The present invention relates to a process of producing nano-porous hydrated
alumina in powder form or in the form of aqueous suspension by synthesizing
hydrated alumina material.
More specifically, the present invention deals with the synthesis of nano-porous
hydrated alumina, either in powder form or in the form of its aqueous
suspension/sol with defined pore size and specific surface area characteristics.
BACKGROUND OF THE INVENTION
Aluminium usually forms two types of hydroxides - tri-hydroxtde and mono-
hydroxide. Some of those hydroxides are well-characterised crystalline
compounds, whilst others are ill-defined amorphous compounds. The most
common tri-hydroxides are gibbsite, bayerite and nordstrandite, whilst the more
common mono-oxide hydroxide forms ere boehmite and diaspora. The above
hydrated aluminas often loosely called aluminum trihydrate, popularly known as
ATH. Besides the above hydroxides, the material also has several other
synonyms like, hydrated alumina, alumina hydrate. ATH is a versatile material
that has been catering a wide range of across many interdisciplinary areas,
starting metallurgical to ceramic processing industries. It is used as filler in FRLS
(fire retardant low smoke) in electrical/electronic cable industries, as catalyst
supports and absorbents in chemical industries, as functional additives in
polymers, paints, composites, automotive and in pharmaceutical industries. ATH
is commercially available as a coarse-grained material however, with the
development of nanoscience and technology, lice any other material, ATH is also
finding new applications using its ultra fine or nano-sized material. Ultra fine-
grained or nano-grained ATH has advantages over the coarse grained ATH in
many applications. Nano ATH could significantly reduce the filler load in the
composition, thus making the body more light in all areas of composites, which
significantly improve the thermal stability, mechanical strength and electrical
properties and hence much better ability to promote flame retardancy in case of
FRLS applications. Nano sized ATH also known to improve arc-track resistance In
plastics for electrical applications. As filler in fine printing papers, it increases
opacity and brightness, and in paper coatings, it imparts brightness, gloss and
high ink receptivity. Finer grained or nano ATH could be better reinforcing
pigment in adhesives, where it improves cold flow properties and cohesion
besides the stabilization in pH. Finer ATH also improves the surface properties
those exploited in polishing applications, cleansing agent, mould wash and
separating agent. Finer or nano ATH with high purity finds new applications in
pharmaceuticals, high purity chemical manufacturing and in paint industries.
Aluminium hydroxides produced from the Bayer process are generally yellowish
in color and relatively coarse in size. There are different ways of synthesizing
finer ATH, for example, either by grinding the Bayer precipitates followed by the
size classification or by direct precipitation of a finer-sized ATH from Bayer liquor.
Various authors Csige et al, Sleppy et al, have worked on the development of
finer grade hydrates by grinding coarse ATH powders, but achieving finer-sized
(<2 micron) particles by such a method is highly energy intensive and hence did
not draw much attention.
Therefore, attempts were made for direct precipitation for finer-sized particles.
Baksa et al. produced several fine hydrates in the air jet mill by precipitation in
the presence of a modifier namely, aluminium sulfate. According to the authors,
a lower precipitation temperature and a lower pregnant liquor concentration are
advantageous, and the addition of at least 1.0% Al2(SO4)3 is required as AI2O3.
They also studied the production of 0.5 micron of ATH particles by decomposing
gallium-aluminium alloy with NaOH and or H2O.
Satapathy and Pattanaik produced aluminium trihydrate of 4 micron in size by a
two-stage precipitation process using a liquor having alumina/caustic ratio of 1
with the corresponding soda content of 0.28% and seed of 10-15 micron in size
thereby.
Martinswerk, Germany had also produced SGA (special grade alumina) since
1970 and is the supplier of one micron precipitated ATH all over the world.
Interestingly, Shibue et al. found that the size of primary particles is not affected
significantly by the mechanical stirring and the size of the particle decreases with
an increase in the tank size. However, the authors found that the particle size
distribution is sharper in the batch than in continuous precipitation.
T.Tran et al. studied the effect of 3,4 Dihydroxy Benzoic Acid (3,4 DHBA), an
organic acid on the precipitation of ATH particles. In this study, a second order
rate equation was established in order to understand the rate of nucleation and
crystal growth during the precipitation of ATH. In another study, the same group
M J. Kim et al. studied the precipitation of radial ATH particles with an aim to
investigate the various factors affecting the synthesis. Precise control of
supersaturation condition and temperature as a function of time is reported to be
important in producing such ATH particles with desirable shape and size.
J.K. Pradhan et al studied the various factors affecting the quality of ATH
precipitates. The effect of a number of parameters such as precipitation
temperature, amount of seed, seed surface area, precipitation time, soda content
of pregnant liquor, modifier additives, etc. on the precipitation of ATH were
studied to achieve the required fineness.
Packed bed reactors have also been used for synthesis of ATH J. Chen et al.
developed a route for synthesizing so-called nano-fibrillar ATH by carbonation in
a rotating packed bed at room temperature. Pseudoboehmite fibers of 1-10 nm
in diameter and 50-300 nm in length are obtained. Factors affecting carbonation
process such as high gravity level, gas/liquid ratio and solution concentration are
discussed.
Z. Peng-yuan et al worked on the preparation of ultrafine ATH via a new method
combined with high gravity reaction and thermal hydrolysis. Z. peng-yuan et al
prepared nano-ATH by the RPB method, in which industrial ATH was used as raw
materials for the preparation of sodium aluminate which was precipitated by
carbonation and ATH of below 50 nm was prepared.
Sonochemistry is involved with the application of ultrasound to chemical
reaction/s and take advantage the process related to crystallization or
precipitation reactions. It is known that the decomposition fraction of
precipitation process of diluted sodium aluminate solution can be enhanced by
ultrasonic treatment. Liu Ji-bo et al. studied the effect of ultrasound frequency
on the precipitation process of supersaturated sodium aluminate solution under
the conditions similar to those in industry. The result indicate that the ultrasonic
treatment at 16 kHz can enhance the decomposition rate of sodium aluminate
solutions and also has effects on particle morphology and particle size
distribution of ATH precipitates.

