Van Krevelen Model Research Articles

Extraction of Silica from Different Solid Wastes and Its Application as Catalyst Support for Vapour Phase Oxidation of Aromatic Hydrocarbon

In present study, the research work has been divided into two sections viz. Firstly, amorphous silica was extracted from different silica rich agricultural and industrial waste materials (fly ash, perlite and rice husk ash) via sol-gel method. The extracted silica (RHA-Si, Perlite-Si, FA-Si) was characterized using BET surface area, FESEM, EDS, XRD, NMR and FTIR techniques. The surface area and particle size of extracted silica was found to be 23.06-255.5 m2/g and 61-267 nm. Secondly, fly ash silica was used as catalyst support and modified desirably by loading different molybdenum weight percentages via acid impregnation method. The presence of MoO3 on the surface of extracted silica were confirmed by different characterization techniques. The kinetic studies of the FASi/Mo-15 catalyst for the vapour phase oxidation of benzene was also investigated by employing the Mars Van Krevelen model to analyze the reaction mechanism and rate of the process. The vapour phase oxidation reaction follows zero order kinetics, with faster benzene conversion occurring at lower intake concentrations and higher reaction temperatures.

Bimetallic Fe-Mn loaded H-ZSM-5 zeolites for excellent VOCs catalytic oxidation at low-temperatures: Synergistic effects and catalytic mechanisms

The development of catalysts with high adsorption and low-temperature catalytic characteristics for the decomposition of volatile organic compounds (VOCs) is of great importance for air purification. Herein, H-ZSM-5 (proton form Zeolite Socony Mobil-5) zeolite -supported Fe-Mn elements were synthesized via a two-step procedure (hydrothermal and precipitation methods). These zeolite-supported bimetal Fe-Mn catalysts exhibited a mesoporous structure, uniform distribution of Fe-Mn elements, excellent catalytic-oxidation capacity for toluene at low-temperatures, and high stability. Due to interactive synergies between the bimetallic Fe and Mn, the Fe2-Mn1/H-ZSM-5 (ratio of Fe and Mn was 2:1 with a total amount of 15 % to H-ZSM-5) exhibited 100 % toluene-conversion at 200 °C and T90 = 195 °C (temperature of toluene-conversion at 90 %) with GHSV = 20,000 mL/(g∙h). Its distinguishing catalytic-oxidation abilities at low temperatures were due to abundant oxygen-vacancies, adequate acidic-sites and various active oxygen-species, which improved the adsorption of toluene and expedited its degradation. The Fe2-Mn1/H-ZSM-5 catalyst also demonstrated a good resistance to humidity and renewable catalytic properties over five cyclic tests. The in-situ DRIFTS and LC-MS results revealed that toluene was decomposed to benzyl-alcohol, benzaldehyde, benzoate-acid, formates or/and acetate step by step, and finally CO2 and H2O. The mechanism of toluene oxidation over Fe2-Mn1/H-ZSM-5 catalysts was proposed and elucidated via the Mars–van-Krevelen (MVK) model. This study comprehensively explored the adsorption and decomposition kinetics of VOCs over bimetal decorated H-ZSM-5, which was useful toward the design of highly efficient catalysts for the removal of VOCs.

Hydrothermal synthesis and characterization of Cu-MnOx catalysts for CO oxidation: Effect of Cu:Mn molar ratio on their structure and catalytic activity

A comprehensive study was conducted on the synthesis of Cu-MnOx catalysts using a one-pot hydrothermal method followed by a calcination step. These catalysts were thoroughly characterized using techniques such as X-ray diffraction (XRD), X-ray fluorescence (XRF), N2 physisorption, X-ray photoelectron spectroscopy (XPS), temperature programmed desorption and reduction (TPD and TPR), and advanced transmission electron microscopy (TEM). The results revealed that the Cu:Mn molar ratio of the precursors, Cu(NO3)2 and KMnO4, had a significant impact on the crystalline structure and morphology of the synthesized samples. A Cu-doped cryptomelane with a nanorod morphology was observed when the Cu:Mn molar ratio was 0.05. Conversely, a mixture of Cu-doped cryptomelane nanorods, Mn2O3, and Cu1.5Mn1.5O4 spinel nanoparticles was formed at molar ratios of 0.1 and 0.25. The catalytic activities of these catalysts for CO oxidation followed the order: 0.25Cu-MnOx ≈ 0.1Cu-MnOx > 0.05Cu-MnOx > cryptomelane. A good correlation was found between the catalytic performance and reducibility of the catalysts under CO atmosphere that can be related to the ability of the samples to activate the CO molecule. The high reducibility of the sample even at room temperature suggest that the CO oxidation over the synthesized Cu-MnOx catalysts may follow a Mars van Krevelen model.

