WO2014099043A1

WO2014099043A1 - Zeolite catalyst for hydroisomerization of light paraffins to produce high octane gasoline

Info

Publication number: WO2014099043A1
Application number: PCT/US2013/053778
Authority: WO
Inventors: Cong-Yan Chen; Xiaoying Ouyang; Stacey Ian Zones
Original assignee: Chevron U.S.A. Inc.
Priority date: 2012-12-19
Filing date: 2013-08-06
Publication date: 2014-06-26

Abstract

A process for isomerizing light paraffins using a catalyst comprising an SSZ-82 zeolite and at least one Group VIII metal. Such catalysts show enhanced selectivity in the conversion of n-hexane to the higher octane C₆ isomer 2,3-dimethylbutane over the lower octane C₆ isomer 2,2-dimethylbutane when compared to other large pore zeolites. Moreover, isomerization can be performed at a lower temperature for maximum isomer yield.

Description

ZEOLITE CATALYST FOR HYDROISOMERIZATION OF LIGHT PARAFFINS TO PRODUCE HIGH OCTANE GASOLINE

TECHNICAL FIELD

[001 ] The application generally relates to a process for isomerizing light paraffins by using a catalyst comprising an SSZ-82 zeolite and at least one Group VIII metal. Such catalysts show enhanced selectivity in the conversion of n-hexane to the higher octane Ce isomer 2,3-dimethylbutane over the lower octane Ce isomer 2,2-dimethylbutane when compared to large pore zeolites Y and mordenite.

BACKGROUND

[002] Modern automobile engines require high octane gasoline for efficient operation. Previously, lead and oxygenates, such as methyl-t-butyl ether (MTBE), were added to gasoline to increase the octane number. Furthermore, several high octane components normally present in gasoline, such as benzene, aromatics, and olefins, must now be reduced. Obviously, a process for increasing the octane of gasoline without the addition of toxic or environmentally adverse substances would be highly desirable.

[003] Gasoline is generally prepared from a number of blend streams, including light naphthas, full range naphthas, heavier naphtha fractions, and heavy gasoline fractions. The gasoline pool typically includes butanes, light straight run, isomerate, FCC cracked products, hydrocracked naphtha, coker gasoline, alkylate, reformate, added ethers, etc. Of these, gasoline blend stocks from the FCC, the reformer and the alkylation unit account for a major portion of the gasoline pool.

[004] For a given carbon number of a light paraffin component in naphtha, the shortest, most branched isomer tends to have the highest octane number. For example, the singly and doubly branched isomers of hexane, mono-methylpentane and dimethylbutane respectively, have octane numbers that are significantly higher than that of n-hexane, with dimethylbutane having the highest research octane number (RON). Likewise, the singly branched isomer of pentane, 2-methylbutane, has a significantly higher RON than n-pentane. By increasing the proportion of these high octane isomers in the gasoline pool, satisfactory octane numbers can be achieved for gasoline without additional additives.

[005] Two types of octane numbers are currently being used, the motor octane number (MON) determined using ASTM D2700-12 ("Standard Test Method for Motor Octane Number of Spark-Ignition Engine Fuel") and the RON determined using ASTM D2699-12 ("Standard Test Method for Research Octane Number of Spark-Ignition Engine Fuel"). The two methods both employ the standard Cooperative Fuel Research (CFR) knock- test engine. Sometimes, the MON and RON are averaged, (MON + RON)/2, to obtain an octane number. Therefore, when referring to an octane number, it is essential to know which one is being discussed. In this disclosure, unless clearly stated otherwise, octane number will refer to the RON. For comparative purposes, the RON for isomers of pentane and hexane are listed in Table 1.

TABLE 1

[006] Gasoline suitable for use as fuel in an automobile engine should have a RON of at least 80, e.g., at least 85, or at least 90. High performance engines generally require a fuel having a RON of about 100. Most gasoline blending streams have a RON generally ranging from 55 to 95, with the majority typically falling between 80 and 90. Obviously, it is desirable to maximize the amount of dimethylbutane in light paraffins of the gasoline pool in order to increase the overall RON.

