Synthesis and characterization of copper deposited on MCM-41 synthesized by co-condensation and post-synthesis grafting methods for the synthesis of 5‑substituted 1H‑tetrazoles in water

doi:10.21203/rs.3.rs-1421816/v1

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Synthesis and characterization of copper deposited on MCM-41 synthesized by co-condensation and post-synthesis grafting methods for the synthesis of 5‑substituted 1H‑tetrazoles in water

https://doi.org/10.21203/rs.3.rs-1421816/v1

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Cu-functionalized mesoporous MCM-41 materials were synthesized via two different types of post-functionalization modification of MCM-41, without the ligand (MCM-41@Cu) and with ligand (MCM-41@Histidine@Cu). Cu-functionalized mesoporous MCM-41 sample was also prepared directly via co-condensation of CuCl₂ and decyltrimethylammonium bromide (DTAB) with tetraethyl orthosilicate (TEOS) (Cu-MCM-41). The textural properties of the samples were characterized by powder X-ray diffraction (XRD), Fourier transforms infrared (FTIR) spectra, nitrogen adsorption − desorption, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The results show that the structure of MCM-41 persisted after modification. These catalysts are efficient catalysts for the synthesis of 5-substituted 1Htetrazoles in water. Cu-functionalized mesoporous MCM-41 materials displayed high catalytic performance at low-temperature. Cu-functionalized MCM-41 sample synthesized with direct co-condensation method displayed better catalytic activity compared to Cu-functionalized mesoporous MCM-41 materials prepared with post-synthesis grafting methods. These catalysts can be easily recovered.

Cu-functionalized

Mesoporous MCM-41

Synthesis of 5-substituted 1H tetrazoles.

Mesoporous silica materials have received considerable attention in the field of catalysis and separation [1–4]. The acidity, reactivity, and ion-exchange capacity of pure mesoporous silica materials are weak due to the absence of active sites in their structure [5–7]. The high concentration of the silanol groups on the pore walls of mesoporous silica are helpful to the insertion of active sites [7–9]. MCM-41 as mesoporous silica materials are ideal solid supports for insertion of active sites due to their highly ordered pore structure with diameters of 2 − 10 nm and large surface areas [7, 10–13].

Ordered mesoporous silica may be synthesized in the presence of different polymers and surfactants. Alkyl trimethylammonium bromides (ATABs) were widely used as a surfactants [14]. The length of the ATAB alkyl chain affects the pore size [15–16]. Heavier surfactants were seldom used because they were difficult to dissolve. ATAB surfactants with lower molecular weights appear to be more difficult to organize themselves, so they produce less ordered materials with wider pore size distributions [17].

There are two overall methods for the modification or derivatization of mesoporous silica: direct co-condensation and post-synthesis grafting method [18–20]. Post-functionalization modification of MCM-41 has gained considerable interest for the fabrication of favorable catalysts [21]. In this method, after the formation of the ordered mesoporous silica the grafting of functional groups has occurred. The post-functionalization modification method uses the silanol groups present on the silica surface [19, 22]. Specific parts of the ordered mesoporous materials surface (pore entrances, pore surface, external surface) can be modified and high functionalization degrees are possible without destroying the mesoscopic order [19, 23]. Functionalization of mesoporous silica with both inorganic and organic materials is attractive because of the possibility of conjoining the various groups with mesoporous materials [19, 24–25]. Asefa et al described the preparation of catalysts that include organ amine and silanol groups for the Henry reaction [26]. Cu-functionalized MCM-41 were synthesized with DL-2-Pyrrodidone ligand for the synthesis of symmetrical disulfides and thioether formation reaction by Molaei et al [27]. An exciting choice is the presentation of active species via direct co-condensation technique while the mesoporous materials are being prepared [28]. Co-condensation is a one-step approach in which the functional molecules are introduced into the materials during mesoporous development [29]. This quite simple and inexpensive approach may lead to the completely homogenous distribution of active species in the structure of silica [30].

