A. J. DANDY
334 disagreement with our results. The low-pressure data at 71.1" are shown in Figure 4, and suggest a slope intermediate between our results at 25 and 50". At 50 atm, their data are about 4% higher than those derived from Our values, and the discrepancy at 37.8" is larger. In view of the smoothness of our data, and the irregularities previously noted in those of Olds,
Sage, and Lacey, the new results are probably more reliable. Acknowledgment, The authors are grateful to the Office of Saline Water, U. S. Department of the Interior, for financial support and to the Computer Center, Universitlt of California, Berkeley, for the use of its facilities.
Sorption of Vapors by Sepiolite by A. J. Dandy Makerere University College, University of East Africa, Kampala, Uganda, East Africa Accepted and Transmitted by The Faraday Society
(March SO,1967)
The sorption by outgassed sepiolite of nitrogen at - 197 and - 184", of oxygen at - 184", and of ammonia, methylamine, and ethylamine at 0, 25, and 41" has been studied. Uptakes of nitrogen and oxygen were measured volumetrically; uptakes of the other three vapors were measured by means of a silica helix. The BET surface area of sepiolite is increased by outgassing at 70 instead of at 25". The areas calculated from isotherms for Nz at -197 and -184", for Oz at -184", and for CHaNHzand CZH5NH2at 0" all lie in the range 354-383 m2/g, but that calculated from NH3 sorption data at 0" is 747 m2/g. Prior heating of sepiolite in air or in vacuo at 300" considerably reduces the area available to nitrogen. The isosteric heat of Nz sorption is 2.2 kcal/mole at "monolayer" coverage; values for the polar vapors increase in the order of increasing basic strength of sorbate. Differential entropies of sorption have been calculated. There are sudden changes in these functions at corresponding coverages. The results are interpreted in terms of adsorption on the external surface of sepiolite and partial penetration of the lattice "channels" of sepiolite.
Introduction Sepiolite possesses properties which make it important as a molecular sieve and as a sorbent of gases and vapors.' It is a magnesium trisilicate of ideal formula HrMg9SilzO~o(OH)~o. 6Hz0 and has structural similarities to attapulgite (palygorskite), which is also an important industrial sorbent. The main deposits of sepiolite are at Eski shehir (Turkey), Amboseli (Tanzania), and Vallecas (Spain). Nagy and Bradley2 have proposed a structure for sepiolite. An important feature of this structure is the existence of "channels" which traverse the crystals. These channels are thought to be wider in sepiolite than in attapulgite, the dimensions being about 5.6 X 11.0 A. Sepiolite samples used in the present work were obtained from Amboseli and have a chemical constitution very similar to that of Vallecas sepiolite, of approximate formula The Journal of Physical Chemistry
Hs(Mg1',Fe1',Fe111,A13111)* X
-
(Si,AllI1)lzOao(OH)lo 6Hz0 Barrer and R/lackenziea found that large amounts of nitrogen, n-pentane, and isopentane were rapidly sorbed by sepiolite at temperatures near their respective boiling points, but that neopentane was sorbed to a lesser extent. They concluded that lattice penetration by these molecules was not important or was very limited. Muller and Koltermann4 investigated the flow of various gases through a column of sepiolite at 20" and atmospheric pressure. While COz, SO2, n-C4HI0,CCla(1) R. H. S. Robertson, Chem. Ind. (London), 1492 (1957). (2) B. Nagy and W. F. Bradley, Am. Mineralogist, 40, 855 (1955). (3) R.M. Barrer and N. Maokenzie, J . Phys. Chem., 58, 560 (1954). (4) K. P. Muller and M, Koltermann, Z . Anorg. Allgem. Chem., 36, 341 (1965).
