Aqueous Phase Hydrodeoxygenation of Phenol over Ni3P-CePO4

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Kinetics, Catalysis, and Reaction Engineering

Aqueous Phase Hydrodeoxygenation of Phenol over Ni3P-CePO4 Catalysts Zhiquan Yu, Yao Wang, Shan Liu, Yunlong Yao, Zhichao Sun, Xiang Li, YingYa Liu, Wei Wang, Anjie Wang, Donald M. Camaioni, and Johannes A. Lercher Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01606 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 17, 2018

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Industrial & Engineering Chemistry Research

Aqueous Phase Hydrodeoxygenation of Phenol over Ni3P-CePO4 Catalysts Zhiquan Yu,†‡ Yao Wang,†‡ Shan Liu,†‡ Yunlong Yao,†‡ Zhichao Sun,†‡ Xiang Li,†‡ Yingya Liu,†‡ Wei Wang,†§ Anjie Wang,†‡* Donald M. Camaioni,ǁ and Johannes A. Lercherǁ⊥ †

State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University

of Technology, Dalian 116024, China ‡

Liaoning Key Laboratory of Petrochemical Technology and Equipment, Dalian University of

Technology, Dalian 116024, China §

ǁ

Yinchuan Energy Institute, Yongning Wangtaibu, Yinchuan 750105, China Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999,

Richland, Washington 99352, USA ⊥

Department of Chemistry and Catalysis Research Center, Technische Universität München,

Lichtenbergstrasse 4, 85748 Garching, Germany

Corresponding author: Prof. Anjie Wang

E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Unsupported Ni3P-CePO4 catalysts were prepared by co-precipitation, followed by drying, calcination, and temperature-programmed reduction. The prepared catalysts were characterized by XRD, N2 adsorption-desorption, TEM, STEM-EDS elemental mapping, XPS, NH3-TPD, FT-IR of adsorbed pyridine and H2-TPR. Their catalytic performance in hydrodeoxygenation (HDO) were investigated using aqueous solution of phenol (5.0 wt%) as the feed. CePO4 was generated in the co-precipitation, and stable in the subsequent drying, calcination and temperature-programmed reduction (final temperature 500 °C). It is shown that the addition of CePO4 resulted in enhanced HDO activity and a maximum activity appeared at Ce/Ni ratio of 0.3. The presence of CePO4 improved the dispersion of Ni3P significantly, leading to enhanced hydrogenation activity. CePO4 served as the major dehydration sites as well, because of its surface acidity (mainly Lewis acid). In addition, the kinetics of aqueous phase HDO of phenol and cyclohexanol catalyzed by Ni3P and by Ni3P-CePO4 with Ce/Ni ratio of 0.3 were investigated.

KEYWORDS: Ni3P; CePO4; Aqueous phase; Hydrodeoxygenation; Phenol; Kinetics

1. Introduction With the dwindling of fossil fuels, increasing attention has been paid on the research of the production of fuel and bulk chemicals from renewable biomass. Compared with the petroleum fuel, the high oxygen content of bio-oils imparts serious disadvantages, such as low energy density, high viscosity, poor thermal and chemical stability.1-3 As a result, bio-oils must be updated by removing oxygen through deoxygenation process. Hydrodeoxygenation (HDO) is regarded as one of the promising method to upgrade bio-oils. Bio-oils from lignin-derived


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biomass contain around 30 wt% phenolic compounds, including phenol and its substituted derivatives, guaiacol and syringol.2,3 In addition, phenols are frequently found as the intermediates in the products in HDO of other oxygen-containing components, such as benzofuran, dibenzofuran and aryl alkyl ethers,4-6 suggesting that the deoxygenation of phenol and its derivatives are rate-determining. Therefore, phenol and its substituted derivatives (such as catechol, cresol, guaiacol) are often selected as the model compounds in HDO. Metal phosphides are reported to be active in HDO of various oxygen-containing compounds.4,7-12 Recently, we found that unsupported Ni3P was active and stable in catalyzing phenol HDO both in aqueous phase and in organic phase.13 The stability of the catalyst in aqueous phase is essential in HDO, because water is the by-product and high concentration of water is generally present in bio-oils. In previous study, although unsupported Ni3P showed high hydrogenation activity at temperature as low as 150 ºC, the catalyst exhibited low deoxygenation activity because the predominant product was cyclohexanol. The conversion of cyclohexanol to oxygen-free cyclohexene through acid-catalyzed dehydration becomes the key to enhance the overall HDO efficiency. Apparently, an introduction of solid acid materials will help to construct a bi-functional catalyst with both hydrogenation and dehydration sites, which are involved in phenol HDO. Proton-type zeolites are among the industrially important acid carriers, and were investigated in the supported HDO catalysts.7,9,14,15 However, the strong surface acid sites catalyze the side reactions such as cracking and isomerization, in addition to the dehydration in HDO network. Cerium phosphate (CePO4) is used in many fields as a functional material, and has received considerable attention in the catalysis area.16-19 Li et al. studied the selective catalytic reduction of NO with NH3 over CePO4, and found that CePO4 prepared by co-precipitation method


