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Aerosol route synthesis of Ni-CeO2-Al2O3 hybrid nanoparticle cluster for catalysis of reductive amination of polypropylene glycol
Advanced Powder Technology 30 (2019) 2293–2298
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Aerosol route synthesis of Ni-CeO2-Al2O3 hybrid nanoparticle cluster for catalysis of reductive amination of polypropylene glycol Hung-Yen Chang, Guan-Hung Lai, De-Hao Tsai ⇑ Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 22 May 2019 Received in revised form 4 July 2019 Accepted 11 July 2019 Available online 23 July 2019 Keywords: Nanoparticle Nickel Cerium Aluminum Amine
a b s t r a c t We demonstrated an aerosol-based approach to synthesize Ni-CeO2-Al2O3 hybrid nanostructure as a potent nanopowder catalyst for the production of polyetheramine via reductive amination of polypropylene glycol. The method combines a gas-phase evaporation-induced self-assembly with two-stage thermal treatments of the aerosol particles. The hybrid Ni-CeO2 nanoparticles (NPs) composed of ultrafine, homogeneously-distributed nanocrystallites of metallic Ni and ceria were shown to uniformly decorate on the surface of Al2O3 nanoparticle cluster (NPC). The composition, physical size and surface state of the hybrid nanostructure were tunable by design. It was found that hybridization with Al2O3 or CeO2 enhanced catalytic activity of the Ni catalyst. A high yield of 77% of the desired PEA and a high selectivity to primary amine (100%) achieved simultaneously. The surface nitridation of Ni catalyst was effectively suppressed via the incorporation with CeO2 NPs. An enhanced operation stability was observed by using the Ni-CeO2-Al2O3 hybrid nanostructure as catalyst in comparison to the Ni-only NP. The work demonstrated a facile route for controlled gas-phase synthesis of hybrid nanopowder catalysts using Al2O3 NPC as the support matrix and CeO2 NP as the promoter to further enhance the performance of Ni catalyst toward reductive amination. Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Reductive amination of polypropylene glycol (PPG) has shown to be an attractive route for the industrial production of polyetheramines (PEA) [1–4], an important intermediate of petrochemicals and pharmaceutics [1,4,5]. Heterogeneous catalysis using Ni-based hybrid catalysts offers cost-effective access and allows a continuous operation mode for the reductive amination of PPG to PEA [1,6–8]. Aerosol-based synthesis of the Ni-based hybrid nanostructure as the heterogeneous catalyst has shown a promise [9–13]. In comparison to solution-based approach, the major advantages of using the aerosol-based synthetic approach are that (1) it avoids the issues related to the restrictions arouse from solution-based chemistry, for example, substantial limitations in physical and chemical properties of the solvent (e.g., boiling point, solubility to precursor), and (2) using surfactant or other additive for the size and morphological control is not required [11,14–17]. The objective of the study is to use an aerosol-based synthetic method, with two stages of gas-phase thermal treatments, to ⇑ Corresponding author. E-mail address: [email protected] (D.-H. Tsai).
fabricate a Ni-based hybrid nanostructure as catalyst. The design of the hybrid nanostructure is that Al2O3 nanoparticle cluster (NPC) and CeO2 nanoparticle (NP) are chosen as the support material and the promoting additive to the Ni-based catalyst, respectively. During the gas-phase evaporation-induced self-assembly (EISA; as depicted in Scheme 1a), ultrafine Ni crystallites are homogenously dispersed with ceria crystallites in the Ni-CeO2 hybrid NPs [13,18], which are deposited uniformly on the alumina support in form of mesoporous NPC. The Ni dispersion and the accompanied metal-support interface are able to increase significantly [19–21], providing a synergistic route for the catalysis of reductive amination of PPG to PEA (see Scheme 1b) [2–5,7]. A suite of multiple measurement techniques, including scanning electron microscopy (SEM), x-ray diffractometry (XRD), x-ray photoelectron spectroscopy (XPS), NH3 temperature-programmed desorption, Brunauer-Emmett-Teller (BET) surface area and metal surface area analyses are employed to provide a robust and comprehensive analysis of the material properties of the synthesized hybrid nanostructures. Activity, selectivity and cyclic operation stability are investigated, which are correlated with material properties of the synthesized catalysts. The study also aims to provide the mechanistic understanding of the catalysis in the hybrid nanostructure for further enhancing the activity, selectivity and operation
https://doi.org/10.1016/j.apt.2019.07.009 0921-8831/Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
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Nebulizer
Pre-heater
Diffusion Dryer
1st Tube Furnace
2nd Tube Furnace
H2 250 mL/min
800 oC
500 oC Compressed N2 1.5 L/min
NiO Ni precursor Precursor soluon
CeO2
Ni
Ce precursor +NH3 & PPG
Scheme 1. Cartoon depiction of (a) aerosol-based synthesis of Ni-CeO2-Al2O3 hybrid nanoparticle cluster for (b) synergistic catalysis of reductive amination of polypropylene glycol.
stability. To our knowledge, the work reports the first study of synergistic catalysis of reductive amination of PPG using Ni-based hybrid nanostructure synthesized via an aerosol route.
