J. Phys. Chem. 1991, 95, 4596-4598
4596
scriptions are strictly correct in this situation because the MD calculation, with periodic boundary conditions, suppresses longran e fluctuations. The thermodynamic potential can be described by = 6,+ &S ; E 3 , / 2 - E?/S?r, where P, is the z component of polarization, a0is a smooth varying part (approximately independent of E,), and &S is a singular part which depends on order parameters. Using the simplest choice of bs, minimizing the potential 4 with respect to order parameters at constant E,, and eliminating the order parameters from 4 by standard proceduresz7 gives the following equation
8
1 E,2 & = &, - -KE$ -+ al(Eo- E,)”%(Eo - E,) + 2 87 az(E, - Eo)2-a29(E,- Eo) (6)
where alra2,a i ,and al are constant and depend on the symmetry and the number of order parameters of the system, K is a nonsingular part of the susceptibility of the system, 9 is a step function, and Eo is the critical electric field where phase transition occurs. Based on the Landau theory, al and azare, in general, equal to half-integer values. The functional form of this equation does not depend on the classification of order parameters. This equation implies Xn =
ap,
aE,
= K - al(2 - a l ) ( l - a l ) ( E o- E,)-ule(Eo- E,) -az(2 - az) X (1 - a 2 ) ( 4- E o ) ” ’ W , - Eo) (7)
s = - -a4
aT
840 = -aT
1
aK
+ -(-)E$ 2 aT
(z)
- al(2 - a I ) ( E o- E,)’-”l9(E0 - E,) X
(2)
+ a2(2 - a2)(Ez- EO)i-az9(EZ- E,) -
(8)
+
where P, = -a(& E,2/8?r)/aEZ,&/aT 2 0, and aEo/aT 5 0. The simulation data, presented in Figure 2b, are fitted by the abme model equations. K = 0.16, a1 = 0, az = 0.04, and a2= -1 /2 are found from the “best fit”, which is also plotted in Figure 2b. This choice of the parameters describes an entropy and derivatives of
the free energy resembling an antifemelectric type behavior,which has a continuous second derivative (but not third) of the free energy, xz, with sharp change near the phase transition point. In Figure 2b, the rapid and systematic decrease in fluctuations of dielectric responses may also be understood from the Landau theory and is thus not an artifact of our simulations. Although fluctuations are not fully treated in this Letter, we note that their thermodynamics and dynamics may be included by adding the terms containing gradients of order parameters to as. These may be obtained from MD simulation by analysis of stability with respeci to small perturbation of periodic boundary conditions. But the critical indices can be found directly from the calculations of the type presented here, which allow determination of the universality classz3 of the investigated system. In summary, structural, dielectric, and thermodynamic properties of water between conducting surfaces indicate the possibility of spontaneous symmetry breaking and existence of a continuous phase transition induced by an external field. One phase transition occurs at the entropy maximum coincident with a sharp change in the slope of the second derivative of the free energy. The other transition takes a place at E, = 0 and is associated with the entropy minimum. This behavior confirms a previous experimental obs e r v a t i ~ n . ~ IAlso, , ’ ~ two types of order parameters, connected to M~ and plI,could describe the recent computer simulation results, the possibility of the crystal-like orientation near the surface,Is and two distinct components of dipole moment correlation functions.I6 Furthermore, the present results indicate strong nonlinear and anisotropic behavior of dielectric response function, transport coefficient, and dielectric relaxation time as a function of an external field and temperature which will be important for understanding many dynamic and thermodynamic properties of water in the double layer.32 Acknowledgment. The present work was supported, in part, by the National Science Foundation through Grants C H E W 19436, DMR88-19885,and CHE88-15130 (W.P. Reinhardt and M.L. Klein instrumentation grant) and by the donors of the Petroleum Research Fund, administered by the American Chemical Society, which are the most gratefully acknowledged. (32) Watanabe, M.; Brodsky, A. M.; Reinhardt, W. P. Manuscript in preparation.
