Lewis Acid Catalysis by Zeolites.pdf

Chapter 7
Lewis Acid Catalysis by Zeolites
Guanna Li1
, Chong Liu1
, Roderigh Rohling1
, Emiel J.M. Hensen1
and Evgeny A. Pidko1,2
1
Eindhoven University of Technology, Eindhoven, The Netherlands, 2
ITMO University,
St. Petersburg, Russia
Chapter Outline
7.1 Introduction 229
7.2 Structural Problems in Lewis Acid
Zeolite Catalysis 231
7.2.1 “Single-Site” Lewis Acid Lattice
Sites 232
7.2.2 Extraframework Lewis Acid
Sites 235
7.3 Synergistic Effects in Lewis Acid
Zeolite Catalysis 243
7.3.1 Catalytic Properties Due to
Lewis AcidBase Pairs
(Mδ1
2 Oδ2
) 243
7.3.2 Lewis AcidBrønsted Acid
Synergy 245
7.4 Reactivity in Confined Space 250
7.4.1 Molecular Recognition in
Cation-Exchanged Zeolites 251
7.4.2 Confinement Effects and Entropy
Effects in Zeolite Catalysis 253
7.4.3 Molecular Recognition and
Confinement-Driven Reactivity
of Alkali-Exchanged
Faujasite 256
7.5 Conclusions 257
Acknowledgments 259
References 260
7.1 INTRODUCTION
Acidity belongs to one of the fundamental concepts in chemistry. In 1923,
Gilbert N. Lewis formulated the electron-pair theory of acidbase reactions
[1]. The Lewis acid (LA) and Lewis base (LB) were defined as the electron
pair-acceptor and electron pair-donor, respectively. The breakthrough of the
Lewis conceptual approach was that such a definition of acids and bases
could now be applied to all compounds and not only to those containing pro-
tons. Catalysis by LA is widely employed to promote various chemical

These authors contributed equally.
229
Modelling and Simulation in the Science of Micro- and Meso-Porous Materials.
DOI: http://dx.doi.org/10.1016/B978-0-12-805057-6.00007-7
© 2018 Elsevier Inc. All rights reserved.

transformations including different types of isomerization processes, CC
coupling reactions, selective hydrogenation, and functionalization of organic
molecules [2]. In nature, the reductive transformations by enzymes involve
LA catalysis [3,4], i.e., in a vast contrast to noble metal catalysis commonly
employed in chemical synthesis. Among the different types of LA catalysts
developed by chemists so far, zeolites represent one of the most important
class of acidic inorganic materials extensively utilized as heterogeneous cata-
lysts for a wide range of important processes [5].
As discussed in Chapter 1, Structure Prediction of Microporous
Materials, and elsewhere in the book, zeolites are crystalline silicate-based
materials with open 3D frameworks.The isomorphous substitution of a part
of silicon atoms with other ions creates local defects, which define the
catalytic properties of zeolites. Zeolite structures are composed of cages and
channels of molecular size, inside which the active centers as well as various
adsorbed species including water and reactive intermediates can reside. Such
a multifunctional reaction environment confined inside the zeolite micro-
pores can be viewed as a conceptual mimic of the reactive environments in
nature’s catalysts—enzymes [6]. In enzymes the catalytic LA sites are
incorporated in a tailor-made protein microreaction environment, which act
cooperatively with the reactive site to promote chemical transformations
with a high selectivity along a predefined reaction path. Similarly, the
intrinsic properties of the active sites in zeolite micropores are comple-
mented by features resembling those found in enzymes such as shape
selectivity, confinement effects, and molecular recognition.
The introduction of Lewis acidic extraframework or framework sites into
the zeolite structure gives rise to highly active LA catalysts for various
chemical reactions including hydroxylation, oxidation, hydrogenation, and
isomerization of organic compounds. The utilization of these catalyst sys-
tems for the conversion of new feedstocks including biomass, carbon diox-
ide, and natural gas is currently being extensively explored as a basis for
future more sustainable chemical industry. Molecular insights into the
mechanisms of LA-catalyzed reactions and understanding of the role of the
complex reaction environment inside the zeolite micropores are necessary to
design new and improved catalytic systems for new more efficient and sus-
tainable chemical processes. Such understanding can be obtained by using
modern computational chemistry methodologies. Molecular simulations have
become indispensable for mechanistic studies in zeolite catalysis. They are
extensively used to shed light onto the nature and structure of intrazeolite
active sites and to address the complexity of catalytic reaction mechanism.
In this chapter, we present an overview of recent reactivity concepts in
LA zeolite catalysis relevant to sustainable chemistry applications. With a
selection of recent representative examples from our group, we illustrate the
utility of modern computational techniques to reveal fundamental aspects of
catalytic phenomena in zeolite micropores, which are not directly accessible
230 Modelling and Simulation in the Science of Micro- and Meso-Porous Materials

to experimental techniques. This chapter is organized as follows. In the first
part, we will focus on the structural aspects of Lewis acidic zeolites and
show how computations can be used to address the structural problem of
zeolite catalysis. This will be followed by a section on the catalytic role of
cooperative effects and synergistic action between different active sites
inside the zeolite micropores. In the final section, we will discuss the role of
confinement and molecular recognition effects in zeolite catalysis and pres-
ent our views on the future role of calculations in the field of LA catalysis
by zeolites.
7.2 STRUCTURAL PROBLEMS IN LEWIS ACID ZEOLITE
CATALYSIS
Lewis acidity of zeolites usually stems from the presence of specific cationic
sites inside their micropores. There are two main strategies for the introduction
of LA sites in zeolites. The LA functionality can be created directly within the
zeolite lattice through the isomorphous substitution of lattice Si atoms with tet-
ravalent metal cations such as Ti41
, Sn41
, and Zr41
(Scheme 7.1A). This strat-
egy allows forming highly dispersed and well-defined single-site LA sites with
unique catalytic characteristics. For example, the incorporation of titanium
into the medium-pore MFI-type zeolites (TS-1) produces highly active cata-
lysts for the selective epoxidation of olefins with H2O2 [7,8]. Stannosilicates
form another important class of Lewis acidic materials of this type that in
recent years have found many important applications as catalysts for biomass
valorization [9,10]. In particular, large-pore BEA zeolites modified with Sn are
highly active catalysts in reactions such as the isomerization and epimerization
of carbohydrates, MeerweinPonndorfVerley reduction of aldehydes and
ketones, Oppenauer oxidation of alcohols, and BaeyerVilliger oxidation of
ketones [1116].
An alternative and generally more versatile approach for the formation of
Lewis acid sites (LAS) in zeolites employs conventional Al-substituted zeo-
lites acting as ion-exchange materials to stabilize the reactive cationic spe-
cies (Scheme 7.1B). The substitution of a tetravalent Si atom in the lattice
with the trivalent Al generates local negative charges, which are then com-
pensated by extraframework cations. The charge-compensation by protons
results in strong Brønsted acid sites (BAS). In a more general case, the lat-
tice negative charge is balanced by metal cations showing a substantial
(A) (B)
Si O
Al+3
O
O
O
O Si
O
O
O
O
O
Si O
M4+
O
O
O
O Si
O
O
O
O
O
δ− δ− M
SCHEME 7.1 (A) Framework and (B) extraframework LA sites in zeolites.
Lewis Acid Catalysis by Zeolites Chapter | 7 231

