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Pt/CeO2@MOF Core@Shell Nanoreactor for Selective Hydrogenation of Furfural via the Channel Screening Effect Yan Long, Shuyan Song, Jian Li, Lanlan Wu, Qishun Wang, Yu Liu, Rongchao Jin, and Hongjie Zhang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01851 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 2, 2018
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ACS Catalysis
Type of manuscript: Article
Pt/CeO2@MOF Core@Shell Nanoreactor for Selective Hydrogenation of Furfural via the Channel Screening Effect Yan Long,†,§ Shuyan Song,*,† Jian Li,† Lanlan Wu,†,§ Qishun Wang,† Yu Liu,†,§ Rongchao Jin,‡ Hongjie Zhang*,† †
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China §
Graduate School of the Chinese Academy of Sciences, Beijing 100039, P. R. China
‡
Department of Chemistry, Carnegie Mellon University, Pittsburgh PA 15213, USA
ABSTRACT: Designing metal-organic framework (MOF)-encapsulated hybrid catalysts is considered as an effective way to realize catalytic selectivity due to their unique
channels.
Here,
a
sodium
polystyrenesulfonate
(PSS)-induced,
microwave-assisted route was developed to controllably construct Pt/CeO2@MOF core@shell hybrids. Using PSS as a modifying agent and followed by microwave assistance, MOFs could be continuously grown in an oriented manner on Pt/CeO2 nanospheres. The obtained Pt-CeO2@UIO-66-NH2 exhibited high conversion (99.3%) with high selectivity (>99%) for selective hydrogenation of furfural to furfuryl alcohol. It showed that the CeO2 would promote the catalytic activity while the size 1 ACS Paragon Plus Environment
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confinement effect of the UIO-66-NH2 channels can enhance the catalytic selectivity. This work highlights a useful strategy toward the universal synthesis of highly active, stable and selective catalysts for further utilization.
KEYWORDS: Pt/CeO2, MOFs, heterogeneous catalysis, selective hydrogenation of furfural, microwave synthesis
INTRODUCTION
Pt/CeO2 has been extensively researched recently owing to their significant roles in the catalysis industry, such as environment protection, new energy development and fine organic synthesis.1-3 CeO2 not only acts as a support for metal nanoparticles (NPs) but also is recognized as an active site to improve the activity and stability of CeO2-based materials.4 The reducibility of CeO2 would modulate the d-band center of metal NPs which is an important parameter for the catalytic activity.5 In addition, the Ce3+ contained in CeO2 can work as a Lewis base to influence the substrate adsorption and thus improve the selectivity of catalysts.6 Intensive research efforts have focused on reducing the size of Pt NPs, enhancing the interfacial effects between Pt NPs and CeO2, and controlling the hybrid structure to improve the activity and stability in catalytic reactions.7-9 Furthermore, Pt/CeO2 was also combined with other functional materials, such as graphene, mesoporous SiO2, to enhance its catalytic performance and/or to develop new properties.10,11 Even so, it is not easy for simple noble
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metal-CeO2 hybrids to achieve high conversion with long-term stable selectivity because the catalytic reaction directly occurs on the exposed active sites.
The construction of a core@shell structure to encapsulate the catalytically active substances into a porous shell is a straightforward and effective pathway for this problem. As a unique type of porous materials, metal-organic frameworks (MOFs) were considered as a suitable shell material for excellent core@shell catalysts for their structural flexibility, large surface areas, and especially well-defined pore structures.12-14 The cavities and small pore windows of MOFs will show size-sieving behavior to obstruct the diffusion of large reactants to active sites thus providing specific selectivity.15 Meanwhile, wrapping with MOFs can avoid the agglomeration and detachment of active components, enhancing the stability of catalysts under long-term and harsh catalytic processes. In a word, MOFs can provide a well-defined microenvironment for catalytic reactions to influence the activity and especially provide possible selectivity towards certain reactions. Up to now, remarkable progress has been made in encapsulating noble metal nanoparticles (NPs), such as Pt,14,16 Pd,17 Au16,18 and Ag16,19, in MOFs to develop new functional materials for catalytic applications. However, only a few metal-oxide@MOF core@shell nanostructures have been reported,20 and most of them coat the MOFs on metal oxides with the help of SiO2 or using metal oxide acted as sacrificial templates.21,22 It is still a great challenge to develop a universally applicable way to directly grow MOF outside a heterogeneous metal oxide due to the high interface energy from topology mismatch.
