Olefins, in particular, branched olefins such as isobutene are valuable industrial products and are often used as the starting materials for the production of other desirable chemicals. Isobutene may be polymerised to provide polyisobutene which is a valuable chemical in the detergents and fuel industry. In particular, polyisobutene is used as a fuel additive and a lubricant. Isobutene may be produced via various reaction schemes including the catalytic conversion of oxygenates such as methanol as disclosed in our European Patent Applications 485145 and 489497. Additionally, isobutene may be produced from the cracking of the petroleum additive tertiary butyl alcohol (TBA) . This may be achieved using an alumina catalyst.

We have now discovered that a high selectivity to olefins such as isobutene can be obtained under less severe conditions when a feed containing an oxygenate is passed over a zeolite catalyst.

Accordingly, the present invention provides a process for the production of olefins which comprises passing a feed containing an oxygenate of the general formul I

R-O-R ¹ (I) where R is an alkyl of 4 or 5 carbon atoms and R^- is H or alkyl optionally substituted with an ether group, over a zeolite catalyst whose framework includes a 10- or 12- member channel not intersected

by another 10- or 12- member channel with the proviso that when R^- is H, the zeolite has a framework which includes a 10- member channel not intersected by another 10- or 12- member channel.

The present invention provides a method of producing olefins from an oxygenate containing feed which requires a lower operating temperature than known prior art processes. Furthermore, by-product yield is minimised,thus providing an improvement in the selectivity to the desired product.

The process of the present invention provides a method of cracking an oxygenate to produce an olefin. The oxygenate is of general formula R-O-R^-.

R of general formula I is an alkyl of 4 or 5 carbon atoms and is preferably a secondary or tertiary alkyl. It is especially preferred that R is tertiary butyl. ^- of general formula. I is H or alkyl optionally substituted with an ether group. Suitably ^ is C^ to C alkyl, especially methyl or ethyl. Where ^- is an alkyl substituted with an ether group suitably R ¹ is -(CH2-) _n -O ² where R ² is C2 to C4 alkyl

The process of the present invention is particularly suitable for the production of isobutene from methyl tertiary butyl ether

(MTBE) , 2-methoxy butane, tertiary butyl alcohol (TBA) and secondary butyl alcohol (SBA) .

The feed may be obtained from any suitable source and may be fed into the reaction chamber either with or without a diluent. If it is desired to co-feed a diluent, suitable diluents include steam or an inert gas, e.g. nitrogen, hydrogen, or an alkane. The mole 1 of the diluent gas present may suitably be, for example, up to 50X, preferably up to 25Z, especially from 5 to 10X.

Where the feed contains an alcohol, for example TBA., it is preferred to co-feed water into the reaction chamber. The percent weight of water present in the feed will affect the selectivity to isobutene. Suitably, the feedstream may contain from up to 30X weight of water, preferably up to 20X weight and especially up to 10X weight water. Zeolites which may be used in the present invention include TON

(Theta-1, Nu-10, ZSM-22, KZ-2, ISI-1) , MTT (ZSM-23, EU-13, ISI-4, KZ-1), ZSM-48, FER (FU-9, Nu-23, ISI-6, ZSM-35) and EUO (EU-1, TPZ-3, ZSM-50), all of which contain a 10- membered channel that is not intersected by another 10- or 12- membered channel. MTW (ZSM-12, CZH-5, Theta-3, TPZ-12) and MOR (mordenite) which contain a 12- membered channel that is not intersected by another 10- or 12- membered channel may also be used in the present invention where the feed does not contain an alcohol. The preferred zeolite is TON. Information on zeolite structures is given in the Atlas of Zeolite Structure Types by Meier WM and Olsen DH, 1987, distributed by Polycrystal Book Service, Pittsburgh, USA. All of these known zeolite structure types can be prepared by published literature methods. Typical general methods are given, for example, in "Synthesis of High Silica Aluminosilicate Zeolites" by PA Jacobs and JA Martens; "Studies in Surface Science and Catalysis" vol. 33, Elsevier, 1987; and "Zeolite Molecular Sieves" by DW Breck, John Wiley, 1974.

