Niobium (V) Halide/Oxide-Metal Oxide Compositions and a Process for _Their Preparation

This invention relates to novel metal oxide compositions having niobium (V) halide/oxide bound to the surface thereof. These compositions are useful as catalysts.

The niobium pentahalides are known to be useful as catalysts in various organic reactions. See for example, Friedel-Crafts and Related Reactions, Volume 1, G. A. Olah, Ed., Interscience, 1963, New York. A means of providing niobium pentahalide distributed over a "support" would be very useful, both from an economic point of view and from a handling point of view. The main problem to date in supporting niobium (V) halides on a support is that the niobium pentahalides are basically only soluble in solvents in which they react and thus the usual techniques of impregnation to obtain a metal loading of niobium (V) halide can not be utilized. The instant invention provides a solution to this problem. The compositions of the instant invention provide for niobium (V) halide/oxide bound to the surface a metal oxide substrate.

This invention relates to novel halogen-containing metal oxide compositions. These compositions comprise a solid metal oxide substrate having niobium (V) halide(s)/oxide(s) bound to the surface of the substrate. These compositions are useful as catalysts. In general, they are prepared by reacting a substantially anhydrous metal oxide having surface hydroxyl groups with niobium pentahalide in the vapor state. A preferred method of preparation is to sublime the desired niobium pentahalide and react the vapor therefrom with a substantially anhydrous hydroxyl-containing metal oxide. Preferred oxides are aluminum oxide gels, silicon oxide gels and silicon-aluminum oxide gels.

The compositions of the instant invention comprise pentavalent niobium (also written as niobium (V)), halogen (or halide), oxygen (or oxide) and a solid metal oxide substrate wherein at least one valence of niobium is bound to oxygen, which oxygen is bound to the substrate, at least one valence of the niobium is bound to halogen and the remaining niobium valences are bound to halogen and/or oxygen, which oxygen may or may not be bound to the substrate. The halogens are fluorine, chlorine, bromine, iodine and mixtures thereof. Preferred halogens are fluorine and chlorine.

The metal oxides that are useful as substrates to prepare the compositions of the instant invention are those inorganic oxides which have hydroxyl groups attached to the surface of the substrate. The hydroxyl groups provide the means by which the niobium pentahalides are bound by reaction to the surface of the substrate. The scope of the invention is broad and any metal oxides which have surface hydroxyl (or oxyhydroxyl) groups can be utilized in preparing the compositions.

The term "metal oxide", although used herein in the singular tense, is meant to include the single oxides such as silica, or alumina as well as plural and complex oxides such as silica-alumina, silica-alumina-thoria, zeolites and clays.

The preferred metal oxide substrates used to prepare the instant compositions are the porous, solid inorganic oxides which contain surface hydroxyl groups and which are conventionally used as catalysts and catalyst supports. Non-limiting examples of these types of materials include those having a major component of silica or alumina or both, such as, for example, alumina and aluminous materials; silica and siliceous materials; clays, particularly open lattice clays; and crystalline aluminosilicates (zeolites). Non-limiting examples of aluminous and siliceous materials include, for example, silica-alumina, silica-magnesia, silica-zirconia, silica-titania, alumina-chromia, alumina-ferric oxide, alumina-titania as well as ternary compositions such as, for example, silica-alumina-thoria, silica-alumina-zirconia, etc. Non-limiting examples of crystalline aluminosilicates useful as substrates include synthetic zeolites such as, for example, A, X, Y, L and ZSM types such as ZSM-5 and others and naturally occurring zeolites, such as erionite, faujasite, mordenite, sodalite, cancrinite and others. Non-limiting examples of open lattice clays useful as substrates include bentonite, montmorillonite and others. In a preferred embodiment, the metal oxide should have a major component of silica or alumina or both.

Particularly suitable as substrates for preparing the instant compositions are those solid inorganic oxide compositions known as metal oxide gels or gel oxides.

