Adsorption of Cyclohexane and Methanol on Iron Oxide - American

Adsorption of Cyclohexane and Methanol on Iron Oxide R. I. RAZOUK, R. SH. MIKHAIL, and B. S. GIRGIS

Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0033.ch007

Faculty of Science, Ain Shams University, Abbassia, National Research Centre, Dokki, Cairo, Egypt, U. A. R.

The adsorption isotherms of cyclohexane and methanol vapors on synthetic lepidocrocite and goethite, and on their decomposition products prepared by heating at temperatures varying between 150° and 500°C. for different durations, have been determined. The isotherms are in general Type II (Brunauer classification) and not Type IV common to hydrous ferric oxide gels. The adsorption of cyclohexane is physical, whereas that of methanol is partly physical and partly chemical. The surface areas calculated from the total adsorption of methanol agree in most cases with those estimated from cyclohexane adsorption. Surface area-time of heating curves show that at low temperatures the area increases with time, whereas at intermediate temperatures it increases to a maximum value and then decreases; and at higher temperatures the area decreases regularly. The final limiting surface area-temperature of decomposition curve passes in both cases through a maximum at about 2 0 0 ° , above which temperature sintering is appreciable.

^ he adsorption of gases and vapors on ferric oxide and the hydrous oxide gels of iron has been the subject of numerous investigations (6, 10, 11). The surface properties of active solids prepared by thermal decomposition depend to a great extent on the physical structure of the parent material and how closely the lattice of the latter is related to that of the product (9). Iron oxide provides an interesting case, for it may be prepared from amorphous hydrous gels of indefinite composition or from well-defined crystalline hydroxides—e.g., lepidocrocite and goethite. A l most all the work on the adsorptive properties of ferric oxide has been done on products prepared from hydrous gels containing an indefinite number of water molecules and liable to aging or recrystallization. Recently Goodman and Gregg (8) studied the surface properties of iron oxide prepared from crystalline lepido42

In SOLID SURFACES; Copeland, L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

RAZOUK ET AL.

Adsorption of Cyclohexane and Methanol

43


crocite, but even then their starting material contained a high amount of adsorbed water and only near 2 5 0 ° was there an inflection in their thermogravimetric analysis curve corresponding to the formula F e O . O H . Furthermore, these measurements were mainly confined to the study of the effect of the temperature of thermal treatment, and the decomposition was carried out in the presence of air. But it is now believed that both the temperature of heating and its duration contribute to the development of the surface area of active solids. Also, the presence of air during decomposition generally gives rise to products possessing lower surface areas (18), resulting from sintering processes operative in air at temperatures lower than is required by the Hiittig mechanism (12). The object of the present investigation was to study the adsorption of cyclohexane and methanol vapors on ferric hydroxide in the form of synthetic lepidocrocite and goethite, and on the oxides prepared in vacuo by their thermal decomposition at different temperatures and for varying lengths of time. Experimental Apparatus. The adsorption of cyclohexane and methanol vapors was measured with the aid of a simple volumetric apparatus similar to that described earlier (19). Materials. Lepidocrocite was prepared by the method described by Baudisch and Hartung ( 1 ) by first forming tetrapyridinoferrous chloride and then oxidizing it and precipitating the yellow ferric oxide monohydrate. The latter was dried in an evacuated desiccator over calcium chloride and left in an electric oven at 1 0 5 ° C . to constant weight. The structure of lepidocrocite (y-Fe 0o.H 0) was ascertained from the x-ray diffraction patterns obtained with the aid of a Philips x-ray diffraction unit Type PW 1010, using a cobalt target with iron filter. Loss on ignition of the hydroxide was 11.12% (theoretical value 10.14%), and the oxide formed was hematite a - F e 0 . Goethite was prepared according to Geith (7) by treating ferrous chloride solution with a solution of ammonium carbonate, oxidizing the ferrous carbonate with hydrogen peroxide, and leaving the gel to age for 12 months. The hydroxide was dried in the manner described above. X-ray diffraction patterns proved that the material was goethite ( a - F e 0 H 0 ) . Loss on ignition was 12.71%, and the oxide formed was also a - F e 0 . The preparation of cyclohexane and methanol has been described (15, 16). 2

