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Active and regioselective rhodium catalyst supported on reduced graphene oxide for Cite this: DOI: 10.1039/c5cy01355k 1-hexene hydroformylation†

Minghui Tan,ab Guohui Yang,b Tiejun Wang,d Tharapong Vitidsant,e Jie Li,b Qinhong Wei,b Peipei Ai,b Mingbo Wu,*a Jingtang Zheng*a and Noritatsu Tsubaki*bc

Alkene hydroformylation with syngas (CO + H2) to produce aldehydes is one of the most important chemical reactions. However, designing heterogeneous catalysts to realize comparable performance with mature homogeneous catalysts is challenging. In this report, a reduced graphene oxide (RGO) supported rhodium nanoparticle (Rh/RGO) catalyst was successfully prepared via a one-pot liquid-phase reduction method and first applied in 1-hexene hydroformylation. 1-Hexene hydroformylation reaction under different reaction conditions with this Rh/RGO catalyst was investigated in detail. Low reaction temperature and short reaction time effectively enhanced the n/i (normal to iso) ratio of heptanal in the products. The catalytic performance of the Rh/RGO catalyst was also compared with those of Rh supported on other carbon materials, including activated carbon and carbon nanotubes (Rh/AC and Rh/CNTs). The results showed that Received 18th August 2015, the Rh/RGO catalyst exhibited the highest 1-hexene conversion and the largest n/i ratio of 4.0 among the Accepted 15th September 2015 tested catalysts. The special 2D nanosheet structure of the Rh/RGO catalyst, rather than the 3D porous and 1D nanotube structures of Rh/AC and Rh/CNTs, respectively, principally contributed to its excellent cata- DOI: 10.1039/c5cy01355k lytic performance. These findings disclosed that reduced graphene oxide could be a promising catalyst www.rsc.org/catalysis support for designing heterogeneous hydroformylation catalysts.

1. Introduction hydroformylation. Especially, ligand-modified Rh-based catalysts show very excellent hydroformylation activity and regio- 5 Alkene hydroformylation, the addition of CO/H2 to the car- selectivity to linear products under mild reaction conditions.

Published on 16 September 2015. Downloaded by Beijing University 21/12/2015 05:49:39. bon–carbon double bond of alkene to form aldehyde, is one Even though the traditional homogeneously catalyzed of the main homogeneously metal-catalyzed reactions, as hydroformylation reaction is effective, it is necessary to – shown in Scheme 1.1 3 Generally, the hydroformylation of develop heterogeneous catalytic systems as it is always a chal- 1-alkenes results in mixed products of linear n-aldehydes (n) lenge to overcome the drawbacks of homogeneous catalytic and branched iso-aldehydes (i), in which linear n-aldehydes systems, such as high pressure, metal loss, catalyst separa- are required in large quantities for commodity applications.4 tion and recovery. Both cobalt (Co) and rhodium (Rh) complexes are usually Until now, Rh supported on many solid supports, like 6–8 9,10 11 12,13 used as catalysts in industry for homogeneously catalyzed SiO2, Al2O3, zeolites, MOFs, or carbon materials,11,14 as a catalyst has been studied for heterogeneous alkene hydroformylation. Due to their stable physical and a State Key Laboratory of Heavy Oil Processing, China University of Petroleum, chemical properties, such as large specific surface area and Qingdao 266580, PR China. E-mail: [email protected], [email protected] easy recovery of noble metals from spent catalysts, carbon b Department of Applied Chemistry, School of Engineering, University of Toyama, materials are commonly used as supports for noble metal cat- Gofuku 3190, Toyama 930-8555, Japan. E-mail: [email protected]; 15 14 Fax: +81 76 445 6846; Tel: +81 76 445 6846 alyst preparation. Activated carbon (AC) and carbon c JST, CREST, Gobancho 7, Chiyoda-ku, Tokyo 102-0076, Japan d CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, 510640 Guangzhou, PR China e Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand † Electronic supplementary information (ESI) available: Experimental section (preparation of graphite oxide, pre-treatment of AC and CNTs and CO-TPD), FTIR, XPS and TEM results for Rh/AC and Rh/CNTs. See DOI: 10.1039/ c5cy01355k Scheme 1 Hydroformylation of 1-alkene to aldehydes.

