Effect of partially neutralized aluminum solutions on the texture and pore structure of silica

COLLOIDS AND ELSEVIER

Colloids and Surfaces A: Physicochemical and Engineering Aspects 117 (1996) 131-141

A

SURFACES

Effect of partially neutralized aluminum solutions on the texture and pore structure of silica S.A. Selim a,,, G.M.S. E1 Shafei a, M. Mekewi a W.E.E. Stone b, L. Vielvoye b a Chemistry Dept., Faculty of Science, Ain Shams University, Cairo, Egypt b Unitk de Physico-Chimie Minbrale (MARC), Place Goix du Sud 2/18, 1348 Louvain-La-Neuve, Belgium Received 29 November 1995; accepted 26 March 1996

Abstract N 2 adsorption measurements at 77 K were performed on two types of silica of different porosities, and on their products obtained by soaking in partially neutralized aluminum solutions of varying R = [OH]/[A1] = 0.5-2.0 (pH 3-4). TG analysis showed that the A1 taken by the mesoporous silica was accompanied by its hydroxyls, but with the predominantly microporous silica it was stripped of some of them, where a strong potential field in the narrow pores compensated for any charge differences. IR spectral analysis showed that for microporous silica the peak (due to Si-O stretching of surface OH) is located at 950 cm- 1 instead of approximately 970 cm- 1 in the case of mesoporous silica, which seems to result from the strong perturbation between hydroxyls at close range in the narrow pores. A reduction in this peak takes place upon A1 treatment that increases with R, with the probable formation of a surface complex of the type --Si O-AI. Uptake of A1 ions produced a narrowing of the pores, that increased with the increase in A1 content. From pore structure analysis, the micropore fraction could be sorted into ultramicropores (~< 10 ,~) and supermicropores (10-20 ~,). The volume of the former is almost unaffected by any change in A1 content. The sizes of the supermicropores and mesopores (/> 20 ,~) are crucial to the A1 uptake. Changes in pore sizes at R/> 1.0 facilitate the initial attack of the large A1 polymeric species.

Keywords: Aluminium; Pore structure; Silica; Texture

1. Introduction Silica-alumina systems are used in many applications such as water purification, chromatography, and as catalysts either in the pure phase or as supports for active oxides of metals or their complexes. Many studies have been carried out to define the type of interaction that occurs between solutions of aluminum salts and silica [1 3], and the coagulation [4], precipitation [5,6], and * Corresponding author. 0927-7757/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved P I I S0927-7757(96) 03646-1

polymerization of silica [ 7 - 9 ] . However, the interactions in acidic media (pH 3-4) and the aftereffects on textural characteristics have not been treated in the literature in spite of their importance. In the present investigation, surface and textural features of silica soaked in partially neutralized A1 solutions of different ratios R = [-OH]:[A1] (pH 3-4) were studied with the aid of volumetric a d s o r p t i o n (N2) , thermogravimetric analysis, and infrared spectroscopy. An assessment of the surface features of silica and thermally-treated silica using two types of porous silica, namely mesoporous and

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microporous, was obtained by experiments. The interaction of A1 species with these specific silicas has been presented in a previous publication [ 10]. In the present study their surface and texture are characterized in detail.

2. Experimental 2.1. Silica samples The parent silicas used comprise two amorphous silica samples (purity 99.5%, SiO2 - - dry base), one mesoporous (GS), the other microporous (IS). Their characteristics are summarized in Table 1. Effects due to dehydroxylation of the GS sample were studied. This was achieved by preheating the samples either in air at 660°C at a rate of 10°C min ~ for 5 h under a flow of nitrogen gas, or in a vacuum (2 x 10 -5 mbar) for 3 h. The preheated samples are designated GS(660)air and GS(660)wc.

