CA2089982A1

CA2089982A1 - Preparation of ethylene oxide

Info

Publication number: CA2089982A1
Application number: CA002089982A
Authority: CA
Inventors: Klaus Herzog; Stefan Boeck; Wolf Dieter Mross; Juergen Plueckhan; Karl-Heinz Boehning; Matthias Schwarzmann
Original assignee: Individual
Current assignee: BASF SE
Priority date: 1992-02-20
Filing date: 1993-02-19
Publication date: 1993-08-21
Also published as: EP0557833B1; DE59303461D1; JPH069590A; EP0557833A1

Abstract

O.Z. 0050/43029 Abstract of the Disclosure: Ethylene oxide is prepared by oxidizing ethylene with oxygen in the presence of silver-and promoter-containing catalysts using at least two silver catalysts of different selectivity and activity in a combined catalyst bed, wherein the gaseous reaction mixture comes into contact with silver catalysts of increasing activity and decreasing selectivity on passage through the reactor.

Description

2 0 ~ ~ 9 ~ ~ o. z . 0050/43029 The Preparation of ethy~ x1~
The present invention relates to a process for preparing ethylene oxide by oxidizing ethylene with oxygen in the presence of silver- and promoter-containing cataly~ts using at least two silver catalysts of different selectivity and activity in a combined catalyst bed.
Ethylene oxide is an important basic chemlcal and i5 prepaxed industrially by oxidizing ethylene with oxygen in the presence of silver- and promoter-containing catalysts, using at least two silver catalysts of different selectivity and activity in a combined catalyst bed. Supported catalysts are conventionally used for this, with the catalytically active metallic silver being applied by a suitable process (cf., for example, US-A 2 294 383, US-A 3 423 328 and US-A 3 172 893). The carrier generally used is ~-alumina, but it is also possible in principle to employ other porous materials such as active carbon, dioxides of titanium, zirconium or silicon, or other ceramic compositions. Auxiliaries applied in small amounts to the carrier to improve the catalytic properties are, in particular, alkali metal or alkaline earth metal compounds, particularly preferably cesium compounds (cf., for example, DE-A 23 00 512, DE-A 25 21 906 and DE-A 27 53 359). Some publications disclose the use of other additives such as tungsten and molybdenum (D2-A 24 54 972, JP-A 105 750 (1981), EP-A 357 293 and EP-A 11 356). ~nother substance used for doping silver catalysts to influence their activity and selectivity is rhenium (EP-A 266 015).
JP-A 5471 (1981) describes a process for preparing ethylene oxide which uses silver catalysts which contain alkali metals and which are loaded with differen~ amounts of alkali metals along the length of the catalyst bed in such a way that there is an alkali metal concentration gradient along the bed. According to this publication, the alkali metal content of the silver 2 ~
- 2 - O.Z. 0050t43029 catalyst should be lower at the upper end of the bed, ie.
where the reactant gas enters, than at the lower end, ie.
where the gas exits. Since the doping of the silver cataly~t with alkali metal results in a decrease in S activity and an increase in selectivity for the formation of ethylene oxide, the gaseous reaction mixture comes into contact with increasingly selective and decreasingly active silver catalyst on passage through the reactor.
The selectivities which can be achieved with this process are about 70~, which is thoroughly unsatisfactory and the reason why this process has not become industrially important.
Catalysts which contain rhenium or other tran-sition metal promoters have no~ to date been used industrially as widely as was expected on the basis of the good selectivities achieved with these novel cata-lysts. The reason for this is that additives to the catalytically active silver lower its activity so that higher temperatures are necessary in the reactor to achieve an adequate conversion and thus an economically satisfactory ethylene oxide production, and these tem-peratures are in a range which is difficult to achieve in existing industrial plant for design reasons.
Another serious disad~antage of the use of high temperatures is that they increase the rate of catalyst aging. The consequence is a reduction in the useful life of the catalyst, which makes it necessary~to change the catalyst more often, and this not only increases the catalyst costs but also leads ts large losses of production. These adverse side effects are particularly pronounced with catalysts doped with molybdenum, tungsten and/or rhenium which are particularly recommended because of the improvement in selectivity they produce.
It is an object of the present invention to find 3S a process for preparing ethylene oxide which makes it possible to utilize the selectivity advantages of cata-lysts with reduced activity without the n~cessity fsr 208~9~2 - 3 - O.Z. 0050/43029 disadvantageous high tempera~ures and which improves the economics of ethylene oxide preparation.
We have found that this object is achieved by a process or preparing ethylene oxide by oxidizing ethy-S lene with oxygen in the presence of silver- and promoter-containing catalysts using at least two silver catalysts of different selectivity and activity in a combined catalyst bed, wherein the gaseous reaction mixture comes into contact with silver catalysts of increasing activity and decreasing selectivity on passage through the reactor.
Thus ~ according to the invention more selective but less active silver catalysts are combined with less selective but more active silver catalysts in the reac-tor. The sequence of the catalysts through which thereactant gases pass is from the more selective to the less selective. It is possible to use as many different silver catalysts as required in a combined bed. The arrangement of many catalyst layers or the use of more 2~ selective and less selective silver catalysts in defined mixing ratios which change along the length of the reaction tube, ie. along the length of the catalyst bed, also make it possible to construct a gradient in the catalyst bed so that the reactant gases come into contact on passage through the catalyst bed with silver catalysts of continuou ly decreasing selectivity.
Th~ simplest and most preferred embodiment of the process according to the invention is to use only two different silver catalys~s. These silver catalysts may di~fer in- their chemical composition, but it is also advantageous to combine beds of silver catalysts which differ only in their activity and selectivity with regard to the formation of ethylene oxide, for example by combining a used silver catalyst which has lost selectivity and/or activity during its use for preparing ethylene oxide with a freshly produced silver catalyst which has the same chemical composition and has been 2~99~

