Molecular sieve abstract
This invention relates to the use of pore mouth control of zeolite
NaA for developing a novel molecular sieve adsorbents and their
potential in the separation and purification of gaseous mixtures
by the size/shape selective adsorption. More specifically,the invention
relates to the manufacture and use of a molecular sieve adsorbent,
which is selective towards oxygen from its gaseous mixture with
nitrogen and argon by pore mouth control of zeolite NaA with liquid
phase alkoxide deposition on the external surface at ambient conditions
of temperature and pressure. Thus prepared adsorbent is useful for
the separation and purification of nitrogen and argon from its mixture
with oxygen.
Molecular sieve claims
1. A process for the preparation of a molecular sieve adsorbent
for the size/shape selective separation of air, which comprises
(1) activating a commercially available zeolite A at a temperature
in the range of from about 350 to about 450.degree. C. to eliminate
physically adsorbed water, for a period of time ranging from about
3 to about 6 hours; (2) cooling the activated zeolite in a desiccator
under vacuum in the range of from about 1.times.10.sup.-2 to about
1.times.10.sup.-4 mm Hg; (3) treating the cooled zeolite with tetra
alkyl ortho silicate dissolved in a dry solvent at a concentration
ranging from about 0.1 to about 1.0 wt. %/volume for a specified
period of time in the range of from about 4 to about 8 hours under
substantially continuous stirring; (4) recovering the solvent by
conventional techniques for re-use; (5) drying the treated zeolite
in air in substantially static condition at an ambient temperature
in the range of from about 20 to about 35.degree. Celsius to provide
a modified zeolite; (6) heating the modified zeolite to a temperature
in the range of from about 450 to about 600.degree. Celsius for
a period of time ranging from about 3 to about 8 hours; and (7)
cooling the zeolite at ambient temperature in substantially static
conditions to provide the molecular sieve adsorbent having a preferential
oxygen adsorption selectivity over nitrogen and argon.
2. The process as claimed in claim 1 wherein from about 0.10 to
about 1.00 weight percent of tetra alkyl ortho silicate was deposited
uniformly on the zeolite surface from its dry solution in the dry
solvent.
3. The process as claimed in claim 2 wherein the said tetra alkyl
ortho silicate deposition on the zeolite surface was carried out
in a simple liquid phase reaction at ambient temperature and pressure
conditions.
4. The process as claimed in claim 3 wherein the tetra alkyl ortho
silicate deposited on the zeolite surface was converted into silica
by calcination in air at about 500 to about 650.degree. C. for about
3 to about 6 hours.
5. The process as claimed in claim 4 wherein the adsorbent as prepared
is useful for the separation and purification of nitrogen and argon
from its mixture with oxygen.
6. The process as claimed in claim 5 wherein the adsorbent as prepared
is useful for the chromatographic separation of oxygen, nitrogen
and argon from its mixture.
7. The process as claimed in claim 1 wherein the said tetra alkyl
ortho silicate deposition on the zeolite surface was carried out
in a simple liquid phase reaction at ambient temperature and pressure
conditions.
8. The process as claimed in claim 1 wherein the tetra alkyl ortho
silicate deposited on the zeolite surface was converted into silica
by calcination in air at about 500 to about 650.degree. C. for about
3 to about 6 hours.
9. The process as claimed in claim 2 wherein the tetra alkyl ortho
silicate deposited on the zeolite surface was converted into silica
by calcination in air at about 500 to about 650.degree. C. for about
3 to about 6 hours.
10. The process as claimed in claim 1 wherein the adsorbent as
prepared is useful for the separation and purification of nitrogen
and argon from its mixture with oxygen.
11. The process as claimed in claim 3 wherein the adsorbent as
prepared is useful for the separation and purification of nitrogen
and argon from its mixture with oxygen.
12. The process as claimed in claim 1 wherein the adsorbent as
prepared is useful for the chromatographic separation of oxygen,
nitrogen and argon from its mixture.
13. The process as claimed in claim 3 wherein the adsorbent as
prepared is useful for the chromatographic separation of oxygen,
nitrogen and argon from its mixture.
14. The process as claimed in claim 1 wherein the tetra alkyl ortho
silicate deposited on the zeolite surface was converted into silica
by calcination in air at about 550 for about 4 hours.
15. The process as claimed in claim 1 wherein the commercially
available zeolite A comprises a sodium zeolite A having the general
formula, (Na.sub.2O).sub.6.(Al.sub.2O.sub.3).sub.6.(SiO.sub.2).sub.12+x.
wH.sub.2O.sub.n where the value of x varies from 0.001 to 0.1 and
w is the number of moles of water.
16. A method for separation and purification of nitrogen and argon
from a mixture of nitrogen, argon, and oxygen, comprising providing
a molecular sieve adsorbent made by the method of claim 1 activating
the molecular sieve adsorbent by heating the adsorbent at a temperature
in the range of from about 350 to about 450.degree. C., and exposing
the adsorbent to the mixture of nitrogen, argon and oxygen.
Molecular sieve description
[0001] The present invention relates to a process for the preparation
of a molecular sieve adsorbent for the size/shape selective separation
of air.
[0002] The invention relates to the use of pore engineered zeolites
as size/shape selective adsorbents in separation of gases having
closely related physical properties. More specifically, the invention
relates to the preparation and use of a molecular sieve adsorbent,
which is selective towards oxygen from its gaseous mixture with
nitrogen and/or argon.
FIELD OF THE INVENTION
[0003] The use of adsorption techniques to separate a gaseous component
from a gaseous stream was initially developed for the removal of
carbon dioxide and water from air. Gas adsorption techniques are
now conventionally employed in processes for the enrichment of hydrogen,
helium, argon, carbon monoxide, carbon dioxide, nitrous oxide, oxygen
and nitrogen.
