568
R. M. BARRER, N. MACKENZIE AND D. M. MACLEOD
VOl. 58
between parallel ridges. The top of each ridge will tions of the crystal structure2 nevertheless result in tend to be covered later when each valley has been additional cations being sparsely distributed along largely occupied by sorbate molecules. the channels, which, together with water molecules, As in the case of several other polar s0rbents~~r~9 provide high energy barriers opposing diffusion. the heat of sorption of nitrogen is initially appre- Moreover, unlike those in many zeolites, the chanciably higher than that of oxygen. This is also true nels in attapulgite are not interconnected. Where of the initial entropy decrease following sorption. this situation occurs the high energy barriers may, even if few in number, largely or almost wholly inDiscussion hibit entry into the crystal, by reducing the diffuIt now appears that in addition to substantial ad- sion rate to a negligible value.21 Sorption of nonsorption upon external surfaces H20, NH, and pos- polar molecules (Nz, 0,)and of COzmay neverthesibly (although to a lesser extent) CH30Hand CzHs- less occur a t or just within the entrance points to OH may enter the intracrystalline channels in atta- channels, before the first barrier is encountered, but pulgite. These channels are not however equally this, while providing some extra sites, does not available to Nz,O2 or CO2. Thus polarity rather amount to real lattice penetration.ls than molecular dimensions governs the effect. A Throughout the isotherm measurements a very rather similar behavior is found with the zeolites small but sometimes reproducible hysteresis was analcite or harmotome, which accommodate only observed. The slight extent of hysteresis is almost small polar species. Sorption is measurable within certainly associated with the physical texture of the a crystal only if there is a free energy decrease when crystals. These are in the form of thin lath-like occlusion occurs. &ken in channels so narrow that crystals, often in parallel, twinned configurations appreciable molecular distortion must result from as compact crystal clusters, but also in loose aggresorption the endothermic nature of this distortion gates. With such a texture wedge-shaped capilmay be exceeded by the exothermic interaction be- laries will be common which will fill and empty tween polar molecule and polar environment, and reversibly. Substantial hysteresis will be found the net value of AG is still negative for the polar spe- not with materials such as sepiolite or attapulgite, cies while it is positive for a non-polar which both have the same texture, but rather with However, the channels in attapulgite are, as spongy xerogels (porous glass,zz and some silica noted in the introduction, of considerable dimen- gelsz3). Flaky crystals such as montmorillonite sions, and in a perfect crystal should be able to ac- should tend to be intermediate in behavior between commodate small non-polar species. Imperfec- attapulgite and porous glass. (18) L. (1953).
E. Drain and J. A. Morrison, Trans. Faraday Xoc.,
49, 654
(19) R. M. Barrer, ibid., 40, 555 (1944). (20) R. M. Barrer, "Colloquea internationaux du CNRS," No. 19, "Adsorption et Cinetique heterogene," 1949, p. 27.
(21) R. M. Barrer and L. V. Rees. in preparation. (22) E.#.,R. M. Barrer and J. A. Barrie, Proc. Roy. Xoc. (London). S U A . 250 (1952). (23) E.#., "Advances in Catalysis," Vol. 4, Aoademic Press, New York, N. Y.,1952, p. 87, el 8eq.
