Mechanochemically-assisted solvent-free and template-free synthesis of zeolites ZSM-5 and mordenite.

Aluminosilicate-based zeolite materials, such as ZSM-5 and mordenite, are well-studied as catalysts. Typical approaches to synthesize these zeolites require either templates or seeds to direct ordered crystal growth and both of these are expensive and add to the complexity of zeolite synthesis. In this paper, we describe a solvent-free and template-free method to synthesize crystalline ZSM-5 and mordenite zeolites without any added seed crystals. Key to the success of this approach is a mechanochemical precursor pre-reaction step. High-energy ball-milling is used to initiate a solid-state metathesis (exchange) reaction between Na2SiO3 and Al2(SO4)3 reagents, forming crystalline Na2SO4 and well-mixed aluminosilicate precursor. The solid precursor mixture is thermally converted to crystalline ZSM-5 or mordenite at moderate 180 °C temperatures without solvents or an organic amine structure directing template. Variations in Si/Al ratios in the precursor mixture and additions of solid NaOH to the mechanochemical reaction were found to influence the subsequent growth of either crystalline ZSM-5 or mordenite zeolites. The crystalline zeolites from this solvent-free and template free method have high ∼300 m2 g-1 surface areas directly from the synthesis without requiring high-temperature calcination. These materials are also comparably active to their commercial counterparts in cellulose and glucose biomass catalytic conversion to hydroxymethylfurfural.

Surface Adsorption of Suwannee River Humic Acid on TiO2 Nanoparticles: A Study of pH and Particle Size.

TiO2 nanoparticles are some of the most widely used metal oxide nanomaterials mainly because of their diverse industrial applications. Increasing usage of these nanoparticles raises concerns about the potential adverse effects on the environment. Humic acid is a ubiquitous component of the natural organic matter in the environment that is known to get adsorbed onto nanoparticle surfaces. In this study, adsorption of humic acid on TiO2 nanoparticles of two different sizes (5 and 22 nm) is studied at different environmentally relevant pH values using attenuated total reflectance Fourier transformation infrared spectroscopy. These vibrational spectra provide insights into the nature of the adsorption process (extent of adsorption and reversibility) as a function of pH as well as information about the bonding to the surface. Additionally, the impact of humic acid adsorption on surface charge and agglomeration has been investigated. Interestingly, the results show that the humic acid adsorption is strongly pH-dependent and that adsorption of humic acid on TiO2 nanoparticles alters the extent of agglomeration and modifies the zeta potential and surface charges depending on the pH, thus potentially increasing the bioavailability of TiO2 nanoparticles in the environment.

Sequestration of U(VI) from Acidic, Alkaline, and High Ionic-Strength Aqueous Media by Functionalized Magnetic Mesoporous Silica Nanoparticles: Capacity and Binding Mechanisms.

Uranium(VI) exhibits little adsorption onto sediment minerals in acidic, alkaline or high ionic-strength aqueous media that often occur in U mining or contaminated sites, which makes U(VI) very mobile and difficult to sequester. In this work, magnetic mesoporous silica nanoparticles (MMSNs) were functionalized with several organic ligands. The functionalized MMSNs were highly effective and had large binding capacity for U sequestration from high salt water (HSW) simulant (54 mg U/g sorbent). The functionalized MMSNs, after U exposure in HSW simulant, pH 3.5 and 9.6 artificial groundwater (AGW), were characterized by a host of spectroscopic methods. Among the key novel findings in this work was that in the HSW simulant or high pH AGW, the dominant U species bound to the functionalized MMSNs were uranyl or uranyl hydroxide, rather than uranyl carbonates as expected. The surface functional groups appear to be out-competing the carbonate ligands associated with the aqueous U species. The uranyl-like species were bound with N ligand as η2 bound motifs or phosphonate ligand as a monodentate, as well as on tetrahedral Si sites as an edge-sharing bidentate. The N and phosphonate ligand-functionalized MMSNs hold promise as effective sorbents for sequestering U from acidic, alkaline or high ionic-strength contaminated aqueous media.

