Alumina of high macroporosity and stability

A calcined and shaped alumina, exhibiting a stabilized, high pore volume and a low surace area together with high strength, is made from an alumina, such as pseudoboehmite or chi-rho-eta alumina or mixtures of different alumina species. The stabilized product exhibits a total porosity of at least about 0.4 cc/g, a surface area within the range of about 2-20 m.sup.2 /g. The porosity is stabilized by treatment of the alumina, prior to calcination, with a combination of silica and an inorganic fluoride. The stabilized alumina product can be readily utilized as an adsorbent or a catalyst support, particularly in applications where the catalyst is subjected to high temperatures.

BACKGROUND OF THE INVENTION 
This invention relates to a calcined, shaped alumina possessing a 
stabilized, high pore volume and a low surface area. More particularly, 
the present invention concerns a stabilized alumina shape of high total 
pore volume and low surface area prepared by treatment of an alumina with 
a stabilizing combination of silica and an inorganic fluoride, followed by 
calcination at high temperature. 
For many catalyst applications it is important to provide a catalyst 
substrate or support which exhibits a high pore volume. The high porosity 
of the catalyst support or substrate allows the incorporation of catalytic 
promoters into the substrate and thus provide an active catalyst. 
Alumina-based catalyst supports or substrates are commonly employed since 
many aluminas possess the desired pore volume and pore size distribution 
which allow their combination with catalytic promoters. In addition, the 
alumina support itself does, in many instances, exhibit catalytic 
properties due to the active surface area of the alumina support. Narrow 
pore size distribution is also a preferred property of aluminas, in case 
of being employed as an adsorbent, selective adsorption can be achieved, 
and materials, having a size outside of the pore size range of the 
adsorbent will not be able to penetrate the pores of the adsorbent. 
Low surface aluminas with controlled volume distribution in the 1 micron 
pore diameter range, such as prepared by the method of the invention, are 
especially useful for treatment of impure air. In air purification 
applications the alumina can be impregnated with a chemical composition, 
such as potassium permanganate, which removes contaminants by oxidation. 
Alternatively, aluminas, as produced by the instant invention, possessing 
low surface area and narrow pore size distribution, can be employed as 
supports for deodorizers or air freshening compounds, releasing air 
purifying compounds at a slow, controllable rate due to the narrow pore 
size distribution in the above-mentioned range. 
In the event the catalyst is to be utilized at high temperatures or the 
catalyst is exposed to high temperature excursions, the stability of the 
alumina substrate becomes of major importance. When exposed to high 
temperatures for example in excess of about 1000.degree. C., alumina 
substrates not only alter their crystalline phases, but they also undergo 
other physical changes, for example the surface area, the porosity and/or 
the pore volume of the alumina also change, generally in an undesired 
direction. Activity of the catalyst can be significantly affected when the 
surface area decreases due to high temperature exposure. To alter the pore 
size distribution in a beneficial manner, i.e. to eliminate pores of very 
small diameter, it is customary to subject the alumina substrate to 
temperatures in excess of about 1500.degree. C. This however, as mentioned 
before, causes changes in the pore volume, it generally shrinks, which 
further reduces the activity of the alumina-based catalyst. Consequently, 
in order to provide an alumina-based catalyst support which performs in a 
desired manner, the substrate has to be treated to stabilize certain of 
its required properties. 
In U.S. Pat. No. 2,630,617 alumina pebbles, used as heat exchange media at 
temperatures in excess of about 1650.degree. C., were stabilized against 
breakage and attrition by addition of small quantities of alkaline earth 
fluorides. The incorporation of the alkaline earth metal fluorides in the 
lightly calcined starting material, consisting of alpha corundum crystals, 
inhibited the further growth of the alpha corundum crystals when pebbles 
were made from the mixture and the pebbles were exposed to the 
above-mentioned high temperatures. No stabilization of the pore volume, 
the surface area or the pore size distribution were achieved. As a matter 
of fact, the addition of the alkaline earth metal fluoride to the alpha 
corundum acted, as shown in the patent, acts as a sintering agent or 
densifier resulting in the elimination of the pores and voids in the 
pebbles. 