Lewis acidity. The anionic centers due to lattice Al substitutions together
with the surrounding basic oxygen atoms can be viewed as a ligand environ-
ment stabilizing the exchangeable cationic species and defining its chemical
properties in a manner similar to organic ligands in molecular coordination
chemistry and homogeneous catalysis.
Unlike the well-defined lattice LA sites of the first type, the structural
properties and the nature of extraframework cations are not uniquely defined.
Depending on the chemical composition of the zeolite matrix, chemistry of
the metal ions, conditions of the catalyst preparation, and its postsynthetic
activation procedure, a wide range of possible species including isolated
metal cations (M1
/M21
), functionalized mononuclear cationic species, and
multinuclear clusters as well as bulk oxide aggregates confined in the zeolite
micropores can be formed. The catalytic properties of the resulting LA-
containing zeolites depend strongly on the nature and chemical reactivity of
the extraframework species stabilized inside the zeolite micropores.
Important examples of this class of materials include steam-calcined zeolites,
in which extraframework Lewis acidic aluminum complexes enhance
Brønsted acidity of vicinal zeolitic protons, and high-silica zeolites modified
with Ga or Zn cations active for the dehydrogenation and aromatization of
light alkanes [17,18].
The structural heterogeneity of the actual zeolite materials and the fact
that conventional catalyst preparation methodologies usually give rise to a
variety of different intrazeolite species make it very challenging to unambig-
uously identify the active sites using solely the experimental techniques.
Modern computational methodologies provide practical tools to model differ-
ent species inside the zeolite pores, to investigate their stability and to ulti-
mately address the structural problem of the zeolite catalysis.
7.2.1 “Single-Site” Lewis Acid Lattice Sites
The generation of the reactive centers via the isomorphous substitution of
lattice sites is most well known with the incorporation of aluminums to
impart Brønsted acidity. Lewis acidity of lattice sites is generated upon the
incorporation into the pure silica frameworks of other heteroelements such as
Ti, Zr, and Sn, which do not lead to charge imbalance. In particular, the
introduction of isolated tin atoms in the zeolite lattice gives rise to highly
electron-deficient active sites capable of coordinating and activating various
electron donor substrates such as carbonyl oxygen atoms of carbohydrates.
Such stanosilicate materials behave as water-tolerant LAs rendering them
highly attractive catalytic materials for biomass valorization [19,20].
Lattice-substituted zeolites are commonly referred to as single-site cata-
lysts, which are the materials containing uniform isolated well-defined spe-
cies with specific chemical properties. However, growing experimental
evidence is becoming available in the recent years on a substantial site

heterogeneity in such materials [2125]. Minor modifications in the syn-
thetic procedure employed for the preparation of the catalyst can have a dra-
matic effect on the structural properties of the intrazeolite sites. For example,
a very recent dynamic nuclear polarization (DNP) 119
Sn solid-state nuclear
magnetic resonance (NMR) spectroscopic investigation of a series of highly
active Sn-beta zeolite catalysts prepared in different laboratories revealed a
striking difference in the spectroscopic characteristic (Fig. 7.1) and hence the
chemical environment of the intrazeolite Sn species [26]. These findings cre-
ate a certain ambiguity in defining the catalytically active species in such
materials.
In addition to the direct isomorphous substitution of silicon, the guest
atoms can be incorporated in the lattice as partially hydrolyzed species,
which in the case of Sn-beta are often claimed to be the catalytic sites [27].
The chemical modifications of the guest atoms upon synthesis (e.g., partial
hydrolysis) are not the only source of site heterogeneity. The local coordina-
tion environment around lattice heteroatoms determined by the topological
properties of the host matrix can substantially affect their Lewis acidity and,
therefore, catalytic reactivity. Therefore, it is important to investigate the
relationship between the location, stability, and properties of heteroatom sub-
stitutions in zeolite lattices. The sitting of Ti in TS-1 material with MFI
topology has been thoroughly investigated using both experimental and theo-
retical approaches [2830]. However, no definitive results have been
(A) (B)
–654 –699
–743
–615
TUE_1
TUE_3
TUE_4
KUL_1
TUE_2
Purdue_1
Purdue_3
ETH/UW_1
Purdue_2
–692 –707
–718
–727
–692
–649
–604
–724
–500 –550 –600 –650 –700 –750 –800 –500 –550 –600 –650 –700 –750 –800
–614
–614
–663
–720
–722
–693
–677
–642
–700
–700
–634
–659
–685
FIGURE 7.1 DNP 119
Sn CP Hahn-echo magic angle spinning (MAS) spectra of different Sn-
beta zeolite materials prepared via (A) direct hydrothermal and (B) postsynthetic Sn-
modification strategies by different research groups in different institutions [26].

provided [31]. It has been suggested that Ti sitting in MFI lattice may be not
determined by the intrinsic stability of the specific substitutions but rather by
other kinetic and thermodynamic factors affecting the zeolite growth process
under the hydrothermal conditions [32].
Several detailed computational studies have analyzed the sitting of het-
eroatoms in beta zeolite. A periodic density functional theory (DFT) study
by Shetty et al. [33] demonstrated that the incorporation of titanium atoms in
beta zeolite is more energetically favorable than tin. The lower intrinsic sta-
bility of the Sn-modified beta has been correlated with the enhanced Lewis
acidity of this material. A similar conclusion has been drawn by Yang et al.
[34] who employed periodic DFT calculations to identify the preferred loca-
tions of lattice substitutions and investigate the Lewis acidity of Ti, Sn, and
Zr heteroatoms in the framework of beta zeolite. In general, Sn and Zr form
stronger LAS than Ti, while the acidity of particular species is strongly site
dependent (Fig. 7.2). This implies that the reactivity of a lattice-modified
zeolite material should be affected by the particular distribution of lattice
heteroatoms. Furthermore, theoretical calculations indicated a unique feature
of Sn-containing beta zeolite, in which the formation of paired lattice sites
with enhanced reactivity was predicted to be favorable. It was hypothesized
that such sites may be important for catalytic reactivity in carbohydrate con-
version reactions.
Despite the apparent simplicity of the concept of framework LA sites in
zeolites, the identification of the catalytically active species and understand-
ing their transformations under the reaction conditions represent truly com-
plex problems. The LA sites can be present as perfect tetrahedral sites
embedded in the framework or partially hydrolyzed species with open coor-
dination sites and reactive vicinal hydroxyl groups. Furthermore, synthetic
procedures can lead to the formation of either loosely bound heteroatom
FIGURE 7.2 (A) The structure and location of the nine distinguishable T sites in zeolite beta.
(B) Correlation between the relative stabilities (ΔE) of different Ti-, Sn-, and Zr-substitutions in
beta and the adsorption energy of water (Eads) used as a Lewis acidity probe [34].

structures or extraframework species. The catalytic role of these species and
the fundamental factors that determine their stability, formation, and reactiv-
ity have not been addressed yet. Further computational studies are required
to support and rationalize the growing volume of experimental literature on
these systems.
7.2.2 Extraframework Lewis Acid Sites
A more versatile and therefore more often used approach for tailoring zeolite
reactivity is based on the ability of zeolite lattices to act as ligands stabiliz-
ing cationic species in their pores. The presence of aluminum ions in the
zeolite lattice induces net negative charges on the framework, which require
counter ions for overall charge neutrality. Chemical properties of zeolite
materials including, in particular, their Lewis acidity can be tailored by intro-
ducing reactive extraframework cations or cationic complexes. These reac-
tive centers are usually stabilized at specific zeolite sites and their
distributions are to a large extent controlled by the distribution of lattice
anions and therefore by the sitting of framework Al. Following the general
electrostatic considerations, the stabilization of metal cations or cationic
complexes with a formal charge of 11 can be readily accomplished via a
direct charge-compensation mechanism through the direct coordination of
the cationic species with framework [AlO2]2
tetrahedral site. However for
cations with a charge larger than 11, such a direct charge-compensation
scheme would require the presence of several vicinal lattice Al atoms, i.e.,
seldom encountered in practice and especially in high-silica zeolites. As a
result, the structural models of the active sites in the systems with a rela-
tively high dispersion of lattice aluminum often imply the formation of oxo-
and hydroxo-functionalized species with a low overall charge (e.g., M 5 O1
and M(OH)n
1
) suitable for the direct charge-compensation mechanism. The
increasing structural complexity of the extraframework species may make
other factors such as coordinative unsaturation and basicity of the extrafra-
mework ligands even more important for the overall stability of the zeolite
system than the electrostatic charge-compensation considerations. For low-
silica zeolites the high density of lattice anions available for the stabilization
of extraframework species does not solve this structural problem. The
increased local concentration of cations may lead to the formation of aggre-
gated and clustered structures with varied chemical composition and
increased overall charge.
In this section we will focus on discussing how modern computational
chemistry can help addressing the structural complexity of extraframework
species in zeolites. We will first introduce the concepts of indirect charge-
compensation and self-organization of extraframework cations in zeolites by
discussing the chemistry of Ga-containing zeolites as a showcase example.
Then we will introduce an ab initio thermodynamic analysis approach that

