Ligand Design by Ionomers. ESR of MoV in Perfluorinated Ionomer

This was the case of MoV introduced as MoCl5 in poly(acrylic acid) (PAA) matrices.14 Two types of MoV ESR signals have been identified for this system...
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J. Phys. Chem. 1996, 100, 2229-2236

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Ligand Design by Ionomers. ESR of MoV in Perfluorinated Ionomer Supports Krzysztof Kruczala,† Zhan Gao,‡ Krystyna Dyrek,§ and Shulamith Schlick*,‡ Department of Chemistry, UniVersity of Detroit Mercy, Detroit, Michigan 48219, and Faculty of Chemistry, Jagiellonian UniVersity, Ingardena 3, 30-060 Cracow, Poland ReceiVed: July 11, 1995; In Final Form: October 31, 1995X

Electron spin resonance (ESR) spectra of Mo5+ in perfluorinated membranes (Nafion) neutralized by MoCl5 suggest that the ionomer acts as an ion-selective medium and replaces the chlorine ligands of the molybdenum center by oxygen ligands from the sulfonic groups. This conclusion is based on an analysis of the ESR parameters that characterize the paramagnetic center and is supported by the detection of only one major paramagnetic molybdenum site, Mo(A) in the bulk ionomer, in the absence of solvents. Ligand design via the ionomer was thus achieved. We propose that the main molybdenum species is MoO(SO3-)5, where the unique oxygen ligand is from an sSdO group of the sulfonic moiety. Exposure of Mo/Nafion to water (up to 5 Torr) has no significant effect on the ESR parameters and on the intensity of the ESR signal. Exposure to 25 Torr of water leads to the reversible disappearance of the ESR signal. Brief exposure of Mo/Nafion to gaseous acetonitrile (as CD3CN) results in the appearance of two new Mo species, Mo(B) and Mo(C), which are thought to be formed via replacement of one equatorial or one axial sSdO ligand, respectively, based on an analysis of the effect on the corresponding g| values. Soaking of Mo/Nafion samples with acetonitrile results in the reversible disappearance of the ESR signals. All transformations of the ESR signals are explained by the replacement of one L-type ligand (donating two electrons), sSdO in the case of Nafion, by oxygen from water or by nitrogen from acetonitrile. The loss of the signal in the presence of adsorbates is explained by a replacement of one or more of the -S-O- ligands by ligands from the adsorbates. This process is a reversible reductive replacement of the ionic ligands and leads to the concomittant loss of the paramagnetism. The control of ligands around the paramagnetic center, the involvement of the molybdenum in redox processes, and the accessibility of the center to adsorbates are promising features for catalysis. Initial results for the oxidation of ethanol on Nafion and on Mo/Nafion supports are presented. Correlation plots (A| vs g|) in paramagnetic molybdenum complexes, constructed from data in the literature and measured in this study, show trends that can be used for identification of the number and types of ligands in paramagnetic molybdenum centers.

Introduction Molybdenum catalysts, in the form of oxides MoO3 or various molybdates, are used extensively in redox processes and acidbase catalysis.1-3 Molybdenum oxides and sulfides supported on oxide carriers such as SiO2 and on polymeric supports have been the subject of extensive studies recently.1 Molybdenumcontaining catalysts on oxide supports are widely used in hydrodesulfurization of petroleum, coal liquefaction, and selective oxidation of olefins. The most common process to date on polymeric supports is the hydroformylation reaction in the presence of Mo(CO)6 on polystyrene matrices.2,3 Most recently, the catalytic activity of molybdenum on polyacetylene, polypyrrole, and polyaniline supports has been investigated, and the efficiency and selectivity of the catalysts for the conversion of ethanol to various products have been assessed.4-6 The ability of molybdenum to participate in redox and ligand exchange reactions in species containing many ligand types is crucial to the catalytic properties of this center.1-3 The presence of paramagnetic Mo5+ cations in catalytic systems is an important advantage because it allows the study by electron spin resonance (ESR) of the various steps of sample preparation and catalysis. ESR spectra of Mo5+, a 5d1 cation, consist of strong * Correspondence author. E-mail address: SCHLICKS@UDMERCY. EDU. † University of Detroit Mercy (on leave from the Jagiellonian University). ‡ University of Detroit Mercy. § Jagiellonian University. X Abstract published in AdVance ACS Abstracts, January 1, 1996.

