Ind. Eng. Chem. Res. 1998, 37, 4253-4261
4253
MATERIALS AND INTERFACES OsHCl(CO)(O2)(PCy3)2-Catalyzed Hydrogenation of Acrylonitrile-Butadiene Copolymers J. Scott Parent,† Neil T. McManus, and Garry L. Rempel* Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
A new homogeneous catalyst precursor, OsHCl(CO)(L)(PCy3)2 (1, L ) vacant; 2, L ) O2), has been identified for the selective hydrogenation of olefin within acrylonitrile-butadiene copolymers. This report details a study of its catalytic behavior at hydrogenation conditions approaching those used industrially. Carefully acquired kinetic data, collected within a statistical design framework, define the influence of [Os], [RCN], [H2], [PCy3], and temperature on the catalytic activity. Reversible coordination of nitrile to complex 1 not only reduces the hydrogenation rate but creates an unprecedented sensitivity of the process to pressure. Unique to this system is an apparent second-order dependence of the hydrogenation rate on [H2], which diminishes toward zero order as pressures exceed 60 bar. In contrast, hydrogenation of substrates that lack nitrile is virtually indifferent to [H2] at all pressures. These kinetic measurements, coupled with recently acquired knowledge of the structures and reactivities of complexes 1 and 2, provide the basis of a plausible reaction mechanism. Introduction The chemical modification of diene-based polymers is a versatile synthetic route for preparing materials that cannot be manufactured by conventional polymerization techniques.1 Using this approach, a commodity elastomer such as nitrile-butadiene rubber (NBR) can be transformed into a specialty material (HNBR) that demonstrates superior resistance to degradation by heat, oxygen, and ozone (Scheme 1). If selective for olefin hydrogenation over nitrile reduction, the hydrogenation process can eliminate a reactive site within the polymer backbone while retaining the material’s oil resistance.2 At present, this apparent ethylene-butylene-acrylonitrile terpolymer cannot be made from its constituent monomers, leaving the NBR modification route as the only means available to commercial HNBR producers. Of the innumerable transition-metal complexes capable of catalyzing olefin hydrogenation, those recognized as viable HNBR catalysts have been based on rhodium, ruthenium, and palladium.3 While mononuclear osmium complexes have been largely overlooked, new research has demonstrated their ability to facilitate a variety of reactions, including the selective hydrogenation of compounds containing sensitive functional groups.4 We have recently discovered the remarkable efficiency of the osmium(II) complexes, OsHCl(CO)(L)(PCy3)2 (1, L ) vacant; 2, L ) O2). as catalysts for the selective hydrogenation of NBR.5 Remarkably, these catalysts are more active at industrial conditions (P > 20 bar, T > 100 °C) than the current generation of Rh, Ru, and Pd systems.6 * Author for correspondence. † Present address: Department of Chemical Engineering, Queen’s University, Kingston, Ontario, Canada K7L 3N6.
