US3644510A - Propylene oxidation in the presence of iridium metal - Google Patents

Abstract

PROPYLENE IS OXIDIZED WITH OXYGEN TO ACETIC ACID, ACROLEIN, ETC. BY OXIDATION IN THE CONTACT PRESENCE OF IRIDIUM METAL.

Description

United States Patent Oflice 3,644,510 Patented Feb. 22, 1972 3,644,510 PROPYLENE OXIDATION IN THE PRESENCE OF IRIDIUM METAL Noel W. Cant and William K. Hall, Pittsburgh, Pa., as-

signors to Gulf Research & Development Company, Pittsburgh, Pa. No Drawing. Filed Nov. 25, 1968, Ser. No. 778,805 Int. Cl. C07c 53/08 US. Cl. 260-533 R 8 Claims ABSTRACT OF THE DISCLOSURE Propylene is oxidized with oxygen to acetic acid, acrolein, etc. by oxidation in the contact presence of iridium metal.

This invention relates to a process for oxidizing propylene to obtain a useful mixture of organic compounds comprising acetic acid, acrolein and acetone.

In addition to the above desired compounds, by-products such as carbon dioxide and water are also formed. It was expected that the partial oxidation of propylene would yield primarily organic acids containing three carbon atoms, i.e. propionic acid. Quite unexpectedly, it has been found that the main product of the oxidation reaction is acetic acid so long as the oxidation reaction is carried out in the presence of iridium metal.

In accordance with the invention, therefore, propylene is oxidized to a product comprising acetic acid by reacting propylene with a gas containing free molecular oxygen in the contact presence of iridium metal.

In order to obtain good selectivity to the desired oxygenated compounds it is important that the propylene together with a gas containing free molecular oxygen be contacted with the iridium metal either supported or unsupported at a temperature within the range of about 50 C. to about 300 C., and preferably within the range of about 80 C. to about 200 C. Since the reaction is exothermic, means must be provided to control the temperature of the reaction within the limits defined above. Below the lower temperature limits defined above the reaction rate becomes too low to be economically feasible, whereas at temperatures above the upper limits, the yield of desired oxygenated compounds decreases with the concurrent production of excessive amounts of water and carbon dioxide. The temperature can be controlled by any suitable means, and one method of at least partially controlling the temperature is to dilute the iridium metal by distending it on a suitable support material such as silica, magnesia, alumina, thoria or mixtures thereof. The iridium metal can, of course, be used unsupported and when this is done the metal can be in any suitable form, such as sponge form. When the iridium is distended on a support, the surface area of the support is not critical and can suitably be between 0.1 and 600 square meters per gram. In addition to the above named supports, materials such as carbon, kiesulguhr, pumice, the natural clays, mullite and alundum are also suitable. When the iridium metal is supported, suitable amounts of iridium metal are from 0.2 to 30 weight percent of the total catalyst with preferred amounts from one of ten weight percent and the more preferred amounts from one to five weight percent of the total catalyst.

The method of preparing the supported or unsupported catalysts is not critical. Suitable methods of preparing the supported catalysts, for example, include the method of incipient wetness using aqueous solutions of suitable iridium salts such as H IrCl followed by drying and reduction in hydrogen to obtain the metal.

Another method of controlling the temperature is to dilute the propylene-oxygen mixture with an inert gas such as nitrogen or helium. Yet another method of controlling the reaction temperature is to pass the admixture of propylene and oxygen over the iridium metal catalyst at very high space velocities. Suitable gaseous space velocities are within the range of about one to about 2000 volumes of propylene measured at standard temperature and pressure per volume of catalyst per hour, and the preferred space velocities are from about five to about 200.

A total operating pressure of about one atmosphere is the desired operating pressure. Higher or lower pressures can be used; for example, pressures of from 0.5 to 15 atmospheres or more can suitably be employed.

The ratio of the partial pressure of oxygen to the partial pressure of propylene can suitably be between 0.2 and 100 and is preferably between two and 30. The partial pressure of propylene should be at least 0.05 p.s.i.a. and is preferably from 0.1 to one p.s.i.a. when the total pressure is atmospheric (14 p.s.i.a.). Correspondingly higher partial pressures of propylene would be employed at correspondingly higher total operating pressures.

The propylene is oxidized in the presence of a gas containing free molecular oxygen. Pure oxygen can be used, but this creates problems of temperature control as noted above. It is preferred that the free molecular oxygen be diluted with an inert gas such as nitrogen or helium. The volume percent of free molecular oxygen in the gas containing it can suitably be between one and 100 and is preferably between one and 20. When propylene is mixed with this gas containing free molecular oxygen, the partial pressure of oxygen is usually between 0.5 and ten p.s.i.a. and is preferably between two and five p.s.i.a. If the partial pressure of oxygen is below about 2.0 pounds per square inch absolute, selectivity to the desired oxygenated products decreases, whereas above about two pounds per square inch absolute selectivity to the desired oxygenated compounds remains substantially constant.