SORPTION BY ATTAPULGITE. PART 11. SELECTIVITY SHOWN BY ATTAPULGITE, SEPIOLITE AND MONTMORILLONITE FOR TZ-PARAFFINS BY R. M. BARRER, N. MACKENZIE AND
(IN PART)
D. M. MACLEOD
The Chemistry Department, The University of Aberdeen, Old Aberdeen, Scotland Recsioed March 8, 1964
A study has been made of the sorption of n-heptane, iso8ctane and n-, iso- and neopentane on attapulgite; and of the pentanes on montmorillonite and sepiolite. Attapulgite and montmorillonite show selective sorption in the direction n-heptane > isooctane; and n-pentane > isopentane > neopentane. Sepiolite outgassed at 20 or 140" sorbed n- and isopentane equally strongly, but neopentane less strongly. I n the case of attapulgite selectivity has been investigated as a function of outgassing temperature, and has been found to decrease and disappear when the degassing occurs above about 88". selectivity reappears to an extent dependent upon the initial outgassing temperature on treatment with saturated water va or at room temperature. An analysis of the isotherms for attapulgite has been made in terms of changes in v, values a n a o f affinityconstants C, and in terms of free energies, heats and entropies of sorption. urn and C drop sharply for n-heptane and n-pentane when outgassing occurs above 88",but are little dependent upon outgassing temperature for neopentane and isooctane. Curves of AG,AH and A S against amount sorbed show the sorbent to be heterogeneous, and that all these uantities are initially greater in numerical magnitude for n-paraffins than for bran.m'ied chain paraffins. The selectivity hasxeen di6cussed in terms of these observations.
When mixtures of saturated n-paraffins such as n-hexadecane and n-tetracosane with analogous branched chain paraffins were dissolved in pentane and passed throughga column of attapulgite, Nederbragt and de Jong1v2found that the n- araffin'constituents were preferentially retained y the crys-
tals, while the isoparaffins were carried through. It was suggested by them that the n-paraffins might be entrained in the channels present in attapulgitesv4 but that the branched chain paraffins were not. However, the results of the previous paper4 indicate that even quite small non-polar molecules
(1) G. W. Nederbragt and J. J. de Jong, Rec. l m v . chim., 66, 081 (1946). ( 8 ) Q $W eNadar8ra#tl 4l-w Min, Rwll., 0 , 78 (aosa),
(3) C/. "X-Ray Identification and Structure of Clay Mitierah' blineraloirical SOC.,London, 1951 Chap. 9 and p. 237. (? R . MdBarrekRad N. Mrn&,,ii&?,THI.JOVRWAL,I S , ,569(1964Ja
E
July, 1954
SELECTIVITY SHOWN BY
ATTAPULGITE FOR n-PARAFFINS
such as nitrogen and oxygen are not appreciably occluded within crystals of attapulgfte. The sorption of single pure n- and isoparaffins has accordingly been studied to determine what measure of selectivity arises among their sorption isotherms and to find if possible what factors govern such selectivity. Experimental The gravimetric apparatus of Part I4 was used, according
569
d
to the procedure there described. The same attapulgite and sepiolite crystals were employed, and also crystals of montmorillonite obtained from the Macaulay Institute for Soil Research. The montmorillonite had a base exchange capacity of about 100 meq. per 100 g. The main exchangeable cation was sodium, but there was also about 15% of calcium. Before use the attapulgite and sepiolite were outgassed at various temperatures, while the montmorillonite was outgassed a t 50°, a temperature adequate for removal of ractically all the inter-laminar water.' %e sorbates studied were n-heptane (from hevea pine, of engine test standard), is&ctane (2,2,4-trimethylpentane) of the same standard; and also n-, iso- and neopentane. The latter three hydrocarbons were first obtained as Phillips research grade pure chemicals and ultimately from the Chemical Research Laboratory, Teddington. The purity was then above 99.5%.
Sorption of n-Heptane and Isotktane.-Although the boiling points of n-heptane and of isooctane are nearly the same it was soon apparent that n-heptane showed the greater affinity for attapulgite. Figures l a and b give the isotherms of nheptane and of isooctane at 50 and 65" on attapulgite initially outgassed at 70". There is not a large temperature coefficient in the sorption, so that the heat of transfer from liquid n-heptane or isooctane to the surface of the crystals is small. Equilibrium was established quickly as with nitrogen or oxygent4 and hysteresis was only slight. Last traces of nheptane were less easily removed than were those of isooctane. The isotherm curves at 50" were thoroughly investigated in a comparative manner, using various theoretical isotherms as extrapolation formulas to obtain the monolayer values vm and the affinity constant C , both these quantities being defined just as in the BET theory.E The results are seen in Table I and show that vm and C are both larger for the nparaffin than for the isooctane. TABLE I Isotherm desigoationa
n-Heptane urn, cm.8 a t H.T.P./g.
