PROCESS FOR CATALYTIC CONVERSION OF O EFINS The present invention relates to a process for conversion of an n-olefin to another unsaturated compound. Some unsaturated compounds are readily available e.g. from fossil fuel sources. However the demand for certain unsaturated compounds is greater than for others. There iε therefore a need to interconvert unsaturated compounds. In particular, n-olefins may be converted e.g. by skeletal isomerization or by dimerization optionally followed by cracking. Iso-butene is present in the C ₄ products obtained from steam-cracking of naphthas or from catalytic cracking of high boiling fuel cuts. Other C ₄ products such as n-butene are also present. Demand for iso-butene is increasing and there is therefore a need for a process for converting n-butene efficiently to iso-butene.

Several processes are known for the skeletal isomerization of n-olefins e.g. the production of iso- butene from n-butene. Such conversion has previously been carried out using an amorphous catalyst. For example, EP-A-152918 describes a disproportionation and isomerization catalyst for olefins which comprises silica and molybdenum oxide together with titanium dioxide as promoter.

US 2217252 describes the use of natural and synthetic zeolites such as analcime, chabazite, heulandite, natrolite and thomsonite in the interconversion of olefins.

A process potentially of commercial use for the skeletal isomerization of an olefin is the so-called "Snamprogetti process". In this process the olefin is in contact with an activated alumina catalyst at high temperature (about 450°C) .

The WHSV (weight hourly space velocity) is a measure of the throughput of olefin which is used in continuous use of the catalyst. At 450°C a WHSV of 2 hr" ¹ is common in the Snamprogetti process. Under these

conditions n-butene yields approximately 30% iso-butene with a selectivity of 75%. The catalyst has a life of at least 8 hours before it is necessary to regenerate it.

If the WHSV is increased to more than 12 hr ^-1 the isobutene yield drops to around 8%.

Another fluorine- or chlorine-containing "activated" alumina catalyst is described in EP-A-0071199. The catalytic activity iε comparable with the Snamprogetti catalyst; the isobutene selectivity is higher. US 4503282 describes the use of AMS-1B boro silicate, a boron containing ZSM-5 type zeolite, in the interconversion of olefins. This catalyst operates at low . WHSV (commonly 2 hr ^-1 or less) and high temperature (400- 600°C) . The reported selectivities are lower in general than those of the amorphous alumina based catalysts.

Catalysts are also used in the dimerization of n-olefins, but these are not without their problems. It is known to use as a catalyst solid phosphorous acid on kieselguhr. The catalyst cannot be regenerated and disposal of large quantities of catalyst as land-fill material is no longer acceptable.

The applicants sought a catalytic process for the skeletal isomerization or dimerization and optional cracking of olefins, particularly n-olefinε of 4 to 6 carbon atoms, which had a good selectivity and yield of the desired olefin for a given WHSV, which had an acceptable catalytic life and which used a catalyst which could be easily regenerated.

Previous research aimed at producing catalysts for the interconversion of unsaturated compounds has been directed to compositions, generally amorphous, which contain Bronsted acid sites i.e. sites which are capable of donating a proton, H+. It was believed that interaction between the unsaturated compound and a proton on the surface of the catalyst facilitated the desired conversion.

The present inventors have surprisingly found that a molecular sieve which has been ion exchanged with a cation to give a Lewis acid site is a particularly good catalyst for the interconversion of unsaturated compounds. The present invention provides a process for conversion of an n-olefin of 3 to 9 carbon atoms to another unsaturated compound comprising contacting the olefin in the liquid or vapour phase with a catalyst which comprises a molecular sieve which has been ion-exchanged with a cation to provide a Lewis acid site.

The catalytic process of the present invention has the advantage that it may be used to perform either skeletal isomerization or dimerization and that the dimerization may optionally be combined with cracking. The dominant reaction may be selected by choosing the appropriate reaction conditions bearing in mind the particular catalyst being used.

The catalyst is a molecular sieve, such as a zeolite or a silica/alumina phosphate (SAPO) which should contain a Lewis acid cation site.

The zeolite may be, for example, zeolite Y, zeolite beta, ZK-5, ZSM-5, ZSM-12, ZSM-22 or ZSM-23. Other suitable molecular sieves included SAPO's.

