Bulk Ni-Mo Composites Prepared by Solid Reaction Method and Their Hydrodeoxygenation Performance

Ji Daoyu; Liu Di; Zhang Zongliang; Xu Dongmei; Gao Peng

(1. College of Chemical and Environmental Engineering, Shandong University of Science and Technology,Qingdao 266590; 2. State Key Laboratory of Bioactive Seaweed Substances, Qingdao Brightmoon Seaweed Group Co., Ltd., Qingdao 266400)

Abstract: Bulk Ni-Mo composites were prepared by a simple solid reaction method and the hydrodeoxygenation activity of samples was examined. The test results showed that the Ni-Mo catalysts possessed high catalytic activity for hydrogenation of p-cresol under mild conditions. The XRD, N2 isothermal adsorption, NH3-TPD characterization analyses indicated that the excellent hydrogenation performance of Ni-Mo catalysts could be attributed to their incorporated Mo metal, the developed pore system, and the strong acidity.

Key words: bulk Ni-Mo composites; solid reaction; catalyst; hydrodeoxygenation

1 Introduction

Great efforts have been made on the exploitation of renewable energy due to the lack of fossil fuels and the increase of global energy consumption requirements in the past few decades[1-3]. Biomass has attracted more attention as a sort of abundant and renewable energy[4-6].However, bio-oils are rich in oxygenated molecules, such as phenols, aldehydes, ketones, alcohols, and carboxylic acids, which are responsible for some drawbacks such as low heating value, high viscosity, low volatility, and corrosiveness, so it is necessary to improve the quality of bio-oil[7-9].

Bio-oil could be upgraded by hydrodeoxygenation(HDO) reaction. Catalyst systems are crucial for the HDO process[5,10-13]. The Ni or Co promoted Mo- or W-based sulfides are widely used as catalysts in the bio-oil HDO reaction[14-17]. Unfortunately, the sulfided catalysts are quickly deactivated because of sulfur removal from the catalysts under the circumstance of low content of sulfur, while the final products are contaminated with sulfur[18]. Noble metal possesses a high hydrogenation activity[19]. However, low selectivity for C-O bond cleavage and the high cost of noble-metal catalyst prevent their wide practical application[20]. Moreover, Zhao, et al.[21]investigated the HDO performance of transition metal phosphides.And the results showed that transition metal phosphides were unstable due to the loss of P element during the HDO process, although metal phosphides exhibited good activity. Also, the above catalysts usually required severe reaction conditions (high reaction temperature and pressure). Recently, the transition metals or partially reduced transition metal oxides were employed to catalyze HDO of oxygenated compounds[22-26].Especially, Bykova[26]reported the HDO of guaiacol over the supported Ni-based catalyst, and found that the Ni-based catalyst had high HDO activity, and addition of copper could further improve the catalytic activity,so transition metals or partially reduced transition metal oxides are considered as a type of promising hydrotreatment catalysts.

Based on the current literature reports and our previous studies[27-29], we prepared a bulk Ni-Mo composite with high surface area by the solid reaction method and investigated its HDO activity using p-cresol as a model compound in this work.

2 Experimental

2.1 Catalyst preparation

The bulk Ni-Mo catalysts were prepared by the solid reaction method. Ammonium heptamolybdate[(NH4)6Mo7O24·4H2O] was dissolved in deionized water to form a solution, and then the basic nickel carbonate [NiCO3·2Ni(OH)2·4H2O] was added to this solution. Subsequently, the suspension was added into a magnetically agitated autoclave, sealed and held at 150 °C for 6 h under vigorous stirring. After the termination of reaction, the resulting suspension was filtered, and the obtained precipitate was washed with water and absolute ethanol to remove the water-soluble impurities,followed by subjecting to drying at 60 °C, and 120 °C for 2 h, respectively. Finally, the Ni-Mo composition was obtained by calcining the precipitate at 375 °C for 2 h.The composition of the samples was adjusted by changing the initial Ni/Mo molar ratio. The samples were named as NiMo-R, where R represented the charge ratio of Ni/Mo.In addition, the pure Ni sample was prepared by the same method, albeit without the addition of molybdenum salt in the course of preparation.

