Size-selective hydrogenation at the subnanometer scale over platinum nanoparticles encapsulated in silicalite-1 single crystal hollow shells

Article abstract of DOI:10.1039/c3cc48648f

Highly controlled "ship-in-a-bottle" platinum nanoparticles in silicalite-1 hollow single crystals have been prepared. This catalyst is highly active for toluene hydrogenation but shows no activity for the hydrogenation of 1,3,5-trimethylbenzene.

Full text of DOI:10.1039/c3cc48648f

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ARTICLE TYPE
Size-Selective Hydrogenation at the Subnanometer Scale over Platinum
Nanoparticles Encapsulated in Silicalite-1 Single Crystal Hollow Shells.
Shiwen Li, Tommy Boucheron, Alain Tuel, David Farrusseng and Frederic Meunier*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
The preparation of Pt@silicaliteꢀ1 materials is a multiꢀstep
process involving minor modifications of our previously
published method for Auꢀbased samples9. In brief, a calcined
50 silicaliteꢀ1 is impregnated with an aqueous solution of
Pt(NO3)2(NH3)4 and the suspension is continuously stirred until
the solvent is totally evaporated. The resulting solid is treated
with a tetraꢀnꢀpropylammonium hydroxide (TPAOH) solution in
an autoclave at 170°C for 24 hours. It is during this step that the
55 hollow core of the silicaliteꢀ1 is formed, due to dissolution of the
defectꢀrich core. Finally, the Pt@silicaliteꢀ1 composite is calcined
at 450°C and reduced under H2 at 300°C.
Highly controlled “ship-in-a-bottle” platinum nanoparticles
in silicalite-1 hollow single crystals have been prepared. This
catalyst is highly active for toluene hydrogenation but shows
no activity for the hydrogenation of 1,3,5-trimethylbenzene.
10 The possibility to selectively react molecules based on size or
shape selectivity reached its pinnacle with the use of zeolites as
catalysts1,2. Acid or base catalysed reactions have thus been able
to target specific compounds in mixtures. The possibility to
extent this concept to metalꢀcatalysed reactions has been pursued
15 for several decades with success. Various types of hybrid
nanostructures were prepared, typically comprising a metallic
particles embedded in1,3ꢀ5 or covered by5ꢀ8 porous solids. The
latter solids span a wide variety of structures from zeolites6 to
amorphous carbons7 or oxides8.
Mesoporous shells cannot carry out molecular sieveꢀtype
separation of molecules smaller than about a nanometer. In
contrast, zeolites are crystalline microporous solids, the wellꢀ
defined pores of which enabling a sharp discrimination of the
reactants based on size and/or shape. In an ideal zeoliteꢀprotected
High Resolution TEM pictures clearly show the single
crystalline nature of the hollow silicaliteꢀ1 without apparent
60 crystalline defects such as twinning planes, grain boundary zones
or pinholes (Fig. 1, left).
20
25 metal catalyst, only reactants with sufficiently small kinetic
diameters would be able to cross the zeolite barrier to reach the
metallic constituent. The limit of this approach is yet governed by
the presence of unprotected metal particles (either located on the
external surface of the zeolite constituent or due to the presence
30 of mesoporosity), resulting in imperfect size discrimination.
Inadequate encapsulation may not sufficiently limit the diffusion
of the metallic atoms and may ultimately result in particle
sintering.
Some of us have recently developed an original and scalable
35 synthesis pathway that enables the preparation of gold
nanoparticles located in hollow single crystals of silicalite9. A
desired particle size is achieved by adjusting the concentration of
gold solution used for the silicaliteꢀ1 impregnation and
subsequent dissolution/recrystallization of the core of the crystals.
40 This novel synthesis method is extended here, for the first time,
to the case of platinum nanoparticles, which is formed in hollow
single crystals of silicaliteꢀ1. The outstanding catalytic properties
in terms of size selectivity are assessed using the hydrogenation
of toluene and mesitylene (i.e. 1,3,5ꢀtrimethylbenzene), which are
45 similar in size, since the kinetic diameter of toluene and
mesitylene are 0.61 and 0.87 nm, respectively.
