CA2152073A1 - Production of olefins - Google Patents

Abstract

A process for cracking or decomposing a feedstream containing a major proportion of at least dialkyl ether to produce the corresponding olefins comprises contacting the feedstream with a faujasite aluminosilicate catalyst, wherein at least about 50 wt.% of the alkali metal content orginally present in said faujasite has been exchanged by at least one alkaline earth metal, characterized in that prior to the contacting the catalyst is pretreated with steam which enhances its selectivity and reduces the undesirable by-product yields.

Description

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PRODUCTION OF OLEFINS

The present invention relates to a process for the production of olefins. More particularly, but not exclusively, it relates to a method for the production of pure tertiary olefins by the decomposition of alkyl tert-alkyl ethers in the presence of a new and inl~roved catalyst based on an ~lk~line earth exch~nged faujasite. ~(l-lition~lly, the selectivity of the new and oved catalyst is enh~nced and the llndesirable by-product yields are reduced, by pretreating the catalyst with steam.
0 Olefins, particularly tertiary olefins, may be commercially produced by the sulfuric acid extraction of such oleffns from ~ res cont~ining them obtained e.g., by steam cracking of petroleum feeds. Since this method uses sulfuric acid of high concentration, the use of expensive materials in the fabrication of the extraction apparatus is es~enti~l Also, dilution of the acid to promote olefin recovery and reconcentrating the acid prior to recycling are required and are expensive. In ~d~lition, this method is not always advantageous industrially because tertiary olefins are subject to side reactionssuch as polymerization, hydration and the like during extraction with concentrated sulfuric acid.
It is also known that tertiary olefins may be prepared by re~cting them selectively from such feeds with a primary alcohol in the presence of an acid catalyst to produce the corresponding alkyl tert-alkyl ethers. The tert-alkyl ethers are primarily formed, since the secondary olefins react very slowly and the primary olefins are completely inert. Such alkyl tert-alkyl ethers may then be easily separated and subsequently decomposed back to the tertiary olefins and the primary alcohol.
For producing tertiary olefins from allyl tert-alkyl ethers, there have been proposed methods using various catalysts: For example ~h~",i"ll", compounds supported on silica or other carriers (US-A-4,398,051);
phosphoric acid on various supports (US-A-4,320,232); and metal cont~ining weakly acidic components on a carrier of >20 m2/gm surface area (GB-A-1,173,128). In addition, inferior results are disclosed as being obtained utilizing carriers alone in the decomposition of methyl tertiary butyl ether WO 9S/11209 PCT/US9~/1193~

