Mixed metal oxide catalyst useful for paraffin dehydrogenation

Abstract

A catalyst, methods of making, and process of dehydrogenating paraffins utilizing the catalyst. The catalyst includes at least 20 mass % Zn, a catalyst support and a catalyst stabilizer. The catalyst is further characterizable by physical properties such as activity parameter measured under specified conditions. The catalyst may also be disposed on a porous support in an attrition-resistant form and used in a fluidized bed reactor.

Claims

1. A process for continuous dehydrogenation of paraffins having 2-8 carbon atoms, preferably propane or isobutane, comprising: contacting said paraffins with a catalyst composition at a reaction temperature of 500-800° C., a space velocity of 0.1-25 hr.sup.−1 and a pressure of 0.01-0.2 MPa for a reaction period in the range of 0.05 seconds to 10 minutes; regenerating the said catalyst with an oxygen-containing gas wherein said catalyst regeneration is performed at a reaction temperature of 500-800° C., a pressure of 0.01-0.2 MPa and a regeneration period ranging from 0.05 seconds to 10 minutes; wherein the catalyst composition comprises: a mixed metal oxide catalyst suitable for the dehydrogenation of paraffins having 2-8 carbon atoms, comprising a catalyst composition of the general formula (AC) (CS) (ST) wherein a) AC (Active Catalyst) represents oxides of zinc (Zn) wherein the catalyst comprises at least 20 mass % Zn, b) CS (Catalyst Support) represents oxides of aluminum (Al), silicon (Si), and titanium (Ti) or mixtures thereof, c) ST (Support Stabilizer) represents oxides of metals selected from the group of cerium (Ce), dysprosium (Dy), erbium (Er), europium (Eu), gadolinium (Gd), lanthanum (La), neodymium (Nd), praseodymium (Pr), samarium (Sm), terbium (Tb), ytterbium (Yb), yttrium (Y), tungsten (W), zirconium (Zr), or mixtures thereof, and characterizable by a Activity Parameter>90,000, Selectivity Parameter<0.5 and a stability parameter<0.005 using a test where the mixed metal oxide catalyst is loaded in a fixed-bed reactor such that the 50>dT/dP>10 (diameter of tube to diameter of catalyst particles) and 200>L/dP>50 (length of catalyst bed to diameter of catalyst particles) and 2>dP>0.5 mm exposed to a feed stream of propane at a temperature of 650° C., atmospheric pressure and a feed rate of 10 hr.sup.−1 weight hourly space velocity.

2. The process of claim 1 wherein said contacting is carried out in a fluidized bed reactor or a fixed-bed swing reactor.

3. The process according to any of claims 1-2 wherein the catalyst is characterizable by an activity parameter of between 90,000 and about 125,000 using a test where the mixed metal oxide catalyst is loaded in a fixed-bed reactor such that the 50>dT/dP>10 (diameter of tube to diameter of catalyst particles) and 200>L/dP>50 (length of catalyst bed to diameter of catalyst particles) and 2>dP>0.5 mm exposed to a feed stream of propane at a temperature of 650° C., atmospheric pressure and a feed rate of 10 hr.sup.−1 weight hourly space velocity.

4. The process according to any of claims 1-3 wherein the active catalyst (AC) species (Zn) comprises 30 to 70 or 40 to 60 wt % of the total weight of the catalyst.

5. The process according to any of claims 1-4 wherein the catalyst support (CS) comprises 20 to 70 or 25 to 70 or 30 to 70 or no more than 60 or no more than 50 wt % of the total weight of the catalyst.

6. The process according to any of claims 1-5 wherein the support stabilizer (ST) comprises 0.1 to 20 or 0.2 to 15 or 0.3 to 10 wt % of the total weight of the catalyst.

7. The process according to any of claims 1-6, wherein the catalyst composition has less than 100 ppm by weight of either platinum (Pt) or chromium (Cr).

8. The process according to any of claims 1-7 wherein the BET surface area is at least 5 m.sup.2/g.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 shows illustrates a reaction scheme for a dehydrogenation. Paraffins 1 enter reactor A. The products 2 pass into separator B wherein olefins 3 are separated and catalyst 4 is regenerated in Regenerator C and regenerated catalyst 7 is returned to the reactor. Oxygen-containing gas 5 passes into the Regenerator and stream 6 exits the Regenerator.

