Bifunctional Hierarchical Zeolite-Supported Silver Catalysts for the Conversion of Glycerol to Allyl Alcohol

Article abstract of DOI:10.1002/cctc.201601635

The establishment of suitable processes for the conversion of glycerol into allyl alcohol is hindered by the fast deactivation of solid acids in the dehydration of the substrate to acrolein and by the requirement of hydrogen donors to enhance the selectivity of the subsequent reduction step. In this work, silver nanoparticles deposited onto a hierarchical ZSM-5 zeolite are proved to be an effective bifunctional catalyst to conduct the two reactions in the gas phase and in the presence of hydrogen by using a continuous fixed-bed reactor. The acidic function was accomplished by using a ZSM-5 zeolite modified by facile alkaline and acid treatments, which decreased the amount of Lewis acid centers while preserving the amount of Br?nsted acid centers, and introduced an auxiliary network of intracrystalline mesopores, thus boosting the selectivity to acrolein (62 %) and the resistance to coking. Upon screening of various metals supported on the aluminosilicate, silver was identified as a superior hydrogenation catalyst, enabling a relatively high activity with >50 % allyl alcohol selectivity. Tuning of the metal loading, temperature, pressure, and contact time led to 15 % yield of allyl alcohol, thus approaching the state-of-the-art transfer hydrogenation systems, and stable behavior for 100 h on stream. Our results highlight the advantage of conducting the two transformations over a bifunctional material rather than over two separate single-function solids.

Full text of DOI:10.1002/cctc.201601635

Accepted Article
Title: Bifunctional Hierarchical Zeolite-Supported Silver Catalysts for
the Conversion of Glycerol to Allyl Alcohol
Authors: Giacomo M Lari, Zupeng Chen, Cecilia Mondelli, and Javier
Pérez-Ramírez
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Bifunctional Hierarchical Zeolite-Supported Silver Catalysts for
the Conversion of Glycerol to Allyl Alcohol
Giacomo M. Lari, Zupeng Chen, Cecilia Mondelli,* and Javier Pérez-Ramírez*
Abstract: The establishment of suitable processes for the conversion
of glycerol into allyl alcohol is hindered by the fast deactivation of solid
acids in the dehydration of the substrate to acrolein and by the
requirement of hydrogen donors to enhance the selectivity of the
subsequent reduction step. In this work, silver nanoparticles
deposited onto a hierarchical ZSM-5 zeolite prove as an effective
bifunctional catalyst to conduct the two reactions in the gas phase and
in the presence of hydrogen using a continuous fixed-bed reactor. The
acidic function was accomplished using a ZSM-5 zeolite modified by
facile alkaline and acid treatments, which decreased the amount of
Lewis-acid centers while preserving the amount of Brønsted-acid
centers, and introduced an auxiliary network of intracrystalline
mesopores, thus boosting the selectivity to acrolein (62%) and the
resistance to coking. Upon screening of various metals supported
In the initial dehydration, two water molecules are removed from
glycerol, typically in the gas phase.[7] Specifically, the secondary
hydroxyl group of the substrate is protonated and eliminated,
leading to the formation of 3-hydroxypropanal, which is quickly
converted into acrolein under the conditions (T > 450 K)
employed.[7] Brønsted-acid sites have been identified as selective,
while Lewis-acid and basic functionalities alternatively activate
the primary hydroxyl group, hence forming hydroxyacetone.[5]
Thus, strong solid acids, like heteropolyacids, sulfated zirconia,
and zeolites, belong to the state-of-the-art heterogeneous
catalysts.[8] In particular, microporous crystalline aluminosilicates
have been the materials of choice, since the possibility to tune
their porous properties enables to address one of the main
drawbacks of this reaction, i.e. fouling, which is due to the high
reactivity of the reactant, intermediate, and product at the reaction
temperature.[9] Indeed, different approaches have been applied to
increase the mesoporous surface of zeolites, which has been
demonstrated as the main parameter correlating with lifetime in
coke-forming reactions.[10] Amongst these, post-synthetic
modification via alkaline treatment led to mesoporous crystals
with improved durability.[10a] However, due to the increase in
Lewis acidity upon desilication, the selectivity to acrolein was
reduced in favor to that to acetol.
onto the aluminosilicate, silver was identified as
a superior
hydrogenation phase, enabling a relatively high activity with a >50%
allyl alcohol selectivity. Tuning of the metal loading, temperature,
pressure, and contact time led to 15% yield of allyl alcohol, thus
approaching the state-of-the-art transfer hydrogenation systems, and
stable behavior for 100 h on stream. Our results highlight the
advantage of conducting the two transformations over a bifunctional
material rather than over two separate single-function solids.
