J. Phys. Chem. 1993,97, 9196-9200
9196
Catalysis on Polymer Supports. ESR of Mo(V) Dispersed in Poly(acry1ic acid) Matrices Krystyna Dyrek, Krzysztof Kruczala, and Zbigniew Sojka Faculty of Chemistry, Jagiellonian University, Zngardena 3, 30-060 Cracow, Poland
Shulamith Schlick' Department of Chemistry, University of Detroit Mercy, Detroit, Michigan 4821 9 Received: March 19, 1993; Zn Final Form: June 24, 1993'
ESR spectra of molybdenum complexes obtained by dispersion of MoCl5 in a poly(acry1ic acid) (PAA) matrix were interpreted and lead to a detailed description of the ligands around the paramagnetic Mo(V) center. The specific bonding of the cation to the support is sensitive to the details of sample preparation, thermal treatment, and exposure to adsorbates. The most important factor that controls the type of complex formed is the temperature. The five different Mo(V) complexes that were identified reflect the progressive replacement of chlorine ligands by oxygen ligands from the polymer in activated samples, and by water in samples exposed to water vapor after activation of the polymer containing MoCls. After activation at 373 K molybdenum is linked to the polymer matrix via mono- and bidentate oxygen ligands, but still contains chlorine ligands. After activation a t 523 K, however, all chlorine ligands appear to be replaced by oxygen ligands from the polymer support. Exposure to water vapor leads to replacement of one oxygen ligand around the cation by oxygen from water. In the PAA matrix the Mo-Cl bond is resistant to hydrolysis and to dioxygen, indicating the strong bonding to the matrix and the presence of coordinatively saturated octahedral Mo(V) complexes. Introduction The advantages of homogeneous and heterogeneous catalysis can be combined by attaching catalysts, in most cases transitionmetal cation complexes, to insolublesupports. Polymeric supports are especially convenient, because of the great variety of functional groups that can be prepared for binding the catalytic group and the convenient way of separating the reaction products from the insoluble support. Natural polymers such as cellulose as well as syntheticmaterials such as polystyrene have been used extensively as catalytic supports in many reactions.' Molybdenum catalysts, such as Mo(V1) or mixtures of Mo(VI) and Mo(V), have been used in oxidation and hydrogenation processes on polymeric supports. The most common process to date is the hydroformylation reaction in the presence of Mo(CO)6 as catalyst on polystyrene matrices.2.3 The presence of paramagnetic M o ~ cations + is very convenient, because it allows the study of the various steps of sample preparation and catalytic reaction. ESR spectra of MoS+, a 5d1 cation, consist of strong signals from nonmagnetic nuclei 92M0, 94M0, 96M0, 98M0, and "Mo (total natural abundance 74.32%),which reflect theg-tensor anisotropy. The g values measured in the ESR spectrum of this center arevery sensitiveto the ligands and can provide information on the symmetry of the catalytic site, the immediate environment of the catalytic center, and the changes that occur during r e a ~ t i o n .The ~ ~ ~value of gll is especially affected by the ligands, through the spin orbit coupling XL of the ligand.6 Additional structural information can be obtained from an analysis of the hyperfine interaction of magnetic isotopes 95Mo and 97Mo,with natural abundances 15.78% and 9.698, respectively. Both magnetic nuclei have Z = 5 / 2 and very similar nuclear magnetic moments; separate signals from these isotopes are not normally detected in X-band ESR spectra. The presence of more than one MoS+ site has been detected experimentally by measuring ESR spectra at two microwave frequencies' and confirmed by spectral simulations.* Recently we have investigated molybdenum catalysts on polyacetylene and polypyrrole supports and have assessed the
* To whom all correspondence should be a d d r d .
* Abstract published in Advance ACS Abstracts, August 15, 1993. 0022-3654/93/2097-9196$04.00/0
efficiency and selectivity of the supported catalyst for the conversion of ethanol tovarious products, using GC, ESCA, ESR, X-ray diffraction, conductivity, and chemical analysis.9JO The results have indicated that polyacetylene doped with phosphomolybdic anions is very efficient for the reaction of ethanol to acetaldehyde, ethylene, and diethyl ether, compared to the same reaction in the presence of the unsupported polyacid H3PMo12040.~ On polypyrrole supports, by contrast, no change in the catalytic activity, but significant change in selectivity has been detected.10 In this paper we present an ESR study of MoS+ dispersed in poly(acry1ic acid) (PAA) matrices. The main objective of this study was to identify the type of bonding between Mo5+ and the polymer in the absence and presence of adsorbates and for different activation conditions of the catalytic support. As will become evident, several paramagnetic molybdenum specieswere detected in the course of thermal activation and water adsorption and identified by ESR spectroscopy at X-band and by spectra simulations. Additional support for some assignments was obtained from infrared spectra.