The catalyzed reaction is carried out in the presence of Lewis acid and base sites. This combination of sites may be provided suitably by a molecular sieve. The base sites are present inherently in the molecular sieve e.g. the 0 ² ~ ions in the zeolite framework. Cations present to provide electroneutrality, e.g. divalent cations such as alkaline earth metal cations, provide Lewis acid sites of the desired strength.

It is particularly useful to use as a catalyst a molecular sieve and to ion exchange it with cations. The ion exchange cations present in the sieve do not form an integral part of the framework i.e. they are not covalently

bound into the Si/Al/O network of a zeolite. Thus when taking part in the olefin conversion it is not necessary for cations to be removed from the framework and the framework is not weakened. The catalyst composition may be easily controlled by selecting the particular molecular sieve and exchange cations.

In a sieve such as a zeolite the framework is composed of tetrahedra of Si0 ₄ and A10 ₄ . The presence of the aluminium produces a charge imbalance in the framework. In a SAPO, some of the (A1,P)0 ₄ tetrahedra are replaced by (Si,Al)0 ₄ tetrahedra, which also produces a charge imbalance. To achieve electroneutrality cations are introduced into the sieve. In synthetic zeolites these cations are often alkali metals such as sodium or potassium, or protons.

Such cations may be ion exchanged with other cations which provide stronger Lewis acid sites. Although trivalent cations may be used in the ion exchange the Lewis acid sites produced are generally too strong and it is preferred to use divalent cations. Suitable cations are those whose inclusion in zeolites is known to those of skill in the art, for example, magnesium, calcium, strontium and barium ions. Other divalent cations may also be used to provide Lewis acid strength, for example copper, nickel, and cobalt.

The molecular sieves containing cations will be referred to hereafter by the cation hyphenated to the type of molecular sieve e.g. Mg-beta will be used to denote a zeolite beta containing magnesium ions. The electronegativity of a zeolite (a function of the composition and the structure type) determines the Lewis acid and base site strength. The electronegativity of a molecular sieve may be varied by methods known to the skilled person, such as ion exchange and/or by changing the Si/Al ₂ ratio.

In a typical ion exchange the solid material is mixed with an aqueous solution containing sufficient amount or excess of cation to be introduced. On an experimental scale lOg of solid, for example, may be mixed with 15-25 mis of solution. The mixture is stirred eventually at elevated temperature and the solid is then washed prior to use.

The particular catalyst which will be suitable for any process will depend on the features required for the process. Certain processes will need shorter contact times than others.

When selecting a catalyst for the present process, certain physical attributes of the catalyst should be considered. In general, the higher the ratio of Si/Al the greater the average electronegativity of the catalyst.

A Si/Al ₂ ratio of 120 such as may be present in a molecular sieve will result in a higher electronegativity than a Si/Al ratio of 70. A "high" electronegativity tends to lead to a catalyst which is more active or stable than a catalyst of lower electronegativity. A high Si/Al ₂ ratio is therefore preferred in the molecular sieve.

The catalyst becomes deactivated during use by the deposition of coke. If the catalytically active sites are far apart then the deactivation may be reasonably slow. Active sites which are far apart i.e. a low active site density are therefore preferred.

One of the factors relating to the density of Lewis acid sites is the cation exchange capacity of the sieve. A sieve which requires the presence of cations for its electroneutrality will have a cation exchange capacity (CEC) generally expressed in meq/g (dried sieve) . The higher the CEC, the more closely positioned the active sites for this process. Sieves with a low CEC are therefore preferred in general. The following table gives

an indication of CEC values for some of the sieves which may be used in the present process.

When the silica - alumina ratio increases this results in a decrease in the- cation exchange capacity.

Improved stability of catalyst is also seen when the channel dimensions i.e. pore size/void space of the catalyst are reduced. By decreasing the internal volume, coke precursors are prevented from reaching the critical size which would render them unable to leave the pore system. Coke formation is reduced and the deactivation rate of the catalyst will thereby be reduced. If the pore size is too small the catalytic activity is adversely affected. A medium pore size which appears to be the optimum is a T atom ring size of 8 to 12.

A particularly preferred catalyst which has active sites which are far apart, and limited channel dimensions is SAPO-11 i.e. a silica/alumina phosphate which can be synthesized by known methods.

The catalyst may be used in the form of a powder or an extrudate, although the extrudate is usually preferred in industrial processes.