Figure 1. Left: HRTEM picture of sample Pt@silicaliteꢀ1 showing the
singleꢀcrystal nature of the hollow zeolite calcined at 450°C and reduced
65
in H2 at 300°C. Right: HRTEM picture showing the sample after
reduction in H2 at 750°C.
Hence, Pt nanoparticles cannot diffuse out of the internal
cavity, preventing the formation of metal particles at the zeolite
outer surface. Pt particles are known to migrate and coalesce
70 under H2 between 600 and 700°C10. As a matter of fact, the
Pt@silicaliteꢀ1 material is resistant to a calcination at 750°C (Fig.
1, right) and the particle size distribution of the Pt was hardly
changed from that obtained at 600°C, with a slight increase from
10.3 to 10.4 nm (Fig. 2). The increase was probably due to the
75 aggregation of very small Pt molecules present in the cavity,
rather than transport and coalescence through the microporous
walls of the silicaliteꢀ1 crystals.
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DOI: 10.1039/C3CC48648F
(A)
30
(B)
Figure 2. Pt particle size distribution on Pt@silicaliteꢀ1 after reduction in
H2 at 600 and 750°C.
Figure 3. Conversion during the hydrogenation of (A) toluene and (B)
nmesitylene over the Pt@silicaliteꢀ1 and a Pt/SiO2. Toluene pressure =
903 Pa, balance H2. Mesitylene pressure = 61 Pa, balance H2.
5
The catalytic activity of the Pt@silicaliteꢀ1 for the
hydrogenation of toluene and mesitylene to the corresponding
saturated compounds was investigated (see experimental details
in the Supplementary Information). The activity of a conventional
Pt sample supported on high surface area silica was also
35
The case of mesitylene was dramatically different. Mesitylene
was readily fully hydrogenated to trimethylcyclohexane over the
conventional SiO2ꢀsupported Pt, while a conversion lower than
2% was measured in the case of the Pt@silicaliteꢀ1, that was a
value within our experimental error (Fig. 3.B).
10 measured for these two reactants.
The Pt@silicaliteꢀ1 and the Pt/SiO2 led to
a similar
40
The turnꢀover frequency for mesitylene hydrogenation on the
Pt/SiO2 was only calculated for the data point displaying a
conversion lower than 15% and was found to 7.6 x 10ꢀ3 sꢀ1 at
41°C. Note that the TOF for toluene hydrogenation would be
expected to be around 4 x 10ꢀ3 sꢀ1 at 41°C on the same catalyst.
conversion versus temperature plot for toluene (Fig. 3.A). Since
the Pt loading was twice higher on the Pt@silicaliteꢀ1 and the
corresponding dispersion was twoꢀfold lower, as compared to the
15 Pt/SiO2, essentially identical turnꢀover frequencies were obtained
for these two catalysts as far as toluene hydrogenation was
concerned (see Fig. S3, Supplementary Information).
The values of turnꢀover frequencies (e.g. 6 x 10ꢀ2 sꢀ1 at 80°C)
are consistent with those obtained for toluene hydrogenation on
20 other Ptꢀbased catalysts using similar experimental
45 Therefore, the TOFs for hydrogenation of toluene and mesitylene
on the Pt/SiO2 were of the same order of magnitude.
The data reported above indicate that the rate of mesitylene
hydrogenation on the Pt@silicaliteꢀ1 was at least two orders of
magnitude lower than that on the Pt/SiO2. We can therefore
50 conclude that the vast majority (at least 99%) of the observed Pt
particles were indeed inside the hollow boxes.
conditions11,12
.
Similarly, the apparent activation energies
measured here (ca. 55 ± 3 kJ molꢀ1, Fig. S4 and S5, ESI) are
similar to those reported elsewhere11,13
.
The origin of this inhibition was the size exclusion of the
mesitylene from the silicaliteꢀ1 pores. High resolution TEM
images revealed that the zeolite hollow boxes were essentially
These data therefore indicates that the silicateꢀ1 hollow box
25 did not significantly hinder neither the transport of toluene nor
that of the reaction product methylcyclohexane.
55 monocrystaline materials.
