AL UMINA COATED SILICA
PROEFSCHRIFT
TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAP AAN DE TECHNISCHE HOGESCHOOL TE DELFT OP GEZAG VAN DE RECTOR MAG-NIFICUS DR. R. KRONlG, HOOGLERAAR IN DE AFDELING DER TECHNISCHE NATUUR-KUNDE, VOOR EEN COMMISSIE UIT DE SENAAT TE VERDEDIGEN OP WOENSDAG 29 MAART DES NAMIDDAGS TE 2 UUR
DOOR
WILLEM HUBERT MEIJS
scheikundig ingenieurgeboren te Brunssum
DIT PROEFSCHRIFT IS GOEDGEKEURD DOOR DE PROMOTOR PROF. DR. J. H. DE BOER
Ter nagedachtenis aan mijn Vader Aan mijn Moeder
DE DIRECTIE VAN DE KO TINKLIJKE ZWAVELZUUR FABRIEKEN V/H KETJEN N.V. DANK IK VOOR HAAR BIJZONDERE MEDEWERKING BIJ HET TOT STAND KOMEN VAN DIT PROEFSCHRIFT EN VOOR DE TOESTEMMING DE RESULTATEN VAN HET ONDERZOEK IN DEZE VORM TE PUBLICEREN.
PREFACE
Many studies have already been published concerning the properties of silica-alumina catalysts, which are out-standing for their activity in heterogeneous proton catalysis at high temperatures and have therefore arous-ed great interest also in the industry. Vet the nearous-ed was felt of a systematic study of the influence of alumina con-tent on surface properties of colloidal silica-alumina systems. We hope that the investigation described in the present thesis is a successful attempt in this direction, and that it will contribute to a greater knowledge of surface chemistry.
CHAPTER I. A.
B. CHAPTER 11.
CONTENTS
The material and its surface area Determination of the surface area. . . I. The nitrogen isotherm . . . .
2. Determination of the nitrogen isotherm. 3. The molecular surface area of nitrogen Preparation of the samples. . . .
Investigation of the surface by physical adsorption methods page 11 11 14 17 18
A. Surface area measurements on catalysts . . 20 I. The surface areas of non-heated samples 20 2. Influence of rehydration on surface area 23 B. Adsorption of lauric acid . . . 25 I. The adsorption of lauric acid on y-AI.O. 25 2. The adsorption of lauric acid on silica . 27 3. The adsorption oflauric acid on non-heated catalyst samples 27 4. Lauric acid adsorption on heated and rehydrated samples. 29
CHAPTER 111. The texture of the catalysts and the parent substances A.
B. C.
Porosity and texture of catalysts I. Development of porosity . . .
2. The textures of SiO. and AI.0 3 Measurement of the pore volume Changes in texture . . . .
I. Sin tering of silica . . . . . 2. Sintering of cracking catalysts 3. Sin tering phenomena of AI.03 •
CHAPTER IV. The retention of water
A.
B.
C. CHAPTER V.
Water content of the catalyst samples . . . . I. Water content of non-heated silica and Al hydroxide 2. Water content of non-heated catalyst samples 3. The water coverage of the surf ace
The adsorption of water vapour I. Water retention by y-AI.Oa. . .
2. The adsorption of water on catalyst samples. The density of catalyst samples. . .
The chemisorption of ammonia Introduction. . . . . I. Experimental. . . 2. Experimental results 32 32 33 35 37 37 38 41 SI SI SI 56 58 58 62 66 70 71 73
CHAPTER VI. Extraction of A120 3 with acetylacetone
Introduction. . . 80
I. Proper ties of extracted non-heated samples 80 2. Influence of the time of extraction . . . . 83 3. The extractability ofheated (non-technical) catalysts 84 4. The extract ion of Ket jen F.C.C. . . . 89
CHAPTER VII. Structure of Si02-Al20 3-catalysts
Introduction. . . .
I. Proper ties of the non-heated samples (dried at 120°C) 2. Sintering of silica-alumina catalysts
3. Some remarks on cracking activity. . . .
Literature references Summary Samenvatting 91 91 97 100 · 105 · 107 · 109
CHAPTER I
THE MATERlAL AND lTS SURFACE AREA
A. DETERMINATION OF THE SURFACE AREA
Measuring the surf ace area of a substance by means of adsorption really means determining the amount of adsorbed matter that is bound in
a monolayer. To this end it is necessary to determine an adsorption
isotherm; in many cases, however, measuring one or more points of this isotherm will suffice. The only thing further needed is a careful estimate
of the surface area occupied byeach molecule of the adsorbed substance.
The complete coverage of the surface can sometimes be concluded from
a characteristic shape of the isotherm. In other cases it is calculated with
the aid of equations that have been derived theoretically. Section 1. The nitrogen isotherlD (-195°C)
The isotherm (FIG. 1-1) represents
the amount of nitrogen adsorbed (V)
as a function of the relative vapour pressure (x). The first part of the isotherm (from x = 0.05 to x = 0.30)
is generally used for calculating the
surface area, preferably starting from
the equation of BRUNAUER, EMMETT
and TELLER [1]:
x = _l_ + (C- l).x (la) (I- x). V V",G V",C
where x = relative vapour pressure V m
(P
/
Po);
C = constant; V = amount of N2 adsorbed at a relative vapourpressure x; V", = amount of N 2
bound in a monolayer. o Xm
- x
These authors derived this equa- F JG. I 1 - . E l f xamp e 0 mtrogen lsot . . h erm tion on the basis of the concept of
multilayer adsorption, according to which adsorption occurs simultane-ously in more than one layer. In that case G can be considered to
repre-sent the ratio between the residence times of the nitrogen molecules which are bound in the first and the subsequent layers respectively (order of magnitude of C = 70-150). The mean monomo1ecu1ar coverage is reached at a relative vapour pressure of
1 .
Xm = - - (order of magmtude of Xm = 0.1)
l
+VC
x By measuring some points of the isotherm and then plotting
against x, Vm can be calcu1ated. (I-x)· V
Uwe write formu1a (la) as follows:
( I
-
X)
Vm =V(l-x) 1+~
..
. . .
(lb) we see that, if the values of C and x are not too low, a slight variation of the value of C (C=
about 100) has relatively 1ittle influence on the value of V m' For tentative determinations it is therefore advantageous to meas-ure only one point of the isotherm (e.g. at x = 0.2). Cis then assigned avalue obtained from exact determinations made with analogous samples. As mentioned before, the adsorption isotherm is formulated as the result of multilayer adsorption. However, this line of thought is hardly
compatible with the formation of a hysteresis loop, as of ten observed when
also the desorption branch is determined. This formation fits in far more readily with the theory of capillary condensation, by which is understood the phenomenon th at owing to the presence of concavely curved sur-faces, condensation in the pores may occur even before the vapour is
saturated. For this phenomenon KELVIN derived the following formula: -2 y M cos a
rK =
e
RTln XK • • • • • • • • • • • • • • • • (2a) where M=
molecular weight; e=
density of the liquid; y=
surface tension of the liquid; X K = relative vapour pressure at which conden -sation occurs; a=
angle of contact of the liquid with the surface; rK = radius of curvature of the surface; R = gas constant; T = absolutetemperature.
