Fossil Fuels Utilization - American Chemical Society

5 Molten Hydroxide Coal Desulfurization Using Model Systems 1

Downloaded by UNIV LAVAL on May 5, 2016 | http://pubs.acs.org Publication Date: September 18, 1986 | doi: 10.1021/bk-1986-0319.ch005

Bruce R. Utz, Sidney Friedman, and Steven K. Soboczenski

Pittsburgh Energy Technology Center, U.S. Department of Energy, Pittsburgh, PA 15236

The chemistry f o r the removal o f organosulfur from c o a l , using fused caustics (molten hydroxides), was examined. Benzothiophene and dibenzothiophene were chosen as model compounds that simulate organosulfur compounds in c o a l . Results indicate that the desulf u r i z a t i o n o f thiophene-ring systems involves initial r i n g opening t o form an intermediate aromatic thiol, followed by a slower s u l f u r - e l i m i n a t i o n step that produces the desulfurized product. The reactivities of the two hydroxides (NaOH and KOH) used were also studied. The r e l a t i v e amount o f each hydroxide in the mixture may be critical in removing organosulfur, and potassium hydroxide is the reactive species. This suggests that the rate o f decomposition/ desulfurization may be enhanced by hydroxides containing the larger alkali c a t i o n , potassium. The s u l f u r - e l i m i n a t i o n step was further examined by conducting decomposition experiments with thiophenol, sodium thiophenolate, or potassium thiophenolate. These compounds were chosen because they have the same f u n c t i o n a l i t y as the intermediate aromatic t h i o l formed from the decomposition o f benzothiophene. Preliminary r e s u l t s show that potassium thiophenolate is more r e a d i l y desulfurized than sodium thiophenol a t e and provides further evidence to support the important r o l e o f the potassium cation. Additional evidence f o r the role o f the potassium cation was obtained by replacing KOH with other potassium s a l t s . S i m i l a r rates o f benzothiophene d e s u l f u r i z a t i o n were obtained with the K CO -NaOH fused s a l t mixture. The 2

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'Current address: E.I. du Pont de Nemours & Co., Inc., LaPorte, TX 77571 This chapter not subject to U.S. copyright. Published 1986, American Chemical Society

Markuszewski and Blaustein; Fossil Fuels Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

