Modeling of carbonic acid pretreatment process using ASPEN-plusŖ

ASPEN-Plus^Ŗ process modeling software is used to model carbonic acid pretreatment of biomass. ASPEN-Plus was used because of the thorough treatment of thermodynamic interactions and its status as a widely accepted process simulator. Because most of the physical property data for many of the key components used in the simulation of pretreatment processes are not available in the standard ASPEN-Plus property databases, values from an in-house database (INHSPCD) developed by the National Renewable Energy Laboratory were used. The standard non-random-two-liquid (NRTL) or renon route was used as the main property method because of the need to distill ethanol and to handle dissolved gases. The pretreatment reactor was modeled as a “black box” stoichiometric reactor owing to the unavailability of reaction kinetics. The ASPEN-Plus model was used to calculate the process equipment costs, power requirements, and heating and cooling loads. Equipment costs were derived from published modeling studies. Wall thickness calculations were used to predict construction costs for the high-pressure pretreatment reactor. Published laboratory data were used to determine a suitable severity range for the operation of the carbonic acid reactor. The results indicate that combined capital and operating costs of the carbonic acid system are slightly higher than on H₂SO₄-based system and highly sensitive to reactor pressure and solids concentration.     

Collection efficiency of scattered light in single-ended optical fiber sensors

Optical fibers allow a variety of spectroscopic sensing methods to be implemented in a single-ended backscattering geometry. Taking multimode fibers with surface-enhanced Raman scattering active tips as a model system, it is shown that the remote single-ended collection geometry can be relatively inefficient in comparison to the performance of the underlying sensor structure. Therefore the performance of the single-ended geometry has been compared to the analogous sensor structure on a nonguiding silica glass substrate. While part of the reduction in collection efficiency can be attributed to mismatches between the numerical aperture of the collection optics and that of the fiber, this study suggests that there can be an additional loss due to a mismatch between the confocal area of the collection optics and the area of the fiber core. This effect is most significant for high numerical aperture objectives. However, the collection efficiency is somewhat higher than would be expected from a simple area ratio analysis. This can be attributed to the graded-index fiber used in the model system and the relaxation of confocal requirements in the longitudinal direction.     

Influence of electric field on SERS: frequency effects, intensity changes, and susceptible bonds

The fundamental mechanism proposed to explain surface-enhanced Raman scattering (SERS) relies on electromagnetic field enhancement at optical frequencies. In this work, we demonstrate the use of microfabricated, silver nanotextured electrode pairs to study, in situ, the influence of low frequency (5 mHz to 1 kHz) oscillating electric fields on the SERS spectra of thiophenol. This applied electric field is shown to affect SERS peak intensities and influence specific vibrational modes of the analyte. The applied electric field perturbs the polar analyte, thereby altering the scattering cross section. Peaks related to the sulfurous bond which binds the molecule to the silver nanotexture exhibit strong and distinguishable responses to the applied field, due to varying bending and stretching mechanics. Density functional theory simulations are used to qualitatively verify the experimental observations. Our experimental and simulation results demonstrate that the SERS spectral changes relate to electric field induced molecular reorientation, with dependence on applied field strength and frequency. This demonstration creates new opportunities for external dynamic tuning and multivariate control of SERS measurements.     

Insertion of Isocyanides into N−Si Bonds: Multicomponent Reactions with Azines Leading to Potent Antiparasitic Compounds

Trimethylsilyl chloride is an efficient activating agent for azines in isocyanide‐based reactions, which then proceed through a key insertion of the isocyanide into a N?Si bond. The reaction is initiated by N activation of the azine, followed by nucleophilic attack of an isocyanide in a Reissert‐type process. Finally, a second equivalent of the same or a different isocyanide inserts into the N?Si bond leading to the final adduct. The use of distinct nucleophiles leads to a variety of α‐substituted dihydroazines after a selective cascade process. Based on computational studies, a mechanistic hypothesis for the course of these reactions was proposed. The resulting products exhibit significant activity against Trypanosoma brucei and T. cruzi, featuring favorable drug‐like properties and safety profiles.     

A graph-theoretical approach for the analysis and model reduction of complex-balanced chemical reaction networks

In this paper we derive a compact mathematical formulation describing the dynamics of chemical reaction networks that are complex-balanced and are governed by mass action kinetics. The formulation is based on the graph of (substrate and product) complexes and the stoichiometric information of these complexes, and crucially uses a balanced weighted Laplacian matrix. It is shown that this formulation leads to elegant methods for characterizing the space of all equilibria for complex-balanced networks and for deriving stability properties of such networks. We propose a method for model reduction of complex-balanced networks, which is similar to the Kron reduction method for electrical networks and involves the computation of Schur complements of the balanced weighted Laplacian matrix.