For nitrogen (assuming complete wetting, so a = 0°) one obtains at
78 OK [2]:
rK = -4.05 A
log XK . . . (2b) According to the above criterion, the adsorption branch ofthe isotherm is due to condensation governed by the radius of curvature of the surface or - more exactly - by the radius of curvature of the nitrogen layer
already adsorbed. The evaporation from the capillaries on the other hand correlates with the radius of curvature of the liquid meniscus as determined by the pores filled with liquid nitrogen. Thus, by studying the hysteresis it is possible to obtain further semi-quantitative data on the texture of the sample under investigation.
By way ofillustration the isotherms of two cracking catalysts are given. The first (FIG. 1-2a) is that of a synthetic product (Ket jen cracking catalyst). It has a skeleton of spherical partides and contains pores
"->-' ~ -i. 300 300 Ë 200 200 100 100 00L---~1 00L---~ _ _ x
FIG. I-2a. Nitrogen isotherm of heated cracking catalyst (78 OK) ("Ket jen Fluid
Cr. Cat.")
_ _ x
FIG. I-2b. Nitrogen isotherm of acti-vated clay-catalyst (78.1 OK) [3]
which, if supposed to be cylindrical, have a mean diameter of 45 Á. The other isotherm (FIG. 1-2b) belongs to a day-base catalyst [3]. As far as we know the type of hysteresis found here, which is characterized by a very wide loop, only occurs with products built up of lamellar parti-des [4, 5]. If the arrangement of the lamellae is parallel, condensation phenomena do not occur until in the region where x
=
l.O. The strong desorption at x=
0.50 is determined by the width of the slit between the lamellae (27 Á).can be calculated from the isotherms. For the synthetic catalyst this should preferably be done by the method of BARRETT, ]OYNER and HALENDA [6], which was derived for tubular pores. lts application to the isotherm of the activated clay is not very successful. The model of slit shaped pores as suggested by STEGGERDA [7] and by INNEs [8] would seem to be far more realistic. The latter author in a way leaves open the question whether the adsorption or the desorption branch should be considered for the calculation. However, in accordance with the picture given above, we are inclined to prefer the desorption isotherm for analysing the pore distribution.
Section 2. DeterDlÏnation of the nitrogen isothertn
To determine the adsorbed amount of nitrogen we have used two different methods:
A. A certain amount of gas is admitted to a calibrated apparatus and the pressure at which the adsorption equilibrium is established is meas-ured. The adsorbed volume of gas can be calculated with the aid of the reference values.
B. The system is allowed to attain equilibrium at a desired pressure. Subsequently the nitrogen present in the vessel is measured in a gas burette, after desorption.
The amount of nitrogen adsorbed is given as millilitres of (gaseous) nitrogen of76 cm Hg and 0 o
e
(mlN2 (N.T.P.)) or as millilitres ofliquidnitrogen: 1000 mI N2 (N.T.P.) ~ l.547 mI N2 (liq.).
Method A
FIG. I-3a shows the main parts of the gas adsorption apparatus. It consists of a burette (B), a manometer (M) and an adsorption vessel (A)
which is immersed in a bath of liquid nitrogen (purity 99
%).
B is composed of a series of glass bulbs, the volumes of which have been determined by weighing with mercury before mounting of the apparatus.
M is provided with an indicator needie (i). When pressures are meas-ured, the mercury meniscus is so adjusted as to touch the point of the needie, the total volume of the measuring space thus being constant.
In C nitrogen is condens ed. Traces of non-condensable gases are re-moved by suction. The nitrogen thus purified is used for the adsorption experiments.
of A and of the rest of the apparatus (B and lines to cock K) must be known. These parts are calibrated under the same conditions as pre-vailing during the experiments, i.e. by introducing calibration gas into the evacuated apparatus and filling B with mercury.
UP
,'
,l~qU;d
sample N, D K p M ;-\ compressed air
FIG. 1-3a. Apparatus ror gas adsorption
From the changes in pressure and volume (,1 V) the calibration volumes
are calculated with the aid of Boyle's law. Initially helium was used
as the calibration gas, a calibration being made before each experiment.
Later only one calibration with nitrogen was done, followed by an inci-dental check calibration. (Obviously, in this case A must not yet contain the catalyst sample. For the volume occupied by the sample a correction is calculated and applied. The decrease in volume of A during the
experi-ment as a result of nitrogen adsorption is negligible in many cases).
Mter evacuation of the apparatus (p
<
10-3 mm Hg) at roomtempera-ture pure nitrogen is admitted into B. After reading of the pressure (P) the cock of A is opened and the system is allowed to attain equilibrium
(15 to 30 minutes). The equilibrium pressure is then measured, A is closed, nitrogen is admitted into B, etc.
Finally the N2 saturation pressure is measured at the bath tempe-rature. To this end the Dewar D is placed around the condensing vessel C which contains liquid nitrogen. The pressure (about 800 mm Hg) was measured with a differential manometer.
As a result of the bath being enriched with oxygen, the variations in quality of the liquid nitrogen available and the changes in atmospheric pressure, small variations occur in the temperature of the bath. If long-term measurements are carried out, e.g. the determination of the entire isotherm, the saturation pressure should be determined at regular inter-vals during the experiment. Also the room temperature must be kept as nearly constant as possible.
Method B (FIG. I-3b)
The samples under investigation have been placed in a number of adsorption vessels (A) arranged in a circle and suspended in a bath of liquid nitrogen. The vessels are connected with a manometer. Af ter evacuation so much pure ni trog en is admitted that a predetermined final pressure is reached. When equilibrium has been attained A is with-drawn from the nitrogen bath and connected to a Töppler pump with
f high vacuum manomecer A vacuum
"
Töpplerpumpadsorption [-195°C] desorption [20°C] FIG. I-3b. Apparatus for the determination of pore volumes
the aid of which the (cautiously) desorbed gas can be pumped quantita-tively into a gas burette, where its volume is measured. The de ad space of A has been determined previously. For the rest, the procedure is anal-ogous to th at followed for method A.
For measurements of one point only, such as the determination of thr pore volume, this method is to be preferred to method A. It is less suit. able, however, for measuring the desorption branch of the isotherm.
Section 3. The Jnolecular surf ace area of nitrogen
When nitrogen is bound to Al20 3 or Si02 no chemisorption occurs, as
may be the case when it is adsorbed on metals (Fe, V). The bonding is weak and almost unaffected by the surface whether this is in a hydrated or in a dehydrated state. The surface area of a molecule in a monolayer is determined mainly by intermolecular Van der Waals forces and it will not differ much from th at of a molecule at the surface ofliquid nitrogen. If one considers a liquid to be the closest packing of spherical molecules, the area a.N occupied by a single molecule is given by:
( M)2
1
aN
=
1.091 Ne 3 = 16.25 Á2where M
=
molecular weight; N=
6.023x
1023 Imol.;e
=
densityof liquid N2 (0.808 gimi).
LIVINGSTON [9] has calculated about the same value from critical quantities:
aN
=
15.4 Á2.In reality the nitrogen molecule should not be looked upon as a sphere, but rather as an ellipsoid having different Van der Waals constants for the two axial directions, viz. 12.3 and 17.1 Á 2 respectively i.e. an ave rage
value of 15.5 Á2 [10].
