DOI:
10.1039/C6RA21218B
(Paper)
RSC Adv., 2016,
6, 95252-95262
Highly efficient continuous-flow oxidative coupling of amines using promising nanoscale CeO2–M/SiO2 (M = MoO3 and WO3) solid acid catalysts†
Received
23rd August 2016
, Accepted 28th September 2016
First published on 29th September 2016
Abstract
The development of promising solid acid catalysts alternative to hazardous liquid acids is essential towards a sustainable chemical industry. This work reports the synthesis of nanostructured CeO2–MoO3/SiO2 and CeO2–WO3/SiO2 solid acids, along with CeO2–MoO3, CeO2–WO3 and CeO2 for continuous-flow oxidative coupling of benzylamine using O2 as a green oxidant. A systematic physicochemical characterization has been undertaken using XRD, Raman, N2 adsorption–desorption, TEM, NH3-TPD, and XPS techniques. It was found that the dispersion of CeO2–MoO3 and CeO2–WO3 species on the SiO2 support leads to remarkable structural and acidic properties, due to the synergetic effect of the respective components. TEM analysis reveals the presence of highly dispersed WO3 (0.8–1.2 nm) and MoO3 (0.8–1 nm) nanoparticles in the synthesized catalysts. Among the various catalysts developed, the CeO2–MoO3/SiO2 sample exhibited higher BET surface area (248 m2 g−1), abundant oxygen vacancy defects, and large amounts of strong acidic sites. Owing to improved properties, the CeO2–MoO3/SiO2 solid-acid showed a superior catalytic performance in the continuous-flow oxidative coupling of benzylamine: the obtained benzylamine conversions for 1 h are ∼11.8, 55, 70, 76, and 96%, respectively, for CeO2, CeO2–WO3, CeO2–WO3/SiO2, CeO2–MoO3, and CeO2–MoO3/SiO2 catalysts. Importantly, the CeO2–MoO3/SiO2 solid acid exhibited a remarkable steady performance in terms of benzylamine conversion (∼88–96%) and selectivity of N-benzylbenzaldimine product (∼96–97.8%) up to 6 h. The outstanding catalytic performance of CeO2–MoO3/SiO2 solid acid coupled with the application of continuous-flow synthesis, economical benefits of the respective oxides, and eco-friendly oxidant is expected to bring new opportunities in the design of industrially-favourable chemical processes.
1. Introduction
Over the past few decades, several chemical processes have been developed using conventional liquid acids, such as H2SO4, HCl, HNO3, and HF.1 These liquid acids have numerous disadvantages that include corrosive nature, high toxicity, produce large amounts of hazardous waste, difficulty in recovery and reuse, and tedious work-up procedure for the separation and purification of the products resulting in various environmental and economical problems.2,3 Therefore, significant efforts have been made to replace liquid acid catalysts with efficient and eco-friendly heterogeneous solid acid catalysts in the chemical industry.4,5 Solid acid catalysts possess high stability, strong acid sites, and excellent hydrophobic surface, which create favourable conditions to perform the chemical reactions under milder reaction conditions.6 Moreover, solid acid catalysts are accompanied with various advantages over the liquid acid catalysts, such as non-corrosive nature, low toxicity, tunable acidic strength, high surface area, facile recovery and reuse, less disposal problem, high purity of the products and their easy isolation.7–9
A variety of solid acid catalysts have been developed, such as zeolites, heteropoly acids, ion exchange resins, clays, and promoted metal oxides for various potential applications in the chemical industry.10–14 Among them, promoted metal oxides have attracted much attention due to their beneficial physicochemical properties, including high surface area, remarkable thermal stability, excellent water-resistance capacity, and strong acidic properties.15 Owing to unique redox properties, abundant structural defects (e.g., oxygen vacancies), and adequate acid–base properties, CeO2-based metal oxides act as efficient catalytic components for various industrial applications, such as three-way catalysis, soot oxidation, water-gas-shift reaction, oxidation of amines, etc.16–20 On the other hand, molybdenum oxide (MoO3)- and tungsten oxide (WO3)-based catalysts play a key role in various acid-catalyzed reactions due to their outstanding acidic properties.4,9,15 It is therefore expected that the combination of CeO2 with MoO3 or WO3 may result in unusual and interesting properties, resulting in exceptional catalytic activities. Support is an important component in heterogeneous catalytic systems, which enhances the dispersion of active phase on the catalyst surface, where most of the catalytic reactions take place.21–23 Moreover, the dispersion of CeO2–MoO3 and CeO2–WO3 on the surface of support (e.g., SiO2, an extensively used support due to its high surface area and strong thermal durability) may lead to the formation of synergistic interfaces with attractive physicochemical properties due to co-operative effect of the respective components, which is favourable for various heterogeneous catalytic applications, including oxidation reactions.