SORPTION OF VAPORS BY SEPIOLITE
335
NOz, and n-C4H90Hwere strongly sorbed, no sorption of CO and Nz could be observed. They state that Nz and GO can pass freely through the channels of the sepiolite structure and conclude that the latter have a diameter of 6-8 8. It was of interest to extend these investigations by studying the sorption of molecules of differing sizes and polarities by sepiolite at various temperatures. The results yield information concerning the nature and availability of the surface. Experimental Section Sepiolite was obtained from Amboseli, Tanzania, and was identified by X-ray and chemical analysis. In all cases, the analyses show the mineral to be a very pure sepiolite. The DTA curves were similar to those found for Vallecas sepiolite, although a sharp peak at 850" was not observed. Samples were ground to between 30 and 40 BS mesh. Cylinder nitrogen and oxygen were purified and dried. Ammonia, methylamine, and ethylamine were prepared by heating NH4C1, CH3NH2.HC1, and CzHsNHz. HC1, respectively, with soda lime. All solids were previously degassed in vacuo. The vapors were purified by passage over CaO, soda lime, and NaOH, and by repeated fractionation; they were stored over fresh CaO. Vapor pressure measurements showed that the temperatures of liquid N2 and liquid 0 2 were - 197 and - 184O, respectively, at the prevailing atmospheric pressure (ca. 66 cm). The sorption isotherms of Nz at - 197 and at - 184", and of O2 at --184" were determined in a standard volumetric apparatus. Pressures were measured by means of a mercury manometer connected to the sorption vessel via a solid COZ-acetone cold trap. A silica helix was used to measure gravimetrically the sorption of NH3, C&n"2, and CzHsNHz on sepiolite. It had an extensioin of 63.7 cm when fully loaded a t 2.5 g. The sensitivity over the measured range was about 17.95 cm/g and was checked between runs. The extensions were independent of temperature in the range 0-70", and buoyancy corrections were negligible, except at pressures higher than 30 cm, when they were very small. A cathetometer, reading to 0.001 cm, was used to measure the extension of the helix and the mercury manometer levels. The balance case was kept at a constant temperature by immersion in a thermostated water bath (constant to *0.1") or in ice. An oil diffusion pump backed by a Speedivac rotary oil pump was used to evacuate the apparatus to about 5 X mm, as measured by a McLeod gauge (bulb capacity, 310 ml; capillary, 1 mm) and a Speedivac Pirani gauge. Results and Discussion Surface Area and Water Removal. Figure 1 illustrates typical Naland 0 2 isotherms on outgassed sepiolite samples. Variation of the outgassing periods from 1
01
10
0
20 30 40 Pressure, om.
50
60
70
-
Figure 1. 1, Sorption of 02 on sepiolite a t 184": V, sorption; A, desorption; outgassed 1 hr at 96"; 2, sorption of Ne on sepiolite a t -197': outgassed 24 hr at 70'; 3, sorption of Nz on sepiolite a t 184': 0, outgassed 1 hr a t 96'; $,outgassed 24 hr a t 70'; 4, sorption of Ne on sepiolite a t - 184': outgassed 1 hr a t 300°,dotted line represents Pofor curves 1 and 2; inset, V , [ml(STP)/g] us. t plots for curves 3 and 4.
-
to 24 hr had negligible effect on the reproducibility of isotherms. The outgassing procedure was standardized to 24 hr at 70" for the majority of the later experiments. The isotherms were type I1 in the Brunauer classification and yielded good straight lines up to PIPo = 0.2, when plotted according to the BET equation. BET surface areas were calculated from Nz isotherms at 197 and 184", and from O2isotherms a t - 184", on sepiolite samples subjected to various outgassing procedures. The results are presented in Table I. The cross-sectional area of a Nz omolecule was taken as 16.2 Az at - 197" and as 16.9 A2 at 184", and that of O2 was taken as 14.1 iz at 184". Caution is required in interpreting these results, particularly as micropore filling may be occurring in the initial stages of the isotherms; this will be discussed later. However, they are expressed as BET surface areas for convenience, and some general conclusions can be drawn from a comparison of the values. The areas calculated for samples outgassed at 70 and 96" were reproducible, and those derived from Nzisotherms are in good agreement with each other; it is not possible to make a true comparison of these with the result
-
-
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Volume 78, Number 1 January 1968
A. J. DANDY
336
Table I: BET Surface Areas of Sepiolite Samples Preheating in air
... ... ... ...
