The Fischer–Tropsch Synthesis converts synthesis gas from alternative carbon resources, including natural gas, coal, and biomass, to hydrocarbons used as fuels or chemicals. In particular, iron-based ...catalysts at elevated temperatures favor the selective production of C2–C4 olefins, which are important building blocks for the chemical industry. Bulk iron catalysts (with promoters) were conventionally used, but these deactivate due to either phase transformation or carbon deposition resulting in disintegration of the catalyst particles. For supported iron catalysts, iron particle growth may result in loss of catalytic activity over time. In this work, the effects of promoters and particle size on the stability of supported iron nanoparticles (initial sizes of 3–9 nm) were investigated at industrially relevant conditions (340 °C, 20 bar, H2/CO = 1). Upon addition of sodium and sulfur promoters to iron nanoparticles supported on carbon nanofibers, initial catalytic activities were high, but substantial deactivation was observed over a period of 100 h. In situ Mössbauer spectroscopy revealed that after 20 h time-on-stream, promoted catalysts attained 100% carbidization, whereas for unpromoted catalysts, this was around 25%. In situ carbon deposition studies were carried out using a tapered element oscillating microbalance (TEOM). No carbon laydown was detected for the unpromoted catalysts, whereas for promoted catalysts, carbon deposition occurred mainly over the first 4 h and thus did not play a pivotal role in deactivation over 100 h. Instead, the loss of catalytic activity coincided with the increase in Fe particle size to 20–50 nm, thereby supporting the proposal that the loss of active Fe surface area was the main cause of deactivation.
Catalysts promoted with low amounts of sodium and sulfur exhibited higher selectivity to lower olefins and lower methane production. Promoted bulk and α-Al2O3-supported catalysts showed similar ...selectivities; however, bulk catalysts displayed lower catalytic activity and extensive coke formation.
•Supported iron catalysts promoted with Na and S exhibited high selectivity to lower olefins and low methane production.•Na- plus S-promoted bulk catalysts displayed high lower olefins selectivity but also a low mechanical stability.•Na increased alpha and coke formation while S presumably blocked chain termination through hydrogenation in a selective way.
The Fischer–Tropsch synthesis of lower olefins (FTO) is an alternative process for the production of major chemical building blocks from natural gas, coal, or biomass-derived synthesis gas. The addition of low concentrations of sulfur plus sodium to Fe/α-Al2O3 resulted in catalysts with high C2–C4 olefins selectivity (∼50%C), enhanced catalytic activity, and decreased methane production (<20%C) when the reaction was carried out at 340°C, 20bar and H2/CO=1. Sodium reduced methane selectivity by increasing the chain growth probability while sulfur probably reduced the hydrogen coverage of the catalyst resulting in even lower methane selectivities and higher olefin content of the products. The addition of extra sodium resulted in a detrimental effect on catalytic activity while favoring the formation of carbon deposits. Our results show that the nature and concentration of the promoters play a key role in the design of FTO catalysts with optimum catalytic performance.
Depletion of crude oil resources and environmental concerns have driven a worldwide research on alternative processes for the production of commodity chemicals. Fischer-Tropsch synthesis is a process ...for flexible production of key chemicals from synthesis gas originating from non-petroleum-based sources. Although the use of iron-based catalysts would be preferred over the widely used cobalt, manufacturing methods that prevent their fast deactivation because of sintering, carbon deposition and phase changes have proven challenging. Here we present a strategy to produce highly dispersed iron carbides embedded in a matrix of porous carbon. Very high iron loadings (>40 wt %) are achieved while maintaining an optimal dispersion of the active iron carbide phase when a metal organic framework is used as catalyst precursor. The unique iron spatial confinement and the absence of large iron particles in the obtained solids minimize catalyst deactivation, resulting in high active and stable operation.
In this combined in situ XAFS, DRIFTS, and Mössbauer study, we elucidate the changes in structural, electronic, and local environments of Fe during pyrolysis of the metal organic framework Fe-BTC ...toward highly active and stable Fischer–Tropsch synthesis (FTS) catalysts (Fe@C). Fe-BTC framework decomposition is characterized by decarboxylation of its trimesic acid linker, generating a carbon matrix around Fe nanoparticles. Pyrolysis of Fe-BTC at 400 °C (Fe@C-400) favors the formation of highly dispersed epsilon carbides (ε′-Fe2.2C, d p = 2.5 nm), while at temperatures of 600 °C (Fe@C-600), mainly Hägg carbides are formed (χ-Fe5C2, d p = 6.0 nm). Extensive carburization and sintering occur above these temperatures, as at 900 °C the predominant phase is cementite (θ-Fe3C, d p = 28.4 nm). Thus, the loading, average particle size, and degree of carburization of Fe@C catalysts can be tuned by varying the pyrolysis temperature. Performance testing in high-temperature FTS (HT-FTS) showed that the initial turnover frequency (TOF) of Fe@C catalysts does not change significantly for pyrolysis temperatures up to 600 °C. However, methane formation is minimized when higher pyrolysis temperatures are applied. The material pyrolyzed at 900 °C showed longer induction periods and did not reach steady state conversion under the conditions studied. None of the catalysts showed deactivation during 80 h time on stream, while maintaining high Fe time yield (FTY) in the range of 0.19–0.38 mmolCO gFe –1 s–1, confirming the outstanding activity and stability of this family of Fe-based FTS catalysts.
