Synergistic promotion of ultra-small Pt nanoparticles and oxygen vacancy in MOF catalyst for ethyl levulinate to valerolactone at room temperature
0 Abstract
Currently, designing highly efficient catalysts for biomass hydrogenation at low temperatures remains a significant challenge. This paper proposes a straightforward solvent-treatment strategy to create rich oxygen vacancies (OV), facilitating the loading of ultra-small (1.6 nm) Pt nanoparticles (NPs) onto a metal-organic framework (MOF) (LaQS) with rich OV (LaOV-r). Consequently, a bifunctional Pt2/LaOV-r catalyst, featuring Lewis acid and metal hydrogenation sites, was synthesized. Under mild conditions (80 °C), the Pt2/LaOV-r catalyst exhibited a catalytic yield of >99% in converting biobased ethyl acetylpropionate [ethyl levulinate (EL)] to valerolactone [γ-valerolactone (GVL)]. This yield was 3.2 and 13.3 times higher than those measured by Pt2/LaQS and commercial Pt/C catalysts, respectively. Specifically, Pt2/LaOV-r catalyzed the full conversion of EL to GVL even at room temperature. The results revealed that the synergistic effect between ultra-small Pt NPs and OV in the MOF catalyst is important for the efficient conversion of EL into GVL. Especially, the abundant OV defects in LaOV-r not only enhanced the electron cloud density of Pt NPs at active sites of hydrogenation, but also elevated the content of moderately-strong acidic sites. This enhances the ability to activate H2 and EL, and promotes the intra-molecular dehydration of intermediates to GVL. The synergistic catalytic mechanism of OV and ultra-small Pt NPs in MOFs is proposed. This study presents an effective strategy for defect engineering aimed at enhancing catalytic biomass conversion using MOFs-loaded metal NPs.
Keywords
INTRODUCTION
Hydrogenation to produce fuels and chemicals is an important way to realize the value of biomass[1,2]. Levulinic acid (LA) is considered one of the twelve most important biomass platform molecules[3,4]. LA can be transformed into many high-value derivatives[2,3,5]. Among these, γ-valerolactone (GVL) stands out as a pivotal bioplatform molecule, offering versatility for diverse applications[6-10]. The catalytic hydrogenation of LA and ethyl acetylpropionate [ethyl levulinate (EL)] is considered the primary route for GVL production[9,11-15]. The relatively low boiling point of EL compared to LA enhances catalyst stability and reduces the risk of reaction vessel corrosion. However, the hydrogenation process from EL to GVL still faces problems such as complicated catalyst preparation and poor cycle stability. In particular, the catalytic efficiency is low at lower temperatures. Therefore, a thorough study is needed to find an efficient catalytic system for preparing GVL.
Precious metal catalysts (such as Pt, Pd, Ru, etc.), especially Pt-based catalysts have excellent capabilities of activating hydrogen and carbonyl[16-21]. However, currently Pt-based catalysts typically require higher hydrogen pressure (2.5-6 MPa)[22-24] and/or elevated reaction temperature (≥150 °C)[25,26]. Therefore, it is necessary to develop Pt-based catalytic systems for GVL preparation under mild conditions. The development of bifunctional catalysts is crucial in light of the fact that EL undergoes reduction followed by dehydration to generate GVL. Among them, the metal site facilitates C=O reduction, and the acidic site influences intermediate conversion and reactant adsorption[17,27-29]. Therefore, simultaneous tuning of both hydrogenation and Lewis acidic sites in Pt-based catalysts is imperative.
Generally, the sizes of metal nanoparticles (NPs) and the interaction of metal-support play a significant role in determining the catalytic performance[30,31]. However, the small metal particle size does not significantly enhance catalyst performance in hydrogenation reactions, particularly at low temperatures[32]. Therefore, controlling the interaction between metal and support becomes a crucial factor in regulating hydrogenation activity. In recent years, metal-organic frameworks (MOFs) have garnered considerable attention as catalyst supports due to their high specific surface areas, diverse structures and customizable functionalities[33]. Although the functional synergy between metal NPs and MOFs has been extensively investigated, resulting in remarkable catalytic performance. However, it is regrettable that the majority of activity originates from the metal NPs alone, which undermines the potential contribution of MOFs[34]. MOFs are considered to help reactant molecules to activate/regulate surface properties of metal NPs[35]. Therefore, the integration of MOFs with metal NPs holds great promise for harnessing synergistic catalytic effects.
