Linear a-olefins are produced on a large scale for example by ethylene oligomerisation, Fischer-Tropsch synthesis or, as in the case of 1-hexene, by controlled trimerisation. The ethylene oligomerisation can be performed for example according to Ziegler by a two-stage process, i. e. by a building-up reaction using triethyl aluminium, followed by'short-time high-temperature displacement' (termed ALFEN process), or by a single-stage high-temperature process with triethyl aluminium catalysis.

Another process for producing a-olefins is the dehydration of alcohols, which has been described in detail for example by Knözinger, H. , Angew. Chem. (Applied Chemistry), 17it. Ed., vol. 7, 1968, no. 10, p. 791-805.

Short-chain linear a-olefins are especially employed as co-monomers for making polyethylene copolymers. High purity is essential for this use, particularly as regards the 1-olefins content, plus the lowest possible amount of impurities, such as internal, branched, or cyclic alkenes, and dienes or alkines. It is known that few ppm of impurities already result in rapid deactivation of the polymerisation catalyst. Owing to the small boiling-point difference of the double-bond isomers and structural isomers of the olefins, which makes splitting by distillation expensive, the a-selectivity of the dehydration step is most important.

By the term'a-selectivity'as used herein is meant the ratio of the a-olefin formed to the total olefins formed, i. e. the total of a-olefins, internal, branched, and cyclic olefins.

Suitable alumina catalysts for this application comprise for example zinc-and zirconium-doped aluminas (cf. e. g. U. S. patent 4, 260, 845 ; EP patent 0 150 832-B1, respectively). The a-selectivity can also be improved by addition of bases, which, however, has an adverse effect on the reactivity. For example, base-doped alumina

catalysts, such as barium-doped alumina, require significantly higher temperatures than the corresponding undoped catalysts. Acidic aluminas can be used at considerably lower temperatures causing, however, more isomerisation reactions and hence a decrease in a-selectivity.

Furthermore, it is known that there are processes, wherein prior to use the bases are homogeneously mixed with the alcohol. German patent DE 3915 493 C2 describes the improvement of a-selectivity in the dehydration of fatty alcohols on y-alumina by addition of 20 to 300 ppm of ammonia. However, it is a disadvantage of said process to add an additional component, namely ammonia, which later must be separated.

The commonly known processes of alumina-catalysed dehydration have the disadvantage of poor a-selectivity obtained with the customary catalysts. The selectivity can be improved at the expense of the turnover or by costly modification of the catalyst or by additon of further substances, which later must be separated.

Moreover, reaction temperatures of greater than 350°C are necessary in order to achieve a fairly good conversion.

It is, therefore, an object of the present invention to provide a process for producing a-olefins by dehydration of alcohols, which process yields a high a-selectivity and good conversion at a low reaction temperature of less than 350 °C, particularly 320 °C and lower, without addition of bases or acids or without catalyst doping. Another object of the present invention is to provide a novel dehydration catalyst.

According to the present invention, the problem has been solved by a process for producing cc-olefins by dehydration of alcohols in the presence of y-alumina, wherein the alcohol is brought into contact with at least one y-alumina having - a pore volume of greater than 0.9 ml/g, preferably greater than 1.0 ml/g (measured in accordance with DIN 66133, contact angle 131°C), - at least one pore radii maximum in the mesopores range (10 to 250 A) from 20 to 90 A, preferably 30 to 80 A, and at least one additional pore radii maximum in the macropores range (greater than 250 A).

The dehydration is preferably carried out at temperatures in the range from 260 to 350 °C, most preferably from 280 to 320 °C. The preferred embodiments of the subject invention are set out in the subordinate claims or hereinbelow.

According to the invention, the preferable alcohols are comprised of linear or branched 1-alkanols having 4 to 14 carbon atoms. Examples of suitable 1-alkanols include 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol, 1-nonanol, 1- <BR> <BR> <BR> decanol9 l-undecanol, l-dodecanol, l-tridecanol9 l-tetradecanol9 2-ethyl-l-hexanol9 and 2-butyl octanol, of which 1-hexanol and 1-octanol are particularly preferred.

In addition to all-y-phase alumina, catalyst blends composed of y-and 6-phases may be used as well. The y-alumina according to the invention is mostly comprised of the y-phase, namely, more than 50 wt%, preferably more than 90 wt%, and it has a bimodal pore radii distribution comprising at least one pore radius maximum in the mesopores range (pore radius 10 to 250 A) and one in the macropores range (pore radius > 250 A) with a total pore volume of greater than 0.9 ml/g and an amount of mesopores of preferably greater than 0.6 ml/g (determined in accordance with DIN 66133 by the mercury intrusion method at a contact angle of 131°). At least one maximum of the mesopores radii distribution of the y-alumina of the invention is in the range from 20 to 90 A, preferably 30 to 80 A, whereas the preferable maximum of the macropores is greater than 1,000 A.

