WO2013186789A1

WO2013186789A1 - A catalyst composition and a process for selective hydrogenation of methyl acetylene and propadiene

Info

Publication number: WO2013186789A1
Application number: PCT/IN2013/000359
Authority: WO
Inventors: Nagesh Sharma; Ajay Kumar; Sharad Vasudeorao LANDE; Kalpana Gopalakrishnan; Raksh Vir Jasra
Original assignee: Reliance Industries Limited
Priority date: 2012-06-11
Filing date: 2013-06-06
Publication date: 2013-12-19
Also published as: EP2858752A1

Abstract

In the present invention a selective hydrogenation catalyst composition comprising (a) an inorganic oxide carrier; and (b) fine-alloy particles of an active metal and a promoter metal components dispersed on the surface of the inorganic oxide carrier is disclosed. The improved dispersion of the active component is found to be around 30 % of surface area of the carrier surface as measured by H₂ Chemisorption method. The improved dispersion of fine alloy particles of the present invention is accomplished by employing an equilibrium adsorption impregnation method.

Description

A CATALYST COMPOSITION AND A PROCESS FOR SELECTIVE

HYDROGENATION OF METHYL ACETYLENE AND PROPADIENE

FIELD OF THE DISCLOSURE:

The present disclosure relates to a catalyst composition for the selective hydrogenation of alkylenes and/or di-olefins present in a light olefin stream and a process for preparation thereof. The present disclosure further relates to a process for the selective hydrogenation of alkylenes and/or di-olefins.

BACKGROUND:

The large scale production of light olefins such as ethylene, propylene and butylenes is accomplished by number of processes such as steam cracking, fluid catalytic cracking, conversion of methanol to olefin, paraffin dehydrogenation, alcohol dehydration, methane coupling and Fisher Tropsch reactions. These processes .are usually accompanied by production of varying level of acetylenic and/or di-olefinic by-products such as acetylene, methyl acetylene, propadiene, butyne and butadiene. The presence of these by-products in the light olefins stream often acts as poisons to downstream processing catalysts, such as polymerization catalyst. Therefore, their removal from the light olefin stream is an utmost requirement in the petrochemical refinery industries. The preferred method for the removal of these by-products is a selective catalytic hydrogenation in which, highly unsaturated compounds such as alkylenes and/or di-olefins are reduced to less unsaturated compounds. For example acetylene is converted to ethylene,, methyl acetylene and propadiene are converted to propylene, and butyne and butadiene are converted to butylenes.

However, this selective hydrogenation is often accompanied by some other competing reactions such as oligomerization of two or more alkylene/di-olefin molecules to produce heavier unsaturated compounds (generally referred to as 'Green oil') or the generation of saturates for example ethane, propane or butanes as a result of over-hydrogenation. Both of these reactions are undesirable reactions as they reduce selectivity for the required light olefins. Other than reduced selectivity, the decreased life of hydrogenation catalyst is also observed due to the side-production of green-oil. The selective hydrogenation of unsaturated hydrocarbons which are mostly present in a feedstock as an impurity (usually in small quantity of about 3-5%) is a extremely important process for the petroleum/petrochemical industries which involve large scale commercial productions. For example, acetylene remains up to 3 % in ethylene stream. Similarly, methyl acetylene and propadiene remain up to 3-5% in propylene stream. Therefore, the selective hydrogenation is advantageous not only for the purpose of the purification of feed stock but also for increasing the yield of feed stock. Even a small increase in ethylene or propylene selectivity or yield will be proven economical to a greater extent for the large scale industrial process.

EXISTING KNOWLEDGE:

Hitherto reported catalysts for the selective hydrogenation of alkylenes and di-olefins comprise Group 8-10 metals such as palladium and rhodium, often in combination with other • metals such as silver and indium, wherein metals are dispersed on a support, such as alumina, The activity and selectivity of such catalysts depend not only on the type of metal (s) employed but also on the ability to disperse or alloy small particles of the desired metals, followed by drying and calcinations. The impregnation methods practiced so far for the impregnation of metal components on the solid support are allied with certain anomalies such as non-homogenous metal dispersion or alloying of metal particles particularly at the conditions of high metal loading.

Therefore, a need for an improved method of dispersing and alloying metal components on a catalyst support to prepare a catalyst with improved ... selectivity for the selective hydrogenation of alkylenes and/or diolefins is highly desired.

OBJECTS:

Some of the objects of the present disclosure are described herein as:

It is object of the present disclosure to ameliorate one or more problems of the prior art or to at least provide a useful alternative.

Another object of the present invention is to provide a catalyst composition for the selective hydrogenation of unsaturated impurities present in a light olefin stream. Still another object of the present invention is to provide a highly selective catalyst for the selective hydrogenation of unsaturated impurities present in a light olefin stream.