Experimental Section Sample Preparation. PAA of molecular weight in the range 100 000-580 000, obtained from Aldrich or by polymerization of acrylic acid (Reachim), and MoC15 (Aldrich) in a molar COOH/Mo ratio 6:l were dissolved in dry dimethylformamide (DMF). After 2 hat ambient temperature, the Mo-PAAcatalyst was precipitated by pouring the DMF solution into freshly distilled acetone. The precipitate was filtered and dried at 323 Kin vacuo. Additional details will be published.ll All samples were activated in the temperature range 373-523 K in vacuo prior to admission of the adsorbate. The adsorption of water at a pressure of 25 Torr was performed in the temperature range 373-523 K. The adsorbate was in contact with the supportedcatalyst for 30 min at each temperature unless specified otherwise. Spectroscopic Measurements. ESR spectra were measured at 77 and 300 K using an ESR-220 Adlershof spectrometer in Cracow and a Bruker 200D SRC spectrometer in Detroit, both with 100kHz modulation. The spectrometerswere interfaced respectively Q 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 36, 1993 9197
Catalysis on Polymer Supports gl,o= 1.946 A,,o=
51.6 G
b g,,
=
1.968
A,, = 81.9 G
Mi
g, = 1.037 A,
= 36.7 G
C
gls0= 1.953
A,,.,=
52 C
H 50 Cs
Figure 1. X-band ESR spectra of MoCIs in DMF at 300 K (a), at 77 K (b), and in cyclohexane at 300 K (c). Experimental spectra (thick line), simulated spectra (thin line).
H 50 G
w
I q, Mo IC)
Figure 2. X-band ESR spcctra at 77 K of Mo-PAA activated in vacuo
at373 K(a),473K(b)and523K(c). Expcrimentalspectra(thickline),
simulated spectra (thin line). with data acquisition software DPR developed in Cracow and EPRDAS from Mega Systems Solutions, Rochester, NY. All spectra were recorded using a microwave power of 2 mW. As a g-factor standard we used 2,2-diphenyl-1-piqlhydrazyl (DPPH, g = 2.0036). ESR spectra were simulated with the program SIM14A. The original program was modified by J. M. Lagan (Cracow), who introduced a users’ interface and optimization procedures (grid search with adjustable step, the Nelder-Mead simplex method with no restrictions, and Monte Carlo). Infrared spectra of samples in KBr pellets were recorded at 293 K using a Digilab FTS-60V spectrometer.
TABLE I: ESR Parameters for MoJ+Complexes species
Results and Discussion In this section we will discuss the results obtained for MoCl5 in solution, dispersed in PAA matrices at different activation temperatures (Mo-PAA samples) and in activated PAA matrices in contact with water vapor. MoClS in DMF and Cyclohexane Solutions. X-band ESR spectra of MoCl5 in DMF at 300 and at 77 K are given in Figure 1. The spectrum at 300 K consists of an isotropic signal from the nonmagnetic molybdenum nuclei, with gi, = 1.946. The isotropic sextet from the magnetic isotopes 95Mo and 97Mo indicates a hyperfine constant A h = 51.6 G. At 77 K an asymmetric ESR spectrum is obtained, with 81= 1.968, gl = 1.937, All = 81.9 G, and Al = 36.7 G. These parameters were deduced from the simulated spectrum, also given in Figure 1. The g-tensor values are almost identical to those of the center M o O C ~ ~suggesting ~-,~ that in DMF the paramagnetic cation Mo5+ exists as ((CH3)2NHC=O)MoClS, due to the attachment of one molecule of DMF through the oxygen to the cation. This assignment seems reasonable, especially if we compare the gh values of MoC15 in DMF and in cyclohexane, Figure la,c, respectively. The gh value measured in cyclohexane is higher (1.953), most probably due to lack of coordination of this solvent and weaker donor capability, compared to DMF. The g h value for MoCl5 in DMF is similar to that measured for MoOClS2- in DMF, where the dominant signal (295%) was assigned to ((CH3)2NHC=O)MoOC&-.13 However,it is difficult to make an assignment based on gi, values alone: data in the
AH gl (G) 1.948 1.9632 1.94 74.7 46.6 1.947 1.970 1.935 1.947 1.947 1.968 1.937 81.9 36.7 1.953 1.947 1.967 1.937 81.9 36.7 1.942 1.957 1.934 82.0 -37 1.931 1.902 1.946 94.3 40.0 1.928 1.903 1.94 1.932 1.907 1.945 FUO
ref 5 ref 5 ref 13 this work this work this work this work this work this work this work
literature document a variety of molybdenum paramagnetic species with similar values of E ~but , quite different g-tensor components.5JJ3 Relevant ESR parameters for molybdenum species, from the literature and measured in this study, are collected in Table I. MoCls Dispersed in PAA (Mo-PAA Samples). X-band ESR spectra at 77 K of the Mo-PAA sample activated at 373, 473, and 523 K in vacuo are given in Figure 2. All ESR spectra measured for the activated samples, Figure 2, were retained after extraction of the sample with acetone, indicating bond formation between the polymer matrix and the molybdenum cations. Interpretation of the complex spectrum in Figure 2a (activation at 373 K) is facilitated by the hyperfine splitting detected in the low-field part of the spectrum, clearly seen in the vertically expanded spectrum in Figure 2a; two Mo5+ species with comparable intensities, which we label Mo(A) and Mo(B), are suggested by the results. Simulation of the spectra indicated the following ESR parameters: for Mo(A): 811 = 1.967, g, = 1.937, All
81.9 G,A , = 36.7 G
for Mo(B): gll = 1.957, 8, = 1.934, All = 82.0 G, A,
37 G
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Dyrek et al.
The Journal of Physical Chemistry, Vol. 97,No. 36, 1993
n
in comparison with Mo(A) and Mo(B) (Figure 2b). The signal detected after heating to 523 K is axially symmetric with reversed gvalues (gll< gL), Figure 2c; this is species Mo(C). Simulations indicate the following ESR parameters for Mo(C): gl = 1.902, g l 1.946,All = 94.3 G, and A L = 40.0 G. Computer simulations also indicate that the ESR spectrum in Figure 2b, for the intermediate activation temperature of 473 K, can be reproduced by assuminga superpositionof contributionsfrom Mo(A), Mo(B), and Mo(C) species with relative intensities 2:3:5. The ESR parameters of species Mo(C) are consistent with Mo5+in a dI,ground stateand a tetragonally distortedoctahedral coordination of symmetry C4y formed by oxygen ligands.7 The extent of the tetragonal distortion can be evaluated by calculating the distortion factor a from the expression given in eq 1, where 6 is the tetragonal distortion, A is the octahedral crystal field and g, = 2.0023.
+-t 1750
2000
1500
Cm-'
Figure 3. FTIR spectra at 293 K of PAA (a) and of Mc-PAA (b), in KBr pellets.
The presence of two Mo5+ species after activation at 373 K indicatesthat the molybdenumcomplexis bonded to the polymeric matrix via two types of coordination. The presence of chlorine ligands is suggested by the g-tensor components: gl > gl. For Mo5+surrounded by oxygen ligands the reverse relationship, gl < gL,was deduced.6~'~Since the species corresponding to signal Mo(A) is unleachableby acetone and has ESR parameters similar to those detected for molybdenum specieshaving a M A bond: we assign Mo(A) to a MoC15 species attached to the polymer matrix via a carboxylic group. The attachment is formed by the ligand displacement mechanism given below, where we use the notation P for the polymer matrix and COOH for one of the carboxylic groups: ((CH3)2NHC=O)MoCI5
+
PCOOH
-
CH-CH2
(LOOH
),
-CH-CH2-
+ PCOOH
( 8)
(0oCP)X PC< 0>MoCI4, 1
+ xHCl
(LOOH
)
-
4
+
HCI
-
(4-x)PCOOH
PC< 0>Mo(OOCP)4
+
4HCI
0
(C)
I
CH-CH2
LOOH
(CH&NHC=O
+ XPCOOH
0
+ PC=O:MoCb
The parameters of the signal Mo(B), especially the lower value of 41, compared to Mo(A) (Table I), indicates further replacement of chlorine ligands by oxygens from the polymer matrix. Comparison of the FTIR spectra of PAA (Figure 3a) and MoPAA (Figure 3b) reveals that besides a band at 1724 cm-l (corresponding to the C-0 vibration of the monodentate carboxylic groups), a stronger band appears at 1652 cm-l for Mo-PAA, which suggests the presence of bidentate carboxyl groups.I5 A mechanism that is consistent with ESR and FTIR data should therefore involve the release of C1- ligands and formation of chelating molybdenum4arboxyl group bonding. For this reason we propose that the species Mo(B) are formed according to the following reaction scheme: ((CH&NHC=O)MoCIs
PC,Y O ,,MoC14
0
-D
(CH3)2NHC=O
PCOOH E
For site Mo(C) we obtain a = 1.78, indicating a relatively weak tetragonal distortion. By comparison, typical a values in the literature for pentocoordinated Mo~+,which obviously has a high tetragonal distortion, are 2.65,* 2.6916 and 2.58.17 The reversal of the g-tensor values of Mo(C) compared to Mo(A) and Mo(B) is due to further replacement of chlorineby oxygen ligands (see below) and to the different ligand fieldsplitting contributions to the g-value shifts; for chlorine b l = 586 cm-l and for oxygen = 152 cm-1:6
+
PC
MoC4 0 (6)
The PCOO- group is attached to the MoC14 group via a bidentate chelate bonding. Changes in the ESR signals on thermal activation of MoPAA samples in the temperature range 473-523 K are shown in Figure 2b,c. At 473 K the ESR signal becomes more symmetrical
In this way, at the highest temperature of activation (523 K) the Mas+ ions are coordinated to the PAA matrix via four monodentatecarboxyl bonds and onechelating COO group, giving a charge-balanced and coordinatively saturated structure of the surface complex. Adsorption of Water on Activated Mo-PAA Samples. Upon adsorption of water vapor at 25 Torr on Mo-PAA samples activated at 373 K, a decrease in the intensity of signals corresponding to Mo(A) and Mo(B) species was observed. Additional changes in the spectra were detected for longer contact times with water vapor. The ESR spectrum obtained for exposure of activated Mo-PAA samples to water for 4 days is shown in Figure 4a; the new signal detected, Mo(D), is characterized by gl = 1.903, gl 1.94 and a = 1.59. Evacuation of the sample leads to the disappearance of the signal corresponding to Mo(D) and to a spectrum similar to the original signal obtained after activation, as seen in Figure 4b. The intensity of the Mo(A) species is not completely recovered, however, and the intensity ratio Mo(A)/Mo(B) decreased;these changes were reproducible. Because evacuation restored the original spin concentration, we suggest that three processes are involved in the changes that occur on water adsorption. First, the overall decrease in signal intensity could be due to reversible transformation of the Mo(A) species into an ESR-silent species, most likely by dimerization.l8Jg Second, we propose that a small fraction of species Mo(A) is irreversibly transformed into Mo(B); this step is suggested by the continuous decay of Mo(A) species and increase of Mo(B) species after each adsorption-desorptioncycle. Third, Mo(B) transforms reversibly into Mo(D). Because the 81value for Mo(D) is lower than for Mo(B), it is likely that the transformation Mo(B) Mo(D) involves a displacement of an additional chlorine ligand
-
-
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The Journal of Physical Chemistry, Vol. 97, No. 36, 1993 9199
/ Mo (El
b
J
I/-----
H
50 G
Figure 4. X-band ESR spectra at 77 K of Mo-PAA activated at 373 K, after adsorption of water vapor during 4 days (a), followed by desorption (b).
I
C
by a water molecule: H 50
c
Figure 5. X-band ESR spectra at 77 K of Mo-PAA activated at 523 K after 30 min of contact with water vapor (a), 11 days of contact with watervapor (b), followed bydesorption (c). Experimental spectra (thick line), simulated spectra (thin line).
