J . Phys. Chem. 1990, 94, 8019-8020
mixtures do not form solid alloys, they do form small clusters with heteronuclear bonding strong enough to survive photodissociation. Product channels and branching ratios suggest that both elements in these mixed clusters are integrated more or less equally into the cluster bonding. Assuming thermodynamic control of dissociation channels, lower limits on the dissociation energies of BiCr (2.58 eV), Bi-Cr+ (2.06 eV), BiFe (1.85 eV), and Bi+-Fe (1.85 eV) are obtained. Bi atoms, with a 6s26p3electron configuration, are expected to have an "inert pair" of s electrons, forming bonds through the valence p 0rbita1s.l~The interactions of these p orbitals with either the s or d transition-metal orbitals are presumably the source of
8019
the bonding observed here. Theoretical studies on these mixedmetal systems would be valuable to determine the exact orbital nature of bonding in these systems and the bonding energetics. Further studies on larger clusters and on their tunable laser spectroscopy are currently under way in our laboratory to obtain more detailed data on these bimetallic systems. Acknowledgment. We acknowledge the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work. M.B.B. received support from the NSF Visiting Professorships for Women Program (NSF/VPW).
Application of Dlfferential Scanning Calorimetry in the Study of Intrazeolite Metailophthalocyanines Kenneth J. Balkus, Jr.,* and John P. Ferraris Department of Chemistry, University of Texas at Dallas, Richardson, Texas 75083-0688 (Receiued: June 18. 1990; In Final Form: July 25, 1990)
We report a differential scanning calorimetric method for the study of Co(I1) phthalocyanine formation in zeolite Nay.
Zeolite ship-in-a-bottle complexes are exemplified by the zeolite
X and Y encapsulated metallophthalocyanines. These intrazeolite complexes are generally prepared by a template synthesis which involves the condensation of 1,2-dicyanobenzene within the metal-exchanged zeolite to form the entrapped metallophthalocyanine. This is pictured below for Co(I1)-exchanged NaY which was used in this study.
+ CoNaY
There have been several reports of zeolite-encapsulated metallophthalocyanine^.'-^ The usual evidence presented for the intrazeolite location of the metal complexes includes changes in the electronic spectra relative to solution analogues, scanning electron microscopy before and after extractions, failure to extract the ( I ) Meyer, G.;Worhle, D.; Mohl, M.; Schulz-Ekloff, G.Zeolites 1984, 4, 30. (2) Diegruber, H.; Plath, P. J.; Schultz-Ekloff, G.J. Mol. Catal. 1984, 24, 115. (3) Balkus, Jr., K. J.; Welch, A. A,; Gnade, B. E, J. Inclusion Phenom., in press. (4) Gabrielov. A. D.; Zakharov, A. N.; Romanovsky, B. V.; Tkachenko, 0.P.; Shpiro, E. S.;Minachev, Kh. M. Koord. Khim. 1988, 14, 821. ( 5 ) Zakharov, A. N.; Romanovsky, B. V.; Luca, D.; Sokolov, V. I. Metalloorg. Khim. 1988, /, 119. (6) Kimura, T.;Fukuoka, A.; Ichikawa, M. Shokubai 1988, 30, 444. (7) Zakharov, A. N.; Romanovsky, B. V. J. Inclusion Phenom. 1985, 3, 389. (8) Romanovsky. 8. V. Acra Phys. Chem. 1985, 31, 215. (9) Herron, N. J. Coord. Chem. 1988, 19, 25. (IO) Zakharov, A. N.; Gabrielov, A. G.;Romanovsky, B. V.; Sokolov, V. 1. Vestn. Mosk. Uniu.. Ser. 2: Khim. 1989, 30, 234.
0022-3654/90/2094-8019$02.50/0
complexes with strong donor solvents, and recovery of the complexes after dissolution of the zeolite with acid. It has been proposed that differences in the electronic spectra of entrapped and solution species arise from distortion of the phthalocyanine ring from planarity inside the zeolite. This suggests there may be an energetic difference between formation of the metallophthalocyanine inside versus outside the zeolite. This prompted us to explore differential scanning calorimetry as a technique to study the intrazeolite formation of these complexes. The thermal behavior of our zeolite samples was studied with an indium-calibrated Perkin-Elmer DSC-2 differential scanning calorimeter in the temperature range 29C-560 K. Samples (3-10 mg) were sealed in stainless steel (SS) capsules and heated at a rate of 20 K/min. A nitrogen purge of 50 mL/min was maintained. Open pans are not appropriate for this type of study since the reactants can volatilize. Furthermore, for partially hydrated zeolites the SS capsules are essential because sealed aluminum pans cannot withstand the vapor pressure generated by the volatization of zeolitic water. (Although Perkin-Elmer aluminum capsules can withstand pressures between 2 and 3 atm, the vapor pressure of 10 mg of hydrated NaY far exceeds this limit at 433 K.) On the other hand, stainless steel capsules are reliable to -23 atm and 570 K. Hydrated NaY samples in SS capsules exhibit no thermal events to 560 K, whereas NaY in open SS or aluminum pans produces a very broad endotherm in the range -373-500 K. These latter results confirm the observations of Aboul-Gheit et a].," who studied thermally induced dehydration of a number of zeolites by DSC and TGA. The hydrated (-20 wt% water) cobalt(I1)-exchanged NaY zeolite (4 wt% Co) used in this study also does not exhibit any thermal events to 560 K in sealed SS capsules. lntrazeolite metallophthalocyanines can be prepared by heating a mixture of 1,2-dicyanobenzene and the metal ion exchanged zeolite in a sealed ~ e s s e l . ' - ~We , ~ have simulated these conditions by sealing intimately mixed CoNaY and dicyanobenzene (1:l by ( 1 1 ) Aboul-Gheit, A. K.; Summan, A. M.; Ahmed, M. A.; Mousa, M. A. Thermochim. Acta 1990, 158, 5 3 .
