Method for preparing catalyst for selective hydrogenation of diolefins

Abstract

The present invention relates to a catalyst and a method for preparation of that catalyst for the selective hydrogenation of diolefins present in gasoline streams along with the shifting of lighter sulfur compounds in the feed stock to heavier sulfur compound by the reaction with olefinic compounds.

Claims

1. A catalyst for simultaneously carrying out selective hydrogenation of diolefins and conversion of light sulfur compounds to heavier sulfur compounds in the gasoline streams, wherein the catalyst comprises: a metal complex impregnated over a first impregnated product, wherein the first impregnated product comprises at least one metal from group VIB metals impregnated over an alumina support, wherein the metal complex comprises a group VIII metal or a mixture of a group VIII metal and a group VIB metal and an organic additive, wherein the organic additive and the group VIII metal are in a molar ratio of 0.1-2.0, wherein and the catalyst has two metal active sites, wherein one metal active site comprises the group VIB metals and other metal active site comprises the mixture of the group VIII metal and the group VIB metal, wherein the group VIB metals in the catalyst is in a range of 5% to 15% by weight as a metal oxide with respect to total dry weight of the catalyst and the group VIII metal is in a range of about 0.5% to 5% by weight as a metal oxide with respect to the total dry weight of the catalyst.

2. The catalyst as claimed in claim 1, wherein the catalyst has at least 60% pore volume from pores of diameter in a range of 60-120 Å and has 10 to 20% pore volume from pores of a diameter >120 Å out of total pore volume of the catalyst.

3. The catalyst as claimed in claim 1, wherein the active site of the catalyst comprising group VIB metal is configured to combine lighter sulfur compound with olefins compounds and the active site of the catalyst comprising a mixture of group VIII and group VIB metals is configured to promote selective hydrogenation of the diolefins.

4. A method for preparation of a catalyst for simultaneously carrying out selective hydrogenation of diolefins in the gasoline streams and conversion of light sulfur compounds to heavier compounds, wherein the method comprises two consecutive impregnations steps of: (a) impregnating a solution of group VIB metal over an alumina support to obtain a first impregnated product; (b) adding a solution of either group VIII metal or a mixture of group VIII metal and group VIB metal to an organic additive to obtain a metal complex, wherein the organic additive and group VIII metal are in a molar ratio in a range of 0.1-2.0; and (c) impregnating the metal complex over the first impregnated product to obtain the catalyst.

5. The method as claimed in claim 4, wherein the group VIB metal is selected from molybdenum and tungsten and the group VIII metal is selected from nickel and cobalt.

6. The method as claimed in claim 4, wherein the first impregnated product of step (a) is dried at a temperature in a range of 100 to 150° C. for 8 to 12 hours.

7. The method as claimed in claim 4, wherein the step (b) is carried out a temperature in a range of 60-70° C.

8. The method as claimed in claim 4, wherein the catalyst obtained in step (c) is dried at a temperature in a range of 100 to 150° C. for 8 to 12 hours and then calcined at a temperature in a range of 400 to 500° C. for 2 to 4 hours.

9. The method as claimed in claim 4, wherein the organic additive is selected from nitrilotriacetic acid (NTA), ethylene diamine tetraacetic acid (EDTA) and ethylene dinitrilotetraacetic acid.

10. The method as claimed in claim 4, wherein the solution of group VIB metal is prepared by mixing a source of group VIB metal into a solvent, wherein: the source of group VIB metal is selected from ammonium heptamolybdate, molybdenum trioxide and molybdic acid for molybdenum; and ammonium tungstate, tungstic acid and phosphotungstic acid for tungsten, and the solvent is selected from water, aqueous solution of ammonia, phosphoric acid and an organic amine, wherein the organic amine is selected from monoethanol amine and diethyl amine.

