effect of thermal treatment on the nanostructure of sio2al2o3 xerogels

ELSEVIER Journal of Non-Crystalline Solids 215 ( 1997 ) 146-154

JOUI~NAL OF

Effect of thermal treatment on the nanostructure of SIO2-A1203 xerogels

Chuan Lin, James A. Ritter *, Michael D. Amiridis Department of Chemical Engineering, Swearingen Engineering Center, UnitJersi~ of South Carolina, Columbia, SC 29208. USA

Received 29 August 1996; revised 17 February 1997

Abstract

SIO2-A1203 xerogels of various compositions were prepared using the sol-gel process. Aluminum nitrate was mixed with prehydrolyzed tetraethylorthosilicate (TEOS), and gelation was initiated with NH4OH, with a final gelation pH of 8.5. The effect of calcination protocol on the surface area, pore volume, pore size distribution, skeletal density, and pore and particle structures of the resulting xerogels was studied. The results show that essentially mesoporous materials are produced by this synthesis route, and that it is possible to thermally treat these mixed oxide gels to tailor the pore size distribution without severely affecting surface area or pore volume. Low Ai203 content gels remain amorphous and undergo viscous sintering with heat treatment, whereas heat treatment of high AlzO 3 content gels causes crystallization which delays sintering. Thus it is possible to add small amounts of A1203 to impart thermal stability in SIO2-A120 3 xerogels. © 1997 Elsevier Science B.V.

I. Introduction

There has been increasing interest in synthesizing novel porous materials via the sol-gel process [1-6]. Porous oxides derived from the sol-gel process have found numerous applications in filtration, separa- tions, catalysis and chromatography [7]. This interest stems from the fact that the sol-gel process can be utilized to produce a variety of porous oxide gels varying not only in composition but also in structure, including variation in the surface area (SA), pore volume (PV), pore size distribution (PSD), mean pore size and pore texture. The single oxide silica gel system has been studied extensively to gain an un- derstanding of the evolution of sol-gel derived pore

* Corresponding author. Tel.: + 1-803 777 3590; fax: + 1-803 777 8265; e-mail: [email protected].

structures [8-12]. Mixed oxide, SiO2-A120 3 has also received considerable attention [13-32].

Physical, chemical and spectroscopic methods, have been utilized in probing the structure of SiO 2- A120 3 gels as a function of calcination temperature and composition. Much of this work has focused on high calcination temperatures, under which the pore structure, depending on the composition, collapses into either an amorphous glass or a well known crystal structure such as mullite or cordierite [14,15,20,24-30]. Other studies have used spectroscopic techniques at or near ambient conditions to follow the gelation kinetics of these systems [13,16,17,21,31]. Limited work has been done, however, with the SiO2-A120 3 system at temperatures between 250 and 800°C [18,19,22,23,32], where materials with a controlled pore structure can be produced and used as catalyst supports and adsorbents.

0022-3093/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0022-3093(97)00105-1

C. Lin et al. / Journal of Non-Crystalline Solids 215 (1997) 146-154 147

The authors [32] studied, the effect of gelation pH and composition on the pore structure of SIO2-A1203 xerogels; in this work, the effect of thermal treatment was examined. SA, PV and PSDs were analyzed as functions of the calcination temperature. In addition, scanning electron microscopy (SEM), pycnometry, X-ray diffraction (XRD) and thermogravimetric analysis (TGA) were utilized to monitor the structural changes taking place during calcination.

2. Experimental

Reagent-grade tetraethylorthosilicate (TEOS, 99%, Alfa), aluminum nitrate (AI(NO3) 3 • 9H20, 98 + %, Alfa), nitric acid (HNO 3, 70.4 wt% Mallinck- rodt), and ammonium hydroxide (NH4OH, 50 vol.%, Alfa) were used as received. SIO2-A1203 gels containing different amounts of A1203 were synthesized at a constant gelation pH of 8.5, using a sequential route [32]. The SiO 2 sol was prepared by adding one part (by volume) of TEOS to two parts of distilled water, which was adjusted to a pH of 2 with nitric acid. The TEOS was then allowed to hydrolyze overnight under continuous agitation at room temperature to produce a homogeneous sol. Saturated aluminum nitrate was added next, the amount depending on the desired A1203 content. Mixing was continued for 1 h. Finally, a 2 M solution of ammonium hydroxide was added dropwise to form a hydrogel at a pH of 8.5, while agitation was continued for one additional hour.

