24 research outputs found
Reactivity and Selectivity Descriptors for the Activation of CâH Bonds in Hydrocarbons and Oxygenates on Metal Oxides
CâH bond activation
at lattice O atoms on oxides mediates
some of the most important chemical transformations of small organic
molecules. The relations between molecular and catalyst properties
and CâH activation energies are discerned in this study for
the diverse CâH bonds prevalent in C<sub>1</sub>âC<sub>4</sub> hydrocarbons and oxygenates using lattice O atoms with a
broad range of H atom abstraction properties. These activation energies
determine, in turn, attainable selectivities and yields of desired
oxidation products, which differ from reactants in their CâH
bond strength. Brønsted-EvansâPolanyi (BEP) linear scaling
relations predict that CâH activation energies depend solely
and linearly on the CâH bond dissociation energies (BDE) in
molecules and on the H-atom addition energies (HAE) of the lattice
oxygen abstractors. These relations omit critical interactions between
organic radicals and surface OH groups that form at transition states
that mediate the H atom transfer, which depend on both molecular and
catalyst properties; they also neglect deviations from linear relations
caused by the lateness of transition states. Thus, HAE and BDE values,
properties that are specific to a catalyst and a molecule in isolation,
represent incomplete descriptors of reactivity and selectivity in
oxidation catalysis. These effects are included here through crossing
potential formalisms that account for the lateness in transition states
in estimates of activation energies from HAE and BDE and by estimates
of molecule-dependent but catalyst-independent parameters that account
for diradical interactions that differ markedly for allylic and nonallylic
CâH bonds. The systematic ensemble-averaging of activation
energies for all CâH bonds in a given molecule show how strong
abstractors and high temperatures decrease an otherwise ubiquitous
preference for activating the weakest CâH bonds in molecules,
thus allowing higher yields of products with CâH bonds weaker
than in reactants than predicted from linear scaling relations based
on molecule and abstractor properties. Such conclusions contradict
the prevailing guidance to improve such yields by softer oxidants
and lower temperatures, a self-contradictory strategy, given the lower
reactivity of such weaker H-abstractors. The diradical-type interactions,
not previously considered as essential reactivity descriptors in catalytic
oxidations, may expand the narrow yield limits imposed by linear free
energy relations by guiding the design of solids with surfaces that
preferentially destabilize allylic radicals relative to those formed
from saturated reactants at CâH activation transition states
Methanol Oxidative Dehydrogenation on Oxide Catalysts: Molecular and Dissociative Routes and Hydrogen Addition Energies as Descriptors of Reactivity
The oxidative dehydrogenation (ODH)
of alkanols on oxide catalysts
is generally described as involving H-abstraction from alkoxy species
formed via OâH dissociation. Kinetic and isotopic data cannot
discern between such routes and those involving kinetically-relevant
H-abstraction from undissociated alkanols. Here, we combine such experiments
with theoretical estimates of activation energies and entropies to
show that the latter molecular routes prevail over dissociative routes
for methanol reactions on polyoxometalate (POM) clusters at all practical
reaction temperatures. The stability of the late transition states
that mediate H-abstraction depend predominantly on the stability of
the OâH bond formed, making H-addition energies (HAE) accurate
and single-valued descriptors of reactivity. Density functional theory-derived
activation energies depend linearly on HAE values at each O-atom location
on clusters with a range of composition (H<sub>3</sub>PMo<sub>12</sub>, H<sub>4</sub>SiMo<sub>12</sub>, H<sub>3</sub>PW<sub>12</sub>, H<sub>4</sub>PV<sub>1</sub>Mo<sub>11</sub>, and H<sub>4</sub>PV<sub>1</sub>W<sub>11</sub>); both barriers and HAE values reflect the lowest
unoccupied molecular orbital energy of metal centers that accept the
electron and the protonation energy of O-atoms that accept the proton
involved in the H-atom transfer. Bridging O-atoms form OâH
bonds that are stronger than those of terminal atoms and therefore
exhibit more negative HAE values and higher ODH reactivity on all
POM clusters. For each cluster composition, ODH turnover rates reflect
the reactivity-averaged HAE of all accessible O-atoms, which can be
evaluated for each cluster composition to provide a rigorous and accurate
predictor of ODH reactivity for catalysts with known structure. These
relations together with oxidation reactivity measurements can then
be used to estimate HAE values and to infer plausible structures for
catalysts with uncertain active site structures
Toward More Complete Descriptors of Reactivity in Catalysis by Solid Acids
Density functional theory and classical
electrostatics are used
to develop reactivity descriptors for catalysis by solid acids. Acid
strength, as deprotonation energies (DPE), reflects the charge reorganization
required to disrupt covalent OH bonds in inorganic acids and the electrostatic
forces that resist the separation of protons from conjugate anions.
