Method for the preparation of monoliths of zeolite lta

Abstract

The present invention relates to a method for the preparation of monoliths of zeolite A, allowing the 3D printing of zeolite precursor gels, the product crystallization taking place contemporary to the component solidification, and to form monolithic objects with optimized designs in terms of the properties required for each application.

Description

[0001] METHOD FOR THE PREPARATION OF MONOLITHS OF ZEOLITE LTA

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a method for the preparation of monoliths of zeolite A, allowing the 3D printing of zeolite precursor gels, the product crystallization taking place contemporary to the component solidification, and to form monolithic objects with optimized designs in terms of the properties required for each application.

[0004] STATE OF ART

[0005] Zeolites are aluminosilicates of natural or synthetic origin provided with an extremely regular and repetitive three-dimensional structure, that is containing ordered microcavities having variable size: such feature gives the zeolites a high specific surface, thus making them extremely suitable to the use in phenomena of industrial interest such as adsorption and catalysis, in the latter case only if suitable modified so as to give them an essential feature such as acidity. The porosity size is comparable to that of simple molecules such as carbon dioxide or water, which then can interact with zeolite differently from other molecules having greater sizes: for this reason the zeolites are also called "molecular sieves". Considering the existence of several zeolitic structures (more than 200) which have been identified or synthetized over the years, each one thereof with a different size and shape of the micro-cavities, it is possible to select a specific zeolitic structure so as to obtain a high selectivity for a determined molecule or group of molecules. Moreover, thanks to their inorganic nature, zeolites are stable at high temperatures. This feature, together with the high specific surface and their peculiar selectivity, makes these materials easily applicable in many industrial chemical processes.

[0006] In particular, zeolite A (officially known as LTA), has been one of the first synthetic zeolites due to the related simplicity of the synthesis method which does not provide the use of critical raw materials. The main feature of LTA is its three-dimensional structure of micropores with size of about 0.4 nm: such size makes that it has an extremely high selectivity towards molecules with sufficiently reduced kinetic radius so as to be able to penetrate inside the ordered microstructure, thereamong the carbon dioxide. Optionally, with wholly reversible post-synthesis treatments, the accessible internal volume can be reduced to 0.3 nm or increased to 0.5 nm, by improving (in the latter case) the capability of adsorbing CO2for applications such as the capture and sequestration of carbon dioxide.

[0007] Zeolites are mainly produced through hydrothermal synthesis processes, where the precursors are placed inside high pressure and temperature reactors for several hours or days until the complete crystallization of the product. Such processes have high costs due to the required conditions and the need for specialized reactors.

[0008] Moreover, in a traditional hydrothermal synthesis of zeolite, it is necessary to use huge amounts of solvent with respect to the mass of the material precursors. This involves that the reaction yield, in terms of product mass obtained with respect to the mass of the used reagents, is quite low and that the zeolites, obtained under the form of powder inside the synthesis mixture, are to be separated from the mother water through long and expensive filtration and / or centrifugation processes.

[0009] Another problem is linked to the fact that the zeolites obtained from the hydrothermal processes as fine powders are not suitable to the immediate use in applications of industrial type, for safety factors and ease of use, and then are to be subjected to forming processes such as grinding or extrusion through insertion of a foreign binder which, apart from diluting the active phase, can limit the mass transportation and the performances of the finished product.

[0010] Moreover, the forming processes most commonly used at industrial level often require high temperature treatments for consolidating the binder, feature which apart from constituting an energy cost, often causes a porosity variation in the material affecting the final properties of the product.

[0011] SUMMARY OF THE INVENTION

[0012] The present invention relates to a method for the production of monoliths of zeolite A which, with respects to the methods known in the prior art, results to be less expensive in terms of time and costs, since the crystallization method allows to use only a furnace working at low temperatures and any not necessarily hermetic container. The method of the present invention does not require intermediate passage of solidifying or aging the synthesis gel before the crystallization treatment. In fact, generally when the zeolitebinder compound is not dried before conversion, they are usually used as clay material binders or other silicates or aluminosilicates in solid form, mixed with water and the zeolitic powder so as to form a paste. On the contrary, in the present invention, the used binder is the same mixture of sodium aluminate and colloidal silica therewith the zeolites A to be bound were produced, which however does not involve a dilution of the active phase in the finished product.

[0013] Moreover, the method of the present invention allows to obtain the zeolite crystallization directly from the synthesis mixture, without having to go through an intermediate solidification step, as well as the removal of the stabilizing agent with treatments of few hours at lower temperatures than those used in the known methods (400°C instead of 550°C).

