Overall chemical kinetics model for partial oxidation of methane in inert porous media

An overall three-step six-component chemical kinetics model which includes CH₄ + O₂ → CO + H₂O + H₂ and reversible 2CO + O₂ ←→ 2CO₂ and CH₄ + H₂O ←→ CO + 3H₂ reactions is elaborated for the simulation of partial oxidation of methane in inert porous media. Procedure of the model adjusting to the experimental data is described. Kinetic parameters of the model are derived on the basis of temperature–flow rate, H₂ and CO output concentration–flow rate and temperature–pressure experimental correlations. It is found that extremely slow solid body temperature growth with flow rate T_s,max(G) reported in the works on partial oxidation of methane (and other hydrocarbons) in inert porous media may be reproduced by the model. The model is designed for optimization, scale up and design assistance of the reactors of partial oxidation of methane.It is demonstrated that the overall chemical kinetics model can be combined with detailed gas-phase kinetics model for the investigation of detailed composition of syngas and intermediary components.     

Catalytic properties of Rh, Ni, Pd and Ce supported on Al-pillared montmorillonites in dry reforming of methane

A natural Maghnia clay was pillared by Al₁₃ and impregnated by 3–10 wt.% Me (Me = Rh, Ni, Pd, Ce) to be used as catalysts in the reforming of methane with carbon dioxide to synthesis gas. The structural and textural properties of materials calcined at 450 °C were determined by several techniques (XRD, FT-IR, ²⁷Al magic angle spinning (MAS) NMR, X-ray photoelectron spectroscopy (XPS), BET, thermogravimetric analysis (TGA)–DSC, H₂-temperature programmed reduction (TPR) and NH₃-TPR). Although impurities are present in the Al-pillared layered clay (PILC) support, most properties are close to those of pure Al-pillared Na-montmorillonite. Impregnation and calcination leads to the plugging of most micropores by clusters or microparticles of oxides. The NMR resonances of Al_VI and Al_IV specie are not modified after impregnation, and Al_VI/Al_IV ratio only varies on loading when compared to Al-PILC. Catalytic experiments show that the most active catalyst is 3% Rh/Al-PILC on which 88 mol.% of methane is converted at 650 °C with a minimum amount of carbon deposit. The conversions decrease along the 3% Rh ≈ 10% Ni > 3% Pd > 3% Ni > 3% Ce series. The H₂/CO ratio amounts to 1.1 with Rh and to 0.85 with Pd which are metallic at the temperature of reaction, but it has a lower value with Ni and Ce due to the RWGS reaction known to proceed in the presence of oxides.     

Acid-base properties and the directions of oxidative transformation of methane over nickel-based catalysts

The reactions of dry (CO₂) reforming and partial oxidation of methane have been investigated in a membrane reactor. The membrane is composed of a dense thin silica (SiO₂) film supported on porous Vycor tubes and was synthesized by chemical vapor deposition. The hydrogen permeance of the membrane was 0.2–0.3 cm³/(cm² min atm) at 600°C combined with a H₂/N₂ selectivity of 200–300. Significant increases in methane conversion were attained in both reactions at 500–750°C, albeit at very low space velocities. The membrane permeance declined by 50% after exposure to feeds containing H₂O, but otherwise exhibited excellent stability under reaction conditions.     

Oxy-CO2 reforming of methane to syngas over CoOx/MgO/SA-5205 catalyst

The oxy-CO₂ methane reforming reaction (OCRM) has been investigated over CoO_x supported on a MgO precoated highly macroporous silica–alumina catalyst carrier (SA-5205) at different reaction temperatures (700–900 °C), O₂/CH₄ ratios (0.3–0.45) and space velocites (20,000–100,000 cc/g/h). The reaction temperature had a profound influence on the OCRM performance over the CoO/MgO/SA-5205 catalyst; the methane conversion, CO₂ conversion and H₂ selectivity increased while the H₂/CO ratio decreased markedly with increasing reaction temperature. While the O₂/CH₄ ratio did not strongly affect the CH₄ and CO₂ conversion and H₂ selectivity, it had an intense influence on the H₂/CO ratio. The CH₄ and CO₂ conversion and the H₂ selectivity decreased while the H₂/CO increased with increasing space velocity. The O₂/CH₄ ratio and the reaction temperature could be used to manipulate the heat of the reaction for the OCRM process. Depending on the O₂/CH₄ ratio and temperature the OCRM process could be operated in a mildly exothermic, thermal neutral or mildly endothermic mode. The OCRM reaction became almost thermoneutral at an OCRM reaction temperature of 850 °C, O₂/CH₄ ratio of 0.45 and space velocity of 46,000 cc/g/h. The CH₄ conversion and H₂ selectivity over the CoO/MgO/SA-5205 catalyst corresponding to thermoneutral conditions were excellent: 95% and 97%, respectively with a H₂/CO ratio of 1.8.     