Both charge reorganization (covalent) and electrostatic (ionic) components
vary monotonically with DPE on solid acids with different heteroatoms
within a given type of oxide framework, but their relative contributions
differ among different acid types. Ion-pair transition states recover
predominantly the ionic part of the DPE, and the extent to which they
recover each component is a unique property of a transition state
and thus of an acid-catalyzed reaction, independent of the acid strength
or type. These fractional recoveries, together with the ionic and
covalent DPE components, a unique property of a solid acid, provide
a general and complete descriptor of reactivity, which we illustrate
here for diverse reactions (proton shuttling, H<sub>2</sub>O elimination,
methyl shift, ring contraction) on several types of solid acids (Mo-
and W-based polyoxometalate clusters with S, P, Si, Al, and Co central
atoms and MFI type heterosilicates with Al, Ga, Fe, and B heteroatoms).
For protons confined within small voids of heterosilicates, the transition
state stabilization and reactivity depend additionally on van der
Waals interactions that are unrelated to acid strength
Kinetic and Theoretical Insights into the Mechanism of Alkanol Dehydration on Solid Brønsted Acid Catalysts
Elementary steps that mediate ethanol
dehydration to alkenes and
ethers are determined here from rate and selectivity data on solid
acids of diverse acid strength and known structure and free energies
derived from density functional theory (DFT). Measured ethene and
ether formation rates that differed from those expected from accepted
monomolecular and bimolecular routes led to our systematic enumeration
of plausible dehydration routes and to a rigorous assessment of their
contributions to the products formed. H-bonded monomers, protonated
alkanol dimers, and alkoxides are the prevalent bound intermediates
at conditions relevant to the practice of dehydration catalysis. We
conclude that direct and sequential (alkoxide-mediated) routes contribute
to ether formation via S<sub>N</sub>2-type reactions; alkenes form
preferentially from sequential routes via monomolecular and bimolecular
syn-E2-type eliminations; and alkoxides form via bimolecular S<sub>N</sub>2-type substitutions. The prevalence of these elementary steps
and their kinetic relevance are consistent with measured kinetic and
thermodynamic parameters, which agree with values from DFT-derived
free energies and with the effects of acid strength on rates, selectivities,
and rate constants; such effects reflect the relative charges in transition
states and their relevant precursors. Dehydration turnover rates,
but not selectivities, depend on acid strength because transition
states are more highly charged than their relevant precursors, but
similar in charge for transition states that mediate the competing
pathways responsible for selectivity
Mechanistic Evidence for Sequential DisplacementâReduction Routes in the Synthesis of PdâAu Clusters with Uniform Size and Clean Surfaces
Bimetallic
PdâAu clusters with (Pd/Au)<sub>at</sub> compositions
of 0.5, 1.0, and 2.0 narrowly distributed in size were prepared using
colloidal methods with reagents containing only C, H, and O atoms,
specifically polyvinyl alcohol (PVA) as protecting species and ethanol
as the organic reductant. Synthesis protocols involved contacting
a solution of Au precursors with nearly monodisperse Pd clusters.