The oxidation reactions play a pivotal role in the chemical industry, which contribute to ∼30% of total chemicals production.24 Particularly, the selective oxidation of amines into imines is an important functional group transformation due to many applications of the imines. For example, imines are key building blocks for the production of dyes, pharmaceuticals, agrochemicals, fine chemicals, anti-cancer agents, anti-inflammatory agents, and biologically active compounds.25–27 In addition, imines act as crucial intermediates for several organic reactions, such as addition, reduction, and cycloaddition. Liquid-phase synthesis of imines via condensation of aldehyde or ketone with primary amine is conventionally used route.28 However, this protocol requires hazardous Lewis acids, highly active carbonyl compounds, and dehydrating agents, which often result in tedious workup procedure and the formation of unwanted products. In contrast, the liquid-phase direct oxidative coupling of amines into imines has been conducted using several inorganic oxidants, such as chromate, permanganate, and peroxides, with the expensive catalysts and harmful organic solvents.29 These methods usually lead to generation of large amounts of metal waste, showing a negative effect on the environment. As a consequence, the development of alternative efficient methods to liquid-phase processes using environmentally benign oxidants and inexpensive catalysts is needed towards a sustainable chemical industry. In this context, continuous-flow oxidative coupling of amines using molecular oxygen seems to be a promising approach to overcome the above drawbacks. Continuous-flow process shows a number of advantages, such as solvent-free conditions, excellent flexibility to modify the conditions during the course of reaction to obtain high yields of the desired products, easy separation and purification of the products.30 In addition, continuous-flow processes can be easily scaled up, which is a key aspect from industrial point of view.31 On the other hand, the use of molecular oxygen for the oxidative coupling of amines is of great interest due to its high abundance, inexpensive, and eco-friendly nature.32
Therefore, the present work investigates the continuous-flow oxidative coupling of amines by selecting benzylamine as a model compound with O2 as a green oxidant over CeO2–MoO3/SiO2 and CeO2–WO3/SiO2 solid acid catalysts. For comparison, the catalytic efficiency of CeO2, CeO2–MoO3, and CeO2–WO3 solid acids was also investigated for continuous-flow oxidative coupling of amines under identical reaction conditions. Various characterization techniques, such as X-ray diffraction (XRD), Brunauer–Emmet–Teller (BET) surface area, Barrett–Joyner–Halenda (BJH) pore size distribution, Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma optical emission spectroscopy (ICP-OES) and ammonia-temperature programmed desorption (NH3-TPD) were employed to analyse the physicochemical and acid properties of the catalysts.
2. Experimental section
2.1 Catalyst preparation
The CeO2–WO3 and CeO2–MoO3 catalysts were synthesized by a facile and economical co-precipitation method. In this method, (NH4)6Mo7O24·4H2O (Sigma Aldrich, AR grade), (NH4)6H2W12O40·xH2O (Sigma Aldrich, AR grade), and Ce(NO3)3·6H2O (Sigma Aldrich, AR grade) were employed as metal precursors and dilute aq. NH3 solution (25 v/v%) is used as the precipitating agent. In a typical experiment, to synthesize CeO2–WO3 and CeO2–MoO3 samples (90
:
10; molar ratio based on metal oxides), the requisite quantities of corresponding metal precursors were dissolved in double distilled water and then stirred for 1 h under vigorous conditions. Afterwards, aq. NH3 solution was added to the above solution by drop-wise until the pH of the solution reached ∼8.5. The obtained slurry was decanted, filtered off, and washed with double distilled water several times until free from the impurities. The collected precipitates were oven-dried at 393 K for 12 h and calcined at 773 K for 5 h at a heating rate of 5 K min−1 in air atmosphere. Pure CeO2 was also prepared using the same preparation procedure.
Deposition co-precipitation method was employed to prepare the CeO2–WO3 and CeO2–MoO3 mixed oxides supported on SiO2 (90 + 10
:
100; molar ratio based on metal oxides). In a typical experiment, an appropriate quantity of colloidal silica was dispersed in double distilled water and then stirred for 1 h under vigorous conditions. Afterwards, the requisite amounts of cerium, molybdenum, and tungsten precursors were dissolved in double distilled water separately and mixed together with the above silica solution under stirring conditions. Aq. NH3 solution was then added to the above solution by drop-wise over a period until the pH of the solution reached ∼8.5. The resulting precipitates were collected, filtered off, and washed with double distilled water several times until free from the impurities. Subsequently, the solid product was oven-dried at 393 K for 12 h. The obtained solid cake was crushed with the help of mortar and then subjected to calcination at 773 K for 5 h in a muffle furnace with a heating rate of 5 K min−1 in air atmosphere.
2.2 Catalyst characterization
The structural features and phase composition of the prepared catalysts were investigated with the help of XRD technique. The XRD patterns were recorded on a Rigaku diffractometer using Cu Kα radiation (0.1540 nm), operated at 40 kV and 40 mA in the 2θ range of 2–80° with a step size of 0.02° and a step time of 2 s. The XRD phases present in the synthesized samples were identified with the help of Powder Diffraction File-International Centre for Diffraction Data (PDF-ICDD). The average crystalline size of the samples was estimated using Debye–Scherrer equation and lattice parameters of the samples were estimated by a standard cubic indexation method. The ICP-OES analysis (Thermo Jarrel Ash model IRIS Intrepid II XDL, USA) has been undertaken to estimate weight percentage of the elements in the developed catalysts.
Raman spectra of the samples were recorded on a Horiba Jobin-Yvon HR800 Raman spectrometer equipped with a liquid-nitrogen cooled charge coupled device (CCD) detector and a confocal microscope. The emission line at 632.81 nm from an Ar+ ion laser was focused on the sample under microscope. The textural properties, such as surface area, pore volume, and pore size distribution of synthesized samples are determined by N2-adsorption–desorption analysis using Micromeritics ASAP 2010 instrument. Prior to the analysis, the samples were evacuated at 423 K for 12 h to remove the residual moisture and flushed with argon gas for 2 h.
The TEM-HRTEM studies were made on a TECNAIG2 TEM microscope equipped with a slow-scan CCD camera and at an accelerating voltage of 200 kV. For the TEM analysis, few milligrams of sample was taken and dispersed in ethyl alcohol. Further, the sample was subjected for ultra-sonication until sample completely dispersed in the ethanol solution. After well dispersion, a droplet was deposited on a copper grid supporting a perforated carbon film and allowed to dry. The specimen was examined under vacuum at room temperature.