Scheme 1
Our understanding of the reactions that constitute an NBR hydrogenation has been furthered by 31P and 1H NMR studies of complexes 1 and 2.7 Scheme 2 illustrates the structures and reactivities of the O2, RCN, and H2 complexes that are of particular interest to our present application. When employed as a catalyst precursor, 2 is activated by O2 dissociation to generate the five-coordinate analogue, 1. Owing to its coordinative unsaturation, complex 1 readily adds small Lewis bases to form isolable complexes such as the nitrile adduct, complex 4. Molecular hydrogen adds to the metal center by η2 coordination, wherein the H-H bond remains intact.8 Under 24 bar of H2 at 130 °C, the predominant osmium complexes during the hydrogena-
10.1021/ie980405g CCC: $15.00 © 1998 American Chemical Society Published on Web 10/15/1998
4254 Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 Scheme 2
tion of NBR are known to be the dihydrogen and nitrile complexes 3 and 4, respectively. In this paper, we present kinetic data for the selective hydrogenation of NBR by complex 2 at reaction conditions that approach those used in commercial operations. These data define the functional relationship between hydrogenation activity and process factors such as [Os], [RCN], [H2], [PCy3], and temperature. This information, combined with our recent spectroscopic studies of complexes 1 and 2, provides the foundation of a plausible reaction mechanism. Given the scarcity of literature devoted to the catalytic chemistry of osmium and the commercial potential of this class of complexes, we hope that this contribution advances both academic and industrial research interests. Experimental Section Complex 1, OsHCl(CO)(PCy3)2, was prepared by refluxing OsCl3‚3H2O with PCy3 (both from Strem Chemicals) in methoxyethanol according to the method of Esteruelas and Werner.9 The corresponding dioxygen adduct, OsHCl(CO)(O2)(PCy3)2 (2), was prepared, exposing a suspension of complex 1 in hexane to pure O2 as detailed by Esteruelas et al.10 Oxygen-free H2 and D2 of 99.99% purity (Linde-Union Carbide) and reagentgrade monochlorobenzene (Fischer Chemicals) were used as received. The nitrile-butadiene rubber provided by Bayer Inc. (Krynac 38.50) contained 62 wt % butadiene (80% trans, 15% cis, 5% vinyl isomerization) and possessed an Mn ) 70 000 and a polydispersity of 3.6. A styrene-butadiene copolymer (Finaprene 410 from Petrofina) having an 18 mol % styrene content and an Mw ) 160 000 was used to study the process in the absence of nitrile. Fresh samples of 1-decene and 1-heptene as well as the cis and trans isomers of 2-heptene (Aldrich) were used immediately following purification by passing through an alumina column. Polymer solutions were prepared in the dark under an argon atmosphere and vigorously degassed in the autoclave before the addition of catalyst. The hydrogenation apparatus, a high-pressure variation of that developed by Mohammadi and Rempel,11 controlled temperature to (1 °C and pressure to (0.02 bar. Once the system was brought to the desired pressure and temperature, the reaction was initiated by releasing into the solution a known mass of powdered complex 2 along with any required phosphine. Real-time measurements of the hydrogen consumed by the reaction and the
solution temperature were then recorded. Each experiment was allowed to proceed until gas consumption ceased, whereupon the final conversion (X ) 1 - [Cd C]/[CdC]0) provided by H2 uptake was confirmed using the infrared spectroscopic technique of Marshall et al.12 IR spectra of solvent-cast films were collected on a Nicolet 520 FT-IR spectrophotometer. Certain samples were analyzed by 1H and 13C{1H} NMR on a Bruker AC300 spectrometer. Experimental Design. Experimental conditions were assigned according to a central composite structure, composed of a two-level factorial design and a series of univariate experiments.13 The first component consisted of a 23 factorial design in the principal factors of interest ([Os], [H2], and [RCN]). This established the significance of joint factor interactions in which two or more factors act in combination to influence the hydrogenation activity. The univariate or “one-at-a-time” experiments in [Os], [H2], [RCN], [PCy3], and temperature examined the influence of each factor acting alone. By varying a single factor in isolation, these studies defined the functional relationship between the catalyst activity and the process factor in greater detail. The selection of an appropriate range for each factor considered the catalyst weighing precision, the polymer solution viscosity, and the reaction rate that the apparatus could control and monitor effectively. Gas-Liquid Interfacial Mass Transfer. Once catalyst is charged to the reactor, the hydrogenation reaction depletes the liquid phase of dissolved hydrogen, thereby initiating H2 transfer across the gas-liquid interface. The rate of this H2 transfer is governed by the well-known mass transport equation describing absorption into agitated fluid
d[H2] k1A ) ([H2]* - [H2]) dt V
(1)
where [H2] ) bulk H2 concentration, M; [H2]* ) equilibrium bulk H2 concentration, M; kl) gas-liquid-mass transfer coefficient, m/s; A ) gas-liquid interfacial area; m2; V ) liquid-phase volume, m3. While it is clear that [H2]* cannot be maintained during hydrogenation reactions, efficient agitation can minimize the deficit hydrogen and prevent kinetic data from becoming confounded by a mass-transfer limitation. The interfacial mass-transfer rate generated by the apparatus was measured to ensure that the H2 consumption measurements equated to the inherent rate
Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 4255
Figure 1. Physical absorption of H2 into a NBR solution. [RCN] ) 172 mM; T ) 25 °C; (() 400 rpm, (9) 1200 rpm, (2) 1000 rpm, (b) 800 rpm, (s) model.