The question whether the assumptions in deriving Vm with the aid of
the B.E.T. equation and in the choice of the molecular surface area are also valid quantitatively can be answered by other, independent, measure-ments of the surface. An ingenious method is based on the measurement of the heat of imbibition [11]. According to experiments made on tita-nium white aN would be about 16 Á2. For porous substances this method
is unsuitable.
The surface area of y-A1203 can under certain conditions be
deter-mined by adsorption of lauric acid. By comparing the values found by adsorption oflauric acid and ofN 2 we arrive at an area ofjust over 16 Á2
(CHAPTER lil). In accordance with literature data we have adhered to
the value indicated above: aN = 16.25 Á2.
B. PREPARATION OF THE SAMPLES
Thecolloidal system alumina-silica-water presents many possibilities of variation. For a systematic study of this complex system it is a first requisite to reduce the number of variables to a minimum. This, we thought, could best be achieved by choosing a common silica gel as a carrier and precipitating on it various amounts of Al hydroxide. This method has the additional advantage of being fairly similar to the procedures used for the commercial production of synthetic cracking catalysts. As regards the preparation we shall restrict ourselves to only a few remarks :
a. When a water glass solution (Na20. 3 Si02) is neutralized by the addition of sulphuric acid a sol is formed, probably via a real silicic acid solution. Within the sol condensation reactions occur of the type:
which cause the particle size to increase. The rate of the condensation strongly depends on the conditions. Under otherwise equal conditions it is highest when the pH
=
7 and lowest at pH=
2 [12].Af ter a certain time the sol passes into a gel. This does not mean that the condensation reactions have co me to an end. For it is known that by allowing silica gel to remain in the mother liquor - i.e. the so-called ageing or ripening - one can change the properties of the dry product eventually obtained. A high degree of condensation, resulting in a low surface area and a large pore volume of the dried silica gel, is favoured by a high pH (= 7), a long ageing period, a high temperature and a high Si02 concentration of the gel.
b. If a silica gel (5% Si02 ) is prepared at room temperature and further
processed without any special ripening at a higher temperature, the Al20 3-Si02 samples obtained from it have small pores (0 25 Á). This entails the serious drawback that methods of determination using the adsorption of large molecules cannot be applied because part of the sur-face is not accessible for these molecules. In our investigation, for in-stance, we have to deal with the adsorption of lauric acid. In view of the length of the lauric acid molecule (18 Á), it is understandable th at we have aimed at samples having as large a pore volume as possible.
causes the pore volume of the A120 3-containing samples to increase.
Ageing for some hours at 60 °C appeared sufficient to obtain finished
samples with a pore volume of about 1.0 ml/g. This prolonged treatment was the more attractive because it would minimize the effect of possible
continued condensation during working up, which was done at a lower temperature.
c. Owing to the addition of different amounts of A120 3 - by means of an Al2(S04)3 solution, followed by neutralisation with NH3 - the
conditions in the various solutions are not exactly identical. The samples were prepared in such a way, however, that the most important varia bie
of the colloidal system, the pH, was as nearly the same as possible in all cases.
Procedure
The following solutions we re used as starting materials : a. water glass (Na20· 3.45 Si02, 17% Si02)
b. AI2(SO 4) 3 (8 g A120 3 per 100 grams of solution) c. NH3 (14%)
d. Sulphuric acid (25.5%)
e. Washing water (pH = 2.8)
By simultaneous spraying of sulphuric acid and water glass in a calculated volume of water a 5% Si02 sol was prepared at pH = 2.
With the aid of NH3 the pH of the sol was raised then to 6.5 at which
value gelation occurred.
The gel was broken up into fine particIes by stirring, heated to 60 °C
and kept at this temperature for six hours.
Af ter ageing the gel was cooled down to 18 cC and acidified to a pH =
=
3. Eleven portions of the gel we re then mixed with various amounts of AI2(S04)3 solution (while stirring for some minutes), brought to apH = 4.9 by slow addition ofNH 3 (20 to 30 minutes), filtered and washed salt-free with lukewarm water. The filter cake was removed from the filter and suspended in water. Then the pH of the suspension was raised to 8.0 with NHa. The product was kept at this pH for 15 minutes and then washed free from sulphate with water. The washed cake was dried
at 120 °C.
The above method of preparation was used for the series of catalyst samples most thoroughly investigated. Unless indicated otherwise, by
"catalyst samples" representatives from this series are invariably meant.
According to their A120 3 contents they are indicated by the Roman
figures II to XII incIusive. The grain size was 0.3
<
d<
3 mmoCHAPTER II
INVESTIGATION OF THE SURFACE BY PHYSICAL
ADSORPTION METHODS
A. SURFACE AREA MEASUREMENTS ON CATALYSTS
Section 1. The surf ace areas of non-heated samples
If we take a flat surface as a carrier and allow a substance to be ad-sorbed on it, the total surface area remains constant. In the preparation of the catalysts Al hydroxide is precipitated on silica gel. It might be supposed, for instance, that the hydroxide is bound - chemically or physically - to the Si02 surface. It is then possible to measure the surface
area of such an Al20 3-containing sample and find out to what extent the
experimental data are in accordance with the general rule given above. This is the principle we have applied to the series of catalysts prepared from one batch of silica gel.
TABLE U-I shows the surface area (SSi) and the composition of the various samples. For obvious reasons both quantities have been ealculated for I g of Si02 •
TABLE U-I. Surfaee area and eomposition of non-heated eatalyst samples
Sample Composition SSi Sample Composition SSi No. mg A120 a/g Si02 m2/g SiO. No. mg A120a/g Si02 m2/g Si02
II 32 533 VIII 164 475 III 55 510 IX 190 479 IV 87 487 X 207 472 V 109 475 XI 270 499 VI 121 476 XII 448 557 VII 145 475
The results listed in this table have been plotted in FIG. U-I. In
this figure three different ranges ean be distinguished: One is charae
-terized by a decrease of SSi (a), in the seeond the surface area is constant (b), while in the third SSi increases again (e).
artie-ularly since the A1 20 3 contents concerned are relatively low, a
con-stant value of SSi would have seemed quite norm al. The decrease is systematic and so large (> 10 %) that it is far beyond the measuring accuracy. Also such fortuitous circumstances as a continued condensation of the silica gel during working up of the samples cannot explain this unexpected effect. For, when the A120 3 is extracted with acetyl acetone
(CHAPTER VI, section 1), one finds that the extract- '8600
ed 'samples all have the ': same surface area of about 1550
550 m2jg Si02 • The same Vf
value is obtained when
t
extrapolating SSi to Al 20 3- 1 500free silica. (FIG. U-I). Ob-viously, therefore, the "car-rier" has remained
un-450
o o
changed and is the same for all samples. Conse-quently, the decrease in surface area must be as-sociated with the presence
400 '---_ _ ..L-_ _ --L _ _ ----'-_ _ - - - ' ' - - _ - - - - '
of the alumina.