Quantification of gabapentin polymorphs in gabapentin/excipient mixtures using solid state 13C NMR spectroscopy and X-ray powder diffraction.

Gabapentin was used as a model pharmaceutical compound with susceptibility to polymorphic transformation as a function of environmental and mechanical stress. The utility of 13C CP/MAS NMR and XRPD as stability-indicating methods to quantify polymorphic transformation kinetics was investigated. Polymorphic Form II and III were distinguishable based on their chemical shift and distinct diffraction peak differences. Reproducible and accurate quantification of polymorphic composition in the presence of selected excipients was demonstrated using both signals from 13C CP/MAS NMR spectra and XRPD patterns. The effect of excipients on polymorphic transformations (Form II→III) was determined by measuring the transformation after co-milling. Both 13C CP/MAS NMR and XRPD were capable of measuring polymorphic composition in co-milled excipient mixtures without excipient peak interference. The amounts of Form III present in co-milled mixtures containing colloidal silicon dioxide, starch, hydroxy propyl cellulose and dibasic calcium phosphate were 8.7, 21, 33, and 39mol%, respectively. A quenching procedure for obtaining 13C CP/MAS NMR spectra and environmentally-controlled XRPD were devised to determine polymorphic transformation kinetics of co-milled excipient mixtures during storage.

Functionalized magnetic mesoporous silica nanoparticles for U removal from low and high pH groundwater.

U(VI) species display limited adsorption onto sediment minerals and synthetic sorbents in pH <4 or pH >8 groundwater. In this work, magnetic mesoporous silica nanoparticles (MMSNs) with magnetite nanoparticle cores were functionalized with various organic molecules using post-synthetic methods. The functionalized MMSNs were characterized using N2 adsorption-desorption isotherms, thermogravimetric analysis (TGA), transmission electron microscopy (TEM), (13)C cross polarization and magic angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectroscopy, and powder X-ray diffraction (XRD), which indicated that mesoporous silica (MCM-41) particles of 100-200nm formed around a core of magnetic iron oxide, and the functional groups were primarily grafted into the mesopores of ∼3.0nm in size. The functionalized MMSNs were effective for U removal from pH 3.5 and 9.6 artificial groundwater (AGW). Functionalized MMSNs removed U from the pH 3.5 AGW by as much as 6 orders of magnitude more than unfunctionalized nanoparticles or silica and had adsorption capacities as high as 38mg/g. They removed U from the pH 9.6 AGW as much as 4 orders of magnitude greater than silica and 2 orders of magnitude greater than the unfunctionalized nanoparticles with adsorption capacities as high as 133mg/g. These results provide an applied solution for treating U contamination that occurs at extreme pH environments and a scientific foundation for solving critical industrial issues related to environmental stewardship and nuclear power production.

Silica Nanoparticle-Generated ROS as a Predictor of Cellular Toxicity: Mechanistic Insights and Safety by Design.

Evaluating toxicological responses of engineered nanomaterials such as silica nanoparticles is critical in assessing health risks and exposure limits. Biological assays can be used to evaluate cytotoxicity of individual materials, but specific nano-bio interactions-which govern its physiological response-cannot currently be predicted from materials characterization and physicochemical properties. Understanding the role of free radical generation from nanomaterial surfaces facilitates understanding of a potential toxicity mechanism and provides insight into how toxic effects can be assessed. Size-matched mesoporous and nonporous silica nanoparticles in aminopropyl-functionalized and native forms were investigated to analyze the effects of porosity and surface functionalization on the observed cytotoxicity. In vitro cell viability data in a murine macrophage cell line (RAW 264.7) provides a model for what might be observed in terms of cellular toxicity upon an environmental or industrial exposure to silica nanoparticles. Electron paramagnetic resonance spectroscopy was implemented to study free radical species generated from the surface of these nanomaterials and the signal intensity was correlated with cellular toxicity. In addition, in vitro assay of intracellular reactive oxygen species (ROS) matched well with both the EPR and cell viability data. Overall, spectroscopic and in vitro studies correlate well and implicate production of ROS from a surface-catalyzed reaction as a predictor of cellular toxicity. The data demonstrate that mesoporous materials are intrinsically less toxic than nonporous materials, and that surface functionalization can mitigate toxicity in nonporous materials by reducing free radical production. The broader implications are in terms of safety by design of nanomaterials, which can only be extracted by mechanistic studies such as the ones reported here.