of NBR hydrogenation. The reactor was charged with a 2.5 wt % solution of NBR in monochlorobenzene and purged of atmospheric gases using H2. The agitator was then stopped, and the system was pressurized under stagnant conditions. The physical H2 absorption that was initiated by starting the agitator and maintaining a fixed rpm is illustrated in Figure 1. At 1200 rpm (used throughout the kinetic studies), a near-equilibrium condition was established within 10 s. Fitting the [H2] versus time profile to the integrated form of eq 1 yielded an overall mass-transfer coefficient, klA/V ) 0.31 s-1. Given that the maximum hydrogenation rate constant recorded in this study was an order of magnitude less than klA/V, interfacial H2 transport did not limit the rates of hydrogenation. Gas uptake measurements therefore reflected the inherent kinetics of the catalyst system. Results Complex 2 is extremely robust as a solid and shows no signs of degradation in air over the course of months. This unique stability, coupled with a rapid transformation to the dihydrogen adduct (3) when exposed to hydrogen conditions, makes 2 an ideal catalyst precursor.7 It has therefore been used exclusively in the kinetic studies, with the expectation that its catalytic behavior parallels that of its five-coordinate analogue, 1. We have great confidence in the precision of the kinetic data, given that five replicates of the central reaction conditions ([2] ) 80 µM, [RCN] ) 172 mM, PH2 ) 23.7 bar, T ) 130 °C) yielded a narrow 95% confidence interval of k′ ) (3.57 ( 0.19) × 10-3 s-1. Selectivity of NBR Hydrogenation. Complex 2 was inoperative in ketone solvents such as acetone and 2-butanone. However, in monochlorobenzene, it acted as an efficient catalyst for the quantitative hydrogenation of olefin resident within NBR. 1H and 13C NMR spectra of HNBR produced using 2 were consistent with those published by Mohammadi and Rempel14 and Bhatachjaree et al.15 for a RhCl(PPh3)3 product. No evidence of nitrile hydrogenation was found in either spectra, suggesting that the apolar solvent resistance of the material was not compromised. While complex 2 quantitatively hydrogenates NBR, the data presented in Figure 2 show a kinetic dependence on the structure of the olefin. Observed reaction rates for isomers of heptene decreased in the order
Figure 2. Hydrogenation of heptene isomers. [2] ) 20 µM; PH2 ) 24.2 bar; [CdC]0 ) 210 mM; T ) 115 °C; (2) 1-heptene, (9) cis2-heptene, (b) trans-2-heptene.
Figure 3. NBR conversion versus time and first-order log plots. [2] ) 110 µM; [RCN] ) 172 mM; PH2 ) 14 bar; T ) 130 °C; (b) conversion, (O) ln(1 - conversion).
