A Graphene Oxide Supported Catalyst with Thermo- Responsive

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A Graphene Oxide Supported Catalyst with Thermo-Responsive Smart Surface for Selective Hydrogenation of Cinnamaldehyde Jie Zhu, Xuejie Ding, Dan Li, Mengdi Dou, Mohong Lu, Yongxin Li, and Faliang Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19594 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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ACS Applied Materials & Interfaces

A Graphene Oxide Supported Catalyst with ThermoResponsive

Smart

Surface

for

Selective

Hydrogenation of Cinnamaldehyde Jie Zhu, 1 Xuejie Ding, 1 Dan Li, 1 Mengdi Dou, 1 Mohong Lu, 1 Yongxin Li,*,1 and Faliang Luo ,2

*

Yongxin Li ([email protected]); Faliang Luo ([email protected])

1

Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Changzhou University, Changzhou 213164, China. 2 School of Chemistry & Chemical Engineering, Ningxia University, Yinchuan 750021, China. 1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract In this study, a graphene oxide (GO) based thermoresponsive smart catalytic material with a phase transition temperature at approximately 37 oC was developed by growing poly(N-isopropylacrylamide) (PNIPAM) on GO sheets (i.e. GO-PNIPAM). The composite was characterized by FT-IR, N2 adsorption, TGA, OEA, DSC and XPS. GO-PNIPAM supported Ru catalysts (i.e. Ru/GO-PNIPAM) were then prepared for cinnamaldehyde (CAL) hydrogenation. The influence of thermo-sensitive smart surface on the reaction was investigated. Results indicated that GO-PNIPAM exhibited the hydrophilic surface at 25 oC that resulted in the highly dispersed Ru nanoparticles on the composite. Afterwards the surface wettability of Ru catalyst was spontaneously changed to hydrophobicity at 70 oC that greatly improved CAL sorption on the catalyst in the reaction. The synergistic effect between Ru and GO-PNIPAM as well as the great adsorption ability to reactants on Ru/GO-PNIPAM jointly resulted in the enhancement of catalytic activity over it in comparison to that over GO supported Ru catalyst (Ru/GO). Meanwhile, the hydrophobic surface of Ru/GO-PNIPAM at a high temperature preferred C=O adsorption mode, yielding a higher cinnamyl alcohol (COL) selectivity than Ru/GO did. Keywords: wettability controllable surface; poly(N-isopropylacrylamide); graphene oxide; cinnamaldehyde hydrogenation

2


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Table of Contents graphic

3



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1 Introduction Surface chemistry is an important issue in developing supported catalyst for selective hydrogenation.1 It produces great effects on the reaction mainly in two aspects: active sites and adsorption performance to reactants on the catalyst.2−6 Specifically in selective hydrogenation of cinnamaldehyde (CAL) (Figure 1),7−9 for example, a hydrophilic surface of a support enhances the metal dispersion in catalyst preparation, using conventional aqueous wetness impregnation technique. On the other hand, a hydrophobic surface of as-prepared catalyst not only helps the fast adsorption of CAL molecules on it in the reaction, but also prefers the adsorption mode of carbonyl group on metal nanoparticles, contributing to the high cinnamyl alcohol (COL) selectivity. 6, 10, 11

The hydrogenation of carbonyl group to COL is often the research hotspot in the

reaction because carbonyl group has a higher thermodynamic stability than C=C bonds that hampers the production of COL.2, 3, 10, 12−15 It is thus evident that, controlling the hydrophilic surface of the support for catalyst preparation as well as the hydrophobic surface of as-prepared catalyst for the reaction will jointly result in a high catalytic performance in CAL hydrogenation. At present, some carbon nanomaterials (CNMs) such as graphene and carbon nanofibers (CNFs) are often employed as attractive catalyst supports in CAL hydrogenation due to their several significant characteristics such as resistance to acids and bases, electronic transfer property and minimal mass transfer limitations.2, 3, 6, 16 For achieving high catalytic performance, CNM and its supported catalyst traditionally undergoes the complicated surface treatments. Briefly, CNM is initially treated with strong oxidants (e.g. concentrated HNO3) to obtain a hydrophilic surface for enhancing metal dispersion on it. While after catalyst preparation, some previously introduced hydrophilic groups on CNM are often required to be eliminated further to regain the hydrophobic surface for improving its adsorption performance in the reaction.11, 17 In comparison, developing a smart surface with controllable wettability on CNM might provide a potential alternative. By chemical modification of some thermoresponsive polymers

such

as

poly(N-isopropylacrylamide)