2.2 Catalyst characterization

The X-ray powder diffraction (XRD) analysis was carried out with a D/Max2500P diffractometer using CuKα radiation (λ=0.154060 nm) operating at 40 kV and 40 mA. The 2θ was scanned over the range of 10°—80° at a rate of 10(°)/min. The BET specific surface area (SSA),average pore diameter, and pore volume of the catalysts were determined by N2isothermal adsorption using a Quantachrome Quadrasorb surface area analyzer. The pore-size distributions of the samples were determined from the isotherms by the Barrett-Joyner-Hallenda (BJH)method. The elemental composition was measured on a PANalytical Axios X-ray fluorescence spectrometer(XRF). The ammonia temperature-programmed desorption (NH3-TPD) measurement of the catalysts was carried out on an AutoChem 2950 HP analyzer. Firstly,the catalyst (100 mg) was reduced in a flow of H2at 375 °C for 1 h, and was purged with He (50 cm3/min)at 450 °C for 1 h, prior to being cooled down to 100 °C.The adsorption of ammonia (20 min) at this temperature was performed. Subsequently, a He flow (at a rate of 35 cm3/min) was passed to eliminate the physisorbed ammonia. Finally, the NH3-TPD analysis was carried out by heating the samples from 100 °C to 800 °C at a temperature increase rate of 10 °C/min. The evolved ammonia was analyzed by a TCD detector.

2.3 Catalyst testing

The HDO reaction was carried out in a fixed-bed reactor.Before the reaction, the catalysts were activated with a H2stream at a flowrate of 0.02 L/min and a temperature of 375 °C for 2 h. After the pretreatment, H2and the reaction mixture (15% of p-cresol in decalin) were fed into the reactor. The reaction was carried out under the conditions covering: a reaction temperature of 200 °C under atmospheric pressure, a H2flowrate of 0.03 L/min,a WHSV of 1.5 h-1, and a H2/oil volume ratio of 150. The liquid products collected from a gas-liquid separator were analyzed by a GC-FID chromatograph (Shimadzu GC 2010plus) and a GC-MS system (Agilent 6890/5973N).The conversion and deoxygenation rate (D.D.) for each experiment are calculated as follows:

3 Results and Discussion

The bulk Ni-Mo composites with different Ni/Mo ratio were prepared by the solid reaction method. Firstly the composition of the samples was measured (as shown in Table 1) with the results indicating that the Ni/Mo atomic ratios (R) of the four catalysts were 8.61, 4.45,3.04, and 2.42, respectively. The ratios were higher than those expected (the expected R = 8, 4, 2.67, and 2,respectively), which indicated that there was a greater loss for Mo than Ni in the course of the solid reaction.The HDO catalytic activity of the samples using p-cresol as the model substance was investigated, with the results shown in Figure 1. On the pure Ni catalyst, a p-cresol conversion of 45.4% and a deoxygenation rate of 27.3% revealed the low HDO activity of the pure Ni catalyst. As expected, the activity of the Ni-based catalyst increased significantly when molybdenum was introduced. The NiMo-8 catalyst showed the highest activity (with a conversion of 100%, and a deoxygenation rate of 91.4%) among all samples. As the Mo content in the catalysts further increased, the activity of catalysts gradually decreased. In general, the bulk Ni-Mo composites exhibited excellent HDO activity under mild reaction conditions (a reaction temperature of 200°C under atmospheric pressure). In order to clarify the mechanism of different catalytic performance of these catalysts, the textual properties of Ni-Mo catalysts were determined by the N2isothermal adsorption technique.