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nanotube (CNT)11 supported Rh catalysts were investigated reported as a catalyst support to prepare heterogeneous cata- for alkene hydroformylation to obtain aldehydes. Although lysts for alkene hydroformylation. these catalysts exhibited high conversion of alkene, the In this work, we prepared a Rh nanoparticle-loaded RGO aldehyde selectivity and n/i ratios were low in the formed (Rh/RGO) catalyst via a one-pot liquid-phase reduction products. As a novel carbon material, graphene has attracted method. The as-prepared Rh/RGO catalyst was first tested as much attention because of its unique 2-dimensional (2D) a heterogeneous catalyst for 1-hexene hydroformylation. The structure and mechanical, electronic and electrochemical catalytic performance of this Rh/RGO catalyst under varied properties.16,17 Besides the above properties, graphene oxide reaction conditions, such as reaction temperature or time, (GO), derived from graphene, has more advantages due to was investigated in detail. The hydroformylation activity and the abundant oxygen-containing functional groups on its regioselectivity to n-heptanal over the Rh/RGO catalyst were sheet surface,18 which can improve the hydrophilicity and also compared with other carbon material supported Rh cata- stability of GO, therefore ensuring its utilization in liquid- lysts (Rh/AC and Rh/CNTs). phase reaction systems. Meanwhile, these functional groups on the surface of GO can act as nucleation sites to anchor 2. Experimental metal ions.19,20 Thus, GO has been regarded as an excellent support to develop a variety of heterogeneous catalysts.21 Fur- 2.1 Catalyst preparation thermore, GO combined with metal ions on its surface can The reduced graphene oxide supported Rh nanoparticle (Rh/ be simultaneously reduced to form reduced graphene oxide RGO) catalyst was prepared by a one-pot method. The prepa- (RGO) supported metal nanoparticles for various applica- ration process of the Rh/RGO catalyst is shown in Scheme 2. tions.22 However, to our knowledge, until now no RGO is First, 0.5 g of graphite oxide obtained from graphite powder Published on 16 September 2015. Downloaded by Beijing University 21/12/2015 05:49:39.

Scheme 2 Illustration of the preparation process of the Rh/RGO catalyst.

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23 † by a modified Hummers' method (see the ESI ) was added H2 = 1 : 1. After purging with 1.0 MPa syngas three times to into 120 mL of glycol–water solution (20 mL of deionized remove residual air, another 5.0 MPa syngas containing 80 mmol

water, 100 mL of ethylene glycol) that was beforehand mixed of CO and 80 mmol of H2 was sealed in the reactor at room Ĳ – with 0.25 g of Rh NO3)3 water solution containing 5.1 mg of temperature. The hydroformylation reaction was conducted Rh3+. After ultrasonic treatment for 2 h, the mixed solution at various temperatures in the range of 70–110 °C, for differ- was heated in a 105 °C oil bath for 10 h with magnetic stir- ent times of 1–10 h, under continuous stirring. After the ring and water reflux. Then the mixture was filtered and reaction, the reactor was cooled down in an ice water mix- washed with deionized water and ethanol, respectively, sev- ture for 20 min and then depressurized to atmospheric pres- eral times. Finally, the catalyst was dried under vacuum at 60 sure. The liquid products were analyzed quantitatively with °C for 12 h. The obtained sample was the Rh/RGO catalyst a gas chromatograph (Shimadzu GC-2014, Japan) equipped with a Rh loading amount of 1.07 wt% detected by using an with a capillary column (InertCap 5, length: 30 m) and a inductively coupled plasma atomic emission spectrometer flame ionization detector (FID). (ICP-AES, Optima 7300DV, PerkinElmer Inc., USA). The AC and CNT supported Rh catalysts (Rh/AC and Rh/ 3. Results and discussion CNTs) were also prepared by the same method and used as reference catalysts of the Rh/RGO catalyst in the following 3.1 Catalyst characterization sections. For further details, please see the ESI.† The metal The XRD patterns of graphite, graphite oxide and Rh/RGO loading amount of Rh was about 1.02 wt% and 1.04 wt% are compared in Fig. 1. The diffraction pattern of graphite (detected by ICP-AES) for Rh/AC and Rh/CNTs, respectively. shows two peaks at about 26.6° and 54.7°, which can be ascribed to the (002) and (004) reflections of graphite (JCPDS 2.2 Catalyst characterization no. 41-1487), respectively. For graphite oxide, the peak (001) at 10.7° is the characteristic peak of graphite oxide obtained The crystal structure of the samples was confirmed by X-ray through liquid oxidation from graphite. Because of the intro- diffraction (XRD) (RINT 2400 diffractometer, Rigaku, Japan) duction of abundant oxygen-containing functional groups, with a Cu Kα radiation source (λ = 1.54 Å, a scanning rate of − the interlayer spacing of graphite oxide reaches 0.84 nm, 0.02° s 1 in the range of 5–80°) at 40 kV and 20 mA. Raman much larger than 0.34 nm of the original graphite. Due to spectroscopy was performed using an Ar+ ion laser at 514.5 the exfoliation of 3D graphite oxide to 2D GO sheets and then nm (Renishaw inVia 2000 Raman microscope, Renishaw, UK) the reduction of GO by ethylene glycol to RGO, the peak to characterize the graphitic structure of the samples. The around 10.7° of graphite oxide will shift to a large angle for catalyst morphology was characterized by transmission RGO. Therefore, the Rh/RGO catalyst has two peaks at 24° electron microscopy (TEM) (JEM-2100UHR, JEOL, Japan) with and 43°, belonging to the crystalline structures (002 and 100) an accelerating voltage of 200 kV. The oxygen-containing of RGO (JCPDS no. 01-0646), respectively. The peak at 24° is functional groups on the catalysts were characterized by Fou- much broader, which indicates that the catalyst has some rier transform infrared spectroscopy (FTIR) (Nicolet NEXUS micro-structural defects and less crystallite carbon atoms, 670, Thermo Scientific, USA). X-ray photoelectron spectro-

