WO1998052880A1

WO1998052880A1 - Microporous and mesoporous ceramic material compositions and methods

Info

Publication number: WO1998052880A1
Application number: PCT/US1998/007565
Authority: WO
Inventors: Isabel M. Tejedor; Marc A. Anderson
Original assignee: Tejedor Isabel M; Anderson Marc A
Priority date: 1997-04-14
Filing date: 1998-04-14
Publication date: 1998-11-26
Also published as: AU7466898A

Abstract

The present invention concerns microporous and mesoporous ceramic materials and methods of making and using them. The ceramic materials are made of metal or metalloid oxide particles, preferably nanoparticles, on which hydrolyzable metal atoms, or ions thereof, are absorbed.

Description

MICROPOROUS AND MESOPOROUS CERAMIC MATERIAL COMPOSITIONS AND METHODS

TECHNICAL FIELD

The present invention concerns novel microporous and mesoporous ceramic materials and novel methods of making and using same.

More particlularly the invention relates to such ceramic materials, and methods of making and using them, which are made of metal or metalloid oxide particles, preferably nanoparticles, on which hydrolyzable metal atoms, or ions thereof, are adsorbed.

BACKGROUND OF THE INVENTION

Prior art background that is especially relevant to the present invention, and mesoporous or microporous ceramic materials made of metal or metalloid oxide particles, methods of making them and methods of using them, is provided in United States Patent Nos. 5,610,109 and 5,439,624, which are incorporated herein by reference. Reference may also be had to, for example, United States Patent Nos. 5,487,774; 5,194,200; 5,028,568; and 5,006,248, which also are incorporated herein by reference.

In particular, porous ceramic materials are known in the art that are made from silicon oxides, aluminum oxides, combinations of silicon oxides and aluminum oxides, titanium oxides, zirconium oxides, and other metal oxides. Sintering of a xerogel of the metal or metalloid oxide material which is used to fabricate a ceramic material results in a durable rigid material which can be either unsupported or supported by being coated on a substrate. The substrate can be either porous or non-porous and metallic or non- metallic, depending on the particular application. Reference herein to "ceramic material" is to the material apart from any substrate on which it may be coated. The ceramic material may be in any physical form, such as membranes, pellets, ribbons, or films.

Porous ceramic materials are known in which atoms or ions of metals, other than the metal or metals that are part of the metal or metalloid oxide(s) which comprises the bulk of the ceramic material, are incorporated or "doped" into the ceramic material. This incorporation might involve either or both (1) incorporation of atoms or ions of the other metal(s) into the metal or metalloid oxide particles or (2) adsorption of atoms or ions of the other metal(s) on the surfaces of such particles. Porous ceramic materials find many uses that are known in the art. These uses include, among others, separating gases in mixtures thereof (e.g., separating oxygen and nitrogen from air), filtering impurities from gases (e.g., air) or fluids (e.g., water), catalyzing various chemical reactions, storage of electrical energy in batteries or ultracapacitors, and, in the case of porous ceramics that are conducting, providing electrodes for various electrochemical reactions, such as making hydrogen and oxygen from water or vice-versa.

The suitability of a porous ceramic material for any particular use depends on a number of factors. These factors have presented major problems for the porous ceramic material art because they have been difficult to control.

In the case of such materials to which the present invention relates, in which the metal or metalloid oxide particles of the materials have atoms or ions of a "dopant" metal adsorbed to the partcles' surfaces, two of these factors are (1) the amount of metal atom or ionic adsorbant on the particles and (2) the porous structure, and in particular the pore size, of the xerogel (and ceramic material) after the dopant metal atoms or ions have been adsorbed by the oxide particles. The present invention provides solutions to the problem of controlling these two factors and, thereby, provides porous ceramic materials that are substantially improved and methods of making and using such improved membranes.

