Synthesis of thin film zeolite membrane

Abstract

Synthesis of crystalline CHA Zeolite powder and highly oriented defect-free CHA Zeolite membrane is described herein by the secondary growth of small pore silicoaluminophosphate powder having chabazite (CHA) type framework crystals from economic raw material Zeolite films are formed through the gradual growth and integration of individual zeolite seed crystals within a designated seed layer and through the secondary growth method by synthesizing the membrane of the zeolites using a hydrothermal process. Thereby generating defect-free membranes with improved permeance by utilizing the smaller seed crystals typically ranging from 2 to 5 μm. Ultrasound has been used to accelerate the crystallization process by enhancing mass transfer, promoting uniform nucleation, and facilitating the dispersion of precursor materials as well as to improve productivity in terms of enhanced adherence of seed crystals to support surfaces, leading to crystal quality and morphology that leads to faster synthesis times, as evidenced by the rapid generation of seed crystals.

Description

SYNTHESIS OF THIN FILM ZEOLITE MEMBRANETECHNICAL FIELD OF THE INVENTION

[0001] The present disclosure relates to the synthesis of small pore silicoaluminophosphate powder having chabazite (CHA) type framework and thin-film layer membrane.BACKGROUND OF THE INVENTION

[0002] The field of membrane technology is constantly looking for better materials to improve the efficiency of separation operations. Selecting suitable starting materials for membrane production is critical. To qualify as membrane-grade material, the criteria include being easily processable, inexpensive, and stable. According to recent research, N-aryl-linked spirocyclic polymers have shown promise in meeting these requirements for the separation of complex hydrocarbon mixtures (Finn et al., 2020). However, broader application of polymeric membrane is hindered by poor chemical and thermal durability, such as plasticization and aging under rigorous conditions (e.g., high pressure), as well as limited selectivity and flux (Tin et al., 2004).

[0003] Zeolite-based materials (such as molecular sieves, membranes, and catalysts) are widely utilized for separation purposes due to their exceptional properties. Their large surface area, variable pore diameters, and remarkable thermal stability enable highly selective and efficient molecular separation. These qualities make zeolites suitable for various industries, including petrochemicals, gas separation, and environmental technology.

[0004] Zeolites, known for their microporous aluminosilicate structure, have a unique three-dimensional network where tetrahedra are joined by oxygen atoms, creating numerous intermolecular cavities and channels. These holes and channels are sized to accommodate small molecules (such as water and carbon dioxide) allowing for selective adsorption, separation, and catalysis.

[0005] Zeolite membranes, in particular, leverage these properties to achieve highly selective and efficient molecular separation. In petrochemicals, zeolite membranes are used forPage No. 1 / 24refining and processing hydrocarbons. In gas separation, they help to isolate specific gases from mixtures. In environmental technology, zeolite membranes play a crucial role in removing pollutants from air and water, significantly enhancing the efficiency and effectiveness of these industrial processes.

[0006] Specifically, thin-film zeolite membranes are highly sought after for gas mixture separation due to their high permeance, selectivity, shelf-life, surpassing other types of membranes like polymeric ones.

[0007] The most effective molecular sieving membrane among zeolite membranes is silicoaluminophosphate, which has a chabazite (CHA) type framework and a pore diameter of 0.38 nm. It has been used for a variety of applications and, most notably, demonstrated excellent performance in the separation of H2 gas from other light gases like CO2, N2, CH4, etc. CHA zeolite is composed of double-six-membered rings (d6r) and CHA cages (CHA cages refer to the specific structural units within the zeolite framework), which form sinusoidal pore networks. Sinusoidal pore networks refer to a structure where the pores or channels within the zeolite framework exhibit a sinusoidal or wave-like shape. CHA type zeolite offers substantial advantages in membrane applications, especially for selective separation of small compounds (e.g. CO2 from biogas).

[0008] However, controlling the orientation of the zeolite crystals within the membrane, defect generation, and non-uniformity in pore distribution pose a significant challenge. In this regard, Pham et al., (2013) studied gel-free secondary growth techniques, for uniformly oriented silica MFI zeolite films, addressing the challenges related to continuous zeolite film growth on porous supports, aiming to replace energy-inefficient distillation methods. This may be seen in “Gel-Free Secondary Growth of Uniformly Oriented Silica MFI Zeolite Films and Application for Xylene Separation" (Pham, T. C. T, Nguyen, T. H., Yoon, K. B., Angew. Chem. Int. Ed., 2013, h i s: ZdQLorgZlQ 0Q2 / an 10,201301766). Their work emphasized growing films with highly oriented channels to maximize permeance, achieving thin films to enhance permeance further, and ensuring a pinhole-free (defect-free) structure to optimize separation factors. The zeolites should ideally be highly orientated, or aligned in a certain orientation, in order to maximize molecular transport channels and enhancePage No. 2 / 24permeability and selectivity. High crystalline membranes have better mechanical strength and stability due to their well-organized structure, which increases the effectiveness of their separation.

[0009] Techniques have been devised to produce preferential orientation, in which the majority of crystals align in the desired pattern. Techniques such as templating, magnetic field, electric field, and shear flow approaches provide accurate crystal orientation but are complex, expensive, and difficult to scale when compared to seeding for zeolite membrane synthesis.

