160
PETERCANNON
VOl. 63
ADSORPTION OF FLUORIKATED METHANES BY LINDE MOLECULAR SIEVES BY PETER CANNON General Electric Research Laboratory, Scheneclady, N e w York Received September 86, 1868
The adsorption of CHClFz on Linde 4A Molecular Sieve has been measured, and the effect of the gas on the Sieve has heen defined by CO, ndsor tion measurements and by thermogravimetric, X-ray, mass spectrometric and classical analyses. The observed sorption of 8HClFz is m a l l and irreversible, and the CHClF, decomposes upon heating the system, leaving C o nin the lattice. The isotherms are type 1 (Brunauer’s classification). Extended calculations have been made for some of the isotherms. CHCIFz sorbs very strongly on Linde Sieve 5A (type 1 isotherm). On reducing the pressure of CHClF, over the solid, anomalous desor tion results are obtained, explicable only on the basis that CHCIFz decomposes on the Sieve a t 25”, to form more vola& materials. The results are consistent with the formation of small but significant amounts of c:wbon dioxide. Continued evacuation of the system results in loss of adsorbent weight but no gross collapse of the solid lattice is seen by X-ray diffraction. Analytic and thermogravimetric studies of the system have also been made. CHCIFz sorbs reversibly on an Alcoa F-1 alumina sam le, in marked contrast with the behavior on Liitde Sieve 5A a t the same temperature and pressure. The adsorption of e)CIzFz by Molecular Sieve 5.4 is slow a t pressures of the order 100 mm. No decomposition of CClZFz is seen in these ex eriments, in strict comparison with those results obtained from CHCIFz systems. It was found that CClzF, is strongly gouiid to the sieve substrate a t 25-50’, and is not removed readily by pumping. The non-equilibrium results are meaningful for an interpretation of the kinetics of CC12F2adsorption. The adsorption is thought to occur in two stages, the second of which is controlled by a pore field screening effect. Approximate energies for the adsorption process have been estimated from some of the data. The second stage adsorption gives heats in the range 0-10 ked. per mole adsorbed, the first stage involving even higher values.
Introduction 4A Molecular Sieve is n synthetic crystalline zeolite of the general formula Na2O.Al203.2Si0z. nHz0.l The synthetic zeolites differ from the natural zeolites (e.g., chnbazite) principally in their lattice geometry. Both the nat,ural and synthetic materials exhibit a remarkable selectivity of sorptive commonly supposed to be due to a very narrow range of physical dimensions and energetic properties of the pores. These may be altered, to a certain extent, a t will, by substituting the free cations in the pores. Thus, replacement of two Na+ ions by a Ca++ ion results in a net increase of the minimum cross sectional area of the pore. By such methods, 4 or 5A pores may be obtained in the zeolites. The data issued by the Linde Companya show the adsorption of various gases on solid materials, among the latter being Linde sieves 4A a,nd SA. Separations are chimed for CCIZF2-CC13F and for CClzFz-CzC12F4 mixture^.^ The minimum ratio of molecular radii for which separation is claimed is 1: 1.08 (CC12Fz,.CzClzF4). However, not only t’he relative dimensions but also the absolute size of the diameters are of importance. Thus, the ratio of the diameters of CClZFz and CCIzF,, and CClzF:, and CClsF are 1: 1.08 and 1: 1.11, respectively (calculated from refraction ,data), and the critical dimension of CC13Fis 4.9 A. Here then R separation is claimed of the molecyles CC12F2and C c 1 8 (with diameters 4.4 and 4.9 A.) on a 5A sieve. It is logical to suppose from this that not one but. both molecular diameters influence the separation. The diameters of the molecules of CCIZFZapd CHClF, have been calculnt,ed as 4.4 aiid 4.0 A., respectively, n s measured from plane projecttions of the moleculnr configurations, based on the n tomic (1) D. W.B r ~ l i ai, a!., . I . A m . Chew, ,SOT.. 7 8 , 5!ili3 (19aO). (2) R. ilI. Batrer nnil D. IV. Brook, Trans. Faradnu Soc., 49, 040 (1953). (3) hIoleoular Sieve D a t a Sliects. Linde Co., Tonnwoiida, N. Y. (4) Chemical Week, p. GO, Nov. 20, 1954.
