Title of Invention	''A PROCESS FOR THE SYNTHESIS GAMMA ALUMINA POWDER''
Abstract	A process for the synthesis of gamma alumina powder comprising the steps of preparing a solution of an aluminum nitrate hydrate compound with aminoacetic acid (glycine), with a molar ratio of aluminum nitrate hydrate to glycine in the range of 0.09 to 0.15.

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FIELD OF THE INVENTION This invention relates to a method for the production of gamma alumina powder. This invention further relates to a method for the production of gamma alumina powder by solution combustion reaction technique. BACKGROUND OF THE INVENTION Aluminum oxide (alumina) is a ceramic material mat finds diverse applications from conventional refractories to advanced electronics to space research, due to its extra-ordinary physical, chemical, mechanical, structural and functional properties. Out of more than fifteen distinct crystallographic modifications of alumina, majority of applications are covered primarily by two modifications, ie. the alpha and gamma forms. The alpha form, for eg. is widely used as a chief raw material for most engineering applications. The superior properties of alpha alumina and its numerous compositions that can be achieved are high mechanical strength, wear resistance, corrosion resistance, capability to withstand high temperature and thermal stresses, high electrical insulation and improved dielectric properties. The gamma alumina, on the other hand, are largely used as catalyst carriers (support), bi-functional carriers (serves as support and also contribute to the catalytic performance), various kinds of fillers, modifiers (for modifying rheological properties), adsorbents, porous filters (membranes for separation) etc. either in monolith form or its pelleted form. Gamma alumina can be tailor made containing a wide range of surface hydroxyls, specific surface area, pore structure etc. and hence is a promising candidate for the above applications. Conventionally, thermal dehydration of aluminum hydroxides (depending on hydroxide modification, ie. bayrite, gibbsite or boehmite) produce different low temperature modifications, eg. chi, eta, rho, gamma (250-900°C) and delta, kappa, theta alumina in the high temperature range (900-1150°C). These intermediate phases of alumina in the above two temperature ranges are manytimes collectively called as 'transition alumina' and industrial production of transition alumina by thermal dehydration of aluminum hydroxides are well known. As the free energy separations of those low temperature forms of alumina are close and thus synthesis conditions plays crucial role when a particular phase in purer form (free from other phases) is attempted to synthesize. As the alpha modification is the thermodynamic phase in the alumina system, all the low temperature phases of alumina ultimately stabilise to alpha phase at higher temperatures. There are numerous examples of producing transition alumina using different wet chemical methods. Among the precipitation routes, precipitation of aluminum hydroxides as bohemite using common sources of aluminum salts like nitrates and sulfates as the mother liquors and liquor ammonia as the precipitating agent is well evident In this process, after the precipitation reaction is over, the precipitates are thoroughly washed in order to remove the adsorbed anionic or molecular species from the precipitates and then the precipitates are calcined in air at a given temperature for promoting first dehydration and then crystallisation into gamma alumina phase or other transition alumina phase/s. Among numerous wet chemical methods, MacKenzie, KJD et al, investigated preparation of transition alumina by thermal dehydration of mechanically activated gibbsite. Gibbsite is also used by Mista W et al, for the preparation of transition alumina by flash calcination method In a sol-gel process, attempts have been made by Varma H K et al, to spray dry the bohemite sols so as to avoid the agglomeration during the gelation step. The spray dried sols were then calcined for the preparation of gamma alumina powder. Spray pyrolysis of aluminum sulfates is also evident for synthesizing gamma alumina powder. Suh D J et al, investigated a fast sol-gel route to prepare high surface area gamma alumina powders via a precursor so-called alumina aerogels. Another technique for the preparation of gamma alumina powders by the hydrolysis of aluminum triisoproxide under the power ultrasound (100 W/cm2 in presence of formic acid or oxalic acid as peptizers, investigated by Ramesh S et al. Synthesis of nanopfaase gamma alumina powder using electron beam heating to vaporize materials in inert or reactive environments has also been tried by Eastman J A et al and named as electron beam evaporation method The relationship between the imperfection structure of gamma alumina that is formed by dehydration of boehmite and the dehydration temperatures is established by Jian-Min H, by analysing the integral width of X-ray reflections. Korean patent publication number 2000040938 describes a production process of gamma alumina powder that comprises several steps, starting from kaolinite, which is reacted with sulphuric acid prior to heating it in the temperature range using 800-900°C and then the resultant solution is filtered to remove impurities which is subsequently purified as hydrous aluminum sulphate by crystallisation process and finally the crystals were heat treated in the temperature 900-1000°C to produce gamma alumina. Chinese patent publication number 1298915 describes the preparation of high-purity gamma alumina nanoparticles mat emits blue light, by the reaction with aluminum alkoxide solution and water vapor, following atomisation, hydrolysis and further calcining the hydrolysed product at 600-700°C. Japanese patent publication number 2000189744 describes the synthesis of both gamma-AlOOH and gamma alumina - described therein as humidity control materials, by heating aluminum hydroxide gels. Chinese patent publication number 1184078 decribes the manufacture of gamma alumina powder from different solutions of aluminum salts like aluminum nitrate, aluminum sulphate, aluminum chloride and sodium meta-aluminate, to produce a gel by adjusting pH and then calcining the gel at a temperature in the range of 450-700°C. Another Chinese patent publication number 1164563 also describes the preparation of gamma alumina from different aluminum salts like aluminum sulphate, aluminum chloride or aluminum nitrate following precipitation reactions in different pH conditions and then filtering, drying and calcining the precipitates at different temperatures to produce gamma alumina. Russian patent publication number 2038304 describes Hie production of a low density gamma alumina by electroerosion dispersion of aluminum in cabronated water and aging of aluminum hydroxide dispersion in ammoniacal solution. Japanese patent publication number 07267632 describes me manufacture of gamma alumina porous bodies by heating kaolin minerals and/or alumina silica gels and then treating with alkalies or HF toelute amorphous silica and thus the formation of porous gamma alumina body. European patent publication number 518106 describes the manufacture and application of partially crystalline, transition-type aluminum oxides and gamma alumina by shock-calcination of fine-acicular boehmite. Chinese patent publication number 1062124 describes the preparation of gamma alumina from aluminum alkoxides by hydrolysis and aging and then calcination of the aged-product US laid-open patent application number 20010055558 describes a gas phase method for obtaining transition alumina particles (hose suitable for polishing. These particles as described are either amorphous or transition alumina with gamma, delta or theta crystalline modifications. However, the process for preparing a metastable phase like gamma alumina following a combustion route has not so for been disclosed. OBJECTS OF THE INVENTION ft is therefore an object of mis invention to propose a method for the production of gamma alumina powder by solution combustion reaction technique. It is a further object of this invention to propose a method for the production of gamma alumina powder by solution combustion reaction technique which is rapid. Further objects and advantages of the invention will be apparent from the ensuing description. At the outset of the description which follows, it is to be understood that the ensuing description only illustrates a particular form of this invention. However, such a particular form is only an exemplary embodiment and the teachings of the invention is not intended to be taken restrictively. BRIEF DESCRIPTION OF THE INVENTION Thus according to this invention is provided a process for the synthesis of gamma alumina powder comprising the steps of preparing a solution of an aluminum nitrate hydrate compound with aminoacetic acid followed by heating said solution to obtain a dry mass; combusting said dry mass to obtain a powder; calcining said powder to obtain gamma alumina powder. In accordance with this invention, the process essentially comprises three steps of mixing, heating and calcination. In the step of mixing, aluminum nitrate hydrate and aminoacetic acid (glycine) are mixed in a ratio of 0.9 to 0.15 and thoroughly stirred by a mechanical stirrer to form an aqueous solution, using preferably deionised water. The quantity of deionised water that is to be taken should be sufficient to dissolve the reactants. The solution needs to be filtered in case appearance of any foreign residue thereby. The solution is then heated to promote dehydration of the solution. Any source of heat can be used in the present invention. It can be a simple electrical heater or an electrical oven. A temperature in the range of 70 to 95 °C of the solution is to be maintained during the dehydration process. The dehydrated solution upon further heating produces a gel and then a dry mass. As soon as the dry mass is formed, the temperature of the dry mass is to be increased to ISO to 300°C and maintained for a little while. The dry mass soon undergoes a combustion reaction (no flame or no fire) from the surface associated with the evolution of gases and with the yield of a voluminous powder. The heating process is continued until the combustion reaction is complete and the combusted powder is to be collected after the combustion reaction is over. The combusted powder is amorphous and voluminous and because of its high surface area, may get contaminated with certain amounts of evolved gases in the course of combustion reaction, following either absorption or adsorption mechanism. In order to obtain the gamma alumina phase from the combusted amorphous powder, a heat treatment at a particular temperature for a given period of soaking time is necessary. In this calcination step, the absorbed gases, if any, are also removed from the combusted powder, besides its crystallisation into gamma alumina phase. In order to obtain the gamma alumina powder with highest specific area, the calcination of the combusted powder is carried out at different temperatures in the range of 500-1200°C in air for a soaking period of, for eg. one hour by keeping the powder in a sintered alumina crucible. The calcination reaction can be carried out in a muffle furnace or any similar furnace by electricity, gas or any other source of fuel/s by firing. After the calcination treatment, the furnace is to be air-cooled and the calcined powder is to be collected and checked in each case for the formation of gamma phase by XRD and the corresponding values of specific surface area by BET method. The invention will now be explained in greater detail with the help of the following non-limiting examples. EXAMPLE 1 In a glass beaker or any similar container, 3.7514 gms of Al(NO3)3. 9H2O (AR grade) and 0.2252 gms of amino acetic acid, CH2NH2COOH (glycine) (AR grade) was taken. 50 cm3 of deionised water was added to it and allowed the whole mixture for stirring until a clear solution was resulted. As mentioned in the previous section, the quantity of deionised water to be added is not so strict - any quantity can be added, however, the quantity should be sufficient to dissolve the reactants. The solution is filtered in case of appearance of any foreign materials or colloidal particles. The filtered solution was then heated at 90°C on a hot plate with constant stirring. The solution turned into a syrupy liquid in 15 min and then to a transparent gel and finally to a dry white mass. As soon as the dry white mass resulted, the temperature of the hot plate was increased to such that temperature of the white dry mass reaches to approximately to 200°C. Within a minute time, the dry mass undergoes a mild combustion reaction from the surface and yielded a white voluminous powder with the evolution of gases. The heating process at 200°C continued for another five minutes so that the combustion reaction of the dry mass was complete. The glass beaker was allowed to cool down under normal cooling and the resultant combusted powder was collected and volume and weight of the corresponding powder was recorded. The combusted powder was then heat treated at different temperatures in air as atmosphere, starting 500°C until 1200°C (an interval of 100°) for a soaking of one hour at the set temperature in an electrical muffle furnace and the powder in each case, was then collected and analyzed by X-ray powder diffractometry by for phase analysis and multi-point BET analysis for measurement of specific surface area. Powders produced at different calcination temperature show different levels of specific surface area and crystallinity. The phase composition of the combusted and all the calcined powders was carried out by X-ray powder diffraction (XRD) in a Siemens D5000 diffractometer using CuK„ radiation. The BET specific surface area of the powders was determined by nitrogen absorption ( Micromeritics, Gemini 2375) at 77K with the BET method. Simultaneous thermogravimetry (TGA) and differential thermal analysis (DTA) of the transparent gel in nitrogen atmosphere was carried out in a Setaram Thermal Analyzer (TG-DTA 92, France) using alpha alumina as a reference material and with a heating rate of 10°/min. Differential scanning calorimetry (DSC) of the combusted powder was carried out in a Setaram DSC with a heating rate 107min. The morphology of calcined powder was examined in a DSM Gemini 982 scanning electron microscope. The crystallinity and crystallite size of the gamma alumina powders were examined by using a Transmission electron microscope (JEM-2000 FX, Japan). The infrared spectra of the combusted and calcined powders in KBr pellets were recorded using a BRUKER IFS infrared spectrophotometer. Table 1 illustrates the typical composition of the precursor that allows complete combustion of aluminum nitrate at a low temperature for the synthesis of gamma alumina powder described in the present invention. Table 2 illustrates decomposition behaviors (peak temperatures of endothermic or exothermic reactions and corresponding mass loss) of the precursor during the combustion reaction, as recorded in the differential thermal analysis (DTA) and corresponding mass loss in the thermogravimetry analysis (TGA) of the precursor. TABLE 1 (Table Removed) TABLE 2 (Table Removed) Example 2 Using the same procedure, apparatus described in Example 1, bulk quantities of combusted powders is obtained by proportionately increasing the batch size of the reactants, in several folds, so as to make a batch size of a few hundred grams to a few kilograms of gamma alumina powder. The combusted powders thus obtained were calcined in air at different set temperatures in the range of 500°C - 1200°C with an interval of 100°C