o 100 200 300 400 500 _ compos;t;on [mg AI,O,/g S;O,]
FIG. II-l. Surface area and composition of non-heated catalysts
The first remark that should be made in explanation of the phenome-non is th at a silica surface may definitely not be regarded as being plane,
. unless perhaps over very small areas. When larger distances are consid
-ered it should rather be called very rugged. This "ruggedness" must be attributed especially to the contact points between the primary silica particles, at which points disturbances of atomic dimensions of ten occur. The adsorption of one or a few aluminium hydroxide groups may perhaps suffice to seal such an "atomic" irregularity, resulting in a decrease in area. We mayalso conceive another kind of steric hindrance that might occur:
When a capillary wall is covered by a layer of adsorbed matter, its surface area decreases. Now, in the case of non-dried silica gel one could hardly speak of pores, because they do not form until during drying. However, recent electron-microscopic investigations [1] on (wet) silica gel suggest that, especially in an aged gel, the elementary silica particles are not floating about individually, but occur in small, fairly dense agglom-erations (200 to 1500
A
in diameter).In that case the adsorption of aluminium hydroxide could certainly cause a steric effect, since the elementary particles of the silica (SSi =
550 mZ) have a diameter of about 60 Á. From this it can be calculated that for a cubic packing of the particles in an aggregate the narrowest passages have a width of ab out 25 Á. This is sufficiently small to make
steric effects possible (see also CHAPTER VII, section 1).
b. When the sample contains from 100 to 200 mg Alz0 3 per gram of
Si02 the area remains constant (SSi = 475 m2). The conclusion to be
drawn from this observation is naturally related directly to the two possibilities indicated above in explanation of the decrease of SSi at very
low Al20 3 contents.
In the former case, when the sample contains 10% of Al20 3 , the "atomic" cavities or capillaries would all be sealed, making the surface
behave as a plane one. In the latter case, assuming agglomeration within
the silica gel, the constancy of SSi would rather mean that from 10% of A120 3 onwards the Al hydroxide is no longer bound to the interior of the
agglomerations, but only to their outer surface. This would also mean that the bound Al hydroxide is not uniformly distributed over the
sur-face, a possibility that should certainly be taken account of.
c. After a range of constant SSi' further addition of Al20 3 causes a regular increase in surface area. This can best be explained if we assume that in this range a new Al hydroxide phase develops. The
addition of 250 mg A120 3 makes SSi increase by 75 m2• The area of the independent new phase ("free alumina") would therefore be about
300 m2/g A1 203 •
It should be remarked here that, chemically speaking, the same Al
hydroxide could certainly be formed in range b. The two ranges would
then differ mainly in that in b a coalescence with the surface takes place,
whereas at high A120 3 contents Al hydroxide occurs also beside the
original Si02 surface, even to such an extent as to form a marked
con-tribution to the area of the total surface.
To complete the picture of the formation of "free A120 3" it may be useful to mention some phenomena that were observed during the pre-paration of the samples (CHAPTER I B):
1. Of the non-washed gels (pH = 4.9) samples we re drawn and
examined. The products having the highest Al203 contents (XI and
XII) clearly showed two distinct gel phases. The other samples did not. Obviously, in the former case Al hydroxide had visibly precip-itated outside the silica gel phase.
2. When the fractions 0.3-3 mm, which we re destined for further in-vestigation, we re sieved, especially sample IX was conspicuous. The fraction
<
0.3 mm contained a small amount of very fine whitepowder ("free Al203"?), marked by its relatively strong adherence
to the sieve wall. Further, this powder could not be removed
quan-titatively from the grains (> 0.3 mm) because it stuck to them. The
grains were therefore blown "dust" -free by air sifting.
Also other samples, having an even higher Al20 3 content, we re found
to contain a similar powder, although this "pollen" character was
the most striking in sample IX.
Section 2. InHuence of rehydration on surface area
When silica or alumina is heated to high temperatures, changes occur which become apparent, for instance, from a decrease in surface area and
in ignition loss. The binding of selective adsorptives (H20, lauric acid)
depends, at least for silica, not only on the capacity (area) but very
strongly on the degree of hydration of the surface. In order to eli mi na te as far as possible the influence of the temperature on the degree of
hy-dration, the samples we re rehydrated. Any differences in water coverage
between the various samples may then be ascribed to variations in
structure.
Rehydration was carried out by the procedure applied by VLEESKENS
[2a] to silica. Rehydration procedure:
A few grams of a heated sample were heated for 3 to 4 hours in distilled water
(I = 80 to 90 Oe) with occasion al stirring. Af ter rehydration the product is dried [or
20 to 24 hours in a drying oven (120°C).
TABLE II-2. Surface areas of heated samples before and af ter
rehy-dration
Sample Sn 7250
I
SR 7250I
Sn/SR Sn 8750I
SR 8750 I Sn/SR m 2/g Si02 m2/g Si02 7250 m2/g Si02 m2/g Si0 2 8750 II 488 490 1.00 394 400 0.98 III 460 463 0.99 345 348 0.99 IV 439 446 0.98 313 312 1.00 V 446 443 1.01 362 356 1.02 VI 437 443 0.99 358 364 0.98 VII 432 438 0.99 361 357 1.01 VIII 433 451 0.96 330 347 0.95 IX 443 463 0.96 364 384 0.95 X 427 460 0.93 349 363 0.96 XI 461 483 0.96 367 386 0.95 XII 503 518 0.97 384 404 0.95
We shall not discuss here the sintering phenomena (see CHAPTER lIl).
We shall only deal with the influence of rehydration on the surface area. TABLE I1-2 gives the are as before and af ter rehydration (SD and SR) of samples heated for 24 hours at 725 °C and 875°C. SD and SR have been calculated per g of Si02 ; also the ratio SD/SR is mentioned.
When heated samples are rehydrated their surface areas remain prac-tically unchanged. This behaviour is entirely analogous to that of silica [2a]. Any steric hindrance, theoretically possible owing to water being bound, is found to remain within the accuracy of the measure-ments and can therefore be neglected.
This regularity, however, obtains only when the Al20 3 contents are not too high. With samples VIII to XII inclusive rehydration produces an increase in area. This is true for both series . This effect, though very small, is so pronounced and occurs so regularly that there is sufficient reason to investigate how (y-)A120 a behaves when rehydrated.
Heated y-Al20a, unlike silica, is rehydrated already at a low degree of
humidity. If it adsorbs water under this condition and the "wet" product is dried, the area of the rehydrated sample is found to be smaller than th at of the starting material.
To investigate rehydration in (liquid) water we have prepared two samples of y-A120a by heating böhmite H.S. *) for 24 hours at 800 °C and
900 °C respectively, conditioning at a relative humidity of 23% and
drying at 120
o
e
("rehydrated vap."). Part of the dried samples was then kept for 3.5 hours in hot water (90°C) and again dried (" rehy-drated liq.").T ABLE I1-3 shows the surface area S (calculated per g A1 20 3 ) of the rehydrated products.
TABLE I1-3. Influence of the rehydration method on surf ace area of y-Al203
Rehydration method Saooo
non-rehydrated rehydrated vap. rehydrated liq. 182* m2/g A1 203 175 147* m2/g Al 203 145 " 212 168 " "
*) These values were estimated with the aid of the data of TABLE IV -3.
As ean be seen from the tabie, there exists a considerable difference between the two methods of rehydration. With water adsorption from
the vapour phase only rehydration "in situ" occurs, whereas in liquid water transport of matter may take place through the liquid. As aresult, Al20a may be detached from the surface by the action of water, the total area increasing considerably. It is also quite possible that y-A1 20 a is dissolved, followed by its precipitation as a hydroxide. The term "transport of matter" should therefore be taken in its widest sense.
