Mesoporous ZrO2 Nanoframes for Biomass ... - ACS Publications

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Mesoporous ZrO2 Nanoframes for Biomass Upgrading Haiqing Wang, Hao Chen, Bing Ni, Kai Wang, Ting He, Yulong Wu, and Xun Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07567 • Publication Date (Web): 27 Jul 2017 Downloaded from http://pubs.acs.org on July 30, 2017

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ACS Applied Materials & Interfaces

Mesoporous ZrO2 Nanoframes for Biomass Upgrading Haiqing Wang,† Hao Chen,‡ Bing Ni,† Kai Wang,† Ting He,† Yulong Wu,*§ and Xun Wang*† † Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China. ‡ Department of Chemical Engineering, School of Chemical Engineering and Technology, Xi'an jiaotong University, Xi'an 710049, China. § Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China. KEYWORDS zirconia/ZrO2 • hollow structure • zirconocene dichloride • controllable synthesis • biomass upgrading

ABSTRACT: The rational design and preparation of high-performance catalyst for biomass upgrading are of great significance and remain a great challenge. In this work, mesoporous ZrO2 nanoframe, hollow ring, sphere and core-shell nanostructures have been developed through a surfactant-free route for upgrading biomass acids into liquid alkane fuels. The obtained ZrO2 nanostructures possess well-defined hollow features, high surface areas and mesopores. The diversity of resultant ZrO2 nanostructures should be arising from the discrepant hydrolysis of two different ligands in zirconocene dichloride (Cp2ZrCl2) as zirconium precursor. The timedependent experiments indicate that Ostwald ripening and salt-crystal-template (SCT) formation

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mechanisms should account for hollow spheres and nanoframes, respectively. Impressively, compared with hollow sphere, commercial nanoparticle, and ever reported typical results, ZrO2 nanoframe promoted Ni catalyst exhibits greatly enhanced catalytic activity in upgrading of biomass acids into liquid alkane fuels, which should be ascribed to the hollow feature, large active surface area, highly dispersed Ni and strong metal-support interactions arising from the structural advantages of naoframes. The nanoframes also possess excellent solvothermal and thermal stability. Our findings here can be expected to offer new perspectives in materials chemistry and ZrO2-based catalytic and other applications.

INTRODUCTION The accelerated depletion and exhaustion of non-renewable fossil fuels have raised great concerns over the issues of climate change and social instability. Bio-fuels derived from biomass are regarded as an alternative sustainable energy resource. Fatty acids i.e. octanoic acid (OA, C8) and stearic acid (SA, C18) exist extensively in vegetable and microalgae oils, respectively. However, their upgrading into liquid fuels needs development of sulfur-free catalysts to replace the traditional sulfided NiMo/Al2O3 catalysts. Lercher and co-works have found that sulfur-free supported metal catalyst i.e. ZrO2-promoted Ni (Ni/ZrO2) exhibited good performance for transforming crude microalgae oils into diesel-range alkanes as compared with other supports, such as Al2O3, and SiO2.1 However, it is still of great challenge to further improve the catalytic efficiency of Ni/ZrO2 catalysts. The metallic Ni and the synergistic effect between Ni and ZrO2 support derived from the adsorption of carboxylic group at the oxygen vacancy of ZrO2 can efficiently catalyze the rate-determining step, i.e. the hydrogenation of the fatty acid into aldehyde. Active particles with smaller size and higher dispersion are conducive to enhancing


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catalytic activity,2-6 which are greatly depending on the morphology, porosity, surface area, and crystal phase of catalyst supports.7-11 Therefore, delicate design and synthesis of ZrO2 catalyst support with proper architectures are of great significance and remain a great challenge to achieve high-performance catalyst for biomass upgrading. To date, although the rich synthetic chemistry of ZrO2 nanocrystals has been reported,12-16 we must consider the many unfavorable restrictions to the emerging applications, such as fuel-cell electrolytes,17 gate dielectrics,18 optical materials,19 memory technology,20 gas sensor and storage,21 biocarrier,22-24 nanogrowth,25 and catalysis.26-31. By contrast, mesoporous transitionmetal oxide hollow structures are becoming much highly desired, because of their unique characteristics such as shell permeability, high surface area, and low density for reactant adsorption and diffusion, the mechanical stress buffer, and light harvesting.22, 31-36 Moreover, the hollow feature as well as well-controlled shape and size can endow materials with specific functions for designated purposes. The currently reported methods for ZrO2 hollow spheres by Zheng22 and Yin33 groups are both based on sacrificial silica particles. Unfortunately, the methods are constrained with hard template. Meanwhile, the subsequent template removal or annealing process could lead to the collapse of the hollow nanostructures.37 Therefore, it is of great interest and still remains a great challenge to develop a flexible strategy for fabricating mesoporous ZrO2 hollow nanostructures with well-controlled morphology, size and stability. Previous reports found that the nanosynthesis and nanostructure of materials are greatly related to the type and hydrolysis of precursors.14,