Published on 16 September 2015. Downloaded by Beijing University 21/12/2015 05:49:39. possibly generated by the reduction process with ethylene gly- scopy (XPS) was conducted on an ESCALAB 250Xi spectrome- col and the utilized Rh nanoparticle loading. There is no ter (Thermo Scientific, USA). obvious Rh peak except for the weak peak (111) of Rh The metal surface area and metal dispersion were deter- (denoted by ∇ in Fig. 1, JCPDS no. 05-0685) at 41°, because of mined by a H2 chemical adsorption experiment with a Quantachrome Autosorb-1 vacuum apparatus (Quantachrome Instruments, USA). The samples were degassed at 200 °C

under vacuum for 1 h and then reduced in H2 at 400 °C for

2 h. The chemisorption of H2 was carried out at 100 °C for 30 min to equilibrium. Rh dispersions, as well as exposed Rh surface areas and sizes, were determined by assuming a Rh/H adsorption stoichiometry equal to 1 and a spherical geometry. Carbon monoxide temperature programmed − desorption (CO-TPD) at a heating rate of 5 °C min 1 was performed on a BELCAT-B-TT apparatus (Bel Japan Inc.) equipped with a mass spectrometer (BELMass).

2.3 Catalytic activity test of 1-hexene hydroformylation The 1-hexene hydroformylation reaction was carried out in an autoclave with an inner volume of 85 mL. 0.10 g of the catalyst and 3.73 g of 1-hexene (40 mmol) were loaded in the autoclave. The molar ratio of the utilized syngas was CO : Fig. 1 XRD patterns of graphite, graphite oxide and Rh/RGO.

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the low loading amount and fine dispersion of Rh nanoparticles on the RGO support. For carbon materials, Raman spectroscopy is well recog- nized as a sensitive probe to detect their defects and disor- der.23 Fig. 2 presents the Raman spectra of graphite, graphite oxide and Rh/RGO. The two strong bands observed in graph- − ite are the G band at 1580 cm 1 and the 2D band at 2700 − cm 1.24 The G band is the first-order spectrum due to the bond stretching of all pairs of sp2 atoms in both rings and chains.25 For graphite oxide and Rh/RGO, two noticeable − − broad bands (D band at 1340 cm 1 and G band at 1580 cm 1) are observed, and the D band is ascribed to the defects or imperfections of graphite oxide and RGO. The intensity ratio

