Mechanism of the Oxidation of Heptafulvenes to Tropones Studied by Online Mass Spectrometry and Density Functional Theory Calculations.

We have identified the most likely reaction mechanism for oxidizing heptafulvenes to the corresponding tropones by experimental and theoretical investigations. The experimental studies were done by coupling a three-dimensional printed miniaturized reactor with an integrated electrospray ionization needle to a mass spectrometer. Using the experimentally observed ions as a basis, nine alternative reaction pathways were investigated with density functional theory calculations. The lowest energy reaction pathway starts with the formation of an epoxide that is opened upon the addition of a second equivalent of the oxidizing species meta-chloroperoxybenzoic acid. The adduct formed then undergoes a Criegee-like rearrangement to yield a positively charged hemiketal, which on deprotonation dissociates into acetone and tropone. Overall, the reaction mechanism resembles a Hock-like rearrangement.

Simple 3D printed stainless steel microreactors for online mass spectrometric analysis.

A simple flow chemistry microreactor with an electrospray ionization tip for real time mass spectrometric reaction monitoring is introduced. The microreactor was fabricated by a laser-based additive manufacturing technique from acid-resistant stainless steel 316L. The functionality of the microreactor was investigated by using an inverse electron demand Diels-Alder and subsequent retro Diels-Alder reaction for testing. Challenges and problems encountered are discussed and improvements proposed. Adsorption of reagents to the rough stainless steel channel walls, short length of the reaction channel, and making a proper ESI tip present challenges, but the microreactor is potentially useful as a disposable device.

Efficient Catalytic Microreactors with Atomic-Layer-Deposited Platinum Nanoparticles on Oxide Support.

Microreactors attract a significant interest for chemical synthesis due to the benefits of small scales such as high surface to volume ratio, rapid thermal ramping, and well-understood laminar flows. The suitability of atomic layer deposition for application of both the nanoparticle catalyst and the support material on the surfaces of channels of microfabricated silicon microreactors is demonstrated in this research. Continuous-flow hydrogenation of propene into propane at low temperatures with TiO2 -supported catalytic Pt nanoparticles was used as a model reaction. Reaction yield and mass transport were monitored by high-sensitivity microcoil NMR spectroscopy as well as time-of-flight remote detection NMR imaging. The microreactors were shown to be very efficient in propene conversion into propane. The yield of 100 % was achieved at 50 °C with a reactor decorated with Pt nanoparticles of average size of roughly 1 nm and surface coverage of 3.2 % in 20 mm long reaction channels with a residence time of 1100 ms. The activity of the Pt catalyst surfaces was on the order of several to tens of mmol s-1 m-2 .

A Simple Method for Improving the Spatial Resolution in Infrared Laser Ablation Mass Spectrometry Imaging.

In mass spectrometry imaging of tissues, the size of structures that can be distinguished is determined by the spatial resolution of the imaging technique. Here, the spatial resolution of IR laser ablation is markedly improved by increasing the distance between the laser and the focusing lens. As the distance between the laser and the lens is increased from 1 to 18 m, the ablation spot size decreases from 440 to 44 µm. This way, only the collimated center of the divergent laser beam is directed on the focusing lens, which results in better focusing of the beam. Part of the laser energy is lost at longer distance, but this is compensated by focusing of the radiation to a smaller area on the sample surface. The long distance can also be achieved by a set of mirrors, between which the radiation travels before it is directed to the focusing lens and the sample. This method for improving the spatial resolution can be utilized in mass spectrometry imaging of tissues by techniques that utilize IR laser ablation, such as laser ablation electrospray ionization, laser ablation atmospheric pressure photoionization, and matrix-assisted laser desorption electrospray ionization. Graphical Abstract ᅟ.

Lab-on-a-chip reactor imaging with unprecedented chemical resolution by Hadamard-encoded remote detection NMR.

The development of microfluidic processes requires information-rich detection methods. Here we introduce the concept of remote detection exchange NMR spectroscopy (RD-EXSY), and show that, along with indirect spatial information extracted from time-of-flight data, it provides unique information about the active regions, reaction pathways, and intermediate products in a lab-on-a-chip reactor. Furthermore, we demonstrate that direct spatial resolution can be added to RD-EXSY efficiently by applying the principles of Hadamard spectroscopy.

Remote detection NMR imaging of gas phase hydrogenation in microfluidic chips.

The heterogeneous hydrogenation reaction of propene into propane in microreactors is studied by remote detection (RD) nuclear magnetic resonance (NMR). The reactors consist of 36 parallel microchannels (50 × 50 µm(2) cross sections) coated with a platinum catalyst. We show that RD NMR is capable of monitoring reactions with sub-millimeter spatial resolution over a field-of-view of 30 × 8 mm(2) with a steady-state time-of-flight time resolution in the tens of milliseconds range. The method enables the visualization of active zones in the reactors, and time-of-flight is used to image the flow velocity variations inside the reactor. The overall reaction yields determined by NMR varied from 10% to 50%, depending on the flow rate, temperature and length of the reaction channels. The reaction yield was highest for the channels with the lowest flow velocity. Propane T1 relaxation time in the channels, estimated by means of RD NMR images, was 270 ± 18 ms. No parahydrogen-induced polarization (PHIP) was observed in experiments carried out using parahydrogen-enriched H2, indicating fast spreading of the hydrogen atoms on the sputtered Pt surface. In spite of the low concentration of gases, RD NMR made imaging of gas phase hydrogenation of propene in microreactors feasible, and it is a highly versatile method for characterizing on-chip chemical reactions.