The XPS studies of the synthesized catalysts were performed using a Shimadzu (ESCA-3400) spectrometer at a pressure better than 1 × 10−9 Torr. The X-ray source utilized was Mg Kα (1253.6 eV) radiation. The core level binding energies (BEs) were charge corrected with respect to the adventitious carbon (C 1s) peak at 284.6 eV. The NH3-TPD experiments of the samples were performed on a Micromeritics AutoChem 2910 instrument. Prior to TPD analysis, the catalyst was degassed up to 573 K under the flow of He. Then, the NH3 gas was passed through the catalyst for 30 min and subsequently flushed with He gas to remove the physisorbed NH3 gas. A thermal conductivity detector was used for continuous monitoring of the desorbed gas, and the areas under the peaks were integrated. The chemisorbed amount of NH3 was measured by flowing He gas with a rate of 20 mL min−1 from 323 to 1073 K at a heating rate of 10 K min−1.
2.3 Catalytic activity test
The continuous-flow oxidation of benzylamine was carried out in a fixed bed vertical down flow glass reactor inserted into double zone furnace under atmospheric pressure. The required amount (300 mg) of catalyst was made in to pellets with appropriate mess size and mixed with the same amount of quartz grains of similar size. The catalyst was tightly packed in glass reactor by introducing quartz wool on both the ends of the reactor. The upper portion of the glass reactor was filled with glass beads which serve as a pre-heater for reactant molecules. Before the catalytic run, the catalyst was activated at 673 K for 1 h in the presence of oxygen. The oxidation reaction was initiated by feeding benzylamine into the reactor with the help of syringe pump with a fixed flow rate (1 mL h−1) of benzylamine along with oxygen flow (60 mL min−1). The reactions were performed at different temperatures in the range of 433–513 K. After the reaction, the products were collected in an ice cold trap at the bottom of the reactor for every 1 h. The products were confirmed by GC-MS equipped with a DB-5 capillary column. Samples were taken periodically and analysed by GC equipped with BP-20 (wax) capillary column and a FID. In addition to the GC-MS the reaction products (Scheme 1) namely, N-benzylbenzaldimine, benzaldehyde, and benzonitrile are also confirmed by comparing the retention times of the authentic chemicals with that of reaction mixture in a GC with BP-20 (wax) capillary column and a FID.
 |
| Scheme 1 Continuous-flow oxidative coupling of benzylamine. | |
3. Results and discussion
3.1 XRD analysis
XRD is an extensive probe technique to analyse the composition and phase purity of the prepared samples. Fig. 1 represents the XRD patterns of CeO2, CeO2–WO3, CeO2–MoO3, CeO2–WO3/SiO2, and CeO2–MoO3/SiO2. As shown in Fig. 1, the XRD peaks of all samples can be assigned to (111), (200), (220), (311), (222), (400), (331), and (420) planes, which are characteristic reflections of cubic CeO2 with fluorite structure.33,34 In contrast, the diffraction patterns belonging to MoO3 and WO3 are not detected in the XRD profiles of CeO2–WO3, CeO2–MoO3, CeO2–WO3/SiO2, and CeO2–MoO3/SiO2 samples. This observation can be explained by three factors: (1) complete incorporation of molybdenum and tungsten ions into the CeO2 lattice, forming cerium oxide solid solution, (2) amorphous nature of MoOx and WOx, and (3) high dispersion of MoOx and WOx species in the synthesized catalysts.35
 |
| Fig. 1 Powder X-ray diffraction patterns of CeO2, CeO2–WO3, CeO2–MoO3, CeO2–WO3/SiO2, and CeO2–MoO3/SiO2 catalysts. | |
Moreover, the XRD lines corresponding to SiO2 support are not found for CeO2–WO3/SiO2 and CeO2–MoO3/SiO2 samples, which could be due to the amorphous nature of SiO2. As can be noticed from Fig. 1, the XRD patterns of CeO2–WO3, CeO2–MoO3, and corresponding silica supported catalysts are considerably different from pristine CeO2 in terms of peak positions and their intensity. These unusual characteristics obviously indicate the structural modifications in the ceria lattice, which is mostly due to the incorporation of MoOx and WOx species. To understand this, the ceria lattice parameter of the samples was estimated (Table 1). Low lattice parameter values were found for CeO2–WO3, CeO2–MoO3, CeO2–WO3/SiO2, and CeO2–MoO3/SiO2 samples compared with that of pure CeO2. It is a well-known fact that doping of smaller sized ions into the CeO2 lattice leads to a lattice contraction. The ionic radii of Mo6+ (0.062 nm) and W6+ (0.056 nm) ions are low compared with that of Ce4+ (0.097 nm). Hence, the replacement of Ce4+ by smaller sized Mo6+ and W6+ in the CeO2 lattice results in lattice contraction i.e. low lattice parameter (Table 1). The lattice parameter values of CeO2, CeO2–WO3, CeO2–MoO3, CeO2–WO3/SiO2, and CeO2–MoO3/SiO2 samples were found to be ∼0.541, 0.538, 0.537, 0.538 and 0.539 nm, respectively.
