Effect of ammonia co-feeding on oxygenates over K-Mo2C in the Fischer-Tropsch synthesis

Master Thesis


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University of Cape Town

The Fischer-Tropsch (FT) process, producing long chained waxes and transportation fuels, is competing with fuels derived from crude oils and its profitability is therefore dependent on the global oil price. However, increasing the value of synthesized products could render the profitability of the FTS independent of fluctuations in the oil price (which are mostly due to global political trends). One way to achieve this, is to target fine chemicals instead of fuels. At the Catalysis Institute, this has been investigated by adding ammonia to the feed gas stream and obtaining highly valuable amines, amides and nitriles. It has been shown that the so-called nitrogen containing compounds are formed instead of the Fischer-Tropsch typical albeit minor products alcohols, aldehydes and carboxylic acids, i.e. oxygenates. Increasing the oxygenate selectivity was investigated in numerous studies as no commercial FT based process exists which produces oxygenates at a significant yield. Typically, transition metals such as Fe, Co, Rh and Ni are active for the FT synthesis. Based on reaction conditions employed, commercial Fe and Co based catalysts have been shown to produce between 6 and 12 C% oxygenates. Rh has been shown to have a high oxygenate selectivity, but the associated high raw material cost becomes prohibitive for use as a commercial FT catalyst. Catalysts other than the traditionally known FT active transition metals have shown promising results in terms of oxygenate selectivity. Transition metal carbides such as Mo2C, have been investigated under Fischer-Tropsch conditions. While the bare catalyst produces mainly methane and other hydrocarbons, upon promotion with potassium the selectivity shows a significant shift towards oxygenates. This project investigates the use of potassium promoted molybdenum carbide as a catalyst for high oxygenate selectivity in the Fischer-Tropsch synthesis. β-Mo2C was synthesized and subsequently promoted with different levels of potassium and its Fischer-Tropsch synthesis performance was evaluated in a stainless steel fixed bed reactor. The influence of catalyst synthesis protocols, reactor pressure and temperature, feed gas space velocity, and K/Mo wt.% promotion on catalyst activity and selectivity were studied. At a stable CO conversion (±10%) and its related oxygenate selectivity (±35 C%) ammonia was co-fed to the catalyst to study the conversion of oxygenates to nitrogen containing compounds. In summary, an unpromoted β-Mo2C catalyst reached CO conversions to ±40% at the conditions applied. Initial promotion of the catalyst with potassium showed a significant drop in catalyst activity, however, an increase in potassium content did not further decrease catalyst activity. The selectivity towards oxygenates was greatly enhanced from 10 C% up to 42 C% (CO2-free) at similar reaction conditions. Simultaneously, the oxygenate distribution shifted towards higher alcohols. The initial methanol content in the total oxygenate slate was around 60 C%, decreasing to about 20 C% upon potassium promotion. During co-feeding of ammonia, N-containing compounds were observed in the form of nitriles (±9 C%, CO2-free) and small traces of amides (±0.1 C%, CO2-free). Acetonitrile was the most dominating formed N-containing compound (≥58 C%). Upon the co-feeding of ammonia, the oxygenate selectivity decreased by roughly 10 C% points (CO2-free) but did not reach zero. Catalyst activity was slightly affected but recovered with time on stream. A slowly building up blockage appeared after 1-3 hours TOS simultaneously with a decreasing CO2 selectivity, suggesting the reaction with NH3 forming ammonium carbonate. This could however not be confirmed. The benefits of producing N-containing compounds using a potassium promoted β-Mo2C needs to be further investigated, trying to avoid the blockage by suppressing the WGS-activity of the catalyst. It is promising that the activity is hardly affected and that in the short period of time on stream N-containing compounds were observed.