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(US-A-4,398,051) and lltili~in~ H2S04 treated clay in the decomposition of t-alkyl ether ~lk~nol~ (US-A~,254,290).
One of the main disadvantages of such processes is that the disclosed catalysts do not have good catalyst life in that higher and higher 5 temperatures, which eventually become limiting, are required to m~int~in high conversion of the alkyl tert-alkyl ethers. Additionally, larger amounts of the dialkyl ether by-product are produced as the catalyst ages with the disadvantage of a reduction in yield of the desired tertiary olefin. This lack of good catalyst life may be due to the instability of the catalyst, to high 10 temperatures being required for good conversion thus promoting fouling, to the catalyst itself promoting fouling or to any or all of these. Also, a number of the catalysts such as ion ~Ych~nge resins cannot be regenerated after use.
More recently, processes have been discovered which provide improved yields of tertiary olefin product. For example, US-A~,691,073 5 discloses a process for preparing tertiary olefins from alkyl tertiary alkyl ethers comprising contacting the ether with a catalyst which has been epared by reacting a clay with HF and/or HCl and calcining the resultant clay product. Although the process produces very high yields and selectivity towards the production of tertiary olefin products, these catalysts often tend 20 to become deactivated as a consequence of coke and/or polymeric build up in a relatively short on-stream time. Also, the ~ mino~ilic~te structure of many clays is not sufficiently stable to wiLh.cl~nd repeated high temperature regenerations required to remove catalyst deposits.
Natural and synthetic f~nj~cite catalysts are known for use in the 2s conversion or pyrolysis of ethers and alcohols into olefins or distillate range hydrocarbons. For example, US-A-4,467,133 (Chang) discloses the collvel~ion of methanol into a distillate range hydrocarbon ll~Ll~L~lre by passing methanol over a rare earth ç~rch~nged faujasite (such as zeolite X or Y) at a temperature below 315C (600F). More particularly, Chang discloses a 30 process for converting lower alcohols (Cl to C4 alcohols) or lower dialkyl ethers to distillate range (C1o+ ) hydrocarbons suitable for use as diesel fuels.
The process involves passing the feedstream over one of numerous disclosed ~ mino~ cate catalysts the alkali metal content of which has been e~ch~nged with hydrogen or a Group IB - VIII metal. The focus of the ~ ~i52073 t1i~rlos--re is on zeolite Y as the preferred zeolite and a rare earth metal as the preferred eYçh~3nge action. Example 1 on colnmn 6 shows the conversion of methanol to C1o-C2g+ olefins using REY zeolite. Quite clearly the process leads to dehydrogenation and oligomerization of the components in s the feed streams, as evidenced by conversion of methanol to olefins having a minimllm of 10 carbon atoms. This is quite distinct from the process of the present invention, where an ~lkzlline earth metal eYsh~nged faujasite, e.g., zeolite Y, is used as the catalyst which does not produce significant amounts of ~ till~te range hydrocarbons (as does Chang) but rather selectively COllv~ alkyl ethers to their corresponding olefins.
US-A-5,254,785, equivalent to co-pending PCI appliciqtion~
PCr/US93/05399, te~the~ selectivity illlprovelllent in the process of converting an alkyl ether to its colles~onding olefin using a catalyst based on an ~lk~line earth eYch~nged faujasite with absolutely no mention of steam.
It would be desirable to be able to further i~ rove the selectivity of this catalyst, while at the same time reduce the llndecirable by-product make, when using this catalyst to make olefins. By adding steam to pretreat the catalyst, additional selectivity illlplovelllents are achieved with reduced by-product make.

SUMM~RY 0~ I'HE INVENTION
According to the present invention, there is provided a process for selectively converting an alkyl ether to its corresponding olefin which comprises cont~cting the ether with a faujasite catalyst wherein at least 50~o 2~ by weight of the original alkali metal content has been eY~h~nged with at least one ~lk~line earth metal, characterized in that prior to the cont~cting~
the catalyst is pretreated with steam to enh~nce catalyst selectivity and to reduce the Im~lesirable by-product yields.
The alkyl ether may be one having for example at least 5 carbon atoms, preferably from 5 to 12 carbon atoms and more preferably from 5 to 8 carbon atoms. The process is particularly but not exclusively suited to conversion of tertiary alkyl ethers, such as tertiary butyl methyl ether or tertiary amyl methyl ether, to their corresponding olefins.

WO 95/11209 . ~ PCT/US9~111934 In accordance with the invention, one or a rnixture of ethers may be converted to their corresponding olefins. The ether(s) may comprise a component of a feed which is contacted with the specified catalyst. Thus, the invention also provides a process for cracking or decomposing a feedstream cont~ining a major proportion of at least one dialkyl ether having at least about 5 carbon atoms to produce the collesponding olefirls.
In accordance with the invention, the catalyst is pretreated with stearn until the catalyst is saturated. Typically the ste~ming is cont1llçted at a temperature from 100 to 400C (212 to 752F), ~refelably from 100 to 23ZC
0 (212 to 450 F), and most preferably to 121 to 177~C (250 to 350F), at a space velocity of from 0.1 to 15 hr~l WHSV, preferably from 0.3 to 5 hr~l WSHV, and most preferably from 0.5 to 3 hr~l WHSV, and at a ~les~ule of from atmospheric pres~ule to 4140 kPag (600 psig), preferably from atmospheric ~res~uie to 1720 kPag (250 psig), and most preferably 103 to 1030 kPag (15 to 150 psig).
The conversion, cracking or decomposition is preferably condllcted in the vapor phase. The preferred temperatures for the contact between ether and catalyst are in the range of from 51 to 315C (12S~ to 600F).
The process generally offers the advantages of longer catalyst life coupled with high yield and selectivity rates towards production of the olefin which corresponds to the starting ether. Additionally, the production of undesirable by-products, such as methyl secondary butyl ethers, dimethyl ether, and isobutane, is reduced, by at least 20%, preferably 30%, and most preferably 40%. The contact between the ether and the catalyst is generally 2s performed under conditions of temperature and pres~ure sufficient to convert a substantial proportion of the ether to its corresponding olefin. Rates of conversion of ether to olefin in excess of 60% by weight may be obtained, preferably in excess of 90% by weight.