Yet another object of the present invention is to provide a highly active and thermally stable catalyst for the selective hydrogenation of unsaturated impurities present in a light olefin stream wherein very low target production of green oil is achieved.

Further object of the present invention is to provide a process for the preparation of a catalyst composition employed for the selective hydrogenation of unsaturated impurities present in a light olefin stream.

Another object of the present disclosure is to provide a process for the selective hydrogenation of unsaturated impurities present in a light olefin stream.

Other objects and advantages of the present invention will be more apparent from the following description when read in conjunction with the accompanying figures, which are not intended to limit the scope of the present invention.

SUMMARY:

In accordance with the present invention there is provided a selective hydrogenation catalyst composition with an improved active metal dispersion; said catalyst composition comprising (a) an inorganic oxide carrier and (b) fine-alloy particles of an active metal component and a promoter dispersed thereon, wherein the dispersion of said active metal component on the carrier surface is at least 30 % of surface area of the carrier surface as measured by H₂ Chemisorption method.

Typically, the average crystallite size of the active metal component dispersed on the alumina surface varies between 3 nm to 12 rim.

Typically, the catalyst composition is characterized by X-ray powder diffraction pattern having characteristic peaks at the 2Θ values of 38 to 40 Typically, the active metal is at least one metal selected from the group the consisting of palladium, platinum and nickel:

Typically, the promoter is at least one metal selected from the group consisting of silver, gold and copper.

Typically, the weight proportion of the active metal component dispersed on the surface of the inorganic oxide carrier varies between 0.01 and 5.0 % with respect to the mass of the carrier.

Typically, the weight proportion of the promoter dispersed on the surface of the inorganic oxide carrier varies between 0.05% and 0.3% with respect to the mass of the carrier.

Typically, the inorganic oxide carrier is alumina selected from the group consisting of alpha- alumina, theta alumina, delta-alumina, gamma alumina and combinations thereof.

Typically, the morphological form of alumina is selected from the group consisting of spheres, pills, cakes, extrudates, powders, and granules.

Preferably, the alumina is spheroidal alumina characterized by the surface area ranging between 25 to 200 m²/g, pore volume ranging between 0.25 ml/g to 0.4 ml/g, and particle size ranging between 1.4 mm to 4mm

Typically, the catalyst composition further comprises at least one metal selected from the group consisting of tin, lead, rhenium gallium, indium, thallium and combinations thereof in a weight proportion varying between 0.01 wt % to 0.5 wt %.

In accordance with the present invention there is provided a process for the preparation of a selective hydrogenation catalyst with improved dispersion of active metal component; said process comprising: ^v

i. preparing fine alloy particles containing at least one active metal component and at least one promoter metal component by dissolving at least one active metal precursor and at least one promoter metal precursor in an aqueous acidic solution having pH between 1.2 and 1.4 to obtain a solution containing the fine alloy particles; ii. soaking an inorganic oxide carrier in the solution containing fine alloy particles for a time period ranging between 2 to 24 hours to obtain inorganic oxide carrier dispersed with said fine alloy particles; and

iii. drying and calcining the inorganic oxide carrier dispersed with said fine alloy particles to obtain said hydrogenation catalyst.

Typically, the aqueous acidic solution containing active and promoter metal precursors in method step (i) is heated at a temperature of 70 °C for a time period ranging between 5 to 15 min.

Typically, the active metal precursor is at least one metal precursor selected from the group consisting of palladium containing compounds, platinum containing compounds, and nickel containing compounds.

Typically, the active metal precursor is palladium containing compound selected from the group consisting of palladium nitrate and palladium chloride; preferably palladium nitrate.

Typically, the weight proportion of the palladium containing compound dissolved in the aqueous acidic solution varies between 0.0005 mol/1 to 0.025 mol/1.

Typically, the promoter metal precursor is at least one metal precursor selected from the group consisting of copper containing compounds, silver containing compounds and gold containing compounds.

Typically, the promoter metal precursor is silver containing compound selected from the group consisting of silver nitrate and silver chloride; preferably silver nitrate.

Typically, the weight proportion of the silver containing compound dissolved in the aqueous acidic solution varies between 0.0025 mol/1 to 0.015 mol/1.

Typically, the inorganic oxide carrier is alumina selected from the group consisting of alpha- alumina, delta-alumina, theta-alumina, gamma alumina and combinations thereof. Typically, the morphological form of alumina is selected from the group consisting of spheres, pills, cakes, extrudates, powders, and granules.

Typically, the alumina is spheroidal alumina characterized by the surface area ranging between 25 to 200 m²/g, pore volume ranging between 0.25 ml/g and 0.4 ml/g, and particle size ranging between 1.4 mm to 4mm.

