Process for the manufacture of unsaturated aldehydes and acids from the corresponding olefins

A process for the catalytic oxidation of olefins to unsaturated aldehydes and acids and the ammoxidation of olefins to unsaturated nitriles in which the catalyst comprises a promoted, reduced, antimony oxidemolybdenum oxide-containing catalyst.

This invention relates to a process for the catalytic oxidation of olefins 
to unsaturated aldehydes and acids and to the oxidation of olefin-ammonia 
mixtures to unsaturated nitriles. More specifically this invention relates 
to a process for the catalytic oxidation of olefins such as propylene and 
isobutylene to acrolein, acrylic acid, methacrolein, and methacrylic acid, 
respectively, and the ammoxidation of propylene and isobutylene, 
respectively, to acrylonitrile and methacrylonitrile. 
The catalyst of this invention is composed of the oxides of molybdenum and 
antimony and preferably contains other metal oxides. The catalyst 
compositions most useful in this invention are represented by the 
following formula: 
EQU A.sub.a B.sub.b Sb.sub.c Mo.sub.d O.sub.e 
wherein A comprises one or more of the promoting elements selected from the 
group consisting of tellurium, tungsten, titanium, manganese, nickel, 
iron, copper, lead, rhenium, bismuth, tin, uranium, chromium, phosphorus 
and boron, and B is a member selected from the group consisting of 
molybdenum, tungsten, aluminum, nickel and sulfur, and wherein a is a 
number of from 0.001 to 1.0, b is a number of from 0 to 2.0 c is a number 
from 1 to 9, d is a number from 1 to 9, and e is a number dependent upon 
the valence requirements of the combined metals. The preferred catalysts 
include those compositions wherein a is 0.005 to 0.5, b is 0.001 to 1.0, c 
is 1 to 8, d is 1 to 8 and e is 4 to 40. 
The method employed in preparing the catalyst of this invention is critical 
to the oxidation process described herein. In the empirical formula 
designating the composition of the catalyst of this invention, A in the 
formula represents a promoter element and B represents a reducing element. 
The method employed in preparing the catalyst departs from the usual 
classical procedures involving co-precipitation or impregnation techniques 
and involves the simple mixing of the respective metal oxides of antimony 
and molybdenum, the reducing agent and the compound of the promoter 
element or elements as a slurry in water. 
In a preferred procedure for combining the essential elements of the 
catalyst composition, an aqueous suspension of molybdenum trioxide is 
pre-reduced in a controlled manner so that at least some of the molybdenum 
is reduced to a valence state below +6 before the molybdenum oxide is 
mixed with a lower oxide of antimony, antimony trioxide. A wide range of 
reducing agents can be employed for this purpose including finely divided 
or colloidal metals such as molybdenum, tungsten, magnesium, aluminum, 
nickel, bismuth, antimony, chromium, cobalt, zinc, cadmium, tin, or iron, 
sulfur, hydrogen sulfide, sulfur dioxide, hydrazine hydrate, ammonia, 
hydroxylamine, organic reducing agents, such as, sugars, pyrogallol, and 
the like. Most preferred is finely divided metal in the amount of from 
about 0.01 to 0.2 atoms of metal per mole of molybdenum trioxide present. 
It is also preferred that the promoter element be added in a non-oxidizing 
form. 
On refluxing the aqueous suspension of molybdenum trioxide with the 
reducing agent, at least a part of the normally insoluble molybdenum 
trioxide is solublized forming an intense deep blue coloration. It is 
hypothesized that this blue color which develops is the result of the 
reduction of molybdenum, at least in part, to a lower oxidation state in 
the oxidation-reduction reaction occurring between hexavalent molybdenum 
and the reducing metal. Although preferredly the molybdenum trioxide is 
pre-reduced before reaction with the antimony trioxide, beneficial results 
are also obtained by first reacting the molybdenum trioxide with antimony 
trioxide followed by reaction with the reducing agent, or by reacting the 
three components simultaneously. It is also contemplated to be within the 
scope of this invention to employ a combination of a lower oxide of 
molybdenum with a higher oxide of antimony, as for example antimony 
tetroxide or antimony pentoxide in preparation of the catalyst. 
