WO2003042287A2

WO2003042287A2 - Porus materials based on template-forming microorganisms

Info

Publication number: WO2003042287A2
Application number: PCT/EP2002/012628
Authority: WO
Inventors: Stephan Andreas Schunk
Original assignee: Hte Aktiengesellschaft The High Throughput Experimentation Company
Priority date: 2001-11-12
Filing date: 2002-11-12
Publication date: 2003-05-22
Also published as: AU2002363761A1; DE10155469A1; WO2003042287A3

Abstract

The present invention relates to a process for producing a porous material from a mixture comprising at least one microorganism, at least one nutrient and at least one skeleton-forming substance, characterized in that the microorganisms act, in conjunction with the nutrient, in a combined growth and drying process as template-former for the skeleton-forming substance. The present invention further relates to a material produced by this process wHich is distinguished, in particular, by a consecutive or complex, that is to say not simple or random, distribution of pore sizes.

Description

Porous materials based on template-forming microorganisms

The present invention relates to a process for producing a monolithic or powderlike porous material from a mixture comprising at least one microorganism, at least one nutrient and at least one skeleton-forming substance, characterized in that the microorganisms act in combination with the nutrient, in a combined, fermentation, growth and drying process, as template-formers for the skeleton-forming substance. The present invention further relates to a material prepared by this process which is distinguished, in particular, by a consecutive or complex, that is to say not a simple or random, distribution of pore sizes.

Porous materials are characterized, inter alia, by their pore size, pore size distribution, wall thickness and pore volume (porosity). Regarding pore size, generally a distinction is made between microporous materials, mesoporous materials and macroporous materials. The terms "microporous", "mesoporous" and "macropo- rous" are used in the context of the present invention as they are defined in Pure Appl. Chem., 45, p. 79 (1976), that is to say as pores whose diameters are greater than 50 nm (macroporous) or are between 2 nm and 50 nm (mesoporous) or are less than 2 nm (microporous). The pore size distribution can be, inter alia, narrow or broad, unimodal (one prevailing diameter) or bimodal (two co-existing diameters). A system of potentially particular interest, although not achieved by the current prior art, is a consecutively built up pore system in which the pore diameter becomes successively smaller or greater from the exterior to the interior of the porous material. Other complex pore structures, for example pore systems which become greater or smaller periodically from the interior to the exterior of the porous material are also conceivable. In pore systems which are arranged in a complex manner, consecutively or in other ways, the transport properties within the materials, in particular the diffusion properties, are affected advantageously. At the same time, the porous shaped body attains maximum stability. Wall thickness and pore volume are essential parameters which, in their interaction, establish the mechanical stability and free surface area of the porous material. Ideally, in a synthetically produced porous material the abovementioned parameters, and other parameters of importance for the specific application, can be set in a controlled manner and also varied in as broad a range as possible.

According to the prior art, porous inorganic materials are produced essentially by "foaming" the desired skeleton-forming material, for example a silicaceous composition, on or in a template. The differing processes for producing porous materi- als are essentially delimited from one another by varying the template.

Conventional processes for producing porous inorganic materials, in particular silica gels, make use of surface-active substances, for example ionic surfactants, as structure-forming template formers and use possibly inert oils as swelling agents (see, for example, US 5 057 296). However, these processes are associated with some disadvantages, for example the inherently low mechanical strength necessitated by the high pore volume, the deficient structural control during the process, due to phase transformations, and also due to the fact that a shaped body is not obtained, but only a fine powder.

An important further development is represented by sol-gel processes (see, for example, US 6 228 340), in particular those in which low-molecular-weight, non- ionic amphiphilic surface-active agents are used, as are described, for example, in Attard et al, Nature 378 (1995). The template is formed in these processes from the droplets of an emulsion and the ceramic composition is deposited on the droplets in a sol-gel process. The porous material is finally obtained by drying, ageing and calcining (burning). By means of this process, monolithic porous shaped bodies may also be produced, but only limited pore diameters are accessible. In particular, macropores are not obtainable. Expanded clays and expanded rocks (Poroton or Trolit materials, see, inter alia, EP 0 417 583 Bl) are produced by a further pore-forming process in which one or more components, which can break down thermally resulting in the formation of an expanding gaseous component are added to a ceramic composition with binding capacity. During the thermal curing of the composition with binding capacity, which takes place at comparatively high temperatures the pore formation is achieved via the decomposition of, for example, polymers or inorganic or organic peroxides. Typically the added component decomposes either before curing the mixture (in the case of Trolit materials) or when the melting or softening point of the composition is reached (in the case of Poroton materials). With the materials described, macroporous systems are obtained, which, however, generally do not have sufficient micropore or mesopore volumes for uses as catalysts or sorbents.