The selective hydrogenation of acrolein to allyl alcohol is
particularly challenging due to the p-conjugation of the C=O bond
with the C=C bond. Palladium, ruthenium, rhodium, and platinum
are fully selective to propanal,[11] while silver and gold catalysts
show an allyl alcohol selectivity of 50–60% in gas-phase
experiments.[12] Alloying of silver with cadmium and zinc
Introduction
The development of chemocatalytic technologies to convert
biomass into chemicals continues to receive great attention as a
means to limit our reliance on fossil sources. In this context,
glycerol, the main by-product in the manufacture of biodiesel,
comprises a versatile starting material for the preparation of
various compounds possessing a C3 backbone.[1] Besides the
widely studied acrylates, lactates, carbonates, diols, etc.,[2] few
works have focused on allyl alcohol, which is a high added-value
chemical used for the synthesis of specialty polymers, glycidol,
glycidyl esters, and amines.[3] Since its industrial production is
based on the hydrogenation of propene-derived acrolein[4] and the
latter can be attained from glycerol through dehydration,[5] a two-
step transformation has been proposed to form allyl alcohol from
the triol (Scheme 1).[6]
G. M. Lari, Dr. Z. Chen, Dr. C. Mondelli,*
Prof. Dr. J. Pérez-Ramírez*
Institute for Chemical and Bioengineering
Department of Chemistry and Applied Biosciences
ETH Zurich
Scheme 1. Simplified reaction network for the transformation of glycerol into
allyl alcohol (solid arrows) and the main side products (dashed arrows).
Vladimir-Prelog-Weg 1, 8093 Zurich (Switzerland)
E-mails: cecilia.mondelli@chem.ethz.ch, jpr@chem.ethz.ch
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10.1002/cctc.201601635
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increased the yield up to 70% and this technology is industrially
exploited.[11c] In addition to the nature of the metal, the support
was shown to have great effect on its performance, as the
deposition of platinum on a strongly acidic carrier increased the
allyl alcohol selectivity to 60%.[11b] Transfer hydrogenation in the
liquid phase has been proposed as an alternative for the
preparation of allyl alcohol, using isopropanol and 2-butanol over
Cd-Zn, MgO-ZnO, or lanthanide-based catalysts.[13] While this
approach slightly enhanced the selectivity, the generation of low-
value ketones from the alcohols in equimolar amount to allyl
alcohol represents a strong drawback toward an industrial
process.
The combination of the two reactions over a single catalyst has
been tackled infrequently and was typically performed in the
presence of an organic hydrogen donor. Bulk and ceria-supported
rhenium catalysts achieved high allyl alcohol yields in batch liquid-
phase tests with pentanols or benzyl alcohol.[14] Nevertheless, the
scarce availability and extremely high price of this metal strongly
limit the future commercialization of these materials. More viable
systems include platinum or iron oxide supported onto an acidic
carrier, which featured allyl alcohol yields of 10 or 20%,
respectively, in the presence of propanols as hydrogen donors.[15]
Nevertheless, by-products formation and the loss of part of the
donor due to dehydration over the acidic material emerged as
additional disadvantages to the formation of the ketones. It should
be noted that, although the production of allyl alcohol in the
presence of molecular hydrogen has never been addressed
directly, a yield of 5% in this chemical has been observed over
copper or nickel on solid acids, which were used to transform
glycerol into propan(di)ol.[16]
Figure 1. X-ray diffractograms of the acid catalysts investigated.
and selectivity towards the acrolein intermediate. The former
property was improved contacting the aluminosilicate with 0.3 M
aqueous NaOH, which leads to the extraction of silicon atoms
from the framework, generating
a secondary network of
interconnected mesopores.[18] Upon this treatment, the Si/Al ratio
decreased to 26 (H-Z40-h, Table 1), the mesoporous surface
area increased to 303 m2 g−1, and ca. 45% of the initial crystallinity
(Figure 1) and 31% of the original microporous volume were lost.