A unique crystallographic plan
orientation was observed on the whole side of a box (data not
2
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DOI: 10.1039/C3CC48648F
shown). N2 adsorption isotherm revealed that the hollow zeolites
7
8
C. Galeano, R. Guttel, M. Paul, P. Arnal, A.ꢀH. Lu, F. Schuth,
Chem.ꢀ Europ. J. 2011, 17, 8434.
were free of mesoporous pinholes9. The channel dimensions of
silicaliteꢀ1 are about 0.56 x 0.53 nm and toluene can enter those
while mesitylene is too large and is therefore excluded from the
micropores. Adsorption isotherms of toluene and mesitylene on
the silicaliteꢀ1 hollow boxes (free of Pt) confirm the uptake of
toluene and the size exclusion of the larger hydrocarbon from the
pores (see Fig. S6, ESI). The hollow zeolite crystal operates as a
zeolite oriented membrane with a cutꢀoff at about 0.7 nm.
R. N. Devi, F. C. Meunier, T. Le Goaziou, C. Hardacre, P.J. Collier,
S. E. Golunski, L. F. Gladden, M. D. Mantle, J. Phys. Chem. C,
2008, 112, 10968.
60
5
9
S.W. Li, L. Burel, C. Aquino, A. Tuel, F. Morfin, J.L. Rousset, D.
Farrusseng, Chem. Commun., 2013, 49, 8507.
10 A.K. Datye, Q. Xu, K.C. Kharas, J.M. McCarty, Catal. Today, 206,
111, 59.
65
10
Since mesitylene cannot enter the pores of the hollow box, it
is reasonable to propose that each hollow box entirely consists of
the MFIꢀtype structure, without any significant mesoporosity.
Therefore, the cutꢀoff size to access the embedded Pt particles
should be above the kinetic diameter of pꢀxylene (= 0.585 nm),
11 J.L. Rousset, L. Stievano, F.J. Cadete Santos Aires, C. Geantet, A.J.
Renouprez, M. Pellariny, J. Catal. 2001, 197, 335.
12 A.A. Taimor, I. Pitault, F.C. Meunier, J. Catal. 2011, 278, 153.
13 S.D. Lin, M.A. Vannice, J. Catal. 1991, 143, 554.
70 14 S.F. Garcia, P. B. Weisz, J. Catal., 1990, 121, 294.
15 F.C. Meunier, D. Verboekend, J.ꢀP. Gilson, J.C. Groen, J. Pérezꢀ
Ramírez, Microporous Mesoporous Mater. 2012, 148, 115.
15 which is known to readily diffuse in MFIꢀtype materials and
around that of oꢀ and mꢀxylene (ca. 0.68 nm), which still can
enter MFI pores, but whose transport are greatly hindered14,15
.
In conclusion, we have been able to prepare a new type of
catalytic materials consisting in single Pt particles embedded in a
20 silicaliteꢀ1 monocrystal without measurable Pt particles at the
outer surface. This type of wellꢀdefined materials paves the way
for more academic studies and practical applications of shape,
size and product selectivity for a wide range of catalytic systems
in terms of catalyst composition and reaction mixtures. Of
25 particular interest is the fact that the size of the metal particles
contained within the hollow crystal can be much larger than the
cavities of the sieving element, contrary to the materials prepared
75
by the methods reported earlier1,3,4
.
30
The authors thank the scientific services of IRCELYON. This
study has been supported by the European Union Seventh
Framework Programme FP7ꢀNMPꢀ2010, under Grant Agreement
n° 263007 (acronym CARENA).
Notes and references
35 S. Li, T. Boucheron, Dr. A. Tuel, Dr. D. Farrusseng, Dr F.C. Meunier,
Institut de Recherches sur la Catalyse et l’Environnement de Lyon
(IRCELYON), Université Lyon 1, CNRS
2, Av. Albert Einstein F-69626 Villeurbanne, Fax: (+)33 4 72 44 53 65
E-mail: frederic.meunier@ircelyon.univ-lyon1.fr
40 † Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
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DOI: 10.1039/C3CC48648F
Graphical abstract
A newly developed method enables preparing a single particle of platinum
encapsulated in a hollow single crystal of a microporous silicalite-1. The sample was
able to hydrogenate toluene unimpeded, while the hydrogenation of mesitylene was
totally prevented, proving that the level of defects in the structure was negligible.
Pt@Silicalite-1
+ H2