Performance of a catalytically activated ceramic hot gas filter for catalytic tar removal from biomass gasification gas

Silicon carbide-based filter elements were catalytically activated to provide filter elements for catalytic tar removal from biomass-derived syngas. The filter element support was coated with CeO₂, CaO–Al₂O₃ and MgO with a specific surface of 7.4, 15.9 and 21.9 m²/g synthesized by exo-templating with activated carbon. Doping of a MgO coated filter element with 60 wt% NiO has led to an increase of the specific surface from 0.15 to 0.21 m²/g, whereas in case of a MgO–Al₂O₃ coated filter element a decrease from 1.18 to 0.91 m²/g was found. An increase of the NiO loading from 6 to 60 wt% on a MgO coated filter element resulted in an increase of the naphthalene conversion from 91 to 100% at 800 °C and a face velocity of 2.5 cm/s at a naphthalene concentration of 5 g/Nm³ in model biomass gasification gas. In case of a MgO–Al₂O₃ coated filter element with 60 wt% NiO in addition to complete naphthalene conversion in the absence of H₂S, a higher conversion of 66% was found in the presence of 100 ppmv H₂S compared to 49% of the MgO–NiO coated filter element. After scaling up of the catalytic activation procedure to a 1520 mm long filter candle, which shows an acceptable differential pressure of 54.9 mbar, 58 and 97% naphthalene conversion was achieved in the presence and absence of H₂S, respectively. The calculated WHSV value of 209.6 Nm³ h⁻¹ kg⁻¹ indicates the technical feasibility of a further increase of the catalytic performance by an increase of the NiO loading.     

Single- and double-stage catalytic preferential CO oxidation in H2-rich stream over an α-Fe2O3-promoted CuO–CeO2 catalyst

Single- and double-stage catalytic preferential CO oxidation (CO-PrOx) over-Fe₂O₃-promoted CuO–CeO₂ in a H₂-rich stream has been investigated in this work. The catalyst was prepared by the urea-nitrate combustion method and was characterized by X-ray diffractometer (XRD), X-ray fluorescence (XRF), Brunauer–Emmet–Teller (BET), transmission electron microscope (TEM), and scanning electron microscope (SEM). The catalytic activity tests were carried out in the temperature range of 50–225 °C under atmospheric pressure. The results of the single-stage reaction indicated that complete CO oxidation was obtained when operating at a O₂/CO ratio of 1.5, W/F ratio of 0.36 g s/cm³, and at a reaction temperature of 175 °C. At these conditions, H₂ consumption in the oxidation was estimated at 58.4%. Applying the same conditions to the double-stage reaction, complete CO oxidation was found and H₂ consumption in the oxidation was reduced about 4.9%. When decreasing the double-stage reaction temperature to 150 °C, the results elucidated that CO could be converted to CO₂ completely while H₂ consumption in the oxidation was further reduced to 33.5%. A temperature blocking 2² factorial design has been used to describe the importance of the factors influencing the catalytic activity. The factorial design was according to the experimental results. When adding CO₂ and H₂O in feed, reduction of CO conversion for single- and double-stage reaction is obtained due to a blocking of CO₂ and H₂O at a catalytic active site. Comparing CO conversion obtained when operating with/without CO₂ and H₂O in feed for single- and double-stage reaction, less reduction is achieved when operating in double-stage reaction.     

