GB2185908A - Process for the preparation of catalyst supports - Google Patents

Abstract

Diatomite or perlite, an inorganic binder, a solvent, and up to 12 wt.% of an organic burnout material are mixed, extruded, pelleted, dried and calcined to form porous catalyst supports comprising diatomite or perlite and an inorganic binder. The resulting catalyst support has a mean pore diameter of 1-25 microns and is very useful for immobilizing microbial cells thereon.

Description

SPECIFICATION Processforthe preparation of catalyst supports and materials produced thereby This invention relates to a processforthe production of catalyst supports. It also relates to the catalyst supports produced by the above process. Additionally, it relates to cata lystsupports containing catalytical- ly active substances such as microbial cells immobilized thereon.

The use ofvarious su bstances 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 thermodyna mically possible or economically practical in many instances, it has become increasingly important to lookforwaysto efficiently utilize and maintain such catalytically active materials. Furthermore, since the cost ofthecatalytically active materials must itself be an active consideration in deciding whetherto commercializea 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. Forexample,theyfind use in industrial applications in the preparation of food such as cheese or bread swell 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, subjectto deactivation, they are difficu it to remove for reuse from solutions in which they are utilized and they may not retain their catalytic activity over extended periods oftime.These difficulties lead to an increased cost in the use of enzymes in commercial scale operations due to the necessity forfrequent replacement ofthe enzyme. In orderto reduce the high cost of enzyme replacement, various methods to immobilize enzymes priorto their use have been devised. This immobilization ofthe 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 ofthe substrate which is being biochemically reacted.

Apartfrom immobilization of enzymesthemselves, various substances and techniques have been put forward by which the enzymes could be immobilized without isolation. In particular, whole cells of microorganisms can be immobilized, thus using the microbial cell as a carrierforthe enzyme and obviating the need for extraction of the enzyme from the cell.

One commonly used support or entrapement mate rialformicrobial cell immobilization isa gel, usually an alginate gel. Essentiallythe cells aretrapped in a three-dimensional polymernetworkwith 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-organismsto proliferate. It is a relatively routine matterto 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 contaminent moulds, bacteria, and the like. Such gels, of course are not reusable either.

Anothertype of material used to immobilize catalytic agents such as enzymes and microbial cells is a porous pellet composed primarily of a high silica contentormixtures of 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 of making the pellet. On the porous surfaces ofthe pellets are deposited small amounts of the catalyticallyactive 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 reactions it isto 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 ofthe 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. Poreswhich are one to five times the size ofthe largest microbial cell typically provide the highest production rates. In microbial cell immobilization, the pore diameters are based upon the major cell dimensions. Living systems require additional care to insure adequate space for cell reproduction.

A big disadvantage with the use of conventional high silica-based catalyst supports is thattheir average pore diameter is too small for accomodating microbial cells. Theirtypical average pore diameter is much less than 1 micron. Typically, diamaters of 1 to 25 microns are needed to accomodate the microbial cells. Of course, when it becomes difficultto immobilize an effective number of microbial cells on a typical silica-based catalyst support, the economic attractive- nessofsuchasupport 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.During the course of such research, it was discovered that an efficient catalyst su pport made by the process of forming a mixture comprising an inorganic binder, an organic burnout material, a solvent, and either expanded perlite, calcined diatomite, orflux-calcined diatomite, followed byforming an extrudate from the above mixture and then drying and calcining the extrudate results in an extremely economical, efficient supportfor 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 ofthe present invention, the average pore diameter is conveniently controlled so that it falls in the range of from 1 -25 microns.This contrasts sharplywiththe more conventional catalyst supports wherein the resulting average pore diameter is much smallerandthereforedifficultto immobilize microbial cells on. Thus our invention results in the production of a catalystsupportwhose average pore diameter is in a range which is neithertoo large ortoo small for efficiently immobilizing microbial cells as in the case of silica-based supports.

Ourcatalystsupportalso does not provide for an environment where a decline in microbial activity occurs and microbial contaminants proliferate as in the case of gels. Furthermore our supports are inert, rigid, and are reusable which greatly enhances their economic attractiveness. Additi onally, our supports are made by an inventive process which employs economical ingredients and is easy to conduct.

