The role of bulk H and C species in the chain lengthening of Fischer-Tropsch synthesis over nickel

This work documents the possible role of bulk species (dissolved H and C atoms) in the Fischer-Tropsch reaction. This has led us to study the sorption (adsorption or absorption) of hydrogen when increasing the pressure from 0.1 to 5 MPa, the selectivity toward C₂₊ hydrocarbons in carbon monoxide hydrogenation at 5 MPa total pressure, and the amount of nickel carbide formed in the course of the reaction over unsupported nickel, Ni/SiO₂, Ni/TiO₂ and Ni/Cr₂O₃ catalysts. Increasing the hydrogen pressure results in a subsequent sorption which is found to be proportional to the nickel dispersion, and can be attributed either to a completion of the surface hydrogen adlayer (8%) or to a formation of a hydrogen sublayer. No relationship could be found between the selectivity towards C₂₊ hydrocarbons and the quantity of sorbed hydrogen. A correlation between this selectivity and the amount of nickel carbide formation was demonstrated, suggesting that the active phase which leads to C₂₊ hydrocarbons is nickel carbide, or that carbon atoms of subsurface sites participate in the homologation reaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Conversion of methane to higher hydrocarbons in pulsed DC barrier discharge at atmospheric pressure

Conversion of methane to C₂/C₃ or higher hydrocarbons in a pulsed DC barrier discharge at atmospheric pressure was studied. Non-equilibrium plasma was generated in the barrier discharge reactor. In this plasma, electrons which had sufficient energy collided with the molecules of methane, which were then activated and coupled to C₂/C₃ or higher hydrocarbons. The effect of the change of applied voltage, pulse frequency and methane flow rate on methane conversion, selectivities and yields of products was studied. Methane conversion to higher hydrocarbons was about 25% as the maximum. Ethane, propane and ethylene were produced as primary products, including a small amount of unidentified C₄ hydrocarbons. The selectivity and yield of ethane as a main product came to about 80% and 17% as the highest, respectively. The selectivities of ethane and ethylene were influenced not by the change of pulse frequency but by the change of applied voltage and methane flow rate. However, in case of propane, the selectivity was independent of those condition changes. The effect of the packing materials such as glass and A1₂O₃ bead on methane conversion was also considered, showing that A1₂O₃ played a role in enhancing the selectivity of ethane remarkably as a catalyst.     

Selective Conversion of Methane to C2 Hydrocarbons Using Carbon Dioxide over Mn-SrCO3 Catalysts

The combination of Mn with SrCO₃ leads to effective catalysts for the selective conversion of CH₄ to C₂ hydrocarbons using CO₂ as an oxidant; C₂ selectivities approach 88 and 79.1% with a C₂ yield of 4.3 and 4.5% over catalysts with an Mn/Sr ratio of 0.1 and 0.2, respectively. It is assumed that the Mn³⁺/Mn²⁺ couple formed in the reaction plays an important role in the activation of CO₂ and CH₄.     

Synergic enhancement effect of zirconium oxide,cerium oxide and iron or cobalt oxide on the formation of isobutene from CO and H2

Fe₂O₃ or CoO modified CeO₂-ZrO₂ catalysts lead to four times higher yield of hydrocarbons than ZrO₂ with C₄ hydrocarbons selectivity of more than 50% and isobutene selectivity of more than 80%. XRD and XPS measurements suggested that the interaction of Fe or Co oxide with CeO₂ causes the higher Ce³⁺ concentration with their higher oxidation state. Their combination with ZrO₂ synergistically causes the active and selective formation of isobutene.     

Steady-state conversion of methane to C4+ aliphatic products in high yields using an integrated recycle reactor system

Conversion of methane in high yields to C₄₊ nonaromatic hydrocarbons was demonstrated in a recycle system. The principal components of the recycle system included an oxidative coupling reactor with a Mn/Na₂WO₄/SiO₂ catalyst at 800°C for conversion of methane to ethylene, and a reactor with an H-ZSM-5 zeolite at 275°C for subsequent conversion of ethylene to higher hydrocarbons. Total yields of C₄₊ products were in the range of 60–80%, and yields of C₄₊ nonaromatic hydrocarbons were in the range of 50–60%. This revised version was published online in July 2006 with corrections to the Cover Date.     