(PNIPAM)

and

poly(N4


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vinylcaprolactam) (PVCL) on CNM, thermoresponsive smart surface could be constructed on it. PNIPAM, for example, has a lower critical solution temperature (LCST) at about 32 oC and a phase transition would happen if the temperature were below or beyond this point. Based on the scheme, PNIPAM grafted CNM displays hydrophilic surface at room temperature for catalyst preparation and afterwards automatically switches its surface wettability to the hydrophobic one at high temperature for the reaction. PNIPAM has extensive use in several applications such as cell adhesion ,18 sensors, 19

filtration membrane

20

and recently catalysis.21−25 Q. Wang et al.,24 for instance,

prepared recyclable palladium nanoparticles which were stabilized by PNIPAM (i.e. Pd/PNIPAM) for aqueous hydrodechlorination of 4-chlorophenol. This catalytic system realized homogenous reaction at a low temperature as well as heterogeneous recycling at a high one. Herein, graphene oxide (GO) was employed to develop smart catalytic material by directly growing PNIPAM on GO sheets (i.e. GO-PNIPAM) because GO is endowed with abundant epoxy (C-O) and hydroxyl (C-OH) groups that provide potential chemical modifications on GO sheets by introducing some moieties such as ionic liquids (ILs) and polymers.26−28 The corresponding thermoresponsive Ru catalyst (i.e. Ru/GO-PNIPAM) exhibited enhanced catalytic performance in CAL hydrogenation in comparison to GO supported Ru catalyst (i.e. Ru/GO). Apparently, this study constructed a competitive smart carbon nanomaterial for catalytic hydrogenation, which further broaden its application in relevant academic and industrial fields.

2 Experimental 2.1 PNIPAM grafting from GO surface (GO-PNIPAM composite) Before PNIPAM grafting, GO was prepared through a modified Hummers’ method

25, 29

and N-isopropylacrylamide (NIPAM, 98 %, Aladdin) was recrystallized

twice in hexane. After that, PNIPAM chain was grown on GO surface, using ammonium persulfate (APS) as the polymerization initiator. In brief, 4.5 g of purified 5



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NIPAM was firstly dissolved in 200 ml of deionized water and 2.0 g of GO was then dispersed in it. After the temperature elevated to 75 oC, 0.3 g of APS was immediately added to the mixture, kept stirring for about 40 min under an argon atmosphere. After polymerization completed, the synthesized GO-PNIPAM was purified by filtration and rinsing in sequence at room temperature to remove impurities. Finally, GO-PNIPAM was dried under vacuum at 60 oC overnight. 2.2 Ruthenium (Ru) catalysts supported on GO-PNIPAM Three GO-PNIPAM supported Ru catalysts were prepared at different temperatures by KBH4 reduction method. Ru loading over catalysts in this work kept constant at 4.0 wt. %. In a typical experiment, GO-PNIPAM (1.0 g) was dispersed in aqueous RuCl3 solution (Ru ≥ 37%, Sinopharm, China; 20 mM, 20.0 mL) at room temperature. 1 ml of aqueous KBH4 solution (20 mM) was then dropped to the mixture with vigorous stirring. Reduction proceeded for 1 h at the same temperature. After that, as-prepared Ru catalyst was purified by filtration and rinsing in sequence at room temperature. After drying under vacuum at 60 oC overnight, the catalyst was finally obtained and marked as Ru/GO-PNIPAM (L). Another catalyst prepared at 50 oC was named Ru/GO-PNIPAM (H). The third one prepared by depositing half of Ru nanoparticles at room temperature and the rest at 50 oC was denoted as Ru/GOPNIPAM (LH). In addition, GO supported Ru catalyst (i.e. Ru/GO) that prepared at room temperature served as the control. 2.3 Characterization of GO-PNIPAM and Ru/GO-PNIPAM Structural properties of GO-PNIPAM and Ru catalysts were fully characterized and the effect of support surface chemistry on catalyst preparation was investigated. Fourier transform infrared spectroscopy (FT-IR) were carried out on a Bruker Tensor 27. BET surface area of the materials were measured by a Micromeritics ASAP 2010 apparatus. Transmission electron microscopy (TEM) images was acquired by JEM2100. X-ray photoelectron spectroscopy (XPS) was operated on a Perkin-Elmer PHI 5000C. Thermogravimetric (TG) and organic elemental analysis (OEA) were tested using a TASDT Q600 instrument and a Thermo Scientific FLASH 2000 Analyzer, 6


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respectively. Differential scanning calorimeter (DSC) were conducted on a PerkinElmer DSC8000. 2.4 Catalytic Performance of Ru/GO-PNIPAM on selective hydrogenation of cinnamaldehyde The catalytic performance of Ru/GO-PNIPAM was estimated in CAL hydrogenation and the results were compared to that of Ru/GO. The tests were operated in a 100 ml autoclave at 70 oC and 2.0 MPa hydrogen pressure. The substrate cinnamaldehyde (A.R., Sinopharm, China) was firstly dissolved in isopropanol (IPA, A.R., Sinopharm, China), keeping CAL concentration at 5.0 wt. %. 0.2 g of the catalyst was then dispersed in 40 g of CAL solution. Afterwards, the autoclave was sealed, flushing with N2 flow adequately for hampering the reaction. As the temperature ascended to 70 oC, H2 was filled into the system to replace N2, maintaining the reaction pressure of 2.0 MPa. The reaction proceeded at a stirring speed of 1000 rpm for eliminating external diffusion limitation. Table S1 (Electronic Supplementary Information, ESI) summarized the reaction conditions in this study. A few reaction liquid were removed from the autoclave at different reaction times. They were further analyzed by a VARIAN CP3800 GC equipped with a HP-5 capillary column and FID. CAL conversion (X), the selectivity towards the product (Si, i= HCAL, COL, HCOL or PB) were calculated by Equation (1)-(2). 𝑋=

