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Structural and Functional Trends in Dehydrating Bimodules from trans-Acyltransferase Polyketide Synthases
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Effects of Phosphonic Acid Monolayers on the Dehydration Mechanism of Aliphatic Alcohols on TiO 2
The kinetics for surface-catalyzed alcohol dehydration reactions often depend on the structure of the alcohol. Studies of structure–activity relations across primary, secondary, and tertiary alcohols can provide fundamental information on the nature of active sites on the surface. Here in this paper, we investigated the dehydration of 1-butanol, 2-butanol, and tert-butanol over TiO 2 anatase catalysts modified with various phosphonic acid (PA) self-assembled monolayers (SAMs). As a response to the presence of PAs, the three C4 alcohol isomers showed different dehydration rates, with 1-butanol dehydration being enhanced to the greatest extent by PA modification. Furthermore, the fluorinated, more polar 4-fluorobenzylphosphonic acid outperformed alkyl PAs across all alcohols. Steady-state kinetic measurements and temperature-programmed desorption studies indicated that PA SAMs significantly lowered the dehydration activation barrier; the extent of reduction in the barrier was sensitive to both the substitution of the alcohol and the charge distribution on the PA in a way that was consistent with stabilization of a carbenium-like transition state. Overall, the effect of PA modifiers on alcohol dehydration rates was found to be determined from a balance between transition state stabilization and active site blocking effects, with the potential to tune activity and selectivity based on the structure and coverage of the SAM.
Dehydration-driven stress transfer triggers intermediate-depth earthquakes
Intermediate-depth earthquakes (30–300 km) have been extensively documented within subducting oceanic slabs, but their mechanics remains enigmatic. Here in this paper we decipher the mechanism of these earthquakes by performing deformation experiments on dehydrating serpentinized peridotites (synthetic antigorite-olivine aggregates, minerals representative of subduction zones lithologies) at upper mantle conditions. At a pressure of 1.1 gigapascals, dehydration of deforming samples containing only 5 vol% of antigorite suffices to trigger acoustic emissions, a laboratory-scale analogue of earthquakes. At 3.5 gigapascals, acoustic emissions are recorded from samples with up to 50 vol% of antigorite. Experimentally produced faults, observed post-mortem, are sealed by fluid-bearing micro-pseudotachylytes. Microstructural observations demonstrate that antigorite dehydration triggered dynamic shear failure of the olivine load-bearing network. These laboratory analogues of intermediatedepth earthquakes demonstrate that little dehydration is required to trigger embrittlement. We propose an alternative model to dehydration-embrittlement in which dehydration-driven stress transfer, rather than fluid overpressure, causes embrittlement.
Differential response of the photosynthetic machinery to dehydration in older and younger resurrection plants
Abstract A group of vascular plants called homoiochlorophyllous resurrection plants evolved unique capabilities to protect their photosynthetic machinery against desiccation-induced damage. This study examined whether the ontogenetic status of the resurrection plant Craterostigma pumilum has an impact on how the plant responds to dehydration at the thylakoid membrane level to prepare cells for the desiccated state. Thus, younger plants (<4 months) were compared with their older (>6 months) counterparts. Ultrastructural analysis provided evidence that younger plants suppressed senescence-like programs that are realized in older plants. During dehydration, older plants degrade specific subunits of the photosynthetic apparatus such as the D1 subunit of PSII and subunits of the cytochrome b6f complex. The latter leads to a controlled down-regulation of linear electron transport. In contrast, younger plants increased photoprotective high-energy quenching mechanisms and maintained a high capability to replace damaged D1 subunits. It follows that depending on the ontogenetic state, either more degradation-based or more photoprotective mechanisms are employed during dehydration of Craterostigma pumilum.