BRIEF DESCRIPrION OF DR~VVINGS
Figure 1 is a plot of the effective CaY catalyst activity with (the invention) and without (comparative) pretreatment with steam.

WO 95111209 PCT/US9~/11934 . ' . .
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Figure 2 is a plot of the corresponding methyl secondary butyl ethers (%) col,vel~ion with and without pretreatment with steam using CaY catalyst.
Figure 3 is a plot of the col~es~onding dimethyl ether (mol %) with and without pretreatment with steam using CaY catalyst.
Figure 4 is a plot of the corresponding isobutane levels (wt. ppm) with and without pretreatment with steam using CaY catalyst.
Figure S is a plot of the effective comparative ZSM5 catalyst activity with and without pretreatment with steam.
0 Figure 6 is a plot of the methyl secondary butyl ethers (%) collv~l~ion with and without pretre~tme~t with steam using comparative ZSM5 catalyst.
Figure 7 is a plot of the dimethyl ether (mol %) with and without pretre~tment with steam using comparative ZSM5 catalyst.
5 Figure 8 is a plot of the isobutane levels (wt. ppm) with and without pretreatment with steam using comparative ZSM5 catalyst.

DETAILED DESCRIPTION OF THE INVENTION
The catalyst which may be used in the process of this invention is based on a natural or synthetic f~ ite, for example zeolite Y. This is an alkali metal col-t~ g crystalline ~lllmino~ilicate well known in the art and is described in US-A-3,130,007. The preferred f~ cite has a silica to ~lllmin~
molar ratio in the range of from 3 to 1 to 6 to 1 and/or pore dimensions greater than about 6 Angstroms.
This zeolite material may be activated for the ether decomposition (conversion) reaction by base eYch~nging the alkali metal originally present in the zeolite, e.g. sodium, with one or a mixture of alkaline earth metals suchthat at least 50% by weight of the alkali metal is replaced by the ~lk~line earth metal. It is prefel,ed to conduct the eYch~nge such that as many as possible of the original alkali metal ions are so eYçh~nged, e.g., at least 75%
by weight and more preferably at least 85% by weight. Most preferably the eYçh~nge is such that the original alkali metal content of the zeolite is reduced to a level below about 1% by weight and the degree of eYrh~nge is 5?.~3 about 90% or above. Suitable ~lk~line earth eYrh~nge metals are calcium, barium and strontium, with calcium being most preferred.
Base çYch~nge may be condllcted for example by cont~ctinf~ the zeolite (which has been preferably previously calcined) one or more times 5 with an aqueous solution cont~inin~ an ~lk~line earth metal salt dissolved therein, preferably at a temperature ranging from ambient up to about 85C
(185F). A wide variety of salts may be employed such as the chlorides, bromides, carbonates, snlf~tes or nitrates, so long as such salts are soluble inwater such that ion transfer can take place. Calcium chloride is the preferred 0 salt. The concentration of the salt in solution may for example range from about 0.1 to about 25~o by weight. Preferably the concentration is sufficient to provide a slight excess of the stoichiometric amount of eYch~n~e cation.
After an eY(~h~n~e cont~ct period which may range for example from about 60 minl~tes to about 24 hours, the eYch~nged zeolite is separated from 15 the exchange solution, washed and dried. The eYçh~n~e can be repeated one or more times if necessary in order to replace the m~xi".ll~l number of alkali metal ions with ~lk~line earth metal ions.
Other e~rch~n~e processes may also be employed, such as the so called incipient wetness method, wherein the zeolite is infused with exchange 20 solution to form a paste which is then dried.