[0010] Typically zeolites are synthesized by using commercially available expensive chemical reagents like sodium silicates, aluminum salts (alumina source) or colloidal silica in a highly alkaline media, fumed silica, water glass (silica source) etc. Some synthesis mixtures may also include quite expensive organic structure-directing agents or seeding to obtain highly crystalline products. The primary challenges to be addressed include the necessity for high accuracy in product creation and the associated costs of a complex fabrication process. Indeed, high-selectivity separations are possible with zeolite membranes. Specifically, for gas phase separations, this requires nearly perfect membranes, with a very low concentration of defects, the separation selectivity plummets as the number of defects (inter-crystalline gaps a few nanometers in size) increases. In recent studies, Korelskiy et al., (2017) demonstrated that high-quality MFI (mobil-type five) zeolite membranes can still contain defects comprising up to 0.5-0.7 % of the membrane area. These defects can significantly impact membrane performance and longevity. Among the various types of defects identified, grain boundary defects and cracks are the most commonly reported. Grain boundary defects occur at the interfaces between different crystal grains, while cracks can form due to mechanical stresses or thermal cycling. These defects can lead to reduced selectivity and increased permeability for unwanted molecules, compromising the overall efficiency of the membrane in separation processes. Presence of the defects in membranes alter performance by decreasing the separation factor. High defect densities can dominate separation, reducing the effectiveness of membranes. Liu et al., (2021) studied MFI membranes for para / ortho-xylene separation, estimating parameters like thickness, defect density, and pore blockage. They found that at a defect porosity of 10’7, separation factors forPage No. 3 / 24para-xylene and ortho-xylene were ranging from 7000 to 10000. However, at higher defect porosity (IO-6), the separation factor dropped sharply to 2000, indicating reduced efficiency. Additionally, flux of para-xylene varied with defect porosity, ranging from 10’5mol m’2s at 10’7defect porosity to 10’4mol m’2s at 10’6defect porosity.

[0011] A US Patent numbered US 2016 / 0101415 provides for methods of enhancing the catalytic activities of 8-MR zeolites, the methods comprising treating a precursor 8-MR zeolite that has been prepared without the use of an organic structure directing agent and having an Si / Al ratio of less than 5, with high temperature steam for a period of time sufficient to extract at least a portion of the aluminum from the precursor zeolite framework to form a steam-treated zeolite having an Si / tetrahedral Al ratio of greater than 5, wherein the steam has a temperature in a range of from about 350°C to about 850°C. The compositions produced by these methods and their use in catalytic reactions are discussed.

[0012] Despite the potential for high separation performance in ideal zeolite membranes (an ideal zeolite membrane features high selectivity and permeability due to uniform pore sizes and low defect density, while maintaining stability across various thermal, chemical, and mechanical conditions), defects introduced during synthesis can impair separation efficiency. Defects such as thermal cracks arise from thermal expansion mismatches and grain boundary (crystalline regions) defects originate due to mismatched crystalline orientation. Minimizing these defects poses challenges due to the polycrystalline nature of zeolite films, arising from discrete and randomly oriented seed crystals.

[0013] Further, there is a need for defect free membranes with improved permeance enhancing membrane efficiency and performance. As a person skilled in the art may realize “Permeance” of a zeolite membrane refers to the rate at which a specific substance (e.g. gas or liquid) permeates through the membrane per unit area and per unit pressure difference and selectivity may refer to the ratio of permeances of two different species. Thin films enhance membrane separation efficiency by allowing smoother permeate flow through zeolite pores. Whereas, thin films synthesized using larger seed crystals may lead to membranes with defects due to inadequate coverage.Page No. 4 / 24

[0014] Further, conventional seed synthesis methods typically involve longer synthesis times, leading to slower nucleation and crystal growth kinetics, potentially resulting in variations in crystal size and distribution. Additionally, these methods may yield (“yield” denotes the amount of desired product obtained) zeolite crystals with lower purity and irregular morphology, along with poorer adherence to support surfaces.

[0015] The present invention provides an economical and environment friendly way of said synthesis.SUMMARY

[0016] The present invention provides an economical and environment friendly method for synthesizing a highly oriented, defect-free chabazite (CHA) type zeolite thin film membrane. In one embodiment, the method comprises obtaining, cleaning and drying a support material and modifying surface of the support material with silica and depositing silica layer onto the inner side of the support material. Further, drying and then heating takes place. Further steps involve depositing a CHA zeolite seed crystals on the support material having a silica layer and subjecting the seeded support to secondary growth hydrothermal synthesis under controlled temperature and time conditions to obtain a continuous CHA zeolite membrane layer. Further, the steps of calcining the CHA zeolite membrane under controlled heating and cooling rates may take place wherein the membrane exhibits improved permeance and selectivity in molecular separation.

[0017] In an embodiment herein the support material is cleaned with acetone in a sonication bath for 10-15 min to remove any impurities and drying the support materials in an oven for 12 h at 100 -120 °C. Further, the support material may comprise alumina, clay-alumina, zirconia, titania, silica, or combinations thereof, having a porosity of 10-30%.

[0018] Further, in an embodiment, the silica intermediate layer is formed by hydrolyzing tetraethyl orthosilicate (TEOS) in ethanol in the presence of water and acid, and depositing the silica layer by dip-coating.Page No. 5 / 24

[0019] In an embodiment, the reaction mixture has a final molar TEOS: ethanol: water: acid ratio of 1 : 3.8: 6.4: 0.085. Further, the support material having a silica intermediate layer may be coated by dipping, rolling, or combinations thereof to obtain a seeded support.

