ZrO₂基固体酸碱催化水解低浓度氟利昂的研究

【作者】 刘天成；

【导师】 宁平；

【作者基本信息】 昆明理工大学 ， 环境工程， 2010， 博士

【摘要】 氟利昂是一类温室气体,对人类健康、地球生态环境造成了严重破坏。氟利昂无害化和资源化处理已成为当今环保技术研究的热点之一,将低浓度氟利昂在排放到大气之前彻底分解技术开发有重要现实意义。本文利用ZrO2负载金属氧化物制备固体酸、固体碱催化剂催化水解低浓度氟利昂CFCs(以CFC-12为例),重点研究了固体酸、固体碱催化剂的制备条件,低浓度氟利昂催化水解工艺条件,水解机理,水解反应的热力学和动力学。催化剂制备优化实验：浸渍法制备固体酸MoO3/ZrO2的单因素最佳制备条件为：1次过饱和浸渍,浸渍液(NH4)6Mo7O24·4H2O浓度为0.5 mol.L-1,液固比1.5,浸渍温度80℃,浸渍时间4 h,ZrO2的质量分数为20-40%,焙烧温度为450℃。湿混法制备固体碱Na2O/ZrO2为：Zr:Na的摩尔比为1:0.35,焙烧温度为600℃；溶胶-凝胶法制备的固体碱CaO/ZrO2为：Zr:Ca的摩尔比为1:0.35,焙烧温度为650℃。低浓度CFC-12在固体酸碱催化剂水解工艺条件考察：固体酸MoO3/ZrO2为：水解温度250℃,反应气体组成(mol%)：1.0 CFC-12,40.0 H2O(g),10.0 O2,其余为N2,具有较强的稳定性和选择性,可采用加热与浸渍联合再生。固体碱Na2O/ZrO2和CaO/ZrO2为：水解温度260℃,反应气体组成(mol%):1.0 CFC-12,50.0 H2O(g),8.0 O2,其余为N2,空速小于1000 h-1,或者气体流量小于10.0 cm3min-1, CFC-12的转化率达到90.0%以上,空气中的水分易导致催化剂失活,可采用加热再生。低浓度CFC-12在固体酸碱催化剂水解机理分析：在实验研究并借助BET、TEM、TPD、XRD、FT-IR等方法表征催化剂的基础上提出了低浓度CFC-12水解机理。(1)固体酸MoO3/ZrO2水解低浓度CFC-12。催化剂孔径分布在1.5-6.0nm,主要集中和起主要催化作用的孔径为3.90 nm；晶粒大小随着焙烧温度的升高而增大,晶相随着焙烧温度的升高而增加；弱酸位强度相当,而中等强度酸位和强酸位随焙烧温度的增加而增加,NH。脱附峰从低温向高温发生位移；CFC-12水解是弱酸位、中强酸位和强酸位协同催化的共同结果,CFC-12水解产生的Cl-进入到催化剂表面或内部,导致催化剂在开始使用的10 h内活性降低；催化剂中MoO3。与ZrO2骨架以一种较强的相互作用力结合在一起形成强酸位。提出了固体酸酸中心形成模型和CFC-12水解机理模型。(2)固体碱Na2O/ZrO2和CaO/ZrO2水解低浓度CFC-12。催化剂总孔体积和平均孔径随着焙烧温度升高而增加,而比表面积随着温度升高是先增加后降低；ZrO2晶粒与Na2O或CaO纳米颗粒相互嵌合,堆积成孔径为15-35 nm的一种类似海绵状的介孔复合物,具有较高的比表面积和活性；Na2O/Zr0。在75-425℃温度范围内存在脱附峰,而CaO/ZrO2在425-635℃温度范围内存在脱附峰,且随着焙烧温度的升高,脱附峰的位置向高温方向漂移；Na2O/ZrO2(600℃)主要物相为四方和单斜氧化锆晶相,而CaO/ZrO2(650℃)为四方氧化锆晶相和少量的单斜氧化锆晶相,起主要催化作用的物相是单斜相ZrO2(m-ZrO2)。提出了固体碱碱中心形成模型和CFC-12水解反应机理模型。热力学分析：利用弗伦德里希等温方程和克劳修斯-克拉佩龙方程对CFC-12吸附等温线和吸附等容线进行拟合,计算了固体酸MoO3/ZrO2对CFC-12的反应吸附热-⊿HAM在56.30～73.22 kJ.mol-1内,为放热反应,属于化学吸附。计算了固体碱Na2O/ZrO2和CaO/ZrO2对CFC-12的反应吸附热和在水解温度260℃时标准摩尔反应吉布斯函数和平衡常数。动力学分析：利用班厄姆吸附速率公式和阿累尼乌斯方程,由吸附容量曲线计算了固体酸MoO3/ZrO2和固体碱Na2O/ZrO2催化水解CFC-12的反应活化能、反应速率方程和反应级数。结果表明固体酸MoO3/ZrO2反应级数n随水解温度的升高而降低,反应速率常数K随水解温度的升高而升高,反应活化能为123.12 kJ.mol-1;固体碱Na2O/ZrO2在催化反应初期阶段,反应级数n随水解温度的升高而降低,反应速率常数K随水解温度的升高而升高,反应活化能为51.18 kJ.mol-1。在催化反应后期阶段,n随水解温度的升高而降低,而K随水解温度升高而升高,随反应时间延长而降低,反应活化能为312.74 kJ.mol-1。更多还原

