An Experiment on Heterogeneous Catalysis Juan Bussi, Carlos C o m a and Juan A. Coch Frugonil Facultad de Quimlca, Gral. Flores 2124, Montevideo, Uruguay
An experiment on the homogeneous catalysis of the reac- was retion HnOz(aq) = HzO(1) %Oz(g) by C r ~ 0 7 ~ions ported in this Journal (I).We have found i t to he of interest to carrv out the same reaction with Cr2072- ions fixed on a solid. Such heterogeneous catalysts are of great importance in the commercial production of chemicals and fuels (2). Heterogeneous catalysts.are in general more stable than their homogeneous counterparts and are easier to use in practice because they are readily separated from reactants and products. A large area of research involves the inmobilization of homogeneous catalysts. In this study we demonstrate the conversion of a homogeneous catalyst into a heterogeneous one. We also show the importance of the support in obtaining an active material.
+
Catalyst Preparation The fixation of Cr.0~2- ions on a solid surface can be easily performed by usingihe adsorption properties of solid COGpounds (3). Thus a preferential adsorption of C1207~-from i n aqueous s o ~ u t i o ~ c take a n place by the use of a suitable solid adsorbent. For our purposes care must be taken to ensure that no Cr2O72- will desorh from the solid and pass into the solution under the conditions employed in the catalysis experiments. Otherwise homogeneous reactions would interfere. Procedure Thesolids testedascautlyticsupports were the ion exchange resin Amherlite IRA 402, magnesium oxide-aluminum oxide (4), and an All of them were kept in a desiccator over P10r acid alumina 6,6). before and after the adsorption treatment. The adsorption of CrsO?2- ions was carried out hy contact of the solid (2.00 g) with a 0.0100 M KrCr207 solution (10.00mL)for one day at room temperature followed by filtration,washing and drying at 80 'C. No elution of adsorbed C r ~ 0 7 ~ions - into solution was detected in any case when the solids were kept in distilled water at temperatures be~~tween 20 OC and 80 OC for 1h. The amount of adsorbed C r ~ 0 was calculated from the concentration of C r ~ 0 7 ~in- the supernatant solution measured by atomic absorption spectroscopy. In all cases, resulta indicate almost complete removal of Cm072- ions from solution (>99.5%). Final weight percent of CrzO?- on the supports are reoorted ~ . - in the table. ~~. The catalyticartivity was determined by measurin~the volumeof O>evolved when 15.0 mL of 0.415 M HIOI were mixed with the catalyst under stirring. In order torompare theactivities of coo-‘under conditions of homogeneous and heterogeneous catalysis, its concentration, expressed as moles of C r 2 0 Ton the solid phase per liter of solution, was the same as in homoeeneous catalysis experi~~~~
the reaction. This is probably due to the action of the oxygen or other intermediate product of the reaction on the orzanic matrix of the resin. Over Mg-A1 oxide, C1207~- ions showed a low activity compared bith that of the homogeneous catalysts. This can be explained by the basic character of the oxide, which probably affects the stability of the adsorbed C r ~ 0 7 ~ -In. effect, for a C r ~ 0 7 ~solution having the same pH as an aaueous susoension of this oxide (DH = 9.5). oracticallv all tde dichromite passes to the chromate fo&,*and the measured rate was also verv low (0.14 umol s-' at 297 K). Acid alumina was the best cataiyst giving a rate o€ about half that of the homoeeneous catalvst a t 297.0 0.5 K. Several runs were perfzrmed with this catalyst in order to observe its behavior. No CrsO? ions were detected in the reaction solution, which indicates that only heterogeneous catalysis took place. During the reaction, the color of the solid (light yellow) turned to the same brownish color observed in the homogeneous reaction, suggesting that the s formed ( I ) . The measame intermediate active s ~ e c i e are sured rate is roughly proportional to the amount of catalyst as was found for the homoeeneous reaction (see table). The lower activity of t