of the D band to G band (ID/IG) is related to the degree of dis- 26 order and defects. The ID/IG ratio increases obviously from graphite oxide to Rh/RGO, indicating the increasing amount of defects on the RGO support through a series of treatment Fig. 3 FTIR spectra of graphite oxide and Rh/RGO. processes. The increased defects here are beneficial to the formation of more catalytic active sites on the Rh/RGO catalyst for the following 1-hexene hydroformylation. The reduction process can be confirmed by comparing the X-ray photoelectron spectroscopy (XPS) analysis is also oxygen-containing functional groups between graphite oxide conducted to elucidate the surface change and reduction and Rh/RGO by FTIR, as shown in Fig. 3. Graphite oxide degree of Rh/RGO from graphite oxide. As shown in exhibits obvious peaks of oxygen-containing functional Fig. 4(a) and (b), after the reduction process, the oxygen con- − groups, such as –OH (at 3000–3500 cm 1), CO (carbonyl/ tent of Rh/RGO decreases obviously from 29.16% of graphite − − carboxy at 1733 cm 1) and C–O (carboxy at 1418 cm 1, epoxy oxide to 12.41%. The C1s peak can be fitted to investigate the − − at 1225 cm 1, and alkoxy at 1054 cm 1) groups,27,28 which can species and the number of oxygen-containing functional act as nucleation sites to anchor Rh ions. After reduction, the groups. For graphite oxide in Fig. 4(a), the C1s peak consists − peaks of CO (1733 cm 1) and some C–O (epoxy at 1225 of five components assigned to CC (284.4 eV), C–C (285.2 − − cm 1 and alkoxy at 1054 cm 1) have disappeared on the Rh/ eV), C–O (286.7 eV), CO (287.2 eV) and OC–O (288.5 − RGO catalyst; however, the peaks of the –OH (3450 cm 1) and eV),29 and the two strong peaks (C–O and CO) indicate the − C–O (carboxy at 1418 cm 1) groups still remain. It indicates presence of abundant oxygen-containing functional groups that the oxygen-containing functional groups on GO have not on graphite oxide. Compared with graphite oxide, the CO been fully removed and GO is partly reduced to RGO. peak disappears and the C–O and OC–O peaks of Rh/RGO

Published on 16 September 2015. Downloaded by Beijing University 21/12/2015 05:49:39. decrease significantly in Fig. 4(b), indicating that most of the oxygen-containing functional groups have been removed after reduction except for some C–O and OC–O groups. The result is consistent with the FTIR results. Due to the high electronegativity of GO, it is difficult to remove all oxygen-containing functional groups and reduce Rh3+ to Rh0 completely under our mild preparation conditions.30 These C–O and OC–O groups on Rh/RGO are stable and still remain after reaction, as shown in Fig. 3 and 4(c). But for Rh/CNTs and Rh/AC, these groups are changed after reaction and less stable than those on Rh/RGO as shown in Fig. S1.† The residual groups on Rh/RGO can be used for anchoring Rh nanoparticles and are beneficial to the stability of the catalyst in heterogeneous reaction. Usually, in Rh– ligand homogenous catalyzed hydroformylation, the electron- withdrawing ability of the substituents can influence the selectivity to linear aldehydes.31 Therefore, as a kind of electron-withdrawing group, the carbonyl group on the surface of the catalyst may be in favour of n-heptanal produc-

tion. As shown in Fig. 4(d), the binding energy of Rh 3d5/2 was 308.6 eV which was assigned to a non-stoichiometric Fig. 2 Raman spectra of graphite, graphite oxide and Rh/RGO. state Rhn+ between Rh3+ and Rh0.32,33 This result indicated

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Fig. 4 XPS spectra of graphite oxide and Rh/RGO: (a) survey spectra and C1s spectra of graphite oxide, (b) survey spectra and C1s spectra of fresh Rh/RGO, (c) survey spectra and C1s spectra of spent Rh/RGO, (d) Rh 3d region XPS spectra of fresh and spent Rh/RGO. Published on 16 September 2015. Downloaded by Beijing University 21/12/2015 05:49:39. that Rh3+ was partly reduced on RGO because of the high electronegativity of GO. A similar result was also obtained for Rh/CNTs as shown in Fig. S2.† Without electronegativity of the AC support, the Rh/AC catalyst exhibits a higher reduction state Rh0 (Fig. S3†) under the same preparation conditions, which also verifies the effect of electronegativity on Rh3+ reduction. Rhn+ as an electro-deficient species indicated that oxygen linkages exist between Rh nanoparticles and 22,34 RGO. After reaction, the binding energy of Rh 3d5/2 decreased slightly to 308.3 eV and was still maintained at the Rhn+ state, demonstrating the high stability of the Rh/RGO catalyst. Fig. 5 shows the TEM images of the Rh/RGO catalyst at different magnifications. It seems that the Rh particles have a large size, 40–60 nm, as exhibited at low magnification in Fig. 5(a). However, in Fig. 5(b) and (c), the magnified TEM analysis of the catalyst discloses that the large Rh particle in fact has a cluster-like structure comprising dozens of smaller Rh nanoparticles with a size of 6 nm. This cluster-like structure has proved its significant potential and prospects in cat- Fig. 5 TEM images of the Rh/RGO catalyst obtained with different alytic reactions.35 The cluster-like structure of Rh particles scale bars: (a) 200 nm, (b) 100 nm, and (c) 5 nm.