Table 1 BET surface area (S), average crystallite size (D), lattice parameter (LP), pore volume (V), and pore size (P) of CeO2, CeO2–WO3, CeO2–MoO3, CeO2–WO3/SiO2, and CeO2–MoO3/SiO2 catalysts
Sample |
Sa (m2 g−1) |
Db (nm) |
LPb (nm) |
Vc (cm3 g−1) |
Pc (nm) |
From BET analysis. From XRD spectra. From BJH analysis. |
CeO2 |
41 ± 2 |
8.9 ± 0.5 |
0.541 ± 0.05 |
0.113 ± 0.01 |
9.82 ± 1 |
CeO2–WO3 |
45 ± 2 |
7.2 ± 0.5 |
0.538 ± 0.05 |
0.292 ± 0.03 |
16.11 ± 1 |
CeO2–MoO3 |
50 ± 2 |
6.5 ± 0.5 |
0.543 ± 0.05 |
0.083 ± 0.01 |
4.37 ± 0.5 |
CeO2–WO3/SiO2 |
145 ± 3 |
6.7 ± 0.5 |
0.538 ± 0.05 |
0.434 ± 0.02 |
8.9 ± 0.4 |
CeO2–MoO3/SiO2 |
248 ± 3 |
5.3 ± 0.5 |
0.539 ± 0.05 |
0.482 ± 0.03 |
6.15 ± 0.6 |
The average crystallite size of the prepared samples is calculated using Scherer equation and the obtained values are presented in Table 1. It was found that the addition of Mo or W and SiO2 to CeO2 results in smaller crystallite size. The strong interaction between the dopant and support with CeO2 can efficiently suppress the growth of the particles, thereby decreased ceria crystallite size. The average crystallite size of CeO2, CeO2–WO3, CeO2–MoO3, CeO2–WO3/SiO2, and CeO2–MoO3/SiO2 samples were found to be ∼8.9, 7.2, 6.5, 6.7 and 5.3 nm, respectively. The estimated ratio of Ce/Mo in CeO2–MoO3 and CeO2–MoO3/SiO2 by ICP-OES analysis was found to be ∼88/12 and 89/11, while the ratio of Ce/W in CeO2–WO3 and CeO2–WO3/SiO2 catalyst was 91/09 and 89/11, respectively. These values are very close to the expected values of Ce/Mo or Ce/W (90/10).
3.2 N2 adsorption–desorption analysis
In order to estimate the textural properties of the prepared samples, such as specific surface area, pore diameter, and pore volume, the N2 adsorption–desorption studies were undertaken and the achieved textural properties are summarized in Table 1. The N2 adsorption–desorption isotherms of prepared samples are shown in Fig. S1 of the ESI.† The CeO2, CeO2–WO3, and CeO2–MoO3/SiO2 samples show a typical type IV isotherm with H1 hysteresis loop, indicating the mesoporous nature of the materials. In contrast, the CeO2–MoO3 sample shows a type IV isotherm with H2 hysteresis loop.36,37 This indicates that the CeO2–MoO3 sample has complex mesoporous structure, i.e. the distribution of pore size and shape is not well defined. On the other hand, the CeO2–WO3/SiO2 sample shows a typical type IV isotherm with H3 hysteresis loop.38 The pore size distribution profiles of the samples are shown in Fig. S2 of the ESI.† Interestingly, the CeO2, CeO2–WO3, and CeO2–MoO3 samples show unimodal size distribution curve, whereas the CeO2–WO3/SiO2 and CeO2–WO3/SiO2 samples show multimodal size distribution curve. The obtained pore volume and pore diameter of the pure CeO2 were ∼0.04 cm3 g−1 and 3.9 nm, respectively (Table 1). The pore volume and pore diameter values for doped and supported samples are considerably increased when compared to that of pure CeO2 (Table 1). It can be noted from Table 1 that the addition of Mo/W and SiO2 to CeO2 remarkably enhances its specific surface area. Among the catalysts prepared, the CeO2–MoO3/SiO2 catalyst exhibited highest specific surface area (Table 1). This observation indicates the existence of strong synergetic interaction between the Ce–Mo mixed oxide and SiO2 that inhibits the crystal growth, hence high specific surface area. The specific surface areas of CeO2, CeO2–WO3, CeO2–MoO3, CeO2–WO3/SiO2, and CeO2–MoO3/SiO2 were found to be ∼41, 45, 50, 145, and 248 m2 g−1, respectively.
3.3 Raman spectroscopy analysis
Raman spectroscopy is a powerful characterization tool to estimate the structural features and structural defects (e.g., oxygen vacancies) in the ceria-based materials. Fig. 2 shows Raman profiles of the samples. A prominent peak was found at about ∼463.6 cm−1 for bare CeO2, indicating the F2g active mode of fluorite structured CeO2 with space group Fm3m.39,40 As shown in Fig. 2, the doped and supported ceria catalysts also exhibited F2g Raman active mode. Interestingly, no Raman bands related to WO3, MoO3, and SiO2 were identified. This observation further confirms the formation of ceria-based solid solutions with cubic fluorite structure, in line with the XRD results (Fig. 1). Interestingly, the shifting of F2g peak position to lower wavenumber with peak broadening was observed after the addition of Mo, W, and SiO2 support to CeO2. This observation clearly indicates the variation in Ce–O vibration frequency after the addition of Mo/W and SiO2, revealing the existence of strong interactions in the respective catalysts, which is highly beneficial for achieving good results in heterogeneous catalysis.41 The above unusual characteristics are more pronounced in the case of CeO2–MoO3/SiO2 catalyst. Surprisingly, only CeO2–MoO3/SiO2 catalyst exhibited an additional broad peak at ∼594.2 cm−1. This peak is attributed to lattice defect sites i.e. oxygen vacancies.42 It has been shown that these point defects play an outstanding role in the activation of oxygen.43–45 It is therefore expected that these oxygen vacancies could act as adsorption sites for oxygen molecules in the oxidation of benzylamine, hence better results could be achieved with the CeO2–MoO3/SiO2 catalyst.
