PROCESS FOR THE SYNTHESIS OF METAL CATALYSTS

FIELD OF THE INVENTION:

The present invention provides a process for the synthesis of metal catalysts with support. More particularly, the invention provides a process for the synthesis of highly dispersed metal catalysts on a support with isoelectric point in the range of 1-5 and use of the supported metal catalyst for catalyzing various reactions thereof.

BACKGROUND AND PRIOR ART OF THE INVENTION:

Metal particles on support are one of the most prolifically used catalysts, due to their excellent activity, even under ambient conditions. The most desired characteristics in metal nanoparticles on a suitable support are recyclability, high dispersion, low particle size (as small as possible), and no compromise in activity.

Usual prior processes for supporting metal nanoparticles on metal oxides employ deposition precipitation method in aqueous medium. Specifically, in the case of supported gold nanocatalysts an aqueous solution of gold precursor is deposited on the support at pH around 8-10 to get nanoparticles of gold in about 2-3 hours or greater time lines. This process is generally good for supports which have high interaction with gold nanoparticles like Ti0 ₂ , Ce0 ₂ etc. But this method cannot be used for a support like silica. The main reason for this is the isoelectric point (IEP) or point of zero charge (PZC) of Si0 _2. which is very low. It falls in the range of 2-3 and so is not useful for supporting gold via deposition precipitation method. The term deposition precipitation means precipitation of active precursor of metal usually in the form of its hydroxides which after a certain time gets deposited on the surface of support. If the interaction of support with the metal precursor is high then small nanoparticles with high dispersions on the support will be obtained. Supporting gold on the metal oxides using deposition precipitation method requires adsorption of [Au(OH) ₄ ] species which is formed from precursor HAuCl ₄ .3H ₂ 0 at a pH 8- 10 on the surface of support. Under these pH conditions surface of supports like silica becomes negatively charged and adsorption of [Au(OH) ₄ ] species becomes difficult due to negative-negative charge repulsions. This results in the poor loading of gold precursor on the silica surface. Further treatments like drying and calcinations results in large gold nanoparticles supported on silica which are inactive for catalytic reactions. Therefore deposition precipitation method cannot be used for supporting gold on silica. One of the methods of supporting gold on silica is by utilization of silanes. They are organic linkers having alkoxy groups and one or more carbon chain with a functional group (eg: -NH ₂ , -COOH, -Cl, -SH...) attached to silicon. For the synthesis of gold NPs/ Si0 ₂ , Si0 ₂ is modified by these organic linkers ( Appl . Catal. A, 2003, 254 289-296, J. Mol. Catal. A: Chem., 2015, 404—405, 83-91). The alkoxy group of silane interacts with hydroxyl groups present in silica and functional groups remain free. In this case, when gold precursor (HAuCU· 3H ₂ 0) is used, the gold ions formed in aqueous solution has much improved interaction with the functional groups present on the modified silica. But these silanes are costly and require 12-24 hr stirring under reflux conditions. Further, calcination step is an absolute requirement to expose the active sites of gold nanoparticles. This results in the loss of functional groups and agglomeration of metal is possible with concomitant loss in catalytic activity.

Another method of supporting gold NPs on silica is by using dilute solution of ammonia. The use of ammonia has been explored to remove excess chloride ions during the synthesis stage of the supported catalyst. Here, precursor is mixed with the support (Si0 ₂ /Ti0 ₂ ) for 1 hr at room temperature followed by washing with ammonia solution to remove excess of chloride ions (J. Phys. Chem. B, 2006, 110, 22471). The CO oxidation activity on Au/Ti0 ₂ is high, whereas Au/Si0 ₂ is inactive at room temperature. Ammonia solution has been used for precipitating the amine modified gold complexes on the silica. The synthesis involved mixing of gold precursor (HAuCU) and silica at 75 deg C for 1 hr (Appl. Catal. A: Gen., 2008, 347, 216). After cooling down to room temperature, NH ₄ OH solution is added and stirred for 1 hr. The final catalyst is washed several times and dried. The CO oxidation activity of this catalyst shows the material to be active at room temperature giving approx 50 % conversion with full conversion achieved after 200 °C. The stability of the catalyst deteriorated with time, losing almost 50 % of activity after 1 hr. Au nanoparticles have been deposited on Si0 ₂ and CoO modified Si0 ₂ by using ammonia solution. But the activity of Au/Si0 ₂ was similar to normal Au supported Silica (J. Mol. Catal. A: Chem, 2010, 320, 97-105).

