Al2O3 catalyst derived from layered double hydroxides precursor for selective hydrogenation of pyrolysis gasoline

Applied Catalysis A: General 468 (2013) 204–215

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An egg-shell type Ni/Al2 O3 catalyst derived from layered double hydroxides precursor for selective hydrogenation of pyrolysis gasoline Xin Wen, Rushi Li, Yixuan Yang, Jiali Chen, Fazhi Zhang ∗ State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China

a r t i c l e

i n f o

Article history: Received 1 April 2013 Received in revised form 19 August 2013 Accepted 23 August 2013 Available online 31 August 2013 Keywords: Ethylene industry Pyrolysis gasoline Selective hydrogenation Egg-shell catalyst Layered double hydroxide

a b s t r a c t Egg-shell type catalysts, in which a thin layer of catalytically active component is distributed on the outer surface of the support particle, have been theoretically and experimentally proved to be useful in processes where the reaction has a very high rate and the intraparticle diffusion becomes the limiting step. Herein we report the preparation of an egg-shell Al2 O3 -supported nickel (Ni/Al2 O3 ) catalyst derived from layered double hydroxides (LDHs) precursor, and its catalytic performance for selective hydrogenation of pyrolysis gasoline (PyGas), an important by-product of ethylene industry from thermal decomposition of heavier oil fractions, which was carried out in the liquid phase. Firstly, a Ni2+ Al3+ -containing LDHs (NiAl-LDHs) precursor was in situ grown on the surface of ␥-Al2 O3 spheres by using a diluted ammonia solution as precipitator. Then, egg-shell Al2 O3 -supported nickel oxide (NiO/Al2 O3 ) sample with NiO crystallites highly dispersed on the external edge of the Al2 O3 support was obtained after calcination at 450 ◦ C. The experimental study of selective hydrogenation of styrene (PyGas model 1) for revealing the intrinsic hydrogenation kinetics was carried out in a batch reactor at 60 ◦ C using the egg-shell Ni/Al2 O3 catalyst (denoted LP-Ni/Al2 O3 ; LP is expressed as layered precursor) fabricated by subsequent ex situ presufidation and final H2 reduction at 500 ◦ C of the NiO/Al2 O3 sample. In addition, an Ni/Al2 O3 catalyst with uniform distribution of Ni in the catalyst was prepared by a conventional wet impregnation method (denoted IM-Ni/Al2 O3 ; IM is expressed as impregnation method) with the consistent Ni loading amounts and given into the consideration for comparison. The estimated effectiveness factor () of the LP-Ni/Al2 O3 catalyst was higher than that of the IM-Ni/Al2 O3 one, demonstrating the catalyst with eggshell structure has a lower intraparticule mass transfer resistance. The catalytic hydrogenation activities of both Ni-based catalysts were further evaluated by selective hydrogenation of diolefins (PyGas model 2), along with selective hydrogenation of PyGas model 1, in a micro-flow reactor. The egg-shell Ni/Al2 O3 catalyst derived from LDHs precursor exhibited a superior catalytic hydrogenation performance, which mainly be due to Ni metal being deposited on the Al2 O3 support in a much thinner outer layer, as well as a smaller average size of nickel particles with stronger interaction between the nickel species and support. Several characterization techniques including powder X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), low temperature N2 adsorption–desorption, X-ray photoelectron spectroscopy (XPS), and temperature programmed reduction of hydrogen (TPR)/temperature programmed desorption of hydrogen (H2 -TPD) were adopted to investigate the physical–chemical properties of the two supported Ni catalysts in detail. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Pyrolysis gasoline (PyGas, typically containing C5 C12 hydrocarbons) is an important by-product of ethylene industry from the thermal decomposition of heavier oil fractions. It is unstable for

∗ Corresponding author. Tel.: +86 10 6442 5105; fax: +86 10 6442 5385. E-mail address: [email protected] (F. Zhang). 0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2013.08.040

there are up to 15 wt% of gum-forming agents, mostly styrene and diolefins. In order to yield high octane value gasoline blending stock and make it a high potential feedstock for extraction of aromatics, PyGas must be stabilized by selective hydrogenation of styrene and diolefins into ethylbenzene and monoolefins, respectively, under mild temperature over supported metal catalysts [1–4]. Presently, the catalysts most frequently used in this process are aluminasupported nickel [5–7] or palladium [3,4,8–10] catalysts. Compared to Pd-based catalysts, Ni catalysts have received extensive

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attention due to their low cost, poison and gum resistance, as well as their suitable properties in hydrotreating, hydrogenation and hydrogenolysis of hydrocarbons [11–13]. Several fabrication strategies, such as adding promoters [6,13], modifying catalyst support [14], and introducing new presulfidation process [7], have been reported for improving the catalytic performance of Ni catalysts for PyGas selective hydrogenation. With the purpose of retaining acceptable pressure gradients, the commercial fixed-bed reactor for selective hydrogenation generally requires the pelleted catalysts with 1–3 mm diameter [3,15,16]. For this size range of catalyst particles, intraparticle diffusive limitations may cause insufficient reactants to the inner parts of the catalyst particles, leading to the decreased catalytic activity and selectivity. Accordingly, the optimal distribution of the catalytically active component would maximize the catalyst effectiveness and improve the activity, selectivity, as well as poison resistance [17–19]. Due to the advantages of short transport or diffusion paths and a better heat transport, egg-shell type catalysts, in which a thin layer of active component is placed on the outer surface of the catalyst particles, have been theoretically and experimentally proved to be useful in processes where the reaction has a very high rate and the intraparticle diffusion becomes the limiting step, such as selective hydrogenation reaction [3,10,15,16,20–27], Fischer-Tropsch synthesis [28], methane partial oxidation [29], and hydrodesulfurization [30]. For the reaction system of selective hydrogenation of PyGas, the reaction rate is relatively high. And the reaction proceeds in the liquid phase under mild temperature. Thus, the effect of mass transfer on the kinetics for egg-shell catalyst should not be ignored [23]. The use of egg-shell Pd catalysts to significantly improve the catalytic performance in the selective hydrogenation of the styrene derivatives and dialkenes has been reported previously [3,20–24]. However, to the best of our knowledge, few researchers have investigated the egg-shell Ni catalysts for the PyGas hydrogenation process. It has been reported that fabrication of egg-shell Ni catalysts with common Al2 O3 support spheres is an intricate because there are many variables and parameters in the impregnation course that have to be carefully controlled, such as viscosity, pH value and nature of the impregnating solution, contact time of the impregnating solution with the support, and activation of the required phase (drying, calcination and reduction) [14,29–35]. Nowadays, it remains a challenge to develop the reliable fabrication method for the egg-shell alumina-support Ni catalysts for the actual practical applications. Layered double hydroxides (LDHs, also known as hydrotalcitelike materials) are a family of anionic clays which can be expressed by the general formula [M2+ 1−x M3+ x (OH)2 ]x+ [An− ]x/n ·yH2 O, where metal cations M2+ and M3+ occupy the octahedral holes in a brucite-like layer; the value of the coefficient x is equal to the molar ratio of M3+ /(M2+ + M3+ ); and inorganic or organic anion An− is located in the hydrated interlayer galleries [36–38]. Calcination of a Ni2+ Al3+ -containing LDHs has been reported to afford NiAl-mixed metal oxides, which have displayed high catalytic performance [39], largely owing to the ability of the LDHs matrix to host the metal elements distributed homogeneously at the atomic level [40]. Recently, supported Ni/Al2 O3 catalyst with uniform distribution of Ni in the catalyst derived from NiAl-LDHs precursor was reported in our laboratory [41]. Micro-spherical ␥Al2 O3 was used as the catalyst support and the sole source of Al3+ for the in situ growth of NiAl-LDHs precursor in the pore canals of the material. Formation of the LDHs was thought to be resulted from the decomposition of urea precipitator dissolved in an aqueous solution of Ni2+ impregnated into the ␥-Al2 O3 . Herein we report the preparation of an egg-shell Ni/Al2 O3 catalyst derived from NiAl-LDHs precursor by using a diluted ammonia solution as precipitator. For comparison, a Ni/Al2 O3 catalyst with