It is remarkable that the area increase due to rehydration is found to occur for samples which are - approximately - in the region of "free AI20 a". The mechanism of this increase cannot readily be eXplained for a system with various phases. One hypothesis is that an "alumina" particle fixed to the silica as a result of heating is detached (" de-sint-ered") owing to rehydration. Clearly, calculations using the increase in surface area to provide information on, for instance, the amount of "free A120a" in the sample must be rejected.
B. AnsORPTION OF LAURIC ACID
Section 1. The adsorption of lauric acid on y-A1203
a. The mechanism of the adsorption is thought to be as follows:
When a solution of lauric acid (Cl lH 2aCOOH) in pentane is contacted
with activated Al 20 3 a monolayer is formed already at a low equilibrium concentration [3] (see FIG. I1-2).
Bonding deprives the surf ace of ~ 0.7 (;
its polarity, which makes further
1
adsorption impossible. Accord- ~ingly the isotherm has a pro-nounced saturation character. The surface area occupied by fatty acids when spread on water
u
.~ -= 0.5
-:l
is at least 20.5 Á 2 per molecule 0.05 _ _ concentratien [moIe0.1 li]
[4]. This value does not ne ces- FIG. II-2. Adsorption of lauric acid on ac
-sarily obtain also when they are tivated Al,03 [3] bound to AI 20a. For, in an
ad-sorbed layer the fatty acid molecule does not occupy its "own" area, but it adapts itself to the structure of the surface. HOUBEN [3] assumes that the most probable plane at the surface is the (111) -plane of the y-A1203 spinel and that a lauric acid molecule is bound to four oxygen atoms of this plane. The molecular area oflauric acid then is (J L
=
4 X 6.73Procedure
20 mI of a solution of lauric acid in pentane of known normality
(0.1 N) was pipetted into a small sealing bottle containing a weighed amount of sample. Af ter the bottle had been sealed by fusion it was shaken for 16 hours in a rotator. The solid matter was th en allowed to settle, af ter which 10 mI of the supernatant clear liquid was pipetted off and titrated with a 0.05 N alcohol ic potassium hydroxide solution.
The pentane was purified by extraction with strong sulphuric acid followed by distillation.
b. Steric hindrance
The phenomenon of steric hindrance invariably occurs when part of the surface becomes inaccessible owing to lack of space for the molecule to be adsorbed. The volume of the amount of adsorbate - in the case of
adsorption on the "interior surface" - can on no account be larger than the pore volume. The magnitude of the steric effect depends on the geometry and the dimensions of the pores.
In the solid state a lauric acid molecule occupies a volume of 336
A3.
If the fatty acid is introduced into a cylindrical capillary (lengthLA,
Lxnr2
radius rA), the pore can contain a maximum of 336 molecules. The area measurable with the aid of lauric acid is then certainly
Lxnr2
smaller than 336 X aL· If the adsorption of the fatty acid is to be used for area determinations, round pores should have a radius of at least 25
A.
This value must even be considered too low.The extent to which the steric effect may interfere and how neglecting ~1.0 9-:;;: . " .~ 0.5 u
]
it may lead to false conclusions
can be illustrated as follows (FIG. 11-3). When activated Al203 takes
up water, the "lauric acid surface" ' -_ _ _ _ _ -'-P.;:..S.:;.;.H:....:.6=30 decreases until the water layer
ad-sorbed has an average thickness of 6
A.
This finding led FORTUIN[5] to the conclusion that the A120 3 contained (round)
capil-laries with a radius of about 21.5
A
° 0 ' - - - - ' - - - : ' 1 0 ' : - - -_ _ _ ---'---%-H-:,~ (and hence of 15.5 A af ter the
ad-sorption of water). If we calculate
FIG. II-3. Effect of water content of AI,03
would be taken up by one lauric acid molecule in the hydrated pore, we find a value of VL = just over 200
A3
.
It is improbable that the fatty acid molecule should öccupy so small a volume. The method of calculation used by FORTUIN therefore seems incorrect.The decrease in "surface area" must certainly not be regarded as evidence of the presence of cylindrical pores. Also if the pores are slit-shaped part of the surface may become inaccessible owing to adsorption of water (causing the slit width to decrease by 12 A!).
However, in the above case probably still another effect plays a part. The shape of the adsorption isotherm [5] tends to indicate that the lauric acid is not bound as strongly to the wate"!:' layer as to the "water-free" surface of the activated A120 3 and has - perhaps - a different density of packing. The results of some other experiments may point in this direction (CHAPTER III C, Section 3 : 4b).
Section 2. The adsorption of laurie acid on siliea [6]
The adsorption of fatty acids on silica is of another type than that on A120 3 • It can be strongly influenced by variation of the OH coverage of the surface. H, for instance, silica is rehydrated with water af ter heating to 800 °C, it takes up more than 1.5 times as much lauric acid as before rehydration. The difference between silica and A120 3 becomes even more pronounced if we compare the amount of the fatty acid bound with the surface area. Per 100 m2 ofarea more than 0.6 mmole ofC
llH23COOH is bound to A120 3 (CHAPTER 111 C, Section 3) For heated and rehy-drated silica this figure is 0.25 mmole/100 m2
•
In contrast with the crystalline nature of y-A1203 , the surface of silica is
presumably more irregular and rugged as a result of locally inhibited condensation reactions. In addition, VLEESKENS [6] calculated that steric effects at the points of contact between the elementary particles can be very large.
Nevertheless, the values of the specific adsorption indicated above for various types of silica lie within relatively narrow limits. This fact enabled us to use the adsorption of lauric acid for characterizing the average structure of the surface in the mixed Si02-A1203 system of the catalyst
samples.
Seetion 3. The adsorption of laurie acid on non-heated catalyst saDlples
With respect to composition and texture they resembie silica, whereas with increasing Al20 3 content their surface structure becomes more and
more like that of Al hydroxide. Both features will therefore be refiected in the adsorption of lauric acid.
The results of the measurements of lauric acid adsorption on non-heated catalyst samples are listed in T ABLE II-4. The values have been calculated per gram of Si02 • With the aid of the area values we have also
calculated the specific adsorption (f) of the various samples. (For surface area and composition of the samples see T ABLE II-I).
TABLE II-4. Lauric acid adsorption on non-heated catalyst samples
Sample II 111 IV V VI VII VIII IX X XI XII
Amount of lauric acid adsorbed
1.58 mmolefg Si02 1.66 1.76 1.77 1.82 1.87 2.00 2.07 2.06 2.20 2.44 f 0.296 mmoleflOO m" 0.326 0.361 0.373 0.382 0.394 0.421 0.432 0.436 0.440 0.438
These figures show th at the presence of Al20 3 causes the lauric acid
adsorption to increase. This observation permits no conclusion as to whether Al203 is bound to the silica surf ace or whether it is present as
separate particles. Except for the very high alumina samples, the latter .,... E 0.45 8 ';-(; E .s 0.40
r
0.35 100 200 300 400 500FIG. 11-4. Specific adsorption of lauric acid on non-heated catalysts
alternative certainly does not apply. For if it did, the area would increase proportionally. It seems therefore justified to at-tribute the increased lau-ric acid adsorption mainl y to a change in surface na-ture. This change can best be described by the specif-ic adsorption
f
In FIG. II-4 the values off
have been plotted as a function of sample composition.As is shown in the graph,jincreases regularly with the Al203 content
until, rather suddenly, it becomes constant. The highest value of j is reached at a composition of about 200 mg A120 3/g Si02 •
It will be remembered that at about the same composition the surface area (Ss;) began to increase and that we have attributed this phenomenon to the formation of "free AI20 3" . Apparently, when a certain percentage of Al20 3 has been adsorbed the silica surface is, in some way or other, "saturated" with Al hydroxide.
