Effect of acidic properties of mesoporous zeolites supporting Pt nanoparticles on hydrogenative conversion of methylcyclopentane

Article abstract of DOI:10.1021/ja509273h

The effect of acidic properties of mesoporous zeolites on the control of product selectivity during the hydrogenative isomerization of methylcyclopentane has been investigated. A series of mesoporous zeolites with controlled acidic properties were prepared by postdealumination process with hydrochloric acid under hydrothermal conditions, and the resultant zeolites used for supporting colloidal Pt nanoparticles (NPs) with a mean size of 2.5 nm (±0.6 nm). As compared to the pure Pt NPs supported on catalytically inert mesoporous silica (MCF-17) as the reference catalyst that can produce isomers most selectively (～80%), the Pt NPs supported on mesoporous zeolites produced C₆-cyclic hydrocarbons (i.e., cyclohexane and benzene) most dominantly. The type and strength of the Br?nsted (B) and Lewis (L) acid sites of those zeolites with a controlled Al amount are analyzed by using FT-IR after the adsorption of pyridine and NH₃ temperature-programmed desorption measurements, and they are correlated with the selectivity change between cyclohexane and benzene. From this investigation, we found a linear relationship between the number of Br?nsted acid sites and the formation rate for cyclohexane. In addition, we revealed that more Lewis acidic zeolite having relatively smaller B/L ratio is effective for the cyclohexane formation, whereas more Br?nsted acidic zeolite having relatively larger B/L ratio is effective for the benzene formation.

Full text of DOI:10.1021/ja509273h

Article
pubs.acs.org/JACS
Effect of Acidic Properties of Mesoporous Zeolites Supporting Pt
Nanoparticles on Hydrogenative Conversion of Methylcyclopentane
†
,‡
,‡
†,‡
,†,‡
Kyungsu Na, Selim Alayoglu,* Rong Ye, and Gabor A. Somorjai*
†
Department of Chemistry, University of California, Berkeley, California 94720, United States
‡
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
*
S Supporting Information
ABSTRACT: The effect of acidic properties of mesoporous zeolites on the control of
product selectivity during the hydrogenative isomerization of methylcyclopentane has
been investigated. A series of mesoporous zeolites with controlled acidic properties were
prepared by postdealumination process with hydrochloric acid under hydrothermal
conditions, and the resultant zeolites used for supporting colloidal Pt nanoparticles (NPs)
with a mean size of 2.5 nm (±0.6 nm). As compared to the pure Pt NPs supported on
catalytically inert mesoporous silica (MCF-17) as the reference catalyst that can produce
isomers most selectively (∼80%), the Pt NPs supported on mesoporous zeolites
̈
produced C -cyclic hydrocarbons (i.e., cyclohexane and benzene) most dominantly. The type and strength of the Bronsted (B)
6
and Lewis (L) acid sites of those zeolites with a controlled Al amount are analyzed by using FT-IR after the adsorption of
pyridine and NH temperature-programmed desorption measurements, and they are correlated with the selectivity change
3
between cyclohexane and benzene. From this investigation, we found a linear relationship between the number of Bro
sites and the formation rate for cyclohexane. In addition, we revealed that more Lewis acidic zeolite having relatively smaller B/L
̈
nsted acid
ratio is effective for the cyclohexane formation, whereas more Bro
for the benzene formation.
̈
nsted acidic zeolite having relatively larger B/L ratio is effective
1
. INTRODUCTION
metal NPs are supported on them. Such combinations of metal
NPs with zeolites make an interface between them, which can
sometimes make considerable changes in catalytic activity and
Supported metal nanoparticle (NP) catalysts are the typical
form of heterogeneous catalysts used ubiquitously in the
current chemical industries. The surface of a metal NP is the
catalytic site on which the reactant can be adsorbed and
converted to various product molecules. The molecular
interaction to the surface of metal NPs can affect the overall
catalytic activity and the product selectivity. Size, shape, and
composition of metal NPs are also crucial factors for controlling
the reaction processes.
11
1
−5
product selectivity. Particularly in the viewpoint of product
selectivity, achieving 100% product selectivity to a single
desired product would become much more feasible through the
23−25
rational design of catalyst system.