It is preferable to use alumina having an A1203 content of >99 wt%, which unlike the aluminas described in US 4,260, 845 or EP 0 150 832-B 1, is not doped with zinc or zirconium or other additives, a process-conditioned amount of zinc and zirconium of typically less than 10 ppm notwithstanding. It is furthermore preferable not to employ any other type of dehydration catalysts, including co- catalysts or active carriers.

An example of a typical composition of the catalysts of the invention is given in Table I hereinbelow.

Table I Catalyst Composition PURALTM KR 1 Al wt% 53.6 Si ppm 553 Ca ppm 176 Na ppm 159 Fe ppm 156 Mg ppm 78 Ga ppm 70 Ti ppm 14 Pb ppm 14 Cr ppm 11 K ppm <10 Li, Zr, B, Mo, Ni, Mn, Zn, Cu, Co each < 5 ppm In principle, any commercially available y-alumina catalyst can be employed for the dehydration of 1-alcohols, but their use is less advantageous, compared to the catalysts of the invention. The y-Al203-catalysts of the invention differ from conventional catalysts in their physical properties, such as surface, porosity, pore geometry, and chemical composition. The catalysts of the invention excel by their high pore volume of greater than 0.9 ml/g and pore radii distribution which is at least bimodal and has at least one maximum in the mesopores range from 30 to 80 A and one in the macropores range of preferably >1, 000 A, wherein the pore volume of the mesopores preferably makes up more than 40 % of the total pore volume, whereas independently thereof the pore volume of the macropores preferably constitutes more than 20 %. In the dehydration of 1-alcohol the special combination of physical properties of this alumina surprisingly yields a crude product having a significantly higher a-selectivity with unimpaired conversion (cf.

Table II).

The y-alumina catalysts of the invention are different from the conventional catalysts usually employed for the dehydration of 1-alcohols yielding 1-olefins, because they have been modified by the manufacturing method, especially with respect to their physical properties, such that they have turned out to be

surprisingly selective in the dehydration process, particularly as regards the purity of the desired l-olefin.

The process of the invention is preferably carried out at a pressure ranging from 10 to 2,000 mbar. The high-porosity alumina is preferably employed for example in the form of beads, extrudate clippings, granules, or pellets. The dehydration is preferably carried out continuously, most preferably in a continuous tube reactor.

The process of the invention can be carried out for example as follows: The preferably gaseous alcohol is passed preferably at 280 to 320 °C through a reactor, especially a tube reactor, packed with the alumina of the invention.

Pressure and temperature are such that at the reaction conditions of choice the catalyst is not contacted by liquid, e. g. the higher-boiling ether, which may theoretically be formed as an intermediate during the reaction. The alcohol may optionally be mixed with inert gas, e. g. nitrogen. The alumina is present for the most part as a g-phase material, preferably in the form of granules, extrudate, or beads.

The reaction product is condensed, the aqueous phase is separated, and the organic phase is analysed by gas chromatography in order to examine the reaction. With the purpose of evaluating the catalyst activity, the reactor. is operated at steady conditions until a constant composition/stationary state is reached. The organic phase obtained after phase separation is distilled and both the unreacted alcohol and the dialkyl ether formed can be recycled to the process. Alternatively, the alcohol/dialkyl ether mixture can be separated from the olefin/water mixture by condensation immediately after the reaction, followed by direct recycling to the process.

The dihexyl ether which is usually formed as a by-product during the reaction of 1- hexanol can be distilled off and recycled to the process. The compositions of the crude educts/products and the test conditions for various catalysts have been compiled in Table II.

PURALT"" KR1 granules and extrudates are exemplary of the high-prosity bimodal y-aluminas of the invention. When comparing the a-selectivities at 300 °C and a feed rate of 2 ml/min, their remarkable superiority over the

Sudchemie catalyst is obvious. This superiority is yet more evident at 325 °C.

PURALT KR29 too, is a high-porosity y-alumina, but with a monomodal pore radii distribution. The P 180 catalyst is a high-porosity alumina, but compared with the catalyst of the invention, it has a considerably greater average pore radius in the mesopores range.

It has become apparent that the process for producing 1-hexene in the co-monomer mode utilising the alumina according to the invention is much more cost-effective. For example, working-up of the crude product by distillation in order to obtain a 1-hexene content of > 99 % can be accomplished by means of a column with fewer separation stages. The turnover can be affected by temperature, feed quantity (MHSV), and catalyst bed length, but the a-selectivity decreases as the turnover increases.

Figure 1 shows the pore radii distributions of PURAL KR1 according to example 3 and of PURAL KR2 according to comparative example 4. The pore radii were determined by the Hg intrusion method according to DIN 66133 at a contact angle of 13 1'using a measuring apparatus of micromeritics company.