Therefore, there is a need in the field of catalysis to provide a suitable process for the synthesis of small size metal nano particles on supports with high monodispersity where the process should be cost effective and environment friendly with good reusable catalytic activity. OBJECTS OF THE INVENTION:

Main objective of the present invention is to provide a process for the synthesis of highly dispersed metal catalysts on a support with isoelectric point (point of zero charge) in the range of 1-5, wherein said process allows 100% metal loading without loss of metal precursor in solution. More particularly, a process for supporting gold or palladium nanoparticles on metal oxides supports with low isoelectric point in the range of 1-5 and that allows 100% metal loading without loss of metal.

Another objective of the present invention is to provide an efficient supported metal catalyst for catalyzing a wide range of reactions such as CO oxidation, alcohol oxidation, alkene oxidation, unsaturated alcohol oxidation, preferential oxidation, methane oxidation, H ₂ 0 ₂ synthesis, C-C coupling reaction, transfer hydrogenation, steam reforming of methane, Fischer-Tropsch reaction, C0 ₂ hydrogenation etc.

ACRONYMS USED TO DESCRIBE THE INVENTION:

FWHM: full width at half maximum

NPs: nanoparticles

AS x mm: Au supported on SBA-15 with x=l-4 mmole NH ₄ Cl

SBA-15: a mesoporous silica sieve based on uniform hexagonal pores with a narrow pore size distribution and a tunable pore diameter between 5 to 15 nm

IEP: Iso-electric point

PZC: Point of zero charge

SUMMARY OF THE INVENTION:

Accordingly, a process for the synthesis of a metal catalyst on a support with low isoelectric point (IEP) in the range of 1 to 5 at the pH in the range of 8 to 12 is provided. The metal catalyst so synthesized has no metal agglomeration and is in the size range of 1 to 5 nm.

A process for the synthesis of metal catalyst on a support with low isoelectric point (IEP) in the range of 1 to 5 comprises the steps of:

(a) preparing a solution of metal precursor in water; (b) preparing a uniform dispersion of metal oxides support in water, adding ammonium chloride to this dispersion and adjusting its pH at 9-10 using the solution of alkali;

(c) adding solution of step (a) to dispersion of step (b) with stirring; and

(d) collecting the catalyst formed, drying and calcining at a temperature range of 400-600 °C for 3-6 hr at the ramp rate of 2°C per minute to obtain the catalyst.

In an embodiment, the process for the synthesis of the metal catalyst allows 100% metal loading and no loss of metal is observed.

In an aspect of the invention, a process for the oxidation of CO, alcohol oxidation, alkene oxidation, unsaturated alcohol oxidation, preferential oxidation of CO, methane oxidation, H ₂ 0 ₂ synthesis, C-C coupling reaction, transfer hydrogenation, steam reforming of methane, Fischer-Tropsch reaction, C0 ₂ hydrogenation etc. has been provided using the supported metal nano catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1: UV-visible spectrum of Gold precursor HAuCU and HAuCU + NH ₄ Cl. Arrow head in the inset points towards the descending order of amount of NH ₄ CI used i.e. from 5 mmoles to 4, 3, 2, 1.5, 1 and 0 mmole (HAuCU only).

Figure 2: (a) XRD spectrum of (A) as synthesized and (B) calcined AS x mm catalysts (a) AS lmm (b) AS 1.5 mm (c) AS 2 mm (d) AS 3 mm (e) AS 4 mm.

Figure 3: TEM images of (A, a) AS 1 mm (B, b) AS 1.5 mm (C, c) AS 2 mm (D, d) AS 3 mm (E,e) AS 4 mm. Scale Bar (A-E) 50 nm and (a-e) 20 nm.