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supported on RGO possesses not only a large metallic surface area and high dispersion, but also abundant active crystal defects and synergetic effects between small building units,36 which can provide more active sites and higher activity for the targeted hydroformylation. Moreover, because the residual groups can be used for anchoring Rh nanoparticles, the cluster-like structure of Rh particles supported on RGO is stable and no obvious aggregation of Rh particles is detected (Fig. S4†).

3.2 Catalyst performance in 1-hexene hydroformylation reaction using the Rh/RGO catalyst The major products of 1-hexene hydroformylation are Fig. 6 Influence of reaction temperature on 1-hexene hydro- heptanals, including n-heptanal and i-heptanal, as given in formylation over the Rh/RGO catalyst. Reaction conditions: 0.10 g of

Scheme 3. The side reactions of 1-hexene hydroformylation the Rh/RGO catalyst, 3.73 g of 1-hexene, 5 MPa syngas (CO : H2 = 1 : 1), mainly include 1-hexene isomerization through its carbon– 1h. carbon double bond shift to generate internal hexene and further hydroformylation of internal hexene on the same catalyst producing i-heptanal. Other side reactions, like hexene internal hexene produces only i-heptanal, thus leading to a hydrogenation, can produce some hexane. lower n/i ratio of heptanal at higher reaction temperature. Fig. 6 shows the effect of reaction temperature on the cat- Higher reaction temperature promotes the isomerization alytic properties of the Rh/RGO catalyst. The 1-hexene conver- and hydrogenation of 1-hexene to produce internal hexene sion increases with increasing reaction temperature and and hexane, instead of the desired 1-hexene hydro- reaches up to 100% when the reaction temperature is higher formylation reaction to generate n-heptanal. Moreover, the than 90 °C. The heptanal yield (n-heptanal and i-heptanal) following hydroformylation of internal hexene will be acceler- also exhibits a similar trend with a maximum yield of 71.9% ated at high reaction temperature, which also produces more at 100 °C. Moreover, the hydrogenation of hexene is pro- i-heptanal and decreases the n/i ratio directly. Therefore, moted slightly at high reaction temperature (>80 °C), produc- here, we can conclude that lower reaction temperature is ben- ing more hexane. eficial to the higher n/i ratio of heptanal over the Rh/RGO cat- Fig. 7 gives the selectivity and n/i ratio of heptanal at dif- alyst. In order to avoid the undesired isomerization and ferent reaction temperatures. Although the total heptanal hydrogenation of 1-hexene, low reaction temperature is yield increases with increasing reaction temperature, the essential to 1-hexene hydroformylation over the Rh/RGO cata- selectivity to n-heptanal decreases obviously, as shown in lyst. To further investigate the catalytic performance of the

Published on 16 September 2015. Downloaded by Beijing University 21/12/2015 05:49:39. Fig. 7. It can be seen that, due to the accelerated production Rh/RGO catalyst, we performed more catalytic tests at a of i-heptanal, the selectivity to n-heptanal decreases from 46.6 to 29.6% and the n/i ratio of heptanal decreases from 4.0 to 0.8. Higher reaction temperature usually stimulates the shift of the carbon–carbon double bond of 1-hexene and then improves the formation of internal hexene (refer to Scheme 3).30 Moreover, higher reaction temperature also significantly accelerates the hydroformylation of internal hexene to form i-heptanal. As given in Fig. 6, the yield of internal hexene increases initially and then decreases rapidly with increasing reaction temperature. The hydroformylation of

Fig. 7 Influence of the reaction temperature on the selectivity to n-heptanal and i-heptanal and the n/i ratio of heptanal over the Rh/ Scheme 3 1-Hexene hydroformylation and the co-existing side RGO catalyst. Reaction conditions: 0.10 g of the Rh/RGO catalyst,

reactions. 3.73 g of 1-hexene, 5 MPa syngas (CO : H2 = 1 : 1), 1 h.

