Highly Porous Nitrogen- and Phosphorus-Codoped Graphene: An

Research Article pubs.acs.org/journal/ascecg

Highly Porous Nitrogen- and Phosphorus-Codoped Graphene: An Outstanding Support for Pd Catalysts to Oxidize 5‑Hydroxymethylfurfural into 2,5-Furandicarboxylic Acid Chunlin Chen,*,† Xingtao Li,†,‡ Lingchen Wang,† Ting Liang,†,‡ Lei Wang,*,† Yajie Zhang,† and Jian Zhang*,† †

Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, 1219 Zhongguan West Road, Ningbo 315201, China ‡ Ningbo University, 818 Fenghua Road, Ningbo 315211, China S Supporting Information *

ABSTRACT: The oxidation of 5-hydroxylmethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is a sustainable and promising route to bioderived aromatic polyesters. So far, the design of catalyst has been restricted by the unclear working mechanism, and thus most of the supported noble metal catalysts cannot provide a remarkable reaction rate under atmospheric pressures and room temperature. Here we report a new mechanistic insight into the structure−performance correlation of graphene-supported Pd catalysts. It is demonstrated that a new kind of highly porous nitrogen- and phosphorus-codoped graphene sheets (HPGSs) will enhance the fraction of surface Pd2+ species, which plays a determining role to reduce the activation energies of both HMF conversion and FDCA formation. Such a support effect may assist in developing highly active catalysts for FDCA synthesis under mild conditions. KEYWORDS: Graphene, Palladium, 5-Hydroxymethylfurfural, Oxidation, Activation energy

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INTRODUCTION

phase boundary area, which greatly depended on the properties of catalyst support. Graphene, a rising-star 2D carbon material in combination with extensive theoretical surface area and versatile surface chemistry, has been considered as an excellent support for metal catalysts. A high-surface-area graphene monolith can be produced via assembling fragments of graphene oxide or reduced graphene, but unfortunately, it is difficult to maintain such a highly porous structure after a continuously stirring and recycling procedure. Recently, direct synthesis of porous graphene aggregate5 has been demonstrated as an effective strategy to prevent restacking and thus to enhance the dispersion of metal particles. For instance, Antonietti, Li, and their co-workers6 reported a sacrificing-template technique by using glucose as the major carbon source to in situ fabricate monolayer-patched graphene sheets by thermal removal of the interlayer spacer like dicyandiamide or boric acid. The freestanding graphene sheets showed an attractive performance in metal-free oxidative coupling of acetonitrile,6,7 even though its surface area is still 8 by NaOH adjusting), and it was reduced by

RESULTS AND DISCUSSION Characterization of HPGS and Pd/HPGS. The morphological properties of HPGS in Figure 1 were detailed by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). As an assembly of curved layers with abundant interlayer spaces, the morphology of HPGS is significantly different from the reported graphene aerogels and 3D graphene aggregates of chemically reduced graphene or graphene oxides. Owning to the in situ growth and solid-phase route, the HPGS comprised continuous but wrinkled sheets with thin walls consisting of a few layers of graphene sheets (Figure 1A). Such an open and 11301

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calculation model in CO chemisorption,18 matching very well with the mean size of pores for tight immobilization. Morphological properties of Pd catalysts were detailed by HRTEM and scanning transmission electron microscopy (STEM) techniques. As shown in Figure 2A, Pd nanoparticles

Figure 1. Morphological and structural characteristics of HPGS. (A) SEM, (B) high-resolution TEM (HRTEM), (C) AFM measurement, and (D, E, F) dispersion in water.