2. Experimental 2.1. Materials & nanoparticle synthesis Nickel (II) nitrate hexahydrate (98%; Showa Chemical Industry Co., Ltd., Tokyo, Japan), cerium (III) nitrate hexahydrate (99%. Showa Chemical Industry Co.), and aluminum oxide nanopowder (13 nm; Uni-Onward Corp., Taipei, Taiwan, ROC) were used as precursors of Ni, CeO2, and Al2O3 in the hybrid nanostructure, respectively. The precursor solution was prepared by dissolving Ni and CeO2 precursors in an aqueous dispersion of Al2O3 nanopowder at pH 3. Glacial acetic acid (>80%; J. T. Baker, Avantor, Allentown, PA, USA) was used for the pH adjustment of the precursor solution. As depicted in Scheme 1a, the Ni-CeO2-Al2O3 hybrid nanostructure is prepared using a customized nebulizer with N2 at a flow rate of 1.5 L/min. The aqueous precursor solution was aerosolized in the form of submicron droplets, which were firstly delivered to an aerosol diffusion drying unit and converted to dried precursor crystallites at their molecular level homogeneity in solutions via the EISA [12,22–25]. Followed by a gas-phase thermal decomposition of precursor particles in the 1st flow reactor at a temperature of 500 °C for 4 s [10,12,26], the dried precursor crystallites were thermally decomposed to hybrid oxide NPs. Here, NiO and CeO2 crystallites were homogeneously dispersed in NPs and uniformly decorated on the surface of the Al2O3 nanoparticle cluster. Selective reduction of NiO to Ni was performed via a H2-based thermal
reduction, where the H2 at a flow rate of 250 mL/min was introduced to mix with the aerosol flow prior to the 2nd stage flow reactor. The temperature of the 2nd stage flow reactor was 800 °C and the retention time was 13 s [26,27]. As a result, the crystallite size and the surface state of the nanocatalyst can be tuned by design in the continuous gas-phase synthesis process [24–27]. Table 1 summarizes the sample identification having different relative weight fractions of Ni, Al2O3 and CeO2 in the hybrid nanostructure (denoted as CNi, CAl and CCe, respectively). 2.2. Material characterization A field emission scanning electron microscope (SEM, Hitachi SU8010, Hitachi, Japan; operated at 10 kV) equipped with an energy dispersive X-ray spectrometer (EDS) was employed for the analyses of morphology, primary diameter, and spatial elemental distribution of the synthesized hybrid nanostructures. A X-ray powder diffractometer (D2 Phaser, Bruker, Massachusetts, USA), with Cu-Ka radiation (k = 1.5406 Å) was used to perform a X-ray diffraction (XRD) analysis operated at 30 kV, 10 mA, and a scanning rate of 2 °/min. A Surface Area Analyzer (ASAP 2060, Micromeritics, Norcross, GA, USA) was employed to obtain the specific surface area (SBET) of the synthesized catalyst, which was calculated by the Brunauer-Emmett-Teller (BET) method in the relative pressure range of 0.05 < P/P0 < 0.3; A CO pulse chemisorption was carried out on a temperature-programmed chemisorption system (ASAP 2920, Micrometrics) to obtain metal surface area (Smsa) and metal dispersion (D) of the sample. The sample was firstly purged with helium at 35C. Then CO pulse of 0.5 cm3 was repeatedly injected
Table 1 Notation of the samples. CNi, CCe and CAl are the relative weight fractions of Ni, CeO2 and Al2O3 in the precursor solution, respectively. Sample
CNi (wt%)
CCe (wt%)
CAl (wt%)
CCe/CNi
CAl/CNi
Al2O3 powder 1Ni-0Ce-0Al 1Ni-0Ce-1Al 1Ni-0.5Ce-0Al 1Ni-0.5Ce-1Al 1Ni-0.5Ce-5Al
0 100 76.2 62.0 52.0 31.5
0 0 0 38.0 31.8 19.3
100 0 23.8 0 16.2 49.1
N/A 0 0 0.61 0.61 0.61
N/A 0 0.31 0 0.31 1.56
H.-Y. Chang et al. / Advanced Powder Technology 30 (2019) 2293–2298
to the sample until no further CO uptake after consecutive injections; The NH3-based temperature-programmed desorption (NH3-TPD) was studied on the same temperature-programmed chemisorption system (ASAP 2920, Micrometrics) for the analysis of the acidity of the synthesized catalyst, which was conducted by starting with a 1-h NH3 adsorption followed by 0.5 h of the gas sweeping using He to remove weakly-bound species. Then desorption of NH3 was carried out up to 900C with a heating rate of 10C/min in a helium flow. The amount of sample used for the above analyses was 100 mg. 2.3. Catalytic performance tests Activity tests of reductive amination of PPG were carried out in a customized 250 mL-autoclave reactor at 230C and 10.34 MPa for 2 h. (0.225–0.713) g of the catalyst powder (i.e., on the basis of the same amount of Ni) was dispersed with 5 g of PPG in the reactor. The partial pressure of H2 was 0.83 MPa at room temperature. The corresponding molar ratios of H2 to PPG and NH3 to PPG were 1.55 and 40.6, respectively. Eq. (1) shows the determination of the conversion ratio of PPG to PEA (XPPG) in auto-titrating the final product (877 Titrino plus, Metrohm Ltd., Herisau, Switzerland) [7],
X PPG ¼ ½ðA 0:1Þ = ð8:6 BÞ
ð1Þ
Here A is the amount of 0.1 mol/L HCl used in the auto-titration (unit: mL), and B is the sample weight of the final product (unit: g). Using Equation (2), the conversion ratio of PPG to the higher degree of amine of PEA (i.e., not primary amine) was obtained,
X PPG
SA
¼ ½ðC 0:1Þ=ð8:6 BÞ
ð2Þ
Here C was the amount of 0.1 mol/L HCl used in this titration (unit: mL), which was determined by adding 0.5 g of salicylaldehyde (SA) with 3 mL of final product and 50 mL of DI water [28]. The selectiv-
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ity of primary amine, ZPA, was then determined using Eq. (3) based on XPPG and XPPG-SA,
Z PA ¼ ð1 X PPG - SA =X PPG Þ 100%
ð3Þ
3. Results and discussion 3.1. Catalyst characterization Fig. 1 shows the representative SEM images with EDS elemental mapping of 1Ni-0.5Ce-0Al and 1Ni-0.5Ce-5Al (additional SEM images with histogram-based analyses were shown in Supplementary Material). Clearly, the morphology of the synthesized catalysts were relatively spherical with homogeneous elemental distributions of Ni and Ce in the hybrid nanostructure. The results imply a successful creation of the Ni-Ce-O interfaces in both of the hybrid nanostructures. The mesoporous structure of 1Ni-0.5Ce-5Al was observed, indicating a successful formation of Al2O3 nanoparticle cluster (NPC) as support material of Ni-CeO2 hybrid nanoparticle (NP) via gas-phase EISA. Fig. 2 shows the XRD patterns of the synthesized catalysts. For 1Ni-0Ce-0Al, only metallic Ni crystalline was shown in the XRD pattern, indicating a successful H2-reduction of NiO to Ni in the gas phase. After the addition of Ce precursor (1Ni-0.5Ce-0Al), the crystalline of CeO2 was shown with the crystalline of metallic Ni in the diffractogram. With a further addition of Al2O3 NP in the precursor solution (i.e., as a NP dispersion), the metallic Ni presented altogether with the crystalline CeO2 and Al2O3, suggesting that the selective reduction of NiO to Ni achieved in the Ni-CeO2 and Ni-CeO2-Al2O3 hybrid nanostructures (i.e., without altering the oxidation states of Ce and Al). By calculation, the crystallite size of Ni (dc,Ni) of 1Ni-0Ce-0Al, 1Ni-0Ce-1Al, 1Ni-0.5Ce-0Al 1Ni-0.5Ce-1Al, and 1Ni-0.5Ce-5Al were 21.3 nm, 17.0 nm, 14.3 nm, 15.6 nm and
Fig. 1. Representative SEM images with EDS elemental mapping. (a) 1Ni-0.5Ce-0Al; (b) 1Ni-0.5Ce-5Al.
