Description:PROCESS FOR FABRICATING HIGHLY DISPERSED METAL LOADED NANOCATALYSTS FOR SELECTIVE HYDROGENATION

FIELD OF INVENTION

The present disclosure relates to heterogenous catalysts and more particularly relates to a process for fabricating highly dispersed metal loaded nanocatalysts for selective hydrogenation of polyunsaturated compounds.

BACKGROUND

Heterogeneous catalysis is a fundamentally important and industrially critical process which creates the basis of sustainable future with minimal utilization of energy, time, waste generation and resources. It offers several potential advantages such as recyclability, tunable selectivity, and facile scale up. However, a major challenge lies in achieving higher selectivity and faster kinetics while improving recyclability by inhibiting deactivation of the catalyst. The difficulty lies in overall optimization of the multi-variable catalytic process and at the same time aiming for greater sustainability.

Conventional approaches in this direction have focused on utilization of nanoparticles, including single active metals/alloys to create doped nanostructures with precisely tuned crystal facets to achieve greater density of active sites, resulting to higher selectivity of reaction with faster kinetics. A persistent drawback of all such strategies is the high surface free energy of nanostructured materials that results in intense aggregation and higher propensity for catalytic poisoning, leading to rapid loss of initial high activity.

Various carbon derivatives (such as: carbon black, graphite, graphene, carbon nanotubes) have been employed to overcome such challenges, that provide favourable adhesion energies overcoming the cohesive forces causing aggregation or deactivation of catalysts. Herein, while nanocarbon supports provide large accessible surface area and electrical conductivity, their soft graphitizable nature reduces the catalytic activity upon both continued cycling and prolonged exposure.

A viable solution for overcoming such disadvantages is utilizing chemically and thermally stable hard carbon materials as catalyst supports. However, there is lack of synthetic design principles to prepare monodispersed and accessible surfaces with such hard carbons which is a major roadblock for their potential application as active catalytic supports.

The existing transition metal-based catalysts exhibit good catalytic activity for hydrogenation reactions, yet most of them show poor stability with a common issue of sintering/agglomeration of the active sites which strongly depends on the interfacial interaction between the support and the catalyst.

Thus, there remains a need in the art to develop a highly dispersed metal nanocatalysts secured by high surface area and porous support material which can provide thermal, chemical stability and durability for commercial hydrogenation applications.

OBJECT OF THE INVENTION

It is the primary object of the present disclosure to provide a process for fabricating highly dispersed metal loaded nanocatalysts stabilized by specially designed support material for selective hydrogenation of polyunsaturated compounds.

SUMMARY OF THE INVENTION

In an aspect of the present disclosure, a process for fabricating highly dispersed metal loaded nanocatalysts for selective hydrogenation of polyunsaturated compounds. In particular, a process for fabrication of metal loaded nanocatalysts tunably anchored on a nanostructured hard carbon support for selective hydrogenation of polyunsaturated compounds is disclosed. The process for fabricating metal loaded nanocatalysts on the nanostructured hard carbon support by embedding a plurality of metal nanoparticles on and within the matrix of a high surface area porous nanostructured hard carbon material is been disclosed.

In an aspect of the present disclosure, a process for fabricating highly dispersed heterogenous metal loaded nanocatalysts for selective hydrogenation of polyunsaturated compounds by embedding the plurality of metal nanoparticles within the matrix of porous hard-carbon nanostructure support is been disclosed. The step of embedding the plurality of metal nanoparticles within the matrix of porous hard-carbon nanostructure support comprises the steps of:
adding a metal precursor solution with a concentration in the range of 0.1mM - 0.5 mM to 10 mg – 15mg of silica-based template;
stirring the resultant solution of step (a) continuously at a speed in the range of 900 rpm to 1100 rpm for a time period of 5h to 7h for the uniform distribution of the plurality of metal nanoparticles on the surface of silica-based template;
washing the metal nanoparticles loaded silica-based template solution obtained in step (b) with de-ionised water;
air-drying the washed metal nanoparticles loaded silica-based template overnight or heating the washed nanoparticles loaded silica-based template at a temperature in the range of 80°C to 85°C, thereby forming a powder;
reducing the powder obtained in step (d) in a tube furnace at a temperature in the range of 295°C to 305°C for a time period in the range of 2h to 2.5h in presence of oxygen free 98% to 99.9% pure hydrogen atmosphere at a flow rate in the range of 100 sccm to 300 sccm;
collecting the reduced powder obtained in step (e) in an alumina boat and heating the reduced powder in a tube furnace at a temperature in the range of 735°C to 745°C with a ramp rate of 10°Cmin-1 in presence of helium or any equivalent inert, non-reducing, non-oxidising atmosphere at a flow rate in the range of 100 sccm to 300 sccm;
subjecting the hot powder obtained in step (f) to gas-phase chemical vapor deposition of carbon over the metal loaded silica-based template in presence of a carbon precursor at a flowrate in the range of 100 sccm to 300 sccm at a temperature in the range of 735°C to 745°C for a time period in the range of 3 minutes to 13 minutes thereby forming powder; and
etching out the silica-based template in the powder catalysts of step (g) in presence of an alkali thereby resulting in the formation of highly dispersed heterogenous metal loaded nanocatalysts.