The gel oxides which are particularly suitable for use in preparing the compositions of the instant invention are any of the metal oxide gels that are well known in the catalytic art useful as either catalyst base materials or as promoting materials in catalyst compositions. Additionally, the term "metal oxide gel" or "gel oxide" as used herein shall also include the plural oxide gels, i.e., those that contain mixtures or compounds of two or more metal oxides. A metal oxide gel is basically a metal oxide that contains chemically bound water in the form of hydroxyl groups or oxyhydroxyl groups as opposed to adsorbed water and water of hydration, although adsorbed water and water of hydration may also be present. They are typically prepared by the precipitation of the metal component(s) in an aqueous medium. Upon calcination at sufficiently elevated temperatures, water is given off and the gel is converted to the oxide with two hydroxyl moieties giving one molecule of water and an oxygen is attached to a metal ion. Illustrative of gel oxide base materials used to prepare the composition of this invention are aluminas, silicas, alumina-silicas, alumina-zirconias, silicazirconias and the like, including naturally occurring hydrous oxide minerals such as clays, such as, for example kaolinites, the montmorillonites, and the like. Among the clays, the open lattice clays are particularly desirable. Also, included are the zeolites, both natural and synthetic. The structure of the gel oxides can range from amorphous to highly crystalline. Preferred oxide gel materials are selected from the group consisting of alumina, silica, alumina-silica, crystalline aluminosilicates (zeolites) and open lattice clays.

Since the niobium (V) halide/oxide is bound to the surface of the metal oxide substrate by a reaction of the halide with the metal oxide substrate through a hydroxyl moiety, the metal oxide substrate must have pendant surface hydroxyl groups attached to the surface. Before reaction, the metal oxide substrate must have pendant surface hydroxyl groups, whereas, after reaction, the metal oxide may or may not have surface hydroxyl groups, depending on the degree of reaction with the niobium pentahalide.

Prior to use in preparing the instant compositions the hydroxyl-containing metal oxide substrate should be substantially free of absorbed water, i.e., "substantially dehydrated or anhydrous". The absorbed or free water is removed by heating the substrate at temperatures ranging from about 100.degree. C. to about 900.degree. C. prior to contact with the niobium pentahalide vapor. Any environment that provides for drying is suitable such as air, vacuum, inert gas such as nitrogen, etc. The dried metal oxide substrates should be kept away from a humid atmosphere after drying. It is understood that a dried metal oxide substrate prior to use in preparing the instant composition will still contain chemically bound water in the form of hydroxide and oxyhydroxide.

An aluminum oxide gel is one of the preferred substrates. This alumina can be any of the variety of available aluminas. These are commercially available under various names such as alumina gels, activated aluminas, gamma aluminas, etc. Regarding purity of the alumina, it may be stated that small amounts of impurities are not generally detrimental, and may be beneficial when the impurity is present as a cogel. In fact "impurities" may be purposely added for catalytic effects. The following table lists several commercial aluminas and their properties which are found suitable.

Silica gel is also another preferred substrate. These are readily available commercially and are essentially substantially dehydrated amorphous silica. These materials are available in various density grades, from low density with surface areas ranging from about 100-300 m.sup.2 /g to regular density with surface areas up to about 800 m.sup.2 /g. The commercially available materials are used as dessicants, selective absorbents, catalysts and catalyst supports. Regarding purity of the silica, it may be stated that small amounts of impurities are not generally detrimental and may be beneficial when the impurity is present as a co-gel. In fact, "impurities" may be purposely added for catalytic effects. The following table lists several commercial silicas and their properties which are found suitable.

Other preferred substrates are the aluminosilicates. These materials contain various mixtures of aluminum and silicon oxides. They are readily available commercially and are generally employed as cracking catalysts. Typically they contain from about 50 to about 95, preferably from about 70 to about 90 percent by weight of silica. Illustrations of commercially available alumina-silicas are Davison Grade 980-25 (manufactured by Davison Chemical Division, W. R. Grace & Co.) which contains about 75% SiO.sub.2 and 25% Al.sub.2 O.sub.3 and Davison Grade 980-13 which contains about 87% SiO.sub.2 and 13% Al.sub.2 O.sub.3. These materials can be prepared in a conventional fashion, as for example by co-precipitation, co-gellation, or by spray drying.

Encompassed within the term "aluminosilicates" are most of the zeolites. The zeolites are found to be specifically useful as substrates. Zeolites are ordered, porous crystalline aluminosilicates having a definite crystalline structure within which there are a large number of small cavities which are interconnected by a number of still smaller channels. Zeolites useful as substrates may be either synthetic or natural. At least 34 species of zeolite minerals are known and the synthetic zeolites number in the hundreds. Any zeolite will be useful as a substrate provided that the zeolite, prior to reaction with tantalum pentahalide, contains chemically bound water in the form of hydroxyl groups. Depending on the state of reaction, the reacted product may contain no hydroxyl groups, if all such groups were reacted with the niobium pentahalide, or there may be unreacted hydroxyl groups still present.

