To boost function-led discovery of new zeolites with desired pores and properties, millions of hypothetical zeolite structures have been predicted via various computational approaches. It is now well accepted that most of these predicted structures are experimentally unrealisable under conventional synthetic conditions. Many structure evaluation criteria have been proposed to screen out unfeasible structures, among which the framework density–framework energy correlation criterion and the local interatomic distances criteria are the most frequently used. However, many hypothetical structures passing these criteria have been found unfeasible because of the existence of highly distorted framework rings. Here, we propose a new set of structure evaluation criteria to screen out such unfeasible structures. After optimising all existing zeolite structures as silica polymorphs, we find that the closest non-adjacent O⋯O distances in existing zeolite rings generally show a normal distribution. By comparing the closest non-adjacent O⋯O distances between existing and hypothetical zeolite structures, we are able to screen out many unfeasible hypothetical zeolite structures with distorted rings that are deemed feasible according to previous structure evaluation methods.

In silico prediction of potential synthetic targets is the prerequisite for function-led discovery of new zeolites. Millions of hypothetical zeolitic structures have been predicted via various computational methods, but most of them are experimentally inaccessible under conventional synthetic conditions. Screening out unfeasible structures is crucial for the selection of synthetic targets with desired functions. The local interatomic distance (LID) criteria are a set of structure rules strictly obeyed by all existing zeolite framework types. Using these criteria, many unfeasible hypothetical structures have been detected. However, to calculate their LIDs, all hypothetical structures need to be fully optimized without symmetry constraints. When evaluating a large number of hypothetical structures, such calculations may become too computationally expensive due to the forbiddingly high degree of freedom. Here, we propose calculating LIDs among structures optimized with symmetry constraints and using them as new structure evaluation criteria, i.e., the LIDsym criteria, to screen out unfeasible hypothetical structures. We find that the LIDsym criteria can detect unfeasible structures as many as the original non-symmetric LID criteria do, yet require at least one order of magnitude less computation at the initial geometry optimization stage.The symmetric LIDsym criteria detect unfeasible hypothetical zeolite structures one order of magnitude faster than the original non-symmetric LID criteria.Download high-res image (174KB)Download full-size image

AbstractThe side-chain alkylation of toluene with methanol over alkali-cation-containing zeolite Y is an important reaction for industrial production of styrene, but the exact mechanism of this reaction is still unclear. The most accepted opinion is that the Lewis acid–base sites in zeolite Y activate the transformation from methanol to formaldehyde, the side-chain alkylation of toluene with formaldehyde, and the formation of 2-phenylethanol and styrene afterwards. In this study, we investigate the roles of various types of hydroxyl groups that could possibly exist in zeolite Na-Y during this reaction, including the Brönsted acid sites and the terminal Al—OH and Si—OH groups, respectively. Through density functional theory (DFT) calculations, we find that the Brönsted acid sites in Na-Y may catalyze the ring alkylation of toluene and be responsible for the formation of xylene, a side product discovered in experiments. More importantly, we find, for the first time, a new reaction pathway from 2-phenylethanol to styrene over various types of hydroxyl groups in Na-Y, which is kinetically more favorable than the conventional pathway. According to our calculation results, the most possible mechanism for this styrene production process may involve reactions over both the Lewis acid–base sites and the hydroxyl groups in Na-Y.

The confinement effect of zeolite cavities on the methanol-to-olefin (MTO) conversion is investigated through density functional theory calculations. According to the side-chain mechanism, we select several hydrocarbon pool (HP) intermediates that may exist during the MTO conversion process and optimize their structures within the cluster models of zeolite cavities cha, lev, and lta, respectively. The transition states during methylation, deprotonation, methyl shift, and olefin production are also located within these cavities. According to our results, all of the HP intermediates are stabilized in zeolite cavities, especially in cha and lta. Moreover, the cha cavity displays the lowest intrinsic free-energy barriers for all of the methylation and olefin-production steps, indicating its high MTO catalytic activity. We find that the differences in reaction barriers and reaction energies are highly related to the different confinement effects of zeolite cavities. In comparison with lev and lta, the cha cavity matches the dimensions of HP species very well, so it is able to provide the most suitable confinement to HP species. Our discovery will provide further understanding of the side-chain mechanism, which is important for finding new catalysts for MTO conversion.

Two divalent-metal-containing aluminophosphates, (C5H14N2)[Co2Al4P6O24] and (C5H14N2)[Zn2Al4P6O24] (denoted as MAPO-CJ62; M = Co, Zn), have been hydrothermally synthesized by using N-methylpiperazine as the structure directing agent. Their structures are determined by single crystal X-ray diffraction and further characterized by powder X-ray diffraction, inductively coupled plasma, and thermogravimetric and diffuse reflectance spectroscopy analyses. Both of these two compounds exhibit a new zeolite framework topology. This new zeolite framework contains 1-dimensional 8-ring channels running along the [010] direction. All the metal and P atoms are tetrahedrally coordinated and alternately connected to each other through bridging O atoms. Inductively coupled plasma analysis shows that the molar ratio of M:Al in MAPO-CJ62 is 1:2. The M2+ ions in MAPO-CJ62 selectively occupy two of the three possible crystallographically distinct positions. A pure aluminophosphate analogue of MAPO-CJ62 without M2+-incorporation, denoted as AlPO-CJ62, has not been obtained in our experiment so far. The necessity of introducing M2+ ions and their ordered distribution in MAPO-CJ62 has been elucidated by analyzing the distortions of Al-centered tetrahedra in the hypothetical framework of AlPO-CJ62.

