Catalytic consequences of micropore topology, mesoporosity, and acidity on the hydrolysis of sucrose over zeolite catalysts

Article abstract of DOI:10.1039/c4cy00360h

The effects of zeolite micropore topology, mesoporosity, and acidity on the hydrolysis of polysaccharides were probed in reactions of sucrose over a variety of zeolite catalysts (FER, MFI, MOR, MWW, BEA, FAU, pillared MFI (PMFI), and pillared MWW (PMWW)). The measured rate of sucrose hydrolysis over microporous zeolites varied by a factor of ～100 following a trend of FER < MOR ～ MFI < BEA ～ MWW < FAU, indicating that the hydrolysis of sucrose increases with increasing zeolite micropore sizes. The presence of mesoporosity in PMFI and PMWW zeolites enhanced the rates of sucrose reactions by a factor of ～2 in comparison with the microporous MWW and MFI zeolites, which may result from the enhanced acid site accessibility and mitigated diffusion constraints. The examination on the effects of zeolite acidity on the hydrolysis of sucrose by employing MFI, PMFI, BEA, MWW, and FAU zeolites with a range of Si/Al ratios showed that a Si/Al ratio of ～70-150 provides a maximal rate constant per acid site in the catalysts. The measured activation energies of the catalytic reactions in all zeolites were similar. The measured entropies, however, increased abruptly with increasing micropore sizes of zeolites and slightly with increasing mesoporosity in the zeolites. The present study suggests that the hydrolysis of sucrose is driven primarily by the reaction entropies that are dominated consecutively by the micropore topology, acidity, and mesoporosity of the zeolite catalysts. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cy00360h

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Catalytic consequences of micropore topology,
mesoporosity, and acidity on the hydrolysis of
sucrose over zeolite catalysts†
Cite this: DOI: 10.1039/c4cy00360h
Yao He,ab Thomas C. Hoff,a Laleh Emdadi,a Yiqing Wu,a Judicael Bouraimaa
a
*
and Dongxia Liu
The effects of zeolite micropore topology, mesoporosity, and acidity on the hydrolysis of
polysaccharides were probed in reactions of sucrose over a variety of zeolite catalysts (FER, MFI, MOR,
MWW, BEA, FAU, pillared MFI (PMFI), and pillared MWW (PMWW)). The measured rate of sucrose
hydrolysis over microporous zeolites varied by a factor of ~100 following a trend of FER < MOR ~ MFI <
BEA ~ MWW < FAU, indicating that the hydrolysis of sucrose increases with increasing zeolite micropore
sizes. The presence of mesoporosity in PMFI and PMWW zeolites enhanced the rates of sucrose
reactions by a factor of ~2 in comparison with the microporous MWW and MFI zeolites, which may result
from the enhanced acid site accessibility and mitigated diffusion constraints. The examination on the
effects of zeolite acidity on the hydrolysis of sucrose by employing MFI, PMFI, BEA, MWW, and FAU
zeolites with a range of Si/Al ratios showed that a Si/Al ratio of ~70–150 provides a maximal rate constant
per acid site in the catalysts. The measured activation energies of the catalytic reactions in all zeolites
were similar. The measured entropies, however, increased abruptly with increasing micropore sizes of
zeolites and slightly with increasing mesoporosity in the zeolites. The present study suggests that the
hydrolysis of sucrose is driven primarily by the reaction entropies that are dominated consecutively by
the micropore topology, acidity, and mesoporosity of the zeolite catalysts.
Received 22nd March 2014,
Accepted 2nd May 2014
DOI: 10.1039/c4cy00360h
www.rsc.org/catalysis
fructose sugars, representing the starting point for the syn-
thesis of chemicals or fuels by acid-catalyzed reactions from
1. Introduction
The dwindling crude oil reserves and environmental issues
drive the research for alternative fuel and chemical sources
that are currently derived from petroleum.1,2 Biomass attracts
considerable attention as a renewable feedstock due to its
huge reservoir of renewable carbon and “CO2 moderate”
impact on the environment.3,4 Sucrose biomass constitutes
the main carbohydrate reserve in plant biomass.5 The hydro-
lysis of sucrose allows its conversion into glucose and
polysaccharides.3,6
Enzymes,7–9 mineral acids,10,11 and solid acids4,12–17 have
been employed as catalysts for the hydrolysis of sucrose. The
sucrose conversion over enzyme catalysts is generally efficient,
several times faster than with mineral or solid acids. The
enzyme stability and narrow operating temperature range,
however, are barriers in the enzymatic hydrolysis of sucrose.18,19
The homogeneous mineral acids catalyze sucrose conversion
in a wide temperature range, while the considerable energy
consumption in the product separation, formation of large
amount of acid waste, and corrosion problems are the main
drawbacks. The development of efficient, easily separable,
and environmentally friendly solid acid catalysts is therefore
considered for the sucrose hydrolysis reaction. Acid ion-
exchange resins,12,20–22 polyheteroacids,15 layered transition-
metal oxides,23 organic–inorganic hybrid sulfonic mesoporous
silicas or carbons,12,13,24 proton-form zeolites,22,25–28 etc. have
been used as solid acids in the sucrose hydrolysis reactions.
