6396
J. Phys. Chem. 1987, 91, 6396-6400
Some Observations on the Effect of Pressure on a Five-Component Anionic Microemuision Per Fotland Reservoir Department, Norsk Hydro Research Center, 5000 Bergen, N o w a y (Received: January 27, 1987; In Final Form: June 17, 1987) Pressure is applied to a number of microemulsion samples located in a pseudoplane (at atmospheric conditions). All multiphase samples show an increasing water solubilization and/or a decreasing oil solubilization. The observed phase transitions were of the kind 1-phase WI and WIII WI. In addition, several liquid liquid + solid transitions were recorded. Precipitation of the solid phase occurred at pressures of a few hundred bar, depending on the overall composition. The appearance of this solid phase seems to be independent of the particular phase behavior at which it occurs.
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Introduction Translucent, isotropic solutions consisting of water, salt, surfactant, oil, and alcohol are often defined as microemulsions. These solutions must also be in thermodynamical equilibrium. In contrast to this definition are the macroemulsions which are kinetically stable. The above definition of microemulsions will be used in this paper. It should be noted that alcohol and salt are not a necessary condition for the formation of microemulsions. In enhanced oil recovery (EOR) by chemical means, microemulsion phase behavior is of vital importance.’ In oil reservoirs extreme conditions of temperature and pressure are often met. Every aspect of microemulsions has to be considered in order to have a successful oil recovery by surfactant flooding. A system composed of five components where two or more are only partially miscible will show complicated phase behavior. There are a vast amount of papers dealing with the phase behavior of micro emulsion^.^-^ A five-component system (keeping temperature and pressure constant) needs four independent axes for representation of its phase diagram. This complicates the graphical presentation of the phase diagram, which is vital for the physical understanding of the system. In some domain of this 4-dimensional space, microemulsions display three-phase behavior. In these domains it is possible to treat the microemulsion as consisting of three components of which two are immiscible. The result is the pseudophase model: which is probably the most applicable microemulsion model there is. In this model salt, water and alcohol are grouped as pseudophase W’, oil and alcohol are named pseudophase 0’, and surfactant and alcohol are included in pseudophase S’. The partitioning of alcohol must be experimentally determined for each set of components. Upon mixing, these pseudocomponents display 1, 2, or 3 phases in equilibrium. A microemulsion phase in equilibrium with an excess 0’ phase or W’ phase is called a W I or WII phase, respectively. A microemulsion in equilibrium with both an excess W’ and an excess 0’ phase is called a WIII.’ When a pseudoplane is determined, the phase behavior (of the pseudophases 0’,W’, and S’) can be recorded in a standard triangular diagram where the tie lines indicate pseudocompositions. Recently, van Nieuwkoop et a].* have outlined an alternative approach for representation of a quinary system where all the components are treated as real ones (not pseudo). This latter approach is probably the most exact (1) Shah, D. 0. In Surface Phenomena in Enhanced Oil Recovery; Plenum: New York, 1981. (2) Bellocq, A. J.; Biais, J.; Bothorel, P.; Clin, B.; Fourche, G.; Lalanne, P.; Lemaire, B.; Lemanceau, B.; Roux, D. Adv. Colloid Interface Sci. 1984, 20, 167. (3) Bourrel, M.; Salager, J. L.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci, 1980, 75, 45 1. (4) Kahlweit, M.; Strey, R.; Firman, P. J . Phys. Chem. 1986, 90,671. (5) Van Nieukoop, J.; Snoei, G . Presented at the Second European Symposium on Enhanced Oil Recovery, Paris, Nov 8-10, 1982. (6) Biais, J.; Bothorel, P.; Clin, B.; Lalanne, P. J . Dispersion Sci. Technol. 1981, 2, 67. (7) Winsor, P. A. Solvent Properties of Amphiphilic Compounds; Butterworth Scientific Publication: London, 1954. (8) Van Nieuwkoop, J.; de Boer, R. B.; Snoei, G. J . Colloid Interface Sci. 1987, 109, 350.
