ADSORPTION OF FORMIC AND ACETICACIDSBY SPHERON 6
Sept., 1963
1789
PHYSICAL AND CHEMICAL ADSORPTION OF FORMIC AND ACETIC ACIDS BY SPHERON 6 BY J. J. KIPLING AND E. H. M. WRIGHT Department of Chemistry, The University, Hull, England Received February 4, 196s The vapors of acetic acid and formic acid renct slowly with the carbon black, Spheron 6, a t room temperature. Desorption a t 60" yields acetic acid in the former case, but a mixture of carbon dioxide and water vapor in the latter case. Adsorption on Spheron 6 from the liquid mixtures acetic acid-carbon tetrachloride, acetic acidcyclohexane, and formic acid-cyclohexane involves both chemisorption and physical adsorption. The individual isotherms for physical adsorption of each component have been calculated. Acetic acid appears to be physically adsorbed in a double liiyer from cyclohexane but in a single layer from carbon tetrachloride. The onset of multilayer adsorption of formic acid occurs at low relative concentrations in both solvents. Results of adsorption on Graphon, which has a surface almost free from oxygen complexes, are included for comparison.
Introduction I n an investigation of adsorption by solids from liquid mixtures, it was found that the lower fatty alcohols were strongly attracted to the oxygen sites on the surface of a coconut shell charcoal. Subsequently it was suggested2 that methyl alcohol might be chemisorbed by some of the oxygen groups on the surface of the carbon black, Spheron 6. When we examined adsorption on Spheron G from mixtures involving the lower fatty acids, it therefore seemed appropriate to examine the possibility that chemisorption of the acids took place to some extent, in addition to the expected physical adsorption. To this end, vapor phase experiments were carried out to supplement the results obtained from liquid-phase adsorption. The readiness with which adsorption occurred was examined, and the products of desorption were identified. Experimental Materials.-Spheron 6 is a medium-processing channel blwk manufnctured by the Cabot Corporation. For purposes of comparison, some results were obtained with Spheron 6 (1000") and Spheron 6 ( 2 7 0 0 O ) [Graphon], as no chemisorption of f a t Q :acids on these materials would he expected to occur. The specific surface areas of Spheron 6 and Graphon were taken to be 115 and 84 m.2/g., re~pectively.~ The purification of the acids and solvents has been described previously.4~5 The limiting solubilities of formic acid in cyclohexane and in carbon tetrachloride a t 20" were fdund to be 1.03 and 2.69 g./lO0 g. of solvent, corrcsponding, respectively, to mole fractions (of dimeric acid) of 0.0092 and 0.0413. Desorption.-For the desorption experiments, the solid was first left, in contnct with the saturated vapor for a known time a t 20". A sample of about 2 g. was then placed in a 20-ml. bulb which was evacuated to a pressure of 10-2 to 10-8 mm. The sample was weighed a t intervals. When desorption became very RIOW a t 20°, it was continued a t 60". The method has been used previously in a similar investigation.6 Typical results are shown in Table I; the remainder are summarized in Table 11. I n some of the desorption experiments, the gaseous products formcd a t 60" were collect,ed in a cell a t a pressure of about 8 mm. The infrared spectra were then measured in a Tinicani SI' 100 spectrometer. Adsorption from Solution.-AdForption from the liquid phase was carried out a t 20". The acetic acid mixtures were analyzed by means of the Rayleigh-Hilger int.erferometer.8 The formic arid mixtures were analyned by titration wit,h aqueous potassium hydroxide in the presence of cold, neutral rtlcohol.3 (1) C. G. Gasser and J. J. Kipling. J . I'lq/s. Chem., 64, 710 (1960). (2) C . G. Gasser and ,J. J. Kipling, "l'rocoedings of the Fourth Conference on Carbon." Pergarnon Press, London and New York, 1960. P. 55. (3) J. J. Kipling and E. I f . I f . Wright, J . Cliem. Soc., 855 (1962). (4) A. Blackburn and J. J. Kipling, i b i d . . 3819 ( 1 9 % ) ; 1493 (1955); A. Blackburn, J. J. Kipling, and D. A . Tester. ibid., 2373 (1957). ( 5 ) J. J. Kipling and D. B. Peakall, (bid., 834 (1957).