Defect engineering is an effective strategy for tuning the performance of MOFs-based catalysts[36,37]. Previous studies have demonstrated that the introduction of defects in MOFs not only enhanced porosity and generated many open metal sites, but also facilitated the anchoring of metal NPs, regulating electronic structure and increasing active sites of the catalyst[38,39]. MOFs-based catalysts with abundant defects exhibit enhanced adsorption and activation capabilities towards reactants and intermediates[40]. Additionally, facile synthetic methods and improved durability are pivotal factors for practical applications of NPs/MOFs catalysts[34]. However, the current materials primarily consist of a limited combination of metal NPs and well-known MOFs, such as UiO-66 and ZIF-8[35]. So, it is still a big challenge to develop hybrid materials of MOFs with high thermal stability and metal NPs. Moreover, the methods reported for MOFs defect regulation are relatively few and complex. Therefore, there is an urgent need to develop a simple and effective method to solve the above problems.
This study presents a concise NaBH4 room-temperature treatment strategy for preparing bifunctional catalyst (Pt2/LaOV-r) with ultra-small metal NPs and rich oxygen vacancies (OV). When Pt2/LaOV-r was used to convert EL to GVL, excellent catalytic results were obtained. In particular, complete conversion was also achieved at room temperature. The synergistic promotion of metal NPs and OV in MOFs support was explored through characterization and theoretical calculations.
EXPERIMENTAL
Materials
Methanol (99.7%), ethanol (99.7%), 2-propanol (99.7%), ethylenediamine (99.7%), dichloromethane (99.7%), NaBH4 (99.7%) and N,N-dimethylformamide (DMF) (99.7%) were purchased from Sinopharm Chemical Reagent Co. EL (99.0%) and LaCl3·xH2O (99.0%) were purchased from Adamas Reagent Co. H2PtCl6·6H2O and 8-hydroxyquinoline-5-sulfonic acid (H2QS) were procured from TCI (Shanghai).
Catalyst characterization
Powder X-ray diffraction (XRD) patterns were obtained using a Rigaku Ultima XRD spectrometer with Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 40 mA. The specific surface areas and pore properties of catalysts were determined using the Brunauer-Emmett-Teller (BET) method at 77 K with a Micromeritics ASAP 2460 physical adsorption instrument for N2 adsorption-desorption measurements. Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) were conducted using a JEM-2100 instrument at 200 kV. X-ray photoelectron spectroscopy (XPS) analysis was performed with a monochromatic Al Kα
Synthesis of catalysts
The catalysts were prepared by loading ultra-small Pt NPs using the impregnation method. The LaQS support was synthesized by reacting LaCl3·xH2O and H2QS under solvothermal conditions. The 1.22 g
Method for catalytic performance evaluation
The hydrogenation of EL is carried out in a 25 mL reactor with high pressure. Normally, 25 mg of
At the end of the reaction, the mixture was cooled to a lower temperature and the catalyst particles were filtered out of the solution using a filter membrane. The reacted catalyst was placed on a filtration unit and washed with ethanol, and dried at the end of filtration for subsequent cycling experiments. Reactants in the liquid phase were identified by gas chromatography-mass spectrometry (GC-MS, GCMS-QP2010 SE, Shimadzu) while the reaction samples were analyzed by GC (GC-7890A, Agilent). The reactant conversion and product selectivity are calculated as follows:
RESULTS AND DISCUSSION
Material characterization
Synthesis of LaOV-r anchored Pt NPs
Figure 1A is a schematic diagram of the preparation process of Pt2/LaOV-r. Firstly, a highly stable La-MOF (LaQS) was synthesized by hydrothermal method[41]. Then, LaQS support was treated by simple NaBH4 solution at room temperature to form OV-rich LaQS (LaOV-r). Then, LaOV-r was impregnated by Pt ions with subsequent NaBH4 reduction at room temperature to give Pt2/LaOV-r with ultrafine Pt NPs. Meanwhile, a control sample (Pt2/LaQS) was prepared by the same method using pristine LaQS.