Flower-like Pt/Fe2O3–CeO2 Catalysts for Highly Efficient Low-Temperature Catalytic Oxidation of Toluene

A series of porous Pt/FeₓCey microflowers have been synthesized through a solvothermal method. The well-defined mesoporous structure promotes gas distribution as well as even dispersion of active species, thus exposing more active sites and improving the catalytic efficiency for low-temperature toluene removal. The catalyst composition can be regulated by changing the Fe/Ce molar ratio and Pt loading content. Among the samples, 1 wt % Pt/Fe₃Ce₁ performed the best, with a reaction temperature for 90% toluene conversion of 192 °C at a WHSV of 36,000 mL/(h·g). The synergistic effect of Fe₂O₃ and CeO₂ promotes electron transfer and migration of oxygen species. Addition of Pt enhances the mobility of lattice oxygen, generating a high concentration of active surface lattice oxygen, which facilitates toluene removal. The Mars–van Krevelen model could be applied to explain the mechanism of toluene oxidation over these Fe-based catalysts.

Oxidation kinetics of 2‐methyl‐1,3‐propanediol to methacrylic acid in a fluidized bed reactor

AbstractMethacrylic acid (MAA) is a specialty monomer for poly methyl methacrylate (PMMA). Partial oxidation of 2‐methyl‐1,3‐propanediol (2MDPO) to MAA is an alternative to commercial technology with fewer process steps, less cost and toxic feedstocks, and longer catalyst lifetime. Here, we evaluated the effect of 2MPDO and oxygen concentrations, reaction temperature, and contact time on product selectivity and 2MPDO conversion over VOCu0.5/Cs(NH4)2PMo12O40. Higher temperature increases selectivity and conversion; MAA selectivity reached 46% at 69% 2MPDO conversion. Higher 2MPDO to O2 ratio favours higher MAA selectivity due to methacrolein (intermediate) oxidation to MAA (a higher ratio favours consecutive oxidation) or lower combustion of MAA to COx. Shorter contact times decrease MAA selectivity. The Mars van Krevelen model characterizes the experimental data better than both the Langmuir‐Hinshelwood or Eley‐Rideal models; The reaction sequence involves both direct and indirect reactions in which 2MPDO reacts to methacrolein (MAC) as an intermediate and then it oxidizes to MAA (indirect) and/or 2MPDO directly oxidizes to MAA but the indirect reaction rate to MAA is 50 times faster than the direct reaction rate. The reaction is first order with respect 2MPDO and oxygen, and the rate‐limiting step is 2MPDO activation on oxidized sites.

First discernments for NO storage and reduction (NSR) on lithium cuprate (Li2CuO2) at moderate temperatures (100 ≤ T ≤ 400 °C)

The present study was carried out to analyze the NO storage and reduction (NSR) on Li2CuO2. The objective of this investigation was to determine the first insights of NSR, while using a new material. Li2CuO2 was initially evaluated as NO sorbent (2 mL/min) in the presence of different oxygen contents. These results were used to verify the NO consumption and chemical sorption and to propose a reaction mechanism for the lithium nitrate formation. The best NSR results were obtained between 200 and 300 °C and Li2CuO2 showed high stability during cycling processes. A reaction mechanism, based on the Mars–Van Krevelen model was presented. After that, similar experiments were performed but using a lower NO concentration (777 ppm), where the results were equivalent to those obtained with higher NO concentrations. Thus, results establish interesting bases to analyze and allow to a double NO NSR-SCR process in the future, using alkaline ceramics.