H.-Y. Chang et al. / Advanced Powder Technology 30 (2019) 2293–2298
Ni
Al2O3
CeO2
1Ni-0Ce-0Al
Intensity (a.u.)
1Ni-0Ce-1Al
1Ni-0.5Ce-0Al
NH3 desorption (a.u.)
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1Ni-0.5Ce-5Al 1Ni-0.5Ce-1Al 1Ni-0.5Ce-0Al 1Ni-0Ce-1Al 1Ni-0Ce-0Al
0
100
200
300
400
500
600
700
800
900
Temperature ( )
1Ni-0.5Ce-1Al
Fig. 3. NH3-TPD analyses of the synthesized catalysts.
1Ni-0.5Ce-5Al
30
40
50
2 theta
60
70
80
(o)
Fig. 2. XRD patterns of the synthesized catalysts.
6.8 nm, respectively (i.e., using the peak of 2h = 44) [7,26]. The results illustrate that utilization of Al2O3 NPC effectively reduced the dc,Ni of Ni catalyst, and the effect was possibly attributed to the improvement of Ni dispersion and also the inhibition of the sintering of Ni crystallites during the two stages of thermal treatments in the gas phase (i.e., thermal decomposition and H2-reduction). Table 2 summarizes SBET, Smsa and D of the synthesized samples. In comparison to 1Ni-0Ce-0Al (Ni-only sample) having a SBET of 32.6 m2/g and a Smsa of 0.1 m2/g, both SBET and Smsa of the samples increased after the hybridization with the Al2O3 NPC and/or CeO2 NP. At a constant CCe/CNi (=0.61), SBET increased from 55.8 m2/g to 74.9 m2/g and 101.9 m2/g, respectively, by increasing CAl/CNi from 0 to 0.31 and 1.56, respectively. Simultaneously, Smsa increased from 2.8 m2/g to 3.7 m2/g and 10.9 m2/g by increasing CAl/CNi from 0 to 0.31 and 1.56, respectively. The results show that adding CeO2 or Al2O3 increases surface area of the synthesized catalyst, which was attributed to the inhibition of the sintering of metallic Ni during the aerosol-based synthesis process (i.e., two stages of thermal treatments). Therefore, both CeO2 and Al2O3 are beneficial to improve the metal dispersion (D) in the hybrid nanostructure. Fig. 3 shows the NH3-TPD analyses of the synthesized catalysts. Except for the 1Ni-0Ce-0Al, three distinct bands, a, b and c peaks, were identified in the other four samples. The a and b peaks were considered as the low acidic site, and the intensity of the peak were mainly correlated to surface area of the catalyst; c peak repre-
3.2. Catalytic activity test Fig. 4 shows the conversion ratios of PPG (XPPG) and the selectivity of primary amine (ZPA) catalyzed by the synthesized samples after 2-h reaction. The XPPG was only 34.3% for the 1Ni-0Ce-0Al (i.e., the Ni-only sample), and it surged to 42.0% and 62.3% after addition of Al2O3 NPC (1Ni-0Ce-1Al) and CeO2 (1Ni-0.5Ce-0Al), respectively. At a constant CCe/CNi (=0.61), the XPPG further increased from 62.3% to 74.2% and 77.9% by increasing CAl/CNi from 0 to 0.31 and 1.56, respectively. The results indicate that addition of CeO2 and Al2O3 NPC to the Ni-based catalysts was beneficial to catalytic activity, which was shown to be proportional to CAl. Note that ZPA was shown to be very high (98.5% to 99.7%) catalysed by the synthesized catalysts. The uncertainties of XPPG and ZPA were < 1.9% and <0.8% to the mean values, respectively.
90 80 70 60 50
Table 2 BET surface area (SBET), metal surface area (Smsa) and metal dispersion of the catalysts. Sample
SBET (m2/g)
Smsa (m2/g)
D (%)
Al2O3 powder 1Ni-0Ce-0Al 1Ni-0Ce-1Al 1Ni-0.5Ce-0Al 1Ni-0.5Ce-1Al 1Ni-0.5Ce-5Al
95.6 32.6 67.8 55.8 74.9 101.9
N/A 0.1 6.3 2.8 3.7 10.9
N/A 0.02 1.24 0.69 1.16 7.28
100
XPPG ZPA
80 60 40
40
20
30
0
Z PA (%)
20
sented the relative high acidic site, and its peak temperature was strongly affected by the acidity of the catalyst surface. The results show that (1) addition of CeO2 and/or Al2O3 NPC increased the acidity of the Ni catalyst, (2) CeO2 was more acidic than Al2O3 due to the fact that 1Ni-0.5Ce-0Al has a higher c peak temperature than 1Ni-0Ce-1Al, and (3) surface area of the catalyst increased with CAl in the Ni-CeO2 hybrid nanostructure, as evidenced by the increase of the peak areas of a, b and c peaks. Note that no distinct desorption band was identified for 1Ni-0Ce-0Al, which was possibly attributed to the low surface area.
X PPG (%)
Al2O3
Fig. 4. The conversion ratios of PPG (XPPG) and the selectivity of primary amine (ZPA) catalyzed by the synthesized samples after one cycle of 2-h reaction.
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Fig. 5 shows the XRD patterns of the samples after one cycle of the 2-h reaction. Only metallic Ni crystalline was identified for all samples after the activity tests, indicating that the amount of oxidized Ni was insignificant. The dc,Ni were 21.6 nm, 20.7 nm, 15.1 nm, 14.4 nm and 6.3 nm for 1Ni-0Ce-0Al, 1Ni-0Ce-1Al, 1Ni-0.5Ce-0Al, 1Ni-0.5Ce-1Al, and 1Ni-0.5Ce-5Al, respectively (as summarized in Table 3), illustrating that the sintering of Ni crystallites during the catalysis was negligible.
Ni
Al2O3
Ni3N
CeO2
Intensity (a.u.)
1Ni-0Ce-0Al 1Ni-0Ce-1Al 1Ni-0.5Ce-0Al
3.3. Cyclic operation stability
1Ni-0.5Ce-1Al 1Ni-0.5Ce-5Al
30
40
50
2 theta
60
70
80
(o)
Fig. 5. XRD patterns of the catalysts after one cycle of the 2-h reaction.