In an aspect of the present invention, the step of etching out the silica-based template in presence of an alkali further comprises the steps of:
dispersing the powder obtained in step (g) in an etching solution of concentration ranging from 1M to 5 M for a time period in the range of 5h to 6h;
centrifuging the resultant solution and washing metal loaded porous hard-carbon nanostructure with deionized water till the pH turns neutral resulting in a wet black precipitate;
drying the wet black precipitate at a temperature of 80°C to 85°C in a hot air oven followed by drying under super-critical CO2.

In another aspect of the present disclosure, a process for fabricating highly dispersed heterogenous metal loaded nanocatalysts for selective hydrogenation of polyunsaturated compounds by embedding the plurality of metal nanoparticles on the external surface of porous hard-carbon nanostructure support. The step of embedding the plurality of metal nanoparticles further comprises the steps of:
adding a metal precursor solution with a concentration in the range of 0.1 mM - 0.5 mM to 10 mg -15 mg of porous hard-carbon nanostructure support;
stirring the resultant solution of step (a) continuously at a speed in the range of 600 rpm to 800 rpm for time period of 5 h to 7 h for the uniform loading of the plurality of metal nanoparticles on the surface of porous hard-carbon nanostructure support;
washing the metal nanoparticles loaded porous hard-carbon nanostructure support solution obtained in step (b) with de-ionized water;
air-drying the washed metal nanoparticles loaded porous hard-carbon nanostructure support overnight or heating the washed metal nanoparticles loaded porous hard-carbon nanostructure support at a temperature in the range of 80°C to 85°C, thereby forming a powder;
collecting the powder obtained in step (d) in an alumina boat and reducing the powder in a tube furnace at a temperature in the range of 295°C to 305°C for a time period of 2h to 2.5h in presence of oxygen free 98% to 99.9% pure hydrogen atmosphere at a flow rate in the range of 100 sccm to 300 sccm.
cooling the reduced powder obtained in step (e) to room temperature resulting in the formation of highly dispersed heterogenous metal loaded nanocatalysts.

In an aspect of the present invention, the porous hard-carbon nanostructure as a catalyst support is nanocarbon florets (NCF).

In an aspect of the present invention, the metal nanoparticle comprises of metals selected from platinum, palladium, nickel, cobalt, iron, gold, silver, ruthenium, rhodium, iridium, and copper.

In an aspect of the present invention, the etching solution is selected from a group comprising of sodium hydroxide (NaOH), potassium hydroxide (KOH), cesium hydroxide (CsOH), buffered hydrogen fluoride (BHF), and concentrated hydrofluoric acid (HF).

In an aspect of the present invention, the carbon precursor is selected from a group comprising of acetylene, methane, carbon dioxide, carbon monoxide, ethanol, isopropanol, butane, and isobutane.

In an aspect of the present invention, the silica-based template is dendritic fibrous nanosilica (DFNS) of varying textural and surface properties.

In an aspect of the present disclosure, the metal precursor is selected from a group comprising of metal chlorides, metal chlorides salts, metal acetates, metal nitrates, metal sulphates, metal carbonates that are water soluble.

These and other objects, features, and advantages will become apparent to those of ordinary skill in the art from a reading of the following detailed description.

BRIEF DESCRIPTION OF DRAWINGS

The exemplary embodiments of the present invention have been described with reference to the accompanying drawings below:

Figure 1 illustrates the SEM images of pristine NCF in accordance with an embodiment of the present invention.

Figures 2 (a) - 2 (e) illustrate the surface area analysis, FT-IR analysis, Raman analysis, NEXAFS analysis and XRD analysis of pristine NCF respectively in accordance with an embodiment of the present invention.

Figure 3 (a) illustrates the SEM and TEM images of PON, in accordance with an embodiment of the present invention.

Figure 3(b) illustrates the SEM images of PIN 3, PIN 10 and PIN 13 catalysts in accordance with an embodiment of the present invention.

Figure 3(c) illustrates the TEM images of PIN 3, PIN 10 and PIN 13 catalysts respectively in accordance with an embodiment of the present invention.

Figures 4 (a) –4 (b) illustrate the surface area analysis of PON and PIN 3 catalysts in accordance with an embodiment of the present invention.

Figures 4 (c) – 4 (d) illustrate the XPS analysis of PON and PIN 3 catalysts, respectively in accordance with an embodiment of the present invention.

Figure 5 (a) illustrates the schematic of styrene hydrogenation reaction in accordance with an embodiment of the present invention.

Figure 5 (b) illustrates a 50ml custom-built reactor in accordance with an embodiment of the present invention.

Figures 5 (c) – 5 (d) illustrate the kinetic trends for various Pt-NCF catalysts in comparison with commercial Pt/C catalyst in accordance with an embodiment of the present invention.

Figures 6 (a) – 6 (b) illustrate the catalytic activity and recyclability of PON catalysts for styrene hydrogenation reaction in accordance with an embodiment of the present invention.

Figures 6 (c) – 6 (d) illustrate the TEM images of spent PON catalysts after cycle 1 and cycle 6 respectively in accordance with an embodiment of the present invention.

Figure 6 (e) illustrates the Raman analysis of spent PON catalysts for 6 cycles in accordance with an embodiment of the present invention.

