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R. W . Glass and R . A. Ross
(14) C. Kemball, Advan. Catal., 2, 237 (1950). (15) See paragraph at end of paper regarding supplementary material. (16) "Handbook of Chemistry and Physics," Chemical Rubber Publishing Co., Cleveland, Ohio, 1970, p F 156. (17) A . V. Kiselev and V. I . Lygin, Kolloid. Zh., 21, 581 (1959), (18) J. Uytterhoeven, M. Sleex, and J. J. Fripiat, Bull. SOC. Chim. fr., 6, 1800 (1965). (19) A. V. Kiselev, "Structure and Properties of Porous Materials," D. H. Everett and F. S. Stone, Ed., Butterworths, London, 1958, p 21 0. (20) M. L. Hair, Ed., "Infrared Spectroscopy of Surface Chemistry," Edward Arnold, Ltd., London, 1967.
(21) S. G . Ash, A. V . Kiselev, and B. V. Kuznetsov, Trans. Faraday soc., 67, 3118 (1971). (22) E. Robinson and R. A. Ross, J. Chem. SOC.A, 84 (1970). (23) J. M. Thomas and W. J. Thomas, "Introduction to the Principles of Heterogeneous Catalysis," Academic Press, London, 1967. (24) C. Kemball and E. K. Rideal, Proc. Roy. Soc., Ser. A, 187, 53 (1946). (25) A. Clark and V . C. F. Holm, J. Catal., 2, 21 (1963). (26) A. B. Burg, "Organic Sulphur Compounds," N. Kharasch, Ed., Pergamon Press, London, 1961, p 30. (27) M. L. Hair, "Infrared Spectroscopy in Surface Chemistry," Edward Arnold, Ltd., London, 1967.
Surface Studies of the Adsorption of Sulfur-Containing Gases at 423°K on Porous Adsorbents. I I. The Adsorption of Hydrogen Sulfide, Methanethiol, Et hanethiol, and Dimethyl Sulfide on y-Alumina R. W. Glass"' and R. A. Ross Department of Chemistry, iakehead University, Thunder Bay, Ontario, Canada (Received February 26, 1973)
Calorimetric heats of adsorption have been determined for hydrogen sulfide, methanethiol, ethanethiol, and dimethyl sulfide adsorbed a t 423°K on y-alumina at coverages up to 0 = 0.5. Adsorption isotherms have been measured and the entropies of the adsorbed species have been calculated. Heats of adsorption and adsorption capacities a t a given coverage increased with increasing methylation of the adsorbate. A similar type of adsorbed species involving hydrogen-bond type interactions between the sulfur atoms of the adsorbate and hydroxyl hydrogen atoms on the alumina surface is proposed to account for all the results. Differences in heats, entropies, and adsorption capacities among the systems are explained by the increasing inductive effect on the sulfur atoms with increasing methylation of the adsorbate which results in the formation of stronger hydrogen bonds with the adsorbent.
