Oxidation of Alcohols to Carbonyl Compounds, Benzylic Carbons to their Ketones, and Arenes to their Quinones with Molecular Oxygen in Subcritical Water

Summary. Molecular oxygen is used as an efficient oxidant for the conversion of alcohols into carbonyl compounds, benzylic carbons to their ketones, and arenes to their quinones in subcritical water in the absence of catalysts. The procedure utilizes water and does not require support materials and metal salts.     

Oxidation of Thiols to Disulfides with Molecular Oxygen in Subcritical Water

Summary. Molecular oxygen is used as an efficient oxidant for the oxidative coupling of thiols to disulfides in subcritical water in the absence of catalysts. The procedure utilizes water and does not require support materials and metal salts providing high yields (>90%).     

Evolution inclusions governed by the difference of two subdifferentials in reflexive Banach spaces

The existence of strong solutions of Cauchy problem for the following evolution equation du(t)/dt+∂?¹(u(t))-∂?²(u(t))∋f(t) is considered in a real reflexive Banach space V, where ∂?¹ and ∂?² are subdifferential operators from V into its dual V^*. The study for this type of problems has been done by several authors in the Hilbert space setting.The scope of our study is extended to the V-V^* setting. The main tool employed here is a certain approximation argument in a Hilbert space and for this purpose we need to assume that there exists a Hilbert space H such that V⊂H≡H^*⊂V^* with densely defined continuous injections.The applicability of our abstract framework will be exemplified in discussing the existence of solutions for the nonlinear heat equation: where Ω is a bounded domain in R^N. In particular, the existence of local (in time) weak solution is shown under the subcritical growth condition q<p^* (Sobolev's critical exponent) for all initial data This fact has been conjectured but left as an open problem through many years.     

Flux difference splitting for open-channel flows

A finite difference scheme based on flux difference splitting is presented for the solution of the one-dimensional shallow-water equations in open channels, together with an extension to two-dimensional flows. A linearized problem, analogous to that of Riemann for gas dynamics, is defined and a scheme, based on numerical characteristic decomposition, is presented for obtaining approximate solutions to the linearized problem. The method of upwind differencing is used for the resulting scalar problems, together with a flux limiter for obtaining a second-order scheme which avoids non-physical, spurious oscillations. The scheme is applied to a one-dimensional dam-break problem, and to a problem of flow in a river whose geometry induces a region of supercritical flow. The scheme is also applied to a two-dimensional dam-break problem. The numerical results are compared with the exact solution, or other numerical results, where available.     

Continuous subcritical water extraction of medicinal plant essential oil: comparison with conventional techniques

A subcritical extractor equipped with a three-way inlet valve and an on/off outlet valve has been used for performing subcritical water extractions (SWE) in a continuous manner for the isolation of the essential oil of fennel, a medicinal plant. The target compounds were removed from the aqueous extract by a single extraction with 5 ml hexane, determined by gas-chromatography-flame ionization (GC-FID) and identified by mass spectrometry (MS). The proposed extraction method has been compared with both hydrodistillation and dichloromethane manual extraction. Better results have been obtained with the proposed method in terms of rapidity, efficiency, cleanliness and possibility of manipulating the composition of the extract.     

Bonded stationary phases for reversed phase liquid chromatography with a water mobile phase: application to subcritical water extraction

