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Thermolysis of organic compounds under H2 (D2)
Journal of Analytical and Applied Pyrolysis 54 (2000) 89 – 107 www.elsevier.com/locate/jaap
Thermolysis of organic compounds under H2 (D2) Robert D. Guthrie * Department of Chemistry, Uni6ersity of Kentucky, Lexington, KY 40506, USA
Abstract This paper is a review of our experiments and those of other authors which relate to chemical processes taking place when organic compounds are heated under hydrogen or deuterium gas. For the most part, coverage is confined to reactions believed to directly involve H2, D2 or H and D atoms generated therefrom. However, discussion of some follow-up reactions is required to explain the observed distribution of deuterium in products from reactions under D2. Reactions wherein hydrogen is activated by added inorganic catalysts are avoided except as relevant for understanding uncatalyzed examples. Reactions with surface-immoblilized substrates are discussed and related to reactions of coal with hydrogen gas (‘hydroliquefaction’). © 2000 Elsevier Science B.V. All rights reserved. Keywords: Hydrocracking; Hydrothermolysis; Coal; Liquefaction; Hydroliquefaction; Pyrolysis
1. Introduction Much of the initial interest in the high temperature reactions of organic compounds with H2 was stimulated by their apparent benefit in processes for coal liquefaction. Early in the evolution of coal liquefaction methodology, increased yields of useful products were recognized to result from carrying out the thermolysis in an environment of H2, usually at elevated pressure at temperatures in the range of 400 – 500°C. Early efforts at molecular-level understanding seized upon the potential for obtaining mechanistic details through the substitution of D2 for H2 and analysis of the resulting products. Kershaw and Barrass [1] claim to be the first to use D2 to study the mechanism of coal hydrogenation. Their procedure utilized a catalyst, stannous chloride (1% by weight of coal) and the reaction was carried * Tel.: +1-606-257-7068; fax: + 1-606-323-1069. 0165-2370/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 3 7 0 ( 9 9 ) 0 0 0 8 0 - 7
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out at 450°C for 1 h. Examination of the products by mass spectrometry and nuclear magnetic resonance spectroscopy showed deuterium to be widely distributed in both aromatic and aliphatic positions in the products. Subsequent literature contains ample evidence that deuterium is incorporated into reaction products when thermolysis of coal is carried out under D2 gas even in the absence of deliberately added catalysts [2 – 5]. While these experiments clearly showed that H2 could participate in the thermochemistry of coal, it seemed clear that the complexity of coal’s composition would severely limit the resolution of mechanistic questions, particularly considering that breaking of the strong bond in H2 was likely to be slower than the subsequent processes which distributed H (or D) throughout the coal and its thermolysis products. The problematic energy barrier was removed when deuterium was introduced into the liquefaction milieu in the form of a labeled ‘donor solvent’ [6–10]. However, the details of deuterium distribution were still necessarily very complex. Research thus emphasized the study of structurally homogeneous systems picked to model the internal structures of coal. Much valuable mechanistic delineation of high temperature radical chemistry was justified through its relation to coal liquefaction. A thorough review has been prepared by Poutsma [11]. The contribution which follows is generally restricted to a summary of the direct, uncatalyzed reactions of organic compounds with H2 or D2. The hydrogen-transfer processes which are likely to follow under conditions necessary to produce the initial interaction are discussed to the extent necessary for understanding the fate of the hydrogen introduced. 2. Experimental Hydrogenation experiments carried out by the author were performed in the device shown in Fig. 1. It consists of a thick glass reaction bulb with a long capillary neck. The reaction bulb has a volume of approximately 12 ml and the 1–2 mm i.d. capillary section is approximately 16 cm long. The glass reactor tube is inverted and several glass beads followed by solid reactant are added through the end opposite to the capillary. The reaction vessel is sealed at the constriction as shown in the second drawing in Fig. 1. Liquid reactants may then be added through the capillary end via a long syringe needle. The vessel is then suspended in glass wool in the interior of a stainless steel reactor tube (third drawing in Fig. 1). The capillary section of the tube extends into a separate stainless steel neck (fourth drawing in Fig. 1) which is topped by a valve to admit hydrogen gas. In the case of stilbene hydrogenations, a sealed thin-walled ampule containing 2–4 mg of diphenylethane (DPE) is imbedded in the glass wool cladding. The thermolysis of this DPE is then used as secondary indicator of reaction conditions. The assembled apparatus and reaction vessel is then evacuated, pressured with D2 or H2 gas and shaken at the desired temperature in a fluidized sand bath. When the reaction is complete, the reactor is cooled and vented. Reactants and products are removed from the reaction vessel by either solvent extraction or direct distillation, including a measured amount of internal standard for gas chromatographic analysis.
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3. Discussion
3.1. Generation of H atoms The bond dissociation energy of H2 is 104 kcal/mol [12], clearly too high to allow unimolecular dissociation. There exists a general consensus among workers in this field that the initiating process leading to the incorporation of hydrogen from H2 requires reaction of a thermolytically generated radical with H2 to form a hydrogen atom as shown in Eq. (1). R +H2 R – H +H
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Convincing demonstration of the involvement of this reaction was initially provided by Vernon [13] who found that when the thermolysis of 1,2diphenylethane, DPE, a reaction known to produce benzyl radicals, was carried out in the presence of H2 a secondary reaction producing benzene and ethylbenzene was observed. As this outcome required the presence of H2, Vernon proposed the sequence of Eqs. (2) – (5). The key steps here were reactions 3 through 5 in which benzyl radicals reacted with H2 to produce H atoms which then attacked the benzene ring of the starting DPE to displace the 2-phenylethyl radical as shown in Eqs. 4 and 5.
Fig. 1. Apparatus used for reactions of organic compounds with H2 or D2.
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(2)
(3)
(4)
(5) This process has been called ‘hydrocracking’. The mechanistic sequence of Eqs. (4) and (5) had been proposed earlier as a path for dealkylation in the thermolysis of toluene [14 – 16] at higher temperatures. When 1,2-dinaphthylethane was substituted for DPE, the relative amount of hydrocracking was drastically reduced, reflecting a less favorable thermochemistry for the reaction analogous to Eq. (3). However, somewhat more competitive hydrocracking with this system was observed by Wei and co-workers [17]. Vernon also showed that biphenyl, a molecule which by itself was inert at the 450°C temperature of these experiments was cleaved to give benzene in the presence of H2 and 1,2-DPE. Vernon’s conclusions were confirmed by Shin, Baldwin and Miller [18] and also by Burr and Javeri [19]. Competition experiments were carried out by Bockrath, Schroeder and Keldsen [20] which showed that H atoms attacked DPE and methylnaphthalene molecules unselectively or, if number of sites for attack were considered, preferred methylnaphthalenes by a 2:1 ratio. These authors also examined the selectivity for methyl group removal within the single molecule, 1,3-dimethylnaphthalene. They found that the H atom showed a 4:1 preference for attack at the 1-position. H atoms are also held responsible for the opening of tetralin to give butylbenzene [21]. Thermolysis of DPE under D2 pressure suggested refinements and additions to Vernon’s scheme [22]. Benzyl radicals generated via Eq. (2) are most likely to react with starting DPE to produce 1,2-diphenylethyl radicals by the well documented process of Eq. (6) [23,24] rather than with D2 as shown in Eq. (3) because of the greater bond dissociation energy for the HH bond compared to the CH bond in either toluene or diphenylethane (104 vs. 88.6 and 86.1 kcal/mol) [25]. However, Vernon has suggested that the smaller prexponential (A factor) would partially compensate for the unfavorable enthalpic difference, making the reaction with H2 competitive [13]. Poutsma [11], has suggested a rate preference factor of 10–15 for the reaction with DPE when it is present at equal concentration with H2. However, McMillen, Malhotra and Nigenda [26] make comparisons suggesting a factor of at
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least 100. We estimate from our experiments, assuming the measured cold pressure is a valid measure of D2 concentration, that the dominant reaction producing D atoms is that of Eq. (7) wherein 1,2-diphenylethyl radicals carry the main assault on D2. (6)
(7) The viability of Eq. (7) was demonstrated by generating the 1,2-diphenylethyl radical through the dissociation of independently synthesized 1,2,3,4-tetraphenylbutane in the presence of deuterium. Under these circumstances the process of Eq. (7) competes with disproportionation, Eq. (8), such that the yield of DPE is increased under D2 and the product contains a substantial amount of D.
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3.2. D-atom-pro6oked H– D substitution These experiments also showed that D atoms not only dealkylate alkyl aromatic compounds as shown earlier, Eqs. (4) and (5), but also attack unsubstituted aromatic carbons to provoke D – H exchange as shown in Eqs. (9) and (10). It was found that the D content of the various components of the DPE thermolysis mixture gained aromatic deuterium at rates roughly proportional to the number of available aromatic C – H sites.