11. The method as claimed in claim 4, wherein the solution of group VIII metal is prepared by mixing a source of group VIII metal into a solvent; and the solution of the mixture of group VIII metal and group VIB metal is prepared by mixing the source of group VIII metal and group VIB metal into the solvent, wherein: the source of group VIII metal is selected from nickel nitrate, nickel carbonate and nickel chloride for nickel; and cobalt nitrate, cobalt acetate and cobalt carbonate for cobalt; the source of group VIB metal is selected from ammonium heptamolybdate, molybdenum trioxide and molybdic acid for molybdenum; and ammonium tungstate, tungstic acid and phosphotungstic acid for tungsten; and the solvent is selected from water, aqueous solution of ammonia, phosphoric acid and an organic amine wherein the organic amine is selected from monoethanol amine and diethyl amine.

12. The method as claimed in claim 4, wherein the alumina support is gamma-alumina support and the alumina support is prepared by a process comprising the steps of: (a) milling and peptizing an alumina powder with a dilute solution of a mineral acid to obtain a dough; (b) extruding the dough in an extruder machine to obtain a wet extrudate; and (c) drying the wet extrudate at a temperature in a range of 100 to 150° C. for 10 to 16 hours and then calcining at a temperature in a range of 400 to 600° C. for 4 to 8 hours to obtain an extrudate of the alumina support.

13. The method as claimed in claim 12, wherein the extrudate is in a cylindrical, trilobe or quadralobe shape and has a diameter in a range of 1.2-1.5 mm.

14. The method as claimed in claim 12, wherein the alumina is pseudo-boehmite alumina having more than 85% crystalline phases and a surface area in a range of 250-300 m.sup.2/g and a pore volume of 0.8 ml/g measured by low-temperature nitrogen adsorption after dehydrating the pseudo-boehmite alumina under vacuum.

15. The method as claimed in claim 12, wherein the mineral acid is selected from nitric acid, sulfuric acid, and hydrochloric acid.

16. The method as claimed in claim 12, wherein the alumina support has a bimodal porous network with pores having a diameter in a range 60-120 Å and >120 Å, with at least 20% of the pore volume from the pores having a diameter in a range of 60-120 Å and at least 40% of the pore volume from the pores having a diameter >120 Å out of the total pore volume of the alumina support.

17. The method as claimed in claim 4, wherein the alumina support comprises two alumina materials in equal proportions with a different porosity, wherein one alumina powder has 60-65% pores having a diameter in a range of 60-120 Å, and other alumina powder has 60-65% pores having a diameter >120 Å.

Description

BRIEF DESCRIPTION OF DRAWING

(1) FIG. 1: illustrates relative distribution of sulphur compounds in feed and product stream.

DETAILED DESCRIPTION OF THE INVENTION

(2) The present invention relates to a method of preparation of a catalyst for simultaneously carrying out the selective hydrogenation of diolefins and combination reaction of light sulfur compounds like mercaptans with olefinic compounds to form a heavier sulfur compounds in the gasoline streams. The catalyst comprises of at least one metal from group VIB and at least one metal from group VIII mounted on the surface of a gamma-alumina support. The catalyst prepared according to the current invention is having two types of metal active sites, one comprising of only group VIB metals such as molybdenum and tungsten, whereas other type of active site is a group VIB metal promoted with group VIII metal like nickel and cobalt. The resultant varying metal profile enable to use this catalyst for two simultaneous reactions, wherein, the active sites of group VIB metal is responsible for the combination reactions of lighter sulfur compounds with olefinic compounds, whereas the group VIB metal promoted with group VIII metal are responsible for the hydrogenation of the diolefins.

(3) The catalyst, according to present invention, is derived from commercially available cheap raw materials of each component. The gamma-alumina support is derived from one or more pseudo-boehmite alumina which is a precursor material mainly comprises of aluminium hydroxide and having alumina content in the range of 70-75 wt. % as oxide. The pseudo-boehmite alumina should be having more than 85% crystalline phases and surface area in the range 250-300 m.sup.2/g and pore volume 0.8 ml/g measured by low-temperature nitrogen adsorption after dehydrating the sample under vacuum.

