EFFICIENT BENZYL ALCOHOL OXIDATION CATALYSED BY RECYCLABLE VANADIUM AND MANGANESE BOUND POLYSTYRENE-ANCHORED THIOPHENE-2-CARBOXALDEHYDE CATALYST

Abstract

The present invention relates to a recyclable polymer-supported catalyst for the oxidation of benzyl alcohol to benzaldehyde, using manganese or vanadium bound to a polystyrene-anchored thiophene-2-carboxaldehyde ligand. The catalyst exhibits high catalytic efficiency and selectivity for benzaldehyde production when used with hydrogen peroxide or tert-butyl hydroperoxide at an optimized temperature of 60°C. The catalyst can be reused for multiple cycles without significant loss of activity, making it both cost-effective and environmentally friendly. The invention also describes a method for performing the oxidation reaction using this catalyst under optimized conditions. This innovation addresses challenges associated with conventional homogeneous catalysts, offering a practical solution for selective oxidation processes in industrial applications.

Specification

Description:The following specification particularly describes the invention and the manner in which it is to be performed:
TECHNICAL FIELD

[001] The present invention relates to the field of catalysis, more specifically to the development of recyclable heterogeneous polymer-supported metal catalysts.

BACKGROUND
[002] Conventional homogeneous catalysts cause challenges in separation and recycling, resulting in more waste and environmental damage. These catalysts often exhibit lower conversion rates and selectivity, making the process less efficient and economical.
[003] Furthermore, many existing catalysts degrade rapidly or lose activity after a few cycles, limiting their practical use.
[004] Some processes also require harsh reaction conditions, which are energy-intensive and less sustainable.
[005] To address these issues inherent in homogeneous catalytic systems, a heterogeneous catalytic system is well required.

SUMMARY
[006] The invention presents a novel approach to the oxidation of benzyl alcohol using a recyclable polymer-supported catalyst, specifically a polystyrene-anchored thiophene-2-carboxaldehyde ligand bound to either manganese or vanadium metal ions. Conventional homogeneous catalysts often present difficulties in terms of separation, recycling, and environmental sustainability. This heterogeneous catalyst overcomes these challenges by providing excellent catalytic efficiency, selectivity for benzaldehyde, and reusability
[007] The object of the invention is to develop a recyclable, heterogeneous catalyst system that overcomes the limitations of conventional homogeneous catalysts in the oxidation of benzyl alcohol.
[008] Another object of invention is to achieve high conversion rates and selectivity for benzaldehyde production under optimized reaction conditions.
[009] Another object of invention is to create a catalyst that is environmentally friendly, reusable, and cost-effective for industrial-scale oxidation processes
[0010] In one of the implementations, the catalyst can be easily separated from the reaction mixture and reused for at least four cycles with minimal loss in performance.
[0011] In another implementation, the oxidation reaction is carried out under optimized conditions, including the use of hydrogen peroxide or tert-butyl hydroperoxide as oxidants, at a temperature of 60°C, with a reaction time of 6 hours.
[0012] In another implementation, the catalyst offers a high conversion rate of up to 73% and selectivity for benzaldehyde up to 89%, making it ideal for industrial applications where cost-effectiveness and environmental sustainability are priorities.
[0013] In another impelematation, the invention is applicable in the chemical and pharmaceutical industries, particularly for processes requiring selective oxidation of alcohols
[0014] In another impelematation, the invention can be used in the production of fine chemicals like benzaldehyde, a key intermediate in the synthesis of perfumes, dyes, and pharmaceuticals.