2.2. Soaking procedure Silica was first soaked by placing 1.6 g in 50 ml of a 0.2 M solution of AI(NO3)3'9HzO (Merck) at room temperature (22 + 2°C). The mixture was then partially neutralized to R = [OH] : [A1] = 0.5, 1.0, 1.5 or 2.0 (pH 3-4), by adding respectively 50 ml of 0.1, 0.2, 0.3 or 0.4 M solution of NaOH (Merck) at a rate of 1.0 ml min-~ through a peri-

staltic feeding pump under continuous magnetic stirring. The slow addition of the base to the A1 salt solution is known to produce polymeric species, and clear solutions up to R = 2.7 have been reported [11]. All preparations were aged for 60 days at room temperature under continuous rocking. Preliminary experiments showed that this time interval is sufficient for equilibrium to be achieved. Polyethylene cells and bottles were used throughout the course of this work. A bottle containing 1.6 g silica in 100 ml distilled water (adjusted to p H i 4 ) was also prepared as a reference, and conserved under agitation for an identical period.

2.3. Techniques Analysis of the A1, Si and Na contents of the soaked silica samples was carried out adopting the method of Bernas [12], i.e. dissolution by HF, followed by the determination of the elements present by atomic absorption. Chemical analysis results estimated as percentage w/w (A1203:SIO2) and (Na20:SiO2) in the different samples are given in Table 2. Thermogravimetric analysis (TGA) of solid samples was carried out under static air condition using the SETARAM automatic electric balance 670, in line with a linear temperature programmer SETARAM PRT 540, set to a heating rate of 10°C min -~. The thermograms were recorded through

Table 1 Characteristics of silica samples Item

Mesoporous

Microporous

Comments

Origin Designation Appearance Major impurities (ppm) Specific surface area (m2 g - l ) Total pore volume (ml g - l ) Average pore radius

Grace Xerogel product (254) GS Beads (3-4 mm) Al(800), Na(600), Ca(40) and Fe(60) 570

Silicaa prepared by acid leaching of mica IS Fluffy flakes A1(920), Na(590) and Ca(1800)

Atomic absorption

714

ABEVfrom N 2 adsorption (77 K)

0.9800

0.4854

Vp~o99~as liq. volume

34

13.6

rn = 2Vp...... / ABET

5.5

4.1

1.6 g/100 ml (H20) at (22 + 2 ° )

(A) pH in water a Prepared by CATA

Univ. Cath. de Louvain/Louvain-La-Neuve, Belgium.

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S.A. Selim et aL / Colloids Surfaces A: Physicochem. Eng. Aspects 117 (1996) 131-141

Table 2 Aluminum and sodium contents of different Al-treated samples R

0.5 1.0 1.5 2.0

GS

GS(660)..

IS

GS(660)v.~

AP

Na b

AP

Nab

AI"

Na b

AP

Na b

0.0500 0.0670 0.0800 0.1070

0.0020 0.0030 0.0020 0.0020

0.0570 0.0650 0.0800 0.1100

0.0016 0.0022 0.0021 0.0021

0.0470 0.067 0.0760 0.0980

0.0003 0.0003 0.0004 0.0006

0.0720 0.0770 0.1170 0.1520

0.0016 0.0014 0.0015 0.0011

%A12O3/SiO 2. b %Na20/SiO2"

a V I T A T R O N recorder. The percentage weight loss used in these thermograms was calculated relative to the sample weight of the respective silica after correction for physically adsorbed water, which is believed to be evolved at a temperature of 200°C for samples GS and IS and at 135°C and 115 ° C for GS(660)vac and GS(660),i~ respectively. Infrared spectra of solid samples, presented as KBr films, were obtained in the range 4000-400 cm -1 on a Fourier Transform Bruker IFS 88 spectrophotometer (flushed with dry air), with a D T G S detector. Each spectrum is the result of 200 scans ( ~ 2 0 m i n . ) with 1.0cm -a spectral resolution. N 2 adsorption-desorption isotherms were obtained volumetrically at 77 K with a computerinterfaced Sorptomatic ASAP 2000, using nitrogen gas of high purity (99%).

