Study On The Reaction Process Of Benzene Oxidation To Phenol With N₂O Over Fe-ZSM-5 Zeolite

Fe-ZSM-5 zeolite, as a catalyst, was widely used in the oxidation of benzene with N2O to phenol with good initial activity and selectivity. However, the rapid deactivation of the catalyst leads that the industrialization of the process route was restricted. While most researches were focused in the preparation and modification of the catalyst, it failed to solve the problem of high deactivation rate. Based on the use of Fe-ZSM-5 zeolite in circulating fluidized bed, a series of experimental and theoretical studies were carried out on the characteristic of the oxidation of benzene to phenol with N2O, including reaction conditions, reaction network and steady or unsteady state performance of the catalyst in the reaction. The purpose of this paper was to provide the theoretical instructions for the development of the circulating fluidized bed process and a new methods for the solution of the catalyst deactivation.At first, based on an isothermal fixed bed reactor (SizeΦ21×3mm, L450mm), the effects of temperature (623-773K), space velocity (2500～10000h-1) and benzene/N2O molar ratio (2-12) at the entrance of the reactor on the reaction process and the distribution of products during the oxidation of benzene with N2O to phenol over Fe-ZSM-5 zeolite were investigated by the single-factor test method, respectively. The results revealed that, with the increase of temperature, the conversion of N2O increased and the selectivity of phenol decreased, the productivity and yield of phenol first increased and then decreased; With the increase of space velocity, the conversion of N2O decreased and the selectivity of phenol increased. A appropriate space velocity was beneficial to increase the productivity of phenol. With the increase of benzene/N2O molar ratio, the conversion of N2O and the selectivity of phenol both increased. But overhigh benzene/N2O molar ratio would result in a reduction in the productivity of phenol. Finally, based on the results of the single-factor experiments above, the global influence of temperature, space velocity and benzene/N2O molar ratio on the reaction process was investigated by means of the orthogonal test of Li6(45). The optimal reaction conditions were determined as follows:temperature of 673K, space velocity of 6000 h-1 and benzene/N2O molar ratio of 5:1.The coking behavior of the Fe-ZSM-5 zeolite catalyst during the oxidation of benzene with N2O to phenol was studied in an isothermal fixed bed reactor using thermogravimetric analyzer. The effect of reaction time, temperature, molar ratio of reactants, and space velocity on coking was investigated. Based on the informations obtained from experiments and the gas chromatography-mass spectrometry analysis of the coke on the catalyst, a mechanism for coke formation was suggested. The results showed that the formed coke mainly came from the further oxidation or polymerization of phenol as product, revealing that sequential deactivation occurred in the reaction process. Based on the suggested mechanism and the experimental data, a kinetic model for coke formation that describes coke deposition over the Fe-ZSM-5 zeolite catalyst was developed. A statistical test and a residual distribution analysis showed that the established kinetic model was in good agreement with the experimental data and reliable.Based on the experimental results of the isothermal fixed bed reactor and the various space velocities, as well as the mechanism of coke formation, the thermodynamic properties and the reaction network of the reaction system were studied systematically. The main reaction and main side reactions were determined and the kinetic reaction network for the oxidation of benzene with N2O to phenol was proposed, which can reasonably explain the experimental phenomena. The results of the thermodynamic analysis showed that when the temperature was above 500 K, the adiabatic temperature rise of the main reaction, which phenol was produced in, tended to zero. However, there was a remarkable temperature rise in the reactor actually. Accordingly, the reaction heat in the oxidation process mainly came from side reactions. In the temperature range of 500 K to 900 K, the reaction equilibrium constants were extremely high so that the effect of counterreactions on reaction conversion was not considered.Based on the thermmdynamic analysis and the kinetic reaction network proposed, the power function type and the mechanism type kinetic models, which described the kinetic characteristic of the reaction system, were established. The intrinsic kinetic experiments of the oxidation of benzene with N2O to phenol over Fe-ZSM-5 zeolite were carried out in an isothermal integral reactor (sizeΦ6×2 mm, L300 mm) on condition that internal and external diffusions were excluded. The kinetic