Imines are important nitrogen compounds; they contain a reactive C=N moiety [1, 2]. Imines are nitrogen sources and can undergo various transformations. They are widely used in biological, agricultural, and pharmaceutical synthetic processes [3-6]. The condensation of aldehydes or ketones with amines is the traditional method for imine synthesis, and is usually performed in the presence of an acidic catalyst [7-9]. Recently, the direct oxidative coupling of an alcohol and amine to give an imine has attracted increasing attention because of its environmentally friendly properties [10-16]. Air or molecular oxygen can be used as the oxygen source for the production of various types of desirable imines from appropriate alcohols and amines as starting reagents, with water as the only by‐product. Effective noble‐metal‐based homogeneous and heterogeneous catalysts such as supported Au, Ru, Pd, and Pt have been reported [17-24]. However, the high price of these noble metals and the need to use inorganic or organic bases as additives in the reaction system limit their large‐scale application in imine synthesis.
Recently, a few metal oxide catalysts that can catalyze this imine synthesis have been reported [25-27]; for example, CeO2 is an effective catalyst for imine formation from benzyl alcohol and aniline at low temperature (60 ℃), with a high yield of 96% after 24 h [25]. Manganese oxides supported on hydroxyapatite (MnOx/HAP) also show high activity in the oxidative coupling of alcohols and amines [26]. When benzyl alcohol and aniline were used, a 92% yield of the imine was obtained after reaction for 24 h at 80 ℃. The catalytic activity, especially the capacity to activate molecular oxygen, is highly dependent on the redox properties of the metal oxides. They determine the catalytic behavior of the metal oxide in the oxidative dehydrogenation step. Tuning the redox properties of metal oxide catalysts should therefore be an efficient method for developing high-performance catalysts for imine synthesis via oxidative coupling.
Among various metal oxides, iron oxides have the advantages of ready availability, low cost, and low toxicity [28-32]. However, iron oxides show low activities in the oxidative coupling of alcohols and amines to imines [25]. This could be related to the intrinsic inert properties of a bulk material, such as a low surface‐to‐bulk atomic ratio and low surface energy. We recently reported that the redox properties of iron oxides can be tuned by forming highly dispersed iron oxide species on carbon supports [33, 34]. The catalysts show high redox activities in O2 activation. This property of these iron oxide species might enable them to catalyze oxidative coupling of alcohols and amines to imines.
In this work, carbon-supported FeOx (FeOx/HCMK-3) was used as a catalyst for oxidative coupling of alcohols and amines to imines. A 98.8% yield of imine was obtained in the oxidative coupling of benzyl alcohol and aniline after reaction for 6 h at 80 ℃. The FeOx/HCMK-3 catalyst was efficient in the synthesis of a series of imine compounds. The correlations between the activity and physicochemical properties of the catalyst were investigated based on a series of characterization results. The reaction mechanism of imine synthesis over FeOx/HCMK-3 was also explored.
All chemicals were analytical grade and used without further purification. Double-distilled water was used in all experiments.
SBA-15 templates were prepared using a triblock copolymer, EO20-PO70-EO20 (P123; EO: ethylene oxide; PO: propylene oxide), as the surfactant and tetraethyl orthosilicate (TEOS) as the silica source, using the procedure reported by Zhao et al. [35]. Typically, TEOS (4.68 mL) was added to 2 mol/L HCl (60 mL) containing P123 (2 g) at 35 ℃. After stirring in a water bath (35 ℃) for 24 h, the mixture was transferred to Teflon-lined stainless steel autoclaves (100 mL) and heated to 100 ℃ for 3 d. The resultant solid product was separated by filtration, dried at 100 ℃ for 12 h, and calcined at 550 ℃ for 6 h.
CMK-3 was prepared using SBA-15 as a hard template and sucrose as the carbon source [36]. Typically, SBA-15 silica (1.0 g) was added to a solution containing sucrose (1.25 g), sulfuric acid (0.14 g), and distilled water (5.0 g). The mixture was kept in an oven for 6 h at 80 ℃. The temperature was then increased to 160 ℃ and maintained for 6 h. The heating procedure was repeated after addition of the carbon precursor (0.8 g of sucrose, 0.09 g of H2SO4, and 5.0 g of H2O) to achieve complete infiltration of the internal pores of the SBA-15 silica. The carbon-silica composite was obtained by pyrolysis at 900 ℃ for 6 h under an Ar flow and then washing in 10 wt% HF aqueous solution to remove the silica template.