Many studies have been recognized in the incorporation of transition metal elements in the silicate framework for the preparation of catalysts. Copper is a very appealing element because of its polarizability and special redox properties [31]. Therefore, copper-based catalysts have been the focus of recent studies [27, 32–35]. The Cu/ZnO on the PMS (periodic mesoporous silicates) have been synthesized by Grunert and coworkers [35]. Zinc and copper-containing MCM-48 and MCM-41 materials were prepared by Hartmann et al. [36]. Cu-Al MCM-41 was studied by Kevan and coworkers [37].

The aim of this report is to investigate the efficiency of the grafting and co-condensation methods for the synthesis of Cu-functionalized mesoporous MCM-41 materials. In this way, Cu-functionalized mesoporous MCM-41 were synthesized by two different types of post-functionalization modification methods including: with or without ligand (MCM-41@Cu and MCM-41@Histidine@Cu). Cu-functionalized MCM-41 sample was also synthesized directly by co-condensation of CuCl₂ and decyltrimethylammonium bromide (DTAB) with tetraethyl orthosilicate (TEOS) (Cu-MCM-41) for the synthesis of 5-substituted 1Htetrazoles.

2.1. Preparation of mesoporous MCM-41

1 g of decyltrimethylammonium bromide (DTAB) was introduced to 480 mL of distilled water with stirring to get a homogeneous solution. 3.5 mL of sodium hydroxide (2M) was introduced to this solution and stirring was continued for 5 minutes at 80°C. After that, 5 mL of tetraethyl orthosilicate (TEOS) was introduced to the solution and stirred for 2 hours at 80 ° C. The obtained product was collected and washed with distilled water after two hours. The MCM-41 was dried at 60°C and calcinated at 550°C for 5 hours to completely remove the surfactant [5] (Scheme 1.)

2.2. synthesis of Cu-functionalized mesoporous MCM-41

2.2.1. Post-synthetic modification of MCM-41 with Histidine

1 g of MCM-41 mesoporous silica and 1.5 g of Histidine ligand in deionized water (40 mL) were completely dispersed. The resulting mixture was refluxed for 48 hours. After that, the precipitate was collected by filter paper and washed with water. The MCM-41@Histidine was dried in an oven at 60°C.

2.2.2. Cu-functionalized MCM-41 via post-synthesis grafting method (with ligand)

In this step, 1 g of the previous step precipitate (MCM-41@Histidine) with 2.5 mmol of CuCl₂. in ethanol solvent (40 mL) was refluxed for 16 hours. After that, the resulting precipitate was collected and washed several times with ethanol, and finally dried in an oven at 50°C. The obtained Cu-functionalized MCM-41 were designated as MCM-41@Histidine@Cu (Scheme 2.).

2.2.3. Cu-functionalized mesoporous MCM-41 via post-synthesis grafting method (without ligand)

Afterward, the obtained MCM-41 (1 g) with 2.5 mmol of CuCl₂ in 30 mL of ethanol solvent was refluxed for 16 hours. After that, the resulting precipitate was collected and washed several times with ethanol, and finally dried in an oven at 50°C. The obtained Cu-functionalized MCM-41 were designated as MCM-41@Cu (Scheme 3.).

2.2.4. Cu-functionalized mesoporous MCM-41 via direct co-condensation method

The method for preparation of Cu-functionalized mesoporous MCM-41 by direct co-condensation technique was reported previously [38]. 5 g of decyltrimethylammonium bromide (DTAB) was introduced to 66 mL of a solution containing 1.0 g of NaOH. After that, TEOS (20 mL) was introduced to the solution and stirred until a clear gel was obtained. Then the solution of CuCl₂ was added slowly and stirred for 2 hours. Then the final mixture was transferred to a Teflon bottle and subjected to autogenous pressure for 7 days at 90°C. The resulting precipitate was collected and rinsed with distilled water. Then, the product was dried at 50°C and calcinated at 550°C for 24 hours to completely remove the surfactant. The obtained Cu-functionalized mesoporous MCM-41 were designated as Cu-MCM-41(Cu: Si = 1:50) (Scheme 4.).