[0004] Adsorbents most often effect separations by adsorbing one
or more component strongly than another. The various interaction
forces engaged in adsorption process are van der Waals interactions,
acid-base interactions, hydrogen bond, electrostatic, chelation,
clathration and covalent bond. Two important separation mechanisms
are exclusion of certain molecules in the feed because they are
too large to fit into the pores of the adsorbent (molecular sieving
effect, size/shape selective separation) and differences in the
diffusion rates of the adsorbing species in the pores of the adsorbent.
[0005] The four types of adsorbents dominating in usage are activated
carbon, zeolite molecular sieves, silica gel and activated alumina.
Carbon molecular sieves (CMS), which exhibits very narrow pore size
distribution, facilitates separation based on different inter particle
diffusion rates. The efficient separation of air to recover nitrogen
has provided a secure and somewhat growing market for carbon molecular
sieve.
[0006] Adsorption processes for the separation of oxygen and nitrogen
from air are being increasingly used for commercial purposes for
the last three decades. Oxygen requirements in sewage treatment,
fermentation, cutting and welding, fish breeding, electric furnaces,
pulp bleaching, glass blowing, medical purposes and in the steel
industries particularly when the required oxygen purity between
90 to 95% are being largely met by adsorption based pressure swing
or vacuum swing processes. It is estimated that at present, around
20% of the world's oxygen demand is met by adsorptive separation
of air. However, the maximum attainable purity by adsorption processes
is around 95% with separation of 0.934 mole percent argon present
in the air being a limiting factor to achieve 100% oxygen purity.
Furthermore, the adsorption-based production of oxygen from air
is economically not competitive to cryogenic fractionation of air
for production levels more than 200 tonne per day.
[0007] Adsorption capacity of the adsorbent is defined as the amount
in terms of volume or weight of the desired component adsorbed per
unit volume or weight of the adsorbent. The higher the adsorbent's
capacity for the desired components the better is the adsorbent
as the increased adsorption capacity of a particular adsorbent helps
to reduce the amount of adsorbent required to separate a specific
amount of a component from a mixture of particular concentration.
Such a reduction in adsorbent quantity in a specific adsorption
process brings down the cost of a separation process.
[0008] The adsorption selectivity of a component over other is
calculated as the ratio of the volumes of gas adsorbed at any given
pressure and temperature. The adsorption selectivity of a component
results from steric factors such as the differences in the size
and shape of the adsorbate molecules; equilibrium effect, i.e. when
the adsorption isotherms of components of a gas mixture differ appreciably;
kinetic effect, when the components have substantially different
adsorption rates.
BACKGROUND OF THE INVENTION
[0009] The principal characteristic of the separation, removal
or concentration of oxygen from the air is that usually there is
no cost for the starting material, which is air. The cost of the
oxygen produced or removed, depends essentially upon the following
factors.
[0010] (a) Costs of equipment necessary for separating or concentrating
oxygen,
[0011] (b) Costs of energy necessary for operating the equipment,
[0012] (c) When purified oxygen is needed, the cost of the purification
step has to be taken into account.
[0013] Another characteristic is that separation or concentration
of oxygen can be achieved either by separating oxygen or by separating
nitrogen from air as a starting material.
[0014] Taking into consideration the above-described factors, various
economically advantageous processes have heretofore been proposed.
These include, for example, the process in which the air is liquefied
at low temperatures to separate oxygen or nitrogen making use of
difference in the boiling point between liquid oxygen (-182.9.degree.
C.) and liquid nitrogen (-195.8.degree. C.). The apparatus employed
is suited for producing large amounts of oxygen and the production
of most of the oxygen and nitrogen in the world is based on this
procedure. One disadvantage of the process is that it requires large
amounts of power. Another is that large-scale equipment is necessary
site specific and portability is very less. Another is that it takes
hours for switching on and switching off the plant.
[0015] In another approach the membrane separation system has employed
for the separation of oxygen and nitrogen from air [U.S. Pat. No.
5091216 (1992) to Hayes et.al. U.S. Pat. No. 5004482 (1991)
to Haas et.al, U.S. patent application 20020038602 (2002), to Katz;
et al.]. The main drawbacks of this method is the thin polymeric
films used in the separation process are too weak to withstand the
high differential gas pressures required for the separation and
the purity of the product gas is only around 50%.
[0016] In the prior art, adsorbent which are selective for nitrogen
from its mixture with oxygen and argon have been reported [U.S.
Pat. No. 4481018 (1984) to Coe et.al, U.S. Pat. No. 4557736
(1985) to Sircar et.al, U.S. Pat. No. 4859217 (1989) to Chao;
Chien-Chung, U.S. Pat. No. 4943304 (1990) to Coe et.al, U.S. Pat.
No. 4964889 (1990) to Chao; Chien-Chung, U.S. Pat. No. 5114440
(1992) to Reiss; Gerhard, U.S. Pat. No. 5152813 (1992) to Coe
et.al, U.S. Pat. No. 5174979 (1992) to Chao; Chien-Chung et.al,
U.S. Pat. No. 5454857 (1995) to Chao; Chien-Chung, U.S. Pat. No.
5464467 (1995) to Fitch et.al., U.S. Pat. No. 5698013 (1997)
to Chao; Chien-Chung., U.S. Pat. No. 5868818 (1999) to Ogawa et.al.,
U.S. Pat. No. 6030916 (2000) to Choudary et.al., U.S. Pat. No.