c
Isotjctane om.' at S.T.P./g.
urn,
C
BET6 Huttig' A B D
68 5.86 11.6 8.90 9.88 40 7.43 8.1 9.65 45 6.51 9.7 7.86 100 4.92 13.8 64 4.76 13.0 9.09 H 9.55 42 6.43 9.3 10.51 24 7.15 7.5 K TheimeId 10.80 24 8.08 6.6 Chu Liango 8.99 .. 5.43 .. a Isotherms A to K are those correspondingly designated by Barrer, Mackenzie and MacLeod.10 (6) R. M. Barrer and D. M. MaoLeod, in preparation. ( 6 ) 9. Brunauer, "Physical Adsorption of Gases and Vapore," O.U.P., 1945, Chap. 6. (7) G. F. Huttig, Monatsh., 78, 177 (1948). (8) 0.Theimer, TTanr. FaTadav Soc., 48, 326 (1952). (9) 8. Chu Liang, Tms JOURNAL, 66, 1410 (1961). (10) R. M. Barrer, N. Mackenzie and 8,M, MrraLeod, J , Chmm &ier., a718 (IO~P);4184 ( & o m ) .
0.2 0.3 0.4 0.5 0.6 0.7 0.8 Relative pressure. Fig. 1.-Sorption isotherms of (a) n-heptane and (b) isooctane on attapulgite outgassed at 70': 0 , sorption a t 50'; 0 , desorption at 50"; A, sorption at 50". 0.1
The area per molecule of sorbed heptane has been given as 6411and as 5912 sq. A. Using scale models, areas of 54 sq. A. for n-heptane and 52.5 sq. A. for isooctane were obtained. Such areas tend to be low; multiplying each by 1.1 gives an area of 59.4 sq. A. for n-heptane (in line with Livingston's figure12) and 57.8 sq. A. for isooctane. These areas are very similar, and the marked differences in om in Table I therefore suggest that a larger surface is available for n-heptane than for isooctane, owing to some configurational peculiarity of the surface. Apart from this the C constants of Table I show that n-heptane molecules in the first layer have the greater affinity for the crystals. Two factors therefore increase the selectivity toward the n-paraffin. The Pentane Isomers.-The sorption of the three pentane isomers confirmed the results obtained with n-heptane and isooctane, the order of selectivity in sorption being n-pentane > isopentane > neopentane, provided the initial outgassing was carried out below 88" (see below). If however the crystals were outgassed at higher temperatures the selectivity was reduced almost to nothing (Fig. 2a). It also was found that the same order of selectivity occurred when montmorillonite (outgassed at 50") was used as sorbent at 50" (Fig. 2b). The quantity of each isomer sorbed ia however very much smaller, corresponding to a much reduced surface of the sorbent. When sepiolite was the sorbent, whether water removal had occurred by outgassing at 20" or a t (11) E. H. Loeaer and W. D. Harkina, J . Am. Chem. Soc., '71,3427 (1960). (12) R, K.bivinwtou, J. Colloid Boi., 1,447 (1949).
