ADSORPTIONOF CYCLOHEXANE ON ALUMINAS
1805
Adsorption of Cyclohexane on Aluminas Prepared by Thermal Decomposition of Aluminum Hydroxide in Vucuo and in Presence of Air
by R. I. Razouk, R. Sh. Mikhail, and G. R. Iskander Chemistry Department, Faculty of Science, Ain Shams Univerdy, Abbassia, Cairo, U.A.R.
(Received M a y 12,1964)
The adsorption of cyclohexane vapor was measured on the products of thermal decomposition of two parent crystalline hydroxides of aluminum. The effect of temperature of preparation, duration of heating, and presence of air during decomposition was studied. In agreement with the results obtained with similar systems previously investigated, decomposition at low temperatures results in a marked increase in specific surface area, but sintering develops a t higher temperatures. The latter is enhanced by rise of temperature, by increase of duration of heating, and also by presence of air during the decomposition process.
Introduction Several factors are known to affect markedly the surface properties of active solids prepared by thermal decomposition.’ The presence of air during the heat treatment is of particular interest owing to the relatively limited work cited in the literature in this connection although active solids are usually prepared under this condition. In an earlier investigation,2 it was found that the presence of air during the preparation of magnesium oxide by thermal decomposition of the hydroxide and carbonate invariably gives rise to products possessing much lower surface areas which in certain cases are only one-tenth of the surface of the product prepared in vacuo under similar conditions. Similar behavior was recently observed in the production of calcium oxide by thermal decomposition of the hydroxide.a The present work has been undertaken to investigate the dependence of the adsorptive properties on the absence or presence of air during the dehydration of two crystalline preparations of aluminum hydroxide. Both the effect of temperature of dehydration and duration of thermal treatment were studied. The nature of the solid phases was also investigated by means of X-ray analysis.
Experimental Apparatus and Technique. A conventional volumetric system was used for the adsorption measure-
ments. X-Ray diffraction patterns were obtained with the aid of a Philips 114.23 Debye-Schemer powder camera with Ni-filtered Cu radiation. Materials. Adsorption measurements were carried out on the products obtained from two parent preparations of aluminum hydroxide by dehydration in vacuo and in presence of air at 110, 200, 350,500, 650,800, and 950”. The term ‘(in vacuo” means here while pumping off air and gaseous products, keeping the pressure less than 10-3 mm., and the term (‘in presence of air” means while the material is in contact with “nondry” air containing water vapor formed by dehydration of the hydroxide. Preparation G was obtained by the slow addition of ammonia (20.3%) to a solution of aluminum chloride until the supernatant liquid had pH ca. 8. The precipitate was washed with dilute ammonia and then dried in a vacuum oven at 50”. The water content was 34.4% as compared with 34.65 for the trihydrate. X-Ray diffraction patterns possessed clear and sharp lines which gave d and I values characteristic of gibbsite. Preparation B was obtained by adding ammonia to a (1) See, e.g., 8. J. Gregg, “The Surface Chemistry of Solids,” 2nd Ed., Reinhold Publishing Corp., New York, N. Y.,1961, p. 300 et seq. (2) R. I. Razouk and R. Sh. Mikhail, Actes C o w . Intwn. Catalyse, 9, Par&, 1860,2023 (1961). (3) R. Sh. Mikhail, J . Phys. Chem., 67, 2060 (1963).
Volume 69, Number 6
June 1966
1806
boiling solution of alum. The precipitate was washed with ammonia and dried in a vacuum oven a t 40". The water content was 33.6%. X-Ray powder photographs gave clear patterns, and the d and I values corresponded to those of bayerite. Cyclohexane was prepared in a pure state by the method described earlier.4
R. I. RAZOUK, R. SH. MIKHAIL, AND G. R. ISKANDER
0.3
r-7
Results Adsorption Measurements. Preliminary experiments on the adsorption of cyclohexane on the parent hydroxide preparations did not give concordant results, but the data obtained with the products of dehydration formed a t and above 110" were well reproducible. The present work is therefore confined to these products. In all cases the adsorption is rapid, and thorough outgassing a t room temperature removes all the adsorbate. The isotherms are type I1 of Brunauer's clas~ification.~Desorption isotherms form with adsorption isotherms small hysteresis loops which join together at relative vapor pressures depending on the temperature of dehydration. The hysteresis loops are of type A combined with a little of type E of de Boer's classification,6and the pore spectrum of all specimens is very broad as might be judged from the broad region of relative vapor pressure over which the hysteresis loop extends. A typical set of results is represented in Figure 1, which shows the adsorption isotherms of cyclohexane on the products of dehydration of preparation G formed at 350, 500, 650, 800, and 950" by heating in vacuo for 5 hr. Representive desorption isotherms are also drawn for products formed a t 350,650, and 950". The isotherms are found to obey the B.E.T. equation7 in the normal range of pressure. The specific surface areas of the various products were calculated from the monolayer capacity and by taking the crosssectional area of the cyclohexane molecule as 39 .&.2.8 Previous work in this laboratory has shown that the adsorption of cyclohexane a t ordinary temperature could be successfully used to measure the surface area of magnesium oxide4and iron oxide. For the purpose of comparison, the specific surface areas of certain products were estimated by the standard low temperature nitrogen adsorption and were found to be in satisfactory agreement with present values obtained from the adsorption of cyclohexane. Thus, the specific surface areas calculated from cyclohexane and nitrogen adsorption were 244 and 220 m.a/g. for the product obtained from preparation G by dehydration in vacuo a t 350", 246 and 228 m.2/g. for the product obtained a t 650", 143 and 134 m.2/g. for the product formed by The J O U Tof ~Physical Chmi8try
P/PO.
