In the Laboratory
Alkene Isomerization Using a Solid Acid as Activator and Support for a Homogeneous Catalyst
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Andrew J. Seen School of Chemistry, University of Tasmania, Locked Bag 1371, Launceston, TAS, Australia;
[email protected] The use of supported reagents and catalysts for chemical synthesis allows the development of “greener” chemical processes by immobilizing potentially corrosive and toxic reagents or catalysts on a solid support, thus providing a simple means for recovery of these reagents or catalysts (1). There has been renewed interest in this area in recent years as the focus on green chemistry builds, with several journals devoting complete issues to supported reagents and catalysts (2, 3). We have developed an upper-level undergraduate experiment that, in addition to introducing students to catalysis using an air sensitive transition-metal complex, introduces the use of a solid acid as an activator and support for the catalyst. The experiment is based on the Ni[P(OEt)3]4– H2SO4 alkene isomerization catalyst system reported in the literature (4–6). A similar homogeneous experiment has been previously reported in this Journal (7). Substitution of the H2SO4 with a solid acid, such as the ion exchange polymer Nafion–H+, not only activates the catalyst but also immobilizes the cationic species within the solid acid:
Ni[P(OEt)3]4 + Nafion –H+ Nafion– HNi[P(OEt)3]4
{
Nafion– HNi[P(OEt)3]4
{
}
}
+
+
Nafion– HNi[P(OEt)3]3
{
}
+
+ P(OEt)3
While the Nafion supported catalyst shows a lower isomerization rate than the homogeneous species owing to diffusion limitations of 1-octene in Nafion (8), immobilization of the active species within the Nafion provides greater protection to the active species from decomposition by oxygen. This increased stability affords the opportunity to characterize the immobilized species by UV–vis spectroscopy without the need for working under an inert atmosphere and ensures that students will find the catalysis more reproducible than a homogeneous system.
Equipment required for this experiment includes a vacuum distillation apparatus, a 3-neck 100-mL round bottom flask fitted with septum and gas inlet–outlet, a magnetic stirrer, and cylinder of nitrogen with regulator. A GC with a nonpolar capillary column (we used a BPX5 column) and FID detector, and a scanning UV–vis spectrometer are also required.
Procedure Ni[P(OEt)3]4 is prepared according to previously reported procedures in air (7, 9), but must be vacuum dried and stored under nitrogen: NiCl2·6H2O + 5P(OEt)3 + 2HNEt2
Ni[P(OEt)3]4 + 2 (H2NEt2)Cl + (EtO)3P(O) + 5H2O
The immobilized species is characterized by UV–vis spectroscopy with a sample of Nafion–H+ film immersed in a solution of Ni[P(OEt)3]4, deoxygenated 1-octene, and methanol and then placed directly in the beam of a UV–vis spectrometer. Catalysis is performed under nitrogen at room temperature by combining Nafion–H+ granules with Ni[P(OEt)3]4, methanol, and 1-octene. A stopwatch is started on addition of the 1-octene and samples are taken by syringe at intervals during the reaction (such as 5, 10, … minutes) and analyzed by GC. The catalyst can not be reused owing to decomposition of the catalytic species, but the Nafion can be cleaned and regenerated for further use. The experiment can either be completed in two three or four hour sessions (with assistance from teaching staff to distill the P(OEt)3 and prepare the Nafion–H+), or it can be undertaken as an extended experiment or small project. Hazards Triethylphosphite is a strong lachrymator and must be used in the fumehood. Use gloves when handling the strong acid Nafion beads or film. Ensure organics have been washed out of the Nafion prior to regenerating the Nafion in HNO3. Results and Discussion
Experiment
Reagents and Equipment The chemicals required for this experiment are available from Aldrich Chemical Co. A source of high purity nitrogen is also required. All chemicals and solvents were used as purchased, with the exception of triethylphosphite, which was vacuum distilled prior to use, and Nafion, which was converted to the acid form. www.JCE.DivCHED.org
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Nafion films are transparent in the UV–vis range. The presence of Ni[P(OEt)3]4 and {HNi[P(OEt)3]4}+, which absorb at 238 and 325 nm, respectively (5), can be confirmed by UV–vis as shown in Figure 1. While solutions of the activated Ni[P(OEt)3]4 decompose quite readily in the presence of air, simple purging of the experimental setup with N2 has been found adequate to maintain catalyst activity for several hours, as is shown in Figure 2.
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Journal of Chemical Education
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In the Laboratory 100
1-octene 4
Composition / wt %
80
Absorbance
3
2
1
trans -2-octene cis -2-octene
60
40
20
0 250
300
350
0
400
0
30
Wavelength / nm
60
90
120
150
180
Time / min
Figure 1. UV–vis spectrum of Ni[P(OEt) 3 ] 4 (238 nm) and {HNi[P(OEt)3]4}+ (325 nm) in Nafion–H+ film.
Figure 2. Isomerization of 1-octene using Ni[P(OEt)3]4 and Nafion– H+ in methanol.
The predominant products observed are trans- and cis2-octene and a small quantity of octane. The trans-2-octene to cis-2-octene ratio changes through the course of the reaction, from 1.4:1 at 5 minutes to 3.4:1 at 180 minutes, illustrating that while formation of the cis isomer is favored kinetically, the trans isomer is thermodynamically more stable. The expected trans-2-octene to cis-2-octene equilibrium distribution is 5.5:1 (10). The formation of octane during catalysis is consistent with Tolman’s observations of butane forming during isomerization of 1-butene and is consistent with the catalyst deactivation pathway proposed by Tolman (6).
ization of 1-octene, instructions for the students, and notes for the instructor are available in this issue of JCE Online.
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Supplemental Material
Details of the preparation of Ni[P(OEt)3]4, characterization of the Nafion immobilized species, catalytic isomer-
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Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Sherrington, D. C. Chem. and Ind. 1991, 1, 15–19. J. Mol. Catal. A: Chem. 2001, 177, 1–170. Chem. Rev. 2002, 102, 3215–3892. Cramer, R.; Lindsey, R. V., Jr. J. Am. Chem. Soc. 1966, 88, 3534–3544. Tolman, C. A. J. Am. Chem. Soc. 1970, 92, 4217–4222. Tolman, C. A. J. Am. Chem. Soc. 1972, 94, 2994–2999. Birdwhistell, K. R.; Lanza, J. J. Chem. Educ. 1997, 74, 579–581. Seen, A. J.; Cavell, K. J.; Hodges, A. M.; Mau, A. W.-H. J. Chem. Soc., Dalton Trans. 1992, 1381–1385. Meier, M.; Basolo, F. Inorg. Synth. 1972, 13, 112–113. Alberty, R. A.; Gehrig, C. A. J. Phys. Chem. Ref. Data 1985, 14, 803–820.
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