570
R. M. BARRER, N. MACKENZIE AND D. M. MACLEOD
Vol. 58
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0.3 0.4 0.5 0.6 0.7 0.05 0.1 0.15 0.2 0.25 0.30 0.35 Relative pressure. Relative pressure. Fig. 2.-(a) WCSHIZ (a), iso-CbH12 ( A ) and neo;CoHlz (a)at 20" on attapul+giteoutgassed at70". Also n-CbHl? (01, iso-CaHl2 (A) and neo-CaHlz(m) at 20' on attapulote outgassed at 215", shomng loss of selectivity. (b) Sorption of isomeric pentanes at 50" on montmorillonite outgassedat 50'. 0 , A and 8 ,sorption points; 0 , A and m,desorptionpoints. 1, n-CsH12; 2, iso-C6H12;3, neo-CsHlz. 0.1
0.2
140", the isotherms showed no selectivity as between ?%- and isopentanel but both these paraffins were a t given p / p o more copiously sorbed than neopentane. Sorption equilibria were rapidly established, suggesting that, as with n i t r ~ g e n ,lattice ~ penetration was not important. The amounts sorbed were however large, as with attapulgite. Sorption and Outgassing Temperatures.-Sorption isotherms of each pentane isomer were measured a t 20" on the attapulgite crystals outgassed a t 20, 70, 88, 110, 124, 130, 260 and 360°, a selection of the data being given in Fig. 2a. Surface areas were next determined from all the isotherms using the BET linear plot to find urn and C for each isomer (Table 11). In these calculations the surfaceoarem used were 49.7 sq. A, (?-pentane), 50.4 sq. A. (isopentane) and 45.5 sq. A. (neopentane), Scale models of each molecule were used to find the relative molecular areas, and then each area was multiplied by the factor required to bring the +pentane area up to the value of 49.7 sq. A., as given by Corrin.18 According to Table 11there is a sharp drop in the area available for sorption of npentane and of isopentanel but not in that available to neopentane, for outgassing temperatures between g8 and lloo* This is paralleled by changes in the affinity constant, C, for n- and isopentanes. The close relationship between the temperature coefficient of water 10% affinity constants ci surface areas and amounts sorbed a t relative pressures of 0.1 is brought out h Fig. 3, where each of these quantities is s h o w in relation t o outgassing tern(la) M.L. Coria, Tare JOWBNA&,66, 812 (1961).
EFFECTOF
y2$,s$Ef 20 70 88 llo 124 130 260 380
TABLE I1 TEMPERATURE ON SURFACE SORBENT
OUTaASSINQ n-Pentane
172 170 170 114
60 58 39 18.0 9.6
Isopentane
m.z,;. Area
155 153 154 115
ol 111
95
11,6 11.1
21.4 16.8 18.3 12,6
103
11.5 9.7 8.2
OF
Neopentane
m.z,g, Area,
112 105 104 98 97 92 95 93
14.8 14.9 13.6 13.1 13.5 11.8 9.9
perature. It is only the low temperature stage of water loss which is coupled with marked changes in the other quantities. Similar behavior was also o b m ~ e dwhen the sorbates were Nz,0 2 and co2 (Part 14) Moreover, as shown Fig- 41 the pulgite recovered its Selectivity if Crystah3 initially degassed above 88" were exposed to saturated water vapor and then again degassed a t a temperature below 88". The extent of recovery was less the higher the initial o u t g a s s ~ g temperature above 88". Functions.-The for transthermodynamic functions A@, AH and fer of hydrocarbons from the liquid t o the sorbed state were calculated as in part 1.4 The relationships between these functions and the surface coverage on attapulgite crystals are shown in Fig. 5 for n-heptane and isoactane, and jn Fig, 6 for the isomeric pentanes. me sorbent was here outgassed below 88".
July, 1954
SELECTIVITY SHOWN BY ATTAPULGITE FOR %-PARAFFINS
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50 100 150 200 250 300 350 Outgassing temperature, "C. Fig. 3.-Correlation of various properties in pentane sorption with outgassing temperature: 0 , n-CsH12; A, iso-C6H12; 8 , neo-CsHl2: 1, water loss from attapulgite; 2, C value; 3, surface area; 4, amount sorbed a t relative pressure 0.1.
The free energy, heat and entropy changes on sorption are clearly much greater for n-heptane than for isooctane. I n particular the entropy decrease on transferring isooctane from liquid to adsorbed film is very small, while the corresponding decrease for n-heptane is initially extremely large. This suggests that for n-heptane the configurations of the molecule on the surface may be very restricted. This will be the case if the n-heptane molecule closely fits its environment in the surface. Large initial heat, entropy and free energy changes are associated also with the uptake of n-pentane. On the other hand for iso- and neopentanes, like isooctane, the three thermodynamic functions are initially smaller. The branched chain molecules studied have fewer possible configurations in liquid or vapor states than have their more flexible straight chain counterparts. If all molecules when sorbed are constrained to one configuration the entropy decrease for the n-paraffin will on this count exceed that for the branched chain species. However the constraint on sorption of branched chain molecules does not in fact seem very large, as shown for instance by the small entropy change for isooctane. It seems therefore that the first n-paraffin molecules sorbed in the process lose a relatively greater fraction of their already more numerous gaseous and liquid phase configurations than do branched chain params.
