Claims:CLAIMS
We claim;
[CLAIM 1]. A zirconia supported stabilized palladium nanocluster by mono lacunary tungstophosphoric acid preparation method comprises steps as:
i. dissolving sodium salt of mono lacunary tungstophosphoric acid Na7LTPA in minimum amount of distilled water followed by the addition of a stoichiometric amount of aqueous palladium chloride PdCl2 solution dropwise;
ii. aging of resulting mixture and evaporation of the excess water to dryness on the water bath;
iii. drying of the resulting material formed in step ii overnight in an oven, formed brown coloured palladium nanoclusters Na5Pd(II)LTPA;
iv. treating material formed in step iii under H2 pressure in Parr reactor obtains black coloured material was Na5PdH2LTPA.
v. impregnation of zirconia ZrO2 with aqueous solution of Pd(II)LTPA in double distilled water
vi. drying material formed in step v at 80-100 °C
vii. treating material formed in step vi under pressure in Parr reactor to obtain a catalyst zirconia supported stabilized palladium nanocluster by mono lacunary tungstophosphoric acid PdLTPA/ZrO2.
[CLAIM 2]. The zirconia supported stabilized palladium nanocluster as claimed in claim 1, wherein said catalyst is sustainable up to five catalytic runs without any degradation or leaching.

[CLAIM 3]. The zirconia supported stabilized palladium nanocluster as claimed in claim 1, wherein said catalyst in C-C coupling reaction of SM coupling reaction gives 99% conversion and Heck coupling reaction gives 98% conversion.

[CLAIM 4]. The zirconia supported stabilized palladium nanocluster as claimed in claim 1, wherein said 99% conversion was achieved with high substrate to catalyst ratio of 5150/1 for SM coupling reaction.

[CLAIM 5]. The zirconia supported stabilized palladium nanocluster as claimed in claim 1, wherein said 98% conversion was achieved with high substrate to catalyst ratio of 858/1 for Heck coupling reaction.

[CLAIM 6]. The zirconia supported stabilized palladium nanocluster as claimed in claim 1, wherein concentration for Pd in said catalyst is very law 0.0194 mol% for SM coupling reaction and 0.117 mol% for SM coupling reaction Heck coupling reaction.

[CLAIM 7]. The zirconia supported stabilized palladium nanocluster as claimed in claim 1, wherein Turnover Number TON for said catalyst is 5099 and Turn Over Frequency TOF for said catalyst is 10198 h-1 is in SM coupling reaction.

[CLAIM 8]. The zirconia supported stabilized palladium nanocluster as claimed in claim 1, wherein Turnover Number TON for said catalyst is 841 and Turn over Frequency TOF for said catalyst is 140 h-1 is in SM coupling reaction.

[CLAIM 9]. The zirconia supported stabilized palladium nanocluster as claimed in claim 1, wherein analytical evaluation through EDX, FT-IR, XRD, BET
and XPS of regenerated catalyst confirm the sustainability of the catalyst.

Dated this 31st day of January 2022.

, Description:FIELD OF THE INVENTION:
The present invention relates to chemical process of nanocluster preparation and application thereof in chemical synthesis. More particularly the present invention relates to novel method for preparation of palladium nanoclusters and its stabilization by mono lacunary tungstophosphoric acid and its heterogenization onto zirconia. The present invention further relates to characterization of nanocluster formed through novel pocess and application of nanocluster formed as an efficient catalyst for water mediated C-C cross coupling reactions [Suzuki-Miyaura and Heck].

