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Octahedral MnO2 molecular sieve decorated Meso-ZSM-5 catalyst for eco-friendly synthesis of pyrazoles and carbamates Bhaskar Sarmah, and Rajendra Srivastava Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03993 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on December 9, 2017

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Industrial & Engineering Chemistry Research

Octahedral MnO2 molecular sieve decorated Meso-ZSM-5 catalyst for eco-friendly synthesis of pyrazoles and carbamates Bhaskar Sarmah and Rajendra Srivastava*

Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, Punjab-140001, India. E-mail: [email protected] Phone: +91-1881-242175; Fax: +91-1881-223395

1

ACS Paragon Plus Environment


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Abstract Octahedral MnO2 molecular sieve (hereafter designated as OMS) was prepared in the presence of urea and mesoporous ZSM-5 (designated as Meso-ZSM-5) was prepared using propyltriethoxysilane as additive under hydrothermal synthesis condition. OMS decorated MesoZSM-5 was prepared by heating a grounded mixture of OMS and Meso-ZSM-5. Powder X-ray diffraction, N2-sorption, SEM, TEM, thermogravimetric analysis, FT-IR, diffuse reflectance UVvisible spectroscopy, and NH3 & CO2 temperature programmed desorption techniques were used to characterize the material. The catalyst was demonstrated in one-pot, one-step synthesis of pyrazoles via cyclization followed by dehydrogenation as the key reaction steps. Synthesis of carbamates by the reaction of cyclic and acyclic carbonates/di-tert-butyl dicarbonate with amines was another important application of this catalyst. Catalyst exhibited efficient recyclability. No leaching of active species took place even after five recycles. A strong interface between OMS and Meso-ZSM-5 imparts strong acidity and distinguished activity in the synthesis of pyrazole heterocycles and organic carbamates. Keywords: Octahedral MnO2, mesoporous zeolite, heterogeneous catalyst, pyrazole, carbamate.

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Introduction Most of the industrial fine/bulk chemical syntheses are based on single-step organic transformations.1 Several such single-step strategies lead to the production of the final synthetic drugs and other organic compounds. Catalysis and industrial researchers are making significant efforts to develop catalysts and reagents in order to improve the efficiency of such single-step processes.2 The 21st century is currently witnessing consistent development of environmental friendly synthesis processes (with high Eco Score and E-factor) to get the desired products in high yield.3 An exponential growth in the synthesis and use of heterogeneous catalysts are taking place all over the world.4 We are also contributing in this direction by developing suitable zeolites

and

transformations.

metal

oxides

based

sustainable

heterogeneous

catalysts

for

organic

5-10

Synthesis of heterocyclic compounds and amine derivatives are important with respect to pharmaceutical industry. Recently, we reported a sustainable synthetic strategy for the preparation of a wide range of heterocyclic compounds using nanocrystalline zeolites.3 Most of these syntheses are based on acid-base catalysts. However, in several cases a suitable metal/metal oxide is required to catalyze organic transformations. Most of the synthetic protocols for the preparation of heterocyclic compounds involve condensation, cyclization, and oxidation/dehydrogenation steps.11 Synthesis of heterocyclic compounds such as pyrazoles is very important and interesting with respect to industrial and academic points of view.12 Pyrazoles exhibit important biological activities, viz., an estrogen receptor, anticancer, anti-inflammatory, and molluscicidal drug.13 Therefore, their sustainable synthesis is very important. Similarly, organic carbamates are required in large scale because they are synthetic precursors for the production of pharmaceuticals, fungicides, herbicides, pesticides and other industrially important chemicals.14 Organic carbamates are synthesized using toxic chemicals such as phosgene and isocyanate. Therefore, several alternative routes based on oxidative carbonylation of aromatic amines, catalytic reductive carbonylation of nitro aromatics, coupling reaction of amines and carbon dioxide etc. have been developed.15 However, some of these processes are cumbersome & hazardous and require special setup to carry out the reactions. Therefore, the development of an eco-friendly and straight forward synthesis would be highly appreciable. Efforts have been made to synthesize carbamates by the reaction of amines and eco-friendly dimethyl 3



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carbonate/cyclic carbonates.16 Another simple method to produce selected carbamate is the reaction between amine and di-tert-butyl dicarbonate.17 This method is commonly used by organic chemists to protect amine functionality during multi-step organic syntheses. Synthesis of pyrazoles requires cyclization and dehydrogenation as key reaction steps. Therefore, their synthesis can be facilitated by using multi-functional catalysts. Another important point to be considered here is that their synthesis involves large organic molecules and precursor compounds which are difficult to catalyze by the microporous zeolites and metal oxides. Therefore, it is important to select mesoporous zeolites. In the last 10 years, efforts have been made to catalyze large organic molecular transformations using nanocrystalline zeolites.1823

Similarly, octahedral molecular sieve MnO2 (hereafter represented as OMS) is being used in

selective organic transformations.24-26 OMS attracts researchers due to its ion-exchange capacity, negligible toxicity, low cost, excellent stability/activity, and a potential candidate for the catalysis due to the coexistence of multi-valent Mn species. OMS has the capability to oxidize aromatic alcohol to aromatic aldehyde.27 Novel method for the synthesis of OMS and its applications in catalysis are being explored by a few selected researchers across the globe.