Preferredly, the metal promoter of the catalyst is subsequently added to 
the aqueous suspension of mixed oxides of antimony, molybdenum and the 
reducing metal, in the form of a non-oxidizing compound such as, for 
example, the metal oxide, hydrous metal oxide, the hydroxide, the halide, 
the acid, a salt of the acid, a salt of an organic acid, an organometallic 
compound and the like. Satisfactory results are also obtained, however, by 
adding the promoter element to the component mixture at any stage of the 
catalyst preparation. 
A highly reproducible method for combining the components of the catalyst 
of this invention comprises refluxing an aqueous suspension of molybdenum 
trioxide and a finely divided metal for a period of about 1 to 3 hours at 
100.degree. C. until the deep blue coloration characteristic of a lower 
oxidation state of molybdenum appears. Antimony trioxide is then added to 
the aqueous suspension of the reduced molybdenum oxide and the reducing 
metal and this mixture is again refluxed at 100.degree. C. for a period of 
about 1 to 5 hours. To this mixture is added the metal promoter in a form 
disclosed hereinabove, and the entire mixture is further refluxed for 
about 1 to 5 hours at the same temperature. The aqueous slurry is then 
evaporated to dryness, and final drying is accomplished by placing the 
catalyst in an oven at a temperature of about 120.degree. to 130.degree. 
C. for a period of from about 2 to 24 hours. 
The catalyst of this invention may be supported on a carrier material such 
as for example, silica, zirconia, calcium stabilized-zirconia, titanis, 
alumina, thoria, silicon carbide, clay, pumice, diatomaceous earth and the 
like, or it may be employed satisfactorily in an unsupported form. If a 
carrier is utilized it may be employed in amounts of up to 95 percent by 
weight of the total catalyst composition. 
The catalyst system herein described is useful in the oxidation of olefins 
to corresponding oxygenated compounds, such as unsaturated aldehydes and 
acids, and in the ammoxidation of olefins to unsaturated nitriles. 
Nitriles and oxygenated compounds such as aldehydes and acids can be 
produced simultaneously using process conditions within the overlapping 
ranges for these reactions, as set forth in detail below. The relative 
proportions of each that are obtainable will depend on the catalyst and on 
the olefin employed. It is also contemplated to be within the scope of 
this invention, that with the catalyst system employed herein, the 
unsaturated aldehyde may be further oxidized in a second step to the 
corresponding unsaturated acid. The unsaturated aldehyde need not be 
isolated from the other reaction products and can be further oxidized to 
the unsaturated acid while remaining in the reaction mixture. The term 
"oxidation" as used in this specification and claims encompasses the 
oxidation to aldehydes and acids and to nitriles, all of which conversions 
require oxygen as a reactant. 
Oxidation of Olefins to Aldehydes and Acids 
The reactants used in the oxidation to obtain oxygenated compounds are 
oxygen and an olefin such as propylene or isobutylene, or their mixtures. 
The olefins may be in admixture with paraffinic hydrocarbons, such as 
ethane, propane, butane and pentane, as for example, a propylene-propane 
mixture may constitute the feed. This makes it possible to use ordinary 
refinery streams without special preparation. 
The temperature at which this oxidation is conducted may vary considerably 
depending upon the catalyst, the particular olefin being oxidized and the 
correlated conditions of the rate of throughput or contact time and the 
ratio of olefin to oxygen. In general, when operating at pressures near 
atmospheric, i.e., --10 to 100 p.s.i.g., temperatures in the range of 
250.degree. to 600.degree. C. may be advantageously employed. However, the 
process may be conducted at other pressures, and in the case where 
superatmospheric pressures, e.g., above 100 p.s.i.g. are employed somewhat 
lower temperatures are feasible. In the case where this process is 
employed to convert propylene to acrolein and acrylic acid, or isobutylene 
to methacrolein and methacrylic acid, a temperature range of from about 
300.degree. to 500.degree. C. has been found to be optimum at atmospheric 
pressure. 
While pressures other than atmospheric may be employed it is generally 
preferred to operate at or near atmospheric pressure, since the reaction 
proceeds well at such pressures and the use of expensive high pressure 
equipment is avoided. Pressures of between atmospheric and 30 p.s.i.g. are 
most preferred. 
The apparent contact time employed in the process is not critical and may 
be selected from a broad operable range which may vary from 0.1 to 50 
seconds. The apparent contact time may be defined as the length of time in 
seconds which the unit volume of gas measured under the conditions of 
reaction is in contact with the apparent unit volume of the catalyst. It 
may be calculated, for example, from the apparent volume of the catalyst 
bed, the average temperature and pressure of the reactor, and the flow 
rates of the several components of the reaction mixture. 