[0014] Advantageously, the present invention allows the direct 3D printing of zeolite precursor gels, the product crystallization taking place contemporary to the component solidification. The possibility of forming directly the synthesis gel allows to produce net- shape monolithic objects with optimized designs in terms of the properties required for the particular application (for example sizes, porosity, mechanical properties, load losses, conductivity) by exploiting the flexibility of the preparation method. Moreover, such designs can be varied each time without the need for modifying the productive line since, as in the traditional forming processes, the use of moulds or matrixes, which have to be changed in every design modification, is not required.

[0015] Moreover, the synthetized 3D printed materials were tested in the capture of CO2at low temperature and pressure, then under capture conditions similar to post-combustion processes. The obtained capture data are extremely interesting results, by obtaining in the case of the 3D-printed sample LTA a capability of capturing pure CO2at 25°C and 1 bar equal to 3.5 mmol / g. Under the same conditions the capability of capturing CO2of LTA synthetized in form of power is equal to 3.7 mmol / g. The sample then is capable to keep almost entirely the capture capability by going from powder to monolith. This piece of data represents a particularly high and advantageous value since one of the difficulties most encountered in the forming processes relates to the strong decrease in the performances of the formed material with respect to the powdery one.

[0016] In the field of the materials for capturing CO2, zeolite 13X is often used as reference. In fact, it has a high capture capability at low temperature (about 7 mmol / g at 25°C and 1 bar), but it has, apart from a high cost, even a rapid decrease in the performances as the temperature increases (at 90°C it loses about 50% of the capture capabilities). The zeolite proposed in the present patent application, instead, shows better capture performances at high temperatures (at 90°C it keeps about 65% of the capability of capturing CO2) by making it exploitable in a much wider range of temperatures. It is also possible to evaluate post-synthetic treatments of monoliths of zeolite A (or in case during the synthesis phase) as ionic exchange procedures with the purpose of adapting the features of the printed element to the possible industrial applications, with the purpose of further improving the properties of capturing carbon dioxide, even possibly in presence of humidity.

[0017] Moreover, the zeolite of the present invention can be used in processes for capturing and / or separating small molecules, in particular for capturing CO2from anhydrous flows and separating the latter from dimensionally bigger molecules such as N2and CH4.

[0018] Another application is the one related to the capture of water molecules (as drying agent), or for systems for capturing flows of humid carbon dioxide at industrial level, since the possibility of modelling a monolith with the wished shape and sizes allows to assembly a system suitable to capture in series water and carbon dioxide, then by using the same material firstly to anhydrify the exhaust gas and then to capture therefrom the carbon dioxide.

[0019] Moreover, due to the potentiality of the method which allows an optimization of component design inaccessible to traditional forming techniques, the monoliths would be particularly useful for adsorption processes in gas flow. In fact, it is possible to obtain resistant structure, easy to handle, but which at the same time are highly porous and permeable to the gas by reducing the operating costs of the plant and the material waste.

[0020] Therefore the present invention relates to: a method for the preparation of monoliths of zeolite A (LT A) comprising the following steps: i) preparing a gel by mixing a silica precursor, a sodium precursor and an alumina precursor in aqueous solution, so that the concentration of silica in said gel is comprised between 13% and 19% by weight and wherein said mixing takes place at a speed so as to obtain a homogeneous mixture; ii) forming, preferably by pouring into a mould, and subsequently crystallizing the gel obtained in step i) for at least 12 hours, at a temperature of at least 60°C, so as to obtain a monolith of zeolite A; iii) drying the product obtained in step ii) for at least 12 hours, at a temperature between 75 and 500°C.

[0021] In an additional subject of the present invention, the method comprises the following steps: i) preparing a gel by mixing a silica precursor, a sodium precursor and an alumina precursor in aqueous solution, so that the concentration of silica in said gel is comprised between 13% and 19% by weight and wherein said mixing takes place at a speed such as to obtain a homogeneous mixture; i-a) adding a rheological additive and / or powder of zeolite A (LT A) to said gel, so as to obtain a slurry; ii) forming with a 3D printing process performed by extruding the slurry obtained in step i-a) and subsequently crystallizing the gel obtained in step i) for at least 12 hours, at a temperature of at least 60°C, so as to obtain a monolith of zeolite A; iii) drying the product obtained in step ii) for a period of time of at least 12 hours, at a temperature between 75 and 500°C; iv) optionally removing the rheological additive from the zeolite A monolith.

[0022] In an additional subject of the present invention, the method comprises the following steps: i) preparing a gel by mixing a silica precursor, a sodium precursor and an alumina precursor in aqueous solution, so that the concentration of silica in said gel is comprised between 13% and 19% by weight and wherein said mixing takes place at a speed such as to obtain a homogeneous mixture; i-x) incubating the gel obtained in step i) for a period of time comprised between 8 and 16 hours, at a temperature from 60 to 90°C, so as to obtain a slurry; ii) forming with a 3D printing process performed by extrusion, and subsequently crystallizing the gel obtained in step i) for at least 12 hours, at a temperature of at least 60°C, so as to obtain a monolith of zeolite A; iii) drying the product obtained in step ii) for a period of time of at least 12 hours, at a temperature between 75 and 500°C.