𝐶0𝐶𝐴𝐿 ― 𝐶𝐶𝐴𝐿 𝐶0𝐶𝐴𝐿

𝑆𝑖 = 𝐶0

𝐶𝐴𝐿

𝐶𝑖 ― 𝐶𝐶𝐴𝐿

× 100%

(1)

× 100%

(2)

where CCAL and Ci are the instant molar concentration of the reactant CAL and the product in the reaction; C0CAL is the initial molar concentration of CAL. The mass balance was close to or even better than 97%. The initial reaction rates in CAL hydrogenation over different Ru catalysts were further calculated using turnover frequencies (initial TOFs) by Equation (3). 𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑇𝑂𝐹 = 𝑘 × 𝑛0𝐶𝐴𝐿 × (𝑁𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑅𝑢) ―1

(3)

where k , n0CAL and Nsurface Ru represent reaction rate constant, initial mole of CAL and 7



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total moles of surface Ru on catalyst, respectively. Moreover, activation energy for CAL hydrogenation over a catalyst was determined using Arrhenius equation in Equation (4). 𝐸𝑎

𝑙𝑛𝑘 = 𝑙𝑛𝐴 ― 𝑅𝑇

(4)

where Ea is activation energy, T is reaction temperature, A and R are constants on behalf of frequency factor and ideal gas constant (8.314 J·mol-1·K-1), respectively. In addition, Ru/GO-PNIPAM (LH) was recycled across five runs. After each run, the catalyst was recycled by filtration and rinsing with ethanol and deionized water successively at room temperature for removing residuals. Finally, it was dried under vacuum at 60 oC overnight. The recycled Ru/GO-PNIPAM (LH) would be used in the next run.

3 Results and discussion 3.1 Synthesis of PNIPAM on GO sheet Initially, the prepared GO was confirmed by XRD and TEM. Figure S1 showed a typical XRD spectrum of GO with a strong characteristic peak at 10.5 o on behalf of GO (002). Its crystalline interplanar spacing was calculated to be 0.82 nm, similar to the reported sheet spacing of GO. It indicated the successful preparation of GO in combination with its TEM image (Figure S2). FT-IR was then measured for identifying several functional groups on GO and GO-PNIPAM (Figure 2). Clearly, pristine GO exhibited some typical groups including sp2 carbon ring (C-C), epoxy group (C-O) and hydroxyl group (C-OH) with their characteristic peaks at 1590, 1228 and 1060 cm-1 on its FT-IR spectrum, respectively. In comparison, after polymer synthesis, the resulting composite showed some distinctive groups from those of GO in its spectrum, including conjugated carbonyl (C=O, 1648 cm−1), Amide (-NH, 1533 cm−1), isopropyl (1375 and 1385 cm−1), methylene (-CH2, 2924 cm−1) and methyl (-CH3, 2973 cm−1) groups, which were characteristic in PNIPAM structure. More important, the appearance of ether bond (CO-C, 1128 cm−1) in the spectrum of GO-PNIPAM hinted that PNIPAM was very 8


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possibly grafted from GO via covalent combinations with ether linkage. FT-IR results provided the direct evidence for the successful synthesis of PNIPAM on GO. After that, TGA and OEA was employed to estimate the grafting amount of PNIPAM on GO. TG curve of GO (Figure 3 and Figure S3a in ESI) showed signiﬁcant mass loss before 300 oC. The one below 140 oC (approximately 18.5 % of the total) could be explained by water vaporization. Another one (approximately 29.5% of the total) was found in the temperature range of 140-260 oC with DTG peak at 202 oC, owing to the decomposition of some thermal unstable functional groups (e.g. -OH and -COOH) on GO sheets. In comparison, TG curve of pure PNIPAM (Figure 3 and Figure S3c in ESI) exhibited a dominant mass loss in the temperature range of 260-450 oC, different to that of GO. Obviously, it was completely decomposed in this temperature range. Hence, the amount of grafting PNIPAM on GO could be decided through calculating the mass loss in the temperature range of 260-450 oC. TG curve of GOPNIPAM (Figure 3 and Figure S3b in ESI) hinted two distinct mass losses in the range of 140-260 oC with DTG peak at 223 oC as well as 260-450 oC with DTG peak at 401 oC,

in accordance with those of GO and PNIPAM. Thus, it was reasonable to conclude

that PNIPAM was successfully grafted from GO surface and its grafting amount was approximately 60% of the total. Furthermore, independent to the TGA results, elemental analysis from OEA (Table S2 in ESI) confirmed the mass fraction of element C, N and H in GO material. Results indicated that GO-PNIPAM possessed a high mass fraction of element nitrogen (7.21%), hinting the introduction of PNIPAM on GO sheet. Based on the content of nitrogen element, the growing amount of polymer accounted for about 58.1 % of the total in the composite, in agreement with that on TG. XPS data acquired from Figure 4 and Table S3 (ESI) offered further clues to the composition of surface functional groups on GO and GO-PNIPAM composite. Clearly, C1s spectrum of GO (Figure 4a) was separated in five typical peaks at 288.8, 287.2, 286.7, 284.9 and 284.6 eV, representing carboxyl (COOH), epoxy (C-O), hydroxyl (COH), C-C (sp3) and C-C (sp2) groups, respectively. However, three new surface functional groups including amide (C-N, 285.7 eV), ether (C-O-C, 286.4 eV) and 9