allows predicting stabilities of extraframework species in zeolites under
experimental conditions. The success of this methodology will be illustrated
by relevant catalytic systems based on Cu-, Fe-, and Al-containing zeolites.
7.2.2.1 Self-Organization of Extraframework Cations
We first consider the formation and transformation of gallium-containing
extraframework cations in high-silica zeolites as a representative example.
Zeolites modified with gallium are efficient Lewis acidic catalysts for pro-
cesses such as dehydrogenation and dehydroaromatization of light alkanes
[17,18] and the conversion of biomass-derived furanics to aromatic com-
pounds [35,36]. Understanding the nature of the reactive Ga sites in zeolites
is therefore important for further optimization and improvement of these
important catalytic processes.
Hensen and coworkers carried out a series of experimental studies on the
reactivity of well-defined Ga sites in ZSM-5 prepared via chemical vapor
deposition of trimethylgallium onto acidic zeolites [37,38]. It was shown that
the oxygenation of isolated Ga1
with N2O enhances strongly the alkane
dehydrogenation activity [39]. In view of the low density of Al in the zeolite
matrix (Si/Al 5 20) and the 1:1 Ga:N2O stoichiometry of the oxygenation
reaction, the increased reactivity was attributed to the formation of
[Ga 5 O]1
extraframework species. However, subsequent mechanistic
studies could not reveal a favorable catalytic mechanism involving such
complexes [40].
To resolve this apparent inconsistency between the theory and experi-
ment, the transformations of Ga species inside zeolite micropores were stud-
ied in detail by periodic DFT calculations [41,42]. A relatively small unit
cell of mordenite-type zeolite with a representative Si/Al ratio of 23 was
used to construct periodic zeolite models with varied lattice Al distribution.
The intrinsic stability of zeolite models containing univalent isolated Ga1
ions
and their oxygenated [GaO]1
counterparts does not depend on the relative
location of Al in the framework. The latter species represent metastable -
configurations. Their self-organization into binuclear [(GaO)2]21
clusters
decreases the total energy of the zeolite models by as much as 437 kJ/mol
(Fig. 7.3). Such a high exothermicity of the dimerization process is associated
with the excessively high basicity of the terminal oxo-ligand and unfavorable
trigonal coordination of the Ga site in the mononuclear [GaO]1
species.
Calculations reveal the crucial role of the latter factor on the intrinsic stability
of the clustered oxygenated species. Higher exothermicity was predicted for
the reaction paths resulting in complexes with favorable tetrahedral coordina-
tion of Ga centers in [(GaO)2]21
even when such configurations could not
provide a geometrical possibility for the direct charge-compensation between
the extraframework cation and the lattice anionic centers (see, e.g., MOR-II-SP
and MOR-III-SP configurations in Fig. 7.3). These data indicated that the

location and the stability of cationic Ga complexes in high-silica zeolites are
predominantly controlled by the coordination properties of the metal centers.
Furthermore, it was clearly shown that the presence of multiply charged bi- or
oligonuclear metal oxide species in zeolites does not require the immediate
proximity of an equivalent number of [AlO2]2
negative framework charges.
Such phenomena of self-organization of oxygenated cationic metal complexes
in zeolites and a favorable mechanism of indirect charge-compensation were
later reported for a wide range of other systems such as zeolites MOR, FAU,
and ZSM-5 modified with Zn, Al, La, Cu, and Fe. For all these systems,
periodic DFT calculations showed a general tendency for mononuclear oxy-
genated and hydroxylated complexes to self-organize into bi- and oligonuclear
cationic clusters [43].
7.2.2.2 Stability of Cationic Complexes in Zeolite Voids
Electronic structure methods and, particularly, periodic DFT calculations are
instrumental to studying intrinsic stability and reactivity of extraframework
species in zeolites. Simulations allow to assess the thermodynamics of the
interconversion between different hypothetic intrazeolite species so that their
stabilities could be assessed. To illustrate this we will consider the question
FIGURE 7.3 Self-organization of [GaO]1
cations in high-silica mordenite [4143].

of the speciation of extraframework iron in ZSM-5. High-silica zeolites mod-
ified with iron (Fe/ZSM-5) are important catalysts for chemical processes
such as the decomposition of N2O, selective catalytic reduction of NOx, the
direct oxidation of benzene to phenol, and of methane to methanol [4446].
The nature of the catalytic Fe complexes in these systems and, accordingly,
their catalytic properties depend on the conditions of catalyst preparation
conditions, i.e., the method of Fe introduction, postsynthetic activation pro-
cedure and loading of Fe [4749]. Iron in ZSM-5 can be present in a wide
variety of forms ranging from isolated Fe21
and Fe31
cations and oligonuc-
lear Fe clusters to larger iron oxide agglomerates confined in the zeolite
pores [50,51].
The nature of the catalytic Fe sites in Fe/ZSM-5 has been a subject of
many experimental and computational studies. Mechanistic proposals involv-
ing the mononuclear Fe1
, Fe21
, and [FeO]1
as well as the binuclear [Fe
(μO)Fe]21
, [HOFe(μO)FeOH]21
, [Fe(μO)2Fe]21
, and [Fe(μO)
(μOH)Fe]1
complexes as the active sites have been made [5257]. The
relative stability of such extraframework complexes in a realistic ZSM-5
zeolite model has been studied by periodic DFT calculations [58]. The
computational results were used to construct a reaction scheme for the inter-
conversion of different intrazeolite Fe complexes (Fig. 7.4) involving besides
the Fe complex itself zeolitic BAS, water, and O2 as co-reactants or by-
products of particular transformations. A neutral cubic [Fe4O6] cluster inside
the straight pore of ZSM-5 was chosen as a starting point of the reaction net-
work. When located near zeolite BAS, such a cluster is readily protonated to
form cationic species, which can further be converted to hydroxylated and
oxygenated binuclear clusters.
Electronic structure calculations provide information regarding the intrin-
sic stability of the extraframework metal complexes at 0K. However, under
practical conditions, finite temperature effects and the presence of gaseous
environment during the catalyst synthesis and preactivation introduce a sub-
stantial entropic contribution that affects the stability of the different extra-
framework species. These factors can be taken into account by using a
statistical thermodynamic approach, i.e., often referred to as ab initio
Thermodynamic Analysis (aiTA) [59,60]. With this method, stabilities of dif-
ferent systems containing extraframework complexes of different chemical
composition can be investigated as a function of the experimental conditions.
Within the aiTA, the interconversion of different chemical systems
should be defined with a generic reaction equilibrium. In the case of the Fe-
containing ZSM-5 zeolite, such an equilibrium was defined as
x
2
Fe2O3 1 H ZSM-5 1
n 2 2
2
H2O2FexOmHn=ZSM-5
1
n 1 3x 2 2m 2 2
4
O2
ð7:1Þ

FIGURE 7.4 Reaction paths for interconversion of Fe-containing complexes confined in ZSM-5 zeolites. Reaction energies are given in kJ/mol [58].

The Gibbs free energy (ΔG(T,p)) of formation of a particular intrazeolite
Fe complex is then defined as
ΔG5GFexOmHn=ZSM5 1
n 1 3x 2 2m 2 2
4
GO2
2
n 2 2
2
GH2O
2
x
2
GFe2O3
2 GHZSM5
ð7:2Þ
GFexOmHn=ZSM-5 and GH-ZSM-5 are the Gibbs free energies of a Fe-
containing ZSM-5 model and the H-form of ZSM-5 model. GFe2O3
, GO2
, and
GH2O correspond to the Gibbs free energy of bulk α-Fe2O3, gaseous water,
and oxygen, respectively. The factor x denotes the number of Fe atoms in
the unit cell of the FexOmHn/ZSM-5 system. Such a definition implies that
the relative stabilities of different Fe-containing zeolite systems are analyzed
with respect to the bulk iron oxide and the proton form of the ZSM-5 zeolite.
The entropy change of the solids is assumed to be negligible compared with
that of the other reactants, and the calculated electronic DFT energy (E) is
used to represent the Gibbs free energy of the solids. For gaseous reagents,
the Gi equals their chemical potential (μi). Then, the Gibbs free energy can
be rewritten as
ΔGðT; pÞ 5 ΔE 1
n 1 3x 2 2m 2 2
2
ΔμO 2
n 2 2
2
ΔμH2O ð7:3Þ
The chemical potential differences (μi) are defined as
ΔμOðT; pÞ 5
1
2
ΔμO2
ðT; p0
Þ 1 RT lnðpO2
=p0
Þ