[0023] Additional advantages and / or embodiments of the present invention will be evident from the following detailed description.

[0024] DETAILED DESCRIPTION OF FIGURES

[0025] Figure 1 shows a monolith LTA obtained by pouring according to the procedure described in section examples of the present invention.

[0026] Figure 2 SEM analyses show a microstructure consisting of partially agglomerated homogeneous crystals of zeolite LTA, without the detectable presence of extraneous phases (Figure 2A). The crystals (Figure 2B) have size of about 500 nm with rhombicuboctahedron morphology (Figure 2C).

[0027] Figure 3 The XRD pattern of the monolith shows that the product consists of zeolite LTA with high crystallinity considering the absence of amorphous bands, by confirming what emerged from SEM analyses.

[0028] Figure 4 Physisorption isotherm of CO2at 25°C of the powder LTA obtained by grinding a poured monolith compared with a commercial powder LTA.

[0029] Figure 5 IR spectrum of the interaction between the probe molecule (carbon monoxide) and the active sites responsible for adsorption. Dashed line: room temperature, activated sample; solid line: interaction of CO with the active sites at -196°C.

[0030] Figure 6 CO2adsorption isotherms at 22 C Full squares: adsorption isotherm on powder LTA obtained by grinding a monolith poured according to the method described in example 1 ; full stars: adsorption isotherm on monolith LTA obtained by DI W according to the method described in the example 2. The empty symbols represent the desorption isotherms.

[0031] Figure 7 Thermogravimetric analysis in air on the printed monolith. The derivative of weight loss shows that a complete loss of CMC can be obtained only at approximately 370°C.

[0032] Figure 8 IR spectra of the interaction between the probe molecule (carbon monoxide) and the active sites responsible for the adsorption in the several samples after different attempts at removing CMC in static muffle. Dotted line: RT, activated sample; solid line: interaction of CO with the active sites at -196°C.

[0033] Figure 9 Image of some monoliths LTA with complex geometries manufactured by Direct Ink Writing. The monoliths have cylindrical shape with 17-mm diameter and 28-mm height.

[0034] Figure 10 XRD pattern after heat treatment of a monolith LTA obtained by DIW compared with the theoretical pattern of zeolite LTA.

[0035] Figure 11 CO2physisorption tests at 25°C on a monolith LTA obtained by DIW compared with commercial granules LTA.

[0036] Figure 12 Statistical analysis of the compression resistance according to Weibuil method for monoliths LTA with log-pile geometry and geometric porosity of 50%. Figure 13 Image of a monolith LTA obtained by DIW without adding filler. The monolith has squared base with 20-mm side and 8-mm height.

[0037] Figure 14 XRD Pattern for a monolith LTA obtained by DIW without adding filler compared with the theoretical pattern of zeolite LTA.

[0038] GLOSSARY

[0039] In the present invention, under the term “monolith of zeolite A” one relates to a solid and continuous structure mainly consisting of crystals of zeolite A arranged three- dimensionally.

[0040] In the present invention, under the term “alumina, silica or sodium precursor” one relates to a chemical compound used to provide aluminate ions, silica and sodium ions, respectively, during the synthesis of monoliths of zeolite A.

[0041] In the present invention, under the expression “crystallizing the gel” one relates to the process of promoting the formation and growth of the crystals of zeolite A inside a gelatinous matrix.

[0042] In the present invention, under the expression “pouring gel into a mould” one relates to the action of pouring the gel containing the alumina precursor and silica in a suitable form or mould, with the purpose of giving a wished shape to the zeolite A monolith.

[0043] In the present invention, under the term “rheological additive” one relates to a substance added to the gel to verify and modify its rheological properties, for example viscosity and threshold stress to sliding.

[0044] In the present invention, under the term “slurry” one relates to a fluid mixture comprising the precursor gel of zeolite A and in case zeolite A powder and / or a rheological additive, used as raw material for manufacturing monoliths.

[0045] In the present invention, under the expression “homogenization of slurry” one relates to one or more procedures performed on the slurry to guarantee a uniform distribution of the different species inside the liquid.

[0046] In the present invention, under the term “superabsorbent polymer” one relates to a type of polymeric material having the capability of absorbing and keeping huge amounts of liquids with respect to its own mass, and mainly consists of long and cross-linked molecular chains which form a porous structure at microscopic level. This porous structure allows the material to absorb water or other liquids inside its internal spaces, by forming a gel. The superabsorbent polymers can absorb from 10 to 100 times their mass in liquid, depending upon the type of polymer and the environmental conditions.

[0047] In the present invention, under the term “sonication” one relates to the process for applying high-frequency ultrasounds to stir or disperse the particles in the slurry and to improve their homogeneous distribution.