[007] Hydroisomerization is an important refining process whereby the RON of a refinery's gasoline pool can be increased by converting straight chain normal or singly branched light paraffins into their more branched isomers. The hydroisomerization reaction is controlled by thermodynamic equilibrium. At higher reaction temperatures, the equilibrium shifts towards the lower octane isomers (e.g., from dimethylbutanes via methylpentanes to n- hexane). Since the high octane components (e.g., 2,3-dimethylbutane with a RON = 101.0) are the target products in this process, it is desirable to develop a more active catalyst to perform this reaction at a lower temperature. [008] There is a need for new and improved hydrocarbon hydroisomerization catalysts and processes that provide high selectivity for producing high octane isomers of light paraffins, wherein the catalysts are also highly active, environmentally benign, and readily regenerable.

SUMMARY

[009] There is provided a hydroisomerization process which comprises contacting a hydrocarbon feed stream comprising normal and singly branched C₄ to C7 paraffins, under hydroisomerization conditions, with a catalyst comprising an aluminosilicate SSZ-82 zeolite and at least one Group VIII metal to form an isomerized product having a higher

concentration of doubly and singly branched paraffins than the feed stream and having a 2,3- dimethylbutane to 2,2-dimethylbutane mole ratio of at least 0.7.

DETAILED DESCRIPTION

[010] The following terms will be used throughout the specification and will have the following meanings unless otherwise indicated.

[011] "Hydroisomerization" refers to a process in which paraffins are isomerized to their more branched counterparts in the presence of hydrogen over a catalyst.

Hydroisomerization is intended to provide a product stream enriched in high octane paraffin isomers from a feed stream comprised of normal and singly branched C₄ to C7 paraffins by the selective addition of branching into the molecular structure of the feed stream paraffins. Hydroisomerization ideally will achieve high conversion levels of the normal and singly branched light paraffins to more highly branched paraffins while at the same time minimizing the conversion by cracking. Hydroisomerization can be achieved by contacting the feed with a hydroisomerization catalyst in an isomerization zone under hydroisomerizing conditions.

[012] Zeolites can be broadly described as crystalline microporous molecular sieves that possess three-dimensional frameworks composed of tetrahedral units (T0₄/2, where T = Si, Al, or other tetrahedrally coordinated atom) linked through oxygen atoms. The cage or pore size of these materials is denoted by the number of oxygen atoms (likewise the number of tetrahedral atoms) circumscribing the pore or cavity, e.g., a pore circumscribed by n oxygen atoms is referred to as an n-membered ring pore, or more simply, n-MR. Depending on the largest pore openings that they possess, zeolites are usually categorized as small (8- MR), medium (10-MR), large (12-MR), or extra-large (> 14-MR) pore zeolites. As used herein, "large-pore zeolite" refers to a zeolite delimited by 12-membered rings wherein the pore aperture measures from about 6.5 A to 8 A. Examples of large pore zeolites include beta zeolite, faujasite, mordenite, and zeoliteY.

[013] The zeolite designated "SSZ-82" and methods for making it are disclosed in U.S. Patent No. 7,820, 141. SSZ-82 possesses a novel two-dimensional channel system composed of intersecting 12- and 10-MR pores with effective pore widths of 5.2 x 8.0 A and 4.9 x 5.5 A, respectively. Details of the structure of SSZ-82 are further described by D. Xie et al. in J. Am. Chem. Soc. 201 1, 133, 20604-20610. In one embodiment, the SSZ-82 zeolite has a Si0₂/Al₂0₃ mole ratio of from 20 to 200.

[014] "C_n" describes a hydrocarbon molecule wherein "n" denotes the number of carbon atoms in the molecule.

[015] "Paraffin" refers to any saturated hydrocarbon compound, i.e., a hydrocarbon having the formula C_nH(2_n + 2) where n is a positive non-zero integer.

[016] "Normal paraffin" refers to a saturated straight chain hydrocarbon.

[017] "Singly branched paraffin" refers to a saturated hydrocarbon having the molecular structure

R

R¹ C R²

H

where R, R¹ and R² are independent alkyl groups; and wherein R is a normal alkyl group (e.g., methyl) as a branch and R¹ and R² represent portions of the normal paraffin chain or backbone.