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uniform distribution of Ni was prepared by conventional wet impregnation method with consistent Ni loading amounts under identical treatment conditions of calcination, presufidation, and reduction. The physical–chemical properties of the two kinds of supported Ni catalysts were investigated by several characterization techniques including powder X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), low temperature N2 adsorption–desorption, X-ray photoelectron spectroscopy (XPS), and temperature programmed reduction of hydrogen (TPR)/temperature programmed desorption of hydrogen (H2 -TPD). Experimental study of selective hydrogenation of styrene (PyGas model 1) was first carried out in a batch reactor for revealing the intrinsic hydrogenation kinetics of both Ni-based catalysts. Then, catalytic hydrogenation activities were further evaluated by selective hydrogenation of diolefins (PyGas model 2), along with PyGas model 1, in a micro-flow reactor. 2. Experimental 2.1. Materials Ni(NO3 )2 ·6H2 O, NH4 NO3 , ammonia, dimethyl disulfide (DMDS), styrene, 1-hexene, toluene, 1,7-octadiene, and n-heptane are all of A.R. grade and used without further purification. The porous ␥-Al2 O3 spheres with a purity of 99.5% prepared through a reduction-magnetic separation process in our laboratory [42], were crushed into particles with an average particle size of 20–40 mesh and calcined at 600 ◦ C for 4 h before used as support. The deionized water with a conductance below 10−6 S cm−1 is used in all synthesis and washing processes. 2.2. Preparation of catalyst samples The egg-shell Al2 O3 -supported nickel oxide (NiO/Al2 O3 ) samples, with different Ni loading amounts 5, 8, 12, and 15 wt%, were fabricated as follows. Firstly, a certain amounts of Ni(NO3 )2 ·6H2 O and NH4 NO3 were dissolved in deionized water to obtain a mixed salt solution of the concentration of nickel ions about 0.15 mol L−1 . The resulting solution was added to a conical flask with 6 g ␥-Al2 O3 particles and 2 wt% of aqueous ammonia solution was added dropwise to the mixed salt solution with stirring simultaneously until the pH value of the mixed solution reached 7.25. Subsequently, the flask was placed into the bath oscillator with the oscillation frequency of about 140 beats per minute at 70 ◦ C for 24 h. The particles in the flask were separated with the residual solution and washed thoroughly with deionized water until the pH value of the washings reached 7.0. Finally, the sample was dried at 70 ◦ C for about 12 h (denoted NiAl-LDHs/Al2 O3 ). Before presulfidation, NiAl-LDHs/Al2 O3 sample was calcined in air at 450 ◦ C for 4 h with a ramping rate of 5 ◦ C min−1 for obtaining the nickel oxide-based sample (denoted LP-NiO/Al2 O3 ). The ex situ presulfided catalyst sample was obtained by impregnation with a solution of DMDS as an organosulfide agent in an excess of n-heptane [1]. The ex situ presulfidation process consists of the following steps: The obtained LP-NiO/Al2 O3 catalyst (5.0 g) was contacted for 1 h at 30 ◦ C with 40 mL of a solution of DMDS in an excess of n-heptane in a rotary evaporator. The concentration of the impregnating solution was chosen to incorporate an amount of sulfur exceeding that required by the stoichiometry Ni3 S2 in the final catalyst, i.e., 5.0 wt%. Subsequently, the obtained sample was moved into a micro flow reactor (see Supporting Information Fig. S1) flushing with N2 at room temperature at 0.5 MPa, and heated at a given temperature 500 ◦ C for 20 min, with a heating rate of 5 ◦ C min−1 . The presulfided sample is denoted LP-NiS/Al2 O3 . Then,

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the presulfided sample was reduced at 500 ◦ C and 0.5 MPa for 2 h in flowing H2 (50 mL min−1 ) and N2 (30 mL min−1 ) mixture before lowering to the setting temperature for hydrogenation reaction in the micro flow reactor or shifting out for hydrogenation reaction in a batch reactor. The obtained catalyst sample is denoted LP-Ni/Al2 O3 . For comparison, Ni/Al2 O3 catalyst with uniform distribution of Ni was obtained by a conventional wet impregnation method with the consistent Ni loadings. The sample was prepared by impregnating 6 g ␥-Al2 O3 particles in 15 mL Ni(NO)3 ·6H2 O solution at room temperature for 24 h and then dried at 70 ◦ C for 12 h. The resulting sample is denoted Ni(NO3 )2 /Al2 O3 . Before presulfidation, Ni(NO3 )2 /Al2 O3 sample was calcined in air at 450 ◦ C for 4 h with a ramping rate of 5 ◦ C min−1 for obtaining the nickel oxide-based catalyst (denoted IM-NiO/Al2 O3 ). The ex situ presulfidation and reduction in the micro flow reactor was carried out in succession, and the resulting sample is denoted IM-NiS/Al2 O3 and IM-Ni/Al2 O3 , respectively. 2.3. Preparation of NiAl-LDHs powder As a reference sample, NiAl-LDHs powder was prepared by means of a method using separate nucleation and aging steps (SNAS) [43]. Ni(NO3 )2 ·6H2 O and Al(NO3 )3 ·9H2 O with a Ni2+ /Al3+ molar ratio of 2:1 were dissolved in deionized water to make a mixed salt solution ([Ni2+ ] + [Al3+ ] = 0.66 mol L−1 ). NaOH and Na2 CO3 were dissolved in deionized water to make an alkali solution ([NaOH] = 1.5 mol L−1 and [Na2 CO3 ] = 0.7 mol L−1 ). The two solutions were added simultaneously to a colloid mill operating with a rotation speed of about 4000 r min−1 . The resulting slurry was transferred to a four-necked flask and aged at 40 ◦ C for 15 h with vigorous stirring. The precipitate was centrifuged and thoroughly washed with deionized water until the pH value of the filtrate reached 7.0. The final NiAl-LDHs were obtained after drying at 70 ◦ C for 12 h. 2.4. Analysis and characterization Powder X-ray diffraction (XRD) patterns were recorded on a Shimadzu XRD-6000 X-ray powder diffractometer (Cu K␣ radiation,  = 0.15406 nm) at 40 kV, 30 mA, a 2 angle ranging from 3◦ to 80◦ , with a scan speed of 10◦ min−1 . The weight fraction of Ni in the sample was determined by a Shimadzu ICPS-7500 inductively coupled plasma emission spectrometer (ICP-ES) after the samples were dissolved using chloroazotic acid. The elemental analysis for carbon was carried out using a Vario EL cube elemental analyzer (Elementar Analysensysteme GmbH). The morphology of the samples and the Ni elemental distribution were investigated by using a scanning electron microscope with an EDX attachment (SEM, Hitachi S-4700, EDX Zeiss Supra55). The catalyst was imbedded in a thermoplastic resin. After solidification, grinding machine was used to grind off the catalyst particles so as to expose the cross section. The accelerating voltage applied was 20 kV. High-resolution transmission electron microscopy (HRTEM) was carried out on JEOL-2100 operated at an accelerating voltage of 200 kV. The low temperature N2 adsorption–desorption experiments were carried out using a Quantachrome Autosorb-1 system. The Barrett–Joyner–Halenda (BJH) method was used to calculate pore volume and the pore size distribution. X-ray photoelectron spectroscopy (XPS) was recorded using an ESCALAB250 X-ray photoelectron spectrometer equipped with a monochromatized Mg K␣ X-ray radiation (1253.6 eV photons). The depth profiling experiment was conducted with high energy Ar+ ions beam. Binding energies were calibrated based on the graphite C 1s peak at 284.6 eV. Temperature programmed reduction (TPR) and hydrogen temperature programmed desorption (H2 -TPD) of the catalysts were conducted on a Micromeritics

Table 1 The compositions of two types of PyGas model feed. Compound

PyGas model 1 (wt%)

PyGas model 2 (wt%)

