Rigid/Flexible Organic Structure Directing Agents for Directing the Synthesis of Multipore Zeolites: A Computational Approach

Abstract

The preferential crystallization of pure silica ITQ-39 and MFI zeolites using SDAEt and SDAPr, respectively, as organic structure directing agents (SDA) is investigated here by theoretical methods using a periodic DFT-D model. For this purpose, the templating roles of SDAEt and SDAPr have been analyzed following a systematic study of the interaction of these two SDAs with the above described zeolites through SDA stabilization energies (balance of intermolecular SDA dispersive interactions and SDA strain). From theoretical studies, it has been found that SDAEt located inside ITQ-39 zeolite and SDAPr placed within MFI show the largest stabilizations. These theoretical results agree with previous experimental observations for the preferential crystallization of pure silica ITQ-39 and MFI using SDAEt and SDAPr molecules, respectively.Despite there are many variables influencing the nucleation and crystallization processes in zeolite synthesis, 1 different rationalized inorganic and organic directing effects have allowed the formation of zeolites with specific pore topologies. 2,3 On one hand, the introduction of some inorganic elements in the synthesis gel directs the formation of specific small cages in zeolites, named secondary building units (SBUs), as for instance, germanium atoms directing the formation of double-4-rings (D4R) or double-3-rings (D3R). 4 The presence of those small cages do not define the full zeolitic framework topology by themselves, but as we have reported in the last years, the presence of Ge atoms systematically allowed the crystallization of extra-large pore zeolites with pores larger than 10 Å. 5-9 These results are in clear agreement with the predictions performed by Brunner and Meier, who proposed that structures with large amount of 4-and 3-rings in their structure would present low framework densities (FD, measured as T-atoms/1000 Å 3 ), and consequently, large void volume or very open structures. 10On the other hand, it is broadly described in the literature that the size, shape and rigidity of the organic molecules introduced in the synthesis gels acting as structure directing agents (SDAs), clearly influence the pore topology of the crystallized zeolites. 3,11 As general trends, the increase of the size and rigidity of the organic SDA directs towards larger pore openings, while changing from linear to branched organic SDAs permits the synthesis of three-dimensional zeolites. 3,11 The most common organic molecules used in zeolite synthesis as organic SDAs are amines and quaternary ammonium cations, 1,11,12 but very recently, phosphonium and phosphazenes have also been introduced as efficient organic SDAs. 13,14 From all described zeolite topologies, some of the most attractive are those called multipore zeolites, which combine interconnected pores with different apertures. 15 The ability of interconnecting different sized channels allows preferential diffusion of reactants and products through these pores, which may induce unique activities and selectivities by molecular traffic cont...

Locating Organic Guests in Inorganic Host Materials from X-ray Powder Diffraction Data

“…2−5 Indeed, understanding the host−guest interaction of the SDA with the zeolite framework is crucial, if we are to understand why and how these materials form. This fact has fueled efforts to use and improve computational models to predict the host− guest interaction 6 and thereby direct the synthesis, 7 with the ultimate goal of using this information for the rational design of zeolites. 8 It would be beneficial, therefore, if the locations of the organic guest species could also be retrieved experimentally, and X-ray diffraction methods would be ideal for this.…”

“…Several groups have performed large systematic studies on the effect of the charge, shape, size and composition of the SDA on the host–guest interaction. − Indeed, understanding the host–guest interaction of the SDA with the zeolite framework is crucial, if we are to understand why and how these materials form. This fact has fueled efforts to use and improve computational models to predict the host–guest interaction and thereby direct the synthesis, with the ultimate goal of using this information for the rational design of zeolites …”

Could Mesophases Play a Role in the Nucleation and Polymorph Selection of Zeolites?

“…Silica zeolites consist of four-coordinated Si bridged by oxygen atoms . A variety of organic cations, typically tetraalkyammonium, are used as structure-directing agents (SDA) in the synthesis of zeolites, with the goal of producing specific crystal polymorphs. − It has been proposed that the size and shape of SDAs direct the synthesis toward a given polymorph. ,,,− Interestingly, the same SDA can lead to different zeolites under distinct synthetic conditions, and identical zeolites can be obtained with more than one SDA . The molecular mechanism by which cations direct the nucleation toward a specific zeolite structure has not yet been resolved …”

Retrosynthetic Co-Templating Method for the Preparation of Silicoaluminophosphate Molecular Sieves

“…[10][11][12][13][14][15][16][17] The short-range nonbonding interaction energies of these with the oxygen atoms of the framework are found to be crucial in determining which phase crystallises. [18][19][20][21] In most cases, new zeolites have resulted from exploratory crystallisation in the presence of a wide range of OSDAs which are either available commercially or prepared by customised syntheses, with screening of conditions and inorganic gel composition for each OSDA tried.…”

“…There has therefore been a tremendous synthetic effort toward the direct synthesis of novel structures. The most successful approach to new structures, particularly for materials with relatively low framework charge densities, is to use organic structure-directing agents, OSDAs, which are typically alkylammonium cations or amines. − The short-range nonbonding interaction energies of these with the oxygen atoms of the framework are found to be crucial in determining which phase crystallizes. − …”