Controlled fabrication of mesoporous ZSM-5 zeolite-supported PdCu alloy nanoparticles for complete oxidation of toluene

Mesoporous ZSM-5 zeolite-supported PdCu alloy nanoparticle catalysts are synthesized via a facile and environmentally friendly method. The mesoporous structure is formed through the loss of carbon black particles encapsulated by zeolite crystals during calcination. The ZSM-5-supported PdCu nanoparticles are shown to possess mesoporous structure, evenly distributed PdCu alloy nanoparticles, and excellent catalytic performance in toluene oxidation. The mesoporous structure of the ZSM-5-supported PdCu nanoparticles provides a large specific surface area and pore size, resulting in numerous accessible active sites. The alloying of Pd with Cu both increases the catalytic performance of the catalyst and lowers its cost. Overall, the catalyst with a Pd/Cu ratio of 8:2 showed the highest catalytic activity of the fabricated materials, achieving 100 % toluene conversion at 160 °C. The mechanism of toluene oxidation over these catalysts can be explained by the Mars–van Krevelen model.

Preparation, characterization and catalytic performance of Pt supported on porous carbonaceous materials in the oxidation of toluene as a volatile organic compound

Platinum-carbonaceous catalysts were prepared by the wet impregnation method and tested for catalytic oxidation of toluene as a volatile organic compound. The textural properties of the constructed catalysts were considered by X-ray diffraction, X-ray fluorescence, inductively coupled plasma – optical emission spectroscopy, Fourier transform infrared, scanning electron microscope and N2 adsorption–desorption analysis. The catalytic assessments showed that the best activity (>99%) and high stability and selectivity to CO2 (>99%) are related to platinum-supported carbon nanotube. The curves of the conversion and selectivity demonstrate that the performance of catalysts to eliminate the volatile organic compound and turn it into CO2 conforms to the following descending order: platinum-supported carbon nanotube >platinum-supported graphene >platinum-supported activated carbon >platinum-supported carbon nanofibre. The kinetic of toluene oxidation has been evaluated as a function of toluene and oxygen partial pressures in different temperatures. Two kinetic models (Power Law and Mars–van Krevelen mechanisms) were applied to the reaction and compared with the experimental data. Mars–van Krevelen model is more appropriate than the Power Law model for this reaction as Mars–van Krevelen model showed better prediction of the behaviour of the reaction.

Oxygen Activation and Pathways in High-Temperature Reactions of Light Alkane Oxidation: A Seeming Simplicity of Kinetic Description

The kinetic regularities of the catalytic oxidation processes that proceed via a stepwise redox mechanism are considered. The main assumptions of the Mars–van-Krevelen model widely used to describe such processes are analyzed, and their correspondence to actual catalytic systems—typical catalysts for the partial oxidation of light alkanes (oxidative coupling of methane, oxidative dehydrogenation of C2+ alkanes, oxidative cracking of C3+ alkanes)—is examined. Certain features of the composition and operating conditions of these catalysts require some revision of existing notions about the activation of molecular oxygen, including factors that determine its interaction with active sites and participation in the catalytic cycle. Particular attention is paid to: the thermodynamic and kinetic conditions for the realization of the redox mechanism (thermochemistry of oxide systems, the lifetime of oxygen in a bound state); suitability of the redox-type mechanisms in the case of catalysts that do not contain cations with a variable oxidation state; chemical and phase transformations, which are part of the catalytic redox cycle; interpretation of the results of kinetic experiments in the study of heterogeneous-homogeneous processes and the influence of free-radical reactions on the values of experimentally determined kinetic parameters.