BACKGROUND OF THE INVENTION:
Palladium nanocatalysts (PdNCs) are historically dominant over other metals and extensively investigated for organic transformations. Still they are serving as a central tool for numerous important organic transformations as well as total synthesis. However, Pd is very much prone to form inactive aggregates during their synthesis or catalytic reactions, and hence, stabilization of Pd is necessary.
Numbers of efforts have been made for the same using different stabilizing agents such as dendrimers, phosphine base ligands, surfactants, ionic liquids, etc. As the reported stabilizing scaffolds are mostly organic, toxic and expensive, it is important to replace the same by some alternatives. Heteropoly acids (HPAs) are the excellent candidate for the same. HPAs are discrete early transition metal–oxide cluster anions and comprise a class of inorganic complexes having unrivalled versatility and structural variation in both symmetry and size. In addition, they have following advantages: (i) Robust oxoanionic nature which greatly enhance the stability power, (ii) reducing capacity, favors the stability of Pd into its most stable oxidation state zero, (iii) large relative sizes which sterically hindered Pd and prevent it from agglomeration during the synthesis as well as its catalytic reaction and (iv) it avoids the use of external ligands for stabilization of Pd, which are mostly organic-toxic materials.
Hitherto there have been increasing efforts toward the designing of unsupported as well as supported PdNCs via heteropoly acid stabilization.
US 2014/0235893 A1 relates to the synthesis of polyethylene glycol stabilized catalyst comprising the oxides of Mo, V, Nb and nano metallic Pd together with 12-tungstophosphoric acid and/or Sb and Ca. This catalyst set the benchmark for the partial oxidation of ethane to selectively produce acetic acid with minimum production of side products ethylene and CO. Moreover, the designed catalyst was supported on titania and used as heterogeneous catalyst for the same without compromise in activity.
Papaconstantinou et al. (Angew. Chem. Int. Ed. 2002, 41, 1911-1914) first time reported the use of 12-tungstosilisic acid (H4SiW12O40) as reducing as well as stabilizing agent for the synthesis of Pd nanoparticles using isopropyl alcohol as sacrificial solvent. This photochemically synthesized material was characterized by TEM. In the same year, Neumann et al. (Org. Lett. 2002, 4, 3529-3532) derived monolacunary tungstophosphate stabilized Pd nanoparticles by reduction of potassium salt of palladium substituted monolacunary tungstophosphoric acid K5[PPdW11O39]. The synthesized material was characterized by TEM, electron diffraction measurement and 31P NMR. Its catalytic efficiency was evaluated for Suzuki-Miyaura (SM), Heck, Stille-type carbon-carbon coupling and carbon-nitrogen coupling reactions of bromoarenes in aqueous media. Furthermore, they supported K5[PdPW11O39].12H2O on ?-Al2O3 by impregnation method and obtained catalyst, [K5[PdPW11O39].12H2O/?-Al2O3] applied for SM reaction using chlorobenzene as one of the substrates under solvent free condition.
Liu et al. (Langmuir 2008, 24, 5277-5283) synthesized Dawson-type potassium salt of V- substituted tungstophosphate cluster (K9[H4PVW17O62]) and used as reducing as well as stabilizing agent for Pd nanoparticles. Synthesized material was characterized by XPS, TEM and zeta potential analysis.
Corma et al. (J. Phys. Chem. C 2010, 114, 8828-8836) applied tetra butylmethylimidazolium molybdovanadophosphate ([bmim]4HPO40V2Mo10) ionic liquid for stabilization of Pd nanoparticles and characterized by elemental analysis, AAS, TGA, 1H NMR, HAADF-STEM, HRTEM, XPS, XANES and EXAFS. The catalytic activity of the material was studied for Heck coupling.
Yamashita et al. (RSC Adv. 2012, 2, 1047-1054) synthesized Pd nanoparticles stabilized via silica supported cesium salt of 12-tungstophosphoric acid (Pd/Cs2.5H0.5PW12O40/SiO2) by photo-assisted deposition method and characterized by ICP, XRD, UV-vis spectroscopy, BET surface area and FT-EXAFS. It was found to be active catalyst for the direct synthesis of hydrogen peroxide.
Thorimbert et al. (Tetrahedron 2013, 69, 5772-5779) reported the DFT study for phosphovanadotungstate polyanion [P2W15V3O62]9- as a powerful support to stabilize palladacycles conjugated to the inorganic framework via an organic ligand as well as its application towards Heck coupling. In the same year, Kortz et al. (J. Colloid Interface Sci. 2013, 394, 157-165) reported the wet chemical synthetic method to prepare Pd metal nanoclusters stabilized by tetrabutylammonium salts of tungstophosphate with the well-known Keggin [a-PW12O40]3-, Wells–Dawson [P2W18O62]6- and their lacunary derivatives [a-PW11O39]7- and [P2W15O56]12- structures. These stabilized catalysts were characterized by EDX, TEM, UV-vis spectroscopy, FT-IR, XRD and XPS. The activity of the synthesized materials was evaluated for 1-hexene hydrogenation and comparison showed the exceeding activity of [P2W18O62]6-stabilized Pd nanoclusters compare to all others. They also reported (Appl. Catal. A 2013, 453, 262-271) the use of novel Krebs type tetrabutylammonium salt of various polyoxoanions [(TBA)4H4[M4(H2O)10(XW9O33)2], where M = Mn, Ni, Zn, X = Te, Se; M = Fe, X = Sb, As] as stabilizers for Pd(0) metal clusters and characterized them by EDX, TEM, UV-vis spectroscopy, FT-IR, XRD and XPS. Catalytic activity of the designed materials was assessed for hydrogenation of cyclohexene and 1- hexene, in the same year. Simultaneously, Yuan et al. (Int. J. Hydrogen Energy 2013, 38, 11074-11079) reported a novel method for the synthesis of bimetallic Pt and Pd nanoparticles deposited on multiwalled carbon nanotubes (MWCNTs) using 12-molybdophosphoric acid (H3PMo12O40) as stabilizer. This H3PMo12O40/PtPd/MWCNT nanotubes was characterized by FT-IR, XPS, TEM, ICP-AES and explored as electrocatalysts for the methanol electrooxidation.