The formation of PdâAu clusters was inferred from the monotonic
growth of clusters with increasing Au content and confirmed by the
in situ detection of Au plasmon bands in their UVâvisible spectra
during synthesis. Specifically, transmission electron microscopy (TEM)
showed that growth rates were proportional to the surface area of
the clusters, and rigorous deconvolution and background subtraction
allowed for determination of the intensity and energy of Au-derived
plasmon bands. This feature emerged during initial contact between
Au precursors and Pd clusters apparently because Au<sup>3+</sup> species
deposit as Au<sup>0</sup> using Pd<sup>0</sup> as the reductant in
a fast galvanic displacement process consistent with their respective
redox potentials. The plasmon band ultimately disappeared as a result
of the subsequent slower reduction of the displaced Pd<sup>2+</sup> species by ethanol and of their deposition onto the bimetallic clusters.
Such displacementâreduction pathways are consistent with the
thermodynamic redox tendencies of Au, Pd, and ethanol and lead to
the conclusion that such triads (two metals and an organic reductant)
can be chosen from thermodynamic data and applied generally to the
synthesis of bimetallic clusters with other compositions. These bimetallic
clusters were dispersed on mesoporous Îł-Al<sub>2</sub>O<sub>3</sub> supports, and PVA was removed by treatment in ozone at near-ambient
temperature without any detectable changes in cluster size. The absence
of strongly bound heteroatoms, ubiquitous in many other colloidal
synthesis protocols, led to Al<sub>2</sub>O<sub>3</sub>-dispersed
clusters with chemisorption uptakes consistent with their TEM-derived
cluster size, thus demonstrating that cluster surfaces are accessible
and free of synthetic debris. The infrared spectra of chemisorbed
CO indicated that both Pd and Au were present at such clean surfaces
but that any coreâshell intracluster structure conferred by
synthesis was rapidly destroyed by adsorption of catalytically relevant
species, even at ambient temperature; this merely reflects the thermodynamic
tendency and kinetic ability of an element to segregate and to decrease
surface energies when it binds an adsorbate more strongly than another
element in bimetallic particles
Mechanism of IsobutanalâIsobutene Prins Condensation Reactions on Solid Brønsted Acids
The
selectivity to 2,5-dimethyl-hexadiene isomers (2,5-DMH) via
acid-catalyzed isobutanalâisobutene Prins condensation is limited
by isobutene oligomerization reactions (to 2,4,4-trimethyl-pentene
isomers) and by skeletal isomerization and cyclization of the primary
2,5-DMH products of Prins condensation. Experiment and theory are
used here to assess and interpret acid strength effects on the reactivity
and selectivity for isobutanalâisobutene Prins condensation
routes to 2,5-DMH, useful as precursors to <i>p</i>-xylene.
Non-coordinating 2,6-di-<i>tert</i>-butylpyridine titrants
fully suppress reactivity on Keggin heteropolyacids, niobic acid,
and mesoporous and microporous aluminosilicates, indicating that Prins
condensation, parallel isobutene oligomerization, and secondary skeletal
isomerization and cyclization of primary 2,5-DMH products occur exclusively
on Brønsted acid sites. The number of titrants required to suppress
rates allows site counts for active protons, a requirement for comparing
reactivity among solid acids as turnover rates, as well as for the
rigorous benchmarking of mechanistic proposals by theory and experiment.