2.3. General process for the synthesis of 1H- tetrazoles

1 mmol of nitrile, 1.2 mmol of sodium azide, and 70 mg of catalyst were weighed separately and poured into a 25 ml flask. 3 ml of H₂O was added to the reaction mixture and placed at 80°C. Progression of the reaction was followed by TLC with a suitable ratio of n-hexane and ethyl acetate solvent (3:2). After the end of the reaction, the catalyst was filtered and the reaction mixture was treated with ethyl acetate and acidified with 10 mL of HCl (5 N). After cooling the reaction solution, the precipitate (tetrazole) formed with 2–5 mL of cold water was washed and the yield of the product was determined using the weight of the precipitate obtained.

2.4. Spectral data

5-(3-Nitrophenyl)-1 H -tetrazole

¹HNMR (400 MHz, DMSO, ppm): δ 7.93–7.95 (m, 1H), 8.45–8.50 (m, 2H), 8.85 (s, 1H); FT-IR (KBr) γ cm^− 1: 3418, 3084, 3040, 2930, 2860, 1625, 1540, 1483, 1361, 1201, 1170, 1109, 931, 821, 794, 741, 673 (Table 3, Entry1).

5-(4-Nitrophenyl)-1 H -tetrazole

¹HNMR (400 MHz, DMSO, ppm): δ 8.35–8.40 (d, 2H), 8.47–8.49 (d, 2H); FT-IR (KBr) γ cm^− 1: 3418, 3242, 2932, 2861, 2030, 1620, 1535, 1352, 1166, 1082, 1001, 866, 756, 623, 484 (Table 3, Entry 2).

5-(4-Bromophenyl))-1 H -tetrazole

¹HNMR (400 MHz, DMSO, ppm): δ 7.83–7.86 (m, 4H); FT-IR (KBr) γ cm^− 1: 3421, 3002, 2931, 2859, 2758, 1609, 1459, 1369, 1231, 1178, 1048, 988, 713, 626, 485 (Table 3, Entry 14)

3.1. Characterization

3.1.1. FT-IR

The FT-IR analysis of MCM-41, Cu-MCM-41, MCM-41@Cu, and MCM-41@Histidine@Cu are shown in Fig. 1. The FT-IR spectra of as-synthesized vibration bands of samples with MCM-41 support, including Si − O−Si asymmetric stretching vibration at 1076–1095 cm^− 1, Si − O−Si symmetric stretching vibration at 796–810 cm^− 1, and Si − O−Si bending vibration at 460–470 cm^− 1 [39] were detected. The broad bands around 3450 cm^− 1 are associated to the − OH stretching vibration of the H₂O molecule [31], the band around 1630 cm^− 1 is associated to the − OH bending vibration from adsorbed water [31].

For MCM-41, the presence of decyltrimethylammonium bromide (DTAB) lead to the bands appearing at 2850, and 2920 cm^− 1, which are associated to the symmetric and asymmetric stretching of − CH₂−, respectively. The band around 3740 cm^− 1 is associated to the stretching vibration of − OH from terminal silanol groups (Si-OH), which is invisible after the coordination with cupric ions or condensation of silanol groups with the organic group.

For as-synthesized Cu-MCM-41, The vibration bands at 959–970 cm^− 1 are due to the incorporation of metal into the silica framework. This band is due to the stretching vibrations of Si-OH or Si-O-M (M = metal) [39–40], indicating the heteroatom incorporation into the framework of mesoporous materials [41]. This band is weak for parent MCM-41[40].

For as-synthesized MCM-41@Cu, there is a band observed at 958 cm^− 1, which corresponds to the vibration of Si-O-Cu⁺ in the MCM-41@Cu sample. These results suggest that copper species have been effectively applied on the MCM-41 framework.

For the Histidine, the bands at 676–721 cm^− 1 are associated to the bending vibration of the sp² CH group, 1141 cm^− 1 is associated to the stretching vibration of CN in the imidazole group, 1411 cm^− 1 is associated to symmetric COO group stretching vibration, 1452 cm^− 1 is due to symmetric NH bending vibration, 1613 cm^− 1 is associated to C=C stretching vibrations, 2034 cm^− 1 is associated to Ν−Η stretching vibrations, 3200–3500 cm^− 1 are associated to the stretching vibrations of Ο−Η of H₂O [42].