The carbon-supported iron oxide catalyst was prepared using a wet impregnation method. Typically, an Fe-containing solution was obtained by dissolving Fe(NO3)3·9H2O in a certain amount of water. Before added to the iron nitrate solution, the CMK-3 support was treated in 4 mol/L HNO3 solution at 60 ℃ for 6 h (denoted by HCMK-3). After stirring for about 3 h at room temperature, the mixture was heated at 80 ℃ under atmospheric pressure to evaporate the water. The as-synthesized catalyst was thermally treated at 400 ℃ for 4 h under an Ar flow. The iron oxide loading was 5 wt% (calculated based on Fe2O3) and the resultant material was denoted by FeOx/HCMK-3. For comparison, a sample consisting of Fe3+ ions supported on HCMK-3 (denoted by Fe-HCMK-3) was also prepared by the same impregnation method and using HNO3-treated CMK-3 as a support. The difference was that after impregnation the sample was dried in an oven at 80 ℃ for 12 h without further thermal treatment at 400 ℃ in an Ar flow for 4 h. The Fe species loading was the same as that on FeOx/HCMK-3. Bulk Fe2O3 was also prepared using a conventional precipitation method with Fe(NO)3·9H2O as the Fe source and ammonia as the precipitating agent (pH 9). The resultant material was calcined at 400 ℃ for 4 h in air.
Powder X-ray diffraction (XRD) patterns were recorded with a Rigaku X-ray diffractometer using Cu Kα radiation (λ = 1.5418 Å). Transmission electron microscopy (TEM) images and high-angle annular dark field scanning TEM (HAADF-STEM) images were obtained using a FEI Tecnai F20 instrument with an accelerating voltage of 200 kV, equipped with an energy-dispersive X-ray spectroscopy analyzer. X-ray photoelectron spectroscopy (XPS) was performed using a Thermo ESCA LAB 250 system with an Mg Kα source (1254.6 eV). N2 adsorption-desorption isotherms were recorded at −196 ℃ using a Micromeritics ASAP 2010N analyzer. Raman spectra were recorded using a Bruker RFS 100 Raman spectrometer with an Ar laser (532 nm) as the excitation source. Temperature-programmed reduction (TPR) was performed using a Tianjin Xianquan TP-5079 adsorption analyzer. Before examination, the catalysts (30 mg of FeOx/HCMK-3, 10 mg of Fe2O3) were treated in an Ar (99.99%) flow at 400 ℃ for 30 min. The zeta-potential curve was plotted as a function of pH, from 2 to 8 (Zeta PALS analyzer, Brookhaven Instruments Corporation, USA). The sample was suspended in aqueous 0.01 mol/L NaCl solution. The pH was adjusted using 1.0 mol/L NaOH and HCl solutions. The Fe content was estimated using inductively coupled plasma atomic emission spectroscopy (ICP-AES; Perkin-Elmer emission spectrometer). Fourier transform infrared (FT-IR) spectra were recorded using a Thermo Nicolet 6700 FT-IR spectrometer. The Boehm titration method was used to determine the amounts of surface groups. The carbon support (100 mg) was placed in 10 mL of a 0.05 mol/L solution containing NaOH, Na2CO3, and NaHCO3. The vials were sealed and shaken for 24 h, and the contents were filtered. A known quantity of HCl was added to each filtrate. The excess acid left in the solution was titrated with NaOH using an automatic potentiometer titrator (LeiCi ZDT-4A). The amounts of acidic sites were calculated based on the assumption that NaOH neutralizes carboxylic, phenolic, and lactonic groups, NaHCO3 neutralizes carboxylic groups, and Na2CO3 neutralizes carboxylic and lactonic groups.
The aerobic oxidative coupling of an alcohol and amine was performed in a 50-mL two-necked flask at atmospheric pressure. A quantity of catalyst (0.3 g) was added to the reactor containing alcohol (1.0 mmol), amine (2.0 mmol), and toluene (10 mL). The mixture was in contact with air, and the reaction temperature was kept at 80 ℃. The reaction mixture was extracted via a sampling pipe with a filtrator and then transferred to a vial. The products were analyzed using a gas chromatograph equipped with an HP-5 column and a flame ionization detector. The conversion and yield of the imine product were calculated based on the alcohol to imine ratio. The products were identified using standard compounds and gas chromatography-mass spectrometry. The turnover frequencies (TOFs) were calculated based on benzyl alcohol conversion in the initial stage (60 min) and the amount of FeOx in the catalyst.