0022-3654 I87 I209 1-6396$01 S O ,I O I
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since no need for pseudoization is necessary. For the present purpose, we have chosen the pseudo composition approach, which is much used in the modeling of phase behavior of chemical flooding processes. The phase transition WI * WII can be induced by a number of parameter variations (salinity, alcohol concentration, temperature, pressure, etc.). The nature of the transition depends upon the particular set of components studied. Little work has been done to elucidate the effects of pressure on microemulsions. Previous work in this field has focused on the change in optimum salinity with pressure and the effect of gas dissolved in the oil phase. Skauge and Fotland9 have shown that, for anionic commercial surfactants, increasing pressure leads to increasing optimal salinity or equivalently (keeping salinity fixed) increasing water solubilization and decreasing oil solubilization. Rossen and Kohn’O measured the compressibility of single-phase microemulsions. They reported several phase transitions but did not detect the nature of the transition itself. Nelson” pressurized a WIII sample to 2500 psi but did not detect any significant changes in phase behavior. H e also pressurized the sample with methane, and due to compositional changes in the oil (decreasing ACN) he obtained a change in phase behavior. The main purpose of this paper is to investigate the pressure effect on microemulsions located in a pseudoplane. The pseudoplane is determined at atmospheric conditions. Pressure is applied to a number of samples at various locations in the pseudoplane. In this way the effect of pressure can be displayed in a phase prism, where pressure represents the ordinate.
Experimental Section Materials. Our model system was composed of the following components: sodium dodecyl sulfate (SDS), 1-butanol, cyclohexane, water, and sodium chloride. SDS was obtained from BDH chemicals as specially pure (purity >99.9%); 1-butanol, cyclohexane, and sodium chloride were obtained from Merck (purity >99.5%). Water was ion exchanged and filtered by a Millipore filtration unit (0.22 rm). The resistance was better than 18 MQ. The samples were weighed directly into 18-mL glass vials with a screw cap. Pressure Arrangement. After preparation, approximately 5 mL of the same was transferred to a glass syringe, which was obtained from Henke Sass, Wolf GMBH. This syringe was placed within a Ruska standard PVT cell, rated to a maximum pressure of 680 bar (10 000 psi). The remaining volume of the cell was filled with pure water, and the pressure was controlled by pumping water into the cell. The details, of the pressure setup, are described el~ewhere.~ While increasing the pressure, the cell was continuously agitated in order to have a large oil/water interface in the sample. This (9) Skauge, A.; Fotland, P. Presented at the sPE/DOE Fifth Symposium on Enhanced Oil Recovery, SPE14932, 1986. (10) Rossen, W. R.; Kohn, J. P. SOC.Pet. Eng. J . 1984, 536. (1 1) Nelson, R. C. Presented at the SPE/DOE Third Joint Symposium on Enhanced Oil Recovery, SPE10677, 1982.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 25, 1987 6397
Five-Component Anionic Microemulsion
55
4
ci
W
4
0
2a,
251
s
4
20
0
!.......................... 0.00
0.04
0.08
0.12
0.16
0.20
0.24
Wt. alcohol
W t . a l c o h o l added(g) Figure 1. Determination of amount of alcohol to be associated with the oleic pseudophase. The horizontal lines are the initial phase volumes of
added(g)
Figure 2. Same as for Figure 1, but the excess oil phase contains significant amounts of other components as well as alcohol and oil.
the lower and middle phases. is necessary to allow the system a rapid response to the increase of pressure (Le., mass transfer across the interfaces). After the desired pressure was reached, the cell was thoroughly shaken and the phase volumes were recorded with a cathetometer. When subsequent readings (12-h intervals) showed no significant change in phase volume, the microemulsion was taken to be at equilibrium. All measurements were done a t 20 O C . Determination of the Pseudoplane. The surfactant was taken to be located entirely in the microemulsion phase where it constitutes the membrane phase between the oil and water phases. The alcohol in the W' pseudophase was determined by liquid chromatography. Alcohol partitioning to the 0' pseudophase was determined by a dilution technique. The phase volumes of a WIII sample were recorded at equilibrium. A certain amount of pure oil was added, and the new equilibrium was allowed to settle. This new equilibrium represents possibly another pseudoplane and lies outside our initial pseudoplane. By addition of alcohol, the sample can now be made to cross the desired pseudoplane. The sample was titrated with alcohol, and after each step the new phase volumes were recorded (at equilibrium). Somewhere along this path the phase volumes of the lower and middle phase equal those of the original sample. This is at the exact point of the pseudoplane. The amount of alcohol needed to reach this point together with the added amount of oil yields directly the alcohol partitioning into the oleic phase. Any change in composition that results in movement within the same tie triangle does not affect the equilibrium; only the relative volumes of the phases are changed. The above-described method results in a net movement directly toward the 0' vertex which, according to the model, coincides with the CY comer of the total phase diagram. Consequently, the only effect is to increase the volume of the oil excess phase; the other two phases remain identical in both volume and composition. Figure 1 shows an example of an alcohol titration. The horizontal lines are the initial phase volumes of the lower and middle phase. The addition of pure oil leads to a new equilibrium which lies below the desired plane. Adding alcohol bring us stepwise up to the desired pseudoplane. As one can see in the figure both the lower and the middle phase cross the initial volumes at the same abscissa value (amount of added alcohol). This is also an indirect test of the validity of the pseudophase model, at least from the oleic side. In search of pseudoplanes we came across several samples were the above method could not be used due to the fact that the oil excess phase contained significant amounts of both water and surfactant. This is most readily seen during an alcohol titration because the phase volumes of the lower and middle phase do not become equal to the initial volumes at the same alcohol concentration (see Figure 2). The partitioning of water to the oil phase has been neglected as has the partitioning of oil to the water
S'
One-phase area
w
0'
Figure 3. Experimentally determined pseudoplane. The dotted lines indicated that the phase border is not exactly determined. The tie lines are calculated by using phase volumes and A, 7,and u.
