Jason R. Hyde

Continuous catalytic reactions in supercritical fluids

Abstract Recent heightened awareness of the environmental impacts associated with a large proportion of established chemical processes has led to the application of considerable pressures on the chemical industry, both regulatory and consumer driven, to adopt a cleaner and greener approach to manufacture. The economies of scale and associated efficiencies of continuous processes have long been a contributing factor in the design and efficient running of many large-scale industrial plants. When successfully combined with a versatile and environmentally benign solvent system such as supercritical fluids (SCFs), continuous processing can be seen to be suitable for a wide variety of reactions (hydrogenation, hydroformylation, alkylation, etc.). which can be conducted efficiently in an environmentally sensitive way. This review article aims to show the reader how the marriage of these two technologies is helping chemistry to achieve this goal.

Electrodeposition of metals from supercritical fluids

Electrodeposition is a widely used materials-deposition technology with a number of unique features, in particular, the efficient use of starting materials, conformal, and directed coating. The properties of the solvent medium for electrodeposition are critical to the techniques applicability. Supercritical fluids are unique solvents which give a wide range of advantages for chemistry in general, and materials processing in particular. However, a widely applicable approach to electrodeposition from supercritical fluids has not yet been developed. We present here a method that allows electrodeposition of a range of metals from supercritical carbon dioxide, using acetonitrile as a co-solvent and supercritical difluoromethane. This method is based on a careful selection of reagent and supporting electrolyte. There are no obvious barriers preventing this method being applied to deposit a range of materials from many different supercritical fluids. We present the deposition of 3-nm diameter nanowires in mesoporous silica templates using this methodology.

Supercritical fluids: A route to palladium-aerogel nanocomposites

Supercritical carbon dioxide (scCO2) is used to prepare novel silica aerogel composites containing nano-particles of palladium. The material produced has been found to exhibit a Pd loading of 8% by wt. The particles deposited fit within two discrete size ranges of <6 nm and 15–20 nm as analysed by XRD and TEM. We demonstrate that scCO2 may be successfully applied to prepare catalytic support materials based upon aerogels and that the catalyst is active in a continuous flow reactor leading to effective hydrogenation of cyclohexene.

Two-dimensional correlation analysis of Raman optical activity data on the α-helix-to-β-sheet transition in poly(L-lysine)

Raman optical activity (ROA) has evolved into an incisive probe of structure and conformational transitions in polypeptides and proteins revealing many signal patterns characteristic of specific secondary structural elements. In order to further facilitate analysis of ROA spectral intensity variations, two-dimensional correlation methods are applied to ROA and Raman spectra monitoring the α-helix-to-β-sheet transition in poly(L-lysine) as a function of temperature. Pretreatment of data using background subtraction, normalization and gentle smoothing is essential for the successful generation of 2D ROA correlations, 2D Raman correlations and 2D Raman/ROA heterocorrelations. The pseudoscalar nature of ROA spectra results in detailed 2D correlation analyses providing extensive interpretation of spectral intensity variations. Synchronous plots indicate band assignments consistent with established assignments in poly(L-lysine) together with possible new assignments. Corresponding asynchronous plots probe the temporal sequence of the conformational transition indicating distinct temporal phases while monitoring aggregation through a small amount of β-structure present at the start of the experiment ahead of α-helix unfolding. This study demonstrates the potential of 2D correlation analysis as a valuable technique for the extraction of detailed information about aggregation and conformational transitions in polypeptides and proteins from associated ROA and Raman spectra. Results indicate that aggregation of poly(L-lysine) monomers precedes intramolecular conversion of α-helix to β-sheet, which is then followed by fibril formation.

In situ generation of hydrogen for continuous hydrogenation reactions in high temperature water

The thermal decomposition of HCO2H or preferably, HCO2X (X = Na or NH4) can be used to generate H2 for the continuous hydrogenation of aromatic and cyclic aldehydes, ketones and nitroaromatics in high temperature pressurised water (HTPW). This means that hydrogenation reactions can be carried out in exactly the same equipment as has previously been used for selective oxidation in HTPW, thus facilitating relatively simple application of these reactions for non-specialists.

Supercritical hydrogenation and acid-catalysed reactions "without gases"

The high temperature catalytic decomposition of HCO2H and HCO2Et are used to generate the high pressure H2 and the supercritical fluids needed for micro-scale hydrogenation of organic compounds; our approach overcomes the problems and limitations of handling high pressure gases on a small-scale and opens the way to the widespread use of continuous supercritical reactions in the laboratory.

Continuous hydrogenation reactions in supercritical CO2“without gases”

Continuous fixed-bed hydrogenation reactions are one of the most promising reactions studied under supercritical conditions. A reactor and supporting equipment has been developed in a collaboration between the University of Nottingham and HEL Ltd. to provide the means for small-scale experimental research. The high pressure gases required to achieve the supercritical state are not supplied by bottled or liquefied gases, but by the in situ decomposition of formic acid, HCO2H, which can be selectively decomposed to produce CO2 and H2. These gases can be used directly as both the supercritical solvent and reagent gases. Further control of the H2 concentration can be achieved by the parallel decomposition of other liquid precursors, namely ethyl formate HCO2Et, which can produce C2H6 + CO2. We report the hydrogenation of several organic substrates to demonstrate this approach, its application in research and its potential as a development tool.

Continuous reactions in supercritical fluids; a cleaner, more selective synthesis of thymol in supercritical CO2

Continuous fixed-bed catalytic Friedel–Crafts alkylation of m-cresol with different alkylating agents, isopropanol (IPA) and propylene, has been carried out using supercritical CO2, scCO2, as an alternative and more environmentally friendly reaction medium, for the synthesis of the fine chemical thymol. Both a solid Lewis acid catalyst (γ-Al2O3) and a solid Bronsted acid catalyst (Nafion® SAC-13) have been investigated over a range of reaction conditions to optimise yield and selectivity for thymol. The reaction product distribution was found to be related to the type of catalyst employed. This is likely to have been due to the different reaction pathways through which the reaction occurred, a direct Friedel–Crafts alkylation in the case of Bronsted type acids and a Fries rearrangement when employing the Lewis catalyst. The new technique of 2DCOR-GC analysis was employed to establish the order of formation of the different species generated in the reaction over the two catalysts in scCO2.

A non-oxide sol–gel route to synthesise silicon imidonitride monolithic gels and high surface area aerogels

Monolithic gels have been produced by reaction of Si(NHMe)(4) with ammonia in THF solution and converted to high surface area aerogels by critical point drying with ammonia saturated diethylamine solvent.

The automation of continuous reactions in supercritical CO2: the acid-catalysed etherification of short chain alcohols

We describe a new, automated apparatus for continuous catalytic reactions in supercritical carbon dioxide, scCO2. The reactor incorporates on-line sampling, enabling us to quantify gaseous products such as light alkenes, which would normally be lost in the decompression stage of more conventional supercritical reactors. The on-line sampling is coupled to a gas chromatograph running an isothermal program, which provides automated analysis of the reaction products. We illustrate the operation of the reactor by studying the acid-catalysed etherification of linear alcohols, C1 to C5, both individually and in pairs, over a range of temperatures and pressures equating to a total of 180 different experiments.