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presented acidity.20 The medium Lewis acid and weak Brønsted acid in rare earth phosphates are originated from rare earth cations and HnPO4-3+n species, respectively.21 Wang et al. correlated well the Lewis acid content with the surface Ce4+ amount in CePO4, and found that a linear relationship was displayed between the Lewis acid amount and the catalytic performance in the dehydration of glucose to HMF.22 Kanai et al. reported that the Lewis acid sites in CePO4 was responsible for the chemo-selective acetalization between 5-hydroxymethylfurfural (HMF) and alcohols by enhancing the electrophilicity of the reactant.23 Addition of Ce species not only decreased the particle size of catalyst significantly, but also enhanced the hydrogenation and C-N bond cleavage activities of Ni2P in the hydrodenitrogenation (HDN) of quinoline.24 In the present study, Ni3P-CePO4 catalysts were prepared by temperature programmed reduction (TPR) method from phosphate precursors, which were prepared via co-precipitation. Their catalytic performance was tested in the aqueous phase HDO of phenol. 2. Experimental Section 2.1. Catalyst Preparation. The precursors of Ni3P-CePO4 catalysts were prepared by a coprecipitation method, with an initial Ni/P molar ratios of 3/(1+3x) (x represented the Ce/Ni molar ratio). A typical preparation procedure for the precursor with x = 0.3 is as follows. 6.61 g Ni(NO3)2·6H2O and 2.96 Ce(NO3)3·6H2O were dissolved in 15 mL de-ionized water under stirring to obtain Solution A. 1.9 g (NH4)2HPO4 was dissolved in 5 mL de-ionized water to form Solution B. Solution B was added dropwise into Solution A under stirring to yield a slurry. The slurry was heated over an electric oven to evaporate water, until a solid product was obtained. The solid product was dried at 120 °C for 12 h, and then calcined at 500 °C for 3 h to obtain the catalyst precursor. The precursor was transformed into nickel phosphide by in situ temperature


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programmed reduction (TPR) in H2.25 For comparison, unsupported Ni3P was prepared following the same procedure with an initial Ni/P molar ratio of 3. CePO4 was prepared by a co-precipitation method as follows. 6.58 g Ce(NO3)3·6H2O was dissolved in 10 mL de-ionized water under stirring to form Solution A. 2.0 g (NH4)2HPO4 was dissolved in 5 mL de-ionized water to form Solution B. Solution B was added dropwise into Solution A under stirring. The resultant slurry was heated to evaporate water to obtain the product CePO4. To test the thermal stability of CePO4, it was calcined at 500 °C for 3 h prior to XRD characterization and HDO reaction. 2.2. Catalyst characterization. Prior to characterization, the prepared catalysts by TPR method were passivated in a 20 mL·min-1 O2/Ar flow (0.5 vol% O2) for 2 h at ambient temperature. XRD patterns were collected on a Rigaku D/Max 2400 diffractometer with nickel-filtered CuKα radiation at 40 kV and 100 mA. The BET specific surface area was calculated from the N2 adsorption-desorption isotherms, which were measured at -196 °C on a Micrometritics Tristar II 3020 model. Transmission electron microscopy (TEM) and scanning TEM (STEM)-EDX elemental mapping were observed on a Tecnai G2 F30 transmission microscope operated at 300 kV. The sample for TEM observation was prepared by depositing a drop of an ultrasonic-treated ethanol suspension of the solid material onto a carbon-coated Cu grid. XPS spectra were measured on a Multilab2000 X-ray photoelectron spectrometer, using an Mg-Kα source. All binding energies were referenced to the C 1s peak at 284.6 eV.