A further related processing route is taken in the production of foam ceramics. Here a foamed polymer forms the template for a ceramic composition. A significant disadvantage of this process is the poor environmental tolerance, due to the calcination, combined with the burning of polymers. In particular when polyure- thane foam is used, firstly the relatively high material cost must be taken into account and, secondly, the fact that during the process compounds with cyano-groups and nitrous gases are formed. In addition, this process is also significantly limited with respect to the variety of pore sizes and pore size distributions which can be obtained. In particular, microporous or mesoporous fractions are not obtainable.

Porous materials of particular interest, but also having particularly great require- ments with respect to the properties of the pore system, are zeolites, in particular zeolites used for catalytic processes. A disadvantage in industrial zeolite production according to the current prior art is the fact that amines must frequently be used as template formers. During the thermal decomposition as part of the detem- plating of the pore system, nitrous gases are produced. Said processes therefore are not environmentally acceptable. In addition, the zeolites produced by the prior art are frequently of no use in industrial processes as such, since they occur as powders and not as shaped bodies. These zeolite powders must therefore be trans- formed into shapeable compositions, e.g. by adding binders. After that, the resulting mixture is extruded to form shaped bodies.

A further disadvantage of the zeolites produced by these processes is that complex pore structures, for example hierarchically structured pores, which are desirable for many applications, cannot be obtained intrinsically during pore formation, but instead must be formed by means of secondary pore structures using pore formers, for example carbonaceous materials.

The prior art further describes general processes which involve the use of microorganisms, comprising bacteria and fungi, in porous materials, comprising catalysts. In most cases the prior art, however, merely describes the immobilization of microorganisms, for example yeasts, on a porous support, for example silicates or resins, which have been foamed using polyurethane or other templates not based on microorganisms. Chia et al., describe, for example, introducing live yeast cells into a silicate matrix (Chia et al, J Am. Chem. Soc, 122, 6488 (2000)). The purpose of these studies was to fix living cells in a porous matrix and not to produce porous materials using living cells. Zhang et al. describe the use of bacteria as surface templates (Zhang et al., Chem. Commun., 781 (2000)). Fibres of bacterial ori- gin are used in this work to induce crystal growth on the fibre surface. However, no indication is given of the use of microbial growth within the meaning of the present invention, that is to say for producing a porous shaped body. Furthermore, adding microorganisms, for example photosynthetic bacteria or yeast, to concrete with the purpose of producing biologically active cement ("bioconcrete"), is de- scribed in the literature (see JP 8169745). In this case also, the microorganisms, however, are only added as an admixture and do not represent the actual template former for producing the pore structure.

The co-use of suitable microorganisms, in particular yeast, and their fermentation ability and in particular for metabolically-induced release of gases which can lead to pore formation is described in WO 94/17137. However, this publication limits itself to the production of spongy materials from hydrocolloids by foaming gels. Herein, the microorganisms are additives but not the main template formers. In addition, this process, as are all other processes of the prior art which comprise the use of microorganisms, is limited by the fact that there is no disclosure as to how, using the processes described, macroporous structures or systems having hierarchically ordered pore structures can be produced. In addition, the use of renewable resources, cheap and non-sensitive materials or even waste products is not described in the prior art.

In summary, it may be stated that there is a multiplicity of processes for producing porous materials including the production of porous inorganic materials, but that all of these processes are limited by at least one of the following disadvantages: (i) only a fine powder can be produced, but not a porous monolithic shaped body; (ii) only one type of pores of a defined diameter is obtainable, for example micro- pores, mesopores or macropores, and the pore diameter may not be set by process parameters; (iii) even if the pore diameter can be controlled and unimodal or bi- modal pore diameter distributions can be set, it is not possible, however, to set complex pore structures, for example hierarchically ordered pore structures, intrinsically, that is to say during pore formation on the template; (iv) the materials used as template are frequently either expensive or else must be classified as of concern toxicologically or with respect to their biodegradability, in particular in connection with uses in the food industry, the pharmaceutical industry, biotechnology and medical technology.