Cronin et al. (Inorg. Chem. Front. 2014, 1, 178-185) developed one-pot strategy for the synthesis of stabilized palladium via potassium salt of selenotungstic acid isomers K28[H12Pd10Se10W52O206].65H2O and K26[H14Pd10Se10W52O206].68H2O as well as sodium salt of tellurotungstic acid cluster Na40[Pd6Te19W42O190]·76H2O and characterized by SCXRD. In the same year, Proust et al. (RSC Adv. 2014, 4, 26491-26498) also invented simple procedure for vacant tungstophosphates ([PW11O39]7- and [P2W19O69(H2O)]14-) and tungstoarsenate ([As2W19O67(H2O)]12-) stabilized palladium (0) nanoparticles in water by hydrogenation with H2 at ambient temperature and atmospheric pressure. These crystalline materials were characterized by EDX, SCXRD, FT-IR, Raman, 31P NMR, TEM, HRTEM and XPS.
Han et al. (Chem. Eur. J. 2015, 21, 5387-5394) reported one-pot synthesis for 12- tungstophosphoric acid stabilized Pd nanocrystals (H3PW12O40 Pd NCs). This stabilized NCs was characterized by SEM-EDX, TEM, UV-vis spectroscopy, FT-IR, ICP, CV and XPS. The material was utilized for chemical (Suzuki coupling) and electrochemical catalysis (formic acid oxidation). In the same year, Ahmadpour et al. (RSC Adv. 2015, 5, 24319-24326) demonstrated the use of 12-molybophosphoric acid as stabilizing agent for Pd nanoparticles supported on graphene nanosheets (Pd/H3PMo12O40/GNSs). The synthesized material was characterized by EDX, SEM, TEM, HAADF-STEM, XRD and proved its synergistic behavior for enhanced ethanol electrooxidation reaction and shows better tolerance to poisoning species. During the same period, Wei et al. (J. Mater. Chem. A 2015, 3, 13962-13969) prepared 12- molybdophosphoric acid stabilized supported palladium nanoparticles (Pd/PMo12O40) by in situ reduction method and characterized by BET surface area, XRD, SEM, TEM, HRTEM and XPS. Moreover, they proved it as a superior nanocatalyst to carbon supported platinum for promotion of the oxygen reduction reaction, i.e. serves as an assistant catalyst, facilitating the decomposition of the harmful hydrogen peroxide intermediates.
Chen et al. (Appl. Catal. A 2016, 523, 304-311) designed one pot- method for tetra metal substituted sandwich phosphorotungstate [M4(H2O)2(PW9O34)2]10- (where, M = Fe2+, Co2+, Mn2+, Cu2+ and VO2+) stabilized Pd nanoparticles, encapsulated in situ in mesoporous aluminum phosphate (mAPO). Synthesized material was characterized by XRD, TEM, BET surface area, ICP, FT-IR, UV-vis spectroscopy and their catalytic efficiency were evaluated toward aerobic oxidation of alcohol in water. Simultaneously, Zhang et al. (RSC Adv. 2016, 6, 39618-39626) developed facile and green one-pot synthetic method for 12-tungstophosphoric acid stabilized tri-component catalyst comprising of Pd nanoparticles on macroporous carbon (Pd@ H3PMo12O40/MPC) and characterized by XRD, SEM, TEM, EDX, XPS and CV. The synthesized catalyst was to promote the development of new electrochemical sensors and electrode materials. In the same year, Han et al. (CrystEngComm 2016, 18, 6029-6034) reported one-pot synthesis for 12-molybdophosphoric acid stabilized bimetallic Pt and Pd based core- shell nanocrystals (Pd/Pt@H3PMo12O40) and characterized by SEM, TEM, HAADF-STEM, EDX, ICP-AES, XPS and XRD. The efficiency of the material was evaluated for electrooxidation of methanol.
Zhao et al. (Electrochem. Commun. 2017, 83, 56-60) fabricated 12-tungstophosphoric acid stabilized palladium nanoparticles on reduced graphene oxide (Pd/H3PW12O40/RGO) by a simple one-pot photoreduction method and characterized by TEM, EDX, XRD, XPS and ICP. The synthesized catalyst was used for electrooxidation of ethylene glycol and glycerol. Parallelly, Zhang et al. (J. Colloid Interface Sci. 2017, 505, 615-621) designed ex-situ decorated ordered mesoporous carbon with palladium nanoparticles via 12-tungstophosphoric acid stabilization (Pd-H3PW12O40-OMC) and characterized by SEM, TEM, EDX, BET surface area, XRD and XPS. The efficiency of this tri-component system was evaluated for sensitive detection of acetaminophen in pharmaceutical products.
Leng and Dai (ChemSusChem 2018, 11, 3396-3401) anchored palladium nanoparticles on a 12- tungstophosphoric acid attached to melem (C6H6N10; 2,5,8-triamino-heptazine) porous hybrid (H3PW12O40/melem, as stabilizing agent) by hybridization and post chemical reduction, and characterized by FT-IR, SEM, ICP-AES, UV-vis spectroscopy, TEM, HRTEM, H2-TPR, XRD and XPS. The efficiency of the catalyst was studied toward formic acid dehydrogenation. In the same year, Yuan et al. (ACS Appl. Energy Mater. 2018, 1, 2, 411–420) reported the self-assembled 12-phosphomolybdic acid stabilized pristine graphene supported Pd nanoflowers and characterized by FE-SEM, EDX, TEM, HRTEM and XRD. This self-assembled catalyst was found to be superior for formic acid oxidation. In addition, Patel and co- worker (Catal. Lett. 2018, 148, 3534–3547) developed stabilized Pd(0) nanoparticles by 12- tungstophosphoric acid supported on zirconia (Pd-TPA/ZrO2) and characterized by EDX, TGA, FT-IR, XPS, TEM, BET and XRD. The catalyst was found to be highly active for water mediated Suzuki-Miyaura cross coupling reaction.
