Fourier Transform Infrared Spectroscopic Characterization of Olefin

Sep 15, 1999 - cm-1 for 4,4-dimethyl-1-pentene, and 57 cm-1 for 3-meth- yl-1-butene .... by the fraction of complexed olefin ([1 - A1640/A1640,0] or. ...
0 downloads 0 Views 99KB Size
Download PDF
Ind. Eng. Chem. Res. 1999, 38, 4051-4059

4051

Fourier Transform Infrared Spectroscopic Characterization of Olefin Complexation by Silver Salts in Solution S. Sunderrajan, B. D. Freeman,* and C. K. Hall Department of Chemical Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905

Complex formation between 1-hexene and silver salts in chloroform solution has been characterized using Fourier transform infrared spectroscopy. Silver tetrafluoroborate, AgBF4, and silver triflate, AgCF3SO3, form 1:2 Ag+:1-hexene complexes in solution. The olefinic CdC vibrational stretching frequency at 1640 cm-1 in 1-hexene shifts to approximately 1580 cm-1 upon complexation with these silver salts. Silver trifluoroacetate, AgCF3CO2, exhibits limited complexation and silver nitrate, AgNO3, exhibits no measurable complexation. Salts with the larger BF4- and CF3SO3- anions have a much stronger tendency to form complexes than those with the smaller NO3- and CF3CO2- anions. The tendency of these salts to form complexes in solution was most strongly correlated with anion size and was not well-correlated with measures of anion-donor electron density or softness parameter. The effect of ether-containing compounds, such as triethylene glycol (TEG) and poly(ethylene glycol) (PEG), on the silver-olefin interaction in chloroform solution was also studied. The formation of a silver ion-ether oxygen-olefin complex is consistent with the data. For TEG added to a solution of AgBF4 and 1-hexene in chloroform, there are approximately 2.5 oxygens per silver ion in the complex. For PEG added to a solution of silver triflate and 1-hexene in chloroform, there are approximately 1.8 oxygens per silver ion. Introduction Olefin/paraffin separation is the most energy intensive separation in the petrochemical industry.1 The conventional method for separating olefin/paraffin mixtures is fractional distillation. In addition to being energy intensive, this method is capital intensive, requiring a large number of contacting stages (over 100) and high reflux ratios (on the order of 10), due to the similar volatilities of olefins and paraffins.1,2 Alternative separation routes that are less capital and energy intensive are desirable. An alternative method for separating olefins and paraffins is chemical adsorption or chemical absorption using group I-B metal salts, such as silver tetrafluoroborate, AgBF4.3 These salts form complexes with unsaturated compounds such as olefins and acetylenes but not with saturated compounds such as paraffins.4 Complexes between group I-B metal salts and olefins form because of the favorable interaction between olefinic π-bonds and the group I-B metal σ- and π-bonds. Among group I-B metal salts, silver salts exhibit high olefin absorption.1 For example, Hughes et al. observed that an aqueous 4.0 M solution of silver nitrate, AgNO3, can absorb up to 4.4 g of ethylene/100 g of AgNO3 at an ethylene partial pressure of 1 atm, while the same solution absorbs approximately 200 times less ethane under the same experimental conditions. Chemical adsorption/absorption-based olefin/paraffin separation techniques, however, suffer from the disadvantage that water must be added to the process stream in order to activate the silver salt (solvate the silver ions) before silver ion-olefin complexation will occur.1 Water can be added to the system via humidified process streams,1 * Corresponding author. Telephone: (919) 515-2460. Fax: (919) 515-3465. E-mail: [email protected]. URL: www.che.ncsu.edu/membrane.

aqueous liquid membranes,5 or water-swollen facilitated transport membranes.1 However, these processes require undesirable humidification and dehumidification steps.1 If the process streams to aqueous liquid membranes and water-swollen facilitated transport membranes are not humidified, water evaporation leads to salt precipitation and decreased membrane performance over time.1 Solid polymer electrolytes made from blends of poly(ethylene oxide) [PEO] with silver salts, such as AgBF4, exhibit both high olefin solubility and high olefin/ paraffin selectivity even in the absence of water.6 Solid polymer electrolytes are polymer/salt solid solutions. The salt dissolves in the polymer through the interaction of salt cations with electron pairs on a heteroatom (e.g., the oxygen in PEO) in the polymer backbone. Silver salts such as AgBF4 dissolved in polymers such as PEO become solvated and are active for olefin complexation. For example, a PEO/AgBF4 film containing 80 wt % AgBF4 (which corresponds to a 1:1 molar ratio of oxygen atoms to silver atoms) exhibits a propylene solubility of 8.5 g/100 g of polymer electrolyte, which is 70 times greater than propane solubility (0.12 g/100 g) in the same film.6,7 In a membrane, the same mixture is more than 1000 times more permeable to propylene than to propane.8 Anion properties influence olefin solubility in PEO/ silver salt films. For example, PEO/AgBF4 and PEO/ AgCF3SO3 sorb 8.5 and 1.76 g of C3H6 per 100 g of polymer electrolyte, respectively, at 35 °C and 70 cmHg pressure. In contrast, PEO/AgNO3 only sorbs 0.52 g of propylene per 100 g of polymer electrolyte.6 These films contained 1 mol of oxygen atoms in PEO for each mole of silver atoms. Therefore, salt anion characteristics are important factors in determining olefin solubility. In this study, Fourier transform infrared (FTIR) spectroscopy is used to probe the effect of anion type on

10.1021/ie9900667 CCC: $18.00 © 1999 American Chemical Society Published on Web 09/15/1999

4052

Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999

Figure 1. Schematic of silver ion-olefin complexation. σ-bond results from the donation of electrons from the 2p bonding orbital of olefin to the vacant 5s-orbital of the silver ion, and π-bond results from the back-donation of electrons from the 4d-orbital of the silver ion to the unoccupied π*-2p antibonding orbital of the olefin.4

silver ion-olefin complexation in chloroform solution. The olefin considered is 1-hexene, and the silver salts were silver tetrafluoroborate, AgBF4; silver triflate, AgCF3SO3; silver trifluoroacetate, AgCF3CO2; and silver nitrate, AgNO3. Silver ion-olefin complexation is monitored through peak area measurements at 1640 cm-1, associated with the CdC stretching vibration of 1-hexene, and at 1585 cm-1, associated with the CdC stretching vibration of the olefin-silver complex, as increasing amounts of silver salt are added to a solution of 1-hexene and chloroform. FTIR spectroscopy is also used to study how the introduction of ether oxygencontaining species [such as triethylene glycol (TEG) and poly(ethylene glycol) (PEG)], which compete with the olefin for the silver ion, affects the complexation of the silver salt and olefin in solution.