FOSSIL FUELS UTILIZATION: ENVIRONMENTAL CONCERNS

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possible r o l e o f water in molten hydroxide desulf u r i z a t i o n was examined by varying the amount of water present in c e r t a i n hydroxides. Results demonstrate that water does not play an important r o l e in the removal o f s u l f u r from organosulfur moieties. The removal of mineral matter, including p y r i t i c s u l f u r , and some of the organic s u l f u r with molten hydroxides is a known technique Q ) . The TRW "Gravimelt" Molten Hydroxide D e s u l f u r i z a t i o n Process is now being developed by the Department of Energy to improve the removal o f s u l f u r from coal ( 2 ^ ) . Other methods have been used ( 4 ) , but none have been as e f f e c t i v e as the Gravimelt Process. This p a r t i c u l a r process is a t t r a c t i v e because it removes much of the organic s u l f u r as w e l l as p y r i t i c s u l f u r . While the chemistry of inorganic s u l f u r removal (primarily p y r i t e ) by c a u s t i c is being examined a c t i v e l y (§), l i t t l e research has been done on the corresponding removal of s u l f u r from organosulfur moieties in c o a l . Two model compounds that are being used i n i t i a l l y to examine the chemistry of organosulfur removal are benzothiophene and dibenzothiophene. Characterizations of organosulfur in coal (6-8) suggest that thiophene-type r i n g structures are the prevalent organosulfur forms in many coals. The objective of t h i s study is to determine how molten hydroxides remove organosulfur from these model compounds and to r e l a t e these r e s u l t s to coal. Results of studying the chemistry of molten-hydroxide d e s u l f u r i z a t i o n , using thiophene-type r i n g systems, may lead to innovative approaches to u t i l i z a t i o n of t h i s h i t h e r t o unexplored area of organosulfur chemistry, such as, the use of other fused s a l t s to enhance desulfurization. Experimental A l l molten hydroxide d e s u l f u r i z a t i o n reactions were performed using 1/2-inch Monel Swagelok unions as reactors. Similar 316s t a i n l e s s - s t e e l reactors developed cracks and leaks, r e s u l t i n g in loss of v o l a t i l e components. In a t y p i c a l reaction, 3.1 to 4.0 g of powdered sodium hydroxide and/or potassium hydroxide, 0.3 to 0.6 g of an organosulfur compound, and a 1/4-inch-diameter Monel b a l l (to ensure adequate mixing) were added to the reactor under a nitrogen atmosphere in a glove box. In some cases, the hydroxide mixture was added in two steps to reduce the free volume of the reactor, which ensured that the reactant would be in the condensed phase and in intimate contact with the molten hydroxide. This was accomplished by melting most of the powdered base in the reactor at 350°C, followed by cooling the reactor to ambient temperature and subsequently adding the organosulfur compound and 0.3 to 0.5 g of a d d i t i o n a l base. The end caps were tightened to 35 f t l b ; and f o r each a d d i t i o n a l experiment, the caps were tightened an a d d i t i o n a l 3 to 5 f t l b u n t i l 75-80 f t l b was reached. The reactor was bolted to a bracket assembly and immersed in a Tescom SBL-2 f l u i d i z e d sand bath that was preheated to reaction temperatures. Reaction temperatures ranged from 350°C to 400°C, and reaction times varied from 10 minutes to 3 hours. Vigorous mixing (s240 shakes/min) was effected by using a B u r r e l l mechanical



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w r i s t - a c t i o n shaker. The reactors were cooled r a p i d l y by immersion in ambient-temperature water. After r e a c t i o n , the vessels were opened, placed in 50 mL of C H 2 C I 2 , and shaken f o r 5 minutes. This p a r t i a l l y removed neutral organics and minimized evaporative losses of v o l a t i l e organics. The C H 2 C I 2 s o l u t i o n was decanted into a f l a s k containing 0.1 g of an i n t e r n a l standard. The reactor was then shaken with 50 mL of deionized water u n t i l the base was dissolved (20-30 minutes). The strongly b a s i c s o l u t i o n was decanted and the reactor was washed with an a d d i t i o n a l 10 mL of water to ensure complete removal o f its contents. The aqueous solutions were combined and extracted with two 25-mL portions o f C H 2 C I 2 to remove any o f the remaining neutral and basic organics. Using litmus paper as the i n d i c a t o r , the water layer was a c i d i f i e d to a pH o f 4-5 with 4 to 6 mL o f concentrated HC1. The a c i d i f i e d s o l u t i o n contained a c i d i c products that had previously remained in the aqueous phase as soluble s a l t s . The n e u t r a l i z e d water layer was then extracted with two 25-mL portions of C H 2 C I 2 . I n t e r n a l standards were added to each portion to determine q u a n t i t a t i v e l y the i n d i v i d u a l components and permit c a l c u l a t i o n of the material balances. Material balances of approximately 85J-95J were achieved. The three methylene chloride portions were i n d i v i d u a l l y analyzed using an HP 5740 gas chromatograph with a 50-meter SE-54 phenylmethylsilicone c a p i l l a r y column. In most cases, flame i o n i zation was s u f f i c i e n t to detect the i n d i v i d u a l components of the e x t r a c t s , although in c e r t a i n instances a mass s e l e c t i v e detector (MSD) was a l s o used to a s s i s t in the i d e n t i f i c a t i o n o f products. The C H 2 C I 2 was removed using a rotoevaporator, and the products were further characterized by proton NMR, FTIR, and low-voltage, high-resolution mass spectrometry. Some loss of the more v o l a t i l e components was observed. Thermal decomposition reactions were conducted using t h i o phenol and its s a l t s . The aromatic t h i o l (0.2-0.6 g) and a s t a i n l e s s - s t e e l b a l l were added to the reactor, and the reactor was heated to 375°C f o r 30 minutes. Sodium thiophenolate and potassium thiophenolate were prepared by mixing equivalent amounts of a 1 M NaOH or KOH s o l u t i o n and thiophenol in a round-bottomed f l a s k sparged with nitrogen. The s o l u t i o n was s t i r r e d f o r approximately 1 minute and then rotoevaporated u n t i l the water had been removed. Residual water was removed by adding absolute ethanol and distill i n g the water-ethanol azeotrope. The s o l i d product was dried in a vacuum oven f o r 1 hour a t 100°C. Experiments designed to examine the decomposition c h a r a c t e r i s t i c s of the thiophenolate s a l t s were performed with the "neat" s a l t s , as w e l l as with the free a c i d (thiophenol). Experiments to examine decomposition c h a r a c t e r i s t i c s of thiophenolate s a l t s were also performed using 0.6 g of thiophenol and 3.0-3.5 g of the KOH:NaOH mixture (60:40). Results and Discussion Reaction of Benzothiophene. A weight r a t i o of 1:1 (KOH:NaOH) was chosen f o r the i n i t i a l model-compound study, which was based on one of the r a t i o s that the TRW process is using f o r bench-scale experiments with coal ( 2 ) . Preliminary experiments with benzothiophene at 375°C and 30-minute reaction times indicated that benzothiophene