ð7:4Þ
ΔμH2OðT; pÞ 5 ΔμH2OðT; p0
Þ 1 RT lnðpH2O=p0
Þ ð7:5Þ
Free energies of different Fe-containing systems are therefore expressed
as the functions of the chemical potentials of water and oxygen, which are,
in turn, directly related to the temperature and respective partial pressures.
Fig. 7.5 illustrates how Gibbs free energies (ΔG(T,p)) for different Fe-
containing species in ZSM-5 zeolite change with varying chemical potentials
of O2 and H2O. Such dependencies can be used to bridge experiments and
calculations. One can see that for all chemical potentials, Fe/ZSM-5 system
is dominated by three binuclear extraframework Fe cations, namely the
oxygen-bridged [Fe(μO)2Fe]21
and [Fe(μO)Fe]21
clusters as well as the
hydroxylated [HOFe(μO) Fe]OH21
complex that becomes stabilized only
at very high H2O partial pressures. The [Fe(μO)Fe]21
complexes contain-
ing bivalent iron centers are mainly present in Fe/ZSM-5 catalyst activated
at low oxygen chemical potential and H2O-free conditions, whereas the for-
mation of its Fe31
-containing counterpart [Fe(μO)2Fe]21
is favored upon
the high-temperature calcination in an O2-rich environment. Unfortunately,
for the Fe-containing zeolites, most of the extraframework species corre-
spond to metastable phases with respect to the bulk iron oxides. This makes

the preparation and, more importantly, maintaining the well-defined nature
of single-site Fe/ZSM-5 materials under catalytic conditions virtually
impossible.
On contrary, for copper-containing zeolites, extraframework Cu cations
represent the most thermodynamically stable configurations. This creates a
principle possibility to construct well-defined Cu-containing zeolite materials.
aiTM approach has been employed to investigate the copper speciation in
Cu/ZSM-5 and Cu/MOR zeolites [61,62], which are promising catalysts for
the direct selective oxidation of methane to methanol [63]. Computational
analysis revealed that at the conditions of the high-temperature calcination
FIGURE 7.5 (A) Gibbs free energy of formation of FexOmHn in ZSM-5 (ΔG), cf. Eq. (7.3), as
a function of oxygen chemical potential (ΔμO) and water chemical potential (ΔμH2O). (B) ΔμO
and ΔμH2O are translated into pressure scales at T 5 1100K [58].

procedure commonly employed for catalyst activation, the most stable -
configurations correspond to the trinuclear [Cu3(μO)3]21
cations, which
have not been considered before (Fig. 7.6). In view of the limited cation
mobility at lower temperatures, such species can become kinetically
stabilized in the activated materials. These theoretical predictions were
confirmed by complementary kinetic and spectroscopic experimental
studies. Importantly, Cu/MOR zeolites with exclusive trinuclear Cu specia-
tion allow reaching record high-methanol productivities in the selective
methane oxidation with O2 [61].
Recently, the aiTA approach has been successfully employed to shed
light onto one of the long-standing scientific problems in zeolite catalysis—
the nature of extraframework aluminum (EFAl) in faujasite-type zeolites.
Zeolites with faujasite topology constitute one of the key component of the
industrial fluidized catalytic cracking catalysts. Prior to the catalytic applica-
tion, the catalyst undergoes a steam-calcination treatment, by which both its
acidity and hydrothermal stability are substantially improved. Upon this pre-
activation step a part of the lattice aluminum is extracted into the extraframe-
work position resulting in the formation of Lewis acidic sites inside the
zeolite pores. The nature of such EFAl complexes in faujasite zeolites was
studied by a combination of periodic DFT calculations and aiTA analysis
[64]. Theory pointed to multinuclear cationic complexes inside the small
sodalite cages of faujasite as the dominant EFAl species in partially dealumi-
nated faujasite-type materials. The presence of such cationic EFAl clusters
inside the inaccessible sodalite cages strongly enhances the protolytic
propane cracking activity of vicinal supercage BAS [64].
FIGURE 7.6 Structure and location of [Cu3(μO)3]21
cluster in mordenite predicted by
DFT [61].

In summary, molecular modeling is a powerful tool useful to study and
predict structural and thermodynamic properties of LA sites in zeolites.
Calculations suggest that the self-organization of oxygenated and hydroxyl-
ated extraframework cations is a general phenomenon common for a wide
range of zeolite systems. The stability and location of cationic clusters are
mainly controlled by the favored coordination environment, while the direct
coordination of the cationic centers with the lattice anions is usually less
important. When constructing zeolite models to be used in mechanistic stud-
ies, the formation of multinuclear charged complexes should be considered.
Another important aspect is the relativity of the stability concept. The “sta-
bility” of a chemical system in a general case is defined by its free energy,
which, in turn, is a function of the reaction conditions. These factors have to
be taken into account when the structural aspects of zeolite catalysts are
investigated.
7.3 SYNERGISTIC EFFECTS IN LEWIS
ACID ZEOLITE CATALYSIS
7.3.1 Catalytic Properties Due to Lewis
AcidBase Pairs (Mδ1
2 Oδ2
)
The catalytic reactivity of Lewis acidic zeolites is controlled by both the
properties of the LA centers and the conjugated base. In Section 7.2.2.2, we
discussed the structural complexity of Lewis acidic lattice and extraframe-
work sites. In all these structures, the electropositive cations—Lewis acidic
metal centers—are accompanied by electronegative counterparts, i.e., basic
framework or extraframework oxygen centers. Their close proximity gives
rise to cooperative effects where both species contribute to the catalytic
cycle. They cannot therefore be considered as separate entities, but should
rather be viewed as a synergistically cooperating active site ensemble.
The dehydrogenation of alkanes is an important reaction in the petro-
chemical industry. High-silica zeolites of Zn/ZSM-5 are excellent LA cata-
lysts being able to activate C 2 H bond of light alkanes [65,66]. The
structure of the active site has been a subject of hot debate in the past
decade. Various Zn-containing species including mononuclear Zn21
, binuc-
lear [ZnOZn]21
, and oligonuclear Zn-oxo complexes have been assigned to
be the active sites for ethane dehydrogenation [6769]. Theoretical studies
revealed that the activation of the ethane C 2 H bond is promoted by the
Zn21
cation and one of the nearby basic lattice oxygen atoms, which acts as
a proton acceptor [70]. This heterolytic cleavage of the ethane C 2 H bond
takes place over the conjugate LALB pair resulting in the formation of a
hydroxyl group and zinc-alkyl fragment (Scheme 7.2). A similar synergistic
effect of LALB pair has also been observed for ethane dehydrogenation
catalyzed by gallium modified ZSM-5 zeolites. The exact cooperation pattern

depends on the basicity of the proton-accepting site connected to Ga, which
can be tuned by modifying the local coordination environment of Ga or by
changing the nature of the base site. As a result, the reactivity of the catalytic
center as a whole is also modified [71].
The catalytic activity of the binuclear gallium-oxo ([Ga2O2]21
) clusters
in the dehydrogenation of light alkanes is much higher than that of the
mononuclear gallyl [GaO]1
species [40,41]. The formation of the binuclear
complex results in the decrease of both the Lewis acidity of the Ga center
and basicity of the bridging extraframework oxygens. Although this
decreases somewhat the reactivity of the Ga site toward the initial CH
bond activation, such a modification dramatically lowers the barrier for the
next steps along the catalytic cycle. In particular, the regeneration of the
[Ga2O2]21
active site via H2 recombination from [HGa(OH)OGa]21
is a
much more favorable process compared to the analogous conversion of
mononuclear [HGaOH]1
and [GaO]1
species.
Although Fe-exchanged ZSM-5 zeolites are commonly considered as
redox catalysts, the pronounced LALB characteristics of their intrazeolite
Fe sites strongly influences their catalytic performance, in particular, in the
selective oxidation of benzene to phenol. DFT calculations revealed that iso-
lated mononuclear Fe21
and [FeO]21
cations in such materials can only be
stabilized in six-membered rings with symmetrically distributed framework
Al ions. In practice, the number of such positions is low and the dominant
part of the zeolite sites is occupied by more complex iron species such as
[Fe(μO)Fe]21
, [Fe(μO)2Fe]21
, and [HOFe(μO)2FeOH]21
representing
the thermodynamically preferred configurations in Fe-rich catalysts [58].
Calculations reveal that only the isolated mononuclear cations can promote
the selective oxidation of benzene to phenol in a catalytic manner [57]. Very
recent experimental studies employing modern advanced spectroscopic tech-
niques have confirmed this prediction [72]. The activation of CH bonds of
benzene over the alternative oxygen-containing binuclear sites results in the
SCHEME 7.2 Ethane dehydrogenation by the synergetic effect of LALB pair in Zn/ZSM-5 [70].