Selective methanation of CO over supported Ru catalysts

The catalytic performance of supported ruthenium catalysts for the selective methanation of CO in the presence of excess CO₂ has been investigated with respect to the loading (0.5–5.0 wt.%) and mean crystallite size (1.3–13.6 nm) of the metallic phase as well as with respect to the nature of the support (Al₂O₃, TiO₂, YSZ, CeO₂ and SiO₂). Experiments were conducted in the temperature range of 170–470 °C using a feed composition consisting of 1%CO, 50% H₂ 15% CO₂ and 0–30% H₂O (balance He). It has been found that, for all catalysts investigated, conversion of CO₂ is completely suppressed until conversion of CO reaches its maximum value. Selectivity toward methane, which is typically higher than 70%, increases with increasing temperature and becomes 100% when the CO₂ methanation reaction is initiated. Increasing metal loading results in a significant shift of the CO conversion curve toward lower temperatures, where the undesired reverse water–gas shift reaction becomes less significant. Results of kinetic measurements show that CO/CO₂ hydrogenation reactions over Ru catalysts are structure sensitive, i.e., the reaction rate per surface metal atom (turnover frequency, TOF) depends on metal crystallite size. In particular, for Ru/TiO₂ catalysts, TOFs of both CO (at 215 °C) and CO₂ (at 330 °C) increase by a factor of 40 and 25, respectively, with increasing mean crystallite size of Ru from 2.1 to 4.5 nm, which is accompanied by an increase of selectivity to methane. Qualitatively similar results were obtained from Ru catalysts supported on Al₂O₃. Experiments conducted with the use of Ru catalyst of the same metal loading (5 wt.%) and comparable crystallite size show that the nature of the metal oxide support affects significantly catalytic performance. In particular, the turnover frequency of CO is 1–2 orders of magnitude higher when Ru is supported on TiO₂, compared to YSZ or SiO₂, whereas CeO₂- and Al₂O₃-supported catalysts exhibit intermediate performance. Optimal results were obtained over the 5%Ru/TiO₂ catalyst, which is able to completely and selectively convert CO at temperatures around 230 °C. Addition of water vapor in the feed does not affect CO hydrogenation but shifts the CO₂ conversion curve toward higher temperatures, thereby further improving the performance of this catalyst for the title reaction. In addition, long-term stability tests conducted under realistic reaction conditions show that the 5%Ru/TiO₂ catalyst is very stable and, therefore, is a promising candidate for use in the selective methanation of CO for fuel cell applications.     

Effect of methane concentration on reaction behavior in direct-methane solid oxide fuel cells with (Ce,Gd)O2−x-impregnated FeCr layer for fuel processing

A solid oxide fuel cell (SOFC) with a Ni-yttria-stabilized zirconia anode of 1 cm² area was set up with a porous disk of gadolinia-doped ceria-impregnated FeCr as a gas diffusion layer (GDL) under direct-methane feeding. In this setup of SOFC plus GDL, the tests at 800 °C and ambient pressure show that the current density, the methane conversion rate, the product formation rates, and the CO₂ selectivity increased with increasing methane concentration. The major reaction in the GDL is CO₂ reforming of methane to produce the syngas (CO plus H₂). The anodic electrochemical oxidation of CO from GDL results in an overall rate of CO₂ formation being much larger than that of CO formation. There is a synergy between the rate of reaction in the GDL and that over the anode.     

Plasma catalytic methane conversion over sol–gel derived Ru/TiO2 catalyst in a dielectric-barrier discharge reactor

Plasma catalytic methane conversion was carried out in the presence of sol–gel derived Ru/TiO₂ catalysts within a dielectric-barrier discharge (DBD) reactor. Plasma-assisted reduction (PAR) was applied to reduce the prepared Ru/TiO₂ catalysts in DBD reactor, and most of the catalysts were successively reduced by PAR within 15 min. The highest methane conversion was obtained when 5 wt% Ru/TiO₂ catalysts were used after calcination at 400 °C. The selectivities of light alkanes (C₂H₆, C₃H₈, C₄H₁₀) were highly increased when Ru/TiO₂ catalysts were used in DBD reactor.     

