Increasing the level of 2-methyl-2-butene in isoamylene

A method for increasing the ratio of 2-methyl-2-butene (2MB2) to 2-methyl-1-butene (2MB1) in isoamylenes involves fractionating a feedstream containing tertiary amyl methyl ether (TAME) and isoamylenes including 2MB2 and 2MB1 in a ratio of about 2:1 to effect a separation between an overhead hydrocarbon fraction of isoamylenes including 2MB2 and 2MB1 in a ratio of about 1:1, a bottoms fraction including TAME, and a sidestream hydrocarbon fraction consisting essentially of isoamylenes including 2MB2 and 2MB1 in a ratio of about 6 to 12:1, recovering the sidestream hydrocarbon fraction, and recycling the overhead hydrocarbon fraction of isoamylenes to form a mixture which is subsequently reacted to form the feedstream. Prior to fractionation, the feedstream is formed by passing isoamylene, and optionally TAME, in a vapor phase over an ether cracking catalyst which isomerizes isoamylene and converts 2MB1 to 2MB2, i.e., the feedstream for fractionating, which contains 2MB2 and 2MB1 in a ratio of 2 to 5:1. A method for converting 2-methyl-1-butene to 2-methyl-2-butene which may be used to form the feedstream for fractionating involves providing a hydrocarbon stream comprising isoamylenes including 2MB1 and 2MB2 in a ratio of within the range of 1:1 to 5, adding about TAME to the isoamylenes to form a mixture which is passed in the liquid phase at a temperature and a LHSV which favors isomerization over an acidic ion exchange resin catalyst to produce a reaction product including 2MB1, 2MB2 in a ratio of 1:6 to 12, and TAME.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to methods for increasing the level of 
2-methyl-2- butene in isoamylene, and particularly for converting 
2-methyl-1-butene isoamylene into 2-methyl-2-butene isoamylene. 
2. Discussion of Background and Material Information 
Isoamylene is a component of a C.sub.5 refinery stream. The C.sub.5 portion 
of such hydrocarbon streams typically contain at least two isoamylene 
monomers, i.e., 2-methyl-1-butene and 2-methyl-2-butene in a weight ratio 
of about 1:1 to about 1:4, and most often about 1:2, respectively. 
The separation of isoamylene from other C.sub.5 components by fractionation 
is somewhat difficult because of the closeness of their boiling points, 
i.e., less than about 10.degree. F. for many of these components. In order 
to recover the isoamylene content of such a mixture by conventional 
fractionation, a plurality of steps, for example as disclosed in U.S. Pat. 
No. 3,236,908, are typically reqired. Each of the fractionations resulting 
from the multi-step process, however, must be subjected to a further 
separation of components boiling within a relatively narrow range which 
requires the employment of complex and expensive equipment. 
U.S. Pat. No. 3,236,908, SANFORD et al., attempt to obviate the need for 
such complex and expensive fractionation equipment by providing a method 
for producing isoamylene, which is predominantly 2-methyl-2-butene, from 
the 2-methyl-1-butene present in catalytic gasoline in which a 
liquid-phase, ambient-temperature, selected isomerization step is used. In 
their process, the effluent from a catalytic cracking of gas oil is 
fractionated to produce an overhead fraction consisting essentially of 
2-methyl-1-butene and lower boiling C.sub.5 hydrocarbons substantially 
free of higher boiling materials. The fraction thus obtained is then 
admixed with sulfuric acid of from 60 to 70% by weight concentration with 
respect to water in order to isomerize 2-methyl-1-butene to 
2-methyl-2-butene. The sulfuric acid phase is separated from the 
hydrocarbon phase and the hydrocarbon phase is then fractionated to 
recover 2-methyl-2-butene as product. 
U.S. Pat. No. 4,447,668, SMITH, Jr. et al., are directed to a method for 
producing high purity tertiary C.sub.4 and C.sub.5 olefins by the 
disassociation of corresponding alkyl ethers and the subsequent 
dimerization of the olefins to produce high purity dimers thereof. In one 
embodiment of their process, a feed stream of C.sub.1 through C.sub.6 
alkyl tertiary amyl ether is vaporized and a feed stream in a vaporized 
state is passed through a fixed bed cationic acidic exchange resin whereby 
the ether is at least partially disassociated and the disassociation 
product stream from the catalyst bed contains isoamylene, alcohol 
corresponding to the alkyl radical and unreacted alcohol tertiary amyl 
ether. The alcohol is then removed from the disassociation product stream 
prior to fractionating the condensed stream, which is predominantly 
isoamylene and unreacted ether feed, to recover isoamylene. 