The heterogeneous solid acid catalyst systems, however,
show concerns on their microenvironment that lead to the
diffusion constraint on reactants/products and restricted
a Department of Chemical and Biomolecular Engineering, University of Maryland,
College Park, MD, 20742, USA. E-mail: liud@umd.edu; Fax: +1 301 405 0523;
Tel: +1 301 405 3522
b Institute of Clean Energy Utilization, School of Electric Power, South China
University of Technology, Guangzhou, 510640, China
† Electronic supplementary information (ESI) available: Rate constants of
hydrolysis of sucrose in zeolites with variable micropore topology,
mesoporosity, and acidity (Si/Al ratio); plots for sucrose conversion as a
function of reaction time and rate constant determination for sucrose
hydrolysis reactions over each catalyst; and plots for determination of measured
activation energy and entropy in sucrose hydrolysis reactions over zeolite
catalysts. See DOI: 10.1039/c4cy00360h
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accessibility to acid sites, considering that the dimensions
of sucrose molecules are relatively big (kinetic diameter of
~0.88 nm).28–30
Zeolites are crystalline microporous aluminosilicates, with
channel and pocket dimensions typically less than 1 nm, which
enable shape-selective catalysis and thus are good candidates
for selective conversions of biomass-derived products.31 The
zeolite topology can promote reaction rates and selectivity
based on spatial constraints and attractive or repulsive interac-
tions between adsorbed molecules and pore walls and by alter-
in sucrose hydrolysis reactions in an aqueous solvent have
not been studied. We aim to reveal the catalytic performance
of PMFI and PMWW zeolites in these reactions and to
directly compare their catalytic performance to that of con-
ventional microporous zeolite catalysts. The present study
will lead to a systematic understanding of the effects of zeo-
lite microenvironment (topology, mesoporosity, and acidity)
on conversion of sucrose and other related biomass mole-
cules under environmentally friendly conditions.
ing the relative stability of surface-bound intermediates inside 2. Experimental
micropores. Meso-/microporous zeolites contain both meso-
2.1 Preparation of zeolite catalysts
and micropores, which allow the facile pore access and fast
transport of bulky molecules and thereby enable beneficial
effects on the activity, selectivity, and/or stability in a wide
range of catalyzed reactions.32–35 Studies of sucrose hydrolysis
have been conducted on faujasite (FAU), beta polymorph A
(BEA), mordenite (MOR), and mordenite framework inverted
(MFI) structure type zeolites.26,28,30,36,37 Moreau et al.26
reported that FAU is the most efficient zeolite catalyst in the
reactions. Studies on the dealuminated FAU catalysts by
Buttersack et al.28 showed that two or less protons per unit cell
in FAU have the maximum activity per acid site. A systematic
study on the zeolite microenvironment (micropore topology,
mesoporosity, and acidity) effects of the conventional micro-
porous zeolites and the emerging meso-/microporous zeolites
on the sucrose hydrolysis reaction has not been reported.
In the present study, we assessed the catalytic consequences
of micropore topology, mesoporosity, and acidity of a range of
microporous and meso-/microporous zeolite catalysts on the
hydrolysis of sucrose. The commercially available zeolite
catalysts, ferrierite (FER), MOR, BEA, MFI, and FAU with a
range of Si/Al ratios, were employed to examine the effects of
zeolite micropore topology and acidity on the sucrose hydrolysis
reactions. The effect of mesoporosity on the sucrose hydrolysis
reactions was investigated by using the meso-/microporous
pillared MWW (PMWW) and pillared MFI (PMFI) zeolite cata-
lysts. PMWW, derived from a layered precursor, MWW (P), is
the first pillared zeolite material with microporous layers and
mesoporous interlayer spaces.38–41 The synthesis of PMWW
was done by expanding layers of MWW (P) by using a surfac-
tant and then intercalating by silica species which converted
into inorganic pillars upon condensation and hold the layers
apart creating interlayer mesopores. PMFI is a material
reported in recent years, which was created by pillaring the
multilamellar MFI nanosheets by silica-based species using a
similar procedure to that of PMWW.42,43 Our previous study
showed that the intrinsic catalytic behavior of Brønsted acid
sites in the meso-/microporous PMWW and PMFI zeolites
is similar to their microporous analogues in the ethanol
dehydration and monomolecular conversion of propane and
isobutane as probe reactions,44 while they showed much effi-
cient catalysis in diffusion constrained catalytic reactions
when the self-etherification of benzyl alcohol and alkylation
of mesitylene were used as the probe reactions.45 The cata-
lytic consequences of the mesoporosity of PMWW and PMFI
The conventional microporous FER, MFI, MOR, BEA, and FAU
with different acidity (Si/Al ratio) were purchased from Zeolyst.
