【作者】 程鹏；

【导师】 于吉红；

【作者基本信息】 吉林大学 ， 无机化学， 2016， 博士

【摘要】 沸石分子筛材料具有规则的孔道结构和较大的比表面积,在各个领域被广泛地应用,尤其在石油炼制,石油化工,精细化工中作为催化、吸附分离和离子交换材料发挥着极其重要的作用。随着沸石分子筛在化学工业和人们的日常生活中所需比重的逐渐增加,如何更快、更高效地合成分子筛也成为了人们关注的焦点。沸石分子筛大多数是在强碱性的介质中通过硅酸根离子及硅铝酸根离子的解聚和缩聚重排而形成的。在前期的工作中我们发现,沸石分子筛的水热合成体系中存在羟基自由基,通过紫外辐射沸石分子筛反应溶液可以产生大量的羟基自由基并促进硅酸根离子及硅铝酸根离子的解聚和缩聚,从而显著加速了沸石分子筛的晶化过程,并缩短了沸石合成所需的时间。理论计算模拟证明了羟基自由基相对于氢氧根阴离子在硅酸根的解聚和缩聚中具有更高的催化活性。本文针对硅酸盐及硅铝酸盐分子筛体系,采用几种不同的方法向沸石分子筛合成体系中引入羟基自由基,即紫外光辐射、向反应溶液中加入Fenton试剂、加入过硫酸根离子和加入自由基化的晶种等,研究其对沸石分子筛合成体系的晶化过程的影响,并通过电子顺磁共振技术确定羟基自由基的存在。通过这些方法引入羟基自由基可以显著加快沸石分子筛的晶化过程,这一研究为沸石分子筛的高效合成提供了新的途径。本论文所得到的主要结果概述如下:1.在以四丙基氢氧化铵为有机模板剂的Silicalite-1合成体系中,通过紫外辐射引入了羟基自由基,在70 oC条件下加速了Silicalite-1的晶化过程。电子顺磁共振测试中确认了羟基自由基,硅基自由基,乙醇自由基的特征信号。通过不同的表征手段(如X-射线衍射,氮气吸附,扫描电子显微镜,透射电子显微镜),研究表明紫外辐射下80小时的产物与正常合成100小时的产物在结晶性,微孔体积和产率等方面相近。进一步将能够产生大量羟基自由基的Fenton试剂加入到Silicalite-1沸石分子筛的合成体系中,在电子顺磁共振测试中发现了极强的羟基自由基的特征信号。发现在相同的晶化时间内,其产物在产率和结晶性方面相对于紫外辐射的产物更加优异。我们又研究了羟基自由基掩蔽剂的存在对沸石分子筛晶化过程的影响。实验结果表明,反应体系中羟基自由基的掩蔽剂—乙醇的存在会显著降低沸石分子筛的晶化速率。2.过二硫酸盐可以通过热活化产生硫酸根自由基,而硫酸根自由基可以很容易在任何p H值的条件下与水反应生成羟基自由基和硫酸根。通过定量地加入不同浓度的过硫酸钠加速了Silicalite-1的晶化过程。电子顺磁共振测试确认了乙醇自由基的特征信号,且过硫酸钠的浓度越大,乙醇自由基的信号越强,说明产生的羟基自由基越多。在电子顺磁共振的测试中,发现硫酸钠的盐溶液也可以产生羟基自由基,这也为前期的研究人员用酸根加速沸石分子筛的合成提供了理论依据。通过各种表征手段(如X-射线衍射,动态光散射,扫描电子显微镜,氮气吸附)研究表明,加入过硫酸钠的反应溶液16小时晶化的产物与正常合成24小时晶化的产物在结晶性、微孔体积和产率等方面相近。在适宜的浓度范围内,过硫酸钠的浓度越大,加速的效果越好。用一定浓度的过硫酸钠代替部分有机模板剂,与正常合成Silicalite-1的效果相同。减少模板剂的用量,可降低环境污染,使沸石分子筛的合成更为绿色,节省成本。同时,用过硫酸钠也加速了方钠石的合成,过硫酸钠的存在减少了Na A沸石和方钠石共存的时间,对混合共存的相有一定的选择性。3.通过煅烧和研磨的方法对Silicalite-1晶种进行预处理产生自由基化的晶种,使Silicalite-1表面邻近的硅羟基(Si-OH)脱水或使Silicalite-1中的硅氧硅(Si-O-Si)键均裂从而产生非桥连氧原子空穴中心(NBOHC,Si-O?)。通过电子顺磁共振技术确认了自由基化晶种非桥连氧原子空穴中心的特征峰及自由基化晶种水溶液羟基自由基的特征峰。在Silicalite-1的初始反应溶液中加入不同类型的晶种加速了Silicalite-1的晶化过程,通过不同的表征手段(如X-射线衍射,动态光散射,扫描电子显微镜)研究表明,加入自由基化的晶种的反应溶液在相同晶化时间的产物在结晶性、单分散性和产率等方面优于加入正常晶种和未加入晶种的反应溶液的产物。更多还原

【Abstract】 Zeolites are microporous crystalline aluminosilicates that are widely used as ion-exchangers in the detergent industry, catalysts in the petrochemical and chemical industry, and adsorbents in massive air separation. So far, zeolites have been the most important solid catalysts in the chemical industry. Since their introduction as a new class of industrial materials, the annual market for synthetic zeolites has grown immensely, to several million tons worldwide per year.In the hydrothermal crystallization of zeolites from basic media, it has been well accepted that the hydroxide ions(OH–) catalyze the depolymerization of the aluminosilicate gel by breaking the Si,Al–O–Si,Al bonds and catalyze the polymerization of the aluminosilicate anions around the hydrated cation species by remaking the Si,Al–O–Si,Al bonds. Recently, it has been discovered that the hydroxyl free radicals(?OH) are involved in the zeolite crystallization under hydrothermal conditions. The hydroxyl free radicals generated by ultraviolet irradiation can significantly accelerate the crystallization processes of zeolites—such as Na–A, Na–X and Na Z–21. The discovery that the zeolite synthesis mechanism can be promoted through free radicals sheds a new light on zeolite crystallization and will create new routes for the synthesis of zeolite materials that are largely demanded in the chemical industry.In this thesis, several methods were adopted to introduce hydroxyl free radicals to the synthetic systems of zeolites of Silicalite-1 and Na A, by means of, for example, UV irradiation, addition of Fenton’s reagent, addition of persulfate, and utilization of radicalized seeds. We investigated the influence of radicals on the crystallization of these zeolites, characterized the radicals by electron paramagnetic resonance(EPR) technique. The radicals introduced by above methods accelerated the crystallization of zeolites remarkably. Our work will provide novel approaches for efficient synthesis of zeolites by utilizing hydroxyl free radicals. The main results are summarized as follows:1. Hydroxyl radicals generated by ultraviolet irradiation were introduced to Silicalite-1 synthetic system, in which tetrapropylammonium hydroxide was used as the organic template. The synthesis was conducted at 70 ?C. The hydroxyl, silicon-based, and ethanol radicals were detected by electron paramagnetic resonance(EPR). The hydroxyl free radicals involved crystallization process was shortened for 20 hours than the normal crystallization process.Fenton’s reagent, which can generate large amounts of hydroxyl radicals, was also added to the synthetic system of Silicalite-1. The Fenton’s reagent had a much better acceleration effect than the UV irradiation.Inhibiting ?OH may slow down the zeolite crystallization. Because ethanol is an effective ?OH scavenger, we crystallized Silicalite-1 under UV conditions by evaporating the ethanol in the mixture. The crystallization process was accelerated when the ethanol was removed.2. Persulfate can produce sulfate radicals under heating, which would react with H2 O easily to generate hydroxyl radicals and sulfate at any p Hs. The crystallization of Silicalite-1 was remarkably accelerated by adding sodium persulfate. The EPR signal of ethanol radicals was observed during the synthesis and the intensity of the signal was increased with the increase of the concentration of sodium persulfate, indicating the increase of the concentration of hydroxyl free radicals. The crystallinity of the products at the reaction time of 16 hours is the same as that under the normal condition at the reaction time of 24 hours. We also found that sodium sulfate solution can also produce hydroxyl radicals. Increasing the concentration of sodium persulfate can enhance the acceleration effect on the crystallization process. The introduction of the sodium persulfate can decrease the usage of the organic template, thus reduce the cost of synthesizing zeolites and the pollution of environment. In addition, the crystallization process of zeolite Na A and sodalite can also be accelerated by the sodium persulfate. The hydroxyl free radicals generated from the sodium persulfate may also affect the phase selectivity of the synthetic systems.3. A new seeding-synthesis-related strategy has been developed to accelerate the crystallization of zeolites. By means of milling and heating, the Silicalite-1 zeolite crystals can be radicalized forming surface nonbridging oxygen hole center(NBOHC, ≡Si-O?). The corresponding g values of 2.0043 and 2.0048 unambiguously confirmed the existence of the ≡Si-O? radicals in the milled and heated crystals by EPR. And the signal of hydroxyl free radicals was detected by EPR in aqueous solution of radicalized seeds. Radicalized crystals were used as seeds that showed obvious accelerating effect in the crystallization of nanosized Silicalite-1 than the non-radicalized seeds. It can be expected that the application of this strategy will further improve the efficiency of the zeolite synthesis in industry.更多还原