[018] "Doubly branched paraffin" refers to a saturated hydrocarbon such as

R R R

R¹ C R² or R1 c C R²

R H H

where R, R¹ and R² are independent alkyl groups; and wherein R is a normal alkyl group (e.g., methyl) as a branch and R¹ and R² represent portions of the normal paraffin chain or backbone. Thus, a singly branched paraffin has one R group per paraffin molecule while a doubly branched paraffin has two R groups per molecule where the two R groups can be the same alkyl groups or different ones. [019] "Mono-methylpentane" refers to 2-methylpentane, 3-methylpentane, or mixtures of these isomers. Similarly, "dimethylbutane" refers to 2,2-dimethylbutane, 2,3- dimethylbutane, or mixtures of these isomers.

[020] The isomers of C₄ to Ce paraffins are included in the light naphtha fraction of the gasoline pool. One skilled in the art will recognize that some isomers of C7 paraffin can also be present in the light naphtha fraction. However, heptane and its isomers are generally present only in minor amounts.

[021] When used herein, the Periodic Table of the Elements refers to the version published by CRC Press in the CRC Handbook of Chemistry and Physics, 88th Edition (2007-2008). The names for families of the elements in the Periodic Table are given here in the Chemical Abstracts Service (CAS) notation.

Feed stream

[022] A refinery feed stream referred to as light paraffins typically contains mainly normal and singly branched C₄ to C7 hydrocarbons and has a relatively low octane number because it contains substantial amounts of C₄ to Ce normal paraffins. Typically, the feed stream has a RON of less than 80 (e.g., less than 75, less than 70, less than 65, less than 60, or less than 55).

[023] In one embodiment, the feed stream contains predominantly normal and singly branched C₄ to Ce paraffins. The singly branched C₄ to Ce paraffins can be singly branched C5 to Ce paraffins. Generally, the feed stream contains at least 10 wt. % normal C₄ to Ce paraffins (e.g., at least 20 wt. %, at least 30 wt. %, at least 40 wt. %, at least 50 wt. %, at least 60 wt. %, at least 70 wt. %, at least 80 wt. %, or at least 90 wt. % normal C₄ to Ce paraffins). In another embodiment, the feed stream contains predominantly normal and singly branched C5 to Ce paraffins. In a sub-embodiment, the feed stream contains predominantly n-pentane and n-hexane. In yet another embodiment, the feed stream contains at least 10 wt. % n- hexane (e.g., at least 20 wt. %, at least 30 wt. %, at least 40 wt. %, at least 50 wt. %, at least 60 wt. %, at least 70 wt. %, at least 80 wt. %, or at least 90 wt. % n-hexane). As used herein, the term "predominantly" means an amount of 50 wt. % or more of the substance in question as a fraction of the total feed.

[024] Optionally, the feed can be hydrotreated in a hydrotreating process to remove any excess sulfur and/or nitrogen content, prior to the hydroisomerization process.

Optionally, the feed contains benzene which can be hydrogenated to cyclohexane in the hydroisomerization process to reduce the benzene content in the gasoline product. Hydroisomerization Catalyst

[025] Catalysts useful for hydroisomerization processes are generally bifunctional catalysts that include a hydrogenation/dehydrogenation component and an acidic component. The hydroisomerization catalyst usually contains at least one Group VIII metal (e.g., platinum or palladium) on a porous inorganic oxide support (e.g., alumina, silica-alumina or a zeolite). If the support itself does not have sufficient acidity to promote the needed isomerization reactions, such acidity can be added. Examples of a useful acid component include a zeolite, a halogenated alumina component, or a silica-alumina component.

[026] Catalysts useful for hydroisomerization processes described herein contain at least one Group VIII metal on an SSZ-82 zeolite, typically in the aluminosilicate form. The at least one Group VIII metal compound can be present in an amount to provide sufficient activity for the catalyst to have commercial use. By Group VIII metal compound, as used herein, is meant the metal itself or a compound thereof. Non-limiting examples of Group VIII metals include platinum, palladium, and combinations thereof.