2

2

H

2

2

: v

2

3

Results and Discussion Adsorption of Cyclohexane. The adsorption of cyclohexane was determined on synthetic lepidocrocite and its decomposition products prepared by heating it in vacuo for varying intervals of time at 1 9 0 ° , 3 0 0 ° , 4 0 0 ° , and 5 0 0 ° , and also on synthetic goethite and its decomposition products obtained in a similar manner by heating at 1 5 0 ° , 1 8 0 ° , 2 5 0 ° , 3 0 0 ° , and 5 0 0 ° . The adsorption was found to be physical in nature, and the isotherms are Type II of the Brunauer classification (3) in all cases except on iron oxide prepared from goethite at 3 0 0 ° and 5 0 0 ° ; here the isotherms are Type III. This finding is in contrast to the Type IV isotherms common to ferric oxide gels; the difference may be due to the crystalline nature of the parent material. Typical results are shown in Figures 1 and 2. Thus Figure 1 represents the isotherms of cyclohexane on the two parent materials lepidocrocite and goethite at 3 5 ° , while Figure 2 represents the isotherms at the same temperature on iron oxide prepared from lepidocrocite by heating in vacuo at 3 0 0 ° for 0.5, 1, 3, 8, and 18 hours. The results agree with the equation of Brunauer, Emmett, and Teller (4) : In SOLID SURFACES; Copeland, L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

44

ADVANCES IN CHEMISTRY SERIES Ρ

=

±

+

(c

~

1) £

as may be judged from the linear plots of p/s(p — p) against p/po shown in the insets of Figures 1 and 2. Here s is the amount adsorbed at equilibrium pressure, p, p is the saturation vapor pressure, s is the monolayer capacity, and c is a constant related to the heat of adsorption of the first layer. It was thus possible to calculate the specific surface area of the adsorbent from the slope and the intercept of the linear plot, taking the cross-sectional area of cyclohexane as 39 sq. A. (20). 0


0

m

Figure 1.

Adsorption isotherms of cyclohexane at 35° C. I. Lepidocrocite II. Goethite

The relation between the specific surface area and time of heating at various temperatures is represented in Figures 3 and 4 in the case of goethite and lepido crocite and their decomposition products, respectively. In general, the surface area of the product increases with the duration of heating at the lower temperatures of preparation as a result of further decomposition. At intermediate temperatures, the specific surface area rises with time of heating up to a maximum and then falls to lower values, while at higher temperatures of decomposition the surface area falls regularly with the duration of heating. Thus when goethite is decomposed at 1 5 0 ° , the product possesses a surface area very close to that of the parent material. Furthermore, this area does not vary with the duration of heating and subsequently with the amount of water loss; In SOLID SURFACES; Copeland, L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.


RAZOUK ET AL.

0


0.2

0.4

0.6

P/P Figure 2.

45

0.8

0

Adsorption isotherms of cyclohexane on ferric oxide prepared from lepidocrocite by decomposition at 300° C.

I, II, III, IV, V.

Isotherms at 35° on products obtained by heating for 0.5, 1, 3, 8, and 18 hours

for decomposition at this temperature is slow, and even after heating for 7 hours only 41% of the water content is eliminated. X-ray measurements show that the diffraction patterns of goethite and its product of decomposition in vacuo at 1 5 0 ° are identical, so that the water is driven off from the crystal, leaving a pseudolattice and also a pseudomorph (5) possessing the same surface area. The remaining water may act as a stabilizer for the pseudomorph. Decomposition at 1 8 0 ° , however, produces a product possessing the patterns of hematite ( a-Fe O ) with broadened lines indicating the existence of strain in the lattice which results in the observed increase in the surface area. At 2 5 0 ° , the pattern of the normal lattice of a-Fe 03 is obtained, and the surface area diminishes regularly as a result of sintering. The behavior of lepidocrocite on decomposition is slightly different. Heating in vacuo at 1 9 0 ° leads to an increase in surface area with time of heating during the first 2 hours when the water loss amounts to 3.3%, and then the area remains constant, although prolonged heat treatment produces further loss of water, attaining 7.7% after 60 hours and 10.5% after 400 hours. X-ray measurements show that on heating at this temperature, the pattern of lepidocrocite disappears and that of maghemite (y-Fe 0 ) makes its appearance, in agreement with the results of Bernai, Dasgupta, and MacKay (2), although the lines are broadened. The independence of the surface area upon the water content at this temperature may 2

s

2

2

3



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0

5

10

15

TIME, HOURS Figure 3. Effect of duration cf heating on specific surface area of products prepared from goethite at various temperatures I. Goethite II, III, IV, V, VI.