Figure 7 illustrates the turnover frequency (TOF) comparison of various commercially available Pt-based catalysts with the specific heterogeneous catalysts in accordance with an embodiment of the present invention.

Figures 8 (a) – 8 (b) illustrate the H2-TPD analysis of various Pt-NCF catalysts in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described in detail below with reference to the drawings and specific embodiments. This embodiment is implemented on the premise of the technical solution of the present invention, and detailed implementation manners and specific operation procedures are given, but the scope of protection of the present invention is not limited to the following embodiments.

In the present invention, a process for fabricating metal loaded nanocatalysts on a nanostructured hard carbon support for selective hydrogenation of polyunsaturated compounds is disclosed.

In an embodiment of the present invention, the process for fabricating metal loaded nanocatalysts on the nanostructured hard carbon support by embedding a plurality of metal nanoparticles on and below a high surface area porous nanostructured hard carbon material, called Nanocarbon Florets (NCF), as a catalyst support is disclosed. The three-dimensional well-defined lamellae of NCF forms open-ended conical microcavities providing a graded pore structure that facilitates faster diffusional access of the reactants and products.

In an embodiment of the present invention, process for fabricating highly dispersed heterogenous metal loaded nanocatalysts for selective hydrogenation of polyunsaturated compounds by embedding the plurality of metal nanoparticles within the matrix of porous hard-carbon nanostructure support. The step of embedding the plurality of metal nanoparticles comprises the steps of:
adding a metal precursor solution with a concentration in the range of 0.1mM - 0.5 mM to 10 mg – 15mg of silica-based template;
stirring the resultant solution of step (a) continuously at a speed in the range of 900 rpm to 1100 rpm for a time period of 5h to 7h for the uniform distribution of the plurality of metal nanoparticles on the surface of silica-based template;
washing the metal nanoparticles loaded silica-based template solution obtained in step (b) with de-ionised water;
air-drying the washed metal nanoparticles loaded silica-based template overnight or heating the washed nanoparticles loaded silica-based template at a temperature in the range of 80°C to 85°C, thereby forming a powder;
reducing the powder obtained in step (d) in a tube furnace at a temperature in the range of 295°C to 305°C for a time period in the range of 2h to 2.5h in presence of oxygen free 98% to 99.9% pure hydrogen atmosphere at a flow rate in the range of 100 sccm to 300 sccm;
collecting the reduced powder obtained in step (e) in an alumina boat and heating the reduced powder in a tube furnace at a temperature in the range of 735°C to 745°C with a ramp rate of 10°Cmin-1 in presence of helium or any equivalent inert, non-reducing, non-oxidising atmosphere at a flow rate in the range of 100 sccm to 300 sccm;
subjecting the hot powder obtained in step (f) to gas-phase chemical vapor deposition of carbon over the metal loaded silica-based template in presence of a carbon precursor at a flowrate in the range of 100 sccm to 300 sccm at a temperature in the range of 735°C to 745°C for a time period in the range of 3 minutes to 13 minutes thereby forming powder; and
etching out the silica-based template in the powder catalysts of step (g) in presence of an alkali thereby resulting in the formation of highly dispersed heterogenous metal loaded nanocatalysts.

In an embodiment of the present invention, the step of etching out the silica-based template in presence of an alkali further comprises the steps of:
dispersing the powder obtained in step (g) in an etching solution of concentration ranging from 1M to 5 M for a time period in the range of 5h to 6h;
centrifuging the resultant solution and washing metal loaded porous hard-carbon nanostructure with deionized water till the pH turns neutral resulting in a wet black precipitate;
drying the wet black precipitate at a temperature of 80°C to 85°C in a hot air oven followed by drying under super-critical CO2.

In an embodiment of the present invention, a process for fabricating highly dispersed heterogenous metal loaded nanocatalysts for selective hydrogenation of polyunsaturated compounds by embedding the plurality of metal nanoparticles on the external surface of porous hard-carbon nanostructure support. The step of embedding the plurality of metal nanoparticles further comprises the steps of:
adding a metal precursor solution with a concentration in the range of 0.1mM - 0.5 mM to 10 mg -15 mg of porous hard-carbon nanostructure support;
stirring the resultant solution of step (a) continuously at a speed in the range of 600 rpm to 800 rpm for time period of 5 h to 7 h for the uniform loading of the plurality of metal nanoparticles on the surface of porous hard-carbon nanostructure support;
washing the metal nanoparticles loaded porous hard-carbon nanostructure support solution obtained in step (b) with de-ionized water;
air-drying the washed metal nanoparticles loaded porous hard-carbon nanostructure support overnight or heating the washed metal nanoparticles loaded porous hard-carbon nanostructure support at a temperature in the range of 80°C to 85°C, thereby forming a powder;
collecting the powder obtained in step (d) in an alumina boat and reducing the powder in a tube furnace at a temperature in the range of 295°C to 305°C for a time period of 2h to 2.5h in presence of oxygen free 98% to 99.9% pure hydrogen atmosphere at a flow rate in the range of 100 sccm- to 300 sccm.
cooling the reduced powder obtained in step (e) to room temperature resulting in the formation of highly dispersed heterogenous metal loaded nanocatalysts.

In an embodiment of the present invention, the porous hard-carbon nanostructure as a catalyst support is nanocarbon florets (NCF).

