Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR TXE PREPARATION OF CATALYST SUPPORTS AND
MATERIALS PRODUCED THER~BY
This invention relates to a process for the production of
catalyst supports. It also relates to the catalyst supports
produced by the above process. Additionally, it relates to catalyst
supports containing catalytically active substances such as
microbial cells immobilized thereon.
The use of various substances to support, and in some
instances immobilize, catalytically active materials is well known
to those skilled in the art. Since catalytically active substances
help to make reactions proceed which would otherwise not be
thermodynamically possible or economically practical in many
instances, it has become increasingly important to look for ways to
efficiently utilize and maintain such catalytically active
materials. Furthermore, since the cost of the catalytically active
materials must itself be an active consideration in deciding whether
to commercialize a process using the catalyst, there is even more
reason to look at utilizing the catalyst as desirably as possible.
One of the most important classes of catalytically active
materials or agents currently being studied and utilized in both
theoretical and commercial settings are enzymes. It is known that
enzymes, which are proteinaceous in nature and which are commonly
water soluble, act as biocatalysts which serve to regulate many and
varied chemical reactions which occur in living organisms. The
enzymes may also be isolated and used in analytical, medical, and
industrial applications. For example, they find use in industrial
applications in the preparation of food such as cheese or bread as
well as being used in the preparation of alcoholic beverages. The
enzyme glucose isomerase is extensively used to convert glucose to
fructose in the manufacture of high fructose corn syrup.
Since enzymes are commonly water soluble as well as being
generally unstable and, therefore, subject to deactivation, they are
- difficult to remove for reuse from solutions in which they are
utilized and they may not retain their catalytic activity over
extended periods of time. These difficulties lead to an increased
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cost in the use of enzymes in commercial scale operations due to the
necessity for frequent replacement of the enzyme. In order to
reduce the high cost of enzyme replacement, various methods to
immobilize enzymes prior to their use have been devised. This
immobilization of the enzyme permits its reuse, whereas it might
otherwise undergo deactivation or be lost in the reaction medium in
which it is used. These immobilized enzyme systems may be employed
in various reactor systems, for example, in packed columns and
stirred tank reactors, depending on the nature of the substrate
which is being biochemically reacted.
Apsrt from immobilization of enzymes themselves, various
substances and techniques have been put forward by which the enzymes
could be immobilized without isolation. In psrticular, whole cells
of micro-organisms can be immobilized, thus using the microbial cell
as a carrier for the enzyme and obviating the need for extraction of
the enzyme from the cell.
One commonly used support or entrapement material for
microbial cell immobilization is a gel, usually an alginate gel.
Essentially the cells are trapped in a three-dimensional polymer
network with relatively large interstitial spaces in the gel. The
use of such gels has not been without problems though.
One problem with immobilizing microbial cells in a gel is
their marked tendency to lose their activity during storage or other
- periods of non-use, for instance during transportation. An
accompanying difficulty during non-use is the tendency for
contaminating micro-organisms to proliferate. It is a relatively
routine matter to prepare gel-immobilized cells which have high
activity upon immediate use, but the activity tends to decay
relatively quickly if the gel-immobilized microbial cells are not
used. A basic disadvantage of gel is that it has a high water
activity and probably provides a good environment for growth of
contaminant moulds, bacteria, and the like. Such gels, of course,
are not reusable either.
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Another type oE material used to immobilize catalytic
agents such as enzymes and microbial cells i5 a porous pellet
composed primarily of a high silica content or mixtures oE silica
and alumina. The high silica content is derived from the addition
of a high purity siliceous material to the reaction mixture in the
process oE making the pellet. On the porous surfaces of the pellets
are deposited small amounts of the catalytically active agent. In
general, the use of such a support can be advantageous because it
greatly increases the efficiency of the use of the catalyst. By
spreading the catalyst material over a large support surface area
much more of its catalytically active surface is exposed to the
chemicals whose reaction it is to catalyze.
