【作者】 刘芝平；

【导师】 李瑞丰；

【作者基本信息】 太原理工大学 ， 化学工程与技术， 2013， 博士

【摘要】 沸石分子筛以其发达规整的孔道结构,高的比表面积,较强的酸性和较好的水热稳定性在吸附、分离和催化领域有着广泛的应用。然而由于微孔沸石的孔径与涉及到的许多碳氢化合物分子的动力学直径非常接近,受微孔孔径的制约,分子在晶内的传质阻力较大,一定程度上限制了其在工业中的应用。为此,研究者在保留沸石固有的微孔结构的基础上,又构建了中孔和／或大孔,形成了多级孔沸石。这类沸石兼有微中和／或大孔材料两者的优点,可以有效缩短客体分子的扩散距离,增加客体分子的扩散途径,提高沸石表面活性位的有效利用率,因而受到人们的广泛重视。目前有关多级孔沸石的研究主要集中在制备,表征以及催化性能的评价方面。然而,对多级孔沸石的具体应用进行分析后不难发现,吸附质或扩散分子在沸石内的吸附和扩散是影响过程性能的主要因素。为此,本文选取不同多级孔结构的5A和ZSM-5沸石为研究对象,构型和动力学直径不同的碳氢化合物(正庚烷,正辛烷,正癸烷,甲苯,枯烯)为探针分子,利用智能重量分析仪IGA(Intelligent Gravimetric Analyser)和自行组建的零长柱装置ZLC(Zero Length Column)对这些碳氢化合物在多级孔沸石上的吸附与扩散性能进行了研究。研究方法是通过碳氢化合物在沸石上吸附等温线和ZLC脱附曲线的测定,并采用各种吸附模型和相应的数学模型方程对其进行拟合处理,从而获得和计算相关的热力学和动力学参数,如：吸附量,亨利常数,吸附热,有效扩散时间常数,活化能等；从中分析吸附量随外比表面积的变化关系；孔结构变化前后亨利常数和吸附热的变化规律；吸附热随负载量的关系；中孔孔隙度对有效扩散时间常数和活化能的影响以及不同的孔结构对扩散机理的影响。研究内容和得到的主要结论如下：1.直链烷烃(正庚烷,正辛烷和正癸烷)在多级孔5A沸石中的吸附和扩散烷烃分子在微孔5A沸石内的吸附符合Langmuir模型,而在多级孔5A沸石上的吸附可用Langmuir-Fraudlich和Toth二种吸附模型得到好的拟合；在同一温度下,三种直链烷烃在多级孔5A沸石内的最大吸附量均大于各自在微孔5A内的最大吸附量,但吸附质与吸附剂相互作用力随中孔的引入而减小,并由于微中孔吸附的共存,使得表面吸附多相性增加；基于吸附数据计算获得的烷烃分子在沸石表面的吸附亨利常数和初始吸附热数据,结果表明,沸石晶内中孔的存在,使其亨利常数和初始吸附热均减小；由Clausius-Clapeyron方程计算得到的等量吸附热随表面吸附量变化的趋势表明,三种直链烷烃在多级孔5A沸石内吸附热均随吸附量的增加而减小,其中以癸烷的变化最为明显。另外,由于沸石中中孔的存在,使的分子扩散路径缩短,空间位阻减小,减小了反应物分子的传质阻力；随中孔孔隙度的增加,活化能减小,有效扩散时间常数增大。2.庚烷和甲苯在具有晶间中孔的纳米ZSM-5沸石中的吸附和扩散纳米ZSM-5沸石中晶间中孔的形成,外表面积增加,提高了吸附质的吸附能力,同时也削弱了吸附质与吸附剂表面的相互作用,造成了亨利常数和初始吸附热的下降。由于纳米沸石ZSM-5中沸石粒径的减小,极大的缩短了客体分子的扩散路径,分子的扩散速率得到大幅度的提高,活化能明显减小,即中孔对扩散起到了促进作用。与庚烷相比,甲苯在扩散中有较大的位阻,故有效扩散时间常数小于庚烷,而活化能大于庚烷。3.庚烷和甲苯在具有晶内中孔的ZSM-5沸石中的吸附和扩散中孔ZSM-5沸石的吸附能力和扩散系数较传统ZSM-5沸石有极大的提高,同时,活化能和吸附热均随着中孔孔隙度的增加而明显降低。结果表明,沸石中中孔的引入,为反应物和产物分子提供了一个短的扩散路径,高的扩散速率,从而使中孔ZSM-5在MTG催化反应中表现出高的燃油产率,增强了催化剂的抗失活能力。庚烷和甲苯在H-和Na-ZSM-5沸石中的吸附和扩散结果的比较表明,酸强度的增加导致吸附质与吸附剂表面的相互作用力增强,对应的亨利常数和吸附热数值,H型大于Na型；高的酸强度也导致大的扩散阻力,造成活化能增加,有效扩散时间常数减小。4.碳氢化合物在具有晶间和晶内中孔的ZSM-5沸石微球中的吸附和扩散由于中微孔孔道的相互交错连接,削弱了吸附质与吸附剂表面的相互作用,造成了亨利常数和吸附热的下降,碳氢化合物分子在不同中孔孔隙度ZSM-5样品中的吸附热随着中孔孔隙度的增加而减小,而且,吸附热随着负载量的增加而减小。对于相同的ZSM-5样品,由于吸附质分子自身物化性质的影响,吸附热大小顺序为：枯烯>甲苯>正庚烷。由于中孔ZSM-5沸石微球中沸石粒径的减小,以及晶体内中孔的出现,改变了客体分子的扩散路径,分子的扩散速率得到大幅度的提高,扩散过程需要的活化能减小。庚烷在ZSM-5的中微孔中同时进行扩散,而甲苯以中孔扩散为主,不同的扩散途径导致甲苯的有效扩散时间常数大于庚烷,而活化能比庚烷要小。更多还原

【Abstract】 Zeolite molecular sieves are widely used in adsorption, separation and catalyzation because of their developed ordered porous structures, high specific surface area, strong acidity, and better hydrothermal stability. However, their use was hampered by their small microporous channels, which are subject to diffusional limitations on reaction rates, because of the similarity between the size of the involved guest molecules and the micropore diameter. Therefore, the researchers create secondary mesopores on the basis of retaining the zeolite’s microporous structure and named hierarchical porous zeolite. This kind of zeolites with a micro/mesostructure are of particular interest because they combine the advantages of both microporous crystalline zeolites and those of mesoporous materials, which largely shorten the diffusion length of the guest molecules, increase the diffusion path of the guest molecules, and improve the effective utilization of the active sites of the zeolite surface, thus highly valued by people.Currently, studies of the hierarchical porous zeolite mainly focus on the preparation, characterization, and evaluation of its catalytic performance. However, based on the analysis of the specific application of the hierarchical porous zeolite, it is not difficult to find that adsorption and diffusion of the adsorbate (diffusion molecules) in the crystal are the main factors affecting the process performance. To this end, the paper conducts a systematic and detailed analysis of the adsorption and diffusion properties of hydrocarbons in the hierarchical porous zeolite. These studies are crucial for adsorption^ separation and catalytic dynamics and will play an important guiding role in developing