In order to remove the ammonium nitrate from the hydrogel, the gel was washed with distilled water and vacuum filtered three times, using ten times the theoretical weight of the dried gel per washing. Ethanol, in the same proportion, was used for the final washing and filtering step. It is believed that a final washing with ethanol allows the pore structure to expand [ 10].

Temperature-programmed drying in air was used in the final preparation step. With a heating rate of 0.5°C/min, the gel was heated to 65°C and held there for 2 h. It was then heated to 110°C and held there for another 2 h. Finally, it was heated at a rate of 5°C/min to the final calcination temperature (varied as a parameter in this study), and held there fo r3 h.

The surface areas (SAs) and pore volumes (PVs) were obtained with a Micromeritics Pulse Chemisorb 2700 Analyzer. The N 2 adsorption and desorption (ads-des) isotherms were obtained at 77 K with a Coulter Omnisorp 610, and the corresponding pore size distributions (PSDs) were calculated from the desorption branch of the isotherms, based on the Barrett-Joyner-Halenda (BTH) method [33]. A Perkin-Elmer TGA (TGA-7) was used to determine the weight loss of the gels in helium at a heating rate of 10°C/min. Prior to the TGA analysis, the samples were pre-dried in air at l l0°C. A Quantachrome Ultrapycnometer 1000 was used to obtain the skeletal densities. Scanning electron micrographs were obtained with a Hitachi S-2500 Delta SEM and XRD patterns were collected using a Rigaku D-max B diffractometer equipped with a Cu source.

3. Results

The SAs and PVs of the SIO2-A1203 xerogels containing 15 and 75 wt% A1203 are shown in Fig. 1 for various heat treatments. For the gels containing 15 wt% A1203, both the SA and PV decreased almost linearly with increasing calcination temperature between 250 and 800°C, and then dropped sharply to zero at 1000°C. In contrast, the SA and PV of the gels containing 75 wt% A1203 did not depend as strongly on the calcination temperature, as both remained relatively constant at temperatures to

500 0.30

4 0 0

g 300

2 0 0

1 0 0

i 4 15 wt% 1d203

0 '~ ' ' ' J ' ' ' ' ' ' 10~00 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0

Temperature (oC)

0.25

0.20

0 .15 v

0 . 1 0 ~

0 .05 a ,

0.00

Fig. 1. Effect of calcination temperature on the surface areas and

pore volumes of SiO 2 -A1203 xerogels containing 15 and 75 wt% A1203. The error associated with these measurements was + 3% over the range investigated.

148 C. Lin et al. / Journal of Non-Crystalline Solids 215 (1997) 146-154

800°C. However, a weak maximum in the PV is seen at around 450°C, which was also observed in the independently measured N 2 isotherms. When the temperature was raised to 1000°C, a portion of the pore structure remained intact; the SA and PV in this case were about 100 m2/g and 0.17 cm 3 STP/g.

Figs. 2 and 3 display the N 2 ads-des isotherms and the corresponding PSDs of the 15 and 75 wt% A1203 gels for several heat treatments. All of the

"3' O

* T=250 o112 - = - T = 4 5 0 oC

- - * - - T = 8 0 0 °C

.0 1.5 2.0 2.5 3.0 3.5 4.0 Pore Radius, r (nm)

0.3

0.2

~d

0.1

/ / I I ~

~ / . r ¸

o , j

J . /

B

0.0 , I , i , I i I i I , i i I , I L I i L

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

p/po

Fig. 2. Effect of calcination temperature on the pore size distributions (A) and N 2 adsorption-desorption isotherms at 77 K (B) of SiO 2 -A1203 xerogels containing 15 wt% AI203. The error associated with these measurements was + 2% over the range investigated.