(6) J. J. Kipling, Hilger J . , 3, 35 (1957).
All experiments involving formic acid were carried out with test tubes which had been treated with a silicone on the inner walls. This was to prevent reaction between formic acid arid the glass. Monolayer Values.--Rxperimerital monolayer values were obt:tiried by applying the €3 .E.T. equation to radsorptiori isotherms of the respective vapors determined a t 120". The separate values were self-consistent.
TABLE I DESORPTION UNDER VACUUM FROM SITERON 6 Temp.
Adsoibrnt
Adsolbate
Durat w n of Residual drsorp- drsorpw t . of bcforr tion, tion, adsorbate, clrsorptlon 'C. he. mg./g.
Time of contact
of
Spheron 6
Water
7days
20
Spheron 6
Acetic acid
3 hr.
20
5 24 48 1 3 28
2.0 0.6 0.0 34.8 23.8 17.5 11.8 11.0
45 52 75 100
9.3 9.2 8.3 8.2
5 21
Results Desorption of Vapors.-Table I1 shows that the simple nonpolar cyclohexane is very readily desorbed from Spheron 6 and Graphon. Acetic acid is initially desorbed rapidly from Graphon, but a longer time is required for complete desorption. I n part this effect might he due to the lower vapor pressure of acetic acid and to its stronger adsorption by Graphon, probably in the form of dimeric molecules, each having eight main centers of contact with the surface. The desorption of acetic acid from Spheron 6 is much slower, and much greater quantities are retained after a given period of desorption. As water can be desorbed completely, retention of acetic acid cannot bc attributed solely to the polarity of the molecule. The most significant result is that the quantity retained a t a given time depends on the time of initial contact before desorption was started. This indicates that adsorption probably consists, in part, of a slow chemical reaction with the surface. Similar results are found in the experiments on formic acid. The small residual retention by Graphon is possibly due to chemical reaction of the formic acid with the very small number of polar groups believed to be present on the s ~ r f a c e . ~
1790
J. J. KIPLIXG AND E. H. AI. WRIGHT
Vol. 67
TABLEI1 RESIDUAL X7EIGHTS O F ADSORBATE RETAINED A T THE LIYIT O F DEsoRmIos
Graphon
Cyclohexane Acetic acid Formic acid
7 -
Spheron 6 (1000")
Cyclohexane Acetic acid
7
Formic acid
7
Cyclohexane Acetic acid
7
Adsorbent
Spheron 6
a
Time of contact before desorption, daya
Adsorbate
*I
c
Acetic acid Formic acid
* I
Formic acid
1
3 hr.
Formic acid 3 hr. Thrse times are additional t o the periods of desorption a t 20'.
0.6
.i. e"
c
6
. 0.4
2
3 0.3
v
P 0.2
0.1
0.2 0.4 0.6 0.8 Mole fraction of dimeric acid in liquid phase a t equilibrium.
Fig. 1.-Adsorption (composite isotherms) from acetic acidcarbon tretrachloride mixtures on Spheron 6 a t 20". The broken line i s the isotherm for adsorption on Graphon. I
bi --.
2 0.5
fi
-
0.4
I
\
4
0.3
d
I 0.2 0.1 0
I
I I I I 0.2 0.4 0.6 08 Mole fraction of dimeric acid in liquid phase at equbllbrium.
1
Fig. 2.-Adsorption (composite isotherms) from acetic acidcyclohexane mixtures on Spheron 6 at 20". The broken line i s the isotherm for adsorption on Graphon.
Temp. of desorption, OC.
