Process for making inorganic gels

A process for making inorganic gels by reaction of tetraalkoxy orthosilicates, tetraalkoxy titanates and tetraalkoxy zirconates with strong carboxylic acids. Water need not be present initially as a reactant. Optically clear, very small pore size, narrow pore size distribution, and high specific area inorganic gels useful for abrasion-resistant coatings, optical applications, catalyst or enzyme support, gas separation, or chromatography packing are thus produced.

BACKGROUND OF THE INVENTION 
This invention relates to a rapid, nonaqueous process for producing 
optically clear, inorganic gels of silicon, titanium and zirconium. 
The above-described inorganic gels can be dried to form strong, hard, 
optically clear compositions that are useful as abrasion-resistant and 
controlled refractive index coatings in optical applications such as, for 
example, lenses and windows. They may be used as dielectric coatings and 
anti-corrosive coatings. Additionally, the gels have a pore structure of 
extremely small size, high specific surface area and narrow pore size 
distribution which makes them useful for catalyst or enzyme support, gas 
separation and in chromatography as column packing. 
Inorganic gels of silicon, titanium and zirconium are typically created by 
hydrolysis of organo-silicon,-titanium and -zirconium compounds, and 
subsequent condensation of the hydrolysis products. For silica gels, 
hydrolysis, (I), and condensation, (II) and (III), are represented by the 
following equations: 
EQU --SiOR+H.sub.2 O.rarw..fwdarw.--SiOH+ROH (I) 
EQU 2(--SiOH).rarw..fwdarw.--SiOSi--+H.sub.2 O (II) 
EQU --SiOH+--SiOR.fwdarw.--SiOSi--+ROH (III) 
where R is an alkyl group. 
Hydrolysis and condensation reactions are normally catalyzed by acids or 
bases. 
Existing methods of producing small pore size, high specific surface area, 
inorganic gels suffer from a number of problems. As discussed by C. Plank 
et al. in J. Colloid Sci., 2, 399 (1947) small pore, high surface area gel 
is ideally produced near the isoelectric point which for silica is about 
pH of 2. However, acid-catalyzed condensation reaction is so slow that 
several weeks or longer may be required for gelation to occur at about 
25.degree. C. at this pH. If condensation is conducted at elevated 
temperature, complete gelation can occur in less time; however, pore size 
will increase and surface area will decrease. In general, alkaline or 
fluoride ion catalyzed gelation is faster but produces larger pore size 
gels than acid catalyzed gelation and may lead to opaque gels. Another 
problem is that many conventional processes use alcohol as solvent for 
reactants. Presence of alcohol enhances the reverse of reactions (I) and 
(III) which hinders production of gel. 
U.S. Pat. No. 4,950,779 (Wengrovius, et al.) discloses a process for making 
organosilicon oligomers by reacting polyalkoxysilanes or polyaminosilanes 
with formic acid without addition of water. Wengrovius' products are 
organosilicon oligomers of low molecular weight, in contrast to gels of 
silicon dioxide which are completely crosslinked networks of unlimited 
molecular weight. Wengrovius' silicone oligomers contain at least one 
non-hydrolyzable organic substituent on each silicon atom which limits the 
ability of the oligomers to form gel. 
Coltrain et al. demonstrate in a paper presented at a 1989 conference and 
published in Ultrastruct. Process. Adv. Mater., Wiley, New York, N.Y., 
1992, pp. 69-76, that the rate of acid-catalyzed hydrolysis of 
tetraalkoxysilanes and formation of gels depends on the pH of the medium. 
For a strong organic acid such as trifluoroacetic acid, the rate of 
reaction in an aqueous medium slows down as the concentration of the acid 
increases. 
It is desirable to provide a rapid process requiring only as much water as 
necessary to propagate reaction, for the synthesis of open pore, inorganic 
gels having fine pore structure. 