It is an object of the present invention, therefore, to develop a process which makes it possible, starting from inexpensive base substances which virtually arise as waste materials and at the same time are toxicologically safe and biodegradable, to produce porous, preferably inorganic, materials which, in particular, but not exclusively, are characterized in that micropores, mesopores and macropores can be produced continuously and specifically by simple variation of process parameters. In addition the present invention makes it possible to produce pore structures having clearly different pore diameters, for example a structure in which micro- pores and macropores are present simultaneously in the same shaped body. The invention also makes it possible to produce coexisting micro- and macroporous structures in situ, in particular also hierarchically ordered pore structures, in the primary pore-formation process, that is to say pore systems in which the pore diameter becomes successively smaller or larger from the exterior to the interior of the material.

The inventive object is achieved by a process suitable for producing a porous material from a mixture comprising at least one microorganism, at least one nutrient and at least one skeleton-forming substance and characterized in that the microorganisms act in combination with the nutrient in a combined growth and drying process as template formers for the skeleton-forming substance. The addition of an additional template former which can be known from the prior art, and other additives, is also conceivable. The process for producing this material class is, in par- ticular, but not exclusively, characterized in that compared with the prior art, consecutive or complex pore structures can be produced in situ, in that the pore size can be varied, and in that pores of all sizes and, in particular the macropores which are obtainable by few other processes are accessible, and in that finally micropores and macropores can also be present simultaneously in the shaped body as obtained in the end, e.g. by means of calcination. In addition, the process is characterized in that it can be carried out at temperatures which, compared with the prior art, are significantly reduced, and in that it can be effected in an environmentally sustainable manner.

Skeleton-forming materials in the context of the present invention are taken to mean all substances which, in interaction with template-forming substances and, optionally with further substances, and after a drying, fermentation and/or calcination phase, are able to form a porous shaped body.

The skeleton-forming material can be in principle any material having the above defined properties which does not significantly inhibit the function of the at least one microorganism. The skeleton-forming material may be selected from the group consisting of inorganic, organic or organometallic substances, or a mixture of two or more thereof. Preferably, a ceramic slurry (ceramic casting composition, slip) is used which can comprise, for example, silicates, aluminosilicates, alumino- phosphates, mixed oxides of main group and subgroup elements and, in particular, the subgroup elements, alkali metal oxides or alkaline earth metal oxides and other glass-forming oxides or mixtures thereof, with mixtures of Al₂O₃ and SiO being particularly preferably used. The slurry is preferably produced by mixing the ceramic solids, which are preferably present as powder, with water and possibly, but not necessarily, with surfactants or other surface-active agents, as a result of which a colloidal dispersion can be obtained.

In addition, the introduction of fibrous or sheet-like constituents can be advantageous, and it is conceivable that the skeleton-forming materials are introduced as precursors which result in the actual skeleton-forming agent under processing conditions. As an example, the addition of TEOS (tetraethylorthosilicate) may be mentioned and its hydrolysis to silicic acid under conditions which lead to condensation. In a preferred embodiment, in addition, microporous solids such as zeolites or other porous substances can be added as constituent or main component to the skeleton-forming substance. This embodiment permits, in a particularly simple manner, a composite material to be produced having simultaneously present mi- cropores (from the zeolite or other added microporous substance) and macropores (as a result of the pore formation induced by the fermentation and drying process of the microorganisms in contact with nutrients).

In principle, it is also conceivable to use skeleton-forming substances which are not of inorganic nature, for example lipids, amphiphiles, colloids or polymers and mixtures of these and other substances, or mixtures of these or one or more of the abovementioned substances. Microorganisms in the context of the present invention are taken to mean all biological units including, but not restricted to, unicellular or multicellular and pro- karyotic or eukaryotic forms which are able to convert a nutrient with release of metabolic products which are gaseous or expand in other ways, that is to say are pore-forming. With respect to the biological functionality, no further restrictions are made. Preferably, bacteria or fungi are used. Bacteria which are known from biotechnology and are thus also relevant for the present invention are preferably, but not exclusively, selected from the following group: Pseudomonas, Acetobacter, Methylomonas, Escherichia, Aerobacter, Lactobacillus or Mycobacterium.

Fungi which are known from biotechnology and thus relevant for the present invention are preferably, but not exclusively, selected from the following group of classic subjects of the fermentation industry: representatives of the genera Penicil- lium, Aspergillus or Cephalosporium. Particular preference is given in the context of the invention to yeast fungi (from the class of Endomycetales) and here, further preference is given to culture yeasts, such as baker's or brewer's yeast (Saccharo- myces cerevisiae). The capacity or specific suitability of the yeast for achieving a desired amount of released metabolic products can be improved in a targeted manner by selection or genetic techniques. Algae are generally considered as colour- bearing fungi and are also conceivable as microorganisms to be used. Finally, the use of polypeptides or enzymes is also conceivable, in which case these microorganisms as defined in the context of the present invention are to participate in at least one reaction resulting in the release of potentially pore-forming templates.