6231644 (2001)] to Jain et.al., wherein the zeolites of type A,
faujasite, mordenite, clinoptilites, chabazite and monolith have
been used. The efforts to enhance the adsorption capacity and selectivity
have been reported by exchanging the extra framework cations with
alkali and/or alkaline earth metal cations and increasing the number
of extra framework cations of the zeolite structure by modifying
the chemical composition. The adsorption selectivity for nitrogen
has also been substantially enhanced by exchanging the zeolite with
cations like lithium and/or calcium in some zeolite types. They
have been employed in processes for the separation or concentration
of oxygen by removing nitrogen selectively from the air. However,
the molecular sieves of these types have an isotherm, which follows
Langmuir adsorption isotherm. As a result, when the pressure reaches
1.5 atmospheres absolute (ata) the increase in the adsorptivity
is not large compared with the increase in the pressure. Moreover,
a very large amount of nitrogen must be separated since the molar
ratio of N.sub.2/O.sub.2 in the air is 4. Therefore, the advantage
achieved by enlargement of the apparatus to permit the use of high
pressure is rather small. This limits the application of this process
to small volume installations. The maximum attainable oxygen purity
by adsorption processes is around 95%, with separation of 0.934-mole
percent argon present in the air being a limiting factor to achieve
100% oxygen purity. These adsorbents highly moisture sensitive and
the adsorption capacity and selectivity will decay in the presence
of moisture. The chromatographic separation of oxygen and argon
is also possible by using these adsorbents.
[0017] U.S. Pat. No. 4453952 (1984) to Izmi et.al. discloses
the manufacture of an oxygen selective adsorbent by substituting
the Na cations of zeolite A with K and Fe (II). The adsorbent shows
oxygen selectivity only at low temperature and its preparation requires
multistage cation exchange, adding to the cost of preparation. Cation
exchange is carried out at around 80.degree. C. using aqueous salt
solutions of metal ions to be exchanged. This results into higher
energy requirement as well as generation of effluents during exchange
process. Furthermore, potassium exchange in zeolite leads to lower
thermal and hydrothermal stability of the adsorbent.
[0018] Carbon molecular sieves are effective for separating oxygen
from nitrogen because the rate of adsorption of oxygen is higher
than that of nitrogen. The difference in rates of adsorption is
due to the difference in size of the oxygen and nitrogen molecules.
Since the difference in size is quite small, approximately 0.2 A.degree.,
the pore structure of the carbon molecular sieve must be tightly
controlled in order to effectively separate the two molecules. In
order to improve the performance of carbon molecular sieves, various
techniques have been used to modify pore size. The most common method
is the deposit of carbon on carbon molecular sieves. For example,
U.S. Pat. No. 3979330 to Munzner et.al discloses the preparation
of carbon containing molecular sieves in which coke containing up
to 5% volatile components is treated at 600.degree. C.-900.degree.
C. in order to split off carbon from a hydrocarbon. The split-off
carbon is deposited in the carbon framework of the coke to narrow
the existing pores. U.S. Pat. Nos. 4528281; 4540678; 4627857
and 4629476 to Jr. Robert, S. F. disclose various preparations
of carbon molecular sieves for use in separation of gases.
[0019] U.S. Pat. No. 4742040 to Ohsaki et.al. discloses a process
for making a carbon molecular sieve having increased adsorption
capacity and selectivity by pelletising powder coconut shell charcoal
containing small amounts of coal tar as a binder, carbonising, washing
in mineral acid solution to remove soluble ingredients, adding specified
amounts of creosote or other aromatic compounds, heating at 950.degree.
C. -1000.degree. C., and then cooling in an inert gas. U.S. Pat.
No. 4880765 to Knoblauch et.al., discloses a process for producing
carbon molecular sieves with uniform quality and good separating
properties by treating a carbonaceous product with inert gas and
steam in a vibrating oven and further treating it with benzene at
high temperatures to thereby narrow existing pores. Preparation
of carbon molecular sieve is a multistep process with utmost care
at each stage to get totally reproducible carbon molecular sieve.
Additionally, the process is very high temperature process, which
results into higher cost of the adsorbent.
[0020] U.S. Pat. No. 5081097 to Sharma et.al., discloses copper
modified carbon molecular sieves for selective removal of oxygen.
The sieve is prepared by pyrolysis of a mixture of a copper-containing
material and a polyfunctional alcohol to form a sorbent precursor.
The sorbent precursor is then heated and reduced to produce a copper
modified carbon molecular sieve. Pyrolysis is a high temperature
process making the whole process of preparation of the adsorbent
an energy intensive process.
[0021] Another process uses a transient metal-based organic complex
capable of selectively absorbing oxygen [U.S. Pat. No. 4477418
(1984) to Mullhaupt Joseph et.al.; U.S. Pat. No. 5126466(1992)
to Ramprasad et.al.; U.S. Pat. No. 5141725(1992) to Ramprasad
et.al.; U.S. Pat. No. 5294418(1994) to Ramprasad et.al.; U.S.
Pat. No. 5945079 (1999) to Mullhaupt Joseph et.al; U.S. patent
application 20010003950 (2001), to Zhang, Delang et al.]. The absorption
by these complexes is reversible with changes in temperature and
pressure so that it is theoretically possible to achieve separation
or concentration of oxygen by means of a temperature swing or a
pressure swing cycle of the air.
[0022] However, in practice, severe deterioration of the organic
complex occurs with repeated cycles of absorption and liberation
of oxygen. Moreover, the organic complex itself is expensive. Therefore,
the use of this process is limited to special situations. The main
drawback of this process lies in air and moisture sensitivity of
the metal complexes used which lowers the stability of the adsorbent
produced. Additionally, the cost of the metal complexes used in
preparation of the adsorbent is very high.