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carbonyl (C=O, 288 eV) groups were further separated from C1s spectrum of GOPNIPAM (Figure 4c), in agreement with the element composition analysis from its O1s and N1s spectra (Figure S4 in ESI). Clearly, C-N and C=O groups were characteristic in PNIPAM structure. Also, GO-PNIPAM exhibited an increased amount of C-C (sp3) species (38.3 %) in comparison to that of GO (18.8 %) (Table S3). It was reasonably originated from the carbon backbone of the polymer. Hence, these evidences also suggested the successful synthesis of PNIPAM on GO. Furthermore, in contrast to C1s composition on GO, that on GO-PNIPAM exhibited a pronounced C-O-C species (9.5 %, Table S3). However, the proportion of C-O and C-OH on it greatly decreased to 7.7 % and 6.2 %, respectively. It indicated that a large amount of C-O and C-OH groups on GO participated in the grafting of PNIPAM, leading to the generation of C-O-C, in accordance with FT-IR results mentioned above. For further proving this opinion, hydroiodic acid (HI) was employed here to identify ethers due to its characteristic reaction to break ether bonds. The detailed experiment process was shown in ESI. FT-IR spectrum of GO-PNIPAM after HI treatment exhibited some weak peaks on behalf of typical groups on GO such as C-C (1590 cm-1) and C-OH (1060 cm-1), while some characteristic groups representing those on PNIPAM including CH3, CH2, C=O and NH2 groups disappeared (Figure S5). It hinted the separation of the polymer from GO surface. Noted that, the characteristic peak contributing to ether bond (about 1128 cm-1) was missing as well, hinting the break of ether bonds on the composite. In addition, TG curve of GO-PNIPAM after HI treatment (Figure S3d) showed a small weight loss between 260-450 oC (approximately 15 % of the total), far less than that before HI treatment (approximately 60 % of the total). Logically, most PNIPAM were removed from GO surface after HI treatment, in agreement to FT-IR analysis. Therefore, based on above characterizations, PNIPAM were successfully grafted from GO sheet by covalent combinations with ether linkage. The possible APS initiated polymerization mechanism of PNIPAM on GO was described in Scheme S1 (ESI). Specifically, APS was firstly thermally decomposed to sulfate radical (HO3SO·), which 10


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could be further converted to hydroxyl radicals (·OH) in aqueous medium. This process is widely recognized as Scheme S1a and S1b. ·OH, a labile and highly reactive radical, then attacks hydroxyl and epoxy groups on GO sheet, resulting in the formation of oxyradicals (C-O·) on it (Scheme S1c). After that, introduced NIPAM monomers are continuously polymerized to PNIPAM chains on GO surface (Scheme S1d). Thus, the composite GO-PNIPAM was obtained. Some reports supported this mechanism. 30−32 3.2 Thermoresponsive properties of GO-PNIPAM materials GO is highly dispersible in protic (e.g. water and isopropanol) and aprotic polar (e.g. tetrahydrofuran) solvents to form stable colloids due to its hydrophilic surface while agglomerate in aprotic apolar solvents (e.g. toluene and n-hexane).27, 33, 34 Figure S6a-S6f in ESI indicated that temperature took no effect on GO dispersion in solvents. However, after introducing PNIPAM on GO, the composite exhibited evident thermosensitivity on its dispersion (Figure S6g-S6l). For example, GO-PNIPAM was highly dispersible in aqueous solution at 25 oC (Figure S6g), while it agglomerated at 50 oC (Figure S6j). Apparently, the hydrophilic surface of the composite at low temperature changed to the hydrophobic one with the increase of temperature, resulting in its agglomeration in water at high temperature. Noted that the composite seems to be dispersible in IPA both at 25 oC and 50 oC, very possibly attributing to its partially nonpolar nature. It renders IPA a good solvent for GO-PNIPAM well dispersion in it at high temperature. Results indicated the successful construction of thermoresponsive smart surface on GO in this work. Furthermore, LCST of GO-PNIPAM was tested through DSC, as shown in Figure 5. Compared to GO, GO-PNIPAM showed a clear endothermic peak at 36.7 oC, hinting its LCST at this temperature. Additionally, the samilar LCST of Ru/GO-PNIPAM (36.9 oC)

to that of GO-PNIPAM indicated that the process of catalyst preparation in this

work had no impacts on PNIPAM stability. 3.3 Catalytic performance Several Ru/GO-PNIPAM catalysts were prepared at different temperatures, investigating the effects of thermoresponsive GO-PNIPAM on catalyst preparation and 11