S.F. McGrath et al. studied the sonochemical processing of bauxite using
Resonant sonics. The device Resonant sonics addresses all of these issues such
as ability to significantly increase the rate of alumina precipitation, the ability to
increase thermal efficiency and relatively high yield, either directly or indirectly.
The principle of Biomimetic Mineralisation (BMS) has also been used in synthesis
of ATH. Yaacob I.I. et. Al. worked on this Using unilamellar vesicles that have
been used as reactors for the aqueous phase precipitation of nanometer sized
ATH particles within their inner cores. ATH particles thus obtained were of
nanometer-sized with nearly spherical morphology.
Tadafumi Adschiri et al. studied Hydrothermal synthesis of AIO(OH) using a flow
type apparatus over the range of temperature from 523 to 673 K at 30 MPa.
Nano-sized crystals were formed at supercritical condition. The mechanism of
nano particle formation at supercritical conditions was discussed based on the
metal oxide solubility and kinetics of the hydrothermal synthesis reaction.

US Patent No. 4,511,542 has disclosed Bayer process sodium aluminate liquor
being seeded with a relatively small seed charge in a first precipitator at a
relatively low temperature and the produced slurry being transferred to a second
precipitator, wherein solid content of the slurry is allowed to be increased to
about 250-700 g/l and a coarse, strong product hydrate is recovered exceeding
80 g/l based on the alumina (AI2O3) content of the supersaturated sodium
aluminate liquor.
US Patent No. 3,954,958 has disclosed production of powdery alumina hydrate
from an alkyl aluminium compound by hydrolysis of the alkyl aluminium
compound by water.
US Patent No. 6,890,647 has disclosed about highly transparent alumina hydrate
particles having an average particle diameters of 0.02 to 0.2pm, a total pare
volume of 0.5 to 1.5 ml/g and a volume of pores whose diameter is from 15 to
30nm ranging from 0.3 to 1.00 ml/g. The said particles are formed into a high
concentration dispersion sol, exhibiting a low viscosity.

US Patent No. 5,378,753 has disclosed an alumina hydrate particles having high
surface area and low dispersity and a low soluble soda content. They are made
by milling a liquid suspension of alumina hydrate, subjecting the milled
suspension to classification into coarse and fine fractions, recycling the coarse
fraction to the mill and recycling the fine fraction to the classification stage and a
narrow particle size distribution is obtained.
US Patent No. 6,048,470 has disclosed a process for producing alumina sol by
stirring a dispersion of an alumina hydrate having a solid content from 1 to 40 wt
% at a PH of from 7 to 12 and then adding an acid thereto for peptization
treatment.
Chemical methods / techniques for synthesizing nano-porous hydrated alumina
with defined pore size has not been disclosed so far. It was in this context, this
invention was made. The invention provides the precursors to be used, reaction
conditions, experimental parameters for the synthesis of nano-porous hydrated
alumina powder both in its powder form or in its aqueous suspension form with
defined characteristics of pore size, pore volume, and specific surface area.