[0023] U.S. Pat. No. 6087289 (2000) to Choudary et al. discloses
a process for the preparation of a zeolte-based adsorbent containing
cerium cations for the selective adsorption of oxygen from a gas
mixture. Cerium exchange into zeolite is carried out under reflux
conditions using aqueous solution of cerium salt at around 80.degree.
C. for 4-8 hours and repeating the exchange process several times.
The main drawbacks of this adsorbent lie in observation of oxygen
selectivity only in the low-pressure region. Furthermore, adsorbent
preparation is a multi-step ion exchange process, which also generates
liquid effluent.
[0024] European Patent 0218403 to Greenbank discloses a dense
gas pack of coarse and fine adsorbent particles wherein the size
of the largest fine particles is less than one-third of the coarse
particles and sixty percent of all particles are larger than sixty
mesh. Although not specifically stated, it is evident from the examples
that these percentages are by volume. This system is designed primarily
for enhancing gas volume to be stored in a storage cylinder. It
is mentioned, however, that it can be utilized for molecular sieves.
There is nothing in this application, however, which would give
insight into the fact that significantly enhanced PSA efficiency
could be obtained by combining coarse and fine particles of kinetically-selective
sieve material in a single bed. It has been found in accordance
with the present invention that, within certain limits as will be
defined, a mixture of coarse and fine kinetically selective sieve
particles will unexpectedly give enhanced PSA performance.
[0025] In another approach chemical vapour deposition technique
was used for controlling the pore opening size of the zeolites by
the deposition of silicon alkoxide [M. Niwa et al., J C S Faraday
Trans. I, 1984 80 3135-3145; M. Niwa et al., M. Niwa et al., J.
Phys. Chem., 1986 90 6233-6237; Chemistry Letters, 1989 441-442;
M. Niwa et al., Ind. Eng. Chem. Res., 1991 30 38-42; D. Ohayon
et al., Applied Catalysis A-General, 2001 217 241-251]. Chemical
vapour deposition is carried out by taking a requisite quantity
of zeolite in a glass reactor, which is thermally activated at 450.degree.
C. in situ under inert gas like nitrogen flow. The vapours of silicon
alkoxide are continuously injected into inert gas stream, which
carries the vapours to zeolite surface where alkoxide chemically
reacts with silanol groups of the zeolite. Once the desired quantity
of alkoxide is deposited on the zeolite, sample is heated to 550.degree.
C. in air for 4-6 hours after which it is brought down to ambient
temperature and used for adsorption. The major disadvantages of
this technique are (i) Chemical vapour deposition, which leads to
non-uniform coating of alkoxide which in turn results in non-uniform
pore mouth closure, (ii) The process has to be carried out at elevated
temperature where the alkoxide gets vaporised, (iii) The deposition
of the alkoxide requires to be done at a slow rate for better diffusion
and (iv) This method is expensive and lack of a commercial level
at higher scale will be difficult.
[0026] At present nitrogen and argon containing less than 10 ppm
oxygen is produced by using a deoxo hybrid system in which the oxygen
is removed by reducing it to water over a catalyst with hydrogen.
OBJECT OF THE INVENTION
[0027] The main object of the present invention is to provide a
process for the preparation of a molecular sieve adsorbent for the
size/shape selective separation of air, which obviates the drawbacks
as detailed above.
[0028] Still another object of the present invention is to provide
an oxygen selective zeolite based adsorbent from its gaseous mixture
with nitrogen and argon.
[0029] Still another object of the present invention is to provide
an adsorbent, which can be prepared by the external surface modification
of the zeolite A.
[0030] Yet another object of the present invention is to provide
an oxygen selective adsorbent by a simple liquid phase surface modification
of zeolite A.
[0031] Yet another object of the present invention is to have a
uniform deposition of alkoxide on the surface of zeolite A.
[0032] Yet another object of the present invention is to provide
an adsorbent with high thermal and hydrothermal stability.
[0033] Yet another object of the present invention is to provide
an adsorbent, which is selective to oxygen over nitrogen and argon
and can be used commercially for the separation and purification
of nitrogen and argon.
[0034] In the drawings accompanying this specification,
[0035] FIG. 1 represents the adsorption isotherms of nitrogen,
argon and oxygen at 15.degree. C. on the adsorbent obtained from
example-1.
[0036] FIG. 1 represents the adsorption isotherms of nitrogen,
argon and oxygen at 15.degree. C. on the adsorbent obtained from
example-6.
SUMMARY OF THE INVENTION
[0037] Accordingly, the present invention provides a process for
the preparation of a molecular sieve adsorbent for the size/shape
selective separation of air, which comprises of a molecular sieve
adsorbent represented by the general formula, (Na.sub.2O).sub.6.(Al.sub.2O.sub.3).s-
ub.6.(SiO.sub.2).sub.12+x. wH.sub.2O where the values of x varies
from 0.001 to 0.1 w being the number of moles of water, which comprises
(1) activating the commercially available zeolite A in the temperature
range of 350 to 450.degree. Celsius to eliminate physically adsorbed
water, for a period ranging from 3 to 6 hours; (2) cooling the activated
zeolite in a desiccator under vacuum in the range of 1.times.10.sup.-2
to 1.times.10.sup.-4 mm Hg; (3) treating the cooled zeolite with
tetra alkyl ortho silicate dissolved in a dry solvent in the concentration
range of 0.1 to 1.0 wt %/volume for a specified period in the range
of 4 to 8 hours under continuous stirring; (4) recovering the solvent
by conventional techniques for re-use; (5) drying the treated zeolite
in air in static condition at ambient temperature in the range of
20 to 35.degree. Celsius; (6) heating the modified zeolite in the
temperature range of 450 to 600.degree. Celsius for a period ranging
from 3 to 8 hours; (7) cooling the zeolite at ambient temperature
in static condition; (8) measuring the adsorption of oxygen, nitrogen
and argon by a static volumetric system, prior to it the zeolite
samples were activated in the temperature range of 350 to 450.degree.