Amine modification of nonporous silica nanoparticles reduces inflammatory response following intratracheal instillation in murine lungs.

Amorphous silica nanoparticles (NPs) possess unique material properties that make them ideal for many different applications. However, the impact of these materials on human and environmental health needs to be established. We investigated nonporous silica NPs both bare and modified with amine functional groups (3-aminopropyltriethoxysilane (APTES)) in order to evaluate the effect of surface chemistry on biocompatibility. In vitro data showed there to be little to no cytotoxicity in a human lung cancer epithelial cell line (A549) for bare silica NPs and amine-functionalized NPs using doses based on both mass concentration (below 200µg/mL) and exposed total surface area (below 14m(2)/L). To assess lung inflammation, C57BL/6 mice were administered bare or amine-functionalized silica NPs via intra-tracheal instillation. Two doses (0.1 and 0.5mg NPs/mouse) were tested using the in vivo model. At the higher dose used, bare silica NPs elicited a significantly higher inflammatory response, as evidence by increased neutrophils and total protein in bronchoalveolar lavage (BAL) fluid compared to amine-functionalized NPs. From this study, we conclude that functionalization of nonporous silica NPs with APTES molecules reduces murine lung inflammation and improves the overall biocompatibility of the nanomaterial.

Nano-Bio Interactions of Porous and Nonporous Silica Nanoparticles of Varied Surface Chemistry: A Structural, Kinetic, and Thermodynamic Study of Protein Adsorption from RPMI Culture Medium.

Understanding complex chemical changes that take place at nano-bio interfaces is of great concern for being able to sustainably implement nanomaterials in key applications such as drug delivery, imaging, and environmental remediation. Typical in vitro assays use cell viability as a proxy to understanding nanotoxicity but often neglect how the nanomaterial surface can be altered by adsorption of solution-phase components in the medium. Protein coronas form on the nanomaterial surface when incubated in proteinaceous solutions. Herein, we apply a broad array of techniques to characterize and quantify protein corona formation on silica nanoparticle surfaces. The porosity and surface chemistry of the silica nanoparticles have been systematically varied. Using spectroscopic tools such as FTIR and circular dichroism, structural changes and kinetic processes involved in protein adsorption were evaluated. Additionally, by implementing thermogravimetric analysis, quantitative protein adsorption measurements allowed for the direct comparison between samples. Taken together, these measurements enabled the extraction of useful chemical information on protein binding onto nanoparticles in solution. Overall, we demonstrate that small alkylamines can increase protein adsorption and that even large polymeric molecules such as poly(ethylene glycol) (PEG) cannot prevent protein adsorption in these systems. The implications of these results as they relate to further understanding nano-bio interactions are discussed.

Chemical Insight into the Adsorption of Chromium(III) on Iron Oxide/Mesoporous Silica Nanocomposites.

Magnetic iron oxide/mesoporous silica nanocomposites consisting of iron oxide nanoparticles embedded within mesoporous silica (MCM-41) and modified with aminopropyl functional groups were prepared for application to Cr(III) adsorption followed by magnetic recovery of the nanocomposite materials from aqueous solution. The composite materials were extensively characterized using physicochemical techniques, such as powder X-ray diffraction, thermogravimetric and elemental analysis, nitrogen adsorption, and zeta potential measurements. For aqueous Cr(III) at pH 5.4, the iron oxide/mesoporous silica nanocomposite exhibited a superior equilibrium adsorption capacity of 0.71 mmol/g, relative to 0.17 mmol/g for unmodified mesoporous silica. The aminopropyl-functionalized iron oxide/mesoporous silica nanocomposites displayed an equilibrium adsorption capacity of 2.08 mmol/g, the highest adsorption capacity for Cr(III) of all the materials evaluated in this study. Energy-dispersive spectroscopy (EDS) with transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) experiments provided insight into the chemical nature of the adsorbed chromium species.