Styrene Toluene 1-Hexene 1,7-Octadiene n-Heptane

10.0 35.0 – – 55.0

10.0 35.0 3.5 1.5 50.0

ChemiSorb 2720 with a quartz reactor which was loaded with about 100 mg of sample. The procedure was as follows: The sample was cleaned at 300 ◦ C for 2 h in argon in order to remove any physisorbed molecules, and then heated with a heating rate of 5 ◦ C min−1 up to 1000 ◦ C in the stream of 10% H2 in Ar with the total flow rate of 40 mL min−1 . The outlet gas was passed through a cold trap to remove the moisture produced during reduction, then the adsorption of the reduced samples was finished by 10% H2 in Ar. Chemisorbed H2 was desorbed by programmed heating at a rate of 10 ◦ C min−1 in the stream of Ar with the flow rate of 40 mL min−1 . The amount of hydrogen consumed was recorded using a thermal conductivity detector. The recorded data was quantified using a calibration sample Ag2 O as a standard. The metal specific surface area and specific dispersion of Ni was calculated based on the H2 chemisorbed using the following simplified equation [41]: Ni metal specific surface area (SANi ): SANi (m2 g−1 -Ni) =

Vad SF N · · · RA Ws FNi Vm

(1)

Ni dispersion (D): D (%) =

Vad FW SF · · · 100 Ws FNi Vm

(2)

where Vad = chemisorbed volume of H2 at standard temperature and pressure (STP) conditions to form a monolayer, Ws = weight of the sample (g), SF = stoichiometric factor (the Ni:H molar ratio in the chemisorption) which is taken as 1, FNi = weight fraction of Ni in the sample as determined by ICP, N = 6.023 × 1023 atoms mol−1 , Vm = molar volume of H2 (22,414 mL mol−1 ) at STP, RA = atomic cross-sectional area of Ni (0.0649 nm2 ), FW = formula weight of Ni (58.71 g mol−1 ). 2.5. Catalytic testing Two types of PyGas model feed were employed for evaluation the catalytic performance of the resulting Ni/Al2 O3 catalysts and their compositions are shown in Table 1. Styrene was chosen as the model reactant to be hydrogenated with an excess of n-heptane and toluene (PyGas model 1), for it is main component in PyGas (5–10 wt%) and can form gum which should be removed [3,44]. Further, a PyGas model feed containing styrene, toluene, 1-hexene, 1,7-octadiene, and n-heptane (PyGas model 2) was adopted for evaluating comprehensively the catalytic performance for the industrially widely applied hydrogenation of PyGas. This synthetic feedstock has been investigated as a useful representative model feed for a broader group of hydrocarbon blends subjected to hydroprocessing like PyGas [9,44]. The most important reactions that may occur during the hydrogenation of the PyGas model feed are summarized in Supporting Information Fig. S2. Styrene conversion, 1,7-octadiene removal, and maximum yield of internal octenes by isomerization of 1-octene are primary objectives. Whilst, aromatic ring hydrogenation, forming corresponding cyclohexanes, and complete hydrogenation of 1,7-octadiene and 1-hexene to corresponding n-alkanes should be minimal.

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Selective hydrogenation of styrene (PyGas model 1) was first carried out in a batch, stainless steel, stirred tank reactor for revealing the intrinsic hydrogenation kinetics of the two kinds of supported Ni catalysts. This batch reactor was charged with styrene (47.5 mL) in toluene (172.1 mL) and n-heptane (350.0 mL, as an internal standard). Catalytic tests were performed using both pelletized and powdery (finely milled) catalysts. The reactor was operated at 60 ◦ C, 1000 rpm, and a constant H2 pressure of 2.0 MPa. In a typical run, pellets (0.5 g) or powder (0.2 g) were used. The reaction effectiveness factor () for each catalyst was determined using a procedure reported by other groups when investigating the kinetics of selective hydrogenation on supported Pd catalyst with egg-shell structure [4,24]. Catalytic hydrogenation activities were further evaluated by selective hydrogenation of diolefins (PyGas model 2), along with selective hydrogenation of PyGas model 1, in a micro-flow reactor over the temperatures range 40–80 ◦ C. The catalyst bed in constant temperature zone consisted of 1.5 mL catalyst sample diluted with 1.5 mL SiC (60–80 mesh) to prevent maldistribution of heat and mass in the reactor. The other part of trickle-bed reactor was filled with quartz sand. Activated alumina (5 g) was placed on top of the catalyst bed for the adsorption of 4-tert-butylcatechol (TBC), usually used as inhibitor to prevent gumming during storage in styrene [7]. The hydrogenation reactions were performed at temperatures from 40 to 80 ◦ C with a weight hourly space velocity (WHSV) at 30 h−1 . H2 pressure was varied from 0.4 to 3.0 MPa, keeping a constant PyGas pressure of 3.0 MPa and using inert N2 , and a hydrogen/oil volume ratio at 80. Reactants and products were analyzed using a Shimadzu GC-2014C chromatographic instrument by means of a flame ionization detector and a 9006-PONA capillary column. Catalytic activity and selectivity were calculated using the following simplified equation, based on chromatographic data by area normalization method and n-heptane was used as standard sample to analyze other data: Conversion (%) =

mi − mo × 100 mi

(3)

where m denotes the number of moles of reactant entering (subscript i) and exiting (subscript o) the reactor per unit time. m selectivity (%) =

m m (ethylbenzene) + m (ethylcyclohexane) × 100

m selectivity (%) =

m × 100 m (1-octene) + m (internal-octene) + m (n-octane)

(4)

(5)

where m and m denotes the number of moles of reactant (m: ethylbenzene and ethylcyclohexane; m : 1-octene, internal-octene and n-octane) exiting the reactor per unit time. 3. Results and discussion 3.1. Preparation and characterization of NiO/Al2 O3 samples Porous ␥-alumina spheres with a purity of 99.5% were prepared in our laboratory through a reduction-magnetic separation process and used as a support [42]. The XRD reflections (3 1 1), (4 0 0) and (4 4 0) of ␥-Al2 O3 (JCPDS No. 29-1486) can be observed in Fig. 1a. The XRD pattern of the reference sample NiAl-LDHs powder (Fig. 1f), which was prepared by means of a method using separate nucleation and aging steps [43], is similar to those reported in the literature for LDHs phase [36]. The peaks at low angle arise from the basal (0 0 3) and higher order (0 0 6 and 0 0 9) reflections, and the peak around 60◦ 2 arises from the (1 1 0) reflection. After in situ growth of LDHs on the ␥-Al2 O3 spheres by using a diluted ammonia solution as precipitator, the resulting NiAl-LDHs/Al2 O3

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Fig. 1. XRD patterns of ␥-Al2 O3 support (a), NiAl-LDHs/Al2 O3 (b), LP-NiO/Al2 O3 (c), Ni(NO3 )2 /Al2 O3 (d), and IM-NiO/Al2 O3 (e). NiAl-LDHs powder prepared by SNAS method was included for comparison (f).

sample shows a superposition of the characteristic XRD reflections of ␥-Al2 O3 and those of LDHs as shown in Fig. 1b, confirming the deposition of LDHs crystallines on the ␥-Al2 O3 support. After calcination at 450 ◦ C, LP-NiO/Al2 O3 sample exhibits similar diffraction pattern to that of the Al2 O3 support (Fig. 1c). Except for those of Al2 O3 support, the resulting IM-NiO/Al2 O3 sample by impregnation and subsequent calcination at 450 ◦ C shows two obvious peaks at 2 43.29◦ and 62.85◦ , corresponding to the (2 0 0) and (2 2 0) reflection of bulk NiO crystallites (JCPDS No. 47-1049), respectively, signifying that larger NiO particles were formed on the support surface (Fig. 1e). It is noted that the peak intensity of the two NiO characteristic reflections for LP-NiO/Al2 O3 is relatively lower than that of IM-NiO/Al2 O3 , demonstrating a higher dispersion of NiO on the support for the former. The Ni loading of the two NiO/Al2 O3 sample is substantially the same amount (7.98 wt% for LP-NiO/Al2 O3 and 8.05 wt% for IM-NiO/Al2 O3 ), measured by ICP. Representative SEM images of NiAl-LDHs/Al2 O3 and LPNiO/Al2 O3 are shown in Fig. 2, respectively. NiAl-LDHs/Al2 O3 has a well-developed two-dimensional network grown homogeneously on the surface of Al2 O3 support. After calcination at 450 ◦ C, LPNiO/Al2 O3 can keep the network architecture which may benefit for the dispersion of Ni species on the support surface. The distribution of Ni species in LP-NiO/Al2 O3 and IM-NiO/Al2 O3 samples was compared and shown in Fig. 3. In case of IM-NiO/␥-Al2 O3 (Fig. 3b), the corresponding EDX map- and line-scan (Fig. 3d and f) of particle

Fig. 2. Representative SEM images of NiAl-LDHs/Al2 O3 (a) and LP-NiO/Al2 O3 (b).