Table 3 Summary of dc,Ni before and after one cycle of catalysis. Sample
dc,Ni (nm) before catalysis
dc,Ni (nm) after catalysis
1Ni-0Ce-0Al 1Ni-0Ce-1Al 1Ni-0.5Ce-0Al 1Ni-0.5Ce-1Al 1Ni-0.5Ce-5Al
21.3 17.0 14.3 15.6 6.8
21.6 20.7 15.1 14.4 6.3
90
Fig. 6 shows the XPPG and ZPA of three cycles of activity tests catalyzed by Ni-only NP (1Ni-0Ce-0Al) and Ni-CeO2-Al2O3 NPC (1Ni-0.5Ce-5Al). The XPPG catalyzed by 1Ni-0Ce-0Al was 34.2% for the 2nd cycle and 30.7% for the 3rd cycle tests, which was shown to be 11.2% and 20.3% lower than the 1st cycle test (XPPG = 38.5%), respectively. In comparison, the XPPG catalyzed by 1Ni-0.5Ce-5Al remained remarkably higher than by 1Ni-0Ce-0Al over three cycles of activity tests. The XPPG was 70.3% in the 2nd cycle and was 68.0% in the 3rd cycle tests catalyzed by 1Ni-0.5Ce-5Al, which was 7.7% and 10.8% lower than the 1st cycle, respectively. The results show that the cyclic operation stability was shown to be higher by using the Ni-CeO2-Al2O3 NPC as catalyst than by the Ni-only NP. Note that the dc,Ni were 21.4 nm and 11.0 nm for 1Ni-0Ce-0Al and 1Ni-0.5Ce-5Al, respectively, after 3-cycle catalysis, indicating that the sintering of Ni crystallites was negligible (see XRD patterns in Supplementary Material). The formation of Ni3N crystalline was also identified for 1Ni-0Ce-0Al, and the nitridation of Ni catalyst was shown to be negligible catalyzed by 1Ni-0.5Ce-5Al. The ZPA was also shown to slightly decrease over the three cycles of activity tests. For 1Ni-0Ce-0Al, the ZPA decreased to
100
80
XPPG (%)
70 60 50
80 60 40
Z PA (%)
20
For 1Ni-0Ce-0Al and 1Ni-0Ce-1Al, the formation of Ni3N crystalline was identified in the XRD patterns [i.e., 2h = 39.5, 41.9, 58.7, 71.4and 78.3; using JCPDS 89-5144] [7]. The nitridation was attributed to strong adsorption of ammonia on the surface of Ni catalysts, which was identified as a major deactivation route of the Ni catalyst [4,7,29–31]. In contrast, the Ni3N crystalline was unable to be identified in the XRD patterns of 1Ni-0.5Ce-0Al, 1Ni-0.5Ce-1Al, and 1Ni-0.5Ce-5Al. The results indicate that addition of CeO2 suppressed the surface nitridation of Ni catalyst during the activity tests, and the enhancement in the material stability of Ni catalyst was possibly attributed to the modification of its surface acidity [4]. As indicated in the NH3-TPD study (Fig. 3), CeO2 NP increased the surface acidity of the Ni-only NP and the Ni-Al2O3 NPC. As a consequence, the binding of ammonia on Ni surface was weakened especially at the Ni-Ce-O interface, which prevented irreversible adsorption of ammonia and amines in the hybrid nanostructure. To our knowledge, the work reports the first time that the enhanced catalytic activity in the reductive amination was correlated with material stability via the creation of Ni-Ce-O interfaces uniformly decorated on the mesoporous Al2O3 NPC.
40 30 20
20 0
Fig. 6. Activity tests over three cycles of the reductive amination of PPG using 1Ni-0Ce-0Al and 1Ni-0.5Ce-5Al as catalysts.
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97.3% and 94.6% at the 2nd and the 3rd cycles of activity tests, respectively. For 1Ni-0Ce-0Al, the ZPA was also slightly decreased to 98.4% and 95.4% over the two and three cycles of activity tests, respectively. The decline of ZPA was possibly attributed to the adsorbed products in the 1st cycle turning to be further aminated in the following 2nd and 3rd cycles. The presence of the adsorbed amine product were identified by the XPS study (i.e., the presence of nitrogen compound after the activity test; see Supplementary Material) and confirmed by the SEM analyses (i.e., formation of fibres on the surface of catalyst; see Supplementary Material). 4. Conclusion A hybrid nanostructure, Ni-CeO2-Al2O3 nanoparticle cluster, is successfully developed using an aerosol-based synthesis approach: gas-phase evaporation-induced self-assembly followed by twostage gas-phase thermal treatments. Using Al2O3 nanoparticle cluster as the support matrix and CeO2 nanoparticle as the promoting additive, ultrafine Ni crystallite size, large surface area, controlled chemical composition, physical size and surface state achieve simultaneously via the proposed aerosol-phase route. A superior catalytic performance is observed for the Ni-CeO2-Al2O3 hybrid nanoparticle cluster toward reductive amination of polypropylene glycol to polyetheramine: high activity (78%) and selectivity (100%). The strong metal-support interface reaction at the Ni-Ce-O interface is proposed to effectively suppress surface nitridation of Ni and subsequently to improve catalytic activity and operation stability of the Ni catalyst. The work demonstrated a promising strategy for the synthesis of Ni-based hybrid nanoparticle cluster as a high-performance catalyst with mechanistic understanding of the catalysis of reductive amination of polypropylene glycol in the hybrid nanostructure by design. Acknowledgements The authors thank Ministry of Science and Technology (MOST) of Taiwan, R.O.C. for financial support under Contract MOST 1062622-8-007-017 & MOST 107-2628-E-007-002-MY3. The authors also thank Huan-Ming Chang, Chuen-Lih Hwang, Chun-Yu Lee, and Chih-Cheng Chia of Chang Chun PetroChemical Co. Ltd., Taiwan, R.O.C. for helpful discussion and experimental support. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.apt.2019.07.009. References [1] K.-I. Shimizu, K. Kon, W. Onodera, H. Yamazaki, J.N. Kondo, Heterogeneous Ni catalyst for direct synthesis of primary amines from alcohols and ammonia, ACS Catal. 3 (2012) 112–117. [2] Y. Liu, K. Zhou, H. Shu, H. Liu, J. Lou, D. Guo, Z. Wei, X. Li, Switchable synthesis of furfurylamine and tetrahydrofurfurylamine from furfuryl alcohol over Ò RANEY nickel, Catal. Sci. Technol. 7 (2017) 4129–4135. [3] D. Ruiz, A. Aho, P. Mäki-Arvela, N. Kumar, H. Oliva, D.Y. Murzin, Direct amination of dodecanol over noble and transition metal supported silica catalysts, Ind. Eng. Chem. Res. 56 (2017) 12878–12887. [4] K. Kim, Y. Choi, H. Lee, J.W. Lee, Y2O3-Inserted Co-Pd/zeolite catalysts for reductive amination of polypropylene glycol, Appl. Catal. A 568 (2018) 114– 122. [5] D. Ruiz, A. Aho, T. Saloranta, K. Eränen, J. Wärnå, R. Leino, D.Y. Murzin, Direct amination of dodecanol with NH 3 over heterogeneous catalysts catalyst screening and kinetic modelling, Chem. Eng. J. 307 (2017) 739–749.
