Three-dimensional graphene aerogel supported Ir nanocomposite as a highly efficient catalyst for chemoselective cinnamaldehyde hydrogenation

Diamond & Related Materials 91 (2019) 272–282

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Three-dimensional graphene aerogel supported Ir nanocomposite as a highly efficient catalyst for chemoselective cinnamaldehyde hydrogenation Ling Li, Ge Gao, Jia Zheng, Xin Shi, Zhi Liu

T

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Faculty of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Cinnamaldehyde hydrogenation Graphene aerogel Heterogeneous catalysis Iridium Self-assembly

A three-dimensional graphene aerogel-supported Ir (Ir/GA) nanocomposite with interconnected porous framework and high dispersion of Ir nanoparticles is synthesized via a simple yet effective “co-reduction self-assembly” strategy, in which the chemical co-reduction of graphene oxide and H2IrCl6 occurs in the presence of glycerol and Ir is directly incorporated onto the reduced graphene oxide nanosheets. The Ir/GA possesses rich hierarchical porosity, a low amount of oxygen-containing functional groups, and a certain amount of Irδ- species, which notably improve the accessibility to both sides of the resulting reduced graphene oxide nanosheets, number of active Ir sites, electron transfer ability, and affinity of active Ir with C]O band. As a catalyst in chemoselective cinnamaldehyde (CMA) hydrogenation towards cinnamyl alcohol (CMO), the Ir/GA shows satisfactory CMA conversion of 85.8% and CMO selectivity of 83.2% with a high TOF activity of yielding CMO up to 17.5 h−1 at reaction time of 1 h. Moreover, the Ir/GA delivers good catalytic durability even after seven successive cycles without pronounced activity deterioration. This study highlights the exciting possibility to synthesize highly efficient graphene-supported precious metal catalysts with multiscale control by overall consideration in mass transportation, metal dispersion, metal-support interactions, and electronic density of active sites for chemoselective CMA hydrogenation.

1. Introduction As an important reaction in pharmaceutical and fragrance industries, chemoselective cinnamaldehyde (CMA) hydrogenation towards cinnamal alcohol (CMO) has been being the hot subject in heterogeneous catalysis over the past decades. Also, it is considered as a representative model in both correlating the catalytic behaviors with microstructures of heterogeneous catalysts and investigating the intrinsic interactions of active component with support [1]. Accompanying with the yield of CMO, the undesired products, hydrocinnamaldeyde (HCMA) and hydrocinnamyl alcohol (HCMO) are often unavoidably formed via parallel and consecutive pathways in this reaction due to the thermodynamical and kinetical priorities of C]C bond over C]O bond (Fig. 1). It is well established that noble metal with large d-band can give rise to strongly attractive interaction with the C]O bond, which is beneficial to bring about a high CMO selectivity (Ir > Pt > Ru > Rh > Pd) [2,3]. According to this principle, noble metal Ir seems to be the most efficient one for the CMA hydrogenation towards CMO. In comparison with the extensive investigation of Pt-based catalysts, there are few reports to date dealing with Ir-based catalysts for this reaction [4–11]. For instance, Breen

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et al. prepared a Ir/C (5 wt% Ir) catalyst by a homogeneous dispersion method with a low specific surface area of graphite (97 m2 g−1) as the support. The average particle size of Ir was as large as 35 nm. After 5 h reaction, the catalyst gave a medium CMA conversion of 60% and a high CMO selectivity of 97% [4]. Lin et al. synthesized a Ir/TiO2 (2 wt% Ir) catalyst by deposition-precipitation using Na2CO3 as the precipitating agent. After 6 h reaction, the catalyst only showed a low CMA conversion of 16.9% and a little CMO selectivity of 4.1% [5]. With the same strategy, Zhao et al. also fabricated a Ir/TiO2 (2 wt% Ir) catalyst using ammonia as the precipitating agent. After 1 h reaction, the catalyst exhibited a low CMA conversion of 10.9% and a medium CMO selectivity of 55.6% [6]. Rojas et al. reported the synthesis of Ir/SiO2 (1 wt% Ir) catalyst by traditional impregnation-H2 reduction with an average Ir particle size of 3.1 nm. When used as the CMA hydrogenation catalyst, it delivered a low CMA conversion of 21.6% and a medium 57% CMO selectivity after 3 h reaction [7]. Machado et al. utilized [Ir (μ-SC(CH3)3)(CO)2]2 to prepare a multiwalled carbon nanotubes-supported Ir (Ir/MWCNT) catalyst via wet impregnation with a post-reduction treatment. When the CMA conversion got to 50%, the CMO selectivity of the catalyst reached a somewhat better value of 68% [8]. Evidently, the potentials of CMA hydrogenation towards CMO in Ir-

Corresponding author. E-mail address: [email protected] (Z. Liu).

https://doi.org/10.1016/j.diamond.2018.12.018 Received 12 August 2018; Received in revised form 17 December 2018; Accepted 17 December 2018 Available online 18 December 2018 0925-9635/ © 2018 Elsevier B.V. All rights reserved.