radii cited by Neuberger.6 This model does not include possible polarization effects, but calculations based on the molar refraction values of the C-H, C-F and C-C1 bonds appearing in GILzsstonea do include these effects and give a ratio of molecular radii of 1.13: 1. This relative agreement shows the smallness of permanent dipole formation. A separation might then be expected for CClzFz a i d CHClF, on a sieve material with a pore size of 4.54.9 A. Even so, the precise pore sizes of the 4A and 5A sieves cannot be used to predict the exact adsorption behavior of gases on these zeolites. It is therefore necessary to make direct measurements of adsorption to determine whether a separation of CHClF, and CClzF2might be obtained using sieves 4A and 5A. There are few data in the literature on this subject, except for an observation by Barrer2 that CHClF? decomposes on natural chabazite whereas CClzFz does not sorb to a comparable extent. Breck, et uL,l have reported single points for CClzFzadsorption on Sieves 4A and 5A. Experimental Apparatus.--All the adsorption experiments were done on an automatic recording electromagnetic sorption balance. This instrument is operable over the pressure range 1 p-20 atmospheres. It is to be described in full elsewhere. Its sensitivity in the configuration used in this work was 0.05 mg./g., and the experiments were planned to produce a change in weight of the specimen of the order of 1 g. Other experiments involving storage of gases over solids were conducted in “Hoke” stainless steel flasks, equipped with “Hoke” RB231 or 2RB281 needle valves and stainless steel or prime brass pipe. Chemicals.-Carbon dioxide gas was freeze dried and vacuum distilled. Chlorodifluoromethane was prepared from tank grade material by low temperature distillation and stripping. The product purity was 99.3+% molar (determined by gas chromatography). Dichlorodifluoromethane was prepared from tank grade material by differential hydrolysis, freeze drying and vacuum distillation. The product purity was 99.8+% molar. 15) A. Neuberger, 2. Krzst., 93, 1 (1030) ( 5 ) S. Glasstone, “Textbook of Physical Cheiiiistry,” 2nd edition. p. 538.
Feb., 1950
ADSORPTION OF FLUORINATED METHASES BY LIXDEMOLECULAR SIEVES
The Linde Molecular Sieve samples were carefully quarter-sampled from single batches. Determination of the Surface Area of the 4A Sieve.The isotherm (Fig. 1) of the adsorption of carbon dioxide on 4A sieve a t 25' after 8 hours evacuation (10-2 p ) a t 350' is of a simple type. No adsorption hysteresis was observed. A Langmuir plot of the solption points was linear from 2 to 8510 mm. COZpressure (Fig. 2). Below 5 mm. pressure on desorption, a branch of the Langmuir plot was obtained corresponding to a lower available specific surface area than the main line (inset, Fig. 2). The value for the principal available specific surface area (calculated on the basis that carbon dioxide is present on the surface as a liquid two-dimensional film) is 566.6 sq. m./g. This is less than the value of 810 sq. ni./g. obtained by D. W. Breck, et d . , l using oxygen at low pressure and a t -183'. The observed adsorption isotherm is in agreement with the more limited data of the Linde C O . over ~ the relevant pressure range. Adsorption of Freon-22 (CHCLF?) on Sieve 4A.-The sample of Sieve 4A previously used for the COI determination of surface area was outgassed a t 350" for 15 hours. CHClF? gas was admitted to the system and a small type I adsorption wns observed a t the lowest preesures. Subsequent increases iii pressure to 8000 mm. gave slight further increases i n weight. The isotherm, corrected for buoyancy effects, is shown on Fig. 3 together with the Langmuir plot. A small, permanent, positive residual adsorption was observed 011 pumping the system a t 26' for GO hours. A "step" of constant pressure was observed at 23 p for 10 hows during t,lie pump down. A portion of the fouled sample, when analyzed for volatile matter, gave only carbon dioxide i n a niass spectroFetric run during which the sample wn,s heated to 1000 The amount of COn corresponded \vit'li 15 mg. COz/g. Sieve 4A, compared with a residual adsorption of 26 nig./g. No hydrochloric or hydrofluoric acid was seen. Halogen