An oxygen-free experiment was performed in a Schlenk tube. First, the FeOx/HCMK-3 catalyst was put in a Schlenk tube, and degassed at 80 ℃ for 1.5 h under vacuum to remove adsorbed O2. High-purity N2 (99.99%) was injected into the Schlenk tube until the pressure was 1 atm. The degassed toluene, benzyl alcohol, and aniline were added to the Schlenk tube. The system was further degassed and balanced with high purity N2. The Schlenk tube was then put in an oil bath (80 ℃) to start the reaction. After each sample extraction, the system was degassed and balanced with high purity N2. All these treatments were to ensure the removal of molecular O2 from the system. In the recycling experiments, the solid catalyst was separated by filtration and treated at 400 ℃ for 1 h before the next cycle.
Fig. 1 shows the preparation of the carbon-supported FeOx (5 wt%, calculated based on Fe2O3) catalyst. The catalyst support was ordered mesoporous carbon CMK-3 prepared via a hard-template method. The CMK-3 was treated with 4 mol/L HNO3 at 60 ℃ for 6 h before use as the catalyst support. The resultant carbon was denoted by HCMK-3. This was a key step in preparing a catalyst with highly dispersed FeOx species. A large amount of oxygen-containing functional groups formed on the HCMK-3 surface. Table 1 shows that the amounts of functional groups on HCMK-3, including carboxylic, lactonic, and phenolic groups, were all higher than those on CMK-3. The total amount of functional groups was about 1.47 mmol/g. The abundant functional groups endow the HCMK-3 support with a hydrophilic surface. This facilitates introduction of iron nitrate solution into the mesopores of the carbon support. The dried solid was calcined at 400 ℃ for 4 h in an Ar flow to give the FeOx/HCMK-3 catalyst.
The XRD patterns show that FeOx/HCMK-3 retains the ordered arrangement of the parent HCMK-3 support (Fig. 2a). The three diffraction peaks in the range 2θ = 0.75°-3° can be indexed to the (100), (110), and (200) reflections of the HCMK-3 hexagonal space group. Wide-angle XRD patterns show that both samples have two broad diffraction peaks, centered at 24.5° and 44° (Fig. 2b), which are assigned to diffractions from the (002) and (100) graphite planes of the carbon supports [37]. No crystalline phase related to iron oxides was observed, indicating that the iron oxide particles are small or the degree of crystallization is low. The Raman spectra of FeOx/HCMK-3 and HCMK-3 show two peaks, at 1358 and 1598 cm−1, which are assigned to the D and G bands of the carbon support (Fig. 2c) [38, 39]. No peak below 1000 cm−1 attributable to crystalline iron oxides can be observed [40, 39]; this also indicates that the iron oxide particles in FeOx/HCMK-3 are small. The N2 adsorption-desorption isotherms show that FeOx/HCMK-3 gives isotherms typical of an ordered mesostructure (type IV) with a well-defined step in the adsorption curve near p/p0 = 0.5 (Fig. 2d). The surface area, pore volume, and pore size of FeOx/HCMK-3 are similar to those of HCMK-3, indicating that the iron oxides do not block the mesopores of the CMK-3 support.
TEM images confirm that FeOx/HCMK-3 retains the ordered structure of the CMK-3 support (Fig. 3a). No Fe particles were observed in the region examined, even at high resolution (Fig. 3b and c). STEM and energy-dispersive X-ray mapping were performed to investigate iron oxide dispersion (Fig. 3d and e). The results show that Fe species are nearly homogeneously dispersed on the HCMK-3 surface, but the particle size still cannot be ascertained based on these measurements. A sample with a low FeOx loading (0.25 wt%) was prepared to clarify the FeOx dispersed state on the HCMK-3 surface. FeOx particles still cannot be observed in the HRTEM image (not shown), but some light spots (about 2-3 nm) can be distinguished in the HAADF-STEM image (Fig. 3f). These results suggest that FeOx species are present as two-dimensional or even sub-nano clusters. These clusters could be the constituent units of the highly dispersed FeOx on the carbon support. The HRTEM and HAADF-STEM images together confirm that the FeOx species are highly dispersed on the HCMK-3 support surface.
