Catalyst and process for production of an alpha, beta-ethylenically unsaturated monocarboxylic acid

FIELD OF THE INVENTION
This invention relates to a catalyst and process for the production of an alpha, beta-ethylenically unsaturated monocarboxylic acid. More specifically, this invention relates to a catalyst and process for synthesizing methacrylic acid by the vapor-phase condensation of propionic acid with formaldehyde.
BACKGROUND OF THE INVENTION
Unsaturated carboxylic acids such as methacrylic acid, and the esters of such acids such as methyl methacrylate, are widely used for the production of corresponding polymers, resins and the like. Typically, a saturated monocarboxylic acid, such as propionic acid (PA), can be catalytically reacted with formaldehyde (FA) to produce an alpha, beta-ethylenically unsaturated monocarboxylic acid, such as methacrylic acid (MA), and water as a co-product. The produced alpha, beta-ethylenically unsaturated monocarboxylic acid can be esterified to a polymerizable, alpha, beta-ethylenically unsaturated monocarboxylic acid ester, such as methyl methacrylate (MMA).
MMA is a monomer containing a carbon-carbon double bond (>C.dbd.C<) and a carbonyl group ##STR1## Polymers derived from MMA are sometimes also referred to as "acrylic" or "acrylic-type" polymers. The MMA-type polymers have desirable transparency, weatherability and physical strength properties. Typical end-uses for MMA-derived polymers include acrylic sheet that can be fabricated into signs, advertising displays, lighting fixtures, glazing materials, structural panels and the like, molding resins for automobile, bus, truck and other vehicular tail-light lenses, plumbing and electrical fixtures and the like, as well as constituents of a variety of surface coatings, adhesives, inks, floor polishes and the like.
Generally, the condensation reaction to synthesize an alpha, beta-ethylenically unsaturated aliphatic monocarboxylic acid, such as MA, takes place in the vapor or gaseous phase and in the presence of a catalyst which can be basic, acidic, or substantially neutral. In the absence of the catalyst, reactants typically require addition of heat energy to overcome an "energy of activation" of the reaction, which can be a barrier to formation of the desired products. Also, in the instance where the reactants are chemically converted to a variety of products, a catalyst may tend to increase the rate of formation of one product relative to one or more of the other products. Such a catalyst is said to possess increased selectivity qualities, often a consideration when choosing a catalyst for commercial production purposes.
Reaction temperature plays an important role in the activity of a catalyst, another important consideration. At a particular temperature, for example, a commercially-acceptable percentage of the reactants might be converted to a desired product, with only a relatively minor percentage of the reactants being converted to undesired by-products. Typically, an increase in the temperature of the reaction not only tends to increase the rate at which the reactants are converted to the desired product or products, but also tends to increase the rate at which undesired by-products are produced as well.
Catalysts commonly used for reacting PA with FA to produce MA are alkali metals supported on silica. Typical catalysts of this type are disclosed in U.S. Pat. No. 4,147,718 to Gaenzler et al., U.S. Pat. No. 3,933,888 to Schlaefer, U.S. Pat. No. 3,840,587 to Pearson, U.S. Pat. No. 3,247,248 (see also Canadian Pat. No. 721,773) to Sims et al., and U.S. Pat. No. 3,014,958 to Koch et al.
These prior-art catalysts, while effecting condensation of PA with FA to produce MA, unfortunately also generate undesirable by-products that have to be separated from the produced MA. Relatively low conversion and/or selectivity performance values, together with relatively low catalyst useful-life values, are additional drawbacks.
Generally, when PA and FA are reacted in the vapor phase, and in the presence of a catalyst, to produce desired product MA and co-product H.sub.2 O, a variety of undesirable by-products are simultaneously produced as well. The more common of these undesirables are hereinafter referred to as by-product A (2,5-dimethyl-2-cylopenten-1-one), by-product B (2,4,4-trimethyl-gamma-butyrolactone), and by-product 3-P (3-pentanone). The presence of these by-products is generally undesirable because current MA-esterification and MMA-polymerization technology requires separation of these by-products either from the MA before it is esterified to MMA, or before the produced MMA is polymerized. It is additionally desirable to remove by-product A from the MA prior to esterification as the presence of this by-product tends to interfere with the desired formation of MMA. In particular, the presence of by-product A in the MA tends to cause an undesirable polymerization of MA and attendant separation problems. Loss of product also may become significant.
Accordingly, it would be desirable to have a catalyst which provides improved PA conversion, which decreases undesirable by-product generation, and which enhances useful catalyst life. The catalyst, and the catalyst support, of the present invention meet the foregoing desires.
Not only has the catalyst of the present invention been observed to be more active than conventional catalysts (i.e. the present catalyst has been observed to enable the MA-synthesizing, gas-phase, condensation reaction of PA with FA to take place at a relatively lower temperature for a given conversion); but the catalyst has also been observed to exhibit increased selectivity toward production of MA as well. While reduction of reaction temperature tends to increase the useful life of the catalyst per se, a reduced operating temperature may reduce overall operating costs as an added benefit. The reduction in the amounts of the undesirable by-products, moreover, tends to reduce, and may even eliminate, the costs attendant to (1) the removal of the undesirable by-products from the MA prior to esterification, and (2) the purification of the MMA prior to polymerization.
SUMMARY OF THE INVENTION
The particulate catalyst of the present invention is suitable for synthesis of an unsaturated carboxylic acid. This catalyst is especially suitable for condensation of a saturated monocarboxylic acid, such as propionic acid, with formaldehyde to produce an alpha, beta-ethylenically unsaturated monocarboxylic acid, such as methacrylic acid.
The present catalyst comprises SiO.sub.2, SnO.sub.2 (together referred to below as "SiO.sub.2 -SnO.sub.2 mixed-oxide") and cesium ions. The cesium ions, the primary catalytically active ingredient of the present catalyst, are present on the catalyst in the +1 oxidation state and in an amount of about 1 to about 15 percent by weight, preferably in an amount of about 4 to about 10 percent by weight, and more preferably in an amount of about 7 to about 10 percent by weight, based on the weight of the catalyst. The present catalyst may also include a relatively small amount of boron.
The present SiO.sub.2 -SnO.sub.2 mixed-oxide comprises a porous silica gel, and particulate SnO.sub.2 having a particle size of no more than about 10,000 microns. This SiO.sub.2 -SnO.sub.2 mixed-oxide contains SiO.sub.2 in an amount of about 5 to about 85 percent by weight of the total, and SnO.sub.2 in an amount of about 95 to about 15 percent by weight of the total. The present SiO.sub.2 -SnO.sub.2 mixed-oxide has a surface area of about 10 to about 300 m.sup.2 / gram, preferably about 50 to about 135 m.sup.2 / gram, a porosity of less than about 5 cm.sup.3 / gram, preferably less than about 1 cm.sup.3 / gram, and a pore size distribution such that less than about 10 percent of the pores present in the catalyst have a pore diameter greater than about 750 Angstroms.
Preferably, the relative amounts of SiO.sub.2 and SnO.sub.2, based on the weight of the SiO.sub.2 -SnO.sub.2 mixed-oxide, are about 30 to about 70 percent by weight SiO.sub.2 and about 70 to about 30 percent by weight SnO.sub.2, more preferably about 40 to about 60 percent by weight SiO.sub.2 and about 60 to about 40 percent by weight SnO.sub.2. Particularly preferred are mixtures of tin and silicon oxides containing about 40 to about 50 percent by weight SiO.sub.2 and about 60 to about 50 percent by weight SnO.sub.2. Most preferred is a mixture of oxides containing about 50 percent by weight of each of SiO.sub.2 and SnO.sub.2.
Of the pores present in the SiO.sub.2 -SnO.sub.2 mixed-oxide, a major portion preferably has a pore diameter of about 50 to about 500 Angstroms, and more preferably has a pore diameter of about 80 to about 300 Angstroms.
Average cesium ion site density on the SiO.sub.2 -SnO.sub.2 mixed-oxide surface is preferably about 1 to about 10 cesium ions, in the +1 oxidation state, per square nanometer of the catalyst support surface area. More preferably, the average cesium ion site density on the catalyst support surface is about 2 to about 7 cesium ions per square nanometer of the SiO.sub.2 -SnO.sub.2 mixed-oxide surface area.
A cesium compound can be placed on either oxide of the SiO.sub.2 -SnO.sub.2 mixed-oxide, or both, before they are admixed or afterwards. It is likely that the cesium compound on the surface of the SnO.sub.2 component, if present initially, migrates to the SiO.sub.2 component of the SiO.sub.2 -SnO.sub.2 mixed-oxide under reaction conditions.