U.S. Pat. No. 4,003,851 discloses an alumina catalyst support free of 
thermal shrinkage and of phase change due to stabilization by exposure for 
a period of 24 hours to temperatures at about 980.degree. C. The process 
as disclosed avoids the incorporation of stabilizers in the alumina 
support for fear that such stabilizers may interfere with the performance 
of the catalyst made from the stabilized support. However, the high 
temperature-long term treatment disclosed in the reference admittedly 
results in shrinkage and consequent loss of pore volume. Also, due to the 
shrinkage, an unfavorable pore size distribution can be expected. These 
detrimental properties, derived by the high temperature treatment 
disclosed, counterbalance any beneficial effect that a stabilizer may have 
on the catalytic activity of a substrate stabilized with a chemical agent. 
In U.S. Pat. No. 4,220,559 a high temperature-stable, alumina-based 
catalyst is described. The catalyst, used for catalyzing combustion 
reactions at 1000.degree.-1400.degree. C., is stabilized against phase 
transformation at high temperatures by incorporation of certain 
stabilizers, such as mixtures of the oxides of strontium or barium with 
silica, zirconia or stannous oxide. These stabilizers will assure that, at 
the high application temperatures, alpha alumina formation will be 
minimized and thus the active surface area of the catalyst will be 
retained for extended periods. The stabilizer mixtures employed in this 
reference reduce the surface area loss of the catalyst at the high 
temperatures employed in the combustion reactions. However, shrinkage of 
the pore volume and change in the pore size distribution are not readily 
achieved since the mixtures used may impart mineralizing effects which 
allow retention of the surface area, but not the pore volume. 
It has now been discovered that an alumina adsorbent, support or substrate 
can be readily stabilized against high temperature deterioration, such as 
surface area loss and pore volume shrinkage by stabilizing the substrate 
with a synergistically acting mixture of silica and an inorganic fluoride. 
DETAILED DESCRIPTION OF THE INVENTION 
This invention relates to an alumina stabilized against pore volume and 
pore size distribution changes at high temperatures. More particularly, it 
concerns a calcined alumina shape, stabilized by treating the alumina, 
prior to calcination, with a stabilizing quantity of a synergistically 
acting mixture of silica and an inorganic fluoride. 
In the present process an alumina species, such as pseudoboehmite, 
chi-rho-eta transitional alumina, gibbsite or the like, or mixtures of 
these aluminas, are utilized for the preparation of a shaped, calcined and 
alumina product. In a preferred embodiment a mixture of aluminas are 
utilized and one alumina constituent of the mixture is a pseudoboehmitic 
alumina, the structure of which can be determined by X-ray diffraction 
using copper K.alpha. radiation. For the pseudoboehmitic alumina used in 
this invention the diffraction peak of greatest intensity appears at 
6.5-6.8 angstroms and its pseudoboehmite content can be readily 
established by measuring the area under the 14.5.degree.-2.THETA. 
diffraction peak. Generally, it is advantageous to employ a 
pseudoboehmitic alumina which has a pseudoboehmite content of not less 
than about 25% by weight. This type of pseudoboehmite can be readily 
prepared in accordance with the manufacturing method disclosed in detail 
in U.S. Pat. No. 3,630,670 (Bell et al). Other methods, resulting in the 
production of pseudoboehmite of the structure defined hereinabove, can 
also be employed. 
In the preferred embodiment the other alumina constituent of the mixture, 
utilized in the preparation of the stabilized alumina of the present 
invention, is gibbsite or an alumina exhibiting, prior to calcination, a 
crystalline structure composed of chi, rho and eta phases. This type of 
alumina is generally prepared from alumina hydrate (Al.sub.2 
O.sub.3.3H.sub.2 O) by subjecting the alumina hydrate to flash 
calcination. Flash calcination is accomplished by contacting the alumina 
hydrate particles with a high temperature gas for a short period of time, 
generally for a period of less than about a minute, usually only for a few 
seconds. U.S. Pat. No. 3,226,191 (H. E. Osment et al) discloses a suitable 
method for the preparation of alumina having chi-rho-eta transitional 
phases using the flash calcination process. The transitional alumina, used 
in the present invention, can be prepared by any conventional method which 
provides the desired chi-rho-eta transitional structure for the alumina. 
For the preparation of the preferred alumina composition of the present 
invention generally about 50-95%, preferably 60-90% by weight 
pseudoboehmitic alumina is utilized, the balance being the chi-rho-eta 
transitional alumina. The differing aluminas, having particle size 
distributions which allow their ready admixture, are then combined. It is 
of importance that a uniform admixture is obtained in the mixing step, 
since the uniformity of the admixture affects the properties of the shapes 
made from the mixture. 
