general routes to porous metal oxides via inorganic and organic templates

Journal of Sol-Gel Science and Technology, 2, 67-72 (1994) © 1994 Kluwer Academic Publishers, Boston. Manufactured in The Netherlands.

General Routes to Porous Metal Oxides via Inorganic and Organic Templates

Code: AP10

CHRISTOPHE ROGER AND MARK J. HAMPDEN-SMITH Department of Chemistry and Center for Micro-Engineered Ceramics, University of New Mexico, Albuquerque, NM 87131

DALE W. SCHAEFER AND GREG B. BEAUCAGE Sandia National Laboratories, Albuquerque, NM 87185

Abstract. The generation of porous metal oxides by removal of template molecules from inorganic polymers formed by sol-gel type hydrolysis and condensation of metal alkoxides is described. The template molecules include organic polymers, copper (II) ions in hybrid copper oxide/silica sols and copper (II) bis(hexafluorocetylacetonate) (hfac). Neutron scattering experiments on the system in which polyacrylic acid (Mw = 2,000 Daltons) is used as an organic template to generate microporous tin oxide show that removal of the template generates skeletal voids. In a second series of experiments, mixed copper/silicon oxide xerogels were prepared by hydrolysis of mixtures of Si(OEt)4 and Cu(OCH2CH(CHa)N(CH3)H)2 in the ratios of Si:Cu = 2:1, 4:1, 9:1. Selective removal (etching) of the copper component generates porous silica. Neutron scattering data and BET surface area measurements are consistent with the creation of pores with molecular dimensions (micropores, 10 A. or less). In the third strategy, Si(OEt)4 is hydrolyzed in the presence of Cu(hfac)2, a volatile, inert inorganic template, in a 4 to 1 molar ratio. Removal of the template from the xerogel at 100°C in vacuo affords microporous silica.

Keywords: sol-gel, porous silica

1. Introduction

The preparation of dense and porous metal oxide films for applications in ceramic membrane technology, es- pecially gas separation and oxygen ion transport, is a rapidly expanding field [1, 2]. Metal oxides are suitable for gas separations due to their intrinsic physical and chemical properties. The phenomena upon which gas separations are based in porous and dense materials include; (i) the ratio of molecular weights of the gases, (ii) size exclusion and (iii) chemical interaction either between molecules and pore surfaces or via atomic, ionic or molecular diffusion through dense materials. These phenomena are not discussed in detail here because they have been reviewed elsewhere [3]. Figure 1 describes the factors which contribute to the ability to separate a mixture of two gases with different kinetic dimensions and molecular weights as a function of the dimensions of the pore.

Porous materials that offer the optimum potential for gas separation are those with pore diameters in the

range of molecular dimensions (<2 nm) provided the molecules exhibit a significant difference in either po- larity or size. In addition, fully dense materials are capable of atomic, ionic or molecular diffusion and are often extremely specific to a particular gas and can exhibit high selectivity under the correct conditions. Under these conditions any porosity associated with the membrane is extremely detrimental and destroys the selectivity.

Sol-gel chemistry offers the potential to prepare metal oxide films for gas separation applications at low temperatures, with controlled composition and sto- ichiometry. However, while control over evolution of microstructure has been exhibited in the synthesis of silicate materials, a similar level of control has yet to be demonstrated in non-silicate metal oxide systems [4]. The origin of this difference lies in the different kinetic and thermodynamic behavior of metal-oxygen bonds compared to silicon-oxygen bonds. However, the ability to control microstructure is a crucial aspect of membrane technology. As a result, we have been in- terested in developing a general synthetic strategy for

68 Roger, Hampden-Smith, Schaefer and Beaucage

Phenomenon

i

Viscous Flow ', Knudsen diffusion No Separation ', Separation

' according ', to 1/'4Mw

i

i

Diagrammat ic Representat ion

i

(i) Macroporous

Surface Diffusion Enhanced selectivity

for adsorbing species

(ii) Mesoporous

Capillary condensation Liquid flow,

pervaporation

Molecular sieving

Based on size exclusion

Ionic or atomic

diffusion

(iii) Microporous (iv) Dense

o o

o 0

0 o o 0

0 o

<< dpore ~. >> dpore

i

i L1 i i

i 00

i

U °0 Qo

Qo Qo o oQ o O

Fig. 1. Schematic representation of membrane-based gas transport and diffusion mechanism,

the formation of porous metal oxides that is applicable to a wide variety of metal oxide materials. This strategy is illustrated in Figure 2 and involves the incorporation of a templating molecule which is intended to define the pore size in the final material.

This strategy is designed to work as follows. A templating ligand is added to a metal alkoxide compound solution. The templating molecule may be either inert in this system or react to displace an alkoxide ligand (a). Hydrolysis (b) or (c) generates an inorganic polymer (I or II) in which the template ligand is trapped. In the final step (d), the template is removed to generate porosity. Clearly it is important that the materials produced in steps (b) or (c) do not possess porosity so that the only porosity is that generated in the final step. We have demonstrated previously that this strategy can be used to prepare relatively high surface area, microporous silica, titania and tin oxide via removal of polyacrylic acid (Mw = 2,000) as a template [5]. The rationale for choice of polyacrylic acid was based on the processing advantages of organic/inorganic hybrid polymers [6, 7]. This strategy is analogous to that used in the synthesis of high silica zeolites where large tetraalkyl ammonium ions are used as templates for crystal growth and then removed upon heating in order to tailor the pores of the final materials [8].

Here we present neutron scattering data for the tin polyacrylate system which provides further mi- crostructural information. In addition, a new varia- tion on the template strategy focusing on the synthesis and characterization of porous silica as a model system using metal-organic Cu(II) molecules as templates is described.

2. Results and Discussion

2.1. Organic Template

We have demonstrated previously that modified metal alkoxides can be used as precursors for porous metal oxides [3, 5, 9]. In particular, metal alkoxide complexes react with one equivalent of polyacrylic acid (Mw = 2,000) to yield the corresponding metal polyacrylate alkoxide compounds [5] (Fig. 2, step (a)). A first hydrolysis at neutral pH followed by a second under acidic conditions affords porous metal oxides, see Figure 2, steps (b) and (d), respectively. In the case of a polyacrylate tin oxide, acid hydrolysis reduces the surface area from 130 to 103 m2/g but the pore volume increases and the average pore radius decreases from 62 to 16 ]k [5].

General Routes to Porous Metal Oxides 69

(RO)4M +

(a)

( R O ) 3 M ~ + RO-

(c)

(b)

\ / O,,-M--o~ M /

_ . / • 0 o ' \ M--O ~. ~O.. /

/ ~ ~ M~7

~.. I ~ M / O ~ "M.~

/ O ~ ; / M Q O x M / O / \

l

\ / / M - - o ~ . /

M--O ~ ~ 0 . /

/ XO.vM../.MTO \ ,O / \ O ", M

/ N I1

0 / M - - O ~ M / / I Pore / ~"

.~ M _ o I M ~0.. / 0 ~/ d Pore .M~7 u \ / O ,

~ ]M Pore M~gPore ~1~

/ "O--M. v ( o , ,o M 7 \ o - - / \

/ ~ ( d ) Ill

~ : Templating molecule

Fig. 2. Illustration of the organic/inorganic template strategy.

The neutron scattering data show substantial

changes after acid treatment. The scattered intensity

(plotted as the scattering cross section per unit sample

volume) for samples of density 0.73 g/cm 3) is shown

'7

E 0 0

v

> M

O O T -

O

O

O

O T -

e D

0.001

• . | • m • | | • , u m m m • ~

0. l 0

~6 A

-2.5

• • m l • • • | | • i | • l l | | • ~ • n l m l l

0.01 0.1

o(a ")

Fig. 3. Scattering cross section per unit volume in the case of the polyacrylate tin system.

