PROCESS FOR MAKING PROPYLENE AND ETHYLENE

Abstract: Propylene and ethylene are made by heating a mixture of C4 and/or C5 olefins with a zeolite catalyst that has channels large enough to admit the C4 and C5 olefins and large enough to allow propylene and ethylene to diffuse out, but small enough to retard diffusion of dimerized products from the channels and small enough to minimize formation of hydrocarbon coke precursors within the channels. The catalyst has a pore diameter greater than 3.5 ANGSTROM and one-dimensional, non-interconnecting channels having a pore size index within the range of 14 to 28. Also valuable are zeolite catalysts that have one or more interconnecting channels, including (1) a primary channel that has a pore diameter greater than 3.5 ANGSTROM and a pore size index within the range of 14 to 28, and (2) a secondary channel interconnected therewith that has a pore size index less than 20.

Description: PROCESS FOR MAKING PROPYLENE AND ETHYLENE FIELD OF THE INVENTION The invention relates to a process for making propylene and ethylene from higher (C4 and C5) olefins using zeolite catalysts that have particular channel dimensions. BACKGROUND OF THE INVENTION Propylene and ethylene are produced commercially by a number of methods, including, for example, steam cracking or pyrolysis of paraffinic materials, petroleum refinery cracking, and alkane dehydrogenation. U.S. Pat. No. 5,043,522 explains some of the problems with these methods for making propylene. Disproportionation of olefin mixtures to make ethylene or propylene is also known, and is described, for example, in U.S. Pat. Nos. 4,180,524, 4,499,328, and 4,517,401. These processes generally use amorphous catalysts such as tungsten or molybdenum oxides with other compounds that help to promote the disproportionation. Steam crackers optimized for naptha feedstocks produce more C4 and higher olefins than steam crackers using lighter feedstocks such as ethane and propane. However, the amount of C4 olefins generated from steam cracking may exceed the need for these olefins in current uses such as paraffin alkylation and methyl tert- butyl ether production. Moreover, naphtha-based steam crackers also produce C5 olefins, which have limited high-value uses. In sum, there is a need for processes that can upgrade C4 and C5 olefins, when market conditions require, to propylene and ethylene. Zeolites are a well-known class of natural and synthetic crystalline aluminosilicates that comprise networks of SiO 4 and AIO 4 tetrahedra in which the silicon and aluminum atoms are crosslinked by shared oxygen atoms. Zeolites contain channels or voids of characteristic dimensions. The channel openings, or "pores," can be rather circular, but more often they are elliptical or irregular in shape. In some zeolites, the channels are one-dimensional and do not interconnect, as in a series of parallel tunnels. In others, the channels intersect and form large cavities at the intersections. The channels normally contain cations such as sodium, potassium, magnesium, ammonium, or the like, and may contain protons or water molecules. Water can be removed by heating, leaving an active site within the catalyst. The cations can be exchanged, in whole or part, by other different cations, or by protons to make the "H" form of the zeolite. Zeolites have been used for many types of hydrocarbon transformations, including, for example, toluene disproportionation (see, e.g., U.S. Pat. No. 4,160,788), hydrocarbon oil dewaxing (U.S. Pat. No. 5,000,840), selective sorption of hydrocarbons (U.S. Pat. No. 4,423,280), olefin oligomerization (U.S. Pat. No. 4,855,527) or isomerization (U.S. Pat. No. 5,177,281 ), gasoline upgrading (U.S. Pat. No. 5,298,150), desulfurization of hydrocarbons (U.S. Pat. No. 5,401 ,391 ), and conversion of C5 linear olefins to tert-alkyl ethers (U.S. Pat. No. 5,420,360). Zeolites have also been used in processes for making propylene or ethylene from mixtures of olefins and paraffinic hydrocarbons. For example, U.S. Pat. Nos. 5,043,522 and 5,026,936 describe a process for making propylene in which a mixture of 40-95 wt.% paraffinic hydrocarbons (C4 and higher) and 5-60 wt.