(solution) literature review summary of the project "Selective

(solution) literature review summary of the project "Selective

literature review summary of the project ?Selective hydrogenation of acetylene?. It?s 8-12 pages double spaced with font size of 12. The reference section will single spaced.  Review examples are attached.

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Review
pubs.acs.org/acscatalysis Modern Trends in Catalyst and Process Design for Alkyne
Hydrogenations
Micaela Crespo-Quesada, Fernando Ca?rdenas-Lizana, Anne-Laure Dessimoz, and Lioubov Kiwi-Minsker*
Group of Catalytic Reaction Engineering, E?cole Polytechnique Fe?de?rale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
ABSTRACT: This review provides an overview of the recent
achievements in catalytic process development for alkyne hydrogenations. It underlines the necessity of simultaneous optimization
over di?erent length scales from molecular/nanoscale of active
phase, up-to macro-scale of catalytic reactor design. One case
study, the hydrogenation of 2-methyl-3-butyn-2-ol, is analyzed in
detail to illustrate the practical application of this approach. Finally,
it presents the personal view of the authors concerning the new
trends and paths available in the ?eld.
KEYWORDS: alkyne, hydrogenation, catalyst design, support e?ect, reactor design 1. INTRODUCTION
Sustainable processing with minimal environmental impact has
been recognized as one of the major challenges of this century.1
As a result of the severe restrictions in environmental
legislation, the chemical industry is now undergoing a
progressive rede?nition. Catalytic technology, as a fundamental
tool for green chemistry, has an unprecedented enabling
potential for sustainable production. Heterogeneous catalysts
are of utmost importance in the ?ne chemical industry. The
typical design in these systems is based on an active phase, main
responsible for the catalytic performance (activity and
selectivity), immobilized on a suitable support. This avoids
agglomeration of the active species during chemical reaction
and enables an easy catalyst recovery. The conventional
methodology applied for catalyst optimization has been an
empirical ?trial-and-error? approach which results, at best, in
slight or incremental improvements of their performance.
Moreover, it is greatly based on speculation and is, in practice,
laborious and time-consuming. With the concomitant advance
in theoretical understanding and the development of computational power, a new era of rational catalyst design (RCD) is
dawning.2 This approach is based on a multidisciplinary
combination of new advances in synthesis, characterization,
and modeling with the ultimate aim of predicting the catalyst?s
behavior based on chemical composition, molecular structure,
and morphology.3
Given the multicomponent nature of an heterogeneous
catalyst, it is necessary to bear in mind that its overall
performance depends not only on the contribution of the active
component, but also on other factors such as the interplay
between the catalytically active species and the surrounding
environment, and the type of chemical reactor where the
process is carried out (Figure 1). Therefore, RCD must span
over multiple levels of complexity, from the molecular or
nanoscale involving the design of the active sites to the macroscale design of the industrial reactor where the catalyst is bound