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had reacted with the molten hydroxide to form two p r i n c i p a l products. C a p i l l a r y GC, high resolution mass spectrometry, FTIR, and NMR techniques i d e n t i f i e d the major product as o-thiocresol and the minor product as toluene. Extending the reaction times to 3 hours gave a considerably d i f f e r e n t d i s t r i b u t i o n o f products. Not only was the benzothio phene consumed, but the major product was toluene. These r e s u l t s i n d i c a t e that the o v e r a l l reaction o f benzothiophene with molten hydroxide involves a r i n g opening and e l i m i n a t i o n o f a one-carbon fragment t o form o-thiocresol (Equation 1), followed by a slower s u l f u r e l i m i n a t i o n to form toluene (Equation 2 ) .


_

ΚΟΗ:ΝαΟΗ

...

These r e s u l t s are supported by the observations o f Weissgerber and Seidler who i d e n t i f i e d small amounts o f o-thiocresol and formic a c i d when reacting benzothiophene with KOH a t 300°C-310°C. Preliminary r e s u l t s indicate that the one-carbon fragment may be carbon monoxide or formic a c i d . Apparently higher temperature, as w e l l as extended reaction time, are e f f e c t i v e in removing s u l f u r from the species completely. E f f e c t o f KOH:NaOH Ratio. Because o f inconsistencies in product y i e l d , a s e r i e s o f experiments was conducted to determine the r e a c t i v i t y o f each hydroxide. Mixtures having d i f f e r e n t r a t i o s o f KOH and NaOH were prepared, and t h e i r a c t i v i t i e s were compared by determining the extent o f benzothiophene conversion to the i n i t i a l product, o - t h i o c r e s o l , or to the subsequent product, toluene. Weight percents o f KOH:NaOH used f o r t h i s study were as f o l l o w s : 0:100, 40:60, 45:55, 48:52, 50:50, 52:48, 55:45, 60:40, 75:25, 90:10, and 100:0. Figure 1 shows the r e s u l t s o f these experiments. Note that a t approximately a 45:55 r a t i o o f KOH:NaOH, the conver sion o f benzothiophene to o-thiocresol increases dramatically. The lack o f any s i g n i f i c a n t reaction a t lower percentages of KOH was unexpected. Increasing the weight percent of KOH in the hydroxide mixture beyond approximately 50J resulted in a decrease in the con centration o f o-thiocresol and a corresponding increase in toluene formation, consistent with the proposed mechanism where toluene is a decomposition product o f the intermediate ( o - t h i o c r e s o l ) . Maximum y i e l d s o f desulfurized product (toluene) were obtained by using pure KOH. Conversions varied dramatically a t approximately a 45:55 r a t i o ; therefore a l l subsequent experiments were done with a 60:40 r a t i o o f KOH:NaOH, since reactions were occurring a t a s i g n i f i c a n t rate in t h i s mixture. The 60:40 r a t i o was used as the "standard" r a t i o f o r most of the study. The lack o f r e a c t i v i t y f o r hydroxide mixtures containing l e s s than 40 wt% o f KOH was believed to be caused by the existence o f an