The catalyst may be used in the process without additional binder or it may be formulated with a binder or carrier material such as ~lllmin~, silica, clay or an ~ min~/silica ~ ure. Bound catalyst may be prepared by mixing the powdered catalyst with water and preferably from 5 to 40% by weight 25 binder to form a paste, and extruding and drying the paste to form small pellets. The bound catalyst is then preferably further activated by calcination, 343-593C (6S0~-1100F) and preferably for a period of about 10 minllteS up to a period of hours, e.g., 24 hours. The ion toYch~nge process may be condllçte-l prior to or subsequent to the formulation of such bound 30 zeolites, preferably subsequent to such formnl~tion Ethers which may be cracked (converted) using the specified catalyst in the process of this invention preferably contain from 5 to 12 carbon atoms, more preferably from 5 to g carbon atoms and most preferably from 5 to 8 carbon atoms. Preferred ethers include tertiary allyl ethers such as tertiary 2ls~ ~3 - butyl methyl ether and tertiary butyl ethyl ether, and ~ertiary amyl counterparts inçllltling the methyl and ethyl ethers. Feedstreams which may typically be employed in commercial applications of the process preferably contain at least 70% up to 1005~o by weight of the ether, for example tertiary alkyl ether. The balance (if any) of the feedstream may co~ -ise for example, primarily a -lL~ure of saturated and Im~t~lrated hydrocarbons and alcohols such as methanol or tertiary alkylalcohols.
The decomposition (conversion) reaction may be conducted in any suitable reactor which is packed with one or more beds of the ~lk~line earth 0 exchanged catalyst.
Prior to the introduction of feed, steam is fed to the reactor at conducted at a temperature from 100 to 400 C (212 to 752F), preferably from 100 to 23ZC (212 to 450F), and most preferably to 121 to 177DC (250 to 350F), at a space velocity of from 0.1 to 15 hr~l WHSV, preferably from 0.3 to 5 hr~l WSHV, and most preferably from 0.5 to 3 hr~l WHSV, and at a pres~ure of from ~tmospheric pressùle to 4140 kPag (600 psig), preferably from ~tmospheric pressure to 1720 kPag (250 psig), and most preferably 103 to 1030 kPag (15 to 150 psig). The flow is m~int~ined until conclen~te is seen in the outlet of the reactor. At this point, the catalyst is saturated and ste~ming is disco~ led. This same ste~ming method can be applied to other zeolite catalyst systems which have very good catalyst activity, but poor selectivity.
Ether is then fed to the reactor at normal operating cQ~llition~
Reactor operating temperatures for this process are generally relatively low, 2s preferably ranging from 51 to 315C (125 to 600F), more preferably from 115 to 260C (240 to 500F) and most preferably from 137~ to 193C (280 to 380F). Operating pressure may range for example from atmospheric to about 1.72 MPag (250 psig), with 344 to 862 kPag (50 to 125 psig) being preferred. Pressure is preferably such that the reaction occurs subst~nti~lly in the vapor phase. The reactor may be equipped with a suitable temperature controlling means such that the desired operating temperatures can be m~int~ined or adjusted in the reactor.
In a collLilluous process the reaction is preferably carried out at a spatial velocity expressed in terms of weight of organic feed per unit weight of WO 95/11209 PCT/US9~111934 catalyst per hour in the range of from 0.5 to 100 WHSV, more preferably from 1 to 20 WHSV.
The process is especially suited for the conversion of fractions cont~ining tertiary amylmethyl ether into corresponding isopentene olefins such as 2-methyl-2-butene or 2-methyl-1-butene; and for conversion of fractions cont~ining methyl tertiary butyl ether into isobutylene. A particular advantage of the process is that the decomposition (conversion) product generally contains only a very low content of the corresponding by-product ~lk~nes, such as isobutane or isopentane, which are very difficult to separate o from their olefin counterparts.
Even though the catalyst could be used without the steam pretreatment, the selectivity is enhanced and the by-product yields are reduced with this pretreatment step. By using the steam pretreatment, the by-product yields are reduced by at least 20%, preferably at least 30%, and 5 most preferably at least 40%. By-products, include, for example, in the conversion reaction of tertiary butyl methyl ether to isobutylene, methyl secondary butyl ethers, dimethyl ether, and isobutane.

