 111" and P. F. Britt Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6197

D. V. Franco, I. I. Maes, and J. Yperman" Laboratory of Inorganic and Physical Chemistry, Limburgs Universitair Centrum, B-3590 Diepenbeek, Belgium Received February 8, 1995. Revised Manuscript Received March 30, 1995@

For the well-swept fixed-bed reactors used in temperature-programmed reduction (TPR) t o specify the organic sulfur forms present in coals and kerogens, calibrants must neither melt nor evaporate before the onset of thermal decomposition. In this respect, nonmelting silicaimmobilized substrates are suitable with the Si-0-C linkage being stable up t o ca. 500 "C. Silica-immobilized samples of dibenzothiophene, diphenyl sulfide, phenyl benzyl sulfide, and thioanisole have been synthesized and noncatalytic tests have been conducted in atmospheric and high-pressure TPR reactors. The characteristic reduction temperatures of the non-thiophenic compounds investigated are well resolved from that of dibenzothiophene for both techniques and the results have validated previous findings by TPR on coals. The use of high hydrogen pressure (150 bar) lowered the reduction temperatures substantially. The H2S recoveries from the atmospheric experiments are low suggesting that, for the non-thiophenic compounds, secondary reactions occur yielding refractory thiophenes which are not detected. Although sulfur recoveries are greatly improved, such reactions are still evident with 150 bar hydrogen pressure especially in the case of the phenyl benzyl sulfide, possibly as a consequence of the high surface coverages used. Insights into the retrogressive chemistry occurring for the immobilized phenyl benzyl sulfide have been provided from GC-MS analysis of hydrolyzed TPR residues obtained at different temperatures, and from vacuum pyrolysis experiments conducted as a function of surface coverage.

Introduction Temperature-programmed reduction (TPR) is based on the principle that different organic sulfur forms present in solid fuels have different characteristic reduction temperatures at which hydrogen sulfide (H2S) evolves. However, only limited success has been achieved thus far in specifying organic sulfur forms in coals, primarily because only the labile non-thiophenic forms have actually been observed resulting in poor sulfur balances with virtually all the thiophenic sulfur remaining in the char due to the low pressures and the low boiling reducing agents ~ s e d . l -Further, ~ little account

* Corresponding authors. Abstract published in Advance A C S Abstracts, J u n e 1, 1995. (1)Attar, A. In Analytical Methods for Coal and Coal Products; Academic: New York, 1979; Vol. 111, Chapter 56 and DOE/PC/30145TI Technical ReDort. (2) Majchrbwicz, B. B.; Yperman, J.; Reggers, G.; Gelan, J . M.; Martens, H. J.; Mullens, J.; van Poucke, L. C. Fuel Process. Technol. 1987. 1.5. - ,- , 363-376. --(3) Majchrowicz, B. B.; Franco, D. V.; Yperman, J.; Reggers, G.; Gelan, J . M.; Martens, H. J . ; Mullens, J.;Van Poucke, L. C. Fuel 1991, 70, 434-441. @

-~

has been taken of the reduction of pyrite to pyrrhotite, and retrogressive reactions including the conversion of sulfides into thiophenes. These drawbacks have recently been overcome by using a well-swept, fured-bed reactor at relatively high hydrogen pressures (up t o 150 bar). Typically over 80% of the organic sulfur is reduced to H2S with the remainder being released in the tars, and catalysts such as sulfided molybdenum can be used to further aid hydrodesulf~rization.~,~ Self-consistent results were obtained for a suite of coals and kerogen^.^ Recent improvements in the design of atmospheric pressure reactors, particularly in using higher sweep gas velocities, suggest that the extent of retrogressive reactions should be reduced and sulfur recoveries i m p r ~ v e d . Indeed, ~,~ although TPR is a thermal technique and clearly may suffer interferences from retro(4)Dunstan, B. T.; Walker, L. V. Final Report to Australian Natl. Energy Res. Dev. and Dem. Council (1988). ( 5 ) Kumar, A,; Srivastava, S. K. Fuel 1992,71, 718-719. (6) Lafferty, C. J.; Mitchell, S. C.; Garcia, R.; Snape, C. E. Fuel 1993, 72. - - ,367-371 --(7) Mitchell, S. C.; Snape, C. E.; Garcia, R.; Ismail, K.; Bartle, K. D. Fuel 1994,73,1159-1166.

0887-0624/95/2509-0707$09.00/0 0 1995 American Chemical Society

708 Energy & Fuels, Vol. 9, No. 4, 1995

Ismail et al.