6
−9
In this work, we investigated the effect of acidic properties of
zeolite on the control of product selectivity and catalytic activity
during model hydrogenative isomerization of methylcyclopen-
tane (MCP). We used mesoporous BEA zeolites having
secondary mesopores within nanocrystalline zeolite particles
6
−9
In practical reaction processes, the
metal NPs are used as the catalytic sites after supporting on the
robust inorganic support materials of porous materials with
high surface area and large pore volume.
1
0−13
Therefore, the
̈
to control the amount and strength of Bronsted and Lewis acid
sites for elucidating effects of acidic properties on the selectivity
structural properties of supporting materials also possess
prominent functions that can critically influence the reac-
tions.
Among the various porous inorganic materials, zeolites are
control during the MCP/H reaction under ambient pressure at
2
1
4−16
150 °C. The acidic properties of mesoporous zeolite were
controlled by systematic postdealumination with different
concentrations of hydrochloric acid and hydrothermal con-
greatly contributing to the heterogeneous catalytic reac-
1
7−19
26
tions.
In current chemical industries, zeolites occupy
dition, where 2.5 nm Pt NPs were supported inside mesopore
voids with the mean diameter of 15 nm to build catalytic
architectures with Pt/zeolite interfaces.
more than 40% among the whole solid catalysts, which is the
17
largest portion, followed by metal oxides. Zeolites have a
charge-localized surface due to the presence of isolated
heteroatoms such as Al, Sn, V, and Ti that are covalently
surrounded by crystalline microporous silicon oxide frame-
2
. EXPERIMENTAL SECTION
2
0−22
Mesoporous BEA zeolite and mesoporous silica MCF-17 were
work.
Zeolites can catalyze many reactions over the
1
0,27
synthesized by following the literature reported elsewhere.
The
charge-localized atomic sites on the zeolite surface that can
induce the catalytic reactions through forming an acid−base
pair with reactant molecules. Zeolites can usually do the
catalysis alone, but they can also make catalytic synergies when
Received: September 8, 2014
Published: November 14, 2014
©
2014 American Chemical Society
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Article
Table 1. Quantification of Acid Sites According to the FT-IR and NH TPD Experiments
3
IR
NH TPD
3
a
b
c
d
e
f
g
h
i
j
k
BET surface area
755
pore size
20
pore volume
0.85
Pt
B
L
B/L
LW
BW
BM
BS
BEA-10
BEA-4
0.49
0.51
0.52
0.48
0.98
1545)
0.43
1548)
0.12
1550)
0.07
1552)
0.58
1.69
2.05
3.00
7.00
0.568
(130)
0.228
(123)
0.042
(105)
0.011
0.388
(196)
0.158
(174)
0.038
(157)
0.024
(140)
0.359
(274)
0.145
(227)
0.042
(187)
0.013
(198)
0.247
l
l
l
l
l
l
l
l
m
m
m
m
m
m
m
m
m
m
m
m
(
(1451)
0.21
(522)
0.109
(550)
0.038
(660)
0.032
(707)
815
770
865
18
11
13
0.93
0.86
1.23
m
m
m
(
(1449)
0.04
BEA-1
(
(1448)
0.01
BEA-0.5
m
(
(1447)
(89)
a
2
b
c
BET surface area calculated by Brunauer−Emmett−Teller method (m /g). Average size of mesopore (nm). Total pore volume calculated by BJH
3
d
e and f
method (cm /g). Pt loading (wt %) of Pt NPs supported zeolites determined by ICP-AES.
i−k
(e) Bro
nsted acid sites with (i) weak, (j) medium, and (k) strong strength. Center of
the IR peak (cm ). Center of the deconvoluted TPD profile (°C).
̈
nsted and (f) Lewis acid sites (mmol/g).
g
h
l
Relative ratio of B/L. Lewis acid sites with weak strength,
Bro
̈
−1
m
Figure 1. (A) FT-IR spectra after the adsorption of pyridine followed with desorption at 150 °C of BEA-10, 4, 1, and 0.5 (blue, pink, red and green,
−
1
respectively), where Bro
̈ ̈
nsted (B) and Lewis (L) acid sites are observed at around 1550 and 1450 cm , respectively. (B) Quantification of Bronsted
and Lewis acid sites of zeolites from FT-IR spectra in (A). (C) NH temperature-programmed desorption (TPD) profiles for BEA-10, 4, 1, and 0.5
3
(
from bottom to top), where each TPD profile was deconvoluted into four distinct peaks of weak Lewis acid sites (L , blue) and Bro
̈
nsted acid sites
W
with weak (B , green), medium (B , pink) and strong (B , red) strengths. The fitted signals from four peaks were also drawn together (cyan).