Synthesis and characterization of a novel isocoumarin derived polymer and its thermal decomposition kinetics

A novel isocumarin derived polymer poly(2-(isocoumarin-3-yl)-2-oxoethyl methacrylate) poly(ICEMA) was synthesized by free radical polymerization. The spectral characterization was performed with FTIR and 1H,13C-NMR techniques. The glass transition temperature of poly(ICEMA) was measured to be 161.69 °C by DSC technique. The initial decomposition temperatures obtained from TGA showed a change in the positive direction from 256.59 °C to 286.10 °C as the heating rate increased to 20 °C/min. Thermal decomposition activation energies of poly(ICEMA) in the conversion range of 7% - 19% were found to be 136.12 kJ/mol and 134.83 kJ/mol by Flynn–Wall–Ozawa and Kissinger’s models, respectively. In addition, various integral models such as Coats-Redfern, Tang, Madhusudanan and Van-Krevelen models were used to determine the thermal decomposition mechanism of poly(2-(isocoumarin-3-yl)-2-oxoethyl methacrylate)which showed that it proceeded at the optimum heating rate of 5 ºC/min over the D1 one-dimensional diffusion type deceleration mechanism

Study of HMS Modified ZrO2 Supported Platinum Catalysts for Toluene Removal: Catalytic Combustion and Kinetics Study

Reaction behaviors and kinetics of catalytic oxidation of toluene with different feed flows over Pt/Zr(x)-HMS catalysts with Si/Zr ratio equal to 5, 10, 20 and 35 were investigated over a wide temperature range (200 – 500 oC). Results show that Pt/Zr(x)-HMS performs more easily toluene oxidation. The kinetic data were fitted by the Power-law and Mars–van Krevelen kinetic models. The fitting results show that the Mars–van Krevelen model (R2 > 0.99) is more suitable for predicting the conversion of toluene than the Power law model (R2 = 0.53), and the Mars–van Krevelen model can accurately express the reaction rate of this process.

Insight into Copper Oxide‐Tin Oxide Catalysts for the Catalytic Oxidation of Carbon Monoxide: Identification of Active Copper Species and a Reaction Mechanism

AbstractHerein, we report the high activity of CuO‐SnO2 catalysts for the catalytic oxidation of CO. In particular, SnCu30 shows the highest activity and a high water resistance. If we compare the XRD, X‐ray absorption fine structure, and H2 temperature‐programmed reduction results of SnCu30 before and after HNO3 treatment, we find the existence of three kinds of Cu species in the catalyst, that is, highly dispersed CuO, bulk CuO, and Cu incorporated in the SnO2 lattice. The highly dispersed CuO and the surface lattice oxygen species are the active sites for the catalytic oxidation of CO. We used X‐ray photoelectron spectroscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy to confirm the existence of Cu+ species on the surface of the CuO‐SnO2 catalysts, which can provide the adsorption sites for CO. Our results show that the reaction pathways of the catalytic oxidation of CO over the CuO‐SnO2 catalysts follow the Mars–van Krevelen model.

Catalytic removal of phenol from gas streams by perovskite-type catalysts

Three perovskite-type catalysts prepared by citric acid method are applied to remove phenol from gas streams with the total flow rate of 300mL/min, corresponding to a GHSV of 10,000/hr. LaMnO3 catalyst is first prepared and further partially substituted with Sr and Cu to prepare La0.8Sr0.2MnO3 and La0.8Sr0.2Mn0.8Cu0.2O3, and catalytic activities and fundamental characteristics of these three catalysts are compared. The results show that phenol removal efficiency achieved with La0.8Sr0.2Mn0.8Cu0.2O3 reaches 100% with the operating temperature of 200°C and the rate of mineralization at 300°C is up to 100%, while the phenol removal efficiencies achieved with La0.8Sr0.2MnO3 and LaMnO3 are up to 100% with the operating temperature of 300°C and 400°C, respectively. X-ray photoelectron spectroscopy (XPS) analysis shows that the addition of Sr and Cu increases the lattice oxygen of La0.8Sr0.2Mn0.8Cu0.2O3, and further increases mobility or availability of lattice oxygen. The results indicate that La0.8Sr0.2Mn0.8Cu0.2O3 has the best activity for phenol removal among three catalysts prepared and the catalytic activity of phenol oxidation is enhanced by the introduction of Sr and Cu into LaMnO3. Apparent activation energy of 48kJ/mol is calculated by Mars–Van Krevelen Model for phenol oxidation with La0.8Sr0.2Mn0.8Cu0.2O3 as catalyst.

