Energy & Fuels 1995,9, 1097-1103
1097
Hydrothermolysis of a Silica-Immobilized Diphenylethane Robert D. Guthrie" and Sreekumar Ramakrishnan Department of Chemistry University of Kentucky, Lexington, Kentucky 40506
Phillip F. Britt and A. C. Buchanan, I11 Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831
Burtron H. Davis Kentucky Center for Applied Energy Research, 3572 Iron Works Pike, Lexington, Kentucky 4051 1 Received July 10, 1995. Revised Manuscript Received September 8, 1995@
The compound 1-(4'-hydroxypheny1)-2-phenylethaneattached through an Si-0-Ar linkage to fumed silica was subjected to thermolysis (390-410 "C) under Dz and NZ(14 MPa). Product distributions under DZare similar to those previously observed in vacuum except for increases in yields of benzene and phenol expected as a consequence of hydrocracking. The high pressure of Dz does not inhibit retrogressive reactions (rearrangement, cyclization) previously found t o be promoted by restricted mass transport conditions. A demonstrated silica-promoted H-D exchange of small amounts of phenolic compounds present in equilibrium with surface-attached materials under the closed, high-pressure reactor conditions required for DZ experiments complicates interpretation of deuterium distribution information. However, non-hydroxylated aromatic rings are immune t o this process and measurement of their D incorporation shows that the normal D-atom-promoted exchange is more efficient in the gas phase than on the silica surface.
Introduction Recently, we have examined the hydrothermolysis of several coal-model compounds with DZat temperatures from 300 to 450 "C and pressures of 14 MPa in glass tubes in the absence of catalysts. Studies have been carried out with l,2-diphenylethane (bibenzyl), DPE, various deuterium-labeled diphenylethanes, and 1,2,3,4tetrapheny1butane.l In the absence of high pressures of Hz or Dz, DPE decomposes by the free-radical pathway shown in eqs 1-9. The main products are
e
PhCH2CHzPh 2PhCHi DPE PhCH:+ DPE PhCHCH2Ph+ PhCH3 DPE* 2DPE* PhCH2C-CCHzPh
triphenylpropane, with the latter two compounds undergoing slow thermolysis under the reaction conditions.2 In the liquid phase, 1,l-diphenylethane yield becomes significant.2 In the presence of a high pressure of Dz, the results fit a mechanism wherein thermolysis produced radicals, eq 1,react via an energetically uphill reaction with Dz, for example, eq 10, and the D atoms DPE'
+ D, - PhCHDCH,Ph + D'
(10)
produced add reversibly to aromatic rings, eq 11, to
(1)
---.)
2DPE*
-
DPE* + PhCH; DPE* + PhCH; Ph2CHCH;
I
1
Ph Ph PhCHZCHPh + DPE STB PhCHz-CH-CH2Ph
-
I
Ph STB + PhCH3
-
DPE* *Ph2CHCH; + DPE PhzCHCH3+ DPE*
DPE*
-
(3)
produce D/Hexchange and also displace alkyl radicals, eq 12, in the manner suggested by V e r n ~ n .Similar ~
(4) (5)
(6) (7) (8)
fi +H*
toluene, stilbene, 1,2,3,4-tetraphenylbutane, and 1,2,3@Abstractpublished in Advance ACS Abstracts, October 15, 1995.
processes explain the uncatalyzed reaction of 14442'phenylethy1)benzyllnaphthalenewith Dz.4 In the absence of a termination process, the sequences of eqs 11 ( l ) ( a ) Guthrie, R. D.; Shi, B.; Sharipov, R.; Davis, B. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1993, 38, 526-533. (b) Rajagopal, V.; Guthrie, R. D.; Shi, B.; Davis, B. H. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1993,38, 1114. (c) Ramakrishnan, S.; Guthrie, R. D.; Shi, B.; Davis, B. H. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1993,38,1122. (d) Guthrie, R. D.; Shi, B.; Rajagopal, V.; Ramakrishnan, S.; Davis, B. H. J. Org. Chem. 1994,59,7426-7432. (2) Poutsma, M. L. Fuel 1980,59,335. (3) Vernon, L. W. Fuel 1980,59,102-106. (4)Shi, B.; Ying, J.; Guthrie, R. D.; Davis, B. H. Energy Fuels 1994, 8, 1268-1275.
0 1995 American Chemical Society 0887-0624/95/2509-1097$09.00/0
1098 Energy & Fuels, Vol. 9, No. 6, 1995
Guthrie et al.
and 13 or 12 and 13 constitute a kinetic chain reaction
H or R'
+ D,-
HD or RD
+ D'
(13)
which could completely exchange all aromatic ring hydrogens present or "hydrocrack all aryl-alkyl bonds. However, chain termination interferes, we believe, mainly by radical disproportionation, eqs 4 and 6, resulting in a situation where exchange and "hydrocracking'' reactions remain comparable in rate to the initial cleavage, eq 1. In an effort to limit termination by radical disproportionation, we studied 2,2,5,5-tetramethyl-3,4-di~henylhexane,~ a compound for which a reaction analogous to eq 1 produces phenylneopentyl radicals. These, having no ,!?-hydrogens,cannot serve as H-donor radicals in disproportionation reactions. However, even the reaction of this substrate finds a route to termination through formation of alkenes which react with D atoms t o give radicals which can disproportionate. When hydrothermolysis is used for conversion of nonvolatile materials such as coals to liquid fuel, a major difference between these "real" hydroliquefaction reactions and those involved in all of the model studies to date is that the "real" reactions take place largely in condensed phases. In the initial stages of coal hydroliquefaction, it seems likely that reactions occur at solid surfaces or in amorphous polymeric phases where the substrate molecules have limited diffusional freedom. We became interested in attempting t o model such systems. Previous studies by Buchanan, Poutsma, Britt, and their co-workers had investigated the role of restricted mobility in thermolysis by attaching model compounds to silica through Si-0-Ar linkagese6These authors demonstrated that for surface-attached radicals, rearrangement and cyclization reactions became more competitive with termination steps. It therefore seemed logical and important to examine the effect of restricted mobility on the hydrothennolysis reaction. For this initial s t ~ d y we , ~ used materials prepared by the original procedure devised by Buchanan and and techniques for hydrothennolysis developed by Guthrie and co-workers.'
