Chemistry of inorganic compounds – Zeolite – Organic compound used to form zeolite
Patent
1997-06-20
2000-02-01
Bell, Mark L.
Chemistry of inorganic compounds
Zeolite
Organic compound used to form zeolite
423709, 423716, 423DIG38, C01B 3930
Patent
active
060199563
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the zeolite Offretite, to processes for its manufacture, and to the use of the zeolite as catalyst.
A traditional synthesis of Offretite obtains the zeolite from synthesis mixtures relatively rich in potassium and containing the tetramethylammonium cation as template or structure directing agent. The resulting product has a relatively large particle size.
EP-A-400961 describes a synthesis of Offretite using a synthesis mixture containing metakaolin obtained by calcination at a temperature of at least 550.degree. C., the resulting product having an average particle size in the range of 0.1 to 10 .mu.m.
WO 92/14680 describes the use of additional, divalent, cations to facilitate manufacture of Offretite of small particle size. U.S. Pat. No. 3,578,398 describes a procedure for producing "sub-micron" Offretite.
WO 93/08125 describes the preparation of MFI, MEL and Beta zeolites of particle size sufficiently small to enable a colloidal suspension to be formed; in general for this purpose the largest dimension of the particles is required to be at most 100 nm.
Products of small particle size have advantages over larger particle size products, for example, when used as a catalyst, e.g., in hydrocarbon conversions, they have an enhanced ratio of surface area to mass, high diffusion rates, reactivities and resistance to deactivation by pore plugging and surface contamination. Similarly, they have advantages in organic separations, and are also valuable in the manufacture of supported layers, especially membranes, as described in WO 94/25151. In certain of the procedures described in that patent application, the disclosure of which is incorporated herein by reference, the zeolite layer is deposited from a colloidal suspension onto a support; instability in the suspension is deleterious. Accordingly, it would be desirable to be able to manufacture Offretite capable of forming a stable suspension, and for this purpose a particle size of at most 100 nm, and advantageously at most 75 nm, is desirable. A stable suspension is one in which no settlement takes place at all, or one in which settlement takes place so slowly as to be insignificant over the time scale concerned. Such a suspension is referred to herein as colloidal and particles capable of forming such a suspension may be referred to as colloidal size particles.
The present invention is based on the observation that if the concentration of inorganic cations, especially potassium, in the synthesis mixture is reduced from that typically previously used a colloidal suspension of Offretite may be obtained. It has surprisingly been found also that it is not in fact essential for the synthesis mixture to be clear and homogeneous as previously suggested, e.g., in WO 93/08125, as necessary for colloidal zeolite production. Similarly WO 94/05597 indicates that though a clear synthesis mixture is a necessary condition for the manufacture of colloidal zeolites, it is not a sufficient one. Surprisingly, mixture in which, for example, the source of silicon is incompletely dissolved, is capable of yielding uniform colloidal-size Offretite crystals.
The present invention accordingly provides a process for the manufacture of a colloidal suspension of Offretite, wherein a synthesis mixture having a molar composition, when calculated in terms of oxides, in the following ranges:
______________________________________ K.sub.2 O:Al.sub.2 O.sub.3
0.25 to 0.5:1
(TMA).sub.2 O:Al.sub.2 O.sub.3 at least 2.0:1
SiO.sub.2 :Al.sub.2 O.sub.3 8 to 10:1
H.sub.2 O:Al.sub.2 O.sub.3 80 to 100:1
______________________________________
thermal treatment at a temperature and for a tine sufficient to form a
colloidal suspension of Offretite.
Advantageously, the molar ratio of (TMA).sub.2 O:Al.sub.2 O.sub.3 is 2.0 to 3.5:1.
The process according to this aspect of the invention has the advantage of being capable of providing a stable suspension free from unreacted solid starting material.
The invention further provides a process for the manufacture of the z
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Inui et al., Chem. Ab. vol. 96 No. 162052c, pp. 695-696, 1982.
Bell Mark L.
Exxon Chemical Patents Inc.
Sample David
Sherer Edward F.
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