In this study, we present a sustainable synthetic process with high eco scale score for the preparation of pyrazoles and carbamates using OMS supported Meso-ZSM-5 catalyst. MesoZSM-5 also produced pyrazole but with low yield. Pyrazole synthesis was facilitated when OMS was supported on the external surface of Meso-ZSM-5. An optimum loading of OMS was required to achieve excellent activity and selectivity for the desired product. Applicability of this catalyst was further demonstrated in the sustainable synthesis of carbamates by the reaction of dialkyl carbonate/cyclic carbonate/di-tert-butyl dicarbonate (boc-anhydride) and amines.

Experimental Materials and methods Potassium permanganate, manganese sulphate mono hydrate, urea, and nitric acid were procured from Loba Chemie Pvt. Ltd., Mumbai, India. Tetraethoxysilane (TEOS), n-propyltriethoxysilane, tetrapropylammonium hydroxide (40% aqueous), and sodium aluminate were procured from 4


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Sigma–Aldrich India Pvt. Ltd. Solvents and reagents were procured from Merck and Spectrochem India Pvt. Ltd.

Catalyst synthesis Meso-ZSM-5 (Si/Al ratio =20) was synthesized by following a reported procedure.28 Hform of Meso-ZSM-5 was used in the catalysis. To exchange Na-form of Meso-ZSM-5, calcined Meso-ZSM-5 (Na-form) (2 g) was added to 1 M aqueous solution of NH4Cl (80 mL), and stirred at 343 K for 6 h. After 6 h, it was filtered and washed with copious of distilled water. This process was repeated two more times (total three times). Finally, the solid product was dried in an oven at 373 K and then calcined in a programmable furnace at 823 K for 6 h (heating ramp 1 K /min) to obtain the H-form of Meso-ZSM-5. OMS was prepared by following a reported procedure.27 For the synthesis of OMS/MesoZSM-5 nanocomposites, OMS and Meso-ZSM-5 with different weight ratios (such as 20, 30 and 40

are

represented

as

OMS(20%)/Meso-ZSM-5,

OMS(30%)/Meso-ZSM-5,

and

OMS(40%)/Meso-ZSM-5, respectively) were grounded in a mortar-pestle. While grounding, ethanol (2 mL) was used to make the mixture homogeneous. Most of the ethanol was evaporated during the grinding process. Finally, the mixture was placed in an oven at 373 K for 2 h to remove the residual ethanol and then heated in a programmable furnace at 573 K for 12 h (heating rate 1 K/min) to obtain OMS/Meso-ZSM-5 nanocomposites.

Procedure of catalytic reactions Synthesis of substituted pyrazoles In a typical synthesis, chalcone (1 mmol), phenylhydrazine (1.5 mmol), catalyst (90 mg), and dichloromethane (4 mL) were added in a round-bottom flask and magnetically stirred for 10 minutes. Dichloromethane was evaporated and the solid mass was heated at 443 K for 24 h. TLC (Ethyl acetate: hexane =15:85; v/v) was performed to monitor the progress of reaction. After completion of the reaction, it was cooled to ambient temperature, and the solid mass was diluted with 5 mL of acetone. Catalyst was removed using centrifuge machine. Acetone was evaporated using rota evaporator and the product was purified using column chromatography [eluent (ethyl

5



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acetate: hexane =5:95; v/v) and silica gel (mesh size 100-200) as the stationary phase]. Products were confirmed using 1H NMR spectroscopy that matched well with the reported data.

Synthesis of N-carbamates Carbonate (2 mmol), amine (2 mmol), and catalyst (50 mg) were magnetically stirred at 353 K for desired time under argon atmosphere. After completion of reaction, the reaction mixture was diluted with 5 mL dichloromethane and the catalyst was removed with the help of centrifuge machine. The organic fraction was subjected to GC (Yonglin 6100; BP-5; 30 m × 0.25 mm × 0.25 µm) and the products were identified using GC-MS (Schimadzu GCMS-QP 2010 Ultra; Rtx-5 Sil Ms; 30 m × 0.25 mm × 0.25 µm). In some cases, products were confirmed by 1H NMR and COSY analysis.

N-Boc protected organic carbamates In a typical reaction procedure, (Boc)2O (1 mmol), primary amine (1 mmol), and catalyst (20 mg) were magnetically stirred at 300 K. Reaction was performed for 10-30 min. After the reaction, the reaction mixture was diluted with 5 mL dichloromethane and the catalyst was removed by centrifugation. The organic fraction was subjected to GC-MS for the identification of products. To purify the crude mixture, column chromatography was performed [using eluent (ethyl acetate: hexane =5:95; v/v) and silica gel (mesh size 100-200) as the stationary phase]. Products were also confirmed by 1H NMR.