In selecting such a porous, inorganic material to
immobilize micro-organisms careful consideration must be given to
the pore diameter of the carrier. Production rates are greatly
affected by concentration of the enzymes or microbial cells and by
the ease of diffusion to them. It has been generally recognized
that by maximizing the concentration of microbial cells and
accepting the resulting diffusion rates gives the best performance~
The highest loading of microbial cells are obtained when the pore
diameters are based upon the microbial cell diameters. Pores which
are one to five times the size oE the largest microbial cell
typically provide the highest production rates. In microbial cell
immobilization, the pore diamaters are based upon the major cell
dimensions. ~iving systems re~uire additional care to insure
adequate space for cell reproduction.
A big disadvantage with the use of conventional high
silica-based catalyst supports is that their average pore diameter
is too small for accomodating microbial cells. Their typical
average pore diameter is much less than 1 micron. Typically,
diamaters of 1 to 25 microns are needed to accomodate the microbial
cells. ~E course, when it becomes difficult to immobilize an
efEective number of microbial cells on a typical silica-based
catalyst support, the economic attractiveness of such a support in
commercial processes is greatly reduced.
Because of the above limitations to both the gel and
silica-based inorganic supports for immobilization of microbial
cells, research was conducted to find a support which would overcome
all the above disadvantages as well as offer other advantages.
S During the course of such research, it was discovered that an
efficient catalyst support made by the process of formin~ a mixture
comprising an inorganic binder~ an organic burnout material, a
solvent, and either expanded perlite, calcined diatomite, or
flux-calcined diatomite, followed by formin~ an extrudate from the
above mixture and then drying and calcining the extrudate results in
an extremely economical, efficient support for immobilizing
catalytic agents, in particular microbial cells. In the present
invention~ we believe that by eliminating a high purity silica
source from the reaction mixture of the present invention, the
average pore diameter is conveniently controlled so that it falls in
the range of from 1-25 microns. This contrasts sharply with the
more conventional catalyst supports wherein the resulting average
pore diameter is much smaller and therefore difficult to immobilize
microbial cells on. Thus our invention results in the production of
a catalyst support whose average pore diameter is in a range which
is neither too large or too small for efficiently immobilizing
microbial cells as in the case of silica-based supports.
Our catalyst support also does not provide for an
environment where a decline in microbial activity occurs and
- 25 microbial contaminants proliferate as in the case of gels.
Furthermore our supports are inert, rigid, and are reusable which
greatly enhances their economic attractiveness. Additionally, our
supports are made by an inventive process which employs economical
ingredients and is easy to conduct.
Therefore, it is an object of the present invention to
provide a novel process for the production of an inorganic catalyst
support especially useful in the immobilization of microbial cells.
It is another object of the present invention to provide a
catalyst support made by the above novel process.
Other aspects, objects, and the several advantages of the
present invention are apparent from the specification and the
appended claims.
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In accordance with one embodiment of the present invention
we have discovered a novel process ~or the production of an
inorganic catalyst support which is especially useful for
immobili2ing microbial cells. Our inventive process involves the
steps of:
(a) forming an extrudable mixture comprising:
(i) 20-70 wt.% of one material selected from the
~roup consisting of calcined diatomite, flux
calcined diatomite and expanded perlite;
(ii) 5-30 wt.~ inorganic binder;
(iii) 0-12 wt.~ organic burnout material; and
(iv) 20-50 wt.% solvent;
(b) extruding the mixture through a die to form an
extrudate and then separating the extrudate into a plurality oE
pellets;
(c) drying the pellets at a temperature in the range of
about 200 to 500F for about 5-30 minutes; and thereafter
(d) calcining the dried pellets at a temperature in the
range of about 700 to 2,000F for about 10-45 minutes.
Preferably, the mixture in the inventive process will
comprise 30 - 45 wt.% of the diatomite or perlite used, 10 - 15 wt.%
inorganic binder, O - 12 wt.% organic burnout material, and 35 - 50
wt.% solvent.
Diatomite is a chalky sedimentary material composed of the
skeletal remains of single celled aquatic water plants called
diatoms. Many modern diatomite deposits were laid down by
sedimentation in shallow waters years ago. Subsequent geologic
uplift has raised these beds to positions where they can be mined by
conventional methods. Deposits are found in numerous parts of the
world, with one of the largest and purest deposits being located on
the central California coast. In other locations, there are
currently shallow bodies of water where diatomite deposition has
occurred and/or is currently occurrin~. Such deposits are presently
mined by dredging. A typical dry diatomite analysis is shown in
Table I below.