© 2012 American Chemical Society Figure 1. Rational catalyst design spans over several levels of scale and
complexity.2 to operate, since they are in direct interaction in?uencing the
overall performance.
The catalytic partial hydrogenation (semi-hydrogenation) of
alkynes is of special relevance in the bulk and ?ne chemical
industries4 since it is an e?cient method for production of
alkenes. This process is conventionally performed over
quinoline-promoted CaCO3 supported Pd catalyst partially
poisoned with lead, which is known as Lindlar?s catalyst.5,6
However, the application of this conventional catalytic system
to alkyne hydrogenations is sometimes problematic because of
selectivity issues (overhydrogenation to alkanes), limited
catalyst robustness, and reuse. Moreover, catalyst deactivation,
the presence of toxic lead, and the need for the addition of an
Received: May 4, 2012
Revised: June 29, 2012
Published: July 11, 2012
1773 dx.doi.org/10.1021/cs300284r | ACS Catal. 2012, 2, 1773?1786 1774 10?100; 0?100
100; 85
2?100; 0?100 1 ; 278?323 1 ; 348
1; 348
0.9 × 10?3?1 ; 308?373 0?100; 0?92
5?100; 30?100 1 ; 373?523
1 ; 423?523
3?100; 20?99.5
65?69; 40?74
58?88, 10?100
3?100; <5?90
35?100; 25?60 100;
24
15?80 ; 25?98 10?100 ; 48?100
0?63; 18?100 1 ; 273?298
1 ; 363?623 1 ; 363?553
0.06?0.20; 293?333
0.07?0.11; 313?493
1; 255?293
1 ; 373?513 5?100; 2?98 100; 25?80
50; 49?76
0?100; >95 1 ; 523
1 ; 298
1 ; 283?353 1 ; 423 10?25 ; 35?85 1 ; 343?423 <5?99; 60?85 <5?100; 0?100 1?16 ; 303?453 1 ; 298?613 0?100; 45?100 X (%); SCC (%) 1 ; 298?673 P (bar) ; T (K)
powder vs [structured] support(s) reactor 26 24,25 17
18
19?23 14?16 11?13 27,28
29,30 ?xed-bed microreactor
?xed-bed microreactor
continuous-?ow microreactor
(CO) ?-Al2O3; CaCO3; HT
CaCO3 (Lindlar)
?-Al2O3; ZnO; SiO2; Al2O3; CNTh Pd; Pd?Pb; Cu; Ni ; Cu?Ni
Pd?Pb
Pd black; Pd ?-Al2O3 Pd-Xg 43
43
44?47 42 ?ow reactor Al2O3; SiO2
SiO2 d 34
35
36
37?39
40,41 dynamic ?ow
static constant volume system
static constant volume system
glass ?ow
?xed-bed ?ow microreactor
SiO2
pumice 31,32
17,33 ?xed-bed microreactor
?xed-bed microreactor Ni?Al ; Cu
Cu ; Cu?Fe; Cu?Al ; Cu?
Ni?Fe; Cu?Ni?Al
Cu
Ni
Ni; Cu; Ni?Cuf
Pt ; Pt?Ru
Cu ZrO2; SiO2; Al2O3
SiO2; MgO; (?-,?)Al2O3; ZrO2; TiO2;
Fe2O3; SiO2?Al2O3
HTd
HTd; SiO2; Al2O3 ref.
8?10 pulse-?ow reactor
dynamic ?ow/pulse-?ow reactor Pd ; Pt
Cu ; Auf
(CO) ?ow reactor ?-Al2O3 Au ICe; Al2O3 d [CNF; ACF]c Pd?Gae ; Pd ; Pd?Age gas circulation system/single pass ?ow
reactor/?xed-bed microreactor
?ow reactor/microreactor HT
active carbon
pumice; SiO2; Al2O3; MgAl2O4 TiO2; Al2O3 Al2O3; ?-Fe2O3; CeO2 Cu?Fe;Cu?Ni?Fe
Pd
Pd ; Ni?Zn ILa, PVPb,
KBr stabilizer
(modi?er) meso-level/macro-level pulse-?ow reactor/jacketed tubular ?xed-bed
reactor
?xed-bed microreactor
nongradient ?ow
gradientless microreactor/tubular ?xed-bed
reactor
pulse-?ow reactor Au; Ag ; Pd; Pd?Au; Pd?Ag;
Pd?Cu
Pd Au metal nanolevel a IL = ionic liquid. bPVP = poly(vinylpyrrolidone). cCNF = carbon nano?bers, ACF = activated carbon ?bers. dHT = hydrotalcite. eIC = intermetallic unsupported compound. fcatalysts provided by the
World Gold Council. gX = Ge, Sb, Sn, Pb. hCNT = cabon nanotubes. 1-Pentyne but-1-yne; 1-butene3-yne
2-methyl-1-buten-3yne propyne+propadiene
but-1-yne acetylene and
acetylene+ethylene
phenylacetylene
+Styrene
propyne acetylene+ethylene acetylene reactant(s) reaction Table 1. Compilation of Literature on Gas-Phase Alkyne Hydrogenation ACS Catalysis
Review dx.doi.org/10.1021/cs300284r | ACS Catal. 2012, 2, 1773?1786 [100]