Kinetic and theoretical treatments show that both reactions involve
kinetically relevant CâC bond formation elementary steps mediated
by cationic CâC coupling transition states. Transition state
charges increase with increasing acid strength for Prins condensation,
becoming full carbenium-ions only on the stronger acids. Oligomerization
transition state structures, in contrast, remain full ion-pairs, irrespective
of acid strength. Turnover rates for both reactions increase with
acid strength, but oligomerization transition states preferentially
benefit from the greater stability of the conjugate anions in the
stronger acids, leading to higher 2,5-DMH selectivities on weaker
acids (niobic acid, aluminosilicates). These trends and findings are
consistent with theoretical estimates of activation free energies
for Prins condensation and oligomerization elementary steps on aluminosilicate
slab and Keggin heteropolyacid cluster models. High 2,5-DMH selectivities
require weak acids, which do not form a full ion-pair at transition
states and thus benefit from significant stabilization by residual
covalency. These trends demonstrate the previously unrecognized consequences
of incomplete proton transfer at oxygen-containing transition states
in dampening the effects of acid strength, which contrast the full
ion-pair transition states and stronger acid strength effects in hydrocarbon
rearrangements on solids acids of catalytic relevance. These mechanistic
conclusions and the specific example used to illustrate them led us
to conclude that reaction routes involving O-containing molecules
become prevalent over hydrocarbon rearrangements on weak acids when
parallel routes are accessible in mixtures of oxygenate and hydrocarbon
reactants
The Roles of Entropy and Enthalpy in Stabilizing Ion-Pairs at Transition States in Zeolite Acid Catalysis
Acidic zeolites are indispensable catalysts in the petrochemical industry because they select reactants and their chemical pathways based on size and shape. Voids of molecular dimensions confine reactive intermediates and transition states that mediate chemical reactions, stabilizing them by van der Waals interactions. This behavior is reminiscent of the solvation effects prevalent within enzyme pockets and has analogous consequences for catalytic specificity. Voids provide the âright fitâ for certain transition states, reflected in their lower free energies, thus extending the catalytic diversity of zeolites well beyond simple size discrimination. This catalytic diversity is even more remarkable because acid strength is essentially unaffected by confinement among known crystalline aluminosilicates. In this Account, we discuss factors that determine the âright fitâ for a specific chemical reaction, exploring predictive criteria that extend the prevailing discourse based on size and shape. We link the structures of reactants, transition states, and confining voids to chemical reactivity and selectivity.Confinement mediates enthalpyâentropy compromises that determine the Gibbs free energies of transition states and relevant reactants; these activation free energies determine turnover rates via transition state theory. At low temperatures (400â500 K), dimethyl ether carbonylation occurs with high specificity within small eight-membered ring (8-MR) voids in FER and MOR zeolite structures, but at undetectable rates within larger voids (MFI, BEA, FAU, and SiO<sub>2</sub>âAl<sub>2</sub>O<sub>3</sub>). More effective van der Waals stabilization within 8-MR voids leads to lower ion-pair enthalpies but also lower entropies; taken together, carbonylation activation free energies are lower within 8-MR voids. The âright fitâ is a âtight fitâ at low temperatures, a consequence of how temperature appears in the defining equation for Gibbs free energy.In contrast, entropy effects dominate in high-temperature alkane activation (700â800 K), for which the âright fitâ becomes a âloose fitâ. Alkane activation turnovers are still faster on 8-MR MOR protons because these transition states are confined only partially within shallow 8-MR pockets; they retain higher entropies than ion-pairs fully confined within 12-MR channels at the expense of enthalpic stability. Selectivities for <i>n</i>-alkane dehydrogenation (relative to cracking) and isoalkane cracking (relative to dehydrogenation) are higher on 8-MR than 12-MR sites because partial confinement preferentially stabilizes looser ion-pair structures; these structures occur later along reaction coordinates and are higher in energy, consistent with Marcus theory for charge-transfer reactions. Enthalpy differences between cracking and dehydrogenation ion-pairs for a given reactant are independent of zeolite structure (FAU, FER, MFI, or MOR) and predominantly reflect the different gas-phase proton affinities of alkane CâC and CâH bonds, as expected from BornâHaber thermochemical cycles. These thermochemical relations, together with statistical mechanics-based treatments, predict that rotational entropy differences between intact reactants and ion-pair transition states cause intrinsic cracking rates to increase with <i>n</i>-alkane size.Through these illustrative examples, we highlight the effects of reactant and catalyst structures on ion-pair transition state enthalpies and entropies. Our discussion underscores the role of temperature in mediating enthalpic and entropic contributions to free energies and, in turn, to rates and selectivities in zeolite acid catalysis