1-heptene > cis-2-heptene > trans-2-heptene, which correlates with the ease of olefin coordination to the metal center. As a result, NBR hydrogenation profiles are complicated by the mixture of isomers within the copolymer (approximately 80% trans, 15% cis, 5% vinyl). As shown in Figure 3, an initially rapid hydrogenation, during which cis and vinyl isomers are preferentially saturated, declines to the moderate activity supported by the remaining trans functionality. The commercial demand for a highly saturated product requires that all three isomers be hydrogenated efficiently. Given the relatively slow rate of trans isomer reduction and its 80% abundance within NBR, we have focused our study on the hydrogenation of this structural entity. All of the heptene conversion profiles plotted in Figure 2 are first order with respect to olefin. That is, the hydrogenation rate is proportional to the concentration of double bonds, according to
-
d[CdC] ) k′[CdC] dt
(2)
This relationship applies equally to the latter stages of NBR hydrogenation, when all remaining olefin is trans disposed. Plotting the conversion data as a first-order log plot (Figure 3) reveals two distinct periods. The cis and vinyl double bonds are consumed within 150 s, after which the ln(1 - X) versus time profile is linear with a slope of -k′, the apparent first-order rate constant for trans isomer hydrogenation. Because k′ uniquely rep-
4256 Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 Table 1. 23 Factorial Design Data for NBR Hydrogenation, T ) 130 °C expt
[Os], mM
[RCN], mM
PH2, mM
[H2], mM
103 k′, s-1
14 6 10 7 15 2 4 11 8 12 9 13 3 16 1 5
0.0701 0.0699 0.0897 0.0904 0.0702 0.0699 0.0905 0.0910 0.0701 0.0702 0.0902 0.0906 0.0899 0.0896 0.0698 0.0698
188 188 156 157 188 188 188 189 156 156 156 156 188 188 156 156
20.7 20.7 20.7 20.7 27.6 27.6 20.7 20.7 20.7 20.7 27.6 27.6 27.6 27.6 27.6 27.6
84.2 84.2 84.2 84.2 112.2 112.2 84.2 84.2 84.2 84.2 112.2 112.2 112.2 112.2 112.2 112.2
1.81 1.72 3.39 3.37 4.20 3.86 2.43 2.29 2.12 2.33 6.52 6.73 5.19 5.02 5.13 5.43
resents the catalytic activity, it was applied in all cases where first-order kinetics were observed. Factorial Design Experiments. A thorough kinetic study improves our knowledge not only of how each factor affects k′ alone (as probed by the univariate experiments) but whether factors act in combination. The results of the factorial experiments (Table 1) provide a means of assessing the significance of such joint factor interactions. An analysis of variance (ANOVA) of the rate constants derived from the study is summarized in Table 2. In addition to strong maingroup influences ([Os], [H2], and [RCN]), highly significant [Os]‚[H2], [Os]‚[RCN], and [H2]‚[RCN] binary interactions operate within the catalytic mechanism. This indicates that the effect of a factor such as [H2] on the reaction rate depends on the specific catalyst and nitrile loading employed. While the existence of a three-factor interaction ([Os]‚[H2]‚[RCN]) is not supported by the data, a valid kinetic mechanism must account for these binary interaction terms. Univariate Kinetic Experiments. The ANOVA treatment of the factorial experiments established the significance of joint factor interactions without determining their functional form. The univariate components of the central composite design augment the factorial study by exploring how each factor influences the hydrogenation rate in isolation. The influence of the amount of 2 charged to the system is illustrated in Figure 4. Irrespective of the pressure, the reaction rate is linearly proportional to the total concentration of osmium, [Os]T. This is consistent with the observations of Andriollo et al.16 for styrene hydrogenation by the isopropyl analogue of 1, OsHCl(CO)(P-i-Pr3)2. A firstorder response of k′ to [2] suggests that the active complex is a mononuclear species. Conversion profiles for hydrogenations that are first order with respect to [CdC] are, by definition, independent of the amount of olefin charged to the reactor. Therefore, the activity of 2 is expected to be independent of the amount of a simple olefin such as 1-decene that is charged to the system. Studies of butadiene copolymers must, however, consider the influence of the functional groups of the comonomer. In the case of a material such as styrene-butadiene rubber (SBR), the pendant phenyl groups do not interact with the metal center. Consequently, experiments 1-3 of Table 3 exhibit an activity that was unaffected by the amount of SBR hydrogenated. This was not the case for hydrogenations of the