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Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Aerosol route synthesis of Ni-CeO2-Al2O3 hybrid nanoparticle cluster for catalysis of reductive amination of polypropylene glycol Hung-Yen Chang, Guan-Hung Lai, De-Hao Tsai ⇑ Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 22 May 2019 Received in revised form 4 July 2019 Accepted 11 July 2019 Available online 23 July 2019 Keywords: Nanoparticle Nickel Cerium Aluminum Amine
a b s t r a c t We demonstrated an aerosol-based approach to synthesize Ni-CeO2-Al2O3 hybrid nanostructure as a potent nanopowder catalyst for the production of polyetheramine via reductive amination of polypropylene glycol. The method combines a gas-phase evaporation-induced self-assembly with two-stage thermal treatments of the aerosol particles. The hybrid Ni-CeO2 nanoparticles (NPs) composed of ultrafine, homogeneously-distributed nanocrystallites of metallic Ni and ceria were shown to uniformly decorate on the surface of Al2O3 nanoparticle cluster (NPC). The composition, physical size and surface state of the hybrid nanostructure were tunable by design. It was found that hybridization with Al2O3 or CeO2 enhanced catalytic activity of the Ni catalyst. A high yield of 77% of the desired PEA and a high selectivity to primary amine (100%) achieved simultaneously. The surface nitridation of Ni catalyst was effectively suppressed via the incorporation with CeO2 NPs. An enhanced operation stability was observed by using the Ni-CeO2-Al2O3 hybrid nanostructure as catalyst in comparison to the Ni-only NP. The work demonstrated a facile route for controlled gas-phase synthesis of hybrid nanopowder catalysts using Al2O3 NPC as the support matrix and CeO2 NP as the promoter to further enhance the performance of Ni catalyst toward reductive amination. Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Reductive amination of polypropylene glycol (PPG) has shown to be an attractive route for the industrial production of polyetheramines (PEA) [1–4], an important intermediate of petrochemicals and pharmaceutics [1,4,5]. Heterogeneous catalysis using Ni-based hybrid catalysts offers cost-effective access and allows a continuous operation mode for the reductive amination of PPG to PEA [1,6–8]. Aerosol-based synthesis of the Ni-based hybrid nanostructure as the heterogeneous catalyst has shown a promise [9–13]. In comparison to solution-based approach, the major advantages of using the aerosol-based synthetic approach are that (1) it avoids the issues related to the restrictions arouse from solution-based chemistry, for example, substantial limitations in physical and chemical properties of the solvent (e.g., boiling point, solubility to precursor), and (2) using surfactant or other additive for the size and morphological control is not required [11,14–17]. The objective of the study is to use an aerosol-based synthetic method, with two stages of gas-phase thermal treatments, to ⇑ Corresponding author. E-mail address: [email protected] (D.-H. Tsai).
fabricate a Ni-based hybrid nanostructure as catalyst. The design of the hybrid nanostructure is that Al2O3 nanoparticle cluster (NPC) and CeO2 nanoparticle (NP) are chosen as the support material and the promoting additive to the Ni-based catalyst, respectively. During the gas-phase evaporation-induced self-assembly (EISA; as depicted in Scheme 1a), ultrafine Ni crystallites are homogenously dispersed with ceria crystallites in the Ni-CeO2 hybrid NPs [13,18], which are deposited uniformly on the alumina support in form of mesoporous NPC. The Ni dispersion and the accompanied metal-support interface are able to increase significantly [19–21], providing a synergistic route for the catalysis of reductive amination of PPG to PEA (see Scheme 1b) [2–5,7]. A suite of multiple measurement techniques, including scanning electron microscopy (SEM), x-ray diffractometry (XRD), x-ray photoelectron spectroscopy (XPS), NH3 temperature-programmed desorption, Brunauer-Emmett-Teller (BET) surface area and metal surface area analyses are employed to provide a robust and comprehensive analysis of the material properties of the synthesized hybrid nanostructures. Activity, selectivity and cyclic operation stability are investigated, which are correlated with material properties of the synthesized catalysts. The study also aims to provide the mechanistic understanding of the catalysis in the hybrid nanostructure for further enhancing the activity, selectivity and operation
https://doi.org/10.1016/j.apt.2019.07.009 0921-8831/Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
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Nebulizer
Pre-heater
Diffusion Dryer
1st Tube Furnace
2nd Tube Furnace
H2 250 mL/min
800 oC
500 oC Compressed N2 1.5 L/min
NiO Ni precursor Precursor soluon
CeO2
Ni
Ce precursor +NH3 & PPG
Scheme 1. Cartoon depiction of (a) aerosol-based synthesis of Ni-CeO2-Al2O3 hybrid nanoparticle cluster for (b) synergistic catalysis of reductive amination of polypropylene glycol.
stability. To our knowledge, the work reports the first study of synergistic catalysis of reductive amination of PPG using Ni-based hybrid nanostructure synthesized via an aerosol route.