SUMMARY OF THE INVENTION 
According to the present invention, there is now provided a process for 
making open pore, inorganic gel, comprising: 
(1) intimately mixing together the following components (A) and (B), either 
in a liquid state or in solution in an organic liquid: 
(A) at least one compound selected from the group consisting of 
Si(OR.sup.1).sub.4, Ti(OR.sup.2).sub.4, Zr(OR.sup.3).sub.4, Ti.sup.a 
(OR.sup.4).sub.n X.sup.d.sub.p, and Zr.sup.b (OR.sup.5).sub.n 
X.sup.d.sub.p 
wherein 
each one of R.sup.1, R.sup.2, R.sup.3, R.sup.4, and R.sup.5 independently 
is a C.sub.1 -C.sub.8 alkyl; 
X is a chelating ligand; 
each one of a and b is independently a coordination number having an 
integer value of 4-6; 
d is a number corresponding to the chelating ability of ligand X, being 2 
for bidentate chelating ligands and 3 for tridentate chelating ligands; 
p is either 1 or 2, with the proviso that p is 1 for tridentate chelating 
ligands; 
EQU n=a-d.p; and 
(B) a strong carboxylic acid, having a pK.sub.a value not higher than about 
4.0, and containing from 0 to 40 mole % water, the amount of carboxylic 
acid being at least 1.5 moles of --COOH groups per mole of compound (A), 
and the total amount of water from any source initially present in the 
reaction medium being less than 5 moles, and preferably less than 1 mole, 
per mole of Component (A); 
(2) agitating the above mixture while maintaining the mixture at a 
temperature within the range of about 0.degree. to 100.degree. C.; and 
(3) isolating the inorganic gel. 
There is also provided a process for synthesis of inorganic gel having 
extremely fine pore structure.

DETAILED DESCRIPTION OF THE INVENTION 
The process of this invention can produce an open pore, very small pore 
size, inorganic gel structure, occasionally hereinafter referred to as 
"ultrafine pore structure". The term "open pore" means a structure having 
voids which are interconnected and are accessible by an unobstructed path 
to the surface. Pore size of ultrafine pore structure has not been 
measured directly but is believed to be smaller than about 2 nm. Ultrafine 
pore structure is identified by specific surface area and density 
measurements. More specifically, ultrafine pore structure is detected by 
subjecting a gel to adsorption analysis by ASTM standard C1069-86 which 
measures nitrogen adsorption at -196.degree. C. in four hours. This 
method, sometimes hereinafter referred to as "low-temperature nitrogen 
analysis", is suitable for determining the specific surface area of gel 
having open pores into which nitrogen readily diffuses at -196.degree. C. 
Such pores are usually larger than about 1.5 nm. An open pore gel which 
indicates less than 10 m.sup.2 /g specific surface area by low-temperature 
nitrogen analysis has ultrafine pore structure. 
To determine that a gel which indicates less than 10 m.sup.2 /g has open 
porosity, it should be tested by both low pressure mercury porosimetry 
bulk density analysis according to ASTM standard C493(15.01), and helium 
pycnometry skeletal density analysis as described by T. Woigner et al. in 
J. Non-cryst. Solids 93, 17 (1987). Bulk density analysis provides the 
mass per unit volume of gel including the volume of void space of open 
pores. Skeletal density analysis provides the density of solid gel 
excluding open pore volume. If skeletal density of a given gel is higher 
than its bulk density, then the gel has open porosity. However, if 
skeletal density is equal to bulk density, the gel may either be nonporous 
or have closed pores. 
Gel made by the process of this invention which indicates greater than 10 
m.sup.2 /g specific surface area by low-temperature nitrogen analysis may 
have a pore size distribution which includes ultrafine pore structure as 
well as pores larger than about 1.5 nm. A high diffusion rate adsorption 
analysis can sometimes be used to identify ultrafine pore structure in 
such gel. A suitable high diffusion rate adsorption analysis is provided 
by modifying the, ASTM C1069-86 test procedure, for example, by conducting 
the procedure at temperature higher than -196.degree. C., (hereinafter, 
"high-temperature nitrogen analysis") or by substituting carbon dioxide 
for nitrogen and conducting the procedure at -78.degree. C. or higher 
(hereinafter, "CO.sub.2 analysis"). Under such conditions, diffusion of 
adsorbate into ultrafine pores occurs more rapidly than during 
low-temperature nitrogen analysis. The BET method described in the Journal 
of the American Chemical Society, S. Brunauer et al., 60, page 309 (1938), 
can be used to calculate specific surface area of gel tested by adsorption 
analyses. Because the BET method relies on certain assumptions about pore 
geometry which may not be true for ultrafine pore structure, it may not 
accurately calculate pore sizes and specific surface area of gel 
containing such structure. Nevertheless a gel is believed to have 
ultrafine pore structure when CO.sub.2 or high-temperature nitrogen 
analysis indicates specific surface area more than that indicated by 
low-temperature nitrogen analysis. If CO.sub.2 or high-temperature 
nitrogen analysis does not exceed specific surface area indicated by 
low-temperature nitrogen analysis, no conclusion can be drawn that the gel 
possesses ultrafine pore structure. 
