592
S. J. GREGG AND K. S. W. SISO
T H E EFFECT OF HEAT TREATMEKT ON T H E SURFACE PROPERTIES OF GIBBSITE. I ADSORPTION ISOTHERMS OF NITROGEN AND
OF
OXYGENAT
- 182.7"C.
S. J. GREGG A N D K . S. W . SING Washington Singer Laboratoraes, Prince of Wales Road, Exeter, England Received January 30, 1960
The effect of heat treatment on the surface properties of hydrated alumina has been studied by a number of workers, using adsorption methods. Precipitated alumina (3, 18, 19) and commercial Activated alumina (13), both of unknown composition, have been adopted as starting materials, as well as natural gibbsite (y-Alt03.3H20) (15) ; adsorbates for vapor adsorption have included ethylene (3), carbon tetrachloride (18),pyridine (19), hydrogen sulfide (2), nitrogen (13, 15), and various alcohols (l), whilst the dye Waxoline Blue (22) has been used as adsorbate for adsorption from solution. Comparison of the results reveals a wide variation both in the temperature and in the residual mater content which correspond to maximum '"activity"; figures given for the optimum temperature vary between 340°C. and 950°C. and for the optimum residual water content between 1 and 9.5 per cent. This variation is perhaps not unexpected for a number of reasons: (a) the optimum conditions depend on factors frequently unspecified, such as the exact method of preparing the starting material, the weight of the sample (20), and the type of furnace; (b) the time of heating is an important yariable (23); ( c ) the adsorption capacity is often derived from some arbitrary point on the isotherm; moreover, in most cases the detailed isotherm is lacking. It mas accordingly decided to undertake a systematic investigation with pure precipitated Bayer Hydrate as starting material, determining reasonably detailed isotherms of nitrogen and of oxygen a t - 182.7"C., and of carbon tetrachloride papor a t around 25"C., after heating to controlled temperatures for a definite time under standard conditions. Other properties measured included heat of immersion in carbon tetrachloride, mater content, specific gravity, and bulk density. Besides providing a picture of the chemical and physical changes occurring during the thermal decomposition of gibbsite, it was hoped that the results would yield useful information concerning the nature of adsorption on porous solids. For comparative purposes a number of experiments were also done with commercial activated alumina. EXPERIMESTAL
Materials The gibbsite was taken from a batch (Batch I) of pure Bayer Hydrate powder supplied by Messrs. Peter Spence and Sons Ltd. and made by the Bayer process. Analysis showed the impurities to be 0.016 per cent ferric oxide and 0.17 per
EFFECT OF HE.11' O S SCRF.ICE PROPERTIES OF GIBBSITE. I
593
cent sodium oxide. For temperatures up to 1000°C. samples, each of about 10 g., were heated in a platinum crucible suspended in a vertical electric furnace; for higher temperatures a platinum boat in a horizontal electric furnace \vas used. I n each case the furnace was adjusted to the required temperature, and the sample then inserted and kept there for 5 hr. ; the temperature was generally constant to ivithin =t5"C. On removal from the furnace a portion of the sample was immediately introduced into the iveighed bulb D (figure 1 !.
FIG. 1 . Volumetric adsorption apparatus. A , gas buret; B, manometer; C, three-way t:rp D , I,ulh containing adsorbent; E, Dewar flnsk containing liquid osygen.
The commercial activated alumina was supplied by the makers (llessrs. Peter Ppence and Sons Ltd.) in granular form. It is stated by them t o be boehmite (y-Al,Oy.H,O~of fairly high purity. An ayerage analysis of the product gives 12 per cent water, 3 per cent sulfur trioxide, and ca. 0.1 per cent ferric oside. This material was heated for 5 hr. at one temperature only, ti?., 430°C. Sitrogen and osygen were obtained from commercial cylinders, several liters being stored in glass reseri-oirs attached o,t the apparatus. The gases were not further purified, except for drying. The nitrogen was reported ( 5 ) to be about 99.9 per cent pure, with argon as the impurity. The efiect of drying the nkrogen
594
S. J . GREGG .\SD E(.
9. IV. S I S G
was nearly but not, quite negligible, leading only t o a slight increase in adsorption a t higher pressures. Helium, spectrally pure, was obtained from the makers in a soda glass container, and this was fused on to the apparatus.
