Energy & Fuels 1997, 11, 61-75
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Hydrogen Transfer Induced Cleavage of Biaryl Bonds Bruce R. Cook,* Bernadette B. Wilkinson, Claude C. Culross, Stephen M. Holmes,† and Luis E. Martinez‡ Exxon Research and Development Laboratories, P.O. Box 2226, Baton Rouge, Louisiana 70821-2226 Received April 26, 1996X
Biaryl bonds are the strongest carbon-carbon single bonds in fossil fuels. This paper examines hydrogenolysis and alkane cohydrogenolysis of biphenyl and dimethylbiphenyls, in detail. Biphenyl cleavage was found to be enhanced by copyrolysis and cohydrogenolysis with small amounts of 2,2,3,3-tetramethylbutane (hexamethylethane, HME). Much smaller enhancements were found for cohydrogenolysis with other alkanes. Increased biaryl cleavage rates, in HME cohydrogenolysis, were found to be a direct consequence of initiation by hydrogen atom generated during HME decomposition. Both biphenyl pyrolysis and hydrogenolysis mechanisms involve ipso hydrogen atom attack, followed by ejection of phenyl radical and the formation of benzene. Hydrogen atom is regenerated either through the phenylation of starting biphenyl or through the direct reaction of phenyl radical with H2. Both propagation reactions are very fast, leading to highly efficient chain transfer. Biaryl bond hydrogenolysis was found to be first-order in biphenyl and half-order in H2. Dimethylbiphenyls were found to undergo both demethylation and biaryl cleavage reactions during either neat hydrogenolysis or HME cohydrogenolysis. Cleavage of the much weaker aromatic methyl bond was found to be only slightly favored over biaryl cleavage in dimethylbiphenyl/HME cohydrogenolysis. The branching ratio for biaryl cleavage versus demethylation was also found to be sensitive to dimethylbiphenyl structure. The similar rates for biaryl cleavage and demethylation, as well as the structure sensitivity of the branching ratio, indicate the rate of formation and stability of the ipso hydrogen atom adduct are important in determining the rates of aromatic displacement reactions.
Introduction Facile strong bond cleavage is a key goal for the efficient conversion of heavy hydrocarbon resources. Biaryl bonds are the strongest carbon-carbon single bonds in fossil fuels, with bond strengths calculated to be in the range from 4731 to 4982 kJ/mol. Biaryl cleavage is also a key limiting reaction in the catalytic3,4 and thermal5 center ring cracking of multiring aromatics, such as phenanthrene. Therefore, the ability to cleave strong biaryl bonds would have a major beneficial impact on direct coal liquefaction, or upgrading coal,6 or petroleum-derived aromatic streams. The simple pyrolysis chemistry of biphenyl, in inert media, has been extensively studied. Significantly fewer studies of simple biphenyl hydrogenolysis have been reported. Pure biphenyl pyrolyzes very slowly at temperatures below 500 °C, producing terphenyl and * Address correspondence to this author at Exxon Research & Engineering, Route 22 East, Annandale, NJ 08801. † 1992 ERDL summer intern. Present address: University of Illinois at Urbana-Champaign. ‡ 1991 ERDL summer intern. Present address: Harvard University. X Abstract published in Advance ACS Abstracts, November 15, 1996. (1) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982, 33, 493-532. (2) Robaugh, D.; Tsang, W. J. Phys. Chem. 1986, 90, 5363-5367. (3) Wu, W. L.; Haynes, H. W. Hydrocracking and Hydrotreating; ACS Sypmposium Series 20; American Chemical Society: Washington, DC, 1975; pp 65-81. (4) Lemberton, J. L.; Guisnet, M. Appl. Catal. 1984, 13, 181-192. (5) Penninger, J. M. L.; Slotboom, H. W. Industrial and Laboratory Pyrolysis; ACS Symposium Series 32; American Chemical Society: Washington, DC, 1975; pp 444-456. (6) Poutsma, M. L. Energy Fuels 1990, 4, 113-131.
S0887-0624(96)00066-7 CCC: $14.00
quaterphenyl oligomers and a minor amount of benzene.7,8 Proksch et al.7 have reported that holding times of 5-20 h are required at 420-465 °C to produce 99
3.2 ( 0.1 0.4 ( 0.3 1.9 ( 0.3 4.0 ( 0.3
4.5 ( 0.6 0.2 ( 0.2 1.7 ( 0.4 1.4 ( 0.4 7.9 ( 1.7
11.4 ( 0.4 0.5 ( 0.1 3.8 ( 0.5 0.3 ( 0.2 13.3 ( 0.8
87.4 ( 0.4 3.1 ( 0.4
83.3 ( 1.0 3.1 ( 0.4
69.1 ( 0.8 2.3 ( 0.4
a Conversion is the wt % disappearance of HME starting material. Selectivity is 100 times the wt % yield of each product divided by the wt % conversion. Results represent averages of multiple independent experiments.