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NH3-TPD profile was obtained on a Chembet-3000 analyzer. Before measurements, 0.2 g sample was re-reduced in H2 for 1 h at 500 ºC, and then cooled to 40 ºC and exposed to NH3 for 30 min. In a He flow, the temperature was raised to 500 ºC at 10 ºC·min-1. The desorption signal was recorded by a TCD. A standard sample (HZSM-5) with known acid site concentration was used to calibrate the acid amount. The FT-IR spectra of adsorbed pyridine were recorded using an Equinox 55 spectrophotometer operated at a resolution of 4 cm-1. Prior to pyridine adsorption, the sample was treated in the cell in vacuum (p = 7×10-4 Pa) at 450 °C for 1 h, and then exposed to pyridine at 40 °C until saturation. The cell was evacuated at 7×10-4 Pa for 30 min to remove the physically adsorbed pyridine. The cell was evacuated sequentially at 150 °C for 1 h, 300 °C for 45 min, and 450 °C for 30 min, and the IR spectra of the chemisorbed pyridine were recorded, respectively. H2-TPR was carried out on a Chembet-3000 analyzer with a TCD. The calcined sample (0.2 g) was reduced in a stream of H2/Ar (10 vol.% H2, flow rate: 50 mL·min-1) at a heating rate of 10 ºC·min-1 up to 700 ºC. 2.3. HDO performance. The HDO of phenol was carried out in a stainless-steel tubular reactor (10.0 mm i.d.). The precursor was pelleted, crushed, and sieved to 0.4-0.8 mm, and 0.2 g was charged in the middle of the reactor. Prior to HDO reaction, the precursor was transformed into metal phosphide by in situ H2 TPR. The reduction conditions were as follows: H2 flowrate: 150 mL·min-1, H2 pressure: 1.0 MPa, temperature program: room temperature to 400 °C at 2 °C·min-1, 400 °C to 500 °C at 1 °C·min-1, and then kept for 2 h. Afterwards, the reactor was cooled to the reaction temperature, and was pressurized with hydrogen (99.99%) (100 mL·min-1)


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and kept at 4.0 MPa by means of a back-pressure regulator at the outlet. An aqueous solution of 5.0 wt% phenol was fed by a high-pressure LC pump. After the reaction system had achieved a steady state (in 2 h), the liquid product was sampled from the gas-liquid separator. The organic product was extracted by dichloromethane and then dehydrated by anhydrous magnesium sulfate. The dried organic phase was analyzed by a gas chromatograph (Agilent 6890, FID) with a HP-INNOWax capillary column (30 m×320 µm×0.5 µm). The gas product was analyzed at different temperatures by means of gas chromatograph (Agilent, HP-AL/S capillary column and FID). It is shown that no gaseous hydrocarbon was detectable. The conversions and product selectivity were determined on carbon mole basis. Though phenol was readily hydrogenated, the predominant product at low temperatures was oxygencontaining compound (i.e. cyclohexanol). In other words, the conversion of phenol does not reflect the performance of HDO. Therefore, HDO conversion was used in the investigation:

=

[] [][][] []

× 100%

(1)

where [phenol]0, [phenol], [cyclohexanone], and [cyclohexanol] represent the concentration of phenol in the feed, and those of phenol, cyclohexanone, and cyclohexanol in the product. [%&'(]

! "# #$ = ∑[%&'(]) × 100%

(2)

)

where [product]i (mol·L-1) represents the concertation of product i. 2.4. HDO Kinetic study. Weight time is defined as:

+=

,-./

(3)

0112


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where wcat denotes the catalyst weight (0.1 g) and nfeed the total mole flow rate of the feed.26 τ was adjusted by varying the flow rates of the liquid (0.1-0.3 mL⋅min-1) and gas (100-300 mL⋅min-1), while keeping their ratio constant. The absence of internal mass-transfer limitation at the investigated conditions was confirmed by the calculation of the Weisz-Prater criteria (Table S1).10,27 In addition, the preliminary test showed that the conversion of phenol was constant when the total flow rate and the catalyst weight were varied while keeping τ constant. It suggests that there was no external diffusion limitation either. The HDO reaction of phenol in large excess of hydrogen was generally treated as a pseudo first order to the concentration of phenol.28-30 If the fixed-bed reactor is treated as a plug flow reactor, then − ln61 − x8 = kτ (4) where x was the conversion of phenol. 3. Results and discussion 3.1. Synthesis and Characterization. The XRD patterns of Ni3P and Ni3P-CePO4 with various Ce/Ni ratios are presented in Figure 1. All patterns showed diffraction peaks at 2θ = 41.8, 43.6, 46.6, and 52.8 º, attributable to the (321), (112), (141), and (312) planes of Ni3P (PDF 34-0501), respectively. It suggested that the addition of Ce species did not affect the formation of Ni3P phase. With increasing the Ce/Ni ratio, the diffraction peaks at 2θ = 14.5, 20.0, 28.4, and 31.4 º, which are attributed to the (100), (101), (120), and (112) planes of CePO4, became increasingly stronger. Moreover, neither CeO2 phase nor cerium phosphide was detectable. It is apparent that