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Fig. 3. Representative SEM images of cross-section of LP-NiO/Al2 O3 (a) and IM-NiO/Al2 O3 (b). The corresponding EDS images collected in map- and line-scan (correspond to red line) mode for LP-NiO/Al2 O3 (c,e) and IM-NiO/Al2 O3 (d,f) are shown. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

shows a very uniform distribution of nickel throughout the sample, which may be attributed to the migration of Ni2+ during impregnating inside the pore canals of Al2 O3 support. However, LP-NiO/Al2 O3 sample exhibits a remarkable egg-shell profile with Ni segregation (0.10–0.13 mm) toward the outer surface of the support (Fig. 3c and e). It is interesting to find that a thin shell with a thickness about 8.6 ␮m (pointed out by red arrow in Fig. 3a inset) is located on the external surface of Al2 O3 particle for LP-NiO/Al2 O3 sample by the in situ growth of LDHs on the support surface. However, for the IM-NiO/Al2 O3 the profile line where the particle and the resin material meet (Fig. 3b, inset) is due to the exposed area of the resin material, rather than a thin shell of specimen. It is known that the layer thickness of the catalytically active phase plays a key role in enhancement the catalytic activity and selectivity for the eggshell catalysts, by overcoming the intraparticle diffusion limitation [24,25,27,29,33]. Iglesia et al. revealed that the layer thickness of active phase about 0.1–0.2 mm for egg-shell Co catalyst was feasible to obtain a higher catalytic activity and C5 + selectivity for the Fischer-Tropsch synthesis due to their small diffusion restriction in the presence of CO [33]. Qiu et al. demonstrated that nickel component distribution for the supported Ni catalyst had a remarkable effect on the catalytic performance in methane partial oxidation (POM), rather than the particle size of nickel crystallite and the reducibility of NiO species [29]. They found that egg-shell type Ni catalyst with the layer thickness of the active phase about 0.25 mm exhibited an improved catalytic activity for POM.

N2 adsorption–desorption isotherms and pore size distribution of the two supported nickel oxide-based samples, as well as those of ␥-Al2 O3 support, are shown in Fig. 4. The corresponding textural properties are listed in Supporting Information Table S1. All

Fig. 4. N2 adsorption–desorption isotherms of two NiO/Al2 O3 samples. Inset shows the pore size distribution curves of the samples. ␥-Al2 O3 support is included for comparison.

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Fig. 5. TPR profiles of nickel oxide- and sulfide-based samples (region I: 100–300 ◦ C; region II: 300–670 ◦ C; region III: 670–850 ◦ C). Thermal treatment at an elevated temperature 500 ◦ C in N2 was carried out for the two NiS/Al2 O3 samples.

the isotherms of the samples are Type IV with an obvious hysteresis loop. The shape of the hysteresis loop is a superposition of Type H1 and H3, which is generally taken to indicate that samples have both parallel and tubular slit-shaped capillary pores which are caused by the gas escaping during calcination and the stacking of alumina and NiAl-LDHs micro-crystallites [45]. Compared to those of the porous ␥-Al2 O3 support, the volume values of LPNiO/Al2 O3 and IM-NiO/Al2 O3 sample move toward slightly lower at the same P/P0 , which may be resulted from the deposition of nickel species, leading to the aggregate block for some pores in Al2 O3 . In addition, Table S1 shows that both supported nickel oxide-based samples have a lower specific surface area, total pore volume, and a narrower pore size distribution, in comparison with the ␥-Al2 O3 support. It is worthy noting that LP-NiO/Al2 O3 sample derived from LDHs precursor exhibits a relatively higher surface area and a narrower pore size distribution than IM-NiO/Al2 O3 , which could be related to the formation of NiAl-LDHs microcrystallites with regular size and well-developed two-dimensional pores in situ grown on the surface of support (Fig. 2). To obtain information about the nickel metal-alumina support interaction, TPR profiles of the two nickel oxide-based samples were measured (Fig. 5). According to the literature, the reduction peaks around 350, 520, 650, and 800 ◦ C corresponding to Ni species in nickel catalysts are related to the amorphous NiO, crystal NiO, NiAl2 O4 , and NiAlx Oy (x > 2), respectively [11]. Generally speaking, the amorphous and crystal NiO are assigned to the surface Ni species (on the surface of the alumina support at region II); while NiAl2 O4 and NiAlx Oy (Ni Al spinel at region III) are ascribed to the skeleton Ni species (in the lattice of the support). The maximum reduction peak assigned to the reduction of crystal NiO in LP-NiO/Al2 O3 and IM-NiO/Al2 O3 are located at 536 and 526 ◦ C, respectively, illustrating the stronger interaction between the nickel oxide species and alumina in the former. HRTEM analyses have been carried out in order to directly investigate the morphology and size dispersion of Ni particles on the two kinds of supported Ni catalyst samples before and after reduction (Fig. 6). For the two NiO/Al2 O3 samples, many small particles with the morphology of sphere-like manner, potentially based on the minimization of surface energy, can be found in the HRTEM images (Fig. 6a and b). Particles in the two Ni/Al2 O3 catalyst samples, fabricated by subsequent ex situ presufidation and final H2 reduction at 500 ◦ C of the corresponding NiO/Al2 O3 samples, exhibit lattice fringes at 2.03 and 2.02 A˚ (Fig. 6e and f) for LP-Ni/Al2 O3 and IMNi/Al2 O3 , respectively, which should be ascribed to (1 1 1) planes

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of Ni0 crystallite [46]. It can be found that nickel particle size of the IM sample is much larger than that of the LP sample, irrespective whether NiO or Ni0 crystallites. Supporting Information Fig. S3 displays that histogram of the particle size distribution for LP-Ni/Al2 O3 presents a narrow distribution of nanoparticles with the surface area-weighted diameter of about 4–6 nm, while IMNi/Al2 O3 results in a wide range of larger particles about 5–12 nm. The results give the direct evidence of the presence of highly dispersed Ni nanoparticles with small particle size in the LP-Ni/Al2 O3 sample, which may have an essential effect on the selective hydrogenation performance. Supported nickel catalysts can be prepared conventionally by the wet impregnation method [47]. The uncontrollable evaporation of the impregnating solution and the decomposition of the metal salt precursor often occur in the subsequent drying and final calcination step, leading to a greater crystallite formation of active species on support surface. This may give rise to the decreased dispersion of active species and plugging or even completely closuring the support pores in the resulting catalysts [47,48]. Moreover, because of the weak adsorption between Ni(NO3 )2 and ␥-Al2 O3 support, impregnated Ni species may re-equilibrate quickly during the drying and calcination treatment, which may result in an uniform distributions of Ni components in catalyst. It had been demonstrated that strong absorbing of catalytically active species on the support surface may overcome these problems and usually lead to the egg-shell type configuration [15,49]. Some research revealed that the high viscosity of impregnating solution by adding some organic solvents (such as hydroxyethyl cellulose, ethylene glycol, acetone, and n-undecane) can reduce the impregnation rate for the preparation of egg-shell catalysts [24,29,33,50]. However, the use of organic solvents with high cost causes the environmental problems and a water solution is more suitable for industrial application. Egg-shell type catalyst was also fabricated by using a porous hollow silica support [51]. In the present report, the egg-shell Al2 O3 -supported Ni catalyst was prepared from NiAl-LDHs precursor which was in situ grown on the support surface. Micro-spherical ␥-Al2 O3 was used as a support and sole source of Al3+ for the synthesis of NiAl-LDHs. Instead of urea, which was used as precipitator for the fabrication of a uniform Ni/Al2 O3 catalyst derived from NiAl-LDHs precursor in our laboratory [41], we adopt a diluted ammonia aqueous solution as precipitator for the fabrication of the egg-shell type Ni/Al2 O3 catalyst. As reported above, a large quantity of NiAl-LDHs crystallites were congregated on the Al2 O3 surface, and the concentration profile of nickel metal layer in LP-NiO/␥-Al2 O3 is about 0.10–0.13 mm. The choice of ammonia aqueous solution as precipitator had been tested previously by our group to fabricate NiAl-LDHs film by reaction of an alkaline solution of Ni2+ ions with porous anodic alumina (PAO) as both substrate and sole source of Al3+ cations [52]. A heterogeneous nucleation mechanism for the growth process of the film was proposed [53], which involving the formation of [Ni(NH3 )n ]2+ complexes by Ni2+ and a large excess of ammonium ions in solution and the adsorption of Ni2+ species on the PAO substrate forming a gel. Then, NiAl-LDHs nucleation occurs only at the gel-solution interface where Ni2+ and Al3+ species are present in abundance. In the case of the fabrication of egg-shell Ni/Al2 O3 catalyst with ammonia aqueous solution as precipitator, we suggest that the dissolution of Al3+ cations from alumina support may play an important role in the deposition of the formed NiAl-LDHs crystallites on the external edge of support. Nevertheless, due to the thermal activation of urea decomposition at ambient pressure requires a long reaction time [54], the releasing rate of Al3+ species is relatively slow when using urea as precipitator, which may contribute to the diffusion of Ni2+ species from the exterior to the pore canals of ␥-Al2 O3 , where the LDHs nucleation and crystal growth take place.

