Process for Para-Ethyltoluene Dehydrogenation

1. Field of the Invention

This invention relates to improved catalysts for the selective dehydrogenation of dialkyl aromatic hydrocarbons to produce alkyl vinyl aromatic hydrocarbons, more particularly to catalysts for the production of para-methylstyrene (PMS) via the dehydrogenation of para-ethyltoluene (PET).

2. The Prior Art

The vinyl benzenes play a particularly important role in the preparation of synthetic plastics and resins. The polymerization of styrenes, for example, to produce polystyrene resins is well known.

Styrene and styrene derivatives are typically produced from ethylbenzene materials by dehydrogenation over solid catalysts in the presence of co-fed steam, and at temperatures ranging from 500.degree. C. to 700.degree. C. The catalysts found to be the most effective for this process are those which are based on potassium oxide (carbonate) promoted, chromium oxide stabilized, iron oxide material. Catalysts of this type are said to be self-regenerative inasmuch as, in addition to their effectiveness in promoting dehydrogenation, they also promote the water gas reaction in the presence of the steam co-feed, to thereby remove coke which would otherwise build up on and deactivate the catalyst. The lifetime of such self-regenerative catalysts is thus determined by the effectiveness of the catalyst in maintaining its activity for conversion of ethybenzene materials such as para-ethyltoluene for any given steam/hydrocarbon ratio in the feed. Catalysts of this type which can maintain such activity at generally lower steam/hydrocarbon ratios are, of course, more economically desirable.

Considerable research has been directed toward attempts to improve the activity and selectivity of iron oxide-potassium carbonate-chromium oxide-based dehyrogenation catalysts. Any improvement which results in either increasing the selectivity (moles of desired product per mole of reactant reacted) or the conversion (moles of reactant reacted per mole of starting material) without lowering the other is economically attractive since the result is that the yield (moles of desired product produced per mole of reactant) of the product has been increased. Any increase in the numerical value of the yield results in a more efficient operation with more reactant being converted into the desired product. In commercial operations, many of which produce millions of pounds of product per year, a trade-off is frequently effected between selectivity and conversion. An increase of only 1 or 2 percentage points in the selectivity can result in a substantial savings of starting materials. An increase in conversion can substantially reduce capital expenditure and energy consumption. The trade-off may vary depending on raw materials costs, energy costs, and the age of the plant.

Attempts have been made to improve the conversion effectiveness and selectivity of iron oxide type dehydrogenation catalysts for use in various alkylaromatic dehydrogenation reactions. Riesser; U.S. Pat. No. 4,152,300; issued May 1, 1979, for example, discloses that an improvement in ethylbenzene dehydrogenation catalyst selectivity can be realized by incorporating small amounts of certain metal oxide materials into dehydrogenation catalyst compositions comprising mixtures of iron oxide, potassium oxide, vanadium oxide and, optionally, chromium oxide.

Courty; U.S. Pat. No. 4,134,858; issued Jan. 19, 1979, discloses an iron oxide based dehydrogenation catalyst containing particular amounts of clay to improve the conversion, selectivity and yield of styrene and divinylbenzenes produced by dehydrogenation of ethyl- or diethylbenzene. This U.S. Pat. No. '858 patent also notes that oxides of copper, vanadium, zinc, manganese, magnesium, nickel, cobalt, bismuth, tin and antimony can be added to the disclosed dehydrogenation catalysts.

Notwithstanding such attempts to improve iron oxide based dehydrogenation catalysts, there is a continuing need to formulate catalysts of this type which can be used to realize improved conversion and/or yield in the dehydrogenation of other types of alkylaromatic materials such as, for example, in the production of para-methylstyrene from para-ethyltoluene.

Accordingly, it is an object of the present invention to provide an improved iron oxide based dehydrogenation catalyst especially useful for the dehydrogenation of para-ethyltoluene to produce para-methylstyrene.

It is a further object of the present invention to provide a para-ethyltoluene dehydrogenation process employing a catalyst which provides desirably high conversion of para-ethyltoluene to p-methylstyrene even at relatively low steam to hydrocarbon ratios in the charge to the reaction zone.

These and other objectives can be achieved by means of the invention described and claimed herein.

The present invention relates to an improved dehydrogenation catalyst composition especially useful for the selective dehydrogenation of para-ethyltoluene to produce para-methylstyrene. Such a catalyst comprises from about 30% to 60% by weight of an iron oxide component, calculated as ferric oxide, from about 13% to 48% by weight of a potassium compound component, calculated as potassium oxide, from about 0% to about 5% by weight of a chromium compound component, calculated as chromic oxide, and from about 1% to 15% by weight of a magnesium compound calculated as magnesium oxide.