products subsequently were found i n tlic solid residuum where they presumably are present in a fully ioiiized form. I t seemed liiglily desirable to determine whether the carbon clioxide was formed as a result of heating the fouled sample, or was already present a.s such on the sieve 4A as a I,esult,of CHClFZ sorption. A sample of Sieve 4A (4 g.) was therefore outgassed at 350" for 15 hr. in a small st.ainless steel (31G) bomb. It was allowed to cool, and CHClF2 was distilled in (100 g.), A separate sample of CHClFz also was taken in a clean, empt'y, stainless steel homh. Both vessels were maintained a t 25" for three weeks. Infrared spectra of the gas phase in each homb were taken (the gas pressure was the saturation pressure of CHClF2 in each case). The treatcd sample showed a band a t 4.30 p (absorbance 0.64) which was n o t present in the untreated sample (adsorbance a t 4.30 p , 0.252). In addition, the treated sample showed a pair of less well marked bands a t 14.66 and 15.08 p which did not appear i n the spectrum of the clean material. The first band is assignahle to carboii dioxide: the other two have not been assigned, but they may represent some carbon-multiple chlorine grouping (-CCl?, -CC13) which was not present in the original material. Quantitative analyses of the gases were obtained by gns phase chromatography. The apparatus was a Perkin-Elmer "Vapor Fractonieter" ("J" column at loo"), and the sample size was 2 cc. of gas. C:Lrl,on dioxide was found to be present to the esteiit of less tlian 0.1% i r i the untreated sample, b u t was 0.4% of the t,reatcd CHClFz gas phase. Also found i n the rewtcd CHClFZ were II?O, O.l%, and CCl?F?,the content of which decreased froin 0.20 to 0.15% over Sieve 4A. The solid residue from the above experiment w a ~once more stored with purified CHCIFz for three weeks at, 2 6 O . No decomposition products at all w r e seen on this occasion. The tlecomposit,ion is thewfore nppnrently a +f-lilnit,ing surface renction. Thermogrnvinietric analysts of the CHCIFz-reacted saniple int1ic:ited thzt it took up less wnter on exposure t'o the :ttmosphcre lhan a clexn poi,tion used as a control. 'rho total mnterial volatilized between 100400" was less f o the ~ fouled sample than for the clenii material, indicating an over-all loss in surface activity. An X-ray diffraction pattern of the fouled material showed that no gross structural change had taken place in the sample fouled with CEIClFz a t room temperature. Hence it appears likely that the oxygen in the CO, product was in this case not t,emoved froni Si-0-Si links but froin
161 'INUES
0
ADSORPTION DESORPTION
t
# d
I
1000
250
CO,
Fig. 1.-Adsorption
500 P R E S S U R E . M M Hg.
730
of carbon dioxide on Sieve 4A a t 25".
4 .-c
_.
I
I
i
.
i i -A
1
2
co,
1 - 1 1 3 4 5 P R E S S U R E , YH ~g x 10-3
Fig. Z.-Lnngmuir
6
7
8
isotherm of COZ 011 Sieve 4A a t 25".
the constitutional water of the Sieve, present as terminal silanol or aluminol groups. COz Isotherms on CHClFz Treated Sieve 4A.--A portion of the sample which has been in conlinct with CHCIFl was restudied using carbon dioxide a t 25". The sample was pumped in the cold for more than one week t)o clean it as well as possible without baking it. 'The residual gas pressure was 0.5 p . Sorption was slow and the equilibrium values are less than those for COZon the clean 4A sieve. The Lnngmuir plot is shown with the isotherm on Fig. 4 . The isotherms with the reacted material showed apparent hysteresis loops when less than 8-24 hr. were allowed for each point. The extent of these apparent hysteresis loops was dependent 011 the previous maximum pressure of COZ whicli had Ileen attained. This behavior suggests the presence of small traces of permanent gas formed during the earlier sorption of CHClFz. The surface arcas corresponding witjh the two branches of the Langmuir plot are 220 and 200 sq. m./g., respectively, (assuming a liquid 2-dimensional model for the sorbed carbon dioxide). The thermodynamic work of compression to achieve a certain surface coverage has been calculated for
162
PETER CANNON
,026 ,024 -
I
I
I
I
1
1
C H C I F ~ PRESSURE, MMX 10-3
Fig. 3.-I,
h
adsorption isotherm of CHClFz on Sieve 4A a t 25"; 11, Langmuir plot of same.
