Process for the production of tertiary alkyl ethers

Abstract: A process is disclosed for the production of alkyl tertiary alkyl ethers in C₄+ hydrocarbon streams rich in isoolefins, typically containing catalyst deactivating amounts of dienes and/or compounds containing heteroatoms. The process is especially advantageous in extending the cycle length for the zeolite catalyzed etherification of isoolefins in C₄+ FCC gasoline by reducing catalyst aging. It has been discovered that if hydrogen is co-fed with the alkanol and C₄+ isoolefin rich feedstreams to an etherification reaction catalyzed by acidic zeolite wherein the zeolite has been impregnated with a noble metal the rate of catalyst aging or deactivation is stantially lowered. The process is especially effective, i.e., catalyst aging is particularly reduced, when hydrogen is co-fed to an etherification reaction using acidic zeolite Beta catalyst containing palladium.

Description: This invention relates to a process for increasing the oxygen content of C 5 + olefinic gasoline by the conversion of the isoolefin components of gasoline to high octane value alkyl tertiary alkyl ethers. The invention particularly relates to an etherification process for the production of these tertiary alkyl ethers which uses a noble metal modified regenerable acidic zeolite catalyst concurrently with co-fed hydrogen. The combination of zeolite catalyst modification and hydrogen cofeeding in the etherification step results in a highly advantageous decrease in the rate of catalyst deactivation while enhancing the octane value of the gasoline. It is known that isobutylene and isoamylenes, and other isoalkenes or isoolefins, produced by hydrocarbon cracking may be reacted with methanol and other C 1 -C 4 lower aliphatic alcohols, or alkanol, over an acidic catalyst to provide methyl tertiary butyl ether (MTBE) or the like. Generally, it is known that asymmetrical ethers having the formula (CH 3 ) 3 C-O-R, where R is a C 1 -C 4 alkyl radical, are particularly useful as octane improvers for liquid fuels, especially gasoline. MTBE, ethyl t-butyl ether (ETBE), tert-amyl methyl ether (TAME) and isopropyl t-butyl ether (IPTBE) are known to be high octane ethers. The article by J.D. Chase, et al., Oil and Gas Journal, April 9, 1979, discusses the advantages one can achieve by using such materials to enhance gasoline octane. The octane blending number of MTBE when 10% is added to a base fuel (R+O = 91) is about 120. For a fuel with a low motor rating (M+O = 83) octane, the blending value of MTBE at the 10% level is about 103. On the other hand, for an (R+O) of 95 octane fuel, the blending value of 10% MTBE is about 114. The liquid phase reaction of methanol with isobutylene and isoamylenes at moderate conditions with a resin catalyst is known technology, as provided by R. W. Reynolds, et al., The Oil and Gas Journal, June 16, 1975, and S. Pecci and T. Floris, Hydrocarbon Processing, December 1977. An article entitled "MTBE and TAME - A Good Octane Boosting Combo," by J.D. Chase, et al., The Oil and Gas Journal, April 9, 1979, pages 149-152, discusses the technology. Preferred catalysts are polymeric sulfonic acid exchange resin such as Amberlyst 15 and zeolites such as zeolite Beta and ZSM-5. The acid resin catalysts are effective catalysts at temperatures below 90°C. At higher temperatures the resin catalyst is unstable. Typically, with acid resin catalyst the etherification reaction is carried out in liquid phase. However, mixed phase etherification is known, particularly where the catalyst is contained as a fixed bed in a fractionator which serves to both separate the reaction products and operate as a vessel to contain the catalyst under etherification conditions. U. S. Patent 4,978,807 to Smith describes an etherification catalyst reaction zone contained within a distillation tower. Typical hydrocarbon feedstock materials for etherification reactions include olefinic streams, such as cracking process light gas containing butene isomers in mixture with stantial amounts of paraffins including n-butane and isobutane. The C 4 components usually contain a major amount of unsaturated compounds, such as 10-40% isobutylene, 20-55% linear butenes, and small amounts of butadiene. Also, C 4 + heavier olefinic hydrocarbon streams may be used, particularly mixtures of isobutylene and isoamylene. C 5 + olefinic hydrocarbon streams containing isoamylene comprising fluid catalytic cracking (FCC) gasoline are an especially