pseudophase. The method is simple and requires little work but is very time-consuming and therefore practical only to samples which equilibrate fairly rapidly. In the notation of Biais6 the following quantities are defined Y = Va0/V0
(la)
x = V,W/V,
(1b)
u = V,S/V, €
=
V,O/V,O
where Vao,Vaw,and V,S are the alcohol volumes in the oil, water, and surfactant pseudophases, respectively. V,, V,, and V, are the volumes of oil, water, and surfactant in the respective pseudophases. Results and Discussion
Figure 3 shows the determined pseudoplane in the tetrahedron; the corners do not represent real phases but the pseudophases. The pseudoplane in this work is defined by X = 0.032, = 0.036, u = 0.86, and e = 0 (eq 1). The density of the pure components has been used, and the densities of water and surfactant are both set equal to 1 g/cm3. The tie lines are calculated by using the measured phase volumes and the above-mentioned alcohol partitioning. Above the multiphase area (at s' concentration above 20%) there is a large one-phase area and a region of solid/liquid
6398 The Journal of Physical Chemistry, Vol. 91, No. 25, 1987
Fotland
S
S
W
0
t Figure 4. Pressure-composition diagram of the pseudoplane. The diagram is a five-step idealized version of the pressure effect. Note that the pseudocompositions may change with pressure and overall composition.
two-phase compositions. As the figure indicates, there are also found some two-phase samples where an oil excess phase is in equilibrium with a very viscous liquid crystalline water phase, probably containing most of the surfactant. The dotted lines indicate that these borders have not been exactly determined. Figure 4 shows an idealized version of the effect of pressure. The starting point (base area of the prism) contains a WIII emulsion. As the pressure increases, the middle phase composition turns counterclockwise (toward the W’ corner). Further, increasing the pressure leads to mass transfer of surfactant and probably alcohol, across the lower interface. This means that the lower “leg” of the trinodal is tilted upward from the W’ corner. The pseudophase model, as we have used it, is no longer valid because we now have one excess oil phase and two microemulsion phases in equilibrium. However, one may still define a plane given by the trinodal and compute new pseudocompositions. Finally, the trinodal seems to collapse at a critical tie line and the transition to WI is completed. Close to the critical point the volumes of the two phases should become equal. Since this is not the case (see below), this point is not a critical point but in the close vicinity of one. As the pressure increases, the first signs of a solid phase (i.e., from a WI phase) appear. However, there is still a microemulsion phase and an excess oil phase in equilibrium. These three phases represent three points in the phase diagram and define a new trinodal (dark gray shaded). Further increase in pressure leads to a more solid phase and the emulsion point of the trinodal moves downward, possibly along the emulsion/oil binodal. As the phase diagram in Figure 3 shows, there is a solid/liquid (s/l) two-phase region in the close vicinity of the oil/surfactant side and to some extent the water/surfactant side. One possible route to the formation of solid/liquid/liquid (s/l/l) trinodal is the overlapping of a liquid/liquid (1/1) with a s/l region. This is schematically shown in Figure 5. At low pressure (base area of the prism) there is one l/l-WI region at low surfactant concentration and a s/l area along the 0’-S’-(W’) line. ’The binodal of the s/l region can be interpreted as the solubility limit of SDS at the given pressure, temperature, and composition. As the pressure increases, the solubility of SDS most probably decreases, which means that the binodal stretches downward approaching the 0’-W’ side of the phase diagram. At the same time the WI binodal stretches upward approaching the S’ corner. Eventually, the tie lines from the two 2-phase areas overlap and a tie triangle is formed. At even higher pressures the emulsion point of this
Figure 5. Proposed mechanism of precipitation displayed in a pressurecomposition diagram. These six stages may be thought to be between stages 5 and 6 of Figure 4. TABLE I: Pseudocompositions (wt ’36) Together with Type of Phase Behavior and Pressure of Precipitation (POP) sample S’, 7% 0’, % ’ W’, 7% type POP, bar A 2 20 78 WIII 333 f 5 B 2 49 49 WIII 322f 10 20 70 WII 365 f 5 C 10 350f 5 D 10 70 20 1-F 70 10 1-F 365 f 5 E 20