In an embodiment of the present invention, the metal nanoparticle comprises of metals selected from platinum, palladium, nickel, cobalt, iron, gold, silver, ruthenium, rhodium, iridium, and copper.

In an embodiment of the present invention, the etching solution is selected from a group comprising of sodium hydroxide (NaOH), potassium hydroxide (KOH), cesium hydroxide (CsOH), buffered hydrogen fluoride (BHF), and concentrated hydrofluoric acid (HF).

In an embodiment of the present invention, the carbon precursor is selected from a group comprising of acetylene, methane, carbon dioxide, carbon monoxide, ethanol, isopropanol, butane, and isobutane.

In an embodiment of the present invention, the silica-based template is dendritic fibrous nanosilica (DFNS) of varying textural and surface properties.

In an embodiment of the present disclosure, the metal precursor is selected from a group comprising of metal chlorides, metal chlorides salts, metal acetates, metal nitrates, metal sulphates, metal carbonates that are water soluble.

The high diffusion coefficient (~0.9 cm2 s-1 at 1013 K) of the gas-phase carbon precursors obtained through the thermal CVD (chemical vapor deposition) route ensures complete infiltration into the micro and mesopores of DFNS during the deposition.

In an embodiment of the present invention, the process for fabricating metal loaded nanocatalysts by embedding the plurality of metal nanoparticles on the surface of the hard nanostructured NCF has been disclosed, thereby the present invention provides an efficient accessibility of the reactants.

In another embodiment of the present invention, the process for fabricating metal loaded nanocatalysts by embedding the plurality of metal nanoparticles within the hard nanostructured NCF matrix has been disclosed, thereby the present invention tunes the thickness of the carbon matrix by varying the carbon deposition time.

In an embodiment of the present invention, the hydroxy terminations of the NCF are configured as an anchoring point for the fabrication of the metal loaded nanocatalysts, in particular, for the fabrication of metal loaded nanocatalysts.

Working Example 1: Design of synergistic Support-catalyst systems:

The high surface area porous nanostructured carbon support NCF is manufactured through a templated synthesis process comprising the steps of heating a pre-defined amount of open-ended dendritic fibrous nano-silica (DFNS) in an alumina boat, placed inside a quartz tube furnace. The DFNS is heated upto a temperature of 740°C with a ramp rate of 10°Cmin-1 in presence of helium atmosphere at a flow rate of 100 sccm. Further, subjecting the hot DFNS to gas-phase chemical vapor deposition at 740°C, thereby adding acetylene as a source of carbon at a flowrate of 100 sccm for 10 minutes. Subsequently, cooling the resultant solution to room temperature and removing the silica template through constant stirring in 1M NaOH solution for a time period of 5h – 6h. Centrifuging the cooled resultant solution and washing the precipitate with deionized (DI) water until the pH becomes neutral. Finally drying the wet black precipitate at 80°C in a hot-air oven and subsequently in a super-critical CO2 drier thereby forming Nanocarbon Florets (NCF).

Working Example 2: Synthesis of platinum on NCF (PON) catalyst:

The process for fabricating metal loaded nanocatalysts by embedding the plurality of metal nanoparticles on the external surface of the NCF comprises the steps of preparing a platinum precursor solution comprising of 0.1mM of chloroplatinic acid hydrate (H2PtCl6, xH2O). Then adding a pre-defined volume of the platinum precursor to a pre-defined amount of NCF. Stirring the resultant mixture continuously at 700 rpm for 5h for uniform loading of the plurality of the platinum nanoparticles on the surface of NCF. Further, washing the platinum nanoparticles loaded NCF solution in de-ionised water and air drying the washed platinum nanoparticles loaded NCF at 80°C for overnight thereby forming a powder. Collecting the dried powder on an alumina boat and reducing the dried powder in a tube furnace at a temperature of 300°C for 2h under a constant flow of hydrogen at a flow rate of 100 sccm. Cooling the reduced dried powder to room temperature thereby forming Platinum on NCF (PON) catalyst with a pre-defined amount of metal loading.

Working Example 3: Synthesis of platinum in NCF (PIN) catalyst:

The process for fabricating metal loaded nanocatalysts by embedding the plurality of metal nanoparticles within the NCF matrix comprises the steps of adding a pre-defined amount of DFNS to a pre-defined amount of 0.1 mM solution of chloroplatinic acid and stirring the resultant mixture continuously at 1000 rpm for 5h for uniform distribution of small platinum nanoparticles on the surface of DFNS. Further, washing the platinum nanoparticles loaded DFNS solution in de-ionised water and air-drying the washed platinum nanoparticles loaded DFNS. Reducing the dried powder sample in a tube furnace at a temperature of 300°C for 2h under a constant flow of hydrogen at a flow rate of 100 sccm. Collecting the reduced powder sample on an alumina boat and heating the reduced powder sample up to a temperature of 740°C with a ramp rate of 10°Cmin-1 under a constant flow of helium at a flow rate of 100 SCCM. Further, subjecting the hot powder sample to gas-phase chemical vapor deposition of carbon with acetylene as a carbon precursor at a flowrate of 100 sccm at a temperature of 740°C for a duration of 3 minutes, 10 minutes and 13 minutes thereby forming Platinum in NCF (PIN) catalysts with varied carbon thickness, i.e., PIN 3, PIN 10 and PIN 13 catalysts respectively. Etching out the silica template through constant stirring in 1M NaOH solution for a time period of 5 h – 6 h. Centrifuging the resultant solution and washing the precipitate with deionized (DI) water until the pH of the supernatant becomes neutral. Finally drying the wet black precipitate at 80°C in a hot-air oven and under super-critical CO2 thereby forming three different variants of PIN catalyst.

