Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

Подождите немного. Документ загружается.

10.4 Epoxidation Reactions 345

Figure 10.6 (a) Transmission electron microscopy image of

the Au/TiO

contact interface; (b) A schematic representation

of the interface. Reproduced with permission from Ref. [97] ;

strong contact with the support at highly dispersed isolated tetrahedral Ti

sites.

Moreover, this catalyst also provided a very high PO selectivity ( > 99%) in a gas

phase containing O

and H

, at ambient pressure. The same group also demon-

strated the formation of PO over an Au - based catalyst to be a typical structure -

sensitive reaction [97] , with the selective production of PO being catalyzed only

by hemispherical Au particles of a suitable size (2 nm < diameter Au nanoparti-

cle < 10 nm). In more recent investigations, these authors also showed an organi-

cally modiﬁ ed mesoporous titanosilicate to be an efﬁ cient support [98] , providing

a reasonably efﬁ cient H

consumption, high yields, and PO selectivities in

excess of 90% [99] .

These studies initiated a growing research interest from both industry and

academia [100 – 103] , with one of the more remarkable investigations being con-

ducted by Hutchings and coworkers [102] . This group used catalytic amounts of

peroxides to initiate the oxidation of alkenes with O

, and making it unnecessary

346 10 Oxidation Catalysis by Nanoscale Gold, Silver, and Copper

to sacriﬁ ce H

in order to activate the O

. Here, Au/graphite was found to be very

active in catalyzing the expoxidation of cyclohexene, styrene, cis - stilbene, and cyclo -

octene, even in a solvent - free system. Although the selectivity could be increased

by using the correct solvent (e.g., toluene), the environment - friendly expoxidation

in solvent - free systems would be much more attractive to the chemical industry

and is certain to become another hot topic of research in the near future.

10.4.2

Silver Catalysts

Silver is considered to be an almost uniquely effective catalyst for heterogeneous

epoxidation reactions, and the mechanism of the epoxidation of ethylene with

oxygen over a silver catalyst has been the subject of extensive investigation [104 –

111] . However, despite such numerous studies and its wide use, a number

of questions remain unanswered regarding this catalytic system, including: “ How

do the supports and promoters affect the reaction? ” ; “ What is the mechanism

of the primary and secondary reactions? ” ; and “ What relationship exists between

the electron and structure factors? ” The interaction of oxygen with metal surfaces

has been suggested as one of the most important elementary steps in heterogeneous

catalysis, and several reviews of oxygen adsorption, active oxygen species, promoter

effects and reaction mechanisms on silver catalysts have been produced [112] . The

oxygen species on silver were found to play a key role in ethylene epoxidation, and

extensive studies have been performed to establish details of the interaction of

oxygen with silver surfaces, namely whether the chemisorbed oxygen is atomic or

molecular [113] . Details of surface molecular, surface atomic, subsurface atomic,

and bulk atomic oxygen species have each been reported in the literature.

Signiﬁ cant efforts to improve selectivity have included the use of different silver

precursors, of different preparation techniques, and of different promoters. Such

information is gathered following the continuous addition of a chlorine - containing

hydrocarbon species to the gaseous reactants as a moderator, which also acts to

depress the overall reaction rates. Campbell reported that small amounts of pro-

moters (e.g., Cl) would increase the ethylene oxide selectivity [114, 115] . For an

industrial oxidation of ethylene, alkali metal ions are an important additive when

using a silver catalyst, and alkali or alkaline earth promoters (e.g., cesium) can

provide further substantial improvements [116 – 118] . Campbell [116] reported the

role of a cesium promoter in silver catalysts for the selective oxidation of ethylene.

The oxidation of ethylene in solution, catalyzed by polymer - protected silver col-

loids, and the promotion effect by alkali metal ions on colloidal silver catalysts,

have also been studied [119] . Colloidal dispersions of silver nanoclusters, when

protected by poly(sodium acrylate), caused increases in the rate of oxidation, the

reaction temperature and also the catalytic activity following the addition of Cs(I)

and Re(VII) ions [120] .