Another aspect of the present invention is directed to methods for making the present catalyst and SiO.sub.2 -SnO.sub.2 mixed-oxide. One way of making the present catalyst is to produce a freely-flowing slurry having a SiO.sub.2 /SnO.sub.2 weight ratio of about 0.05 to about 6 and a Cs/(SiO.sub.2 +SnO.sub.2 +Cs) weight ratio of about 0.01 to about 0.15, solidify the produced slurry to a gel, dry the obtained gel to a crushable solid state, and calcine the dried gel for a time period sufficient to remove most of the adsorbed moisture therefrom. The freely-flowing slurry is produced by combining, with agitation, desired amounts of a silica sol containing silica particles of about 50 to about 200 Angstroms in diameter, a slurry containing SnO.sub.2 particles, and a cesium compound able to provide cesium in the +1 oxidation state on the catalyst surface.
Preferably, the silica sol is aqueous and has a silica content of about 14 to about 34 weight percent.
The cesium compound is preferably a salt selected from the group consisting of cesium carbonate, cesium hydroxide, cesium phosphate, cesium fluoride and cesium nitrate, and more preferably is cesium carbonate.
The produced, freely-flowing slurry can include boric acid as a source of boron, and/or ammonium nitrate as a gelling promoter. The pH of the produced, freely-flowing slurry can be adjusted to a value of about 7, prior to solidification, to reduce the time period required to effect gelling. An inorganic acid, preferably nitric acid, is used for pH-adjustment purposes.
The produced gel can be formed into beads, cylinders or other suitable shapes of a desired configuration, and then dried. Alternatively, the produced gel can be dried first and then subjected to a size-reduction operation. The drying can be carried out either at about atmospheric pressure, or at a subatmospheric pressure and at a relatively lower temperature. Preferably, the gel is dried to substantially constant weight.
Calcining of the dried gel preferably is carried out for a time period sufficient to reduce the weight of the dried gel by at least about 2 weight percent. Preferably, calcination is carried out to a weight loss of about 3 to about 4 wt. %.
The catalyst of the present invention can also be made by first preparing the above-described mixture of silicon and tin (IV) oxides and then contacting (such as by impregnation) the mixture with the cesium compound.
Still another method of making the catalyst of the instant invention is to physically mix the porous silica gel impregnated with a cesium compound with the tin (IV) oxide. This mixture can then be made into a form suitable for use in the reactor chosen.
Yet another way of making the catalyst is to grind the porous silica gel containing the cesium compound with the tin (IV) oxide and to form the result into a catalyst suitable for commercial use.

BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings,
FIG. 1 is a graph showing percent selectivity toward production of by-product A as a function of percent conversion of propionic acid (PA) to methacrylic acid (MA); and
FIG. 2 is a graph comparing the conversion and selectivity performance of the cesium-containing SiO.sub.2 -SnO.sub.2 mixed-oxide catalyst of the present invention to that of a cesium-containing control catalyst having silica as the catalyst support.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The catalyst of the present invention is particularly well-suited for the gas-phase synthesis of methacrylic acid (MA) via condensation of formaldehyde (FA) with propionic acid (PA). The catalyst of the present invention, a three-component catalyst containing porous silica gel, tin (IV) oxide and supported cesium ions, catalytically induces this condensation reaction. The cesium ions are present on the catalyst surface in the +1 oxidation state and in amounts discussed hereinbelow.
The vapor-phase condensation reaction takes place in a reactor suitable for effecting desired heterogeneous catalysis. The reactor effluent commonly includes not only the desired synthesis product, MA, and water as a co-product of the desired condensation reaction, but also unreacted PA and FA, a variety of organic by-products and certain relatively volatile gases as well. The thus-synthesized MA can be separated from the unconverted PA and FA reactants and the above-identified co-product and by-products, then esterified with a suitable C.sub.1 to C.sub.4 aliphatic alcohol such as methanol (MeOH), and thereafter can be purified to obtain a polymer-grade ester such as methyl methacrylate (MMA).
In the ensuing detailed description, certain terms will be utilized for purposes of conciseness, and for purposes of elucidating the features and advantages of the present invention. These terms are defined hereinbelow.
The term "activity of a catalyst" as used herein means the relative ease or difficulty of the catalyst at a given temperature to effect chemical conversion of the reactants to the desired product or products.
The terms "average pore radius" and "average pore diameter" as used herein mean the respective dimensions as determined utilizing the well-known BET nitrogen desorption method. [See, e.g., S. Brunauer et al., J.A.C.S., 60, 309, (1938).] The reported values are those representing a pore radius or diameter where one-half of the pore volume present exhibits a radius or diameter smaller than the reported value and one-half of the pore volume present exhibits a radius or diameter greater than the reported value, respectively.
The term "catalyst" as used herein broadly means a substance which increases the rate at which a thermodynamically-allowable chemical reaction takes place. Typically, relatively small percentages of catalyst markedly affect the rate of a given chemical reaction.
The term "calcining" as used herein means subjecting dried material to a temperature of at least about 250.degree. C. (about 480.degree. F.).
The term "colloid" as used herein means a suspension of finely divided particles that do not settle out of, and cannot be readily filtered from, the medium in which they are suspended.
The term "colloidal silica" as used herein describes a dispersion of typically millimicron-size SiO.sub.2 particles in an aqueous or organic solvent. Colloidal silica is also sometimes referred to as "silica sol." Illustrative silica sols are those commercially available from the Nalco Chemical Company, Oakbrook, Ill. 60521, under the designations Nalco 1034-A and Nalco 2326 silica sols.
Nalco 1034-A silica sol has a silica concentration of about 34 weight percent, based on the weight of the silica sol, a mean silica particle size of about 20 nanometers, a pH of about 3.2, a particulate surface area of about 150 m.sup.2 / gram of SiO.sub.2, a viscosity of about 10 centipoises, and a Na.sub.2 O content of less than about 0.05 wt. %, based on the weight of the silica sol.
Nalco 2326 silica sol is an ammonium-stabilized silica sol, has a silica concentration of about 14.5 weight percent, based on the weight of the silica sol, has a mean silica particle size of about 5 nanometers, exhibits a pH of about 9, has a particulate surface area of about 600 m.sup.2 / gram of SiO.sub.2, has a viscosity of about 5 centipoises, and has a Na.sub.2 O content of less than about 0.05 wt. %, based on the weight of the silica sol.
The term "drying" as used herein means subjecting the material to be dried to a temperature of no more than about 250.degree. C. (about 480.degree. F.).
The term "silica gel" as used herein describes a coherent, rigid, continuous three-dimensional network of spherical particles of colloidal silica. Silica gel has not been observed to possess an ordered crystal structure, but rather, has been observed to be constituted primarily by silica (SiO.sub.2) in amorphous state. Silica gel per se is known to catalyze certain chemical reactions, and is used as a catalyst carrier or support in a number of commercial catalytic processes.
The term "WHSV" as used herein means weight hourly space velocity, and is expressed as grams of feed per gram of catalyst per hour.
Additional definitions include the following equations. ##EQU1## Similar terminology will be utilized to describe the yield, selectivity and conversion of the organic by-products when the performance of the present catalyst is discussed.
The catalyst suitable for the purposes of the present invention is constituted principally by porous silica gel particles and tin (IV) oxide particles and has the catalytically active ingredient, cesium ion, on the surface thereof. Neither porous silica gel nor tin (IV) oxide exerts a noticeable catalytic effect vis-a-vis the condensation reaction; however, the combination of these constituents provides certain advantages with respect to the overall performance of the present catalyst under process conditions as will be discussed in greater detail hereinbelow.
The relative amounts of SiO.sub.2 and SnO.sub.2 in the present SiO.sub.2 -SnO.sub.2 mixed-oxide can vary over a relatively wide range. In particular, it can contain about 5 to about 85 weight percent SiO.sub.2 and about 95 to about 15 weight percent SnO.sub.2. The catalyst of the present invention comprises the foregoing SiO.sub.2 -SnO.sub.2 mixed-oxide and cesium ions.
The cesium ions, the catalytically active ingredient of the present invention, are present on the present catalyst support surface in the +1 oxidation state and in an amount of about 1 to about 15 percent by weight, based on the weight of the catalyst. Preferably, the cesium ions are present in an amount of about 4 to about 10 percent by weight, and more preferably in an amount of about 7 to about 10 percent by weight, based on the weight of the catalyst. The average cesium ion site density on the catalyst surface is preferably about 1 to about 10 cesium ions per square nanometer of the catalyst surface area, and more preferably is about 2 to about 7 cesium ions per square nanometer of the catalyst surface area. The average cesium ion site density in any given instance is determined by the relative amount of cesium compound used in making the present catalyst, together with the surface area of the present catalyst.