... 1 hr 1 hr 1 hr 1 hr 1 hr
at at at at at
100' 200' 300' 500' 700"
BET area,
Outgassing procedure
mz/g
48 hr a t 25' 24 hr a t 70' 24 hr a t 70' 1 hr a t 96' 3 hr a t 96' 48 hr at 25' 1 hr a t 96' 1 hr at 300" 1 hr a t 400' 5 rnin a t 25' 5 rnin a t 25' 5 rnin a t 25' 5 rnin at 25' 5 min a t 25'
354 380 375 375 378 310 352 195 196 330 328 158 158 157
Isotherm
Nz a t N2 a t NZa t N2a t N2a t O2 a t Oz a t NZat Nz a t Nza t Nza t Nza t Saat N2a t
- 184' -184' - 197' -184' -184' -184' -184' - 184' -184' -184' - 184' - 184' - 184' -184'
calculated from the oxygen isotherm, owing to the uncertainty in the values of Nz and Oz molecular crosssectional areas. The highest surface areas reported here are higher than those previously reported for sepiolite obtained from other source^.'^^^^ Nevertheless, the relationship between surface area and heat pretreatment (water loss) is similar to that found by Barrer and Ma~kenzie,~ who heated sepiolite samples in vucuo; the surface area is increased by outgassing at 70" rather than at 25" and is markedly decreased by heating in air o r i n vucuo above 200". (The low area obtained for samples preheated in air at 300", compared with that for sepiolite heated at 300" in vucuo, may be partly due to sorption of atmospheric moisture during transfer to the sorption apparatus in the former case.) The density of sepiolite samples as received was 2.086 g/ml. This rose to about 2.7 g/ml on outgassing at 70", and was accompanied by a loss in weight of about 19% (based on the outgassed weight). Heating in air at 100" caused a loss in weight of about 14.5%. It has been reported5 that six molecules of adsorbed water and one molecule of coordinated water per half-cell are lost easily on heating. Between 250 and 350", a further two half-molecules of coordinated water are lost, and a structure transformation takes place at about 350". Outgassing or heating sepiolite at temperatures up to 200" thus removes water and makes more surface available for the adsorption of nitrogen. This may be interpreted in terms of at least partial penetration of the lattice channels by N2. The O2 molecules appear to penetrate these channels to about the same extent. The decrease in the BET surface area of sepiolite, which occurs when samples are heated above 200", may be due to sealing of the channel entrances, though Barrera has tentatively suggested that the cause may be the smoothing of surface corrugations. Muller4 has shown that sorption data for the large molecules n-C4Hlo(estimated The Journal of Physical Chemistry
cross-sectional area 37.6 A2) yield a low value for the surface area of sepiolite (163 mz/g, compared with a value of 331 mz/g derived from the uptakes of COz (16.9 A2) and SO2 (24.4 d2). This low value may represent the "external" surface, excluding the surface in the channels. It compares with the value of 196 mZ/g for sepiolite heated in uucuo at 300 or 400" (Table I). At such temperatures, therefore, the microporous structure may partially collapse, leaving only the external surface accessible to nitrogen. This view is supported by a comparison of the NZisotherms at - 184" for sepiolite outgassed at 96" and sepiolite preheated at 300" (curves 3 and 4 in Figure 1). The former curve exhibits a much steeper slope in the initial stages than does curve 4; it has been suggested6 that a steep slope in this initial region indicates micropore filling by the adsorbate on microporous solids. The application of the BET equation may then evaluate external surface plus micropore volume, though the application of the BET equation itself in these circumstances is of uncertain validity. de Boer, el UL,' have suggested that plots of V,, (volume sorbed, ml (STP)/g) us. the thickness of the adsorbed layer, t, may yield information concerning the pores of sorbents. The thickness of the sorbed layer, t, may be put equal to 16.43 V , / A for N2a t -184", assuming a liquid density of 0.76 g/ml. The inset in Figure 1 is t plots based on the ideal shape for Nz sorption at - 184" on sepiolite outgassed at 300" (plot 4'); the assumption is made that for this sample the BET surface area, A , of 195 mz/g represents the external surface. Plot 3' for Nzsorption on sepiolite outgassed at 70 or 96" is linear after an initial steep rise; the linear section cuts the ordinate at V , = 32 ml. No physical significance can be given to the t plot values in this case, but the shape of plot 3' suggests that micopores are being filled in the initial section.*~g de Boer relates the slopes of t plots to the external surface areas of sorbents. The slope of the linear portion of 3' is greater than that of 4'; hence large pores or surface corrugations, in addition to micropores, may be destroyed by heating sepiolite samples at 300". Dubinin has modified the Polanyi potential theorylO for microporous solids, and has derived" the isotherm equation (5) J. L. Martin-Vivaldi and J. Cano-Ruiz, National Academy of Science of Spain, National Research Council Publication No. 456, 1956,p 177. (6) S. J. Gregg and K. 8. W. Sing, "Adsorption, Surface Area, and Porosity," Academic Press, London, 1967, p 208. (7) B. C. Lippens, B. G. Linsen, and J. H. de Boer, J . Catalysis, 3, 32 (1964); B. C. Lippens and J. H. de Boer, ibid., 4, 319 (1965); J. H. de Boer, B. G. Linsen, and Th. J. Osinga, ibid., 4, 643 (1965); J. H. de Boer, et al., J. Colloid Interface Sci., 21, 405 (1966). (8) R. E. Day and G. D. Parfitt, Trans. Faraday Soc., 63, 708 (1967). (9) R. F. Horlock and P. J. Anderson, ibid., 63, 717 (1967). (10) M.Polanyi, {bid., 28, 316 (1932). (11) M.M.Dubinin, Chem. Rev., 60, 235 (1960).
