Diquaternary Ammonium Compounds in Zeolite Synthesis: Cyclic and Polycyclic N-Heterocycles Connected by Methylene Chains

An additional dimension has been added to our long-standing studies in high silica zeolite synthesis via a guest/host synergism. We have created and studied the impact of making symmetric diquaternary ammonium compounds, by varying the chain length between nitrogen charge centers, and the heterocycle size and geometry containing the nitrogen. This allows the introduction of a second spatial parameter in the use of the charged organo-cation guest in the zeolite synthesis. The series of 15 diquaternary ammonium compounds (5 heterocycles synthesized onto chain lengths of C4−C6) were tested in a total of 135 zeolite syntheses reactions. Nine screening reactions were employed for each guest molecule, and the conditions built upon past successes in finding novel high silica zeolites via introduction of boron, aluminum, or germanium as substituting tetrahedral framework atoms for silicon. Eighteen different zeolite structures emerged from the studies. The use of specific chain lengths for derivatives of the pyrrolidine ring system produced novel zeolite materials SSZ-74 and 75

Amplitude death in a ring of nonidentical nonlinear oscillators with unidirectional coupling

We study the collective behaviors in a ring of coupled nonidentical nonlinear oscillators with unidirectional coupling, of which natural frequencies are distributed in a random way. We find the amplitude death phenomena in the case of unidirectional couplings and discuss the differences between the cases of bidirectional and unidirectional couplings. There are three main differences; there exists neither partial amplitude death nor local clustering behavior but oblique line structure which represents directional signal flow on the spatio-temporal patterns in the unidirectional coupling case. The unidirectional coupling has the advantage of easily obtaining global amplitude death in a ring of coupled oscillators with randomly distributed natural frequency. Finally, we explain the results using the eigenvalue analysis of Jacobian matrix at the origin and also discuss the transition of dynamical behavior coming from connection structure as coupling strength increases.Comment: 14 pages, 11 figure

SBE-type metal-substituted aluminophosphates: detemplation and coordination chemistry

The detemplation process in Me-SBE (Me = Co^2+, Mg^2+, and Mn^2+) aluminophosphates was studied to elucidate materials stability and framework characteristics. In addition, the hydrothermal synthesis conditions were optimized to obtain materials with minimal phase impurities. This was accomplished by means of decreasing reaction temperature and increasing aging periods. Scanning electron microscopy analysis of the Mg- and Mn-SBE as-synthesized samples revealed square plates with truncated corner morphologies grown in aggregated fashion and contrasting with the previously reported hexagonal platelike morphology of Co-SBE. Cautious detemplation in vacuum, using an evacuation rate of 10 mmHg/s and a temperature of 648 K, resulted in surface areas of about 700, 500, and 130 m^2/g for Mg-, Co-, and Mn-SBE, respectively. Thermal gravimetric analysis and in situ high-temperature powder X-ray diffraction analyses indicate the frameworks for all of the SBE variants experienced collapse upon treatment with helium at temperatures above 700 K and subsequently formed an aluminophosphate trydimite dense phase. Detemplation in air at all times resulted in framework destruction during detemplation. In situ differential scanning calorimetry−powder X-ray diffraction data showed that the SBE frameworks experience breathing modes related to specific endothermic and exothermic scenarios during air treatment. Decomposition and elimination of the organic template during vacuum treatment was verified by Fourier transform infrared spectroscopy. X-ray photoelectron spectroscopy revealed that most of the Co atoms in vacuum-treated samples are in tetrahedral coordination, while the Mn atoms exhibit various coordination states. Ultraviolet-visible, electron paramagnetic resonance, and magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy corroborated the latter result in addition to providing evidence for the formation of Mn extra framework species. ^(27)Al MAS NMR spectra for vacuum-detemplated Mg-SBE samples prior to and after dehydration confirmed the reversible formation of aluminum octahedral sites. This, however, did not affect the porous nature of detemplated Mg-SBE samples as these are capable of adsorbing 19 water molecules per super cage at 298 K

Reversibility and Improved Hydrogen Release of Magnesium Borohydride

Desorption and subsequent rehydrogenation of Mg(BH_4)_2 with and without 5 mol % TiF_3 and ScCl_3 have been investigated. Temperature programmed desorption (TPD) experiments revealed a significant increase in the rate of desorption as well as the weight percentage of hydrogen released with additives upon heating to 300 °C. Stable Mg(B_xH_y)_n intermediates were formed at 300 °C, whereas MgB_2 was the major product when heated to 600 °C. These samples were then rehydrogenated and subsequently characterized with powder X-ray diffraction (pXRD), Raman, and NMR spectroscopy. We confirmed significant conversion of MgB_2 to fully hydrogenated Mg(BH_4)_2 for the sample with and without additives. TPD and NMR studies revealed that the additives have a significant effect on the reaction pathway during both dehydrogenation and rehydrogenation reactions. This work suggests that the use of additives may provide a valid pathway for improving intrinsic hydrogen storage properties of magnesium borohydride