40?100 ; 85?100
10?99 ; 77?99 [>95]
96?100 ; 81?97
22?48h
95 ; 97.5
80 ; 15?59 [94]
100 ; 91?99 100 ; 65?85 2?10 ; 308?348 1 ; 328?337 5 ; 333
2?6.7 ; 308?343
5 ; 343
1?6 ; 303?323
2.4 ; 323 1775 15 ; 80
8?100 ; 83?91 1 ; 283
7 ; 333 metal PVPc, Surfactants, ILi (Rh, Ru,
Ag, Cu, Pb)
PVPc, Na2MoO4, AOTa, CTABe
(S-CCl, quinoline)
(pyridine) Pt
Rh (Sn)
PVPc (PPh3) reactor semibatch microreactor
semibatch vibration reactor
semibatch silica, [monoliths], CNFd, SiO2
graphite oxide, C, HTp, Al2O3, Montmorillonite
C, CNTq, ACr [Si foam] ?-Al2O3, MCM-41f
Montmorillonite
Nylon, C semibatch
Fisher?Porter bottle microreactor semibatch
SBCRm
semibatch
semibatch
semibatch multibatch
membrane semibatch
semibatch
semibatch
semibatch Fisher?Porter
bottle vibration reactor
semibatch semibatch
semibatch
vibration reactor TiO2
[ZnO/SMF]d
CaCO3 (Lindlar)
?-Al2O3, C
– CaCO3, C C, SiO2, Al2O3, TiO2
TiO2, AMMSiTig
TiO2, Al2O3, CaCO3
CaCO3 (Lindlar)
C, CaCO3, Al2O3, CeO2, SBA-15j, MCM-48j,
MSU ?-Al2O3k, graphite oxide
[ZnO/SMF]d, ZnO CTABe
PVPc
PVPc
phenanthroline, PVPc (Bi, Pb) powder vs [structured] support(s)
[CNF/SMF]d, Al2O3,
CaCO3 (Lindlar)
MCM-41f stabilizer (modi?er) meso-level/macro-level

AOTa, bipyb, PVPc (Bi, Pb) Pd PdZn
Pd
Pd
Pd
PEO-b-P2VPn
Pt, Rh, Ru, Pd PVPc
and Ni
Pd
(Cu, quinoline, KOH)
Pd
CTABe, SDSo ((nC4H9)4NBH4)
Pd Pd PdOxHy
Pt
Pd
Pd
Pd Pd
Pd
Pt nanolevel ref. 84
85 78?83 72?75
59,76?78 67
68
62
69,70
71 61?66 53
54,55
50,51,56
50
51,57?60 48?51
49
52 f a AOT = sodium di-2-ethylhexylsulfosuccinate. bbipy = bipyridine. cPVP = poly(vinylpyrrolidone). dCNF = carbon nano?bers, SMF = sintered metal ?bers. eCTAB = cetyltrimethylammonium bromide.
Mesoporous silica. gAMMSiTi = amorphous microporous titania-silica mixed oxide. hInformation not provided. iIonic liquids. jMesoporous silica. kMesoporous gamma alumina. lS-CC = Sulfur containing
compound. mSBCR = staged bubble column reactor. nPEO-b-P2VP = poly(ethylene oxide)-block-poly-2-vinylpyridine. oSDS=Sodium dodecyl sulfate. pHT= hydrotalcite. qCNT=carbon nanotubes.
r
AC=activated carbon. 4?100 ; 10?100 [>95] 1?20 ; 283?333 phenylacetylene 6 ; 95 [54?97]
4?100 ; 95?100 [91?99] 1?2.3 ; 298
1?8 ; 283?303 h 10?100 ; 20?95
7?90 ; 63?99 5 ; 353
1 ; 333?393
1 ; 298
1 ; 298
1?2.8 ; 283?298 X (%) ; SCC (%) [Y(%)]
85?100 ; 98.5
99 ; 87
33?78 ; 96?99 P (bar);T (K) 1?10.5 ; 298?303
10.5 ; 303
1 ; 298 MPY
4-octyne 2-butyne-1,4-diol 3-hexyne 3hexyn-1-ol
MBY 1-hexyne 1pentyne
2-hexyne 1-hexyne reactant reaction Table 2. Compilation of Literature on Liquid-Phase Alkyne Hydrogenation ACS Catalysis
Review dx.doi.org/10.1021/cs300284r | ACS Catal. 2012, 2, 1773?1786 ACS Catalysis Review amine modi?er are the main drawbacks of this catalytic system.7
Therefore, during the past few years a signi?cant number of
publications were devoted to alkyne hydrogenation processes.
Given the requirement of the simultaneous consideration of
di?erent levels for optimum catalytic process design, this review
is focused on the analysis of the di?erent scale lengths (nano,
micro, and macro) applied to the catalytic selective CC to
CC hydrogenation with special emphasis on the literature
published over the past decade.
Instead of merely providing an enumeration of the articles
dealing with catalytic semi-hydrogenation over the di?erent
scale lengths, a representative compilation of studies is given in
Tables 1 and 2 for gas and liquid phase operations, respectively.