Results and discussion Physicochemical characterization Figure 1 shows the XRD pattern of Meso-ZSM-5 that belongs to the MFI framework structure.28 Reflections in the 2θ range (5−80°) for OMS correspond to the (110), (200), (310), (211), (301), (411), (600), (002), and (541) planes (Figure 1a) that matches well with the octahedral manganese oxide reported in the literature (ICDD No. 00-29-1020).27 In the XRD pattern of OMS(30%)/Meso-ZSM-5, both OMS and Meso-ZSM-5 phases are present which confirms that during the heat treatment process, framework structures of OMS and Meso-ZSM-5 are not disturbed and their individual characteristics are preserved. XRD patterns of 6


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OMS(20%)/Meso-ZSM-5 and

OMS(40%)/Meso-ZSM-5

are very similar

to that

of

OMS(30%)/Meso-ZSM-5 which confirms the framework stability of individual materials at different loading (Figure S1, ESI).

Textural properties of the materials were determined from N2-adsorption desorption measurements. OMS isotherm shows that there is a gradual increase in the adsorption volume in the pressure range of 0.2-0.85 (P/P0), which is followed by the steep increase at a pressure range higher than 0.85 (P/P0) (Figure 2). This Type II adsorption isotherm and H3 hysteresis show that the condensation occurs in the inter-crystalline mesopores in the pressure range of 0.2-0.85 (P/P0) whereas condensation occurs in the macropores above this pressure. Further, the non local density functional theory (NLDFT) and BJH methods were used to determine the porosity. NLDFT pore size distribution confirms the presence of micropores (with micropore diameter of 0.5 nm). BJH analysis shows the bimodal pore size distribution. First pore size distribution falls in the rage of 2-10 nm (mesopores) whereas the second falls in the range of 10-120 nm (mesoporous-macroporous overlapping domain) with a peak maximum of 33 nm. Meso-ZSM-5 shows Type IV isotherm with an increase in N2 adsorption in the pressure region 0.4-0.9 (P/P0), which is due to the capillary condensation in the inter-crystalline mesopores (Figure 2). NLDFT pore size distribution confirms the presence of micropores (with micropore diameter of 0.55 nm). BJH pore size distribution for Meso-ZSM-5 is observed in the range of 3.0–10 nm (with peak maximum of 3.6 nm) (Figure 2, inset). Isotherm and pore size distribution for OMS(20%)/MesoZSM-5 are somewhat similar to that of Meso-ZSM-5 but with less adsorbed volume. Further, the surfae area and pore volume for OMS(20%)/Meso-ZSM-5 are comparatively lower than that of parent Meso-ZSM-5 (Table 1). OMS(30%)/Meso-ZSM-5 exhibits similar isotherm which represents inter-crystalline mesoporosity. But it shows distinct increase of nitrogen adsorption above 0.9 (P/P0). NLDFT pore size distribution shows the presence of micropores similar to that of Meso-ZSM-5. Furthermore, a bimodal BJH pore size distribution with mesopores in the range of 3.0–15 nm (with a peak maximum of 4.0 nm) and 15-120 nm is obtained. Isotherm for OMS(40%)/Meso-ZSM-5 is a combination of Type-II and Type IV. It shows sloping adsorption in the range of 0.4-0.85 (P/P0), followed by the steep adsorption profile similar to that of OMS in the pressure range of 0.85-0.99 (P/P0). NLDFT pore size distribution shows the presence of 7



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micropores similar to that of Meso-ZSM-5. Furthermore, a bimodal BJH pore size distribution with mesopores in the range of 3.0–15 nm and 15-120 nm is obtained. Textural properties such as surface area and total pore volume are obtained from N2-adsorption desorption measurements and the values are summarized in Table 1. It may be noted that surface area (especially external surface area) and pore volume (especially mesopore volume) are decreased after the loading of OMS on the external surface of Meso-ZSM-5.