SUMMARY OF THE INVENTION

We have now discovered that ionic species can be adsorbed from aqueous solution to load the ions on to the surface of metal/metalloid oxide particles and have discovered how to control the quantity of ion adsorbed and the porous structure of the xerogel, and resulting porous ceramic material, doped with the ions. Especially advantageously, we have discovered how to maximize the quantity of adsorbed material and still preserve the microporosity of the porous ceramic material that comprises the adsorbed material. Thus, we have discovered novel porous ceramic materials that have high loadings of dopant and remain microporous (pore diameter less than about 20 Angstroms.) The metal ions, in the porous ceramic materials of the invention that are doped with such ions, can be reduced in order to provide porous ceramic material that is doped with zero-valent metal atoms corresponding to the ionic dopant. The invention thus also includes novel porous ceramic materials that remain microporous but are doped heavily with atomic metal. These microporous ceramic materials will be extremely highly conducting and find application in many electrical devices, including, among others, batteries, ultracapacitors, fuel cells, and the like.

The xerogels, that are doped in accordance with the invention and ultimately further processed by known methods to prepare the porous ceramic materials of the invention, are also part of the invention.

More generally, the metal-including dopant, be it metal ion per se or an ionic species that comprises the metal ion, in the doped xerogels of the invention, that ultimately may be further processed into porous ceramic materials of the invention, will ultimately yield metal that is atomically incorporated into the metal/metalloid oxide particles of the material, adsorbed metal interspersed between these particles, or adsorbed oxide of the metal dopant, depending on the amount of dopant incorporated into the doped material and the firing conditions used to transform the doped xerogel into the porous ceramic material.

DETAILED DESCRIPTION OF THE INVENTION

We have discovered that adsorption from aqueous solution may be used as a means to load ions, in particular ions that are hydrolyzable metal ions per se or that include such metal ions, on to the surface of metal/metalloid oxide particles and thereby prepare metal-ion-doped (or, after reduction of the metal ion, metal- atom-doped) porous xerogels and,, ultimately after further processing, porous ceramic materials which comprise dopant corresponding to the metal-ion dopant and metal or metalloid oxide particles, preferably nanoparticles.

Furthermore, and especially advantageously, we have discovered how to control the quantity of metal-ion dopant that is adsorbed from solution on to the metal or metalloid oxide particles and how to control how the porous structure of the xerogel of metal or metalloid oxide particles is affected by the doping with metal ion. Thereby we have discovered novel, improved, and highly advantageous xerogels and corresponding porous ceramic materials which remain microporous and are doped to a degree not heretofore achieved in material that is microporous and, in fact, doped to the maximum amount possible while retaining microporosity (pore diameter less than about 20 Angstroms).

Our invention thus encompasses methods of doping ceramic membranes made of metal and metalloid oxide particles with ions and atoms of hydrolyzable metals in order to make metal-ion or metal-atom doped porous ceramic membranes.

Further, the invention encompasses the novel, improved metal- ion and metal-atom doped ceramic membranes that can be made by the methods of the invention.

Still further, the invention encompasses methods of using the metal- ion and metal- atom doped membranes of the invention in the various uses, understood in the art, to which such membranes may be put.

As indicated above, the invention relates to doping with ions in aqueous solution that are hydrolyzable metal ions per se or that include ions of hydrolyzable metals. Also, the invention relates to doping with protolyzable anions. Among the metals of interest are the transition metals and others. Thus, doping in accordance with the invention may be with, among many others, aluminum ions or ions that include aluminum, iron ions or ions that include iron, chromium ions or ions that include chromium, cobalt ions or ions that include chromium. Other metals of interest include platinum, gold, copper, zinc, cadmium, nickel, zirconium, palladium, molybdenum, manganese, tin and mercury. Some exemplary protolyzable anions for use in accordance with the invention are sulfate ions, phosphate ions, arsenate ions, and vanadate ions.

We have found that the conditions of pH and concentration of the dopant ions of interest in aqueous solution, for any ionic dopant/metal oxide system, are critical in determining both the extent of dopant adsorption and the porous structure of the resulting xerogel (i.e. does it retain microporosity or become mesoporous). Ultimately, then, the pH and concentration of the dopant ions are critical in determining the loading of metal and porous structure of the porous ceramic material made with the xerogel.