important feedstock. Improvements in the etherification of isoamylenes to TAME in C 5 + FCC gasoline are very desirable to meet the amended requirements of the Clean Air Act with respect to gasoline oxygen content while avoiding C 4 - hydrocarbon evaporative emissions. These amendments specify that gasoline sold in CO nonattainment areas during winter months will have 2.7 wt% oxygen by 1992 while in ozone nonattainment areas 2.0 wt% year round must be achieved by 1995. As noted above, the use of zeolite catalyst for the etherification reaction of lower alkanol with isoolefins to produce MTBE and/or or TAME is well known in the art. Among the advantages in employing acidic zeolite for the catalysis of etherification is the fact that it is much more readily regenerable than acidic resin catalyst. While sulfonated resin catalyst such as Amberlyst-15 are highly effective as etherification catalysts the fact that they are organic resins limits the temperatures to which they can be exposed without degradation. Zeolites, on the other hand, are stable at high temperatures which allows the repeated regeneration of deactivated catalyst. High temperature catalyst regeneration is by far the preferred route for regeneration to remove carbonaceous deposits, particularly produced by diene contaminates in the etherification hydrocarbon feedstream. DE-OS-3813689.9 discloses a process for the reaction of branched olefins with alkanols in the presence of hydrogen and catalytically active layered clays at elevated temperatures and pressure offers particular advantages, especially from an economic point of view, if, instead of the hitherto customary catalyst-charged ion-exchanger resins, layered clays that have been charged with catalytically active amounts of hydrogenating metals are used. EP-A-55045 discloses synthesis of tertiary alkyl ethers using as catalyst zeolite beta which may be associated with a noble metal. The present invention relates to a process for the production of alkyl tertiary alkyl ethers in C 4 -C 5 + hydrocarbon streams rich in isoolefins, typically containing catalyst deactivating amounts of dienes. The process is especially advantageous in extending the cycle length for the zeolite catalyzed etherification of isoolefins in C 5 + FCC gasoline by reducing catalyst aging. It has been found that if hydrogen is co-fed with the alkanol and C 4 -C 5 + isoolefin rich feedstreams to an etherification reaction catalyzed by acidic zeolite wherein the zeolite has been impregnated with a noble metal the rate of catalyst aging or deactivation is stantially lowered. The process is especially effective, i.e., catalyst aging is particularly reduced, when hydrogen is co-fed to an etherification reaction using acidic zeolite Beta catalyst containing palladium. It has also been found that the present process results in an advantageous isomerization of monoolefins, especially alpha olefins, in the feedstream to produce more useful internal olefins. The invention provides a process for the production of high octane value alkyl tertiary alkyl ether with a reduced rate of catalyst deactivation which process comprises contacting a feedstream comprising C 4 + hydrocarbons rich in isoolefins and containing dienes, an alkanol feedstream and a co-fed hydrogen feedstream with regenerable, acidic metallosilicate catalyst particles, the catalyst containing Group VIIIA metal of the Periodic Table of the Elements, in an etherification zone under etherification conditions whereby an effluent stream is produced containing the alkyl tertiary alkyl ether, unconverted isoolefin and unconverted alkanol characterised in that the metallosilicate catalyst comprises zeolite beta. In a preferred embodiment the feedstream comprises C 5 + hydrocarbons rich in isoolefins and containing dienes and/or alpha olefins, an alkanol feedstream and a co-fed hydrogen feedstream, whereby gasoline rich in TAME is produced. Figure 1 is a graphical representation of the catalyst aging rate in etherification for a preferred catalyst of the invention with and without hydrogen co-feed. Figure 2 is graphical representation comparing the etherification process of the invention under different process conditions. Figure 3 is a schematic drawing of a two stage process of the invention employing zeolite and acidic resin catalyst. Isoolefins or isoalkenes in this invention are those having the formula R 2 C=CH 2 or R 2 C=CHR, particularly C 4 -C 7 isoolefins. Alkanols which may be used in the present