Ind. Eng. Chem. Fundam., Vol. 18, No. 4, 1979 419
Development of a Microreactor System to Measure Catalytic Activity on Ion-Implanted, Monocrystalline Supports Wojtek A. Smigiel, Ian H. B. Halning, Paul M. Rabette,’ and Mlchel Che Laboratoire de Chimie des Solides, ER 133-CNRS,
Tour 54, Universit6 Pierre et Marie Curie, 75230 Paris Cedex 05
Three microreactor systems (flow system for work at atmospheric pressure; flow system with a facility to work at elevated pressures; closed system with a facility to pretreat under flowing conditions) have been developed to study the catalytic activity of platinum implanted into monocrystalline supports. Features include the ease of conversion between the different systems,automated sampling devices, use of recirculation, and the design of a reaction vessel suitable to study the activity of these catalysts.
Introduction Investigations using microreactors are well known (Celler, 1970; Germain, 1966; Doraiswamy and Tajbl, 1974; Smigiel and Siedlewski, 1976) and can be divided into two basic sections: (1)those involving batch (or static) reaction systems and (2) those involving flow systems. We wished to make a comparative study of the catalytic activity of some low-area platinum implanted into monocrystalline oxides, using ethylene hydrogenation as the test reaction. These catalysts have low surface areas so that it was found to be difficult to use a classic flow system (Rabette et al., 1979) for the feebly implanted catalysts. It was decided to use systems involving recirculation of reactants over the catalysts, giving a higher probability of contact of the reactants with the catalyst surface and a faster approach to steady-state conditions than with a batch system. T o this end three systems were designed: (a) a flow system with recirculation for work at atmospheric pressure, (b) a flow system with recirculation with the facility to work at elevated pressure, and (c) a closed, or batch, system with recirculation with the facility to pretreat under flowing conditions. The latter was found to be the best system for our catalysts. An important feature is that the systems are easily interconverted. Advances have been made on a previously described system (Aronow et al., 1970) inasmuch as a complex automatic sampling unit and a new reaction vessel have been designed. The new reaction vessel has the advantages that catalyst samples are easily changed, the reactant gases are preheated before access to the catalyst, and the reactants pass over the catalyst only. A reaction vessel for single crystal metal catalysts has already been described (Dalmai-Imelik and Massardier, 1976). Catalysts The catalyst supports were plates of MgO, A1203,and SiOz. They were implanted with platinum with the Mark IV E M separator of the Atomic Energy Research Establishment Harwell using monoenergetic ions accelerated to 30 keV with a beam current not exceeding 100 KAcm-2. Under these conditions platinum was deposited almost entirely in the first 150 A below the surface. Implantation doses of 1014,1015,10l6, and 10’’ Pt+ ions cm-2 were used, corresponding approximately to 0.07, 0.7, 7, and 70 monolayers on the geometric area assuming a hexagonal close-packed arrangement. Description of the Microreactor Systems Table I lists the commercial sources of the major components used in all the systems, and Figure 1 shows 0019-7874/79/1018-0419$01 .OO/O
Table I. Sources of the Major Components Used in t h e Reaction Systems component 2 electrovalves, GR-10 1electrovalve, GR-8 restrictor for reaction circuit flowmeters, flowmeter regulators
source Erba Science (France)
Emerson Electric (France) S A . , Division Brooks Prolabo, 11, rue Pelee thermostat control for sample loop Paris, France Metal Bellows Corp., p u m p for reaction system, MB 4 1 Sharon, Mass. 02067 variable speed motor for this p u m p Societe Proservice, Asnieres 92600, France Perkin-Elmer Ltd gas chromatograph F 11 (France) programmer for automatic sampler Crouzet (Paris)
schematically the pump-driven flow system which was initially constructed. It comprises four independent parts as follows. (1) Gas Regulation and Mixing Unit. This unit is composed of four highly compressed gases, Gl-G4, with corresponding two-stage reducing valves, Pl-P4, restrictors, Rl-R4, and flow-meters, Fl-F4. A gas mixer, M, consisting of a glass tube containing glass particles, was included for the three gases (symbolized by the numbers 1-3) involved in the reaction mixture. Normally two gases are reactants (G1 and G2) and the third (G3) is a diluent. G4 is the carrier gas for the chromatograph. (2) Reaction System. This part consists of the reactor, variable speed circulating pump MB 41 (Table I) and coil, CC, which is cooled by a ventilator during an experiment. The microreactor (volume 280 cm3),which is presented in detail in Figure 2, is composed of two parts, one fixed and one removable. The fixed part consists of two concentric tubes, serving the dual purpose of preheating and acting as an inlet and outlet for the reactant gases. The second part, which is removable so that catalysts can be changed easily, is made of a head (the reactor seal), a hollow tube (thermocouple well), and the catalyst basket having exactly the same external diameter as the internal diameter of the inner tube of the fixed part. The catalyst (0.25 cm2 in geometric area) is normally fixed in the basket with the aid of quartz wool. (3) Multiport Valve System (Figure 1). This part is the chromatographic sampler for this system. Valve 1 0 1979 American Chemical Society
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Ind. Eng. Chem. Fundam., Vol. 18, No. 4, 1979
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I Figure 1. General reactor and gas chromatography system for flowing pretreatments and reactions with recirculation, without ability to vary pressure. Abbreviations are as explained in the text.