The present invention also relates to a dehydrogenation process wherein para-ethyltoluene, along with steam, is passed over the magnesium-containing catalyst composition at a temperature from about 500.degree. C. to 700.degree. C. with a LHSV of from about 0.3 to 3 and steam to hydrocarbon ratios of from about 1:1 to 5:1, to selectively produce para-methylstyrene.

The dehyrogenation catalyst compositions of the present invention contain as an essential catalytic component one or more iron compounds, generally in the form of iron oxide. Many forms of iron oxide can be used in the catalyst compositions of this invention. Typically, iron oxides employed in catalyst preparations of this sort are a synthetically produced, powdered red, red-brown, yellow or black pigment. The red or red-brown pigments are highly pure ferric oxide, while the black pigment is the magnetic form, ferrosoferric oxide (Fe.sub.3 O.sub.4), which is usually found in the catalyst under various reaction conditions. The yellow iron oxides consist of the monohydrated form of ferric oxide. These oxides are prepared by various methods, e.g., oxidation of iron compounds, roasting, precipitation, calcination, etc. A suitable form of iron compound is the monohydrated yellow iron oxide used in the preparation of catalysts according to U.S. Pat. Nos. 3,360,597, issued Dec. 26, 1967, and 3,364,277; issued Jan. 16, 1968. Particularly suitable are pigment grade red iron oxides of purities exceeding 98% weight. These red oxides have surface areas ranging from 2 to 50 m.sup. 2 /gram and particle sizes from 0.1 to 2 microns. The iron compound is present in the catalyst in either one or a mixture of both of its possible oxidation states, i.e., as ferrous iron or ferric iron or mixtures thereof, as for example, ferrosoferric iron.

The catalyst compositions herein generally comprise from about 30% to 60% by weight, preferably from about 35% to 55% by weight, of iron oxide calculated as ferric oxide. Alternatively stated, the catalyst compositions herein generally comprise from about 21% to 42% by weight, and preferably from about 24% to 39% by weight, of iron oxide, calculated as iron metal.

The dehydrogenation catalyst compositions of the present invention also essentially comprise, as a catalyst promoter, one or more potassium compounds. The potassium promoter material can be added to the catalyst in various forms. For example, it may be added as the oxide, or as other compounds which are convertible, at least in part, under calcination conditions, to the oxides, such as the hydroxides, the carbonates, the bicarbonates, the phosphates, the borates, the acetates, and the like. A particularly preferred potassium compound is potassium carbonate. The potassium compound is generally present in the catalyst as a potassium oxide, a potassium carbonate or a mixture thereof. High carbon dioxide partial pressures in the reaction gases will favor high carbonate to oxide ratios and vice versa within the potassium component.

The catalyst compositions herein generally comprise from about 13% to 48% by weight, and preferably from about 27% to 41% by weight, of potassium promoter compound, calculated as potassium oxide. Alternatively stated, the catalyst compositions herein generally contain from about 11% to 40% by weight, and preferably from about 22% to 34% by weight, of potassium oxide, calculated as potassium metal.

An optional, but frequently utilized, third component of the present catalyst composition is a chromium compound which serves as a stabilizer for the active catalytic components. Chromium compounds have, in fact, typically been added to alkali-promoted iron oxide catalysts to extend their life. Chromium, as used in the compositions of this invention, can be added to the catalyst in the form of a chromium oxide or in the form of chromium compounds which decompose upon calcination to chromium oxides, as for example, chromium nitrates, hydroxides, acetates, and the like. Chromium can also be added in the form of alkali metal chromates. If potassium chromates are used, such materials can, of course, also contribute to the requisite concentration of potassium essentially present in the dehydrogenation catalyst compositions as hereinbefore discussed.

Thus, the catalyst compositions herein can comprise from about 0% to about 5% by weight, and preferably from about 1% to 4% by weight chromium compound, calculated as chromic oxide. Alternatively stated, the present composition can comprise from about 0% to 3.5% by weight, preferably from about 1.4% to 2.8% by weight, of a chromium oxide calculated as chromium metal.

