Process for cleaving alkyl tert.-alkyl ethers

Alkyl tert.-alkyl ethers can be cleaved into the underlying alkanols and tert.-olefins in the presence of strongly acidic substances in a column apparatus, the strongly acidic substance being made available at the foot of the column. A particularly advantageous process design results when a water stream is fed below the top column tray in countercurrent to the tert.-olefin flowing upwards and is taken off again from the column above the bottom circulation; in the case of using an insoluble, strongly acidic substance, the latter is located in a catalyst bed at the foot of the column, and the alkyl tert.-alkyl ether is preferably fed in between the catalyst bed and the bottom circulation evaporator.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for cleaving alkyl tert.-alkyl 
ethers into the underlying alkanols and tert.-olefins in a column 
apparatus. 
It is already known (DE-OS (German Published Specification) 3,210,435) to 
cleave methyl tert.-butyl ether (MTBE) in a column apparatus into methanol 
and i-butene, a cation exchanger based on sulphonated 
styrene/divinylbenzene resins being arranged as a catalyst in various 
column sections, between each of which column sections with 6 distillation 
trays but without cation exchanger are located. The MTBE is fed above the 
top catalyst bed. The flow of the liquid MTBE fed in and of the methanol 
produced during the reaction is downwards through the catalyst bed, and 
the gaseous i-butene formed, if appropriate as an azeotrope with methanol, 
flows upwards through the catalyst bed. With respect to the reaction 
conditions, the above DE-OS (German Published Specification) gives an 
indication of 5 bar and a column temperature between 45.degree. C. (top 
temperature) and 100.degree. C. (bottom temperature). In such a process, 
it is to be expected that 1. the column will easily flood due to the dense 
catalyst packing and hence severely lose separation effect and 2. the 
catalyst bed is continuously turned over by the liquid and gaseous streams 
flowing through in opposite directions, channel formation in the catalyst 
beds being highly probable. As a result of such channel formation, the 
major part of the catalyst mass would not come into contact with the 
substances flowing through, and its utilization would thus be completely 
unsatisfactory. The assembly of such a column apparatus, the introduction 
of the catalyst into the envisaged column sections and, of course, any 
replacement of the catalyst are involved and hence expensive. i-Butene, 
which is first formed in the lower catalyst layers, must flow on its 
upward path through all the catalyst layers and beds located above; during 
this, undesired dimerization to give diisobutylene cannot be excluded, 
which reduces the i-butene yield and causes additional difficulties in 
working up the bottom product. Similarly, methanol produced by cleavage in 
the upper catalyst layers is subject to the risk or formation of dimethyl 
ether on the acidic catalyst resin on its path to the column bottom. The 
formation of undesired by-products is further promoted by the high 
temperature in the lower part of the column. 
SUMMARY OF THE INVENTION 
A process for cleaving alkyl tert.-alkyl ethers into the underlying 
alkanols and tert.-olefins in the presence of strongly acidic substances 
in a column apparatus has been found, which is characterized in that the 
strongly acidic substance is made available at the foot of the column. 
BRIEF DESCRIPTION OF THE DRAWING 
The invention may be carried out in a column (1) provided with distillation 
and or extraction devices and further provided in the bottom circulation 
with a bed of a polymeric, strongly acidic substance. The alkyl 
tert.-alkyl ether to be cleaved is fed via the line (10); the tert.-olefin 
produced is taken off al the column top via (11). 
DETAILED DESCRIPTION OF THE INVENTION 
The primary or secondary C.sub.1 -C.sub.4 -alkanols such as methanol, 
ethanol, propanol, i-propanol, n-butanol or sec.-butanol may be mentioned 
as examples of underlying alkanols. The primary alkanols and, amongst 
these, methanol or ethanol in turn may be mentioned as being preferred; 
methanol is particularly preferred. 
C.sub.4 -C.sub.7 -tert.-Olefins such as i-butene, i-amylenes, i-hexenes or 
i-heptenes may be mentioned as examples of underlying tert.-olefins. 
i-Butene and i-amylenes may be mentioned as being preferred. 