GLOSSARY

(2) Arrhenius Activation Energy (E): The Arrhenius equation gives the quantitative basis of the relationship between the activation energy and the rate at which a reaction proceeds. Arrhenius activation energy term from the Arrhenius equation is as an experimentally determined parameter that indicates the sensitivity of the reaction rate to temperature. From the equation, the activation energy can be found through the relation
k=k.sub.0e.sup.−E/RT
Where k is the rate constant (units 1/sec) of a reaction at temperature T, k.sub.0 is pre-exponential factor, E is activation energy for the reaction, R the universal gas constant, T the reaction temperature (in Kelvin). k is calculated from conversion (x) and residence time (τ)(measured in seconds) as follows
k=−ln(1−x)/τ
Activity Parameter—The catalyst activity is quantified by the activity parameter which is the pre-exponential factor in the Arrhenius equation k.sub.0 for the dehydrogenation reaction, an empirical relationship between temperature and rate coefficient using a value of 81.7 kJ/mole for E the Activation Energy for the dehydrogenation reaction using for Titania based catalyst.
Selectivity Parameter—Since selectivity of propylene varies with propane conversion, a method is required to compare selectivity obtained by different catalysts under various conversions. Selectivity parameter is calculated from the ratio of the first-order rate constant of the propylene cracking reaction (k.sub.C) to the first-order rate constant of the propane dehydrogenation reaction (k.sub.D) and remains constant irrespective of the propane conversion. A catalyst producing propylene with a high selectivity will have a selectivity parameter<0.2.
The selectivity parameter (=k.sub.C/k.sub.D) is calculated from propane conversion (x) and propylene yield (y) by solving the equations shown below:
k.sub.D=−ln(1−x)/τ
y=[k.sub.D/(k.sub.C−k.sub.D)][(e.sup.(−kDτ)−e.sup.(kct)]
Stability Parameter—The loss of catalyst activity with time is quantified by the stability parameter which measures the rate of change of catalyst activity with time. A catalyst with high stability will have a low stability parameter value<0.005.
Stability Parameter=(k.sub.D0−k.sub.Dt)/t
Where k.sub.Dt is the rate constant of dehydrogenation of propane at time t, k.sub.D0 is the rate constant of dehydrogenation of propane at time 0 and t is the time on stream (in hours)
For instance, at a reaction temperature of T=625° C. (898 K) and residence time τ=1 second, for a propane conversion of x=46% and propylene yield of y=42% Activity Parameter=35,028 Selectivity Parameter=0.23

(3) Characterization of the catalyst is conducted in the absence of a diluent gas such as nitrogen, hydrogen, steam or helium.

(4) Attrition Index: The attrition resistance of catalysts used in fluidized reactor systems are characterized by the Attrition Index determined by ASTM tests such as AJI—Air Jet Index which is the percent attrition loss at 5 hours (ASTM D5757—Standard Test Method for Determination of Attrition of FCC Catalysts by Air Jets).

(5) Calcination Temperature—The term “calcination temperature” refers to the maximum temperature utilized as an intermediate step in the catalyst synthesis procedure intended to convert the metal salts to their oxide form.

(6) Conversion—The term “conversion of a reactant” refers to the reactant mole or mass change between a material flowing into a reactor and a material flowing out of the reactor divided by the moles or mass of reactant in the material flowing into the reactor. For propane dehydrogenation, conversion is the mass of propane reacted divided by the mass of propane fed.

(7) Geldart Classification: Catalyst particles can be characterized by four so-called “Geldart Groups”. The groups are defined by their locations on a diagram of solid-fluid density difference and particle size. For Group A the particle size is between 20 and 100 μm, and the particle density is typically less than 1.4 g/cm.sup.3. For Group B, the particle size lies between 40 and 500 μm and the particle density between 1.4-4 g/cm.sup.3. Most particles used in fluidized beds are Geldart Group A powders. In some embodiments, catalysts of the present invention have the characteristics of Group A; in some embodiments, catalysts of the present invention have the characteristics of Group B.

(8) “Particle size” is number-average particle size, and, for non-spherical particles, is based on the largest dimension.

(9) Pore size—Pore size relates to the size of a molecule or atom that can penetrate into the pores of a material. As used herein, the term “pore size” for zeolites and similar catalyst compositions refers to the Norman radii adjusted pore size well known to those skilled in the art. Determination of Norman radii adjusted pore size is described, for example, in Cook, M.; Conner, W. C., “How big are the pores of zeolites?” Proceedings of the International Zeolite Conference, 12th, Baltimore, Jul. 5-10, 1998; (1999), 1, pp 409-414.