It is to be understood, that for the preparation of the stabilized alumina 
shape of the present invention it is not necessary to employ a mixture of 
the pseudoboemitic and chi-rho-eta aluminas. The calcined, stabilized 
shape may be readily made from pseudoboehmite or chi-rho-eta alumina 
alone, or as a matter of fact, from gibbsite or other alumina species 
taken alone or in combination. The embodiments described hereinbefore and 
in the following description only indicate the preferred alumina 
compositions utilized in the process. 
The mixture of the pseudoboehmitic alumina and chi-rho-eta alumina is then 
combined with the stabilizing composition of the instant invention. The 
stabilizing composition used to impart high temperature stability to the 
alumina composition consist of a mixture of a silica-containing compound 
and an inorganic fluoride selected from the group consisting essentially 
of sodium fluoride (NaF), ammonium fluoride (NH.sub.4 F), aluminum 
fluoride (AlF.sub.3), hydrogen fluoride de (HF), fluosilicic acid (H.sub.2 
SiF.sub.6) or mixtures of these fluorides. The silica-containing 
constituent of the stabilizing composition can be a sodium silicate or a 
variation thereof, for example the commonly utilized water glass Na.sub.2 
O.xSiO.sub.2 where x=3-5, or sodium metasilicate (Na.sub.2 SiO.sub.3). It 
is also possible to incorporate the desired silica content in the alumina 
compound by use of a silica sol or other well-dispersed silica source. 
The quantity of silica-containing material found to be an effective 
stabilizer for the calcined alumina shape is in the range from about 0.2 
to about 5.0% by weight of the stabilized composition. Thus, in order to 
achieve this stabilizing quantity in the calcined alumina shape, one has 
to calculate the silica content of the starting material and thus 
determine the amount to be incorporated in the mixture of the 
pseudoboehmitic and chi-rho-eta alumina. For example, if an aqueous 
solution of sodium silicate is utilized, then the water and the sodium 
contents of the silica-containing stabilizer must be first determined in 
order to obtain the final, desired SiO.sub.2 content in the calcined 
alumina shape. 
The fluoridic salts, which were found to impart stabilizing properties to 
the calcined alumina shapes in combination with the SiO.sub.2 -containing 
compounds, as mentioned before, can be selected from the group consisting 
essentially of NaF, NH.sub.4 F, HF, AlF.sub.3, H.sub.2 SiF.sub.6 and their 
mixtures. In the case of these salts, the stabilizing quantity of fluoride 
(F) to be present in the calcined alumina shape is in the range from about 
0.02 to about 1.00% by weight of the stabilized composition. If a volatile 
fluoride compound, such as NH.sub.4 F, is used, the volatility of the 
ammonia derivative during the calcination step must be taken into account. 
It was found that unless a mixture of both a silica-containing salt and a 
fluoridic compound is utilized for stabilizing the alumina composition, 
the stabilizing effect will be lower. This clearly indicates a synergistic 
effect between the fluoride and the SiO.sub.2. The reason for such 
synergism is not known and it has not been demonstrated previously for the 
preparation of stabilized alumina products. 
Incorporation of the stabilizing composition in the aluminas to be 
stabilized can be accomplished either during the mixing of the gibbsitic 
or pseudoboehmitic and chi-rhoeta aluminas or during the shaping step of 
the already mixed aluminas. It is also possible to add the stabilizing 
composition to the already shaped alumina mixture. In this case the 
stabilizing composition is generally used as an impregnant. In the event 
the incorporation of the stabilizing composition occurs during the mixing 
of the pseudoboehmitic and transitional aluminas, the stabilizing 
composition can be added either individually or in combination, in 
solution or in solid form. Again, uniform admixture is a requirement. If 
the stabilizing composition is incorporated during the shaping of the 
mixture of aluminas, then it is preferable to add the composition either 
as a solution or as a dispersion. The dispersion or solution can be used 
as a wetting agent for the shaping step. 