A highly active silica(silicon)-supported vanadia catalyst for C1 oxygenates and hydrocarbon production from partial oxidation of methane

A very low surface area silica-silicon substrate has been used as a support for vanadium oxide and has been tested in the partial oxidation of methane. Use of a reactor with variable dead volume ahead of the bed of the catalyst allows determining the relevance of gas phase reactions in initiating methane conversion. Experimental evidence supports that at atmospheric pressure C₁ oxygenates are essentially produced on the catalyst surface rather than in the gas phase. Comparison with a high surface area silica-supported vanadium oxide catalyst clearly highlights the double role of surface area in promoting catalytic activity, but also in promoting non-selective further oxidation of reaction products. It is shown that a reaction system combining dead volume upstream the bed of the catalyst and a very low surface area is very promising to activate methane conversion to C₁ oxygenates and C₂₊ hydrocarbons at remarkable TOF number preventing further non-selective oxidation. In addition, production of C₂₊ hydrocarbons is observed at temperatures as low as 750 K.     

The role of C2 intermediates in Fischer-Tropsch synthesis over ruthenium

The C₂ products formed over Ru during Fischer-Tropsch synthesis often lie well below the Anderson-Schulz-Flory line describing the C₄₊ products. This has led to speculation that either the surface precursor to C₂ hydrocarbons is exceptionally long lived, or that the ethylene formed by CO hydrogenation readsorbs and thereby reenters the chain growth process. In this study, the role of ethylene readsorption on the dynamics of chain initiation and growth is investigated using¹³CO/H₂ and¹²C₂H₄ to differentiate between the carbon sources. Ethylene addition is found to suppress the rate of methanation and increase the rates of formation of C₃₊ hydrocarbons. Ethylene serves as an effective chain initiator, as well as a source of C₁ monomer species which participate in chain propagation. No evidence is seen, though, for the participation of C₂ species in chain propagation.     

Selective oxidation of methane with air over silica catalysts

Partial oxidation of methane by oxygen to form formaldehyde, carbon oxides, and C₂ products (ethane and ethene) has been studied over silica catalyst supports (fumed Cabosil and Grace 636 silica gel) in the 630–780 °C temperature range under ambient pressure. The silica catalysts exhibit high space time yields (at low conversions) for methane partial oxidation to formaldehyde, and the C₂ hydrocarbons were found to be parallel products with formaldehyde. Short residence times enhanced both the C₂ hydrocarbons and formaldehyde selectivities over the carbon oxides even within the differential reactor regime at 780 °C. This suggests that the formaldehyde did not originate from methyl radicals, but rather from methoxy complexes formed upon the direct chemisorption of methane at the silica surface at high temperature. Very high formaldehyde space time yields (e.g., 812 g/kg cat h at the gas hourly space velocity = 560 000 (NTP)/kg cat h) could be obtained over the silica gel catalyst at 780 °C with a methane/air mixture of 1.5/1. These yields greatly surpass those reported for silicas earlier, as well as those over many other catalysts. Low CO₂ yields were observed under these reaction conditions, and the selectivities to formaldehyde and C₂ hydrocarbons were 28.0 and 38.8%, respectively, at a methane conversion of 0.7%. A reaction mechanism was proposed for the methane activation over the silica surface based on the present studies, which can explain the product distribution patterns (specifically the parallel formation of formaldehyde and C₂ hydrocarbons).     

Mixed alcohols synthesis from carbon monoxide hydrogenation over potassium promoted β-Mo2C catalysts

Potassium-promoted β-Mo₂C catalysts were prepared and their performances in CO hydrogenation were investigated. The main products over β-Mo₂C catalyst were C₁-C₄ hydrocarbons, only ∼4 C-atom% alcohols were obtained. The products of hydrocarbons and alcohols obeyed traditional linear Anderson-Schultz-Flory (A-S-F) distribution. However, modification with K₂CO₃ resulted in a remarkable selectivity shift from hydrocarbons to alcohols. Moreover, it was found that potassium promoter enhanced the ability of chain propagation of β-Mo₂C catalysts and resulted in a higher selectivity to C₂₊OH. For K/β-Mo₂C catalysts, the hydrocarbon products also obeyed traditional linear A-S-F plots, whereas alcohols gave a unique linear A-S-F distribution with remarkable deviation of methanol compared with that on β-Mo₂C catalyst. It could be concluded that potassium promoter might exert a prominent function on the whole chain propagation to produce alcohols. A surface phase on the K/β-Mo₂C catalysts such as the “K-Mo-C” explained the higher value for C₂₊OH, especially could promote the step of C₁OH to C₂OH, or could have a role in producing directly C₂OH, but again this would be speculative. At the same time, the influence of the loadings of K₂CO₃ on the performances of β-Mo₂C catalyst was investigated and the results revealed that the maximum yield of alcohol was obtained at K/Mo molar ratio of 0.2.     

