Catalyst for the Preparation of Ethylene and Vinyl Chloride From Ethane

Vinyl chloride (CH.sub.2 .dbd.CHCl) is prepared using a number of well known processes. Two familiar processes are (1) the hydrochlorination of acetylene and (2) the oxychlorination of ethylene to form dichloroethane which in turn is dehydrohalogenated to form vinyl chloride (see C. A. Schildknecht, Vinyl and Related Polymers, John Wiley and Sons, Inc., N.Y., N.Y. (1952), pages 388-390, and U.S. Pat. No. 2,847,483). As acetylene is more expensive then ethylene, the latter process is economically favored, and much activity is noted in this art area (see U.S. Pat. Nos. 3,634,330; 3,454,663; 3,448,057; and 3,624,170). Ethylene, in turn, can be prepared by the oxydehydrogenation of ethane (see U.S. Pat. No. 3,769,362). Although high yields of ethylene are particularly desired, processes which use ethane as a feed stock can produce not only ethylene but also can directly produce vinyl chloride and other valuable products such as ethylene dichloride, ethyl chloride, and the like. The ethylene, ethylene dichloride, and ethyl chloride can be readily reacted to form more vinyl chloride.

The present invention is directed to an improved process for the preparation of ethylene and vinyl chloride from ethane which process employs a solid solution catalyst containing iron stabilized with lanthanum and/or lanthanides. This catalyst exhibits improved activity and/or long lifetime. Catalysts containing iron or lanthanum are known to the art; see U.S. Pat. Nos. 3,907,713; 3,849,339; 3,769,362; 3,703,593; 3,658,934; 3,658,933; 3,207,809; 2,847,483; and 2,674,633; and British Pat. No. 1,039,369. However, none of these patents discloses a solid solution catalyst. An article in the Journal of The American Ceramic Society, Vol. 43, No. 7 (1960), page 367, discloses compounds of lanthanum and iron; recently issued U.S. Pat. No. 3,904,553 discloses certain specific solid solutions as having activity as catalysts.

The invention comprises an improved catalyst and process for the preparation of ethylene and vinyl chloride from ethane. The catalyst is a solid solution catalyst containing iron cations substituted for cations of the host lattice, which catalyst is stabilized with lanthanum and/or lanthanides. Use of the catalyst in the process of the invention produces yields of up to 90 mole percent of ethylene and up to 95 combined mole percent of ethylene, ethyl chloride, and vinyl chloride. The catalysts can remain active for up to 200 hours without substantial loss of iron.

Ethane is reacted with oxygen and a chlorine source and in the presence of a solid solution catalyst containing iron stabilized with lanthanum and/or lanthanides to prepare ethylene, vinyl chloride, and other valuable by-products. Depending upon feed and reactor conditions, about 25 to 90 mole percent yield of ethylene and up to a 95 mole percent total yield of ethylene and vinyl chloride can be obtained. Conversion of ethane to products can approach 100 mole percent.

In the process, ethane, oxygen, and a chlorine source are placed into a reactor vessel containing a solid solution catalyst of the invention. The process contemplates the use of standard techniques concerning the type of operation, reactor size and design, and the like. The process can be operated as a batch process, but is preferably conducted as a continuous process wherein reactants and products are continuously added and withdrawn. The solid solution catalyst can be fixed in a bed, it can be supported, or it can be present as particles that can readily fluidize during operation. A preferred embodiment of the process is to employ the solid solution catalyst in particulate form that will fluidize in the process thereby establishing maximum contact with the reactants. Such processes are known as fluid bed processes, and the reactors designed for such as known as fluid bed reactors. A typical reactor is designed such that one or more gaseous reactants is introduced in the reactor below the catalyst bed, and the gas pressurized through the bed lifting and suspending the catalyst in the reactor volume. Other reactants can be added at appropriate levels above, below, or any point in the fluid catalyst bed. Normally, products are withdrawn from the top portion of the reactor and collected or further treated as desired.

