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Catalytic Alkane Metathesis by Tandem Alkane Dehydrogenation-Olefin Metathesis

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Science  14 Apr 2006:
Vol. 312, Issue 5771, pp. 257-261
DOI: 10.1126/science.1123787

Abstract

With petroleum supplies dwindling, there is increasing interest in selective methods for transforming other carbon feedstocks into hydrocarbons suitable for transportation fuel. We report the development of highly productive, well-defined, tandem catalytic systems for the metathesis of n-alkanes. Each system comprises one molecular catalyst (a “pincer”-ligated iridium complex) that effects alkane dehydrogenation and olefin hydrogenation, plus a second catalyst (molecular or solid-phase) for olefin metathesis. The systems all show complete selectivity for linear (n-alkane) product. We report one example that achieves selectivity with respect to the distribution of product molecular weights, in which n-decane is the predominant high-molecular-weight product of the metathesis of two moles of n-hexane.

The interconversion of alkanes via alkane metathesis is a reaction with enormous potential applicability: Embedded Image(1) Embedded Image Alkanes are the major constituents of petroleum. As oil reserves dwindle, the world will increasingly rely on the Fischer-Tropsch process (reductive oligomerization of CO and H2) to produce liquid hydrocarbons—specifically n-alkanes—from the vast reserves of coal, natural gas, oil shale, and tar sands, or from biomass. The energy content of U.S. coal reserves alone, for example, is about 40 times that of U.S. petroleum reserves and is comparable to that of the entire world's petroleum reserves (1).

Unfortunately, neither natural sources nor Fischer-Tropsch production yield alkane mixtures with a tightly controlled molecular weight (MW) distribution, which is important for varied applications. For example, n-alkanes in the range of ∼C9 to C20 constitute the ideal fuel for a diesel engine (which runs ∼30% more efficiently than a gasoline engine); the absence of aromatic impurities results in cleaner burning than that of conventional diesel fuel or even gasoline (2, 3). n-Alkanes lower than ∼C9, however, suffer from high volatility and lower ignition quality (cetane number) (4). In addition to F-T product mixtures, low–carbon number, low-MW alkanes are also major constituents of a variety of refinery and petrochemical streams. In general, there is currently no practical method for the interconversion of alkanes to give products of higher MW; this challenge provides extremely large-scale potential applications of alkane metathesis (Eq. 1). Additionally, Eq. 1 might be applied to the formation of low-MW products from high-MW reactants (e.g., by reaction with ethane). Although hydrocracking is already a well-established process for this purpose, Eq. 1 might offer an advantage, for some applications, of higher selectivity and/or less severe conditions.

We report two systems in which the metathesis of linear alkanes is achieved efficiently and selectively at moderate temperatures via a tandem combination of two independent catalysts, one with activity for alkane dehydrogenation and the other for olefin metathesis. In particular, we exploit highly selective, soluble molecular catalysts developed for each of these reactions, as well as solid-phase olefin metathesis catalysts.

The basic tandem catalytic process is outlined in Fig. 1 for metathesis of an alkane of carbon number n (Cn) to give ethane and C(2n–2). A dehydrogenation catalyst, M, reacts with the alkane to give the corresponding Cn terminal alkene and MH2. Olefin metathesis of the 1-alkene generates ethylene and an internal C(2n–2) alkene. The alkenes thus produced serve as hydrogen acceptors and generate the two new alkanes via reaction with MH2, regenerating M and closing the catalytic cycle.

Fig. 1.

Alkane metathesis via tandem transfer dehydrogenation–olefin metathesis illustrated with the formation and metathesis of two molecules of 1-alkene. M, active catalyst in the transfer dehydrogenation cycle [e.g., (pincer)Ir].

