‘High Conductivity’ Monolith Catalysts for Gas/Solid Exothermic Reactions

The replacement of conventional packed beds of catalyst pellets with novel ‘high conductivity’ honeycomb catalysts in industrial externally cooled multitubular fixed‐bed reactors for exothermic gas/solid processes offers potential advantages besides reduced pressure drops. Near‐isothermal operation could result in safer reaction operation, better catalyst thermal stability, improved selectivities, higher throughputs and more economical reactors with larger tubes. We report herein an experimental and theoretical investigation of this concept.     

Experimental and modeling analysis of the effect of catalyst aging on the performance of a short contact time adiabatic CH4-CPO reactor

Catalytic partial oxidation of methane at short contact time was studied in a lab-scale packed bed reactor over a 0.5 wt% Rh/A₂O₃ catalyst. Experiments were focused on the investigation of catalyst stability and durability upon repeated start-up/shut-down tests at different inlet temperatures and flow rates. Measurements of the axial temperature profiles evidenced a high sensitivity of the steady state thermal behavior of the reactor on catalyst activity: a decrease of the intrinsic catalytic activity was interpreted as the cause of a progressive over-heating of the bed which, in turn, moderated the loss of methane conversion and syngas productivity. At sufficiently high flow rate the observed temperature rise spread along the whole catalytic bed. Under such conditions both steady state and dynamic reactor performances were affected by the progressive decay of catalyst activity. A rationalization of the observed results was pursued by applying a one dimensional (1D) heterogeneous model of the reactor to the quantitative analysis of experimental results. Model predictions revealed the occurrence of operating surface temperatures up to 1100 °C and allowed to quantify the progressive worsening of reactor performances in terms of a loss of reforming activity localized in correspondence of the catalyst hot spot.     

Addition of propene to carbon monoxide-hydrogen in higher alcohol synthesis over unpromoted and caesium-promoted ZnCrO catalysts

Catalytic activity tests in higher alcohol synthesis over unpromoted and caesium-promoted ZnCrO catalysts with the addition of propene to CO-H₂ have been performed to clarify the role of alkenes in the chain growth to higher oxygenates. Over the unpromoted catalyst, the observed increments of selected products (1-butanol, 2-methyl-1-butanol, 1-methoxy-2-methyl-butane, C₇ ketone) upon the addition of propene are consistent with the occurrence of a hydrocarbonylation reaction of propylene to an aldehydic linear C₄ intermediate, which is successively transformed according to previously established reaction paths of higher alcohol synthesis over ZnCrO catalysts. Hydrocarbonylation is slower than aldol condensation, and the influence of propene addition is greatly reduced over the caesium-promoted catalyst as well as at higher reaction temperatures. We conclude that the contribution of alkenes to the reaction scheme of higher alcohol synthesis over alkali-promoted ZnCrO catalysts is apparently unimportant.     

Dominant Reaction Pathways in the Catalytic Partial Oxidation of CH4 on Rh

Reforming technologies are at the heart of converting fossil fuels and biofuels to syngas and hydrogen for novel energy applications and, among reforming technologies, catalytic partial oxidation is appealing for decentralized energy production due to the compactness of reactors. Yet, the mechanisms of these reactions are poorly understood. Here we combine fundamental surface chemistry and detailed reactor models to elucidate the pathways leading to syngas and propose strategies for optimizing the process.     

NH3–NO/NO2 SCR for Diesel Exhausts Aftertreatment: Reactivity, Mechanism and Kinetic Modelling of Commercial Fe- and Cu-Promoted Zeolite Catalysts

The activity and the mechanism of the main reactions in the NO/NO₂–NH₃ SCR reacting system were comparatively investigated over a Fe- and a Cu-promoted commercial zeolite catalyst for the aftertreatment of Diesel exhausts. A dynamic micro-kinetic model in close agreement with all the details of the SCR catalytic chemistry was also developed.     

A C1 microkinetic model for methane conversion to syngas on Rh/Al2O3

A microkinetic model capable of describing multiple processes related to the conversion of natural gas to syngas and hydrogen on Rh is derived. The parameters of microkinetic models are subject to (intrinsic) uncertainty arising from estimation. It is shown that intrinsic uncertainty could markedly affect even qualitative model predictions (e.g., the rate‐determining step). In order to render kinetic models predictive, we propose a hierarchical, data‐driven methodology, where microkinetic model analysis is combined with a comprehensive, kinetically relevant set of nearly isothermal experimental data. The new, thermodynamically consistent model is capable of predicting several processes, including methane steam and dry reforming, catalytic partial oxidation, H₂ and CO rich combustion, water‐gas shift and its reverse at different temperatures, space velocities, compositions and reactant dilutions, using the measured Rh dispersion as an input. Comparison with other microkinetic models is undertaken. Finally, an uncertainty analysis assesses the effect of intrinsic uncertainty and catalyst heterogeneity on the overall model predictions. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Catalytic combustion of gasified biomasses over Mn-substituted hexaaluminates for gas turbine applications

The catalytic combustion of gasified biomasses over Mn-substituted hexaaluminates with high thermal stability is addressed. Combustion activity tests of the main fuel components, i.e. CO, H₂, C₂H₄ and CH₄, have been performed, and the effects of H₂O and CO₂ on the fuel combustion have been investigated. The reactivity of NH₃ in the catalytic combustion has also been studied in view of its potential source of undesired fuel-NO_x. Lab-scale data have been preliminarily scaled up through mathematical modeling.     

NOx Adsorption Study over Pt–Ba/Alumina Catalysts: FT-IR and Reactivity Study

The NO _x storage process over Ba/Al₂O₃ and Pt–Ba/Al₂O₃ NSR catalysts has been analyzed in this study by performing experiments at 350 °C with NO₂ and NO/O₂ mixtures using different complementary techniques (Transient Response Method, in situ FT–IR and DRIFT spectroscopies). The collected data suggest that over the Pt–Ba/Al₂O₃ catalyst the NO _x storage process from NO/O₂ mixtures occurs forming at first nitrite species, which progressively evolve to nitrates. In addition, a parallel nitrate formation via disproportionation of NO₂ (formed upon NO oxidation) cannot be excluded.     

Comparison among structured and packed-bed reactors for the catalytic partial oxidation of CH4 at short contact times

Three types of catalyst support (foams, honeycomb monoliths with square channels and spheres with approximately equal values of specific geometric surface a_v) were examined and compared by simulation with a 1D, dynamic heterogeneous mathematical model for application to the autothermal partial oxidation of methane. Both cold start-up and steady-state behaviours were investigated.

It was found that mass and, particularly, heat transfer properties markedly affect the reactor behaviour, both at start up and at steady state. Thus, the choice of the catalyst support can lead to greatly different reactor performances. Concerning the reactor start-up, simulations revealed that better interphase heat transport properties and lower bed heat capacity are useful to minimize the total start-up time; on the other hand, more favourable transport properties reduce the maximum flow rate which allows to achieve and maintain an ignited steady state. At steady state, oxygen conversion is strictly governed by interphase mass transfer, while methane conversion depends on a more complex, mixed chemical-diffusional regime.