Intrinsic kinetics of Fischer–Tropsch reactions over an industrial Co–Ru/γ-Al2O3 catalyst in slurry phase reactor

The rate of Fischer–Tropsch synthesis over an industrial well-characterized Co–Ru/γ-Al₂O₃ catalyst was studied in a laboratory well mixed, continuous flow, slurry reactor under the conditions relevant to industrial operations as follows: temperature of 200–240 °C, pressure of 20–35 bar, H₂/CO feed ratio of 1.0–2.5, gas hourly space velocity of 500–1500 N cm³ g_cat^− 1 h^− 1 and conversions of 10–84% of carbon monoxide and 13–89% of hydrogen. The ranges of partial pressures of CO and H₂ have been chosen as 5–15 and 10–25 bar respectively. Five kinetic models are considered: one empirical power law model and four variations of the Langmuir–Hinshelwood–Hougen–Watson representation. All models considered incorporate a strong inhibition due to CO adsorption. The data of this study are fitted fairly well by a simple LHHW form − R_H2 + CO = ap_H2^0.988p_CO^0.508 / (1 + bp_CO^0.508)² in comparison to fits of the same data by several other representative LHHW rate forms proposed in other works. The apparent activation energy was 94–103 kJ/mol. Kinetic parameters are determined using the genetic algorithm approach (GA), followed by the Levenberg–Marquardt (LM) method to make refined optimization, and are validated by means of statistical analysis. Also, the performance of the catalyst for Fischer–Tropsch synthesis and the hydrocarbon product distributions were investigated under different reaction conditions.     

Intensified Fischer–Tropsch synthesis process with microchannel catalytic reactors

A microchannel catalytic reactor with improved heat and mass transport has been used for Fischer–Tropsch synthesis. It was demonstrated that this microchannel reactor based process can be carried out at gas hourly space velocity (GHSV) as high as 60,000 h⁻¹ to achieve greater than 60% of single-pass CO conversion while maintaining relatively low methane selectivity (<10%) and high chain growth probability (>0.9). In this study, performance data were obtained over a wide range of pressure (10–35 atm) and hydrogen-to-carbon monoxide ratio (1–2.5). The catalytic materials were characterized using BET, scanning electron microcopy (SEM), transmission electron microcopy (TEM), and H₂ chemisorption. A three-dimensional pseudo-homogeneous model was used to simulate temperature profiles in the exothermic reaction system in order to optimize the reactor design. Intraparticle non-isothermal characteristics are also analyzed for the FT synthesis catalyst.     

Surface changes in Ru/KL supported catalysts induced by the preparation method and their effect on the selective hydrogenation of citral

Several 2 wt.% Ru/KL supported catalysts were prepared by various methods with different ruthenium precursors and characterized by CO and H₂ chemisorption, N₂ adsorption, TPD of NH₃, TEM and XPS. Furthermore, CO chemisorbed species have been studied by FT-IR and microcalorimetry. Characterization measurements of catalyst IWI-Ru, prepared by incipient wetness impregnation from ruthenium acetylacetonate, evidence metal nanoparticles of 1 nm placed inside the zeolite channels, thus blocking the accessibility to part of ruthenium loading inside the micropores. Catalyst prepared by treating the KL zeolite with RuCl₃·xH₂O aqueous solution (I-Ru) exhibits nanoparticles in the range 6–8 nm at the external surface and clusters smaller that 1 nm, inside the micropores. These latter do not significantly affect the diffusion of probe molecules through the channels. Catalytic performances in the selective hydrogenation of citral in the liquid phase, at 323 K and 5 MPa, show that IWI-Ru is less active than I-Ru, but more selective towards unsaturated alcohols. Furthermore, for IWI-Ru, selectivity increases with the increasing conversion. On the other hand, removal of acid sites of the I-Ru catalyst enhances the hydrogenation activity and increases the selectivity towards citronellal. All these results are analyzed and discussed in terms of the size, shape and location of ruthenium particles in the catalysts, as well as of the metal–support interaction.     

Chemically activated fungi-based porous carbons for hydrogen storage

A set of porous carbons has been prepared by chemical activation of various fungi-based chars with KOH. The resulting carbon materials have high surface areas (1600–2500 m²/g) and pore volumes (0.80–1.56 cm³/g), regardless of the char precursors. The porosities mainly derived from micropores in activated carbons strongly depend on the activation parameters (temperature and KOH amount). All activated carbons have uniform micropores with pore size of 0.8–0.9 nm, but some have a second set of micropores (1.3–1.4 nm pore size), further broadened to 1.9–2.1 nm as a result of increasing either the activation temperature to 750 °C or KOH/char mass ratio to 5/1. These fungi-based porous carbons achieve an excellent H₂ uptake of up to 2.4 wt% at 1 bar and −196 °C, being in agreement with results from other porous carbonaceous adsorbents reported in the literature. At high pressure (ca. 35 bar), the saturated H₂ uptake reaches 4.2–4.7 wt% at −196 °C for these fungi-based porous carbons. The results imply a great potential of these fungi-based porous carbons as H₂ on-board storage media.     