I 100
Fig. 4.-I,
I
I
200 300 %PRESSURE,
I 400 YM. H I .
I
I
500
adsorption isot,herm of COz on CHClFz-treated 4A a t 25"; 11, Langmuir plot of same.
both substrates, aLid is less for the CHCIFZ-treated 4A a t all coverages. The available surface of Sieve 4A is apparently decreased and de-energized after contact with CHClFz. Sorption of CHClFz on Sieve 5A.-Since the data reported above for the adsorption of carbon dioxide on clean Sieve 4A were in substantial agreement with those issued by the Linde Company,6 it was unnecessary to make new measurements of the adsorption of carbon dioxide on Sieve 5A, and the surface area of the 5A sieve to carbon dioxide a t 25" was calculated from the published isotherm! A value of 592 sq. m./g. was obtained, using the Langmuir equation. A sample of Sieve 5A was outgassed a t 350°, as with 4A.
Vol. 63
The sorption of CHClFz on Sieve 5A was measured a t 25" (Fig. 5 ) . The results are highly unusual. The general form (Type I ) of the isotherm is similar to that with Sieve 4A: however, the adsorption of CHCIFz is much larger than with 4A. Equilibrium was attained quite rapidly: 90% of the weight increase for each dose was reached in less than one hour, and four hours only were required for experimentally steady values of weight increase to be reached. The isotherm was run up to the flat portion, and a t 163.9 mm. CHClFZ pressure a desorption cycle was commenced. Upon removing a little of the system gas, however, a large loss in weight was seen, placing the desorption curve below the adsorption plot. The divergence of the two curves became even more marked down t o a pressure of approximately 20 mm. Each desorption point took more than one day to equilibrate. Below 20 mm. pressure the desorption curve approached the adsorption curve, but still lay below and to the right of it. In this pressure region, the system equilibrium pi'essures "bunched" together, with continuously dropping sample weights. These "steps" in the isotherm were seen a t 11 and 6 mm. pressure. None of the plausible reaction products have a vapor pressure at 25" which corresponds with either of these values. At still lower system pressures, the apparent adsorbent weight became less than the initial sample weight. On torching the cold traps between Dumpings, a substantial amount of a white solid which melted well below 0" was removed; however, no etching of glass or attack on the brass lines leading in and out of the traps was seen. The absence of such degradative attack was taken to imply that water and acid fluoride products are not co-formed in the desorption. The experiment was discontinued when the sample had lost about 160 mg. of its initial weight. At' this stage, the system was still very gassy, the lowest pressure reached a t equilibrium after any one pumping being 23 p. The results have been plotted as observed (Fig. 5 ) and according to the Langmuir equation (Fig. 6). Using a liquid CHClFz model, the adsorption branch gives a value for the specific surface area of Sieve 5 4 to CHCIFz or 340.4 sq. m./g. This may be compared with the value found for CHCIFzon Sieve 4A of only 15.1 s m./g. When the gas evolved from a t e a t e d sample of the reacted material was dissolved in alkali, no halide ions were However, carbonate,. and found in the r e ~ u l t ~ a solution. nt only carbonate was found: the amount corresponds with a carbon dioxide content in the pumped-out solid of 8.3 mg. C%/ g. Sieve. comparison of the X-ray diffraction patterns of the snmple and a portion of clean material showed no significant differences in the over-all structure of the Sieve as a result of the CHClFz treatment, though an increase in amorphous background was seen. Therinogravimetric analysis curves indicated that the reacted sample evolved considerably less volatile material on heating to 400' after exposure to the atmosphere than a clean sample used as a control. It appears that the surface has been modified in such a way as to permit little water to be taken up from the atmosphere. The profiles of the TG.4 curves differ; that for the reacted material drops linearly from the very beginning of the heating cycle (25') to 400+" suggesti;g the presence on the lattice of materials volatile below 100 . The profile for the unreac1,ed material shows no drop in weight, commences until n temperature of 100" is