XPS shows that there are no obvious differences between the Fe 2p binding energies of FeOx/HCMK-3 and bulk Fe2O3 (Fig. 4a). The Fe 2p1/2 and Fe 2p3/2 peaks appear at 724.2 and 710.9 eV, respectively. This indicates that most of the Fe species are present in the iron oxides in the +3 valence state [42, 43]. Clear differences can be observed in the O 1s spectra. The binding energies in the deconvoluted O 1s XPS spectrum are in according with those reported in the literature [44, 45]. The O 1s spectrum of bulk Fe2O3 can be deconvoluted into two peaks (Fig. 4b). Peak I, in the range 529.5-530.5 eV, is assigned to O2− ions in the iron oxide lattice. Peak II, centered at about 532 eV, is related to oxygen in surface OH groups or oxygen vacancies in Fe2O3. The peak II/peak I area ratio is 0.21 for bulk Fe2O3. The FeOx/HCMK-3 O 1s spectrum can be deconvoluted into three peaks. The new peak (peak III) at 534 eV is attributed to oxygen functional groups remaining on the catalyst surface. The peak II/peak I area ratio is 3.54 for FeOx/HCMK-3, which is much higher than that for bulk Fe2O3. This is caused by the small size of the iron oxide particles and interactions between iron oxides and the carbon support.
Diffuse reflectance infrared Fourier transform (DRIFT) spectra show that some oxygen-containing groups remain on the FeOx/HCMK-3 surface (Fig. 5a). Further FT-IR spectra were obtained to clarify whether the Fe species were located in iron oxides or present as Fe3+ ions coordinated with these functional groups. A sample consisting of Fe3+ ions supported on HCMK-3 (denoted by Fe-HCMK-3) was prepared via the same impregnation method as was used to prepare FeOx/HCMK-3, with HNO3-treated CMK-3 as a support. The Fe species loading was the same as that for FeOx/HCMK-3. The difference is that after impregnation, the sample was dried in an oven at 80 ℃ for 12 h to obtain Fe-HCMK-3, whereas further thermal treatment at 400 ℃ in an Ar flow for 4 h is needed to obtain FeOx/HCMK-3. The peak at 469 cm−1 in the FT-IR spectrum of FeOx/HCMK-3 (Fig. 5b) can be attributed to the stretching vibration of Fe-O, similar to that of Fe2O3. However, this signal is different in the Fe-HCMK-3 spectrum. A combination of the FT-IR and XPS results confirms that Fe is mainly located in iron oxide species (i.e., FeOx). The formation of FeOx species in FeOx/HCMK-3 is a result of the high treatment temperature. The oxygen-containing functional groups in HCMK-3 can interact with these two-dimensional FeOx species.
The H2-TPR profiles (Fig. 6) show that the iron oxide species in FeOx/HCMK-3 are more reducible than those in bulk Fe2O3. There are three reduction peaks present in the profile of bulk Fe2O3. The narrow peak (centered at 430 ℃) can be ascribed to the reduction of Fe2O3 to Fe3O4. The two overlapping peaks in the range 450-800 ℃ can be ascribed to further reduction to FeO/Fe species [46, 47]. For FeOx/HCMK-3, only one broad peak is present in the range 200-600 ℃. The start and center of this peak are clearly at lower temperatures than those for bulk Fe2O3, confirming the high reducibility of FeOx/HCMK-3. The amounts of H2 consumed by FeOx/HCMK-3 and Fe2O3 during TPR were 2.13 and 18.10 mmol/g, respectively. The FeOx content in FeOx/HCMK-3 was 5 wt%, therefore the stoichiometric consumption of H2 by FeOx should be 0.93 mmol/g, which is lower than the actual amount of H2 consumed (2.13 mmol/g). A combination of the H2-TPR, XPS, and DRIFT results shows that additional H2 is consumed (1.2 mmol/g) because of the presence of oxygen-containing functional groups in FeOx/HCMK-3. The quantity of reduced functional groups in FeOx/HCMK-3 was determined based on H2-TPR of HCMK-3. The results show that H2 consumption by HCMK-3 was about 1.08 mmol/g, which is consistent with H2 consumption by the carbon support in FeOx/HCMK-3 (1.2 mmol/g).
