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离子热法合成分子筛的研究进展

来源：抵帆知识网

2012

文章编号: 0253-9837(2012)01-0039-12

Chinese Journal of Catalysis

国际版DOI: 10.1016/S1872-2067(11)60343-4

Vol. 33 No. 1

综述: 39~50

离子热法合成分子筛的研究进展

王亚松1,3, 徐云鹏1, 田志坚1,2,*, 林励吾2

中国科学院大连化学物理研究所洁净能源国家实验室, 辽宁大连 116023

中国科学院大连化学物理研究所催化基础国家重点实验室, 辽宁大连 116023

中国科学院研究生院, 北京 100049

摘要: 离子热法是以离子液体或低共熔混合物为介质的一种新型的分子筛合成方法, 它提供了一种离子态的独特合成环境, 为合成新型分子筛及研究分子筛的生成机理提供了机会. 本文综述了离子热法在分子筛合成方面取得的一些进展, 包括合成方法的创新、合成机理的研究、新材料的合成以及新型催化剂的制备等, 并展望了其发展前景. 关键词: 离子热合成; 分子筛; 微波加热; 催化剂; 结构导向剂; 离子液体中图分类号: TQ031.2; O3 文献标识码: A 收稿日期: 2011-09-22. 接受日期: 2011-11-07.

*通讯联系人. 电话/传真: (0411)84379151; 电子信箱: tianz@dicp.ac.cn 基金来源: 国家自然科学基金 (20903092, 21001102).

本文的英文电子版(国际版)由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).

Research Progress in Ionothermal Synthesis of Molecular Sieves

WANG Yasong1,3, XU Yunpeng1, TIAN Zhijian1,2,*, LIN Liwu2

Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,

Dalian 116023, Liaoning, China

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China

Graduate University of Chinese Academy of Sciences, Beijing 100049, China Abstract: Ionothermal synthesis is a new method for the synthesis of molecular sieves and it takes advantage of an ionic liquid or a deep eutectic mixture as a medium. Ionothermal synthesis offers an ionic environment and an opportunity to obtain novel molecular sieves and determine the mechanism of molecular sieve synthesis. A brief review about the progress in the ionothermal synthesis of molecular sieves is summarized and includes innovations in the synthesis method, determination of the synthesis mechanism and the preparation of novel mo-lecular sieves and new catalysts. Future development in the ionothermal synthesis of molecular sieves is also discussed. Key words: ionothermal synthesis; molecular sieves; microwave heating; catalyst; structure directing agent; ionic liquids Received 22 September 2011. Accepted 7 November 2011.

*Corresponding author. Tel/Fax: +86-411-84379151; E-mail: tianz@dicp.ac.cn

This work was supported by the National Natural Science Foundation of China (20903092, 21001102).

English edition available online at Elsevier ScienceDirect (http://www.sciencedirect.com/science/journal/18722067).

分子筛材料具有规则的孔道结构和离子交换性能, 广泛应用于催化、吸附分离等领域, 其合成主要采用水热法和溶剂热法[1~3]. 为了合成新结构、新性能的分子筛, 探索新的合成方法一直是分子筛材料科学的重要研究方向. 2004 年, Cooper 等[4]报道了一种以离子液体或低共熔混合物为介质的分子筛合

成新方法——离子热合成法, 为新型分子筛材料合成及其合成机理的研究开辟了新途径.

离子液体作为一类新型的环境友好的“绿色溶剂”, 具有许多独特的性质: (1) 离子液体液程宽、挥发性低、不易燃, 因此离子热合成可以在常压下进行, 从而降低了分子筛合成的压力风险, 为合成机理

40 催化学报 Chin. J. Catal., 2012, 33: 39–50

的研究提供了便利, 有利于进行因水热合成的高压而无法进行的原位表征; (2) 离子液体有机阳离子与分子筛合成常用的有机胺结构导向剂结构相近, 可以兼作溶剂和结构导向剂, 而且其种类繁多, 分子结构可设计, 增加了合成的可控变量, 为创制新材料提供了新平台; (3) 离子液体具有强极性和导电性, 不但提供了一种与分子溶剂不同的离子态新型合成环境, 而且离子热合成更易于与微波等电磁技术结合, 可以赋予合成以新特性.

目前, 离子热法在合成磷酸盐分子筛材料方面取得了较大进展. 除了一些已知结构的磷酸铝[4~13]和杂原子磷酸铝分子筛[14~18], 一系列新结构的磷酸亚磷酸盐[28~30]和有机膦酸盐[31~32]等分子筛盐[19~27]、

或类分子筛空旷骨架材料也相继被合成出来. 离子热法合成磷酸盐分子筛材料的研究也加深了人们对分子筛合成机理的认识[33~36]. 本文主要针对离子热法在分子筛晶化机理和新材料合成等方面的进展进行综述.

离子液体兼具溶剂和结构导向剂的作用. 进一步还研究了在不同链长烷基取代基的咪唑溴离子液体中 CHA 结构的磷酸铝分子筛的合成, 认为离子液体阳离子在 HF 酸存在下分解, 生成的 1,3-二甲基咪唑阳离子导向了 CHA 结构的生成[5]. 1.1 胺的结构导向作用

和溴化 1-甲基Wang 等[7]对比研究了[Emim]Br

-3-丁基咪唑离子液体 ([Bmim]Br) 中磷酸铝分子筛的合成, 发现离子液体阳离子尺寸影响所合成分子筛的结构, 1-甲基-3-乙基咪唑导向生成十元环结构的 AEL (AlPO4-11) 分子筛, 而阳离子尺寸较大的 1-甲基-3-丁基咪唑离子液体同时导向生成 AEL 和十离子二元环的 AFI (AlPO4-5) 分子筛. 向[Bmim]Br 热体系中添加正二丙胺等有机胺可以改变晶化动力学, 合成出纯相的 AFI 分子筛 (见图 1). 固体核磁 (NMR) 结果发现, 离子液体与胺协同发挥结构导向作用, 共存于分子筛孔道之中. 1H NMR 结果表明, 离子液体阳离子与胺分子之间存在氢键作用, 从而改变了离子液体阳离子与无机骨架物种间的非键作用, 影响晶化动力学, 同时二者也可能形成超分子导向大孔或低骨架密度的产物. Pei 等[9]继续增加胺的浓度, 得到了骨架密度更低的三维交叉孔道的 LTA 结构磷酸铝分子筛. Xu 等[34]通过 NMR 技术结合理论计算研究了添加有机胺的 1-甲基-3-丁基咪唑体系中分子筛晶化的演变规律, 也证实了离子液体阳离子与胺分子之间存在相互作用, 胺在离子液体中的浓度变化导致不同结构的磷酸铝分子筛成核晶

1 离子热合成中结构导向作用的研究

在水热或溶剂热法合成分子筛过程中, 经常需要添加胺、季铵盐、有机大分子等结构导向剂[37]. 结构导向作用是分子筛合成研究的主要问题. Cooper 等[4]和 Parnham 等[21]首次在溴化 1-甲基-3-乙基咪唑 ([Emim]Br) 离子液体中合成出了两种新结构的 SIZ-1 和 SIZ-6, 以及两种已知结构的 SIZ-3 (AEL) 和 SIZ-4 (CHA) 等多种微孔磷酸铝分子筛, 并指出

10090Relative crystallinity (%)80706050403020100010203040Time (min)

5060 AlPO4-5 AlPO4-11 (a)Relative crystallinity (%)100806040200(b) AlPO4-5 AlPO4-11010203040Time (min)

5060图 1 [Bmim]Br 离子液体中微波加热合成分子筛的晶化曲线

Fig. 1. Crystallization curves of AlPO4-5 and AlPO4-11 in [Bmim]Br upon microwave heating using synthesis mixtures with the following composi-tion: Al2O3:2.55P2O5:0.6HF:20[Bmim]Br:xn-DPA. (a) x = 0; (b) x = 1.2.

www.chxb.cn 王亚松等: 离子热法合成分子筛的研究进展 41

化.

Wang 等[7]还利用离子液体液程宽的特点, 研究了 280

°C 高温晶化条件下分子筛晶化的演变, 发现随着晶化时间的延长, 晶化产物由 AFI 结构转为 AEL 和 ATV 结构. 结果表明, 有机胺的结构导向作用除了表现在可以提高分子筛的生成速率, 而且与离子液体共同填充于分子筛孔道中, 起稳定晶体结构、抑制亚稳态的分子筛结构向热力学稳定的无机致密相转变的作用.

综上可见, 通过调节离子液体和有机胺的种类和用量以控制无机物种的组装, 从而得到不同拓扑结构的产物, 不但证明了离子液体本身以及有机胺等添加物在离子热合成中的结构导向作用, 而且为分子筛合成过程中结构导向作用的理论研究以及新结构分子筛材料的离子热合成奠定了基础. 1.2 离子液体的结构导向作用

Ma 等[22]研究了含氟的[CnMIm]Br (n = 2–6) 系列离子液体体系中 LTA 结构磷酸镓分子筛的合成, 发现离子液体阳离子烷基取代基的链长还能影响磷酸镓分子筛晶体尺寸和形貌,表现出“外模板作用”.

在[Emim]Br 离子液体中合成的 GaPO4-LTA 分子筛为棱长约 200 μm 的正八面体单晶颗粒, 而随离子液体烷基取代基链长的增加, GaPO4-LTA 产物晶粒逐渐变小并且结晶度下降, 形貌也由正八面体转为立方体[22]. 研究表明, 这种“外模板作用”与离子液体阳离子和 GaPO4-LTA 分子筛骨架之间内在的相互作用有关. 13C CP/MAS NMR 结果显示, 离子液体阳离子存在于 GaPO4-LTA 分子筛孔道中, 但除了离子液体阳离子的特征峰外, 在[C2MIm]Br 中合成的 GaPO4-LTA 还在 δ = 49.8 和 57.8 处出现了两个新的谱峰, 表明离子液体阳离子与 LTA 骨架间存在强的相互作用, 产生新的化学位移, 而随离子液体阳离子烷基取代基碳链的增长, 这种相互作用逐渐减弱, 相应的这两个峰减弱并最终消失. 71Ga MAS NMR 结果显示, F−进入分子筛骨架中部分双四元环结构单元, 与 Ga 配位, 形成五配位 Ga, 并导致 GaPO4-LTA 骨架带有负电荷而与离子液体阳离子相互作用. 这种相互作用的强度与离子液体阳离子的电荷密度相关, 短碳链取代基阳离子的电荷密度较高, 与分子筛骨架相互作用强; 随烷基取代基碳链增长, 阳离子正电荷变得分散, 与 LTA 骨架作用

PC+0.18O+0.41Ga+0.21NF+0.23-0.17

图 2 GaPO4-LTA 骨架与离子液体阳离子间的相互作用

Fig. 2. Interaction between the GaPO4-LTA framework and the ionic

liquid cations.

减弱 (见图 2). 强的相互作用可能导致强的结构导向作用, 稳定分子筛晶体, 导致大单晶的形成. 同时结合 X 射线衍射 (XRD) 的结果还可以发现, 这种强的相互作用抑制了 LTA(200) 晶面的生长, 得到正八面体形貌的产物; 随相互作用逐渐减弱, LTA 各晶面的生长速率趋于相等, 产物形貌逐渐向立方体转变. 另外, 离子液体阳离子的烷基取代基的碳链长不同, 使其极性发生变化, 导致体系内极性区与非极性区分布不均匀, 也可能影响分子筛晶化动力学, 致使产物的形貌尺寸发生变化.

上述研究为分子筛合成中的结构导向作用增加了新的解释, 同时也提供了一种通过选择结构导向剂、调变结构导向剂与分子筛骨架相互作用、控制分子筛晶粒的尺寸和形貌的合成新策略.

2 分子筛合成过程中水的影响

水在分子筛的合成过程中具有至关重要的作用. Oliver 等[38]曾在溶剂热条件下考察了水对分子筛结构的影响, 认为水的加入量影响原料的水解程度, 从而影响产物的结构. 但是由于水热和溶剂热体系本身含有大量水, 水对分子筛晶化过程的影响一直难以研究, 缺乏明确的实验证据. 而离子热体系中离子态的新型合成环境为研究水在分子筛合成过程中的作用提供了机会. Ma 等[33]巧妙选择合成起始原料, 设计了完全不含水的 AlOOH-NH4H2PO4- NH4F-[BMIm]Br/[EMIm]Br 离子热合成磷酸铝分子筛体系, 通过向此体系中加入反应剂量的水, 考察了水在分子筛合成过程中的作用.

在无水条件下, 分子筛的成核和晶化都很缓慢, 并呈自催化现象; 而向合成体系中添加反应剂量的

42 催化学报 Chin. J. Catal., 2012, 33: 39–50

水, 晶化诱导期缩短, 分子筛的成核和晶化速率显著提高[36]. 水解和缩聚是分子筛的合成过程中两个最基本的反应, 而质子和羟基能够诱导这两个反应的发生[39]; 水不但是 H+和 OH−离子的来源, 而且也是 H+和 OH−以水合离子形式传递的载体[40,41]. 无水情况下, 经过相对较长的诱导过程, 原料间缓慢反应生成微量的水, 生成的水作为催化剂进一步促进了水解和缩聚反应, 导致分子筛产物的生成. 反应剂量的水或其它极性物种的加入可以极大地促进分子筛的成核和晶化过程. Ng 等[36]详细明确地提供了水对分子筛晶化极度重要作用的实验证据, 加深了对分子筛合成机理的认识. Wragg 等[35]也系统研究了在离子热法合成分子筛过程中水浓度对产物结构的影响, 发现少量水的加入有利于分子筛结构的生成.

离子热合成是在离子态这种独特的环境下进行的, 它为研究各种反应组分以及添加剂对分子筛合成的影响提供了新的视角, 推动了分子筛合成机理研究的发展.

法, 合成了新型层状磷酸镍材料 DRM-1. 3.1 离子热法合成超大微孔-CLO 磷酸铝分子筛

基于通过选择离子液体种类及调节有机胺添加种类和用量所合成分子筛产物骨架密度的合成离子液体中, 通过调节设想, Wei 等[20]在[Emim]Br

1,6-己二胺的添加量, 合成出了迄今为止最大孔径的磷酸铝分子筛 DNL-1 (Dalian National Laboratory Number One). Rietveld 全谱拟合结构精修法联合 NMR 等技术解析结构得出该分子筛为-CLO 结构, 具有 20 元环的超大孔开口 (见图 3).

以往超大孔结晶磷酸铝分子筛只有 18 元环 VFI 结构的 VPI-5 和 14 元环 AET 结构的 AlPO4-8[44,45], 而已知的-CLO 结构分子筛只有磷酸镓分子筛 Cloverite 以及 Zn, Mn 等杂原子取代的 Cloverite[46,47]. DNL-1 属立方晶系, 晶胞参数 a = 5.1363 nm, Al–O 键长 0.174 nm, P–O 键长 0.159 nm (磷酸镓分子筛 Cloverite Ga–O 键长 0.181 nm, P–O 键长 0.152 nm). 13C CP/MAS NMR 结果显示, 离子液体阳离子和 1,6-己二胺存在于 -CLO 结构中并保持完整, 19F MAS NMR 结果表明, F–存在于分子筛产物中, 所以离子热法合成 DNL-1 分子筛是在离子液体、胺和 F–共同结构导向作用下生成的. 热重和原位 XRD 结果表明, DNL-1 分子筛具有良好的热稳定性, 在 950 oC 左右转晶为致密相, 脱除模板剂的 DNL-1 在 60% 湿度的空气中放置可达 6 d. DNL-1 还具有较高的比表面积 (631 m2/g)、微孔孔体积 (0.20 m3/g) 和介孔孔体积 (0.22 m3/g), 预期 DNL-1 在大分子催化以及气体分离、储存等领域有着广阔的应用前景.

3 离子热法合成新结构的分子筛

在离子热合成体系中, 改变离子液体和添加 F−、胺等物质, 调变合成变量均能够影响产物的结构. Morris 等先后合成了含有阻断结构的 SIZ-1 和层状结构的 SIZ-6[21]、SIV 结构的 SIZ-7[20]、SIZ-2[4]、

链状磷酸铝化合物 Al(H2PO4)2F[42]以及一系列的磷酸铝和磷酸镓[19]等新型结构的磷酸盐分子筛材料. Xing 等[43]在离子热体系中通过添加 N-甲基咪唑合成了 Al/P 比为 6/7 的新型阴离子骨架结构的磷酸铝材料 JIS-1. Wei 等[28]通过添加氨水采用离子热

lta cagecubic supercage(3 nm)clocagetwo unintersecting three-dimensional channel system with 20-ring and 8-ring图 3 DNL-1 分子筛的结构图

Fig. 3. The framework structure of DNL-1

www.chxb.cn 王亚松等: 离子热法合成分子筛的研究进展 43

3.2 离子热法合成杂原子磷酸铝分子筛及其催化性能

磷酸铝分子筛骨架由 AlO4 和 PO4 四面体严格交替组成, 呈电中性[48], 不具有离子交换性能和酸性. 只有在其骨架中的 Al 和 P 部分被 Si 以及其它金属元素 (Mg, Co 等) 取代后, 生成相应的杂原子取代磷酸铝分子筛, 才能应用于催化和吸附分离等领域[49,50]. Parnham 等[21]在添加氢氧化钴的条件下,

合成了 SOD, AEI 和新型 SIV 结构的 CoAPOs 分子筛. Wang 等[15]利用离子热法进行了杂原子磷酸铝分子筛的合成, 得到了含 Mg 的 MgAPOs 分子筛, 并以其为载备了 Pt/MgAPO-11 催化剂,研究了其催化应用.

以醋酸镁为镁源, 在 1-甲基-3-丁基咪唑溴离子热体系中可以合成 AFI 和 AEL 结构的 MgAPOs 分子筛. 加入适量的胺, 合成了纯相的 MgAPO-11 分子筛. 胺作为助结构导向剂提高了 AEL 结构产物的选择性, 并且能够减少杂原子物种在孔道内的沉积, 改善 MgAPO-11 分子筛的酸性. 醋酸镁的添加量对分子筛产物的形貌、比表面积、孔体积以及酸性等也都具有显著的影响. 将 Pt 负载在离子热制备的 MgAPO-11 分子筛上, 所得的 Pt/MgAPO-11 催化剂在十二烷烃临氢异构化反应中表现了高的异构化活性和选择性.

离子液体作为一种离子溶剂, 对无机盐的溶解有新的特性, 而且离子热合成在无水体系中进行, 能够避免某些过渡金属盐在水存在条件下的快速水解而形成沉淀难以进入骨架的现象, 因此离子热法在合成杂原子分子筛方面应该有大的发展空间.

4 微波促进离子热法合成分子筛

微波加热具有快速、均匀等特点, 应用于分子筛的水热合成表现出可以改变晶化选择性, 提高晶化速率的优点[51]. 但是微波透明的材料, 如聚四氟乙烯、石英、玻璃等, 往往不能承受水热过程的自生压力, 因此了微波水热合成方法的应用.

由于离子液体本身是一种离子导体, 与电磁波相互作用比水更强, 而且离子液体蒸汽压极低, 如果将微波加热与离子热合成分子筛结合, 可以降低对反应容器材质承压能力的要求. 因此, Xu 等[6]首次将微波加热应用于离子热合成, 报道了一种快速、安

全的合成方法——微波促进离子热合成法. 结果显示, 微波离子热法合成提高了分子筛的晶化速率, 使合成时间从常规加热的数十小时迅速减少到几分钟, 并且得到与常规加热条件下不同形貌和更高结晶度的产物. 微波与离子液体作用是一种电阻加热形式, 与常规的传导式加热相比, 升温速率高且更均匀, 因而可能促进原料在离子热体系中的溶解, 从而加速了原料间的反应和分子筛结构的形成. Wragg 等[52]采用微波离子热法合成了具有 CHA 结构的 SIZ-4 分子筛, 并计算出产物的生成速率常数大约是常规加热条件下的 10 倍, 也证明了该方法快速合成的特点. Lin 等[53]利用微波离子热合成法合成了金属有机骨架, 与常规加热的条件相比提高了产物的产率和纯度. 基于微波离子热合成法的常压合成和较快的合成速率, Cai 等[]在微波加热条件下于[Emim]Br 离子液体中合成了 AEL 结构的分子筛膜, 该分子筛膜具有优异的耐腐蚀性.

微波离子热合成法是以“绿色”溶剂为介质的一种“绿色”、安全、高效的新型合成方法, 在合成分子筛材料、分子筛功能膜等方面有很强的应用潜力.

5 离子热法合成硅铝分子筛

离子热合成研究在磷铝分子筛合成方面取得了诸多成果, 但是在硅铝分子筛合成方面的进展有限. 可能原因有: 首先, 在常用的咪唑基离子液体体系中, 部分硅物种溶解性较差[55,56], 一些能够溶解的硅物种发生水解和缩聚反应的速率极其缓慢, 不利于分子筛结构的形成; 其次, 硅铝分子筛的水热合成一般是在强碱性条件下进行的[57], 若向常用离子液体体系中添加诸如 NaOH 等具有强碱性的物质, 会导致离子液体分解[58]. 但是, 通过向常用离子液体体系中引入水或将常用离子液体功能化, 则能够在离子液体中合成出硅铝分子筛.

马英冲等[59]首先将铝酸钠和硅酸钠在碱性溶液中混合成硅铝胶, 然后将此硅铝胶与[Emim]Br 混合, 在敞口容器中 50~150 °C 条件下晶化, 制备出了 SOD 结构的硅铝分子筛. 在该合成体系中, 水和[Emim]Br 离子液体的摩尔比约为 2.5, 所以这并不是严格意义上的离子热合成[35]. 离子液体作为溶剂并未起到明显的空间填充作用或结构导向作用. 由于所用离子液体具有亲水性, 硅铝胶加入到离子液

44 催化学报 Chin. J. Catal., 2012, 33: 39–50

体后, 其中的水向离子液体扩散, 相对来说, 晶化前驱体中水的含量减少, 硅铝胶浓度增大, 导致骨架密度大、孔小的 SOD 结构硅铝分子筛的生成. Cai 等

[60]

将由硅酸乙酯、四丙基氢氧化铵、水和乙醇制

得的溶胶通过冷冻干燥制备成干凝胶, 然后将此干凝胶分散于[Bmim]Br 离子液体-水体系中, 采用微波加热, 175 °C 条件下晶化, 常压合成出了 MFI 结构的全硅分子筛. 在该合成体系中, 可能是由于[Bmim]Br 离子液体的亲水性, 减少了水分的蒸发, 保持了干凝胶晶化所需的水量, 从而促进分子筛结构的形成. 上述两个在离子液体中合成硅铝分子筛的例子均表明水具有重要的作用, 但是对其机理的理解还需要对离子液体和水之间具体的热力学相互作用进行深入的研究. Wheatley 等[61]通过离子交换[Bmim]Br 离子液体, 制得[Bmim]OH0.65Br0.35 功能化离子液体, 再添加适量的硅酸乙酯、氢氟酸和反应计量的水, 于 170 °C 晶化制得 MFI 和 TON 结构的全硅分子筛. 该体系的合成环境与水热合成全硅分子筛时相似, 均含有大量的 OH−, 从而有可能促进硅源在离子液体中的溶解、水解和缩聚反应, 进而导向生成 MFI 和 TON 型全硅分子筛.

离子液体的种类超过百万, 而当前应用于离子热合成研究的离子液体仅以咪唑基或吡啶基离子液体为主. 所以开发更多种类的离子液体、探索合适的反应条件, 离子热法合成硅铝分子筛有可能实现.

6 离子热法合成分子筛的展望

离子热法作为一种高效、安全的分子筛合成新方法, 显示了强大的合成优势, 为分子筛合成的研究带来新的机会. 虽然离子热合成的研究取得了一些进展, 但是总体来说还处于不成熟阶段, 例如, 离子液体的种类很多, 而目前使用的种类很少, 所以开发更多种类的离子液体用于新结构、新性能的分子筛、杂原子分子筛等材料的合成还有待于进一步探索; 由于离子液体在合成过程中既是溶剂又是模板剂, 所以使用具有手性特征的离子液体来合成手性分子筛材料是有可能的; 使用两种或几种不同性质 (例如, 亲水性和疏水性) 的离子液体共同作为介质, 有可能合成出新颖的分子筛材料; 硅铝分子筛在工业上的应用比较广泛, 但很少采用离子热法合成, 所以离子热法合成硅铝分子筛也值得探索. 总之, 进一

步发掘离子热合成的特点, 以拓展离子热合成的应用范围, 是今后离子热合成法的发展方向.

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英译文 English Text

Molecular sieves are widely applied in catalysis, adsorp-tion, and separation, as these processes benefit from their highly ordered channels and ion exchange abilities. Mo-lecular sieves are usually prepared using hydrothermal or solvothermal synthesis methods [1–3]. Moreover, the chal-lenges associated with novel approaches for the synthesis of molecular sieves with exotic structures and properties con-tinue to be of great importance in the field of molecular sieve materials. In 2004, Cooper et al. [4] reported a new method referred to as ionothermal synthesis, which was the use of an ionic liquid or a deep eutectic mixture as both the solvent and the structure directing agent. This opened up an alternative route for the synthesis of novel molecular sieves and for the determination of the mechanism of molecular sieve synthe-sis.

Ionic liquids have various peculiar properties such as a wide liquidus range and a low measurable vapor pressure and are nonflammable. Therefore, ionothermal synthesis can be carried out at ambient pressure, which alleviates the safety concerns associated with potential hydrothermal pressure risks. The crystallization mechanism may thus be determined easily and in situ characterization may be carried out during

46 催化学报 Chin. J. Catal., 2012, 33: 39–50

synthesis of molecular sieves and this cannot be achieved using the hydrothermal process because of the autogenous high pressure. Ionic liquids can act as a solvent and provide a template at the same time because the chemical structures of ionic liquids are similar to those of the organic amines that are used as structure directing agents (SDAs) in the hydro-thermal synthesis of molecular sieves. A large variety of ionic liquids exist and the chemical structures of these ionic liquids are designable while the controllable synthesis vari-ables used in an ionothermal system increase upon the in-troduction of organic amines. Therefore, ionothermal syn-thesis offers a new platform for the creation of novel mate-rials. Ionic liquids exhibit strong polarity and conductivity and they provide an ionic synthesis environment, which differentiates it from other solvent molecules. They are also amenable to being combined with electromagnetic wave technologies such as microwave heating, which endows ionothermal synthesis with new characteristics.

Much progress has been made mainly in phosphate mo-lecular sieve materials using ionothermal synthesis. Apart from aluminophosphate [4–13] and metal-containing alu-minophosphate [14–18] with known structures, a range of novel phosphate [19–27], phosphite [28–30], and organo-phosphate [31–32] etc. molecular sieves or zeolitic analogue open framework materials have been synthesized by the ionothermal method. Research into the ionothermal synthesis of aluminophosphate molecular sieves also increases the understanding of the synthesis mechanism [33–36]. This review mainly concentrates on summarizing progress into the investigation of the crystallization mechanism and the ionothermal synthesis of novel molecular sieve materials.

1 The structure directing effect in the ionothermal synthesis of molecular sieves

In the hydrothermal or solvothermal process, organic amines, quaternary ammonium salts, and organic macro-molecules are usually added as SDAs [37]. The structure directing effect is always a major problem in the synthesis of molecular sieves. Morris et al. [4,21] first prepared alu-minophosphate molecular sieves in 1-ethyl-3-methylimi-dazolium bromide ([Emim]Br) ionic liquids including the known AEL and CHA structures and two novel materials SIZ-1 and SIZ-6. The authors pointed out that onic liquids acted as a solvent and a template simultaneously. CHA alu-minophosphate molecular sieves were then synthesized using 1-alkyl-3-methyl imidazolium bromide ionic liquids with different length alkyl substituent chains. The 1-alkyl-3-methyl imidazolium cations broke down in the presence of F– and formed 1,3-dimethyl imidazolium cations to direct the CHA structure [5].

1.1 Structure directing effect of amines in ionothermal synthesis

Wang et al. [7] synthesized AEL (10-ring, AlPO4-11) type aluminophosphate molecular sieves in [Emim]Br ionic liq-uids and AEL and AFI (12-ring, AlPO4-5) topologies in 1-butyl-3-methylimidazolium bromide ([Bmim]Br) ionic liquids separately. The results imply that the architecture of the final product is influenced by the size of the imidazolium cation. When n-dipropylamine etc. organic amines were added to a [Bmim]Br ionic liquids system the crystallization dynamics changed and pure AFI was obtained (see Fig. 1). Solid-state NMR spectroscopy shows that the amine exerts its structure directing effect and co-exists in the channel of the molecular sieves together with ionic liquid cations. 1H NMR spectra give strong evidence for the formation of hy-drogen bonds between the ionic liquid and the amine. Hy-drogen bonds between the amine and the ionic liquid may disrupt the nonbonding interactions established between the ionic liquid cations and the inorganic framework and influ-ence the dynamics of the crystallization process. Ionic liquid cations and organic amines may form supermolecular structures to direct the formation of molecular sieves with large pores or with a low framework density through hy-drogen bond interactions. Pei et al. [9] studied the effect of organic amine quantity and LTA molecular sieves with a low framework density and three-dimensional cross channels were obtained. By a combination of solid-state NMR spec-troscopy and density functional theory calculations, Xu et al. [34] studied the crystallization evolution of molecular sieves in [Bmim]Br ionic liquids upon the addition of organic amines. Their work confirms that amines interact with ionic liquids by hydrogen bonds and the concentration of amines could affect the nucleation and crystallization of the alu-minophosphate molecular sieves with different topological structures.

Taking advantage of the wide liquidus range of ionic liq-uids, Wang et al. [7] investigated the crystallization evolution of molecular sieves in an ionothermal system at a high crystallizing temperature of 280 °C and found that the structure of the products transformed from AFI to AEL and ATV with an increase in the crystallization time. Their results show that organic amines as SDAs improve the crystalliza-tion rate and the selectivity of molecular sieves, and also cooperate with the ionic liquids that remain in the channels of the products to stabilize the crystal structure and inhibit the transformation of the AlPO4 molecular sieve framework to an ultimately dense phase.

The above conclusions show that a different topology structure of the products can be obtained by adjusting the varieties and amounts of ionic liquids and organic amines to control the assembly of inorganic species. This confirms the

www.chxb.cn 王亚松等: 离子热法合成分子筛的研究进展 47

structure directing role of ionic liquids and organic amine additives and establishes a theoretical basis for an iono-thermal study into the structure directing effect and synthesis of novel molecular sieves.

1.2 Structure directing effect of ionic liquids in ionothermal synthesis

Ma et al. [22] studied the ionothermal synthesis of LTA gallophosphate molecular sieves (GaPO4) in a range of [CnMim]Br (n = 2–6) ionic liquids in the presence of fluo-ride. They found that the length of the alkyl substituent chain of the ionic liquid cations could influence the sizes and mor-phologies of the GaPO4-LTA products. The ionic liquids represent a “shape and size-directing template” effect.

The GaPO4 molecular sieves synthesized in [C2Mim]Br ionic liquids are octahedral single crystal with a size of about 200 µm. However, with an increase in the alkyl chain length of the ionic liquids the sizes and crystallinity of the crystals decrease and the morphologies of the GaPO4 single crystals gradually transform from octahedral to cubic [22]. This re-search shows that the “shape and size-directing template” effect may be related to the inner-interactions between ionic liquid cations and GaPO4-LTA frameworks. The 13C CP/MAS NMR spectrum of GaPO4-LTA shows that all the distinct resonances of [C2Mim]+ are observed which implies that [C2Mim]+ remains in the channels of the GaPO4 prod-ucts. In addition, two more peaks are observed at δ = 49.8 and 57.8, which indicates that strong interactions exist between the ionic liquid cations and the GaPO4-LTA framework and this is responsible for the new chemical shift. With an in-crease in the alkyl chain length of the ionic liquids the in-teractions gradually weaken and these two peaks also de-crease in intensity and ultimately disappear. The 71Ga MAS NMR spectrum shows that F– is trapped in the D4R units of molecular sieve framework. F– coordinates with Ga to form penta-coordinated Ga and, therefore, the GaPO4-LTA framework has negative charges that can interact with the ionic liquid cations. The intensity of the interaction is related to the electron density of the ionic liquid cations. On the short alkyl substituent chain of ionic liquid cations the intensity of the interaction is strong because of the high electron density. With an increase in the length of the alkyl chain of the ionic liquid cations the intensity of the interaction weakens be-cause the positive charges gradually disperse (Fig. 2). The strong interaction may induce a powerful structure directing effect that stabilizes the molecular sieve framework and directs the formation of a large single crystal. By combining the XRD results the strong interaction evidently inhibits the growth of the (200) plane of the LTA framework and, there-fore, GaPO4-LTA with octahedral morphology is obtained. With the interaction attenuating the growth rate of every

lattice plane of the LTA framework is similar and the mor-phology gradually transforms to a cubic morphology. In addition, with an increase in the alkyl chain length of the ionic liquid cations the nonpolar domain becomes more connected while the polar domain becomes more permeated. This may change the crystallization kinetics of the molecular sieves, which leads to a variation in the morphologies and the sizes of GaPO4-LTA.

This research further explains the structure directing effect and the “shape and size-directing template” effect during the synthesis of molecular sieves, and a novel synthesis strategy is found to control the sizes and the morphologies of the molecular sieves by selecting proper SDAs and adjusting the interactions between the SDAs and the molecular sieve frameworks.

2 The role of water in the synthesis of molecular sieves

Water plays an important role in the synthesis of molecular sieves. Oliver et al. [38] investigated the effect of water on the structure of molecular sieves in a solvothermal system. They suggested that the additional water determined the extent of hydrolysis for the starting materials and then the resulting structure type. However, the influence of water on the crystallization process of the molecular sieve is difficult to study and there is a lack of definitive experiment evidence because of the excess water present during the hydrothermal and solvothermal syntheses. The novel ionic environment of ionothermal system allows for the study of the role of water in the synthesis of molecular sieves. Ma et al. [33] skillfully selected original materials and designed a completely anhy-drous AlOOH-NH4H2PO4-NH4F-[BMIm]Br/[EMIm]Br ionothermal system to explore the effect of water on the synthesis of molecular sieves by adding reagent quantities of water.

In the absence of water, the rate of nucleation and crystal-lization of the molecular sieves is slow and it appears to be an autocatalytic process. Once the reagent quantity of water is added the induction time is dramatically reduced and the rate of nucleation and crystallization of the molecular sieves improves distinctly [36]. Hydrolysis and condensation are two of the most significant basic reactions in the synthesis of molecular sieves and H+ as well as OH– are expected to induce these two important reactions [39]. The added water produces and transports H+ and OH– hydrates [40,41]. In an anhydrous system, a trace amount of water can be produced slowly in the reaction between AlOOH and NH4H2PO4 after a long induction time. The produced water, as a catalyst, promotes hydrolysis and condensation to lead to the forma-tion of molecular sieve frameworks. The reagent quantities of water or other polar species can thus thoroughly accelerate

48 催化学报 Chin. J. Catal., 2012, 33: 39–50

the nucleation and crystallization processes of molecular sieves. Ma’s work explicitly provides experimental evidence of the most important role of water in the synthesis of mo-lecular sieves and provides a deeper understanding of the synthesis mechanism of molecular sieves. Wragg et al. [35] also systematically studied the effect of water concentration on the structure of the products from the ionothermal syn-thesis of molecular sieves and found that low amounts of water were an advantage in the formation of molecular sieve frameworks.

The unique ionic environment of ionothermal synthesis offers a novel perspective for an investigation into the in-fluence of various reaction species and additives on the synthesis of molecular sieves and it promotes study into the mechanism of molecular sieve synthesis.

3 Ionothermal synthesis of novel molecular sieves

In ionothermal synthesis the architecture of the products can be affected by a change in the ionic liquids, the addition of F– and amines etc., and by adjusting other variables. Morris et al. synthesized the interrupt frameworks SIZ-1 and SIZ-2 [4], the layered SIZ-6 [21], SIV topology SIZ-7 [20], a chained aluminophosphate compound Al(H2PO4)2F [42] and a series of aluminophosphate and gallophosphate [19] novel phosphate molecular sieves. Xing et al. [43] prepared a new anion aluminophosphate framework at a Al/P ratio of 6/7 (JIS-1) using an ionothermal system by adding proper amounts of N-methylimidazole. Wei et al. [28] synthesized new layered nickel diphosphates (DRM-1) by the ionother-mal method with the addition of ammonia.

3.1 Ionothermal synthesis of aluminophosphate molecular sieves with 20-ring pore openings

On the basis of synthesis strategy to control the framework density of molecular sieves by selecting different types of ionic liquids and modulating the sorts and amounts of amine, Wei et al. [20] synthesized the largest known pore alu-minophosphate molecular sieves, which was denoted as DNL-1 (Dalian National Laboratory Number One) in [Emim]Br ionic liquids by adjusting the amount of 1,6-hexanediamine. A combination of the Rietveld refine-ment method and solid-state NMR spectroscopy confirms that DNL-1 is a single phase aluminophosphate molecular sieve with a -CLO topology structure and 20-ring pore openings (see Fig. 3).

Previously reported aluminophosphate molecular sieves with extralarge pores were VPI-5 (VFI framework, 18-ring) and AlPO4-8 (AET framework, 14-ring) [44,45]. However, the known -CLO topology structure is merely gallophosphate

cloverite and Zn or Mn-containing cloverite where some Ga atoms of gallophosphate are replaced by Zn or Mn [46,47]. DNL-1 has a cubic crystal system with a refined unit cell parameter a = 5.1363 nm, an Al–O bond length of 0.174 nm, and a P–O bond length of 0.159 nm (Ga–O bond length of 0.181 nm and a P–O bond length of 0.152 nm in gallophos-phate cloverite). The 13C CP/MAS NMR spectrum of DNL-1 shows that both [Emim]+ and HDA remain intact upon oc-clusion within the -CLO structure. The 19F MAS NMR spectrum of DNL-1 shows that F– is trapped in the -CLO structure. Therefore, DNL-1 is directed by ionic liquid cations, F–, and amine as a co-template. TG and in-situ XRD show that DNL-1 has excellent thermal stability. It does not transform to the dense phase AlPO4 tridymite until 950 °C. After calcination at 850 °C, DNL-1 remains stable for about 6 d in an atmosphere with 60% humidity. DNL-1 also has a high specific area (631 m2/g), micropore volume (0.2 m3/g), and mesopore volume (0.22 m3/g). DNL-1 is expected to have many potential applications in catalysis, separation, and gas storage.

3.2 Ionothermal synthesis of metal-containing molecular sieves and their catalytic performance

Aluminophosphate molecular sieve frameworks are com-posed of strictly alternating AlO4 and PO4 tetrahedrons [48]. Because aluminophosphate frameworks are neutral, alu-minophosphate molecular sieves lack ion exchange ability and acidity. Only the Al and/or P atoms in the alumino-phosphate frameworks are partially substituted by silicon or other metal elements to form the corresponding metal-containing molecular sieve frameworks and these molecular sieves can be used in catalysis, adsorption, and separation [49,50]. Parnham et al. [21] ionothermally syn-thesized CoAlPOs molecular sieves with SOD, AEI, and SIV topology with the addition of cobalt hydroxide. Wang et al. [15] tried to synthesize metal-containing molecular sieves and MgAlPOs molecular sieves were obtained. Using MgAlPO-11 as a support, the Pt/MgAlPO-11 catalyst was prepared and the catalytic performance of the Pt/MgAlPO-11 catalyst was investigated.

MgAlPOs with AEL and AFI structure were synthesized in [Bmim]Br ionic liquids by adding magnesium acetate as the Mg starting material. Pure AEL type MgAlPO-11 was obtained upon the addition of appropriate amounts of amine. The additional amine as SDAs can improve the selectivity of the AEL topology, reduce the Mg species deposited in the channels and modify the acidity of MgAPO-11. The amount of added magnesium acetate can dramatically affect the morphology, specific surface area, porous volume, and acid-ity of the products. Noble-metal Pt was loaded onto MgAlPO-11 to prepare the Pt/MgAlPO-11 catalyst. The

www.chxb.cn 王亚松等: 离子热法合成分子筛的研究进展 49

catalyst is exceptional for isomerization selectivity in the hydroisomerization of n-dodecane.

Ionothermal synthesis occurs in an anhydrous environ-ment and it prevents the quick hydrolysis of some transition metal salts before they enter the framework, which would produce sediment in the presence of water. The ionothermal approach shows great promise for the development of syn-thesis routes for metal-containing molecular sieves.

4 Microwave-enhanced ionothermal synthesis of molecular sieves

Microwave heating is rapid and uniform. It has been used in the hydrothermal synthesis of molecular sieves and has advantages of rapid crystal growth rate and high product selectivity [51]. However, microwave-transparent materials such as PTEF, quartz, and glass become flexible at high temperature and cannot withstand the resultant autogenous high pressure. This limits the use of the microwave hydro-thermal technique in the synthesis of molecular sieve mate-rials.

Ionic liquids are ion conductors and the interaction be-tween ionic liquids and microwaves are much stronger than that between water and microwaves. Because of the low vapor pressure of ionic liquids the material requirement for pressure-resistant vessels is reduced if microwave heating is combined with ionothermal synthesis. Xu et al. [6] were the first to report a fast and safe synthesis route that they referred to as the microwave-enhanced ionothermal method in which microwave heating was applied to an ionothermal synthesis. Their results show that the crystallization rate improves dramatically and the crystallization time is reduced from tens of hours to several minutes by the microwave-enhanced ionothermal synthesis of molecular sieves. Furthermore, the products synthesized by this approach have different mor-phologies and higher crystallinity compared with synthesis by conventional heating for ionothermal synthesis [6]. The interaction between ionic liquids and microwaves is in the form of resistance heating. The heating rate of microwave heating is faster and more uniform than that of conventional conduction heating. Therefore, the digestion and the reaction of reagents are facilitated and a rapid crystallization rate is obtained in the ionothermal system. Wragg et al. [52] syn-thesized SIZ-4 with CHA topology structures by this ap-proach and calculated the rate constant, which was found to be ten times than that of conventional heating. They also confirmed the rapid crystallization rate of the micro-wave-enhanced ionothermal method. Lin et al. [53] iono-thermally synthesized [Co3(TMA)2(OAc)2] and [Ni3(TMA)2- (OAc)2] anion frameworks under microwave heating. The yield and purity of the product were better than that obtained by conventional heating. On the basis of the ambient pressure

synthesis and the fast crystallization rate of the micro-wave-enhanced ionothermal synthesis, Cai et al. [] pre-pared a highly oriented SAPO-11 zeolite film with excep-tional anti-corrosion properties in [Emim]Br ionic liquids. The microwave-enhanced ionothermal method is a “green”, simple, and highly efficient approach as it exploits green solvents as a medium. This method offers many pos-sibilities for the preparation of molecular sieves and func-tional molecular sieve coatings.

5 Ionothermal synthesis of silicon-based zeolites

The ionothermal method has resulted in many achieve-ments in the synthesis of aluminophosphates. However, the synthesis of silicon-based zeolites has been far more chal-lenging. The primary problem with silicon-based zeolite synthesis is the poor solubility of some silica species in commonly used ionic liquids [55,56]. Although some silica species dissolve, the hydrolysis and condensation rate of the silica starting materials are quite slow. On the other hand, silicon-based zeolites are usually synthesized in a hydro-thermal system with a strongly alkaline environment [57]. If strong alkaline materials such as NaOH were added to the ionothermal system the ionic liquid would break down [58]. These two factors may inhibit the development of iono-thermal synthesis for silicon-based zeolites. However, the introduction of water into the ionothermal system or the use of task specific ionic liquids as a medium may allow sili-con-based zeolites to be prepared.

Ma et al. [59] prepared the SOD aluminosilicate zeolite from an aqueous basic aluminosilicate colloid that crystal-lized in [Emim]Br ionic liquids. First, the sodium aluminate and sodium silicate were mixed in a sodium hydroxide and potassium hydroxide solution to form an aluminosilicate colloid. The aluminosilicate colloid was then mixed with [Emim]Br ionic liquids and the mixture was crystallized at 50–150 °C over a certain time in an open vessel to obtain the SOD structure. In this synthesis system the molar ratio of water to ionic liquids is about 2.5. This can strictly not be attributed to the ionothermal synthesis wherein the molar ratio of water to ionic liquids is less than 1 [35]. The ionic liquid as solvent does not work as a space filling agent or a SDA. Because the ionic liquid is hydroscopic water diffuses to the ionic liquid once the aluminosilicate colloid is within the ionic liquid. The water content in the aluminosilicate colloid decreases and the concentration of the aluminosili-cate colloid increases, which leads to the formation of a SOD structure with a large framework density and small pores. Cai et al. [60] prepared the MFI silicate zeolite by dry-gel con-version using an ionic liquid and microwave heating. The dry precursor gels (DGP) were prepared from a synthesis solu-tion composed of tetraethylorthosilicate, tetrapropylammo-

50 催化学报 Chin. J. Catal., 2012, 33: 39–50

nium hydroxide, water, and ethanol. The mixture was then dried by lyophilization to obtain the DGP. Next, the DGP was dispersed in a [Bmim]Br-water system. The synthesis system was crystallized at 175 °C under microwave heating. Finally, the MFI silicate zeolite was produced at ambient pressure. This process is similar to that of the synthesis of the SOD aluminosilicate zeolite in [Emim]Br ionic liquids. Because the ionic liquid is hydrophilic which may reduce the evapo-ration of water, it retains adequate water for the crystalliza-tion of DGP in the ionic liquids, which promotes the forma-tion of a MFI structure. The two above-mentioned examples for the synthesis of silicon-base zeolites in an ionic liq-uid-water system indicates that water plays a critical role. However, the thermodynamic interaction between ionic liquids and water needs to be further studied for a better understanding of this synthesis mechanism. Wheatley et al. [61] synthesized MFI and TON silicate zeolites by the ionothermal method in [Bmim]OH0.65Br0.35 ionic liquids prepared from [Bmim]Br ionic liquids by anion exchange. Tetraethylorthosilicate, hydrofluoric acid, and water were added to the ionic liquid and the mixture was crystallized at 170 °C to obtain the MFI and TON zeolites. The synthesis environment of this system resembles that of the hydro-thermal process where both systems contain excess OH–. This may accelerate the dissolution, hydrolysis, and con-densation of the silicate species and direct the MFI and TON topology structures.

Around 1 million binary ionic liquids are available but only a few of the easily available ionic liquids, which are mainly imidazole- and pyridine-base ionic liquids, have been studied for use in ionothermal synthesis. The preparation of silicon-base zeolites by the ionothermal approach will be realized by exploring the application of many more ionic liquids in ionothermal synthesis and searching for the ap-

propriate reacting conditions.

6 Prospect of the ionothermal synthesis of molecular sieves

Ionothermal synthesis has excellent advantages for the preparation of molecular sieves as it is a highly efficient, safe, and novel method, which offers new opportunities to study the synthesis of molecular sieves. Although some progress has been made, ionothermal synthesis is still in a preliminary stage in general. For example, many types of ionic liquids exist but only a few ionic liquids have been studied and, therefore, the development of many more va-rieties of ionic liquids as media for the synthesis of molecular sieves with new structures, properties as well as metal-containing molecular sieves needs to be explored. Ionic liquids can act as solvents and templates in the iono-thermal system and the chirality of molecular sieves can be influenced by chiral ionic liquids. It is possible to synthesize novel molecular sieve materials using mixed ionic liquids by mixing two or more miscible ionic liquids to produce a new solvent with different properties (hydrophobic or hydro-philic) as a cooperative medium. Aluminosilicate zeolites are widely used in industry but few reports exist about the ionothermal synthesis of aluminosilicate zeolites. The iono-thermal synthesis of aluminosilicate molecular sieves is a worthwhile research topic. In conclusion, the further explo-ration of particular characteristics of ionothermal synthesis to expand the application of the ionothermal method is a pos-sible development direction for ionothermal synthesis in future.

Full-text paper available online at Elsevier ScienceDirect http://www.sciencedirect.com/science/journal/18722067

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