Catalyst Deactivation Process Research Articles

Studies on bimetallic catalyst behavior during naphtha reforming

Reforming of naphtha over Pt–Sn/Al2O3 and Pt–Re/Al2O3 catalysts has been investigated. Addition of nitrogen accelerated the catalyst deactivation process, enabling the study of activity decline and subsequent regeneration. Effect of presulfiding on the activity, selectivity and regeneration capacity of the catalyst was also examined. Presence of nitrogen promoted the dealkylation of C7 hydrocarbons, resulting in a higher initial benzene yield which declined with loss in catalyst activity. Sulfiding lowered the aromatics yield by blocking some of the active sites but deactivation rate declined, prolonging catalyst life between regenerations. Performance of Pt–Sn catalyst was similar to that of Pt–Re catalyst except that the aromatics yield was somewhat higher in the latter case.

Targeting by Comparison with Laboratory Experiments the SCR Catalyst Deactivation Process by Potassium and Zinc Salts in a Large-Scale Biomass Combustion Boiler

The deactivation of a commercial selective catalytic reduction (SCR) catalyst of type V2O5−WO3/TiO2 has been studied in this work through comparisons of results from a full-scale biomass combustion plant with those from laboratory experiments. In the latter, the catalyst was exposed to KCl, K2SO4, and ZnCl2 by both wet impregnation with diluted salt solutions and deposition of generated submicrometer aerosol particles by means of an electrostatic field. The reactivity of freshly prepared and deactivated catalyst samples was examined in the SCR reaction, for which the influence of the different salts and the method of exposure were explored. Chemical and physical characterizations of the catalyst samples were carried out focusing on surface area, pore volume, pore size, chemical composition, and the penetration profiles of potassium and zinc. Particle-deposition deactivation as well as commercially exposed catalyst samples were shown to impact surface area and catalyst activity similarly and to have penetration profiles with pronounced peaks. Salt impregnation influenced pore sizes and catalyst activity more strongly and showed flat penetration profiles. Deposition of submicrometer-sized particles on the monolithic SCR catalyst has been shown to induce deactivation of the catalyst with characteristics resembling those obtained in a commercial biomass combustion plant; the laboratory process can be used to further assess the deactivation mechanism by biomass combustion.

Kinetics of the catalytic hydrogenation of d-fructose over a CuO-ZnO catalyst

Abstract Kinetics of d -fructose hydrogenation over a copper catalyst (61 wt% CuO and 39 wt% ZnO) in aqueous solutions was studied. The hydrogenation experiments were carried out batchwise in a three-phase laboratory-scale reactor (300 ml, Parr Co.), operating at 35–65 bar and between 90 and 130 °C. The main hydrogenation products were mannitol and its epimer, sorbitol. Also, a minor isomerization of fructose to glucose was observed. In the operating regime studied, the reaction rate showed a second order dependency with respect to the hydrogen pressure. Mannitol selectivity at the experimental range varied from 60 to 68%. The selectivity values improved slightly, as the hydrogen pressure increased or the reaction temperature decreased. The effect of catalyst loading and catalyst deactivation during consecutive hydrogenation batches was also studied. Catalyst characterization studies (nitrogen adsorption BET, XPS, SEM and particle size analysis) were carried out for a better understanding of the catalyst deactivation and reduction processes. The fitting of the experimental data to the kinetic model was carried out by Modest software using a combined Simplex–Levenberg–Marquardt method. The proposed kinetic model was able to predict the experimental concentrations of fructose and mannitol as well as the by-products sorbitol and glucose with a better than 95% degree of explanation.

Propene polymerization using ansa-metallocenium ions: Catalyst deactivation processes during monomer consumption and molecular structures of the products formed

