【作者】 刘莹；

【导师】 辛峰；

【作者基本信息】 天津大学 ， 化学工程， 2008， 博士

【摘要】 整体式催化剂与传统颗粒状催化剂相比具有床层压降低、传质效率高、放大效应小等特点,不仅能提高催化效率和反应选择性,还有助于实现低能耗、零排放和安全操作的工艺过程。TS-1钛硅分子筛是具有MFI拓扑结构的一种催化新材料,在与H2O2组成的体系中表现出优异的温和与清洁催化氧化性能,引起了国内外研究者的广泛关注。但由于TS-1的原粉的粒径小,且生产成本高,当使用浆态床反应器时,催化剂的分离与回收难度大,不可避免地造成催化剂损失。本文运用水热合成技术在酸处理过的堇青石蜂窝陶瓷载体上制备了TS-1分子筛整体式催化剂,旨在解决分子筛的分离与回收问题。研究工作包括三部分:一是对整体式载体的表面处理;二是在载体表面生长TS-1分子筛;三是对整体式催化剂进行活性评价。载体堇青石表面预处理的结果表明,盐酸处理过的堇青石比硝酸处理的堇青石比表面积大2倍左右。比较TS-1催化剂的负载情况,盐酸处理过的堇青石载体更有利于分子筛晶体的生长。TS-1的水热合成中发现,当采用混合模板剂合成TS-1催化剂时,获得的催化剂晶体颗粒为微米级颗粒。当混合模板剂中四丁基氢氧化铵(TBAOH)和四乙基氢氧化铵(TEAOH)的比例发生变化时,会影响TS-1晶体的纯度。若产生TS-1和TS-2混和晶体,则XRD谱图的2θ=45.5°左右就会出现三个峰。通过对堇青石上的分子筛涂层进行FT-IR分析,发现1050 cm-1处的肩峰是由分子筛骨架外的Si-O键振动所引起的,而与分子筛的晶粒大小和晶体形状关系不大。TG-DTA的结果显示,催化剂焙烧去除模板剂的温度控制在550~800℃之间都是可行的。实践中,对TS-1粉末催化剂的焙烧均采用550℃的温度,而整体式催化剂的焙烧温度则选择为600℃。TS-1分子筛焙烧前后的XRD图表明焙烧对低角度衍射峰的影响较大,使部分衍射峰发生了重叠和裂分,但是对高角度的衍射峰几乎没有影响。根据相对结晶度的计算结果,焙烧后钛硅分子筛的结晶度变大,焙烧的方式能使分子筛晶体结构趋于完美,使其具有良好的稳定性。以四丙基氢氧化铵(TPAOH)为单一模板剂合成TS-1催化剂时,随着钛含量增加,分子筛的颗粒从圆柱形逐渐变为椭圆形;而适当增加模板剂的用量或水硅比,都会有利于分子筛晶体的成型。晶化温度是TS-1分子筛膜合成过程中的关键因素,当温度较低时堇青石上会出现分子筛未晶化或晶化不完全的情况,170℃的温度比较适合分子筛的晶化。为了改进整体式催化剂上密集的分子筛膜由于重力的影响呈现厚度分布不均匀的状况,采用水热合成和分步老化的方法,通过分子自组装行为,在堇青石载体上制备了树枝状的分子筛膜,膜内晶体颗粒均匀地保持在200nm左右,实现了分子筛膜在载体上的均匀连续分布,焙烧过程没有使该分子筛膜形态产生变化。催化剂评价在自行设计的固定床整体式催化剂反应器中进行。首先对TS-1催化剂作了酸活化,活化后的分子筛对环己酮的转化率有所降低,但是对环己酮肟的选择性增加。比较不同制备方法下分子筛的催化活性后发现,用混合模板剂制备的催化剂活性低于单一模板剂制备的催化剂,原因在于前者的分子筛粒径较大。将树枝状膜和密集膜的整体式催化剂与粉末催化剂进行了活性比较,虽然密集膜整体催化剂的活性低于粉末催化剂,但无需催化剂分离;而树枝状膜因利于反应物在整体催化剂上的扩散,进而提高了催化剂的利用率,其活性达到与粉末催化剂相当的水平。更多还原

【Abstract】 Monolithic catalyst possesses distinguish advantages over traditional particle catalysts in small pressure-drop, low mass-transfer resistances, unremarkable scale-up effect and so on. Not only improving the efficiency and selectivity of catalytic reaction, it is also contributed to the achievement of low energy, zero emission and safe operation process.As an incoming catalyst with MFI topological structure, Titanium silicalite-1 molecular sieve （TS-1）, has been drawn more attention of domestic and abroad researchers because of its excellent catalytic-oxidation performance in H2O2/TS-1 system with mild reaction conditions. Since using powder TS-1 in slurry reactor, it is difficult to separate such a small crystal from liquid, the unavoidable loss of expensive catalyst increases the operation cost. In this paper the TS-1 molecular sieve was synthesized on honeycomb-shaped cordierites pretreated by acid liquid with hydrothermal method, the aim was at solving the problem mentioned.My research was focus on pretreatment of cordierite support, growth of TS-1 on cordierite and evaluation of catalytic activity in cyclohexanone ammoximation.First of all, the cordierite supports were pretreated by acid solution. The results showed that the specific surface area of support pretreated with hydrochloride acid was larger 2 times than that pretreated by nitric acid, and the former was superior in planting the TS-1 molecular sieve on cordierite. Then the mixed templates were used to synthesize TS-1 monolithic catalysts, crystal size of molecular sieves was larger in micron scale. The mole ratio of tetrabutylammonium hydroxide （TBAOH） to tetraethylammonium hydroxide （TEAOH） in mixed templates affected the purity of TS-1 crystal obviously. When mixed TS-1 and TS-2 crystals were formed, a triplet-peak would appear at 2θ=45.5°in XRD patterns. Based on the FT-IR analysis of molecular sieve film on the cordierite supports, a shoulder peak at 1050cm-1 should be the vibration of out framework Si-O, and not strong interrelated with the size and shape of TS-1 crystal.After investigating the calcination process of catalyst with TG-DTA, the temperature range, 550 800℃is suitable to removal of the templates. Actual calcination temperatures of TS-1 powder and monolithic catalyst were chosen as 550℃and 600℃respectively. Comparing the XRD patterns of TS-1 catalysts before and after calcination, the low angle diffraction peaks were evidently changed to partial overlap and split, however, almost no impact was observed in higher angle. According to the calculation TS-1 relative crystallinity became larger after calcination, which meant that the calcination could lead to perfect structure and good stability of crystal.When tetrapropylammonium hydroxide （TPAOH） was used as template, TS-1 crystal shape gradually changed from cylinder to ellipse with the increase of Ti content, and increase in the amount of template or H₂O/SiO₂ ratio would benefit the formation of TS-1 crystal. Crystallization temperature was a key factor for synthesis of TS-1 monolithic catalysts, proper one is 170℃, below it the precursor had not been completely crystallized on cordierite supports. The dense film of TS-1 catalysts on support was uneven because of the effects of gravity. When the TS-1 catalyst was crystallized step by step on a honeycomb cordierite surface by using hydrothermal method, a branch-like TS-1 film was formed evenly, which was self-assembly of crystalline rather than oriented grown of crystalline.A monolithic reactor self designed was used to test the catalytic performance of TS-1 monolithic catalysts in the cyclohexanone ammoximation reaction. After TS-1 catalyst was activated by acid, the cyclohexanone conversion decreased, but oxime selectivity increased. Comparing the activity of TS-1 catalysts synthesized by different templates, the TS-1 catalyst by single template had higher activity than that by mixed templates, which maybe caused by different particle size. Compared to TS-1 powder, the reuse of monolithic catalysts could be convenient although its activity was somewhat low. When the branch-like TS-1 film on supports, dense film on supports and TS-1 powder were used to catalyze cyclohexanone ammoximation reaction, the branch-like TS-1 film was easy to diffuse reactants, thus improve the utilization of monolithic catalyst, and finally made catalytic activity almost as the same as the TS-1 powder.更多还原