acrylonitrile-butadiene copolymer. The nitrile within this material is known to reversibly coordinate to complex 1 at these reaction conditions (Scheme 2). Through σ-donation from the nitrogen lone pair, nitrile coordinates trans to the hydride to bind the metal center in a catalytically inactive form. As a result, the factorial study and the univariate experiments illustrated in Figure 5 demonstrated lower hydrogenation activity as the concentration of NBR was increased. This behavior is not unique to complex 2, having also been established for RhCl(PPh3)3- and RuHCl(CO)(PCy3)2-catalyzed NBR hydrogenations.17,18 The influence of hydrogen pressure on the behavior of 2 is remarkable. Esteruelas et al.19 have shown the rate of benzilideneacetone hydrogenation by OsHCl(CO)(PMe-t-Bu2)2 to be independent of pressure. While their reaction conditions (PH2 ) 0.70-1.26 bar, T ) 60 °C, and [Os]T ) 2.5 mM) were considerably milder, this observation is consistent with our study of 1-decene hydrogenation by complex 2. Figure 6 illustrates the activity of 2 toward 1-decene to be virtually indifferent to hydrogen pressure over a considerable range. This zero-order response of k′ to [H2] applies to noncoordinating butadiene copolymers as well. Experiments 4-8 of Table 3 confirm that at 130 °C, the activity of 2 for SBR hydrogenation is constant from 5.24 to 38.2 bar. The effect of pressure on the hydrogenation of coordinating polymers such as NBR departs from this trend drastically. Indeed, the observed response of 2 to variations in hydrogen pressure is unprecedented (Figure 7). The RhCl(PPh3)3 system is known to shift from first to zero order with increasing hydrogen pressure, while strict first-order behavior is maintained by the ruthenium analogue of 1, RuHCl(CO)(PCy3)2.17,18 In contrast, the rate of NBR hydrogenation by 2 at 130 °C and pressures below 41 bar is second order with respect to hydrogen; k′ ∝ [H2]2. This discovery suggests 2 molecules of H2 generate the active complex or are involved in the rate-determining step. The propensity of nitrile to generate such a unique response is explored in the discussion. To further examine this result, the univariate series of [2] ) 30 µM was extended to a H2 pressure of 80 bar. Due to the sensitivity of the system to pressure, only this low-catalyst series could be monitored and controlled effectively. Representative NBR conversion profiles from these experiments are plotted in Figure 8. Each hydrogenation began with a period of preferential cis and vinyl hydrogenation. Beyond this initial period, the trans-olefin hydrogenation rates at 20.6 and 29.6 bar yielded the expected response, first order in olefin, second order with respect to [H2]. However, raising the pressure to 51.7 bar produced less than the second-order enhancement in activity observed at lower pressures. The influence of H2 continued to decline with increasing pressure until little difference was observed between the 62 and 80 bar experiments. Whereas SBR and 1-decene hydrogenations are consistently zero order with respect to [H2], the hydrogenation of NBR shifts from second order to zero order as the system pressure increases. This shift in [H2] order is accompanied by changes in the influence of [CdC]. With each increase in pressure, the conversion versus time plots became increasingly linear, until at 80 bar the rate of olefin reduction was no longer directly proportional to its concentration. Only at elevated conversions did high-pressure profiles
Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 4257 Table 2. ANOVA results of the 23 Factorial Experiments analysis of variance source
sum of squares
DF
mean square
F ratio
P
[Os] PH2 [RCN] [Os]PH2 [Os][RCN] PH2[RCN] [Os]PH2[RCN] error
4.35 × 10-6 3.20 × 10-5 4.52 × 10-6 1.12 × 10-7 1.72 × 10-7 4.16 × 10-7 2.10 × 10-8 1.75 × 10-7
1 1 1 1 1 1 1 8
4.35 × 10-6 3.20 × 10-5 4.52 × 10-6 1.12× 10-7 1.72 × 10-7 4.16 × 10-7 2.10× 10-8 2.19 × 10-8
198 1460 206 5.12 7.86 19.0 0.959
6.28 × 10-7 2.43 × 10-10 5.43 × 10-7 5.45 × 10-2 2.31 × 10-2 2.43 × 10-3 0.356
Figure 4. Influence of [2] on the hydrogenation rate. [RCN] ) 172 mM; T ) 130 °C; (b) 13.8 bar, (9) 24.2 bar, (2) 34.5 bar.
