Elsevier

Applied Catalysis A: General

Volume 480, 20 June 2014, Pages 93-99
Applied Catalysis A: General

K-promoted Mo/Co- and Mo/Ni-catalyzed Fischer–Tropsch synthesis of aromatic hydrocarbons with and without a Cu water gas shift catalyst

https://doi.org/10.1016/j.apcata.2014.04.044Get rights and content

Highlights

  • Catalysts were developed for the conversion of H2/CO into aromatic hydrocarbons.

  • Mo/Ni or Mo/Co, both alone and used with a WGS catalyst, exhibited good activity.

  • Tetra- and penta-methylbenzene made up over 80% of the collected liquid products.

  • The catalyst recyclability was good through three cycles.

  • The WGS catalyst improves CO conversion and H2 use efficiency.

Abstract

The catalyst systems Mo/Co/K/ZSM-5 and Mo/Ni/K/ZSM-5, alone and with the added copper-based water gas shift catalyst, were used for the conversion of two CO/H2 ratios in a batch reactor. GC analysis of the gas phase was used to determine CO conversion while GCMS and NMR studies were used to characterize the liquid products formed and liquid product selectivities. The liquids were hydrocarbons consisting mainly of alkyl substituted benzenes. Methyl substitution in the alkyl benzenes in the product liquid ranged from an average of 1.3 to 4.5 methyls per ring depending on reaction conditions and reactant gas mole ratios. The additional presence of the WGS catalyst significantly increased CO conversion in the reactions taking place at 280 °C from ∼25% to ∼90% while increasing selectivity toward higher average methyl substitution. Similar conversions and selectivities were observed with both a bio-syngas and a 50/50 mixture of H2 and CO.

Introduction

With increasing population, economic development and limited supplies of fossil fuels, new approaches are needed to make renewable liquid fuels [1]. The catalytic conversion of biomass-derived synthesis gas (CO + H2) to liquid hydrocarbons is an approach that has been intensely studied for several decades [2], [3]. The major focus in gas-to-liquids (GTL) technology has been Fischer Tropsch (FT) synthesis [4], which converts synthesis gas into liquid hydrocarbon fuels (Eq. (1)). Traditionally, commercial catalysts used cobalt, iron and ruthenium [4], [5] though a variety of catalysts can be used [4]. Molybdenum, on a zeolite (ZSM-5) support, has shown promise for the conversion of synthesis gas to liquid hydrocarbons [6] as have cobalt and nickel [2]. However these systems can produce a wide range of hydrocarbons (C1–C60) during the reaction [6].(2n + 1) H2 + nCO  CnH(2n+2) + nH2O

Molybdenum-zeolite FT catalysts have been used to produce aromatics and branched/cyclized alkanes, olefins and oxygenates from synthesis gas [6]. Similar mixed metal ZSM-5 catalyst systems have been used to enhance selectivity to gasoline through increased production of high-octane aromatic hydrocarbons by promoting oligomerization, cracking, isomerization, and aromatization reactions on ZSM-5 (Si/Al = 50) [2], [7], [8], [9], [10], [11]. Anderson–Schulz–Flory (ASF) polymerization kinetics is generally observed in FT product distributions [12]. Most FT catalysts produce aromatic hydrocarbons by chain propagation reactions of initially formed low molecule weight aliphatic hydrocarbons or alcohols followed by dehydrations, cyclizations and dehydrogenations [13], [14], [15]. Thus, multistage catalysis processes govern FT product distribution.

Many studies identified advantages of using bi-functional catalysts to convert synthesis gas to high carbon number gasoline range hydrocarbons [16], [17], [18], [19]. Alkali metal promotion can enhance catalytic activity. For example, potassium has long been used as the promoter with several FT catalysts [20]. Water gas shift (WGS) catalysts can decrease H2O while increasing the effective H2/CO ratio [21] (Eq. (2)). Cu-based WGS catalysts have been widely employed commercially since the early 1960s and exhibit high activity and selectivity [22]. Some are reported to maintain catalyst activity under reaction conditions required for FT conversions [23], [24], [25]. The commercial Cu-based catalyst (HiFUEL W220 used in this study) was previously used to catalyze the WGS reaction at low temperatures (170–250 °C) and low carbon monoxide concentrations, in the ‘low-temperature’ CO-shift process [26], [27], [28]. Furthermore, another study reports high CO conversion activity extends up to 500 °C using this same W220 catalyst [29].H2O + CO  H2 + CO2

Here we present novel mixed catalyst systems combining a Mo/Co/K/ZSM-5 or Mo/Ni/K/ZSM-5 catalyst with this Cu-based W220 low temperature WGS catalyst for the FT converting both bio-syngas and a H2/CO (50/50) ratio syngas with remarkable selectivity to methylated benzene ring products.

Section snippets

Catalyst preparation

The Mo/Co/K zeolite catalyst was prepared using incipient wetness impregnation [6]. The ammonium form of ZSM-5 (SiO2/Al2O3 = 50) (50.0 g) obtained from Zeolyst International was dried for 24 h and then impregnated with an aqueous solution containing 4.60 g of (NH4)6Mo7O24·4H2O (Sigma Aldrich). This Mo/ZSM-5 was then impregnated with an aqueous solution with 12.35 g of Co(NO3)2·6H2O (Sigma Aldrich). Finally this Mo/Co/ZSM-5 was impregnated with an aqueous solution with 1.25 g of K2CO3 (Sigma Aldrich).

Characterization of the catalyst samples

The catalyst surface areas, metal and carbon compositions are listed in Table 2. Impregnating ZSM-5 with metals significantly reduced the surface area from 422 m2/g to approximately 300 m2/g, indicating that the added metals block some zeolite channels resulting in lower surface area. No significant surface area change was observed when comparing metal-impregnated zeolite catalysts before and after the catalytic reactions. This implies negligible surface coking of the FT catalysts. In contrast,

Conclusions

Novel catalyst systems have been developed for the conversion of 1/1 H2/CO and bio-syngas into aromatic hydrocarbons. Potassium-activated Mo/Ni or Mo/Co on a ZSM-5 support, both alone and with a WGS catalyst present, exhibited good batch CO conversions and remarkable selectivities to alkyl benzenes. The CO conversions for the FT/WGS catalyst systems were ∼90% at 280 °C, a surprising increase verses using the FT catalyst alone (∼25%). Remarkable selectivities were found for tetra- and

Acknowledgments

This work was performed through the Sustainable Energy Research Center at Mississippi State University and is supported by the U.S. Department of Energy under Awards (DE-FG3606GO86025) and U.S. Department of Agriculture under Award (AB567370MSU).

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