Structure-directing Agent Molecules Research Articles

Location of 6-azonia-spiro- [5,5]-undecane molecules in ZSM-12 using X-ray synchrotron powder diffraction data

High-silica ZSM-12 (MTW topology), synthesized with 6-azonia-spiro- (Rimer, 2018; Rimer, 2018) [5,5]-undecane as structure directing agent (SDA), was carefully characterized by using X-ray synchrotron powder diffraction. The structure refinement led to a monoclinic unit cell (space group Cc, Z = 4, a = 25.0814(13) Å, b = 15.0987(8) Å, c = 24.4419(17) Å, β = 107.844(6), V = 8810.8(8) Å3) with a three times enlargement of the unit cell along the b axis with respect to the calcined starting ZSM-12 (space group C2/c, Z = 1, a = 24.8633(3) Å, b = 5.01238(7) Å, c = 24.3275(7) Å, β = 107.7215(6)°). The SDAs conformation and position in the framework structure did not maintain the twofold axes coinciding with the crystallographic twofold axis thus changing the space group symmetry from C2/c to Cc. The structure refinement gave an extraframework content 6.13 SDA molecules per unit cell (corresponding to ~9.0 wt%) in good agreement with the weight loss given by the thermogravimetric analysis. The refined bond distances revealed the occurrence of non-bonding interactions of both azonia molecules with the MTW pore wall. AZO-1 and AZO-2 molecules are H-bonded between them thus forming clusters aligned along the b axis. This information is crucial to tailor the physicochemical properties of zeolites as well as to address their hydrophobicity, pore dimensions and cavities thus optimizing the selectivity of zeolite catalysts.

High Ethylene Selectivity in Methanol‐to‐Olefin (MTO) Reaction over MOR‐Zeolite Nanosheets

AbstractPrecisely controlled crystal growth endows zeolites with special textural and catalytic properties. A nanosheet mordenite zeolite with a thickness of ca. 11 nm, named as MOR‐NS, has been prepared using a well‐designed gemini‐type amphiphilic surfactant as bifunctional structure‐directing agent (SDA). Its benzyl diquarternary ammonium cations structurally directed the formation of MOR topology, whereas the long and hydrophobic hexadecyl tailing group prevented the extensive crystal growth along b axis. This kind of orientated crystallization took place through the inorganic–organic interaction between silica species and SDA molecules present in the whole process. The thin MOR nanosheets, with highly exposed (010) planes and 8‐membered ring (MR) windows, exhibited a much improved ethylene selectivity (42.1 %) for methanol‐to‐olefin (MTO) reactions when compared with conventional bulk MOR crystals (3.3 %).

High Ethylene Selectivity in Methanol-to-Olefin (MTO) Reaction over MOR-Zeolite Nanosheets.

Precisely controlled crystal growth endows zeolites with special textural and catalytic properties. A nanosheet mordenite zeolite with a thickness of ca. 11 nm, named as MOR-NS, has been prepared using a well-designed gemini-type amphiphilic surfactant as bifunctional structure-directing agent (SDA). Its benzyl diquarternary ammonium cations structurally directed the formation of MOR topology, whereas the long and hydrophobic hexadecyl tailing group prevented the extensive crystal growth along b axis. This kind of orientated crystallization took place through the inorganic-organic interaction between silica species and SDA molecules present in the whole process. The thin MOR nanosheets, with highly exposed (010) planes and 8-membered ring (MR) windows, exhibited a much improved ethylene selectivity (42.1 %) for methanol-to-olefin (MTO) reactions when compared with conventional bulk MOR crystals (3.3 %).

Three-Dimensional Open-Framework Germanate Built from a Novel Ge13 Cluster and Containing Two Types of Chiral Layers

A new open-framework germanate [Ge15O30(OH)4]·2(H2tren), denoted SU-69, was synthesized under hydrothermal conditions with tris(2-aminoethyl)-amine (tren) as a structure directing agent (SDA). SU-69 crystallizes in a monoclinic space group (C2/c, No. 15) with a = 20.2656(7) A, b = 11.6250(4) A, c = 18.5602(10) A, and β = 90.528(4)°. The framework of SU-69 is built from a novel Ge13O27(OH)2 (Ge13) cluster with two additional GeO3(OH) tetrahedra. Two types of chiral 3,6-net building layers are found in the framework, which alternately stack and connect to form a three-dimensional achiral framework with a two-dimensional 10 × 12-ring channel system. The SDA molecules interact with the framework via H-bonds. The thermal stability of as-synthesized SU-69 has also been investigated.