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Molten Hydroxide Coal Desulfurization

induction or i n h i b i t i o n period. Experiments were conducted using the standard KOHrNaOH mixture a t reaction temperatures o f 375°C and r e a c t i o n times between 5 and 30 minutes in order t o determine i f an induction or i n h i b i t i o n period e x i s t s . The r e s u l t s (Figure 2) demonstrate that a t shorter reaction times, no reaction occurs. The existence o f an induction or i n h i b i t i o n period would explain why benzothiophene was unreactive when using hydroxide mixtures containing l e s s than 40 wtjt KOH. In the previous s e t o f e x p e r i ments (Figure 1), a l l experiments were conducted f o r 30 minutes. At lower KOH concentrations, 30-minute reaction times f e l l w i t h i n the induction or i n h i b i t i o n period, and therefore no reaction occurred. A hydroxide mixture containing l e s s than 40 w t j o f KOH was chosen a r b i t r a r i l y (30 wtj) and allowed to react a t 375°C f o r 1 hour to determine i f benzothiophene would decompose a t longer react i o n times. After 1 hour, benzothiophene completely decomposed to o - t h i o c r e s o l and toluene. These r e s u l t s demonstrate that hydroxide mixtures containing lesser amounts o f KOH have an induction or inh i b i t i o n period longer than 30 minutes. Additional evidence to support t h i s hypothesis was obtained by conducting reactions with benzothiophene and pure KOH a t d i f f e r e n t reaction times. These r e s u l t s (Figure 2) show that the induction or i n h i b i t i o n period decreases s i g n i f i c a n t l y when KOH alone is used and further suggests that the r e l a t i v e amount o f KOH can influence the length o f the induction or i n h i b i t i o n period. Most important, the existence o f an induction or i n h i b i t i o n period suggests a f r e e - r a d i c a l step in the decomposition o f the thiophene r i n g . Further evidence f o r the f r e e - r a d i c a l nature o f the reaction was obtained from experiments conducted under l e s s severe conditions in order to i s o l a t e the i n i t i a l ring-opened intermediate before subsequent loss o f the one-carbon fragment. Efforts to i s o l a t e the i n i t i a l decomposition product were unsuccessful. Apparently, the loss o f the one-carbon fragment occurs r a p i d l y , consistent with a f r e e - r a d i c a l chain reaction o f some type. While some s p e c i f i c r o l e o f the K cation may account f o r the increased r e a c t i v i t y o f benzothiophene with increasing amounts o f KOH in the hydroxide mixture, the possible r o l e o f the t o t a l base, KOH, cannot be neglected. Potassium hydroxide is a stronger base and nucleophile in t h i s system than NaOH is (JO), and the increased b a s i c i t y and n u c l e o p h i l i c i t y could account f o r increased r e a c t i v i t y toward benzothiophene decomposition. The chemical nature o f i o n i c melts is not f u l l y understood. While the hydroxide melts are believed to be f u l l y dissociated ( V O , explanations have a l s o been given f o r the formation o f " q u a s i c r y s t a l l i n e states" ( J 2 ) , where order within the melt e x i s t s and d i s s o c i a t i o n is not complete. I t is d i f f i c u l t to deduce how much independent freedom K and OH" have and i f e i t h e r the K cation or KOH or both are the important species. To determine the p o t e n t i a l r o l e o f the potassium c a t i o n , experiments were conducted in which d i f f e r e n t potassium s a l t s were substituted f o r KOH. The two s a l t s examined were KC1 and K 2 C O 3 . The possible r o l e o f KOH cannot be eliminated because o f the react i o n s shown in Equations 3 and 4. +

+

+



56


Figure 1. Reactions o f benzothiophene with molten KOH/NaOH mixtures a t 375°C f o r 30 minutes, benzothiophene, — * ; o-thiocresol, · ; toluene, ····•····.

