The Synthesis Of Open-framework Aluminophosphates And The Structure-directing Effect Of Piperazine Molecule

Zeolites and related microporous crystalline materials, which have periodicthree-dimensional (3D) frameworks and well-defined pore structures, have attracted muchinterest due to their wide applications in catalysis, ion exchange, separation and adsorption. Thedesign of new structural topologies and the elucidation of structural design principles haveserved to increase the chemist's control over the properties of the microporous crystallinematerials. However, because of the limited understanding in their formation mechanism, it isdifficult in rational synthesis of these materials. In a sense, the synthesis is blindness. In order toachieve the goal of rational synthesis of microporous materials, it is essential to investigate thecrystallization process and discover their formation mechanism.Most of the earlier work in microporous materials concentrated on aluminosilicate zeolitesuntil Wilson and co-workers discovered the aluminophosphate structures (AlPO_4-n) in the early1980s. The formation of aluminophosphates with zeolitic and related structures has given rise toa flurry of activity resulting in many new structures with different dimensionality and richstoichiometries. Ruren Xu's group at Jilin University did a lot of work in this area; a largenumber new open-framework aluminophosphates with different stoichiometries weresynthesized. The compounds exhibiting1D,2D, and3D extended structures were formed bystrict alternation of Al polyhedra (tetrahedra, trigonal bypyramid, and octahedra) and Ptetrahedra via sharing their vertices. Compared to microporous aluminosilicate,aluminophosphates give great varieties in structure and composition.Organic structure-directing agents play an important role in the synthesis ofopen-framework aluminophosphates. However, the understanding of the structure-directingeffect at molecular level is still very limited. This thesis focuses on the synthesis of newopen-framework aluminophosphates and the structure-directing effect of piperazine played indirecting the open-framework aluminophosphates. Main achievements are as follow:1. A new3D anionic open-framework aluminophosphate with multi-directional intersecting12Rchannel system has been hydrothermally synthesized by using piperazine as structure-directingagent. The structure is build by stacking of a new type of super-cage [4⁶12⁸] along [001],[241],[2-21], and [-4-21] directions. The super-cage [4⁶12⁸] is composed by the primarystructure unit denoted4=1by international zeolite association which can also been found in inNAT, EDI, and THO type zeolites. Because there exist3-fold rotoinversion axes along the [001]direction, the compound exhibits racemic left-hand and right-hand3₁helical chains. Theformation of helical chains is derived from the cis-cis connection of4=1building units. In thiscompound,22mol%of the protonated piperazine can be exchanged by sodium cations. 2. A novel3D chiral open-framework fluoroaluminophosphate Al₆P₃O₁₂F6(OH)6C₄N₃H₁₆hasbeen hydrothermally synthesized by using diethylenetriamine as structure-directing agent underbasic condition. The compound gives an unusual Al/P ratio of2. The framework of AlPO-JU89is constructed by hexagonal array helical chains sharing F-F edge of cis-AlO₂(OH)₂F₂octohedral units between adjacent helical chains, PO₄tetrahedrons between adjacent helicalchains connect them tightly. This kind of helical chain winding3₁axis along [001] direction isconstructed by Δ-shape cis-AlO₂(OH)₂F₂octahedrons through sharing OH vitexes. Theconnections between hexaganol array3₁helical chains give a new3₁helical chain with largehelical radius along [001]. This new3₁helical chain is made of alternation of PO₄tetrahedra anddimered cis-AlO₂(OH)₂F₂octahedra by sharing F-F edges.1D10-ring channel is formed byconvolving of the new helical chain. The formation of the multi-helical structure attribute to thepresence of Δ-shape cis-AlO₂(OH)₂F₂octohedral units. The chiral unit needs three kinds of μ-ligand to build, the basic gel containing F under hydrothermal condition give a chance to formthe chiral units. The synthesis of AlPO-JU89gives a hint for constructiong non-centrosymmetricstructure. The VCD measurement shows the as-synthesized compounds have optical activitywith enantiomeric excess.3. The crystallization processes of layered aluminophosphates AP2pip and AlPO-CJ9frominitial mixtures compositions of Al₂O₃: P₂O₅:1.1Piperazine:277H₂O and Al₂O₃: P₂O₅:4.0Piperazine:277H₂O, respectively, were thoroughly characterized by XRD, pH, elemental, andNMR analyses. The intermediates were obtained by ex-situ centrifugal separation