Zhang et al. (Anal. Chim. Act 2019, 1047, 28-35) developed tricomponent stabilized nanohybrid comprising of Pd, 12-tungstophosphoric acid and nitrogen-doped hollow carbon spheres (Pd/H3PW12O40/NHCS). Synthesized nanohybrid material was characterized by XRD, SEM, TEM and XPS, and assessed its activity as selective electrochemical sensor for acetaminophen. In the same year, Ahmadpour et al. (RSC Adv. 2019, 9, 37537) synthesized grapheme oxide supported bimetallic nanocomposite based on Au and Pd using 12-phosphomolybdic acid as stabilizing as well as reducing agent (Au@Pd/PMo12/rGO) via one-pot route. Designed catalyst was characterized by HAADF-STEM, TEM, SEM, EDS, ICP-MS and powder XRD. Moreover, it was found to be a dual functional electrocatalyst for ethanol electro-oxidation and hydrogen evolution reactions. Parallel, Bagherzadeh and group successfully supported Pd nanoparticles on polyoxometalate ([Mo368O1032(H2O)240 (SO4)48]n-.(TBA+)n) and characterized by XRD, FE-SEM, TEM, FT-IR, TGA and BET. The material was found to be an efficient catalyst for Heck reaction in PEG medium. Simultaneously, Patel and co-worker (Catal. Lett. 2019, 149, 1476–1485) reported the synthesis of stabilized Pd(0) nanoparticles by 12-tungstophosphoric acid supported on zirconia (Pd-TPA/ZrO2) and characterized by EDX, TGA, FT-IR, XPS, TEM, BET and XRD. The catalyst was found to be highly active and selective for C=C hydrogenation of unsaturated hydrocarbons in neat water.
Zhang and He et al. (Sens. Actuators B Chem. 2020, 323, 128647) developed nanohybrid of polyoxometalate-derived MoS2 nanosheets tightly and vertically grown over ß-FeOOH nanorods (pd- MoS2@ß-FeOOH), and characterized by TEM and XPS. The designed material was exploited as the platform to immobilize the complementary DNA strands of microRNA-21 for further detection. In the same year, Patel and co-worker (Catal. Lett. 2021, 151, 803–820) synthesized silica encapsulated 12- tungstophosphoric acid stabilized palladium nanoclusters (PdTPA/SiO2) and characterized by EDX, CV, TGA, BET, FT-IR, 31P MAS NMR, XRD, XPS, TEM and HRTEM. The catalyst was found highly active and sustainable for water mediated cyclohexene hydrogenation.
Patel and co-worker (Catal. Lett. 2021, DOI: 10.1007/s10562-021-03658-w) designed zirconia supported palladium nanocluster stabilized by 12-tungstophosphoric acid (PdTPA/ZrO2) and characterized by EDX, TGA, FT-IR, 31P MAS NMR, XRD, BET, XPS, TEM and HRTEM. The efficiency of the catalyst was evaluated for C-C cross coupling (SM and Heck) reactions. In the same year, same group reported (RSC Adv. 2021, 11, 8218-8227) zirconia supported vacant phosphotungstate stabilized Pd nanoparticles (Pd-PW11/ZrO2) and characterized by EDX, FT-IR, BET, XRD, XPS, TEM and HRTEM. The efficacy of the catalyst was studied for low temperature water mediated cyclohexene hydrogenation.
However still there is a need of palladium nanoclusters and its stabilization by Heteropoly acids, turn out to be highly efficient for both Suzuki- Miyaura (SM) and Heck coupling.
Therefore, one object of the present invention is to provide an improved process for the preparation of palladium nanoclusters and its stabilization by mono lacunary tungstophosphoric acid and its heterogenization onto zirconia.
However, up to now HPA stabilized PdNCs have not been turn out to be highly efficient for both Suzuki- Miyaura (SM) and Heck coupling.
Therefore, one object of the present invention is to provide improved the catalyst based on stabilized Pd nanoclusters having upgraded activity for water mediated SM and Heck coupling with high substrate/catalyst ratio as well as TON and TOF along with low active amount of Pd under mild reaction condition.
Embodiment of the present invention can ameliorate one or more of the above mentioned problems.
Inventors of the present invention have surprisingly found that the novel present invention further relates to characterization of nanocluster formed through novel pocess and application of nanocluster formed as an efficient catalyst for water mediated C-C cross coupling reactions [Suzuki-Miyaura and Heck].
SUMMARY OF THE INVENTION
Therefore an aspect of the present invention provides an improved process for the preparation of palladium nanoclusters and its stabilization by Heteropoly acids, turn out to be highly efficient for both Suzuki- Miyaura (SM) and Heck coupling.
Therefore a further aspect of the present invention provides an improved process for the preparation of palladium nanoclusters and its stabilization by mono lacunary tungstophosphoric acid and its heterogenization onto zirconia.
Another aspect of the present invention provides an improved process for the preparation of the supported stabilized palladium nanoclusters by mono lacunary tungstophosphoric acid for water mediated C-C cross coupling reaction (SM and Heck) with high substrate/catalyst ratio, Turnover Number (TON) as well as Turnover Frequency (TOF).
Further aspect of the present invention provides an industrially viable, zirconia supported stabilized palladium nanoclusters by mono lacunary tungstophosphoric acid (PdPW11/ZrO2) and preparation thereof using a simple incipient-wet impregnation and post reduction method.
In another aspect of the present invention is to provide characterization of the catalyst by various instrumental techniques such as EDX, TGA, BET, FT-IR, powder XRD, XPS and HRTEM.
In another aspect the invention provides the catalyst with highly efficient catalytic activity for water mediated SM and Heck cross coupling reactions. Further catalytic activity in SM coupling reaction and Heck coupling reaction the catalyst shows High substrate/catalyst ratio; Very low Pd Concentration and Very high TON as well as TOF.
Another aspect of the catalyst is found to be truly heterogeneous in nature and recyclable with stable catalytic activity up to five cycles and can be used for more. Further characterizarion of the same through EDX, FT-IR, BET and XPS of regenerated catalyst confirmed the sustainability of the catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS:
Figure 1: Illustrates the effect of loading on SM and Heck coupling reactions and evaluation thereof by varying % loading of PdLTPA (10-40 %) onto ZrO2.
Figure 2 EDX elemental mapping of the catalysts (a) PdLTPA and (b) PdLTPA/ZrO2.
Figure 3: TGA curves of PdLTPA and PdLTPA/ZrO2.
Figure 4: XPS spectra of PdLTPA (a, b and c) and PdLTPA/ZrO2 (d, e and f).
Figure 5: HRTEM images of PdLTPA/ZrO2.
Figure 6: EDX mapping of regenerated PdLTPA/ZrO2.
Figure 7: XPS spectra of regenerated PdLTPA/ZrO2.