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Figure 3. Modification to the valve system of Figure 1 nec sary to perform reactions at pressures greater than 1 atm. ~
Figure 2. Reaction vessel.
facilitates the choice between products (continuous line) and reactants (broken line) for the gas analysis. Valve 2 (kept thermostatted at 30 " C ) is the classic sampling valve equipped with a sampling loop, SL, attached to ports 3 and 6. Both valves are equipped with pneumatic pistons which are controlled by electromagnets. The solenoid controller for the electromagnet of valve 2 is automatic and thus samples can be taken at predetermined time intervals (between 1 and 60 min). It might be pointed out that an eight-port valve is sufficient for sampling and that an
eight-port valve (with the configuration of two independent four-port valves) can substitute for the ten-port mode switching valve. Restrictors R5 and R6 ensure pressure equilibrium in the circuit. (4) Gas Chromatographic Analyzer. A conventional gas chromatograph using nitrogen as carrier gas and a flame ionization detector is employed as the analyzer of the gaseous products. To convert the above flow system to the one previously mentioned with a facility to work at elevated pressures or to the static system with a facility to pretreat under flowing conditions, modifications are necessary only to the valves and valve connections. Figure 3 shows a more sophisticated valve system which can be used with the recirculating flow system of Figure 1. It enables work to be carried out at elevated pressures
Ind. Eng. Chem. Fundam., Vol. 18, No. 4, 1979
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Table 11. Review of t h e Relations between t h e System Functions and Valve Positions for t h e Circuits Presented in Figures 1, 3 and 4 figure 1
possible methods of use
function used in one cycle
flow method with recirculation without facility for pressure pressure
analysis of products
analysis of reactants
3
as Figure 1, b u t with facility t o work under controlled and elevated pressures
analysis of products
analysis of reactants
4
valve arrangement for flowing pretreatments and static reactions
for work in static conditions (and product analysis) for work in flowing conditions (analysis of reactants or pretreating gases)
position of valvea 1
2
0 0 0 0 0
0 1 0 1 0
1 1 1 1 1
0
0 0 0 0 0 0 0
0 0
1 1 1 1 1 1 1 0 0 0 0 0
1 1 1 1 1
3
operation carried o u t b
cycle no. 1
2
3
1 0
1
4
0
0
5
0 0
1 1 0 1
6
1
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0 0 0 1 0 0 1
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7
8
9
1 0
1
10
0 0 1
11
0 1 0
12
0, position of valve in which gases flow through valve by way of the continuous line; n o current flows through the electrovalve; 1 position of valve in which gases flow through valve by way of the dotted line; current flows through the electrovalve. E, equalization of pressure in the circuit to 1 atm (5-10 s ) ; I, injection of gases t o be analyzed into the carrier gas (0.5-2 rnin); A, analysis of sample introduced during operation 1(3-60 min); N, the purging of the sample loop by the gases forming the next sample t o be analyzed ( > 3 min); S, starting point of the analytical process; R, reactants, P, products.
(up to 8 atm) by the incorporation of a third valve, valve 3, and regulator, P5. The pressure in the sampling loop, SL, is equalized with atmospheric pressure by means of this valve and is measured by the manometer, MA. Figure 4 shows the modifications necessary to the basic circuit of Figure 1 in order to carry out investigations using a closed, static system. In this circuit, valve 1permits the establishment of a system by which we can pretreat the catalysts under flowing conditions (broken line) and afterwards perform reactions with a closed system (continuous line). Valve 2 acts in its usual capacity as chromatographic sampler. Table I1 reviews the programming details for all the reaction systems described.