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Herein, we integrated the Pt/CeO2 with MOF to develop core@shell-structured Pt/CeO2@MOF hybrids. As illustrated in Scheme 1, the continuous and integrated MOF layer was grown around Pt/CeO2 nanospheres with the induction of sodium polystyrenesulfonate (PSS) and microwave assistance. In this process, PSS functioned as a molecular linker altering the surface charge of Pt/CeO2 and inspiring the adsorption of metal ions to the Pt/CeO2 while microwave heating was introduced to control the nucleation and growth of MOFs in order to avoid their self-growth. By this way, core@shell-structured Pt/CeO2@MOF hybrids with changeable cores, and different MOF shells with a tunable sheath thickness were synthesized with excellent control. As expected, the Pt-CeO2@UIO-66-NH2 exhibited high selectivity with long-term stability in the hydrogenation of biomass derived furfural, giving furfuryl alcohol selectivity > 99% with furfural conversion of 99.3%.
Scheme 1. Schematic illustration for the formation of Pt-CeO2@UIO-66-NH2.
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RESULT AND DISSCUSION
The Pt-CeO2@UIO-66-NH2 was fabricated by a series of processes, including polyol-mediated synthesis of hollow Pt-loaded CeO2 nanospheres, surface modification of Pt-CeO2 and microwave-assistant synthesis of UIO-66-NH2, as schematically illustrated in Scheme 1. The structures and morphologies of as-prepared samples were examined by powder X-ray diffraction (XRD) and transmission electron microscopy
(TEM).
As
shown
in
Figure
1a,
the
XRD
pattern
of
Pt-CeO2@UIO-66-NH2 exhibiting peaks at 2θ = 28.5o, 33.1o, 47.5o and 56.5o correspond to the (111), (200), (220) and (311) faces of fluorite-phase CeO2 (JCPDS 34-0394), respectively. And others are all matched well with the UIO-66-NH2 reflections (Figure S1). However, no clear peak of the Pt NPs was observed due to the low content (< 1wt%) and small size (< 3 nm) of the Pt NPs. The TEM images (Figure 1b, 1c) show uniform and monodisperse hollow nanospheres, and no scattered UIO-66-NH2 NPs or uncoated Pt-CeO2 nanospheres were found. From Figure 1c, a typical core@shell structure, distinguished by their contrast difference between the core and shell, was revealed. In each hybrid nanosphere, the core was self-assembled by numerous tiny CeO2 NPs to form a hollow nanosphere about 120 nm which loaded with Pt NPs (Figure S2b, S2d) and encased with UIO-66-NH2 which formed a continuous external sheath. The lattice spacing of the core, d=0.327 nm and d=0.231 nm, are consistent with the (111) planes of fluorite-phase CeO2 and the (111) planes of fcc Pt, respectively.23 In addition, TEM elemental mapping clearly exhibits that Ce and Pt only exist in the core, while Zr exists in the shell of the nanospheres (Figure 5 ACS Paragon Plus Environment
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1e-1f), further confirming the core@shell nanostructure of Pt-CeO2@UIO-66-NH2. X-ray photoelectron spectroscopy (XPS) also provided core@shell nanostructures with indirect evidence (Figure S3(purple)). There are C, N, O and Zr peaks, but no Ce or Pt peaks were observed in the XPS spectra of Pt-CeO2@UIO-66-NH2, because the XPS can only probe the element information within a thickness of only several atomic layers.24
Furthermore,
the
specific
surface
areas
of
Pt-CeO2
and
Pt-CeO2@UIO-66-NH2 were calculated from the BET curves (Figure S4). The BET surface area of Pt-CeO2@UIO-66-NH2 was up to nearly four times (232.9 m2·g-1) that of Pt-CeO2 (59.5 m2·g-1). Such a large BET surface difference resulted from the high porosity of UIO-66-NH2.