DETAILED DESCRIPTION OF THE INVENTION:
Therefore, an aspect of the present invention provides an improved process for the preparation of palladium nanoclusters and its stabilization by heteropoly acids.
Further aspect of the present invention is to provide an improved process for the preparation of palladium nanoclusters and its stabilization by mono lacunary tungstophosphoric acid and its heterogenization onto zirconia.
Another aspect of the present invention provides an improved process for the preparation of the supported stabilized palladium nanoclusters by mono lacunary tungstophosphoric acid for water mediated C-C cross coupling reaction (SM and Heck) with high substrate/catalyst ratio, Turnover Number (TON) as well as Turnover Frequency (TOF).
Further aspect of the present invention provides an industrially viable, zirconia supported stabilized palladium nanoclusters by mono lacunary tungstophosphoric acid (PdPW11/ZrO2) and preparation thereof using a simple incipient-wet impregnation and post reduction method.
In another aspect of the present invention is to provide characterization of the catalyst by various instrumental techniques such as EDX, TGA, BET, FT-IR, powder XRD, XPS and HRTEM.
In another aspect the invention provides the catalyst with highly efficient catalytic activity for water mediated SM and Heck cross coupling reactions. Further catalytic activity in SM coupling reaction and Heck coupling reaction the catalyst shows High substrate/catalyst ratio; Very low Pd Concentration and Very high TON as well as TOF.
Another aspect of the catalyst is found to be truly heterogeneous in nature and recyclable with stable catalytic activity up to five cycles and can be used for more. Further characterizarion of the same through EDX, FT-IR, BET and XPS of regenerated catalyst confirmed the sustainability of the catalyst.
Zirconia supported stabilized palladium nanocluster by mono lacunary tungstophosphoric acid (PdPW11/ZrO2) was synthesized in by top-down approach.
Synthesis of stabilized palladium nanoclusters by mono lacunary tungstophosphoric acid (PdLTPA):
Process of Preparation:
1 g Na7LTPA (sodium salt of mono lacunary tungstophosphoric acid) was dissolved in minimum amount of distilled water followed by the addition of a stoichiometric amount of aqueous PdCl2 (Palladium Chloride) (57.8 mg) solution dropwise. The resulting mixture was aged for 1 h at 80 °C and the excess water was evaporated to dryness on the water bath. The resulting material was oven dried at 120 °C overnight, obtained brown coloured Na5Pd(II)LTPA [palladium nanoclusters by mono lacunary tungstophosphoric acid] (Later, Pd(II)LTPA), finally treated under 1 bar with H2 at 40 °C for 30 min in Parr reactor, obtained (black coloured) material was designated as Na5PdH2LTPA (Later, PdLTPA). Here, no reduction of counter cation (sodium) is in good agreement with reported one (Org. Lett. 2002, 4, 3529-3532). Synthetic reaction scheme is shown in scheme 1.