3. Results and discussion 3.1. Thermogravimetric analysis 3.1.1. The untreated silica The thermogram of sample GS shows an initial stage of weight loss, due to physically adsorbed water, which concludes in a higher percentage weight loss for sample IS, and a lower one for preheated GS samples, with a shift to lower temperatures in the latter case (see Fig. 1). The microporosity of sample IS is behind [13] this increase in weight loss, whereas in the case of preheated GS samples, the decrease in the extent of

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temperature %; Fig. 1. Thermograms of the parent silicas. hydroxylation reduced the amount of physically adsorbed water and weakened its binding to the surface. Dehydroxylation (200°C < T < 500°C) proceeds through condensation of surface hydroxyls [ 14,15] and at higher temperatures condensation of inner bulk hydroxyls takes place [ 14]. This is reflected as a stage of weight loss at T > 300°C in the case of sample GS, which is absent in the case of preheated GS samples which show only a small gradual weight loss up to 1040°C. Probably bulk rehydroxylation is rather more enhanced than surface rehydroxylation, being more effective for the vacuum-heated sample, since the estimated number of O H species per nm 2 [16] is in line with

134

S.A. Selim et al./Colloids Surfaces A: Physicoehem. Eng. Aspects 117 (1996) 131-141

reported data [17] (except for sample GS(660)vac that shows an exceptionally higher value (see Table 3)). Presumably bulk rehydroxylation in this case has occurred nearly homogeneously, until the case of the air-heated counterpart; in the latter case the evolved water, during heating, is believed to form a screen of steam about the solid that enhances its sintering. 3.1.2. The Al-treated silica The weight loss observed at temperatures above 200°C (see Table 4) and the amount of A1 found on the treated samples (see Table 2) are found to increase with the value of R. The Al-treated silicas show higher values of weight loss than those of their parent counterparts (see Table 3), and the differences observed between the parent GS samples are nearly wiped out on treatment with A1. This indicates that now it is the water associated with A1 which has become the dominant source of weight loss, and this interpretation finds support from the electron spectroscopy for chemical analysis [18] and cross-polarization magic angle spinning nuclear magnetic resonance [-10] results (which have shown

Table 3 Silanol densityfor differentparent silicas as determinedby TGA Sample

GS GS(660)~ir GS(660)vac IS

(meg-l)"

A~Ev

wt. loss excluding adsorbedH/O

OH (mm 2)

570 498 535 714

3.86 1.29 2.64 5.42

4.5 1.7 3.3 5.1

From Table 5. Table 4 Percentage of weight loss at high temperature for different silica samples R

0.5 1.0 1.5 2.0

%wt. loss at temperature >200°C and < 1040°C GS

GS(660)alr

GS(660)va~

IS

4.09 4.30 4.50 5.50

4.00 4.30 5.70 5.90

3.60 4.01 4.40 5.20

6.30 6.80 6.60 7.10

that the A1 taken by silica is accompanied by OH and H 2 0 groups). It should be noted (see Table 4) that for GS samples the weight losses increased between R = 0.5 and 2.0 by around 40%, whereas for the IS samples this increase is only around 10%. This may be attributed to the narrowness of pores in the latter case, which caused the attacking A1 species to be first stripped of some of its OH groups [13] and water to enable its penetration. 3.2. Infrared spectroscopy

The effect of heating sample GS is reflected in the IR spectra (see Fig. 2), where the lower frequency end of the peak at 1095 cm-1 is enhanced and the peak at ~ 8 0 0 c m -1 is slightly shifted toward a higher frequency. These two peaks are assigned [19] to the asymmetric and symmetric vibrations of bulk Si-O Si respectively, and the observed spectral changes are attributed [20] to the presence of additional, more distorted Si-O-Si links which are formed after dehydroxylation. Also, a decrease in the intensity of the peak at 969 c m - 1, assigned to the Si-O stretching vibration of surface silanols [21], is recorded in the spectra of samples GS(660)air and GS(660)v,c. This decrease has to be attributed to the dehydroxylation resulting from the heat treatment. However, the spectrum of sample IS (Fig. 4) displays this peak at 950 cm -1, which is probably due to strong perturbations between O H groups present at close range in the narrow pores. Aluminum treatment caused a decrease in the intensity of this peak (Figs. 3 and 4) which is in line with what we have detected by 298i CP/MAS N M R [10], i.e. a decrease of the number of O H groups as R increases. A decrease or disappearance of the Si O H peak (in the IR spectrum) has been reported upon methoxylation [21] or in the presence of another hydrogensequestering reagent [22]. In our case this decrease is interpreted as resulting from the formation of an inner sphere surface complex of the type - S i O-A1 (where - S i represents a surface silicon atom) which involves H + release from the hydroxyl group. 3.3.