The possible alkyl tert.-alkyl ethers are therefore: methyl tert.-butyl 
ether (MTBE), tert.-amyl methyl ether (TAME), methyl tert.-hexyl ether, 
methyl tert.-heptyl ether, ethyl tert.-butyl ether, ethyl tert.-amyl 
ether, ethyl tert.-hexyl ether, ethyl tert.-heptyl ether and others. 
The strongly acidic substances which can be used are both soluble, 
low-molecular substances of inorganic or organic nature and insoluble, 
highly polymeric substances of inorganic or organic nature. The first 
group includes, for example, sulphuric acid, phosphoric acid, 
benzenesulphonic acid, toluenesulphonic acid, formic acid, 
trifluoromethanesulphonic acid and the like; within the range of their 
solubility, they effect the homogeneous catalytic cleavage of the said 
ethers. The second group includes substances such as acidic SiO.sub.2, 
acidic Al.sub.2 O.sub.3, polyphosphoric acids, certain fluorides, acidic 
zeolites such as mordenites and the like, and also neutral substances 
which have been treated with strong acids, such as silicas impregnated 
with H.sub.2 SO.sub.4 or H.sub.3 PO.sub.4, aluminas or other inorganic 
support materials, as well as sulphonated coals, and also organic, 
strongly acidic cation exchangers, all the substances mentioned being 
understood as the H.sup.+ form; they effect a heterogeneous catalytic 
cleavage of the said ethers. 
Amongst the said substances, the insoluble, highly polymeric substances are 
preferred, and particularly the strongly acidic cation exchangers in the 
H.sup.+ form. 
Examples of strongly acidic cation exchangers which can be used are those 
based on styrene/divinylbenzene resins, phenol/formaldehyde resins or 
coumarone/indene resins, the aromatic nuclei of which carry sulphonic acid 
groups. Preferably, the sulphonated styrene/divinylbenzene resins which 
are obtainable under various trade names can be used. These cation 
exchangers are used in the H.sup.+ form. 
The column apparatus used can be an apparatus such as is known to those 
skilled in the art for distillation and/or extraction purposes and is 
appropriately equipped. 
According to the invention, the strongly acidic substance is made available 
at the foot of the column. Preferably, the strongly acidic substance is 
located in the bottom heater circulation of the column. 
Soluble, low-molecular, strongly acidic substances can, for example, be fed 
into the lower part of the column, separately from or together with the 
ether which is to be cleaved. Theoretically, a single feed of the strongly 
acidic substance, which remains at the foot of the column, suffices, while 
the ether is decomposed into its cleaveage products and leaves the column 
in this form over the top thereof or at points above the bottom of the 
column; in practice, however, in order to remove undesired byproducts, a 
small purification stream is taken off at the foot of the column, and this 
always contains a little strongly acidic substance as a loss. Such losses 
are compensated by re-addition of the strongly acidic substance. 
Insoluble polymeric substances of the said type do not show any losses via 
a purification stream and are therefore preferred. This applied to a 
particular extent to strongly acidic cation exchangers, which are 
therefore taken below as examples for the further description of the 
process according to the invention. 
According to the invention, the bottom circulation thus always flows 
alternately through the cation exchanger as an example of an insoluble, 
polymeric, strongly acidic substance and through the indirect column 
heater which is operated electrically or by means of a separate heat 
carrier stream (steam, heat transfer fluid). The bottom circulation can 
here be effected by the convection caused by the bottom heater or as 
forced circulation by means of pumps. The forced circulation effected by a 
pump can here also be against the convection effect, so that it is 
possible in principle to recycle the bottom circulation into the column 
below its take-off point, so that the flow to the cation exchanger 
location in the bottom is from below. It is preferred, however, for the 
flow to the cation exchanger to be from above, and this can be effected by 
convection or by forced circulation. 