20 20 20 Continued a t GO 20 20 Continued a t GO 20 Continued a t 60 20 20 Continued a t 60 20 20 Continued at 60 20 Continued a t 60 20
Duration of desorption, hr.
Residual wt. of adsorbate, mdg.
3 168 93 24" 3 96 144" 101
0.0 0 .0 0.5 0.0 0.0 3.0 1.1 3.5 0.2 0.0 15.7 2.1 8.2 29.2 1.2 16.8 0 .0 10.3
505
3 96 120" 100 101 120" 101 57" 100
Infrared Analysis.-The gaseous product obtained from Spheron 6 after 7-days' contact with acetic acid showed prominent absorption bands at 1747,1783,1303, 1380 and 1456, and 955 cm.-l These correspond, respectively, to typical frequencies of the acetic acid spectrum, associated with the groups C=O (associated acid), C=0 (monomeric acid), C-0, CH3,and 0-H. The only product identified was thus considered to be acetic acid. The corresponding desorbate from Spheron 6 on which formic acid had been adsorbed gave an absorption band a t 2340 +5 em.-' characteristic of carbon dioxide, and a further band at 3400 cm.-I, characteristic of water vapor. Absorption a t wave numbers characteristic of the formic acid spectrum8 [C=O (associated acid), 1740; C=O (monomeric acid) , 1780; C-0 or 0-H, 1360; other vibrations, 3050 and 2870 was not observed. This is clear evidence of dissociative chemisorption, followed by desorption of carbon dioxide and water as the products of surface oxidation, Adsorption from the Liquid Phase.-The results for liquid-phase adsorption on Spheron 6 are given in Fig. 1-4. There is strong preferential adsorption of the acids i4 each case. The preferential adsorption on Graphon is very much lower, which suggests a t least that the relatively polar surface of Spheron 6 has a significant effect. Data are plotted in terms of millimoles of dimeric acid to afford comparison with results for higher members of the homologous series.3g Discussion The Nature of Chemisorption.-A complete quantitative analysis of the surface groups on Spheron 6 has not yet been made. An attempt to suggest the nature of the chemisorption processes therefore must be speculative. It is clear, from the spectral evidence, that formic acid is oxidized by Spheron 6. As formic (7) R . N. Smith, J. Duffield, R. A . Pierotti, and J . hfooi, J . Phus. Chem., 60, 496 (1956). (8) J. Fahrenfort and H. F. Hazebroek, 2. phystk Ckem. (Frankfurt), 20, 105 (1959). (9) J, J. Kipling a n d E. H, M Wrighti J 4C h e m &e,, 3812 (1963).
Sept., 1963
ADSORPTION OF FORMIC AND ACETICACIDSBY SPHERON 6
acid is known to be oxidized by triphenylcarbinol, a probable reaction a t the carbon black surface is =C.OH
+ HeCOOH --+= C . H + HzO + COz
though it is likely that other reactions also occur. The carbon dioxide and water vapor were desorbed only at 60’ under reduced pressure, and then only in very small quantities. The reaction therefore must be preceded by strong chemisorption, perhaps by a dissociative process at a multiple bond. So far, investigations of the surface complexes of carbons have not identified specific oxidizing groups present on the surface of Spheron 6 in quantities sufficient to achieve this type of oxidation. Acetic acid, on the other hand, is not oxidized, and can be recovered a t 60’. The nature of its reaction with the surface is not certain. One possibility, however, can be suggested in terms of the “n-lactone” structured postulated by Garten, Weiss, and Willis. lo This is that the lactone group is attacked by the acetic acid as
ir
1.0
B 8.0.8
h
E $ 0.6 \ v
I 0.4 0.2
0.006 0.010 0.015 Mole fraction of dimeric acid in liquid phase a t equilibrium.
Fig. 3.-Adsorption (composite isotherm) from formic acidcarbon tetrachloride mixtures on Spheron 6 a t 20”. The broken line is the isotherm for adsorption on Graphon.