Although the process contemplates the use of known operating techniques and reaction conditions, certain conditions are herein stated as useful and practical. The reactants comprise ethane, oxygen (usually used in the form of air), and a chlorine source. The chlorine source is preferably hydrogen chloride gas. Using one mole of ethane as a basis, the hydrogen chloride is used at from about 0.1 mole to about 10 moles or more. More preferably, the hydrogen chloride is used at a level of from about 0.5 mole to 5 moles per mole of ethane. In general, as a higher ratio of hydrogen chloride to ethane is used, the yield of vinyl chloride and other chlorinated products increases and the yield of ethylene decreases. However, levels of use of hydrogen chloride above 5 moles per mole of ethane also increase the amount of hydrogen chloride to recycle. Excellent results have been obtained using about 1 to about 4 moles of hydrogen chloride per mole of ethane. As both ethylene and vinyl chloride can be prepared in significant amounts using the catalysts and as the yield of ethylene to vinyl chloride is highly dependent upon the hydrogen chloride to ethane ratio in the reactant feed, the process can be termed either an oxydehydrochlorination process to prepare ethylene or an oxychlorination process to prepare vinyl chloride.

Oxygen, preferably in the form of dry air, is used at from about 0.1 mole to about 1.5 moles of oxygen to one mole of ethane. A more preferred level is from about 0.5 mole to about 1 mole. The use of levels of oxygen of about 1 mole per mole of ethane is preferred in an oxychlorination process. In an oxydehydrochlorination process, excellent results have been obtained using a level of oxygen of about 0.5 to 0.6 mole per mole of ethane.

Ethane, oxygen, and hydrogen chloride are put into the reactor as reactants. Temperature of the reaction ranges from about 400.degree. C. to about 650.degree. C., and more preferably from about 475.degree. C. to about 600.degree. C. Materials withdrawn from the reactor in the product stream comprise ethylene, vinyl chloride, chlorinated products such as ethylene dichloride and ethyl chloride, carbon oxides (CO and CO.sub.2), water, and unreacted ethane and hydrogen chloride.

The improved feature of the process of the invention is the use of a solid solution catalyst containing iron cations substituted for cations in the host lattice which catalyst is stabilized with lanthanum and/or lanthanides. The catalyst is basically a solid solution of iron cations in a host lattice. This is in contrast to catalysts wherein an active ingredient such as cupric chloride or iron oxide is merely absorbed onto the surface of a support structure or material. The difference is crucial and can be distinguished both in the physical state of the catalyst and in the activity of the catalyst.

The solid solution catalyst is a true solution wherein iron cations are substituted for host lattice ions in the catalyst structure. An X-ray diffraction pattern of a solid solution catalyst is characteristic of the diffraction pattern of the host lattice. For example, a solid solution catalyst of Fe.sub.2 O.sub.3 in .alpha.-Al.sub.2 O.sub.3 will exhibit an X-ray diffraction pattern characteristic of .alpha.-Al.sub.2 O.sub.3. In contrast, if Fe.sub.2 O.sub.3 is merely absorbed onto .alpha.-Al.sub.2 O.sub.3, the X-ray diffraction pattern will show the presence of both Fe.sub.2 O.sub.3 and .alpha.-Al.sub.2 O.sub.3.

A distinguishing feature of the solid solution catalysts of the invention, i.e., solid solution catalysts containing iron and stabilized with lanthanum and/or lanthanides, is in the increased retention of iron upon use. For example, an .alpha.-Al.sub.2 O.sub.3 solid solution catalyst containing iron cations and stabilized with lanthanum cations used at reaction conditions of 1 mole ethane/0.6 mole oxygen/1.5 mole hydrogen chloride lost about 3% by weight of its original iron content after about 100 hours of use. In contrast, a catalyst which is a simple solid solution of iron in .alpha.-Al.sub.2 O.sub.3 lost about 4% by weight of its iron content under the same conditions. In further contrast, a catalyst comprised of ferric oxide merely absorbed onto Al.sub.2 O.sub.3, operating under the same set of conditions, lost over 8% by weight of its original iron content after about 100 hours of use.

The solid solution catalysts containing iron cations can be of different types. The iron exists as ferric (Fe.sup.+3) and/or ferrous (Fe.sup.+2) ions. The ferric ion is the active ion in the catalyst. However, as the ferrous ion can oxidize to a ferric ion in the process, the use of solid solution catalysts containing ferrous ions are within the scope of the invention.

In the solid solution catalyst containing iron cations there is direct substitution of iron ions for host lattice ions. An example of this catalyst is (Fe.sub.x.sup.+3 M.sub.2-x.sup.+3)O.sub.3 wherein x is greater than 0 and less than 2 and M is a metal such as Al or Cr. An example of this is a solid solution catalyst of ferric oxide (Fe.sub.2 O.sub.3) in aluminum oxide (Al.sub.2 O.sub.3). As the ferric ion is much greater in size than an aluminum +3 ion, the solubility of the ferric ion in aluminum oxide is limited. Hence, the solid solution catalysts of the example wherein M is aluminum encompass materials of the formula wherein x has an upper limit of about 0.15.

The solid solution catalyst containing iron is stabilized with lanthanum and/or a lanthanide. Although the lanthanum or lanthanide is an integral part of the catalyst, it is believed that the lanthanum or lanthanide does not enter into solid solution with the host lattice as does the iron. Characterization of the catalysts of this invention will be discussed further in a subsequent section of the application.

The lanthanum and lanthanides can be employed in the solid solution catalysts singly or as mixtures of the metals. The lanthanides are elements 58 to 71 of the Periodic Table. More preferably, the lanthanides used are Cerium, Praeseodymium, Neodymium, and Erbium. A preferred catalyst consists of a solid solution catalyst containing iron and stabilized with lanthanum, praeseodymium or neodymium. Excellent results have been obtained using a catalyst of Fe.sub.2 O.sub.3 in .alpha.-Al.sub.2 O.sub.3 stabilized with lanthanum.

The solid solution catalysts of the invention contain iron and have X-ray diffraction patterns characteristic of the host lattice material. Solid solutions are known to exist (see C. S. Barrett, Structure of Metals, Crystallographic Methods and Principles, and Data, 2nd Ed., McGraw-Hill Book Co., Inc., N.Y., N.Y. (1952), at pages 220 et seq.).

The catalyst is first identified and characterized by analyzing it to determine what elements it contains. This can be done using well known techniques such as chemical analysis, atomic absorption spectroscopy, X-ray fluorescence spectroscopy, and optical microscopy. For example, the solid solution catalyst of iron oxide in aluminum oxide, stabilized with lanthanum would show iron, lanthanum, aluminum, and oxygen to be present in the catalyst. The presence and quantity of iron in the catalyst can be readily determined using a standard method of chemical analysis such as the dichromate method for the determination of iron. The amount of iron in the solid solution catalysts is limited by the solubility of the ions in the host lattice. The solid solution catalysts can contain from about 0.1 percent to 20 percent by weight and more preferably from about 0.5 percent to about 10 percent by weight of iron in the catalyst expressed as iron oxide. The catalyst can contain a similar level of lanthanum and/or lanthanide expressed as the oxide.

The second step of identification and characterization involves running an X-ray diffraction scan on the catalyst. The X-ray diffraction scan will show a pattern of peaks, which peaks have positions and intensities distinctive of the crystalline phases which are present. The X-ray diffraction peak positions and intensities of the catalyst can be compared to peak positions and intensities of known crystalline phases that are published (in the ASTM Powder Diffraction File, for example), or that are experimentally obtained. For example, a catalyst comprised of iron oxide merely impregnated on aluminum oxide will have an X-ray diffraction pattern of peak positions showing the distinct peak positions and intensities of iron oxide and aluminum oxide crystalline phases.

In contrast, the X-ray diffraction pattern of a solid solution catalyst containing iron shows the positions of the X-ray diffraction peaks in the solid solution catalyst to be shifted from the peak positions in the X-ray diffraction pattern of the host lattice. The shift in peak positions may be accompanied by changes in the relative intensities of the peaks, but the intensity changes are generally small.

The shift in X-ray diffraction peak positions when solid solutions are formed results from the expansion (or contraction) of the dimensions of the unit cell of the crystalline phase of the host lattice. The dimensions of the unit cell of the host lattice are changed due to the substitution of iron cations for cations of the host lattice. If the cation is larger than the cation it displaces, the unit cell dimensions will increase in size to accommodate the larger cation. The amount of expansion (or contraction if the iron cation is smaller than the host lattice cation it displaces) of the unit cell dimensions can be determined by calculating the lattice parameters of the unit cell of the solid solution phase and comparing these lattice parameters to the lattice parameters of the unit cell of the host. A change in lattice parameters due to iron substitution in a crystalline host lattice is frequently in accordance with Vegard's law (see page 221 of the above-cited reference). Since a change in the lattice parameters causes a change in the X-ray diffraction peak positions, a quick comparison of the X-ray diffraction pattern of the catalyst and the pattern of the host lattice will show whether a solid solution catalyst has been prepared.

Alternately, a more accurate method of confirming the preparation of a solid solution catalyst is to experimentally run X-ray diffraction scans of the prepared catalyst and of the host lattice and then calculate the lattice parameters of each. If the values obtained for the lattice parameters of the catalyst and host lattice are different, a solid solution catalyst has been prepared. If the geometry and dimensions (lattice parameters) of the unit cell of the host lattice is not known, it can be determined using established methods for indexing and interpreting X-ray diffraction patterns (see L. V. Azaroff and M. J. Buerger, The Powder Method In X-Ray Crystallography, McGraw-Hill Book Co., Inc., N.Y., N.Y. (1958), chapters 6 to 13). The high 2.theta. values (where .theta. is the Bragg angle) are normally used to calculate the lattice parameters.

In the case of a solid solution catalyst stabilized with lanthanum and/or a lanthanide, the X-ray diffraction pattern will clearly show the presence of the solid solution, which is the primary crystalline phase, and will additionally show the presence of crystalline lanthanum and/or lanthanide compounds which are present in detectable amounts. For example, in the case of a solid solution catalyst of Fe.sub.2 O.sub.3 in .alpha.-Al.sub.2 O.sub.3 stabilized with lanthanum, the X-ray diffraction pattern will show the presence of the Fe.sub.2 O.sub.3 in .alpha.-Al.sub.2 O.sub.3 solid solution crystalline phase and crystalline compounds of lanthanum such as La.sub.2 O.sub.3 and LaAlO.sub.3.

In summary, the solid solution catalysts of the invention can be identified and characterized by (1) the presence of iron and lanthanum and/or lanthanides in the catalyst, and (2) the X-ray diffraction pattern of the catalyst. The iron is present as cations substituted in the host lattice for cations of the host lattice. The iron content can be measured using standard analysis techniques. The X-ray diffraction pattern of the solid solution catalyst will exhibit peak positions characteristic of the host lattice but shifted due to the presence of the iron cations in the host lattice. Lattice parameters calculated for the host lattice and the solid solution catalyst will differ. The X-ray diffraction pattern of the solid solution catalysts of the invention will exhibit extraneous peaks in the pattern due to formation of crystalline compounds other than the solid solution catalyst itself, such as lanthanum oxide or lanthanide oxides.

The solid solution catalysts used in the Examples were prepared by first impregnating a host lattice precursor with an iron salt and a lanthanum salt and/or a salt of a lanthanide that yields the oxides upon heating, then heating the impregnated host lattice precursor to about 550.degree. C. followed by heating to 1200.degree. C. or more. The first heat treatment converts the salts to oxides, and initiates conversion of the host lattice precursor to the host lattice. The second heat treatment completes the formation of the host lattice and produces a rearrangement of the metal atoms between the metal ions in the host lattice and the iron ions. The catalyst prepared is a solid solution catalyst containing iron, stabilized with lanthanum and/or lanthanides, having a distinctive X-ray diffraction pattern.

The solid solution catalyst can be prepared in other different ways. Another method is to physically admix iron oxide, lanthanum or a lanthanide oxide, and the host lattice material and heat the mix to allow dissolution and substitution of the iron ions for those of the host lattice, and formation of the stabilized catalyst. Heating conditions vary for the nature of the host lattice employed, but typically are above about 1100.degree. C.

A third method of preparation is to use the so-called sol-gel process wherein an iron salt, lanthanum and/or lanthanide salt, and a salt precursor of the host lattice are mixed together as solutions and a base is added to co-precipitate out a mixture of the corresponding hydrated oxides. For example, ferric nitrate, lanthanum nitrate, and aluminum nitrate can be dissolved in water and ammonium hydroxide added to the solution to co-precipitate a mixture of hydrated iron, lanthanum, and aluminum oxides. The mix is then heated to above about 1100.degree. C. to perfect dissolution and substitution of the iron ions for aluminum ions.

A fourth method is to dissolve a lanthanum or lanthanide salt in a solvent such as water or ethanol and use the solution to impregnate a preformed solid solution catalyst then dry and heat the mix to cause the metal salt to decompose upon heating to yield the oxide.

In all of these methods a metal oxide precursor can be used in place of the metal oxide per se. The precursor, which is typically a salt of the metal, decomposes on heating to yield the oxide form of the metal. Examples of iron oxide precursors are iron chloride, iron sulfate, iron formate, iron oxalate, iron citrate, iron nitrate, and the like. Precursors of the oxides of lanthanum or lanthanides and of the selected metals can also be employed. Examples of lanthanum oxide precursors are lanthanum nitrate, lanthanum chloride, lanthanum sulfate, lanthanum oxalate, and the like.

The solid solution catalysts of the invention can be used in the process in the form of a fixed bed, a fluidized bed, on a fixed support, on a fluidized support, or in a number of ways well known to the art. Although in the examples the process used is a fluidized bed process, it is understood that other well known techniques can be employed. The following Examples are given to further illustrate the invention.

Solid solution catalysts were used in an oxydehydrochlorination process to react ethane to ethylene and small amounts of vinyl chloride. The reactions were conducted in a fluid bed reactor wherein the ethane, oxygen used in the form of air, and anhydrous HCl were premixed at a set molar ratio of reactants and the mixture fed into a heated reactor near the bottom. The catalyst used was in the form of particles of a size passing between 80 mesh and 325 mesh screens. Contact times in the reaction were from about 4 seconds to about 10 seconds. Products were withdrawn from the top of the reactor as gases, scrubbed with water and analyzed using a gas chromatograph. The process was run as a continuous process for times of 1 hour up to 300 hours or more per run.

The following examples detail experiments conducted using various mole ratios of reactants, various temperatures and times of reaction, and different solid solution catalysts.

Experiments were conducted to compare ethane conversion and yield of ethylene and vinyl chloride obtained between a catalyst of this invention and an impregnated catalyst. Both catalysts contained about 2% by weight of Fe.sub.2 O.sub.3. The catalyst of the invention was prepared by adding a solution of 31.9 grams of La(NO.sub.3).sub.3 --6H.sub.2 O and 29.75 grams of Fe(NO.sub.3).sub.3 --9H.sub.2 O dissolved in about 400 milliliters of ethanol to 431.1 grams of Al.sub.2 O.sub.3 --3H.sub.2 O (sold by Alcoa Co. as C--31) and evaporating off the ethanol. The mixture was then heated at 550.degree. C. for 16 hours to dehydrate the alumina trihydrate and to decompose the ferric nitrate and lanthanum nitrate to ferric oxide and lanthanum oxide. The catalyst was then further heated at 1200.degree. C. for 16 hours to cause formation of the solid solution catalyst.

The impregnated catalyst employed in the example was prepared by impregnating aluminum oxide with a solution of ferric nitrate, drying the mix and then heating the mix for 16 hours at 550.degree. C. The preparation is similar to the preparation of the solid solution catalyst except that no heat treatment at 1200.degree. C. was done. X-ray diffraction analysis of the impregnated catalyst showed two distinct phases, i.e., the impregnated catalyst was a mixture of Fe.sub.2 O.sub.3 and Al.sub.2 O.sub.3.

The catalysts were placed into the reactor and the reactants fed into the reactor at a mix of 1 mole ethane/0.6 mole of oxygen (as air)/1.5 moles of anhydrous hydrogen chloride. Contact time throughout the runs was about 5 seconds. Temperature of reaction was 550.degree. C. Results are given in the following tables.

The data shows that the use of the solid solution catalysts of the invention results in significantly higher mole percent conversion of ethane to products and higher yield of ethylene and vinyl chloride than the use of the impregnated catalysts.

The solid solution catalysts of the invention have excellent stability, showing activity for long periods of time when in use. The catalyst used above lost only 3.1% of its original iron content after 97 hours and only 4.7% of its original iron content after 203 hours of use (A solid solution catalyst containing iron but no lanthanum lost almost 4% of its iron content in 100 hours of use at the same conditions). In contrast, the simple catalyst of iron oxide impregnated on aluminum oxide lost 8.4% of its original iron content after 90 hours of use.

