Synthesis and properties of microporous solgel silicamembranes

R.F.S. Lenza *, W.L. Vasconcelos

Department of Metallurgical and Materials Engineering, Federal University of Minas Gerais, Rua Esprito Santo 35-2 andar,30160-030 Belo Horizonte, MG, Brazil

Abstract

Unsupported silica membranes were prepared via solgel processing through acid-catalyzed hydrolysis and con-

densation of tetraethyl orthosilicate (TEOS). The possibility of controlling the pore sizes and porosities of silica

membranes were investigated by varying processing parameters. The eect of the type and concentration of the acid

catalyst and molar ratio water/alkoxide on the structure of the membranes was determined by measuring their struc-

tural properties. The membranes catalyzed only with nitric acid have a smaller pore structure, with larger pore-solid

surface areas and solid densities than the membranes catalyzed with a combination of nitric acid and hydrofluoric acid.

The average pore size increases for larger hydrofluoric acid concentrations, in a range from 1:1 0:1 to 22 0:9 nm.Silica membrane obtained with molar ratio water/alkoxide equal to 2 has a pore structure diering from the pore

structures of the other membranes. The connectivity (ca. 1019 cm3) and the diusion coecients (ca. 1:6 0:2 106 to2:8 0:3 105 cm2=s) of the Cu in the pore structure of these silica membranes indicate the feasibility of using thesematerials in impregnation and separation processes. 2000 Elsevier Science B.V. All rights reserved.

1. Introduction

Ceramic membranes prepared by solgel pro-cessing have advantages in comparison withpolymeric membranes. For instance, ceramicmembranes usually have better mechanical resis-tance at higher working temperatures than poly-meric membranes [1]. Another important factor isthe capability of the solgel processing to generatematerials with controlled pore structures between1 and 500 nm [2]. The properties of solgel ma-terials are determined by a number of processparameters, and it is important that the eects of

these parameters become well-known to makesolgel a reliable and practical technology formembrane fabrication. According to other studies[3,4] the connectivity of the pore structure as wellas the permeability of the porous network can becontrolled and modeled. Thus, acting in the pro-cessing conditions, we can change the porestructure and adapt the permeability of themembranes to various dierent separation pro-cesses. The selectivity of the solgel membranescan be controlled through chemical modificationof the pore using impregnation methods, in par-ticular for loading the membranes with metal ca-tions [5]. The loading of cations within a poroussilica gel membrane, producing desired properties,requires an understanding of the transport mech-anisms of metal cations within the liquidporoussilica system [6].

Journal of Non-Crystalline Solids 273 (2000) 164169

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* Corresponding author. Tel.: +55-31 238 1813; fax: +55-31

238 1815.

E-mail address: rubia@demet.ufmg.br (R.F.S. Lenza).

0022-3093/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 - 3 0 9 3 ( 0 0 ) 0 0 1 6 3 - 0

The objective of this work is to study the eectof the type and concentration of the catalyst,molar ratio of H2O/TEOS (tetraethyl orthosili-cate) and gelation kinetics on the structure of themembranes and to determine the pore structure ofthe membranes. Impregnation and diusion ex-periments, using copper sulfate, were performed tostudy the aect of the structure of the membraneson the separation process.

2. Experimental procedure

Solgel silica monoliths were prepared by acidcatalysis of TEOS (Aldrich) and deionized water,with ethanol used as solvent. As catalysts, nitricacid (HNO3, Merck) and hydrofluoric acid (HF,Merck) were used. Samples were prepared in threegroups [3]: Group A: the molar ratios H2O/TEOS and

ethanol/TEOS were fixed; HNO3 was used ascatalyst. The pHs of the acid solutions were0.5, 1.0, 1.5, 2.5 and 3.0.

Group B: the H2O/TEOS and ethanol/TEOSwere fixed; HNO3 was used as catalyst of initialacid solutions (pH 1.5) and the HF/TEOSmolar ratios were 2.1, 3.1, 3.8, 6.2 and 12.4.

Group C: the molar ratio ethanol/TEOS wasfixed; HNO3 was used as catalyst (pH 1.5);the H2O/TEOS molar ratios were 2, 4, 10 and16.The sols obtained with the various mixing

conditions were cast into cylindrical containers.The gelation was carried out at dierent temper-atures. The gelation time (tgel) was recorded foreach of the sols. The gelation time was taken as thetime at which the solution stopped its flow insidethe container when the container was tipped on itsside for 3 s. For some sols, tgel was determinedfrom viscosity measurements. The samples wereaged at 60C during 48 h and dried at 80C during48 h. After drying the samples were analyzed byBET method, using an automatic gas sorptionmachine (Autosorb 1, Quantachrome) for volume,surface area, and sizes of pores within experi-mental error of 5% [3]. The average pore radiuswas estimated from rp 2Vp=Sp [3]. True density(solid phase) measurements were performed using

a helium micropycnometer (Quantachrome). Bulkdensities were determined by mercury pycnometry.Both density measurements are performed on twosamples and repeated six times. Membranes of0.5 mm of thickness and 1.0 cm of diameter wereobtained by cutting the solgel silica monolithswith a diamond saw.