Experimental and Theoretical Evidence for the Reactivity of Bound Intermediates in Ketonization of Carboxylic Acids and Consequences of AcidâBase Properties of Oxide Catalysts
Ketonization
of carboxylic acids on metal oxides enables oxygen
removal and the formation of new CâC bonds for increasing the
energy density and chemical value of biomass-derived streams. Information
about the surface coverages and reactivity of various bound species
derived from acid reactants and the kinetic relevance of the elementary
steps that activate reactants, form CâC bonds, and remove O
atoms and how they depend on acidâbase properties of surfaces
and molecular properties of reactants is required to extend the range
of ketonization catalytic practice. Here, we examine such matters
for ketonization of C<sub>2</sub>âC<sub>4</sub> carboxylic
acids on monoclinic and tetragonal ZrO<sub>2</sub> (ZrO<sub>2</sub>(m), ZrO<sub>2</sub>(t)) materials that are among the most active
and widely used ketonization catalysts by combining kinetic, isotopic,
spectroscopic, and theoretical methods. Ketonization turnovers require
ZrâO acidâbase pairs, and rates, normalized by the number
of active sites determined by titration methods during catalysis,
are slightly higher on ZrO<sub>2</sub>(m) than ZrO<sub>2</sub>(t),
but exhibit similar kinetic dependence and the essential absence of
isotope effects. These rates and isotope effects are consistent with
surfaces nearly saturated with acid-derived species and with kinetically
limited CâC bond formation steps involving 1-hydroxy enolates
formed via Îą-CâH cleavage in bound carboxylates and coadsorbed
acids; these mechanistic conclusions, but not the magnitude of the
rate parameters, are similar to those on anatase TiO<sub>2</sub> (TiO<sub>2</sub>(a)). The forms of bound carboxylic acids at ZrâO pairs
become more stable and evolve from molecular acids to dissociated
carboxylates as the combined acid and base strength of the Zr and
O centers at each type of site pair increases; these binding properties
are estimated from DFT-derived NH<sub>3</sub> and BF<sub>3</sub> affinities.
Infrared spectra during ketonization catalysis show that molecularly
bound acids and monodentate and bidentate carboxylates coexist on
ZrO<sub>2</sub>(m) because of diversity of ZrâO site pairs
that prevails on such surfaces, distinct in coordination and consequently
in acid and base strengths, and that monodentate and bidentate carboxylates
are the most abundant species on saturated ZrO<sub>2</sub> surfaces,
consistent with their DFT-derived binding strengths. Theoretical assessments
of free energies along the reaction coordinate show that monodentate
carboxylates act as precursors to reactive 1-hydroxy enolate intermediates,
while strongly bound bidentate carboxylates are unreactive spectators.
Higher 1-hydroxy enolate coverages, brought forth by stabilization
on the more strongly basic O sites on ZrO<sub>2</sub>(m), account
for the more reactive nature of ZrO<sub>2</sub>(m) than TiO<sub>2</sub>(a). These findings indicate that the elementary steps and site requirements
for ketonization of C<sub>2</sub>âC<sub>4</sub> carboxylic
acids are similar on MâO site pairs at TiO<sub>2</sub> and
ZrO<sub>2</sub> surfaces, a conclusion that seems general to other
metal oxides of comparable acidâbase strength
Synthesis of Bimetallic AuPt Clusters with Clean Surfaces via Sequential Displacement-Reduction Processes
We
report the synthesis of bimetallic AuPt nanoparticles (3.3â4.3
nm) of uniform size and composition using colloidal methods and reagents
containing only C, H, O, and N. These clusters were dispersed onto
SiO<sub>2</sub> and treated at low temperatures in the presence of
reductants to remove all surface residues without concomitant agglomeration,
thus leading to bimetallic structures suitable for mechanistic inquiries
into bimetallic effects on surface reactivity. Synthesis protocols
exploit and generalize galvanic displacement-reduction (GDR) processes
previously used to prepare AuPd clusters; these routes promote bimetallic
mixing but become more challenging for systems (e.g., AuPt) with smaller
reduction potential differences and less favorable mixing enthalpies
than AuPd. These hurdles are addressed here through procedural modifications
that inhibit the formation of large Au-rich clusters, which compromise
size and compositional uniformity. In doing so, we extend GDR techniques
to endothermic alloys with elements of more similar redox properties.