new trinodal moves along the WI binodal and finally covers more or less the whole phase diagram. Table I lists the composition, phase behavior, and pressure of precipitation (POP) for the investigated samples. Samples D and E confirm that the solubility of SDS decreases with pressure. These two samples were 1-phase microemulsions a t 1 bar but precipitated at approximately 365 and 450 bar, respectively. This is not surprising when one considers the increasing density of the oil which causes it to behave more similar to an oil having a higher ACN value.9 The POP data of Table I seem to be in contrast with the foregoing discussion on the solid-phase formation by overlapping. For samples closer to the s/l binodal the POP seems to increase when compared to samples at low S’ concentration. One must bear in mind that the phase diagram on the surfactant side contains much more alcohol (overall composition) than the samples closer to the W’-0’ side. (This is most readily seen from the A, y,and 0 values.) Increasing alcohol content also leads to increasing SDS solubility in the oil phase, and the corresponding POP is likely to increase. The dependence of A, y, and u on pressure is not constant but varies with varying overall composition. The pseudophase model, in its present form, assumes that the surfactant is located in the interface between the 0’and W’ phases; this is not correct when some SDS has precipitated. Since the alcohol partitioning probably changes with pressure, the samples cannot be expected to lie in the same pseudoplane when the pressure is varied. It must be noted that if y, A, and u change with pressure, it is not correct to display the phase behavior in a phase prism. However, for the purpose of illustration it is valuable to use this approach. It is of interest to observe that precipitation occurs both from WI and WII and single phases. Previous work has shown that precipitation may also occur from WIII samples.12 This fact may be of some (12) Fotland, P.; Skauge, A. J. Dispersion Sci. Technol. 1986, 7 , 536.
The Journal of Physical Chemistry, Vol. 91, No. 25, 1987 6399
Five-Component Anionic Microemulsion importance because it means that precipitation occurs at different solubilization values for both water and oil and is not dependent upon the particular phase behavior. The changes in total volume of samples A to E (see Table I) were recorded. For samples C to E, the volume is decreasing linearly with pressure. Samples A and B are initially WIII samples; the pressure-volume data seem to be of poorer accuracy, and nothing definite can be said of these data. We tested one more sample (not listed in Table I) with composition S’ = 40,O’ = 30, and W = 30. Examination of this sample between crossed polarizers revealed the formation of a transparent liquid crystalline phase. Referring to Figure 3, this result indicates that the liquid crystal region is expanding with increasing pressure. Fitting the pressure volume data to a straight line yielded overall compressibilities intermediate to those of water and cyclohexane. The detailed changes in phase volume as a function of pressure are shown in Figure 6 a 4 , where a 4 also refers to samples A-D of Table I. The phase volumes are relative to the total volume (which is also a function of pressure). If the samples were not agitated while the pressure was increased, the microemulsion phase became turbid and excess oil was Seen released into the oleic excess phase. In order to establish equilibrium, the cell must be agitated. If the attainment of equilibrium is dependent on water entering the middle phase by diffusion, one could never be sure of a proper equilibrium to be established. A common feature of curves a and b is the increasing water solubilization with increasing pressure; WI. From a mithat is, the transition is of the type WIII croscopic point of view, an increase in pressure favors a curvature of the interfacial film toward oil. In this aspect the effect of presure is similar to that of temperature (at least for most anionic microemulsions). Further increase of pressure leads to transfer of mass across the lower interface and the excess water phase gets a bluish tinge. At some pressure, indicated in the figure, the interface disappears and the transition to W I is completed. The pressure at which the interface disappears is close to a critical pressure. Further increasing the pressure leads to precipitation of the