IdentHlcatlon, Thermal Stability, and Catalytic Property of Tetracerrbonylchromlum( 0) Encapsulated In NaX Zeolite Yasuaki Okamoto,* Yukiko Inui, Hiroyuki Onimatsu, and Toshinobu Imanaka Department of Chemical Engineering, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: March 15, 1991) It was found for the first time that tetracarbonylchromium(0)is encapsulated in NaX zeolite in high punty only in the presence of gaseous CO (0.8-10 Torr). Cr(CO),/NaX was virtually inactive for the hydrogenationof butadiene at room temperature, whereas Cr(CO),/NaX exhibited an extremely high activity. Cr(CO),/NaX showed photoassisted hydrogenation activity during UV irradiation. Introduction
Transition-metal carbonyls supported on inorganic matrices have been investigated to prepare novel heterogeneous catalysts with well-characterized active species and/or with prominent catalytic properties.Is2 Group 6 metal carbonyls anchored on (1) Bailey, D. C.; Langer, S.H. Chem. Reu. 1981, 81, 109. (2) (a) Yermakov, Yu. I.; Kuznetsov, B. N.; Zakharov, V. A. Caralsis by Supporfcd Complexes; Elsevier: Amsterdam, 1981. (b) Gates, B. C.; Guczi, L.; Knozingcr, H. Mefa/ Clusrers in Cafalysis;Elsevier: Amsterdam, 1986.
inorganic oxides have been studied most extensively among metal carbonyls since the pioneering researches of B ~ r w e l land ~ . ~Howe5 and co-workers on the decarbonylation and catalysis of Mo(CO)~ and derived subcarbonyl species anchored on A1203. The de(3) (a) Burwell, R. L., Jr.; Haller, G.L.; Taylor, K. C.; Read, J. F. Adu. Cutul. 1969, 20, I. (b) Brenner, A.; Burwell, R. L., Jr. J . Carol. 1978. 52, 353. (4) Brenner, A.; Burwell, R. L., Jr. J . Am. Chem. Soc. 1975, 97, 2565. (5) (a) Howe, R. F.; Davison. D. E.; Whan, D. A. J. Chem. Soc.,Faraduy Trans. I 1972, 68, 2266. (b) Smith, J.; Howe, R. F.; Whan, D. A. J . Caral. 1974, 34, 191.
0022~3654/91/2095-4596%02.50/0 0 1991 American Chemical Society
Letters carbonylation of Mo(CO), encaged in zeolite has also received much attention6 with the hope of generation of uniform, welldefined active species. Recently, it has been established that tricarbonylmolybdenum(0) is thermally stabilized in the supercages of Y- and X-type zeolites (faujasite) by evacuating at 373-423 K." It has been demonstrated using a temperature-programmed decarbonylationtechnique,1° XPS," and 1R'O.I' that the thermal stability and the electronic state of tricarbonyl species depend strongly on zeolite cation. Direct bondings between zeolite framework oxygens and the Mo(CO)~moiety were evidenced by EXAFS.9." It has been shown that stereuselective hydrogenations of simple dienes to cis-Zolefins are catalyzed by subcarbonylmolybdenum(u) species encapsulated in zeolite.10J2 The active subcarbonyl species is suggested to be Mo(CO)~on the basis of an IR study.I3 In spite of increasing knowledge about anchored tricarbonyl species, the formation, thermal stability, and catalytic properties of penta- or tetracarbonyl species have never been well established. Only tentative assignments have been made in the IR studies on decarbonylation of Mo(CO), and W(CO)6 encapsulated in ~eolite.6~'*~' With M o ( C O ) ~ / A ~ ~formations O~, of stoichiometric Mo(CO), and MO(CO)~ species have been claimed by Brenner and Burwell' on the basis of CO/Mo ratio. However, no spectroscopic evidence and catalytic properties have been reported for these subcarbonyl species. In the present IR study, we succeeded for the first time in preparing and identifying tetracarbonylchromium(0) anchored in NaX zeolite in an almost pure form. We examined briefly the thermal stability of the Cr(CO), spectes and its catalytic property for the hydrogenation of butadiene with or without UV irradiation. Experimental Section The composition of NaX zeolite (1 3 X, Gasukuro Kogyo Co. Ltd.) was Na86A186Si1M0384. The zeolite was pressed into selfsupporting wafers (usually 15-25 mg, 2 cm in diameter) for IR studies. The sample was pretreated in a vacuum (600 K (usually 0.5 f 0.1 Cr(CO)6/supercage). In situ DRS-VIS spectra (350-800 nm) of chromium carbonyl species encaged in NaX were obtained at room temperature after various treatments of Cr(CO),/NaX on a Hitachi 200-20 spectrophotometer against a pure alumina disk as a reference. Hydrogenation of 1,3-butadiene (BD) was carried out over a (6) Howe, R. F. Tailored Metal Caralysrs; Iwasawa, Y., Ed.; D. Reidel: Dordrecht, 1986; p 141. (7) Okamoto, Y.; Maezawa, A.; Kane, H.; Mitsushima, I.; Imanaka, T. J . Chem. SOC.,Faraday Trans. I 1988,84, 851. (8) Okamoto, Y.; Maezawa, A.; Kane, H.; Imanaka, T. J . Caral. 1988, 112. 585. (9) Ozkar, S.; Ozin, G. A.; Moller, K.;Bein, T. J. Am. Chem. Soc. 1990, 112,9575. (IO) Okamoto, Y.; Maezawa, A.; Kane, H.; Imanaka, T. Proceedings of rhe 9rh htrmatioyl Congress on aralysis;Phillips, M. J., Ternan, M.. Eds.; The Chemical Institute of Canada: Ottawa, 1988; Vol. 1, p 1 1. ( I 1) Okamoto, Y.; Imanaka, T.;Asakura, K.; Iwasawa. Y. J . Phys. Chem.,
in press. (12) (a) Okamoto, Y.; Maezawa, A.; Kane, H.; Imanaka, T. J . Chem. Soc., Chem. Commun. 1988,380. (b) Okamoto, Y.; Kane, H.; Imanaka, T. Catal. Len. 1989, 2, 335. (13) Okamoto, Y.; Kane, H.; Imanaka, T. Chem. Leu. 1988, 2005. (14) (a) Gallezot, P.; Couderier, G.; Primet, M.; Imelik, E. ACS Symp. Ser. 1977, 40, 144. (b) Gallezot, P.; Couderier, G.; Primet, M.; Imelik, B. C. R. Acad. Sci. Paris 1976,282C 31 1. (c) Abdo, S.; Howe, R. F. J . Phys. Chem. 1983,87, 1713,1722. (d) You-Sing. Y.; Howe, R. F. J. Chem. Soc., Faraday Trans. I 1986, 82, 2887.
The Journal of Physical Chemistry, Vol. 95, No. 12, I991 4591
v
X I %
c
z :
EIlc
2
Figure 1. IR spectra of chromium carbonyls encapsulated in N a X zeolite: (a) adsorption of Cr(C0)6 into NaX, (b) heat treatment of (a) at 373 K for 30 min in an in situ IR cell without evacuation, (c) evacuation of (b) at room temperature for 30 min, and (d) admission of butadiene (22 Torr) to (b) at room temperature. The spectra were measured at room temperature.
zeolite wafer in the IR cell (100 cm3) at 295 K. The state of chromium carbonyl was sometimes monitored by IR during the reaction. After admission of BD/H2, chromium carbonyl/NaX wafers in the IR cell were irradiated with UV rays by using a high-pressure mercury lamp (SHL- lOOUVQ, Toshiba Lighting & Technology Co. 100 W) through water filter (8.6 cm).