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reaction temperature of 70 °C with varied reaction times, and the reaction results are compared in Fig. 8. The 1-hexene conversion over the Rh/RGO catalyst increases quickly with increasing reaction time and reaches up to 100% at 4 h. The heptanal (n-heptanal and i-heptanal) yield increases sharply before 4 h, similar to that of 1-hexene conversion, but rises slowly after 4 h. At the initial reaction stage before 4 h, the formed heptanal mainly comes from 1-hexene hydroformylation. But after 4 h of reaction, the hydroformylation of internal hexene derived from 1-hexene isomerization contributes to the rest of the heptanal yields. This hypothesis is well supported by the change in internal hexene yield as shown in Fig. 8. It is clear that the internal hexene yield increases before 3 h and then decreases abruptly. The used low reaction temperature here can neither absolutely restrain 1-hexene isomerization to form internal Fig. 9 Effect of reaction time on yields of n-heptanal and i-heptanal hexene nor suppress the hydroformylation of internal hexene and the n/i ratio of heptanal over the Rh/RGO catalyst. Reaction conditions: 0.10 g of the Rh/RGO catalyst, 3.73 g of 1-hexene, 5 MPa syngas to generate i-heptanal. Although the formation of internal (CO : H2 = 1 : 1), 70 °C. hexene is not inhibited completely,37 the isomerization rate of 1-hexene at 70 °C is obviously lower than that at high reaction temperature; therefore more 1-hexene is converted to produce n-heptanal though hydroformylation reaction. produced only by the hydroformylation of the newly formed The total heptanal yield increases with increasing reaction 1-hexene from the reverse isomerization of internal hexene. time as given in Fig. 8. The yield profiles of n-heptanal and After the reaction for 10 h, the n/i ratio of heptanal on the i-heptanal are shown in Fig. 9. Both heptanal yields increase Rh/RGO catalyst still remains at 1.1, similar to that of with increasing reaction time, and the n-heptanal yield is unmodified Rh complexes in a homogeneous catalytic sys- always higher than the i-heptanal yield, that is, the n/i ratio tem,38 but higher than those in the reactions performed at of heptanal is larger than 1.0 on this Rh/RGO catalyst at the high temperatures or some other Rh-based heterogeneous used low reaction temperature. systems.39,40 At the initial stage of reaction, the n/i ratio of heptanal is Considering the thermodynamic equilibrium between as high as 4.0, which means that n-heptanal formation is the 1-hexene and internal hexene, it is not easy to completely major reaction route of 1-hexene hydroformylation. However, avoid 1-hexene isomerization and the subsequent hydro- as the reaction proceeds, the 1-hexene isomerization forming formylation of internal hexene, regardless of reaction temper- internal hexene and the following internal hexene hydro- ature and reaction time. Higher selectivity to n-heptanal can

Published on 16 September 2015. Downloaded by Beijing University 21/12/2015 05:49:39. formylation generating i-heptanal dominate the reaction probe obtained only at low temperature for a short reaction cess, which therefore decease the n/i ratio of heptanal in the time. Therefore, the catalytic activity and regioselectivity to products. After the reaction for 4 h, the original 1-hexene is n-heptanal by 1-hexene hydroformylation over different cata- depleted, and the subsequent n-heptanal in theory may be lysts should be evaluated at low temperature for a short time, ensuring that only 1-hexene takes part in the reaction.

3.3 Comparison of Rh/RGO with Rh/CNT and Rh/AC catalysts In addition to the presented Rh/RGO catalyst, we also prepared other carbon material supported Rh catalysts, like Rh/ AC and Rh/CNTs, as reference catalysts. The XRD patterns of these catalysts are compared in Fig. 10. All the catalysts exhibit no obvious Rh peaks except for the weak peak (111) of Rh at 41° due to the low loading of Rh. After reaction, the XRD patterns of these spent catalysts are similar to their fresh samples. Table 1 compares the detailed catalytic performances for 1-hexene hydroformylation over these carbon material supported Rh catalysts at 70 °C for 1 h. All the pure carbon

Fig. 8 Effect of reaction time on 1-hexene hydroformylation over the supports (RGO, AC and CNTs), without Rh, were also tested Rh/RGO catalyst. Reaction conditions: 0.10 g of the Rh/RGO catalyst, under the same reaction conditions and found to have no

3.73 g of 1-hexene, 5 MPa syngas (CO : H2 = 1 : 1), 70 °C. activity for 1-hexene hydroformylation or other side reactions.