Celsius.
[0038] In an embodiment of the present invention commercially available
zeolite A may used for the preparation of the molecular sieve adsorbent.
[0039] In another embodiment of the present invention the zeolite-A
was activated at 350 to 550.degree. C. for 3-6 hours followed by
cooling under inert or vacuum condition.
[0040] In another embodiment of the present invention the tetra
alkyl ortho silicate was dissolved in dry solvent, which may be
selected from like toluene, benzene, xylene and cyclohexane.
[0041] In another embodiment of the present invention 0.10 to 1.00
weight percentage of tetra alkyl ortho silicate may be deposited
onto the zeolite in a single step by treating the activated zeolite
with a solution of tetra alkyl ortho silicate in dry solvent for
4 to 8 hours.
[0042] In still another embodiment of the present invention the
said tetra alkyl ortho silicate may be deposited in the range of
was carried out at tetra alkyl ortho silicate concentration of 0.10
to 1.00% by weight of the zeolite.
[0043] In still another embodiment of the present invention the
alkoxide deposition may be carried out in liquid phase for a period
ranging from 4 to 8 hours under continuous stirring at ambient temperature.
[0044] In still another embodiment of the present invention the
alkoxide deposition may be uniform on the surface of the zeolite.
[0045] In still another embodiment of the present invention the
solvent was recovered by distillation method preferably under vacuum
distillation and can be re-used
[0046] In still another embodiment of the present invention the
adsorbents are dried in air or under vacuum conditions.
[0047] In still another embodiment of the present invention the
adsorbent is calcind in the temperature range 500 to 600.degree.
C. preferably at 550.degree. C.
DESCRIPTION OF THE INVENTION
[0048] In the present invention, we report a novel process to control
the pore size of zeolite A, which has oxygen adsorption selectivity
over nitrogen and argon. Furthermore this adsorbent displays high
thermal and hydrothermal stability.
[0049] Zeolites, which are microporous crystalline alumna-silicates,
are finding increased applications as adsorbents for separating
mixtures of closely related compounds. Zeolites have a three dimensional
network of basic structural units consisting SiO.sub.4 and AlO.sub.4
tetrahedrons linked to each other by sharing apical oxygen atoms.
Silicon and aluminium atoms lie in the centre of the tetrahedral.
The resulting alumno-silicate structure, which is generally highly
porous, possesses three-dimensional pores the access to which is
through molecular sized windows. In a hydrated form, the preferred
zeolites are generally represented by the following formula, M.sub.2/nO.Al.sub.2O.sub.3.xSiO.sub-
.2.wH.sub.2Owhere M is a cation, which balances the electrovalence
of the tetrahedral and is generally referred to as extra framework
exchangeable cation, n represents the valency of the cation and
x and w represents the moles of SiO.sub.2 and water respectively.
[0050] The attributes which makes the zeolites attractive for separation
include, an unusually high thermal and hydrothermal stability, uniform
pore structure, easy pore aperture modification and substantial
adsorption capacity even at low adsorbate pressures. Furthermore,
zeolites can be produced synthetically under relatively moderate
hydrothermal conditions.
[0051] Structural analysis of the samples was done by X-ray diffraction
where in the crystallinity of the zeolites are measured from the
intensity of the well-defined peaks. The in situ X-ray powder diffraction
measurements at 30.degree. C., 100.degree. C., 200.degree. C., 300.degree.
C., 400.degree. C., 500.degree. C., 600.degree. C., 650.degree.
C., 700.degree. C., 750.degree. C., 800.degree. C. and 850.degree.
C. shows that the newly developed adsorbent have high thermal stability.
X-ray powder diffraction was measured using PHILIPS X'pert MPD system
equipped with XRK 900 reaction chamber.
[0052] The zeolite NaA powder [Na.sub.12(AlO.sub.2).sub.12.(SiO.sub.2).sub-
.12.wH.sub.2O] was used as the starting material. X-ray diffraction
data showed that the starting material was highly crystalline. A
known amount of the zeolite NaA powder [Na.sub.12(AlO.sub.2).sub.12.(SiO.sub.2).sub.12-
.wH.sub.2O] was activated at 400.degree. C. to remove the water
adsorbed in the zeolite and mixed thoroughly with a solution having
known amount of tetra alkyl orthosilicate in 100 ml dry solvent,
the sample was dried by evaporating solvent under reduced pressure
and the tetra alkyl ortho silicate species deposited on the zeolite
surface was converted into silica by calcinations of the zeolite
at 550.degree. C.
[0053] Oxygen, nitrogen and argon adsorption at 15.degree. C. was
measured using a static volumetric system (Micromeritics ASAP 2010),
after activating the sample at 350.degree. C. to 450.degree. C.
under vacuum for 4 hours as described in the Examples. Addition
of the adsorbate gas was made at volumes required to achieve a targeted
set of pressures ranging from 0.5 to 850 mmHg. A minimum equilibrium
interval of 5 seconds with a relative target tolerance of 5.0% of
the targeted pressure and an absolute target tolerance of 5.000
mmHg were used to determine equilibrium for each measurement point.
[0054] The selectivity of pure components of two gases A and B
is given by the equation,
.alpha..sub.A/B=[V.sub.A/V.sub.B].sub.P,T
[0055] where V.sub.A and V.sub.B are the volumes of gas A and B
adsorbed at any given pressure P and temperature T.