Typically, the process of the present invention further comprises a step of impregnating at least one metal component selected from the group consisting of tin, lead, rhenium, gallium, indium, thallium, and rhodium on the inorganic oxide carrier.

Typically, the calcination in method step (iii) is carried out at a temperature ranging between 450 °C to 550 °C and for a time period varying between 4 hrs to 8 hrs.

In accordance with the present invention there is provided a process for selectively hydrogenating alkylenes and/or di-olefins present in a light olefin stream by employing the selective hydrogenation catalyst of the present invention; said process comprising the following steps:

i. providing a mono-olefin stream containing alkylene and/or di-olefin components; and

ii. selectively hydrogenating said alkylene and/or diolefin components by contacting the mono-olefin stream with the selective hydrogenation catalyst of the present invention under hydrogenation conditions effective to hydrogenate said alkylene and di-olefins.

Typically, the mono-olefin stream is C₂ to C₄ containing stream; preferably C₃ containing stream.

Typically, the alkylene is at least one selected from the group consisting of methyl acetylene and acetylene; preferably methyl acetylene.

Typically, the di-olefin is at least one selected from the group consisting of propadiene and vinyl-acetylene; preferably propadiene. Typically, the selective hydrogenation is carried out at a temperature in the range of from 10 °C to 40 °C, at a pressure in the range of from ίθ to 40 atmospheres and with a liquid hourly space velocity varying in the range of from 10 to 30 h^"1.

BRIEF DESCRIPTION OF THE ACCOMPONAYING DRAWINGS:

Figure 1 illustrates X-Ray diffractograms of (A) commercial hydrogenation catalyst; and (B) catalyst of the present disclosure;

Figure 2 illustrates UV-DRS spectrum of (A) alumina; (B) silver on alumina; and (C) catalyst of the present disclosure comprising Pd-Ag complex dispersed on alumina surface;

Figure 3 illustrates comparative analysis of methyl acetylene percent conversion carried out in the presence of a commercial hydrogenation catalyst and catalyst in accordance with the present disclosure;

Figure 4 illustrates comparative analysis of propadiene percent conversion carried out in the presence of a commercial hydrogenation catalyst and catalyst in accordance with the present disclosure;

Figure 5 illustrates comparative analysis of propylene selectivity during selective hydrogenation of methyl acetylene and propadiene carried out in the presence of a commercial hydrogenation catalyst and catalyst of the present disclosure; and

Figure 6 illustrates comparative analysis of C₆ oligomer contents (%) during selective hydrogenation of methyl acetylene and propadiene carried out in the presence of a commercial hydrogenation catalyst and catalyst of the present disclosure.

DETAILED DESCRIPTION:

The present disclosure is accomplished taking into account the above described goals and objects of the present disclosure. Accordingly, the present invention envisages a catalyst composition with improved selectivity for the selective hydrogenation of alkylenes and/or diolefins present in a light olefin stream. Also, envisaged in accordance with the present invention is a process which selectively converts alkylenes and/or diolefins present in a light olefin feed to corresponding less un-saturated olefins (C₂-C₄) thereby improving the overall yield of a given light olefin stream.

Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

The use of the expression "at least" or "at least one" suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the invention to achieve one or more of the desired objects or results.

The term "less unsaturated olefins (C₂-C₄)" as used in context of the present disclosure refers to olefins containing single double bond such as ethylene, propylene and butylenes.

In a first aspect of the present disclosure there is provided a catalyst composition comprising an inorganic oxide carrier, at least one active metal component and at least one metal promoter. The active and the promoter metal components present in the catalyst composition of the present disclosure are substantially dispersed over the entire surface of the inorganic oxide carrier.

The inorganic oxide carrier present in the catalyst composition of the present disclosure is preferably alumina and various polymorphic forms thereof which include alpha-alumina, delta-alumina, theta alumina, gamma alumina and any combinations thereof. Further the oxide carrier material may be in any morphological forms that include spheres, pills, cakes, extrudates, powders, granules and the like.

In accordance with one of the embodiments of the present disclosure, the alumina is spheroidal alumina characterized by the surface are raging between 25 to 200 m²/g, pore volume ranging between 0.25 mi/g to 0.4 ml/g and particle size in the varying in the range of from 1.4 mm to 4.0 mm.

The active metal component dispersed over the surface of the inorganic oxide carrier is a transition metal component selected from group VIII elements of Periodic Classification.

The active metal component as employed in the present context includes palladium, platinum and nickel. The most preferred active metal component is palladium. The selectivity of the catalyst is in part governed by the proportion of the active metal component and accordingly, the catalyst composition of the present invention comprises varying amounts of the active metal component, ranging from a narrower range of 0.01 to 5 wt %.