The impregnation experiments were carried outby immersion of the membranes in CuSO4 0.25 Mfor various times (from 10 to 10 000 min), afterwhich, the copper concentrations in the solutionsand membranes were measured. A schematicdrawing of the apparatus used in the diusionexperiments is given in Fig. 1. The apparatusconsists of two glass flasks, one containing CuSO40.25 M (flask 1) and the other (flask 2) with thesolvent (deionized water), connected by a tubeholding the membrane. After seven days, bothflasks were sampled and the concentrations of thesolutions were measured by atomic absorptionspectroscopy. Errors were determined by repeatingthe experiments twice for each sample and aver-aging the data.

3. Results

3.1. Eect of the type and concentration of thecatalyst

The tgels and the physical properties such asspecific surface area (Sp), specific pore volume(Vp), average pore radius (rp), and bulk density(qv), for the silica membranes are listed in Table 1.The variation of rp with the molar ratio, HF/TEOS, is shown in Fig. 2. The gelation time

Fig. 1. Schematic drawing of the apparatus used in the

diusion experiments.

R.F.S. Lenza, W.L. Vasconcelos / Journal of Non-Crystalline Solids 273 (2000) 164169 165

observed for samples catalyzed only with HNO3was minimum around pH 0.5. The gelation timewas maximum at pHs around 2:0 0:5. We notethe eect of the fluoride anion in the gelation time.A HF concentration of 0.18 mol/l decreased thegelation time from 3880 60 min (with no HF) to2 0:1 min. It can be seen from Table 1 that themembranes catalyzed only with HNO3 have asmaller pore structure, with larger Svs and qvs thanthe membranes catalyzed with a combination ofHNO3 and HF. The membranes catalyzed withHF have larger Vps. It can be seen from Fig. 2 thatthe average pore size increases for larger HFconcentrations, in a range from 1.1 (with no HF)to 22 nm (HF/TEOS 12.4).

3.2. Eect of the molar ratio H2O/TeOS

The physical properties of the silica membranesobtained with the various H2O/TEOS molar ratios

are shown in Table 2. From Table 2, we note thatas the H2O/TEOS ratio decreases the gelationtimes become larger. No significant dierence wasobserved in the gelation times using a H2O/TEOSmolar ratio of 10 or 16. Similar results were ob-tained by Colby et al. [7]. As shown in Table 2, theSps varied from 789 24 m2=g (H2O/TEOS 10)to 0:6 0:02 m2=g (H2O/TEOS 2).

3.3. Impregnation experiments

The metric and topological properties such asvolume fraction of pores (Vv), surface area of poresper unit volume (Sv), pore connectivities (Gv), andpermeabilities (Kp) of the silica membranes and theCu content incorporated in the silica gel mem-branes are shown in Table 3. The Gvs were ob-tained by using a geometric model proposed byVasconcelos [8], and the Kps were estimated by ageneral equation in the literature [9]. We observefrom Table 3 that the pore connectivities (Gv)varies from (0:02 0:01 1019 to (20 5 1019 cm3. The permeabilities vary from (0:8 0:1 1019 to (54 8 1019 m2. We note that thepore connectivity decreases as the pore size in-creases, while the permeability increases for largerpore sizes. The quantity of copper cations loadedinside the pores is larger for membranes with largerpore sizes, as shown in Table 3.

3.4. Diusion experiments

The results of the diusion experiments arepresented in Table 4. In experiment 1 sample A

Table 1

Structural properties of silica membranes

HNO3 pH HF (mol/l) tgel (min) Sp (m2/g) Vp (cm

3/g) rp (nm) qv (g/cm3)

0.5 0 2880 60 727 22 0.42 0.02 1.2 0.1 1.62 0.04

1.0 0 3600 60 949 28 0.58 0.03 1.2 0.1 1.46 0.03

1.5 0 3880 60 789 24 0.44 0.02 1.1 0.1 1.47 0.05

1.5 0.031 140 7 360 10 1.21 0.05 6.8 0.3 0.65 0.01

1.5 0.045 49 2.5 305 9 1.23 0.06 8.1 0.3 0.54 0.01

1.5 0.055 33 1.7 344 10 1.49 0.07 8.7 0.3 0.54 0.03

1.5 0.090 6 0.3 268 8 1.61 0.08 12.0 0.5 0.55 0.01

1.5 0.180 2 0.1 148 4 1.60 0.08 22 0.9 0.51 0.01

2.5 0 4300 60 873 26 0.53 0.03 1.2 0.1 1.38 0.05

3.0 0 3800 60 1029 31 0.66 0.03 1.3 0.1 1.15 0.03

Fig. 2. Average pore radius versus molar concentration of HF.