W
M
S
detailed synthesis procedure was described in the Supporting
Information.
3
. RESULTS AND DISCUSSION
.1. Pt Metal NPs Supported Mesoporous BEA
3
For postdealumation of mesoporous BEA zeolite, 0.5 g of H+-
exchanged mesoporous BEA zeolite was diluted in 20 mL of 1−2 M
HCl solutions, and heated for 30−90 min under reflux condition. After
treatment, the zeolite was collected by filtration, washed with distilled
water three times, and dried in an oven at 120 °C. The resultant
mesoporous BEA zeolite was denoted as BEA-x, where x is the content
Zeolites with Controlled Acidity. Upon postdealumination,
Al contents in mesoporous BEA zeolites were systematically
controlled. The resultant mesoporous BEA zeolites with
controlled amount of Al were further characterized with ICP-
AES, XRD, FT-IR, and NH TPD before supporting Pt metal
3
nanoparticles. According to the ICP-AES, the total Al content
decreased from 10% to 4%, 1% and 0.5% upon postdealumi-
nation. No significant collapse of zeolite framework was
observed according to the XRD analysis (see Supporting
(
%) of Al in the zeolite framework.
Poly(vinylpyrrolidone) (PVP)-capped Pt NPs with an average size
of 2.5 nm were synthesized by following the literature reported
16
Information Figure S1A). N adsorption analysis also proved
elsewhere (see Supporting Information for details). The PVP-capped
Pt NPs were supported on the mesoporous BEA zeolites with
controlled Al amount and mesoporous silica MCF-17.
2
that the series of dealuminated BEA zeolites possessed similar
pore properties to the pristine BEA zeolite (see Table 1 for
pore analysis data).
The acidic properties of mesoporous BEA zeolites were
investigated by FT-IR after the adsorption of pyridine as a
probe base molecule and TPD experiments after the adsorption
of NH (Figure 1). The IR data shows the stretching bands of
̈
pyridine adsorbed on the Bronsted (B) and Lewis (L) acid sites
at 1550 and 1450 cm , respectively (Figure 1A and Supporting
The catalytic materials synthesized in this work were characterized
with X-ray diffraction (XRD), transmission electron microscope
(
(
TEM), inductively coupled plasma atomic emission spectroscopy
ICP-AES), N2 physisorption analysis, FT-IR analysis and NH3
3
temperature-programmed desorption (TPD). The catalytic testing
was performed using lab-build plug-flow reactor connected to a gas
chromatograph (GC) equipped with FID detector (see Supporting
Information for the details).
−1
Information Figure S2). The selective quantification of both
acid sites was carried out with adsorption coefficients, which
28
1
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Journal of the American Chemical Society
Article
shows that the number of acid sites decreased proportionally to
the total Al contents upon postdealumination (Figure 1B and
Table 1). The pristine mesoporous BEA zeolite (BEA-10)
Scheme 1. Schematic Drawing for the Reaction of
Methylcyclopentane Reforming
showed the largest amount of Bro
g) with comparable amount of Lewis acid sites (0.58 mmol/g).
The ratio between Bronsted and Lewis acid sites (B/L ratio) is
.69. The B/L ratio was progressively increased with the
decrease of Al contents in the dealuminated BEA zeolites
Table 1). This result indicates that the Lewis acid sites are
dealuminated more preferably than the Bronsted acid sites
̈
nsted acid sites (0.98 mmol/
̈
1
(
̈
upon hydrochloric acid treatment. As already reported in
literatures, the Lewis acid sites are mostly originated from the
extraframework Al Species that can be leached out by aqueous
acid more easily than Bronsted acid sites surrounded
̈
26
tetrahedrally by −O−Si bonds.
Figure 1C shows NH TPD profiles of mesoporous BEA
3
zeolites, showing the desorption phenomena of NH molecules
3
adsorbed on the acid sites upon increase of temperature. The
TPD profiles can be deconvoluted further to four peaks
according to the desorption temperature. For example, NH3
TPD profile of pristine zeolite BEA-10 can be deconvoluted to
four peaks at 130, 196, 274, and 522 °C that can be assigned by
desorption of pyridine from Lewis acid sites with weak strength
(
L ), Bronsted acid sites with weak (B ), medium (B ), and
̈
W W M
strong (B ) strengths, respectively. Table 1 summarized the
S
quantification results of the series of BEA zeolites from NH3
TPD profiles in comparison with values calculated by FT-IR.