long-term catalyst deactivation [57]. Such a strikingly different behavior of
Fe sites is determined solely by the differences in their acidbase character-
istics. The presence of highly basic extraframework oxygen ligands in the
binuclear sites opens a path for secondary conversions of the phenol product
leading to the active site deactivation. An acidic OH group of adsorbed phe-
nol can heterolytically dissociate over the Feδ1
?Oδ
acidbase pair yield-
ing a phenolate species ([C5H6O]
) and an OH group grafted to iron sites.
This reaction is thermodynamically favored and proceeds with very low acti-
vation energies for all sites containing basic extraframework oxygens. The
bulky grafted phenolate species block the zeolite channels and decrease the
accessibility of the active sites. They are argued to be coke precursors caus-
ing the fast deactivation of Fe/ZSM-5 zeolite catalysts during the benzene to
phenol oxidation. The ability of isolated Fe21
sites to promote the oxidation
process in a catalytic manner is due to the combination of low Lewis acidity
of Fe cations stabilized at the chelating zeolite sites and low basicity of the
high-silica zeolite framework. This makes the heterolytic dissociation of phe-
nol highly unfavorable allowing thus the regeneration of the active site via
the release of the reaction product from the zeolite voids.
7.3.2 Lewis AcidBrønsted Acid Synergy
The LALB cooperativity discussed above is the example of a synergistic
action of the components of a single reactive site. Zeolite voids can poten-
tially contain a variety of sites of different nature acting synergistically dur-
ing the catalytic transformations. Besides Lewis acidic cations, zeolite pores
often contain weakly acidic silanol groups at lattice defects and strong (resid-
ual) BAS. The interaction and cooperation between these sites can substan-
tially alter the catalytic performance of zeolite materials.
A representative example of such phenomena is the acidity enhancement
of steam-calcined faujasite-type zeolites [73]. Experimental studies showed
that the steam-calcination activation procedure of Y zeolites with faujasite
topology results in the formation of Lewis acidic EFAl species inside the
zeolite pores and a concomitant increase of the zeolite Brønsted acidity
[7476]. In particular it was found that the increase in the ratio between the
cationic EFAl species and BAS in zeolite Y results in a pronounced increase
of the rate of monomolecular propane cracking [77]. The enhanced catalytic
reactivity has been related to the synergy between the cationic EFAl com-
plexes and vicinal BAS [78]. Although there is a substantial evidence for the
promoting effect of EFAl species on the zeolite Brønsted acidity as well as
the associated catalytic properties, the exact mechanism of this phenomenon
is still under debate. Alternative hypotheses emphasize the role of zeolite
pore structure and confinement on the cracking reactivity than variation in
intrinsic strength of the acid sites [7981].

The lack of the direct structural information about the EFAl species gener-
ated by the steam-calcination procedure and limited computational capabilities
represented until recently the key hurdles in the development of a consistent
theory to rationalize the phenomenon of acidity enhancement due to the
presence of EFAl species. Earlier DFT studies employed cluster modeling
approach, in which only a small fraction of the zeolite framework representing
the first coordination sphere of the active site was implicitly considered in
the calculations [82]. Such models were not able to adequately represent the
structural complexity of the low-silica faujasite zeolites, in which because of
the high density of extraframework species nonlocal effects become very
important. Furthermore, the choice of the lattice fragment for the zeolite
cluster model imposes severe limitations on the size and overall charge of
EFAl species that can be potentially considered in the computational studies.
Cluster DFT calculations have mostly focused on the analysis of the
structure and promoting effect of small, mononuclear EFAl species. Mota
and coworkers carried out DFT calculations to compare the stability of six
different types of mononuclear EFAls interacting with a T6 cluster zeolite
model (T stands for the tetrahedral lattice Si or Al atom) [83]. They identi-
fied [Al(OH)]21
as the preferred EFAl structure able to enhance the acidity
of vicinal zeolite protons [84]. The proximity of EFAl species and BAS in
dealuminated zeolite Y has been investigated by a combination of solid-state
NMR and DFT calculations [78]. The promoting effect of the Lewis acidic
neutral Al(OH)3 and cationic [Al(OH)]21
species on zeolite acidity was dis-
cussed. It was proposed that EFAl can be stabilized in the immediate vicinity
of BAS without directly interacting with them. The acidity enhancement in
steam-calcined zeolite Y was attributed to the indirect stabilization of the lat-
tice by mononuclear EFAl species upon the BAS deprotonation. It was
hypothesized that EFAls partially compensate the negative charge of the lat-
tice anionic sites bearing an acidic proton. This weakens the OH bond
resulting in the increased acidity of the associated BAS (Scheme 7.3).
An adequate representation of the chemical composition and coordination
environment of the low-silica faujasite catalysts can only be achieved by using
periodic modeling approach. Note that even in this case the zeolite structure is
assumed to be perfectly periodic and essentially defect-free system, in which
long-range dynamic features and relaxations are essentially omitted.
Nevertheless, it is generally assumed that these factors play only a minor if
any role for the catalytic properties of intrazeolite reaction environments.
Si
O O
O O
Al
O
O
Si
O
O
Si
O
O
H Al(OH)3 or AlOH2+
O O
SCHEME 7.3 A schematic representation of the BrønstedLA synergy between a model
mononuclear EFAl and BAS in dealuminated faujasite HY zeolite [78].

We recently carried out a detailed study of the stability and catalytic role
of different hypothetic EFAl species on low-silica zeolite models [64].
Calculations revealed the importance of larger multinuclear multiple-charged
cationic EFAl species, which could not be considered within the cluster
modeling approximation. These EFAl aggregates are preferentially stabilized
inside small inaccessible to most reactants sodalite cages supporting thus the
hypothesis on the indirect mechanism of the acidity enhancement by EFAl
species.
By using more realistic structural models for the EFAl-containing fauja-
sites it became possible to shed light onto the catalytic role of EFAl species.
Fig. 7.7 summarizes the key results of periodic DFT calculations on propane
cracking as a model reaction over an EFAl-free faujasite model (H-FAU)
and a representative EFAl-containing structure featuring a trinuclear
[Al3O4H3]41
complex confined in the sodalite cage (EFAl/H-FAU,
Fig. 7.7A). Calculations were carried out on the first step of the cracking
reaction, i.e., the protonation of propane by zeolite BAS to form a carbonium
ion intermediate Int (Fig. 7.7B), which is the rate-determining step of the
overall catalytic process. It was found that the presence of the EFAl at the
sodalite cage of the zeolite strongly stabilizes the structures of TS and Int,
thereby providing the means to lower the reaction barrier for the rate-
limiting protonation step (Fig. 7.7D). Based on the DFT results, we attributed
this promoting effect to the interaction of the strongly Lewis acidic multiple-
FIGURE 7.7 (A) The EFAl-free (H-FAU) and EFAl-containing (EFAl/H-FAU) faujasite models.
(B) Mechanism for protonation of the CC bond during monomolecular propane cracking.
(C) Local structures of adsorption complexes (Ads), transition states (TS), and reaction
intermediates (Int). (D) Reaction energy diagram of propane cracking in faujasite zeolites [64].

charged cationic EFAl with the zeolite lattice, which allows stabilizing the
anionic zeolite lattice upon the deprotonation of the BAS.
We propose that such an indirect mechanism of Lewis/Brønsted acid
cooperativity is a unique feature of faujasite-type lattice topology providing
sodalite cages capable of efficient stabilization of the cationic large multinu-
clear cationic aggregates [64,85]. In other zeolite topologies, cooperative
effect featuring a concerted interaction of zeolite BAS and LAS with the
substrate resulting in its efficient activation and transformations along the
desirable reaction path can potentially realize. Such a mechanism involving a
synergistic action of a Lewis acidic EFAl and a zeolite BAS has been pro-
posed for catalytic conversion of methylcyclohexane over EFAl-containing
ZSM-5 zeolites (Scheme 7.4) [86]. The protonation of an adsorbed methylcy-
clohexane molecule results in a carbonium ion polarized by a neighboring
EFAl LAS site. The interaction of an activated intermediate with LAS
facilitates the ring-opening reaction resulting in the higher rate of the overall
reaction. Besides the concerted action of LAS and BAS, catalytic transfor-
mations over zeolites can make use of individual promoting effect of each of
the sites at particular elementary step of the catalytic reaction. A relevant
example is illustrated in Scheme 7.4B for catalytic glycerol dehydration by
Al/H-ZSM-5 zeolite [87]. In this case, the catalytic process starts with the
dehydration of an internal secondary hydroxyl group of glycerol by zeolite
BAS to produce an unsaturated diol 4, which then enters a separate LAS-
catalyzed sequence of transformations involving the tautomerization and
dehydration reactions.
Besides the LA-promotion of Brønsted acid reactivity, the catalytic per-
formance in LA-catalyzed reactions can be promoted through the coopera-
tion with BAS. Such effects have been recently discovered to play an
(A)
Si
Si
Si
Si
Si
Si
Si
Al
Si
Si
O
O
O O
O
O
O
O
O
O
H
Al
O
Protolysis
Polarization
(B)
OH
OH
2
3
1
5 4
6 7
OH2
+
H2
O
H+
OH–
+ H+
–H+
OH
OH Tautomerization OH
LAS
OH
HO
HO
HO
O
O O
HO
H2O
BAS
SCHEME 7.4 Polarization and protonation effects provided the synergistic effect between LA
and BA (left) [86], and stepwise glycerol dehydration promoted by the cooperativity of BAS and
LAS (right) [87].