A combined experimental and computational study of the mechanism of fructose dehydration to 5-hydroxymethylfurfural in dimethylsulfoxide using Amberlyst 70, PO 4 3- /niobic acid, or sulfuric acid catalysts
We report on a combined experimental and theoretical study of the acid catalyzed dehydration of d-fructose in dimethylsulfoxide (DMSO) using; Amberlyst 70, PO 4 3- /niobic acid, and sulfuric acid as catalysts. The reaction has been studied and intermediates characterized using; 13C, 1H, and 17O NMR, and high resolution electrospray ionization mass spectrometry (HR ESI–MS). High level G4MP2 theory calculations are used to understand the thermodynamic landscape for the reaction mechanism in DMSO. We have experimentally identified two key intermediates in the dehydration of fructose to form HMF that were also identified, using theory, as local minima on the potential surface for reaction. A third intermediate, a species capable of undergoing keto–enol tautomerism, was also experimentally detected. However, it was not possible to experimentally distinguish between the keto and the enol forms. These data with different catalysts are consistent with common intermediates along the reaction pathway from fructose to HMF in DMSO. The role of oxygen in producing acidic species in reactions carried out in DMSO in presence of air is also discussed.
Formation and Dehydration Enthalpy of Potassium Hexaniobate
The formation energetics of hydrous and dehydrated potassium hexaniobates are investigated using high‐temperature oxide melt solution calorimetry. The enthalpies of formation of K 4 Nb 6 O 17 and K 4 Nb 6 O 17 ·3H 2 O from oxides are (−864.42 ± 10.63) and (−899.32 ± 11.48) kJ/mol, respectively. The formation enthalpy of K 4 Nb 6 O 17 from elements is (−7289.64 ± 12.50) kJ/mol, and of K 4 Nb 6 O 17 ·3H 2 O is (−8181.94 ± 13.24) kJ/mol. The enthalpy of dehydration (Δ H dehy ) for the reaction K 4 Nb 6 O 17 ·3H 2 O (xl, 25°C) = K 4 Nb 6 O 17 (xl, 25°C) + 3H 2 O (l, 25°C) is endothermic and is 34.60 ± 7.56 kJ/mol. The Δ H dehy per mole of water, 11.53 ± 2.52 kJ/mol, indicates the water molecules in K 4 Nb 6 O 17 ·3H 2 O are not just physically adsorbed, but loosely bonded in the K 4 Nb 6 O 17 phase, presumably in specific interlayer sites. The loss of this water near 100°C on heating is consistent with the weak bonding of water.
Reversible Dehydration Behavior Reveals Coordinatively Unsaturated Metal Sites in Microporous Aluminum Phosphonates
Abstract not provided
Dehydration of fatty alcohols on zirconia supported tungstate catalysts
Zirconia supported tungstates catalyze octadecanol dehydration, an important step in the carbon preserving conversion of triglycerides to fuels.
WO x supported on γ-Al 2 O 3 with different morphologies as model catalysts for alkanol dehydration
The distinctive morphological and surface characteristics of platelet-like γ-Al2O3 were compared to a regular, commercial γ-Al2O3. γ-Al2O3 platelets display dominant (110) surface facets and higher densities of coordinative unsaturated penta-coordinate Al3+ (Al3+penta) sites than regular γ-Al2O3, as measured by solid-state magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). Such Al3+penta sites are also the preferred surface anchoring sites for tungsten oxide (WOx) species consistent with NMR analysis indicating that these sites are consumed upon WOx adsorption. The higher Al3+penta density on γ-Al2O3 platelets leads to greater WOx dispersion (or smaller WOx clusters), as demonstrated by scanning transmission electron microscopy and ultraviolet–visible spectroscopy, and WOx species at intermediate WOx surface concentration are the most active for the probe reaction of 2-butanol dehydration. WOx on γ-Al2O3 platelets approaches the highest turnover rates at higher surface densities than WOx on regular γ-Al2O3, yet with similar highest rate values for both series of catalysts. This indicates that different Al2O3 supports mainly affect the dispersion of supported WOx rather than the intrinsic reactivity of individual WOx clusters with similar size.