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Interestingly, the upper branch of Eq. (10) produces an H atom which apparently persists long enough to participate in the process of Eq. (4) with the result that benzene produced under D2 pressure is initially 50% undeuterated. Data indicate a minimum preference of 4:1 for attack of D at unsubstituted versus substituted aromatic ring positions. For the thermolysis of 300 mg of DPE under 2000 psi of D2 in a 12 ml vessel, D is incorporated about 1.4 times faster in aromatic positions versus aliphatic positions in recovered DPE. This ratio changes in favor of aromatic positions at lower initial DPE concentrations, presumably reflecting lower termination rates, Eq. (8), and consequently greater average numbers of repetitions for the chain sequence of Eq. (9), the upper branch of Eqs. (10) and (11). As might be predicted, there is also a greater yield of hydrocracking products, benzene and ethylbenzene which would arise by the sequence of Eqs. (4), (5) and (1) with the R referred to in Eq. (1) being the 2-phenylethyl radical, arising in Eq. (5) when the substrate is 1,2-DPE. D2 + H D +HD
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Deuterium incorporation in the ethylbenzene produced has been shown to be relatively high and to be concentrated in the methyl group. It would be expected that the selectivity of the 2-phenylethyl radical would be less than that of the 1,2-diphenylethyl radical and therefore more likely to pick up D via Eq. (1) than H via the analogs of Eqs. (6) or (8).
3.3. Radical hydrogen transfer Although the hydrogen atom has been accepted by most authors as the reactive agent for transfer of hydrogen to aromatic ring sites, an alternative mechanism, named radical hydrogen transfer (RHT), has been proposed by McMillen [27]. It is a suggested mechanistic sequence where hydrogen is being transferred from an organic radical species serving as a ‘hydrogen donor’ to some hydrogenunsaturated site in a closed-shell molecule. An example of the type of process to which McMillen refers is shown in Eq. (12). (12) McMillen considers a number of alternative mechanisms for hydrogen transfer processes including hydrogen-atom generation and finds evidence in many situations consistent with involvement of Eq. (12). His strongest case concerns the hydrogenolysis of 1-(2-naphthylmethyl)naphthalene by various hydrogen donor solvents as shown in Eq. (13). The substantial and changing selectivity observed when different hydrogen donors were employed militates against a common hydrogen source, e.g. the hydrogen atom, being responsible for all cases.
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3.4. Re6erse radical disproportionation Franz and co-workers [28], however, suggest that the observations of McMillen can be explained without invoking RHT and have carried out calculations which indicate that the energy barrier for the RHT reaction shown in Eq. (12) and in simpler cases is too high to allow RHT to compete with other mechanisms of hydrogenolysis. Reverse radical disproportionation, RRD, in which hydrogenolysis is initiated by direct H-atom transfer from closed-shell donor molecule to closedshell substrate giving rise to two radical species is suggested. Camaioni, Autrey and Franz [29] have also questioned interpretations by Billmers, Griffith and Stein [30] suggesting that the 2-ethyl-9-hydroanthryl radical exhibits RHT in hydrogen transfer to anthracene. Resolution of this debate is to some extent peripheral to the focus of this review, in that it questions the mechanism of hydrogen transfer from ‘donor solvents’ and not from H2. When H2 is the only source of hydrogen, it seems necessary to consider H-atom extraction by thermally-generated radicals to be the most likely initiation step (Eq. (1)). As these reactions necessarily generate H atoms, at least part of the hydrogenolysis-type products must arise via the clearly exothermic combination of the H atoms with organic compounds. Nonetheless, it is possible that subsequent to this initiation, RHT provides an alternate interpretation for the sequence of Eqs. (9) and (10) or of (4) and (5). Once an H atom reacts with an unsaturated site, as for example in Eq. (9), it is legitimate to ask whether the resultant radical loses an H atom by unimolecular dissociation or transfers it to another molecule by RHT. At least in the case where initial H atom attack occurs at a site ipso to a substituent as for example in Eq. (4), a bimolecular path for transfer of the R radical seems overwhelmingly improbable. As far as we are aware, no one has suggested radical alkyl transfer. As to the question of how H atoms are transferred between adducts it is relevant to note Franz’s calculation that the activation barrier to RHT is somewhat lower than the experimental bond energy values for the CH bond in the adduct, which are, in effect, the energies for H-atom production. However, the energy differences between the two paths are not large. As example, the estimated activation energy for Eq. (12) is 22 kcal/mol whereas the enthalpy change for Eq. (10) is 24 kcal/mol, a value which must be
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somewhat lower than the activation energy. However, because of the fact that this compares a bimolecular reaction (Eq. (12)) to one that is unimolecular (Eq. (10)), the difference in preexponential factors between the two paths will compensate in favor of Eq. (10). In fact, Autrey and co-workers estimate that RHT will generally be one thousand times less probable than H atom production [31]. The design of experimental tests of the relative importance of RHT is not easy. McMillen’s method of comparing hydrogenolysis sensitivities for different substrates and donors seems a good approach. However, this will have to be carried out in circumstances where it can be proven that the hydrogenolyzing reagent is truly the adduct radical.
3.5. Hydrogenolysis of 1 -[4 -(2 -phenylethyl)benzyl]naphthalene One of the fascinating mysteries of hydrogenolysis selectivity centers around the coal-model compound, 1-[4-(2-phenylethyl)benzyl]naphthalene, NMBB, designed by Farcasiu and co-workers [32] and shown as reactant in Eq. (14). They studied the thermolysis of this compound and also its reaction with the hydrogen donor solvent, 9,10-dihydrophenanthrene. It was found, as expected, that cleavage of the thermodynamically favored bibenzylic ‘d’ bond occurred fastest, followed by one or more possible sequences including those analogous to Eqs. (3),(4) and (5) or Eqs. (7),(4) and (5), leading to
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hydrogenolysis of the ‘a’ bond. However with certain carbon-black catalysts, notably the active carbon designated ‘Black Pearls 2000’, high selectivity for ‘a’ bond cleavage was observed. Farcasiu proposed that the observed catalytic phenomenon resulted from oxidation of this compound on the surface of the catalyst to produce a radical cation which then underwent preferential cleavage of the ‘a’ bond. Theoretical calculations have provided support for a mechanism explaining ‘a’ bond cleavage on this basis [33]. However Penn and Wang [34] have offered experimental analogies suggesting that the envisioned radical cation would undergo ‘b’ bond cleavage. A number of workers have now studied the reaction of NMBB both with [35] and without [36] catalysts. A variant of this mechanism wherein the radical cation accepts a hydrogen atom at its ipso position prior to cleavage was proposed by Franz and co-workers [37] to explain similar hydrocracking selectivity observed with iron catalysts. This suggestion was later withdrawn in light of selectivities observed in the catalyst-promoted scission of benzylphenyl phenyl ethers and replaced by an argument favoring reversible H-atom transfer from catalyst to substrate [38]. In this mechanism the catalyst simply serves as a source of H atoms and the rest of the sequence mimics those discussed above.
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A study of the reaction of NMBB with D2 has been carried out in the presence and absence of catalysts and donor solvent (dihydrophenanthrene) [39]. The D2 studies demonstrate that in the presence of typical coal hydroliquefaction catalysts deuterium is distributed extensively throughout both the intended target molecule and the donor solvent molecules. It seems clear that in these circumstances, reversible hydrogen transfer between catalyst and all hydrogen containing molecules cannot be avoided. Thus the sequence shown in Eq. (15) must be considered as a viable alternative in the hydrocracking of NMBB. Moreover, the anticipated thermochemical preference for hydrogenation of the naphthyl ring would explain favored hydrogenolysis of the ‘a’ bond. In support of this suggestion, tetrahydro-NMBBs were found in significant amounts, even in runs involving no added catalyst.
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3.6. Termination of H-atom chains Seeing that there is a general consensus that H-atom chains such as Eqs. (1), (3), (4), (5) or (1), (6), (7), (4), (5) constitute viable paths for hydrocracking of alkylaromatic compounds, it is logical to ask why such chains have limited length and do not permit efficient stripping of alkyl moieties, a transformation which would be particularly valuable in treatment of fossil fuels. The answer must be that the propagation steps in these chains are too slow relative to the competing termination steps. It seems likely that disproportionation reactions such as that of Eq. (8) are the principle termination processes and thus constitute the main problem, Stein has shown that coupling dominates for benzylic radicals at lower temperatures [40]. But, at the temperatures used for most liquefaction model studies, radical couplings of benzylic radicals are reversible and do not provide viable termination routes. It therefore appears that the most important termination step in the DPE hydrogenolysis, and probably for most processess involving 1,2-diaryl radicals, is disproportionation of two 1,2-diaryl radicals to give 1,2-diarylalkene and diarylalkane as shown in Eq. (8). The search for reactions which could support more efficient H-atom chains is thus directed to systems which might limit disproportionation. Central bond
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scission in 2,2,5,5-tetramethyl-3,4-diphenylhexane, TMDH, Eq. (17), and related compounds had been shown by Ru¨chardt [41] to occur under relatively mild conditions, offering the opportunity to see whether reaction with D2 could be induced at temperatures below those normally used in hydroliquefaction, Moreover, the phenylneopentyl radical produced would lack the b-hydrogen atom necessary for normal disproportionation.
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Thermolyses of TMDH [42] in 14 Mpa of D2 by comparison to runs carried out under N2 lead to a 4- to 5-fold increase in the yield of the reductive cleavage product, neopentylbenzene. This evidence for the variant of Eq. (1), Eq. (18), is a clear example of the capacity of benzylic radicals to remove H atoms from H2. With 50 mg of meso-TMDH in a 12 ml tube, only 40% of the phenylneopentyl radicals generated from the reaction of Eq. (17) managed to avoid reacting with D2. When the amount of substrate was reduced to 5 mg, less than 15% of the neopentylbenzene produced avoided deuteration.