To date, two heterogeneous catalyst systems have been reported to effect the interconversion of alkanes. Burnett and Hughes showed that passage of butane over a mix of platinum on alumina (a dehydrogenation catalyst) and tungsten oxide on silica (an olefin metathesis catalyst) at high temperatures (399°C) results in formation of lower and higher MW alkanes, predominantly propane and pentane (24.7% and 15.9%, respectively, with 37.6% n-butane unreacted) (5, 6). In addition to linear saturated hydrocarbons, small quantities of branched C4 and C5 alkanes, methane, and alkenes were also formed. Supported Ta and W hydride catalysts that function as alkane metathesis catalysts at much lower temperatures were reported by Basset (79), but product yields were low. For example, a turnover number (mol of propane transformed per mol of catalyst) of 60 and conversion of 6% was observed for metathesis of propane to give C1 to C6 alkanes with the use of a supported Ta hydride species (120 hours, 150°C) (8). More recent work showed that an alumina-supported tungsten hydride species gave somewhat increased turnover numbers (8). Basset has shown that these systems operate via the reaction of metal centers with alkanes to give metal-carbene complexes (via α-H elimination) as well as free olefins (via β-H elimination and β-alkyl transfer) (10). Such pathways, unlike the one outlined in Fig. 1, are consistent with the reported formation, from linear alkanes, of both branched and linear products, as well as catalytic methane production (710).

Our investigation was largely based on the use of Ir-based pincer complexes, first reported by Jensen and Kaska (11, 12) and explored extensively in our own laboratories (1317); specifically, complexes 1, 2a, and 2b were used (Fig. 2). These systems exhibit high stability, but their dehydrogenation activity is inhibited by buildup of even moderate concentrations of alkene product. The dual catalytic system (Fig. 1) would require only a very low steady-state concentration of alkenes during catalysis; thus, inhibition of catalysis by product could be avoided. Numerous olefin metathesis catalysts are available (1820); we examined the Schrock-type catalyst 3 (21, 22) after determining that the widely used Grubbs-type catalysts (19) react with and deactivate the iridium-based dehydrogenation catalysts.

Fig. 2.

Dehydrogenation catalysts 1 and 2 and Schrock-type metathesis catalyst 3.

Initial experiments combining 3 with Ir-based dehydrogenation catalysts in solution proved successful. An n-hexane solution containing 10 mM dehydrogenation catalyst precursor 1-C2H4 (0.14 mol % relative to hexane) and 16 mM Schrock catalyst 3 was heated at 125°C under argon in a sealed glass vessel for 24 hours. This process converted 135 equivalents (relative to Ir) of n-hexane to a range of C2 to C15 n-alkanes. No branched or cyclic alkanes were detected. Products were monitored by gas chromatography (GC) using mesitylene as an internal standard. The product distribution was concentrated in the C2 to C5 and C7 to C10 ranges (Table 1, entry 1). Heating for longer times resulted in few additional turnovers. However, upon addition of more olefin metathesis catalyst 3, alkane metathesis was reinitiated, indicating that decomposition of 3 is responsible for the deactivation of the system under these conditions. Using 1-H2 and two equivalents of t-butylethylene (TBE) as a hydrogen acceptor, along with catalyst 3, similar results were obtained (Table 1, entry 2). The reaction of the dihydride catalyst precursor 1-H2 with one equivalent of TBE is presumed to generate the catalytically active species (tBuPOCOP)Ir (1). The use of two equivalents of TBE per mol of 1-H2 is expected to give results most comparable to those obtained with 1-C2H4, because 1-C2H4 constitutes the same catalytically active species plus 1 mol of olefin.]

Table 1.

Representative examples of the metathesis of n-hexane (7.6 M) by 1 or 2 (10 mM) and 3 (16 mM): distribution of C2 to C15 n-alkane products.