0022-3654/96/20100-2229$12.00/0

signals from nonmagnetic nuclei 92Mo, 94Mo, 96Mo, 98Mo, and (total natural abundance of 74.32%), from which the g-tensor components can be deduced. The g values measured in the ESR spectra of this center are sensitive to 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 reaction.7-12 The value of g| is especially affected by the ligands through the spin-orbit coupling λL (of the ligand).13 Additional structural information can be obtained from an analysis of the hyperfine interaction of the magnetic isotopes 95Mo and 97Mo, with natural abundances of 15.78% and 9.69%, respectively. These nuclei have I ) 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 several Mo5+ sites has been detected experimentally by measuring ESR spectra at two microwave frequencies9 and confirmed by spectral simulations.12 The development of powerful simulation programs by the Cracow group6,12 has become essential for the interpretation of complex spectra consisting of a superposition of contributions from multiple molybdenum centers. The decrease, and disappearance, of the paramagnetism is also an important indication of the ligand exchange reactions that take place. In favorable cases ESR spectra from molybdenum can provide information on the nature of ligands coordinated to the metal. This was the case of MoV introduced as MoCl5 in poly(acrylic acid) (PAA) matrices.14 Two types of MoV ESR signals have been identified for this system, depending on pretreatment con100Mo

© 1996 American Chemical Society

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Kruczala et al.

SCHEME 1. Nafion Ionomer, Acid Form

temperature range 333-573 K. The notation we use is Mo/ Nafion-x, where x is the percent neutralization. Adsorption Experiments. To study the interaction of Mo/ Nafion with adsorbates, samples activated at 373 K in vacuo (to 2 × 10-5 Torr) were degassed by the freeze-pump-thaw method and exposed at ambient temperature to water vapor (125 Torr) or to DAN (40 Torr) for 30 min, or soaked with DAN. Weight Loss. Thermogravimetric analysis (TGA) was performed with a DuPont 2100 instrument in the temperature range 300-900 K. Spectroscopic Measurements. ESR spectra were measured with a Bruker ECS 106 X-band spectrometer equipped with the ESP 3220 data system for spectra acquisition and manipulation and with the ER 4111 variable temperature unit. Spectra (2K points) were recorded with a 100 kHz magnetic field modulation. The microwave power was 2 mW unless indicated otherwise. For g-factor standards, we used 2,2-diphenyl-1picrylhydrazyl (DPPH, g ) 2.0036) or Cr3+ in a single crystal of MgO (g ) 1.9796).29 Simulations. ESR spectra were simulated with the program SIM14A.30 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). The program calculates the energy levels of the spin system to second order and the transition probabilities to first order. Simulations were performed on a NEC computer equipped with a 486/66 processor. Catalytic Performance. The catalytic performance of Nafion and Mo/Nafion-30 for the oxidation of ethanol was measured with the fully automated test unit at the Jagiellonian University.6 The system consists of a 5-channel mass flow controller (Omega) for gas reactants blending, 6-port switching, and 10-port injection valves (Valco) for continuous 2-column analysis of reagents before and after the reaction using thermal conductivity and flame ionization detectors (TCD and FID, respectively). The gas components are dosed with a glass saturator connected to a Cole-Palmer cryostat. The reaction is carried out in a tubular quartz reactor in a split furnace heated in the proportioning-integral-derivative control (PID) regime by an Omega controller. The gas sampling and the simultaneous data acquisition on two channels (TCD and FID) are performed on-line using an IBM PC 386 computer.