DESCRIPTION OF THE INVENTION
The main objective of the present invention is to provide a wet precipitation
reaction technology for the synthesis of nano-porous hydrated alumina both in
powder form or in the form of aqueous suspension/sol that possess defined pore
size in the range of 3-7 nanometer in the material, using aqueous solution of
aluminum nitrate as the precursor and ammonia or sodium hydroxide/potassium
hydroxide as the precipitating agent/s under defined experimental conditions and
parameters like PH in the range of 9-10, concentration, heat treatment
procedure etc.
Another objective of the present invention is to provide a precipitation technique
for synthesizing nano-porous hydrated alumina both in powders form and
suspension/sol form, other than sol-gel route, chemical gelation, solution or
colloidal route, calcinations or thermal decomposition methods.

A further objective of the present invention is to synthesize nano-porous
hydrated alumina powder and Hs suspension/sol in a cost-effective manner using
an aqueous based liquid precursor, which is actually an aqueous solution of
aluminum nitrate in a particular range of concentration.
A still further objective of the invention is to provide a synthesis technology for
generating reproducible nano-porous hydrated alumina powder and
suspension/sol with defined pore size in the range of 3-7 nanometer.
A still yet further objection of the invention is to provide a synthesis technology
for generating hydrated alumina powders with specific surface area in the range
of 75-400 m2/g.
An yet another objective of the present invention is to define various process
parameters such as maintaining a PH of the final solution of precursor and
precipitating agent in the range of 9-10 for producing the above hydrated
alumina powder and its suspension/sol, which are very significant for the
synthesis.

A still yet another objective of the present invention is to provide nano porous
hydrated alumina powder and its suspension/sol that has great applications in
seramic membrane coating, in-situ ceramic processing, polymer composites,
additives, fillers etc.
Other objects, novel features, advantages and applications of the present
invention will be set forth in the description that follows further. The objectives
and the advantages of the present invention may be realized and attained to a
particular material by means of permutations and combinations of the derived
nano-porous hydrated alumina powders or suspension/sols.
According to the invention there is provided a process of producing nano-porous
hydrated alumina in powder form or in the form of aqueous suspension by
synthesizing hydrated alumina material comprising the steps of
a) preparing an aqueous solution by dissolving solid aluminium nitrate
hydrate [AI(N03)3.9H2O] in distilled/de-tonwed water in a container
and obtaining a clear solution of concentration in the range of 0.1 -
0.3 moles/litre on purifying the aqueous solution on filtration, called
as precursor and pouring the solution in a precipitating tank fitted
with an agitator/stirror,
b) preparing a precipitating agent by dissolving either ammonia or
solid sodium hydroxide or potassium hydroxide in de-
ionised/distilled water to obtain a final concentration in the range of
0.1 to 1.5 moles/litre and adding slowly the precipitating agent
prepared to the tank containing the precursor, wherein in the tank
the precursor and the precipitating agent are uniformed mixed by
the stirrer.
c) carrying out precipitation reactions between the precursor and
precipitating agent till the PH of the solution in the tank reaches in
the range of 9-10, to yield a white gelatinous precipitate, and when
addition of precipitating agent is ceased.
d) the stirring of the solution/mixture of suspension is continued for a
while and the entire solution/mixture is filtrated to filter the
precipitate to separate unreacted precursor/precipitating agent, the
filtered precipate is then washed continuously with distilled/de-
ionized water to remove absorbed precursor or precipitating agent
till the PH of the filtrate reaches 7.

e) the washed precipitates resulted from the step (d) being further
treated in two alternative course of treatments by either preparing
a suspension/sol of any of the washed precipitates by mixing it with
de-ionized/distitled water in desired ratio, the resultant aqueous/sol
being used for different coating and processing applications for
generating nano-porous structure with a pore size in the range of
3-7 nanometers in the coated layer of a host matrix by heat
treating the matrix in the temperature range of 110-350°C or the
washed precipitates being dried at a temperature range of 110°C
±10°C, until consistence in weight of the dried precipitate is
attained at a particular temperature, pulverizing/milling the dried
precipitates in nylon pots or alumina lined pots using alumina balls
as grinding media for a period of 12-24 hours to result
pulverized/milled powders of the dried precipitate, sieving the
resulted powders through 100-200 micron sieve, collecting the
sieved powders and characterize evaluating the sieved nano-porous
powders.

DETAILED DESCRIPTION OF THE PRESENT INVENTION:
A. Preparation of mother liquor and the precipitating agent/s:
The present invention describes the synthesis route for synthesizing nano-porous
hydrated alumina both in powder form and suspension/sol form, by adopting a
precipitation reaction technique. For the said purpose, hydrated aluminmum
nitrate (solid) is to be dissolved in de-ionized/distilled water maintaining the
solution concentration in the range of 0.1- 0.3 moles/litre, which is termed as
'precursor'. The precipitating agent may vary and could be from simple liquor
ammonia to sodium hydroxide to potassium hydroxide. In case of liquor
ammonia, concentrated ammonia solution of 25% (weight/volume) needs to be
diluted with deionized water in the volume ratto of 1:10 and the diluted solution
needs to be used as the precipitating agent. In case of sodium hydroxide,
analytical grade (>99% chemical purity) sodium hydroxide needs to be dissolved
in de -ionized/distilled water so as to make the final concentration of 0.5
moles/litre. Similarly, analytical grade of potassium hydroxide could also be used
as precipitating agent maintaining the same concentration to that of the sodium
hydroxide using distilled/de-ionized water.

