Elsevier

Fuel

Volume 111, September 2013, Pages 81-87
Fuel

Support effects on hydrotreating of soybean oil over NiMo carbide catalyst

https://doi.org/10.1016/j.fuel.2013.04.066Get rights and content

Highlights

  • Supported bimetallic oxide precursors were prepared by impregnation method.

  • Carbides of the catalysts were formed by temperature-programmed reduction (TPR).

  • NiMoC/Al-SBA-15 shows the highest surface area and largest pore volume.

  • NiMoC/Al-SBA-15 gave the highest yield to diesel range hydrocarbons.

Abstract

As an alternative to conventional sulfide catalysts, NiMo carbide catalysts were prepared by impregnation method and studied for the hydrotreating of soybean oil to produce hydrocarbons in the gasoline to diesel range. The effect of the catalyst supports on activity and selectivity was investigated by using three different types of materials: mesoporous material (Al-SBA-15), alumina (γ-Al2O3) and zeolites (ZSM-5, Zeolite β and USY). The structural characterization of the catalysts was performed using XRD, BET and TEM. Catalytic tests were carried out in a bench scale flow reactor at 400 °C and 650 psi. The results showed that the Al-SBA-15 supported catalyst has the highest surface area of 711.5 m2 g−1 and largest pore volume of 0.71 cm3 g−1. Among the five catalysts, hydrotreating on NiMoC/Al-SBA-15 gave the highest yield of organic liquid product (96%) and highest selectivity (97%) to hydrocarbons in the boiling range of the diesel fraction. For the three zeolite-supported catalysts, hydrotreating of soybean oil produced more hydrocarbon products in the boiling range of green gasoline (about 15–40%).

Introduction

Due to volatility in the price of fossil fuels, world energy security and environmental concerns, economical techniques for the production of biofuels from alternative and renewable sources must be developed. It is well known that triglyceride based vegetable oils, animal fats, and recycled grease have the potential to be suitable sources of fuel under the right processing conditions.

Currently, lipid-based feedstocks can be converted into liquid hydrocarbon fuels by hydrotreating processes similar to those found in the oil and gas refining industry [1], [2]. Conventional γ-Al2O3 supported sulfided bimetallic catalysts (usually Mo- or W-based sulfides promoted with Ni or Co) are presently used for desulfurization of fossil diesel streams in a process requiring high energy consumption, high temperature, high pressure, and with large amounts of hydrogen [3]. The products obtained are essentially n-paraffins (n-C15 up to n-C18) solidifying at subzero temperatures and as such are unsuitable for high quality diesel fuels, kerosene and gasoline compounds [4]. The process is costly and the yield of product can be low because of coke formation, which causes catalyst deactivation and pressure build-up in the reactor [5]. More importantly, the base metals in these hydrocracking catalysts need to be maintained in their sulfided form in order to be active at process conditions, and therefore, a sulfurization co-feed needs to be added to the feedstock [6].

In order to address the above issues, a number of studies have been carried out to develop non-sulfided catalysts with high activity, good selectivity and long lifetime in a hydrotreating process [7], [8], [9], [10], [11], [12], [13]. Among them, the nitrides and carbides of early transition metals have been identified as a new class of hydrotreating catalysts that are competitive with the conventional bimetallic sulfided catalysts. These catalysts exhibit high activity similar to the noble metals because the introduction of carbon or nitrogen into the lattice of the early transition metals results in an increase of the lattice parameter a0 and an increase in the d-electron density [14]. Han et al. [11] reported that a transition metal carbide catalyst, Mo2C, supported on multi-walled carbon nanotubes, showed 90% conversion and 91% hydrocarbon selectivity for one-step conversion of vegetable oils into branched diesel-like hydrocarbons. Most recently, Han et al. [15] successfully prepared ordered mesoporous carbon (OMC)-supported molybdenum carbide catalysts. The catalysts exhibited high product selectivity and excellent resistance to leaching in the conversion of vegetable oils into diesel-like hydrocarbons. Nitrides of molybdenum, tungsten and vanadium supported on γ-Al2O3 were also used for hydrodeoxygenation of oleic acid and canola oil [12]. The oxygen removal exceeded 90% over the supported molybdenum catalyst for a long reaction duration (450 h) and the yield of middle distillate hydrocarbons (diesel fuel) ranged between 38 and 48 wt%. Moreover, bimetallic nitride and carbide catalysts were found to be much more active and stable than the mono-metallic ones [16], even though no application in the biomass hydrotreating process has been reported.

While the nitrides and carbides of early transition metals have been evaluated as hydrotreating catalysts to convert vegetable oils to biofuels, no systematic study exists on the effect of the support on the hydrotreating activity of the catalysts, even though the support plays an important role in the cracking function for hydrotreating catalysts [17], [18]. The support contributes to the cracking of the C–O or C–C bond and to the isomerization of the resultant n-olefins, that, after hydrogenation, are transformed into isoparaffins [10], [19]. Hence, the aim of this work is to investigate the effects of support on hydrotreating activity of bimetallic (NiMo) carbide catalysts.

In this study, the preparation of Al-SBA-15 with Si/Al = 80 and hydrotreating catalysts based on this mesoporous material along with commercialized γ-Al2O3, ZSM-5 (Si/Al ratio = 80), Zeolite β (Si/Al ratio = 38) and USY zeolite (Si/Al ratio = 80) are presented. Nickel and molybdenum were impregnated as active metals. The carbides of the catalysts were evaluated for hydrotreating of soybean oil in a bench-scale plug flow reactor.

Section snippets

Preparation of Al-SBA-15

Al-SBA-15 with Si/Al = 80 was synthesized following the procedure of Wu et al. [20]. A typical synthesis procedure was followed: 20 g of commercialized SBA-15 powder (ACS Materials, LLC, Medford, MA) were dispersed in 150 mL hexane. Then, 0.067 g aluminum isopropoxide dispersed in a small amount of hexane was added with stirring. After 10 min, the solution was diluted by adding more hexane (150 mL) and the stirring was continued for another 24 h at room temperature. The mixture was filtered and the

Catalyst characterization

Fig. 1 exhibits the nitrogen adsorption–desorption isotherms of the five supported NiMoC catalysts. According to De Boer’s theory the isotherm curve of NiMoC/Al-SBA-15 is type IV and the adsorption hysteresis loop is type A, which means that NiMoC/Al-SBA-15 has a meso porous structure with uniform regular channel distribution. The specific adsorption capacity is as high as 450 m2/g. NiMoC/γ-Al2O3 catalyst shows a type IV isotherm curve and the adsorption hysteresis loop is type E, indicating

Conclusions

The hydrotreating of soybean oils on supported NiMo carbide catalysts makes possible the production of gasoline to diesel range liquid hydrocarbons. Because of specific pore structures, all of the zeolite-supported catalysts have a strong cracking activity, producing more gaseous and gasoline products. The meso-porous γ-Al2O3 and Al-SBA-15 supported catalysts led to a higher production of green diesel, containing mostly C15–C18 hydrocarbons, which were mainly formed by

Acknowledgments

Financial support from the Department of Energy (Grant DE-FG36-05GO85005) and National Institute of Food and Agriculture (Grant MICW-2010-01534) for this research are gratefully acknowledged.

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