The inﬂ uence of silver nanoparticle size on catalytic activity is due not only to an

enhanced surface area but also to particular electronic properties, which differ from

those of bulk silver. The effect of silver particle size on the reaction rate is a well -

10.5 Selective Oxidation of Hydrocarbons 347

known property of supported silver catalysts [121, 122] . For example, when monitor-

ing the distribution of Ag supported on alumina, silver particles of 30 – 70 Å on

alumina or silica showed higher activities. Previously, small silver clusters had been

shown to be the most effective, probably due to the electronic and other properties

of silver (atomic environment, electron work function, electric conductance, etc.),

and differed considerably from bulk silver in this respect. Although the majority of

studies have used bulk silver samples, uncertainty remains as to the nature of the

active sites for ethylene epoxidation on a commercial catalyst. Enlargement of the

silver particles has been found to decrease the amount of subsurface oxygen, and

result in the appearance of nucleophilic oxygen. These ﬁ ndings have been used to

provide a possible explanation for the size effect in ethylene epoxidation over Ag/

catalysts (Figure 10.7 ) [123 – 126] . The kinetic study and shape - controlled cata-

lytic epoxidation of oleﬁ ns by these nanoparticles on several supports such as α -

, CaCO

and spherical particles of TiO

(all obtained using the St ö ber method)

were investigated for a non - allylic oleﬁ n (e.g., styrene) and for an allylic oleﬁ n (e.g.,

propene), using molecular oxygen and N

O as oxidants.

Silver supported on titania was found to be active for propene epoxidation using

hydrogen/oxygen mixtures at 50 ° C [127, 128] . The direct aerobic oxidation of

various alkenes catalyzed by H

polyoxometalate - stabilized silver nano-

particles supported on Al

showed a higher selectivity towards epoxide in the

liquid phase [129] . Styrene represents a useful alkene model for studying the reac-

tion mechanism of terminal alkene epoxidation; the size and morphology of the

nanoparticles was reported to affect the catalytic behavior of silver catalysts sup-

ported on α - Al

and MgO, in the selective oxidation of styrene in the gas phase

[130 – 134] . The epoxidation of styrene to its oxide by molecular oxygen was studied

using a Cs - loaded silver nanowire catalyst, and resulted in the desired product with

greater selectivity.

10.5

Selective Oxidation of Hydrocarbons

The selective oxidation of alkanes with molecular oxygen represents a major chal-

lenge [135, 136] , as the production of the more valuable oxidized products relative

to the low cost of the raw materials is of economic interest. The chemical inertness

of hydrocarbons makes the activation of C – H bonds especially difﬁ cult, and usually

requires dramatic reaction conditions such as high temperature and pressure. The

oxidation of cyclohexane is of special interest industrially, because the process

produces KA - oil as an intermediate; this is a mixture of cyclohexanone and

cyclohexanol that is important in the petroleum chemical industry. KA - oil is used

for the production of adipic acid and ε - caprolactam, which are key materials in the

respective manufacture of 6,6 - nylon and 6 - nylon [137] .

Modern industrial methods typically require both high pressure and tempera-

ture when using a soluble cobalt catalyst. A high selectivity for cyclohexanone

and cyclohexanol can only be achieved at a low conversion, as these products are

348 10 Oxidation Catalysis by Nanoscale Gold, Silver, and Copper

Figure 10.7 (a) Conversion of ethylene; (b)

Selectivity and conversion of Ag on γ - Al

with catalysts prepared by different methods:

A, precipitation; B, modiﬁ ed precipitation; C,

water – alcohol; D, microemulsion silver

loading (7.5 – 8.6%). Reproduced with

B.V., Amsterdam.

10.5 Selective Oxidation of Hydrocarbons 349

substantially more reactive than the cyclohexane reactant. Thus, under mild condi-

tions it is difﬁ cult to achieve, simultaneously, a high conversion and selectivity.

Whilst it is desirable to identify a good catalyst in order to activate the reaction

between oxygen and cyclohexane, the technology by which cyclohexane is oxidized

by O

to produce KA - oil has not yet been improved [138] . In an effort to reduce

the use of environmentally harmful elements, heterogeneous catalysts prepared

by immobilizing Mn

III

, Co

III

, Cr

III

and Fe

III

ions on metal oxides have been

developed for the oxidation of cyclohexane [139 – 141] , although these systems were

subsequently found to suffer from leaching under the reaction conditions used.

Mesoporous materials were reported as efﬁ cient support materials because in

the pores, cyclohexane is oxidized more readily than cyclohexanol, such that the

selectivity is enhanced.