interconnected holey characteristic was also confirmed in a lowmagnification TEM image (Figure S1), which may greatly favor the mass transfer during the catalytic processes. Thin areas of HPGS are presented in the high-resolution TEM image in Figure 1B. The skeleton walls consisted of several curved graphitic layers that were chemically and mechanically stable to prevent the collapse of hierarchical pores and layers restacking. AFM images of HPGS on an ultraflat mica in Figure 1C clearly showed an unfolded graphene flake with a lateral dimension ∼300 nm and thickness ∼1 nm. This indicated the presence of approximately single or double layers of graphene if we took account of the fact that the apparent thickness obtained by AFM includes the van der Waals forces between the graphene sheets and substrate, as well as the increased interlayer spacing.12−14 The X-ray photoelectron spectroscopy (XPS) results revealed that the surface is highly functionalized by heteroatoms. The relative contents of N, P, and O atoms were 5.07%, 0.99%, and 5.89%, respectively. Deconvolution of asymmetric N1s and P2p spectra identified that the incorporated N and P species were pyridinic N, quaternary N, and P−C components, centering at ∼398.3, ∼401.0, and ∼130.4 eV,15−17 respectively (Figure S2). The heteroatomic functionalities endowed HPGS with a superior hydrophilicity, as reflected by a low water contact angle of 7.09° and a good dispersing stability in water over 31 days (Figure 1D). Outstanding structural and surface properties inspired us to employ HPGS as noble-metal catalyst support. Pd catalysts were conveniently synthesized using formaldehyde reduction of palladium chloride in alkaline solution and then immobilized on HPGS. The Pd/HPGS sample featured a Brunauer−Emmett− Teller (BET) surface area (SA) up to 2218 m2 g−1 and a total pore volume as high as 4.64 cm3 g−1 with mean pore diameter at 8.3 nm (detailed in Figure S3). Such a mesoporous characteristic is absolutely beneficial to a highly efficient transport of the reactant molecules in the three-phase boundary area. The composition analysis revealed the Pd loading of 4.18 ± 0.05 wt % with a metal dispersion of 19.97%, and thus an average size of Pd particles of 5.6 nm based on a hemisphere

Figure 2. Morphological and structural characterization of Pd/HPGS. (A) TEM, (B) HRTEM, (C) STEM HAADF image, and (D) elemental maps.

were dispersed thoroughly in the 3D framework of porous HPGS, and the statistical analysis reported that the diameter of >67% Pd particles ranged from 3.0 to 7.0 nm to give an average size of 5.8 nm, in accordance with the result of CO chemisorption. The difference is attributed to the fact that tiny particles were somewhat shaded by the wrinkled graphene sheets and cannot be included in size statistics. The HRTEM image in Figure 2B clearly displayed a particle of ∼6 nm with two sets of lattices with the spacing distances of 0.194 and 0.224 nm, which were assigned to (200) and (111) facets of metallic Pd, respectively, being confirmed by the X-ray diffraction patterns in Figure S4. The fuzzy boundaries between Pd nanoparticles and HPGS support hampered the analysis of smaller particles, while the high-angle annular dark-field (HAADF) STEM with a high sensitivity to atomic number contrast provided more overviewed information with a high spatial resolution (Figure 2C). Elemental maps in Figure 2D showed that N and P atoms homogeneously dispersed throughout the carbon support, whereas O ones often stayed at the boundary of the graphene sheets.19 Pd particles were likely distributed in the areas with abundant heteroatoms, which may originate from the stabilization effect of N- or Ocontaining functionalities on metal nanoparticles.20−22 Catalytic Performance. The catalytic performance of Pd/ HPGS for selective oxidation of HMF into FDCA was evaluated in comparison with other Pd catalysts supported on rGO, CNTs, and AC (Figure S5). It is noted that Pd/rGO and Pd/CNT were synthesized via the same immobilization process by using rGO and CNT as catalyst supports, while Pd/AC was commercially purchased. Although the four catalysts appeared 11302

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ACS Sustainable Chemistry & Engineering Table 1. Summary of Structural Parameters of Supported Pd Catalystsa catalyst

SA (m2 g−1)

Pd/HPGS Pd/rGO Pd/CNT Pd/AC

2218 1704 225 1223

loading (wt %)

D (%)

Pd size (nm)

Pd2+ (%)

± ± ± ±

19.97 21.97 20.75 20.54

5.6 5.1 5.4 5.5

79.6 33.3 46.8 61.5

4.18 4.08 4.10 3.99

0.05 0.06 0.07 0.04

WF (eV) 4.68 4.85 4.59 4.53

± ± ± ±

0.05 0.00 0.02 0.00

a

Pd loading was derived from ICP analysis. Atomic Pd dispersion (D) and particle size were calculated from CO chemisorption. Surface Pd2+ fraction was determined by XPS deconvolution of Pd 3d5/2, as depicted in Figure S6. Work function (WF) was measured by ultraviolet photoemission spectroscopy (UPS).