The techniques for the preparation of the niobium pentahalides utilized to prepare the instant compositions are well known in the art. Typically they are prepared by passing a dry halogen gas over niobium metal at elevated temperatures. By way of illustration, niobium pentachloride is prepared by passing dry chlorine over niobium metal sheet at about 300.degree. C. to 350.degree. C. (Alexander & Fairbrother, The Journal, 1949, 5223).

The niobium pentahalides utilized will comprise niobium pentafluoride, niobium pentachloride, niobium pentabromide and niobium pentaiodide. A mixture of halides can be utilized. Preferred halides are chloride and fluoride.

The metal oxide-niobium (V) halide/oxide compositions of the instant invention are prepared by a process comprising reacting under substantially anhydrous and oxygen-free conditions a suitable metal oxide which has water chemically bound as hydroxyl and which is substantially free from absorbed water with niobium pentahalide vapor and thereafter recovering the product.

Thus are produced metal oxide compositions having niobium (V) halide/oxide bound to the surface thereof. By the term "bound" it is meant that the pentavalent niobium has at least one valence bound to an oxygen which is part of the metal oxide substrate. By the term "surface" it is meant both the external and internal pore surfaces which are accessible to the niobium pentahalide vapor during preparation process.

The niobium pentahalides readily sublime and thus lend themselves to a preferred method of preparation which is called "reactive sublimation" wherein niobium pentahalide is sublimed into an anhydrous, non-oxidizing atmosphere and allowed to contact and react with the hydroxyl-containing metal oxide substrate. In the preparation of the instant compositions by reactive sublimation, it is important that the reaction be carried out under substantially anhydrous conditions and in a neutral or reducing environment to prevent decomposition of the niobium halide.

In this preferred method of preparation, the niobium pentahalide is sublimed by suitable application of temperature and/or vacuum into an essentially anhydrous and oxygen-free atmosphere where it is allowed to contact and react with a substantially anhydrous, hydroxyl-containing metal oxide substrate. Any temperature and/or vacuum which causes the niobium pentahalide to sublime is suitable. Temperatures up to about 200.degree. C. are suitable. Sublimations conditions will vary somewhat depending on which niobium halide is being utilized. Frequently the metal oxide substrate is heated during reaction, say up to about 200.degree. C. This heating is not critical to this invention, but it has been found that by so heating, a more even distribution of the niobium pentahalide on the metal oxide substrate is effected. After reaction, the metal oxide composition is frequently subjected to an additional period of time at sublimation conditions without the presence of a niobium pentahalide source. This extra step allows for any unreacted niobium pentahalide to be sublimed off of the metal oxide composition. As noted above, the metal oxide before use is frequently subjected to a heat treatment to remove absorbed water. Vacuum can also be applied. Generally, if this pretreatment temperature is too low, free water will remain, and, if the temperature is too high, sintering of the metal oxide substrate will occur, both of which will adversely affect the properties of the composition. Generally, the most desirable pretreatent temperatures range from about 200.degree. C. to about 400.degree. C.

It is postulated that when niobium pentahalide reacts with the hydroxyl group of a metal oxide gel, that the reaction may be illustrated variously as follows (using chloride as an illustrative halide): ##STR1##

In the final composition a mixture of the above described reaction products will exist. The distribution of the products is believed to be affected by reaction conditions such as temperature.

Thus, depending on the niobium content desired in the final composition, a niobium pentahalide vapor is reacted with the hydroxyl-containing metal oxide substrate until a part or the whole of the hydroxyl group population of the metal oxide substrate is exhausted.

It will be apparent that products having varying niobium contents will be suited for various purposes. Thus, the niobium pentahalide may be reacted with the hydroxyl-containing metal oxide substrate in such amounts as is necessary to prepare products which are suitable for the particular purposes.

The reaction between the niobium pentahalide vapor and the hydroxyl-containing metal oxide substrate is carried out at temperatures ranging from room temperature to elevated temperatures say 150.degree.-200.degree. C. or higher. The reaction is normally carried out in an anhydrous, i.e., free from water vapor, atmosphere. The atmosphere should further be a neutral or reducing atmosphere, i.e., oxygen-free. Dispersal of the niobium pentahalide vapor in a vacuum provides a quite suitable atmosphere for reaction with the metal oxide gel.

The metal oxide-niobium (V) halide/oxide compositions of the instant invention may be produced in virtually any physical form, as for example, they may be pellets, beads, extrudates, microspheres and in other particular forms, as for example rings, saddles and the like, and in porous or non-porous form.

