Continuous flow synthesis of pyridinium salts accelerated by multi-objective Bayesian optimization with active learning.

We report a human-in-the-loop implementation of the multi-objective experimental design via a Bayesian optimization platform (EDBO+) towards the optimization of butylpyridinium bromide synthesis under continuous flow conditions. The algorithm simultaneously optimized reaction yield and production rate (or space-time yield) and generated a well defined Pareto front. The versatility of EDBO+ was demonstrated by expanding the reaction space mid-campaign by increasing the upper temperature limit. Incorporation of continuous flow techniques enabled improved control over reaction parameters compared to common batch chemistry processes, while providing a route towards future automated syntheses and improved scalability. To that end, we applied the open-source Python module, nmrglue, for semi-automated nuclear magnetic resonance (NMR) spectroscopy analysis, and compared the acquired outputs against those obtained through manual processing methods from spectra collected on both low-field (60 MHz) and high-field (400 MHz) NMR spectrometers. The EDBO+ based model was retrained with these four different datasets and the resulting Pareto front predictions provided insight into the effect of data analysis on model predictions. Finally, quaternization of poly(4-vinylpyridine) with bromobutane illustrated the extension of continuous flow chemistry to synthesize functional materials.

Bioinspired High-Performance Energetic Materials Using Heme-Containing Crystals.

Synthetic hemozoin crystals (ß-hematin) are assembled with aluminium nanoparticles (nAl) to create a nanomaterial composite that is highly energetic and reactive. The results here demonstrate that hemozoin rapidly oxidizes the nAl fuel to release large amounts of energy (+12.5 ± 2.4 kJ g(-1) ).

Single walled carbon nanotube growth and chirality dependence on catalyst composition.

Vertical arrays of single walled carbon nanotubes (VA-SWNTs) were grown using bi-metallic nanoparticle pro-catalysts. Iron oxide particles were doped with varying quantities of first row transition metals (Mn, Co, Ni, and Cu) for a comparative study of the growth of nanotubes. VA-CNT samples were verified using scanning electron microscopy, and characterized using resonance Raman spectroscopy. The length of the VA-CNTs is used as a measure of catalyst activity: the presence of dopants results in a change in the CNT length and length distribution. Cross correlation of the Raman spectra reveal variations in the distribution of radial breathing mode peaks according to the pro-catalyst composition. The formation of various chirality nanotubes is constant between repetitive runs with a particular catalyst, but may be controlled by the identity and concentration of the metal dopants within the iron catalyst. These results demonstrate that the composition of the catalyst is a major driving force toward type selective growth of nanotubes.

Biologically tunable reactivity of energetic nanomaterials using protein cages.

The performance of aluminum nanomaterial based energetic formulations is dependent on the mass transport, diffusion distance, and stability of reactive components. Here we use a biologically inspired approach to direct the assembly of oxidizer loaded protein cages onto the surface of aluminum nanoparticles to improve reaction kinetics by reducing the diffusion distance between the reactants. Ferritin protein cages were loaded with ammonium perchlorate (AP) or iron oxide and assembled with nAl to create an oxidation-reduction based energetic reaction and the first demonstration of a nanoscale biobased thermite material. Both materials showed enhanced exothermic behavior in comparison to nanothermite mixtures of bulk free AP or synthesized iron oxide nanopowders prepared without the use of ferritin. In addition, by utilizing a layer-by-layer (LbL) process to build multiple layers of protein cages containing iron oxide and iron oxide/AP on nAl, stoichiometric conditions and energetic performance can be optimized.

Influencing solvent miscibility and aqueous stability of aluminum nanoparticles through surface functionalization with acrylic monomers.