In a surprisingly advantageous manner, the arrangement, according to the 
invention, of the cation exchanger as an example of an insoluble, 
polymeric, strongly acidic substance permits rapid removal of the cleavage 
products from the cation exchanger, so that secondary reactions of the 
tert.-olefin (to give the dimer) or of the alkanol (to give the dialkyl 
ether) can be effectively suppressed. Particularly in the case of the 
cleavage of methyl tert.-alkyl ethers, the formation of dimethyl ether, 
which makes the preparation of pure cleavage products very difficult, is 
suppressed. Furthermore, the immediate vicinity of the cation exchanger 
provided and the bottom heater makes it possible to set the temperature 
required for the cleavage of the alkyl tert.-alkyl ethers with very much 
greater accuracy and thus to avoid unnecessary thermal stresses on the 
reaction products. 
The foot of the column, preferably the bottom circulation, is at a 
temperature of, for example, 50.degree.-100.degree. C., preferably 
55.degree.-85.degree. C., particularly preferably 60.degree.-80.degree. C. 
In the case that alkyl tert.-alkyl ethers having more than 6 C atoms in 
total are cleaved, it may be necessary to raise the upper limit of the 
said temperature ranges by 10.degree.-20.degree. C. When the 
abovementioned inorganic, insoluble, strongly acidic substances are used, 
it can also be advantageous to employ temperatures of up to 200.degree. 
C., preferably up to 160.degree. C. In the case of the lower-boiling 
ethers amongst those mentioned above, the process can then be carried out, 
in the manner known to those skilled in the art, under such a pressure 
that a liquid phase is maintained at the foot of the column. 
It is preferred to operate within the said temperature range, which may be 
the range of 50.degree.-100.degree. C. or, in the case of higher ethers, a 
range extended upwards by 10.degree.-20.degree. C. or, finally, the range 
extending up the 200.degree. C., as described, at the maximum boiling 
point of the reaction mixture circulating in the bottom, which temperature 
is established as a result of the remaining column conditions. This 
ensures the most rapid degassing possible of the gaseous cleavage products 
flowing off upwards. Under the column conditions to be taken into account, 
special reference may be made to the pressure to be established. The 
process according to the invention can in principle also be carried out 
under a reduced pressure, but such a reduced pressure is less preferred 
because of the difficult condensation of the tert.-olefin at the column 
top, and it is used only in the case of tertiary olefins having more than 
6 C atoms. The column is therefore preferably operated under normal or 
slightly elevated pressure, for example 1-5 bar. When operating under 
normal pressure, however, the column bottom is subject at least to the 
differential pressure known to those skilled in the art and depending on 
the column height. 
The arrangement, according to the invention, of the cation exchanger allows 
flow velocities through the bottom circulation within a very wide range. 
These flow velocities are defined as LHSV (Liquid hourly space velocity) 
and assume values of a=1-100, preferably 10-50, liters of bottom 
circulation per liter of cation exchanger per hour. The ether to be 
cleaved is added to the bottom circulation at a rate of 0.5-10, preferably 
3-8, l of ether per l of cation exchanger per hour. 
The tert.-olefin formed by the cleavage is taken off from the process 
according to the invention at the top of the column, if appropriate as an 
azeotrope with the alkanol produced at the same time. The major part of 
the alkanol produced by the cleavage is taken from the bottom outflow, 
together with small quantities of uncleaved ether and small quantities of 
by-products. 
In an advantageous variant of the process according to the invention, a 
water stream is fed to the column below the upper column trays. This water 
stream effects washing of the tert.-olefin taken off at the column top and 
allows the alkanol produced in the cleavage to be taken off as a mixture 
with water from one of the lower trays above the column bottom. The feed 
of the alkyl tert.-alkyl ether to be cleaved is preferably below the 
take-off of the said water stream. The water rate here amounts to 2-6 
parts by weight per 1 part by weight of the quantity of alkanol to be 
washed out. 
In this advantageous process variant, only a small purification stream is 
taken from the bottom outflow, and this consists essentially of 
unconverted alkyl tert.-alkyl ether and small quantities of high boilers.