estrutura mesoporosa de Al2O3 preparados de Poli (N-vinilpirrolidona)

estrutura mesoporosa de Al2O3 preparados de Poli (N-vinilpirrolidona)

(Parte 1 de 2)

ISSN 0020-1685, Inorganic Materials, 2008, Vol. 4, No. 2, p. 146–153. © Pleiades Publishing, Ltd., 2008. Original Russian Text © T.M. Zima, N.I. Baklanova, N.Z. Lyakhov, 2008, published in Neorganicheskie Materialy, 2008, Vol. 4, No. 2, p. 189–197.

The ability to produce porous, permeable ceramics with a tailored size of nanopores is of key importance in designing novel effective mesoporous (pore diameter from 2 to 50 nm according to IUPAC recommendations) membrane devices for the fine purification and separation of biological systems, hot gases, and liquids and for performing a number of chemical reactions in various industrial processes. Mesopores are known to play an important role in primary transformations of substances [1, 2]. They can be used to control the kinetics and efficiency of adsorption, catalytic, and other processes. In connection with this, mesopore formation and the microstructure (specific surface, porosity, particle size, pore size, distribution of active components, etc.) of mesoporous materials are now receiving particular attention.

One of the most widely used approaches for fabricating mesoporous oxide materials is sol–gel processing, which offers the possibility of producing one- and multicomponent systems of complex structure and composition, with a tailored volume and size of nanopores.

The formation and properties of mesoporous oxides and oxide mixtures prepared from sols of metalorganic compounds have been studied extensively [3–1]. Mesoporous structure can be created in a variety of oxides, but only a limited number of them can have a highly ordered microstructure.

The microstructure of SiO 2 has been the subject of many studies. In particular, mesoporous silica was shown to have a very large specific surface, considerable pore volume, and narrow pore size distribution [4]. Increasing the concentration of organic additives improves the microstructure of SiO 2 . The microstruc- ture of SiO 2 –ZrO 2 mixtures depends on the ZrO 2 con- centration [5]. On the whole, mesoporous ZrO 2 has a rather small specific surface. After introducing 23 mol %

ZrO 2 into an SiO 2 + ZrO 2 mixture, its components may react at temperatures above 500 ° C, the reaction being accompanied by an increase in the total volume and specific surface of mesopores. As reported by Par- vulescu et al. [6], the pore size in mesoporous ZrO 2 is determined by the type of surfactant used and the length of its chain. The best microstructural characteristics of zirconia were achieved at 380 ° C. Raising the calcina- tion temperature was reported to destroy the mesoporous structure of zirconia. Comparative analysis of the microstructural characteristics of Al 2 O 3 –SnO 2 materi- als prepared via coprecipitation and sol–gel processing

[7] indicated that uncalcined sol–gel derived Al 2 O 3 had a narrower pore size distribution, from 2 to 10 nm.

Most oxides ( ZrO 2 , TiO 2 , SnO 2 , and others) have small specific surfaces and mesopore volumes even at

350°C [8]. According to De Farias et al. [8], the critical temperature is 600°C . The effects of calcination tem- perature and TiO 2 content on the specific surface of binary and mixed oxides were studied by Montoya et al. [9]. At temperatures from 500 to 900°C and TiO 2 contents from 6 to 4 wt %, they obtained a unimodal pore size distribution in mesoporous materials. Moreover, raising the calcination temperature to above

Mesoporous Structure of Al 2 O 3 Prepared from Poly( N -vinylpyrrolidone)-Modified Sols of Hydrous Metal Oxides

T. M. Zima, N. I. Baklanova, and N. Z. Lyakhov

Institute of Solid-State Chemistry and Mechanochemistry, Siberian Division, Russian Academy of Sciences, ul. Kutateladze 18, Novosibirsk, 630128 Russia e-mail: zima@solid.nsc.ru Received December 29, 2006; in final form, April 1, 2007

Abstract —Mesoporous alumina with a narrow effective pore diameter distribution has been prepared using poly( N

-vinylpyrrolidone)-modified sols of hydrous Al 2 O 3 and Al 2 O 3 –ZrO 2

. We compare the microstructures of nanoporous aluminas prepared from electrochemically produced unmodified and modified sols of hydrous oxides and describe the formation of highly ordered mesoporous structures from a mixture of modified sols of hydrous metal oxides differing in chemical nature. Microstructural studies of uni- and biporous permeable nanosystems during heat treatment demonstrate that the pore diameter distribution in the mesoporous oxides prepared from the modified sols remains unchanged at calcination temperatures of 700 ° C and lower. The micro- structure and phase composition of the oxides depend on the initial properties of the sols.

DOI: 10.1134/S002016850802012X

Vol. 4No. 2 2008

MESOPOROUS STRUCTURE OF Al 2 O 3 PREPARED147

500°C produced significant changes in the principal microstructural characteristics of the mixed oxides. In this context, the ability to produce permeable oxide materials with a thermally stable mesoporous structure is of particular importance.