1.2 ’~ 1.4
cII
\d
1.2 1.4
0. 4.
1791
“0
The significance of this suggestion is that Spheron 6 is estimated to contain 228 ,uequiv./g. of n-lactone structures and, according to our experiments, chemisorbs approximately 260 pequiv./g. of acetic acid. The evidence is not (conclusivebecause we have shown that sodium fluoresceinate takes up approximately 3 molecules of acetic acid per molecule irreversibly under the conditions used in the experiments on Spheron 6. It is not clear whether the oxygen groups present in fluorescein are present on the Spheron 6 surface unless they have been converted t o n-lactone groups. Adsorption from the Liquid Phase.-In order to compare adsorption from the liquid phase for the lower and higher members of the homologous series of fatty acids, it is desirable to evaluate not only the composite (or “surface excess”) l1 isotherms, but also the isotherms for the actual adsorption of the individual components. If only physical adsorption occurs, we can, as before, use the equations no(Ax/m) = n?(l - x) - ncx
_ _ _ _J_ _ _ _ _ _ _ 4 __..---- +-------
,
__--
0.001 0.002 0.003 0.004 Mole fraction of dimeric acid in liquid phase a t equilibrium.
Fig. 4.-Adsorption (composite isotherm) from formic acidcyclohexane mixtures on Spheron 6 a t 20”. The broken line is the isotherm for adsorption on Graphon.
(1)
and
where Ax is the decrease in mole fraction of component 1 in a liquid mixture containing initially a total of no moles, when it is brought into contact with rn g. of solid; x is the mole fraction of component 1in the bulk liquid a t equilibrium; nls and naSare the numbers of moles of components 1 and 2, respectively, adsorbed by 1 g. of solid, and (nls), and (nzs>, the corresponding (10) V. A . Garten, D. E. Weiss, and J. B. Willis, Australian J. Chem., 10, 295 (1957). (11) J. J. Kipling, “Proceedings of the Third International Congress of Surface Aotivity,” 1960, Verlag der Universitiltsdruckerei, Mainz, Vol. 11, p. 77.
0.2 0.4 0.6 0.8 Mole fraction of dimeric acid in liquid phase at equilibrium.
Fig. L-Ind ividual isotherms for physical adsorption on Spheron 6 a t 20” from acetic acid-cyclohexane mixtures (full lines) and from acetic acid-carbon tetrachloride mixtures (broken lines).
quantities required to give complete monolayer coverage. These equations can be solved for nls and nas if physical adsorption only occurs and if it is assumed t o be restricted to a single molecular layer. If chemisorption of component 1 occurs in addition t o physical adsorption, eq, 1 must be modified to eq. 3
J. J. KIPLING AN E. H. XI. WIZIGIIT
1792
I
I
~
0.010 0.015 hlole f r u o t i i i n in liquid phase a t equilibriuin. 0.005
0.12
0.24
0.3G
t/m.
Fig. 6.--lndividual isotherins for pliysicd adsorption from formic ack-mrbon ttitrac:liloridc niixt.urt,s on Splic~on6 at 20". x / x o shows the mole fraction of a givm solution relative t o that :it tlit: solubility limit. The arrow s1ion.s the nionolayor value for physical adsorption of formic acid.
$ - 1.0
t
I
I
I
2 0.8 c
acid
'5 0.6
3z 0.4
0.2
0001 0.002 0003 0004 Mole fiaction in liquid phase at equilibrium. 0 11 0 22 0.33 0 44 x/m
Fig. 7.-Individual isotherms for physical adsorption from forniica acwl-cyc 1ohex:tnc niixtures on Spheron 6 at 20". 5/20 S t i o H b t l i v molv fraction of a given solution rchtive t o that at the solubilitv limit The. arrow shows the monolayer value for ph\ si( :il adsorption of formic acid.