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Fig. 1. Reaction pathways of CMA hydrogenation.

homogeneous for 30 min. Then, 5 mL of glycerol was added to the above mixture and stirred for another 30 min. Afterwards, the resulting solution was transferred to a stainless steel autoclave with a Teflon inner layer and put into a drying oven at 120 °C and kept standing for 2 h. In this procedure, H2IrCl6 was reduced to Ir by glycerol at the initial stage. Then, the reduced Ir conversely catalyzed glycerol to decompose and produce more active hydrogen in the present of water [21]. The asgenerated active hydrogen thus further hastened the reduction of IrCl62− and began to further reduce GO. With the increasing reaction time, H2IrCl6 was gradually reduced to Ir which was adhered on the surface of the partially reduced GO nanosheets. Accompanying with the continuous emergence of active hydrogen, the partially reduced GO sheets were deeply reduced to graphene nanosheets and consequently formed a Ir nanoparticles/3D graphene hydrogels monolith. The monolith was washed with acetone and di-distilled water for at least five times to remove residual impurities. Finally, the monolith was freeze-dried for 3 days to obtain the Ir/GA catalyst. In comparison with the Ir/GA synthesized by the CRSA strategy, commercial microporous carbon (AC) and self-synthesized mesoporous carbon (MC) were used as supports to prepare Ir/AC and Ir/MC catalysts by the conventional support impregnation-H2 reduction (see Supporting information for details). Ir loadings were determined by inductively coupled plasma spectrometer (ICP) on an IRIS Intrepid II XSP instrument. ICP analysis indicated that Ir loadings in the Ir/GA, Ir/MC and Ir/AC catalysts are 2.7, 3.0, and 2.9 wt%, respectively.

based catalysts are not fully released, and there is still large room in Ircatalyst preparation and optimization of active Ir to further boost the catalytic performance. On the other hand, the CMA hydrogenation is a structure-sensitive reaction, and the category and property of support play an important role on determining the activity and selectivity of catalyst [12]. Recently, He et al. fabricated a series of Ir-based catalysts (Ir/H-MoOx, Ir/MoO3, Ir/TiO2, Ir/ZrO2, Ir/AC, and Ir/SiO2) by conventional impregnation-H2 reduction, and found that the Ir/H-MoOx possessed the highest CMA conversion (> 99%) and CMO selectivity (93%) after 2 h reaction. They attributed the pronounced enhancements on both conversion and selectivity of the catalyst to the strong metalsupport interaction, during which the reported novel support (H-MoOx) could donate much more electrons to Ir and lead to striking increase of electronic density in Ir, and thereby giving rise to superior catalytic performance [13]. Based on the previous reports, the exploration of new methods to design Ir-based catalysts with overall consideration in mass transportation, metal dispersion, metal-support interactions, and electronic density of active sites is highly desired and attractive for obtaining efficient chemoselective CMA hydrogenation towards CMO. Recently, three-dimensional (3D) graphene aerogel (GA) has triggered increasing interests in catalysis owing to its hierarchically porous architectures and inheriting the excellent intrinsic merits of grapheme [14–18]. Compared to the restacked two-dimensional graphene sheets, the 3D interconnected porous GA would not only potentially provide more accessible sides with a larger specific surface area for higher active species dispersion, but it also would provide multi-dimensional pathways for the efficient mass transportations of the reactant/product molecules, thus resulting in a good catalytic performance. Moreover, its abundant surface functional groups, defects, and vacancies can offer favorable sites to anchor the active species and prevent them from detaching during reaction [19]. Inspired by these interests, as well as the further activity-pursuing of Ir-based catalysts in chemoselective CMA hydrogenation, herein we design a facile and environmentally friendly strategy to synthesize a 3D GA-supported Ir nanoparticles (Ir/ GA) catalyst under a mild condition via a direct co-reduction self-assembly (CRSA) of chloroiridic aicd (H2IrCl6) and graphene oxide (GO) with glycerol as initiating reducing agent. Compared with the conventional support dispersion or impregnation, the CRSA strategy is more atom-economic and can make a high Ir dispersion and extremely small particle size onto the resulting GA. When used as the CMA hydrogenation catalyst, the Ir/GA exhibits superior CMA conversion and CMO selectivity with higher TOF activity of yielding CMO than those of the single-sized mesoporous carbon and microporous carbon-supported Ir counterparts.

2.2. Characterization X-ray diffraction (XRD) patterns of the samples were collected with D/Max-βb diffractometer using a Cu Kα radiation source (λ] 0.15432 nm). The average crystallite sizes (D) of Ir were calculated with the Debye-Scherrer's equation:

D=

kλ β cos θ

Raman spectra of the samples were obtained from 1000 to 2000 cm−1 on a Lab Ram Infinity Raman spectrometer with a 514.5 nm laser. Textural properties of the samples were measured by N2-sorption at −196 °C with a Micromeritics ASAP 2010 apparatus. The specific surface areas (SBET), pore size distributions (PSD), mesopore volumes (Vmeso), total pore volumes (VP), and micropore volumes (Vmic) of samples were determined by employing the Brunauer–Emmett–Teller equation, Barrett–Joyner–Halenda method with adsorption branches, and t-plot method, respectively. The microstructures, morphologies, particle sizes, and Ir dispersions of the samples were observed by using a JSM 6360-LV scanning electron microscope (SEM) and a JEOL 2000EX transmission electron microscope (TEM), respectively. Ir particle size distributions were obtained from measuring 200 randomly selected particles with a special counting-software. X-ray photoelectron spectra (XPS) of the samples were conducted on PHI Quantum 2000 equipped with an Al Kα radiation source. All binding energies (BE) were calibrated with graphitic carbon C1s peak at BE of 284.5 eV as a reference. CO chemisorption data of the samples were preformed on a Micromeritics AutoChem II 2920 instrument. Before measurement, the as-synthesized sample (0.1 g) was purged with pure Ar at 200 °C for