The invention will be further described with reference to the following experimental work. In all of the examples to follow the following procedure was employed. A single pass flow system was used wherein a feed mixture of propylene, oxygen and helium was passed through a bed (approximately two parts by volume of catalyst) of a supported iridium catalyst at a given temperature between 50 C. and 200 C. at a flow rate of between 2400 and 3000 volumes of total feed per hour. The contact time was about 2.3 seconds and the space velocity based on the total feed was about 1500 volumes of feed per volume of catalyst per hour. The propylene space velocities for any particular run can be calculated by multiplying 1500 by the ratio of the propylene partial pressure in millimeters to the total pressure of about 740 millimeters. The reaction products were cooled to C. by indirect cooling to condense the reaction product which was mostly acetic acid with minor amounts of acetone, acrolein, acrylic and/or propionic acids. Water and CO were formed as by-products.

EXAMPLE 1 In this run for this example, the feed mixture was passed through a bed of an alpha-alumina supported iridium catalyst containing 1.5 weight percent iridium. The results of this run are summarized in Table I below.

EXAMPLE 2 In the run for this example, the feed mixture was passed through a bed of a silica supported iridium catalyst containing five weight percent iridium. The silica support was a commercial Cab-O-Sil material obtained from the Cabot Company. The catalyst was prepared by 3 the method of incipient wetness by contacting the Cab- O-Sil with an aqueous solution of an appropriate amount of an iridium salt, i.e. H IrCl drying the material and reducing at 300 C. in hydrogen to convert the salt to metallic iridium. The results of this run are summarized in Table I below.

Referring to Table I below, the percent selectivity to the useful oxygenated products Was about the same (30.5 v. 28) for the alpha-alumina supported iridium catalyst (Example 1) as with the silica supported iridium catalyst (Example 2) at about the same oxidation rate.

A series of runs was made by passing a feed mixture containing varying amounts of oxygen, propylene and helium over the catalyst of Example 1 at varying conditions to determine the effect of changes in temperature and feed composition on reaction rate and selectivity to the production of acetic acid, acetone and acrolein. The results of this series of runs are given in Table II below.

Referring to Table II below, it can be seen that the selectivity to the production of acetic acid, acetone and acrolein decreases slightly in going from 100 to 123 C.

propylene with a gas containing free molecular oxygen in the contact presence of iridium metal.

2. A process according to claim 1 wherein the reaction occurs at a temperature from 50 to 300 C., a partial pressure of oxygen above about 0.5 pounds .per square inch absolute and a propylene space velocity from about one to about 2000.

3. The process of claim 2 wherein the temperature in the reaction zone is maintained in the range of about 80 to about 200 C.

4. A process according to claim 2 wherein the partial pressure of oxygen is within the range of about 0.5 to about 10 pounds per square inch absolute.

5. The process of claim 1 wherein the iridium metal is deposited on a support.

6. A process according to claim 5 wherein the reaction occurs at a temperature from 50 to 300 C. and the amount of iridium metal is from 0.2 to 30 weight percent of the total catalyst.

TABLE I.-PRODUCTS OF PROPYLENE OXIDATION OVER SUPPORTED IRIDIUM AT A TOTAL PRESSURE OF ABOUT 740 Percent selectivity I to Total percent Oxidation 1 Acrylic selectivity Oxygen Propylene Rate us Acrolcin to useful Wt. percent pressure pressure Temp. Volumes, Acetic proplonic plus oxygenated Example No. Metal Support metal (mm) (mm.) C.) C3H6/1'HII1. acid acid acetone products 1 Ir a-Alzog 1. 5 131 7 135 0. 15 0. 5 6. 0 30. 5 2 Ir SiO 5.0 61 21 135 0.18 21 3 4 28 1 The oxidation rate is defined as the rate at which propylene is converted to all products in volumes (STP) per minute, e.g. if the propylene is passed over the catalyst at two volumes per minute and it is found that 1.8 volumes per minute is recovered unchanged, then the oxidation rate is 2.0

minus 1.8, or 0.2 volumes per minute.

a The prrcent selectivity is defined as the percent of the propylene oxidized which is converted to the given product.

TABLE II.DEPENDENDENCE OF RATE AND SELECTIVI'IY OF PRO- PYLENE OXIDATION OVER 1.5% Ir/a-AlzOa 0N PROCESS VARIABLES Percent selectivity to- Oxygen Propylene Oxidation Ex. Temp. pressure pressure rate. Acetic No. 0.) (mm.) (mm.) vols/min. acid Acetone Acrolein A See Table I footnotes.

(compare Examples 3-8). The rate, however, increases considerably with the increase in temperature (again compare Examples 3-8). The rate of oxidation increases with propylene pressure as can be seen by comparing EX- amples 9-13. The oxidation rate is not greatly affected by the oxygen partial pressure but the percent selectivity to useful oxygenated products increases with an increase in oxygen partial pressure as can be seen by a comparison of Runs 14-18.

Obviously, many modifications and variations of the invention as hereinabove can be made Without departing from the spirit and the scope thereof, and such modifications and variations are intended to be included within the scope of this invention.

We claim:

1. A process for the production of a product comprising acetic acid, which process comprises reacting 7. A process according to claim 6 wherein the support is alpha-alumina.

8. A process according to claim 6 wherein the support LORRAINE A. WEINBERGER, Primary Examiner R. D. KELLY, Assistant Examiner U.S. Cl. X.R.