Compounds (A) are tetraalkyl orthosilicates Si(OR.sup.1).sub.4, tetraalkyl 
titanates Ti(OR.sup.2).sub.4, tetraalkyl zirconates Zr(OR.sup.3).sub.4, 
chelated titanates Ti.sup.a (OR.sup.4).sub.n X.sup.d.sub.p, and chelated 
zirconates Zr.sup.b (OR.sup.5).sub.n X.sup.d.sub.p, sometimes hereinafter 
collectively referred to as "tetraalkoxy compounds". The present invention 
is based on the discovery that tetraalkoxy compounds, which either are 
soluble in strong carboxylic acids or are soluble in nonaqueous solvents 
for strong carboxylic acids, readily react in the presence of 
significantly less than stoichiometric amounts of water or initially 
without any water to produce inorganic gels. 
R.sup.1, R.sup.2, R.sup.3, R.sup.4 and R.sup.5 are linear, branched, 
alicyclic groups. Illustrative of such acyclic groups are ethyl, propyl, 
butyl, hexyl, 1-methylethyl, 2-methylpropyl, 1-methylpropyl, 
1,1-dimethylethyl and 2,2-dimethylpropyl groups. Illustrative of such 
alicyclic groups are cyclobutyl, cyclopentyl and cyclohexyl groups. 
Chelating compounds suitable for use in the present invention are chemicals 
containing chelating ligands capable of bonding to titanium or zirconium 
through two or more oxygen atoms. Illustrative of compounds containing 
bidentate chelating ligands, X.sup.2, is acetylacetone. Illustrative of 
compounds containing tridentate chelating ligands X.sup.3 is 
triethanolamine. 
Examples of tetraalkyl orthosilicates suitable for use in this invention 
include: tetramethyl orthosilicate, tetraethyl orthosilicate (TEOS), and 
tetra(n-propyl) orthosilicate. It is to be noted that the above lo 
organosilicon compounds can be named in two different ways, and both 
nomenclatures are generally accepted and understood by those skilled in 
the art. The alternative nomenclature is "tetraalkoxysilanes", for 
example, tetraethoxysilane and tetramethoxysilane. Examples of suitable 
tetraalkyl titanates include: tetraisopropyl titanate, tetra(n-butyl) 
titanate and tetra(2-ethylhexyl) titanate. Representative of bidentate 
chelated titanales is titanium triisopropoxide- 2,4-pentanedionate and 
representative of tridentate chelated titanates is 
isopropoxy(triethanolaminato-)titanium. An example of tetraalkyl 
zirconates is tetra(n-propyl) zirconate and an example of chelated 
zirconates is zirconium bis(acetoacetonate)diisopropylate. 
Inorganic gel of either exclusively one of silicon, titanium and zirconium 
or a mixture of them, can be produced by hydrolyzing and condensing the 
corresponding tetraalkoxy compound or appropriate mixture of tetraalkoxy 
compounds. In general, titanium- and zirconium-containing inorganic gel 
has higher refractive index than silicon gel. Gels can be made within 
broad ranges of silicon/titanium/zirconium compositions by controlling the 
reactant mixture ratios of silicon:titanium:zirconium tetraalkoxy 
compounds accordingly. Gels made by the process of this invention 
typically may have a small amount of unreacted alkoxy and hydroxy groups 
on the silicon, titanium and zirconium atoms. 
To obtain rapid gel formation, strong carboxylic acids suitable for use in 
the present invention should be capable of attaining homogeneous, liquid 
phase contact with tetraalkoxy compounds. Consequently, such strong 
carboxylic acids are either liquid and miscible with tetraalkoxy compounds 
under reaction conditions or soluble in solvents for tetraalkoxy compounds 
at reaction conditions. Either strong carboxylic acid or tetraalkoxy 
compound can be first dissolved in solvent and the other reactant added to 
the resulting solution. Alternatively, both reactants can be separately 
dissolved in either the same or different solvents and the two solutions 
combined. Suitable solvents for use in the present invention are 
nonaqueous, polar and aprotic. Illustrative of such solvents are 
tetrahydrofuran, acetone, acetonitrile and dichloromethane. Presence of 
solvent dilutes the concentrations of carboxylic acid and tetraalkoxy 
compound, which generally slows gel formation, but gel formation rate can 
be increased by raising reaction temperature. Solvents capable of forming 
hydrogen bonds with silanol (Si--OH) groups, such as, for example, 
tetrahydrofuran and alcohols, also can slow the gelation rate by impeding 
the condensation reaction of the silanol groups 
Strong carboxylic acid which is solid at room temperature can be used 
without a solvent if heated above the melting point in order to mix it 
with tetraalkoxy compound in the liquid state. Such high temperatures can 
increase reaction rates dramatically but can inhibit formation of 
ultrafine pore structure. Therefore, to avoid using too high a 
temperature, it is preferred to dissolve solid acids in suitable solvents 
at about room temperature when production of ultrafine pore structure gel 
is desired. 