The process of the present invention may be used to isomerize or dimerize and optionally crack n-olefins of 3 to 9 carbon atoms particularly n-olefins of 4 to 6 carbon atoms i.e. n-butene, n-pentene and n-hexene, especially n- butene.

The process may be carried out by charging a reactor such as an SS reactor with the catalyst and then activating the catalyst. This is generally achieved by continuously passing an inert gas such as nitrogen through the catalyst for 1-3 hours at elevated temperature. The temperature and pressure in the reactor are then adjusted to the conditions under which it is desired to carry out the reaction. The olefin is then fed through the catalyst, optionally with an inert diluent. When the reaction temperature is in the range 250- 500°C especially 375-475°C and the reaction pressure iε 0.08-0.12 MPa, especially atmospheric presεure, the major reaction will be the skeletal isomerization of the n- olefin. Thus n-butene will be isomerized to give iso- butene. At higher temperatures the secondary C reactions such as alkylation, dimerization, hydrogenation and cracking are suppressed but the formation of the iso-butene is also less. The above ranges are believed to provide the optimum amount of product with acceptably small levels of side products.

Reducing the temperature and increasing the pressure will tend to favour formation of the dimerized products. When the reaction temperature is 80-250°C, especially around 200°C, and the reaction pressure is relatively high e.g. greater than 10 bar g, the major reaction will be the dimerization of the n-olefin. Thus n- butene will be dimerized to give unsaturated C ₈ compounds.

On the other hand if the reaction temperature is maintained at a temperature comparable with that used for isomerization i.e. 250-500°C but the presεure iε increaεed from isomerisation pressure to 0.12-2 MPa, this favours the formation of disproportionated dimerization products; for example n-butene under these reaction conditions will form propene and pentenes. Using the present process the skilled person can control the form of catalyst used by selecting a particular molecular sieve structure and pore size and ion exchanging the sieve with the desired cation to provide Lewis acid sites in the sieve. Once the catalyst has been selected, the reaction conditions appropriate for the desired product using that catalyst can be established by adjusting the temperature and pressure based on the knowledge of which products are favoured by increasing or decreasing the temperature or pressure. The catalyst will eventually require regeneration to rid it of the coke deposits which gradually build up the

pores. The regeneration may be easily carried out by a coke burn in air at a temperature of 350-700°C, preferably 450-550°C.

The following examples illustrate the invention. KYAMPT.F 1 - Skeletal isomerization

A SS reactor was charged with 1.0 g of catalyst, particles size 210 μm < d < 600 μm.

Prior to use the catalyst was activated at 500°C for 2 hours under a continuous flow of nitrogen (50 ml/min) . After activation the reactor temperature was changed to the desired temperature.

In all experiments 1-butene was used as a feed. If, necessary the feed stream was diluted with nitrogen. The reactions were run for different timeε, depending on the deactivation rate of the catalyεt.

Samples of the product stream were analyzed every hour by gas chromatography (GC) on line, using an AI2O3/KCI plot column.

The following test conditions were used: Temperature 375°C

Pressure atmospheric

WHSV 1

HC/Diluent 1 : 3

Diluent nitrogen The results are shown in Tables I and II.

The time on stream taken to reach a conversion level of 10% can be uεed to measure the stability of the catalyst; the longer this time, the more stable the catalyst. With decreasing Lewis acid strength (in the order of Mg(II), Ca(II), Sr(II) and Ba(II)), the following observations were made: increase in isobutene selectivity; increase in catalyst life; - increase in the heavies selectivity; decrease in lights selectivity and decrease in hydrogen transfer (ratio of C4 and C3=) .

TABLE I RESULTS FOR ZEOLITE Y

sel. = Selectivity for * Time in Hours

(1) Lights are defined as Cτ_, C ₂ , 2~, C3 ⁼ and C ₄

(2) Heavies are defined as C5 ^+, s

TABLE II

( 1) Lights are defined as C _j , -2 ' C ₃ , C ₃ ^~ and C ₄

( 2 ) Heavies are defined as C ₅ ^{+ ,} s

The deactivation rate of zeolite Y is high. It is known from the open literature that coke formation increases with the radiuε of the zeolite channels and cages. Zeolite Y iε one of the larger pore zeolites, therefore showing a fast deactivation. Zeolite Beta in comparison with zeolite Y, has fewer acid siteε and alεo haε εmaller channelε.

