WO1993011092A1 - Process of oxidizing aliphatic hydrocarbons employing a molybdate catalyst encapsulated in a hard, glassy silica matrix - Google Patents

Abstract

A process for preparing olefins and diolefins in high productivity which involves contacting an aliphatic hydrocarbon, such as butane, with a heterogeneous catalyst composition containing reactive oxygen under reaction conditions sufficient to produce a more highly unsaturated aliphatic hydrocarbon, such as 1,3-butadiene. The catalyst composition contains a glassy silica matrix of specified surface area and macroporosity into which are encapsulated domains of a catalyst component containing oxides of magnesium and molybdenum. The catalyst has high crush strength and is useful in transport reactors.

Description

PROCESS OF OXIDIZING ALIPHATIC HYDROCARBONS EMPLOYING A MOLYBDATE CATALYST ENCAPSULATEDIN A HARD, GLASSY SILICA MATRIX

This invention pertains to the oxidation of aliphatic hydrocarbons, such as alkanes and monooiefins, in the presence of a molybdate catalyst to form more highly unsaturated aliphatic hydrocarbons.

Unsaturated aliphatic hydrocarbons, such as monooiefins and diolefins, are useful as monomers and comonomers in the preparation of polyolefin plastics.

U.S. Patent 3,862,256 discloses a process for the oxidative dehydrogenation of paraffin hydrocarbons, such as butane, over a catalyst containing oxy compounds of molybdenum and magnesium and up to 20 weight percent of a third oxy compound, such as, silicon oxide. When butane is contacted with the catalyst, the products include butenes and butadiene; however, the selectivity and space-time yield of butadiene are lower than desired In addition, the feed contains hydrocarbon and oxygen, which is not desirable for safety reasons. Finally, the magnesium oxide support does not possess the strength and attrition resistance needed for fluid bed or transport reactors.

U.S. Patent 4,229,604 discloses a process for the oxidative dehydrogenation of a paraffin, such as butane, to unsaturated hydrocarbons, such as butenes and butadiene The catalyst contains molybdenum and magnesium oxides which may be impregnated into a carrier consisting of granulated porous crystalline silica modified with alkali carbonate. The catalyst may comprise up to 20 percent by weight carrier, it is taught that during carrier preparation silicates of the alkali metals are formed. It is further taught that on the surface of the catalyst there exists an active magnesium molybdate. Disadvantageously, the catalyst produces a selec- tivity and space-time yield of butadiene which are too low for industrial use.

Whilethe oxidation of aliphatic hydrocarbons is well researched in the prior art, the selectivity and space-time yield to particular unsaturated hydrocarbons, such as diolefins, fall short of those which are desired fo- commercial exploitation. Moreover, the catalysts employed in the prior art do not possess the strength and attrition resistance required for use in industrial fluid bed ortransport reactors. Accordingly, it would be desirable to have a selective, direct oxidation of an aliphatic hydrocarbon, such as an alkane or monoolefin, to the corresponding unsaturated aliphatic hydrocarbons, specificallythe diolefin. It would be more desirable if such an oxidation produced a high selectivity and high productivity of the diolefin and other olefϊns, and correspondingly low selectivities to deep oxidation products, such as carbon dioxide. Finally, it would be most desirable if the above-identified process could be accomplished with a catalyst having a high strength and attrition resistance so as to be useful in a commercial scale fluid bed ortransport reactor. In one aspect, this invention is a process of preparing an unsaturated aliphatic hydrocarbon comprising contacting an aliphatic hydrocarbon having three or more.carbon atoms with a catalyst of this invention, described hereinafter Under the reaction conditions of the process of this invention more unsaturated aliphatic hydrocarbons, such as diolefins, are formed in a productivity equal to or greater than 0.15 gram per gram catalyst per hour (g/g cat- hr).

Advantageously, aliphatic hydrocarbons can be oxidized directly to more highly unsaturated aliphatic hydrocarbons by the process of this invention. Surprisingly, the process of this invention produces a high selectivity and high productivity of more highly unsaturated aliphatic hydrocarbons, especially diolefins, and low selectivities and low yields of undesirable deep oxidation products, such as carbon monoxide and carbon dioxide. In a preferred aspect, butadiene can be produced directly from butane in high selectivity and high productivity by the process of this invention while maintaining low selectivities of deep oxidation products. For the purposes of this invention, the "productivity" is defined as the grams of unsaturated aliphatic hydrocarbon(s) produced per gram catalyst per hour-

In a second aspect, this invention is a solid heterogeneous catalyst composition containing reactive oxygen. The composition comprises a glassy silica matrix having a Brunauer-Emmett-Telier (BET) surface area no greaterthan 20 m2/g and having macropores in the range from 500 A to 4000 A in diameter, as determined by methods described in detail hereinafter. The silica matrix comprises from 25 to 90 weight percent of the catalyst composition. Encapsulated into the silica matrix are domains of a catalyst component comprising magnesium oxide and molybdenum oxide. The above-identified catalyst composition exhibits a crush strength greaterthan 0.60 lb-

The catalyst composition of this invention is useful in the above-identified process of oxidizing aliphatic hydrocarbons to more unsaturated aliphatic hydrocarbons

Advantageously, the catalyst composition of this invention achieves a high productivity to unsaturated aliphatic hydrocarbons when compared with catalysts of the prior art More advantageously, the catalyst of this invention is strong and hard. Consequently, the catalyst composition disclosed herein possesses the activity and strength required for use in commercial fluid bed and transport reactors, such as riser reactors.

In a third aspect, this invention is a process of preparing the above-identified 5 catalyst composition comprising (a) treating a source of magnesium oxide with a blocking agent, (b) adding the treated source of magnesium oxide to an alkali metal silicate solution, the silicate being present in a concentration sufficient to provide silica in an amount ranging from 25 to 90 weight percent of the catalyst composition, (c) polymerizing the silicate to form a composite material comprising a glassy silica matrix having a BET surface area no greater than ¹⁰ 20 m2/g and having macropores ranging from 500 A to 4000 A in diameter, the matrix containing domains of the treated source of magnesium oxide, (d) ion-exchanging the composite material with an ammonium salt to reduce the concentration of alkali metal ions, (e) drying and calcining the composite material under conditions sufficient to remove the blocking agent and sufficient to convert the source of magnesium oxide into magnesium oxide, (f) 15 impregnating the domains of magnesium oxide with a source of an oxide of molybdenum and optionally a promoting amount of a source of an oxide of alkali metal, (g) calcining the resulting impregnated composite material under conditions sufficient to convert the sources of an oxide of molybdenum and oxide of alkali metal to an oxide of molybdenum and an oxide of alkali metal. 0

In a fourth aspect, this invention is a process of preparing a hard composite material comprising a glassy silica matrix having a BET surface area no greater than 20 m²/g and having macropores ranging from 500 A to 4000 A in diameter, the silica matrix having encapsulated therein domains of a metal oxide phase. The process comprises (a) treating a source of the metal oxide with a blocking agent, the metal oxide being selected from those 5 which are reactive with an alkali metal silicate, (b) adding the treated source of the metal oxide to an alkali metal silicate solution, (c) polymerizing the silicate to form a composite material comprising a glassy silica matrix having a BET surface area no greater than 20 m²/g and having macropores ranging from 500 A to 4000 A in diameter, the matrix containing domains of the treated source of metal oxide phase, and (d) calcining the composite material under conditions 0 sufficient to remove the blocking agent and sufficient to convert the source of metal oxide into metal oxide. In this mannerthe above-identified hard composite material is produced having a crush strength greaterthan 0.60 lb.

The above-identified process of preparing a hard composite material is useful for 5 preparing a metal oxide encapsulated in silica without forming a significant quantity of unwanted metal silicate. Thus, the process is especially useful when the metal oxide and silica are reactive and, without the blocking agent, would form significant quantities of metal silicate. The composite materials are useful as strong and hard catalysts or catalyst supports.