The SiO.sub.2 -SnO.sub.2 mixed-oxide of the present invention comprises a porous silica gel, and SnO.sub.2 particles having a particle size of up to about 1 centimeter (10,000 microns) distributed substantially uniformly throughout the silica gel. Preferably, the SnO.sub.2 particles are less than about 100 microns (0.1 mm) in diameter; more preferably, the SnO.sub.2 particles are less than about 10 microns (0.01 mm) in diameter. They should not be smaller than about 0.03 microns (about 300 Angstroms) in diameter. In preparing the SiO.sub.2 -SnO.sub.2 mixed-oxide, any suitable SnO.sub.2 -containing media can be utilized. However, use of SnO.sub.2 colloids is not preferred.
The catalyst has a surface area of about 10 to about 300 m.sup.2 / gram, preferably about 50 to about 135 m.sup.2 / gram, a porosity of less than about 5 cm.sup.3 / gram, preferably less than about 1 cm.sup.3 / gram, and a pore size distribution such that less than about 10 percent of the pores present in the catalyst have a pore diameter less than about 50 Angstroms, and such that less than about 10 percent of the pores present in the catalyst have a pore diameter greater than about 750 Angstroms.
The pore size distribution can be controlled by appropriate selection of the silica particle size and particle size distribution in the silica sol that is used as one of the starting materials for the present catalyst support. For example, a silica particle size of about 20 nanometers (about 200 Angstroms) will provide a relatively larger pore size than a silica particle size of about 5 nanometers (about 50 Angstroms).
The relative amounts of SiO.sub.2 and SnO.sub.2 present, based on the weight of the SiO.sub.2 -SnO.sub.2 mixed-oxide of the present invention, are preferably about 30 to about 70 percent by weight SiO.sub.2 and about 70 to about 30 percent by weight SnO.sub.2, and more preferably about 40 to about 60 percent by weight SiO.sub.2 and about 60 to about 40 percent by weight SnO.sub.2. Particularly preferred is a SiO.sub.2 -SnO.sub.2 mixed-oxide containing about 40 to about 50 percent by weight SiO.sub.2 and about 60 to about 50 percent by weight SnO.sub.2. Most preferred is a material containing about 50 percent by weight of each of SiO.sub.2 and SnO.sub.2.
Of the pores present in the catalyst, a major portion preferably has a pore diameter of about 50 to about 500 Angstroms, and more preferably has a pore diameter of about 80 to about 300 Angstroms. Most preferably, the average pore diameter is about 90 to about 200 Angstroms.
The SiO.sub.2 -SnO.sub.2 mixed-oxide-and-Cs-containing catalysts of the present invention can be prepared in a preferred manner by forming a freely-flowing slurry containing silica sol, SnO.sub.2, and a cesium compound able to provide cesium (Cs) in the +1 oxidation state. The slurry is then gelled, dried, and calcined.
A preferred method for making the catalyst of the present invention comprises the steps of producing a freely-flowing slurry having a SiO.sub.2 / SnO.sub.2 weight ratio of about 0.05 to about 6 and a Cs/(SiO.sub.2 +SnO.sub.2 +Cs) weight ratio of about 0.01 to about 0.15, solidifying the produced slurry to a gel, drying the obtained gel to a crushable solid state, and calcining the dried gel for a time period sufficient to remove primarily adsorbed moisture therefrom. This freely-flowing slurry is produced by combining the respective amounts of (1) a silica sol containing silica particles of about 50 to about 200 Angstroms in diameter, (2) a slurry containing SnO.sub.2 particles less than about 10 microns in diameter, and (3) a cesium compound able to provide cesium ions in the +1 oxidation state on the catalyst support surface. The foregoing slurry constituents are combined with agitation to produce a substantially uniform admixture.
Preferably, the silica sol that is used in the production of the freely-flowing slurry is aqueous and has a silica content of about 14 to about 34 weight percent, based on the weight of the silica sol; and the contained silica has a mean particle size of about 5 to about 20 nanometers (about 50 to about 200 Angstroms). As pointed out above, however, the pore size of the produced catalyst can be adjusted by modulating the particle size distribution of the silica sol utilized. For example, silica sols having different mean particle sizes can be commingled in various proportions prior to gelling.
The cesium compound, for purposes of the present invention, can be relatively volatile, water or solvent soluble, or thermally decomposable.
Illustrative of the thermally decomposable cesium compounds that can be utilized are cesium borofluoride (CsBF.sub.4), cesium bromate (CsBrO.sub.3), cesium bromochloride iodide (CsIBrCl), cesium dibromoiodide (CsIBr.sub.2), cesium perchlorate (CsClO.sub.4), cesium chloroiodide (CsICl.sub.2), cesium dichloroiodide (CsICl.sub.2), cesium hydride (CsH), cesium permanganate (CsMnO.sub.4), cesium nitrate (CsNO.sub.3), cesium oxide (Cs.sub.2 O), and the like.
Illustrative of the relatively volatile cesium compounds that can be utilized are cesium dibromochloride (CsBr.sub.2 Cl), cesium formate [Cs(CHO.sub.2 .multidot.H.sub.2 O)], cesium hydrofluoride (CsF.multidot.HF), cesium hydrogencarbide (CsHC.sub.2), cesium hydroxide (CsOH), cesium pentaiodide (CsI.sub.5), cesium triiodide (CsI.sub.3), cesium hydrogen nitrate (CsNO.sub.3 .multidot.NHO.sub.3), cesium dihydrogen nitrate (CsNO.sub.3 .multidot.2HNO.sub.3), cesium peroxide (Cs.sub.2 O.sub.2), cesium trioxide (Cs.sub.2 O.sub.3), cesium propionate [Cs(C.sub.3 H.sub.5 O.sub.2)], and the like.
Illustrative of the water-soluble cesium compounds that can be utilized are cesium acetate [Cs(C.sub.2 H.sub.3 O.sub.2)], cesium azide (CsN.sub.3), cesium benzoate [Cs(C.sub.7 H.sub.5 O.sub.2)], cesium monobromide (CsBr), cesium carbonate (Cs.sub.2 CO.sub.3), cesium hydrogen carbonate (CsHCO.sub.3), cesium chlorate (CsClO.sub.3), cesium chloride (CsCl), cesium chromate (Cs.sub.2 CrO.sub.4), cesium fluoride (CsF), cesium fluosilicate (Cs.sub.2 SiF.sub.6), cesium formate [Cs(CHO.sub.2)], cesium hydroxide (CsOH), cesium iodide (CsI), cesium nitrate (CsNO.sub.3), cesium oxalate [Cs.sub.2 (C.sub.2 O.sub.4)], cesium salicylate [Cs(C.sub.7 H.sub.5 O.sub.3)], cesium selenate (Cs.sub.2 SeO.sub.4), cesium hydrogen tartrate [CsH(C.sub.4 H.sub.4 O.sub.6)], and the like. The cesium compound can also be soluble in a water-miscible or water-immiscible organic solvent.
Specifically, the cesium compound is preferably selected from the group consisting of cesium carbonate, cesium hydroxide, cesium phosphate, cesium propionate, cesium fluoride and cesium nitrate, and more preferably is cesium carbonate or cesium propionate.
More particularly, the cesium is preferably contained in an aqueous solution of relatively high cesium concentration, usually approaching saturation for the particular compound that is utilized. More dilute solutions can be used, if desired.
The produced, freely-flowing slurry can include boric acid as a boron source and/or ammonium nitrate as a gelling promoter. To promote gelling rate, the pH of the produced, freely-flowing slurry can be adjusted to about 7. An inorganic acid, preferably nitric acid, is used for such a desired pH-adjustment purpose.
Many of the cesium salts disclosed herein, as well as SnO.sub.2, per se, can act as gelling promoters, at least to a limited extent. It is preferable, however, to add a specific gelling promoter, so as to provide the present catalyst support, or the present catalyst per se, with the desired herein-described physical properties within a given time period. A suitable gelling promoter for this purpose is ammonium nitrate (NH.sub.4 NO.sub.3). The concentration of the gelling promoter in the slurry can be in the range of about 0.5 to about 1.5 wt. %, based on the weight of the slurry.