Figure 3. Reaction data. (A) Catalytic performance of Pd/HPGS and the contrast catalysts. Conditions: HMF/Pd = 60 mol/mol, 1 equiv of NaOH, 50 °C, 500 sccm O2. (B) Catalytic recyclability of Pd/HPGS and Pd/AC. Reaction conditions: HMF/Pd = 120 mol/mol, 1 equiv of NaOH, 50 °C, 500 sccm O2.

Figure 4. Kinetic measurements and analysis. (A) TOF (h−1) and (B) Ea (kJ mol−1) obtained within 1 h reaction time. (C) Correlation between activation energies and surface Pd2+ fractions over the four catalysts. Conditions: HMF/Pd = 1440 mol/mol, 1 equiv of NaOH, 500 sccm O2. Reaction temperature: 50 °C for TOF measurements and 20−60 °C for Ea measurements, respectively.

HMF and FDCA (Figure S7). This phenomenon can be wellunderstood by the parallel-consecutive reaction mechanism in HMF oxidation into FDCA.23,24 In this reaction network, three partially oxidized intermediates play a reservoir role in the route to the final product. Because the active component was surface Pd species, the time course for the FDCA selectivity displayed a similar uprising tendency over the four catalysts (Figure S8), suggesting that the physicochemical properties of carbon supports may have no obvious effect on moderating the relative distribution of oxidation products. Nevertheless, Pd/ HPGS showed a great capacity to speed up the oxidation of HMF into intermediates and FDCA. Toward the potential application in an industrial scale, the recyclability of Pd/HPGS was examined with an inferior dosage of the catalyst. Pd/HPGS performed quite well, and the initial FDCA yield slightly decreased from 83.1% to 72.2% because we enhanced the HMF/Pd ratio from 60 to 120. As illustrated in Figure 3B, the high productivity of FDCA can be stably maintained for 20 consecutive cycles with a remarkable retention ratio of 96.7%

distinctly different in SA and pore volume, as listed in Table 1, they showed almost the same Pd loading and particle size. A large variation of Pd2+ ratio and work function of catalysts demonstrated that both chemical and electronic states of the active centers were significantly affected by the distinct surface and structural properties of carbon supports via the metal− support interaction. The temporal evolution of HMF conversion and FDCA formation over the catalysts was carried out under the ambient pressure and temperature no higher than 60 °C. A relatively low amount of NaOH with 1 equiv of HMF was used. As presented in Figure 3A, the Pd/HPGS catalyst exhibited the highest HMF conversion and FDCA yield within 6 h. Temporal trends of HMF conversion and FDCA formation over the four catalysts were different. With the increasing reaction time, the increment of HMF conversion became slower while that of FDCA yield improved particularly over Pd/ HPGS. Reaction rate normalized by the mass of catalysts or the amount of Pd revealed more clearly the opponent trends of 11303

