Effect of Calcined Temperature on Coke Deposition for Autothermal Reforming of Methane with Ni-Cu Catalyst

Ni-Cu/ZrO2-CeO2-Al2O3 catalysts were prepared by co-precipitation method at pH=9 and using Na2CO3 as the precipitant. The Ni loading （mass fraction） of the catalysts was 10%. The catalysts were characterized by X-ray diffraction, temperature-programmed oxidation （TPO）, scanning electron microscope （SEM）, and X-ray photoelectron spectroscopy （XPS）. The effects of calcined temperature of support on coke deposition were studied. TPO, SEM and XPS results indicated there was no peak of higher temperature oxygen consumption on Ni-Cu/ZrO2-CeO2-Al2O3 catalyst （support was calcined at 800 ℃）, which could lead to the deactivation of the catalyst. The carbon species were carbonate and inactive carbon （filamentous carbon species） on the surface of catalyst reacting for 40 h which perhaps led to the deactivation of the catalyst.

Comparative study on stability and coke deposition over supported Rh and FePO_4 catalysts for oxy-bromination of methane

Rhodium- and iron phosphate-based catalysts are by far the most promising catalysts for oxy-bromination of methane(OBM) reaction. However, most literature reported either Rh- or FePO4-based catalysts, and the results were rarely studied in a uniform environmental condition. In this report, comparative study was conducted on silica- and silicon carbide-supported rhodium and iron phosphate catalysts with the main focuses on stability performance and coke deposition. The catalytic results demonstrated that the stability of both Rh- and FePO4-based catalysts was greatly influenced by the supports used, and silicon carbide-supported catalysts showed much better anti-coking ability as compared with silica-supported ones. Temperature-programmed oxidation over the used catalysts further indicated that the coke formation mechanisms were completely different between silica-supported rhodium and iron phosphate catalysts. While cokes might be caused by condensation of CH2Br2over supported iron phosphate, methane decomposition might be the reason for coke formation over silica-supported rhodium catalyst. These findings might pave the way for designing highly efficient and stable catalysts of the OBM reaction.

Hierarchical Porous ZSM-5 Zeolite Synthesized by in situ Zeolitization and Its Coke Deposition Resistance in Aromatization Reaction

Hierarchical ZSM-5 zeolites with micro-, meso- and macroporosity were prepared from diatomite zeolitization through a vapor-phase transport process on solid surfaces. The aromatization performance of the catalysts was in- vestigated on a fixed bed reactor by using FCC gasoline as feedstock. The crystal phase, morphology, pore struc- tures, acidity and coke depositions of the hierarchical ZSM-5 zeolites were characterized by means of X-ray diffrac- tion （XRD）, scanning electron microscope （SEM）, N2 adsorption/desorption, Fourier transform infrared （FT-IR） and thermogravimetry-mass spectrogram （TG-MS）, respectively. The results show that the prepared hierarchical ZSM-5 zeolite possesses excellent porosity and high crystallinity, displaying an improved aromatization performance and carbon deposition resistance due to its meso- and macroporous structures.

Effects of specific surface area of metallic nickel particles on carbon deposition kinetics

Carbon deposition on nickel powders in methane involves three stages in different reaction temperature ranges. Temperature programing oxidation test and Raman spectrum results indicated the formation of complex and ordered carbon structures at high deposition temperatures. The values of I（D）/I（G） of the deposited carbon reached 1.86, 1.30, and 1.22 in the first, second, and third stages, respectively. The structure of carbon in the second stage was similar to that in the third stage. Carbon deposited in the first stage rarely contained homogeneous pyrolytic deposit layers. A kinetic model was developed to analyze the carbon deposition behavior in the first stage. The rate-determining step of the first stage is supposed to be interfacial reaction. Based on the investigation of carbon deposition kinetics on nickel powders from different resources, carbon deposition rate is suggested to have a linear relation with the square of specific surface area of nickel particles.

Coking of Pt/γ-Al_(2)O_(3) catalyst in landfill gas deoxygen and its effects on catalytic performance

Catalytic oxidation of CH_(4) has been proved to be an attractive option for landfill gas(LFG) upgrading.However, coking of catalysts in catalytic LFG deoxygen has been clearly observed in industrial applications. In this regard, it is necessary to investigate whether coke deposition originates from CH_(4) or volatile organic compounds present in LFG, and the influence of coke deposition on catalytic performance. Herein,we evaluate the LFG deoxygen on Pt/γ-Al_(2)O_(3) catalyst in simulated LFG(CH_(4), CO_(2), O_(2), N_(2)) and its co-feed with representative volatile organic compounds, ethylbenzene, toluene, benzene and cyclohexane. The results show that the coking of the catalyst is originated from volatile organic compounds rather than CH_(4). The Pt/γ-Al_(2)O_(3) catalyst does not deactivate during LFG deoxygen process, even significant amount of coke deposited, up to 18.15%(mass). Characterization analyses reveal that although coke deposition overall covers the catalyst surface, resulting in mesopores blockage and a reduced number of accessible Pt sites, however, the coke formed, H-rich carbonaceous components, behaves as counterpart for O_(2) elimination. Besides, the coke deposited is mainly filamentous. Thus, coke formation has little negative effect on the overall catalytic performance of Pt/γ-Al_(2)O_(3) catalyst ultimately. The results obtained in this work are helpful for the rational design of robust Pt based catalysts for LFG deoxygen without undue attention to their coking properties, and also favor the innovation of more attractive purification scheme configurations.

