Polymer nanocomposite membranes and their application for flow catalysis and photocatalytic degradation of organic pollutants

The modern world essentially needs a chemical industry that can operate with reduced production costs, and produce high-quality products with low environmental impact. The polymer nanocomposite-based flow catalytic membrane reactor where the reaction and separation can be amalgamated in one unit is considered as one of the new alternative solutions to solve these problems. In this review, we have discussed state-of-the-art flow-through catalytic reactors based on polymer nanocomposite membranes. The unique advantages of flow catalysis include uninterrupted operation, good recyclability, and reaction product without contamination that leads to simple purification. Various catalytic model reactions such as coupling, hydrogenation, esterification in the flow system are presented. We have also presented an overview of methods adopted for preparing such nanocomposite membranes. In the last section, a discussion has been made on the recent advances on polymer-based nanocomposite membranes for the degradation and separation of organic pollutants.     

Wenn die Rührgeschwindigkeit die Produktverteilung bestimmt: Selektivität mischungsmaskierter Reaktionen

Many chemical reactions are so fast relative to mixing that significant conversion takes place before homogenization down to the molecular scale is reached. In such cases, the bond‐breaking and bond‐forming events mainly occur in regions where concentrations gradients are still present. Consequently the rate of mixing can have a pronounced effect on the kinetics and the product distribution in competitive chemical reactions. This review gives an account of such mixing‐sensitive single‐phase liquid‐liquid chemical reactions where all reactants and products are soluble, because a better understanding of the coupling between mixing and chemical reaction is needed to optimize the product distribution by choosing the appropriate reactor geometry and mixing conditions. Additionally these reactions have the potential to be employed as an exploratory microprobe to investigate the local flow characteristics or the mixing process at the molecular level and to evaluate the mechanism of chemical reactions.     

Use of a membrane reactor to improve selectivity to intermediate products in consecutive catalytic reactions

Through modeling it has been shown that a concentric-tube catalytic membrane reactor can be used to increase the selectivity for the intermediate products of a consecutive reaction scheme. The reactants are fed to the tube-side of the reactor where the catalyst is also located. The wall of the tube is permeable, allowing the intermediate products to pass through to the annular space instead of undergoing further reaction. The annular space is swept by an inert gas flow and contains no catalyst. Both permselective and non-permselective membranes have been considered in both co-current and counter-current flow regimes. In contrast to most catalytic membrane reactor applications where reactions are reversible and thermodynamically limited, in the present study the reactions considered are irreversible and are under kinetic control.     

Microwave Flow: A Perspective on Reactor and Microwave Configurations and the Emergence of Tunable Single‐Mode Heating Toward Large‐Scale Applications

Microwave heating in chemical reactions was first reported in 1986. There have since been many reports employing microwave heating in organic chemistry, where microwave heating has afforded higher yields of products in shorter time periods. However, such reactions are challenging to scale in batch due to the limited penetration depth of microwaves as well as the wave propagation dependence on cavity size. Continuous flow has addressed both these issues, enabling scalability of microwave processes. As such, a host of reports employing microwave flow chemistry have emerged, employing various microwave heating and reactor configurations in the context of either custom‐built or commercial apparatus. The focus of this review is to present the benefits of microwave heating in the context of continuous flow and to characterize the different types of microwave flow apparatus by their design (oscillator, cavity type and reactor vessel). We advocate the adoption of tunable, solid‐state oscillator single‐mode microwave flow reactors which are more versatile heaters, impart better process control and energy efficiency toward laboratory and larger‐scale synthetic chemistry applications.     

Fitting isoperibolic calorimeter data for reactions with pseudo-first order chemical kinetics

The non-linear least squares fitting of the chemical heat flow and the reactor temperature are compared for reactions with pseudo-first order chemical kinetics carried out in an isoperibolic calorimeter operating quasi-isothermally. Both methods give very similar results for the reaction rate constant and enthalpy of reaction but fitting the reactor temperature appears to have some advantages especially when there is an enthalpy of mixing of the reagents.     