N 2

adsorption

3.3.1. Mesoporous samples All the isotherms obtained are of Type IV according to Brunauer's classification [23], exhibiting

135

S.A. Selim et al./Colloids Surfaces A: Physicochem. Eng. Aspects 117 (1996) 131-141

q I /~

I ....

I GS (660)vat

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I

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II

~ ~~,.,o GS

f

U C2

v] .£2 o 03 <3

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o

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400

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sloping hysteresis loops of type H2 [ 2 4 ] , characteristic for pores with n a r r o w entrances, over a P : P ° range of 0.51-0.85. N o change in the general shape of the isotherm is observed for samples

heated to 660°C in air or a vacuum. The total pore volume, Vp, taken at P : P ° = 0.99 as liquid volume, is however reduced c o m p a r e d to the nonheated sample, and to a larger extent for the air-

S.A. Selim et al./Colloids Surfaces A." Physicochem. Eng. Aspects 117 (1996) 131-141

136

......

is

average pore radius, r H (assuming cylindrical pore shape), was calculated. Practically, rH remained almost constant because a decrease in Vpis accompanied by a decrease in ABET(see Table 5). Surface area values, At, could also be extracted from the V t plots drawn using the t curve of Lecloux and Pirard [26] depending on the value of the BET C constant [27]. The agreement between At and ABET(see Table 5) can be used to assess the correct choice of the applied reference data. Fig. 5 illustrates the V-t plots for sample GS and its Al-treated ones. The mesoporous character reflected from the type IV isotherm of sample GS is manifested in the upward deviation that commences at t=4.8 A (p:po =0.3) which is below the hysteresis closure point (~0.5). This indicates the presence of a region of capillary condensation without hysteresis that may be called "enhanced adsorption", attributed [-28] to pores in the range of wide micropores and/or narrow mesopores. This effect is still observed for sample GS-H20. The pore system of sample GS can therefore be described as consisting of narrow openings that communicate with wider bodies that may also comprise narrower regions. The narrow constrictions governing the pore system represent preferred positions at which "erosion" takes place through dissolution. However, for GS-H20, by examination at high t values (10-12 A) of the corresponding V-t plot, widening of some existing narrow pores is predicted. This leads to the accessibility of some additional pores t o N 2 , resulting in an increase in the values of Vp, ABET and rn. It is expected that upon heating some sintering

1

L) C (3 .0 tn .D o

~ooo

900

800

700

600

500

t, oo

'~/AVENUMI3ERS ( crlq~l

Fig. 4. IR spectra of the Al-treated IS samples at different R values.

heated sample (see Table 5). Upon soaking in water, the loss in Vp is less recovered in the latter case than when the sample is heated in a vacuum. It is also observed (see Table 5) that when sample GS is maintained in water, its Vp value increases remarkably. Finally, all Al-treated samples present a lower total pore volume than their respective parent samples. The specific surface areas were calculated using the BET equation [25] in its normal range of applicability adopting the value of 16.2 ~2 for the cross-sectional area of N2. Using the above Vp values and the corresponding ABET values, an Table 5 Surface parameters of the different mesoporous samplesa

P H20 0.5 1.0 1.5 2.0

BET C const,

rH ('~-)

3

1

2

3

1

2

3

0.910 0.980 0.890 0.850 0.850 0.780

101 100 99 99 101 99

70 84 104 104 103 105

70 98 75 77 100 100

34.0 35.5 34.5 34.0 34.5 34.5

34.0 34.0 32.5 32.5 33.5 33.0

34.0 0.5l 35.0 0 . 5 1 34.0 0 . 5 1 33.0 0.57 3 4 . 5 0.57 34.0 0 . 5 1

Vp(o.99 ) (ml g l)