Figure 4. Principal decomposition reactions for minibomb HME pyrolysis.
pyrolysis: the elimination of hydrogen atom from tertbutyl radical (Figure 2, reaction 3). In minibomb pyrolysis, bimolecular hydrogen transfer reactions involving tert-butyl radical and starting HME slow hydrogen atom production by consuming tert-butyl radical before it can undergo the elimination reaction. In hydrogenolysis, bimolecular hydrogen transfer increases the production of hydrogen atom through the direct reaction of tert-butyl radical with molecular hydrogen. This direct reaction with hydrogen is the dominant radical pathway for HME hydrogenolysis. This reaction pathway becomes even more favored at high H2/HME feed ratios. The principal reactions involved in minibomb pyrolysis of HME are shown in Figure 4. Chain initiation occurs through reaction 2, the symmetric cleavage of HME to form two tert-butyl radicals. Hydrogen atom is then formed through unimolecular elimination from tert-butyl radical, reaction 3. Product H2 is formed through the rapid reaction of hydrogen atom with available hydrocarbons such as starting HME (reaction 5). Product H2 is, therefore, a qualitative and perhaps semiquantitative measure for hydrogen atom production. The first-order rate constant of reaction 3 at 450 °C is 3.7 × 102 s-1, as calculated by extrapolating Tsang’s shock wave rate parameters to current conditions.22 No literature values are available for reaction (22) Tsang, W. J. Am. Chem. Soc. 1985, 107, 2872-2880.
Figure 5. Key side reactions for minibomb HME pyrolysis.
5, but a second-order rate constant can be approximated by adjusting the pre-exponential term for Tsang and Hampson’s23 rate for hydrogen atom abstraction from ethane. This results in an estimated second-order rate constant for reaction 5 of approximately 4.5 × 108 M-1 s-1 at 450 °C. Initial HME concentration must be considered for comparison to competitive unimolecular reactions. This is accomplished by calculating an initial pseudo-first-order rate constant by multiplying by the intial HME concentration. The initial pseudo-first-order rate constant for reaction 5 is 1.8 × 107 s-1. Hydrogen atoms, generated through reaction 3, will obviously quickly react with HME to form molecular hydrogen. Although reaction 5, per se, should decrease as HME is consumed during the reaction, it can be expected that similar reactions with product hydrocarbons, such as reaction 8 in Figure 5, will become more important as conversion increases. In addition to producing hydrogen atom, tert-butyl radical will also be subject to hydrogen transfer reactions. The most important hydrogen transfer reaction (23) Tsang, W.; Hampson, R. F. J. Phys. Chem. Ref. Data 1986, 15, 1087-1279.
Cleavage of Biaryl Bonds
at early conversion would be the abstraction of hydrogen from starting HME, reaction 4. This reaction forms isobutane, the major product in minibomb pyrolysis, and HME radical. HME radical can be expected to undergo relatively rapid beta scission, as described in reaction 6, producing isobutene and re-forming tert-butyl radical. This bimolecular hydrogen transfer pathway regenerates the active intermediate for producing hydrogen atom but, in effect, lowers efficiency for hydrogen atom production by decomposing HME unproductively. No literature kinetic data are available for either reaction 4 or 6, but reasonable estimates can be made using literature values for similar reactions. Zhang and Back24 have reported a detailed kinetic study for the reaction of methyl radical with isobutane, including the abstraction of isobutane’s tertiary hydrogen. The calculated second-order rate constant for this reaction at 450 °C is 5.1 × 105 M-1 s-1. The rate for tert-butyl radical abstraction of hydrogen from methane can be calculated using this rate constant and the equilibrium constant at 450 °C. Standard temperature and pressure thermodynamic parameters for methane and isobutane were taken from Stull, Westrum, and Sinke.25 Benson’s26 parameters were used for methyl and tert-butyl radical. The equilibrium constant at 450 °C was calculated using the change in constant pressure heat capacities to extrapolate from 25 °C to reaction conditions. This results in a calculated equilibrium constant at 450 °C of 2.0 × 10-3 for methyl radical abstraction of isobutane’s tertiary hydrogen and a calculated second-order rate constant of 2.6 × 102 M-1 s-1 for tert-butyl abstraction from methane. A secondorder rate constant for reaction 4 can now be estimated by lowering the activation energy 6.9 kcal to account for the weaker carbon hydrogen bond strength in HME vs methane and adjusting the pre-exponential factor for the higher number of hydrogens in HME. These assumptions lead to an approximate second-order rate constant of 1.4 × 105 M-1 s-1 and initial pseudo-firstorder rate constant of 5.5 × 103 s-1. The rate for reaction 6 should be relatively fast in comparison. An estimated first-order rate constant of 1 × 106 