[0020] In an embodiment, the calcination may be performed at 500-600 °C with heating and cooling rates of 0.02 °C / min and 0.5 °C / min, respectively. As per an embodiment, the CHA Zeolite crystals may be formed by a process comprising the steps of preparing a first mixture (mixture A) by adding a phosphorus source selected from phosphoric acid, phosphonic acid or phosphate esters to deionized water, followed by addition of an aluminium source, and stirring to obtain a homogeneous mixture. Further step involves preparing a second mixture (mixture B) by mixing a silica source selected from colloidal silica, silica derived from silica sand, or water glass, with a structure-directing agent selected from morpholine, tetraethylammonium hydroxide (TEAOH), or a mixture thereof, and water, and stirring to obtain a homogeneous mixture. Further, steps involve combining mixture B with mixture A under stirring while applying ultrasonic irradiation to promote nucleation and control seed crystal size. Further, subjecting the combined mixture to hydrothermal crystallization to obtain CHA zeolite seed crystals having a wherein the CHA zeolite crystals having a particle size in the range of 2 to 5 pm, a chabazite-type framework, and a pore size of about 0.38 nm, is undertaken.

[0021] Further, in one embodiment, the ultrasonic irradiation with frequencies ranging from 20 kHz to 10 MHz is applied during at least one stage selected from during addition of the silica source to mixture A, after completion of silica addition; and after formation of the aluminosilicate mixture. Further, the hydrothermal crystallization for seed synthesis may be conducted at 170-220 °C for 24-120 hours.

[0022] The invention herein also discloses a chabazite (CHA) type zeolite thin film membrane obtained by the method of any preceding claim, the membrane comprising, a porous ceramic support coated with a silica intermediate layer; and continuous polycrystalline CHA zeolite layer having preferential crystal orientation, substantially free of intercrystalline defects, and having a thickness of 1 to 20 pm. Further disclosed is a system for synthesizing CHA zeolite seed crystals, comprising a crystallization vessel configured to hold an aluminosilicate reaction mixture, an ultrasonic probe configured to deliver ultrasonic irradiation at a power of 50-150 W and duty cycle of 30-70% to thePage No. 6 / 24reaction mixture, an agitator for uniform mixing; and a temperature control arrangement for maintaining crystallization conditions in the range of 170-220 °C.

[0023] Further, in one embodiment, the phosphorus source comprises phosphonic acid or phosphate esters to enhance control over material properties and reduce environmental impact. Further, the preferential orientation is achieved by matching hydroxyl group sites on the silica layer and specific crystallographic facets of the CHA crystals.

[0024] Further, using this approach disclosed is a defect-free ( or low defects) composite membrane comprising a polymeric support (here active material is CHA zeolite) or polymer-zeolite composite fibers, wherein a thin and defect free CHA zeolite layer is formed either by roll-to-roll coating of a polymer membrane or by roll-to-roll electrospinning of a CHA-polymer precursor gel (liquid) into fine fibers (solid ID / OD: upto 400 micron), followed by drying to obtain a flexible hollow fiber membrane with molecular sieving properties. Please Note: ID / OD of the fiber is dependent on design of spinnerate nozzle.BRIEF DESCRIPTION OF DRAWINGS

[0025] The illustrations / drawings included in the specification are an integral part of the present invention and aid in better understanding the information provided. These illustrations / drawings showcase practical applications of the current disclosure and when combined with the written description, help to clarify its main concepts. The drawings are illustrated by the means of flow diagrams, schematic representations and wherein:

[0026] Figure 1 illustrates a schematic representation of the setup used for the ultrasound-assisted seed synthesis as per an embodiment herein;

[0027] Figure 2 illustrates the process flow diagram for the synthesis of CHA zeolite thin film membrane, as per an embodiment herein;

[0028] Figure 3 illustrates a schematic representation feasible binding mechanism between zeolite seed crystals and the support surface via SiO2 as an intermediate linker, as per an embodiment herein; and

[0029] Figure 4a illustrates a schematic of top view of the zeolite membrane seeded with CHA zeolite on inner surface of the support tube with silica intermediate layers as per anPage No. 7 / 24embodiment herein; and Figure 4b illustrates a schematic representation for the cross section of the zeolite membrane seeded with CHA zeolite on inner surface of the support tube with silica intermediate layers as per an embodiment herein;

[0030] Figure 5 illustrates the particle size distribution analysis of the CHA seeds prepared by ultrasound assisted method as per an embodiment herein.DETAILED DESCRIPTION

[0031] Various embodiments herein provide a process for the synthesis of Crystalline CHA Zeolite Powder and Highly oriented defect-free CHA Zeolite Membrane. The following description provides specific details of certain embodiments of the invention illustrated in the drawings to provide a thorough understanding of those embodiments. Further, in one embodiment, the secondary growth method, utilizing small-sized seed crystals synthesized using ultrasound, is employed to mitigate defect formation.

[0032] In various embodiments herein a seeding strategy may be used, in which a thin layer of zeolite seeds is deposited on a substrate to ensure the formation of high-quality membranes and to promote homogeneous crystal development.