DETAILED DESCRIPTION
[0015] Synthesis of polymer-anchored ligand : Two grams of aminomethylated polystyrene (PS-CH2-NH2) were soaked in 20 mL of methanol for 1 hour. Thiophene-2-carboxaldehyde (TCA) (9.0 mmol) was dissolved in 50 mL of methanol and added to the polymer suspension. The mixture was refluxed with stirring at 80°C for 37 hours in a water bath. Afterward, it was vacuum filtered and washed several times with hot ethanol, hot methanol, and acetone. Finally, it was dried in an air oven at 70°C. The attachment of the ligand to the polymer was primarily confirmed using the ninhydrin test at regular intervals. (Scheme 1).
[0016] Referring to figure 2, Polymer-anchored ligand PSCH2-TCA (4.5-9.0 mmol) was allowed to soaked in methanol (15 mL) for 1 hr. VOSO4.5H2O/Mn(CH3COO)2.4H2O was dissolved in methanol and then added to the above suspension. The color of the resin changed as the reaction progressed. After refluxing for 20 hours, the mixture was cooled to room temperature, filtered under vacuum, and washed multiple times with hot methanol, ethanol, and acetone. The resin was then dried in an oven at 80°C
[0017] Catalytic oxidation reaction : A benzyl oxidation catalyzed by polymer-anchored metal catalysts was performed in a two-neck flask placed in a thermostatted oil bath. To assess the catalytic efficiency of polystyrene-anchored metal catalysts, the oxidation of benzyl alcohol was carried out using an oxidizing agent. Various reaction conditions were tested, including different oxidants (t-BuOOH and H2O2), temperatures (60°C), reaction times (4, 5, 6, 7 and 8 hours), and catalyst amounts (0.05 and 0.1 g). The catalyst was pre-swollen in 20 mL of CH3CN solvent for 30 minutes in a 50 mL two-neck round-bottom flask. Subsequently, benzyl alcohol (10 mmol) and an oxidant (10 mmol) were added. The mixture was stirred at the specified temperature, and aliquots were taken at different time intervals. Thin layer chromatography (TLC) was employed to monitor the reaction's progress. A control experiment without the catalyst was also conducted. Product formation was confirmed using GC-MS. The recovered catalyst beads were washed, dried under vacuum, and reused.
[0018] Referring figure 2, Aalytical and spectral data of the synthesized compounds:
Polymer-supported ligand [PSCH2-TCA]
Cream beads; C 81.14, H 7.40, N 3.41, S 8.01. Ligand incorporation: 2.43 mmol/g of resin. FTIR (cm-1): ?(C=N) 1652, ?(C-S-C) 824. DRS (cm-1): p ? p* 39062, n ? p* 30303.

P olymer-supported vanadium catalyst [PSCH2-TCA-V]
Olive green beads; C 57.68, H 6.95, N 2.46, S 11.25, V 6.8, Ligand incorporation: 1.75 mmol/g of resin. Vanadium incorporation: 1.34 mmol/g of resin. FTIR (cm-1): ?(C=N) 1639, ?sy(SO4) 1515, ?asy(SO4) 1120, ?(V=O) 966, ?(M-N) 485, ?(C-S-C) 840 , ?(M-S) 443, ?(M-O) 590. DRS (d-d band in cm-1): 2B2?2E 12269, 2B2?2B1 15873, 2B2?2A1 23923; EPR: g? 1.98, g? 1.92; A? 66 G and A? 166 G.

Polymer-supported vanadium catalyst [PSCH2-TCA-Mn]
Light brown; C 72.13, H 6.64, N 3.02, S 6.61, Mn 5.4, Ligand incorporation: 2.16 mmol/g of resin. Manganese incorporation: 0.98 mmol/g of resin. FTIR (cm-1): ?(C=N) 1640, ?sy(OAc) 1450, ?asy(OAc) 1595, ?(M-N) 473, ?(C-S-C) 838, ?(M-S) 433, ?(M-O) 540. DRS (d-d band in cm-1): 6A1g?4T1g(G) 16638, 6A1g?4T2g(G) 22675, EPR: g 1.9, A 85 G
[0019] Referring figure 3, EDX analysis
EDX analysis was used to confirm both the loading of the ligand and the incorporation of the metal ion into the polymer support (Figure 2). The appearance of a vanadium and manganese signal in the catalysts indicates that vanadium and manganese ions have successfully bound to [PSCH2-TCA].
[0020] Referring figure 4, FTIR Analysis
The [PSCH2-TCA] compound displays an absorption band at 1652 cm?¹, attributed to the v(C=N) stretching, indicating the presence of azomethine functionality. In the metal catalysts, this v(C=N) band shifts to a lower frequency of 1639-1640 cm?¹, which signifies the coordination of the azomethine nitrogen to the metal ions. The binding of the metal ion with [PSCH2-TCA] is further confirmed by the presence of C-S-C, M-N, M-S, M-O and M=O bands at 838-840 cm?¹, 473-485 cm?¹ , 433-443 cm?¹, 540 cm?¹ and 966 cm?¹