[0048] In the present invention, under the term “defoaming” one relates to the action of removing air bubbles or foam from the slurry to avoid the formation of imperfections or defects in the monolith during the printing process.

[0049] In the present invention, under the term “Direct Ink Writing (DIW)” one relates to a three- dimensional printing technique which uses in this case a zeolite A-based material in form of ink to deposit and model subsequent layers to obtain the wished shape of the monolith.

[0050] In the present invention, under the expression “maximum size of particles at least 10 times smaller than the diameter of the printing nozzle” one relates to the requirement that the particles of zeolite A powder in the slurry are subjected to a selection, crushing or size reduction so that their maximum size is at least 10 times smaller than the diameter of the nozzle used during printing.

[0051] In the present invention, under the expression “incubating the gel” one relates to the process of keeping the gel containing the alumina precursor and silica under controlled conditions, such as humidity, temperature and time, leading to an increase in viscosity associated to the partial crystallization of the mixture, which allows to obtain the rheological properties required to printing without the need for fillers or additives.

[0052] In the present invention, under the expression “log-pile” one relates to a type of 3D printed monolith, whose structure consists of layers of parallel filaments which are superimposed by alternating the direction thereof at 0° and 90° to create a high degree of interconnected porosity.

[0053] DETAILED DESCRIPTION

[0054] The present invention relates to a method for the preparation of monoliths of zeolite A (LTA) comprising the following steps: i) preparing a gel by mixing a silica precursor, a sodium precursor and an alumina precursor in aqueous solution, so that the concentration of silica in said gel is comprised between 13% and 19% by weight and wherein said mixing takes place at a speed such as to obtain a homogeneous mixture; ii) forming and subsequently crystallizing the gel obtained in step i) for at least 12 hours, at a temperature of at least 60°C, so as to obtain a monolith of zeolite A; iii) drying the product obtained in step ii) for period of time of at least 12 hours, at a temperature between 75 and 500°C.

[0055] In an embodiment, said alumina precursor is selected from sodium aluminate (NaAI02), aluminium hydroxide, aluminium alkoxides, boehmite, alumina, aluminium salts (nitrates and sulphates), kaolin, metakaolin, metallic aluminium.

[0056] In an embodiment, said silica precursor is selected from colloidal silica, (pyrogenic or precipitated) solid phase silica, sodium silicates, silicon alkoxides, kaolin, metakaolin, silicic acid.

[0057] In an embodiment, said sodium precursor is selected from sodium hydroxide, sodium aluminate, sodium silicates, Sodium salts (sulphates, borates, carbonates, fluorides, bromides).

[0058] In a preferred embodiment of the present invention, both sodium and alumina are provided by the same precursor, preferably sodium aluminate (NaAIO2). In an additional embodiment, said alumina precursor is used in solution and is present in a concentration comprised between 0.5M and 0.7M, preferably about 0.6M.

[0059] In an embodiment, said silica is colloidal silica. The colloidal silica is a stable aqueous suspension of nanoparticles of amorphous silica.

[0060] In an embodiment, said silica has size of particles ranging from 5 to 50 nm, preferably 22 nm.

[0061] In a preferred embodiment, said silica is present in a concentration between 16 and 17% by weight, preferably about 16.35% by weight. In a preferred embodiment, said alumina and sodium precursor is NaAIO2, and said silica is colloidal silica.

[0062] In an embodiment said gel obtained in step i) has a molar composition [x SiO2:AI2O3: y Na2O: zH2O], wherein x is a number from 1 .8 a 3, y is a number from 0.8 to 1 .2, and z is a number from 20 to 30.

[0063] In a particularly preferred embodiment, said gel obtained in step i) has a molar composition [x SiO2:AI2O3: y Na2O: zH2O], wherein x is 2, y is 1 , and z is 25.

[0064] In an embodiment, said step i) takes place in a mechanical mixer at a centripetal acceleration range between 0.75g and 210g, preferably between 3 and 16g, still more preferably 8g, with progressive speed increase when adding silica up to 162g. By considering rpm, the mixer acts at a range between 300 and 700 rpm, preferably 500 rpm, with progressive speed increase when adding silica up to 220 rpm.

[0065] In an embodiment, said step ii) takes place in humid environment, with relative humidity (U.R.) of at least 95%, preferably 100%.

[0066] In a preferred embodiment, said step ii) takes place at a temperature of at least 60°C, preferably between 60 and 90°C, still more preferably 75°C. Since at higher temperatures than water boiling temperature high pressures could develop inside the material, in case one wishes to perform said step ii) at higher temperatures than water boiling temperature, thus generally higher than 100°C, it is necessary to perform crystallization in a hydrothermal reactor.

[0067] However, the present invention allows to use lower temperatures than that of water boiling, thus allowing not to have to perform crystallization in a hydrothermal reactor, but in any type of container.