A p p a r a t u s and proccdurr The isotherms of osygen and of nitrogen were determined by a volumetric technique. Since the adsorptions t o be measured Irere fairly large, it was possible t o devise an apparatus of relatively simple design. special feature was the combination of manometer and gas buret in one unit, the essentials of which are shown in figure 1. The gas buret (A), made from an ordinary high-grade "liquid" buret, \vas set upright and formed the left-hand limb of the manometer. Glass tubing of the same diameter was joined on a t the bottom. after cutting off the buret tap, and formed the right-hand limb (B) of the manometer. The complete unit was mounted on a \rooden stand. -1mercury-sealed three-way tap (C), fused t o the top of the gas buret, made connection either to the atlsorbent bulb (D) through a B.10 standard joint or to the gas storage v e n d s (not sho\rn). The adsorbent could be outgassed by loivering the mercury in -4 and in B below the T-bend and pumping by means of a mercury diffusion pump hacked by a Speedivac pump. (The storage vessels etc. could be evacuated through independent lines.) The whole apparatus, except for pumps, was en' closed in a large air thermostat maintained, within iO.l"C., a t 25-25.5'C'. .ifter introduction of the adsorbent, it iras found advantageoue to plug the neck of the bulb with a small amount of asbestos \roo1 to prevent spurting driring desorption; blank tests proved adsorption of nitrogen and of osygen on the asbestos to be negligible. Each sample was outgassed for 2 hr. at 200°C. before the liquid-osygen bath (E) was placed in position around the bulb. I n calculating the amount of gas adsorbed a t each pressure, the usual corrections were made for the deviations of nitrogen and of osygen from the perfect gas laws a t - 163°C. Dead-space determinations were made x i t h helium in the usual way. The temperature of the liquid-osygen bath \vas calculated (0. i ) from the saturated vapor pressure, PO, of the osygen, irhich was determined at intervals; it \Vas found t o be 90.50" =k 0.02"K. (corresponding to PO = i 8 . G i. 0.08 cm.). An apparatus with a gas buret and manometer combined into a single unit has also been described by Iirieger (14). It consists, on the buret side, of a series of bulbs, but since it has no three-\vay tap or valve corresponding to C, only a limited number of points can be determined on the isotherm. A$
RESULTS . I S D DISCUSSION
The isotherms of nitrogen are sho\rn in figure 2. (all isotherms will be given in graphical form, plotting the amount 21 (in cubic centimeters a t S.T.P.) adsorbed per gram of sample against thc relative pressure, p p o , a t equilibrium.) They appear t o be both reproducible and reversible: desorption points and further atltorption points after a second outgassing at 200OC. a!1 lie on the s:ime
EFFECT O F H E i T O S SURF.ICE PROPERTIED O F GIBB*ITC. I
595
smooth curve, so that hysteresis is absent up to the highest pressures attainable (p,'po = ca. 0.25). I n each case an initial steep portion, where the amount adsorbed increases rapidly with increase of pressure, is followed by n long practically linear portion of lesser slope, the exact value of which varies from one sample t o another. The sample prepared a t 410'C. is the best adsorbent for nitrogen a t all relative pressures up to 0.25; that prepared at 200°C., as \vel1 a s those at llOO°C.and above, adsorbed negligible amounts.