In spite of the lower Si/Al ratio, the overall density of acid sites
(Table 1) as well as the Brønsted-/Lewis-acid sites ratio
decreased, in agreement with previous observations that a good
fraction of the aluminum atoms in the modified zeolite are present
in the form of hexacoordinated species with Lewis-acid
character.[19] To remove these unselective sites, an acid washing
was performed, which produced H-Z40-haw. This step decreased
the aluminum content by ca. 25%. Coherently with the removal of
amorphous portions of the material, the microporous volume of
the zeolite was partially (by ca. 10%) restored and the Lewis
acidity halved, while the Brønsted acidity remained unchanged.
The parent and modified zeolites were tested in the gas-phase
conversion of glycerol at 673 K for 2 h on stream. As expected,
the selectivity to acrolein was found to depend on the Brønsted
acidity of the materials (Figure 2a), being 49.2, 31.8, and 62.1%
for H-Z40, H-Z40-h, and H-Z40-haw, respectively. Acetol and coke
formed as by-products (Table 2), as indicated in earlier literature
reports, while compounds that could originate from oxidation and
reduction reactions were not observed, in line with the very
modest activity of aluminosilicates in these transformations and
the inert atmosphere employed.[20] The extent of acetol generation
followed the Lewis acidity of the zeolites: H-Z40-h > H-Z40 > H-
Z40-haw. The role of this property was confirmed by testing Lewis-
acidic g-alumina, which displayed the highest selectivity to this
chemical. The behavior of H-Z40, H-Z40-h, and H-Z40-haw was
investigated over prolonged runs (Figure 2b). In line with
previous studies indicating an augmented Smeso as the main
parameter determining catalyst lifetime in this reaction,[10a] the
modified zeolites displayed higher stability than H-Z40.
Nevertheless, H-Z40-h exhibited a lower deactivation rate than H-
Z40-haw. These results can be rationalized considering that, in
Herein, we introduce silver nanoparticles supported on
a
hierarchical zeolite as an efficient bifunctional catalyst for the H2-
mediated transformation of glycerol into allyl alcohol in a
continuous gas-phase process. The catalyst design firstly
encompassed the tailoring of the porous and acidic properties of
MFI-type aluminosilicates through post-synthetic base and acid
treatments to increase their selectivity and long-term stability in
the dehydration of the substrate to acrolein. Secondly, different
metals deposited onto the optimized zeolite were tested to identify
the best hydrogenation phase for the reduction of the intermediate
to allyl alcohol. Thereafter, the supporting procedure and the
metal loading were tuned along with the operating conditions to
boost the productivity and on-stream durability of this novel
catalytic technology.
Results and Discussion
Design of the dehydration phase
The development of suitable catalysts for the conversion of
glycerol into allyl alcohol in the presence of molecular hydrogen
involved the design of acid and redox components. Based on its
previous preferred use in this reaction,[10a,17] a commercially
available zeolite with MFI framework topology and a nominal bulk
Si/Al = 40 (coded as H-Z40) was selected for the first phase and
modified through post-synthetic treatments to enhance its stability
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Table 1. Characterization data for the acid and bifunctional catalysts.
b
c
d
c
f
f
Ma
wt.%
Si/Ala
Naa
wt.%
SBET
Smeso
Vpore
Vmicro
Cryst.e
%
CB
CL
Catalyst
mol mol−1
m2 g−1
m2 g−1
cm3 g−1
0.50
0.26
0.78
0.90
0.76
0.79
0.81
0.85
0.86
cm3 g−1
0.00
0.16
0.11
0.12
0.09
0.08
0.11
0.11
0.10
µmol g−1
µmol g−1
g-Al2O3
-
0
0.0
0.4
0.4
0.4
2.7
0.6
0.3
0.6
0.3
85
83
-
4
81
H-Z40
-
42
26
45
40
43
43
44
43
397
518
594
467
481
544
541
523
97
100
55
62
59
64
61
58
60
177
126
142
121
130
145
129
120
63
H-Z40-h
-
303
366
298
320
337
335
339
67
H-Z40-haw
Pd/H-Z40-haw
Pt/H-Z40-haw
Au/H-Z40-haw
Ru/H-Z40-haw
Ag/H-Z40-haw
-
37
6.8
6.0
6.9
5.7
2.7
51
60
64
59
65
a XRF. b BET method. c t-plot method. d Volume adsorbed at p/p0 = 0.99. e XRD, with respect to H-Z40. f FTIR of adsorbed pyridine.
