Subrata Saha and
Sharifah Bee Abd Hamid*
Nanotechnology and Catalysis Research Center (NANOCAT), University of Malaya, Kuala Lumpur 50603, Malaysia. E-mail: sharifahbee@um.edu.my; Fax: +60 379676956; Tel: +60 379676959
First published on 26th September 2016
A highly crystalline and spinel-structured Cu–Mn mixed oxide prepared via a solvent evaporation technique demonstrated superior catalytic activity for the oxidation of vanillyl alcohol to vanillin using different oxygen sources, air and H2O2, under optimum reaction conditions. The Cu–Mn mixed oxide catalyst contains large concentrations of structural defects (i.e. surface oxygen vacancy sites, defective oxygen species and grain boundaries) which act as active sites to enrich the transformation of vanillyl alcohol to vanillin. The atomic ratio of Cu and Mn in the metal oxides was observed to influence the catalytic performance significantly under similar reaction conditions. Moreover, reaction conditions such as time, solvent, temperature, and pressure were investigated to achieve suitable reaction parameters for the oxidation of vanillyl alcohol. For the catalytic activity, 94% conversion was measured with 99% selectivity for vanillin using H2O2 in the presence of base. Meanwhile, 91% conversion with 81% selectivity for vanillin was obtained by liquid phase aerobic oxidation in base-free conditions. The catalyst also showed high stability for three recycling redox reactions.
Vanillyl alcohol represents the B-0-4 linkage of lignin.5 Vanillyl alcohol has been widely studied as a phenolic model compound of lignin.6–8 Vanillin, the selective oxidative product of vanillyl alcohol, is the major flavor constituent of vanilla. Also, vanillin has great economic value in the pharmaceutical industry, as it is used as an important platform chemical.9–11 The large-scale consumption of vanillin requires an economically viable production protocol to satisfy demand. 85% of the world supply of vanillin is produced via the chemical route of petroleum-derived guaicol transformation, and 15% of vanillin production is based on lignin feedstock.12 Thus, the selective oxidation of lignin is a vital alternative and environmentally feasible method to synthesize vanillin.
Alcohol oxidation is commonly performed using homogenous organometallic complexes. Although this process offers high conversion, it has some serious drawbacks, such as a considerable amount of waste, contamination of the product, separation protocols, poor selectivity and fast deactivation of the catalyst.5,13–17 Thus, heterogeneous catalysts can be a suitable replacement because they possess many advantages compared to their homogeneous counterparts. Extensive studies have been reported of wet air aerobic or liquid phase oxidation of lignin by novel metal-based catalysts.18–21 However, the use of novel metal-based catalysts for these applications raises the questions of economic sustainability and health issues, as food industries also benefit from the flavor properties of vanillin. Thus, from the environmental and economical points of view, designing a heterogeneous transition metal-based catalyst is highly desirable.
Mixed oxide catalysts have been explored in many oxidation reactions, as they show superior catalytic activity due to their synergistic interactions and electronic properties.22 An isolated Co3O4 catalyst has shown promising catalytic activity for vanillyl alcohol oxidation; however, the usage of base (i.e. NaOH) in this oxidation reaction results in a large amount of inorganic waste.23 Various mixed oxides, such as CoTiO3,24 Co–Mn mixed oxide,25 and graphene promoted Co–Mn oxide26 catalysts, have been explored for the oxidation of vanillyl alcohol to vanillin. Although some catalysts have exhibited moderate conversion or selectivity, extensive research is still required to substantially enrich the conversion and selectivity under milder reaction conditions for an economically feasible process.