In one method aspect of this invention, the produced gel can be formed into beads, cylinders or other suitable shapes of a desired configuration, and then dried. For example, the gel can be dried as a sheet-form material.
Preferably, the drying is carried out at about atmospheric pressure. Alternatively, the drying step can be carried out at a subatmospheric pressure and at a relatively lower temperature, e.g. about 150.degree. C. (about 300.degree. F.) or below. The gel is then dried to a crushable solid state. It is preferred to dry the gel to substantially constant weight. Drying can be carried out in ambient atmosphere or in an inert atmosphere, as desired.
When it is desirable to dry the gel as a sheet-form material, the dried gel can be comminuted, such as by crushing, prior to further heat treatment.
Calcining of the dried gel is carried out for a time period sufficient to reduce the weight of the dried gel by at least about 2 weight percent. Preferably, calcination is carried out to a weight loss of about 3 to about 4 wt. %. Calcining preferably is carried out at a temperature in the range from about 450.degree. C. to about 600.degree. C. for a time period of up to about 8 hours. Calcination temperature and time are catalyst preparation parameters which can be used to fine tune catalyst selectivity and activity. For a given cesium loading as calcination temperature increases initial surface area decreases. This changes the density of cesium ions on the surface influencing catalyst selectivity. Longer calcination times can also result in lower initial surface area for the catalyst.
In the examples appearing below, the following conditions were maintained, and the following equipment and procedures were used, unless otherwise indicated.
Reagent-grade trioxane was used as the FA source; however, in the conversion of PA with FA in the presence of the catalyst of this invention to synthesize MA any suitable source of formaldehyde can be used such as formalin, paraformaldehyde, methanolic formaldehyde, substantially anhydrous formaldehyde, and the like.
A laboratory minireactor was used to determine the MA-synthesis performance of each catalyst. All experimental determinations or runs were conducted at a PA/FA mole ratio of about 3/2. The minireactor comprised an elongated 12.7 mm. (millimeter) O.D. quartz tube having an externally-controllable thermowell longitudinally disposed in, and along the longitudinal axis of, the quartz tube. Catalyst to be tested, for MA-synthesis performance-determination purposes, was placed in the quartz tube and about the thermowell, forming an annular catalyst bed. Each bed of catalyst contained about 2 to about 3 grams of catalyst having a particle size of about 20 to about 40 mesh (U.S. Sieve). A spun quartz plug supported each catalyst bed.
The trioxane was thermally cracked by passing the feed through a hot reactor zone, heated to a temperature of about 390.degree. C. (about 735.degree. F.) to about 440.degree. C. (about 825.degree. F.), and located above the catalyst zone, prior to passing the feed through the catalyst bed.
The minireactor was operated at WHSV values between about 1.0 and about 3.2. Variations in the WHSV value within this range were not observed to affect the relationship between conversion and selectivity of the catalysts being tested.
The vapor-phase synthesis of methacrylic acid from propionic acid commonly produces coke, which is observed to deposit on the catalyst surface. Such coke deposits are usually removed from the catalyst by burning off with oxygen utilizing dilute air. Typically, catalyst decoking is effected whenever MA-synthesis performance of the catalyst falls below a predetermined criterion, such as a given value of percent conversion of PA to MA.
Initial performance studies were generally carried out by subjecting each investigated catalyst to appropriate MA-synthesis conditions for about 30 minutes prior to collecting the desired number of aliquot samples for analytical purposes, and thereafter decoking with air before removing additional aliquot samples. This particular catalyst decoking procedure was utilized to reduce the likelihood of a variable build-up of coke upon the catalyst, which might affect evaluation of catalyst performance.
General sampling procedures, for analytical purposes, included collection of about 10 to about 25 grams of the reactor effluent in a tared U-tube, or Erlenmeyer-type receiver containing about 10 to about 25 grams of isopropanol at room temperature (i.e. about 25.degree. C.). Reactor effluent samples were thereafter analyzed via gas chromatography (GC), employing internal-standard techniques. That is, the GC response for each of the organic components in the minireactor effluent was based upon the known response of the GC to an internal standard added to the sample. Actual PA titrations indicated that the propionic acid used in the feed was about 99.6 to about 99.9% pure.
Unless otherwise stated, pore volume, surface area and average pore diameter were determined by the BET nitrogen desorption test.
Each of the Tables appearing below presents the MA-synthesis performance data of a single catalyst over a period of time, unless stated otherwise.
The stated percentages of SiO.sub.2, SnO.sub.2 and certain other materials were based upon the weight of the SiO.sub.2 -SnO.sub.2 mixed-oxides. The stated percentages of cesium were based on the weight of the catalyst.
EXAMPLES
EXAMPLE 1
Catalyst Containing About 4 wt. % Cs+1 on 50-50 wt. % SiO.sub.2 -SnO.sub.2 Mixed-Oxide Support
A slurry was formed by combining, with agitation, particulate SnO.sub.2 (about 50 grams) suspended in deionized water (about 125 milliliters), Cs.sub.2 CO.sub.3 (about 5 grams) dissolved in water (about 10 milliliters) and silica sol (Nalco 1034-A silica sol). Ammonium nitrate (about 5 grams) in water (5 milliliters) was also added. The resulting admixture was left standing and was observed to gel in about 1 hour.
Thereafter, the obtained gel was dried in a microwave oven to constant weight, and then crushed. The dried-and-crushed gel was calcined at a temperature of about 540.degree. C. (about 1000.degree. F.) for about 8 hours. A catalyst containing about 4 wt. % Cs on a SiO.sub.2 -SnO.sub.2 mixed-oxide of about 50-50 wt. % SiO.sub.2 / SnO.sub.2 was obtained.
A Cs+1-on-silica gel "control" catalyst was prepared in substantially the same manner, except that particulate SnO.sub.2 was omitted and replaced by an equivalent amount of silica. In particular, the silica control catalyst was prepared by combining with agitation silica sol (about 280 grams of Nalco 1034-A silica sol), boric acid (about 0.3 grams in about 15 ml. of deionized water), and Cs.sub.2 CO.sub.3 (about 5 grams in about 50 ml. of deionized water). After the combined ingredients had been stirred for about 15 minutes, NH.sub.4 NO.sub.3 (about 2.8 grams) was added to the mixture. The pH of the mixture was adjusted to 7.0 by the addition of concentrated nitric acid (HNO.sub.3). After gelation, the catalyst was dried to constant weight in a microwave oven, and calcined at 540.degree. C. (about 1000.degree. F.) for about 8 hours. A Cs+1-on-silica control catalyst having a Cs content of about 3.75 wt. %, based on the weight of the catalyst, was obtained. While the control catalyst make up composition included about 500 parts by weight of boron per million parts by weight of the catalyst (p.p.m.), the presence of this level of boron was not observed to affect the catalyst performance.
Each of the catalysts thus produced was then subjected to MA-synthesis performance testing in the laboratory minireactor. The MA-synthesis performance data of the control catalyst is presented in Table I, and that of the SiO.sub.2 / SnO.sub.2 mixed-oxide catalyst is presented in Table II, below.
The performance of the control catalyst (Table I) was determined in the laboratory minireactor at a temperature ranging from about 297.degree. C. (about 567.degree. F.) to about 350.degree. C. (about 662.degree. F.), and at a WHSV value ranging from about 1.03 to about 2.60 hr.-1
TABLE I______________________________________MA-Synthesis Performanceof a 3.75 wt. % Cs on Silica CatalystRun % % % S % S % SNo. PAC % PAS PAY T (.degree.C.) WHSV A B 3-P______________________________________1 48 65 31 350 1.45 9.1 4.7 1.12 41 76 31 333 1.46 4.7 2.6 0.73 38 79 30 327 1.46 4.4 2.3 0.84 35 82 29 320 1.46 3.4 1.8 0.75 31 89 27 315 1.46 3.1 1.6 0.86 27 94 25 308 1.46 2.6 1.4 0.47 25 93 23 304 1.47 2.1 1.1 0.88 28 93 26 312 1.46 2.6 1.4 0.89 26 93 24 307 1.46 2.3 1.2 0.610 32 88 28 318 1.47 3.1 1.6 0.611 31 84 26 314 1.47 2.5 1.4 0.712 34 82 28 319 1.49 2.9 1.5 0.713 36 81 29 326 1.48 3.8 1.9 0.714 29 82 24 325 2.55 2.1 1.6 0.815 30 86 26 330 2.55 2.9 2.1 1.116 38 74 28 320 1.04 3.7 1.7 0.817 25 87 22 319 2.53 1.9 1.4 1.018 33 78 26 310 1.04 3.0 1.4 0.819 22 83 18 310 2.57 1.3 1.2 0.920 31 73 23 302 1.04 2.0 0.9 0.621 19 78 15 302 2.60 0.6 0.6 0.922 27 81 22 297 1.03 1.5 0.8 0.623 15 92 14 297 2.55 1.1 0.7 0.924 47 67 32 350 1.50 6.9 3.7 1.125 31 83 26 315 1.50 2.4 1.6 0.726 31 82 26 316 1.50 2.5 1.3 0.727 30 87 26 315 1.47 2.5 1.4 0.728 28 89 25 314 1.48 2.0 1.3 0.8______________________________________
Summarizing, the data of Table I show that at a PA conversion of about 30%, the selectivity to MA was about 86 to about 87%. (Run Nos. 15 and 27.) In particular, a temperature ranging from about 315.degree. C. (about 599.degree. F.) to about 330.degree. C. (about 626.degree. F.) at a WHSV ranging from about 1.47 to about 2.55 hr.-1 resulted in a PA conversion of about 30%.