This invention comprises novel compositions and the method for producing them. The compositions basically comprise metal oxide substrates having niobium (V) halides/oxides reactively bound to the surface of said substrate. The halides are selected from the group consisting of fluoride, chloride, bromide, iodide and mixtures thereof. Preferred halides are fluoride and chloride. The compositions are generally prepared by a process which comprises contacting the hydroxyl-containing metal oxide substrate in a substantially anhydrous state with the niobium pentahalide in the vapor state and allowing the vapor to react with the substrate in an atmosphere which is substantially oxygen- and water-free. In the preferred process, sublimation of the niobium pentahalide is used to put the niobium pentahalide in the vapor state.

A variation of the above process is utilized to produce a composition containing mixed halides, particularly mixed chlorides and fluorides. In this variation a niobium (V) chloride/oxide-metal oxide composition is prepared by reactive sublimation. The niobium (V) chloride/oxide-metal oxide composition is then contacted with an oxygen-containing gas or a chemical compound containing oxygen which is weakly covalently bonded to the compound. It is postulated that oxygen replaces part of the halide of the composition. The material is then reacted with a liquid or gaseous fluorinated hydrocarbon which is believed to react preferentially with the oxygen bound only to the niobium, producing, it is postulated, a composition containing various mixtures of chlorides, fluorides, oxides, oxychlorides, oxyfluorides, oxychlorofluorides, etc., depending on reaction conditions. Analyses of compositions prepared in this fashion show that they contain varying amounts of chlorine and fluorine along with amounts of oxygen (not bound to the substrate) ranging from insignificant to moderate depending on the degree of fluorination obtained using the fluorinated hydrocarbon. The amount of oxygen remaining can be varied by choice of fluorinated hydrocarbon and reaction conditions. Reaction temperatures and pressures for the reaction with the fluorinated hydrocarbon are not critical. Temperatures of room temperature or greater are generally suitable. Different fluorinated hydrocarbons will have different optimum temperatures, pressures and times of contact, and these can readily be determined by routine experimentation. Particularly suitable fluorinated hydrocarbons are the Freons, such as, for example Freon 12 (CF.sub.2 Cl.sub. 2), Freon 14 (CF.sub.4), Freon 23 (CHF.sub.3), Freon 112 (CCl.sub.2 F-CCl.sub.2 F), Freon 116 (CF.sub.3 --CF.sub.3), Freon 142 (chlor-difluor-methyl methane), Freon C138 (octafluorocyclobutane) and similar materials. One particular advantage of this process is that it allows for the preparation of compositions containing higher amounts of fluoride than does the process using reactive sublimation of niobium pentafluoride alone. Compositions containing the fluoride are more resistant to oxygen degradation than the compositions containing chloride alone. Thus, when the mixed chloride/fluoride compositions are used as catalysts, the feeds need not be purged of oxygen and air is no longer a poison. Feeds containing oxygen (e.g., O.sub.2, peroxide, etc.), however, will still compete for catalyst sites and, hence, the observed rates of reaction can be reduced.

As noted above, a modification of the basic composition of the instant invention can be obtained by contacting the niobium (V) halide/oxide-metal oxide compositions with oxygen or a compound containing oxygen which is weakly covalently bonded to said compound. Illustrative of said compounds are the peroxides and peroxy compounds, both organic and inorganic, the hypohalites, etc. It is postulated that contact of the instant composition with oxygen or the indicated oxygen-containing compounds converts part of the halogen of the composition to oxygen which is not bound to the substrate. Thus, there are two possible types of oxygen bound to the pentavalent niobium of the composition. One type is the oxygen(s) which is bound to the niobium and to the substrate. This presence of this type of oxygen is required to produce the compositions of the instant invention. The other type of oxygen which optionally may be present is oxygen bound only to the niobium of the composition. Thus, at least one valence of pentavalent niobium is bound to oxygen which is bound to the substrate, at least one valence of the niobium is bound to halogen and the remaining niobium valences are bound to halogen and/or oxygen which is or is not bound to the substrate. This modification containing the optional oxygen may be effected either inadvertently or purposefully. It may be effected by contact with oxygen or oxygen-containing compounds present as additives or impurities in feed streams when the compositions are used as catalysts. For many reactions the instant compositions provide for very active catalysts. When these very active catalysts are used in packed-bed flow reactors, they can lead to hot spots and reactor runaway. The activity of the catalyst can be moderated by contact with oxygen or oxygen-containing compounds as described above.