Propene polymerization using ansa-metallocene dialkyl complexes [Me 2C(Cp)IndMMe 2] ( 1a; M = Zr) and ( 1b; M = Hf) was studied using B(C 6F 5) 3 as a stoichometric activator. The initial rates of propene uptake as well as the steady-state rates of propene consumption are dramatically increased in the presence of excess borane using 1a and to a lesser extent 1b. 1H and 19F NMR experiments at B:Zr ratios of 1:1 and 1.2:1 reveal that the by-products produced following consumption of monomer are also sensitive to B:Zr stoichiometry. Under the former conditions, initiation is inefficient and unreacted ion-pairs [Me 2C(Cp)IndZrMe][MeB(C 6F 5) 3] ( 2a– b) effect C–H activation of the unsaturated oligomers that form to produce π-allyl complexes [Me 2C(Cp)IndZr(η 3-2-R-C 3H 4)][MeB(C 6F 5) 3] ( 4a) [R = –(CH 2CHMe) n CH 2CH 2CH 3] and methane; these oligomeric complexes were identified by comparison to π-allyl complexes 4b (R = Me) and 4c (R = CH 2CMe 3) prepared from ion-pairs 2a– b and either isobutene or 2,4,4-trimethylpentene. Complex 4b was structurally characterized and presents a highly distorted π-methallyl ligand. In the presence of excess borane, initiation is efficient, and the principal by-products formed are Zr-borohydride complexes Me 2C(Cp)IndZrMe(μ-H)B(C 6F 5) 3 ( 5a– b). The major isomer present was identified by comparison to the spectroscopic data observed for its Hf-analog 6 with could be isolated from these reactions and was structurally characterized. Complexes 5 are detected after consumption of monomer is complete whereas complex 6 was formed competitively during consumption of monomer and both are relatively unreactive towards subsequent monomer insertion. Thus, the formation of these μ-borohydride complexes is a reversible deactivation pathway for some ansa-metallocenium ions, and that excess borane may serve to reactivate them towards further insertion.

Methanol Selective Oxidation to Formaldehyde over Iron‐Molybdate Catalysts

This review deals with the important industrial reaction of formaldehyde manufacture by methanol oxidation over iron molybdate catalysts. Detailed reference is made to the used catalyst, preparation techniques (coprecipitation, sol‐gel like, mechanical mixing, etc.) including unsupported and supported catalysts, promoters and characterization methods. The controversial active phase assignment (stoichiometric versus Mo rich iron molybdate) is discussed. The proposed reaction mechanisms and kinetic laws for the main and side reactions are examined. The catalyst deactivation processes are reviewed and the role of Mo excess on these processes is underlined. Finally conclusions and perspectives are presented.

Simulation, Optimization and Parametric Studies of a Solid Catalyzed Gas Phase Ethylene Polymerization Fluidized Bed Reactor

A steady-state model incorporating interactions between separate bubbles and emulsion phases in a fluidized-bed polyethylene reactor is employed to study the effect of operating conditions on the stability of gas-phase polyethylene reactors. A simple optimization program is developed to find the optimum combination of operating parameters at which the maximum polyethylene production rate is achieved with the constraints only on the reactor temperature, which should not exceed the melting point of polymer. Using the optimum values found, the parametric investigation for the effect of the catalyst deactivation rate constant and process operating parameters, such as the catalyst feed rate and the gas superficial to minimum fluidization velocity on system behavior are investigated. The investigations reveal that, the fluidized bed polyethylene reactor is prone to show unstable behavior and temperature oscillations, and the optimum polyethylene production rate could be achieved when it operates close to its melting point under a suitable controller to avoid polymer melting.

Deactivation of a commercial catalyst in the epoxidation of ethylene to ethylene oxide—basis for accelerated testing

An accelerated deactivation procedure for a commercial Ag-based catalyst for ethylene epoxidation was developed in a gradientless Berty-recycle reactor by increasing temperature and oxygen concentration which, however, were kept constant in the whole reactor for a single experiment. Thus, deactivation kinetics could be studied explicitly. The initial rates of both C 2H 4O and CO 2 formation and of ethylene consumption increase with reaction temperature and oxygen concentration. At 260 °C increasing O 2 concentration favors the rate of ethylene oxide over that of CO 2 formation. Above 260 °C the opposite is true. Catalyst activity decays faster at both higher reaction temperature and oxygen concentration. A correlation exists between the increase of the size of Ag particles and the catalyst activity decay, pointing to sintering as the main deactivation mechanism for this catalyst. Thus, by increasing these variables the catalyst deactivation process is accelerated.