FIG.2. Isotherms of nitrogen n t -182.i"C. on gibbsite heated to the following temperatures f o r 5 hr.: c u r v e I , 290'C.; curve 11, 1010°C.; curve I I I . 9 2 0 T . ; curveIV, 350°C.; curve I-,83O'C.; curve V I , i95'C.; curve VII, 6 9 5 T . curve V I I I , 595°C.; curve I S , 500°C.; curve X , 460OC.; curve S I , 410°C. O A U V , adsorption, first run. O A , desorption, first run. Q q,:idsorption, second run. e, desorption, second run. Abscissa: relative pressure. Ordinate: adsorption in cubic centimeters (S.T.P.) per gram of solid. FIG.3 . Isotherms of osygen at -182.i"C. on gibbsite heated for 5 hr. :it 460°C. (sample L,)and i95"C. (sample SI',. 0 , adsorption for sample LI; 0 , desorption for saml)le L I 0, adsorption, first run; 0 , desorption, first r u n . Q , adsorption, second r u n , for s:rn1ple SI. Ordinate and abscissa a s in figure 2.
The isotherms of oxygen on two of the samples, Ll (4GO"C.) and SI(i95"C.), are given in figure 3. These could be taken to relative pressures close to unity and they show some interesting features; they are reversible only at low relative pressures ( < ca. 0.35 for sample L1 and < ca. 0.5 for sample SI), and a hysteresis loop is present over the whole of the higher pressure range. It is important to note t,hat, although these two isotherms a t the lowpressure end are very similar in relative position t o the nitrogen isotherms on the same samples (figure 2), they cross a t p / p o = ca. 0 . i 0 , so that above this pressure their positions are reversed: sample L1 adsorbs more oxygen per gram a t relative pressures below 0.7, whilst sample SIadsorbs more above this pressure. This clearly emphasizes the
.jOG
S. J. GREGG
.4KD IC. S. W. SIKG
necessity of considerable caution in selecting a single point on an isotherm as a measure of the activity of the adsorbent. The isotherm of nitrogen on commercial activated alumina was very similar in form t o those on decomposed gibbsit,e, but the amounts adsorbed at a given pressure were greater than in any of the curves of figure 2 ; a t relative pressures of 0.10 and 0.20, for example, the adsorption was 51.0 cc. and 59.0 cc., respectively ( c j . references 8, 9, and 13). The isotherm of oxygen on commercial activated alumina was, however, rather different from those on decomposed gibbsite ( c j . figure 3); it was of the "closed-loop" type, obtained, for example, with nitrogen on porous glass (10, 11) a t -195"C., with benzene on ferric oxide gel (16) a t 40°C., and with water on silica gel (17) a t 30°C. The form of the isotherms of oxygen on the decomposed gibbsite is less usual (in that hysteresis persists to the highest pressures reached) but has been obtained, for instance, with water vapor on silica gel (24) and on tantalum pentoxide gel (25) (both prepared under special conditions); it is extremely similar t o that obtained with carbon tetrachloride vapor on similar samples of decomposed gibbsite (12) a t 25'C. An interesting phenomenon noticed in the present work was the difference in the time of equilibration, t,, over the various regions of the adsorption isotherms. with nitrogen, equilibrium was reached between 10 and 30 min. after the adniission of gas; except for the first admission, t, was somewhat longer at higher than a t lower pressures (cf. reference 21). Much more noticeable were the differences in 1, found with the oxygen isotherms: a period of several hours was required on the steep portions of the hysteresis loop (especially on desorption), \\hereas 20 min. or so was sufficient on the remainder of the isotherm. Lambert and Clark (16), on the other hand, using benzene on ferric oxide gel, reported an increase in t , over the whole of the loop. With oxygen the hysteresis loop appeared t,o be real-points on it showed no shift on redetermination after leaving overnight. The absence of hysteresis with nitrogen is not surprising, since with oxygen, likewise, the effect is not found until the pressure exceeds ca. 0.3. A full discussion of the results will be deferred to a later paper in the series, but the curve of surface area, S , of the decomposed gibbsite against the temperature of preparation is given in Part 1.I (see figure 6, curve 11). S has been calculated by the usual B.E.T. procedure (4),with the aid of the equation p 4pa
- p)
-
1 Z'mC
+ -( c. 2 VmC
1)
PQ
where ti,, is the monolayer capacity and c is a constant related to the heat of adsorption; the conversion from v, to S has bee! made by taking t$e customary cross-sectional molecular areas: nitrogen, 17.0 A.2; oxygen, 14.1 The sharp rise in S between 300°C. and 400°C. will be noted and thereafter the rather gradual fall, until a t 1100°C. it has almost reached zero. I t is interesting, for example, that a t the high temperature of 1OOO"C. the surface area is still quite large. The points derived from the osygen isotherm do not fall 011 the same curve as those from the nitrogen isotherm and the discrepancy much exceeds experiment a1 error.
