Direct Detection of Paired Aluminum Heteroatoms in Chabazite Zeolite Catalysts and Their Significance for Methanol Dehydration Reactivity.

The distributions of heteroatoms within zeolite frameworks have important influences on the locations of exchangeable cations, which account for the diverse adsorption and reaction properties of zeolite catalysts. In particular for aluminosilicate zeolites, paired configurations of aluminum atoms separated by one or two tetrahedrally coordinated silicon atoms are important for charge-balancing pairs of H cations, which are active for methanol dehydration, or divalent metal cations, such as Cu, which selectively catalyze the reduction of NO, both technologically important reactions. Such paired heteroatom configurations, however, are challenging to detect and probe, due to the typically nonstoichiometric compositions and nonperiodic distributions of aluminum atoms within aluminosilicate zeolite frameworks.

Cooperative and Competitive Occlusion of Organic and Inorganic Structure-Directing Agents within Chabazite Zeolites Influences Their Aluminum Arrangement.

We combine experiment and theory to investigate the cooperation or competition between organic and inorganic structure-directing agents (SDAs) for occupancy within microporous voids of chabazite (CHA) zeolites and to rationalize the effects of SDA siting on biasing the framework Al arrangement (Al-O(-Si-O)-Al, = 1-3) among CHA zeolites of essentially fixed composition (Si/Al = 15). CHA zeolites crystallized using mixtures of TMAda and Na contain one TMAda occluded per cage and Na co-occluded in an amount linearly proportional to the number of 6-MR paired Al sites, quantified by Co titration. In contrast, CHA zeolites crystallized using mixtures of TMAda and K provide evidence that three K cations, on average, displace one TMAda from occupying a cage and contain predominantly 6-MR isolated Al sites.

Ion distribution in copper exchanged zeolites by using Si-29 spin lattice relaxation analysis.

Transition metal-containing zeolites, particularly those with smaller pore size, have found extensive application in the selective catalytic reduction (SCR) of environmental pollutants containing nitrogen oxides. We report these zeolites have dramatically faster silicon-29 (Si-29) spin lattice relaxation times (T1) compared to their sodium-containing counterparts. Paramagnetic doping allows one to acquire Si-29 MAS spectra in the order of tens of seconds without significantly affecting the spectral resolution.

Mixed domain models for the distribution of aluminum in high silica zeolite SSZ-13.

High silica zeolite SSZ-13 with Si/Al ratios varying from 11 to 17 was characterized by aluminum-27 and silicon-29 NMR spectroscopy. Aluminum-27 MAS and MQMAS NMR data indicated that in addition to tetrahedral aluminum sites, a fraction of aluminum sites are present in distorted tetrahedral environments. Although in samples of SSZ-13 having high Si/Al ratios all aluminum atoms are expected to be isolated, silicon-29 NMR spectra revealed that in addition to isolated aluminum atoms (Si(1Al)), non-isolated aluminum atoms (Si(2Al)) exist in the crystals.

A combined 17O RAPT and MQ-MAS NMR study of L-leucine.

We report the application of rotor-assisted population transfer (RAPT) to measure the quadrupolar coupling constant (C(q)) for spin 5/2 nuclei. Results from numerical simulations are presented on the magnitude of enhancement factor as a function of frequency offsets, i.e.

Enhancing sensitivity of quadrupolar nuclei in solid-state NMR with multiple rotor assisted population transfers.

Rotor-assisted population transfer (RAPT) was developed as a method for enhancing MAS NMR sensitivity of quadrupolar nuclei by transferring polarization associated with satellite transitions to the central m=12-->-12 transition. After a single RAPT transfer, there still remains polarization in the satellite transitions that can be transferred to the central transition. This polarization is available without having to wait for the spin system to return to thermal equilibrium.

Selective suppression and excitation of solid-state NMR resonances based on quadrupole coupling constants.

The dependence of the (Rotor Assisted Population Transfer) RAPT enhancement on offset frequency for nuclei experiencing different quadrupolar couplings has been exploited to design two new spectral editing schemes, pi/2-RAPT and RAPT-pi-RAPT, for the selective excitation or suppression, respectively, of nuclei with large quadrupolar couplings. Both approaches are demonstrated on the 87 Rb spectrum of Rb(2)SO(4), which contains two resonances with C(q) values of 2.6 and 5.

A simple technique for determining nuclear quadrupole coupling constants with RAPT solid-state NMR spectroscopy.

An enhanced Rotor Assisted Population Transfer (RAPT) experiment is presented and used as a simple and fast technique to measure the magnitude of the nuclear quadrupolar coupling constant of half-integer quadrupolar nuclei. The enhanced RAPT sequence consists of a train of Gaussian pulses with alternating off-resonant frequencies of +/-nuoff. Simulated and experimental results demonstrating the method are given in the case of 87Rb (spin 3/2) and 27Al (spin 5/2) nuclei.