The following examples illustrate the invention.
Example 1 - CaY Catalyst Preparation Prior to Steam Pretreatment A calcium exçh~nged zeolite Y catalyst was prepared as follows: 108.3 grams of pellets of zeolite Y (LZY-52, available from UOP) which cont~ined 20% by weight of ~ min~ as a binder were packed into a 45.72 cm (18 inch) 2s glass column. The colllmn was then flushed with 100 ml. of ultra high purity water (pH-6.7) at a temperature of 65.5C (150F).
A solution of 217 g of calcium chloride in 3500 ml. of ultra high purity water was formed and this solution was then passed through the packed zeolite bed at a rate of 2 ml. per minute at 65.5C (150F). The packed 30 zeolite was then washed with additional pure water until the effluent was essentially free of chloride ions as indicated by a negative silver nitrate test.
The exch~nged zeolite was then dried overnight under a vacuum at ambient temperatures and then dried at 100C (212~F) for 8 hours under v~cllllm-WO 95/11209 PCT/US94tll93~
~l~o~3 Analysis showed that about 90% by weight of the original sodium ions present in the zeolite had been exchanged by calcium ions.

Example 2 - ~omparative CaY Catalyst Performance Without Ste~min~
s The exchanged zeolite of Example 1 was crushed and sieved to 20-40 mesh and packed into a 30.48 cm by 0.635 cm (12 inch by 0.25 inch) stainless steel reactor column which was then connected to a feed line. The reactor was placed in a circ~ ting hot air oven and also connected to an effl~le~t collector line.
0 A feed stream cont~ining 95+% of tertiary butyl methyl ether was preheated and passed into the inlet of the reactor at a constant temperature m~int~ined at about 174.9~C (34S~F), at a ~res~ure of 620.5 kPag (90 psig) and at a WHSV in the range of from about 3 to 5. Reaction product removed from the discharge of the reactor showed an initial co~lve~ion rate of greater than 95% of tertiary butyl methyl ether to isobutylene. The process was continued under co"sLant conditions of pressure and temperature until the %
conversion to isobutylene dropped below 90%.

Example 3 - Invention CaY Catalyst Performance With Ste~min~
Example 2 was repeated under the conditions set forth therein except the catalyst was steamed prior to the introduction of the feed steam. The catalyst was steamed to saturation at WHSV=2, 174C (34S~F), and atmospheric pressure.

2~ Example 2 and 3 - Comparisons The results are illustrated in Figures 1-4 showing the effect of catalyst performance with and without ste~ming. For the purpose of comparison, a value was developed to chart the relative performance of each charge of catalyst under varying conditions. The value was normalized to the initial activity of fresh normal hydrogen floride (HF) treated attapulgite clay, as disclosed in US-A-4,691,073, at 360F with tertiary butyl methyl ether. This value called the effective activity was empirically determined to be:
A = C - ln(1 - C/100) - t(T) + n C = % conversion or MTBE