2. Experimental 2.1. Materials & nanoparticle synthesis Nickel (II) nitrate hexahydrate (98%; Showa Chemical Industry Co., Ltd., Tokyo, Japan), cerium (III) nitrate hexahydrate (99%. Showa Chemical Industry Co.), and aluminum oxide nanopowder (13 nm; Uni-Onward Corp., Taipei, Taiwan, ROC) were used as precursors of Ni, CeO2, and Al2O3 in the hybrid nanostructure, respectively. The precursor solution was prepared by dissolving Ni and CeO2 precursors in an aqueous dispersion of Al2O3 nanopowder at pH 3. Glacial acetic acid (>80%; J. T. Baker, Avantor, Allentown, PA, USA) was used for the pH adjustment of the precursor solution. As depicted in Scheme 1a, the Ni-CeO2-Al2O3 hybrid nanostructure is prepared using a customized nebulizer with N2 at a flow rate of 1.5 L/min. The aqueous precursor solution was aerosolized in the form of submicron droplets, which were firstly delivered to an aerosol diffusion drying unit and converted to dried precursor crystallites at their molecular level homogeneity in solutions via the EISA [12,22–25]. Followed by a gas-phase thermal decomposition of precursor particles in the 1st flow reactor at a temperature of 500 °C for 4 s [10,12,26], the dried precursor crystallites were thermally decomposed to hybrid oxide NPs. Here, NiO and CeO2 crystallites were homogeneously dispersed in NPs and uniformly decorated on the surface of the Al2O3 nanoparticle cluster. Selective reduction of NiO to Ni was performed via a H2-based thermal
reduction, where the H2 at a flow rate of 250 mL/min was introduced to mix with the aerosol flow prior to the 2nd stage flow reactor. The temperature of the 2nd stage flow reactor was 800 °C and the retention time was 13 s [26,27]. As a result, the crystallite size and the surface state of the nanocatalyst can be tuned by design in the continuous gas-phase synthesis process [24–27]. Table 1 summarizes the sample identification having different relative weight fractions of Ni, Al2O3 and CeO2 in the hybrid nanostructure (denoted as CNi, CAl and CCe, respectively). 2.2. Material characterization A field emission scanning electron microscope (SEM, Hitachi SU8010, Hitachi, Japan; operated at 10 kV) equipped with an energy dispersive X-ray spectrometer (EDS) was employed for the analyses of morphology, primary diameter, and spatial elemental distribution of the synthesized hybrid nanostructures. A X-ray powder diffractometer (D2 Phaser, Bruker, Massachusetts, USA), with Cu-Ka radiation (k = 1.5406 Å) was used to perform a X-ray diffraction (XRD) analysis operated at 30 kV, 10 mA, and a scanning rate of 2 °/min. A Surface Area Analyzer (ASAP 2060, Micromeritics, Norcross, GA, USA) was employed to obtain the specific surface area (SBET) of the synthesized catalyst, which was calculated by the Brunauer-Emmett-Teller (BET) method in the relative pressure range of 0.05 < P/P0 < 0.3; A CO pulse chemisorption was carried out on a temperature-programmed chemisorption system (ASAP 2920, Micrometrics) to obtain metal surface area (Smsa) and metal dispersion (D) of the sample. The sample was firstly purged with helium at 35C. Then CO pulse of 0.5 cm3 was repeatedly injected
Table 1 Notation of the samples. CNi, CCe and CAl are the relative weight fractions of Ni, CeO2 and Al2O3 in the precursor solution, respectively. Sample
CNi (wt%)
CCe (wt%)
CAl (wt%)
CCe/CNi
CAl/CNi
Al2O3 powder 1Ni-0Ce-0Al 1Ni-0Ce-1Al 1Ni-0.5Ce-0Al 1Ni-0.5Ce-1Al 1Ni-0.5Ce-5Al
0 100 76.2 62.0 52.0 31.5
0 0 0 38.0 31.8 19.3
100 0 23.8 0 16.2 49.1
N/A 0 0 0.61 0.61 0.61
N/A 0 0.31 0 0.31 1.56
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to the sample until no further CO uptake after consecutive injections; The NH3-based temperature-programmed desorption (NH3-TPD) was studied on the same temperature-programmed chemisorption system (ASAP 2920, Micrometrics) for the analysis of the acidity of the synthesized catalyst, which was conducted by starting with a 1-h NH3 adsorption followed by 0.5 h of the gas sweeping using He to remove weakly-bound species. Then desorption of NH3 was carried out up to 900C with a heating rate of 10C/min in a helium flow. The amount of sample used for the above analyses was 100 mg. 2.3. Catalytic performance tests Activity tests of reductive amination of PPG were carried out in a customized 250 mL-autoclave reactor at 230C and 10.34 MPa for 2 h. (0.225–0.713) g of the catalyst powder (i.e., on the basis of the same amount of Ni) was dispersed with 5 g of PPG in the reactor. The partial pressure of H2 was 0.83 MPa at room temperature. The corresponding molar ratios of H2 to PPG and NH3 to PPG were 1.55 and 40.6, respectively. Eq. (1) shows the determination of the conversion ratio of PPG to PEA (XPPG) in auto-titrating the final product (877 Titrino plus, Metrohm Ltd., Herisau, Switzerland) [7],
X PPG ¼ ½ðA 0:1Þ = ð8:6 BÞ
ð1Þ
Here A is the amount of 0.1 mol/L HCl used in the auto-titration (unit: mL), and B is the sample weight of the final product (unit: g). Using Equation (2), the conversion ratio of PPG to the higher degree of amine of PEA (i.e., not primary amine) was obtained,
X PPG
SA
¼ ½ðC 0:1Þ=ð8:6 BÞ
ð2Þ
Here C was the amount of 0.1 mol/L HCl used in this titration (unit: mL), which was determined by adding 0.5 g of salicylaldehyde (SA) with 3 mL of final product and 50 mL of DI water [28]. The selectiv-
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ity of primary amine, ZPA, was then determined using Eq. (3) based on XPPG and XPPG-SA,
Z PA ¼ ð1 X PPG - SA =X PPG Þ 100%
ð3Þ
3. Results and discussion 3.1. Catalyst characterization Fig. 1 shows the representative SEM images with EDS elemental mapping of 1Ni-0.5Ce-0Al and 1Ni-0.5Ce-5Al (additional SEM images with histogram-based analyses were shown in Supplementary Material). Clearly, the morphology of the synthesized catalysts were relatively spherical with homogeneous elemental distributions of Ni and Ce in the hybrid nanostructure. The results imply a successful creation of the Ni-Ce-O interfaces in both of the hybrid nanostructures. The mesoporous structure of 1Ni-0.5Ce-5Al was observed, indicating a successful formation of Al2O3 nanoparticle cluster (NPC) as support material of Ni-CeO2 hybrid nanoparticle (NP) via gas-phase EISA. Fig. 2 shows the XRD patterns of the synthesized catalysts. For 1Ni-0Ce-0Al, only metallic Ni crystalline was shown in the XRD pattern, indicating a successful H2-reduction of NiO to Ni in the gas phase. After the addition of Ce precursor (1Ni-0.5Ce-0Al), the crystalline of CeO2 was shown with the crystalline of metallic Ni in the diffractogram. With a further addition of Al2O3 NP in the precursor solution (i.e., as a NP dispersion), the metallic Ni presented altogether with the crystalline CeO2 and Al2O3, suggesting that the selective reduction of NiO to Ni achieved in the Ni-CeO2 and Ni-CeO2-Al2O3 hybrid nanostructures (i.e., without altering the oxidation states of Ce and Al). By calculation, the crystallite size of Ni (dc,Ni) of 1Ni-0Ce-0Al, 1Ni-0Ce-1Al, 1Ni-0.5Ce-0Al 1Ni-0.5Ce-1Al, and 1Ni-0.5Ce-5Al were 21.3 nm, 17.0 nm, 14.3 nm, 15.6 nm and
Fig. 1. Representative SEM images with EDS elemental mapping. (a) 1Ni-0.5Ce-0Al; (b) 1Ni-0.5Ce-5Al.
H.-Y. Chang et al. / Advanced Powder Technology 30 (2019) 2293–2298
Ni
Al2O3
CeO2
1Ni-0Ce-0Al
Intensity (a.u.)
