ZIRCONIUM PHOSPHATE AND METHOD FOR ITS PREPARATION

Abstract: The invention relates to a zirconium phosphate in particles having sizes comprised within the range of from 1 to 100 µm, a lamellar structure with interlayer dis­ tance, in the anhydrous state, comprised within the range of from 7.9 to 8.2 Å and a surface area comprised within the range of from 9 to 20 m²/g. The method consists in altering the lamellar struc­ ture of a zirconium phosphate with layer structure of alpha type by intercalating into it an organic substance containing a proton-acceptor group and water and by a treatment with ultrasounds, in regenerating the hydrogen form of said zirconium phosphate by an acid and in wash­ ing the same with diluted acids and/or water.

Description: The present invention relates to a particular type of zirconium phosphate and to the method for its preparation. One among the most widely used and studied ion exchangers belonging to the class of acid salts of tetravalent metals is zirconium phosphate with layer structure of alpha type [A. Clearfield, Inorganic Ion Exchange Materials, C.R.C. Press, Boca Raton (FA) USA, 1982]. Such an ion exchanger has been obtained by two different preparation methods. In the first method, known as the "reflux method", zirconium phosphate is first prepared in the amorphous state, and the product is then crystallized by refluxing it in aqueous solutions of phosphoric acid [ A. Clearfield and J.A. Stynes, J. Inorg. Nucl. Chem. 26 , 117 (1964) ]. The degree of crystallinity increases with the reflux time, and with the reflux time being the same, it increases with increasing concentration of the solution of phosphoric acid. Parallelly to the degree of crystallinity the crystal sizes increase. In order to obtain microcrystals having dimensions of µm order and a good crystallinity degree, reflux times exceeding 100 hours in 10-12 M phosphoric acid are required. In the second method, also known as the "method by direct precipitation in the presence of HF", a solution is first prepared containing a zirconium-(IV) fluorocomplex in phosphoric acid [G. Alberti, E. Torraca, J.Inorg. Nucl. Chem. 30 , 317 (1968)]. The crystalline zirconium phosphate is then precipitated by decomposing the fluorocomplex by slowly evaporating hydrofluoric acid. In this method, the crystallinity degree and the crystal size depend on the speed at which HF has been evaporated off. If evaporation is controlled so that precipitation may occur during a time longer than 24 hours, optimum crystalinity degrees and particle sizes of from a few tens to a few hundreds micrometers are obtained. By slow thermal decomposition of Zr-fluorocomplexes crystals having a size of a few millimeters can be prepared.(G.Alberti, R.Giulietti, J.Inorg.Nucl.Chem., 42 , 1062 (1980)). Independently of the method of preparation, a zirconium phosphate having an alpha-type layered structure has the composition: Zr(HPO₄)₂.H₂O and an interlayer distance of 0,76 nm (7,6 angstrom). The water molecule is lost by heating to 110°C and the interlayer distance drops from 0,76 nm to 0,74 nm (7,6 angstrom to 7,4 angstrom). Such a dehydration is irreversible and the anhydrous Zr phosphate cannot be rehydrated even by soaking it in water. The crystalline structure of alpha-Zr(HPO₄)₂.H₂O is well known at present (A.Clearfield and G.D.Smith, Inorg.Chem., 8 , 431 (1969)). The arrangement of the = P-OH groups is such that cavities are formed in the interlayer region (one cavity per Zr atom) connected by windows through which ions or molecules having a cross-section narrower than, or equal to, that of the potassium ion can pass easily, while larger ions, such as Rb+, Cs+, Ba++ or large organic ions cannot enter due to steric hindrance. EP-A-0 094 919 discloses a method for preparing Zr phosphate comprising the steps of intercalating, into a crystalline Zr phosphate alpha-Zr(HPO₄)₂.H₂O an organic substance containing a proton acceptor group and water, thus obtaining a colloidal aqueous suspension of said intercalated Zr phosphate which is then treated with acids, with the formation of a Zr acid phosphate with modified lamellar structure, the precipitate being separated from the mother liquors, and finally washed with diluted acids and/or water. It has been surprisingly found that a type of Zr phosphate permits the passage of and the intercalation of ions larger than the potassium ion and thus the drawbacks of the prior art can be overcome. This invention, therefore, provides a particulate zirconium acid phosphate of