[0056] The important inventive steps involved in the present invention
are that the molecular sieve adsorbent obtained by the control of
the pore mouth of the zeolite (i) by the deposition of silica by
chemically reacting alkoxide with silanol groups present on the
external surface of the zeolite followed by calcination at 500-600.degree.
C. (ii) by liquid phase chemical reaction of tetra alkyl orthosilicate
in moisture free solvent to ensure uniform deposition of silica
on the surface of the zeolite at ambient conditions, (iii) enhancement
of thermal and hydrothermal stability of the adsorbent by silica
deposition on the external surface of the zeolite (iv) to prepare
zeolite based oxygen selective adsorbent based on shape/size selectivity
by a method other than conventionally used cation exchange
[0057] The following examples are given by way of illustration
and therefore should not be constructed to limit the scope of the
present invention.
EXAMPLE-1
[0058] A known amount of zeolite NaA, [(Na.sub.2O).sub.6.(Al.sub.2O.sub.3)-
.sub.6.(SiO.sub.2).sub.12.wH.sub.2O], was activated at 350.degree.
C. under vacuum and adsorption measurements were carried out as
described earlier. The adsorption capacity for oxygen is 3.48 cc/g
at 15.degree. C. and 765 mmHg and selectivity for nitrogen over
oxygen is around 3 to 5 in the pressure range studied, the values
are given in table 1.
EXAMPLE-2
[0059] 10.0 g of the zeolite NaA powder [Na.sub.12(AlO.sub.2).sub.12.(SiO.-
sub.2).sub.12.wH.sub.2O] was activated at 400.degree. C. to remove
the adsorbed water in the zeolite and stirred with 0.10 g tetra
methyl orthosilicate in 100 ml dry toluene. The sample was dried
after 5 hrs by evaporating solvent under reduced pressure. The tetra
methyl ortho silicate species deposited on the zeolite surface was
converted into silica by calcinations of the zeolite at 550.degree.
C. A known amount of the sample was activated at 350.degree. C.
under vacuum and adsorption measurements were carried out as described
earlier. In situ X-ray powder diffraction measurements at various
temperatures up to 850.degree. C. shows that the adsorbent has high
thermal and hydrothermal stability. The adsorption capacity for
oxygen is 3.50 cc/g at 15.degree. C. and 765 mmHg, selectivity for
oxygen over nitrogen is 1.5 to 0.95 and selectivity for oxygen over
argon is 1.3 to 2.1 in the pressure range studied, the values are
given in table-1.
EXAMPLE-3
[0060] 10.0 g of the zeolite NaA powder [Na.sub.12(AlO.sub.2).sub.12.(SiO.-
sub.2).sub.12.wH.sub.2O] was activated at 400.degree. C. to remove
the water adsorbed in the zeolite and stirred with 0.10 g tetra
ethyl orthosilicate in 100 ml dry solvent. The sample was dried
after 5 hrs by evaporating solvent under reduced pressure. The tetra
ethyl ortho silicate species deposited on the zeolite surface was
converted into silica by calcinations of the zeolite at 550.degree.
C. A known amount of the sample was activated at 350.degree. C.
under vacuum and adsorption measurements were carried out as described
earlier. In situ X-ray powder diffraction measurements at various
temperatures up to 850.degree. C. shows that the adsorbent has high
thermal and hydrothermal stability. The adsorption capacity for
oxygen is 3.53 cc/g at 15.degree. C. and 765 mmHg, selectivity for
oxygen over nitrogen is 1.6 to 1.1 and selectivity for oxygen over
argon is 1.3 to 2.2 in the pressure range studied, the values are
given in table 1.
EXAMPLE-4
[0061] 10.0 g of the zeolite NaA powder [Na.sub.12(AlO.sub.2).sub.12.(SiO.-
sub.2).sub.12.wH.sub.2O] was activated at 400.degree. C. to remove
the water adsorbed in the zeolite and stirred with 0.15 g tetra
ethyl orthosilicate in 100 ml dry toluene. The sample was dried
after 5 hrs by evaporating toluene under reduced pressure. The tetra
ethyl ortho silicate species deposited on the zeolite surface was
converted into silica by calcinations of the zeolite at 550.degree.
C. A known amount of the sample was activated at 350.degree. C.
under vacuum and adsorption measurements were carried out as described
earlier. In situ X-ray powder diffraction measurements at various
temperatures up to 850.degree. C. shows that the adsorbent has high
thermal and hydrothermal stability. The adsorption capacity for
oxygen is 3.15 cc/g at 15.degree. C. and 765 mmHg, selectivity for
oxygen over nitrogen is 1.8 to 0.97 and selectivity for oxygen over
argon is 2.8 to 3.2 in the pressure range studied, the values are
given in table 1.
EXAMPLE-5
[0062] 10.0 g of the zeolite NaA powder [Na.sub.12(AlO.sub.2).sub.12.(SiO.-
sub.2).sub.12.wH.sub.2O] was activated at 400.degree. C. to remove
the water adsorbed in the zeolite and stirred with 0.20 g tetra
ethyl orthosilicate in 100 ml dry toluene. The sample was dried
after 5 hrs by evaporating toluene under reduced pressure. The tetra
ethyl ortho silicate species deposited on the zeolite surface was
converted into silica by calcinations of the zeolite at 550.degree.
C. A known amount of the sample was activated at 350.degree. C.
under vacuum and adsorption measurements were carried-out as described
earlier. In situ X-ray powder diffraction measurements at various
temperatures up to 850.degree. C. shows that the adsorbent has high
thermal and hydrothermal stability. The adsorption capacity for
oxygen is 3.78 cc/g at 15.degree. C. and 765 mmHg, selectivity for
oxygen over nitrogen is 2 to 1.1 and selectivity for oxygen over
argon is 3.0 to 3.4 in the pressure range studied, the values are
given in table-1.