in each case for a soaking period of one hour, so as to optimize the conditions of calcination reaction for obtaining a crystalline gamma alumina phase that is associated with maximum specific surface area. As in the smaller batch size, the calcined powders do not show any appreciable changes with respect to their crystallinity and specific surface area in the BET, when calcined at any temperature in a temperature range of 500°C - 700°C. This behavior as also understood with the fact that the calcined powders (calcined in the temperature range of 500°C - 700°C ) remained in amorphous state or in a state with poor crystallinity in the aforesaid temperature range and hence does not change its specific surface area in the BET analysis. As the calcination temperature crosses beyond a temperature of 700°C, the combusted powder slowly starts crystallizing into gamma alumina phase, which remains in gamma phase until a temperature of 925°C and then converts to alpha alumina phase at a temperature of 1140°C, as shown in the corresponding DTA analysis. So, the calcination reaction of the combusted powder in the temperature range of 750°C - 1200°C is very crucial in this invention, as appearance and disappearance of the gamma alumina phase occurs in this range and hence also the variation of specific surface area of the corresponding calcined powders. Among the calcined powders (those calcined at different temperatures in the aforesaid temperature range), the combusted powder that is calcined at 900°C for a soaking period of one hour gives the highest specific surface area in the BET (83 m2/g) with good crystallinity of gamma alumina phase. The specific surface area of the derived gamma alumina powders slowly increases when the calcination temperature increases from 750°C to any temperature up to 900°C and then decreases as the calcination temperature exceeds 900°C. Actually, the crystallization from amorphous alumina to gamma alumina is completed in the temperature range of 750°C - 900°C and hence the specific surface area of the powder increases and the prominence of the peaks in the corresponding XRD patterns is also reflected and also in the electron diffraction in the TEM experiments which corroborate the crystallization behavior. The specific surface area of the calcined powders those calcined beyond 900°C gradually comes down as the temperature of calcination increases, because of coarsening the gamma alumina particles and further coarsens because of formation of alpha alumina phase beyond a temperature of 1140°C (exothermic peak temperature). Using the same procedure, the quantity of the amino acetic acid in the precursor composition, was slowly increased by keeping the counter aluminum nitrate fixed and different batches of powders were made using different levels of aminoacetic acid content in the precursor. All such precursors (aqueous solution of aluminum nitrate and amino-acetic acid) upon heating at close to 200°C produced a transparent gel and men to a dry mass in the same previous fashion, however combusted violently with fires and flames. The resultant combusted powders are also found to be amorphous, however observed to contaminate with different amounts of carbon and nitrogen and hence the color of the combusted powder was not white. As the quantity of amino acetic acid increases in the precursor composition, the color of the combusted powder becomes light brown to dark brown. As mentioned above, the combustion reaction in such cases becomes and is found to be associated with fires and flames. As a general rule, violent nature of the combustion reaction increases as me quantity of the amino acetic acid in the precursor composition is increased. Also, higher temperature (close to 900°C) and higher length of soaking (at least two hours) is required to get rid of carbon, which gets contaminated in such combusted powders. WE CLAIM: 1. A process for the synthesis of gamma alumina powder comprising the steps of preparing a solution of an aluminum nitrate hydrate compound with aminoacetic acid (glycine), with a molar ratio of aluminum nitrate hydrate to glycine In the range of 0.09 to 0.15, followed by heating said solution to obtain a dry mass; combusting said dry mass to obtain a powder; calcining said powder to obtain gamma alumina powder. 2. A process as claimed in claim 1 wherein the aqueous solution Is formed using deionised water. 3. A process as claimed in claim 1, wherein said aqueous solution is heated at a temperature in the range of 70 to 95°C, to obtain a dry mass. 4. A process as claimed in claim 1, wherein said dry mass is combusted at a temperature in the range of 150 to 300°C, 5. A process as claimed in claim 1 wherein said combusted powder Is calcined at a temperature in the range of 500-1200°C. 6. A process as claimed in claim 1 wherein the step of calcination is carried out with an optional step of soaking, where the soaking period Is about 1 hr. 7. A process for the synthesis of gamma alumina powder substantially as herein described and illustrated.

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864-del-2003-abstract.pdf

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864-del-2003-complete specification (granded).pdf

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