modified structure, characterized in that it has a particle size of from 1 micrometre to 100 micrometres, a lamellar structure with interlayer of distance in the anhydrous state of from 0,79 nm to 0,82 nm (7,9 Angstrom to 8,2 Angstrom) and a surface area of from 9 m²/g to 20m²/g. According to another aspect of the present invention, a process is provided for preparing the particulate zirconium phosphate as defined above by intercalating, into crystalline zirconium phosphate alpha-Zr(HPO₄)₂.H₂O having an interplanar distance of 0,76 nm (7,6 Angstrom), an organic substance containing a proton-acceptor group and water until a stable colloidal aqueous suspension of intercalated zirconium phosphate is prepared, characterized in that it further comprises the steps of: treating the colloidal suspension with ultrasounds; treating the suspension thereafter with a non-zirconium-complexing acid until a precipitate of zirconium acid phosphate with modified lamellar strcture is formed,and recovering the precipitate and washing it with dilute acid and/or water. In a preferred embodiment, the treatment with ultrasounds is continued until such time as the optical density of the suspension remains constant and is thenceforth unaffected by the ultrasounds. It should be observed that the zirconium acid phosphate having modified lammelar structure obtained according to the invention and hereunder indicated also as "delaminated-ZPH", has properties of exchange, intercalation and absorption very different from those of the materials of the prior art on the basis of lamellar zirconium phosphate with layer structure of alpha type. As the organic substance containing proton-acceptor groups, an amine is preferably used and this is preferably selected between methylamine and n-propylamine. In the following, reference is made to this latter for easiness' sake, and the phenomena which can be evidenced during the process of the invention are explained. The crystals of α-Zr(HPO₄)₂.H₂O are dispersed in water and n-propylamine is added to the dispersion under mechanical stirring. For example, the amount of propylamine can be varied within the range of from 25% to 75% (preferably 50%) of the number of gram-atoms of acidic protons present in the α-Zr(HPO₄)₂.H₂O amount used; usually, n-propylamine is added at a slow addition rate, but the addition modalities are not crytical. Also the ratio between the amount of crystals and the amount of water used can be varied within a wide range, from a minimum of 0.05 g per 100 g of water up to a maximum value which is essentially a function of the stirring system used and of its efficiency. In fact, while the amine is being progressively added, the mixture viscosity tends to increase, and the higher the zirconium phosphate / water ratio, the greater such an increase in viscosity. The modification in lamellar structure caused by the intercalating of n-propylamine is such, that a very stable colloidal suspension is even formed. Such a suspension can be obtained also by causing the intercalation to take place in a non-aqueous medium (e.g., in an alcohol, in particular in methanol) and then adding water. The suspension of zirconium phosphate with modified lamellar structure obtained by intercalation in water or by dispersion in water of the intercalated product previously prepared in a non-aqueous medium is used for the treatment with ultrasounds. The regeneration of zirconium phosphate in hydrogen form is carried out by acidifying the colloidal suspension up to a pH value comprised within the range of from 0 to 3, preferably to pH 1. The acidification causes also the flocculation of zirconium acid phosphate, which can be recovered by simple filtering, and washing and drying in air at room temperature or by spray-drying or freeze-drying. Differently from α-Zr(HPO₄)₂.H₂O ( d = 7.6 Å), the product according to the present invention exchanges quickly such large-sized ions as Cs⁺ and Ba²⁺, and is furthermore able to absorb amines, alcohols and organic cations of even large size from aqueous or organic solutions. In general, the product according to the present invention can be used as absorbent and intercalating agent for proton-acceptor organic substances having molecular weights up to 500 Daltons. For example, the absorption of Cs⁺ ion from an 0.1 M CsCl aqueous solution maintained at pH = 3 is of 1.25 meq/g, that of methylene blue from aqueous solution arrives up to 1.85 meq/g, bornylamine and quinine can be accomodated inside the interlayer region, whilst alcohols