Figure1. (a) XRD pattern, (b) TEM image, (c) enlarged TEM image, (d) high-magnification TEM image, and EDX mapping analysis of (e) cerium, (f) platinum, (g) zirconium and (h) oxygen of Pt-CeO2@UIO-66-NH2. 6 ACS Paragon Plus Environment
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The surface modification of Pt-CeO2 and the nucleation process of the UIO-66-NH2 were determined as key factors in our synthesis. When the Pt-CeO2 was undecorated with PSS, individual UIO-66-NH2 nanocrystals formed, leaving Pt-CeO2 uncoated (Figure S5a). Nevertheless, with the introduction of PSS, from partially coated Pt-CeO2/UIO-66-NH2 hybrids (Figure S5b) to finally conformably wrapped Pt-CeO2@UIO-66-NH2 (Figure 1a) were obtained. These results indicate that PSS was essential for the formation of a well-integrated UIO-66-NH2 sheath. To elucidate the role of PSS, the surface ξ-potential of Pt-CeO2 nanospheres was determined. It shows that the ξ-potential changed from 19.3 mV to -16.2 mV after modification with PSS, which suggested a change in the surface charge of Pt-CeO2. Therefore, PSS acts as a connector and lowers the interfacial energy between the Pt-CeO2 and UIO-66-NH2, thus inducing the growth of UIO-66-NH2 on Pt-CeO2 via electrostatic interaction.25,26 More interestingly, there were always many scattered UIO-66-NH2 and exposed Pt-CeO2 nanospheres despite the decoration with enough PSS in the traditional
solvent-thermal
method
(Figure
S6a).
However,
only
uniform
Pt-CeO2@UIO-66-NH2 was obtained when microwave energy was employed to heat the reaction mixture while keeping the other experimental conditions constant (Figure S6b). Compared to traditional solvent-thermalization processes, microwaves can evenly heat up the reaction mixture directly at the “molecular” level to form more and smaller nuclei adhering to the surface of Pt-CeO2. This is effective for coating conformal MOFs on other materials.25,26 On the basis of these results, as shown in Scheme 1, we can speculate the mechanism of the coating of UIO-66-NH2 on Pt-CeO2 7 ACS Paragon Plus Environment
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as follows: with the modification of PSS (Figure S7), the electronegative PSS adsorbed on the Pt-CeO2 through electrostatic interaction to change the surface charge of Pt-CeO2. In addition, it also connected with Zr4+ via this interaction. In this way, PSS acts as an adhesive to induce the adsorption of Zr4+ ions onto the Pt-CeO2 nanospheres lowering the interfacial energy. Then, the adsorbed Zr4+ nucleates on the surface of Pt-CeO2 in microwave-assistant heat treatment process. Finally, the nuclei grew and connected with each other to form a full sheath rather than crystallizing via independent nucleation.
Figure 2. TEM images of Pt-CeO2@UIO-66-NH2 with a (a) 180 nm, (b) 120 nm, and (c)
60
nm
hollow
Pt-CeO2
Pt-CeO2@UIO-66-NH2(1),
nanosphere (e)
core;
TEM
images
Pt-CeO2@UIO-66-NH2(2),
of
(d) (f)
Pt-CeO2@UIO-66-NH2(3), (g) Pt@CeO2@UIO-66-NH2, (h) Pt-CeO2@ZIF-8(Zn), and (i) Pt-CeO2@MIL-100(Fe).