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support often acting as an electron reservoir.24 Hence, there is no doubt that the Pd component should play a vital role in the aerobic oxidation of HMF to FDCA. We further attempted to fabricate a quasi-single-atom Pd/HPGS catalyst using the atomic layer deposition method for investigation of the steric effect, as detailed in Figure S13. It is disappointing that the quasi-atomically dispersed catalyst with 0.046 wt % Pd loading exhibited much lower TOFs for HMF conversion and FDCA yield of 209 and 0.8 h−1, respectively, at 60 °C. It is wellaccepted that the catalytic performances are connected with size and facets of active metal catalyst.4,31 However, in the present study, the similar exposed facets with approximate Pd dispersion and particle size (as described in Figures 2B, S4, and S5) could not explain the wide difference in catalytic activity. As a powerful means to study surface chemical state, XPS analysis in Table 1 already showed that different carbon supports tended to induce a widely varied valence state of Pd component. Surface Pd2+ ratios on Pd/HPGS, Pd/rGO, Pd/ CNT, and Pd/AC were 79.6%, 33.3%, 46.8%, and 61.5%, respectively. Although the high valence state Pd2+ was considered less stabile than metallic Pd0, the transformation and equilibrium of Pd2+ and Pd0 coordinated the catalytic cycle in the coexistent system of oxidizing agent and reducing substrate,32 being supported by the excellent stability of Pd/ HPGS during the catalytic recycling experiments. The pivotal role of Pd2+ species was already proved to be active and stable in liquid oxidation of alcohol with O2.33 In this study, the Ea value of FDCA formation showed a linear dependency on the fraction of Pd2+ (PdII, %) on the surface, which can be fitted by the empirical equation Ea,FDCA = 80.69−0.23 × PdII (eq 1) as shown in Figure 4C. Considering the same synthesis route of Pd catalysts as well as the metal dispersion, we concluded that the unique properties of HPGS most probably afforded the extremely high fraction of Pd2+ in Pd/HPGS. On account of the only detectable intermediate 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) and the superior carbon balance up to 98.5 ± 1.9%, the possible mechanism for aerobic oxidation of HMF over the Pd catalysts is proposed in Scheme 2.34 In this mechanism, the surface Pd2+ species act as an electron acceptor to promote the dehydrogenation of the geminal diol intermediate and the oxidation of the alcohol side-chain to form the aldehyde intermediate with the assistance of hydroxide ions, while the metallic Pd0 sites are generally responsible for activation of adsorbed oxygen. Among these steps, the oxidation of hydroxyl group of HMFCA into aldehyde group of 5-formyl-2-furancarboxylic acid (FFCA) is considered to be the rate-determining step and can be efficiently accelerated by the formation of Pd alcoholate species via Pd-hydride shift with the aid of Pd2+ and OH−. Although the working mechanism should be deeply investigated, the synergic effect of doped nitrogen or phosphorus heteroatoms in support cannot be neglected. Considering the electron localization derived from the elemental doping with different electronegative atoms, we speculate that the doped nitrogen element has more effect on the improvement of the Pd2+ fraction as compared with phosphorus. Phosphorus participation in the HPGS synthesis process contributed to reduce the thickness of walls and create micropores or defects in skeletons. Therefore, the interaction between the carbon surface and metal particles could be greatly enhanced by introducing functional groups and creating defects. On the other hand, the doping of nitrogen and phosphorus into carbon lattice was also demonstrated to be an effective strategy to enhance the catalytic performance of metal-

over Pd/HPGS, in contrast to the retention of 54.7% over Pd/ AC. The HMF conversion slightly decayed from 99.7% to 95.2%, and FDCA yield reduced from 72.2% to 69.8% over Pd/ HPGS, while the corresponding conversion reduced from 95.9% to 90.9% and yield reduced from 25.4% to 13.9% over Pd/AC. Kinetic Investigation and Structure−Performance Correlation. The superior performance of Pd/HPGS under the mild conditions allowed us to investigate its intrinsic activity with the assistance of kinetic measurements. Several tests were carried out individually by varying catalyst dosage to achieve different liquid hourly space velocities (LHSVs) and rule out the mass transfer artifact. As depicted in Figure S9, the conversion rate of HMF over Pd/HPGS approached a stable value as the LHSV reached 500 molHMF mol−1Pd h−1, indicating that the mass transfer limitation was negligible. To further ascertain the kinetic details, we chose the conditions at the HMF/Pd ratio of 1440 for 1 h of reaction time to evaluate four catalysts at an appropriate enlarged temperate range from 20 to 60 °C. After being normalized with the dispersion ratio of Pd atoms, the turnover frequency (TOF) revealed the temperature-dependent activities in the order of Pd/HPGS > Pd/ CNT > Pd/AC > Pd/rGO (Figure 4A), being a little different from the result in former order with mass transfer barrier in Figure 3A. We note that all the kinetic data were acquired below 20% of HMF conversion, and the huge difference in reaction rates of HMF and FDCA can be attributed to the short reaction time that cannot provide a sufficient amount of more active intermediates to finally yield FDCA. Pd/HPGS displayed a superior catalytic activity to the other catalysts, leaving a space of 30% and 50% of HMF and FDCA rates, respectively. Such considerable intrinsic rates also surpassed those of most other reported precious Pt, Pd, Au, and Ru catalysts even under relatively lower oxygen pressure, temperature, and amount of bases (Table S1 and Figure S10). A direct comparison of kinetic parameters always provides us a deep insight into the origin of catalytic activity. The Arrhenius relationship of TOF values with the reaction temperatures gave the apparent activation energies (Ea, kJ mol−1). As illustrated in Figure 4B, the derived Ea values of HMF conversion were 32.5−44.8 kJ mol−1 while those of FDCA formation were much higher, i.e., 60.5−71.9 kJ mol−1. Among the four catalysts, Pd/ HPGS exhibited the lowest Ea values of both HMF conversion and FDCA formation. For HMF conversion, the value is much lower than the reported values of Ru/C (51 ± 3 kJ mol−1),25 VOx/TiO2 (64−77 kJ mol−1),26,27 and Pt/Al2O3 (37.2 kJ mol−1)28 catalysts but similar to that of Pt/C (29.0 kJ mol−1).29 For FDCA formation, Ea of Pd/HPGS is up to 12 kJ mol−1 lower than those of the other three Pd/C catalysts, and this fact can well-interpret its superior capacity to deeply oxidize into FDCA. For the Au−Pd alloy catalysts in the literature, surface oxygen-containing groups were emphasized to comprehend the different activities of carbon-supported catalysts.30 However, there is no clear correlation between the atomic fraction of surface functionalities and Ea, as presented in Figure S11 and Table S2. The reason is probably attributed to the diversity of absorbed substrates or intermediates and active oxygen species under basic and nearly neutral conditions. On the other hand, the activation energies showed no obvious correlation with the work function of catalysts as illustrated in Figure S12, suggesting that electron transfer between support and metal has no observable effect on catalytic behavior in spite of carbon 11304