MWW and PMWW were derived from the same precursor,
MWW (P). The hydrothermal synthesis of MWW (P) was car-
ried out by using the method described by Corma et al.39,40
One portion of the crystalline product MWW (P) was dried and
calcined to produce MWW. The other portion of MWW (P) was
swollen according to the method developed by Maheshwari
et al.,41 followed by pillaring of the swollen materials using
the procedure reported by Barth et al.38 The resulting solid
was treated using the same conditions as those for MWW to
produce PMWW. A multilamellar MFI was synthesized using
the method reported by Ryoo and co-workers,43 through a
coherent assembly of the zeolite layer and the structure
directing agent, a diquaternary ammonium surfactant with a
relatively long hydrocarbon chain. Pillaring of multilamellar
MFI was done as reported by Na et al.42 to produce PMFI, using
a similar pillaring procedure to that of swollen MWW (P). The
as-synthesized MWW and MFI zeolites were ion-exchanged four
times using 1 mol L−1 aqueous NH4NO3 (weight ratio of zeolite
to NH4NO3 solution = 1 : 10) at 353 K for 12 h and subsequently
collected by vacuum filtration, washed with deionized (DI)
water three times, and dried at 343 K overnight. No ion-
exchange process was applied to the commercial zeolites since
they were purchased in the NH4+-form. All zeolite samples in
their NH4 -form were treated in dry air (100 mL min−1, ultrapure,
+
Airgas) by increasing the temperature from ambient temperature
to 823 K at 1.45 K min−1 and holding for 4 h to thermally
decompose NH4+ to NH3 and H+. To differentiate the same type
of zeolite with different Si/Al ratios, each catalyst is named by
its structure type and Si/Al ratio in the remainder of this paper.
2.2 Textural and acidity property investigation
A scanning electron microscope (SEM) was employed for direct
visualization of crystal morphologies of meso-/microporous
and microporous zeolite catalysts. SEM images were collected
on a JEOL 6500 SEM without crushing or a metal coating of
the samples. Powder X-ray diffraction (XRD) patterns were
collected on a Bruker AXS D5005 diffractometer using Cu–Kα
radiation. Nitrogen (N2) adsorption–desorption measure-
ments were carried out at 77 K on an Autosorb-iQ analyzer
(Quantachrome Instruments). Prior to the measurement, sam-
ples were evacuated overnight at 573 K and 1 mmHg. Si and Al
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contents were determined by inductively coupled plasma opti-
cal emission spectroscopy (ICP-OES, Galbraith Laboratories).
(MR), respectively.46,47 Table 1 summarizes the types of zeolites
and graphical representation of the pore systems of each zeo-
lite catalyst. FER zeolite contains one-dimensional channels of
10-MR (0.42 × 0.54 nm) and one-dimensional channels of 8-MR
(0.35 × 0.48 nm) that are perpendicularly intersected. MFI zeo-
lite consists of two interconnected 10-MR channel systems: one
is straight running along the b-axis direction (0.53 × 0.56 nm)
and the other is zigzag running parallel to the a-axis direction
(0.51 × 0.55 nm). The PMFI contains mesopores created by the
inorganic pillar species sitting between MFI layers, parallel to
the zigzag channels and perpendicular to the straight channels
within the layers. MWW structure contains two independent
pore systems. One system is defined by sinusoidal 10-MR chan-
nels with dimensions of 0.41 × 0.51 nm and the other system
consists of supercages delimited by 12-MR channels with
dimensions of 0.71 × 0.71 × 1.81 nm. The consecutive super-
cages are connected through slightly distorted elliptical 10-MR
windows (0.40 × 0.55 nm). The PMWW was created by pillaring
the MWW layers by SiO2, containing the 10-MR sinusoidal
channels and hourglass shaped pores (half of the supercages
in MWW) within the intact layers and mesopores between the
layers. Table 1 also shows that MOR zeolite has the 8-MR
pocket (0.34 × 0.48 nm) and 12-MR (0.7 × 0.65 nm) channel
systems, with the 8-MR pockets sitting in the walls of 12-MR
channels. A BEA zeolite structure consists of 12-MR straight
channels of a free aperture of 0.66 × 0.67 nm viewed along the
a-axis and 12-MR zigzag channels of 0.56 × 0.56 nm viewed along
the c-axis. FAU zeolite contains 12-MR pore mouths (0.74 ×
0.74 nm) that are 3-dimensionally connected with internal
supercages (~1.3 nm). The topological analysis above indi-
cates that FER and MFI are medium-pore zeolites, MOR,
MWW, BEA, and FAU are large-pore zeolites, and PMWW and
PMFI are mesoporous zeolites, respectively.