Table 1. Substrate Loadings and HzS Peak Evolution Temperatures in Atmospheric and High-pressureTPR substratea Zdibenzothiophene =diphenyl sulfide (=PhSPh) =thioanisole (zPhSCH3) *phenyl benzyl sulfide (ZPhSCHzPh) sulfur powder cysteine (HSCHZCH(NHZ)CO~H)

loadingb (mmovg) 0.60 0.68 0.60 0.59 0.16 0.077

loadinf (wt %) 11.0 12.6 7.4 11.8 3.2 1.6

-

-

high press. 480 300-320 360 270 d d d

180

low press.Q 710-745 590-610 490-500 375-400 d d

255 195-220

a Silica-immobilizedsubstrates denoted by "z". Millimoles of organic per gram of derivatized silica. Weight of organic per weight of derivatized silica. Not determined. e The range of values represents T,, obtained in the absence or presence of a reducing mixture; see text.

gressive chemistry of the non-thiophenic sulfur forms, the resolution obtained is superior to that from X-ray photoelectron specroscopy (XPS)and X-ray absorption near-edge structure (XANES), which have received considerable attention for specifying the proportions of aliphatic and aromatic carbon-bound sulfur in C O ~ ~ S . ~ J O - ~ ~ For the well-swept fixed-bed reactors now used in temperature-programmed reduction (TPR) at both atm o s p h e r i ~ ~and . ~ high p r e s s ~ r e s , calibrants ~,~ must neither melt nor evaporate before the onset of thermal decomposition of the organic sulfur groups in order that they remain in the reactor zone. In this respect, nonmelting silica-immobilized substrates that employ a Si-O-C,,l linkage are suitable, and it has been demonstrated using immobilized benzene (xSi-0C6H5) that the SiOC linkage is stable up to 500 "C, even in reducing atm0~pheres.l~Immobilized samples of dibenzothiophene, diphenyl sulfide, phenyl benzyl sulfide, and thioanisole have now been synthesized for use as calibrants in TPR using both the well-swept highpressure r e a ~ t o rand ~ , ~latest design of the atmospheric pressure r e a ~ t o r .To ~ demonstrate the potential of the immobilized sulfur compounds for probing the retrogressive chemistry of non-thiophenic forms in the solid state, GC-MS analysis has been carried out on the hydrolyzed residues obtained from the immobilized phenyl benzyl sulfide at different temperatures (300600 "C). Further, to ascertain the effects of surface immobilization, atmospheric pressure TPR tests have also been carried out on the corresponding sulfurcontaining phenols, which are the precursors used to prepare the immobilized substrates. Only the results from noncatalytic tests are reported here. The Si-0-C linkage, although thermally robust, is susceptible to hydrolysis as well as cleavage in other nucleophilic solvents, which limits the catalyst formulations that can be used. Dispersed catalysts, particularly the group VI11 noble metals and sulfided molybdenum, are generally beneficial in TPR, particularly at high hydrogen pre~sure,~ and , ~ this will be the subject of future publications. Experimental Calibrants and Their Synthesis. The silica-immobilized substrates were prepared from the reaction of the appropriate sulfur-containing phenol and Cabosil fumed silica on a gram scale as described previously for the synthesis of silicaimmobilized d i p h e n y l a l k a n e ~ . ' ~ J ~ 4-Hydroxythioanisole (HOCeH4SCH3)was the only commercially-available precursor used (Aldrich Chemical Co.). 3-Hydroxydibenzothiophene was prepared via the base hydrolysis of the corresponding bromo

derivative in a tubing bomb under autogeneous pressure.16 Benzene sulfinic acid was reacted with phenol to yield 4-hydroxydiphenyl sulfide ( H O C ~ H ~ S C ~using H S ) a modification of Hinsberg's method.17 4-Hydroxyphenyl benzyl sulfide (HOC&SCH&&) was prepared from 4-hydroxythiophenol and benzyl bromide in an analogous reaction to Williamson's ether synthesis.18 The sulfur-containing phenols were synthesized and purified to yield ca. 10 g batches to provide sufficient sample for the immobilization reaction. The surface loadings for the substrates listed in Table 1 were determined from gas chromatographic analysis (Hewlet Packard 5890) of the base hydrolysis products with the use of internal standard^.'^"^ Samples of immobilized phenyl benzyl sulfide with three different surface loadings were prepared by adjusting the phenol to silica hydroxyl ratio as previously described.14J5 Cysteine (HSCH&H(NH~)COZH) and sulfur powder were used t o ascertain the reduction temperatures for thiols and polysulfides, respectively, which are both difficult to immobilize on silica by current methods. Further, both cysteine and sulfur powder decompose thermally before the onset of melting and, hence, immobilization is not necessarily required. Atmospheric Pressure TPR. Since the atmospheric pressure TPR setup was last described: the reactor design has changed significantly. Preheating the carrier gas was found to be unneccesary and the cooling system has also been simplified. The reactor is now constructed entirely from glass (Figure 1). The upper part is borosilicate glass which facilitates proper sealing and the lower part is quartz which allows operation a t temperatures up to 1000 "C. Recently, this new reactorQ was modified further. The spiral in the cooling compartment has been replaced by a straight tube and the collector omitted (Figure 1). The reactor length has been reduced to decrease the free volume and, hence, to minimize the time taken for the H2S evolved to reach the ion-selective electrode used for detection. The tests on the immobilized substrates were carried out using 20 mg samples both with and without the reducing mixture of pyrogallol, phenanthrene, (8)Mullens, J.;Yperman, J.; Carleer,R.; Franco, D. V.; Van Poucke, L. C.; Van Der Biest, J. Appl. Clay Sci. 1993, 8 , 91-99. (9)Yperman, J.; Franco, D. V.; Mullens, J.;Reggers,G.; Van Poucke, L. C.; Marinov S. P. In Composition, Geochemistry and Conversion of Oil Shales; Nato AS1 Ser. C, Vol. 455; Snape, C. E., Ed.; Kluwer: Dordrecht, The Netherlands, 1995; pp 449-459. (10) Davidson, R. M. Organic sulphur in coal. I E A Coal Research Report No. C R / 6 0 , August 1993. (11)Calkins, W. H. Fuel 1994, 73, 475-484. (12) Snape, C. E.; Mitchell, S. C.; Ismail, K.; Garcia, R. Euroanalysis Vlll: Reviews on Analytical Chemistry; Royal Society of Chemistry:

London, 1994; pp 103-120. (13)Lafferty, C. J.; Mitchell, S. C.; Garcia, R.; Snape, C. E.; Buchanan 111, A. C.; Britt, P. F.; Klavetter, E. Energy Fuels 1993, 7 , 331-333. (14) Buchanan 111,A. C.; Biggs, C. A. J . Org. Chem. 1989,54,517t52.5

(15)Britt, P. F.; Buchanan 111, A. C. J . Org. Chem. 1991,56,61326140.

(16)Cullinane,N. M.; Davies, C. G.; Davies, G. I. J . Chem. SOC. 1936, 1435-1437. (17) Hilbert, G. E.; Johnson, T. B. J . A m . Chem. SOC.1929,51,15261535. (18) Miller, E.; Read, R. R. J . A m . Chem. SOC.1933,55,1224-1227.

Organic Sulfur Forms in Fossil Fuels

Energy & Fuels, Vol. 9,No. 4, 1995 709

f To dekyor cel

0

H

=I-

Boro-Quartzglass transition

,

Thermocouple

Figure 1. Atmospheric pressure TPR appartus. resorcinol, and 9,lO-dihydroanthracene. Hydrogen was used as the sweep gases with a flow rate of 56 cm3 min-l based on preliminary experiments in nitrogenhydrogen mixtures where the H2S recoveries were much lower. The reactor was heated a t 5 "C min-l to the peak temperature of 1000 "C. The first derivative plots (i.e., instantaneous hydrogen sulfide concentration vs temperature) obtained from the ion-selective electrode output were smoothed using the Savitsky-Golay-Gorry p r ~ c e d u r e , ~and ~ ? ~the ~ total amount of H2S evolved was determined by integrating the peak area. High-pressureTPR. The operation of the high-pressure system shown in Figure 2 has been described previ~usly.~ The tests were conducted mainly a t 150 bar pressure. Typically 0 was diluted in a 1O:l w/w 200 mg of substrate ( ~ 5 pm) mixture with acid-washed sand (75-250 pm) to increase the voidage in the fixed-bed. The samples were heated from 100 to 600 "C a t a rate of 5 "C min-l with the hydrogen volumetric flow rate of 5 dm3 min-l measured a t ambient conditions corresponding to a superficial gas velocity of ca. 0.5 m s-l in the reactor. At the end of each run, liquid products collected in the ice/water cooled trap were recovered using dichloromethane, and the s a d c h a r mixture in the reactor tube was also collected for analysis. H2S was detected using a quadrupole mass spectrometer (Figure 2, mainly with a 100 amu VG instrument, but a more sensitive 300 amu instrument was used in later tests). In addition, for a t least one test on each substrate, the hydrogen sulfide was collected in cadmium acetate solution and the total amount evolved was then determined by titration (modificationof ASTM 2385-81). To assess the overall levels of desulfurization achieved, the sulfur contents of the recovered silicdsand mixtures (chars) were determined by XRF and, in some cases, the liquid products were analysed by GC and GC-MS (Hewlet Packard 5972A). (19) Savitzky, A.; Golay, M. Anal. Chem. 1964,36, 1627-1639. (20)Gorry, P. A. Anal. Chem. 1990, 62, 570-573.

Tests were conducted on the immobilized phenyl benzyl sulfide with the highest surface coverage (0.59 mmoug) in which the maximum temperatures of only 300 and 360 "C were used instead of the standard setting of 600 "C. The chars were hydrolyzed with base to remove the hydrocarbon and sulfurcontaining products from the silica surface for analysis. In addition to these TPR experiments, a series of vacuum pyrolysis tests a t 300 "C were also conducted on the immobilized phenyl benzyl sulfide as a function of surface coverage, as described previously for diphenylalkanes,14J5with reaction times of 5,30, and 120 min being used. Both volatile and residue products were identified using GC and GC-MS. Residue products bound to the silica are listed in Table 5 as the corresponding phenols. The HOPhSH product was found to be difficult to recover quantitatively, with hydrolyses performed under argon atmosphere providing the most reproducible results.