2. Experimental 2.1. Synthesis of Ir/GA GO solution was synthesized from commercial graphite with a modified Hummers method as introduced in the Supplementary Material [20]. The Ir/GA catalyst was synthesized via the CRSA strategy with glycerol as reducing agent. Briefly, 30 mg of GO, 30 mL of water, and 6.5 mg H2IrCl6 solution (10 wt%) were mixed and stirred to be 273

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30 min, then cooled to room temperature. Then, the CO was injected from a calibrated on-line sampling valve with a pulse mode. CO adsorption was assumed to be completed until the thermal conductivity detector signals being constant. A CO/Ir stoichiometry of 1 was used to calculate Ir dispersion [22]. 2.3. Catalytic measurements The activities of catalysts in the CMA hydrogenation were measured in a 100 mL custom-designed stainless steel reactor with a Teflon inner layer. Typically, 0.1 g catalyst was first dispersed in 50.0 mL isopropanol for about 5 min, and then 2 mL CMA was added under stirring. Preliminary measurements in different reaction solvents (methanol, ethanol, acetone, isopropanol, toluene, ethyl acetate, cyclohexane, and THF) showed that the Ir/GA catalyst presented the optimal CMA conversion and COM selectivity in isopropanol (Table S1). After sealing, H2-purging, and pressurizing, the reactor was heated to the desired temperature with a stirring speed of 700 rmp. The reaction was monitored by taking around 1.0 mL sample from the reaction mixture periodically to determine the conversion and selectivity. A gas chromatography (Agilent 6890) equipped with a flame ionization detector and a 0.25 mm × 30 m FFAP capillary column was used to analyze the reaction products. The turnover frequencies (TOF), representing the conventional calculation of turnover frequency based on the metal dispersion of a catalyst, was calculated as the following equation with some modification [23]:

TOF =

Fig. 3. Raman spectra of graphite, GO, and Ir/GA.

reduction of both H2IrCl6 and GO, the position of the (0 0 2) reflection transfers to 2θ = ~25° for the Ir/GA with the emergence of a weak peak, indicating the successful conversion of the initial GO and the presence of few packed nanosheets within the Ir/GA [24,25]. In addition, three broadening diffraction peaks located at 2θ = 40.6°, 47.3° and 69.1° are observed, which can be assigned to the (111), (200), and (220) planes of the face-centered cubic Ir crystal (JCPDS No. 06–0598), respectively. In cases of the Ir/MC and Ir/AC, there are two typical amorphous carbon peaks located at ~23° and ~42°, and the positions of Ir diffraction peaks are identical to those of the Ir/GA. But the diffraction peaks become relatively strong and sharp. Using the Scherrer's equation and the (2 2 0) peaks, the average crystallite sizes of Ir on GA, MC, and AC are calculated to be 2.4, 5.4, and 7.7 nm, respectively. These results indicate that both GO and IrCl62− are reduced effectively when glycerol is used as reducing agent, and highly-dispersed Ir nanoparticles form on the surface of resulting GA. Fig. 3 records the Raman spectra of raw graphite, the as-synthesized GO, and Ir/GA catalyst. For GO, two obvious peaks are observed at ~1357 cm−1 (D band) and ~1580 cm−1 (G band), assignable to the lattice defect-induced disorder features and first-order scattering of the E2g mode for sp2 carbon lattice, respectively. In Ir/GA, the two bands shift towards a lower frequency (D band at ~1350 cm−1and G band at ~1578 cm−1), which is ascribed to the continuous reduction of GO during the form of Ir nanoparticles, suggesting the strong coupling between Ir nanoparticles and graphene in the hybrid architecture [26]. Compared with the strong D bands of GO and Ir/GA, the ordered graphite shows a very weak D band suggesting its high graphitization degree. The intensity ratios of the D and G bands (ID/IG) for GO and Ir/ GA are 0.94 and 1.05, respectively. The increase of ID/IG in Ir/GA is attributable to the decrease in the average size of the sp2 domains during the formation of Ir nanoparticles on the formed graphene and the successive reduction of GO [27], which well verify that glycerol used in our case is effective to reduce IrCl62− and subsequently produce active hydrogen to further reduce GO, thus resulting in the restoration of sp2 network with small and isolated domains of aromatics within the sheets. Fig. 4 shows the N2 adsorption–desorption isotherm as well as the corresponding PSDs of the as-synthesized Ir-based catalysts. It can be seen in Fig. 4a that the Ir/GA exhibits a representative type-II isotherm with combination forms of a distinct H2 type hysteresis loop within the medium P/P0 of 0.2–0.8 for mesoporosity and a well-defined H3 hysteresis loop at the P/P0 > 0.95 with rapid rise and fall of adsorption and desorption branches as well as lack of a clear adsorption plateau at P/P0 ≈ 1.0 for macroporosity [28–30]. In Fig. 4b, its PSD further confirms the existence of mesopores and macropores, which are possibly derived from the interstices, gaps, slits or pores of the randomly piled reduced graphene oxide (RGO) nanosheets. In Fig. 4c, the Ir/MC shows a typical type-IV isotherm with a well-defined H1-type hysteresis loop

CMOyielded t (mcat x / M ) D

where CMOyielded is the molar amount of obtained CMO calculated with CMA conversion and CMO selectivity, t is the reaction time (h), mcat is the amount of catalyst (g), x is the Ir content in the catalyst (wt%), M is the molar weight of Ir (192.2 g mol−1), and D is the Ir dispersion (%) calculated from H2 chemisorption data or average particle size of Ir. 3. Results and discussion 3.1. Structural features of catalysts Fig. 2 compares the XRD patterns of the as-synthesized GO and Irbased catalysts. For GO, a sharp and strong diffraction peak, indicative of the (002) reflection, can be clearly observed at 2θ]9.8°. The interlayer distance of GO calculated from the (0 0 2) reflection is 0.906 nm with the Bragg's Law (2dsinθ]nλ), which is much larger than that of graphite (0.34 nm) implying the formation of hydroxyl, epoxy, and carboxyl groups on both sides and edges of GO sheets.24 After the co-

Fig. 2. XRD patterns of GO, Ir/GA, Ir/MC, and Ir/AC. 274

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Fig. 4. N2-sorption isotherms and corresponding pore size distributions of the Ir/GA (a and b), Ir/MC (c and d), and Pt/AC (e and f) catalysts.