2.3 Catalytic hydrolysis of sucrose
The liquid phase catalytic hydrolysis of sucrose (Sigma-
Aldrich, 99.5% purity) in DI water was carried out in a three-
neck round-bottom flask (100 mL) equipped with a reflux
condenser and heated in a temperature controlled oil bath
under atmospheric pressure and magnetic stirring (1′′ stirring
bar, 500 rpm stirring speed) conditions. The reaction scheme
of sucrose hydrolysis is illustrated in Fig. 1. In a typical exper-
iment, 20 mL of DI water was added to the desired amount
(typically 0.2 g) of zeolite catalyst. The as-obtained mixture
was maintained for 0.5 h under the required reaction temper-
ature and stirring conditions and then 1.0 g of sucrose was
added. This moment of sucrose addition was considered as
the initial reaction time. Liquid samples were withdrawn at
regular intervals and analyzed by a high performance liquid
chromatograph (Agilent 1100 HPLC) equipped with a Bio-Rad
Aminex HPX-87H column connected to an autosampler and a
refractive index detector to calibrate and separate the reac-
tants and products. During the measurement, the column
was kept at 333 K with 0.005 mol L−1 sulfuric acid at a flow
rate of 0.5 mL min−1 as the mobile phase. Under these analy-
sis conditions, the hydrolysis of sucrose in a sulfuric acid
eluent was less than 0.2%, showing that the HPLC analysis
method did not influence the reaction results. The influence
of external mass transfer limitations on the reaction rates
was ruled out by running the reactions at a high enough stir-
ring speed (500 rpm), showing that a further increase in the
stirring speed did not enhance the reaction rate anymore. In
the examined temperature interval, absence of products from
consecutive reactions of glucose was confirmed by running a
control experiment using glucose as the reactant. Conversion
of fructose by the following series reactions was observed by
HPLC analysis. Zeolite colorization was not observed in the
studied reaction conditions.
3.2 Morphology of zeolite catalysts
Fig. 2 shows the SEM images of zeolite catalysts used in the
sucrose hydrolysis reactions. FER (FER-28) contains flake-like
particles that form large aggregates, as shown in Fig. 2(a).
The MFI (MFI-12 and MFI-40), MOR (MOR-45), BEA (BEA-12,
BEA-19, and BEA-100), and FAU (FAU-15 and FAU-40) zeolites
with no specific particle sizes due to their broad range of
sizes are shown in Fig. 2(b)–(c), (e), (h)–(j), (k)–(l), respectively.
3. Results and discussion
3.1 Topological properties of zeolite catalysts
Small, medium, and large pore zeolites are classified according
to their pore openings that are 8-, 10-, and 12-membered rings
Fig. 1 Scheme showing the hydrolysis of sucrose to glucose and fructose in an aqueous solution.
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Table 1 Topology and graphical representation of pore systems of the medium-pore, large-pore, and mesoporous zeolite catalysts used in the
sucrose hydrolysis reactions
Pore structure
Graphical representation
Zeolite
Si/Al ratio
28
Pore shape
Pore size (nm)
of pore topology
Medium-pore
FER
MFI
8 MR
10 MR
0.35 × 0.48
0.42 × 0.54
12
40
70
10 MR
10 MR
0.51 × 0.55
0.53 × 0.56
Large-pore
MOR
45
8 MR
12 MR
0.34 × 0.48
0.65 × 0.7
MWW
20
30
10 MR
10 MR
Supercage
Side pocket
0.41 × 0.55
0.41 × 0.51
0.71 × 1.82
0.71 da × 0.9 hb
BEA
FAU
12
19
100
12 MR
12 MR
0.56 × 0.56
0.66 × 0.67
15
40
12 MR
12 MR
0.74 × 0.74
0.74 × 0.74
Mesopore
PMFI
70
150
200
10 MR
0.51 × 0.55
0.53 × 0.56
2.8
10 MR
Mesoporec
PMWW
30
10 MR
0.41 × 0.51
0.71 × 1.82
0.71 da × 0.9 hb
1.8
Supercage
Side pocket
Mesoporec
a
d represents “diameter”. b h represents “height”. c The mesopore size was determined from the N2 adsorption branch by using the BJH model.