ABET (m2 g - 1)

At (m e g- 1)

1

2

3

1

2

3

1

2

570 640 493 533 490 468

498 499 487 481 464 450

535 563 518 516 490 460

564 634 499 549 502 479

482 492 484 479 461 450

539 555 510 510 484 456

0.980 1.140 0.855 0.915 0.855 0.810

0.858 0.855 0.790 0.780 0.780 0.740

a Key: P = parent sample; 1 = GS; 2 = GS(660)air; 3 = GS(660)v.c; * = hysteresis closure point.

(p/pO), 1

2

3

0.51 0.51 0.51 0.51 0.51 0.51

0.51 0.51 0.51 0.51 0.51 0.51

137

S.A. Selim et al./Colloids Surfaces A." Physicochem. Eng. Aspects 117 (1996) 131 141

0.5 20

i

GS-tI20

1,5

22

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o

>

200.

GS

2

4

6

8

10

17

I

I

,

i

i

2

4

6

8

10

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12

t(l Fig. 5. V t plots of the different GS samples.

of the samples should occur and that it should be larger when conducted in air rather than in a vacuum [29], as has been observed (see Table 5). The effect of heating the samples is also reflected in the V-t plots, whereas for GS(660)vac-H20, the region of enhanced adsorption (discussed above) is no longer present. For the Al-treated samples, it is observed (see Table 5) that, in all cases, the presence of A1 decreases ABET and Vp, as compared to the corresponding sample soaked in water, for a given R, in the following order: GS > GS(660)vac > GS(660)~ir. This decrease should be compared with other observations. Firstly, we have shown [-18] that under our experimental conditions (R = 0.0-2.0, p H - - 3 - 4 ) both the rate of dissolution, and the final solubility value of silica are enhanced, and increase with R (except at R =0.5, where the solubility of the Al-treated sample is lower than the corresponding value found for silica in H20) (see Table 6). Secondly, by solid N M R [-10], we have shown: (1) that part of the A1 is substituted

Table 6 Solubility of silica of different samples (taken from Ref. [18]) R

H20 0.5 1.0 1.5 2.0

Solubilitya [mgl-l(SiOz)] GS

GS(660)air

GS(660)w¢

IS

125 97 353 615 638*

170 98 351 608 676*

155 107 364 694 878

130 75 385 578 556*

a Values at the end of 60 days (equilibrium values except in the cases marked with asterisks).

into the silica framework; (2) that the adsorption of A1 on the surface takes place via an interaction with the silica hydroxyls. For pure silica, the solubility process can be viewed as a condensationdecondensation process between dissolved silicic acid, Si(OH)4, and surface hydroxyls that play the most important role in both directions, i.e. breaking and reforming Si-O bonds. These hydroxyls

138

S.A. Selim et al./Colloids Surfaces A: Physicochem. Eng. Aspects 117 (1996) 131-141

also represent the centers at which adsorption of A1 takes place. The increased solubility (see Table 6) with an increase in R is expected to lead to pore widening; meanwhile, due to the established equilibria, there will also be an increase in deposition or condensation that will take place on the hydroxyls that are free from adsorbed A1, and consequently will cause some pore narrowing. The net result of any change observed in the surface parameters will depend primarily on the initial number of hydroxyls and A1 content, as well as the relative extent of each of the above processes. The increase observed in the values of ABET and Vp on going from R = 0.5 to R = 1.0 in the case of sample GS (see Table 5) is apparently due to the low equilibrium solubility in the former case; the adsorption of A1 at the narrow entrances of some narrow pores has blocked them with respect to N 2 and the adsorption of A1 from solution on this sample seems to exceed the decondensation process. The mechanisms taking place here are different from those observed at much higher values of pH [2], where A1 precipitates on the surface of silica, decreasing its solubility value.