656 m2 g−1 and a Vp of 1.47 cm3 g−1. Its Vmeso and Vmic are 1.36 and 0.04 cm3 g−1, respectively, further indicating its hierarchically porous architecture. In contrast with the Ir/GA, the Ir/AC has a higher SBET of ~1255 m2 g−1 but a smaller Vp of 1.24 cm3 g−1, which are primarily originated from the micropore contribution with a Vmic of 1.23 cm3 g−1 and an almost negligible Vmeso. The Ir/MC possesses typical textural properties of mesoporous materials with a SBET of ~558 m2 g−1 and a Vp of 0.39 cm3 g−1. Its Vmeso and Vmic are 0.38 cm3 g−1 and 0.01 cm3 g−1, respectively, further manifesting its single mesoporosity. Fig. 5a depicts the representative SEM image of the Ir/GA at a lowmagnification. It is clearly seen that the Ir/GA displays a continuous 3D hierarchically interconnected porous framework with randomly-oriented wrinkled RGO nanosheets. At a high-magnification, the RGO nanosheets with winding fold surface and interpenetrating structure is highly visible through the macropores (Fig. 5b), further reflecting the enriched porosity of the Ir/GA. Such result should be attributed to the CRSA strategy used in our case, in which the liquid-phase chemical co-

Table 1 Textural properties of the Ir/GA, Ir/MC, and Ir/AC catalysts. Catalyst

SBET (m2 g−1)

Vmeso (cm3 g−1)

Vmic (cm3 g−1)

Vp (cm3 g−1)

Ir content (wt%)

Ir/AC Ir/MC Ir/GA

1255 558 656

– 0.38 1.36

1.23 0.01 0.04

1.24 0.39 1.47

2.9 3.0 2.7

and a sharp capillary condensation step at a P/P0 = 0.4–0.8, indicative of mesoporous materials with cylindrical channels [31]. The PSD of the Ir/MC is narrowly located between 2 nm and 12 nm with a peak centered at around 5.3 nm (Fig. 4d). In Fig. 4e, the Ir/AC presents a typical type-I isotherm with a high N2-adsorption volume at initial P/P0. The PSD of the Ir/AC is mainly ranged from 0.4 nm to 2 nm, reflecting its predominantly microporous structure (Fig. 4f). Table 1 summarizes the detailed textual properties of the three catalysts. The Ir/GA has a SBET of 275

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Fig. 5. Representative SEM images of the Ir/GA at a low-magnification (a) high-magnification (b). Typical TEM images of the Ir/GA catalyst at a low-magnification (c) and a high-magnification (d); high-resolution TEM image of the Ir/GA catalyst (e); particle size distribution of the Ir/GA catalyst (f).

ranged from 0.4 nm to 2.0 nm. Both reflected the AC was predominantly microporous structure. When the AC was impregnated with 5 wt% H2IrCl6 solution, the low porosity of AC seriously impeded the solution dispersion into the inner pores. After the subsequent H2thermal reduction reaction, the large Ir particles were therefore formed. By contrast, the MC can afford a richer porosity with a better solution dispersion into its inner pores, and the resulting Ir/MC thus exhibited a smaller Ir particle sizes. These observations are in good line with the results obtained from XRD. On the other hand, with the same metal loading content, it is commonly recognized that the small size of metal nanoparticles with high dispersion on a support will afford a high amount of active sites and give rise to a large active surface area for catalytic reaction, which is conductive to increasing the activity of a catalyst. As shown Fig. 5f, S1b, and S1d, the Ir/GA (2.7 wt%) has both smaller Ir nanoparticles and higher dispersion than those of the Ir/MC (3.0 wt%) and Ir/AC (2.9 wt%) even it has a lower Ir loading content. These indicate that the CRSA strategy used in our case is more effective to prevent the aggregation of Ir particles than the conventional support impregnation-H2 reduction method, and can make RGO to be an efficient carrier to support high dispersion, uniform, and small-sized metal nanoparticles under glycerol reduction. XPS is an effective technique to study the elemental composition, heterocarbon components, and metal valence information of the active species in catalysts. The “heterocarbon” in Table 2 means a part of carbon on the sample surface associating with oxygen to form different oxygen-containing functional groups, such as CeO, C]O, and OeC]O in our case. The heterocarbon plus pure carbon functional groups (CeC or/and C]C) equals 1. Fig. 6 presents the high-resolution XPS C1s core level survey of the as-synthesized GO and Ir-based catalysts. The C1s spectrum of GO is typically asymmetric and can be deconvoluted into four individual component peaks at 284.7, 286.8, 287.8 and 288.9 eV (Fig. 6a), which are assigned to the sp2-hybridized C (CeC), the C in