Calculation of thermal conductivity of filled oligomeric compositions

Different models for the calculation of thermal conductivity of oligomeric compositions filled with a dispersed filler (silicon carbide) are studied. It is shown that the calculated values of thermal conductivity of oligomeric compositions with introduced dispersed filler provided by the van Krevelen model developed for the calculation of thermal conductivity of unfilled amorphous polymers with isolated regions of the crystalline phase are the closest to the experimental ones.

Reaction kinetics and mechanism of benzene combustion over the NiMnO3/CeO2/Cordierite catalyst

The kinetics of the catalytic combustion of benzene at different concentrations over the NiMnO3/CeO2/Cordierite catalyst were investigated to gain more insight into the catalytic reaction mechanism. A kinetic study was performed in a packed-bed tubular reactor under different conditions. The catalytic combustion kinetics were modeled using a Power-Law model and the Mars–Van Krevelen model. The results showed that the Mars–Van Krevelen kinetic model provided a significantly better fit to explain the catalytic combustion kinetics of benzene over the NiMnO3/CeO2/Cordierite catalyst. The results showed that the reduction reaction occurred more easily than did the oxidation reaction on the surface of the catalyst. Moreover, the values of the pre-exponential factor for the reduction steps (7.84×1011s−1) is higher than those of the oxidation steps (1.04×109s−1), indicating that the more is the effective collision times between the activated molecules, and the easier for chemical reaction to occur, and the degree and speed are more intense and rapid. Therefore, it can be concluded that the catalyzed surface oxidation reaction is the control step of catalytic benzene combustion. Based on this analysis of the experimental results and the assumptions of the Mars–Van Krevelen model, it was determined that the catalytic combustion of benzene over the NiMnO3/CeO2/Cordierite catalyst obeys the Mars–Van Krevelen mechanism. The catalytic combustion reaction occurred by the interaction between the benzene molecules and the active sites of the NiMnO3/CeO2/Cordierite catalyst. The catalytic oxidation of benzene involves a catalytic redox cycle of adsorption, deoxidation, desorption, oxygen supply and regeneration.

Insight into the improvement effect of the Ce doping into the SnO2 catalyst for the catalytic combustion of methane

Herein, we report the performance of the catalytic combustion of methane over the Ce-doped SnO2 catalysts. Doping with Ce increases the surface areas, decreases the crystallite sizes, and activates both the surface metal cations and surface oxygen species. Upon methane combustion, the surface Sn4+ cations are active sites, and the surface lattice oxygen plays an important role, as well. Kinetics results suggest that the activation energy (Ea) and pre-exponential factor (A) are determined by the reducibility and the area-specific quantity of the surface Sn4+ cations, respectively. The Sn0.7Ce0.3O2 catalyst exhibits the highest area-specific rate because of its lowest Ea and relatively bigger A values. Its turnover frequency is five times higher, as compared with the SnO2. The reaction pathways upon the Sn-rich catalysts (SnO2 phase) follow the Mars–van Krevelen model, while they become more complex upon the Ce-rich ones (CeO2 phase). Additionally, these SnO2-based catalysts display the high water resistance.

Catalytic combustion kinetics of isopropanol over novel porous microfibrous‐structured ZSM‐5 coating/PSSF catalyst

Porous thin‐sheet cobalt–copper–manganese mixed oxides modified microfibrous‐structured ZSM‐5 coating/PSSF catalysts were developed by the papermaking/sintering process, secondary growth process, and incipient wetness impregnating method. Paper‐like sintered stainless steel fibers (PSSF) support with sinter‐locked three‐dimensional networks was built by the papermaking/sintering process, and ZSM‐5 coatings were fabricated on the surface of stainless steel fibers by the secondary growth process. Catalytic combustion performances of isopropanol at different concentrations over the microfibrous‐structured Co–Cu–Mn (1:1:1)/ZSM‐5 coating/PSSF catalysts were measured to obtain kinetics data. The catalytic combustion kinetics was investigated using power–rate law model and Mars–Van Krevelen model. It was found that the Mars–Van Krevelen model provided fairly good fits to the kinetic data. The catalytic combustion reaction occurred by interaction between isopropanol molecule and oxygen‐rich centers of modified microfibrous‐structured ZSM‐5 coating/PSSF catalyst. The reaction activation energies for the reduction and oxidation steps are 60.3 and 57.19 kJ/mol, respectively. © 2014 American Institute of Chemical Engineers AIChE J, 61: 620–630, 2015