The aliphatic hydrocarbons which can be employed in the process of this invention include alkanes and oiefins which have three or more carbon atoms.

The alkanes can be alternatively described as paraffin or saturated hydrocarbons.

As noted hereinbefore, the alkanes contain three or more carbon atoms, and additionally, can have straight-chain or branched structures. Typically, the alkane contains up to 20 carbon atoms. Examples of suitable alkanes include n-butane, n-pentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane, n-dodecaπe, and higher saturated homologues, as well as isobutane, isopentane, neopentane, and likewise branched hexanes, heptanes, octanes, nonanes, decanes, dodecanes, and higher branched homologues. Certain alicyclic hydrocarbons are suitable reactants, and therefore, forthe purposes of this invention are included herein. Some examples of alicyclic hydrocarbons include cyclobutane, cyclopentane, cyclohexane, cycloheptane, cyclooctane, methylcyclopentane, methylcyclohexane and other alkyl-substituted cycloalkanes. Preferably, the alkane is normal or linear.

The oiefins can be further described as aliphatic hydrocarbons containing at least one unsaturated double bond. As noted earlier, the oiefins should also contain three or more carbon atoms, and typically up to 20 carbon atoms. The location of the double bond is not critical; therefore, the double bond can occur at a terminal or internal location along the carbon chain. Preferably, however, the olefin has a normal or linear structure, ratherthan a branched structure. For example, 1-butene is preferred over isobutylene. Thus, some examples of suitable oiefins include, 1-butene, 2-butene, 1-pentene, 2-pentene, 3-pentene, 1-hexene, 2-hexene, 3-hexene, and likewise 1-heptene, 1-octene, 1-nonene, 1-decene, and isomers thereof wherein the unsaturation occurs at any other position along the carbon chain. Oiefins containing more than one double bond, such as 1,3-hexadiene and isoprene, are also acceptable, being converted in the process of this invention to more highly unsaturated hydrocarbons. Certain alicyclic oiefins, such as cyclohexene and vinyl cyclohexene, are also suitable starting materials, and therefore, forthe purposes of this invention are included herein. Preferably, the olefin is a monoolefin. More preferably, the olefin is 1- or 2-butene. Alkynesare notsuitable reactants for the process of this invention.

The many specific examples of aliphatic hydrocarbons, noted hereinabove, are representative of those which are suitable forthe process of this invention, and are not intended to be limiting thereof. The preferred alkanes are normal paraffins which can be represented by the general formula:

CH₃-(CH₂)_n-CH₃ wherein n is an integer from 1 to 8. More preferably, n is an integer from 2 to 6. Most pre¬ ferably, n is 2, and the alkane is n-butane.

Optionally, the aliphatic hydrocarbon reactant can be diluted with a non-reactive gas, such as nitrogen, helium, argon, methane, carbon dioxide or steam. While the type of diluent is determined by prevailing economic considerations, a preferable diluent is nitrogen 0 If a diluent is used, the amount can vary widely depending upon the design of the reactor and the capacity of the solid oxidant. The hydrocarbon content of the hydrocarbon-diluent mixture typically ranges from 1 to 100 mole percent. Preferably, the hydrocarbon content of the mixture ranges from 10 to 100 mole percent, more preferably, from 20 to 100 mole percent.

The catalyst composition of this invention, described in detail hereinbelow, is a ⁵ solid heterogeneous oxide at least a portion of the oxygen of which is reactive. By this it is meant that a labile form of oxygen is present in the catalyst, and that this labile form of oxygen is capable of oxidizing the aliphatic hydrocarbon. Thus, i n one aspect the catalyst of this invention is a solid oxidant. After the labile oxygen is removed through reaction, the catalyst is spent. Moreover, the catalyst may build up over time a carbonaceous residue on its surface. ^u The spent and poisoned catalyst can be regenerated by contact with a source of gaseous oxygen. Thus, in addition to the aliphatic hydrocarbon, oxygen is required for the catalytic process of this invention.

Oxygen is typically supplied from a gaseous source provided as a continuous oxygen-containing feed. Any source of oxygen is acceptable, such as pure gaseous elemental 5 oxygen, air, or nitrous oxide. The preferred source of oxygen is gaseous air. Optionally, the gaseous elemental oxygen can be diluted with a non-reactive gas, such as nitrogen, helium, argon, or carbon dioxide. Preferably, the diluent is nitrogen. If a non-reactive diluent is employed, the oxygen content of the mixture is preferably less than 50 mole percent. More preferably, the oxygen content of the mixture ranges from 0.5 to 30 mole percent, most preferably, from 1 to 20 mole percent.

The amount of oxygen employed in the catalytic process of this invention is any amount which is (1) sufficient to oxidize fully the solid heterogeneous catalyst, and (2) sufficient to remove carbonaceous residues from the catalyst's surface. Preferably, the regeneration of the catalyst is carried out separately from the oxidation of the aliphatic hydrocarbon. Alternatively, it is acceptable to co-feed a small amount of gaseous elemental oxygen with the aliphatic hydrocarbon. The function of the co-feed is to burn off carbonaceous residues on the surface of the catalyst, to replenish to some extent the reactive oxygen of the catalyst, and to burn off any hydrogen which is formed in the process. The concentration of oxygen in the aliphatic hydrocarbon and oxygen feed is limited by the explo¬ sive limits of this mixture. Preferably, the oxygen concentration is maintained outside the lower explosive limit.

The solid heterogeneous catalyst composition of this invention comprises a hard silica matrix and a catalytic component. The silica matrix can be characterized as a glassy silica having a BET surface area no greaterthan 20 m2/g. The term "glassy" means thatthe silica is an amorphous and disordered phase, as determined by X-ray diffraction (XRD). Additionally, the silica can be characterized as a dense phase, meaning that it does not contain a measurable density of micropores or mesopores. A typical micropore ranges in size from 4 A to 20 A, while a typical mesopore ranges from 20 A to 200 A. The silica of this invention does, however, contain a random system of macropores characterized by large pores on the order of 500 A to 4000 A in diameter. In a visual sense, the topology of the silica is best compared to that of a sponge or irregular honeycomb. The catalytic component comprises an oxide of molybdenum and an oxide of magnesium, at least partially combined as magnesium molybdate. Preferably, the catalytic component consists essentially of an oxide of molybdenum and an oxide of magnesium- The catalytic component occurs as discrete domains of magnesium oxide containing molybdenum oxide, the domains being encapsulated in the silica matrix. The domains of the catalyst component range in size from 0-1 μm to 500 μm. Optionally, the catalytic component may also contain a promoting amount of alkali metal and/or an oxide of vanadium.

The silica in the above-identified heterogeneous catalyst acts as an inert and hard matrix, thereby imparting a high crush strength and attrition resistance to the catalyst so that it is suitable for use in fluid bed ortransport reactor. The magnesium oxide functions in a dual role: first, as a support forthe active catalyst component comprising magnesium oxide and molybdenum oxide, and secondly, as a base. It is believed that basicity enhances the desorption of olefinic products in the oxydehydrogenation process. The molybdenum oxide contributes significantly to the catalyst's activity, especially as combined with magnesium oxide in the form of magnesium molybdate The alkali metal promoter functions to increase the basicity of the catalyst thereby increasing the selectivity to higher unsaturates in the process of this invention. The alkali metal promoter is a Group IA metal compound. Small amounts of other elements may be present in the catalyst, provided that these elements do not materially change the performance of the catalyst. As a first step in preparing the catalyst composition of this invention, magnesium oxide is encapsulated into the aforementioned silica matrix. This preparation presents certain challenges. U.S. Patent 3,678,144 teaches a method of preparing a glassy silica body having certain metal oxides bound into the silica network. The patent is silent with respect to magnesium oxide. It has now been discovered that when magnesium oxide powder is blended into an aqueous potassium silicate solution with a gellation agent according to the method of U.S. Patent 3,678,144, the aqueous silicate is readily absorbed onto the surface of the magnesium oxide forming silica and magnesium silicates. The resulting hard composite material exhibits significantly reduced activity in the oxydehydrogenation process of this invention. It is believed that the reduced activity is related to the presence of the surface silicates. Surprisingly, it has now been further discovered that if good phase separation exists between the magnesium oxide and silica, it is possible to maintain an active magnesium oxide surface.