1Ni-0Ce-1Al
1Ni-0.5Ce-0Al
NH3 desorption (a.u.)
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1Ni-0.5Ce-5Al 1Ni-0.5Ce-1Al 1Ni-0.5Ce-0Al 1Ni-0Ce-1Al 1Ni-0Ce-0Al
0
100
200
300
400
500
600
700
800
900
Temperature ( )
1Ni-0.5Ce-1Al
Fig. 3. NH3-TPD analyses of the synthesized catalysts.
1Ni-0.5Ce-5Al
30
40
50
2 theta
60
70
80
(o)
Fig. 2. XRD patterns of the synthesized catalysts.
6.8 nm, respectively (i.e., using the peak of 2h = 44) [7,26]. The results illustrate that utilization of Al2O3 NPC effectively reduced the dc,Ni of Ni catalyst, and the effect was possibly attributed to the improvement of Ni dispersion and also the inhibition of the sintering of Ni crystallites during the two stages of thermal treatments in the gas phase (i.e., thermal decomposition and H2-reduction). Table 2 summarizes SBET, Smsa and D of the synthesized samples. In comparison to 1Ni-0Ce-0Al (Ni-only sample) having a SBET of 32.6 m2/g and a Smsa of 0.1 m2/g, both SBET and Smsa of the samples increased after the hybridization with the Al2O3 NPC and/or CeO2 NP. At a constant CCe/CNi (=0.61), SBET increased from 55.8 m2/g to 74.9 m2/g and 101.9 m2/g, respectively, by increasing CAl/CNi from 0 to 0.31 and 1.56, respectively. Simultaneously, Smsa increased from 2.8 m2/g to 3.7 m2/g and 10.9 m2/g by increasing CAl/CNi from 0 to 0.31 and 1.56, respectively. The results show that adding CeO2 or Al2O3 increases surface area of the synthesized catalyst, which was attributed to the inhibition of the sintering of metallic Ni during the aerosol-based synthesis process (i.e., two stages of thermal treatments). Therefore, both CeO2 and Al2O3 are beneficial to improve the metal dispersion (D) in the hybrid nanostructure. Fig. 3 shows the NH3-TPD analyses of the synthesized catalysts. Except for the 1Ni-0Ce-0Al, three distinct bands, a, b and c peaks, were identified in the other four samples. The a and b peaks were considered as the low acidic site, and the intensity of the peak were mainly correlated to surface area of the catalyst; c peak repre-
3.2. Catalytic activity test Fig. 4 shows the conversion ratios of PPG (XPPG) and the selectivity of primary amine (ZPA) catalyzed by the synthesized samples after 2-h reaction. The XPPG was only 34.3% for the 1Ni-0Ce-0Al (i.e., the Ni-only sample), and it surged to 42.0% and 62.3% after addition of Al2O3 NPC (1Ni-0Ce-1Al) and CeO2 (1Ni-0.5Ce-0Al), respectively. At a constant CCe/CNi (=0.61), the XPPG further increased from 62.3% to 74.2% and 77.9% by increasing CAl/CNi from 0 to 0.31 and 1.56, respectively. The results indicate that addition of CeO2 and Al2O3 NPC to the Ni-based catalysts was beneficial to catalytic activity, which was shown to be proportional to CAl. Note that ZPA was shown to be very high (98.5% to 99.7%) catalysed by the synthesized catalysts. The uncertainties of XPPG and ZPA were < 1.9% and <0.8% to the mean values, respectively.
90 80 70 60 50
Table 2 BET surface area (SBET), metal surface area (Smsa) and metal dispersion of the catalysts. Sample
SBET (m2/g)
Smsa (m2/g)
D (%)
Al2O3 powder 1Ni-0Ce-0Al 1Ni-0Ce-1Al 1Ni-0.5Ce-0Al 1Ni-0.5Ce-1Al 1Ni-0.5Ce-5Al
95.6 32.6 67.8 55.8 74.9 101.9
N/A 0.1 6.3 2.8 3.7 10.9
N/A 0.02 1.24 0.69 1.16 7.28
100
XPPG ZPA
80 60 40
40
20
30
0
Z PA (%)
20
sented the relative high acidic site, and its peak temperature was strongly affected by the acidity of the catalyst surface. The results show that (1) addition of CeO2 and/or Al2O3 NPC increased the acidity of the Ni catalyst, (2) CeO2 was more acidic than Al2O3 due to the fact that 1Ni-0.5Ce-0Al has a higher c peak temperature than 1Ni-0Ce-1Al, and (3) surface area of the catalyst increased with CAl in the Ni-CeO2 hybrid nanostructure, as evidenced by the increase of the peak areas of a, b and c peaks. Note that no distinct desorption band was identified for 1Ni-0Ce-0Al, which was possibly attributed to the low surface area.
X PPG (%)
Al2O3
Fig. 4. The conversion ratios of PPG (XPPG) and the selectivity of primary amine (ZPA) catalyzed by the synthesized samples after one cycle of 2-h reaction.
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Fig. 5 shows the XRD patterns of the samples after one cycle of the 2-h reaction. Only metallic Ni crystalline was identified for all samples after the activity tests, indicating that the amount of oxidized Ni was insignificant. The dc,Ni were 21.6 nm, 20.7 nm, 15.1 nm, 14.4 nm and 6.3 nm for 1Ni-0Ce-0Al, 1Ni-0Ce-1Al, 1Ni-0.5Ce-0Al, 1Ni-0.5Ce-1Al, and 1Ni-0.5Ce-5Al, respectively (as summarized in Table 3), illustrating that the sintering of Ni crystallites during the catalysis was negligible.
Ni
Al2O3
Ni3N
CeO2
Intensity (a.u.)
1Ni-0Ce-0Al 1Ni-0Ce-1Al 1Ni-0.5Ce-0Al
3.3. Cyclic operation stability
1Ni-0.5Ce-1Al 1Ni-0.5Ce-5Al
30
40
50
2 theta
60
70
80
(o)
Fig. 5. XRD patterns of the catalysts after one cycle of the 2-h reaction.