Figure 1. (A) Route for the preparation of LaQS, Pt2/LaOV-r and Pt2/LaQS; (B and C) XRD patterns and N2 adsorption isotherms of LaQS, LaOV-r, Pt2/LaQS and Pt2/LaOV-r; (D) TEM images of LaQS; (E) Lattice fringes and crystalline surfaces of Pt in Pt2/LaOV-r; (F and G) TEM particle images and grain sizes of Pt2/LaOV-r; (H and I) TEM particle images and grain sizes of Pt2/LaQS; (J) HR-TEM image and EDS elemental mapping of Pt2/LaOV-r. XRD: X-ray diffraction; TEM: transmission electron microscopy; HR-TEM: high-resolution TEM; EDS: energy dispersive X-ray spectroscopy.
XRD characterization was performed to confirm the structural integrity of the catalyst support. Figure 1B shows that the characteristic peaks in XRD patterns of Pt2/LaOV-r, Pt2/LaQS and LaOV-r match well with those of the parent LaQS. Additionally, they show good agreement with the simulated XRD characteristic peaks of LaQS. The structure of LaQS remains stable whether loaded after the vacancy treatment or directly loaded. Moreover, the content of the loading metal has little effect on the structure of LaQS [Supplementary Figure 1]. Diffraction peaks attributed to Pt NPs are challenging to be observed in Pt2/LaOV-r and Pt2/LaQS by XRD. This difficulty may arise from the good dispersion of small-sized Pt NPs or low metal Pt content.
N2 sorption tests were employed to compare the porous structures of the catalysts. Figure 1C illustrates that all samples exhibit typical hysteresis loops, indicative of a type IV adsorption-desorption isotherm. This confirms the mesoporous structure. Meanwhile, as shown in Table 1, compared with LaQS (843.37 m2/g), the Langmuir specific surface area of LaOV-r with more vacancies is decreased to 512.25 m2/g. An increase of OV probably destroys some pores. The surface area provides abundant binding sites for Pt NPs. Compared with Pt2/LaQS (603.56 m2/g), the specific surface area of Pt2/LaOV-r after NaBH4 treatment is decreased to 327.03 m2/g, mainly due to the occupation of the pore space by Pt NPs.
The specific surface area, pore volume, pore size, Pt particle size, Pt species proportion, OV content and Pt content of different catalysts
| Sample | SBET (m2·g-1)a | Pore volume (cm3·g-1)a | Pore size (nm)a | Pt particle size (nm) | Pt0 (%)b | Pt4+ (%)b | OV (%)b | Pt (wt%)c |
| LaQS | 843.37 | 0.34 | 6.80 | - | - | - | 24.61 | - |
| Pt2/LaQS | 603.56 | 0.35 | 11.99 | 1.7 ± 0.1 | 75.18 | 24.82 | 37.15 | 1.4 |
| LaOV-r | 512.25 | 0.27 | 8.70 | - | - | - | 31.53 | - |
| Pt2/LaOV-r | 327.03 | 0.27 | 11.44 | 1.6 ± 0.1 | 65.97 | 34.03 | 44.35 | 1.6 |
| Pt/C | 2,745.39 | 1.06 | 3.43 | - | 76.18 | 23.82 | 26.70 | 5.0 |
Firstly, LaQS support with high specific surface area has a sheet structure [Figure 1D], which offers more sites for the adsorption of metallic Pt NPs. Secondly, the abundant OV on LaOV-r support have a positive effect on metal Pt dispersion. Importantly, the large π bond of the quinoline ring on the surface of the LaQS support interacts with Pt NPs. This is further proved by infrared spectroscopy (IR) and XPS analysis. The lattice stripe of 0.226 nm is clearly seen in Figure 1E, which corresponds to the crystal plane of single crystal Pt (111) (PDF#04-0802)[42]. It can be seen from the TEM images of Figure 1F-I that the particle size of the metal on Pt2/LaOV-r with rich OV is only about 1.6 nm, which is almost the same as that of Pt2/LaQS
Interaction between LaOV-r support and ultra-small Pt NPs
Figure 2A displays OV peaks at g = 2.003 for LaQS, Pt2/LaOV-r, Pt2/LaQS and LaOV-r in EPR spectra. Among these, the peak area of Pt2/LaOV-r is larger than that of Pt2/LaQS, indicating a significantly higher number of OV in Pt2/LaOV-r. Similarly, the number of OV in LaOV-r is also higher than that in LaQS. This is further supported by the XPS analysis of oxygen [Figure 2B]. Compared with 532.42 eV on Pt2/LaQS, the binding energy of OV on Pt2/LaOV-r is increased to 532.73 eV. The binding energies of OV on Pt2/LaOV-r and Pt2/LaQS increase by 0.36 and 0.05 eV, compared with 532.37 and 532.31 eV for LaOV-r and LaQS, respectively. This trend is consistent with the binding energy of N 1s in Figure 2C and of S 2P in Figure 2D. This suggests that with OV increase, more electrons are transferred from O/N elements on the support to the metal Pt. Figure 2E demonstrates that the binding energy of La remains constant, suggesting that La–O bond breaking is not responsible for OV generation. Combined with LaQS structure[41], this finding supports that OV are primarily generated by S–O bond breaking.