In view of the above and in another aspect, this invention is a method of preparing a composite material comprising a glassy silica matrix having encapsulated therein domains of magnesium oxide. The aforementioned method is easily generalized for preparing a glassy silica matrix having encapsulated therein discrete domains of a reactive metal oxide phase. The term "reactive" means that the metal oxide or a source of the metal oxide is capable of reacting with the alkali metal silicate from which the silica is derived or reacting with silica itself to form metal silicates. The method of this invention comprises (a) treating a source of a metal oxide with a blocking agent, the metal oxide being selected from those which are reactive with an alkali metal silicate, (b) adding the treated source of metal oxide to an alkali metal silicate solution, (c) polymerizing the silicate to form a composite material comprising a glassy silica matrix having a BET surface area no greater than 20 m²/g and having macropores ranging in size from 500 A to 4000 A, the silica matrix having encapsulated therein domains of the source of metal oxide treated with blocking agent, and (d) calcining the composite material under conditions sufficient to remove the blocking agent and sufficient to convert the source of metal oxide into metal oxide. Optionally, the composite material may be ion-exchanged with an ammonium salt after the polymerization step (Step c) and prior to the calcination step (Step d) to reduce the concentration of alkali metal ions. Advantageously, in this preparative process the formation of deactivating surface silicates is significantly reduced Moreover, good phase separation exists between the metal oxide and silica when compared with the process of U.S. Patent 3,678,144 which does not employ blocking agent.

Any source of metal oxide is suitable for the preparation of the composite material provided that the metal oxide itself is reactive with an alkali metal silicate. The metals of Groups IIA, IIIA, IVA, and VA provide suitable reactive oxides, the group designations (HA, IIIA, etc.) following the recommendations of the former lUPAC Preferably, the metals are selected from the group consisting of magnesium, titanium, zirconium and niobium. More preferably, the metal is magnesium. Aside from the oxides themselves, suitable sources of such oxides include the hydroxides, halides, nitrates, sulfates, acetates, and carbonates of the selected metal- Preferred sources include the metal oxides and hydroxides. Even more preferably, the source of metal oxide is an oxide or hydroxide of magnesium, titanium, niobium or zirconium. Most preferably, the source of metal oxide is magnesium hydroxide or magnesium oxide- >t is also beneficial forthe particle size of the magnesium hydroxide to range from 0-1 μm to 500 μm, preferably, from 1 μm to 250 μm.

The blocking agent may be any organic compound with a plurality of functional groups containing oxygen or nitrogen. Non-limiting examples include polyols, poly(carboxylic acids), polyanhydrides, polyamines, polyamides, polyesters, polyethers,and other polyhydroxylated compounds, such as cellulosics and starches. Polymers based on phenolic or phenolformaldehyde resins may also be used. Preferred blocking agents include poly(vinyl alcohol) and polyacrylic and polymethacrylic acids or salts. More preferred is poly(vinyl alcohol) having a molecular weight ranging from 1000 to 500,000. Most preferred is poly(viπyl alcohol) having a molecular weight ranging from 14,000 to 115,000, available as 75-100 percent hydrolyzed acetate groups.

Typically, the blocking agent is dissolved in a suitable solvent to form a solution, and the source of metal oxide is mixed into the solution to form a second solution or gel or paste. Any solvent is acceptable provided that it is inert with respect to the blocking agent and source of metal oxide. Water is the preferred solvent, but acetone, alcohols, and other common organic solvents are also acceptable. The concentration of the blocking agent in the solvent usually ranges from 1 weight percent to 50 weight percent. The source of metal oxide is generally added slowly and with a high degree of agitation to the solution containing the blocking agent. The amount of blocking agent employed typically ranges from 1 to 20 weight percent of the weight of the source of metal oxide. The resulting solution or gel or paste is dried at a temperature in the range from 50°Cto 200°Cfor a time sufficient to remove the solvent and form a dried solid. Thereafter, the solid is crushed and sieved to a fine powder At this stage, a transmission electron micrograph (TEM) of the powder typically revealsthat some of the particles of the source of metal oxide are coated with a layer of blocking agent, the thickness commonly ranging from 0.1 μm to 1 μm. Other particles, however, do not show any coating, and it is believed that the coating is thinner than the detectable limit, possibly on the order of one monolayer in thickness. Afterthe source of metal oxide istreated with blocking agent, the treated source is blended into an aqueous alkali metal silicate solution and the silicate is polymerized. Suitable alkali metal silicate solutions and polymerization conditions are specified in U Patent 3,678,144. For example, the suitable alkali silicates include lithium silicate, sodium silicate, and potassium silicate. In order to maintain the silica in solution, the concentration of the alkali metal must be sufficient to yield a solution having a pH greater than 10 Preferably, the alkali silicate solution is a potassium silicate solution, more preferably, a commercially available potassium silicate solution containing 8.3 weight percent K₂0 and 20.0 weight percent Si0 , the balance being water. Optionally, colloidal silica may be used in combination with the alkali silicate solution. The amount of colloidal silica which may be blended with the alkali silicate ranges from 0 to about 30 weight percent of the total silica present.

The metal oxide source, treated with blocking agent, is blended into the alkali silicate solution very slowly and with a high degree of agitation to ensure that the solution remains smooth and fluid. The amount of alkali silicate solution, and optional colloidal silica, employed is sufficient to provide silica in the range from 25 to 90 weight percent based on the weight of the calcined composite material, preferably from 35 to 70 weight percent The actual value will vary depending upon the end use of the composite material. In the preferred application involving a catalyst containing magnesium and molybdenum oxides for butane oxidation, the silica concentration ranges from 25 to 90 weight percent based on the weight of the calcined catalyst composition.

A gellation agent is required for the polymerization of the silicate. The gellation agent functions to reduce the pH of the silicate solution by neutralizing the alkali metal ions which are present, and thereafter the silica polymerizes. Suitable gellation agents include formamide, formaldehyde, paraformaldehyde, glyoxal, ethyl acetate, and methyl acetate. Preferably, the gellation agent is formamide. Since the rate of polymerization varies with the specific gellation agent, it may be added to the alkali silicate solution either before or after the addition of the treated metal oxide source. If the gellation agent is added first, then the polymerization should not reach completion before the metal oxide source is fully blended. For example, if the gellation agent is formamide, it is usually added to the silicate solution prior to the addition of metal oxide. If the gellation agent is ethyl acetate, it should be added after the addition of metal oxide. The concentration of gellation agent is related to the concentration of alkali ions present. Typically, the concentration ranges from 1 to 10 weight percent based on the weight of the alkali silicate solution, preferably from 2 to 5 weight percent.

There are different ways of handling the viscous mixture containing the alkali silicate, the treated metal oxide source and the gellation agent. In one method, the mixture is heated in a batch in a drying oven typically under a nitrogen purge at a temperature ranging from 70°Cto 120°C. Normallythe Tiixture sets to a hard mass within 1 hour, at which time it may be broken into smaller pieces and cured. The curing process generally includes heating at a temperature in the range from 100°Cto 225°C for a time ranging from 2 hrto 10 hr. Post cure, the dried composite is usually crushed and sieved to a powder having a particle size in the range from 177 μm to 1190 μm (80 to 14 mesh).