In addition to the foregoing, a number of other methods have been proposed 
for producing tertiary olefins from alkyl tert-alkyl ethers using various 
catalysts. 
For example, U.S. Pat. No. 4,398,051 uses aluminum compounds supported on 
silica or other carriers. U.S. Pat. No. 4,320,232 employs phosphoric acid 
on various supports. British Patent No. 1,173,128 uses metal-containing 
weakly acidic components on a carrier of 20M.sup.2 /gm surface area. U.S. 
Pat. No. 4,398,051 attempts to produce tertiary olefins from alkyl 
tert-alkyl ethers utilizing carriers alone in the decomposition of methyl 
tertiary butyl ether. To this end, U.S. Pat. No. 4,254,290 utilizes 
H.sub.2 SO.sub.4 -treated clay in the decomposition of t-alkyl 
ether-alkynols. 
U.S. Pat. No. 4,691,073, MICHAELSON discovered that high purity olefins are 
obtainable in extremely high yields over a sustained period by bringing 
alkyl tert-alkyl ethers into contact with a specified catalyst, i.e, clays 
treated with hydrofluoric acid and/or hydrochloric acid. 
S.U. 644,767, CHAPLITS, is directed to obtaining increased yields of 
2-methyl-2-butene by isomerization of 2-methyl-1-butene in the presence of 
a catalyst composed of a moulded sulpho-cation exchange resin with a 
thermoplatic material, such as polypropylene and polyethylene, in the 
presence of methyl-tert-amyl ether tert-amyl alcohol, ethyl alcohol 
acetone or mixtures thereof, used as 5-10% by wt. of the initial material 
at a temperature between 60.degree.-80.degree. C., and preferably 
70.degree.-80.degree. C., and a pressure of 2.5-4.5 atmospheres. 
It is known that tertiary olefins may be prepared by reacting them 
selectively from petroleum feeds with a primary alcohol in the presence of 
an acid catalyst to produce the corresponding alkyl tert alkyl ethers. 
Such alkyl tert-alkyl ethers may then be separated and subsequently 
decomposed back to the tertiary olefins and the primary alcohol. 
For example, European Patent Application No. 123,338, GROENEVELD, is 
directed to the process for preparation of methyl tertiary butyl ether 
(MTBE) by reacting isobutene with methanol in the presence of an acid 
catalyst to yield MTBE followed by conversion of normal butenes present in 
the hydrocarbon flow to isobutene followed by passing the mixture thus 
obtained to a reaction zone to form MTBE. 
SUMMARY OF THE INVENTION 
In accordance with the present invention there is provided a method for 
converting 2-methyl-1-butene to 2-methyl-2-butene which involves providing 
a hydrocarbon stream of isoamylenes including 2MB1 and 2MB2 in a ratio 
within the range of 1:1 to 5, to which a tertiary alkyl ether is added to 
form a mixture which is passed at a temperature of less than 60.degree. C. 
and a LHSV which favors isomerization over an acidic ion exchange resin 
catalyst to produce a reaction product including 2MB1, 2MB2, an ether, an 
alcohol and isoamylene dimer, preferably wherein the tertiary alkyl ether 
is a member selected from the group consisting of tertiary amyl methyl 
ether (TAME) and methyl tertiary butyl ether (MTBE), and most preferably 
wherein the alkyl tertiary ether is TAME. When TAME and MTBE are used, the 
alcohol in the reaction product is methanol (MeOH). The 2MB1 and 2MB2 are 
preferably present in the mixture in a ratio of about 1:5 and the mixture 
is preferably in a liquid phase. The temperature at which reaction is 
performed is below 60.degree. C. and preferably within the range of 
30.degree. C. up to 60.degree. C., and the LHSV is within the range of 1 
to 30 hr.sup.-1, preferably wherein the temperature is within the range of 
33.degree. C. to 55.degree. C., and the LHSV is within the range of 5 to 
15 hr.sup.-1. It is preferred that at least 3% by weight TAME, preferably 
in the range of 5 to 10%, be present in the mixture which may also 
include members selected from the group consisting of alkanes, alkenes, 
and alkynes, and preferably wherein the mixture is essentially devoid of 
water and alcohol. 