Time-Resolved Infrared Reflectance Studies of the Dehydration-Induced Transformation of Uranyl Nitrate Hexahydrate to the Trihydrate Form
Uranyl nitrate is a key species in the nuclear fuel cycle. However, this species is known to exist in different states of hydration, including the hexahydrate ([UO2(NO3)2(H2O)6] often called UNH), the trihydrate [UO2(NO3)2(H2O)3 or UNT], and in very dry environments the dihydrate form [UO2(NO3)2(H2O)2]. Their relative stabilities depend on both water vapor pressure and temperature. In the 1950s and 1960s the different phases were studied by infrared transmission spectroscopy, but were limited both by instrumental resolution and by the ability to prepare the samples for transmission. We have revisited this problem using time-resolved reflectance spectroscopy, which requires no sample preparation and allows dynamic analysis while the sample is exposed to a flow of N2 gas. Samples of known hydration state were prepared and confirmed via X-ray diffraction patterns of known species. In reflectance mode the hexahydrate UO2(NO3)2(H2O)6 has a distinct uranyl asymmetric stretch band at 949.0 cm-1 that shifts to shorter wavelengths and broadens as the sample desiccates and recrystallizes to the trihydrate, first as a shoulder growing in on the blue edge but ultimately results in a doublet band with reflectance peaks at 966 and 957 cm-1. The data are consistent with transformation from UNH to UNT as UNT has two inequivalent UO22+ sites. The dehydration of UO2(NO3)2(H2O)6 to UO2(NO3)2(H2O)3 is both a structural and morphological change that has the lustrous lime green UO2(NO3)2(H2O)6 crystals changing to the matte greenish yellow of the trihydrate solid. The phase transformation and crystal structures were confirmed by density functional theory calculations and optical microscopy methods, both of which showed a transformation with two distinct sites for the uranyl cation in the trihydrate, with but one in the hexahydrate.
Following the movement of Cu ions in a SSZ-13 zeolite during dehydration, reduction and adsorption: a combined in situ TP-XRD, XANES/DRIFTS study
Cu-SSZ-13 has been shown to possess high activity and superior N2 formation selectivity in the selective catalytic reduction of NOx under oxygen rich conditions. Here, a combination of synchrotron-based (XRD and XANES) and vibrational (DRIFTS) spectroscopy tools have been used to follow the changes in the location and coordination environment of copper ions in a Cu-SSZ-13 zeolite during calcinations, reduction with CO, and adsorption of CO and H2O. XANES spectra collected during these procedures provides critical information not only on the variation in the oxidation state of the copper species in the zeolite structure, but also on the changes in the coordination environment around these ions as they interact with the framework, and with different adsorbates (H2O and CO). Time-resolved XRD data indicate the movement of copper ions and the consequent variation of the unit cell parameters during dehydration. DRIFT spectra provide information about the adsorbed species present in the zeolite, as well as the oxidation states of and coordination environment around the copper ions. A careful analysis of the asymmetric T-O-T vibrations of the CHA framework perturbed by copper ions in different coordination environments proved to be especially informative. The results of this study will aid the identification of the location, coordination and oxidation states of copper ions obtained during in operando catalytic studies. Financial support was provided by the US Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Program. Part of this work (sample preparation) was performed in the Environmental Molecular Sciences Laboratory (EMSL) at Pacific Northwest National Laboratory (PNNL). The EMSL is a national scientific user facility supported by the US DOE, Office of Biological and Environmental Research. PNNL is a multi-program national laboratory operated for the US DOE by Battelle. All of the spectroscopy work reported here was carried out at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL). NSLS is a national scientific user facility supported by the US DOE.
Surface Reactions and Catalytic Activities for Small Alcohols over LaMnO3(100) and La0.7Sr0.3MnO3(100): Dehydrogenation, Dehydration, and Oxidation
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Luminescence induced by dehydration of kaolin - Association with electron-spin-active centers and with surface activity for dehydration-polymerization of glycine
Experimental data concerning emission of light upon dehydration as a function of preheating and pre-gamma-irradiation are correlated with reported studies of electron-spin resonance (ESR) activity after similar pretreatments. The effect of these pretreatments on the kaolin-promoted incorporation of glycine into peptide oligomers in a wet/cold, hot/dry fluctuating environment is compared to their effect on the ESR and luminescent signals. The existence of spectroscopically active centers appears to be loosely anticorrelated with reaction yield; these yields are increased by increasing the overall energy content of the material. It is concluded that some part of the chemical yield is produced by a mechanism involving intrinsic, excited electronic states of the clay crystal lattice. These states may be derived from thermally, interfacially, and/or mechanically induced charge reorganization within interspersed energy levels in the band structure of the material.
Evaluation of COSMO-SAC method for the prediction of the alcohol-water partition coefficients of the compounds encountered in aqueous phase fructose dehydration
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