Molecular sieve abstract
Disclosed is a product and process for the production of granules
of high mechanical strength comprising mixing a molecular sieve
and a metal oxide into a paste with a silicate, shaping the paste
into granules, subjecting the shaped granules to an exchange of
bases, drying the granules and tempering the granules at 300.degree.
to 400.degree. C.
Molecular sieve claims
What is claimed is:
1. A process for the production of granules of high mechanical
strength and high adsorption capacity comprising forming directly
an aqueous paste, the solids thereof consisting essentially of a
molecular sieve, a metal oxide and a water soluble silicate, shaping
the paste into granules, subjecting the shaped granules to an exchange
of bases, drying the granules, and tempering the granules at 300.degree.
to 400.degree. C.
2. The process of claim 1 wherein the metal oxide is added in a
total amount of from 5% to 94%, the molecular sieve is added in
a total amount of from 1% to 95%, and the water soluble silicate
is added in a total amount of from 5% to 50% calculated as solids.
3. The process of claim 2 wherein the metal oxide is from 9% to
50% by weight, the molecular sieve is from 27% to 79% by weight
and the water soluble silicate is from 12% to 28% by weight.
4. The process of claim 2 wherein the metal oxide is selected from
the group consisting of oxides of copper, barium, zinc, magnesium,
rare earths, titanium, tin, lead, vanadium, antimony, chromium,
manganese, iron, cobalt, nickel and mixtures thereof.
5. The process of claim 1 wherein the silicate is an aqueous solution
of sodium silicate with a mole ratio of Na.sub.2 O:SiO.sub.2 of
0.26 to 0.49.
6. The process of claim 5 wherein the mole ratio of Na.sub.2 O:SiO.sub.2
is 0.27 to 0.30.
7. The process of claim 1 wherein the silicate is a guanidine silicate
solution with a SiO.sub.2 content of from 20% to 40% by weight.
8. The process of claim 7 wherein the SiO.sub.2 content is from
25% to 35% by weight.
9. The process of claim 1 wherein the shaping is performed by the
dripping of the mixture to be granulated into a precipitating solution.
10. The process of claim 1 wherein the shaping is performed by
mechanical means.
11. The process of claim 9 wherein the precipitating solution comprises
an aqueous solution of an ammonium salt having a concentration of
from 12% to 36% by weight.
12. The process of claim 11 wherein the ammonium salt solution
concentration is 15% to 37%.
13. The process of claim 10 wherein the exchange of bases is by
treatment of the shaped granules with an ammonium salt solution
having a concentration of from 5% to 33%.
14. The process of claim 11 wherein the ammonium salt solution
has a concentration of from 15% to 25%.
15. The process of claim 9 wherein the exchange of bases is by
treatment of the granules in an ammonium salt solution having a
concentration of from 2% to 10%.
16. The process of claim 15 wherein the ammonium salt solution
is from 3% to 5%.
17. Granules containing molecular sieves consisting essentially
of from 1 to 95% by weight molecular sieve, from 5 to 94% by weight
magnesium oxide and from 5 to 46% by weight SiO.sub.2 formed by
tempering an admixture of molecular sieve, magnesium oxide and a
water soluble silicate.
18. The granules of claim 17 comprising 27 to 79% by weight molecular
sieve, 9 to 50% by weight magnesium oxide and 10 to 24% by weight
SiO.sub.2 formed by tempering an admixture of molecular sieve, magnesium
oxide and a water soluble silicate.
19. The granules containing molecular sieves produced by a process
comprising forming a paste from a mixture consisting essentially
of a molecular sieve, a metal oxide and a silicate solution, shaping
the paste into granules, subjecting the shaped granules to an exchange
of bases, drying the granules and tempering the granules at 300.degree.
to 400.degree. C.
20. A catalyst composition comprising the granules defined by claim
19.
21. An adsorption medium comprising the granules defined by claim
19.