¢D

• T=250 °(2 I~ -'- T = 4 5 0 oC

t~l • T = 8 0 0 oC _ _ T = I O 0 0 o C

/ / / t '

• • i I i , I

1 .0 1 .5 2 . 0 2 .5 3 . 0 3 . 5 4 . 0

Pore Radius, r (rim)

0.3

j 20 . /

o 0.1 ~ . ~ . / . / ? , / -

* ~ B

0 . 0 , i , i , i , t , i ~ r I , ] , I , J

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

p/po

Fig. 3. Effect of calcination temperature on the pore size distributions (A) and N 2 adsorption-desorption isotherms at 77 K (B) of SiO 2 -A1203 xerogels containing 75 wt% A1203. The error associated with these measurements was + 2% over the range investigated.

gels, regardless of A1203 content and heat treatment, exhibited type IV N 2 isotherms with type H2 hysteresis loops [34]. The PSDs reveal significant differences that depend on the AIzO 3 content. Fig. 2 shows that regardless of the calcination temperature, gels containing 15 wt% AI203 have very similar and distinctly bimodal PSDs. In contrast, the gels containing 75 wt% A1203 (Fig. 3) exhibited only slight

C. Lin et al. / Journal of Non-Crystalline Solids 215 (1997) 146-154 149

bimodal character that disappeared, along with some of the microporosity, with increasing calcination temperature. Both types of PSDs were very narrow, with the radii of the majority of pores ranging between 1 and 3 nm. These notable differences in the PSDs with AI203 content were consistent with the N 2 desorption branches of the isotherms, which were slightly broader for the gels containing 15 wt% AI203 that exhibit bimodal PSDs. Finally, as men- tioned above, the 75 wt% AI203 gels exhibited a weak maximum in the PV at approximately 450°C.

lOO

90

~ s o u

70 0

~ 60 u

~ 50

u 40 > ,

"~ 30

= 2 °

10

0

/ I - - b

w / / - - ~ T = 800 oC

/

A

L i I , I I I I I

1.0 1.5 2.0 2.5 3.0 3.5 4.0 Pore Radius, r (rim)

lOO

90

8o

70

> 60

o 50

u 40

,d 30

S 2o

10

0

/ : / /

/

/

j /

/

/ ,7-/¸

• T = 250 oC - ' - Tffi450 °C

- - ~ T f f i 800 °C • - T = 1000 oC

I k i , L h

1.0 1., 2.° 21, 3.0 31, ,1o Pore Radius, r (nm)

Fig. 4. Effect of calcination temperature on the cumulative pore volumes of SiO 2 -A1203 xerogels containing 15 (A) and 75 (B) wt% AI203.

. ~ 6 0 0 °C

~ r , c 'r~'lwlr'~ 3 5 0 * C

. . . . f , , , , i , , , , i , , , , i . . . .

1 0 26 42 58 74 9 i t

28 (CaKe)

Fig. 5. Effect of calcination temperature on the X-ray diffraction patterns of SiO 2-A1203 xerogels containing 15 wt% AI203 (A) and 75 wt% A1203 (B).

This maximum is seen clearly in Fig. 3 by observing the variation in the N 2 loadings with temperature at P/Po = 1.0. This was not the case for the high SiO 2 content gels, as seen in Fig. 2.

The PSD data were also integrated to provide the cumulative pore volumes (CPVs) shown in Fig. 4. For all of the gels containing 15 wt% AI203, 90% of the PV was accounted for by pores with radii below 2 nm regardless of the calcination temperature. The distribution of pore sizes within this range, however, was affected by the calcination temperature, with the smaller pores (i.e., with radii below 1 nm) being more susceptible to higher temperature treatments. As a result, the CPV of these micropores decreased from 23 to 15% when the calcination temperature was raised from 250 to 800°C. This result is consistent with the overall decrease in SA and PV for these materials (see Fig. 1).