Products obtained by heating at 151°, 180°, 250°, 300°, and 500° C.

be explained by assuming that the development of the surface area resulting from decomposition is just counterbalanced by sintering. When decomposition takes place at 3 0 0 ° , activation occurs only at the very early stages of heating, probably as a result of the transformation of maghemite into hematite about this temperature, as was found from x-ray diffraction patterns and also from earlier observations (2). Beyond this stage and above this temperature sintering becomes dominant. A limiting surface area is always obtained after a sufficiently long heat treatment, and this value is characteristic of each temperature, in agreement with the results obtained with magnesia (17). In Figure 5 the limiting specific surface areas are plotted as a function of the temperature of preparation. The same behavior is encountered in lepidocrocite and goethite. But the area is invariably greater in case of goethite and its products, unless decomposition is carried out at 5 0 0 ° , when the two areas become very small and close to each other. Probably the higher water content in synthetic goethite (12.71%) than in lepidocrocite (11.12%) accounts for the greater activity of the former and its products of decomposition; for the excess of water over the stoichiometric value may be present on either the surface or as a solution in the lattice (21), and in the latter case its presence and removal will induce lattice strains which lead to an increase in the surface area. In both cases, the maximum limiting area is developed at about 2 0 0 ° , and above this temperature sintering becomes appreciable. This temperature is lower than would be expected for ordinary sintering to occur according to the theory of Hiittig (12), but agrees with the results of Goodman and Gregg (8). During the early stages of the sintering process, and particularly at lower ternIn SOLID SURFACES; Copeland, L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

RAZOUK ET AL.


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peratures where there is substantially very little lattice mobility, surface diffusion provides the major contribution to the sintering process. This effect increases rapidly with temperature, as does the depth of the layer over which it operates, and may still persist even at temperatures which permit volume diffusion of the material to take place.

0

5

10

15

60„

TIME, HOURS Figure 4. Effect of duration of heating on specific surface area of products prepared from lepidocrocite at various temperatures I. Lepidocrocite Zi, III, IV, V. Products obtained by heating at 190% 300% 400°, and 500° C. Adsorption of Methanol. The adsorption isotherms of methanol on lepidocrocite and goethite and their decomposition products are also sigmoid-shaped Type II. However, with the exception of lepidocrocite itself, there is considerable hysteresis, and outgassing at room temperature does not completely remove the adsorbate. Subsequent isotherms obtained after thorough degassing of the preexposed solid at room temperature were reversible and invariably lower than the initial isotherms. In the light of the criteria put forward by Kipling and Peakall (13), the adsorption is regarded as partly physical and partly chemical. The difference between the two isotherms corresponds therefore to the amount of methanol chemisorbed. The results of typical experiments are shown in Figure 6. The isotherms of methanol also obey the Brunauer, Emmett, and Teller equation, as may be judged from the linear plots in the inset of Figure 6. The surface areas of the various specimens estimated by applying the Brunauer, Emmett, and Teller equation (4) to the initial adsorption isotherms and using the value of 18 sq. A. (14) are compared with the surface areas calculated from the adsorption of cyclohexane in Table I. The surface areas calculated from the total adsorption of methanol on lepidocrocite as well as its products formed by

American Chemical Society Library



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100°

200°

300°

400

c

TEMPERATURE Figure 5. Effect of temperature of preparation on limiting surface areas of products prepared from lepidocrocite (I) and goethite (II) thermal decomposition at temperatures below 4 0 0 ° are in fair agreement with the areas of the products calculated from the adsorption of cyclohexane. With products formed at 4 0 0 ° and 5 0 0 ° , the surface areas calculated from the adsorption of methanol are appreciably greater than those estimated from the adsorption of cyclohexane. These differences may be attributed to the larger size of the cyclohexane molecule in comparison with that of methanol. At the higher temperatures where sintering is liable to take place readily, most of the pores will have sizes accessible to the molecules of methanol and not to those of cyclohexane. Table I.