For the analysis of the NH -TPD spectra, a protocol was
3
employed in which the calculated B/L ratios form the pyridine-
probed FTIR spectra were used as constraints to slave the peak
deconvolution (Supporting Information Figure S3).
On the series of BEA zeolite with controlled Al contents, Pt
NPs with mean diameter of 2.5 nm were supported. The Pt
NPs were synthesized by colloidal synthesis reaction of H PtCl6
2
precursor salt in ethylene glycol in the presence of PVP as the
capping agent (see Supporting Information for details). Prior to
catalytic runs, the catalysts were conditioned in 150 Torr O at
2
2
50 °C and in 150 Torr H at 150 °C, which were found as the
2
optimum conditions to treat the residual PVP capping and
Figure 2. (A) Product selectivity (%) and (B) mass activity data over
tested catalysts based on the catalyst weight (μmol/(s/g)).
10
activate Pt NPs. The Pt NPs were supported with good
dispersion (Supporting Information Figure S4). In addition, as
proved by TEM, the dealuminated mesoporous BEA zeolites
have very similar morphology with the pristine mesoporous
BEA zeolite (BEA-10) before postdealumination. The crystal-
line lattice fringes from the microporous zeolite framework
were also observed clearly over the zeolite catalysts, which
indicated that no significant structural collapse of zeolite
framework occurred upon postdealumination with HCl.
catalytic activity, and hence Pt/SiO is the catalyst for giving a
2
sole function of Pt NPs for this reaction.
The effect of making catalytic interface between Pt and acid
sites on the zeolite surface was investigated after supporting Pt
NPs on the series of mesoporous BEA zeolites. In contrast to
the Pt/SiO , the major products over Pt/zeolites were C -cyclic
2
6
3.2. Catalytic Hydrogenative Conversion of Methyl-
hydrocarbons such as cyclohexane (blue) and benzene (red)
that are ring-enlarged products from MCP. It is highly
remarkable that such ring-enlarged hydrocarbons were not
cyclopentane. The series of mesoporous BEA zeolites with
and without supporting Pt metal NPs was tested for the MCP/
H reaction. In Scheme 1, MCP can be converted into various
formed over Pt/SiO at the same reaction condition (150 °C).
2
2
hydrocarbon species via reforming pathways such as dehydro-
genation to olefins (1, green), ring-opening isomerization to
acyclic isomers (2, pink), ring-enlargement cyclization to
cyclohexane (3, blue) or benzene (4, red), and cracking to
C −C hydrocarbons (5, gray). Figure 2A shows the product
Benzene can only be produced over Pt/SiO when the reaction
2
temperature approached 250 °C. However, cyclohexane was
not obtained over Pt/SiO2 no matter how the reaction
condition was changed. The favorable formation of C -cyclic
6
hydrocarbons can be attributed to the synergistic catalytic
actions at the interface of Pt and acid sites on the zeolite
surface. Interestingly, the relative selectivity ratio between
cyclohexane and benzene was progressively changed according
to the acidic properties of zeolite supports. Pt/BEA-10 zeolite
produced the largest product selectivity to cyclohexane (58%)
followed with 32% of benzene. The selectivity to cyclohexane
1
5
selectivity during the MCP/H2 reaction over the tested
catalysts. As shown in Figure 2A, 2.5 nm PVP-capped Pt
metal NPs supported on the mesoporous silica (Pt/SiO2)
produced acyclic isomers most dominantly (pink, ∼80%) and a
lesser amount of olefin products (green, ∼20%). Note that the
pure mesoporous silica without supporting Pt NPs has no
1
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−1
Figure 3. (A) Turnover frequency (TOF, s ) of Pt NPs supported catalysts. TOFs were calculated based on the number of Pt atoms exposed at the
surface of 2.5 nm-Pt NPs, assuming that all the surface Pt sites were reduced at 250 °C under H gas. Each bar is divided into product formation rates
2
for olefins (green), acyclic isomers (pink), cyclization to cyclohexane (blue) and cyclization/dehydrogenation to benzene (red). (B) Correlation of
−
1
product formation rates (s ) for cyclohexane (blue) and benzene (red) with the total amount of Bro
̈
nsted acid sites (mmol/g), and (B) with the
relative ratio of Bronsted and Lewis acid sites (B/L).