The information shown serves to illustrate (a) the range of
reactions that have been investigated; (b) the operating
temperatures and pressures; (c) the catalytic performance in
terms of activity (presented as conversion (X)) and product
distribution (in terms of selectivity (Si) or yield (Y)); (d) the
nature of the catalytic systems that have been investigated at
di?erent scale lengths, that is, nano (metal and precursor/
stabilizer), meso (support), and macro-level (reactor).
Of direct relevance to this work, we should mention the
overview on hydrogenation of carbon?carbon multiple bonds
published by Molna?r and co-workers in 2001,86 which has since
been supplemented by the reports of Borodzin?ki and Bond87,88
on selective hydrogenation of ethyne in ethene-rich streams
over palladium catalysts, and the recent examination of the
theoretical work to elucidate the catalytic properties required
for selective alkyne hydrogenation in mixtures by Lo?pez and
Vargas Fuentes.89 The critical role of a combined (catalyst,
process and reactor) design strategy for optimizing heterogeneous catalysis was illustrated in a review by Sie and Krishna in
1998,90 which has been since covered in more recent
publications.2,91
This Review contains two main parts. First, a critical analysis
of the pertinent literature dealing with catalyst design for
selective CC to CC hydrogenations across the three scale
lengths previously reported is provided and new trends are
underlined. In the second part, a case study, the hydrogenation
of 2-methyl-3-butyn-2-ol (MBY) is presented for illustrative
purposes. Finally, the Review ends with a consideration of
directions for the future. gies.93 Indeed, despite the faster ole?n hydrogenation over Pd
with respect to the acetylenic counterpart, the reduction of the
latter is favored as a result of the increased adsorption strength,
that is, the selectivity has a thermodynamic nature.94
Several factors have been proposed to control a catalyst?s
performance in alkyne hydrogenations involving the di?erent
scale lengths of the catalyst architecture:
? Nanoscale, where the e?ect of the morphology of the
active nanoparticles is considered through the observation of
metal dispersion,14,20,47,48,63,64,79,87 shape e?ects,64,71 and/or
the presence of speci?c types of active sites.61 Surface
modi?cation by the reaction medium is also important at this
stage of optimization.16,44,95
? Mesoscale, where the interactions between the active metal
nanoparticle and its intimate environment is assessed through
the modi?cation by alloying, 46,63 the involvement of
additives,32,49,69,96 and/or metal/support interactions.28,30
? Micro/milli/macro-scale design where the support
structure and morphology are tailored according to the
particular requirements of the reaction in conjugation with
the development of a suitable chemical reactor.68
2.1. Active Phase Optimization: Nano/Meso-Level.
One of the key factors a?ecting the catalytic behavior of the
active phase is the interplay between its properties and the
reacting molecules. Therefore, to achieve a fundamental
understanding of catalytic reactions, surface-science experiments44?47 and theoretical calculations23,33,93,95,97 are required
to provide insights into surface dynamics and the nature of the
adsorbed species. A quintessential work steering in this
direction in which density functional theory (DFT) calculations
are used to identify potential new catalysts for the selective
hydrogenation of acetylene was recently published.23 The
authors proposed Ni?Zn as an optimal alternative formulation
with respect to the conventional Pd-based active phase, which
was subsequently corroborated experimentally. Despite the
undeniable usefulness of such work, the results should be
somehow considered carefully since the formation of oligomers
during acetylene hydrogenation was neglected in their
calculations.
Although surface and theoretical analyses can provide an
additional tool for catalyst optimization, there is still a gap in
terms of expected and obtained response when moving to real
catalytic systems. Nanoparticle morphology has been identi?ed
as a key characteristic linked to the active phase for the
hydrogenation of alkynes where di?erences in catalytic
performance have associated to mechanistic, electronic, and/
or geometric e?ects.98?100 The incorporation of a second metal
or speci?c compound has also proved an e?ective means to
in?uence selectivity and activity in CC hydrogenation.
This section reviews the results of nanoscaled studies for
catalytic alkyne hydrogenations considering ?rst the active
phase alone, particularly the issue of structure sensitivity, and of
active phase modi?cation under reaction conditions. Second,
the modi?cation of the active nanoparticles through alloying
and additives is also reviewed.