Surface morphology and microstructure of the materials were studied by SEM. Spheroid like micrograph is obtained for Meso-ZSM-5, whereas aggregated nanowire like morphology is obtained for OMS (Figure 3). SEM images confirm that individual morphologies of OMS and Meso-ZSM-5 are not disturbed in the nanocomposite material even after the heat treatment process, confirming the high stability of the material. EDAX spectra recorded during SEM analysis confirm the presence of various elements in the OMS and nanocomposite material (Figure S2). Micro/nanostructure of the materials was investigated using TEM. High resolution TEM images further confirm the nanowire like morphology for OMS (Figure 4). Each nanowire thickness is less than 10 nm. Selected area electron diffraction pattern further confirms the highly crystalline nature of OMS. TEM investigation confirms that spheroid morphology of MesoZSM-5 is built with 15-20 nm zeolite crystals (Figure 4). Nanowires of OMS are dispersed around the spheroid morphology of Meso-ZSM-5 (Figure 4). Well ordered lattice fringes with an inter-planar spacing of 0.644 nm corresponds to the OMS is observed for nanowires adhered to the surface of Meso-ZSM-5 (Figure 4d, inset).29 Thermograms were recorded in the N2 atmosphere for all the samples. OMS shows three different weight losses in the temperature range of 298-1023 K (Figure 5). Weight loss lower than 573 K is due to the removal of physisorbed water molecules, whereas weight loss in between 573 K - 873 K is due to the removal of chemisorbed water molecules or dehydration of surface -OH groups present in the sample (Figure 5). Weight loss in the temperature range of 873 K - 1023 K can be ascribed to the removal of lattice oxygen from the OMS framework (total weight loss = 13.4 %). TGA profile of Meso-ZSM-5 shows two temperature ranges for weight loss (total weight loss = 3.4 %) (Figure 5). Below 423 K, weight loss is due to the physisorbed 8


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water molecules; however above this temperature, removal of water molecules occurs from the micropores and/or by the dehydration of surface silanol groups. Thermogram of OMS(30%)/Meso-ZSM-5 is the resultant of the thermogram behaviour of OMS and Meso-ZSM5 (Figure 5). Based on the TGA profile and weight loss of OMS(30%)/Meso-ZSM-5 one would clearly say that it is not just a physical mixture of these two components. To confirm this hypothesis, OMS(30%)/Meso-ZSM-5 was also subjected to TGA analysis before heat treatment. Before heat treatment, the OMS(30%)/Meso-ZSM-5 sample exhibits the weight loss of 8.87 %, whereas the weight loss for the heat treated OMS(30%)/Meso-ZSM-5 is 6.06 %. Based on the results obtained it can be concluded that during the heat treatment process, surface –OH groups of OMS and Meso-ZSM-5 react and dehydration takes place which results decrease in the weight loss observed in the case of heat treated OMS(30%)/Meso-ZSM-5 sample. Due to such dehydration reaction, a strong interface between OMS and Meso-ZSM-5 is formed which would impart better activity than its physical mixture. Similar results are also obtained for other two nanocomposites prepared in this study (Figure S3). Diffuse reflectance UV-Vis spectra of OMS, Meso-ZSM-5, and OMS(30%)/Meso-ZSM5 nanocmposites are presented in Figure 6. No significant absorption band is observed for MesoZSM-5. OMS exhibits absorptions in lower wavelength (220−500 nm) as well as in higher wavelength (500−700 nm). The absorption at lower wavelength can be ascribed to O2−→Mnn+ (Mn2+/ Mn3+) charge transfer transition, and the latter can be attributed to d−d absorption band originated from Mnn+ (Mn3+/Mn4+) species [2t2g (↑) 3eg (↑) in Mn4+ and 5E  5T2 transition in Mn3+ species].30 OMS(30%)/Meso-ZSM-5 shows the UV-visible absorption similar to that of OMS but with less intense above 500 nm.

FT-IR spectra of OMS, Meso-ZSM-5, and OMS(30%)/Meso-ZSM-5 are shown in Figure 7. Very less intense FT-IR spectrum is observed for OMS when compared to other two materials. Peaks at 3441 and 1631 cm-1 are due to the stretching and bending modes of water molecules/hydroxyl groups present in the interlayer/surface. Peaks at 524 and 716 cm-1 are atttributed to Mn-O stretching vibrations which is a characteristic band for birnessite.31 MesoZSM-5 shows characterstics peaks that are attributed to the surface –OH groups (3441 cm-1 (stretching), 1631 cm-1 (bending)), asymmetric Si-O-Si (1098 cm-1), symmetric Si-O-Si (798 cm9



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), and MFI pentasil units (550 cm-1), respectively. OMS(30%)/Meso-ZSM-5 exhibit all the