When more Al20 3 is added the "Al hydroxide" primarily formed no
longer has a pronounced preference for the silica and forms an inde-pendent phase having its own surface. The analogy between the silica covered with A120 3 and the "free Al20 3" is even so strong that they
cannot be distinguished by means of lauric acid. This does not necessarily mean, of course, that the two surfaces are identical.
It should also be remarked th at in the region where the area is con-stant (FIG. 11-1), the influence of Al20 3 is still clearly perceptible. We
must therefore assume th at the Al hydroxide bound in this stage is still highly dispersed over the surface.
Finally we shall consider the mean surface coverage of a "completely covered" silica (200 mg A1203/g Si02). The surface area of this sample
is 475 m2/g Si0
2 • An Al atom then occupies on an average an area of
aAl
=
20 Á2. If we take as a reference area th at of Al20 3-free Si02(550 m2/g Si02) then aAl = 23 Á2.
VLEESKENS [6, 7] demonstrated that (heated) Si02 has a coverage of
1 Si-atom per 23.5 Á2.
This means that the capacity of the silica surface is about 1 Al atom per Si atom. Even though no conclusions can as yet be drawn as to the adsorption mechanism proper, this value does indicate that the mean thickness of the "Al hydroxide" layer is ab out equal to that of a mono-layer (...,2.5 À).
Section 4. Lauric acid adsorption on heated and rehydrated
saInples
When the samples are heated the lauric acid adsorption decreases much more strongly than corresponds with the decrease in area. This phenomenon, which also occurs with silica, covers the whole range. The adsorption on silica [2b] can be increased by rehydration. As we shall see, this also obtains for catalyst samples.
TABLE 11-5 gives the results of measurements on rehydrated samples, which had previously been heated for 24 hours at 725
o
e
and 875o
e,
respectively. For the former series also the results obtained with dehy-drated samples are mentioned (D 725°). Besides the amount adsorbed, calculated per gram of Si02 , also the specific adsorption factor
f
is given(for the corresponding surface are as see TABLE 11-2).
T ABLE 11-5. Lauric acid adsorption on heated catalyst samples Sample D 725°
I
R 725° fR 725° R 875° fR 875°No. mmole(g Si02 mmole(g Si02 mmole(IOO m2 mmole(g SiO, mmole(IOO m2
II - 1.31 0.267 1.03 0.258 III 0.94 1.29 0.279 1.01 0.291 IV 0.93 1.34 0.300 0.92 0.296 V - 1.40 0.316 1.18 0.330 VI 0.95 1.47 0.332 1.19 0.326 VII 0.98 1.50 0.343 1.23 0.343 VIII 1.00 1.60 0.354 1.18 0.339 IX - 1.69 0.365 1.31 0.341 X 1.09 1.67 0.363 - -XI 1.19 1.78 0.368 - -XII 1.43 2.07 0.400 1.42 0.351
The first two columns clearly show the influence of rehydration. Not only is the adsorption increased by rehydration, as is the case with silica, but also the enrichment of the surface with Al20 3 is far more conspicuous
than with the non-rehydrated samples. This difference is perceptible only up to about the composition of sample VIII.
At high alumina contents the increase is about the same for the two types. For instance, XIID 7250 adsorbs 0.43 mmole more than VIIID 725°;
with the rehydrated samples this difference is 0.47 mmolejg Si02 • Since 'f 0.45
~
~ 0.40..s
r
0.35 0.30 I , I I ,,
,
, / , / ,"/- ------- ---1200 -I , R 875° 100 200 300 400 500_ _ _ compositton [mg A1203!g SiOz]
FIG. 11-5. Specific adsorption of lauric- acid on re-hydrated catalysts
heating at 725
o
e
has only a slight sintering ef -fect (the area has de-creased by only 5 to 10%), we may assume that the difference (0.47 mmoie) is due mainly to the pres-ence of activated alumina which is formed from "free Al hydroxide" and whose structure is prob-ably closely related to that of y-A1203'-cific adsorption
f
of the R 725° and R 875° series as a function of their composition. For comparison the corresponding values of the non-heated samples (" 120 0") are given.The curves for R 725° and R 875° lie lower on the plot than those for "120°". This may be due to several causes: gene rally the degree of hydration of the surface is lower for rehydrated samples than for the original preparations. (See CHAPTER IV A Sec. 2.) It mayalso be that heating has given rise to considerable structural changes.
For the rest, even in a purely physical mixture of Al 20 3 and Si0 2
f
may be expected to depend on the temperature. In that case the magni-tude of the specific adsorption is determined by the ratio between the "surface areas" of the components. If our system is viewed in this light, we might say th at the surface of the A120 3 disappears more quickly thanthat of the Si02. A far more remarkable feature is the fact th at R 725°
and R 875° al most coincide at the low A120 3 contents. This would mean
that there is no detectable selectivity during sintering and that the average composition of the surface remains the same ("homogeneous sin tering" ).
This "homogeneous sintering" is not found at the higher alumina contents. The effect of the A1 20 3 content on the specific adsorption, which can still be perceived for R 725° at an alumina content higher than 14%, is almost completely eliminated by heating to 875°C. At these compositions therefore the thermal stability of the original system is not very high.
The figure clearly shows that at 875 °C the surface tends to assume a stabie configuration, if a sufficient amount of Al 20 3 is present. The "excess" A1 20 3 seems to be collected in particles which have too small a joint surface to exert any considerable influence on
f
The phenomena observed might also imply that, if up to about 100 mg A1203/g Si0 2 the A1 20 3 is located in the internal texture of the agglomerates, a chemical reaction to form surface silicate already took place there at 725°C, so that continued heating produced no further changes. This would than occur under the influence of the surface energy of the very small particles. It would then have to be assumed that the reaction on the external surface did not occur until the temperature had risen to 875 °C, the result being that the structure becomes independent of the alumina content again. The alumina on the outer surface would th en be found in a form that is less rich in energy (thicker layers?). However, lauric acid adsorption is not sufficiently specific to allow of a decision as to the con -figuration at the surface. This goal can only be reached with such other reagents as H 20 and NH 3 (see CHAPTERS IV and V respectively).CHAPTER 111
THE TEXTURE OF THE CATAL YSTS AND THE PARE NT SUBSTANCES
A. POROSITY AND TEXTURE OF CATALYSTS
Section 1. The developrnent of porosity
The pores in aporous substance can develop in various ways
[lJ.