- 4 - O.Z. 0050/43029 prepared in the sa~e way as the used silver catalyst.
It is, o course, also possible according to the invention to use different mixtures of silver catalysts in combined catalyst beds for preparing ethylene oxide.
The comhined silver catalyst beds according to the inven-tion can be produced in the simplest case by placing two or more layers of the different catalysts in succe~sion in the reaction tube.
It may also prove particularly advantageous for the selectivity of ethylene oxide preparation to keep the different catalysts in the combined catalyst beds in the reactor at different temperatures during the ethylene oxide preparation, so that each silver catalyst can be operated at an individually adjustable temperature.
Structured temperature control of this type can be easily achieved with reactors provided, for example, by DE-A 2~ 01 528 and DE-A 28 30 765.
Conventional industrial reactors for preparing ethylene oxide can be used particularly straightforwardly for that embodiment of the process according to the invention in which only two different catalysts are com-bined in two beds. This embodiment of the process accord-ing to the invention is particularly preferred because of the ease of implementation and the very good results achieved in this way. The arrangement of the different catalysts in the combined bed which is generally par-ticularly preferred in this procedure is such that the more selective but less active catalyst comes in~o con-tact with the reactant gases first, ie. is located nearer the reactant gas inlet in the reactor, whereas the bed of the more active but less selective catalyst is advan-tageously located nearer the reactant gas outlet from the reactor so that the reactant gases pass through it only after they have passed through the layer of less active catalyst.
The advantage of the combined silver catalyst beds according to the invention for preparing ethylene 2~8998~