3.3.2. Microporous samples The isotherms obtained on all the IS samples are of the same shape, which seems to be a composite of types I and IV with a small H2 hysteresis loop covering the P : P ° range 0.4-0.7. The initial P : P ° region of the isotherm up to 0.03 is characterized by a steep enhanced adsorption, this is followed by a less steep and linear region covering the P : P ° range 0.1-0.6, after which adsorption turns towards a nearly constant value until saturation. The variation in the different surface parameters (see Table 7) indicates that soaking in water caused a slight increase in Vp and a small decrease in ABET with some pore widening. The dissolutionprecipitation processes in water seem to have widened some pores and blocked, to some extent, some of the narrower pores, giving rise to the observed changes. Aluminum treatment led in all cases to a decrease in surface parameters with R (except rn, that remained constant) for R ~> 1.0. The apparently abnormal behavior found for R = 0.5 was also present in the case of sample GS

Table 7 Surface parameters of the different microporous samples a R

ABET (meg l)

At (m2g t)

Vp (mlg-1)

rH (~,)

BET C const.

IS H20 0.5 1.0 1.5 2.0

714 681 544 588 509 468

722 690 (554) (596) (521) (470)

0.4854 0.4904 0.3886 0.4259 0.3768 0.3426

13.6 14.4 14.3 14.5 14.8 14.6

146 136 96 79 90 84

a Values between parentheses are calculated from the straight line drawn passing through the origin.

(see Table 5) and is perhaps due to some A1 blocking of narrow pores. The V-t plots of samples IS and I S - H 2 0 , using the reference data for 300 ~< C >/100 1-26], reproduce the particulars expected from the general shape of the adsorption-desorption isotherms. The plots (see Fig. 6) show a downward deviation at t ~ 6.0 .~ (P:P° = 0.53) that adopts a fixed value at t~>8.0,~ (P:P° =0.75), indicating that both samples are predominantly microporous. Moreover, in both cases, all the points up to t ~ 6.0 A lie on the straight line that passes through the origin giving A t values that agree with ABET(see Table 7). This behavior may result from a compensation effect [30], i.e. the fraction of mesopores present are of the lower range sizes so that only limited condensation can take place - - the average pore radius is ~ 14 ~, (see Table 7). For all Al-treated samples, an extrapolated line from the points in the range up to t = 4.3-4.8 ,~ ( P : P ° = 0 . 2 3 - 0 . 3 1 ) does not pass through the origin but gives a positive intercept. This intercept has been considered [31] as being equivalent to the total micropore volume, Vmp. The slope of the straight line is then equivalent to the area, Aex, of all the other pores. The near Type IV character of the isotherm and the constant adsorption values near saturation make the pore structure analysis of the mesopore fraction possible. This is achieved by applying the Modelless method [32] to the adsorption branch, adopting the cylindrical pore idealization. The cumulative areas, A. . . . and pore volumes, Voum, obtainable from such analysis,

S.A. Selim et al./Colloids Surfaces A: Physicochem. Eng. Aspects 117 (1996) 131-141

1.3

1tO

139

¢

2.0

d

Ioo 0 i

dl/

i / "4

~_)

tOO

o

U.5

2o0

,

2/

1.5

~, ~.,

~

o

o

IS

2°

0

t~ i

i

i

a

i

i

2

4

6

8

10

12

J

i

|

|

i

i

2

4

6

8

10

12

t(2&) Fig. 6. V-t plots of the different IS samples.

together with A~x and Vmp values of the different samples are given in Table 8. The cumulative parameters are considered to comprise all the pores except the micropore fraction, and consequently one would expect the following: A~x = Acu m and Vp- Vmp = Veum. However, from Table 8, A~x > A . . . . and Vp- Vmp > V~um,indicate that b o t h A,x a n d Vmp i n c l u d e a certain fraction

of pores which may be masked in view of this way of evaluation. We therefore believe that the micropore fraction must be split into two subgroups: supermicropores and ultramicropores, and we define: Vmp=Vu=volume of ultramicropores; Vp- V ~ - V~ = ~ = volume of the supermicropores; Ae~ - Ac~,~ = As = area of the supermicropores; and