can be calculated using half the activation energy and the same pre-exponential factor as reaction 3. On the basis of this rate analysis, the rate of reaction 4 should be considerably faster than that of reaction 3, and therefore the majority of HME decomposition should occur through the non-hydrogen atom producing bimolecular pathway. It should be noted that though this bimolecular pathway is not terminal because tert-butyl radical is re-formed by reaction 6, the efficiency for hydrogen atom production is lowered by unproductive consumption of HME. The relative efficiency for hydrogen atom production from HME pyrolysis is directly related to the branching ratio for tert-butyl radical undergoing reaction 3 vs reaction 4. Reaction 3 produces hydrogen atoms and ultimately molecular hydrogen through reaction 5. Reaction 4 produces isobutane and ultimately isobutene. (24) Zhang, H. X.; Back, M. H. Int. J. Chem. Kinet. 1990, 22, 537541. (25) Stull, D. R.; Westrum Jr., E. F.; Sinke, G. C. The Chemical Thermodynamics of Organic Compounds; Robert F. Krieger Publishing: Malabar, FL, 1987; reprint with corrections. (26) Benson, S. W. Thermochemical Kinetics, 2nd ed.; Wiley: New York, 1976.
Energy & Fuels, Vol. 11, No. 1, 1997 65
Both paths lead to the regeneration of tert-butyl radical. The molar ratio of molecular hydrogen, the ultimate product of reaction 3, to isobutane, the product of reaction 4, should provide a quantitative measure of this important tert-butyl radical branching ratio and therefore the relative efficiency for hydrogen atom production. The product H2/isobutane mole ratios for shock wave, photolytic, and minibomb pyrolysis of HME are ∞, 2.6, and 0.11, respectively. These values essentially inversely correlate with the experimental HME concentration. Low-concentration shock wave pyrolysis occurs exclusively through reaction 3, intermediate-concentration photolysis occurs predominantly through reaction 3, and high-concentration minibombs occur principally through reaction 4. A literature-predicted product H2/ isobutane mole ratio for minibombs can be calculated using the ratio of literature rate constant for reaction 3 divided by reaction 4’s literature-estimated pseudo-firstorder rate constant, as discussed above. The experimental 0.11 H2/isobutane mole ratio for the minibombs is bracketed by the literature-predicted H2/isobutane mole ratios of 0.07 at initial concentration and 0.17 at 60% HME conversion. Isopentane, propene, and methane are the other significant products of minibomb HME pyrolysis. These compounds are the products of secondary radical reactions of primary isobutane and isobutene products as shown in Figure 5. Propene and methane most likely arise from beta scission of isobutyl radical as shown in reaction 11. Two distinct routes are possible to isobutyl radical. The first route is hydrogen abstraction of isobutane’s primary hydrogen, while the second is hydrogen atom addition to isobutene. Potential hydrogen abstractors for the first route are either tert-butyl radical, as shown in reaction 7, or hydrogen atom, as shown in reaction 8. Although reaction 7 is energetically uphill, large amounts of tert-butyl radicals would probably make this reaction significant. Hydrogen atom abstraction of either primary or tertiary hydrogen from isobutane should have relatively low activation energies (see discussion of reaction 5 above), and although teritiary hydrogen abstraction would be energetically favored, primary hydrogen abstraction should be favored by a larger pre-exponential factor based on the number of abstractable hydrogens. It is difficult, if not impossible, to assess the significance of tertiary hydrogen abstraction reactions because the tert-butyl radical product is also the principal radical chain product of HME decomposition. Hydrogen atom addition to isobutene can form either isobutyl or tert-butyl radicals (reactions 9 and 10). Although tert-butyl is favored energetically, it is once again difficult, if not impossible, to assess the significance of hydrogen atom addition routes to it. Hydrogen abstraction from isobutene should also be a rapid reaction, but the absence of allene product indicates either that the hydrogen atom addition to isobutene is highly favored over abstraction or that the beta scission of the highly stabilized isobutenyl radical is much slower than quenching by hydrogen transfer. HME hydrogenolysis requires consideration of additional reactions involving molecular hydrogen. Figure 6 shows the principal reactions involved in HME hydrogenolysis. The direct reaction of tert-butyl radical with molecular hydrogen, reaction 13, is now the
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Cook et al. Table 4. Conversion and Product Selectivity for Neat Biphenyl Pyrolysis (No HME) and Biphenyl/HME Copyrolysisa selectivity temp time (°C) (min) no HME
450
10 mol 450 % HME
Figure 6. Principal decomposition reactions for minibomb HME hydrogenolysis.