Figure 2. (A) EPR spectra of LaQS, LaOV-r, Pt2/LaQS and Pt2/LaOV-r; (B-E) XPS spectra of O 1s, N 1s, S 2p and La 3d of LaQS, LaOV-r,
FT-IR [Figure 2F] was employed to analyze the interactions of the OV-rich Pt2/LaOV-r and Pt2/LaQS with the support (LaOV-r and LaQS). Compared with LaQS, the 1,660 cm-1 belongs to the C=C stretching vibration of the quinoline ring in LaOV-r. The C=C bond on quinoline rings is destroyed when OV is generated. Further, the C=C bands of Pt2/LaOV-r and Pt2/LaQS are smaller. Another reason may be the interaction of Pt with the C=C conjugated system of the quinolyl ring. Moreover, the bands of 1,564 and 1,461 cm-1 belonging to the C=N stretching vibration of the quinoline ring in Pt2/LaOV-r are decreasing. Meanwhile, compared with LaQS and LaOV-r, the band attributed to S–O vibration (690 and 600 cm-1) on Pt2/LaOV-r and Pt2/LaQS are greatly reduced. The possible reason is that the electron-rich S=O and C=N groups in LaQS and LaOV-r interact with Pt NPs through π–π bonds. This interaction of metal with conjugated quinoline ring leads to the high dispersion of Pt NPs. Compared with Pt2/LaQS, the above characteristic bands of Pt2/LaOV-r decrease more obviously with OV, indicating that OV also interacts with Pt NPs. This finding is consistent with the XPS and TEM analyses.
Catalytic properties
OV promoted catalytic performance of ultrafine Pt NPs
Table 2 displays the comparative results of different catalysts. The catalytic activity of LaOV-r support for EL is 0 at 130 °C, the same as the blank (Entries 1-2). The commercial Pt/C catalyst has a conversion of 50% and a selectivity of only 25% at 80 °C and 2 MPa H2 (Entry 3). The reaction conversion of Pt2/LaQS is only 31%, but its selectivity is increased to 99% (Entry 4). Although the catalytic conversion of Pt2/LaQS is not as high as that of Pt/C, a higher selectivity is obtained, which may be due to the interaction of Pt with the support. Remarkably, under the same conditions, Pt2/LaOV-r has 99% conversion and 99% selectivity in the 4 h reaction (Entry 5); the yield is 13.3 and 3.2 times higher than those of commercial Pt/C and Pt2/LaQS, respectively. The reaction conversion of Pt1/LaOV-r is 70% and the selectivity is 50% (Entry 6), probably because the Pt content reduction affects the LaOV-r support thereby decreasing the catalytic activity of Pt1/LaOV-r [Supplementary Figures 2-6]. The product yield and selectivity of Pt2/LaOV-r remain hardly changed after three cycles (Entry 8). An absence of Pt in the reaction solution measured by ICP suggests that the interaction of Pt with LaOV-r ensures the stability of Pt2/LaOV-r. By comparing the XPS spectra of O 1s after the cycles, the binding energy of OV only decreased by 0.03 eV after cycles [Supplementary Figure 7]. Meanwhile, the OV content only showed a minor decrease. This indicates that OV remain relatively stable during cyclic reactions. Notably, the Pt2/LaOV-r catalyst has >99% yield of GVL after 36 h at room temperature of 25 °C (Entry 7). Compared with the Pt metal catalyst in the catalytic literature
Preparation of GVL from EL catalyzed by different catalystsa
| Entry | Catalyst | Condition | Con (%) | Sel (%) |
| 1 | blank | 130 oC | 0 | 0 |
| 2 | LaOV-r | 130 oC | 0 | 0 |
| 3 | Pt/C | 80 oC | 30 | 25 |
| 4 | Pt2/LaQS | 80 oC | 31 | >99 |
| 5 | Pt2/LaOV-r | 80 oC | >99 | >99 |