(4) The catalyst of the current invention can be prepared in any granular shapes, but more preferably as extrudates in cylindrical, trilobe or quadralobe shapes of 1.2-1.5 mm diameter. To prepare such extrudates, pseudo-boehmite alumina is first peptized with dilute solution of mineral acids like nitric acid, sulfuric acid, hydrochloric acid and then forced through an appropriate die of extruder machine. The resultant wet extrudates are dried 100-150° C. for 10-16 hours and calcined at 400-600° C. for at least 4-10 hours to obtain the Gamma-alumina support.

(5) The alumina support thus obtained as extrudates give rise to a bimodal porous network with majority of pores are having diameter in the range of 60-120 Å and >120 Å. In one of the features of the present invention, the majority of pores of the bimodal porous network are having diameter in the range of 60-120 Å and 120-200 Å. According to the current invention, alumina support is having at least 20% of the total pore volume from the pores having diameter in the range 60-120 Å and at least 40% of the total pore volume from the pores having diameter >120 Å. More preferably, 30% of the total pore volume due to the pores having diameter in the range 60-120 Å and 50% of the total pore volume due to the pores having diameter >120 Å are desired in the alumina support. Alternatively, instead of using a single alumina with bimodal porous network, two alumina materials with different porosity can be admixed in suitable proportions so as to achieve a bimodal pore distribution as above. For instance, one type of pseudo-boehmite with large porous texture which can give rise to pores of diameter >120 Å to the extent of at least 60% of total pore volume in oxide form and another pseudo-boehmite having at least 60% pores in the range 60-120 Å in oxide form can be admixed in suitable proportions to obtain the support material of the present invention.

(6) The catalyst prepared according to the current invention comprises of group VIB metal in the range 5 to 15 wt. % as metal oxide with respect to the total dry weight of final catalyst and the group VIII metal in the range 0.5 to 5 wt. % as metal oxide with respect to the total dry weight of final catalyst. Preferably, the group VIB metal used is molybdenum and tungsten whereas, the preferred group VIII metals are Ni and Co. The common raw materials used as the source of metal are one or more of the group comprising of ammonium hepta molybdate, molybdenum trioxide and molybdic acid for molybdenum, ammonium tungstate, tungstic acid and phosphotungstic acid for tungsten, nickel nitrate, nickel carbonate and nickel chloride for nickel, cobalt nitrate, cobalt acetate and cobalt carbonate for cobalt.

(7) According to the further embodiment of the current invention, the catalyst preparation is carried out through a two consecutive impregnations on the alumina support wherein, in the first impregnation step, a solution of group VIB metal is impregnated on the alumina support and subsequently in the second impregnation step, a solution of either group VIII metal alone or a mixture group VIII and group VIB is impregnated on the product obtained after first impregnation. Prior to impregnation in the second stage, the metal solution used for the second impregnation is added with one or more of compounds selected from a group consisting of Nitrilotriacetic acid (NTA), Ethylene diamine tetraacetic acid (EDTA) and Ethylene dinitrilotetraacetic acid. The molar ratio of the organic additive to group VIII metal is in the range 0.1-2.0, more preferably in the range 0.5-1.0.

(8) The preparation of catalysts comprises of steps for preparing the metal solutions by dissolving the required quantities of metal salts and chemicals in water or aqueous solution of ammonia, phosphoric acid or organic amines like monoethanol amine, diethyl amine etc. The impregnation of resultant solution having appropriate amounts of metals on the extrudate support can be carried out either by wet impregnation method or by incipient wetness method to obtain the final catalyst with desired metal loading. The wet catalyst obtained after first impregnation is dried at 100-150° C. for 8-12 hours and after second impregnation, the catalyst is again dried at 100-150° C. for 8-12 hours and then calcined at 400-500° C. for 2-4 hours to deposit the metal oxides on the support surface. The resultant finished catalyst is found to have pore volume to the extent of at least 60% from the pores of diameter in the range 60-120 Å, more preferably at least 70% from the pores of diameter in the range 60-120 Å out of total pore volume of the catalyst. The extent of pores of diameter >120 Å is in the range 10-20%.

