Heather A. Meylemans

Solvent‐Free Conversion of Linalool to Methylcyclopentadiene Dimers: A Route To Renewable High‐Density Fuels

The development of techniques for the efficient synthesis of custom fuels and chemicals from sustainable natural feedstocks is of fundamental importance to society as the direct and indirect costs of petroleum use continue to increase. For general transportation fuels, complex mixtures of molecules that have somewhat lower utility than petroleum-based analogs may be sufficient; however, for specific applications, such as jet and missile propulsion, a more selective model that produces molecules with defined and specialized properties is required. Well-characterized, single-site catalysis is the basis of elegant synthetic strategies for the production of pure compounds. In particular, ruthenium-based olefin metathesis catalysts are known to catalyze a number of reactions, including self-metathesis, cross-metathesis, ring-closing metathesis (RCM), and ring-opening metathesis polymerization (ROMP). This family of catalysts is ubiquitous in the literature and has been applied in many fields of chemistry, ranging from natural product synthesis to polymer chemistry. The transition of these catalysts to large-scale industrial processes has in the past been hindered by their modest turnover numbers and high cost. To overcome these difficulties, catalytic systems that can efficiently yield pure products while maintaining low catalyst concentrations need to be developed. In this report, we detail a ruthenium-catalyzed method for the synthesis of dimethyldicyclopentadiene from linalool, a linear terpene alcohol. Recent work in our laboratory has focused on the conversion of terpenes into high-density fuel surrogates. Although terpenes are naturally produced by pine trees and a variety of plants, a truly sustainable method may require the utilization of bioengineered microbes to produce specific molecules or families of molecules from waste cellulose. Within the terpene family, linalool is a particularly intriguing feedstock for fuels because of its molecular structure. Although RCM of linalool must proceed through a sterically hindered transition state, the reaction is facilitated by coordination of the allylic alcohol. This results in an efficient method for the synthesis of 1-methylcyclopent-2-enol (1) and isobutylene (Scheme 1). Both products are of significant interest as they can be converted to renewable fuel and polymer products. Isobutylene is a valuable side-product that can be selectively trimerized to produce jet fuel, dimerized or alkylated with C4 raffinate to produce high-octane gasoline, or polymerized to polyisobutylene. Meanwhile, 1 is a promising precursor for the synthesis of methylcyclopentadiene dimer, which can be hydrogenated and isomerized to produce the high-density missile fuel RJ-4 (Scheme 1). NMR-scale conversions of linalool to 1 under dilute conditions and at elevated temperatures have been reported in the literature. Catalysts used for this reaction (Figure 1) have included the first-generation Grubbs catalyst (2), both a second-generation Grubbs (5) and Grubbs–Hoveyda catalyst

Synthesis, Characterization, and Cure Chemistry of Renewable Bis(cyanate) Esters Derived from 2-Methoxy-4-Methylphenol

A series of renewable bis(cyanate) esters have been prepared from bisphenols synthesized by condensation of 2-methoxy-4-methylphenol (creosol) with formaldehyde, acetaldehyde, and propionaldehyde. The cyanate esters have been fully characterized by infrared spectroscopy, (1)H and (13)C NMR spectroscopy, and single crystal X-ray diffraction. These compounds melt from 88 to 143 °C, while cured resins have glass transition temperatures from 219 to 248 °C, water uptake (96 h, 85 °C immersion) in the range of 2.05-3.21%, and wet glass transition temperatures from 174 to 193 °C. These properties suggest that creosol-derived cyanate esters may be useful for a wide variety of military and commercial applications. The cure chemistry of the cyanate esters has been studied with FTIR spectroscopy and differential scanning calorimetry. The results show that cyanate esters with more sterically demanding bridging groups cure more slowly, but also more completely than those with a bridging methylene group. In addition to the structural differences, the purity of the cyanate esters has a significant effect on both the cure chemistry and final Tg of the materials. In some cases, post-cure of the resins at 350 °C resulted in significant decomposition and off-gassing, but cure protocols that terminated at 250-300 °C generated void-free resin pucks without degradation. Thermogravimetric analysis revealed that cured resins were stable up to 400 °C and then rapidly degraded. TGA/FTIR and mass spectrometry results showed that the resins decomposed to phenols, isocyanic acid, and secondary decomposition products, including CO2. Char yields of cured resins under N2 ranged from 27 to 35%, while char yields in air ranged from 8 to 11%. These data suggest that resins of this type may potentially be recycled to parent phenols, creosol, and other alkylated creosols by pyrolysis in the presence of excess water vapor. The ability to synthesize these high temperature resins from a phenol (creosol) that can be derived from lignin, coupled with the potential to recycle the composites, provides a possible route to the production of sustainable, high-performance, thermosetting resins with reduced environmental impact.

Renewable thermosetting resins and thermoplastics from vanillin

Two cyanate ester resins and a polycarbonate thermoplastic have been synthesized from vanillin. The bisphenol precursors were prepared by both an electrochemical route as well as by a McMurry coupling reaction. 1,2-Bis(4-cyanato-3-methoxyphenyl)ethene (6) had a high melting point of 237 °C and did not cure completely under a standard cure protocol. In contrast, the reduced version, 1,2-bis(4-cyanato-3-methoxyphenyl)ethane (7) melted at 190 °C and underwent complete cure to form a thermoset material with Tg = 202 °C. 7 showed thermal stability up to 335 °C and decomposed via formation of phenolics and isocyanic acid. A polycarbonate was then synthesized from the reduced bisphenol by a transesterification reaction with diphenylcarbonate. The polymer had Mn = 3588, Mw/Mn = 1.9, and a Tg of 86 °C. TGA/FTIR data suggested that the polycarbonate decomposed via formation of benzodioxolones with concomitant elimination of methane. The results show that vanillin is a useful precursor to both thermosetting resins and thermoplastics without significant modification.