Reasoning behind the Construction of the Above Systems The circuits presented in Figures 1,3, and 4 are the three stages of the development of a system in which the catalytic activity over the complete range of the platinumimplanted oxide catalysts could be viewed. Figure 4 exhibits the system actually in use a t present. The type of configuration shown in Figure 1 has been recently proposed for the study of the hydrogenation of ethylene on supported platinum (Schlatter and Boudart, 1972) and may be satisfactorily applied to conventional
catalysts with surface area larger than that of the ionimplanted catalysts. It should be preferred whenever kinetic parameters are needed because it can be considered as a differential reactor (Doraiswamy and Tajbl, 1974). The configuration of Figure 1 corresponds to an improvement of the system used recently for ion implanted catalysts (Rabette et al., 1979). Catalysts implanted to a high degree (1017Pt ions cm-2) gave quantitatively useful results for the ethylene hydrogenation reaction, but those feebly implanted (51Ol6 Pt ions cm-2) gave immeasurable conversions. Because the hydrogenation of ethylene involves a pressure decrease as the reaction proceeds, an increase in reactant pressure should increase activity. To this end the circuit of Figure 3 was constructed. The pressure here may be regulated up to 8 atm and the results obtained must be adjusted to similar pressure conditions before comparisons can be made. Here also the conversions obtained using the feebly implanted catalysts were small and could not be used quantitatively. An increase in temperature to cause catalytic activity cannot be used in the case of these catalysts as we wish to cause no migration of the implanted platinum to the surface (Rabette et al., 1979) at this stage in our studies and also the effects of any low-temperature pretreatments would be annulled by the use of higher reaction tem-
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Ind. Eng. Chem. Fundam., Vol. 18, No. 4, 1979
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Figure 4. Modification to the valve system of Figure 1 necessary to perform reactions under static conditions with pretreatments under flowing conditions. Table 111. Rates of Ethylene Hydrogenation (at 2 4 0 "C after Oxygen Pretreatment) over Pt Implanted into MgO (100) Faces rate of C,H, hydrop.,
a
catalyst
% min-' c m - 2 of cat.a
MgO blank 10l4 Pt c m - 2 l O I 5 Pt cm-, 10l6 P t c m - 2 lo" P t c m - 2
2.3 x 4.0 x 10-3 2.4 x 10.' 3.8 x lo-' 1.0
Geometric surface of the catalyst.
peratures. The closed system (Figure 4) was then chosen to measure catalytic activity. This circuit gave sufficiently large measurable differences (Table 111) in the rates of reaction for the whole range of implantation doses, so that comparisons of catalytic activity over the whole range could be made. With this system it is the accumulation of products obtained from each elemental cycle of circulation which gives rise to the measurable and detectable activities. Removing samples from such a system disturbs the reaction, but if the circuit volume is known, corrections can be made. In our case the ratio of circuit volume to sample loop (300 cm3:0.5cm3 = 600:l) is large and since about ten
samples are withdrawn per reaction, this correction is not necessary. Two important points should be made: (a) that the conversion between the flow system of Figure 1 and the batch reactor system of Figure 4 can be carried out in a matter of minutes, and (b) that the conversion between the flowing pretreatment and static-reaction stages in the apparatus of Figure 4 is performed by the simple flicking of a switch. The ease of conversion between the systems of Figures 1, 3, and 4 is obviously useful since different types of results are obtainable from flow (at both atmospheric and elevated pressures) and both systems. It is proposed to use the two systems developed here in a detailed study of the reactions over the 10l6and 10'' P t ions cm-2 catalysts. Future papers, in collaboration with the laboratory of Dr. A. J. Tench of AERE Harwell, will discuss the treatment of results from such systems. Conclusion Three microreactor systems have been designed, each involving reactant recirculation. The closed system with the facility to pretreat under flowing conditions was found to be the best method for viewing the catalytic activity of the whole range of implanted catalysts. The important features of these systems are that (a) they enable studies to be made of the catalytic activity of samples having low specific surface area (of the order of 1 cm2 g-l), (b) they are easily interconverted, and (c) they employ automatic sampling devices, which have obvious advantages. Acknowledgment Thanks are due to the "Action ThBmatique ProgrammBe" (No. 3195) du Centre National de la Recherche Scientifique de France (for I.H.B.H.), to the Ministere des Affaires Etranggres, France (for W.A.S.) for the provision of grants, and to Dr. A. J. Tench, Atomic Energy Research Establishment, Harwell, England for the preparation of the catalysts. We would also like to express our gratitude to Professor C. 0. Bennett (Department of Chemical Engineering, University of Connecticut) for his helpful discussions. Literature Cited Aronow, N. A,, Wanszenker, A. R., Dorochow, F. S., Joffic, I. I., Fuks, I. S., Idlis, G. S., Kinet. Katol., 11, 1048 (1970). Celler, W., Przem. Chem., 49, 68 (1970). Dalmai-Imelik, G., Massardier, J., Proc. Sixth Int. Congr. Catal., London, 1, 90 (1976). Doraiswamy, L. k.,Tajbl, D . G., Cat. Rev. (Sci. Eng.), 10(2), 177 (1974). Germain, J. E., "La Cetalyse au labwatoire et dans I'Industrle", p 45, B. Claudel, Ed., Masson, Paris, 1967. Rabette, P., Deane, M., Tench, A. J., Che, M., Chem. phys. Left., 60, 348 (1979). Schlatter, J. C., Boudart, M., J . Catal., 24, 482 (1972). Smigiel, W., Siedlewski, J., Przem. Chem., 55, 414 (1976).
Received for review July 19, 1978 Accepted March 27, 1979
