16821
J. Phys. Chem. 1995,99, 16821-16823
19F and 27AlMAS NMR Study of the Dehydrofluorination Reaction of Hydrofluorocarbon-134 over Basic Faujasite Zeolites Clare P. Grey* Chemistry Department, S U M Stony Brook, New York 11 794-3400
David R. Corbin' DuPont Central Research and Development, f Wilmington, Delaware 19880-0262 Received: August 16, 1995; In Final Form: October 4, 1995@ Dehydrofluorination of hydrofluorocarbon-134 (CF2HCF2H) occurs over the basic zeolite NaX at 275 "C, to produce HFC- 1 123 (CFzCFW) and tetrahedrally-coordinated aluminum fluoride species. Under moist conditions, the tetrahedral aluminum fluoride species are hydrolyzed and six-coordinate species are formed. The framework of the less basic zeolite NaY is not attacked by HFC-134 under similar conditions.
As a result of the scientific evidence linking chlorine from chlorofluorocarbons (CFCs) in the atmosphere, with the seasonal ozone depletion over Antarctica, the Montreal Protocol and subsequent agreements have called for a complete phaseout of CFC production in developed countries by the year 2000.' Efforts around the world have been focused on developing processes to produce environmentally friendly altematives.1-3 The syntheses of these altematives are more complex than the syntheses of the CFCs involving many more steps, and unwanted hydrofluorocarbon (HFC) and hydrochlorofluorocarbon (HCFC) isomers are often produced. Consequently, the purification of the products remains a concem. In the synthesis of HFC-134a (CF~CFHZ), the replacement for the refrigerant CFC- 12 (CF2Clz), the isomer HFC-134 (CF2HCFzH) is a common byprod~ct.~ We have been studying the interactions of HFC-134a and HFC134 with basic zeolites, using gas chromatography (GC),in order to design methods to separate these two isomers. A correlation between the separation of HFC-134 and -134a and the Sanderson electronegativity of zeolite Y exchanged with monovalent cations has been established: the greater the electronegativity, the poorer the ~eparation.~ Dehydrofluorination of HFC-134 was observed in separations over one of the more basic zeolites studied, NaXe5 HFC-134 was not eluted from the GC column, and the dehydrofluorination product HFC1123 (CF2CFH) was observed. A magic-angle spinning (MAS) NMR study of the dehydrofluorination reaction over zeolites NaY and NaX was commenced, to probe the reaction more fully. The samples were prepared by dehydrating zeolite NaX or NaY under vacuum at 400 "C. A controlled amount of HFC134 was introduced, by syringing a known volume of gaseous HFC-134 into the vacuum line. The sample treatments are listed in Table 1. The excess loading samples were prepared by exposing NaX or NaY to an atmosphere of HFC-134, until no more gas was adsorbed. Some samples were rehydrated by exposing them to moist air from a saline solution for an hour. All samples were transferred in a glovebag into rotors for the NMR experiments. Spectra were acquired with CMX-120 and Bruker MSL-360 spectrometers, and the chemical shifts are quoted relative to CFC13 at 0 ppm. The Hartmann-Hahn condition for I9Fto 27Alcross polarization was determined with AlF3. The I9F MAS NMR of the HFC-l34/NaX system showed that partial decomposition of HFC-134 occurred after heat Contribution No. 7103. @Abstractpublished in Advance ACS Abstracts, November 1, 1995.
0022-365419512099-16821$09.00/0
TABLE 1: Loading Levels and Heat Treatments of the Zeolite Samples sample
zeolite
1Xa 1Xb 4x eXa eXb eY
NaX NaX NaX NaX NaX NaY
loading level (per supercage)
1 1 4
excess excess excess
heat treatment temp ("C) time (h) 275 0.083 275 1 275 4 275 17" 275 48" 275 17
2.8% and 8.3% F by weight for eXa and eXb, respectively, from fluorine elemental analysis.