Alternatively, the viscous mixture containing the treated source of metal oxide, the gellation agent, and the alkali silicate may be suspension polymerized to yield spheroidal

5 beads or balls having a size in the range from 200 μm to 1500 μm. Spheroidal particles are preferred for fluid-bed transport reactors. In this method, the mixture is added to an immiscible liquid, typically a chlorinated hydrocarbon, such as The Dow Chemical Company's DOWTHERM E© o-dichlorobenzene, at a temperature in the range from 5°Cto 100°C, preferably from 10°Cto 80°C. The addition may be effected by simply pouring the mixture into

10 the immiscible liquid with sufficient agitation to disperse the mixture into droplets or by injecting the mixture through a droplet-forming nozzle. In order to prevent coalescence of the spheres, fumed silica may be added as a suspension agent to the chlorinated hydrocarbon. Bead size is controlled by the stirring rate of the shear mixer. Typically, a shear rate of 300 rpm to 725 rpm is used. This method yields hard, spheroidal beads comprising regions of the

15 treated source of metal oxide isolated within the above-described silica matrix.

As a third alternative, the viscous mixture containing the treated source of metal oxide, the gellation agent, and the alkali silicate can be spray-dried to form spheroidal particles ranging in diameterfrom 10 μm to 250 μm. For industrial scale applications the spray-drying method is preferred. Any spray-drying equipment which is conventionally used to produce ^' 20 catalystparticles forfluidized bed reactors maybe employed. Forexample, a Niro Atomizer S-

12.5R/N spray drying apparatus, with a means for controlling the inlet and outlet temperatures, is acceptable.

Analysis of the composite material following polymerization of the silicate reveals 25 good phase separation between the source of metal oxide and the silica matrix. For example, a backscattered electron image of a material produced by the polymerization of silicate in the presence of poly(vinyl alcohol)-blocked magnesium hydroxide reveals a silica/magnesium hydroxide composite. The corresponding elemental Mg map shows areas of high magnesium concentration which are identified as discrete magnesium hydroxide particles. The 30 corresponding elemental Si map reveals that essentially no silicon resides in areas of high magnesium concentration- Additionally, potassium levels are much higher in the silicon rich areas than in areas of high magnesium concentration, as illustrated by elemental K mapping. From these data it is concluded that good separation of the magnesium hydroxide and silica phases is present. Transmission electron micrographs of the above-identified magnesium 35 hydroxide/silica composite show predominantly crystalline magnesium hydroxide bounded by a dense, glassy silica. Again, good phase separation exists for 80 percent of the composite. Up to 20 percent of the silica may appear as crystalline fines, which may contain some magnesium; however, not enough magnesium is present to indicate formation of magnesium silicate.

If desired, the composite can be leached or treated with solvents to remove the metal oxide from the silica matrixto yield a pure silica matrix. This procedure simply requires thatthe composite be soaked in an acid solution. In the absence of the domains of metal oxide, the silica gives the appearance of a sponge or irregular honeycomb. The BET surface area of the silica is no greater than 20 m²/g, preferably no greater than 10 m²/g, more preferably no greater than 5 m²/g. At the lower limit it is possible for the surface area to be as low as 0.2 m²/g. The BET method for determining surface area is described by R. B. Anderson in Experimental Methods in Catalytic Research, pp. 48-66, Academic Press, 1968. As noted hereinbefore, the silica matrix essentially does not contain a microporous or mesoporous structure; however, a large macroporous structure randomly permeates the matrix. The macropores range in diameter from 500 A to 4000 A, as determined by mercury infusion techniques using, for example, a Micromeritics Model 9305 mercury porosimeter.

The composite comprising the silica matrix and the treated metal oxide may contain alkali metal ions derived from the alkali silicate solution. Accordingly, the composite will have basic properties. Should a less basic, neutral or acidic composite be desired, the composite may be ion-exchanged with an acid solution or an ammonium salt, such as ammonium nitrate, to the desired degree of acidity. In the case of the catalyst composition of this invention, the concentration of alkali metal ions may be reduced via ion-exchange to levels less than 0.5 weight percent, preferably, less than 0.1 weight percent. The ion-exchange procedure is conducted after polymerization of the silicate (Step c) and prior to calcination

(Step d). The molarity of the acid or ammonium salt solution is typically low, preferably ranging from 0.1 to 2 M. The pH of the solution is typically in the range from 7.5 to 9.0, preferably in the range from 8.2 to 8.9. The ion-exchange procedure may be carried out simply by stirring the composite in a flask filled with the ion-exchange solution or by passing the solution through a column filled with composite. At least two ion-exchanges are preferred, and more may be beneficial. Following the optional removal of alkali ions, the composite is dried for 2 hrto 10 hr at a temperature between 60°C and 150°C. Thereafter, the composite is calcined at a temperature ranging from 400°C to 800°C for a period from 1 hrto 10 hrto remove the blocking agent and to convert the source of metal oxide to the metal oxide. After calcination a composite material is obtained comprising the above-described silica matrix having encapsulated therein discrete regions of metal oxide phase. Calcination does not significantly change the morphology or surface area of the silica matrix. For the specific case of magnesium oxide, the BET surface area of the magnesium oxide phase ranges from 70 ²/g to 170 m^?/g Accordingly, the calcined composite material has a BET surface area ranging from 30 m²/g to 150 m2/g.

The calcined composite comprising the silica matrix and metal oxide can be impregnated with any catalytic metal or metal compound to form a hard catalyst composition. For example, a composite comprising the silica matrix and magnesium oxide can be impregnated with a solution containing a source of molybdenum oxide to form a strong catalyst composition which is active in the hydrocarbon oxydehydrogenation process of this invention- The impregnation technique is described by Charles N. Satterfield in Heterogeneous Catalysis in Practice, McGraw-Hill Book Company, New York, 1980, pp. 82-83. Any source of molybdenum oxide is acceptable, including for example, Mo0₃, (NH4)₂Mo₂0 ,

(NH₄)_gMo₇0₂₄-4H₂0, and (NH₄)₂Mo0₄. The molybdenum oxidecan also be obtained from a precursor molybdenum compound, such as molybdenum carbonyls, e.g., Mo(CO)₆. Preferably, the molybdenum is in the + 6 oxidation state. Preferably, the source of molybdenum oxide is ammonium heptamolybdate represented by the formula (NH₄)₆Mo₇0₂₄-4H₂0. Generally, the desired quantity of a molybdenum oxide or precursor compound is dissolved in a solvent, preferably water, to make a solution. The solution is brought into contact with the composite material and the resulting slurry is dried to remove solvent. If the solution is aqueous, the drying is conducted in an oven at a temperature in the range from 70^cCto 120°C. Thereafter, the dried slurry is calcined to form a catalytically active composition containing the silica matrix, magnesium oxide and molybdenum oxide. The calcination is typically conducted at a temperature ranging from 300°Cto 900°Cfora time ranging 0.5 hourto 24 hours. Preferably, the calcination is conducted at a temperature in the range from 500°C to 800°C, more preferably, from 550^σCto 650^CC. Alternatively, the dried slurry, described hereinabove, can be employed directly with no prior calcination in the catalytic process of this invention. Since the molybdenum precursor can be converted into molybdenum oxide at300°C, and since the catalyst bed is heated to a temperature higher than 300°C, the dried composition will be converted in situ into the catalytically active magnesium and molybdenum oxides. As noted hereinbefore, calcination essentially does not change the basic morphology of the composite- The molybdenum oxide is associated with the magnesium oxide particles and not with the silica matrix, as shown by TEM-

The elemental analysis of the calcined solid reveals a composition ranging from 3 to 30 weight percent MoO_, from 72 to 7 weight percent MgO, and from 25 to 90 weight percent silica. Preferably, the composition ranges from 5 to 25 weight percent MoO., from 25 to 70 weight percent MgO, and from 25 to 70 weight percent silica- More preferably, the composition ranges from 10 to 20 weight percent MoO., from 30 to 55 weight percent MgO, and from 35 to 50 weight percent silica. It is beneficial to add a promoting amount of at least one alkali metal promoter to the catalyst component. The promoter serves to increase the selectivity and productivity of unsaturated products, e.g. diolefins, in the process of this invention. Such a promoter is typically a compound of lithium, sodium, potassium, rubidium, cesium or francium of sufficient basicity to improve the selectivity to higher unsaturates in the process of this invention.