Nutrients in the context of the present invention are to be taken to mean all substances which can be metabolized by the microorganisms and lead in at least any one step of the process to gaseous or otherwise expanding and thus pore-forming metabolic products. The nutrients can be added in solid, liquid or gaseous form, with liquid nutrient solutions being particularly preferred since they beneficially affect the miscibility, the nutrient uptake and the flowability of the composition overall. Setting an isotonic nutrient solution or a solution otherwise adapted to the microorganisms is particularly preferred in this case. The specific selection of the nutrients obviously depends on the microorganisms used and all of the combinations previously known from biotechnology are preferably used. Microorganisms can be cultured on an industrial scale by all processes known to those skilled in the art, taking into account environmental factors such as temperature, oxygen requirement, light etc. in a genus-specific manner. Bacteria typically require a carbon source and nitrogen source and, in addition, minerals, trace elements and frequently vitamins and other growth factors. All of these requirements are met by the traditional nutrient medium of bacteriology, nutrient broth, which is also commercially available in dried form as a standardized ready to use medium. Of particular interest also, are bacteria which metabolize hydrocarbons and/or other organic molecules, for example bacteria of the Pseudomonas or Mycobacteriaceae type.

A particularly preferred embodiment is the combination of yeast cultures with carbohydrates, where the at least one carbohydrate can be selected from the group consisting of the monosaccharides, disaccharides, oligosaccharides or polysaccha- rides. Particularly preferably, a mixture of glucose, maltose and starch is used, where the ratios (e.g. yeast to saccharide) are determined by the desired growth conditions and thus ultimately the desired parameters for the pore size and the pore size distribution.

For fungi in general and yeasts in particular, natural nutrients, which can also comprise waste products, can also be used. Such natural nutrients for fungi/yeasts can, inter alia, be selected from the following group, without being restricted to this group: maize meal extract, malt extract, yeast extract, peptone, commercial sucrose, impure glucose, wheat hydrolysates, pomace, residues from fruit processing, molasses, syrups or excrement from plant-eating animals (for instance horse manure). These natural media may be prepared without great consumption of time, even occur in part as waste products of other industrial sectors and already contain all of the necessary minerals and trace elements, preferably in the suitable ratios. Thus, the addition of natural nutrients to the microorganisms as used in the context of the invention is a particularly preferred embodiment.

To proceed from the nutrient to the preferred embodiment of a nutrient solution, addition of a liquid medium is necessary. In a particularly preferred embodiment, water is used, in which case an isotonic aqueous solution which is adapted to the desired growth conditions of the microorganisms is further preferred. Alcoholic solutions or solutions which comprise other organic solvents or aqueous-organic emulsions are likewise conceivable.

In addition to the skeleton-forming material, the microorganisms and the nutrient solution or the nutrient, other additives can also be added to the mixture, in particular surfactants, stabilizers and additional template-formers which can already be known from the prior art. These additives can be taken, individually or as a mixture of two or more thereof, from the following group: surfactants, that is to say surface-active substances quite generally and anionic, cationic and nonionic amphiphiles, in particular polyethylene oxide, polyethylene glycol, colloids, lipids, inert oils, polymers, latex particles, liquid crystals or inorganic salts. Additives which beneficially affect, in particular, the growth of microorganisms and/or their stability are also present in this group, in particular amino acids, (poly)peptides, enzymes, coenzymes, vitamins or trace elements.

The term "template former" in the context of the present invention relates to the function of the above specified microorganisms which is, in interaction with the above described nutrients and if appropriate together with other substances, in a fermentation, growth and drying process or a fermentation, growth or drying process, to release metabolic products, particularly preferably gaseous metabolic products, which lead or contribute to pore formation. Therefore, the preferably gaseous metabolic products according to the invention represent the actual template. In the inventive process for producing the porous material, the components may be added in any sequence. In a preferred embodiment, the skeleton-forming substance is -present as inorganic dispersions and the nutrients are added as nutrient solution which already contains the microorganisms. This ensures that the material is present before drying/calcination as a fluid composition which, if desired, can be incorporated into shape-giving bodies.