principal means for forming hydrogen atom. Evans and Walker27 have reported the rate of reaction for tert-butyl and H2, reaction 13, as 1.7 × 104 M-1 s-1 at 450 °C. Although slower by almost an order of magnitude than reaction 4, excess H2 will favor reaction 13. Reaction 13 should become the principal reaction for tert-butyl radical at H2/HME concentrations >8.2 and the dominant atomic hydrogen producer at ratios below that. All biphenyl hydrogenolysis reactions in this study were conducted at H2/HME ratios in excess of 19; therefore, essentially every tert-butyl radical should produce hydrogen atom through reaction 13. In conclusion, high-concentration minibomb pyrolysis of HME produces hydrogen atoms by the same mechanism as observed in high-temperature shock waves. The efficiency of hydrogen atom production in minibombs is considerably lower than shock waves due to competing hydrogen transfer reactions. In contrast to pyrolysis, minibomb hydrogenolysis of HME produces hydrogen atoms efficiently through the direct reaction of tert-butyl radical with molecular hydrogen. Biphenyl Pyrolysis Results. Consistent with literature reports, biphenyl was found to be essentially inert when pyrolyzed for 120 min at 450 °C. Average conversions and selectivities for neat biphenyl pyrolysis are shown in Table 4. Conversion is defined as the weight percent disappearance of starting biphenyl. Selectivity is defined as the yield of each product divided by conversion. Previous studies of biphenyl pyrolysis7 have assumed first-order kinetics, and for ease of comparison to literature rates this convention will be used in this study. Limits for analysis indicate that the first-order rate constant for biphenyl conversion under these conditions is 4,4′ > 3,3′ . biphenyl. Significant levels of toluene and benzene, the expected products of biaryl cleavage, are observed for all three dimethylbiphenyls. The rate of biaryl cleavage is also structure sensitive. The yields of biaryl cleavage products are 3.7, 4.9, and 5.4 mol % for 3,3′-, 2,2′-, and 4,4′-dimethylbiphenyl, respectively, vs 3.2 mol % for biphenyl. In addition to biaryl cleavage, all of the dimethylbiphenyls undergo significant demethylation to form the corresponding methylbiphenyl and biphenyl. In addition to biaryl cleavage and demethylation, the 2,2′-dimethylbiphenyl isomer also undergoes a significant amount of ring closure, forming 4-methylfluorene and fluorene. Selectivity for biaryl cleavage vs demethylation is also sensitive to substitution for the two isomers where these are essentially the only observed reactions. The 3,3′-dimethylbiphenyl isomer shows a higher selectivity for biaryl cleavage than the 4,4′-dimethylbiphenyl isomer, albeit at much lower conversion. The relative ratios of the first-order rates of biaryl cleavage to demethylation, during neat hydrogenolysis, are 0.54, 0.45, and 0.42 for 3,3′-dimethylbiphenyl, 4,4′-dimethylbiphenyl, and 2,2′-dimethylbiphenyl, respectively. The addition of HME increases the overall rate of conversion for all three dimethylbiphenyls (Table 13). The magnitude of the increase is once again dependent on the position of substitution. A complete reversal in relative conversion is observed during HME cohydrogenolysis. 3,3′-Dimethylbiphenyl, the most refractory during neat hydrogenolysis, has the highest conversion during cohydrogenolysis with HME. The first-order rates of conversion are increased 4.3-, 2.1-, and 1.4-fold, respectively, for 3,3′-, 4,4′-, and 2,2′-dimethylbiphenyl. Specifically, the rates of demethylation and biaryl cleavage are both enhanced for all three isomers, and the increases are proportional to the enhancement of overall conversion. The independence of product selectivity on the presense of HME is, once again, strong evidence that HME acts solely as an inititior of hydrogenolysis, impacting the rate of conversion by establishing the number of chains. It also indicates either very long radical chains or that the same radical is acting as chain initiator and as a chain carrier. The relative selectivity for biaryl cleavage vs demethylation exhibits the same structure sensitivity observed in neat hydrogenolysis, even though relative
Cook et al. Table 13. HME Cohydrogenolysis of Dimethylbiphenyls at 450 °C, 2 h, and H2/Dimethylbiphenyl Mole Ratio of 7.8a
a Conversion is mol % disappearance of dimethylbiphenyl starting material. Selectivity is 100 times the mole yield of rings in each product divided by 2 times the number of moles of dimethylbiphenyl converted.
