Method for the manufacture of acrylic acid

A process for the manufacture of acrylic acid comprising reacting propylene and oxygen (preferably in the form of air) in a reaction zone having a catalyst characterized by the following formula: EQU A.sub.a B.sub.b C.sub.c Ca.sub.d Fe.sub.e Bi.sub.f Mo.sub.12 O.sub.x where PA1 A=one or more of Li, Na, K, Rb and Cs PA1 B=one or more of Mg, Sr, Mn, Ni, Co and Zn PA1 C=one or more of Ce, Cr, Al, Sb, P, Ge, Sn, Cu, V and W and PA1 a=0.01 to 1.0; b and e =1.0-10 PA1 c=0 to 5.0, preferably 0.05 to 5.0, especially preferred being 0.05 to 4.0 PA1 d and f=0.05 to 5.0, and x is a number determined by the valence requirements of the other elements present; at an elevated temperature to produce acrylic acid and acrolein.

BACKGROUND OF THE INVENTION
 The present invention is directed to an improved process for the
 manufacture of acrylic acid. Presently, acrylic acid is produced by a
 two-step process. Propylene is first oxidized to acrolein over a mixed
 metal oxide catalyst comprising iron, bismuth and molybdenum promoted with
 suitable elements, and the acrolein is further oxidized to acrylic acid
 over a second catalyst in a separate reactor. Typically, catalysts
 containing oxides of iron, bismuth and molybdenum promoted with suitable
 elements are readily available for the selective oxidation of the
 propylene to acrolein (i.e. this first step in the two-step process in the
 manufacture of acrylic acid). Examples of suitable types of catalysts for
 this first step can be found in U.S. Pat. 4,162,234 and 4,280,929 assigned
 to the assignee of the present application.
 In the second step of the two-step process acrolein is oxidized over the
 second catalyst to acrylic acid. It is always the case that the
 selectivity of the acrolein to acrylic acid is below 100%. However, the
 acrylic acid that is formed in the first step of the two-step process
 passes through the second reactor with no decomposition. Therefore, it is
 advantageous to use catalysts that produce substantially larger amounts of
 acrylic acid during the oxidation of the propylene to acrolein in the
 first reactor, thereby getting higher yields of acrylic acid in the
 two-step process.
 In related patent application U.S. Ser. No. 08/923,878 filed Sep. 2, 1997,
 and assigned to the assignee of the present invention, there is a
 disclosure of a novel catalyst useful in the manufacture of acrylonitrile
 and hydrogen cyanide. The catalyst was specifically disclosed as
 containing a mixed metal oxide of iron, molybdenum and bismuth promoted
 with various metals and useful in the manufacture of acrylonitrile with
 substantially higher yields of co-product hydrogen cyanide. It is the
 discovery of the instant application that the catalyst of co-pending
 application 08/923,878 can not only be used in the first step of the
 two-step process for the manufacture of acrylic acid, but results in
 unexpected high yield of acrylic acid during the first step of the
 process. This high yield of acrylic acid in the first step leads to a
 higher yield of acrylic acid overall being achieved in the two-step
 process.
 SUMMARY OF THE INVENTION
 It is a primary object of the present invention to provide a novel process
 for the production of acrylic acid and selected oxidation of propylene to
 acrolein.
 Additional objects and advantages of the invention will be set forth in
 part in the description which follows and in part will be obvious from the
 description, or may be learned by the practice of the invention. The
 objects and advantages of the invention may be realized and attained by
 means of the instrumentalities and combinations particularly pointed out
 in the appended claims.
 To achieve the foregoing objects and in accordance with the purpose of the
 present invention as embodied and described herein, the process of the
 present invention comprises reacting propylene and oxygen (preferably in
 the form of an oxygen-containing gas such as air) in a reaction zone
 having a catalyst characterized by the following formula:
EQU A.sub.a B.sub.b C.sub.c Ca.sub.d Fe.sub.e Bi.sub.f Mo.sub.12 O.sub.x
 where
 A=one or more of Li, Na, K, Rb and Cs
 B=one or more of Mg, Sr, Mn, Ni, Co and Zn
 C=one or more of Ce, Cr, Al, Sb, P, Ge, Sn, Cu, V and W
 and
 a=0.01 to 1.0; b and e=1.0-10
 c=0 to 5.0, preferably 0.05 to 5.0, especially preferred being 0.05 to 4.0
 d and f=0.05 to 5.0, and x is a number determined by the valence
 requirements of the other elements present;
 at an elevated temperature (e.g. 200.degree. to 600.degree. C.) to produce
 acrylic acid and acrolein.