Detailed Mechanistic Studies using in situ Spectroscopic Analysis: A Look at Little‐Known Regions of the Heck Reaction

AbstractThe mechanism of the Heck reaction using palladium complexes with large phosphoramidite ligands is investigated. The catalyst precursor is an inactive dimer that equilibrates with the active monomeric species. A series of kinetic models is introduced and compared with concentration profiles obtained from FT‐NIR spectroscopy. First, an analytical solution of the differential equations for a simplified mechanism is considered. This fits well at low conversions but deviates at higher conversions with increasing deactivation. Formation of palladium clusters and palladium black is then included, with the simplification that all the deactivation processes are represented by a single first‐order process. This results in a five‐step mechanism that describes the dimer‐monomer equilibrium, the Pd(II)/Pd(0) catalytic cycle, and the catalyst deactivation process. The model can be used to monitor the transient concentrations of the virtual Pd(0), Pd(II), and dimer catalyst species, and helps to explain the effects of water. The high resolution of the measurements and low error levels of the models render this approach a powerful tool for mechanistic studies in homogeneous catalysis.

Kinetics of Photocatalyzed Gas Reactions Using Titanium Dioxide as the Catalyst – Part I: Photocatalyzed Total Oxidation of Olefines with Oxygen

AbstractIn order to obtain a useful expression for the kinetics of the photocatalyzed total oxidation reaction of olefines with TiO2 as the catalyst, the dependence of reaction rate on the concentration of the educt species, the absorbed irradiation intensity, the wavelength of the light and the temperature was investigated. The experiments were performed in a CSTR system. The results obtained for steady‐state conditions showed that besides the photocatalytic activity, the TiO2 powder fixed on one wall of the reactor has, in addition, a thermal activity for higher temperature. Although the reaction behavior of the system is relatively complex, it can be modeled straightforward by a concept proposing a cooperative coadsorption of oxygen and the olefine species mediated by the electron/hole pairs generated by the absorbed light on the semiconductor's surface.

Mg–V–Al mixed oxides with mesoporous properties using layered double hydroxides as precursors: catalytic behavior for the process of ethylbenzene dehydrogenation to styrene under a carbon dioxide flow

A new catalyst for the process of ethylbenzene dehydrogenation to styrene under a CO 2 atmosphere was obtained using vanadium-substituted layered double hydroxides as precursors. After calcination, vanadium-substituted hydrotalcite-like samples gave mixed oxides with a high surface area and strong mesoporous characteristics; the XRD analysis indicates the formation of Mg 3V 2O 8 and Mg 2Al 2O 4 in bulk while the XPS results point to the presence of a mixture of V 5+ and V 3+ ions on the surface. Catalytic tests show that the styrene yield and selectivity increase when the vanadium content of the sample increases. The higher catalytic activity is attributed to the formation of V 5+ on the surface that is promoted by the presence of CO 2 as a cofeed gas. The FTIR adsorption of pyridine shows that aluminum contributes to Lewis-type acidity of the catalyst and is important for establishing features of the catalyst deactivation process.

Deactivation of a Cu/Al2O3 Catalyst in a N2O Decomposition Reaction

Abstract Dinitrogen oxide (nitrous oxide, N2O), a non-CO2 greenhouse gas, is produced as a by-product in the oxidation of cyclohexanone and cyclohexanol with nitric acid in adipic acid production plants. A Cu/Al2O3 catalyst has been used to decompose N2O to nitrogen and oxygen. We investigated the rate of deactivation of Cu/Al2O3 to establish an industrial abatement process for N2O. When Cu/Al2O3 catalyst containing 3 wt% of Cu was used, the apparent activation energy for the N2O decomposition reaction was estimated to be 167 kJ mol−1 at 746–853 K and 113 kJ mol−1 at 853–953 K. The former activation energy has been ascribed to an inhibition effect of oxygen. The rate constants of the N2O decomposition reaction have decreased with time. The decrease in the reaction-rates was analyzed by applying a first-order equation with respect to the rate constants. Simulation of the catalyst deactivation process has revealed that the Cu/Al2O3 catalyst is effective for at least for two years under the condition that 15 m3 of the catalyst is used for an adipic acid plant with a production capacity of 120 kt per year. Thus, we have concluded that the catalyst is suitable for use in industrial processes.