Precipitation reaction is to be conducted using any of the above precipitating
agents with the aforesaid precursor' under defined experimental parameters like,
pH, temperature, concentration of precursor, concentration of precipitating agent
etc.
D. Conducting precipitation reaction:
Precipitation reaction is conducted in a precipitation tank that is fitted with i) a
magnetic stirrer/agitator for mixing the precipitating agent with the precursor, ii)
pH electrode for measuring pH of the solution and iii) thermometer for measuring
the temperature of the solution. A fixed volume of the precursor with defined
concentration is taken in a precipitation tank and then the precipitating agent
with defined concentration is taken in a precipitation tank and then the
precipitating agent with defined concentration is added thereby slowly for
carrying out the precipitation reaction. Both the stirring speed in the magnetic
stirrer and the rate of addition of precipitating agent are maintained at a fixed
rate so that an uniform rate of mixing of the precursor and precipitating agent is

ensured. The rate of addition of the precipitating agent and speed of the stirrer
are to be adjusted such a way that the precipitating agent is almost
instantaneously spreads over the precursor solution and a large concentration
gradient thereby is avoided. Addition of the precipitating agent is continued until
the desired pH level of the reaction medium is reached at equilibrium condition.
After thorough mixing between the precipitating agent and the precursor at a
particular pH with a fixed speed of the stirrer, a white gelatinous precipitate
yields.
The concentrations of both the precursor and precipitating agent may vary
widely in a large range. However, as a thum rule, it has been observed that, as
the concentrations of the precursor increases, particle size of the precipitate
become coarse. The same is true for the precipitating agent as well pH of the
reaction medium has a remarkable effect on the volume of precipitates formed in
a given set of concentration both in the precursor and the precipitating agent. As
a general rule, the precipitates start appearing with a pH of ~ 4 by the addition
of the precipitating agent to the precursor and the volume of the precipitates
slowly increases as pH of the suspension/mixture increases until a value ~ 10.
However, addition of the precipitating agent needs to be continued until the
desired pH level is obtained at an equilibrium condition.

C. Filtering and washing of the precipitates:
After the precipitation reaction is conducted under defined experimental
conditions, the entire suspension/mixture is subjected to filtration by which the
precipitates are to be separated out from the unreacted precursor and
precipitating agent used therein. The precipitates are essentially to be made free
from the contaminants (both ionic and molecular species or either) that would
have originated either from the precursor or the precipitating agent. The
precipitates hence need to be thoroughly washed with distilled/de-ionized water
until they are free from other unreacted products in the reaction medium, which
might be absorbed by the precipitates and contaminate the precipitete. For
washing and filtration, a simple vacuum filtration system or a rotating filter or a
thickener filter press or any of those available from commercial sources, can be
used depending on the batch size of precipitates and other conveniences.
D. Drying of the precipitates:
The thoroughly washed precipitates are obtained in the form of cakes, which
need to be dried for producing in the form of powders. For the purpose of drying

the precipitates, either an electrically-heated oven or microwave oven could be
used, provided the precipitate temperature does not exceed 110°C ± 10°C. The
time duration and rate of drying may vary, depending on me batch size of the
precipitates and also the type of drying equipment, however, importantly, the
drying process needs to be continued until the constancy in weight of the
material is attained at the particular drying temperature. After drying, there
would be substantial decrease in the volume of the precipitates (~ 90%), and
hard solid masses result.
E. Pulverization of the dried precipitates:
In order to reduce the particle of the dried precipitates, a pulverization operation
is introduced. The dried precipitates are to be pulverized preferably by using
nylon (or similar inert material) bowl or even alumina-lined bowls and alumina
balls as the grinding media for a definite period of time depending on efficiency
of the milling apparatus used. However, in a normal condition, using a nylon
bowl with alumina balls, a period of 12-24 hours of millrig is sufficient to get
powders with desired fineness of the dried calces. Excessive pulverization often
results agglomeration of the particles, which adheres so firmly with the grinding

balls that it becomes difficult to remove powders from the balls. After
pulverization operation, the resultant powders are sieved using 100-200 micron
sieves that results free flowing powder. The hard agglomerated mass those have
not passed through the said sieve is rejected. The capacity of the bowl depends
on the quantity of the powders to be pulverized.
F. Testing and characterization of the dried precipitates:
The pulverized powders are characterized in terms of specific surface area in the
BET, pore size profile and pore volume analysis using liquid nitrogen as an
absorbent and nitrogen adsorption/desorption isotherms. The powders are
further characterized in terms of scanning electron microscopy (morphology), X-
Ray Powder diffraction (crystallinity and phase analysis), thermogravimetry and
differential thermal analysis (TGA-DTA) analysis (confirmation of water of
crystallization in the material obtained in the powder form).