spite of a larger Smeso, the latter solid has a slightly higher
concentration of Brønsted-acid sites, which are known to favor
condensation reactions leading to coke.[21] Thus, they suggest
that the density of strong acid sites is an additional parameter
Figure 2. (a) Acrolein to acetol selectivity ratio versus the relative concentration
of Brønsted- and Lewis-acid sites and (b) temporal evolution of the normalized
conversion of glycerol over the dehydration catalysts investigated.
Figure 3. (a) X-ray diffractograms of the bifunctional catalysts studied and (b)
concentration of Brønsted- and Lewis-acid sites versus the Ag content.
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Table 2. Performance acid and bifunctional catalysts in the gas-phase conversion of glycerol.
Xglycerol
%
Sacetol
%
S1,2-propanediol
%
Sallyl alcohol
%
Spropanal
%
Sacrolein
%
Cokea
wt.%
CBb
%
Catalyst
g-Al2O3
24.1
51.7
0.1
0.0
0.0
22.8
0.8
93.9
H-Z40
34.8
48.2
52.2
73.0
84.2
21.0
18.7
53.5
28.8
40.2
16.4
34.9
10.1
20.1
15.1
15.1
1.0
0.1
0.1
0.0
6.2
7.7
4.1
8.1
13.1
0.0
49.2
31.8
62.1
11.4
21.2
67.6
20.2
47.4
2.1
3.4
2.7
1.1
0.5
0.9
1.2
0.8
90.6
90.1
89.1
100.5
96.5
99.9
95.1
96.9
H-Z40-h
1.4
0.0
H-Z40-haw
0.8
0.0
5% Pd/H-Z40-haw
5% Pt/H-Z40-haw
5% Au/H-Z40-haw
5% Ru/H-Z40-haw
5% Ag/H-Z40-haw
10.8
15.4
2.4
36.8
40.9
0.8
7.4
14.9
10.6
8.0
a TGA, determined after 2 h reaction. b Carbon balance. Samples were collected between 1 and 2 h on stream.
determining the on-stream performance. Based on its superior
selectivity to acrolein, H-Z40-haw was selected for further
investigations.
propanal, respectively. While the selectivity towards the diol lied
in a relatively narrow range for all of the catalysts studied, the
choice of the metal and of the loading was demonstrated crucial
to balance between the desired hydrogenation of acrolein to allyl
alcohol and its undesired reduction to propanal. In agreement with
some previous reports,[11] the use of palladium and platinum
generated catalysts that converted acrolein to propanal with
almost full selectivity (86 and 84% based on the amount of
acrolein converted) and a relatively high activity. Conversely, gold,
ruthenium, and silver exhibited a relatively low hydrogenation
activity, but much higher selectivity. In particular, silver provided
Design of the hydrogenation phase
Various metals (Pt, Ru, Pd, Ag, and Au) were chosen based on
the literature dealing with the selective hydrogenation of acrolein
to allyl alcohol and deposited onto H-Z40-haw by incipient wetness
impregnation in an amount corresponding to a 5 wt.% loading.
Elemental analysis (Table 1) revealed that the metal content was
close to the nominal value in all cases and that the composition
of the support was not affected by the catalyst preparation
procedure. X-ray diffraction (XRD, Figure 3a) evidenced the
presence of metallic phases for Ag-, Au-, and Pt-based solids and
of metal oxides for Pd- and Ru-containing catalysts. In all cases,
the Brønsted acidity of the synthesized materials slightly
diminished (<15% variation, Table 1), while the concentration of
Lewis-acid sites increased substantially.
