Weichuan
Xu
a,
Hong
Huang
b,
Lingling
Bi
a,
Haimei
Xu
c,
Zhichao
Miao
b and
Mei
Wu
*a
aFaculty of Chemical Engineering, Huaiyin Institute of Technology, Huai'an, 223003, P. R. China. E-mail: meiwu@hyit.edu.cn
bSchool of Chemistry and Chemical Engineering, Shandong University of Technology, Zibo 255000, P. R. China
cQingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Science, Laoshan District, CN-266101 Qingdao, China
First published on 28th May 2024
Carbonyl compounds serve as essential raw materials in the production of fine chemicals and pharmaceuticals. An economical and environmentally-friendly route to generating carbonyl compounds involves the catalytic oxidation of alcohols and hydrocarbons using heterogeneous catalysts. In this work, we synthesize a series of vanadium-doped ordered mesoporous zirconium oxophosphate (V/M-ZrPO) nanocomposites via a modified one-pot evaporation-induced self-assembly approach. Characterization results confirm the successful embedding of vanadium species into the framework of the M-ZrPO support. Dispersion of vanadium sites generates catalysts exhibiting enhanced performance for the selective oxidation of alcohols and hydrocarbons to carbonyl compounds. In particular, the 20 V/M-ZrPO catalyst demonstrates excellent reusability and broad substrate compatibility. Relative to established catalytic materials, the 20 V/M-ZrPO catalyst demonstrated superior performance across key metrics including product yield, reaction time, and recyclability. These findings highlight the promise of V/M-ZrPO nanocomposites as efficient and durable catalysts for sustainable carbonyl compound production.
Beyond the nature of active sites, the number of accessible sites also critically impacts catalytic performance.21–23 Structured mesoporous materials possess regular pore patterns, consistent pore widths, and large exposed surfaces - features that have generated significant interest for catalytic applications. The extensive porous network enables homogeneous dispersion of active components throughout the support framework, providing abundant active sites for reactant adsorption and conversion. Consequently, the rational design of vanadium-doped mesoporous architectures could yield optimized heterogeneous catalysts.
Herein, we synthesized a series of vanadium-doped ordered mesoporous zirconium phosphate (V/M-ZrPO) nanocomposites using an altered single-step self-assembly approach. The influence of precise vanadium loadings on the textural, chemical, and catalytic properties is investigated systematically. Our findings confirm the successful incorporation of highly dispersed vanadium species into the parent M-ZrPO material. Introduction of accessible and uniform vanadium sites significantly enhances the catalytic performance for selective alcohol and hydrocarbon oxidations to generate carbonyl compounds. Notably, the 20 V/M-ZrPO catalyst demonstrates remarkable reusability alongside broad substrate compatibility. These results signify V/M-ZrPO composites as promising heterogeneous catalysts for sustainable carbonyl production.
Nitrogen physisorption experiments were carried out at −196 °C using a Micromeritics 3Flex surface characterization analyzer. Prior to measurements, samples were degassed at 300 °C for 2 hours under vacuum.
(High-resolution) Transmission electron microscopy ((HR)TEM), energy dispersive X-ray spectroscopy (EDS), as well as elemental mapping were performed on a FEI TECNAI G2 F20 instrument working at 200 kV.
Raman spectra were obtained using a LabRam HR instrument that has a charge-coupled device (CCD) detector under 532 nm laser over the spectral from 100 to 1500 cm−1 at room temperature.
UV-visible (UV-vis) spectra were recorded from 200 to 800 nm using a PE Lambda 650S spectrophotometer at ambient conditions.
Temperature-programmed reduction (TPR) experiments were performed using a Chembet PULSAR TPR/TPD analyzer (Quantachrome Instruments). Typically, 0.1 g Samples were heated from 40–950 °C at a ramp rate of 20 °C min−1 with a flow of 40 mL min−1 5% H2/Ar.
X-ray photoelectron spectroscopy (XPS) was carried out using a Thermo Scientific ESCALAB 250Xi instrument. The obtained P, Zr, V, and O spectra were calibrated according to the C 1s peak (284.8 eV).
Gas chromatography–mass spectroscopy (GC-MS) instrument (Agilent-7890B-7000D) was employed to identify the reaction products.
Inductively coupled plasma (ICP) was conducted on a Thermo Fisher iCAP PRO(OES).