solid phase. No analysis has been performed on this precipitate, but we believe that it is solid SDS perhaps with some associated salt. The solid phase melts quickly when the pressure is lowered. This means that, for analysis, the solid must be extracted at the particular pressure studied. In addition to this solid phase there is also an excess oil phase and a water-rich microemulsion phase. Further increasing the pressure leads to more precipitation of solid; eventually, visual observation of the phases is impossible due to the presence of the precipitate. Figure 4c shows a pressure plot of sample C which is of type WII; also here the water solubilization slightly increases and the oil solubilization is constant. At approximately 365 bar a solid phase appears. Figure 4d shows the changes in phase behavior for sample D; at a pressure of approximately 150 bar an excess oil phase gradually appears. Between 150 and 350 bar the sample is a typical W I phase. At 350 bar the solid phase is first seen. One more single-phase microemulsion was pressurized (use Table I); this sample showed no change in phase behavior until precipitation of SDS. We did not pressurize any W I microemulsions since samples A, B, and D are W I microemulsions within a certain pressure range. It is-perhaps worth mentioning an aspect (although outside the scope of this article) we found fascinating. If the pressure is rapidly increased (especially for single-phase samples) well above the pressure of precipitation, the solution becomes supersaturated. When crystal growth occurs at supersaturation, it appears quite rapidly. Rather large crystals appear within seconds. The solid phase disappears equally rapidly when the pressure is lowered. The rate of growth can be easily controlled by changing the pressure. By this technique it would be possible to study the crystal growth itself.
,Pressure
-
Conclusion 1. The equilibrium state of a microemulsion is sensitive to the hydrostatic pressure applied to the solution. 2. The transition from WIII to W I is found to occur close to
I
“i 0
50
100
150
200
300
250
350
400
450
0
Pressure (bar) 100
~
JII
~
solid/
liquid/
o
J 0
, 50
~
, I00
,
, 150
,
,
200
, 253
.
, 300
,
, 350
I
.
~ 400
. 450
,
~
,
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Pressure
Pressure Figure 6. a-d show variation of phase volume with pressure for samples A-D.
a critical point. 3. Increasing pressure leads to (i) decreasing oil solubilization in the microemulsion phase and/or increasing water solubilization, (ii) a larger solid/liquid 2-phase domain at high surfactant con-
,
6400
J . Phys. Chem. 1987, 91, 6400-6402 a net decrease in the liquid crystal domain. Perhaps the most important result of this investigation is the fact that precipitation is independent of the particular kind of phase behavior present.
centration, (iii) a larger liquid crystal/liquid 2-phase domain, and (iv) overlapping of the solid/liquid binodal with the WI, WII, or WIII binodal to form a new solid/Iiquid/liquid trinodal. Point iii is not in agreement with Figure 6, which does not take into account the liquid crystal region. Presently, we are not able to explain what happens to this region when pressure increases. It is of course possible that this domain increases in some directions and decreases rapidly in other directions which would result in
Acknowledgment. I thank 0.J. Kvammen and A. Skauge for helpful discussions and comments and Norsk Hydro for the permission to publish these results.
Radical Formation upon Butene Adsorption on H-Mordenites Tsuneki Ichikawa,* Masaaki Yamaguchi, and Hiroshi Yoshida Faculty of Engineering, Hokkaido University, Sapporo, 060 Japan (Received: January 27, 1987)
An ESR study has beem carried out to clarify the mechanism of radical formation upon butene adsorption on H-mordenites. Tetramethylethylene cation radicals and their derivatives are selectively generated upon the adsorption of different butenes. Tetramethylethylene molecules generated as a byproduct of acid-catalyzed reactions of butenes react with electron-accepting sites in the mordenites to convert to the tetramethylethylenecation radicals. The cation radicals react with the tetramethylethylene molecules to form the cation radicals of the tetramethylethylene dimer. These dimers further react to convert to 1,1,2trimethylallyl radicals. An explanation is made for the selective formation of the tetramethylethylene cation radicals upon the adsorption of acyclic alkenes.