G. Preparation of Suspension/Sol:
Aqueous suspensions/sols of the washed precipitates with desired concentration
for specific purpose could be prepared by mixing the washed precipitates and de-
ionized/distilled in appropriate ratio. The resultant suspensions/sols could be
used for numerous purposes, e.g., coating applications including generation of
thin membrane layer on supported substrate/s following dip/spin coating
technique/s, in-situ processing for making composites etc. the derived
suspensions/sob give another opportunity to exploit the material in different
applications besides the same material in the powder-form. Instead of water,
other solvents, while preparing the suspension/sol could also be used depending
on the chemical compatibility between the solvent and the precipitate.
The present invention will be better understood from the following description
with examples and with reference to the accompanying drawings in which
Figure 1 represents a schematic flow sheet diagram for the precipitation reaction.

Figure 2 represents specific surface area profile as a function of temperature of
the nano-porous hydrated alumina powders.
Figure 3 represents (A) Pore size profile of nano-porous hydrated alumina
obtained through aluminium nitrate precursor (0.1 Molar) with ammonia (2.27%
(w/l) as the precipitating agent at pH 9.3 ± 0.5, (B) Corresponding
Absorption/Desorption isotherm.
Figure 1 represents the steps involved in the method of production of nano-
porous hydrated alumina on characterization evaluation. The flow chart describes
and define the steps of the invention in a self explanatory mode and need not be
elaborately described.
In figure 2 the change of specific surface area of nano-porous hydrated alumina
powders prepared according to the invention is shown with temperature. It is
observed that specific surface area of the hydrated alumina powders increases
gradually on heating upto 350°C treated in Nitrogen gas atmosphere but no
further increase of the surface is noted on further heating beyond 350°C.

Figure 3A shows the resulted pores of the nano-porous hydrated alumina
powders revealed from the profile developed with nanometer diameter of the
powders against pore volume of the powders. The measurements reveals well
defined pores with an average diameter of 3 to 6 Nanometer with a
corresponding pore volume of 0.2 to 0.25 cm3/g.
Example 1:
Preparation of nano-porous hydrated alumina using ammonia as the precipitation
agent:
0.1 Mole (37.514 g) of aluminium nitrate hydrate [AI(N03)3.9H20] is dissolved in
1 litre of distilled/de-ionized water and the resulting solution is filtered to remove
impurities or other contaminants, wherein a clear solution is resulted which is
termed as precursor solution of 0.1 M. The precursor solution of 0.1 M is poured
into the precipitation tank, which is fitted with an agitator/stirrer (Figure 1).
While the precipitating agent was prepared by taking liquor ammonia of 25%
(weight/volume) which is then diluted to 1:10 by volume using de-
ionised/distilted water, which is then slowly added to the precipitation tank that is

already under constant agitation. As pH of the solution changes from 3 to 9.3
(within a time period of 30 min.), a white gelatinous precipitate results. The
addition of precipitating agent is ceased till the pH of the medium reaches to a
value 9.3 ± 0.5. The stirring was still continued for another 10 minutes and the
entire mixture is now subjected to filtration. A vacuum filtration system is used
for filtration operation. Once the precipitate is filtered from the suspension by
separating unreacted precursor/precipitating agent, the filtered precipitate is
further washed with distilled/de-ionized water to remove absorbed precursor or
precipitating agent. The washing is continued till the pH of filtrate reaches ~ 7.
Washed precipitates are dried in a microwave oven maintaHiing a temperature of
~ 110°C, till there is no change in weight of the precipitates at this drying
temperature. A sharp decrease in the volume (~90%) of the precipitate occur
after drying and a hard solid mass is obtained. The resultant hard mass is
pulverized/milled into finer particles using nylon pot/bowl and alumina balls for a
period of 12 hours. The pulverized/milted powders are sieved through 100-200
micron sieved and the sieved powder was collected. AD the measurements
towards characterizations and testing were carried out using sieved powders
only. Specific surface area of the pulverized precipitates is found to be in the
range of 250-300 m2/g, with a degassing temperature of 120°C (a part of the