22. An adsorption medium as defined in claim 21 containing MgO
as metal oxide.
Molecular sieve description
FIELD OF THE INVENTION
The invention relates to granules of molecular sieves and an improved
process for their production. More specifically, granules of molecular
sieves of good mechanical strength and high adsorption capacity
are produced from metal oxides and water soluble silicates according
to the process of the invention.
BACKGROUND OF THE INVENTION
Molecular sieves are usually obtained from industrial production
in the form of crystalline powders with a fine particle size. These
molecular sieves must, therefore, be shaped into larger agglomerates
for most applications. For their use as drying media, problems in
shaping often arise because granules of high mechanical strength
must be produced without reducing the adsorption capability of the
molecular sieve.
In most of the known processes for the production of shaped bodies
from molecular sieves water glass and/or clay are used as binders.
Even though the bodies obtained may possess adequate mechanical
strength, their water adsorption capability is reduced. Thus, West
German Published Application No. 1 192 164 describes a process for
the manufacture of molecular sieves in a spherical shape, wherein
finely crystalline molecular sieves are mixed with water glass to
form a paste and dripped into an aqueous solution of sodium, alkaline
earth metal, nickel, cobalt or aluminum salts as the solidification
liquid. The spheres obtained by the process have good water absorption
capabilities, but their mechanical strength is entirely inadequate.
According to the process described in West German Published Application
No. 1 165 562 the granulation of molecular sieves is attained by
stirring them with a silicic acid sol into a flowable suspension
having a pH value of 8.0 to 10.0 preferably 8.2 to 9.0 mixing
the suspension with small amounts of a suspension of magnesium oxide
and dripping the mixture into a liquid immiscible with water; the
granules are then dried and hardened by means of a heat treatment.
The adsorption capacity of the granules obtained in this manner
corresponds to the adsorption capacity of the zeolite contained
therein. The difficulty inherent in the process resides in the fact
that the zeolite-silica sol must be adjusted to a narrow pH range.
This is because in the case of excessively low alkalinity, the zeolite
solution converts at a relatively rapid rate into a soft gel; at
only slightly higher pH values, on the other hand, magnesium oxide
loses its effectiveness as a gelling agent completely. Molecular
sieves must therefore be freed of appreciable amounts of alkaline
contamination by means of a thorough washing. In addition, the spheres
of the gel have no wet strength prior to hardening. Special measures
must therefore be taken to prevent their agglomeration and their
adherence to the walls of vessels, a condition which eventually
may lead to incrustations.
DESCRIPTION OF THE INVENTION
A process has now been discovered for the production of granules
containing molecular sieves. The process is free of the aforementioned
disadvantages and the granules produced are characterized by good
mechanical strength and high adsorption capacity. According to the
process of the invention, the components molecular sieve, metal
oxide and silicate are made into a paste, shaped by mechanical means
or by dripping into a precipitating solution and, following an exchange
of bases, dried and tempered. By means of the selection of the oxide
and/or the molecular sieve, granulates are obtained which find applications
as adsorption media, catalyst or catalyst carriers. In another embodiment,
a powder of a molecular sieve and metal oxide are mixed to a paste
in a silicate solution, possibly with the addition of water, the
mixture is shaped, subjected to an exchange of bases, dried at 100.degree.-140.degree.
C. and tempered at 300.degree. to 400.degree. C.
When magnesium oxide is used as the metal oxide, the granules surprisingly
exhibit an adsorption capacity higher than the existing molecular
sieve proportion in an amount up to 123%.
Metal oxides suitable for use in the process of the invention,
in addition to magnesium oxide, are the oxides of copper, barium,
zinc, the rare earths, titanium, tin, lead, vanadium, antimony,
chromium, manganese, iron, cobalt or nickel, or their mixtures,
respectively. The process yields mechanically strong granules which
may also find application as catalysts or catalyst carriers.
