Organometallics 2010, 29, 4001–4003 DOI: 10.1021/om100716t
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Low-Temperature Rhodium-Catalyzed Dehydration of Primary Alcohols Promoted by Tetralkylammonium and Imidazolium Halides George R. M. Dowson, Igor V. Shishkov, and Duncan F. Wass* School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K. Received July 21, 2010 Summary: Rhodium complexes, promoted by imidazolium or tetraalkylammonium halide salts, catalyze the dehydration of primary alcohols with good conversion and selectivity. With the growing scientific and social awareness of environmental issues, chemical and energy companies are increasingly looking to utilize alternative sustainable feedstocks. Ethylene is the key current C2 synthetic intermediate in the petrochemical industry, produced in large scale from nonrenewable resources such as crude oil.1 Biomass-derived ethanol, or bioethanol, is currently used as a fuel,2 but it also has the potential to be used as a versatile C2 feedstock for commodity chemicals3 or in the synthesis of more advanced fuels.4 Dehydration of ethanol to ethylene is well-known and can be achieved in the presence of a range of heterogeneous acidic catalysts such as zeolites,5 polyacids,6 and acidic alumina.7 Such catalysts operate at temperatures of typically 300400 °C, with diethyl ether being a significant side product. One of the most promising systems described to date is a modified H-ZSM-5 zeolite catalyst which dehydrates ethanol to ethylene with 42% yield and 99% selectivity at 170 °C.5d We reasoned that a metal-based homogeneous catalyst system for this seemingly simple transformation would be complementary to existing heterogeneous catalysts and potentially offer advantages in certain applications. In this paper, we describe a new transition-metal-catalyzed, homogeneous approach to the dehydration of ethanol and its higher homologues and a preliminary study into the mechanism by which the catalyst operates. Our approach is depicted in Scheme 1. Initially, the alcohol should react with the acid HX to yield the corresponding alkyl halide, which undergoes steps similar to the mechanism
Scheme 1. Proposed Model Mechanism for Alcohol Dehydration
of the well-known Monsanto process,8 save β-elimination in our case leading to the desired product. In a set of primary screening experiments, n-hexanol was taken as a model for the dehydration of ethanol, since the hexene or dihexyl ether dehydration products, being liquids, could more easily and rapidly be analyzed by GC. Rhodium complexes and hydriodic acid were utilized as components of the catalytic system by analogy with Monsanto chemistry. The results for the dehydration of hexanol are presented in Table 1. We initially speculated that a wide range of simple Rh complexes would show the desired reactivity. However, although formation of hexenes was indeed observed with only [Rh2(CO)4Cl2] and HI, the yields and selectivity were extremely poor (run 1). This disappointing result led us to investigate a range of potential promoters. Halide salts are known to promote carbonylation reactions with related species,9 and we found that addition of 1-butyl-3-methylimidazolium chloride to the reaction mixture gave significantly improved yields of hexenes (run 2). Addition of 2 equiv of PMes3 relative to [Rh2(CO)4Cl2] led to further, modest improvements in catalyst performance (run 3); we attribute this to improved rhodium complex solubility rather than a ligand effect, since in other related conditions no effect within error is observed (for example, compare runs 13 and 18). Although some dihexyl ether was also observed, good selectivity to the olefins is achieved. The hexenes produced consisted of isomeric cis and trans 2- and 3-hexenes with only minor amounts of 1-hexene. This isomeric pattern is in line with what would be expected for a standard metal hydride isomerization mechanism operating