Scheme 1 Reaction scheme of PdLTPA synthesis.
Synthesis of PdLTPA supported onto Zirconia
A series of catalysts, containing 10-40 % of Pd(II)LTPA supported on ZrO2 was synthesized by incipient wet impregnation method. 1 g ZrO2 (Zirconium dioxide /zirconia) was impregnated with aqueous solution of Pd(II)LTPA(0.1/10-0.4/40 gmL-1 of double distilled water), dried at 100 °C for 10 h and finally treated under 1 bar with H2 using Parr reactor. The obtained materials with 10-40 % loading was designated as 10% PdLTPA/ZrO2, 20% PdLTPA/ZrO2, 30% PdLTPA/ZrO2 (Later, PdLTPA/ZrO2) and 40 % PdLTPA/ZrO2, respectively. Reaction scheme for PdLTPA/ZrO2 synthesis is shown in scheme 2.

Scheme 2 Reaction scheme of PdLTPA/ZrO2 synthesis.
The process for preparing the present novel Zirconia supported stabilized palladium nanocluster by mono lacunary tungstophosphoric acid (PdPW11/ZrO2) can be modified accordingly by any person skilled in the art based on the knowledge of the chemical synthesis. However all such variation and modification is still covered by the scope of present invention.
These and other aspects of the invention may become more apparent from the examples set forth herein below. These examples are provided merely as illustrations of the invention and are not intended to be construed as a limitation thereof.
EXAMPLE 1: Catalytic Evaluation:
The C-C coupling reactions were carried out. The effect of loading on SM coupling and Heck coupling was evaluated by varying % loading of PdLTPA (10-40 %) onto ZrO2. The obtained results depicted in figure 1 show that conversion increases with increase in % loading from 10 % to 30 % PdLTPA on ZrO2. However, a further increase in loading from 30 % to 40 % resulted no change in conversion. Hence, 30% PdLTPA/ZrO2 (Later, PdLTPA/ZrO2) catalyst was selected for detailed characterization and catalytic study.
EXAMPLE 2: Catalyst Characterization
EXAMPLE 2a: Characterization of catalyst by EDS elemental analysis
The gravimetric analysis of Pd (3.58 wt %) and W (67.34 wt %) for PdLTPA are in good agreement with the theoretical values (3.54 wt % and 67.26 wt %, respectively) as well as EDX values (3.49 wt % and 67.12 wt %, respectively). The EDS value of Na (3.79 wt %) is also resemble with theoretical one (3.82 wt %), which is equivalent to five sodium atoms. For PdLTPA/ZrO2, EDX values for Pd (0.82 wt %), W (15.47 wt %) and Na (0.86 wt %) are also in good agreement with theoretical values, Pd (0.81 wt %), W (15.52 wt %) and Na (0.86 wt %). EDX elemental mapping of the catalysts is shown in figure 2.
EXAMPLE 2b: Characterization of catalyst by TGA
TGA of PdLTPA (Figure 3) shows 5.10 % weight loss due to adsorbed water, in the temperature range of 60-110 °C. While, 1.36 % weight loss up to 190 °C, indicating the loss of crystalline water molecules. Whereas, PdLTPA/ZrO2 shows an initial weight loss of 3.74 % up to 110 °C, indicating the loss of adsorbed water molecules followed by 1.48 % weight loss up to 190 °C because of removal of crystalline water. Besides this, no significant weight loss up to 500 °C, indicates high thermal stability of the catalyst.
EXAMPLE 2c: Characterization of catalyst by FT-IR
The FT-IR spectra of ZrO2, LTPA, PdLTPA, and PdLTPA/ZrO2 were recorded. FT-IR spectrum of PdLTPA/ZrO2 exhibited bands at 1096, 1042, 957, 902, 810 cm-1 corresponding to P-O, W=O, and W-O-W stretching vibration frequencies, respectively. Here, the retention of all the characteristic peaks correspond to PdLTPA without any significant shift indicates that basic structure of Keggin unit remains intact even after impregnation as well as post reduction.
EXAMPLE 2d: Characterization of catalyst by BET
The value for BET surface area of ZrO2 was found to be 170 m2/g, whereas, that
of PdLTPA/ZrO2 was 204 m2/g. The observed drastic increase in surface area is
due to the presence of PdNCLs onto surface of ZrO2.
EXAMPLE 2e: Characterization of catalyst by XRD
To study the surface morphology and rate of dispersion in synthesized
materials, the XRD patterns of LTPA, PdLTPA, ZrO2 and PdLTPA/ZrO2 were
recorded. Absence of any crystalline peak corresponds to PdLTPA in XRD patterns of PdLTPA/ZrO2, reveals the homogeneous dispersion of PdLTPA onto surface of ZrO2.
Moreover, patterns did not reflect any diffraction corresponds to
PdLTPA, indicates the high degree of dispersion onto surface of ZrO2 as well as
no sintering of Pd was form during the synthesis.
EXAMPLE 2f: Characterization of catalyst by XPS
The oxidation state of the Pd, W and O was confirmed by recording high resolution XPS spectra of PdLTPA and PdLTPA/ZrO2 (Figure 4). High resolution Pd3d and W4f XPS spectra of PdLTPA shows peaks of Pd3d at binding energy 335.2 eV (3d5/2) and 340.5 eV (3d3/2), confirming the presence of Pd(0) (ChemSusChem 2018, 11, 3396-3401; RSC Adv. 2014, 4, 26491-26498; Appl. Catal., A 2013 453, 262-271; J. Colloid Interface Sci. 2013, 394, 157-165). Whereas, PdLTPA/ZrO2 shows peaks at 335.2 eV and 340.4 eV, correspond to Pd3d5/2 and Pd3d3/2, confirming the presence of Pd(0) onto surface of ZrO2.
PdLTPA shows (Figure 4c) peaks of W4f7/2 and W4f5/2 at binding energy 35.7 and 37.8 eV, characteristic of W(VI), confirming the presence of W(VI). Similarly, PdLTPA/ZrO2 also shows (Figure 4f) peaks at binding energy 35.6 and 37.7 eV confirming no reduction of W(VI) during the synthesis (RSC Adv. 2014, 4, 26491 26498; Catal. Sci. Technol. 2012, 2, 979-986).
EXAMPLE 2g: Characterization of catalyst by HRTEM
HRTEM images of PdLTPA/ZrO2 (Figure 5) are presented at various magnifications. Images clearly indicate the presence of PdNCLs of ~2 nm with equidistance homogeneous dispersion onto surface of ZrO2 without any aggregation.
In summary, FT-IR shows the retention of Keggin structure even after
impregnation and post-reduction of the catalyst. The presence of Pd(0) is
confirmed by XPS. Further, it confirms the stability of W(VI), during the post
reduction of the catalyst. HRTEM confirms the presence of homogeneously
dispersed PdNCLs onto surface of ZrO2.
EXAMPLE 3: Catalytic activity in various chemical reaction
EXAMPLE 3a: Catalytic activity in SM Coupling reaction
Iodobenzene and phenylboronic acid were selected as the test substrates and effect of different reaction parameters such as palladium concentration, time, temperature, base, solvent and solvent ratio were studied to optimize the conditions for maximum conversion. We have screened the all parameters thoroughly to achieve maximum % conversion. The effect of catalyst amount (Pd concentration) was screened between 0.0194 mol% - 0.0776 mol% and results show that only 0.0194 mol% of catalyst is capable to achieve 99 % conversion. Similarly, the influence of time on catalytic conversion was screened between 10 min to 40 min and effect of temperature was studied from 60 to 90 °C. Effect of organic and inorganic bases was also studied and K2CO3 was found as the best for maximum 99% conversion. The effect of different solvent (ethanol, acetonitrile, toluene and water) as well as solvent ratio were also evaluated an appropriate solvent for the present system.
From the above study, only 0.0194 mol% of Pd is sufficient for the maximum % conversion (99), along with very high substrate/catalyst ratio (5150/1), TON (5099) as well as TOF (10198 h-1).
EXAMPLE 3b: Catalytic activity in Heck coupling reaction
Similarly, in this case, iodobenzene and styrenewere selected as the test substrates. Effect of different reaction parameters such as palladium concentration, time, temperature, base, solvent (DMF, ethanol, toluene and water) and solvent ratio was studied to optimize the conditions for maximum conversion.
From the above study, only 0.117 mol% of Pd is sufficient for the maximum % conversion (98), along with very high substrate/catalyst ratio (858/1), TON (841) as well as TOF (140 h-1).