In accordance with one of the embodiments, the weight proportion of palladium content dispersed on the surface of the inorganic oxide carrier varies between 0.01 to 5.0 wt%, based on the mass of the inorganic oxide carrier.

The catalyst composition of the present disclosure further comprises at least one metal promoter uniformly dispersed on the surface of the inorganic oxide carrier along with the active metal component. The choice and selection of promoter metal impregnated on the surface of the catalyst composition along with the active metal component depends on their affinity to form fine alloy particles on the surface the inorganic oxide carrier.

The promoter metal present in the catalyst composition of the present disclosure comprises at least one metal selected from group IB elements of Period Classification that comprises copper, silver and gold. The most preferred group IB metal is silver. The weight proportion of the promoter metal typically varies between 0.05 wt % to 0.3 wt%, with respect to the mass of the carrier.

Apart from the active and the promoter metal components, the. catalyst composition of the present disclosure further comprises at least one metal selected from the group consisting of tin, lead, rhenium, gallium, indium and thallium in a weight proportion varying between 0.01 wt% to 0.5 wt%, with respect to the mass of the carrier. The active and the promoter metal components exist in the form of fine-alloy particles uniformly dispersed over the entire surface of the alumina support. In accordance with an exemplary embodiment of the present invention, the catalyst composition comprises fine alloy particles of palladium and silver uniformly dispersed on thereto surface. The dispersion of palladium as measured by H₂ chemisorption method is about 30% of surface area of the carrier surface. Further, the average crystallite size of palladium particles dispersed on the alumina surface varies between 3 nm to 12 nm, as measured by H₂ chemisorption method.

The uniform and improved dispersion of fine alloy particles comprising palladium and silver metals on the alumina support is further established by XRD study. The presence of diffraction peaks at 26-values of 38 to 40° indicates uniform dispersion of alloy particles containing palladium and silver metals on the alumina support.

In another aspect of the present disclosure, there is provided a process for uniformly dispersing fine alloy particles comprising an active and a promoter metal component on the surface of an inorganic oxide carrier to obtain a selective hydrogenation catalyst; said process comprising the following steps:

The dispersion of the active and other metal components on the surface of the inorganic oxide carrier in accordance with the process of the present disclosure is a con-current process.

The con-current dispersion of the active and the promoter metal components on the carrier surface is carried out by employing an equilibrium adsorption impregnation method. The equilibrium adsorption method according to the present disclosure comprises a step of contacting an inorganic oxide carrier with a solution comprising fine alloy particles of active and promoter metal components particularly palladium and silver containing alloy particles.

The fine alloy particle comprising the active and the promoter metal components are prepared by dissolving at least one active metal precursor and at least one promoter metal precursor in an aqueous acidic solution.

The active and the promoter metal precursors are particularly dissolved in water under continuous stirring followed by adding small quantity of concentration hydrochloric acid. The concentrated hydrochloric acid is added slowly in small portions over a period of time so as to maintain pH of the solution varying between 1.2 to 1.4.

At the pH of 1.2 to 1.4, the alloy formation of active and promoter metal components starts. To further enhance the complex formation, the aqueous acidic solution comprising the active and the promoter metal precursors is heated at a temperature of about 70 °C for a time period ranging between 5 to 15 minute. The aqueous acidic solution comprising fine alloy particles of the active and the promoter metal components is then cooled to room temperature. The inorganic oxide carrier, particularly alumina is then dipped in the solution comprising fine alloy particles to impregnate the alloy particles on thereto surface. In accordance with one of the exemplary embodiments of the present invention, the alumina support is dipped for a time period of about 24 hours to effectively impregnate the alloy particles.

After 24 hours the solution is decanted and the alumina impregnated with fine alloy particles is subjected to drying and calcinations.

In accordance with one of the exemplary embodiments of the present invention, the calcinations of the impregnated alumina is carried out a temperature ranging between 450 °C to 550 °C for a time period varying between 4 hours to 8 hours.

The group 8 metal precursor employed for the purpose of the present invention comprises at least one selected from the group consisting of palladium containing compounds, platinum containing compounds and nickel containing compounds.

In accordance with one of the embodiments of the present invention, the active metal precursor is palladium containing compound selected from the group consisting of palladium nitrate, palladium chloride; preferably palladium nitrate.

The weight proportions of the active metal precursor are taken in an effective amount so as to obtain their desired contents within the vicinity of the alumina support. The weight proportion of the palladium containing compound dissolved in the aqueous acidic solution of the present invention varies between 0.0005 mol/1 to 0.025 mol/1. The promoter metal precursor as employed in the context of the present invention comprises at least metal precursor selected from the group consisting of copper containing compounds, silver containing compounds and gold containing compounds.

