Chemistry of inorganic compounds – Zeolite – Organic compound used to form zeolite
Reexamination Certificate
1999-07-12
2001-02-20
Bell, Mark L. (Department: 1755)
Chemistry of inorganic compounds
Zeolite
Organic compound used to form zeolite
C423S705000, C423S328100, C423S328200
Reexamination Certificate
active
06190639
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the preparation of mesoporous molecular sieves and to porous crystalline materials, which may be obtained by the same process. More specifically, the present invention relates to a process for preparing mesoporous molecular sieves of an acidic nature and of high crystallinity and to a certain class of such mesoporous molecular sieves, which are stable at high temperatures, thus making them very suitable for use in catalysts.
BACKGROUND OF THE INVENTION
The expression “mesoporous” as used throughout this specification refers to pores having diameters in the range of from 1.5 to 20.0 nm.
Crystalline mesoporous molecular sieves are known in the art. For instance, in International patent specification No. WO 93/02013 a mesoporous molecular sieve is disclosed, which, after calcination, exhibits an X-ray diffraction pattern with the strongest peak (i.e. relative intensity is 100) at a d-spacing d
1
of at least 1.8 nm and at least one additional weaker peak (relative intensity up to 49) at a d-spacing d
2
such that the ratio d
2
/d
1
has a value of 0.87 0.06. The molecular sieve is suitably prepared by adding a silicon oxide source and optionally an aluminum oxide source to a solution containing an organic templating agent, followed by agitating the mixture thus obtained for 10 minutes to 6 hours at a temperature of from 0 to 50° C. and a pH of 7 to 14 and finally crystallizing the agitated mixture at a temperature of 50 to 200° C., preferably 95 to 150° C., for 4 to 72 hours. The crystallized material recovered is then calcined to eventually arrive at the crystalline mesoporous molecular sieve envisaged.
In International patent specification No. WO 93/01884 a catalyst composition is disclosed of which the support component comprises a non-layered mesoporous molecular sieve which, in calcined form, exhibits an X-ray diffraction pattern with at least one peak having a relative intensity of 100 at a d-spacing of at least 1.8 nm and which has a benzene sorption capacity greater than 15 grams benzene per 100 grams of the material at 6.7 kPa and 25° C. Several methods are disclosed for preparing the mesoporous material, but all methods typically involve preparing a starting mixture containing the necessary components including an organic templating agent and sources of oxides of silicon and optionally aluminum, followed by crystallization of the material at a pH of at least 9 and at a temperature which suitably is above 50° C., but which—as becomes apparent from the working examples—in practice ranges from 95 to 150° C. Crystallization time ranges from 4 hours (at 105° C.) in example 19 to 192 hours plus an additional period of about 12 hours (at 95° C.) in example 3. In most working examples, however, crystallization temperatures of about 100° C. for periods of time of 20 to 90 hours are applied.
In International patent specification No. WO 95/30625 a process for preparing mesoporous molecular sieves similar to those described in WO 93/01884 is disclosed, wherein the molecular sieve is crystallized at a pH from 3 to 8 from a starting mixture containing a specified amount of fluoride in addition to the required oxide sources and organic templating agent(s). Crystallization conditions typically involve temperatures of from 60 to 250° C., preferably from 90 to 200° C., for a period of time of from 2 to 336 hours, but most suitably from 24 to 120 hours.
Although the prior art processes for preparing mesoporous molecular sieves perform satisfactory, there is still room for improvement, particularly in terms of crystallization temperature and crystallization time. It will be appreciated that lower crystallization temperatures and shorter crystallization times are attractive from an economic perspective. Lower temperatures, namely, require less expensive equipment, whilst shorter crystallization times make higher product yields per unit of time possible. In addition, the high temperature resistance and crystallinity of the prior art mesoporous materials are, though acceptable, not yet at an optimum level. Particularly when applied as support material in catalysts, mesoporous molecular sieves being stable at temperatures up to 100° C. and having increased crystallinity are desired.
SUMMARY OF THE INVENTION
The present invention therefore provides a process for preparing mesoporous molecular sieves at relatively low crystallization temperatures and relatively short crystallization times. More specifically, the present invention aims to provide such process involving crystallization temperatures of at most 90° C., and most suitably of at most 60° C., and crystallization times of at most 10 hours. Furthermore, the present invention aims to provide highly crystalline, acidic, mesoporous molecular sieves which have an excellent thermal stability.
These and other objects have been achieved by the present invention. Accordingly, the present invention relates to a process for the preparation of a mesoporous, crystalline, acidic molecular sieve comprising the successive steps of
(a) preparing an aqueous mixture comprising a silicon source, a fluorine source, and an organic template at a temperature in the range of at most 90° C. such that crystallization starts;
(b) adjusting the pH of the solution to such value in the range of from about 5.0 to about 12.5 that a precipitate is formed;
(c) recovering the precipitate; and
(d) calcining the precipitate to obtain the mesoporous, crystalline, acidic molecular sieve,
wherein an aluminum source is added in one or more of steps (a) and (b) and/or between steps (c) and (d) in such amount that after addition of all aluminum source the Si/Al atomic ratio is at least about 3.
Another embodiment of the invention provides for a process for the preparation of a mesoporous, crystalline, acidic molecular sieve comprising the successive steps of
(a) preparing an aqueous mixture comprising a silicon source, a fluorine source, and an organic template at a temperature in the range of at most 90° C. such that crystallization starts;
(b) adjusting the pH of the solution to such value in the range of from about 5.0 to about 12.5 that a precipitate is formed;
(c) recovering the precipitate;
(d) subjecting the precipitate to a hydrothermal treatment; and
(e) calcining the hydrothermally treated precipitate to obtain the mesoporous, crystalline, acidic molecular sieve,
wherein an aluminum source is added in one or more of steps (a) and (b) and/or between steps (c) and (d) in such amount that after addition of all aluminum source the Si/Al atomic ratio is at least about 3.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As the silicon source any such compound containing silicon, which is known in the art to be useful as a source of silicon, can be used. Accordingly, any oxides, alkoxides and/or halides of silicon may be used as well as ammonium compounds and silicate salts. Specific examples, then, include silica powder or colloidal silica, tetramethylorthosilicate (TMOS), tetraethylorthosilicate (TEOS), tetramethylammonium silicate, sodium silicate and ammoniumhexafluorosilicate. It has, however, been found particularly advantageous to use a silicon source, which is soluble in the synthesis mixture at a temperature of at most 60° C. and the appropriate pH conditions. Using such silicon source results in the aqueous mixture prepared in step (a) to be a clear, aqueous solution. This embodiment is preferred. Accordingly, the preferred silicon sources are those which give a clear solution at the start of the synthesis and prior to pH adjustment, so at the conditions of initial pH and temperature for obtaining the desired material. Examples of such soluble silicon sources, then, include the silicon compounds TMOS, TEOS, tetramethylammonium silicate, sodium silicate and ammoniumhexafluorosilicate.
In addition to the silicon source, the mixture prepared in step (a) also comprises an organic template (R) The organic template is suitably used in such amount that the molar ratio of template to silicon (R/Si) has a value in
Guth Jean-Louis
Huve Laurent Georges
Voegtlin Anne-Claude
Bell Mark L.
Sample David
Shell Oil Company
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