invention include methanol, ethanol, 1-propanol, isopropanol, 1-butanol and 2-butanol. Anhydrous methanol is a preferred alkanol. The term lower alkyl refers to C 1 -C 4 alkyl including methyl, ethyl, n-propyl and isopropyl. In the etherification process it is known that alkanol and isoolefins may be reacted in equimolar quantities or either reactant may be in molar excess to influence the complete conversion of the other reactant. Because etherification is an incomplete reaction the etherification effluent comprises unreacted alkanol and unreacted hydrocarbons. On a stoichiometric equivalency basis, equimolar quantities of methanol and isoolefins are advantageous but an excess between 2 and 200% of either component can be passed to the etherification reaction unit. In the present invention, the molar ratio of alkanol to isoolefin, such as methanol to iso-butylene, can be between 0.7 and 2, but preferably the molar ratio is 1 for methanol to isobutylene in liquid phase etherification. Advantageously, the excess methanol may be 40% or more when the hydrocarbon feedstream comprises significant quantity of isoamylenes, but equimolar quantities are preferred when the hydrocarbon feedstream iso-olefin component consists essentially of C 4 hydrocarbons. FCC gasoline is a preferred hydrocarbon feedstock for the process of this invention, although isoolefin rich C 4 or C 4 + hydrocarbon streams can be used. Typically, FCC gasoline comprises predominantly C 5 -C 7 hydrocarbons containing 25% etherifiable isoolefins, particularly isoamylenes. Dienes in small quantities (<1%) are also present in FCC gasoline and are known to contribute to catalyst deactivation in prior art etherification processes. Small quantities of alpha olefins may also be present. The feedstream also contains traces of heteroatoms such as nitrogen and sulfur in quantities sufficient to influence the rate of catalyst aging. Zeolite Beta is described in U.S. Reissue Patent No. 28,341 (of original U.S. Patent No. 3,308,069), to which reference is made for details of this catalyst. The original cations associated with zeolites utilized herein can be replaced by a wide variety of other cations according to techniques well known in the art, e.g., by ion-exchange. Typical replacing cations include hydrogen, ammonium, alkyl ammonium and metal cations, and their mixtures. Metal cations can also be introduced into the zeolite. In the case of metal cations, particular preference is given to metals of Groups IB to VIIIA of the Periodic Table, including, by way of example, iron, nickel, cobalt, copper, zinc, palladium, calcium, chromium, tungsten, molybdenum, rare earth metals. These metals can also be present in the form of their oxides. A typical ion-exchange technique involves contacting the particular zeolite with a salt of the desired replacing cation. Although a wide variety of salts can be employed, particular preference is given to chlorides, nitrates and sulfates. Representative ion-exchange techniques are disclosed in a number of patents including U.S. Patent Nos. 3,140,249; 3,140,251; and 3,140,253. Following contact with a solution of the desired replacing cation, the zeolite is then preferably washed with water and dried at a temperature ranging from 82° to 316° C (150° to about 600°F) and thereafter calcined in air or other inert gas at temperatures ranging from 260° to 816°C (500° to 1500°F) for periods of time ranging from 1 to 48 hours or more. For incorporation into zeolite useful in the instant invention metals of Group VIIIA of the Periodic Table of the Elements are preferred. Zirconia-bound zeolite Beta containing palladium (0.3%) and having a high alpha value (400) is a particularly preferred catalyst for the novel etherification process of the instant invention. Alpha value, or alpha number, is a measure of zeolite acidic functionality and is more fully described together with details of its measurement in U.S. Patent No. 4,016,218, J. Catalysis, 6, pp. 278-287 (1966) and J. Catalysis, 61, pp. 390-396 (1980). It has been discovered that the method of incorporating palladium in zeolite catalyst of the invention can result in a catalysts having widely varying etherification activity. To illustrate this discovery, two methods are described, Examples A and B, to introduce palladium into commercially available zeolite Beta catalyst. According to the etherification activity test described in Example C, catalyst A has an etherification activity of 0.021 grams converted