In accordance with one of the embodiments of the present invention, the promoter metal precursor is silver containing compound selected from the group consisting of silver nitrates and silver chloride; preferably silver nitrate. The weight proportion of silver nitrate mixed in the aqueous acidic solution of the present invention typically varies between 0.0025 mol/1 to 0.015 mol/1.

Apart from promoter and active metal components, some additional metal components particularly the metal components from group III, IV and VII of Periodic classification can also be impregnated on the alumina surface. In accordance with one of the exemplary embodiments of the present invention, at least one metal selected from the group consisting of tin, lead, rhenium, gallium, indium, and thallium is impregnated on alumina surface along with the active and the promoter metal components.

The impregnation of above described additional metal components is accomplished by dissolving corresponding metal precursors in the aqueous acidic solution along with the active and promoter metal precursors. The additional metal precursors are dissolved in the aqueous acidic solution in a weight proportion so as to achieve the content of the additional metal varying between 0.01 wt% to 0.5 wt % on the alumina surface, with respect to the mass of the carrier. -

In another aspect of the present disclosure, there is provided a process for selective hydrogenation of alkylene and/or diolefin present in a light olefin feed by using the catalyst composition of the present disclosure. The selective hydrogenation of alkylenes and/or diolefins present in light olefin feed is carried out under hydrogenation conditions effective to hydrogenate alkylenes and/or diolefins * ^

The light olefin feed containing alkylene and di-olefin in accordance with the present invention is C₂ to C₄ containing feed; particularly propylene feed (C₃) wherein methyl acetylene and propadiene present in C₃ containing feed are selectively reduced to propylene in the presence of the catalyst composition of the present invention. The embodiments herein and the various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.

Example 1:

This example describes a process for the preparation a catalyst composition comprising palladium and sliver particles impregnated on alumina.

The equilibrium adsorption impregnation method was used for the preparation of catalyst composition. Palladium nitrate (0.15 gm) and sliver nitrate (0.047 gm) were dissolved in 50 ml of distilled water by simultaneous addition. Followed to this, 10 ml of cone, hydrochloric acid was slowly added in small proportion for the preparation of Pd-Ag complex formation with pH 1.2-1.4. The solution was heated at 70 °C for 15 minutes to accelerate Pd-Ag complex formation. The complex solution then cooled down to room temperature and used for impregnation. A 30 gm of spheroidal alumina support was dipped in the Pd-Ag containing solution and soaked for 24 hours for adsorption of metals on the alumina support. Gentle manual stirring was given during the soaking period. After 24 hrs the remaining solution was decanted and catalyst dried for 12 hours at 120 °C followed by calcinations at 550 °C for 6 h. The catalyst composite was prepared with the composition of palladium and sliver around 0.2% w/w and 0.1 % w/w, respectively.

The catalyst prepared by employing an equilibrium adsorption method of impregnation as described in example- 1 is analysed by using various physicochemical techniques such as X- Ray diffraction, UV-Vis reflectance spectroscopy and hydrogen chemisorption.

The catalyst composite was prepared with the composition provided in Table- 1 by the process of example -1. Table 1:

COMPARATIVE ANALYSIS:

X-ray diffraction (XRD) analysis is carried out to evaluate the crystalline structure of the catalyst prepared in accordance with the process of example- 1. Additional to this, comparative XRD analysis of the commercial hydrogenation catalyst and the catalyst prepared in accordance with the process of example- 1 is also carried out (refer to Figure 1 of the accompanying drawings). Alumina support used in both catalysts shows similarity to a great extent and particularly comprises mix phase alumina containing mainly delta and theta phases. However the Pd crystallite size as calculated from Pd (111) peak at 2Θ ~ 40° using Scherrer formula was found to be smaller in the catalyst composition prepared in accordance with the process of example-1 (12 nm ) as compared to that of commercial catalyst (14 nm). The smaller (about 15%) crystallite size of Pd is expected to improve Pd dispersion. Because of lower Pd crystallite size as confirmed from XRD analysis, the catalyst composition prepared in accordance with the process of example-1 has high Pd dispersion in comparison to the commercial hydrogenation catalyst.