Background Silver-Olefin Complex Formation. The structure and bonding of silver-olefin coordination complexes have been reviewed by Winstein and Lucas9 and by Beverwijk et al.4 A schematic illustrating the electronic structure of the complex between an olefin molecule and a silver ion (Ag+) is shown in Figure 1. A σ-type bond is formed by the donation of π-electrons from the occupied 2p bonding orbital of the olefin to the vacant 5s orbital of the silver ion. Concurrently, a π-type bond is formed by the back-donation of d-electrons from the occupied 4d-orbitals of the silver ion to the unoccupied π*-2p antibonding orbitals of the olefin. The sharing of electron density between silver and olefin weakens (i.e., lengthens) the CdC bond in the olefin, thereby reducing the CdC stretching frequency.10 This frequency shift can be detected by infrared spectroscopy. The formation and stability of olefin-silver complexes depend on the extent of orbital overlap between the silver ion and the olefin.10 The extent of orbital overlap is governed by the position of the silver ion with respect to the olefinic bond, which is determined by a combination of an electronic effect, a steric effect, and an anion effect.11,12 The electronic effect refers to donation of electron density by the olefin to the silver ion. Electron donation by the olefin and subsequent silver ion-olefin complexation lengthens the CdC bond and reduces the bond angle of olefinic carbons from values typical of planar sp2-hybridization (120°) to values similar to those of

tetrahedrally bonded sp3-hybridized carbons (109.5°).10 Upon complexation, the extent of olefin geometric perturbation increases as olefin π-basicity increases, since π-basicity reflects the ease with which electrons are donated by the olefin. In this regard, Quinn and Glew11 used FTIR spectroscopy to study solid-state fluorolube mulls of silver tetrafluoroborate-olefin complexes (1:2 AgBF4:2 olefin) for a range of olefins from ethylene to trans-2-butene. The double bond stretching vibration frequency change upon complexation increased linearly with decreasing olefin ionization potential. Ionization potential increased linearly with increasing olefin basicity, since basicity reflects the ease of electron donation. For example, the olefin double bond stretching frequency decrease is 40 cm-1 upon complexation with ethylene (which has an ionization potential of 10.5 eV) and 66 cm-1 upon complexation with trans2-butene (which has an ionization potential of 9.2 eV).11 Steric hindrance limits orbital overlap between the olefin and the silver ion and, therefore, makes the olefin-silver complex less stable (i.e., weaker).12 Steric effects oppose π-basicity effects in their impact on olefinic double bond stretching vibration frequency shifts. For example, in the absence of steric effects, the olefinic double bond stretching vibration frequency change upon complexation should increase with increasing branching because the π-basicity of the olefin increases with increasing branching.12 However, upon increasing substitution at the olefinic double bond in 1:2 silver tetrafluoroborate-olefin complexes, the olefinic double bond stretching vibration frequency shift was essentially constant at 56 cm-1 for 1-pentene, 56 cm-1 for 4,4-dimethyl-1-pentene, and 57 cm-1 for 3-methyl-1-butene, despite the increasing degree of branching of the alkyl substituents in this series of olefins.12 Typically, strongly interacting anions and cations form salts with high lattice energies.13 Lattice energy is the amount of energy required to melt the crystal structure of a salt.13 However, strong cation/anion interactions limit other interactions, such as silver ionolefin interactions. The formation of a silver ion-olefin complex is favored by weak cation/anion interactions and, therefore, low salt lattice energies.11 Weaker cation/ anion interactions and lower salt lattice energies are more typical of salts containing large anions.11 Additionally, silver salt-olefin complexation in the solid state is accompanied by lattice expansion to accommodate the olefin.11 Salts with large anions are able to accommodate olefins into the salt crystal structure easier than salts containing small anions.11 Quinn and Glew studied the formation and stability of olefin complexes with solid silver salts such as silver perchlorate, AgClO4; silver tetrafluoroborate, AgBF4; and silver hexafluoroantimonate, AgSbF6.11 Complex stability was measured in terms of the silver ion-olefin complex dissociation pressure. Complex stability increased in the order AgClO4 < AgBF4 < AgSbF6. The increase in complex stability was attributed to increasing anion radius, which is associated with larger lattice structures and lower lattice energies. Silver ion-olefin complex formation in solution is also affected by competing interactions from solvents.14-16 More basic solvents, i.e., solvents with a greater tendency to donate an electron pair, compete more effectively with olefin for coordination with silver than less basic solvents.14 Therefore, the more basic the solvent, the greater the competition between solvent and

Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999 4053

olefin for silver ion coordination. Hartley et al.15 determined the effect of solvent interactions on silver ionolefin complexation for several solvents: water, methanol, ethanol, and propylene carbonate. The equilibrium constant for silver ion-olefin complex formation decreased in the order propylene carbonate > water > methanol > ethanol, which is, as expected, in order of increasing basicity. Silver ion-olefin complex formation in solution is also affected by competing cation-anion interactions. The effect of silver ion-anion interaction on silver ion-olefin complexation was studied by Crookes and Woolf,14 who measured ethylene absorption in aqueous solutions of silver nitrate, silver trifluoroacetate, and silver perchlorate using a gravimetric absorption technique. At the same silver ion concentration, the ethylene absorption efficiency, defined as the ratio of the moles of complexed ethylene to the total moles of silver ion, decreased in the following order: silver perchlorate > silver trifluoroacetate > silver nitrate. Lower ethylene absorption efficiencies were attributed to stronger silver ion-anion interactions. Silver-Olefin Complex Stoichiometry. Silverolefin stoichiometry in the solid state was reported by Quinn et al. to be 1:2 (i.e. one silver atom for every two olefin molecules dissolved in the salt) for all olefins except ethylene, which exhibited 1:1, 2:3, and 1:3 complexes with solid AgBF4.11,12,17,18 The larger range of stoichiometries accessible to ethylene was attributed to its small size. Small ethylene molecules can complex with silver without significantly perturbing the salt lattice while larger olefins such as propylene or butylene significantly perturb the salt lattice structure upon complexation and thus exhibit only a single complex stoichiometry.14 Experimental Section Materials. The salts silver tetrafluoroborate (AgBF4, MW ) 195), silver nitrate (AgNO3, MW ) 170), silver trifluoroacetate (AgCF3CO2, MW ) 221), and silver triflate (AgCF3SO3, MW ) 257) were purchased from Aldrich Chemical Co., Milwaukee, WI, and used as received. Chloroform [CHCl3 (ACS HPLC grade)] and 1-hexene [C6H12 (ACS reagent grade)] were purchased from Aldrich and used as received. 1-Hexene was selected as a model olefin because it is the lowest molecular weight olefin that is a liquid at ambient conditions (normal boiling point ) 63.7 °C), and it has a single olefinic double bond.19 Low molar mass model polymer electrolytes containing ether oxygens were used to study the effect of competing interactions for silver ions between olefins and ether oxygens. The model compounds were TEG and PEG. TEG (MW ) 150, 99% purity) and PEG (average MW ) 600) were purchased from Aldrich and used as received. Sample Preparation. Chloroform was chosen as a solvent to prevent excessive infrared absorption by the olefin-silver salt solution.15,20 None of the silver salts used in this study dissolved appreciably in chloroform at ambient conditions. With the exception of AgNO3, however, the silver salts were readily soluble in a chloroform/1-hexene mixture (chloroform and 1-hexene are completely miscible under ambient conditions), presumably due to silver-olefin complexation. To prepare a sample for FTIR characterization, a known amount of salt was dissolved in a fixed quantity of a

chloroform/1-hexene (typically 1:1 v:v) solution in a 40 mL sample vial capped with a rubber septum. Silver salt was added to the chloroform/1-hexene solution incrementally in a dry nitrogen environment in a darkened glovebag since silver salts are hygroscopic and photosensitive.4,11,12 FTIR spectra were recorded after each addition of silver salt by withdrawing 0.2 mL samples of solution using a syringe and spreading the sample evenly over an ATR (attenuated total reflection) crystal in the spectrometer. After withdrawing each sample, the mother solution was replenished with 0.2 mL of a chloroform/1-hexene mixture of the same concentration as the original solution. Low molar mass model polymer compounds, TEG and PEG, were added to the solution of chloroform, 1-hexene, and silver salt prepared as described above. These experiments were performed by first dissolving the silver salt in a chloroform/1-hexene solution (1:1 v:v) and then adding the polymer. Sufficient silver salt was added to complex the olefin at a stoichiometric ratio of 1:2 Ag+:1-hexene. Infrared Spectra. FTIR spectra were recorded at room temperature using a Nicolet 750-Magna spectrophotometer. An ATR liquid cell with a ZnSe crystal was used. All spectra were obtained by averaging 256 scans at 1 cm-1 resolution. For peak area calculations, the reference peak was the C-H bending deformation peak of chloroform at 1200 cm-1. This peak is readily observed in most spectra and does not usually interfere with other peaks in the spectrum. The spectra for each series of experiments were normalized by matching the reference peak intensity in each spectrum. For the few spectra where the 1200 cm-1 peak was obscured by contributions from other species, the reference peak was a C-H stretching vibration of 1-hexene at 2850 cm-1. This peak remained unaffected by the addition of silver salts. Normalization of the spectra using a subtraction procedure was not necessary since an initial solution of constant concentration (typically 1:1 v:v chloroform and 1-hexene) was used throughout. The peak area calculation was performed by choosing wavenumbers suitably bracketing a peak position (such as the peak at 1585 or 1640 cm-1). For example, for increasing silver tetrafluoroborate concentration in a mixture of 4 mL of chloroform and 5 mL of 1-hexene, the peak area calculation at 1640 cm-1 was performed by measuring peak intensity values at wavenumbers between 1647.5 and 1632.5 cm-1. A linear baseline was constructed between the intensities at these two wavenumbers, and the area of the peak was calculated as the area enclosed between the baseline and the spectral peak. A similar procedure with the same wavelength limits was used in all successive peak area calculations for the same starting solution. These peak areas were then used to estimate complex stoichiometry. Results and Discussion Silver-Olefin Interaction. Figure 2 presents spectra for a mixture of 4 mL of chloroform and 5 mL of 1-hexene as a function of increasing silver tetrafluoroborate concentration. The peak at 1640 cm-1 represents the CdC stretching vibration of 1-hexene.11 With increasing amounts of AgBF4, this peak diminishes while a new peak at approximately 1585 cm-1 appears. This new peak, at lower frequency than the olefinic stretch in the absence of AgBF4, represents the CdC stretching vibration of the olefin-silver ion complex.12

4054

Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999

Table 1. Effect of AgBF4 Concentration on the FTIR Spectra of Mixtures of 4 mL of Chloroform and 5 mL of 1-Hexenea spectrum identification in Figure 2

AgBF4 added to solution (g)

nAg × 104 mol of AgBF4 added to solution

AgBF4 concn (g/cm3 solution)

mol 1-hexene/mol AgBF4 nolefin/nAg

a b c d e

0 0.67 1.27 2.48 3.84

0 34 65 127 197

0 0.07 0.14 0.27 0.43

∞ 11.6 6.12 3.14 2.03

a

R peak area A1640 A1585 3.16 2.63 2.00 0.93 0

eq 1 1640 cm-1

eq 2 1585 cm-1

1.95 2.25 2.22 2.03

2.28 2.12 2.18 2.03

0 1.10 1.94 3.88 5.60

Peak areas are calculated from Figure 2 for the peaks centered around 1640 and 1585 cm-1, respectively.