Synthesis of Renewable Bisphenols from Creosol

A series of renewable bisphenols has been synthesized from creosol (2-methoxy-4-methylphenol) through stoichiometric condensation with short-chain aldehydes. Creosol can be readily produced from lignin, potentially allowing for the large scale synthesis of bisphenol A replacements from abundant waste biomass. The renewable bisphenols were isolated in good yields and purities without resorting to solvent-intense purification methods. Zinc acetate was shown to be a selective catalyst for the ortho-coupling of formaldehyde, but was unreactive when more sterically demanding aldehydes were used. Dilute HCl and HBr solutions were shown to be effective catalysts for the selective coupling of aldehydes in the position meta to the hydroxyl group. The acid solutions could be recycled and reused multiple times without decrease in activity or yield.

1-Hexene: a renewable C6 platform for full-performance jet and diesel fuels

A highly efficient and selective process has been developed for the conversion of 1-hexene to jet and diesel fuels. In combination with commercial processes for the dehydration of ethanol and trimerization of ethylene, this work provides a basis for the synthesis of full-performance hydrocarbon fuels from bio-ethanol. Selective oligomerization of 1-hexene with a Cp2ZrCl2/MAO catalyst at ambient temperature and pressure resulted in 100% conversion of 1-hexene with >80% selectivity to a mixture of the dimer and trimer. The hydrogenated dimer had a −20 °C viscosity of only 3.5 mPa s, an exceptionally low freezing point of −77 °C, and a cetane number of 67 suggesting that it has performance characteristics suitable for both jet and diesel fuels. The hydrogenated trimer had a flash point of 128 °C, a cetane number of 92, a 40 °C viscosity of 3.1 mPa s, and a −20 °C viscosity of 24.5 mPa s. These properties suggest that the trimer has applications as a high-performance diesel fuel. In addition to the fuel-range hydrocarbons, heavier oligomers have potential as biolubricants which improves the carbon yield of useful products to near quantitative levels.

High Tg thermosetting resins from resveratrol

The tricyanate esters of the natural product resveratrol (1) and dihydroresveratrol (2) were synthesized and subjected to thermal curing which gave polycyanurate network polymers which exhibited glass transition temperatures of >340 °C and 334 °C, respectively. Thermal decomposition temperatures of 412 °C and 403 °C for polycyanurates of 1 and 2, respectively, were typical of this class. However, char yields (600 °C) of 71% and 66% for 1 and 2, respectively, were more than double that from the polycyanurate of bisphenol A dicyanate (25%).

Synthesis of renewable plasticizer alcohols by formal anti-Markovnikov hydration of terminal branched chain alkenes via a borane-free oxidation/reduction sequence

An efficient method for the formal anti-Markovnikov hydration of 1,1-disubstituted alkenes has been developed. The utility of the process has been demonstrated by conversion of bio-derived butene oligomers into primary alcohols through initial oxidation to vicinal acetoxy-alcohols, diols, or diacetates, followed by selective dehydration/tautomerization of the diols, and hydrogenation of the intermediary aldehydes. This approach allows for the isolation of important industrial plasticizer alcohols from a renewable source. In a broader context, this pathway, which can be conducted with sustainable, conventional reagents under mild conditions, represents a unique alternative to hydroboration for a challenging subset of hindered olefins.

AgInS2 quantum dots for the detection of trinitrotoluene.

AgInS2 (AIS) quantum dots (QDs) were synthesized via a thermal decomposition reaction with dodecylamine as the ligand to help stabilize the QDs. This reaction procedure is relatively easy to implement, scalable to large batches (up to hundreds of milligrams of QDs are produced), and a convenient method for the synthesis of chalcogenide QDs. Metal powders of AgNO3 and In(NO3)3, were used as the metal precursors while diethyldithiocarbamate was used as the sulfur source. The AIS QDs were characterized via transmission electron microscopy, atomic force microscopy, and energy dispersive x-ray spectroscopy. As an application for these less toxic nanomaterials, we demonstrate the selective detection of Trinitrotoluene (TNT) at concentrations as low as 6 micromolar (μM) and without the functionalization of a ligand that is specifically designed to interact with TNT molecules. We also demonstrate a simple approach to patterning the AIS QDs onto filter paper, for the detection of TNT molecules by eye. Collectively, the ease of the synthesis of the less toxic AIS QDs, and the ability to detect TNT molecules by eye suggest an attractive route to highly sensitive and portable substrates for environmental monitoring, chemical warfare agent detection, and other applications.

Selective solvent-free chromium detection using cadmium-free quantum dots

Abstract. Currently, the method of choice to test for the presence of chromium in water is to submit samples to a lab for testing. We present a simple field-ready test that is selective for the presence of chromium at concentrations of 100 ppb or greater. The Environmental Protection Agency maximum contaminant level (MCL) for total chromium is 100 ppb. This test uses a simple on/off fluorescent screening employing the use of silver indium sulfide (AgInS2) quantum dots (QDs). These QDs were impregnated into cotton pads to simplify field testing without the need for solvents or other liquid chemicals to be present. The change in fluorescence is instant and can be readily observed by eye with the use of a UV flashlight.