 In the preferred embodiment of the present invention, A is selected to be
 one or more of lithium, sodium, potassium and cesium, especially preferred
 being cesium and potassium.
 In another preferred embodiment, B is selected from the group consisting of
 magnesium, manganese, nickel and cobalt, or mixtures thereof.
 In still another preferred embodiment, C is selected from the group
 comprising cerium, chromium, antimony, phosphorus, germanium, tungsten, or
 mixtures thereof, especially preferred being cerium, chromium, phosphorus,
 and germanium.
 In still another preferred embodiment of the present invention, a may range
 from about 0.05 to 0.9, especially preferred being above 0.1 to 0.7.
 In a further preferred embodiment of the present invention, b and e may
 range from about 1 to 10. In still a further preferred embodiment of the
 present invention, c, d and f may range from about 0.05 to 4, especially
 preferred being 0.1 to 3.
 A further preferred embodiment of the present invention comprises
 recovering the acrylic acid and acrolein from the first reaction zone,
 introducing at least acrolein and oxygen into a second reaction zone
 having a second catalyst to react the acrolein and oxygen at an elevated
 temperature to produce acrylic acid, and recovering the acrylic acid from
 the second reaction zone. Any suitable acrolein to acrylic acid catalyst
 may be used in this second step. For example, typical second stage
 catalysts (e.g., 62% Sb.sub.3 Sn.sub.3 V.sub.3 W.sub.1.2 Mo.sub.12
 O.sub.x.multidot.38% SiO.sub.2) as described in U.S. Pat. No. 3,840,595,
 herein incorporated by reference, are suitable in the practice of the
 present invention.
 In another preferred embodiment of the present invention, the first
 reaction from propylene to acrylic acid and acrolein takes place in a
 fluid bed reactor and the second reaction from acrolein to acrylic acid
 takes place in a fixed bed reactor.
 The catalyst of the present invention can be used either supported or
 unsupported. Preferably the catalyst is supported on silica, alumina or
 zirconium or mixtures thereof, especially preferred being silica.
 DETAILED DESCRIPTION OF THE INVENTION
 The catalysts of the present invention may be prepared by any of the
 numerous methods of catalyst preparation which are known to those of skill
 in the art. For example, the catalyst may be manufactured by
 co-precipitating the various ingredients. The co-precipitating mass may
 then be dried and ground to an appropriate size. Alternatively, the
 co-precipitated material may be slurried and spray dried in accordance
 with conventional techniques. The catalyst may be extruded as pellets or
 formed into spears in oil as is well known in the art. Alternatively, the
 catalyst components may be mixed with a support in the form of the slurry
 followed by drying or they may be impregnated on silica or other supports.
 For particular procedures for manufacturing the catalyst, see U.S. Pat.
 Nos. 5,093,299; 4,863,891 and 4,766,232 assigned to the assignee of the
 present invention, herein incorporated by reference.
 Typically, the A component of the catalyst may be introduced into the
 catalyst as an oxide or as a salt which upon calcination will yield the
 oxide. Preferably, salts such as nitrates which are readily available and
 easily soluble are used as the means of incorporating the A element into
 the catalyst.
 Bismuth may be introduced into the catalyst as an oxide or as a salt which
 upon calcination will yield the oxide. The water soluble salts which are
 easily dispersed but form stable oxides upon heat treating are preferred.
 An especially preferred source for introducing bismuth is bismuth nitrate
 which has been dissolved in a solution of nitric acid.
 To introduce the iron component into the catalyst, one may use any compound
 of iron which, upon calcination will result in the oxides. As with the
 other elements, water soluble salts are preferred for the ease with which
 they may be uniformly dispersed within the catalyst. Most preferred is
 ferric nitrate.
