Energy & Fuels 1989,3, 465-468
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Relative Importance of Thermolysis and Hydrogenolysis Processes: "Liquefaction"of a Bibenzyl Polymer Ripudaman Malhotra,* Donald F. McMillen, Doris S. Tse, and Gilbert A. St.John Department of Chemical Kinetics, Chemistry Laboratory, SRI International, Menlo Park, Ca1 i f ornia 94025 - 3493 Received January 5, 1989. Revised Manuscript Received April 17, 1989
A polymeric analogue of bibenzyl, poly( 1,4-dimethylenenaphthalene),has been subjected to coal liquefaction conditions, and the products have been analyzed by GC and field ionization mass spectrometry (FIMS) in order to assess the competition between spontaneous thermal scission of the weak central bonds and hydrogenolysis of the 97 kcal/mol alkyl-aryl linkages. Reaction for 1 h at 400 "C in tetralin, 9,10-dihydrophenanthrene,or 9,lO-dihydroanthracene solvent systems results in complete solubilization, yielding product mixtures consisting primarily of varying amounts of low molecular weight oligomers of the starting polymers. Analysis by FIMS reveals, via the carbon atom substitution on each oligomer, that weak-bond scission and strong-bond hydrogenolysis are roughly competitive notwithstanding the fact that weak-bond scission in monomeric analogues under similar conditions is -30 times faster than observed here, while hydrogenolysis of strong alkyl-aryl linkages is 10 times slower than observed here. This 300-fold change in relative rates is most reasonably attributed to cage recombination and chain hydrogenolysis effects that are enhanced in the polymeric substrate.
-
Introduction Coal conversion research continues to be governed to a large extent by the paradigm that bond cleavage results from thermolysis of weak bonds followed by scavenging of the thermally generated radicals, although there is a growing recognition of other processes that could also lead to bond cleavages.' A few years ago, we presented a scheme for coal liquefaction that included hydrogenolysis of strong bonds resulting from transfer of a hydrogen to the ipso position of a critical linkage.2 The H-transfer step can take place by several pathways, and we have shown that the competition between the various pathways can impact technologicallyimportant aspects of coal conversion such as efficiency of hydrogen ~tilization.~The evidence for the hydrogenolysis of strong bonds has come from studies with simple model compounds, such as diarylmethanes and diaryl ether^,^.^ as well as froxh hybrid studies in which known structures were either mixed with4 or grafted onto6coal. We now wish to report a brief study on the "liquefaction" of a bibenzyl polymer that was originally designed to cleave by the thermolysis of the weak C&-C& bond. The results show that, under liquefaction conditions, comparable amounts of products are formed by hydrogenolysisof the much stronger C,-C, bond, even though previous liquefaction studies with monomeric analogues would suggest that the weak-bond thermolysis should be several hundred times faster. As an analogue of bibenzyl, poly( 1,4-dimethylenenaphthalene) has a weak Celk-Cdk bond (DHO = 55 kcal/mol) that is estimated to have a thermolysis half-life (1) For example, see the Introduction by the editor and review articles in: The Chemistry of Coal Conversion; Schlosberg, R. H., Ed.; Plenum Press: New York, 1985. (2) McMillen, D. F.; Malhotra, R.; Hum, G. P.; Chang, S.-J. Energy Fuels 1987, I , 193. . (3) Malhotra, R.;McMillen, D. F. Prepr. Pup.-Am. Chem. SOC.,Diu. Fuel Chem. 1988,33(3), 339. (4) McMillen, D. F.;Malhotra, R.; Chang, S.-J.;Ogier, W. C.; Nigenda, S. E.; Fleming, R. H. Fuel 1987, 66, 1611. (5) Kamiya, Y.;Koyanagi, S.; Futamra, S. Prepr. Pup.-Am. Chem. SOC.,Diu. Fuel Chem. 1988, 33(3), 300; Fuel 1988, 67, 1436. (6) Choi, C.-Y.; Stock, L. M. J . Org. Chem. 1984, 40, 2871.
of about 5 min at 400 OC.'+ Solomon and co-workers have studied the behavior of this polymer under pyrolysis conditions.1° The products were analyzed by using field ionization mass spectrometry (FIMS), and the relative amounts of monomers, dimers, and other oligomers were used to obtain rate constants for a model of coal pyrolysis that consists of a set of first-order bond dissociation reactions with a distribution of activation energies. However, examination of the FIMS data from that work shows that even under pyrolysis conditions, about 15% of the products resulted from the scission of the stronger C,-C, bond (DHO = 97 kcal/mol). As the results presented below illustrate, this trend toward *unexpected" hydrogenolysis is further enhanced under liquefaction conditions.
Experimental Procedure The poly(l,4-dimethylenenaphthalene)was obtained from Advanced Fuel Research, East Hartford, CT. Tetralin, 9,lOdihydrophenanthrene,9,10-dihydroanthracene,and anthracene were all obtained from Aldrich and used without further purification. The reactions were conducted in evacuated fused silica ampules, which were placed in steel tubing pressure vessels and heated by immersion in a molten-salt bath maintained at 400 (*2) "C. The amounts (mg) of polymerfdihydroaromaticfaromatic were 23/102/0 (dihydrophenanthrene), 12/52/0 (dihydroanthracene),and 10145146 (dihydroanthracenefanthracene). The product mixtures were analyzed by capillary GC and by FIMS. The FIMS analyses were performed on an instrument, constructed at SRI, that is equipped with a highly stable foil-type ionizer. The instrument and procedure have been previously described."J2 (7) McMillen, D. F.; Golden, D. M. Hydrocarbon Bond Dissociation Energies. Annu. Reu. Phys. Chem. 1982, 33, 497. (8) Stein, S. E. A Fundamental Chemical Kinetics Approach to Coal Conversion. In New Approaches in Coal Chemistry; Blaustein, B. D., Bockrath, B. C., Friedman, S., Eds.; Advances in Chemistry 169; American Chemical Society: Washington DC, 1981; p 97. (9) Barton, B. D.; Stein, S. E. J . Phys. Chem. 1980,84, 2141. (IO) (a) Squire, K. R.; Solomon, P. R.; Carangelo, R. M.; DiTaranto, M. B. Fuel 1986,65, 833. (b) Solomon, P. R. Synthesis and Study of Polymer Models Representative of Coal structure. GRI Final Report under Contract No. 5081-260-0582, 1983. (c) Squire, K.R.; Solomon, P. R.; DiTaranto, M. B.; Carengelo, R. M. Prepr. Pup.-Am. Chem. SOC., Diu. Fuel Chem. 1985, 30(1),1985.
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466 Energy & Fuels, Vol. 3, No. 4, 1989 2.30
f
5 d 8
IIIIII 100
2w
XI0
-
/I 1 1 1 1 1 4w
500
600
m
MASS (Wzl
Figure 1. FI mass spectrum of "liquefaction"product of a bibenzyl polymer showing that hydrogenolysis of strong bonds competes effectively with thermolysis of weak bonds during liquefaction. Conversion conditions: 400 "C, 1 h, in dihydro-
phenanthrene.
Results and Discussion The extent of solubilization and bond cleavage of poly(1,4-dimethylenenaphthalene)was determined under liquefaction conditions with several different donor solvents and no added H2 pressure. The starting polymer, which is reported to have a number-average molecular weight of 10000 (corresponding to 64 monomer units),lob is insoluble in tetralin, but after 1 h at 400 "C in donor solvents, the product was completely soluble (i.e., the "liquefaction" was complete). The product was analyzed by GC and FIMS and was shown to consist largely of monomeric, dimeric, trimeric, and tetrameric species; higher homologues accounted for less than 10% of the product. Because tetralin and its artifacts interfered with the mass spectral analysis of the cleavage products, we discontinued the use of this solvent for this study. Further experiments were conducted with dihydroanthracene, dihydrophenanthrene, and a 5050 mixture of anthracene and dihydroanthracene as solvents. Analogous results were obtained with these solvents. As discussed below, results from these solvents also allow us to examine the impact of solvent composition on the relative importance of weak-bond thermolysis and strong-bond hydrogenolysis. Figure 1 shows the FI mass spectrum of the product obtained in dihydrophenanthrene. The spectrum was obtained by heating the product in the probe of the mass spectrometer to a maximum of 250 "C. Independent experiments with the polymer itself were used to establish that the conditions of the analysis do not contribute to any thermal degradation products. The spectrum shows clusters of peaks in the mass ranges for monomeric, dimeric, trimeric, and tetrameric species. The clusters are a result of varying degrees of alkyl substitutions and formation of hydrogenated products. Cleavage resulting from thermolysis of weak bonds and subsequent scavenging of the radicals would result in the formation of bibenzylic oligomers with methyls at each of the terminal naphthalene units; thus the species formed would have the general formula CH3(NapCH2CH2),NapCH3,that is, with two terminal methyls. However, cleavage resulting from hydrogenolysis of the C=-C& bond would lead to species with any number of terminal alkyl groups ranging from 0 to 2, distributed between methyl and ethyl. As can be seen in the figure, the peaks nominally corresponding to the (11)StJohn, G.A.; Buttrill, S. E. Jr.; Anbar, M. In Organic Chemistry of Coal; Larsen, J., Ed.; ACS Symposium Series 71;American Chemical Society: Washington, DC, 1978; p 223. (12) McCormick, R. L.;Baker, J. R.; Haynes, H. W.; Malhotra, R. Energy Fuels 1988, 2, 740.
thermolysis products are flanked by peaks for hydrogenolysis products, which together are even more intense than the central peak.13* In this case, -50% of the product is formed as a result of hydrogenolysis. The spectrum also shows some very intense peaks due to phenanthrene and its hydrogenated analogues, as well as some dimerized solvent species in the vicinity of mass 356. The presence of dimers and higher oligomers of 1,4dimethylnaphthalene in the product mixture is surprising, because the reaction was conducted for over 10 half-lives based on the estimated cleavage rate of 1,2-dinaphth-lylethane. Their presence is very telling evidence for the recombination of the geminate radical pairs, a process that is more likely in a polymeric system (and in coal), and results in a severe attenuation of the net cleavage reaction by thermolysis. From the distribution of monomers, dimers, trimers, and tetramers in the product, which account for about 90% of the starting polymer (ii = 64), we can estimate that about 50% of the linkages in the original polymer have been broken (including those by hydrogen~ l y s i s ) . 'Since ~ ~ the estimated half-life for thermolysis (of each linkage) is about 5 min (based on the measured rates in closely related stru~tures'-~~J~), the net thermolysis rate appears to be attenuated by about 30 times. Squire et al.'O" were also led to invoke recombination of greater than 90% of the radicals in order to model the distribution of the products from the pyrolysis of this polymer. This attenuation in polymeric systems, with and without added scavengers, is consistent with, but decidedly greater than, the attenuation factor of 1.3 reported by Stein for thermolysis of bibenzyl in teralin at 400 "C.14 In contrast to its effect on thermolysis, the heterogeneous nature of the polymeric system was expected to enhance the hydrogenolysis process. First, reversal of the hydrogenolysis is less likely (because it requires trapping of a very short-lived intermediate); therefore, one expects hydrogenolysis to be much less subject to the attenuation effect noted for simple thermolysis. Second, the relative importance of hydrogenolysis over thermolysis could be still greater in polymeric (or heterogeneous) systems to the extent that, after scavenging a polymeric radical, a dihydroaromatic solvent molecule is converted into a cyclohexadienyl radical that would be present in the vicinity of the polymeric backbone-ready to perform hydrogenolysis. Evidence for such enhancement can also be found in the pyrolysis of silica-supported bibenzyl.15J6 (13) (a) Note that while an odd number of carbon atoms on an oligomer cannot result from any combination of weak-bond thermolyses, an even number can result from a combination of two strongbond cleavage reactions. Thus some portion of the central peaks is due to such combinations. Further, some fraction of the oligomers are expected to have single methylene linkages resulting from ipso displacement by benzylic radicals rather than H atoms. These products of course cannot be distinguished from isomeric H atom displacement products merely by their molecular ions. However, since displacement by benzylic radicals will always lead to one product having two naphthalene rings connected by a strong methylene bridge, the dominance of and the carbon-substitution pattern of the monomeric products indicate that displacement by benzylic radicals is minor compared to displacement by H atoms.'& In any case, the general point of this paper, namely the rather surprising importance of addition-elimination reactions cleaving very strong bonds, even for structures where every cluster is joined by a weak bond, would remain unchanged. However, the question of the competition between displacement by H atoms and displacement by carbon-centered radicals is important, since it bears directly on the importance of retrograde reactions in coal liquefaction. We will address this question in future publications. (b) We estimated the fraction of linkages cleaved by simply counting the number of linkages left in the product monomers (none), dimers, trimers, and tetramers obtained from an assembly of 100 polymer molecules (Is = 64) as a starting mixture. (14)Stein, S.E.;Robaugh, D. A.; Alfieri, A. D.; Miller, R. E. J. Am. Chem. SOC. 1982,104,6567. (15) Buchanan, A. C.; Dunstan, T. D. J.; Douglas, E. C.; Poutama, M. L. J. Am. Chem. SOC. 1986, 108, 7703.
"Liquefaction" of a Bibenzyl Polymer
Energy & Fuels, Vol. 3, No. 4, 1989 467
Table I. "Liquefaction"of a Bibenzyl Polymer in Donor Solvents at 400 OC
% dihydro-
solvent dihydrophenanthrene (100%) dihydroanthracene (100%)
anthracene/dihydroanthracene(5050)
aromatic remaining 37 2.5 12
% linkages cleaved 52
39 46
The extent of acceleration of the hydrogenolysis in the polymeric system can be estimated by comparing the observed rate with that for the hydrogenolysis of 1,2'-dinaphthylmethane (DNM). In both cases, the cleavage follows transfer of hydrogen to a naphthalene moiety. We have previously found the cleavage of DNM in a 5050 mixture of anthracene and dihydroanthracene proceeds with a defined first-order rate constant of 7.1 X lo+ s - ~ . ~ Because roughly half of the bonds in.the polymer were cleaved during the 1-h reaction and about half of the product results from hydrogenolysis, the observed rate constant for hydrogenolysis is roughly 8.0 X 10" s-', which translates into approximately an 11-fold acceleration. Thus a hydrogenolysis process expected to be some 300 times slower than the weak-bond thermolysis has, in the polymeric system, become as fast as thermolysis. From the FI mass spectra we can chart the progress of four different liquefaction-related changes in this model system. There are (1) percent linkages cleaved, (2) the ratio of thermolysis to hydrogenlysis, (3) the radio of reduction of the polymer to cleavage of the linkages, and (4) the formation of retrograde products from the solvent components. Table I summarizes these factors for conversion of the polymer in three different solvent systems and illustrates how the various criteria for conversion effectiveness can be countervailing. If we use the percent of linkages cleaved as a measure of the liquefaction ability, dihydrophenanthrene is a more effective solvent than dihydroanthracene. Dilution of the dihydroanthracene with an equal amount of anthracene, a nondonor, appears to increase the overall effectiveness of the solvent and the thermolysis/ hydrogenolysis ratio. While it may seem surprising that either thermolysis or hydrogenolysis rates would increase when the initial dihydroanthracene concentration is decreased, these trends are in fact consistent with previous work on H-transferinduced bond s~ission.'~J~ The key observation is that the presence of PCAH in the initial solvent dramatically slows the wasteful loss of dihydroaromatic, in this case to tetrahydroanthracene and solvent dimers. Four times as much dihydroanthracene remained after 1h of reaction when the dihydroanthracene was diluted with anthracene! Similar crossovers in hydroaromatic content have been observed in other model compound work in this laboratoryla and have also been reported in actual coal liquefaction studies.lg This surprising result is explained by the shifting competition (16) McMillen, D.F.;Malhotra, R.; Nigenda, S. E. Fuel 1989,68,380. (17) McMillen, D. F.;Malhotra, R. 1987Internationol Conference on Coal Science; Moulijn, J. A., Nater, K. A., Chermin, H. A. G., Eds.; Elsevier: Amsterdam, 1987; p 193. (18) McMillen, D.F.;Malhotra, R.; Tse, D. S. Interactive Effects Between Solvent Components: Possible Chemical Origin of Synergy in Liquefaction and Coprocessing. Submitted for publication in Energy Fuels. (19) Mochida, I.; Yufu, A.; Sakanishi, K.; Korai, Y. Fuel 1988,67, 104.