Table 14. Effect of Holding Time on HME Cohydrogenolysis of 3,3′-Dimethylbiphenyl at 450 °Ca selectivity time (min)
total convrn
3-methylbiphenyl
biphenyl
toluene
benzene
30 60 120
15.6 ( 1.6 32.8 ( 2.0 54.4 ( 2.2
67 ( 6 62 ( 2 55 ( 2
2 ( 0.6 4 ( 0.3 9(1
21 ( 3 25 ( 2 25 ( 1
1 ( 0.6 4(1 7(1
a [3,3′-Dimethylbiphenyl] init ) 0.45 M, [HME]init ) 0.045 M, and 7.6 H2/3,3′-dimethylbiphenyl. Conversion is the mol % disappearance of dimethylbiphenyl starting material. Selectivity is 100 times the mole yield of rings in each product divided by 2 times the number of moles of dimethylbiphenyl converted. Results represent averages of several independent experiments.
conversions are reversed. The ratio of first-order rates of biaryl cleavage to demethylation are 0.56, 0.43, and 0.46 for 3,3′-dimethylbiphenyl, 4,4′-dimethylbiphenyl, and 2,2′-dimethylbiphenyl, respectively. This indicates that the relative selectivities observed are fundamental in nature and not simply a reflection of relative conversion. Ring closure to form fluorenes remains a significant reaction for the 2,2′ isomer. The relative selectivity for fluorene vs 4-methylfluorene is essentially the same as for neat hydrogenolysis at lower conversion. This suggests that the observed selectivity is primarily a function of the ring closure step rather than subsequent demethylation of 4-methylfluorene. Fluorene is formed through the ejection of methyl, while 4-methylfluorene is formed through the ejection of hydrogen. The observed selectivities for hydrogen vs methyl displacement are 0.65 for neat hydrogenolysis and 0.60 for HME cohydrogenolysis. The effect of conversion on selectivity was further confirmed by examining 3,3′-dimethylbiphenyl/HME cohydrogenolysis at lower holding times (Table 14). The biaryl to demethylation ratio is essentially the same at all conversion levels. The ratios of first-order rates for biaryl cleavage to demethylation are 0.55 and 0.57 at 30 and 60 min holding times vs 0.56 at 120 min. At a 30 min holding time, the conversion is essentially the same as observed for neat hydrogenolysis at 120 min. At comparable conversion levels selectivity is identical, once again indicating that HME is acting solely as an initiator and does not significantly affect propagation reactions.
Cleavage of Biaryl Bonds
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Table 15. Effect of Temperature on Neat and HME Cohydrogenolysis of 3,3′-Dimethylbiphenyl at 475 °Ca selectivity
neat w/HME
total convrn
3-methylbiphenyl
biphenyl
toluene
benzene
15.2 ( 1.5 60.1 ( 1.1
66 ( 6 51 ( 1
2 ( 0.6 9(1
27 ( 3 26 ( 2
1 ( 0.6 4(1
a [3,3′-Dimethylbiphenyl] init ) 0.45 M; [HME]init ) 0.045 M; 7.6 H2/3,3′-dimethylbiphenyl; 30 min holding time. Conversion is the mol % disappearance of dimethylbiphenyl starting material. Selectivity is 100 times the mole yield of rings in each product divided by 2 times the number of moles of dimethylbiphenyl converted. Results represent averages of several indepedent experiments.