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To explore the universality of this developed method, a series of Pt/CeO2@MOF were built. First, the core can change both in the size and nanostructure. Hollow Pt-CeO2 spheres with size 180 nm (Figure S2a), 120 nm (Figure S2b), 60 nm (Figure S2c), and 50 nm multicore@shell Pt@CeO2 nanospheres (Figure S2e) were all successfully encapsulated in the UIO-66-NH2 shell (Figure 2a-2c, 2g). Furthermore, the thickness of UIO-66-NH2 shell can be easily controlled by repeating the process for UIO-66-NH2 synthesis. Taking 120 nm Pt-CeO2 hollow spheres as an example, the Pt-CeO2@UIO-66-NH2 with shell thickness of 8 nm (Pt-CeO2@UIO-66-NH2(1), Figure
2d),
15
nm
(Pt-CeO2@UIO-66-NH2(2),
Figure
2e),
and
30
nm
(Pt-CeO2@UIO-66-NH2(3), Figure 2f) all were successfully produced, respectively (The XRD and XPS patterns were shown in Figure S1, S3). Finally, because the interaction, induced by PSS, between Pt-CeO2 and Zr4+ is a nonspecific electrostatic interaction, other MOFs consisting of different metal centers and organic linkers, can also be grown on the Pt-CeO2 nanospheres by this strategy. So Pt-CeO2@ZIF-8(Zn) (Figure 2h, S8) and Pt-CeO2@MIL-100(Fe) (Figure 2i, S9) were also created.
Benefitting from the structural confinement effect of the MOFs and sometimes the interaction between the reactants and the ligands of the MOFs, MOF-based catalysts may be suitable candidates for selective hydrogenation of α,β-unsaturated aldehydes.27,28
Therefore,
the
catalytic
potential
of
as-prepared
Pt-CeO2@UIO-66-NH2 for the hydrogenation of furfural was investigated, along with the results of Pt@UIO-66-NH2 and Pt-CeO2 as control. As shown in Figure 3a, Pt-CeO2@UIO-66-NH2(1) gave a 100% conversion within 30 h at almost uniform 9 ACS Paragon Plus Environment
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speed. However, when catalyzed by Pt@UIO-66-NH2, with a similar Pt content (1.16 wt%, Figure S10 and Table S1), furfural decreased 50% quickly and then slowly achieved 100% conversion at 45 h. And both of them exhibited > 99% conversion of furfural with > 99% selectivity for furfuryl alcohol (Figure 3c, 3d). For the Pt@UIO-66-NH2, it shows that the yield of furfuryl alcohol was far below the apparent conversion of furfural at the initial stage of reaction. As the reaction progress, this gap gradually reduced and finally closed. And no other products were detected in the whole process (Figure 3b, Table S2). Besides, the final 99.4% yield of furfuryl alcohol also indicated that there is no furfural was entrapped in the catalyst. On this basis, we can speculate that the sharp decrease of furfural in the initial reaction period was mainly resulted from the adsorption by the channels of UIO-66-NH2
instead
of
translation.
Thus,
compared
to
Pt@UIO-66-NH2,
Pt-CeO2@UIO-66-NH2(1) exhibited superior catalytic activity and remained the same selectivity to furfuryl alcohol, revealing CeO2 could improve the catalytic activity of selective hydrogenation of furfural to furfuryl alcohol. This promotion of CeO2 for the hydrogenation of furfural can be contributed to the strong interaction effects between Pt and CeO2. To characterize this interaction, hydrogen temperature-programmed reductions (H2-TPR) experiments were carried out (Figure S11). The main reduction peak centered at 518 °C for Pt@UIO-66-NH2 is assigned to the reduction of Pt-O species.29,30 While in Pt-CeO2@UIO-66-NH2(1), the presence of CeO2 caused the reduction peak shift to much lower temperature centered at 426 oC owing to the interaction of Pt-O-Ce at the Pt-CeO2 interface. This revealed a strong Pt-CeO2 10 ACS Paragon Plus Environment
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interaction which would affect the d-band center and surface density of Pt and thus enhanced the catalytic activity.5,29,31 What’s more, it was said that the Lewis acidic and basic sites coexisting on the low-vacancy surfaces of CeO2 can favor the dissociation of H-H bond.32,33
Figure3. (a) Plots of the conversion of furfural, (b) the yield of furfuryl alcohol versus the reaction time catalyzed by Pt@UIO-66-NH2 and Pt-CeO2@UIO-66-NH2(1), and time course of the hydrogenation of furfural catalyzed by (c) Pt @UIO-66-NH2 and (d) Pt-CeO2@UIO-66-NH2(1).