Figure 2. The effect of silver tetrafluoroborate concentration on FTIR spectra of mixtures of 4 mL of chloroform and 5 mL of 1-hexene: (a) 0 g of AgBF4, (b) 0.67 g of AgBF4, (c) 1.27 g of AgBF4, (d) 2.48 g of AgBF4, and (e) 3.84 g of AgBF4.

The shift in the olefinic stretching vibration in chloroform/ 1-hexene solution, from 1640 cm-1 to approximately 1585 cm-1 in the presence of AgBF4, is essentially equal to the shift for AgBF4-olefin complexes in the solid state, from 1640 to 1587 cm-1.12 If more AgBF4 was added to the solution than indicated in the caption of Figure 2, a solid precipitate formed in the solution. Additionally, the peak near 1585 cm-1 does not grow with higher AgBF4 concentrations than those indicated in Figure 2, suggesting complete complexation of the olefin by the silver ions. Presumably, any AgBF4 that could not complex with olefin precipitates, since AgBF4 alone is not soluble in chloroform. Table 1 presents peak areas between 1630 and 1647 cm-1 for the peak centered at 1640 cm-1, and between 1565 and 1610 cm-1 for the peak centered near 1585 cm-1 for each spectrum in Figure 2. The area of the peak centered at 1640 cm-1 is proportional to the amount of free 1-hexene in solution, and the area of the peak centered at 1585 cm-1 is proportional to the amount of complexed 1-hexene, consistent with Beer’s law.21,22 The number of moles of complexed olefin per mole of silver salt, R, is calculated as follows:

R ) [1 - A1640/A1640,0](nolefin/nAg)

(1)

R ) [A1585/A1585,f](nolefin/nAg)

(2)

or

where A1640 and A1585 are the peak areas for the peaks at 1640 and 1585 cm-1, respectively. A1640,0 is the area of the peak at 1640 cm-1 before the addition of AgBF4,

Figure 3. The moles of 1-hexene complexed by each silver ion, R, as a function of AgBF4 concentration based on data in Figure 2 and Table 1.

and A1585,f is the area of the peak at 1585 cm-1 after the maximum amount of salt has been added. The moles of silver salt and olefin in the solution are nAg and nolefin, respectively. The ratio A1640/A1640,0 is the concentration of uncomplexed olefin in solution relative to the initial concentration of uncomplexed olefin, since the peak at 1640 cm-1 is attributed to the uncomplexed CdC olefin stretching vibration. Similarly, the ratio A1585/A1585,f is the concentration of complexed olefin in the solution relative to the final concentration of complexed olefin, since the peak at 1585 cm-1 is attributed to the complexed CdC olefin stretching vibration. nolefin/nAg is the ratio of the number of moles of olefin in the solution to the number of moles of silver in the solution. The number of moles of complexed olefin per mole of silver salt is calculated in each case by multiplying the mole ratio (nolefin/nAg) by the fraction of complexed olefin ([1 - A1640/A1640,0] or [A1585/A1585,f]). Figure 3 presents R as a function of AgBF4 concentration. The error bars were calculated using a propagation of errors method.23 Values of R based on the peaks related to uncomplexed olefin and complexed olefin are very consistent, independent of AgBF4 concentration, and suggest that the stoichiometry is two olefin molecules for each silver ion. This result is in excellent agreement with the stoichiometry of solid-state AgBF4/ olefin complexes.12 Figure 4 presents spectra of chloroform/1-hexene mixtures as a function of silver triflate concentration. As AgCF3SO3 concentration increases, the peak at 1640 cm-1 (the CdC stretching vibration of the free olefin) diminishes while a new peak appears at 1587 cm-1. With further addition of AgCF3SO3, the peak at 1640 cm-1 eventually vanishes and the peak at 1587 cm-1

Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999 4055 Table 2. Effect of AgCF3SO3 Concentration on the FTIR Spectra of Mixtures of 4 mL of Chloroform and 4 mL of 1-Hexenea spectrum identification in Figure 4

AgCF3SO3 added to solution (g)

nAg × 104 mol of AgCF3SO3 added to solution

AgCF3SO3 concn (g/cm3 solution)

mol 1-hexene/mol AgCF3SO3 nolefin/nAg

a b c d e f g h i

0 0.4 1.15 1.47 1.85 2.50 3.39 3.89 4.49

0 16 45 57 72 97 132 151 175

0 0.05 0.14 0.18 0.23 0.31 0.42 0.49 0.56

∞ 20.6 7.15 5.59 4.45 3.29 2.43 2.11 1.83

a

R peak area A1640 A1585 2.60 2.20 2.14 1.87 1.18 0.81 0.36 0.23 0

0 0.29 0.49 1.23 1.78 1.97 2.70 2.92 3.04

eq 1 1640 cm-1

eq 2 1585 cm-1

3.09 1.29 1.57 2.45 2.27 2.09 1.92 1.83

2.06 1.14 2.29 2.58 2.14 2.16 2.02 1.83

Peak areas are calculated from Figure 4 for the peaks centered around 1640 and 1585 cm-1, respectively.