The mixture of the gibbsitic or pseudoboehmitic alumina and transitional 
alumina is shaped to obtain a catalyst substrate which can be readily 
employed for catalytic reactions. Shaping of the alumina mixture can be 
accomplished by well-known conventional methods, for example by forming 
spheres in a rotating pan, by extrusion or by any other method resulting 
in spheres, tablets, extrudates of differing shape. In the event spheres 
are to be made from the mixture of the aluminas, the sphere forming 
process described in U.S. Pat. No. 3,222,129 (Osment et al) can be 
utilized with good results. For certain catalyst applications a high 
porosity is required. This can be achieved by the incorporation of a 
combustible material in the mixture of aluminas either during mixing or 
after shaping through impregnation, for example impregnation with a 
polyvinyl alcohol or latex composition. In most of the shaping operations 
moisture is utilized to assist in the shaping, this moisture is generally 
removed by drying the formed shapes. This drying operation, which usually 
takes place at temperatures in excess of about 100.degree. C., can be 
applied to the shapes formed from the mixture of the aluminas. 
The shapes are then subjected to a calcination step to obtain the 
stabilized shapes of the present invention. Calcination is usually 
accomplished in conventional calcining equipment, such as kilns, rotary or 
stationary, muffle furnaces or shaft kilns and the like. The temperatures 
employed for the calcination of the alumina composition of the present 
invention is within the range from about 1200.degree. to about 
1500.degree. C., preferably within the range from about 1300.degree. to 
about 1400.degree. C. for a time at least about an hour, preferably 
between 2-4 hours. 
The resulting calcined shapes exhibit a high stability when exposed to 
temperatures in excess of about 1000.degree. C. for extended periods 
without materially affecting the total pore volume or the pore volume 
distribution of the stabilized substrate. The calcined shapes also exhibit 
a narrow pore size distribution which is preferred for many applications. 
Thus, it has been found that the novel, stabilized calcined alumina shapes 
possess such a narrow pore size distribution where at least about 75% of 
the total pores have a diameter within the range from about 0.2 to about 
1.2 microns.

The following examples further illustrate the novel aspects of the present 
invention. 
EXAMPLE I 
A shaped, high temperature-stable calcined alumina substrate was made by 
first forming a mixture from pseudo-boehmitic and transitional aluminas. 
The mixture contained 33% by weight pseudoboehmitic alumina having a 
pseudo-boehmite content of more than about 90% by weight as determined by 
subjecting the alumina to X-ray diffraction determination under K.alpha. 
radiation. The balance of the mixture was a transitional alumina having a 
chi-rho-eta structure. The transitional alumina was prepared from alumina 
hydrate (Al.sub.2 O.sub.3.3H.sub.2 O) by subjecting the alumina hydrate to 
flash calcination in accordance with the method disclosed in detail in U.S. 
Pat. No. 3,226,191. From the mixture of the pseudoboehmitic alumina and 
transitional alumina spheres were formed in accordance with the process 
disclosed in detail in U.S. Pat. No. 3,222,129. The spheres were activated 
at about 400.degree. C. for a period of about 2 hours to provide an 
activated alumina shape having a total pore volume of 0.83 cc/g. The 
activated shapes were then step-wise impregnated with aqueous solutions of 
Na.sub.2 O.3.3SiO.sub.2 and NaF. The quantity of stabilizing agents 
incorporated in the shapes was calculated to achieve final concentrations 
of 1.0% by weight SiO.sub.2 and 0.1% by weight F (based on the total 
weight of the calcined and stabilized shape) in the calcined shapes. The 
impregnated shapes were calcined in a gas-fired furnace at about 
1350.degree. C. and then subjected to analysis. A comparison was also made 
by calcining alumina shapes made from the same alumina composition without 
addition of the stabilizing compound. The results are shown in Table 1. 
TABLE 1 
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Alumina Samples 
Physical Properties Stabilized 
Control 
______________________________________ 
Total pore volume in cc/g 
0.653 0.490 
Median pore diam. in angstroms 
0.92 0.41 
Crush strength in kg. 
0.72 0.36 
Surface (BET) in m.sup.2 /g 
3.4 8.5 
______________________________________ 
Both the stabilized alumina and the control were then subjected to a high 
temperature test at a temperature of 1450.degree. C. for a time period of 
24 hours. Subsequently, the physical properties of these aluminas were 
compared. The results of the comparison are shown in Table 2. 