Results and Discussion Figures 3-10 show the atmospheric and high pressure H2S evolution profiles for the four sulfur-containing immobilised substrates investigated. Where appropriate, the evolution profiles for methane (monitored a t mlz 15, (M-H)+), benzene (mlz 78, M+) and toluene (mlz 91, (M-H)+) have been included with the high pressure TPR H2S (mlz 34, M+) traces. The peak temperatures in the H2S evolution profiles for both the atmospheric and high-pressure tests are summarized in Table 1, and the proportions of the initial sulfur released as H2S during the TPR experiments are listed in Table 2. For the high-pressure tests, the proportions of the initial sulfur remaining in the recovered silicdsand mixtures are given in Table 3. Dibenzothiophene. For the atmospheric pressure tests on both the immobilized and free hydroxy forms of dibenzothiophene, reduction peaks were observed at 745 and 710 "C with and without the reducing mixture, respectively (Figure 3 and Table 1). However, the sulfur recoveries were low (Table 2) due to the inherent stability of dibenzothiophene toward reduction. In addition, secondary condensation reactions are likely to occur, leading to the formation of more highly condensed thiophenic structures which are not reducible was obtained a t atmospheric pressure. The same "2' for both the immobilized and free phenolic forms of dibenzothiophene under the same experimental conditions. The reducing mixture is clearly ineffective for dibenzothiophene with the-T,, being even higher than when the mixture is not used. When nitrogen was used in the high-pressure reactor, no discernible H2S peak was observed (Figure 4). This is consistent with the results from the atmospheric pressure reactor where little H2S evolved below the peak temperature of 600 "C used in the high-pressure reactor. However, at both 30 and 150 bar of hydrogen pressure, broad peaks centered at ca. 520 "C are evident (Table 1 and Figure 4). As discussed later, this 2"' corresponds closely to the dominant feature found in the traces of most of the coals characterised thus far.7 The level of desulfurizationachieved at 150 bar pressure is virtually 100% (Table 3) with nearly 50% of the sulfur being detected as H2S (duplicateruns, Table 2). However, the remaining ca. 50%of the sulfur cannot be accounted for readily. Even with the use of dry ice instead of icelwater to trap the volatiles, dibenzothiophene was the only significant sulfur-containingproduct recovered from the trap by GC analysis, accounting for an additional ca.

Ismail et al.

710 Energy & Fuels, Vol. 9, No. 4, 1995 Mains Electricty Supply

To \ :nt

L

Post-reactor

1 A

I

Figure 2. Schematic of high-pressure TPR apparatus. Table 2. Hydrogen Sulfide Yields for TPR Experiments on Immobilized Substrates low press. low press. withhigh with reducing out reducing mixture mixture press. substrate (mol %) (mol %) (mol %) xdibenzothiophene 20 27 47a xdiphenyl sulfide 11 12 65 xthioanisole 10 15 57 xphenyl benzyl sulfide 40 40 84" a

2

."

Mean of two determinations, estimated error of ca. 435%.

."3

Table 3. Sulfur Remaining in Substrates after High-pressure TPR Tests Run to 600 "C

3 9 .&

wt%

substrate xdibenzothiophene xdiphenyl sulfide xthioanisole xphenyl benzyl sulfide

sulfur x 10% 1.21 3.94 4.56 1.77

% of initial sulfur remaining in char 0.01 0.02 0.02 0.01

a As determined for sample diluted in sand (1O:l dilution); see Experimental Section.

4% of the initial sulfur. The reason for the poor sulfur

balance is not clear, but could still be attributable to inefficient trapping of volatiles, but more likely to the formation of products that are not volatile or stable enough to be detected by GC. The fact that some dibenzothiophenewas recovered confirms that the Si0-C surface linkage is being cleaved to a modest extent at the relatively high temperatures required for the reduction of the C-S bonds. Diphenyl Sulfide. Reduction peaks at 590 and 610 "C were observed in the atmospheric pressure tests on ,, being the immobilized substrate with the lower T obtained again without the reducing mixture (Figure 5 and Table 1). The Tm, is well resolved from that of dibenzothiophene in common with high-pressure TPR (see below). The sulhr recoveries are low (Table 2 , l l 12%) as for dibenzothiophene, suggesting that retro-

745

L

1

fi 715

silica immobilised without reducing mixture

%

55 Ll

Y

2

1

145

I-

hydroxycompound with reducing mixture

d)

-E,, ,A,,

g .d

hydroxy compound wlthout reducing mixture ,

,

5

Q

0

200

400

600

800

1000

Temperature ("C)

Figure 3. Atmospheric pressure TPR H2S evolution profiles for immobilized dibenzothiophene and 3-hydroxydibenzothiophene with and without the reducing mixture.

gressive reactions dominate. In the absence of the reducing mixture, only one sharp peak is observed (the Tm,, Figure 5), indicating that the secondary reaction products (presumably condensed thiophenes) are not detectable at atmospheric pressure. When the reducing mixture is added, other secondary peaks are observed at higher temperatures (Figure 5). At 150 bar of hydrogen pressure, the HzS evolution profile is broad with the Tm,, occurring in the range 300-320 "C. Some secondary chemistry is occurring as