[0021] Referring figure 5, DRS Analysis
The reflectance spectrum of [PSCH2-TCA-V] displays three distinct transitions at 12269, 15873, and 23923 cm?¹, which fall within the range reported for five-coordinated (C4v symmetry) oxovanadium(IV) compounds. These transitions can be assigned to 2B2g ? 2Eg, 2B2g ? 2B1g and 2B2g ? 2A1g in increasing energy. [PSCH2-TCA-Mn] shows two bands at 16638 and 22675 cm?¹, corresponding to 6A1g ? 4T1g(G) and 6A1g ? 4T2g(G) transitions, suggesting an octahedral geometry
[0022] Referring figure 6, EPR Study
The [PSCH2-TCA-V] shows an axially symmetrical signal typical of V(IV) ions. Eight well-resolved lines (due to 51V; I = 7/2; 2nI + 1 = 8) appear in both the parallel and perpendicular regions, resulting from the coupling of an unpaired electron of V(IV) with its nuclear spin. The presence of distinct hyperfine lines suggests that vanadium ions are well dispersed within the polymer support, with negligible vanadium-vanadium interactions. The spectrum is anisotropic, with Hamiltonian parameters: A? > A? (A ? = 66 and A? =166); g? > g? (g?= 1.98 and g? = 1.92). These values indicate a square pyramidal complex with a V=O bond along the z-axis and C4v symmetry . [PSCH2-TCA-Mn] shows six hyperfine lines due to the interaction between electron spin and nuclear spin (55Mn, I = 5/2, 2nI + 1 = 6). The Hamiltonian parameters, A = 85 G and g = 1.9, are characteristic of Mn(II) compounds with axial symmetry and octahedral geometry.
[0023] In one of the embodiments, to examine how reaction temperature affects the oxidation of styrene using the synthesized catalysts. Reactions were carried out at temperatures ranging from 40 °C to 80 °C, with a 1:1 mmol ratio of benzyl alcohol to either TBHP or H2O2 in an acetonitrile medium over 8 hours. The results, summarized in Tables 1 and 2, show that benzyl alcohol conversion increased as the reaction temperature rose. However, no conversion was detected below 40 °C. Higher temperatures led to rapid peroxide decomposition, reducing the conversion rate. The optimal temperature for the oxidation process was determined to be 60 °C

Table 1. Conversion of Benzyl alcohol and Product Selectivity
Catalyst Amount of catalyst (g) Time (h) % Conversion % Selectivity

Others
Blank test 4 7.7
5 8.8
6 9
[PSCH2-TCA-V]
0.05 4 55 75
5 63 81
6 65 83 10.1 4.4 2.5
0.1 4 62 80
5 66 87
6 69 88 6.3 1.4 4.3
[PSCH2-TCA-Mn]
0.05 4 51 72
5 57 72
6 60 74 12.5 6.1 7.4
0.1 4 55 73
5 59 74
6 62 76 13.5 6.8 3.7
* Conditions: 20 ml CH3CN, 10 mmol benzyl alcohol, 10 mmol H2O2, 60°C; Conversion measured using GC-MS.

Table 2. Conversion of Benzyl alcohol and Product Selectivity

Catalyst Amount of catalyst (g) Time (h) % Conversion % Selectivity

Others
Blank test 4 7.5
5 8
6 8.6 85.6 9.9 2.1 2.4
[PSCH2-TCA-V]
0.05 4 57 77
5 69 82
6 70 85 9.6 3.8 1.6
0.1 4 61 83
5 70 88
6 73 89 5.2 1.3 4.5
[PSCH2-TCA-Mn]
0.05 4 48 71
5 55 71
6 57 75 10.8 5.1 9.1
0.1 4 53 71
5 59 73
6 61 73 11.2 5.8 10
* Conditions: 20 ml CH3CN, 10 mmol benzyl alcohol, 10 mmol H2O2, 60°C; Conversion measured using GC-MS.

Tables 1 and 2 illustrate how the amount of catalyst influences benzyl alcohol oxidation over time. Additionally, an experiment was conducted using benzyl alcohol with TBHP and H2O2 without adding a catalyst, resulting in a benzyl alcohol conversion rate of 7.5 % to 9.0 %. Increasing the catalyst amount to 0.1 g enhances benzyl alcohol conversion rates. This improvement can be attributed to the increased availability of active sites on the catalyst, which allows more substrate and oxidant molecules to interact with the catalyst. However, further increases in catalyst amount did not significantly affect benzyl alcohol conversion, which remained nearly constant.
[0024] Referring to figure 7, the progress of the reaction, benzyl alcohol oxidations with TBHP and H2O2 were conducted in a 1:1 ratio, using 0.05 g and 0.1 g of catalyst at 60 °C with continuous stirring. The reaction showed minimal activity in the first 2 hours, indicating an induction period. After this period, the conversion of benzyl alcohol steadily increased with longer reaction times for all catalysts tested. Tthe highest rate of benzyl alcohol conversion was observed after 6 hours. Additionally, the selectivity for different products remained largely consistent as the reaction time increased.
[0025] Referring to Table 3 and figure 8, to evaluate the reusability of the polymer-anchored metal catalysts, recycling experiments were performed for the oxidation of benzyl alcohol. After each experiment, the catalyst was separated through simple filtration, washed, dried under vacuum, and reused under the same reaction conditions. The catalytic process was repeated with additional substrate added as needed under optimal conditions, yielding final products similar in nature and amount to those from the initial run. As shown in Table 3 and Figure 8, a slight decrease in conversion was observed after four cycles, though product selectivity remained unchanged. The recovered catalyst was analyzed using IR and DRS spectra, which showed no alterations even after multiple reuses. Additionally, the metal content of the catalyst was determined by AAS after filtration. By the fourth recycle, a slight loss in metal content was noted. These results suggest that the catalysts maintain their integrity and effectiveness through multiple uses.