As soon as the composition has the desired consistency and shape, drying of the composition can be started. In contrast to the prior art which usually requires dry- ing at high temperatures, e.g. in the processes of firing or calcination, in the present invention, the simple and cost-effective step of drying of the mixture in the temperature range or just above the temperature range in which the metabolic (fermentation) processes of the microorganisms as used proceed, is sufficient. In a preferred embodiment, wherein yeast is used, the drying can take place, for exam- pie, in the temperature range from 40°C to 100°C. The fact that drying can be carried out at the low temperatures specified does not exclude that an additional firing or calcination step is additionally and subsequently carried out (see Example 2).

For the particularly preferred embodiment using yeast, the preferred temperature range from 5°C to 80°C is sufficient, that is to say the range in which the growth of yeast, the production of gases and the removal of water (and thus the completion of the fermentation and solidification process) can be controlled precisely. At low temperatures, that is to say from about 5°C to about 10°C, although the yeast is active, that is to say gases are produced, drying of the shaped body is retarded. In a medium temperature range, which depending on the yeast genus is between 10°C and 30°C, the yeast is completely activated and growth and thus gas formation, which is responsible for the formation of pores (template effect), is greatest. In this temperature range the control of the porosity and other relevant properties of the composition to be dried can be difficult. In a preferred embodiment, a temperature range of from 30°C to 80°C is employed in which the yeast is optimally activated, that is to say does not grow uncontrollably, or in a particularly preferred manner, grows in accordance with the chosen pore size, and at the same time sufficient water is removed from the system so that the composition can cure to form a porous shaped body.

It is conceivable to carry out the process at temperatures below 5°C, even though the yeast in this case is essentially inactive. It is also conceivable to set the process temperature higher or significantly higher than 80°C, for example, but not re- stricted to, 150°C. This can be advantageous, for example, if rapid curing is desired. Depending on the heat capacity of the above described mixture to be cured, and by temperature gradients, the yeast can still be active even under these conditions, for example in the interior of the mixture, that is to say can release pore- forming gases. At the same time, the exterior of the mixture which is in contact with the furnace wall can cure already and the yeast in this range can be inactive, that is no longer contribute to pore formation. Such a process procedure is therefore a preferred embodiment which permits the pore size distribution to be varied spatially within the monolithic shaped body.

In a further preferred embodiment, the mixture is placed in a tubular furnace having a defined temperature gradient and thus the growth of the microorganisms, and here in particular the yeast, by setting a desired temperature profile is controlled in space and time, or space or time. With the aid of this inventive embodiment, by controlling pore formation in space and/or time, spatially separated different pore structures and pore sizes can coexist in a coherent shaped body.

In addition to the composition of the mixture and the temperature, the oxygen content also proved to be a particularly important process parameter. The growth of microorganisms generally and yeast in particular can be set in a targeted manner by adjusting the oxygen content. Thus, for example, carbohydrates are fermented to ethanol in yeast cultures under anaerobic conditions and the yeast growth virtu- ally stops. Under aerobic conditions, in contrast, and at a high glucose content, the yeast grows continuously and no alcohol is produced. Thus, by choosing the oxygen content the yeast growth in the composition can be set exactly and thus also the amount and distribution of CO₂ which then leads during drying/fermentation to pore formation. In general, any oxygen content or gas composition is conceivable at which the yeast can fulfil its inventive function. In a preferred embodiment, the oxygen content is set in the range of, in each case inclusively, from 10 to 20% by volume of oxygen.

A further essential parameter for controlling the growth of the microorganisms, in particular of yeast, is pressure, which can be set from outside. Thus, for example, the growth or fermentation process can be carried out at constant volume, or in a gas-tight vessel which is set to a pressure which can be set externally via a pressure mediator. In general, the growth of, for example yeast, is retarded under an ele- vated external pressure.

In addition, the growth of microorganisms can also be retarded, interrupted or terminated by adding growth inhibitors and/or metabolic poisons. Such growth inhibitors can be selected from the group consisting of ammonia, ammonium ions, sulphides or alcohols.

The inventive process for producing the porous material is, in a preferred embodiment, characterized in that a targeted and controlled variation of pore size and other important characteristic features of the porous material such as wall thickness or pore size distribution is possible as a function of one or more parameters which can be set in space or time, or in space and time. These parameters are controlled either during the fermentation and drying process or during the fermentation or the drying process.