Potassium and nickel doped β-Mo2C catalysts for mixed alcohols synthesis via syngas

Potassium and nickel doped β-Mo₂C catalysts were prepared and their performances of CO hydrogenation were investigated. The main products over β-Mo₂C catalyst were hydrocarbons, and only few alcohols were obtained. The potassium promoter resulted in remarkable selectivity shift from hydrocarbons to alcohols over β-Mo₂C. Moreover, it was found that the potassium promoter enhanced the ability of chain propagation of β-Mo₂C catalyst and resulted in a higher selectivity to C₂₊OH. When doped by potassium and nickel, β-Mo₂C catalyst showed high activity and selectivity for mixed alcohols synthesis, the Ni promoter further enhanced the whole chain propagation to produce alcohols especially for the step of C₁OH–C₂OH. From the XPS analysis, it had been proved that the formation of higher alcohols might be attributable to the presence of Mo^IV species, whereas the formation of hydrocarbons was closely associated with the presence of Mo^II species on the surface of the catalysts.     

Hydrocarbon synthesis from a model gas of underground coal gasification

Underground coal gasification gases contain CO and H₂; therefore, they can be used as raw materials in the synthesis of higher hydrocarbons. The possibility of obtaining a wide-cut hydrocarbon fraction on a cobalt catalyst was demonstrated with the use of CO-H₂-N₂ and CO-CO₂-H₂-N₂ model mixtures as an example. In this case, the selectivity for C₅₊ hydrocarbons was to 96% at a low yield of methane. The resulting hydrocarbons were characterized by a chain-growth probability to 0.89.     

Synthesis of hydrocarbons from a model gas of the gasification of oil shale

Gases from the gasification of oil shale contain CO and H₂; this fact makes it possible to use them as a raw material in the synthesis of higher hydrocarbons. The possibility of obtaining a long distillate of hydrocarbons on a cobalt catalyst was demonstrated using a CO-CO₂-H₂-N₂ model mixture of the gasification products of Leningrad shale as an example. In this case, to 83.8% selectivity for C₅₊ hydrocarbons with a low yield of methane (7.3%) was reached. The resulting hydrocarbons were characterized by a chain growth probability to 0.84.     

Isoprene formation from CO and H2 over CeO2 catalysts

The CO-H₂ reaction over CeO₂ catalysts at around 623 K and 67 kPa forms isoprene with about 20% and 70% selectivities in total and C₅ hydrocarbons, respectively. The formation of dienes may be due to the low and high activity of CeO₂ for alkene and CO hydrogenation, respectively.     

Selective Transformation of Dimethyl Ether into Small Molecular Hydrocarbons Over Large-pore Beta Zeolite

In dimethyl ether (DME) conversion, large-pore beta zeolite is found possessing a novel performance of selectively synthesizing small molecular hydrocarbons, around 95% selectivity to C₃ + C₄ + C₅ with hardly any large molecular C₆₊ products, which does not follow the shape selectivity of zeolite with large pore system completely.     

Methane transformation into aromatic hydrocarbons by activation with LPG over Zn‐ZSM‐11 zeolite

Methane (C₁) can be activated by interaction with liquefied petroleum gas (LPG) even at very high C₁ molar fractions in the feed (C₁/(C₁ + LPG)=0.85) at temperatures of 450–550°C, GHSV(LPG)=2240 and 810 ml/g h, over Zn‐ZSM‐11 (molar fraction Zn²⁺/(Zn²⁺H⁺=0.86) and total pressure of 1 atm. The isobutane (i‐C₄) of LPG could be the main responsible of this interaction. Aromatic hydrocarbons were the main products in the whole range of C₁ molar fractions (0.4–0.85) studied, reaching excellent levels of 10–45%. This revised version was published online in July 2006 with corrections to the Cover Date.     