Neat hydrogenolysis and HME cohydrogenolysis of 3,3′-dimethylbiphenyl exhibit slightly different activation energies (Table 15). The overall activation energy for neat hydrogenolysis is 236 kJ/mol (56.5 kcal/mol), while the corresponding activation energy for HME cohydrogenolysis is slightly higher at 284 kJ/mol (67.9 kcal/mol). These differences may reflect the different initiation steps for both systems. Increased selectivity for biaryl cleavage vs demethylation is observed as temperature is increased. The ratio of first-order rate constants for biaryl cleavage to demethylation is 0.61 for both neat hydrogenolysis and HME cohydrogenolysis, which reflects a ∼20 kJ/mol higher activation energy for biaryl cleavage vs demethylation. The activation energy for hydrogen atom displacement of phenyl can be calculated to be ∼40 kJ/mol on the basis of Tsang’s activation energy for hydrogen atom displacement of methyl in toluene.17 Dimethylbiphenyl Hydrogenolysis Discussion. Native and process-derived biphenyls will invariably contain other functionalities, including alkyl substitution. Alkyl substitution makes possible competing dealkylation and hydrogen abstraction reactions, in addition to biaryl cleavage. Both of these side reactions are potentially deleterious. Dealkylation results in the undesirable production of light gases, while benzylic abstraction forms stable radicals that could slow chain propagation or lead to undesirable condensation reactions. It is therefore important to assess the effect of alkyl substitution on hydrogen atom induced biaryl hydrogenolysis. Both demethylation and biaryl cleavage are observed in 3,3′-dimethylbiphenyl/HME cohydrogenolysis. Demethylation is faster than biaryl cleavage, but biaryl cleavage is competitive, occurring at greater than half the rate of demethylation. First-order kinetic analysis of overall conversion, demethylation, and biaryl cleavage for 3,3′-dimethylbiphenyl/HME cohydrogenolysis (Figure 13) confirms the expected first-order dependence on dimethylbiphenyl concentration for all three. As shown above, biaryl hydrogenolysis should be half-order in H2. Hydrogen atom induced demethylation is also known to be half-order in H2.6 The rate laws for demethylation and biaryl cleavage of dimethylbiphenyl are, therefore, given by eqs 6 and 7, respectively. At 450 °C, kdemethylation and kbiaryl cleavage for 3,3′-dimethylbiphenyl during HME cohydrogenolysis are 3.0 × 10-5 and 1.6 × 10-5 M-1/2 s-1, respectively. The rate of biaryl cleavage for 3,3′-dimethylbiphenyl/HME cohydrogenolysis is 3.3 times slower than for biphenyl during HME cohydrogenolysis. Although the biaryl cleavage is slowed by methyl substitution, the combined rate of demethy-
Figure 13. First-order kinetics treatment for total conversion, demethylation, and biaryl cleavage of 3,3′-dimethylbiphenyl during cohydrogenolysis with HME at 450 °C and H2/biphenyl ratio of 7.6. The initial concentration of 3,3′-dimethylbiphenyl is 0.45 M.
lation and biaryl cleavage is only 10% slower than for biphenyl. This indicates that initiation is rate determining and that regeneration of hydrogen atom is fast by comparison. The shortfall in combined rate apparently represents the amount of quenching due to abstraction of benzylic hydrogen to form a stabilized benzylic radical. For 3,3′-dimethylbiphenyl, benzylic radical likely only regenerates hydrogen atom through the direct reaction with H2, and so benzylic abstraction will only slow the overall rate of reaction by producing a relatively stable radical.
-d[aryl-CH3 bonds]/dt ) kdemethylation[aryl-CH3 bonds][H2]1/2 (6) -d[biaryl bonds]/dt ) kbiaryl cleavage[biaryl bonds][H2]1/2 (7) A plausible reaction scheme for 3,3′-dimethylbiphenyl/ HME cohydrogenolysis is shown in Figure 14, where hydrogen atom and phenyl and methyl radicals are chain carriers. As with biphenyl/HME cohydrogenolysis, the initial formation of hydrogen atom occurs through reactions 2 and 13. Hydrogen atom is, once again, the entry point into dimethylbiphenyl hydrogenolysis. Hydrogen atom addition can now occur ipso to either phenyl (reaction 17), leading to biaryl cleavage, or methyl (reaction 19), leading to demethylation. Hydrogen atom regeneration occurs through the direct reactions of phenyl and methyl radicals with H2, reactions 18 and 20, respectively. Termination occurs through reaction 14. In addition to ipso addition, hydrogen atom can abstract benzylic hydrogen from starting dimethylbiphenyl (reaction 21). The benzylic radical formed by reaction 21 can undergo few plausible reactions other than the reverse of reaction 21. As discussed above, benzylic hydrogen abstraction from 3,3′-dimethylbiphenyl should slow overall conversion but produce no extra products. Biaryl cleavage is slowed in 3,3′-dimethylbiphenyl from competing abstraction, which globally reduces both demethylation and biaryl cleavage, and by competition with demethylation reactions. The branching ratio for biaryl cleavage vs demethylation favors demethylation, but not as much as might be expected on the basis of the relative bond energies1 for aromatic methyl (426 kJ/mol) vs biaryl (476
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Cook et al.
Figure 15. Hydrogen atom adducts responsible for either biaryl cleavage or demethylation of dimethylbiphenyls.