With growing interest in the development of new composite systems for a variety of applications that require easily processable materials and adequate structural properties with high energy densities, we have pursued the chemical functionalization of oxide-passivated aluminum nanoparticles (nAl) using three acrylic monomers, 3-methacryloxypropyltrimethoxysilane (MPS), 2-carboxyethyl acrylate (CEA), and phosphonic acid 2-hydroxyethyl methacrylate ester (PAM), to provide chemical compatibility within various solvent and polymeric systems. Fourier transform infrared and X-ray photoelectron spectroscopy suggest that attachment of MPS and PAM monomers occurs through the formation of bonds directly to the passivated oxide surface upon reaction with surface hydroxyls, whereas CEA monomers interact through the formation of ionic carboxylate binding to aluminum atoms within the oxide. The coated particles demonstrate enhanced miscibility in common organic solvents and monomers; MPS and PAM coatings are additionally shown to inhibit oxidation of the aluminum particles when exposed to aqueous environments at room temperature, and PAM coatings are stable at even elevated temperatures.

Ultrasmall copper nanoparticles from a hydrophobically immobilized surfactant template.

Ultrasmall copper nanoparticles are produced by N,N,N',N'-tetramethyl-p-phenylenediamine (TMPDA) reduction of aqueous Cu(2+) on a hydrophobically immobilized sodium dodecylbenzenesulfonate (SDBS) surfactant template in the presence of sodium citrate at room temperature. Single-walled carbon nanotubes (SWNTs) act as a scaffold controlling the size of the SDBS micelle, which in turn confines a limited number of copper ions near the nanotube surface. TMPDA reduction forms copper nanoparticles as confirmed by X-ray photoelectron spectroscopy and electron diffraction, whose size was determined by atomic force microscopy and transmission electron microscopy to be approximately 2 nm. Particles formed in the absence of the SWNT immobilizer range from 2 to 150 nm.

Self-assembled monolayer and multilayer films of the nanocluster [HxPMo12O40 subsetH4Mo72Fe30(O2CMe)15O254(H2O)68] on gold.

Films of the molybdenum-iron nanocluster [H x PMo 12O 40 subsetH 4Mo 72Fe 30(O 2CMe) 15O 254(H2O) 68] (FeMoC) were generated on gold via the self-assembly technique using two divergent routes. The first route entails the self-assembly of unfunctionalized FeMoC onto a preprepared carboxyl-terminated SAM on gold. The second route involves the preparation of thiol-terminated functionalized FeMoC clusters, which are then allowed to self-assemble onto bare gold surfaces. Monolayer films of FeMoC clusters are attained via both routes, with the second route requiring shorter immersion times (2 days) than the first route (6 days). Multilayer films of FeMoC are formed via the second route for immersion times longer than 2 days. Characterization of these films using optical ellipsometry, X-ray photoelectron spectroscopy, and atomic force microscopy confirm the self-assembly of the clusters on the surfaces.

Growth, new growth, and amplification of carbon nanotubes as a function of catalyst composition.

Carbon nanotubes (CNTs) have been grown using Fe, Co, Ni, and Co/Fe spin-on-catalyst (SOC) systems, involving the metal salt dispersed with a spin-on-glass precursor. During initial growth runs (CH4/H2/900 degrees C), the CNT yield followed the order Co-SOC > Fe-SOC >> Ni-SOC. The Fe catalysts produced the longest nanotubes at the expense of a larger average CNT diameter and broader diameter distribution than the Co-SOC system. A series of Co/Fe-SOCs were prepared where as the atomic percentage of Co is increased nucleation of CNT increases but the CNT length decreases. The linear relationship between the diameter and length of CNTs grown from the Co/Fe-SOC suggests that slow growth is beneficial with respect to control over CNT diameter. After initial CNT growth, the original samples were subjected to additional growth runs. Four individual reactions were observed in the Fe-SOC and binary Co/Fe-SOC: regrowth (amplification), double growth (a second CNT growing from a previously active catalyst), CNT etching, and nucleation from initially inactive catalysts (new growth). CNT etching was observed for the mixed catalyst systems (Co/Fe-SOC) but not for either Fe-SOC or Co-SOC. During the regrowth experiments, CNTs were observed that were not present after the initial growth run (and were not as a result of amplification or double growth). Thus, catalysts, which were initially inactive toward nucleation of CNTs in the original growth run, are capable of becoming activated when placed back into the furnace and submitted to regrowth under identical conditions.

