Process for the preparation of a binary membrane top layer

Abstract: Preparation of binary membrane ultrafiltration (UF) toplayers (Al₂O₃-CeO₂ and Al₂O₃-TiO₂) to make chemically stabe UF membranes and/or catalytically active membranes. Two stable sols are prepared and mixed under controlled conditions and a top layer is prepared on a support under controlled conditions.

Description: The invention relates to a process for the preparation of a binary membrane top layer in a ceramic ultrafiltration membrane system consisting of a mesoporous top layer and a macroporous inorganic carrier. An ultrafiltration membrane is here understood to be a membrane with a top layer having pores smaller than 0.02 µm, and a carrier having pores larger than 0.1 µm. It is known from the Netherlands Patent Application NL-A-8303079 to prepare a porous membrane top layer from a single component via the sol/gel technique. Inorganic membranes are here prepared by lining a dry macroporous inorganic carrier by dipping in a stable sol of colloidal metal oxide or metal hydroxide particles with a certain particle size, the concentration of the dispersed phase in a non-solvent for the metal oxide or metal hydroxide particles being 0.01 to 25 %w and the sol in the boundary layer being first converted into a lyogel, removing the carrier from the liquid and drying in air to a xerogel and subsequently calcining and possibly sintering. It has, however, been found desirable to change the chemical composition of the membrane top layer for catalytical purposes or for gas/solid adsorption/diffusion. Furthermore, the chemical stability and the possibilities of shaping the top layer have been found to leave room for improvement. According to the invention, this is achieved by the use of a multicomponent membrane top layer free of defects (especially binary defects) and of cracks. The process according to the invention is thereto characterized in that a) a stable boehmite sol and a stable sol of either CeO₂ or TiO₂ is prepared and the sols thus obtained are subsequently mixed to form a binary (Al₂O₃-CeO₂ or Al₂O₃-TiO₂) sol; b) the top layer is applied to the carrier by dipping in the aforesaid sol; c) the top layer thus obtained is dried and subsequently calcined. It is pointed out that binary membranes are known per se from the article by Cot et al in "Industrial Ceramics" 8(3) (1988) 143-8. This discusses the RuO₂-TiO₂ system. The invention also relates to a membrane provided with a binary top layer obtained according to the aforesaid process, as well as to the use of a binary top layer thus obtained. The invention will now be explained by way of example in more detail with reference to the drawing and the examples, the figure schematically showing a cross-section through a binary layer on a carrier. With reference to the figure, an α-Al₂O₃ carrier 1 is shown on which a binary top layer 2 TiO₂ + gamma-Al₂O₃ is applied. The dimension L is here 2 mm, while the dimension l is 1 to 2 µm. Such a binary membrane is prepared in the following way: An α-alumina carrier is dipped for 2 seconds in a dip solution of a binary sol plus 3 %w on a dry solids basis of polyvinyl alcohol (PVA). Binary sols are made by mixing alumina (1 mol/l) and titania (0.2 mol/l) sols. Alumina sols have a pH of 3 to 6 and titania sols 2 to 3. While mixing the sols, the pH must remain below 5, since otherwise flocculation may take place. The isoelectric point of titania is at pH = 5. After dipping, the preparation is dried at 40°C and 60% relative humidity for 15 hours and then calcined at 450°C for 3 hours with a heating and cooling rate of 10°C/hour. There is a relationship between the quantity of added PVA and the dipping time required for obtaining a certain layer thickness. Because only a small quantity of PVA (binder) can be added to the dip solution, since PVA reacts with titania resulting in flocculation, the dipping time is in this case short (thin layer). The heating and cooling rate during calcination is low in order to avoid cracking. According to the invention, two stable sols of alumina (boehmite) and either CeO₂ or TiO₂ are prepared and mixed to form a binary sol. In the case of TiO₂, sulphate ions are added to the sol in order to stabilize the anatase phase of TiO₂ at higher temperatures. The stability of the binary sol must be controlled, since both CeO₂ and TiO₂ sols are more acidic than alumina (boehmite) sols. The pH of stable boehmite sols, TiO₂ sols and CeO₂ sols are 3.5, 2 and 1 respectively. TiO₂ and boehmite sols can be mixed to 65 %w TiO₂. Higher concentrations of TiO₂ lead to a sharp increase in the viscosity of the binary sol. For the preparation of stable boehmite/CeO₂ sol mixtures, nitrate ions must be removed from the CeO₂ sol by dialysis until a pH of approximately 3 is obtained. Membrane top layers can, as has already been pointed out, be obtained from a single component according to the process described in the Netherlands Patent Application NL-A-8303079. It is surprising that the structure and the pore diameter of the binary membrane top layers obtained according to the invention closely resemble those of single component Al₂O₃ membrane material. The porosity varies between 35 and 50%. Microporous membrane top layers of CeO₂ cannot be prepared, since this material coalesces at 450-600°C, accompanied by considerable shrinkage and consequently a high chance of cracking. There is hardly any or no grain growth of the dispersed phase (in contrast with the single component membrane), so that in this way a high degree of dispersion of the minority component can be maintained up to a high temperature, for example 900°C. Surprisingly, these binary membranes are free of cracks up to a thickness of 10 µm after their preparation (measured via gas permeation), despite expansion differences between the Al₂O₃ and TiO₂ components or the Al₂O₃ and CeO₂ components. After calcination at 450 or 600°C, TiO₂ and CeO₂ are present as particles between the Al₂O₃ platelets, i.e. a two-component and two-phase membrane top layer. The invention will now be further explained with reference to some examples: EXAMPLE 1 A ceria sol made by peptizing a Ce-hydroxide (oxide) precipitation with acid has a pH of about 1. By means of dialysis, the pH is brought to a value of about 3-4. The sol is then mixed with a boehmite sol of pH 3.5 in a concentration ratio of Al₂O₃/CeO₂ = 3.5 equal to 35 %w CeO₂. By means of the membrane top layer preparation method, described in the Netherlands patent application NL-A-8303079, a top layer is applied to a 2 mm thick homogenous carrier with a pore diameter of 160 nm. The membrane top layer is dried under controlled conditions (40°C/60% relative humidity) and calcined at 450 or 600°C. The membrane is crack-free, as shown by He gas permeation as a function of pressure up to 500 kPa. EXAMPLE 2 Binary sols are prepared by mixing a boehmite sol (AlOOH 1 mol/l) and a titanium hydroxide sol (O.2 mol/l). The first sols have a pH of 3-6 and the second a pH of 2 to 3. While the sols are being mixed, the pH must remain below 5, since otherwise flocculation will occur, as the isoelectric point of titanium hydroxide sols is 5. 3 %w binder (PVA) is added to the titanium hydroxide sol in order to make the formation and progress less critical. The Al₂O₃/TiO₂ concentration ratio after calcination at 450°C is 35 %w. The top layer is applied in the same manner as in Example 1. Within the examples the concentration can be varied: O< TiO₂/Al₂O₃ ≦ 75 %w TiO₂; O<CeO₂/Al₂O₃ ≦ 60 %w CeO₂. Various types of carriers can be used: homogeneous carriers with pores of between 2 nm and 1 µm; multilayer (composite) carriers (including commercial) with the same pore sizes in the top layer. Calcination is possible at between 400 and 900°C without substantial alteration in the structure. Various modifications of the invention will become apparent to those skilled in the art from the foregoing description, accompanying drawing and examples. Such modifications are intended to fall within the scope of the appended claims.