A¢× = Au = area of the ultramicropores. The different parameters obtained for the different samples are included in Table 8, and the variation with R of the different parameters in the micropore range is represented in Fig. 7. The pronounced decrease in area in all the Al-treated samples can be regarded as the result of pore narrowing or even blocking. The fact that the V - t plots of these samples gave a positive intercept on the adsorption axis indicates that the A1 treatment has created a group of very narrow pores in the ultramicropore range which are filled at low relative pressures. We have to emphasize that in the V - t plots of these samples, if a straight line is drawn that passes through the origin and ABE T --

Table 8 Different parameters obtained from pore size distribution data and micropore analysis of IS series R

IS H20 0.5 1.0 1.5 2.0

Ac~ (m2g l)

cP Ac,~m (m2g-1)

Vmp= Vu (mlg-l)

¢P Vcum (mlg-:)

Vp- Vmp

As

Au

Vs

(mlg l)

(mZg 1)

(m2g-l)

(mlg l)

446 504 427 387

340 350 288 336 304 277

0.0480 0.0450 0.0465 0.0450

0.3038 0.3416 0.2740 0.3184 0.2841 0.2595

0.4854 0.4904 0.3046 0.3809 0.3303 0.2976

158 168 123 110

98 84 82 81

0.0666 0.0625 0.0462 0.0381

140

S.A. Selim et al. / Colloids' Surfaces A: Physicochem. Eng. Aspects 117 (1996) 131 141

~60

).oll 6O

*----~-e~

vs

~0:

2O

6.6

io

i.s

2'.0

R Fig. 7. Variations of the different parameters in the microporous range of IS samples.

includes most of the points located at high t values, similar to the parent silica, we obtain At values that are in agreement with the corresponding ABET values (see Table 7) and the initial points at t values < ~ 4 A fall above that straight line, which is a clear indication of the presence of ultramicropores. The volume of these pores seems to be independent of any change in the A1 content, but the specific area shows an initial decrease upon going from R = 0.5 to R = 1.0, and only negligible changes for further increase in R. This clearly shows that this group of pores is only affected at small A1 contents, where A1 can block some of the pore entrances. Meanwhile, as a result of decondensation (through dissolution) some widening of other ultramicropores takes place giving rise to the appearance of supermicropores (between R -- 0.5 and R = 1.0), as observed from the resulting area changes (see Table 7). However, this widening does not seem to take place along all the pore length, i.e. it occurs more at the upper part of the pore without proceeding throughout the entire depth of the pore. Further increase in decondensation with increase in A1 content (increase in R) will lead to deeper widening, which results in more A1

uptake (see Table 2) and therefore results in lower area and volume of the supermicropore type (see Table 8). In view of the analysis conducted in the micropore range, the apparent constancy of the ultramicropore fraction reflects the inability of A1 species to penetrate into these pores, which leads to the idea that it is the bulky AI species that attack the surface. Such an attack, as proposed, is regarded to be through condensation with surface hydroxyls. The remarkable decrease in BET C constant upon A1 treatment (see Table 7) can thus be attributed to this condensation, where the hydroxyl groups on silica are considered to be the high energy nitrogen adsorption sites [33].

4. Conclusions

Assessment of the pore structure of silica (of different porosities) soaked in A1 solutions partially neutralized (by base addition) to different degrees, indicated that the pore size is an important factor that controls the interaction between the A1 species and the silica surface. This reveals that the attacking

S.A. Selim et al./ Colloids Surfaces A: Physicochem. Eng. Aspects 117 (1996) 131 141

AI species are bulky enough, and therefore their approach to the interaction sites, the hydroxyl groups, on the silica surface is greatly dependent on the pore dimensions. The interaction is seen to proceed by condensation between AI species and surface hydroxyls and results in the formation of an inner sphere surface complex. The sizes of the supermicropores (10-20 ,~) and mesopores (~>20/~) are crucial to the A1 uptake.

Acknowledgement The authors would like to thank Professor P.G. Rouxhet for his continuous interest and support.

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