It is generally desirable to carry out hydrolysis and condensation at a 
temperature in the range of 0.degree.-100.degree. C., and preferably in 
the range of 0.degree.-60.degree. C. Reaction temperature in the range of 
0.degree.-30.degree. C. is most favorable for the formation of ultrafine 
pore structure. Operation in the strong carboxylic acid to tetraalkoxy 
compound mole ratio range from 2:1 to 10:1, and preferably from 3:1 to 
5:1, also favors production of ultrafine pore structure. The process of 
this invention therefore provides the advantage of producing gels having 
ultrafine pore structure that has not been reported for gels made by 
conventional processes. Most favored conditions for producing ultrafine 
pore structure are the combination of low mole ratio of 2:1 to 10:1 and 
low reaction temperature. Wet gel can be dried to produce coatings, 
monoliths and other shaped forms, such as optical lenses. By "dried" it is 
meant that the liquid residue of hydrolysis and condensation, including 
solvents, carboxylic acid, tetraalkoxy compound and byproducts of 
reaction, are removed. Volatile liquid residue can be removed by 
evaporation at elevated temperature and/or subatmospheric pressure. If the 
volatility of the liquid residue is low, high temperatures and vacuum 
conditions may be needed. Shaped articles can fracture if dried under such 
harsh conditions. To avoid the need for harsh drying conditions, volatile 
reactants, such as strong carboxylic acids having atmospheric boiling 
points below about 180.degree. C., are preferred for use in this 
invention. If acids having atmospheric boiling points above 180.degree. C. 
are used, they may be removed from the wet gel by alternative methods such 
as by extraction with a solvent boiling below 180.degree. C. followed by 
evaporation of the low boiling solvent. Alternatively, mild drying 
conditions can be employed, but they usually must be maintained for long 
duration to accomplish drying. 
The process of this invention has the surprising, desirable feature that no 
water need be added to the reactants initially and that the steady state 
water concentration during reaction is quite small. One of the benefits of 
this feature is that clear gels can be made readily without a need to use 
a water-miscible solvent to obtain a homogeneous medium. While water is 
necessary for hydrolysis, a sufficient amount is formed by reaction of 
strong carboxylic acid with alcohol produced by hydrolysis and by 
metathesis reaction (IV): 
EQU HCOOH+--SiOR.rarw..fwdarw.--SiOOCH+ROH (IV) 
The process then continues as shown in equations (V) and (VI), below and 
then as shown above in equation (I). 
EQU HCOOH+ROH.fwdarw.HCOOR+H.sub.2 O (V) 
EQU --SiOOCH+SiOH.fwdarw.SiOSi--+HCOOH (VI) 
Naturally, any water which may be present as diluent in strong carboxylic 
acid can contribute to hydrolysis. Strong carboxylic acid containing at 
most 20 mol % water is preferred for the process of this invention. 
The overall, idealized, stoichiometry of the reactions occurring in the 
process of the present invention requires at least two moles of organic 
acid per mole of compound (A), as shown below in equation (VII), but it 
has been found in practice that satisfactory results can be obtained when 
the amount of organic acid is less than required, but at least 1.5 moles 
per mole of compound (A). 
EQU 2 HCOOH+Si(OR).sub.4 .fwdarw.SiO.sub.2 +2ROH+2ROOCH (VII). 
Examples of strong carboxylic acids effective in this invention include 
formic acid, monochloroacetic acid, dichloroacetic acid, trifluoroacetic 
acid and hydroxyacetic acid. Neat formic acid is chemically unstable. 
Reagent grade formic acid, which is typically supplied commercially with 
about 4-20 wt % (9.6-38.9 mol %) water is stable, however. The process of 
this invention has been found particularly effective with 96 wt %, reagent 
grade formic acid, without further addition of water. Formic acid which 
has been dried to about 99.5 mol % purity is also highly effective. Other 
strong carboxylic acids, including dichloroacetic acid and trifluoroacetic 
acid, for example, can exist in essentially anhydrous form having less 
than about 0.1 mol % water. Weaker acids, such as acetic acid, and strong 
mineral acids such as hydrochloric and nitric acid, are not satisfactory 
for use according to this invention. 