The replacement of chlorine by oxygen without change in the coordination number of species Mo(D) is reflected in the distinct shift of 81,while the value of a is fairly constant. The water-containing molybdenum Mo(D) species and C1are a charge-balancing pair that enable the removal of chlorine as HCI during desorption, so that the process is reversible. The high dielectric constant of water and its ability to solvate ions seem to be important factors in the dissociation of the Mo(B) species, becausenochangein theintensity of this species is detected after adsorption of ethanol.20 It is important to mention that in the system Mo-PAA no appreciable hydrolysis of MoCls is detected, in contrast to the situation in the MoC15SiOzsystem.18 This result is due to the strong attachment of MoS+to the polymer support. Moreover, preliminary results on oxygen adsorption indicate that MoS+in PAA is not sensitive to dioxygen. Because oxygen is expected to attach to a vacancy in the coordination scheme and not to compete with COO-, C1-, or HzO in ligand displacement reactions, this observation confirms our interpretation that the molybdenum complexes we identified are fully saturated, in agreement with the bonding schemes suggested above. ESR spectra at 77 K of Mo-PAA samples activated at 523 K and exposed to water vapor at ambient temperature are presented in Figure 5. After long exposures to water vapor, a new signal Mo(E) dominates the spectrum, with gl = 1.945, gll= 1.907, and a = 1.66. These parameters indicate a hexacoordinated MoS+ species,similar to the initial complex of type Mo(C). Evacuation of the sample restored the original signal, with improved resolution of the parallel signal, Figure 5c. The transformation Mo(C) Mo(E) is therefore reversible. Since both complexes are completely ligand-saturated, the only way in which a water molecule can enter the coordination sphere of the complex Mo(C) is a ligand replacement reaction so that the oxygen ligand from the polymer is replaced by oxygen from water:
-
Conclusions ESR spectra of molybdenum complexesobtained by dispersion of MoCb in a poly(acry1ic acid) (PAA) matrix were interpreted and lead to a detailed description of the ligands around the paramagnetic Mo(V) center. The bonding of the cation to the support is sensitiveto theconditions ofsample preparation,thermal treatment, and exposureto adsorbates. The most important factor that controls the type of complex formed is the temperature. Five different Mo(V) complexes were identified, which reflect the progressive replacement of chlorine ligands by oxygen ligands from the polymer in activated samples, and by water in samples exposed to water vapor after activation of the polymer containing MoC15. After activation at 373 K molybdenum is linked to the polymer matrix via mono- and bidentate oxygen ligands and still contains chlorine ligands. After activation at 523 K all chlorine ligands appear to be replaced by oxygen ligands from the polymer support. Exposure to water vapor leads to replacement of one ligand around the cation by oxygen from water. In the PAA matrix the M d l bond is resistant to hydrolysis and todioxygen, indicating the strong bonding to the matrix, and the presence of saturated (hexacoordinated) Mo(V) complexes.
Acknowledgment. This research was supported by the donors of the Petroleum Research Fund (PRF), administered by the American Chemical Society, by National Science Foundation Grant DMR-8718947 (PolymersProgram) and by KBN (Poland) Grant P/03/055. K.D. and K.K. are grateful to PRF for support of their stay in Detroit. S.S.is grateful the American Association of University Women (AAUW) for the 1991/1992 Founders’ Fellowship. References and Notes (1) Tsonis, C. P. J. Chem. Educ. 1984, 61, 479. (2) Pittman, C. U.;Evans, G. 0. Chem. Tecfinol. 1973, 560. (3) Tsonis, C. P.; Farona, M.F. J. Organomet. Chem. 1976,114, 293. (4) Narayana, M.; Zhan, R. Y.;Kevan, L. J. Phys. Chem. 1985,89,636. ( 5 ) Che, M.; Louis, C.;Sojh, 2.J . Chem. Soc., Faraday Trans. 1 1989, 85. 3939. (6) Manoharan, P. T.; Rogers, M. T. J. Chem. Phys. 1968,49, 5510.
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(7) Che, M.; Fournier, M.; Launay, J. P. J . Chem.Phys. 1979,71,1954. ( 8 ) Dyrek. K.; Labanowska, M.J. Chem. Soc., Faraday Trans. 1 1991, 87, 1003. (9) Pozniczek, J.; Kulszewicz-Bajer, I.; Zagonka, M.; Kruczala, K.; Dyrek, K.; Bielanski, A.; Pron, A. J . Caral. 1991, 132, 31 1. (10) Kulszewicz-Bajer, I.; Zagorska, M.; Pozniczek, J.; Bielanski, A.; Kruczala, K.; Dyrek, K.; Pron, A. Synrh. Met. 1991, 41-43, 39. (11) Bortel, E.; Dyrek, K.; Kruczala, K., to be published. (12) Siml4A Program, QCPE No. 265, written by G. P. Lozos, B. M. Hoffman, and C. G. Franz. (13) Garner, C. D.; Hyde, M. R.; Mabbs, F. E. Inorg. Chem. 1976, 15, 2327.
Dyrek et al. (14) Louis, C.; Che, M. J. Phys. Chem. 1987, 91, 2875. (1 5 ) Pomagailo, A. D. In Polymer-Immobilized Metal Complexes Catalysis; Nauka Publishers: Moscow, 1988 (in Russian). (16) Hanuza, J.; Jezowska-Trzebiatowska, B.; Oganowski, W. Bull. Pol. Acad. Sci., Chim. 1977, 25, 735. (17) Shelimov, B. N.;Pershin, A. N.; Kazansky, V.B. J . Caral. 1980,64, 426. (18) Louis, C.; Che, M. J. Caral., in press. (19) Spacu, P.; Gheorghiu, C.; Constantinescu, M.; Antonescu, L. J. Less Common Mer. 1976, 44, 161. (20) Dyrek, K.; Kruczala, K., Schlick, S.,unpublished results.