Surface-Selective Solution NMR Studies of Functionalized Zeolite Nanoparticles.

The surface chemistry of zeolite nanoparticles functionalized with the organosilane aminopropyldimethylmethoxysilane (APDMMS) was selectively probed using solution (1)H NMR spectroscopy. The use of solution NMR spectroscopy results in high-resolution NMR spectra, and the technique is selective for protons on the surface organic functional groups due to their motional averaging in solution. In this study, (1)H solution NMR spectroscopy was used to investigate the interface of the organic functional groups of APDMMS-functionalized silicalite nanoparticles (∼35 nm) in D2O. The pKa for the amine group of APDMMS-functionalized silicalite nanoparticles in D2O was determined using an NMR-pH titration method based on the variation in the proton chemical shift for the alkyl group protons closest to the amine group with pH. The resulting NMR spectra demonstrate the sensitivity of solution NMR spectroscopy to the electronic environment and structure of the surface functional groups.

Adsorption and Photochemical Properties of a Molecular CO2 Reduction Catalyst in Hierarchical Mesoporous ZSM-5: An In Situ FTIR Study.

As part of our recent effort to attach well-defined molecular photocatalysts to solid-state surfaces, this present study investigates adsorption and photochemical properties of a tricarbonyl rhenium(I) compound, Re(bpy)(CO)3Cl (bpy = 2,2'-bipyridine), in hierarchical mesoporous ZSM-5. The molecular Re(I) catalyst, a Ru(bpy)3(2+) photosensitizer, and an amine-based electron donor were coadsorbed in the mesopores of the hierarchical ZSM-5 through simple liquid-phase adsorption. The functionalized ZSM-5 was then characterized with infrared and UV-visible spectroscopies and was tested in CO2 reduction photocatalysis at the gas-surface interface. In the mesoporous ZSM-5, CO2 molecules were adsorbed on the amine electron-donor molecules as bicarbonate, which would release CO2 upon light irradiation to react with the Re(I) catalyst. The formation of important reaction intermediates, particularly a Re-carboxylato species, was revealed with in situ Fourier transform infrared spectroscopy in combination with isotopic labeling. The experimental results indicate that hierarchical mesoporous zeolites are promising host materials for molecular photocatalysts and that zeolite mesopores are potential "reaction vessels" for CO2 reduction photocatalysis at the gas-solid interface.

Framework stability of nanocrystalline NaY in aqueous solution at varying pH.

Nanocrystalline zeolites are emerging as important materials for a variety of potential applications in industry and medicine. Reducing the particle size to less than 100 nm results in advantages for nanocrystalline zeolites relative to micrometer-sized zeolite crystals, such as very large total and external specific surface areas and reduced diffusion path lengths. Understanding the physical and chemical properties of zeolite nanocrystals is imperative for further development and application of nanocrystalline zeolites. In this study, the framework stability of nanocrystalline NaY zeolite with a crystal size of 66 nm and Si/Al = 1.74 was investigated at pH 7.4, 4, 2, and 1. The solids and solutions were analyzed using several different analytical techniques. The relative crystallinity and crystal size and morphology of the solids were examined by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. The aluminum content, Si/Al, and coordination were monitored by inductively coupled plasma/optical emission spectroscopy (ICP/OES), X-ray photoelectron spectroscopy (XPS), and aluminum-27 solid-state magic-angle spinning NMR. As the acidity of the medium increased, the framework stability of nanocrystalline NaY decreased. Treatment of the zeolite samples at pH 1 resulted in complete degradation of the zeolite framework after 1 h. An increase in Si/Al was also observed, suggesting the selective removal of aluminum at low pH.

DFT calculations of EPR parameters for copper(II)-exchanged zeolites using cluster models.