XRD patterns are shown in Fig. 5 for SiO2-Al203 xerogels containing 15 and 75 wt% A1203, respec-

150 C. Lin et al. / Journal of Non-C~stalline Solids 215 (1997) 146-154

tively. For the 15 wt% A1203 xerogels, only a broad peak at 20 = 23 ° was observed that was associated with amorphous SiO 2. This result indicated that all of the gels containing 15 wt% AleO 3 remained amorphous regardless of the calcination temperature. In contrast, the XRD patterns of the gels containing 75 wt% A1203 show the characteristic peaks of ~-AI~O 3 at 20 = 37.2, 45.8, 61.2 and 67 ° following calcination at high temperatures. Traces of these peaks were first detected in the XRD pattern at approximately 600°C, and their intensity increased

Fig. 7. Scanning electron micrographs of SiO 2 -AI203 xerogels containing 75 wt% A1203 calcined at 250°C (A) and 1000°C (B).

Fig. 6. Scanning electron micrographs of SiO 2-AIzO 3 xerogels containing 15 wt% AI203 calcined at 250°C (A) and 1000°C (B).

with the calcination temperature, indicating the onset of the formation of 8-A1203 crystals.

SEM images of the 15 and 75 wt% A1203 gels calcined at 250 and 1000°C are shown in Figs. 6 and 7, respectively. It is seen that the 15 and 75 wt% gels calcined at 250°C were comprised of loosely packed 100 nm diameter particles, whereas, after heating to 1000°C, the gel particles became tightly packed, indicating that some sintering had occurred in both cases. Both gels also exhibited considerable shrink-

C. Lin et al. / Journal of Non-Crystalline Solids 215 "1997) 146-154 151

age, with the resulting particle diameters reduced to around 50 nm.

Fig. 8 displays the skeletal density as a function of the calcination temperature for both the 15 and 75 wt% A120 3 gels. The skeletal density of the 15 wt% A120 3 gels initially increased slightly with the calcination temperature and then remained relatively constant. In contrast, the skeletal density of the 75 wt% A1203 gels increased linearly with the calcination temperature. This correlates well with the observed linear decrease in SA and PV (Fig. 1) for the same materials.

TGA results for the 15 and 75 wt% A120 3 gels are shown in Fig. 9. The TGA results for the 0 and 100 wt% A120 3 gels are included for comparison. These results are normalized to the mass remaining at 100°C; weight losses at temperatures below 100°C (not shown) can be attributed to the significant amounts of physisorbed water present. Fig. 9B shows that the total weight loss of the gels decreased linearly with an increase in the AIeO 3 content of the gel, with values ranging between 5 and 35%. The temperature at which this weight loss was completed also decreased with increasing AI20 3 content. Simi- lar results have been reported elsewhere [ 18].

The corresponding differential TGA (DTG) results are displayed in Fig. 10; they exhibit a series of peaks and valleys, with the peaks indicating the

o~

3.0

2.9

2.8

2.7

2.6

2.5

2.4

2.3

2.2

2.1

2.0

/ / I

D

- 15wt%AhO3 - ~ - 75 wt% A1203

/

1.9 . / / 1 . S , I , I , : , ~ , I ~ 0 ' k , I

200 300 400 500 600 700 8 0 900 1000

Te mpe ra tu r e (0(2)

Fig. 8. Effect of calcination temperature on the skeletal densities

of S i O 2 - A 1 2 0 3 xerogels containing 15 and 75 wt% A120 3. The

error associated with these measurements was + 1% over the

range investigated.

v

100

90

80

70

60

A

v ×,,

"~'( I ! • • l l ~ l l ' i l 4 1 4 i 4 i t - l i \

× • \

×

×

- 1 5 w t ~ A . 1 2 0 ' J - - - v - - 7 5 w t ~ A 1 2 0 3 /

i t O 0 w t ~ A l a O ~ J , I , I ~ L ~ L ~ I i i , ~ I , I i I i

200 400 600 soo 1000

Temperature (oC)

et0

100

90

80

70

B

60 i , I , I , 3i0 4 1 0 , , , I , J , r , 910 , I 0 10 20 50 60 70 80 100

Ah03 (wt%)