The at least one parameter can be selected from the following group of factors which affect the growth of the microorganisms and/or the curing of the composi- tion to give a porous shaped body, without however being restricted to this group: temperature, temperature gradient, pressure, pressure gradient, oxygen content, type and amount of microorganisms, type, amount and state of the nutrients used, type, amount and state of any metabolic poisons and/or growth inhibitors possibly used, duration of the drying interval at a given temperature and/or given oxygen content, and type and amount of the optionally added additives.

In principle the process according to the invention can optionally be extended by any desired further mechanical, chemical, physical or physicochemical treatment. These optional treatment steps can either be part of the preparation of the materials used or process steps, or take place during or after at least one of the process steps mentioned above (combining the components, fermentation or growth process, drying process, detemplating/calcining).

The term "consecutive pore structure", as it is used according to the invention, denotes a pore structure in which the pore diameter becomes greater or smaller from the interior to the exterior of the shaped body continuously or in steps, but preferably in one direction. The term "complex pore structure", as used according to the invention, denotes a pore structure in which the pore diameter varies within the material in a non-simple manner, that is to say, for example, different, but well- defined, pore diameters coexist, or the pore diameter changes periodically from the interior to the exterior of the resulting shaped body. A complex pore structure is explicitly differentiated from a random pore structure in which, although different pore diameters can also be present, these have not been obtained by targeted proc- ess control and thus also cannot be reproduced. A further feature of consecutive or complex pore structures is that although at least two different pore diameters can be present, the distribution of the pore sizes is narrow, that is to say the deviation from a given pore diameter preferably does not exceed 50%.

In a preferred embodiment, the process is controlled in such a manner that successively changing temperatures or oxygen contents or nutrient contents or all con- ceivable combinations in time and/or place of these and other parameters which affect the growth of the microorganisms and/or the curing of the composition to give a porous shaped body are selected. By successively adjusting process parameters, a consecutive arrangement of different pore sizes, wall thicknesses or other quantities by which the porous shaped body may be obtained. The inventive setting of hierarchically ordered pore structures affects, in particular, the transport properties of the media to be used in the porous shaped body in a beneficial manner (thus, for example, in processes of absorption, catalysis, separation or comparable processes having an underlying pore or channel system). In addition, the po- rous shaped body, compared with the prior art, achieves a stability maximum, in particular in the case of macroporous systems.

In a further preferred embodiment, the inventive process is controlled in such a manner that micropores, mesopores or macropores are present simultaneously in the complete shaped body. Such a complex pore structure can be achieved, for example and particularly simply by the inventively used skeleton-forming substance already comprising intrinsically microporous substances, for example MS- 41 materials or zeolites.

The inventive process is subjected to open-loop/closed-loop control by at least one data processing system, in which case the desired process parameters can be input into the data processing system, and the data processing system controls and monitors the process according to these preset quantities and can read and store essential control parameters.

Of particular interest in the production of porous shaped bodies using a template is also the removal of any interfering residues of the template former and/or its waste products after the complete curing of the skeleton-forming substances of the shaped body. A further essential advantage of the present invention compared with the prior art is the fact that the materials for producing the shaped body, in particular the template-forming microorganisms and their nutrients, are completely biodegradable, and that they are edible and are completely safe with respect to toxicology for humans and the environment. If complete removal of all substances not belonging to the skeleton of the porous shaped body is desired, that is to say in particular the removal of microorganisms, nutrients and metabolic products, this can be performed by simple extraction in a suitable solvent bath, in particular again in an environmentally acceptable aqueous solution, or by firing under mild conditions, which in the case of the present process can proceed without reselasing toxic combustion products such as the unwanted by-products released during the calcination of PU foams.

The inventive process for producing a porous material is, in particular, also characterized in that, compared with the processes which result from the prior art, the inventive process is particularly simple and cheap. This, in addition to other factors, is also caused by the use of cheap and insensitive starting materials some- times even comprising waste products of other industrial processes, but at least consisting of renewable raw materials. Thus, for example, the inventive process, in a preferred embodiment, can also be carried out using an inorganic mixture which can arise or be produced as waste in the building industry, and using yeast fungi which can be taken from the baking or brewing industry and again using nutrient solutions such as maize or malt extracts which in turn can be produced or arise as waste in the brewing industry or in the agriculture sector, for example the sugar industry. The present invention is markedly superior, in particular under this aspect, to customary processes for producing porous materials which use, for example, template formers such as polyurethane foams which are expensive, sensitive, subject to regulation, poorly biodegradable and toxicologically hazardous.