FIG. 1 is a compound plot of gelation times vs. initial reactant 
compositions from two series of experiments, one series of which was run 
by V. Gottardi, et al., and reported in J. Non-cryst. Solids, 63, 71 
(1984). TEOS was the tetraalkoxy compound used in both series. The 
abscissa is the mole ratio of hydrolytic agent to TEOS. The experiments in 
the series conducted according to the present invention are represented by 
square data points. This series was run at 20.degree. C. and the strong 
carboxylic acid used was 96 wt % formic acid. In the acid-rich reactant 
mole ratio range of about 6-20, gelation occurs in from about 30-120 
minutes. In the mole ratio range of 2-6, gelation time is found to 
increase to greater than about 100 hours. The Gottardi data, obtained at 
60.degree. C., are represented by the circular data points. The Gottardi 
experiments used water as the hydrolyric agent and the reaction system was 
catalyzed by hydrochloric acid. Gelation time at 20.degree. C. was 
calculated for FIG. 1 by using 14 kcal/mol (58,700 J/mol) activation 
energy of gelation reported by M. W. Colby, et al., in J. Non-cryst. 
Solids 99, 129 (1988). FIG. 1 shows that gelation according to the present 
invention at each hydrolytic agent:TEOS mole ratio is several hundred 
times faster than accomplished by the conventional process. 
When very highly reactive tetraalkyl titanates, such as, for example, 
tetraisopropyl titanate, are used in the presence of water, titanium 
dioxide tends to precipitate. Clear, homogeneous gels suitable for optical 
applications cannot be made from such highly reactive tetraalkyl titanales 
and stoichiometric amounts of water, as reported in, for example, Sol-gel 
Science, C. J. Brinker et al., Academic Press, 1990, page 53. According to 
the present invention, titanium dioxide precipitate can be avoided by 
using essentially anhydrous, strong carboxylic acids, such as 
trifluoroacetic acid, rather than 96 wt % formic acid. Furthermore, even 
96 wt % formic acid can be used to make clear gel from less reactive 
titanates such as tetrabutyl titanate. Therefore, the process of the 
present invention allows formation of gels not obtainable by conventional 
means. 
The high gelation rate of the process of the present invention is quite 
unexpected. In J. Non-cryst. Solids, 73, 681 (1985), Schmidt reports that 
in the presence of 0.002 M aqueous HCl, TEOS undergoes hydrolysis 
approximately fifty times more slowly than does an alkyltrialkoxysilane, 
(CH.sub.3)Si(OC.sub.2 H.sub.5).sub.3. Wentgrovius discloses that the 
reaction of alkyltrialkoxysilane with formic acid to form silicone 
oligomers is very slow, taking several hours or days. The combined 
disclosures of Schmidt and Wengrovius suggest that formation of gel by 
reaction of tetraalkyl orthosilicates with formic acid would be extremely 
slow since hydrolysis of the silicon-alkoxy bond is an essential step in 
the process. Surprisingly, it has been found that such gel formation 
occurs remarkably quickly. 
Inorganic gel produced according to the present invention can be dried 
slowly, under mild temperature and pressure conditions to a monolithic 
form. Alternatively, the gel can be rapidly dried at up to 300.degree. C. 
and either under vacuum or in a stream of inert gas to make a powder. 
Prior to complete gelation, the solution can also be applied by 
conventional techniques such as dip coating, spray coating and knife 
coating, for example, to provide coatings on various substrates. Wet gels 
can also be dried to produce an aerogel by extraction with supercritical 
fluids such as, for example, CO.sub.2 and methanol, which preserves the 
original pore structure in the product. Such aerogels are useful as 
acoustic or thermal insulating materials, especially when the density of 
the aerogel is very low. Accordingly, it is desirable to generate wet gels 
of very low solids content which can serve as precursors to useful 
aerogels. 
This invention is now illustrated by representative examples of certain 
preferred embodiments thereof, where all parts, proportions, and 
percentages are by weight, unless otherwise indicated. All units of weight 
and measure other than SI units have been converted to SI units. Formic 
acid was commercially available acid solution containing 4% water and 
example quantities are given on the basis of formic acid actually present, 
unless otherwise stated. 