Figure 2. Induction Period - molten hydroxide treatment o f benzothiophene (reaction temperature - 375°C). KOH:NaOH (60:40), ο KOH, — · — . ;


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Molten Hydroxide Coal Desulfurization (3)

NaOH + K C l ^ K O H + N a C l 2 NaOH + K C O ^ 2 K O H + N a C O


2

s

2

s

(4)

By c a l c u l a t i n g the enthalpy o f the reaction and assuming no s i g n i f i c a n t entropy change, the ΔΗ (based on heats o f formation and heats o f fusion) o f Equation 3 is approximately +5.2 kcal/mole and of Equation 4 is approximately +10.5 kcal/mole. While both reac t i o n s are not favored thermodynamically, a t high temperatures ( i . e . , 375°C), these reactions w i l l e s t a b l i s h an equilibrium where s i g n i f i c a n t amounts o f KOH and/or K+ and OH" may e x i s t . Benzothiophene experiments conducted a t 375°C f o r 30 minutes with KCl-NaOH mixtures (70:30 by wt) resulted in no decomposition or d e s u l f u r i z a t i o n . Experiments conducted with K C0s-Na0H mixtures (70:30 by wt) resulted in complete decomposition o f benzothiophene, y i e l d i n g o - t h i o c r e s o l and toluene as products. Relative amounts o f the two products were s i m i l a r to those found in experiments that used the K0H-Na0H mixture. Experiments with the KCl-NaOH mixture were repeated a t longer reaction times (1 and 3 hours). After 1 hour, very l i t t l e decomposition o f benzothiophene had occurred. A f t e r 3-hour reaction times, the majority o f benzothiophene had decomposed to toluene ( 4 % ) , o-thiocresol (26J), and t o l y l d i s u l f i d e (23%). While the y i e l d o f t o l y l d i s u l f i d e (an oxidation product o f o - t h i o c r e s o l ) was somewhat unexpected, the longer reaction times demonstrate that KCl-NaOH mixtures can cause benzothiophene decom position. Again, the induction or i n h i b i t i o n period may account f o r the lack o f KCl-NaOH r e a c t i v i t y using 30-minute reaction times. Results o f experiments with benzothiophene and pure NaOH serve to emphasize the difference between NaOH and KOH. Even a t reaction times o f 6 hours, no benzothiophene decomposition was observed with NaOH. These r e s u l t s not only emphasize the r o l e o f KVK0H but suggest that Κ+/Κ0Η is a necessary part o f the hydroxide mixture f o r decomposition and ultimate d e s u l f u r i z a t i o n o f thiophene-ring systems. From t h i s , it can be seen that the amount o f KOH within the hydroxide mixture would probably be critical in removing organo s u l f u r from c o a l . While the p a r t i c u l a r r o l e o f KOH has not been determined, evidence from the l i t e r a t u r e has shown that the s i z e o f the cation may be important in s t a b i l i z i n g intermediate carbanions. Wallace e t a l . (J^) conducted a s e r i e s o f base- catalyzed, betae l i m i n a t i o n reactions with isopropyl s u l f i d e and measured the amount o f o l e f i n production. The proposed mechanism involved i n i t i a l a b s t r a c t i o n o f a proton by the t-butoxide base, and forma t i o n o f a carbanion, with subsequent e l i m i n a t i o n o f the s u l f u r moiety (which can be considered a good leaving group) to form the o l e f i n (Equation 5 ) . 2

Fossil Fuels Utilization - American Chemical Society

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