or improving hierarchical porous zeolite molecular sieve based catalysts and the use conditions of hierarchical porous zeolite molecular sieves.Considering the differences in pore structure, modification mechanism and application scope, this paper respectively selects meso-zeolite 5A, and meso-zeolite ZSM-5 of different pore structure as the research object. First, the research uses adsorption isotherms and desorption curves obtained by IGA (the Intelligent Gravimetric Analyzer) and ZLC (Zero Length Column) to compare the shapes of adsorption isotherms and desorption curves of different samples; after that, based on the adsorption and desorption data, according to the corresponding equations and models, it calculates the thermodynamics and kinetics parameters which mainly include:adsorbed amount, Henry constant, effective diffusion time constant, activation energy, etc.; Analyzes how adsorbance changes with the specific surface area and the outside specific surface area, how Henry constant and the adsorption heat change with changes in the pore structure, the relationship between the adsorption heat and the loading, the impact of mesopore porosity on the effective diffusion time constant and the activation energy and the impact of different pore structures on the diffusion mechanism. The main conclusions of the paper are as follows:1 Adsorption and Diffusion of n-Paraffins in Hierarchical Porous 5A ZeolitesThe adsorption and intra-crystalline diffusion of n-heptane, n-octane and n-decane in 5A zeolites with an intracrystalline mesopores were studied, and were compared with that of microporous 5 A zeolite. It is shown that the adsorption data from three n-paraffins on mesoporous 5A zeolites might be well correlated by using the Langmuir-Fraudlich and Toth models. Duo to the coexistence of micropores and mesopores on mesoporous 5A zeolites, both the higher maximum adsorption capacity and more energetic heterogeneity on the surfaces of them were present, whereas, the affinity to the adsorbent measured_by the Henry’s constants and the heats of adsorption at zero coverage decreased with the introduction of mesopores in zeolite crystals. Furthermore, the isosteric heat of adsorption by caculated by Clausius-Clapeyron equation decreased with the increasing surface coverage of n-paraffins. The change of decane is most obviously. The mesopores were found to enhance molecule diffusion. Moreover, the effective diffusion time constant (Deff/R2) increased with mesoporosity in the zeolites, whereas the activation energy decreased with increasing mesopore volume. The effective diffusivity values of n-alkanes in mesoporous zeolite 5A were generally higher than that the microporous zeolite 5A sample. This clearly implied the important role of the mesopore in zeolites crystals in facilitating the transport of reaction molecules due to shorter average diffusion path length and less steric hindrance.2 Adsorption and Diffusion of n-heptane and toluene in nano-ZSM-5 ZeolitesMicro and mesoporous ZSM-5 were investigated with regard to their equilibrium and kinetic characteristics using n-heptane and toluene as sorbates by the standard gravimetric technique and ZLC method. The adsorption capacities of nano-ZSM-5 samples were significantly higher than those of microporous ZSM-5, In contrast to that, the isosteric heats of adsorption and Henry constants in the nano-ZSM-5 were found to be much smaller. The Diffusion rates of heptane and toluene depends on the particle size of the nano-ZSM-5 crystals. The diffusion time constants for mesoporous ZSM-5 were found to be higher than for microporous ZSM-5. However, the diffusion activation energies for both C7 hydrocarbons in nano-ZSM-5 zeolite were found to be lower than for microporous ZSM-5, which suggests that the decrease of the diffusion path length within the microporous crystals by reducing the particle size can promote the process of diffusion. Diffusion activation energies of n-heptane were higher in comparison to toluene, which has a larger kinetic diameter.3 Adsorption and Diffusion of n-heptane and toluene in Hierarchical Porous ZSM-5 ZeolitesAdsorption and diffusion properties of n-hetane and toluene in meso-structured ZSM-5 zeolites were studied by high precision intelligent gravimetric analysis (IGA) and ZLC technology. As expected, great increase in adsorption capacity and diffusion efficient of n-heptane and toluene in the mesostructured ZSM-5 zeolites was observed compared with conventional ZSM-5. At the same time, the adsorption activation energy of n-heptane and toluene in the mesostructured ZSM-5 zeolites was significantly decreased. The adsorption heats with low n-heptane and toluene loading showed a clear decline with increase of mesoporosity in the zeolite samples. These results clearly indicate that introduction of mesopores into the zeolites offered a short diffusion path and high diffusion rate for reactants and products, which resulted in a high yield of fuel oil and an enhanced resistance against the catalyst deactivation in the reaction of methanol to gasoline. The effect of the acid properties on the adsorption and diffusion of C7 hydrocarbons n-heptane and toluene in mesostructured ZSM-5 zeolites was further studied. As expected, the interaction with the acid sites causes a increase in the heats of adsorption and Henry constants and a decrease in the diffusivities of both sorbates. The values of the activation energy of diffusion for both C7 hydrocarbons in H-form ZSM-5 zeolites are higher than those in Na-forms.4 Adsorption and Diffusion of hydrocarbon in ZSM-5 Zeolite microspheresThe ZSM-5 microspheres were consisted by nanosized zeolite particles and there were intracrystalline mesopores inside the nanosized particles. Thus hierarchical pore system exists in these materials. The first one is the innate micropore of zeolite ZSM-5, the second one is the intercrystalline mesopores formed owing to the aggregation of nano-crystal, and the third one is the intracrystalline mesopores inside the nano-crystal. The zeolite ZSM-5 microsphere showed enhanced catalytic effect In benzylation of benzene by benzyl alcohol (BA) and MTG. Here, mesoporous ZSM-5 zeolites were further investigated with regard to their adsorption and diffusion characteristics. Such adsorption heats and Henry constants were found to be dependent on the framework mesoporosity and showed a clear decline with increase of mesoporosity in the zeolite samples. Adsorption heats for n-heptane, toluene and cumene on the same samples decreased with increase of mesoporosity. The isosteric heats of adsorption estimated by the Clausius-Clapeyron equation are significantly higher for toluene and cumene in comparison with n-heptane, and they are strongly correlated to the sorbate loadings. The sharp initial decrease of isosteric heats of adsorption with respect to coverage, which shows different behavior for the different sorbate/adsorbent systems. The diffusion activation energies for hydrocarbons in ZSM-5 microsphere were found to be lower than for microporous ZSM-5. which suggests that the decrease of the diffusion path length within the microporous crystals by reducing the particle size and introduction of mesopores into the zeolites can promote the process of diffusion. The transport of n-heptane in zeolite ZSM-5 microsphere was found to be mainly controlled by micro and mesopore diffusion in the pore structure, while that of toluene was dominated by the mesopore diffusion process. Diffusion activation energies of n-heptane are higher in comparison to toluene, which has a larger kinetic diameter.更多还原