Additionally, Pt-CeO2@UIO-66-NH2 showed different selectivity from Pt-CeO2 for the hydrogenation of furfural. When catalyzed by Pt-CeO2@UIO-66-NH2(2), the furfural was almost completely transformed into furfuryl alcohol with 99.3% yield and > 99% selectivity in 54 h. And no noticeable additional product was observed even when the reaction time was prolonged to 70 h (Figure 4a). This is completely 11 ACS Paragon Plus Environment
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different from the result of Pt-CeO2. Pt-CeO2 could achieve only 89.7% selectivity for furfuryl alcohol at maximum. Subsequently, furfuryl alcohol was further hydrogenated
to
tetrahydrofurfuryl
alcohol
and
few
1,2-pentanediol
and
1,5-pentanediol, leading to a gradual decrease of the furfuryl alcohol yield until it was fully converted (Figure 4b). These completely different phenomena of Pt-CeO2 vs Pt-CeO2@UIO-66-NH2(2) clearly indicate the excellent chemo-selectivity of Pt-CeO2@UIO-66-NH2(2) for the hydrogenation of furfural enhanced by the UIO-66-NH2 shell. According to previous studies, two modes, planar form and vertical form, for the adsorption of furfural on the metallic surface were proposed for the catalytic hydrogenation of furfural.34-37 The products can be profoundly affected by the adsorption configurations of the reactants on the catalysts surface. The reaction shows a preference for tetrahydrofurfuryl alcohol when furfural adsorbed in the planar form, in which both C=C bonds in the furan ring and the C=O group can coordinate with active sites. For the other mode, furfural was adsorbed vertically through the C=O group to form furfuryl alcohol. On the Pt surface, furfural prefers the planar configuration. Two C=C bonds and C=O bond sit along three different Pt-Pt bridge sites (Figure S12a).36,38,39 As a result, when catalyzed by Pt-CeO2, both the C=C and C=O of furfural are hydrogenated, and tetrahydrofurfuryl alcohol becomes the final product. However, in the Pt-CeO2@UIO-66-NH2(2) system, confined by the UIO-66-NH2 frame, furfural can be adsorbed only with the vertical geometry (Figure S12b). The framework of UIO-66-NH2 consists of 7.5 Å tetrahedral cages, 12 Å octahedral cages and 6 Å narrow triangular windows.40 The narrow windows 12 ACS Paragon Plus Environment
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compelled the furfural (6.6 Å × 4.9 Å × 1.6 Å, Figure S13) to diffuse across UIO-66-NH2 to the Pt-CeO2 surfaces vertically via the C=O group and, thus, hampered the adsorption and hydrogenation of the C=C bond in furan ring. Therefore, the reason for the high selectivity of Pt-CeO2@UIO-66-NH2(2) was the steric effect imposed by the small size of the UIO-66-NH2 pore structures. This is, in essence, size selectivity. In addition, this size selectivity was also observed in the hydrogenation of olefins (Figure S14). It gave approximately 54.5% conversion for n-hexane (2.5 Å) after 20 h, while only 4.2% for cyclohexene (4.2 Å) within 20 h. The results provide some evidence for the complete coating of Pt-CeO2 with a UIO-66-NH2 layer.
