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The Study Of N2O Catalytic Decomposition And One-Step Oxidation Benzene To Phenol Over Zeolites

Posted on:2008-07-15Degree:DoctorType:Dissertation
Country:ChinaCandidate:Q H GuoFull Text:PDF
GTID:1101360245974854Subject:Chemical Engineering
Abstract/Summary:PDF Full Text Request
N2O has been considered as an environmental pollutant as its destroying the ozone layer and inducing the greenhouse effect.In recent years,the concentration of N2O in the atmosphere is rising,and the adipic acid production is responsible for 10%of the world-wide emissions.Thus, the removal of the N2O from adipic acid waste gases has received much attention.In this paper,the catalytic decomposition of N2O and one-step oxidation benzene to phenol with N2O have been systemically studied over iron modilied ZSM-5,βand MCM-22 zeolites.The metal modified zeolites were prepared through liquid ion-exchanged using for catalytic decomposition of N2O.The effects of zeolite type,Si/Al ratio,modified metal ions species,technological conditions and impurity gas on the catalytic performance of modified zeolites were experimentally investigated.The results showed that:(1) Fe-βzeolite of BEA type loading with iron was more efficient catalyst for direct N2O decomposition than Fe-ZSM-5 zeolite of MFI type and Fe-MCM-22 zeolite of MWW type The zeolite activity enhanced with increasing the iron loading;(2)At the same Fe3+loading,the catalytic activity was decreased with increasing the Si/Al ratio;(3)The catalytic activities of the Fe-ZSM-5 samples prepared with Fe2(C2O4)3·5H2O as Fe precursor were higher than those of the Fe-modified ZSM-5 samples obtained with Fe(NO3)3·9H2O as Fe precursor;(4)The results of single-factor experiments showed that micro-content oxygen did not influence the catalytic performance of zeolite catalysts.However, micro-content stream would result in the quick deactivation of zeolite catalysts.Micro-content NO would obviously increase the catalytic activity.The results of ESR,FT-IR,UV-vis and NH3-TPD indicated that the iron ions located at the Bronsted acid sites were the active component in the direct decomposition of N2O.However,the distribution of iron ions loading on the different type zeolites was different as the distribution of Bronsted acid sites.The iron ions loading on theβzeolite located at the straight channels may be more accessible to N2O molecules and had the higher reducibility.Thus,Fe-βzeolite showed higher catalytic activity than Fe-ZSM-5 zeolite.For Fe-ZSM-5 zeolite,in the case of the Fe-ZSM-5 zeolites prepared with Fe(NO3)3·9H2O as Fe precursor,the iron ions could locate at theβandγsites firstly,but they would also locate at theαsites when the iron loading increased.For the Fe-ZSM-5 samples obtained with Fe2(C2O4)3·5H2O as Fe precursor,iron ions mainly occupied theαsites of the zeolitic straight channels.The iron ions located at theα,βandγ,sites of the ZSM-5 channels had the ability to directly decompose N2O molecules.However,the catalytic activity of iron ions located onαsites of the ZSM-5 zeolite was higher than the iron ions located onβsites andγsites.From the results of in situ DRIFT,it was found that NO scavenged adsorbed oxygen(deposited by N2O during the decomposition)leading to the formation of NO2.This process accelerated the consumption of O atom.Thus,NO significantly enhances the catalytic activity of zeolite catalysts.A series of zeolites for one-step oxidation benzene to phenol with N2O were prepared through liquid ion-exchanged and hydrothermal synthesis.The effects of zeolite type,Si/Al ratio,modified metal species and pretreatment methods on the catalytic performance of catalysts for one-step oxidation benzene to phenol were systemically investigated.And the deactivation and regeneration experiments of the catalyst were also investigated.The results showed that the main factors,influencing the catalytic performance of zeolite,included zeolite type,Si/Al ratio and Fe3+loading, and so on.Fe-ZSM-5 zeolite was more efficient catalyst for one-step oxidation benzene to phenol than Fe-βzeolite and Fe-MCM-22 zeolite. For Fe-ZSM-5 zeolite,the N2O conversion decreased and phenol selectivity increased with increasing the Si/Al at the condition of the same Fe3+loading.However,the N2O conversion increased and phenol selectivity decreased with increasing the Fe3+loading at the condition of the same Si/Al.High-temperature calcination and hydrothermal treatment resulted in the increase of activity-sites for phenol formation because of Fe-ion surface migration.In reaction,the catalysts' activity gradually decreased due to catalyst deactivation by coke.The deactivation catalysts would be regenerated with coke combustion.In the case of regeneration in N2O,catalytic activity could be fully restored.The regeneration conditions were temperature 723 K,Space velocity 2752 h-1and N2O:He (mol ratio)=1:9.In N2O one-step oxidation benzene to phenol reaction,the results of in situ DRIFTS showed that benzene and N2O could be adsorbed on the different activity sites of zeolite surface.Thus,this reaction might be according to the L-H reaction mechanism.However,the phenol molecules,adsorbed on the activity sites,would ulteriorly react to form the coke.These could result in deactivation.The deactivation effect of coke was caused by poisoning of activity sites.Two kinds of coking species(high-temperature carbon and low-temperature carbon)were found through the characterization by TG-DTA,O2-TPO and C/H analysis.The low-temperature carbon formed at the first stage of reaction and occupied the activity sites.This was the reason that resulted in deactivation.
Keywords/Search Tags:N2O, benzene, phenol, Fe-ZSM-5, zeolite, catalytic decomposition, deactivation
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