The Olefin Metathesis Reaction
Dennis W. Bennett' University of Utah Salt Lake City, UT 84112
An experiment in heterogeneous catalysis
During the past few years a renewed interest in olefin metathesis catalysis has led to a reconsideration of its fundamental implications and practical applications (1-4). The eeneral reaction results in the transalkvlidenation of olefins and is believed to occur through a concerted cycloaddition nrocess which is formallv forbidden" (5) ."svmmetrv .
VALVE HOOD
t
I
+
RICH = CHR2 R3CH = CHR4 *, RICH = CH3 + R2CH = CHR4 In the absence of a catalyst the reactants are confronted by a formidable activation barrier and small quantities of products are formed only a t temperatures in excess of 700°C. The addition of a group VI transition metal to the olefin system introduces a new set of rules, providing a "symmetry-allowed" pathway for the reaction to occur catalytically ( 6 ) . The auto-metathesis of propylene is a typical example 2 C H r C H = CHz
d CHz =CHz + CH3--CH =CH-CH3 csta1ys
The means hy which the transition metal catalyst is able to effect this remarkable process has been a topic of considerable discussion, with the most popular mechanism currently favoring a non-concerted process in which each olefin reacts independently with the active site. The reaction is proposed to proceed through metallocarbene and metallacyclobutane transition states (Fig. 1). This scheme was initially proposed by Herrisson and Chauvin and is termed "the carhene mechanism" since the chain carrying intermediate is believed to he a metallocarhene (7). While olefin metathesis catalysis is both theoretically and practically important, conventional homogeneous reaction systems do not lend themselves t o student experimentation since they require difficult inert atmosphere techniques and often incorporate pyrophoric co-catalysts. Fortunately there exist several stable heterogeneous catalysts which effectively demonstrate the reaction and are readily adaptable for student use. The experiment descrihed here illustrates the incorporation of such catalysts in a continuous flow fixed-bed reactor system, providing the student with the additional bonus of exposure to some of the common tools of industrial research. The reactants and products are all gases and are analyzed easily with a simple gas chromatograph equipped with a thermal conductivity detector and a column packed with 20% B,@-oxydipropionitrileon 45-60 mesh Chromasorb P. The tungsten oxide-silica catalyst described in this experiment is easy to prepare, and others (e.g., CoO.MoOs A1203) can he purchased commercially.
-
CH,C.I=C+
H~C-chch,
CHICH=CH2
+ M=CH2
1
4
CH3HC-CH,
+
-
CH,C~=CHCH,
+
CHfCH2
1 I + +
M-CH,
M=CHCH3
Figure 1. Rapylene metathesis via a "carbene" mechanism. 672 1 Journal of Chemical Education
FLOWMETER
Figure 2. Smematic diagram of continuous flow fixed-bed reanw system. Preparation of a WOjSI02 Catalyst A weighed amount of 10-50 mesh anhydrous silica gel (about 10 e) is added to about 25 ml of distilled water. The excess wat& is decanted, the wet silica is weighed, and the volume of water adsorbedner . gram of SiOn is calculated. 100 g of fresh silica gel isthen placed in an evaporating dish and thorouehlv saturated with a solution containinn 8.5 n ammonium &tungstate ((NH~)~[WIZO~S(OH)~]) di&olvd in just enough distilled water to hvdrate the silica as descrihed above. ~ x c e iwater s is then drive" from the catalyst by placing it in a drvinn oven (120°C), after which the solid is cooled and stored & a closed container.
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The Reaction System Figure 2 illustrates the basic components of the continuous flow reactor system. AU connections are made with NPT pipe fittings or stainless steel compression fittings and all pipe joints should be assembled with Teflon" tape. Stainless steel is the material of choice for the comoonents inside the tube furnace, with external ithing and fittings madeeither of hrass or stainless steel. The pressure relief valve should release at 50 psig, with escape gases vented to the hood. Once activated the catalvst is extremelv sensitive t o poisoning, particularly from water and oxygen: I t is very important &the system upstream from the reactor to be completely airtight. I t should be constructed entirely of metal tubing and connections. The use of even short lengths of plastic or rubber tubing will allow enough diffusion to result i n certain poisoning of the catalyst. All . nuree and feed eases must he ourified orior to their exposure to the active catalyst surface. This is conveniently accomolished bv oassinn..the .eases throueh driers contaiuine artivaied 13X &lecular sieves. A typical drier is illustrate^ in Figure 3. The molecular sieves should be dehvdrated IIV them in a borosilirate glass tube and heat/ng them i n a dry nitrogen stream at 100°C in a tuhe furnare for 3 hr (Fig. 4), then aliowing them to cool while continuing the nitrogen purge. The top pipe fitting is then removed from the drier and thetnolecula~sieves areadded under a continuous flow of nitrogen. The top of the drier is replaced and the valves are closed until the drier is attached to the reaction system. A simple stainless steel reactor is illustrated in Figure 5. The Present address: Department of Chemistry and Geology, Clemson . . . University, Clemson, SC 29631.
OARSE FRIT
Figure 3. A lypieal gas drier.
Figure 4. Setup l a h y i n g molecular sieves. To pow sieves into driers transfer nitrogen inlent to ball joint at bottom of tube.
inlet tuhe is coiled to increase the path within the tuhe furnace and serves as a pre-heater for the incoming gas. A small piece of quartz wool is placed in the lower compression union to contain the catalyst particles. The tuhe furnace should be mounted vertically to insure uniform packing and prevent channeling in the reactor and should he fitted with a thermostat or an external temperature controller. The reaction effluent can he oassed throueh a GLC -aas sampling valve connected to a soap bubble flowmeter, which in turn is vented to the hood (alternatively, the entire system can be assembled in a hood). If a gas sampling valve is not available a simple "tee" fitted with a septum can he inserted in the effluent line (short lengths of plastic or rubber tubing are permitted downstream from the reactor). The gas samples canthen be withdrawn into a syringe for analysis. The Experiment
The reactor is disconnected at the top and filled with solid catalyst. The system is reassembled and with the compressed air regulator set a t about 20 psig needle valve A (Fig. 2) is opened gradually until the soap bubble flow meter indicates that about 200 cc of air per minute is flowing through the reactor. The furnace is regulated a t 600°C and the catalyst is activated for 1-2 hr under a continuous flow of air. Dry nitrogen is then substituted for the air (at about the same flow rate), the furnace is reset at 400°C, and the catalyst is treated with nitrogen for another hour. The propylene regulator is set a t 20 psig, needle valve A is closed. and needle valve B is eraduallv ooened until the oro~~~~, pylene is flowing through the system a i a i a t e of 50-100 cEper min. The effluent is then sampled and analyzed for metathesis products. The catalyst can be regenerated simply by treating it with air at 600°C and nitroaen at 400°C as previouslv described and may he left in'the reactor between experiments. Discussion
The chromatograms can he quantitated hy precalibration of the gas chromatograph using a mixture of propylene, eth-
.
Figure 5. Fixed-bed reactor. All metal components are stainless steel.
vlene. and hutenes of known concentration.. or oeak ~.~ . areas can he normalized using published thcmnal response factors (81 to conmensate for diiferences in thermal conductivities uf the varioui products. The resultant mole fractions can then he used to determine the oroximitv of the reaction to eauilihrium ( K , for the metathesisof propdene at 400°C is apprLximately 0.7). The need for contamination-free reactants cannot he overemphasized, and the propylene used in the experiment must he of high purity. Nevertheless, with moderate care to avoid poisoning the active catalyst in this experiment will provide the student with a straightforward and aesthetic illustration of the olefin metathesis reaction. In addition, with relatively simple modifications the reaction system can he adapted to other olefin metathesis reactions, kinetics studies, or other heteroeeneous catalvsts. Although there are relatively few hazards in this experiment. care should he taken to exclude anv ~ossihleleakage of the hidrocarhons into the air, especially lithe vicinity o? the furnace. and the effluent should he vented into a hood. Other precautions include those associated primarily with the routine handline of comoressed gases and high temperature electrical furnaces, both of which have become com&nplace in the advanced undergraduate lahoratory. ~
Acknowledament -
The author wishes to thank F. Pennella of Phillips Petroleum Comoanv for helpful discussions involving olefin metathesis ingeneral andthis experiment in particular. Literature Cited (I) Gmbbs, R. H..in '"ProgressIn Inorganic Chemistry." Vol. 24, S. J. Lippard, (Editor). John Wiley & Sans, N e w York, 1978. pp. 1-50, (2) Csldemn, N.,in "Tho Chemistryof Double Bonded FunctionalGroups." Part 2. Pat& S., (Editor).Wiley, New York. 1977.p~.91334. (3) Banks.R.L.,For(schr. Chom.Forsch.. 25.39 (19721. (4) Hughes, W. B., Orga~mefollieChem Synlh., 1, 341 (1972). (5) Haflmann, R. and Woodward, R. 9.. Amounts Chem. R m , 1.17 (1988). i C.,l Ind. ~ Chem. . Pmd. Re% Dsuelop, 3.170 (1964). (6) ~ ~R. L. and ~ ~k ~ G. ~ ,~ Eng. (7) Herrisson,J.L., andchauvin,Y.,Mokromoi. Chrm., 141,161 (19701. (6) Masner, A. E., Rosie, D. M., and Armbright, P.A,, A d . Chem., 31,230 (1959).
Volume 57. Number 9, September 1980 I 673