Figure 5. Influence of [RCN] on the hydrogenation rate. [2] ) 80 µM; PH2 ) 23.7 bar; T ) 130 °C. Table 3. Styrene-Butadiene Copolymer (SBR) Hydrogenation, T ) 130 °C expt
[2], µM
PH2, bar
[CdC]0, mM
103 k′, s-1
1 2 3 4 5 6 7 8
80 80 80 60 60 60 60 60
24.2 24.2 24.2 5.24 10.4 24.2 31.2 38.2
138 275 400 275 275 275 275 275
4.94 5.39 5.42 3.85 3.60 3.85 3.69 4.08
revert to first-order kinetics. This intermediate reaction order supports the rate expression
-
KA[CdC] d[CdC] ) dt KB + KC[CdC]
(3)
where KA, KB, and KC are functions of [2], [RCN], [H2],
Figure 6. Hydrogenation of 1-decene. [2] ) 20 µM; [CdC]0 ) 210 mM; T ) 110 °C; (2) 8.3 bar, (b) 4.1 bar, (9) 1.0 bar.
Figure 7. Influence of pressure on the rate of NBR hydrogenation. [RCN] ) 172 mM; T ) 130 °C; (2) [Os] ) 200 µM, (9) [Os] ) 80 µM, (b) [Os] ) 30 µM.
and temperature. At low pressures, KC[CdC] < KB to yield an overall order with respect to olefin of one. As the pressure is raised, KC[CdC] approaches KB to reduce the reaction order toward zero. This functional relationship between KC and [H2] is revisited in the Discussion section. Diminished reaction orders such as those illustrated in Figure 8 can result from poor interfacial mass transfer. Without adequate agitation, the absorption of H2 across the gas-liquid interface becomes rate determining and the kinetic data no longer reflect the influence of process factors such as [H2] and [CdC]. The rate of hydrogen absorption across the interface is defined by eq 1. The gas-liquid mass-transfer coefficient, klA/V, for the hydrogenation apparatus was derived from independent studies to be 0.31 s-1. The maximum rate of hydrogen consumption in the 80 bar experiment (Figure 8) was 4.0 mM/s, while [H2]* under
4258 Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998
Figure 8. NBR conversion profiles at high pressure. [2] ) 30 µM; [RCN] ) 172 mM; T ) 130 °C.
Figure 10. Arrhenius plot for the hydrogenation of NBR. [2] ) 80 µM; [RCN] ) 156 mM; PH2 ) 24 bar; T ) 120-140 °C. Table 4. Supplementary Kinetic Data for NBR Hydrogenation catalyst
variation
103 k′, s-1
2 2 2ba 2ba 2ba 2 2 2
24.2 bar H2 24.2 bar D2 24.2 bar H2 29.0 bar H2 33.9 bar H2 2 mol equiv [NEt3] 25 mol equiv octylamine 5.5 mol equiv proton sponge 1,8-bis(dimethylamino)naphthalene 15 mol equiv acetic acid 0.4 mL H2O
3.57 ( 0.19 2.17 ( 0.64 1.41 3.11 4.82 2.91 1.54
2 2
3.55 1.38 3.19
a P-i-Pr analogue, OsHCl(CO)(O )(P-i-Pr ) . [Os] ) 80 µM, 3 2 3 2 T [RCN] ) 172 mM, T ) 130 °C, PH2 ) 24.2 bar unless specified.
Figure 9. Influence of added PCy3 on the hydrogenation rate. [2] ) 80 µM; [RCN] ) 172 mM; PH2 ) 23.7 bar; T ) 145 °C.