Higher temperatures and lower Au<sup>3+</sup> precursor concentrations
promoted metal mixing and inhibited homogeneous and heterogeneous
nucleation. Cluster size and compositional uniformity were confirmed
by UVâvisible spectroscopy during and after colloid formation,
transmission electron microscopy, and high-angle annular dark-field
(HAADF) imaging with energy-dispersive X-ray spectroscopy (EDS). Particle-by-particle
EDS analysis and HAADF imaging demonstrated the prevalence of GDR
processes in AuPd bimetallic cluster assembly. These methods also
showed that size-dependent intracluster diffusion during AuPt cluster
formation, driven by unfavorable AuPt mixing thermodynamics, leads
to Au surface enrichment, thus promoting autocatalytic Au deposition.
This rigorous mechanistic comparison of AuPt and AuPd systems provides
essential guidance and specific control variables and procedures for
the synthesis of other bimetallic systems based on the redox potential
differences and mixing thermodynamics of their two components
Transition-State Enthalpy and Entropy Effects on Reactivity and Selectivity in Hydrogenolysis of <i>n</i>âAlkanes
Statistical
mechanics and transition state (TS) theory describe
rates and selectivities of CâC bond cleavage in C<sub>2</sub>âC<sub>10</sub> <i>n</i>-alkanes on metal catalysts
and provide a general description for the hydrogenolysis of hydrocarbons.
Mechanistic interpretation shows the dominant role of entropy, over
enthalpy, in determining the location and rate of CâC bond
cleavage. Ir, Rh, and Pt clusters cleave CâC bonds at rates
proportional to coverages of intermediates derived by removing 3â4
H-atoms from <i>n</i>-alkanes. Rate constants for CâC
cleavage reflect large activation enthalpies (Î<i>H</i><sup>⧧</sup>, 217â257 kJ mol<sup>â1</sup>) that
are independent of chain length and CâC bond location in C<sub>4+</sub> <i>n</i>-alkanes. CâC bonds cleave because
of large, positive activation entropies (Î<i>S</i><sup>⧧</sup>, 164â259 J mol<sup>â1</sup> K<sup>â1</sup>) provided by H<sub>2</sub> that forms with TS. Kinetic
and independent spectroscopic evidence for the composition and structure
of these TS give accurate estimates of Î<i>S</i><sup>⧧</sup> for cleavage at each CâC bond. Large differences
between rate constants for ethane and n-decane (âź10<sup>8</sup>) reflect an increase in the entropy of gaseous alkanes retained
at the TS. The location of CâC bond cleavage depends solely
on the rotational entropies of alkyl chains attached to the cleaved
CâC bond, which depend on their chain length. Such entropy
considerations account for the ubiquitous, but previously unexplained,
preference for cleaving nonterminal CâC bonds in <i>n</i>-alkanes. This mechanistic analysis and thermodynamic treatment illustrates
the continued utility of such approaches even for hydrogenolysis reactions,
with complexity seemingly beyond the reach of classical treatments,
and applies to catalytic clusters beyond those reported here (0.6â2.7
nm; Ir, Rh, Pt)