are even intercalated by the exchanger also by simple cold washing with them. The dry product has also the property of considerably hydrating when it is contacted with water, with formation of a-Zr(HPO₄)₂.nH₂O, the value of n of which depends on the relative humidity under which the sample is conditioned. The hydration causes an increase of the interplanar distance, which can be easily detected by the powder X-ray diffraction spectrum. For example, a sample conditioned under a relative humidity of 92% has increased its interlayer distance up to 1,01 nm (10.1 Å), and n has resulted equal to 3.5. In some samples the hydration is so high that when they are placed in water, they swell up to yield a gelified mass showing an X-ray diffraction spectrum similar to that of the amorphous substances. However, by carrying out a gradual drying, it has been found, by controlling the X-ray diffraction spectrum of the samples, that the peaks typical of the phases with lower hydration degree gradually reappear, until finally the anhydrous compound having an interlayer distance comprised within the range of 0,79 nm to 0,82 nm (from 7.9 to 8,2 Å) is obtained. Such a behaviour would let think that the interlayer distance of the samples dipped into water may reach even larger values than those obtained by the conditioning under relative moisture of 92% (1,01 nm) (10,1 Å). It should be observed that the method according to the present invention leads also to the obtainment of powders constituted by particles the planar dimensions of which are of the same order of magnitude as of the initial α-Zr(HPO₄)₂. H₂O crystals, whilst the thickness, due to the modification occurred in the lamellar structure, is considerably lower (from 10 to 200 times lower). As a consequence, the surface area of delaminated-ZPH according to the present invention is considerably larger than that of crystalline zirconium phosphate α-Zr(HPO₄)₂.H₂O being used as the starting material. By starting for example from crystals having sizes of some tens of µm and surface areas of 0.1-0.2 m²/g, products with surface areas of 9-20 m²/g can be obtained. The combination of a high surface area with particles having considerable planar distances is a typical characteristic of delaminated-ZPH according to the present invention and such a characteristic, besides to the purpose of the surface absorption of various polar substances, can be advantageously used in all catalytic processes in which zirconium phosphate is used, both as catalyst and as catalyst support. Some examples are now supplied to the purpose of better illustrating the invention, it being understood that the invention must not be considered as limited to them or by them. Because the number of equivalents of acidic protons present per g of delaminated-ZPH depends on the hydration degree, which depends in its turn on the relative humidity under which the sample has been conditioned, in the following examples the absorption capacity, for comparison homogeneousness' sake, is referred, unless otherways specified, to 1 g of anhydrous product. Example 1 Zirconium phosphate with layer structure of alpha type has been prepared by the method of direct precipitation in the presence of hydrofluoric acid. In particular, 85 g of ZrOCl₂.8H₂O was dissolved in 1.2 l of water. To the solution first 80 ml of concentrated solution at 40% by weight of HF and then 715 ml of phosphoric acid at 85% by weight were added. The clear solution placed in a plastics vessel was heated in water bath at 80°C and into it pre-humidified air was bubbled. The complete precipitation was reached over a time of about 4 days. The crystals were separated from the solution, washed with distilled water up to pH 4-4.5 and stored in dryer over phosphoric anhydride. One gram of α-Zr(HPO₄)₂.H₂O (prepared as above said) with average crystal size around 15 µm and surface area of 0.2 m²/g was suspended in 100 ml of water, and 50 ml was added of an 0.066 M n-propylamine aqueous solution. The additions was carried out slowly, dropwise during about 4 hours under vigorous stirring. By keeping the mixture stirred for further about 20 hours a colloidal suspension of product with modified lamellar structure was obtained. This suspension was treated with ultrasounds (frequency of 7 MHz and power of 150 W) for a total time of 14 minutes. The ultrasound treatment was carried out by alternating 2 minutes of treatment to one minute of rest interval, during which the vessel was cooled by dipping into water. After this treatment, the