treatment for only 5 min at 275 "C. New peaks, in addition to the resonance from HFC-134, were observed in the spectrum of sample 1Xa. Three resonances at approximately -110, -130, and -215 ppm contained fine structure and were a doublet, a triplet, and a triplet, respectively. These were assigned to HFC-1123. In addition, a broad peak centered around -195 ppm and sharper resonances at -201 and -254.5 ppm were observed. When the spectrum was acquired without MAS, only the peaks assigned to HFC-134 and HFC-1123 remained, demonstrating that all other species are rigidly bound to the zeolite framework. After heating for 1 h (lxb), the intensity of the I9F resonances that could be assigned to rigid fluorine-containing species increased (Figure la). An additional resonance was seen in this spectrum at -123 ppm. This resonance was not, however, observed in spectra of any other samples. After treatment for 4 h (sample 4X), further resonances became visible at around -153 and -158 ppm. Two distinct resonances were observed at -189 and -203 ppm (Figure lb). The former resonance was now the most intense (apart from the HFC-134 resonance). The sample darkened on heat treatment, indicating that some coking had occurred. The two peaks at - 189 and -204 ppm dominated the spectrum of the sample heated for 17 h eXa (Figure 2a). This sample was pumped down under vacuum at 275 "C (after the initial heat treatment) to remove the HFC-134. Further heat treatment for an additional 21 h ( e m ) did not result in a significantly different I9F spectrum, but the elemental analyses obtained for samples eXa and eXb clearly indicate that further dehydrofluorination had occurred. The resonances at -189 and -203 ppm are close to the chemical shift range observed for A1F4- species.6 I9F resonances have also been observed in this chemical shift range 0 1995 American Chemical Society
Letters
16822 J. Phys. Chem., Vol. 99, No. 46, 1995
DDm
1 " " 1 " " 1 ~ " ' 1 " " 1 " " ~ -50 -100 -150 -200
II
-250
-300
-IS0
DDm
1
-50
'
'
~
'
l
-100
'
"
'
l
- '150'
'
'
1
'
'
-200
'
'
-:50
" " 1
-300
Figure 1. I9F MAS NMR spectra of (a) 1Xb and (b) 4X. The resonance from HFC-134 is truncated in both spectra. The chemical shifts of the isotropic resonances are marked in ppm; *, spinning sideband.
A
-140
-160
,
-180
-180
f
-200
,
-220
-240
-260
Figure 2. Effect of rehydration on HFC-134 treated NaX. (a) I9FMAS NMR spectrum of eXa. (b) I9F spectrum of eXa after rehydration. The chemical shifts of the isotropic resonances are marked in ppm; all other peaks are spinning sidebands.
that have been assigned to F- ions in sodalite cages and to Al-F bonds in aluminum species in the channels of the zeolite.' The resonance at -203 ppm has a larger spinning sideband manifold
than the resonance at - 189 ppm, which suggests either a more distorted '9F environment, or stronger dipolar coupling to nearby nuclei. Assuming an Al(tet1-F bond length of 1.62 A,6 the contribution to the spinning sidebands from A1-F dipolar coupling will be as much as 17.5 kHz. We, therefore, tentatively assign the resonance at -203 ppm to an AI,,,-F species. The resonance at -189 ppm is assigned either to an aluminum fluoride species with considerable motion or to nonframework fluoride ions. The small peaks at -153 to -157, and -123 ppm, most likely result from Si-F bonds and Si&*- species, re~pectively.~ One of the lowest frequency I9F chemical shifts that has been reported as yet is that for NaF, at -225 ppm.* Hence, we tentatively assign the resonance at -251 ppm to a Na+F- ion pair. Sample eXa was hydrated and its I9FNMR spectrum is shown in Figure 2b. The resonance at -203 ppm has disappeared, and the intensity of the resonance at around -189 ppm has diminished considerably. A resonance at -180 ppm dominates the spectrum, and there is a smaller resonance at -172 ppm. All other resonances have disappeared. The resonances at -180 and -172 ppm are intermediate between the chemical shifts generally observed for fluorine atoms coordinated to octahedral and tetrahedral aluminum atoms. It is unlikely that the tetrahedral aluminum fluoride species will be stable in moist conditions, and we assign the resonance at - 180 ppm to fluoride