The present invention is also directed to a method for controlling the 
ratio of 2-methyl-2-butene (2MB2) to 2-methyl-1-butene (2MB1) wherein a 
feedstream is produced by passing a hydrocarbon stream including 
isoamylene in the vapor phase over an acid-treated clay catalyst to 
produce a reaction product including 2MB1, and 2MB2. The temperature at 
which the reaction is within the range of 100.degree. C. up to 250.degree. 
C., and more preferably within the range of 110.degree. C. to 250.degree. 
C. The hydrocarbon stream may also include TAME, in which case the 
reaction product will include unreacted TAME in addition to methanol 
(MeOH). If this is the case, the reaction product may be washed with water 
to remove the MeOH to produce the feedstream. The 2MB2 and 2MB1 are 
preferably present in the reaction product in a ratio of about 1 to 5:1. 
The feedstream may then be subjected to fractionating wherein an overhead 
fraction containing 2MB2 and 2MB1 in a ratio of preferably 1:1, but in any 
event less than the ratio of 2MB2 and 2MB1 in the feedstream, and a 
sidestream fraction consisting essentially of 2MB2 and 2MB1 in a ratio of 
between 6 to 12:1 are separated. It is preferred to recycle the overhead 
hydrocarbon fraction of isoamylene including 2MB2 and 2MB1 in a ratio of 
about 1:1 to the feedstream prior to introducing the feedstream into the 
cracking reactor in which the previously described reaction is performed. 
In accordance with the present invention, a method is provided for 
controlling the ratio of 2-methyl-2-butene (2MB2) to 2-methyl-1-butene 
(2MB1) in isoamylenes which involves fractionating a feedstream containing 
isoamylenes including 2MB2 and 2MB1 in a ratio of about 2 to 5:1 to effect 
a separation between an overhead hydrocarbon fraction of isoamylene 
including 2MB2 and 2MB1 present in a ratio of about 1:1 and a sidestream 
hydrocarbon fraction consisting essentially of isoamylene including 2MB2 
and 2MB1 present in a ratio between about 6 to 12:1, and recovering the 
sidestream hydrocarbon fraction, preferably wherein the ratio of 2MB2 and 
2MB1 in the sidestream hydrocarbon fraction of isoamylene is about 9:1. 
The feedstream including isoamylene may include an alkyl tertiary ether, 
preferably selected from the group consisted of tertiary amyl methyl ether 
(TAME) and methyl tertiary butyl ether (MTBE) in addition to 2MB2 and 
2MB1. The preferred ratio of 2MB2 and 2MB1 in the feedstream is about 5:1. 
The preferred alkyl tertiary ether is TAME. In the embodiment wherein TAME 
is present in the feedstream, the fractionation preferably effects a 
further separation of unreacted TAME from the feedstream as a bottoms 
fraction.

DETAILED DESCRIPTION 
A major use of isoamylenes, i.e. 2-methyl-1-butene and 2-methyl-2-butene, 
is in the manufacture of tackifying resins, alkyl phenols and agricultural 
intermediates, with 2MB2 being the preferred isomer. As previously 
mentioned, however, it is often difficult to convert and then recover 2MB2 
from 2MB1. 
Conventional processes for doing so include isomerization reactions. 
Typically, isomerization reactions are catalyzed by an acidic ion exchange 
resin catalyst. Such a catalyst, however, have been found to be inoperable 
if only isoamylenes or a mixture of isoamylenes with other alkanes, 
alkenes, and alkynes, are passed over it. Although not wishing to be bound 
by any particular theory, it is believed that such components do not 
provide the required environment to bring the catalyst to the necessary 
state of solvation, i.e. swelling; thus, the resin catalyst is 
ineffective. Although it has been suggested to include alcohols and water 
to provide the necessary environment to render the catalyst operable, it 
has been found that alcohols tend to react with the isoamylenes to form 
ethers thereby resulting in a product loss. The presence of water causes 
solubility problems and also tends to react with the isoamylenes to form 
alcohol; thus, water is not a particularly desirable solvent. 