- 5 - O.Z. 0050/43029 oxide is that the increases in selectivity which can be achieved by using more ~elective but less active cata-lysts can be utilized without having to accept the disad-vantage of lower activity compared with less selective but more active catalysts Furthermore, the high optimal temperatures which have to be employed in the preparation of ethylene oxide when moxe selective but less active silver catalysts are used alone can be considerably reduced when the~e ~ilver catalysts are employed in com-bined catalyst beds with more active but less selective silver catalysts. In addition, the use, according to the invention, of combined catalyst beds has the very par-ticular advantage that the selectivity of more selective but less active silver catalysts decreases considerably more slowly during use than when a catalys~ of this type is employed alone, ie. not in a combined bed with another catalyst, which means that the useful life of the rele-vant catalyst is considerably increased. These advantages of using a combin~d catalyst bed lead to a considerable improvement in the economics of the process for ethylene oxide preparation.
Silver catalysts which can be used in the process according to the invention are all silver-containing supported catalysts suitable for preparing ethylene oxide from ethylene and oxygen. The carrier can in principle be any porou~ material which is stable under the conditions of ethylene oxide synthesi , for example active carbon, aluminas, dioxides of ti~anium, zirconium or silicon, or other ceramic compositions.
The geometric shape of the carrier particles is genexally of minor Lmportance, but the carrier particles should expediently have shapes which allow unhindered diffusion of the reactant gases to a maximum proportion of the outer and inner surfaces of the carrier particles which are coated with the catalytically active silver particles which are undoped or doped with additives.
~he ca~alytically active components of the silver 208~2 - 6 - O.Z. 0050/~3~29 catalysts used in the pxocess according to the invention, ie. silver and doping substances if added, can be applled to the carrier using all prior art impregnation and deposition processes for preparing silver catalysts for S preparing ethylene oxide, and these processes can com-prise one or more impregnation and calcination stages.
Examples of processes for preparing silver catalysts are those disclosed in DE-A 23 00 512, DE-A 25 21 906, EP-A 14 457, EP-A 85 237, EP-A 384 312, DE-A 24 54 972, DE-A 33 21 895, EP-A 229 465, DE-A 31 50 205, EP-A 172 565 and EP-A 357 293. There are in principle no restrictiQns on the doping of the silver catalysts which can be employed in the process according to the invention with additives which, inter alia, influence the activity, selecti~ity and useful life of the sil~er catalysts, called promoters, ie. it is possible to use all prior art promoters for doping the silver catalysts. Particularly suitable promoters are alkali metal and alkaline earth metal hydroxides or salts and the compounds of elements of group VIb and VIIb of the periodic table, especially compounds of tungsten, molybdenum and/or rhenium.
There are likewise no restrictions on the anions of the salts of the promoters, for example it is possible for all halides, especially fluoride, chloride, carboxylates, nitrates, sulfur-containing anions, such as sulfate or sulfide, phosphates, cyanide, hydroxide, - carbonat~ or anions of heteropoly acids, especially of heteropoly acids of elements of groups VIb and VIIb of the periodic table, particularly preferably anions of he~eropoly acids of tungsten, molybdenum and/or rhenium, to be anions of these salts.
Examples of silver catalysts which are doped with promo~er~ and which can be employed in the process according to the invention are the silver catalys~s of DE-A 23 00 512, DE-A 25 21 906, EP-A 14 457, DE-A 24 54 972, EP-A 172 565, EP-A 357 293, EP-A ~66 015, EP-A 11 356, 2~98~
_ 7 - OOZ. 0050/43029 EP-A 85 ~37, DE-A 25 60 684 and DE-A 27 53 359. To illus-tra~e catalysts which can be employed in the process according to the invention, mention may be made of the examples of silver cataly~ts with a silver content of from S to 50~, in particular from 6 to 30~, oE the weight of the complete catalyst, a content of the heavy alkali metals rubidium and/or cesium of from 1 to 5000 ppm, a tungsten content of from 1 to 5000 ppm, a molybdenum content of from 1 to 3000 ppm and/or a rhenium content of from 1 to 10,000 ppm of the weight of the complete catalyst. The ~-alumina which is the carrier advantageously used for preparing silver catalysts of this type preferably has a BET surface area, determined by the method of Brunauer et al., J. Am. Chem. Soc. 60 (1938) 309, of from 0.1 to 20 m2/g, a porosity measured by mercury porosimetry of from 10~ to 90~ and a cold water uptake in 5 minutes at 20C of from 0.1 to 0.9 ml/g.