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Fig. 10 XRD patterns of fresh and spent catalysts: Rh/RGO (a), Rh/AC (b) and Rh/CNTs (c). Published on 16 September 2015. Downloaded by Beijing University 21/12/2015 05:49:39.

The Rh/RGO catalyst realizes the highest 1-hexene conversion activities are still lower than that of the Rh/RGO catalyst for of 38.4% among the tested catalysts. For 1 h reaction, the 1 h, as indicated in Fig. 11(b). 1-hexene conversion obtained by the Rh/RGO catalyst is sig- For the Rh/RGO catalyst, the highest n-heptanal selectivity nificantly higher than those of Rh/AC and Rh/CNT catalysts, together with the lowest i-heptanal selectivity is achieved. indicating the excellent activity of the Rh/RGO catalyst. Further, the n/i ratio of heptanal obtained by the Rh/RGO cat- Extending the reaction time from 1 h to 2 h can increase the alyst reaches up to 4.0, the highest one among the tested cat- 1-hexene conversion of Rh/AC and Rh/CNTs, but their alysts. All these findings indicate that this Rh/RGO catalyst not only has excellent activity but also good n-heptanal regioselectivity, by which the highest n/i ratio was obtained. The Rh/RGO catalyst shows distinguished catalytic per- Table 1 Catalytic performances for 1-hexene hydroformylation over formance, better than the reference catalysts Rh/AC and different Rh/carbon catalystsa Rh/CNTs. This result can be explained by the unique struc- Selectivity (%) ture, larger exposed Rh surface area and good nanoparticle Conversion Internal n/i ratio dispersion of the Rh/RGO catalyst. Catalyst (%) hexene Hexane Heptanal of heptanal As well known, the exposed surface area and dispersion of Rh/RGO 38.4 40.1 2.6 57.3 4.0 Rh nanoparticles are very important to the catalytic activity Rh/AC 3.6 36.8 63.2 —— and n-heptanal selectivity in 1-hexene hydroformylation. Rh/CNTs 10.7 24.1 39.7 36.2 2.8 Therefore, H2 chemisorption was utilized to analyze the prop- a Reaction conditions: 0.10 g of the catalyst, 3.73 g of 1-hexene, 5 erties of the tested catalysts, and Table 2 lists the analysis ° MPa syngas (CO : H2 = 1 : 1), 70 C, 1 h. results. The Rh/RGO catalyst has the largest exposed Rh

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Fig. 11 1-Hexene conversion (black bar) and n/i ratio of heptanal (red bar) of Rh/RGO, Rh/AC and Rh/CNT catalysts: (a) under the same reaction time of 1 h; (b) with similar 1-hexene conversion for different reaction times: Rh/RGO, 1 h; Rh/AC, 2 h; and Rh/CNTs, 2 h. Reaction conditions:

0.10 g of the catalyst, 3.73 g of 1-hexene, 5 MPa syngas (CO : H2 = 1 : 1), 70 °C.

