770
Energy I%Fuels 1991,5, 770-771
Aqueous Organic Chemistry. 4. Cleavage of Diary1
Scheme I
Ethers
Sir: Previous studies showed that activated diaryl ethers are unreactive thermally at 350 "C, but aqueous chemistry provides ionic pathways for cleavage of these types of carbon-oxygen-carbon cross-links which are abundant in coals.' We now report that even the unactivated diaryl ethers, l-phenoxynaphthalene and 9-phenoxyphenanthrene, are susceptible to cleavage in water. The reactions are catalyzed by water which is a stronger acid at high temperature (-log K, = 11.20 at 250 "C and 11.30 at 300 "C vs 13.99 at 25 The cleavage reactions in liquid water and subsequent reduction reactions observed in the presence of formic acid and sodium formate are facilitated by the ability of the larger ring systems to undergo C-protonation, which parallels the ability of the corresponding hydroxy derivatives of the polycylic ring systems to tautomerize to the keto form. The importance of keto-enol tautomerism followed by rate-determining homolysis of the cyclohexadienone intermediate has been demonstrated in the thermolysis of an hydroxyphenyl phenyl ether at 400 "C in tetralin.3 There has been speculation concerning the potential ability of water to effect hydrolysis and cleavage of carbon-oxygen-carbon cross-links in coa1.4y6 Graff and Brandese found that steam pretreatment of an Illinois No. 6 bituminous coal between 320 and 360 "C dramatically improved the yield of liquids obtained on subsequent conversion or solvent extraction. The steam modified coals also swelled more in water and contained twice as many hydroxyl groups as the raw coal, leading to the hypothesis that steam reacts with ether linkages in the coal forming hydroxyl groups and thereby substantially hydrolyzes an important covalent cross-link in the coal structure.' These conclusions are consistent with model compound studies on ether reactivity in hot water.' In addition, Bienkowsk? reported that pretreatment of Wyodak subbituminous coal at 240 "C followed by liquefaction at 400 "C in the presence of steam increased conversion by 32%. Khang concluded that steam pretreatment at 320 "C did not enhance volatile yields, but low-rank coals showed increased liquid yields on rapid pyrolysis. Subcritical steam pretreatment of Zap lignite and Wyodak coal over broad temperature and time ranges gave sharp increases in tar yields at shorter pretreatment times. Subsequent liquefaction of the pretreated coals showed no benefit except on samples pretreated for very long (-5 h) times.l0 Mochida" reported an improvement in liquefaction yields in a pyrene solvent system associated with a boiling water pretreatment. Ross and co-workers12 reported improvement in (1) Siskin, M.; Brons, G.; Vaughn, S. N. Energy Fuels 1990, 4 , 488. (2) MarehnU, W. L.; Franck, E. U. J. Phys. Chem. Ref. Data 1981,34, 43. (3) McMillen, D. F.; Ogier, W. C.; Ross, D. S.J. Org. Chem. 1981,46, 3322. (4) Chung, K. E.; Goldberg, I.; Ratto, J. Rockwell Intemational Interim Report No. AP-3889, February 1985. (5) Rosa,D. S.In Coal Science: Gorbaty, M., Larsen, J., Wender, I., Eds.; Academic Prese: New York, 19M Vol. 3. (6) Graff, R. A.; Brandw, S. D. Energy Fuels 1987, 1 , 84. (7) Brandw, 5. D.; Graff, R. A,; Gorbaty, M. L.; Siskin, M. Energy h e & 1989,3,494. (8) Bienkmki, P. R.;Narayan, R.; Greenkom, R. A.; Chao, K. C. Ind. Eng. Chem. Res. 1987,26,202. (9) Khan, M. R.;Chen, W-Y.; Suuberg, E. Energy Fuels 1989,3,223. (10) Serio, M. A,; Solomon, P. R.; Kroo, E.; Charpenay, S. Prep. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1991,36,7. (11) Mochida, I.; Moriguchi, Y.; Iwamoto, K.; Fujitau, H.; Korai, Y.; Takeshita, K. Roc. Jpn./U.S. NSF Chem. Coal Liquefaction Meet. 1986, 25. (12) Rosa, D. S.;Hirechon, A. S. Prep. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1990, S5,37.