For the application of the granules as drying media, the use of
magnesium oxide is primarily recommended, but the oxides of other
metals also produce increased adsorption capacities. Thus, the choice
and dosage of the metal oxide addition permits the adjustment of
the adsorption capacity of the granules. The following series of
decreasing effectiveness has been established:
MgO increase in adsorption capacity of 22-123%,
CuO increase in adsorption capacity of approx. 37%,
Fe.sub.2 O.sub.3 increase in adsorption capacity of approx. 18%,
TiO.sub.2 increase in adsorption capacity of approx. 11%,
Cr.sub.2 O.sub.3 increase in adsorption capacity of approx. 9%,
SnO.sub.2 increase in adsorption capacity of approx. 8%,
Sb.sub.2 O.sub.3 increase in adsorption capacity of approx. 8%,
NiO increase in adsorption capacity of approx. 4%,
ZnO increase in adsorption capacity of approx. 2%,
PbO increase in adsorption capacity of approx. 1%,
MnO.sub.2 decrease in adsorption capacity of approx. 3%,
V.sub.2 O.sub.5 decrease in adsorption capacity of approx. 12%,
Co.sub.2 O.sub.3 decrease in adsorption capacity of approx. 15%,
RE.sub.2 O.sub.3 (rare earths) decrease in adsorption capacity
of approx. 16%,
BaO decrease in adsorption capacity of approx. 39%.
Normally, metal oxides producing a reduction in adsorption capacity,
will not be applied as drying media, but these granules are used
as catalysts or catalyst carriers.
The content of the individual components of the granules may vary
within wide limits. With respect to the total solids content of
the granules, the metal oxides may be employed in total amounts
of about 5% to about 94%, preferably 9% to about 50% by weight,
the molecular sieve in amounts of about 1% to about 95%, preferably
27% to about 79% by weight and the water soluble silicate in total
amounts of about 5% to about 50%, preferably 12% to about 28% by
weight, calculated as the solid, i.e. as SiO.sub.2 +possibly Na.sub.2
O. The tempered granules then have practically the same composition.
If water glass is used, the SiO.sub.2 proportion of the granules
is correspondingly lower. The MgO containing granules used preferably
as drying media contain 1% to 95%, preferably 27% to 79% by weight
of a molecular sieve, 5% to 94%, preferably 9% to 50% by weight
MgO and 5 % to 38%, preferably 9% to 22% by weight SiO.sub.2.
Sodium or potassium water glass or guanidine silicate and their
mixtures are suitable for use as the silicate solution. The use
of guanidine silicate introduces no additional sodium ions into
the granules, because the guanidine component rapidly decomposes
and volatilizes at temperatures beginning at 120.degree. C. Suitable
are guanidine silicate solutions with a SiO.sub.2 content of 20
to 40% by weight, preferably 25 to 35% by weight. In a sodium silicate
solution the mole ratio of Na.sub.2 O to SiO.sub.2 should be 0.26
to 0.49 preferably 0.27 to 0.30.
The most different types of molecular sieves, e.g., the well-known
A, X, Y, SK 20 type of their mixtures may be processed into granules.
They may also be present in the ion exchanged form.
Deformation may take place both mechanically and by means of precipitation
granulation. Mixtures of powders of molecular sieves, metal oxide
and silicate solution may be extruded, tabletted or shaped on a
rotary table, a pelletizing drum or by means of merumerizers.
For the deformation of the mixture by dripping into a precipitating
solution, preferably an aqueous solution of an ammonium salt, such
as ammonium sulfate, ammonium acetate, ammonium chloride, is used.
This provides the advantage that no foreign ions are introduced
into the granules. The concentration of the ammonium salt solution
may be between 12% and 36%, preferably is between 15% and 33%.
To provide a suitable consistency of the mixture, the water content,
including the water originating in the silicate solution, should
amount to 25%-56%, preferably 31% to 53% by weight in the case of
mechanical shaping, and to 45% to 200%, preferably 47% to 163% by
weight for deformation by means of precipitation, always with respect
to the total solids content.