vibrations correspond to OMS and Meso-ZSM-5. Same amount of sample was used for all the FT-IR investigation. OMS(30%)/Meso-ZSM-5 before heat treatment and after heat treatment are subjected to FT-IR analysis. Intensities for stretching and bending modes of –OH frequencies for OMS(30%)/Meso-ZSM-5 are comparatively more reduced for the heat treated sample (at 573 K) when compared to the as-synthesized sample (dried sample). This intensity is discussed with reference to the parent Meso-ZSM-5. Significant less intense absorption peaks at 3441 and 1631 cm-1 for heat treated OMS(30%)/Meso-ZSM-5 confirms the involvement of surface –OH species in the formation of strong interface between OMS and Meso-ZSM-5. Similar results are also obtained for other two nanocomposites prepared in this study (Figure S4). Acid strength and the number of acid sites were determined by ammonia-temperature programmed desorption (TPD) technique (Figure 8). Strong acid sites are known to appear at higher desorption temperature. Reactions are carried out at low temperature, therefore the acidity is presented up to 733 K. Meso-ZSM-5 shows a symmetric desorption profile in the temperature range of 343 – 553 K with a peak maximum of 425 K and a weak broad desorption profile in the range of 593-733 K. First desorption is actually a combination of two acid sites with maxima of 425 K and 490 K. Higher desorption temperature in Meso-ZSM-5 shows that it has acid sites with strong acid strength. Three different types of acid sites are observed for OMS such as weak, medium, and strong acid sites. However, medium strength acid sites are predominant in OMS sample. OMS(30%)/Meso-ZSM-5) exhibits different desorption profile when compared to other two samples. With increase in loading from 20% to 30% OMS in the nanocomposites, more amount of NH3 is desorbed from the higher acid strength site (640 K). However, with further increase in loading to 40%, more NH3 was desorbed from the medium strength acid sites (540 K). In OMS(30%)/Meso-ZSM-5), strong acid sites are predominant that would definitely impart more acidity and activity in the acid catalyzed reaction. This investigation confirms that optimum loading & dispersion of OMS and strong interface between OMS and Meso-ZSM-5 provides higher strength acid sites which is not present in the parent material. Total acidity (mmol/g) present in the materials is provided in Table 1.

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CO2-TPD was performed to evaluate the basic strength and the number of basic sites (Figure 9). CO2-TPD profile of OMS shows that medium strength basic sites are predominant in the sample. CO2 desorption occurs in the range of 373-443 K and 443-643 K for OMS. However, three desorption temperature ranges

are observed

in

OMS(20%)/Meso-ZSM-5 and

OMS(30%)/Meso-ZSM-5 samples. One weak desorption at higher temperature (with peak maximum of 643 K) is observed in OMS(30%)/Meso-ZSM-5 and OMS(20%)/Meso-ZSM-5 samples. Desorption profiles clearly show that the amount of CO2 desorbed from OMS(30%)/Meso-ZSM-5 is comparatively higher than OMS(20%)/Meso-ZSM-5 (Table 1). OMS(40%)/Meso-ZSM-5 exhibits similar desorption profile to that of OMS. However, the peaks maxima are shifted for weak and medium strength basic sites. Based on the CO2-TPD profiles one would conclude that OMS(30%)/Meso-ZSM-5 has more basic strength sites when compared to the parent OMS and other nanocomposites prepared in this study. OMS loading and dispersion on the external surface of Meso-ZSM-5 tailor the total basicity and basic strength that would influence the catalytic activity (Table 1).

Catalytic investigation In this study, emphasis is made to develop a one-pot synthesis protocol for the preparation of pyrazoles. As discussed in the introduction section, cyclization and dehydrogenation are the key steps involved in the synthesis of pyrazoles. In general, solid acid would favour the cyclization, while the suitable metal would dehydrogenates the dihydropyrazole to pyrazole. To optimize the reaction parameters, conventional chalcone is reacted with phenylhydrazine. Catalytic activity data shows that all catalysts investigated in this study are equally active (based on E-chalcone conversion) (Table S1), however, the product selectivity varies. Meso-ZSM-5 is active to produce cyclized product. Using Meso-ZSM-5, equal amounts of dihydropyrazole and pyrazole are obtained (Table S1, entry 1). When OMS is used as a catalyst, large selectivity towards pyrazole is obtained which confirms the involvement of OMS in the dehydrogenation process that leads to the superior selectivity towards pyrazole (Table S1, entry 2). Pyrazole selectivity is further improved when nanocomposites are investigated. Using OMS(30%)/Meso-ZSM-5, 85% selectivity towards pyrazole is obtained (Table S1, entry 3). The influence of chalcone and phenylhydrazine ratio is investigated. 11



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Chalcone conversion and pyrazole yield are increased with increase in the phenylhydrazine to chalcone ratio (Table S1, entries 3-6). The highest chalcone conversion and pyrazole yield are obtained when phenylhydrazine to chalcone ratio is 1.5. With further increase in this ratio, no further improvement in the pyrazole yield is obtained. Reaction temperature has remarkable influence on the product yield. Pyrazole yield is increased with increase in the temperature. Results indicate that temperature of the reaction do not significantly influence the cyclization of chalcone. Instead, the dehydrogenation step is facilitated at higher temperature (Table S1, entries 3, 7-9). At 443 K the highest yield of pyrazole is obtained. Amount of the catalyst is another important parameter to obtain higher yield of pyrazole. Dehydrogenation is facilitated to afford the higher yield of pyrazole with increase in the catalyst amount (Table S1, entries 3, 10-11). It is noteworthy that when OMS loading is increased from 20% to 30%, pyrazole yield is also increased. It may be mentioned that, with increasing the loading of OMS to 40% over MesoZSM-5, yield of pyrazole is decreased (Table S1, entries 3, 12-13). Differences in the catalytic activity obtained using various catalysts investigated in this study can be explained with the help of acidity and textual properties. Higher acidic strength and surface area (especially, the external surface area) of OMS(30%)/Meso-ZSM-5 when compared to OMS(40%)/Meso-ZSM-5 are responsible for the higher catalytic activity. Therefore, the highly dispersed state of OMS in OMS(30%)/Meso-ZSM-5 is responsible for the higher catalytic activity.