EntryIr catalyst[TBE] (mM)Temp. (°C)TimeProduct concentration (mM)Total product (M)
C2C3C4C5C7C8C9C10C11C12C13C14C≥15
1 1-C2H4 0 125 6 hours 123 105 183 131 73 70 47 10 4 2 1 0.3 0.75
24 hours 233 191 319 234 133 122 81 22 9 5 2 1 1.35
2 days 261 215 362 265 147 138 89 25 11 6 3 1 1.52
4 days 264 218 372 276 154 146 95 26 12 6 3 1 1.57
Added additional 3 (8 mM)
5 days 502 436 721 420 239 223 153 56 30 18 10 5 2.81
2 1-H2 20 125 1 day 458 345 547 258 151 139 95 29 13 6 3 2 2.05
3 2a-H2View inline 20 125 23 hours (131) 176 127 306 155 37 49 232 18 4 4 10 2 1.25
Added additional 3 (6.4 mM)
46 hours (189) 255 193 399 208 61 81 343 31 9 9 22 7 1.81
  • View inline* 6.4 mM catalyst 3 added initially. Ethane concentrations for entry 3 are extrapolated as explained in the text. For entries 1 and 2, no separation of C2 and C3 peaks was obtained (values shown are not extrapolated).

  • Controls were conducted, including experiments with (i) 1-C2H4 and no metathesis catalyst; (ii) 2a-H2 and TBE, but with no metathesis catalyst; and (iii) metathesis catalyst 3, but with no iridium-based catalyst. In none of these cases was any alkane metathesis observed after heating the n-hexane solutions for 24 hours at 125°C.

    Pincer-ligated iridium complexes have been reported to dehydrogenate n-alkanes with high kinetic selectivity for the formation of the corresponding 1-alkene (15). Thus, the product distributions indicated in entries 1 and 2 in Table 1 presumably reflect a substantial degree of olefin isomerization before olefin metathesis under these conditions. For example, isomerization of 1-hexene to 2-hexene, followed by cross-metathesis between 2-hexene and 1-hexene, could give 1-pentene plus 2-heptene (Fig. 3) (1820). Alternatively, or in addition, 5-decene (from the cross-metathesis of 2 mol of 1-hexene) could be isomerized to give 4-decene; metathesis with ethylene would then give 1-pentene and 1-heptene. The pincer-iridium complexes are well known to catalyze olefin isomerization (15). Thus, terminal dehydrogenation of n-hexane in tandem with olefin metathesis, when coupled with rapid olefin isomerization, can account for the C3 to C5 and C7 to C9 alkanes (Fig. 3).

    Fig. 3.

    Two possible pathways for the metathesis of n-hexane to give n-pentane and n-heptane, initiated by dehydrogenation at the n-hexane terminal position.

    Alkanes with carbon number greater than 10, produced from hexane, must derive from olefin metathesis of at least one alkene of Cn>6. The Cn>6 alkene may result from dehydrogenation of the corresponding n-alkane primary product, or it may be obtained directly via cross-metathesis of hexenes, before the resulting olefin (e.g., 5-decene) is hydrogenated.

    Consistent with the hypothesis that 1-alkenes are the initial dehydrogenation products, under certain conditions n-decane is the major heavy (Cn>6) product of n-hexane metathesis (the non-degenerate cross-metathesis of 1-hexene can only yield ethene and 5-decene). Combining dehydrogenation catalyst 2a and metathesis catalyst 3 (Table 1, entry 3) leads to this outcome. (The formation of n-tetradecane presumably results from the secondary metathesis reaction of n-decane with n-hexane.) Presumably, under the conditions of this reaction, diphosphine-ligated 2a catalyzes isomerization more slowly (or more slowly relative to hydrogenation) than does the diphosphinite-ligated species 1. This reaction was also monitored by 13C nuclear magnetic resonance spectroscopy (NMR), a method that yields less precise results than GC but facilitates continuous monitoring in a sealed reaction vessel. The NMR results were generally consistent with those obtained by GC, and in particular they revealed that the ratio of the major n-alkane products did not considerably change with time.