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3.7. Stereochemical preference A curious feature of the reactions of TMDH is the loss of tert-butyl groups in formation of stilbene and 1,2-DPE as reaction products. The latter is only observed when D2 is employed, suggesting that it is produced by the hydrogenation of initially formed stilbene. Two diastereoisomers of TMDH exist, both formed by the synthetic procedures employed. The most easily purified, the meso-isomer, gives the highest yield of stilbene and DPE, apparently reflecting a preferred conversion of this isomer to stilbene. Limited experiments carried out with the D,L-isomer indicate its relative reluctance to follow this path. Eqs. (19) and (20) show how the most
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stable conformation of meso-TMDH is most favorably aligned to give the thermodynamically preferred trans-stilbene, whereas D,L-TMDH would either be forced to give the less stable cis-stilbene or to react via an unfavorable conformation.
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Partial progress toward the goal of forcing a reaction between the phenylneopentyl radical and D2 is reflected in the fact that the D content of the benzylic position in neopentylbenzene produced is much higher than observed for toluene from DPE. On the other hand there is no evidence of long kinetic chains. The probable slowness of the top branch of Eq. (18) allows ample opportunity for a variety of cleavage reactions followed by coupling and disproportionative termination processes to develop. The transformation of Eq. (19) was unanticipated as it provides a path to the formation of highly deuterated samples of both stilbene and DPE with the latter showing substantial D content at both aromatic and aliphatic sites. It would appear that the stilbene formed serves as a trap for deuterium atoms leading to diphenylethyl radicals which can disproportionate as discussed earlier. The D,L-isomer of TMDH is less prone to side reactions and would be an interesting substrate for further study as an H-atom generator provided that an efficient synthesis can be designed.
3.8. Surface-immobilized substrate In relating hydrogenolysis studies such as those described above to reactions of fossil fuel sources, particularly coal, it is necessary to consider the possible influence of the two-phase nature of the latter reaction. Most of the reactions described thus far seem likely to take place at least partly in the gas phase. When H2 attacks molecules in coal it must find some of its targets in the solid phase. Both
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advantages and disadvantages of this condition might be perceived. On the one hand, bimolecular processes required for termination steps might be limited by the restricted mobility of substrate-derived radicals. On the other, these radicals might be assumed to have limited access to gas-phase reactants, most importantly, H2. Poutsma, Buchanan, Britt and co-workers have devised a clever method for the attachment of thermolysis substrates to silica rendering them immobile and restricting them to the solid phase [43]. A silica-immobilized 1,2-DPE model was prepared by attaching 1-(4%-hydroxyphenyl)-2-phenylethane via an SiOAr link to the surface of fumed silica as shown in Eq. (21) and heating the resultant material in 14 MPa of D2 [44]. The products were analogous to those observed in the hydrothermolysis of DPE itself producing cleavage products both with and without the anchoring hydroxyl group. Interpretation of the D-incorporation results were complicated by several
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unanticipated problems. It was found that free hydroxyl groups on the silica underwent H – D exchange with the D2 present. The original description of this phenomenon noted the opportunity for this to occur via reaction of phenoxyl radicals with D2 followed by rapid exchange between OD(H) groups of the phenols with the silica. Subseqent experiments showed that fumed silica can exchange its OH groups with D2 even in the absence of phenols [45] (although it has yet to be determined whether this can happen after surface modification.) Whatever the mechanism, oxygen-substituted rings underwent an H–D exchange process unrelated to the characteristic radical chemistry explanations for free substrates. Nevertheless, it is possible to conclude from the results that the presence of H2 did not alter the influence of immobilization in favoring unimolecular radical processes such as rearrangement to surface-attached 1,1-diphenylethane moieties or cyclization. Of particular interest is the fact that the nonoxygenated benzyl moieties (C7H7 − n Dn fragments separately analyzable by mass spectrometry) invariably contained less deuterium when present in 1-(4%-hydroxyphenyl)-2-phenylethane separated from the silica surface after reaction than was measured in the benzyl groups of volatile product molecules. For example, the C7H7 − n Dn fragment in the mass spectrum of ethylbenzene showed only 1/3 to 1/2 of the deuterium content of the C7H7 − n Dn fragment from surface-recovered 1-(4%-hydroxyphenyl)-2phenylethane. In contrast, the fragments showed equal D content when free 1-(4%-hydroxyphenyl)-2-phenylethane was thermolyzed in the presence of D2. This substantiates the reasonable supposition that surface immobilized substrates are less accessible to D atoms. An attempt to circumvent the surface exchange problems with a different attachment procedure is outlined in Eq. (22) [46]. This attachment procedure
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Fig. 2. Disappearance of stilbene (STB) as a function of time when heated at 405°C under 14 Mpa of H2.
proved effective for 4-(2%-phenylethyl)benzoic acid and the results from thermolysis under D2 were generally in concert
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with the experiments on ether-linked material. However, unexpected exchange promoted by the carboxyl group in the benzoate ring along with inexplicable, procedure-dependent variations in yields have made these systems less reliable than had been hoped.
3.9. Reactions of alkenes with H2 In the studies described above, there has been limited evidence for reaction of stilbene with H2. In the reaction of DPE with D2, reactions having 300 mg of substrate in a 12 ml reactor showed stilbene yield as a reaction product rising to a maximum and then declining. In reactions run with smaller starting quantities of
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DPE, production of stilbene was not observed. Also in the described thermolysis of TMDH, stilbene produced under N2 was found to be partly replaced by highlydeuterated DPE when the reaction was carried out under D2. These results suggested the possibility of a direct reaction between stilbene and D2. Initially, treatment of 300 mg of stilbene with H2 in a 12 ml reactor under conditions of the DPE hydrothermolysis showed no reaction. However, continued examination of this reaction showed that reaction of stilbene with H2 or D2 is efficient at 400°C when the concentration of stilbene is lowered under 100 mg/12 ml [47]. Interestingly, it was found that different alkenes responded quite differently to concentration changes. 2-Phenylpropene (a-methylstyrene) gave increased reduction at higher concentrations. Anthracene underwent hydrogenation to 9,10-dihydroanthracene but the reaction showed no significant concentration dependence. Phenanthrene failed to show measurable conversion under conditions where the other three compounds were hydrogenated (410°C and 14 MPa D2). A more detailed kinetic study of the reaction of stilbene with H2 was then carried out confirming the previously observed concentration dependence and demonstrating autocatalytic behavior as shown in the plot of Fig. 2 [48]. A mechanism was proposed which when computer simulated using the kinetic modeling program ‘ACUCHEM’ [49] gave a reasonable fit to the observed kinetic data. The key step in the model sequence is the addition of H atoms to the double bond as shown in Eq. (23). Other necessary processes are Eqs. (2), (3), (6), (7), and (8). For many of these reactions, literature values for the involved rate constants were available and were assumed in the modeling process. Since the most recent publication on this reaction, an improved kinetic fit has been obtained by including the reaction of H atoms with 1,2-DPE to give 1,2-diphenylethyl radicals as shown in Eq. (24), the thermodynamically favored reverse of Eq. (7) [50].
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(24) The proposed model sequence gives a reasonable fit to the observed experimental data for conversion of DPE-contaminated stilbene to DPE. However, there remains an important problem with pure stilbene. An initiation step is needed! If pure stilbene is subjected to the proposed reaction scheme with H2 even the computer-deprived eye can see that no reaction will take place. It was considered that small amounts of DPE, which can be seen to initiate the sequence via Eq. (2) and are invariably present in commercial stilbene might be responsible. However, reactions run with stilbene prepared via a synthesis designed to preclude DPE contamination,
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participated in a manner indistinguishable from that of commercial samples. Although the nature of the requisite initiation step in this reaction has not been proven, successful modeling may be achieved by assuming that the surface of the glass vessel catalyzes the slow conversion of stilbene to DPE. In separate studies it has been found that fumed silica with its high surface area will catalyze the hydrogenation of alkenes and arenes at temperatures well below those required for thermal reaction in a glass tube [51]. In the ‘uncatalyzed’ reaction, once a small amount of DPE is produced, the reaction becomes self-sustaining with Eq. (2) providing initiation. Increasing conversion leads accordingly to faster reaction, although limited at higher conversions by the H-atom-consuming process of Eq. (24). The inverse concentration effect of stilbene results from the bimolecular nature of Eq. (8). Stilbene traps H atoms as indicated in Eq. (23). The resultant diphenylethyl radicals can then either react with H2 as shown in Eq. (7) or disproportionate by Eq. (8). Higher concentration of stilbene will result in higher steady-state concentration of diphenylethyl radicals which will favor the second-order termination process of Eq. (8) relative to the pseudo-first order propagating reaction of Eq. (7). Although limited experimental data are available for the reaction of 2-phenylpropene with H2 it is reasonable that analogous mechanistic steps are involved. In this case it seems likely that the reaction referred to earlier as reverse radical disproportionation, RRD, provides initiation as shown in Eq. (25). In fact, Ru¨chardt has suggested analogous involvement of RRD in the reaction of 2phenylpropene with dihydroanthracene, (Ru¨chardt refers to this by the older designation, ‘molecular disproportionation’) [52]. Subsequent steps in the hydrogenation sequence would then be parallel to those proposed for stilbene. Significantly, a substantial amount of higher molecular weight material is observed in the 2-phenylpropene reaction suggesting that coupling of radicals may be a viable termination process here.