 |
| Fig. 2 Raman spectra of CeO2, CeO2–WO3, CeO2–MoO3, CeO2–WO3/SiO2, and CeO2–MoO3/SiO2 catalysts. Ov: oxygen vacancy. | |
3.4 TEM analysis
The TEM images of CeO2–WO3 and CeO2–MoO3 samples are shown in Fig. 3. It was obvious from Fig. 3 that both samples show highly crystalline nanoparticles with visible lattice fringes. The estimated d-spacings in both samples are ∼0.31 nm, which can be assigned to (111) plane of the CeO2. The average diameter of CeO2 is found to be ∼5–10 and 5–12 nm in CeO2–WO3 and CeO2–MoO3 samples, respectively. A careful observation of Fig. 3 reveals high dispersion of WO3 and MoO3 particles with an average diameter of ∼1–1.2 and 0.8–1 nm in CeO2–WO3 and CeO2–MoO3 samples, respectively. Fig. 4 shows TEM images of CeO2–WO3/SiO2 and CeO2–MoO3/SiO2 samples. Highly crystalline nanoparticles with the estimated d-spacings of 0.31 nm were found in both samples, corresponding to (111) plane of the CeO2. Quite smaller sized CeO2 nanoparticles were found in CeO–WO3/SiO2 (∼4–8 nm) and CeO2–MoO3/SiO2 samples (∼3–7 nm) than on unsupported samples. The estimated particles size of WO3 and MoO3 in CeO2–WO3/SiO2 and CeO2–MoO3/SiO2 samples was 0.8–1 and 0.9–1 nm, respectively.
 |
| Fig. 3 HRTEM images of CeO2–WO3 and CeO2–MoO3 catalysts. | |
 |
| Fig. 4 HRTEM images of CeO2–WO3/SiO2 and CeO2–MoO3/SiO2. | |
3.5 XPS studies
XPS analysis was carried out to know the oxidation state of the elements as well as chemical environment of the elements present in the prepared catalysts. The W 4f core level XP spectra of the CeO2–WO3 and CeO2–WO3/SiO2 catalysts are presented in Fig. S3(A) and (B) of the ESI.† It was found that the CeO2–WO3 sample exhibits binding energies at ∼37.5 and ∼35.5 eV, whereas the CeO2–WO3/SiO2 catalyst shows binding energies at ∼37.4 and ∼35.3 eV for W 4f5/2 and W 4f7/2 contributions, respectively. The appearance of these binding energies suggests the existence of W6+ in the prepared samples.46 Fig. S3(C) and (D)† shows the Mo 3d core level XP spectra of the CeO2–MoO3 and CeO2–MoO3/SiO2 catalysts. The CeO2–MoO3 sample shows two peaks at the binding energies of ∼233.5 and ∼236.6 eV for Mo 3d5/2 and Mo 3d3/2 contributions, respectively. On the other hand, the CeO2–MoO3/SiO2 catalyst showed peaks of Mo 3d5/2 and Mo 3d3/2 at ∼232.6 and 235.7 eV, respectively. Appearance of these peaks reveals the existence of Mo6+ in the synthesized catalysts.47
The Ce 3d core level spectra of CeO2, CeO2–WO3, CeO2–MoO3, CeO2–WO3/SiO2, and CeO2–MoO3/SiO2 catalysts are shown in Fig. 5. The obtained Ce 3d spectra are complex, which is due to hybridization of Ce 4f and O 2p levels and overlap among a series of peaks in the range of ∼880–920 eV. In order to identify the oxidation state of cerium, we labelled the deconvoluted Ce 3d core level spectra as shown in Fig. 5. The Ce 3d spectra are composed with eight peaks, with two sets of spin–orbit multiplets corresponding to Ce 3d3/2 and Ce 3d5/2 contributions. The peaks labelled by u are due to 3d3/2 spin orbit state, while the peaks labelled with v are due to 3d3/2 spin orbit state. As shown in Fig. 5, the peaks categorized by v (881.9 eV), v′′ (888.22 eV), v′′′ (897.76 eV), u (900.25 eV), u′′ (906.89 eV), and u′′′ (916.23 eV) denote the 3d104f0 electronic state of Ce4+, whereas the peaks labelled by u′ (901.81 eV) and v′ (884.16 eV) signify the Ce3+ with the electronic configuration of 3d104f1.48,49 Therefore, both the Ce3+ and Ce4+ ions are presented in all the prepared samples, indicating that the surface of the cerium oxide is reduced to some extent. It is worth noting that the binding energies of CeO2–WO3, CeO2–MoO3, CeO2–WO3/SiO2, and CeO2–MoO3/SiO2 catalysts shift to higher binding energy side compared to pure CeO2, demonstrating the modification of the chemical environment of Ce in doped and supported catalysts. The O 1s core level spectra of the samples are shown in Fig. S4 of the ESI.† It is obvious that all samples exhibit two kinds of oxygen species. Appearance of peak at lower binding energy i.e. ∼528.9 eV indicates the presence of lattice oxygen, whereas the peak at higher binding energy peak at ∼530.5 eV represents the surface adsorbed oxygen species. The obtained Si 2p core level XP spectra of CeO2–WO3/SiO2 and CeO2–MoO3/SiO2 catalysts show a peak centred at ∼102.8 eV, which is attributed to SiO2, in line with the reported values in the literature (Fig. S5 of the ESI†).50
 |
| Fig. 5 The Ce 3d core level XP spectra of (a) CeO2, (b) CeO2–WO3, (c) CeO2–MoO3, (d) CeO2–WO3/SiO2, and (e) CeO2–MoO3/SiO2 catalysts. | |
3.6 NH3-TPD studies
NH3-TPD analysis was undertaken to estimate the acidic properties of the catalysts (Fig. 6). As shown in Fig. 6, various desorption peaks are found for the prepared catalysts, which is due to the variation in the activation energy of NH3 desorbed from different acidic sites present on the catalyst surface. As per the literature, the obtained profiles can be classified as low temperature region (LT) and high temperature region (HT), corresponding to before and after 673 K, respectively.13 Appearance of peaks in LT region indicates the release of NH3 from the weak acidic sites present on the catalyst surface. On the other hand, HT region is attributed to release of strong acidic sites. It was obvious from Fig. 6 that the dispersion of CeO2–MoO3 and CeO2–WO3 on SiO2 support results in improved acidic strength compared with that of respective unsupported catalysts. Among the samples, the CeO2–MoO3/SiO2 sample shows high intensity of HT region, indicating the presence of large amounts of strong acidic sites, which is a key factor in the oxidative coupling of benzylamine as discussed in the following paragraphs.