Introduction anethiol, and dimethyl sulfide a t 423°K on A-alumina heat-treated to 700". Many different types of adsorbents and catalysts are used in industry to remove obnoxious sulfur-containing Experimental Section gases from gas streams. A-Alumina either by itselpa or Materials. ?-Alumina was prepared by heating boehmdoped with NaOH2b or other compounds is one of the most ite in air a t 700" for 20 hr. The boehmite was prepared by commonly used materials and yet few data have been rea method described earlier.3 The surface area, "water conported in the literature on the thermodynamic properties tent," "lump" density by displacement of mercury, and of such systems or on the nature of the adsorbed surface pore size distribution data, determined by the nitrogen complexes formed. adsorption method, of the y-alumina have been reported3 Studies of the adsorption of sulfur dioxide a t relatively along with similar data for other samples heat-treated a t elevated temperatures on y-alumina have been r e p ~ r t e d . ~ other temperatures. In brief, the y-alumina had a surface A recent paper4 on the adsorption of hydrogen sulfide, area of 160 m2 g-l, a "water content" of 18.6 pmol m-2, a methanethiol, ethanethiol, and dimethyl sulfide a t 423°K "lump" density of 0.6864 g cm-3, and a mean pore diameon heat-treated silica gels has shown that for any given ter of 35 A. adsorbate the strongest bonds were formed with the adAfter heat treatment, the adsorbent was cooled over sorbent possessing the highest surface hydroxyl group conphosphoric anhydride, transferred to the appropriate apcentration and that for any given adsorbent the strength paratus, and then outgassed at 150" overnight a t of the adsorbate-adsorbent interaction increased with inTorr. Hydrogen sulfide, methanethiol, ethanethiol, and creased electron-donating power of the adsorbate. dimethyl sulfide were each purified to >99.9% in the In the present work calorimetric heats of adsorption, manner described earlier.4 adsorption isotherms, and entropies of adsorption have A p p a r a t u s and Procedure. Heats of adsorption and adbeen determined for hydrogen sulfide, methanethiol, ethsorption isotherms were determined in apparatus similar The Journal of Physical Chemistry, Vol. 77,
No; 27, 1973
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Adsorption of Sulfur-ContainingGases
I4
to that described earlier,5 and entropies of adsorption calculated from the heat data as b e f ~ r e . ~
Results The heat curves for hydrogen sulfide, methanethiol, ethanethiol, and dimethyl sulfide adsorbed a t 423°K on y-alumina are shown in Figure 1. The curves for methanethiol, ethanet hiol, and dimethyl sulfide, although similar, were separated from each other by approximately 3.0 kcal mol-I above a surface coverage of 0.700 pmol m-2. For these adsorbates the heats of adsorption tended to level out a t a coverage of about 0.700 ymol m - 2 after falling relatively rapidly from 0.400 pmol m-2, The results for hydrogen sulfide adsorption were limited because of the small adsorption capacity of y-alumina for this gas (Table I). However, the results that were obtained showecl a relatively rapid fall in the heats of adsorption from 31.5 kcal mol-' at 0.050 pmol m-2 to 16.3 kcal mol-1 a t 0.340 pmol m-2, and a t coverages above this latter value the heats tended to level out to 16.0 kcal mol-l. For methanethiol, ethanethiol, and dimethyl sulfide the relatively constant heat values at a coverage of 1.40 pmol m-2 were 16.5, 18.4, and 20.7 kcal mol-', respectively. The adsorption isotherms for hydrogen sulfide, methanethiol, ethanethiol, and dimethyl sulfide a t 423°K on Xalumina are shown in Figure 2. All of the isotherms were concave to the pressure axis while the amount of hydrogen sulfide adsorbed was very much smaller than that for the other gases. At a pressure of 150 Torr the amount of gas adsorbed was 0.370, 1.20, 1.51, and 1.72 pmol m-2 for hydrogen sulfide, methanethiol, ethanethiol, and dimethyl sulfide, respectively. Reversibiliky tests6 showed that approximately 100 pg of hydrogen sulfide could not be desorbed from the sample, whereas the adsorption was reversible with the other three gases. In all cases infrared analysis of the gases desorbed from the y-alumina samples on completion of the isotherm showed the absence of foreign components. Analyses of the solid residues showed the presence of sulfur on y-alumina exposed to hydrogen sulfide a t 423°K. Sulfur was not detected on the other residues. Experiment a1 differential molar entropies of hydrogen sulfide, methanethiol, ethanethiol, and dimethyl sulfide adsorbed a t 423°K on y-alumina are shown as a function of surface coverage in Figure 3. TABLE I: Comparison of the Adsorption Characteristics of Hydrogen Sulfide, Methanethiol, Ethanethiol, and Dimethyl Sulfide Adsorbed at 423°K on ?-Alumina Heat-Treated at 700"
AS i= Szt
Adsorbate
"Limiting" heat at 1.40 lmo1r T 2 , kcal mol-'
Amount adsorbed at 150Torr,
pmol m - 2
-
S,) at 0.120 p n o l m - 2 , cal deg-' m o l - l
Hydrogen sulfide
Methanethiol Ethanethiol Dimethyl sulfide
16. Oa 16.5
0.37 1.20
10.0a 17.3
18.4
1.51
25.7
20.7
1.72
29.5
a "Limiting" heats and values for hydrogen sulfide are quoted for a surface coverage of 0.410 wmol m-'
G
Y 10.
0
02
04
06
08
IO
I2
I4
I6
AMOUNT ADSORBED ( p mole m"1
Figure 1. Variation of the heats of adsorption with surface coverage for hydrogen sulfide (o), methanethiol ( X ) , ethanethiol ( A ) , and dimethyl sulfide ( 8 ) adsorbed at 423°K on y-alumina heat-treated at 700".
PRESSURE ( Torr 1
Figure 2. Adsorption isotherms for hydrogen sulfide (o), methanethiol ( X ) , ethanethiol ( A ) , and dimethyl sulfide ( E ) adsorbed at 423°K on y-alumina heat-treated at 700". The entropy curves for methanethiol, ethanethiol, and dimethyl sulfide adsorptions were very similar in shape. They exhibited negative entropy values a t their lowest coverages followed by a rapid increase in positive entropy values between 0.100 and 0.120 pmol m-2. At this latter coverage the entropy values had leveled off to 29.2, 32.3, and 26.0 cal deg-1 mol-I for methanethiol, ethanethiol, and dimethyl sulfide, respectively. The entropy curve of hydrogen sulfide was very similar to that of methanethiol u p to the maximum surface coverage measured. Table I summarizes the principal adsorption characteristics for all of the gases.
Discussion Recently4 it was demonstrated that a correlation appeared to exist between the heats of adsorption of all of these gases on silica gel and the degree of methylation of the adsorbate. A similar correlation can be made for y alumina. The heats of adsorption and adsorption capacities increase, and the entropies of adsorption decrease with increasing methylation of the adsorbate. The "water content" of the y-alumina is apparently equivalent to two monomolecular layers of water. HowThe Journal of Physical Chemistry, Voi. 77, No. 21, 1973
R. W . Glass and R . A. Ross
2578
--
I
n a
I,
'-...--
n
10
05
15
20
AMOUNT ADSORBED ()1 mole rn+)
Figure 3. Experimental differential molar entropies of adsorption for hydrogen sulfide (o), methanethiol ( X ) , ethanethiol ( A ) , and dimethyl sulfide ( 8 )adsorbed at 423°K on y-alumina heattreated at 700'; -----, "mobile" adsorption entropy values. ever, it has been shown7 that water molecules that are not desorbed as such during heat-drying up to 300" react to form surface hydroxyl groups. Thus, it can be assumed that after heat treatment for 20 hr a t 700" a large proportion of this water is present in the bulk of the adsorbent, while not excluding the possibility that a large number of hydroxyl groups exist on the surface. Thus the surface of the 700" y-alumina may be considered to be similar with regard to hydroxyl group distribution to that for silica gel, and similar properties may be expected. I t is pertinent to note that infrared studies of the adsorption of hydrogen sulfide on alumina8 suggest that fairly strong interactions occur involving the formation of hydrogen bonds. A comparison was made of the shifts of the SH stretching frequency to a lower value, and of the HSH bending frequency to a higher value, with the shifts obtained for adsorbed water and it was concluded that a likely representation of adsorbed hydrogen sulfide was
/
I
H H
Al
/ \ A color change from white to bright violet was noted on adsorption of hydrogen sulfide on the y-alumina a t room temperature, and it was postulated that this may have been due to dissociation of the hydrogen sulfide molecule or possibly to the formation of some higher polymeric form of sulfur. More recently, further infrared workg has suggested that decomposition of hydrogen sulfide has taken place on y-alumina. This was confirmed in the present work where the residue was pale yellow and chemical tests showed the presence of -100 pg of sulfur in an unknown state. It should be noted that this amount represents an extremely small fraction of available adsorbent surface and the effect on heat and entropy results can be assumed to be insignificant. The heats of adsorption of hydrogen sulfide a t 423°K fell from 32.5 kcal mol-' at a coverage of 0.020 pmol m to an almost constant value of 16.3 kcal mol-l a t 0.420 pmol m - 2 . These results compare reasonably well with isosteric heats determined for hydrogen sulfide on y-alu-
minalO a t 573 and 737°K when the values were 25 and 38 kcal mol-I, respectively, a t surface coverages of 0.170 and 0.114 pmol m-2. Entropy values calculated subsequently from these latter heats indicated that the mobility of the hydrogen sulfide molecule was highly restricted,1° possibly by reaction as a base a t Lewis acid sites on the oxide surface. While alternative mechanisms have been proposedll to account for infrared investigations on the adsorption of methyl and ethyl alcohols on alumina surfaces a t 543°K it is generally agreedllJ2 that surface alkoxy1 species are formed. Thus, all of these studies indicate that strong interactions may be expected for the adsorption of hydrogen sulfide, methanethiol, ethanethiol, and dimethyl sulfide on a y-alumina heat treated a t 700" on which Lewis acid sites are probably present.13 In the present work, no color changes were observed with the residues obtained after the adsorption of methanethiol, ethanethiol, and dimethyl sulfide and no sulfur was detected in the residues. For these gases adsorption on two different types of sites is proposed: (a) adsorption on a relatively high energy Lewis acid site, probably more significant a t low surface coverages, and (b) adsorption involving hydrogen bonds between the sulfur and hydroxyl hydrogen atoms, which may predominate a t higher surface coverages. The argument used to account for the results for the adsorption of these gases on silica gels can also be applied plausibly to explain the adsorptions a t higher surface coverages on y-alumina. Thus the influence of the methyl groups on the inductive effect and hence the strength of the hydrogen bond formed between the sulfur atom and the hydroxyl hydrogen atom should be reflected in the values of the entropy differences ( A S ) and the heats of adsorption. This is consistent with the results since the A H and A S values are, respectively, 16.0 kcal rno1-l and 10.0 cal deg-1 mol-1 for hydrogen sulfide; 16.5 kcal mol-1 and 17.3 cal deg 1 mol-1 for methanethiol; 18.4 kcal mol-I and 25.7 cal deg-1 mol-1 for ethanethiol; and 20.7 kcal mol-1 and 29.5 cal deg-1 mol--] for dimethyl sulfide. Hence a t a given coverage for a given gas, the heats are higher for adsorption on y-alumina than on silica gel. This feature may be explained by the greater acidity of the hydroxyl hydrogens on alumina14 which would give rise to stronger hydrogen bonds on association with electron donor molecules. References and Notes Present address, Ontario Research Foundation, Sheridan Park, Clarkson, Ontario, Canada. (a) See, for example, W. Strauss, "Air Pollution Control, Part I," W. Strauss, Ed., Wiley-lnterscience. New York, N. Y., 1971; (b) J. I. Paige, J. W. Town, J. H. Russell, and H. J. Kelly, U. S. Govt. Res. Develop. Rep., 70(21), 5 5 (1970). R. W. Glass and R.A. Ross, Can. J. Chem., 50, 2451 (1972). R. W. Glass and R. A . Ross, J. Phys. Chem., 77, 2571 (1973), Part I of this series. R. W.Glass and R . A. Ross, Can. J. Chem.. 49, 2832 (1971) W. J. Jones and R. A. Ross, J. Chem. SOC.A , 1021 (1967). J . B. Peri and R. E. Hannan, J. Phys. Chem., 64, 1526 (1960) A. V . Deo. i . G. Dalla Lana, and H. W . Habgood, J . Catal.. 21, 270 (1971). T. L. Slager and C. H . Amberg. Can. J. Chem., 50(21), 3416 (1971). A. J. de Rosset, C. G. Finstrorn, and C. J. Adams, J . Catai., 1 , 235 (1962). R. G. Greenler, J . Chem. Phys., 37, 2094 (1962). A. V. Kiselev and V . I. Lygin, "Infrared Spectra of Adsorbed Species." L. H. Little, Ed , Academic Press, New York, N. Y., 1966. J. B. Peri, J . Phys. Chem., 69, 220 (1965). M. L. Hair, Ed., "Infrared Spectroscopy in Surface Chemistry," Edward Arnold Ltd., London, 1967.
The Journal of Physical Chemistry, Vol 77. No. 21, 1973