Fig. 9. Weight loss as a function of temperature (TGA) for

SiO2-AI203 xerogels with four different A1203 contents (A); weight remaining following calcination at 1000°C as a function of A1203 content (B). The error associated with these measurements was _+0.5% over the range investigated.

onset of certain events taking place within the gel structures. The first major event occurs between 150 and 200°C, with the initiation temperature being a weak function of the A120 3 content. The onset of the second major event occurs at approximately 250°C and is a strong function of the A120 3 content. Fig. 10 also shows that all o f the processes associated with weight loss were completed in the 550 to 600°C temperature range. At calcination temperatures above 600°C, however, both the SA and PV continued to decrease (see Fig. I) without further weight loss (i.e.,


[" x I I

I ~:: x. " = - - J

100 200 300 400 500 600 700 800 900 1000

Temperature (oC)

Fig. 10. Differential weight loss as a function of temperature (DTG) for SiO 2 -Al203 xerogels with four different Al203 contents.

the structure became more compact with no further reaction).

4. Discussion

The characterization data presented in the previ- ous section lead to some interesting conclusions regarding the effect of thermal treatment on the nanostructure of the low and high A 1 2 0 3 c o n t e n t

SiO2-A1203 xerogels. A comparison between the CPVs for the gels containing 15 and 75 wt% A1203 (Fig. 4A and B), for example, reveals that at temperatures up to 800°C the 15 wt% A1203 gels had significantly more micropore volume than the corresponding 75 wt% Al203 gels. This result is consistent with the 15 wt% Al203 gels having higher SAs but lower PVs. However, at some temperature between 800 and 1000°C, the entire pore structure of the 15 wt% A 1 2 0 3 gels collapsed. In contrast, for the 75 wt% AI203 gels, an increase in temperature resulted in the loss of micropore volume, which totally disappeared at 450°C. The decrease in micropore volume was accompanied by an increase in the mesopore volume, which explains the presence of weak maxima in both the SA and PV results shown in Fig. 1. In contrast to the 15 wt% Al203 gels,

these materials also retained significant pore volume even after calcination at 1000°C, with most of the pores (90%) being in the mesopore region (radii between 2 and 3 rim).

It is noteworthy to compare these results with those reported previously by Sermon et al. [23]. Similar preparation procedures were used in both studies, with the exception of the additional ethanol washing step employed here. Sermon et al. [22] reported that the hysteresis loop changed from type H2 to type H3 to no hysteresis with the A I 2 0 3

content increasing, respectively, from 25 to 50 to 75 wt%. However, in this study, the shape of the hysteresis loop was independent of the A1203 content or calcination temperature. The kinds of isotherms and hysteresis loops displayed in Figs. 2 and 3 are typical of inorganic oxide gels and can be attributed to capillary condensation occurring within mesopores [34]. The type H2 hysteresis loop is very complex and results from a combination of 'ink-bottle' shaped pores and network effects [34,35]. Finally, the SAs and PVs reported by Sermon et al. [23] decreased dramatically with the Al203 content from values similar to those reported here to near zero for the 75 wt% A1203 gels. These marked differences between the results reported by Sermon et al. [23] and those reported here and also by Lin and Ritter [32] demon- strate that slight modifications in the preparation procedure, such as the additional ethanol washing step, can have a significant effect on the resulting pore structure of the gel. Similar results have been reported elsewhere [19,22].