Cu–Mn-based spinel catalyst has also shown promising catalytic activity in many applications, such as steam reforming of methanol, low temperature oxidation of CO and gas phase oxidation of hydrocarbons.27–30 Cu–Mn mixed oxide catalyst has been prepared via different routes, such as sol–gel,31 co-precipitation,32,33 formate decomposition,34 solid state reaction35 and redox reaction.36,37 The most frequently used preparation method is co-precipitation of metal precursors using alkaline NaHCO3 or NaOH. However, the presence of trace metal (i.e. sodium, Na) in the catalyst after the washing procedure poisons the catalytic activity of the catalyst.38 In this work, we synthesized a spinel-structured, highly crystalline, mesoporous Cu–Mn mixed oxide catalyst via a simplified and facile solution method without using alkaline solution. To our knowledge, the catalytic activity of the Cu–Mn mixed oxide catalyst prepared via the solution method was tested for the first time in the oxidation reaction of vanillyl alcohol using environmentally friendly oxidants, such as air and H2O2. Moreover, various analytical tools, such as XRD, HRTEM, SAED, XPS, FESEM, TG/DTG, EDX, H2-TPR, and O2-TPD, were employed to establish the intimate relationships of the properties of the catalyst, such as crystallinity, phase detection, morphology, thermal stability, surface area and redox properties, with its catalytic activity. Furthermore, the influence of reaction parameters such as catalyst loading, H2O2 concentration, time, and the nature of the solvent were explored for vanillyl alcohol oxidation over novel Cu–Mn mixed oxide catalyst. In addition, the stability of the Cu–Mn mixed oxide catalyst was also investigated.
Temperature-programmed reduction and oxidation were carried out on a Micrometrics Chemisorb 1100 Series instrument. 0.2 g sample was treated at 180 °C for 1 hour with N2 gas (20 ml min−1) in a quartz tube for removal of the surface moisture prior to analysis. Thereafter, the temperature was elevated to 900 °C under gas flow of a mixture of 5% H2 with 95% N2 at a heating rate of 10 °C min−1 and was maintained for 10 min. The reduced sample was also analyzed to determine the re-oxidation process of the catalyst with the same temperature programme.
(1) |
(2) |
The Cu1.5Mn1.5O4 phase after calcination at 500 °C observed in the XRD pattern is in harmony with previous literature reports of the phase diagram of Cu–Mn–O.39 the cubic spinel shaped Cu1.5Mn1.5O4 phase detected in this synthesis protocol was reported to be stable at low temperature in the phase diagram. It is worthwhile to mention that this simple and facile synthesis protocol and calcination at 500 °C was very effective and successful for the synthesis of spinel-structured, highly crystalline and stable mixed oxide catalyst. Moreover, the spent catalyst was also analyzed by XRD to detect any changes in the phase and crystallinity. As expected, there was no change in the XRD pattern; it showed a similar pattern at the same Bragg angle as the fresh catalyst. This suggested that the crystallinity and the Cu1.5Mn1.5O4 phase of the catalyst are well preserved. Also, similar reaction conditions was observed to not affect the crystal phase in previous literature reports of CoTiO3 catalyst in vanillyl alcohol oxidation.24 However, the intensity of the Mn3O4 phase was slightly reduced after the catalytic reaction, probably due to amorphization and re-distribution of the corresponding species.40
Fig. 3 Oxidation state analysis of catalyst A (Cu:Mn): [a] wide spectra, [b] Cu 2p spectra, [c] O 1s spectra, [d] Mn 3s spectra. |
Cu (%) | Mn (%) | O (%) |
---|---|---|
5.82 | 20.57 | 73.65 |
However, some shake-up peaks due to charge transfer were also observed at the binding energies of 942 eV and 962 eV. This further confirmed the presence of Cu2+ species in the Cu–Mn crystal cage.46 In addition, the O 1s XPS spectrum is displayed in Fig. 3c. The asymmetric peak of the O 1s spectrum was further resolved into three components by Gaussian fitting. The peak at the lowest binding energy of 529 eV corresponds to the lattice oxygen species (OL) (O2−) of Cu–O and Mn–O. The peak at the binding energy of 531.2 eV could be attributed to surface oxygen vacancies as per previously reported literature.47,48 Surface oxygen vacancies (OV) as surface defects can be caused by the non-stoichiometry of the metal ions in the nanocrystalline domain. Also, the peaks at higher binding energies in the range of 531.8 to 532.9 eV have been reported as chemisorbed (OC) or dissociative species (O2−, O2− or O−) and OH−.49 The atomic percentages of the three types of oxygen species were calculated from the XPS data and are shown in Table 2.