Table 3 Summary of dc,Ni before and after one cycle of catalysis. Sample
dc,Ni (nm) before catalysis
dc,Ni (nm) after catalysis
1Ni-0Ce-0Al 1Ni-0Ce-1Al 1Ni-0.5Ce-0Al 1Ni-0.5Ce-1Al 1Ni-0.5Ce-5Al
21.3 17.0 14.3 15.6 6.8
21.6 20.7 15.1 14.4 6.3
90
Fig. 6 shows the XPPG and ZPA of three cycles of activity tests catalyzed by Ni-only NP (1Ni-0Ce-0Al) and Ni-CeO2-Al2O3 NPC (1Ni-0.5Ce-5Al). The XPPG catalyzed by 1Ni-0Ce-0Al was 34.2% for the 2nd cycle and 30.7% for the 3rd cycle tests, which was shown to be 11.2% and 20.3% lower than the 1st cycle test (XPPG = 38.5%), respectively. In comparison, the XPPG catalyzed by 1Ni-0.5Ce-5Al remained remarkably higher than by 1Ni-0Ce-0Al over three cycles of activity tests. The XPPG was 70.3% in the 2nd cycle and was 68.0% in the 3rd cycle tests catalyzed by 1Ni-0.5Ce-5Al, which was 7.7% and 10.8% lower than the 1st cycle, respectively. The results show that the cyclic operation stability was shown to be higher by using the Ni-CeO2-Al2O3 NPC as catalyst than by the Ni-only NP. Note that the dc,Ni were 21.4 nm and 11.0 nm for 1Ni-0Ce-0Al and 1Ni-0.5Ce-5Al, respectively, after 3-cycle catalysis, indicating that the sintering of Ni crystallites was negligible (see XRD patterns in Supplementary Material). The formation of Ni3N crystalline was also identified for 1Ni-0Ce-0Al, and the nitridation of Ni catalyst was shown to be negligible catalyzed by 1Ni-0.5Ce-5Al. The ZPA was also shown to slightly decrease over the three cycles of activity tests. For 1Ni-0Ce-0Al, the ZPA decreased to
100
80
XPPG (%)
70 60 50
80 60 40
Z PA (%)
20
For 1Ni-0Ce-0Al and 1Ni-0Ce-1Al, the formation of Ni3N crystalline was identified in the XRD patterns [i.e., 2h = 39.5, 41.9, 58.7, 71.4and 78.3; using JCPDS 89-5144] [7]. The nitridation was attributed to strong adsorption of ammonia on the surface of Ni catalysts, which was identified as a major deactivation route of the Ni catalyst [4,7,29–31]. In contrast, the Ni3N crystalline was unable to be identified in the XRD patterns of 1Ni-0.5Ce-0Al, 1Ni-0.5Ce-1Al, and 1Ni-0.5Ce-5Al. The results indicate that addition of CeO2 suppressed the surface nitridation of Ni catalyst during the activity tests, and the enhancement in the material stability of Ni catalyst was possibly attributed to the modification of its surface acidity [4]. As indicated in the NH3-TPD study (Fig. 3), CeO2 NP increased the surface acidity of the Ni-only NP and the Ni-Al2O3 NPC. As a consequence, the binding of ammonia on Ni surface was weakened especially at the Ni-Ce-O interface, which prevented irreversible adsorption of ammonia and amines in the hybrid nanostructure. To our knowledge, the work reports the first time that the enhanced catalytic activity in the reductive amination was correlated with material stability via the creation of Ni-Ce-O interfaces uniformly decorated on the mesoporous Al2O3 NPC.
40 30 20
20 0
Fig. 6. Activity tests over three cycles of the reductive amination of PPG using 1Ni-0Ce-0Al and 1Ni-0.5Ce-5Al as catalysts.
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97.3% and 94.6% at the 2nd and the 3rd cycles of activity tests, respectively. For 1Ni-0Ce-0Al, the ZPA was also slightly decreased to 98.4% and 95.4% over the two and three cycles of activity tests, respectively. The decline of ZPA was possibly attributed to the adsorbed products in the 1st cycle turning to be further aminated in the following 2nd and 3rd cycles. The presence of the adsorbed amine product were identified by the XPS study (i.e., the presence of nitrogen compound after the activity test; see Supplementary Material) and confirmed by the SEM analyses (i.e., formation of fibres on the surface of catalyst; see Supplementary Material). 4. Conclusion A hybrid nanostructure, Ni-CeO2-Al2O3 nanoparticle cluster, is successfully developed using an aerosol-based synthesis approach: gas-phase evaporation-induced self-assembly followed by twostage gas-phase thermal treatments. Using Al2O3 nanoparticle cluster as the support matrix and CeO2 nanoparticle as the promoting additive, ultrafine Ni crystallite size, large surface area, controlled chemical composition, physical size and surface state achieve simultaneously via the proposed aerosol-phase route. A superior catalytic performance is observed for the Ni-CeO2-Al2O3 hybrid nanoparticle cluster toward reductive amination of polypropylene glycol to polyetheramine: high activity (78%) and selectivity (100%). The strong metal-support interface reaction at the Ni-Ce-O interface is proposed to effectively suppress surface nitridation of Ni and subsequently to improve catalytic activity and operation stability of the Ni catalyst. The work demonstrated a promising strategy for the synthesis of Ni-based hybrid nanoparticle cluster as a high-performance catalyst with mechanistic understanding of the catalysis of reductive amination of polypropylene glycol in the hybrid nanostructure by design. Acknowledgements The authors thank Ministry of Science and Technology (MOST) of Taiwan, R.O.C. for financial support under Contract MOST 1062622-8-007-017 & MOST 107-2628-E-007-002-MY3. The authors also thank Huan-Ming Chang, Chuen-Lih Hwang, Chun-Yu Lee, and Chih-Cheng Chia of Chang Chun PetroChemical Co. Ltd., Taiwan, R.O.C. for helpful discussion and experimental support. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.apt.2019.07.009. References [1] K.-I. Shimizu, K. Kon, W. Onodera, H. Yamazaki, J.N. Kondo, Heterogeneous Ni catalyst for direct synthesis of primary amines from alcohols and ammonia, ACS Catal. 3 (2012) 112–117. [2] Y. Liu, K. Zhou, H. Shu, H. Liu, J. Lou, D. Guo, Z. Wei, X. Li, Switchable synthesis of furfurylamine and tetrahydrofurfurylamine from furfuryl alcohol over Ò RANEY nickel, Catal. Sci. Technol. 7 (2017) 4129–4135. [3] D. Ruiz, A. Aho, P. Mäki-Arvela, N. Kumar, H. Oliva, D.Y. Murzin, Direct amination of dodecanol over noble and transition metal supported silica catalysts, Ind. Eng. Chem. Res. 56 (2017) 12878–12887. [4] K. Kim, Y. Choi, H. Lee, J.W. Lee, Y2O3-Inserted Co-Pd/zeolite catalysts for reductive amination of polypropylene glycol, Appl. Catal. A 568 (2018) 114– 122. [5] D. Ruiz, A. Aho, T. Saloranta, K. Eränen, J. Wärnå, R. Leino, D.Y. Murzin, Direct amination of dodecanol with NH 3 over heterogeneous catalysts catalyst screening and kinetic modelling, Chem. Eng. J. 307 (2017) 739–749.
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