reduction of GO and H2IrCl6 occurs in the presence of glycerol (initiating reducing agent) without stirring. When the hydrophilic GO is gradually reduced to hydrophobic RGO, the increasing hydrophobicity of the liquid-phase leads to the aggregation of the formed RGO nanosheets. At the same time, the water is also further expelled out. As a result, the formed RGO with high hydrophobicity and conjugated π–π stacking interactions results in the formation of 3D architecture of the Ir/GA. Fig. 5c illustrates the typical TEM image of the Ir/GA at a lowmagnification. The thin and transparent RGO nanosheets with crumpled silk veil waves and overlap multilayers are well observed, confirming the 3D hierarchically interconnected porous framework with interpenetrating structure of the Ir/GA. The partial magnified image in Fig. 5d reveals that Ir nanoparticles are uniformly mono-dispersed on the surfaces of RGO nanosheets without any aggregation into large clusters, as seen by the tiny, spherical, and black spots. The closer inspection of the high-resolution TEM image in Fig. 5e shows that the interplanar spacing between two adjacent lattice fringes of the nanparticles is ~0.226 nm, which is quite consistent with the (1 1 1) crystal plane of face-centered cubic Ir [32]. From the corresponding size distribution histogram, the average size of the Ir nanoparticles is estimated to be ~2.28 nm (Fig. 5f). Fig. S1a shows TEM image of the Ir/MC. Most of Ir nanoparticles disperse unevenly outside the tunnels of MC, and the detectable Ir particle sizes range widely from 2.0–11.0 nm with an average diameter of 5.78 nm (Fig. S1b). This is likely due to the conventional support impregnation with the as-synthesized MC, which leads to the weak confinement effect of mesopores. On the surface of AC, Ir disperses randomly and partial nanoparticles seem to aggregate to larger clusters (Fig. S1c). The detectable Ir particle sizes on AC are in the range of 5.9–10.0 nm with an average diameter of 7.25 nm (Fig. S1d). Preliminary N2-sorption measurements showed that the AC presented a typical type-I isotherm with a high N2-adsorption volume at initial P/P0. The corresponding pore size distribution was mainly 276

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Table 2 Distribution of functional groups derived from the XPS data. Sample

Relative atomic percentage (%) CeC

GO Ir/AC Ir/MC Ir/GA

39.7 60.8 63.5 80.2

CeO 41.8 24.6 22.6 7.2

Heterocarbon component (%) C]O 11.4 4.2 6.5 8.6

OeC]O

Relative atomic percentage (%) Metallic Ir (0)

7.1 10.4 7.4 4.0

60.3 39.2 36.5 19.8

– 70.5 75.3 71.1

Irδ+ or Irδ– 29.5 (Irδ+) 24.7 (Irδ+) 28.9 (Irδ-)

and 66.5 eV arises from the Ir4+ in IrO2 [35,36]. The binding energies of Ir in the two catalysts with their relative atomic percentage are also listed in Table 2. For the Ir/AC, 70.5% of Ir is present as metallic state and the rest 29.5% is in oxidized state (Irδ+). By contrast, the deconvolution results of Ir/MC show that 75.3% of Ir exists as metallic state and the rest 24.7% exists as Irδ+. The higher content of metallic Ir in the Ir/MC is possibly ascribed to the larger pore diameter of MC which is conductive to the incorporation, dispersion, and further reduction of IrCl62− to Ir (0). While for the Ir/AC, the abundant microporous texture of the AC seriously limits the dispersion of IrCl62− in the pores, and thus leading to a low degree of reduction. Intriguingly for the Ir/GA (Fig. 7c), in addition to the Ir(0) doublet at 60.8 eV (Ir 4f7/2) and 63.8 eV (Ir 4f5/2), there is a red-shifted doublet (olive lines) at 60.3 and 63.3 eV corresponding to the negatively charged Irδ- on the RGO surface. According to the CRSA strategy in our case, the Ir source (H2IrCl6) is firstly reduced to Ir by glycerol, and then the reduced Ir conversely catalyzes glycerol to decompose and produce active hydrogen in the present of water. The as-generated H atoms can adsorb with the oxygen species on the surface of the GO sheets and react with them, during which the increasing H atoms become protonic and electron transport from H 1 s orbital to the O 2p orbital occurs simultaneously.

epoxy and hydroxy groups (CeO), the C in carboxyl groups (C]O), and the C in carboxylate or ester groups (OeC]O), respectively [33]. Compared with the lower relative atomic percentages of the C]O (11.4%) and OeC]O (7.1%) functional groups, the CeO functional group is predominant (41.8%) in GO, as shown in Table 2. After the synthesis of Ir/GA by the CRSA strategy (Fig. 6b), the content of the CeO functional group remarkably lessens from 41.8% to 7.2% with slight content decreases of the C]O and OeC]O functional groups. This suggests that GO is mainly reduced by glycerol through the removal of most epoxy and hydroxy groups with the formation of a mixture of reduced and non-reduced products, which is quite coincident with the previously experimental discovery [34]. For the Ir/MC (Fig. 6c) and Ir/AC (Fig. 6d), most of C emerge in the form of sp2hybridized structure (CeC), and the contents of heterocarbon are much lower than that of GO but still higher than that of Ir/GA (Table 2). Fig. 7 displays the XPS spectra of Ir 4f in the as-synthesized Ir-based catalysts. In the Ir/MC and Ir/AC (Fig. 7a and b), the Ir 4f region can be deconvoluted into three pairs of doublets. The most intense doublet (blue lines) at 60.8 eV (Ir 4f7/2) and 63.8 eV (Ir 4f5/2) is assigned to metallic Ir(0), the next doublet (pink lines) at 61.8 and 65.6 eV is attributed to the Ir2+ in IrO, and the third doublet (green lines) at 63.7