 |
| Fig. 6 NH3-TPD profiles of (a) CeO2–WO3, (b) CeO2–MoO3, (c) CeO2–WO3/SiO2, and (d) CeO2–MoO3/SiO2 catalysts. | |
3.7 Catalyst screening in continuous-flow oxidative coupling of benzylamine
The catalytic efficiency of the prepared solid acids was tested for continuous-flow oxidative coupling of amines by choosing benzylamine as a model substrate. As shown in Scheme 1, three products, such as N-benzylbenzaldimine (DBI), benzonitrile (BN), and benzaldehyde (BA) are possible in the oxidation of benzylamine with O2 as the oxidant. These products are confirmed by GC-MS analysis (Fig. S6 of the ESI†). The catalytic reactions were performed at 493 K with 1 mL feed rate of benzylamine and 60 mL flow rate of oxygen over the catalyst packed in a fixed bed reactor. Fig. 7 shows the conversion of benzylamine and selectivity of the products obtained for the catalysts up to 6 h. With the increase of reaction time, the conversion of benzylamine is slightly increased up to 2 h and then decreased with time. The benzylamine conversions for CeO2 were found to be ∼11.8, ∼17.2, ∼13.1, ∼8.7, ∼8 and ∼7% for 1, 2, 3, 4, 5, and 6 h, respectively. Interestingly, a high selectivity of N-benzylbenzaldimine (∼90.8%) was found for 1 h, however its selectivity drastically decreased to ∼77% at 6 h, indicating that other side products are formed during the reaction. This is evidenced with the increase of benzaldehyde and benzonitrile products from 8.3 and 0.9% to 20.1 and 2.9% for 1 and 6 h, respectively. It can be noted from Fig. 7 that the CeO2–WO3 and CeO2–MoO3 catalysts show better performance in the oxidation of benzylamine compared with that of pure CeO2. For instance, the CeO2–WO3 catalyst showed ∼55% benzylamine conversion for 1 h time, which is approximately five times higher than the catalytic activity of pure CeO2 (11.8% benzylamine conversion) at similar reaction conditions.
 |
| Fig. 7 Continuous-flow oxidative coupling of benzylamine over the solid acid catalysts. Reaction conditions: benzylamine (1 mL h−1), temperature (493 K), catalyst amount (0.3 g), and O2 bubbling rate (60 mL min−1). ( ) CeO2, ( ) CeO2–WO3, ( ) CeO2–MoO3, ( ) CeO2–WO3/SiO2, and ( ) CeO2–MoO3/SiO2. | |
As well, a high selectivity of N-benzylbenzaldimine (∼95%) was found for 1 h with the CeO2–WO3 catalyst. These remarkable results suggest the significance of WO3 addition in the promotion of catalytic performance of CeO2 towards continuous-flow oxidation of benzylamine. As observed in the case of CeO2, the catalytic activity of CeO2–WO3 was found to decrease rapidly with the increase of reaction time. The achieved benzylamine conversion and selectivity of N-benzylbenzaldimine over the CeO2–WO3 catalyst are ∼36 and ∼87%, respectively for 6 h reaction time. Similarly, the CeO2–MoO3 catalyst exhibited ∼76 and 45% benzylamine conversion for 1 and 6 h times, respectively, which are higher than that of the benzylamine conversion obtained for the CeO2 and CeO2–WO3 catalysts. This observation suggests that the Mo species are more active than W species in the enhancement of the catalytic efficiency of CeO2 for the benzylamine oxidation. Another outstanding observation noticed in the case of CeO2–MoO3 catalyst is that this catalyst shows a steady performance towards the selectivity of the N-benzylbenzaldimine (95.9 and 93% for 1 and 6 h, respectively).
In order to improve their catalytic efficiency, the CeO2–WO3 and CeO2–MoO3 species are dispersed on a SiO2 support. As expected, the CeO2–WO3/SiO2 and CeO2–WO3/SiO2 catalysts showed a higher performance compared with that of unsupported respective catalysts. An exceptional result noticed in this study is that the CeO2–MoO3/SiO2 catalyst shows a 96% benzylamine conversion for 1 h. Moreover, the CeO2–MoO3/SiO2 catalyst shows a steady performance with the increase of time: a 87.9% benzylamine conversion was found for 6 h. Both CeO2–WO3/SiO2 and CeO2–MoO3/SiO2 catalysts show a noticeable stable performance towards the selectivity of N-benzylbenzaldimine: 96 and 97.8% are found for 1 h, respectively, which are slightly decreased to 95.8 and 96% for 6 h reaction time.