SORPTION OF VAPORSBY SEPIOLITE
337
(Yo)- D(bg Po/P)' (1) volume uptake of adsorbate, Yo = total
200
log V , = log
where V a = volume of micropores, and D is a constant. Figure 2 illustrates Dubinin plots for N2 sorption at -184 and -197" on sepiolite samples outgassed at 70 or 96". The results satisfy the Dubinin equation in the initial stages of Nz uptake. Extrapolation of the lines gives intercepts which correspond to uptake volumes approximately the same as the volumes at BET monolayer coverage, found by applying the BET equation. However, the interpretation of V oremains uncertain. The fact that the Dubinin equation is obeyed suggests that Nt does have partial access to micropores with overlapping force fields for sepiolite samples outgassed at 96". The isosteric heat of adsorption of N2 on sepiolite was calculated from the adsorption isotherms at -. 197 and - 184", using the Clausius-Clapeyron equation. The value was 2.2 kcal/mole in the region of BET monolayer coverage. Sorption of Polar Vapors. The sorption of NHa, CHaNH2,and CzH6NH2 by sepiolite at 0, 25, and 41" was studied. Fresh samples of sepiolite were outgassed at 70" f o r at least 24 hr before each run. Typical results are shown in Figures 3,4, and 5. Points on any isotherm were reproducible after raising and lowering the temperature through the range 0-41". With the exception of C2H&H2 at 0" at high relative pressures, the isotherms were all reversible, although long periods of evacuation at 70" were necessary to completely remove any of these vapors from sepiolite samples. The CzH5?JH2isotherm at 0" shows definite hysteresis effects. Experimental points were determined after many days to allow the system to reach equilibrium.
150
2 # 9;
3 100 .I.'
I
4
50
0 0
20
40
60
80
Presaure, om.
Figure 3. Sorption of NHs on sepiolite outgassed a t 70'.
200
0'
150
$
3
$-
f: 100
*
8
3
1.20
8
- 1.15
4
- 1.10 3 2E
-
1.05
-
a 1.00 cl
-
0.95
50
N
L4 1.3
- 0.80
tL 0
1
2
01
0
20
40
60
80
Premure., om.
Figure 4.
Sorption of CHsNHs on sepiolite outgassed a t 70'.
0.85
3
4
5
6
7
L o 2 (Po/P).
Figure 2. Dubinin plots of vapor sorption on sepiolite: 1, Nz a t -197'; 2, N f a t -184' (sepiolite outgassed at 70 or 96'); 3, NHI a t 0' (sepiolite outgassed a t 70').
In general, equilibria were achieved only after several hours. This effect indicates lattice penetration by sorbate molecules. The relative pressures achieved with NH3 and CH3NHz at the temperatures studied Volume 78, Number 1 January 1068
338
A. J. DANDY
500
Table I1 : Constants for NHs, CHaNH2, and CgHbNHe, and BET Surface Areas for Sepiolite Calculated from Vapor Sorption Isotherms a t 0"
------___ N Ha
400