The coordination environment of Cu(II) in hydrated copper-exchanged zeolites was explored through the use of density functional theory (DFT) calculations of EPR parameters. Extensive experimental EPR data are available in the literature for hydrated copper-exchanged zeolites. The copper complex in hydrated copper-exchanged zeolites was previously proposed to be [Cu(H(2)O)(5)OH](+) based on empirical trends in tetragonal model complex EPR data. In this study, calculated EPR parameters for the previously proposed copper complex, [Cu(H(2)O)(5)OH](+), were compared to model complexes in which Cu(II) was coordinated to small silicate or aluminosilicate clusters as a first approximation of the impact of the zeolitic environment on the copper complex. Interpretation of the results suggests that Cu(II) is coordinated or closely associated with framework oxygen atoms within the zeolite structure. Additionally, it is proposed that the EPR parameters are dependent on the Si/Al ratio of the parent zeolite.

DFT calculations of the EPR parameters for Cu(ii) DETA imidazole complexes.

The electron paramagnetic resonance (EPR) parameters for Cu(ii) diethylenetriamine imidazole complexes, which serve as empirical models for copper-containing proteins, were calculated using density functional theory (DFT). The orientations of three different types of imidazole ligands, imidazole, 1-methylimidazole and 4-methylimidazole, were investigated by rotating the ligand about the Cu(ii) imidazole bond. The calculated EPR values indicate that the imidazole ligands studied are oriented approximately +/-45 degrees with respect to the ligand plane. EPR parameters calculated using the B3LYP density functional in conjunction with the conductor-like solvent model (COSMO) show the best agreement with the experimentally determined EPR values. Good agreement with electron spin echo envelope modulation (ESEEM) data is achieved when an explicit water molecule is located near the remote nitrogen atom of the imidazole ligand. The implications of these DFT calculations for interpreting experimental pulsed EPR data for copper proteins containing imidazole ligands are discussed.

Effect of crystal size and surface functionalization on the cytotoxicity of silicalite-1 nanoparticles.

In this report, we describe the synthesis and characterization of nanocrystalline silicalite (the purely siliceous form of the zeolite, ZSM-5) of defined crystal size and surface functionalization and determine the effect on the type and degree of cytotoxicity induced in two distinct model cell lines. The silicalite materials were characterized by powder X-ray diffraction, dynamic light scattering and zeta potential, solid state NMR, thermal gravimetric analysis, and nitrogen adsorption using the BET method to determine specific surface area. The silicalite samples were functionalized with amino, thiol, and carboxy groups and had crystal sizes of approximately 30, 150, and 500 nm. The cytotoxicities of the silicalite samples with different crystal sizes and different surface functional groups were investigated using human embryonic kidney 293 (HEK-293) cells and RAW264.7 macrophage cell lines. We used the lactic dehydrogenase release assay to measure damage to the cell membrane, the caspase 3/7 activity assay to measure key molecules involved in apoptosis, and the Annexin V-propidium iodide staining method to provide visual confirmation of the types of cell death induced. We have shown that the impact of size and surface functionalization of silicalite nanoparticles on cell toxicity and mechanism of cell death is cell type-dependent. Thirty nanometer silicalite nanoparticles were nontoxic in RAW264.7 cells relative to untreated controls but caused necrosis in HEK293 cells. Carboxy-functionalized 500 nm silicalite nanoparticles resulted in apoptosis and necrosis in RAW264.7 cells and predominantly activated apoptosis in HEK293 cells.

Density functional theory investigation of EPR parameters for tetragonal Cu(II) model complexes with oxygen ligands.

Density functional theory (DFT) calculations of the electron paramagnetic resonance (EPR) parameters for a series of tetragonal Cu(II) model complexes were conducted. Model complexes containing four oxygen atoms directly coordinated to a Cu(II) metal center were chosen because of their importance in the Peisach-Blumberg truth tables and their frequent use in the interpretation of EPR spectra of Cu(II) proteins and copper-containing catalysts. Molecular g- and copper A-tensors were calculated using the BP86 and B3LYP density functionals. The DFT calculations reproduce the experimentally observed trends in the parallel components of the A- and g-tensors. Important insight into the structural basis for the empirical trends in g( parallel) and A( parallel) was obtained from the DFT calculations. Notably, g( parallel) systematically increases and A( parallel) systematically decreases with increasing Cu-O equatorial bond length. These results have been used to provide structural insight into copper EPR data for copper-exchanged zeolites.