1 H'
- HCOiH product quality from donor-solvent liquefaction of a hydrothermally pretreated coal. Pollack and co-~orkers'~ found that pretreatment with water at 300 "C for 30 min resulted in a small but consistent improvement (243%) in methylene chloride and heptane solubles on liquefaction of Illinois No. 6 and Rawhide coals. Tse" and co-workers found that aqueous pretreatment of a Zap lignite and Rawhide subbituminous coal resulted in increased hexane solubles, and when the pretreatment was followed by coprocessing with a Maya crude at 425 "C at 6-890 increase in hexane solubles was obtained. We find that diphenyl ether, l-phenoxynaphthalene, phenanthrene, 9-hydroxyphenanthrene, and 9-phenoxyphenanthrene are unreactive thermally in cyclohexane at 315 "C over 3 days. Diphenyl ether is also unreactive at 315 "C in water and in 15% formic acid over 3 days, but with 15% sodium formate, it undergoes cleavage at 315 "C to phenol (6.6%). In all aqueous experiments a liquid water phase was maintained (7 mL of solution and 1.0 g of ether in an 11 mL T316SS reactor). Pressures ranged from 1500 psi in water to 3300 psi in formic acid. All conversions are in moles as a percent of the starting material. Acid-catalyzed hydrolysis of diphenyl ether in 15% phosphoric acid yields 92% conversion to phenol. As such, we speculate that the cleavage is acid catalyzedl6 with nucleophilic attack of phosphate or formate ion on ortho-protonated diphenyl ether resulting in the formation of two molecules of phenol (Scheme I). l-Phenoxynaphthalene is more reactive than the bis monocyclic system and is cleaved in water at 315 "C after 3 days (94.6% conversion) to phenol (94.6 mol % ) and l-naphthol(92.4 mol %). Small amounts of naphthalene (0.4 mol %) and l-tetralone (1.8 mol %) are presumably formed via reduction of l-naphthol. Formic acid and sodium formate, which are formed in the reaction of coal in the presence of water and carbon monoxide, are known to be strong reducing agents.1620 Under aqueous conditions
-
-
(13) Pollack, N. R.; Holder, G. D.; Warzinski, R. P. Prep. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1991,36, 15. (14) Tse,D. S.;Hirschon, A. S.; Malhorta, R.; McMillen, D. F.; Rosa, D. S. Prep. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1991,36, 23. (15) Independent NMR observation made by Dr. G.P. Miller indicates diphenyl ether is deuterated at least 2 orders of magnitude faster at the ortho than the para position in D20 at 315 O C . Similarly, 1phenoxynaphthalene is deuterated faster at the 2-position of the naphthalene ring than at the two ortho positions of the phenyl ring; deuteration at these positions being at least an order of magnitude faster than at any other position. (16) Gibson, H. W. Chem. Rev. 1969,69,673. (17) Appell, H. R.; Miller, R. D.; Illig, R. G.; Moroni, R. C.; Steffgen, F. W. Report PETC/TR-79/1 (1979).
0887-0624/91/2505-0770$02.50/00 1991 American Chemical Society
Energy & Fuels 1991,5, 771-773
-
Scheme I1
- PhOH
II
H-
0
I-COn
I
OH
they release hydrogen which is a source for partial hydrogenation of aromatics.21t22However, reaction in 15% formic acid at 315 "C completely converted 1-phenoxynaphthalene to phenol (100 mol %) and 1-naphthol (98.6 mol 9%) over 3 days with only traces of naphthalene (0.8 mol %) and 1-tetralone (0.6 mol 9%)formed. Comparison of conversions in formic acid (36.6%) and water (4.5%) after 2 h clearly shows that acid-catalyzed hydrolysis, rather than any reduction properties of formic acid, is important. With 15% sodium formate at 315 "C over 3 days, however, 1-phenoxynaphthalene underwent 24.6% conversion to phenol (24.6 mol %), 1-naphthol (13.0 mol %), 1,2-dihydronaphthalene(5.8 mol %), and naphthalene (5.8 mol %). Thus,reaction in sodium formate was slower than reaction either in acid or water, but reduction was of greater importance, This result is reinforced by similar studies in 15% phosphoric acid where 1-phenoxynaphthalene gives 91.6% conversion in only 30 min with only traces (1.2 mol 9%)of