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the following CAL hydrogenation. 3.3.1 Ru deposition on GO-PNIPAM Physicochemical properties of GO materials in Table 1 indicated that the synthesized PNIPAM on its sheet was generally conducive to the improvement of structural property on GO. However, the increased BET surface area of GO-PNIPAM uncertainly resulted in a higher metal particles dispersion on it than that on GO. From TEM images and distribution diagrams for Ru nanoparticles on catalysts (Figure 6), Ru/GO-PNIPAM(H), the catalyst prepared at a high temperature (50 oC, > LCST), for example, had the Ru particle size at 7.2 ± 2.5 nm (Figure 6c), larger than that of Ru/GO (4.9 ± 1.5 nm, Figure 6a ). It was explained by its hydrophobic surface against the dispersion of Ru nanoparticles. Hence, besides the surface area of a support, its surface property is another important factor influencing the deposition of metal nanoparticles on it. XPS for Ru3d over Ru/GO and Ru/GO-PNIPAM were further tested for investigating the effects of GO-PNIPAM on Ru deposition. As shown in Figure 4c and 4d, C1s and Ru3d species on catalysts were deconvoluted together due to their partly overlapped spectra. In general, three Ru species (i.e. Ru0, Ruδ+ and Ru4+) were separated in Ru3d spectra of both catalysts.35, 36 Cationic Ru including Ruδ+ and Ru4+on them accounted for approximately 60 % of the total (Table S4, ESI). However, there was a shift toward low binding energy in Ru3d spectrum of Ru/GO-PNIPAM (LH) in contrast to that of Ru/GO. Metallic Ru, for example, had a binding energy at 279.8 eV on Ru/GO-PNIPAM (LH), 0.4 eV lower than that on Ru/GO (Table S4). Ruδ+ on Ru/GOPNIPAM had a similar shift toward low binding energy. Results hinted a strong coordination effect of GO-PNIPAM with Ru nanoparticles. Nitrogen atoms from amide groups on PNIPAM side chains possibly took some positive effects on it. They provided electrons for coordination and anchoring with metal nanoparticles.37-39 Hence, Ru nanoparticles were deposited on GO-PMIPAM very possibly based on the following mechanism (Figure 7). Specifically, thermoresponsive GO-PNIPAM exhibited a hydrophilic surface at 25 oC (< LCST), which contributed to well dispersion 12


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of GO-PNIPAM in aqueous medium (Figure S6g, ESI). The increased surface area of GO-PNIPAM as well as its hydrophilic surface gave rise to a small Ru particle size of GO-PNIPAM (L) (4.5 ± 0.8 nm, Figure 6b). In this case, Ru nanoparticles deposited on both hydrophilic GO sheets and side chains of PNIPAM where gathered a great deal of amide groups, favoring the deposition of metal particles as mentioned above. However, when the temperature ascended to above LCST (50 oC), the extended PNIPAM chain curled up to a coil. According to this conformation, the hydrophobic backbone of the polymer exposed outwards but its hydrophilic part contracted inward. This conformation led to GO-PNIPAM aggregation in aqueous medium (Figure S6j, ESI). Thus, a great amount of Ru nanoparticles deposited on hydrophobic surface of PNIPAM coils, resulting in a relatively low Ru dispersion of Ru/GO-PNIPAM (H) (Figure 6c). Additionally, Ru/GO-PNIPAM (LH) logically had a comparatively small Ru particle size (5.6 ± 2.3 nm, Figure 6d) than Ru/GO-PNIPAM (H) did due to Ru deposition on both the backbone of the polymer and its side chains (Figure 6d) in process of catalyst preparation at 25 oC and 50 oC successively. Noted that, although a low temperature (< LCST) favored the deposition of metal nanoparticles on GO-PNIPAM in aqueous medium, some deposited metal nanoparticles on hydrophilic side chains of PNIPAM might be wrapped in the compressed coil at a high temperature (> LCST). The declined number of Ru sites on Ru/GO-PNIPAM at high temperature might produce some negative effects on its catalytic activity. 3.3.2 Catalytic activity over Ru/GO-PNIPAM Generally, PNIPAM grafting on GO improved the catalytic activity in CAL hydrogenation from Figure 8. The reaction over Ru/GO-PNIPAM (LH), for instance, arrived at nearly complete CAL conversion (about 96 %) after 10 h, much higher than that over Ru/GO (about 70 %) at the same time. The reaction rate constants (k) were estimated in Figure S7 (ESI) for further calculating initial TOFs over Ru/GO and Ru/GO-PNIPAM by Equation (3). Results indicated that, although the high Ru dispersion on Ru/GO (24 %, Table 1), it exhibited a lower initial TOF (about 2.6 min13



1)

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in Table 2 in comparison to Ru/GO-PNIPAM (LH) did (about 6.9 min-1). The low