These results show that oxygen functional groups play important roles in the formation of highly dispersed iron oxide species. They render the HCMK-3 support surface negatively charged. Zeta-potential measurements show that HCMK-3 is negatively charged at pH 1.0-8.0 (Fig. 7). Untreated CMK-3 is positively charged at pH less than 5.1. FeOx/HCMK-3 was prepared by impregnation with Fe(NO3)3 aqueous solution. The pH of the Fe3+-containing solution was 1.65 (Fig. 7). Under these preparation conditions, there is an electrostatic force between the positively charged metal ions and negatively charged carbon surface (Fig. 1). This could be the main reason for the homogeneous dispersion of Fe ions on the carbon surface. In the subsequent thermal treatment, these surface oxygen functional groups provide strongly active sites for anchoring FeOx species, and suppress iron oxide aggregation.
Fig. 8 shows the catalytic activity of FeOx/HCMK-3 in the oxidative coupling of benzyl alcohol and aniline to an imine at 80 ℃. The imine was identified using gas chromatography-mass spectrometry. For comparison, the reaction was also performed using bulk Fe2O3 and HCMK-3. The activity of bulk Fe2O3 was low. Only a 13.2% yield of imine was obtained after reaction for 8 h at 80 ℃. The HCMK-3 support was almost inactive under the test conditions. For FeOx/HCMK-3, the yield of imine after reaction for 6 h was 98.8%. The TOF for FeOx/HCMK-3 in the initial stage was 2.9 h−1. The catalytic performance of FeOx/HCMK-3 at 60 ℃ was also investigated for comparison with those of other reported catalytic systems. The TOF was 0.6 h−1. Comparisons with results reported in the literature [19, 25, 26] show that this TOF is higher than those for non-noble-metal oxide catalysts (e.g., CeO2 and MnOx/HAP), but a little lower than those of supported Au catalysts under similar reaction conditions. The reaction was also performed using equimolar amounts of the alcohol and amine. A 78.4% yield of the imine was obtained after reaction for 8 h. All these results indicate that FeOx/HCMK-3 is an effective catalyst for imine formation.
Multiple aerobic oxidation cycles were performed to examine the recoverability of FeOx/HCMK-3 (Fig. 9). The catalyst retained high activity after five cycles, with only 3% and 5% decreases in the third and fifth cycles compared with the first one. Loss of Fe from the catalyst to the reaction mixture was determined using ICP-AES. Trace amounts of Fe (0.02 ppm, 0.02‰ of total Fe) were detected, indicating that Fe active sites were not lost. The XRD patterns show that there was no aggregation of iron oxide species in the used FeOx/HCMK-3. All these results suggest that iron oxide active sites are stable in the FeOx/HCMK-3 catalyst. The decrease in the imine yield was mainly caused by loss of catalyst in the recycling process. The catalyst was recycled by filtration and then thermally treated at 400 ℃ for 1 h under Ar to remove adsorbed reagents. Catalyst loss (about 7%) is inevitable during this process.
Experiments were performed to identify the active sites in FeOx/HCMK-3 (Table 2). When Fe(NO3)3 was used as the catalyst, the yield of imine after reaction for 8 h was 1.8%. Even in the co-presence of Fe(NO3)3 and HCMK-3, the imine yield was only 5.0%. These results differ from those reported by Zhang et al. [48], mainly because different reaction conditions were used. Under their reaction conditions, Fe(NO3)3 shows moderate activity in imine synthesis and the activity can be greatly enhanced by addition of 2, 2, 6, 6-tetramethyl-1-piperidinyloxy [48]. We also investigated the catalytic performance of Fe-HCMK-3. An imine yield of 24.6% was obtained over Fe-HCMK-3 after reaction for 8 h; this is much lower than the yield achieved using FeOx/HCMK-3. Loss of Fe into the reaction mixture was detected using ICP-AES; the amount was about 33.65 ppm, which is 3.2% of the total amount of Fe loaded on Fe-HCMK-3. All these results suggest that Fe3+ ions or coordinated Fe species were not active in the imine synthesis reaction. Supported FeOx species were the effective and stable active sites. The highly dispersed FeOx species on the HCMK-3 surface could affect the redox properties of the FeOx/HCMK-3 catalyst. The reducibility of these iron oxide species could be responsible for the high activity in this reaction.
