Analyzing ionic liquids using ATR | Spectroscopy Solutions
Research into ionic liquid-based catalyst systems aim to improve the duty cycle, rate of metal loss and yield efficiency of large scale reactions using continuous flow reactors, such as those often used in the pharmaceutical industry.
This article details the issues that are faced by the industry and a recent study that uses FTIR-ATR spectroscopy in the development of palladium poly(ionic liquid) catalyst membranes.
What are ionic liquids?
Ionic Liquids (IL) are generally defined as ionic compounds that are liquid below 100 °C. More usually they have melting points below room temperature.
Compared with conventional ionic compounds, their liquid state is due to a significantly lower symmetry and the charge of the cation and the anion being distributed over a larger volume of the molecule by resonance.
Structures of cations of ionic liquids
Because of this, the solidification of the Ionic Liquid takes place at lower temperatures.
Why are ionic liquids useful?
Ionic Liquids have such a unique collection of thermal, solvent and electrical properties, that they have become interesting to chemists in a number of applications. The flexibility of ILs allows a huge number of possible combinations of organic cations and anions.
Structure of anions of ionic liquids- hexafluorophosphate, tetrafluoroborate, trifluoroacetate, triflate
This allows chemists to design and fine-tune the physical and chemical properties of ILs by introducing or combining structural motifs and, thereby, allowing tailor-made materials. One particular area is in the formation of membrane structures for separations and also to support catalyst materials for high yield chemical reactions in the pharmaceutical industry.
The Suzuki-Miyaura coupling reaction
Suzuki reaction is a coupling reaction between a boronic acid and an organohalide catalysed by a palladium(0) complex. This important reaction was first discovered in 1979 by Akira Suzuki and winning the 2010 Nobel Prize in Chemistry for Suzuki, Heck and Negishi.
Mechanism of Suzuki coupling reaction
The reaction is widely used to synthesise poly-olefins, styrenes, and substituted biphenyls. This type of coupling reaction is used to produce the painkiller Naproxen, a nonsteroidal anti-inflammatory drug related to propionic acid.
Ionic liquid membrane systems have now taken the Suzuki reaction to a new degree of efficiency. This is because the palladium catalyst can be immobilised and supported inside ionic liquid membranes.
These systems using polymer backbones such as poly(vinylbenzyl) have been shown to be highly effective catalyst systems with high yields and reaction selectivity. However, there are also disadvantages including the need for high metal catalyst loadings, metal loss due to leaching, or catalyst breakdown, which can lead to additional processing steps.
Wilson, Kore et al. (2018), Hybrid ionic liquid membrane catalysts
Hybrid catalyst–membrane systems can now address the problems providing high efficiency and excellent yield of products along with low metal loss, and a recyclable catalyst. For the Suzuki–Miyaura coupling reaction, palladium species are immobilised onto Poly(ionic liquids) supported onto membranes.
In a recent study by Wilson, Kore et al. (2018)3, anisotropic palladium–poly(ionic liquid) catalyst membranes supported by polytetrafluoroethylene (PTFE) were produced by pulsed plasma deposition of poly(vinylbenzyl chloride) to form an attached benzyl chloride layer.
This was then followed by the Menshutkin reaction to form surface tethered quaternized N-butylimidazole moieties which are subsequently complexed with palladium (II) chloride to the imidazolium rings. Benzyl chloride groups provide a quick, low cost approach for fabricating anisotropic palladium–poly(ionic liquid) catalyst membrane systems.
Infrared to follow the membrane functionalisation
The pulsed plasma deposition of the benzyl chloride layer is crucial to the formation of the ionic liquid catalyst layer on the PTFE. This reaction was followed by infrared spectroscopy of pulsed plasma deposited benzylchloride films.
This confirmed a high level of benzyl chloride functional group structural retention by monitoring the disappearance of the monoalkyl vinyl CH2 wag vibration mode (906 cm−1). The characteristic Cl-CH2- wag mode at 1263 cm−1 of the benzylchloride group could also be distinguished.
To follow the attachment of the benzylchloride layer to PTFE, attenuated total reflectance (ATR) Infrared spectra were obtained of vinylbenzyl chloride liquid precursor using a Golden Gate accessory from Specac Ltd.
The Golden Gate has the advantage that it allows for many different sampling options in the quantitative and qualitative analysis of solids, liquids, pastes and microsamples and also a number of different viewing modes. The KRS-5 lenses used in this instance allow for a slightly wider spectral transmission range for Mid IR studies allowing more detail of the 906cm-1 and 1263cm-1 signals.
Quaternisation and characterisation of the membrane
Quaternisation of the membrane attached benzylchloride group was also followed successfully using FTIR techniques. Pulsed plasma deposited poly(vinylbenzylchloride) quaternised with N-butylimidazole was identified by characteristic para-substituted benzene ring stretches at 1603 cm−1 and 1490 cm−1 respectively.
In these cases Reflection-Absorption InfraRed (RAIRS) Spectra were attained of pulsed plasma poly(vinylbenzyl chloride) deposited onto silicon wafers. An FTIR spectrometer was used, fitted with a liquid nitrogen cooled MCT detector operating at 4 cm−1 resolution across the 400–4000 cm−1 range.
The instrument included a variable angle reflection-absorption accessory from Specac Ltd, which was set to a grazing angle of 66° for silicon wafer substrates and adjusted for p-polarization. RAIRS is a useful vibrational spectroscopy technique for studying the identity, structure, and orientation of adsorbates on single surfaces such as silicon wafers.
Resulting reaction efficiency
The produced membrane system is then used in a continuous flow anisotropic palladium–poly(ionic liquid) catalyst system. The catalyst coated membrane substrates were evaluated in a heated batch reactor and sonicated for the Suzuki–Miyaura coupling reaction and shown to exhibit an excellent 77 ± 7% product yield (343 K, 0.5 h, 0.06 mol % Pd loading) and >99% selectivity, as well as retention of catalytic activity over an extended period.
The system was also associated over a long period with low levels of palladium catalyst leaching (from an already small (sub 0.1 mol% Pd) catalyst loading). This ionic liquid membrane catalyst system also successfully facilitates the selective separation of the desired Suzuki–Miyaura carbon–carbon coupling reaction biphenyl product from starting materials.
The ionic liquid-based catalyst system has huge potential in large scale reactions in the pharmaceutical industry that use continuous flow reactors. This is because it has high yield efficiency, a low rate of metal loss, a long-duty cycle and the potential to be used with a wider range of precious metal reaction methodologies.