Fig. 1 (a) Low-angle X-ray diffraction, (b) N2-physisorption, and (c) pore size distribution of V/M-ZrPO series. |
Samples | SAXD | N2-physisorption | Contents obtained from XPS (at%) | |||||
---|---|---|---|---|---|---|---|---|
d 100 (nm) | Specific surface areab (m2 g−1) | Pore sizec (nm) | Pore volumed (cm3 g−1) | V | Zr | P | O | |
a Acquired from XRD patterns using Bragg's law. b The specific surface area was calculated via the Brunauer–Emmett–Teller (BET) method with the relative pressure ranging from 0.05 to 0.30. c The pore size was obtained from the adsorption branch of isotherms by using Barrett–Joyner–Halenda (BJH) method. d The pore volume was determined from the amount of nitrogen adsorbed at a relative pressure of 0.990. | ||||||||
0 V/M-ZrPO | 8.83 | 163 | 5.63 | 0.22 | 0.0 | 16.2 | 14.6 | 69.2 |
5 V/M-ZrPO | 9.29 | 190 | 6.64 | 0.26 | 1.6 | 15.1 | 14.0 | 69.3 |
10 V/M-ZrPO | 9.49 | 179 | 6.57 | 0.30 | 2.0 | 15.9 | 13.6 | 68.5 |
15 V/M-ZrPO | 9.70 | 205 | 6.57 | 0.28 | 2.8 | 14.7 | 13.6 | 68.9 |
20 V/M-ZrPO | 9.70 | 175 | 6.55 | 0.29 | 3.4 | 14.7 | 12.1 | 69.8 |
The N2 sorption isotherms shown in Fig. 1b present a type IV behavior with hysteresis loops of H1-type26,27 within 0.4–0.7 P/P0, consistent with capillary condensation in uniform mesopores. The ordered mesochannels are also evidenced in TEM images (Fig. 2) along both parallel and perpendicular directions. Addition of V led to an upward shift of the adsorption branch and hysteresis loop, reflecting an increase in pore size. PSD data in Fig. 1c further verifies a narrow distribution of 2–9 nm sized pores, with the average pore diameter becoming larger at higher V content. Table 1 compiled textural parameters of all samples. Upon V-doping, the specific surface area first increased from 163 m2 g−1 (V/M-ZrPO) to 205 m2 g−1 (15 V/M-ZrPO), before decreasing to 175 m2 g−1 for 20 V/M-ZrPO. A similar trend was observed for pore size and volume. The pore size improves from 5.63 nm to 6.64 nm and gradually decreases to 6.55 nm after 5 V/M-ZrPO, and an increase of pore volume was observed for V doped samples by 18–36%. XPS and EDS analyses also confirmed successful V incorporation, though the surface actual content was lower than nominal values, especially at higher loadings.
Fig. 2 TEM of (a) 0 V/M-ZrPO, (b) 5 V/M-ZrPO, (c) 10 V/M-ZrPO, (d) 15 V/M-ZrPO, (e) 20 V/M-ZrPO, (f) SAED of 20 V/M-ZrPO, and elemental mapping of (g) 20 V/M-ZrPO, (h) O, (i) P, (j) Zr and (k) V. |
TEM was utilized to visually inspect the mesoporous morphology of the V/M-ZrPO materials, as depicted in Fig. 2. Regular striped frameworks representing the characteristic parallel-aligned mesochannels were clearly observed along the [110] orientation in Fig. 2a–e. Additionally, cylindrically-shaped pores aligned perpendicular to the [001] direction were distinctly visible in Fig. 2d, confirming the existence of long-range order in the materials. This periodic mesostructure corroborates the N2 sorption results indicating cylindrical pores. From the micrographs, pore diameters of approximately 6 nm were estimated, consistent with pore size distribution data. Importantly, no detectable V aggregations were found, implying that vanadium species are homogeneously dispersed among mesoporous skeletons. The selected area electron diffraction (SAED) was performed to further investigate the crystallographic characteristic of V/M-ZrPO. The results in Fig. 2f exhibits broad diffraction rings, indicative of the low crystallinity of these mesoporous samples. This finding is aligned with XRD data. Elemental mapping of 20 V/M-ZrPO (Fig. 2g–k) further illustrates the uniform spatial distribution of O, P, Zr, and V, substantiating highly dispersed V species throughout the mesostructure achieved via the one-pot EISA synthesis.
Raman spectroscopy was employed to investigate the bonding characteristics within the V/M-ZrPO materials. As shown in Fig. 3b, samples with low V content (below 5%) displayed no distinctive peaks. However, new peaks emerged at 890 cm−1 and 1030 cm−1 for V loadings of 10% and above, with increasing intensity at higher V concentrations. The peak at 890 cm−1 can be attributed to symmetric V–O–V bridging vibrations28 associated with vanadyl (VO2+) moieties incorporated into the phosphate framework. Meanwhile, the 1030 cm−1 peak likely contains overlapping signal from asymmetric VO terminal stretches29 in vanadyl groups.