Figure 1. Adaorption-desorption isotherms of cyclohexane on the products of dehydration of preparation G formed in vacuo at various temperatures: 0, adsorption; a, desorption.
dehydration in presence of air a t 650", and 37 and 34 m.'J/g. for the product formed a t 950". Experiments on the effect of duration of heating (Figure 2) show that when dehydration is carried out in vacuo below 650°, the specific surface area increases with the duration of thermal treatment as a result of further decomposition although the increase is found to be far less than corresponds to the decrease in the water content. Dehydration a t 800" for 0.5 hr. gives rise to a product of maximum surface area, but heating for longer periods leads to progressive diminution in the specific surface area even though it is accompanied with further slight decomposition. Thermal treatment a t higher temperatures leads to a continuous decrease in surface area. However, when dehydration is conducted in presence of air, prolonged heating results in decreasing the specific surface area even a t low temperatures although the (4) R. I. Razouk and R. Sh. Mikhail, J. Phvs. Chem., 61,886 (1957). (6) 9. Brunauer, "Physical Adsorption of Gms and Vapors," Oxford University Press, London, 1944,pp. 149 et seq. (6) J. H.de Boer, "The Structure and Properties of Porous Materials," D. H. Everett and F. 8. Stone, Ed., Buttemorths Scientific Publications, London, 1968,p. 68 et seq. (7) S. Brunauer, P. H. Emmett, and H. Teller, J. Am. Chem. SOC., 60,309 (1938). (8) R. N.Smith, C. Pierce, and H. Cordes, ibid., 72,5595 (1950). (9) R. I. Razouk, R. Sh. Mikhdl, and B. 8. Girgk, Advances in Chemistry Series, No. 33, American Chemical Society, Washington, D. C., 1961,p. 42.
1807
ADSORPTION OF CYCLOHEXANE ON ALUMINAS
.? -
.
30
E * B 8
v
20
B
10
P
0
200
400
600
800
Temperature of dehydration ("C.). 0
10 0 Duration of heating (hr.).
10
Figure 2. Effect of duration of thermal treatment on the specific surface area (upper curves) and the water content (lower curves) of the products of dehydration of preparation G formed in vacuo (left) and in the presence of air (right) a t various temperatures.
percentage of decomposition niight increase several-fold. The only exception is for an initial period not exceeding 2 hr. at temperatures below 650" when an increase in area accompanies the early stages of dehydration. This emphasizes the onset of sintering in presence of air a t a temperature much lower than in vacuo. The effect of temperature of dehydration on the specific surface area of the products formed by heating preparations G and B for 5 hr. in vacuo and in presence of air is represented in Figure 3. It is evident that dehydration in vacuo is accompanied by development of larger surface area and that the temperature at which maximum activity is obtained is slightly higher in vacuo than in presence of air. However, when comparison is made for equal water contents rather than for equal durations of heating, the diminution in the specific surface area of products formed in presence of air becomes less marked in some cases. It is to be noted, however, that the adsorption values near saturation vapor pressure are, by far, less dependent on the thermal treatment than the specific surface areas themselves. This implies that rise of temperature leads to changes in the average pore radius while the pore volume remains essentially the same. This is in agreement with the observed general tendency of the relative vapor pressure at which the hysteresis loop closes up to be displaced toward higher values as the temperature of thermal treatment is raised. Structural Changes Accompanying Dehydration. XRay diffraction patterns were obtained for both prep-
Figure 3. Specific surface areas of products formed by the dehydration of preparation G ( 0 )and preparation B (0) a t various temperatures in vacuo (continuous lines) and in presence of air (dotted lines).