A series of zeolitic alumino- and gallogermanate compounds, |M(II)(en)3|[M(III)2Ge4O12] (M(II) = Ni, Co, Zn; M(III) = Al, Ga; en = ethylenediamine), has been synthesized using in situ formed [M(II)(en)3]2+ cations as the structure-directing agents. These zeolitic compounds exhibit the same JST framework topology which is built exclusively of 3-rings. Their structures were determined by single-crystal X-ray diffraction. The frameworks of these compounds are constructed from M(III)- and Ge-centred tetrahedra. According to inductively coupled plasma (ICP) analyses, the ratios of M(III)/Ge are 1/2 in all these compounds. The resulting negative charges in these frameworks are compensated by extra-framework [M(II)(en)3]2+ cations. Single crystal structural analyses show that there are two crystallographically distinct atom sites for M3+. According to the ICP results and the unique structural feature of JST, the most reasonable distribution of M3+ cations in these structures was deduced.

Two heteroatom-containing open-framework aluminophosphates, (C3H4N2)2FeAl3P4O16 and (C3H4N2)2CrAl3P4O16 (denoted as MAPO-CJ50, M = Fe, Cr), have been synthesized by using imidazole as the template under solvothermal conditions. The structure of FeAPO-CJ50 is determined by single-crystal X-ray diffraction, and its analogous structure CrAPO-CJ50 is identified by powder X-ray diffraction. The 3-dimensional framework of MAPO-CJ50, constructed by MO4N2 octahedra, AlO4 tetrahedra, and PO4 tetrahedra, contains interconnecting 10- and 8-ring channels. Imidazole molecules are coordinated with framework M3+ ions and interact with each other through π–π stacking interactions in the channels. These two compounds show photoluminescent properties due to ligand-to-metal charge transfer. Magnetic measurements reveal that there are antiferromagnetic interactions between M3+ ions in the frameworks of FeAPO-CJ50 and CrAPO-CJ50.

To boost function-led discovery of new zeolites with desired pores and properties, millions of hypothetical zeolite structures have been predicted via various computational approaches. It is now well accepted that most of these predicted structures are experimentally unrealisable under conventional synthetic conditions. Many structure evaluation criteria have been proposed to screen out unfeasible structures, among which the framework density–framework energy correlation criterion and the local interatomic distances criteria are the most frequently used. However, many hypothetical structures passing these criteria have been found unfeasible because of the existence of highly distorted framework rings. Here, we propose a new set of structure evaluation criteria to screen out such unfeasible structures. After optimising all existing zeolite structures as silica polymorphs, we find that the closest non-adjacent O⋯O distances in existing zeolite rings generally show a normal distribution. By comparing the closest non-adjacent O⋯O distances between existing and hypothetical zeolite structures, we are able to screen out many unfeasible hypothetical zeolite structures with distorted rings that are deemed feasible according to previous structure evaluation methods.

Two heteroatom-containing open-framework aluminophosphates, (C3H4N2)2FeAl3P4O16 and (C3H4N2)2CrAl3P4O16 (denoted as MAPO-CJ50, M = Fe, Cr), have been synthesized by using imidazole as the template under solvothermal conditions. The structure of FeAPO-CJ50 is determined by single-crystal X-ray diffraction, and its analogous structure CrAPO-CJ50 is identified by powder X-ray diffraction. The 3-dimensional framework of MAPO-CJ50, constructed by MO4N2 octahedra, AlO4 tetrahedra, and PO4 tetrahedra, contains interconnecting 10- and 8-ring channels. Imidazole molecules are coordinated with framework M3+ ions and interact with each other through π–π stacking interactions in the channels. These two compounds show photoluminescent properties due to ligand-to-metal charge transfer. Magnetic measurements reveal that there are antiferromagnetic interactions between M3+ ions in the frameworks of FeAPO-CJ50 and CrAPO-CJ50.

A series of zeolitic alumino- and gallogermanate compounds, |M(II)(en)3|[M(III)2Ge4O12] (M(II) = Ni, Co, Zn; M(III) = Al, Ga; en = ethylenediamine), has been synthesized using in situ formed [M(II)(en)3]2+ cations as the structure-directing agents. These zeolitic compounds exhibit the same JST framework topology which is built exclusively of 3-rings. Their structures were determined by single-crystal X-ray diffraction. The frameworks of these compounds are constructed from M(III)- and Ge-centred tetrahedra. According to inductively coupled plasma (ICP) analyses, the ratios of M(III)/Ge are 1/2 in all these compounds. The resulting negative charges in these frameworks are compensated by extra-framework [M(II)(en)3]2+ cations. Single crystal structural analyses show that there are two crystallographically distinct atom sites for M3+. According to the ICP results and the unique structural feature of JST, the most reasonable distribution of M3+ cations in these structures was deduced.