Characterization of NCF support and Platinum-NCF catalysts, Pt-NCF (PON and PIN variants):

Referring to figure 1, illustrates the Scanning Electron Microscope (SEM) images of pristine NCF in accordance with an embodiment of the present invention. The three-dimensional well-defined lamellae of NCF (mean diameter ? 400 ± 5 nm) forming a graded pore structure facilitates faster diffusional access of the reactants and products.

Referring to figures 2 (a)- 2(d), illustrate the surface area analysis, Fourier transform infrared (FT-IR) analysis, Raman analysis, Near Edge X-Ray Absorption Fine Structure (NEXAFS) analysis and X-Ray diffraction (XRD) analysis of pristine NCF respectively in accordance with an embodiment of the present invention.

In particular, figure 2(a) illustrates that the Pt-NCF catalysts of the present invention exhibits a high specific surface area (936 m2/g) that is composed of a combination of micro and mesoporous network, resulting in a pore volume of 0.44 cm3/g.

Figure 2(b) illustrates that during the synthesis process, the alkaline etching introduces hydroxy and carboxy terminations on NCF making it hydrophilic in nature.

Figure 2(c) illustrates the structural analysis of NCF through vibrational spectroscopy and illustrates the presence of prominent D-band (1350 cm-1) and G-band (1630 cm-1) that are characteristic of sp2 carbon framework. While the G-band originates from in-plane tangential vibration modes, the origin of the D-band is attributed to the disorder induced Double Resonance Raman (DRR) scattering process. This is further confirmed from monotonic shift in the spectral position of the D-band on changing the excitation laser from 1350 cm-1 at 532 nm to 1323 cm-1 at 633 nm, while the G-band remains invariant (1593 cm-1). Thus, NCF is composed of short-range graphitic domains that are disordered in the long range.

Figure 2(d) illustrates that the NEXAFS C1s spectra of NCF exhibits characteristic peaks at 285.4 eV and 288.5 eV along with broader features between 292 eV – 307 eV. The intensity of p* C=C are distinctly lower for NCF compared to HOPG (Highly Oriented Pyrolytic Graphene as reference), indicating the discontinuous nature of the graphitic basal planes. Such short-range ordering with long range disorder is further substantiated by the broad s* resonance peaks in NCF, PON and PIN variants. The shoulders at 284.3 eV and 287.2 eV are prominent in all these three samples and are attributed to the edge functionalities and the C-O-C cross-linkages. A quantitative analysis of the p* C=C reveals 65% of graphitic C=C structure with the remaining coming from the structural disorder.

Figure 2(e) illustrates the powder x-ray diffractogram analysis of pristine NCF. In particular, figure 2(e) exhibits reflections corresponding to (002) and (101) facets at 220 and 430, respectively. The broad nature and the lowering in intensity of the (002) reflection planes (FWHM ? 0.86, 2? = 220) confirms the long-range disorder and the expanded d-spacing (0.406 nm), both of which contribute to enhanced pore volume and diffusional accessibility of the catalysts.

Referring to Figure 3(a), illustrates the SEM (scanning electron microscope) and TEM (transmission electron microscopy) images of PON, Figure 3(b), illustrates the SEM images of PIN 3, PIN 10 and PIN 13 catalysts respectively and Figure 3(c), illustrates the TEM images of PIN 3, PIN 10 and PIN 13 catalysts respectively in accordance with an embodiment of the present invention. The Pt-NCF catalysts result in negligible changes in the morphology of NCF with identical dimensions of monodispersed platinum nanoparticles in the range of 5 nm -7 nm in size.

Referring to figures 4 (a) – 4 (b), illustrate the surface area analysis of PON and PIN 3 catalysts. The surface area and pore structure were similar for all the platinum loaded catalysts (both PIN and PON ? 350 m2/g) and significantly lower than the pristine NCF (936 m2/g). The decrease in surface area is attributed to the loading of the platinum nanoparticles.

Referring to figures 4 (c) – 4 (d), illustrate the x-ray photoelectron spectra (XPS) analysis of PON, PIN 3, PIN 10 and PIN 13 catalysts, respectively in accordance with an embodiment of the present invention. In particular figure 4 (c-d) illustrates the variation in spatial distribution of platinum nanoparticles and the difference in chemical environment around the platinum nanoparticles is evidenced through x-ray photoelectron spectra (XPS). The XPS in the platinum region of PON exhibits two strong peaks at values 71.8 eV and 75.3 eV corresponding to 4f7/2 of Pt (0) and 4f5/2 of PtO2, respectively.

For PIN catalysts, a prominent peak at 71.2 eV is observed corresponding to 4f7/2 of Pt (0). In addition, the 4f5/2 of PtO2 is observed due to the anchoring of the Pt nanoparticles through the oxygen functionality of NCF. The shake-up satellite observed at ~76.4 eV corresponds to the metallic state of Pt and is therefore increasingly observed in case of PIN 13 compared to PIN 3.