The MA-synthesis performance data of two separate SiO.sub.2 -SnO.sub.2 mixed-oxide cesium-containing catalysts was combined and is respectively presented in Table II, below. That is, Run Nos. 1-6 of Table II represent the MA-synthesis performance of one 4 wt. % Cs, 50-50 wt. % SiO.sub.2 -SnO.sub.2 catalyst over time, and Run Nos. 7-26 represent the MA-synthesis data of another such catalyst, both prepared in a manner similar to that of EXAMPLE 1. These MA-synthesis catalyst performance data were obtained by employng separate laboratory minireactors.
TABLE II______________________________________MA-Synthesis Performanceof 4 wt. % Cs, 50.about.50 wt. % SiO.sub.2 --SnO.sub.2 CatalystsRun % % % S % S % SNo. PAC % PAS PAY T (.degree.C.) WHSV A B 3-P______________________________________1 48 56 27 353 1.46 2.3 2.4 1.52 44 56 25 329 1.44 1.4 1.4 1.23 36 85 28 306 1.45 0.7 0.7 0.94 27 89 24 282 1.45 0.2 0.2 0.65 17 91 15 259 1.45 0.0 0.0 0.66 14 77 11 247 1.45 0.0 0.0 0.57 36 80 29 314 1.44 1.1 1.3 0.88 27 96 26 290 1.51 0.4 0.5 0.69 34 92 31 304 1.45 0.7 0.9 0.710 40 84 33 315 1.41 1.0 1.3 0.811 34 89 31 303 1.36 0.7 0.8 0.812 37 82 30 303 1.27 0.7 0.8 0.713 32 86 28 294 1.29 0.3 0.5 0.714 26 93 24 294 1.83 0.3 0.4 0.515 22 96 21 294 2.39 0.0 0.3 0.616 33 88 29 320 2.38 0.9 1.1 0.917 32 89 28 326 3.02 0.8 1.0 1.018 35 82 29 330 2.78 0.7 1.0 1.119 33 85 28 333 3.19 0.6 1.0 1.020 34 81 28 337 3.19 0.7 1.0 1.121 36 78 28 342 3.18 0.8 1.2 1.322 39 80 31 341 2.16 1.0 1.2 1.123 41 75 31 329 1.57 0.9 1.1 0.924 34 87 30 314 1.57 0.6 0.7 0.925 25 93 23 290 1.56 0.0 0.2 0.726 32 83 27 302 1.56 0.5 0.4 0.6______________________________________
Summarizing, the data of Table II show that at about 30% conversion of PA, the selectivity to MA is at least about 89%. This value was determined employing a least-squares fit of the data.
The relatively lower temperature demonstrates that the SiO.sub.2 -SnO.sub.2 mixed-oxide catalyst is more active than the silica control catalyst at about the same Cs.sup.+1 level.
The relative selectivity toward the production of by-product A, for each of the SiO.sub.2 -SnO.sub.2 mixed-oxide catalysts (Table II), and for the silica control catalyst (Table I), is shown in FIG. 1 as a functinn of PA conversion. Briefly, the SiO.sub.2 -SnO.sub.2 mixed-oxide catalyst exhibits a reduced selectivity toward production of by-product A, as compared to the control catalyst, with each catalyst containing about 4 wt. % Cs based on the weight of the catalyst. In particular, at 30% conversion of PA to MA, the selectivity toward production of by-product A is about 0.4% when using the SiO.sub.2 -SnO.sub.2 mixed-oxide catalyst of the present invention, as compared to about 2.7% using the control catalyst.
The data of Table II also show that the selectivity toward generation of by-product B is significantly less when using the SiO.sub.2 -SnO.sub.2 mixed-oxide catalyst, as compared to the control catalyst. At 30% conversion of PA, for example, by-product B selectivity was about 0.7% for the SiO.sub.2 -SnO.sub.2 mixed-oxide catalyst, as compared to about 2.2% using the control catalyst.
The overall comparative-performance data show that the 50-50 wt. % SiO.sub.2 -SnO.sub.2 mixed-oxide supported catalyst is a relatively more active MA-synthesis catalyst than a comparable SiO.sub.2 supported catalyst, as demonstrated by the relatively lower temperature needed for similar conversion. Also, the SiO.sub.2 -SnO.sub.2 mixed-oxide supported catalyst exhibited a relatively higher selectivity toward production of MA, relative to production of the undesired by-products A, B and 3-P, than the comparable SiO.sub.2 support catalyst. In particular, the amount of by-product A that was produced using the SiO.sub.2 -SnO.sub.2 mixed-oxide catalyst of the present invention, for a given amount of MA, was less than that produced using the control, by a factor of about 6.
EXAMPLE 2
10 wt. % Cs on 50-50 wt. % SiO.sub.2 -SnO.sub.2 and on 70-30 wt. % SiO.sub.2 -SnO.sub.2 Catalysts
SiO.sub.2 -SnO.sub.2 mixed-oxide catalysts having relatively high cesium loadings (e.g. about 10 wt. % Cs) were prepared in a manner similar to EXAMPLE 1 and were found to possess excellent conversion and selectivity MA-synthesis performance characteristics. The 50-50 wt. % SiO.sub.2 -SnO.sub.2 mixed-oxide catalyst was prepared utilizing Cs.sub.2 CO.sub.3 and Nalco 2326 silica sol. The 70-30 wt. % SiO.sub.2 -SnO.sub.2 mixed-oxide catalyst was prepared utilizing Nalco 1034-A silica sol.
Specifically, the 50-50 wt. % SiO.sub.2 -SnO.sub.2 catalyst and the 70-30 wt. % SiO.sub.2 -SnO.sub.2 catalyst, each at a 10 wt. % Cs loading, demonstrated a selectivity to by-product A, at 30% conversion of PA to MA, of less than about 0.1%. The observed data are presented in FIG. 1. While the observed reduced selectivity toward production of by-product A is somewhat lower than for the about 4 wt. % Cs-loaded SiO.sub.2 -SnO.sub.2 catalyst discussed above in connection with EXAMPLE 1, it is markedly better (by a factor of about 25) than the selectivity of the about 4 wt. % Cs silica control catalyst of EXAMPLE 1.
The individual performance characteristics of these 50-50 wt. % SiO.sub.2 -SnO.sub.2 and 70-30 wt. % SiO.sub.2 -SnO.sub.2, 10 wt. % Cs, catalysts are presented in Tables III and IV, below. Specifically, Table III presents the 10 wt. % Cs, 50-50 wt. % SiO.sub.2 -SnO.sub.2 MA-synthesis catalyst performance data, and Table IV presents the 10 wt. % Cs, 70-30 wt. % SiO.sub.2 -SnO.sub.2 MA-synthesis catalyst performance data.