EXAMPLE 4: Catalytic activity in control, leaching and heterogeneity test
In both reactions, control experiments were carried out with LTPA, ZrO2, PdCl2 and PdLTPA under optimized conditions in order to understand the role of each component. Obtained results show that LTPA and ZrO2 were inactive towards the reactions. Almost same conversion was found in the case of PdCl2, PdLTPA and PdLTPA/ZrO2 in all reactions. This indicates that Pd is real active species responsible for the reactions and we could succeed in supporting of real active species without any alteration in activity.
The leaching of PdNCLs as well as LTPA from ZrO2 was checked and
found no leaching of either Pd or LTPA from ZrO2. In the present case, it was also found that ZrO2 holds PdLTPA very strongly and does not allow to leach it into the reaction mixture, making it a true heterogeneous catalyst.
EXAMPLE 5: Catalytic activity in recyclability and sustainability of the catalyst
Recyclability and sustainability for PdLTPA and PdLTPA/ZrO2 were studied. PdLTPA exhibited gradual decrease in % conversion due to leaching of active PdNCLs, whereas PdLTPA/ZrO2 displayed constant % conversion up to five cycles for both reactions, confirming the important role played by the support.
EXAMPLE 6: Characterization of Regenerated catalyst
The stability of the regenerated catalyst was studied by its characterization,
such as EDX, FT-IR, BET and XPS.
For regenerated PdLTPA/ZrO2, the EDX values of Pd (0.81 wt %), W (15.44 % wt) and Na (0.84 wt %) are in good agreement with the fresh one (0.82 wt % Pd, 15.47 wt % W and 0.86 wt % Na), confirming no emission of Pd, W as well as Na from the catalyst during the reaction, confirming no emission of Pd and LTPA from ZrO2 during the reaction. Elemental mapping is shown in figure 6.
The FT-IR spectra spectrum of regenerated catalyst shows bands at 1096, 1049, 957, 903, 810 cm-1 corresponding to P-O, W=O, and W-O-W stretching vibration frequencies, respectively. Here, the retention of all the characteristic peaks of PdLTPA without any significant shift indicates that basic structure of Keggin unit
remains intact even after its repeated use confirming the sustainability of the
catalyst.
Identical BET surface area of fresh (206 m2/g) and regenerated (204 m2/g)
catalysts indicates that PdNCLs sites remains intact during the reaction, do not
undergo sintering or aggregation.
XPS spectra of regenerated PdLTPA/ZrO2 are displayed in figure 7. The
spectra of regenerated catalyst are found to be identical with fresh one (Figure
4), confirms the retention of Pd(0) active species as well as W(VI), which did
not undergo reduction during the reaction, indicating the sustainability of the
catalyst.
In conclusion, FT-IR shows the retention of LTPA structure even after its reuse number of times. XPS confirm the retention of the oxidation state of Pd(0). While BET surface area reveals the unaltered uniform dispersion of PdNCLs without sintering.
EXAMPLE 7: Viability of the catalyst
Under the optimized condition, the scope and limitations of substrates were investigated for SM coupling by using different halobenzenes (Table 1).
Table 1 Substrate study for SM coupling