measurement process). Prior measuring specific surface area, the samples are
heated at a specific temperature for a definite period of time in nitrogen gas
atmosphere, in order to record the true dry weight of the sample, which is an
essential parameter for the measurement. This specific temperature for heat
treating the sample in nitrogen gas atmosphere is called degassing temperature.
It is further observed that as the heat-treatment temperature increases during
the de-gassing process, there is an increase in specific surface area of the
powders and it reaches up to a value of 408 m2/g at 350°C, when the sample is
soaked for a period of 4 hours at this temperature (fig.2). Nitrogen gas
adsorption and desorption pattern by the BET measurements reveals the
presence of well defined pores with an average diameter of ~ 3 nanometer with
a corresponding pore volume of ~ 0.25 cm3/g (Fig.3). X Ray Powder diffraction
of the dried precipitate show peaks similar to that of bayerite structure of
alumina hydrates. Peak width reveals the presence of fine primary particles in
the material. Scanning electron microscopy analysis shows irregular morphology
of the material.

Example 2
Preparation of nano-porous hydrated alumina using sodium hydroxide as the
precipitating agent:
Another similar precipitation reaction is conducted as mentioned in the example
1. In this case, sodium hydroxide is used as the precipitating agent which is
prepared by taking 20 grams of analytical grade (chemical purity >99%) sodium
hydroxide flakes and dissolving the same in 1 litre of distilled/de-ionized water.
On the other hand, 0.1 Mole (37.514 g) of aluminium nitrate hydrate [AI(NO3)3
9H2O] is dissolved in 1 litre of distilled/de-ionized water and the resulting
solution is filtered to remove impurities or other contaminants, wherein a clear
solution is resulted which is termed as precursor solution of 0.1 M. The precursor
solution of 0.1 M is poured into the precipitation tank, which is fitted with an
agitator (Figure 1). The precipitating agent was slowly added to the precursor
solution in similar fashion as described in the example 1, until the pH of the
solution in equilibrium condition is reached to a value in the range of 9.8 ± 0.5.

The precipitates are washed similarly as explained in the example 1 and further
dried in a microwave oven maintaining a temperature of 110°C. These dried
precipitates are further pulverized into finer sizes using nylon pot/bowl and
alumina bails for a period of 12 hours. The pulverized powders are sieved
through 100 - 200 micron sieve and the sieved powder was collected. All the
measurements towards characterizations and testing were carried out using
sieved powders only. Specific surface area of the dried precipitates is in the
range of 250-300 m2/g with degassing temperature of 120°C. The gas adsorption
and desorption analysis in the BET reveals the presence of well defined nano-
pores with a pore diameter in the range of 3 - 6 nanometer with a
corresponding pore volume of ~ 0.25 cc/g. X Ray Powder diffraction of the dried
precipitate shows similar peaks to that of bayerite the width of the peaks reveals
the presence of fine crystallites in the material. The precipitates under scanning
electron microscopy shows defined morphology.
Example 3
Preparation of nano-porous hydrated alumina using 0.3 molar precursor using
ammonia as precipitating agent 0.3 Mole (112.542 g) of aluminium nitrate
hydrate [Al(NO3)3. 9H2O] is dissolved in 1 litre of distilled /de-ionized water and

the resulting solution is filtered to remove impurities or other contaminants,
wherein a clear solution is resulted which is termed as precursor solution with a
concentration of 0.3 molar. Precipitation reaction is conducted as mentioned in
example 1, with the same concentration of precipitating agent (ammonia) and
maintaining the same PH of the solution. The precipitates obtained in this case
were relatively coarse in size as compared to those obtained in example 1. The
precipitates are washed similarly as explained in the example 1 and further dried
in a microwave oven maintaining a temperature of 110°C. These dried
precipitates are further pulverized into finer sizes using nylon pot/bowl and
alumina balls for a period of 12 hours. The pulverized powders are sieved
through 100-200 micron sieve and the sieved powder was collected. All the
measurements towards characterizations and testing were carried out using
sieved powders only. Specific surface area of the dried and milled precipitates is
found to be in the range of 78-85 m2/g with a degassing temperature of 120°C.
The gas adsorption and desorption analysis in the BET of the sample reveals the
presence of well defined nano-pores of 3- 6 nanometer with a corresponding

pore volume ~ 0.2 cc/g. X ray powder diffraction of the pulverized powder shows
similar peaks to that of bayerite in the hydrated alumina. However, the narrow
width of the peaks in this sample reveals the presence of not so fine crystallites
as compared to that obtained in example 1. The precipitates under scanning
electron microscopy shows defined morphology of the sample.
Example 4
Preparation of nano-porous hydrated alumina using 0.3 molar precursor
concentration with sodium hydroxide as precipitating agent.
With the same precursor concentration with the same procedure as said in
example 3, precipitation reaction is conducted, taking sodium hydroxide as the
precipitating agent. The precipitating agent is prepared by taking 20 g of
analytical grade (chemical purity > 99%) sodium hydroxide flakes and dissolving
it into 1 litre of distilled/de-ionized water. Similar precipitation reaction is carried
as described in the previous examples, until the pH of the solution reaches at a
value of 9.8 ± 0.5 in equilibrium condition. The precipitates are washed similarly
as explained in the example 1 and further dried in a microwave oven maintaining

a temperature of 110°C. These dried precipitates are further pulverized into fmer
sizes using nylon pot and alumina balls for a period of 12 hours. The pulverized
powders are sieved through 100-200 micron sieve and the sieved powder was
collected. All the measurements towards characterizations and testing were
carried out using sieved powders only. The precipitates obtained were coarse in
size as compared to those obtained in example 1. BET surface area of the dried
precipitates is in the range of 78-85 m2/g with a degassing temperature of
120°C. The gas adsorption and desorption analysis in the BET reveals the
presence of well defined nano-pores with a pore diameter in the range of 3 - 6
nm with a corresponding pore volume ~ 0.2 cc/g in the sample. X ray powder
diffraction of the powder shows similar peaks to that of bayerite of hydrated
alumina. However, the narrow width of the peaks reveals the presence of
relatively larger crystallites as compared to that obtained in example 1. The
precipitates under scanning electron microscopy shows defined morphology.
Example 5
Preparation of hydrated alumina suspension/sol

Precipitates those obtained in the previous examples, gives another opportunity
to prepare various suspensions/sols with variable concentration/s by mixing the
washed precipitate/s (by volume) and distilled/de-ionized water (by volume) in
desired volume ratio that gives rise hydrated alumina Isuspension/sol. These so-
derived suspensions/sols offer further opportunity to exploit the aforesaid nano-
porous hydrated alumina material in different applications, e.g., i) generating
nano-porous layers (defined pore size to that of the original material) with
variable thickness on supported ceramic substrates or ii) in-situ incorporation of
the suspension/sol to another matrix/phase, A series of suspensions/sols of the
nano-porous hydrated alumina by increasing the precipitate concentration, e.g.,
i) 5 vol%, ii) 10 vol%, iii) 15vol%, iv) 20 vol% and v) 25 vol% in the same
volume of distilled/de-ionized water are prepared. All the above suspensions/sols
are found to be stable at room temperature for a period of at least 2 days under
ambient conditions and no sedimentation was observed which suggests the
suspensions/sols are good candidates for further applications, particularly for
coating. These derived suspensions/sols give another opportunity for undertaking
coating applications on various substrates or for generating in-situ incorporation
of the suspension/sol to another matrix/phase. Besides, the above concentration
of the suspensions/sols, any intermediate strength of the suspension/sol other
than the specific ranges could also be prepared, depending on the area of
applications.
The invention as herein exemplified should not be read and interpreted in a
restrictive manner as various adaptations, modifications, changes are possible
within the scope and limit of the invention as defined in the encampused
appended claims.
WE CLAIM
1. A process of producing nano-porous hydrated alumina in powder form or in the
for m of aqueous suspension by synthesizing hydrated alumina material
comprising the steps of:
a) preparing an aqueous solution by dissolving solid aluminum nitrate
hydrate [(AI(NO3)3.9H2O] in distilled/de-ionised water in a container and
obtaining a clear solution of concentration in the range of 0.1-0.3
moles/litre on purifying the aqueous solution on filtration, called as
precursor and pouring the solution in a precipitating tank fitted with an
agitator/stirrer.
b) preparing a precipitating agent by dissolving either ammonia or solid
sodium hydroxide or potassium hydroxide in de-ionised/distilled water to
obtain a final concentration in the range of 0.1 to 0.5 mole/litre and
adding slowly the precipitating agent prepared to the tank containing the
precursor, wherein in the tank the precursor and the precipitating agent
are uniformly mixed by the stirrer.
c) carrying out precipitation reactions between the precursor and
precipitating agent till the pH of the solution in the tank reaches in the
range of 9-10, to yield a white gelatinous precipitate, and when addition
of precipitating agent is ceased.
d) the stirring of the solution/mixture of suspension is continued for a while
and the entire solution/mixture is filtrated to filter the precipitate to
separate unreacted precursor/precipitating agent, the filtered precipitate
is then washed continuously with distilled/deionized water to remove
absorbed precursor or precipitating agent till the pH of the filtrate reaches
7.
e) the washed precipitates resulted from the step (d) being further treated in
two alternative course of treatments by either preparing a suspension/sol
of any of the washed precipitates by mixing it with de-ionized/distilled
water in desired ration, the resultant aqueous/sol being used for different
coating and processing applications for generating nano-porous structure
with a pore size in the range of 3-7 nanometers in the coated layer of a
host matrix by heat treating the matrix in the temperature range of 110°C
± 10°C, until consistency in weight of the dried precipitate is attained at a
particular temperature, pulverizing/milling the dried precipitates in nylon
pots or alumina lined pots using alumina balls as grinding media for a
period of 12-24 hours to result pulverized/milled powders of the dried
precipitate, sieving the resulted powders through 100-200 micron sieve,
collecting the sieved powders and characterize evaluating the sieved nano-
porous powders.
2. A process of producing nano-porous hydrated alumina as claimed in claim 1
wherein the precursor is analytical grade/s of aluminum nitrate or hydrated
aluminum nitrate having chemical purity > 99 wt% and the resultant sieved
powder is nano-porous hydrated alumina which shows well defined pores with a
diameter in the range of 3-7 nanometer and its specific surface area increases
linearly as a function of temperature in the range of 100-350°C and specific
surface area in the range of 75-400 m2/g is attained.
3. A process a claimed in claim 1 wherein the precipitation reaction is conducted at
room temperature in the precipitation tank with a magnetic stirrer/agitator, pH
electrode for measuring pH of the mixed solution, a thermometer for measuring
the temperature of the solution and in the tank.
4. A process as claimed in the preceeding claims wherein as the concentrations of
the precursor and the precipitating agent increases particle size of the
precipitates becomes coarser.
5. A process as claimed in the preceeding claims wherein the precipitates start
appearing at a pH of 4 by the addition of the precipitating agent to the precursor
and the volume of the precipitates slowly increases until pH of the
suspension/mixture increases to reach a value of 10.
6. A process as claimed in the preceeding claims wherein during washing and
filtration, a known vacuum filtration system or a rotating filter or a thickner filter
press is employed.
7. A process as claimed in the preceeding claims wherein the precipitates
are dried in an electrically heated oven or in a microwave oven and on
drying there is substantial decreases upto 90% in the volume of the
precipitates and hard solid mass result.
8. A process as claimed in the preceeding claims wherein the pulverized
powders are characterized in terms of surface area in the BET, pore
size profile and pore volume analysis using liquid nitrogen
adsorption/desorption isotherms.
9. A process as claimed in claim 8 wherein the said powders being further
characterized in terms of scanning electron microcopy (morphology), X
ray powder diffraction (crystallinity and phase analysis),
thermogravimetry and differential thermal analysis (TGA-DTA)
(confirmation of water of crystallization in the material obtained in the
powder form).
10. A process of producing nano-porous hydrated alumina in powder form
or in the form of aqueous suspension as herein described and
illustrated.