Suitable compounds include the alkali oxides, hydroxides and carbonates. Compounds which decompose on heating to the oxides are also suitable, such as alkali metal acetates and oxalates. Alkali metal salts may be found which are also suitable, although typically, the alkali metal halides and alkali metal silicates are not preferred due to their lower basicity. Preferably, the alkali metal promoter is an alkali metal oxide, hydroxide, carbonate, acetate, or oxalate More preferably, the alkali metal promoter is an oxide or hydroxide of potassium or cesium. Most preferably, the alkali metal promoter is an oxide or hydroxide of potassium.

The amount of alkali metal promoter significantly affects the selectivity of the catalyst. Generally, any amount of alkali metal promoter is acceptable which is sufficient to increase the selectivity and the productivity of unsaturated products, such as diolefins, in the process of this invention. Typically, the amount of alkali metal promoter calculated as the alkali hydroxide is in the range from 0.01 to 5 weight percent based on the combined weights of silica, magnesium oxide and molybdenum oxide. Preferably, the amount of alkali metal promoter calculated as the alkali metal hydroxide is in the range from 0.02 to 2 weight percent, more preferably, in the range from 0.1 to 1.0 weight percent, based on the combined weights of silica, magnesium oxide and molybdenum oxide. Below the lower preferred amount of alkali metal promoter the selectivity to diolefin is reduced while the selectivity to deep oxidation products is increased. Above the upper preferred amount of alkali metal promoter the selectivity and productivity to diolefin are also reduced.

The alkali metal promoter can be added to the catalyst component in a variety of ways known to those in the art. For example, the promoter can be applied by the impregnation technique, noted hereinbefore. In this technique the molybdenum-impregnated composite is immersed in a solution of the alkali metal promoter, for example, a methanolic solution of the alkali metal oxide or hydroxide. The alkali-impregnated composite is then drained of excess solution, dried in an oven to remove residual solvent, and calcined at a temperature in the range from 550°Cto 650°C. Alternatively, the alkali metal compound can be impregnated from the same solution as the molybdenum compound.

Optionally, the catalyst component of this invention can contain an activator which functions to increase the activity of the catalyst at any given temperature. Preferably, the activator does not decrease significantly the selectivity to diolefins and monooiefins. Preferably, the activator allows the reaction to be run at a lower temperature, while achieving high selectivity and high productivity of diolefins. Activators which are suitable for incorporation into the catalyst include the oxides of vanadium, preferably V₂0_s Any amount of vanadium oxide can be added to the catalyst provided that (1) the activity of the catalyst is increased, and (2) the selectivity for alkenes, including mono- and diolefins, is not significantly decreased. Generally, if an activator is used, the concentration ranges from 0.05 to 10 weight percent based on the total weight of the catalyst composition- Preferably, the concentration of activator ranges from 0.10 to 5.0 weight percent, more preferably, from 0.15 to 2.0 weight percent. The activator can also be applied to the composite by the impregnation technique.

The process of this invention can be carried out in any suitable reactor, including batch reactors, continuous fixed-bed reactors, slurry reactors, fluidized bed reactors, and riser reactors. Preferably, the reactor is a continuous flow reactor, such as a continuous fixed-bed reactor or a transport reactor of the type described hereinafter.

The preferred commercial reactor for the process of this invention is a transport bed reactor, such as a riser reactor. In such reactors the catalyst particles are subjected to constant impact with other catalyst particles and with the walls of the reactor. Such forces gradually reduce the size of the catalyst particles to small fines which are lost in the reaction products; thus, the useful lifetime of the catalyst is greatly limited. Consequently, it is required forthe catalyst to be prepared in a form which is able to withstand high impact and erosion forces. The catalyst composition of this invention possesses the strength and attrition resist¬ ance required for commercial use.

Typically, the riser reactor comprises an upright vessel of relatively low ratio of diameter to length. The catalyst is continuously charged into the bottom of the riser reactor. Likewise, the aliphatic hydrocarbon feedstream is delivered concurrently to the bottom of the riser reactor as a vapor phase feed or as a liquid phase feed. Preferably, the alkane is delivered asa vapor phase feed pre-mixed with an inert, gaseous diluent, and optionally, a small concentration of oxygen. The feed moves upward through the reactor, thereby contacting the catalyst. Upon contacting the catalyst, the feed is converted into a mixture of products, including monooiefins, diolefins, higher unsaturated oiefins, cracking products, deep oxidation products, such as carbon monoxide and carbon dioxide, and heavies, such as benzene and furan in the case of a butane feed. The product stream exits the riser reactor and is separated by known methods, such as distillation, to recover the desired products, typically the diolefins. Unreacted alkanes and monoolef in products are recycled to the riser reactor for further oxidation.

Riser reactor technology is advantageous forthe process of this invention, because (1) the hazard of using a feedstream containing a mixture of alkane and/or olefin and elemental oxygen is eliminated, and (2) the selectivity for diolefins is enhanced, especially at the high temperatures required for this process. In contrast, if a feedstream of alkane and oxygen is employed at a high temperature and a high oxygen/alkane mole ratio, there is a tendency to produce more deep oxidation products, such as carbon monoxide and carbon diox¬ ide. In addition, the danger of a run-away reaction is greater.

The operation of a riser reactor can be simulated by employing a method of alternating pulses. Thus, a pulse of the hydrocarbon-containing feed is passed through the catalyst bed where it is oxidized to form the desired olefin products. Next, a pulse of inert gas is passed through the catalyst bed to purge the bed of residual alkanes and alkenes. After purging, a pulse of oxygen-containing feed is passed through the catalyst bed to regenerate the catalyst. Finally, a second pulse of inert gas is passed through the catalyst bed to purge the bed of oxygen, after which the cycle is repeated. Such a procedure is employed in the illustra¬ tive embodiments, described hereinafter.

The aliphatic hydrocarbon reactant is contacted with the catalyst at any operable temperature which promotes the oxidation process of this invention and yields the desired unsaturated products. Typically, the temperature is in the range from 400°C to 700°C Prefer¬ ably, the temperature is in the range from 500°Cto 650°C. More preferably, the temperature is in the range from 530°C to 600°C. Below the preferred lower temperature the conversion of reactant may be low. Above the preferred upper temperature the selectivity and productivity of diolefin products may decrease. Likewise, the aliphatic hydrocarbon reactant is contacted with the catalyst at any operable pressure which promotes the oxidation process of this invention and yields the desired unsaturated products. Typically, the partial pressure of the reactant is adjusted to maintain the reactant in the vapor state at the operating temperature. Preferably, the partial pressure of the aliphatic hydrocarbon is in the range from subatmospheric to 100 psig. More preferably, the partial pressure is in the range from 1 psig to 30 psig, most preferably, from 3 psig to 15 psig.

When the process of this invention is conducted in a continuous flow reactor, described hereinbefore, the flow rate of the reactants can be varied. Generally, in the process of this invention the aliphatic hydrocarbon reactant is fed into the reactor at any operable flow rate which promotes the oxidation reaction and yields the desired conversion and selectivity of unsaturated products. The flow rate is expressed as the gas hourly space velocity (GHSV) and is given in units of volume of aliphatic hydrocarbon-containing gaseous feed per total reactor volume per hour or simply hr^'1. Typical values vary from 100 hr^"1 to 20,000 hr^"1. Preferably, the GHSV ranges from 100 hr¹ to 500 hr¹. It should be understood that the space velocity controls the residence time of the reactants. In a riser reactor, for example, a gas residence time less than 10 seconds is preferred, while times less than 5 seconds are more preferred and less than 1 second are most preferred. Forthe case of the riser reactor, the spent catalyst leaves the top of the reactor and is transported into a second reactor for regeneration. Regeneration is effected by contact with oxygen. Typically, a preheated oxygen source, like that described hereinbefore, is fed into the bottom of the second reactor. The spent catalyst is contacted with the oxygen source at any operable temperature, pressure, and oxygen-source flow rate which are sufficient to regenerate the catalyst. The process variables should be controlled, however, so as to prevent a runaway reaction or the buildup of excessive heat. Preferably, the temperature is in the range from 500°Cto 700°C, more preferably, in the range from 550°Cto 650°C. Preferably, the pressure is in the range from subatmospheric to 100 psig, more preferably, in the range from 2 psig to 50 psig. The oxygen-source flow rate will depend upon the heattransfer properties of the particular reactor. For example, at some high flow rates the temperature may rise dramatically resulting in an uncontrolled reaction.