This line represents a function fitted to the data by linear

regression.

166 R.F.S. Lenza, W.L. Vasconcelos / Journal of Non-Crystalline Solids 273 (2000) 164169

was used and in experiment 2, sample B was used.As shown in Table 3, these samples have dierentpore structures. To obtain information about thetransport process of the solute through the poresof the silica gel membranes, we used the dia-phragm cell method described by Cussller [10].According to the literature, this method gives ex-cellent results, being considered ecient for de-termining of the diusion coecient ofmembranes. The equation used for the calculationof the diusion coecient (D) is [10]

D 1bt

ln

C1;1 C1;2C01;1 C01;2

!" #; 1

where b is a partition coecient. The diusioncoecient of the Cu in the pores of these silica

membranes is ca. 106 cm2/s for membranes withsmaller pores and ca. 105 cm2/s for membraneswith HF, as can be seen in Table 4.

4. Discussion

As shown in Table 1, using smaller pH in theinitial solution, produces larger specific surfaceareas, smaller porosities and increased bulk den-sities. The average pore radius did not changewithin errors of measurement with the pH of ini-tial solution. The gelation time of silica mem-branes depend on the pH of the reactant solution,as shown in Table 1. The gelation time observedfor these samples was maximum at the isoeletricpoint of silica, as expected. The HF concentration

Table 3

Metric and topological properties of silica membranes and Cu content in the silica membranes

Sample Sv 106(cm1)

Vv rp(nm)

Gv 1019(cm3)

Kp 1019(m2)

Time

(min)

Cu

(%)

A 1.6 0.1 0.66 0.04 8.1 0.3 0.020 0.001 54 8 10 0.565 0.002

100 0.593 0.002

10 000 0.715 0.003

B 11.6 0.7 0.65 0.05 1.1 0.1 7 2 1.0 0.2 10 0.029 0.001

100 0.074 0.001

10 000 0.096 0.001

C 13.9 0.7 0.85 0.06 1.2 0.1 20 5 1.60 0.3 10 000 0.048 0.001

D 11.8 0.7 0.68 0.05 1.2 0.1 8 2 1.10 0.2 10 000 0.054 0.001

E 5.9 0.4 0.42 0.04 1.5 0.1 0.9 0.2 1.10 0.2 10 000 0.193 0.001

F 8.5 0.5 0.48 0.04 1.1 0.1 2.4 0.6 0.80 0.1 10 000 0.064 0.001

Table 2

Structural properties of silica membranes

H2O/TEOS tgel (min) Sp (m2/g) Vp (cm

3/g) rp (nm) qv (g/cm3)

2 30200 60 0.60 0.02 (8.6 0.4) 104 3.0 0.2 1.38 0.024 10100 60 610 18 0.34 0.01 1.1 0.1 1.30 0.02

10 4320 60 789 24 0.44 0.02 1.1 0.1 1.46 0.05

16 4320 60 748 22 0.44 0.02 1.2 0.1 1.51 0.07

Table 4

Cu content in the solutions (g//l) and diusion coecient

Experiment Flask 1

(C1;1)

Flask 2

(C1;2)

Initial solution

C01;1Solvent

C01;2t (s) D (cm2/s)

1 12.53 0.05 0.0873 0.005 12.68 0.05

aected the gelation time and the structure of thesilica membranes. Iler [11] proposed that the po-lymerization process involves, temporarily, theexpansion of coordination number of silicon from4 to 5 or 6, and the eectiveness in the polymer-ization process of the fluorine anion is due to itssmaller ionic radius compared to that of the hy-droxyl molecule ion. According to the Iler theory,the polymerization reaction is catalyzed for hy-droxyl at pHs greater than 2, while for smallerpH, the polymerization rates are proportional tothe H and F concentrations. As shown in Table1, the structure obtained with the larger HF con-centrations, have the largest rps, Vps and smallerSps. The range of rp and porosity obtained forthese membranes are adequate for applications inultrafiltration processes [12].

The results shown in Table 2 show that themembrane obtained with molar ratio H2O/TEOSequal to 2 has a pore structure diering from thestructure of other membranes, which is due to aneect of water concentration on the gelation ki-netics [7]. For complete hydrolysis, the H2O/TEOSmolar ratio must be at least 2. When H2O/TEOSmolar ratio

feasibility of using these membranes in the im-pregnation and separation processes.

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R.F.S. Lenza, W.L. Vasconcelos / Journal of Non-Crystalline Solids 273 (2000) 164169 169