Lattice oxygen (OL) | Surface oxygen vacancy (OV) | Defective oxygen (OD) |
---|---|---|
46.23% | 38.36% | 15.40% |
Fig. 4 Morphologies of the catalysts. [a] Catalyst A (Cu:Mn), [b] catalyst B (Cu:2Mn), [c] catalyst C (2Cu:Mn), [d] spent catalyst A. |
HRTEM images were captured to further confirm the particle size and the formation of Cu1.5Mn1.5O4 phase in terms of the d spacing of the phase (Fig. 5). The d spacing was measured as 4.74 Å, which confirmed the (111) plane of Cu1.5Mn1.5O4 (Fig. 5b). Also, the crystallite size was measured in the range of 8 to 13 nm. A high resolution insight into the interface of the plane revealed the presence of surface defects in the form of numerous grain boundaries associated by partial dislocations. As shown in Fig. 5c, partial dislocation occurred at the interface of two (111) planes of Cu1.5Mn1.5O4; this was labelled as T. The observed grain boundaries notably contribute to the high catalytic activity of the Cu1.5Mn1.5O4 catalyst. Furthermore, a selective area electron diffractogram (SAED) was also obtained to confirm the crystallinity and the formation of Cu1.5Mn1.5O4 phase in the synthesized catalyst (Fig. 5d). The diffusive ring observed in the SAED pattern confirmed that the material was polycrystalline. The bright spots noted in the SAED pattern confirmed each plane of Cu1.5Mn1.5O4. Fig. 5d shows the formation of the (111) and (220) planes of Cu1.5Mn1.5O4 phase in terms of the measured d spacings of 4.74 Å and 2.91 Å.
Fig. 5 HRTEM images of catalyst A (Cu:Mn). [a] Overview image, [b] lattice parameter, [c] grain boundary, [d] SAED pattern. |
Moreover, the elemental composition of the prepared material was determined by EDX analysis (Fig. 6). The atomic ratios observed in the EDX spectra of the three catalysts tentatively matched the primary ratio of metal loading in the synthesis protocol. Also, the area mappings clearly show the high degree of homogeneity of both metals in the catalyst (Fig. 6d).
Fig. 6 EDX spectra of the synthesized catalysts. [a] Catalyst A, [b] catalyst B, [c] catalyst C, [d] homogeneity of the catalyst, [e] Cu atoms, [f] Mn atoms, [g] O atoms. |
Moreover, a comprehensive study of the TPR profiles of the synthesized catalysts revealed that the catalyst containing an equimolar ratio of Cu and Mn had the first peak maxima at the lowest reduction temperature. The catalyst A (Cu:Mn) reduction started at 275 °C, while those of the other two catalysts started at 350 °C and 360 °C, respectively. One explanation of the low reduction temperature of catalyst A is the presence of a large concentration of structural defects associated with defective oxygen species; this was also suggested by the O 1s XPS spectra as well as the small particle size. Another possible reason was the formation of a high degree of Cu–O–Mn species, which causes an electronic interaction between Cu and Mn.
In order to study the oxygen vacancies in the catalyst, temperature programme oxygen desorption was used (Fig. 8). In the TPD profile, four distinctive oxygen desorption peaks were noted at temperature maxima around 230 °C, 340 °C, 540 °C and 680 °C. The first oxygen desorption peak at low temperature was attributed to surface oxygen vacancies. Desorption of oxygen at the lattice oxygen vacant sites of Cu occurred at a temperature of 340 °C, which was indicated by the prominent peak of catalyst C with a high loading of Cu. A similar trend was observed in the case of catalyst B with high content of Mn at a temperature of 540 °C. Therefore, the third oxygen desorption peak corresponds to the oxidation of Mn2+ to Mn3+. A previous literature study reported the presence of defective oxygen species in the lattice at high temperatures of 796 K to 863 K for supported MnOx species.55 A similar observation was made by another group of researchers for Al2O3 and SiO2-promoted MnOx species.56 Therefore, in our work, the oxygen desorption peak at high temperature around 680 °C originated from structural defects associated with defective oxygen species. The most intense peak at high temperature for catalyst A indicated the presence of a high degree of defective oxygen species among all three catalysts. Thus, the reduction temperature of catalyst A (Cu:Mn) was noticeably lower, as observed in the TPR profile.