*To whom correspondence should be addressed. E-mail: duncan.wass@ bristol.ac.uk. (1) Arpe, H. J. In Industrielle Organische Chemie: Bedeutende VorUnd Zwischenprodukte, 6th ed.; Wiley-VCH: Weinheim, Germany, 2007. (2) (a) Demirbas, M. F.; Balat, M. Energy Convers. Manag. 2006, 47, 2371. (b) Gnansounou, E.; Dauriat, A. J. Sci. Ind. Res. 2005, 64, 809. (c) Hansen, A. C.; Zhang, Q.; Lyne, P. W. L. Bioresour. Technol. 2005, 96, 277. (3) (a) Danner, H.; Braun, R. Chem. Soc. Rev. 1999, 28, 395. (b) Kvisle, S.; Aguero, A.; Sneeden, R. P. A. Appl. Catal. 1988, 43, 117. (c) Ndou, A. S.; Plint, N.; Coville, N. J. Appl. Catal. A: Gen. 2003, 251, 337. (c) Gucbilmez, Y.; Dogu, T. Ind. Eng. Chem. Res. 2006, 45, 3496. (d) Inaba, M; Murata, M.; Saito, M.; Takahara, I. React. Kinet. Catal. Lett. 2006, 88, 135. (4) Tsuchida, T.; Yoshioka, T.; Sakuma, S.; Takeguchi, T.; Ueda, W. Ind. Eng. Chem. Res. 2008, 47, 1443. (5) (a) Talukdar, A. K.; Bhattacharyya, K. G.; Sivasanker, S. Appl. Catal. A: Gen. 1997, 148, 357. (b) Takahara, I.; Saito, M.; Inaba, M.; Murata, K. Catal. Lett. 2005, 105, 249. (c) Phillips, C. B.; Datta, R. Ind. Eng. Chem. Res. 1997, 36, 4466. (d) van Mao, R. L.; Nguyen, T. M.; Maclaughlin, G. P. Appl. Catal. 1989, 48, 265. (6) Lee, K. Y.; Arai, T.; Nakata, S.; Asaoka, S.; Okuhara, T.; Misono, M. J. Am. Chem. Soc. 1992, 114, 2836. (7) Knoezinger, H. Angew. Chem., Int. Ed. Engl. 1968, 7, 791.
(8) (a) Yoneda, N.; Kusano, S.; Yasui, M.; Pujado, P.; Wilcher, S. Appl. Catal., A 2001, 221, 253. (b) Ellis, P. R.; Pearson, J. M.; Haynes, A.; Adams, H.; Bailey, N. A.; Maitlis, P. M. Organometallics 1994, 13, 3215 and references cited therein. (9) (a) Maitlis, P. M.; Haynes, A.; James, B. R.; Catellani, M.; Chiusoli, G. P. Dalton Trans. 2004, 3409. (b) Smith, B. L.; Torrence, G. P.; Murphy, M. A.; Aguilo, A. J. Mol. Catal. 1987, 39, 115. (c) Murphy, M. A.; Smith, B. L.; Torrence, G. P.; Aguilo, A. J. Mol. Catal. 1986, 303, 257.
r 2010 American Chemical Society
Published on Web 08/25/2010
pubs.acs.org/Organometallics
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Dowson et al.
Table 1. Dehydration of 1-Hexanola
Table 2. Dehydration of Primary Alcoholsa run
run
salt (amt (mol %))b
d
none ImCl (14.4) ImCl (14.4) ImCl (21.6) ImCl (28.8) ImCl (43.1) ImBr (14.4) ImBr (21.6) ImBr (43.1) Im(PF6) (14.4) ImMeCl (14.4) Bu4NBr (14.4) Bu4NBr (21.6) Bu4NCl (14.4) Bu4NI (14.4) Bu4NBr(21.6) Bu4NCl (5) Bu4NBr (21.6) Bu4NBr (21.6) ImCl (14.4) LiBr (21.6) Bu4NBr (21.6)
1 2d 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17e 18d 19f 20 21g
conversn of hexanol (%)
selectivity to hexenes (%)c
4 13 22 24 19 5 48 49 80 11 18 65 78 18 34 72
50 92 92 >99 93 >99 63 62 60 28 94 71 79 96 29 84
20 83 4 4 70
0 74 78 60 56
a Reaction conditions: 3 mL of hexanol, 0.065 mol % of [Rh2(CO)4Cl2], 0.13 mol % of PMes3, 6 mol % of 57% aqueous HI, 110 °C, 48 h. The molar percentage is given relative to hexanol. Conversion and selectivity were determined by GC. b Im = 1-methyl-3-butylimidazolium. c The other product is di-n-hexyl ether. d No ligand used. e No Rh catalyst used. f 0.13 mol % PPh3 added. g 6 mol % hexyl iodide in place of HI.