10.5.1

Gold Catalysts

Zhao and coworkers were the ﬁ rst to apply gold nanoparticles for this application,

and reported that a supported gold catalyst could activate cyclohexane at 150 ° C,

with selectivities in the region of 90% [142, 143] . Under similar reaction condi-

tions, Kake Zhu and coworkers found that gold nanoparticles immobilized by a

variety of methods in the channels of SBA - 15 showed a good performance in cata-

lyzing the aerobic oxidation of cyclohexane in a solvent - free system (Figure 10.8 )

[144] , with the highest conversion being reported as 32%. In order to enhance

selectivity, the aerobic oxidation of cyclohexane was performed below 100 ° C and

catalyzed by supported Au, Pt, and Pd catalysts [145] . Although, the selectivity for

cylohexanone and cyclohexanol was found to decline rapidly with the enhanced

conversion and longer reaction times, the gold catalysts provided an identical

performance to the Pt and Pd catalysts.

Figure 10.8 Representative transmission electron microscopy

images. (a) Au – APS/SBA - 15; (b, c) Au – SH/SBA - 15 at low

(b) and high (c) magniﬁ cation. Reproduced with permission

350 10 Oxidation Catalysis by Nanoscale Gold, Silver, and Copper

10.5.2

Silver Catalysts

Silver catalysts have also found use in hydrocarbon oxidation reactions. Since

the granting of the ﬁ rst patent in 1931 for the manufacture of ethylene oxide with

an Ag catalyst [146] , the industrial production of ethylene oxide by direct oxidation

in gas phase has become widely used, with ethylene being converted into ethylene

glycol or a variety of other derivatives. In fact, the selective oxidation of ethylene

over a supported material represents one of the few uses of silver as an industrially

important catalyst, whilst also providing fundamental interests in the surface

sciences [112, 147] .

Silver has, however, rarely been considered as a catalyst for the selective oxida-

tion of saturated hydrocarbons. Yet, nanoscale silver supported on MCM - 41 for

the liquid - phase oxidation of cyclohexane was found to be an effective catalyst

in the absence of a solvent [148] , with a higher turnover number and improved

selectivity compared to other support systems of Ag/TS - 1 and Ag/Al

10.5.3

Copper Catalysts

Copper nanoparticles supported on natural zeolites of different structure and

origin have been utilized for the complete oxidation of hydrocarbons over a tem-

perature range of 170 to 250 ° C. Although a complete conversion was reported,

this was shown to depend on the Si/Al ratio of the zeolite matrix and the different

nanospecies of copper present [149] .

10.6

Oxidation of Alcohols and Aldehydes

The oxidations of alcohols and polyols are important processes in industrial chem-

istry, and it is not surprising that a signiﬁ cant effort is currently being expended

within the scientiﬁ c community to improve present - day technologies, and in par-

ticular to create processes that are more “ green. ” The objective of these research

investigations has been the development of a catalytic system that would compete

against a stoichiometric approach involving concentrated and toxic oxidizing

agents. Supported platinum and palladium catalysts are well known as effective

catalysts for the oxidation of polyols under acidic or basic conditions. Yet, sup-

ported gold nanoparticles were also found to be very effective for the oxidation of

alcohols, including diols, in the presence of a base [150 – 157] . These catalysts could

also be used in the oxidation of sugars, glucose, and sorbitol [158, 159] . When

using dioxygen as the oxidant, Carrettin et al . reported a 100% selectivity for the

oxidation of glycerol to glycerate, this being catalyzed by an Au catalyst supported

on graphite under relatively mild conditions, and with yields approaching 60%

[160 – 162] . In these studies the presence of a base was also found to be essential

for both activity and selectivity.

10.6 Oxidation of Alcohols and Aldehydes 351

10.6.1

Gold Catalysts

It has been commonly observed that the oxidation of alcohols catalyzed by

supported gold catalysts show better performances under basic conditions in terms

of high selectivities and reasonable activities, although the mechanism behind this

chemistry remains unclear. Another interesting observation is that the types of

support used for the gold catalysts, which have important roles in the oxidation of

CO and epoxidation, appear to have little effect in the oxidation of alcohols. In fact

Rossi and coworkers [163] , when monitoring the oxidation of glucose to gluconic

acid, found that unsupported colloidal AU particles were equally active as Au nano-

particles supported on active carbon, under the same conditions. A subsequent

study extended the application of colloidal gold catalysts to the oxidation of 1,2 - diols

[164, 165] . The disadvantage of an unsupported colloidal gold catalyst is its poor

long - term performance compared to a supported gold nanocatalyst, due to prob-

lems of aggregation. Recently, gold nanoclusters stabilized by polymers [166]

showed good activity in the aerobic oxidation of benzyl alcohol in aqueous media.