Table 1 Elemental analysis of bulk Ni-Mo samples

Figure 1 HDO of p-cresol on the bulk Ni-Mo catalysts

The NiMo samples exhibited the type IV isotherms with a developed hysteresis loop characteristic of mesoporous materials, which could be attributed to the adsorption occurring in the pores of the Ni-Mo composition except for the Ni sample (Figure 2). The textural properties of the catalysts were summarized in Table 2. It was found that the sample with a Ni/Mo molar ratio of 8 had the most developed pore system (261.8 m2/g, 0.67 mL/g),and the SSA and pore volume of the samples decreased as the Mo content further increased, while the SSA and pore volume of Ni catalyst were only 79.8 m2/g and 0.12 mL/g, respectively. The result of N2isothermal adsorption showed that the SSA and pore volume of the samples increased significantly when molybdenum was incorporated. The trend of variation in the pore system of the catalysts was consistent with that of their catalytic activity (as shown in Figure 1). It was understandable that the developed porous structure was beneficial to the exposure of more catalytic active sites, resulting in the improvement of the catalyst performance.

Figure 2 N2 adsorption-desorption profiles of the bulk Ni-Mo catalysts

Table 2 Textural properties of the bulk Ni-Mo catalysts

The XRD measurements were carried out to further investigate the structure of the materials (Figure 3). The XRD patterns of pure Ni, NiMo-8, and NiMo-4 samples exhibited four diffraction peaks at 2θ=37.10°, 43.10°,62.59°, and 75.04°, which were assigned to the (111),(200), (220) and (311) planes of NiO (ICDD PDF 65-2901), respectively. And the diffraction peak intensity of NiO gradually became weak as the Ni/Mo molar ratio decreased. No diffraction peak of separated Mo was observed in the XRD patterns of NiMo-8 and NiMo-4,which indicated that Mo was well dispersed on the surface of NiO, making the Mo species undetectable by the XRD method. The NiMoO4crystal was detected at around 44.2°in addition to NiO in the XRD patterns of NiMo-2.67 and NiMo-2. The production of NiMoO4crystals had led to the decreased SSA and pore volume of the catalysts, which should be responsible for the low catalytic activity.

Figure 3 XRD patterns of the bulk Ni-Mo catalysts

We also found that the selectivity of deoxygenation products declined as the Ni/Mo molar ratio increased as shown in Figure 1. It was well known that the acidity of the catalysts had a significant effect on the cleavage of C-O bonds. Thus, the NH3temperature-programmed desorption measurements were performed to monitor the acidity of the Ni-Mo catalysts (Figure 4). In the spectra of NiMo-2 catalyst, two small humps centered at 450 °C and 635 °C were observed, which demonstrated that this catalyst possessed a very weak acidity and a small amount of acidic sites. As for NiMo-8, NiMo-4, and NiMo-2.67 catalysts, a broad peak at about 450 °C and a distinct peak at about 635 °C appeared, showing that the medium and strong acidic sites existed simultaneously on these catalyst surface with the dominance of strong acidity[10,30-31]. As the Ni/Mo molar ratio increased, the intensity of NH3desorption peak at about 635 °C increased gradually and the center position of the peak moved to the right slightly,which meant that the amount of strong acid sites and acid strength increased. Apparently, the acidity of the catalysts was beneficial to the deoxygenation of the oxygencontaining compounds, so the NiMo-8 catalyst possessed the highest selectivity of deoxygenation products.

In general, the bulk Ni-Mo composites prepared by the solid reaction method had the developed pore structure.The introduction of molybdenum could greatly increase the SSA and pore volume of the samples, but the mechanism for this was unclear. The Ni-Mo catalysts exhibited excellent catalytic activity for hydrogenation reaction of p-cresol under mild conditions. According to the above-mentioned analytical results, the excellent hydrogenation performance of Ni-Mo catalysts could be attributed to the incorporated Mo and the developed pore system along with the strong acidity.

Figure 4 NH3-TPD profiles of the bulk Ni-Mo catalysts

4 Conclusions

In this paper, we prepared a bulk Ni-Mo catalyst with the developed pore structure via the simple solid reaction method. The Ni-Mo catalyst showed the advantages of excellent catalytic activity and simple preparation method. More work is under way to further explore its physicochemical property and catalytic performance for treating the real bio-oils.

Acknowledgements:This work was supported by grants from the National Natural Science Foundation of China(No. 21306106) and the Open Foundation of the State Key Laboratory of Bioactive Seaweed Substances, Qingdao Brightmoon Seaweed Group Co., Ltd. (No. SKL-BASS1723).