Discussion The loss of selectivity when the crystals of atta-
0
0.2 0.3 0.4 Relative pressure. Fig. 4.-Regeneration of selectivity in sorption of pentanes a t 20" by treatment of attapulgite outgassed a t 130 and 360" with saturated water vapor a t 20". For comparison the isotherm of n-Cs&2 on a fresh sample outgassed a t Z O O , and isotherms of the three pentanes on a sample outgassed a t 360°, are shown: 0 , 0 ) n-pentane; A , A, isopentane; 8, M, neopentane.
0.1
pulgite are outgassed above 88' is a process associated with decreasing vm and C for n-pentane, but with little change in these quantities for neopentane, while isopentane behaves in an intermediate manner (Fig, 3). A reason for the higher energy and greater entropy decrease on sorption of n-paraffins as well as of the decrease in vm and C may be sought in terms of the structure of the crystals. These according to Bradley's model14should have surface corrugations4 of molecular dimensions, in which it may be that only n-parafflns may fit closely provided the crystals are degassed below 88". Moreover, a very limited entry into the mouths of the intracrystalline channels may be possible for simple non-polar molecules and for n-parafflns thereby providing some additional energetically sorbing sites for these species, whereas more globular branched chain molecules cannot avail themselves of these sites. Such a process of entry would be limited to positions just a t or very near the surface because of permanent adventitious cations, arising from crystal defects and causing barriers in the channels. The entrance points to channels have then to become blocked to n-paraffins and the availability of corrugations also be impaired if outgassing is above 88". This could result from a process of surface sintering and reaction, for example if some hydroxyl water were eliminated and caused (14) W.
F. Bradley, Am.
MSnerdogW. 98, 406 (1040).
R. M. BARRER, N. MACKENZIE AND D. M. MACLEOD
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and for some ion-exchanged montmorillonites.6 In the former case a. similar explanation to that above VIVIn. has been offered. The difficulty which this view Fig. 5.-Some thermodynamic functions for (a) n-he tan: and (b) isooctane, calculated from isotherms at 50 a n 8 6 5 . encounters as an interpretation also for the data obtained with attapulgite is the drop in this case in Si-0-Si bonds to form across the narrow dimensions v, and C for n-pentane on outgassing above 88", of the channels or corrugations (-3.7 A. across). while there is no appreciable change in urn or C for Nevertheless such a process must be partially re- neopentane, so that selectivity vanishes. According versed on treatment with water. Although objec- to arguments based only on the distance from the tions can be raised to some aspects of the above surface of molecular centers of gravity the selectivview it seems to offer the best interpretation so far ity would always remain. The order of selectivity in sepiolite (n-pentane of Fig. 3 and of the thermodynamic data. Selectivity shown by the layer lattice crystals of isopentane > neopentane) also cannot be related montmorillonite toward the n-pentane cannot be simply to distances of molecular centers of gravity explained in similar terms. However it is to be from the sorbing surface, because n-pentane and expected that if flattened on to the basal surfaces of isopentane are not differentiated by this sorbent. montmorillonite the center of gravity of n-pentane The possible structural similarity between attapulwill be nearest and that of neopentane farthest gite and sepiolite4J may mean that an explanation from these surfaces. This means that n-pentane in terms of surface corrugations must be sought also will tend to be sorbed with the greatest heat and for sepiolite. It may be concluded that one gendecrease in entropy. The same order of selectivity eral explanation of the influence of molecular strucamong the pentanes has been observed for carbonls ture upon selectivity toward isomeric paraffins is not possible for attapulgite, montmorillonite and (18) R.A. Beebe, G.Kington, M. H.Polley and W.R.Smith, J . Am. sepiolite. Chsm. SOL, 71,40 (1950).
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