Fig. 6. XPS spectra of C1s of the as-synthesized GO (a), Ir/GA (b), Ir/MC (c), and It/AC (d) catalysts. 277

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Fig. 7. XPS spectra of Ir 4f of the as-synthesized Ir/MC (a), Ir/AC (b), and Ir/GA (c) catalysts.

the successful synthesis of Ir/GA catalyst, we can investigate the effect of catalyst structures on the reactivity in the selective hydrogenation of CMA. For comparison, the identical catalytic measurements are also performed on the Ir/MC and Ir/AC catalysts, respectively. For the three samples, the main hydrogenation products of CMA, CMO, HCMA and HCMO are totally found. The products from condensation of isopropanol and CMA are also found but with negligible amount. In addition, products from decarbonylation or cracking reactions are not detected. Table 3 lists the detailed hydrogenation results of the as-synthesized Ir-based catalysts. At 1 h, the Ir/GA exhibits a high CMA conversion of 85.8%, which is much higher than those of the Ir/MC (47.2%) and Ir/ AC (39.6%) counterparts. Furthermore, the Ir/GA possesses the highest CMO selectivity (83.2%) amongst the three catalysts, whereas the selectivities towards HCMA and HCMO are as low as 8.5% and 6.6%, respectively. By contrast, the Ir/AC has a lowest CMO selectivity of 25.8% but its HCMO selectivity is the highest (52.3%). This is stemmed presumably from the abundant micropores in the AC (Table 1), which enormously inhibit the effective diffusion of intermediate CMO, and resulting in the successive hydrogenation of CMO to HCMO. Owing to the typical mesoporous structure, the Ir/MC can provide relatively smoother paths for mass transportation, and thereby presenting a slightly better CMO selectivity of 51.5%. According to these data, we calculate the TOFs of the three catalysts in terms of the amount of CMO yield at reaction time of 1 h. As shown in Table 3, TOF of the Ir/GA is 17.5 h−1, 1.7 and 3.1 times of the Ir/MC (10.5 h−1) and Ir/AC (5.73 h−1), respectively. Evidently, the Ir/GA catalyst strikingly transcends the Ir/MC and Ir/AC catalysts in the selective hydrogenation of CMA towards CMO. In addition, we further compare the CMA conversion, CMO selectivity, and TOF over the Ir/GA with those over the

Subsequently, the terminal oxygen atoms on the surface of the GO sheets transfer charges, thereby causing the partial reduction of GO and producing delocalized electrons at the surface of the partially reduced GO. As a result, the delocalized electrons would inevitably interact with the part of reduced Ir and lead to the formation of the Irδ- species. In addition to the XPS measurements, we perform supplementary competitive hydrogenation experiments of toluene (T) and benzene (B) to further reveal the electron transport of the catalysts (see Supplementary Material for details). Since the electron donation is easier for toluene compared to benzene, a larger KT/B value suggests a lower electron density of Ir nanoparticle with weaker electron transport ability [37]. As shown in Fig. S2, the lower KT/B value (3.28) of the Ir/GA when compared to those of the Ir/MC (3.53) and Ir/AC (3.66) implies that much more amount of electrons transport occurred in the Ir/GA. The increased electron density of Ir nanoparticles attenuates the binding energy, and favors the hydrogenation of C]O [1]. This is well coincident with the XPS results. Based on these facts, the enhancement on electronic density of Ir is advantageous both to the d-electron feedback into polar C]O band and the repulsive four-electron interactions with C]C bond [1,38,39], we can therefore envisage that the Ir/GA with a certain amount of Irδ- species will be potentially favorable to the chemoselective hydrogenation of CMA.

3.2. Catalytic hydrogenation of CMA Over the past years, the factors that influence the CMO selectivity, including active metal category, metal particle size, support nature, promoters adjunction, and so forth, have been investigated considerably [12], however, there are scarce reports concerning the effect of catalyst structures on the catalytic performances. In this work, with 278

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Table 3 Results of CMA hydrogenation over the Ir/GA, Ir/MC, and Ir/AC catalysts (Reaction conditions: 0.1 g of catalyst, 2 mL of CMA in 50 mL of isopropanol, 2 MPa of H2 pressure, 70 °C, and 1 h reaction time.) and the comparison of catalytic performance with the previously-reported Ir-based catalysts. Catalyst

Ir/GA Ir/MC Ir/AC Ir/SiO2 Ir/H-MoOx Ir/TiO2 Ir/TiO2 Ir/graphite Ir/Mg3Al0.75Fe0.25 Ir/FeOx/SiO2

Ir content (wt%)

2.7 3.0 2.9 1.0 3.4 2.0 2.0 5.0 3.0 1.0

CMA conversion (%) at 1 h

85.8 47.2 39.6 5 (0.5 h) 100 (2 h) 10.9 (1 h) 16.9 (6 h) 7.9 (1 h) 59.8 (1 h) 5 (10 min)

Selectivities (%) CMO

HCMA

HCMO

83.2 51.5 25.8 57 93 55.6 4.1 9.6 68.2 83

8.5 13.8 17.6 40 Not provide 38.0 90.6 73.2 27.1 13

6.6 25.5 52.3 0 Not provide 6.4 5.3 6.8 4.7 4

TOF (h−1) at1h

References

17.5 10.5 5.73 0.39 (0.5 h) 4.78 (2 h) 8.84 (1 h) 0.664 (6 h) 1.47 (1 h) 1.61 (1 h) 2.78 (10 min)

This work This work This work [7] [13] [6] [5] [4] [11] [9]

Others 1.7 9.2 4.3 3 0 0 0 10.4 0 0

Bold data highlights the best catalytic performance amongst the counterparts in Table 3.