The catalytic properties of Pt@UIO-66-NH2 with different thickness of UIO-66-NH2 shell in the selective hydrogenation of furfural (Figure 4c, Table S2, S3) was also investigated. All of the Pt-CeO2@UIO-66-NH2 exhibited improved selectivity but decreased activity compared to Pt-CeO2. And it shows that the thicker the UIO-66-NH2 shell, the longer the time needed to achieve 100% conversion of furfural. In addition, all the yield of products and the conversion of furfural are nearly equal with no mass losses at any stage of reactions. Thus, the decreased activity of Pt-CeO2@UIO-66-NH2 can be contributed to the diffusion resistance of UIO-66-NH2 frame which hinder the approaching of furfural molecules to Pt-CeO2 in Pt-CeO2@UIO-66-NH2 catalysts. Therefore, an increase in the diffusion length leads to a decrease in the reaction rate. Furthermore, the stability of the catalyst is a determining factor for its practical applications. Thus, the reusability of Pt-CeOt2@UIO-66-NH2(2) was further investigated by detecting the conversion of 13 ACS Paragon Plus Environment
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furfural in the same period. As shown in Figure 4d, for the first run, Pt-CeO2@UIO-66-NH2(2) gives 50.5% conversion of furfural and 99.6% selectivity to furfuryl alcohol in 20 h. It was used five times without observing any sign of deactivation or decrease in furfuryl alcohol selectivity. At the fifth run, a 100.7% selectivity to furfuryl alcohol at 49.6% conversion of furfural was obtained. Moreover, further characterizations show no observable changes in the morphology, structure and the elements content of the spent catalysts, after two cycles of catalytic reactions (Figure S15). And the BET surface area of Pt-CeO2@UIO-66-NH2(2) before and after catalysis were 143.6 m2·g-1 and 142.3 m2·g-1, respectively. These indicate the excellent stability of as-prepared Pt-CeO2@UIO-66-NH2.
Figure 4. Time course of the hydrogenation of furfural catalyzed by (a) Pt-CeO2@UIO-66-NH2(2) and (b) Pt-CeO2, (c) plots of the conversion of furfural versus the reaction time catalyzed by Pt-CeO2 and Pt-CeO2@UIO-66-NH2(1-3), and
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ACS Catalysis
(d) the recyclability of Pt-CeO2@UIO-66-NH2(2) in the selective hydrogenation of furfural.
CONCLUSION
In summary, we have demonstrated a simple and effective strategy to achieve the growth of a MOF coating on Pt/CeO2 nanospheres in a controllable manner. In this synthesis process, PSS was used as a modifying agent to alter the surface charge of Pt/CeO2 and microwave heat-treatment was introduced to adjust the nucleation process of the MOFs. The whole synthesis could be precisely controlled both for the Pt/CeO2 hybrid cores with changeable sizes and nanostructures and for the MOF shells with tunable sheath thicknesses including UIO-66-NH2, ZIF-8(Zn), and MIL-100(Fe). Based on the characterization of the integrated core@shell structures, their catalytic properties for the hydrogenation of furfural were further explored. Experimental results revealed that Pt-CeO2@UIO-66-NH2 showed superior catalytic activity than Pt@UIO-66-NH2, indicating CeO2 was beneficial to the hydrogenation of furfural. This promotion can be contributed to (1) the modulation of the d-band center and surface density of Pt by the interaction between Pt and CeO2 and (2) the favor for the dissociation of H-H bond by Lewis acidic and basic sites coexisting on CeO2 might. This may extend the application of CeO2 to hydrogenation reactions involving
dissociation
of
H−H
bonds
processes.
More
importantly,
Pt-CeO2@UIO-66-NH2 provided a thickness-dependent reaction rate and high selectivity with long-term stability for the hydrogenation of furfural. It is expected 15 ACS Paragon Plus Environment
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that, in a catalytic reaction, the activity and products can be controlled through rational catalyst design. In addition, the synthesis route reported here may provide new
opportunities
for
preparing
core@shell-structured
metal-oxide@MOF
nanomaterials with various compositions.
EXPERIMENTAL SECTION
Chemicals and Materials. Cerium (III) nitrate hexahydrate (Ce(NO3)3·6H2O, Aladdin, 99.95%), Polyvinylpyrrolidone (PVP, Mw~58000, Aladdin), Sodium hydroxide (NaOH, Beijing Chemical Works, 96.0%), Chloroplatinic acid hexahydrate (H2PtCl6·6H2O, Aladdin, ≥37.0% Pt basis), Sodium polystyrenesulfonate (PSS, Mw~70000, Sigma-Aldrich), Zirconium(IV) chloridenitrate (ZrCl4, Aladdin, 98%), 2-aminoterephthalic acid (NH2-BDC, Alfa Aesar, 99%), Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Aladdin, 99%), Ferric chloride hexahydrate (FeCl3·6H2O, Aladdin, 99%), 2-Methylimidazole (Aladdin, 98%), 1,3,5-Trimesic acid (H3BTC, Aladdin, 98%), Furfural (Aladdin, ≥ 99.5% (GC)), Furfuryl alcohol (Aladdin, 98%) Tetrahydrofurfuryl alcohol (Aladdin, > 98% (GC)). The ethylene glycol, N,N-dimethylformamide (DMF), isopropyl alcohol, methanol and ethanol are all obtained from Beijing Chemical Works. All starting materials and solvents, excepted DMF, were used without further purification. And DMF was dried with 4A molecular sieve before used.