(9) Alternative to above preparation approach, pseudo-boehmite powders with different pore size distribution can be separately supported with different active metals species and then the mixture of these metal incorporated alumina powders is extruded to obtain a catalyst with the desired metal profile. In such case, one type of pseudo-boehmite, wherein at least 60% pores are in the range 60-120 Å in the oxide, was loaded with group VIB metal and the other pseudo-boehmite having larger porous texture, wherein pores of diameter >120 Å (or 120-200 Å) amounts to the extent of at least 60% of total pore volume in oxide form, was loaded with a mixture group VIII and group VIB. The resultant metal incorporated pseudo-boehmite powders are mixed thoroughly, extruded in cylindrical, trilobe or quadralobe shapes of 1.2-1.5 mm diameter, dried at 100-150° C. for 8-12 hours and then calcined at 400-500° C. for 2-4 hours.

(10) The methodology of preparation employed according to current invention is suitable for giving a unique metal profile in the catalyst wherein the pores of alumina support in the diameter range 60-120 Å are deposited preferentially with group VIB metal whereas pore of diameter >120 Å are deposited preferentially with a combination of group VIB and VIII metals. Therefore, the catalyst of current invention is having two types of metal active sites, one comprising of mainly group VIB metals such as molybdenum and tungsten, whereas other type of active site is a group VIB metal promoted with group VIII metal like nickel and cobalt. The presence of two different types of metal active sites enables the catalyst to have multi-functionality in catalytic applications.

(11) The catalyst of current invention is particularly useful for pretreatment of gasoline streams before subjecting it to hydrotreatment, wherein the gasoline streams comprise of monoolefins, diolefins, and light sulfur compounds like mercaptans and sulfides along with other hydrocarbons such as paraffins, aromatics, naphthenes etc. Typically, the diolefin content in gasoline varies in the range 1-6% and sulfur content in the range 0.1-0.7%. The content of olefinic compounds in gasoline streams are generally in the range 40-75%. The RON of refinery feed streams generally available is in the range 70-80. The catalyst of present invention is particularly useful for treating gasoline streams obtained from fluidized catalytic cracking (FCC) units, Resid fluidized catalytic cracking like INDMAX technology, Coker units and other heavy cut naphthas, which has a boiling range typically in the range 100-275° C.

(12) The pretreatment of gasoline primarily includes the hydrogenation of diolefins as per the reaction pathway shown in Scheme 1. According to current invention, the active sites of catalyst comprising of group VIB metal promoted with group VIII metal are responsible for the transformation of 1,3-hexadiene to 3-hexene by the addition of hydrogen molecule as exemplified in Eq-1. Further hydrogenation of 3-hexene to n-hexane as represented by Eq-2 is undesired and is restricted with the special features of the invention in terms of pore size distribution and metal concentration profile. Therefore, advantageously, according to present invention, the active sites of the catalyst are highly efficient for selectively hydrogenating the diolefins to mono-olefins and thereby maximizing the retention of mono-olefins in the process and thereby to limit the loss of octane number of the gasoline.

(13) ##STR00001##

(14) Simultaneously to above hydrogenation reaction, the pretreatment process of gasoline also involves transformation of light sulfur compounds to heavier sulfur compounds as shown in Scheme 2. According to present invention, the active sites comprising preferentially of group VIB metal are responsible for the combination reactions of lighter sulfur compounds with mono-olefins to form heavier sulfur compounds. The light sulfur compounds of mercaptans and sulfides having boiling points lower than that of thiophene like methanethiol, ethanethiol, dimethylsulfide, methylethylsulfide, diethylsulfide etc, may be transformed into corresponding mercaptans or sulfides with higher molecular weight. The combination reaction of propane-2-thiol with 2-hexene to form a thioether molecule namely propylhexylsulfide is exemplified in Eq. 3. The shift of lighter sulfur compounds to heavier compounds enables to produce a desulphurized light gasoline fraction without any loss of mono-olefins, whereas the fraction with heavier sulfur compounds can be separated out and desulfurized in the subsequent hydrotreatment process.