Having found the suitable catalyst and optimum reaction condition, scope of the substrates was investigated. In this study, several chalcones and phenylhydrazine/hydrazine were selected and reactions were carried out to synthesize a variety of substituted pyrazoles (Table 2). In all the cases, reactions take place efficiently and afford excellent selectivity for the pyrazoles under optimum reaction condition. Electron withdrawing group present in the chalcone aromatic ring affords higher yield of the product when compared to the electron donating group (Table 2, entries 1-3). The minor amount of the by-product usually includes the intermediate dihydropyrazole. Hydrazine is also found to be successful in producing corresponding pyrazole. However, the product yield is somewhat lower when compared to the phenylhydrazine (Table 2, entry 4). When hydrazine is used as a substrate, pyrazole is exclusively formed in the reaction.

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Catalyst was subjected to the recycling study. Catalytic activity data reveals that the catalyst was stable and exhibited consistent activity even after five recycles (Figure 10a). After completion of each cycle, the catalyst was separated from the reaction mixture and thoroughly washed with dichloromethane. The recovered catalyst was dried in an electric oven at 373 K for 6 h and then used in the next cycle. Recovered catalyst was analyzed using XRD, N2-adsorption, and elemental analysis. XRD patterns of fresh and recovered catalysts are similar which confirms the framework stability of the nanocomposite even after five recycles (Figure S5). Only a marginal decrease in the textural properties is obtained for the recovered catalyst when compared to the fresh one (Table 1). Elemental analysis confirms that the content of Mn in the recovered catalyst is similar to that of fresh catalyst (Table 1, entry 4). Characterizations confirm that no significant change in the physico-chemical properties of the recycled catalyst is observed, which confirms the stability of the catalyst after five recycles (Table 1, entry 4). In order to demonstrate the scalability of this process, 5 times excess reactants are reacted to obtain the desired pyrazole product with 90% yield in 16 h instead of 12 h, which suggests its possibility for further scaleup. It would be interesting to note that this reaction is possible to be accomplished without using a costly metal like Palladium. Literature reports suggest that this reaction can proceed well only in the presence of Pd metal (Table S2).12,

32-33

Comparative catalytic activity shows that the

present catalyst exhibits better activity when compared to various Pd or non-Pd based catalyst reported in the literature (Table S2).12, 32-40

Results show that the one-pot synthesis of pyrazole is accomplished using OMS(30%)/Meso-ZSM-5 nanocomposite. The catalytic activity data also shows that both MesoZSM-5 and OMS are required to achieve high yield of the pyrazole. Meso-ZSM-5 has shown its capability as strong acid catalyst in a wide range of acid catalyzed reactions. In this case also, it is playing a crucial role in the cyclization process (Scheme 1). Carbonyl group of chalcone is activated in the presence of acid sites located in the catalyst, which results in the generation of a carbocation (Scheme 1). NH2 group of the phenylhydrazine attacks the carbocation and produce another cation (benzylic cation) and hydrazone (Scheme 1). Acid sites of the catalyst facilitate the cyclization reaction between benzylic cation and the hydrazone to form dihydropyrazole. The final dehydrogenation step is facilitated by the OMS sites present in the bi-functional 13



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nanocomposite material to afford the final product pyrazole (Scheme 1). High selectivity towards pyrazole can also be correlated with the strong acid sites present in the nanocomposite catalyst when compared to the parent materials.

Another interesting and important molecule which fascinates chemists is carbamate. The sustainable synthesis of carbamates can be achieved by the reaction of amines with organic carbonates. In this study, nanocomposite materials are explored for the preparation of a wide range of carbamates (Table 3 and Table S3). Reaction condition is optimized using benzylamine and dimethyl carbonate as model reactants. In addition to the desired carbamate product, substantial amount of (E)-N-benzylidene-1-phenylmethanamine is obtained using OMS catalyst (Table S3, entry 1). This product is obtained due to the oxidative coupling of benzylamine. In order to confirm the origin of this product, one control experiment is performed. When only benzylamine is reacted with OMS, (E)-N-benzylidene-1-phenylmethanamine is exclusively obtained but with low yield (Table S3, entry 2). Using Meso-ZSM-5, low conversion for benzylamine is observed, however, selectivity for the carbamate product is high (Table S3, entry 3). The highest amine conversion and the carbamate selectivity are observed over OMS(30%)/Meso-ZSM-5 (Table S3, entry 4). Literature reports suggest that basicity of the catalyst is important for higher selectivity towards the production of carbamate.41 Since OMS(30%)/Meso-ZSM-5 has good basicity, it imparts better selectivity towards the desired carbamate product. Various solvents are investigated in this study to obtain a high yield of the desired product. Nonpolar solvent toluene is found to be better when compared to the polar solvents, such as DMF, DMSO, THF, and even acetonitrile (Table S3, entries 4-8). Reaction proceeds much better in the neat condition; therefore further optimization is made in the neat condition. In this case also, OMS(30%)/Meso-ZSM-5 exhibits better activity than other two nanocomposites investigated in this study, which confirms that the high dispersion and better basicity and textural properties are favorable to achieve better activity (Table S3, entries 4-8).

Highly active catalyst was investigated in the synthesis of a wide range of carbamates and results are summarized in Table 3. Benzylamines having different substituent influence the chemical reactivity. With electron donating group (-OCH3), less selectivity for the carbamate is 14


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obtained (Table 3, entry 2). Electron donating group facilitates the oxidative coupling of benzylamine and therefore more selectivity for the carbamate is obtained when the electron withdrawing group is present in the benzylamine (Table 3, entry 3). Further, reaction with electron withdrawing group is more sluggish and exhibits less benzylamine conversion. Results demonstrate that only carbamate is formed when aliphatic amines are reacted with DMC. Primary amine (butyl amine) reacts completely and quantitative yield of the product is obtained (Table 3, entry 4). The reaction with secondary amine (piperidine) is sluggish but produces carbamate exclusively (Table 3, entry 5). It is interesting to observe that quantitative yield of the di-carbamate compound is obtained when, 1,6-hexanediamine is reacted (Table 3, entry 6). However, reaction is slow when ethylenediamine is reacted and produces only 54% yield for the di-carbamate (Table 3, entry 7). Further, the reaction with ethanolamine is very facile and quantitative yield of the carbamate is obtained exclusively, which confirms that under the optimum condition, transesterification has not taken place (Table 3, entry 7). In addition to amines, carbonates are also varied. Reaction proceeds well with diethyl carbonate but the yield of the product is slightly lower under the optimum reaction condition when compared to dimethyl carbonate (Table 3, entries 10-12). When cyclic carbonate is reacted with benzylamine, in this case also, reactions proceed well. Only one product is obtained when symmetric cyclic carbonate (ethylene carbonate) is reacted (Table 3, entry 12) (Figure S6). However, two products (regioisomers) are obtained when substituted cyclic carbonate (propylene carbonate) is reacted. 1

H NMR and COSY NMR (Figure S7, S8) confirms these products. Although various

benzylamine and aliphatic amines have been converted to the corresponding carbamates, but aniline and its derivative could not be converted to carbamate using dialkyl carbonate. Literature reports suggest that the synthesis of carbamate from benzylamine can proceeds using metal complex and organobase functionalized on the heterogeneous support (Table S4).41-43 However, in this study a truly heterogeneous and recyclable catalyst based on OMS/Meso-ZSM-5 is demonstrated which exhibits similar activity to that of heterogenized systems. Reusability and recyclability of the catalyst OMS(30%)/Meso-ZSM-5 is also investigated in the reaction where benzylamine and dimethyl carbonate are reacted in the neat condition. After completion of each cycle, catalyst was separated from the reaction mixture. Recovered catalyst was washed thoroughly with dichloromethane, dried in an electric oven at 373 K for 6 h and then used in the 15



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next cycle. Even after 5th recycle, catalytic activity was retained (Figure 10 b). Comparative data suggests that the present catalyst exhibits better or similar activity to that of various heterogeneous catalysts reported in literature (Table S4).41-43

Organic carbamates have their own significance but most often they are produced as a synthetic intermediate in the multi-step organic synthesis. Amines are protected in the form of carbamate (N-Boc protected product) and then it is de-protected to get original amine functionality. Therefore, it is not only important to get selective protection but it should be easily de-protected to get the original amines. Most commonly, di-tert-butyl dicarbonate (Boc anhydride) is used for the protection of amine to form carbamate. In order to demonstrate the versatility of this catalyst in the selective protection of a wide range of amines, amines are reacted with di-tert-butyl dicarbonate to form N-boc protected amines (Table 4). Since the ditert-butyl dicarbonate is very reactive when compared to organic carbonates, protection is carried out at ambient temperature using 20 mg of the catalyst in just 30 minutes. In contrast to reaction of aniline with carbonates, in this case, reaction goes very well with aniline and its derivatives to produce N-boc protected aniline derivatives (Table 4). Substituent’s present in the aromatic ring in the aniline influences the reactivity (Table 4, entries 2-5). Table 4 reveals that substrate bearing electron donating groups afford higher yields compared to the substrate having electron withdrawing groups. Selective protection of amine group takes place when aminophenol is reacted and O-Boc derivative is not obtained (Table 4, entry 4). Several aliphatic amines and cyclic amines are easily converted to N-boc protected amines as demonstrated in Table 4. In this case, when one equivalent of diamine is reacted with one equivalent of di-tert-butyl dicarbonate, mono protection of the amine is obtained (Table 4, entry 5). In order to carry out di-protection, two equivalents of di-tert-butyl dicarbonate are required. Chemoselectivity towards N-boc protection is further observed when ethanolamine is reacted with the excess of di-tert-butyl dicarbonate. In this case, selective N-boc protected ethanolamine is obtained. In order to demonstrate the scalability of this process, 10 times excess reactants are reacted to obtain the desired N-boc protected aniline with 93% yield in 1 h instead of 30 min, which suggests its possibility for further scale up. Comparative data suggest that the present catalyst exhibits better or similar activity to that of various heterogeneous catalysts reported in literature (Table S5).44-52 16