parameters were estimated by means of Simplex optimal method. Statistical test showed that the two kinetic models established were in good agreement with the experimental data and reliable. By comparison between the two models, the mechanmism type kinetic model showed better agreement with the experimental data than the power function type. Therefore, the mechanism type kinetic model, which was used for describing the catalyst kinetics, was more reliable.It can be seen from the above results that Fe-ZSM-5 zeolite catalyst possessed favorable initial activity and selectivity in the oxidation of benzene with N2O to phenol, whereas deactivated rapidly. Accordingly, the rule of the catalyst performance changing with time should be considered when the reaction process was investigated. The experimental apparatus and the analysis method of dynamic online mass spectrograph were established. The reaction mechanism and the dynamic characteristics of the oxidation of benzene with N2O to phenol over Fe-ZSM-5 zeolite were studied by the temperature programmed dynamic experiments. The sequence of the elementary reactions, which showed well agreement with the experimental data, was proposed. The first step corresponded to a week chemical adsorption of N2O on the catalyst surface followed by the dissociation of N2O on the condition that the temperature was above 634 K. The surface atomic oxygen and nitrogen adsorbed upon two different active sites. The dissociation of N2O was the most crucial step, dominating the overall reaction process. Moreover, the surface atomic oxygen had a remarkable higher activity than the surface atomic nitrogen. Benzene was oxidated by the chemisorbed atomic oxygen to phenol. Furthermore, the formation of the strongly adsorbed NO on the Fe-ZSM-5 by temperature programmed desorption of N2O during the reaction process was discovered. A one-dimensional heterogeneous transient kinetic model for the integral fixed bed reactor based on the elementary reaction steps above was developed. The corresponding parameters of this model were evaluated and statistically tested using the transient experimental data. The results showed that the constructed transient kinetic model can reasonably describe the transient reaction behavior for the oxidation of benzene with N2O to phenol. The results also provided important theoretical references for not only understanding the relative rates and the rate controlling step of elementary reactions thoroughly, but also taking further measures to optimize reaction efficiency.On the basis of the mechanism of coke formation on the catalyst, the steady/unsteady state reaction kinetics, and the solution of the catalyst deactivation by fuildized bed reactor due to realizing easily the periodic operation of reaction and regeneration, the rule of the effect of periodic operation conditions and the catalytic performance of Fe-ZSM-5 zeolite during the oxidation of benzene with N2O to phenol were investigated in a fixed fluidized bed equipment in practice. The results showed that the periodic operation can improve evidently the reaction performance of the catalyst, and that after the deactivation, the catalyst was regenerated again and again, and the activity of the deactivation catalyst can be basically recovered to the level of fresh catalyst. When reaction conditions were reaction temperature of 703 K, benzene/N2O molar ratio of 9:1 and operational gas velocity of 0.050 m·s-1, and regeneration conditions were regeneration temperature of 703 K, N2O concentration of 12.5% and regeneration gas velocity of 0.059 m·s-1, and periodic operation conditions were cycling period of 6 minutes and cycling split of 0.4, the yield of phenol increased by 10.46% than that operated under steady state. The results provided directly basic data for the development of circulating fluidized bed reactor.Finally, based on reaction network, transient kinetics and the experiments in the fixed fluidized bed, a one-dimensional heterogeneous transient mathematical model for the fixed fluidized bed reactor was proposed on some acceptable assumptions. The utilization efficiency of the catalyst and the total heat transfer coefficient were corrected using steady state experimental data in the fixed bed. A Crank-Nicolson finite difference algorithm and an improved second order Rosenbrock method were used to solve the mathematical model established. The simulation program was programmed in MATLAB software. The effects of operation period and cycling split on the reaction performance were analyzed by simulation study. The simulation results were in general agreement with the experimental phenomena. Shortening operation period and augmenting cycling split both increased the yield of phenol remarkably. It suggested that circulating fluidized bed reactor was more promising than fixed bed reactor in economy and technology...