these conditions is 325 mM. Assuming the existence of a pseudosteady state, eq 1 predicts a bulk H2 concentration at this stage of the reaction equal to
4.0 mM/s ) 0.31 s-1 (325 mM - [H2]) [H2] ) 310 mM To support such a reaction rate, the steady-state concentration of hydrogen would deviate from equilibrium by 15 mM, a difference of just 4.7%. Therefore, [H2] was invariant throughout the high-pressure experiments, and the observations cannot be attributed to insufficient mass transfer. Rather, the diminished reaction orders reflect the inherent behavior of the hydrogenation process. One of the advantages of the new osmium technology is that, unlike the RhCl(PPh3)3 system, no additional phosphine is required to maintain the stability of the complex. Nevertheless, we have examined the effect of free PCy3 on hydrogenation activity as it relates to the underlying catalytic mechanism. A recent study of the RuHCl(CO)(PCy3)2 system revealed that just 1 equiv of PCy3 relative to catalyst reduced the NBR hydrogenation activity by half.18 The influence of free phosphine on 2 appears to be equally severe (Figure 9). In this case, 1 M equiv diminished the activity by an order of magnitude, which is more extreme than the effect produced by nitrile, a good ligand for complex 1. Besides affecting the hydrogenation rate, additional PCy3 appears to reduce the reaction order with respect
to olefin through a presently ill-defined [CdC]‚[PCy3] interaction. The last univariate series examined the influence of temperature on the activity from 120 to 140 °C. An Arrhenius treatment of the data is illustrated in Figure 10. The plot is linear (R2 ) 0.985) with residuals distributed randomly about the predicted response. This suggests that a single rate-determining step is operative within the kinetic mechanism. Were two or more mechanisms functioning in parallel, k′ would respond to multiple sets of activation parameters to yield a nonlinear Arrhenius relationship. The slope of ln(k′) versus 1/T provided an apparent activation energy of 96 kJ/mol, which provides further evidence that the kinetic data were acquired without mass-transfer limitation. Supplementary Kinetic Data. The effect of specific perturbations of the HNBR process on the activity of 2 is summarized in Table 4. These controlled variations of the process conditions were designed to gain further insight into the reaction mechanism. For instance, should bond breaking/formation be directly involved in the rate-determining step, the replacement of a constituent atom by a heavier isotope is expected to raise the relative activation energy of the transition state, thereby reducing the rate of reaction. For hydrogenations, the ratio of hydrogen to deuterium (k′H2/k′D2) is a simple indicator of a cleavage of a bond to hydrogen in the reaction’s rate-determining step. The two experiments employing D2 were relatively slow, revealing a kinetic isotope effect of k′H2/k′D2 ) 1.65 ( 0.51. This result signifies the involvement of an Os-H bond in the rate-limiting step of the hydrogenation mechanism.
Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 4259
The P-i-Pr3 analogue of 2 was studied to appreciate the influence of the phosphine ligands on catalyst activity. The data in Table 4 show that OsHCl(CO)(O2)(P-i-Pr3)2 is less active than its PCy3 analogue at all pressures. Note that this system is also second order with respect to [H2] over equivalent reaction conditions. In each of the experiments, a preferential hydrogenation of the cis and vinyl isomers of NBR was observed. Organic acids and bases have been prescribed for use with Ru and Os catalysts to improve their selectivity.3 Common examples are acetic acid and octylamine, both of which have been screened for an effect on the activity of 2. The data show that additives capable of associating with the metal center are detrimental to the hydrogenation rate. However, the strong noncoordinating base 1,8-bis(dimethylamino)naphthalene produced no such effect.
Scheme 3
Discussion Scheme 2 illustrates the predominant complexes and identifiable reactions of an NBR hydrogenation mechanism. The stability of these complexes permits them to be characterized by spectroscopic techniques. As a result, these equilibria must act only on the periphery of the catalytic cycle and cannot directly involve the catalytically active species. The reactions that constitute the hydrogenation cycle must therefore be inferred from kinetic data. In addition to the measured response of k′ to the process factors, two observations relate directly to the rate-determining step of the reaction. The response of k′ to temperature suggested that a single reaction mechanism is operative, and the kinetic isotope effect k′H2/k′D2 implicated a cleavage of a bond to hydrogen. The conceptually difficult element of the NBR hydrogenation data is the apparent