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4. One-step hydrogenation A particularly interesting case and one deserving of further study is the reaction of anthracene with H2. The uncatalyzed reaction has been reported in 1987 by Chiba and co-workers as an intermediate process in the anthracene-promoted hydrogenation of benzophenone [53]. It has been shown that the dihydroanthracene produced contains 2.1 – 2.6 atoms of D when the reaction is carried out with D2 [47]. Most importantly, there appears to be no concentration dependence, no exchange at aromatic sites and no higher molecular weight products, characteristics which do not fit the type of H-atom chain mechanisms described for most of reactions covered in this review. Although more extensive documentation including
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a careful kinetic study would be required for support, it is necessary to consider the possibility that the reaction of anthracene with H2 takes place via a symmetry-allowed [4+ 2]-cycloaddition. As anthracene is the only system reported where this reaction is viable, it seems a real possibility. Some years ago Fleming and Wildsmith [54] showed that trans-3,6-dideuteriocyclohexa-1,4-diene when heated at 340°C gave monodeuteriobenzene with very little benzene or dideuteriobenzene. They also showed that cis-5,6-dideuteriocyclohexa-1,3-diene failed to give stereospecific loss of either H2 or D2 on thermolysis to give benzene. This agreed with an earlier study by Frey, Krantz and Stevens [55], which showed that cis-3,6-dimethylcyclohexa-1,4-diene lost hydrogen much more rapidly than the trans-isomer. All of this is consistent with the mechanism of Eq. (26) which pictures a single-step concerted process for the addition of H2 to a 1,3-diene or reversedly, the loss of H2 from the 3 and 6 positions of a 1,4-cyclohexadiene. Addition of H2 would be disallowed by orbital symmetry for 9,10-H2 addition to phenanthrene or other vicinal additions and also for H2 removal from a 1,3-cyclohexadiene.
(26)
5. Summary The noncatalytic (or thermal) reaction of H2 or D2 with organic compounds can generally be explained by invoking radical-producing reactions of the substrates followed by reaction of the radicals with H2 (or D2) to remove H (or D), the process generating an H-atom (or D atom). The radical-producing reactions reported have usually been homolytic dissociations of thermolytically labile bonds. In a few cases reverse radical disproportionation (RRD) has been invoked. Once H atoms have been generated by this route they react with organic molecules by hydrogen atom abstraction or by addition to unsaturated sites in substrate molecules. The latter processes produce new radicals which are capable of further reaction with H2 either directly or after some characteristic sequence of radical– radical transformations. A potentially important example is the displacement of substituent groups from aromatic rings, the favored explanation for hydrocracking. In principle, radical chain processes could continue indefinitely. But because the reaction with H2 is normally endothermic and consequently relatively slow, radical–radical reactions terminate these sequences via coupling and disproportionation thus limiting the average number of repetitions and the efficiency of the overall processes. Some interesting reactions with H2 are observed in systems where initiating radical formation cannot be specified. In some cases, unidentified catalysts may be involved. It seems likely that many catalyst-initiated reactions which require temperatures in excess of 300°C involve radical sub-paths similar to those discussed
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above. In a few special cases, direct, nonradical [4+ 2]-cycloaddition of H2 to substrates could be occurring. References [1] J.R. Kershaw, G. Barrass, Fuel 56 (1977) 455. [2] (a) R.P. Skowronski, J.J. Ratto, I.B. Goldberg, L.A. Heredy, Fuel 63 (1984) 440 – 448. (b) R.P. Skowronski, J.J. Ratto, L.A. Heredy, in: R.H. Filby (Ed.), At. Nuc. Methods Fossil Energy Res., 1982, pp. 207–228. (c) I.B. Goldberg, H.R. Crowe, J.J. Ratto, R.P. Skowronski, L.A. Heredy, Fuel 59 (1980) 133. (d) J.J. Ratto, ACS Fuel Chem. Preprints 24 (1979) 155. (e) R.P. Skowronski, L.A. Heredy, J.J. Ratto, ACS Fuel Chem. Preprints, 23 (1978) 155. [3] N.S. Noor, A.F. Gaines, J.M. Abbott, Fuel 65 (1986) 67 – 73. [4] H.A. Dabbagh, B. Shi, C.G. Hughes, B.H. Davis, Energy Fuels 8 (1994) 219 – 226. [5] L.A. Heredy, L.R. Mcoy, I.B. Goldberg, Proc. Int. Conf. on Coal Science, (1983) 695. [6] (a) H.-H. King, L.M. Stock, Fuel, 61 (1982) 447 – 449. (b) H.-H. King, L.M. Stock, Fuel, 59 (1980) 447 – 449. (c) H.-H. King, L.M. Stock, Fuel, 61 (1982) 257 – 264. [7] T.R. Aulich, C.L. Knudson, S.B. Hawthorne, Prepr. Pap. Am. Chem. Soc. Div. Fuel Chem. 33 (1988) 368–379. [8] (a) J.A. Franz, D.M. Camaioni, Fuel, 63 (1984) 213 – 229. (b) J.A. Franz, Fuel, 58 (1979) 405 – 412. [9] D.C. Cronauer, R.I. McNeil, D.C. Young, R.G. Ruberto, Fuel 59 (1982) 610 – 619. [10] A.F. Gaines, Y. Yurum, Fuel 55 (1976) 129 – 137. [11] M.L. Poutsma, Energy Fuels 4 (1990) 114. [12] S.W. Benson, J. Chem. Ed. 42 (1965) 502. [13] L.W. Vernon, Fuel 59 (1980) 102–106. [14] M. Szwarc, J. Chem. Phys. 16 (1948) 128. [15] (a) H. Blades, A.T. Blades, E.W.R. Steacie, Can. J. Chem., 32 (1954) 298 – 311. (b) A.T. Blades, E.W.R. Steacie, Can. J. Chem., 32 (1954) 1142 – 1145. [16] S.J. Price, Can. J. Chem. 40 (1962) 1310–1317. [17] Z.-M. Zong, X.-Y. Wei, Fuel Processing Technology 41 (1994) 79 – 85. [18] S.-C. Shin, R.M. Baldwin, R.L. Miller, Energy and Fuels 3 (1989) 71 – 76. [19] J.G. Burr, I. Javeri, Fuel 63 (1984) 854–857. [20] B.C. Bockrath, K.T. Schroeder, G.L. Keldsen, Proceedings 1985 International Conference on Coal Science, 28–31 October, 1985, N.S.W. Sydney, Pergamon Press, NY, 1985, pp. 83 – 86. [21] J.J. de Vlieger, A.P.G. Kieboon, H. van Bekkum, Fuel 63 (1984) 334. [22] (a) R.D. Guthrie, B. Shi, B. Rajagopal, S. Ramakrishnan, B.H. Davis, J. Org. Chem., 59 (1994) 7426-7432. (b) R.D. Guthrie, B. Shi, R. Sharipov, B. Davis, Prep. Pap. Am. Chem. Soc. Div. Fuel Chem. 38 (1993) 526–533. (c) V. Rajagopal, R.D. Guthrie, B. Shi, B.H. Davis, Prep. Pap. Am. Chem. Soc. Div. Fuel Chem. 38 (1993) 1114. (d) S. Ramakrishnan, R.D. Guthrie, B. Shi, B.H. Davis, Prep. Pap. Am. Chem. Soc. Div. Fuel Chem., 38 (1993) 1122. [23] (a) M.L. Poutsma, Fuel, 59 (1980) 335. (b) M.L. Poutsma, Prepr. Div. Pet. Chem., Am. Chem. Soc., 25 (1980) 30. [24] H. Zeldes, R. Livingston, Fuel 61 (1982) 1254. [25] D.A. Robaugh, S.E. Stein, Int. J. Chem. Kinetics 13 (1981) 335. [26] D.F. McMillen, R. Malhotra, S.E. Nigenda, Prepr. Pap. Am. Chem. Soc. Div. Fuel Chem. 37 (1982) 180. [27] (a) D.F. McMillen, R. Malhotra, S.-J. Chang, W.C. Ogier, S.E. Nigenda, R.H. Fleming, Fuel, 66 (1987) 1611–1620. (b) R. Malhotra, D.F. McMillen, Energy Fuels, 4 (1990) 184. (c) D.F. McMillen, R. Malhotra, S.E. Nigenda, Fuel, 68 (1989) 380. (d) D.F. McMillen, R. Malhotra, G.P. Hum, S.-J. Chang, Energy Fuels, 1 (1987) 193. (e) D.G. McMillen, R. Malhotra, Proc. Int. Conf. Coal Sci., (1987) 193. [28] J.A. Franz, D.M. Ferris, D.M. Camaioni, S.T. Autrey, Energy and Fuels 8 (1994) 1016 – 1019. [29] D.M. Camioni, S.T. Autrey, J.A. Franz, J. Phys. Chem. 97 (1993) 5791 – 5792.
