负载型M/SBA-15催化剂在双戊烯脱氢裂解与N₂O催化分解中的应用研究

【作者】 杜俊明；

【导师】 赵东元； 徐华龙；

【作者基本信息】 复旦大学 ， 物理化学， 2008， 博士

【摘要】 SBA-15是一种新颖的介孔分子筛材料,具有较大的比表面积、孔体积和均一的孔径（5～30 nm）等特点,同时具有良好的水热稳定性,因此它在催化领域具有更加广泛的应用前景,受到人们的普遍关注。另一方面绿色化学概念日益受到世人关注,研究与开发环境友好的催化剂及催化反应工艺,对化学工业发展将产生重大影响。本文的主要工作是以SBA-15作为催化剂载体,采用浸渍、嫁接等方法将金属组分负载于介孔分子筛表面,并将其应用于两类绿色化学催化反应中,一类是工业双戊烯的催化脱氢裂解反应,另一类是温室气体N₂O的催化分解反应,研究金属氧化物负载的介孔分子筛催化剂的构效关系以及在不同反应体系中的催化反应规律。一.M/SBA-15催化剂的合成及其在双戊烯脱氢裂解中的应用工业双戊烯主要是由单环单萜烯的异构体混合物组成,通用分子式为C₁₀H₁₆,是多种化工生产中的副产品。由于双戊烯是可再生资源,理论上可由它代替石油生产目前以石油为基础的大部分烃类产品。以SBA-15作为载体,不同金属盐溶液作为前驱体,通过浸渍法制得负载型M/SBA-15催化剂。表征结果表明:当硅与金属摩尔比Si/M在10以上时,所得催化剂负载组分分散性较好,且都可保持SBA-15二维六角的介孔结构,而孔径、孔容和比表面等物理性质随负载量上升略微有所下降。将M/SBA-15催化剂应用于双戊烯的催化转化过程中发现:以Zn/SBA-15为催化剂可高选择性制得对伞花烃,副产物主要为双戊烯歧化产物对孟烯,反应活性稳定。以Si/Zn=28的Zn/SBA-15为催化剂,在723 K,GHSV=800 h^-1,LHSV=0.2 h^-1的反应条件下,双戊烯转化率可达98.2%,对伞花烃选择性为88.3%。在Al/SBA-15催化剂上,双戊烯也可脱氢制得对伞花烃,同时伴有裂解产物甲苯生成,在反应过程中甲苯和对伞花烃选择性比值在反应过程中逐渐下降;在M/SBA-15（M=Fe、Cr、Ni）催化剂上,反应产物分布介于两者之间,对伞花烃的选择性略低。双戊烯的催化脱氢裂解与催化剂表面酸性质有密切关系,为了研究M/SBA-15催化剂表面酸性与催化活性的关系,采用碱金属元素K对Al/SBA-15催化剂的表面酸性进行修饰,同时选用具有较强表面酸性的HZSM-5（Si/Al=25）分子筛为催化剂以作比较。研究结果表明,在KAl/SBA-15催化剂上,随K含量增加,双戊烯转化率明显下降,同时甲苯的生成很大程度地受到抑制;以HZSM-5为催化剂,反应初期双戊烯转化率可达95%以上,产物中以甲苯为主,但随反应的进行,转化率与甲苯选择性明显下降,产物中对伞花烃选择性显著提高。采用吡啶吸附脱附红外对以上各催化剂表面酸性能进行了表征。结果表明:HZSM-5酸性位以Br（?）nsted酸为主,Al/SBA-15以Lewis酸为主,Zn/SBA-15表面只有Lewis酸中心。综合催化活性与催化剂表面酸性表征结果,推测双戊烯在Lewis酸位进行双键异构后脱氢生成对伞花烃,而Br（?）nsted酸位可使对伞花烃进一步裂解生成甲苯。Br（?）nsted酸位上易发生烯烃聚合产生结焦是造成HZSM-5催化剂活性降低的主要原因,Zn/SBA-15表面只有Lewis酸,稳定性有显著提高,而K的修饰能有效减少Al/SBA-15催化剂表面B酸中心数,促进催化的活性及稳定性的提高。双戊烯的催化脱氢反应活性受酸强度的影响较小,较弱酸性下即能发生,而与酸性位数量呈正比关系。采用Silicalite-1、MCM-41、SBA-15、KIT-6等具有不同孔径、结构的载体成功制备了含5%金属氧化物的负载型催化剂。研究结果表明:催化剂的孔径增加对于促进催化反应活性和稳定性的提高有显著作用,载体孔径的增加有利于提高催化剂传质性能,有效抑制失活现象发生。二、M/SBA-15催化剂的合成及其在N₂O催化分解反应过程研究N₂O是一种温室效应气体,对臭氧层也有潜在的破坏作用。目前大气中N₂O主要来源于硝酸工业尾气和汽车尾气排放,通过催化分解的手段有效去除废气中的N₂O已经成为环境催化中的重点研究课题。研究结果表明:Rh、Ru对N₂O催化分解具有很高的活性。使用SBA-15作为载体可以为活性组分提供较大的载体表面,通过不同合成手段也可显著改善金属元素在催化剂表面的分散性能,同时引入Zn、Zr、Ce等修饰组分可以进一步改变活性组分的分散状态,促进其活性的提高。本部分的工作着重于研究Rh、Ru这两种元素在SBA-15上的分散性能及其与修饰元素相互之间作用关系,评价该类催化剂在N₂O催化分解的活性。（一）不同Rh前驱体制备Rh/SBA-15上的N₂O分解反应采用系列Rh的有机金属络合物（配位体包括羰基、乙酰丙酮、三苯基膦等）作为前驱体,通过浸渍与嫁接的方法将Rh引入SBA-15。结构表征与TEM结果表明:Rh在SBA-15表面的分散与前驱体分子大小及亲疏水性质相关,亲水性小分子在SBA-15表面能较好分散,而疏水大分子较容易形成大颗粒聚集体。N₂O催化分解反应结果表明:Rh分散性较好的以二聚二羰基氯化铑作为前驱体的Rh/SBA-15-CDCR上N₂O分解率较高,N₂O的50%转化率温度T₅₀比其它催化剂低50 K以上。（二） Ru/SBA-15与RuM/SBA-15上的N₂O分解反应通过浸渍手段将Zn、Zr、Ce等修饰元素引入SBA-15作为载体负载Ru用于N₂O分解。研究结果表明:经过适当修饰的RuM/SBA-15催化剂对于N₂O分解活性要高于Ru/SBA-15。其中Zr对Ru/SBA-15催化剂的修饰作用最为显著,且在Zr修饰含量5%-20%时基本相同,N₂O完全分解温度同比下降100 K。RuCe/SBA-15在Ce载量10%的情况下其活性与RuZr/SBA-15活性相当,推测Zr和Ce能提供氧空穴,帮助Ru上吸附氧迁移,从而增加反应活性。但CeO₂较易聚集,负载量较大时容易造成孔道堵塞,引起活性下降。更多还原

【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.更多还原