Constraining molecules at the closest approach: chemistry at high pressure

The purpose of this tutorial review is to illustrate the effects that the application of high pressures can have on chemical reactions involving highly compressible molecular materials. The essentials of the high-pressure technology (generation and in situ control of high pressures) are described with particular attention to the versatile diamond anvil cell (DAC) apparatus. The general effects of pressure on chemical equilibrium, reaction rate and reaction mechanism are discussed. The motivation for application of high-pressure methods (in the 1-300 MPa range) to chemical synthesis and in biochemistry are illustrated focusing the attention on environmental effects and with an excursus on developing biotechnological applications. The peculiarities and the unexpected outcomes of chemical reactions occurring at very high pressures (>or=300 MPa) are discussed considering the extraordinary results obtained in polymerization and amorphization of simple molecules and of unsaturated hydrocarbons. The possible connection of the high temperature-high pressure thresholds for chemical reactions with microscopic counterparts (intermolecular distances, molecular orientations) is also discussed.     

Photon Equivalents as a Parameter for Scaling Photoredox Reactions in Flow: Translation of Photocatalytic C−N Cross‐Coupling from Lab Scale to Multikilogram Scale

With the development of new photocatalytic methods over recent decades, the translation of these chemical reactions to industrial‐production scales using continuous‐flow reactors has become a topic of increasing interest. In this context, we describe our studies toward elucidating an empirically derived parameter for scaling photocatalytic reactions in flow. By evaluating the performance of a photocatalytic C?N cross‐coupling reaction across multiple reactor sizes and geometries, it was demonstrated that expressing product yield as a function of the absorbed photon equivalents provides a predictive, empirical scaling parameter. Through the use of this scaling factor and characterization of the photonic flux within each reactor, the cross‐coupling was scaled successfully from the milligram scale in batch to a multi‐kilogram reaction in flow.     

Consideration of some sampling problems in the on-line analysis of batch processes by low-field NMR spectrometry

A low-field medium-resolution NMR spectrometer, with an operating frequency of 29 MHz for 1H, has been assessed for on-line process analysis. A flow cell that incorporates a pre-magnetisation region has been developed to minimise the decrease in the signal owing to incomplete polarisation effects. The homogeneous esterification reaction of crotonic acid and 2-butanol was monitored using a simple sampling loop; it was possible to monitor the progression of the reaction through changes in CH signal areas of butanol and butyl crotonate. On-line analysis of heterogeneous water-toluene mixtures proved more challenging and a fast sampling loop system was devised for use with a 5 L reactor. The fast sampling loop operated at a flow rate of 8 L min(-1) and a secondary sampling loop was used to pass a sub-sample through the NMR analyser at a slower (mL min(-1)) rate. It was shown that even with super-isokinetic sampling conditions, unrepresentative sampling could occur owing to inadequate mixing in the reactor. However, it was still possible to relate the 1H NMR signal obtained at a flow rate of 60 mL min(-1) to the composition of the reactor contents.     

Development of an NMR interface microchip "MICCS" for direct detection of reaction products and intermediates of micro-syntheses using a "MICCS-NMR".

The development of an NMR interface microchip and its applications to the real-time monitoring of chemical reactions are described. The microchip device was named "MICCS" (MIcro Channeled Cell for Synthesis monitoring), and the method using it was named "MICCS-NMR". MICCS was inserted into a 5 mm Phi NMR sample tube. Thus standard solution NMR probes without any modifications can be used in MICCS-NMR measurements. A gap between MICCS and the sample tube was filled with a deuterated solvent for an NMR lock. The reaction temperature and reaction time in MICCS can be easily changed by adjusting the temperature of the NMR probe and changing the flow rates, respectively. The effectiveness of the MICCS-NMR was verified in the real-time monitoring of the Wittig reaction. Preliminary data on the direct detection of intermediates of the Grignard reaction is also reported. Besides real-time monitoring of chemical reactions, MICCS-NMR would be useful as a qualitative detection method for microchip-based synthesis.     