optical density of the suspension at 620 nm varied from the initial value of 0.41 to an end value of 0.10. The solution was then acidified with 6 M HCl up to pH 0.7. The so-obtained precipitate was filtered off, washed three times each time with 100 cc of 0.1 M HCl, and finally with water until the washing liquors reached a pH ≧ 3.5. To the moist product obtained water was added up to a total volume of 60 ml, and stirred, to obtain a suspension which was poured by small portions into liquid nitrogen. The frozen granules were freeze-dried. The solid product obtained was further dried at 70°C and under a pressure of 19.95 mbar (15 mm Hg) (End yield 0.96 g). The obtained product, after conditioning at room temperature over phosphoric anhydride, had an interlayer distance,measured from the powder X-ray dispersion spectrum, of 0,81 nm (8.1 Å). (It should be observed that during the determination of the diffraction spectrum the sample must be protected against atmospheric humidity, otherways phenomena of rehydrating occur, which cause an increase of the interlayer distance). The surface area of the product, computated from the nitrogen absorption data by the BET method, was of 19 m²/g. In Fig. 1A a photomicrograph of the product taken with by a scanning electron microscope (Philips Mod. 501/B) at 791X magnification, and in Fig 1B the photomicrograph of the same product at 6330X magnification, are reported. In the two photomicrographs the white segments represent 10 µm and 1 µm respectively. From the two photomicrographs the dimensions and the morphological characteristics of the individual particles can be easily seen. The absorption power of the product for methylene blue from aqueous solution has resulted of 1.1 mmol of dye per g of product. Example 2 One gram of α-Zr(HPO₄)₂.H₂O of Example 1 was suspended in 50 ml of water and to it 25 ml of 0.13 M n-propylamine aqueous solution was added. The addition modalities and all other operations were carried out exactly as described in Example 1. The end yield in dry delaminated-ZPH was of 0.97 g, its interlayer distance (at room temperature and under relative humidity = 60%) was 0,87 nm (8.7 Å). Its absorption power for methylene blue was 1.2 mmol/g. Example 3 The process was carried out exactly as described in Example 2, with the difference that the recovery of the product in the dry state was carried out by spray-drying. The yield was 0.8 g, the interlayer distance of the product obtained under the same conditions as of Example 2 was 0,86 nm (8.6 Å), and the absorption power for methylene blue was 0.9 mmol/g. Example 4 The process was carried out exactly as described in Example 2. The recovery of the end product in the dry state was carried out by direct drying of the moist product at 70°C and under a pressure of 19.95 mbar (15 mm Hg ). The yield was 0.92 g, the interlayer distance was 0,88 nm (8.8 Å), (relative humidity of 60%) and the absorption power for methylene blue 1.3 mmol/g. Example 5 One gram of α-Zr(HPO₄)₂.H₂O, the same sample as of Example 1, was suspended in 50 ml of absolute methanol containing 0.28 ml (0.0033 mol) of n-propylamine and was kept under stirring for about 10 hours. The intercalary amine - containing product, after filtering and drying, was suspended in 150 ml of water and stirred for 30 minutes. The so-obtained colloidal suspension was submitted to treatment with ultrasounds by the same modalities as described in Example 1. The optical density of the ultrasound-treated end solution was of 0.099 at 620 nm. By proceeding exactly as described in Example 1, 0.94 g of product having and interlayer distance of 0,86 nm (8.6 Å) (relative humidity = 60%) and an absorption power for methylene blue of 0.9 mmol/g of product conditioned over P₂O₅ was obtained. The surface area of this product (BET) was of 15 m²/g. Example 6 An amount of 1.000 g of product obtained as described in Example 1 was brought to constant weight in vacuum dryer over phosphoric anhydride and subsequently equilibrated over saturated solutions of magnesium nitrate [relative humidity (R.H.) 60%], of sodium chloride (R.H. 75%), and barium chloride (R.H. 92%). The following % weight increases and the following values of interlayer distance were recorded: R.H. % Weight Increase d (Å) 0 0 0,81nm (8.1) 60 7.9 0,86nm (8.6) 75 14.1 0,94nm (9.4) 92 17.5 1,01nm (10.1) The sample reconditioned over phosphoric anhydride returned back to its starting weight of 1.000 g. Another sample of 0.284 g of delaminated-ZPH, prepared according to Example 2, stored