[0033] In the present disclosure, the seed layer, comprising CHA zeolite crystals aligned in a specific desired orientation, guides the subsequent growth process, leading to a highly oriented CHA zeolite membrane with the potential for superior performance. The usual problems faced such as improper crystallization conditions, impurities in the synthesis mixture, thermal expansion mismatches, or structural distortions that cause cracks and pinholes in the membrane are overcome. Defect-free zeolite membranes are imperative for accurate separation, ensuring maximum selectivity and permeability. Defects can be reduced through synthesis, optimization and post-treatment procedures, hence improvement in the membrane performance for a variety of industrial separation applications may be made possible as elaborated herein.

[0034] The disclosure thus emphasizes the development of defect free CHA zeolite membranes with high crystallinity and preferential orientation, achieved through the strategic use of seed layers to significantly improve the effectiveness and efficiency of separation processes across a wide range of industrial applications. Disclosure hereinPage No. 8 / 24also discusses the possible use of the ultrasound (US) for the synthesis of the highly crystalline, tailored CHA seeds. Ultrasound is utilized effectively in CHA seed synthesis across various stages due to its ability to induce cavitation, a process where rapid formation, growth and collapse of bubbles create intense local heating and pressure changes. This phenomenon enhances reaction kinetics, promotes nucleation, and improves crystalline uniformity by facilitating homogeneous mixing and dispersion of reactants. Additionally, US enables control over particle size, morphology, and crystal structure, thereby contributing to the production of advanced CHA seeds with tailored properties which are subsequently used for the synthesis of the defect free and highly oriented CHA zeolite membrane.

[0035] It is understood that the terminologies used to describe a particular embodiment are intended to describe it and not to limit the scope of the invention. As used herein, the term “and / or” included any and all combinations of one or more of the associated listed items. The language used is descriptive and not intended to limit the scope of the subject matter. Phrases such as “including”, “comprising”, “having”, “containing” or “involving” are broad and encompass any related subject matter, equivalents or additional elements not explicitly mentioned.

[0036] The present invention is explained with various examples and accompanying illustrations, where the numbers used in the drawing refer to the same elements mentioned in the description. Throughout the detailed description, numeric values and ranges are given for the different aspects of the implementations discussed.

[0037] According to an embodiment of the present disclosure, the Zeolite films are formed through the gradual growth and integration of individual zeolite seed crystals within a designated seed layer and the process continues until the zeolite film reaches a critical thickness (1 to 20 pm). The critical thickness ensures that interfacial voids are completely filled, resulting in a continuous and uniform zeolite film structure. According to another embodiment of the present disclosure, the final structure typically comprises a micron-sized polycrystalline zeolite film layer on porous supports.

[0038] According to an embodiment of the present disclosure, a secondary growth method may be used in creating highly oriented, defect-free, and highly selective thin filmPage No. 9 / 24zeolite membranes. The secondary growth method involves a nucleation process resulting from the presence of seed crystals in the solution.

[0039] As may be understood with reference to Figure 3, which illustrates a schematic representing the working principle of the invention, as per an embodiment herein. More particularly, a feasible binding mechanism between zeolite seed crystals and the support surface via SiO2 as an intermediate linker, as per an embodiment herein is shown.

[0040] In one embodiment, this may involve coating the support surface with zeolite seeds followed by synthesizing the membrane using a hydrothermal process. A person skilled in the art may realize that seeding involves the introduction of either homogeneous or heterogeneous crystals into a solution undergoing crystallization to initiate and promote the nucleation and growth of additional crystals. By precisely controlling the nucleation and growth of zeolite crystals on the support surface, this method ensures formation of uniform thin layer membrane with enhanced reproducibility. The presence of seeds inhibits bulk nucleation in the synthesis mixture, directing crystal growth to the desired location and promoting the development of a continuous membrane layer. Proper orientation of seed crystals is crucial for achieving uniform membrane properties. A person skilled in the art may realize that seed crystal as used herein, refers to a small, pre-existing crystal that serves as a starting point or nucleus for the growth of a larger crystal structure. Ultrasound (US), with frequencies ranging from 20 kHz to 10 MHz, induces acoustic cavitation in liquid-solid systems, generating bubbles due to pressure fluctuations. The collapse of these bubbles releases stored energy, resulting in high temperatures and pressures conducive to crystallization. This phenomenon accelerates crystal growth rates and nucleation, reducing crystallization time and producing smaller zeolite crystals. US has been demonstrated to accelerate the crystallization process by enhancing mass transfer, promoting uniform nucleation, and facilitating the dispersion of precursor materials as well as to improve productivity in terms of enhanced adherence of seed crystals to support surfaces, leading to crystal quality and morphology. These effects lead to faster synthesis times, as evidenced by the rapid generation of seed crystals within 2 days in our study.

[0041] In one embodiment, the Zeolites synthesis process involves formation of a homogeneous gel using dispersion / mixing of suitable sources of silica and alumina in aPage No. 10 / 24solvent (usually water), and a structure-directing agent (SDA). Subsequently, synthesized gel is processed for hydrothermal crystallization at high pH and at 200-220 °C. The reaction time can vary from a few hours to several days. Finally, the solid products are washed and separated, by centrifugation or filtration process. US can be irradiated at different stages such as: 1) when the silica solution is being added dropwise to the alumina source, 2) once the addition of the silica source is completed and 3) when the mixture of the aluminosilicate is formed.