This invention relates to a process of producing nano-porous hydrated alumina in a
powder form or in the form of aqueous suspension by synthesizing hydrated alumina
material following a precipitation reaction between aqueous solution of aluminum nitrate
(precursor) and aqueous solution/s of liquor ammonia or sodium hydroxide
(precipitating agent) under defined experimental condition. The precipitation reaction is
carried out by mixing both the precursor (concentration range 0.1 - 0.3 moles/liter) and
the precipitating agent/s (0.1 - 1.5 moles/liter) at a given pH range of 9-10 under
stirring conditions at ambient temperature. Gelatinous precipitate results when both the
precursor and precipitating agent is mixed together in the pH range of 9-10, irrespective
of the concentration used in the precursor and either of the precipitating agent in the
aforesaid range. After the precipitation reaction, the resultant precipitate is filtered end
then washed several times with distilled/de-ionized water so as to make the precipitate
free from the contaminants arising out of either from the precursor or the precipitating
agent/s. The washed precipitates are dried either in conventional electrical oven or
microwave oven maintaining a temperature in the range of 100-120°C. The resultant
dried powder is called as nano-porous hydrated alumina powder. The powder possesses
a pore size in the range of 3-7 nanometer. The specific surface area of the powder
increases linearly in the range of 75-400 m2/g, when the same is heated in air or
nitrogen atmosphere in the temperature range of 100-350°C. Apart from the said nano-
porous alumina powder, different aqueous suspensions/sols are prepared by mixing the
washed-precipitate with distilled/de-ionized water in desired weight/volume ratio, which
offers further opportunities to use the resultant suspensions/sols for various coating
applications. X-ray powder diffraction (XRD) pattern of the nano-porous hydrated
alumina powder shows its crystalline structure and has similarity with the well-known
Bayerite structure of hydrated alumina. The powder showed coarse particles with
irregular morphology in the scanning electron microscopy (SEM) investigations.