2.1.1. Structure Sensitivity. The morphology (shape and
size) of the active phase (metal nanoparticles) is among the
structural features that have a greater impact on catalytic
performance in alkyne hydrogenation.101 In the 1960s it had
become clear that the rate of certain catalytic reactions,
expressed per unit area, or turnover frequency (TOF), was
independent of the metal particle size and were de?ned as
structure-insensitive. On the other hand, if a correlation between 2. MULTI-LEVEL APPROACH FOR CATALYTIC ALKYNE
HYDROGENATION
The selective CC hydrogenation (to the correspondent
ole?n) is an important process in industry for both the
production of intermediates in the manufacture of ?ne
chemicals,63 and in bulk chemistry, for example, ethylene
hydrogenation during the synthesis of polyethylene (global
annual production of 50 million tons92) and the puri?cation of
alkene streams for the upgrade of low weight fractions from
stream crackers.
The open literature on selective alkyne hydrogenation
reports the process carried out in both gas (see Table 1) and
liquid-phase (see Table 2). It can be seen that this type of
reaction has been primarily carried out over powdered catalysts
based on monometallic Pd and, to a lesser extent, Pt-, Ni-, Cu-,
and Au-based catalysts where the last two metals show promise
in terms of achieving high selectivity.9,17,32 Focusing on Pd as
the metal with the best performance for this type of
reaction,5?7 the increased alkene selectivity over Pd can be
associated with the distinct alkyne/alkene adsorption ener1776 dx.doi.org/10.1021/cs300284r | ACS Catal. 2012, 2, 1773?1786 ACS Catalysis Review (nanohexagons vs nanospheres) but dependent on the number
of Pd atoms located on (111) planes.64
We can therefore conclude that an accurate identi?cation of
the active sites responsible for the catalytic performance would
actually imply a redef inition of the terms ?size? and ?shape
e?ects? to ?structure e?ect? arising from the relative amount of
active sites on the nanoparticle surface regardless of the shape
or size of the crystallite. This is illustrated in recent studies for
the hydrogenation of 2-methyl-3-butyn-2-ol where it has been
shown that the dependency between TOF and particle size
disappeared when only Pd111, that is, the active sites for the
reaction, is taken into account.64,65 This ?nding is complemented by our recent work61 where, for the same reaction, we
have shown for a series of Pd nanoparticles with well-de?ned
shapes and sizes (see Figure 2) that two type of active sites, that TOF and metal dispersion could be established, the reaction
was then referred to as structure-sensitive. It is however more
likely that every reaction will show a degree of structure
sensitivity, depending on the stringency of its requirements for
an active center.102 It is worth mentioning that a variation in
shape also implies important morphological di?erences: cubes
only present (100) plane atoms, octahedra solely (111) plane
atoms whereas a mixture of both can be found in cubeoctahedra (sphere). This will provoke another type of
structure-induced e?ect, that is, a shape e?ect, particularly if
each type of surface atom possesses a di?erent reactivity.
The ?rst study tackling the issue of nanoparticle size effects
was published in 1983 by Boitiaux et al.103 Since then, we have
come to realize that a key requirement for structure sensitivity
studies calls for the preparation of catalysts which di?er only in
particle size and/or shape. Typical catalysts are based on
supported metal nanoparticles where size control of the metal
phase has been achieved by modi?cations in the nature of the
precursors, supports or preparation conditions. As a result, not
only size but other important chemical and structural properties
of the catalyst that a?ect catalytic performance are also
modi?ed. Therefore, the early data published on size e?ects
in the selective hydrogenation of multiple carbon?carbon
bonds are rather controversial, although some consensus
emerges pointing toward higher activity for larger particles,
that is, an increase in metal dispersion decreases
TOF.14,18,20,47,48,64,65,79 Small nanoparticles (less than 2 nm)
are characterized by a predominance of surface atoms of low
coordination number characterized by an electron density
de?ciency. The low activity can be explained on the basis of
strong complexation of the surface atoms by the highly
unsaturated electron-rich alkyne. Furthermore, it is known that
Pd can absorb hydrogen at room temperature when the partial
pressure exceeds 0.02 atm resulting in the formation of ?palladium hydride.104 Hydride formation is the result of
hydrogen di?usion in the Pd crystallite structure to occupy
the available octahedral ?vacancies? in the metal lattice. The
relationship between the number of Pd atoms in the bulk
crystal with respect to those on the surface, decreases with
decreasing particle size to attain a limiting value (<2.5 nm),
where Hab/Pd is close to zero.105,106 The in?uence of this phase