Crystallization Mechanism of Cage-Based, Small-Pore Molecular Sieves: A Case Study of CHA and LEV Structures

Here we have investigated the crystallization mechanisms of SSZ-13 and SAPO-34 with the CHA topology and NU-3 and SAPO-35 with the LEV topology using 1H–13C CP MAS NMR and IR spectroscopies. The nucleation of these cage-based, small-pore molecular sieves begins with the formation of large 20-hedral cha or 17-hedral lev cages, with incorporation of organic structure-directing agents (SDAs) alone or together with inorganic cations, in both aluminosilicate and silicoaluminophosphate compositions. The next two steps are the construction of multiple-cha or multiple-lev cages in an appropriate arrangement by sharing 8-rings and their subsequent coupling to form smaller 8-hedral double 6-ring units, leading to viable nuclei of CHA or LEV molecular sieves. The initial formation of the large cages, especially in the presence of large SDA molecules, can in our view be generalized to other cage-based zeolite systems.

Multi-scale theory and simulation of shape-selective nanocrystal growth

Capping agents, or structure-directing agents (SDAs), play a central role in the shape-selective, solution-phase synthesis of metal nanocrystals – although their exact role in many syntheses remains elusive. In this Highlight, we discuss, using the example of PVP-mediated synthesis of Ag nanocrystals in the polyol process, how multi-scale theory and simulation can lead to an understanding of how SDAs actuate nanocrystal growth. First-principles DFT calculations provide insight into the adsorption energetics of SDA molecules on metal surfaces and a comparison of DFT to experiment confirms the adage that SDA molecules promote nanocrystals with facets to which they bind the most strongly. To study aspects of solution-phase syntheses under experimental conditions, we introduced the metal–organic many body (MOMB) force field, which has high fidelity to DFT results. Using molecular dynamics simulations based on the MOMB force field, we showed how stronger PVP binding to Ag(100) leads to a lower Ag flux to this surface relative to Ag(111) and how this promotes {100}-facetted kinetic Wulff shapes, consistent with experiment.

Hydrophobicity enhancement of Ti-MWW catalyst and its improvement in oxidation activity

The effect of the preparation conditions of the Ti-MWW catalyst on the hydrophobicity of the catalyst and its catalytic activity was investigated. Our findings demonstrated that the condensation of interlayer hydroxyl groups was greatly affected by the preparation conditions, in particular washing conditions of the as-synthesized lamellar precursor of Ti-MWW, then further controlling the final hydrophobicity and oxidation properties. Using organic solvents, especially EAOH, instead of water to wash the wet lamellar precursor would synchronize the interlayer hydroxyl condensation with the decrease of interlayer distance mainly caused by the leaching of piperidine, which was used as structure-directing agent (SDA). After acid-treatment, less SDA was kept in the EAOH washed sample and 3D-MWW with less defects was formed by calcination. Both drying temperature and acid-treatment would also affect the amount of SDA occluded in the interlayer void space of the acid-treated samples and then further affect the final interlayer hydroxyl condensation upon the following calcination. The lower both drying temperature and acid-treatment temperature were favorite to unequal interlayer dehydroxylation to form MCM-56, while higher drying temperature such as 150°C not only caused the anatase phase in the calcined samples but also occluded more SDA molecules in the acid-treated samples which greatly affect the further interlayer hydroxyl condensation upon the calcination. Ti-MWW-OH-100 containing smallest amount of silanols and less defect sites showed the best hydrophobicity and the highest catalytic activity in 1-hexene oxidation.

Structure analysis of a BEC-type germanosilicate zeolite including the location of the flexible organic cations in the channels

A BEC-type zeolite (polymorph C of zeolite beta) with low germanium content (Si : Ge = 5.1) has been synthesised using the flexible linear diquaternary cationic form of pentamethyldiethylenetriamine (2,2′-(methylazanediyl)bis(N,N,N-trimethylethanammonium) as the organic structure-directing agent (SDA). The distribution of germanium within the framework and the location of the SDA within the pores have been determined by analysing synchrotron X-ray powder diffraction data collected on the as-synthesised form. The findings, which are corroborated by 13C and 19F NMR data, indicate that the structure of the SDA is retained during the synthesis and that the Ge atoms occupy only sites in the double 4-ring. Although the BEC framework structure has a three-dimensional 12-ring channel system, the SDA molecules are found to orient themselves in two dimensions along the channels running parallel to the a and b axes.