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[30] R. Billmers, L.L. Griffith, S.E. Stein, J. Phys. Chem. 90 (1986) 517 – 523. [31] (a) T. Autrey, E.A. Alborn, J.A. Franz, D.M. Camaioni, Energy and Fuels, 9 (1995) 420 – 428. (b) D.M. Camaioni, S.T. Autrey, T.B. Salinas, J.A. Franz, J. Am. Chem. Soc. 119 (1996) 2013 – 2022. [32] (a) M. Farcasiu, C. Smith, Energy and Fuels, 5 (1991) 83 – 87. (b) M. Farcasiu, C.M. Smith, E.P. Ladner, A.P. Sylwester, Prepr. Div. Fuel Chem. Am. Chem. Soc., 36 (1991) 1869. (c) M. Farcasiu, C. Smith, E.A. Hunter, Proceedings of Int. Conf. on Coal Sci., 1991, p. 166. (d) M. Farcasiu, C. Smith, V. Pradhan, I Wender, Fuel Process. Technol., 29 (1991) 199. [33] (a) H.B. Ades, A.L. Companion, K.R. Subbaswamy, Energy Fuels, 8 (1994) 71. (b) H.B. Ades, A.L. Companion, K.R.Subbaswamy, J. Phys. Chem., 95 (1991) 6502 – 6507. (c) H.B. Ades, A.L. Companion, K.R. Subbaswamy, J. Phys. Chem., 95 (1991) 2226. (d) H.B. Ades, A.L. Companion, K.R. Subbaswamy, Prepr., Div. Fuel Chem., Am. Chem. Soc., 36 (1991) 420. [34] J.H. Penn, J-H. Wang, Energy Fuels 8 (1994) 421 – 425. [35] (a) J.C. Linehan, B.M. Bean, D.W. Matson, J.L. Fulton, A.E. Crump, Prepr., Div. Fuel Chem. Am. Chem. Soc., 37 (1992) 488. (b) J.C. Linehan, J.G. Darab, D.W. Matson, Prepr. Div. Fuel Chem. Am. Chem. Soc., 38 (1993) 66. [36] T.D. Walter, S.M. Casey, M.T. Klein, H.C. Foley, Prepr. Pap. Am. Chem. Soc. Div. Fuel Chem. 38 (1993) 46. [37] J.A. Franz, D.M. Camaioni, M.S. Alnajjar, T. Autrey, J.C. Linehan, Am. Chem. Soc. Div. Fuel Chem. Preprints 40 (1995) 203. [38] (a) T. Autrey, J.C. Linehan, D.M. Camaioni, T.R. Powers, E.F. McMillan, J.A. Franz, Am. Chem. Soc., Div. Fuel Chem. Preprints, 40 (1995) 973 – 977. (b) T. Autrey, J.C. Linehan, D.M. Camaioni, L.E. Kaune, H.M.Watrob, J.A. Franz, Catalysis Today, 31(1996)105-111. [39] B. Shi, Y. Ji, R.D. Guthrie, B.H. Davis, Energy Fuels 8 (1994) 1268 – 1275. [40] M.J. Manka, S.E. Stein, J. Phys. Chem. 88 (1984) 1514 – 1519. [41] (a) K.-H. Eichin, H.-D. Beckhaus, S. Hellmann, H. Fritz, E.-M. Peters, K. Peters, H.-G. von Schnering, C. Ru¨chardt, Chemische Berichte, 116 (1983) 1787 – 1821. (b) G. Hellmann, H.-D. Beckhaus, C. Ru¨chardt, Chemishe Berichte, 112 (1979) 1808 – 1834. (c) H.D. Beckhaus, C. Ru¨chardt, Chemishe Berichte, 110 (1977) 878 – 895. (d) G. Kratt, H.-D. Beckhaus, H.J. Lindner, C. Ru¨chardt, Chemishe Berichte, 116 (1983) 3265 – 3263. (e) G. Kratt, H.-D. Beckhaus, H.J. Lindner, C. Ru¨chardt, Chemishe Berichte, 117 (1984) 1748 – 1764. [42] (a) R.D. Guthrie, R.V. Sharipov, S. Ramakrishnan, B. Shi, B.H. Davis, J. Org. Chem., 60 (1995) 4504 – 4509. (b) R. Sharipov, R.D. Guthrie, B. Shi, B.H. Davis, Am. Chem. Soc. Div. Fuel Chem. Preprints, 38 (1993) 1129. [43] (a) A.C. Buchanan, III, T.D.J. Dunstan, E.C. Douglas, M.L. Poutsma, J. Am. Chem. Soc., 108 (1986) 7703–7715. (b) A.C. Buchanan, III, P.F. Britt, C.A. Biggs, Energy and Fuels, 4 (1990) 415 – 417. (c) A.C. Buchanan, III, C.A. Biggs, J. Org. Chem., 54 (1989) 517 – 525. (d) P.F. Britt, A.C. Buchanan, III, J. Org. Chem., 56 (1991) 6132 – 6140. (e) P.F. Britt, A.C. Buchanan, III, E.A. Malcolm, J. Org. Chem., 60 (1995) 6523. [44] R.D. Guthrie, S. Ramakrishnan, P.F. Britt, A.C. Buchanan, III, B.H. Davis, Energy and Fuels, 9 (1995) 1097–1103. [45] (a) V.K. Rajagopal, R.D. Guthrie, T. Fields, B.H. Davis, Catalysis Today, 31 (1996) 57 – 63. (b) V. Rajagopal, R.D. Guthrie, B.H. Davis, prep. Am. Chem. Soc. Div. Fuel Chem., 40 (1995) 945 – 949. (c) T. Fields, V. Rajagopal, B.H. Davis, prep. Pap. Am. Chem. Soc. Div. Fuel Chem., 60 (1995) 6523. [46] S. Ramakrishnan, R.D. Guthrie, P.F. Britt, A.C. Buchanan, III, Prep. Pap. Am. Chem. Soc. Div. Fuel Chem., 40 (1995) 555–559. [47] V. Rajagopal, R.D. Guthrie, B. Shi, B.H. Davis, Prep. Pap. Am. Chem. Soc. Div. Fuel Chem. 39 (1994) 673. [48] V. Rajagopal, R.D. Guthrie, B.H. Davis, Prep. Pap. Am. Chem. Soc. Div. Fuel Chem. 42 (1997) 84. [49] Available from: W. Braun, J.T. Herron, D.K. Kahaner, NIST, Gaithersburg, MD, 20899. See Int J. Chem. Kinet., 29 (1988) 51. [50] R.D. Guthrie, Unpublished results presented at the April 1997 meeting of the American Chemical Society, San Francisco, CA. .
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Thermolysis of organic compounds under H2 (D2) Robert D. Guthrie * Department of Chemistry, Uni6ersity of Kentucky, Lexington, KY 40506, USA
Abstract This paper is a review of our experiments and those of other authors which relate to chemical processes taking place when organic compounds are heated under hydrogen or deuterium gas. For the most part, coverage is confined to reactions believed to directly involve H2, D2 or H and D atoms generated therefrom. However, discussion of some follow-up reactions is required to explain the observed distribution of deuterium in products from reactions under D2. Reactions wherein hydrogen is activated by added inorganic catalysts are avoided except as relevant for understanding uncatalyzed examples. Reactions with surface-immoblilized substrates are discussed and related to reactions of coal with hydrogen gas (‘hydroliquefaction’). © 2000 Elsevier Science B.V. All rights reserved. Keywords: Hydrocracking; Hydrothermolysis; Coal; Liquefaction; Hydroliquefaction; Pyrolysis
1. Introduction Much of the initial interest in the high temperature reactions of organic compounds with H2 was stimulated by their apparent benefit in processes for coal liquefaction. Early in the evolution of coal liquefaction methodology, increased yields of useful products were recognized to result from carrying out the thermolysis in an environment of H2, usually at elevated pressure at temperatures in the range of 400 – 500°C. Early efforts at molecular-level understanding seized upon the potential for obtaining mechanistic details through the substitution of D2 for H2 and analysis of the resulting products. Kershaw and Barrass [1] claim to be the first to use D2 to study the mechanism of coal hydrogenation. Their procedure utilized a catalyst, stannous chloride (1% by weight of coal) and the reaction was carried * Tel.: +1-606-257-7068; fax: + 1-606-323-1069. 0165-2370/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 3 7 0 ( 9 9 ) 0 0 0 8 0 - 7
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out at 450°C for 1 h. Examination of the products by mass spectrometry and nuclear magnetic resonance spectroscopy showed deuterium to be widely distributed in both aromatic and aliphatic positions in the products. Subsequent literature contains ample evidence that deuterium is incorporated into reaction products when thermolysis of coal is carried out under D2 gas even in the absence of deliberately added catalysts [2 – 5]. While these experiments clearly showed that H2 could participate in the thermochemistry of coal, it seemed clear that the complexity of coal’s composition would severely limit the resolution of mechanistic questions, particularly considering that breaking of the strong bond in H2 was likely to be slower than the subsequent processes which distributed H (or D) throughout the coal and its thermolysis products. The problematic energy barrier was removed when deuterium was introduced into the liquefaction milieu in the form of a labeled ‘donor solvent’ [6–10]. However, the details of deuterium distribution were still necessarily very complex. Research thus emphasized the study of structurally homogeneous systems picked to model the internal structures of coal. Much valuable mechanistic delineation of high temperature radical chemistry was justified through its relation to coal liquefaction. A thorough review has been prepared by Poutsma [11]. The contribution which follows is generally restricted to a summary of the direct, uncatalyzed reactions of organic compounds with H2 or D2. The hydrogen-transfer processes which are likely to follow under conditions necessary to produce the initial interaction are discussed to the extent necessary for understanding the fate of the hydrogen introduced. 2. Experimental Hydrogenation experiments carried out by the author were performed in the device shown in Fig. 1. It consists of a thick glass reaction bulb with a long capillary neck. The reaction bulb has a volume of approximately 12 ml and the 1–2 mm i.d. capillary section is approximately 16 cm long. The glass reactor tube is inverted and several glass beads followed by solid reactant are added through the end opposite to the capillary. The reaction vessel is sealed at the constriction as shown in the second drawing in Fig. 1. Liquid reactants may then be added through the capillary end via a long syringe needle. The vessel is then suspended in glass wool in the interior of a stainless steel reactor tube (third drawing in Fig. 1). The capillary section of the tube extends into a separate stainless steel neck (fourth drawing in Fig. 1) which is topped by a valve to admit hydrogen gas. In the case of stilbene hydrogenations, a sealed thin-walled ampule containing 2–4 mg of diphenylethane (DPE) is imbedded in the glass wool cladding. The thermolysis of this DPE is then used as secondary indicator of reaction conditions. The assembled apparatus and reaction vessel is then evacuated, pressured with D2 or H2 gas and shaken at the desired temperature in a fluidized sand bath. When the reaction is complete, the reactor is cooled and vented. Reactants and products are removed from the reaction vessel by either solvent extraction or direct distillation, including a measured amount of internal standard for gas chromatographic analysis.