UV-vis spectroscopy was utilized to probe the oxidation states and coordination environments of V species within the V/M-ZrPO samples. As shown in Fig. 3c, the undoped 0 V/M-ZrPO material displayed a characteristic absorption around 230 nm from Zr–O–P bonds.30 Upon incorporating V, a broad peak emerged at 250–550 nm, whose intensities increased progressively with higher V content. The peak can be ascribed to ligand-to-metal charge transfer transitions of tetrahedrally coordinated V5+ centers in VO43− units and d–d transitions associated with octahedral V4+ sites in VO2+ moieties. The presence of this broad peak confirms the integration of V5+ and V4+ species into Zr4+ phosphate sites during synthesis.
The redox behaviors of V species within the samples were investigated via H2 temperature-programmed reduction experiments, with profiles shown in Fig. 3d. The undoped material displayed no reduction peaks, while introduction of V led to emerging signals and increased with higher V content, attributed to the greater number of reducible V centers. Additionally, the reduction peak shifted to lower temperatures from 610 °C down to 583 °C, suggesting decreased V-framework interactions and increased reducibility of V sites.
X-ray photoelectron spectroscopy (XPS) was utilized to characterize the chemical states, compositions, and V4+/V5+ ratios of surface V species. The V 2p spectra in Fig. 4c exhibit peaks around 525.2 eV and 517.7 eV whose intensities strengthen with increasing V content. The 525.2 eV peak corresponds to V5+ centers present as tetrahedral VO43− units, while the 517.7 eV peak originates from octahedral V4+ sites in VO2+ moieties. With the increased doping of V, the V4+/V5+ ratios gradually rose from 4.0 to 4.6 calculated from peak areas, probably due to an increased number of V atoms competing for a limited amount of lattice oxygen.
Fig. 4 XPS narrow spectra of the (a) Zr 3d, (b) P 2p, (c) V 2p, and (d) O 1s orbitals, respectively. |
The O 1s XPS data in Fig. 4d shows two peaks at 532.8 eV and 531.9 eV for 0 V/M-ZrPO from coordinated oxygen ligands attached to Zr metal centers and phosphate oxygen species, respectively. After V addition, the 532.8 eV peak disappeared, reflecting the replacement of Zr–O bonds with new V–O and P–O linkages. Meanwhile, the phosphate O peak gradually shifted to lower binding energies down to 531.5 eV for 20 V/M-ZrPO, indicating electron enrichment of the phosphates due to coordination with lower charge V4+ centers instead of V5+.
The findings of the catalytic performance of V/M-ZrPO in the oxidation of this substrate are displayed in Fig. 5. All components are confirmed by GC-MS. Without introducing vanadium species, the 0 V/M-ZrPO sample could achieve a yield of acetophenone of 21.2%, similar to the uncatalyzed reaction. Nevertheless, when adding vanadium elements, the yield of acetophenone dramatically increased by almost 2-fold and achieved 59.4% for 5 V/M-ZrPO, indicating the prime output was attributed to the introduced vanadium species in the mesoporous structure of V/M-ZrPO. Also, by elevating vanadium contents progressively, the electrochemical catalytic capability gradually enhanced and attained 94.5% yield for 20 V/M-ZrPO. Furthermore, the yield of acetophenone depending on 20 V/M-ZrPO catalyst load was investigated. The results showed an initial increase in yield with increasing amount, plateauing at ∼94% at 100 mg. However, the yield decreased slightly if the catalyst load increase continues. This might be because of over-adsorption of 1-phenylethanol on the catalyst surface that blocks active sites and inhibit electron transfer for the oxidation reaction.
To further investigate reaction parameters on catalytic performance, oxidant concentration and reaction temperature effect were considered for 1-phenylethanol electro-oxidation by 20 V/M-ZrPO in Fig. 6. Fig. 6a showed that the increasing oxidant level has a positive effect on product yield, but the effect was not that pronounced after the TBHP level reached 3 mmol. Fig. 6b indicated that the yield increased to 92.8% when the reaction temperature increased to 80 °C, after which the value almost stalled.
Fig. 6 Effect of (a) TBHP oxidant concentration and (b) reaction temperature on 1-phenylethanol electro-oxidation by 20 V/M-ZrPO. |
To optimize the reaction time of the catalyst for the liquid-phase electro oxidation of 1-phenylethanol, 20 V/M-ZrPO was tested for 10 h. As displayed in Fig. 7a, kinetic experiments identified 6 hours as the ideal reaction time for 20 V/M-ZrPO. Hot filtration was performed following 1 h of reaction time. The drastically reduced rate of acetophenone yield after removing the catalyst indicates the heterogeneity of 20 V/M-ZrPO catalyst. Also, to find out the stability of the catalyst, five successive runs were conducted, and excellent reusability was demonstrated over these catalytic cycles with only 4% deactivation.