Energy & Fuels, Vol. 9, No. 4, 1995 711


A

30 bar H 2

: 150 bar N z

,

I

zoo

100

,

400

300

b

0

. I

k m *

._

3

-

d

b

-

.-0

-

590

and 500 "C in the atmospheric pressure reactor for the immobilized substrate with and without the reducing mixture, respectively (Figure 7 and Table 1). The 2'" is clearly significantly lower than that of diphenyl sulfide at 600 "C. However, the use of the mixture is a disadvantage in that it gave rise to a much lower H2S recovery (Table 2), an enhanced peak at 640 "C, and higher temperature peaks up to 700 "C, an indication that secondary reaction pathways are becoming more important.

silica immobilised wthout reducing mixture

g- $ 1 l

.

l

,

I

600

ization was well in excess of 99% (Table 3), but much more of the sulfur evolved as H2S (65%,Table 2). Sulfur compounds detected in the trap by GC accounted for an additional 2%of the sulfur, the major compounds being identified as mono- and dihydroxydiphenyl sulfide (HOPhSPh and HOPhSPhOH, accounting for 62%of the sulfur in the compounds trapped) from hydrolysis of the SiOC linkage.14J5 Smaller quantities of benzenethiol (15%),diphenyl disulfide (5%),and dibenzothiophene (6%) were also detected, the latter indicating that cyclization had occurred.

silica immobilised with reducing mixture

.-t; e

1

500

l

,

l

.

l

a

l

,

Ismail et al.

712 Energy & Fuels, Vol. 9, No. 4, 1995

t

100

I

I

I

I

I

200

300

400

500

600

Temperature ("C)

Figure 6. High-pressure TPR evolution profiles of H2S and benzene from the immobilized diphenyl sulfide.

il

490


.-

t-

-0

t: d


1

0

,

1

200

,

1

400

500

,

1

600

,

1

800

1

1

,

1000

Temperature ("C) Figure 7. Atmospheric pressure TPR H2S evolution profiles from immobilized thioanisole with and without the reducing mixture. to ePhCH(SH)Ph; see eq 3 below. Benzene, and to a lesser extent toluene, evolve from 380 "C with the bulk evolving between 480 and 600 "C from hydrocracking of the SiO-C bonds as found for the other substrates (Figure 8). The H2S recovery of ca. 60%was again much higher than ir+ the atmospheric pressure experiments and slightly lower than for the high-pressure experiments for the immobilized diphenyl sulfide. Trapped liquid products accounted for an additional 4% of the initial sulfur in the thioanisole and included the corresponding sulfoxide and sulfone arising from oxidation of the starting material. This suggested that the low sulfur recovery could be, in part, due to oxidation. However, the absence of distinct peaks from sulfones

and sulfoxides in XANES spectra of all the immobilized substrates indicated that oxidation had not occurred to a significant extent during their preparation and, hence, oxidation of the thioanisole must have taken place subsequently. The low sulfur balance again must result from incomplete trapping of volatile products and/or the formation of products not detectable by GC. Phenyl Benzyl Sulfide. The atmospheric pressure traces of both the immobilized substrate (0.59 mmol g-l) and free hydroxy compound are complex with three distinct bands occurring at 375-385, 490-520, and 630-640 "C followed by a tail extending up to 900 "C (Figure 9). However, the primary reduction temperatures are again the same for the free and immobilized phenol. Use of the reducing mixture increased the intensity of the low-temperature peak from the primary reduction of the central C-S bond (Figure 91, indicating that it is now effective, but the overall recovery was no higher (Table 2). Interestingly, despite the extensive secondary chemistry giving rise to the two higher temperature peaks at 490-520 and 630-640 "C possibly arising from a combination of thioanisoles and diphenyl sulfides plus benzothiophenes, respectively (the possible assignment of the peak at 630-640 "C to benzothiophenes is based on the result for dibenzothiophenes with a T,,, of 700 "C), the HzS recovery of 40 mol % (Table 2, without the reducing mixture) is the highest for the substrates investigated. Despite the primary reduction peak at 375-385 "C being more pronounced, the HzS recoveries for the free phenol are lower (Figure 9), suggesting that immobilization may retard the bimolecular reaction pathways leading to ring condensation. At 150 bar of hydrogen pressure, the primary reduction event occurs at ca. 270 "C where the initial HzS evolution profile correlates with that for toluene (Figure 10). However, ca. 70% of the HzS evolves after the initial cleavage of the CH2-S bond in the temperature range 350-500 "C due to the formation of diary1 sulfides and possibly thiophenes (see following). Nonetheless, complete desulfurization was achieved (Table 3) with greater than 80% of the sulfur evolving as H2S (Table



I

,

1

100

300

200

4

I

I

400

500

600

Toluene

Temperature ("C)

Figure 8. High-pressure TPR evolution profiles of HzS, methane and toluene from the immobilized thioanisole. 490

desulfurization achieved (Table 3). The 360 "C char shows that although the xPBS conversion is 95.5%, substantial amounts of sulfur compounds remain in the char. The major sulfur-containing compound found is dihydroxydiphenyl sulfide (from the corresponding diattached species, xPhSPhx). The formation of this retrogressive product can be explained by eqs 1 and 2.