ions associated with a hydrated zeolite framework and the resonance at -172 ppm to octahedral A1-F species. In contrast to NaX, the sample of NaY (eY) shows very little conversion of HFC-134 after heat treatment for up to 17 h at 275 "C. Some HFC-1123 is visible in the I9F MAS NMR spectrum but the only observable resonance from a rigid species, is a resonance at -254 ppm, close to the low-frequency peak in the NaX system (which we assigned to a Na+F- ion pair). No additional 27Al resonances, other than those from the tetrahedral aluminum atoms in the zeolite framework, were observed in the 27AlMAS NMR spectra of the HFC-l34/NaX samples. Presumably, the line width of the resonance from the tetrahedral aluminum framework atoms is so broad that any other intense resonances are obscured. 27Alresonances become visible in the octahedral region for aluminum at 1.3 and -36.4 ppm, however, in the spectrum of hydrated eXa. Integration of the spectrum gives a ratio of Al(oct):Al(tet) of 1.1:98.9. Assuming that all the leached out aluminum species are present as octahedral species and that they are visible by NMR, then 1.1% of the aluminum atoms in the framework have leached out on heat treatment for 17 h. In contrast, no resonances were observed from octahedral aluminum species in the 27Alspectrum of hydrated eY. Cross polarization (CP) from I9Fto 27Alwas performed, and a resonance was detected in the *'A1 CP spectrum of eXa at +47 ppm (Figure 3a). This resonance is slightly shifted from the resonance from framework aluminum atoms (at 60.5 ppm) in the 27Alblock decay spectrum of eXa and must arise from nuclei in close proximity to a I9Fnucleus. The chemical shift of this resonance is close to that observed in the solution NMR spectrum of A1F4- (49.2 ppm).6 We therefore assign this resonance to a tetrahedral aluminum fluoride species. This is consistent with the 19Fspectrum of this material. The *'A1 CP spectrum of the hydrated eXa shows two peaks in the octahedral region of the aluminum chemical shift range, at 0 and 9.1 ppm, which can be assigned to A1(H@)6-,FX species (x L 1, Figure 3b). The noticeable improvement in the signal-to-noise in (b) as compared to (a) is due to the much less distorted aluminum environments and consequently smaller 27Alquadrupolar coupling constants in the hydrated sample, which under MAS
J. Phys. Chem., Vol. 99,No. 46, 1995 16823
Letters 41
9
I'
PPm I
300
I
I
200
100
0
-100
-200
-300
Figure 3. 'W'Al CP MAS NMR spectra of (a) eXa and (b) rehydrated eXa, acquired with spinning speeds of 10 lcHz and short CP contact times of 50 ps (Le., half a rotor period). The chemical shifts of the
isotropic resonances are marked in ppm. conditions results in more efficient CP? The 2 7 A l quadrupole coupling constant is now sufficiently small that spinning sidebands, resulting from transitions other than the central transition, can be seen. The 29SiNMR of NaX and hydrated eXa shows that a small increase in the SUA1 ratio of the zeolite has occurred on treatment with HFC-134. The SUA1 ratios are 1.285(f0.01) and 1.32(&0.01), for NaX and eXa, respectively. This is equivalent to a loss of framework aluminum of 2.7%, which is more than was calculated from the 27Al NMR spectra. One possible reason for this is the overlap of Si(OH)(OSi)3-,(OA1), and Si(OSi)4-,(OAl),+l resonances.I0 Presence of significant concentrations of Si(OH)(OSi)3-,(OAl), groups will result in an apparently higher SUA1 ratio. In addition, some of the nonframework aluminum species may be tetrahedrally coordinated or may be invisible in the 27AlMAS NMR spectrum due to distorted aluminum environments. The SUM ratio of sample eXa, therefore, lies between the extremes of the two different measurements (i.e., 1.31 f 0.02). X-ray powder diffraction of eXa and eXb showed that a significant loss in crystallinity has occurred after HFC- 134 treatment, the crystallinity decreasing from samples eXa to eXb. HFC-134 is strongly bound to NaX at room temperature, presumably via interactions between the protons in HFC- 134 and the basic oxygen sites in the supercages of NaX and via an interaction of the fluoride atoms with the sodium cations in the supercages (sites I1 and In). The relative