TABLE I
Component Wt.%
Si02 ( ) 86.0
A1203 3.6
2 3 1.3
Group I Oxides 1.2
Group II Oxides 1.1
Other 0-5
Water 3.0
Loss on Ignition 3.6
Note:
~a) predominantly in amorphous form
Calcined diatomite is diatomite which is mined, dried,
granulated, and passed through a kiln which is operated at a
temperature in the range of about 1600F to 2400F. The calcinstion
causes the diatomite particles to shrink and harden and, to a
certain extent, to agglomerate themselves into larger clusters.
Flux calcined diatomite is produced by adding a flux to the
diatomite. The flux can be added as a solution dissolved in a water
spray or mixing water. Alternatively, dry flux powder can be
incorporated into the mass of diatomite particles either during air
conveying of the diatomite or by dry mixing of the flux and
diatomite in conventional dry mixing devices such as tumblers.
Normally there will be from about 3 to about 10 weight percent flux
- based on the weight of the dry diatomite. Typical fluxes include
alkali metal salts such as sodium carbonate ('`soda ash"), sodium
chloride, sodium hydroxide, and sodium silicate. Those skilled in
the art will be well aware of the appropriate quantity of flux to
use for any particular type of flux and diatomite.
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A commercially available form of expanded perlite may be
used in the present invention. Perlite is a mineral of volcanic
origin which generally alls into the rhyolitic class. The unique
feature of perlite is that it contains several percent water of
hydration. If the perlite is rapidly heated to a temperature on the
order of 1600F (870C) the water is converted to steam and the
perlite "pops", i.e. it rapidly expands to a much lower density.
The amount of expansion is usually on the order of 4 to 20 times the
original volume. AEter expansion, the perlite is milled and
classified to produce a specified particle distribution size.
Preferably the expanded perlite utilized in this invention will have
a density o about 5 - 20 pcf, most preferably about 12 - 16 pcf.
Whatever perlite or diatomite is used in the present
invention is one which preferably forms a cake having a permeability
between about 10 and 2000 darcies. The cake is formed by flowing a
slurry of 20 grams of the perlite or diatomite used mixed with 980
grams of water through a 325-mesh screen at a rate of 1.0 gallons
per square foot per minute.
Another component of the present invention is an inorganic
binder. Generally any commercially available inorganic binder may
be used in the present invention. Of course, it must have the
requisite strength to bind with the mixture ingredients, especially
the diatomite or expanded perlite.
One class of inorganic binder which may be used in the
present invention are clays. Examples include kaolin clays and
bentonite clays. Kaolin clays, sometimes referred to as white or
porcelain clays are a white-burning clay, which due to their great
purity, have a high fusion point. Kaolin clays are also the most
refractory of all clays.
Bentonite clays are a form of montmorillonite clays.
Bentonite clays are hydrous alumina silicates normally containing
significant portions of sodium, magnesium, and calcium oxides.
Another class of inorganic binders which can be used are
monovalent silicates. Examples of monovalent silicates include but
are not limited to sodium silicate and potassium silicate with
sodium silicate preferred.
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Other inorganic binders which may be utilized include
phosphoric acid based binders such as aluminum phosphate and
colloidal suspensions including colloidal silica, colloidal alumina,
and colloidal zirconia.
Suitable combinations of the above inorganic binders may be
utilized. However, clay based binder systems are presently
preferred.
Suitable organic burnout materials for use in the present
invention include but are not limited to starches, cellulose fibers,
corn meal, and powdered carbons. Examples of the cellulose fibers
include kraft fiber, wood fiber, straw fibers, and others which are
a well opened fiber. Short fiber lengths are preferred for ease in
mixing and extrudin~.
Any commercially available solvent can be used in the
present invention which will cause the mixture of the solid
components to take on an extrudable consistency. These solvents may
be organic or aqueous in nature however an aqueous solvent is
presently preferred.
Examples of suitable organic solvents include but are not
limited to kerosene, diesel fuels, and alcohols.