The present invention is based in part on the discovery that the presence 
of ether with the isoamylenes provides the necessary environment for resin 
catalyst operability. Thus, one embodiment of the present invention is 
directed to a method for rendering acidic ion exchange resin catalysts 
operable in the isomerization reaction to convert 2MB1 to 2MB2. Although 
numerous ethers may be used to bring the catalyst to the required state of 
solvation, it has been discovered that tertiary amyl methyl ether (TAME) 
and methyl tertiary butyl ether (MTBE) are preferred, with TAME being most 
preferred. 
In accordance with the present invention, therefore, TAME is introduced in 
a mixture with isoamylenes over the resin catalyst. The iscamylenes being 
subjected to the isomerization reaction normally contains 2MB2 and 2MB1 in 
a ratio of about 2:1. The mixture in a liquid phase is passed over the 
acidic ion exchange resin catalyst at temperatures below 60.degree. C. and 
preferably within the range of 30.degree. C. up to 60.degree. C. and a 
LHSV within the range of about 1 to 30 hr.sup.-1 based on an empty reactor 
volume. It has been discovered that more preferred results are achieved 
when the isomerization reaction is performed at a temperature within the 
range of 33.degree. C. to 55.degree. C. and a LHSV within the range of 5 
to 15 hr.sup.-1, wherein the level of TAME introduced with the isoamylene 
over the catalyst is present in the amount of 5% by total weight of the 
reactor feed. The resultant reaction product contains 2MB2 and 2MB1 in a 
ratio which approaches the equilibrium for the isomerization reaction, 
i.e. over 12:1 at the lower temperatures, down to about 11:1 at the higher 
temperature. 
Catalysts which have been found to be suitable for use in this process of 
the present invention include cation exchange resins. A preferred catalyst 
for purposes of the present invention is a macroreticular sulfonic acid 
cation exchange resin, such as Amberlyst 15 (trademark) and the like. 
As previously indicated, this process of the present invention is conducted 
at a preferred reaction temperature of about 30.degree. C. up to 
60.degree. C., and more preferably within the range of 33.degree. C. to 
55.degree. C. It has been found that such temperature ranges are critical 
to this process of the present invention. Lower temperatures have been 
found to be inefficient because reaction rates are too low and, therefore, 
larger reactors are required for commercial production. Higher 
temperatures, however, tend to result in more undesirable by products, 
particularly isoamylene dimer is product loss, and methanol which 
contributes to product contamination. Notwithstanding performing the 
reaction at lower temperatures, it has been found that a gradual catalyst 
activity loss may be experienced because low levels of diolefins, and 
particularly cyclopentadiene, tend to foul the catalyst, thereby reducing 
its activity. Therefore, although the equilibrium ratio of 2MB2 to 2MB1 
within the range of about 12:1 may be achieved when the reaction is 
performed at 33.degree. C. at LHSV=20 hr.sup.-1 on fresh catalyst, it has 
been found that where a catalyst is used that had been running for 
extended periods of time, such as about 1 month, the reaction must be 
carried out at a temperature of about 45.degree. C. to produce an 
equilibrium ratio of about 11:1. 
As previously indicated, LHSV should be maintained within the range of 
about 1 to 30 hr.sup.-1 and more preferably within the range of 5 to about 
15 hr.sup.-1, based on standard conditions and empty reactor volume. 
The process of the present invention is most preferably practiced at a high 
enough pressure to maintain the hydrocarbon in the liquid phase, 
preferably subcooled. Pressures employed are 10 psig or higher depending 
on temperature, and preferably from about 20 to about 100 psig. 
Therefore, by careful selection of operating conditions, i.e., minimizing 
temperature and maximizing LHSV, it has been found that the methanol and 
dimer formation can be minimized. 
In contrast to the discovery of the present invention, attempts to 
isomerize a hydrocarbon feedstream containing isoamylene which did not 
include TAME were not successful in achieving a similarly high product 
2MB2 to 2MB1 ratio even by raising the temperature to increase the rate of 
the isomerization reaction. 