in Figure 3. The abscissa is the momentum transfer, Q, which is related to the scattering angle 0 and wave-

length ), as Q = (4rr/),) sin(0/2). The polyacrylate tin oxide (species II. Fig. 2; black circles, Fig. 3) shows a limiting slope at small Q of -3 .6 . This value is consistent with scattering from uniformly dense objects. Although this slope might be interpreted as scattering from fractally rough surfaces, the limiting region is rather small, insufficient to rule out a slope of -4 .0

corresponding to smooth surfaces. This scattering is typical of xerogels and represents scattering from the granular structure that results from fracture of the sample during drying and exists on length scales exceeding 500 A. Although there is a hint to some structure at large Q, corresponding to smaller distances, the dom- inant large-Q feature is a limiting flat background that we attribute to incoherent scattering from protons with a calculated density of 3 × 1022 cm -3. These protons are removed by acid treatment and clearly correspond to the hydrogens in the sacrificial organic phase.

After acid treatment, the large-scale, small-Q scattering from the granular structure is unchanged. A new feature appears, however, corresponding to scattering from domains with a radius of gyration, Ra, of 76 A as illustrated in Figure 4. We believe these domains are the voids left by the departing organic polymer. Ra is the correlation range of density fluctuations and is not simply related to the pore size or BET surface area.


Fig. 4. Schematic representation of the tin oxide structure formed by removal of polyacrylic acid illustrating the domain structure at- tributed to RG.

We gain some information of the morphology of the voids from the high-Q limiting region of the scattering curve. The power-law profile and limiting slope of -2 .5 are consistent with scattering from volume- fractals. It is as if the polyacrylate molecules clustered into a branched network very similar to that observed in polymer gels. Although scattering cannot distinguish between a void network and a polymer network, the method of synthesis favors a solid matrix and branched void network. To our knowledge such a structure has not been observed previously.

2.2. Inorganic Templates

2.2.1 Selective Removal of One Component of a Mixed Metal Oxide. Recently it has been demonstrated [10] that metal oxides can be dry-etched at relatively high rates and low temperatures, with formation of discrete metal-organic complexes. This dry etching process involves the reaction metal oxide such as copper (II) oxide with hexafluoroacetylace- tone (where CFaC(O)CH = C(OH)CF 3 = hfac-H) to form Cu(hfac)2 and water according to equation 1.

CuO + 2hfac-H --+ Cu(hfac) 2 + HzO (1) The reaction is performed under a partial pressure of hfac-H and both volatile products (Cu(hfac)2 and water) are transported out of the system in the gas phase. Furthermore, it was also demonstrated that in complex mixed metal oxides such as PbTiOa and

YBa2CuaOT_x, certain metal components could be selectively etched. As a result of these observations, we decided to investigate this technique for the formation of a porous material derived from a mixed metal oxide where one component (metal ion) could be selectively removed, completely. The mixture CuO/SiO2 was chosen as a model system where it was anticipated that CuO will be removed selectively.

The compound Cu(OCH2CH(CHa)N(CHa)H)2 [11] was chosen as a precursor to CuO because it is soluble in common solvents and does not precipi- tate upon addition of water. Hydrolysis (with an excess of water, approximately 24 equivalents per metal atom (Cu + Si)) of ethanol solutions of Si(OEt)4 and Cu(OCH2CH(CH3)N(CHa)H)2 in ratios 2:1, 4:1 and 9:1, respectively, afforded purple sols. Removal of the volatile components in vacuo and drying at 110°C yielded blue or green powders depending on the ini- tial concentration of copper. Copper could then be removed from the xerogels according to equation 1 by either a dry or wet process.

In the dry-etching process, the xerogels were loaded in a furnace tube heated at 110 °C under a reduced pressure of hfac-H for 2 to 4 hours. White powders were obtained after washing the materials with water and acetone followed by drying at 110 °C before analysis. BET measurements show an increase in surface area and pore volume after the etching process. Species 1-3 (Table 1) have surface areas of 2,187 and 279 m2/g respectively; after dry etching, the surface area of these materials 4-6 (Table 1) increased to 11,588 and 515 m2/g, respectively. A decrease in the average pore radius was also observed from species 2 and 3 to 5 and 6 (Table 1) (11 and 107 ]~ to 10 and 73 ~, respectively).