Heterogeneous catalytic activity of PON and PIN.

Referring to Figure 5 (a), illustrates the schematic of styrene hydrogenation reaction. The similarity in surface area, porosity and chemical constitution permits direct comparison of the heterogeneous catalytic activity of various Pt-NCF catalysts towards a model reaction with industrial relevance comprising styrene hydrogenation.

Referring to Figure 5 (b), illustrates a 50ml custom-built reactor. In an embodiment of the present invention the PON, PIN 3, PIN 10 and PIN 13 catalysts were evaluated for their heterogenous catalytic activities in the custom-built reactor illustrated in Figure 5 (b). The Teflon lined inner cavity of the custom-built reactor is fitted with solenoid valve regulator configured for controlling the pressure of hydrogen present inside the reaction chamber. A pressure regulator, thermocouple and vent valve are coupled to the hermetically sealed SS304 reaction chamber configured for controlling the pressure, monitoring the temperature, and accessing the progress of the reaction.

In a standard experiment of styrene hydrogenation reaction the steps comprise of, adding a pre-defined amount of as prepared PON or PIN catalyst and a pre-define volume of 20% styrene solution in isopropanol into the custom-built reactor with a substrate to catalyst ratio of 10000:1. Then sealing the custom built reactor and flushing with H2 (99.99% purity) gas three times thereby removing the air inside the custom-built reactor and maintaining a constant pressure of 7 bar throughout styrene hydrogenation reaction inside the custom-built reactor. Subsequently, heating the custom-built reactor to a temperature of 100°C and subjecting the reactants to a constant stirring at 550 rpm throughout the reaction. Monitoring the progress of the reaction by withdrawing aliquots from the reaction chamber at arbitrary time durations and subjecting the reaction mixture to Gas chromatography–mass spectrometry (GC-MS) and Nuclear magnetic resonance (NMR) spectroscopy analyses. Cooling the custom-built reactor in an ice-water bath after the completion of the reaction time and depressurize slowly. Collecting the liquid product and catalyst mixture and separating by centrifugation at 12000 rpm with subsequent washing with a solvent.

The total conversion of styrene and selectivity of ethylbenzene are calculated using the following formulae:

Styrene conversion (%) = ((Moles of Styrene)in-(Moles of Styrene)out)/((Moles of Styrene)in)*100….(1)

Ethylbenzene selectivity (%) =(Moles of ethylbenzene)/(?"(Moles of products)" )*100……………… (2)

The Turnover Frequency (TOF) is estimated as:
Turnover Frequency (h-1) = (Moles of ethylbenzene)/"(Moles of active metal) * (reaction time (h))" ……...…(3)

Referring to figures 5 (c) – 5 (d), illustrate the kinetic trends for various Pt-NCF catalysts in comparison with commercial Pt/C catalyst in accordance with an embodiment of the present invention. The PIN 3, PIN 10 and PIN 13 catalysts of the present invention exhibit superior kinetics as compared to the commercial Pt/C catalyst. Particularly the PIN 3 catalyst illustrates superior kinetics of 4.4 M.min-1, for 20 min-90 min of reaction and a selectivity of 99% towards ethylbenzene. The PON, PIN 10, PIN 13 catalysts and commercial Pt/C illustrated a kinetics of 3.7 M.min-1, 4.2 M.min-1, 3.6 M.min-1and 4.1 M.min-1 respectively. Thereby, the kinetic order among the PIN variants follows PIN 3> PIN 10> PIN 13.

The PIN 3 catalyst of the present invention exhibits a unique combination of both high kinetic facility and superior selectivity towards ethylbenzene, among all the catalysts probed including commercial Pt/C. This observation indicates the active role of NCF in driving the hydrogenation reaction towards both faster kinetics and higher selectivity.

The higher kinetics of the catalysts of the present invention is evident from the attainment of faster saturation and completion of reaction as illustrated in figure 5 (d) of the present invention. In particular, figure 5(d) illustrates the overall kinetics order of the PIN3, PIN 10, PIN 13 and PON catalysts of the present invention including commercial Pt/C. Specifically the PIN 3 catalyst of the present invention attains faster saturation (within 40 minutes of the reaction). The kinetics order illustrated in figure 5(d) is PIN 3 > PIN 10 > commercial Pt/C ? PIN 13 ? PON. This kinetics order of the catalyst indicates that an optimal overlayer deposition of hard carbon is critical to achieve the desired kinetics and catalytic selectivity. The selectivity of commercial Pt/C catalyst is found to decrease with time leading to the fully hydrogenated product ethylcyclohexane (ECH) beyond 60 min of reaction. The progress of reaction towards ethylcyclohexane (ECH) is not observed with any of the PIN catalyst variants of the present invention.

Referring to figures 6 (a) – 6 (b), illustrate the catalytic activity and recyclability of PON catalysts for styrene hydrogenation reaction respectively. In particular, figures 6 (a) – 6 (b) illustrate that the conversion towards ethylbenzene exhibits a time-dependent increase for PON catalyst and the selectivity of the reaction is consistently higher throughout the observed time scale. Further, the kinetics of catalytic conversion follows a first order kinetics with negligible by-products formation over the entire course of the reaction.

