David E. M e a d and John E. Bensonz Department of Chemical Engineering Stanford University Stanford, California
Liquid Phase Dehydrogenation
I
of lsopropanol Heterogeneous catalysis experiment
Attempts to develop a heterogeneous catalytic experiment amenable to simple kinetic studies are generally blocked by requirements of extensive catalyst pretreatment, elaborate high vacuum apparatus, and expensive instrumentation for analyzing the reaction mixture. Further difficulties often encountered are catalyst deactivation or poisoning, undesirable side reactions, and masking of the true kinetics by heat and mass transfer effects. Hence, heterogeneous catalysis has commonly been neglected in physical chemistry and chemical engineering laboratory experiments. An easily performed undergraduate experiment, which is unhindered by the above limitations, is the liquid-phase dehydrogenation of isopropanol to acetone on porous nickel catalysts. This reaction, represented by the equation CH,-CHOH-CH,(l)
-
CH-CO-CHdC
+ Hdg)
has readily measurable rates a t the boiling point of the alcohol, 82'C, and at atmospheric pressure. Consequently, inexpensive glass equipment can be used. The volume of hydrogen evolved as a function of time is measured in gas burets, and rates are obtained by graphical differentiation. Since the acetone and alcohol concentrations are stoichiometrically related to the amount of hydrogen evolved, time-consuming chemical analysis is eliminated. This experiment is also quite safe since the reaction is endothermic. The dehydrogenation experiment illustrates a common feature of many heterogeneous catalytic reactions: inhibition by a product, in this case acetone. It also demonstrates an important principle: a reaction that is thermodvnamicallv unfavorable mav be carried out bv continuously removing one of the products, in this case hydrogen. The experiment provides a good introduction to the field without overwhelming the student with the difficulties usually inherent in catalytic systems. Active Raney nickel (obtainable from Raney Catalyst Co., Inc., Chattanooga, Tenn.) may be employed as a catalyst. However, a more active catalyst is easily prepared by sodium borohydride reduction of nickel acetate to NiZB. This may be done in advance, if desired, in quantities sufficient for a class. With this catalyst, reaction rates are surprisingly reproducible even for catalysts from different batches. The dehydrogenation reaction is clean, and catalyst deactiva-
1
Union Oil Company of California, Brea, California. Diokieson College, Carlisle, Pa.
tion during the course of a run is negligible after the first few minutes. The Experiment The catalyst preparation is based on the work of Paul, Buisson, and J ~ s e p h . ~ An aqueous 4 wt yo solution of Ni(C2H30z)2.4Hz0is charged into a magnetically stirred flask. For purposes of increasing the surface area, sufficient CrCla.6H20is added to give a chromiumto-nickel weight ratio of 0.02. The catalyst is precipitated and reduced by slowly adding an aqueous 10 wt % solution of sodium borohydride to the nickelchromium solution. A 3: 1 M ratio of sodium borohydride to nickel acetate should be used to ensure complete reduction. The catalyst is obtained as a fine precipitate with an average particle diameter of 50p and a surface area of about 80 m2/g. Following reduction, the catalyst is washed three times with oxygen-free distilled water and three times with isopropanol. Although not pyrophoric, the catalyst should be stored under isopropanol until used to prevent oxidation of the surface.
Figure 1. Apporatvr for dehydrogenation of iroproponol.
The dehydrogenation apparatus, shown in Figure 1, is a simplified version of an apparatus used in the studies of Claes and Jungers4 The reactor consists of a twonecked 500 ml round bottom flask equipped with a Teflon-coated magnetic stirring bar. An egg-shaped stirring bar and a strong driving magnet are recommended to provide good agitation for uniform catalyst suspension. The means of heating must permit close " A U ~ R., burs so^, P., AND JOSEPH, N., Ind. Eng. C h a . , 44, 1006 (1952). CLAES,F.,AND JUNGERB, J. C., Bull. Soe. Chirn. Fmme, Ser. 5,25, 1167 (1958).