Therefore, it is an object ofthe present invention to provide a novel process forthe production of an inorganic catalystsupport especially useful in the immobilization of microbial cells.

It is another object ofthe 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 apparentfrom the specification and the appended claims.

In accordance with one embodimentofthe present invention we have discovered a novel process for the production of an inorganic catalyst support which is especially useful for immobilizing 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 group consisting of calcined diatomite,fluxcalcined diatomite and expanded perlite; (ii) 5-30wt.% inorganic binder; (iii) 0-12wt.% organic burnout material; and (iv) 20-50wt.% solvent; (b) extruding the mixture th rough a die to form an extrudate and then separating the extrudate into a plurality of pellets; (c) drying the pellets at a temperature in the range of about 200"to 5009Ffor about 5-30 minutes; and thereafter (d) calcining the dried pellets at a temperature in the range of about 700 to 2,000 Ffor about 10-45 minutes.

Preferably, the mixture in the inventive process will comprise 30-45 wt.% ofthe diatomite or perlite used, 10--15 wt.% inorganicbinder,0-12wt.% organic burnout material, and 35-50 wt.% solvent.

Diatomite is a chalky sedimentary material composed ofthe 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 ofthe 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 ir currently occurring. Such deposits are presently mined by dredging. Atypical dry diatomite analysis is shown in Table I beiow.

TABLE I Component Wt% Si02(a) 86.0 Air203 3.6 Fe203 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 1 600"F to 2400"F. The calcination 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 ofthe diatomite or by dry mixing of the flux and diatomite in conventional dry mixing devices such as tumblers. Normally there will be from about3to about 10 weight percentflux 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.

Thoseskilled intheartwill bewell awareofthe appropriatequantityoffluxtouseforanyparticular type of flux and diatomite.

A commercially available form of expanded perlite may be used in the present invention. Perlite is a mineral of volcanic origin which generally falls into the rhyolitic class. The uniquefeature of perlite is that it contains several percent water of hydration. lithe perlite is rapidly heated to a temperature on the order of 1600 F (870"C) the water is converted to steam and the perlite "pops", i.e. it rapidly expands two a much lower density. The amount of expansion is usually on the order of 4 to 20timesthe original volume. After 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 of about 5-20 pcf, most preferably about 12--16 pcf.

Whatever perlite ordiatomite 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 ofthe 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, espe ciallythe diatomite or expanded perlite.

Oneclassofinorganicbinderwhich 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 dueto their great purity, have a high fusion point. Kaolin clays are also the most refractoryofall 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.

Other inorganic binders which may be utilized include phosphoric acid based binders such as aluminium phosphate and colloidal suspensions including colloidal silica, colloidal alumina, and colloidal zirconia.

Suitable combinations ofthe above inorganic binders may be utilized. However, clay based binder systems are presently preferred.

Suitable organic burnoutmaterialsfor use inthe 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 extruding.

Any commercially available solvent can be used in the present invention which will causethe 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, aialcoholsq After the mixture ofthesolids and solvent is formed into the extrusion feed, it is extruded in conventional extrusion up eq & mentthrough a dietoform an extrudatefrom which individual pellets may he separated It is frequently desirableto incorporate a lubricant or similar extrusion aid into the mixture to facilitate the extrusion; such material will be burned out ofthe product during the subsequent drying and/orcalcining. Most commonly the extrudate is an elongated rod-like material of circular, oval, our square cross-sections. Circular cross-sections are preferred to minimize attrition ofthe pellets in subsequent handling.Normallythe extruded rod is approximately 0.06to 0.25 inches in width or diameter and preferably approximately 0.11 5to 0.135 inches.

The extruded rod is commonly severed at intervals approximately equal to the diameterorwidth ofthe rod such thatgenerally cylindrical or cubical pellets having approximately equal dimensions in all dimensions are formed. Conventional severing equipment such as wire knives can be used.

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 zone dryer in which the temperature generally ranges between about2000-5000F, preferably between about 2500-4500F. Drying will generally beforabout5-30 minutes, preferably about 1015 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 underthedesired size range.