important role for the catalytic properties of lattice-modified Lewis acidic
zeolites such as Sn, Ti, and Zr-containing microporous silicates [5,88,89].
Large-pore BEA zeolites containing well-defined lattice Sn sites are consid-
ered unique catalysts for the isomerization of glucose to fructose in water.
This reaction is crucial for establishing favorable conversion paths from cel-
lulosic biomass to various platform chemicals [16].
There is a general consensus in the scientific literature that Sn-BEA zeo-
lites are dominated by two major species, namely the partially hydrolyzed
so-called open Sn site (Scheme 7.5A), in which the Sn center forms three
bonds with the surrounding zeolite lattice and one coordination site is occu-
pied by an OH ligand, and a “closed” Sn site (Scheme 7.5B), i.e., the perfect
tetrahedral lattice center [27]. It is proposed that the open site configuration
allows for a higher flexibility of the Sn center resulting in a higher Lewis
acidity and reactivity toward glucose activation [14,24,9092].
Computational studies on the mechanism of glucose to fructose isomeri-
zation by Sn-containing zeolites revealed the crucial role of secondary
hydrogen bonding interactions between the carbohydrate substrate and vari-
ous OH sites neighboring the catalytic Sn species. It has been demonstrated
independently by several research groups that a favorable reaction channel
for glucose to fructose isomerization can only be established when the cata-
lytic action of the LA Sn site during the rate-determining step is accompa-
nied by protonation reactions within the hydrogen-bonded moiety [23,94,96].
Specifically, the barrier for the rate-determining H-shift reaction catalyzed
by the Lewis acidic Sn site decreases dramatically when this step is accom-
panied by the simultaneous protonation of the aldehyde functional of the
adsorbed sugar group by a proton donor of the vicinal SiOH silanol or water
co-adsorbed to the LA Sn site (Fig. 7.8B) [94]. Such a cooperative action of
the LA center and the proton donor promoting the H-shift step in glucose to
fructose isomerization over Sn-BEA conceptually resembles the promoting
effect of the transient formation of binuclear Cr21
complexes in ionic liquids
(Fig. 7.8A) and the cooperative action of catalytic water molecules within
the reaction environment of xylose isomerase enzyme containing binuclear
Sn OH
O
O
O
Si
Si
Si
Sn O
O
O
O
Si
Si
Si
Si
(A) (B)
SCHEME 7.5 The representative lattice Sn sites in Sn-BEA zeolite: (A)a partially hydroxylated
SnOH open site and (B) and perfect tetrahedral closed Sn site [27].

LA sites [93,95]. We propose that such a synergistic mechanism involving a
cooperation between BA and LA functionalities within a single reaction
environment is a general phenomenon for homogeneous, heterogeneous, and
enzymatic catalyst systems for aldose isomerization.
The examples above clearly demonstrate the importance of multiple-site
environment for the catalytic reactivity of zeolites. Secondary interactions
can affect the properties of the main catalytic species and even alter the reac-
tion mechanisms. A realistic representation of the active site environment is
necessary to adequately analyze the respective mechanistic pathways and
therefore get an insight into the complexity of multifunctional catalysis.
7.4 REACTIVITY IN CONFINED SPACE
Besides the cooperation between the sites of different chemical nature such
as the BASLAS cooperativity discussed in Section 7.3.2, the presence of
multiple reactive sites of the same type can also affect the catalytic proper-
ties of zeolites. Such multiple-site reactivity is expected to be particularly
important for zeolites with a low Si/Al ratio in the framework resulting in a
high density of exchangeable cations in the micropores. The adequate
description of such systems requires the construction of realistic models that
explicitly take into account the details of the multifunctional reaction envir-
onments. The theory of cooperative catalysis in confined space is currently
in development. Thanks to the progress in IT technologies, we are now able
to model the chemical transformations in such systems with an appreciable
accuracy. The computational findings reported so far in this area indicate
unexpected and conceptually mechanistic similarities between catalysis by
zeolites, supramolecular systems, and enzymes.
The availability of well-defined active sites inside the molecular-sized
micropores motivated researchers to refer to these systems as so-called rigid
FIGURE 7.8 Cooperative effects play a key role in glucose to fructose isomerization. (A)
Transient self-organization of Cr centers is necessary to lower the activation energy of the rate-
determining H-shift reaction by ionic liquid-mediated LA catalysts [93]. (B) A favorable reaction
path in the case of the heterogeneous Lewis acidic Sn-beta catalyst is established only
when multiple-center interactions and cooperativity between different components of the
active site are taken into account [94]. (C) Both these effects are efficiently utilized by xylose
isomerase enzyme to promote the glucose to fructose isomerization at a low temperature with
high selectivity [95].

enzymes. Indeed, the catalytic properties of enzymes are often related to the
availability of well-defined active sites confined within a specific reaction
environment formed by the surrounding proteins. The term “rigid” is used
here because unlike the supramolecular ensembles around the active sites
created by the proteins, zeolite micropores cannot utilize structural dynamics
to promote chemical transformations. In enzymatic catalysis, it is proposed
that the selective conversions of substrates can be facilitated by such
dynamic effects as the coupling of enzyme vibrations to the vibrational
modes of the substrate or the motion of protein chains to affect the catalytic
properties of the reactive centers [9799]. This however cannot be directly
translated to zeolite catalysis as the lattice vibrations of the inorganic materi-
als are too minor to influence chemical conversion paths. A conceptual
bridge between the fields of enzymatic and zeolite catalysis can be created
by other concepts well established in biocatalysis such as molecular recogni-
tion and confinement-driven reactivity [100,101].
7.4.1 Molecular Recognition in Cation-Exchanged Zeolites
The multi-site concept in zeolite catalysis can directly be linked to the
molecular recognition phenomena found in enzymatic systems. Molecular
recognition of specific substrates selectively bound by an active center
occurs through the formation of multiple noncovalent interactions such as
electrostatic interactions, dipoledipole interactions, and H-bonding interac-
tions [102,103]. The effect of molecular recognition on the catalytic transfor-
mations can be viewed as the prealignment of substrates and the subsequent
stabilization of the transition states [103105]. Although the exact definition
in the enzymatic catalysis is somewhat stricter in a sense that molecular rec-
ognition features allow discriminating between different substrates, the gen-
eral concept of a catalytic reactivity induced through multiple interactions
can commonly be found in zeolite-based systems. These effects are particu-
larly important for low-silica alkali-exchanged zeolites.
Conventionally, the catalytic properties of alkali-exchanged low-silica
zeolites are attributed to the high basicity of their aluminum-rich frameworks
and the presence of hard Lewis acidic centers. The LA strength of the active
sites decreases in the order of Li1
. Na1
. K1
. Rb1
. Cs1
following
the increase of the ionic radius with a concomitant increase of the zeolite lat-
tice basicity [106108]. The substratezeolite interactions inside such low-
silica zeolites can therefore be dominated by those with the Lewis acidic
cations or the basic oxygen centers or even be the result of the concerted
action of these different sites.
The simultaneous interaction of the guest molecules with both the
exchangeable cations and the basic lattice sites can be illustrated by consid-
ering the adsorption of different molecules in alkali-exchanged faujasites.
Fig. 7.9 shows a structure of an adsorption complex formed between para-
xylene and an exchangeable cation at the SII site located at the six-

membered ring facing the supercage site in zeolite Y (faujasite). At low
loading (23 molecules per supercage), aromatic compounds adsorb prefer-
entially face-on onto the cationic sites at the SII crystallographic position
[109111]. It has been observed that the unit cell of the zeolite contracts
upon adsorption. This effect was attributed to both the changes in the
cationframework interaction due to adsorption and the formation of new
direct adsorbentframework interactions [109]. This proposal has been con-
firmed by the results of FT-Raman spectroscopy. Xylene adsorption was
accompanied by the red shift of the bands corresponding to the CC stretch-
ing vibrations due to the formation of the π complex with the alkali cations.
Furthermore, confinement of xylene in the zeolite cages leads to the forma-
tion of CH?O contacts with the basic lattice as is evidenced by the blue
shift of the methyl CH stretching vibrations [110]. Whereas toluene and
xylenes can only form π-complexes with individual cations or form stacked
aggregates within the supercage at higher loadings, benzene can adopt a
higher symmetry position in the 12-membered ring window of the supercage
[111,112]. This alignment is attributed to van der Waals interactions between
benzene CH groups and the framework oxygen centers. Similarly, the geome-
try of the adsorption complexes of ferrocene in NaY zeolite is predominantly
determined by the weak van der Waals interactions between the cyclopentadie-
nyl ligands and pore walls [113]. This illustrates the importance of weak
van der Waals interactions on the molecular recognition properties of zeolite
materials. In spite of being quite weak individually, the formation of multiple
interactions of that type often provides a driving force sufficiently strong
to induce specific conformations of the adsorbed molecules and therefore
determine the geometry of the adsorption complexes.
Adsorption of molecules in low-silica zeolites can involve interaction
with more than one exchangeable cation. Such a dual-site adsorption mode
requires an optimal match between the size of the adsorbent and the zeolite
FIGURE 7.9 Adsorbed para-xylene on the exchangeable cation located at the six-membered
ring via a cation-π interactions. Weak CH?O contacts between the protons of the aromatic
guest molecule and the basic framework oxygen atoms are shown by dashed lines [109].

cations at the adjacent sites. Such a dual-site adsorption has been reported
for the interaction of different small molecular probes including CO, CO2,
and CH4 with alkali-exchanged faujasites [108,114116]. Dual-site adsorp-
tion gives rise to specific shifts of characteristic absorption bands of the
adsorbed molecules in the infrared spectra. The extent of the frequency shift
is proposed to reflect the geometric properties of the adsorption complexes,
i.e., in essence a function of cation size, site occupancy, and the framework
basicity [114,117,118]. Secondary interactions with the basic framework sites
also affect the spectroscopic characteristics of the adsorbed molecules
[114,118]. The role of such interactions with the framework has been demon-
strated for the adsorption of CO2 on K1
-, Rb1
-, or Cs1
-exchanged zeolites
Y. Being a weak acid, CO2 can react with the basic lattice sites of faujasite
framework [108,115]. Molecular adsorption of carbon dioxide involves the
formation of both cationOCO and OframeworkCO2 interactions to yield
surface carbonate species [108]. Because the CO2 adsorption strength follows
the order expected for the zeolite basicity (Cs1
. Rb1
. K1
. Na1
. Li1
),
the interaction with the basic site has been proposed to be the dominant
factor in carbon dioxide adsorption on alkali-exchanged faujasties.
Nevertheless, the cooperative nature of the interactions with both LA and LB
sites resulting in the formation of surface carbonates should not be
neglected.
7.4.2 Confinement Effects and Entropy
Effects in Zeolite Catalysis
Another concept directly related to the above molecular recognition phenom-
enon is the confinement effect. When a species is placed into a narrow space,
many short-range interactions will be formed between the confined substrate
and the confining matrix. This may lead to either the chemical confinement,
i.e., preorganization of the substrate molecules so that their transformations
become favored, or physical confinement, i.e., nothing else but the increase
of the local concentration of the substrate. Both effects can strongly affect
the rate of the catalytic reaction and therefore were extensively discussed in
previous literature. Derouane and coworkers [119121] in their earlier works
highlighted the importance of the geometrical match between the zeolite
micropores and the guest molecules. It was proposed that the so-called float-
ing of molecules could explain the rapid diffusion of some molecules
through the zeolite pores. A perfect match of the van der Waals radii of the
pores and the guest molecules would result in a very tight fit of the mole-
cules inside the pores and give rise to an exact force cancelation so that a
facile diffusion could be achieved [120]. Apart from the rapid diffusion, an
optimal fit between the micropores and the guest molecules was used to
explain the dependency of the rate of n-pentane cracking reaction on the
structure of the zeolite catalyst [119].

Enzymes and zeolites are not the only examples of catalysts, whose proper-
ties are controlled by the geometry of the confinement space. An illustrative
example of the supramolecular assembly acting as a confining space is the
system developed by Fujita and coworkers [122,123]. The supramolecular
catalysts reported by them were able to catalyze DielsAlder cycloaddition
reactions with naphthalene derivatives, which are quite deactivated substrates.
These new catalysts allowed achieving very high reaction yields with a
remarkable regioselectivity. Their supramolecular architecture was used to
align the reactants with a geometry resembling an early transition state. After
the reaction, the geometry of the product did not match that of the supramolec-
ular host system anymore, which consequently expelled it from the cage lead-
ing to regeneration of the catalyst. Similar confinement-driven DielsAlder
cycloaddition catalysis has been recently discovered by us for faujasite-based
catalysts. These results will be discussed in more detail in Section 7.4.3.
Confinement in zeolite micropores results in substantial restrictions of the
degrees of freedom of the adsorbed molecules affecting the entropy of the
system. The resulting entropic effects may play a decisive role in the cata-
lytic properties of zeolite-based materials. Entropic effects were discussed in
detail in the previous chapter. The thermodynamics and kinetics of chemical
transformations are defined by the underlying free energy changes that
consist of both enthalpic and entropic contributions. Enthalpic contributions
to the ΔG (Gibbs free energy change) and ΔG#
(free energy of activation)
reflect the intrinsic chemistry of the system and include the changes in bond-
ing as well as various interactions such as the electrostatics, hydrogen
bonding, and dispersion. Entropic contributions essentially arise from the
changes in the degrees of freedom of the molecules related to their ability to
rotate, vibrate, or translate in space. To understand zeolite catalysis and the
effects of molecular recognition and confinement both the enthalpic and
entropic effects of catalytic transformations in zeolite micropores have to be
carefully accounted for. This is particularly important for theoretical studies
commonly employing static ab initio calculations, in which the information
about the reaction mechanism and its energetics is obtained from the analysis
of potential energy surfaces, while the entropic factors are usually neglected.
To properly account for the entropy, various degrees of freedom and confor-
mations of the adsorbed molecules have to be sampled extensively. Ab initio
molecular dynamic simulations [124] and particularly those augmented by
advanced transition-state sampling techniques [125,126] can be used to sam-
ple the free energy surfaces of catalytic reactions. However these methods
are commonly associated with very high computational costs and can
therefore not be routinely be applied to analyzing complex catalytic transfor-
mations in multifunctional systems.
The adsorption in zeolites is characterized by a favorable enthalpy
[124,127,128] that is counteracted by the entropy penalty due to the partial
loss of translational and rotational degrees of freedom [124,128]. This leads
to an effective trade-off between enthalpy and entropy resulting in a lower

effective adsorption free energy that in turn may have a strong influence on
the predicted reactivity trends [129]. Consider a hypothetical bimolecular
reaction of a Compound A with a Compound B taking place in a zeolite
micropore. The energy diagram for this chemical reaction is schematically
illustrated in Fig. 7.10. The red line indicates the potential energy profile
produced directly from DFT calculations and the black line corresponds
to the free energy profile that also accounts for the entropy effects at a
particular temperature. Let us assume that adsorption to exchangeable cations
inside the zeolite cage completely eliminates all three translational degrees
of freedom of the reactants, while the changes in all other entropic contribu-
tions are negligible. Upon strong adsorption (ΔEads 5 ΔHads, red line), the
molecular motions, in particular the translation of the adsorbed molecules
are severely restricted. This leads to a counteracting effect of the entropy
on the free energy of adsorption (ΔGads 5 ΔHads 2 TΔSads, black line)
effectively reducing the stabilizing effect of the favorable interaction with
the components of the zeolite matrix.
In spite of the decreased stabilization of the adsorbed reactants, the ham-
pered motion of the adsorbed species may be very beneficial for the catalytic
transformation. The reaction kinetics is defined by the Gibbs free energy of
activation ΔGact 5 ΔEact 2 TΔSact (here for simplicity we neglect finite tem-
perature and zero-point energy corrections to potential energy and assume
reaction enthalpy ΔH to be equal to the respective potential energy change
ΔE). Because the activation entropy of the reaction is influenced by the dif-
ference between the entropies of the initial and transition states, it can have a
very strong effect on the overall reaction rate. A highly ordered initial state
does not lose much entropy upon the formation of the highly ordered
transition-state complex. The term ΔΔStr,react in Fig. 7.10 vanishes and the
free energy barrier (ΔGact,1) is then equal to the activation barrier (ΔEact,1).
When only one of the reactants is strongly adsorbed and the other one is
moving in the cage relatively freely, the formation of an activated complex
FIGURE 7.10 Schematic reaction profiles of a hypothetical bimolecular reaction A 1 B yield-
ing C in a zeolite cage. The red line (gray in print versions) is the potential energy profile and
the black line shows a respective Gibbs free energy diagram.