An essential step of the inventive process is drying and curing which, compared with the prior art, takes place under drastically lower temperatures. This makes further simplification of the process possible, since the same complex protective, insulating, monitoring or control measures are not necessary as are required at high temperatures, in particular at temperatures above 500°C. The porous material produced by the inventive process can be used for all purposes for which those skilled in the art would use porous materials in general. This relates, in particular, to the sectors described below, without being restricted to these applications.

Porous materials, in particular porous inorganic materials, are of particular importance as support materials for catalysts or are, for example in the case of zeolites, themselves catalytically active. The inventive porous materials are distinguished for these applications, in particular, by means of the fact that consecutive pore structures or pore structures made up in other complex manners can be produced by the inventive process. Such complex or consecutive pore structures can advantageously serve to increase the selectivity and/or to improve the transport in and out of reaction starting materials, reaction products and substances relevant in other ways.

Porous shaped bodies are, furthermore, frequently used as absorbent or ion- exchanging media, in particular in the fields of chromatography, descaling/water softening or for various embodiments of ion exchangers. Here also, in turn, one of the characteristic features of the inventive material, that is to say consecutive or complex pore structure, becomes advantageously noticeable, since it is conceivable that in this case the co-absorption of molecules or atoms or ions of different sizes becomes possible. This also includes the possibility of synthesizing a considerably more comprehensive chelating agent which is able to complex harmful or other- wise unwanted metal ions of different sizes.

In addition, porous (inorganic) shaped bodies are used in the construction material industry, in particular as actual construction material, but also as insulating material and as fillers in the plastics industry or automotive industry. The consecutive pore structure of the inventive material not only has the advantage owing to the increased wall thickness of being more stable, that is to say needing less material for the same stability, but is also characterized by improved insulation properties based on the fact that the successive combination of different conductance values (here heat flux or longitudinal pressure wave transport due to differently sized pores) always leads to a drastically increased resistance, that is to say to improved heat insulation and/or sound insulation.

The advantages of the material of complex or consecutively structured porosity are obviously also applicable to uses for ceramic high technology materials, for example in the areas of high-performance dielectrics, magnetic materials, magnetooptic materials, nonlinear optic materials or high-temperature superconductors.

In the food industry, porous support materials are of particular interest, in particular when the pore size can be set specifically, to take up desired nutrients and, in particular, when macropores are available. The inventive production process for such support materials in the food industry is characterized in particular, by the use of food-compatible constituents, and furthermore by the fact that the porous support material can be produced at temperatures which do not lead to the destruction of any additional components also present, such as vitamins or other sensitive substances. The statements made with respect to the use of the inventively produced porous material in the food industry also apply correspondingly to the animal feed industry and the food and feed supplement industry.

Specifically for the food industry, it may be added that the inventively produced porous materials are also suitable, in particular, for low-calorie or calorie-free products due to the (i) minimal caloric nutritional value by using inorganic skeleton substances, that is to say not utilizable by humans, and (ii) by the cfood- compatibility of the components and (iii) by a pleasant eating experience caused by the crispy bread-like consistency.

A further particularly highly promising area of application of the inventively produced porous materials is materials for use in up-taking and absorption of chemi- cals, but also, for example, in the sector of animal waste absorption. In chemical applications this is the safe absorption of potentially hazardous vapours and/or liquids, as can occur, for example, in accidents or unintentional spillings. Here, the particularly complex or consecutive pore structure of the inventive materials is advantageously noticeable, since in the presence of different pore sizes, different materials can be absorbed. Similar advantages may also be formulated for materials for the up-taking of animal wastes, for example cat litter.

A further area of application for the inventively produced porous materials is the cosmetics industry, in which porous, spongy substances can frequently be used as support materials for peeling, makeup or other products, but in particular for products having an absorbent or active-compound-releasing function.

Particularly topical and certainly not yet completely exhausted areas of application of porous inorganic materials are, for example, in the pharmaceutical industry and in medical technology. An important problem in application of active ingredient in a complex medically relevant body, for example the human body, is the targeted and/or time-delayed release of an active compound in the intended organ. The more targeted the release, the lower is usually the necessary dose and thus the risk of unwanted side effects. It is conceivable that pore size and pore structure of the porous support material containing the active compound can be set in such a manner, for example by using a consecutive pore system, that the inventive porous material is particularly suitable for the absorption and targeted and/or time-delayed release of a given active compound.

In medical technology, porous shaped bodies are of importance, in particular, for the production of artificial bone implants. Obviously, the inventive materials, having their stability improved with respect to the prior art, owing to the greater wall thickness given by the consecutive pore structure, have a clear market advan- tage. To what extent the production of the porous substance by clearly nontoxic processes, using microbiologically active substances is advantageous, will have to be proven in clinical trials.