Figure 4: Histograms of particles size distribution of AS catalysts (a) AS 1 mm (b) AS 1.5 mm (c) AS 2 mm (d) AS 3 mm (e) AS 4 mm

Figure 5: Comparative activity plot of AS 1 to 4 mm catalysts. Reaction Conditions: 50 mg catalyst, 1 CO: 5 0 ₂ : 19 N ₂ , 25 mL min \ GHSV: 30,000 mL (g _cat h) ¹

Figure 6: Time on stream (TOS) plots of AS 4mm catalyst at (a) room temperature and (b) at 200 °C.

Figure 7: CO oxidation activity plot of AS 4 mm catalyst at doubled GHSV (60,000 mL

(gcath) ^{· 1} )

Figure 8: XRD pattern of Pd/SBA-l5 1 mm calcined catalyst Figure 9: TEM images of Pd/SBA-l5 1 mm catalyst, (a) AS l.5mm and (b) AS 2mm. Scale Bar: 20 nm

Figure 10: (a) CO oxidation plot of Pd/SBA-l5 (b) Time on stream plot at 160 °C. Reaction Conditions: 50 mg catalyst, 1 CO: 5 0 ₂ : 19 N ₂ , 25 mL min ¹ , GHSV: 30,000 mL (g _cat h) ¹

Figure 11: C0 ₂ hydrogenation activity of Pd/SB A- 15. Reaction conditions: 100 mg catalyst, lC0 ₂ : 3H ₂ , 9 mL min ¹ flow rate.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in detail in connection with certain preferred and optional embodiments, so that various aspects thereof may be more fully understood and appreciated.

A process for the synthesis of a metal catalyst on a support with low isoelectric point (IEP) or point of zero charge (PZC) in the range of 1 to 5 at the pH in the range of 8 to 12 is provided.

The process for the synthesis of metal catalyst on a support with low iso-electric point (IEP) in the range of 1 to 5 comprises the steps of:

(a) preparing a solution of metal precursor in water;

(b) preparing a uniform dispersion of metal oxides support in water, adding ammonium salt to this dispersion and adjusting its pH at 9-10 using the solution of alkali;

(c) adding solution of step (a) to dispersion of step (b) under stirring; and

(d) collecting the catalyst formed, drying and calcining at a temperature of 400-600 °C for 3-6 hours at a ramp rate of 2 °C per minute to obtain the catalyst.

In a preferred embodiment, the metal in the catalyst is selected from, but not limited to noble metals or transition metals.

Metal precursor at step a) is selected from, but not limited to gold, silver, platinum, palladium, cobalt, copper, nickel, etc. The synthesis protocol can also be applied to other metals such as ruthenium, rhodium, iridium, iron, chromium etc.

The metal oxide support with IEP in the range of 1-5 at step b) is selected from silica with long hexagonal channels, SBA-15, mesoporous silica, commercial silica, aluminosilicate. The ammonium salt at step b) is selected from ammonium halides, ammonium carbonates.

Alkali at step b) is selected from sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH) ₂ ), calcium carbonate (CaC0 ₃ ), and magnesium hydroxide (Mg(OH) ₂ .

In an embodiment, the process of synthesis of the metal catalyst allows 100% metal loading and no loss of metal is observed.

In another embodiment of the invention, the particle size of the metal catalyst is in the range of 1 to 5 nm.

An ammonium salt (by varying the amount) in-situ modifies the metal precursor during the synthesis of said catalyst in the deposition precipitation method and helps to deposit smaller metal nanoparticles on metal oxide support and also achieves higher dispersion. The process results in the metal precursor being deposited in the channels of the metal oxide support. The calcination of the as synthesized material results in the formation of a colored powder which is the final catalyst.

Another embodiment of the present invention provides a metal catalyst on a support with low isoelectric point (IEP) in the range of 1 to 5, wherein said metal catalyst catalyzes various reactions like oxidation of CO, alcohol oxidation, alkene oxidation, unsaturated alcohol oxidation, preferential oxidation of CO, methane oxidation, H ₂ 0 ₂ synthesis, C-C coupling reaction, transfer hydrogenation, steam reforming of methane, Fischer-Tropsch reaction, C0 ₂ hydrogenation etc.