Ferrocene as a precursor reagent for metal-catalyzed carbon nanotubes: competing effects

Single-walled nanotube (SWNT) synthesis is studied in a pyrolysis flame configuration seeded with ferrocene as a catalyst precursor reagent. The concentration of ferrocene is prominent among the factors that determine SWNT inception, growth, and catalyst particle deactivation. This reflects ferrocene’s role as metal catalyst precursor and as a source of cyclopentadiene species upon its thermal decomposition. Results with different reactant gas mixtures and flows are presented and interpreted in light of a SWNT synthesis mechanism and catalyst deactivation processes.

Consequences of catalyst deactivation for process design and operation

Catalyst deactivation has important consequences for the design of a process and the way it is operated. The nature of the deactivation, and in particular the question whether it can be reversed under conditions which are compatible with the normal operation or whether a separate regeneration treatment of the catalyst is required to restore its activity, as well as the time-scale of the deactivation determine the type of technology that is feasible and process options like reactor type and process configuration. This relationship between deactivation behaviour and process lay-out forms the subject matter of the present paper. The general principles that guide the choices of process type and parameters are illustrated in more detail with examples from the fields of catalytic reforming of petroleum naphtha and hydroprocessing of petroleum residues. In these fields, different catalyst deactivation mechanisms are operative and catalyst deactivation rates can vary widely depending upon feedstock and process parameters. Consequently, different reactor technologies and process configurational choices are possible. The relation between catalyst deactivation behaviour and process design and operation can be viewed from two sides: on the one hand, the deactivation behaviour may dictate the choice between viable process options and may provide an incentive for the development of novel technology that can cope optimally with the demands set by the deactivation of the catalyst. On the other hand, the introduction of novel technological options may widen the scope of a process, e.g. by opening the possibilities to apply novel catalysts or to operate under unconventional conditions that lead to a more economic or otherwise better process, possibilities that were previously barred by catalyst deactivation.

Preparation and Activity of Cu–Al Mixed Oxides via Hydrotalcite-like Precursors for the Oxidation of Phenol Aqueous Solutions

We performed thermogravimetric analysis (TGA), X-ray diffraction (XRD), BET areas, and FT-IR spectroscopy to characterize copper–aluminium mixed-oxide samples with Cu/Al ratios between 0.5 and 3.0. The thermal stability, crystallinity, and purity of the materials depended on the Cu/Al atomic ratio. The FT-IR and TG detected carbonate (mainly) and nitrate as counteranions which interact in the interlayer region. We found loosely bound carbonate and nitrate anions and one strongly bound type of carbonate. We used dynamic XRD experiments to study the evolution of phases during calcination. All the samples after calcination showed well-dispersed CuO and/or CuAl2O4 phases. We also tested their catalytic behaviour for the oxidation of 5 g/l phenol aqueous solutions using a triphasic tubular reactor working in a trickle-bed regime and air with an oxygen partial pressure of 0.9 MPa at a temperature reaction of 413 K. Phenol conversion decreased continuously over time for the samples calcined at lower temperatures (673 K). This is because of continuous loss of the CuO phase by elution and the formation of a new phase like copper oxalate on the surface of the copper catalysts which also elutes with time XRD shows that samples calcined at higher temperatures (1073 K) and after HCl treatment (0.1 M) to avoid the CuO phase, have a pure copper aluminate phase. This CuAl2O4 phase reaches steady activity plateaus in the 55–65% range of phenol conversion. The triphasic tubular reactor using trickle-bed regime largely avoids polymer formation as a catalyst-deactivation process.

Characterization and activity of copper and nickel catalysts for the oxidation of phenol aqueous solutions