The synthesized MFI (MFI-70) zeolite has a uniform particle size
of ~200 nm (Fig. 2(d)). The PMFI (PMFI-70, PMFI-150, and
PMFI-200) zeolites in Fig. 2(m)–(o) are composed of aggregated
platelet-like particles, similar to the PMFI reported by Na et al.42
The synthesized MWW (MWW-20 and MWW-30) zeolites
crystallize as thin rounded flakes, which are 500–1000 nm in
diameter and 50–100 nm in thickness, as shown in Fig. 2(f)–(g).
PMWW (PWW-30) resembles the disc-like crystal morphology of
MWW, as evidenced by comparing the morphology of PMWW
in Fig. 2(p) to that of MWW. The SEM images reveal that no
obvious trend on zeolite geometrical properties is observable in
all the investigated catalysts. The trends of catalytic behaviors of
the zeolite catalysts, discussed below, are predominately domi-
nated by the zeolite microenvironments of the catalysts. The
XRD patterns of the synthesized MWW, MFI, PMWW, and PMFI
confirm that the samples are highly crystalline and the data
have been reported in our previous publication.44
3.3 N2 adsorption–desorption isotherm analysis
N2 adsorption–desorption measurements were used to reveal
the porosity features of the medium-pore, large-pore, and
mesoporous zeolite catalysts. Table 2 summarizes the micro-
pore volume, cumulative pore volume, and Brunauer–
Emmett–Teller (BET) surface area of each catalyst analyzed
from N2 adsorption–desorption isotherms. Among all the
investigated microporous catalysts, FAU-type zeolites have the
highest surface area, cumulative pore volume, and micropore
volume, followed by the BEA-type zeolites. MWW-, MOR-, and
MFI-type zeolites have similar surface areas and micropore
and cumulative pore volumes, all of which are smaller than
those of FAU and BEA zeolites. FER has the lowest surface
area and cumulative pore volume among all the investigated
microporous zeolites. The meso-/microporous PMFI and
PMWW zeolites have similar surface areas and cumulative
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Fig. 2 SEM images of (a) FER-28, (b) MFI-12, (c) MFI-40, (d) MFI-70, (e) MOR-45, (f) MWW-20, (g) MWW-30, (h) BEA-12, (i) BEA-19, (j) BEA-100,
(k) FAU-15, (l) FAU-40, (m) PMFI-70, (n) PMFI-150, (o) PMFI-200, and (p) PMWW-30.
pore volumes that are larger than those of microporous BEA
but smaller than those of FAU zeolites. The acidity of the zeo-
lite catalysts does not influence their porosity properties, as
shown by the similar surface areas and pore volumes across
the same type of zeolites with different Si/Al ratios. The sur-
face areas and porosities of these zeolites are consistent with
their topological properties discussed in section 3.1.
where CC is the glucose concentration in solution (mol L−1),
t is the reaction time (s), khydro is the specific rate constant
(per Brønsted acid site, [mol H+]−1 s−1), MB (mol H+) is the
total moles of Brønsted acid sites present in the batch reac-
tor, and CA is the sucrose concentration in solution (mol L−1).
The number of acid sites of each catalyst is shown in Table 2.
By integrating eqn (1), the rate equation becomes
0
3.4 Sucrose hydrolysis reaction




CA
CA
ln
 khydroMBt
(2)


In water solvent, sucrose (A) was consumed simultaneously
via hydrolysis (A + H2O → B + C) reactions to form fructose
(B) and glucose (C). Because an excess amount of water
was used, the hydrolysis reaction can be approximated as
pseudo-first order in the limiting reactant A, sucrose. The
rate equation is
where C0A is the initial sucrose concentration in solution
(mol L−1).
The profiles of sucrose conversion as a function of reac-
tion time and ln(C0A/CA) as a function of reaction time in the
hydrolysis of sucrose for different temperatures over the
investigated catalysts with variable micropore topology, meso-
porosity, and acidity are reported in Fig. S1.† The linear
dCC
(1)
 khydroMBCA
dt
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Table 2 Acidity and porosity characteristics of the medium-pore, large-pore, and mesoporous zeolites used in sucrose hydrolysis reactions
Number of Brønsted acid sitesb
Cumulative pore volc
Micropore vold
BET surface areae
Zeolite
Si/Al ratioa
(mmol g−1
)
(cc g−1
)
(cc g−1
)
(m2 g−1
)
Medium-pore
FER
MFI
28
12
40
70
45
20
30
12
19
100
15
40
70
150
200
30
0.575
1.282
0.406
0.235
0.362
0.794
0.538
1.282
0.833
0.165
1.042
0.407
0.235
0.110
0.083
0.538
0.144
0.168
0.199
0.173
0.214
0.196
0.210
0.254
0.260
0.267
0.350
0.359
0.349
0.317
0.307
0.359
0.103
0.132
0.137
0.136
0.162
0.143
0.120
0.152
0.160
0.169
0.198
0.201
0.118
0.121
0.125
0.131
368
410
466
427
552
384
442
611
642
662
871
877
750
674
642
771
Large-pore
MOR
MWW
BEA
FAU
Mesopore
PMFI
PMWW
a
b
c
Determined from elemental analysis (ICP-OES, Galbraith Laboratories). Calculated based on Si/Al ratio in each sample. Cumulative pore
d
e
volume determined using the Saito–Foley method. Micropore volume determined by the t-plot method. Surface area calculated from the
multi-point BET model.