We illustrate hereinbelow the pH and dopant concentration conditions for a number of specific systems. From this information it will be routine for the skilled person to ascertain the conditions to achieve heavy, and indeed maximum loading, while retaining microporosity in any system of interest. We have found surprisingly that certain principles concerning adsorption of ions from aqueous solution apply as well to adsortion on metal/metalloid oxide particles, even nanoparticles, from which porous ceramic materials are made. For example, in adsorption of ions from aqueous solution, the pH is the master variable controlling the extent of the adsorption in a particular adsorbent/adsorbate system. In the particular case of polyvalent cations, the pH influences the adsorption mainly by changing the speciation of the metal ion in the bulk solution. Baes and Mesmer. The Hydrolysis of Cations, John Wiley & Sons: New York, 1976. Nevertheless, changing the speciation of multivalent cations in solution may not only change the extent of adsorption but the nature of cationic surface species as well. We have discovered then that we could apply these principles to prepare metal doped oxide sols having different gelation properties, to provide xerogels with the desired type of porosity (preferably microporosity) and metal- loading (preferably higher than ever achieved heretofore with microporous materials).

By increasing the pH and the concentration of metal ions in the suspension we increase the adsorption density of the metal ion. Furthermore, the number of polymeric surface species and the average degree of polymerization also rise. The presence of polymeric species in any colloidal system will affect its tendency to aggregate and will, as well, influence the structure of the aggregate (=gelation properties). Void et al.. Colloid and Interface Chemistry. Addison- Wesley: London, 1983. Matijevic et al., J. Colloid Interface Sci. 1971, 35, 560-68.

In addition, the gelation properties of the doped sols will be influenced by the gelation properties of the adsorbent at the pH of the sol. The extent of this effect should decrease with increasing adsorption density. Furthermore, the pH of adsorption will determine the protonation state of these surfaces, the higher the pH, the higher the degree of hydrolysis of the cationic surface species and the higher the tendency for condensation when particles collide. Hence, the concentration of metal ion in the system in conjunction with pH will affect the gelation properties of the sols in three ways: 1) by establishing the adsorption density; 2) by controlling the degree of polymerization of the surface species; 3) and by influencing the state of protonation. As mentioned above, different combinations of pH and concentration of metal ions in solution should provide a variety of porous structures in resulting xerogels. This hypothesis has been probed by studying several Siθ2/polyvalent metal systems. The results reported in the following sections will confirm the validity of this assumption.

While this disclosure specifically refers to the doping of SiO₂ particles, the methods described here can be equally applied to other oxides such as TiO₂, A1₂0₃, and others well known in the art. The reason for this rational is that hydrolyzable cations exhibit an unusual feature with respect to adsorption onto oxide surfaces. For these systems the adsorption density does not seem to be affected by the nature of the adsorbent, neither by the magnitude nor the sign of its surface charge. That is because the level of adsorption of metal ions unto oxide surfaces and the type of the surface species formed relates largely to the difference between the pK, of the hydrated metal ion and the pH of the suspension. Kinniburgh and Jackson, Adsorption of Inorganics at Solid-Liquid Interfaces; Anderson et al., eds., Ann Arbor Science, Ann Arbor, Michigan, USA, 1981. The nature of the oxide adsorbent has very little effect on the adsorption reaction.

The following Example illustrates the invention in some detail with respect to certain specific examples. However, the informationin the Example is not intended to be limiting, in fact is not limiting, and should not be construed to limit the scope of the invention.

Among other things, the Example teaches novel, very high loadings of Co at 0.05 to 5 atoms per.nm², Fe at 0.85 to 5 atoms per nm², Al at 0.05 to 10 atoms per nm², and Cr at 0.05 to 5 atoms per nm² on silicon dioxide particles while still preserving microporosity.