The silver catalysts which are disclosed in the publications mentioned by way of example in the previous paragraph and which have the contents of silver and promoters stated therein and were prepared by the impreg-nation, drying, silver-decomposition and calcination processes stated therein can all be successfully used in the process according to the invention. Thus, it is not the nature of the catalysts employed in each case which is critical for success of the process according to the invention but solely the use of combined ~beds of cata-lyst~ which differ in their selectivity and activity.
Only by this measure is it possible to achieve the above-mentioned advantages, eg. the increase in the useful lifeof the relevant silver catalysts.
The combined silver catalys~ beds according to the invention can be used to produce ethylene oxide by the direct oxidation of ethylene with oxygen by conven-tional methods. All the reactors which can be used in theprior art processes for ethylene oxide preparation can be employed for this, for exampls the conventional 208~2 - 8 - O.Z. 0050/43029 industxial externally cooled multitubular reactors (cf.
Ullmann's Encyclopedia of Industrial Chemistry; 5th Ed.;
Vol. A10; 117-135, 123-125; VCH V~rlagsgesellschaft;
Weinheim 1987) and reactors with a loose catalyst bed and cooling pipes, for example the reactors disclosed in DE-A 34 14 717, EP-A 82 609 and EP-A 339 748. The com-bined catalyst bed is produced by simply introducing the different catalysts in the .required sequence into the relevant reactor.
The preparation of ethylene oxide from ethylene and oxygen u~ing the catalyst beds according to the invention can take place under conventional conditions as described, for example, in DE-A ~5 21 906, EP-A 14 457, DE-A 23 00 512, EP-A 172 565, DE-A 24 54 972, EP-A 357 293, EP-A 266 015, EP-A 85 237, EP-A 82 609 and EP-A 339 748. The reactant gas containing ethylene and molecular oxygen can moreover be mixed with inert gases such as nitrogen or gases which are inert under the reaction conditions, such as steam, methane and, if required, reaction moderators (inhibitors), for example halohydrocarbons such as vinyl chloride or 1,2-dichloro-ethane. The oxygen content of the reactant gas is ex-pediently in a range such that the gas mixture is not explosive~ An example of a suitable reactant gas com-position for preparing ethylene oxide is about 30% by volume ethylene, about 8% by volume oxygen, ~rom 0.5 to 5 ppm of a chlorine-containing inhibitor such as vinyl chloride or dichloroethane, with the remainder of the reactant gas usually being composed of hydrocarbons such as methane or ethane or else of inert gases such as nitrogen. It is also possible for other substances such as steam, carbon dîoxide or noble gases to be present in the reactant gas. The oxidation is generally carried out at ~rom 165 to 300C.
The preparation of ethylene oxide from ethylene and oxygen can advantageously be carried out in a cir-culating process in which the reactant gas mixture is _ 9 2 ~ 8 9 9 8 2 o. z . 0050/43029 circulated through the reactor, and the ethylene oxide and byproducts ~ormed in the reaction are removed from the product gas stream after each passage, and the latter is then replenished with the required amounts of ethy-lene, oxygen and xeaction moderators and returned to thereactor. The ethylene oxide can be removed from the product gas stream and worked up by the conventional prior art processes (cf. Ullmann's Encyclopedia of Industrial Chemistry; 5th Ed.; Vol. AlO; 117-135, 123-125; VCH Verlagsgesellschaft; Weinheim 1987).
EXANPLES
The comparative experiments to determine the selectivity and activity of the individual silver cata-lysts and of the combined catalyst beds according to the invention were carried out in the following way:
The uncomminuted catalysts were packed in a total amount of 13 dm3 into a steel reactor. The reactor was equipped with a cooling ~acket through which the temperature-controlling liquid was passed. The reactant 0 gas had the following composition:
% by volume ethylene 8 ~ by volume oxygen ca. 6.5 ~ by volume carbon clioxide ca. 4 % by volume argon ca. 4 % by volume steam ca. 3 ppm vinyl chloride remainder methane.
The term "ca." means that the content of these components in the reactant gas s~ream may vary by ~ 20%
during the test period.
The pressure in the reactor was adjusted to 16 bar. The temperature of the cooling liguid was adjus-ted so that at a space velocity of 3300 m3(STP)/m3 of catalyst x h an oxygen co~version of 35~ was achieved.
Samples were taken after continuous operation for 4 and 50 days, and the selectivity of the oxidation of ethylene to ethylene oxide and the activity of the catalyst, lo 2 ~ 8 9 9 8 ~7~o . Z . 0o5o/43o2g expressed a~ the temperature required for 35~ oxygen conversion, were determined.
CATALYSTS:
The carrier employed for all the catalysts was ~-alumina with a purity grea~er than 99~. The carrier was employed in the form of rings with the dLmensions 8 x 8 x 2 mm (total diameter x height x wall thickness). The characterlstic data for the carrier are listed in Table I.

