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