Experimental Section General Procedure. The procedure for attachment of 1-(4'-hydroxyphenyl)-2-phenylethane to fumed silica (Cab-0Si1 M-5, Cabot Corp., 200 m2/g,ca. 1.5 mmol OWg) to produce SiODPE has been described earlier.6a Batches were prepared with surface coverages of 0.51,0.32,and 0.09 mmovg. The techniques for thermolysis of these materials under Nz or Dz are basically those used for free substrates, described previously in which the material to be subjected to hydrothermolysis is placed in a ca. 12 mL glass bulb connected t o a ca. 17 cm capillary (1-2 mm i.d.1.l In the case of derivatized Cab-OSil, 400-500 mg of solid was placed in the tube with a plug of glass wool to prevent the solids from being lost through the (5)(a) Sharipov, R.; Guthrie, R. D.; Shi, B.; Davis, B. H. Prepr. Pap.-Am. Chem. SOC.,Diu.Fuel Chem. 1993,38, 1129. (b) Guthrie, R. D.; Sharipov, R. V.; Ramakrishnan, S.; Shi, B.; Davis, B. H. J . O g . Chem. 1995,60,4504-4509. (6) (a) Buchanan, A. C., 111, Dunstan, T. D. J.; Douglas, E. C.; Poutsma, M. L. J . Am. Chem. SOC. 1986, 108, 7703-7715. (b) Buchanan, A. C., 111; Britt, P. F.; Biggs, C. A. Energy Fuels 1990,4, 415-417. (c) Buchanan, A. C., 111.; Biggs, C. A. J . Org. Chem. 1989, 54, 517-525. ( c ) Britt, P. F.; Buchanan, A. C., I11 J.Org.Chem. 1991, 56, 6132-6140. (7) Guthrie, R. D.; Ramakrishnan, S.; Britt, P. F.; Buchanan, III., A. C.; Davis, B. H. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1994, 39, 668.
capillary. A few glass beads were added to provide agitation when the tube was shaken. The tubes were placed inside a stainless-steel tube reactor and, after being placed under a cold pressure of gas of ca. 5.6 MPa, were shaken a t reaction temperature in a fluidized sand bath. After reaction, volatile materials were removed under vacuum (2 x Torr and 80100 "C) into a liquid Nz-cooled trap and surface-attached materials were removed by basic hydrolysis, extraction, and trimethylsilylation as described p r e v i o ~ s l y . ~An " ~ ~external standard, 4-hydroxybiphenyl or biphenyl, was added t o each fraction and analysis carried out by gas chromatography with FID detection (HP5890) using experimentally determined response factors where possible. Recovery of total products ranged from 75 to 90%. While there may have been some net loss of volatile products either through the capillary tube extension during the reaction or during workup due to loss of volatiles during pumping, there was no evidence for any major selective loss of volatile products in that, for the most part, the yield of benzene was comparable to that of its surface counterpart, ethylphenol, etc. Products from reactions run under Dz were analyzed for deuterium content by GUMS, using parent ions (M+)or major fragment ions (M+ - 15 was particularly helpful for trimethylsilylated materials) taking into account normal isotopic abundances of 13C, etc., and M+ - 1fragments where significant as determined using authentic samples. Control experiments in which phenol-de was subjected to workup conditions (treatment with 1 M NaOH for 14 h followed by acidification and extraction) showed no loss of deuterium from ring positions in recovered phenol, analyzed after trimethylsilylation. One difference between product distributions in these reactions carried out under Dz and previous studies of vacuum thermolysis of SiODPE was the presence of significant quantities of free phenolic compounds after the reaction. Although small amounts of free phenolics were present in the vacuum thermolysis of SiODPE, amounts were significantly larger under Dz. One possibility is that water, which is known to be released continuously as fumed silica is heated (product literature indicates 0.5% weight loss as uncoated silica is heated from 110 to 400 "C), remains in the reaction zone under our high-pressure conditions but is pumped out of the evacuated vessel used in simple thermolysis experiments. Experiments wherein the reaction tubes were evacuated to remove surface moisture prior to hydrothermolysis were indistinguishable from those for which this was not done. It is also possible that Dz is involved in the process, possibly by adding to siloxanes formed when HO-Si-0-Si-OAr groups equilibrate with siloxanes and HOAr. The data in Tables 2-4 have been separated for HOAr (free phenols from the volatile fraction) and TMSOAr (phenols isolated and trimethylsilylated after hydrolysis of recovered silica). Exchange of Phenol-& with Cab-0-Sil. The following is a typical procedure. A mixture of Cab-0-Si1 (dried a t 200 "C for 24 h, 3.27 g) and phenol-de (32.5mg, 0.322mmol) was heated in a sealed tube a t 399 "C for 2 h. The deuterium distribution in the recovered phenol after trimethylsilylation dz = 39.6%,d~ = 41.6%, (TMSOPh)was do =