Results and Discussion Figure 1 shows the IR spectra of Cr(C0)6/NaX variously treated. After adsorption of Cr(CO)6, spectrum l a was obtained. The characteristic features of the spectrum, e.g., the appearance of the infrared-inactive v1 band at 2117 cm-' and a red shift (Av(C0) = -64 cm-I) of the v6 band (1936 cm-I) indicate that Cr (CO)6 molecules encapsulated in NaX zeolite strongly interact with the zeolite surface, probably with extra-framework cations (Na+) as in Y-type ~eolite.9.'~When Cr(CO),/NaX was heated in the cell at 373 K for 30 min (without evacuation), a partial decarbopylation of Cr(CO)6 took place (CO pressure 1.5-1.8 Torr, depending on the sample weight) and after cooling to room temperature, spectrum l b was observed at the expense of the bands for Cr(C0)6. When the sample was left in the presence of CO (ca.2 Torr) at room temperature, the four bands in spectrum 1b were gradually and simultaneously replaced with the bands due to Cr(C0)6. After 17.5 h, 23% of the original amount of Cr(C0)6 was calculated to be restored on the basis of the intensity of v I band. . The IR spectrum lb rapidly decreased in intensity on evacuation at room temperature and a new set of bands (1917 and 1767 cm-I) in Figure I C replaced the spectrum lb. Species I C was thermally stable at 423 K under vacuum and at 473 K in the presence of CO (3.6 Torr). It was extremely air-sensitive at room temperature. Species IC is unambiguously assigned to tricarbonylchromium(0) in a trigonal pyramidal symmetry on the basis of similarity of the spectrum to those for Mo(CO), anchored in X- and Y-type ~ e o l i t e s . ~ ~More ~ J ' recent IR studies by Ozkar et ale9confirmed the above assignment. Simultaneous intensity decreases of the four bands of spectrum l b upon evacuation and reaction with gaseous CO suggest that the four bands in spectrum 1b are attributed t o a single chromium subcarbonyl species and that the CO/Cr ratio of species 1b is 3 < CO/Cr < 6. Species 1b was found to be thermally stabilized in NaX zeolite even at 425 K so long as CO was present in the gas phase (5.5-10 Torr). With increasing temperature at 300-400 K, the proportion of Cr(C0)6 decreased, whereas that of species 1b increased and reached a maximum at 360-390 K, depending on the CO pressure (15) Zecchina, A.; Bordiga, S.;Platero, E.; Arean, C. 0. J . Caral. 1990, 125, 568.
4598 The Journal of Physical Chemistry, Vol. 95, No. 12, 1991
Letters
TABLE I: IR Wavenumbem for Cbromium Carbonvls Emmadated in NaX Zeolite d Related C
species la lb IC
Id
assignment Cr(C0)6/NaX Cr(CO)4/NaX Cr(CO),/NaX (?4-C4H6)Cr(CO)4/NaX
m
d
wavenumbers/cm-I 2117 (w), 2055 (sh), 2012 (sh), 1973 (sh), 1936 (s) 2042 (w) (A,), 1909 (SI (Blh 1831 (m) (Al), 1784 (SI (Bz) 1917 (s)(AI), 1767 (s) (E) 2021 (w) ( A A 1897 (4 (B1L 1836 ( 4 (Al), 1776 (SI (Bz)
Cr(CO)* (CHI matrix, 20 K) C ~ S - ( C ~ H , ) ~ C ~ ((gas, C O )295 ~ K) t r a n ~ - ( C ~ H , ) ~ C r ( C(Xe, 0 ) ~ 195 K) (q4-C4H6)Cr(CO)4 (n-heptane) Cr(CO)5 (C4J (Ar, 20 K)
2051, 2045, 1953 2035, 2093,
ref this work this work this work this work
1935, 1929, 1888 1949, 1913 1979, 1943, 1932 1965, 1936
'Perutz, R. N.;Turner, J. J. J . Am. Chem. SOC.1975,97,4800. (b) Weiler, B. H.; Grant, E. R. J. Phys. Chem. 1988, 92, 1458. (c) Gregory, M. F.; Jackson, S. A.; Poliakoff, M.; Turner, J. J. J . Chem. SOC.,Chem. Commun. 1986, 1175. (d) Dixon, D. T.; Burkinshaw, P. M.; Howel, J. S . J. Chem. SOC.,Dalton Trans. 1980, 2237. (e) Graham, M. A.; Poliakoff, M.; Turner, J. J. J . Chem. SOC.A 1971, 2939. TABLE II: Catalytic Activity and Selectivity of Chromium Carbonyl Swcies Encapsulated in NaX for the Hydrogenation of Butadieme at 295 K '
species Cr(CO), Cr(CO),
zeolite weight/mg 4.2 11 4.2 11
total press./Torr 155
235 154 248
HZ/BD ratio 3.7 3.3 3.6 3.9
reaction time/h 16.7 3.0 16.7 3.gd
conversion of BD/% 67 15 0.8 1.44
seIectivity/%b cis-2-butene I-butene 97.8 2.2 97.0 3.0 ca. 100 C ca. 100 C
'Chromium content 0.5 Cr(CO),/supercage and the volume of IR cell; 100 cm3. * N o formations of trans-2-butene and butane were observed. (The amount of I-butene was too small to ,kdetected. dUV-irradiated for 3 h.