(10) One of ordinary skill in the art will understand how to determine the pore size (e.g., minimum pore size, average of minimum pore sizes) in a catalyst. For example, x-ray diffraction (XRD) can be used to determine atomic coordinates. XRD techniques for the determination of pore size are described, for example, in Pecharsky, V. K. et at, “Fundamentals of Powder Diffraction and Structural Characterization of Materials,” Springer Science+Business Media, Inc., New York, 2005. Other techniques that may be useful in determining pore sizes (e.g., zeolite pore sizes) include, for example, helium pycnometry or low-pressure argon adsorption techniques. These and other techniques are described in Magee, J. S. et at, “Fluid Catalytic Cracking: Science and Technology,” Elsevier Publishing Company, Jul. 1, 1993, pp. 185-195. Pore sizes of mesoporous catalysts may be determined using, for example, nitrogen adsorption techniques, as described in Gregg, S. J. at al, “Adsorption, Surface Area and Porosity,” 2nd Ed., Academic Press Inc., New York, 1982 and Rouquerol, F. et al, “Adsorption by powders and porous materials. Principles, Methodology and Applications,” Academic Press Inc., New York, 1998.

(11) Regeneration Temperature—The catalyst may be regenerated under flowing air gas at elevated temperatures in order to remove heavier hydrocarbons (coke) from the active catalyst structure. The maximum temperature used in this step is referred to as the “regeneration temperature.”

(12) Residence Time (τ)—Residence time is the time a substance is in the reaction vessel. It can be defined as the volume of the catalyst bed divided by the flow rate (by volume per second) of gases into the reactor. τ=volume of Catalyst bed (m.sup.3)/volumetric flow of reactants (m.sup.3/s).

(13) Selectivity—The term “selectivity” refers to the amount of production of a particular product (or products) as a percent of all products resulting from a reaction. For example, if 100 grams of products are produced in a reaction and 80 grams of olefins are found in these products, the selectivity to olefins amongst all products is 80/100=80%. Selectivity can be calculated on a mass basis, as in the aforementioned example, or it can be calculated on a molar basis, where the selectivity is calculated by dividing the moles a particular product by the moles of all products. Unless specified otherwise, selectivity is on a mass basis. For propane dehydrogenation, selectivity is the mass of propylene produced divided by the mass of all products.

(14) WHSV—The term WHSV refers to the Weight Hourly Space Velocity and is defined as the weight of reactant fed to a reactor per hour divided by the weight of the catalyst in the reactor

(15) Yield—The term “yield” is used herein to refer to the amount of a product flowing out of a reactor divided by the amount of reactant flowing into the reactor, usually expressed as a percentage or fraction. Mass yield is the mass of a particular product divided by the mass of feed used to prepare that product. When unspecified, “%” refers to mass % which is synonymous with weight %. Ideal gas behavior is assumed so that mole % is the same as volume % in the gas phase. For propane dehydrogenation, mass yield is the mass of propylene produced divided by the mass of propane fed. Mass yield of the inventive processes are preferably at least 50% in a single pass, preferably at least 70%.

(16) As is standard patent terminology, the term “comprising” means “including” and does not exclude additional components. Any of the inventive aspects described in conjunction with the term “comprising” also include narrower embodiments in which the term “comprising” is replaced by the narrower terms “consisting essentially of” or “consisting of.” As used in this specification, the terms “includes” or “including” should not be read as limiting the invention but, rather, listing exemplary components. As is standard terminology, “systems” include to apparatus and materials (such as reactants and products) and conditions within the apparatus.

EXAMPLES

Example 1

(17) 1.2 gm of Zinc Nitrate Hexahydrate, 11 gm of Titanium Oxy-Sulfate and 0.9 gm of Zirconium Tetrachloride inorganic salts were dissolved in 500 ml of deionized water. The salt solution was heated to 55° C. while stirring. When the desired temperature was reached, 2 molar aqueous solution of ammonium hydroxide was added to the inorganic salts solution drop-wise till the mixture reached the pH of 7.3. The resulting precipitate was isolated by filtration and stirred in an aqueous solution of 0.1 M ammonium hydroxide for 30 minutes at room temperature followed by filtration. The precipitate was subsequently isolated by filtration and stirred in an aqueous solution of 0.25 M ammonium nitrate for 30 minutes at room temperature, filtered and dried overnight and then calcined to a temperature of 815° C. for 4 hours to produce the mixed-metal oxide catalyst. This catalyst is designated as Catalyst A.

Example 2

(18) The catalyst was prepared as in Example 1 with the difference being that the amount of Zinc Nitrate Hexahydrate dissolved was 18 gm. This catalyst is designated as Catalyst B

Example 3

(19) The catalyst was prepared as in Example 1 with the difference being that the amount of Zinc Nitrate Hexahydrate dissolved was 27.45 gm. This catalyst is designated as Catalyst C.

Example 4

(20) The catalyst was prepared as in Example 1 with the difference being that the amount of Zinc Nitrate Hexahydrate dissolved was 33.6 gm. This catalyst is designated as Catalyst D.