Here only a simplified picture will be given of the genesis of pores according to the two types we are mainly dealing with in the case of silica and alumina, while a distinction will be made between the preparation of adsorbents from gels and from non-plastic materiais.a. Gels
By way of example, let us consider a hydrogel with 5% Si02, which we
shall here take to be a physical mixture of water and Si02 (specific
'"
!.1.0
o
0.43
volumes 1.0 and 0.43 ml/g respec-tively) (FIG. lIl-la). This gel has still a strongly plastic character. lts volume corresponds to point A in the figure.
When the sample is dried the gel shrinks under the influence of the surface tension to such an ex-tent that for every gram of water evaporated the volume decreases by about one mI (section AB of the curve). At a certain moment (point B) the rigidity of the grain
100 is so large that it can resist the
surface tension ; the evaporation
FIG. lIl-Ia. Change ofthe particIe volume
during drying of a hydrogei of water then no longer gives rise
~%H20
to shrinkage of the grain. The grain volume calculated per gram of material increases (BC). In this way aporous mass is formed, which is sometimes referred to as a xerogel. lt will be clear that by changing the drying conditions the porosity of the
xerogel can be varied considerably. This process is irreversible: wetting the dry gel does not produce any peptization.
This mechanism also fits in with VAN BEMMELEN'S classic figure [2J
(FIG. III-lb). In this graph the water content of a hydrogel is plotted as a 0
Ï function of the relative humidity at ~ which it was conditioned (ABCO) as well as the isotherm of the xe rog el (OCDB'C). A striking feature is the sharp break in B where we assume capillaries begin to form. The figure further shows clearly th at the drying process is irreversible.
b. Non-plastic substances
With these substances we have mainly to deal with the phenomenon of pseudomorphism: a component of a solid compound escapes, mostly in a gaseous state, without causing any considerable change in the shape and the external volume of the grains.
This phenomenon is quite of ten ob
-served, e.g. with the aluminium hy-o
1.0
_ relative humidity
Fig. 111-1 b. Isotherms of a silica-h y-drogel and of the corre-spon ding xerogel [2J
droxides. However, not all inorganic solids that decompose with formation of gaseous components possess this property. In this respect vermiculite forms a spectacular example: rapid heating of this lamellar mineral causes it to expand to more than ten times its original volume [3].
Section 2. The textures of Si02 and Al203
The texture in aporous grain is determined by the shape, the dimen-sions· and the mutual arrangement of the elementary particles or, put otherwise, by the shape and the dimensions of the pores.
The micro-measures found in colloid chemistry may be related to one, two or three dimensions. A distinction is made between, respectively, lamellae (sheets), fibrils (fibres) and particles not having a pronounced direction of growth (spheres, irregular polyhedrons).
Si02 Silica particles seem to occur preferentially in a form which, to
a first approximation, can be described as spheres. A po rous silica grain can therefore weIl be considered to have a spherical packing [4].
A certain way of packing gives a silica of a certain porosity. The closest
sphere packing of particles of the same size, which would correspond to
a grain volume of about 0.6 mI per
gram is hardly ever obtained. Silicas
mostly have a grain volume of
be-tween 0.75 and 1.5 mI per gram.
Obviously, it is difficult to des cri be
the shape of the interglobular spaces
(pores) in aporous substance so built
up. In the case of a packing of small
cylindrical bodies the capillary
con-densation (of nitrogen) is governed
by the radius of the inscribed circle
of the pore (rv in FIG. lIl-Ic). The
FIG. lIl-Ic. remaining space between the cylin
-ders is already filled with adsorbed
nitrogen at a lower relative vapour pressure. Similarly, the pores in
silica are also considered to be tubular.
Al203 Activated alumina [5] can be obtained by heating well-
crystal-Iized gibbsite or bayerite (Al(OH) 3). Another method for
producing y-Al203 consists in heating böhmite (AlOOH) prepared from
a gel. Between these two types there is a characteristic difference:
After the pseudomorphous decomposition of the tri hydroxides the
particles in the grain have been arranged very regularly. As with most
polyvalent hydroxides the aluminium trihydroxide crystals can in
prin-ciple be considered to consist of plate lattices. The pronounced
orienta-tion of these lattices is preserved upon decomposition so that activated
Al203 is approximately composed of lamellae with intermediate parallel
slits [6].
The particles in gelatinous böhmite are of a sheet-like nature [7]. In
this case there is no rigidly parallel orientation as is found for
pseudo-morphous decomposition products. Owing to the relatively random
packing of the particles the pores cannot a priori be thought of as slits.
However, the particle shape has still so much effect that a böhmite
grain may certainly not be considered a spherical packing, as with silica.
This effect is manifest also in the appearance of the sample: silica of ten
makes the impression of being some special kind of glass (by its trans
-parency, hardness, shell-shaped surface of fracture), whereas gelatinous
böhmite in many cases rather resembles tuff or clay (opaque, looser
Af ter the preceding remarks a detailed discussion of the texture of cracking catalysts would be superfluous. The synthetic types with a low or moderately high A120a content conform weIl to silica. Catalysts composed of activated day on the contrary present a texture that is still reminescent of the lamellar structure of the day-earths and IS therefore related rather to that of activated A120 a (from gibbsite).
B. 11EASUREMENT OF THE PORE VOLUME
a. A direct method of measuring the pore volume consists in filling the por es with liquid (by means of capillary condensation of nitrogen or water vapour *) and determining the amount adsorbed.
Condensation between the grains should be avoided by working at an equilibrium pressure slightly under the saturation pressure of the liquid (relative vapour pressure x
=
97 to 99%). The radius of the largest pores that are then filled can be calculated with the aid of Kelvin's formula (CHAPTER IA). For x = 98% it is ab out 500 A (N2 , 78 OK).Determining the pore volume just means determining a point on the adsorption isotherm at a high relative pressure. For the measurement to be correct this should be a point on the desorption branch ofthe isotherm. However, if no wide or slit-shaped pores are present the, less laborious, measurement of a point on the adsorption branch will of ten suffice. (For the experimental procedure see CHAPTER IA Section 2).
b. Another, indirect method of determining the pore volume is based on measurement of the grain volume. This is the sum of the skeletal volume and the pore volume. If the skeletal volume is known, the pore volume can be calculated from the grain volume.
1. The grain volume
The volume of the grains is measured using mercury which can pene-trate between the grains but not into very narrow pores. The radius of capillaries that are just acces si bie to mercury can be calculated from the formula
p·r
= -2 YHg cos {}where p
=
pressure (dynes per sq. cm); r=
pore radius (cm); YHg=
surface tension of mercury (475 dynes/cm, 20 °C); {} = contact angle 0 mercury with the wall ({)
=
140 0)If
P
is expressed in atmospheres and r in microns the equation reads 7.25r = - - fJ. p
*) Assuming that the measurement IS not accompanied by swelling or chemica I
At a pressure of 1 atm. pores with a diameter smaller than about 15 fL
are inaccessible to mercury. By changing
p
the mercury can be made topenetrate into smaller pores. For
pores varying in width between
1000
A
and 10 fL mercurypenetra-D tion at elevated pressure is the more
attractive method [8J.
