microreactors for single catalyst particle diagnostics: measuring catalytic activity at high-throughput in multi-phase flows

Jeroen Vollenbroek is a PhD student in the research group Biomedical and Environmental Sensorsystems (BIOS). His supervisors are prof.dr.ir. M. Odijk from the Faculty of Electrical Engineering, Mathematics and Computer Science and prof.dr.ir. B.M. Weckhuysen from Utrecht University.

In this thesis, the work performed on a ‘Droplet Microreactor Platform’, which is a project in the ‘single Catalyst Particle Diagnostics’ project within the Multiscale Catalytic Energy Conversion (MCEC) gravitation program. The aim of this thesis was to develop microreactors using droplet microfluidics for the high-throughput screening of single catalyst microparticles. Microreactors were developed in which catalyst particles can be screened for their catalytic activity and the most active particles are sorted out.

In chapter 2 the characterization methods that exist for on-line and in-situ characterization of catalysts in microreactors were reviewed. The main conclusion is that there is a whole range of spectroscopic methods available that are well suited for studying catalyst microparticles and catalyst layers inside microreactors. The mainstream available spectroscopic techniques are Ultraviolet-Visible (UV-VIS), Infrared (IR), Raman Spectroscopy (RS), Nuclear Magnetic Resonance (NMR), and X-Ray Absorption Spectroscopy (XAS). Locally formed products, differences in activity along a catalyst bed, turnover frequencies, and even the chemical state of the catalyst themselves are measurable using these techniques. However, it was concluded that the characterization of the activity and deactivation of catalyst microparticles and catalyst layers is mostly done in packed-bed reactors with multiple particles, or wall-coated reactors. By studying multiple particles or layers at the same time, only ensemble averages are produced. It is expected that there are still many unexplored research applications for the high-throughput analysis of single catalyst particles, similar to what has been done to single cell analysis in the biological field.

Chapter 3 focusses on the design, fabrication and validation of a droplet microreactor operating at elevated temperatures and pressures. The developed microreactor made out of silicon and glass and contains integrated thin film microheaters and temperature sensors. By using Joule heating in the thin film structures, the microreactor is capable of operating at temperatures up to 120 °C. With the use of a backpressure regulator it is possible to work at a pressure of 7 bar inside the microreactor. Stable and monodisperse droplets were created and used as nanoreactors for chemical analysis, where the temperature dependent quenching of Rhodamine B is monitored at multiple locations in the microreactor as a proof of concept. Nanoparticles which exhibit temperature dependent luminescence are used for validation of the temperature inside the fluidic channels. Measurements close to the integrated temperature sensors have a temperature difference of 0.86 °C between the two methods, showing that the temperature can accurately be measured and controlled using these integrated sensors. With the developed system, possibilities for the analysis of homogeneous catalytic reactions can be explored, using either the droplet or the continuous phase as the catalyst.

In chapter 4 the silicon/glass droplet microreactor that was developed in chapter 3 is used to study the catalytic activity of single Fluid Catalytic Cracking (FCC) Equilibrium Catalyst (ECAT) particles. It is known that these particles deactivate over time due to metal accumulation and dealumination. By performing an oligomerization reaction with 4-methoxystyrene which forms a fluorescent product, the activity of the particle can be determined by the measured fluorescence intensity. The reaction is catalysed by the zeolite domains, which are the main active components, inside the particles. Particles are encapsulated in droplets and by changing the temperature of the microreactor, it is found that the reaction occurs on-chip at temperatures of 95 °C or higher. In-situ experiments are performed at this temperature and the fluorescence intensity produced by the particles is measured after a residence time of 37 seconds, where highly active particles show a strong fluorescence signal. In total ~1000 particles were analysed with an average rate of 1 particle per 2.4 seconds. Data analysis on all particles shows a distribution in particle activity, displaying a majority of bulk particles that exhibit a low-to-moderate fluorescence signal and a minority of highly active particles showing a high amount of fluorescence signal. To find the cause of this activity distribution, the particles need to be analysed ex-situ. However, to only analyse the most active ones and compare them to deactivated particles, it is necessary to separate these highly active particles from the rest. This is not possible yet with the system developed in chapter 4.

