胆碱和*BEA分子筛在硅锗铝IWR分子筛制备中的协同导向效应（英文）

Cooperative structure-directing effect of choline cation and *BEA zeolite in the synthesis of aluminogermanosilicate IWR zeolite

分子筛的制备过程一般需要结构导向剂的参与.结构导向剂主要包括碱(土)金属离子、有机胺或季铵盐为代表的有机模板剂和固体晶种等三类.研究合成凝胶中不同结构导向剂之间的作用对于理解分子筛晶化机理意义重大.以往的研究大都集中在模板剂-模板剂、模板剂-碱(土)金属离子和模板剂-同晶之间的相互作用,迄今尚未有模板剂-异晶之间相互作用研究的报道.我们研究组首次发现模板剂和异晶在分子筛制备中存在协同导向效应.在硅锗铝IWR分子筛的合成中,我们考察了不同的季铵碱模板剂和铝源,发现只有使用胆碱作模板剂、*BEA分子筛作铝源才可以成功制得目标分子筛,相同条件下不加入*BEA分子筛或换用MOR、ZSM-5、MCM-22、USY分子筛及异丙醇铝等铝源都无法合成得到IWR分子筛.这说明*BEA分子筛在IWR分子筛的制备中起到了其他铝源所不具备的结构导向作用,因此我们称之为合成IWR分子筛的"异晶".为了研究胆碱和*BEA分子筛之间的相互作用,进而揭示模板剂-异晶协同导向制备分子筛的一般规律与反应机理,我们制备了不同晶化时间的IWR分子筛样品,使用X射线粉末衍射(XRD)、扫描电子显微镜(SEM)、紫外-拉曼光谱(UV-Raman)、固体核磁共振(MAS NMR)和N2物理吸附等手段对这些样品进行了表征.结果显示,加热过程中*BEA分子筛首先发生溶解解离, 72 h后该分子筛相完全消失;12 h开始出现CDO分子筛相,随着晶化时间延长, CDO分子筛先增多后减少,在24 h时达到最大值, 120 h时消失,说明该分子筛是晶化过程的一个中间相;IWR分子筛在12 h开始出现,此后结晶度不断升高,至168 h晶化完全.在上述表征的基础上,我们提出了模板剂-异晶协同导向制备IWR分子筛的晶化机理.IWR分子筛的晶化分为以下5个步骤:(1)*BEA分子筛溶解解离得到硅铝4元环、5元环和6元环等结构单元;(2)胆碱诱导形成硅锗5元环结构单元;(3)硅锗5元环组装生成CDO分子筛;(4)CDO分子筛溶解解离形成硅锗5元环;(5)*BEA分子筛导向的结构单元与胆碱导向的结构单元重组生成IWR分子筛.其中,步骤3和4是一对可逆反应,且可与步骤5同时进行;随着反应物原料的不断消耗,步骤5占据主导,从而导致CDO分子筛完全消失和IWR分子筛的结晶完成.由于胆碱只导向生成5元环结构单元,而构成IWR分子筛骨架结构的4元环和6元环只能由*BEA分子筛提供,因而模板剂和异晶在反应中都不可或缺,二者起到协同导向的作用.分析发现,*BEA分子筛与IWR分子筛具有共同的结构单元(4、5、6元环)和一定的结构相似性是其可以成为异晶、发挥协同导向作用的关键.上述协同导向法的关键在于模板剂和异晶分别导向生成部分结构单元,再经重组得到目标分子筛.这种方法有望用于合成通过传统水热法难以得到的分子筛,并且为新结构分子筛的开发提供新思路.使用协同导向法制备更多种类分子筛的研究正在进行中.

In this contribution,we report the cooperative structure-directing effect of choline hydroxide and aluminosilicate*BEA zeolite in the synthesis of aluminogermanosilicate IWR zeolites for the first time.*BEA zeolites,at variance with any other aluminosilicate zeolites,can serve as heterogeneous seeds for the growth of IWR zeolites and play a cooperative structure-directing role.The crystallization process was investigated using multiple techniques to characterize a series of solid products obtained with various crystallization times.The experiments clearly showed the dissolution of the*BEA zeolite and of an intermediate CDO-type structure.A plausible mechanism for the novel cooperative synthesis has been proposed.The crystallization of the IWR zeolite involves several steps,among which the crucial one is believed to be the reassembly of the building units produced from the decomposition of*BEA zeolite seeds,induced by choline molecules.Having similar structure and common building units(four-,five-,and six-membered rings)with the IWR zeolite,the*BEA zeolite is capable of promoting the reassembly of the building units and can thus play a cooperative structure-directing role.By highlighting the cooperative structure-directing effect of organic molecules and heterogeneous seeds,this study opens up new perspectives for the synthesis of target zeolites that are difficult to prepare by traditional methods.This new synthetic route is also expected to shed light on the discovery of novel zeolites.