However, Corma and coworkers [167, 168] proposed that the support of Au/CeO

catalysts could help to stabilize a reactive peroxy intermediate from O

, and thus

could enhance the activities of gold catalysts in the selective oxidation of alcohols

to aldehydes/ketones, and of aldehydes to acids. One further interesting point here

was that the gold catalysts functioned well in solvent - free systems without any

additional base, a point which differed considerably from earlier ﬁ ndings.

One of the most signiﬁ cant advances in the ﬁ eld of alcohol oxidation, provided

by the group of Hutchings [169] , indicated that an Au/Pt alloy supported on TiO

was a highly active catalyst for the oxidation of benzyl alcohol, cinnamyl alcohol,

and vanillyl alcohol. In particular, for the oxidation of benzyl alcohol, the perform-

ance of the Au - Pd/TiO

catalyst was remarkably superior to that of Au/TiO

and

Pd /TiO

in terms of both conversion and selectivity (Figure 10.9 ).

10.6.2

Silver Catalysts

The silver - catalyzed partial oxidation of methanol to formaldehyde, which is an

industrially important chemical transformation of alcohols to carbonyls in the gas

phase, and is signiﬁ cant in the synthesis of drugs, vitamins, fragrances, and many

complex syntheses, was ﬁ rst employed on an industrial scale by BASF AG in 1905

[170, 171] . Since that time, several silver - based catalysts, including bulk and sup-

ported systems, have been developed for the oxidation of alcohols. The main

problems encountered problems with bulk silver are low catalytic activities at low

temperatures, or the products of cracking and/or overoxidation at higher reaction

temperatures. Various attempts have been made to improve the catalytic perform-

ance of silver - based catalysts, either by adding additives to the bulk silver catalysts

or by dispersing the silver particles on supports [172 – 175] . Supported silver would

be expected to enhance the dispersion and stability of silver, and thus also enhance

its catalytic activity at relatively low temperatures. The design of a catalyst combin-

352 10 Oxidation Catalysis by Nanoscale Gold, Silver, and Copper

Figure 10.9 (a) Benzyl alcohol conversion and

selectivity in benzaldehyde with the reaction

time at 373 K and 0.1 MPa p O

. Squares

indicate Au/TiO

; circles indicate Pd/TiO

;

triangles indicate Au – Pd/TiO

. Solid symbols

indicate conversion, open symbols indicate

selectivity; (b) Au – Pd/TiO

- catalyzed reactions

at 363 K, 0.1 MPa p O

, for cinnamyl alcohol

(squares) and vanillyl alcohol (circles). Solid

symbols indicate conversion, open symbols

indicate selectivity to the corresponding

aldehydes. Reproduced with permission from

Cambridge.

ing the advantages of both bulk silver catalysts (conventional electrolytic silver)

and supported silver catalysts, with better performance, remains a major research

challenge in the area of alcohol catalytic oxidation [176] .

Supported platinum and palladium nanoparticles are generally acknowledged

as effective catalysts for the oxidation of polyols [177] . In situ electrolytic nanosil-

ver/zeolite ﬁ lm/copper grid catalysts have shown higher catalytic oxidation proper-

ties towards mono, di, and other types of alcohols, and with higher selectivities

10.7 Direct Synthesis of Hydrogen Peroxide 353

[176, 178] . The silver nanoparticles, when generated in situ , highly dispersed on

the zeolite ﬁ lm and then precoated on a copper grid, demonstrated excellent activi-

ties towards polyhydric alcohol at low temperature. Such performance effectively

avoids the problems of both overoxidation and C – C bond cracking at high tem-

peratures, or mild oxidation at low temperatures (as in the case of the practical

electrolytic silver catalyst). Au – Ag alloy clusters (size range 1.6 – 2.2 nm) with

various Ag contents (5 – 30%) and prepared using a coreduction method in the

presence of poly( N - vinyl - 2 - pyrrolidone), were investigated in the aerobic oxidation

of p - hydroxybenzyl alcohol as a model reaction to understand the effect of Ag on

the catalytic activity of Au clusters [179] . The rate constants per unit surface area

for the Au – Ag:PVP clusters with a small Ag content ( < 10%) were larger than those

of monometallic Au:PVP clusters of comparable size. Spherical nanoparticles

anchored on the external walls of a multiwalled carbon nanotube (Ag/MWNT)

composite electrode exhibited a high catalytic activity for the electro - oxidation of

methanol [180] .