[0042] Figure 1 illustrates a schematic representation of the setup used for the ultrasound-assisted seed synthesis as per an embodiment herein. The system may include an Ultrasonic Probe 1, a glass crystallizer 2, baffles 3, formed seed crystals 4, Agitator (Propeller type) 5, a Crystallizing Solution (Aluminosilicate Solution) 6 that is maintained at the desired temperature. Figure 2 illustrates the process flow diagram for the synthesis of CHA zeolite thin film membrane, as per an embodiment herein. Understanding the working of the invention may be better understood by viewing figure 1 in conjunction with figure 2.

[0043] Followed by this, synthesized gel is processed for hydrothermal crystallization at high pH and at 200-220 °C wherein the reaction time can vary from a few hours to several days. Resulting in the formation of solid products which are washed and separated, by centrifugation or filtration process further on the Ultrasound can be irradiated at different stages such as during the first step - when the silica solution is added dropwise to the alumina source, and then during the second step when - once the addition of the silica source is completed and finally during the third step - when the mixture of the aluminosilicate is formed.

[0044] The present disclosure as per Figure 2 describes a flow diagram for the synthesis of highly oriented defect-free CHA zeolite thin film membrane, wherein the synthesis of CHA Zeolite Crystals or Synthesis of small pore CHA zeolite powder involves the following steps for its synthesis wherein the first step is - In mixture A, phosphorus sources such as phosphoric acid (85%) and other sources of phosphorus such as phosphonic acid, esters of phosphates are added to deionized water and stirred for an hour followed by the addition of aluminium iso-propoxide (98 %) powder. This mixture is stirred overnight.Page No. 11 / 24

[0045] In a further embodiment of the present disclosure, the esters of phosphates may be added in mixture A for the CHA powder synthesis. Further, in addition to the dipping procedure, a rolling technique can also be used to deposit the seed crystals on the support.Synthesis small pore CHA zeolites powder

[0046] In one embodiment, the method may involve various steps as outlined hereinbelow. The method may start with preparing two mixtures.

[0047] In mixture A, phosphorous source such as phosphoric acid (85 %) and other sources of phosphorus such as phosphonic acid, esters of phosphates may be added to deionized water and stirred for an hour followed by the addition of aluminum iso-propoxide (98 %) powder. This mixture may be stirred overnight.

[0048] In an embodiment, for mixture B, colloidal silica (40 %) or (silica derived from silica sand) and structure directing agent morpholine (99 %) / TEAOH (35 %) or mixture of the morpholine and TEAOH and the water may be mixed and stirred for an hour.

[0049] The further step may involve adding mixture B to mixture A with constant stirring. Ultrasound with power: 100 W; duty cycle: 50 % may then be used for the 30 min, while the addition of the silica is being carried out.

[0050] In another exemplary embodiment, the ultrasound may be applied after 30 min of the addition of mixture B in A. In yet another embodiment, the Ultrasound may be applied after the completion of the synthesis time.

[0051] In the synthesis CHA zeolite, silicon, aluminium, and phosphorus species react in an alkaline environment. Under ultrasonic irradiation, water molecules thermally split into free radicals (H+ and HO ). These radicals catalyse the condensation reactions between silicon, aluminium, and phosphorus species, leading to the formation of intermediate species with Si-O-P and Si-O-Al bonds. Subsequent removal of water molecules results in the formation of Si-O-P and Si-O-Al bonds, essential for nucleation. One proposed reaction mechanism involves the condensation of silicon species (e.g., Si (OH)4) with aluminium and phosphorus species to form pentacoordinate intermediates. The ultrasonic irradiation enhances these reactions, promoting faster polymerization-depolymerization cycles and accelerating the crystallization process.Page No. 12 / 24

[0052] It may be noted that, the phosphorus sources used, are phosphonic acid or phosphate esters instead of conventional phosphoric acid in the synthesis, offers the potential for improved control over material properties, enhanced catalytic performance, reduced environmental impact, and diversification of synthetic routes.

[0053] In an embodiment, representative reaction mechanism for the synthesis of CHA zeolite under ultrasonic irradiation may be as shown below:Formation of Free Radicals:H2O + US^> H- + OH-Condensation reaction:Si (0H)4 + Al [(OH)4] / '- -)^Si - O - Al + 2H2OSi 0H~)4 + (PO4)A(3 -)^Si - O - P + 2H2OPolymerization:(Si - O - Al) m + (Si - O - P) n — >■ CHA Zeolite

[0054] The further step may involve stirring the resulting mixture vigorously at room temperature and stirring further overnight to make a homogeneous sol. In one exemplary scenario, the stirring of the resulting mixture may happen at rpm: 800-1000 for 15-30 min, further step of overnight stirring may be carried out at 600 rpm. A person skilled in the art may realize that the stirring revolution per minute and time associated may vary, as long as the intended purpose is met.

[0055] In a further step, the homogeneous sol may be transferred to the Teflon-lined stainless steel hydrothermal synthesis reactor and kept in the oven at 170-220 °C for a time ranging from 24 h to 120 h. The synthesis temperature significantly affects the crystallization process, impacting nucleation, crystal growth, and final phase formation. Lower temperatures, such as 170 °C, result in slow crystallization rates, yielding amorphous solids with small particle sizes. At higher temperatures like 200 to 220 °C, fully crystalline CHA forms, but at the same time, higher temperatures above 200 °C may induce phase transformation to denser phases indicating optimal conditions for complete conversion. Synthesis of CHA zeolite is also governed by crystallization time,Page No. 13 / 24which guides the transformation from amorphous precursors to crystalline structures. Shorter durations primarily yield amorphous solids, while longer durations foster nucleation and growth of CHA crystals. Variation in temperature and time may depend on desired crystallinity, particle size, and structural properties required for the synthesized material.