trends in ln(C0A/CA) as a function of reaction time are satisfac-
tory in all the cases, confirming that the sucrose hydrolysis is
the pseudo-first-order reaction. From the observed rate con-
stants (khydroMB), calculated from the slope of the lines in
Fig. S1,† khydro can be obtained for each catalyst at a desig-
nated reaction temperature. The as-obtained khydro values
have been listed in Table S1 with their confidence intervals.†
The temperature dependence of the rate constants of the
sucrose hydrolysis over the zeolite catalysts was evaluated by
the Arrhenius plot (the natural logarithm of regressed rate
constants versus the inverse temperature) approach (refer to
Fig. S2 in the ESI†) to assess the kinetic parameters of the
reaction. The slope of the Arrhenius plot was used to cal-
Fig. 3 Rate constant of sucrose hydrolysis over zeolite catalysts with a
similar Si/Al ratio and different micropore topology (T = 352 K).
culate the measured reaction activation energies (ΔEmeas
,
kJ mol−1). The Eyring equation (eqn (3)) was used to calculate
the measured reaction entropies (ΔSmeas, J mol−1 K−1),
zeolite micropore sizes from 8-MR × 10-MR channels in FER
to 10-MR channels × 12-MR supercages in MWW. The
increase in reaction rate with increasing micropore sizes of
zeolites is also observed for the hydrolysis reaction over zeo-
lite MFI, MOR, and FAU with a Si/Al ratio of ~40, zeolite MFI
and BEA with a Si/Al ratio of ~12, and BEA, MWW, and FAU
with a Si/Al ratio of ~20, respectively. These results clearly
indicate that the hydrolysis of sucrose preferentially occurs in
large-pore zeolites. In contrast, spatial constraints imposed
by small pores of zeolites may mainly allow surface catalysis
to occur.
The difference in activity of the zeolite catalysts can be
understood from the molecular size of sucrose and the pore
size of the zeolite catalysts. In the aqueous solution, the
sucrose molecule is characterized by an approximately spheri-
cal shape with a relevant diameter of ~0.88 nm.29 Although
the largest pore size of the investigated catalysts is 0.74 nm
in the FAU catalyst, the sucrose molecule can enter the zeo-
lite pore by adopting a more elliptic form. The larger pore
k T
1






B
(3)
ln k

 ln
 

H
TSmeas
meas


hydro 





h
RT
where kB is the Boltzmann constant (m2 kg s−2 K−1), T is
the absolute temperature (K), h is the Planck's constant
(m2 kg s−1), R is the ideal gas constant (J mol−1 K−1), and
ΔHmeas is the measured reaction enthalpy (kJ mol−1), respec-
tively. From eqn (3), the measured reaction entropy equals
R[intercept in Arrhenius plot − ln(kBT/h)]. Below, the effect of
zeolite micropore topology, acidity, and mesoporosity on the
hydrolysis of sucrose reactions and the corresponding kinetic
parameters (measured activation energies and entropies) are
discussed in sequence.
3.4.1 Zeolite micropore topology effect. Fig. 3 shows the
rate constants (khydro) of sucrose hydrolysis at 352 K over
zeolites with similar Si/Al ratio but different micropore
topology. The sucrose hydrolysis rate on FER-28 and MWW-30
(with a similar Si/Al ratio of ~30) increases with increasing
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sizes of the catalyst, the easier it is for the sucrose molecules
to enter the zeolite micropore. As a result, the labile protons
on the active sites of the catalysts have more mobility to
reach the oxygen of the acetalic bond in sucrose and to pro-
mote the hydrolysis reaction. The FAU-type zeolite has the
highest activity in the investigated catalysts, followed by BEA-
and MWW-type zeolites, which are consistent with their
topology features and porosity properties of the catalysts
shown in Tables 1 and 2, respectively. MOR is ascribed to the
large-pore zeolite category. The pore channels of MOR,
however, run along a one-dimensional direction. MFI belongs
to the medium-pore zeolite category, while the 10-MR channels
run along 3-dimensional directions that may facilitate the
transport of sucrose molecules in the pore systems compared
to MOR-type catalysts. As a result, the MFI has comparable
reaction activity in sucrose hydrolysis to that of the large-pore
MOR zeolite. FER consists of 8-MR × 10-MR pore channels,
the smallest pore sizes among the investigated catalysts.
