Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2002-02-07
2003-06-24
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C096S011000, C423S328200, C502S077000, C502S078000, C502S079000
Reexamination Certificate
active
06582495
ABSTRACT:
The invention relates to a process for controlled production of supported zeolite membranes, to the membranes obtained and to their use in separation processes.
Zeolites have the principal advantages of having a crystalline structure and a defined pore size, of having modifiable surface properties (in particular in terms of hydrophilic nature/organophilic nature and acidity) and linked to the chemical composition of the framework. The particular topology of zeolites and their cation exchange properties means that they can be used for applications of separation by selective adsorption or for catalytic reactions. For a given structural type, connected to the crystalline family to which each zeolite belongs (these structure types have been described in the work by Meier, W. M., Olson, D. H., in “Atlas of Zeolite Structure Types” (1992), Butterworth-Heinemann, Ed.), separation of molecules present in a mixture can take place by selective adsorption and/or size exclusion in particular. However, separation on a powdered zeolite is a discontinuous process. In contrast, a zeolite membrane offers the possibility of separating molecules by a continuous process, which may be particularly advantageous from a technological and economical viewpoint.
A variety of processes for producing zeolite membranes have already been described. The hydrothermal route using porous supports has the advantage of stabilising the zeolite crystals in a porous matrix (alumina, stainless steel, for example) and at the surface thereof. European patent application EP-A-0 778 075 describes a process for producing zeolite membranes supported by porous glass. United States patent U.S. Pat. No. 5,429,743 and International patent application WO-A-95/29751 describe protocols for producing composite membranes supported by an inorganic macroporous matrix. Reference can also be made to documents U.S. Pat. No. 4,099,692, WO-A-93/19840, U.S. Pat. No. 5,567,664 and WO-A-96/01683. International patent application WO-A-00/33948 describes a process for producing composite zeolite membranes supported on tubular solids that are optionally multi-channelled. Such composite zeolite-based membrane materials are formed from a zeolitic phase deposited on a support.
Such preparations of membrane materials are carried out by isothermal heat treatment of a mixture containing precursors of the zeolitic phase, which is active for separation. Crystallisation is carried out at a fixed temperature maintained during that step of the synthesis (isothermal treatment). The zeolite crystallisation step can also be repeated a number of times. The synthesis is then reproduced after optionally cooling the material to ambient temperature, washing and drying of said material. The operations are identical and enable successive layers and/or zeolite crystals to be deposited that fill the interparticular spaces. In such cases, the preparation period is considerably prolonged. That mode of multi-step synthesis also encourages the production of thick layers of zeolites, which can crack when calcining the membrane (Vroon, Z. A. E. P., Keizer, K., Burggraaf, A. J., Verweij, H.,
J. Membr. Sci.
144 (1998) 65-76). Further, increasing the thickness can considerably limit the transfer of material through the membrane during the separation operation, thus reducing the technical and economic advantage of the membrane separation operation, due to a reduction in the productivity of the separation step. Further, this mode of multi-step synthesis requires a large quantity of precursors for the zeolitic phase, which considerably increases the cost of the starting materials and the precursors used. It also has the disadvantage of prolonging the period for producing the membrane material and augmenting the operating cost of the separation operation.
Apart from the membrane material preparation field, zeolite crystals (powdered material) have been obtained during polythermal preparations, carried out at different temperatures as the reaction progresses, in the absence of a support. U.S. Pat. No. 5,089,243 describes preparing alumino-silicate powders using a two-step crystallisation process in the absence of an organic agent. The first step is carried out at 240° C. to 325° C. for 1 to 20 minutes. The second step is carried out between 120° C. and 225° C. for 1 to 100 hours. Recently, non-supported zeolite crystals have been obtained by raising the temperature during synthesis (Li, Q., Creaser, D., Sterte,
J. Microporous and Mesoporous Mater.
31 (1999) 141-150). A first step is carried out at 60° C. or 80° C. and enables the number and density of the crystals to be controlled. Raising the temperature to 100° C. encourages crystallite growth. In those two types of preparation, the materials obtained are powders, in the form of divided solids, and in no case constitute membrane materials with a solid continuous layer for use in separation.
One of the difficulties of preparing zeolite-based membranes resides in controlling zeolite crystallisation to obtain zeolite crystals that are properly bound to the support, principally localised in the pores of the support, forming a continuous composite zeolite/support layer (obtained by obstructing the voids in the support with crystals of the zeolitic phase) and preferably sufficiently fine to limit the resistance to transfer through the membrane material. The majority localisation of the zeolitic phase in the pores of the support endows it with very good thermal resistance and mechanical resistance of the membrane material. However, it is not excluded that a minor portion of the zeolitic phase is localised on the external surface of the support. One of the essential aims of the present invention is to provide a method for controlled production of supported zeolite membranes in which the zeolitic phase has the characteristics described above. The zeolitic membranes obtained by the process of the invention have better separating power than membranes synthesised in accordance with prior art methods. They also have very high structural integrity, i.e., an absence of defects in the structure of the zeolitic phase and an absence of interparticular spaces, i.e., the voids present between the zeolite crystals.
The present invention concerns a process for preparing a supported zeolite membrane constituted by a composite continuous zeolite/support layer of controlled thickness, wherein the zeolite crystals are principally localised in the pores of a porous support and optionally on the external surface thereof. This preparation method comprises at least the formation of a precursor gel of said zeolite, bringing said gel into contact with said support and crystallising the zeolite from said gel. The preparation process of the invention is characterized in that said zeolite is crystallised by carrying out a non-isothermal thermal programme comprising at least three steps in succession constituted by a first constant temperature stage carried out at a temperature in the range 50° C. to 300° C. followed by cooling to a temperature of strictly less than 50° C. followed by a second constant temperature stage carried out at a temperature in the range 50° C. to 300° C. This preparation process of the invention can produce high performance materials for separation in a single step (one-pot crystallisation).
The first constant temperature stage is preferably carried out at a temperature in the range 80° C. to 220° C. This first constant temperature stage is maintained for a period in the range 1 hour to 15 days, preferably in the range 3 hours to 72 hours.
The reduction in temperature from the first constant temperature stage is such that the mixture is cooled to a temperature that is strictly less than 50° C., preferably less than 40° C. and is maintained at that temperature for a period of 1 minute to 72 hours, preferably in the range 20 minutes to 9 hours, more preferably 30 minutes to 5 hours.
The second constant temperature stage is preferably carried out at a temperature in the range 80° C. to 220° C., more preferably carried out at a temperatur
Chau Christophe
Dalmon Jean-Alain
Miachon Sylvain
Prevost Isabelle
Institut Francais du Pe'trole
Millen White Zelano & Branigan P.C.
Spitzer Robert H.
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