Claims: Process for the preparation of a binary membrane top layer (2) in a ceramic ultrafiltration membrane system consisting of a mesoporous top layer (2) and a macroporous inorganic carrier (1), characterized in that (a) a stable boehmite sol and a stable sol of either CeO₂ or TiO₂ are prepared and the sols thus obtained are subsequently mixed to form a binary Al₂O₃-CeO₂ or Al₂O₃-TiO₂Sol; (b) the top layer is applied to the carrier (1) by dipping in the aforesaid binary sol; and (c) the top layer (2) thus obtained is dried and subsequently calcined. Process according to claim 1, characterized in that a ceria sol is made by peptizing a Ce-hydroxide or Ce-Oxide precipitate with acid and mixing this sol with a boehmite sol of pH 3.5 in a concentration ratio or Al₂O₃/CeO₂ = 3.5 equivalent to 35 %w CeO₂. Process according to claim 1, characterized in that a binary sol is prepared by mixing a boehmite sol and a titanium hydroxide sol, the pH remaining below 5 during the mixing of the sols. Process according to claims 1-3, characterized in that the carrier (1) has pores of between 2 nm and 1 µm. Process according to claim 4, characterized in that the carrier (1) consists of several layers. Process according to claim 4, characterized in that the carrier (1) is homogeneous. Process according to claim 2, characterized in that the top layer (2) is applied to a 2 mm thick homogeneous carrier (1) with a pore diameter of 160 nm. Process according to claims 1-7, characterized in that drying is carried out at 40 °C and 60% relative humidity. Process according to claims 1-8, characterized in that calcination is carried out at between 400 and 900 °C, and in particular at 450 °C or at 600 °C. Process according to claims 1-9, characterized in that O< TiO₂/Al₂O₃ concentration ratio ≦ 75 %w TiO₂. Process according to claims 1-9, characterized in that O< CeO₂/Al₂O₃ concentration ratio ≦ 60 %w CeO₂.

Inventors: BURGGRAAF ANTHONIE JAN, KEIZER KLAAS, ZASPALIS CASSILIS THOMAS, BURGGRAAF, ANTHONIE JAN, KEIZER, KLAAS, ZASPALIS, CASSILIS THOMAS
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1990
Main classification: B01D71
IPC codes: B01D71