Although this process of the present invention has been described with 
respect to the isomerization of 2MB1 to 2MB2, it is believed that the 
isomerization reaction over acidic ion exchange resin catalyst can in 
general be improved by the presence of an ether, and may be applied to the 
isomerization of numerous hydrocarbon feed compositions. Thus, hydrocarbon 
feeds which may be suitable for purposes of this process of the present 
invention include feed streams containing 2MB1 and 2MB2 in a mixture with 
saturated hydrocarbons, other straight chain and branched olefins, and 
small amounts of certain diolefins. One example of such a feed is the 
naphtha fraction from a refinery catalytic cracking unit. It should be 
noted, however, that high levels of diolefins, and even low-levels in the 
case of cyclopentadiene, have been found to foul the cation exchange 
resin, reducing its activity, and therefore its ability to catalyze the 
isomerization reaction. U.S. patent application Ser. No. 885,528, filed 
July 14, 1986 commonly owned with the present application, the disclosure 
of which is hereby incorporated by reference thereto, addresses the 
control of levels of diolefins in such process streams. 
This process of the present invention, carried out within the ranges as 
described herein before, is illustrated in the following example. The 
examples, adapted to the isomerization of 2MB1 to 2MB2, are presented as 
illustrative of the general applicability of the process to feed stocks 
previously discussed without being limitative of the invention. 
EXAMPLE I 
A hydrocarbon stream including TAME was isomerized in accordance with the 
previously discussed method under the operating conditions indicated below 
using a macroreticular highly cross-linked cation exchange resin catalyst 
at a temperature of 43.degree. C. 
TABLE I 
______________________________________ 
Run Process Pressure psig - 90 
Process Temperature .degree.C. - 43 
LHSV hr.sup.-1 - 10 
Component Feed Stream Effluent Composition 
______________________________________ 
2MB2:2MB1 67/21 wt % 69/6 wt % 
ratio 3:1 12:1 
2MB1 conversion 71% 
By Product 
dimer 13.8 wt % 
MeOH 240 ppm 
______________________________________ 
The above example shows that at a temperature of 43.degree. C. the presence 
of TAME effectively increases the 2MB2:2MB1 ratio and results with a high 
2MB1 conversion. 
EXAMPLE II 
In contrast to the run described in Example I, attempts were made to 
isomerize a hydrocarbon stream not including TAME following the procedure 
otherwise in accordance with the present invention under the conditions 
indicated below using a macroreticular highly cross-linked cation exchange 
resin catalyst with the following results: 
TABLE II 
______________________________________ 
Conditions 2MB2:2MB1 
Run Temperature Pressure LHSV Ratio 
______________________________________ 
A.sub.1 
55.degree. C. 
90 6 hr.sup.-1 
4.8:1 
______________________________________ 
The above example shows by way of contrast with Example I that the presence 
of TAME is necessary in order to achieve a high 2MB2 to 2MB1 ratio, and 
that increasing the temperature at which isomerization is performed does 
not make up for the absence of TAME. 
EXAMPLE III 
The importance of the presence of TAME for purposes of the present 
invention was further borne out when TAME was subsequently added to the 
hydrocarbon stream processed in Example II. 
TABLE III 
______________________________________ 
Conditions 2MB2:2MB1 
Run Temperature 
Pressure LHSV Ratio 
______________________________________ 
A.sub.2 55.degree. C. 
90 6 hr.sup.-1 
11.1 
(with TAME) 
A.sub.3 43.degree. C. 
90 6 hr.sup.-1 
11.1 
(with TAME) 
______________________________________ 
As shown, upon introducing TAME to the hydrocarbon stream being isomerized, 
the 2MB2 to 2MB1 ratio of the recovered product increased dramatically and 
was maintained independent of the temperature at which the reaction was 
performed. 
EXAMPLE IV 
The purpose of this example is to show that isomerization of hydrocarbon 
streams including TAME, as in Example I, is effectively performed at low 
temperatures and results with a reduced by-product production. 
TABLE IV 
______________________________________ 
Conditions 
Tem- Pres- 2MB2:2MB1 
By-Products 
Run perature sure LHSV Ratio dimer MeOH 
______________________________________ 
A.sub.4 
32.degree. C. 
90 10 hr.sup.-1 
12.8:1 3.5 100 
wt % ppm 
______________________________________ 
Thus, the presence of TAME has been discovered not only as being necessary 
in the production of high ratios of 2MB2 to 2MB1 but also in permitting 
isomerization to proceed under conditions which yield a higher purity 
product. 