[027] The at least one Group VIII metal can be combined with or incorporated into the SSZ-82 zeolite by any one of numerous procedures, for example, by co-milling, impregnation, or ion exchange. Processes which are suitable for these purposes are known to those skilled in the art. The at least one Group VIII metal can be present in the SSZ-82 zeolite in an amount suitable for catalysis of light paraffins. The metal-loaded zeolite catalyst can be sufficiently active and selective under hydroisomerization conditions so as to provide a substantial increase in high octane doubly branched light paraffins during a single pass through a hydroisomerization zone or reactor. Generally, the amount of metal component combined with the zeolite can be in the range from 0.05 wt. % to 5.0 wt. % (e.g., from 0.1 wt. % to 3.0 wt. %, or from 0.1 wt. % to 1.0 wt. %) wherein the given wt. % is based on the weight of the zeolite.

[028] Optionally, the catalyst can be pre-sulfided to lower the hydrogenolysis activity. Procedures that are suitable for pre-sulfiding metal-loaded zeolite catalysts are known to those skilled in the art.

[029] In situations where the catalyst is deactivated by coke deposit or other poisons, the catalyst activity can be rejuvenated via catalyst regeneration. Procedures suitable for the regeneration of zeolite catalysts are known in the art. In addition, the zeolite catalyst is environmentally benign since it is not chlorinated to boost its acidity.

[030] Catalysts based on the SSZ-82 zeolites described herein have high levels of activity for the hydroisomerization of light paraffins and also show enhanced selectivity in the conversion of n-hexane to the higher octane Ce isomer 2,3-dimethylbutane over the lower octane Ce isomer 2,2-dimethylbutane when compared to large pore zeolites Y and mordenite.

Process Conditions

[031 ] The catalytic hydroisomerization conditions employed depend on the feed used for the hydroisomerization and the desired properties of the product. Typical hydroisomerization conditions which can be employed include a temperature of from 150°F to 700°F (66°C to 371°C), e.g., from 400°F to 650°F (204°C to 343°C), from 450°F to 600°F (232°C to 316°C), or from 480°C to 520°C (249°C to 271°C); a pressure of from 50 psig to 2000 psig (0.45 MPa to 13.89 MPa), e.g., from 100 psig to 1000 psig (0.79 MPa to 7.00 MPa), or from 150 psig to 450 psig (1.14 MPa to 3.20 MPa); a hydrocarbon feed liquid hourly space velocity (LHSV) of from 0.5 If¹ to 5 If¹, e.g., from 0.5 If¹ to 3 If¹, or from 0.75 If¹ to 2.5 If¹; and a hydrogen to hydrocarbon (H2/HC) mole ratio of from 0.5 to 10, e.g., from 1 to 10, or from 2 to 8. Exemplary hydroisomerization conditions include a temperature of from 480°F to 520°F (249°C to 271°C), a pressure of from 150 psig to 450 psig (1.14 MPa to 3.20 MPa), a LHSV of from 0.5 IT¹ to 3 If¹, and a H₂/HC mole ratio of from 2 to 8.

[032] In one embodiment, the hydroisomerization conditions can include a temperature at or about the temperature for maximum isomer yield of one or more light paraffins. The temperature for maximum isomer yield from a particular feed stream (e.g., containing one or more light normal paraffins) can be determined empirically for a given zeolite catalyst, e.g., by performing hydroisomerization of the feed stream over a range of temperatures under defined conditions, and analyzing the composition of the product stream for each hydroisomerization temperature. The product analysis can be conducted, for example, by on-line GC analysis. Hydroisomerization temperatures can be successively increased, e.g., in 5°F to 10°F (2.8°C to 5.6°C) increments from a starting hydroisomerization temperature (e.g., about 400°F, 204°C), until isomer yields in the product stream from the reactor have peaked. Naturally, the temperature for maximum isomer yield can vary depending on the composition and activity of the zeolite catalyst, and on other factors.