ditions: signals with g| > g⊥ (Mo(A) and Mo(B)) and signals with g⊥ > g| (Mo(C)). Species Mo(A) and Mo(B) were obtained after evacuation of the catalyst at 373 K, while species Mo(C) appeared only after activation in vacuo at 523 K. On the basis of ESR and IR data, species Mo(A) and Mo(B) have been assigned to MoV coordinated to chlorine ligands and also attached to the polymer matrix via oxygens of the carboxylic groups and species Mo(C) has been assigned to MoV coordinated to oxygen ligands only. The reversal of the g-tensor values of Mo(C) compared to those of Mo(A) and Mo(B) was related to the different spin-orbit coupling constants of oxygen and chlorine (λO ) 152 cm-1 15 and λCl ) 586 cm-1 16, which influence the value of g|.17 A diagram that summarizes the relation between g| and g⊥, as well as the relative ionic character of the Mo-ligand bonds, has been proposed by Che et al. and is a useful indicator of the ligation scheme around molybdenum.9,11 In this paper we present an ESR study of Mo5+, dispersed in Nafion perfluorinated ionomers, in the bulk ionomer and in the presence of water and acetonitrile as adsorbates. The main objective of this study was to identify the type of bonding between Mo5+, the polymer and the adsorbates, for different activation conditions. The present investigation was motivated by results of catalytic studies of molybdenum (as Keggin units, H3PMo12O40) on polymeric matrices such as polyacetylene and polypyrrole4-6 and by similar studies with Mo5+/PAA,14 which have suggested that actiVation of a molybdenum catalyst initially present as MoCl5 essentially means the stripping of the chlorine ligands and their replacement by oxygen ligands. Our idea was to study the possibility that Nafion, a perfluorinated ion-selective ionomer membrane that has been studied extensively in our group,18-26 might effectively block the access of the chlorine anions. The superselectivity of perfluorinated membranes such as Nafion is due to the greatly reduced rate of transport of anions across the membrane.27,28 The results presented below suggest that the ionomers can be used for ligand design, in this case replacement of chlorine by oxygen ligands from the sulfonic groups or solvents present in the system, without the need to activate the catalytic support at high temperatures. Experimental Section Materials. Nafion membranes (Scheme 1) with an equivalent weight (EW) of 1100 g/mol SO3H and a thickness of 0.178 mm were obtained from DuPont. MoCl5 (99.9%) was obtained from Aldrich. Deuteriated acetonitrile (DAN, 99% D enrichment) was reagent grade from Aldrich. Ethanol (EtOH) was dried with molecular sieves (type 3A from Fisher) and kept in a glovebox. Sample Preparation. Treatment of the membranes has been described.19,20,26 The treated membranes in the acid form (Nafion/H) swollen by water were dried in vacuo at ambient temperature for 1 d and at 350 K for 1 h and swollen in ethanol for 24 h; this process was then repeated. Mo/Nafion was prepared by treating the membranes with a solution of MoCl5 in ethanol for 24 h. The amounts of MoCl5 corresponded to neutralization of 2, 10, and 30% of the sulfonic groups in the membrane. All manipulations involving molybdenum were carried out in the glovebox or in vacuo, with no exposure to air. Mo/Nafion samples were thermally activated in the

Results Mo/Nafion System. ESR spectra of Mo/Nafion were measured in the temperature range 125-300 K as a function of Mo5+ content (2, 10, and 30% neutralization) and activation temperature (in the range 333-573 K). Selected ESR spectra of Mo/Nafion-2 at 125 K for the indicated activation temperatures (1 h at each temperature) are given in Figure 1. All samples were positioned similarly in the ESR cavity and, therefore, the intensities could be directly compared. The weak Mo5+ signal obtained immediately after neutralization increases dramatically for activation in vacuo at and above 340 K. At high activation temperatures, 523 K, a signal at g ) 2.003 appears and is assigned to a perfluorinated alkyl radical (Vide infra), but the basic Mo5+ signal remains the same. At an activation temperature of 553 K the signal from the alkyl radical increases and the signal from the Mo5+ center changes significantly. Above this temperature the Mo5+ signal decreases and that from the alkyl radical dominates the spectrum. The signal at g ) 2.003 can be easily saturated by microwave power, and a combination of lower temperatures and higher microwave powers leads to the almost complete disappearance of this signal

Ligand Design by Ionomers

Figure 1. X-band ESR spectra at 125 K of Mo/Nafion-2 for the indicated activation temperatures (1 h at each temperature). Upward arrow indicates the signal from alkyl radicals obtained by thermal decomposition of the ionomer, which is visible at activation temperatures g523 K. Asterisks show a MnII impurity.

Figure 2. Weight vs temperature plots: (A) for Nafion; (B) for Mo/ Nafion-30.

in some samples, as expected for alkyl radicals. The changes at high activation temperatures are well correlated with the weight loss of samples, as seen in the TGA data for Nafion/H and Mo/Nafion-30 presented in Figure 2. The weight losses for Nafion/H at different temperatures are in line with data reported for Nafion/H films.31 The ESR signals from Mo5+ measured in Mo/Nafion-10 and -30 were more intense but similar in shape to those measured for Mo/Nafion-2. In Figure 3 we present ESR spectra of Mo/ Nafion-30 at 125 K, including the vertical expansion (×30) of the signals in the high magnetic field range, together with simulated spectra calculated with Gaussian line shapes. The simulated spectra are excellent fits to the experimental spectra,

J. Phys. Chem., Vol. 100, No. 6, 1996 2231

Figure 3. Experimental (s) and simulated (- - -) X-band ESR spectrum of Mo/Nafion-30 at 125 K. Stick diagrams for the parallel and perpendicular spectral components of species Mo(A) are also indicated.