Fig. 7 (a) Reaction time optimization with hot filtration results after 1 h reaction (b) reusability test of 20 V/M-ZrPO. |
To validate the resilience of the 20 V/M-ZrPO catalyst, N2 adsorption, UV spectroscopy, XPS, TEM with elemental mapping and ICP are performed subsequent to the reaction. In Fig. 8a and b, a sustained specific surface area of 136 m2 g−1 and a reduced primary pore diameter to 4.79 nm are observed, suggesting the preserved microstructure. In Fig. 8c, a conspicuous absorption band near 300 nm in the UV-visible spectra implies the incorporated vanadium into the phosphate position as detected in pristine samples. Further substantiation is derived from XPS data, which unambiguously demonstrates the consistent valence states of Zr, P, and V, attesting to structural integrity. Complementary TEM imagery (Fig. 8g) reveals that the mesochannels remain parallel and undisturbed, affirming the retention of an ordered mesoporous architecture post-catalytic process. Furthermore, elemental mapping conclusively verifies the homogeneous dispersion of V alongside O, P, and Zr following the catalytic reaction. Lastly, ICP analysis of the post-reaction solution discloses a minimal vanadium concentration of merely 15.3 mg L−1, testifying to insignificant leaching of vanadium during the catalytic event. Collectively, these exhaustive analyses serve as compelling evidence supporting the remarkable robustness of the 20 V/M-ZrPO catalyst material.
The catalytic performance of 20 V/M-ZrPO for the oxidation of 1-phenylethanol was compared to other catalysts reported in the literature, as summarized in Table 2. These benchmark catalysts consist of those based on transition metals as well as noble metals. The results indicate that even with microwave irradiation, some of the transition metal-based catalysts were unable to achieve higher acetophenone yields than 20 V/M-ZrPO. Furthermore, these catalysts usually exhibited poor recyclability. Comparable yields were obtained using other transition metal catalysts, but required reaction times twice as long. The noble metal-based catalysts generally gave higher yields but had limitations such as prolonged reaction times or inadequate reusability. Taken together, these findings suggest 20 V/M-ZrPO is an effective catalyst for the catalytic oxidation of 1-phenylethanol, without apparent drawbacks in terms of yield, reaction time, or recyclability.
Catalyst | Oxidant | Reaction time (h) | Temperature (°C) | Yield (%) | Recyclabilitya | Ref. |
---|---|---|---|---|---|---|
a Recyclability refers to cycles when the yield loss is within 5%. b Microwave irradiation. c Reaction pressure is at 10 Atm. | ||||||
20 V/M-ZrPO | TBHP | 6 | 80 | 92.8 | >5 | This work |
FeO(OH), Fe3+O(OH), Fe3O4 | t-BuOOH | 3MWb | 80 | 82 | 2 | 31 |
Cu(II)@10% CatMP-1 | TBHP | 12 | 70 | 94 | >10 | 32 |
[Cu(H2R)(HL)]·H2O | TEMPO | 0.25MWb | 80 | 85 | / | 33 |
CoCl2 −5% CNTs | t-BuOOH | 1MWb | 80 | 85 | 1 | 34 |
1.0 wt% Pd/60 wt% PK–SiO2 | O2c | 12 | 100 | 100 | / | 35 |
1 wt% Au/carbon xerogel | TBHP | 2MWb | 150 | 90 | 1 | 36 |
MnO2 commercial | TBHP | 7 | RT | 84 | 1 | 37 |
Expanding the substrate scope revealed 20 V/M-ZrPO could efficiently oxidize a range of alcohols (benzyl alcohol, 4-methoxybenzyl alcohol, diphenylmethanol, cyclohexanol, cyclopentanol, cyclohexanemethanol) along with hydrocarbons (ethylbenzene, 1,4-diethylbenzene, diphenylmethane) with high yields of desired carbonyl products, as listed in Table 3. This broad applicability highlights 20 V/M-ZrPO as a promising heterogeneous catalyst in selective oxidation reactions, enabled by highly dispersed V active sites incorporated within the stable mesoporous backbone structure.
Entry | Substrates | Con. (%) | Major products | Sel.(%) |
---|---|---|---|---|
a CH3CN 15 mL, substrates 0.01 mol, TBHP 0.03 mol, catalyst mass 0.1 g, 80 °C, 4 h. b CH3CN 15 mL, substrates 0.01 mol, TBHP 0.03 mol, catalyst mass 0.1 g, 80 °C, 12 h. c CH3CN 15 mL, substrates 0.01 mol, TBHP 0.03 mol, catalyst mass 0.1 g, 80 °C, 6 h. | ||||
1a | 57 | 66:34 | ||
2b | 96 | 11:89 | ||
3c | 98 | 18:82 | ||
4c | 92 | 100 | ||
5b | 94 | 100 | ||
6b | 74 | 100 | ||
7b | 65 | 30:70 | ||
8b | 95 | 75 | ||
9b | 86 | 44:56 | ||
10b | 79 | 100 |
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