The compositions of the instant invention find particular utility as catalysts for hydrocarbon conversions, such as, for example, alkylation reactions, isomerization reactions, oligomerization reactions, etc.

The compositions of the instant invention and an illustrative use of these compositions as a catalyst will be further described below by the following Illustrative Embodiments which are provided for illustration, and are not to be construed as limiting the invention.

The following illustrates the preparation of a composition of the instant invention utilizing niobium pentachloride as an illustrative halide. A 200-ml Schlenk flask containing 50-75g of silica gel (Davison 57, 60-100 mesh) was heated to 300.degree. C. under a vacuum of ca. 0.1 torr for 16-20h. The flask was moved into a dry box whereupon 6.5g of the silica was placed on one side of a fritted Schlenk tube. Niobium pentachloride (6.0g) was placed on the other side of the frit. The NbCl.sub.5 end of the tube was wrapped with heating tape and then an insulation wrap was installed along with a Thermocouple wire. A vacuum of ca. 0.1 torr was applied at the end of the silica-containing section. The deposition was carried out overnight (16-20h) at 100.degree. C. with the tube mounted horizontally. The siliceous material was removed in a dry box and then subjected to a vertical sublimation in order to remove any condensed but unreacted NbCl.sub.5.

The following technique has been found to result is a somewhat more homogeneous catalyst than the above described technique and is utilized where uniformity is important. In this preparative technique a glass scrubbing bottle was modified by internally adding a coarse fritted disc which divided the bottle into an upper section and lower section. The lower section was fitted with a stoppered connection which allowed it to be charged with niobium pentahalide and the upper section was fitted with a vacuum stopcock connection which allowed it either to be closed off or connected to a vacuum. To the modified gas-scrubbing bottle were added about 20g of NbCl.sub.5 to the bottom section and about 60g of Davison 57 silica (-20+30 mesh, pretreated at 300.degree. C. under 0.1 torr vacuum for 12-24 h) to the top section. Both sections were loaded in a dry box containing a nitrogen atmosphere. The bottom section was stoppered and the top section had the vacuum stopcock closed before removing from the dry box. The bottom section of the bottle was immersed into an oil bath and heated at about 150.degree. C. The top section was wrapped with heating tape and heated to about 150.degree. C. A vacuum (about 0.1 torr) was applied at the top of the bottle. The heating and vacuum phase of the preparation was simultaneous and carried out over a period of 18h. At the end of 18h, the bottle (vacuum stopcock closed) was put back into the dry box and 20g a fresh NbCl.sub.5 was added to the bottom section. The rest of the procedure was then repeated for another 18h. Then, the silica was removed, in a nitrogen-filled dry box, and vertically sublimed at 150.degree. C. and 0.1 torr for 18h. This step was employed to remove any deposited but unreacted NbCl.sub.5 on the silica surface. A small (<200 mg) of NbCl.sub.5 was generally collected on the cold finger of the sublimator. The resulting material can be used as a catalyst without further preparation. The material contained on the average about 2-4 wt % Nb with a mole ratio of chlorine to niobium of about 2.5 to 3.5:1.

Similar niobium (V) chloride-containing compositions may be prepared in a fashion similar to above using as supports a silica-alumina (such as Davison Grade 980-25 or an alumina (such as Kaiser alumina KA-201).

Niobium (V) fluoride-containing, niobium (V) bromide-containing and niobium (V) iodide-containing compositions may be prepared using the reactive sublimation techniques described above and niobium pentafluoride, niobium pentabromide and niodium pentaiodide, respectively as reactive sublimation agents.

The following example illustrates the use of a composition of the instant process as an olefin alkylation catalyst.

In a dry box, to a 50-ml Erlenmeyer flask containing a stirring bar were added 10 ml of benzene, 0.5 ml of 2-pentene, 0.5 ml of iso-pentane (as an internal standard) and 1 g of the composition prepared as described above and containing about 2 wt % of niobium. The reaction was stirred at ambient temperature under atmospheric pressure for 30 minutes. The reaction was then filtered, and the solution analyzed directly. Analysis of the product showed almost complete conversion of the 2-pentene with a 99+% selectivity to pentyl benzene.

The above experiment was repeated using 5 ml of benzene, 1 ml of 2-pentene, 1 ml of iso-pentane and 0.84 g of the niobium (V) chloride-containing silica composition (above 2 wt % Nb). Analyses of the reaction product at various time intervals gave the following results: