Claims
- 1. A process for producing tertiary olefins comprising: decomposing an alkyl-tertiary alkyl ether in the presence of an alumina-modified silica catalyst at a pressure of 1 to 10 kg/cm.sup.2, a temperature of up to 500.degree. C., and a Liquid Hourly Space Velocity of 0.5 to 200 h.sup.-1, wherein the catalyst has an alumina content of about 0.5% by weight relative to the silica content and is prepared from high purity silica, containing not more than 0.05% by weight Na.sub.2 O, not more than 0.15% by weight SO.sub.4, and not more than 0.30% by weight Al.sub.2 O.sub.3 initial impurities.
Priority Claims (1)
| Number |
Date |
Country |
Kind |
| 21450 A/85 |
Jul 1985 |
ITX |
|
Parent Case Info
This application is a continuation of U.S. patent application Ser. No. 07/337,337, filed Apr. 13, 1989, which is a continuation of U.S. patent application Ser. No. 07/026,909, filed Feb. 27, 1987, both of which are now abandoned.
The present invention relates to a process for the production of pure tertiary olefins, by the decomposition of the corresponding alkyl-tert.alkyl-ethers.
Several methods are known for the obtainment of tertiary olefins.
For example, some of such methods are based on the use of H.sub.2 SO.sub.4 which however, besides the corrosion and pollution problems, show several drawbacks, among which the need for concentrating the acid before recycling it. Other methods are based on the decomposition of the corresponding methyl-ethers in the presence of suitable catalytic systems.
However, the use of the herein mentioned catalysts causes, in the most of the cases, the formation of dialkyl-ethers due to the dehydration of the corresponding primary alcohols.
The higher the operating temperature, the more easily this reaction occurs; some of the catalysts of the prior art require rather high temperatures to be used, which leads to a loss of alcohol, with the consequent need for further alcohol to be fed to the initial etherification reaction.
Furthermore, the formation of dialkylether requires a greater complexity at the level of reaction facilities, because e separation of the dialkyl ether from the tertiary olefin becomes necessary. And still furthermore, the formation of a considerable amount of dialkyl-ether renders necessary also the primary alcohol to be hydrated before being recycled, otherwise in the etherification reaction phase separation and possibility of formation of tertiary alcohols would occur.
A further drawback, to be faced when the reaction is carried out beyond certain temperature limits is given by the occurrence of dimerization and trimerization of the tertiary olefin recovered from the decomposition of the ethers.
Some drawbacks are overcome when the reaction of decomposition of the tert.alkyl ethers is carried out in the presence of a catalytic system constituted by activated alumina modified by the partial substitution of superficial --OH groups with silanol groups, according to as disclosed in Italian patents Nos. 1,001,614 and 1,017,878, in the name of the present Applicant.
Unfortunately, the activated alumina modified according to as disclosed in the above mentioned patents causes, already in case of a small increase in reaction temperature, the formation of dialkyl-ether, with the consequent reduction of the recovery of the primary alcohol to be recycled.
In the U.S. Pat. No. 4,254,296 always in the name of the same Applicant, to the contrary, a catalyst is used, selected among a crystalline silica modified with oxides of metal cations, such as aluminium and boron, by which much higher performances are achieved than by the activated alumina modified by silanolic groups.
Such a material has however an extremely high production cost, and is difficult to prepare.
Furthermore, the catalyst constituted by silicized alumina does not have a long life, because it does not succeed in limiting the side products within such limits as to allow financially interesting quantitative recoveries of the products obtained. Greater amounts of dimethyl-ether correspond to greater amounts of pure methanol, and to greater amounts of isobutene going lost during the distillation and separation step.
We have surprisingly found that it is possible to overcome the drawbacks of the prior art by using a catalyst which, besides having a low cost and being simple to prepare, has good catalytic performance.
The process of the present invention for the production of tertiary olefins consists of reacting the corresponding alkyl-tert.alkyl-ethers in the presence of a catalyst comprising silica modified by the addition of alumina in an amount comprised within the range of from 0.1 to 1.5% by weight relative to the silica.
In particular, said process can be used for the purpose of obtaining isobutene from the breakage of methyl-tert.butyl-ether.
The reaction of decomposition of the alkyl-tert.alkyl-ethers is carried out at temperatures equal to or lower than 500.degree. C., preferably within a range of from 130.degree. to 350.degree. C.
The reaction is generally carried out under pressures comprised within the range of from 1 to 10 kg/cm.sup.2, preferably under a pressure at least equal to the vapour pressure of the olefin to be recovered at the condensation temperature provided.
The space speed expressed as the volume of liquid per volume of catalyst per hour (LHSV), at which the reaction is carried out, is comprised within the range of from 0.5 to 200 h.sup.-1, preferably of from 1 to 50 h.sup.-1.
The catalyst can be simply prepared by starting from a preformed silica having the necessary purity (which is a product available from the market), then limiting the operations to the impregnation, drying and calcination. The impregnation is carried out with a solution of a salt of aluminium (for example, a nitrate or an isopropoxide), so to give the desired end content of alumina.
The primary alcohols which can be recovered at the end of the decomposition process of the invention contain preferably from 1 to 6 carbon atoms.
The process of the present invention can be used for recovering the tertiary olefins from mixtures of olefins from C.sub.4 to C.sub.7 such as, for example, those outcoming from thermal cracking, steam cracking or cat cracking.
Among the various tertiary olefins which can be obtained in the pure state, isobutylene, isoamylenes such as 2-methyl-2-butene and 2-methyl-1-butene, isohexenes, such as 2,3-dimethyl-1-butene, 2,3-dimethyl-2-butene, 2-methyl-1-pentene, 2-methyl-2-pentene, (cis and trans) 3-methyl-2-pentene, 2-ethyl-1-butene and 1-methyl-cyclopentene, or finally the tertiary isoheptenes.
The conversion of the tert.alkyl ether into primary alcohol or tertiary olefin is practically quantitative.
The formation is observed of very minor amounts of dimers and trimers of the tertiary olefin recovered, while no tertiary alcohol is formed.
The operating modalities and the advantages of the process according to the present invention shall be more evident from the examination of the following illustrative Examples, by which however the invention is not to be considered as being limited.
US Referenced Citations (1)
| Number |
Name |
Date |
Kind |
| 4254296 |
Manera et al. |
Mar 1981 |
|
Continuations (2)
|
Number |
Date |
Country |
| Parent |
337337 |
Apr 1989 |
|
| Parent |
26909 |
Feb 1987 |
|
Patent Grant
5043519