Figure 2 shows the l-hexane content in the crude product versus the a-selectivity as a function of feed quantity and temperature. The data points plotted for a 1- hexene content of less than 30 % have been obtained at 300 °C, whereas the data collected for a 1-hexene content of greater than 30 % are based on 325 °C (legend: (1) 300°C, 2 ml/min; (2) 325°C, 3 ml/min ; (3) 325°C, 2 ml/min; (4) 325°C, 1 ml/min). With an increasing turnover the differences in the a-selectivities of various catalysts become particularly plain.

Examples Preparation of t-Al203 Extrudates 2 kgs of alumina and 2 kgs of water were mixed for 20 minutes in a Z-type kneader mixer. The resultant paste was extruded through a 2-mm strainer of a single-screw extruder and then was dried and calcined onto the desired surface.

Preparation of y-Al203 Granules Alumina and water were continuously mixed in a double-screw extruder at a feed rate of 15 kgs/h. The resultant granules were dried and then calcined onto the desired surface.

Examples 1 through 5 Dehydration of Hexanol on Alumina (for the specification, see Table 11) In a continuously operated tube reactor (23 mm in diameter, 100 mm in length), 2 ml/min of hexanol vapour having a purity of 99.4 % and 0.25 1/min of nitrogen were passed at 300 °C/325 °C and 2 bar through a 50-mm catalyst bed comprised of 7.2 to 9.9 grams of catalyst, depending on the bulk density.

When using the monomodal y-alumina of Sudchemie (CS 331-1; pore volume 0.82 ml/g) at 300°C reaction temperature, the organic phase obtained after phase separation of the crude product comprised 44.2 % hexanol, 32.2 % dihexyl ether, and 20.7 % 1-hexene (comparative example 4). The a-selectivity was 96.5 %.

When using PURAL KR1 at 300°C reaction temperature, the organic phase obtained after phase separation of the crude product comprised 35.2 % hexanol, 38.7 % dihexyl ether, and 24.5 % 1-hexene. The a-selectivity of the hexenes was 97. 7 %.

Table II Gas-Phase Dehydration of 1-Hexanol on Al2O3 Catalysts Examples 1 2 3 4 Catalyst AlO3 CS 331-1 PURAL KR1 PURAL PURAL I<R2 P180 KR1 (Sudchemie) (Sasol) (Sasol) (Sasol) (Sasol) Type Extrudates Granules Extrudates Extrudates Extrudates Surface m2/g) 235 184 232 215 157 Pore volume (Hg) 0. 83 1.31 1. 28 0. 89 0.99 [ml/g], thereof pore radii of <300A 0.69 (83%) 0.79 (60%) 0.77 (60%) 0.84 (94%) 0.77 (78%) 300-1, OOOA 0. 04 (5%) 0.05 (4%) 0.03 (2%) 0.01 (2%) 0. 03 (3%) > 1, 000 Å 0. (11%) 0. 47 (36%) 0.48 38% 0.04 4%) 0. 19 (19%) Pore radius maximum 1,000 2,500 5, 000 5, 000 (large dis- > 1,000 Å tribution) Pore radius maximum 50 68/49 50/35 70/40 117 < 1, 000 Å Temperature [°C] 300 325 300 325 325 325 325 Feed quantity [ml/min] 2 2 2 2 2 2 2 Composition [wt% l 1-Hexanol 44.22 24.77 35.21 19.03 21.14 21.83 24.25 (educt 99. 40) Dihexyl ether 32. 23 14.78 38. 65 18.75 15.40 14. 33 13. 76 1-Hexene 20. 704 52.551 24.517 58.421 58. 840 58.653 55.90 cis-2-Hexene 0. 518 4.210 0.412 1.839 2.320 2.668 3. 49 trans-2-Hexene 0. 180 1.483 0. 116 0.530 0.826 1. 007 1.32 cis-3-Hexene 0. 013 0. 118 0. 010 0.046 0.066 0.085 0.08 trans-3-Hexene 0. 011 0.103 0.008 0. 037 0. 056 0. 076 0. 19 2-Ethyl-1-butene 0.013 0.015 0.016 0.025 0.024 0.022 n. a. 3-+4-Methyl-1-pentene 0.003 0.009 0. 003 0.008 0.009 0.009 n. a. 3-Methyl-2-pentene 0.003 0.012 0.002 0.009 0.008 0.010 n. a. Total of other hexenes------0. 31 Total of C6 olefins 21. 45 58. 50 25.08 60.92 62.15 62. 53 61. 29 Conversion [%] 55.52 75.08 64.58 80.86 78.74 78.04 75.60 α-Selectivity [%] 96.54 89.83 97.74 95.91 94.68 93.80 91.21