The cycle durability is a fundamentally important merit for precious metal catalysts from the economic point of view. By an ethanol washing-centrifugation-drying process, the Ir/GA can be easily recovered and reused at least seven cycles without distinct deterioration in both CMA conversion and CMO selectivity (Fig. 8b). All of the above results apparently indicate that the Ir/GA is an extremely active and stable catalyst for the chemoselective hydrogenation of CMA towards CMO. The excellent catalytic performance of the Ir/GA catalyst for the chemoselective hydrogenation of CMA towards CMO can be attributed to the following factors. Fig. 9 illustrates the proposed mechanism: (1) On the macroscopic scale, the continuous 3D hierarchically interconnected porous RGO nanosheet framework in the Ir/GA can not only act as scaffolds to support and stabilize Ir nanoparticles but also provide more accessible both sides of RGO as much as possible for the monodispersion of Ir nanoparticles. To verify this point, we perform CO chemisorption measurements on the as-synthesized Ir-based catalysts. As tabulated in Table 4, the measured CO uptake and calculated Ir dispersion (DIr) on the Ir/GA are 29.6 μmol gcat−1 and 46.1%, which are

Fig. 8. Time course of CMA hydrogenation over the Ir/GA catalyst (a). Reaction conditions: 0.1 g of catalyst, 2 mL of CMA in 50 mL of isopropanol, 2 MPa of H2 pressure, and 70 °C. Reusability of the Ir/GA catalyst (b). Reaction conditions: 0.1 g of catalyst, 2 mL of CMA in 50 mL of isopropanol, 2 MPa of H2 pressure, 70 °C, 2 h.

previously reported Ir-based catalysts. At the reaction time of 1 h, our results clearly outperform most of the reported counterparts with varied solvents, H2 pressures, or extended reaction time (Table 3). Fig. 8a demonstrates the time course of CMA hydrogenation over the Ir/GA catalyst. It is noted that the CMA conversion of the Ir/GA increases remarkably within the initial reaction time of 80 min. At 80 min, the Ir/ GA exhibits a high CMA conversion of 96.3%. After 80 min, the CMA conversion of the Ir/GA reaches nearly 100%. During the whole reaction proceeding (0–300 min), the desired product of CMO is predominant consistently with a stable selectivity of ~84%, which is far higher than those of the by-products of HCMA and HCMO (< 10%).

Fig. 9. The proposed mechanism for the excellent catalytic performance of chemoselective CMA hydrogenation towards CMO with the Ir/GA catalyst. 279

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theoretical reaction order of “1” for mass diffusion, and further confirming our conjecture. (2) On the nanoscale, the high dispersion of Ir nanoparticles on both sides of RGO in the 3D GA can afford a high utilization efficiency of Ir with rich active sites. In addition, as discussed in the XPS section (Figs. 6 and 7), the Ir/GA is synthesized by glycerol reduction, which can eliminate most of the oxygen-containing functional groups on the formed surfaces of RGO nanosheets (Table 2). It is commonly believed that removal of oxygen-containing functional groups from carbon surfaces could enhance the electron transfer from carbon to active metal sites, restrain the coordination of C]C to active metal sites, and significantly elevate both CMA conversion and CMO selectivity [40,41]. In Table 2 and Table S2, it is seen that the Ir/GA contains less oxygen-containing functional groups than the Ir/MC and Ir/AC. We can therefore infer that the Ir/GA would possess stronger electron transfer ability between active Ir and carbon scaffold than the other counterparts, which favors the back-bonding interactions with πC=O to a larger extent than πC=C, and thereby increasing the CMO selectivity [3,42]. In addition, He et al. proved that Irδ- species are beneficial for the selective hydrogenation of C=O moiety in α,β-unsaturated aldehydes, owing to the strengthened the four-electron repulsive interactions with non-polar of C]C and enhanced electrostatic interactions of polar Cδ+ = Oδ− [13]. In our case the Ir/GA synthesized via the CRSA strategy can produce a certain amount of Irδ- species on the RGO surface (Fig. 7c and Table 2), which can also make contribution to the high CMO selectivity in some degree. All of the macroscopic and microscopic features in positive of the Ir/GA should be responsible for the excellent performance of chemoselective CMA hydrogenation to CMO. Enlightened by the good results of the Ir/GA catalyst in selective CMA hydrogenation towards CMO, we further explore its universality

Table 4 CO chemisorption uptakes and the corresponding Ir dispersions. Sample

CO chemisorption (μmol/gcat)

DIra(%)

DTEMb (%)

Ir/AC Ir/MC Ir/GA

12.2 15.3 29.6

18.7 23.5 46.1

15.2 19.0 43.9

a DIr was calculated by CO chemisorption data with a stoichiometry of CO/ Ir = 1/1. b DTEM was estimated according to DTEM = 1.099/dTEM.

