A. Iulian Dugulan

Supported Iron Nanoparticles as Catalysts for Sustainable Production of Lower Olefins

From Plant to Plastic Petroleum is primarily used as fuel, but it is also used in the production of plastics. Thus, if biomass were to replace petroleum as societys carbon feedstock, a means of deriving ethylene and propylene—the principal building blocks of todays commodity plastics— would be helpful. Well-known Fischer-Tropsch (FT) catalysts can transform gasified biomass into a range of hydrocarbon derivatives, but ethylene and propylene tend to constitute a small fraction of the overall product distribution. Torres Galvis et al. (p. 835) now demonstrate a class of iron catalysts on relatively passive supports (carbon nanofibers or α-alumina) that robustly directed the FT process toward light olefins. A class of iron catalysts selectively transforms gasified biomass into the building blocks of common plastics. Lower olefins are key building blocks for the manufacture of plastics, cosmetics, and drugs. Traditionally, olefins with two to four carbons are produced by steam cracking of crude oil–derived naphtha, but there is a pressing need for alternative feedstocks and processes in view of supply limitations and of environmental issues. Although the Fischer-Tropsch synthesis has long offered a means to convert coal, biomass, and natural gas into hydrocarbon derivatives through the intermediacy of synthesis gas (a mixture of molecular hydrogen and carbon monoxide), selectivity toward lower olefins tends to be low. We report on the conversion of synthesis gas to C2 through C4 olefins with selectivity up to 60 weight percent, using catalysts that constitute iron nanoparticles (promoted by sulfur plus sodium) homogeneously dispersed on weakly interactive α-alumina or carbon nanofiber supports.

Iron Particle Size Effects for Direct Production of Lower Olefins from Synthesis Gas

The Fischer-Tropsch synthesis of lower olefins (FTO) is an alternative process for the production of key chemical building blocks from non-petroleum-based sources such as natural gas, coal, or biomass. The influence of the iron carbide particle size of promoted and unpromoted carbon nanofiber supported catalysts on the conversion of synthesis gas has been investigated at 340-350 °C, H(2)/CO = 1, and pressures of 1 and 20 bar. The surface-specific activity (apparent TOF) based on the initial activity of unpromoted catalysts at 1 bar increased 6-8-fold when the average iron carbide size decreased from 7 to 2 nm, while methane and lower olefins selectivity were not affected. The same decrease in particle size for catalysts promoted by Na plus S resulted at 20 bar in a 2-fold increase of the apparent TOF based on initial activity which was mainly caused by a higher yield of methane for the smallest particles. Presumably, methane formation takes place at highly active low coordination sites residing at corners and edges, which are more abundant on small iron carbide particles. Lower olefins are produced at promoted (stepped) terrace sites that are available and active, quite independent of size. These results demonstrate that the iron carbide particle size plays a crucial role in the design of active and selective FTO catalysts.

Metal organic framework-mediated synthesis of highly active and stable Fischer-Tropsch catalysts

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.

Direct Evidence of Water-Assisted Sintering of Cobalt on Carbon Nanofiber Catalysts during Simulated Fischer−Tropsch Conditions Revealed with in Situ Mössbauer Spectroscopy

Cobalt on carbon nanofiber model catalysts with very small dispersed cobalt particles of 5 nm were subjected to H(2)O/H(2) treatments at 20 bar and 220 degrees C. Using in situ Mossbauer spectroscopy we could unambiguously prove that oxidation of the nanoparticles by water will not occur when hydrogen is present. Only in a water/argon atmosphere did oxidation take place. This rules out oxidation as the deactivation mechanism in Fischer-Tropsch synthesis. Even more important, we define the relative humidity (RH) as a key parameter to understanding deactivation by water. At a RH below 25% sintering was absent even when measuring for 4 weeks, whereas at a high RH of 62% as much as half of the small super paramagnetic cobalt particles (<5 nm) sintered into larger particles in 1 week. Activity loss as measured at Fischer-Tropsch conditions amounted to 73%, which could be directly related to the metal dispersion loss 77% due to sintering as evidenced by detailed TEM analysis of the spent sample.