The synthetic zeolite immediately after synthesis contains cations which, depending upon the precise synthesis method used, may be hydrogen, aluminium, alkali metals, organic nitrogen containing cations or any combination thereof.

The zeolite is preferably used in the present process in the hydrogen form. The hydrogen form may be achieved by, in the case of organic containing zeolite, calcination to remove the organics followed by either ammonium ion exchange followed by calcination, proton exchange with an acid solution or a combination of both. In the case of the zeolite synthesised in the absence of organic nitrogen containing compound, the hydrogen form could, if desired, be prepared by either direct ammonium exchange followed by calcination or proton exchange with acid solution or a combination of both. If so desired, the hydrogen form of the zeolite also may be partially exchanged or impregnated with a metal such as Ga or Mg and used in the present process.

The zeolite may be modified to alter its acidity or shape selectivity in such a way to improve the catalytic performance. The

modifications may include a calcination regime, steam treatment, chemical treatment, e.g. with a dealu inating agent such as SiCl , EDTA, etc or an aluminating agent such as sodium aluminate, AICI3 inclusion of phosphorus compound, Lewis base, HF etc. A combination of treatments may also be carried out. The treatment step may be carried out during the preparation of the H-form or be carried out after preparation of the H-form.

The zeolite, if desired, may be bound in a suitable binding material. The binder may suitably be one of the conventional alumina, silica, clay or aluminophosphate binders or a combination of binders.

The process according to the invention may suitably be carried out at a temperature of from 100 to 400 ^β C, preferably 150 to 300"C, especially 150 - 200'C, and is preferably carried out at atmospheric pressure, although other pressures may be used if desired, eg up to 15 barg.

The oxygenate feed may be fed into the reaction chamber either with or without diluents at a rate of suitably 0.1 to 50, preferably 0.9 to 10, especially 0.9 to 4.5 liquid hourly space velocity (LHSV) . For the purposes of the present invention, it is understood that liquid hourly space velocity is defined as the volume of feed fed per volume of catalyst per hour.

The process of the present invention may be carried out in any suitable reactor, for example a fixed bed, fluid bed, a reactive distillation column, a slurry reactor or a continuous catalyst regeneration reactor. The preferred reactor is a fixed bed reactor. The reactor may be made from any suitable material, e.g. steel or quartz.

The product of the process of the present invention will, of course, be dependent upon the feed. Where R is butyl, the product comprises butenes, e.g. n-butene and iso-butene. Where R is pentyl, the product comprises pentenes, e.g. n-pentene and iso-pentene. The product stream will also comprise water. Additionally, small amounts of other alkenes such as ethene, propene, hexene, octene, and the corresponding alcohols may also be present.

The process will now be described with reference to the following examples. Example 1 - Synthesis of Theta-1 Zeolite

Theta-1 was synthesised using ammonia as the templating agent. Sodium aluminate (19.67g, 61wtX AI2O3, 38wtX a2θ) and sodium hydroxide (17.58g ex BDH) were dissolved in distilled water (240g) . Ammonia solution (1400g, SG 0.90° containing 25% ammonia) was added with gentle mixing. Ludox AS40 (Trade Mark) (1200g) silica gel which contained 40wtZ silica was added over 20 minutes with stirring to maintain a homogeneous hydrogel. The molar composition of the hydro el was:

2.9 Na ₂ 0 : 175 NH3 : 1.0 A1 ₂ 0 ₃ : 68 Si0 ₂ : 950 H ₂ 0 The mixture was then loaded into a 5 litre Parr autoclave and crystallised at 175 ^β C for 29 hours under autogeneous pressure whilst mixing by a mechanical stirring action at 150 revs/min. The total time included time for the autoclave to reach the reaction temperature from ambient (about 3 hours) . At the end of the crystallisation period, the autoclave was cooled and the product filtered, washed and dried in an air oven at 100 ^β C. The crystallinity and the purity of the zeolite were determined by X-ray powder diffraction (XRD) . The sample contained Theta-1 zeolite with estimated amount of cristobalite of less than 52. Example 2- Preparation of the H-Form Theta-1 Zeolite