Synthesis, characterization, and carbon dioxide adsorption of covalently attached polyethyleneimine-functionalized single-wall carbon nanotubes.

The reaction between fluorinated single-wall carbon nanotubes (F-SWNTs) and branched (M(w) = 600, 1800, 10000, and 25000 Da) or linear (M(w) = 25000 Da) polyethyleneimine (PEI) yields the covalent attachment of the polymer to the sidewalls of the nanotubes. The resulting PEI-functionalized SWNTs (PEI-SWNTs) were characterized by solid-state (13)C NMR, Raman spectroscopy, X-ray photoelectron spectroscopy, UV-vis spectroscopy, atomic force microscopy, transmission electron microscopy, and thermal gravimetric analysis studies. As expected, the number of polymer molecules per SWNT is larger for low molecular weight PEI than for high molecular weight PEI. However, above 1800 Da, the number of polymer molecules per SWNT does not vary as much. This is supported by Raman spectral data that shows the D:G ratio is relatively insensitive of the molecular weight for M(w) > 1800 Da. The PEI-SWNTs are shown to have solubility in aqueous media of up to 0.4 mg x mL(-1). Solid-state (13)C NMR shows the presence of carboxylate substituents that have been attributed to carbamate formation as a consequence of the reversable CO(2) absorption to the primary amine substituents of the PEI. Desorption of CO(2) is accomplished by heating under argon at 75 degrees C, while the dependence of the quantity of CO(2) absorbed on temperature and the molecular weight of the PEI is reported. Under the conditions investigated the maximum absorption of 9.2% w/w is observed for PEI(25000)-SWNT at 27 degrees C. The possible CO(2) absorption applications of the PEI-SWNTs is discussed.

A study of the formation, purification and application as a SWNT growth catalyst of the nanocluster [HxPMo12O40[subset]H4Mo72Fe30(O2CMe)15O254(H2O)98].

The synthetic conditions for the isolation of the iron-molybdenum nanocluster FeMoC [HxPMo12O40 [subset]H4Mo72Fe30(O2CMe)15O254(H2O)98], along with its application as a catalyst precursor for VLS growth of SWNTs have been studied. As-prepared FeMoC is contaminated with the Keplerate cage [H4Mo72Fe30(O2CMe)15O254(H2O)98] without the Keggin [HxPMo12O40]n- template, however, isolation of pure FeMoC may be accomplished by Soxhlet extraction with EtOH. The resulting EtOH solvate is consistent with the replacement of the water ligands coordinated to Fe being substituted by EtOH. FeMoC-EtOH has been characterized by IR, UV-vis spectroscopy, MS, XPS and 31P NMR. The solid-state 31P NMR spectrum for FeMoC-EtOH (delta-5.3 ppm) suggests little effect of the paramagnetic Fe3+ centers in the Keplerate cage on the Keggin ion's phosphorous. The high chemical shift anisotropy, and calculated T1 (35 ms) and T2 (8 ms) values are consistent with a weak magnetic interaction between the Keggin ion's phosphorus symmetrically located within the Keplerate cage. Increasing the FeCl2 concentration and decreasing the pH of the reaction mixture optimizes the yield of FeMoC. The solubility and stability of FeMoC in H2O and MeOH-H2O is investigated. The TGA of FeMoC-EtOH under air, Ar and H2 (in combination with XPS) shows that upon thermolysis the resulting Fe : Mo ratio is highly dependent on the reaction atmosphere: thermolysis in air results in significant loss of volatile molybdenum components. Pure FeMoC-EtOH is found to be essentially inactive as a pre-catalyst for the VLS growth of single-walled carbon nanotubes (SWNTs) irrespective of the substrate or reaction conditions. However, reaction of FeMoC with pyrazine (pyz) results in the formation of aggregates that are found to be active catalysts for the growth of SWNTs. Activation of FeMoC may also be accomplished by the addition of excess iron. The observation of prior work's reported growth of SWNTs from FeMoC is discussed with respect to these results.