WO 95/11209 PCT/US9~/1193 ~

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T = temperature in F
t = temperature norIn~li7~tion factor (delta C/delta T) n = constant to normalize to 100%
The initial activity of the steam treated Ca-Y declined a~p-oxi~ tely 7 percent over an unsteamed catalyst, but the primary by-products of dimethyl ether (DME), butenes, and isobutenes were significantly reduced. DME
production was reduced by 75 percent, methyl secondary butyl ether (MSBE) conversion (used as a measure of butene formation) was reduced 42 percent, and isobutane was reduced 100 percent (was not detectable < 10 ppm). The 0 ether decomposition activity of the steam treated catalyst returned to the same level as the unsteamed catalyst after 75 hours of operation. From this point on, however, the same slope of deactivation was observed, but by-product formation was m~int~ined at the greatly reduced level.

Examples 4 and 5 - Comparative ZSM5 Catalyst Performance With and Without Steaming Examples 2 and 3 were repeated under the conditions set forth therein except the catalyst used was ZSM5. The catalyst was steamed to saturation at WHSV=2, 174C (345F), and atmospheric ~re~ ,ure.
Example 4 and 5 - Comparisons The results are illustrated in Figures 5-8 showing the effect of ZSM5 catalyst performance with and without ste~min~. For the purpose of comparison, the effective activity was determined as denoted above.
The initial activity of the steam treated ZSM5 declined a~rcx;.l.~tely 3 percent over an unsteamed catalyst, but the primary by-products of dimethyl ether (DME), butenes, and isobutenes were significantly reduced.
DME production was reduced by 59 percent, methyl secondary butyl ether (MSBE) conversion (used as a measure of butene formation) was reduced 19 percent, and isobutane was reduced 69 percent. The ether decomposition activity of the steam treated catalyst returned to the same level as the unsteamed catalyst after 7 hours of operation. From this point on, the same slope of deactivation was observed, but by-product formation was m~in~ined at the greatly reduced level.

Claims (12)

CLAIMS:

1. A process for selectively converting an alkyl ether to its corresponding olefin which comprises contacting the ether with a catalyst comprising a faujasite, wherein at least 50% by weight of the original alkali metal content of the catalyst has been exchanged with at least one alkaline earth metal comprising calcium, barium or strontium, characterized in that prior to the contacting the catalyst has been pretreated with steam.

2. The process of Claim 1 wherein the catalyst is zeolite Y.

3. The process of any preceding claim, wherein the catalyst has been pretreated with steam at a temperature in the range of from 100 to 400 °C (212 to 752 °F), and/or at pressure from atmospheric to 4140 kPag (600 psig), and/or at a space velocity of from 0.1 to 15 hr-1 WHSV.

4. The process of any preceding claim wherein the catalyst is saturated with steam.

5. The process of any preceding claim wherein the ether has from 5 to 12 carbon atoms, preferably from 5 to 8 carbon atoms.

6. The process of any preceding claim wherein the ether is a tertiary alkyl ether.

7. The process of Claim 6 wherein the ether comprises tertiary butyl methyl ether or tertiary amyl methyl ether.

8. The process of any preceding claim wherein the selectivity of the alkyl ether to its corresponding olefin is such that the rate of conversion of ether to olefin is at least 60% by weight, preferably at least 90% by weight, most preferably at least 95% by weight.

9. The use of a faujasite catalyst, at least 50% by weight of the original alkali metal content of which has been exchanged with at least one alkaline earth metal and which has been pretreated with steam, for conversion of an alkyl ether to its corresponding olefin.

10. The use of a faujasite catalyst as starting material in a procedure which comprises exchanging at least 50% by weight of the original alkali metal content of the catalyst with at least one alkaline earth metal and pretreating the catalyst with steam to form a catalyst for use in conversion of an alkyl ether to its corresponding olefin.

11. The process of any preceding claim wherein the byproduct make from the conversion reaction is reduced by at least 20%, preferably at least 30%, as compared to that make using the catalyst which has not been pretreated with steam.

12. The process of Claim 7 wherein the by-product yields of methyl secondary butyl ethers, dimethyl ether, and isobutane are reduced by at least 20%, preferably at least 30%.