To evaluate the catalytic activity, catalyst A (Cu:Mn) was considered to optimize the reaction variables. No conversion was observed in acetonitrile in the presence or absence of catalyst. However, in the presence of equimolar NaOH base relative to vanillyl alcohol in isopropanol, the oxidation reaction occurred. Interestingly, surprising catalytic activity was achieved. To determine the role of the base, the reaction without catalyst in the presence of NaOH and H2O2 in isopropanol solvent was also considered. No conversion of vanillyl alcohol was found. Therefore, it should be mentioned that the presence of base in the liquid phase oxidation of vanillyl alcohol by Cu1.5Mn1.5O4 is crucial, as it probably deprotonates the phenoxy ion and favors coordination with the catalyst. It is a well-known fact that H2O2 coupled with NaOH favors lignin valorization over metal oxides.23,57,58
To monitor the progress of the oxidation reaction, HPLC analysis was performed after every 0.5 h interval of reaction time (reaction conditions: 1 mmol vanillyl alcohol, 1 mmol NaOH (0.15 g), 3 mmol H2O2, 20 ml isopropanol, 0.0037 g cm−3 catalyst) (Fig. 9). A steeply progressive trend for the amount of conversion was observed. After two and a half hours of reaction time, the conversion reached 94% with 99% selectivity for vanillin. With increasing reaction time, the conversion remained almost constant (99%). Based on these results, 2.5 h was chosen as the optimum time for vanillyl alcohol oxidation by Cu1.5Mn1.5O4 catalyst using H2O2 oxidant.
Fig. 9 Reaction progress with time. Reaction conditions: 1 mmol vanillyl alcohol, 3 mmol H2O2, 1 mmol NaOH, 85 °C, 20 ml isopropanol, catalyst A (Cu:Mn) mass 0.0037 g cm−3. |
To determine the optimum loading of catalyst in the reaction, four different loadings, 0.0012 g cm−3, 0.0025 g cm−3, 0.0037 g cm−3 and 0.0050 g cm−3, were considered (reaction conditions: 1 mmol vanillyl alcohol, 1 mmol NaOH (0.15 g), 3 mmol H2O2, 20 ml isopropanol, 85 °C, 2.5 h) (Fig. 10). A sharp increase to 94% from 56% was observed in the amount of conversion when the catalyst loading was changed from 0.0012 g cm−3 to 0.0037 g cm−3, owing to a significant mass transfer effect. However, in the case of 0.0050 g cm−3 catalyst loading, the conversion was further enriched by 2%. This was possibly due to insignificant mass transfer.59 Surprisingly, the selectivity towards vanillin remained constant at 99% for all loadings of the catalyst. Thus, 0.0037 g cm−3 was the best catalyst mass for liquid phase selective oxidation of vanillyl alcohol to vanillin using H2O2 oxidant.
Fig. 10 Effects of catalyst A (Cu:Mn) mass on conversion and selectivity. Reaction conditions: 1 mmol vanillyl alcohol, 3 mmol H2O2, 1 mmol NaOH, 85 °C, 20 ml isopropanol, 2.5 h. |
Four different molar ratios of H2O2 concentration to vanillyl alcohol, 1 mmol, 2 mmol, 3 mmol and 4 mmol, were injected in order to scrutinize the effect of oxidant concentration in the reaction (reaction conditions: 1 mmol vanillyl alcohol, 1 mmol NaOH (0.15 g), 20 ml isopropanol, 2 h, 85 °C, 0.0037 g cm−3 catalyst) (Fig. 11). The conversion markedly improved when the concentration of H2O2 was increased. The conversion reached a maximum at 94% in the case of 3 mmol. An increased amount of H2O2 accelerates the rate of reaction. However, 4 mmol showed a slight decline in the conversion (92%). This was possibly because the degradation of H2O2 became predominant over the oxidation of vanillyl alcohol.17 Notably, the selectivity was not affected for the abovementioned ratios of H2O2. Based on the experimental data, 3 mmol was chosen as the best condition for liquid phase oxidation of vanillyl alcohol by H2O2.