TABLE III______________________________________10 wt. % Cs on 50.about.50 wt. % SiO.sub.2 --SnO.sub.2Run % % % S % S % SNo. PAC % PAS PAY T (.degree.C.) WHSV A B 3-P______________________________________1 26 86 22 298 1.55 0.2 0.5 0.42 34 83 28 307 1.54 0.2 0.8 0.43 38 80 31 317 1.54 0.4 1.2 0.64 45 68 31 327 1.55 0.5 1.5 0.65 28 92 26 292 1.54 0.0 0.5 0.56 32 88 28 304 1.55 0.2 0.6 0.57 32 89 28 306 1.63 0.6 0.7 0.68 28 86 24 305 2.48 0.0 0.5 0.49 32 82 26 316 2.46 0.0 0.7 0.610 28 90 25 298 1.55 0.0 0.5 0.411 28 90 25 297 1.56 0.0 0.5 0.412 33 84 28 307 1.55 0.2 0.7 0.413 35 86 30 316 1.55 0.3 1.0 0.714 41 76 32 327 1.53 0.5 1.4 0.715 26 94 24 292 1.52 0.0 0.4 0.616 32 89 29 303 1.28 0.2 0.8 0.617 31 90 28 304 1.50 0.2 0.7 0.618 32 90 29 308 1.54 0.2 0.8 0.6______________________________________
TABLE IV______________________________________10 wt. % Cs on 70- 30 wt. % SiO.sub.2 --SnO.sub.2Run % % % S % S % SNo. PAC % PAS PAY T (.degree.C.) WHSV A B 3-P______________________________________1 21 85 18 296 1.62 0.0 0.8 0.42 28 89 25 305 1.60 0.0 0.6 0.63 32 88 28 310 1.58 0.2 0.7 0.64 39 77 30 320 1.47 0.3 1.0 0.65 40 77 31 326 1.54 0.4 1.3 0.86 35 80 28 326 2.39 0.2 0.8 0.87 35 83 29 332 2.37 0.3 1.1 0.98 32 88 28 306 1.42 0.0 0.6 0.79 34 87 30 315 1.43 0.2 0.8 0.810 27 94 25 298 1.33 0.0 0.4 0.611 30 91 28 306 1.33 0.3 0.6 0.612 30 90 27 311 1.64 0.0 0.6 0.713 36 80 29 320 1.64 0.2 0.8 0.614 37 84 31 327 1.63 0.3 1.0 0.815 30 87 26 326 2.72 0.0 0.6 0.816 30 90 27 306 1.60 0.0 0.5 0.6______________________________________
Briefly, each of these catalysts is seen to exhibit an average selectivity toward production of MA, at about 30% conversion of PA, of about 89%.
EXAMPLE 3
Effect of Varying the Relative Amounts of SiO.sub.2 -to-SnO.sub.2, at about 7 wt. % Cs
A series of catalysts containing about 7 wt. % Cs, and varying relative amounts of SiO.sub.2 -to-SnO.sub.2, were investigated. Catalysts containing SnO.sub.2 at levels of about 30%, about 40%, about 50%, about 60%, and about 70%, were each prepared using Nalco 2326 silica sol in a manner similar to EXAMPLE 1. The MA-synthesis performance data of these catalysts are presented below in Tables V-IX, below. The 7 wt. % Cs, 40-60 wt. % SiO.sub.2 -SnO.sub.2 catalyst MA-synthesis performance data is presented in Table V, and the data of the 30-70 wt. % SiO.sub.2 -SnO.sub.2 catalyst is presented in Table VI. The 7 wt. % Cs, 70-30 wt. % SiO.sub.2 -SnO.sub.2 catalyst MA-synthesis performance data is presented in Table VII, while the data of the 60-40 wt. % SiO.sub.2 -SnO.sub.2 catalyst is presented in Table VIII. The 7 wt. % Cs, 50-50 wt. % SiO.sub. 2 -SnO.sub.2 catalyst MA-synthesis performance data is presented in Table IX.
TABLE V______________________________________7 wt. % Cs, 40-60 wt. % SiO.sub.2 --SnO.sub.2Run % % % S % S % SNo. PAC % PAS PAY T (.degree.C.) WHSV A B 3-P______________________________________1 28.3 91 26 284 1.46 0.5 0.6 1.52 37.8 87 33 294 1.43 0.7 0.8 1.03 40.8 86 35 302 1.55 0.8 1.0 1.04 42.5 80 34 309 1.39 1.4 1.6 1.15 45.2 72 32 320 1.47 1.5 1.9 1.26 22.8 93 21 267 1.44 0.0 0.2 1.07 30.2 92 28 285 1.46 0.2 0.3 1.08 27.4 93 26 278 1.36 0.2 0.3 1.09 39.8 84 34 302 1.38 0.7 0.9 1.0______________________________________
TABLE VI______________________________________7 wt. % Cs, 30- 70 wt. % SiO.sub.2 --SnO.sub.2Run % % % S % S % SNo. PAC % PAS PAY T (.degree.C.) WHSV A B 3-P______________________________________1 22.0 85 19 283 1.52 0.3 0.3 0.82 30.3 89 27 293 1.46 0.0 0.4 0.73 35.8 85 31 306 1.45 0.2 0.8 0.74 40.2 80 32 317 1.38 0.3 1.2 1.05 43.5 72 31 328 1.50 0.5 1.8 1.16 20.6 93 19 268 1.47 0.0 0.4 0.77 34.3 83 28 300 1.46 0.0 0.5 0.7______________________________________
TABLE VII______________________________________7 wt. % Cs, 70- 30 wt. % SiO.sub.2 --SnO.sub.2Run % % % S % S % SNo. PAC % PAS PAY T (.degree.C.) WHSV A B 3-P______________________________________1 26.7 90 24 297 1.66 1.8 1.2 1.02 35.0 85 30 306 1.68 2.6 2.0 0.83 41.5 74 31 314 1.71 3.0 2.6 0.94 44.4 71 32 323 1.71 3.6 3.3 1.15 29.6 85 25 288 1.74 0.9 0.9 0.86 33.6 85 29 298 1.66 1.5 1.4 0.97 37.9 78 30 306 1.76 2.0 1.9 1.08 23.9 88 21 280 1.74 0.4 0.4 0.89 28.0 82 23 286 1.74 0.7 0.7 0.810 29.8 88 26 292 1.72 1.0 0.9 0.911 35.2 78 28 298 1.76 1.4 1.4 0.9______________________________________
TABLE VIII______________________________________7 wt. % Cs, 60- 40 wt. % SiO.sub.2 --SnO.sub.2Run % % % S % S % SNo. PAC % PAS PAY T (.degree.C.) WHSV A B 3-P______________________________________1 22.9 94 21 286 1.58 1.0 0.7 0.92 32.0 90 29 296 1.58 1.5 1.2 0.93 39.5 83 33 305 1.58 2.0 1.6 1.14 44.8 75 34 314 1.58 2.6 2.3 1.25 24.9 91 23 277 1.59 0.3 0.3 1.06 29.8 89 26 287 1.59 0.8 0.7 1.07 34.0 85 29 292 1.58 0.9 0.8 1.18 36.4 86 31 298 1.58 1.4 1.3 1.0______________________________________
TABLE IX______________________________________7 wt. % Cs, 50-- 50 wt. % SiO.sub.2 --SnO.sub.2Run % % % S % S % SNo. PAC PAS % PAY T (.degree.C.) WHSV A B 3-P______________________________________1 22.7 85 19 282 1.56 0.4 0.5 0.92 32.5 90 29 293 1.56 0.7 0.8 0.83 39.4 85 34 303 1.56 1.0 1.1 0.84 43.7 80 35 312 1.56 1.1 1.3 0.95 32.7 77 25 282 1.55 0.2 0.2 0.76 33.6 89 30 290 1.58 0.4 0.5 0.87 37.3 89 33 299 1.50 0.6 0.9 0.98 40.3 86 35 304 1.43 0.9 1.1 0.89 22.7 94 21 271 1.57 0.0 0.3 0.510 25.4 93 24 278 1.58 0.0 0.3 0.711 26.5 94 25 282 1.58 0.3 0.3 0.912 32.9 90 30 292 1.56 0.4 0.5 0.913 37.4 87 33 300 1.48 0.6 0.9 0.914 48.3 72 34 318 1.41 1.3 1.6 1.0______________________________________
Also presented below is Table X which summarizes the percent cesium present on the catalyst, the surface area (S.A.), and the cesium ion site density for a number of catalysts produced and tested in the manner set forth herein. In particular, the above-discussed catalysts of Tables V-IX, and the 10 wt. % Cs, 50-50 wt. % SiO.sub.2 -SnO.sub.2 mixed-oxide catalyst discussed above in connection with Table III of EXAMPLE 2, as well as a catalyst presented in Table XV of EXAMPLE 5, below, are all compared at a conversion value of about 35% PAC.
