Chemistry of hydrocarbon compounds – Unsaturated compound synthesis – By skeletal isomerization
Reexamination Certificate
1991-06-05
2001-11-27
Venkat, Jyothsna (Department: 1627)
Chemistry of hydrocarbon compounds
Unsaturated compound synthesis
By skeletal isomerization
C502S064000
Reexamination Certificate
active
06323384
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to structural isomerization of linear olefins to isoolefins using zeolite compositions as isomerizing catalysts.
BACKGROUND OF THE INVENTION
Increasing demand for high octane gasoline blended with lower aliphatic alkyl ethers such as octane boosters and supplementary fuels has created a significant demand for isoalkylethers, especially the C
5
to C
7
methyl, ethyl and isopropyl-t-alkyl ethers. Consequently, there is an increasing demand for the corresponding isoalkene starting materials such as isobutene, isoamylenes and isohexenes.
In many instances, it is desirable to convert an alkene such as normal butene, to a methyl branched alkene, for example isobutylene, by mechanisms such as structural isomerization. Such converted isoalkenes then can be reacted further, such as by polymerization or oxidation, to form useful products. Normal alkenes containing four carbon atoms (1-butene, trans-2-butene and cis-2-butene) and five carbon atoms (1-pentene, trans-2-pentene, and cis-2-pentene) are relatively inexpensive starting compounds. Conventionally, butenes and amylenes, including to a minor extent isobutylene and isoamylene, are obtained as a by-product from refinery and petrochemical processes such as catalytic and thermal cracking units.
Zeolite materials, both natural and synthetic, are known to have catalytic properties for many hydrocarbon processes. Zeolites typically are ordered porous crystalline aluminosilicates having a definite structure with cavities interconnected by channels. The cavities and channels throughout the crystalline material generally can be of such a size to allow selective separation of hydrocarbons. Such a hydrocarbon separation by the crystalline aluminosilicates essentially depends on discrimination between molecular dimensions. Consequently, these materials in many instances are known in the art as “molecular sieves” and are used, in addition to catalytic properties, for certain selective adsorptive processes. Zeolite molecular sieves are discussed in great detail in D. W. Breck,
Zeolite Molecular Sieves,
Robert E. Krieger Publishing Company, Malabar, Fla. (1984)
Generally, the term “zeolite” includes a wide variety of both natural and synthetic positive ion-containing crystalline aluminosilicate materials, including molecular sieves. They generally are characterized as crystalline aluminosilicates which comprise networks of SiO
4
and AlO
4
tetrahedra in which silicon and aluminum atoms are cross-linked in a three-dimensional framework by sharing of oxygen atoms. This framework structure contains channels or interconnected voids that are occupied by cations, such as sodium, potassium, ammonium, hydrogen, magnesium, calcium, and water molecules. The water may be removed reversibly, such as by heating, which leaves a crystalline host structure available for catalytic activity. The term “zeolite” in this specification is not limited to crystalline aluminosilicates. The term as used herein also includes silicoaluminophosphates (SAPO), metal integrated aluminophosphates (MeAPO and ELAPO), metal integrated silicoaluminophosphates (MeAPSO and ELAPSO). The MeAPO, MeAPSO, ELAPO, and ELAPSO families have additional elements included in their framework. For example, Me represents the elements Co, Fe, Mg, Mn, or Zn, and El represents the elements Li, Be, Ga, Ge, As, or Ti. An alternative definition would be “zeolitic type molecular sieve” to encompass the materials useful for this invention.
Developments in the art have resulted in formation of many synthetic zeolitic crystalline materials. Crystalline aluminosilicates are the most prevalent and, as described in the patent literature and in the published journals, are designated by letters or other convenient symbols. Zeolites have been specifically named and described as Zeolite A (U.S. Pat. No. 2,882,243), Zeolite X (U.S. Pat. No. 2,882,244), Zeolite Y (U.S. Pat. No. 3,130,007), Zeolite ZSM-5 (U.S. Pat. No. 3,702,886), Zeolite ZSM-11 (U.S. Pat. No. 3,709,979), Zeolite ZSM-12 (U.S. Pat. No. 3,832,449), Zeolite ZSM-23 (U.S. Pat. No. 4,076,842), Zeolite ZSM-35 (U.S. Pat. No. 4,016,245), Zeolite ZSM-48 (U.S. Pat. No. 4,375,573), Zeolite NU-1 (U.S. Pat. No. 4,060,590) and others. Various ferrierite zeolites, including the hydrogen form of ferrierite, are described in U.S. Pat. Nos. 3,933,974, 4,000,248 and 4,942,007 and patents cited therein. SAPO-type catalysts are described in U.S. Pat. No. 4,440,871. MeAPO type catalysts are described in U.S. Pat. Nos. 4,544,143 and 4,567,029; ELAPO catalysts are described in U.S. Pat. No. 4,500,651, and ELAPSO catalysts are described in European Patent Application 159,624.
Up until now, catalysts for structurally isomerizing alkenes, particularly butene to isobutene, have utilized large pore zeolites having two or three-dimensional interconnecting channels, together with an associated catalytic metal such as platinum, palladium, boron or gallium. Continuing problems with present methods using such zeolites are coking of the catalyst pore spaces and undesirable by-product formation, in particular dimers, trimers and aromatics.
SUMMARY OF THE INVENTION
In the methods of this invention, a linear olefin is converted under isomerizing conditions to a methyl branched olefin using an isomerizing catalyst composition made up of a zeolite having only in one dimension a pore structure having a pore size small enough to retard by-products and retard the formation of coke and its precursors and large enough to permit entry of the linear olefin and diffusion of the isoolefin product. Generally, zeolites having in one dimension a pore structure with a pore size ranging from more than about 0.42 nm to less than about 0.7 nm are useful for the processes of this invention. Zeolites with this specified pore size are typically referred to as medium or intermediate pore zeolites and typically have a 10-member (or puckered 12-member) ring channel structure in one dimension and an 8-member or less (small pore) in the other dimensions, if any. For purposes of this invention, a one-dimensional pore structure is considered one in which the channels having the desired pore size do not interconnect with other channels of similar or larger dimensions; it may also be considered alternatively as a channel pore structure (see U.S. Pat. No. 3,864,283) or uni-directional sieve. Such one-dimensional, intermediate pore size catalyst compositions provide increased selectivity of the isomerization reaction, decreased dimer and trimer by-product formation, and decreased coking of the catalyst.
Zeolites that contain small pores (i.e., less than about 0.42 nm) do not allow for diffusion of the methyl branched isoolefin product, e.g., isobutylene; while zeolites that contain large pores (i.e., greater than about 0.7 nm) in any one dimension are subject to substantial by-product formation and coking. Coking is believed to be the result of oligmerization and polymerization, aromatization, or alkylation of the feed paraffin or olefin hydrocarbons. Zeolites of this invention should contain at least one pore dimension with the specified pore size. A two or three-dimensional pore structure having the specified pore size would permit substantial contact of the isomerizing olefins and thereby facilitate unwanted dimerization and trimerization reactions.
The processes of this invention are characterized by selectivities which range from about 70% to 99% over run lengths of 48 to 120 hours, and isoolefin yields of between about 30% to 37% at the beginning of a run and 12% to 20% at the end of run conditions.
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Murray Brendan D.
Powers Donald H.
Winquist Bruce H. C.
Burgess Tim L.
Equistar Chemicals LP
Garcia Maurie E.
Venkat Jyothsna
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