strength of the these two interactions, will effect the mechanism for the dehydrofluorination of the HFC-134. The Lewis acidity of the cation has been shown to be an important factor in governing the reaction mechanism and reactivity of dehydrochlorination reactions over X and Y zeolites." The reactivity of methyhalides (I, Br, C1) toward hydrocarbon formation over alkali-metal exchanged zeolites has been correlated with framework basicity; higher reactivity was, however, observed over alkali-earthexchanged zeolites, again suggesting that cation interactions play a significant role in controlling reactivity.12 Our preliminary
GC and NMR results have shown that dehydrofluorination occurs more readily over CsX than NaX and less readily over LiX.5 Hence, the basicity of the zeolite appears to be more important than cation polarizability in lowering the activation energy of the dehydrofluorination reactions over alkali-metal X zeolites. This suggests that the protonation of a basic oxygen site on the zeolite framework is the rate limiting step in the dehydrofluorination reaction over NaX: the aluminum atoms in the zeolite framework are then attacked by the fluoride ions, forming the tetrahedral aluminum fluoride species AlF,04-, ( x L 1). Attack of the zeolite framework does not occur in the case of NaY at these temperatures. 4-Coordinate aluminum fluoride species are less stable than 6-coordinate aluminum fluoride group^,^ and yet there is considerable evidence for the formation of the former species. However if x < 4, the aluminum fluoride species AlF,o4-, may still be coordinated to the zeolite framework, which may stabilize the 4-coordinated species. Altematively, the formation of an AlF63- anion may not be as favorable as the formation of the AlF4- anion given the negative charge on the zeolite framework. The tetrahedral aluminum fluoride species are no longer stable under moist conditions, and nonframework octahedral aluminum fluorides are formed. Our NMR results demonstrate that when studying the dealumination of zeolites, or the modification of surfaces via methods using, for example, fluorinated gases or Sick, extreme care must be taken in preventing moisture from entering the samples if correct dealumination mechanisms are to be deduced. Finally, we were able to detect aluminum fluoride species, present in low concentrations, that could not be detected in the block decay 27Alspectrum, with 19F-27AlCP NMR. This suggests that 19F27Al CP NMR will also prove useful in the study of other catalytic aluminum fluoride systems. Acknowledgment. G . C. Campbell and D. W. Reutter are thanked for their help in collecting the 19FNMR spectra and gas chromatography data, respectively. C.P.G. wishes to acknowledge financial support from E. I. DuPont de Nemourq and Co., and the National Science Foundation through the National Young Investigator program (DMR-9458017). References and Notes (1) Manzer, L. E. Science 1990, 249, 31. (2) Manzer, L. E.; Rao, V. N. M. Adv. Card. 1993, 39, 329. (3) Webb G.; Winfield, J. Chem. Brit. 1992, 28, 996. (4) Corbin D. R.; Mahler, B. A. World Patent, W.O. 94/02440, Feb
1994. ( 5 ) Corbin D. R.; Reutter, D. W.; Grey, C . P., unpublished results. (6) Herron, N.; Thom, D. L.; Harlow, R. L.; Davidson, F. J. Am. Chem. SOC. 1993, 115, 3028. (7) Delmotte L.; Soulard, M.; Guth, F.; Seive, A,; Lopez, A.; Guth, J. L. Zeolites 1990, 10, 778. Guth, J. L.; Delmotte, L.; Soulard, M.: Brunard, N.; Joly, J. F.; Espinat, D. Ibid. 1992, 12, 929. Klock, E.; Delmotte, L.; Soulard, M.; Guth, J. L.; Proceed. 9th Int. 2 0 1 . ConJ Montreal., von Ballmoos, R., Higgins, J. B., Treacy, M. M. J., Eds., 1992; Vol. 1, p 611. (8) Burum, D. P.; Elleman, D. D.; Rhim, W.-K. J. Chem. Phys. 1978, 68, 1164. (9) Vega, A. J. Solid State Nucl. Magn. Reson. 1992, I , 17. (10) Engelhardt, G.; Lohse, U.; Samoson, A,; Magi, M.; Tarmak, M.; Lippmaa, E. Zeolites, 1982, 2 , 59. (11) Klading, W.; Noller, H. J. Catal. 1973, 29, 385. (12) Murray, D. K.; Chang, J.-W.; Haw, J. F. J. Am. Chem. SOC.1993, 115, 4732. Murray, D. K.; Howard, T.; Goguen, P. W.; Krawietz, T. R.; Haw, J. F. J. Am. Chem. SOC.1994, 116, 6354.
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