The exchange of bases takes place in a known manner such as by
means of a solution of an ammonium salt. It is advantageous to apply
the solution of the ammonium salt after mechanical forming in a
concentration of 5% to 33%, preferably 15% to 25%, and after shaping
by means of precipitation in a concentration of 2% to 10%, preferably
3% to 5%.
Drying may be performed at room temperature or elevated temperatures;
temperatures of 100.degree. to 140.degree. C. are recommended.
The process of the invention offers numerous advantages over the
state of the art:
1. It is not necessary to maintain a narrowly limited pH range;
there is no danger of premature gelling and a strongly alkaline
pH value poses no interference.
2. Due to the possibility of shaping by mechanical means, a substantially
higher fracture hardness may be obtained.
3. By using an aqueous solidification solution in place of an organic
precipitating liquid, the contamination of the granules by an organic
substance and thus their dark coloring may be prevented; organic
substances are decomposed during tempering leading to the formation
of carbon inclusions. In contrast, an exchange of bases already
takes place in the ammonium salt precipitating solution; no foreign
ions are introduced.
4. Water glass is considerably less expensive than stabilized silica
sol, the preparation of the latter by the conventional process using
ion exchangers is difficult and expensive. No additional sodium
ions, to be removed later, are introduced when a guanidine silicate
solution is used.
5. A commercial grade of magnesium oxide may be utilized. It is
not necessary to use magnesia usta extra light initially in order
to obtain a suitable hydrated magnesium oxide.
In spite of this simplification of the process and the reduction
of its cost, the granules obtained represent substantial improvement
in technical progress over the prior art:
1. An increase in the adsorption capacity at a relative humidity
of 20% at 25.degree. C., by up to a maximum of 123% over the molecular
sieve content, combined with good mechanical strength of the granules.
Simultaneously, a saving in the expensive molecular sieve material
by the substitution of less expensive materials, such as, e.g.,
magnesium oxide, is obtained.
2. In the case of shaping by precipitation, the gel pearls have
good wet strength, i.e. the pearls do not fracture during handling
and during the base exchange; rejection waste is thus slight.
3. The granules are water proof; i.e. they do not burst when in
contact with spray water or placed in water.
4. Shaped bodies and spherical granules with different diameters
may be produced.
For the production of spherical granulates of medium to the smallest
dimensions, the precipitation method is preferred; mechanical forming
is particularly suitable for the manufacture of large bodies.
5. Molecular sieves of the most varied types, e.g. types A, X,
Y, SK 20 also ion exchanged molecular sieves may be shaped into
granules by the process of the invention.
With the molecular sieve X it is even possible to obtain an increase
in the water vapor adsorption capacity at a relative humidity of
20%, at 25.degree. C., to 182% over the capacity of the proportion
of the molecular sieve present.
EXAMPLES
A molecular sieve powder, metal oxide powder and silicate solution,
possibly together with water, are intermixed in the amounts shown
in the tables presented hereinafter in an arbitrary sequence. The
water glass used has a density of 1.362 the mole ratio of Na.sub.2
O:SiO.sub.2 is 0.2943; the quanidine silicate solution has a concentration
of 25% by weight, calculated as SiO.sub.2.
The initial mixture is shaped
(a) in Examples 1 to 4 mechanically by means of extrusion and
comminution, followed by rounding on a rotary plate with standing
side walls;
(b) in Example 5 by dripping into an aqueous solution of an ammonium
salt of the concentration given in the tables.
To facilitate formation of the spherical shape, the ammonium salt
solution has a top layer of mineral oil, with a thickness of a few
cm.
The solidified granules are taken from the precipitating solution
after 10 minutes.
Shaping is followed by a 4 hour base exchange at room temperature,
rinsing with water, drying at 120.degree. C. and tempering at 350.degree.
to 400.degree. C. |