EXAMPLE 1 
TEOS was added to formic acid in a 3.00:1 mole ratio of formic acid:TEOS at 
room temperature and stirred intermittently. A mildly exothermic reaction 
ensued during which the temperature rose to a maximum slightly above room 
temperature. A transparent, wet gel was produced after 23 hours at 
20.degree. C. The wet gel was dried within minutes of its formation in a 
rotary evaporator at 20 kPa for 30 min at 60.degree. C. Thereafter, 
pressure was lowered to 15 Pa and drying was completed at 50.degree. C. 
for 16 hours to produce granular, dry gel. 
The dry gel was pulverized and subjected to surface area measurement by 
low-temperature nitrogen analysis. The dry gel sample did not show 
appreciable nitrogen adsorption after 4 hours, which indicated that the 
specific surface area was less than 10 m.sup.2 /g. Dry gel was analyzed by 
low pressure mercury porosimetry which indicated the bulk density to be 
1.64 g/cm.sup.3. Measurement by helium pycnometry revealed that skeletal 
density was 1.83 g/cm.sup.3. Experimental error of the density 
measurements was .+-.0.02 g/cm.sup.3. Because skeletal density was greater 
than bulk density, the gel had open porosity. The findings of less than 10 
m.sup.2 /g specific surface area and open porosity show that the gel 
contained ultrafine pore structure. 
The same sample was then subjected to CO.sub.2 analysis at 25.degree. C. as 
well as at -78.degree. C. Each CO.sub.2 analysis indicated appreciable 
adsorption of CO.sub.2, from which minimum specific surface area of 250 
m.sup.2 /g was calculated. Results of CO.sub.2 analyses thus confirmed 
that the gel contained ultrafine pore structure. 
EXAMPLE 2 
The procedure of Example 1, was repeated with formic acid and TEOS combined 
in a mole ratio of 7.74:1. A clear gel formed in 41 minutes at 23.degree. 
C. The gel was dried as in Example 1 then subjected to low- temperature 
nitrogen analysis which indicated that nitrogen had not been adsorbed to 
significant extent. Bulk density and skeletal density of the gel were 
determined to be 1.21 and 1.89 g/cm.sup.3, respectively, showing that it 
had open porosity. Taken with the low nitrogen adsorption result, open 
porosity indicated that the gel had ultrafine pore structure. 
Pieces of dried gel were submerged in water. The gel fractured 
spontaneously and bubbles of air evolved. It was concluded that the air 
had been trapped within the pores and was liberated by gel fracture, 
additionally confirming that the gel had open porosity and ultrafine pore 
structure. 
EXAMPLE 3 
The procedure of Example 2 was repeated with formic acid to TEOS mole ratio 
of 2.31:1. A clear wet gel was formed in approximately 120 hours at 
23.degree. C. Again, insignificant nitrogen was adsorbed after 4 hours at 
-196.degree. C. Bulk and skeletal densities were determined to be 1.41 and 
1.64 g/cm.sup.3, respectively, indicating that the gel had open porosity. 
Pieces of dried gel fractured and liberated air bubbles when submerged in 
water. 
Examples 1-3 suggest that low to intermediate mole ratios of acid to 
tetraalkoxy compound favor production of gel having ultrafine pore 
structure. 
EXAMPLE 4 
At room temperature, 4.54 g of TEOS was added to 31.39 g of formic acid and 
stirred intermittently to produce a 31.3:1 mole ratio acid:TEOS mixture. A 
mildly exothermic reaction ensued during which the temperature rose to a 
maximum of 34.8.degree. C. A transparent, wet gel was produced after about 
120 minutes. The flask was evacuated to a pressure of 13.33 Pa for 24 
hours and a powdered, dry gel resulted. 
The pore size distribution of the dry gel was determined by low-temperature 
nitrogen analysis. The results are plotted in FIG. 2 as percent pore 
volume vs. pore size. FIG. 2 shows that 52% percent of total detected pore 
volume of the sample had pore size less than 2.0 nm. The fraction of pore 
volume drops dramatically as pore size increases above 2.0 nm. FIG. 2 also 
indicates that the pore size distribution produced by the process of this 
invention is narrow. Specific surface area of the powdered gel was also 
determined by nitrogen adsorption analysis to be 570 m.sup.2 /g. 
EXAMPLE 5 
Formic acid and TEOS in initial mole ratio of 2.16 were placed in a flask. 