When the aliphatic hydrocarbon is contacted with the catalyst of this invention, an oxidation of the aliphatic hydrocarbon occurs resulting in the loss of at least two hydrogen atoms from the hydrocarbon reactant with formation of by-product water. The organic prod¬ ucts which are produced are predominantly unsaturated aliphatic hydrocarbons, such as monooiefins and diolefins. These unsaturated products usually contain the same number of carbon atoms as the reactant aliphatic hydrocarbon. Thus, these products are not products of cracking, which would contain fewer carbon atoms than the starting hydrocarbon. Generally, also, the unsaturated products possess a higher degree of unsaturation than the reactant hydrocarbon. For example, alkanes, such as butane, can lose two hydrogen atoms to yield monooiefins, such as 1-buteπe, trans-2-butene, and cis-2-butene. In turn, monooiefins, such as the butenes previously cited, can lose two hydrogen atoms to form 1 ,3-butadiene.

The preferred diolefin products can be represented by the general formula: CH 2 = CH-CH = CH-( ^vCH 2'm -H wherein m is an integerfrom Oto 6. Preferably, m is an integerfrom 0 to 2. More preferably, m is 0 and the unsaturated product is 1,3-butadiene. Isomers of the formula shown hereinabove can also be formed wherein the unsaturation occurs at any other location along the carbon chain. Preferably, the unsaturation occurs in a conjugated fashion, as exemplified in the product 1,3-butadiene. Even more unsaturated variants of the general formula can be formed wherein further oxidation has occurred to yield more than two ethyienic double bonds. Alkynes, however, are notformed in significant amounts.

In addition to alkenes, the product stream can contain by-products of various types. For example, when the saturated alkane is n-butane, small quantities of cracking products, such as propylene and ethylene, can be formed, as well as heavies, such as benzene and furan, and deep oxidation products, such as carbon monoxide and carbon dioxide Unexpectedly, however, these by-products, especially the deep oxidation products, are significantly reduced over the prior art processes.

Forthe purposes of this invention, "conversion" is defined as the mole percentage of aliphatic hydrocarbon reactant lost from the feed stream as a result of reaction. The conversion can vary widely depending upon the reactants, the form of the catalyst, and the process conditions such as temperature, pressure, flow rate, and catalyst residence time. Within the preferred temperature range, as the temperature increases the conversion gen¬ erally increases. Within the preferred gas hourly space velocity range, as the space velocity increases the conversion generally decreases. Typically, the conversion of the aliphatic hydrocarbon is equal to or greater than 10 mole percent. Preferably, the conversion is greater than 20 mole percent; more preferably, greater than 30 mole percent; even more preferably, greaterthan 40 mole percent; and most preferably, greaterthan 50 mole percent.

Likewise, forthe purposes of this invention "selectivity" is defined as the mole percentage of converted carbon which forms a particular product. Selectivities also vary widely depending upon the reactants, the form of the catalyst, and the process conditions. Typically, the process of this invention achieves high selectivities to diolefins. Within the preferred temperature range, as the temperature increases the selectivity for alkenes generally decreases. Within the preferred space velocity range, as the space velocity increases the selectivity for alkenes generally increases. Preferably, the combined selectivity to all alkenes is equal to or greater than 50 mole percent; more preferably, greaterthan 60 mole percent; even more preferably, greaterthan 70 mole percent; most preferably, greaterthan 80 mole percent.

Typically, the selectivity to diolefins is greaterthan 40 mole percent. Preferably, the selectivity to diolefins is greater than 50 mole percent, more preferably, greater than 60 mole percent, most preferably, greaterthan 70 mole percent.

The concept of simultaneous high conversion and high selectivity can be conveniently expressed in terms of yield. Forthe purposes of this invention, the term "yield" refers to the numerical product of the single-pass conversion and selectivity. For example, a process according to the present invention operating at a conversion of 0.65, or 65 mole percent, and a selectivity to diolefin of 0.75, or 75 mole percent, would have a diolefin yield of 0.49, or 49 mole percent. Typically, the yield of diolefin achieved in the process of this invention is equal to or greater than 8 mole percent. Preferably, the yield of diolefin achieved in the process of this invention is greaterthan 18 mole percent, more preferably greater than 28 mole percent, most preferably, greaterthan 35 mole percent. Typically, in the oxidation of butane the yield of total C, oiefins is equal to or greater than 20 mole percent. Preferably, in the oxidation of butane the yield of total C₄ oiefins is greaterthan 30 mole percent, more preferably, greaterthan 35 mole percent, most preferably, greaterthan 40 mole percent. The rate at which a desired product is produced in the process of this invention can be expressed in the concept of space-time yield. Forthe purposes of this invention the "space-time yield" is defined as the mole percentage yield of a given product per hour (yield hr-¹), and it is the numerical product of the single-pass conversion, the selectivity, the gas hourly space velocity, and the concentration of the aliphatic hydrocarbon in the feedstream, wherein the conversion, selectivity and concentration are expressed as decimal fractions. Preferably, the space-time yield of diolefin in the process of this invention for a 20 volume percent alkane feed is greaterthan 30 percent per hour, more preferably, greaterthan 60 percent per hour, and most preferably, greater than 80 percent per hour. Another measure of the rate at which a desired productis produced is the

"productivity," defined as the grams unsaturated aliphatic hydrocarbon(s) formed per gram catalyst per hour (g/g cat-hr). Preferably, the productivity of butadiene in this process is equal to or greaterthan 0.10 g/g cat-hr, more preferably, greaterthan 0.25 g/g cat-hr. Preferably, the combined productivities of all of the unsaturated aliphatic hydrocarbons, such as C4 oiefins, is equal to or greaterthan 0.15 g/g cat-hr, more preferably, greaterthan 0.20 g/g cat-hr, most preferably, greaterthan 0.30 g/g cat-hr.

Testing the attrition resistance of a catalyst requires having on hand a large amount of catalyst sample. It is desirable to have a simple test procedure for small catalyst samples which gives an indication of attrition resistance. A test of crush strength is such a procedure, because increased crush strength suggests better attrition resistance.

Crush strength can be tested on any conventional equipment designed for such a purpose, however, a materials testing frame capable of providing a constant crosshead movement rate and a load capacity of greaterthan 50 lb is preferred. For example, a suitable testing frame is an Instron 1125 instrument with a 20,000 lb capacity. This frame can be equipped with a 200 lb compression load cell with a stainless steel compression platen. A 1 cm diameter compression jig is designed and built to screw in directly to the bottom portion of the machine crosshead. A strip chart or computer data acquisition system is suitable for monitoring the load versus crosshead displacement. Prior to testing, the load cell is balanced and calibrated. This is completed with the cell/platen in the compression testing configuration. The load cell is allowed to equilibrate for at least 15 minutes priorto calibration. Preferred instrument settings are the following: crosshead speed, 0.02 inches/min; chart speed, 2.0 inches/min load cell range setting, 0-10 lb full scale. The specimen is centered on the load cell platen just belowthe compression jig. The crosshead iscarefullylowered by manual control until minimal clearance between the fixture and specimen is achieved. Each specimen is tested at room temperature until the first sign of failure is observed (drop in load). The maximum load observed by the specimen is determined by the strip chart or computer data system.