    Although it is difficult to precisely quantify ethane production under our conditions, the solution concentration for the first run in entry 3 was measured by GC as 128 mM. GC analysis of the gas phase indicated the presence of 1.7 μmol of ethane; if this quantity were also in solution, the total ethane concentration would have been 131 mM. (Likewise, a small amount of propane in the gas phase brings the effective concentration from 175 mM to 176 mM.) The average carbon number of all products was found to be ∼6.0 (measured as 5.95), an indication that no significant loss of lighter alkanes had occurred (which would raise the observed value above 6.0). The “effective ethane concentration” of 131 mM is substantially lower than the concentration of n-decane observed (232 mM). This discrepancy between ethane and n-decane production is probably largely attributable to secondary metathesis of the ethene product with 2- or 3-hexenes (which would then contribute to the formation of propene, butene, and pentene). It is also worth noting in the context of this experiment that only trace quantities of methane were observed [0.5 mM effective concentration, probably formed via decomposition of the expected molybdenum methylidene intermediate (1820)]. This result contrasts sharply with the Basset systems, in which methane is formed catalytically; this difference is in accord with the key role played by α-H elimination and β-alkyl elimination in the Basset systems (710). The catalytic cycle in the systems reported here does not involve either of these elimination steps.

    In addition to the alkane disproportionation (self-metathesis) illustrated above, the catalyst system may be used for alkane comproportionation (cross-metathesis)—that is, the production of intermediate-MW alkanes from low-MW and high-MW reactants. Table 2 shows the result of treating a mixture of 4:1 (mol:mol) n-hexane and the C20 alkane, n-eicosane, with catalysts 1 and 3.

    Table 2.

    Concentrations of C2 to C38 n-alkane products resulting from the metathesis of n-hexane (4.36 M) and eicosane (n-C20H42; 1.09 M) by 1-C2H4 (7.14 mM) and 3 (11.43 mM) at 125°C.

    TimeProduct concentration (M)Total product
    C2-5C7-10C11-14C15-19C21-24C25-38
    1 day 0.44 0.36 0.24 0.31 0.14 0.066 1.56
    6 days 0.56 0.64 0.31 0.27 0.12 0.070 1.97

    Monitoring the reactions by NMR spectroscopy affords insight into the resting state(s) of the catalysts as well as the extent of alkane metathesis. 31P and 1H NMR spectroscopy indicates that some loss of catalyst 2a-H2 occurs during the reaction of n-hexane solution. However, a substantial concentration of 2a-H2 remains, even when the reaction is no longer progressing. The olefin metathesis catalyst is less easily monitored because its resting state depends on the nature of the olefins present in solution; the loss of the 2-methyl-2-phenylpropylidene ligand and the expected formation of 2-methyl-2-phenylbutane (resulting from hydrogenation of 3-methyl-3-phenylbutene) are observed early in the course of the reaction. However, the appearance of free 2,6-diisopropylaniline is observed to approximately coincide with the decreased reaction rate; thus, decomposition of 3 appears to be an important factor limiting turnover numbers in this case. This conclusion is in accord with the experiments in which addition of 3 reinitiates catalytic activity.

    In contrast to (tBuPCP)Ir-based 2a-H2, the resting state of the (tBuPOCOP)Ir-based catalysts (1) is the Ir(I)(alkene) complex. For example, monitoring by 31P NMR spectroscopy of n-hexane metathesis by 1-C2H4 and 3 indicates that at early stages of the reaction (∼150 min), 1-C2H4 is the major Ir resting state, with 1-(1-propene), 1-(1-butene), 1-(1-pentene), 1-(1-hexene), and 1-(internal-hexene) present in small concentrations. However, at later stages of the reaction (4 days), 1-(1-hexene) is observed as the major iridium species in solution. The resting state of the Ir species appears to reflect the activity of the metathesis catalyst. Because the metathesis catalyst decays at the late stage of the reaction, ethylene is no longer produced, although it continues to be consumed as a hydrogen acceptor. As a result, the dehydrogenation of hexane results in the gradual conversion of 1-C2H4 to 1-(1-hexene).