The Theta-1 as synthesised in Example 1 which contained both Na ⁺ and H4 ⁺ ions was directly ion exchanged in order to remove the Na ⁺ ions. The zeolite was mixed for 1 hour at room temperature with an aqueous ammonium nitrate solution (1M, zeolite to solution weight ratio of 1:20). The zeolite was filtered, washed and the ion exchange treatment repeated twice. The ammonium form of the zeolite was then dried at 100 ^β C and calcined overnight in air at 550°C to convert it to the hydrogen form. The X-ray diffraction pattern of the H-form is shown in Table 1. Example 3

The zeolite powder (H-form) was pressed into tablets at 10 tonnes. The tablets were broken and sieved into granules to pass

through 850 micron but not 600 micron sieves. An 7.8ml volume of the catalyst weight 3.15 was loaded into a quartz reactor with a 35ml preheater zone in an isothermal Carbolite furnace, activated in air at a rate of 600ml per hour following the temperature profile: room temperature l°C/minutev 120°C (2 hours) l°C/minute. 500C° (14- hours.-10°C/minute-.150°C initial test temperature.

Tertiary butyl alcohol (TBA) was pumped into the reactor using a perfusor syringe driver fitted with a 50ml syringe. On entering the reactor, the TBA was vapourised and mixed with nitrogen (gas flow of 590ml per hour) . The products were identified using gas chromatography. Table 2 provides the product stream analysis obtained for the reaction. Example 4

The zeolite powder (H-form) was pressed into tablets at 10 tonnes. The tablets were broken and sieved into granules to pass through 850 micron but not 600 micron sieves. An 8.2ml volume of the catalyst weight 4.34g was loaded into a quartz reactor with a 25ml preheater zone in an isothermal Carbolite furnace, activated in air at a rate of 600ml per hour following the temperature profile: room temperature 2°C/minute 300 ^β C (12 hours)-10°C/hour >. 175°C.

Methyl tertiary butyl ether was pumped into the reactor using a perfusor syringe driver fitted with a 50ml syringe. On entering the reactor, the MTBE was vapourised and mixed with nitrogen (gas flow of 680ml per hour). Table 3 provides the product stream analyses obtained for the reaction. Comparative Example 1

The process of Example 3 was repeated using a commercial alumina catalyst of surface area 184 ± 4m'/g and mean pore volume of 19nm. A 7.8ml volume of the catalyst (weight 5.13g) was used. The catalyst was purchased from ARCO under the trade name of UOP CAB 2L and came in the form of 3mm spheres. Table 3 provides the product stream analysis obtained for the reaction. It can be seen that conversion of tertiary butyl alcohol is considerably less when an alumina catalyst is used in the process. Selectivity to isobutene is also less than in the corresponding process using the zeolite

catalyst. Comparative Example 2

The process of Comparative Example 1 was repeated using methyl tertiary butyl ether as the feed. The product stream analysis is given in Table 4. It can be seen that conversion of MTBE is considerably less when an alumina catalyst is used in the process. Selectivity to dimethyl ether is also greater than in the corresponding process using the zeolite catalyst.

TABLE 1 XRD OF PRODUCT OF EXAMPLE 2

Variation in intensities of ± 20%. Variation in 2 theta positions of ± 0.2* with corresponding variations in D spacings. Peak below 10% of I _max excluded. Copper alpha 1 wavelength, 1.54060.

X-Ray Diffractometer Philips PW 1820/00

Slits 1/4*, 0.2\ l/4 ^»

2 Theta Scan 2 ^~ - 32*

Step Size 0.025-

Time 4 seconds

TABLE 2

TABLE 3 PRODUCT STREAM ANALYSIS FROM CRACKING MTBE USING THETA-1

DME - dimethyl ether

TABLE 4

TABLE 5

PRODUCT STREAM ANALYSIS FROM CRACKING MTBE USING ALUMINA CATLAYST

DME - dimethyl ether