Chemistry of inorganic compounds – Zeolite – Organic compound used to form zeolite
Patent
1994-06-03
1997-09-30
Bell, Mark L.
Chemistry of inorganic compounds
Zeolite
Organic compound used to form zeolite
423705, 423709, 423DIG22, 502 77, C01B 3940
Patent
active
056723318
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for preparing crystals of an MFI-type zeolite whose crystals size is uniform and controlled, and to catalysts, catalyst carriers and adsorbents comprising zeolites when produced by this process.
2. Description of Related Art
Zeolites, molecular sieves having a crystalline structure which is typically an alumino silicate, find many uses as catalysts or catalysts carriers or as adsorbents. The habit and size of crystals is an important factor in the applicability of a zeolite. Generally the intrinsic quality of a zeolite is better when the crystals are small and uniform, than when the crystals are large and less uniform. Small, uniform crystals have a relatively low mass transfer resistance, and short pathways for molecules to enter and leave the zeolite structure.
As with many types of zeolite, MFI zeolites may be produced in many different crystal sizes and shapes. Indeed, within one batch of crystals there can be a significant variation in crystal habit and size. It is therefore desirable to be able to control reliably the size and size distribution of the crystals.
The present applicants have found that the use of a surprisingly small amount of seed crystals of size less than 100 nm average diameter may be used to produce accurately and reproducibly MFI-type crystals of uniform size.
Whilst the use of seed crystals when crystallising zeolites is a well known technique, previous publications recommended the use of a relatively large amount of seed. For example, EP-A-172068 describes a process in which a "complementary salt" or finely crushed crystals of a zeolite are added to the zeolite synthesis mixture. The examples suggest a weight percent of seed crystals of at least 0.8 weight percent based on the total weight of the synthesis mixture. EP-A-170486 suggests seeding a synthesis mixture with crystals of ZSM-50 or zeolite b. The examples use at least 0.66 weight percent of seed based on the total weight of the synthesis mixture. EP-A-110650 also suggests using seed crystals in zeolite synthesis. However, the examples all suggest an amount of seed of at least 0.6 weight percent based on the total weight of synthesis mixture.
GB 1567948 suggests the use of seeds in the synthesis of ZSM-5 zeolite. The amount of seed zeolite may be from 0.01 to 100 grams per 100 grams of SiO.sub.2. However, the alkalinity of this synthesis mixture is 0.1 to 0.4 expressed as a molar ratio of OH.sup.- /SiO.sub.2 and the effect of the seeds on the crystal size and uniformity is obscured in a synthesis mixture with such a high alkalinity.
SUMMARY OF THE INVENTION
The present applicants have found that if the seed crystals are sufficiently small, possibly present in the synthesis mixture in the form of a colloidal suspension, and if the synthesis mixture has a sufficiently low alkalinity then as little as 1700 ppm or less (0.17 wt % or less) of seed crystals may be used, based on the weight of the synthesis mixture.
The present invention thus provides a process for the preparation of an MFI-type zeolite comprising mixing: an average diameter of 1 .mu.m or less; in the form of a colloidal suspension; of 0.05 to 1700 ppm by weight of the synthesis mixture, and the synthesis mixture having an alkalinity, expressed as a molar ratio of OH.sup.- /SiO.sub.2, of less than 0.1; and allowing the synthesis mixture to crystallise.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A shows a X-ray diffractogram of the colloidal seeds used in accordance with this invention.
FIG. 1B shows a SEM micrograph of the colloidal seeds used in accordance with this invention.
FIG. 2 is a graph showing the effect of the quantity of seed crystals used in the synthesis mixture of some of Examples 2-9 and the crystalline diameter of the resultant ZSM-5 crystals produced.
FIGS. 3A, 3B and 3C show SEM micrographs of unseeded crystals and the seeded crystals produced in some of Examples 2-9.
FIG. 4 is a graph showing the relationship between the log of amount of seed
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Bell Mark L.
Exxon Chemical Patents Inc. (ECPI)
Sample David
Sherer Edward F.
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