The complex equilibria associated with the hydrogenation of CO2 to form valuable products are of considerable interest for the remediation of CO2. Due to the potential usefulness of these reactions, alternative methods of driving them more favorably using alternative energy sources, such as microwave radiation, are also of interest. We report here a study of the methanation reaction that yields CH4 , which was studied under microwave and conventional convective heating conditions. An essential part of the science that arises from such studies is characterizing the microwave-specific effects on the reaction. From the determined equilibrium constants, we found that microwave radiation strongly inhibited the formation of CH4. The effective thermodynamic parameters obtained from a van't Hoff plot reflect this inhibition. The enthalpy under microwave conditions is -38.63 kJ/mol, compared to -180.02 kJ/mol under conventional heating methods. Similarly, the free energy across the temperature range is less negative, suggesting a decrease in spontaneity under microwave conditions. We also observed a significant difference in the entropy, with entropy of -52 J/mol under microwave conditions and -206 J/mol under conventional conditions. This is consistent with prior observations that microwaves do not accelerate exothermic reactions. As discussed, this difference may arise from the microwave-driven dissociation of the reactant before CH4 bonds are made. Analysis of this system is of interest in understanding how the microwave inhibits the formation of the products of exothermic reactions. These results are of significance for understanding the broad area of microwave driven heterogeneous catalysis.

Macroscopically homogeneous mixtures of p-nitroanisole (pNA) and mesitylene (MES) can be selectively heated using microwave (MW) energy. The pNA solutes agglomerate into distinct phase domains on the attoliter-scale (1 aL = 10(-18) L), and these agglomerates can be MW-heated selectively to temperatures that far exceed the boiling point of the surrounding MES solvent. Here, a 1 : 20 mixture of pNA : MES is used as a mixed solvent for aryl Claisen rearrangement of allyl naphthyl ether (ANE). ANE itself does not heat effectively in the MW, but selective MW heating of pNA allows for transfer of thermal energy to ANE to accelerate rearrangement kinetics above what would be expected based on Arrhenius kinetics and the measured bulk solution temperature. This focused study builds on prior work and highlights 1 : 20 pNA : MES as a mixed solvent system to consider for strategically exploiting MW-specific thermal effects.

Agglomerates of polar molecules in nonpolar solvents are selectively heated by microwave radiation. The magnitude of the selective heating was directly measured by using the temperature dependence of the intensities of the Stokes and anti-Stokes bands in the Raman spectra of p-nitroanisole (pNA) and mesitylene. Under dynamic heating conditions, a large apparent temperature difference (ΔT) of over 100 °C was observed between the polar pNA solute and the nonpolar mesitylene solvent. This represents the first direct measurement of the selective microwave heating process. The magnitude of the selective microwave heating was affected by the properties of the agglomerated pNA. As the concentration of the pNA increases, the magnitude of the selective heating of the pNA was observed to decrease. This is explained by the tendency of the pNA dipoles to orient in an antiparallel fashion in the aggregates as measured by the Kirkwood g value, which decreased with increasing concentration. This effect reduces the net dipole moment of the agglomerates, which decreases the microwave absorption. After the radiation was terminated, the effective temperature of the dipolar molecules returned slowly to that of the medium. The slow heat transfer was modeled successfully by treating the solutions as a biphasic solvent/solute system. Based on modeling and the fact that the agglomerate can be heated above the boiling temperature of the solvent, an insulating layer of solvent vapor is suggested to form around the heated agglomerate, slowing convective heat transfer out of the agglomerate. This is an effect unique to microwave heating.

A high-temperature retro-Diels-Alder reaction is accelerated by microwave (MW) heating to rates higher than expected based on Arrhenius kinetics and the measured temperature of the reaction mixture. Observations are consistent with selective MW heating of the polar reactant relative to other, less polar components of the reaction mixture.

A series of hard, transparent, thermoset polymer samples containing tetravinylsilane (TVS) and 1,3-benzenedithiol (BDT) with varying loadings of zirconium oxide clusters Zr-6(OH)(4)O-4(OMc)(12)(ZOC) were synthesized. Resulting polymers exhibited a higher refractive index (n) than the parent polymer containing only the monomers TVS and BDT. The refractive index reached a maximum value of 1.711 at a ZOC loading of 3 wt% and then decreased as the ZOC concentration in the polymer matrix increased. The refractive index of ZOC was determined to be 1.540 using the Becke line method. Because the refractive index of ZOC is lower than that of the TVS-BDT polymer matrix, the finding that the incorporation of small quantities of ZOC can increase the refractive index of the TVS-BDT polymer composite was unexpected and is accounted for by the effects of ZOC on the packing efficiency of the composite.