Multilayer structured MFI-type titanosilicate: Synthesis and catalytic properties in selective epoxidation of bulky molecules

A lamellar titanosilicate (LTS-1) was hydrothermally synthesized by employing a bifunctional surfactant as the structure-directing agent (SDA). Highly crystalline LTS-1 was obtained at an optimal SDA/Si molar ratio of 0.04. As-synthesized LTS-1 possessed a multilayer structure, which was constructed from a collection of 2-nm zeolite nanosheets and interlayer SDA molecules. Removing the intercalated organic species induced irregular layer stacking to a certain extent, leading to intracrystal mesopores of ca. 3.2nm in diameter. The catalytic performance of LTS-1 was investigated in the epoxidation of various bulky alkenes with tert-butyl hydroperoxide, cumene hydroperoxide, or aqueous H2O2. LTS-1 was more active than conventional titanosilicates for reactions involving bulky alkenes and oxidants, and it was immune to Ti leaching and irreversible deactivation.

Molecular Modeling, Multinuclear NMR, and Diffraction Studies in the Templated Synthesis and Characterization of the Aluminophosphate Molecular Sieve STA-2

Molecular modeling has been used to assist in the design of a new structure directing agent (SDA) for the synthesis of the AlPO(4) form of STA-2, bis-cliazabicyclooctane-butane (BDAB). This is incorporated as a divalent cation within the large cages of STA-2, as determined via a combination of solid-state (13)C and (15)N MAS NMR, supported by (14)N and (1)H-(15)N HMQC solution NMR and density functional calculations. As prepared AlPO4 STA-2 containing cationic SDA molecules achieves neutrality by the inclusion of hydroxide ions bridging between 5-fold coordinated framework Al atoms. Synchrotron X-ray powder diffraction data of the dehydrated as-prepared form indicates triclinic symmetry (Al(12)P(12)O(48)(OH)(2)center dot BDAB, P1, a = 12.3821(2) angstrom, b = 12.3795(2) angstrom, c = 12.3797(3) angstrom, alpha = 63.3585(8)degrees, 63.4830(7)degrees, gamma = 63.4218(7)) with the distortion from rhombohedral R (3) over bar symmetry resulting from the partial order of hydroxide ions in bridging Al-OH-Al sites within cancrinite cages. Upon calcination in oxygen, the organic SDA is removed, leaving AlPO(4) STA-2 with a pore volume of 0.22 cm(3) g(-1) (R (3) over bar, Al(36)P(36)O(144), a = 12.9270(2) angstrom, c = 30.7976(4) angstrom). Dehydrated calcined AlPO(4) STA-2 has two crystallographically distinct P and Al sites: (31)P MAS NMR resolves the two distinct P sites, and although 27AI MAS NMR only partially resolves the two Al sites, they are separated by MQMAS. Furthermore, 2D (27)Al -> (31)P MQ-J-HETCOR correlation spectroscopy confirms that each framework Al is linked to the two different P sites via AI-O-P connections in a 3:1 ratio (and vice versa for P linked to different Al). The (27)Al and (31)P resonances are assigned to the crystallographic Al and P sites by calculation of the NMR parameters using the CASTEP DFT program for an energy-minimized AlPO(4)(SAT) framework.

Synthesis, characterization and enhanced photoconductivity from a mesoporous titania on dye doping

New wormhole-like mesoporous TiO 2 material has been synthesized through a convenient sol–gel method in the presence of a Schiff base secondary amine hexadecyl-2-pyrrole-methylamine (HPMA) containing chelating donor sites as template or structure directing agent (SDA). SDA molecules can be easily removed from the composite to generate mesoporosity and upon removal of the SDA molecule, this mesoporous TiO 2 material showed very high surface area (480 ± 10 m 2/g) with an average pore diameter of 2.57 ± 0.05 nm. When Rose Bengal dye is entrapped inside the nanopores of this material, it showed a drastic enhancement ( ca. 40-folds) in the photoconductivity vis-à-vis mesoporous TiO 2 alone under white light illumination.