0 1990 American Chemical Society
Letters
8020 The Journal of Physical Chemistry, Vol. 94, No. 21, 1990
~
E
' 1
300
400
500
-~
300
TEMPERATURE (K)
Figure 1. DSC thermogram for the reaction of CoCI2.6H20 with 1,2dicyanobenzene.
weight) in SS DSC capsules. To compare the thermal behavior of cobalt(l1) phthalocyanine formation without zeolite, we also examined a mixture of CoCI2.6H20and dicyanobenzene in a 4: 1 molar ratio. Differential scanning calorimetry was recently used to investigate the mechanism of metallophthalocyanine formation from dicyanobenzene using a series of copper salts.I2 In the case of CuCI2.2H2O a single sharp exotherm was observed with an onset temperature of 525 K and a peak at 533 K. Figure 1 shows the thermogram for the reaction of CoCI2.6H20and dicyanobenzene. We assign the sharp exotherm (onset 523 K, peak 527 f 1 K) to the formation of cobalt phthalocyanine. The endothermic loss of water of crystallization occurs at 336 K, and the endotherm at 414 K corresponds to the melting of dicyanobenzene (mp 412-414 K). There is also a weak exotherm at -475 K which appears to arise from the interaction of water with dicyanobenzene. A thermogram of dry dicyanobenzene exhibits only an endotherm (416 K) for melting while a dicyanobenzene sample sealed with 2 p L of water shows both the endotherm for melting (409 K) and a broad exotherm at 478 K. Further study is required to better define the origin of this exothermic reaction and how it might affect the formation of phthalocyanine. It should also be noted, however, that the endothermic peak for dicyanobenzene melting shifts to lower temperatures as the amount of water in the sample increases. A thermogram for the reaction of CoNaY and dicyanobenzene is shown in Figure 2. The endotherm at 413 K and the broad exotherm at -485 K are associated with dicyanobenzene. The (12) Christie, R. M.: Deans, D. D. J . Chem. SOC.,Perkin Trans. 2 1989, 193.
400
500
TEMPERATURE (IC)
Figure 2. DSC thermogram for the reaction of CoNaY with 1,2-dicyanobenzene.
single exotherm (onset 536 K, peak 542 f 1 K) for intrazeolite metallophthalocyanine formation occurs at a higher temperature than with CoCl2.6H2O. It was anticipated that encapsulation of the complex might require a higher temperature because of the steric constraints of the zeolite supercage, and our results suggest this may be the case. It is highly unlikely that the shift in peak temperatures is the result of changes in dicyanobenzene diffusion, since dicyanobenzene readily fits in the zeolite micropores. If there are more cation sites in the zeolite than there are cations, the Co(I1) will move to different sites, especially upon heating. We cannot exclude the possibility that some surface species might be formed due to Co(1I) ion migration to surface sites. To the extent that this occurs, we would expect surface complexes to form at a lower temperature, similar to CoCI2.6H20 whose exotherm occurs at 527 K. A closer examination of the exotherm in the zeolite case reveals an unsymmetrical peak where the shoulder is centered at -527 K. This suggests we may be able to distinguish between complexes formed inside and outside the zeolite, but this must await more detailed modeling studies of this peak shape. Schulz-Ekloff reports the optimum temperature for the synthesis of Co(I1) phthalocyanine in zeolite X to be 523 K based on trial and error.'q2 This is quite close to what we observe in the Y-type zeolite, which suggests that DSC could prove to be a valuable technique for the study and optimization of intrazeolite reactions. We are currently extending this work to other metal systems to assess the generality of the technique. Acknowledgment. We thank the Robert A. Welch Foundation and the donors of the Petroleum Research Fund, administered by the American Chemical Society, for their support of this work.