(15) ##STR00002##

(16) The catalyst prepared according to the current invention, as a result of having varying metal profile in the unique porous texture of the support, is found to exhibit high catalytic function for simultaneously carrying out the hydrogenation of diolefins and combination reactions of lighter sulfur compounds with olefins in the gasoline streams to form heavier sulfur compounds. Accordingly, the use of catalyst of current invention is highly advantageous to upgrade the gasoline feedstock and thereby effectively processed further to obtain gasoline fuel meeting the environmental regulations.

EXAMPLES

(17) The disclosure will now be illustrated with working examples, which is intended to illustrate the working of disclosure and not intended to take restrictively to imply any limitations on the scope of the present disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice of the disclosed methods, the exemplary methods, devices and materials are described herein. It is to be understood that this disclosure is not limited to particular methods, and experimental conditions described, as such methods and conditions may vary.

Example 1

(18) A. Preparation of Alumina Support:

(19) Pseudo-boehmite alumina powder (1180 g, Al.sub.2O.sub.3, 72% w) is ball milled for 30 minutes. The powder is then mixed with a dilute solution of nitric acid (12 g, 520 ml de-mineralized water) and then mulled for 30 minutes in a mix-muller. The resultant dough is then extruded using an extruder machine with 1.2 mm trilobe die. The wet extrudates are dried at 120° C. for 12 hours and then calcined at 500° C. for 6 hours to obtain the gamma-alumina support.

(20) B. Preparation of Catalyst:

(21) According to current invention, the catalyst is prepared through a two stage impregnation process. For first impregnation, ammonium heptamolybdate (60 g, 98 wt. % purity) was dissolved in 800 mL of de-mineralized water and this solution was poured over the gamma-alumina support with continuous mixing in a rotating impregnation vessel in 15 minutes and continued mixing for another 15 minutes so as the extrudates to absorb the solution completely. The wet extrudates are dried at 120° C. for 10 hours.

(22) For second impregnation, ammonium heptamolybdate (90 g, 98 wt. % purity) and nickel nitrate hexahydrate (120.9 g, 98% purity) were dissolved in 560 ml of de-mineralized water under stirring. To this solution added Nitrilotriacetic acid (NTA) (38.8 g, 99% purity) under stirring and warmed to 60-70° C. to obtain a clear solution. This solution was poured over the gamma-alumina support with continuous mixing in a rotating impregnation vessel in 15 minutes and continued mixing for another 15 minutes so as the extrudates of the first impregnation step to absorb the solution completely. The wet extrudates are dried at 120° C. for 10 hours and then calcined at 450° C. for 3 hours.

(23) The physico-chemical properties of the alumina extrudate support and catalyst prepared are given in Table 1.

(24) TABLE-US-00001 TABLE 1 Physico-chemical properties of support and catalyst of current invention Support Catalyst Parameters BET Surface area, m.sup.2/g 262 245 Pore volume, ml/g 0.8 0.7 BJH Desorption Pore diameter 122 80 % Pore volume in pore dia. range .sup. >120 Å 58 10 60-120 Å 33 80   <60 Å 9 10

Example 2

(25) The catalyst is prepared as described in Example 1 except that entire quantity of the molybdenum source Ammonium heptamolybdate is dissolved in water and incorporated into the support in the first impregnation step and second impregnation is carried out with a solution having only nickel, wherein nickel nitrate hexahydrate is dissolved in water and is added with Nitrilotriacetic acid.

Example 3

(26) The catalyst is prepared as described in Example 1 except that two alumina powders with different pore distribution are separately loaded with metals through the first and second impregnation steps. Each alumina is taken in equal proportions of quantities and total amount of the support material is kept the same as that of Example 1 to obtain identical overall metal loading in the catalyst. The first impregnation is carried out on alumina powder with 60-65% pores in the diameter range 60-120 Å, whereas second impregnation is performed on a different alumina powder having 60-65% pores in the diameter range >120 Å, employing impregnation solution having both Mo and Ni. Both aluminas have only <10% pores below the diameter 60 Å.