For as-synthesized MCM-41@Histidine@Cu, the specific band at 1452 cm^− 1 in Histidine is absent in the spectrum of MCM-41@Histidine@Cu, revealing that the binding of Histidine with cupric ions occurred through the amine group [42].

3.1.2. XRD

Figure 2 shows the small-angle XRD (SXRD) patterns calcined MCM-41, Cu-MCM-41, MCM-41@Cu, and MCM-41@Histidine@Cu. The X-ray diffraction of MCM-41 shows a strong peak at 2θ = 1.5° corresponding to the reflections of the (100) plane and two weak peaks at 2θ = 3.3°, 3.9° corresponding to the reflections of the (110) and (200) plane, indicating a well-ordered two dimensional hexagonal (p6mm) formation of channels [39, 43]. Therefore, after the introduction of copper species, the structure of the MCM-41 material remains unchanged. However, the intensity is less than pure MCM-41. The reduction of the intensity of the reflections of the (d₁₀₀) plane (except for MCM-41@Histidine@Cu) and the disappearance of d₁₁₀ and d₂₀₀ plane reflections can be attributed to the fall in the ordering of the channels after impregnation the copper particles [39, 44].

The wide-angle XRD (WXRD) analysis of Cu-MCM-41, MCM-41@Cu, and MCM-41@Histidine@Cu samples are illustrated in Fig. 3. WXRD analysis of Cu-MCM-41 displays peaks at 2θ = 36.2 and 39.1°, which correspond to monoclinic CuO [45]. The diffraction peak at 2θ = 75.1° can be due to metallic copper species [46]. But these diffraction peaks are absent in MCM-41@Cu, and MCM-41@Histidine@Cu samples because of low copper content. In addition, the feature at 2θ = 23° was due to amorphous silica [46].

3.1.3. N₂ adsorption-desorption isotherms

Figure 4 depicts the N₂ adsorption-desorption isotherms of the Cu-MCM-41, and MCM-41@Histidine@Cu samples. All the copper-containing MCM-41 samples display type IV isotherms, which are characteristic of mesoporous materials with 2-dimensional cylindrical channels according to the IUPAC nomenclature [47]. It can be concluded that mesopores of the particles are retained even after functionalization. Textural parameters of the MCM-41, Cu-MCM-41, and MCM-41@Histidine@Cu samples are summarized in Table 1. The incorporation of Cu species inside the mesoporous channels reduced textural parameters. As can be seen, a gradual reduction in the surface area of the modified samples was found. In other words, the addition groups block the N₂ adsorption by decreasing the available sites. The decrease in pore volume and pore size in comparison with pure MCM-41 is due to the dispersion of copper into the silica walls [31]. The increase in pore size of the Cu-MCM-41 sample in comparison with parent MCM-41 material is due to the higher amount of copper species in the former materials.

3.1.4. TEM, SEM, and EDS

To characterize the array of mesoporous materials, TEM images of Cu-MCM-41 are illustrated in Fig. 5. It can be observed that the Cu-MCM-41 sample exhibits parallel one-dimensional mesoporous channels, indicating copper species does not affect the structure of MCM-41.

The SEM images of Cu-MCM-41, MCM-41@Cu, and MCM-41@Histidine@Cu are illustrated in Fig. 6. It can be observed that the particles are spherical with size ranges from 17–42 nm for Cu-MCM-41, 80–118 nm for MCM-41@Cu, and 87–113 nm for MCM-41@Histidine@Cu. The EDS spectra of Cu-MCM-41, MCM-41@Cu, and MCM-41@Histidine@Cu confirmed the presence of Cu, O, Si, N atoms in the structure of the catalysts. From EDS spectra, sample synthesized via co-condensation technique (Cu-MCM-41) displays a higher amount of copper in framework compared to samples synthesized via post-functionalization method.