Heat was applied to control reaction temperature at 65.degree. C. A 
transparent wet gel was produced after 21 hours. The gel was dried as in 
Example 4 for 3 hours at 70.degree. C. but did not crumble into powder. As 
measured by low-temperature nitrogen analysis, average pore diameter of 
the dried gel was 1.9 nm and specific surface area was 577 m.sup.2 /g. 
Solids content of the gel was determined by calculation to be 19.4 
percent. Liquids that were removed from the wet gel during drying were 
analyzed by gas chromatography which revealed the presence of ethyl 
formate, ethyl alcohol, and a small amount of water. No formic acid was 
detected because it had reacted with ethyl alcohol to form ethyl formate. 
As compared to Examples 1 and 2, this example suggests that higher 
reaction temperature produces larger pore size. 
EXAMPLE 6 (Comparative) 
At room temperature, TEOS was added to glacial acetic acid and stirred 
intermittently to produce a 9.66:1 mole ratio acid:TEOS mixture. No gel 
formation was detected after several weeks at room temperature. 
EXAMPLE 7 
Into a vessel was placed formic acid and tetra(n-propyl) orthosilicate in a 
mole ratio of 19.8:1. Initially, reactants were only partially miscible 
but a homogeneous solution was obtained after 15 minutes of stirring. 
After standing without further agitation for 16 hours, a transparent wet 
gel was produced. 
EXAMPLE 8 
The method of Example 4 was repeated, except that initial formic acid:TEOS 
mole ratio was 2.16:1. An clear gel was formed after 7 days at room 
temperature. Bulk density of the dried gel as determined by mercury 
porosimetry was 1.72 g/cm.sup.3 which is high compared to that of 
sol-derived gel made conventionally, in the presence of stoichiometric 
amounts of water. High bulk density .gels usually have higher hardness 
than low density gels and are useful for example, for scratch-resistant 
coatings. 
EXAMPLE 9 
Reagent grade, 97.7% dichloroacetic acid was combined with TEOS in 
acid:TEOS mole: ratio of 7.19:1 to form a solution. After standing 
overnight, at room temperature, a clear gel was formed. 
EXAMPLE 10 
TEOS (7.52 g, 36.1 mmol) was allowed to react with 5.98 g of formic acid 
(125 mmol) fix 10 minutes in a flask. Tetra(2-ethylhexyl) titanate (1.124 
g, 1.99 mmol) was then added. A clear solution resulted which formed a gel 
in 90 minutes. The gel was dried for approximately 16 hours at 125.degree. 
C. and at a pressure of 20 kPa, and then analyzed for titanium and silicon 
content by the inductively coupled plasma method of ASTM standard 
E1277-91. This analysis revealed that the weight ratio of titanium to 
silicon was 0.127 indicating that both elements had been incorporated in 
the gel network. Complete incorporation of titanium and silicon in 
reactants would have produced a titanium/silicon ratio of 0.094, which 
suggests that titanium somewhat more efficiently incorporates into the 
network than does silicon. 
EXAMPLE 11 
The method of Example 4 was repeated, except that acid was diluted with 
water to 72 wt % formic acid (50 mol %). Acid to TEOS mole ratio was 
8.15:1. The solution was initially clear and formed a translucent gel 
after 55 minutes at room temperature. After standing for several days, the 
gel became opaque. Opacity indicates the presence, due to the large amount 
of water used, of components having particles large enough to scatter 
visible light; nevertheless, this example demonstrates that strong 
carboxylic acid having as much as 50 mol % water can produce gel rapidly 
according to the present invention. 
EXAMPLE 12 
By dropwise addition of 0.643 g trifluoroacetic anhydride to 6.416 g 
reagent grade trifluoroacetic acid, any water initially present was 
converted to the acid. The anhydride added was in excess of the amount 
needed to convert the small amount of water. To the anhydrous 
trifluoroacetic acid mixture was added 1.674 g of TEOS with stirring. Mole 
ratio of acid to TEOS was 7.00:1. The reaction container was placed in a 
temperature controlled bath maintained at 26.degree. C. A clear gel formed 
97 minutes thereafter, demonstrating that the process of this invention 
can operate without any water initially present. Trifluoroacetic anhydride 
was not observed to react with TEOS in the absence of trifluoroacetic 
acid. 
EXAMPLE 13 
Into a well thermally insulated vessel was placed formic acid and TEOS in 
initial mole ratio of 7.97:1. Heat of reaction raised internal temperature 
to a maximum of 53.degree. C. After 26 minutes, a transparent wet gel was 
produced. 