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Fig. 6. Representative HRTEM images of LP-NiO/Al2 O3 (a), IM-NiO/Al2 O3 (b), LP-Ni/Al2 O3 (c), and IM-Ni/Al2 O3 (d). (e and f) shows a high-magnification view of (c) and (d), respectively.

3.2. Ex situ presulfidation of NiO/Al2 O3 samples Presulfidation procedure is proved to be necessary for the Al2 O3 -supported nickel catalysts for PyGas hydrogenation [55]. This pretreatment can not only allow a safe start-up but also lead to an increase in selectivity to the desired mono-olefins. An ex situ presulfidation process had been adopted for the preparation of Ni/Al2 O3 catalyst, in which the catalyst is presulfided off site by some organic sulfide agents into a non-toxic, stable, and odorless catalyst [1,7]. This catalyst can be activated in situ during start-up under hydrogen pressure in the feedstock. DMDS has been revealed to be an efficient sulfiding molecule and can be decomposed easily at low temperature, producing various species able to initiate the sulfidation of the oxidic catalyst [56]. Herein, the ex situ presulfidation process was carried out by impregnation of the two kinds of NiO/Al2 O3 samples with DMDS in a rotary evaporator at 30 ◦ C and thermal treatment at an elevated

treatment temperature 500 ◦ C in N2 subsequently. XRD patterns of the nickel sulfide-based samples are shown in Supporting Information Fig. S4. LP-NiS/Al2 O3 sample derived from LDHs precursor shows a decreased NiO diffraction peaks (2 37.18◦ and 42.86◦ ) after sulfidation at 30 ◦ C. It is surprised that NiO diffraction peaks are visibly diminished for IM-NiS/Al2 O3 sample. Considering the reduction of the presulfided sample was carried out in flowing H2 and N2 mixture at 500 ◦ C before catalytic reaction, we performed an thermal treatment of the 30 ◦ C-sulfidated NiS/Al2 O3 sample with N2 at temperature 500 ◦ C (see Section 2). The two 500 ◦ C-sulfidated NiS/Al2 O3 samples shows an identical XRD patterns, without characteristic NiO diffraction peaks. It is reported that metal sulfides can be formed under the sulfidation conditions via O S exchange [55,57]. The above results indicate that the geometric structure of NiO on the two sulfided samples is different which may lead to the different sulfidation degree. The interaction between the nickel species and Al2 O3 support for LP-NiS/Al2 O3 is relatively stronger

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Table 2 Hydrogen adsorption properties of the two NiS/Al2 O3 catalysts. Sample

Ni loading (%)a

H2 uptake (mL g−1 STP )b

Dispersion (%)b

Ni specific surface area (m2 g−1 )b

IM-NiS/Al2 O3 LP-NiS/Al2 O3

7.98 8.05

1.49 1.06

9.78 6.90

65.12 45.93

a b

As determined by ICP before H2 reduction. As determined by H2 -TPD.

due to the confinement effect of the “Ni O Al” in the LDHs main layer [36]. Thus, O S exchange in the LP sample is more difficult than that in the IM sample without the 500 ◦ C thermal treatment. The textural properties of the two sulfided nickel samples are investigated by the low temperature N2 adsorption–desorption technique and the results are listed in Table S1. A decrease in specific surface area and total pore volume can be seen for the two NiS/Al2 O3 samples after sulfidation. To investigate the surface properties of the catalyst sample after sulfidation, we characterize the two NiS/Al2 O3 samples by TPR (Fig. 5). Compared with the corresponding NiO/Al2 O3 sample, the two sulfided samples display an obvious shift of the reduction peak in region to a lower temperature. According to the literature, Ni species which are located in subsurface positions on the oxidic precursor could transfer to the surface even at low sulfidation temperatures, suggesting that the interaction between Ni species and the support is weakened by sulfidation [58,59]. In addition, a new peak can be found in region, which may be attributed to the reduction of NiS1+x or oxysulfides (partial sulfidation) [59]. The above TPR results demonstrate that the sulfidation step can lower the threshold reduction temperature, allowing the employ of a relatively mild temperature during in situ activation of the Ni-based catalyst sample. Hoffer et al. reported that for the partially sulfided Ni catalysts, the surface of Ni particles was covered with a small amount of sulfur, which may leads to the desired selectivity for PyGas hydrogenation [1]. It can be seen from Fig. 5 that, despite the smaller Ni particle size, LP-NiS/Al2 O3 sample shows the higher reduction temperature in region than IM-NiS/Al2 O3 sample, which may be resulted from the restriction by the “Ni O Al” structure of NiAl-LDHs precursor. Hydrogen uptake values and other related properties calculated from the H2 -TPD measurement on the two NiS/Al2 O3 catalysts are given in Table 2, along with the nickel loading as determined by ICP before H2 reduction. Irrespective of the consistent value of the Ni loading for the two samples, Ni dispersion and specific surface area in LP sample are higher than that in IM sample. This is consistent with the obvious decrease of the H2 consumption peak of LP sample in the high reduction temperature (600–700 ◦ C) and the distinct shift of reduction peak at region II (reduced from 536 ◦ C to 506 ◦ C) in Fig. 5. In addition, the value of surface S/Ni atomic ratio in LP sample (0.13) is significantly higher than that in IM sample (0.07), determined by SEM-EDS technique. According to the literature, complete saturation of Ni monolayer on the alumina support has been reached with 1.0–2.0 wt% S, corresponding to an S/Ni atomic ratio of 0.2–0.5 [1]. The higher S/Ni atomic ratio in LP sample indicates a deposition of Ni species on the support surface. The binding energies of electrons determined by XPS can provide useful information about the nickel species state and the chemical composition on the samples, as well as the distribution of surface Ni species. Fig. 7 shows XPS spectra of the oxidized, sulfided, and reduced Ni-based samples prepared by two different methods in the Ni 2p3/2 region. The peaks located around 855.6 and 856.7 eV can be attributed to the presence of surface NiO and NiO particles interacting more strongly with the alumina support, i.e., Nio 2+ -low and Nio 2+ -high, respectively [41,60]. The quantified percentages of the different Ni species, identified by decomposition of Ni 2p3/2 XPS emission lines are shown in Supporting Information