An aspect of an embodiment provides 100% conversion of CO at room temperature with Au on SBA-15 (4 mm) catalyst, which is prepared by using 4 millimoles of ammonium chloride. The catalyst maintained the 100% conversion even after 48 h time on stream.

UV- Visible Spectroscopy

UV visible analysis has been done for HAuCl ₄ and HAuC + NH ₄ Cl precursors to see the change occurring in the peak position of Au ³⁺ ions. It can be seen in Figure 1 that the peak at 300 nm has been shifted towards the higher wavelength after the addition of NH4CI of different moles. This peak appears due to ligand to metal charge transfer between AU ³⁺ and Cl ions. Addition of NH4CI can result in exchange of Cl ligands with NH ₃ ligands which can cause the red shift of this peak. This exchange of ligands indicates that gold precursor has been modified after addition of NH4CI.

XRD Analysis

The wide angle XRD spectra of as synthesized (A) and calcined catalysts (B) have been shown in Figure 2. The as synthesized catalysts showed only a broad peak centered at 23° corresponding to amorphous silica. No reflections of metallic gold can be seen. After calcination, all the catalysts show peak at 38.2° which appears due to metallic Au (111) plane. The full width at half maximum (FWHM) of this peak increases with an increase in the amount of NH4CI indicating the reduction in Au NPs size. The other reflections of Au NPs cannot be seen which is due to small size of Au NPs which are fully dispersed in channels of SBA 15.

TEM Analysis

The TEM analysis as shown in Figure 3 of all the AS x mm catalysts shows that particle size of gold nanoparticles is very small. Comparison of particle size of all the samples clarifies that with an increase in the amount of NH4CI the gold nanoparticles size decreases and AS 4mm catalyst possess the smallest gold nanoparticles.

The histograms of the nanoparticles as shown in Figure 4 indicates that AS 4 mm catalyst has the maximum no. of particles which fall in the size range of 2-4 nm. AS 1 and 1.5 mm catalysts show minimum frequency for 2-4 nm sized particles and maximum frequency for particles of size more than 6 nm which ultimately affect their catalytic activity also.

The AS series of gold catalysts synthesized in line with examples 1-5 with varying quantities of ammonium chloride are tested for CO oxidation after calcination, without further pre-treatment. Figure 5 shows the comparative activity of AS catalysts for CO oxidation synthesized using different amounts of NH ₄ Cl. This comparison plot shows that AS 4 mm catalyst shows the best activity. This catalyst shows 100% conversion at room temperature and maintains this activity throughout the temperature range. This is due to presence of high percentage of smaller gold nanoparticles (2-4 nm) in AS 4 mm catalyst (as evident by particle size histogram). It is already well established that gold nanoparticles of size 1-5 nm are active for catalysis especially for CO oxidation. For an inert support like silica this criteria becomes even more crucial for getting high activity. Most of the particles in the size range of 1-5 nm are obtained by using 4mmoles of NH ₄ Cl without losing any precursor during the synthesis. The presence of these small gold nanoparticles makes AS 4 mm catalyst highly active and stable catalyst. Other AS catalysts also show more than 50% conversion at 25-30°C which is quite high if compared with already reported gold silica based catalyst.

Time on stream (TOS) plots of AS 4 mm catalyst at low as well as high temperature as shown in Figure 6a and 6b show that catalyst is highly stable and does not deactivate under reaction conditions. This can be attributed to the encapsulation of small Au NPs inside the channels of SB A- 15 which prevents the sintering of Au NPs under reaction conditions since sintering is one of the major causes of deactivation for Au based catalysts.

Further the effect of Gas Hourly Space Velocity (GHSV) has also been seen for AS 4 mm catalyst and it has been observed that even after doubling the GHSV the catalyst still maintains very high activity at room temperature for several hours as shown in Figure 7.

Pd/SBA-l5 catalyst (5 wt% Pd metal loading) has been synthesised by modified deposition precipitation (DP) method using NH4CI as an in situ modifier of Pd precursor. The synthesised catalyst has been tested for C0 ₂ hydrogenation. The XRD analysis of this catalyst shows only a broad peak at 23° as shown in Figure 8 which represents the amorphous silica. XRD analysis of this catalyst shows no reflections of either Pd or PdO which indicates the presence of highly dispersed NPs of very small size.