EXAMPLE 14 
Commercially available trifluoroacetic acid containing 0.4 wt % water as 
determined by Karl-Fischer analysis was reacted with TEOS at an acid to 
TEOS mole ratio of 5.88:1. Gelation was observed after 38 minutes at 
23.degree. C. The gel was dried as in Example 4. Low-temperature nitrogen 
analysis indicated specific surface area of 510 m.sup.2 /g and average 
pore size of 2.4 nm. 
EXAMPLE 15 
The method of Example 4 was repeated with formic acid to TEOS mole ratio of 
16.4:1. Reactants were placed in a cylindrical vial and a few mg of 
rhodamine G dye was added. A bright orange solution was produced which 
gelled on standing overnight at room temperature. Drying at room 
temperature and atmospheric pressure for 24 days produced an orange 
cylinder approximately 6 mm in diameter and 12 mm high of gel of 1.52 
g/cm.sup.3 bulk density. The color was uniform throughout except for a 
slightly darkened region at one end of the cylinder. This example suggests 
the compatibility of gel produced by the process of this invention with 
materials which may be employed in optical applications, such as dyes for 
lenses or filters. 
EXAMPLE 16 
Formic acid and TEOS were each cooled to 2.degree. C., then mixed in 
initial mole ratio of 7.69:1. Throughout mixing and reaction, temperature 
was maintained at 2.degree. C. After 5 hours, a clear gel was observed. 
EXAMPLE 17 
A solution was prepared by dissolving 1.25 g monochloroacetic acid (13.2 
mmol) in 0.73 g tetrahydrofuran (10 mmol) over a 5-minute period. While 
the acid solution was stirred, 0.61 g TEOS (2.9 mmol) was added and a 
clear solution resulted. The solution was allowed to stand at room 
temperature for 3 days, after which it had not gelled, illustrating that 
dilution by solvent can slow reaction rate. After heating to 60.degree. C. 
for 22 hours the solution did form a clear gel. Thus the rate reduction 
was offset by increased reaction temperature. 
EXAMPLE 18 
Tetrabutyl titanate was placed in a small flask under inert atmosphere. 
Over a two-minute period, 96% formic acid was added to the flask at room 
temperature to make a 4.89:1 mole ratio acid:titanate mixture. An 
exothermic reaction was apparent, but no precipitation of titanate was 
observed. After 15 minutes, a clear, pale yellow gel had formed. The gel 
was dried at atmospheric pressure and room temperature for several weeks, 
after which it was dried under vacuum at 100.degree. C. for approximately 
16 hours. Translucent pieces of gel several millimeters in size were 
produced. Low- temperature nitrogen analysis indicated specific surface 
area of 301 m.sup.2 /g and average pore size of 4.2 nm. 
EXAMPLE 19 
Inside an inert atmosphere glove box 4.25 g tetraisopropyl titanate (19.3 
mmol) was placed into a small flask. While contents of the flask were 
stirred at room temperature, 7.33 g anhydrous trifluoroacetic acid (64.3 
mmol) was slowly added. An exothermic reaction was apparent, but no 
precipitation of titanium dioxide was observed. After 17 hours, a clear, 
pale yellow gel had formed. The gel remained clear after drying at room 
temperature and atmospheric pressure for several weeks. This example 
demonstrates that highly reactive tetraalkyl titanate, which typically 
precipitates titanium dioxide during hydrolysis in the presence of 
stoichiometric amounts of water used in conventional gel formation was 
capable of producing a homogeneous gel by the process of this invention. 
EXAMPLE 20 (Comparative) 
Water, TEOS, and ethanol were combined in a 2:1:5 mole ratio. Ethanol was 
necessary to maintain miscibility. The resulting solution was divided into 
several portions, to which varying amounts of 96% formic acid were added. 
Those samples were placed in sealed containers and thermostatted at 
60.degree. C. The gelation times of those samples are reported in the 
table below: 
______________________________________ 
Mole ratio Gel time, 
HCOOH/TEOS hrs. 
______________________________________ 
0.010 31 
0.056 193 
0.101 214 
0.201 173 
______________________________________ 
The above data confirm the observation made by Coltrain, loc. cit., that 
with an acid as a catalyst in an aqueous medium, gelation times increase 
with acid concentration. The process of the present invention, which 
requires larger amounts of acid and can operate in an initially nonaqueous 
medium or in the presence of a small amount of water, produces gelation in 
very short times, usually minutes, rather than hours.