thermolysis/ hvdrogenolvsis 0.84 1.10 1.28
reduction/ cleavage
0.38 0.04
0.02
solvent dimers/solvent monomers 0.01 1.27 0.20
between free H atoms and radical hydrogen transfer as the PCAH content of the solvent increases.16J8 The result of this crossover in [AnH2]in the present case is that both the steady state concentration of the carrier species, which is proportional to ([An][AnH2])1/2,and the scavenging ability, which is proportional to [AnH2],should be a factor of 2 larger at the end of the 1-h experiment. Therefore, it is entirely reasonable that the overall cleavage effectiveness should have risen significantly upon replacement of half the initial AnH2 by its parent PCAH. Two other measures of the solvent system's performance are the yield of reduction products and the extent of solvent dimerization. The fraction of the tetrahydro analogues of the total cleavage products is also given in Table I. Dihydrophenanthrene is a stronger reducing agent than dihydroanthracene (phenanthrene being a thermodynamically more stable aromatic than anthracene), and hence, the greater amounts of the tetrahydrooligomers are as expected. Addition of anthracene to dihydroanthracene decreases the net reducing ability of the system, and this fact is reflected in the somewhat lower amounts of the tetrahydro analogues of the polymer cleavage products. A measure of the solvent's propensity to undergo coupliig reactions in general can be derived from the observed extent of solvent dimerization. As is well recognized, anthracene systems tend to show much greater coupling than the phenanthrene system. In the anthracene systems, it appears that the presence of anthracene reduces the solvent's tendency to couple. From the fact that greater amounts of tetrahydro derivatives of cleavage products are formed when the reaction is started with neat dihydroanthracene, we expected to fiid greater amounts of tetrahydroanthracene also in the product mixture from this solvent than in the product from the anthracene-dihydroanthracenesolvent system. However, less tetrahydroanthracene is actually present in the product when the reaction is started with neat dihydroanthracene. Resolution of this paradoxical result is possible if tetrahydroanthracene was indeed formed but further reacted to give other products. This surmise is consistent with the reported intermediacy of tetrahydroaromatics in coupling reactionsmand with the 6-fold higher ratio of solvent dimers/solvent monomers seen here when the reaction was started with neat dihydroanthracene as the solvent. Conclusions The results described here show that, in polymeric systems, there is a marked tendency for thermolysis reactions to be slower, and hydrogenolysis reactions to be faster, than would be expected on the basis of a simple transfer of rates measured for analogous monomeric substrates. Thus, a combination of attenuation of weak-bond (20) Senthilnathan, V. P.; Stein, S. E. J. Org. Chem. 1988,53, 3000.
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thermolysis by facile recombination and acceleration of alkyl-aryl bond cleavage through chain hydrogenolysis leads, in the cases studied here, to a relative enhancement of the hydrogenolysis process by a factor of about 300. Similar attenuation and acceleration factors would be expected to operate in coals as well. The results also substantiate the claim that the PCAH initially present (in solvents that are not "overhydrogenated") tend to retard the wasteful loss of hydrogen to H2 and to the formation of tetrahydro rings in the substrate and solvent structure. In the case of dihydroanthracene solvent, the initial presence of PCAH also markedly retards the formation of
solvent dimers, which evidently are produced through the intermediacy of tetrahydroanthracene.
Acknowledgment. We gratefully acknowledge financial support for this work by the U.S. Department of Energy under Contract No. DE-FG-2286PC90908. We also thank P. R. Solomon of Advanced Fuel Research for supplying the polymer, which was originally prepared under a grant from the Gas Research Institute. Registry No. CH3(NapCH2CHJ,NapCH3,120638-03-1; h H 2 , 613-31-0; An, 120-12-7; tetralin, 119-64-2; 9,lO-dihydrophenanthrene, 776-35-2.
