Sina Sartipi

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.

Breaking the Fischer–Tropsch synthesis selectivity: direct conversion of syngas to gasoline over hierarchical Co/H-ZSM-5 catalysts

We report the combination of Fischer–Tropsch catalyst with acid functionality in one single catalyst particle. The resulting bifunctional catalyst is capable of producing gasoline range hydrocarbons from synthesis gas in one catalytic step with outstanding activities and selectivities.

Catalysis engineering of bifunctional solids for the one-step synthesis of liquid fuels from syngas: a review

The combination of acidic zeolites and Fischer–Tropsch synthesis (FTS) catalysts for one-step production of liquid fuels from syngas is critically reviewed. Bifunctional systems are classified by the proximity between FTS and acid functionalities on three levels: reactor, catalyst particle, and active phase. A thorough analysis of the published literature on this topic reveals that efficiency in the production of liquid fuels correlates well with the proximity of FTS and acid sites. Moreover, possible side reactions over the FTS metal, including direct CO hydrogenation and hydrocarbon hydrogenolysis, are addressed. The contribution of these side reactions should carefully be considered and separated from that of the zeolite function when evaluating the performance and product spectrum of zeolite-containing catalysts.

Towards Liquid Fuels from Biosyngas: Effect of Zeolite Structure in Hierarchical-Zeolite-Supported Cobalt Catalysts

Wax on, wax off: Bifunctional cobalt-based catalysts on zeolite supports are applied for the valorization of biosyngas through Fischer-Tropsch chemistry. By using these catalysts, waxes can be hydrocracked to shorter-chain hydrocarbons, increasing the selectivity towards the C5 -C11 (gasoline) fraction. The zeolite topology and the amount and strength of acid sites are key parameters to maximize the performance of these bifunctional catalysts, steering Fischer-Tropsch product selectivity towards liquid hydrocarbons.

Insights into the Catalytic Performance of Mesoporous H-ZSM-5-Supported Cobalt in Fischer–Tropsch Synthesis

Mesoporous H‐ZSM‐5 (mesoH‐ZSM‐5) was used as a carrier for a series of bifunctional Co‐based catalysts for Fischer–Tropsch synthesis with ZrO2 and/or Ru added as promoters. The reducibility of the catalysts was studied in detail by using temperature‐programmed reduction and X‐ray absorption spectroscopy. A comparison of the catalytic performance of Co/mesoH‐ZSM‐5 and Co/SiO2 (a conventional catalyst), after 140 h on stream, reveals that the former is two times more active and three times more selective to the C5–C11 fraction with a large content of unsaturated hydrocarbons, which is next to α‐olefins. The acid‐catalyzed conversion of n‐hexane and 1‐hexene, as model reactions, demonstrates that the improvement in the selectivity toward gasoline range hydrocarbons is due to the acid‐catalyzed reactions of the Fischer–Tropsch α‐olefins over the acidic zeolite. The formation of methane over the zeolite‐supported Co catalysts originates from direct CO hydrogenation and hydrocarbon hydrogenolysis on coordinatively unsaturated Co sites, which are stabilized as a consequence of a strong metal–zeolite interaction. Although the addition of either ZrO2 or Ru increases the catalyst reducibility considerably, it does not affect the product selectivity significantly.

Effect of pretreatment atmosphere on the activity and selectivity of Co/mesoHZSM-5 for Fischer–Tropsch synthesis

The structure and catalytic performance of bifunctional 10 wt% Co/mesoHZSM-5 catalysts pretreated under different conditions, i.e. in stagnant air, or in a flow of air, N2, or 1 vol% NO/Ar, were investigated for the Fischer–Tropsch synthesis (FTS) under fixed operating conditions of T = 513 K, P = 15 bar, H2/CO = 1. The combination of acid sites and FTS functionality leads to the direct formation of gasoline range hydrocarbons and suppresses the formation of C20+ products. The highest activity, C5–C11 selectivity and lowest CH4 selectivity were obtained for Co/mesoHZSM-5 catalyst pretreated in stagnant air. Pretreatment in gas flow resulted in a lower activity and C5–C11 selectivity, and in a higher CH4 selectivity, in particular for samples pretreated with NO. Characterization shows that this underperformance is due to changes in the Co3O4 particle size distribution and cobalt reducibility, and is related to the cobalt loading relative to the mesopore area. Pretreatment in air or N2 flow increased the number of small Co3O4 particles and increased cobalt reducibility by suppressing the formation of highly dispersed cobalt, e.g. cobalt silicates, in strong interaction with mesoHZSM-5. Pretreatment in a 1 vol% NO/Ar flow significantly increased cobalt dispersion further, decreasing the cobalt reducibility due to the strong interaction between cobalt and mesoHZSM-5. Based on both TEM and in situ DRIFTS studies, the optimum performance of Co/mesoHZSM-5 pretreated in stagnant air could be attributed to a lower fraction of small cobalt particles, known to promote the formation of CH4via hydrogenolysis or direct methanation. Additionally, small cobalt particles are more susceptible to be oxidized under FT conditions, thereby decreasing FT activity and indirectly increasing CH4 selectivity by increasing the H2/CO ratio through the water gas shift reaction.

Dynamic Release–Immobilization of a Homogeneous Rhodium Hydroformylation Catalyst by a Polyoxometalate Metal–Organic Framework Composite

Thermal treatment of phosphotungstic acid (PTA)‐MIL‐101(Cr) composites in the presence of hydroformylation catalyst RhH(CO)(PPh3)3 leads to immobilization of the homogeneous Rh complex within the metal–organic framework (MOF) scaffold by coordination of PTA‐Rh. The Rh complex‐containing MOFs are tested in the hydroformylation of 1‐octene in which PTA competes with CO during ligand association. In the presence of the carbonyl ligand, the Rh complex is released from the MOF and behaves as a homogeneous catalyst. Therefore, the product spectra and selectivities of the Rh complex‐containing MOFs are similar to those of RhH(CO)(PPh3)3. Upon CO evacuation, Rh recoordinates to PTA, allowing for easy recycling of this new pseudo‐heterogeneous catalyst.

Carbon/H-ZSM-5 composites as supports for bi-functional Fischer–Tropsch synthesis catalysts

Mesoporous H-ZSM-5–carbon composites, prepared via tetrapropylammonium hydroxide (TPAOH) post treatment of H-ZSM-5 followed by deposition of pyrolytic carbon, have been used as the support for the preparation of Co-based Fischer–Tropsch catalysts. The resulting catalysts display an improved performance during Fischer–Tropsch synthesis (FTS), with higher activity, higher selectivity towards C5–C9 (gasoline range) hydrocarbons and lower selectivity towards C1 (and C2) than Co/mesoH-ZSM5 (without pyrolytic carbon). This is due to the weaker metal–support interaction caused by the deposited carbon (as revealed by XPS) leading to a higher reducibility of the Co species. Further, the partial deactivation of the Bronsted acid sites by pyrolytic carbon deposition, as was observed by NH3-TPD, allows the modification of the zeolite acidity. Both the olefin to paraffin (O/P) and the isoparaffin to normal paraffin (I/N) ratios decrease with the increase in the carbon content, opening the door to further tune the catalytic performance in multifunctional FTS operations.