Figure 4. The effect of silver triflate concentration on the FTIR spectra of mixtures of 4 mL of chloroform and 4 mL of 1-hexene: (a) 0 g of AgCF3SO3, (b) 0.40 g of AgCF3SO3, (c) 1.15 g of AgCF3SO3, (d) 1.47 g of AgCF3SO3, (e) 1.85 g of AgCF3SO3, (f) 2.50 g of AgCF3SO3, (g) 3.39 g of AgCF3SO3, (h) 3.89 g of AgCF3SO3, and (i) 4.49 g of AgCF3SO3.

stops growing. The peak at 1587 cm-1 is due to the Cd C stretching vibration of the olefin-silver ion complex. Table 2 presents the peak areas between 1630 and 1647 cm-1 for the peak centered at 1640 cm-1, and between 1565 and 1610 cm-1 for the peak centered at 1587 cm-1 for each spectrum presented in Figure 4. Figure 5 presents R, the moles of complexed olefin per mole of AgCF3SO3, as a function of AgCF3SO3 concentration. The error bars were calculated using a propagation of errors method. AgCF3SO3 complexes with 1-hexene in the ratio 2:1 olefin:silver ion, which agrees with the reported solid-state stoichiometry of this complex.12 As with AgBF4, the complexation stoichiometry is independent of salt concentration. Similar studies were performed using silver trifluoroacetate and silver nitrate. While silver trifluoroacetate does exhibit limited interaction with the olefin, it does not clearly exhibit the disappearance of the uncomplexed CdC peak or the corresponding growth in the complexed CdC peak. The FTIR spectrum of silver trifluoroacetate overlaps both the uncomplexed CdC peak and the complexed CdC peak, which compromised a quantitative estimation of the olefin/silver complexation stoichiometry. AgNO3 did not dissolve in a dry chloroform/1-hexene mixture, even upon stirring overnight. The FTIR spectrum of the supernatant liquid was identical to that of the solvent mixture before the

Figure 5. The moles of 1-hexene complexed by each silver ion, R, as a function of AgCF3SO3 concentration in a 1:1 v:v mixture of chloroform and 1-hexene.

addition of AgNO3, so 1-hexene is not removed from the solution as a precipitated complex. Therefore, silver nitrate, unlike the other silver salts studied, does not complex with 1-hexene in chloroform solution. On the basis of these results, anion nature influences the formation and stability of olefin-silver complexes. Steric, electronic, and other anion characteristics such as anion softness may contribute to the observed differences in complexation behavior. Because these experiments are performed in chloroform solution, the overall equilibrium of the olefin-silver complex will involve contributions from the lattice energies of the salts as well as solvation energies for the various species (olefin, salt) in solution. However, the complexation stoichiometry for AgBF4 and AgCF3SO3 is exactly the same in solution as in the solid state and the IR spectral shifts observed upon complexation in solution are quite consistent with those observed in the solid state. On this basis, the solvent does not measurably influence the olefin/silver ion complex structure. Among the salts considered in this study, anion radius increases in the following order:24 CF3CO2- (0.156 nm) < NO3- (0.189 nm) < BF4- (0.232 nm) < CF3SO3- (0.256 nm). AgCF3CO2 appears to exhibit limited complexation behavior (though this conclusion is not certain given the spectral overlap difficulties), while AgNO3 exhibits no measurable complexation with 1-hexene. AgBF4 and AgCF3SO3 exhibit strong complexation. Thus, silver salt-olefin complexation appears to be favored by large anions, which are more easily solvated,13 a factor that

4056

Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999

increases the availability of silver ions for olefin complexation. In the solid state, silver-olefin complex stability increases with increasing anion radius, and larger anions typically yield a lower salt lattice energy, which, in turn, facilitates silver-olefin complexation.11,12 The distance between the silver ion center and the center of the closest anion atom (oxygen for NO3-, CF3CO2-, and CF3SO3- and fluorine for BF4-) in a vacuum was estimated using ZINDO-S quantum mechanics-based calculations. These distances increase as follows: 0.225 nm (Ag-O distance in Ag CF3CO2) < 0.228 nm (Ag-O distance in AgNO3) < 0.233 nm (Ag-F distance in AgBF4) < 0.237 nm (Ag-O distance in Ag CF3SO3). This order is consistent with anion size. AgBF4 and AgCF3SO3 exhibit strong silver-olefin complexation and the other two salts do not. Thus, the center to center distance between the silver atom and the closest anion atom appears to correlate to a certain extent with silver ion-olefin complexation. From this point of view, the larger the center to center distance, the easier the approach of the olefin to the silver atom, and the higher the propensity for silver-olefin complexation. Crookes and Woolf14 suggested that silver ion-anion interactions increase as anion-donor atom electron density decreases. The electron density at the donor oxygen (or fluorine) sites was estimated using resonance contributions by Pauling’s method.14,25 Electron density (in units of electronvolts) at donor oxygen (or fluorine) sites decreases in the following order: NO3- (-0.44) > BF4- (-0.72) > CF3SO3- (-1.13) > CF3CO2- (-1.58). Since silver ion-anion interactions increase as aniondonor electron density decreases and electron density decreases in the order given above, silver-olefin complexation should increase as silver anion donor electron density decreases. This, however, does not correlate well with our experimental results since silver triflate and silver tetrafluoroborate exhibit more complexation than silver trifluoroacetate. However, since silver trifluoroacetate exhibits limited complexation with 1-hexene (which is not consistent with the steric arguments given above but is consistent with the electronic arguments presented here), a combination of electronic effects and steric factors may influence olefin-silver complexation behavior. Anion properties are often described using a softness parameter, σ,13,24,26,27 a concept based on Pearson’s hard-soft acid-base (HSAB) theory. The HSAB theory has been used to rationalize lithium salt complexation with the ether units of polymer electrolyte hosts such as PEO.24,28 According to the HSAB principle, acids and bases that are large, easily polarizable, easy to oxidize, and have low electronegativity are soft while those that are small, less polarizable, difficult to oxidize, and highly electronegative are hard. The anion softness parameter is defined as27