The excellent catalytic activity of CeO2–MoO3/SiO2 solid acid in continuous-flow oxidative coupling of benzylamine is due to the combination of various favourable physicochemical properties. It is widely accepted that catalysts with high surface area play a key role in heterogeneous catalytic reactions. High surface area catalysts contain more number of active sites and efficiently promote mass transfer, leading to improved catalytic activities. It was obvious from Table 1 that the CeO2–MoO3/SiO2 catalyst has higher surface area (248 m2 g−1) compared with that of the synthesized catalysts. It has been shown that catalysts having high concentration of acidic sites show high catalytic activity for benzylamine oxidation.54,55 The reason is that the acidic sites activate the amine to give the imine intermediate as shown in Scheme 2. It was found from NH3-TPD studies that the CeO2–MoO3/SiO2 catalyst exhibit more number of strong acidic sites compared to other prepared catalysts (Fig. 6).
 |
| Scheme 2 A plausible reaction mechanism for oxidative coupling of benzylamine. | |
As shown in Scheme 2, oxygen vacancies could play a key role in the activation of O2 during the course of reaction, which can assist to enhance the oxidation of benzylamine. Raman studies reveal that only the CeO2–MoO3/SiO2 catalyst has oxygen vacancies (Fig. 2). Therefore, the outstanding catalytic efficiency of CeO2–MoO3/SiO2 solid acid in the continuous-flow oxidative coupling of benzylamine is due to high surface area, abundant oxygen vacancies, and large amounts of strong acidic sites.
3.8 Reaction mechanism
A plausible mechanism was proposed for the continuous-flow oxidative coupling of benzylamine over the CeO2–MoO3/SiO2 solid acid (Scheme 2). As shown in Scheme 2, an imine intermediate is formed from the oxidative dehydrogenation of benzylamine, with the release of water.39,51–53 Due to instability of the produced intermediate, it transforms to target product through two reaction pathway's.13 In path A, the formed intermediate readily reacts with H2O molecule to yield the benzaldehyde, which subsequently reacts with another benzylamine to give the final coupled imine product. In path B, the benzylamine attacks the intermediate due to availability of lone pair of electrons present on the nitrogen atom of the benzylamine, giving an aminal product, which subsequently looses NH3 molecule to give the target imine product.31 Our results reveal the formation of benzaldehyde (Fig. 7), suggesting that path A is the favourable route to achieve the target imine product. As shown in Scheme 2, the successive oxidative dehydrogenation of imine intermediate yields the benzonitrile, which is evident from the obtained results (Fig. 7).
3.9 Effect of reaction temperature
To understand the influence of temperature on the continuous-flow oxidative coupling of benzylamine, various experiments were carried out using high efficient CeO2–MoO3/SiO2 solid acid at different temperatures (433–513 K) by keeping other parameters constant. The obtained results are shown in Fig. 8 when reaction was performed at 433 K, which is lower than the boiling point of benzylamine, the activity of the catalyst was very low i.e. a ∼48% benzylamine conversion was found for 1 h time. This could be due to low vaporization rate of benzylamine at 433 K, hence less number of reactant molecules to interact with the catalyst and thereby low benzylamine conversion. Moreover, the benzylamine conversion was found to decrease with the increase of time at 433 K, which could be due to the accumulation of reactant molecules on the active sites of the catalyst. As well, a 61.7% selectivity to N-benzylbenzaldimine was found, along with high quantity of benzonitrile (∼32.6%) and small amount of benzaldehyde (6%). With the increase of reaction temperature to 453 K, the benzylamine conversion was found to increase from ∼48 to ∼65% for 1 h time. Interestingly, the selectivity of N-benzylbenzaldimine (∼75–90.6%) and benzonitrile (∼8.7–24.8%) was considerably improved compared to previous temperature and show a poor selectivity to benzaldehyde. Further increase of reaction temperature to 473 K, the conversion of benzylamine was significantly improved (∼88.1%) at 1 h, which is decreased to 53% for 6 h. In contrast, the selectivity to N-benzylbenzaldimine is significantly increased from 79.7 to 90.5% with the increase of time from 1 to 6 h, respectively, at 473 K. When the temperature is increased to 493 K, the conversion of benzylamine markedly increases and reached to maximum conversion of ∼96% for 1 h.
 |
| Fig. 8 Effect of reaction temperature in continuous-flow oxidative coupling of benzylamine over CeO2–MoO3/SiO2 solid acid catalyst. Reaction conditions: benzylamine (1 mL h−1), catalyst amount (0.3 g), and O2 bubbling rate (60 mL min−1). ( ) 433 K, ( ) 453 K, ( ) 473 K, ( ) 493 K, and ( ) 513 K. | |
The high conversion of benzylamine is probably due to the fact that high temperature may help to accelerate the vaporization rate of benzylamine in order to provide more number of reactant molecules to interact with the active sites of the catalyst, resulting in improved amine conversion at 493 K. Noticeably, the benzylamine conversions (∼87.9–96%) were consistent up to 6 h at 493 K. Remarkably, a superior and steady selectivity to desired N-benzylbenzaldimine product (∼90–97.8%) was found at 493 K at all reaction times. When the reaction was performed at 513 K, the conversion of benzylamine is drastically decreased from 93 to 70% with the increase of time from 1 to 6 h, respectively. This could be attributed to low contact time between the reactant and catalyst caused by high diffusion rate of benzylamine over the catalyst surface. Moreover, the selectivity of N-benzylbenzaldimine (75%) is drastically decreased, with the increase of benzonitrile selectivity (24.8%) at 513 K for 1 h. Therefore, the optimum temperature was found to be 493 K for continuous-flow oxidative coupling of benzylamine over the CeO2–MoO3/SiO2 solid acid catalyst.