Therefore, the lowest reaction rate of sucrose hydrolysis is
observed for the FER-type zeolite. The present study system-
atically examined the effect of zeolite micropore topology on
the reaction rate of sucrose hydrolysis, showing a trend of
FER < MOR ~ MFI < BEA ~ MWW < FAU.
3.4.2 Zeolite acidity effect. Fig. 4 shows the rate constants
of the hydrolysis of sucrose over MFI, BEA, MWW, and FAU
zeolites with variable Si/Al ratios. The rate constant per acid
site generally increases sharply with increasing Si/Al ratio
to ~50 in each type of catalyst and then reaches a plateau
when the Si/Al ratio increases further to ~100, as shown in
BEA-type zeolite catalysts. These data are consistent with that
of hydrolysis of sucrose on FAU zeolite catalysts reported by
Buttersack et al.28 It has been shown that the catalytic activity
of FAU zeolite increased strongly with increasing Si/Al ratio
from 27 to 55 and then slightly with Si/Al ratio from 55 to
110. The present study indicates that this catalytic behavior
is generalizable to MFI-, BEA-, and MWW-type zeolite cata-
lysts. Fig. 4 also indicates that the zeolite activity trend of
MFI < BEA ~ MWW < FAU in sucrose hydrolysis reactions is
independent of their Si/Al ratio. The comparison of the
zeolite micropore topology and acidity effects on the sucrose
hydrolysis reactions shows that zeolite topological properties
play a more important role in determining the zeolite cata-
lytic activity.
The changes in the catalytic activity as a function of the
zeolite acidity (Si/Al ratios) can be understood from the inter-
actions of sucrose and the active sites in the zeolite, which is
related to the local concentration of sucrose and acid sites in
the zeolite catalysts. The variation on the Si/Al ratios changes
the hydrophilicity–hydrophobicity balances of the zeolite
catalysts.48
A higher Si/Al ratio induces more hydro-
phobicity of the zeolites and thus stronger adsorption of sac-
charide molecules onto zeolite catalysts. The adsorption of
saccharide molecules on the zeolites can be expressed by the
empirical correlation, K = b − a ln(x), where K is the adsorp-
tion equilibrium constant, a and b are the parameters specify-
ing the hydrophilicity–hydrophobicity of the adsorbed saccharides,
and x is the fraction of ionic sites in the zeolites.49,50 An
increase in Si/Al ratio of zeolites results in a decrease in x
and an increase in the adsorption equilibrium constant of
sucrose molecules, and the resultant increased affinity to the
zeolite catalysts. Higher affinity of the sucrose molecule
towards the zeolite with higher Si/Al ratios was confirmed for
the Na+-form FAU zeolites.49 The increase in affinity of the
sucrose molecules towards the zeolites with higher Si/Al
ratios results in the enhancement of catalytic activity per acid
site of the catalysts.
3.4.3 Zeolite mesoporosity effect. Fig. 5 shows the effect of
the zeolite mesoporosity on the reaction rate of sucrose
hydrolysis. Two pairs of zeolite catalysts, MWW-30 and
PMWW-30, MFI-70 and PMFI-70, respectively, were employed
for this study. The inclusion of mesopores by pillaring
the lamellar MWW and lamellar MFI resulted in the enhance-
ment on the reaction rates by a factor of ~2, consistent
with their porosity properties discussed in section 3.2. The
increase in the reaction rates with increasing zeolite meso-
porosity may result from the enhanced acid site accessibility
and mass transport in meso-/microporous PMWW and PMFI
Fig. 5 Rate constant of sucrose hydrolysis over MWW, PMWW, MFI,
and PMFI zeolite catalysts (T = 352 K). (MWW and PMWW both have
Si/Al of 30; MFI and PMFI both have Si/Al of 70.)
Fig. 4 Rate constant of sucrose hydrolysis over MFI, BEA, MWW, and
FAU with different Si/Al ratios (T = 352 K).