Kinetics of surface reactions in carbon deposition from light hydrocarbons

Carbon deposition from ethene, ethine and propene as a function of pressure was studied at various temperatures and two different surface area/volume ratios. Deposition rates as a function of pressure of all hydrocarbons indicate Langmuir-Hinshelwood kinetics which suggests that the deposition process is controlled by the heterogeneous surface reactions (growth mechanism). These kinetics are favored at decreasing reactivity (C₃H₆>C₂H₂>C₂H₄), decreasing temperature and residence time as well as increasing surface area/volume ratio. A linear rate increase at high pressures suggests that carbon is additionally or preferentially deposited by aromatic condensation reactions between polycyclic aromatic hydrocarbons large enough to be physisorbed or condensed on the substrate surface (nucleation mechanism). The results completely agree with earlier results obtained with methane.     

Direct partial oxidation of methane over ZSM-5 catalyst: Zn-ZSM-5 catalyst studies

Extended studies on Zn-ZSM-5 catalyst for the production of liquid hydrocarbons in the direct partial oxidation (DPO) of CH₄ with O₂ are reported. Previously, it was reported that metal-containing ZSM-5 catalysts could produce C₅₊ hydrocarbons from pure CH₄/O₂ feeds without feed additives. Zn-ZSM-5 produced the highest C₅₊ yields of the catalysts tested. This work shows that the method of introducing Zn onto the catalyst, ion-exchange versus impregnation, does not significantly alter C₅₊ yields if low Zn content is maintained ( 0.4–0.5 wt%). Liquid hydrocarbon yields in this system doubled after 8 h on stream while overall C₂₊ yields increased by over 300%. Mechanistic implications of these findings are discussed. Finally, processing a natural gas feed over Zn-ZSM-5 gave higher C₅₊ yields over CH₄ feed but these yields were not improved over previously published results using HZSM-5.     

Production of C4 hydrocarbons from Fischer–Tropsch synthesis in a follow bed reactor consisting of Co–Ni–ZrO2 and sulfated-ZrO2 catalyst beds

Fischer–Tropsch synthesis was performed in a fixed-bed microreactor over a single bed consisting of Co–Ni–ZrO₂ catalyst as well as over a follow bed configuration consisting of Co–Ni–ZrO₂ and sulfated-ZrO₂ catalyst beds for the selective production of C₄ hydrocarbons. A maximum C₄ hydrocarbon selectivity of 14.6 wt.% was obtained using the single bed approach at 250°C and weight hourly space velocities (WHSV) of 15 h⁻¹. When a follow bed approach was used, there was an impressive increase in the selectivity for C₄ hydrocarbons to a maximum of 24 wt.% and that for iso-C₄ hydrocarbons to a maximum of 13.8 wt.% from 14.6 and 5.5 wt.%, respectively. However, there was a rapid deactivation of the sulfated-ZrO₂ catalyst due to coking and sulfate reduction.     

Effect of the nature of the support of a cobalt catalyst on the synthesis of hydrocarbons from CO,H<Subscript>2</Subscript>, and C<Subscript>2</Subscript>H<Subscript>4</Subscript>

The effect of the nature of the support of a Co catalyst on the synthesis of hydrocarbons from CO, H₂, and C₂H₄ was studied in this work. It was found that the introduction of ethylene into synthesis gas resulted in an increase in the yield of liquid hydrocarbons. In this case, the conversion of C₂H₄ was complete and the degree of its involvement into the synthesis of C₅₊ hydrocarbons depended on the concentration of this component in the starting mixture and the nature of the support. Specific features of the adsorption of CO and C₂H₄ on the used Co catalysts were determined using a temperature-programmed desorption method.     

Higher Hydrocarbon Production through Oxidative Coupling of Methane Combined with Hydrogenation of Carbon Oxides

In the production of higher hydrocarbons, combining oxidative coupling of methane (OCM) with hydrogenation of the formed carbon oxides in a separate reactor provides an alternative to the currently applied methane conversion to syngas followed by Fischer‐Tropsch synthesis. The effects of CH₄:O₂ feed ratio in the OCM reactor and partial pressures of H₂ or/and H₂O in the hydrogenation reactor were analyzed to maximize production of C₂₊ hydrocarbons and reduce CO_x formation. The highest C₂₊ yield was achieved with low CH₄:O₂ feed ratio for OCM and removal of the formed water before entering the hydrogenation reactor.