Catalysts based on CuO/γ-alumina, CuAl 2O 4/γ-alumina, NiO/γ-alumina, NiAl 2O 4/γ-alumina and bulk CuAl 2O 4 have been structurally characterized by BET, porosimetry, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Their catalytic behaviors have also been tested for the oxidation of 5 g/l phenol aqueous solutions using a triphasic tubular reactor working in a trickle-bed regime and air with an oxygen partial pressure of 0.9 MPa at a temperature of 413 K. The copper and nickel catalysts supported on γ-alumina have surface areas of the same order as the support γ-alumina of ca. 190 m 2/g and high active phase dispersions which were also confirmed by SEM, whereas the bulk copper aluminate spinel has a surface area of ca. 30 m 2/g. XRD detects the phases present and shows a continuous loss of CuO by elution and the formation of a copper oxalate phase on the surface of the copper catalysts which also elutes with time. The NiO was also eluted but less than the copper catalysts. Only the copper and nickel spinel catalysts were stable throughout the reaction. Phenol conversion vs. time shows a continuous overall decrease in activity for the CuO/γ-alumina and NiO/γ-alumina catalysts. In turn, the copper and nickel spinel catalysts reach steady activity plateaus of 40 and 10%, respectively, of phenol conversion. The bulk copper aluminate spinel shows an activity plateau of 20% of the conversion which is lower than that from the copper aluminate/γ-alumina catalyst due to its lower surface area. Nickel catalysts always have lower activities than the copper catalysts for the phenol oxidation reaction. The copper catalysts drive a mechanism of partial phenol oxidation to carboxylic acids and quinone-related products with very high specific rates, and the nickel catalysts mainly drive a mechanism of CO 2 formation with lower conversion but with a potential higher catalyst life. The triphasic tubular reactor using trickle-bed regime largely avoids the mechanism of polymer formation as a catalyst deactivation process.

Facile α-C–H activation in 14-electron zirconium half-sandwich compounds: evidence for a new catalyst deactivation pathway

The electron-deficient zirconium diene complex [Zr(Cp″)Me(C 4 H 4 Me 2 -2,3)] [Cp″ = C 5 H 3 (SiMe 3 ) 2 -1,3] cleanly eliminates methane to give an unusual dinuclear dienyl complex [Zr(Cp″)(µ-η 1 :η 4 -C 4 H 3 Me 2 -2,3)] 2 , characterised by a new type of µ-η 1 :η 4 -butadienyl coordination mode; the complex and its zwitterionic reaction product with B(C 6 F 5 ) 3 provide support for catalyst deactivation processes involving alkane elimination reactions.

Direct Oxidation of L-Sorbose to 2-Keto-L-gulonic Acid with Molecular Oxygen on Platinum- and Palladium-Based Catalysts

The selective oxidation of the C1 hydroxyl group of L-sorbose to a carboxylic group without protection of the four other hydroxyl functions was investigated. The reactions were performed in slightly alkaline aqueous solutions with molecular oxygen over various alumina- and carbon-supported Pt and Pd catalysts. Optimum reaction conditions were 50°C, pH 7.3, and a catalyst : reactant ratio of 1 : 4 (wt/wt). The lower the pH and the temperature, the higher the selectivity toward 2-keto-L-gulonic acid. Catalyst deactivation was also found to increase with lower pH and temperature. A 5 wt% Pt/alumina catalyst showed the best catalytic performance (67% selectivity at 58% conversion). Promotion with Bi or Pb had a detrimental effect on selectivity for 2-keto-L-gulonic acid. Electrochemical measurements indicated that the reaction occurs in a rather narrow potential range, which corresponds to a moderate oxygen coverage of Pt or Pd. Four types of catalyst deactivation processes were identified, based on XPS and ICP-AES analysis and on the in situ determination of the oxidation state by monitoring the catalyst potential during reaction. A significant chemical poisoning of the active noble metal sites occurred during the initial, destructive adsorption of L-sorbose and during the oxidation reaction. The successive contamination of active sites resulted in overoxidation (too high oxygen coverage of Pt or Pd). The partially oxidized promoters and noble metals were corroded and dissolved in the presence of 2-keto-L-gulonic acid, resulting in an irreversible deactivation.

Transformation of propylene on molybdenum oxide supported on alumina

The transformation of propylene in the presence of molybdenum oxide on alumina was studied in a flow system over the range of temperatures 250–500 °C and contact times of 6–60 sec. Hydrogen, propane, butenes, and a highly unsaturated complex deposited on the catalyst surface were found to be the main reaction products. The decomposition of propylene as well as the formation of the major reaction products were shown to be of the first order. The rate constants and activation energies have been determined. The reaction mechanism has been postulated, involving dissociative adsorption of propylene, hydrogenation of propylene with chemisorbed hydrogen, addition of the methyl group to the complexes formed, and desorption of the products. Kinetics of the process based on this reaction scheme was discussed. Two empirical equations for the catalyst deactivation process have been proposed.