reduction products formed, and in basic calcium carbonate where cleavage and reduction reactions are completely inhibited. Mechanistically (Scheme 11),hot water can act as an acid to protonate 1-phenoxynaphthalene. The protonated intermediate reacts with water to give phenol and 1naphthol. The tautomeric forms of 1-naphthol may undergo reduction to furnish 1,2-dihydro-and 3,4-dihydronaphthol. The former can either lose water to form naphthalene or undergo further reduction to form 1,2dihydronaphthalene. The latter would tautomerize to tetralone. Formic acid and sodium formate are most likely the hydride ion sources for the reduction pathway as shown. (18)Rou, D. S.; N uyen, Q. C. Fluid Phase Equilib. 1983, IO, 319. (19) Rou,D. 9.; I+ cMillsn, ! D. F.; Hum, G. P.; Miin, T. C. DOE/ PC 70811-9, Janua 1987. (20) HorvAth, I. Sirkin, M., rubmittad for publication. (21) Stainberg, V. I.; Wang, J.; Baltisberger, R. J.; Van Buren, R.; W o o b y , N. F. J. Org. Chem. 1978,43, 2991. (22) Baltbbeger, R. J.; Stainbag, V. I.; Wang, J.; Woolsey, N. F. Prep. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1979,24,74.
'%;
77 1
The larger ring system, 9-phenoxyphenanthrene,is much more reactive than 1-phenoxynaphthalene and is cleaved in water at 315 "C in 3 h (97% conversion) to form primarily 9-hydroxyphenanthrene and phenol. A small amount of phenanthrene (4.5 mol %) is also formed. Addition of 15% formic acid resulta in complete conversion to yield phenol (100 mol %), 9-hydroxyphenanthrene (87 mol %), and some additional phenanthrene (13 mol %), most likely by a reductive dehydroxylation pathway. In the presence of sodium formate, the acid-catalyzed cleavage was retarded, yielding phenol and phenanthrene as the only products after 3 days in 47.8% yield. Mechanistically, reduction of the 9-hydroxyphenanthrene product probably proceeds in a manner similar to that as described for 1-naphthol (produced from 1-phenoxynaphthalene). Aquathermolysis of 9-hydroxyphenanthrene at 315 OC for 3 days resulted in the formation of only small amounts of phenanthrene (2.2%). In the presence of reducing systems such as 15% formic acid or 15% sodium formate, however, reductive dehydroxylation was increased to give phenanthrene (63.4 and 98.5%, respectively). Phenanthrene itself was unreactive in water, but in 15% formic acid or 15% sodium formate over 3 days, small amounts of 9,lO-dihydrophenanthrene(0.5% and 1.070, respectively) were observed. This is analogous to previous work on coal related aromatic model compounds carried out in water and carbon monoxide.21i22 Thus, the present results on cleavage of diary1 ethers reinforce our previous conclusion that the presence of water during a coal pretreatment or conversion step will facilitate depolymerization of the macromolecuar structure to give an increased proportion of liquids by cleaving important thermally stable covalent cross-links in the coal structure. Registry No. 1-Phenoxynaphthalene, 3402-76-4; phenol, 108-95-2; 1-naphthol,90-153;%phenoxyphenanthrene,52978957; 9-hydroxyphenanthrene, 484-17-3; water, 7732-18-5. Michael Siskin* Corporate Research Science Laboratory Exxon Research and Engineering Company Route 22 East, Clinton Township Annandale, New Jersey 08801 -0998 Alan R.Katritzky,* Marudai Balasubramanian Department of Chemistry University of Florida Gainesville, Florida 32611 -2046 Received April 17, 1991 Revised Manuscript Received June 7, 1991
XANES Evidence for Selective Organic Sulfur Removal from Illinois No.6 Coal Sir: We have continued our investigation of new methods for the desulfurization of the organic compounds in coal and have used XANES spectroscopic analysis to distinguish between two different classes of organically bound sulfur forms and to follow qualitatively the chemistry of organic sulfur removal in a sample of Illinois No. 6 coal from the Argonne Premium Coal Sample Program (APCSP 3).l (1) Vorres, K.S . Energy Fuela 1990, 4, 420.
0887-0624I91 l2505-0771%02.60/0 0 1991 American Chemical Societv