Obviously, for a complete pen-etration of the mercury between the grains the intergranular spaces of a grain aggregate must be at least
15 fL in width. The narrowest
open-ing (diameter d) in a packing of
spheres of diameter D is given by
FIG. III-2a. d= (2/3y3- 1) D = O.15D
d = 0.15 D (FIG. III-2a)
Since d must be equa1 to 15 fL or 1arger, the grains must be at least
100 fL. In practice we used samples with a partide size of at least 0.3 mmo
Procedure A sketch of the apparatus for measuring the grain volume is
given in FIG. III-2b. It consists of a measuring pipette A (provided with
a ground joint), a calibrated capillary C and a beaker B filled with
mercury. The apparatus has three cocks, D, E and F. To reduce the de ad
volume A can be filled with glass beads. The
meas-urement proceeds as follows:
After evacuation of the apparatus, mercury is
al-lowed to rise in the capillary via F to level p. D and
F are then dosed. Next, by opening E for a brief
moment, some air is admitted and cock F is opened
again. Consequently, the mercury in the capillary
drops to a certain level. The gas pressure above the
mercury is then equal to 1 atmosphere (76 cm Hg)
minus the difference in height between the mercury levels in Band C. The pressure on the sample in A, therefore, is actually somewhat lower than 1 atmos-phere. Af ter dosing F, reading the mercury level in
C and weighing B, this procedure - i.e. opening E,
etc. - is repeated a few times.
Af ter this calibration the pipette is filled with a
weighed amount of sample and the calibration
proce-air
c
sample
FIG. III-2b.
change in weight of B the amount of mercury displaced, and therefore the grain volume of the sample, can be calculated.
2. The specific (skelelal) volume
To calculate the pore volume we must know, besides the grain volume,
also the specific volume (the reciprocal value of the density).
In our experiments we determined the density with the aid of an
imbibition liquid (water). A prerequisite for a reliable and reproducible
measurement is that the sample must not be attacked by the medium
and does no longer contain any gas (air).
Procedure [9] About one gram of sample is weighed into a calibrated
pycnometer. The latter is then partly filled with water and carefuUy
evacuated in a vacuum desiccator. Next, the pycnometer is replenished, brought to 20 °C in a thermostat and weighed. This procedure is weU
reproducible (±0.3 %).
Remark If the sample under test contains narrow pores only (and has
the required grain size) the measurements according to the two methods (a and b) yield about the same value for the pore volume. From the foregoing it is easy to see that a difference in results indicates the presence
ofpores with a width of 0.1
<
d<
15 (1. [10].C. CHANGES IN TEXTURE
Section 1. Sintering of silica
The behaviour of silica [10, 11] during sintering is iUustrated in FIG. 1II-3. When
silica is heated at atmospheric conditions
("dry sintering") the grain shrinkage has a
fixed relationship with the decrease in area. For almost any silica type the ratio between
the pore volume (VI» and the area (S) is
approximately constant (curve a). If for the
average pore radius (r p) we take the relation
2 Vp •
rp =
S
this phenomenon can be formula-ted in such a way that the pore radius remains practicaUy constant. Dry sintering at tem
-peratures bel ow 500 °C almost exdusively
I
~ surface area FIG.III-3.
Sjntering of silica a "dry" sin tering
b "steam" sintering c hydrothermal sin tering
occurs through the presence of OH groups at the surface. When during heating the surf ace has reached a certain degree of dehydration, sintering stops. To produce a further decrease in area at the same temperature the silica must be rehydrated [12].
The change in texture can be strongly influenced by a variation of the conditions. Heating silica in steam, for instance, accelerates the area decrease, also if compared to the decrease of the pore volume: the pores get wider (curve b).
The hydrothermal treatment of silica (230°C) may be considered an extreme case of "steam sintering" . The decrease in pore volume then only amounts to a few per cent of the initial value, while the specific area, which first was about 500 m 2jg falls back to a value of 80 m2jg
(curve c) [12J.
An effect opposite to that of hydrothermal treatment is produced by "sintering" of aerogels: The pore volume of an aerogel sample prepared according to Kistler's method decreases from 4.8 to 0.7 mljg when it is put in water and dried at room temperature. On the other hand, the specific area remains practically constant (curve d).
Section 2. Sin tering of cracking catalysts
TABLE III-l gives the pore volumes (V'IJ) and the pore radii (r'IJ) of non-heated samples (dried at 120°C) and of samples that have been heated at 725 °C and 875 °C respectively.
For V'IJ we have taken the difference between the grain volume and
the specific volume. The latter value of the heated samples was deter-mined for rehydrated samples (for quantitative data see CHAPTER IV-C). As regards the grain volume it makes little difference whether we take rehydrated samples or not. Tentative experiments on this effect showed th at rehydration leaves the grain volume, calculated per gram of water-free product, virtually unchanged.
The pore volumes of the non-heated samples were also measured directly with nitrogen (-195 °C, relative vapour pressure about 98
%)
.
The values obtained are indicated as VpN (pore radius rN ).The average pore radius was calculated with the aid of the formula
2V'IJ
r'IJ =
S
(for the values of S: see TABLES II-l and II-2).All pore volumes have been calculated per g Si0 2.
Before going into details of the sintering phenomena proper (b) we shall first discuss the influence of the A120 3 content on the porosity of
TABLE lIl-I. Pore volumes and pore radii of heated and non-heated catalyst samples.
VpN
I
V p TN TpSample (120°)
mlfg SiO. (120°) R 725° R 875° (120°) (120°) R 725° R 8750
II 1.04 1.06 0.97 0.80 39A 40A 40 A 40A
III 1.08 1.09 1.01 0.75 42 43 44 44 IV
*
l.l4 (1.25) (l.l2) (0.74) 47 (51) (50) (47) V 1.19 1.20 1.09 0.83 50 SI 49 47 VI 1.23 1.25 l.ll 0.89 52 53 50 49 VII 1.27 1.30 1.14 0.91 53 55 52 51 VIII 1.34 1.37 1.25 0.93 56 58 55 57 IX 1.37 1.42 1.28 1.02 57 59 55 53 X 1.36 1.47 1.28 0.98 58 62 56 54 XI - 1.47 1.33 1.06 - 59 55 55 XII 1.58 1.76 1.49 1.25 57 63 58 62*) Sample IV had a heterogeneous aspect. Part of the grains was of a glassy nature;
the other part was opaq ue.
a. Porosity of non-heated samples
In FIG. I11-4 we have plotted the values of Vp and VpN of the
non-heated samples against their compositions.
The direct measurement of the pore volume with nitrogen yields slight-ly lower values than the indirect method with mercury. This difference is due to the presence of wide pores (1000
A
<
d<
15 fL). The plotsshow that the contribution of these pores is negligible for samples with
a relatively low A1203 content. The grain therefore contains only
capil-laries narrower than 1000
A
.
This is not true for
samples with a high alu
-mina content. To explain
this fact we must consider
that the grains are
form-ed by drying of a wet
gel:
The particles *) in a
hydrogel have si zes of
say, 0.01 to 1 mmo When
*) These are not the
ele-mentary SiO.-particles (
dia-meter about 100 A), but the
gel particles obtained by
breaking up a gel mass. (See
CHAPTER I-B). 6' 2.00 in '" I
r
1.50 C>---<) Vp -+--+ VpN _.%:f -1.00 0~--1C:-00"'---:-:20~0 ----:3,.:,00,..---:-"40':-0 - - - - = - '500FIG. III--4. Relation between pore volume and
