BACKGROUND OF THE INVENTION

Field of the Invention

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 improved catalyst based on an alkaline earth exchanged faujasite.

Description of Related Art

Olefins, particularly tertiary olefins, may be commercially produced by the sulfuric acid extraction of such olefins from mixtures containing 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 essential. Also, dilution of the acid to promote olefin recovery and reconcentrating the acid prior to recycling are required and are expensive. In addition, this method is not always advantageous industrially because tertiary olefins are subject to side reactions such as polymerization, hydration and the like during extraction with concentrated sulfuric acid.

It is also known that tertiary olefins may be prepared by reacting 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 alkyl tert-alkyl ethers, there have been proposed methods using various catalysts: For example aluminum compounds supported on silica or other carriers (US-A-4,398,051) ; phosphoric acid on various supports (US-A-4,320,232) ; and metal containing 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 (US-A-4,398,051) and utilizing H2SO4 treated clay in the decomposition of t-alkyl ether alkanols (US-A-4,254,290) .

One of the main disadvantages of such processes is that the disclosed catalysts do not have good catalyst life in that higher and higher temperatures, which eventually become limiting, are required to maintain 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 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 exchange resins cannot be regenerated after use.

More recently, processes have been discovered which provide improved yields of tertiary olefin product. For example, US-A-4,691,073 discloses a process for preparing tertiary olefins from alkyl tertiary alkyl ethers comprising contacting the ether with a catalyst which has been prepared by reacting a clay with HF and/or HC1 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 to become deactivated as a consequence of coke and/or polymeric build up in a relatively short on-stream time. Also, the aluminosilicate structure of many clays is not sufficiently stable to withstand repeated high temperature regenerations required to remove catalyst deposits.

Natural and synthetic faujasite catalysts are known for use in the conversion or pyrolysis of ethers and alcohols into olefins or distillate range hydrocarbons. For example, US-A-4,544,793 discloses a process using an exchanged aluminosilicate catalyst which has a specific X-ray diffraction pattern as shown in Table 1 of that patent. US-A-4,467,133 (Chang) discloses the conversion of methanol into a distillate range hydrocarbon mixture by passing methanol over a rare earth exchanged faujasite (such as zeolite X or Y) at a temperature below 315°C

(600°F) . More particularly, Chang discloses a process for converting lower alcohols (C^ to C ₄ alcohols) or lower dialkyl ethers to distillate range ( ^ _Q ⁺ ) hydrocarbons suitable for use as diesel fuels. The process involves passing the feedstream over one of numerous disclosed aluminosilicate catalysts the alkali metal content of which has been exchanged with hydrogen or a Group IB - VIII metal. The focus of the disclosure is on zeolite Y as the preferred zeolite and a rare earth metal as the preferred exchange cation. Example 1 on column 6 shows the conversion of methanol to CIQ~ ^C 29 ⁺ olefins using REY zeolite. Quite clearly the process leads to dehydrogenation and oligomerization of the components in the feed streams, as evidenced by conversion of methanol to olefins having a minimum of 10 carbon atoms. This is quite distinct from the process of the present invention, where an alkaline earth metal exchanged faujasite, e.g., zeolite Y, is used as the catalyst which does not produce significant amounts of distillate range hydrocarbons (as does Chang) but rather selectively converts alkyl ethers to their corresponding olefins.

SUMMARY OF THE INVENTION

According to the present invention there is provided a process for selectively converting an alkyl ether to its corresponding olefin which comprises contacting the ether with 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.

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.

In accordance with the invention one or a mixture 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 containing a major proportion of at least one dialkyl ether having at least about 5 carbon atoms to produce the corresponding olefins.

The conversion, cracking or decomposition is preferably conducted in the vapor phase. The preferred temperatures for the contact between ether and catalyst are in the range of from 51° to 315°C < (125° to 600°F) .

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. The contact between the ether and the catalyst is generally performed under conditions of temperature and pressure 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.

DETAILED DESCRIPTION OF THE INVENTION

The catalyst which may be used in the process of this invention is based on a synthetic faujasite, for example zeolite Y. This is an alkali metal containing crystalline aluminosilicate well known in the art and is described in US-A-3,130,007. The preferred faujasite has a silica to alumina 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 exchanging the alkali metal originally present in the zeolite, e.g. sodium, with one or a mixture of alkaline earth metals such that at least 50% by weight of the alkali metal is replaced by the alkaline earth metal. It is preferred to conduct the exchange such that as many as possible of the original alkali metal ions are so exchanged, e.g., at least 75% by weight and more preferably at least 85% by weight. _. Most preferably the exchange 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 exchange is about 90% or above. Suitable alkaline earth exchange metals are calcium, barium and strontium, with calcium being most preferred.

Base exchange may be conducted for example by contacting the zeolite (which has been preferably

previously calcined) one or more times with an aqueous solution containing an alkaline earth metal salt dissolved therein, preferably at a temperature ranging from ambient up to about 85°C (185°F) . A wide variety of salts may be employed such as the chlorides, bromides, carbonates, sulfates or nitrates, so long as such salts are soluble in water such that ion transfer can take place. Calcium chloride is the preferred salt. The concentration of the salt in solution may for example range from about 0.1 to about 25% by weight. Preferably the concentration is sufficient to provide a slight excess of the stoichiometric amount of exchange cation.