| 6 | Pt1/LaOV-r | 80 oC | 70 | 50 |
| 7 | Pt2/LaOV-r | 25 oC | >99 | >99 |
| 8 | Pt2/LaOV-rb | 70 oC | 80 | >99 |
The synergy of OV and Pt NPs for enhanced H2 activation
H2-TPD curves were examined to assess the H2 adsorption capabilities of different catalysts [Figure 3A]. Compared to Pt2/LaQS, Pt2/LaOV-r has a larger desorption peak at 217 °C. This suggests that the OV rise significantly enhances H2 adsorption and the support with rich OV in Pt2/LaOV-r exhibits a stronger interaction with the metal Pt NPs. The peak at 217 °C is notably pronounced in the Pt2/LaOV-r catalyst compared to the Pt1/LaOV-r catalyst with 1 wt% loading [Supplementary Figure 3], indicating an enhanced interaction between the support and Pt NPs with an increase of Pt loading, as supported by the Pt 4f XPS analysis in Figure 3B. Compared with 72.58 eV in Pt/C, the binding energy of Pt in Pt2/LaQS is notably stronger [Figure 3B]. In Pt2/LaQS, the binding energy of Pt is decreased to 72.09 eV, representing a reduction of 0.49 eV. This implies that electrons transfer from the LaQS to Pt, which increases the electron density of Pt NPs on Pt2/LaQS. While in Pt2/LaOV-r, the binding energy is decreased to 71.29 eV, a substantial reduction of 0.8 eV. This suggests that an increase of OV content causes more electron transfer from the LaQS to Pt, resulting in the highest electron density of Pt NPs on Pt2/LaOV-r. This enhances the Pt feedback to the H–H back-bonding orbitals, thereby reducing the hydrogen activation energy for adsorption. This improvement enhances the ability of H2 to activate. The reduction behaviors of Pt2/LaOV-r and Pt2/LaQS were assessed using H2-TPR [Figure 3C]. Compared with 138.9 and 206.1 °C for Pt2/LaQS, the reduction peak temperature of Pt2/LaOV-r is lower shifted to 137.2 and 203.2 °C. This may be because the increase of OV promotes the migration of oxygen[43], thus affecting the reduction performance of the catalyst. This elevated OV in MOFs facilitates the interaction of Pt NPs with LaOV-r support, consistent with the XPS analysis. Thus, the primary reason for the higher catalytic activity of Pt2/LaOV-r than Pt2/LaQS is the elevated electron density resulting from the rich OV.
Figure 3. (A) H2-TPD spectra of Pt2/LaQS and Pt2/LaOV-r; (B) XPS spectra of Pt 4f of Pt/C, Pt2/LaQS and Pt2/LaOV-r; (C) H2-TPR spectra of Pt2/LaQS and Pt2/LaOV-r; (D-F) H2 adsorbed on Pt NPs, Pt sites on Pt2/LaOV-r and Pt2/LaQS. TPD: Temperature-programmed desorption; XPS: X-ray photoelectron spectroscopy; TPR: temperature-programmed reduction; NPs: nanoparticles.
The density-functional theory (DFT) calculation confirms the corollary that oxygen-rich vacancies promoted the invigoration of H2 by the Pt NPs. Figure 3D illustrates the adsorption energy of pure Pt NPs for hydrogen (-0.491 eV). The modeled activation of H2 by Pt NPs alone, as well as in Pt2/LaQS, is shown in Figure 3E and F. The adsorption energies of Pt2/LaOV-r and Pt2/LaQS for hydrogen (-0.558 and -0.706 eV) are smaller than that of the Pt NPs (-0.491 eV), suggesting that the association between the supporters (LaOV-r and LaQS) and Pt NPs facilitates the adsorption and activation of H2 with the Pt NPs. This also indicates that the interaction of OV with Pt NPs in Pt2/LaOV-r promotes H2 activation, which agrees with the characterization and the catalysis findings.