Pressure leverage: A tapered‐element oscillating microbalance was used to evaluate carbon deposition on a highly selective and active supported iron catalyst for the production of lower olefins. With ...increasing pressure, the H2/CO ratio had a profound effect on the carbon deposition rate and accordingly, conditions leading to minimal carbon deposition, low methane selectivity, and high olefin selectivity were identified.
Solute hydrophobicity, polarizability, aromaticity and the presence of H-bond donor/acceptor groups have been identified as important solute properties that affect the adsorption on activated carbon. ...However, the adsorption mechanisms related to these properties occur in parallel, and their respective dominance depends on the solute properties as well as carbon characteristics. In this paper, a model based on multivariate linear regression is described that was developed to predict equilibrium carbon loading on a specific activated carbon (F400) for solutes reflecting a wide range of solute properties. In order to improve prediction accuracy, groups (bins) of solutes with similar solute properties were defined and solute removals were predicted for each bin separately. With these individual linear models, coefficients of determination (
R
2) values ranging from 0.61 to 0.84 were obtained. With the mechanistic approach used in developing this predictive model, a strong relation with adsorption mechanisms is established, improving the interpretation and, ultimately, acceptance of the model.
•A more uniform distribution of iron particles was achieved using AIC as precursor.•High coke levels were observed for catalysts with highly aggregated iron particles.•Large iron particles fragmented ...due to carbon deposition during the FTO.
Lower olefins are traditionally produced from cracking of naphtha and other crude oil fractions. The Fischer–Tropsch-to-Olefins process (FTO) enables the direct synthesis of lower olefins from synthesis gas (CO+H2) derived from alternative feedstocks such as natural gas, coal or biomass. A catalyst suitable for this process must comply with different requirements: high selectivity for C2C4 olefins, low methane selectivity, high catalytic activity and excellent mechanical and chemical stability under demanding reaction conditions (high temperatures and low H2/CO ratios). These features have been reported for a catalyst consisting of iron-containing nanoparticles promoted with sodium and sulfur dispersed on a weakly interactive support. In this study, Na plus S promoted α-alumina supported catalysts with loadings of 1–20wt% Fe have been prepared using different iron precursor salts to investigate their effects on catalytic performance. The catalysts prepared from iron nitrate or ammonium iron citrate both consisted of iron nanoparticles of 15-20nm and displayed high selectivity to lower olefins (>50% C) in combination with low methane selectivity (<20% C) when tested under industrially relevant conditions (340°C, 20bar and H2/CO=1v/v). The catalyst synthesized with ammonium iron citrate exhibited a higher catalytic activity and a much lower rate of carbon lay-down (factor 4–6) compared to the sample prepared with iron nitrate. Tentatively, the differences in catalytic performance are attributed to a more uniform distribution of the iron particles observed when ammonium iron citrate was used as precursor. These results suggest that the extent of aggregation of iron (carbide) nanoparticles affects their catalytic performance.
Natural organic matter (NOM) can influence pharmaceutical adsorption onto granular activated carbon (GAC) by direct adsorption competition and pore blocking. However, in the literature there is ...limited information on which of these mechanisms is more important and how this is related to NOM and pharmaceutical properties. Adsorption batch experiments were carried out in ultrapure, waste- and surface water and fresh and NOM preloaded GAC was used. Twenty-one pharmaceuticals were selected with varying hydrophobicity and with neutral, negative or positive charge. The influence of NOM competition and pore blocking could not be separated. However, while reduction in surface area was similar for both preloaded GACs, up to 50% lower pharmaceutical removal was observed on wastewater preloaded GAC. This was attributed to higher hydrophobicity of wastewater NOM, indicating that NOM competition may influence pharmaceutical removal more than pore blocking. Preloaded GAC was negatively charged, which influenced removal of charged pharmaceuticals significantly. At a GAC dose of 6.7 mg/L, negatively charged pharmaceuticals were removed for 0-58%, while removal of positively charged pharmaceuticals was between 32-98%. Charge effects were more pronounced in ultrapure water, as it contained no ions to shield the surface charge. Solutes with higher log D could compete better with NOM, resulting in higher removal.
Background
Obesity is highly prevalent in African American women, especially those in the rural southern USA, resulting in persistent health disparities.
Objective
To test the effectiveness of an ...evidence‐based behavioural weight loss intervention delivered by community health advisors to African American women in the rural south.
Design and Methods
Overweight or obese African American women (30–70 years) from eight counties in Mississippi and Alabama participated in a 24‐month randomized controlled trial of an evidence‐based behavioural weight loss programme augmented with community strategies to support healthy lifestyles (Weight Loss Plus, N = 154) compared to the weight loss programme alone (Weight Loss Only, N = 255). This study reports on 6‐month outcomes on primary (weight change) and secondary (waist circumference, blood pressure, lipids, fasting blood glucose) outcomes, coinciding with the completion of the intensive weight loss phase.
Results
Weight Loss Only participants lost an average of 2.2 kg (P < 0.001). Weight Loss Plus participants lost an average of 3.2 kg (P < 0.001). The proportion of the total sample that lost at least 5% of their body weight was 27.1% with no difference between treatment groups. Similarly, we observed statistically significant reductions in blood pressure, waist circumference and triglycerides in each treatment group, with no statistical differences between groups.
Conclusion
Trained lay health staff and volunteers from the rural southern USA were able to deliver a translation of a high‐intensity behavioural intervention targeted to African American women, resulting in clinically meaningful weight loss and improvement in other metabolic outcomes in a significant proportion of participants.
PDF
