Solid Solutions Involving MGO-V.sub.2 O.sub.3 -TiO.sub.2

This invention relates to substitutional solid solutions involving MgO-V.sub.2 O.sub.3 -TiO.sub.2.

The present invention pertains to a novel oxidation catalyst and the method for its preparation. The material comprises substitutional solid solutions involving MgO, V.sub.2 O.sub.3 and TiO.sub.2 and having compositions that fall within a polygon in a ternary composition diagram of MgO, V.sub.2 O.sub.3 and TiO.sub.2 as described in the drawing herein; wherein the polygon has the vertices and corresponding coordinates in weight percent as shown in Table I.

The substitutional solid solutions involving MgO, V.sub.2 O.sub.3 and TiO.sub.2 having compositions falling within the polygon shaped shaded area in the ternary composition diagram of MgO V.sub.2 O.sub.3 and TiO.sub.2 as shown in the drawing are made by the process comprising the steps of:

(1) heating V.sub.2 O.sub.5 powder having a particle size of less than about 50 microns in an atmosphere in which the partial pressure of oxygen is in the range of about 10.sup.-8 to 10.sup.-10 atmosphere and the CO.sub.2 /H.sub.2 volume ratio is in the range of about 10/1 to 1/1 while gradually increasing the temperature stepwise over a temperature range of about 600.degree. C. to 1300.degree. C. and holding the final temperature for a period of at least about 24 hrs. to ensure complete reduction of all vanadium to V.sup.3+ ;

(2) heating TiO.sub.2 and MgO powders each having a particle size of less than about 50 microns for a period of about 10 to 14 hrs. at a temperature in the range of about 600.degree. C. to 800.degree. C. for TiO.sub.2 and about 1000.degree. C. to 1200.degree. C. for MgO;

(3) thoroughly grinding together about 97 TO 16 wt.% of V.sub.2 O.sub.3 from (1), with about 0 TO 70 wt.% of TiO.sub.2 and about 3 to 42 wt.% of MgO from (2) to produce a mixture having a grain size of less than about 50 microns,

(4) pelletizing the mixture from (3) at a pressure of about 5,000 psi; and

(5) heating the pellets from (4) at a temperature in the range of about 1100.degree. C. to 500.degree. C. for a period in the range of about 12 to 48 hrs. in an atmosphere in which the partial pressure of oxygen is in the range of 10.sup.-10 to 10.sup.-5 atmosphere to produce said solid solutions involving MgO, V.sub.2 O.sub.3 and TiO.sub.2.

In one embodiment, the pellets from step (5) are cooled to ambient temperature while at substantially the same partial pressure of oxygen as in step (5). Preferably, to prevent undue oxidation and moisture pickup, the cooled pellets are stored in a sealed container.

The drawing depicts a ternary composition diagram showing a polygon within which useful substitutional solid solutions involving MgO-V.sub.2 O.sub.3 -TiO.sub.2 are found.

New solid solutions have been synthesized by the subject invention in which the nature and extent of the formation of substitutional solid solutions involving vanadium oxide, notably V.sub.2 O.sub.3, as component have been characterized. This work has led to synthesizing new phases in which vanadium in several oxidation states are accommodated such as to render crystalline phases thermodynamically stable over a wide range of oxygen pressures at elevated temperatures.

The newly synthesized substitutional solid solutions in the MgO-V.sub.2 O.sub.3 -TiO.sub.2 system are shown as falling within the polygon shaped shaded area in the ternary composition diagram in the drawing. The oxidation states of vanadium in the substitutional solid solutions are mainly +3 and +4. The polygon has the following vertices and corresponding coordinates in weight percent:

A typical newly synthesized material consisting of two solid solutions, viz, a spinel having the formula Mg.sub.1.21 V.sub.1.58 Ti.sub.0.21 O.sub.4 and pseudobrookite having the formula Mg.sub.0.20 V.sub.1.60 Ti.sub.1.20 O.sub.5 is illustrated by point H in the drawing having the following coordinates in wt.%: MgO 17, V.sub.2 O.sub.3 61, and TiO.sub.2 22.

Substitutional solid solutions in the MgO-V.sub.2 O.sub.3 -TiO.sub.2 system are synthesized in the following manner: First, V.sub.2 O.sub.3 is prepared by heating commercially available analyticalgrade V.sub.2 O.sub.5 having a particle size of less than about 50 microns in a vertical tube furnace in an atmosphere of carefully selected oxygen pressure within the stability range of V.sub.2 O.sub.3 e.g. in the range of 10.sup.-8 to 10.sup.-10 atmospheres. This is accomplished by using a gas mixture of high-purity CO.sub.2 and H.sub.2 in controlled proportions. The CO.sub.2 /H.sub.2 ratio by volume is in the range of 10/1 to 1/1. The relatively low melting point of the starting vanadium oxide (V.sub.2 O.sub.5), e.g. about 690.degree. C., necessitates heating the oxide slowly. Starting at a temperature of about 600.degree. C., the temperature is gradually increased stepwise over a period of about 12 to 24 hrs. to a final temperature of about 1300.degree. C. At that temperature the oxide is held at least about 24 hrs. e.g. 24 to 30 hrs. to ensure complete reduction of all vanadium to V.sup.3+ (V.sub.2 O.sub.3). The identity of the reaction product may be confirmed by X-ray diffraction analysis.