3.2. Catalytic activity

The Cu-functionalized mesoporous MCM-41 samples were tested for the synthesis of 5substituted 1Htetrazoles. To optimize the experimental conditions, 4-chlorobenzonitrile and NaN₃ in the presence of the catalyst were selected as trial substrates. The influence of several operating conditions like catalyst content, solvent, the temperature was studied and the results are summarized in Table 2.

3.2.1. Effect of solvent.

The synthesis of 5substituted 1Htetrazoles was performed using different solvents such as DMF, DMSO, acetonitrile, dioxane, toluene, EtOH, EtOH/water, and water. The yield of product was increased in water. The solvent dielectric constant shows an important role in stabilizing the intermediate of the reaction. At a higher dielectric constant of the solvent, the stronger ionic forces stabilize the intermediate of the reaction [41, 48]. Among the solvents, water had the highest dielectric constant (80), after that DMSO (47.2), DMF (38.3), acetonitrile (36.6), EtOH (24.3), and hexane (2.0). The solvent influence can be described according to the extent of the reactants solubility. Sodium azide derivatives are better soluble in water. The overall order of reactivity of all the solvents used in the synthesis of 5substituted 1Htetrazoles using Cu-MCM41 is as follows: water > DMSO > DMF > acetonitrile > EtOH > hexane

3.2.2. Effect of temperature

The product yield decreased with the decline of temperature from 100°C to room temperature. According to the Arrhenius equation, the increase of reaction rate with temperature was expected [40]. For this reaction, the best result was obtained at 80°C.

3.2.3. Effect of amounts of catalyst

The influence of catalyst amounts on the synthesis of 5substituted 1Htetrazoles was studied by using different amounts of the Cu-MCM-41 as the catalyst (30 to 100 mg) at 80°C using water as solvent. From Table 2, the maximum yield was taken when 100 mg of catalyst was utilized. The reason for the maximum yield is a large number of active sites available in the framework of the samples.

3.2.4. Effect of different catalysts

Parent MCM-41 and Cu-functionalized mesoporous MCM-41 samples were tested for the synthesis of 5substituted 1Htetrazoles. Without any metal loading, provided no 1H-tetrazole yield. Cu-MCM-41 provided 77% 1H-tetrazole yield in 15 min. In the case of MCM-41@Cu, 75% 1H-tetrazole yield was provided in 45 min. In the case of MCM-41@Histidine@Cu sample 70% 1H-tetrazole yield was provided in 60 min. The highly dispersed copper ions in the Cu-MCM-41 strongly adsorb the more reactants [41].

Overall, the ideal reaction conditions for the synthesis of 5-substituted 1Htetrazoles were: H₂O as the solvent, 100 mg of catalyst at 80°C (Table 1, entry 10). To generalize the scope of the Cu-MCM-41, MCM-41@Cu, and MCM-41@Histidine@Cu for the synthesis of 5-substituted 1Htetrazoles, under optimal conditions a series of substituted nitriles were used, and the results are shown in Tables 3, 4 and 5. As these Tables, the favorable product was gained under mild conditions, and in short reaction times, the 1H-tetrazoles were obtained with good yields. Due to highly dispersed copper ions, the Cu-MCM-41 shows better activity for the synthesis of 5substituted 1Htetrazoles in comparison with MCM-41@Cu, and MCM-41@Histidine@Cu samples.

3.3. Recyclability study

The recyclability of the Cu-MCM-41, MCM-41@Cu, and MCM-41@Histidine@Cu was considered in the synthesis of 1H-tetrazole. 4-bromobenzonitrile was chosen as a model substrate. At the end of the reaction, the catalyst was separated, washed with EtOAc, dried at 50°C. Then, it was spent in the synthesis of 1H-tetrazole with a fresh reaction mixture. In the regenerated sample after six cycles, catalytic activity was not significantly decrease (Fig. 7).