The optimum contact time will, of course, vary depending upon the olefin 
being treated, but in the case of propylene and isobutylene the preferred 
apparent contact time is 0.5 to 15 seconds. 
A molar ratio of oxygen to olefin between about 0.5:1 to 10:1 generally 
gives the most satisfactory results. For the conversion of propylene to 
acrolein, and isobutylene to methacrolein and methacrylic acid, a 
preferred ratio of oxygen to olefin is from about 1:1 to about 5:1. The 
oxygen used in the process may be derived from any source; however, air is 
the least expensive source of oxygen, and is preferred. 
The addition of water to the reaction mixture has a marked beneficial 
influence on the course of the reaction in that it improves the conversion 
and the yield of the desired product. Accordingly, we prefer to include 
water in the reaction mixture. Generally, a ratio of olefin to water in 
the reaction mixture of from 1:0.5 to 1:10 will give very satisfactory 
results, and a ratio of from 1:1 to 1:6 has been found to be optimum when 
converting propylene to acrolein and acrylic acid, and isobutylene to 
methacrolein and methacrylic acid. The water, of course, will be in the 
vapor phase during the reaction. 
Inert diluents such as nitrogen and carbon dioxide may be present in the 
reaction mixture. 
Oxidation of Olefins to Nitriles 
The reactants used are the same as those employed in the production of 
aldehydes and acids above, plus ammonia. Any of the olefins described can 
be used. 
In its preferred aspect, the process comprises contacting a mixture 
comprising propylene or isobutylene, ammonia and oxygen with the catalyst 
at an elevated temperature and at atmospheric or near atmospheric 
pressure. 
Any source of oxygen may be employed in this process. For economic reasons, 
however, it is preferred that air be employed as the source of oxygen. 
From a purely technical viewpoint, relatively pure molecular oxygen will 
give equivalent results. The molar ratio of oxygen to the olefin in the 
feed to the reaction vessel should be in the range of 0.5:1 to 10:1 and a 
ratio of about 1:1 to 5:1 is preferred. 
Low molecular weight saturated hydrocarbons do not appear to influence the 
reaction to an appreciable degree, and these materials can be present. 
Consequently, the addition of saturated hydrocarbons to the feed to the 
reaction is contemplated within the scope of this invention. Similarly 
diluents such as nitrogen and the oxides of carbon may be present in the 
reaction mixture without deleterous effect. 
The molar ratio of ammonia to olefin in the feed to the reaction may vary 
between about 0.05:1 to 5:1. There is no real upper limit for the 
ammonia-olefin ratio, but there is generally no reason to exceed the 5:1 
ratio. At ammonia-olefin ratios appreciably less than the stoichiometric 
ratio of 1:1, various amounts of oxygenated derivates of the olefin will 
be formed. 
Significant amounts of unsaturated aldehydes and even unsaturated acids as 
well as nitriles will be obtained at ammonia-olefin ratios substantially 
below 1:1, i.e., in the range of 0.15:1 to 0.75:1, particularly in the 
case of higher olefins such as isobutylene. Outside the upper limit of 
this range only insignificant amounts of aldehydes and acids will be 
produced, and only small amounts of nitriles will be produced at 
ammonia-olefin ratios below the lower limit of this range. It is generally 
possible to recycle any unreacted olefin and unconverted ammonia. 
We have found that in many cases water in the mixture fed to the reaction 
vessel improves the selectivity of the reaction and yield of nitrile. 
However, reactions not including water in the feed are not to be excluded 
from this invention, inasmuch as water is formed in the course of the 
reaction. Sometimes it is desirable to add some water to the reaction 
mixture, and in general, molar ratios of added water to olefin, when water 
is added, on the order of 1:1 to 4:1 are particularly desirable. However 
higher ratios may be employed, i.e., ratios of up to about 10:1 are 
feasible. 
The reaction is carried out at a temperature within the range from about 
250.degree. to about 600.degree. C. The preferred temperature range is 
from about 350.degree. to 500.degree. C. 
The pressure at which the reaction is conducted is not critical, and the 
reaction should be carried out at about atmospheric pressure or pressures 
up to about 5 atmospheres. In general, high pressures, i.e. about 15 
atmospheres, are not suitable, since higher pressures tend to favor the 
formation of undesirable by-products. 