To summarize, the foregoing test results evidence that the presence of TAME 
in the hydrocarbon stream result in a higher ratio of 2MB2 to 2MB1 in 
contrast to performing the isomerization with no TAME in the reactor feed 
wherein the product 2MB2 to 2MB1 ratio dropped rapidly despite raising the 
temperature from 43.degree. C. to 55.degree. C. Surprisingly, introducing 
a feedstream containing TAME at this time increased the 2MB2 to 2MB1 
product ratio to about 11:1, and that when the temperature was 
subsequently reduced to 43.degree. C., the 2MB2 to 2MB1 ratio remained at 
11:1 or higher. Although not wishing to be bound by any particular theory, 
it is believed that a higher ratio of 2MB2 to 2MB1 was achieved at 
43.degree. C. than at 55.degree. C. because equilibrium favors higher 
ratios at low temperatures. Thus, it has been unexpectedly discovered that 
when isomerizing a hydrocarbon stream containing TAME, better results were 
obtained at lower temperatures, i.e. isomerization performed at 32.degree. 
C. resulted with a product having a high 2MB2 and 2MB1 ratio of 12.8 and 
small amounts of by products, i.e., 3.5 wt % dimer and 100 ppm MeOH. 
In another embodiment of the present invention, a method is provided to 
produce TAME which may be used in the isomerization and fractionating 
embodiments of the present invention. In this embodiment, TAME is provided 
by first recovering isoamylenes from a C.sub.5 hydrocarbon stream by 
reacting the hydrocarbon stream with methanol over an acidic catalyst. 
Suitable catalysts for this purpose include acidic cation exchange resin 
catalysts. The reaction is carried out preferably at a temperature within 
the range of 40.degree. C. to 80.degree. C. and a LHSV of 0.5 to 4. The 
catalyst converts the 2MB1 and the 2MB2 contained in this C.sub.5 
hydrocarbon stream to tert amyl methyl ether (TAME) which may subsequently 
be recovered from the hydrocarbon stream by distillation. The TAME may be 
used, as previously described, to provide the requisite environment for 
the isomerization catalyst, or may be converted to isoamylene, i.e., 2MB2 
and 2MB1, and methanol over an acidic catalyst in the vapor phase in the 
subsequently described cracking process, in which case the methanol is 
removed from the isoamylene and unreacted TAME by washing with water prior 
to fractionating. 
Another embodiment of the present invention is directed to admixing 
isoamylene with TAME feed to an ether cracking reactor. 
More specifically, in this embodiment a feedstream including isoamylene, 
and optionally tertiary amyl methyl ether (TAME), for example produced in 
accordance with the above-described procedure or recycled from the 
distillation tower as described hereinbelow, is passed in the vapor phase 
through an acid-treated clay cracking catalyst to produce an effluent 
product stream exiting from the catalyst bed which contains isoamylene. 
The product stream may also include an alcohol corresponding to the alkyl 
radical, i.e. methanol, in addition to unreacted TAME, if TAME is 
initially introduced in the feedstream. The latter being the case, the 
alcohol is first removed from the product stream, for example by washing 
with water, before passing the washed stream which is predominantly 
isoamylene, i.e, 2MB1 and 2MB2, and unreacted TAME, to a fractionation 
column to further improve the purity of the isoamylene by separating an 
isoamylene fraction having a desired ratio of 2MB2 to 2MB1 as a 
sidestream. 
The preparation of the ether, i.e., TAME from isoamylene and its subsequent 
disassociation according to the present process is an important 
characteristic of the present invention. As previously discussed, prior 
art separating isoamylene from hydrocarbon streams directly by 
fractionation, because of the closeness of the boiling points of the 
components, is extremely difficult and has been found to be even more so 
if extremely high purity isoamylene is desired. However, it has been 
discovered that if isoamylene is first reacted with C.sub.1 -C.sub.6 
alcohols, i.e., methanol, to form ethers, such as TAME, the TAME can be 
separated from the other C.sub.5 components by an otherwise conventional 
distillation technique. Thus, when TAME is disassociated according to the 
present invention, extremely high purity isoamylene may be produced, i.e., 
isoamylene with very little of any other C.sub.5 present and a high ratio 
of 2MB2 to 2MB1 within the range of 6 to 12:1. 
As previously mentioned, the most preferred tertiary alkyl ether for this 
purpose is tertiary amyl methyl ether, i.e., TAME, although other tertiary 
amyl alkyl ethers may be used. Depending on the particular ether, the 
alcohol which is derived from the disassociation of the ethers may be 
ethanol, isopropanol, tertiary butanol and the like, although methanol 
results when TAME is processed. 
Suitable catalysts and conditions used in the cracking step of this stage 
of the process are disclosed in U.S. Pat. No. 4,691,073 MICHAELSON, 
commonly owned with this application, the disclosure of which is hereby 
incorporated by reference thereto. Briefly, the catalyst utilized in the 
present invention may be prepared by reacting a naturally occuring or 
synthetic clay with hydrofluoric acid (HF) or hydrochloric acid (HCl) 
followed by calcining. The reacting or incorporation of HF or the HCl with 
the clay can be accompanied by any means, such as contacting the clay with 
anhydrous HF or HCl or by impregnation of the clay with an aqueous acid, 
for example, a mixing method equilibrium absorption method, 
evaporation-to-dryness method, spray drying and the like. Preferably the 
clay is reacted with 1.0 to 70 wt %, preferably 20 to 50 wt % hydrofluoric 
acid or 1.0 to 30% to 37%, preferably 20 to 30 wt % hydrochloric acid at 
temperatures of 0.degree. C. to 50.degree. C., preferably 10.degree. C. to 
30.degree. C. for 30-120 minutes. The amount of the acid is 0.001 to 1.0, 
and preferably 0.01 to 0.10gm anhydrous acid/gram clay. Following the 
reaction, the fluid is decanted and the clay is then preferably washed 
first with water and then with alcohol before calcining. The calcining 
temperature is selected so as to achieve a highly active high-surface area 
catalyst of a moisture content of less than 5 wt %. Preferably 
temperatures are 250.degree. C. to 1,000.degree. C., and more preferably 
400.degree. C. to 700.degree. C. The calcination is generally carried out 
in air, but an atmosphere of an inert gas, for example nitrogen, carbon 
dioxide, and argon, in addition to steam or mixtures thereof may also be 
used. The time for calcination is generally 0.1 to 24 hours, and 
preferably 0.5 to 10 hours, although the time depends upon the calcination 
temperature. The amount of the flourine or chlorine compounds supported on 
the carrier is 0.1 to 100 parts by weight of the carrier and preferably 
1.5% to 6.0%. Examples of the carrier containing silicon oxides include 
silica, montmorillonite, kaolinite, attapulgite, bentonite and acid clay, 
in addition to silica alumina, silica-zirconia, silica-magnesia and their 
mixtures. The silica may be either in the form of the gel or sol. A 
particularly preferred carrier is one prepared from attapulgite or 
montmorillonite-type minerals. The surface area of the carrier is 
preferably more than 1.sub.m.sup.2 /gm, and more preferably above 
40.sub.m.sup.2 /gm. Preferred surface areas after calcination are in the 
range of 100.sub.m.sup.2 /gm to 400.sub.m.sup.2 /gm. 
The reaction of decomposition of the tert-alkyl ethers takes place with 
good yields under atmospheric pressures, but it is preferred to operate 
under slightly superatmospheric pressures so as to permit the use of 
cooling water without any other expedient to carry out the condensation of 
the products which are obtained. 
The working pressures are generally ranging from 1 to 20 kilograms/sq.cmm 
absolute; and preferably under a pressure which is at least. equal to the 
vapor pressure of isoamylenes and TAME at the condensation temperature 
which is foreseen. 
The reaction is carried out at a temperature below 250.degree. C., and 
preferably in the range of 100.degree. C.-250.degree. C., and more 
preferably in the range of 110.degree. C. -230.degree. C. The reaction is 
carried out at a spacial velocity, expressed in terms of volume of liquid 
per volume of catalyst per hour (LHSV) ranging between 0.5 and 30, and 
preferably of 1 to 5. Preferably, conditions are selected to obtain 
conversions of the isoamylenes and tert-alkyl ethers of 80% and preferably 
90%. With this in mind, the normal operating temperature of the cracker 
reactor should be maintained within the range of 120.degree. F. to 
170.degree. F. 