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3. Discussion
3.1. Generation of H atoms The bond dissociation energy of H2 is 104 kcal/mol [12], clearly too high to allow unimolecular dissociation. There exists a general consensus among workers in this field that the initiating process leading to the incorporation of hydrogen from H2 requires reaction of a thermolytically generated radical with H2 to form a hydrogen atom as shown in Eq. (1). R +H2 R – H +H
(1)
Convincing demonstration of the involvement of this reaction was initially provided by Vernon [13] who found that when the thermolysis of 1,2diphenylethane, DPE, a reaction known to produce benzyl radicals, was carried out in the presence of H2 a secondary reaction producing benzene and ethylbenzene was observed. As this outcome required the presence of H2, Vernon proposed the sequence of Eqs. (2) – (5). The key steps here were reactions 3 through 5 in which benzyl radicals reacted with H2 to produce H atoms which then attacked the benzene ring of the starting DPE to displace the 2-phenylethyl radical as shown in Eqs. 4 and 5.
Fig. 1. Apparatus used for reactions of organic compounds with H2 or D2.
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(2)
(3)
(4)
(5) This process has been called ‘hydrocracking’. The mechanistic sequence of Eqs. (4) and (5) had been proposed earlier as a path for dealkylation in the thermolysis of toluene [14 – 16] at higher temperatures. When 1,2-dinaphthylethane was substituted for DPE, the relative amount of hydrocracking was drastically reduced, reflecting a less favorable thermochemistry for the reaction analogous to Eq. (3). However, somewhat more competitive hydrocracking with this system was observed by Wei and co-workers [17]. Vernon also showed that biphenyl, a molecule which by itself was inert at the 450°C temperature of these experiments was cleaved to give benzene in the presence of H2 and 1,2-DPE. Vernon’s conclusions were confirmed by Shin, Baldwin and Miller [18] and also by Burr and Javeri [19]. Competition experiments were carried out by Bockrath, Schroeder and Keldsen [20] which showed that H atoms attacked DPE and methylnaphthalene molecules unselectively or, if number of sites for attack were considered, preferred methylnaphthalenes by a 2:1 ratio. These authors also examined the selectivity for methyl group removal within the single molecule, 1,3-dimethylnaphthalene. They found that the H atom showed a 4:1 preference for attack at the 1-position. H atoms are also held responsible for the opening of tetralin to give butylbenzene [21]. Thermolysis of DPE under D2 pressure suggested refinements and additions to Vernon’s scheme [22]. Benzyl radicals generated via Eq. (2) are most likely to react with starting DPE to produce 1,2-diphenylethyl radicals by the well documented process of Eq. (6) [23,24] rather than with D2 as shown in Eq. (3) because of the greater bond dissociation energy for the HH bond compared to the CH bond in either toluene or diphenylethane (104 vs. 88.6 and 86.1 kcal/mol) [25]. However, Vernon has suggested that the smaller prexponential (A factor) would partially compensate for the unfavorable enthalpic difference, making the reaction with H2 competitive [13]. Poutsma [11], has suggested a rate preference factor of 10–15 for the reaction with DPE when it is present at equal concentration with H2. However, McMillen, Malhotra and Nigenda [26] make comparisons suggesting a factor of at
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least 100. We estimate from our experiments, assuming the measured cold pressure is a valid measure of D2 concentration, that the dominant reaction producing D atoms is that of Eq. (7) wherein 1,2-diphenylethyl radicals carry the main assault on D2. (6)
(7) The viability of Eq. (7) was demonstrated by generating the 1,2-diphenylethyl radical through the dissociation of independently synthesized 1,2,3,4-tetraphenylbutane in the presence of deuterium. Under these circumstances the process of Eq. (7) competes with disproportionation, Eq. (8), such that the yield of DPE is increased under D2 and the product contains a substantial amount of D.
(8)
3.2. D-atom-pro6oked H– D substitution These experiments also showed that D atoms not only dealkylate alkyl aromatic compounds as shown earlier, Eqs. (4) and (5), but also attack unsubstituted aromatic carbons to provoke D – H exchange as shown in Eqs. (9) and (10). It was found that the D content of the various components of the DPE thermolysis mixture gained aromatic deuterium at rates roughly proportional to the number of available aromatic C – H sites.
(9)
(10)
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Interestingly, the upper branch of Eq. (10) produces an H atom which apparently persists long enough to participate in the process of Eq. (4) with the result that benzene produced under D2 pressure is initially 50% undeuterated. Data indicate a minimum preference of 4:1 for attack of D at unsubstituted versus substituted aromatic ring positions. For the thermolysis of 300 mg of DPE under 2000 psi of D2 in a 12 ml vessel, D is incorporated about 1.4 times faster in aromatic positions versus aliphatic positions in recovered DPE. This ratio changes in favor of aromatic positions at lower initial DPE concentrations, presumably reflecting lower termination rates, Eq. (8), and consequently greater average numbers of repetitions for the chain sequence of Eq. (9), the upper branch of Eqs. (10) and (11). As might be predicted, there is also a greater yield of hydrocracking products, benzene and ethylbenzene which would arise by the sequence of Eqs. (4), (5) and (1) with the R referred to in Eq. (1) being the 2-phenylethyl radical, arising in Eq. (5) when the substrate is 1,2-DPE. D2 + H D +HD
(11)
Deuterium incorporation in the ethylbenzene produced has been shown to be relatively high and to be concentrated in the methyl group. It would be expected that the selectivity of the 2-phenylethyl radical would be less than that of the 1,2-diphenylethyl radical and therefore more likely to pick up D via Eq. (1) than H via the analogs of Eqs. (6) or (8).
3.3. Radical hydrogen transfer Although the hydrogen atom has been accepted by most authors as the reactive agent for transfer of hydrogen to aromatic ring sites, an alternative mechanism, named radical hydrogen transfer (RHT), has been proposed by McMillen [27]. It is a suggested mechanistic sequence where hydrogen is being transferred from an organic radical species serving as a ‘hydrogen donor’ to some hydrogenunsaturated site in a closed-shell molecule. An example of the type of process to which McMillen refers is shown in Eq. (12). (12) McMillen considers a number of alternative mechanisms for hydrogen transfer processes including hydrogen-atom generation and finds evidence in many situations consistent with involvement of Eq. (12). His strongest case concerns the hydrogenolysis of 1-(2-naphthylmethyl)naphthalene by various hydrogen donor solvents as shown in Eq. (13). The substantial and changing selectivity observed when different hydrogen donors were employed militates against a common hydrogen source, e.g. the hydrogen atom, being responsible for all cases.
