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(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.

I will give you 80 $ and if you do good on it I am willing to give twice more then 80$


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...

 


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