Synthesis of molecular sieves of CHA framework type

Details Synthesis of molecular sieves of CHA framework type Synthesis of molecular sieves of CHA framework type

2003-08-06

2004-12-28

Sample, David (Department: 1755)

Chemistry of inorganic compounds

Phosphorus or compound thereof

Oxygen containing

C423S306000, C423SDIG003, C502S208000, C502S214000, C208S046000

Reexamination Certificate

active

06835363

ABSTRACT:

FIELD OF INVENTION
This invention relates to the synthesis of aluminophosphate and silicoaluminophosphate molecular sieves of the CHA framework type. In particular the present invention relates to the synthesis of aluminophosphate and silicoaluminophosphate molecular sieves of the CHA framework type using synthesis templates that contain two dimethylamino moieties in combination with hydrogen fluoride.
BACKGROUND OF THE INVENTION
Olefins are traditionally produced from petroleum feedstock by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefin(s) such as ethylene and/or propylene from a variety of hydrocarbon feedstock. It has been known for some time that oxygenates, especially alcohols, e.g. methanol, are convertible into light olefin(s). The preferred methanol conversion process is generally referred to as methanol-to-olefin(s) (MTO) process, where methanol is converted to primarily ethylene and propylene in the presence of a molecular sieve.
Some of the most useful molecular sieves for converting methanol to olefin(s) are the metalloalluminophosphates such as the silicoaluminophosphates (SAPO's). There are a wide variety of SAPO molecular sieves known in the art, of these the more important examples include SAPO-5, SAPO-11, SAPO-18, SAPO-34, SAPO-35, SAPO-41, and SAPO-56. For the methanol-to-olefins process SAPO molecular sieves having the CHA framework type (“Atlas of Zeolite Framework Types”, 2001, 5th Edition, p. 96) and especially SAPO-34 are particularly important catalysts. The CHA framework type has a double six-ring structure in an ABC stacking arrangement. The pore openings of the structure are defined by eight member rings that have a diameter of about 4.0 Å, and cylindrical cages within the structure of approximately 10×6.7 Å. Other SAPO molecular sieves of CHA framework type include SAPO-44, SAPO-47 and ZYT-6.
The synthesis of AlPO
4
and SAPO molecular sieves is a complicated process. There are a number of variables, which need to be controlled in order to optimise the synthesis in terms of the purity, yield, and quality of the molecular sieve produced. Of these variables the choice of synthesis template is usually one of the most important in determining which framework type is obtained.
One desirable group of silicoaluminophosphate molecular sieves are those, which have low silicon contents. Silicoaluminophosphates of the CHA framework type with low silicon contents are particularly desirable for use in the methanol-to-olefins process. Wilson, et al., reported that it is beneficial to have lower Si content for methanol-to-olefins reaction (
Microporous and Mesoporous Materials,
29, 117-126, 1999). Low Si content has the effect of reducing propane formation and decreasing catalyst deactivation.
In U.S. Pat. No. 4,440,871 (Lok et al.) the synthesis of a wide variety of SAPO materials of various framework types are described with a number of specific examples. Also disclosed are a large number of possible organic templates, with some specific examples. In the specific examples a number of CHA framework type materials are described. SAPO-34 is prepared utilising tetraethylammonium hydroxide (TEAOH), or isopropylamine, or mixtures of TEAOH and dipropylamine (DPA). This is believed to be the first reported synthesis of a SAPO-34 of CHA framework type. Also disclosed in this patent is a specific example that utilises cyclohexylamine in the preparation of SAPO-44. Although other template materials are described in this patent there are no other templates indicated as being suitable for preparing SAPO's of CHA framework type. Certain aminoalcohols are mentioned, including triethanolamine, N-methyldiethanolamine, N-methylethanolamine, N,N-dimethylethanolamine and N,N-diethylethanolamine as possible templates for SAPO molecular sieves. Of these materials N,N-diethylethanolamine is shown to produce SAPO-5, which is of framework type AFI. For the other aminoalcohols no indication is provided as to which SAPO or which framework type may be obtained through their use.