The coordinated, cooperative use of microwave heating with conventional heating can provide advantages in chemical synthesis. Here, heterogeneous mixtures comprising ionic, highly microwave-absorbing organic reagents and nearly microwave-transparent arene solvents are heated conventionally and/or with microwaves, resulting in faster and, in some cases, higher yielding reactions when the two heating methods are applied cooperatively as compared to either method independently. Control experiments in more polar arene solvents show no advantage of cooperative heating, consistent with selective microwave heating phenomena. The experiments are facilitated by reactor technology that regulates internal reaction temperature and coordinates the application of conventional and microwave heating. The positive outcomes in this initial exploratory system suggest that cooperative heating can offer benefits in other systems designed for selective microwave heating.

This paper presents an innovative approach to producing energy-dense, carbon–neutral liquid ammonia as a means for carrying energy. This approach synergistically integrates microwave reaction chemistry with novel heterogeneous catalysis that decouples dinitrogen activation from high-temperature and high-pressure reactions, altering reaction pathways and increasing ammonia formation rate. Results presented here demonstrate that ammonia synthesis can be conducted at 280 ℃ and ambient pressure to achieve ~1 mmol ammonia/g cat/h over supported ruthenium catalyst systems utilizing microwave irradiation. It is further shown that adding promoter ions such as potassium, cerium, and barium significantly improves the ammonia production rate over undoped ruthenium-based catalysts. This effect could be attributed to enhanced dielectric loss processes that lead to stronger microwave absorption by the catalyst. Measurement of the equilibrium constant under microwave conditions showed a higher ammonia yield than under thermal equilibrium conditions for both the iron- and ruthenium-based catalysts. Finally, this study also illustrates the advantages of using a variable-frequency microwave reactor for ambient-pressure ammonia synthesis. Mechanistically, investigators believed that the oscillating electric fields of the radiation can couple with adsorbed nitrogen on the surface and accelerate its dissociation. Since dinitrogen dissociation on the surface is rate limiting, this effectively accelerates the reaction. Overall, the study provides an in-depth analysis of the parameters affecting the use of microwaves in catalyzed ammonia synthesis. This process is fundamentally different from the commercial Haber–Bosch process and provides an alternative method of ammonia synthesis for certain applications, as it is tolerant to intermittent supplies of renewable energy, therefore effectively operating at variable rates of production.

Microwave (MW) heating is more effective than conventional (CONV) heating for promoting a high-temperature oxidative cycloisomerization reaction that was previously reported as a key step in a total synthesis of the natural product illudinine. The thermal reaction pathway as envisioned is an inverse electron-demand dehydro-Diels-Alder reaction with in situ oxidation to generate a substituted isoquinoline, which itself is unstable to the reaction conditions. Observed reaction yields were higher at a measured bulk temperature of 200 °C than at 180 °C or 220 °C; at 24 hours than at earlier or later time points; and when the reaction solution was heated using MW energy as opposed to CONV heating with a metal heat block. Selective MW heating of polar solute aggregates is postulated to explain these observations.

A new class of high refractive index polymers was developed through the thiol ene coupling reaction of trivinylphosphine chalcogenides, [(CH2=CH)(3)P=X (X = S, Se)], with 1,2-ethane dithiol and 1,3-benzene dithiol. The polymers were obtained through a thermal initiation and polymerization process which yielded robust monolithic materials. The polymers have good transmission properties in the visible region of the spectrum with very high refractive indices ranging from 1.66 to 1.75, placing them in the top echelon of high refractive index polymers, with Abbe numbers between 22 and 31. The high refractive index is due, in large part, to the presence of the highly polarizable P=S and P=Se groups, which have molar refractivities of 13.84 and 17.33 mol/cm(3), respectively. The polymers tend to have a high cross-link density that varies with the composition but results in rigid materials for most compositions. The glass-transition temperatures (T-g) for all but one composition were above 70 degrees C, and the storage moduli ranged from 1.5 to 3.8 GPa. The 1,2-ethane dithiol compositions had lower values of T-g than the 1,3-benzene dithiol materials. This was attributed to a lower cross-link density in the 1,2-ethane dithiol polymers, as determined from the residual unreacted vinyl groups measured by Raman spectroscopy and P-31 solid-state nuclear magnetic resonance spectroscopy. The P=Se polymers were slower to react and required higher temperatures; they also produced the hardest materials with the highest refractive indices.

The electronic structure of isolated Cr(VI) sites supported on silica was reinvestigated using multiple, complementary electronic spectroscopies applied to transparent xerogel monoliths. The absorption spectrum exhibits three previously reported peaks, at 22 800, 29 100, and 41 500 cm(-1), as well as a previously unresolved band at ca. 36 900 cm(-1). The emission is a longlived red luminescence with lambda(max) = 13 600 cm(-1), emanating from the lowest excited state. Assignment of the excited states was facilitated using time dependent density functional theory (TD-DFT) calculations performed on cluster models. All of the observed electronic transitions and their energies are accounted for by dioxoCr(VI) sites. The lowest energy observed excitation at 22 800 cm(-1) populates a singlet excited state, while the emitting state is the corresponding triplet state, accessed by intersystem crossing from the singlet state. Spectroscopic bands observed at 29 100, 36 900, and 41 500 cm(-1) were assigned, based on the TD-DFT calculation, to spin-allowed transitions that are consistent with emission polarization anisotropy measurements. Small variations in site symmetry at Cr result principally in inhomogeneous broadening of the spectral bands, as well as a red-edge effect in the photoemission spectrum. There is no evidence for a significant contribution from five-coordinate mono-oxoCr(VI) sites.