TEM image, Figure 9, shows that mesoporous structure of SBA-15 has been intact after Pd loading. Also the particle size of Pd NPs is 1-2 nm corroborating the XRD result. This shows the efficacy of the synthesis protocol to maintain the particle size even after increased metal loading. The high dispersion as well as very small size of the Pd NPs supported on SB A 15 is evidenced by TEM image.

The catalytic activity of this catalyst is tested for CO oxidation reaction. The CO oxidation conditions are identical as mentioned in Example 7. The CO oxidation plot shows that activity for Pd/SBA-l5 catalyst starts from 90 °C with a full conversion at 120 °C (Figure 10a). Activity is still maintained after further increasing the temperature. The time on stream plot at 160 °C shows no signs of deactivation even after 24 hr which shows the stability of the catalyst under reaction conditions (Figure 10b). Figure 11 depicts C0 ₂ hydrogenation activity of Pd/SBA-l5. Reaction conditions: 100 mg catalyst, lC0 ₂ : 3H ₂ , 9 mL min ¹ flow rate. The catalyst shows maximum 10% C0 ₂ conversion at 673 K temperature with almost 100% selectivity to CO at all the temperature range. CO is an important feedstock chemical which is used in Fischer-Tropsch synthesis to produce higher hydrocarbons and oxygenates.

The C0 ₂ hydrogenation activity of Pd/SBA-l5 catalyst synthesized via modified DP method shows the versatility of the catalyst both for oxidation and hydrogenation reactions.

EXAMPLES

Following examples are given by way of illustration therefore should not be construed to limit the scope of the invention.

The synthesis of AS x mm (gold) catalyst has been done in following steps:

Example 1: Synthesis of AS 1 mm catalyst

300 pL of 0.25 M HAuCl ₄ .3H ₂ 0 (hereafter gold precursor) was added to 25 mL of Millipore water and filled in a burette. 500 mg of SBA-15 support was dispersed in 50 mL of Millipore water in a beaker and sonicated for 5 min to get homogeneous dispersion of support in water. After this the SBA-15 support solution was stirred at 400 rpm and 53.5 mg of NH4CI (1 millimole) was added to this under constant stirring. After addition of NH4CI, the pH of the support solution was maintained at 9.45 by using 0.1 M NaOH solution (400mg in 100 mL water). After maintaining the desired pH, gold precursor solution was added drop wise in a time span of 15-20 min while maintaining the pH of 9.45. After complete addition of gold precursor the solution was stirred for another 1 hr. All the steps were performed at room temperature under constant stirring of 400 rpm at the pH of 9.45. After this the supported gold catalyst was collected by centrifugation at 10000 rpm for 10 min and washed thrice with Millipore water at same centrifugation speed (10000 rpm, 10 min). The collected catalyst was dried at 70 °C overnight. The as synthesized yellow colored powder was calcined at 400 °C for 4 hr at the ramp rate of 2°C /min. The color of the final catalyst was reddish brown and designated as AS 1 mm.

Example 2: Synthesis of AS 1.5 mm catalyst

The synthesis of AS 1.5 mm was exactly same as that of AS 1 mm catalyst. The only difference was amount of NH4CI used. 80 mg of NH4CI (1.5 millimoles) was added to form AS 1.5 mm catalyst. Rest of the procedure was same as that of AS 1 mm catalyst. The color of the final catalyst was reddish brown.

Example 3: Synthesis of AS 2 mm catalyst

107 mg of NH4CI (2 millimoles) was added to get AS 2 mm catalyst. Rest of the procedure was same as that of AS 1 mm catalyst. The color of the final catalyst was brown.

Example 4: Synthesis of AS 3 mm catalyst

161 mg of NH4CI (3 millimoles) was added to obtain AS 3 mm catalyst. Rest of the procedure was exactly same as that of AS 1 mm catalyst. The final obtained catalyst was brown in color.