Further, hydrogen chemisorption study was carried out to evaluate dispersion percentage and average crystallite size of palladium particles supported on alumina support. Both, commercial hydrogenation catalyst and the catalyst prepared in accordance with the process of example-1 are analyzed by H₂ chemisorption method. The amounts of chemisorbed hydrogen were determined at 35 °C on Quantachrome-Autosorb I instrument. The instrument was subjected to preliminary evacuation followed by heating. The heating of the instrument was carrier out in twp steps: first, the instrument was heated at 120 °C for 1 h under He flow and then for another 1 hour at 120 °C under hydrogen flow (Praxair, 99. %). Followed to heating, the instrument was further evacuated at 480 °C for 4 h. After that, the instrument is cooling down to 35 °C, and finally measurement of hydrogen uptake for the metal dispersion analysis done at 35 °C. An adsorption isotherm was measured, which includes both physisorption and chemisorptions. The isotherm was determined in the pressure range from 0.5 to 700 mm Hg in order to complete the adsorption isotherm covering a monolayer of reacted molecule over the sample. The purpose of the chemisorptions methods is to evaluate the amount of the gas used to cover a monolayer over the free active Pd sites. The monolayer can be expressed in moles or volumes (NTP) of gas referred to a sample mass or the amount metal present in the catalyst. An average crystallite size d (given in nanometers) was calculated based on irreversible adsorption isotherm of hydrogen, according to the following: d- 6000/Sp, where S is the surface area (of the fraction of reduced palladium, given in m² g- 1) and p is the palladium density (12.02 cm³ g-1). 2S was calculated assuming H/Pd) 1 and a surface area of 7.87 A² per palladium atom. The monolayer uptake, metal dispersion and average crystallite size for commercial catalyst and the catalyst prepared in accordance with the process of example- 1 is provided Table 2.

Table 2: Palladium dispersion of commercial and invention catalyst

It is observed that, the monolayer uptake is higher in the catalyst of the present invention (7.0 umol/g) as compared to the commercial catalyst (4.6 umol/g) which corresponds to more number of palladium active site available for hydrogenation. The average crystallite size of palladium metal in the catalyst of the present invention (3.6 nm) is lower than commercial catalyst (6.4 ηη Due to lower average metal crystallite in the catalyst of the present invention, the metal dispersion is higher than commercial catalyst. It is well-recognized that the size of metal particle determines the dispersion which enhances the activity and selectivity of the catalyst. The size of metal particles depends upon the method of catalyst preparation. In the present invention, the inventors have achieved average metal crystallite size of 3.6 nm and higher dispersion by using unique catalyst preparation approach. The Pd dispersion in commercial hydrogenation catalyst was determined as 17.58% by H₂ chemisorption method whereas in the catalyst prepared in accordance with the process of example- 1 is 30.52 %, which is attributed due to adoption of novel catalyst preparation approach. Numbers of active Pd site available on the catalyst surface are higher which corresponds to good activity, selectivity and stability for hydrogenation catalyst as well as lower green oil formation over commercial catalyst during selective hydrogenation of unsaturated impurities present in a light olefin stream.

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Further, figure 2 of the accompanying drawings shows UV-Vis Diffuse Reflectance spectroscopy analysis of the catalyst prepared in accordance with the process of example- 1. It is observed that alumina support exhibits an absorbance band in the region of 208 -308 nm with its maximum intensity at 230 nm. This band corresponds to ligand to metal charge transfer transitions from 0^2_to Al³⁺. The Ag+ ion bands at 220-250 nm can not be determined due to strong absorption of A1₂C>3 at these wavelengths. The catalyst composition of example- 1 shows a featureless large and wide band whose maximum is observed at 400-450 nm, which is attributed either to PdO or to Pd²⁺ ions as well as to Pd-Ag interaction.

ExampIe-2:

This example describes a process for selective hydrogenation of methyl acetylene and propadiene present in a light olefin stream, particularly, a C₃ light olefin stream in the presence of a catalyst composition prepared in accordance with the process of example- 1. The typical process parameter employed while carrying out the liquid phase hydrogenation of methyl acetylene and propadiene are tabulated in Table 4.

Table 4: Typical process parameters for selective hydrogenation of methyl acetylene and propadiene.

Mol. % MAPD in Feed 1.9 - 4.2

H₂ requirement 5-10% in excess of stoichiometry of total olefins

The olefinic stream containing hydrogenation products is then analyzed by off-line Gas Chromatograph (GC). The percent conversion of methyl acetylene and propadiene is calculated as follows:

MA content in feed - MA content in product

% MA conversion = x 100

MA content in Feed

PD content in feed - PD content in product

% PD conversion = x 100

PD content in Feed

(MA+PD) in feed - (MA+PD) in product

Total MAPD Conversion = — x 100

(MA+PD) in feed

The propylene selectivity is determined as follows:

Weight of Propylene at reactor outlet

% Propylene Selectivity =

Weight of MA+ PD at reactor outlet

The percent conversion of methyl acetylene and propadiene, and propylene selectivity is plotted as function of hours on stream (HOS) (refer to Figure 3, 4 and 5 of the accompanying drawings). TEST EXAMPLE 1:

The selective hydrogenation of methyl acetylene and propadiene present in C₃ olefin stream was also carried out by using commercial hydrogenation catalyst under similar process conditions as employed in example-2. Methyl acetylene and propadiene percent conversion, and propylene selectivity is also calculated.