arations and their dehydration products. Comparison with the A.S.T.M. cardslo shows that preparations G and B are mainly gibbsite (a-A1&3Hz0) and bayerite (/3-A1203.3Hz0),respectively. Drying at 110" in presence of air leaves the patterns unaltered although the water content falls by approximately 15 and 30%, respectively, below the stoichiometric water content in the trihydrate. Similar retainment of a pseudo-structure on dehydration at low temperatures was observed in the thermal decomposition of goethite and lepidocr~cite.~ The changes observed in the patterns of the various products of dehydration are found to be in general agreement with the results of other investigators.ll Thus, when preparation G is heated in vacuo at 200" for 2 hr., the water content falls to 24.91%, and most of the lines which give the characteristic d spacings of gibbsite still remain clear. These lines, however, disappear on dehydration at 350". The product formed between 350 and 800" possess diffuse patterns which are difficult to identify, but the product formed at 950" possesses a pattern of sharp lines corresponding to 6and 8-alumina. The product obtained by dehydration of preparation B at 200" in vacuo for 5 hr. (containing 7.16% water) gives a pattern in which the characteristic lines of bayerite disappear, and some of the lines of boehmite develop although in diffuse form. Products formed be(10) J. V. Smith, Ed., "X-ray Powder Data File," American Society for Testing Materials, Philadelphia, Pa., 1960. (11) See, e.g., M. K. B. Day and V. J. Hill, J. Phys. Chem., 57, 946 (1953); R. Tertian and D. PapBe, J . chim. phys., 55, 341 (1958).
Volunze 69, Number 6 June 1966
1808
tween 350 and 800" give diffuse patterns, but the product formed at 950" possesses a pattern in which some lines characteristic of q- and 0-alumina develop. It is interesting to note that the patterns of the products formed at high temperatures in presence of air were found to exhibit sharper lines than in case of corresponding products formed in vacuo, indicating that the development of the crystalline structure proceeds more readily when dehydration is conducted in presence of air.
Discussion Crystalline hydroxides are frequently used as starting materials for the preparation of active solids because their specific surface areas increase considerably upon heating. Generally, dehydration gives rise to pseudo-morphosis, and the volume of the solid does not vary much. Meantime, water is expelled during dehydration, and space will be created in the porous product. On heating to higher temperatures, however, the surface area of the solid decreases again. I n the course of study of the surface properties of oxides of magnesium, iron, and calcium produced by thermal decomposition of hydroxides and carbonates,2-4~9~12 it has been postulated that during thermal treatment, the mechanism which determines the surface area-temperature relationship is governed by three processes, namely, (i) decomposition, (ii) recrystallization, and (iii) sintering. For crystalline parent materials which decompose to yield ultimately crystalline products, a maximum in the surface areatemperature of dehydration curve might be expected, the ascending branch of the curve being due mainly to the first two processes, while the descending branch to sintering. The three processes overlap in the neighborhood of the maximum. The results obtained in the present investigation using two crystalline hydroxides of aluminum are in good agreement with this picture. Besides the important contribution of temperature and duration of heating in determining the specific surface area of the products of thermal dehydration, the presence of air during the preparation of active solids affects considerably the surface area-temperature curves, giving rise to smaller areas and the displacement of the maximum toward lower temperatures. The diminution of the surface area observed when dehydration is conducted in presence of air is very probably associated with the formation of crystallites of larger grain size. Investigation by the electron microscope on both series of products has shown that thermal
The Journal of Physical Chemistry
R. I. RAZOUK, R. SH. MIKHAIL, AND G. R. ISKANDER
dehydration in presence of air is accompanied by (i) the development of larger grain size, the higher the temperature the greater is the difference between the sizes of particles formed in vacuo and in presence of air, and (ii) the development of better crystalline morphology.la The lower surface areas of the products prepared by dehydration in "nondry" air are ascribed mainly to the presence of water vapor in contact with the decomposing material. The enhanced sintering by water formed during catalytic cracking14 and by steam15 is known. de Boer and Vleeskens16found that surface hydration leads to pore volume and surface area decrease. Anderson and Morgan1' have recently shown that, in the sintering of magnesium oxide, crystal agglomeration and crystal growth are accelerated by the presence of water vapor during thermal treatment and that mere water adsorption does not promote the sintering effects but that a certain mobility and rapidity of exchange of water molecules on the surface is required. Nevertheless, the effect of dry air on reducing the surface area during thermal treatment cannot be overlooked, for heating in presence of dry air has been found to reduce considerably the specific surface area of products formed in vacuo and possessing maximum activity. A similar behavior was observed also with magnesium oxide.4 However, as some water is still retained by these products, it is not possible before further experimentation under better defined conditions to find out the contribution of water vapor and of air separately nor to ascertain whether the diminution in surface area and the increase in grain size take place during the act of decomposition itself or after its completion.
Acknowledgment. The authors wish to express their thanks to Dr. S. Brunauer for his kind permission to R. Sh. M, to carry out the experiments on the low temperature nitrogen adsorption in the Research and Development Laboratories of Portland Cement Assocn., Skokie, Ill., and to Dr. L. E. Copeland for performing and interpreting the electron microscope work. (12) R. I. Razouk and R. Sh. Mikhail, J. Phys. Chem., 63, 1050 (1959). (13) See ref. 3,Figure 5. (14) H.E.Ries, Jr., Advun. C a t d y s b , 4,87 (1950). (15) C.R. Adams, J . Phys. Chenz., 67,313 (1963). (16) J. H.de Boer and J. M. Vleeskens, Koninkl. Ned. Akad. Wetenschap., Proc., B66, 234 (1957). (17) P.J. Anderson and P. L. Morgan, Trans. Faraday SOC.,60,930 (1964).
