Magnetocapacitance effect and magnetoelectric coupling in type-II multiferroic HoFeWO6

We have investigated the multiferroicity and magnetoelectric (ME) coupling in HoFeWO6. With a noncentrosymmetric polar structure (space group Pna21) at room temperature, this compound shows an onset of electric polarization with an antiferromagnetic ordering at the Néel temperature (TN ) of 17.8 K. The magnetic properties of the polycrystalline samples were studied by DC and AC magnetization and heat capacity measurements. The metamagnetic behavior at low temperatures was found to be directly related to the dielectric properties of the compound. In particular, field-dependent measurements of capacitance show a magnetocapacitance (MC) effect with double-hysteresis loop behavior in direct correspondence with the magnetization. Our x-ray diffraction results show the Pna21 structure down to 8 K and suggest the absence of a structural phase transition across TN . Soft x-ray absorption spectroscopy and soft x-ray magnetic circular dichroism (XMCD) measurements at the Fe L2,3 and Ho M4,5 edges revealed the oxidation state of Fe and Ho cations to be 3+. Fe L2,3 XMCD further shows that Fe3+ cations are antiferromagnetically ordered in a noncollinear fashion with spins arranged 90◦ with respect to each other. Our findings show that HoFeWO6 is a type-II multiferroic exhibiting a MC effect. The observed MC effect and the change in polarization by the magnetic field, as well as their direct correspondence with magnetization, further support the strong ME coupling in this compound.The work at University of Houston (UH) is supported by U. S. Air Force Office of Scientific Research Grants FA9550-15-1-0236 and FA9550-20-1-0068, the T. L. L. Temple Foundation, the John J. and Rebecca Moores Endowment, and the State of Texas through the Texas Center for Superconductivity at the University of Houston. The XRD patterns were collected at the National Synchrotron Radiation Research Center at Taiwan. The synchrotron XAS/XMCD experiments were performed at the BOREAS beamline of the ALBA Synchrotron Light Facility in collaboration with ALBA staff. Computational resources were provided by the Extreme Science and Engineering Discovery Environment (XSEDE) [55] supported by the National Science Foundation (ACI-1548562) and the National Energy Research Scientific Computing (NERSC) Center, a DOE Office of Science User Facility supported by the Office of Science, U. S. Department of Energy, under Contract No. DE-AC02-05CH11231. Additional support for this work was provided through resources of the uHPC cluster managed by UH and acquired through NSF Award 1531814. The authors acknowledge the use of the Maxwell/Opuntia/Sabine Cluster and the advanced support from the Research Computing Data Core at UH. The work at National Sun Yat-Sen University was partially supported by the Ministry of Science and Technology of Taiwan under Grant No. MOST 109-2112-M-110-019.Peer reviewe