A combination of the SEM images, skeletal density, and SA and PV data also reveals some very interesting trends. For the 15 wt% A1203 gels, for example, the SA and PV decreased monotonically as a function of the calcination temperature, while the skeletal density initially increased and then remained constant. On the contrary, for the 75 wt% Al203 gels, the SA and PV exhibited weak and broad maxima, while the skeletal density increased monotonically with the calcination temperature. In gen- eral, decreases in the SA and PV can be explained in terms of viscous sintering accompanied by particle shrinkage of the porous structure. Increases in skeletal density can have two possible explanations: either densification of the non-porous structure or the opening of blocked pores [18,19]. In the case of the 15

c Lin et al./Journal of Non-Crystalline Solids 215 (1997) 146-154 153

wt% A1203 gels, the PSDs and the CPVs did not change much upon heat treatment until the entire pore volume collapsed (see Fig. 2A and Fig. 4A), suggesting that viscous sintering and densification caused the observed trends with these materials. Such a model is also consistent with the SEM micrographs, which revealed partial shrinkage of the particles with an increase in the calcination temperature.

On the contrary, the data suggest a different model for the 75 wt% A1203 gels. The small increase in the SA and PV, occurring simultaneous with an increase in the skeletal density, can be explained in terms of a heat induced structural rearrangement that results in the opening of blocked pores. Such a model is consistent with the PSDs and CPVs shown in Fig. 3A and Fig. 4B, which reveal that significant mesopore volume is replacing micropore volume upon heat treatment. At higher temperatures, the decrease in the SA and PV can be explained in terms of viscous sintering accompanied by particle shrinkage of the porous structure, similarly to the 15 wt% A1203 gels and consistent with the SEM images shown in Fig. 7. However, in this case, the skeletal density continuously increased, which can be explained either by the densification of the non-porous structure or by the continuous opening of blocked pores. Both explanations are quite plausible in this case, since the formation of crystals in the structure suggests further densification, and the existence of blocked pores is common in these kinds of xerogel structures [18,19].

Finally, DTG results shown in Fig. 10 display some interesting features that are also indicative of the effect of thermal treatment on low and high A1203 content SiO2-A1203 xerogels. The weight loss in the case of the pure SiO 2 gels can be attributed to a single event, namely the desorption of chemisorbed water. On the contrary, two independent events can be distinguished for all of the gels containing A1203. The magnitude of the total weight loss in this case increased linearly with the A1203 content. Weight losses from these materials can be attributed to the desorption of chemisorbed water, which is expected and appears to be more tightly bound to the AI203. It could also be attributed to the decomposition and desorption of the nitrate precur- sor, but there is no direct evidence to support the latter claim.

Moreover, the SA and PV continued to decrease with an increase in the calcination temperature, even though all of the events that were causing weight loss were completed at 550°C. This phenomenon is generally referred to as structural relaxation [36], a process by which excess free volume is removed with no associated weight loss. In the case of the 15 wt% A1203 gels, which remained amorphous during the heat treatment (see Fig. 5A), this structural relaxation can be attributed to viscous sintering. In the case of the 75 wt% A1203 gels, however, this structural relaxation is attributed to a combination of the viscous sintering of the amorphous SiO2-rich regions and the crystallization of AI203 (see Fig. 5B). Re- suits in the literature support these models, as they show that in this system, and below 800°C, no Si-O-A1 bonds can be formed and that the A1203 is simply dispersed throughout the silica gel network [27].

5. Conclusions

When small amounts of A1203 are added to the silica gel matrix, they remain dispersed and are not crystallized even after calcination at high temperatures. Consequently, low A1203 gels undergo only viscous sintering. In contrast, heat treatment of gels with large amounts of A1203 causes crystallization of the alumina. The presence of the A1203 crystals in this case delays the sintering process. Hence, high A1203 content gels are not densified even after calcination at 1000°C. As a result, the controlling factor of the nanostructure is different depending on the A1203 content. In the temperature range of interest (250 to 800°C), essentially amorphous, mesoporous SiO2-A1203 xerogels can be produced by this synthesis route, and they can be stabilized by the addition of small amounts of AI203. These SiO 2- A1203 xerogels can also be thermally treated to tailor the PSD, without severely affecting the SA or PV.

Acknowledgements

The authors gratefully acknowledge partial support from the US National Science Foundation under


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effect of thermal treatment on the nanostructure of sio2al2o3 xerogels

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