attained. Adsorption of CHCIFz on Alumina -This system was studied for comparative purposes. The adsorption isotherm (25') of CHCIFz on alumina (Alcoa F-1), which had been outgassed a t 350" for 15 hours, is shown on Fig. 7, curve I , and the corresponding Langmuir isotherm is curve I1 of the same figure. The isotherm continued to rise with increasing CHCIF, pressure and did not become flat. This suggests strongly that the isotherm would actually become roncave toward the y-axis a t even higher pressures (i.e., of t,ype I1 or I V rather than type I ) . The Langinuii. plot is linear over a wide inteimediate rsrige of pressures, but not over all the observed pt'essure range, owing to this curvnture. The specific surfncc: area of the alumina sninple to CHClF, over the interrnediizte range of pressure IS 182 sq. m./g. The desorption curve was found to coincide with the adsorption branch. Thus this alumina does not decompose CHClF2 at 25". Sorption of CClZF2on Sieve SA.-D. W. Breck, et d . , l have published single adsorption points for the adsorption
Feb., 1959
ADSORPTION OF FLUORINATED METHANES BY LINDEMOLECULAR SIEVES
of CCIzF2on 4A and 5A sieves a t room temperature. The adsorptions, expressed as x / m values in g./g. were 0.003 on 4A and 0.084 on 5'4, a t $00 mm. pressure and "room tem1)erature." In t,he absence of any more definite information it was essential that these systems be studied further. However, the reported adsorption of CCI?Fx on 4A is so small that it was decided to study adsorption on 5A only, so that any peculiarities in the surface interactions in this t,ype of system might be magnified. The Sieve samples were degassed in the same way as before, and CC12F2w t s purified to better than 99.8% (mole ba.si8) before use. 1Yhc.n 1-2 hr. only was allowed for each point to be taken, spurious type I V isotherms were obtoined (Fig. 8, 25 :tiid 35"). Equilibration was subsequently found t80be vci'y slow between 5 and 800 mm. pressure. In each case the desorption behavior was different from that seen before. A hard vacuum f i ) was realized in a short time, but no great decrease in sample weight was seen. 111 fact, the total decrease in weight that was seen was of the order 0.01 g./g. On attempting to remove the CC12Fi from the 5A by heating and pumping, it vas found that heat had no effect u p to about 200". Subsequently some gas began to be removed but it was still difficult to remove any large amount until 300" was reached. Adsorption isotherms were also measured a t 29.6 and 40.0" (Fig. 8). In these experiments, considerably more time was allowed a t lower pressures, and the points came to equilibrium after 2 days each. As a result, these two isotherms tend more to a type 1 form than do the 25.0 and 35.1" measurements. The higher pressure regions of all four isotherms (pi more than 750 mm.) have a ready rationale, since they show the expected negative temperature coefficient of adsorption. also was studied. The saturation of the Sieve with CCI~FZ A samnle of Linde Sieve 5A was eauilibrated with CClzFz to and x ) m value of 0.193 by allowhg the sample to r&&i seven days in contact with the vapor at more than 2000 mni. pressure at room t,emperature. This sample was pumped down hnrd a t room temperature to a pressure of 0.01 p , brought to 35" and 3138.9 mm. pressure of CClZFz admitted. After one day, a small change in weight had taken plsce: the change in the sorption A x / m , between 0.01 and 3138.9 mm., was 0.0031, corresponding with extracrystalline sorpt'ion only. I t is clew that the internal surface of the Sieve was snturat,ed with respect to CCIZF, by that quantit>yof gas which had been adsorbed earlier. The system was once more pumped down in the cold, for a day. The sample n-eight returned to its previous value, corresponding wit,h ail x / n i of 0.103. In calculatiiig z / m values from the experimental data, buoyancy corrections were made for the volume of gas displacing by the solid higher pressures. These buoyancy and the density determinations i~evealrdthat aMiougli the sieve had been snturated with CClnFt, there was still an appreciable void space within it. The residual gas from the 29.G' experiment was analyzed by gas phase c:hromatography. It was identical with the stwting material: CCIzF2 is not deconiposed by Sieve 5A (nnd, by inference, not by Sieve 4A).