under thermodynamic control. Consistent with this hypothesis, the same isomer distribution within error was observed when pure 1-hexene was refluxed with [Rh2(CO)4Cl2] and HI. Further screening revealed that the optimum amount of the imidazolium salt is 20-30 mol % relative to the alcohol (runs 3-6); the yield of hexenes was decreased if higher salt amounts were used. The use of 1-butyl-3-methylimidazolium bromide gave improved conversion (runs 7-9), although selectivity to olefins is reduced by around 30%. The role of halide seems crucial so that only very poor results are obtained with Im(PF6), despite this being a more hygroscopic additive (run 10); this suggests a coordinative role for the halide, in line with the proposed role of such species in carbonylation catalysis.9 We believe N-heterocyclic carbene ligated species formed via the imidazolium salts can also be ruled out: 1-butyl-2,3-dimethylimidazolium chloride (run 11) cannot form such species but gives very similar results to the monomethylated derivative.10 Good results are also obtained if simple tetraalkylammonium halides are utilized. For example, tetrabutylammonium bromide gives an increase in yield and selectivity compared to imidazolium bromides (compare runs 8 and 13). Again, a halide anion effect is observed so that tetrabutylammonium chloride gives a lower yield but higher selectivity to olefin; in contrast, for tetrabutylammonium iodide, a lower yield is accompanied by a preference in selectivity toward dihexyl ether. In general, salts containing (10) So-called “abnormal’” carbenes cannot be ruled out by this observation but, given the very similar performance of tetraalkylammonium halides, are unlikely to be important for these catalysts. Abnormal carbenes: Schuster, O.; Yang, L.; Raubenheimer, H. G.; Albrecht, M. Chem. Rev. 2009, 109, 3445.
alcohol
22 23 24
ethanol ethanolc ethanol
25
ethanold
26
ethanole
27 28
n-propanol n-butanol
salt (amt (mol %))
conversn of alcohol (%)
selectivity to alkenes (%)b
Bu4NBr (21.6) Bu4NBr (21.6) Bu4NBr (21.6) Bu4NCl (2.7) Bu4NBr (21.6) Bu4NCl (2.7) Bu4NBr (21.6) Bu4NCl (2.7) Bu4NBr (21.6) Bu4NBr (21.6)
83 80 51
53 56 67
68
88
67
86
77 95
96 >99
a Reaction conditions: 3 mL of alcohol, 0.065 mol % of [Rh2(CO)4Cl2], 0.13 mol % of PMes3, 6 mol% of 57% aqueous HI, 48 h, 110 °C. Conversion and selectivity were determined by GC analysis of products or brominated products. b The other product is di-n-alkyl ether; 1-2% butenes was also observed in the case of ethanol (see main text). c 6 mol % ethyl iodide was employed instead of HI. d 0.13% of [Rh2(CO)4Cl2] and 0.26% PMes3. e No PMes3 added.
bromide are more active for dehydration than are those with chloride, although the latter give better selectivity. Intriguingly, a mixture of bromide and chloride salts can be used to improve selectivity while maintaining conversion (run 16). Addition of a phosphine ligand has less effect for these salts; indeed, triphenylphosphine actually acts as a catalyst poison under the same conditions (run 19). Neither dihexyl ether nor olefinic products were observed in control runs when no HI was employed, whereas the absence of the rhodium complex in the reaction (run 17) led to formation of dihexyl ether (20% yield) as the only product, presumably by simple acid catalysis. The use of LiBr in place of imidazolium or tetralkylammonium salts (run 20) gave very low yields of the hexenes. Dehydration of hexanol can be also carried out using 6 mol % of n-hexyl iodide in place of aqueous HI (run 21), supporting the mechanism depicted in Scheme 1. Monitoring an ongoing run based on conditions in run 14 shows smooth conversion of hexanol consistent with first-order kinetics for an 18 h period, the final conversion figure actually being achieved within this time (see the Supporting Information for a typical graph). The selectivity vs time to hexenes compared to dihexyl ether is invariant within error. The optimized catalytic system from these primary screening experiments was extended to other alcohols (Table 2). Dehydration of n-propyl alcohol and n-butyl alcohol proceeds smoothly, affording good yields of olefins with excellent selectivity. Results for ethanol were initially more disappointing, with poor selectivity being observed for tetrabutylammonium bromide (runs 22 and 23). However, using a mixture of bromide and chloride salts, as before, gave improved selectivity (run 24) and yields were improved by using higher catalyst loadings (run 25). It is not clear why the dehydration of ethanol gave lower yields of olefin as compared to n-propanol or n-butanol. A possible explanation could be product inhibition via stronger binding of the π-acidic ethylene at the rhodium center; rhodium ethylene complexes are observed by NMR spectroscopy in a typical run (vide infra). Added trimesitylphosphine has no effect within error for ethanol dehydration (run 26). Studies are underway to define the mechanism of this reaction, and our preliminary investigations point to a number of (11) (a) Ionic Liquids in Synthesis; Wasserscheid, P., Welton, T., Eds.; Wiley-VCH: Weinheim, Germany, 2002. (b) Weldon, T. Chem. Rev. 1999, 99, 2071. (c) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. Rev. 2002, 102, 3667. (d) Parvulescu, V. I.; Hardacre, C. Chem. Rev. 2007, 107, 2615.