There is great interest in mesoporous oxides with a narrow bimodal pore size distribution. As pointed out by Ermolenko and Efros [12], a transition from uni- to biporous nanostructures in some adsorbents and catalysts increases their activity by a factor of 3–9. Moreover, tuning the relationship between the volumes of small and larger pores in bimodal systems makes it possible to increase the volume of the pores of a particular size in a given mesoporous material, without changing the pore size or the effective pore size distribution. One extremely attractive, radically new approach to the sol– gel synthesis of nanoporous inorganic materials with a highly ordered structure is self-assembly in the presence of organic surfactants [13–15].

The objective of this work was to synthesize and characterize mesoporous alumina at temperatures above 500°C

. The alumina was prepared from poly( N - vinylpyrrolidone)-modified sols of hydrous metal oxides.

Porous Al 2 O 3 is known to be a versatile material with high thermal stability and low thermal conductivity. The physicochemical properties of polyvinylpyrrolidone (PVP) [16, 17] suggest that the participation of this surfactant in micelle formation and molecular assembly on the micellar surface of the colloidal hydrous alumina particles being synthesized will be favorable not only for a uniform particle distribution over the system but also for the formation of an ordered microstructure of the oxide, which will persist after the removal of the polymer.

Porous oxide materials were prepared using unmodified and PVP-modified sols of hydrous alumina and a mixture of PVP-modified sols of hydrous alumina and zirconia (2 : 1 volume ratio). The sols were prepared electrochemically, using aqueous 1 M solutions of appropriate metal chlorides. To synthesize the modified sols of the hydrous metal oxides, an aqueous PVP

( M x = 10000) solution was added dropwise in the ini- tial stages of the polycondensation of colloidal particles until its content was 40 wt %. As electrodes, we used platinum gauze electrodes (Fischer electrodes). Xerogels were prepared by drying the sols at room tempera- ture ( 18–25°C ) to a constant weight. To obtain powder samples, the xerogels were calcined for 2 h in air at 500°C and atmospheric pressure.

t , °C

V , cm 3 /g t , °C

S , m 2 /g t , °C

15 D , nm

3 (a) (b)

(c)

Fig. 1. (a) Pore volume, (b) specific surface, and (c) average diameter as functions of calcination temperature for samples prepared from ( 1 ) unmodified and ( 2 ) PVP-modified sols of hydrous alumina and ( 3

) a mixture of PVP-modified sols of hydrous alumina and zirconia.

dV / dD

Fig. 2. Differential effective pore diameter distributions of the samples prepared from ( 1 ) unmodified and ( 2, 3 ) PVP- modified sols of hydrous alumina and calcined at 500 ° C; thermal cycling time of ( 2 ) 2 and ( 3 ) 40 h.

INORGANIC MATERIALS
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ZIMA et al.

The powders calcined at 500°C were pressed at

10 kPa (steel dies, F-12 LEGAT mechanical press) into disks 15 m in diameter and 3 m in thickness, which were then calcined at 500, 700, and 1000°C in a KO-14 muffle furnace. To this end, the samples were placed in the furnace at room temperature, heated to the set temperature, held there for 2 h, and then furnace-cooled.

Pore volumes and specific surfaces were determined by adsorption measurements with a Micromeritics ASAP 2400 analyzer. Microstructures were examined by scanning electron microscopy (SEM) on a JEOL JSM-6700F EX-23000 BU equipped with an energydispersive x-ray (EDX) spectrometer. The phase composition of the pressed materials was determined by x-ray diffraction (XRD) on a DRON-4 powder diffrac- tometer with Cu

K α radiation.

Figure 1 illustrates the effect of calcination temperature on the volume, specific surface, and average diameter of pores in samples prepared from unmodified and PVP-modified sols of hydrous alumina and from a mixture of PVP-modified sols of hydrous alumina and zirconia. At calcination temperatures below 700°C , the

Al 2 O 3 and Al 2 O 3 –ZrO 2 samples prepared from the

PVP-modified sols have larger pore volumes and spe- cific surfaces in comparison with the Al 2 O 3 samples prepared from the unmodified sol. Above 700°C , the total volume and specific surface of mesopores decrease markedly in the samples prepared from the PVP-modified sols and vary insignificantly in the samples prepared from the unmodified sol. The average pore diameter increases with calcination temperature in all of the samples (Fig. 1c).

Figure 2 shows the differential effective pore diameter distributions of the samples prepared from the unmodified and PVP-modified sols of hydrous alumina and calcined at 500°C . According to adsorption analy- sis data, the pressed samples prepared from the unmodified alumina sol had a very narrow pore size distribu- tion before calcination at 500°C . The pore diameter dis- tribution had a maximum at 3.7 nm. After calcination at

500°C , the pore size distribution was also narrow, with a peak at 3.5–4.5 nm (Fig. 2). At the same time, the samples prepared from the modified sol of hydrous alumina, which were pressed and calcined under similar conditions, had a broader pore diameter distribution, with a maximum at 3.5–6.5 nm. The differential pore volume was considerably larger than that in the samples prepared from the unmodified sol. It can also be seen from Fig. 2 that long-term exposure of the samples prepared from the modified alumina sol to air at 500°C leads to the formation of a microstructure with a narrower pore size distribution. In particular, thermal cycling of these samples for 40 h shifts the peak to 3.5– 5.5 nm, but the maximum of the peak in the pore size distribution remains at ~4.6 nm.