Figure 14. Principal reactions for 3,3′-dimethylbiphenyl/HME cohydrogenolysis.
kJ/mol) bonds. The branching ratio is also not statistical, as both demethylation and biaryl cleavage have two potential hydrogen atom addition sites. Similar effects have been observed in shock wave experiments for which measured rates for hydrogen atom displacement of Cl, OH, NH2, and methyl in monosubstituted singlering aromatics also do not reflect the large disparity in bond energies. The relative displacement rates34 at 812 °C for chlorobenzene, phenol, aniline, and toluene are 0.31, 0.51, 0.64, and 1.00, respectively, whereas the bond strengths are 400, 464, 434, and 422 kJ/mol.2 If the energy barrier for carbon-carbon bond breaking were very large relative to the energy barrier for hydrogen atom addition, then the relative rates of displacement should vary by several orders of magnitude. Chemical activation of the hydrogen atom adduct is one possible explanation for the small differences in relative displacement rates. Although chemical activation is possible in dilute shock waves, it is highly unlikely in highpressure minibomb experiments. The small differences in displacement rates indicate that the rate of hydrogen atom addition must be significant in determining the observed rate of displacement. If the rate of formation of the ipso hydrogen atom adduct is important in determining the relative rate of displacement, then the relative stability of the hydrogen atom adduct should affect the biaryl cleavage vs demethylation branching ratio. The branching ratios observed in the various dimethylbiphenyls appear to reflect this. The higher branching ratio exhibited by the 3,3′ isomer vs either the 2,2′ or 4,4′ isomer is consistent with greater resonance stabilization of the demethylation hydrogen atom adduct for the 2,2′ and 4,4′ isomers. As shown in Figure 15, the ipso adduct responsible for biaryl cleavage is stabilized in only one (34) Cui, J. P.; He, Y. Z.; Tsang, W. J. Phys. Chem. 1989, 93, 724727.
ring regardless of alkyl substitution. The ipso adduct responsible for demethylation is stabilized by only one ring for the 3,3′ isomer but is potentially stabilized over two rings for the 2,2′ and 4,4′ isomers. While a significant energy barrier for hydrogen atom addition is consistent with both current and literature data, the structure sensitivity of the branching ratio could also reflect minor changes in the barrier height for the carbon-carbon bond-breaking step. Changes in the barrier height for carbon-carbon bond breaking would result from slight thermodynamic differences in the respective molecular and radical products of reaction 17 or 19. The molecular product of reaction 17 is toluene for all three dimethylbiphenyls, but different methyl-substituted phenyl radicals are formed depending on starting dimethylbiphenyl structure. These methyl-substituted phenyl radicals could vary slightly in thermodynamic stability; however, the observation of similar selectivity in terphenyl dephenylation, for which biphenyl and phenyl radicals are common products, indicates that the relative stability of the methylsubstituted phenyl radical is not important in determining the biaryl cleavage demethylation branching ratio. Methyl radical is the common product of reaction 19 for all three dimethylbiphenyl structures, so any thermodynamic differences for reaction 19 would have to reflect the relative energetics of the monoomethylbiphenyl product vs the starting dimethylbiphenyl. No experimental thermodynamic data are available for all of these compounds, but it is reasonable to assume that both the 2,2′ and 4,4′ isomers would be less stable than the 3,3′ isomer, by analogy to xylene isomer stability. This relative stability difference might favor the demethylation vs the biaryl cleavage reaction, but if the relative stability of the starting dimethylbiphenyl is important in determining the branching ratio, the 2,2′ isomer should have the lowest branching ratio. This is not observed experimentally, where the observed branching ratios for the 2,2′ and 4,4′ isomers are essentially equivalent. Methyl substitution also increases the rate of biaryl cleavage during neat hydrogenolysis. As with the relative biaryl cleavage and demethylation rates, the magnitude of the rate enhancements is structure sensitive. The rate constants, based on eq 7, for biaryl cleavage are 5.7 × 10-6, 5.3 × 10-6, and 3.8 × 10-6 M-1/2 s-1 for the 4,4′, 2,2′, and 3,3′ isomers, respectively. These rates constants are all larger than 2.8 × 10-6
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reactions is highly dependent on the rate of formation and stability of the ipso hydrogen atom adduct. Conclusions
Figure 16. Potential initiation reactions for neat hydrogenolysis of 2,2′-, 3,3′-, and 4,4′-dimethylbiphenyl.
Figure 17. Ring-closure reactions for 2,2′-dimethylbiphenyl during either neat hydrogenolysis or HME cohydrogenolysis.
