GB2096165A - Process for the catalytic isomerization of tetrahydrodimethyldicyclopentadiene - Google Patents

Abstract

Tetrahydrodimethyldicyclopentadiene is catalytically isomerized to a liquid isomeric mixture having a suitable low temperature viscosity making it suitable as a missile fuel. The catalyst comprises an acidic clay as the active catalytic component. Hydrogen is also present and in an amount sufficient to maintain the isomerization activity of the clay. The elevated temperature is sufficient to cause isomerization.

Description

SPECIFICATION Process for the catalytic isomerization of tetrahydrodimethyldicyclopentadiene This invention relates to the isomerization of tetrahydrodimethyldicyclopentadiene, hereinafter referred to as THDMDCP. More particularly, the invention relates to the preparation of an isomeric liquid mixture from THDMDCP. Still more particularly, the invention relates to the catalytic isomerization of THDMDCP to an isomeric liquid mixture. Generally the invention relates to the isomerization of endo-THDMDCP to exo-THDMDCP.

The aforementioned isomeric liquid mixture can be used as high energy missile fuel. Such fuels can be used in either jet or rocket propulsion. Jet propulsion includes a jet engine which can be used for a missile, an aircraft and others and includes the three basic types, i.e., ramjet, turbojet and pulse jet. The term rocket generally refers to a device containing its own oxygen or oxidizing agent. An article in Aviation Week and Space Technology, January 26, 1976, pages 111-113, discloses some of the high density hydrocarbon fuels that are under consideration as missile fuels.

A precursor, dimethyldicyclopentadiene, hereinafter referred to as DMDCP, is prepared by dimerizing methylcyclopentadiene. The resulting dimethyl dimer mixture contains many isomers some of which can be represented by the following structures:

The foregoing structures are predominantly of endo stereo chemical configuration.

DMDCP can be hydrogenated to THDMDCP using hydrogen and a hydrogenation metal such as nickel, Raney Ni, palladium, and the like. The metals can be supported by such supports as carbon, silica-alumina and the like. The hydrogenation proceeds smoothly at a temperature of about 1004000C and with a hydrogen pressure of about 200-5000 psig.

Some of the isomers are known to have substantially different melting points. For example the following two isomers are a clear liquid at ambient temperature,

whereas the following two isomers are solid at room temperature and therefore have an adverse effect on the freezing point of the mixture.

Thus, the problem is how to isomerize the isomeric mixture from one which is not suitable as a missile fuel to an isomeric liquid mixture that is suitable.

U.S. Patent No. 3,381,046 discloses the hydrogenation of DMDCP to THDMDCP using a variety of hydrogenation catalysts including palladium and nickel on Kieselguhr. It further discloses that the DMDCP can be partially hydrogenated and then thermally treated to convert the endo dihydro isomer to the exo dihydro isomer before finally hydrogenating the dihydro exo isomer to the tetrahydro exo isomer. Also, the patent discloses that the endo dimer can be fully hydrogenated and then the tetra endo isomer can be thermally treated in the presence of the hydrogenation catalyst to effect thermal isomerization to the tetrahydro exo isomer. The patent further discloses the use of acidic clay for isomerization but is silent as to the use of hydrogen with the clay.

However, the applicants have discovered the following. If DMDCP is hydrogenated, at a suitable temperature, using a hydrogenation catalyst such as Kieselguhr, the olefinic materials deactivate the isomerization sites on the Kieselguhr. This deactivation is believed to be caused by the formation of polymers and/or carbon. Thus, when the temperature of the mixture of hydrogenated DMDCP and the partially deactivated catalyst is raised to a-conversion temperature, the conversion from endo to exo is caused by a thermal effect rather than a catalytic one. Applicants have also discovered that, to avoid this deactivation, fresh catalyst should be used and that the catalytic isomerization is facilitated by the presence of hydrogen.