Kinetic study of CO oxidation over CuO/MO2 (M = Si, Ti and Ce) catalysts

A series of supported CuO/MO2 catalysts (M=Si, Ti and Ce) were prepared using a chemisorption-hydrolysis method and tested for CO oxidation. Kinetic study was carried out to investigate reaction pathways on these catalysts. The power-rate law reaction expressions over CuO/SiO2, CuO/TiO2 and CuO/CeO2 were r=1.78×10−7Pco0.85Po20.22, r=1.62×10−7Pco0.65Po20.19 and r=1.5×10−7Pco0.7Po20, respectively. Activation energies were calculated to be 74.8, 53.4 and 46.3kJmol−1 for the CuO/SiO2, CuO/TiO2 and CuO/CeO2 catalysts respectively. Elementary steps of CO oxidation were also proposed. For the CuO/SiO2 catalysts, a Langmuir–Hinshelwood (L–H) model involving chemisorption of CO and O2 on surface Cu atoms was proposed; for the CuO/TiO2 catalysts, a L–H model involving chemisorption of CO on Cu atoms and chemisorption of O2 on TiO2 was proposed, which resulted in a interfacial reaction between the two species. For the CuO/CeO2 catalysts, a Mars van–Krevelen model involving chemisorbed CO on Cu atoms reacting with lattice oxygen in CeO2 was proposed. Effect of CuO particle size was also evaluated. It was found that the active site on the periphery of the CuO–TiO2 and CuO–CeO2 interface was more active on large CuO crystallite than that on small one. The enhanced activity could be interpreted by the higher concentration of chemisorbed CO on the active site on the larger CuO crystallite.

Kinetic study of selective propane oxidation to acrylic acid over Mo1V0.3Te0.23Nb0.12Ox using the genetic algorithm

The kinetics of selective propane oxidation to acrylic acid over Mo1V0.3Te0.23Nb0.12Ox was investigated using the experimental data of a micro catalytic fixed bed reactor. Experimental data were obtained under different operating conditions (T = 380–490 °C, GHSV = 50, 33.3 ml (min gcat)−1, (O2)/(C3H8) = 1, 2, 3 and (steam)/(C3H8) = 5, 7.5). Power law, Eley–Rideal and Mars–Van Krevelen models for the prediction the catalytic performance were employed using the genetic algorithm. The reaction orders obtained by the power law model determined that the propane oxidation is dependent on the gas phase oxygen concentration. The Eley–Rideal model, in which surface oxygen is in equilibrium with the gas phase, could not predict this dependency on oxygen concentration. So, it was proposed that selective oxidation of propane over this catalyst follows the Mars–Van Krevelen mechanism. Kinetic analysis allowed determination the critical role of water in the catalyst-reaction system. The absence of water in the reaction feed induces some structural defects which decreases propane activation.

Kinetic Study of Combustion of Isopropanol and Ethyl Acetate on Monolith-Supported CeO2 Catalyst

The complete catalytic combustion kinetics of isopropanol and ethyl acetate were investigated using a CeO2 catalyst coated on a monolith support. The combustions were carried out at low temperatures and atmospheric pressure, on a fixed-bed monolithic reactor. The Mars and van Krevelen model, power rate law model and Langmuir – Hinshelwood model were used to analyse the experimental results. The differential reactor assumption was used to fit the kinetic models. Due to the low activation energies of the reactions, both the Mars – Van Krevelen and power rate law models showed good agreement between the measured and calculated reaction rates, with the Mars – Van Krevelen model being slightly better.