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Scheme 2. Aerobic Oxidation of HMF into FDCA over Pd Catalysts

Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Chunlin Chen: 0000-0001-9204-450X Author Contributions

J.Z. proposed and supervised the overall project as well as revised the manuscript. C.C. carried out the design of HPGS, structural characterization and analysis, and the manuscript preparation. X.L. and L.C.W. performed the catalytic reaction and TEM characterization. T.L. synthesized HPGS. L.W. conducted the Pd-loading processes. Y.Z. supplied reactant and helped with the product analysis. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the technical assistance from Prof. Dr. Bingsen Zhang (Institute of Metal Research, CAS) for HRTEM characterization and Yan Liu and Guoxin Chen (Ningbo Institute of Materials Technology & Engineering, CAS) for element mapping analysis, as well as the financial support from National Natural Science Foundation of China (51422212 and 21403261), Chinese Academy of Sciences (QYZDB-SSW-JSC037 and ZDRW-CN-2016-1), Science Technology Department of Zhejiang Province (2015C31118), Zhejiang Provincial Natural Science Foundation of China (LR16B030001), International Science and Technology Cooperation Program of Ningbo City (2014D10004), Natural Science Foundation of Ningbo City (2014A610108), and K. C. Wong Education Foundation (rczx0800).

free nanocarbons. For instance, Peng and co-workers reported that the embedded nitrogen and phosphorus atoms in graphitic matrix can assist the cyclohexane aerobic oxidation35 and oxygen reduction reaction,9 respectively. Our previous work also showed a determining role of graphitic nitrogen component on the oxidative dehydrogenation of propane.8

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CONCLUSIONS In summary, we have successfully demonstrated that highly porous nitrogen- and phosphorus-codoped graphene sheets featuring free-standing, hydrophilic, heteroatoms incorporated and hierarchically porous characteristics acted as a highperformance support for Pd catalysts. For aerobic oxidation of HMF into FDCA, the Pd/HPGS catalyst showed the highest conversion and yield as compared to other carbon-supported Pd ones. The outstanding performance of Pd/HPGS originated from its minimum Ea in both HMF conversion and FDCA formation, while the structure−activity correlation revealed a key role of Pd2+ species with a high valence state. Our findings will provide a new method to synthesize a highly active carbonsupported Pd catalyst toward the energy-saving production of FDCA under the ambient conditions. A superior retention ratio of activity after 20 consecutive cycles may announce a great potential to large-scale synthesis of FDCA if people can deal well with the engineering problems in the near future.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02049. SEM and TEM images, XPS and XRD analyses, nitrogen physisorption isotherms and pore-size distribution, reaction and kinetic data, and comparison of catalytic performance and kinetic TOF of HMF oxidation into FDCA with references (PDF) 11305

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