Discussion The Surface Chemistry of the CHC1F2-4A System.-The result8sare consisteiit wit'h a Langmuir t'ype adsorpt~ioiiof CHClFz on separate, non-interacting sites. The system appears to be alniost a model for this type of adsorption, except for the decomposition of CHClFz which sets in when the last sorbed residues are removed. The mngiiitutle of the observed adsorption makes it seem likeljr t h t J the process occurs oiily on the outsside of the niicrocrystnls. Consideration of the physicnl size of the OHCIFz molecule also makes this likely. Ilon.e\.er, the gradual increase in sorption a t higher pressures ninkes slow reaction or lattice peiietiat'ioii also possible. The reduced adsorpt'ioii of COZ 011 cold-pumped Sieve 4A after CHClF, coiit):ict ~ ~ ~ o t,hen u l d be explicable in either of two v'ays. First, some pores may be blocked a t all entrances by tightly bound CHClF,. I n this
163
2
r
sz i 2
,I
I -____i__---.-100
1
CHCIF2 P R E S S U R E . M Y . H a .
Fig 5.-..\dsorptioir : L i d clesorption isotherms of CHClFz on Sieve 5A a t 25". The anomalous desorption plot shows clearly the breakdown of CHCIFz o n the surface. 100, I
1 ~~~
I
I
1
ADSORPTION OF CHCIF? 0 DESORPTION OFCHClFz PRODUCTS Dlo.mb DESORPTION OF COn A IIo,llb
v v
I
100 CHCIFz P R E S S U " E . MM. Hg.
200
Fig. 6.-Langmuir plots for COz adsorption, CHCIFg adsorption and CHClFz desorption on Sieve 5A at 25
.
case, there would still be sufficient accessible pores that COZ adsorption a t lower pressures would be the same as on the clean sieve (Le., incomplete occupatioii of any given set of sites). Secondly, the sorbed CHCIF, may be coupled with t8heniolecularly inhomogeneous ionic field of the substrate, giving strong dipole formation, or have reacted to give a reduced internal field. This would also account for the permunent weight change observed 011 desorption, which would be due only t o those sites covered by the non-remov:ible GHClF, residues and would modify not only the rnagnitude of the
PETERCANNON
164
Vol. 63
the same temperature. This contrast emphasizes the extreme selectivity of these adsorbents, and the drastic changes that may be induced in them by cation exchange. The results of the adsorption experiment are not consistent with any type of system metastability involving only one gas component. Calculations made from the Langmuir plots of the adsorption and desorption isotherms (Fig. 7) indicate that slightly lower surface area is available on desorption if the species being removed is only CHClF,; assuming it to be wholly C02, then the calculation gives a higher area than with C 0 2on a clean 5A surface. The isotherms also indicate that the amount of material actually present on the surface is less on desorption than would be expected for CHCIFz but more than for COz alone. PRESSURE, Mu H p . The appearance of the desorption loop is more Fig. 7.-I, adsorption isotherm of CHClFz on Alcoa F-1 alu- nearly like that for Cot adsorption than for CHCIFz mina; 11, Langmuir plot of same. adsorption. The presence of CO, a t the solid-gas interface probably controls the desorption energies, and hence the downward isotherm. Sufficient COz RESIWAL WEIGHT ,------25"G ISOTHERM is present in the reacted system to cause the break in the Langmuir plot a t ca. 20 nim. pressure, which is also present in the isotherm of COZ on 4A. The Decomposition Reaction between CIJCIFz and Molecular Sieves.