A comparison of the resultε for Ca-Beta and Ca-Y εhowε that smaller zeolite channels and lower acid site density resulted in a lower deactivation rate and a higher isobutene selectivity. For Ca-Beta with Si0 ₂ /Al ₂ θ ₃ ratio of 51 the acid site density is lower than that of Ca-Beta with an Si0 ₂ /Al ₂ 0 ₃ ratio of 28. Therefore, the deactivation rate of Ca-Beta(51) is lower and the isobutene yield is higher than that of Ca-Beta(28) . For the SAPO-11 molecular sieves, the acid site density is very low and the channel radius is smaller than for zeolite Y and zeolite Beta. The results for Ca(II) exchanged SAPO-11 as indicated in Table II showed an isobutene yield of 24 wt % and a very low deactivation rate.

Thus the smaller pores in zeolite Beta in

comparison with zeolite Y led to a lower deactivation rate of the catalyst, i.e. a suppression of side reactions.

The even lower deactivation rate of the SAPO-11 catalyst was also due to the small pore size and also the isolated acidic sites.

Varying the WHSV does not affect the initial activity or selectivity of the catalyst. However, deactivation is affected by WHSV; the lower the WHSV, the longer the catalyst life. Regeneration

The catalysts were each regenerated by treating with 5% oxygen in nitrogen at 550°C. When re-used in the isomerization process they showed the same initial activity/deactivation rate but a slight change in selectivities. The Ca-SAPO-11 selectivity for isobutene was slightly down (31 to 28) , for lights was slightly lower (24 to 18) and for heavies slightly up (45 to 54) .

Figure 1 shows the variation in the selectivity for isobutene with varying temperature and pressure using a Ca-SAPO-11 catalyst. It can be seen from this that high temperatures and low pressures favour production of the skeletally isomerized product.

EXAMPLE 2 - Skeletal isomerization using SAPO-11 extrudates. Commercially available SAPO-11 extrudates (as received, Mg(II)- and Ca(II) exchanged forms) were tested for the skeletal isomerization of n-butenes. In all experiments the selectivity for isobutene is somewhat higher than in the powder experiments. The deactivation rate for the extrudates is slightly higher. The time required to reach the maximum yield of isobutene is 24 hours for the Mg(II)- and Ca(II) exchanged extrudates, 48 hours for the as received extrudates and 1 - 2 hours for the powders. The catalyst life of the SAPO-11 extrudates is over 140 hours run length and the isobutene selectivity

is around 125 % of the thermodynamic equilibrium. The high yield of isobutene over molecular sieve catalysts (125 % of thermodynamic equilibrium) can be explained as follows: 1) one part of isobutene is formed via the skeletal isomerization and 2) the other part of isobutene is formed via a dimerization of buteneε to octeneε and followed by selective cracking of these octenes to propene, pentenes and especially isobutene (and other butenes) . The results are shown in Table III.

TABLE III

(1) Lights are defined as C _lr C ₂ , C ₂ =, C ₃ , C ₃ = and C . (2) Heavies are defined as C ₅ +'ε.

EXAMPLE 3 - Production of Propene using SAPO-11 extrudateε.

A SS reactor waε charged with 1.0 gram of commercially available SAPO-11 extrudates (as received, Ca(II) exchanged form), particle size 210 μm <d< 600 μm. Prior to use the catalyst was activated at 500°C for 2 hours under a continous flow of dried nitrogen (50 ml/min) . After activation the reactor temperature was lowered to the desired temperature.

In the experiments 1-butene was used as a feed, diluted with nitrogen. The pressure drop over the reactor was < 0.1 barg. Product analysis was done by gas chromatography on line, using an A1 ₂ 0 ₂ /KC1 plot column.

The following test conditions were used:

Temperature 475°C Catalyst Ca-SAPO-11 extrudates WHSV 1 HC/Diluent 1:3 Diluent nitrogen

The resultε are shown in Table IV.

TABLE IV

(1) Lights are defined as C _lf C ₂ , C ₂ ^:= , C ₃ , C ₃ ⁼ and C 's.

(2) C ₅ 's are defined as C ₅ ^=, s and 0 _$ ' s .

(3) Heavieε are defined aε C ₅ ^+, s.