In accordance with the present invention, the dehydrogenation catalyst compositions containing iron, potassium and optional chromium compounds, as described, also essentially contain particular selected amounts of a magnesium compound which can provide magnesium oxide in the catalyst compositions herein after calcination. Addition of magnesium compounds to the particular iron-potassium-chromium dehydrogenation catalysts utilized herein serves to enhance conversion of aromatic reactants when such catalyst compositions are used to promote the preferential formation of para-methylstyrene from para-ethyltoluene.

Magnesium as used in the catalyst compositions of the present invention can be added to the catalyst in the form of magnesium oxide, MgO, or in the form of other magnesium compounds which decompose upon calcination to form magnesium oxide, as for example, magnesium carbonate, magnesium hydroxide, magnesium sulfate, magnesium nitrate and magnesium acetate. Magnesium oxide itself, i.e., magnesite, is the preferred magnesium compound for use in formulating the compositions herein. Magnesium compounds are added to the catalyst compositions of the present invention to the extent of from about 1% to 15% by weight, more preferably from about 2% to 10% by weight, calculated as MgO.

In addition to the foregoing materials, the catalyst compositions of the present invention can optionally contain a wide variety of materials suitable for altering, adjusting or modifying the catalytic and/or physical properties of such compositions. Materials, for example, which can act as stabilizers, activators, and promoters for dehydrogenation catalysts of the type herein contemplated include, cobalt, cadmium, aluminum, nickel, cesium, and rare earths. Such additives can be incorporated in various forms including their elemental form or in the form of their oxides. If employed, such stabilizers, activators and/or promoters generally comprise from about 1% to 15% by weight of the catalyst compositions herein.

It should be noted that the compositions of the present invention need not contain materials such as potassium aluminosilicate, e.g., kaliophyllite, in order to enhance catalyst activity and/or selectivity to production of p-methylstyrene products. The magnesium oxide-containing catalysts of the present invention, in fact, can be maintained substantially free of clays or clay-like material without adversely affecting catalyst dehydrogenation activity or p-methylstyrene selectivity when used to promote dehydrogenation of p-ethyltoluene.

The physical strength of the catalyst compositions of the present invention can be improved, if desired, by adding any of a variety of optional insoluble binding agents. Binding agents can include, for example, calcium aluminate and Portland cement. It should be noted that binding agents can contribute to the requisite magnesium content of the catalyst compositions herein in the event that such binding agents contain materials which form magnesium oxide upon calcination of composites containing them. The density of the catalyst compositions herein can likewise be modified by the addition of various filler substances, for example, combustible materials such as sawdust, carbon, wood flour, etc. Such materials can be added to the compositions during preparation and thereafter burned out after the catalyst pellets have been formed. Other porosity promoting aids include graphite and aqueous solutions of methylcellulose, which also facilitate extrusion of catalyst pellets as hereinafter described. If employed, binders and other fillers generally can comprise up to about 20% by weight of the catalyst composition.

The catalyst compositions of the present invention are in general prepared by admixing the essential and desired optional components as hereinbefore described and by thereafter drying and optionally calcining the resulting mixture. Calcination temperatures can thus range from about 100.degree. C. to 600.degree. C., preferably from about 150.degree. C. to 550.degree. C. The compounds of the catalyst compositions herein can be admixed in various ways. One method comprises ballmilling together a mixture of the desired oxides and/or compounds decomposable upon calcination to oxides, adding a small amount of water, and extruding the paste formed to produce small pellets, which are then dried and calcined. Another method is to dissolve the components together, spray dry these components to form a resulting powder, calcine the powder into the resultant oxides, and then add sufficient water to form a paste which is extruded into pellets, dried and calcined. Another procedure involves precipitating those materials which are precipitatable, such as iron, chromium and magnesium, as the resultant hydroxides, partially dewatering the resultant precipitate, adding soluble salts of the other desired metals, and then subsequently extruding, drying and calcining the resulting pellets. A preferred method involves dry-blend powdering of oxides and/or compounds decomposable upon calcination to the oxides, adding water, optionally containing dissolved therein soluble compounds decomposable upon calcination to the oxides, then mixing and/or mulling the resultant paste, pelletizing the mixture, subsequently substantially drying at a temperature from about 50.degree. C. to about 300.degree. C., followed by calcining the pellets to form the final product. The drying and calcining could be carried out stepwise in the same furnace by suitable programming of the furnace temperature. Alternatively, water-insoluble dry powders of oxides and/or compounds decomposable upon calcination to the oxides are dry-mixed, and the balance of the other materials needed are dissolved in water and the resultant solution is used to form the paste with the dry powders. There are many variations of the mixing of dry powders, water and water soluble compounds that give equivalent results and fall within the scope of this invention.