The oxidation reaction was carried out at four different temperatures, 65 °C, 75 °C, 85 °C and 90 °C, under optimized reaction conditions to investigate the probable response (reaction conditions: 1 mmol vanillyl alcohol, 1 mmol NaOH (0.15 g), 3 mmol H2O2, 20 ml isopropanol, 2 h, catalyst 0.0037 g cm−3) (Fig. 12). When the temperature was increased to 85 °C from 65 °C, the catalytic conversion improved to 94% from 56%, as expected. However, at a temperature of 90 °C, the conversion was slightly reduced to 92%. In a previous literature study, it was reported that H2O2 degradation was higher above a temperature of 353 K, thus resulting in lower conversion.60 A similar observation was made in our study. Thus, 85 °C was considered to be the best temperature for the maximum amount of conversion in the liquid phase oxidation of vanillyl alcohol using H2O2.
Fig. 12 Effects of temperature on conversion and selectivity. Reaction conditions: 1 mmol vanillyl alcohol, 3 mmol H2O2, 1 mmol NaOH, 20 ml isopropanol, catalyst A (Cu:Mn) mass 0.0037 g cm−3, 2.5 h. |
Two solvents, acetic acid and isopropanol, were considered to study the influence of the nature of the solvent, as NaOH is highly soluble in both media (reaction conditions: 1 mmol vanillyl alcohol, 1 mmol NaOH (0.15 g), 3 mmol H2O2, 2 h, 85 °C, catalyst 0.0037 g cm−3) (Fig. 13). For isopropanol, the highest catalytic activity of 94% conversion was obtained, with 99% selectivity for vanillin. In the case of acetic acid, there was a marginal decrement in the catalytic conversion of 3%. However, the selectivity was remarkably increased to 61%. This was probably due to both the polarity and the pH of the reaction. At lower pH, the formation of peracetic acid was expected, which could further generate peroxy (−OOH) radicals.61 This results in further oxidation of vanillin to vanillic acid in acetic acid medium. Therefore, isopropanol is the best solvent to obtain maximum conversion with excellent selectivity for vanillin.
The catalyst composition is also a vital parameter, and its effects on the catalytic conversion were surveyed (reaction conditions: 1 mmol vanillyl alcohol, 1 mmol NaOH (0.15 g), 3 mmol H2O2, 2 h, 85 °C, 20 ml isopropanol, 0.0037 g cm−3 catalyst) (Fig. 14). The catalyst with equivalent loadings of Cu and Mn was found to be the most effective in the oxidation of vanillyl alcohol using H2O2. Catalytic conversion of 94% with 99% selectivity in the presence of NaOH base was obtained for catalyst A under the optimized reaction conditions. Inferior catalytic conversions were noted for catalysts B and C (67% and 36%, respectively). Notably, the selectivity was not affected by the ratio of the loaded metals. This suggested that the reaction pathways were similar for all the catalysts, as Cu1.5Mn1.5O4 phase was formed in all the catalysts. The above experimental data imply that the catalytic activity was highly influenced by the chemical composition of the metal loading in the synthesis protocol. The metal loading in the bulk phase of the catalyst could significantly assist in the alteration of the concentration of surface reactive oxygen species. Thus, it could be concluded that the formation of the Cu–O–Mn linkage in catalyst A was higher, as was expected due to the key active sites in the catalyst. The superiority of catalyst A was further confirmed by the reducing properties of the catalyst in the H2-TPR profile. Hence, the control of the surface composition in Cu–Mn mixed oxide is vital to achieve improved catalytic activity in the oxidation of vanillyl alcohol. In addition, it is a well-recognized fact that the origin of the excellent catalytic activity of Cu–Mn mixed oxide catalysts is due to the presence of two Jahn–Teller ions, Cu2+ and Mn3+, and their solid state charge transfer redox cycle.62,63
The best catalyst in terms of conversion, A (Cu:Mn), was further considered for aerobic oxidation of vanillyl alcohol to avoid the use of NaOH base. Acetonitrile was chosen as the solvent, as it was found to be a suitable solvent as per a previous report.25 Furthermore, the reaction variables were optimized using air. A blank reaction in the absence of catalyst was performed for the aerobic oxidation of vanillyl alcohol. No conversion was noted. To inspect the progress, the aerobic oxidation reaction in the presence of catalyst was carried out (reaction conditions: 3 mmol vanillyl alcohol, T = 120 °C, P = 21 bar air, 60 ml acetonitrile, 0.0025 g cm−3 catalyst) (Fig. 15). The substrate conversion was sharply enriched with increasing reaction time. It reached a maximum at 97% conversion with selectivity for vanillin after 2 h of reaction time. After that, the reaction did not proceed with appreciable improvement of the conversion. Also, the selectivity decreased owing to overoxidation of vanillin to the corresponding acid. Hence, 2 h was the best reaction time to achieve optimum catalytic activity for liquid phase aerobic oxidation in base-free conditions.