Influences of regeneration atmospheres on structural transformation and renderability of fluidized catalytic cracking catalyst

The regeneration of fluidized catalytic cracking(FCC)catalysts is an essential process in petroleum processing.The current study focused the regeneration reaction characteristics of spent fluidized catalytic cracking catalyst(SFCC)at different atmospheres with influences on pore evolution and activity,for a potential way to reduce emission,produce moderate chemical product(CO),and maintain catalyst activity.The results show that regeneration in air indicates a satisfaction on removing coke on the catalyst surface while giving a poor effect on eliminating the coke inside micropores.This is attributed that the combustion in air led to a higher temperature and further transformed kaolinite phase to silicaaluminum spinel crystals,which tended to collapse and block small pores or expand large pores,with similar results observed in pure O_(2)atmosphere.Nevertheless,catalysts regenerated in O_(2)/CO_(2)diminished the combustion damage to the pore structure,of which the micro porosity after regeneration increased by 32.4% and the total acid volume rose to 27.1%.The regeneration in pure CO_(2)displayed low conversion rate due to the endothermic reaction and low reactivity.The coexistence of gasification and partial oxidation can promote regeneration and maintain the original structure and good reactivity.Finally,a mechanism of the regeneration reaction at different atmospheres was revealed.

MONTE-CARLOSIMULATION OF CATALYST DEACTIVATION

Catalyst deactivation due to coking is microscopically analyzed,then a model is presented,based upon the analogy between coke deposition and solid aggregation.The Monte Carlo simulation results show that the model can fit the experimental data in all cases.With this model,the mechanism of formation of coke with different shapes is derived and the relation between the catalytic activity and coke shape is theoretically demonstrated.In addition,the model described in this paper can also be used to simulate the catalyst preparation so as to make more useful and efficient catalysts.The model in this paper is very simple,with only two parameters that indicate the nature of catalyst deactivation.The extension of the model to more complicated systems is also discussed.

Methane dehydroaromatization with periodic CH_4-H_2 switch:A promising process for aromatics and hydrogen

Long-term stability test of Mo/HZSM-5-N catalysts（HZSM-5-N stands for nano-sized HZSM-5） in methane dehydroaromatization（MDA）reaction has been performed with periodic CH4-H2 switch at 1033-1073 K for more than 1000 h.During this test,methane conversion ranges from 13% to 16%,and mean yield to aromatics（i.e.benzene and naphthalene） exceeds 10%.N2-physisorption,XRD,NMR and TPO measurements were performed for the used Mo/HZSM-5 catalysts and coke deposition,and the results revealed that the periodic hydrogenation can effectively suppress coke deposition by removing the inert aromatic-type coke,thus ensuring Mo/HZSM-5 partly maintained its activity even in the presence of large amount of coke deposition.The effect of zeolite particle size on the catalytic activity was also explored,and the results showed that the nano-sized zeolite with low diffusion resistance performed better.It is recognized that the size effect was enhanced by reaction time,and it became more remarkable in a long-term MDA reaction even at a low space velocity.

Effect of Ca promoter on LPG synthesis from syngas over hybrid catalyst

Direct synthesis of liquefied petroleum gas （LPG） from syngas was carried out over hybrid catalyst consisting of methanol synthesis catalyst and modified Y zeolite with Pd and Ca by different methods. The decrease of CO conversion was mostly attributable to the sintering of Cu in methanol synthesis catalyst. On the other hand, coke deposition on Y zeolite was the reason for the decrease of LPG selectivity. The introduction of Ca decreased the strong acid sites of Y zeolite, suppressed coke formation, and thus improved the stability of hybrid catalyst.

Effects of CO_2 content on the activity and stability of nickel catalyst supported on mesoporous nanocrystalline zirconia

The effects of carbon dioxide content on the catalytic performance and coke formation of nickel catalyst supported on mesoporous nanocrystalline zirconia with high surface area and pure tetragonal crystalline phase were investigated in methane reforming with carbon dioxide. The samples were characterized by XRD, BET, TPR, TPO, TPH, TEM, and SEM techniques. The catalyst prepared showed high surface area and a mesoporous structure with a narrow pore size distribution. The obtained results revealed that the increase in CO2 content increased the methane conversion and stability of the catalyst and significantly reduced the coke deposition. The TPH analysis showed that several species of carbon with different reactivities toward hydrogenation were deposited on the spent catalysts employed under different CO2 contents.