Comparative performance:

The comparative performance of both catalysts for the selective hydrogenation of methyl acetylene and propadiene present in C₃ olefin stream is provided as follows:

The initial percent conversion of methyl acetylene is higher in case of commercial hydrogenation catalyst as compared to the selective hydrogenation catalyst prepared in accordance with the process of example- 1, however with the progress of hours on stream, particularly beyond 80 hours on stream, the methyl acetylene percent conversion increases in case of selective hydrogenation carried out in the presence of catalyst prepared in accordance with the process of example- 1, thus proving its superior activity in terms of methyl acetylene percent conversion over the commercial hydrogenation catalyst ( refer to Figure 3 of the accompanying drawings). The catalyst prepared in accordance with the process of example- 1 shows an increase in methyl acetylene percent conversions from about 63% in the initial 20 hours to about 70 % towards the end of about 200 hours of run, thus showing better performance over commercial hydrogenation catalyst. Similarly the catalyst of the present invention prepared in accordance with the process of example- 1 shows higher propadiene conversion as compared to the commercial hydrogenation catalyst (refer to the figure 4 of the accompanying drawings). The catalyst performance is further evaluated with respect to propylene selectivity. The selectivity comparison for the catalyst of example- 1 and commercial hydrogenation catalyst is plotted as a function of hours on stream (HOS) (refer to figure 5 of the accompanying drawings). The catalyst prepared in accordance with the process of example- 1 shows better selectivity over commercial hydrogenation catalyst for the run of 200 hrs.

Further, C₆ Oligomer (green oil) formation is also monitored during selective hydrogenation process. The experiments were conducted for both the catalysts, i.e. catalyst prepared in accordance with the process of example- 1 and the commercial hydrogenation catalyst. The green oil formation during selective hydrogenation of methyl acetylene and propadiene is monitored at reaction temperature of 60 °C, pressure of 25 Kg/cm² and WHSV of 16 h^"1. The reactions were carried out for a time period of about 25 hours. Figure 6 of the accompanying drawings shows the comparative analysis of the green oil formation during selective hydrogenation carried out in the presence of catalyst of the present disclosure and the commercial hydrogenation catalyst. The C₆ oligomer formation is on the lower side in case of selective hydrogenation catalyst of example- 1 as compared to the commercial hydrogenation catalyst.

TECHNICAL ADVANTAGES:

Technical advantages of the present disclosure pertaining to a catalyst composition and process for the selective hydrogenation of methyl acetylene and propadiene lies in providing:

1. a con-current process for impregnating palladium, silver and other metal components on the solid inorganic oxide carrier;

2. a selective hydrogenation catalyst for the selective hydrogenation of unsaturated impurities present in the light olefin stream thereby without effecting the main olefin stream ;

3. a selective hydrogenation catalyst having uniformly dispersed palladium and silver metal components on the solid catalyst support having better complex forming or alloy forming property; and

4. a selective hydrogenation catalyst having improved dispersion of palladium metals thereby rendering the catalyst with high activity, selectivity, stability and low green oil formation.

The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the invention, unless there is a statement in the specification specific to the contrary.

Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.

While considerable emphasis has been placed herein on the preferred embodiments, it will be appreciated that many embodiments can be made and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. These and other changes in the preferred embodiments as well as other embodiments of the disclosure will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the forgoing descriptive matter to be implemented merely as illustrative of the disclosure and not as limitation.

The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

Claims

Claims:

1. A selective hydrogenation catalyst composition with an improved active metal dispersion; said catalyst composition comprising (a) an inorganic oxide carrier and (b) fine-alloy particles of an active metal component and a promoter dispersed thereon, wherein the dispersion of said active metal component on the carrier surface is at least 30 % of surface area of the carrier surface as measured by H₂ Chemisorption method.

2. The catalyst composition as claimed in claim 1, wherein average crystallite size of active metal component dispersed on the alumina surface ranges between 3 nm to 12 nm.

3. The catalyst composition as claimed in claim 1 characterized by X-ray powder diffraction pattern having characteristic peaks at the 2Θ values of 38 to 40.

4. The catalyst composition as claimed in claim 1, wherein the active metal is at least one metal selected from the group the consisting of palladium, platinum and nickel.

5. The catalyst composition as claimed in claim 1, wherein the promoter is at least one metal selected from the group consisting of silver, gold and copper.

6. The catalyst composition as claimed in claim 1, wherein the weight proportion of the active metal component dispersed on the surface of the inorganic oxide carrier varies between 0.01 and 5.0 % with respect to the mass of the carrier.

7. The catalyst composition as claimed in claim 1, wherein the weight proportion of the promoter dispersed on the surface of the inorganic oxide carrier varies between 0.05 and 0.3 % with respect to the mass of the carrier.