n(l(&/nz)
=
[(n?)phys
+
(?~?)chcm][(1
- x) - 7228x1 (3)
where (nls)yhysand (nls)cllem refer to the numbers of moles of component 1 adsorbed physically and chemically, respectively, by 1 g. of solid. Although not quoted specifically, eq. 3 mas used in our earlier work with alcoliols and oxide adsorbents,12 it being assumed was independent of 2 . These that thc value of (nls)chpm equations can be used for the fatty acid systems. (i) The Acetic Acid Systems.-As the solutions were analyzed after being shaken for 7 days with Spheron 6, was taken (from Table I) as 15.7 mg. per g., Le., the limiting amount of acetic acid held irreversibly by Spheron 6 a t 20'. This corresponds to 0.13 mniole of adsorbed dimer per g. of Spheron 6. The value of (nls)lnfor physical adsorption cannot be determined in the usual way from adsorption of the vapor, as this involves chemical as well as physical adsorption. The monolayer value for Spheron 6 (1000°) therefore was determined experimentally and the value for Sphrron 6 calculated from this on the (12) J. J. Kijrling and I) 1%.Peakall, J. Chem. Sor , 405.1 (1957); 4828
(1956).
Vol. 67
basis of the respective surface area9 to nitrogen. This gave sl value of 0.36 nimole of dimeric acid per g. of Sphcron 6. [It was assiuncd that any chcmixorption occurring on Spheron G (1000O) during the time required to mcasurc thc adsorption isotherm for the vapor (about 3 hr.) could bc neglected. On Sphcrori 6, chemisorption after 3 hr. is about one-half of that occurring after 7 days. On this basis, chcmisorption on Spheron G (1000O) after 3 hr. would be about 0.01 mmolc/g.] With these figures, individual isothcmns for physicd adsorption have been calculated for the acetic acidcarbon tetrachloride system; tlie results are sliown in Fig. 5 . These arc typical of competitive adsorption betwcen two compoiients, oiie of wliich is adsorbed much more strongly than the other. For the acetic acid-cyclohexanc systein, it is apparent from iiispcction of tlie coii~positcisotlicrin that more acetic acid is adsorbed than corresponds to a combiiiation of the amount chemisorbcd and that which would be found in a complete monolayer of pliysically adsorbrd material. We therefore have considwcd thc possibility that thc physically adsorbed layer cxtends to two molecular layers. This possibility seems a likely one when the composite isotherm is analyzed by Schay's method.I3 As the latter section of the isotlicrm is approximately linear, extrapolation of this section to zero mole fraction of acetic acid should givc the maximum quantity of acetic acid adsorbed. The extrapolated value is 0.85 mmole of dimer per gram, which corresponds to the sum of the amount chemisorbed (0.13 mmole/g.) plus two layers of physically adsorbed acetic acid (0.73 nimolc/g.). On this basis, individual isotherms have been calculated and are givcii in I'ig. 5 . Although these isotliernis cannot be put forward with complete certainty, they give as reasonable a reprcseiitation of adsorption from this system as we think can be offered a t prcsent. Examination of tlie total vapor pressurc curve for mixtures of acetic acid and cyclohexane shows that, the system is very far from ideal. We have shown clsewhere'l that tlie adsorbed layer from very nonidcal systems can be more than one molccular layer thick even at thc liquid-vapor interface. It is therefore not remarkable that the same pheiiornenon should occur at the liquid-solid interface. (ii) The Formic Acid Systems.-The extent to which carbon dioxide and \vat cr ran be desorbed from Sphcron G covered with formic acid is so small at 00" that me think it can be iic.rlccdrd at 20". The limiting increase in weight of Sphtwii t i after 7-days' contaclt with formic acid at 20' tlicrdore was taken to be the extent of clicniisorption; this was 27 n,g./g. (0.29 mmole/g.). The value of (n,\),,, for physical adsorption was calculated, as described above, to bc. 0.4