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catalysts. Our previous studies on Brønsted acid site accessi-
bility to bulky molecules by 2,6-di-tertbutyl pyridine (DTBP)
titrations has shown that PMWW and PMFI zeolites had ~10
times higher accessibility to DTBP molecules compared to
microporous MFI and MWW zeolites.44 Also, the PMWW and
PMFI catalysts showed much more efficient catalysis in diffu-
sion constrained catalytic reactions when the self-etherification
of benzyl alcohol and alkylation of mesitylene were used as the
probe reactions.45
The comparison of sucrose hydrolysis reaction rates of the
meso-/microporous zeolites (PMFI and PMWW) and the
large-pore zeolites (BEA and FAU) in Table S1† shows that
the meso-/microporous zeolites have a comparable activity to
that of BEA-type zeolites but a lower activity (~1 order of mag-
nitude) than that of FAU-type zeolites. This result indicates
that the hydrolysis of sucrose is dominated by the zeolite
microporous topological properties. PMFI and PMWW con-
tain mesopores between the zeolitic layers and micropores
within the zeolite layers. The pillaring causes the increase in
mesoporosity but not in zeolite microporosity. Table 2 shows
that PMFI and PMWW have similar micropore volumes to
those of MFI and MWW zeolite catalysts. The active sites in
mesopores, however, might experience the sucrose molecules
with a large hydration layer, which limit the reaction rate in
comparison with large micropores in FAU that might only
contain one water molecule around each sucrose molecule to
facilitate the reaction rate.28 The comparison of the zeolite
catalyst with Amberlyst-15, the industrial standard catalyst,
shows that Amberlyst-15 has comparable reaction rate con-
stants to BEA zeolite, as illustrated in Table S1 and Fig. S1 in
the ESI.†
molecules to the active sites, considering that the hydrolysis
of sucrose is conducted in aqueous and mild reaction tem-
perature conditions.
3.4.4 Assessment of kinetic parameters. Fig. 7 compares
the activation energies (ΔEmeas) and entropies (ΔSmeas) for
the sucrose hydrolysis over all the investigated zeolite
catalysts. Activation energies are comparable for the medium-
pore, large-pore, and mesoporous zeolites, in the range of
102–138 kJ mol−1 as shown in Fig. 7(a). The activation
energies are consistent with those of sucrose hydrolysis in
FAU zeolites reported by Buttersack28 and the more general
hydrolysis of other disaccharides in acid solutions. The
activation entropies, shown in Fig. 7(b), increase from 15 to
135 J mol−1 K−1 as the zeolite category changes from the
medium-pore to large-pore zeolites. The increases in Si/Al
ratios in the same type of zeolite resulted in the increase in
activation entropies by a factor ~2. The presence of meso-
pores in the meso-/microporous PMWW and PMFI zeolite
leads to the increase in activation entropies by a factor ~1.5
compared to their microporous analogues. The changes in
Fig. 6 shows the acidity dependence of the sucrose hydro-
lysis reaction in meso-/microporous zeolites. Similar to that
of medium- and large-pore conventional microporous zeolite
catalysts, the activity per acid site increases with increasing
Si/Al ratio from 70 to 150. The further increase in Si/Al ratio
to ~200, however, decreases the activity per acid site. A possi-
ble explanation is that the hydrophobicity of the zeolite with
very high Si/Al ratios restricts the access of the water
Fig. 7 The measured activation energy (ΔEmeas) (a) and measured
entropy (ΔSmeas) at 352 K (b) of sucrose hydrolysis reactions over
zeolite catalysts with different micropore topology, mesoporosity, and
acidity (Si/Al ratio).
Fig. 6 Rate constant of sucrose hydrolysis over mesoporous PMFI
zeolites with different Si/Al ratios (T = 352 K).
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the reaction entropies are consistent with the variation on
the zeolite microenvironment (micropore topology, meso-
porosity, and acidity), indicating that the hydrolysis of
sucrose over zeolite catalysts is primarily driven by the activa-
tion entropies, in accordance with reports by BeMiller et al.51
Adsorption and diffusion should contribute to the observed
reaction kinetics. The absence of adsorption equilibrium con-
stants and diffusion coefficients of sucrose from each zeolite
catalyst limits the further analysis on the intrinsic kinetic
parameters of sucrose hydrolysis in the microenvironments
of the zeolite catalysts.
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4. Conclusions
The catalytic consequences of micropore topology, meso-
porosity, and acidity on the hydrolysis of sucrose over zeolite
catalysts were examined systematically. The calculated rate
constants, activation energies, and entropies of the hydrolysis
reactions over each catalyst have been used to quantify the
effects of microenvironment of the catalysts on the reaction.
The results confirmed that the large-pore zeolites have higher
activity than medium-pore zeolites. Meso-/microporous zeo-
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logues, possibly due to the mitigated diffusion constraints
and enhanced acid site accessibility, but still lower than that
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The comparison of the effects of zeolite microenvironments
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Acknowledgements
The authors gratefully acknowledge the support from the PI's
start-up fund at the University of Maryland, College Park.
Portions of this work were supported by the ACS-Petroleum
Research Fund (ACS-PRF). We acknowledge the support of
the Maryland NanoCenter and its NispLab. The NispLab is
supported in part by the NSF as a MRSEC Shared Experimental
Facility. Y. H. acknowledges the Chinese Scholarship Council
(CSC) and Prof. Xiaoqian Ma at the South China University of
Technology.
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