630



xPhS' hydroxy compound with reducing mixture

hydroxy compound without reducing mixture

0

200

-

xPhSCHzPh

400

600

800

1000

Temperature ("C)

Figure 9. Atmospheric pressure TPR H2S evolution profiles from immobilized phenyl benzyl sulfide (0.59 mmol g-l) and 4-hydroxyphenyl benzyl sulfide with and without the reducing mixture.

2). Only 0.1% of the sulfur was recovered in the liquid from the trap, with 4-hydroxythiophenol and unreacted hydroxyphenyl benzyl sulfide released from the surface being the only two sulfur compounds identified by GC-

MS. Secondary Reactions. High-pressure TPR experiments were conducted on the immobilized phenyl benzyl sulfide (ePBS) in which maximum temperatures of 300 and 360 "C (rather than the customary 600 "C) were used to help identify potential secondary reaction products containing sulfur. As anticipated, the 600 "C char contained only negligible quantities of products (Table 4) consistent with the extremely high level of

+ xPhSCHzPh

xPhS'

+ PhCH,'

xPhSPhx

+ PhCH,S'

(1) (2)

Thermolysis of the weak central S-C bond in =PBS (eq 1) produces the silica-immobilized phenylthiyl radical (=PhS'). This radical can abstract hydrogen from Hz or xPBS (abstraction from xPBS leads to ePhCHzPh formation as described below) to form xPhSH which is also a significant product found in the char. However, it can also react with =PBS via a displacement reaction (eq 2) leading to the diphenyl sulfide formation. This retrogressive chemistry, involving the reaction of neighboring species on the silica surface, should be most important for the high xPBS surface coverages employed in the TPR experiments, which was confirmed in the vacuum pyrolysis experiments described below. However, the formation of diphenyl sulfide has also been reported to occur during fluid-phase pyrolysis of phenyl benzyl sulfide in the hydrogen-donating solvent tetralin at 388 "CZ1and, thus, is a typical retrogressive pathway for this sulfur species. Analysis of the 300 "C char indicates that desulfurization has occurred to only about 53%. This is reasonably consistent with the analysis of the TPR profile where no more than half of the HzS is evolved in the primary reduction event with peak maximum a t 270 "C (Figure 10). Several vacuum pyrolysis experiments14J5were conducted isothermally at 300 "C to determine if the product compositions would be comparable to those reported in fluid phases and to examine the influence of surface coverage on the retrogressive pathway that ~

~~

(21) Ignasiak, T.M.; Strausz, 0.P.Fuel 1979, 57, 617-621.

Ismail et al.

714 Energy &Fuels, Vol. 9,No. 4, 1995

I

Toluene

I

,

1

I

300

400

1

zoo

100

1

500

600

Temperature (“C)

Figure 10. High-pressure TPR evolution profiles of H2S and toluene from immobilized phenyl benzyl sulfide (0.59 mmol g-l). Table 4. Analysis of F’roducts in TPR Chars from Immobilized Phenyl Benzyl Sulfide0 char temp (“C) =PBS conversion (%Ib desulfurization (%) compounds identified (moll100 mol %PBST HOPh HOPhCHn HOPhCHiPh (H0Ph)zCHPh HOPhSH HOPhSCH3 HOPhSPh HOPhSPhOH HOPhSSPhOH HOPhSCHzPhd

600 99.9 99.9

360 300 95.5 58.4 83.4 53.2

0.068 0.029

2.82 0.24

-

10.2

0.74 3.43 0.51 0.030 7.23 0.48 0.078 4.50

1.09 0.74 3.66 0.043 0.30 0.37 0.43 41.6

a Surface coverage of 0.59 mmollg. Based on recovered starting material. Phenols represent silica-immobilized products in residues. d Recovered starting material.

100 80

7

t

+ 0

0

0 ’ 0

+

.59 mmol/g

0

.16 mmoi/g

0

,077 mmol/g

I

30

60

90

120

150

Time (min)

Figure 11. Conversions of phenyl benzyl sulfide in vacuum pyrolysis experiments at 300 “C.

as HOPhSH). At low temperatures and in the absence of hydrogen pressure, xPhSH exhibits much greater stability. Under these conditions, radical coupling and dehydrogenation reactions are important, leading to products such as PhCHzCH2Ph, PhCH=CHPh, and HOPhSSPhOH. When compared to the TPR chars, hydrocracking to form products such as xPhH is less evident as expected. The major hydrocarbon product found in the vacuum pyrolysis residue, xPhCHzPh, was also found in the TPR chars (see 360 “C char), and diphenylmethane has also been reported t o be formed in fluid phase p y r o l y s e ~ . This ~ ~ , ~product ~ results from a free-radical rearrangement process (eq 31, in which =PhSCH,Ph