TABLE X______________________________________Cesium Ion Site Densities % CsCatalyst.sup.a Fresh/ S.A. Cs/nm.sup.2 Ave..sup.b Sel.of Ave..sup.b Fresh/Ave..sup.b Fresh/Ave..sup.b at 35% PAC______________________________________Table III 9.4/8.5 63/58 6.8/6.6 84Table V 6.7/6.5 136/108 2.2/2.9 91Table VI 7.2/7.0 64/54 5.1/6.0 85Table VII 6.8/6.6 210/170 1.5/1.9 81Table VIII 6.6/6.5 171/137 1.8/2.3 86Table IX 6.6/6.6 134/106 2.2/3.0 90Table XV.sup.c 6.7/7.8 311/222 1.0/2.0 69______________________________________ .sup.a All catalysts were prepared from Nalco 2326 silica sol. .sup.b Average = ("fresh" or initial value + used value)/2. .sup.c Presented in EXAMPLE 5, below.
For these various catalysts, briefly, a cesium ion site density of about 2.2 Cs/nm.sup.2 (catalyst of Table V) appears to be optimal from the standpoint of selectivity at about 35% PAC, although a site density ranging from about 1.5 to at least about 6.8 cesium ions per square nanometer of the catalyst support surface area is also seen to result in the production of a MA-synthesis catalyst having markedly superior MA-synthesis performance characteristics.
Some of the 7 wt. % Cs catalyst performance data, i.e. that of Tables V and IX, were plotted and compared to the MA-synthesis performance data obtained from a 7 wt. % Cs catalyst containing no SnO.sub.2. This comparison is shown in FIG. 2 from which it can be seen that the presence of SnO.sub.2 in the present catalyst support noticeably enhances the MA-synthesis performance of the Cs-bearing catalyst support of the present invention, as distinguished from a control catalyst in which Cs is supported on silica only. Also, FIG. 2 generally illustrates the relationship between conversion and selectivity of the catalysts being tested.
EXAMPLE 4
Other Mixed-Oxide Supports
The inclusion of Bi.sub.2 O.sub.3, GeO.sub.2 and TiO.sub.2 into a silica catalyst support was also investigated. It was found that Cs catalysts supported on SiO.sub.2 /Bi.sub.2 O.sub.3 and SiO.sub.2 /TiO.sub.2 performed generally poorly as MA-synthesis catalysts. Cs catalysts supported on SiO.sub.2 /GeO.sub.2 exhibited an unacceptably short life under process conditions investigated. The observed performance data are presented in Tables XI-XIV, below. The MA-synthesis performance data of the catalyst containing Bi.sub.2 O.sub.3 is presented in Table XI, while the data of the catalyst containing TiO.sub.2 is presented in Table XIV. The MA-synthesis performance data of the catalyst containing GeO.sub.2 is presented in Tables XII and XIII.
Except as noted below, these catalysts were made in substantially the same manner as the SiO.sub.2 / SnO.sub.2 mixed-oxide supported catalyst of EXAMPLE 1. The 10 wt. % Cs, SiO.sub.2 -Bi.sub.2 O.sub.3 catalyst (Table XI) was made using Nalco 1034-A silica sol. Each of the 7 wt. % Cs, SiO.sub.2 -GeO.sub.2 catalysts (Tables XII and XIII) was made using Nalco 2326 silica sol. One of the SiO.sub.2 -GeO.sub.2 catalysts (Table XII) was calcined at about 350.degree. C. (about 660.degree. F.); the other (Table XIII) was calcined at about 540.degree. C. (about 1000.degree. F.). The 4 wt. % Cs, SiO.sub.2 -TiO.sub.2 catalyst (Table XIV) was made using Nalco 1034-A silica sol.
TABLE XI______________________________________10 wt. % Cs, 50--50 wt. % SiO.sub.2 --Bi.sub.2 O.sub.3Run % % % S % S % SNo. PAC PAS % PAY T (.degree.C.) WHSV A B 3-P______________________________________1 14 88 12 289 1.48 0.0 0.6 0.72 15 79 12 298 1.50 0.0 0.6 0.53 18 88 15 308 1.45 0.0 1.0 0.84 22 76 17 319 1.42 0.2 1.3 0.85 25 73 18 325 1.45 0.3 1.5 0.96 30 63 19 332 1.52 0.4 1.6 0.7______________________________________
TABLE XII______________________________________7 wt. % Cs, 50-- 50 wt. % SiO.sub.2 --GeO.sub.2,Calcined at 350.degree. C.Run % % % S % S % SNo. PAC PAS % PAY T (.degree.C.) WHSV A B 3-P______________________________________1 39.2 86 34 310 1.42 0.5 0.7 1.22 33.9 82 28 320 1.44 0.2 0.6 0.83 29.0 80 23 334 1.46 0.3 1.1 0.84 31.8 81 26 340 1.37 0.4 1.1 1.25 21.6 88 19 298 1.43 0.0 0.2 0.96 25.3 84 21 309 1.39 0.0 0.3 0.9______________________________________
TABLE XIII______________________________________7 wt. % Cs, 50--50 wt. % SiO.sub.2 --GeO.sub.2,Calcined at 540.degree. C.Run % % % S % S % SNo. PAC PAS % PAY T (.degree.C.) WHSV A B 3-P______________________________________1 34.7 82 29 300 1.47 0.4 0.5 1.52 31.8 94 30 312 1.51 0.2 0.4 1.43 31.2 88 28 327 1.44 0.2 0.7 1.44 30.3 85 26 342 1.42 0.3 1.1 1.55 20.4 100 20 293 1.47 0.0 0.5 1.3______________________________________
TABLE XIV______________________________________4 wt. % Cs, 50--50 wt. % SiO.sub.2 --TiO.sub.2Run % % % S % S % SNo. PAC PAS % PAY T (.degree.C.) WHSV A B 3-P______________________________________1 29.5 43 13 305 1.54 2.7 3.5 5.22 30.0 47 14 313 1.53 3.6 4.2 4.63 32.8 46 15 322 1.53 4.1 5.3 4.64 24.0 52 12 303 1.52 2.7 2.4 4.3______________________________________
The 10 wt. % Cs, Bi.sub.2 O.sub.3 -containing mixed-oxide catalyst exhibited generally poor selectivity to MA, over a PA conversion range of about 14 to about 30%, as is shown by the performance data of Table XI. The overall MA-synthesis performance, considering either % PAC or % PAY, in particular, is noticeably poor. The 4 wt. % Cs, TiO.sub.2 -containing mixed-oxide catalyst likewise exhibited a generally poor selectivity to MA (Table XIV).
While preliminary screening of the catalysts containing about 7 wt. % Cs on a 50-50 wt. % SiO.sub.2 -GeO.sub.2 mixed-oxide catalyst appear to indicate relatively favorable initial-performance characteristics (Tables XII and XIII), a rapid decrease in activity over the first day-on-feed was observed when employing these particular catalysts to synthesize MA from PA. One of the GeO.sub.2 -containing catalyst supports was calcined at about 350.degree. C. (Table XII), whereas the other was calcined at about 540.degree. C. (Table XIII) as mentioned above, with no noticeable difference in catalyst performance. In light of the rapid decrease in catalyst activity over time, other GeO.sub.2 -containing catalysts were not investigated further.
A catalyst, having a catalyst support that consisted essentially of equal amounts by weight of SiO.sub.2 -SnO.sub.2, but contained no Cs, was also investigated. The MA-synthesis performance of this catalyst was found to be very poor.
EXAMPLE 5
7 wt. % Cs, 40-60wt. % SiO.sub.2 -SnO.sub.2 Catalyst Having Relatively Small Average Pore Diameter
Also investigated, under MA-synthesis conditions utilizing the minireactor, was a catalyst comprising about 7 wt. % cesium, based on the weight of the catalyst, and about 40-60 wt. % SiO.sub.2 -SnO.sub.2, based on the weight of the SiO.sub.2 -SnO.sub.2 mixed-oxide. This catalyst had a "fresh" or initial average pore diameter of about 36 Angstroms and a relatively high "fresh" or initial surface area of about 311 m.sup.2 / gram. The catalyst was prepared utilizing Cs.sub.2 CO.sub.2, Nalco 2326 silica sol, and Nalco SnO.sub.2 sol (TX-2146; about 21 wt. % SnO.sub.2 ; and containing SnO.sub.2 particles having an average particle diameter of about 40 Angstroms), in a manner similar to that of EXAMPLE 1. The MA-synthesis performance data from that investigation are presented in Table XV, below.
Summarizing, this catalyst generally exhibited poorer performance than the above-discussed comparable 7 wt. % Cs, 40-60 wt. % SiO.sub.2 -SnO.sub.2 catalyst (Table V), which was found to have a relatively greater "fresh" or initial average pore diameter, i.e. about 94 Angstroms, and a relatively lower "fresh" or initial surface area, i.e. about 136 m.sup.2 / gram (see Table X, above).