EXAMPLE Adsorbent was SiO2 sols with particles having an average diameter of 6 nm. These particles were formed by the hydrolysis of TEOS in NH4OH solution, followed by dialyzing the suspension in H20 until the pH lowers to 8.6. See United States Patent No. 5,194,200. Under these conditions the sols are fairly stable for months.

Adsorbates were acidic aqueous solutions of Fe(III), Cr(III), Al(III), Co(II) and Ni(II) nitrates. Metal loading of sols was carried out as follows: A desired volume of solution containing the metal ion is added drop-wise to a stable sol at room temperature (19-23 °C). The system is magnetically stirred and the pH monitored and kept at the desired values throughout the metal addition process. The tendency for the pH to drop with the addition of metal makes it necessary to keep adding KOH during the titration. Following this addition process, the samples are left to equilibrate 1 hour at 25 ^C in a shaker. After one hour, the pH is measured and readjusted with base.

Since the adsorption of multivalent cations almost always involves a net release of protons (Schindler, Adsorption of Inorganics at Solid-Liquid Interfaces; Anderson et al., eds., Ann Arbor Science: Ann Arbor, Michigan, USA, 1981), a lowering in the pH value of the sols, shortly after titration, is expected. The magnitude of the pH drop, for a given adsorbent/adsorbate system, depends on the extent of adsorption and the buffer capacity of the system. Under the experimental conditions used in this study, the pH changes during the first hour after ending the titration were larger than one unit. The fact that the pH range of increasing adsorption may be even smaller than a pH unit (United States Patent No. 5,194,200) makes it necessary to check and adjust this variable soon after a significant fraction of the adsorption takes place. The kinetics of adsorption reactions are fast enough to expect the adsorption reaction to be near steady state after one hour (Matejevic et al., above).

After the pH adjustment, samples are put back in the shaker and left to equilibrate for 23 more hours. Then the pH is measured again and readjusted if necessary. If the pH is readjusted the system is left in the shaker to react one more hour and the pH measured again. This latter pH value will be regarded as the "pH of adsorption".

Formation of xerogels was carried out as follows: The metal- loaded silica particles are separated from the solution using centrifugal ultrafilters (CentriCell of 10,000 N.M.W.L.). The sols are centrifuged at 2500 r.p.m. for approximately 1.5 hours. During this process some sols gelled on the filter. The hydrogel volume for these sols

(Gieselmann and Anderson, J. Am. Ceram. Soc. 1989, 72 6, 980-985) varies between 1/30 and 1/6 of the original volume of a sol 0.35 M in Si0₂. Other sols, however, remained as a fluid even when the concentration of solids is 10 M in Si0₂. These more concentrated sols become xerogels after only a few hours of being placed in a polyethylene dish at 40% RH. Regardless of the gelling behavior of the sols all the samples were dried at 40 % R.H. after separation from the filtrate. Xerogels from sols that remained fluid during centrifugation and sols that gel during this process with the smaller hydrogel volume, appeared as transparent chips. Xerogel chips from samples with higher gelation volumes had some cloudiness. Results were as follows: Data reported in Table I shows the relationship between the microstructure of xerogels (as characterized by the specific surface area (SSA), % of porosity, and mean pore radii (R_m)(referred to also as "pore size" hereinabove), level of metal loading (atoms of metal ion per nm2) and the precise conditions of preparation (adsorption pH and metal ion concentration in solution at equilibrium). The operational "pH of adsorption" has been already defined in the previous section. The metal density on the surface of these xerogels was calculated from the difference between Mⁿ⁺ added to the sol and the Mⁿ' in the filtrate. This quantity is normalized with respect to the concentration of solids and specific surface area of the Si02 sol. For all the systems in Table 1 600 m-^/g is used as the nominal SSA . This surface area was calculated for a primary particle size of 6 nm (from TEM images), and the density of amorphous silica (2.02 g/1). The metal ion concentration in the filtrates was determined either by AA, colorimetry or ICP.