    Given the instability of the molybdenum alkylidene catalysts, we turned to investigation of the supported Re metathesis catalyst Re2O7/Al2O3, which exhibits greater stability at high temperatures (23). Reactions were conducted at 175°C with n-decane as the solvent/substrate (Table 3) (24). (Control experiments with Re2O7/Al2O3, with no iridium catalyst present, gave no alkane metathesis after 3 days at 175°C.) The (PCP)Ir catalysts (2) proved more effective than the (POCOP)Ir systems (1). In a typical experiment, an n-decane (2.5 ml, 12.8 mmol) solution of 2b-H4 (12.8 mg, 0.0227 mmol), TBE (10 μl, 0.078 mmol), and hexamethylbenzene (10 mg, internal standard) was heated over Re2O7/Al2O3 (535 mg, 3 wt % Re2O7) at 175°C under argon and monitored by GC. After 3 hours, C2 to C28 alkanes were observed with total product concentration estimated at 1.6 M (corresponding to 180 mol of product per mol of Ir). Catalysis slowed, but after 9 days, product concentrations reached 4.4 M. Remarkably, at 9 days, the n-decane starting material was comparable in molar quantity to n-nonane and n-undecane products, with measured C9:C10:C11 molar ratios of 0.6:1:0.6 (Table 3 and Fig. 4).

    Fig. 4.

    GC trace of product mixture resulting from the metathesis of n-decane (solvent) by 2b-H4 and Re2O7/Al2O3 after 9 days at 175°C (see Table 3).

    Table 3.

    Distribution of C2 to C34 n-alkane products from the metathesis of n-decane (5.1 M) by Ir-based catalysts (9.0 to 9.5 mM) and Re2O7/Al2O3 (16 mM effective Re2O7 concentration) at 175°C.

    Ir catalyst[TBE] (mM)Timen-Alkane concentration (mM)Total product (M)
    C2C3C4C5C6C7C8C9C10C11C12C13C14C15C16C17C18[>C18]
    1-C2H4 (9.5 mM) 0 3 hours 3.9 2.8 8.3 10 12 12 13 16 4980 15 11 9.3 7.2 6.0 4.6 2.1 1.3 1.9 0.14
    18 hours 5.4 9.7 39 43 43 48 55 64 4580 61 46 38 28 23 17 6.9 3.7 5.4 0.54
    7 days 26 101 117 118 115 140 163 3760 154 115 94 71 58 43 18 9.8 16.3 1.36
    2a-H2 (9.0 mM) 18 3 hours 16 61 86 98 122 142 152 3990 137 104 78 53 37 23 9.3 5.2 6.3 1.13
    11 days 39 207 299 327 382 427 446 1500 408 314 245 174 129 87 48 32 58 3.62
    2b-H4 (9.1 mM) 35 3 hours 15 81 117 134 146 172 181 3490 177 147 120 91 72 52 34 26 63 1.63
    18 hours 39 160 234 265 280 318 324 1870 317 271 226 176 145 110 76 61 194 3.20
    9 days 44 220 332 346 405 456 457 753 429 362 300 233 195 151 108 88 241 4.37

    Because decomposition of the olefin metathesis catalysts appears to limit conversions, we expect that more robust olefin metathesis catalysts will yield higher turnover numbers in future studies. We also expect that more active dehydrogenation catalysts will give more turnovers before decomposition of the metathesis catalyst, and that catalysts that are less prone to isomerize the olefin intermediates will yield greater selectivity for C(2n–2) products. The nature of the tandem system permits the detailed investigation of the component catalysts individually, and the development of more suitable catalysts for both dehydrogenation and olefin metathesis (25) is under way.

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    Materials and Methods

    Figs. S1 and S2

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