Table S2. Compared with IM-NiO/Al2 O3 , LP-NiO/Al2 O3 exhibits a higher content of Nio 2+ -high species than that of Nio 2+ -low, indicating a stronger bonding between Ni and Al2 O3 support for the sample derived from LDHs precursor. After sulfidation, the peak corresponding to NiO for the two NiO/Al2 O3 samples display a slightly left shift, illustrating that the interaction of NiO and support is weakened by the incorporation of sulfur in the NiO structure, which is in agreement with the results of TPR (Fig. 5). In addition, Fig. 7 displays a new peak at 853.5 and 854.0 eV for IM-NiS/Al2 O3 and LP-NiS/Al2 O3 , respectively, indicating the appearance of sulfided Ni species during sulfidation treatment. Reinhoudt et al. [58] demonstrated that upon mild calcination of Ni/Al2 O3 sample, Ni species may be partly present as surface aluminate, i.e., Ni(Al) structure, which forms Nis 2+ (Al) group after sulfidation by 10% H2 S in H2 above 330 ◦ C. The emergence of different Nis 2+ (Al) peak in Fig. 7 illustrates different chemical environment of the sulfided Ni species for the two NiS/Al2 O3 samples. It is proposed that, for LPNiS/Al2 O3 sample derived from LDH precursor, the confinement effect of the “Ni O Al” structure in NiAl-LDHs may result in a relatively stronger interaction between Ni species and Al2 O3 support. The above TPR results of Fig. 5 indicate that the sulfided Ni species in LP sample is more difficult to be reduced in comparison with those in IM sample. After a H2 reduction process at 500 ◦ C, the peak corresponding to Nis 2+ (Al) for the two NiS/Al2 O3 samples shifts to a low binding energy (852.5 eV, Fig. 7), which is attributed to Ni0 species [61]. Table S2 also shows a decline of the quantified percentage for Nio 2+ -low species. However, Nio 2+ -high species are hardly reduced with the employed experimental conditions. It is worthy noting that, after sulfidation-reduction process, LP-Ni/Al2 O3 sample possesses a larger percentage of metallic Ni0 compared with that of IM-Ni/Al2 O3 , which is in accordance with the results of H2 -TPD which shows much H2 uptake for the former (Table 2). In addition, it can be found in Table S2 that the surface Ni/Al atomic ratios of three LP samples, detected by XPS, are higher than those of the corresponding IM samples, ascribing to the deposition of Ni species on the out surface of catalyst, i.e., the formation of egg-shell structure. Moreover, we can see a decrease of surface Ni/Al atomic ratio for the two Ni/Al2 O3 samples after H2 reduction, indicating the prompted agglomeration of surface Ni species during the reduction process. 3.3. Catalytic performance for selective hydrogenation of PyGas For revealing the intrinsic hydrogenation kinetics of both supported Ni catalysts, experimental study of selective hydrogenation reaction was first carried out in a batch reactor. Styrene was chosen as the model reactant to be hydrogenated with an excess of n-heptane and toluene (PyGas model 1). The only product detected by gas chromatography was ethylbenzene in all the catalytic tests without the product of ethylcyclohexane. The final selectivity as calculated with the internal standard method was higher than 99%. Supporting Information Fig. S5a shows the results of styrene conversion as a function of time-on-stream obtained with the pelletized Ni catalysts, showing that the LP catalyst derived from LDHs precursor has a higher styrene conversion than the IM catalyst. Table 3 shows the values of initial reaction rate (ri pe ) obtained with

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Fig. 7. Decomposition of Ni 2p3/2 XPS emission lines of the oxidized, sulfided, and reduced Ni-based samples prepared by two different methods.

the pelletized catalysts. A larger value of ri pe can be seen for the LP catalyst. It was confirmed that the catalysts ground to less than 80 meshes had no meaningful diffusive constraints [24]. Herein we adopted the powdery Ni catalysts for representing the hydrogenation under kinetic regime. Fig. S5b shows the values of styrene conversion as a function of time-on-stream for the two powdery Ni catalysts. The initial reaction rate with the powdery Ni catalysts (ri po ) is shown in Table 3. In order to determine the extent of the mass diffusive constraints for the pelletized Ni catalysts, the reaction effectiveness factor () is roughly estimated from the initial reaction rates. The powdery Ni catalyst signifies the chemical control, whereas internal diffusion affects the rate observed on the pelletized catalyst. Thus,  values are obtained by comparing the initial reaction rates of the pelletized and powdery Ni catalysts, ri pe /ri po . It is shown form Table 3 that the estimated  value of the LP catalyst is higher than that of the IM one, demonstrating the catalyst with egg-shell structure has a lower intraparticule mass transfer resistance. Catalytic hydrogenation activities were further evaluated by selective hydrogenation of styrene (PyGas model 1) and diolefins (PyGas model 2) in a micro-flow reactor. To optimize the

Table 3 Catalytic properties of the two supported Ni catalysts for selective hydrogenation of styrene in a batch reactor. Sample

ri pe

ri po



LP-Ni/Al2 O3 IM-Ni/Al2 O3

0.003317 0.001545

0.005539 0.004553

0.5988 0.3393

ri pe : initial reaction rate with the pelletized Ni catalysts; ri po : initial reaction rate with the powdery Ni catalysts; : reaction effectiveness factor, ri pe /ri po .

hydrogenation conditions, the conversion of styrene was investigated by changing four reaction parameters separately, i.e., reaction temperature (T), H2 pressure (PH2 ), Ni loading, and reaction time (Fig. 8). Except for the samples with the highest nickel content (>12 wt%), the selectivity of styrene to ethylbenzene for all the tests performed was higher than 98%, and toluene had hardly any change at all. Generally speaking, LP-Ni/Al2 O3 exhibits a higher catalytic activity than IM-Ni/Al2 O3 upon the employed reaction conditions. In detail, with the reaction temperature increase from 40 to 60 ◦ C, styrene conversion over the two Ni/Al2 O3 catalysts is rapidly increased. With the further increase of reaction temperature to 80 ◦ C, styrene conversion shows a slow rise tendency. Apparently, LP sample displays much higher catalytic activity under the same reaction temperature. In addition, we can see that hydrogen partial pressure has an obvious effect on the hydrogenation of styrene. Under low hydrogen partial pressure, styrene conversion is significantly decreased for the two Ni/Al2 O3 catalysts. It is revealed that polymerization of the PyGas feed may take place on the Pdbased catalyst with the low hydrogen partial pressure, leading to the catalyst deactivation [8]. Furthermore, it is observed that the selectivity of styrene hydrogenation toward ethylcyclohexane increases slightly at high hydrogen partial pressures of 3.0 MPa (about 1.0 mol%). Consequently, the hydrogen partial pressure of 2.0 MPa is found to be suitable for the selective hydrogenation of PyGas model 1 under the employed reaction conditions. Fig. 8 also shows that styrene conversion over two catalyst samples is enhanced rapidly with the increase of Ni content from 5% to 8 wt%. Moreover, the results of time-on-stream analyses for LP-Ni/Al2 O3 and IM-Ni/Al2 O3 catalysts shows that two samples have a high initial activity, approaching 99.8% conversion of styrene. LP sample exhibits a better stability, maintaining 98.0% conversion after attaining a steady state within a period of 8 h time on stream.

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Fig. 8. Styrene conversion versus various reaction conditions for two Ni/Al2 O3 catalysts with PyGas model 1 (10 wt% styrene and 35 wt% toluene in n-heptane).