initial TOF over Ru/GO was related to its hydrophilic surface that hampered CAL adsorption on it in the reaction. In contrast, after the synthesis of PNIPAM, as-prepared Ru/GO-PNIPAM exhibited the hydrophobic surface at high temperature. It helped the improvement of adsorption ability to CAL on its surface in the reaction, finally resulting in an enhanced activity over it. Activation energies of CAL hydrogenation over Ru/GO and Ru/GO-PNIPAM (LH) were further estimated in Figure S8 and summarized in Table 2. That over Ru/GO was estimated to be 40 kJ/mol, higher than that over Ru/GO-PNIPAM (LH) (31 kJ/mol). The comparatively low activation energy over Ru/GO-PNIPAM (LH) was logically ascribed to the strong synergistic effect between Ru and GO-PNIPAM composite that suppressed the activation barrier of CAL hydrogenation. Therefore, enhanced activity as well as decreased activation energy over Ru/GO-PNIPAM showed its superiority to Ru/GO. Moreover, among three Ru/GO-PNIPAM samples, Ru/GO-PNIPAM (L) was discovered to possess the lower activity than the other two samples did in CAL hydrogenation (Figure 8) in spite of its highest dispersion of Ru nanoparticles (26 %, Table 1). The initial TOF over Ru/GO-PNIPAM (L) was further calculated to be approximately 3.1 min-1, much lower than those of Ru/GO-PNIPAM (H) and Ru/GOPNIPAM (LH) (approximately 6.9 min-1 both). In consideration of the similar surface wettability over three Ru/GO-PNIPAM samples in the reaction, we deemed that comparatively low catalytic activity over Ru/GO-PNIPAM (L) was largely relied on the compressed coil-like conformation of PNIPAM at high temperature, covering some Ru sites on the catalyst. The declined number of active sites on Ru/GO-PNIPAM (L) at high temperature led to its relatively low catalytic activity. Results supported the deposition mechanism of metal nanoparticles on the polymer (Figure 7). 3.3.3 Product Selectivity over Ru/GO-PNIPAM Product distribution in selective hydrogenation of CAL over Ru/GO and Ru/GOPNIPAM were given in Figure 9. Generally, as-prepared Ru catalysts showed 14


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comparatively higher HCAL selectivity than that to COL because carbonyl group has a higher thermodynamic stability than C=C bonds that hampers the production of COL. In addition, the intermediates including HCAL and COL were predominant in the reaction over the final product HCOL. It hinted insignificant internal diffusion limitation on GO materials due to their sheet structures. However, product selectivity over Ru/GO-PNIPAM was greatly different from that over Ru/GO. From Figure 9, Ru/GO exhibited the lowest selectivity to COL among four catalysts. For instance, COL selectivity over Ru/GO was only 8 % at CAL conversion of 95 %, much lower than that over Ru/GO-PNIPAM (LH) (about 32 %) at the same conversion. Enhanced COL selectivity over Ru/GO-PNIPAM was ascribed to its preferred C=O adsorption mode on the hydrophobic surface. Specifically, under the reaction temperature (70 oC, > LCST), hydrophobic backbone of PNIPAM exposed outwards, favoring the adsorption to the phenyl ring. Accordingly, C=O group in CAL molecule was allowed to adsorb on the periphery of a Ru nanoparticle adjacent to it on PNIPAM coil (Figure 10), finally resulting in comparatively high selectivity to COL over Ru/GO-PNIPAM samples. Furthermore, in Ru/GO-PNIPAM catalysts, Ru/GO-PNIPAM (LH) had the highest COL selectivity but Ru/GO-PNIPAM (L) did the lowest. For example, COL selectivity over Ru/GO-PNIPAM(LH) arrived at about 32 % at CAL conversion of 90 %, 10 % higher than that over Ru/GO-PNIPAM (L) at the same CAL conversion. Compared to Ru/GO-PNIPAM (L), the other two catalysts possessed more Ru depositions on PNIPAM backbone that were propitious to C=O hydrogenation, rendering higher COL selectivities over them. Additionally, catalytic performance of some carbon materials supported catalysts in CAL hydrogenation were summarized for comparison in Table 3. Clearly, it was greatly affected by several factors such as active site and surface of the catalyst. According to active sites, Pd catalysts exhibited the higher activities than Ru and Pt ones did. The initial TOF over Pd/CNF,5 for instance, arrived at 12 min-1, while that over Ru/CNTs (a)-ht was only 0.8 min-1,40 in spite of their similar supports. However, 15



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COL selectivity over them abided by the sequence Pt > Ru > Pd, which was closely related to d-band width of the metal (Pd < Ru < Pt).15, 41 On the other hand, at the viewpoint of surface chemistry, CNM supported catalyst with thermoresponsive smart surface exhibited enhanced catalytic performance in contrast to those traditional ones. For example, among four Ru catalysts listed in Table 3, Ru/GO-PNIPAM (LH) possessed the highest reaction rate and selectivity to COL thanks to the strong synergistic effect between metal nanoparticles and GO-PNIPAM as well as its hydrophobic surface for the reaction. Hence, CNMs with thermoresponsive smart surface (e.g. GO-PNIPAM) had enormous advantages over other traditional CNMs supports (e.g. CNT, GO and graphene) in developing metalbased supported catalyst for selective hydrogenation. 3.3.4 Recycling of Ru/GO-PNIPAM Ru/GO-PNIPAM (LH) was employed to exam its recyclability in the light of its heterogeneous nature. The results in Figure 11 indicated that CAL conversion after 7 hours’ reaction over Ru/GO-PNIPAM (LH) was only a slight decline from 86 % to 79 % while COL selectivity maintaining unchanged (about 36 %) in ﬁve runs. The actual Ru loading on recycled Ru/GO-PNIPAM (LH) was measured to be 3.79 wt. %, 0.13 wt. % less than the fresh one (Table S5). Hence, the mass loss of Ru nanoparticles was calculated to be 3.3 % of the total on recycled Ru/GO-PNIPAM across five runs. In comparison, that on recycled Ru/GO arrived at 7.6 %. Results proved the strong anchoring effects of GO-PNIPAM on Ru nanoparticles, in accordance with XPS analysis for Ru3d. It indicated the stability of as-prepared Ru/GO-PNIPAM (LH).

4 Conclusions In summary, we developed a graphene oxide supported catalyst with thermoresponsive smart surface, i.e., Ru/GO-PNIPAM. In comparison to Ru/GO, Ru/GO-PNIPAM exhibited an enhanced catalytic performance in CAL hydrogenation thanks to both the strong synergistic effect between GO-PNIPAM and Ru nanoparticles and the hydrophobic surface of the catalyst in the reaction. This study constructed a 16


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competitive smart carbon nanomaterial for catalytic hydrogenation that extended its applications in academia and industry.

Acknowledgements This study is financially supported by the National Natural Science Foundation of China (21676029 and 21673024). The financial support from the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) is also acknowledged.

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21



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Table 1. Physicochemical properties of GO materials supported Ru catalysts Sample

BET Surface Area (m2/g)

Actual Ru loading (wt. %) d

Average Ru size (nm) e

Ru dispersion (%) f

Ru/GO a Ru/GO-PNIPAM (L) a Ru/GO-PNIPAM (H) b Ru/GO-PNIPAM (LH) c

12.3 29.7 31.2 27.5

3.81 4.03 3.76 3.92

4.9 ± 1.5 4.5 ± 0.8 7.2 ± 2.5 5.6 ± 2.3

24 26 17 21

Note: (a) Prepared by impregnating and reducing Ru precursors at the room temperature (< LCST); (b) Prepared by impregnating and reducing Ru precursors at 50 oC; (C) Prepared by impregnating and reducing half of Ru precursors at the room temperature (< LCST) first and then the other half at 50 oC; (d) Determined with ICP-AES; (e) Randomly measured 50 Ru nanoparticles in TEM images (Figure 6) and calculated their average sizes as well as standard deviations; (f) Based on the hemispherical model of metal particles on surface of the catalyst.

Table 2 Intrinsic activity and activation energy over catalysts Sample

Rate constant, k (h-1) a

Initial TOF (min-1) b

Activation energy (kJ/mol) c, d

0.20 0.25 0.35 0.45

2.6 3.1 6.9 6.9

40 31

Ru/GO Ru/GO-PNIPAM (L) Ru/GO-PNIPAM (H) Ru/GO-PNIPAM (LH)

Note: (a) Determined by Figure S7 (ESI) based on Rate equation; (b) Calculated using Equation (3); (c) Determined by Figure S8 (ESI) based on Arenius equation (Equation (4)); (d) Compared Ru/GO-PNIPAM (LH) to Ru/GO.

Table 3 Cinnamaldehyde hydrogenation over several carbon materials supported catalysts Sample Ru/GO Ru/GO-PNIPAM (LH) Ru/TEGO (h) b Ru/CNTs (a)-ht c Pd/AC d Pd/CNF d Pt/G e Pt/CNF-973 (HDP) f

Temperature (oC)

H2 Pressure (MPa)

Initial TOF (min-1)

Selectivity to COL (%) a

70 70 70 70 80 80 60 40

2 2 1 1 2 2 1 0.12

2.6 6.9 2.0 0.8 15 12 5.1 4.4

14 39 31 35 3 6 82 57

Note: (a) Recorded at CAL conversion of 70 %; (b) Reference 2 (TEGO: thermally exfoliated graphite oxide; TEGO (h): H2 thermally treated 22


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TEGO); (c) Reference 40 (CNTs(a): concentrated HNO3 treated CNTs; Ru/CNTs(a)-ht: treated in N2flow at 700 oC for 2 h); (d) Reference 5 (AC: activated carbon; CNF: carbon nanofiber) (e) Reference 15 (G: graphene) (f) Reference 11 (HDP: catalyst preparation by homogeneous deposition precipitation; Pt/CNF-973: catalyst treatment in N2-flow at 700 oC for 2 h)

23



Page 24 of 35

Figure legends 1. Figure 1. Consecutive reaction pathways of cinnamaldehyde hydrogenation. 2. Figure 2. FT-IR spectra of GO (a) and GO-PNIPAM (b). 3. Figure 3. TG curves of PNIPAM, GO and GO-PNIPAM. 4. Figure 4. C1s and Ru3d XPS spectra of GO materials: (a) C1s of GO; (b) C1s of GOPMIPAM; (c) Ru3d of Ru/GO; (d) Ru3d of Ru/GO-PMIPAM (LH). 5. Figure 5. DSC spectra of GO, GO-PNIPAM and Ru/GO-PNIPAM. 6. Figure 6. TEM images with Ru particle size distribution: Ru/GO (a), Ru/GO-PNIPAM (L) (b), Ru/GO-PNIPAM (H) (c) and Ru/GO-PNIPAM (LH) (d). 7. Figure 7. Possible mechanism of Ru deposition on GO-PNIPAM at different temperatures. 8. Figure 8. CAL conversion versus reaction time over Ru/GO and a series of Ru/GOPNIPAM in CAL hydrogenation. 9. Figure 9. Product distribution versus CAL conversion in selective hydrogenation of CAL over Ru/GO (a), Ru/GO-PNIPAM (L) (b), Ru/GO-PNIPAM (H) (c) and Ru/GOPNIPAM (LH) (d). 10. Figure 10. Adsorption of CAL on the surface of PNIPAM coil in the reaction over Ru/GO-PNIPAM (L) (a), Ru/GO-PNIPAM (H) (b) and Ru/GO-PNIPAM (LH) (c). 11. Figure 11. Recycling of Ru/GO-PNIPAM (LH) across 5 runs.