including all hyperfine signals. The species detected in Nafion at various activation temperatures and degrees of neutralization is indicated by Mo(A), and the corresponding parameters used in the simulations are given in Table 1. Spectra at 300 K were simulated with parameters similar to those given in Table 1 for Mo(A): the g values are within (0.001 and the hyperfine splittings within (0.5 G. Adsorption of Water. Contact of all Mo/Nafion samples with water at 25 Torr leads to the complete and immediate disappearance of the Mo5+ signal, but the disappearance is reversible and the signal reappears on desorption. For a water pressure of 1-5 Torr, however, the ESR signal is essentially the same and could be simulated with the parameters used for species Mo(A); the total intensity at 5 Torr was only ≈75% of the initial intensity. For a water pressure of 10 Torr, the signal decreases to ≈40% of the initial intensity after 30 min and to ≈5% after 1d. The very weak spectrum detected after prolonged contact is identified with a new site, Mo(A′), as shown in Figure 4. Although the hyperfine structure in the low field is obscured by the MnII impurity, the main signals and the parallel components in the high-field range are prominent and the ESR spectrum in Figure 4 could be simulated with the parameters given in Table 1. Adsorption of Acetonitrile. In initial experiments, Mo/ Nafion-30 was soaked with DAN, as was done previously for Cu/Nafion.19,20 In soaked samples kept at ambient temperature, however, no new signals were detected and the ESR spectrum disappeared within 15-20 min. In samples measured at 125 K 5-10 min after soaking, two new and very weak signals appear; these are species Mo(B) and Mo(C). In order to stabilize the new signals, we exposed Mo/Nafion to lower DAN pressures. In Figure 5A we present ESR spectra at 300 K obtained on exposure of Mo/Nafion-30 to 40 Torr of DAN as a function of contact time. The spectra have significantly different line shapes even after a short exposure (0.5 h) to DAN.

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Kruczala et al.

TABLE 1: ESR Parameters for Mo5+ Centers

a

species

g|

g⊥

A| (G)

A⊥ (G)

∆H| (G)

∆H⊥ (G)

comments

Mo(A)/Nafion Mo(A′)/Nafion Mo(B)/Nafion Mo(C)/Nafion MoO6 Mo/SiO2 MoO(HSO4)52Mo/PAA Mo/SiO2/CH3CN

1.877 1.907 1.903 1.892 1.877 1.892 1.881 1.902 1.887

1.940 1.941 1.953 1.946 1.940 1.944 1.938 1.946 1.947a 1.929a

104.5 92.5 94.8 98.4 104.4

49.2 43.0 43.7 45.7 48.8

17.4 27.0 16.4 16.7

11.0 21.0 11.1 11.9

104.8 94.3

44.2 40.0

this study this study this study this study ref 12 ref 11 ref 33 Mo(C) in ref 14 ref 8

Rhombic symmetry has been suggested.

detected compared to spectra at 300 K, and good fits to the experimental spectra were obtained with the same sets of parameters. As in the case of water as adsorbant, the disappearance of the ESR signals upon soaking Mo/Nafion with DAN is reversible, and all signals reappear on desorption. Catalytic Activity. The oxidation of ethanol (100 Torr) in the presence of oxygen (also 100 Torr) on Nafion and Mo/ Nafion-30 supports was examined in the temperature range 448-553 K. The conversions and selectivities (in %) are presented for the two types of support in parts A and B of Figure 6, respectively. In the case of Nafion, the two products are ethylene and diethyl ether, both produced by the catalytically active sulfonic groups of the ionomer. The amount of ethylene increases with increasing temperature, and ethylene is the dominant component above ≈470 K. For the Mo/Nafion-30 support, ethanal is detected in addition to the other two products above ≈490 K, and the relative amount increases with temperature. Based on these initial results, we plan a series of catalytic tests on Mo/Nafion with neutralization above 30% and on Nafion neutralized by Mo5+ and Na+ (to decrease the catalytic activity of the acid groups). Discussion

Figure 4. Experimental (s) and simulated (- - -) X-band ESR spectra at 300 K of Mo/Nafion-30 after 4d contact with water at 10 Torr. The total signal is only 5% of the original signal intensity. The simulated spectrum was obtained with the parameters listed in Table 1 for species Mo(A′). Asterisks show the six main signals of the MnII impurity.

Additional spectral changes were observed on a longer time scale, typically 10-90 h. Over long exposure times the total integrated intensity of the signal decreases, as indicated in Figure 5A. The high-field portions of the ESR spectra given in Figure 5A are expanded vertically and presented in Figure 5B, together with the simulated spectra. These results indicate clearly the spectral changes and the presence of three Mo5+ species, Mo(A), Mo(B), and Mo(C), where Mo(A) is the basic signal detected in the system with no adsorbates. For the various times of exposure to DAN, the total spectrum was simulated with different ratios of the three species, as indicated in Figure 5B. The changes in the relative heights of the parallel hyperfine components seen in Figure 5B for species Mo(B) and Mo(C) clearly reinforce the assignment of these signals to different species, and not to a rhombic symmetry, as suggested for acetonitrile adsorbed on Mo/SiO2.8 Species Mo(B) is barely visible for exposure times g20 h, and species Mo(C) is dominant for exposure times g36 h. The ESR parameters for these additional species are also indicated in Table 1. ESR spectra were also measured at 125 K, but no significant differences were