TABLE 2 
______________________________________ 
Alumina Samples 
Physical Properties Stabilized 
Control 
______________________________________ 
Total pore volume in cc/g 
0.635 0.350 
Median pore diam. in angstroms 
0.95 0.68 
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EXAMPLE II 
A stabilized alumina was made by forming a mixture from pseudoboehmitic 
alumina (pseudoboehmite content in excess of 95% by weight) and 
flash-calcined chi-rho-eta alumina. The mixture contained 80% by weight 
pseudoboehmitic alumina, balance chi-rho-eta alumina. The stabilizing 
composition, containing Na.sub.2 O.3.3SiO.sub.2 and NH.sub.4 F, was 
incorporated in the alumina admixture during the shaping step in such an 
amount as to provide 0.075% F and 0.6% by weight SiO.sub.2 in the calcined 
shapes. The shapes containing the stabilizing composition were then dried 
and calcined at the following temperatures: 1300.degree., 1350.degree., 
1400.degree. and 1450.degree. C. The shapes were kept at each temperature 
for a period of 2 hours and the total pore volume, the median pore 
diameter and the strength were determined for each temperature. The 
results were tabulated and are shown in Table 3. 
TABLE 3 
______________________________________ 
Alumina Samples 
Temperature .degree.C. 
Physical Properties 
1300.degree. 
1350.degree. 
1400.degree. 
1450.degree. 
______________________________________ 
Total pore vol. cc/g 
0.631 0.617 0.579 0.579 
Median pore diam. angstroms 
0.92 0.89 0.86 0.96 
Strength in kg. 0.494 0.531 0.735 0.844 
______________________________________ 
EXAMPLE III 
A shaped, spherical, alumina substrate was made from pseudoboehmitic 
alumina containing in excess of about 95% by weight of pseudoboehmite. The 
spheres were thermally-treated at about 400.degree. C. for a period of 
about 2 hours to provide an activated shape having a total pore volume of 
0.95 cc/g. The activated shapes were then step-wise impregnated with 
aqueous solutions of Na.sub.2 O.3.3SiO.sub.2 and NH.sub.4 F. The quantity 
of stabilizing agents incorporated in the shapes was calculated to achieve 
final concentrations of 1.5% SiO.sub.2 and 0.1% F in the calcined 
stabilized shapes, based on the total weight of the calcined, stabilized 
shapes. The impregnated shapes were then calcined in a gas-fired furnace 
at about 1350.degree. C. and then subjected to analysis. A comparison was 
also made by calcining alumina shapes made from the same pseudoboehmitic 
alumina without the addition of the stabilizing compositions. The results 
were tabulated and are shown in Table 4. 
From the results obtained it can be clearly observed that the control 
sample exhibits a significantly lower pore volume than the stabilized 
sample and in addition the median pore diameter of the control falls in a 
generally undesirable range. 
TABLE 4 
______________________________________ 
Alumina Samples 
Physical Properties Stabilized 
Control 
______________________________________ 
Total pore volume cc/g 
0.500 0.321 
Median pore diam. in angstroms 
0.96 0.15 
Crush strength in kg/cm.sup.2 
53.7 89.5 
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EXMPLE IV 
Stabilized, calcined alumina shapes were made from gibbsite. The gibbsite 
was first activated at about 400.degree. C. and then cellulosic fibers, in 
an amount equivalent to about 12% by weight of the total weight of the 
mixture, were added to the activated gibbsite. The mixture was formed into 
spheres and the spheres were cured in a moist atmosphere, then dried and 
reactivated in air at a temperature of about 500.degree. C. The 
temperature employed was sufficient to result in the total combustion of 
the cellulosic fiber content of the shapes. The alumina shapes had a total 
pore volume of 0.75 cc/g. The shapes were impregnated with aqueous 
solutions of Na.sub.2 O0.3.3SiO.sub.2 and NH.sub.4 F to yield a calcined 
product containing 1.0% SiO.sub.2 and 0.1% F, based on the total weight of 
the stabilized spheres. The alumina shapes were then calcined at about 
1350.degree. C. and analysed. The results are shown in Table 5. From the 
results it can be observed that the stabilized shapes have high pore 
volume and a desirable median pore size distribution. 
TABLE 5 
______________________________________ 
Physical Properties Alumina Sample 
______________________________________ 
Total pore volume in cc/g 
0.65 
Median pore diam. in angstroms 
0.92 
Crush strength in kg. 
0.63 
______________________________________ 
The stabilized aluminas prepared by the instant invention have remarkably 
constant pore volume distribution up to an operational range of 
1350.degree. C. Thereafter, the pore volume declines in a very small 
degree, however, the median pore diameter remains essentially the same. 
For this reason the novel, stabilized alumina can be readily employed as a 
catalyst substrate for high temperature operations without experiencing 
significant loss in catalytic activity due to the change in total pore 
volume and median pore diameter. These reasons also apply to an adsorbent 
made from the novel, stabilized alumina composition.