In chapter 5 a glass/glass droplet microreactor is developed that is used in the fluorescence activated catalyst sorting of FCC ECAT particles using dielectrophoresis. Particles encapsulated in droplets and stained with two different fluorescent probe molecules (4-methoxystyrene and 4-fluorostyrene), can be detected and sorted using an optical setup with a photomultiplier tube for fast feedback on the sorting system. By applying dielectrophoresis, with an electric pulse of 600V DC for 95 ms, the droplets containing highly active particles can be manipulated into a ‘sorted’ outlet that leads to a collection vessel containing only the most active particles. The less active particles with a low fluorescence signal as well as empty droplets flow unmanipulated into the ‘non-sorted’ outlet. Particles stained with 4-fluorostyrene can be collected in separate containers, enabling further ex-situ analysis. In total 10 particles stained with 4-fluorostyrene are sorted out and found in the ‘sorted’ collection container. Analysis with confocal fluorescence microscopy confirm that the most fluorescent particles are indeed sorted out and with micro X-ray fluorescence spectroscopy it is shown that these particles have less metal atoms (Ni and Fe) accumulated in them with respect to the unsorted particles. Especially the presence of Ni in the non-sorted particles indicates that the most active particles have been sorted out. Furthermore, these are probably the particles that have spent the least amount of time in the industrial reactor as fresh FCC particles do not contain any nickel. With the demonstrated fluorescence activated catalyst sorting inside a microreactor, it is possible to sort and investigate other catalysts in the future. Screening of freshly synthesized catalyst particles can already contribute valuable information on synthesis parameters. Furthermore, investigating used catalyst which has been deactivated can contribute in finding the source of deactivation. By changing the threshold of this system to only sort out the particles showing the absolute highest fluorescence peaks it could also be possible to sort zeolite ZSM-5 from zeolite-Y, because ZSM-5 has stronger acid sites than zeolite Y, making the staining with 4-fluorostyrene possibly selective for the zeolite type. Because the sorting is directly related to the activity, this sorting method can be a valuable addition to other methods such as density sorting and magnetic sorting of FCC particles. As of now, more work is needed on the sorting and analysis of particles stained with 4-fluorostyrene to make the conclusions more statistically sound.

Because catalytic reactions often consist not only out of a liquid and a solid phase, but also a gas phase is included. Therefore, chapter 6 describes the development of a multi-phase PDMS microreactor for monitoring the hydrogenation of methylene blue (MB) to leuco-methylene blue on an in-house synthesized Pd/SiO2 catalyst. A liquid channel with Pd/SiO2 particles of 40 µm encapsulated in droplets containing 20 ppm MB in ethanol are flanked by two gas channels with H2, separated with 50 µm PDMS walls. Due to the high permeability of PDMS, the hydrogen diffuses to the liquid channels and facilitates the Pd catalyzed hydrogenation of MB at room temperature. In approximately 8 s the reaction completes, as concluded from the discoloration of the particles in droplets going from blue (due to adsorbed MB) to brown (their original color). Droplets containing single particles could be analyzed individually to allow for the diagnostics of multiple heterogeneous catalyst particles, as opposed to the use of packed-bed or wall-coated reactors. This newly developed microreactor has therefore proven to be useful for the single particle diagnostics of catalyst particles in complex multiphase reactions and with this we have a new diagnostics tool in hand to improve the statistical relevance of single catalyst particle analysis and bridge the gap towards bulk characterization. Furthermore, a safe and relatively easy method has been demonstrated to work with hydrogen, as no hydrogen leak was detected by a commercial sensor during all the experiments. Furthermore, by placing the PDMS chip on a heating stage, reactions at elevated temperatures can be performed. In addition, the optical properties of PDMS allow for monitoring reactions with visible (laser) light, therefore allowing to perform in-line UV-vis or Raman spectroscopy. These techniques should be optimized in a way that they can work fast and can detect individual droplets at high flow rates. More sophisticated microreactor designs with microfluidic droplet sorters can be implemented to sort the catalyst particles of interest. This microreactor can be used in research in biomass catalysts, as hydrogenation reactions play an important role in biomass conversion.

In chapter 7 low residual stress silicon-rich silicon nitride (SiRN) membranes are fabricated for operando X-ray Absorption Spectroscopy (XAS), using hard X-rays. The use and functionality of the fabricated SiRN membranes as substrates for XAS operando measurements has been shown by the results obtained by our collaborators (Ahmed Ismail, Inorganic Chemistry and Catalysis group, Utrecht University), where the membranes are functionalized and successfully used in uncovering the interaction between NiFeOOH and the underlying hematite layer during electrocatalysis. The membrane functionalization steps and handling did not result in breakdown of the membranes. Furthermore, an outlook is given for the development of a microreactor using the same SiRN membranes that is capable of operando XAS experiments using soft X-rays. Soft X-rays enables the study of lighter elements, compared to hard X-rays and this new microreactor can therefore be used to study reaction mechanisms at a catalyst surface.