Synergistic promotion of EL activation and intermediate cyclization by OV and Pt NPs
To explore the influence of acidic and basic sites on the EL hydrogenation reaction, Pt2/LaOV-r catalysis poisoning experiments were conducted. In Figure 4A, the addition of benzoic acid to poison the basic site results in a decrease of conversion and selectivity from 100% to 1% and 20%, respectively. This underscores the crucial role of the basic site as a promoter. Furthermore, the addition of potassium thiocyanate (KSCN) to poison the acidic site leads to an even more substantial reduction of conversion and selectivity to 0%. This indicates the essential nature of the acidic site as a catalytic site. In order to further study the effects of acidic and basic sites, NH3-TPD and CO2-TPD analyses were performed. The acidic sites were additionally analyzed through NH3-TPD [Figure 4B]. As depicted, a new desorption peak emerges around 225 °C for
Figure 4. (A) Pt2/LaOV-r catalyzed poisoning experiments for EL hydrogenation; (B) CO2-TPD curves of LaOV-r, Pt2/LaQS and
To explore the activation effect of Pt loading on EL, DFT calculations were performed for Pt NPs and OV in Pt2/LaOV-r. The adsorption of Pt NPs on EL in Pt2/LaQS, and OV and La3+ in LaOV-r were performed
Synergistic catalytic mechanism of OV and Pt NPs
To explore the catalytic mechanism, in situ CO IR tests were conducted on Pt2/LaQS and Pt2/LaOV-r catalysts. The absence of peaks below 2,000 cm-1 [Figure 5A and B] attributed to CO bridge adsorption on Pt0[44,45] further supports the dispersion of Pt in NPs. This is due to the propensity for dipole-dipole interactions among neighboring CO adsorbed species on NPs. Simultaneously, the intensity of CO adsorption peaks on Pt2/LaOV-r is lower than that on Pt2/LaQS, possibly due to the lower Pt0 content in Pt2/LaOV-r. This aligns with the XPS analysis, where Pt0 content in Pt2/LaOV-r (65.97%) is smaller than that in Pt2/LaQS (73.18%). Compared with CO adsorption in Pt2/LaQS catalyst (2,077 cm-1)[45], the adsorption peak in Pt2/LaOV-r is shifted to 2,083 cm-1. This shift may be attributed to an increase in OV in LaOV-r, causing a decrease in the binding energy of Pt from 72.09 eV in Pt2/LaQS to 71.29 eV in Pt2/LaOV-r. This implies that the increase in OV causes LaOV-r support to transfer more electrons onto the Pt NPs. The higher the electron density on Pt NPs means its stronger interaction with CO adsorption, resulting in a shift of the outgoing peak position to a lower wavenumber. Peak positions at higher wavenumbers of 2,188 and
Figure 5. (A and B) In situ infrared spectra of CO desorption in Pt2/LaQS and Pt2/LaOV-r; (C-E) 4-HPE adsorbed on La, OV and Pt sites on Pt2/LaOV-r; (F) a proposed hydrogenation mechanism of EL to GVL. 4-HPE: 4-hydroxyvalerate; EL: ethyl levulinate; GVL: γ-valerolactone.
Lewis acid plays a significant part in the molecular dehydrogenation reactions during the hydrogenation of ethyl acetylpropionate[47,48]. XPS and NH3-TPD analyses reveal that the higher Pt4+ content in Pt2//LaOV-r corresponds to higher acidic strength, contributing significantly to the intra-molecular dehydrogenation of the intermediate 4-hydroxyvalerate (4-HPE). DFT calculations were performed for Pt4+, La3+ and OV adsorption on Pt2/LaOV-r for 4-HPE in order to further investigate the acidic site role in the dehydrogenation process of the intermediate [Figure 5C-E]. With excluding the effects of Pt for OV and La elements in LaOV-r, the effect of OV for the Pt elements in Pt2/LaQS is also excluded [Supplementary Figure 9]. For C–O adsorption in 4-HPE, compared with -1.386 and -1.316 eV of La3+ and Pt4+ on Pt2/LaOV-r, the adsorption energy of OV on Pt2/LaOV-r is increased to -1.676 eV. This implies that the 4-HPE is predominantly adsorbed on OV in Pt2/LaOV-r. This contrasts with the EL analysis of Pt4+ adsorption and activation in the medium-strong acidic site. This suggests that OV in Pt2/LaOV-r exhibiting stronger affinity for 4-HPE promotes its adsorption and favors the subsequent dehydrogenation reaction. The above analysis also shows that OV once again functions as acidic sites for catalysis. Consequently, the catalytic effect of the Pt2/LaOV-r catalyst is noteworthy with OV playing a crucial role in enhancing the adsorption capacity of the intermediate.