Ordered Mesoporous Materials as Supports for Stable Iron Catalysts in the Fischer–Tropsch Synthesis of Lower Olefins

The Fe‐catalyzed Fischer–Tropsch to olefins (FTO) synthesis is a non‐oil‐based route for the production of C2–C4 olefins. The understanding of the interplay between the catalytically active species, promoters, and support materials has improved over the last years, but the nanostructures of the various supports used are often not comparable. Several ordered mesoporous materials with a comparable pore size and pore symmetry are used as model supports for Fe‐based FTO catalysts. Ammonium iron citrate is used as the Fe source for all supports, and Na and S are added as promoters. The formation of catalytically active iron carbide species is suppressed within the strongly interacting mesoporous silica support, but the weakly interacting carbon and silicon carbide supports yield highly active FTO catalysts. Carbon‐supported catalysts show a high selectivity towards lower olefins, low methane production, and stable operation for up to 140 h under industrially relevant FTO conditions.

Promoted Iron Nanocrystals Obtained via Ligand Exchange as Active and Selective Catalysts for Synthesis Gas Conversion

Colloidal synthesis routes have been recently used to fabricate heterogeneous catalysts with more controllable and homogeneous properties. Herein a method was developed to modify the surface composition of colloidal nanocrystal catalysts and to purposely introduce specific atoms via ligands and change the catalyst reactivity. Organic ligands adsorbed on the surface of iron oxide catalysts were exchanged with inorganic species such as Na2S, not only to provide an active surface but also to introduce controlled amounts of Na and S acting as promoters for the catalytic process. The catalyst composition was optimized for the Fischer–Tropsch direct conversion of synthesis gas into lower olefins. At industrially relevant conditions, these nanocrystal-based catalysts with controlled composition were more active, selective, and stable than catalysts with similar composition but synthesized using conventional methods, possibly due to their homogeneity of properties and synergic interaction of iron and promoters.

Synthesis and activation for catalysis of Fe-SAPO-34 prepared using iron polyamine complexes as structure directing agents

The use of transition metal cations complexed by polyamines as structure directing agents (SDAs) for silicoaluminophosphate (SAPO) zeotypes provides a route, via removal of the organic by calcination, to microporous solids with well-distributed, catalytically-active extra-framework cations and avoids the need for post-synthesis aqueous cation exchange. Iron(II) complexed with tetraethylenepentamine (TEPA) is found to be an effective SDA for SAPO-34, giving as-prepared solids where Fe2+–TEPA complexes reside within the cha cages, as indicated by Mossbauer, optical and X-ray absorption near edge spectroscopies. By contrast, when non-coordinating tetraethylammonium ions are used as the SDAs in Fe-SAPO-34 preparations, iron is included as octahedral Fe3+ within the framework. The complex-containing Fe-SAPO-34(TEPA) materials give a characteristic visible absorption band at 550 nm (and purple colouration) when dried in air that is attributed to oxygen chemisorption. Some other Fe2+ polyamine complexes (diethylenetriamine, triethylenetetramine and pentaethylenehexamine) show similar behaviour. After calcination in flowing oxygen at 550 °C, ‘one-pot’ Fe(TEPA) materials possess Fe3+ cations and a characteristic UV-visible spectrum: they also show appreciable activity in the selective catalytic reduction of NO with NH3.

Synthesis of stable and low-CO2 selective ε-iron carbide Fischer-Tropsch catalysts

Phase-pure ε-iron carbide catalysts amenable to scale-up exhibit low CO2 selectivity and high stability in the FT reaction. The Fe-catalyzed Fischer-Tropsch (FT) reaction constitutes the core of the coal-to-liquids (CTL) process, which converts coal into liquid fuels. Conventional Fe-based catalysts typically convert 30% of the CO feed to CO2 in the FT unit. Decreasing the CO2 release in the FT step will reduce costs and enhance productivity of the overall process. In this context, we synthesize phase-pure ε(′)-Fe2C catalysts exhibiting low CO2 selectivity by carefully controlling the pretreatment and carburization conditions. Kinetic data reveal that liquid fuels can be obtained free from primary CO2. These catalysts displayed stable FT performance at 23 bar and 235°C for at least 150 hours. Notably, in situ characterization emphasizes the high durability of pure ε(′)-Fe2C in an industrial pilot test. These findings contribute to the development of new Fe-based FT catalysts for next-generation CTL processes.