in air (R.H. = 60%), dried in oven at 150°C for 24 hours showed a weight loss of 7.6%. This dry product, kept resting in air on the analytical balance returned to its initial weight within of about 2 hours. Example 7 One gram of zirconium phosphate with modified lamellar structure and conditioned over P₂O₅ prepared as described in Example 2 was dispersed with strong stirring in 100 ml of 0.100 M CsCl solution having pH = 3.00. An immediate decrease in the pH value was observed in the solution, which value, about 1 minute after the addition of delaminated-ZPH resulted equal to 1.79. In order to raise the pH to a value of 3.00, 12.5 ml of a 0.100 M CsOH solution was added. It can be inferred that under these conditions 1 g of delaminated-ZPH exchanges 1.25 mmol of its acidic protons with 1.25 mmol of Cs⁺. The same test has been carried out by using 1 g of α-Zr(HPO₄)₂.H₂O (starting material of Example 1 used to obtain the delaminated-ZPH). In this case, no appreciable changes in pH were observed in solution and hence no absorption of Cs had occurred. Example 8 One gram of delaminated-ZPH conditioned over P₂O₅ prepared as described in Example 2 was dispersed under strong stirring in 150 ml of a 0.050 M BaCl₂ solution having pH = 5.25. An immediate decrease in pH value was observed in solution, which value, about 1 minute after the addition, resulted equal to 1.75. The analysis of the supernatant solution for Ba content showed the presence of residual 6.45 mmol of Ba⁺⁺ ions, as compared to the 7.50 mmol initially present. It can be inferred that under these conditions 1 g of delaminated-ZPH exchanges 2.1 mmol of its acidic protons with 2.1 mmol of Ba⁺⁺ ions. Example 9 One g of α-Zr(HPO₄)₂.H₂O was treated as described in Example 1, until the colloidal solution was obtained. This latter, after having undergone the treatment with ultrasounds was acidified. The obtained precipitate was washed first with 0.1 M HCl and then with water as described in Example 1. The moist product which was obtained was washed three times with absolute ethanol. Its interlayer distance resulted to be 1,42 nm (14.2 Å), similar to that of crystalline zirconium phosphate with two intercalary alcohol molecules. Another aliquot of the product was washed with n-propyl alcohol (three times). In this case, the moist product showed on X-ray analysis an interlayer distance of 1,66 nm (16.6 Å), similar to that of the intercalation compound Zr(HPO₄)₂.n-propanol [U. Costantino, J. Chem. S. Dalton, 402 (1979)]. Example 10 One g of α-Zr(HPO₄)₂.H₂O underwent the lamellar structure modification, ultrasound-treatment and washing as described in Example 1, until a moist product was obtained. This latter was dispersed again in water up to a total volume of 100 ml. This suspension was analyzed for its content in zirconium phosphate by drawing, under stirring, 5-ml portions, which, placed inside weighing bottles, were kept at 70°C and under a pressure of 19,95 mbar (15 mm Hg ) until most of the water had evaporated off, and finally were conditioned to constant weight over phosphoric anhydride in a vacuum dryer. The content of the suspension, so determined, resulted equal to 9.5 mg of anhydrous material per ml. To 10 ml of suspension 25 ml was added of a 0.0102 M aqueous solution of methylene blue (equivalent to 0.255 mmol). After letting the equilibrium to be reached under stirring for 48 hours, the supernatant solution was analyzed for its content in methylene blue. The residual content was 0.088 mmol. It can be inferred that the product had absorbed 1.85 mmol of dye per g of product conditioned over phosphoric anhydride. Example 11 To 15 ml of the aqueous suspension of Example 10, containing 142.5 mg of anhydrous product, 55 ml of absolute ethanol and 30 ml of a 0.073 M solution of bornylamine in alcohol were added. After 24 hours of equilibration, 10 ml of supernatant solution was drawn and titrated with 3.16 ml of 0.05 M HCl, from which it can be inferred that the intercalary bornylamine was equal to 4.28 mmol/g. Example 12 To 8 ml of the aqueous suspension of Example 10, containing 76 mg of anhydrous product, 89.5 ml of absolute ethanol and 2.5 ml of a 0.052 N solution of quinine in alcohol were added. After 24 hours of contact time, the supernatant solution was analyzed for its content in residual quinine. Such a content resulted equal to 0.0809 mmol as compared to the 0.130 mmol initially added to react. It can be inferred that the product capacity to absorb quinine was equal to 0.31 mmol of quinine per g.