method. Theresults indicate that the local environments of the final structures are the same as those in thesmall building fragments formed in the initial mixtures. However, the type and number of thesmall building fragments were observed to be different for each initial mixture. These fragmentsunderwent different evolution pathways, and two different structures were ultimately obtained,making the observed structure-directing role of piperazine molecules unique to each initialmixture. The results from the control experiments clearly show that the initial mixturepossessing the same or similar pH may possess similar small building fragments; however, theevolution pathways of the initial mixture may be greatly influenced by the negatively andpositively charged species. In addition, the analysis of the close contacts in the piperazine layerindicates that the structure-directing ability is not solely determined by species type but also areadensity and symmetry of piperazine area.4. The products of piperazine-directed synthesis of two-dimensional (2D) aluminophosphates,AP2pip and AlPO-CJ9, that were crystallized under different conditions for different periods oftime were freeze-dried and characterized using XRD and NMR techniques. The core units of thelayered aluminophosphates including AP₂pip, AlPO-CJ9, and L-DAP, which washydrothermally synthesized in the presence of racemic1,2-diaminopropane, were obtained byapplying a "reverse evolution" process to their structures. The factors affecting thestructure-directing ability of piperazine under acidic and basic conditions were analyzed. Thecooperative role of water in directing the layered aluminophosphate of AP2pip under acidic conditions is discussed. After a thorough analysis of the core units of AlPO-CJ9and L-DAP, anew phenomenon of a "topological-structure-directing effect" or a "topological-templatingeffect" was observed. The observation of the new phenomenon of atopological-structure-directing effect or a topological-templating effect may contribute to adeeper understanding of the nature or origin of the "structure-directing or templating effect".5. The crystallization behavior of the initial mixtures with the composition of Al₂O₃:3.0P₂O₅: xPiperazine:277H₂O (x=2.4-12) was studied. After heating3days at180C, four pure phases ofopen-framework aluminophosphates of AlPO-CJB2(3D), AP2pip (2D), AlPO-JU88(3D), andAlPO-CJ9(2D) were obtained from the initial mixtures with the composition of Al₂O₃:3.0P₂O₅:3.0Piperazine:277H₂O, Al₂O₃:3.0P₂O₅:3.4Piperazine:277H₂O, Al₂O₃:3.0P₂O₅:6.5Piperazine:277H₂O, and Al₂O₃:3.0P₂O₅:12Piperazine:277H₂O, respectively. The productsincluding liquid and solid phases of AlPO-CJB2, AP2pip, and AlPO-JU88for different periodsof time were characterized using ICP, pH, XRD, and NMR analyses. In addition, crystallizationof pH-regulating gel is provided to understand the structure-directing effect of piperazine deeply.The results indicate that: The gel environment in acid and basic condition give different type ofstructure fragments, This lead to the different structure-directing ability, but this is not unique. Ingel of P/Al=3, initial gel corresponding AP2pip andAlPO-CJB2is both acidic, andconcentration of piperazine play a important role in formation of them. In crystallization ofAlPO-CJB2, a transition of AP2pip→AlPO-CJB2can be found, a kind of six-coordinated Alspecies(-10.7ppm in27Al MAS NMR) result in the transiton. In addition, anoin species with largeelectronegativity can reduce active concentration of piperazine in gel.6. The crystallization behavior of the initial mixtures with the composition of Al₂O₃: x P₂O₅:6.4Piperazine:277H₂O (x=1-8) was studied. AlPO-CJ9, AlPO-JU88and AP2pip can be wereobtained from the initial mixtures with the composition of Al₂O₃:2.0P₂O₅:6.4Piperazine:277H₂O, Al₂O₃:3.0P₂O₅:6.4Piperazine:277H₂O and Al₂O₃:5.5P₂O₅:6.4Piperazine:277H₂O,respectively. The crystallization process of AlPO-CJ9, AlPO-JU88and AP2pip wascharacterized by XRD, pH, ICP and NMR analyses. In crystallization AlPO-CJ9, AlPO-CJ9andAlPO-JU88are two competition phases at the early stage of crystallization. Prolonging theheating time, AlPO-JU88completely disappeared and pure phase of AlPO-CJ9was obtained.This AlPO-JU88→AlPO-CJ9transition is attributed to the formation of Al-OH-Al from AlO₄ina large pH increased basic environment throughout crystallization of AlPO-CJ9. Incrystallization of AlPO-JU88, the increase of pH is very limited, the crystallization completed ina very short time. The crystallization rate of AP2pip is much faster than that studied above,suggesting that the concentration of solvable piperazine and inorganic fragment play animportant role on the crystallization rate.