[033] In some embodiments, where the conversion of the hydrocarbon feedstock is lower than targeted, or the yield of the preferred product, e.g., 2,3-dimethylbutane, is lower than targeted, the process can optionally include a separation stage for recovering at least a portion of the unconverted feedstock. Optionally, at least a portion of the feed stream including any unconverted feedstock can be recycled to the hydroisomerization unit or zone.

[034] The hydroisomerization of light paraffins can be performed in a

hydroisomerization zone or reactor. Various reactor types can be used. For example, a hydrocarbon feed (e.g., containing substantial amounts of light paraffins) can be contacted with the zeolite catalyst in a fixed bed system, a moving bed system, a fluidized system, a batch system, or combinations thereof. In a fixed bed system, the preheated feed is passed into at least one reactor that contains a fixed bed of the catalyst prepared from material comprising the zeolite catalyst. The flow of the feed can be upward, downward or radial. The reactors can be equipped with instrumentation to monitor and control temperatures, pressures, and flow rates. Multiple beds can also be used, wherein two or more beds can each contain a different catalytic composition, at least one of which can comprise an SSZ-82 zeolite.

[035] In one embodiment, the feed stream can be contacted with the zeolite catalyst during a single pass of the feed stream through the hydroisomerization zone or reactor to provide an isomerized product comprising at least 15 mole % of dimethylbutane.

Products

[036] The hydroisomerization processes described herein yield an isomerized product enriched in more highly branched C₄ to C7 paraffins, and primarily branched C5 to Ce isomers at maximum isomer yield.

[037] In one embodiment, the isomerized product generally contains at least 25 mole % of dimethylbutane. In another embodiment, the isomerized product contains at least 10 mole % of 2,3 -dimethylbutane. The isomerized product can further contain 2-methylpentane and 3-methylpentane.

[038] In embodiments, the isomerized product has an RON of at least 85 (e.g., at least 90, or at least 95).

EXAMPLES

[039] The following illustrative examples are intended to be non-limiting.

EXAMPLE 1

Preparation of Hydroisomerization Catalyst

[040] Aluminosilicate SSZ-82 (Al-SSZ-82) was prepared as described in Example 7 of U.S. Patent No. 7,820, 141. The Al-SSZ-82 material was separately ion exchanged three times under reflux with an aqueous NH₄NO₃ solution to create the NH₄ ⁺ form of the zeolite. The zeolite was then separately ion exchanged with an aqueous (NH₃)₄Pt(N03)₂ solution to load the zeolite with 0.5 wt. % Pt. The resulting catalyst was subsequently calcined by heating in air at 700°F for 5 hours. The Pt-loaded zeolite was reduced with hydrogen prior to hydroisomerization studies. EXAMPLE 2

Hydroisomerization of n-Hexane over

Pt-Exchanged Al-SSZ-82

[041 ] The catalytic hydroisomerization of n-hexane was carried out using the Al- SSZ-82 catalyst of Example 1 in a flow type fixed bed reactor with pure n-hexane as feed, at a temperature corresponding to the maximum isomer yield for the catalyst. The temperature for maximum isomer yield for the catalyst was determined by product analysis (on-line GC) over a range of successively increased temperatures (10°F increments) starting at a temperature of 400°F, until isomer yields in the product stream of the catalyst sample reached a maximum. The temperature for maximum isomer yield for the catalyst is presented in Table 2. The hydroisomerization conditions included a pressure of 200 psig, a LHSV of 1 If¹, and a molar ¾ to hydrocarbon ratio of 6: 1. The reaction products were analyzed with an on-line GC to quantify each of the Ce alkane isomers, and the results are set forth in Table 2.

EXAMPLE 3

Hydroisomerization of n-Hexane over

Pd-Exchanged Zeolite Y, Mordenite, ZSM-5 and SSZ-32

[042] The hydroisomerization of n-hexane was carried out over Pd/Y, Pd/mordenite, Pd/ZSM-5 and Pd/SSZ-32 in a flow type fixed bed reactor with pure n-hexane as feed at the temperature, pressure, LHSV, and molar H₂ to hydrocarbon ratio as described in Example 2. These catalysts were prepared as described in Example 1 for Pt/Al-SSZ-82. The results at the respective temperatures corresponding to maximum isomer yield are also set forth in Table 2.