38

Organic ligand has been an efficiently proved

strategy for nanostructure morphology and size control via a strong coordination to metal ions.3839

Based on these considerations above, we are intrigued by unique zirconocene dichloridethe

(Cp2ZrCl2) consisted of central zirconium atom and two kinds of ligands (i.e. chloride anion and


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organic cyclopentadiene) with different bonding strength. Here, we demonstrate that a series of mesoporous ZrO2 hollow nanostructures (nanoframe, hollow ring, sphere and core-shell) were fabricated using zirconocene dichloride as zirconium precursor, acetone as solvent and ammonia as basic source with ‘one-step’ and ‘surfactant-free’ synthesis. The morphology, size and hollow feature of ZrO2 nanostructures can be well modulated. The special flexibility of Cp2ZrCl2 for controllable synthesis of hollow ZrO2 nanostructures and the formation mechanisms for hollow sphere and nanoframe have been investigated. In order to demonstrate the structural advantages, the mesoporous ZrO2 hollow nanostructures promoted Ni catalysts were studied and compared in catalytic upgrading of octanoic acid (OA, C8) and stearic acid (SA, C18) into gasoline/dieselrange liquid alkane fuels via a tandem hydrogenation-decarbonylation route (HDC). EXPERIMENTAL SECTION Materials. Zirconocene dichloride (>97.0%) was purchased from TCI. Nano-ZrO2 (50 nm) was purchased from Shanghai Macklin Biochemical Co., Ltd. ZrCl4, acetone (AR), ammonia solution (25 wt.%) and other chemicals were purchased from Sinopharm Chemical Reagent Co. Synthesis of ZrO2 Nanoframes (ZFs). Zirconocene dichloride (0.1 g) was dissolved in acetone (30 mL) to form a colorless solution. After being stirred for 5 min, ammonia was added dropwise to the acetone solution of zirconocene dichloride. The obtained suspension was transferred to a 40-ml Teflon-lined stainless steel autoclave. The autoclave was sealed and heated in the oven at 200 oC for 12 h, and then was allowed to cool down to room temperature naturally. The precipitate was recovered using centrifugation at 10000 rpm for 3 min, followed by washing with acetone, ethanol and deionized water several times. Control experiments were conducted to study the effects of synthesis parameters (zirconium precursor, ammonia, time and temperature)


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on the nanostructures of the ZFs. Moreover, the role of solvent was demonstrated by replacing acetone with deionized water, ethanol, glycol, dimethylacetamide, formamide and acetic acid respectively. NaOH was used as basic source to study the role of ammonia. The zirconium source ZrCl4 was used to demonstrate the unique role of zirconocene dichloride. Synthesis of ZrO2 Hollow Ring, Sphere, and Core-shell. The hollow ZrO2 ring and sphere, and core-shell shared a similar synthetic procedure with above ZFs but without ammonia. The amounts of zirconocene dichloride were used with 0.05, 0.80, 0.10, and 0.15g, respectively. Synthesis of ZrO2-promoted Ni catalysts, Ni/ZrO2 Nanoframe (Ni/ZrO2-F), Hollow Sphere (Ni/ZrO2-H), and Commercial Nanoparticle (Ni/ZrO2-C) Catalysts. The catalyst supports of ZrO2-F and ZrO2-H were calcined at 500 oC for 2 h before Ni impregnation to stabilize the support avoiding potential structural change during the preparation of supported Ni catalysts. The Ni/ZrO2-F, Ni/ZrO2-H and Ni/ZrO2-C (Ni loading, 10 wt.%) were obtained with wetness impregnation method by mixing ZrO2 power with Ni(NO3)2 aqueous solution. 10 wt.% Ni/ZrO2-F was used to study the effects of reaction conditions and to compare with other catalysts. The wetness impregnation was treated overnight. After that, the solid powder was collected by drying the mixture in oven at 100 ºC for 12 h. Then the NiO/ZrO2 powder was obtained with calcination at 550 ºC for 4 h. After a reduction procedure in H2 at 450 ºC for 3 h, the Ni/ZrO2 catalysts was finally obtained. Material Characterization. X-ray diffraction (XRD) data were measured on a Bruker D8 Advance X-ray diffractometer equipped with Cu Kα radiation (λ=1.5418 Å). Transmission electron microscopy (TEM) graphs were observed by using a Hitachi H-7700 TEM operating at 100 kV. High-resolution TEM (HRTEM), dark-field scanning transmission electron microscopy


