Please refer to published version for the most recent bibliographic citation information. If a published version is known of, the repository item page linked to above, will contain details on accessing it.
Please refer to published version for the most recent bibliographic citation information. If a published version is known of, the repository item page linked to above, will contain details on accessing it.
The ratio of NMR relaxation time constants T1/T2 provides a non-destructive indication of the relative surface affinities exhibited by adsorbates within liquid-saturated mesoporous catalysts. In the present work we provide supporting evidence for the existence of a quantitative relationship between such measurements and adsorption energetics. As a prototypical example with relevance to green chemical processes we examine and contrast the relaxation characteristics of primary alcohols and cyclohexane within an industrial silica catalyst support. T1/T2 values obtained at intermediate magnetic field strength are in good agreement with DFT adsorption energy calculations performed on single molecules interacting with an idealised silica surface. Our results demonstrate the remarkable ability of this metric to quantify surface affinities within systems of relevance to liquid-phase heterogeneous catalysis, and highlight NMR relaxation as a powerful method for the determination of adsorption phenomena within mesoporous solids.
Comprehensive
mechanistic insights into the aqueous-phase hydrogenolysis of glycerol
by the ReO
x
–Ir catalyst were obtained
by combining density functional theory (DFT) calculations with batch
reaction experiments and detailed characterization of the catalysts
using X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier
transform infrared techniques. The role and contribution of the aqueous
acidic reaction medium were investigated using NMR relaxometry studies
complemented with molecular dynamics and DFT calculations. At higher
glycerol concentration, the enhanced competitive interaction of glycerol
with the catalyst improved the conversion of glycerol. Sulfuric acid
increased the concentration of glycerol within the pores of the catalyst
and enhanced the propensity for dissociative adsorption of glycerol
on the catalyst, explaining the promotional effect of acid during
hydrogenolysis. Partially reduced and dispersed Brønsted acidic
ReO
x
clusters on metallic Ir nanoparticles
facilitated dissociative attachment of glycerol and preferential formation
of the primary propoxide. The formation of the dominant product, 1,3-propanediol
(1,3-PDO), results from the selective removal of the secondary hydroxyl
of glycerol, with a comparatively low activation barrier of 123.3
kJ mol–1 in the solid Brønsted acid-catalyzed
protonation–dehydration mechanism or 165.2 kJ mol–1 in the direct dehydroxylation mechanism. The formation of 1-propanol
(1-PO) is likely to follow a successive dehydroxylation pathway in
the early stages of the reaction. Although 1,3-PDO is less reactive
than 1,2-propanediol (1,2-PDO), it preferentially adsorbs on the catalyst
in a mixture containing glycerol to form 1-PO. The thermodynamically
favorable pathway involving dehydrogenation, dehydroxylation, and
hydrogenation elementary steps led to the dominant production of 1,2-PDO
on pure Ir catalyst with a high C–O bond cleavage barrier of
207.4 kJ mol–1. The optimum ReO
x
–Ir catalyst with an Ir/Re ratio of 1 exploits the synergy
of the sites of both the components. The detailed insights presented
here would guide the rational selection of catalysts for the hydrogenolysis
of polyols and the optimization of reaction parameters.
Complex chemical reaction environments,
such as those found in
combustion engines, the upper atmosphere, or the interstellar medium,
can contain large numbers of different reactive species participating
in similarly large numbers of different chemical reactions. In such
settings, identifying the most-likely multistep reaction mechanisms
which lead to the production of a particular defined product species
is an extremely challenging problem, requiring search and evaluation
over a large number of different possible candidate mechanisms while
also addressing the permutational challenges posed when considering
a large number of reaction routes available to sets of identical molecular
species. In this article, the problem of generating candidate reaction
mechanisms which form a defined product from a diverse set of reactive
molecules is cast as a discrete optimization of a permutationally
invariant cost function describing similarity between the target product
and the product generated by a trial reaction mechanism. This approach
is demonstrated by generating 2230 candidate reaction mechanisms which
form benzene from diverse sets of reactive molecules which have been
experimentally identified in the interstellar medium. By screening
this set of autogenerated mechanisms, using dispersion-corrected DFT
to evaluate reaction energies and activation barriers, we identify
several candidate barrierless reaction mechanisms (both previously
proposed and new) for benzene formation which may operate in the low
temperatures found in the interstellar medium and could be investigated
further to supplement existing microkinetic models.
Numerous different algorithms have been developed over the last few years which are capable of generating large, dense chemical reaction networks describing the inherent chemical reactivity of a collection of discrete molecules. For all elementary reactions in a given reaction network, reaction rate calculations, followed by direct micro-kinetic modelling, enables one to predict macroscopic outcomes (e. g. rate laws, product selectivity) based on atomistic input data. However, for chemical reaction networks containing thousands of reactant molecules, such simulations can be extremely time-consuming; in addition, the complex coupled time-dependence of molecular concentrations can present challenges when seeking essential mechanistic features. In this Article, we instead present an algorithm which seeks to predict the "most likely" reaction mechanism, or competing mechanisms, connecting any two user-selected reactant and product species, given a previouslygenerated reaction network as input. The approach is successfully tested for reaction networks (containing tens of thousands of possible reactions) describing the carbon monoxide oxidation on platinum nanoparticles.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.