EFFECT OF HEAT ON SURFACE PROPERTIES OF GIBBSITE. I1
597
REFERESCES (1) ;\LEKSEEVSKIi: J. RUSS. Phys. Chem. S O C . 62, 221 (1950). (2) BYLEY: Can. J. Research 10. 19 (1934). (3) BOSWELL A N D DILWORTH: J. l'hys. Chem. 29. 1469 (1925). ( 4 ) BRUSAUER, E~~MET A ST D , TELLEII: J . Ani. Cliem. SOC. 60, 303 (1938). OXYGEN Co. LTD.:Private communication. (5) BRITISH (6) DODGE A S D D.AVIS: J. h m . Chem. Soc. 49, 610 (1927) ( 7 ) EMMETT ASD BRCSAUER: J. Am. Cheni. Soc. 59, 310 (193i). (8) EMMETT ASD BRTSACER: J . Am. Chem. Soc. 59, 1553 (1937). (9) EJI~IETT A X D BRCSIUER: J . Am, Chem. SOC.59, 2652 (193i). ( I O ) EAIMETT A S D CINES: J . Phys. 8L Colloid Chem. 51, 1246 (194i). (11) EMMETTA S D DEWITr: J. .%m. Chem. SOC. 65, 1253 (1943). (12) GREGG ASD SISG:J. Phys. 8L Colloid Cheni. 56, 597 (1951: J. .4m. Chem. SOC.63, 2712 (1041). (13) KRIEGER: (14) KRIEGER: I n d . Eng. Chem., Anal. Ed. 16, 398 (1944). CARTER, ASL)S.ANBORS: Ind. Eng. Chem. 36, 99 (1944). (15) L.~LANDE, (16) 12.4.IIBERT A S D CL.ZRK: PrOC. ROY S O C . (London) Al22, 497 (1929). (17) Rko: J . Phys. Chem. 45, 517 (1941). ( I S ) RAOA S D RAO:Proc. Indian Acad. Sci. 4, 562 (1936). (19) Rho ASD RAO:Proc. Indian Acad. Sci. 6, 221 (1937). (20) ROOKSBY: Trans. Cernm. Soc. (Engl.) 28, 399 (1820). THORSHILL, A N D BR.AY: I n d . Eng. Chern. 33, 1303 (1941 I . c21 I SMITH, (22) T.AYLOR: J . SOC.Cheni. I n d . 68, 23 (1949). (23) WEISERA N D RIILLIG J. Phys. Cheni. 38, 1175 (1931). (24) WEISER,hIILLIoas, A N D HOLMES: J. Phys. Chem. 46, 586 (1942) (25) WEISER,S ' f I L L I G A S , ASD S r e i ~ s o s J: . Phys. Chem. 46. 1051 (1912).
T H E EFFECT OF HE-AT T R E d T M E S T O S T H E SURFACE PROPERTIES OF GIBBSITE. I1
ISOTHERMS OF CARBON TETRACHLORIDE VAPORAT 25°C. 8 . J. GREGG
ASD
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