M-1/2 s-1 for biphenyl itself. Because biaryl cleavage has already been shown to be initiation and not propagation limited, the rate increase due to methyl substitution is likely from more efficient initiation. Likely initiation reactions are the homolytic cleavage of the benzylic carbon-hydrogen bond as shown in reactions 22-24 in Figure 16. Increased resonance stabilization of the benzylic radical should lower the bond energy and thereby increase the observed rate of reaction. The increased rates of both the 2,2′ and 4,4′ isomers are therefore due to the fact that the benzylic radical derived from the 2,2′ and 4,4′ isomers can be stabilized over both rings, while the 3,3′ isomer is only stabilized by one ring. In addition to demethylation and biaryl cleavage, ring closure is a significant reaction for 2,2′-dimethylbiphenyl. Approximately 40% of the reacting 2,2′-dimethylbiphenyl undergoes ring closure to form fluorene and 4-methylfluorene. The ring-closure products are likely derived from the benzylic radical intermediate, formed either by homolytic cleavage of the carbon-hydrogen bond during initiation or by bimolecular hydrogen abstraction reactions. Ring closure through displacement of either methyl (reaction 25, Figure 17) or hydrogen (reaction 26) will lead to fluorene or 4-methylfluorene, respectively. Assuming that only a minor fraction of the fluorene is the product of subsequent demethylation of methylfluorene, the ratio of hydrogen to methyl displacement during ring closure is ∼0.6. As with demethylation vs biaryl cleavage, this rate difference is smaller than might be expected from relative bond energies for aromatic-hydrogen (464 kJ/mol) vs aromatic-methyl (422 kJ/mol) bonds. This indicates that the rate of radical addition also plays a major role in determining the rate of displacement by relatively low-energy benzylic radicals. In conclusion, the rate of biaryl cleavage by hydrogen atom occurs at rates competitive with other aromatic displacement reactions, such as demethylation. The actual observed rate of such aromatic displacement
Hydrogen atoms are produced during both pyrolysis and hydrogenolysis of HME in high concentration minibomb reactors. Hydrogen atoms are produced by different mechanisms for each system. During pyrolysis, hydrogen atoms are produced through the unimolecular elimination of atomic hydrogen from tert-butyl radical. This is the same reaction pathway responsible for hydrogen atom production in both high-temperature shock waves and pulse photolysis. The hydrogen atom production efficiency in minibomb pyrolysis is quite low compared to shock wave and photolytic pyrolysis due to competitive hydrogen transfer reactions. During minibomb hydrogenolysis, hydrogen atoms are produced by the direct bimolecular reaction of tert-butyl radical with H2. This bimolecular path is very efficient in highpressure minibombs. Hydrogen atoms, produced during HME pyrolysis and hydrogenolysis, effect the low-temperature cleavage of biaryl bonds. Biaryl cleavage, for both biphenyl pyrolysis and hydrogenolysis, is accomplished via the formation of an ipso hydrogen atom adduct followed by the elimination of phenyl radical. In pyrolysis, hydrogen atom is regenerated through the phenylation of starting biphenyl. In hydrogenolysis, hydrogen atom is regenerated though the direct reaction of phenyl radical with H2. Once initiated, both propagation reactions are very fast, leading to highly efficient chain transfer. Biaryl bond hydrogenolysis was found to be first-order in biphenyl and half-order in H2. The high chain transfer efficiency for biaryl hydrogenolysis makes the observed rate very sensitive to the presence of initiating coreactants or substituents. Coreactants or substituents that produce hydrogen atoms or other high-energy radicals during decomposition will initiate biaryl cleavage. HME, a well-known shock wave source for hydrogen atoms, is a very effective initiator of biphenyl pyrolysis and hydrogenolysis. Biaryl cleavage is also initiated to a lesser extent during cohydrogenolysis with other alkanes. Aromatic methyl substituents also provide effective initiation of biaryl cleavage through homolysis of benzylic carbon-hydrogen bonds. The effect of methyl substitution is structure sensitive, with the observed rate correlating with the degree of resonance stabilization in the benzylic radical product from initiation through side-chain carbonhydrogen homolysis. Methyl substitution slows by two-thirds the observed rate of hydrogen atom induced biaryl cleavage, primarily through competition with demethylation reactions, also induced by hydrogen atom. Although slower than demethylation, the rate of biaryl cleavage is competitive. The observed branching ratio for biaryl cleavage vs demethylation ranges from ∼0.4 for 2,2′- and 4,4′dimethylbiphenyl to 0.56 for 3,3′-dimethylbiphenyl. The surprisingly similar rates for biaryl cleavage and demethylation and the structure sensitivity of this ratio indicate the relative rate of formation and stability of the ipso hydrogen atom adduct are important in determining the observed rate of displacement. EF9600669