[0056] Further step may involve cooling the mixture to a room temperature and centrifuging the precipitated product at 6000 rpm for 10 min. This may be further followed by washing the product four / five times with deionized water until the pH of the washing liquid is neutral. Further step involves drying the resultant powder overnight at 100 °C followed by calcination at 550 °C for 4-5 hours to eradicate the structure directing agent. Further step involves carrying out particle size analysis of the obtained CHA zeolite powder to precisely determine its particle dimensions, revealing approximately 90% of the total volume of the CHA particles is made up of smaller particles than the 5.59 pm which may be further taken for the seeding purpose. The seed synthesized and seed used for the seeding purpose may need to be smaller in size so as to create a uniform and homogeneous layer. This leads to avoid the void formation and indeed the defect reduction in the film layer of the membrane. The desired shape and size of the seed to be used are obtained by employing US at various stages of the seed synthesis and tailoring the seed properties via optimum condition.Synthesis of membranes from synthesized CHA zeolite powder

[0057] Referring to Figure 2 the different stages involved in the membrane synthesis process may be described as follows:Cleaning of the membrane support material

[0058] Porous alumina or clay-A12O3 (Porosity 10 to 30 %) tube of 10 mm outer diameter and 3 mm thickness and length of 100 mm is selected as a support material for the synthesis of the prototype membrane. Prior to the seeding, tubes are cleaned with acetone in a sonication bath for 10-15 min to remove any impurities. The next step entails drying the support materials in an oven for 12 h at 100 -120 °C.Surface modification of support materialsPage No. 14 / 24

[0059] Cleaned substrate is modified with silica through the hydrolysis and condensation of tetraethyl orthosilicate (TEOS 98 %) in ethanol (99.9 %). The reaction mixture used has a final molar TEOS: ethanol: water: acid ratio of 1: 3.8: 6.4: 0.085. Then the silica layer is deposited onto the inner side of the support surface by a dip-coating technique. The possible binding mechanism between zeolite seed crystals and the support surface via SiO2 as an intermediate linker is depicted in Figure 3. The formation of a CHA zeolite membrane involves a precise binding process between zeolite seed crystals and the support surface, utilizing an intermediate silica (SiO2) layer.

[0060] Using a silica layer as a foundational seed layer on the support surface prior to zeolite membrane growth may significantly mitigate thermal coefficient variations, thereby minimizing defects attributed to thermal expansion discrepancies during membrane formation.

[0061] Initially, the support surface is activated, and an amorphous silica layer is deposited, rich in hydroxyl groups (-OH) on its surface and connected internally by siloxane groups (Si-O-Si) as shown in Figure 3. This silica modification promotes the preferential orientation of CHA zeolite seed crystals on the support surface. The interaction between surface -OH groups of the silica-modified support and the unsaturated -OH groups on the seed crystals facilitates this alignment. Matching reaction energy further promotes uniform nuclei formation. Subsequent secondary growth hydrothermal processes utilize the oriented seed monolayer for membrane growth, with the silica layer ensuring stability.

[0062] Following this, it underwent drying at 60 °C for 1 hour to remove excess solvent and then heated at 400 °C for 1 h to eliminate organic residues. This process ensured the formation of a stable and pure silica layer on the substrate surface, providing a suitable foundation for subsequent seeding CHA crystals. The layer has the additional benefit of serving as a diffusion barrier, preventing the zeolite layer from penetrating into the support.Seeding of the support

[0063] CHA zeolite seed crystals are dispersed in deionized water under ultrasonication for 2 h. The treated support substrate is dipped in a 1-3 % zeolite seed suspension in deionized water 5 times for a duration of 15 sec. Subsequently, it is heated to 100 °C forPage No. 15 / 2412 h and calcined at 550 °C for 4-5 h in a furnace. For the calcination, the heating / cooling rate is adjusted to 0.02 °C / min and 0.5 °C / min for this process.

[0064] Besides the dipping procedure, a rolling technique may be used to deposit seed crystals on the support. This may be done in addition or as an alternative.Growth of CHA zeolite membranes

[0065] The seeded support is placed vertically in an autoclave filled with a reaction mixture having a proposed composition A12O3:SiO2: P2O5: H2O 1 :0.3: 1:66 at 170-220 °C for 24-120 h. After the hydrothermal reaction, the samples are taken out from the autoclave after cooling to room temperature. Then the membrane is washed thoroughly with deionized water until the pH of the washing liquid becomes neutral. This is followed by a calcination step to remove any organic compounds from zeolite pores. During the calcination step, heating and cooling rates are very vital to avoid the cracks and defects. Yang et al., (2019) demonstrated that calcination atmosphere significantly impacts Pd / SiO2 membrane properties, enhancing H2 permeability and selectivity. Calcination in H2 facilitated the transformation to metallic PdO, resulting in smaller pore size, higher porosity, and lower H2 permeance activation energy (2.51 kJ / mol) compared to N2. This led to superior H2 / CO2 permselectivity and improved hydrothermal stability crucial for practical applications. Membranes calcined under H2 showed higher gas permeances due to greater total pore volume, with notable increases in H2 permeance and H2 / CO2 permselectivity: at 200 °C, by 6.88 % and 2.17 %, respectively. Furthermore, Rapid temperature changes create uneven thermal expansion within the material, leading to stress and cracks. Dong et al., (2000) investigated the microstructural evolution of MFI zeolite membranes on a-alumina and yttria-doped zirconia (YZ) substrates during template removal. Heating tetrapropylammonium hydroxide (TPAOH) to 350-500 °C caused significant shrinkage in the zeolite framework, with subsequent cooling leading to zeolite crystal expansion and support shrinkage. This induced compressive stress and potential cracking in YZ-supported films without proper annealing. Intercrystalline gaps formed during template removal enlarged, creating microporous non-zeolitic pores