【Abstract】 Chlorofluorocarbons (CFCs) have been brought about many destroys on humanity health, ecological and globosity environment because CFCs are green house gases. The harmless and resources disposition of CFCs has been a hot topic in recent years, and it is significantly important to develop low concentration CFCs decomposition technique before being liberated into the atmosphere. Low concentration CFCs, dichlorodifluorocarbons (CFC-12), as an example, were catalytic hydrolyzed over solid acid and alkali catalysts, which were prepared by metal oxide supported ZrO2. Factors of catalyst preparation, techniques of catalytic hydrolysis, mechanism, thermodynamics and dynamics were emphasis studied in this paper.From the optimization experimentation, the catalysts prepared by optimum conditions of solid acid MoO3/ZrO2 are, one times superaturation dipping, the concentration of (NH4)6Mo7O24·4H2O 0.5 mol.L-1, the ratio of liquid to solid 1.5, the dipping temperature 80℃and 4 hours, the quality percent of ZrO2 20-40%, and the calcination temperature 450℃. Solid base Na2O/ZrO2 were, Zr:Na=1:0.35, the calcination temperature 600℃. Solid base CaO/ZrO2 were, Zr:Ca=1:0.35, the calcination temperature 650℃.Catalytic hydrolyzed of low concentration CFC-12 was investigated over solid acid-base catalysts. The optimum hydrolytic conditions of solid acid MoO3/ZrO2 were, the hydrolytic temperature 250℃, the composition of reaction gases (mol%),1.0 CFC-12,40.0 H2O(g),10.0 O2, others N2, high stability and selectivity, and regenerated by heat or dipping. The optimum hydrolytic conditions of solid base Na2O/ZrO2 and CaO/ZrO2 were, the hydrolytic temperature 260℃, the composition of reaction gases 1.0 CFC-12,50.0 H2O(g),8.0 O2, others N2, the space velocity 1000 h-1, and the humidity in air brought on catalyst poisoning and regenerated by heat. Catalytic hydrolysis mechanisms for low concentration CFC-12 were studied. Catalysts were characterized by BET, TEM, TPD, XRD and FT-IR. (1) Solid acid MoO3/ZrO2. Distribution of pore size was narrowly range from 1.5 to 6.0 nm, mainly in and the main active pore size was 3.90 nm. Crystalline size increased with the calcination temperature rise and it was in the main form of crystalline phase. Calcination temperature affected acid species of MoO3/ZrO2, which has corresponded weak acid sites, and middle intensity and strong acid sites increased with the calcination temperature. The weak, middle intensity and strong acid sites jointly catalyzed the CFC-12 hydrolysis. MoO3 and ZrO2 framework in catalysts formed strong acids in the form of strong interaction. (2) Solid base Na2O/ZrO2 and CaO/ZrO2.The total pore volume and average pore size were raised with calcination temperature rising, and the specific area was raised and declined with calcination temperature. Mesoporous complex substance was mutual chimeras by ZrO2 crystalline grain, and Na2O or CaO nanometer grain, which has bore diameter 15-35 nm, high specific area and activation. Solid base Na2O/ZrO2 had desorption peaks in the temperature range from 75 to 425℃and solid alkali CaO/ZrO2 in the temperature range from 425 to 625℃. These peaks drift to high temperature with the calcination temperature rise. Solid alkali Na2O/ZrO2 (calcinated at 600℃) had much t-ZrO2 and m-ZrO2, however CaO/ZrO2 (calcinated at 650℃) had much t-ZrO2 and little m-ZrO2. Absorption peaks of solid alkalis decreased with the calcination time prolonged and temperature rise, and the m-ZrO2 was the main active ingredient. The formation model of acid-base center and the reaction mechanism model of low concentration CFC-12 catalytic hydrolysis over acid-base catalysts were brought forward.Adsorption isotherm of CFC-12 was imitated by Freundich isothermal equation, and adsorption isochore was imitated by Clausius-Clapeyron equation. The adsorption heat-(?)HAM of CFC-12 over solid acid was calculated as 56.30-73.22 kJ.mol-1, belong to chemical adsorption. The adsorption heat of CFC-12 over solid base Na2O/ZrO2 and CaO/ZrO2 were calculated. The equilibrium constant and the activation energy were calculated under reaction temperature 260℃.The activation energy, reaction rate equation and reaction order were calculated from the adsorption capacity curve of low concentration CFC-12 by Bangham adsorption rate formula and Arrhenius equation. The reaction order declined and the reaction rate constant K has gone up with reaction temperature rising, the activation energy 123.12 kJ.mol-1 over solid acid MoO3/ZrO2. The activation energy, reaction rate equation and reaction order were calculated of CFC-12 over solid base Na2O/ZrO2. The reaction order declined and the reaction rate constant K has gone up with reaction temperature rising, the activation energy 51.18 kJ.mol-1 in the primary stage. The reaction order declined and the reaction rate constant K has gone up with reaction temperature and declined with the reaction time, the activation energy reached to 312.74 kJ.mol-1 in later stage.更多还原