Molecular sieves and process for their manufacture

Details Molecular sieves and process for their manufacture Molecular sieves and process for their manufacture

1998-10-19

2001-02-20

Bell, Mark L. (Department: 1755)

Chemistry of inorganic compounds

Zeolite

Organic compound used to form zeolite

C423S705000, C423S707000, C423S709000, C423S710000, C423S716000, C423SDIG002, C502S004000, C427S301000, C427S327000, C427S376200

Reexamination Certificate

active

06190638

ABSTRACT:

This invention relates to molecular sieves, to processes for the manufacture of the molecular sieves themselves and to the uses of such molecular sieves.
Molecular sieves find many uses in physical, physicochemical, and chemical processes, more especially as selective sorbents, effecting separations of mixtures of different molecular species, and as catalysts or catalyst precursors. In these applications, the crystallographically-defined open pore structure within a crystalline molecular sieve normally plays an important part.
There have been numerous proposals for the manufacture of molecular sieve layers, usually on a substrate. When the substrate is continuous and porous, and the pores of the substrate are covered to the extent that they are effectively closed, a molecular sieve membrane results; such a membrane has the advantage of being able to perform catalysis and separation simultaneously if required.
Most such layers have the individual particles, e.g., crystallites if the sieve is crystalline, or agglomerates, arranged randomly within the layer. As a result, a molecule that is intended to pass between opposite faces of the layer through a crystallographically-defined pore of a given shape and size must follow a tortuous path, thereby reducing the flux through the layer. Further, when the particles are arranged randomly, there are likely to be numerous voids extending through the thickness of the layer that are of cross-section sufficiently large to allow undiscriminated passage of all the species of molecule in a mixture through them, thereby preventing effective separation of the mixture's components.
In our International Application No. WO 96/101683, the disclosure of which is incorporated by reference herein, we have described a structure comprising a substrate and a crystalline molecular sieve layer (main layer) in which the crystals extend predominantly in a single direction, especially a direction transverse to the plane of the main layer, there being provided between the main layer and substrate an additional relatively thin layer. It will be understood that the main layer is not necessarily planar since a molecular sieve layer may be formed, for example, on an internal or external surface of a tubular substrate, but for simplicity a layer is described herein as if it were planar.
The relatively thin layer may be formed by applying to or forming on a surface of the substrate an initial, thin layer of “seed” crystals, from which the ordered crystals of the main layer may be grown. In the case of an MFI type zeolite, the main layer tends to form in such a way that it is the c-axis or the a-axis of the crystals that extends through the main layer thickness, that is to say, generally perpendicular to the main layer plane.
In the described process, the crystals in the seed layer are substantially spherical colloidal seeds, and the observed orientation of the crystals in the main layer results from growth of the main layer crystals in the direction of the c-axis and/or a-axis being favored. In certain applications, however, it is desired that molecules should pass through the sieve layer in the pores extending in the direction of the b-axis, in which case their path is less tortuous through the ordered layer than through a layer in which the crystals are arranged randomly.
It is know in the art that molecular sieve particles may be used for seeding the synthesis solutions for the preparation of larger molecular sieve particles. For such application of molecular sieve seeds it is desirable to be able to provide new molecular sieve seeds which have controlled particle size and/or particle size distribution and/or particle shape in order to be able to use such seeds for the manufacture of new molecular sieve materials. Furthermore new processes for the manufacture of such molecular sieve seeds are required.
It has now been unexpectedly found that MFI crystals may be formed which may be capable of forming stable colloidal suspension and which have a shape that is non-spherical. These MFI crystals are suitable for use inter alia as seed crystals in the manufacture of a structure comprising a substrate and a molecular sieve layer and as seeds for the manufacture of molecular sieve materials such as particulate molecular sieve materials.
The present invention provides a process for the manufacture of an MFI zeolite which comprises mixing (a) an aqueous alkaline synthesis mixture comprising a silica source and an organic template, the synthesis mixture having a molar OH—:SiO
2
ratio of at most 1.4:1, with (b) an active synthesis mixture being either
(i) an aged aqueous alkaline synthesis mixture comprising a silica source and an organic template, the synthesis mixture having a molar OH—:SiO
2
ratio of at least 0.1:1 and molar organic template: SiO
2
ratio of at least 0.1,
or
(ii) an active mother liquor which is derived from an aqueous synthesis mixture which has been used for at least one crystallisation and the formed crystals having been removed,
and subjecting the mixture of (a) and (b) to hydrothermal treatment to induce crystallisation.
By the process of the invention, it is possible to provide an MFI product in which the crystals are flat, i.e., having one dimension substantially smaller than the other two, while producing crystals that are sufficiently small to be used with advantage as seed crystals in a number of different processes. Advantageously one dimension is at most 50% of the larger of the other two. By control of the reaction conditions, in particular the ratio of component (a) to component (b), it is possible to control the particle size. At lower particle sizes, for example up to about 200 nm, the dimensions are typically in ratios within the range of from 0.3 to 0.4:0.7 to 0.8:1; at the higher particle sizes, for example between 200 nm and 550 nm, the dimensions are typically in ratios within the range of from 0.2 to 0.3:0.6 to 0.7:1. In this specification references to particle sizes are to the mean length of the largest dimension of the particles, as determined by inspection of scanning electron microphotographs (SEM) of the product crystals. In each case one dimension is about half the smaller of the remaining two dimensions, if they are different.
As indicated above, the particle size of the product may be controlled by varying the proportion of components (a) and (b), particle size varying inversely with the proportion of component (b). For example, in a typical case, a synthesis mixture containing about 10% by weight of component (b), based on the total weight of components (a) an (b), yields crystals in the range of from 110 to 200 nm, the crystals forming a suspension in the reaction mixture. Reducing the proportion of (b) to between 5 and 6% yields crystals of from 200 to 250 nm, which settle to the bottom of the reaction vessel, while reducing the proportion to about 0.5% yields still larger crystals, from 450 to 550 nm. It is preferred that the proportion of (b) is greater than 0.5% by weight.
Accordingly, in one preferred embodiment of the invention, a process is provided in which a suspension of zeolite crystals is formed, the crystals having a particle size of at most 200 nm, by employing a mixture of (a) and (b) in which (b) is an active mother liquor and the proportion of (b) is at least 5 % most preferably 6% by weight.
In a second preferred embodiment, a process is provided in which zeolite crystals of particle size of at least 200 nm are formed, by employing a mixture of (a) and (b) in which (b) is an active mother liquor and the proportion of (b) is at most 6% preferably at most 5% by weight.
If (b) is an aged aqueous alkaline synthesis mixture then less may be required compared to that needed when (b) is an active mother liquor, to achieve the same particle size or particle size distribution.
Surprisingly it has also been found that the near-colloidal non-spherical MFI zeolites of the present invention exhibit relatively low levels of twinning. This can be determined from an analysis of the SEM's of the materials. The

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