Synthesis of hollow Pt-CeO2 nanospheres. Pt-CeO2 was prepared by combining the two processes described in previous works with a small change.8,41 Typically, for 16 ACS Paragon Plus Environment
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120 nm Pt-CeO2 hollow nanoshpheres, 0.3333 g Ce(NO3)3·6H2O and 0.1333 g PVP were dissolved in 10 mL ethylene glycol, then 350 µL deionized water was added. After continuous stirring for 30 min, the clear solution was transferred into a Teflon-lined autoclave of 15 mL capacity then heated for 8 h at 160 oC. When the autoclave was cooled at room temperature, the suspension was transferred into a round-bottom flask. Then, 3 mmol of NaOH was dissolved in 10 mL of ethylene glycol and 1 mL of deionized water and added into above suspension and followed up with addition of 5 mL H2PtCl6 (0.0193 mol/L) aqueous solutions. The mixture was stirring for 30 min and then again heated to 160 oC and refluxed at this temperature for 1 h, which yielded a black suspension. After cooled to room temperature, the suspension was collected by centrifugation at 8000 rpm for 5 minutes and washed with deionized water and absolute ethanol sequentially. Finally, the products were dried in vacuum oven at 60 oC overnight.
The synthesis of 60 nm and 180 nm Pt-CeO2 was the same as the above process, except 200 µL and 500 µL deionized water were added in the first step, for 60 nm and 180 nm Pt-CeO2, respectively.
ModifidedPt-CeO2 with PSS. 10 mg as-prepared Pt-CeO2 was added to a mixture of 5 mL ethanol and 5 mL deionized water containing of 200 mg PSS and sonicated dispersion. Then, the mixture was magnetic stirred overnight at room temperature. The solid product (denoted as Pt-CeO2@PSS) was collected using centrifugation at
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10000 rpm for 5 minutes and washed once in sequence with distilled water and DMF (or methanol) and finally dispersed in a 5 mL DMF (or methanol) for further use.
Synthesis of Pt-CeO2@UIO-66-NH2. The UIO-66-NH2 was synthesized according to literature.42 For the synthesis of Pt-CeO2@UIO-66-NH2(1), 5.4 mg ZrCl4 was dissolved in 2.5 mL DMF, then added to 5 mL DMF of Pt-CeO2@PSS, and the mixed solution was further stirred at room temperature for 30 min. After that, 2.5 mL NH2-BDC (4.2 mg) DMF solution was added and continue to stir for another 30 min. Subsequently, the solution was placed in a microwave vessel and sealed. The reaction was then heated with microwave to 120 oC for 30 min. After cooling to room temperature, the solid products were collected by centrifugation at 8000 rpm for 5 minutes and washed with ethanol. Finally, the obtained Pt-CeO2@UIO-66-NH2(1) was dried in vacuum oven at 80 oC overnight or re-dispersed in 5 mL DMF for further use.
Pt-CeO2@UIO-66-NH2(2), Pt-CeO2@UIO-66-NH2(3) can be synthesized by repeating the synthesis process of Pt-CeO2@UIO-66-NH2(1).