[0068] In a preferred embodiment, said step iii) takes place at a temperature between 75°C and 500°C, preferably 75°C.

[0069] In an embodiment, the forming process in step ii) of the method of the present invention is performed by pouring the gel obtained in step i) into a mould. The mould can be any mould known to the person skilled in the art, having any shape and made of any material suitable to the contact with strongly alkaline pHs.

[0070] Therefore, in an embodiment, the method of the present invention comprises the following steps: i) preparing a gel by mixing a silica precursor, a sodium precursor and an alumina precursor in aqueous solution, so that the concentration of silica in said gel is comprised between 13% and 19% by weight and wherein said mixing takes place at a speed such as to obtain a homogeneous mixture; ii) forming by pouring into a mould, and subsequently crystallizing the gel obtained in step i) for at least 12 hours, at a temperature at least of 60°C, so as to obtain a monolith of zeolite A; iii) drying the product obtained in step ii) for a period of time of at least 12 hours, at a temperature between 75 and 500°C. In an additional embodiment, the method of the present invention comprises after said step i) a step i-a) wherein a rheological additive and / or powder of zeolite A (LTA) is added to said gel, so as to obtain a slurry.

[0071] In an embodiment, the concentration of said rheological additive in the slurry is comprised between 0 and 5% by weight, preferably 2%.

[0072] In an embodiment, the concentration of said powder of zeolite A (LTA) is comprised between 30% and 50% by weight, preferably 40%. Preferably a powder of zeolite A (LTA) with maximum size of particles at least 10 times smaller than the diameter of the printing nozzle will be used. The powder of zeolite A (LTA) for example could be obtained by means of a step of grinding a monolith of zeolite A (LTA).

[0073] In a preferred embodiment, said rheological additive is a superabsorbent polymer.

[0074] In a preferred embodiment, said superabsorbent polymer is a derivative of cellulose, preferably selected from carboxymethylcellulose (CMC), methylcellulose, ethylcellulose, hydroxypropylcellulose, and / or an organic compound such as polyacrylic acid.

[0075] In a preferred embodiment, the method further comprises after said step i-a) a step i-b) of homogenization of the slurry obtained in step i-a). The homogenization step i-b) preferably will comprise at least three mixing cycles of at least 5 minutes, interspaced with at least two sonication cycles of at least 5 minutes, and subsequent defoaming of at least 5 minutes.

[0076] In an embodiment, the forming process in step ii) of the method of the present invention is performed by extrusion of the slurry. The extrusion techniques usable for the object of the present invention are known to the person skilled in the art. Preferably said extrusion process is performed with Direct Ink Writing (DIW) method.

[0077] In a preferred embodiment, the method of the invention further comprises a step of eliminating the rheological additive.

[0078] In an embodiment, said step of eliminating the rheological additive comprises subjecting the zeolite A monolith to a heat treatment at a temperature from 350 to 500°C, preferably 400°C, for a period of time of at least 10 hours, with a heating ramp lower than 20°C / minute in air, so as to eliminate the rheological additive. Previously to the heat treatment one or more steps could be provided for washing the monolith in water, preferably washing by immersion in distilled water, at room temperature and slight stirring. In an additional alternative embodiment, the method of the present invention, after step i), provides a step i-x) wherein the gel obtained in step i) is incubated for a period of time comprised between 8 and 16 hours, at a temperature from 60 to 90°C, so as to obtain a slurry. The incubation will be performed preferably in a hermetically closed environment, for example in a closed container.

[0079] In any part of the present description and claims, the term comprising can be replaced by the term "consisting of".

[0080] Example are reported hereinafter having the purpose of better illustrating the methods disclosed in the present description, such examples are in no way to be considered a limitation of the previous description and of the subsequent claims.

[0081] EXAMPLES

[0082] 1 Preparation of powder or monoliths by pouring

[0083] 1.1 Procedure

[0084] A. 111 .60 g of sodium aluminate (NaAIO2, Sigma-Aldrich) are dissolved in 224.79 g of deionized water, by keeping the recipient cooled down by means of running water. The solution then is left to homogenize for 30 minutes on a roller mixer at 20 rpm and afterwards sonicated for 10 minutes.

[0085] B. The NaAIC>2 solution is placed under stirring through a mechanical mixer at 500 rpm and during 5 minutes 163.60 g of colloidal silica (LUDOX TM50, Sigma-Aldrich) are added gradually. Upon adding silica the mixing speed is progressively increased up to 2200 rpm. The concentration of the NaAIO2 solution and the proportions of the two reagents are so as to create a synthesis gel with molar composition [2 SiO2: AI2O3: Na2O : 25 H2O],

[0086] C. The gel is mixed vigorously for 60 minutes at 2200 rpm through a mechanical mixer.