[0066] In another embodiment, by employing controlled rates, like 0.02 °C / min for heating and 0.5 °C / min for cooling, a uniform temperature distribution is achieved, minimizing stress and preventing crack formation. These cracks can significantly compromise thePage No. 16 / 24material's strength, durability, and overall functionality. The calcination procedure used herein may be precise and controlled. Such as, the positioning of the membrane in the furnace to avoid the crack formation.

[0067] Figure 4a illustrates a schematic of top view of the zeolite membrane seeded with CHA zeolite on inner surface of the support tube with silica intermediate layers as per an embodiment herein; and Figure 4b illustrates a schematic representation for the cross section of the zeolite membrane seeded with CHA zeolite on inner surface of the support tube with silica intermediate layers as per an embodiment herein.

[0068] From the exemplary schematic of the CHA zeolite membrane (Figure 4b) it may be observed that the seeds generated by ultrasound assisted synthesis served as the foundation for CHA membrane layers that were grown over silica-coated support by the secondary growth hydrothermal approach. The support substrate may be covered with a silica layer containing strongly hydroxylated sites (-OH) and a network of siloxane units (Si-O-Si). As a result, the abundance of hydroxyl groups on its surface may encourage the creation of a dense monolayer formed of CHA seeds oriented preferentially on the support surface, which is critical for membrane performance optimization. CHA zeolite crystals are distinguished by their surfaces possessing an abundance of unsaturated hydroxy groups (-OH), which interact with silicon hydroxyl groups (Si-OH) found in silica-coated substrates. This interaction leads in hydrogen bonding, which further anchors the seed crystals to the substrate's surface. There is an energy matching between surface -OH on silica modified support and specific crystallographic facets of CHA crystals, which allows them to be attached together via chemical bonds.

[0069] The seeds prepared by ultrasound assisted synthesis that are used as base to generate the highly oriented films of the membrane were analyzed to determine the particle size of the seeds. The obtained results as per an embodiment are depicted in Figure 5. The sample's particle size distribution shows a skew toward larger particles, with a surface- weighted mean diameter of 2.07 pm and a volume- weighted mean diameter of 3.18 pm. The cumulative distribution values provide additional insights: Dv (10) is 1.09 pm, indicating that 10% of the sample volume has particles less than this size; Dv (50) is 2.44 pm, reflecting the median particle size where 50% of the sample volume is smaller; andDv(90)is5.59pm. The particle size distribution affects the membrane's pore size,Page No. 17 / 24thickness, permeability, selectivity, and mechanical properties. Achieving a balanced particle size distribution is critical for enhancing membrane performance and ensuring the desired properties. The optimal particle size for the synthesis of efficient zeolite membranes ranges from nanometers to micrometers. It is also true that small particles promote nucleation and improve membrane homogeneity, but large particles may hasten crystal development. Nonetheless, very small particles might produce agglomeration or poor dispersion, whereas larger ones prevent membrane development. As a result, producing high-quality zeolite membranes with the appropriate qualities requires a careful balance of these many aspects.

[0070] Further, using this approach disclosed is a defect-free ( or low defects) composite membrane comprising a polymeric support (here active material is CHA zeolite) or polymer-zeolite composite fibers, wherein a thin and defect free CHA zeolite layer is formed either by roll-to-roll coating of a polymer membrane or by roll-to-roll electrospinning of a CHA-polymer precursor gel (liquid) into fine fibers (solid ID / OD: upto 400 micron), followed by drying to obtain a flexible hollow fiber membrane with molecular sieving properties. Please Note: ID / OD of the fiber is dependent on design of spinnerate nozzle.

[0071] Based on the disclosure provided herein the CHA zeolite membranes produced are extremely adaptable and have numerous uses, including gas separation, catalytic membrane reactors, water desalination, water treatment, etc. These membranes, with specific pore size and strong chemical stability, are critical for separating CO2 and N2 from CH4, which is required for natural gas purification in industries such as natural gas purification and biogas upgradation. They provide effective separation, increasing the calorific value of the gas while using less energy than traditional processes. CHA zeolites' strong affinity for CO2 makes it ideal for CO2 / N2 separation with great selectivity. Advances in membrane synthesis, such as preferentially orientated membranes and quick heating processes, improve their performance in CO2 / CH4 separation. Furthermore, CHA zeolite membranes are expected to demonstrate exceptional performance in hydrogen purification applications. These membranes also show potential in molecular sieving applications, allowing for H2 / C3Hx and H2 / C4Hx separations. Furthermore, CHA zeolite membranes serve an important role in membranePage No. 18 / 24reactors, particularly in dehydrogenation reactions like propane dehydrogenation (PDH), where zeolite membranes increase reaction yields by selectively separating hydrogen. They also help to increase DMC yield in urea methanolysis to dimethyl carbonate (UM-to-DMC) processes by efficiently separating side products such as NH3. In conclusion, CHA zeolite membranes have a wide range of applications across numerous industrial processes, proving their versatility and ability to improve efficiency and cost-effectiveness.