in the transition state would additionally be associated with a substantial loss
of the respective degrees of freedom. In this case, the substantial ΔΔTStr,act
contribution to the free energy barrier (ΔGact,2) makes it effectively larger
compared to the free energy barrier in the situation of the tight confinement
of both reactants.
The effects of the enthalpyentropy trade-offs for catalytic properties of
zeolites have been illustrated with the example of propane cracking by acidic
zeolites [128]. The adsorption of propane in the narrow ferrierite pores
shows an enthalpy of 249 kJ/mol, while the adsorption to wider pores of
mordenite is somewhat weaker by 8 kJ/mol (241 kJ/mol). Accordingly, the
adsorption to more spacious channels in the latter case results in a smaller
entropy loss that was computed to be 285 and 2108 J/(mol K) respectively,
for propane adsorption to mordenite and ferrierite zeolites [128]. The pro-
moting effect of the confinement on the activation entropy of propane crack-
ing has been demonstrated by Hafner and coworkers [124]. They reported
that propane cracking within the 12- and 8-membered rings of mordenite
proceeds with entropies of activation (ΔSact) of 279 and 225 kJ/(mol K),
respectively. This reflects the importance of the high degree of preorganiza-
tion in the adsorbed initial state inside the narrow space of the mordenite
side pockets.
7.4.3 Molecular Recognition and Confinement-Driven
Reactivity of Alkali-Exchanged Faujasite
The importance of confinement-driven reactivity and molecular recognition
effects for catalytic properties of alkali-exchanged low-silica zeolites has
been shown earlier by us on several examples. In our works on the mecha-
nism of N2O4 disproportionation over alkali-exchanged faujasites, we con-
vincingly demonstrated that the catalytic performance of the zeolite
materials in these processes does not depend on the individual properties of
the reactive sites such as basicity or acidity of the zeolites. It is rather deter-
mined by the possibility to form an optimal coordination environment for the
anionic product of the disproportionation reaction, i.e., in turn controlled by
the size and mobility of the alkali cations inside the faujasite structure
[116,130,131].
More recently, we discovered a similar phenomenon in an important
catalytic process that may potentially give rise to new technologies for the
production of aromatic products from renewable cellulosic biomass feed-
stock. We have carried out a comprehensive periodic DFT study on the
mechanism of the DielsAlder cycloaddition/dehydration reaction between
the biomass-derived furanic compounds and ethylene to form substituted aro-
matic molecules. Let us consider here for simplicity the reaction of a model
reactant—dimethylfuran. Computational studies were carried out by using
two models of the alkali-exchanged zeolites, namely the high- (Si/Al 5 47)

and low- (Si/Al 5 2.4) silica alkali (Li1
, Na1
, K1
, Rb1
, Cs1
)-exchanged
faujasites. The former structure contained a single reactive center per zeolite
unit cell, whereas the structures and chemical composition of the latter fam-
ily of zeolite models corresponded perfectly to the zeolite Y utilized in the
experimental studies. The comparative analysis of the results obtained with
these two distinct sets of models allowed to directly asses the role of multi-
site interactions for the catalytic reaction (Fig. 7.11). Our results clearly dem-
onstrated that the composition of the zeolite models has a dramatic impact
on the predicted reactivity of the zeolite system. The more realistic low-
silica models allowed establishing more complex interaction patterns
between the framework and guest molecules as well as the cations and guest
molecules, which were found to play a crucial role in the mechanism of the
catalytic reaction. The concerted action of the numerous accessible cations in
the micropores of the low-silica zeolite effectively aligned the reactants in a
preactivated state resulting in an effective decrease of the activation barrier
for the reaction. The confinement of the reactants in a specific configuration
inside the faujasite micropore has been shown to be particularly important
for the first DielsAlder cycloaddition step. Because the reactivity in this
case was not controlled by single-site interactions, the trend in the activation
barrier for the coupling reaction did not follow the conventional trend of the
increasing Lewis acidity of the exchangeable cation. Besides the first cyclo-
addition step, subsequent elementary reactions of the catalytic cycle such as
the isomerization and dehydration steps were also severely affected by the
presence of multiple reaction sites within the zeolite cage. They showed a
much lower activation energy compared to that predicted with the single-site
model. These results demonstrate the importance of a realistic description of
the chemical environment of the zeolite catalysts.
7.5 CONCLUSIONS
In this chapter, we presented an overview of recent important findings
related to the theory of LA catalysis by zeolites and illustrated the power
and capabilities of modern computational approaches for addressing the
structural complexity of heterogeneous porous catalysts. Thanks to the
advances in computational hardware and software that made detailed compu-
tational studies on realistic and highly complex chemical systems possible,
we were able to achieve great progress in understanding of chemical reactiv-
ity of zeolite catalysts. Not only are such computational studies used to ratio-
nalize experimental observations, but increasingly they become the key
ingredient of computations-aided rational catalyst design strategies. The role
of computations in elucidating mechanistic complexities of chemical trans-
formations inside the zeolite pores is highlighted by representative examples
relevant to the topic of catalysis for sustainability.

FIGURE 7.11 Reaction profiles of low-silica (Si/Al 5 2.4) Na-exchanged faujasite vs that of high-silica (Si/Al 5 47) Na-exchanged faujasite (A) and those of
high-silica (Si/Al 5 2.4) Na-exchanged faujasite vs high-silica (Si/Al 5 2.4) Rb-exchanged faujasite (B).

The examples discussed in this chapter highlight the crucial role of
model accuracy for the adequate analysis of chemical process inside the
zeolite micropores. A correct representation of the confinement space,
chemical composition, and structural characteristics of the intrazeolite
active species is necessary to obtain mechanistic insights suitable for ratio-
nalization of the experimental data. The apparent agreement between
the numerical results of computational studies employing oversimplified
models and highly complex experiments, which are often encountered in
the literature, may not be considered as a validation to the computational
model, which may effectively produce a correct answer but for a wrong
reason. The development of more realistic models necessitates a thorough
analysis of the fundamental factors determining the speciation of the active
components inside the zeolite catalysts. Here, the combination of electronic
structure calculations with statistic thermodynamic analysis within the
ab initio thermodynamic analysis approaches is particularly useful as it
allows to directly analyze stabilities of different potential active complexes
as a function of the conditions of catalyst preparation. This approach is
useful in identifying the nature of intrazeolite active species and even
in guiding the design and synthesis of desired catalyst with specific
active species.
Finally, the utilization of complex realistic zeolite models opens an avenue
toward novel reactivity concepts. In this chapter we have put an emphasis
on the most recent concepts of active site cooperativity and confinement-
driven reactivity that allow establishing a conceptual bridge between
the fields of zeolite and enzymatic catalysis. With the example of zeolite-
catalyzed conversion of biomass-derived furanics to aromatic compounds
by faujasite-type zeolites, we demonstrated that the catalytic properties of
zeolites often cannot be rationalized by using the conventional single-site
approximation. The complexity of the reactive environment inside the
zeolite cages gives rise to phenomena such as molecular recognition and
confinement significantly affecting the reaction profiles both quantitatively
and qualitatively.
ACKNOWLEDGMENTS
We thank the support from Netherlands Center for Multiscale Catalytic Energy
Conversion (MCEC), an NWO Gravitation programme funded by the Ministry of
Education, Culture, and Science of the government of the Netherlands. E.A.P. gratefully
acknowledges the the support from the Ministry of Education and Science of the Russian
Federation (Project 11.1706.2017/4.6). C.L. thanks China Scholarship Council (CSC) for
financial support. NWO is acknowledged for providing access to the supercomputer
facilities.

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Lewis Acid Catalysis by Zeolites.pdf