This particular example underscores the desirability of having an initial average pore diameter of at least about 50 Angstroms, and an initial surface area of less than about 300 m.sup.2 / gram.
TABLE XV______________________________________7 wt. % Cs, 40-60 wt. % SiO.sub.2 --SnO.sub.2Catalyst, Small Average Pore DiameterRun % % % S % S % SNo. PAC PAS % PAY T (.degree.C.) WHSV A B 3-P______________________________________1 20.9 68 14 293 1.58 1.4 0.7 2.82 21.7 66 14 303 1.57 1.6 0.8 2.73 16.4 86 14 314 1.58 0.3 1.1 4.84 28.8 59 17 326 1.58 2.4 1.6 2.35 31.7 59 19 338 1.58 3.1 2.2 2.06 35.3 56 20 349 1.59 3.5 2.7 1.97 25.0 82 20 293 1.57 0.7 0.5 1.08 30.0 78 24 304 1.58 1.2 1.0 1.29 33.3 72 24 317 1.58 1.7 1.4 1.210 37.0 65 24 328 1.58 2.0 1.7 1.111 39.0 60 23 340 1.58 2.4 2.1 1.212 40.3 60 24 346 1.58 3.3 2.8 1.413 33.6 74 25 303 1.59 1.0 0.9 0.914 25.2 71 18 288 1.59 0.7 0.5 0.815 29.2 76 22 293 1.58 0.7 0.5 0.816 31.7 80 25 306 1.58 1.0 0.8 0.8______________________________________
EXAMPLE 6
5 wt. % Cs, 49-51 wt. % SiO.sub.2 -SnO.sub.2 Catalyst Having Separate Cs/SiO.sub.2 and SnO.sub.2 Granules
A two-component catalyst was prepared by mixing Cs/SiO.sub.2 granules with SnO.sub.2 granules. The Cs/SiO.sub.2 component was prepared by combining Cs.sub.2 CO.sub.3 (31.3 g, dissolved in 62 g of water) with Nalco 2326 silica sol (1379 g). After mixing thoroughly, three fourths of the solution was set aside. The remaining solution (368.2 g) was gelled by addition of 3.44 g of ammonium nitrate in a few ml of water. The gel was dried in a microwave oven to substantially constant weight, crushed, and calcined at 538.degree. C. for 8 hours. The resulting material was found by atomic absorption spectroscopy to contain 9.7% Cs by weight and had a BET surface area of 157 m.sup.2 / g. It was ground to a 18/40 mesh material.
The SnO.sub.2 component was prepared by mixing commercial SnO.sub.2 powder (40 g) with water (30 g) to form a homogeneous paste. The paste was dried in a microwave oven to a mass of 48.5 g, crushed, and calcined at 538.degree. C. for 8 hours. The resulting material has a BET surface area of 5 m.sup.2 / g. It was ground to 18/40 mesh.
For evaluation, 1.03 g of the Cs/SiO.sub.2 material and 0.97 g of the SnO.sub.2 material were placed in the microreactor described above such that the particles were distributed evenly. The composition of the total catalyst load was 5.00% Cs with the remainder being 49:51 SiO.sub.2 :SnO.sub.2 by weight. The MA-synthesis performance of the catalyst was determined at temperatures between 269.degree. C. and 348.degree. C. at a WHSV value of 1.51. The performance data is presented in Table XVI below.
Runs 1-9 in Table XVI show performance which is similar to that of catalysts of similar overall composition set out in Example 3. Runs 10-13 in Table XVI show inferior performance to those of Example 3, however, indicating that the useful lifetime of this catalyst may be shorter.
TABLE XVI______________________________________5 wt. % Cs, 49-51 wt. % SiO.sub.2 --SnO.sub.2Catalyst Having Separate Cs/SiO.sub.2 and SnO.sub.2 GranulesRun % % % S % S % SNo. PAC PAS % PAY T (.degree.C.) WHSV A B 3 P______________________________________1 37.2 92 34 311 1.52 1.7 1.5 1.12 39.4 91 36 310 1.51 1.5 1.5 1.03 48.8 66 32 348 1.51 3.4 1.44 38.3 91 35 310 1.51 0.9 1.0 1.05 20.8 91 19 269 1.51 0.0 0.0 1.86 47.1 73 34 340 1.51 2.8 1.67 25.0 86 22 278 1.51 0.0 0.0 1.18 44.7 78 35 330 1.51 1.9 1.7 1.59 29.0 89 26 290 1.51 0.2 0.2 2.210 42.9 78 33 320 1.51 1.1 1.0 1.211 34.8 83 29 300 1.51 0.4 0.4 1.512 38.5 82 32 310 1.51 0.8 0.6 1.613 33.1 78 26 296 1.51 0.3 0.3 1.6______________________________________
EXAMPLE 7
5 wt. % Cs, 49-51 wt. % SiO.sub.2 -SnO.sub.2 Catalyst Having Cs/SiO.sub.2 and SnO.sub.2 as Separate Particles Within the Same Granule
The two components, 9.7% by weight Cs/SiO.sub.2 and the SnO.sub.2 described in Example 6, were mixed together in a 1.03:0.97 weight ratio and ground finely with a mortar and pestle. The resulting powder was compacted into tablets in a hydraulic press at 12,000 pounds per square inch. The tablets were then crushed to 18/40 mesh. For evaluation, 2.00 g of the catalyst was placed in a microreactor and its performance was determined at temperatures between 270.degree. C. and 340.degree. C. at a WHSV value of 1.51. The performance data is presented in Table XVII below.
TABLE XVII______________________________________5 wt. % Cs, 49-51 wt. % SiO.sub.2 --SnO.sub.2Catalyst Having Cs/SiO.sub.2 and SnO.sub.2 as SeparateParticles Within the Same GranuleRun % % % S % S % SNo. PAC PAS % PAY T (.degree.C.) WHSV A B 3 P______________________________________1 29.4 86 25 300 1.52 1.2 0.9 1.22 34.2 86 29 300 1.52 1.2 1.2 1.03 49.0 66 32 340 1.51 3.9 1.7 1.34 22.0 88 19 270 1.51 0.0 0.0 1.35 47.1 73 35 330 1.49 2.7 1.6 1.16 25.5 91 23 280 1.52 0.2 0.2 1.47 42.8 83 35 320 1.51 1.7 1.6 1.08 37.3 86 32 305 1.53 0.8 0.8 0.89 38.1 87 33 310 1.52 1.1 1.0 0.910 35.2 85 30 300 1.51 0.6 0.6 1.2______________________________________
A novel SiO.sub.2 -SnO.sub.2 mixed-oxide, Cs-containing catalyst has been described hereinabove. Preferred methods for the production of such a catalyst and for its use also are described hereinabove. While the catalyst, its use and methods for making the catalyst have been described with reference to preferred embodiments, the present invention is not to be limited to these embodiments. On the contrary, alternative methods of making and using the catalyst will become apparent to those skilled in the art upon reading the foregoing description. For example, while the above-described methods set forth in detail for making the catalyst illustrate one aspect of the present invention, which results in the co-formation of the novel catalyst the SiO.sub.2 -SnO.sub.2 mixed-oxide can be formed first, and thereafter, cesium ions in the +1 oxidation state deposited onto the SiO.sub.2 -SnO.sub.2 mixed-oxide surface. To that end, the SiO.sub.2 -SnO.sub.2 mixed-oxide of the present invention can be contacted with an effective amount of a cesium ion-containing compound so that cesium ions are not only deposited onto the catalyst support surface in the +1 oxidation state, but so that the cesium ions are present on the catalyst support in an amount ranging between about 1 to about 15 percent by weight, based on the weight of the catalyst, as well. In other methods of preparation porous silica gel can be treated with a cesium compound and then mixed physically with tin (IV) oxide or porous silica gel can be ground with tin (IV) oxide and then compacted and made into proper size catalyst particles for commercial use. Still other variations within the spirit and scope of the present invention are possible and will readily present themselves to one skilled in the art.

Number	Name	Date
1879022	Barclay	Sep 1932
3520915	Kominami et al.	Jul 1970
3840587	Pearson	Oct 1974
3840588	Pearson	Oct 1974
3933888	Schlaefer	Jan 1976
4147718	Gaenzler et al.	Apr 1979
4567030	Yuasa et al.	Jan 1986

Catalyst and process for production of an alpha, beta-ethylenically unsaturated monocarboxylic acid

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