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Volume 43, Number 6, June 1966 / 325
proximity of the driving magnet to the bottom of the flask. It is suggested that the flask be wound with heating tape or resistance wire covered with asbestos cement so that the bottom of the flask is free. . The ground joints are assembled with Teflon sleeves (obtainable from most chemical supply companies) or a stopcock grease insoluble in isopropanol at 8 2 T . About 250 ml of reagent grade isopropanol is charged into the reactor and brought to boiling. Operation a t the boiling point provides good temperature control,
and rate data are plotted as l / r versus [A] as shown in Figure 2. The rate constant is obtained from the intercept and the inhibition coefficient from the slope.
.
"
to the reactor through the addition funnel; the volume of suspension used should be such that about 0.7 to 0.8 g of NizB is added. It is desirable to use a Tefonplug separatory funnel with an enlarged hole to prevent blockage by catalyst particles during the addition step. The chromium-containing catalyst prepared by the above procedure is desirable because of its small particle size and consequently less blockage of the separatory funnel. Hydrogen produced by the reaction passes through the reflux condenser and a dry ice-acetone cold trap, which isolates the reactor and removes any alcohol vapor in the hydrogen. It is then collected in one of the 250-nd gas burets, which are fitted with three-way stopcocks so that one may be filled while the other is in use. Leveling bulbs filled with water and attached to the bottom of each buret are used to refill the buret which is not in use. Quantities may be cut in half to permit the use of 100 ml burets, such as those commonly used in the Victor Meyer molecular weight determination. Volume readings are taken every 30 sec during the first 10 min and at longer intervals as the reaction rate decreases. The reaction is continued until about two 1 of hydrogen have been evolved; this corresponds to a conversion of about 2%. Interpretation of Results
Grapliical differentiation has proved quite adequate for obtaining the rate during the initial stages of the reaction. A plot of the total volume of hydrogen evolved as a function of time is made, and the rate of reaction, T , is obtained a t various times by taking tangent slopes of this plot. The acetone concentrations (for the times at which the rates were calculated) may he obtained by dividing the moles of acetone formed, equal to the total moles of hydrogen evolved, by the initial total volume of isopropanol used. The rate of reaction, r, follows a rate expression of the form:
0.00 0.00
0.10
0.20
0.30
[A] Moler/Liter
I t is desirable to express the rate constant per g of catalyst in order t o compare student results. The catalyst weight may be obtained by allowing the reaction mixture to settle, decanting most of the supernatant liquid, and washing all of the catalyst into a small weighed flask. The remaining liquid is removed by careful evacuation or by a steam bath, and the flask reweighed to obtain the catalyst weight. For the nickel-chromium catalyst described above, a typical value for the rate constant is about 150 ml HP at ambient temperature and pressnrelg min; this corresponds to about 6 X l O F mole Ht/g min. At high agitation intensity, inhibition coefficients of 8 to 9 l/mole are found. It is instructive for the student to realize that the empirical rate expression above is consistent with a reasonable sequence of simple chemical steps occurring at the catalyst surface. It is also important that he realizes that there may be other paths by which the reaction can take place that will yield the same rate expression. A detailed sequence of elementary steps, which is based on one proposed by Kemhall and Stoddart,"= may be found in a paper by Mears andBoudart? Acknowledgment
The helpful suggestions and discussions with Professor M. Boudart are gratefully acknowledged. This work was supported by NSF Grant GP-2305. STODDART, C. T. H.,
AND
K E M B A LC., ~ J . Colloid Sei., 11,
532 (1956).
where lc is a rate constant, /3 is an inhibition coefficient, and [A] is the acetone concentration. This expression may he rearranged to the linearized form:
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Journal o f Chemical Education
8 KEMBALL, C., AND STODDART, C. T. H., PTOC. Roy. SOC. ( h d o n ) , A241,208 (1957). 1 MEARS,D. E., AND BOUDART, M., "The Dehydrogenation of Isopropanol on Catalysts prepared by Sodium Borohydride R e duetion," A.I.Ch.E.J., in press.