R X R´ Product % Conversion TON/TOF (h-1)
H I H 99 5099/10198
H Br H 63
94 (5 h) 3243/6486
4841/968
H Cl H 8
82 (10 h) 412/824
4223/422
OH Br H 91 4687/9374
NO2 Br H 99 5099/10198
COCH3 Br H 93 4790/9580
Reaction conditions: Conc. of Pd(0) (0.0194 mol%), substrate/catalyst ratio (5150/1).
Similarly, under optimized conditions, scope and limitations of substrates for
Heck coupling were also investigated by using different halobenzenes and
styrene derivatives, the obtained results are presented in table 2.
Table 2: Substrate study for Heck coupling

X R Product % Conversion TON/TOF (h-1)
I H 98 841/140
Br H 71
96 (10 h) 609/102
824/82
Cl H 11
48 (10 h) 94/16
412/41
I CH3 72
82 (10 h) 618/103
704/70
Reaction conditions: Conc. of Pd (0.117 mol%), substrate/catalyst ratio (858/1).
Comparison with reported catalyst
Catalytic activity of the present catalyst is also compared with reported catalysts for C-C coupling reactions (Table 3 & 4) in terms of iodobenzene as one of the substrates.
It is seen for SM coupling (Table 3), that the present catalyst is superior in terms of mol% of Pd as well as reaction time as compared to all reported catalytic systems.
Table 3: Comparison of catalytic activity for SM coupling with reported catalysts in organic-water solvent mixture with respect to iodobenzene
Catalyst Pd (mol %) Solvent Temp. (°C)/Time (h) % Conversion/TON/TOF (h-1)
Pd-ScBTC NMOFs
(Tetrahedron 2013, 69, 9237-9244)
0.5 C2H5OH: H2O
(1:1 mL) 40/0.5 99/194/388
Pd/C
(Open Mater. Sci. J. 2015, 9, 173-177) 0.37 C2H5OH: H2O
(1:1 mL) 40/0.5 99/268/535
Oximepalladacycle catalyst
(RSC Adv. 2015, 5, 49568-49576) 0.3 C2H5OH: H2O
(1:1 mL) RT/0.3 95/317/1057
Fe3O4/Ethyl-CN/Pd
(Organomet. Chem. 2015, 29, 259-265) 0.2 C2H5OH: H2O
(1:1 mL) RT/0.2 98/49/245
G-BI-Pd
(Appl. Organomet. Chem., 31, e3667) 0.45 C2H5OH: H2O
(1:1 mL) 80/0.084 98/219/2613
PdLTPA/ZrO2
(Present catalyst) 0.0194 C2H5OH: H2O
(3:7 mL) 90/0.5 99/5099/10198
In case of Heck coupling (Table 4) also, present catalyst is found superior in
terms of the used solvent medium, mol% of Pd, % conversion as well as high
TON/TOF.
Table 4: Comparison of catalytic activity for Heck reaction with reported catalysts with respect to iodobenzene
Catalyst Pd (mol %) Solvent Temp. (°C)/Time (h) % Conversion/TON/TOF (h-1)
PdTSPc@KP–GO (RSC Adv. 2016, 6, 98956-98967) 0.792 H2O (10 mL) reflux/9 89/111/12
Pd/CNCs
(Catal. Sci. Technol. 2016, 6, 7738-7743) 1.412 DMF (10 mL) 40/5 93/65/13
NO2-NHC-Pd@Fe3O4
(New J. Chem. 2017, 41, 9531-9545) 1.0 CH3CN (5 mL) 80/5 96/96/19
5% Pd/CM
(RSC Adv. 2017, 7, 1833-1840) 0.2 DMA 80/24 61/3050/127
PFG–Pd (New J. Chem. 2016, 40, 1287-1296) 1.7 DMF (3 mL) 120/6 95/56/9
PdLTPA/ZrO2
(Present catalyst) 0.117 DMF: H2O
(3:2 mL) 100/6 98/841/140
In the present invention, the novel catalyst, zirconia supported stabilized palladium nanoclusters by mono lacunary tungstophosphoric acid (PdPW11/ZrO2) prepared by wet chemistry method. The presence of nanoclusters as active sites was exposed by XPS and HRTEM analysis. The efficiency of the catalyst was evaluated for C-C cross coupling (SM and Heck) reactions, superiority of the present work lies in obtaining high conversion (99%) with high substrate/catalyst ratio (5150/1) using very low concentration of Pd (0.0194 mol%) and water as a green solvent at low temperature along with high TON (5099) as well as TOF (10198 h-1) for SM coupling. Similarly for Heck coupling reaction superiority of the present work lies in obtaining high conversion (98%) with high substrate/catalyst ratio (858/1) using very low concentration of Pd (0.117 mol%) and water as a green solvent at low temperature along with high TON (841) as well as TOF (140 h-1). The present catalyst is sustainable up to five catalytic runs and can be used for more without any degradation or leaching. The present stabilization as well as synthetic strategy of the catalyst can be extended for designing of other precious metal-based nanocatalysts for various organic transformations.