-

-

=PhCH(SH)Ph xPhCH2Ph (+H,S) (3)

the key step is a sulfur to carbon phenyl shift for the intermediate radical, xPhSCH*Ph, which is generated by hydrogen atom transfers to other radicals (e.g., those formed by homolysis in eq 1). This neophyl-like rearrangement has recently been observed directly in fluid phases.24 It can be seen from Table 5 that in the absence of hydrogen, the degree of desulfurization is low even at relatively high xPBS conversions. As in the TPR chars, a key reaction is the radical displacement chemistry (eq 2) that leads to diphenyl sulfide formation. Related displacement chemistry on xPhSSPhx would similarly produce the diphenyl sulfide species and has been reported t o occur in the pyrolysis of fluid-phase PhSSPh.22 The selectivity for this retrogressive pathway (as measured by the yield of HOPhSPhOH in mole percent) is found to increase both with increasing aPBS surface coverage and reaction extent as illustrated in Figure 12. Hence, the retrogressive reactions identified

forms wPhSPhx. The conversions of xPBS were found to decrease substantially with decreasing surface cover(22)Alnajjar, M. S.; Franz, J. A. Prepr. Pap.-Am. Chem. SOC.,Diu. age (Figure 11). The product distributions shown in Fuel Chem. 1986.31 (1). 287-295. ~ Abraham,’M.A.;’Klein,M. T. Fuel Sci. Technol. Int. 1988,6, Table 5 are similar to those reported in fluid p h a ~ e s . ~ ~ ? ~(23) 633-662. . . ~ Homolysis (eq 1)followed by hydrogen abstraction leads (24)Alnajjar,M. A.; Franz, J. A. J.Am. Chem.SOC.1992,114,10521058. to the major products, PhCH3 and xPhSH (identified ~

~~

~

_

_

_

_

_



Table 5. Vacuum Pyrolysis of Silica-ImmobilizedPhenyl Benzyl Sulfide at 300 "C coverage (mmollg) 0.59 0.59 0.59 0.16 0.16 0.16 0.077 time (min) 5 30 120 5 30 120 5 sample size ( g ) 0.329 0.347 0.297 0.499 0.333 0.309 0.443 xPBS conversion (%Y 22.3 44.7 88.4 12.7 38.6 75.1 11.5 mass balance (%) 85.7 78.1 83.0 93.3 91.7 85.3 92.3 desulfurization (%) 15.1 39.8 36.3 6.0 11.8 26.9 7.8 HOPhSPhOH yield (mol %) 1.06 5.54 6.23 0.78 1.85 3.69 0.77 product distribution (moll100 mol xPBS) PhH 0.012 0.017 0.035 0.041 0.007 0.021 0.050 PhCH3 15.8 34.2 2.056 19.4 43.6 2.59 5.12 PhCH2SH 0.10 0.28 0.98 1.32 0.10 0.66 0.98 PhCH2CH2Ph 0.21 0.39 0.005 0.009 0.055 0.23 0.61 PhCH=CHPh 0.022 2.33 4.84 0.49 2.28 6.99 0.062 0.006 PhCH2SCH2Ph 0.34 0.011 0.097 0.11 0.11 0.28 HOPh 0.11 0.046 0.084 0.070 0.082 0.027 0.095 0.12 HOPhCH3 0.10 0.11 0.080 0.081 0.094 0.14 HOPhSH 2.41 39.8 5.74 23.8 40.8 6.07 0.75b 0.11 0.097 0.18 HOPhSCH3 0.10 0.17 0.16 HOPhCHzPh 0.66 5.58 1.18 4.61 7.04 0.94 4.52 0.18 0.073 HOPhCH(Ph)SH HOPhSPhOH 0.14 1.76 6.83 0.082 0.93 3.56 0.048 HOPhSSPhOH 0.46 0.60 1.27 0.25 0.49 1.71 0.29 0.020 HOPh(Ph)C=CHPh 0.67 2.25 0.065 0.62 1.85 0.046 0.35 0.096 0.33 HOPhCH(Ph)PhOH total products 13.7 31.7 109.6 10.5 50.0 96.2 6.24 a

0.077 30 0.301 25.3 89.0 12.4 1.31

0.077 120 0.304 54.2 81.5 23.9 2.48

0.055 8.48 0.43 0.037 0.21 0.012 0.17 0.12 11.1 0.17 2.75

0.32 0.48 0.19 -

0.060 22.0 0.64 0.085 0.64 0.051 0.13 0.11 26.3 0.18 6.35 0.025 1.48 0.79 0.84 -

24.5

59.7

Based on recovered xPBS. Value appears to be an outlier and is too low based on the other values.

0.59 mmollg

FARRR 0.16 mmollg 0.077 mmollg

1

O1

Figure 12. Selectivity for formation of ZPhSPhx as a function of surface coverage and conversion at 300 "C.