The zeolite-supported metal catalysts were tested in the gas-
phase conversion of glycerol at 673 K in the presence of hydrogen
(40 bar). In line with the decrease of the Brønsted-/Lewis-acid
sites ratio upon metal deposition, the relative acrolein/acetol
selectivity slightly decreased (Table 2). To corroborate the
importance of the preservation of the active acid sites of the
support during the incorporation of the metal phase, an alternative
synthesis procedure was applied, in which the metal precursor
was reduced by addition of NaBH4 after impregnation. In fact,
hydride ions strongly react with protons at the zeolite surface,
leading to the exchange of protonic sites with sodium. Due to its
poor Brønsted (15 µmol g−1) and moderate Lewis (83 µmol g−1)
acidity, this material displayed a low glycerol conversion (24.6%)
Figure 4. TEM micrographs and corresponding particle size distributions of the
(a) 1, (b) 2, and (c) 10 wt.% Ag/H-Z40-haw catalysts and of the 5 wt.% Ag/H-
Z40-haw catalyst (d) as-prepared, (e) reduced, and (f) after use.
and
a
high acetol selectivity (78.3%). Regarding the
hydrogenation activity, part of both the acetol and acrolein formed
were converted into 1,2-propanediol and into allyl alcohol and
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Figure 5. Glycerol yield and product selectivity versus the Ag loading of the H-
Z40-haw-supported catalysts.
the highest allyl alcohol yield (13.1% at 53.5% glycerol
conversion), which was further optimized by tuning of its metal
loading. For this purpose, additional catalysts with a metal content
in the 1-10 wt.% range were prepared. When the silver loading
was lower than 5 wt.%, the reflections at 2q = 38.2 and 44.4°,
assigned to the (111) and (200) lattice planes of Ag, were not
detected (Figure 3a), pointing to a high dispersion of the metal.
The latter was supported by high-angle annular dark field
scanning transmission electron microscopy (HAADF-STEM,
Figure 4). Only small Ag clusters (ca. 1-2 nm) were visualized in
the as-prepared 1 and 2 wt.% Ag/H-Z40-haw catalysts (Figure 4a
and b), while particles of ca. 10 nm were visible in the
10 wt.% Ag/H-Z40-haw sample (Figure 4c). A broad particle size
distribution, featuring small silver clusters and larger aggregates,
was measured for the as-prepared 5 wt.% Ag/H-Z40-haw
(Figure 4d). Onto this material, further clustering was observed
after reduction with hydrogen (Figure 4e). This indicates that the
particle size depended on both the metal loading and the
conditions to which the catalysts were subjected. Investigation of
the acidic properties of these solids revealed a moderate
depletion of the density of Brønsted sites (Table 1) and an
increase in the concentration of Lewis centres with respect to the
bare zeolite. These effects likely arose from the ion exchange of
some of the acidic protons with Ag cations upon impregnation of
the precursor. Although most of these are displaced from these
positions and reduced to nanoparticles, a part proportional to the
metal loaded remains in its Lewis-acidic cationic form at the
expense of part of the protons (Figure 3b). Higher and lower
silver contents did not lead to better performances. In fact, at
higher metal loadings (Figure 5), the increased selectivity of the
hydrogenation reaction originating from the larger amount of
metal centres was counterbalanced by the lower formation of the
dehydration product to be reduced, i.e., acrolein. The latter likely
arose from the decrease of the acidity and accessibility of portions
of the zeolite crystals due to the bulky metal particles blocking the
pores. At lower silver loadings, the situation was reversed, i.e.,
more acrolein was formed but it could be hardly transformed into
allyl alcohol due to the insufficient amount of redox centres. The
catalytic performance was optimized by tuning the temperature,
Figure 6. Glycerol conversion and product selectivity over 5 wt.%Ag/H-Z40-haw
versus the (a) temperature, (b) pressure, (c) weight hourly space velocity, and
(d) time-on-stream.
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pressure, and space velocity. With respect to the first parameter,
the conversion was moderate at low values and monotonously
incremented at progressively higher temperatures, reaching
100% at 773 K (Figure 6a). The selectivity to acrolein and to its
hydrogenation products increased up to 673 K and then
diminished at higher temperatures. Under the latter conditions, a
low value was determined for the mass balance, which was
correlated with the stronger relevance of condensation, coking,
and unselective hydrogenation reactions forming solid deposits
on the catalyst surface. Based on these results, the hydrogen
pressure was varied in the 0-40 bar range at 673 K (Figure 6b).
In the absence of hydrogen, similar results were obtained as upon
the use of H-Z40-haw, except for a slightly lower selectivity to
acrolein, which was attributed to the decreased amount of strong
acid sites. By raising the hydrogen pressure, the conversion
augmented, as well as the selectivity to acrolein hydrogenation
products. While the second observation is explained in a
straightforward manner by the higher amount of hydrogen
available for the transformation, the first is likely due to the fact
that the acid sites required for the conversion of glycerol into
acrolein are partly saturated by the adsorbed product and become
more available when this is removed through reduction.