in alkyne hydrogenation is still rather controversial with reports
in the literature suggesting that it is responsible for the direct
alkane formation99 while others did not observe this
detrimental e?ect,100 although a recent publication reports
theoretical calculations showing the great importance of
subsurface species in alkyne hydrogenations.95
In terms of Pd nanoparticle shape ef fects, there is limited
work available in the open literature, particularly for alkyne
hydrogenation.107,108 Unlike size, shape control was only
recently achieved in a straightforward manner thanks to
colloidal techniques.107?109 The advances in colloidal preparation of metal nanostructures open new opportunities in the
study of structure-sensitive reactions since they provide
catalytic metal particles with size or shape variation without
other perturbations, thus rendering them excellent materials
suitable for catalytic investigations. For example, Telkar et al.71
concluded a dependency between shape and activity (in terms
of TOF) for cubic and spherical nanoparticles in the
hydrogenation of 2-butyne-1,4-diol. In contrast, in the
hydrogenation of 2-methyl-3-butyn-2-ol catalytic performance
was found to be insensitive to the nanoparticle shape Figure 2. Types of active sites in the hydrogenation of 2-methyl-3butyn-2-ol (MBY). Plane atoms, regardless of their crystallographic
orientation, ?1 and low coordination or edge atoms, ?2. Reprinted with
permission from ref 61. Copyright (2011) American Chemical Society. is, plane and edge atoms, are responsible for the catalytic
performance. Indeed, the existence of two or more di?erent
kinds of active sites responsible for observed size and/or shape
e?ects has not been thoroughly discussed in the literature.102
This fundamental knowledge has a signi?cant potential for
catalyst optimization for industrially important hydrogenations.
The study of the reaction on well-de?ned catalysts allows the
full kinetic description of the system, and it therefore enables
the prediction of the size and shape of the active phase to
maximize catalytic e?ciency.61 Only with the new developments in the nanoscaled architecture of the catalyst with the
blooming of simple and versatile colloidal methods of
nanoparticle preparation were these achievements possible.
Additional complications can arise for reactions in gas
(relative to liquid) phase operation as a result of the more
demanding conditions, for example, increased reaction temperatures, which can in?uence the surface chemistry of the
nanoparticles. Much e?ort has been devoted in this direction to
the investigation of the structure sensitivity of acetylene
hydrogenation as illustrated in Table 1.14,15,110 However, it is
now almost unanimously accepted that the observed di?erences
are linked to the formation of a carbonaceous overlayer on the
surface of Pd.28,39,44,47 This has been con?rmed by both
experimental (XPS44) and theoretical (DFT calculations111)
analyses. It was ?rst suggested that the deposited carbon was
only a selectivity modi?er through site isolation envisaging sites
of di?erent sizes between carbon deposits.20?22,39,112 However,
this has been recently revoked in studies showing the formation
of carbide species in the subsurface region of the crystallite that
prevent the dissolution of hydrogen in the bulk of the
nanoparticle,44,45,47,95 that is, eliminates the source of
unselective hydrogenation (Figure 3).44,47 It must be however
1777 dx.doi.org/10.1021/cs300284r | ACS Catal. 2012, 2, 1773?1786 ACS Catalysis Review 2.1.2.1. Catalyst Modi?ers. The incorporation of stabilizing
agents, for example, surfactants, polymers, and dendrimers107?109,116 (see Table 2) during the preparation process
is required for the stabilization of the inherently thermodynamically and kinetically unstable solutions of nanoparticles
subsequently used for catalysis. Moreover, shape control
implies the use of molecular capping agents which selectively
adsorb to one speci?c crystal plane, thus favoring the addition
of metal on the weakly bonded facet and directing the growth
of the nanoparticle.109 Despite the extensive cleaning
procedures applied to the obtained nanoparticles, it is common
to ?nd traces of the stabilizing and capping agents which
modify the true catalytic behavior of the metal.
The presence of PVP as stabilizer for Pd,16,51 Pt,55 and Rh85
has been associated with improved selectivity toward the target
alkene because of the electronic modi?cations of Pd induced by
N-containing species. This is in good agreement with results
obtained over catalysts permanently modi?ed with bipyridinebased ligands,49 phenanthroline-based ligands,56 and copolymers.69
Surfactant stabilizing agents58,64,117 have also been used in
alkyne hydrogenations where catalytic performance has been
correlated with the charge and the alkyl chain length.58
Equivalent activity was obtained with CTAB and AOT, in the
selective hydrogenation of MBY64 while lower rates were
reported for PVP (relative to AOT)101 and ascribed to the
stronger interaction of the latter wi…