EXAMPLE-6
[0063] 10.0 g of the zeolite NaA powder [Na.sub.12(AlO.sub.2).sub.12.(SiO.-
sub.2).sub.12.wH.sub.2O] was activated at 400.degree. C. to remove
the water adsorbed in the zeolite and stirred with 0.25 g tetra
ethyl orthosilicate in 100 ml dry toluene. The sample was dried
after 5 hrs by evaporating toluene under reduced pressure. The tetra
ethyl ortho silicate species deposited on the zeolite surface was
converted into silica by calcinations of the zeolite at 500.degree.
C. A known amount of the sample was activated at 350.degree. C.
under vacuum and adsorption measurements were carried out as described
earlier. In situ X-ray powder diffraction measurements at various
temperatures up to 850.degree. C. shows that the adsorbent has high
thermal and hydrothermal stability. The adsorption capacity for
oxygen is 2.42 cc/g at 15.degree. C. and 765 mmHg, selectivity for
oxygen over nitrogen is around 2.1 to 1.5 and selectivity for oxygen
over argon is 3.5 to 3.8 in the pressure range studied, the values
are given in table 1.
EXAMPLE-7
[0064] 10.0 g of the zeolite NaA powder [Na.sub.12(AlO.sub.2).sub.12.(SiO.-
sub.2).sub.12.wH.sub.2O] was activated at 400.degree. C. to remove
the water adsorbed in the zeolite and stirred with 0.30 g tetra
ethyl orthosilicate in 100 ml dry toluene. The sample was dried
after 5 hrs by evaporating toluene under reduced pressure. The tetra
ethyl ortho silicate species deposited on the zeolite surface was,
converted into silica by calcinations of the zeolite at 550.degree.
C. A known amount of the sample was activated at 350.degree. C.
under vacuum and adsorption measurements were carried out as described
earlier. In situ X-ray powder diffraction measurements at various
temperatures up to 850.degree. C. shows that the adsorbent has high
thermal and hydrothermal stability. The adsorption capacity for
oxygen is 2.63 cc/g at 15.degree. C. and 765 mmHg, selectivity for
oxygen over nitrogen is 2.5 to 1.6 and selectivity for oxygen over
argon is 4.6 to 4.7 in the pressure range studied, the values are
given in table-1.
EXAMPLE-8
[0065] 10.0 g of the zeolite NaA powder [Na.sub.12(AlO.sub.2).sub.12.(SiO.-
sub.2).sub.12.wH.sub.2O] was activated at 400.degree. C. remove
the water adsorbed in the zeolite and stirred with 1.00 g tetra
ethyl orthosilicate in 100 ml dry toluene. The sample was dried
after 5 hrs by evaporating toluene under reduced pressure. The tetra
ethyl ortho silicate species deposited on the zeolite surface was
converted into silica by calcinations of the zeolite at 550.degree.
C. A known amount of the sample was activated at 350.degree. C.
under vacuum and adsorption measurements were carried out as described
earlier. In situ X-ray powder diffraction measurements at various
temperatures up to 850.degree. C. shows that the adsorbent has high
thermal and hydrothermal stability. The adsorption capacity for
oxygen is 1.32 cc/g at 15.degree. C. and 765 mmHg, selectivity for
oxygen over nitrogen is 2.5 to 1.4 and selectivity for oxygen over
argon is 2.5 to 3.5 in the pressure range studied, the values are
given in table-1.
EXAMPLE-9
[0066] 10.0 g of the zeolite NaA powder [Na.sub.12(AlO.sub.2).sub.12.(SiO.-
sub.2).sub.12.wH.sub.2O] was activated at 400.degree. C. to remove
the water adsorbed in the zeolite and stirred with 0.20 g tetra
methyl orthosilicate in 100 ml dry toluene. The sample was dried
after 5 hrs by evaporating toluene under reduced pressure. The tetra
methyl ortho silicate species deposited on the zeolite surface was
converted into silica by calcinations of the zeolite at 550.degree.
C. A known amount of the sample was activated at 350.degree. C.
under vacuum and adsorption measurements were carried out as described
earlier. In situ X-ray powder diffraction measurements at various
temperatures up to 850.degree. C. shows that the adsorbent has high
thermal and hydrothermal stability. The adsorption capacity for
oxygen is 3.77 cc/g at 15.degree. C. and 765 mmHg, selectivity for
oxygen over nitrogen is 2.6 to 1.4 and selectivity for oxygen over
argon is 3.3 to 4.1 in the pressure range studied, the values are
given in table-1.
EXAMPLE-10
[0067] 10.0 g of the zeolite NaA powder [Na.sub.12(AlO.sub.2).sub.12.(SiO.-
sub.2).sub.12.wH.sub.2O] was activated at 400.degree. C. to remove
the water adsorbed in the zeolite and stirred with 0.25 g tetra
methyl orthosilicate in 100 ml dry benzene. The sample was dried
after 5 hrs by evaporating benzene under reduced pressure. The tetra
methyl ortho silicate species deposited on the zeolite surface was
converted into silica by calcinations of the zeolite at 500.degree.
C. A known amount of the sample was activated at 350.degree. C.
under vacuum and adsorption measurements were carried out as described
earlier. In situ X-ray powder diffraction measurements at various
temperatures up to 850.degree. C. shows that the adsorbent has high
thermal and hydrothermal stability. The adsorption capacity for
oxygen is 2.85 cc/g at 15.degree. C. and 765 mmHg, selectivity for
oxygen over nitrogen is 2.4 to 1.3 and selectivity for oxygen over
argon is 4.0 to 4.3 in the pressure range studied, the values are
given in table-1.