(27) The individual metal incorporated alumina powders obtained after first and second impregnation are dried at 120° C. for 10 hours and then mixed. The mixture is homogenized through ball milling and extruded. The wet extrudates are dried at 120° C. for 10 hours and then calcined at 450° C. for 3 hours.

Example 4

(28) Performance Evaluation of Catalysts

(29) The catalytic activity of the catalysts is evaluated in the fixed bed tubular reactor with loading of 20 ml of the catalyst and using coker heavy naphtha. The process conditions employed for the test are given in Table 2. The product stream was analyzed for its composition and other characteristics for ascertaining the efficacy of the catalysts. The analysis results are given in Table 3. The feed and product gasoline streams are also analyzed using GC equipped with Sulfur Chemiluminescence Detector (SCD) for determining the relative distribution of sulfur compounds against elution temperature and the data are plotted in FIG. 1.

(30) TABLE-US-00002 TABLE 2 Feed properties and Process conditions used for catalyst evaluation Process conditions Value Temperature ° C. 170 Pressure bar 20 LHSV 1/h 2 H.sub.2/Oil ratio Nm.sup.3/m.sup.3 50

(31) TABLE-US-00003 TABLE 3 Typical Performance data of catalyst of current invention Product fractions Feed Product C5-65° C. 65-90° C. 90° C.+ Diolefins wt % 5.6 0.03 0.01 0.01 0.01 Total Sulfur ppmw 4110 4110 30 1200 2880 Octane Number (RON) 79.1 — 86.4 — — Density at 15° C. g/ml 0.6862 0.6843 0.6715 0.7102 0.749 NMR Analysis Paraffins + Naphthenes wt % 29.6 39.3 33.5 41.3 52.5 Aromatics wt % 5.6 5.7 2.6 7.6 11.8 Olefins wt % 64.8 55 63.9 51.1 35.7

(32) The hydrogenation of diolefins is a very fast reaction and is generally carried out using a suitable catalyst system at higher feed throughput and lower temperatures as compared to other hydrotreatment process. The reactions generally occur at the process conditions of temperature 160-200° C., pressure 20-25 bar, Liquid hourly space velocity (LHSV) of 2-4 h.sup.−1 and hydrogen/hydrocarbon ratio of approximately 50 Nm.sup.3/m.sup.3. The catalyst of current invention is evaluated at typical operating conditions for ascertaining its performance. The performance data show that the catalyst of current invention is highly efficient to reduce the diolefin content to very low levels (as low as <0.05%) from 5.6 wt % in the feed. The product streams are fractionated and analyzed for its composition and properties. The lower cut fraction (C5-65° C.) is found to have only 30 ppmw of sulphur. The relative distribution of sulphur compounds in the feed and products is shown in FIG. 1. The sulphur compounds in the lower boiling fractions of the feed are found to be shifted to higher boiling heavier fractions in the product. Further, the olefin content as well as RON of this fraction is also quite promising to be used as a gasoline component. In summary, the ability of the catalyst prepared according to current invention to selectively hydrogenate diolefins without seriously reducing the olefins that present in lower fractions and to enable converting of light sulphur compounds into heavier higher boiling compounds is clearly evident from the properties of gasoline product obtained with the use of current catalyst. The lower cut fraction obtained with catalyst of current invention having lower sulphur and higher RON can be directly routed to the gasoline pool of the refinery without subjecting to any further upgradation.

Advantages of the Present Invention

(33) The following are the technical advantages of the present invention over the prior art as disclosed above: Offers a method for preparing a catalyst for hydrogenation of diolefins in gasoline streams along with combination reactions of olefins with light sulfur compounds. Enables to create different types of active sites in the same catalyst so as to enable the catalyst to have multi-functions for simultaneously carrying out different reactions. To upgrade the gasoline feedstock and thereby effectively processed further to obtain fuels meeting the environmental regulations.