Whereas, IM sample reaches styrene conversion only 92.0% after 12 h. As reaction time goes on, there is a certain reduction of catalytic activity for IM sample. The catalytic activities of the two Ni/Al2 O3 catalysts for selective hydrogenation of PyGas model 2 are summarized in Supporting Information Fig. S6. With regard to the compositions of PyGas model 2, 1,7-octadiene and 1-hexene have been shown to improve the buildup of coke through the polymerization of olefins [62], allowing the lower conversion of styrene during PyGas hydrogenation. It can be seen from Fig. 8 that LP-Ni/Al2 O3 exhibits a higher catalytic activity compared to IM-Ni/Al2 O3 , which is accordance with the results of Fig. S6. After 12 h time on stream, ethylbenzene is the main product from the conversion of styrene with about 0.6 mol% of ethylcyclohexane. Under the employed reaction conditions, conversion of 1,7-octadiene yields predominantly octane, with a small quantity of 1-octene (about 6.0 mol%) and internal octenes (<0.5 mol%). While, 1-hexene is translated to n-hexane completely. Fig. S6 displays that conversions of various reactants, styrene, 1-hexene, and 1,7-octadiene, over the two Ni/Al2 O3 catalysts decrease significantly within 8 h time on stream. The stability of catalytic performance is improved for the LP sample with the further increase of reaction time. In addition, Fig. S6 demonstrates a faster conversion rate for styrene than 1,7-octadiene and 1-hexene, indicating a competitive hydrogenation of the three reactants occurs on the active sites of the catalyst which may be explained by the different adsorption ability of reactants. A much stronger adsorption of styrene compared to 1,7-octadiene and 1-hexene on the Pd-based catalyst has been revealed by Gaspar et al. [9]. The competitive hydrogenation of styrene, diolefins and olefins on the Ni-based catalyst was also reported by Hoffer et al. [7]. They found that the hydrogenation of 1-octene was depressed in the presence of styrene due to strong adsorption of styrene on the surface of the catalyst. This type of behavior can be appropriately explained by Langmuir–Hinshelwood kinetic model [2,4], according to which

the lower estimated activation energies and the higher adsorption parameters of styrene and cyclopentadiene compared to the corresponding one of 1-hexene and cyclopentene are in agreement with the order of those reaction rates. Several research groups had employed the egg-shell Pd catalysts in the selective hydrogenation of the styrene derivatives and dialkenes which shows significant enhancement of catalytic activity, selectivity, and stability [3,10,20–24]. An investigation of kinetic behaviors of selective hydrogenation of PyGas over the eggshell Pd catalyst demonstrated that only 27% of the active region of the catalyst in the particles can be used because of the influence of internal diffusion limitations [23]. Lin et al. reported that the eggshell Pd catalyst for the selective hydrogenation of isoprene (one component of PyGas feed) obtains better catalytic performance than the uniform profile catalyst [22]. The above reaction results demonstrate that LP-Ni/Al2 O3 catalyst derived form LDHs precursor is much more active and stable than IM-Ni/Al2 O3 prepared by the wet impregnation method, indicating that the egg-shell type distribution of nickel component produces a remarkable effect on the catalytic performance in PyGas hydrogenation. Catalyst deactivation is generally a problem in the PyGas hydrogenation, and it has been attributed mainly to the presence of polymerization, which leads to poisoning or pore mouth plugging of catalyst [62,63]. For the egg-shell catalyst, a lower internal diffusion resistance for the reactants may result in a shorter residence time, limiting the formation of the detrimental polymer products. The amounts of coke for the two Ni/Al2 O3 catalysts were determined by elemental analysis after hydrogenation reaction, showing the value of about 1.3 wt% for LP-Ni/Al2 O3 and 2.0 wt% for IM-Ni/Al2 O3 , which indicates that LP sample exhibits a better anti-carbon ability than IM one. On the other hand, the effect of particle size of active component on the catalyst stability is not negligible. Increase of adsorption strength of isoprene on the smaller Pd particles has been revealed to be beneficial to inhibit the diffusion of isoprene into alumina

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core for further coke formation [21]. Larger Ni particles have been disclosed to catalyze effectively the coke formation reaction during methane reforming reaction, while smaller Ni particles has a higher saturation concentration of carbon [64,65]. The above HRTEM results (Fig. 6) declare the presence of highly dispersed Ni particles with small particle size for LP-Ni/Al2 O3 catalyst (4–6 nm compared with 5–12 nm for IM-Ni/Al2 O3 ), which may be benefit to the improvement of the catalytic performance. Supporting Information Fig. S7 demonstrates that Ni particles in LP-Ni/Al2 O3 can remain a high dispersion with particle size of about 5 nm after reaction time 24 h. The particle size of metallic Ni and the active component content (determined by ICP analysis) have not changed, indicating the excellent stability for LP sample.

spheres and Professor Hui Zhang at Beijing University of Chemical Technology who helped us resolve some technique questions about the catalytic microreactor. This work was supported by the National Natural Science Foundation of China and the 973 Program (No. 2011CBA00506).

4. Conclusions

[1] B.W. Hoffer, F. Devred, P.J. Kooyman, A.D. van Langeveld, R.L.C. Bonné, C. Griffiths, C.M. Lok, J.A. Moulijn, J. Catal. 209 (2002) 245–255. [2] N. Mostoufi, R. Sotudeh-Gharebagh, M. Ahmadpour, J. Eyvani, Chem. Eng. Technol. 28 (2005) 174–181. [3] T.A. Nijhuis, F.M. Dautzenberg, J.A. Moulijn, Chem. Eng. Sci. 58 (2003) 1113–1124. [4] Z.M. Zhou, Z.M. Cheng, Y.N. Cao, J.C. Zhang, D. Yang, W.K. Yuan, Chem. Eng. Technol. 30 (2007) 105–111. ˜ B. Pawelec, J.L.G. Fierro, J.M. Arandes, J. Bilbao, Fuel 86 (2007) [5] P. Castano, 2262–2274. [6] Y. Qian, S. Liang, T. Wang, Z. Wang, W. Xie, X. Xu, Catal. Commun. 12 (2011) 851–853. [7] B.W. Hoffer, R.L.C. Bonné, A.D. van Langeveld, C. Griffiths, C.M. Lok, J.A. Moulijn, Fuel 83 (2004) 1–8. [8] Y.-M. Cheng, J.-R. Chang, J.-C. Wu, Appl. Catal. 24 (1986) 273–285. [9] A.B. Gaspar, G.R. dos Santos, R. de Souza Costa, M.A.P. da Silva, Catal. Today 133–135 (2008) 400–405. [10] W.-B. Su, W.-R. Chen, J.-R. Chang, Ind. Eng. Chem. Res. 39 (2000) 4063–4069. [11] R. Yang, X. Li, J. Wu, X. Zhang, Z. Zhang, Y. Cheng, J. Guo, Appl. Catal. A: Gen. 368 (2009) 105–112. [12] S.S. Au, J.S. Dranoff, J.B. Butt, Chem. Eng. Sci. 50 (1995) 3801–3812. [13] C. Park, M.A. Keane, J. Catal. 221 (2004) 386–399. [14] Y. Qiu, J. Chen, J. Zhang, React. Kinet. Catal. Lett. 94 (2008) 149–155. [15] T. Vergunst, F. Kapteijn, J.A. Moulijn, Appl. Catal. A: Gen. 213 (2001) 179–187. [16] T.A. Nijhuis, M.T. Kreutzer, A.C.J. Romijn, F. Kapteijn, J.A. Moulijn, Chem. Eng. Sci. 56 (2001) 823–829. [17] F. Shadman-Yazdi, E.E. Petersen, Chem. Eng. Sci. 27 (1972) 227–237. [18] R.C. Dougherty, X.E. Verykios, Catal. Rev. 29 (1987) 101–150. [19] A. Gavriilidis, A. Varma, M. Morbidelli, Catal. Rev. 35 (1993) 399–456. [20] T.-B. Lin, T.-C. Chou, Appl. Catal. A: Gen. 108 (1994) 7–19. [21] J.-C. Chang, T.-C. Chou, Appl. Catal. A: Gen. 156 (1997) 193–205. [22] T.-B. Lin, T.-C. Chou, Ind. Eng. Chem. Res. 34 (1995) 128–134. [23] Z. Zhou, T. Zeng, Z. Cheng, W. Yuan, Chem. Eng. Sci. 65 (2010) 1832–1839. [24] J.M. Badano, C. Betti, I. Rintoul, J. Vich-Berlanga, E. Cagnola, G. Torres, C. Vera, J. Yori, M. Quiroga, Appl. Catal. A: Gen. 390 (2010) 166–174. [25] Z. Shao, C. Li, X. Chen, M. Pang, X. Wang, C. Liang, ChemCatChem 2 (2010) 1555–1558. [26] T. Vergunst, F. Kapteijn, J.A. Moulijn, Ind. Eng. Chem. Res. 40 (2001) 2801–2809. [27] Y. Uemura, Y. Hatate, J. Chem. Eng. Jpn. 22 (1989) 287–291. [28] S.A. Gardezi, L. Landrigan, B. Joseph, J.T. Wolan, Ind. Eng. Chem. Res. 51 (2011) 1703–1712. [29] Y. Qiu, J. Chen, J. Zhang, Catal. Commun. 8 (2007) 508–512. [30] B. Liu, Y. Chai, Y. Liu, Y. Wang, Y. Liu, C. Liu, Fuel 95 (2012) 457–463. [31] X. Liu, J.G. Khinast, B.J. Glasser, Chem. Eng. Sci. 63 (2008) 4517–4530. [32] D. Gulková, L. Kaluˇza, Z. Vít, M. Zdraˇzil, Catal. Commun. 7 (2006) 276–280. [33] E. Iglesia, S.L. Soled, J.E. Baumgartner, S.C. Reyes, J. Catal. 153 (1995) 108–122. [34] W.D. Li, Y.W. Li, Z.F. Qin, S.Y. Chen, Chem. Eng. Sci. 49 (1994) 4889–4895. [35] L. Espinosa-Alonso, K.P. de Jong, B.M. Weckhuysen, J. Phys. Chem. C 112 (2008) 7201–7209. [36] P. Braterman, Z. Xu, F. Yarberry, S. Auerbach, K. Carrado, P. Dutta, Marcel Dekker, Handbook of Layered Materials., New York, 2004, pp. 373–474 (Chapter 8). [37] D.G. Evans, X. Duan, Chem. Commun. (2006) 485–496. [38] G.R. Williams, D. O’Hare, J. Mater. Chem. 16 (2006) 3065–3074. [39] D. Tichit, B. Coq, Cattech 7 (2003) 206–217. [40] P.J. Sideris, U.G. Nielsen, Z. Gan, C.P. Grey, Science 321 (2008) 113–117. [41] J.-T. Feng, Y.-J. Lin, D.G. Evans, X. Duan, D.-Q. Li, J. Catal. 266 (2009) 351–358. [42] P. Liu, J. Feng, X. Zhang, Y. Lin, D.G. Evans, D. Li, J. Phys. Chem. Solids 69 (2008) 799–804. [43] Y. Zhao, F. Li, R. Zhang, D.G. Evans, X. Duan, Chem. Mater. 14 (2002) 4286–4291. [44] M.C. Sze, US Patent 3,493,492 (1970). [45] S. Gregg, K. Sing, Adsorption, Surface Area and Porosity, Academic Press, New York, 1982. [46] Y. Wu, S. Cai, D. Wang, W. He, Y. Li, J. Am. Chem. Soc. 134 (2012) 8975–8981. [47] J. Geus, G. Poncelet, P. Grange, P. Jacobs, Preparation of Catalysts III, Elsevier, Amsterdam, 1983, p. 6. [48] H. Wang, Y. Fan, G. Shi, H. Liu, X. Bao, J. Catal. 260 (2008) 119–127. [49] P.P. Robinson, V. Arun, K.K.A. Rashid, C.U. Aniz, K.K.M. Yusuff, Chem. Eng. Commun. 199 (2012) 321–334.

