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Supported M/SBA-15 catalyst in the dipentene dehydrogenation of cracking and N_2O catalytic decomposition of application

Author: DuJunMing
Tutor: ZhaoDongYuan；XuHuaLong
School: Fudan University
Course: Physical and chemical
Keywords: SBA-15 dipentene dehydrogenation cracking acidity N2O decomposition Rh Ru dispersion precursor
CLC: O643.36
Type: PhD thesis
Year: 2008
Downloads: 524
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Abstract


SBA-15 is a mesoporous silica material with large surface area,pore size,pore volume and high hydrothermal stability,thus its advantage of application in catalysis as catalysts support are fully anticipated.On the other hand,green chemistry becomes a widely accepted idea and a lot of efforts have been made in environmental benign catalysts and catalytic process.In this paper,active phases are introduced into SBA-15 by various method and test for the activity in two different green catalytic processes,the catalytic dehydrogenation of dipentene to cymene and the direct decomposition of N2O,a green house effect gas.The aim of the paper is to grasp further understanding of the relation between catalytic activities and the catalysts mesoporous structures in different catalytic processes,so that constructive patterns can be drawn on practical catalysts preparation.Section 1 Catalytic dehydrogenation of dipentene over M/SBA-15Industrial dipentene is a cheap mixture of different terpenes as the byproduct of camphor preparation and pulp-paper industry,it can serve as a natural and renewable feedstock for fine chemical industries.Owing to the similarity of the molecular structure,p-cymene is the most promising and valuable product obtained by the dehydrogenation of industrial dipentene.Fe,Cr,Ni,Zn,Al were introduced into SBA-15 by impregnation method. Charaterization results suggest that the dispersion of the metal species over SBA-15 is fine when Si/M ratio is above 10 and the two dimension hexagonal structure of SBA-15 is remained,though the specific area,pore volume and size will be smaller than before.Activity test showed that dipentene is converted to p-cymene on these catalysts at different rate and with different by-products.P-cymene was cracked to toluene over Al/SBA-15 catalysts,while on Zn/SBA-15,the major by-product is menthene disproportionated from dipentene,and the stability of Zn/SBA-15 is better than Al/SBA-15.Other catalysts were similar to these two but hold less selectivity to cymene.The yield of p-cymene is maximized on Zn/SBA-15 at 723 K,GHSV = 800 h-1,LHSV=0.2 h-1,whereas the conversion of dipeneene is 98.2%and the selectivity ofp-cymene is 88.3%. The performances of the M/SBA-15 in catalytic dehydrogenation of dipentene are highly related to the acidity of the catalysts.In order to have a better understading, alkali doped KAl/SBA-15 catalysts and microporous zeolite HZSM-5 are compared with original catalysts.Activity tests show that the conversion of dipentene dropped drastically with doping of K and cracking process was inhibited.High conversioin rate was observed in HZSM-5 in the starting period of the reaction and toluene was the main product,however,the stability was not as good as the mesoporous catalysts. The acidities of the catalysts were studied by pyridine adsorption-desorption FT-IR. The result suggests that only Lewis acidity exists on Zn/SBA-15,both Br（?）nsted and Lewis acidity were found on Al/SBA-15,and HZSM-5 owns the strongest Br（?）nsted acidity above all.Together with the activity results,it is concluded that,the acidity plays the vital role in the catalytic process.Dehydrogenation of dipentene to cymene happens on both kinds of acidic sites and cymene will be further cracked on Br（?）nsted sites.Also the Br（?）nsted acidic site is easy to coke and deactivate with TOS.The strengthe of the acidity affect little to the reaction.The effect of the support on the catalytic process of dipentene dehydrogenation was also checked.With different supporting materials of silica（Silicalite-l,MCM-41, SBA-15,KIT-6）,the catalysts show different acitivties.It is proved that the mesoporous materials are better than the microporous Silicalite-1 in terms of conversion rate and stability.This is due to the improved mass transfer with increasing pore size.The structure of the mesopores may affect the contact time between the reactants and the active sites so that the activity and stability is changed accordingly.Section 2 Catalytic decomposition of N2O1.Research on Rh/SBA-15 prepared from different precursorsDifferent Rh precursors with carbonyl,acetylacetone or triphenylphosphine ligands were synthesized and used to introduce Rh into SBA-15.Characterization suggests that the dispersion status were closely related to the nature of precursors. Small hydrophilic molecules tend to have better dispersion on SBA-15 by impregnation,while by grafting,it will diffuse into the micro-meso window on the siliceous wall.Big hydrophobic precursors are easier to aggregate into big particles inside the mesoporous tunnel or on the outer surface. Catalytic results showed that the higher activity was reached on the better dispersed Rh/SBA-15.Rh/SBA-15-CDCR prepared from[（CO）2RhCl]2 shows the best activity of N2O decomposition and the best Rh dispersion.The activity is also affect by some other factors such as residue of Cl.2.Direct decomposition of N2O over RuM/SBA-15Zn,Zr and Ce were introduced into SBA-15 as additives to improve the Ru activity on N2O decomposition.It is found the activity can be improved at certain concentration of additives and RuZr/SBA-15 gave out a nice resuIt by decreasing the N2O total conversion temperature from 573 K to 473 K.The performance of RuCe/SBA-15 is also remarkable at proper Ce loading.However,CeO2 will block the runnel at high loading and affect the decomposition ability.The improvement of the catalytic performance was supposed to connect with the surface characteristics of ZrO2 and CeO2.

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CLC: > Mathematical sciences and chemical > Chemistry > Physical Chemistry ( theoretical chemistry ),chemical physics > Chemical kinetics,catalysis > Catalytic > Catalyst
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