Pore volume V , diameter D , and specific surface S in Al 2 O 3 prepared from the PVP-modified sol and subjected to long- term thermal cycling at 500 ° C

Holding time, h V , cm 3/gD, nmS, m2/g

dV / dD dV / dD

Fig. 3. Differential effective pore diameter distributions of the samples prepared from (1) unmodified and (2) PVP- modified sols of hydrous alumina and calcined at 700°C.

Fig. 4. Differential effective pore diameter distributions of the samples prepared from a mixture of PVP-modified sols of hydrous alumina and zirconia and calcined at (1) 500, (2) 700, and (3) 1000°C.

INORGANIC MATERIALS Vol. 4 No. 2 2008

The table summarizes our data on the effect of heat treatment on the principal microstructural characteristics of the material.

Figure 3 shows the differential effective pore diameter distributions of the samples prepared from the unmodified and PVP-modified sols of hydrous alumina and calcined at 700°C. The pore diameter distributions of the samples prepared from the unmodified sol are seen to be broader than those of the samples prepared from the PVP-modified alumina sol. Moreover, the curve of the alumina prepared from the modified sol has two peaks, in the ranges 3.5–4.0 nm and 4.0–8.5 nm, with maxima at 3.7 and 5.8 nm, respectively.

The differential effective pore diameter distributions of the samples prepared from a mixture of the PVP- modified sols of hydrous alumina and zirconia and calcined at different temperatures are presented in Fig. 4. Characteristically, the mesoporous materials prepared from a mixture of the PVP-modified sols of different hydrous metal oxides have bimodal pore diameter distributions. The samples calcined below 700°C have two well-defined peaks with maxima at 3.7 and 5.4 nm. With increasing calcination temperature, the differential pore volume decreases, but the ratio of the maximum differential pore volume in the first peak to that in the second peak is the same at 500 and 700°C: ~1.35. Calcination at 1000°C reduces the differential pore volume and changes the shape of the distribution.

Figure 5 illustrates the morphology of a fracture surface of the material prepared from a mixture of the PVP-modified sols of hydrous alumina and zirconia and calcined at 700°C. The fracture surface is seen to be formed by spherical particles ranging in size from 70 to 100 nm as determined by SEM. The particles are uniformly, densely packed, and some of them form larger aggregates. At higher magnifications, sufficient for examining individual particles, one can see similar arrangement of smaller sized crystallites. EDX analysis indicated that the aluminum and zirconium were evenly distributed over the spherical particles. The pores were located between the fine crystallites in the spherical particles and their aggregates.

Figure 6 shows the XRD patterns of the samples prepared from the unmodified and PVP-modified sols of hydrous alumina and from a mixture of the modified sols of hydrous alumina and zirconia and calcined at 500, 700, and 1000°C. The samples prepared from the unmodified and modified alumina sols and calcined at 500 and 700°C are seen to differ in phase composition. The XRD patterns of the samples prepared from the unmodified alumina sol and calcined at 500 and 700°C (Fig. 6a) show broad reflections corresponding to poorly crystallized α- and γ-Al2O3. After calcination at 1000°C, the reflections from these phases are sharper.

The XRD patterns of the samples prepared from the PVP-modified alumina sol and calcined at 500 and 700°C (Fig. 6b) show only broad reflections from γ-Al2O3. The sample calcined at 1000°C contained, in addition to the γ-phase, well-crystallized α-Al2O3. The XRD patterns of the samples prepared from a mixture of the PVP-modified alumina and zirconia sols and calcined at 500 and 700°C (Fig. 6c) shows reflections cor- responding to tetragonal ZrO2. After calcination at 1000°C, the XRD pattern shows, along with the above reflections, broad reflections attributable to poorly crys-

Thus, it follows from the present experimental data that the presence of poly(N-vinylpyrrolidone), an organic surfactant, has a significant effect on the formation of a highly ordered mesoporous structure of alumina. PVP is known to readily form complexes with many inorganic molecules owing to the presence of peptide bonds in its structure [16, 17]. The strong adhesion of this polymer to polar surfaces results in the formation of stable colloidal particles with interfacial adlayers of surfactant molecules, which produce high structural/mechanical barriers between the particles and prevent them from coming into direct contact. Slow removal of the excess PVP (decomposition temperature of 270°C in air [16]) from the xerogel does not change the pore structure of the oxide but has a marked effect on its specific surface. As can be seen from Figs. 2–4, the differential mesopore volume in the alumina prepared from the PVP-modified sol remains large after calcination at 500°C. The pore diameter in this material is somewhat larger in comparison with the alumina prepared under similar conditions but with no surfactant. At the same time, after 500°C calcination these materials are close in average pore diameter (Fig. 1c).

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