σ ) (EAan - ∆Han) - (EAOH - ∆HOH)/(IPH - ∆HH) (3) where EAan is the anion electron affinity, ∆Han is the anion enthalpy of hydration, EAOH is the hydroxide ion electron affinity, ∆HOH is the hydroxide ion enthalpy of hydration, IPH is the hydrogen atom ionization potential, and ∆HH is the hydrogen ion enthalpy of hydration. Typically, softness parameters of hard anions are negative while those of soft anions are positive. For example, the softness parameters of the hard anions F-, Cl-, and

Table 3. Summary of Predicted and Experimental Silver-Olefin Complex Strength as a Function of Salt Properties property

silver-olefin complex strength

anion size Ag-anion separation in a vacuum electron density at the donor atom softness parameter experimental observation

CF3SO3- > BF4- > NO3- > CF3CO2CF3SO3- > BF4- > NO3- > CF3CO2CF3CO2- > CF3SO3- > BF4- > NO3NO3- > BF4CF3SO3 ≈ BF4 > CF3CO2- > NO3-

OH- are -0.71, -0.16, and 0.0, respectively.13 The softness parameters of soft anions such as SH-, SCN-, and B(C6H5)4- are 0.63, 0.84, and 6.86, respectively.13 The softness parameters of the anions used in this study are -0.41 for NO3- and -0.30 for BF4-.26 The softness parameter for CF3CO2- is not available, because of the absence of electron affinity data for this anion. However, the softness parameter of its isomorph, CH3CO2-, is -0.48.26 The softness parameter for CF3CO2- may be expected to be somewhat more negative than that of CH3CO2- because of the high electronegativity of the fluorine atoms. The softness parameter for CF3SO3- is unavailable, because of the lack of enthalpy of hydration data for this anion. Thus, the softness parameter values are expected to increase in the following order: CF3CO2- < NO3- < BF4-. Silver is a soft cation with a softness parameter of 0.126.24 According to the HSAB theory, soft cations form strong complexes with soft anions and vice versa. The anions in this study are all hard with hardness increasing in the order BF4- < NO3- < CF3CO2-. According to the HSAB principle, therefore, the weakest complexes would be formed between silver and CF3CO2-, followed by silver and NO3-, and finally silver and BF4-. The stronger the complex between the anion and the silver ion, the weaker the complex between silver and olefin. Thus, results based on the HSAB theory are inconsistent with our experimental results. The disagreement between HSAB theory and experiment may be due to steric effects interfering with effects modeled by HSAB theory. In the undissociated state, silver is closer to the nearest anion atom for the harder anions, CF3CO2- and NO3-, than for BF4-, and steric effects may play a more important role than softness parameter in inhibiting salt dissociation and subsequent olefin complexation with silver for silver trifluoroacetate and silver nitrate. Table 3 presents a summary of the complexation predictions according to steric, electronic, and anion softness effects and compares these to experimental observation. Steric effects (represented by anion size and silver-anion separation) are the best indicator of olefin-silver complexation, but since some limited complexation activity exists with CF3CO2-, this effect may not be the only factor influencing olefin-silver complexation. In this regard, the favorable tendency for complexation by AgCF3CO2 suggested by the electron density results may compensate for the much less favorable tendency to complex based upon steric effects. Anion softness does not appear to correlate with olefinsilver complexation. Silver Ion-Olefin Interaction in the Presence of Glycols. The addition of low molar mass model polymer electrolytes containing ether oxygens (such as triethylene glycol) to solutions of 1-hexene and silver salt in chloroform solution introduces competing interactions between the olefin and the ether oxygens to

Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999 4057

Figure 6. The effect of 1 mL of TEG on the FTIR spectra of mixtures of 4 mL of chloroform, 4 mL of 1-hexene, and 4.2 g of AgCF3SO3: (a) 0 g of AgCF3SO3, (b) 4.2 g of AgCF3SO3, and (c) 4.2 g of AgCF3SO3 + 1 mL of TEG.

Figure 7. The effect of 1 mL of PEG on the FTIR spectra of mixtures of 4 mL of chloroform, 4 mL of 1-hexene, and 2.98 g of AgBF4: (a) 0 g of AgBF4, (b) 2.98 g of AgBF4, and (c) 2.98 g of AgBF4 + 1 mL of PEG.

complex with silver ions. Figure 6 presents the FTIR spectra of a chloroform/1-hexene mixture containing silver triflate and TEG in the spectral region between 1650 and 1560 cm-1. Spectrum a in Figure 6 shows the uncomplexed olefin stretching vibration at 1640 cm-1 for a chloroform/1-hexene (1:1 v:v) mixture. Upon addition of sufficient silver triflate (4.2 g) to complex all of the 1-hexene in a 1:2 silver:olefin ratio, the peak at 1640 cm-1 disappears, while a new peak, ascribed to the silver-olefin complex, appears at 1587 cm-1, as shown in spectrum b in Figure 6. The addition of 1 mL of TEG to this solution (spectrum c) causes a decrease in peak intensity at 1587 cm-1 and the reappearance of a peak at 1640 cm-1. The addition of TEG forms uncomplexed olefin, suggesting that ether oxygens compete with olefins for silver ions. The addition of more TEG to the solution results in the formation of two immiscible liquid layers and a white precipitate. The clear, less dense phase contains chloroform and 1-hexene, while the cloudy, more dense phase contained TEG, silver salt, 1-hexene, and chloroform. The precipitate is believed to be a silver ion-ether oxygen-olefin complex, although we have no independent verification of this hypothesis. On the basis of the area increase in the peak centered at 1640 cm-1, approximately 10% of the olefin initially complexed with silver is decomplexed upon addition of TEG. Accordingly, this should correspond to a decrease in the peak area at 1587 cm-1, corresponding to the presence of the silver-olefin complex, of 10%. However, on the basis of peak area calculations at 1587 cm-1, approximately 30% of the silver-olefin complex is lost upon addition of the glycol. The difference between the apparent decrease in olefin concentration on the basis of total peak areas at 1640 cm-1 (10%) and 1587 cm-1 (30%) is attributed to the formation of a silver ion-ether oxygen-olefin complex that is insoluble in the solution mixture and precipitates from solution. If 20% (30%-10%) of the olefin, all of the added glycol, and 30% of the silver ions that are not complexed with the olefin alone (initially all of the silver is complexed; after glycol addition, only 70% appears to remain complexed) result in the formation of a silver ion-ether oxygen-olefin complex, one can obtain an approximate measure of the number of ether oxygens that complex with silver. On this basis, there are approximately 2.5