The apparent contact time is an important variable, and contact time in the 
range of from 0.1 to about 50 seconds may be employed. The optimum contact 
time will, of course, vary, depending upon the olefin being treated, but 
in general, a contact time of from 1 to 15 seconds is preferred. 
In general, any apparatus of the type suitable for carrying out oxidation 
reactions in the vapor phase may be employed in the execution of this 
process. The processes may be conducted either continuously or 
intermittently. The catalyst bed may be a fixed bed employing a large 
particulate or pelleted catalyst, or in the alternative, a so-called 
"fluidized" bed of catalyst may be employed. 
The reactor may be brought to the reaction temperature before or after the 
introduction of the reaction feed mixture. However, in a large scale 
operation, it is preferred to carry out the process in a continuous 
manner, and in such a system the recirculation of the unreacted olefin is 
contemplated. Periodic regeneration or reactivation of the catalyst is 
also contemplated, and this may be accomplished, for example, by 
contacting the catalyst with air at an elevated temperature. 
The products of the reaction may be recovered by any of the methods known 
to those skilled in the art. One such method involves scrubbing the 
effluent gases from the reactor with cold water or an appropriate solvent 
to remove the products of the reaction. In the recovery of nitrile 
products it may be desirable to employ acidified water to absorb the 
products reaction and neutralize unconverted ammonia. The ultimate 
recovery of the products may be accomplished by conventional means, such 
as by distillation or solvent extraction. The efficiency of the scrubbing 
operation may be improved when water is employed as the scrubbing agent by 
adding a suitable wetting agent to the water. Where molecular oxygen is 
employed as the oxidizing agent in this process, the resulting product 
mixture remaining after the removal of the aldehydes, acids and nitriles 
may be treated to remove carbon dioxide with the remainder of the mixture 
containing the unreacted olefin and oxygen being recycled through the 
reactor. In the case where air is employed as the oxidizing agent in lieu 
of molecular oxygen, the residual product after separation of the nitriles 
and other carbonyl products may be scrubbed with a non-polar solvent, 
e.g., a hydrocarbon fraction, in order to recover unreacted olefin, and in 
this case the remaining gases may be discarded. The addition of a suitable 
inhibitor to prevent polymerization of the unsaturated products during the 
recovery steps is also contemplated. 
The following examples are representative of the process conditions and 
catalyst compositions that are suitable for the process of this invention, 
however, the scope of the invention is not to be limited by these 
examples. 
In the examples, the activity of the catalysts was determined using a 
fixed-bed microreactor composed of a feed induction system, a molten salt 
bath furnace, a scrubber and a vapor phase chromatograph. The reactor was 
constructed from a 5 inches length of pipe having a 3/8 inch I.D., and a 
catalyst capacity of approximately 4 cc of catalyst. 
The catalyst employed had a particle size of 20-32 mesh. The reaction 
product obtained from the oxidation reaction was absorbed in a water 
scrubber and the ammoxidation product was absorbed in a water-hydrochloric 
acid scrubber solution. An aliquot of the scrubber liquid was subsequently 
injected into a Hewlett and Packard gas chromatograph Model No. 5750 for 
analysis. The chromatograph contained a Porapak-Q column, 2 meters in 
length and 1/8 inch in diameter. 
The column was maintained at a temperature of 180.degree. C. for the 
analysis of acrolein, methacrolein, acrylonitrile, methacrylonitrile and 
acetic acid, and at 230.degree. C. for the analysis of acrylic acid and 
methacrylic acid. The unabsorbed gaseous product, consisting essentially 
of carbon monoxide, carbon dioxide, oxygen, nitrogen and unreacted 
hydrocarbon, was analyzed by means of a Fisher Gas Partitioner. Hydrogen 
cyanide and ammonia when present were determined by titration. 
The reaction conditions employed and the conversions obtained utilizing the 
various hydrocarbon feeds and catalyst compositions described in the 
invention are summarized in Tables 1 to 5. In these experiments, the 
results are reported as the mole percent per pass conversion to the 
desired product which is defined as: 
##EQU1## 
and selectivity on a molar basis is defined as: 
##EQU2## 
The catalysts employed in Examples 1 to 40 (Tables 2 to 5) were prepared 
according to the following procedures: