DEHYDROISOMERIZATION OF HYDROCARBONS

Abstract: Dehydroisomerization of normal C₄ and/or C₅ paraffins to corresponding iso-olefins is carried out directly in a single step by using a novel catalyst consisting essentially of Cr₂O₃ and Nb₂O₅ on a carrier having dehydrogenation activity, preferably ZrO₂.

Description: DEHYDROISOMERIZATION OF HYDROCARBONS TECHNICAL FIELD The present invention relates to catalytic dehydroisomerization of hydrocarbons and is more specifically concerned with method and catalyst for the direct production of iso-olefins from the corresponding normal paraffins. BACKGROUND OF THE INVENTION Among the increasing industrial demands for isobutylene is that for use in reaction with methanol to produce methyl-t-butyl ether (HTBE), valuable as a high octane blending component for automotive fuel. The increasing demand is straining the normal supply of isobutylene obtained from catalytic cracking and steam cracking processes. Of course, isobutylene can be readily obtained by dehydrogenation of isobutane by known processes. such as by the commercially available CATOFIN- route (Gussow. S.. et al.. Oil and Gas J., Dec. 1980, pp 96-101) but the demand for isobutane as such as a high octane blending stock, has limited its supply and pushed up its market price because of the cost involved in isomerizing normal butane to meet the demand for isobutane. In typical prior art processes a two step operation is employed, wherein normal butane is first isomerized under particular operating conditions and appropriate catalyst. and the thus obtained isobutane is subjected to dehydrogenation to obtain the desired isobutylene. While the direct conversion of normal butane to isobutylene in a single step is obviously desirable from economic considerations, a primary deterrent to successful operation of this scheme has been the thermodynamic incompatibility of the isomerization and dehydrogenation steps as carried out in existing processes. In the two step process for the conversion of n-butane to isobutylene, the initial isomerization of n-butane to isobutane would likely employ the typical commercially available first step using platinum catalyst on an acidic support, in the presence of added hydrogen at moderate total pressure and at temperatures above about 300"F. The resulting isobutane produced, separated from other reaction products. can be separately dehydrogenated by one of the known methods, such as the CATOFIN process for example, using a chrome-alumina catalyst at high temperatures (550-600"C) and at low pressure (sub-atmospheric). Among recent patents disclosing conversion of hydrocarbons by multi-step operation, including dehydrogenation and isomerization, are: U.S. 4,725,255; 4,324,937; 4,329,516, 4,341,911; and 4,367,357. According to U.S. Patent 3,702,876 selective dehydrogenation of hydrocarbons can be achieved by pretreatment of a Pd catalyst with the desired olefin using a hollow tube palladium catalyst as a hydrogen permeable membrane. Only olefins which have the same skeleton as the charge hydrocarbon can be formed thereby. While in certain reforming processes applied to various streams of mixed hydrocarbons a certain degree of dehydrogenation may take place accompanied by isomerization, none of these reforming processes have been commercially considered as a route for conversion of normal paraffin components in a C4 or C5 hydrocarbon stream to the corresponding iso-olefins. Of interest from an academic viewpoint are the disclosure (1) by Nahm, J. J. West, et al., in Proc. Int. Clean Air Cony. 4th, 491 (1977), of the Photocatalytic conversion of hydrocarbons (1 ppm) adsorbed on airborne particulate containing trace metals (CA 88: 157695v); and that (2) by Matsuoka, S. et al., on the radiation sensitized thermal cracking of n-butane to isobutane and isobutylene (CA 83: 50623q). It has been recently proposed in published European patent application 42,252 (96 CA 180741f) to prepare isobutylene by dehydroisomerization of normal butane at 350-700QC over catalyst comprised of a Group IIIA metal or compound, specifically gallium oxide, deposited on a support having low acidity, such as 01203, SiO2, or an aluminosilicate zeolite. In the given example the hydrocarbon feedstock containing n-butane was passed over the catalyst, comprising Ga zeolite L, at atmospheric pressure, at 5500C and at a contact time of 6 seconds. A conversion of 55 wt% is reported to have been obtained at a yield of 2.5 wt% aromatics and 28.9% butenes fraction comprising about 33% isobutylene (= about 9.5% isobutylene yield). Among the objects of the present invention is the provision of an operative single step process for direct conversion of C4 and/or C5 paraffins to the corresponding iso-olefins, and to provide a novel catalyst promoting such dehydroisomerization. SUMMARY OF THE INVENTION In accordance with the present invention normal butane, alone or in a mixed C4 hydrocarbon-containing charge stream, is converted to isobutylene by contact of the C4 charge with a catalyst comprising from about 1 to 20% by weight of chromia (cur203) and from about 2 to 10% by weight of niobium pentoxide supported on a carrier having dehydrogenation activity, under operating conditions including temperature in the range of about 450-650 C, and at a C4 pressure of from about 0.01 to about 0.5 atmospheres. The butane contact time is preferably about 0.4 to 0.8 seconds. The process is also applicable to the dehydro-isomerization of normal pentane alone or in a mixed hydrocarbon stream which may also comprise normal butane. Any form of pressure-retaining catalytic reactor of the type employed in dehydrogenation of hydrocarbons may be employed in practice of the invention, including suitable means for fractional separation of the reaction products and provision for recycling of selected fractions. DETAILED DESCRIPTION In a laboratory program directed to investigation of activity of various catalysts tested for dehydrogenation of isobutane it was discovered that Nb205 supported on ZrO2 added substantial isomerization activity to the intrinsic dehydrogenation activity of the zirconia support. As a result of this initial discovery further laboratory work was initiated which led to the development of the novel dehydroisomerization catalyst of the present invention, comprised essentially of Cur203 and Nb2O5 supported on a carrier having dehydrogenation activity, preferably on zirconia (ZrO2). EXAMPLE A In five minute onstream runs carried out for the dehydrogenation of 17% isobutane in a helium carrier gas at 6000C, zirconia alone showed high dehydrogenation selectivity at comparatively low iC4 conversion, while under the same operating conditions a run made using Nub205 supported on the zirconia carrier, showed significant isomerization activity but substantially less dehydrogenation activity. The results are reported in Table 1. TABLE 1 iC4 iC4= *nC4= nC4 Run Conversion Selectivity Selectivity Selectivity No. CATALYST wtS wt% wt% wt% 1 ZrO2 10 97 < 1 < 1 2 Nb2O5/ZrO2 30 45 22 1 3 Cr2 3/Nb2 S/Zr 2 66 63 7 0 * all isomers of n-butylene EXAMPLE 1 A catalyst composed of Cr203/Nb205/ZrO2 (0.1/0.04/1 moles) was prepared and tested on the same iC4 charge stock and under conditions of Example A above. The results are reported in Table 1. The foregoing run (3) on the Cr2O3 /Nb2O5/Zro2 catalyst indicated that it was an active catalyst for dehydrogenation of paraffins while retaining substantial oleo in isomerization activity. Although the reported initial results indicated that the isomerization activity had somewhat decreased, it was recognized that the proportions of the components in the catalyst and the process conditions had not been optimized. It will be noted from the results of Run 3 of Table 1, that none of the starting isobutylene was isomerized to n-butane, The catalyst employed in Example 1 was prepared as follows: (a) In the preparation of the ZrO2 substrate, 600 grams of zirconyl nitrate (19.9 wt% ZrO2, 21.9% HNO3) were diluted to 3 liters with distilled water. Precipitation was effected by slow addition of 250 ml of concentrated NH40H with stirring during a 20 minute period. The product was let to set for 3 hours and filtered. The precipitate was washed 3 times with distilled water and dried at 1200C for 24 hours; after which the product was ground and sieved through a 20 mesh screen. (b) 80.7 grams of niobium oxalate were added to an aqueous solution of oxalic acid comprised of 27 grams oxalic acid in 300 ml distilled water. After sitting over the weekend, the niobium oxalate was not completely dissolved. 25 ml of the supernatant liquid (considered to be a saturated solution of niobium oxalate in 1 molar aqueous oxalic acid) was added to 15 grams of the screened ZrO2 substrate from (a) above. After 30 minute contact of the substrate with the niobium oxalate solution with occasional stirring, the impregnated solid was filtered off and dried on a watchglass at 1200C for 24 hours. It was then heat treated in a muffle furnace at 650"C for 2 hours, obtaining a product comprised of Nb2O5 incorporated in ZrO2 substrate. (c) 15 grams of the Nb2O5/ZrO2 product from (b) above was added to 25 ml of a solution containing 60 grams of Cr03 dissolved in 100 ml H2O. After sitting for 30 minutes with occasional stirring, the material was filtered and dried on a watchglass at 1200C for 15 hours; then heat treated in a muffle furnace at 650"C for 2 hours and 20 minutes. The obtained Wb2O5/Cr 203/ZrO2 product was used in the form of a 40 x 60 mesh powder; the surface area is about 35 m /g. While not being bound to any particular theory as to the reaction mechanism involved it is reasonable to assume from the observed experimental results that the dehydroisomerization of normal butane would follow the mechanistic route indicated in equation (I) below: (I) nC4 4 tnC4 = " iC4 5 = = mlxture of normal C4 olefins) The maximum selectivity to isobutylene is determined by the equilibria involving the various C4 mono-olefin isomers. Table 2 below shows the equilibria among these isomers (in mole fractions) as a function of temperature. TABLE 2 Olefln 450"C 5500C 650"C l-butene 0.12 0.15 0.18 2-butene (ois) 0.16 0.17 0.17 2-butene (trans) 0.24 0.25 0.25 isobutylene 0.48 0.44 0.41 In considering the equilibria among the various C4 mono-olefin isomers, pressure was ruled out as a process variable, since neither the total pressure nor the partial pressure of hydrogen has a significant effect on the equilibria obtaininq and thus pressure could not be used as EMI6.1 a means ot optimizing isobutylene EMI6.2 Maximum obtainable conversion at thermodynamic equilibrium, however, is a function of pressure and, therefore, governs total yield of iso-olefins. From the equilibrium data in Table 2, it will be seen that the formation of isobutylene is favored as the temperature is lowered. Temperatures below about 450"C, however, would not be sufficient to further the reaction at a desired reasonable rate. Accordingly, selectivity to isobutylene formation for an optimized process appears to be at about 45%, obtainable at temperatures in the range of 450-550"C. To maximize production of isobutylene, about 55% of the mono-olefin products obtained can be recycled to the dehydroisomerization reaction. Moreover, In establishing the process conditions to be utilized for the desired product yield, consideration needs to be given to avoiding the kinetic routes favoring isomerization of butane to isobutane, formation of cracked products, high yields of coke, and the like. The effect of pressure on C4 dehydrogenation equilibria at various temperatures is shown in Table 3. TABLE 3 C4 Dehydrogenation Equilibria (Mole Fractions) Temperature ( C) 650 550 450 Pressure (atm) 0.1 0.1 0.1 H2 0.607 0.516 0.375 n - C4 0.003 0.035 0.257 1 - C4 = 0.031 0.056 0.042 2C - C4 = 0.030 0.063 0.058 2T - C4 = 0.043 0.093 0.088 1,3 - BD 0.217 0.070 0.066 iC4 = 0.071 0.166 0.174 Pressure (atm) 0.3 0.3 0.3 H2 0.560 0.464 0.280 n - C4 0.014 0.101 0.441 1 - C4 = 0.052 0.060 0.032 2C - C4 = 0.050 0.068 0.044 2T - C4 = 0.072 0.100 0.067 1,3 - BD 0.132 0.028 0.022 iC4 = 0.120 0.179 0.133 In the single step conversion of n-butane to isobutylene the following operating conditions may be employed. RANGE PREFERRED TEMPERATURE, OC 450-650 550-600 C4 PRESSURE, atm. 0.01-0.5 0.1-0.2 H2Xfeed mole ratio 0-10 0-4 Contact time, sec. 0.1-1.0 0.4-0.8 *Space rate, v/hr/V 120-560 240-480 *at STPn-C4 The catalyst to be employed comprises Cr203 and Nb205 supported on a carrier having dehydrogenation activity. The catalyst comprises in calcined form essentially 0.02 to 0.10 parts Cr2O3 and 0.03 to 0.07 parts Nb205 on a support consisting essentially of Zero2; preferably 0.07 parts Cr203 and 0.05 parts Nb205 on said support. The reaction products are separated to recover isobutylene; products boiling above isobutylene being recycled to join the fresh butane feed. In the dehydroisomerization of normal pentane the reaction conditions may be essentially the same as above set out for normal butane. The catalyst may be prepared by impregnating the zirconia or other selected carrier with an aqueous solution containing niobium, drying and/or calcining, and impregnating the resulting solid with an aqueous solution containing chromium, followed by drying and calcining.

Claims: Vhat is Claimed: 1. A process for the production of iso-olefins from normal paraffins of 4 to 5 carbon atoms which comprises contacting the normal paraffin charge with a catalyst comprising from about 1 to 20% by weight of Cr2O3 and from about 2 to about 108 by weight Nb2O5 supported on a carrier having dehydrogenation activity, under operating conditions including temperature in the range of 450-650"C, a gas hourly space velocity from about 120 to 560 volumes of normal paraffin per volume of catalyst per hour, and at a normal paraffin pressure of from about 0.01 to 0.5 atmospheres. 2. The process as defined in Claim 1 wherein said catalyst carrier is selected from the group consisting of ZrO2, A1203, EMI9.1 mixtures these. EMI9.2 3. The process as defined in Claim 1 wherein said catalyst carrier consists essentially of ZrO2. 4. The process as defined in Claim 1 wherein said normal pzraEEin charge consists essentially of n-butane. 5. The process as defined in Claim 4 wherein said carrier consists essentially of a metallic oxide selected from the group consisting of Zoo2, A1203. Ta205 and mixtures of these oxides. 6. The process as defined in Claim 4 wherein said catalyst carrier consists essentially of ZrO2. 7. The process as defined in Claim 4 wherein said catalyst comprises in parts by weight in calclned form 1 ZrO2, 0.02 to 0.10 parts Cr2O3 and 0.03 to 0.07 parts Nb205. 8. The process as defined in Claim 4 wherein said contacting is carried out at temperature in the range of 550-600 C. paraffin pressure in the range of 0.1-0.2 atmospheres and at a gas velocity of 240-480 volumes normal paraffin at STP per volume catalyst per hour. 9. The process as defined in Claim 1 wherein said normal paraffin charge comprises n-pentane. 10. Dehydroisomerization catalyst comprising Cr203 and Nb205 on a carrier having dehydrogenation activity. 11. Catalyst composition according to Claim 10 wherein said carrier consists essentially of a metallic oxide selected from the group consisting of ZrO2, A1203. Ta2O5 and mixtures of these oxides. 12. Catalyst composition according to Claim 10 wherein said carrier consists essentially of ZrO2. 13. Catalyst composition according to Claim 10 consisting by weight of total catalyst essentially of 0.02 to 0.10 parts Cr2O3 and 0.03 to 0.07 parts Nb205 on a support consisting essentially of ZrO2. 14. The method of preparing a dehydroisomerization catalyst which comprises impregnating a substrate carrier comprising ZrO2 with an aqueous solution of a soluble niobium compound, drying and/or calcining, and impregnating the resulting solid with an aqueous solution of a soluble chromium compound, followed by drying and calcining.

Inventors: KIRNER, JOHN FRANCIS
Applicants: AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1986
Main classification: C07C11
IPC codes: B01J23, C07C1, C07C11, C07C5, C07C67, C07B61