Kinetic modelling of methylcyclohexane ring-opening over a HZSM-5 zeolite catalyst

A kinetic model is proposed in order to quantify product distribution in the ring-opening (using high hydrogen concentration in the reaction medium) of methylcyclohexane (MCH) over a catalyst based on HZSM-5 zeolite. The model is based on a reaction scheme proposed by Cerqueira et al. for methylcyclohexane cracking at atmospheric pressure, which has been modified in order to include the effect of hydrogen over the individual reaction steps. The experimental results used for estimating the kinetic constant were obtained in a fixed bed isothermal reactor in a wide range of conditions, i.e. 250–450 °C; WHSV = 0.5–10.5 h⁻¹ (τ = 0.095–2 g_cat h g_MCH⁻¹); pressure = 5–80 bar; H₂/MCH molar flow ratio = 4–79; conversion = 0–100%. The kinetic model proposed can be regarded as a basis for the proposal of models for ring-opening reactions of more complex naphthenic feedstock from a prior hydrogenation step involving aromatic refinery streams of secondary interest.     

Study of ruthenium supported on Ta2O5–ZrO2 and Nb2O5–ZrO2 as catalysts for the partial oxidation of methane

Ru-based catalysts supported on Ta₂O₅–ZrO₂ and Nb₂O₅–ZrO₂ are studied in the partial oxidation of methane at 673–873 K. Supports with different Ta₂O₅ or Nb₂O₅ content were prepared by a sol–gel method, and RuCl₃ and RuNO(NO₃)₃ were used as precursors to prepare the catalysts (ca. 2 wt.% Ru). At 673 K high selectivity to CO₂ was found. An increase of temperature up to 773 K produced an increase in the selectivity to syngas (H₂/CO = 2.2–3.1), and this is related with the transformation of RuO₂ to metallic Ru as was determined from XRD and XPS results. At 873 K and with co-fed CO₂ an increase of the catalytic activity and CO selectivity was found. A TOF value of 5.7 s⁻¹ and H₂/CO ratio ca. 1 was achieved over Ru(Cl)/6TaZr. Catalytic results are discussed as a function of the support composition and characteristics of Ru-based phases.     

Mesoporous H3PW12O40-silica composite: Efficient and reusable solid acid catalyst for the synthesis of diphenolic acid from levulinic acid

Mesoporous H₃PW₁₂O₄₀-silica composite catalysts with controllable H₃PW₁₂O₄₀ loadings (4.0–65.1%) were prepared by a direct sol–gel–hydrothermal technique in the presence of triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) copolymer. Powder X-ray diffraction (XRD) patterns and nitrogen sorption analysis indicate the formation of well-defined mesoporous materials. With H₃PW₁₂O₄₀ loading lower than 20%, the materials exhibit larger BET surface area (604.5–753.0 m² g⁻¹), larger and well-distributed pore size (6.1–8.6 nm), larger pore volume (0.75–1.2 cm³ g⁻¹), and highly dispersed Keggin unit throughout the materials. Raman scattering spectroscopy studies confirm that the primary Keggin structure remained intact after formation of the composites. As a novel kind of reusable solid acid catalyst, as-prepared H₃PW₁₂O₄₀-silica composite was applied for the synthesis of diphenolic acid (DPA) from biomass platform molecule, levulinic acid (LA), under solvent-free condition, and remarkably high catalytic activity and stability were observed.     