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Ce species reacted with phosphate in the course of catalyst preparation. In order to verify the reaction step of CePO4 formation, XRD characterization was used to check the phases of the precursors after drying and after calcination, in comparison of bulk CePO4 (Figure 2). It is illustrated that CePO4 was produced during the co-precipitation, and survived from the high temperature calcination and subsequent temperature-programmed reduction in the preparation of Ni3P catalysts. For Ni3P-CePO4 precursors after calcination, the diffraction peaks characteristic of NiO decreased with increasing Ce/Ni molar ratio, indicating that the introduction of Ce species led to decreased particle size of NiO in the precursors. On the other hand, Figure 1 shows that, with increasing Ce/Ni molar ratio, the peaks of Ni3P phase became increasingly broadened and less intense. The crystal sizes of Ni3P and Ni3P-CePO4 catalysts calculated by Scherrer’s equation are presented in Table 1. It is indicated that the introduction of Ce species reduced the sizes of Ni3P crystallites. The N2 adsorption and desorption isotherms of Ni3P and Ni3P-CePO4 are shown in Figure S1, and the BET specific surface areas (SBET) (Table 1) evaluated from the N2 adsorption-desorption isotherms are summarized in Table 1. The SBET of unsupported Ni3P was 3.2 m2·g-1, whereas those of Ni3P-CePO4(x) catalysts ranged from 34.9 to 75.1 m2·g-1. Apparently, it is indicated that the introduction of Ce species to unsupported Ni3P resulted in increased SBET of the catalysts, indicating the improvement of Ni3P dispersion, which further verified the decreased particle size.


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Figure 1. The XRD patterns of Ni3P and Ni3P-CePO4 with various Ce/Ni ratios.

Figure 2. The XRD patterns of the precursors and CePO4 after drying (a) and after calcination (b).

Table 1. The physical properties of Ni3P and Ni3P-CePO4 with various Ce/Ni ratios. catalyst SBET (m2·g-1) crystallite size (nm) a particle size (nm) b Ni3P 3.2 78.1 85.8


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34.9 Ce/Ni=0.05 Ce/Ni=0.1 47.6 Ce/Ni=0.2 62.7 Ce/Ni=0.3 68.0 Ce/Ni=0.4 75.1 a Calculated by Scherrer’s equation. b Measured by TEM.

28.3 25.2 23.6 7.9 5.1

30.2 23.9 18.7 15.0

The TEM images of unsupported Ni3P and Ni3P-CePO4 catalysts are shown in Figure 3, together with their particle size distribution. It could be seen that the particle size of the catalysts ranged from approximately 15 to 86 nm and the unsupported Ni3P had the largest particle size. In agreement with the results of XRD characterization, the particle size of Ni3P in Ni3P-CePO4 catalysts decreased with increasing the Ce/Ni ratio. EDS mapping (Figure S2) revealed that CePO4 was dispersed homogeneously in the Ni3P-CePO4 catalyst with Ce/Ni ratio of 0.3.


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Figure 3. TEM images and particle size distributions of Ni3P (a and f), and Ni3P-CePO4 with various Ce/Ni ratios: 0.05 (b and g), 0.2 (c and h), 0.3 (d and i) and 0.4 (e and j).

The XPS spectra of Ni3P and Ni3P-CePO4 catalysts are illustrated in Figure 4. Three broad peaks were observed in the Ni 2p region. The peak at 854.5-852.1 eV was related to the reduced Niδ+ (0 < δ < 2), whereas the two others are assigned to Ni2+.31 In the P 2p spectra, the peaks at 129.2-129.6 and 132.0-134.0 eV can be attributed to Pδ- (0 < δ

Aqueous Phase Hydrodeoxygenation of Phenol over Ni3P-CePO4

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