Thus the feedstream may also be obtained via decomposition of TAME as 
described in U.S. Pat. No. 4,691,073, and controlling the TAME conversion 
such that the desired amount of TAME remains in the isoamylene stream 
after water Washing to remove methane. Alternatively, a C.sub.5 
hydrocarbon stream containing isoamylenes may be reacted with methanol 
over an acidic catalyst to convert 2MB1 and 2MB2 to TAME for use in 
forming the mixture. 
Referring now to FIG. 1, a schematic system is shown, which can be used to 
produce high purity isoamylene. 
A feed stream 10 containing 90 wt % tertiary amyl methyl ether (TAME) is 
introduced together with isoamylene through feed stream 12 to a cracking 
reactor 14. As illustrated, the isoamylenes and TAME may be recycled from 
distillation column 20 as top and bottom fractions, respectively, to make 
up at least a portion of feedstreams 12 and 10. Alternatively or 
additionally, isoamylenes and TAME may be provided from a separate source 
of supply. For example, the TAME may be recovered from a C.sub.5 
hydrocarbon stream by reacting the C.sub.5 hydrocarbon stream with 
methanol over an acidic catalyst to convert the 2-methyl-1-butene and the 
2-methyl-2-butene contained in the C.sub.5 hydrocarbon stream to tert-amyl 
methyl ether (TAME), as described above. 
The cracking reactor 14 is provided with an acid-treated clay catalyst, as 
previously described herein, and is heated to a temperature within the 
range of 120.degree. C. to 170.degree. C. The effluent or product stream 
leaving the cracking reactor is composed of isoamylenes, i.e. 2MB1 and 
2MB2, in a ratio of between 1:2 to 5 and preferably in a ratio of 1:5, 
unreacted TAME, and methanol (MeOH). The product stream is then washed 
with water to separate the methanol from the isoamylene and unreacted TAME 
in water wash stage 16. The resultant feedstream for the distillation 
column consists essentially of isoamylene, i.e. 2MB1 and 2MB2 in a ratio 
between 1:2 to 5 and preferably 1:5, and unreacted TAME and is then fed to 
a distillation column 20 which is preferably operated to vaporize the 
isoamylene. The vaporized overhead 22 is composed of isoamylene including 
2MB1 and 2MB2 in a ratio which is less than the ratio of 2MB1 to 2MB2 in 
the feedstream and preferably about 1:1 which, as previously mentioned, is 
recycled through line 12a to be reintroduced to the cracking reactor 14 in 
feedstream 12. In accordance with the present invention, however, a side 
stream 24 is drawn off which consists essentially of isoamylene including 
2MB1 and 2MB2 in a ratio of 1:9. The unreacted TAME is then withdrawn as a 
bottoms fraction 26 and either recycled through line 10a to be 
reintroduced to the cracking reactor 14 in feedstream 10, or may be used 
to provide the necessary environment for resin catalyst operability in the 
previously described isomerization reaction for converting 2MB1 to 2MB2. 
EXAMPLE II 
A feedstream of isoamylene with a 2MB2 to 2MB1 ratio of 1:1 in addition to 
TAME was fed in the gas phase to a reactor containing an acidic cracking 
catalyst, as described above, operated at 125.degree. C. The reactor 
outlet ratio of 2MB2 to 2MB1 was increased as shown below: 
______________________________________ 
Component Feed Stream Reactor effluent 
______________________________________ 
Isoamylene/TAME 36/64 wt % 76/2 wt % 
2MB2:2MB1 18/18 wt % 51/25 
2MB2 to 2MB1 ratio 
1:1 2:1 
______________________________________ 
Computer simulations using fractionation design computer programs were used 
based on the reactor product composition from the above test as the feed 
to a distillation tower, to determine a design of a distillation tower 
that would separate this reactor products into three streams: a bottoms 
product consisting mainly of the unreacted TAME; an overhead stream 
consisting of isoamylene in a 2MB2 to 2MB1 ratio of 1:1; and a high purity 
isoamylene sidestream with a 2MB2 to 2MB1 ratio of 6:1 or higher. 
It is further understood that although the invention has been specifically 
described with reference to particular means and embodiments, the 
foregoing description is that of preferred embodiments of the invention. 
The invention, however, is not limited to the particulars disclosed but 
extends to all equivalents, and various changes and modifications may be 
made in the invention without departing from the spirit and scope thereof.