Thereafterthe dried pellets are calcined orfired in calcining equipment such as a rotary kiln generally at a temperature in the range ofabout 700"-- 2,000"F for about 10-45 minutes, preferably at a temperature in the range of about 1,400 1,800 Fforabout2030 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 ofthe pellets leaving a highly porous composite ofdiatomiteor perlite and inorganic binder.If desired, additional air injection can be made at approximately the mid-point ofthe calcination kiln to enhance the calcination; an airlanceisquitesuitableforsuchairinjection.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 porediameterverysuitableforimmobilizing microbial cells is provided. This inorganic catalyst support is made by the above described inventive process.

The average pore diameter ofthe inventive catalyst supportwill be between about 1 and 25 microns. The resulting catalystsupport has a mean pore diameter which isidealforimmobilizing microbial cells.

Generally, the inventive catalyst supportwill have a surface area in the range ofabout3-20 m2/g, a pore volume in the range of a bout 0.6-1 .2 cc/g, and a crush strength ofabout 1-10 kg.

The catalyst supports ofthe present invention are useful for supporting any suitable catalytic substance.

In particular, the inventive catalyst supports are useful for immobilizing biocatalysts, especially microbial cells. Awide variety of microbial cells to include bacterial and fungus-like microbes can be immobil ized on the support by any method knowntothose skilled in the art. Typically, the immobilization occurs by simply contacting the support with an aqueous suspension of microbrial cel Is to be immobilized thereon. There is a natural attraction between the microbial cell walls and the carrier as produced.

EXAMPLE This example illustrates the preparation of an inventive catalyst support.

300 Ibs. offlux-calcined diatomite (CELITE HYFLO SUPER CELfrom Manville ProductCorporation) were mixed with 100 Ibs. of bentonite clay, 100 Ibs. of cellulosefiber, and 45 gallons of water and mixed thoroughlyto produce an extrudable consistency. The mixture was then fed to a screw extruderwith 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 350 F 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 ofthe pellets, useful as a support for immobilizing microbial cells, was as follows: mean pore diameter, 2.0 microns; surface area, 4.0 m2/g; porevolume, 0.9 cm3/g; and crush strength, kg.

Reasonable modifications and variations are possi byte from the foregoing without departing from the spirit or scope ofthe present invention.

Claims (13)

1. A process for the formation of a catalyst support useful for immobilizing microbial cells comprising the steps of: (a) forming an extrudable mixture comprising: (i) 20-70wt.% ofonematerialselected from the group consisting of calcined diatomite, flux calcined diatomite, and expanded perlite; (ii) 5-30wt.% inorganic binder; (iii) 0-12wt.% organic burnout material; and (iv) 20-50wt.% solvent; (b) extruding the mixture through a die to form an extrudate andthen separating the extrudate into a plurality of pellets; (c) drying the pellets at a temperature in the range of about200"to S000Fforabout5-30 minutes; and thereafter (d) calcining the dried pellets at a temperature in the range of a bout 700" to 2,0000for about 10-45 minutes.

2. A process according to Claim 1 wherein the extrudable mixture in 1 (a) comprises: (a) 3045 wt.% of said material selected from the group consisting of calcined diatomite, flux calcined diatomite, and expanded perlite; (b) 1015wt.% ofsaid inorganic binder; (c) 0-1 2wt.% of said organic burnout material; and (d) 3550wt.% ofsaidsolvent.

3. A process according to Claim 1 wherein said inorganic binder is bentonite clay.

4. A process according to Claim 1 wherein said organic burnout material is cellulose.

5. A process according to Claim 1 wherein said solvent is water.

6. A process according to Claim 1 wherein said drying is conducted at a temperature in the range of 250"to 4500for about 10-15 minutes.

7. A process according to Claim 1 wherein said calcining is conducted at a temperature in the range of 1,400"to 1,8000Fforabout20-30 minutes.

8. A catalyst support made by the process of Claim 1.

9. A catalyst support made by the process of Claim 1 and having an average pore diameter of between about 1 and 25 microns.

10. A catalyst support according to Claim 8 having at least one catalytically active substance thereon.

11. A catalyst support according to Claim 10 wherein said catalytically active substance is a microbial cell.

12. A catalyst support according to Claim 9 having at least one catalytically active substance thereon.

13. A catalyst support according to Claim 12 wherein said catalytically active substance is a microbial cell.