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(STEM), energy dispersive X-ray (EDX) element mapping tests were obtained on a FEI Tecnai G2 F20 STwin microscope at 200 kV. After degassing treatment of samples in vacuum at 150 oC for 2 h, nitrogen adsorption-desorption isotherms at 77 K were recorded using a Micromeritics ASAP 2010 surface area and porosity analyzer. The desorption branch data were used to achieve surface areas and pore size distributions of samples based on the calculations of BrunauerEmmett-Teller (BET) and BarrettJoyner-Halenda (BJH) respectively. X-ray photoelectron spectroscopy (XPS) were performed on a Thermo Fisher ESCALAB 250Xi spectrometer equipped with monochromatic Al Kα X-ray sources (1486.6 eV) at 2.0 kV and 20 mA. Temperature-programmed reduction of hydrogen (H2-TPR) was tested in the temperature range of room temperature to 700◦C with 5% H2/Ar as reducing gas and TCD as the detector. Catalytic Test. The upgrading of biomass acids into liquid alkane fuels was carried out in a stainless autoclave of 250 mL with an external electric heating. In a typical run, 0.1 g of Ni/ZrO2 catalyst, 1 g of octanoic acid, and 100 mL of decane were mixed in autoclave. The magnetic stirring rate was set to be 500 rpm. The hydrogen pressure (reaction temperature and time) was studied in the range of 1 to 5 MPa (240 to 330 oC, and 0.5 to 4 h, respectively). After the reactor autoclave was cooled naturally, the resulting solution was analysed on gas chromatograph (Agilent GC6820) equipped with capillary column (GsBP-Inowax, 30 m×0.32 mm×0.25 µm) and FID as detector. The detailed analysis parameters (injection volume, inlet and detect temperature, and split ratio) for gas chromatograph are 1 µL, 280 oC, 280 oC, and 10:1, respectively.

The

conversion

rate

is

calculated

using

the

equation

of

C%=(Rconverted/Rinitial)×100%, where C, Rconverted, and Rinitial represent conversion rate, converted and initial reactant respectively. The yield data are collected by Y%=(Pproduct/Pinitial)×100%, where Y, Pproduct, and Pinitial denote as yield, carbon atoms in products and initial reactant


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respectively. The turnover frenquency (TOF) is calculated by TOF=Mconverted/(MNi·tR), where Mconverted, MNi and tR represent the molar number of converted reactant, the molar number of total Ni and reaction time. RESULTS AND DISCUSSION

Figure 1. a) TEM images of hollow cubic ZrO2 nanoframes (ZFs) in different views, b) TEM image in highmagnification, c and d) HRTEM images, e) STEM image, f) XRD pattern, g) and h) EDS line scanning profiles in different directions, and i) N2 physisorption isotherm of ZFs (inset, pore size distribution).

Transmission electron microscopy (TEM) images show well-defined cubic ZrO2 nanoframes (ZFs, Figure 1a) synthesized with ammonia introduction in different views, which assume an edge length of ~250 nm and thickness of ~60 nm. The absence of interior core and facets of ZFs implies good permeability in three-dimentional space. Close observations with highmagnification TEM (Figure 1b) demonstrate that ZFs possess highly rough surfaces. Meanwhile,


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as shown in high-resolution TEM (HRTEM) images (Figure 1c and 1d), the primary ZrO2 nanocrystals exhibit an average size of

Mesoporous ZrO2 Nanoframes for Biomass ... - ACS Publications

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