Since the synthesis of SAPO-34 was reported in U.S. Pat. No. 4,440,871, tetraethylammonium hydroxide (TEAOH) either alone, or in combination with dipropyl amine (DPA), has been the template of choice for preparing SAPO-34. However, there are problems associated with the use of TEAOH and DPA. When used alone, TEAOH affords a limited range of synthesis parameters. For example, under certain conditions TEAOH will also template the synthesis of SAPO-18, which has the AEI framework type. TEAOH is thus relatively intolerant to synthesis condition variations. TEAOH is sometimes combined with DPA. However, DPA has a low boiling point (110° C.) resulting in the need for production plants that can handle high pressures. In certain countries, the use of DPA requires special regulatory authorizations due to its toxicity. Also, DPA is an aggressive template and is often implicated in re-dissolution of the silicoaluminophosphate molecular sieve during its synthesis, resulting in poor quality crystalline product due to surface pitting of the crystals. Finally, it has proved difficult up to now to make pure phase CHA silicoaluminophosphate molecular sieves with low silica to alumina ratio. Although there are problems associated with TEAOH and DPA, no completely satisfactory alternative template materials have been reported yet for the preparation of silicoaluminophosphate molecular sieves with the CHA framework type. A further problem associated with the use of TEAOH is that silicoaluminophosphate molecular sieves with the CHA framework produced using this template are relatively expensive due to long crystallization times that are typically between 24 to 72 hours. In a Ph.D. thesis (E. H. Halvorsen, University of Oslo, 1996), it was reported that low silica SAPO-34, designated as UiO-S4, was produced using TEAOH template in combination with HF. When TEAOH is used in combination with hydrogen fluoride the crystallization times may be reduced to between 16 to 24 hours; whilst this is an improvement the resultant molecular sieves are still expensive.
In U.S. Pat. No. 4,440,871, it was reported that SAPO-44 was obtained “as the major phase” using cyclohexylamine as template. In U.S. Pat. No. 6,162,415 (Liu, et al.), relatively pure CHA SAPO-44 was obtained using the same template but with control of the ratio of template to aluminium component and the ratio of phosphorous component to aluminium component. In European Patent Publication No. 0,993,867, it was reported that the use of methylbutylamine resulted in SAPO-47 and the use of cyclohexylamine resulted in impure SAPO-44. Methylbutylamine has an even lower boiling point, at 91° C., than DPA.
When attempts have been made to utilise other types of template compounds such as aminoalcohols, silicoaluminophosphates of framework type other than CHA have been obtained. In U.S. Pat. No. 4,861,739 (Pellet, et al.), Example 102, it was reported that the use of N,N-diethylethanolamine produced CoAPSO-47, having Si concentrated on the peripheries of the crystal and Co at the centre. In U.S. Pat. No. 4,310,440 (Wilson et al.), triethanolamine, N,N-dimethylethanolamine, N,N-diethylethanolamine, N-methyldiethanolamine, and N-methylethanolamine, were all used to prepare AlPO
4
-5, aluminophosphates of framework type AFI. N-methylethanolamine was also reported to produce AlPO
4
-21 of framework type AWO. In European Patent Publication No. 0,993,867, it was reported that diethanolamine produced SAPO-34 and SAPO-5 under different synthesis conditions. To-date all attempts to reproduce this SAPO-34 synthesis have failed.
In the art various attempts have been made to improve the synthesis of AlPO
4
or SAPO molecular sieves. One approach has been the use of fluoride addition to the synthesis. In U.S. Pat. No. 5,096,684 (Guth et al.), morpholine and tetraethylammonium hydroxide were found to template the production of SAPO-34 when in the presence of HF. According to Guth et al., the use of HF in combination

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