Effect of chemical and thermal environments on structures of mesoporous silica formed in aerosol phase

Spheres of mesoporous silica were prepared in aerosol phase of acidic TEOS sols having structure-directing agents (SDA) such as cetyltrimethylammonium bromide (CTAB) and Pluronic P-123. Their structural characteristics were investigated by systematically varying the chemical and thermal conditions of preparation with TEM, FT-IR, BET and SAXS. Both SDAs produced well-assembled 2-D hexagonal structures. Alcoholic solvent left better order with smaller d values than aqueous one. The upper SDA concentration for mesoporosity was much lower in the former than the latter. To get the highest order, it was important to obtain the complete self-assembly of SDA molecules without any deterioration prior to calcination. Well self-assembled mesophase appeared for the product spray dried at 100 °C and subsequently calcined at 400 °C. One-hour calcination yielded the best order. The number of domains increased with the calcination period up to 5 hrs and then decreased in further extended period. CTAB was bonded to silica surface to preserve the affinity to OH after it disappeared while P123 was not the case.

Structure Directing Effect of (1S,2S)-2-Hydroxymethyl-1-benzyl-1-methylpyrrolidinium in the Synthesis of AlPO-5

A pure diastereoisomer of a new chiral organic molecule with two asymmetric atoms, (1S, 2S)-2-hydroxymethyl-1-benzyl-1-methylpyrrolidinium, has been prepared and used as structure directing agent (SDA) for the crystallization of the one-dimensional AlPO-5 microporous aluminophosphate. Our main objective is to induce chiral supramolecular arrangements of the SDA molecules within the microporous structure. The SDA molecules can be arranged as monomers or dimers within the nanochannels of AlPO-5, the aggregation being driven by π−π type interactions between the aromatic rings. It has been found that increasing the crystallization time leads to a higher organic content (and lower water incorporation) together with a higher formation of dimers, which indicates that the incorporation of the organic molecules as dimers is thermodynamically favored. We also observe a partially reversible photoinduced rearrangement of the SDAs; the aggregation is enhanced by exciting the occluded monomers with the appropriate wave...

Soft-templating approach for the synthesis of high surface area and superparamagnetic mesoporous iron oxide materials

Designing pores of nanoscale dimension into a magnetic material can engineer the surface nanostructure, resulting in a mesoporous material with unique physicochemical properties. Here we report a simple soft-templating approach for the synthesis of new mesoporous iron oxide materials having semi-crystalline pore wall by using an anionic surfactant sodiumdodecylsulfate as the structure-directing agent (SDA) or template at low temperature. On removal of the SDA molecules through solvent extraction the mesostructure is preserved and the material shows considerably high BET surface area and type IV isotherms corresponding to the mesopores. This mesoporous Fe 2O 3 material showed superparamagnetic behavior and hence this template-assisted synthesis of mesoporous Fe 2O 3 can find its potential utility in designing magnetic nanostructured materials.

Preparation and characterization of SSZ-54: A family of MTT/ TON intergrowth materials

In this paper we report two different methods to prepare zeolites with framework structures that are intergrowths of the MTT and TON topologies. In one route we use a single structure-directing agent ( N-isopropylethylenediamine) in hydroxide mediated syntheses. The second method uses a mixture of structure-directing agent (SDA) molecules to prepare the MTT/ TON intergrowths. In this case, one molecule alone is selective for MTT-type zeolites and the other molecule alone is selective for TON-type zeolites. By varying the ratio of the individual molecules in the syntheses, we are able to systematically control the MTT or TON character of the intergrowth product. We find that one of the SDA molecules often has a stronger structure-directing or templating effect than the other molecule in the pair. For example, for the system involving mixtures of N-isopropyl-1,3-propanediamine and 1-methylbutylamine, the ratio of N-isopropyl-1,3-propanediamine:1-methylbutylamine in the gel must be below 0.25 before intergrowths are observed; otherwise, an MTT product is observed. We refer to this ability of one SDA molecule to dominate the phase selectivity (in a mixture) as a “dominant templating effect.” We have characterized the structural nature of the MTT/ TON intergrowths by a combination of techniques: comparison of experimental powder diffraction patterns with DIFFaX simulations, observations of systematic peak broadening in the experimental diffraction patterns, comparison of experimental diffraction patterns with those of small-crystallite samples of SSZ-32 ( MTT), and direct observation of stacking faults by high resolution transmission electron microscopy.