-This reaction has been noted previously7and would appear to be similar to that observed by Park, et al.,s who obtained COz on heating wet CHClFZ, the water in the present case being the constitutional water of the zeolite. However, the Sieve 5A/CHClF, system gives COz without heating. An additional anomaly exists since CHClFz would normally be expected to break down via hydrolysis to CO via an orthoformic acid intermediate, Plentiful CO is indeed formed a t higher temperatures in this system by heating CHCIFz and 5A Sieve in a bomb, but none was seen in the 25" sorption experiment. The mechanism appears not yet known since Barrer and Brook2 auggest dehydrofluoriiiation to explain the attack of CHClF2 011 natural zeolites, and Ayscough and Emeleusg remark the formation of COz and SiFr when CFI radicals are studied in quartz apparatus. The recent evidence of Neilson and White'" for I I I the nwociation of liquid CHCIF2, and the peculiar 500 1000 C C l z F 2 PRESSURE, MM Hp geometry of the Molecular Sieve lattice2 make it Fig. 8.-Adsorption isotherms of CC12F2 on Sieve 5A at 25 impossible to rule out an exchange mechanism and 3 5 O (2 hour points) arid a t 30 and 40" ( 2 day points). based 011 polarization of the CHClF2 in the prescase. Adsorbed CHCIF, behaves as a liquid on adsorption but also the energies involved. The ent Moleculnr Sieves, and hence will be slightly polarsecond model is consistent with the results. The Surface Chemistry of the CHCIFn-Sieve 5A ized. Also, in the process of enteTing a pore, a System.-The specific surface area of Sieve 5A CHCIFz molecule will pass within 2 A. of a calcium available to CHCIFZ (340.4 sq. m./g.) and the dif- ion and considerable additional polarization will ference between the liquid densities of COz and result. Even if formal charge separation is never CHCIFz a t 25" indicate that this CHClFz-available attained in sorbed CHCIFz, the unbalanced field of area should correspond with 572 sq. m./g. for Con. the aluminosilicate surfRce is sufficient to abstract a This result differs from that calculated from the fluoride ion, because of the very small distances Linde Company data for GOZ6by only 3%, the involved (M. J. Pryorll has shown that y-alumina liquid volumes of the two materials adsorbed are the surfaces possess sufficient electrostatic inhomogensame, the slight difference arising presumably from eity to dehydrate all hydrated halide ioiis). 111 packing discrepnncics. In the case of Sieve 5A, (7) P. Cannon, J. A m . Chem. Soc., 80, 17GG (1958). ( 8 ) J. D. Ptxrk, et d., Ind. Eng. Chem., 99, 354 (1947). the same amount of adsorption space is available to (9) P. B. Ayscough and H. J. Emeleus, .I. C h ~ mSoc., . 3381 (1954). either CHClFz or COZ, in contrast with Sieve 4A, (IO) E. F. Neilson and r). White, J . A m . Cheni. Soc., 79, 5620 where the specific surface area to CHCIF:! is only 15 (1957). sq. m./g. compared with 544 sq. m./g. for COZ a t (11) AI. J. Pryor, Z.f. Elsklrochsmis, t o be published. CHCIF,
ADSORPTION OF FLUORINATED ~ ~ I E T H ABYNLINDE E S ~VOLECULAR SIEVES
Feh., 1959
close proximity to a Molecular Sieve substrate, CHCIFZ can be sufficiently polarized that the fluorines can illidergo exchange reac tiona. The halogen has beeii fouiid on the lattice in ail ionized form by aiialysis. A conibinatioii of the dehydrohalogeiiation and polnrizntion mechanisms enubles oiie to arrive a t the CO, product, thus --..il--o-di-O--Al+ I 0 I
‘
-+
-11
c~-di-o-Al+
I
I --t
-11 ‘
OH
c1-
C~ICIFz
I
OH
I
COF,
(ii)
OH
E!i-o-dl+
F I
0)
I
Goz
+ H20
(iii)
F
The decomposition is small, of the order