− surface area of 1.60 m2 g 1 and the highest dispersion of catalyst exhibit higher activity than Rh/AC as shown in 18.12% among these catalysts. A similar result is also Table 1. detected from the TEM images shown in Fig. S4.† The layered In addition, the catalyst structure should also be consid- structure and surface functional groups of the GO support ered to elucidate their different catalytic performances. The should be beneficial to the uniform loading of the Rh nano- used AC and CNT supports have complicated physical struc- particles during the Rh/RGO catalyst preparation. The large tures, which in fact suppress the diffusion of reactants and exposed surface area, good dispersion and smaller size of Rh products, especially in the liquid-phase reaction system nanoparticles favor CO adsorption and make CO insertion described here. However, the RGO support differs with AC into CC double bonds more feasible, therefore leading to and CNTs in structure. It has only a 2D structure, that is, a the excellent catalytic performance of the Rh/RGO catalyst in flat nanosheet. Due to the unique sheet structure and abun- 1-hexene hydroformylation. Furthermore, the abundant crys- dant surface functional groups of GO, Rh nanoparticles can tal defects of the Rh/RGO catalyst and synergetic effects be loaded on its surface uniformly, at the same time with a between small building units of Rh nanoparticles in the smaller nanoparticle size, whereby the prepared Rh/RGO cat- cluster-like loose structure (Fig. 5) also provide more active alyst has a larger exposed Rh surface area and fine disper- sites and ensure its higher activity. sion. On the other hand, the 2D structure of the Rh/RGO cat- CO adsorption and the subsequent CO insertion into the alyst will facilitate the diffusion of reactants and products, Published on 16 September 2015. Downloaded by Beijing University 21/12/2015 05:49:39. CC double bonds are the key steps for alkene hydro- considerably better than the Rh/AC and Rh/CNT catalysts. formylation. The activities of the catalysts in CO insertion – can be investigated by CO-TPD.41 43 In this report, CO-TPD was employed to analyze all catalysts, and the results are compared in Fig. 12. The CO desorption peaks are located at 61, 60 and 80 °C for Rh/RGO, Rh/CNT and Rh/AC catalysts, respectively. The desorption temperatures of CO on Rh/RGO and Rh/CNT catalysts are clearly lower than that on Rh/AC, indicating that the CO activation is performed easier on Rh/ RGO and Rh/CNT catalysts. The CO activation is closely related to the catalyst activity for 1-hexene hydroformylation.44 Therefore, the Rh/RGO catalyst and Rh/CNT

Table 2 Textural properties of different carbon material supported Rh catalysts

Exposed Rh surface area Rh dispersion Size − Catalyst (m2 g 1) (%) (nm) Rh/RGO 1.60 18.12 6.1 Rh/AC 0.68 7.76 14.2 Fig. 12 CO-TPD curves of different carbon material supported Rh Rh/CNTs 1.25 14.16 7.8 catalysts.

Paper Catalysis Science & Technology

catalysts indicated that Rh/RGO had the largest exposed Rh surface area, the highest dispersion and the smallest nanoparticle size. 1-Hexene hydroformylation was also performed on these three kinds of carbon material supported Rh cata- Scheme 4 Different schematics of 1-hexene hydroformylation lysts, and their catalytic performances were compared and between the Rh/RGO catalyst and the Rh/AC or Rh/CNT catalyst. discussed. The Rh/RGO catalyst showed the highest catalytic activity of 38.4% and n/i ratio of 4.0, much better than those of the Rh/AC and Rh/CNT catalysts. The excellent catalytic The unsuppressed diffusion of reactants and products on performance of the Rh/RGO catalyst should be attributed to this Rh/RGO catalyst will in part contribute to its good activ- its unique 2D nanosheet structure which determined its larg- ity and regioselectivity.45 1-Hexene can reach the active sites est exposed Rh surface area, highest nanoparticle dispersion of the Rh/RGO catalyst more easily than those of Rh/AC and and smallest nanoparticle size, as well as the unhindered dif- Rh/CNT catalysts, which is beneficial to the improvement of fusion of reactants and products in the liquid-phase 1-hexene

the reaction rate and production of more n-heptanal. More- hydroformylation, along with the lower H2/CO ratio near the over, the products, especially internal hexenes, can diffuse Rh sites. and move away from the Rh/RGO catalyst immediately, reducing the chances of side reactions (internal hexene Acknowledgements hydroformylation) as shown in Scheme 4. Conversely, for Rh/ AC and Rh/CNT catalysts, when these internal hexenes dif- Minghui Tan thanks the China Scholarship Council (CSC) for fuse out of the pores, they will be in contact with other active financial support. sites to produce more i-heptanal leading to a lower n/i ratio of heptanal. References Another important factor in enhancing the n/i heptanal ratio of the Rh/RGO catalyst is the difference between the dif- 1 R. Franke, D. Selent and A. Börner, Chem. Rev., 2012, 112, – fusion rates of H2 and CO on each Rh catalyst. On Rh/AC and 5675 5732. Rh/CNTs with a lot of nanopores filled with liquid medium, 2 P. N. M. van Leeuwen, in Rhodium Catalyzed

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