3 research outputs found
Development of Magnetic Nanoparticles as Microwave-Specific Catalysts for the Rapid, Low-Temperature Synthesis of Formalin Solutions
A series of heterogeneous catalyst
materials possessing good microwave
absorption properties were investigated for their activity as oxidation
catalysts under microwave irradiation. These catalysts, a series of
nanoscale magnetic spinel oxides of the composition MCr<sub>2</sub>O<sub>4</sub> (M = Cu, Co, Fe), were irradiated in aqueous methanol
solution (1:1 MeOH:H<sub>2</sub>O v:v). This resulted in rapid conversion
of methanol to formaldehyde, directly generating aqueous formalin
solutions. The catalytic reaction occurred under relatively mild conditions
(1 atm O<sub>2</sub>, 60 °C), with irradiation times of 80 min
converting 24.5%, 17.7%, and 13.2% of the available methanol to formaldehyde
by the Cu, Fe, and Co chromite spinel catalysts, respectively. Importantly,
reactions run under identical conditions of concentration, time, and
temperature using traditional convective heating yielded dramatically
lower amounts of conversions; specifically, 1.0% and 0.21% conversions
were observed with Cu and Co spinels, and no observable thermal products
were obtained from the Fe spinels. This work provides a clear demonstration
that microwave-driven catalysis can yield enhanced reactivity and
can afford new catalytic pathways
Microwave-Specific Enhancement of the CarbonâCarbon Dioxide (Boudouard) Reaction
The Boudouard reaction, which is
the reaction of carbon and carbon
dioxide to produce carbon monoxide, represents a simple and straightforward
method for the remediation of carbon dioxide in the environment through
reduction: CO<sub>2</sub>(g) + C(s) â 2CO. However, due to
the large positive enthalpy, typically reported to be 172 kJ/mol under
standard conditions at 298 K, the equilibrium does not favor CO production
until temperatures >700 °C, when the entropic term, â<i>T</i>Î<i>S</i>, begins to dominate and the free
energy becomes negative. We have found that, under microwave irradiation
to selectively heat the carbon, dramatically different thermodynamics
for the reaction are observed. During kinetic studies of the reaction
under conditions of flowing CO<sub>2</sub>, the apparent activation
energy dropped from 118.4 kJ/mol under conventional convective heating
to 38.5 kJ/mol under microwave irradiation. From measurement of the
equilibrium constants as a function of temperature, the enthalpy of
the reaction dropped from 183.3 kJ/mol at âź1100 K to 33.4 kJ/mol
at the same temperature under microwave irradiation. This changes
the position of the equilibrium so that the temperature at which CO
becomes the major product drops from 643 °C in the conventional
thermal reaction to 213 °C in the microwave. The observed reduction
in the apparent enthalpy of the microwave driven reaction, compared
to what is determined for the thermal reaction from standard heats
of formation, can be thought of as arising from additional energy
being put into the carbon by the microwaves, effectively increasing
its apparent standard enthalpy. Mechanistically, it is hypothesized
that the enhanced reactivity arises from the interaction of CO<sub>2</sub> with the steady-state concentration of electronâhole
pairs that are present at the surface of the carbon due to the space-charge
mechanism, by which microwaves are known to heat carbon. Such a mechanism
is unique to microwave-induced heating and, given the effect it has
on the thermodynamics of the Boudouard reaction, suggests that its
use may yield energy savings in driving the general class of gasâcarbon
reactions
Microwave-Specific Acceleration of a FriedelâCrafts Reaction: Evidence for Selective Heating in Homogeneous Solution
Thermally
promoted FriedelâCrafts benzylation of arene solvents
has been examined under both conventional convective heating with
an oil bath and heating using microwave (MW) energy. Bulk solution
temperaturesî¸as measured by internal and external temperature
probes and as defined by solvent refluxî¸were comparable in
both sets of experiments. MW-specific rate enhancements were documented
under certain conditions and not others. The observed rate enhancements
at a given temperature are proposed to arise from selective MW heating
of polar solutes, perturbing thermal equilibrium between the solute
and bulk solution. <i>Central to MW-specific thermal phenomena
is the difference between heat and temperature</i>. Temperature
is a measure of the ensemble average kinetic molecular energy of all
solution components, but temperature does not provide information
about solute-specific energy differences that may arise as a consequence
of selective MW heating. Enhanced chemical reactivity of the MW-absorbing
solute can be described as a MW-specific âextra-temperature
thermal effectâ, because the measurable solution temperature
only captures a portion of the solute kinetic molecular energy. Experimental
factors that favor MW-specific rate enhancements are discussed with
an eye toward future development of MW-actuated organic reactions,
in which the observed thermal reactivity exceeds what is predicted
from temperature-based Arrhenius calculations