Four different loadings of catalyst, 0.0006 g cm−3, 0.0012 g cm−3, 0.0025 g cm−3 and 0.0037 g cm−3, were used to study the apparent effects of the catalyst on the aerobic oxidation of vanillyl alcohol (reaction conditions: 3 mmol vanillyl alcohol, T = 120 °C, P = 21 bar air, 60 ml acetonitrile) (Fig. 16). The best catalytic activity was obtained for the catalyst loading of 0.0025 g cm−3 due to significant mass transfer. The worst catalytic activity (48%) was found for the loading of 0.0006 g cm−3. In the case of 0.0037 g cm−3 loading, there was no significant increase in the conversion, possibly owing to insignificant mass transfer.59 It should be mentioned that the selectivity was not changed by changing the loading of the catalyst.
Fig. 16 Influence of catalyst mass A (Cu:Mn) on conversion and selectivity. Reaction conditions: 3 mmol vanillyl alcohol, 120 °C, 21 bar air pressure, 60 ml acetonitrile, 2 h. |
The temperature in the range of 85 °C to 140 °C was found to have a vital role in the catalytic activity for the liquid phase aerobic oxidation of vanillyl alcohol (reaction conditions: 3 mmol vanillyl alcohol, P = 21 bar air, 60 ml acetonitrile, 0.0025 g cm−3 catalyst) (Fig. 17). At 85 °C, the lowest catalytic activity was measured, with 64% conversion. As the temperature increased to 120 °C, excellent catalytic activity, 91% conversion with 81% selectivity for vanillin, was obtained. A further increase in the temperature to 140 °C showed a slight increment in the conversion; however, the selectivity was reduced. This was possibly due to the overoxidation of vanillin to vanillic acid at higher temperature. However, char formation was also observed at 140 °C. Thus, it should be noted that 120 °C is a suitable temperature for the liquid phase aerobic oxidation of vanillyl alcohol.
Fig. 17 Effects of temperature on conversion and selectivity. Reaction conditions: 3 mmol vanillyl alcohol, 21 bar air pressure, 60 ml acetonitrile, catalyst A (Cu:Mn) mass 0.0025 g cm−3, 2 h. |
To investigate the influence of air pressure, the reaction was carried out at four different pressures: 7 bar, 14 bar, 21 bar and 28 bar (reaction conditions: 3 mmol vanillyl alcohol, 120 °C, 60 ml acetonitrile, 0.0025 g cm−3 catalyst) (Fig. 18). The catalytic conversion was found to increase with increasing pressure in the order of 7 bar < 14 bar < 21 bar < 28 bar, whereas the selectivity followed a descending order. The selectivity decreased due to the presence of more desorbed oxygen in the reaction, which caused overoxidation of the product vanillin. The optimum catalytic conversion of 91% conversion with 81% selectivity for vanillin was attained for 21 bar air pressure, while the worst conversion (61%) was measured at 7 bar air pressure. Based on the observed results, one can conclude that 21 bar air pressure is the optimum condition for the aerobic oxidation of vanillyl alcohol.
Fig. 18 Effects of air pressure on conversion and selectivity. Reaction conditions: 3 mmol vanillyl alcohol, 120 °C, 60 ml acetonitrile, catalyst A (Cu:Mn) mass 0.0025 g cm−3, 2 h. |
Furthermore, to detect any leaching of the metals in the reaction, leaching tests were also performed. After 30 min reaction time, the conversion was measured at 28%. After that, the catalyst was filtered and the reaction was carried out in the absence of the catalyst. No further conversion was noticed. This confirms that no metal leached from the catalyst during the catalytic conversion.
Fig. 19 Recyclability study on conversion and selectivity. Reaction conditions: 3 mmol vanillyl alcohol, 120 °C, 21 bar air pressure, 60 ml acetonitrile, catalyst A (Cu:Mn) mass 0.0025 g cm−3, 2 h. |
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