Crystalline molecular sieve layers and processes for their...

Details Crystalline molecular sieve layers and processes for their... Crystalline molecular sieve layers and processes for their...

2002-06-20

2004-02-10

Dunn, Tom (Department: 1725)

Gas separation: processes

Solid sorption

Organic gas or liquid particle sorbed

C095S147000, C502S004000, C502S060000, C502S064000, C502S062000, C502S071000, C502S077000

Reexamination Certificate

active

06689195

ABSTRACT:

This invention relates to crystalline molecular sieve layers, to processes for their manufacture, and to their use.
Molecular sieves find many uses in physical, physicochemical, and chemical processes; most notably as selective sorbents, effecting separation of components in mixtures, and as catalysts. In these applications the crystallographically-defined pore structure within the molecular sieve material is normally required to be open; it is then a prerequisite that any structure-directing agent, or template, that has been employed in the manufacture of the molecular sieve be removed, usually by calcination. Numerous materials are known to act as molecular sieves, among which zeolites form a well-known class.
In International Application WO 94/25151 is described a supported inorganic layer comprising optionally contiguous particles of a crystalline molecular sieve, the mean particle size being within the range of from 20 nm to 1 &mgr;m. The support is advantageously porous. When the pores of the support are covered to the extent that they are effectively closed, and the support is continuous, a molecular sieve membrane results; such membranes have the advantage that they may perform catalysis and separation simultaneously if desired. A number of processes are described in WO 94/25151 for the manufacture of the inorganic layers disclosed therein. WO94/25151 describes the use of a barrier layer which prevents the water in the aqueous coating suspension used from preferentially entering the pores of the support to an extent such that the silica and zeolite particles form a thick gel layer on the support. The barrier layer may be temporary or permanent; temporary barrier layers are fluids such as water or glycol. The membranes of WO 94/25151 exhibited selectivities of para-xylene over ortho-xylene of 20.76 to 60.10 and para-xylene permeances of 1.09×10
−8
mole(px)/m
2
.s.Pa(px) (10 kg(px)/m
2
.day.bar(px)) when measured at low temperature and pressure.
In International Application WO 96/01683 a structure is described which comprises a support, a seed layer, and an upper layer, the seed layer comprising a crystalline molecular sieve having a crystal size of at most 1 &mgr;m, and the upper layer comprising a crystalline molecular sieve of crystals having at least one dimension greater than the dimensions of the crystals of the seed layer. There are a number of processes described in WO 96/01683 for the manufacture of these layers.
In International Application WO 97/25129 a structure is described which comprises a crystalline molecular sieve layer on a substrate and an additional layer of refractory material to occlude voids in the molecular sieve layer. The structures described in the examples have para-xylene over meta-xylene selectivities of between 2 to 8.
In International Application WO 96/01686 a structure is described which comprises a substrate, a zeolite or zeolite-like layer, a selectivity enhancing coating in contact with the zeolite layer and optionally a permeable intermediate layer in contact with the substrate. Examples of these structures are given which have para-xylene over meta-xylene selectivities of between 1 to 10.
Xomeritikas and Tsapatsis in Chemical Materials, 1999, 11, 875-878, describe orientated MFI-type zeolite membranes which have been manufactured using secondary growth a process which requires two successive hydrothermal growths and produces membranes of 25 to 40 &mgr;m thickness. These membranes exhibited para-xylene over ortho-xylene selectivities of 18 when measured at a total aromatic hydrocarbon partial pressure of 27.5 Pa [=15 Pa pX+12.5 Pa oX] and 100° C. and 3.8 at a total aromatic hydrocarbon partial pressure of 550 Pa [=300 Pa pX+250 Pa oX] and 100° C., and permeances for para-xylene of 2.0 to 5.2×10
−8
mole/m
2
.s.Pa [18 to 48 kg
px
/m
2
.day.bar
px
], when tested at temperatures up to 200° C. and at low hydrocarbon partial pressures. The selectivity decreased with increasing partial pressure of para-xylene and it was observed by the authors that the membranes would not be suitable for separation of xylene isomers at elevated temperatures due to the 20 fold reduction in flux ratio at 200° C. compared to that observed at 100° C.
Many commercial petrochemical processes operate at elevated temperature and pressure. Whilst the molecular sieve layers of the prior art may exhibit good selectivity and permeance results when tested at low temperatures, pressures and/or hydrocarbon partial pressures, this is not repeated when tested at high temperatures and high hydrocarbon partial pressures. Thus, there is a need for molecular sieve layers with improved properties for catalytic and/or membrane applications, especially improved properties at elevated temperatures e.g. >250° C. and/or elevated hydrocarbon feed partial pressures >10×10
3
Pa.
The present invention is concerned with crystalline molecular sieve layers which have improved properties compared to crystalline molecular sieve layers in the art, especially for membrane applications. It has surprisingly been found that the control of a number of synthesis parameters for the manufacture of crystalline molecular sieve layers in conjunction with impregnation of the support onto which the crystalline molecular sieve layer is to be deposited during its synthesis, results in crystalline molecular sieve layers with properties, which hitherto have not been achieved.
The present invention in a first aspect provides a process for the manufacture of a crystalline molecular sieve layer, which process comprises:
a) providing a porous support having deposited thereon seeds of molecular sieve crystals of average particle size of 200 nm or less,
b) impregnating the support with an impregnating material before or after deposition of the seeds of molecular sieve,
c) contacting the impregnated support having seeds deposited thereon with a molecular sieve synthesis mixture,
d) subjecting, the impregnated support having seeds deposited thereon, to hydrothermal treatment whilst in contact with the molecular sieve synthesis mixture to form a crystalline molecular sieve layer on the support, and
e) removing the impregnating material from the support.
As examples of porous supports, there may be mentioned porous glass, sintered porous metals, e.g., steel or nickel, inorganic oxides, e.g., alpha-alumina, titania, cordierite, zeolite as herein defined, or zirconia and mixtures of any of these materials. In this context porous supports include supports which have pores which are occluded; such supports, whilst having pores which are not suitable for membrane separation applications, may be used for catalytic applications or separation processes which are not membrane separation processes such as for example adsorption or absorption.
The pore size and porosity of the support should be compatible with the process employed for depositing the molecular sieve seeds. The porous support may be any material compatible with the coating and synthesis techniques utilised in the process of the present invention. For example porous alpha-alumina with a surface pore size within the range of 0.08 to 1 &mgr;m, most preferably from 0.08 to 0.16 &mgr;m, and advantageously with a narrow pore size. Ideally the support should have a relatively high degree of porosity so that the support exerts an insignificant effect on flux through the finished product. Preferably the porosity of the support is 30% by volume or greater; ideally and preferably greater than 33%, and preferably within the range 33 and 40% by volume. The support may be multilayered; for example, to improve the mass transfer characteristics of the support; in this context the support may be an asymmetric support. In such a support the surface region which is in contact with the molecular sieve seeds may have small diameter pores, while the bulk of the support, toward the surface remote from the molecular sieve seeds, may have larger diameter pores. An example of such a multilayered asy

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