Referring to figures 6 (c) – 6 (d) illustrate the high-angle annular dark-field imaging of PON catalysts in the first and sixth cycle of catalysis respectively in accordance with an embodiment of the present invention. In particular, figures 6 (c) – figure 6 (d), illustrate the negligible leaching and aggregation of platinum nanoparticles in the catalyst of the present invention during the catalysis and is supported by the stability of conversion and selectivity observed with repeated cycling in the catalysts of the present invention.

Referring to figure 6 (e), illustrates the Raman analysis of spent catalysts in accordance with an embodiment of the present invention. the Raman spectra of the spent catalyst illustrated by figure 6(e) illustrates the presence of prominent D-band at 1312 cm-1 and G-band at 1568 cm-1. Figure 6(e) further illustrates that the peak position and the peak structure in the Raman spectra are constant for all the samples thereby confirming the integrity of NCF and the catalysts of the present invention (PON, PIN3, PIN 10 and PIN 13) during the repeated catalytic cycles.

Referring to figure 7, illustrates the turnover frequency (TOF) comparison of various commercially available Pt-based catalysts with the specific heterogeneous catalysts in accordance with an embodiment of the present invention. In particular, figure 7 illustrates that the TOF obtained with PIN 3 is 360062 h-1 at ambient pressure and constitutes the highest among all the known catalytic materials for olefin hydrogenation including commercial Pt/C

Referring to Figures 8 (a) – 8 (b), illustrate the H2-TPD analysis of various Pt-NCF catalysts in accordance with an embodiment of the present invention.

In an embodiment of the present invention, the hydrogen uptake by the PON, PIN3, PIN 10 and PIN 13 catalysts have been analyzed thereby illustrating the role of carbon overlayer in establishing higher catalytic activity. Figure 8(a) illustrates that the PIN 3 catalyst exhibits the highest hydrogen uptake of 0.61 mmol H2 gcat-1 among all the other catalyst variants. Further, figure 8(b) illustrates that the hydrogen uptake by the catalyst is proportional with the thickness of carbon layer. The hydrogen uptake by PIN 13 catalyst is 1.75 mmol H2 gcat-1 which is lower than that of PON catalyst 1.95 mmol H2 gcat-1.

In an embodiment of the present invention, discloses the green chemistry matrices for the styrene hydrogenation reaction on Pt-NCF catalysts, under the optimized reaction condition of constant temperature of 373 K, constant pressure of 7 bar and 1.5 hours of reaction time. The three important parameters for the quantifying the green chemistry metrices are:

E-factor: E-factor represents the environmental impact on the sustainability of a chemical conversion and measures the amount of waste generated by a chemical process relative to the amount of product obtained. E-factor is quantified by the following formula. The ideal value of E-factor is 0, corresponding to zero waste production through the process.

(?"Mass of reactant -" ?"Mass of product " )/(?"Mass of product" ) ………………………...…… (4)

Process Mass Intensity (PMI): PMI measures the efficiency of conversion of the reactants into desired products. PMI is quantified by the following formula and the ideal value of PMI is 1.

………………………...…… (5)

Carbon efficiency (CE): CE is the extent to which a given level of output is produced with minimum feasible carbon emissions relative to direct sector peers.

…. (6)

In another embodiment of the present invention, quantitative comparison of the PIN 3 and PON catalysts yield consistently near-ideal values of E-factor, process mass intensity (PMI) and carbon efficiency (CE) for both PIN and PON catalysts. The E-factor, process mass intensity (PMI) and carbon efficiency (CE) of PIN 3 catalyst is 0.20, 1.18 and 92% respectively. The E-factor, process mass intensity (PMI) and carbon efficiency (CE) of PON catalysts are 0.22, 1.22 and 89% respectively.

The quantitative comparison of the PIN 3 and PON catalysts of the present invention establishes the sustainability of the PIN and PON catalysts in the NCF based catalytic conversions.

The present invention is advantages over existing process as the platinum loaded hard carbon catalysts of the present invention has an unprecedented turnover frequency (TOF) of 360062 h-1 and a selectivity of 99% towards mono-hydrogenation of styrene to ethylbenzene at an ambient pressure and a temperature of 100°C. The nanostructured platinum catalysts embedded within the NCF matrix (PIN 3) exhibits significantly improved catalytic activity over the PON catalyst embedded on the surface of the NCF and the commercial Pt/C. The engineered heterogenous catalysts of the present invention are reusable and subscribed to the green matrices of chemical conversions.

The catalysts of the present invention play a vital role in process technology, in particular hydrogenation catalysts contributing significantly for a large variety of industrial applications in oil, gas, refining and pharmacy.

Further the platinum loaded hard carbon based PON and PIN catalysts of the present invention combine the mutually exclusive properties of high catalytic kinetics with high yield and selectivity as compared to commercial state-of-art Pt/C catalysts for styrene hydrogenation reaction.

While the present invention has been described above in terms of specific embodiments, it is to be understood that the invention is not intended to be confined or limited to the embodiment disclosed herein.
, Claims:We Claim,

1. A process for fabricating highly dispersed heterogenous metal loaded nanocatalysts for selective hydrogenation of polyunsaturated compounds, the process comprising the steps of:
a) embedding the plurality of metal nanoparticles within the matrix of porous hard-carbon nanostructure support; and
b) embedding the plurality of metal nanoparticles on the external surface of porous hard-carbon nanostructure support.