̈
decreased upon decrease of Al content in the mesoporous BEA
zeolite, while the benzene selectivity increased at the expense of
cyclohexane decrease. Hence, Pt/BEA-0.5 zeolite produced the
largest amount of benzene (65%) with 12% of cyclohexane.
This is in similar trend with our previous findings over Pt/MFI
catalyst with similar ratios of Al/Si and Pt/Al as the Pt/BEA-0.5
aluminosilicate framework of zeolite could render modeling
based on simple geometric variables and physical constraints
(i.e., surface area, %Pt loading, NH -TPD results) possible (see
3
Supporting Information for details). With this model, the
̈
numbers of different acid sites (Lewis and various Bronsted
sites) were calculated along the surface plane and in concentric
domains away from Pt NP surfaces (Supporting Information
Figure S5). It was found that there is, in average, 1 Lewis and 7
1
0
catalyst.
The four pure mesoporous BEA zeolites without supporting
Pt NPs were investigated in order to elucidate the sole roles of
mesoporous BEA zeolites in the absence of Pt NPs. The results
̈
Bronsted acid sites within approximately 5 nm of Pt NP
surfaces for the BEA-0.5 support (Supporting Information
Figure S5A). It should be noted that these numbers are
negligibly small as compared to the large number of surface Pt
atoms per NP (approximately 250). However, 60 Lewis and
show that these pure zeolites behave like Pt/SiO . They could
2
also produce both acyclic isomers (pink) and olefin products
(
green). The result demonstrated that the role of acidic sites on
the zeolite surface is quite similar to the role of Pt NPs, both of
which are responsible for dehydrogenation via C−H bond
activation and ring-opened isomerization via activations of C−
C and C−H bonds. However, the product selectivities toward
olefins and acyclic isomers were different depending on the
catalyst. BEA-10 zeolite could produce more acyclic isomers,
whereas BEA-0.5 zeolite produced more olefins. In addition,
small amount of benzene (2.7−12.6%) was also produced over
BEA-4, 1, and 0.5 zeolites, where its amount was progressively
increased upon the decrease of Al content.
Figure 2B shows the mass activity data for the tested catalysts
based on the catalyst weight. The mass activity decreased upon
the decrease of Al contents. Pt/BEA-10 showed 0.61 μmol/s/g
of total mass activity, which was about six times higher activity
than the pure zeolite (i.e., BEA-10). The mass activity
decreased to 0.41 μmol/(s/g) for Pt/BEA-0.5, which was also
about five times higher activity than pure zeolite (i.e., BEA-0.5).
The fact that acyclic isomers and olefin products did not change
at any significant rate (see horizontal line in Figure 3A), and the
absence of cyclohexane for pure zeolite supports and pure Pt
̈
110 Bronsted acid sites were calculated using the same model
and identical radial distance for the BEA-10 support
(Supporting Information Figure S5D). The implications of
this observation are 2-fold. First, the probability that a surface
Pt atom would see an acid side of any kind within the bonding
vicinity (0−3 Å) is very small in general, and approaches to
zero in the case of BEA-0.5. This suggests Pt surface
intermediate should spillover to zeolite surface and diffuse
several nanometers before encountering, at any significant
probability, with any acid site. Second, the changes in overall
activity, both mass-based and surface-normalized, are within
30% for the Pt/BEA catalyst for Al concentrations in the 0.5−
10% range. This indicates that the ring enlargement chemistry
over the zeolites should be the rate-limiting step while the
spillover and diffusion processes should be relatively fast. This
also implies that the cyclization reaction might still take place
on the zeolite support within the bonding proximity of surface
Pt atoms.
3.3. Correlation of Acidic Properties with Product
Formation Rates. Given the systematic changes in product
selectivity between cyclohexane and benzene over the series of
Pt/BEA catalysts, one should turn into particular structure−
reactivity correlations based on the acidic properties of zeolite
(i.e., SiO2 is inert) indicates that the reaction could be
decomposed into two major pathways: (i) hydrogen extraction
and hydrogenative ring-opening over individual Pt and acidic
zeolite sites, decoupled from one another; and (ii) hydro-
genative ring closure that led to cyclohexene intermediate, not
observed in the effluent gas mixture only to be revealed by SFG
−1
supports. To this end, the product formation rates (s ) were
correlated with the number distributions of various acidic sites
of different types and strengths; this is illustrated in Figure 3B.