EXAMPLE-11
[0068] 10.0 g of the zeolite NaA powder [Na.sub.12(AlO.sub.2).sub.12.(SiO.-
sub.2).sub.12.wH.sub.2O] was activated at 400.degree. C. to remove
the water adsorbed in the zeolite and stirred with 0.20 g tetra
ethyl orthosilicate in 100 ml dry benzene. The sample was dried
after 5 hrs by evaporating benzene under reduced pressure. The tetra
ethyl ortho silicate species deposited on the zeolite surface was
converted into silica by calcinations of the zeolite at 550.degree.
C. A known amount of the sample was activated at 350.degree. C.
under vacuum and adsorption measurements were carried out as described
earlier. In situ X-ray powder diffraction measurements at various
temperatures up to 850.degree. C. shows that the adsorbent has high
thermal and hydrothermal stability. The adsorption capacity for
oxygen is 3.79 cc/g at 15.degree. C. and 765 mmHg, selectivity for
oxygen over nitrogen is 2.4 to 1.2 and selectivity for oxygen over
argon is 3.6 to 4.0 in the pressure range studied, the values are
given in table-1.
EXAMPLE-12
[0069] 10.0 g of the zeolite NaA powder [Na.sub.12(AlO.sub.2).sub.12.(SiO.-
sub.2).sub.12.wH.sub.2O] was activated at 400.degree. C. to remove
the water adsorbed in the zeolite and stirred with 0.25 g tetra
ethyl orthosilicate in 100 ml dry cyclohexane. The sample was dried
after 5 hrs by evaporating cyclohexane under reduced pressure. The
tetra ethyl ortho silicate species deposited on the zeolite surface
was converted into silica by calcinations of the zeolite at 600.degree.
C. A known amount of the sample was activated at 350.degree. C.
under vacuum and adsorption measurements were carried out as described
earlier. In situ X-ray powder diffraction measurements at various
temperatures up to 850.degree. C. shows that the adsorbent has high
thermal and hydrothermal stability. The adsorption capacity for
oxygen is 2.92 cc/g at 15.degree. C. and 765 mmHg, selectivity for
oxygen over nitrogen 2.6 to 1.5 and selectivity for oxygen over
argon is 4.5 to 4.8 in the pressure range studied, the values are
given in table-1.
EXAMPLE-13
[0070] 10.0 g of the zeolite NaA powder [Na.sub.12(AlO.sub.2).sub.12.(SiO.-
sub.2).sub.12.wH.sub.2O] was activated at 400.degree. C. to remove
the water adsorbed in the zeolite and stirred with 0.25 g tetra
methyl orthosilicate in 100 ml dry cyclohexane. The sample was dried
after 5 hrs by evaporating-cyclohexane under reduced pressure. The
tetra methyl ortho silicate species deposited on the zeolite surface
was converted into silica by calcinations of the zeolite at 550.degree.
C. A known amount of the sample was activated at 350.degree. C.
under vacuum and adsorption measurements were carried out as described
earlier. In situ X-ray powder diffraction measurements at various
temperatures up to 850.degree. C. shows that the adsorbent has high
thermal and hydrothermal stability. The adsorption capacity for
oxygen is 2.87 cc/g at 15.degree. C. and 765 mmHg, selectivity for
oxygen over nitrogen is 2.8 to 1.5 and selectivity for oxygen over
argon is 4.7 to 4.9 in the pressure range studied, the values are
given in table-1.
EXAMPLE-14
[0071] 10.0 g of the zeolite NaA powder [Na.sub.12(AlO.sub.2).sub.12.(SiO.-
sub.2).sub.12.wH.sub.2O] was activated at 400.degree. C. to remove
the water adsorbed in the zeolite and stirred with 0.25 g tetra
ethyl orthosilicate in 100 ml dry xylene. The sample was dried after
5 hrs by evaporating xylene under reduced pressure. The tetra ethyl
ortho silicate species deposited on the zeolite surface was converted
into silica by calcinations of the zeolite at 550.degree. C. A known
amount of the sample was activated at 350.degree. C. under vacuum
and adsorption measurements were carried out as described earlier.
In situ X-ray powder diffraction measurements at various temperatures
up to 850.degree. C. shows that the adsorbent has high thermal and
hydrothermal stability. The adsorption capacity for oxygen is 2.93
cc/g at 15.degree. C. and 765 mmHg, selectivity for oxygen over
nitrogen is 2.4 to 1.4 and selectivity for oxygen over argon is
4.3 to 4.5 in the pressure range studied, the values are given in
table-1.
EXAMPLE-15
[0072] 10.0 g of the zeolite NaA powder [Na.sub.12(AlO.sub.2).sub.12.(SiO.-
sub.2).sub.12.wH.sub.2O] was activated at 400.degree. C. remove
the water adsorbed in the zeolite and stirred with 0.25 g tetra
methyl orthosilicate in 100 ml dry xylene. The sample was dried
after 5 hrs by evaporating xylene under reduced pressure. The tetra
methyl ortho silicate species deposited on the zeolite surface was
converted into silica by calcinations of the zeolite at 650.degree.
C. A known amount of the sample was activated at 350.degree. C.
under vacuum and adsorption measurements were carried out as described
earlier. In situ X-ray powder diffraction measurements at various
temperatures up to 850.degree. C. shows that the adsorbent has high
thermal and hydrothermal stability. The adsorption capacity for
oxygen is 2.77 cc/g at 15.degree. C. and 765 mmHg, selectivity for
oxygen over nitrogen is 2.3 to 1.3 and selectivity for oxygen over
argon is 4.7 to 4.8 in the pressure range studied, the values are
given in table-1. |