second-to-zero-order dependence of the reaction rate with respect to [H2]. A second-order response requires 2 molecules of H2 to either produce an active complex or participate in the rate-determining step. The coordination of 1 molecule of H2 to 1 to form the dihydrogen complex 3 is well established (Scheme 2). It is also known that the η2H2 ligand resists cleavage of the H-H bond which would generate a classical trihydride complex, OsH3Cl(CO)(PCy3)2. Bakhmutov et al.8 identified an exchange between the apical hydride and the trans-coordinated dihydrogen ligand but concluded that the trihydride could at most be a reactive intermediate. The addition of a second molecule of H2 to 3 has not been observed. Another observation that is difficult to rationalize is the severe inhibition of hydrogenation activity by additional PCy3. The coordination of a third phosphine to the coordinatively unsaturated complex 1 is a plausible explanation but is unlikely. Although the triphenylphosphine analogue of 1 is a tris-phosphine complex, OsHCl(CO)(PPh3)3, all attempts by Moers to prepare OsHCl(CO)(PCy3)3 were unsuccessful.20 Esteruelas et al. isolated products of PMe3 and P(OMe)3 addition to OsHCl(CO)(P-i-Pr3)2 but observed no similar coordination of a third bulky phosphine.9 These results, combined with the limited capacity of a good ligand such as nitrile to diminish the hydrogenation activity to the same extent, have led us to dismiss an associative-type inhibition mechanism. If phosphine dissociation were required to generate a catalytic complex, additional PCy3 would influence an already unfavorable equilibrium, thereby decreasing the
rate of reaction. However, variable-temperature 31P NMR spectra of 1 under N2 or H2 showed no evidence of a monophosphine complex nor any signs of exchange broadening that could be attributed to PCy3 loss. Consequently, a monophosphine complex would necessarily be extremely reactive and/or present in trace quantities. In support of a PCy3 dissociation mechanism are the supplementary experiments using the less sterically encumbered P-i-Pr3 analogue of complex 2 (Table 4). Were the formation of an active center to be initiated by PR3 dissociation, a smaller phosphine of similar electron-donating capacity could be expected to yield an inferior hydrogenation rate, as shown by experiment. A feasible hydrogenation mechanism that is consistent with all of the kinetic data is illustrated in Scheme 3. Due to the unusual second-order dependence of k′ on [H2], it is proposed that the dihydrogen ligand of 3 does not add oxidatively to the metal in such a manner to permit either the insertion of olefin or the elimination of an alkyl ligand. While the η2-H2 ligand may indeed participate in olefin hydrogenation, it is proposed that it cannot do so in the absence of a second molecule of hydrogen. This unconventional assumption is required to account for the second-order behavior observed for NBR hydrogenation. Without it, a mechanism containing a single rate-determining step cannot be derived. The observed kinetic isotope effect implicates cleavage of a bond to hydrogen in the rate-limiting reaction. This could result from the insertion of olefin into an Os-H bond or by a reductive elimination of an osmium-alkyl to yield the saturated product. The proposed mechanism does not discriminate between these possibilities. Rather, it assumes one of these processes is rapid relative to its rate-determining counterpart. Accordingly, olefin hydrogenation could be governed by the rate expression
-
d[CdC] ) krds[OsH3(H2)(CdC)P] dt
(4)
4260 Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998
Applying a steady-state assumption to each of the equilibria that lead to the formation of [OsH3(H2)(Cd C)P] provides a means of relating the concentrations of every species to the rate-determining step. A mass balance on osmium yields the concentration of the active center as a function of the total amount of osmium ([Os]T) charged to the system and results in the rate expression
Scheme 4
d[CdC] ) krds[OsH3(H2)(CdC)P] ) dt krds[KH2KPK4K5[Os]T[H2]2[CdC]]/[[P](1 + KCN[CN] +
-
KH2[H2]) + KH2KP[H2] + KH2KPK4[H2]2(1 + K5[CdC])] (5) The rate expression derived from the mechanism is consistent with the kinetic data. The key element of the proposed catalytic cycle is the response of the system to nitrile. Under conditions where KCN[P][CN] is the predominant term of eq 5, the rate expression reduces to that given by 2 d[CdC] KA[Os]T[H2] [CdC] ) dt [P](1 + KCN[RCN])
-
(6)
which is first order with respect to olefin and second with respect to [H2]. Being the dominant equilibrium in the mechanism, nitrile coordination to complex 1 accounts not only for RCN inhibition of activity but its observed second-order dependence with respect to hydrogen. The second-order response of k′ to [H2] was seen to hold only at relatively low pressures (
