A 2021 online symposium hosted by H.E.L Group invited four specialist flow chemistry professionals to discuss the subject in-depth. This article will look at the speakers’ pain points of working in flow chemistry.
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The online/offline analysis argument explored
The first problem highlighted by Dr. Russel Taylor (EPSRC Fellow and lecturer in synthetic inorganic chemistry at Durham University) and Durham University Ph.D. student Samuel Raynes is analyzing online gas phase.
The main challenge with online analysis is that all substances being analyzed must be kept in the gas phase. This is especially important when working with GC-MS-BID. Achieving certain temperatures in research can be challenging.
Taking propane dehydrogenation and methane aromatization as an example, the high temperatures needed for online analysis (600 oC and 800 oC, respectively) can cause issues with keeping all products in the appropriate phase during analysis.
Both speakers agreed that online analysis is preferred when comparing online and offline analysis. It is beneficial where studies require parameters to be changed mid-experiment, such as step increases in temperature.
The main advantage of online analysis is that results can be investigated in real-time, whereas offline analysis cannot adequately account for everything in real time. For instance, researchers may need to ask themselves if there is a risk of contamination in the sampling procedure or if there was loss of sample.
Online analysis has the benefit of knowing that everything that comes out of the reactor goes directly into the GC. However, the detection method must be set up correctly, and online analysis cannot be used for helium or neon due to their inertness.
Parallel reactors reduce screening challenges
Taylor and Raynes also recognized that it is typical to use batch catalysis screening techniques. Employing this approach allows multiple catalysts and reactors to run simultaneously on the same system.
Flow processes usually adopt single reactor experiments which investigate catalysts one at a time to provide differentiation. The ideal flow process would involve more than one flow reactor, especially for high-volume screening. Using parallel reactors can help significantly reduce the time needed for analyses.
Parallel flow catalysis brings with it its own set of engineering challenges. A main challenge is the need to extend the time needed for testing for data quality reasons. Fewer data points are created when multiple flow reactors run through one analysis system.
Dr. Taylor highlighted that this requires compromise when using a single or limited analysis system. Experiments either yield many data points for one catalyst or fewer data points per catalyst in a parallel screening mode. The experimental needs drive researches to choose the most suitable approach.
The problem of particle size
Questions were put to chemist Lee Edwards, GSK Associate Fellow and investigator. Edwards felt that the key problem his work faced was the catalysts themselves.
Pharmaceutical companies need to buy in catalysts with very specific particle sizes for their flow applications, usually between 50 to 400 microns.
Finding a sufficient quantity of catalysts within this range is a main challenge. GSK invest heavily in sourcing specific catalysts from several catalyst suppliers and working with academics to generate new ones. In a bid to combat this, GSK has formed an informal cooperative with other pharma companies to improve their buying power and petition the catalyst suppliers for improved supply.
Edwards is hopeful that a collective approach will improve supply across the pharma industry as a whole.
Dr. Joshua Barham, Sofja Kovalevskaja Group Leader at the University of Regensburg, agreed with Edwards about the importance of particle size.
The issue of flow catalysis techniques, especially to achieve online characterization of events in a heterogenous suspended flow, is critical.
Dr. Barham noted that the Max Planck Institutes have characterized particle size distribution in photocatalysts they have supplied to his group. However, there are further challenges.
When handling the catalysts in flow, they can conglomerate, leading to significant pressure drops that need to be mitigated. To immobilize the catalyst would mean that light could not penetrate, which is a significant issue for a batch heterogeneous reaction using a photocatalytic system.
Improving the problem of gas/liquid solubility
Dr. Barham spoke about several gas/liquid solubility issues. Specifically, challenges arise with maximizing the amount of oxygen in solution when working with organic solvents.
Issues exist in balancing gas/liquid flow rates, pressure drops, and reactions that evolve gas, or where there is not enough gas in solution – especially at the smaller scale where Dr. Barham’s team operates.
Increasing pressure, slowing down feed rates, and other factors could help resolve these issues, but these can create an additional problem. Higher flow rates to increase productivity and mass transfer would mean losing out on residence time, which is an important element in photochemistry.
Flow chemistry is providing both academia and industry with innovative solutions, but several key challenges and pain points still exist. To hear more about current developments in flow chemistry, watch the full webinar recording of the H.E.L Flow Chemistry Mini-Symposium 2021.
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