Example 5

(21) The catalyst was prepared as in Example 1 with the difference being that the amount of Zinc Nitrate Hexahydrate dissolved was 42 gm. This catalyst is designated as Catalyst E.

Example 7

(22) The catalyst was prepared as in Example 2 with the difference being that after the calcination at 815° C., 4 gm of alkaline earth metals modified Dispal binder (T25N4-80) dispersed in 10 gm of deionized water for 30 minutes, was mixed with the mixed-metal oxide catalyst followed by drying and calcining at 650° C. to improve the mechanical strength of the catalyst. This catalyst is designated as Catalyst G.

Example 8

(23) 3.7 gm of Zinc Nitrate Hexahydrate, 2 gm of Titanium Oxy-Sulfate and 0.4 gm of Zirconium Tetrachloride inorganic salts were dissolved in 15 ml of deionized water. The salt solution was heated to 55° C. while stirring. When the desired temperature was reached, the solution was added to 18 gm of alumina support (Catalox SBA 150) via incipient wetness impregnation technique followed by drying and calcination at 815° C. to produce mixed-metal oxide catalyst on alumina support. The catalyst was tested for mechanical strength. This catalyst is designated as Catalyst H.

Example 9

(24) 6 gm of Zinc Oxide, 6 gm of Titanium Oxide, and 1.4 gm of Zirconium Oxide were slurried in 55 ml of deionized water for 30 minutes followed by drying and calcining at 815° C. to produce mixed-metal oxide catalyst. This catalyst is designated as Catalyst I.

Example 10

(25) 6 gm of Zinc Oxide, 6 gm of Titanium Oxide, and 1.4 gm of Zirconium Oxide were slurried in 55 ml of deionized water for 30 minutes. 5.7 gm of alkaline earth metals modified Dispal (T25N4-80) was added to the slurry and mixed for 30 minutes followed by drying and calcining at 815° C. to produce mixed-metal oxide catalyst. This catalyst is designated as Catalyst J.

Example 11

(26) Propane dehydrogenation experiments were performed using a fixed-bed reactor such that the d.sub.T/d.sub.P>10 (ratio of diameter of reactor tube to diameter of catalyst particles) and L/d.sub.P>50 (ratio of length of catalyst bed to diameter of catalyst particles) to ensure plug-flow behavior. The catalyst of interest was first loaded into a quartz glass lined reactor. The catalyst was activated in dry air at atmospheric pressure at a temperature of 600° C. for 4 hours. Following activation, the reactor was allowed to heat up to reaction temperature of 650° C., then purged with dry nitrogen for 0.5 hours. Propane was fed to the reactor at a WHSV equal to 10 hr.sup.−1. The flow rate was controlled by a Brooks mass flow controller. Product samples were taken 7 seconds after the start of reaction and were analyzed on GCs having Petrocol DH and Plot Q columns. The catalyst was regenerated at 650° C. by first purging the reactor with nitrogen and then passing air over the catalyst. The results are shown in Table 1.

(27) TABLE-US-00001 TABLE 1 Catalytic performance of mixed-metal oxides for propane dehydrogenation As shown in the data table, we discovered, surprisingly, that catalysts containing relatively high Zn loading (believed to be at least about 20% or at least about 30% to about 70 mass % Zn) exhibited a substantially superior activity parameter along with high selectivity for olefins. Activity Selectivity Stability Catalyst Zn (wt %) Parameter parameter Parameter A 4 69623 0.24 not measured B 36 98538 0.26 0 C 44.8 123711 0.21 not measured D 48 98538 0.13 not measured E 52.8 92851 0.14 not measured F 60 64913 0.23 not measured G 25.2 95668 0.24 0 I 36 20985 4 not measured J 26.8 113772 0.19 0

Example 12

(28) Chemical composition of Catalyst B was measured using Inductively coupled plasma mass spectrometry. The results are shown in Table 2.

(29) TABLE-US-00002 TABLE 2 Chemical composition of mixed-metal oxide Sample B. Oxide Weight (%) ZnO 48.87% TiO.sub.2 41.58% ZrO.sub.2 9.35% SO.sub.3 0.20% Total 100.00%

Example 13

(30) Physical properties of Catalyst G were measured to characterize its surface area, bulk density, average particle diameter and attrition strength. The results are shown in Table 2.

(31) TABLE-US-00003 TABLE 3 Physical properties of mixed-metal oxide Sample G. Parameter Value Units BET Surface Area 11 m.sup.2/gm Bulk Density 810 kg/m.sup.3 Average Particle 65 μm Diameter Attrition Index 1.3 wt %/hr