per gram of catalyst per hour while the activity of catalyst B is 0.004 grams converted per gram of catalyst per hour. The method for impregnating zeolite Beta catalyst with palladium described in Example A is preferred for the process of this invention. Example A A physical mixture of 70 parts zeolite Beta, 24 parts zirconium oxide and 30 parts AZC (Magnesium Elektron Ammonium Zirconium Carbonate 20 % ZrO2) was mulled to form a uniform mixture. Sufficient amount of deionized water (DI) was added to form an extrudable paste. The mixture was auger extruded to 1.59 mm (1/16") quadrulobe shaped extrudates and dried in an oven at 120°C. The extrudates were then nitrogen calcined at 480°C for 3 hours followed by a 6 hour calcination at 538°C. The extrudates were first humidified and then immersed in a circulating aqueous solution (4ml/g catalyst). Then 0.069 M palladium tetraamine (II) chloride solution (1ml/g catalyst) was added to the aqueous solution over a four hour period. After eight hours of circulation the extrudates were dried in a 120°C oven. The dried extrudates were air calcined for 2 hours at 288°C and four 3 hours at 350°C. The finished palladium/Beta/ZrO2 catalyst had 0.30 wt % Pd. Example B A physical mixture of 70 parts zeolite Beta, 24 parts zirconium oxide and 30 parts AZC (Magnesium Elektron Ammonium Zirconium Carbonate 20% ZrO2) was mulled to form a uniform mixture. Sufficient amount of deionized water (DI) was added to form an extrudable paste. The mixture was auger extruded to 1.59 mm (1/16") extrudates and dried in an oven at 120°C. The extrudates were then nitrogen calcined at 480°C for 3 hours followed by a 6 hour air calcination at 538°C. The extrudates were first humidified and than immersed in a circulating 0.2 M NH4NO3 solution (3.5 ml/g catalyst). Then 0.62 M palladium tetraamine (II) chloride solution was added to the NH4NO3 solution over a four hour period. After eight hours of circulation, the extrudates were washed with de-ionized water and dried in an oven at 120°C. The extrudates were air calcined for three hours at 288°C. The finished palladium/Beta/ZrO2 catalyst had 0.32 wt% Pd. Example C Etherification Activity Test Procedure Etherification activity is conducted in a 150 ml autoclave using 6.54 grams of crushed Pd/zeolite Beta/ ZrO2 and a solution consisting of 2-methyl-1-pentene (25.2 grams), n-hexane (25.8 grams), and absolute methanol (14.4 grams). The reactor is heated to 82°C (180°F) at 448 kPa (50 psig) over 15 minutes. Two hours after the initiation of heating, samples of the liquid within the reactor are obtained and analyzed by gas chromatography. The etherification activity of the catalyst is taken to be grams of converted (to C 6 methyl ether) per gram of catalyst per hour. The process of the invention which involves co-feeding hydrogen to an isoolefin etherification reaction, especially one using FCC feedstream and employing regenerable acidic zeolite catalyst particles containing palladium or other Group VIII metal, provides the following advantages: a reduced rate of catalyst deactivation or aging; isomerization of alpha olefins to more useful internal olefins; reduction in diene content; and an enhancement in the ease of catalyst reactivation with hydrogen. The effectiveness of hydrogen co-feeding for decreasing the aging rate of acidic zeolite Beta catalyst containing palladium and enhancing desirable side reactions was demonstrated in an accelerated aging test. As shown in Figure 1, the catalyst activity, measured as the amount of oxygen incorporated into FCC gasoline through the formation of methyl ethers, remained higher when hydrogen was added to the feedstream. Also, the use of hydrogen co-feed promoted desirable side reactions such as olefin isomerization as shown in Table 1. These desirable side reactions increase the extent of oxygen incorporation into the gasoline. Diene removal also reduces gum formation in the hydrocarbon product. The following Examples illustrate the process of the present invention and the effect of that process on catalyst aging. Example 1 The etherification of FCC gasoline is performed in a fixed bed reactor. In a 6.35mm (1/4") I.D. stainless steel reactor, four grams of palladium impregnated acidic zeolite Beta catalyst particles (20/60 mesh) is mixed with 30 vol% sand (60/120 mesh), and heated to 205°C (400°F) at 2800kPa (400 psig) under hydrogen and maintained under these conditions for 16 hours, after which time the temperature is decreased to 93°C (200°F). The hydrocarbon feed comprising C 5 -215°F (102°C) FCC gasoline is blended with absolute methanol in a three to one ratio by weight (75 wt% hydrocarbon/25 wt% alcohol). For the case of only liquid feed, which is performed in a downflow reactor, the reaction is commenced by stopping the hydrogen flow and feeding the FCC gasoline/-methanol blend to the reactor at a rate of 10.0 grams of liquid per gram of catalyst per hour. The mixed liquid/hydrogen experiment is run upflow, using the same liquid feed rate and 5 ml/minute gas feed rate. In both cases, the total liquid product is contacted with water to remove unreacted methanol from the hydrocarbons. Example 2 As shown by the results of this Example, the rate of hydrogen addition during etherification can affect the catalytic performance of the Pd/zeolite Beta catalyst. Following in-situ treatment of the catalyst with hydrogen at 399°C (750°F) for 72 hours to restore its initial activity after etherification according to the process of Example 1, the catalyst is evaluated under etherification conditions at higher overall pressure and higher hydrogen flow rate. As shown in Figure 2 the rate of deactivation decreased at these new conditions. Example 2A etherification conditions were 104°C (220°F) reaction temperature at 2860 kPa (400 psig) and 5ml/minute hydrogen feed; Example 2B etherification conditions were 104°C (220°F) reaction temperature at 4240 kPa (600 psig) and 10ml/minute hydrogen feed. The extent of pentene isomerization is also increased by the different operating parameters as shown in Table 1. Effect of Hydrogen Co-feed on Hydroisomerization of Linear Pentenes During FCC Etherification Over Pd/Zeolite Beta Example Feed 2C 2D 2E Reaction Conditions Temperature °C (°F) - 97 (206) 107 (224) 112 (233) Pressure kPa (psig) - 2861 (400) 2861 (400) 4240 (600) Liquid Feed (WHSV) - 10.0 10.0 10.0 Hydrogen Feed (SCCM) - 0 5.0 10.0 Linear Pentene Dist. 1-Pentene 17.5 17.4 5.0 4.7 cis-2-pentene 29.9 29.8 70.0 71.5 trans-2-pentene 52.5 52.7 25.0 23.8 Since terminal linear olefins tend to have lower octane values than internal olefins, reducing the concentration of terminal olefins as depicted in Table 1 for the invention is highly advantageous and, along with the elimination of dienes, provides benefits in downstream alkylation. As shown in Table 2, methanol etherification of branched olefins in light FCC gasoline carried out by the process of the invention increases the oxygen content of the hydrocarbon to 1.9 wt%, significantly lowers the bromine and reduces Reid vapor pressure (RVP) with no change in (R+M)/2 octane value. Effect of Etherification on FCC Gasoline Properties Treated Feedstock Etherified Product Oxygen Content, Wt% 0.0 1.9 TAME, wt% 0.0 5.8 C 6 Methyl Ethers, Wt% 0.0 5.4 C 7 Methyl Ethers, Wt% 0.0 2.0 Bromine Number 85 69 RVP kPa (psi) 152 (7.0) 147 (6.3) (R+M)/2 86.2 86.1 Although single-pass fixed bed reactors are used in the foregoing Examples of the process of the invention, other configurations (e.g. fixed bed reactors with recycle, etc.) may be used for the process. Referring to Figure 3, a multi-stage process is illustrated for implementation of the process of this invention in conjunction with acid resin catalyzed etherification. Since palladium-impregnated zeolite Beta performs a number of important catalytic functions such as olefin isomerization and diene removal, multi-stage processes using this catalyst and synthetic resins provides a unique and efficacious route for upgrading olefinic feedstreams for "clean fuels" applications. In Figure 3, a feedstream 301 comprising FCC gasoline, methanol and hydrogen is passed to etherification reactor 302 containing zeolite Beta catalyst impregnated with palladium. Etherification is carried out in this stage at moderate to high temperatures to complete the etherification reaction as well as olefin isomerization and diene saturation. The effluent from the etherification zone containing unconverted alkanol and isoolefins is passed 303 to a second etherification zone 304 containing Amberlyst-15 etherification catalyst. Etherification in this second stage reactor is carried out at a lower temperature. The effluent 305 from the reactor is passed to a debutanizer or depentanizer for product recovery of C 5 + gasoline rich in alkyl tertiary alkyl ethers. The etherification conditions useful in the present invention for either C 4 + or C 5 + feedstreams containing isoalkenes, or isoolefins, include temperature between 38 