Claims: Claims for the following Contracting States : BE, CH, DE, FR, GB, LI, LU, NL, SE Particulate zirconium acid phosphate of modified structure, characterized in that it has a particle size of from 1 micrometre to 100 micrometres, a lamellar structure with interlayer of distance in the anhydrous state of from 0,79 nm to 0,82 nm (7,9 Angstrom to 8,2 Angstrom) and a surface area of from 9 m²/g to 20m²/g. Particulate zirconium acid phosphate according to Claim 1, wherein its increase of the interlayer distance upon hydration reaches 1,01 nm (10,1 Angstrom) under the condition of relative humidity of 92%. A process for preparing a particulate zirconium acid phosphate as defined in Claim 1 by intercalating, into crystalline zirconium phosphate alpha-Zr(HPO₄)₂.H₂O having an interplanar distance of 0,76 nm (7,6 Angstrom), an organic substance containing a proton-acceptor group and water until a stable colloidal aqueous suspension of intercalated zirconium phosphate is prepared, characterized in that it further comprises the steps of: treating the colloidal suspension with ultrasounds; treating the suspension thereafter with a non-zirconium-complexing acid until a precipitate of zirconium acid phosphate with modified lamellar structure is formed,and recovering the precipitate and washing it with dilute acid and/or water. Process according to Claim 3, wherein the non-zirconium-complexing acid is a mineral acid selected from HCl, H₂SO₄ and HNO₃. Process according to Claim 3, wherein the acid treatment of the colloidal suspension is carried out at a pH of from 0 to 3. Process according to Claim 3, wherein the treatment with ultrasounds is continued until such time as the optical density of the colloidal suspension remains constant. Process according to Claim 3, wherein the organic substance containing a proton-acceptor group is an amine selected from methylamine and n-propylamine. Process according to Claim 7, wherein n-propylamine is added in an amount of from 25% to 75% of the number of gram-atoms of the acidic protons present in the amount of zirconium phosphate alpha-Zr(HPO₄)₂.H₂O used. Process according to Claim 3, wherein the stable colloidal aqueous suspension of intercalated zirconium phosphate is obtained by first intercalating the zirconium phosphate in a non-aqueous medium, and then dispersing it in water. Use of the particulate zirconium acid phosphate of modified structure according to Claim 1 or 2 as an ion-exchanger in the recovery of Cs⁺ and Ba⁺⁺ ions. Use of the particulate zirconium acid phosphate of modified structure according to Claim 1 or 2 as a catalyst or as a catalyst-supporting substrate. Claims for the following Contracting State : AT Process for preparing a particulate zirconium acid phosphate having a particle size of from 1 micrometre to 100 micrometres, a lamellar structure with interlayer of distance in the anhydrous state of from 0,79 nm to 0,82 nm (7,9 Angstrom to 8,2 Angstrom) and a surface area of from 9 m²/g to 20m²/g, by intercalating, into crystalline zirconium phosphate alpha-Zr(HPO₄)₂.H₂O having an interplanar distance of 0,76 nm (7,6 Angstrom), an organic substance containing a proton-acceptor group and water until a stable colloidal aqueous suspension of intercalated zirconium phosphate is prepared, characterized in that it further comprises the steps of: treating the colloidal suspension with ultrasounds; treating the suspension thereafter with a non-zirconium-complexing acid until a precipitate of zirconium acid phosphate with modified lamellar structure is formed,and recovering the precipitate and washing it with dilute acid and/or water. Process according to Claim 1, characterized in that the non-zirconium-complexing acid is a mineral acid selected from HCl, H₂SO₄ and HNO₃. Process according to Claim 1, characterized in that the acid treatment of the colloidal suspension is carried out at a pH of from 0 to 3. Process according to Claim 1, characterized in that the treatment with ultrasounds is continued until such time as the optical density of the colloidal suspension remains constant. Process according to Claim 1, characterized in that the organic substance containing a proton-acceptor group is an amine selected from methylamine and n-propylamine. Process according to Claim 5, characterized in that n-propylamine is added in an amount of from 25% to 75% of the number of gram-atoms of the acidic protons present in the amount of zirconium phosphate alpha-Zr(HPO₄)₂.H₂O used. Process according to Claim 1, characterized in that the stable colloidal aqueous suspension of intercalated zirconium phosphate is obtained by first intercalating the zirconium phosphate in a non-aqueous medium, and then dispersing it in water.

Inventors: ALBERTI, GIULIO, BARTOLI, FRANCESCO, COSTANTINO, UMBERTO, DI GREGORIO, FRANCESCO, VALENTINI, CLAUDIO
Applicants: ENIRICERCHE S.P.A.
Application year: 1986
Main classification: C01B25
IPC codes: C07D453, C01B25, B01J39