The composite material or catalyst composition to be tested is sized into particles ranging from 500 μm to800 μm. These particles are calcined at 600°C for 2 hours prior to testing. Care should be taken to select particles of similar size for testing, and regular shaped particles are preferred. Typically, a minimum of ten specimens is tested for each sample. The crush strength of the catalyst of this invention is typically greaterthan 0.60 lb, preferably, greaterthan 0.80 lb, more preferably, greater than 1.00 1b, and most preferably, greater than 1.25 1b, as measured on a particle having a size in the range from 500 μm to δOO μm.

The following examples are illustrative of the process and catalyst of this invention, but are not intended to be limiting thereof. All percentages are given in mole percent carbon, unless noted otherwise.

EXAMPLE 1 - COMPOSITE MATERIAL AND CATALYST PREPARATION A. Preparation of the Composite Material

A 5 weight percent poly(vinyl alcohol) (P A) solution is prepared by adding PVA (26 g; MW 1 15,000; 100 percent hydrolyzed ester) to cold water (500 g) with rapid stirring and heating to 90°C. Magnesium hydroxide powder (90 g) is added to the PVA solution (200 g) with rapid mechanical stirring to form a creamy suspension. The suspension is dried in a nitrogen- purged oven at 80°Cfor 18 hr, and the resulting PVA-treated magnesium hydroxide solid is rough crushed and heated further at 125°C for 4 hr. The dried solid is fine crushed to pass a 170 mesh screen (88 μm).

With rapid mechanical stirring, formamide (3 g) is added slowly to a potassium silicate solution (100 g; 20.8 weight percent Si0₂, 8.3 weight percent K₂0) to form a clear solution free of gel clusters. The PVA-treated magnesium hydroxide powder (50 g), prepared hereinabove, is added gradually to the silicate solution to form a well-mixed slurry. The slurry is poured into a plastic beaker, covered with a watch glass to slow evaporation, and placed in an oven at 80°C for about 45 minutes. During this time, the silicate polymerizes in the batch taking the form of the beaker. The polymerized material is removed and cut into chunks which are cured and dried for 18 hr at 80°C. The hardened chunks are crushed to a size ranging between 177 μm and 1 190 μm (80 - 14 mesh). The crushed particles (70 ml) are loaded into a column and washed four times with 150 ml portions of an aqueous ammonium nitrate solution (1 M; pH 8). The wet particles are then slurried twice in 1 M ammonium nitrate, filtered, slurried twice with acetone, and filtered again. This procedure is designed to remove water located in the pores which could fracture the particles during heating. The filtered particles are air dried at room temperature and dried further at 80^CC for 6 hr. Elemental analysis of the particles indicates that the potassium level is less than 0.1 weight percent. The particles are furtherdried and calcined as follows: 2 hrat 100-150°C, 4 hrat 150-300°C, 1 hr at 300-400°C, 4 hr at 400-450°C, 2 hr at 450-600°C, and 4 hr at 600-610°C. A composite material is obtained comprising a silica matrix having domains therein of magnesium oxide, as determined by TEM. The silica matrix is characterized as having a BET surface area of 1 m²/g and a random macropore system wherein the diameter of the pores is in the range from 3000 A to 4000 A. The domains of magnesium oxide exhibita BET surface area of 140 m²/g-

B- Preparation of the Catalyst

An aqueous solution containing 25 weight percent ammonium heptamolybdate (AHM) (23 g, 20 weight percent as M0O3) adjusted to pH 8.5 is added to the composite material prepared hereinabove (30 g). The wetted material is dried overnight in flowing nitrogen at 80°C and then calcined in air as fol lows: 2 hr at 100-150°C, 4 hr at 150-600°C, and 4 hr at 600- 610°Cto yield a catalyst composition comprising the above-identified silica matrix having domainstherein of magnesium oxide containing molybdenum oxide. The catalyst contains 40.00 weight percentSϊ0₂, 16.67 weight percent M0O3, the remainder being MgO. The crush strength of the catalyst, as measured on an Instron #IV crush strength instrument, gives a maximum load of 1.38 _+_ 0.44 lb for spheroidal particles of 600 μm size. By comparison, commercial alumina beads of approximately the same size, which are suitable for use in a transport reactor, exhibit a maximum load of 1.53 +_ 0.64 lbs. Thus, the strength of the catalyst composition of this invention is sufficient for use in a transport reactor.

EXAMPLE 2 - BUTANE OXIDATION

A catalyst similar to the one prepared in Example 1(B) is employed in the oxidation of butane in the following manner: approximately 15 cc of catalyst are loaded into a Vycor^® reactortube (18 mm OD x 7.6 cm length). The temperature of the reaction is measured from a stainless steel thermowel I (1/8 inch OD) embedded in the catalyst sample. Afeedstream containing butane (10-20 volume percent) and helium (90-80 volume percent) is passed over the catalyst for about 5-10 seconds. The flow of the feedstream isstopped and a purge stream comprising pure helium is passed over the catalyst at the same flow rate for 1 minute. The purge stream is stopped and a stream of oxygen (20 volume percent) in helium is passed over the catalyst at the same flow rate for 1 minute, followed by another purge stream of helium for 1 minute. This cycle is repeated and the combined products are collected in a Saran^® polyvinylidene chloride plastic bag for analysis. Analysis is performed on a Carle gas chromatograph designed to analyze for C_j-C.. alkanes, alkenes and alkadieπes, as well as permanent gases such as N_2# 0₂, CO, C0₂, H₂, and heavier products including furan, benzene, and C₆ compounds. Isobutane is mixed with the feed or products as a standard. "Unknowns" are obtained from the difference between the carbon balance and 100 percent- The process conditions and results are set forth in Table I. TABLE I^®

^® Butane, 20 vol. %; Rxn. temperature, 580°C. ^® BD is butadiene.

It is seen that the catalyst composition containing the above-described silica matrix and oxides of magnesium and molybdenum is highly active and selective in the oxidation of butane to butenes and butadiene (BD).

EXAMPLE 3 - CATALYST PREPARATION AND BUTANE OXIDATION

A catalyst composition is prepared as in Example 1 , with the exception that magnesium oxide (90 g) is used instead of magnesium hydroxide during PVA treatment and PVA-treated magnesium oxide powder (34 g) is added to the potassium silicate solution. The composition thus prepared is essentially identical to the composition of Example 1. Moreover, the catalyst composition prepared with magnesium oxide exhibits a crush strength comparable to the crush strength of the catalyst composition in Example 1 and is therefore suitable for use in a riser reactor. The catalyst prepared with magnesium oxide is tested in the oxidation of butane according to the procedure of Example 2 with the results set forth in Table I. It is seen thatthe catalyst is highly selective and active in the oxidation of butane to butenes and butadiene.

EXAMPLE 4 - CATALYST PREPARATION AND BUTANE OXIDATION

Magnesium oxide (60 g) is added with mixing to a solution containing water (1 0 g) and 21 weight percent polyacrylic acid (50 g; 90,000 MW). The mixture is dried in a nitrogen- purged oven at 80°Cfor 18 hr. The resulting polyacrylic acid-treated magnesium oxide is rough crushed, heated further at 125°C for 4 hr, and crushed again to pass a 170 mesh screen (88 μm). The solid obtained is blended into a potassium silicate solution which is polymerized as in Example 1. The resulting composite is washed with ammonium nitrate, impregnated with a solution of ammonium heptamolybdate and calcined, per Example 1, to yield a catalyst composition of adequate hardness for use in a riser reactor. The catalyst composition is essentially identical to that of Example 1 and contains the above-identified silica matrix and domains of a catalyst component comprising magnesium oxide and molybdenum oxide.