In this section we will identify the main Mo5+ species formed in the perfluorinated ionomer, Mo(A), and (1) propose a ligation scheme that can explain the temperature variation of the signal intensity, (2) present our interpretation of the changes that occur on water adsorption, as well as the disappearance of the signal in the presence of a large amount of water, (3) attempt to explain the spectral changes in the ESR signal of the Mo5+ center upon adsorption of acetonitrile and (4) rationalize the reversible disappearance of the signal in the presence of water and acetonitrile. Mo5+ in Nafion. Only one major ESR signal, characterized by sharp lines, is obtained by activation of Mo/Nafion in the temperature range 333-523 K, irrespective of the degree of neutralization (up to 30%). Moreover, the same signal, albeit less intense, is present even in nonactivated samples, as seen in Figure 1 (top spectrum). Because the preparation method starts with Nafion swollen by EtOH, a solvent effective in penetrating into, and plasticizing, the perfluorinated ionomer,22,23,25 we expect molybdenum ions to be dispersed in the ionomer matrix; indeed, very careful examination of the line shape as a function of cation concentration indicated only very minor changes in the line widths and line shapes at higher degrees of neutralization, and these changes were visible only in the wings. The signal is not affected by temperature, indicating a rigid trapping site of Mo5+. In addition, Mo(A) is detected even at 523 K; under these conditions no remaining solvent (either water or EtOH) is expected on the basis of a 1H NMR study of the water content of Nafion, which has concluded

Ligand Design by Ionomers

J. Phys. Chem., Vol. 100, No. 6, 1996 2233

Figure 5. (A) Experimental (s) and simulated (- - -) X-band ESR spectra at 300 K of Mo/Nafion-30 for the indicated time of contact with deuteriated acetonitrile at 40 Torr. The intensities relative to the original signal (taken as unit intensity) and the stick diagram corresponding to species Mo(C) are indicated. (B) Experimental (s) and simulated (- - -) vertically expanded X-band ESR spectra in the high-field range at 300 K of Mo/Nafion-30 for the indicated time of contact with deuteriated acetonitrile at 40 Torr. Extreme high-field parallel hyperfine components for species Mo(A), Mo(B), and Mo(C) and the A:B:C ratios used in the simulated spectra are indicated.

Figure 6. Oxidation of ethanol in the presence of oxygen on Nafion supports (A) and on Mo/Nafion-30 (B). For each support, the percent conversion and selectivity for the various reaction products are given.

that Nafion dried in the range 373-473 K contains the free sulfonic acid.32 We must conclude therefore that all ligands in Mo(A) are from the ionomer and, most specifically, from the sulfonic groups. The ESR parameters used for simulating species Mo(A) (Table 1) are typical of those of oxygen ligands, most notably g| < g⊥. Moreover, the g-tensor components are essentially identical to those deduced for hexacoordinated Mo5+ in reduced and oxidized MoO312 and in MoO(HSO4)52- 33 and are similar to Mo/silica catalysts,11 and to species Mo(C)/PAA, coordinated to four monodentate carboxyl bonds and one chelating COO group.14 In view of this evidence and the additional results explained below, we propose that in Mo(A) the cation is stabilized by five negative charges from the sulfonic groups in the ionomer

and an oxygen ligand from a chelating -SO3- group as shown in Scheme 2. In this way the most stable hexacoordination is obtained. In Scheme 2 we have suggested chelating groups in two possible positions, Mo(A)-I and Mo(A)-II, because we have no way to identify these two species separately. For the catalytic systems cited in Table 1,11,12,14,33 addition of water resulted in the transformation of the ESR signal to other signals. In the case of Nafion, however, addition of a small amount of water (1-5 Torr) has no significant effect, while the signal disappears completely when the water pressure is 25 Torr. At intermediate pressures (10 Torr) the gradual disappearance of the signal was observed and a new signal that accounted for e5% of the original signal was identified as Mo(A′). We propose that in the formation of species Mo(A′) a water molecule removes a chelating oxygen in the axial