[0072] Further, the invention herein satisfies a highly sought-after requirement to grow a uniform seed layer with proper binding on the support structure and the well seeded support helps to create highly selective membranes. A person skilled in the art may realize that the membrane with proper thickness and defect minimization via proper crystal orientation are necessary to create a highly selective membrane (high permeability and selectivity).Page No. 19 / 24

Claims

I / We Claim:

1. A method for synthesizing a highly oriented, defect-free chabazite (CHA) type zeolite thin film membrane, wherein the method comprises: a. obtaining, cleaning and drying a support material; b. modifying surface of the support material with silica and depositing silica layer onto the inner side of the support material; c. drying and then heating; d. depositing a CHA zeolite seed crystals on the support material having a silica layer; e. subjecting the seeded support to secondary growth hydrothermal synthesis under controlled temperature and time conditions to obtain a continuous CHA zeolite membrane layer; and f. calcining the CHA zeolite membrane under controlled heating and cooling rates, g. wherein the membrane exhibits improved permeance and selectivity in molecular separation.

2. The method as claimed in claim 1, wherein the support material is cleaned with acetone in a sonication bath for 10-15 min to remove any impurities and drying the support materials in an oven for 12 h at 100 -120 °C.

3. The method as claimed in claim 1, wherein the support material comprises alumina, clay-alumina, zirconia, titania, silica, or combinations thereof, having a porosity of 10-30%.

4. The method of claim 1, wherein the silica intermediate layer is formed by hydrolyzing tetraethyl orthosilicate (TEOS) in ethanol in the presence of water and acid, and depositing the silica layer by dip-coating.

5. The method as claimed in claim 4, wherein the reaction mixture has a final molar TEOS: ethanol: water: acid ratio of 1: 3.8: 6.4: 0.085.Page No. 20 / 246. The method as claimed in claim 1, wherein the support material having a silica intermediate layer is coated by dipping, rolling, or combinations thereof to obtain a seeded support.

7. The method of any preceding claim, wherein calcination is performed at 500-600 °C with heating and cooling rates of 0.02 °C / min and 0.5 °C / min, respectively.

8. The method as claimed in claim 1, wherein the CHA Zeolite crystals are formed by a process comprising the steps of : a. preparing a first mixture (mixture A) by adding a phosphorus source selected from phosphoric acid, phosphonic acid or phosphate esters to deionized water, followed by addition of an aluminium source, and stirring to obtain a homogeneous mixture; b. preparing a second mixture (mixture B) by mixing a silica source selected from colloidal silica, silica derived from silica sand, or water glass, with a structure- directing agent selected from morpholine, tetraethylammonium hydroxide (TEAOH), or a mixture thereof, and water, and stirring to obtain a homogeneous mixture; c. combining mixture B with mixture A under stirring while applying ultrasonic irradiation to promote nucleation and control seed crystal size; d. subjecting the combined mixture to hydrothermal crystallization to obtain CHA zeolite seed crystals having a wherein the CHA zeolite crystals having a particle size in the range of 2 to 5 pm, a chabazite-type framework, and a pore size of about 0.38 nm.

9. The method as claimed in claims 1-7, wherein ultrasonic irradiation with frequencies ranging from 20 kHz to 10 MHz is applied during at least one stage selected from: a. during addition of the silica source to mixture A; b. after completion of silica addition; and c. after formation of the aluminosilicate mixture.Page No. 21 / 2410. The method as claimed in claim 7, wherein the hydrothermal crystallization for seed synthesis is conducted at 170-220 °C for 24-120 hours.

11. The method of any of claim 1 to 8, wherein the phosphorus source comprises phosphonic acid or phosphate esters to enhance control over material properties and reduce environmental impact.

12. A composite membrane constructed using method as in claim 1, comprising a polymeric support with active material as CHA zeolite or polymer-zeolite composite fibers, wherein a thin and defect free CHA zeolite layer is formed either by roll-to-roll coating of a polymer membrane or by roll-to-roll electrospinning of a CHA-polymer precursor gel in liquid state into fine fibers in solid state with inner diameter to outer diameter ratio as 400 micron, further comprising steps of drying to obtain a flexible hollow fiber membrane with molecular sieving properties.

13. The composite membrane as in claim 12, wherein the ratio of inner diameter to outer diameter of the fiber is dependent on design of spinnerate nozzle.

14. A chabazite (CHA) type zeolite thin film membrane obtained by the method of any preceding claim, the membrane comprising: a. a porous ceramic support coated with a silica intermediate layer; and b. continuous polycrystalline CHA zeolite layer having preferential crystal orientation, substantially free of intercrystalline defects, and having a thickness of 1 to 20 pm.

15. The membrane of claim 11, wherein the preferential orientation is achieved by matching hydroxyl group sites on the silica layer and specific crystallographic facets of the CHA crystals.

16. A system for synthesizing CHA zeolite seed crystals, comprising: a. a crystallization vessel configured to hold an aluminosilicate reaction mixture;Page No. 22 / 24b. an ultrasonic probe configured to deliver ultrasonic irradiation at a power of 50-150 W and duty cycle of 30-70% to the reaction mixture; c. an agitator for uniform mixing; and d. a temperature control arrangement for maintaining crystallization conditions in the range of 170-220 °C.Page No. 23 / 24

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