% olefins (C4 and higher) are heated in the presence of certain zeolites. The examples use ZSM-5, a zeolite that has interconnecting channels in three dimensions and a pore size index greater than 28. U.S. Pat. No. 5,026,935 uses ZSM-5 to make ethylene from higher hydrocarbons (including butenes and/or propylene) by a cracking process. Still needed in the art are better ways to make propylene and ethylene from higher olefins. Preferably, the process could use the C4 and C5 olefin streams that are readily available from steam cracking. Ideally, the process would retard catalyst coking and deactivation, which hampers productivity in many processes that use heterogeneous catalysts such as zeolites. An ideal process would give valuable ethylene and propylene in favorable yields and selectivities, yet would be suitable for use with fixed-bed reactor systems. SUMMARY OF THE INVENTION The invention is a process for making propylene and ethylene. The process comprises heating a mixture of C4 and/or C5 olefins with a particular zeolite catalyst under conditions effective to produce propylene and ethylene. The catalyst has a pore diameter greater than 3.5 A and one-dimensional, non-interconnecting channels having a pore size index within the range of 14 to 28. Also valuable are zeolite catalysts that have one or more interconnecting channels, including (1 ) a primary channel that has a pore diameter greater than 3.5 A and a pore size index within the range of 14 to 28, and (2) a secondary channel interconnected therewith that has a pore size index less than 20. Catalysts used in the process of the invention are uniquely well suited to the production of propylene and ethylene from C4 and/or C5 olefin streams. First, they have channels large enough to admit the C4 and/or C5 olefins and large enough to allow propylene and ethylene to diffuse out. However, the channels are also small enough to retard diffusion of dimerized products from the channels and small enough to minimize formation of hydrocarbon coke precursors within the channels. DETAILED DESCRIPTION OF THE INVENTION The hydrocarbon feed used as a starting material for the process of the invention is a mixture that contains C4 and/or C5 olefins. By "C4" and "C5" olefins, we mean linear or branched isomers that contain four or five carbons and a single carbon-carbon double bond. These include the various butene and pentene isomers, such as 1 -butene, cis-2-butene, trans-2-butene, isobutene, 1 -pentene, cis- 2-pentene, trans-2-pentene, 3-methyl-1 -butene, 2-methyl-2-butene, 2-methyl-1- butene, and mixtures of these. While the feed can (and often will) contain other types of hydrocarbons, it preferably comprises more than 40 wt.%, more preferably more than 60 wt.%, of C4 and/or C5 olefins. Hydrocarbon streams suitable for use in the process of the invention include C4 and C5 streams (with or without the isobutene component) from fluid catalytic crackers or hydrocarbon pyrolysis units. A preferred hydrocarbon stream consists essentially of C4 and/or C5 olefins. The C4 and/or C5 olefin mixture is converted to propylene and ethylene by contacting it with a particular zeolite catalyst. While many varieties of zeolite catalysts are known, only certain types are useful in the invention. Useful zeolites fall into the general category of medium or intermediate pore zeolites, which typically have a 10-membered ring or puckered 12-membered ring channel structure (see Table 1 below), although numerous 8-membered ring zeolites will also be suitable (see especially Table 2 below). Useful zeolites have a pore diameter greater than 3.5 A. By "pore diameter," we mean the diameter of the ring aperture or pore measured at its narrowest dimension, or the smaller of the two major axes for an elliptical pore. Thus, e.g., a catalyst that has channels with elliptical apertures measuring 3.3 A x 5.0 A would not meet the requirements of the invention because the narrowest dimension (the smaller of the major axes) is not "greater than 3.5 A." Zeolites that have a pore diameter less than or equal to 3.5 Λ are not suitable for use because they are too narrow to permit entry of C4 and/or C5 olefins into the channels. Preferred zeolites have a pore diameter greater than 3.7 A; most preferred are zeolites that have a pore diameter greater than 4.0 A. Zeolites useful in the invention have one-dimensional, non-interconnecting channels. By "one-dimensional, non-interconnecting" channels, we mean ones that are more or less parallel and non-intersecting. Zeolite handbooks such as W.M. Meier et al., Atlas of Zeolite Structure Types. 4th Revised Ed. (1996), hereinafter referred to as "the Atlas," identify such one-dimensional zeolites with a single asterisk ( * ) in their description of the channels. For example, MTT (ZSM-23), a zeolite useful in the invention, has a one-dimensional channel structure. The Atlas describes its channels as follows: [001] 104.5 x 5.2 * The boldface 10 indicates a 10-membered ring structure, the 4.5 and 5.2 refer to pore diameter (in Angstroms; two numbers because of the non-circular apertures), and the asterisk identifies the channels as one-dimensional and non-interconnecting. In contrast, MFI (ZSM-5), a zeolite that is not useful in the invention, has three-dimensional, interconnecting channels. The Atlas describes its channels as