[0087] D. The synthesis gel is poured into a mould made of polyethylene having sizes 20x20x5 cm, then left to crystallize for 48 h at 75 °C in humid environment (U.R. =100%) and at room pressure (by keeping the mould closed with a cover), and at last dried up for 24 h at 75 °C in open mould stove.

[0088] E. Optionally, the monolith can be ground to obtain zeolite LTA in powder.

[0089] 1.2 Characterization Figure 1 shows a monolith LTA obtained by pouring according to the procedure described in section 1.1. SEM analyses show a microstructure consisting of partially agglomerated homogeneous crystals of zeolite LTA, without the detectable presence of extraneous phases (Figure 2A). The crystals (Figure 2B) have size of about 500 nm with rhombicuboctahedron morphology (Figure 2C).

[0090] XRD pattern of the monolith (Figure 3) shows that the product consists of zeolite LTA with high crystallinity considering the absence of amorphous bands, by confirming what resulted from SEM analyses.

[0091] The powder obtained by grinding the monolith was further subjected to volumetric measures for capturing CO2at 25°C. The material shows a slightly lower capture capability at 1 bar than a commercial powder LTA (≈-15%), however keeping the advantages and the greater ease of using the structured adsorbents.

[0092] 2 Preparation of monoliths by extrusion with use of filler

[0093] 2.1 Procedure

[0094] A. 59.45 g of synthesis gel are prepared as described in 1A-1C.

[0095] B. 1.89 g of carboxymethylcellulose (CMC) are added to the gel as rheological additive and powder LTA previously obtained according to point 2E, ground and sieved to obtain a maximum size of the particles about 10 times smaller than the diameter of the printing nozzle. The amount of zeolitic filler depends upon the viscosity required by the printing method (for the extrusion through a 0.58-mm nozzle 39.90 g of filler with maximum size of the particles of 45 pm are added).

[0096] C. The slurry is homogenized by using a high energy planetary mixer in 3 cycles of 5 minutes interspaced with 2 cycles of 5 minutes of sonication. After each mixing or sonication cycle a short cooling in ice bath is applied to bring back the slurry at room temperature.

[0097] D. The slurry is then transferred into a 55 ml syringe and a defoaming cycle of 5 minutes is applied in the same planetary mixer with the purpose of removing air bubbles.

[0098] E. The printing is performed according to Direct Ink Writing (DIW) method. The slurry is pushed through compressed air at a pressure of 2.5 bar in the chamber of a screw extruder which, by depositing it through the nozzle, having 0.58 mm diameter, which moves at a speed of 25 mm / s, reproduces the digital model of the monolith layer layer by layer. F. The monolith crystallization and drying are performed as described in 1 D.

[0099] G. The monolith is subjected to a heat treatment at 400°C for 10h by using a heating ramp of 5°C / minute in air [in order to remove CMC].

[0100] 2.2 Method for removing the printing rheological additive (CMC)

[0101] A. After the printing method, the monolith is treated thermically in muffle at the temperature of 400°C for 10h by using a heating ramp of 5°C / minute in presence of air (static).

[0102] The effectiveness of the removing method is tested by infrared spectroscopy by using probe molecules (carbon monoxide) at -196°C in order to verify the accessibility of the active sites responsible for the CO2adsorption method.

[0103] IR spectrum in Figure 5 shows a very intense signal at 2160 cm-1attributable to the interaction of the probe molecule with the sites responsible for adsorption, index of a total accessibility of the sites.

[0104] B. After the test by infrared spectroscopy the sample is tested for CO2adsorption by using volumetric instruments (Micromeritics - ASAP2020) at the temperature of 25°C in a range comprised between 0 and 1 bar.

[0105] As shown in Figure 6, the printed monolith LTA shows an adsorption capability at 25°C of 3.45 mmol / g, about 19% less with respect to the capture capability of the respective powder. Despite the performance decrease, however the capture capability remains high. Such “negative” aspect, however, is compensated by the possibility of printing the material with sizes and shapes which can be adapted to the potential applications by reducing all problems linked to the industrial use of powders.

[0106] C. The removal of the printing additive represents a crucial parameter for the final applications. In fact, the non-removal or a partial removal of CMC leads to an inaccessibility of the active sites with consequent inapplicability for capturing CO2. The removal method was accurately selected in order to maximize the effectiveness / energy expenditure relationship.

[0107] A first test was then performed on the printed monolith by means of a thermal degradation study performed through thermogravimetric techniques. The results show that a complete removal of CMC can be obtained only around 400°C.

[0108] Figure 7 shows a continuous weight loss until a temperature of about 400°C, temperature which was then selected for the first CMC removal tests. Several removal tests at temperatures comprised between 400 and 600°C (limit temperature for the stability of the crystalline lattice of type LT A) were performed.

[0109] As described in point 4B the thermally treated materials were tested, first of all, by infrared spectroscopy with use of carbon monoxide as probe molecule.