Table I: Characteristic data for the carrier Si content (% by wt.) 0.28 Soluble ions (% by wt.) Al 370 Ca 170 Na 84 BET area (m2/g) 0.8 Cold water uptake (20C, ml/g) 0.42 Porosity (~) 56 Average pore diameter (~m) 13.0 Abrasion (% by wt.) 2.7 The content of soluble ions was determined by accurately weighing about 10 g of carrier particles and subsequently boiling them with fifty percent concentrated nitric acid for 10 minutes. The resulting extract was filtered and then subjected to atomic absorption spec-trometry for quantitative determination of the specified elements. The SiO2 content of the ~-alumina was detenmined by measuring the extinction at 820 nm of ~he blue molybdato silicon complex formed after alkaline digestion of a sample of the carrier and reaction of the resulting solution with ammonium heptamolybdate.
CATALYST A:
100 parts by weight of the specified carrier were 2~99~2 ~ O.Z. 0050/43029 impregnated at room temperature with a solution contain-ing 27.4 parts by weight of silver nitrate, 0.292 part by weight of lithium nitrate, 0~0729 part by weight of cesium hydxo~ide, 24.2 parts by weight of sec-butyl~mine and 5.9 parts by weight oE water. The carrier impregnated in this way was subsequently converted into the finished catalyst in a conveyor furnace at 220C for 1.0 minutes.
CATALYST B:
Catalyst B was prepared by the procedure des-cribed for the preparation of catalyst A using the following starting materials:
100 parts by weight of carrier were impregnated with a solution composed of 27.5 parts by weight of silver nitrate, 0.292 part by weight of lithium nitrate, 0.0729 part by weight of ~esium hydroxide, 0.0106 part by weight of molybdenum trioxide, the latter dissolved in 1 part by weight of a 10~ by weight aqueous ammonia solution, 24.2 parts by weight of sec-butylamine and 4.9 parts by weight of water.
The contents o~ catalytically active metals in catalysts A and B are listed in Table II:

Table II: Contents of catalytically active metals in catalysts ~ and B

Catalyst A B
Ag [% by wt.] 14.B 14.8 Cs [ppm] 550 550 Li tppm] 250 250 Mo [ppm] 60 - -Catalysts A and B were investigated separately as described for their electivity and activity in the preparation of ethylene oxide from ethylene and oxygen.
The results of these experiments are listed in Table III
under headings A and B.

2~g99~2 - 12 - O.Z. 0050/43029 EXAMPLE 1 (ACCORDING TO THE INVENTION) The reaction tube was packed with a layer of catalyst ~ and a layer of the same amount of catalyst B, with catalyst A occupying the half of the catalyst bed on the gas outlet side. The results in the preparation of ethylene oxide using this combined bed are listed in Table III undor heading 1.
EXAMPLE 2 (ACCORDING TO THE INVENTION) The combined catalyst bed employed in this experiment comprised one third catalyst A and two thirds catalyst B, with the catalyst A layer occupying the third of the combined catalyst bed on the gas outlet side. The experimental results obtained with this co~bined bed are listed in Table III under heading 2.

Table III: Test results Catalyst A B 1 2 After operation for four days:
S [%] 80.2 81.4 ~1.0 81.2 a [C] 219 237 222 228 After operation for fifty days:
S [%] 80.0 80.8 80.8 80.9 a [C] 226 245 229 235 Losses of activity and selec~ivity:
S [%] - 0.2 - 0.6 - 0.2 - 0.3 a [C] - 7 - 8 - 7 - 7 S: selectivity a: activity COMPARATIVE EXAMPLE:
In order to show that the advantageous effects of the combined catalyst bed are attributable no~ to the shorter residence time, owing to the shortened bed of 2~89~82 - 13 ~ O.Z. 0050/43029 catalyst A, with, at the same time, an elevated reaction temperature, the comparative experiment described in the following paragraph was carried out. The temperature employed in this comparative experiment was distinctly lower than the optimal temperatu.re for catalyst B (237C) so that if th.is catalyst had been used in place of catalys~ A in this compara~ive experiment its activity would have been entirely inadequate, ie. virtually nonexistent.
The reaction tube was packed with a layer of ~-alumina, which is inert under the reaction conditions, and a bed composed of ca~alyst A. The ~-alumina bed occupied the third of the bed on the gas inlet side, while catalyst A occupied the two thirds of the bed on the gas outlet side.
The temperature required to achieve a 35% oxygen conversion in the preparation of ethylene oxide under the described conditions after operation for 4 days was 228C
(- the activity a), the selectivity for ethylene oxide being 80.4%.
DISCUSSION OF THE RESULTS OF THE EXANPLES:
The advantages of using a combined bed compared with the use of a single catalyst species are clearly evident from Table III:
The advantage of the combined catalyst beds of Examples 1 and 2, which ~re composed partly of a catalyst which contains selectivity-increasing and activity-decreasing promoters and partly of a silver catalyst doped only with alkali metals, compared with a bed composed of the molybdenum-doped catalyst B is that the activity is distinctly improved and the loss o selec-tivity during operation is less. This means that it is possible, as is evident from comparison of catalyst B
with the combined catalyst bed of Example 2, al~o to obtain the selectivity of the more selective but less active catalyst, employed alone, after a certain operation time (50 days) by the combined bed of catalysts , 2ass~2 - 14 O.Z. 0050/43029 of different selectivity and activity, moreover with dis-tinctly higher activity.

Claims

1. A process for preparing ethylene oxide by oxidiz-ing ethylene with oxygen in the presence of silver-containing catalysts using at least two silver catalysts of different selectivity and activity in a combined catalyst bed, wherein the gaseous reaction mixture comes into contact with silver catalysts of increasing activity and decreasing selectivity on passage through the reactor.

2. A process as claimed in claim 1, wherein at least two silver catalysts doped with different promoters are used.

3. A process as claimed in claim 1, wherein one or more of the silver catalysts is doped with at least one element of group VIb or VIIb of the periodic table.

4. A process as claimed in claim 1, wherein at least one of the silver catalysts is doped with tungsten, molybdenum and/or rhenium.

5. A process as claimed in claim 1, wherein at least one of the silver catalysts is doped with at least one alkali metal and/or alkaline earth metal.

6. A process as claimed in claim 1, wherein the various silver catalysts are arranged in at least two layers in the combined catalyst bed in the reactor.

7. A process as claimed in claim 1, wherein the individual silver catalysts in the individual layers in the combined catalyst bed are maintained at different temperatures.