The specific surface area (SSA) and mean pore radii (R,,,) of the xerogels were determined by analyzing N₂ adsorption measurements at 77.3 degK, using the BET and the Kelvin equations, respectively. Since the Kelvin equation can not provide information on the presence of pores having a R,,, below 11 Angstroms we do not know yet the pore size distribution for the microporous systems. The % of porosity was calculated from the uptake of N₂ at saturation pressure (p/p₀ close to 1).

Table I includes as well the microstructure data on SiO2 xerogels from sols titrated at different pH values. This information allows us to evaluate the influence of the adsorbate on the gelation properties of a system.

For the iron doped systems (Fe-1 to Fe-7) pH is the main variable affecting the size of the pore. If the pH is not higher than pH 2.6, adsorption densities as high as almost 1 atom nm-2 produce microporous systems (Fe-3). Similar adsorption densities obtained at only 0.3 units of pH higher will yield mesoporosity (Fe-4). For a fixed pH, an increase in adsorption density seems to increase the pore size (Fe-2 vs. Fe-3; Fe-5 vs.Fe-6)and porosity.

The presence of Cr(III) on the Si02 particles generates a strong tendency for the sols to yield microporous xerogels when compared with both pureSi02 sols or iron doped ones. Unlike the case of iron doped systems, high adsoφtion densities for Cr-doped sols favor the formation of microporosity. The higher the adsorption pH, the higher the metal loading that is needed to produce a microporous xerogel (see Cr-3 vs. Cr-4; Cr-6 vs. Cr-7). One should be careful in comparing this data to the Fe/Si02 system. In the case of Fe, 4 out of 5 of the pH values shown in Table I are below pH=4.0. But on the contrary, the pH values for the systems Cr-2 to Cr-7 are above 4.0. At this pH, pure Si02 xerogels start showing some mesoporosity, and the % of porosity and pore radius increases with increasing pH. Therefore, the results for Cr/Si02 systems can be interpreted as the influence of the pure Si02 adsorbent on the gelation.

The tendency for surface A1(III) to produce open structures is higher than pure silica, especially when comparing systems at low pH (3.5) . What is particularly interesting is that the Al(III) hexaquo complex should not be hydrolysed (pK, = 4.95) at these pH values (Baes and Mesmer, supra). Hence adsorption should be small and gelation properties similar to pure silica. Electrophoretic mobility studies throw some light into the peculiar behavior of the Al-doped systems. Powdered samples of the systems Al-1 (adsorption pH,=2.5; adsorption density = 0.10 atom/nm-2 ) left to equilibrate in a 0.01 M KC1 solution develop a negative charge at pH values of 3 and 3.5. The charge for pure silica xerogels is small but still positive at these values of pH. This electrophoretic behavior indicates that Al (III) is tetrahedrally coordinated in the surface (or subsurface) of the silica particle in place of octahedrally as it is in solution. This finding has the following implications regarding the gelation properties of these sols: 1) the protonation state of the surface complexes do not have to relate to the pK, for the complex in solution in the same way as that for Fe(III) and Cr(III); and 2) the promotion of negative surface charge on the silica particle may change the acidity of the surface silanols. This may, in turn, affect the gelation properties of the sols as a function of pH which may be sifted when compared to pure silica. In systems with adsoφtion pH higher than 4.0, the removal of metal from solution as well as gelation may go through a different mechanism than that in systems for which adsoφtion pH is lower than 4. At these pH values, even at Al(HI) concentrations of 1 mM the Al₁₃O₄(OH)₂₇ ⁷⁺ or -Al(OH)₃ (gibbsite) may be present in the system. Should these species be present, the polymeric species will induce chemical aggregation (flocculation) of the silica particles and the gibbsite (positively charged at these pH values) will provoke heterocoagulation in the silica sols.

Co doped systems show a behavior similar to the Al doped ones, but in different range of pH, since the pK of hydrolysis for Co(II) is close to 9.

TABLE I

Claims

WHAT WE CLAIM IS:

1. A microporous xerogel or ceramic material which comprises metal or metalloid oxide particles and which is doped with at least 0.05 atoms per square nanometer of particle surface of an hydrolyzable metal or protolyzable anion.