(1.4-10 Torr). As the temperature was increased, at >370 K Cr(CO), appeared and overwhelmingly replaced species lb. These processes were completely reversible at 600 K (no appearance of v(C0) bands was confirmed by IR spectra), the CO pressure was measured again at the same temperature as the first equilibration was attained. Neglecting a possible chromium carbide formation during the decomposition, we calculated the CO/Cr ratio for species l b from these pressures to be 4.2 f 0.2 (averaged over three runs). It is accordingly concluded that species l b can be assigned to tetracarbonylchromium(0) encapsulated in NaX. The IR results in Table I support the assignment. This is the first example of an unambiguous spectroscopic identification of Cr(C0)4 anchored in an almost pure form (vide infra) on inorganic materials. DRS-VIS spectra of Cr(CO), and Cr(CO), exhibited main bands at 435 and 530 nm, respectively. On exposure to BD or BD/H2 (BD pressure 10-30 Tmr) at rcom temperature, Cr(C0)4 was found to react readily with BD, forming an air-sensitive BD complex, (~4-C4H6)Cr(C0)4, as shown in Figure Id and Table I. The BD complex was rapidly decomposed on evacuation at room temperature. However, selective removal of BD in the gas phase (in the presence of CO) by trapping at liquid N2 temperature caused no degradation at room temperature but brought about a decomposition to a mixture of Cr(CO), and Cr(CO), at 373 K. Separate experiments indicated that the BD complex was stable even at 383 K so long as CO and BD were present in a gas phase. The tetracarbonylchromium(0) species is considered to be anchored on the zeolite surface by coordination of 2p lone pair
electrons to al + b2 orbitals16of the Cr(CO), moiety. The con,)~ figuration of tetracarbonylchromium(0) is c i ~ - C r ( C O ) ~ ( 0(0, zeolite framework oxygen) on the basis of the appearance of four (C4J is expected to exhibit v(C0) bands. tr~ns-Cr(CO)~(O,), only a single infrared-active band (Table I). The wavenumbers of (BD)Cr(CO), in the supercage are modified by encapsulation. It is surmised that the modifications can be ascribed to Cr-CO bond polarization due to an electric field generated by cations and/or direct bond formation between cation and carbonyl, CrCO-Na+. Table I1 shows the hydrogenation activities of Cr(CO), and Cr(CO), species encapsulated in NaX zeolite. It is revealed that Cr(CO)3/NaX exhibits a remarkable activity for stereoselective hydrogenation of butadiene to cis-2-butene at 295 K. The activity of Cr(C0)3/NaX is 20 times larger than that of M O ( C O ) ~ / L ~ Y . ~ ~ However, it is evident in Table I1 that the activity of Cr(C0)4 is very low at 295 K as compared to that of Cr(CO)3. During the hydrogenation Cr(C0)4 was confirmed to react with BD, forming (BD)Cr(CO),. Taking into consideration the possibility that a small proportion (1-2%) of Cr(CO), is present in the reaction system, it is concluded that Cr(CO),/NaX is virtually inactive for the hydrogenation of butadiene in contrast to Cr(CO),/NaX. The purity of Cr(CO), in Figure 1b is estimated to be >98%. UV irradiation of (BD)Cr(CO), in the presence of BD/H2, however, induced a considerable activity as shown in Table 11. This is consistent with photoassisted hydrogenation in a homogeneous reaction system." Decarbonylation of the (BD)Cr(CO), complex by irradiation is considered to lead to coordinatively unsaturated (BD)Cr(CO), species, followed by an H2 attack and product formation. (16) Elian, M.; Hoffmann, R. Inorg. Chem. 1975, 14, 1058. (17) (a) Wrighton, M.; Ginley, D. S.; Schroeder, M. A.; Morse, D. L. Pure Appl. Chem. 1975,41,671. (b) Jackson, S. A.; Hodge, P. M.; Poliakoff, M.; Turner, J. J.; Grevel, F. W. J . Am. Chem. Soc. 1990, 112, 1221.