The above-identified catalyst is tested in the oxidation of butane according to the procedure of Example 2 with the results set forth in Table I. It is seen that the catalyst composition prepared with a blocking agent of polyacrylic acid instead of po!y(vinyl alcohol) is also highly active and selective in the oxidation of butane to butenes and butadiene (BD). EXAMPLE 5 - CATALYST PREPARATION AND BUTANE OXIDATION

A catalyst composition is prepared as in Example 1 with the exception thatthe slurry containing poly(vinyI alcohol)-treated magnesium hydroxide, formamide and potassium silicate is suspension polymerized into spheroidal particles ratherthan polymerized in batch. The suspension polymerization method involves adding the slurry slowly at 10-12°Cto o- dichlorobeπzene (The Dow Chemical Company Dowtherm E^®), which additionally contains 1 percent by weight fumed silica as a dispersion agent. The mixture isthen agitated using a low shear mixer for a period of time sufficient to break the aqueous phase into droplets. The temperature is then raised to 80°C for 1.5 hr during which time the silicate cures to form spheroidal particles. The particles are washed with acetone to remove the Dowtherm E^®. Thereafter, the particles are aged for 18 hr, washed, dried and calcined as per Example 1. Specifically, the calcination is conducted for 2 hr at 100-150°C, 4 hr at 150-300°C, 1 hr at 300- -400°C, 4 hr at 400-450°C, 2 hr at 450-600°C, and 4 hr at 600-610°C The resulting catalyst composition comprises a silica matrix essentially identical to that described in Example 1. Encapsulated in the matrix are domains of magnesium oxide containing molybdenum oxide. The crush strength of the spheroidal particles is 1.341b _+_ 0.29 lb, as measured on particles of 600 μm size. It is seen that the composition prepared by suspension polymerization is strong enough for use in a riser reactor.

The catalyst prepared hereinabove is tested in the oxidation of butane according to the procedure of Example 2 with the results set forth in Table II. It is seen that the catalyst composition is highly active and selective in the oxidation of butane to butenes and butadiene (BD).

TABLE II ^Θ

© Butane, 20 vol. %; Rxn. T, 580°C; 10 sec pulse. ^@ BD is butadiene. TABLE 11°

© Butane, 20 vol. % ; Rxn. T, 580°C; 10 sec pulse. ^® BD is butadiene.

EXAMPLE 6 - CATALYST PREPARATION AND BUTANE OXIDATION

A catalyst composition (1 1.0 g) prepared as in Example 5 is impregnated with a solution comprising methanol (5.84 g) and potassium hydroxide (0.018 g). The impregnated catalyst is dried and calcined as in Example 5 to yield a catalyst composition having a potassium concentration of 0.1 weight percent. The crush strength of the catalyst gives a maximum load of 1.34 lb _+_ 0.29 for spheroidal particles of 600 μm size. It is seen that the strength of the potassium-doped catalyst is sufficient for use in a transport reactor.

The catalyst istested in the oxidation of butane according to the method of Example 2 with the results set forth in Table II. It is seen that the potassium-promoted catalyst composition achieves high selectivity and productivity for butenes and butadiene. When Example 6 is compared with Example 5 it is seen that the catalyst composition containing potassium achieves a significantly higher selectivity to C₄ oiefins with only a slight reduction in conversion. Example 7 - Catalyst Preparation and Butane Oxidation

A catalyst composition prepared and impregnated with potassium as in Example 6 is impregnated again with a solution comprising methanol (5.84 g) and potassium hydroxide (0.018 g). The impregnated composition is dried overnight and calcined as in Example 5 to yield a composition containing 0.2 weight percent potassium. The crush strength of the spheroidal catalyst particles of 600 μm size is 1.34 _+_ 0.29, therefore the composition is suitable for use in a riser reactor. The catalyst composition is employed in the oxidation of butane with the results shown in Table II. It is seen that the potassium-promoted catalyst composition achieves high selectivity and productivity for butenes and butadiene.

Claims

1. A process of preparing an unsaturated aliphatic hydrocarbon comprising contacting an aliphatic hydrocarbon having three or more carbon atoms with a solid heterogeneous catalyst composition having reactive oxygen and having a crush strength greaterthan 0.60 lb, the catalyst composition comprising a glassy silica matrix having a BET surface area no greaterthan 20 m2/g and having macropores ranging in size from 500 A to 4000 A, the silica matrix comprising from 25 to 90 weight percent of the catalyst composition and having encapsulated therein domains of a catalyst component comprising an oxide of magnesium and an oxide of molybdenum, the contacting occurring under conditions such that an unsaturated aliphatic hydrocarbon is produced in a productivity equal to or greater than 0.15 g/g cat-hr.

2. The process of Claim 1 wherein the aliphatic hydrocarbon is an alkane represented by the general formula:

CH₃-(CH₂)_n-CH₃ wherein n is an integer from 1 to 8.

3. The process of Claim 2 wherein n is 2 and the alkane is n-butane.

4. The process of Claim 1 wherein the temperature is in the range from 400°C to 700°C, the aliphatic hydrocarbon partial pressure is in the range from subatmospheric to 100 psig, and the gas hourly space velocity of the hydrocarbon feedstream is in the range from 100 hr-i to 20,000 hr-.

5. The process of Claim 1 wherein the unsaturated aliphatic hydrocarbon is a diolefin and wherein the diolefin is represented by the general formula:

CH 2 = CH-CH = CH-(CH 2,)'m -H wherein m is an integerfrom 0 to 6.

6. The process of Claim 5 wherein m is 0 and the diolefin is 1 ,3-butadiene.

7. The process of Claim 1 wherein the catalyst component contains a promoting amount of cesium or potassium oxide, hydroxide, carbonate, acetate, or oxalate in a concentration from 0.01 to 5 weight percent, calculated as the alkali hydroxide and based on the combined weights of silica, magnesium oxide, and molybdenum oxide.

8. A solid heterogeneous catalyst composition capable of providing a reactive form of oxygen and having a crush strength greaterthan 0.60 lb, the composition comprising a glassy silica matrix having a BET surface area no greater than 20 m²/g and having macropores ranging in diameter from 500 A to 4000 A, the silica matrix comprising from 25 to 90 weight percent of the catalyst composition and having encapsulated therein domains of a catalytic component comprising magnesia and molybdenum oxide.

9. The composition of Claim 8 wherein the catalytic component contains a promoting amount of potassium or cesium oxide, hydroxide, carbonate, acetate, or oxalate in a concentration from 0.01 to 5 weight percent, based on the combined weights of silica, magnesium oxide, and molybdenum oxide.

10. The process of preparing the catalyst of Claim 8 comprising: (a) treating a source of magnesium oxide with a blocking agent, (b) adding the treated source of magnesium oxide to an alkali metal silicate solution, the silicate being present in a concentration sufficient to provide silica in an amount ranging from 25 to 90 weight percent of the catalyst composition, (c) polymerizing the silicate to form a composite comprising a glassy silica matrix having a BET surface area no greaterthan 20 rπ²/g and having macropores ranging from 500 A to 4000 A in diameter, the matrix containing domains of the treated source of magnesium oxide, (d) ion-exchanging the composite with an ammonium saltto reduce the concentration of alkali metal ions, (e) drying and calcining the composite under conditions sufficient to remove the blocking agent and sufficient to convert the source of magnesium oxide into magnesium oxide, (f) impregnating the domains of magnesium oxide with a source of an oxide of molybdenum, and optionally, a source of alkali metal promoter, and (g) calcining the resulting impregnated composite under conditions sufficient to convert the source of an oxide of molybdenum and optional source of alkali metal promoter into an oxide of molybdenum and an oxide of an alkali metal promoter.

1 1. A process of preparing a composite material comprising a glassy silica matrix having a BET surface area no greater than 20 m /g and having macropores ranging from 500 A to 4000 A in diameter, the silica matrix having encapsulated therein domains of a metal oxide phase, the process comprising: (a) treating a source of the metal oxide with a blocking agent, the metal oxide being selected from those reactive with an alkali metal silicate, (b) adding the treated source of the metal oxide to an alkali metal silicate solution, (c) polymerizing the silicate to form a composite comprising a glassy silica matrix having a BET surface area no greaterthan 20 m²/g and having macropores ranging from 500 Ato 4000 A in diameter, the matrix containing domains of the treated source of metal oxide phase, and (d) calcining the composite under conditions sufficientto remove the blocking agent and to convert the source of metal oxide i nto metal oxide.