[0110] As shown in Figure 8 the only working strategy for the complete removal of CMC is to keep the monolith at the temperature of 400°C for about 10 h. The fact of decreasing the residence time in the muffle or increasing the temperature causes a partial or complete occlusion of the adsorption sites.

[0111] Moreover, some removal attempts were performed, with the help of a gas flow (pure oxygen or air) inside a tubular furnace. Such attempts, however, did not show significant improvements and moreover they result to be disadvantageous from the application point of view, considering the need for an auxiliary gas flow.

[0112] 2.3 Characterization

[0113] Some examples of monoliths LTA obtained according to the procedure in section 2.1 are shown in Figure 9. The flexibility of Direct Ink Writing method allows to implement adsorbents structured with complete control on the internal macroporosity of the monolith. Huge complexity and tortuosity of the network of pores allows a high contact time of the reagent with the monolith which leads to a greater effectiveness, the material being equal.

[0114] Figure 10 shows XRD pattern of a monolith LTA obtained by DIW compared with the theoretical pattern of zeolite LTA, therefrom the presence of significant amounts of extraneous (crystalline or amorphous) phases is noted.

[0115] Monolith fragments were subjected to volumetric tests for capturing CO2at 25°C. As shown in Figure 11 , the material shows a capture capability at 1 bar comparable or slightly higher than commercial granules LTA used as reference. With respect to a granular bed, the properties of a structured adsorbent obtained for processes of Additive Preparation (for example, porosity, load loss, mechanical properties) can be optimized for each specific application with significant method advantages.

[0116] Figure 12 shows the results of compression mechanical tests on monoliths LTA with 50% of design porosity and a log-pile geometry, that is formed by overlapped layers of parallel filaments alternated at 0° and 90°. The statistical feature of the fracture typical of the ceramic materials was evaluated according to the Weibull approach, the distribution thereof of the probability of survival of the samples is shown compared to the experimental data in Figure 12. The calculated parameters of the Weibull distribution, oO and m, are respectively 2.4 MPa and 6.7, giving a probability of survival of the samples higher than 90% for compression efforts lower than about 1.5 MPa. Despite the efforts thereto an adsorbent subjected to a gas flow are very low, a good mechanical resistance is in any case required to allow handling the piece.

[0117] 3 Preparation of monoliths for Additive Manufacturing without using filler

[0118] 3.1 Procedure

[0119] D. 50g of synthesis gel are prepared as described in 1A-1C.

[0120] E. The gel is incubated in a sealed container for about 12 h at 75°C. [Author’s Notes: The increase in viscosity associated to the partial crystallization of the mixture allows to obtain the rheological properties required to printing without the need for fillers or additives.

[0121] F. After aging, the slurry is mixed, printed and crystallized as described in 2C-2F.

[0122] 3.2 Characterization

[0123] An image of a monolith LTA obtained by Direct Ink Writing without adding filler as described in the procedure in section 3.1 is shown in Figure 13.

[0124] Figure 14 shows XRD pattern of the monolith compared with the theoretical one of zeolite LTA. High crystallinity is noted, considering the absence of amorphous bands. The peaks at 24° and 35°, not ascribable to zeolite LTA, are compatible with the presence of small impurities of another zeolite (hydrosodalite), but the absence of other univocal peaks of hydrosodalite makes the identification uncertain.

Claims

CLAIMS1 . A method for the preparation of monoliths of zeolite A (LTA) comprising the following steps: i) preparing a gel by mixing a silica precursor, a sodium precursor and an alumina precursor in aqueous solution, so that the concentration of silica in said gel is comprised between 13% and 19% by weight and wherein said mixing takes place at a speed such as to obtain a homogeneous mixture; ii) forming and subsequently crystallizing the gel obtained in step i) for at least 12 hours, at a temperature of at least 60°C, so as to obtain a monolith of zeolite A; iii) drying the product obtained in step ii) for at least 12 hours, at a temperature between 75 and 500°C.

2. The method according to claim 1 , wherein said alumina precursor is selected from sodium aluminate (NaAIO2), aluminium hydroxide, aluminium alkoxides, boehmite, alumina, aluminium salts (nitrates and sulphates), kaolin, metakaolin, metallic aluminium.

3. The method according to any one of claims 1 or 2, wherein said alumina precursor is used in solution and is present in a concentration comprised between 0.5M and 0.7M, preferably about 0.6M.

4. The method according to any one of claims 1 to 3, wherein said silica precursor is selected from colloidal silica, (pyrogenic or precipitated) solid phase silica, sodium silicates, silicon alkoxides, kaolin, metakaolin, silicic acid.

5. The method according to any one of claims 1 to 4, wherein said silica precursor e colloidal silica.

6. The method according to claim 5, wherein said colloidal silica is present in a concentration between 16 and 17% by weight, preferably about 16.35% by weight.

