Monitoring of chemical reactions in real time

Dr. Neil Canter, Contributing Editor | TLT Tech Beat August 2016

A fluorescence effect potentially may be used to provide real-time data on the condition of a lubricant system.

 

KEY CONCEPTS
• A new technique for monitoring real-time chemical reactions uses nanoparticles prepared from perovskite and cesium lead halides.
• The nanoparticles exchange halides causing a gradual color change in the reaction mixture that can be monitored to follow the progress of the reaction.
• Potentially this approach could monitor the detection and relative concentration of contaminants in lubricant systems.
IN EVALUATING THE CONDITION OF LUBRICANT SYSTEMS, monitoring of specific parameters is invaluable. Typically this is done by taking a sample of the lubricant and running a series of analytical test procedures. The results are then compared to the properties of the lubricant in its original state prior to use.

Real-time monitoring can be a major benefit because it provides instant data that will enable the end-user to determine the condition of the lubricant. In a previous TLT article, a sensor device was developed that can be used to determine the viscosity of a liquid in real time (1). The approach used by the researchers was to measure parameters that can then be converted to viscosity through a mathematical relationship in a process known as an inverse method.

A similar strategy for running chemical reactions also would prove to be beneficial. There are a large number of additive chemistries used in lubricants that are prepared by different processes. In many cases, specific parameters are followed through a sampling technique to determine when the reaction has reached its end point.

Dr. Mathew Maye, associate professor of chemistry at Syracuse University in Syracuse, N.Y., says, “Chemical reactions are difficult to follow over time. Samples need to be taken from a reaction vessel and then evaluated by an analytical method such as nuclear magnetic resonance (NMR) or mass spectroscopy to give the researcher an indication of how close the reaction is to completion.”

There are challenges in monitoring reactions in this fashion according to Maye. He says, “A certain degree of expertise is needed to do analytical techniques such as NMR. The time needed to take a sample from a reaction vessel, travel to the location of the NMR instrument and wait for instrument time also can be problematic.”

A more desirable way to monitor chemical reactions is a sensor that can provide real-time data enabling the researcher to know how far the reaction has progressed. One of the best ways to accomplish this goal is through sensing a change in the color of the reaction mixture.

Maye says, “One of the areas of our research deals with the examination of inorganic nanomaterials like quantum dots. Recently in evaluating a new nanomaterial known as a perovskite, with a unique cesium lead halide composition, we found out by accident that a color change occurred in an experiment where chloride contaminants were present in a solvent. This result prompted us to determine if perovskite nanoparticles might be useful as a means to monitor for halides or other ions in solution, which eventually led us to think about if a similar approach could be used to monitor chemical reactions in real time through colorimetric analysis.”

CESIUM LEAD HALIDE PEROVSKITE NANOPARTICLES
Maye and his research associates have determined that nanoparticles prepared from perovskite and cesium lead halides react with organic compounds containing halides to act as a way to monitor halide exchange reactions in real time. Transmission electron microscopy shows that perovskite nanoparticles based on cesium lead iodides and bromides are spherical and have diameters ranging from 10-15 nanometers.

The researchers initially evaluated how these nanoparticles exchange halides using a series of tetraoctylammonium halides. A gradual color change in the reaction mixture is seen either visually or with a photoluminescence spectroscopy. This color change starts as the perovskite nanoparticles exchange one halide for a different halide.

Further testing of the capabilities of the perovskite nanoparticles was conducted through a halide exchange reaction known as the Finkelstein reactions. A specific example is the reaction of a cesium lead iodide perovskite with 2-bromododecanoic acid to produce the bromide perovskite and 2-iodododecanoic acid.

The reaction is conducted at room temperature under an inert atmosphere. Maye says, “Initially the nanoparticle containing iodide has a red fluorescence color (see Figure 2). As the halide exchange reaction proceeds, the color of the mixture changes over a 90-minute period to orange—then yellow—and ends up green indicating that the perovskite nanoparticle has completely exchanged iodides for bromides.”


Figure 2. Perovskite-based nanoparticles can follow a halide exchange reaction through a gradual color change, taking place over a 90-minute period in the images shown on the right side of the figure. Use of a nonhalide-starting material does not cause a reaction leading to no color change as shown on the left side of the figure. (Figure courtesy of Syracuse University.)

Figure 2 also shows a control where dodecanoic acid is substituted for 2-bromododecanoic acid. In that case, no halide exchange occurs, which means that no color change is observed.

The Finkelstein halide exchange process takes place through an S_N2 mechanism. One concern is that this halide exchange could lead to an elimination process leading to olefin formation. Maye says, “We used NMR analysis to prove that an elimination reaction did not take place.”

The researchers also determined that perovskite nanoparticles can exchange other anions besides halides. Anions that have been evaluated include nitrite and boron tetrafluoride.

Future work will examine if this fluorescence effect can be used to detect cations. Maye says, “We believe that perovskite nanoparticle exchange with cations will cause a color change that can be detected either by the eye or spectroscopy.”

Such a detector could be very useful in a lubricant system as a means to inform a maintenance engineer if there is a contamination problem starting to occur with a specific metal or series of metals. If this information can be provided in a real-time basis, then the chances of taking corrective action to remove the contaminant increase leading to improved lubricant performance and minimizing downtime.

Additional information on this research can be found in a recent article (2) or by contacting Maye at mmmaye@syr.edu.

REFERENCES
1. Canter, N. (2012), “Real-time monitoring of viscosity,” TLT, 68 (6), pp. 16-18.
2. Doane T., Ryan, K., Pathade, L., Cru, K., Zang, H., Cotlet, M. and Maye, M. (2016), “Using perovskite nanoparticles as halide reservoirs in catalysis and as spectrochemical probes of ions in solutions,” ACS Nano. DOI: 10.1021/acsnano.6b00806.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat can be submitted to him at neilcanter@comcast.net.