Exemplary embodiments:

Example 1

7.5 g of yeast (baker's yeast), 1.95 g of Aerosil (A200, Degussa) and 1.5 ml of Ludox AS 40 are added to 25 ml of a maltose solution (50 g/1) and the mixture is stirred for lh at 20°C. The resultant dispersion is poured into a plastic dish and dried at 80°C. A solid porous monolithic body is obtained which follows the shape of the dish. Study under a scanning electron microscope shows the macropores of the material (Figure 1 : length of the reference line: 20 μm; Figure 2: length of the reference line: 2 mm; Figure 3: length of the reference line: 5 μm).

Example 2

7.5 g of yeast (baker's yeast), 1.925 g of Aerosil (A200, Degussa) and 1 ml of Ludox AS 40 are added to 25 ml of a maltose solution (50 g/1) and the mixture is stirred for lh at 20°C. The resultant dispersion is poured into a plastic dish and dried at 80°C. A solid porous monolithic body is obtained which follows the shape of the dish. The body is calcined for 8h at 500°C. The body retains its strength and shape. Study under a scanning electron microscope shows the macropores of the material (Figure 4: length of the reference line: 2 mm; Figure 5: length of the reference line: 200 μm).

Claims

1. Process for producing a porous material comprising contacting at least one microorganism with at least one nutrient and at least one skeleton-forming substance in such a manner that the microorganisms act in combination with the nutrient in a combined growth, fermentation and drying process as template-former for the skeleton-forming substance, wherein the skeleton- forming substance is selected from inorganic, organic or organometallic substances, or a mixture of two or more thereof.

2. Process according to Claim 1, characterized in that the at least one microorganism is selected from the group consisting of algae, fungi, bacteria or enzymes.

3. Process according to Claim 1 or 2, characterized in that the at least one microorganism is selected from the group consisting of the cultured yeasts.

4. Process according to one of the preceding claims, characterized in that the at least one nutrient is selected from the group consisting of carbohydrates or natural nutrient mixtures used in biotechnology and agriculture, or the group of waste products.

5. Process according to one of the preceding claims, characterized in that the at least one nutrient comprises carbohydrates and that yeast is the microor- ganism.

6. Process according to one of the preceding claims, characterized in that the at least one nutrient is present together with the microorganisms in an iso- tonic solution or nutrient solution which is otherwise compatible with the microorganisms.

7. Process according to one of the preceding claims, characterized in that the skeleton-forming substance is used as apart of an inorganic composition.

Process according to one of the preceding claims, characterized in that the at least one constituent of the skeleton-forming substance is selected from the group consisting of silicates, aluminosilicates, aluminophosphates, mixed oxides of the main group and subgroup elements, glass-forming oxides or of mixtures of the abovementioned substances.

9. Process according to one of the preceding claims, characterized in that the essential parameters of the fermentation, growth and drying process or of the fermentation, growth or drying process are set individually or in combination such that the template- forming efficiency of the inventive mixture is controlled in such a manner that a pre-defined pore size or pore size distribution is achieved.

10. Process according to one of trie preceding claims, characterized in that the essential parameters of the fermentation, growth and drying process or of the fermentation, growth or drying process are set individually or in combination, successively, alternately or in an otherwise complex manner in such a manner that the template-forming efficiency of the inventive mixture is controlled in such a manner that a consecutive or otherwise complex pore structure is achieved.

11. Porous material having consecutive or otherwise complex pore structure, obtainable by means of a process according to Claim 10.

2. Use of the material produced by a process according to one of Claims 1 to 10, or of the material according to Claim 11, in the construction material industry, the automotive and plastics industries, the food and feed industry and the food and feed supplement industry, the pharmaceutical and cosmetics industries, for producing support materials and membranes for chromatography and ion exchangers, as sorbent materials, as catalysts, for water softening and complexing, for ceramic high-technology applications, in particular as dielectrics, magnetic or superconducting materials, and as a particularly efficient insulating material or filler.

13. Use of the material produced by a process according to one of Claims 1 to 10 or of the material according to Claim 11 ,in the pharmaceutical industry, as an active ingredient source, and in medical technology for bone implants.

14. Use of the material produced by a process according to one of Claims 1 to 10 or of the material according to Claim 11 in the production, or for the production, of foods, dietetic foods or substitutes.