and 260°C (about 100 and 500°F), pressure between 793 and 7000kPa (about 100 and 1000 psig), hydrocarbon feed rate between 0.1 and 10 weight hourly space velocity (WHSV) based on catalyst, and hydrogen flow rate between 1.8 and 360 v/v (10 and 2000 SCF/Bbl). Preferably, the etherification conditions comprise temperature of 66-204°C (about 150-400°F), pressure of 1482-4240 kPa (about 200-600 psig), hydrocarbon feed rate of about 0.5-5 weight hourly space velocity (WHSV) based on catalyst, and hydrogen flow rate of 9-180 v/v (about 50-1000 SCF/Bbl). Modifications, such as the use of other metals (platinum, etc.) and other catalyst binders (silica, silica-alumina, etc.) are considered within the scope of the invention to enhance the properties of the zeolite-based catalyst. The zeolite catalyst is converted to the hydrogen form prior to use in the process of this invention. Although the process of this invention extends the active useful life of the catalyst, eventually after extended use in the process of this invention the catalyst will require regeneration to restore activity. This may be effected with hydrogen gas at elevated temperature. Optionally, the catalyst can be regenerated or reactivated oxidatively by treatment with air or oxygen to combust and remove carbonaceous deposits. Also, combinations of oxidative regeneration and regeneration by hydrogen gas can be used.

Claims: A process for the production of high octane value alkyl tertiary alkyl ether with a reduced rate of catalyst deactivation, which process comprises: contacting a feedstream comprising C 4 + hydrocarbons rich in isoolefins and containing dienes, an alkanol feedstream and a co-fed hydrogen feedstream with regenerable, acidic metallosilicate catalyst particles, the catalyst containing Group VIIIA metal of the Periodic Table of the Elements, in an etherifcation zone under etherifcation conditions whereby an effluent stream is produced containing an alkyl tertiary alkyl ether, unconverted isoolefin and unconverted alkanol characterised in that the metallosilicate catalyst comprises zeolite beta. The process of claim 1 including the further step of separating the effluent stream in a debutanizer and recovering a debutanizer bottom stream comprising C 5 + hydrocarbons containing alkyl tertiary alkyl ether. The process of claim 1 including the further step of passing the effluent stream to a second etherification zone containing acidic resin catalyst particles under etherification conditions whereby a portion of the unconverted isoolefin and unconverted alkanol is converted to alkyl tertiary alkyl ether; and separating the product stream from the second etherifcation zone in a debutanizer to recover a debutanizer bottom stream comprising C 5 + hydrocarbons rich in alkyl tertiary alkyl ether. The process of any preceding claim including the further step of reactivating spent metallosilicate catalyst particles at temperature between 121°C and 538°C in a stream of hydrogen gas or by oxidative regeneration. The process of any preceding claim wherein the alkanol includes methanol, ethanol, isopropanol and 2-butanol. The process of any preceding claim wherein the isoolefin includes isobutene and isoamylene and the alkanol comprises methanol whereby methyl tertiary butyl ether and methyl tertiary amyl ether are produced. The process of any preceding claim wherein the Group VIIIA metal comprises palladium and/or platinum. The process of claim 7 wherein the catalyst comprises zeolite Beta containing palladium. The process of claim 8 wherein the catalyst comprises zirconia-bound palladium zeolite beta containing about 0.3 weight percent palladium having an alpha value of 400. The process of any preceding claim wherein the feedstream comprises C 5 + hydrocarbons rich in isoolefins and containing dienes and alpha olefins. The process of any preceding claim wherein the etherification conditions comprise temperature between about 38° and 260°C, pressure between about 793 and 7000 kPa, weight hourly space velocity (WHSV) based on catalyst 0.1 to 10, and hydrogen flow rate between 1.8 and 360 v/v. The process of claim 12 wherein temperature, pressure, weight hourly space velocity and hydrogen flow rate are respectively 66-204°C, 1482-4240 kPa, 0.5 to 5, and 9-180 v/v.

Inventors: LE QUANG N, THOMSON ROBERT T, LE, QUANG N., THOMSON, ROBERT T.
Applicants: MOBIL OIL CORP, MOBIL OIL CORPORATION
Application year: 1993
Main classification: C07C41
IPC codes: C10L1, C07C43, C10L10, C07C41