Catalysts for the production of unsaturated acids or esters

Details Catalysts for the production of unsaturated acids or esters Catalysts for the production of unsaturated acids or esters

2000-12-01

2003-04-08

Silverman, Stanley S. (Department: 1754)

Catalyst, solid sorbent, or support therefor: product or process

Catalyst or precursor therefor

Silicon containing or process of making

C502S202000, C502S242000, C502S243000, C502S263000

Reexamination Certificate

active

06544924

ABSTRACT:

This invention relates to the production of ethylenically unsaturated acids or esters thereof, particularly methacrylic acid or alkyl methacrylates, and in particular to novel catalysts therefor. Such acids or esters may be made by reacting an alkanoic acid (or ester) of the formula R′—CH
2
—COOR, where R and R′ are each, independently, hydrogen or an alkyl group, especially a lower alkyl group containing for example 1-4 carbon atoms, with formaldehyde. Thus methacrylic acid or alkyl esters thereof, especially methyl methacrylate, may be made by the catalytic reaction of propionic acid, or the corresponding alkyl ester, e.g. methyl propionate, with formaldehyde in accordance with the reaction sequence:
CH
3
—CH
2
—COOR+HCHO→CH
3
—CH(CH
2
OH)—COOR
CH
3
—CH(CH
2
OH)—COOR→CH
3
—C(:CH
2
)—COOR+H
2
O
The reaction is typically effected at an elevated temperature, usually in the range 250-400° C., using a basic catalyst. Where the desired product is an ester, the reaction is preferably effected in the presence of the relevant alcohol in order to minimise the formation of the corresponding acid through hydrolysis of the ester. Also for convenience it is often desirable to introduce the formaldehyde in the form of formalin. Hence for the production of methyl methacrylate, the reaction mixture fed to the catalyst will generally consist of methyl propionate, methanol, formaldehyde and water.
Suitable catalysts that have been used include alkali metal-doped, especially cesium-doped, silica catalysts. It has been found that certain cesium-doped silica catalysts, i.e. those based upon gel silicas, have an unacceptable service life as they lose their activity and selectivity in a relatively short time. This activity loss may be attributed to two factors.
Firstly the alkali metal compound employed may exhibit appreciable volatility under the reaction conditions employed and so there may be a loss of activity through loss of alkali metal. As described in U.S. Pat. No. 4,990,662, this may be overcome by incorporating a suitable alkali metal compound into the process gas stream so that alkali metal compound is deposited on the catalyst during operation to compensate for any alkali metal compound lost through volatilisation.
Secondly, as may be inferred from U.S. Pat. No. 4,942,258, it is believed that for the alkali metal to be active, the support should have a certain minimum surface area. The requisite area is dependent on the amount of alkali metal in the catalyst: thus it may be inferred that there is a minimum surface area required per unit of alkali metal. During operation, there is a tendency for the silica support to lose surface area. Thus under the reaction conditions there is a risk of hydrolysis of the silica, not only by the water produced by the reaction, but also from water present in the reaction mixture, for example resulting from introduction of the formaldehyde as formalin. We have found that the loss of performance of the gel silica catalysts with time largely results from such hydrolysis causing a decrease in the surface area of the catalyst with time.
Typically the catalyst contains 1-10% by weight of the alkali metal. Preferably at least 2% by weight of alkali metal is employed so that the process can be operated at sufficiently low temperatures that loss of alkali metal through volatilisation can be minimised. The operation at low temperatures has the additional advantage that the rate of deposition of coke, which tends to block the pores of the silica and so reduce activity, is decreased.
We have found that the incorporation of certain modifiers, such as compounds of elements such as boron, aluminium, magnesium, zirconium, or hafnium into the catalysts, in addition to the alkali metal, retards the rate of surface area decrease. In the catalysts of the invention, it is important that the modifier is intimately dispersed in the silica, rather than simply being in the form of particles mixed with the silica particles. It is probable that the metal compounds in whatever form they are added will convert to oxides or (particularly at the surface of the silica) hydroxides before or during drying, calcination or operation of the catalyst and interact either on the surface or in the bulk of the silica structure in that form. Furthermore it is important that the amount of modifier is within certain limits: if there is too little modifier, no significant advantage accrues while if too much modifier is employed the selectivity of the catalyst may be adversely affected. Generally the amount of modifier required is in the range 0.25 to 2 gram atoms of the modifier element per 100 moles of silica.
The aforesaid U.S. Pat. No. 4,990,662 indicates that silicas may contain materials such as aluminium, zirconium, titanium, and iron compounds as trace impurities. That reference however indicates that improved catalysts are obtained if such impurities are removed by acid extraction to give a trace impurity content below 100 ppm.
EP 0 265 964 discloses the use of silica supported catalysts containing antimony as well as the alkali metal. The description indicates that the alumina content is desirably less than 500 ppm. A comparative, antimony-free, example discloses the use of a composition containing 950 ppm alumina. This corresponds to 0.11 gram atoms of aluminium per 100 moles of silica.
U.S. Pat. No. 3,933,888 discloses the production of methyl methacrylate by the above reaction using a catalyst formed by calcining a pyrogenic silica with a base such as a cesium compound, and indicates that the pyrogenic silica may be mixed with 1-10% by weight of pyrogenic zirconia. That reference also discloses the use of a catalyst made from a composition containing cesium as the alkali metal and a small amount of borax. The amount of boron however is about 0.04 gram atoms per 100 moles of silica and so is too small to have any significant stabilising effect. DE 2 349 054 C, which is nominally equivalent to U.S. Pat. No. 3,933,888, exemplifies catalysts containing zirconia or hafnia in admixture with the silica: the results quoted indicate that the zirconia or hafnia containing catalysts give a lower yield based upon the amount of formaldehyde employed.
Yoo discloses in “Applied Catalysis”, 102, (1993) pages 215-232 catalysts of cesium supported on silica doped with various modifiers. While bismuth appeared to be a satisfactory dopant, catalysts doped with lanthanum, lead or thallium gave short term improvements. However high levels of lanthanum gave products of low selectivity while low levels of lanthanum gave catalysts that sintered much faster than the bismuth doped catalysts. The effective additives were all highly toxic heavy metals with appreciable volatility: these considerations preclude their use as catalyst components.
The aforementioned U.S. Pat. No. 3,933,888 indicated that it was important to use a pyrogenic silica and showed that other types of silicas were unsuitable. The pyrogenic silicas said to be suitable are those having a total surface area in the range 150-300 m
2
/g, a total pore volume of 3-15 cm
3
/g and a specified pore size distribution wherein at least 50% of the pore content is in the form of pores of diameter above 10000 Å (1000 nm) and less than 30% is in the form of pores of diameter below 1000 Å (100 nm). In contrast, in the present invention the silicas that may be employed are preferably porous high surface area silicas such as gel silicas, precipitated gel silicas and agglomerated pyrogenic silicas.
Accordingly the present invention provides a catalyst comprising a porous high surface area silica containing 1-10% by weight of an alkali metal (expressed as metal), wherein the catalyst contains a compound of at least one modifier element selected from boron, magnesium, aluminium, zirconium and hafnium in such amount that the catalyst contains a total of 0.25 to 2 gram atoms of said modifier element per 100 moles of silica, said modifier element compound being dispersed in the pores of said silica.
The silica employed in the

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