Daniel F. Shantz

Cation-induced transformation of boron-coordination in zeolites

The coordination of boron atoms and their association with counterions in dehydrated zeolites B-ZSM-5, B-Beta, and B-SSZ-24 have been studied by solid state NMR methods. 11B MAS NMR spectra show that boron occurs in both trigonal (B[3]) and tetrahedral (B[4]) coordination in the zeolite framework. The isotropic 11B chemical shift (δcs) allows one to distinguish between trigonal extra-framework boron species (δcs=18–19 ppm), Bnf[3], and trigonal framework boron (δcs=9.8–10.7 ppm), Bf[3]. Extra-framework boron species can be avoided if the zeolites are completely dehydrated with care. Large counterions such as sodium, lithium and ammonium ions stabilise boron in tetrahedral coordination, whereas protons cause a transformation to trigonal boron in the framework. 11B{23Na} and 11B{1H} rotational echo double resonance (REDOR) NMR reveals that B[4] is selectively associated with sodium ions, and Bf[3] is associated with protons in mixed H,Na zeolites. 1H{11B} REDOR experiments allow one to distinguish between protons in surface or defect silanols (δ=1.7–1.8 ppm) and protons close to Bf[3] (δ=1.9–2.0, 2.2–2.4, 2.7–3.0 ppm).

Guest-host interactions in zeolites as studied by NMR spectroscopy: implications in synthesis, catalysis and separations

The intermolecular forces between occluded organic molecules and the atoms that bound the pores of zeolite structures determine the catalytic activity and diffusivity of the guest molecules. These organic-inorganic forces are also a crucial aspect of the process of structure-direction in zeolite synthesis. A molecular understanding of guest-host interactions can often be deduced by using NMR spectroscopy to study the rotational motion of the occluded molecules. This contribution is an overview of systems where NMR, particularly deuterium NMR, has been used to quantify the forces which exist between occluded organic molecules and the zeolite structure. Systems that are relevant to synthesis, separations, and catalysis are discussed. The works presented show that NMR is a powerful tool for studying guest-host interactions in zeolitic materials. A molecular level understanding of the nature and relative importance of the various forces discussed should lead to a more rational basis for the selection and development of zeolitic materials used in catalytic and adsorption applications.

Phenylboronic Acid Functionalized SBA-15 for Sugar Capture

The synthesis and characterization of organic-inorganic hybrid materials that selectively capture sugars from model biomass hydrolysis mixtures are reported. 3-Aminophenylboronic acid (PBA) groups that can reversibly form cyclic esters with 1,2-diols, and 1,3-diols including sugars are attached to mesoporous SBA-15 via different synthetic protocols. In the first route, a coupling agent is used to link PBA and SBA-15, while in the second route poly(acrylic acid) brushes are first grafted from the surface of SBA-15 by surface-initiated atom transfer radical polymerization and PBA is then immobilized. The changes in pore structure, porosity, and pore size due to the loading of organic content are measured by powder X-ray diffraction and nitrogen porosimetry. The increase in organic content after each synthesis step is monitored by thermal gravimetric analysis. Fourier transform infrared spectroscopy and elemental analysis are used to characterize the chemical compositions of the hybrid materials synthesized. D-(+)-Glucose and D-(+)-xylose, being the most commonly present sugars in biomass, are chosen to evaluate the sugar adsorption capacity of the hybrid materials. It is found that the sugar adsorption capacity is determined by the loading of boronic acid groups on the hybrid materials, and the hybrid material synthesized via route two is much better than that through route one for sugar adsorption. Mathematical modeling of the adsorption data indicates that the Langmuir model best describes the sugar adsorption behavior of the hybrid material synthesized through route one, while the Freundlich model fits the data most satisfactorily for the hybrid material prepared via route two. The adsorption kinetics, reusability, and selectivity toward some typical chemicals in cellulose acidic hydrolysis mixtures are also investigated.

Specific Ion Effects on Nanoparticle Stability and Organocation- Particle Interactions in Tetraalkylammonium-Silica Mixtures

Tetramethylammonium (TMA)- and tetrapropylammonium (TPA)-silica mixtures containing monovalent salts were studied to determine how salt impacts nanoparticle stability and organocation-silica interactions. Dynamic light scattering (DLS) results show that salt concentrations as low as 5 mM can induce nanoparticle aggregation. The extent of aggregation increases with the ionic size of the alkali-metal cations, consistent with the Hoffmeister series. Thus specific ion effects are observed in these mixtures. Pulsed-field gradient (PFG) NMR shows a more obvious increase in the self-diffusion coefficient of TPA than TMA in the presence of salt, indicating TPA is more easily displaced from the nanoparticle surface due to the background electrolyte. A two-site model is used to describe the exchange between tetraalkylammonuim (TAA) adsorbed on the nanoparticles and TAA in solution, from which the binding isotherms of the organocations at low electrolyte concentration was obtained and analyzed using the Langmuir formalism. This analysis also shows specific-ion effects, with the amount of TPA adsorbed to be much smaller than TMA and also much more sensitive to the presence of salt. In the context of the oriented aggregation mechanism proposed previously in the literature, the current work suggests one route for tuning the organocation-particle interaction and thus a route to controlling the rates of some steps in the mechanism.

PFG NMR Investigations of TPA−TMA−Silica Mixtures

Pulsed-field gradient (PFG) NMR studies of tetrapropylammonium (TPA)-tetramethylammonium (TMA)-silica mixtures are presented, and the effect of TMA as a foreign ion on the TPA-silica nanoparticle interactions before and after heating has been studied. Dynamic light scattering (DLS) results suggest that silica nanoparticles in these TPA-TMA systems grow via a ripening mechanism for the first 24 h of heating. PFG NMR of mixtures before heating show that TMA can effectively displace TPA from the nanoparticle surface. The binding isotherms of TPA at room temperature obtained via PFG NMR can be described by Langmuir isotherms, and indicate a decrease in the adsorbed amount of TPA upon addition of TMA. PFG NMR also shows a systematic increase in the self-diffusion coefficient of TPA in both the mixed TPA-TMA systems and pure TPA systems with heating time, indicating an increased amount of TPA in solution upon heating. By contrast, a much smaller amount of TMA is observed to desorb from the nanoparticles upon heating. These results point to the desorption of TPA from the nanoparticles being a kinetically controlled process. The apparent desorption rate constants were calculated from fitting the desorbed amount of TPA with time via a pseudosecond-order kinetic model. This analysis show the rate of TPA desorption in TPA-TMA mixtures increases with increasing TMA content, whereas for pure TPA mixtures the rate of TPA desorption is much less sensitive to the TPA concentration.

Controlling the distribution of framework aluminum in high-silica zeolites

Using CP/MAS, REDOR, and heteronuclear correlation NMR spectroscopy we have investigated the geometrical relationships between the positive charge of organic structure-directing agents and the negative charge in the framework of zeolite ZSM-12. In samples of all-silica and aluminum containing ZSM-12 prepared using benzyltrimethyammonium cations as structure-directing agents (SDA), we have found that the positively charged segment of the SDA is preferentially ordered near the negative framework charge. This result result implies that the distribution of A1 sites in the zeolite is distinct from the random distribution often assumed for high-silica zeolites. Our results suggest that the distribution of acid sites can be controlled using SDAs with different charge distributions.