Quantifying temperature and flow rate effects on the performance of a fixed-bed chromatographic reactor

Chromatographic reactors are based on coupling chemical reactions with chromatographic separation in fixed-beds. Temperature and flow rate are important parameters for the performance of such reactors. Temperature affects mainly adsorption, chemical equilibria, mass transfer and reaction kinetics, whereas flow rate influences residence time and dispersion. In order to evaluate the mentioned effects, the hydrolysis reactions of methyl formate (MF) and methyl acetate (MA) were chosen as case studies. These reactions were performed experimentally in a lab-scale fixed-bed chromatographic reactor packed with a strong acidic ion exchange resin. The chosen reactions can be considered to represent a relative fast (MF) and a relative slow (MA) reaction. The processes which take place inside the reactor were described and simulated using an isothermal equilibrium dispersive model. The essential model parameters were determined experimentally at different temperatures and flow rates. The performance of the chromatographic reactor was evaluated at several discrete constant temperature levels by quantifying product purity, productivity and yield. The work provides insight regarding the influence of temperature and flow rate on values of the model parameters and the performance criteria.     

Development of Flow Analytical Systems for Monitoring Thiocyanate Biodegradation in Waste Waters

ABSTRACT

Metallurgical waste waters have a very complex composition. Different reactions take place in such waters. Among them, the reaction of pyretic materials with free cyanide results in the production of thiocyanate, another hazard to the aqueous media. Biological cleanup (bioremediation) is one of the methods to eliminate SCN of waste waters and, consequently, it is necessary to develop sensitive and convenient methods for thiocyanate analysis in this bioprocess in order to assure environmental quality control.

In the work presented here different spectrophotometric flow systems based on the reaction between Fe(III) and SCN are studied to control thiocyanate in crude and biologically treated coke plant waster waters, using a conventional flow cell and a fibre optical continuous flow system. Chemical and physical parameters of the flow systems were studied. A simple, fast and sensitive continous flow system with optical fibres is proposed for monitoring thiocynate degradation in a waste water biological reactor.     

Non-isothermal two-membrane reactors for reversible gas phase reactions

The performance of a non-isothermal two-membrane reactor for reversible chemical reactions in gas phase has been analyzed by numerical simulation. The analyzed reactions were of the form: aA = bB + cC. Two membranes, that are permeable to all the components of the reaction mixture, are supposed to be the most permeable to one of the two reaction products, satisfying the condition of reverse products permselectivities. The reactant is taken to be the slowest permeating component. A negative temperature influence on the permeabilities of components has been assumed. Co-current plug flow pattern has been accepted. It has been shown that it is possible to enhance reactant conversion above that of a conventional reactor for both endothermic and exothermic reversible reactions, including adiabatic and non-adiabatic case. By using a two-membrane reactor, considerable lowering of feed temperatures is enabled for an endothermic reaction. For endothermic reactions, there is the optimum feed temperature, whereas for exothermic reactions, the higher the temperature, the lower is the attained conversion. In reactor design, the optimal external heat exchange for both endothermic and exothermic reactions can be determinated.     

Dynamic modeling of an immobilized cell reactor

A mathematical model has been developed to describe the dynamic aerobic reaction occurring in a semibatch type of mixed flow reactor, containing cells immobilized in gel beads. This modeling is an extension of that developed in our previous study, for an immobilized cell reactor involving ethanol fermentation. In contrast to anaerobic reactions such as ethanol fermentation, (wherein the influent substrate concentration can be set at any desired level), aeration becomes necessary to provide additional substrate (oxygen) for most aerobic reactions occurring in immobilized cell reactors. Tobacco cell cultivation was chosen as a representative aerobic reaction, and the effect of aeration was assessed in terms of the volumetric coefficient of oxygen from gas to liquid phases.     