It was found that the formation of cyclohexane dramatically
10
vibrational spectroscopy, and subsequent hydrogenation/
dehydrogenation at the interface of Pt and acidic sites of zeolite.
To elaborate this last conclusion that invokes a multipath
chemistry at some dynamic interface, the number densities of
acid sites around Pt NPs were calculated for different BEA
supports. Monodisperse Pt NPs and uniformly distributed
̈
increased with increasing the number of Bronsted sites,
summed over different strengths, whereas benzene formation
showed a slight negative slope. It should be noted that the
strengths of various Bronsted sites (weak, medium and strong)
̈
were defined in the broadest definition of the term, referring to
1
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Article
the low, medium and high temperature NH desorption peaks.
ASSOCIATED CONTENT
Supporting Information
Synthesis of catalytic materials, and their supplementary
characterization. This material is available free of charge via
the Internet at http://pubs.acs.org.
3
■
The relative strengths of various Bro
different for the Pt/BEA catalysts. In general, strong Bro
sites increased in strength (i.e., higher desorption temperature
̈
nsted sites are, however,
nsted
*
S
̈
of NH ) in the order BEA-10 < BEA-4 < BEA-1 < BEA-0.5,
3
̈
while weak and medium Bronsted sites decreased in strength in
AUTHOR INFORMATION
the same order (Figure 1C). Pyridine-probed FT-IR study also
indicated an increase in average vibrational frequency of
■
Corresponding Authors
somorjai@berkeley.edu
salayoglu@lbl.gov
̈
pyridine adsorbed on Bronsted sites from BEA-10 to BEA-0.5
(
Figure 1A) and, therefore, corroborated the conclusions drawn
from the analysis of NH -TPD data. Likewise, the relative
Notes
3
strengths of weak Lewis sites increased in the order BEA-0.5 <
The authors declare no competing financial interest.
BEA-1 < BEA-4 < BEA-10, indicated by the NH -TPD and
3
ACKNOWLEDGMENTS
pyridine-probed FT-IR results. Therefore, the selectivity
switchover from cyclohexane to benzene for the Pt/BEA
catalysts (i.e., the ease of hydrogen extraction) could be single-
■
This work is funded by The Chevron Energy Technology
Company. We acknowledge support from the Director, Office
of Science, Office of Basic Energy Sciences, Division of
Chemical Sciences, Geological and Biosciences of the US DOE
under contract DE-AC02-05CH11231. The authors acknowl-
edge support of the National Center for Electron Microscopy,
Lawrence Berkeley Lab, which is supported by the U.S.
Department of Energy under Contract No. DE-AC02-
̈
handedly delivered by the presence of strong Bronsted sites, in
particular, those that exhibited stronger bonding with the base
molecules as in the case of BEA-0.5. In support of this, it should
be noted that pure BEA-0.5 led to the formation of benzene in
low, yet measurable amounts, while pure BEA-10 did not
produce any measurable cyclic isomers.
0
5CH11231. Work at the Molecular Foundry was supported
Alternative to this view, the Lewis acid sites could interact
with CC double bonds, present in either benzene or
cyclohexene intermediate, by forming π-complex with alumi-
num at Lewis acid sites. If correct, this should lead to stronger
by the Director, Office of Science, Office of Basic Energy
Sciences, Division of Material Sciences and Engineering, of the
U.S. Department of Energy under Contract No. DE-AC02-
0
5CH11231. K. Na thanks the financial support from Basic
(
weak) Lewis acid sites for the Pt/BEA-0.5 catalyst than the Pt/
Science Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of
Education (2012R1A6A3A03039602). We thank Prof. Peidong
Yang and Prof. Omar M. Yaghi for use of the TEM and XRD
instruments, respectively.
BEA-10 catalyst. Our findings, however, indicate the opposite
trend; the desorption temperature of NH over the Pt/BEA-10
3
catalyst was higher than that over the Pt/BEA-0.5 catalyst (130
vs 89 °C in Table 1). Furthermore, the number of weak Lewis
sites over the Pt/BEA-10 outmatched those over the Pt/BEA-
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■
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