According to the present invention, THDMDCP is catalytically isomerized using a catalytic amount of acidic clay having isomeric activity. The isomerization occurs in- the presence of sufficient hydrogen to maintain the catalytic activity of the acidic clay and occurs at a suitable isomerization temperature. The isomerization causes the endo THDMDCP to form exo THDMDOP As a result of the catalytic isomerization, the resulting isomeric mixture has a high density and a suitable low temperature viscosity making it useful as a missile fuel. Furthermore, the product is not contaminated by residues of a metal catalyst and thus the expense and inconvenience normally associated with removal of metal catalyst residues are avoided.

The process is for the catalytic isomerizationof endo THDMDCP. It comprises contacting the endo material with a catalytic amount of acidic clay having isomeric activity. The contacting occurs at a temperature at which isomerization occurs and in the presence of sufficient hydrogen to maintain the isomeric activity of the acidic clay. The contacting is continued until the endo diene is isomerized to its exo isomer. The amount of isomerization can be inferred from the change in properties such as density and/or viscosity.

The isomerization of one of the THDMDCP's via the present invention can be represented by the following formula reaction:

'CH3 C1 H2 CATALYST OH3 0H3 OH3 (A) ENDO EXO It can also be represented by the following formula reaction:

HHH H2 b OH3 HCH3 CATALYST HHH OH3 CH3 H (B) The exo compounds shown are but two of many possible isomers of THDMDCP that result via the isomerization.

While the THDMDCP feed may contain other similar hydrocarbons, such hydrocarbons do not adversely affect the isomerization or the catalyst. Further, the similar hydrocarbons do not adversely influence the desired resulting properties of the isomerized mixture. Thus, for optimum results, the feed consists essentially of THDMDCP which itself can be a mixture of THDMDCP isomers.

The catalyst used to isomerize the THDMDCP is an acidic clay, including synthetic and natural forms. Preferred clays include acidic forms of alumina, silica-alumina, firebrick, Kieselguhr, and fire clay. The isomerization activity of the acidic clay should not have been reduced by carbon and/or polymer formation on its active sites. The amount of catalyst present must be sufficient to catalytically direct, for example, reactions A and B. The preferred catalyst concentration can range from one-half part by weight of catalyst per hundred parts by weight or the THDMDCP to a one to one ratio, while a more preferred range is from 1:20 to 1:10.

The silica-alumina weight ratio may be vary substantially; e.g., in the range of from 1:30 to 30:70. In the catalyst composition, the firebrick, acidic silica-alumina, or acidic alumina, supplies the acidic sites to cause the desired isomerization reaction to occur.

The isomerization temperature needs to be controlled between a narrow range. The lower limit can be determined by the rate of the reaction; i.e., if the temperature is too low, the reaction rate is slow, and such a slow rate makes the process uneconomical. Thus, generally, the lower temperature limit is 1000C with 1250C being preferred. The upper limit is controlled by the formation of undesirable products which adversely affect the properties of the resulting missile fuel. Thus, generally, the upper temperature limit is 4000C with 3500C being preferred.

Hydrogen needs to be present. It keeps the isomerization sites of the catalyst clean and, by doing so, minimizes carbonization and polymerization. It should be present in a sufficient amount to perform its function. Generally, a wide range of hydrogen pressures can be used; however, economic considerations favour the use of as low a pressure as possible.

The properties of the resulting isomerized liquid mixture can vary substantially depending upon the amount of isomerization which occurs. It can depend, in addition to the composition of the initial mixture, on how much each of the particular isomers of THDMDCP is present. Typically, the resulting isomerized THDMDCP mixture will have a density @ 200C 4 in the range of from 0.911 to 0.91 8. The desired density of the missile fuel will depend, in part, on the particular missile design and such factors as the distance the missile is expected to travel.

As to viscosity, the isomeric mixture will typically have a kinematic viscosity at 1000 F in the range of from 2 cst to 3 cst. The desired viscosity of a missile fuel will depend, in part, on the particular missile design and whether the fuel will be heated during the flight and such factors as the altitude at which the missile will fly. The desired freezing point of the isomeric fuel mixture depends, in part, on the design of the missile operating conditions.