The wet-etching process consisted of adding hfac- H to a suspension of the CuO/SiO2 xerogels in THE The mixture was refluxed for two hours and the solution became deeper green with increasing time (Cu(hfac)2QH20 is green). The powders were then filtered, washed and dried as described above to yield colorless solids (Table 1, entries 7,8). BET data show an increase in surface area compared to species 1, 3: from 2 and 279 m2/g to 15 and 429 m2/g for powders 7 and 8, respectively. These results are comparable to those obtained after dry etching.

Neutron scattering data were also obtained on samples 3 and 8. Figure 5 represents the scattering cross section per unit volume divided by the sample density

General Routes to Porous Metal Oxides 71

Table 1. BET surface area and average pore radius of metal-organic Cu(II)/SiO2 systems.

Surface area Average pore Entry. System Treatment (m2/g) radius (A)

1. Si/Cu Oxide (2:1) Hydrolyzed 2 11 2. Si/Cu Oxide (4:1) Hydrolyzed 187 11 3. Si/Cu Oxide (9:1) Hydrolyzed 279 107 4. Si/Cu Oxide (2:1) Dry Etched 1 l 44 5. Si/Cu Oxide (4:1) Dry Etched 588 10 6. Si/Cu Oxide (9:1) Dry Etched 515 73 7. Si/Cu Oxide (2:1) Wet Etched 15 126 8. Si/Cu Oxide (9:1) Wet Etched 429 77 9. Si/Cu (hfac)2 (4:1) As Prepared 187 11

10. Si/Cu (hfac)2 (4:1) 110°C in vacuo 588 10

for xerogels 3 (virgin, black circles) and 8 (treated, open circles).

No difference is observed between the two plots indicating that the porosity created in 8 is at the molecular level (10 A or less) and is not detectable in the Q-range studied.

2.2.2 Removal of a Volatile Inorganic Template. In an analogous fashion to the strategy developed for hybrid polymers, it is possible to trap a species inside an inorganic polymer without having a chemical bond linking the templating molecule to the inorganic oxo network (Fig. 2, species I). The species Cu(hfac)2 is

%

E ",- 0

> cz.

m • • m lm l l |

) -3.2

• • • • lm l l I • • • • |wb

o Treated

• Virgin

T~ | , I I I m t ,H j •

0.001 0.01 0.1 o(A ~)

Fig. 5. Scattering profile for species 3 and 8.

, , , , , , a ~ . , . , , , , ,

a suitable template molecule because it is volatile and does not react with the inorganic polymer or its precursor, Si(OEt)4 as shown by independent control experiments. Si(OEt)4 and Cu(hfac)2 were dissolved in ethanol (in a 4 to 1 ratio) and an excess of water was added. The solvents (ethanol and water) were removed at 0°C (to avoid removal of the volatile copper compound) to yield a green xerogel (9, Table 1). BET surface area measurement showed that the powder had a surface area of 187 m2/g and an average pore size of l 1 ~. Cu(hfac)2 was removed by thermolysis of the xerogel at 110°C in vacuo, filtration, washing and drying, to give a very pale blue powder (10, table 1) indicating that most of the copper was removed. The surface area of the material increased to 588 m2/g, the pore volume tripled (from 0.1 cm a to 0.3 cm 3 for 9 to 10, respectively) and a slight decrease in the average pore size (10 ~ radius) was observed. Micropores have been generated via this strategy.

3. Conclusion

It has been demonstrated that metal-organic Cu (II) compounds can be used as templates to create microporous silicon dioxide. The etching of one component of a mixed metal oxide, or the incorporation of a volatile molecule within an inorganic oxo-network followed by its removal increase the surface area of the material and generate microporosity.

A c k n o w l e d g m e n t s

We thank Lawrence Livermore National Laboratory and the Air Force Office of Scientific Research for fi- nancial support. We thank Dr. Tim Ward for helpful


discuss ion and Mr. W. A c k e r m a n for B E T surface area

measurements . Work at Sandia Nat ional Laborator ies

pe r fo rmed under contract :~DE-AC0476DP00789 .

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general routes to porous metal oxides via inorganic and organic templates

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