The general applicability of the FeOx/HCMK-3 catalyst was also investigated. Eleven imines were obtained by oxidative coupling of amines with various alcohols. Table 3 (entries 1-7) shows that the FeOx/HCMK-3 catalyst is active in reactions using benzyl alcohol derivatives with either electron-rich or electron-poor substituents, especially 4-methylbenzyl alcohol, 4-methoxybenzyl alcohol, and 4-isopropylbenzyl alcohol (the imine yields were 93.2%, 90.7%, and 84.2%, respectively, after reaction for 8 h). FeOx/HCMK-3 also catalyzed the reaction of phenethyl alcohol and allylic alcohols (e.g., trans-2-hexenol) with aniline to give the corresponding imines when the reaction temperature and time were increased (Table 3, entries 8 and 9). Table 3 shows that FeOx/HCMK-3 is active in the aerobic oxidative coupling of aliphatic amines (e.g., cyclohexylamine and n-butylamine) with benzyl alcohol (Table 3, entries 10 and 11). These results clearly show that FeOx/HCMK-3 is an efficient catalyst in oxidative coupling reactions.
Three additional experiments were performed to clarify the reaction mechanism over FeOx/HCMK-3; the results are shown in Fig. 10. Fig. 10a shows that the oxygen species in FeOx/HCMK-3 are active in the aerobic oxidative reaction. Even without the presence of O2, an 8.7% yield of imine was obtained after reaction for 8 h at 80 ℃. This experiment was performed in a Schlenk tube (see the experimental section for details). The FeOx/HCMK-3 catalyst was first degassed at 80 ℃ for 1.5 h under vacuum to remove adsorbed O2. After adding the reactants and solvent, the system was further degassed and then balanced with high-purity N2. All these treatments were performed to ensure the removal of molecular O2 from the system. This result shows that the oxygen species in FeOx/HCMK-3 participate in the reaction. The oxidative coupling of alcohols and amines on FeOx/HCMK-3 occur via a redox mechanism [25, 26]. During imine synthesis over FeOx/HCMK-3, benzaldehyde was detected in the initial stage and the amount slowly decreased with time (Fig. 10b). This suggests that benzaldehyde might be an intermediate and that the reaction proceeds in two consecutive steps: (1) oxidative dehydrogenation of the alcohol to benzaldehyde and (2) imine formation by reaction of benzaldehyde with aniline.
We recently reported that FeOx/HCMK-3 catalyzes the oxidation of benzyl alcohol to benzaldehyde with air as the oxygen source [5]. Benzyl alcohol conversion of 72% was observed after reaction for 8 h. In the imine synthesis, benzyl alcohol conversion improved to 98.8% after reaction for 6 h. The second step in imine formation accelerates the conversion of benzyl alcohol. Fig. 10c shows that FeOx/HCMK-3 efficiently catalyzes imine synthesis from benzaldehyde and aniline. The reaction rate is 372.8 mmol h−1 g−1 (calculated based on iron oxide sites), which is 4.5 times higher than that for imine formation from benzyl alcohol and aniline (82.0 mmol h−1 g−1). This confirms that oxidative dehydrogenation of alcohol is the rate-determining step in the reaction. Based on the above results and related literature reports [49, 50], a plausible reaction mechanism for imine formation from an alcohol and aniline on FeOx/HCMK-3 is proposed (Scheme 1). Oxidative dehydrogenation of benzyl alcohol by oxygen species occurs at the redox sites of FeOx/HCMK-3. Iron is reduced from valence +3 to +2. The produced benzaldehyde reacts with aniline over FeOx/HCMK-3 to afford the corresponding imine. FeOx/HCMK-3 is regenerated by oxidation of the reduced FeOx/HCMK-3 with O2.
FeOx/HCMK-3 with highly dispersed iron oxide species shows high activity and recyclability in aerobic oxidative coupling of alcohols and amines to imines. The excellent catalytic performance of FeOx/HCMK-3 is ascribed to its high reducibility. The lattice oxygens in FeOx/HCMK-3 can participate in imine synthesis, and the reaction follows a redox mechanism over FeOx/HCMK-3. Two consecutive steps are involved in the reaction: oxidative dehydrogenation of an alcohol to an aldehyde and coupling of the aldehyde with an amine to an imine. Oxidative dehydrogenation of alcohol is the rate-determining step in the reaction.
2016,
Vol. 37 Issue (9): 1451-1460 
PDF
2016,
Vol. 37 Issue (9): 1451-1460 PDF