After the mixture of the solids and solvent is formed into
the extrusion feed, it is extruded in conventional extrusion
equipment through a die to form an extrudate from ~hich individual
pellets may be separated. It is frequently desirable to incorporate
a lubricant or similar extrusion aid into the mixture to facilitate
the extrusion; such material will be burned out of the product
during the subsequent drying and/or calcining. Most commonly the
extrudate is an elongated rod-like material of circular, oval, or
square cross-sections. Circular cross-sections are preferred to
minimize attrition of the pellets in subsequent handling. Normally
the extruded rod is appro~imately 0.06 to 0.25 inches in width or
diameter and preferably approximately 0.115 to 0.135 inches.
The extruded rod is commonly severed at intervals
approximately equal to the diameter or width of the rod such that
generally cylindrical or cubical pellets having approximately equal
dimensions in all dimensions are formed. Conventional severing
equipment such as wire knives can be used.
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After the extruded pellets are formed they are dried in
conventional drying units such as continuous belt dryers. Quite
satisfactory materials have been made using a three ~one dryer in
which the temperature generally ranges between about 200 - 500F,
preferably between about 250 - 450F. Drying will generally bP for
about 5 - 30 minutes. preferably about 10 - 15 minutes. The time
and temperature relationships must be such that during the drying
period all moisture is removed. After drying has been completed the
pellets may be allowed to cool and are screened to remove any
pellets which are over or under the desired size range.
Thereafter the dried pellets are calcined or fired in
calcining equipment such as a rotary kiln generally at a temperature
in the range of about 700 - 2,000F for about 10 - 45 minutes,
preferably at a temperature in the range of about 1,400 - 1,800F
Eor about 20 - 30 minutes. The calcining time will normally be at
least about 10 minutes and more on the order of about 20 - 30 minutes
Calcining in an oxygen containing atmosphere should
continue until all the organic burnout material, if any is present,
has been burned out of the pellets leaving a highly porous composite
of diatomite or perlite and inorganic binder. If desired,
additional air injection can be made at approximately the mid-point
of the calcination ~iln to enhance the calcination: an air lance is
quite suitable Eor such air injection. The pellets can then be
screened to remove off-size material.
In accordance with another embodiment of the present
invention, an inorganic catalyst support having a pore diameter very
suitable for immobilizing microbial cells is provid0d. This
inorganic catalyst support is made by the above described inventive
process.
The average pore diameter of the inventive catalyst support
will be between about 1 and 25 microns. The resulting catalyst
support has a mean pore diameter which is ideal for immobilizing
microbial cells.
Generally, the inventive catalyst support will have a
surface area in the range of about 3-20 m /g, a pore volume in the
range of about 0.6-1.2 cc/g, and a crush strength of about 1-10 kg.
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S~ 5~
The catalyst supports of the present invention are useful
for supporting any suitable catalytic substance. In particular, the
inventive catalyst supports are useful for immobiliæing
biocatalysts, especially microbial cells. A wide variety of
microbial cells to include bacterial and fungus-like microbes can be
immobilized on the support by any method known to those skilled in
the art. Typically, the immobilization occurs by simply contacting
the support with an aqueous suspension of microbial cells to be
i~mobilized thereon. There is a natural attraction between the
microbial cell walls ancl the carrier as produced.
EXAMPLE
This example illustrates the preparation of an inventive
catalyst support.
300 lbs. of flux-calcined diatomite tCELITE HYFLO SUPER
CEL~ from ~anville Product Corporation) were mixed with 100 lbs. of
bentonite clay, 100 lbs. of cellulose fiber, and 45 gallons of water
and mixed thoroughly to produce an extrudable consistency. The
mixture was then fed to a screw extruder with 0.13 inch holes in the
die plate. The extruded rod was cut at 0.13 inch intervals into
pellets. These pellets were then dried in a 350F oven for about 20
minutes and were thereafter calcined in a 1,450~F rotary kiln for
approximately 20 minutes. The resulting calcined pellets were
screened to obtain a uniform size. The physical properties of the
pellets, useful as a support for immobilizing microbial cells, was
as follows: mean pore diameter, 2.0 microns; surface area, /l.0
m /g; pore volume, 0.9 cm /g; and crush strengh, 1.5 kg.
Reasonable modifications and variations are possible from
the foregoing without departing from the spirit or scope of the
present invention.