TABLE 2

[043] In the hydroisomerization of n-hexane with an SSZ-82 zeolite catalyst, when compared to large pore zeolites Y and mordenite, enhanced selectivity for the higher octane Ce isomer 2,3-dimethylbutane over the lower octane Ce isomer 2,2-dimethylbutane was observed at maximum isomer yield with about 73 mole % conversion of the n-hexane and with less than about 10 mole % cracking. ZSM-5 and SSZ-32 based catalysts gave high 2,3- dimethylbutane to 2,2-dimethylbutane mole ratios, but the total dimethylbutane produced during n-hexane hydroisomerization by these zeolites was very small (3.2 and 2.2 mole %, respectively, at maximum isomer yield), as compared with the total dimethylbutane production by SSZ-82.

[044] For the purposes of this specification and appended claims, unless otherwise indicated, all numbers expressing quantities, percentages or proportions, and other numerical values used in the specification and claims, are to be understood as being modified in all instances by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained. It is noted that, as used in this specification and the appended claims, the singular forms "a," "an," and "the," include plural references unless expressly and unequivocally limited to one referent. As used herein, the term "include" and its grammatical variants are intended to be non-limiting, such that recitation of items in a list is not to the exclusion of other like items that can be substituted or added to the listed items. As used herein, the term "comprising" means including elements or steps that are identified following that term, but any such elements or steps are not exhaustive, and an embodiment can include other elements or steps.

[045] Unless otherwise specified, the recitation of a genus of elements, materials or other components, from which an individual component or mixture of components can be selected, is intended to include all possible sub-generic combinations of the listed components and mixtures thereof.

[046] The patentable scope is defined by the claims, and can include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. To an extent not inconsistent herewith, all citations referred to herein are hereby incorporated by reference.

Claims

1. A hydroisomerization process, comprising: contacting a hydrocarbon feed stream comprising normal and singly branched C₄ to C7 paraffins, under hydroisomerization conditions, with a catalyst comprising an aluminosilicate SSZ-82 zeolite and at least one Group VIII metal to form an isomerized product having a higher concentration of doubly and singly branched paraffins than the feed stream and having a 2,3- dimethylbutane to 2,2-dimethylbutane mole ratio of at least 0.7.

2. The process of claim 1, wherein the feed stream has a RON of less than 75.

3. The process of claim 1, wherein the feed stream comprises at least 10 wt. % normal C₄ to Ce paraffins.

4. The process of claim 1, wherein the feed stream comprises at least 50 wt. % normal C₄ to Ce paraffins.

5. The process of claim 1, wherein the hydroisomerization conditions comprise a

temperature in the range of from 150°F to 700°F (66°C to 371°C), a pressure of from 50 psig to 2000 psig (0.45 MPa to 13.89 MPa), a hydrocarbon feed LHSV of from 0.5 If¹ to 5 If¹, and a hydrogen to hydrocarbon mole ratio of from 0.5 to 10.

6. The process of claim 5, wherein the hydroisomerization conditions comprise a

temperature of from 480°F to 520°F (249°C to 271°C).

7. The process of claim 5, wherein the hydroisomerization conditions comprise a

pressure of from 150 psig to 450 psig (1.14 MPa to 3.20 MPa).

8. The process of claim 5, wherein the hydroisomerization conditions comprise a LHSV of from 0.5 IT¹ to 3 IT¹.

9. The process of claim 5, wherein the hydroisomerization conditions comprise a

hydrogen to hydrocarbon mole ratio of from 2 to 8.

10. The process of claim 1, wherein the catalyst comprises 0.05 wt. % to 5 wt. % of the at least one Group VIII metal, based on the weight of the zeolite.

11. The process of claim 1, wherein the at least one Group VIII metal is selected from the group consisting of platinum, palladium, and combinations thereof.

12. The process of claim 1, wherein the isomerized product comprises at least 25 mole % of dimethylbutane.

13. The process of claim 1, wherein the isomerized product has a RON of at least 85.