Structure-Directing Role of Amines in the Ionothermal Synthesis

Abstract

Since their discovery, microporous materials have been broadly exploited as shape-selective catalysts and sorbents in different research fields, 1 accompanied by an ongoing search for novel molecular sieve structures and new synthesis methods. For example, Morris and co-workers have recently developed a novel method (ionothermal synthesis) for preparing molecular sieves, which involves the use of ionic liquid as both the solvent and structuredirecting agent. 2 By using this method, Morris et al. have prepared several aluminophosphate and cobalt aluminophosphate structures in an imidazolium-based ionic liquid (1-ethyl-3-methylimidazolium bromide ([emim]Br)). Compared with the traditional hydrothermal or solvothermal methods, 1,3 preparations using the ionothermal method can take place at or near ambient pressure because of the negligible vapor pressure of ionic liquids. 4 This eliminates safety concerns associated with high pressures.The synthesis of molecular sieves often requires the addition of organic molecules, such as amines and quaternary ammonium ions, into the synthesis mixture. The role of these molecules has been described as a structure-directing effect, referring to the complementary shapes of the structure-directing agents and the frameworks they direct. Normally the addition of structure-directing agents to molecular sieve synthesis can affect the rate at which a particular structure is formed. In this work, we report the study of the structure-directing role of amines in the ionothermal synthesis of molecular sieves in 1-butyl-3-methylimidazolium bromide ionic liquid ([bmim]Br). 5 We show that the original crystallization process can be altered by the addition of amine, as summarized in Scheme 1.In the absence of amine, AlPO 4 -11 molecular sieves (AEL-type structure) 6 with minor AlPO 4 -5 (AFI-type structure) 7 were formed after a short induction period at a temperature between 190 and 280°C in [bmim]Br ( Figure 1a and Table 1). Increasing the crystallization time leads to further growth of the AEL structure, while the amount of the AFI structure passed through a maximum and eventually disappeared after 4 h. Both of the two structures are composed of straight one-dimensional channels with AFI containing 12-membered ring (MR) pores (7.3 × 7.3 Å in size) and AEL containing 10-MR pores (6.5 × 4.0 Å). To date, the only ionic liquid used in ionothermal synthesis was [emim]Br. The pore sizes of all the AlPO and CoAlPO products synthesized ionothermally in [emim]Br are no larger than 10-MR. 2,8 The fact that the 12-MR large-pore AFI structure was formed when [bmim]Br was used as the reaction media instead of [emim]Br suggests the possible structure-directing effect of ionic liquid. ( 13 C CP-MAS NMR indicates that the cation of [bmim]Br is occluded in the pores of AlPOs; see Supporting Information.) Moreover, the cation of ionic liquid may have the major influence on the frameworks it directs. Nevertheless, it seems that [bmim] + is still suitable for filling the 10-MR channels because the AEL structu...

Preparation of Inorganic Materials Using Ionic Liquids

“…Tian and co-workers investigated the role of several types of amines added to the ionothermal synthesis of aluminophosphate molecular sieves AlPO 4 -5 and AlPO 4 -11 in [BMim] [Br]. [345] While the addition of the amine appeared to affect the crystallization process and result in a better selectivity, the amines were not occluded into the porous structures. Alternatively, it would be interesting to combine the ionic liquid and organic amine that can cooperatively play a structure-directing role in the formation of zeolitic open-framework structures.…”

Simple and safe synthesis of microporous aluminophosphate molecular sieves by ionothermal approach

“…13 By using this method, some new topological aluminophosphates [13][14][15][16][17] were synthesized by Morris and coworkers that cannot be prepared from hydrothermal or solvothermal system. Also, Tian and coworkers 18 and Xu et al 19 studied the effects of amines and microwave heating on ionothermal synthesis of molecular sieves.…”

Ionothermal Synthesis of Aluminophosphate Molecular Sieve Membranes through Substrate Surface Conversion

“…The organic cation of the IL can act as a structure-directing agent (SDA), [13c] or play a cooperative structure-directing role together with introduced amines and quaternary ammonium compounds. [18][19][20] AEL, AFI, CHA, LTA, and ÀCLO types of aluminophosphate molecular sieves have been successfully synthesized by ionothermal methods.Ionothermal synthesis is a promising method for the preparation of aluminophosphate molecular sieve membranes under ambient pressure. Yan and co-workers prepared AlPO-11 and SAPO-11 films on aluminum alloys as anticorrosion coatings by in situ ionothermal synthesis.…”

“…The organic cation of the IL can act as a structure-directing agent (SDA), [13c] or play a cooperative structure-directing role together with introduced amines and quaternary ammonium compounds. [18][19][20] AEL, AFI, CHA, LTA, and ÀCLO types of aluminophosphate molecular sieves have been successfully synthesized by ionothermal methods.…”