A further experiment using the 2% Fe.sub.2 O.sub.3 in Al.sub.2 O.sub.3 solid solution catalyst stabilized with 4% by weight of La.sub.2 O.sub.3 was run at a reactant feed ratio of 1 mole ethane/0.5 mole oxygen/1.5 mole of hydrogen chloride and with a contact time of about 3 seconds in the reactor. The following results were obtained.

The weight of iron present (expressed as iron oxide) and the weight ratio of iron present (expressed as iron oxide) to lanthanum present (expressed as lanthanum oxide) has an effect upon the mole percent conversion of ethane, the percent yield of ethylene and vinyl chloride, and/or the loss of iron by the catalyst. A series of experiments were run using solid solution catalysts prepared having various weights of iron and weight ratios of iron to lanthanum in the catalyst. The exact catalysts used are described as follows:

The above catalysts were used in a fluidized bed reaction process for conversion of ethane to ethylene. The reactant feed ratio used was 1/0.5/1.5 or 1/0.6/1.5 ethane/O.sub.2 /HCl, temperature of the reaction was 550.degree. C., and contact times ranged from 3 to 5 seconds. Results are given in the following table.

The data shows that conversion of ethane and yields of ethylene and vinyl chloride increase significantly with the increase of iron in the catalyst at least until about 2% by weight iron oxide. Increases with the increase of iron oxide from 4% to 10% by weight are less dramatic. Increasing the weight ratio of La.sub.2 O.sub.3 to Fe.sub.2 O.sub.3 retards iron loss in the catalyst.

The reactant feed ratio used in the conversion process can vary considerably. The main effect of varying this ratio is to change the yields of ethylene and vinyl chloride obtained. Mole percent conversion of ethane is little effected. A series of experiments were conducted using a solid solution catalyst (Catalyst F from Example II) in a fluidized bed process with a contact time of about 4 seconds and at a temperature of 550.degree. C. wherein the reactant feed ratio varied as 1 ethane/0.6 to 1 oxygen/1.5 to 4 hydrogen chloride.

As the amount of HCl and O.sub.2 are increased in a series, conversion of ethane generally increases and yield of vinyl chloride increases, while the yield of ethylene decreases. As the amount of O.sub.2 is increased, the combined yield of ethylene and vinyl chloride remains fairly constant until above about a 0.8/1 oxygen to ethane ratio.

The iron ions are the major active agent in the catalysts of the invention. The major purpose of the lanthanum metal (or lanthanide metal) is to inhibit loss of iron from the catalyst. The catalyst in the previous examples was comprised of iron oxide and lanthanum oxide in aluminum oxide. This example shows the use of lanthanide metals in place of lanthanum in the catalysts. Temperature of the reaction was 550.degree. C. and contact time was 4 to 5 seconds. Reactant feed ratio was 1/0.6/1.5 ethane/O.sub.2 /HCl.

The 2% iron oxide in .alpha.-aluminum oxide stabilized with 4% praeseodymium oxide and 2% iron oxide in .alpha.-aluminum oxide stabilized with 4% neodymium oxide catalysts described in the previous example were used in a series of experiments wherein the reactant feed ratio was varied. The process was a fluidized bed process operating at 550.degree. C. with a contact time of about 5 seconds.

As in Example III which showed the use of the lanthanum stabilized solid solution iron catalyst, as the amount of O.sub.2 and HCl increased the conversion of ethane and yield of vinyl chloride increased and the yield of ethylene decreased.

Combinations of lanthanum and lanthanide metals can be used with the iron oxide and aluminum oxide. Two catalysts were prepared containing 0.5% cerium oxide and 1.3% cerium oxide by weight in a 4% lanthanum oxide stabilized 2% iron oxide in .alpha.-aluminum oxide catalyst. The catalysts were used in the ethane to ethylene conversion process at a temperature of 550.degree. C. and about a 4 second contact time. Results are shown in the following table.

The temperature of the ethane to ethylene conversion reaction has an effect on conversion and yields obtained in the process. Generally, between the temperatures of 500.degree. C. to 600.degree. C., higher temperatures increase the mole % conversion of ethane and somewhat increase the yield of ethylene. A reaction temperature of about 550.degree. C. offers a good balance of mole % conversion of ethane and high ethylene yield and combined yield of ethylene and vinyl chloride. The following results were obtained using a 2% Fe.sub.2 O.sub.3 in Al.sub.2 O.sub.3 catalyst stabilized with 4% Nd.sub.2 O.sub.3.