Reforming-Type Catalysis with Zeolite-Supported PtRe C. Dossi,? C. M. Tsang, and W. M. H. Sachtler* Center for Catalysis and Surface Science, Northwestern University, Evanston, Illinois 60201
R. Psaro and R. Ugo Dipartimento di Chimica Inorganica e Metallorganica, Universitci Milano, Via Venezian, 21, 20133 Milano, Italy Received March 13, 1989. Revised Manuscript Received May 1, 1989
Addition of Re markedly changes the activity, selectivity, and stability of Pt/NaY and Pt/NaHY catalysts for n-heptane conversion at 500 "C and 0.5 MPa. The effect of Re depends strongly on the method of catalyst preparation. With the volatile Re2(CO)locarbonyl as precursor, Re is preferentially deposited in the supercages of the zeolite. In the presence of Pt clusters that act as nucleation Their catalytic signature is a high selectivity sites, bimetallic PtRe clusters are predominantly for deep hydrogenolysis of n-heptane to methane. Formation of PtRe clusters is much lower when a THF solution of the bimetallic carbonyl cluster PtRe2(CO)12is used as the metal precursor. In this case, the product pattern obtained with the reduced catalyst differs little from that of Pt supported on nonacidic Alz03.
Introduction Heterogeneous catalysts containing platinum dispersed on an acidic support, such as chlorided Alz03,have been widely used in naphtha ref~rming.~ The metallic function of these catalysts is improved by adding a second metal, such as Re, Ir, or Sn.4 Pure PtRe alloy catalysts are known to catalyze deep hydrogenolysis of hydrocarbons to methane, but if traces of sulfur are adsorbed on the surface of such alloy particles, the selectivity to aromatics becomes high and formation of graphitic coke is suppressed. The beneficial effect of the combined addition of rhenium and sulfur to supported Pt has been attributed to an ensemble effect; i.e., rhenium binds sulfur strongly (the Re-S bond is stronger than the Pt-S bond) and separates the S-free Pt sites, so that only small metal ensembles are left exposed at the ~ u r f a c e . It ~ has been further proposed that Re or Res, species suppress the deposition of carbonaceous species and/or their conversion to graphitic coke.6 In this model, the metallic function of the catalysts is believed to consist predominantly of bimetallic PtRe particles. In the absence of sulfur or other strongly ad-
* To whom correspondence should be addressed. On leave from Dipartimento di Chimica Inorganica e Metallorganica, Universiti Milano, Via Venezian, 21, 20133 Milano, Italy.
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sorbed modifiers, these PtRe particles can be characterized by their high activity for deep hydrogenolysis to CHI of hydrocarbon^.^,^ The acid sites of the chlorided A1203 support also promote isomerization and aromatization of the catalyst via carbenium ion reaction^.^ Aromatization of linear hydrocarbons can be achieved, however, even in the absence of acidic sites. Selective conversion of C6-C7 paraffins to high-octane aromatics has been recently (1)Tsang, C. M.;Augustine, S. M.; Butt, J. B.; Sachtler, W. M. H. Appl. Catal. 1989,46,45-56. (2)Dossi, C.; Schaefer, J.; Sachtler, W. M. H. J.Mol. Catal. 1989,52, 193-209. (3)Haensel, V. Oil Cas J. 1950,48,82-119. (4)Kluksdahl, H. E. US. Patent 3415737, 1968. Jacobson, R. L.; Kluksdahl, H. E.; McCoy, C. S.; Davis, R. W. Proc. Am. Petr. Inst., Diu. Refin. 1969,504-508. (5) Biloen, P.; Helle, J. N.; Verbeek, H.; Dautzenberg, F. M.; Sachtler, W. M. H. J. Catal. 1980,63,112-118. Sachtler, W. M.H.; Biloen, P. Prep.-Am. Chem. SOC.,Diu. Pet. Chem. 1983,482-490. (6) Shum, V. K.; Butt, J. B.; Sachtler, W. M. H. J. Catal. 1985,96, 371-380;Shum, V. K.; Butt, J. B.; Sachtler, W. M. H. J.Catal. 1986,99, 12fi-129. - - - - - -. (7) Haining, I. H. B.; Kemball, C.; Whan, D. A. J. Chem. Res., Synop. 1977,2056-2077. (8)Augustine, S. M.;Sachtler, W. .M. H. J. Phys. Chem. 1987,91, 5952-595fi. - - - - - - - -. (9)Rek, P. J. M.; den Hartog, A. J.; Ponec, V. Appl. Catal. 1989,46, 213-225.
0 1989 American Chemical Society