8. The catalyst composition as claimed in claim 1, wherein said inorganic oxide carrier : is alumina selected from the group consisting of alpha-alumina, delta-alumina, theta- alumina, gamma alumina and combinations thereof.

9. The catalyst composition as claimed in claim 1, wherein the morphological form of alumina is selected from the group consisting of spheres, pills, cakes, extrudates,

^{* !} powders, and granules.

10. The catalyst composition as claimed in claim 1, wherein the alumina is spheroidal alumina characterized by the surface area ranging between 25 to 200 m²/g, pore volume ranging between 0.25 ml/g to 0.4 ml/g, and particle size ranging between 1.4 mm to 4mm.

11. The catalyst composition as claimed in claim 1 further comprises at least one metal selected from the group consisting of tin, lead, rhenium gallium, indium, thallium and combinations thereof in a weight proportion varying between O.Olwt % to 0.5 wt %, with respect to the mass of the carrier.

12. A process for the preparation of a selective hydrogenation catalyst with improved dispersion of active metal component; said process comprising:

L admixing at least one active metal precursor and at least one promoter metal precursor in an aqueous acidic solution having pH between 1.2 and 1.4 to obtain a solution containing fine alloy particles ;

ii. soaking an inorganic oxide carrier in the solution containing fine alloy particles for a time period ranging between 2 to 24 hours to obtain inorganic oxide carrier dispersed with said fine alloy particles; and ^' . ,

13. The process as claimed in claim 12, wherein the aqueous acidic solution containing active and promoter metal precursors dissolved therein in method step (i) is heated at a temperature of 70 ^oC for a time period ranging between 5 to 15 min,

14. I The process as claimed in claim 12, wherein said active metal precursor is at least one selected from the group consisting of palladium containing compounds, platinum containing compounds, and nickel containing compounds.

15. The process as claimed in claim 12, wherein the active metal precursor is palladium containing compound selected from the group consisting of palladium nitrate, and palladium chloride; preferably palladium nitrate.

16. The process as claimed in claim 12, wherein weight proportion of the palladium containing compound dissolved in the aqueous acidic solution varies between 0.0005 mol/1 to 0.025 mol/1.

17. The process as claimed in claim 12, wherein the promoter metal precursor is at least one metal precursor selected from the group consisting of copper containing compounds, silver containing compounds and gold containing compounds.

18. The process as claimed in claim 17, wherein the .promoter metal precursor is silver containing compound selected from the group consisting of silver nitrate and silver chloride; preferably silver nitrate.

19. The process as claimed in claim 17, wherein weight proportion of said silver containing compound dissolved in the aqueous acidic solution varies between 0.0025 mol/1 to 0.015 mol/1.

20. The process as claimed, in claim 12, wherein said inorganic oxide carrier is alumina selected from the group consisting of alpha-alumina, delta-alumina, theta-alumina, gamma alumina and combinations thereof.

21. The process as claimed in claim 12, wherein the morpho logical form of alumina is selected from the group consisting of spheres, pills, cakes, extrudates, powders, and granules.

22. The. process claimed in claim 12, alumina is spheroidal alumina characterized by the surface area ranging between 25 to 200 m²/g, pore volume ranging between 0.25 ml/g to 0.4 ml/g, and particle size ranging between 1.4 mm to 4 mm.

23. The process as claimed in claim 12 further comprises a step of impregnating at least one metal component selected from the group consisting of tin, lead, rhenium, gallium, indium, thallium, and rhodium.

24. The process as claimed in claim 12, wherein calcinations in method step (iii) is carried out at a temperature ranging between 450 °C to 550 °C and for a time period varying between 4 hrs to 8 hrs.

25. A process for selectively hydrogenating alkylenes and di-olefins present in a light olefin stream by employing the selective hydrogenation catalyst as claimed in claim 1; said process comprising the following steps:

iii. providing a mono-olefin stream containing alkylene and di-olefin components; and

iv. selectively hydrogenating said alkylene and diolefin components by contacting the mono-olefin stream with the selective hydrogenation catalyst of claim. 1 under hydrogenation conditions effective to hydrogenate said alkylene and di- olefins.

26.^■ The process as claimed in claim 25, wherein the mono-olefin stream is C₂ to C₄ containing stream; preferably C₃ containing stream.

27. The process as claimed in claim 25, wherein said alkylene is at least one selected from the group consisting of methyl acetylene, acetylene; preferably methyl acetylene.

28. The process as claimed in claim 25, wherein said di-olefin is at least one selected from the group consisting of propadiene, vinyl-acetylene; preferably propadiene.

29. The process as claimed in claim 25, wherein selective hydrogenation is carried out at a temperature in the range of from 10 °C to 40 °C, at a pressure in the range of from 10 to 40 atmospheres and with a liquid hourly space velocity varying in the range of , from 10 to 30 h^"1.