 Cobalt, nickel and magnesium may also be introduced into the catalyst using
 nitrate salts. However, magnesium may also be introduced into the catalyst
 as an insoluble carbonate or hydroxide which upon heat treating results in
 an oxide.
 The molybdenum component of the catalyst may be introduced from any
 molybdenum oxide such as dioxide, trioxide, pentoxide or heptaoxide.
 However, it is preferred that a hydrolizable or decomposable molybdenum
 salt be utilized as the source of the molybdenum. The most preferred
 starting material is ammonium heptamolybdate.
 Phosphorus may be introduced in the catalyst as an alkaline metal salt or
 alkaline earth metal salt or the ammonium salt but is preferably
 introduced as phosphoric acid. Calcium which is an essential ingredient in
 the catalyst of the present invention can be added via pre-formation of
 calcium molybdate or by impregnation or by other means known in the art.
 (Usually added as Ca-nitrate, along with the other nitrates.)
 The present invention is directed to a process for the production of
 acrylic acid during the oxidation of propylene to acrolein comprising
 reacting oxygen and propylene in a reaction zone in contact with a
 catalyst characterized by the following empirical formula:
EQU A.sub.a B.sub.b C.sub.c Ca.sub.d Fe.sub.e Bi.sub.f Mo.sub.12 O.sub.x
 where
 A=one or more of Li, Na, K, Rb and Cs
 B=one or more of Mg, Sr, Mn, Ni, Co and Zn
 C=one or more of Ce, Cr, Al, Sb, P, Ge, Sn, Cu, V and W
 and
 a=0.01 to 1.0; b and e=1.0-10
 c=0 to 5.0, preferably 0.05 to 5.0, especially preferred being 0.05 to 4.0
 d and f=0.05 to 5.0, and x is a number determined by the valence
 requirements of the other elements present;
 to produce acrylic acid and acrolein. Preferably, the reaction takes place
 between a temperature of 200.degree. to 500.degree. C., preferably
 300.degree. to 400.degree. C.
 The catalysts of the present invention may be prepared by mixing an aqueous
 solution of ammonium heptamolybdate with a silica sol, adding a slurry
 containing the compounds of the other elements to the aqueous solution,
 drying the solution, denitrifying and calcining. The catalyst may be
 spray-dried at a temperature of between 110.degree. C. to 350.degree. C.
 The denitrification temperature may range from 100.degree. C. to
 450.degree. C. Finally, calcination takes place at a temperature of
 between 400.degree. C. to 700.degree. C.
 A further preferred embodiment of the present invention comprising
 recovering the acrylic acid and acrolein produced in the first reaction
 zone, introducing at least the acrolein and oxygen (preferably, air is the
 source for the oxygen) into a second reaction zone at an elevated
 temperature containing a second catalyst suitable for the conversion of
 acrolein to acrylic acid to convert the acrolein to acrylic acid and
 recovering the acrylic acid from the second reaction zone. Suitable
 catalysts for use in the conversion of acrolein to acrylic acid are
 described in previously cited U.S. Pat. No. 3,840,595, herein incorporated
 by reference. Specific examples of catalysts useful in the second reaction
 zone include Mo.sub.9 V.sub.2 W.sub.1 Cu.sub.1 Sn.sub.0.4 O.sub.x ;
 Mo.sub.10 W.sub.1 V.sub.3 Sb.sub.2 Cu.sub.1 Nb.sub.2 O.sub.x ; Mo.sub.12
 V.sub.3 W.sub.1.2 Cu.sub.2 Ti.sub.0.5 O.sub.x ; Mo.sub.9 V.sub.2 W.sub.1
 Cu.sub.1.5 Sn.sub.0.4 P.sub.1 O.sub.x ; Mo.sub.12 V.sub.3 W.sub.1.2
 Cu.sub.2 Sn.sub.0.5 O.sub.x and Sb.sub.3 Sn.sub.3 V.sub.3 W.sub.1.2
 Mo.sub.12 O.sub.x. These catalysts are supported on an inert support such
 as alumina, zirconia or silica, preferably silica. Typically, the
 supported catalyst comprises 70 to 75wt% active phase and 25 to 30wt%
 inert support.