Material Lepidocrocite

Goethite

Surface Areas Calculated from the Adsorption of Cyclohexane and of Methanol Thermal Treatment Temp., Duration, °G. hours 35 — 190 60 300 0.5 3 8 400 0.5 500 0.5 1 3 35 — 150 0.5 7 250 0.5 5 300 0.5 2.5 10

Water Content,

%

11.12 3.4 0 0 0 0 0 0 0 12.7 9.8 7.5 2.1 1.4 0.1 0.1 0.1

Surface Area, Sq. M . / G . Cyclohexane Methanol 81 80 110 117 120 125 81 76 59 49 20 34 7 23 7 17 7 10 122 71 125 71 125 71 122 109 85 70 85 84 63 69 58 65



RAZOUK ET AL.

0


0.2

0.4

0.6 P / P

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0.8

0

Figure 6. Adsorption-resorption isotherms of methanol at 35° C. I. Lepidocrocite II. Goethite, adsorption lia. Goethite, resorption However, in the case of goethite and its products of decomposition formed at 1 5 0 ° , the surface area calculated from the total adsorption of methanol is appreci ably less than that calculated from the adsorption of cyclohexane, whereas with products formed at the higher temperatures the two values are in fair agreement. It is not possible at present to give a satisfactory explanation for this peculiar be havior of goethite and of the products of decomposition formed at 1 5 0 ° . Proba bly the excess of water above the stoichiometric content for goethite (which is eventually greater by 1.6% than for lepidocrocite), as well as the high water con tent of the partly decomposed products obtained at 1 5 0 ° accounts for the lower adsorption of methanol. The assumption is made that some water molecules already occupy sites which otherwise would be occupied by methanol molecules. In all cases, except with lepidocrocite where the adsorption is physical in nature, the fraction of the surface covered by physically adsorbed methanol is approximately 40 to 60% of the total surface, and both types of adsorption take place side by side, presumably on different sites. Literature Cited

(1) Baudisch, O., Hartung, W. H., "Inorganic Syntheses," H . S. Booth, ed., p. 185, McGraw-Hill, New York, 1939. (2) Bernal, J. D., Dasgupta, D.R., MacKay, A. L., Nature 180, 645 (1957). (3) Brunauer, S., "Adsorption of Gases and Vapors," p. 150, Oxford University Press, 1944. (4) Brunauer, S., Emmett, P. H., Teller, E., J. Am. Chem. Soc. 60, 309 (1938). (5) "Dana's System of Mineralogy," 7th ed., p. 683, Wiley, New York, 1958. (6) Foster, A. G., et al., Froc. Roy. Soc. (London) A136, 363 ( 1932); A147, 128 (1934); J. Chem. Soc. 1946, 446; 1952, 1139. (7) Geith, Μ. Α., Am. J. Sci. 250, 677 (1952). (8) Goodman, J. F., Gregg, S. J., J. Chem. Soc. 1956, 3612. In SOLID SURFACES; Copeland, L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

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(9) Gregg, S. J., Ibid., 1953, 3940. (10) Gregg, S. J., et al., Ibid., 1953, 3945. (11) Harkins, W. D., et al., J. Chem. Phys. 14, 117 (1946); J. Am. Chem. Soc. 72, 3427 (1950). (12) Hüttig, G. F., Kolloid-Z. 98, 63 (1942); 99, 362 (1942). (13) Kipling, J. J., Peakall, D. B., J. Chem. Soc. 1957, 834. (14) Razouk, R. I., ElGobeily, Μ. Α., J. Phys. Colloid Chem. 54, 1087 (1950). (15) Razook, R. I., Mikhail, R. Sh., Ain Shams Sci. Bull. 4, 147 (1959). (16) Razouk, R. I., Mikhail, R. Sh., J. Phys. Chem. 61, 886 (1957). (17) Ibid., 63, 1050 (1959). (18) Razouk, R. I., Mikhail, R. Sh., 2nd Intern. Congr. on Catalysis, Paris, July 1960. (19) Razouk, R. I., Salem, A. S., J. Phys. Colloid Chem. 52, 1208 (1948). (20) Smith, N., Pierce, C., Cordes, Ν., J. Am. Chem. Soc. 72, 5595 (1950). (21) Weiser, Η. B., "Inorganic Colloid Chemistry " Vol. II, p. 29, Wiley, New York, 1935. 1961.


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Adsorption of Cyclohexane and Methanol on Iron Oxide - American

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