Comparison of a single grain activated carbon and column adsorption system

Results from a single grain activated carbon adsorption study indicate that the effective diffusion coefficient was from 0.65×10⁻⁶ to 7.4×10⁻⁶ cm²/s for H₂S in the concentration range of 20–300 ppmv at 23 °C for both virgin activated carbon (FAC) and impregnated-regenerative activated carbon (IRAC). The effective diffusivity of the IRAC was nearly two times the FAC for H₂S adsorption. The surface reaction of H₂S-impregnated regenerative activated carbon was faster than that of H₂S and virgin activated carbon. The single grain activated carbon kinetic curve and a time scale conversion method were used to predict the breakthrough curve and the adsorption capacity of the column adsorption system. The single grain activated carbon adsorption system measured the breakthrough curve more efficiently than column adsorption. The prediction error was between 10 and 30%. Improvement can be further achieved by enhanced experimental approaches. It has a great potential for scale-up.     

Bimetallic catalysts for the catalytic combustion of methane using microreactor technology

Pt–W and Pt–Mo based catalysts were evaluated for methane combustion using a sandwich-type microreactor. Alumina washcoated microchannels were impregnated with platinum in combination with and promoted with tungsten and molybdenum and compared with commercially available Pt/Al₂O₃ catalysts. Catalysts were tested in the range of 300–700 °C with flow rates adjusted to GHSV of 74,000 h⁻¹ and WHSV of 316 L h⁻¹ g⁻¹. Catalysts containing tungsten were found to be the most active and the most stable possibly due to a metal interaction effect. A Pt–W/γ-Al₂O₃ containing 4.6 wt% Pt and 9 wt% W displayed the highest activity with full conversion at 600 °C and a selectivity to CO₂ of 99%.     

Thermodynamic-kinetic synergistic separation of CH4/N2 on a robust aluminum-based metal-organic framework

A robust aluminum-based metal–organic framework (Al-MOF) MIL-120Al with 1D rhombic ultra-microporous was reported. The nonpolar porous walls composed of para-benzene rings with a comparable pore size to the kinetic diameter of methane allow it to exhibit a novel thermodynamic-kinetic synergistic separation of CH₄/N₂ mixtures. The CH₄ adsorption capacity was as high as 33.7 cm³/g (298 K, 1 bar), which is the highest uptake value among the Al-MOFs reported to date. The diffusion rates of CH₄ were faster than N₂ in this structure as confirmed by time-dependent kinetic adsorption profiles. Breakthrough experiments confirm that this MOF can completely separate the CH₄/N₂ mixture and the separation performance is not affected in the presence of H₂O. Theoretical calculations reveal that pore centers with more energetically-favorable binding sites for CH₄ than N₂. The results of pressure swing adsorption (PSA) simulations indicate that MIL-120Al is a potential candidate for selective capture coal-mine methane.     

Hydrogen generation and purification in a composite Pd hollow fiber membrane reactor: Experiments and modeling

“Pd nanopore” composite membranes are a novel class of H₂ permselective membranes in which a thin layer of Pd is grown within the pores of a supported nanoporous layer. In this work, Pd nanopore membranes and conventional Pd top-layer membranes were used in the generation of high-purity H₂ from the catalytic decomposition of anhydrous NH₃. An effective 4 μm thick Pd nanopore membrane and 13 μm thick Pd top-layer membrane were synthesized on 2 mm O.D. α-Al₂O₃ hollow fibers. The permeation features of the membranes were determined and the membranes were then employed in a single fiber packed-bed membrane reactor in which Ni-catalyzed NH₃ decomposition served as the test reaction, with conditions spanning a range of conditions (500–600 °C; 3–5 bar total retentate pressure; 60–1200 scc/h g cat space velocity). The NH₃ conversions in both the PBMRs were approximately 10% higher than in a packed-bed reactor (PBR) under similar conditions. The increase in conversion with the PBMR was attributed to the removal of H₂, which has an inhibitory effect on the forward kinetics of the reaction as per the Temkin-Pyzhev type rate mechanism. Reactor productivities in the range of 2 mol/s m³ (at 85% H₂ utilization) to 7 mol/s m³ (at 50% H₂ utilization) were obtained. The permeate stream purity exceeded 99.2% H₂. A two-dimensional pseudo-homogeneous model was successfully used to simulate the experimental results and to interpret the findings. Permeation and kinetic parameters were estimated in permeation and PBR experiments, respectively. Without any data fitting the PBMR model predictions demonstrated very good agreement with experimental trends. Together with an analysis of the characteristic times, the model determined that transverse transport of hydrogen in the catalyst bed limited PBMR performance. The model was used to determine the rate limiting step and to suggest ways in which the reactor productivities could be further improved.