Conversion of Synthesis Gas to Aromatics using Bifunctional Catalysis
J. Lennart Weber1, Daniel Martinez2, Carmen
Martos2, Arturo Vizcaino2, Petra E. de Jongh1,
Krijn P. de Jong1
2 Department of Energy and Chemical
Technology, University Rey Juan Carlos, Campus de Móstoles, Spain.
The production of materials such as plastics, as well as platform
chemicals such as olefins and aromatics is mainly based on processing of crude
oil. In order to reduce the carbon footprint alternative pathways are needed. Synthesis
gas (a mixture of carbon monoxide and hydrogen) can be derived from carbon
containing feedstocks such as natural gas, coal and biomass and therefore
offers a potentially more sustainable pathway for the production of chemicals
and fuels. The Fischer-Tropsch synthesis allows to convert synthesis gas to
hydrocarbons with a wide range of chain length varying from methane to heavy
waxes with more than 30 carbon atoms. These Fischer-Tropsch products can be
further processed to diesel, gasoline, olefins or waxes with very high purity.
Alternatively, synthesis gas can be converted to methanol, which can be further
processed to olefins and other hydrocarbons or dimethyl ether.
In the past years the conversion of synthesis gas to chemicals using
bifunctional catalysts has received great attention. The combination of a metal
oxide and a zeolite into a bifunctional catalyst allows to convert synthesis gas
to short olefins such as ethylene [1] or
aromatics [2] with selectivities as high
as 80% with oxygenates as intermediates. The catalyst bed design plays an
important role for the product distribution and activity of these bifunctional
catalysts [3]. Alternatively,
synthesis gas can be converted into C2-C4 olefins with
high selectivity of 63% using the Fischer-Tropsch to olefins
(FTO) process. Adding sodium and sulfur promoters to an iron (-carbide) based
catalyst suppresses methane formation and increases activity compared to an
unpromoted catalyst [4,5]. Such an FTO
catalyst can be combined with an H-ZSM-5 zeolite to convert the olefins into
aromatics at ~400 C and 1 bar pressure [6].
To reach more industrially relevant conditions, a medium temperature and high
pressure process is required. Here, we show a process to convert synthesis gas
to aromatics operating at temperatures between 250 °C and 300 °C and 20 bar
pressure forming olefins in the C4-C8 range as
intermediates report the impact of temperature and zeolite content in the
catalyst on the formation and distribution of aromatic products. We prepared a bulk iron catalyst (surface area 40 m²/g, pore
volume 0.3 mL/g) by precipitation from a 0.1 mol/L iron nitrate
solution with ammonia solution. After washing, drying at 120 °C and
calcination at 300 °C in static air, the resulting iron oxide was
impregnated with a potassium nitrate solution to achieve a mass ratio of iron
to potassium of 100Fe:3K (denoted as FeK). This potassium promoted bulk iron
catalyst was combined with an H-ZSM-5 zeolite (Si:Al = 15 at/at) in a stacked
bed configuration with ratios of zeolite to FeK between 0.5 m/m and 4 m/m and
tested in a 16-channel high throughput fixed bed unit (Flowrence, Avantium)
under medium alpha conditions (alpha of 0.67 at 250 °C and alpha of 0.60
at 300 °C). After reduction at 350 °C for 2 h in 30 % H2
in N2 at 1 bar, synthesis gas was introduced at temperatures between
250 °C and 300 °C at 20 bar pressure. Here, we show the
influence of temperature and zeolite to FeK ratio in stacked bed mode on the
formation of aromatics and distribution within the aromatic fraction.
The initial activity normalized to the mass of iron (iron time
yield, FTY) was found to be around 5.7 10-5molCO gFe-1
s-1 for all experiments performed at 300 °C and decreased
to a stable FTY of 2.2 10-5molCO gFe-1
s-1 after 40 h on stream, whereas the experiments performed at
250 °C stabilized at an activity of 0.9 10-5molCO
gFe-1 s-1 and at 275 °C at an FTY of
1.1 10-5molCO gFe-1 s-1.
Further, we found that the selectivity to C6-C10
aromatics increased with increasing reaction temperature operating at a zeolite
to FeK ratio of 2 m/m. In stacked bed mode, selectivity to C6-C10
aromatics of 2.3 %C at 250 °C, 2.8 %C at
275 °C and 4.1 %C at 300 °C were observed.
Furthermore, a shift to heavier aromatic products was noticed with on average
7.9 carbon atoms at 250 °C to 8.3 carbon atoms at 300 °C within
the aromatics distribution. The potassium promoted bulk iron catalyst without
zeolite showed a selectivity to C2-C3 olefins of
21 %C and to C4-C8 olefins of 30 %C
at 300 °C and no aromatics were formed (see Figure - top). With increasing
the zeolite to FeK ratio between 0.5 m/m and 1.5 m/m C4-C8
olefins selectivity decreased from initially 30 %C to
18-21 %C, whereas the selectivity to C2-C3
olefins remained constant between 20 %C and 21 %C.
C6-C10 aromatics were formed with 4-6 %C
selectivity (87 % xylenes in C8 aromatics) and an average
carbon number of 8.1 at at a zeolite to FeK ratio of 1 m/m. Increasing the
zeolite to FeK ratio to 2 m/m led to the decrease in C2-C3
olefins selectivity to 7 %C and the formation of heavier
aromatics with an average carbon number of 8.3 at. A further increase in
zeolite to FeK ratio to 2.5 m/m led to the formation of C6-C8
aromatics with 19 %C selectivity, whereas the selectivity
towards both C2-C3 olefins and C4-C8
olefins decreased. At a high zeolite to FeK ratio of 4 m/m we observed a
selectivity to C6-C10 aromatics of 21 %C
and almost no olefins were formed. These results show that C8-aromatics
were formed preferentially from longer C4-C8 olefins when
a low amount of zeolite was present (zeolite to FeK ratio between 0.5 m/m and
1.5 m/m). Increasing the amount of zeolite led to alkylation of light aromatics
such as toluene with ethylene as well as oligomerization of C2-C3
olefins towards C4-C8 olefins, resulting in lower C2-C3
olefins selectivity and increased selectivity to aromatics with higher average
carbon number. Operating with zeolite to FeK ratio of 4 m/m the catalytic data
suggested the aromatization of shorter olefins to form aromatics. In contrast
to a high temperature and low pressure process the aromatization followed a hydrogen
transfer pathway, forming paraffins as side product (data not shown).
We show that combining a potassium promoted bulk iron catalyst with
an H-ZSM-5 zeolite in a stacked-bed enabled a medium temperature route to
convert synthesis gas to olefins and aromatics with adjustable product
composition depending on reaction temperature and catalyst composition. The CO
conversion of this medium temperature process is scalable by changing space
velocity (CO conversion as high as 87 % and 62 % aromatics in the liquid
C5+ fraction, detailed data will be shown at the conference). This demonstrates
an advantage over the processes using oxygenates intermediates to form
aromatics, which seem to be limited to CO conversions of around 20 %. Furthermore,
this medium temperature process shows a long lifetime of the catalysts compared
to the high temperature process operating at 400 °C. These findings allow
to design targeted bifunctional catalyst systems for various applications.
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