Introduction In recent years silica-rich zeolites containing protons as exchangeable cations have attracted considerable attention because of their catalytic activity to convert various organic compounds into saturated and unsaturated hydrocarbons. The active sites of these catalysts are generally considered to be Bransted acid ones. The zeolites are also known to form radical species upon the adsorption of organic compounds. Shih studied the radical formation of 2-butene-adsorbed H-mordenite and concluded that the cation radicals of 2-butene were generated by the reaction of the parent molecules with Lewis acid sites.’ On the basis of this observation, he pointed out the importance of ion radical reactions upon the catalytic reactions of alkenes. However, the ESR spectrum observed is inconsistent with that of 2-butene cation radicals generated in halocarbon matrices by y - i r r a d i a t i ~ n . ~ . ~ Recently, Kucherov and Slinkin studied the radical species generated upon the adsorption of various alkenes on H-mordenite and found that an identical ESR spectrum was observed at 293 K after the adsorption of different acyclic alkenes4” They assigned the spectrum as due to allylic radicals having five equivalent protons with the hyperfine coupling constant of 0.8 mT. However, the equivalent hyperfine coupling cannot be expected for regular allylic radical^.^ Therefore, although the formation of radical species upon the adsorption of alkenes has been known, the mechanism of the radical formation and the role of the radicals upon the catalytic reactions have not been understood. In the present work an ESR study on butene-adsorbed Hmordenites was carried out to clarify the nature of radicals and the mechanism of radical formation. It is shown that the precursor of the radical species is tetramethylethylene molecules formed as a byproduct of the catalytic reaction of alkenes with Bransted acid sites, and the radical species are cation radicals of tetramethylethylene monomer and dimer and 1,1,2-trimethyIallyl radicals generated by the intramolecular proton transfer of the dimer cation radicals. (1) Shih, S. J . Catal. 1975, 36, 238. (2) Shida, T.; Egawa, Y.; Kubodera, H.; Kato, T. J . Chem. Phys. 1980, 73, 5963. (3) Fujisawa, J.; Sato, S.; Shimokoshi, K. J . Phys. Chem. 1985, 89, 5481. (4) Kucherov, A. V.; Slinkin, A. A. Kinet. Karol. 1982, 23, 251. (5) Kucherov, A. V.; Slinkin, A. A. Kfnet. Karol. 1983, 24, 947. (6) Slinkin, A. A.; Kucherov, A. V. Kinet. Karal. 1983, 24, 955.
0022-3654/87/2091-6400$01.50/0
Experimental Section H-mordenites used were JRC-Z HM10, HM15, and HM20 standard catalysts with a Si02/A1203ratio of 10, 15, and 20, respectively. 1-Butene, 2-butene, isobutene, dichlorodifluoromethane (CCI2F2),and tetramethylethylene were obtained from Tokyo Kasei Co. Ltd. These organic compounds were degassed by repeated freezing-pumping-thawing cycles. About 0.2 g of the mordenite in a quartz tube was heated in oxygen at 670 K overnight and then evacuated for 2 h at the same temperature. After the evacuation, 0.3 mmol of butene was introduced to the mordenite at 77 K. The oligomeric product of butene was extracted from the butene-adsorbed mordenite by adding the mordenite into water at 293 K. The gas chromatographic analysis of the product was carried out at 353 K by using Apiezon L. For reducing the concentration of adsorbed butenes, CC12F2 was first adsorbed at 290 K and then 0.03 mmol of butene was introduced a t 77 K after the evacuation of the sample at 290 K. The adsorption of tetramethylethylene was carried out at 290 K. ESR spectra were recorded on a Varian E-line X-band spectrometer equipped with a variable-temperature controller. Results and Discussion Shown in Figure 1 are the ESR spectra of the l-butene-adsorbed mordenite. A broad ESR spectrum started to arise at 200 K. The intensity of the spectrum increased with temperature, accompanying the appearance of hyperfine structures. Above 290 K the spectrum was scarcely changed. The radical concentration was on the order of 1017 spins/g. The spectrum a t 290 K was composed of two components with hyperfine splittings of 0.8 and 1.6 mT, respectively. Cooling of the sample to 77 K caused the reversible broadening of the spectrum. However, the hyperfine structure with a splitting of 1.6 mT was still observed. It can be concluded from these results that there are two radical species with hyperfine coupling constants of 1.6 and 0.8 mT, respectively. The shape and the temperature dependency of the spectrum did not depend on the amount and the kind of butene and the Si02/Al,03 ratio of the mordenite and were the same as those obtained by the adsorption of several acyclic alkene^.^ These evidences indicate that the radical species are generated from an identical molecular product formed by nonradical catalytic reactions of alkenes. The ESR spectra of H-mordenite with a Si02/A1203ratio of 15 obtained at 290 K after the adsorption of several butenes are shown in Figure 2. The radical species
0 1987 American Chemical Society