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Plausible mechanism for the synthesis of carbamate and N-boc protected amine over OMS(30%)/Meso-ZSM-5 by the reaction of amine with dimethyl carbonate/di-tert-butyl dicarbonate is presented in Scheme 2.

Further, the E-factor and eco-score are calculated for various synthesis processes demonstrated above. High value of E-factor indicates hazardous environmental effect. The ecoscore of a chemical process includes the product yield, cost, safety; ease of purification etc. Efactor can be represented as the mass ratio of waste generated to the desired product. The eco score is calculated based on the reported procedure described in our previous study.3 Eco-score and E-factor are calculated for some of the reactions investigated in this study and provided in Table S6 and S7. Table S6 and S7 shows that good eco-score and a low E-factor are observed for pyrazole and carbamate synthesis.

Conclusions This study demonstrated the synthesis of OMS/Meso-ZSM-5 nanocomposites by heating the mixture of OMS and Meso-ZSM-5 in different weight ratios. Optimum loading of OMS (30 wt%) on the external surface of Meso-ZSM-5 provided material with better acidity/basicity and textural properties that were important for the catalytic reactions investigated in this study. OMS nanorods were adhered to the large external surface of spheroid Meso-ZSM-5 as revealed by the TEM investigation. The catalyst exhibited excellent activity in the synthesis of pyrazoles and carbamates. This bi-functional, Pd metal free, solid catalyst efficiently catalyzed one-pot cyclization–dehydrogenation reaction to produce a wide range of pyrazoles by the simple reaction of chalcones and phenylhydrazine/hydrazine. Further OMS(30%)/Meso-ZSM-5 acid– base bi-functional catalyst exhibited excellent activity in the synthesis of a wide range of aromatic and aliphatic carbamates just by the reaction of amines and cyclic/acyclic di-alkyl carbonate or di-tert-butyl dicarbonate under solvent-free and mild reaction condition. Excellent activity of OMS(30%)/Meso-ZSM-5 in these reactions was attributed to the high dispersion of OMS over the surface of Meso-ZSM-5. Catalyst was highly stable and recyclable multiple times with no significant loss in the activity. The protocol was easily scaled up to provide similar yield.

17



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Non-toxic reactants and Pd-free eco-friendly solid catalyst based protocol with easy product isolation make these processes attractive alternative for the industrial applications.

Acknowledgement RS

acknowledge

grateful

to

CSIR, UGC,

New New

Delhi

for

funding

Delhi

for

(01/(2802)/14/EMR-II). fellowship.

Authors

BS

is are

thankful to SAIF, IIT Bombay for HRTEM analysis.

Supporting Information Details of methods and instruments used for materials characterization, XRD patterns, EDAX spectra obtained during SEM analysis of OMS and OMS(30%)/Meso-ZSM-5, 1H and COSY NMR spectra, NMR spectral data, optimization of the reaction parameters in the synthesis of pyrazoles and carbamates, comparative catalytic activities for the synthesis of pyrazole and carbamate, eco-score and e-factor calculations are provided.

18


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Table 1. Physicochemical properties of parent and composite materials investigated in this study.

Catalyst

a

SBET (m2/g)

External surface area (m2/g)

Micro pore volume

Total pore volume

(cc/g)

(cc/g)

b

Mn content (Wt%)

d

Total acid sites NH3desorbed (mmol/g)

d

Total basic sites CO2 desorbed (mmol/g)

Meso-ZSM-5

572

282

0.18

0.56

-

0.191

-

OMS

144

125

0.06

0.38

-

0.167

0.150

OMS(20%)/ Meso-ZSM-5

548

260

0.19

0.54

17.6

0.084

0.048


505

234

0.19

0.50

24.0

0.120

0.106


432

200

0.18

0.56

32.8

0.115

0.097

c

512

238

0.18

0.51

24.2

-

-

OMS(30%)/ Meso-ZSM-5 a

SBET calculated from the adsorption branch in the region of 0.05

Octahedral MnO2 Molecular Sieve-Decorated ... - ACS Publications

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