Reversed phase high-performance liquid chromatography (RP-HPLC) is demonstrated for hydrophobic analytes such as aromatic hydrocarbons on a chemically bonded stationary phase and a mobile phase consisting of only water. Reversed phase liquid chromatography separations using a water-only mobile phase has been termed WRP-LC for water-only reversed phase LC. Reasonable capacity factors are achieved through the use of a non-porous silica substrate resulting in a chromatographic phase volume ratio much lower than usually found in RP-HPLC. Two types of bonded WRP-LC columns have been developed and applied. A brush phase was synthesized from an organochlorosilane. The other phase, synthesized from an organodichlorosilane, is termed a branch phase and results in a polymeric structure of greater thickness than the brush phase. A baseline separation of a mixture containing benzaldehyde, benzene, toluene, and ethyl benzene in less than 5 min is demonstrated using a water mobile phase with 12 000 plates generated for the unretained benzaldehyde peak. The theoretically predicted minimum reduced plate height is also shown to be approached for the unretained analyte using the brush phase. As an application, subcritical water extraction (SWE) at 200°C is combined with WRP-LC. This combination allows for the extraction of organic compounds from solid matrices immediately followed by liquid chromatographic separation of those extracted compounds all using a solvent of 100% water. We demonstrate SWE/WRP-LC by spiking benzene, ethyl benzene, and naphthalene onto sand then extracting the analytes with SWE followed by chromatographic separation on a WRP column. A sand sample contaminated with gasoline was also analyzed using SWE/WRP-LC. This extraction process also provides kinetic information about the rate of analyte extraction from the sand matrix. Under the conditions employed, analytes were extracted at different rates, providing additional selectivity in addition to the WRP-LC separation.     

Fundamental challenges and opportunities for preparative supercritical fluid chromatography

The use of supercritical fluids as mobile phases in chromatography was suggested nearly fifty years ago. In spite of some major potential advantages, this mode of chromatography, generally known as SFC, is only now beginning to be considered by the mainstream community but it still does not yet enjoy a popularity comparable to those of gas or liquid chromatography. This seems to be largely due to a combination of (1) the serious instrumental difficulties that took many years to solve; (2) the complexity of the behavior of supercritical fluids in chromatographic systems when their temperature, pressure, or composition changes; (3) the long-lasting absence of any substantial incentive to use more complex systems, when the simpler and more robust approaches provided by HPLC are available. This situation, however, has begun to significantly change during recent years. The incentive of employing green, sustainable technologies in industrial processes as well as in analyses is increasing. Because mobile phases generally used in SFC tend to be less environmentally harmful and less expensive than those used in HPLC, SFC presents strong economical and regulatory advantages over the latter technique. Added to that, steady advancements in LC techniques in the last three decades has solved many instrumental difficulties related to SFC, which is now taking full advantages of many of these advances. One factor, however, has remained mostly unresolved. A clearer understanding of the physico-chemical behavior of supercritical fluids in preparative chromatographic columns under nonlinear conditions is still needed. This seems to be the main obstacle to the establishment of SFC as a sustainable separation tool. One aim of this review is to highlight these issues in more detail through a survey of the state-of-the-art techniques available for the design and operation of SFC. Another aim is to outline a possible series of investigations, which are necessary to develop a better physical understanding of SFC.     

Decay properties of solutions to the Cauchy problem for the damped wave equation with absorption

We consider the Cauchy problem for the damped wave equation with absorption

     

Enantioseparation of chiral sulfonates by liquid chromatography and subcritical fluid chromatography

Tert‐butylcarbamoyl‐quinine and ‐quinidine weak anion‐exchange chiral stationary phases (Chiralpak® QN‐AX and QD‐AX) have been applied for the separation of sodium β‐ketosulfonates, such as sodium chalconesulfonates and derivatives thereof. The influence of type and amount of co‐ and counterions on retention and enantioresolution was investigated using polar organic mobile phases. Both columns exhibited remarkable enantiodiscrimination properties for the investigated test solutes, in which the quinidine‐based column showed better enantioselectivity and slightly stronger retention for all analytes compared to the quinine‐derived chiral stationary phase. With an optimized mobile phase (MeOH, 50 mM HOAc, 25 mM NH₃), 12 of 13 chiral sulfonates could be baseline separated within 8 min using the quinidine‐derivatized column. Furthermore, subcritical fluid chromatography (SubFC) mode with a CO₂‐based mobile phase using a buffered methanolic modifier was compared to HPLC. Generally, SubFC exhibited slightly inferior enantioselectivities and lower elution power but also provided unique baseline resolution for one compound.