Table 3. Recyclability Test
Catalyst Cycle % Conversion % Selectivity

Others
[PSCH2-TCA-V]
1 69
2 69
3 69.9 88 6.3 1.4 4.3
4 69.7
[PSCH2-PCA-Mn]
1 62
2 62
3 61.8 74 12.5 6.1 7.4
4 61.7
* Conditions: 20 ml CH3CN, 10 mmol benzyl alcohol, 10 mmol H2O2, 60°C; Conversion measured using GC-MS.

[0026] Referring to figure 9, For the oxidation of benzyl alcohol (BzOH), we propose a mechanism involving the formation of a peroxo complex through the reaction of ROOH with the polymer-attached catalyst. In the initial step, a peroxo complex is formed. In the subsequent step, this peroxo complex interacts with BzOH to create an intermediate (I). Finally, the intermediate loses an ROH molecule, resulting in the formation of benzaldehyde and the regeneration of the catalyst.
[0027] In another embodiment, it is advantageous to immobilise the catalyst on an insoluble support since this improves the catalyst's stability, selectivity, recyclability, and ease of separation from the reaction products. The heterogeneous catalyst was reused four times without significant loss of activity
[0028] In another embodiment, wherever possible, solvents and catalysts are recovered and reused, reducing both costs and waste.
[0029] In another embodiment, the invention discloses the synthesis, catalytic activity and characterization of heterogeneous catalyst which was found to be the most effective in producing higher yield oxidation products at 0.1 g concentration of catalyst. The reaction conditions were optimized with respect to temperature, time, oxidant and catalyst amount In another embodiment,
[0030] In another embodiment, reaction conditions are optimized to get maximum yields and reduce the consumption of reagents. This includes adjusting reaction times, temperatures, and the concentrations of reactants to improve the efficiency of the synthesis.
[0031] In another embodiment, Cost of reagents, equipment, analysis, and waste disposal, were minimized throughout the whole process which can further be minimized on bulk scale production
, Claims:1. A recyclable polymer-supported metal catalyst comprising a polystyrene-anchored thiophene-2-carboxaldehyde ligand, wherein, polystyrene-anchored thiophene-2-carboxaldehyde ligand is bound to manganese or vanadium metal ions to form a heterogeneous catalyst for the oxidation of benzyl alcohol
2. The catalyst as claimed in claim 1, wherein the manganese or vanadium incorporation into the polystyrene-anchored thiophene-2-carboxaldehyde ligand is between 0.98 mmol/g and 1.34 mmol/g, providing optimized active sites for benzyl alcohol oxidation.
3. The catalyst as claimed in claim 1, wherein the oxidation reaction occurs at a temperature of 60°C using tert-butyl hydroperoxide (TBHP) or hydrogen peroxide (H2O2) as the oxidizing agent, achieving a conversion rate of at least 73%.
4. The ligand as claimed in claim1, wherein, the ligand is characterized by enhanced catalytic efficiency, high selectivity for benzaldehyde production, and the ability to retain catalytic activity for at least four reuse cycles.
5. A method for oxidizing benzyl alcohol to benzaldehyde using a recyclable polymer-supported metal catalyst, comprising the steps of:
a. pre-swelling the polymer-supported catalyst in a solvent for a predetermined time,
b. adding benzyl alcohol and an oxidizing agent selected from TBHP or H2O2,
c. stirring the mixture at a temperature of 60°C for 6 hours,
d. recovering the catalyst through filtration and reusing it in subsequent oxidation reactions.
6. The method as claimed in claim 5, wherein, the solvent used for pre-swelling the polymer-supported catalyst is acetonitrile, and the reaction is monitored using thin-layer chromatography to track product formation.

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