Example 5: Synthesis of AS 4 mm catalyst

214 mg of NH4CI (4 millimoles) was added to obtain AS 4 mm catalyst. Rest of the procedure was exactly same as that of AS 1 mm catalyst. The final obtained catalyst was dark brown in color.

Example 6: CO oxidation reaction by using AS (Au on SBA-15) x mm catalyst:

The synthesized AS x mm catalysts were tested for gas phase CO oxidation reaction. CO and 0 ₂ were the reactants and final product was C0 ₂ . The reaction was done in a fixed bed continuous flow reactor connected to online GC equipped with Thermal Conductivity Detector (TCD) and Molecular Sieve column. To carry out reaction, 50 mg of the powder AS x mm catalyst was loaded in a fixed bed glass reactor having a diameter of 14 mm. Glass wool was used as the catalyst bed. The reactor was placed in a tubular furnace with a uniform heating zone of 4 cm furnished with temperature controller radix 6400. A K-type thermocouple was placed on the thermo well to measure the temperature of catalyst bed. The reactant gases CO and 0 ₂ were diluted with N ₂ . The ratio of these gases was maintained using Mass Flow Controllers and fixed as 1 CO : 5 0 ₂ : 19 N ₂ . The flow rate of the reactant gases was 25 mL min ¹ with a GHSV (Gas Hourly Space Velocity) of 30,000 mL(g _cat *h) ¹ . The reaction was done at atmospheric pressure. To carry out time on stream reactions (for testing catalyst stability under reaction conditions) the catalyst in the reactor was left undisturbed at the desired temperature for required time. The CO Conversion was calculated by using the formula: [(% area CO _iniuai - % area CO _finai )/ % area COi _nitiai ]*l00 Example 7: Synthesis of Pd/SBA-15 catalyst and characterization

3 wt% Pd supported on SB A 15 was synthesized using 1 mmol of NH ₄ Cl. The palladium precursor used was 564 pL of 0.25 M H ₂ PdC solution. Rest of the procedure was same as that of AS x mm (example 1).

Example 8: synthesis of Ni/SBA-15 catalyst

To expand the applicability of this synthesis first row transition metal supported on SBA-15 has also been synthesized. Ni has been chosen as an example. 3 wt% Ni supported on SBA-15 has been synthesized using 60 mg of N1CI ₂ .6H ₂ O as the Ni precursor. 1 mmole of NH ₄ CI has been used. Ni precursor was dissolved in 25 mL of Millipore water. Rest of the procedure was same as that AS 1 mm and Pd 1 mm catalyst. No leaching of the Ni was observed.

Hence the synthesis protocol proposed here is suitable for the synthesis of noble as well as non noble transition metal supported on low isoelectric point oxides.

Example 9: C0 ₂ hydrogenation over Pd/SBA-15 catalyst

100 mg of the catalyst was loaded in a continuous plug-flow quartz reactor (30 cm in length, 6 mm in i.d.) mounted inside temperature controlled vertical tube furnace. All the catalysts were reduced in ¾ flow at 400 °C for 3 h before starting the reaction. The catalytic activity was evaluated at atmospheric pressure with 23 vol% C0 ₂ and 72 vol% ¾ balanced with N ₂ . The flow rate of feed gases was 9 ml/min with a GHSV of 5400 mL (g _caf h) ¹ and was controlled by individual Alicat mass flow controllers. The concentrations of CO, C0 ₂ , N ₂ and hydrocarbons in the outlet streams were analyzed by an online gas chromatography system (Nucon 5765) equipped with a Thermal Conductivity Detector (TCD) and Flame Ionization Detector (FID) using He as the carrier gas. Permanent gases were analyzed by TCD carbosieve column and hydrocarbons by FID capillary column. The C0 ₂ conversion, X (C0 ₂ ) was calculated as:

The selectivity S for different products was calculated as: Area of product

S(x) = - G - ; -—

Tot&i products

ADVANTAGES OF THE INVENTION:

• no expensive linkers or high end processes used

• one pot, simple and cost effective process conducted within an hour

• no coupling agents or silane needed

• small particle size obtained

• 100% metal loading done

• Excellent catalytic activity