Egg-shell type nickel catalyst LP-Ni/Al2 O3 has been prepared from LDHs precursor with ␥-Al2 O3 sphere as support. Firstly, NiAlLDHs crystal particles were in situ grown on the surface of ␥-Al2 O3 spheres by using a diluted ammonia solution as precipitator. The dissolution of Al3+ cations from alumina support is proposed to play an important role in the deposition of the formed NiAl-LDHs on the external edge of support. Then, egg-shell NiO/Al2 O3 sample was obtained after calcinations of NiAl-LDHs/Al2 O3 sample at 450 ◦ C. Finally, LP-Ni/Al2 O3 catalyst was fabricated by subsequent ex situ presufidation with DMDS as an organosulfide agent and H2 reduction at 500 ◦ C of the NiO/Al2 O3 sample. For comparison, an IM-Ni/Al2 O3 catalyst with uniform distribution of Ni species was prepared by conventional wet impregnation method with consistent Ni loading amounts under identical treatment conditions of calcination, presufidation, and reduction. The physical–chemical properties of the two kinds of supported Ni catalysts were investigated systematically by several characterization techniques. Egg-shell LP-Ni/Al2 O3 catalyst derived from the LDHs precursor, with a smaller average size of nickel particles 4–6 nm, shows a stronger interaction between the nickel species and Al2 O3 support, due to the confinement effect of the “Ni O Al” in the LDHs main layer. The catalytic performance of the two kinds of Ni/Al2 O3 catalysts were evaluated for selective hydrogenation of PyGas, an important by-product of ethylene industry from thermal decomposition of heavier oil fractions. Experimental study of selective hydrogenation of styrene (PyGas model 1) was first carried out in a batch reactor for revealing the intrinsic hydrogenation kinetics of both Ni-based catalysts. The estimated effectiveness factor  of the LP-Ni/Al2 O3 catalyst was higher than that of the IM-Ni/Al2 O3 one, demonstrating the catalyst with egg-shell structure has a lower intraparticule mass transfer resistance. Further, egg-shell catalyst exhibited a superior catalytic hydrogenation performance for the selective hydrogenation of diolefins (PyGas model 2), as well as PyGas model 1, in a micro-flow reactor under various reaction conditions. A lower internal diffusion resistance for the reactants may result in a shorter residence time, limiting the formation of the detrimental polymer products for the egg-shell Ni catalyst. In addition, the smaller particle size of Ni species in this catalyst is suggested to lead to the enhanced catalytic activity and stability. Our investigation about the Ni-based supported catalysts for the selective hydrogenation may provide an evident proof for the control of the distribution of catalytically active component with the purpose of reducing the intraparticle diffusion resistance and enhancing their catalytic properties in industrial practical applications. Acknowledgements We would like to thank Professor Dianqing Li at Beijing University of Chemical Technology who offered us the pristine alumina

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apcata. 2013.08.040. References

X. Wen et al. / Applied Catalysis A: General 468 (2013) 204–215 [50] [51] [52] [53] [54] [55] [56] [57]

T. Fujiyama, M. Otsuka, H. Tsuiki, A. Ueno, J. Catal. 104 (1987) 323–330. J.-X. Wang, J.-F. Chen, Mater. Res. Bull. 43 (2008) 889–896. H. Chen, F. Zhang, S. Fu, X. Duan, Adv. Mater. 18 (2006) 3089–3093. H. Chen, F. Zhang, T. Chen, S. Xu, D.G. Evans, X. Duan, Chem. Eng. Sci. 64 (2009) 2617–2622. P. Benito, M. Herrero, C. Barriga, F.M. Labajos, V. Rives, Inorg. Chem. 47 (2008) 5453–5463. B.W. Hoffer, A. Dick van Langeveld, J.-P. Janssens, R.L.C. Bonné, C.M. Lok, J.A. Moulijn, J. Catal. 192 (2000) 432–440. A.V. Mashkina, Kinet. Catal. 49 (2008) 802–811. P. Arnoldy, J.A.M. van den Heijkant, G.D. de Bok, J.A. Moulijn, J. Catal. 92 (1985) 35–55.

215

[58] H.R. Reinhoudt, E. Crezee, A.D. van Langeveld, P.J. Kooyman, J.A.R. van Veen, J.A. Moulijn, J. Catal. 196 (2000) 315–329. [59] P.J. Mangnus, A. Bos, J.A. Moulijn, J. Catal. 146 (1994) 437–448. [60] L. Zhang, X. Wang, B. Tan, U.S. Ozkan, J. Mol. Catal. A: Chem. 297 (2009) 26–34. [61] G. Poncelet, M.A. Centeno, R. Molina, Appl. Catal. A: Gen. 288 (2005) 232–242. [62] J.A. Moulijn, A.E. van Diepen, F. Kapteijn, Appl. Catal. A: Gen. 212 (2001) 3–16. [63] C.H. Bartholomew, Appl. Catal. A: Gen. 212 (2001) 17–60. [64] K.O. Christensen, D. Chen, R. Lødeng, A. Holmen, Appl. Catal. A: Gen. 314 (2006) 9–22. [65] L. Pelletier, D.D.S. Liu, Appl. Catal. A: Gen. 317 (2007) 293–298.