Concerning the space velocity, the conversion was shown to
decrease at higher values of this parameter, in line with the lower
contact time between the reactants stream and the catalyst. In
view of the consecutive nature of the dehydration and
hydrogenation steps, the selectivity of the intermediates was
enhanced and the one to the products depleted as a function of
this variable (Figure 6c). Finally, the stability of the 5 wt.% Ag/H-
Z40-haw was tested under the optimized reaction conditions for
100 h on stream (Figure 6d). The conversion and selectivity to
acrolein moderately decreased during the first 24 h, which could
be tentatively rationalized by the fact that part of the strong
Brønsted-acid sites were poisoned with adsorbed species, thus
altering the effective Brønsted/Lewis-acid sites ratio. In view of
the reduced selectivity to acrolein, also the selectivity to allyl
alcohol and propanal slightly decreased, although their ratio was
constant. After this initial drop in performance, the behaviour of
the catalyst remained stable till the end of the run. Microscopic
analysis of the used catalyst revealed that the silver particle size
did not change over the course of the reaction (Figure 4f). The
increase in stability observed in this reaction as compared to the
test with the zeolite only (Figure 2b) is remarkable and
demonstrates that adding a hydrogenation functionality to the acid
catalyst is effective in boosting its resistance. This is likely due to
the reduction of coke precursor molecules produced by
dehydration, as speculated in an earlier work,[7] and is in line with
the lower amount of coke detected on the catalyst after reaction
(1.8 wt.%).
acidic properties and porosity of a commercially available zeolite
and selecting a suitable hydrogenation functionality. With respect
to the aluminosilicate, alkaline treatment proved an easy tool to
introduce an auxiliary network of intracrystalline mesopores, thus
increasing the resistance to coking and providing longer lifetime
in the dehydration of glycerol. Furthermore, a subsequent acid
washing was effective in removing residual Lewis acidity, thus
further raising the selectivity to acrolein. Silver was identified as a
better hydrogenation phase than various noble metals based on
its considerable activity in reducing acrolein and its remarkable
selectivity to allyl alcohol in the presence of molecular hydrogen.
The best catalyst, featuring a 5 wt.% Ag loading onto the zeolite,
led to a selectivity to allyl alcohol of 20% at a glycerol conversion
of 80% under optimized conditions of temperature, pressure, and
contact time. Moreover, it displayed a minor activity loss upon a
100-h run. This was attributed to the ability of silver to
hydrogenate coke precursors, which form on the surface under
the reaction conditions and usually lead to a rapid blocking of the
active sites. Our findings provide new insight into the parameters
to be taken into account in the development of bifunctional
catalysts for multistep biomass transformations, showing the
synergistic effects that might arise from coupling components
required for different reactions.
Experimental Section
Catalyst synthesis
NH4-ZSM-5 with a Si/Al ratio of 40 was obtained from Zeolyst International
(CBV8014, referred to as Z40) and was converted into the protonic form
(H-Z40) by calcination at 823 K for 5 h (5 K min−1). g-Al2O3 (Alfa Aesar,
99.997%) was used as received.
Post-synthetic modification of Z40. H-Z40 was subjected to desilication in
an aqueous NaOH solution (0.3 M, 30 min, 100 cm3 per gram of zeolite) at
338 K in an Easymax™ 102 reactor (Mettler Toledo). The resulting
suspension was filtered and the material obtained was ion exchanged
through three consecutive treatments in an aqueous NH4NO3 solution
(0.1 M, 8 h, 100 cm3 per gram of zeolite) at room temperature. The solid
was recovered by filtration, washed with deionized water (100 cm3 per
gram of zeolite, three times), dried at 338 K for 16 h, and calcined at 823 K
for 5 h (5 K min−1). The sample was denoted as H-Z40-h. An aliquot of this
powder was treated three times with HCl (0.1 M, 8 h, 100 cm3) at 338 K,
filtered, washed with deionized water (100 cm3 per gram of zeolite, three
times), and dried at 338 K. The sample obtained was denoted as H-Z40-
h
aw. H-Z40-h (1 g) was further treated with an aqueous solution of NaBH4
(0.05 M, 10 cm3, Sigma Aldrich, >98%) and dried (338 K, 16 h). This
material was denoted as H-Z40-hNaBH4
.