ether oxygens for every silver ion. Silver ion-ether oxygen coordination in solid polymer membranes has been studied by X-ray diffraction, and these studies suggest that silver ion-oxygen coordination obeys a 1:4 stoichiometry.29 Therefore, the stoichiometry of a silver ion-ether oxygen-olefin complex (or that of a silver ion-ether oxygen complex alone) in solution may be somewhat different from that in the solid state. However, the uncertainty in estimating the stoichiometry by this indirect procedure is rather large and more definitive studies should be undertaken to clarify this point. The uncertainty in the amount of silver decomplexed was calculated using a propagation of errors technique. The sources of error are (1) errors in weighing silver salt, 1-hexene, and TEG and (2) errors in peak area calculations. Of these, the uncertainty in the volumes of 1-hexene (0.5 mL out of 4 mL) and TEG (0.20 mL out of 1 mL) contributes significantly to the overall uncertainty in the amount of silver decomplexed. Assuming that all of the added glycol and the 30% of the silver ions that are not complexed with the olefin alone (100% - 70%) result in the formation of a silver ionether oxygen-olefin triple complex, a 20% error in the amount of TEG added can change the ether oxygen: silver ratio from 2.5 ether oxygens for every silver ion to 3.5 ether oxygens for every silver ion. An additional 2% error in the amount of silver ions not complexed with olefin, through a combination of errors in peak area calculations, and amounts of olefin and silver added can change the ether oxygen:silver ratio from 3.5 ether oxygens for every silver ion to approximately 3.8 ether oxygens for every silver ion, which is much closer to the result reported by Antonio and Tsou.29 Figure 7 presents the FTIR spectra of a chloroform/ 1-hexene mixture containing silver tetrafluoroborate, AgBF4, and PEG. Spectrum a in Figure 7 is the uncomplexed olefin stretching vibration at 1640 cm-1 for a chloroform/1-hexene (1:1 v:v) mixture. Upon addition of sufficient silver tetrafluoroborate (2.98 g) to complex all of the olefin at a 1:2 silver-olefin mole ratio, the peak at 1640 cm-1 disappears while a new peak appears at 1585 cm-1 (cf. spectrum b). The addition of 1 mL of PEG to the chloroform/1-hexene-silver salt solution (spectrum c in Figure 7) causes a decrease in peak intensity at 1587 cm-1 and the reappearance of a

4058

Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999

peak at 1640 cm-1. Like the TEG results, addition of more PEG to the solution yields two immiscible layers and a white precipitate. The clear, less dense phase contains chloroform and 1-hexene, while the cloudy, more dense phase contains glycol, silver salt, 1-hexene, and chloroform. The addition of either TEG or PEG to the silver salt solution seems to result in the formation of uncomplexed olefin. On the basis of peak area calculations at 1640 cm-1, approximately 15% of the olefin complexed with AgBF4 is uncomplexed upon addition of the glycol. Accordingly, this should correspond to a decrease in peak area at 1585 cm-1 of 15%. However, on the basis of the area of the peak centered at 1585 cm-1, approximately 40% of the silver-olefin complex disappears upon addition of the glycol. Like with TEG, the apparent decrease in olefin concentration in solution may arise from the formation of an insoluble silver ion-ether oxygen-olefin complex. On the basis of calculations similar to those for TEG, there are 1.8 ether oxygens for every silver ion in the hypothetical silver-olefin-PEG complex. A consideration of the uncertainties in this estimate suggests that the ether oxygen:silver ratio may be as high as 2.3, which is consistent with the TEG results but significantly below the 4:1 ratio reported for silveroxygen complexes in solid polymers.29 Conclusions FTIR spectroscopy was used to characterize silver ion-olefin complexation in chloroform solution. The formation of silver ion-olefin complexes is sensitive to the choice of silver salt. Silver tetrafluoroborate and silver triflate dissolve in chloroform/1-hexene solutions and exhibit silver ion-olefin complexation, as indicated by the strong shift of the CdC stretching vibration from 1640 to approximately 1585 cm-1. Silver trifluoroacetate appears to exhibit limited olefin complexation. Silver nitrate does not dissolve in the chloroform/1-hexene mixture used, and IR results suggest no complexation of olefin by this salt. Silver tetrafluoroborate and silver triflate complex 1-hexene in a 1:2 silver ion:olefin ratio, which is consistent with the stoichiometry for solid-state silver ion-olefin complexes. Anion steric effects, as measured by anion size and silver ion-closest anion atom separation, provide the best correlating parameter to determine if olefin-silver complexation will occur, but these effects are not the sole determinant of complex formation and stability. The favorable tendency for complexation by AgCF3CO2 suggested by an electron density calculation may partially compensate for its much less favorable tendency to complex based upon steric effects. Anion softness characteristics do not correlate with olefin-silver complexation. Low molar mass model polymer electrolytes such as triethylene glycol and poly(ethylene glycol) compete with olefin to bind with silver ions in solution, resulting in the formation of an insoluble silver-olefin-glycol complex. For TEG, the ether oxygen to silver ion ratio in the insoluble complex appears to be approximately 2.5 ether oxygens per silver ion. For PEG, the ratio is 1.8 ether oxygens for every silver ion. These values are lower than from results in solid polymer membranes, where silver ion-oxygen coordination follows a 1:4 stoichiometry.

Acknowledgment This work was supported in part by the National Science Foundation (Young Investigator Award CTS9257911-B.D.F.). We also appreciate helpful discussions with Prof. C. M. Balik. Dr. Angel Lozano kindly performed the ZINDO-S calculations. Literature Cited (1) Eldridge, R. B. Olefin/Paraffin Separation Technology: A Review. Ind. Eng. Chem. Res. 1993, 32, 2208. (2) Jarvelin, H.; Fair, J. R. Adsorptive Separation of Propylene/ Propane Mixtures. Ind. Eng. Chem. Res. 1993, 32, 2201. (3) Way, J. D.; Noble, R. D. Facilitated Transport. In Membrane Handbook; Ho, W. S. W., Sirkar, K. K., Eds.; Van Nostrand Reinhold: New York, 1992. (4) Beverwijk, C. D. M.; Kerk, G. V. D.; Leusink, A. J.; Noltes, J. G. Organosilver Chemistry. Organomet. Chem. Rev. A 1970, 5, 215. (5) Hughes, R. D.; Mahoney, J. A.; Steigelmann, E. F. Olefin Separation by Facilitated Transport Membranes. Recent Dev. Sep. Sci. 1986, 9, 173. (6) Sunderrajan, S.; Freeman, B. D.; Pinnau, I. Sorption and Spectroscopic Analysis of Silver-Olefin Interaction in Polymer Electrolytes. In Proceedings of the American Chemical Society Division of Polymeric Materials: Science and Engineering Amercian Chemical Society: Washington, DC, 1997; pp 267-268. (7) Sunderrajan, S.; Freeman, B. D.; Pinnau, I. Sorption and Permeation of Propylene and Propane in Solid Polymer Electrolytes. J. Polym. Sci.: Polym. Phys. Ed. Submitted. (8) Pinnau, I.; Toy, L. G.; Sunderrajan, S.; Freeman, B. D. Solid Polymer Electrolyte Membranes for Olefin/Paraffin Separation. In Proceedings of the American Chemical Society Division of Polymeric Materials: Science and Engineering; Cocuzzi, D. A., Ed.; American Chemical Society: Washington, DC, 1997. (9) Winstein, S.; Lucas, W. J. Coordination of Silver Salts with Unsaturated Compounds. J. Am. Chem. Soc. 1938, 60, 836. (10) Bochmann, M. Organometallics-2; Oxford Science Publications: Oxford, England, 1994. (11) Quinn, H. W.; Glew, D. N. Coordination Compounds of Olefins with Solid Complex Silver Salts 1. Coordination Compounds with Anhydrous Silver Fluoborate. Can. J. Chem. 1962, 40, 1103. (12) Quinn, H. W.; MacIntyre, J. S.; Peterson, D. J. Coordination Compounds of Olefins with Solid Complex Silver Salts 2. An Infrared and Nuclear Magnetic Resonance Investigation of Silver Tetrafluoroborate-Olefin Complexes. Can. J. Chem. 1965, 43, 2896. (13) Marcus, Y. The HSAB Theory. Israel Journal of Chemistry 1972, 10, 659. (14) Crookes, J. V.; Woolf, A. A. Competitive Interactions in the Complexing of Ethylene with Silver(I) Salt Solutions. J. Chem. Soc., Dalton 1973, 77, 1241. (15) Hartley, F. R.; Searle, G. W.; Alcock, R. M.; Rogers, D. E. Influence of Solvent on the Stability of Silver(I)-Olefin Complexes. J. Chem. Soc., Dalton 1977, 77, 469. (16) Baker, B. B. The Effect of Metal Fluoborates on the Absorption of Ethylene by Silver Ion. Inorg. Chem. 1964, 3, 200. (17) Quinn, H. W. Coordination Compounds of 1,3-Butadiene with Anhydrous Silver Tetrafluoroborate. Can. J. Chem. 1967, 45, 1329. (18) Quinn, H. W.; VanGilder, R. L. Phase Equilibria in the System: Silver Tetrafluoroborate-1-Pentene. Can. J. Chem. 1968, 46, 2707. (19) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids; McGraw-Hill: New York, 1987. (20) Solodar, J.; Petrovich, J. P. Behavior of Silver(I)-Olefin Complexes in Organic Media. Inorg. Chem. 1971, 10, 395. (21) Coleman, P. B. Practical Sampling Techniques for IR Analysis; CRC Press: Ann Arbor, Michigan, 1994. (22) Kendall, D. N. Applied Infrared Spectroscopy; Reinhold: New York, 1966. (23) Bevington, P. R. Data Reduction and Error Analyses for the Physical Sciences; McGraw-Hill: New York, 1969. (24) Gray, F. M. Solid Polymer Electrolytes: Fundamentals and Technological Applications; VCH: New York, 1991.

Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999 4059 (25) Pauling, L. The Nature of the Chemical Bond; Cornell University Press: New York, 1960. (26) Marcus, Y. On Enthalpies of Hydration, Ionization Potentials, and the Softness of Ions. Thermochim. Acta 1986, 104, 389. (27) Pearson, R. G. Hard and Soft Acids and Bases; Dowden, Hutchinson and Ross, Inc.: Stroudburg, PA, 1973. (28) MacCallum, J. R.; Vincent, C. A. Polymer Electrolyte Morphology. In Polymer Electrolyte Reviews; MacCallum, J. R., Vincent, C. A., Eds.; Elsevier Applied Science: New York, 1987.

(29) Antonio, M. R.; Tsou, D. T. Silver Ion Coordination in Membranes for Facilitated Olefin Transport. Ind. Eng. Chem. Rev. 1993, 32, 273.

Received for review January 25, 1999 Revised manuscript received July 20, 1999 Accepted July 26, 1999 IE9900667