Alkyne–Azide Cycloadditions with Copper Powder in a High‐Pressure Continuous‐Flow Reactor: High‐Temperature Conditions versus the Role of Additives

A safe and efficient flow‐chemistry‐based procedure is presented for 1,3‐dipolar cycloaddition reactions between organic azides and acetylenes. This simple and inexpensive technique eliminates the need for costly special apparatus and utilizes Cu powder as a plausible Cu^I source. To maximize the reaction rates, high‐pressure/high‐temperature conditions are utilized; alternatively, the harsh reaction conditions can be moderated at room temperature by the joint application of basic and acidic additives. A comparison of the performance of these two approaches in a series of model reactions has resulted in the formation of useful 1,4‐disubstituted 1,2,3‐triazoles in excellent yields. The risks that are associated with the handling of azides are lowered, thanks to the benefits of flow processing, and gram‐scale production has been safely implemented. The synthetic capability of this continuous‐flow technique is demonstrated by the efficient syntheses of some highly functionalized derivatives of the antifungal cispentacin.     

Optimization criteria for a single bead-string reactor in flow injection analysis based on consecutive kinetic reactions

In flow-injection methods based on chemical reactions, simple one-step reactions are desirable, but more complex kinetics cannot always be avoided. The consecutive reaction considered here is A + reagent (s)→B→C. The response signal depends on the dispersion of the sample zone in the system and on the kinetics of the chemical reaction. The dispersion in a single-bead-string vector is described by the tanks-in-series model. The reaction of paracetamol with hexacyanoferrate (III) and phenol is used for experimental verification. Rules for optimum reactor design are given, the reactor length being used as the optimization parameter. Comparison of the experimental and calculated peak heights at different reactor lengths confirms the optimization rules.     

Applications of the parahydrogen phenomenon in inorganic chemistry

The study of reaction mechanisms by NMR spectroscopy normally suffers from limitations in sensitivity that arise from the physical constraints of the detection method. An overview is presented of how chemical reactions can be studied using parahydrogen assisted NMR spectroscopy where detected signal strengths can exceed those normally seen by factors of over 28,000.     

Online Reaction Monitoring with Fast and Flow-Compatible Diffusion NMR Spectroscopy

Online monitoring by flow NMR spectroscopy is a powerful approach to study chemical reactions and processes, which can provide mechanistic understanding, and drive optimisations. However, some of the most useful methods for mixture analysis and reaction monitoring are not directly applicable in flow conditions. This is the case of classic diffusion-ordered NMR spectroscopy (DOSY) methods, which can be used to separate the spectral information for mixture's components. We describe a fast and flow-compatible diffusion NMR experiment that makes it possible to collect accurate diffusion data for samples flowing at up to 3 mL/min. We use it to monitor the synthesis of a Schiff base with a flow-tube with a time resolution of approximately 2 minutes. The one-shot flow-compatible diffusion NMR described here open many avenues for reaction monitoring applications.     

Reactivity of free clusters

Recent studies of the reactions of free or isolated transition metal clusters with simple molecules will be reviewed. Cluster chemical reactions are carried out in a laser-vaporization cluster source coupled to a continuous-flow reactor. Reactions can be categorized as chemisorptive (or surface) reactions, and bulk reactions. Chemisorptive reactions can be characterized asfacile, in which most cluster sizes of a given metal are equally reactive towards a particular reagent, oractivated, in which strong dependence of reactivity on cluster size is found. Under the normal operating conditions of the cluster reactor, a reaction may be kinetically controlled or at equilibrium. Following chemisorption, adsorbate decomposition and product desorption may occur. Specific reactions to be discussed include the reactions of iron clusters with ammonia and with water and the reactions of nickel clusters with ammonia.     

燃料电池技术在电催化反应领域的应用

燃料电池反应器是一种既能生产有价值化学品,又能同时发电的新型单元操作装置.由于其安全、反应易受控制、无污染,且能源资源利用率高的特点,日益受到各国工业部门的重视.本文评述了几类燃料电池反应器如酸性燃料电池、质子交换膜燃料电池和固体氧化物燃料电池的用途、工作原理及其实现工业化所面临的几个主要问题。