The reaction mechanism is hypothesized for the catalytic hydrogenation of EL by Pt2/LaOV-r[47,48] [Figure 5F]. The catalytic hydrogenation of EL involves the hydrogenation of EL to 4-HPE and the intramolecular dehydrogenation of 4-HPE to GVL. The process necessitates a bifunctional catalyst with both Lewis acid and hydrogenation active sites. The combination of results from experiments and DFT calculations suggests that the synergistic interaction of OV with Pt NPs on the Pt2/LaOV-r surface is essential for efficient transformation of EL to GVL. The characterization shows that OV in Pt2/LaOV-r not only enhances the electron density of the Pt0, but also increases the content of Pt4+ [Table 1]. H2 moieties initially attached to the high-electron-density Pt0 active sites are activated into hydrogen atoms. Both DFT and NH3-TPD show that the Pt4+ sites were effective in adsorbing and activating C=O in EL. Subsequently, the activated hydrogen atoms attack the C=O group in the neighboring EL molecule, producing the 4-HPE. In the 4-HPE to GVL process, the acidic sites of OV play a crucial role[47,48]. The DFT calculations reveal that
CONCLUSION
In this study, a simple solvent-induced OV strategy was used to prepare bifunctional Pt2/LaOV-r catalyst with ultra-small Pt NPs (1.6 nm) and rich OV. At an ambient temperature of 25 °C, the optimal Pt2/LaOV-r catalyst exhibits complete conversion of EL to GVL. At 80 °C, the yield of EL to GVL reached >99%, which is 3.2 and 13.3 fold higher than Pt2/LaOV and commercially available Pt/C catalysts, respectively. The high catalytic performance of Pt2/LaOV-r was attributed to the synergistic interaction between ultra-small Pt NPs and rich OV. OV not only increases Pt0 electron cloud density and Pt4+ content of Pt NPs, but also introduces additional acidic catalytic sites. On the one hand, this enhances the activation of H2 and EL. On the other hand, it also promotes intramolecular dehydration of 4-HPE. Finally, the synergistic catalytic mechanism of OV and Pt NPs on MOFs was also hypothesized through characterization and DFT calculations. This approach is an important guide for the development of vacancy-rich MOFs-based catalysts in the modulation of biomass catalytic conversion.
DECLARATIONS
Authors’ contributions
Experiment performing, data analysis, results interpretation and manuscript drafting: Yan, W.
Data analysis and results discussion: Dao, Z.; Pu, S.; Yang, C.; Zhao, X.; Zhuang, C.
Manuscript revision, theoretical calculations, and administrative, technical and material support: Min, C.; Wang, Y.; Zhao, X. J.; Zou, X.
Availability of data and materials
The raw data supporting the findings of this study are available within this Article and its Supplementary Materials. Further data is available from the corresponding authors upon reasonable request.
Financial support and sponsorship
The authors gratefully acknowledge the supports from the National Natural Science Foundation of China (32360430, 22375031), Science and Technology Planning Project of Yunnan Province (202401BD070001-030, 202101BD070001-007), Young and Middle-aged Academic and Technical Leaders Project in Yunnan Province (202205AC160052), Jilin Natural Science Fund for Excellent Young Scholars (20230508116RC), and Fundamental Research Funds for the Central Universities (JGPY202103, 2412023YQ001).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2025.
Supplementary Materials
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Yan, W.; Wang, Y.; Zhao, X.; Pu, S.; Yang, C.; Dao, Z.; Zhuang, C.; Min, C.; Zhao, X. J.; Zou, X. Synergistic promotion of ultra-small Pt nanoparticles and oxygen vacancy in MOF catalyst for ethyl levulinate to valerolactone at room temperature. Chem. Synth. 2025, 5, 26. http://dx.doi.org/10.20517/cs.2024.35
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