The catalysts of the present invention are especially effective in promoting the dehydrogenation of para-ethyltoluene to selectively produce para-methylstyrene. Such a dehydrogenation reaction is usually carried out at reaction temperatures of about 500.degree. C. -700.degree. C. However, higher or lower temperatures may be used without departing from the scope of this invention. The use of atmospheric, sub-atmospheric, or super-atmospheric pressure is suitable. However, it is preferable to operate at as low a pressure as is feasible, and atmospheric or sub-atmospheric pressure is preferred. The process of the invention may be carried out in batch, semi-continuous, or continuous operation, with continuous operation being preferred. The catalyst is employed in the form of a fixed bed, or in fluidized or suspended form. It is preferable to utilize a fixed bed. The reaction may be carried out in single stage reactors or by staging in series reactors. The reactors may be of various designs, e.g., downflow reactors, radial reactors, etc.

With the use of the catalyst of this invention, it is essential to add steam to the reactant feed to aid in the removal of carbonaceous residues from the catalyst. The reaction feed generally contains from 2-30 moles of steam for every mole of organic feed. Catalysts having higher potassium contents are usually employed at lower steam to feed ratios. Steam to feed weight ratios of from about 1:1 to about 5:1 are desirable. Good results are obtained with steam to feed ratios of about 1.6:1 to about 4:1.

The contact time of the reactant-containing gas with the catalyst is usually defined in terms of liquid-hourly-space velocity (volume of liquid hydrocarbon reactant per volume of catalyst per hour, i.e., LHSV). The LHSV of the organic reactants according to this invention may vary from about 0.3 to 3 and is preferably adjusted within this range to effect the degree of conversion desired for the particular feed in question.

The catalysts of the present invention and their use will be further described by the following illustrative examples which are provided for illustration and are not to be construed as limiting the invention. It should be noted that advantages resulting from increases of selectivity and/or conversion of only one or two percentage points are extremely significant in a commercial process which may produce many hundreds of thousand pounds of product a day.

A reference dehydrogenation catalyst is formulated by admixing the following materials in the following concentrations:

To this mixture is added water to the extent of 20% and the resulting mixture is formed into a paste. The paste is formed into 3/8" disks which are then dried for 16 hours at approximately 180.degree. C. The disks are then crushed to 5-8 mesh particles.

The dehydrogenation catalyst prepared as in Example I is used to dehydrogenate para-ethyltoluene (PET) at atmospheric conditions to form para-methylstyrene (PMS) in a tubular reactor maintained at 620.degree. C. to which PET and steam are introduced. Feed rates and steam/hydrocarbon ratios are varied for several such dehydrogenation runs. Feed rates, steam/hydrocarbon ratios and the resulting conversion of p-ethyltoluene and selectivity to production of p-methylstyrene are set forth in Table I:

The Table I data indicate that the unmodified reference catalyst is useful for promoting dehydrogenation of p-ethyltoluene to p-methylstyrene with relatively high selectivity to p-methylstyrene production.

In a manner similar to that set forth in Example I, another dehydrogenation catalyst is prepared. The following dry materials are admixed in the concentrations shown.

Water is added to the extent of 20% and the resulting paste is again formed into 3/8" disks and dried for 16 hours at 180.degree. C. The dried disks are then crushed into 5-8 mesh particles. The resulting MgO-containing catalyst composition contains the same ratio of iron oxide to potassium carbonate to chromic oxide as does the reference catalyst of Example I.

Using the MgO-containing catalyst of Example III and the general reaction conditions of Example II, para-ethyltoluene (PET) is again dehydrogenated to para-methylstyrene (PMS). The conversion results are set forth in Table II:

A comparison of the Table II data with that of Table I indicates that the modification of the reference catalyst with magnesium oxide in accordance with the present invention gives a catalyst which provides improved conversion of PET with no significant drop in selectivity to p-methylstyrene production.

Using the MgO-containing catalyst as described in Example III and the general reaction procedures of Example II, p-ethyltoluene (PET) is again dehydrogenated to para-methylstyrene (PMS) at 620.degree. C. over an extended period of time with varying reaction conditions. Conversion results are set forth in Table III.

The Table III data indicate that the MgO-containing catalyst of the present invention can effectively convert PET to PMS over an extended period of time on stream at desirably low H.sub.2 O/PET ratios.