Molecular Insights into the Self-Aggregation of Aromatic Molecules in the Synthesis of Nanoporous Aluminophosphates: A Multilevel Approach

Fluorescence spectroscopy and a range of computer simulation techniques are used to study the structure directing effect of benzylpyrrolidine (BP) and (S)-(-)-N-benzylpyrrolidine-2-methanol (BPM) in the synthesis of nanoporous aluminophosphate frameworks with AFI (one-dimensional channels) and SAO (three-dimensional interconnected channels) topologies. We study the supramolecular chemistry of BP and BPM molecules in aqueous solution and compare it with the aggregation state of the molecules found when they are inside the AlPO nanopores after crystallization. The aggregation of the molecules within the structures can be explained by a combination of thermodynamic and kinetic effects. The former are given by the stability of the molecular species interacting with the oxide networks relative to their stability in solution; the latter depend on the aggregation behavior of the molecules in the synthesis gels prior to crystallization. Whereas BPM only forms one type of aggregate in solution, which has the appropriate conformation to match the empty channels of the forming nanoporous frameworks, BP forms aggregates with different molecular orientations, of which only one matches the framework interstices. This different supramolecular chemistry, together with the higher interaction of BPM with the oxide networks, makes BPM a better structure directing agent (SDA); it is also responsible for the higher incorporation of BPM as dimers in the frameworks, especially in the AFI structure, observed experimentally. The concentration of the SDA molecules in the gels, and so the density per volume of the SDAs, determines the exclusion zone from which the pores and/or cavities of the framework will arise, and so the porous network of the formed material. A clear relationship between the SDA density in solution and in the framework is observed, thus enabling an eventual control of the material density by adjusting the SDA concentration in the gels. The topological instability intrinsic to these open framework structures is compensated by a high host-guest interaction energy; the SAO topology is further stabilized by doping with Zn. Our computational results account for and rationalize all the effects observed experimentally, providing a complete picture of the mode of structure direction of these aromatic molecules in the synthesis of nanoporous aluminophosphates.

Template-controlled acidity and catalytic activity of ferrierite crystals

A synthesis strategy to tailor the acid sites location in ferrierite crystals has been developed. The zeolite catalysts were synthesised in fluoride medium using different combinations of organic structure directing agents (SDAs) in the absence of inorganic cations. Therefore, the negative charge associated to the incorporation of aluminium to the framework was compensated exclusively by the positive charge of the organic SDAs. In this way, Al sitting in the zeolite framework was driven by the specific location of the different SDA molecules within the zeolite void volume. Following this synthesis strategy, it has been found that the distribution of strongly acidic hydroxyl groups in the proton form of the zeolites obtained after removal of the organic templates was dependent on the combination of organic molecules used as SDAs. Moreover, the catalytic activity of the zeolites in m-xylene and 1-butene isomerisation increased as the relative population of strong Brönsted acid groups in sterically constrained sites inside the ferrierite cavity decreased.

Tailoring the acid strength of microporous silicoaluminophosphates through the use of mixtures of templates: Control of the silicon incorporation mechanism

SAPO-5 samples have been synthesized with triethylamine (TEA), benzylpyrrolidine (BP) and mixtures of them as structure directing agents (SDAs). It has been observed that in the as-synthesised materials the concentration of SDAs (molecules per unit cell) and the Si content are similar. According to the different molecular size of both SDAs, the samples exhibit higher organic weight and lower water content as the molar fraction of BP in the synthesis gel increases. These differences in selectivity for organic/water incorporation influences the mechanism of Si substitution in the AlPO lattice: the higher organic content obtained at higher BP/TEA ratio leads to the formation of large Si islands, while the higher water content of the sample obtained with TEA enhances the formation of isolated Si(OAl) 4 environments. Interestingly, it has been found that these two opposite trends can be tailored to a certain point by using mixtures of both SDAs, TEA and BP, in the required ratio. The catalytic activity of the samples has been tested in the isomerisation of m-xylene. It has been observed a higher activity of samples obtained with BP as the main SDA, thus evidencing the better performance of large Si islands in this reaction.