Cu-functionalized MCM-41 samples were synthesized via two different types of post-functionalization modification of MCM-41, without the ligand (MCM-41@Cu) and with ligand (MCM-41@Histidine@Cu). Cu-functionalized MCM-41 sample was also prepared directly via co-condensation of CuCl₂ and decyltrimethylammonium bromide (DTAB) with tetraethyl orthosilicate (TEOS) (Cu-MCM-41). FT-IR analysis proved the presence of copper species in the framework of MCM-41. XRD pattern displays the hexagonal mesoporous structure of the MCM-41 material remains unchanged after copper modification. The wide-angle XRD analysis of Cu-MCM-41 shows peaks corresponding to monoclinic CuO and metallic Cu species. The MCM-41@Cu and MCM-41@Histidine@Cu samples displayed no diffraction peaks for copper species because of low copper content. BET studies show that incorporation of copper into the silica walls decreased textural parameters. The SEM images of samples show that the particles are spherical with size ranges from 17–42 nm for Cu-MCM-41, 80–118 nm for MCM-41@Cu, and 87–113 nm for MCM-41@Histidine@Cu. From EDS spectra, sample synthesized via co-condensation technique (Cu-MCM-41) displays a higher amount of copper in framework compared to samples synthesized via post-functionalization method. Due to highly dispersed copper ions, the Cu-MCM-41 shows better activity for the synthesis of 5substituted 1Htetrazoles in comparison with MCM-41@Cu, and MCM-41@Histidine@Cu samples.

ACKNOWLEDGEMENTS

The authors are deeply grateful to the University of Kurdistan for the financial support on this research project.

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Tables 1 to 5 are available in the Supplemental Files section.

Scheme 1 to 5 are available in Supplemental Files section.

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Synthesis and characterization of copper deposited on MCM-41 synthesized by co-condensation and post-synthesis grafting methods for the synthesis of 5‑substituted 1H‑tetrazoles in water

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1. Preparation of mesoporous MCM-41

2.2. synthesis of Cu-functionalized mesoporous MCM-41

2.2.1. Post-synthetic modification of MCM-41 with Histidine

2.2.2. Cu-functionalized MCM-41 via post-synthesis grafting method (with ligand)

2.2.3. Cu-functionalized mesoporous MCM-41 via post-synthesis grafting method (without ligand)

2.2.4. Cu-functionalized mesoporous MCM-41 via direct co-condensation method

2.3. General process for the synthesis of 1H- tetrazoles

2.4. Spectral data

3. Results And Discussion

3.1. Characterization

3.1.1. FT-IR

3.1.2. XRD

3.1.3. N₂ adsorption-desorption isotherms

3.1.4. TEM, SEM, and EDS

3.2. Catalytic activity

3.2.1. Effect of solvent.

3.2.2. Effect of temperature

3.2.3. Effect of amounts of catalyst

3.2.4. Effect of different catalysts

3.3. Recyclability study

4. Conclusion

Declarations

ACKNOWLEDGEMENTS

References

Tables

Scheme

Supplementary Files

Status:

Version 1

Synthesis and characterization of copper deposited on MCM-41 synthesized by co-condensation and post-synthesis grafting methods for the synthesis of 5‑substituted 1H‑tetrazoles in water

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1. Preparation of mesoporous MCM-41

2.2. synthesis of Cu-functionalized mesoporous MCM-41

2.2.1. Post-synthetic modification of MCM-41 with Histidine

2.2.2. Cu-functionalized MCM-41 via post-synthesis grafting method (with ligand)

2.2.3. Cu-functionalized mesoporous MCM-41 via post-synthesis grafting method (without ligand)

2.2.4. Cu-functionalized mesoporous MCM-41 via direct co-condensation method

2.3. General process for the synthesis of 1H- tetrazoles

2.4. Spectral data

3. Results And Discussion

3.1. Characterization

3.1.1. FT-IR

3.1.2. XRD

3.1.3. N2 adsorption-desorption isotherms

3.1.4. TEM, SEM, and EDS

3.2. Catalytic activity

3.2.1. Effect of solvent.

3.2.2. Effect of temperature

3.2.3. Effect of amounts of catalyst

3.2.4. Effect of different catalysts

3.3. Recyclability study

4. Conclusion

Declarations

ACKNOWLEDGEMENTS

References

Tables

Scheme

Supplementary Files

Status:

Version 1

3.1.3. N₂ adsorption-desorption isotherms