Computational studies on the catalyzed conversion of furans to aromatics

Doren, Douglas J.Economic, environmental and political considerations have led to gradually replacement of natural gas for petroleum. One of the deficiencies of this major change of direction in the energy market is a decrease in petrochemical productions such as aromatics, which are important materials for a wide range of chemical processes. It in turn opens up a clear opportunity for renewable sources of energy and chemicals to play an essential role and provide a variety of important chemicals that are traditionally produced from petroleum. ☐ This dissertation includes four projects on the catalyzed conversion of biomass-derived furans to aromatics. Various computational methods are employed to model different systems, investigate catalytic effects and rationalize them from an atomistic point of view. ☐ In the first research project, density functional theory electronic structure calculations are used to explore the mechanism for the Diels-Alder reaction between 2,5-dimethylfuran (DMF) and maleic anhydride (MA). Reaction paths are reported for uncatalyzed, and Lewis and Brønsted acid-catalyzed reactions in vacuum and in a broad range of solvents. The calculations show that while the uncatalyzed Diels-Alder reaction is thermally feasible in vacuum, a Lewis acid (modeled as Na+) lowers the activation barrier by interacting with the dienophile (MA) and decreasing the HOMO-LUMO gap of the reactants. A Brønsted acid (modeled as a proton) can bind to a carbonyl oxygen in MA, changing the reaction mechanism from concerted to step-wise, and eliminating the activation barrier. Solvation effects are investigated with a continuum model of the solvent. This study shows that electrostatic effects play the largest role in determining the solvation energy of the transition state, which tracks the net dipole moment at the transition state. For the uncatalyzed reaction, the dipole moment is largely determined by charge transfer between the reactants, but in the reactions with ionic catalysts, there is no simple relationship between solvation of the transition state and charge transfer between the reactants. Non-electrostatic contributions to solvation of the reactants and transition state also make significant contributions to the activation energy. ☐ Dehydration of the cycloadduct produced from the Diels-Alder reaction between 2,5-dimethylfuran and maleic anhydride to produce 3,6-dimethylphthalic anhydride, which is the topic of the second research project, exemplifies an important step in producing platform chemicals from biomass. The mechanisms of dehydration and catalytic effects of Lewis and Brønsted acids are investigated with density functional theory. The uncatalyzed reaction has a very high activation barrier (68.7 kcal/mol) in the gas phase and it is not significantly affected by solvation. With a Lewis acid catalyst, modeled as an alkali ion, the activation barriers are reduced, but intermediates are also stabilized. The net effect in vacuum is that the energetic span, or apparent activation energy of the catalytic cycle, is 77.9 kcal/mol, even higher than the barrier in the uncatalyzed case. In solution, however, the energetic span is reduced by as much as 20 kcal/mol, due to differences in the solvation energy of the transition states and intermediates. In the case of a Brønsted acid catalyst, modeled as a proton, the gas-phase transition state energies are reduced even more than in the Lewis acid case, and there is no strong stabilization of the intermediates. The energetic span in vacuum is only 13.8 kcal/mol and is reduced even further in solution. Brønsted acid catalysis appears to be the preferred mechanism for dehydration of this cycloadduct. Since the Diels-Alder reaction that produced the molecule has previously been shown to be catalyzed by Brønsted acids, this suggests that a single catalyst could be used to accelerate both steps. ☐ Diels-Alder reactions of furans yield oxanorbornene derivatives which can be converted to a variety of molecules, ranging from molecules of biological interest to naturally occurring organic compounds, and to aromatics via dehydration, a promising alternative for the synthesis of aromatics from renewables. With furan being one of the less reactive dienes, development of Lewis-acidic heterogeneous catalysts, without the shortcomings of the traditional homogeneous ones, is critically important. In the third project, we use computational chemistry to study the Diels-Alder reaction of furan and methyl acrylate in three zeotypic Lewis acids, Sn-, Zr- and Hf-BEA (BEA=zeolite beta). We find that all three exhibit the same ability to enhance the electrophilic character of the dienophile and promote modest charge transfer from the diene. Despite being moderately Lewis-acidic, they still achieve a reduction of about 12.5 kcal/mol in the activation energy relative to the reaction in the absence of catalyst. ☐ In the fourth project, we investigate catalytic effects of transition metal oxides such as HfO2 and ZrO2 on the Diels-Alder reaction of furan and methyl acrylate forming the oxanorbornene carboxylic acid methyl ester. Our theoretical and experimental studies confirm that transition metal oxides exhibit reactivity in the Diels-Alder reaction. Furthermore, we theoretically investigate the effects of surface hydroxylation on the activities for both Langmuir and Eley-Rideal mechanisms. Our calculations suggest that Eley-Rideal is the favorable mechanism for this reaction on all studied surfaces. Clean, partially hydroxylated and fully hydroxylated HfO2 and ZrO2 surfaces lower the activation barrier of the Diels-Alder reaction between furan and methyl acrylate to different degrees. While it turned out that clean oxide surfaces are weak catalysts, surprisingly partially hydroxylated oxide surfaces show higher activities than fully hydroxylated ones. Based on our extensive studies, surface hydroxylation can be used as a means to enhance catalytic effects of transition metal oxides on the Diels-Alder reaction of furan and methyl acrylate. This is quite interesting from application point of view because metal oxides tend to be hydrated. This study shows that there is no need for catalyst dehydration prior to use in a catalytic cycle. Calculated activation barriers are in very good agreement with those obtained from experiments. Although the obtained turnover frequencies are lower than Hf- and Zr-BEA zeolites, this finding paves the road for more applicable and scalable methodologies to derive powerful Diels-Alder intermediates.University of Delaware, Department of Physics and AstronomyPh.D

Catalysis of the Diels–Alder Reaction of Furan and Methyl Acrylate in Lewis Acidic Zeolites

Diels–Alder (DA) reactions of furans yield oxanorbornene derivatives which can be converted to a variety of molecules, ranging from molecules of biological interest to naturally occurring organic compounds, and to aromatics via dehydration, a promising alternative for the synthesis of aromatics from renewables. With furan being one of the less reactive dienes, the development of Lewis acidic heterogeneous catalysts, without the shortcomings of the traditional homogeneous catalysts, is critically important. Herein, we use computational chemistry to study the DA reaction of furan and methyl acrylate in three zeotypic Lewis acids, Sn-, Zr-, and Hf-BEA. We find that all three exhibit the same ability to enhance the electrophilic character of the dienophile and promote modest charge transfer from the diene. Despite being moderately Lewis acidic, they still achieve a reduction of about 12.5 kcal/mol in the activation energy relative to the reaction in the absence of catalyst

Controlling Selectivity in Plasmonic Catalysis: Switching Reaction Pathway from Hydrogenation to Homocoupling Under Visible-Light Irradiation

Plasmonic catalysis enables the use of light to accelerate molecular transformations. Its application to the control reaction selectivity is highly attractive but remains challenging. Here, we have found that the plasmonic properties in AgPd nanoparticles allowed different reaction pathways for tunable product formation under visible-light irradiation. By employing the hydrogenation of phenylacetylene as a model transformation, we demonstrate that visible-light irradiation can be employed to steer the reaction pathway from hydrogenation to homocoupling. Our data showed that the decrease in the concentration of H species at the surface due to plasmon-enhanced H2 desorption led to the control in selectivity. These results provide important insights into the understanding of reaction selectivity with light, paving the way for the application of plasmonic catalysis to the synthesis of 1,3-diynes, and bringing the vision of light-driven transformations with target selectivity one step closer to reality.</p