Optimized Synthesis and Structural Characterization of the Borosilicate MCM-70

A structure analysis of the borosilicate zeolite MCM-70, whose synthesis had been patented in 2003, was reported in 2005. Unfortunately, that structure analysis was somewhat ambiguous. Anisotropic line broadening made it difficult to model the peak shape, some peaks in the electron density map could not be interpreted satisfactorily, the framework geometry was distorted, and MAS NMR results were partially contradictory. In an attempt to resolve some of these points, an optimization of the synthesis was undertaken, and the structure was reinvestigated. The structure was solved from synchrotron powder diffraction data collected on an as-synthesized sample (Pmn2_1, a = 13.3167(1) Å, b = 4.6604(1) Å, c = 8.7000(1) Å) using a powder charge-flipping algorithm. The framework topology, with a 1-dimensional, 10-ring channel system, is identical to the one previously reported. However, the B in this new sample was found to be ordered in the framework, fully occupying one of the four tetrahedral sites. Two extra-framework K^+ ion positions, each coordinated to five framework O atoms and one water molecule, were also found. The solid state ^(29)Si, ^(11)B and ^1H NMR results are fully consistent with this ordered structure

Solid State NMR Characterization of Complex Metal Hydrides systems for Hydrogen Storage Applications

Solid state NMR is widely applied in studies of solid state chemistries for hydrogen storage reactions. Use of ^(11)B MAS NMR in studies of metal borohydrides (BH_4) is mainly focused, revisiting the issue of dodecaborane formation and observation of ^(11)B{^1H} Nuclear Overhauser Effect

Multiple-quantum 13 C solid-state NMR spectroscopy under moderate magic-angle spinning

Abstract A new method for the excitation and detection of high-order 13 C multiple-quantum (MQ) nuclear magnetic resonance (NMR) signals in solids under magic-angle spinning (MAS) condition is presented. In this experimental scheme, high-order 13 C MQ coherences are generated by relaying several sequentially generated 13 C double-quantum (DQ) coherences in dipolar coupled networks. Numerical simulations and experimental implementations are presented to demonstrate the efficiency of this method under a moderate MAS frequency regime. 13 C MQ signals up to 10-quantum coherences are observed experimentally on the model compound 1-13 C-labeled glycine, at an MAS frequency of 12 kHz

Magnesium Borohydride: From Hydrogen Storage to Magnesium Battery

Beyond hydrogen storage: The first example of reversible magnesium deposition/stripping onto/from an inorganic salt was seen for a magnesium borohydride electrolyte. High coulombic efficiency of up to 94 % was achieved in dimethoxyethane solvent. This Mg(BH_4)_2 electrolyte was utilized in a rechargeable magnesium battery

Nonaqueous Fluoride/Chloride Anion-Promoted Delamination of Layered Zeolite Precursors: Synthesis and Characterization of UCB-2

The delamination of layered zeolite precursor PREFER is demonstrated under mild nonaqueous conditions using a mixture of cetyltrimethylammonium bromide, tetrabutylammonium fluoride, and tetrabutylammonium chloride in N,N-dimethylformamide (DMF) as solvent. The delamination proceeds through a swollen material intermediate which is characterized using powder X-ray diffraction (PXRD). Subsequent addition of concentrated HCl at room temperature leads to synthesis of UCB-2 via delamination of the swollen PREFER material and is characterized using PXRD, transmission electron microscopy (TEM), and argon gas physisorption, which shows lack of microporosity in UCB-2. ^(29)Si magic angle spinning (MAS) NMR spectroscopy indicates lack of amorphization during delamination, as indicated by the entire absence of Q^2 resonances, and ^(27)Al MAS NMR spectroscopy shows exclusively tetrahedral aluminum in the framework following delamination. The delamination process requires both chloride and fluoride anions and is sensitive to solvent, working well in DMF. Experiments aimed at synthesizing UCB-2 using aqueous conditions previously used for UCB-1 synthesis leads to partial swelling and lack of delamination upon acidification. A similar lack of delamination is observed upon attempting synthesis of UCB-1 under conditions used for UCB-2 synthesis. The delamination of PREFER is reversible between delaminated and swollen states in the following manner. Treatment of as-made UCB-2 with the same reagents as used here for the swelling of PREFER causes the delaminated UCB-2 material to revert back to swollen PREFER. This causes the delaminated UCB-2 material to revert back to swollen PREFER. Altogether, these results highlight delamination as the reverse of zeolite synthesis and demonstrate the crucial role of noncovalent self-assembly involving the zeolitic framework and cations/anions/structure-directing agent and solvent during the delamination process

Delamination of Layered Zeolite Precursors under Mild Conditions: Synthesis of UCB-1 via Fluoride/Chloride Anion-Promoted Exfoliation

New material UCB-1 is synthesized via the delamination of zeolite precursor MCM-22 (P) at pH 9 using an aqueous solution of cetyltrimethylammonium bromide, tetrabutylammonium fluoride, and tetrabutylammonium chloride at 353 K. Characterization by powder X-ray diffraction, transmission electron microscopy, and nitrogen physisorption at 77 K indicates the same degree of delamination in UCB-1 as previously reported for delaminated zeolite precursors, which require a pH of greater than 13.5 and sonication in order to achieve exfoliation. UCB-1 consists of a high degree of structural integrity via ^(29)Si MAS NMR and Fourier transform infrared spectroscopies, and no detectable formation of amorphous silica phase via transmission electron microscopy. Porosimetry measurements demonstrate a lack of hysteresis in the N_2 adsorption/desorption isotherms and macroporosity in UCB-1. The new method is generalizable to a variety of Si:Al ratios and leads to delaminated zeolite precursor materials lacking amorphization