3.10 Effect of feed rate of benzylamine
In order to explore the effect of reactant feed rate on continuous-flow oxidative coupling of benzylamine, we performed various experiments by varying the feed rate of benzylamine from 0.5 to 2.5 mL h−1 using CeO2–MoO3/SiO2 solid acid at 493 K. The obtained results are presented in Fig. 9. A high conversion of benzylamine (∼91%) with superior selectivity of N-benzylbenzaldimine (∼95%) was found at 0.5 mL h−1 feed rate for 1 h. This observation is due to more contact time between the benzylamine and catalyst at low feed rate, resulting in higher catalytic efficiency. However, low feed rate couldn't provide consistent results with the increase of time, which implies that low feed rate would not be the best choice to perform the reactions. With the increase of feed rate to 1 mL h−1, excellent benzylamine conversion (∼95%) with higher selectivity to N-benzylbenzaldimine (∼97%) was observed. When the feed rate is increased to 1.5 and 2.5 mL h−1, the conversion of benzylamine was drastically dropped to ∼88 and ∼67% for 1 h, respectively. The low catalytic activity at higher feed rate might be due to low contact time between the benzylamine and catalyst. At the same time, the selectivity of N-benzylbenzaldimine was found to decrease with feed rate and the selectivity of side products, like benzonitrile and benzaldehyde was increased when compared to low feed rate. It is therefore concluded that 1 mL h−1 would be the optimized feed rate of reactant to attain better benzylamine conversion with superior selectivity to N-benzylbenzaldimine.
 |
| Fig. 9 Effect of benzylamine feed rate in continuous-flow oxidative coupling of benzylamine over CeO2–MoO3/SiO2 solid acid catalyst. Reaction conditions: reaction temperature (493 K), catalyst amount (0.3 g), and O2 bubbling rate (60 mL min−1). | |
3.11 Continuous-flow oxidation of dibenzylamine
Finally, we investigated the catalytic efficiency of CeO2–MoO3/SiO2 solid acid for continuous-flow oxidation of secondary amine by selecting dibenzylamine (DBA) as a model compound. The obtained results are shown in Fig. 10. The conversion of DBA was found to be ∼93.3% at 1 h, which is considerably decreased with the increase of reaction time. The conversion of DBA over CeO2–MoO3/SiO2 catalyst was found to ∼85, ∼79, ∼63, ∼58 and ∼57% for 2, 3, 4, 5 and 6 h, respectively. A 100% selectivity towards N-benzylbenzaldimine product must be expected because only one step is required to yield the N-benzylbenzaldimine from the oxidation of dibenzylamine (Fig. S7 of the ESI†). However, quite unusual and interesting results were noticed in the continuous-flow oxidation of dibenzylamine with CeO2–MoO3/SiO2 solid acid. The selectivity of N-benzylbenzaldimine was 65.6% for 1 h, which is increased to 81.3% for 6 h. On the other hand, a high selectivity of benzonitrile (∼16.8–32%) was found, along with small quantity of benzaldehyde (2–4%). The key reason for these interesting results is that the formed N-benzylbenzaldimine could undergo hydrolysis with in situ generated water in the oxidation of dibenzylamine, giving benzylamine and benzaldehyde.19 As shown in Scheme 2, these two molecules react with each other to yield the N-benzylbenzaldimine and benzonitrile. On the whole, the CeO2–MoO3/SiO2 solid acid is found to be effective in the continuous-flow oxidation of dibenzylamine.
 |
| Fig. 10 Continuous-flow oxidation of dibenzylamine over CeO2–MoO3/SiO2 solid acid catalyst. Reaction conditions: dibenzylamine (1 mL h−1), reaction temperature (493 K), catalyst amount (0.3 g), and O2 bubbling rate (60 mL min−1). | |
4. Conclusions
In summary, an efficient continuous-flow process for the production of imines was developed over nanostructured CeO2, CeO2–WO3, CeO2–MoO3, CeO2–WO3/SiO2, and CeO2–MoO3/SiO2 solid acid catalysts using O2 as a green oxidant. The catalytic order of the developed solid acids for the continuous-flow oxidative coupling of benzylamine at 1 h was found to be CeO2 < CeO2–WO3 < CeO2–WO3/SiO2 < CeO2–MoO3 < CeO2–MoO3/SiO2. The superior catalytic performance of CeO2–MoO3/SiO2 solid acid is attributed to high BET surface area, more number of oxygen vacancy defects, and high concentration of strong acidic sites. Various reaction parameters, such as time, temperature, and benzylamine feed rate were investigated in continuous-flow oxidative coupling of benzylamine to optimise the reaction conditions. A remarkable observation noticed in this study is that the CeO2–MoO3/SiO2 solid acid shows an outstanding steady performance in terms of benzylamine conversion (∼88–96%) and selectivity of N-benzylbenzaldimine product (∼96–97.8%) up to 6 h. As well, the CeO2–MoO3/SiO2 solid acid was found to be effective in continuous-flow oxidation of dibenzylamine. The outstanding catalytic performance of CeO2–MoO3/SiO2 solid acid coupled with the application of continuous-flow synthesis, cheap metal oxide catalysts, and green oxidant is expected to bring new opportunities in the design of industrially-favourable chemical processes.
Acknowledgements
B. G. and B. M. thank the Council of Scientific and Industrial Research (CSIR), New Delhi for research fellowships. Financial support for this project was received from Department of Science and Technology, New Delhi, under SERB Scheme (SB/S1/PC-106/2012).
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Footnote |
† Electronic supplementary information (ESI) available: N2-adsorption desorption isotherms, pore size distribution profiles, XPS data, oxidative coupling of dibenzylamine and GC-MS data of the products. See DOI: 10.1039/c6ra21218b |
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