24


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H2 O Cinnamaldehyde (CAL)

H2

O

H2

Hydrocinnamic aldehyde (HCAL)

OH Hydrocinnamic alcohol (HCOL)

OH H2 Cinnamyl alcohol (COL)

Figure 1. Consecutive reaction pathways of cinnamaldehyde hydrogenation.

25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Transmittance (%)


Page 26 of 35

GO GO-PNIPAM OH ~3300

CH3 2973

COOH 1720

CH2 2924 C=O 1648

C-C 1590

C-OH 1060 C-O 1228

C-O-C 1128 Amide Isopropyl 1533 1385&1375

3500 3000 2500 2000 1500 1000 Wavenumber (cm-1)

500

Figure 2. FT-IR spectra of GO and GO-PNIPAM.

26


Page 27 of 35

100

GO PMIPAM GO-PNIPAM

80

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 40 20 0

140-260oC 260-450oC

100 200 300 400 500 600 700 800 Temperature (oC)

Figure 3. TG curves of PNIPAM, GO and GO-PNIPAM.

27



GO-PNIPAM (C1s) C-OH

C-C (sp2)

C-O

C-C (sp3)

COOH

292

290

(a)

288 286 284 282 Binding Energy (eV)

Intensity (a.u.)

Intensity (a.u.)

GO (C1s)

280

C-N

(b)

C-OH C-O C=O

290

C-C (sp2)

288 286 284 282 Binding Energy (eV)

280

Ru/GO-PNIPAM (LH) (C1s + Ru3d5/2)

(c)

C-O +

Ru

+

Ru



Ru

290 288 286 284 282 280 278 Binding Energy (eV)

Intensity (a.u.)

C-C C-OH

COOH

C-C (sp3)

C-O-C

292

Ru/GO (C1s + Ru3d5/2) Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 35

C-C

(d)

C-N C-OH&C-O-C C-O C=O

+

Ru + Ru  Ru

290 288 286 284 282 280 278 Binding Energy (eV)

Figure 4. C1s and Ru3d XPS spectra of GO materials: (a) C1s of GO; (b) C1s of GOPMIPAM; (c) Ru3d of Ru/GO; (d) Ru3d of Ru/GO-PMIPAM (LH).

28


Page 29 of 35

GO GO-PNIPAM Ru/GO-PNIPAM

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


36.6 oC

36.9 oC

10 15 20 25 30 35 40 45 50 Temperature (oC) Figure 5. DSC spectra of GO, GO-PNIPAM and Ru/GO-PNIPAM.

29



Page 30 of 35

Figure 6. TEM images with Ru particle size distribution: Ru/GO (a), Ru/GO-PNIPAM (L) (b), Ru/GO-PNIPAM (H) (c) and Ru/GO-PNIPAM (LH) (d).

30


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Figure 7. Possible mechanism of Ru deposition on GO-PNIPAM at different temperatures.

31



100

CAL Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 35

80 60 40 Ru/GO Ru/GO-PNIPAM (LH) Ru/GO-PNIPAM (L) Ru/GO-PNIPAM (H)

20 0

0

2

4

6 8 10 12 14 16 18 20 Reaction Time (h)

Figure 8. CAL conversion versus reaction time over Ru/GO and a series of Ru/GOPNIPAM in CAL hydrogenation.

32


Page 33 of 35

Product Selectivity (%)

80 60

HCAL COL HCOL Others

40 20 0

0

100

20 40 60 80 CAL Conversion (%)


80

Ru/GO-PNIPAM (H)

20 0


100

Ru/GO-PNIPAM (L)

(b)

60 40 20 0

100

(c)

40


80

0

100

60

0

100

(a)

Ru/GO



100


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60




80

Ru/GO-PNIPAM (LH)

100

(d)

60 40 20 0

0


100

Figure 9. Product distribution versus CAL conversion in selective hydrogenation of CAL over Ru/GO (a), Ru/GO-PNIPAM (L) (b), Ru/GO-PNIPAM (H) (c) and Ru/GO-PNIPAM (LH) (d).

33



Page 34 of 35

Figure 10. Adsorption of CAL on the surface of PNIPAM coil in the reaction over Ru/GO-PNIPAM (L) (a), Ru/GO-PNIPAM (H) (b) and Ru/GO-PNIPAM (LH) (c).

34


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CAL conversion & COL selectivity (%)

Page 35 of 35

100

CALconversion COL selectivity

Ru/GO-PNIPAM (LH)

80 60 40 20 0

0

1

2

3 Run

4

5

6

Figure 11. Recycling of Ru/GO-PNIPAM (LH) across 5 runs.

35


A Graphene Oxide Supported Catalyst with Thermo- Responsive

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