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95
(13)
3.4. Re6erse radical disproportionation Franz and co-workers [28], however, suggest that the observations of McMillen can be explained without invoking RHT and have carried out calculations which indicate that the energy barrier for the RHT reaction shown in Eq. (12) and in simpler cases is too high to allow RHT to compete with other mechanisms of hydrogenolysis. Reverse radical disproportionation, RRD, in which hydrogenolysis is initiated by direct H-atom transfer from closed-shell donor molecule to closedshell substrate giving rise to two radical species is suggested. Camaioni, Autrey and Franz [29] have also questioned interpretations by Billmers, Griffith and Stein [30] suggesting that the 2-ethyl-9-hydroanthryl radical exhibits RHT in hydrogen transfer to anthracene. Resolution of this debate is to some extent peripheral to the focus of this review, in that it questions the mechanism of hydrogen transfer from ‘donor solvents’ and not from H2. When H2 is the only source of hydrogen, it seems necessary to consider H-atom extraction by thermally-generated radicals to be the most likely initiation step (Eq. (1)). As these reactions necessarily generate H atoms, at least part of the hydrogenolysis-type products must arise via the clearly exothermic combination of the H atoms with organic compounds. Nonetheless, it is possible that subsequent to this initiation, RHT provides an alternate interpretation for the sequence of Eqs. (9) and (10) or of (4) and (5). Once an H atom reacts with an unsaturated site, as for example in Eq. (9), it is legitimate to ask whether the resultant radical loses an H atom by unimolecular dissociation or transfers it to another molecule by RHT. At least in the case where initial H atom attack occurs at a site ipso to a substituent as for example in Eq. (4), a bimolecular path for transfer of the R radical seems overwhelmingly improbable. As far as we are aware, no one has suggested radical alkyl transfer. As to the question of how H atoms are transferred between adducts it is relevant to note Franz’s calculation that the activation barrier to RHT is somewhat lower than the experimental bond energy values for the CH bond in the adduct, which are, in effect, the energies for H-atom production. However, the energy differences between the two paths are not large. As example, the estimated activation energy for Eq. (12) is 22 kcal/mol whereas the enthalpy change for Eq. (10) is 24 kcal/mol, a value which must be
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somewhat lower than the activation energy. However, because of the fact that this compares a bimolecular reaction (Eq. (12)) to one that is unimolecular (Eq. (10)), the difference in preexponential factors between the two paths will compensate in favor of Eq. (10). In fact, Autrey and co-workers estimate that RHT will generally be one thousand times less probable than H atom production [31]. The design of experimental tests of the relative importance of RHT is not easy. McMillen’s method of comparing hydrogenolysis sensitivities for different substrates and donors seems a good approach. However, this will have to be carried out in circumstances where it can be proven that the hydrogenolyzing reagent is truly the adduct radical.
3.5. Hydrogenolysis of 1 -[4 -(2 -phenylethyl)benzyl]naphthalene One of the fascinating mysteries of hydrogenolysis selectivity centers around the coal-model compound, 1-[4-(2-phenylethyl)benzyl]naphthalene, NMBB, designed by Farcasiu and co-workers [32] and shown as reactant in Eq. (14). They studied the thermolysis of this compound and also its reaction with the hydrogen donor solvent, 9,10-dihydrophenanthrene. It was found, as expected, that cleavage of the thermodynamically favored bibenzylic ‘d’ bond occurred fastest, followed by one or more possible sequences including those analogous to Eqs. (3),(4) and (5) or Eqs. (7),(4) and (5), leading to
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hydrogenolysis of the ‘a’ bond. However with certain carbon-black catalysts, notably the active carbon designated ‘Black Pearls 2000’, high selectivity for ‘a’ bond cleavage was observed. Farcasiu proposed that the observed catalytic phenomenon resulted from oxidation of this compound on the surface of the catalyst to produce a radical cation which then underwent preferential cleavage of the ‘a’ bond. Theoretical calculations have provided support for a mechanism explaining ‘a’ bond cleavage on this basis [33]. However Penn and Wang [34] have offered experimental analogies suggesting that the envisioned radical cation would undergo ‘b’ bond cleavage. A number of workers have now studied the reaction of NMBB both with [35] and without [36] catalysts. A variant of this mechanism wherein the radical cation accepts a hydrogen atom at its ipso position prior to cleavage was proposed by Franz and co-workers [37] to explain similar hydrocracking selectivity observed with iron catalysts. This suggestion was later withdrawn in light of selectivities observed in the catalyst-promoted scission of benzylphenyl phenyl ethers and replaced by an argument favoring reversible H-atom transfer from catalyst to substrate [38]. In this mechanism the catalyst simply serves as a source of H atoms and the rest of the sequence mimics those discussed above.
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A study of the reaction of NMBB with D2 has been carried out in the presence and absence of catalysts and donor solvent (dihydrophenanthrene) [39]. The D2 studies demonstrate that in the presence of typical coal hydroliquefaction catalysts deuterium is distributed extensively throughout both the intended target molecule and the donor solvent molecules. It seems clear that in these circumstances, reversible hydrogen transfer between catalyst and all hydrogen containing molecules cannot be avoided. Thus the sequence shown in Eq. (15) must be considered as a viable alternative in the hydrocracking of NMBB. Moreover, the anticipated thermochemical preference for hydrogenation of the naphthyl ring would explain favored hydrogenolysis of the ‘a’ bond. In support of this suggestion, tetrahydro-NMBBs were found in significant amounts, even in runs involving no added catalyst.
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3.6. Termination of H-atom chains Seeing that there is a general consensus that H-atom chains such as Eqs. (1), (3), (4), (5) or (1), (6), (7), (4), (5) constitute viable paths for hydrocracking of alkylaromatic compounds, it is logical to ask why such chains have limited length and do not permit efficient stripping of alkyl moieties, a transformation which would be particularly valuable in treatment of fossil fuels. The answer must be that the propagation steps in these chains are too slow relative to the competing termination steps. It seems likely that disproportionation reactions such as that of Eq. (8) are the principle termination processes and thus constitute the main problem, Stein has shown that coupling dominates for benzylic radicals at lower temperatures [40]. But, at the temperatures used for most liquefaction model studies, radical couplings of benzylic radicals are reversible and do not provide viable termination routes. It therefore appears that the most important termination step in the DPE hydrogenolysis, and probably for most processess involving 1,2-diaryl radicals, is disproportionation of two 1,2-diaryl radicals to give 1,2-diarylalkene and diarylalkane as shown in Eq. (8). The search for reactions which could support more efficient H-atom chains is thus directed to systems which might limit disproportionation. Central bond
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scission in 2,2,5,5-tetramethyl-3,4-diphenylhexane, TMDH, Eq. (17), and related compounds had been shown by Ru¨chardt [41] to occur under relatively mild conditions, offering the opportunity to see whether reaction with D2 could be induced at temperatures below those normally used in hydroliquefaction, Moreover, the phenylneopentyl radical produced would lack the b-hydrogen atom necessary for normal disproportionation.
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Thermolyses of TMDH [42] in 14 Mpa of D2 by comparison to runs carried out under N2 lead to a 4- to 5-fold increase in the yield of the reductive cleavage product, neopentylbenzene. This evidence for the variant of Eq. (1), Eq. (18), is a clear example of the capacity of benzylic radicals to remove H atoms from H2. With 50 mg of meso-TMDH in a 12 ml tube, only 40% of the phenylneopentyl radicals generated from the reaction of Eq. (17) managed to avoid reacting with D2. When the amount of substrate was reduced to 5 mg, less than 15% of the neopentylbenzene produced avoided deuteration.
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3.7. Stereochemical preference A curious feature of the reactions of TMDH is the loss of tert-butyl groups in formation of stilbene and 1,2-DPE as reaction products. The latter is only observed when D2 is employed, suggesting that it is produced by the hydrogenation of initially formed stilbene. Two diastereoisomers of TMDH exist, both formed by the synthetic procedures employed. The most easily purified, the meso-isomer, gives the highest yield of stilbene and DPE, apparently reflecting a preferred conversion of this isomer to stilbene. Limited experiments carried out with the D,L-isomer indicate its relative reluctance to follow this path. Eqs. (19) and (20) show how the most
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stable conformation of meso-TMDH is most favorably aligned to give the thermodynamically preferred trans-stilbene, whereas D,L-TMDH would either be forced to give the less stable cis-stilbene or to react via an unfavorable conformation.
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Partial progress toward the goal of forcing a reaction between the phenylneopentyl radical and D2 is reflected in the fact that the D content of the benzylic position in neopentylbenzene produced is much higher than observed for toluene from DPE. On the other hand there is no evidence of long kinetic chains. The probable slowness of the top branch of Eq. (18) allows ample opportunity for a variety of cleavage reactions followed by coupling and disproportionative termination processes to develop. The transformation of Eq. (19) was unanticipated as it provides a path to the formation of highly deuterated samples of both stilbene and DPE with the latter showing substantial D content at both aromatic and aliphatic sites. It would appear that the stilbene formed serves as a trap for deuterium atoms leading to diphenylethyl radicals which can disproportionate as discussed earlier. The D,L-isomer of TMDH is less prone to side reactions and would be an interesting substrate for further study as an H-atom generator provided that an efficient synthesis can be designed.