7. The method according to any one of claims 1 to 6, wherein said sodium precursor is selected from sodium hydroxide, sodium aluminate, sodium silicates, sodium salts (sulphates, borates, carbonates, fluorides, bromides).

8. The method according to any one of claims 1 to 7, wherein said sodium precursor and said alumina precursor are the same precursor, preferably sodium aluminate (NaAIO2).

9. The method according to any one of claims 1 to 8, wherein said alumina and sodium precursor is sodium aluminate (NaAIO2), and said silica is colloidal silica.

10. The method according to any one of claims 1 to 9, wherein said gel obtained in step i) has a molar composition [x SiO2:AI2O3: y Na2O: z H2O], wherein x is a number from 1 .8 to 3, y is a number from 0.8 a 1.2, and z is a number from 20 to 30.

11. The method according to any one of claims 1 to 11 , wherein said gel obtained in step i) has a molar composition [x SiO2:AI2O3: y Na2O: zH2O], wherein x is 2, y is 1 , and z is 25.

12. The method according to any one of claims 1 to 11 , wherein said step i) takes place in a mechanical mixer at a range between 300 and 700 rpm, preferably 500 rpm, with progressive increase in speed when adding silica up to 220 rpm.

13. The method according to any one of claims 1 to 12, wherein said step ii) takes place in humid environment, with relative humidity (U.R.) of at least 95%, preferably 100%.

14. The method according to any one of claims 1 to 13, wherein said step ii) takes place at a temperature from 60 to 90°C, preferably 75°C.

15. The method according to any one of claims 1 to 14, wherein iii) takes place at a temperature of 75°C.

16. The method according to any one of claims 1 to 15, further comprising a step iv) of grinding the zeolite A monolith obtained in step iii) so as to obtain powdered zeolite A (LTA).

17. The method according to any one of claims 1 to 16, wherein said said forming step ii) is performed by pouring the gel obtained in step i) into a mould.

18. The method according to any one of claims 1 to 17, comprising after said step i) a step i-a) wherein a rheological additive and / or powder of zeolite A (LTA) is added to said gel, so as to obtain a slurry.

19. The method according to claim 18, wherein the concentration of said rheological additive in the slurry is comprised between 0 and 5% by weight, preferably 2%.

20. The method according to any one of claims 18 to 19, wherein the concentration of said zeolite A powder (LTA) in the slurry is comprised between 30% and 50% by weight, preferably 40%.

21. The method according to any one of claims 18 to 20, wherein said rheological additive is a superabsorbent polymer.

22. The method according to any one of claims 18 to 21 , wherein said superabsorbent polymer is selected from carboxymethylcellulose (CMC), methylcellulose, ethylcellulose, hydroxypropylcellulose, and / or polyacrylic acid.

23. The method according to any one of claims 18 to 22, further comprising after said step i-a) a step i-b) of homogenization of the slurry obtained in step i-a).

24. The method according to any one of claims 18 to 23, wherein said homogenization step i-b) comprises at least three mixing cycles of at least 5 minutes, interspaced with at least two sonication cycles of at least 5 minutes, and subsequent defoaming of at least 5 minutes.

25. The method according to any one of claims 18 to 24, wherein said forming step ii) is performed by extrusion of the slurry.

26. The method according to any one of the preceding claims, wherein said forming process is performed with the Direct Ink Writing (DIW) method.

27. The method according to any one of the preceding claims, wherein said zeolite A powder (LTA) has a maximum particle size at least 10 times smaller than the diameter of the printing nozzle.

28. The method according to any one of claims 18 to 28, further comprising a step of eliminating the rheological additive.

29. The method according to claim 28, wherein said step of eliminating the rheological additive comprises subjecting the zeolite A monolith to a heat treatment at a temperature from 350 to 500°C, preferably 400°C, for a period of time of at least 10 hours, with a heating ramp lower than 20°C / minute in air, so as to eliminate the rheological additive.

30. The method according to any one of the preceding claims, comprising the following steps: i) preparing a gel by mixing in aqueous solution a silica precursor, a sodium precursor and an alumina precursor, so that the concentration of silica in said gel is comprised between 13% and 19% by weight and wherein said mixing takes place at a speed such as to obtain a homogeneous mixture;i-x) incubating the gel obtained in step i) for a period of time comprised between 8 and 16 hours, at a temperature from 60 to 90°C, so as to obtain a slurry; ii) forming with a process carried out by extrusion, and subsequently crystallizing the gel obtained in step i) for at least 12 hours, at a temperature of at least 60°C, so as to obtain a monolith of zeolite A; iii) drying the product obtained in step ii) for at least 12 hours, at a temperature between 75 and 500°C.

31. The method according to claim 30, wherein said incubation takes place in a hermetically closed environment.

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