follows: {[010] 10 5.3 x 5.6 <--> [100] 10 5.1 x 5.5} *** . The triple asterisk denotes a three-dimensional structure, and the double arrow indicates that the channels interconnect. Zeolites useful in the invention have a pore size index within the range of 14 to 28. By "pore size index," we mean the product of the dimensions (in Angstrom units) of the two major axes of the pores. This is the definition used, e.g., by Haag et al. in U.S. Pat. No. 5,177,281 , the teachings of which are incorporated herein by reference. The pore dimensions are simply multiplied together to get the pore size index. For example, TON (ZSM-22), a zeolite catalyst useful in the invention, has elliptical pores measuring 4.4 x 5.5 A. Multiplying these numbers gives a pore size index for TON of 24.2. Cancrinite, on the other hand, a catalyst not useful in the invention, has pores measuring 5.9 A, and a pore size index of 34.8 (i.e., greater than the upper limit of 28 for catalysts useful in the invention). In preferred catalysts of the invention, the channels have a pore size index within the range of 16 to 25. Table 1 summarizes examples of zeolite catalysts useful in the process of the invention, along with their channel descriptions (from the Atlas), minimum pore diameters, and pore size indices. The channel descriptions follow the notation used in the Atlas. As the Atlas explains, each system of equivalent channels is characterized by (1 ) the channel direction (relative to the axes of the type structure), (2) the number of either T- (usually Si or Al) or O- atoms, in bold type, that form the rings controlling diffusion through the channels, and (3) the crystallographic free diameters of the channels in Angstroms, based on the atomic coordinates of the type materials and an oxygen radius of 1.35 A. The number of asterisks indicates whether the channel system is one-, two-, or three-dimensional. Interconnecting channels are separated by a double arrow (<->). A vertical bar (|) means that there is no direct access from one channel system to another. In addition to the zeolites listed in Table 1 , useful zeolites include the topological equivalents of these materials, which are described in the Atlas and on the website listed below. All of the above information about these zeolites is now available "online" courtesy of the Structure Commission of the International Zeolite Association. The website address is: http://www-iza-sc.csb.yale.edu In spite of the foregoing discussion, the process of the invention can be performed using certain zeolites that have one or more interconnecting channels. Such zeolites must have: (1 ) a primary channel that has a pore diameter greater than 3.5 A and a pore size index within the range of 14 to 28; and (2) a secondary channel interconnected with the primary channel that has a pore size index less than 20. Table 2 lists examples of zeolites that meet these criteria. Ferrierite (FER) is a good example of a zeolite having interconnecting channels, yet still suitable for use in the process of the invention. As Table 2 shows, ferrierite has interconnecting channels, as indicated by the double arrow. The primary channels have a pore diameter of 4.2 A (i.e., greater than 3.5 A) and a pore size index of 22.7 (i.e., within the range of 14 to 28). Moreover, the secondary channels have a pore size index of 16.8 (i.e., less than 20). What all of the zeolites useful in the process of the invention have in common are channels large enough to admit the C4 and/or C5 olefins and large enough to allow propylene and ethylene to diffuse out. On the other hand, the channels are generally small enough to retard diffusion of dimerized products from the channels and small enough to minimize formation of hydrocarbon coke precursors within the channels. For comparison purposes, Table 3 lists some zeolites that are not useful in the process of the invention. For example, MTW (ZSM-12) has a one-dimensional, non-interconnecting channel structure, but it has a pore size index of 32.5 (i.e., greater than 28). MEL (ZSM-11) is not suitable for use because it has a three- dimensional, interconnecting channel structure wherein all the channels have a pore size index of 28.6 (i.e., greater than 28 for the "primary" channels and greater than 20 for the "secondary" ones.) Similarly, MFI (ZSM-5), another catalyst with a three-dimensional channel system, would not be suitable because its primary channels have a pore size index of 29.7 (i.e., greater than 28), and its secondary channels have a pore size index of 28.1 (i.e., greater than 20). Mazzite (MAZ) has two one-dimensional channel systems that do not interconnect, and neither channel system meets the requirements of the invention. The smaller channel aperture is too small (3.4 A), and the larger channel has a pore size index of 54.8 (i.e., greater than 28). The zeolites used in the process of the invention are usually powders. To facilitate their use in fixed-bed reactors, the zeolites are optionally combined with one or more binders. Suitable binders are well known in the art and include, for example, natural clays (e.g.,montmorillonite, kaolin, bentonite), silicas, aluminas, and the like. Aluminas and silicas are preferred. When a binder is used, it is typically present in an amount within the range of about 0.5 to about 40 wt.% based on the combined amounts of binder and zeolite catalyst. The binders can be used in any convenient form, including powders, slurries, gels, or the like. If desired, the catalyst powder and/or binder can be combined with water and mulled using commercial mullers such as the Lancaster Mix Muller to produce a catalyst- containing paste. In the alternative, the zeolite catalyst can be used alone in powder or pelletized form. The zeolite catalysts are normally synthesized in the alkali metal form. They are conveniently converted to the hydrogen form by ion exchange with ammonium halide solution, followed by calcination. The original alkali metal can also be replaced by other suitable metal cations, such as other alkali metals, calcium, magnesium, or the like. These metals are generally included to regulate the effective pore size index of the zeolite. For example, converting a zeolite to the hydrogen form generally increases the effective pore size index relative to the alkali metal form of the catalyst, while substituting the alkali metal with a metal tends to decrease the effective pore size index. Whether or not the zeolite catalyst is modified by converting it to the H-form, or by metal substitution, the effective pore size index needs to be within the limits defined herein. Rare earth metals such as lanthanum can be included, if desired, for the purpose of improving yield or selectivity to propylene and ethylene. Trace amounts of an oxidizing metal such as Pd or Pt can be used, if desired, to promote coke removal during catalyst regeneration (see, e.g., U.S. Pat. No. 5,648,585). o I The catalysts can be prepared for use by any number of methods, which are now well known in the art. For example, the patent literature provides synthetic methods for ZSM-23 (U.S. Pat. Nos. 4,076,842 and 4,490,342), ZSM-22 or TON (U.S. Pat. Nos. 4,556,477 and 5,342,596), and ZSM-35 (U.S. Pat. No. 4,016,245). The zeolites can be used essentially "as is." Usually, however, the zeolites are calcined prior to use to remove traces of water, preferably by heating them at a temperature within the range of about 200°C to about 750°C, more preferably from about 200°C to about 650°C, and most preferably from about 300°C to about 600°C. When the zeolites are combined with a binder, calcination usually follows the pelletization process. Pellets are usually made by extrusion. If desired, one or more peptizing acids (e.g., nitric acid, acetic acid) are included with an extrusion aid (e.g., hydroxypropyl methylceilulose) to make a plastic, extrudable material. In practicing the process of the invention, the mixture of C4 and/or C5 olefins is contacted with the zeolite catalyst under conditions effective to produce propylene and ethylene. Preferably, the process is performed in the vapor phase by bringing a heated olefin mixture into contact with the zeolite catalyst. Either the catalyst or the olefin mixture (or both) can be heated. Preferably, the reaction is performed at a temperature within the range of about 200°C to about 750°C, more preferably from about 400°C to about 650°C, and most preferably from about 500°C to about 600°C. While the reactor pressure is not usually critical, it is preferred to perform the process at a total reactor pressure within the range of about 0.5 to about 10 atmospheres, more preferably from about 1 to 3 atmospheres. Any suitable feed rate can be used. Generally, it is preferred to use a hydrocarbon weight hourly space velocity (WHSV) within the range of about 0.5 to about 1000 h ' \ more preferably from about 1 to 50 h '1 . The process of the invention can be practiced in a batch, continuous, semi- batch, or semi-continuous manner. A continuous process is preferred. If necessary, the catalyst can be regenerated using conventional techniques such as treatment with air diluted with an inert gas such as nitrogen. The process can be used with any desired kind of reactor system, including, for example, a fixed-bed, moving-bed, or fluidized-bed reactor system. The catalysts, when pelletized or combined with a binder and extruded, are particularly useful in a fixed-bed reactor system. The following claims define the scope of the invention.

Claims: I claim: 1. A process which comprises contacting a mixture of C4 and/or C5 olefins with a zeolite catalyst under conditions effective to produce propylene and ethylene, wherein the catalyst has a pore diameter greater than 3.5 A and one-dimensional, non-interconnecting channels having a pore size index within the range of 14 to 28. 2. The process of claim 1 wherein the C4/C5 olefin mixture comprises more than about 60 wt.% of C4 and/or C5 olefins. 3. The process of claim 1 wherein the catalyst channels have a pore size index within the range of 16 to 25. 4. The process of claim 1 wherein the catalyst has a pore diameter greater than 3.7 A. 5. The process of claim 1 wherein the catalyst has a pore diameter greater than 4.0 A. 6. The process of claim 1 wherein the catalyst is selected from the group consisting of RON, AEL, EUO, LAU, MTT, TON, CHI, ATN, AWW, JBW, CZP, and RTE. 7. The process of claim 6 wherein the catalyst is selected from the group consisting of RON, LAU, MTT, and TON. 8. The process of claim 1 performed at a temperature within the range of about 200°C to about 750°C, a total reactor pressure within the range of about 0.5 to 10 atmospheres, and a weight hourly space velocity (WHSV) within the range of about 0.5 to 1000 fτ 1 . 9. The process of claim 1 wherein the catalyst further comprises a rare earth metal. 10. A fixed-bed process of claim 1. 11. A process which comprises contacting a mixture of C4 and/or C5 olefins with a zeolite catalyst under conditions effective to produce propylene and ethylene, wherein the catalyst has one or more interconnecting channels, including a primary channel that has a pore diameter greater than 3.5 A and a pore size index within the range of 14 to 28, and a secondary channel interconnected therewith that has a pore size index less than 20. 12. The process of claim 11 wherein the C4/C5 olefin mixture comprises more than about 60 wt.% of C4 and/or C5 olefins. 13. The process of claim 11 wherein the primary channel has a pore diameter greater than 3.8 A. 14. The process of claim 11 wherein the primary channel has a pore diameter greater than 4.0 A. 15. The process of claim 11 wherein the catalyst is selected from the group consisting of DAC, EPI, FER, AEI, ATT, CHA, DDR, EAB, ERI, KFI, LEV, LTA, PAU, RTH, MFS, and ZON. 16. The process of claim 15 wherein the catalyst is selected from the group consisting of FER, ATT, CHA, LTA, and MFS. 17. The process of claim 11 performed at a temperature within the range of about 200°C to about 750°C, a total reactor pressure within the range of about 0.5 to 10 atmospheres, and a weight hourly space velocity (WHSV) within the range of about 0.5 to 1000 h "1 . 18. The process of claim 11 wherein the catalyst further comprises a rare earth metal. 19. A fixed-bed process of claim 11. 20. A process which comprises contacting a mixture of C4 and/or C5 olefins with a zeolite catalyst under conditions effective to produce propylene and ethylene, wherein the catalyst has one-dimensional, non-interconnecting channels large enough to admit the C4 and/or C5 olefins and large enough to allow propylene and ethylene to diffuse out, but small enough to retard diffusion of dimerized products from the channels and small enough to minimize formation of hydrocarbon coke precursors within the channels. 21. The process of claim 20 wherein the catalyst is selected from the group consisting of RON, AEL, EUO, LAU, MTT, TON, CHI, ATN, AWW, JBW, CZP, and RTE. 22. The process of claim 21 wherein the catalyst is selected from the group consisting of RON, LAU, MTT, and TON. 23. A process which comprises heating a mixture of C4 and/or C5 olefins with a zeolite catalyst under conditions effective to produce propylene and ethylene, wherein the catalyst has one or more interconnecting channels large enough to admit the C4 and/or C5 olefins and large enough to allow propylene and ethylene to diffuse out, but small enough to retard diffusion of dimerized products from the channels and small enough to minimize formation of hydrocarbon coke precursors within the channels. 24. The process of claim 23 wherein the catalyst is selected from the group consisting of DAC, EPI, FER, AEI, ATT, CHA, DDR, EAB, ERI, KFI, LEV, LTA, PAU, RTH, MFS, and ZON. 25. The process of claim 24 wherein the catalyst is selected from the group consisting of FER, ATT, CHA, LTA, and MFS.

Inventors: POWERS DONALD H, POWERS, DONALD, H.
Applicants: EQUISTAR CHEM LP, EQUISTAR CHEMICALS, L.P.
Application year: 1999
Main classification: C07C4
IPC codes: C07C4