Process for the preparation of nanocrystalline zeolite beta

Details Process for the preparation of nanocrystalline zeolite beta Process for the preparation of nanocrystalline zeolite beta

2002-11-29

2004-12-07

Sample, David (Department: 1755)

Chemistry of inorganic compounds

Zeolite

Organic compound used to form zeolite

C423S707000, C423S711000, C423S716000, C423SDIG002

Reexamination Certificate

active

06827924

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a process for the preparation of nanocrystalline zeolite beta More particularly, this invention relates to the preparation of nanocrystalline zeolite beta by a modified aerogel protocol comprising four steps, namely, hydrolysis, nucleation, crystallization and supercritcal drying. This improved process gives excellent yields of nanocrystalline zeolite beta with a crystallite size in the range of 10 to 80 nanometer and a broad range of silica to alumina ratio 15 to 200 which shows enhanced activity for nitration of o-xylene to produce 4-nitro o-xylene with higher selectivity.
BACKGROUND OF THE INVENTION
Research has been focused recently on the development of new methods for preparation of zeolites to obtain nanometer size of zeolite crystals. This may be achieved by decreasing the nucleation temperatures, lowering the crystallization times, optimized pH conditions and also in absence of alkali metal cations during the synthesis of zeolites. Zeolite beta, having a three-dimensional large-pore system of a 12-membered ring opening 0.76 nm wide, first described in 1967 in an U.S. Patent, draws much attention because of its unique characteristics, in particular its acidity and potential for acid catalysis. The nanocrystalline zeolite beta offers several advantages over microcrystalline zeolite beta in terms of activity and selectivity due to increased active acidic sites and three dimensional interface with the support and reactant.
Reference is made to U.S. Pat. No. 3,308,069, wherein zeolite beta was described for the first time with a silica-to-alumina ratio from 10 to 150 with crystal size ranging from 0.01 to 0.05 microns in presence of alkali metal cations. The drawbacks are longer crystallization times and also the presence of alkali metal cations makes the zeolite beta inactive acidic catalyst. Reference is also made to Joaquin Perez-Pariente et al,
Applied Catalysis
, 31,1987,35-64 wherein zeolite beta was synthesized from tetraethylorthosilicate, sodium aluminate, tetraethylammonium hydroxide, sodium and potassium hydroxide. They studied the influence of alkali metal cations on the crystallization mechanism. The drawbacks are the presence of alkali metal cations in the synthetic mixture needs longer post-calcination treatment and zeolite prepared is not an acidic catalyst. Larger crystallites form due to longer crystallization times and separation of zeolite crystals require higher centrifugal forces.
Reference is made to Camblor et al,
Zeolites
, 1991, 202 and 792, wherein zeolite beta was synthesized in 30 hours at 135° C. using amorphous silica, a 40% aqueous solution of tetraethylammonium hydroxide, sodium aluminate, aluminum, sodium hydroxide, potassium hydroxide and suggested that the presence of alkali metal cations is essential for the formation of the zeolite. The disadvantages are the presence of alkali metal cations in the synthetic mixture, formation of larger crystallites and further separation of zeolite crystals requires higher centrifugal forces. Reference is also made to U.S. Pat. No. 5,427,765 wherein zeolite beta is synthesized from a mixture of tetraethylammonium hydroxide, an alkali metal silicate and an aqueous solution containing aluminum. The disadvantages is the presence of alkali metal cations in the synthesis mixture and longer crystallization times required even to form larger crystallites.
Reference is made to U.S. Pat. No. 4,923,690 wherein synthesis of highly silicious zeolite beta was described with silica-to-alumina ratio within the range of 20-1000. The drawbacks are to achieve the high silica to alumina ratio, the zeolite has to be partially crystallized. As the zeolite becomes more crystalline, the silica-to-alumina ratio decreases. In this procedure highly silicious zeolite beta was obtained with only 30 to 50% crystallinity.
Reference is made to U.S. Pat. No. 5,989,518 wherein a continuous process was developed to synthesize various molecular sieves, which control both the particle size and particle size distribution. This process involves continuously adding reactive sources of the desired components along with a structure-directing agent into a continuous crystallization reactor. Either interstage backmixing is introduced or the number of stages is adjusted in order to control particle size The disadvantages are the presence of alkali metal cations and the crystallites obtained are in the range of 3 to 20 microns. Reference is also made to U.S. Pat. No. 5,683,673 wherein zeolite beta is synthesized in presence of ethanol. Ethene is evolved during crystallization period for which the pressure developed autogeneously to 50 bar at the end of crystallization. The drawbacks are the longer crystallization times 11 days at 140° C., and separation of zeolite crystals from mother liquor requires higher centrifugation forces of up to 13,000 rpm. Reference is made to M. A. Camblor et al,
Studies in Surface Science and Catalysis
, Volume 105, 341,1997, wherein nanocrystalline zeolite beta was synthesized with crystallite size 10 to 100 nm in the absence of alkali metal cations by using colloidal silica and aluminium metal powder. The disadvantages are this method requires longer crystallization times and the separation of zeolite crystals from the mother liquor require higher centrifugation forces of up to 16,000 rpm. Reference is made to P. R. Hari Prasada Rao et al,
Chemical Communications
1441, 1996 wherein zeolite beta was synthesized by dry gel conversion technique. The drawbacks are this process involves the presence of alkali metal cations in the synthetic mixture, and zeolite prepared will not be an acidic catalyst, longer crystallization times 3-6 days.
OBJECTS OF THE INVENTION
The main object of the present invention is to provide an improved process for the preparation of nanocrystalline zeolite beta by a modified aerogel protocol.
It is another object of the invention to provide a process for the preparation of nanocrystalline zeolite beta with a particle size of in the range of 10 to 80 nanometers with Si:Al molar ratio from 15 to 200 wherein the synthetic mixture is free from alkali metal cations.
It is yet another object of the invention to provide a process for the preparation of nanocrystalline zeolite beta where the crystallization times are low and the nanocrystalline zeolite beta is produced in high yield, is highly crystalline and shows the typical beta zeolite IR absorption bands at 575 and 525 cm.
−1
and X-ray diffraction spectrum.
SUMMARY OF INVENTION
The novelty of present invention is the preparation of nanocrystalline zeolite beta with the crystallite size in the range of 10 to 80 nanometers and in a broad range of silica to alumina ratio 15 to 200 by a modified aerogel protocol. Most of the previously proposed methods for the preparation of zeolite beta have employed synthetic mixtures containing alkali metal cations and suggested that the presence of alkali metal cations is essential for the formation of the zeolite. Hence the preparation of zeolite beta in absence of alkali metal cations with small crystallite sizes is an exciting process. Controlled hydrolysis of the synthetic mixture, aging at room temperature, reducing the crystallization time and also subjecting the crystalline gel to supercritical drying conditions are novel ideas followed to get nanocrystalline zeolite beta in high yield. The obtained nanocrystalline zeolite beta offers several advantages over microcrystalline zeolite beta in terms of activity and selectivity due to increased active acidic sites and three dimensional interface with reactants for example 4-nitro o-xylene is obtained with higher selectivity in the range of 65-75% from nitration of o-xylene.
Accordingly, the present invention provides a process for the preparation of nanocrystalline zeolite beta comprising hydrolysing a silica source and an aluminium source in the presence of a templating agent and in absence of alkali metal cations, nucleating the resulting product under stirring at room temperature followed b

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