Characterization. The X-ray diffraction patterns were collected on a Bruker D8 Focus powder X-ray diffraction with Cu-Kα radiation (λ = 0.15418 nm), with the operation voltage and current maintained at 40 kV and 40 mA, respectively. Scanning electron microscopy (SEM) image and energy dispersive X-ray spectroscopy (EDX) experiments were performed on a Hitachi S4800 field emission scanning electron microscope. Transmission electron microscopic (TEM) images, high-resolution TEM 18 ACS Paragon Plus Environment
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(HR-TEM) image and TEM-mapping images were carried out using FEI Tecnai G2 F20 field emission transmission electron microscope operating at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were taken on an ESCALAB-MKII 250 photoelectron spectrometer (VG Co.) with Al-Kα X-ray radiation as the X-ray source for excitation. Inductively coupled plasma optical emission spectrometer (ICP-OES) analyses and CHN elemental analyzer were performed to determine the elements content. The surface areas were calculated by the Brunauer–Emmett–Teller (BET) method. Zeta-potential of the nanoparticles were determined by Malvern Zeta-sizer Nano, the scattering angle was fixed at 90o and the measurement was carried out at 25 oC. Hydrogen temperature-programmed reductions (H2-TPR) experiments were carried out using a ChemStar TPx chemisorptionanalyzer (USA). 70 mg of catalyst was pretreated in Ar (30 mL/min) at 120 oC for 1 h. Followed by, the TPR was performed over the sample with the temperature increasing from 50 oC to 900 oC at a speed of 5 o/min, by using an H2/Ar flow (10%, 30 mL/min).
Catalytic hydrogenation of furfural. In a typical procedure, the catalysts were first dispersed in ethanol and stirred 8 h for three periods at room temperature and then dried under vacuum for 24 h at 120 oC to remove the solvent in the pore of UIO-66-NH2. Subsequently, the dried catalyst was activated at 120 oC under H2 atmosphere for 1 h. Then, a certain amount of activated catalyst (Pt: 0.5 mol%) was dispersed in 10 mL isopropyl alcohol solution, and 20 µL furfural were added into the above solution. Subsequently, the solution was transferred into a Teflon-lined stainless steel autoclave, the autoclave was purged with H2 for 5 times, and the final 19 ACS Paragon Plus Environment
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H2 pressure of the autoclave was set at 1 MPa. During the catalytic process, the reaction solution was magnetically stirred with the speed of 1000 rpm at 80 oC for the desired time. To study the stability, the catalyst was separated and dried, and then compensated the loss of the catalysts from the catalysts used in parallel experiment. And then it was reused without any reactivation process. The obtained reaction solution was centrifuged, and the analyses were performed on a Bruker 450-GC gas chromatograph equipped with a GSBP-1 NO WAX column (30 m × 0.32 mm × 0.50 µm) and flame ionization detector (FID) with nitrogen as the carrier gas. And the concentration of compounds was determined by external standard according to the peak areas. (The working curve was shown in Figure S16).
Conversion ሺ%ሻ=
Yield ሺ%ሻ=
initial moles of furfural - final moles of furfural ×100% initial moles of furfural
moles of product produced ×100% initial moles of furfural
moles of desired product formed
Selectivity ሺ%ሻ=initial moles of furfural - final moles of furfural×100%
AUTHOR INFORMATION
Corresponding Authors *E-mail for S.S.:
[email protected] *E-mail for H.Z.:
[email protected] ORCID Shuyan Song: http://orcid.org/0000-0002-7758-752X 20 ACS Paragon Plus Environment
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Rongchao Jin: http://orcid.org/0000-0002-2525-8345 Hongjie Zhang: http://orcid.org/0000-0001-5433-8611
Notes The authors declare no competing financial interests.
ASSOCIATED CONTENT
Supporting Information.
The Supporting Information is available free of charge on the ACS Publications website at DOI:10.1021/acscatal.0000000 Details of the additional synthesis methods, structural figures and catalytic data (PDF)
ACKNOWLEDGMENT The authors are grateful for the financial aid from the National Natural Science Foundation of China (21590794, 21771173 and 21521092), Youth Innovation Promotion Association of Chinese Academy of Sciences (2011176), the project development plan of science and technology of JilinProvince (20180101179JC) and CAS-CSIRO project (GJHZ1730).
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Table of Contents (TOC) Artwork A universal sodium polystyrenesulfonate-induced microwave-assisted synthesis method of Pt/CeO2@MOF core@shell nanoreactor was developed to realize the selective hydrogenation of furfural attributed to the suitable size of the UIO-66-NH2 channels.
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