After an exchange contact period which may range for example from about 60 minutes to about 24 hours, the exchanged zeolite is separated from the exchange solution, washed and dried. The exchange can be repeated one or more times if necessary in order to replace the maximum number of alkali metal ions with alkaline earth metal ions.

Other exchange processes may also be employed, such as the so called incipient wetness method, wherein the zeolite is infused with exchange 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 alumina, silica, clay or an alumina/silica mixture. Bound catalyst may be prepared by mixing the powdered catalyst with water and preferably from 5 to 40% by weight 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, preferably at 343°- 593°C (650°-1100°F) and preferably for a period of about 10 minutes up to a period of hours, e.g., 24 hours. The ion exchange process may be conducted prior to or subsequent to the formulation of such bound zeolites, preferably subsequent to such formulation.

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 9 carbon atoms and most preferably from 5 to 8 carbon atoms. Preferred ethers include tertiary alkyl ethers such as tertiary butyl methyl ether and tertiary butyl ethyl ether, and tertiary amyl counterparts including the methyl and ethyl ethers. Feedstreams which may typically be employed in commercial applications of the process preferably contain at least 70% up to 100% by weight of the ether, for example tertiary alkyl ether. The balance (if any) of the feedstream may comprise for example, primarily a mixture of saturated and unsaturated 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 alkaline earth exchanged catalyst. Reactor operating temperatures for this process are generally relatively low, preferably ranging from 51° to 315°C (125° to 600°F) , more

preferably from 115° to 260°C (240° to 500°F) and most preferably from 137° to 193°C (280° to 380°F) . 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 substantially in the vapor phase. The reactor may be equipped with a suitable temperature controlling means such that the desired operating temperatures can be maintained or adjusted in the reactor.

In a continuous process the reaction is preferably carried out at a spatial velocity expressed in terms of weight of organic feed per unit weight of catalyst per hour in the range of from 0.5 to 100 HSV, more preferably from 1 to 20 WHSV.

The process is especially suited for the conversion of fractions containing tertiary amylmethyl ether into corresponding isopentene olefins such as 2-methyl-2- butene or 2-methyl-1-butene; and for conversion of fractions containing 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 alkanes such as isobutane or isopentane which are very difficult to separate from their olefin counterparts.

The following examples illustrate the invention.

Example 1 Catalyst preparation

A calcium exchanged zeolite Y catalyst was prepared as follows: 108.3 grams of pellets of zeolite Y (LZY-52, available from UOP) which contained 20% by weight of alumina as a binder were packed into a 45.72 cm (18 inch) glass column. The column was then flushed with 100 ml. of ultra high purity water (pH-6.7) at a temperature of 65.5°C (150°F) .

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 65.5°C (150°F) . The packed 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 exchanged zeolite was then dried overnight under a vacuum at ambient temperatures and then dried at 100°C (212°F) for 8 hours under vacuum. Analysis showed that about 90% by weight of the original sodium ions present in the zeolite had been exchanged by calcium ions.

Example 2

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 circulating hot air oven and also connected to an effluent collector line.

A feed stream containing 95+% of tertiary butyl methyl ether was preheated and passed into the inlet of the reactor at a constant temperature maintained at about 179.4°C (355°F), at a pressure 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 conversion rate of greater than 95% of tertiary butyl methyl ether to isobutylene. The process was continued under constant conditions of pressure and temperature until the % conversion to isobutylene dropped below 90%. The elapsed time to below 90% conversion was measured at 676 hours.

Example 3

Comparative

Example 2 was repeated under the conditions set forth therein except the catalyst employed was a hydrogen fluoride treated and calcined attapulgite clay as disclosed in US-A-4,691,073. The on stream time for isobutylene conversion to drop below 90% was measured as 46 hours.

A comparison of the results in Examples 2 and 3 demonstrates that the process of this invention provides high yields of olefin over a longer period of time than the HF attapulgite, i.e., 676 hours vs. only 46 hours.

Example 4

In this example, a number of prior art catalysts were evaluated in a pilot plant for their resistance to catalyst deactivation in the ether decomposition (conversion) process. In this test, the initial temperature in the reactor was set at 171.1°C (340°F) and the temperature was gradually raised to 193.3°C (380°F) as needed to maintain 90% conversion of the tertiary butyl methyl ether to isobutylene. The hours on stream before conversion at the peak temperature of 193.3°C (380°F) dropped below 90% were recorded in each case. Some tests were terminated sooner if unacceptable high levels of undesirable by-products were formed. The flow was maintained at about 3.6 WHSV and 620 kPag (90 psig) pressure. Results are shown in Table 1.

The data reported in Table 1 clearly demonstrate the superiority of the preferred process of this invention using the preferred catalyst (CaY) when compared with a process using HF Attapulgite or other catalysts.

Thus the exchanged zeolites Mg-Y, rare earth Y and H-Y are significantly inferior to CaY because they deactivate much more quickly. CaY had the lowest average rate of deactivation of about 0.2°C (0.36°F) per day.

*Test was terminated due to formation of high levels of by-product.

**Test was terminated before the maximum temperature was reached. Ether conversion was still at 90%.