3.8. Surface-immobilized substrate In relating hydrogenolysis studies such as those described above to reactions of fossil fuel sources, particularly coal, it is necessary to consider the possible influence of the two-phase nature of the latter reaction. Most of the reactions described thus far seem likely to take place at least partly in the gas phase. When H2 attacks molecules in coal it must find some of its targets in the solid phase. Both
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advantages and disadvantages of this condition might be perceived. On the one hand, bimolecular processes required for termination steps might be limited by the restricted mobility of substrate-derived radicals. On the other, these radicals might be assumed to have limited access to gas-phase reactants, most importantly, H2. Poutsma, Buchanan, Britt and co-workers have devised a clever method for the attachment of thermolysis substrates to silica rendering them immobile and restricting them to the solid phase [43]. A silica-immobilized 1,2-DPE model was prepared by attaching 1-(4%-hydroxyphenyl)-2-phenylethane via an SiOAr link to the surface of fumed silica as shown in Eq. (21) and heating the resultant material in 14 MPa of D2 [44]. The products were analogous to those observed in the hydrothermolysis of DPE itself producing cleavage products both with and without the anchoring hydroxyl group. Interpretation of the D-incorporation results were complicated by several
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unanticipated problems. It was found that free hydroxyl groups on the silica underwent H – D exchange with the D2 present. The original description of this phenomenon noted the opportunity for this to occur via reaction of phenoxyl radicals with D2 followed by rapid exchange between OD(H) groups of the phenols with the silica. Subseqent experiments showed that fumed silica can exchange its OH groups with D2 even in the absence of phenols [45] (although it has yet to be determined whether this can happen after surface modification.) Whatever the mechanism, oxygen-substituted rings underwent an H–D exchange process unrelated to the characteristic radical chemistry explanations for free substrates. Nevertheless, it is possible to conclude from the results that the presence of H2 did not alter the influence of immobilization in favoring unimolecular radical processes such as rearrangement to surface-attached 1,1-diphenylethane moieties or cyclization. Of particular interest is the fact that the nonoxygenated benzyl moieties (C7H7 − n Dn fragments separately analyzable by mass spectrometry) invariably contained less deuterium when present in 1-(4%-hydroxyphenyl)-2-phenylethane separated from the silica surface after reaction than was measured in the benzyl groups of volatile product molecules. For example, the C7H7 − n Dn fragment in the mass spectrum of ethylbenzene showed only 1/3 to 1/2 of the deuterium content of the C7H7 − n Dn fragment from surface-recovered 1-(4%-hydroxyphenyl)-2phenylethane. In contrast, the fragments showed equal D content when free 1-(4%-hydroxyphenyl)-2-phenylethane was thermolyzed in the presence of D2. This substantiates the reasonable supposition that surface immobilized substrates are less accessible to D atoms. An attempt to circumvent the surface exchange problems with a different attachment procedure is outlined in Eq. (22) [46]. This attachment procedure
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Fig. 2. Disappearance of stilbene (STB) as a function of time when heated at 405°C under 14 Mpa of H2.
proved effective for 4-(2%-phenylethyl)benzoic acid and the results from thermolysis under D2 were generally in concert
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with the experiments on ether-linked material. However, unexpected exchange promoted by the carboxyl group in the benzoate ring along with inexplicable, procedure-dependent variations in yields have made these systems less reliable than had been hoped.
3.9. Reactions of alkenes with H2 In the studies described above, there has been limited evidence for reaction of stilbene with H2. In the reaction of DPE with D2, reactions having 300 mg of substrate in a 12 ml reactor showed stilbene yield as a reaction product rising to a maximum and then declining. In reactions run with smaller starting quantities of
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DPE, production of stilbene was not observed. Also in the described thermolysis of TMDH, stilbene produced under N2 was found to be partly replaced by highlydeuterated DPE when the reaction was carried out under D2. These results suggested the possibility of a direct reaction between stilbene and D2. Initially, treatment of 300 mg of stilbene with H2 in a 12 ml reactor under conditions of the DPE hydrothermolysis showed no reaction. However, continued examination of this reaction showed that reaction of stilbene with H2 or D2 is efficient at 400°C when the concentration of stilbene is lowered under 100 mg/12 ml [47]. Interestingly, it was found that different alkenes responded quite differently to concentration changes. 2-Phenylpropene (a-methylstyrene) gave increased reduction at higher concentrations. Anthracene underwent hydrogenation to 9,10-dihydroanthracene but the reaction showed no significant concentration dependence. Phenanthrene failed to show measurable conversion under conditions where the other three compounds were hydrogenated (410°C and 14 MPa D2). A more detailed kinetic study of the reaction of stilbene with H2 was then carried out confirming the previously observed concentration dependence and demonstrating autocatalytic behavior as shown in the plot of Fig. 2 [48]. A mechanism was proposed which when computer simulated using the kinetic modeling program ‘ACUCHEM’ [49] gave a reasonable fit to the observed kinetic data. The key step in the model sequence is the addition of H atoms to the double bond as shown in Eq. (23). Other necessary processes are Eqs. (2), (3), (6), (7), and (8). For many of these reactions, literature values for the involved rate constants were available and were assumed in the modeling process. Since the most recent publication on this reaction, an improved kinetic fit has been obtained by including the reaction of H atoms with 1,2-DPE to give 1,2-diphenylethyl radicals as shown in Eq. (24), the thermodynamically favored reverse of Eq. (7) [50].
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(24) The proposed model sequence gives a reasonable fit to the observed experimental data for conversion of DPE-contaminated stilbene to DPE. However, there remains an important problem with pure stilbene. An initiation step is needed! If pure stilbene is subjected to the proposed reaction scheme with H2 even the computer-deprived eye can see that no reaction will take place. It was considered that small amounts of DPE, which can be seen to initiate the sequence via Eq. (2) and are invariably present in commercial stilbene might be responsible. However, reactions run with stilbene prepared via a synthesis designed to preclude DPE contamination,
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participated in a manner indistinguishable from that of commercial samples. Although the nature of the requisite initiation step in this reaction has not been proven, successful modeling may be achieved by assuming that the surface of the glass vessel catalyzes the slow conversion of stilbene to DPE. In separate studies it has been found that fumed silica with its high surface area will catalyze the hydrogenation of alkenes and arenes at temperatures well below those required for thermal reaction in a glass tube [51]. In the ‘uncatalyzed’ reaction, once a small amount of DPE is produced, the reaction becomes self-sustaining with Eq. (2) providing initiation. Increasing conversion leads accordingly to faster reaction, although limited at higher conversions by the H-atom-consuming process of Eq. (24). The inverse concentration effect of stilbene results from the bimolecular nature of Eq. (8). Stilbene traps H atoms as indicated in Eq. (23). The resultant diphenylethyl radicals can then either react with H2 as shown in Eq. (7) or disproportionate by Eq. (8). Higher concentration of stilbene will result in higher steady-state concentration of diphenylethyl radicals which will favor the second-order termination process of Eq. (8) relative to the pseudo-first order propagating reaction of Eq. (7). Although limited experimental data are available for the reaction of 2-phenylpropene with H2 it is reasonable that analogous mechanistic steps are involved. In this case it seems likely that the reaction referred to earlier as reverse radical disproportionation, RRD, provides initiation as shown in Eq. (25). In fact, Ru¨chardt has suggested analogous involvement of RRD in the reaction of 2phenylpropene with dihydroanthracene, (Ru¨chardt refers to this by the older designation, ‘molecular disproportionation’) [52]. Subsequent steps in the hydrogenation sequence would then be parallel to those proposed for stilbene. Significantly, a substantial amount of higher molecular weight material is observed in the 2-phenylpropene reaction suggesting that coupling of radicals may be a viable termination process here.
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4. One-step hydrogenation A particularly interesting case and one deserving of further study is the reaction of anthracene with H2. The uncatalyzed reaction has been reported in 1987 by Chiba and co-workers as an intermediate process in the anthracene-promoted hydrogenation of benzophenone [53]. It has been shown that the dihydroanthracene produced contains 2.1 – 2.6 atoms of D when the reaction is carried out with D2 [47]. Most importantly, there appears to be no concentration dependence, no exchange at aromatic sites and no higher molecular weight products, characteristics which do not fit the type of H-atom chain mechanisms described for most of reactions covered in this review. Although more extensive documentation including
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a careful kinetic study would be required for support, it is necessary to consider the possibility that the reaction of anthracene with H2 takes place via a symmetry-allowed [4+ 2]-cycloaddition. As anthracene is the only system reported where this reaction is viable, it seems a real possibility. Some years ago Fleming and Wildsmith [54] showed that trans-3,6-dideuteriocyclohexa-1,4-diene when heated at 340°C gave monodeuteriobenzene with very little benzene or dideuteriobenzene. They also showed that cis-5,6-dideuteriocyclohexa-1,3-diene failed to give stereospecific loss of either H2 or D2 on thermolysis to give benzene. This agreed with an earlier study by Frey, Krantz and Stevens [55], which showed that cis-3,6-dimethylcyclohexa-1,4-diene lost hydrogen much more rapidly than the trans-isomer. All of this is consistent with the mechanism of Eq. (26) which pictures a single-step concerted process for the addition of H2 to a 1,3-diene or reversedly, the loss of H2 from the 3 and 6 positions of a 1,4-cyclohexadiene. Addition of H2 would be disallowed by orbital symmetry for 9,10-H2 addition to phenanthrene or other vicinal additions and also for H2 removal from a 1,3-cyclohexadiene.
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5. Summary The noncatalytic (or thermal) reaction of H2 or D2 with organic compounds can generally be explained by invoking radical-producing reactions of the substrates followed by reaction of the radicals with H2 (or D2) to remove H (or D), the process generating an H-atom (or D atom). The radical-producing reactions reported have usually been homolytic dissociations of thermolytically labile bonds. In a few cases reverse radical disproportionation (RRD) has been invoked. Once H atoms have been generated by this route they react with organic molecules by hydrogen atom abstraction or by addition to unsaturated sites in substrate molecules. The latter processes produce new radicals which are capable of further reaction with H2 either directly or after some characteristic sequence of radical– radical transformations. A potentially important example is the displacement of substituent groups from aromatic rings, the favored explanation for hydrocracking. In principle, radical chain processes could continue indefinitely. But because the reaction with H2 is normally endothermic and consequently relatively slow, radical–radical reactions terminate these sequences via coupling and disproportionation thus limiting the average number of repetitions and the efficiency of the overall processes. Some interesting reactions with H2 are observed in systems where initiating radical formation cannot be specified. In some cases, unidentified catalysts may be involved. It seems likely that many catalyst-initiated reactions which require temperatures in excess of 300°C involve radical sub-paths similar to those discussed
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