An increase in pressure results in selective cracking of the dimerisation-formed octenes into the valuable propene and pentenes. T_YΆ PT.T_ Δ - Dimerization

The dimerization of 1-butene was studied in a stainless steel fixed bed reactor. The temperature was 200°C, pressure was 10 barg and WHSV of 1.5. Prior to use the catalysts were dehydrated and activated at 500°C for 16 hours under dried air at a flow rate of 1000 (ml/ml(cat) x hrs) . The pressure drop over the reactor was in all experiments <0.1 barg. Product analysis was done by gas chromatography using a sgualane column with hydrogen as carrier gas. The average carbon number distribution number was determined off-line with gas chromatography using a 13X molecular sieve column. For both GC analyses, the sample

- 15 - was hydrogenated in situ in the injector εystem of the GC over a platinum on alumina catalyst in order to simplify the analysis, by hydrogenation of the olefins into paraffins. Variouε types of catalyst/zeolites were tested: molecular sieves SAPO-11 extrudates (obtained from Union Carbide, crystal size distribution 0.5 - 5 μm (main distribution 1.3 -2.2 μm) ) and zeolite Beta (silica-alumina ratio of 51) . The catalyst extrudates were crushed to a particle size distribution from 0.2-0.6 mm in order to eliminate diffusion phenomena and reactor wall effects. The results are shown in Tables V to VIII.

TABLE V

Table V: Dimerization of 1-butene over SAPO-11 extrudates.

Temperature: 200°C, Pressure: 10 barg and WHSV: 1.5 Branchiness is defined as

3* tribranched C '8„'s + 2* dibranched C„*s + 1* monobranched C„'s + 0* linear C„'s

"8 8

The random carbon number distribution and the residues on the catalyst indicate that the overall acidity of the SAPO-11 is high (high oligo erization and cracking rates) . Cation exchange of zeolites/molecular sieves can

tailor the strength of the acidity to produce catalysts for the present process.

TABLE VI

Table VI: Dimerization of 1-butene over Mg-SAPO-11 extrudates.

Temperature: 200°C, Pressure: 10 barg and WHSV: 1.5

As can be seen in Table VI, the activity of a magnesium exchanged SAPO-11 extrudates catalyst is the same as for the untreated SAPO-11 extrudates. The branchiness is 1.6. The selectivity to the C8*s is exceptionally high (>98%) .

For the dimerization reaction of butenes over a non-ion exchanged zeolite catalyst the presence of the transition metal nickel its oxidation state on the catalyst support is essential. SAPO-11 extrudates as received were exchanged with nickel and subsequently in situ in the reactor activated at 500°C for 16 hours under dried air flow. The nickel exchange level was not analyzed.

TABLE VII

Table VII: Dimerization of 1-butene over Ni-SAPO-11 extrudates.

Temperature: 200°C, Pressure: 10 barg and WHSV: 1.5.

As can be seen in Table VII the activity of the nickel exchanged SAPO-11 extrudates is, in comparison with the previously tested catalysts (SAPO-11 and Mg-SAPO-ll) , very low.

The activity maintenance of this catalyst is good. Even up to 100 hours on stream the conversion level remains unchanged (2%). The selectivity to C8's and the branchiness of this fraction are comparable to those of Mg- SAPO-ll (>92% and 1.7-1.8, respectively).

In zeolite Beta the number of cation exchangeable sites is higher than in SAPO-11 and it might be expected that nickel exchanged zeolite Beta would be more active. Nickel exchanged zeolite Beta was activated .in situ in the reactor at 500 C for 16 hours under dried air. The nickel exchange level was not analyzed.

TABLE VIII Table VIII : Dimerization of 1-butene over Ni-zeolite Beta .

Temperature: 200 °C, Pressure: 10 barg and WHSV: 1.5.

As can be seen in Table VIII the activity of nickel exchanged zeolite Beta is much higher (14% conversion versus 2% for Ni-SAPO-11) . Although the initial conversion level drops over the first 20 hours, the conversion level remains unchanged up to 130 hours on stream (still 7 % conversion up to 130 hours on stream) . The branchinesε of the C8 fraction is good (1.6-1.8). After the run on the catalyst some amounts of C18 and C22 were observed. The random carbon number distribution and the amounts of residues on the catalyst indicate that the overall acidity of the Beta is rather high (high oligomerization and cracking rates) .