At subsolidus temperatures, this system is dominated by the spinel MgV.sub.2 O.sub.4 -Mg.sub.2 TiO.sub.4) and pseudobrookite (V.sub.2 TiO.sub.5 -MgTi.sub.2 O.sub.5) solid solutions. The former solid solution (spinel) is complete between the two end members at high temperatures (>1400.degree. C.) whereas the latter solid solution (pseudobrookite) extends for most of the composition range from MgTi.sub.2 O.sub.5 toward V.sub.2 TiO.sub.5, but connects to a different phase assemblage(s) near the V.sub.2 O.sub.3 -TiO.sub.2 point.

The rhombohedral solid-solution phases of corundum-type structure extend relatively short distances into the "ternary" system from their respective end members V.sub.2 O.sub.3 and MgTiO.sub.3. In the central part of the "ternary system", the solid solution between these two end members is unstable relative to the phase assemblage spinel solid solution plus pseudobrookite solid solution and hence decomposes to these two phases.

The distribution of vanadium among the phases spinel, pseudobrookite and sesguioxide (=rhombohedral phase) will be relatively sensitive to variations in oxygen pressure. Hence, this distribution will give important information on crystal-chemical and the energetic' aspects of the substitution of vanadium in its various oxidation states, especially in the spinel phase.

Pre-heated analytical-grade oxides having a particle size of less than about 50 microns are used as starting materials for the other components of the solid solutions to be synthesized. For example, TiO.sub.2 is heated for about 10 to 14 hrs. at a temperature in the range of about 600.degree. C. to 800.degree. C. and MgO is heated for about 10 to 14 hrs. at a temperature in the range of about 1000.degree. C. to 1200.degree. C. prior to being used in preparation of the final mixtures.

The mixtures of the three oxide components are mechanically ground together under acetone in an agate mortar to ensure thorough mixing and a sufficiently small grain size e.g. less than 50 microns. For example, about 97 to 61 wt.% of V.sub.2 O.sub.3 is ground together with about 0 to 70 wt.% of TiO.sub.2 and about 3 to 42 wt.% of MgO to produce a mixture having a grain size of less than about 50 microns. Complete formation of the desired compounds in the succeeding heat treatment is thereby promoted. Next, the oxide mixtures are pelletized at a pressure of about 5,000 psi or higher. The pellets may have any conventional size e.g. 1/16" to 1". The pellets are then heated at a temperature in the range of about 1100.degree. C. to 1500.degree. C. for a period in the range of about 12 to 48 hrs. (depending on the nature and solidus temperature of the phase to be synthesized) in a vertical tube furnace with carefully controlled oxygen pressures, in the range of 10.sup.-10 to 10.sup.-5 atmosphere. In the manner described previously for preparing V.sub.2 O.sub.3, the pellets are heated in a furnace atmosphere provided by a gas mixture of CO.sub.2 and H.sub.2 in various desired mixing ratios, typically in the range of about 10/1 to 1/1. These selected ratios are kept constant for the duration of the synthesis by use of a differential manometer. By this method the oxygen pressure at the specified temperature can be controlled to better than .+-.1%. The thermodynamic data for the water-gas reaction (CO.sub.2 +H.sub.2 =CO+H.sub.2 O), on which the calculations were based, are known with extremely high accuracy (better than 0.1%), and hence the method used ensures reliable accurate control of the oxidation state of vanadium during the synthesis. This is extremely important for optimization of the properties of the finished product.

In addition to oxygen pressure, one other parameter influencing the oxidation state of vanadium in oxide phases has been utilized in synthesizing the new phases, viz. the provision of host structures which incorporate vanadium in one (or more) valence state(s) in strong preference to other valence states. In the present case, the extensive ternary solid solution phases of pseudobrookite type and of spinel type structures in the system MgO-V.sub.2 O.sub.3 -TiO.sub.2 serve this purpose. In these solid solutions the vanadium is present partly in the trivalent state, as stabilized by the preferential substitution of V.sup.3+ for Al.sup.3+, and partly in the tetravalent state, as stabilized by the preferential substitution of V.sup.4+ for Ti.sup.4+. Furthermore, MgO in combination with vanadium oxide stabilizes V.sup.5+ in the crystalline phase Mg.sub.3 V.sub.2 O.sub.12 by virtue of the fact that MgO is a base relative to V.sub.2 O.sub.5.

The pellets of the substitutional solid solutions involving MgO-V.sub.2 O.sub.3 -TiO.sub.2 may be used as an oxidation catalyst, offering improved activity and yields and greater stability over a wider temperature range e.g. about 1000.degree. C. or higher than that which is offered by typical oxidation catalysts. For example, as an oxidation catalyst the subject pellets may be used in the conversion of o-xylene to phthalic anhydride, butane to maleic anhydride, or alcohols to aldehydes or organic acids.

As a high temperature refractory or as an ingredient in a high temperature refractory material for lining a gasifier for the partial oxidation of ash-containing liquid hydrocarbonaceous and solid carbonaceous fuels, the subject material would have a long life, withstanding attack from V.sub.2 O.sub.5. For example, a thermal refractory for lining the reaction zone of a partial oxidation gas generator may comprise the previously described solid solutions involving MgO-V.sub.2 O.sub.3 -TiO.sub.2 in the amount of about 1 to 25 wt.%, and the remainder comprising at least one other oxide of one of the elements Cr, Mg, Al, Mn, Si, or mixtures thereof.

The subject solid solutions involving MgO-V.sub.2 O.sub.3 -TiO.sub.2 may also be used as an additive. About 0.01 to 0.2 wt.% (basis weight of ash in the fuel feed) of the additive may be introduced into the refractory-lined partial oxidation gas generator along with the fuel feed to stabilize said refractory.

Various modifications of the invention as herein before set forth may be made without departing from the spirit and scope thereof, and therefore, only such limitations should be made as are indicated in the appended claims.