2234 J. Phys. Chem., Vol. 100, No. 6, 1996 SCHEME 2. Molybdenum Centers in Nafion

position. This interpretation is offered because no substantial changes in the ESR parameters are expected in this case.9 It is also possible that this species was present upon water absorption but was obscured by the more abundant species Mo(A); we note the similarity of the ESR parameters for Mo(A′) to those for species Mo(C) in Mo/PAA, where the presence of a chelating -COO- group was demonstrated by IR.14 Water at larger pressures “unzips” the anions around Mo5+ and forms a diamagnetic molybdenum species. This process is expected to be facilitated and driven by the entropy gain and by the strong ionic interactions between the sulfonic groups and water.18,19 We note that the effect of EtOH is expected to be similar to that of water, and this behavior would explain the lack of an ESR signal in molybdenum-neutralized Nafion obtained by contact of the original membrane with an EtOH solution of MoCl5. In support of this logic are the results of electron spin echo (ESE) studies of Mo5+ on silica:7 only one hydroxyl group or one methanol molecule was detected in the cation coordination sphere on exposure to water or methanol, respectively. This description is in agreement with the recent classification of Mo complexes less in terms of oxidation states and more in terms of the coordination number, which is the number of ligands attached to the central atom.34 In this scheme the most stable and prevalent molybdenum complexes are of the type MoLlXx with l + x ) 6, where L is a ligand that donates two electrons to an empty orbital of the metal and X is a ligand that requires one electron from the molybdenum center to form a bond. In our case L is an oxygen from sSdO or from a chelating sSO3s group and X is the anion sSsO- in the absence of adsorbates. Replacement of the anions around the metal center by water molecules is a reductive replacement process.34 We emphasize that this explanation implies that the species obtained by replacement of two -S-O- ligands, MoL2X4, is expected to be diamagnetic. This is the reason why we do not detect the signal from Mo(A) unless we eliminate the solvent by heating, as clearly seen in Figure 1. The description of the molybdenum center as the cation surrounded by the anionic groups of the ionomer is in accord with the description of ionomers as a phase-separated system of polar and nonpolar domains26 and with the ligation of cations to the sulfonic groups only in the absence of water.18,26,35 The higher temperature necessary for the appearance of the ESR signal in Mo/Nafion might be due not only to the need to eliminate the ligands from the solvents but also to the energy necessary to organize the sulfonic groups around the cation, a process that is expected to be costly in terms of entropy changes. In support of this idea are the results obtained for neutralization of ethylene-methacrylic acid and ethylene-acrylic acid iono-

Kruczala et al. mers by a trivalent cation (La3+): the degree of neutralization achieved is ≈90% at ambient temperature but close to 100% when the materials are heated in vacuo at 490 K.36 Organization of the ligands around the cation in Nafion is most likely facilitated by the presence of the sulfonic at the end of relatively long pendant chains, while in the case of ethylene-acrylic acid ionomers the acid groups are directly connected to the main chain. In a study of Mo/PAA we have observed the disappearance of ESR signals of MoOCl5 in the presence of water due to the formation of an ESR-silent molybdenum dimer.14 In the case of Mo/Nafion it is hard to assume proximity of the cations, especially at the lower degrees of neutralization, because the sulfonic groups are spaced along the ionomer backbone. Moreover, the behavior at 30% neutralization is similar. Adsorption of DAN. The spectral changes that occur upon exposure of Nafion/Mo to DAN can, in principle, be explained in two ways: changes in the g| due to the spin-orbit coupling parameter λ of the ligand and changes in the symmetry of the complex. The spin-orbit coupling for nitrogen is less than that for oxygen, 76 cm-1 vs 152 cm-1,37 and the effect of a nitrogen ligand would be a decrease in g|, contrary to the experimental increase in g| on contact with DAN (Table 1). The effect must be explained therefore in terms of a change in the symmetry of the complex, for instance different bond lengths or bond angles. Of the two ESR signals detected upon exposure to DAN, we assign Mo(C) to the replacement of an L-type oxygen ligand in the axial position because the effect on g| and A| is less pronounced than that for species Mo(B), and Mo(B) is assigned to the replacement of an equatorial oxygen ligand by nitrogen. These assignments are based on the rule that g| and A| are affected by bonding in the equatorial plane and g⊥ and A⊥ by axial ligands.38 We do not rule out a possible rhombic distortion due to ligation to nitrogen, as suggested for Mo/SiO2 exposed to acetonitrile.8 This interpretation is compatible with the ligation schemes Mo(A)-I and Mo(A)-II proposed for Mo(A), (Scheme 2). The decrease and eventual complete disappearance of the signal are due to the same reason as that given for water: the unzipping of the sulfonic groups around the cation center. In support of this suggestion is the observation of ESR signals from Mo(A), Mo(B), and Mo(C) immediately after soaking Mo/ Nafion with DAN and the disappearance of all signals within 15-20 min. The higher values of g| observed for the signals attributable to complexes that contain nitrogen ligands are assigned to a lower ionic character (compared to oxygen), in agreement with data for a large number of molybdenum complexes11 and with the A| vs g| correlation shown in Figure 6B. In the case of Cu/Nafion, we have studied ammonia and acetonitrile ligands and have noticed that although all four inplane ligands in both cases were nitrogen ligands, the two complexes have different ESR parameters for Cu2+, mostly different g| and A| values.39 It is possible that the bulkier acetonitrile dictates a different symmetry in both Cu/Nafion and Mo/Nafion. Reversibility of the ESR Signal from Mo5+. In Mo/Nafion, to which water was adsorbed, we detected no changes in the signal at low water pressures, only the reversible disappearance for a water pressure of 25 Torr. In the Mo/PAA system we have proposed the formation of ESR-silent dimerization; this was possible because PAA is an electrolyte with one COOH group per repeat unit in the polymer. In Mo/Nafion, however, this argument is not valid because the ionic groups in Nafion are far apart and the cations are separated and dispersed. We

Ligand Design by Ionomers

J. Phys. Chem., Vol. 100, No. 6, 1996 2235 TABLE 2: A| vs g| Correlation in MoV Complexes 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

Figure 7. A| vs g| correlation diagrams for the paramagnetic molybdenum centers given in Table 2: (A) halogen ligands; (B) oxygen, sulfur, and selenium ligands. (MoO(NCS)5)2- (no. 18 in Table 2) was also included in part B.

propose to explain the reversible changes by the following process that involves changes in the ligation scheme of Mo:34

A| vs g| in Mo5+ Centers. The effect of oxygen and nitrogen ligands on the ESR parameters of Cu2+ complexes, and in particular the relation between A| and g|, has been considered and plotted in the Peisach-Blumberg diagrams.40 These diagrams are very useful for determining the number and type of in-plane ligands that are affecting the parameters of the parallel components (g| and A|). In the case of molybdenum centers, the existing “correlation” diagram is based on the values of g| and g⊥ because these are the values determined in most studies.9,11 This correlation is limited by the relative insensitivity of g⊥, so most data are spread along the g| axis only. Lately, however, simulation programs have allowed the determination of the hyperfine tensor components, A| and A⊥. In Figure 7 and Table 2 we present the A| vs g| correlation in Mo5+ centers, based on data collected in the literature and in the present study, and indicate the type of ligands. As in the case of paramagnetic Cu2+, the correlation for Mo5+ is very strong and when A| increases, g| decreases. The correlation between A| and g| for halogen ligands, Figure 7A, is very close to being linear. Point 2 in Figure 7A is for a complex with “mixed” ligands, four chlorine and two oxygen ligands, and is slightly below the linear correlation detected for the other ligands. In Figure 7B we present the data for oxygen,

complex

g|

A| × 104 (cm-1)

comments

(MoOF5)2MoO(COO-)Cl4 (MoOCl5)2(MoOCl4)MoOCl5 MoCl5(DMF) (MoOBr5)2(MoOI5)2MoO6 MoO(SO3-)5 (MoO(HSO4)5)2MoO6 (MoO(H2PO4)5)2MoN(SO3-)5 MoO(COO-)5 MoN(SO3-)5 MoO(SO3-)5 (MoO(NCS)5)2(Mo(S-p-tolyl)4)(MoO(SPh)4)(MoO(SEt)4)(MoO(SCH2Ph)4)(MoO(SePh)4)-

1.874 1.934 1.9632 1.967 1.967 1.968 2.090 2.274 1.876 1.877 1.881 1.882 1.891 1.892 1.902 1.903 1.907 1.932 2.016 2.017 2.018 2.021 2.072

92.9 74.0 74.7 79.0 75.1 75.1 66 50 88.5 91.5 92.0 91.3 90.0 86.8 83.7 84.2 82.3 68.6 52.8 52.3 52.9 52.8 48.3

ref 6 in ref 9 ref 14 ref 16,17 ref 42 ref 14 ref 14 ref 20 in ref 9 ref 21 in ref 9 ref 14 in ref 9 this study ref 33 ref 9 (Mo/SiO2) ref 33 this study ref 14 this study this study ref 17 in ref 9 ref 43 ref 43 ref 43 ref 43 ref 43

isothiocyanate groups -NCS, sulfur, and selenium ligands. We observe that complexes containing six oxygen ligands are clustered in the region of large A| and small g|. The effect of replacing four of the oxygen ligands by chlorine ligands is a reduction of A| and an increase of g|. We also notice strong effects when the electronegativity of the ligand changes, in both series: For five fluorine, chlorine, bromine, and iodine ligands, the range of g| is ≈1.85-2.3 and the range of A| is ≈50-95 (in units of 10-4 cm-1). The second series is obtained when oxygen ligands are replaced by nitrogen, sulfur, and selenium ligands. We expect that the correlation shown in Figure 7 be useful for determining the type of ligands in paramagnetic Mo complexes. It is important to mention that for chelating aminobenzenethiol ligands, the g tensor is isotropic and giso ) 1.99 and A| ) 11.9‚10-4 cm-1;41 these complexes have a dz2 ground state, and the symmetry is trigonal prismatic, in contrast to the complexes represented in Figure 7 and Table 2, which have a dxy ground state and a tetragonally distorted octahedral symmetry. Conclusions ESR spectra of Mo5+ in perfluorinated membranes (Nafion) neutralized by MoCl5 suggest that the ionomer acts as an ionselective medium and replaces the chlorine ligands of the cation by oxygen ligands from the sulfonic groups. This conclusion is supported by the detection with ESR of only one major paramagnetic molybdenum site, Mo(A), in the bulk ionomer in the absence of solvents. We have proposed that the signal is MoO(SO3-)5, where the unique oxygen ligand is from a chelating sulfonic moiety. Exposure of Mo/Nafion to water at a pressure of up to 5 Torr has no significant effect on the ESR parameters and on the intensity of the ESR signal from Mo/Nafion. Exposure to higher water pressures leads to the reversible disappearance of the ESR signal. Exposure of Mo/Nafion to acetonitrile (as CD3CN) results in the appearance of two new Mo species, Mo(B) and Mo(C), which are tentatively assigned to the replacement of one equatorial or one axial sSdO ligand, respectively, based on an analysis of the effect on the corresponding g| values. Soaking of Mo/Nafion samples with acetonitrile results in the reversible disappearance of the ESR signals, as detected for water.

2236 J. Phys. Chem., Vol. 100, No. 6, 1996 All transformations of the ESR signals are explained by the replacement of an L-type ligand, sSdO in the case of Mo/ Nafion, by oxygen from water or by nitrogen from acetonitrile. The loss of the signal in the presence of adsorbates was explained by a replacement of the sSdO ligand and of one or more of the sSsO- ligands by ligands from the adsorbates. This process is a reversible reductive replacement of the ionic ligands and leads to the concomittant loss of paramagnetism of the center. A plot of A| vs g| in paramagnetic molybdenum complexes was constructed, from data collected in the literature and in this study, and shows trends that can be used for identification of the number and types of ligands in paramagnetic molybdenum complexes. The control of ligands around the paramagnetic center, the involvement of the Mo/Nafion center in redox processes, and the accessibility of the center to adsorbates are promising features for catalysis. Preliminary catalytic tests on Nafion and Mo/Nafion supports suggest that for the oxidation of ethanol the presence of Mo leads to the appearance of a product, acetaldehyde, that is not detected in the presence of Nafion alone. Acknowledgment. This research was supported by a National Science Foundation grant (Polymers Program). References and Notes (1) Haber, J. In Molybdenum: An Outline of its Chemistry and Uses; Braithwaite, E. R., Haber, J., Eds.; Elsevier: Amsterdam, 1994; Chapter 10, p 477. Table 1 in this chapter is a comprehensive summary of reactions catalyzed by molybdenum compounds. (2) Pittman, C. U.; Evans, G. O. CHEMTECH 1973, 560. (3) Tsonis, C. P.; Farona, M. F. J. Organomet. Chem. 1976, 114, 293. (4) Pozniczek, J.; Kulszewicz-Bajer, I.; Zagorska, M.; Kruczala, K.; Dyrek, K.; Bielanski, A.; Pron, A. J. Catal. 1991, 132, 311. (5) Kulszewicz-Bajer, I.; Zagorska, M.; Pozniczek, J.; Bielanski, A.; Kruczala, K.; Dyrek, K.; Pron, A. Synth. Met. 1991, 41-43, 39. (6) Kruczala, K. Ph.D. Thesis, Jagiellonian University, Cracow, Poland, 1995. (7) Narayana, M.; Zhan, R. Y.; Kevan, L. J. Phys. Chem. 1985, 89, 636. (8) Zhan, R. Y.; Narayana, M.; Kevan, L. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2083. (9) Che, M.; Fournier, M.; Launay, J. P. J. Chem. Phys. 1979, 71, 1954. (10) Che, M.; Louis, C.; Sojka, Z. J. Chem. Soc., Faraday Trans. 1 1989, 85, 3939. (11) Louis, C.; Che, M. J. Phys. Chem. 1987, 91, 2875. (12) Dyrek, K.; Labanowska, M. J. Chem. Soc., Faraday Trans. 1 1991, 87, 1003. (13) Manoharan, P. T.; Rogers, M. T. J. Chem. Phys. 1968, 49, 5510.

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