To obtain an isomerized mixture having a density and viscosity which make it useful as an additive for a high density fuel for an air-breathing missile, the reaction time or contacting time should be sufficient to yield the desired properties. Sufficient time depends inter alia on the amount of the diene isomerized, the amount of stirring, the amount of catalyst used and the configuration of the vessel containing the reaction or contacting mixture. The amount of isomerization can be monitored during the process by measuring, for example, the viscosity; thus, when the desired amount of isomerization has been obtained, the reaction can be stopped.

The following example illustrates an embodiment of the present invention. Also shown is a comparative run.

Example The tetrahydrodimethyldicyclopentadiene is prepared by hydrogenating methylcyclopentadiene dimer over Raney nickel.

1 00 mis of tetrahydrodimethyldicyclopentadiene are mixed with 20 grams of Harshaw alumina 1401-P (an acidic alumina manufactured by Harshaw Chemical Co., Cleveland, Ohio, USA) which has been activated by washing it with dilute H2SO4 and drying it at 3000C in a stream of N2 for 4 hours.

This mixture is charged to an 0.3 liter autoclave-reactor (rocking type) and pressured to 100 psig. with hydrogen. The reactor plus contents are heated to 2500C with rocking for 12 hours, an additional 2 hours at 3500C and an additional 1 hour at 4000 C. Hydrogen pressure is maintained at 100--200 psig. throughout the reaction. The reactor and contents are permitted to cool to room temperature with rocking.

The product is a liquid at room temperature, having a density 200C 4 in the range 0.911 to 0.918 and a Kinematic viscosity at 1000 F in the range 2-3 cst. Considerable isomerization is evident from the increase in area percentage evident from vapour phase chromatographic (vpc) analysis.

Analogous isomerization results are obtained when acidic silica-alumina is used, together with hydrogen, instead of the activated acidic alumina of the above example.

Similar results are also obtained with acid clays generally including Kieselguhr, fire clay and firebrick.

Claims (12)

Claims

1. A process for the catalytic isomerization of endo tetrahydrodimethyldicyclopentadiene comprising: (a) contacting endo tetrahydrodimethyldicyclopentadiene with a catalyst having as its active catalyst component a catalytic amount of an acidic clay having isomeric activity; (b) having the contacting occur at an isomerization temperature and in the presence of sufficient hydrogen to maintain the isomeric activity of the clay; and (c) continuing the contacting until the end6 diene is isomerized to its exo isomer.

2. A process according to Claim 1 wherein the catalyst is an acidic alumina.

3. A process according to Claim 1 wherein the catalyst is acidic silica-alumina.

4. A process as claimed in Claim 1, wherein the catalyst is firebrick.

5. A process as claimed in Claim 1, wherein the catalyst is Kieselguhr.

6. A process as claimed in Claim 1, wherein the catalyst is a fire clay.

7. A process according to any of Claims 1 to 6, wherein the hydrogen pressure is in the range of from 100 to 200 psig.

8. A process according to any one of Claims 1 to 7, wherein the isomerization temperature is in the range of from 1 000C to 4000C.

9. A process according to Claim 8, wherein the temperature range is from 2500C to 4000 C.

10. A process according to Claim 1, wherein the catalyst is an acidic alumina activated by treatment of alumina with sulphuric acid, the temperature is in the range of from 100 to 4000C and the hydrogen has a pressure in the range of from 100 to 200 psig.

11. A process according to any one of Claims 1 to 10, wherein the catalyst to diene ratio is in the range of from one-half part by weight of catalyst per hundred parts by weight of the diene to a one to one ratio.

12. A process according to any one of Claims 1 to 1 wherein the resulting isomer mixture has a density 200C 4 in the range of from 0.911 to 0.91 8.

1 3. A process according to any one of Claims 1 to 12, wherein the resulting isomer mixture has a kinematic viscosity (Z 100 O00F in the range of from 2 cst to 3 cst.

1 4. A process according to Claim 1, substantially as herein described with reference to the specific example.