Metal incorporation. Supported metal (Pt, Ru, Pd, Ag, and Au) catalysts
were prepared by incipient wetness impregnation. The desired amount of
RuCl3∙xH2O (ABCR-Chemicals, 99.9%), Na2PdCl4 (Aldrich-Fine
Chemicals, 98%), AgNO3 (ABCR-Chemicals, 99.9%), K2PtCl4 (Acros
Organics, 99.99%), or HAuCl4∙3H2O (Acros Organics, ≥49.0 wt.% Au) to
obtain a 5 wt.% metal loading was dissolved in deionized water (same
amount as the total pore volume of the support, measured by N2 sorption)
and added to the carrier (475 mg). The resulting powder was mixed
thoroughly, dried at room temperature, and calcined using the protocol
Conclusions
In this study, we uncovered silver nanoparticles supported onto a
hierarchical ZSM-5 zeolite as an efficient bifunctional catalyst for
the continuous dehydration-hydrogenation of glycerol into allyl
alcohol in the gas-phase. The material was designed tuning the
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described above. Ag/H-Z40-haw catalysts containing 1, 2, and 10 wt.% of
the metal were additionally prepared following the same procedure.
number of moles of glycerol reacted and the mole of glycerol fed, the
selectivity to product i as the number of moles of product i formed per mole
of glycerol reacted, and the yield of product i as the product between
glycerol conversion and selectivity to product i. The carbon balance was
calculated as the ratio between the number of moles of carbon in the
condensate and the number of moles of carbon fed. The experimental
error, determined on the basis of three repetitions under selected testing
conditions, was within ±5%.
Catalyst characterization
The metal content in the catalysts was determined by X-ray fluorescence
spectroscopy (XRF) using a Orbis Micro XRF instrument equipped with a
Rh source operated at 35 kV and 500 µA. Powder X-ray diffraction (XRD)
was performed using a PANalytical X’Pert PRO-MPD diffractometer with
Ni-filtered Cu Ka radiation (l = 0.1541 nm), acquiring data in the 10–60°
2q range with an angular step size of 0.05° and a counting time of 2 s per
step. The crystallinity of the zeolite in the catalysts was estimated from the
ratio between the area of selected reflections (23.1, 24.0, and 24.4° 2q) of
the modified and parent samples. N2 sorption at 77 K was conducted using
a Micromeritics TriStar analyzer. Prior to the measurements, the solids
were degassed at 573 K under vacuum for 3 h. Fourier transform infrared
spectroscopy (FTIR) of adsorbed pyridine was conducted using a Bruker
IFS66 spectrometer equipped with a mercury-cadmium-telluride (MCT)
detector. Wafers (ca. 1 cm2, 20 mg) were degassed at 693 K under
vacuum for 4 h, cooled to room temperature and exposed to pyridine
vapors (Sigma-Aldrich, >99%). Thereafter, they were evacuated at room
temperature (15 min) and at 473 K (30 min). Spectra were recorded in the
4000-1300 cm–1 range by accumulation of 32 scans with a resolution of
Acknowledgements
This work was funded by the Swiss National Science Foundation
(Projects 200020-159760 and 200021-169679). The Scientific
Center for Optical and Electron Microscopy at ETH Zurich,
ScopeM, is acknowledged for the use of its facilities. Ms. E.
Vorobjeva is thanked for the STEM analyses.
Keywords: allylic compounds • glycerol valorization •
hierarchical zeolites • hydrogenation • silver
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Entry for the Table of Contents
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Giacomo M. Lari, Zupeng Chen, Cecilia
Mondelli*, Javier Pérez-Ramírez*
Page 1 – Page 8
Bifunctional
Hierarchical
Zeolite-
Supported Silver Catalysts for the
Conversion of Glycerol to Allyl
Alcohol
The conversion of glycerol into allyl alcohol is efficiently catalyzed by a bifunctional
catalyst comprising silver nanoparticles supported on a hierarchical ZSM-5 zeolite.
This material combines the high stability of the zeolite and the remarkable selectivity
of the metal for the dehydration and hydrogenation steps, respectively.
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