Communication
roles for the crucial tetraalkylammonium salts. First, they solubilize the various reaction components; the use of similar salts as ionic liquids11,12 or phase-transfer agents13 is well-known to lead to unique reactivity. No phase separation is observed at the reaction temperature. Second, they generate the active catalytic species. Treatment of [Rh2(CO)4Cl2] with 1 equiv of Bu4NBr in hexanol leads to virtually instantaneous conversion to [Bu4N][Rh(CO)2Br2] (νCO 2064 and 1990 cm-1). Addition of HI leads to equally rapid formation of [Bu4N][Rh(CO)2I2] (νCO 2059 and 1987 cm-1), which persists as the only Rh-CO species observable by IR spectroscopy throughout a 48 h run. Formation of such types of complexes by this reaction has already been described in the literature.14 Addition of 1 equiv of EtI to [Bu4N][Rh(CO)2Br2] in the absence of additional salt leads directly to the production of ethylene; intermediate Rh-Et species are not observed by NMR spectroscopy at room temperature, suggesting that β-elimination of these species is rapid. Both free ethylene at 5.26 ppm and signals consistent with η2-coordinated ethylene at 5.23 ppm (ratio of free to coordinated species 6:4) are observed. Surprisingly, in the absence of (12) Promoting effect of ionic liquids: (a) Sliger, M. D.; P’Pool, S. J.; Traylor, R. K.; McNeil, J., III; Young, S. H.; Hoffmann, N. W.; Klingshirn, M. A.; Rogers, R. D.; Shaughnessy, K. H. J. Organomet. Chem. 2005, 690, 3540. (b) P'Pool, S. J.; Klingshirn, M. A.; Rogers, R. D.; Shaughnessy, K. H. J. Organomet. Chem. 2005, 690, 3522. (13) (a) Yadav, G. D. Top. Catal. 2004, 29, 141. (b) Albanese, D.; Landini, D.; Maia, A.; Penso, M. J. Mol. Catal. A: Chem. 1999, 150, 113. (c) Jin, G.; Morgner, H.; Ido, T.; Goto, S. Catal. Lett. 2003, 86, 207. (d) Jeffery, T. Tetrahedron 1996, 52, 10113. (14) (a) Vallarino, L. M. Inorg. Chem. 1965, 4, 161. (b) Fulford, A.; Bailey, N. A.; Adams, H.; Maitlis, P. M. J. Organomet. Chem. 1991, 417, 139. (15) Roe, D. C.; Sheridan, R. E.; Bunel, E. E. J. Am. Chem. Soc., 1994, 116, 1163.
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additional salt, only one turnover to produce the olefin is observed, even with a large excess of EtI or EtOH. Only upon addition of a minimum of a 4-fold excess of Bu4NBr to rhodium is catalytic turnover achieved. This suggests a third role for the added salt: to regenerate [Bu4N][Rh(CO)2X2] (X = Br, I) by facilitating the reductive elimination of HX from [Bu4N][RhH(CO)2X3] in the final step of our proposed catalytic cycle. This hypothesis is consistent with the report of Bunel and coworkers, which demonstrated the facile disproportionation of [RhH(CO)2X3]- species to [Rh(CO)2X2]-, [Rh(CO)2X4]-, and H2 in the absence of added salt. It also has parallels in halidepromoted rhodium-catalyzed methanol carbonylation at low water concentrations, where halide (usually iodide) is believed to stabilize catalytically active rhodium species from decomposition and precipitation.9b,c It is noteworthy that such stabilization effects are particularly important at low CO pressure, the CO concentration in our case, of course, being effectively zero. Work is ongoing to define the specific mechanism by which salts facilitate catalyst regeneration and the origin of the subtle selectivity changes observed depending on halide. In summary, this new homogeneous catalyst for ethanol or other primary alcohol dehydration gives olefins in good selectivity and opens possibilities for the conversion of bioderived alcohols to higher value products.
Acknowledgment. We thank the EPSRC (to I.V.S.) and BP (to G.D.) for funding. Supporting Information Available: Text and figures giving experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.