here for =PBS have been shown to arise a t least in part from the high surface coverages employed (Table 1)for reasonable sensitivity in the TPR experiments. Thiols and Polysulfides. The traces for cysteine in both reactors comprise a sharp peak from the primary reduction event followed by small tailings, indicating that secondary reactions had largely been avoided. The reducing mixture was effective for both compounds a t atmospheric pressure giving rise to lower reduction temperatures. Similar values of T m w were obtained in both systems (Table 1)and, as anticipated, these are much lower than those obtained for the aliphatic sulfides investigated. General Trends. In both the atmospheric and highpressure systems, all the characteristic reduction temperatures found for the non-thiophenic forms are well resolved from that of dibenzothiophene (Table 1). Further, the reduction temperatures of poly/disulfides and thiols are much lower than those of both aliphatic and aromatic sulfides. Although thiophene and benzo-

thiophene calibrants have not yet been synthesized, their T m a can be anticipated to occur in the range 380450 and 600-700 "C in the high and atmospheric pressure systems, respectively. On the above evidence, these will probably overlap bands from polynuclear diary1 sulfides. Figure 13 shows the high-pressure TPR profiles of the high-sulfur Rasa and Mequinenza lignites. In both cases, the H2S observed accounts for 75%of the organic sulfur with the remaining 25% being released as thiophenic compounds in the tars. These traces are typical of the those for the low-rank and bituminous coals investigated thus far7in that they are comprise a dominant peak with a Tmwin the range 450-500 "C corresponding very closely to that found for the immobilized dibenzothiophene. In addition, a broad shoulder between ca. 200 and 400 "C, attributable to nonthiophenic forms, is most prominent in low-rank coals (acccountingfor ca. 30% of the organic sulfur). The fact that little visible resolution is obtained in this low-

Ismail et al.

716 Energy &Fuels, Vol. 9,No. 4, 1995

remaining in the chars. Thus, Figure 14 indicates that thiophenic sulfur is the dominant form in Rasa lignite (ca. 80% assuming the nonobservable sulfur in the char arises from thiophenic forms) consistent with the highpressure findings summarized above (Figure 13). In contrast, Mequinenza appears to contain a much higher proportion of sulfidic sulfur (ca. 50%, again assuming the nonobservable sulfur is thiophenic, Figure 14). This apparent discrepancy with the high-pressure result would suggest that, in this case, sulfides have been converted to thiophenes to a much lesser extent in atmospheric pressure TPR.

Rasa Lignite

2 Mequinenza Lignite

Conclusions

d

0

200

400

600

800

1000

Temperature ("C) Figure 13. High-pressure TPR traces of Rasa and Mequinenza lignites.

I

IO0

'

I

200

-

,

.

,

400 Temperature ('C)

300

'

,

500

.

,

600

Figure 14. Atmospheric pressure TPR traces of Rasa Mequinenza lignites.

temperature region is most likely attributable to the inherent complexity of the distribution of the nonthiophenic forms. The findings for the immobilized phenyl benzyl sulfide have highlighted that interconversion of sulfur forms can occur even at high hydrogen pressures to a significant extent which could result in the proportions of non-thiophenic sulfur forms in coals being underestimated. The atmospheric pressure TPR profiles of low-rank coals (Figure 14 shows the traces for Rasa and Mequnienza lignites) are also reasonably consistent with the results obtained for the silica-immobilized compounds in that they comprise well-resolved low- and hightemperature peaks separated at 550-600 "C from sulfides and thiophenes (Table 1). The H2S recoveries are 60 and 70% for Rasa and Mequinenza lignites, respectively, with virtually all the sulfur not observed (little tar is obtained from atmospheric pressure TPR)

Sulfur compounds immobilized on silica have been found to be suitable nonvolatile calibrants for TPR and have shown that the characteristic reduction temperatures of the non-thiophenic forms investigated are well resolved from that of dibenzothiophene for both TPR techniques. The use of high hydrogen pressure lowered the reduction temperatures considerably. The characteristic reduction temperatures for both techniques have validated previous findings by TPR on coals. At atmospheric pressure, the reduction temperatures are unaffected by silica immobilization. However, the H2S recoveries from atmospheric experiments are low suggesting that, for the non-thiophenic compounds, secondary reactions occur yielding complex thiophenes or other refractory sulfur forms. Although sulfur recoveries are greatly improved, such reactions are still evident under 150 bar of hydrogen pressure, especially in the case of the phenyl benzyl sulfide, possibly as a consequence of the high surface coverages used. The nonmelting immobilized substrates should also prove to be useful standards for high-vacuum and other thermal techniques, in particular L-edge xANES25and temperatureprogrammed oxidation,26which are also being used to specify organic sulfur forms in coals.

Acknowledgment. The research was supported by (i)the Science and Engineering Research Council (Grant Nos. GWG26600 and GWJ08997) a t the University of Strathclyde, (ii) the European Coal and Steel Community (Contract No. 7220/EC-025), a t the University of Strathclyde and Limburgs University, and (iii) the Division of Chemical Sciences, Office of Basic Energy Systems, U S . DOE at Oak Ridge National Laboratory (Contract No. DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.). In addition, I.I.M. at LUC is supported by a specialization grant from the Vlaams Instituut voor de bevordering van het wetenschappelijktecnnologisch onderzoek in de industrie (I.W.T.). The authors from LUC thank Miss Van Hamel and Mr. J. Kaelen and those from ORNL thank Ms. Kimberly Thomas for technical assistance. EF950032L (25) Brown, J. R.; Kasrai, M.; Bancroft, G. M.; Tan, K. H.; Chen, J-M Fuel 1992, 71, 649-653. (26) 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, 1203-1208.

Silica-Immobilized Sulfur Compounds as Solid Calibrants for

Recommend Documents