2. The process as claimed in claim1, wherein embedding the plurality of metal nanoparticles within the matrix of porous hard-carbon nanostructure support comprises the steps of:
a) adding a metal precursor solution with a concentration in the range of 0.1mM - 0.5 mM to 10 mg – 15mg of silica-based template;
b) stirring the resultant solution of step (a) continuously at a speed in the range of 900 rpm to 1100 rpm for a time period of 5h to 7h for the uniform distribution of the plurality of metal nanoparticles on the surface of silica-based template;
c) washing the metal nanoparticles loaded silica-based template solution obtained in step (b) with de-ionised water;
d) air-drying the washed metal nanoparticles loaded silica-based template overnight or heating the washed nanoparticles loaded silica-based template at a temperature in the range of 80°C to 85°C, thereby forming a powder;
e) reducing the powder obtained in step (d) in a tube furnace at a temperature in the range of 295°C to 305°C for a time period in the range of 2h to 2.5h in presence of oxygen free 98% to 99.9% pure hydrogen atmosphere at a flow rate in the range of 100 sccm to 300 sccm;
f) collecting the reduced powder obtained in step (e) in an alumina boat and heating the reduced powder in a tube furnace at a temperature in the range of 735°C to 745°C with a ramp rate of 10°Cmin-1 in presence of helium or any equivalent inert, non-reducing, non-oxidising atmosphere at a flow rate in the range of 100 sccm to 300 sccm;
g) subjecting the hot powder obtained in step (f) to gas-phase chemical vapor deposition of carbon over the metal loaded silica-based template in presence of a carbon precursor at a flowrate in the range of 100 sccm to 300 sccm at a temperature in the range of 735°C to 745°C for a time period in the range of 3 minutes to 13 minutes thereby forming powder; and
h) etching out the silica-based template in the powder catalysts of step (g) in presence of an alkali thereby resulting in the formation of highly dispersed heterogenous metal loaded nanocatalysts.

3. The process as claimed in claim 2, wherein etching out the silica-based template in presence of an alkali comprises the steps of:
d) dispersing the powder obtained in step (g) in an etching solution of concentration ranging from 1M to 5 M for a time period in the range of 5h to 6h;
e) centrifuging the resultant solution and washing metal loaded porous hard-carbon nanostructure with deionized water till the pH turns neutral resulting in a wet black precipitate;
f) drying the wet black precipitate at a temperature of 80°C to 85°C in a hot air oven followed by drying under super-critical CO2.

4. The process as claimed in claim 1, wherein embedding the plurality of metal nanoparticles on the external surface of porous hard-carbon nanostructure support comprises the steps of:
a) adding a metal precursor solution with a concentration in the range of 0.1mM - 0.5 mM to 10 mg -15 mg of porous hard-carbon nanostructure support;
b) stirring the resultant solution of step (a) continuously at a speed in the range of 600 rpm to 800 rpm for time period of 5 h to 7 h for the uniform loading of the plurality of metal nanoparticles on the surface of porous hard-carbon nanostructure support;
c) washing the metal nanoparticles loaded porous hard-carbon nanostructure support solution obtained in step (b) with de-ionized water;
d) air-drying the washed metal nanoparticles loaded porous hard-carbon nanostructure support overnight or heating the washed metal nanoparticles loaded porous hard-carbon nanostructure support at a temperature in the range of 80°C to 85°C, thereby forming a powder;
e) collecting the powder obtained in step (d) in an alumina boat and reducing the powder in a tube furnace at a temperature in the range of 295°C to 305°C for a time period of 2h to 2.5h in presence of oxygen free 98% to 99.9% pure hydrogen atmosphere at a flow rate in the range of 100 sccm- to 300 sccm.
f) cooling the reduced powder obtained in step (e) to room temperature resulting in the formation of highly dispersed heterogenous metal loaded nanocatalysts.

5. The process as claimed in claim 1, wherein the porous hard-carbon nanostructure is nanocarbon florets (NCF).

6. The process as claimed in claims 1, wherein the metal nanoparticle comprises of metals selected from platinum, palladium, nickel, cobalt, iron, gold, silver, ruthenium, rhodium, iridium, and copper.

7. The process as claimed in claim 2, wherein the etching solution is selected from a group comprising of sodium hydroxide (NaOH), potassium hydroxide (KOH), cesium hydroxide (CsOH), buffered hydrogen fluoride (BHF), and concentrated hydrofluoric acid (HF).

8. The process as claimed in claim 2, wherein the carbon precursor is selected from a group comprising of acetylene, methane, carbon dioxide, carbon monoxide, ethanol, isopropanol, butane, and isobutane.

9. The process as claimed in claim 4, wherein the silica-based template is dendritic fibrous nanosilica (DFNS) of varying textural and surface properties.

10. The process as claimed in claims 2 and 4, wherein the metal precursor is selected from a group comprising of metal chlorides, metal chlorides salts, metal acetates, metal nitrates, metal sulphates, metal carbonates that are water soluble.

Dated this 06th day of September 2023

MAHUA ROY CHOWDHURY
IN/PA - 496
(Authorized agent for the applicant)