Research in the Dudley Lab contributes to the development of emerging and enabling technologies for chemical synthesis. This presentation will feature experiments and observations from a long-standing collaboration with A. E. Stiegman and colleagues at Florida State University on the use of microwave electromagnetic radiation to produce the thermal energy needed to accelerate dynamic chemical processes.

The monomeric, single-atom oxochromium species present on the surface of silica-supported Cr(VI) catalysts was characterized in detail using resonance Raman (RR) spectroscopy over a range of excitation wavelengths corresponding to the primary electronic transitions of Cr(VI)SiO2. The findings resolve a long-standing controversy regarding the possible contribution of mono-oxoCr(VI) sites, (SiO)(4)Cr=O, postulated to coexist with the well-established dioxoCr(VI) sites, (SiO)(2)Cr(=O)(2). Density functional theory (DFT) calculations and a normal coordinate analysis conducted using a chromasiloxane model cluster confirm prior assignments of bands in the nonresonant Raman spectrum at 986 and 1001 cm(-1) to the symmetric and antisymmetric stretching modes, respectively, of the dioxoCr(VI) sites. For all excitation energies, the symmetric stretch shows apparent resonant enhancement. Since all of the electronic transitions are strongly allowed, this finding is consistent with A-term enhancement. UV excitation at 257 nm (into the high energy electronic transition centered at 271 nm) also results in modest resonant enhancement of the antisymmetric stretch, due to the low average symmetry of the surface sites. Excitation at 351 nm (into the electronic transition centered at 343 nm) results in a strong increase in the relative intensity of the antisymmetric stretch, which is likely caused by B-term enhancement. Previously reported evidence for a mono-oxoCr(VI) site consists of a vibrational band observed at ca. 1011 cm(-1) and assigned to its Cr=O stretch. However, the band is observed only upon excitation into the lowest-energy electronic transition, at 439 nm. We show that excitation into this electronic transition causes photoinduced decomposition. The process depends on the laser power and duration of exposure, and it yields the band previously assigned to a mono-oxo species. The resonance Raman study reported here, in combination with our recent rigorous analysis of the corresponding electronic spectra, lead us to conclude that there is no credible spectroscopic evidence for the existence of mono-oxochromate species in highly dispersed Cr/silica materials.

Pyrolysis conditions greatly affect the structure-reactivity relationship of char during coal gasification. This work investigated the effect of temperature and microwave heating on the structural properties of the chars generated during pyrolysis, as well as gaseous and tar products. Results showed that microwave pyrolysis of Mississippi coal produced more gaseous products and less tars compared to conventional pyrolysis. Higher CO/CO2 ratio (> 1) was observed under microwave pyrolysis compared to conventional pyrolysis (CO/CO2 < 1), which may be explained by a greater extent of gasification between solid carbon and the CO2 formed during microwave pyrolysis. Additionally, in microwave pyrolysis, the oil tars generated exhibited lower concentrations of polar oxygenates, while the wax tars showed higher concentrations of non-polar alkanes, as observed from the intensity of C-H vibrations in FTIR. The product compositions and FTIR analysis of the tars (oils and waxes) suggest that the microwave interacted preferentially with these polar species, which have relatively higher dielectric properties compared to alkanes. The structure-reactivity relationship of the chars produced was also investigated using a variety of characterization tools such as XRD, BET, SEM, EDS, and FTIR. Finally, the char reactivity towards combustion suggested that microwave-produced chars have a higher thermal stability, likely due to lower O/C ratios, and could be utilized in the metallurgical industry.

This Personal Account describes collaborative investigations into apocryphal microwave effects in organic chemistry. Focused research on microwave-assisted organic synthesis has been fraught with confusion, controversy, and misinformation. Microwave heating is an undoubtedly useful tactic for organic synthesis, but whether or not it can offer strategic advantages remains an open question in the minds of many people. (Ironically, those who do not consider it an open question are split as to whether it has been resolved affirmatively or negatively.) Our research in this area is guided by the hypothesis that microwave heating can alter reaction kinetics in ways distinct from what is observable under conventional heating. Here we provide a succinct record of the origins of our interests, our initial queries and associated controversies, and recent efforts to identify, quantify, and begin to leverage selective microwave heating for strategic advantage in organic synthesis.