much higher than those on the Ir/MC (15.3 μmol gcat−1 and 23.5%) and Ir/AC (12.2 μmol gcat−1 and 18.7%), respectively. Based on the average size of Ir nanoparticles estimated by TEM, we also calculate the Ir dispersion (DTEM) from DTEM = 1.099/dTEM [8]. It is seen that the values of the DIr are very close to those of the DTEM. Since the stoichiometry of CO chemisorption Ir is 1/1, the Ir/GA has the most number of the active Ir sites amongst the three catalysts, confirming the high dispersion of Ir nanoparticles on the RGO surface of 3D GA. Moreover, the interstices, gaps, slits or pores of the RGO nanosheets can facilitate the diffusion of reactant molecules of CMA (0.90 nm × 0.51 nm × 0.08 nm) and product molecules of CMO (0.98 nm × 0.51 nm × 0.29 nm) and make significant influences on mass distribution and subsequent catalysis. To clarify this issue, we conduct the rate analyses with different initial CMA concentrations (Fig. S3 in the Supporting Information). As for a mass diffusion-controlled hydrogenation of CMA, the reaction order should be one. If the surface-reaction is rate-control, the apparent order should be zero. It is found in Fig. S3 that the slope of fitting linear equation, i.e., the reaction order, is determined to be about 0.83, apparently inclining to the

Table 5 Hydrogenation of other α,β-unsaturated aldehydes to unsaturated alcohol over the Ir/GA catalyst (Reaction conditions: 0.1 g of catalyst, 2 mL of α, β-unsaturated aldehydes, 70 °C, 2.0 MPa of H2 pressure, and 2 h reaction time). Entry

Substrate

Unsaturated alcohol

Conversion (%)

Selectivity(%)

1

100

96.2

2

100

94.5

3

100

94.0

4

94.7

90.7 geraniol + nerol

5

92.5

89.1

6

84.8

87.2

280

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for the other α,β-unsaturated aldehydes with various molecular configurations. As listed in Table 5, crotonaldehyde, leaf aldehyde, and trans-2-methyl-2-pentenal (entries 1–3) show complete conversion with extremely high selectivities (> 94%) towards crotyl alcohol, trans-2hexenol, and trans-2-methyl-2-pentenol, respectively. For citral, α-methylcinnamaldehyde, and α-hexylcinnamaldehyde with relatively complicated molecular configuration possessing longer carbon chain and larger substituents (entries 4–6), their conversion and selectivity towards unsaturated alcohols decrease possibly due to the increasing steric hindrance of the three substrates.

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4. Conclusions We have successfully synthesized a graphene-supported Ir nanoparticles catalyst under a mild condition via a direct co-reduction selfassembly of chloroiridic aicd and graphene oxide with glycerol as initiating reducing agent. Compared with the conventional support impregnation and H2-reduction method, the co-reduction self-assembly strategy enables the resulting Ir/GA to possess three-dimensionally hierarchical porous framework and high dispersion of Ir nanoparticles. When used as a catalyst for chemoselective CMA hydrogenation towards CMO, the Ir/GA exhibits a high TOF activity of yielding CMO up to 17.5 h−1 at reaction time of 1 h, 1.7 and 3.1 times the activity of MC and AC supported counterparts, respectively, and can be reused at least for seven cycles. More importantly, these findings afford a new path way to design graphene-supported precious metal catalysts with high dispersion and efficient mass transportations for broader catalytic applications. Acknowledgements We gratefully acknowledge the support to this research by the National Natural Science Foundation of China (No. U1662125, 21871124), the Postdoctoral Science Foundation of China (No. 2016M590418), the Natural Science Foundation of Liaoning Province (No. 201602457), and the Foundation of Liaoning Province Educational Committee (No. L201683672). We are also thankful for the project sponsored by the Key Laboratory of Functional Materials Physics and Chemistry (Jilin Normal University, No. 2015007), Ministry of Education of China. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.diamond.2018.12.018. References [1] P. Gallezot, D. Richard, Selective hydrogenation of α,β-unsaturated aldehydes, Catal. Rev. Sci. Eng. 40 (1998) 81–126, https://doi.org/10.1080/ 01614949808007106. [2] A. Giroir-Fendler, D. Richard, P. Gazellot, Selectivity in cinnamaldehyde hydrogenation of group-VIII metals supported on graphite and carbon, Stud. Surf. Sci. Catal. 41 (1998) 171–178, https://doi.org/10.1016/S0167-2991(09)60812-0. [3] F. Delbecq, P. Sautet, Competitive C=C and C=O adsorption of α-β-unsaturated aldehydes on Pt and Pd surfaces in relation with the selectivity of hydrogenation reactions: a theoretical approach, J. Catal. 152 (1995) 217–236, https://doi.org/10. 1006/jcat.1995.1077. [4] J.P. Breen, R. Burch, J. Gomez-Lopez, K. Griffin, M. Hayes, Steric effects in the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol using an Ir/C catalyst, Appl. Catal. A Gen. 268 (2004) 267–274, https://doi.org/10.1016/j. apcata.2004.04.002. [5] W.W. Lin, H.Y. Cheng, L.M. He, Y.C. Yu, F.Y. Zhao, High performance of Ir-promoted Ni/TiO2 catalyst toward the selective hydrogenation of cinnamaldehyde, J. Catal. 303 (2013) 110–116, https://doi.org/10.1016/j.jcat.2013.03.002. [6] J. Zhao, J. Ni, J.H. Xu, J.T. Xu, J. Cen, X.N. Li, Ir promotion of TiO2 supported Au catalysts for selective hydrogenation of cinnamaldehyde, Catal. Commun. 54 (2014) 72–76, https://doi.org/10.1016/j.catcom.2014.05.012. [7] H. Rojas, G. Díaz, J.J. Martínez, C. Castañeda, A. Gómez-Cortés, J. Arenas-Alatorre, Hydrogenation of α, β-unsaturated carbonyl compounds over Au and Ir supported on SiO2, J. Mol. Catal. A Chem. 363-364 (2012) 122–128, https://doi.org/10.1016/

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