10.7

Direct Synthesis of Hydrogen Peroxide

As noted above, there has been much recent interest in the design of new hetero-

geneous catalysts for selective oxidation under ambient conditions, and these typi-

cally use hydrogen peroxide as the oxidant [96] . At present, hydrogen peroxide is

produced by the sequential hydrogenation and oxidation of alkyl anthraquinone,

with the annual global production approximating 1.9 × 1 0

tons. However, a

number of problems are associated with the anthraquinone route, including the

cost of the quinone solvent system and a periodic need to replace the anthraqui-

none because of hydrogenation. In view of this, considerable interest has been

expressed in the direct manufacture of hydrogen peroxide from the catalyzed reac-

tion of hydrogen and oxygen. At present, a degree of success has been achieved

using Pd as a catalyst, especially when halides are used as promoters. Typically,

dilute solutions of hydrogen peroxide are produced, with earlier studies indicating

that the Pd catalyst could be combined with an oxidation catalyst, TS - 1, such that

the hydrogen peroxide produced could be used in situ . To date, however, no com-

mercial process exists for the direct manufacture of hydrogen peroxide. Hutchings

and coworkers were the ﬁ rst to show that Au/Al

catalysts were effective for the

direct reaction, and thus far the best catalysts identiﬁ ed have been Pt – Au alloy

supported catalysts (Table 10.2 ); these showed a better performance than pure Pd

or Au catalysts [181, 182] . The suggestion was made that the enhanced activity

observed by the addition of Pd to Au was due to an enhanced activation of hydro-

gen. However, if too much Pd was added, the decomposition activity of the hydro-

gen peroxide was also enhanced, such that the rate declined.

Subsequently, Ishihara et al . [183] have shown that Au/SiO

and Au – Pd/SiO

catalysts are also effective for this reaction at only 10 ° C. In recent studies, Hutch-

ings and coworkers have shown that the selectivity for H

utilization could be

354 10 Oxidation Catalysis by Nanoscale Gold, Silver, and Copper

Table 10.2 Formation of H

from the reaction of H

/ O

over A u and P d catalysts [182] .

Catalyst

Solvent

Temperature

( ° C)

Pressure

(MPa)

(molar ratio)

mmol g (catalyst)

− 1

Au/Al

C H

OH 2 3.7 1.2 1530

Au : Pd (1 : 1)/

OH 2 3.7 1.2 4460

Pd/Al

C H

OH 2 3.7 1.2 370

Au/ZnO scCO

35 9.2 1.0 9

Au : Pd (1 : 3)/

ZnO

scCO

35 9.2 1.1 7

Au : Pd (1 : 1)/

ZnO

scCO

35 9.2 0.8 12

Au : Pd (3 : 1)/

ZnO

scCO

35 9.2 0.9 8

Pd/ZnO scCO

35 9.2 1.3 0

a scCO

= supercritical CO

signiﬁ cantly enhanced when Fe

and TiO

were used as supports [184, 185] .

Indeed, with short reaction times the selectivity may exceed 95% for the reaction

of dilute H

mixtures (1 : 1; 5 vol%) diluted with CO

(95 vol%). Very high rates

of reaction were observed with noncalcined Au – Pd/TiO

catalysts, but these proved

to be unstable as they lost both Au and Pd during the reaction and could not be

successfully reused. However, if the catalysts were calcined at 400 ° C prior to use,

very stable reusable materials were obtained. Detailed structural investigations of

these active stable catalysts, using X - ray photoelectron spectroscopy ( XPS ) and

transmission electron microscopy ( TEM ), showed that the catalysts have a core –

shell structure with a gold - rich core and a palladium - rich shell. It was concluded

that the Au was acting as an electronic promotor for the Pd - rich surface of the

Au – Pd nanocrystals.

10.8

Conclusions

Although the catalytic processes described in this chapter have provided a good

overview of the oxidation catalysis properties of Group 11 metals, this is a far from

comprehensive survey of the ﬁ eld. However, this group of metals has provided