Jackson S.D., Hargreaves J.S.J. Metal Oxide Catalysis

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3.6 The IR Spectroscopy of Adsorbed Probe Molecules for Surface Chemistry Characterization 153

The Oleﬁ n Polymerization Method Oleﬁ ns are very reactive towards the electro-

philic attack of a Br ø nsted acid, and can undergo proton - catalyzed cationic polym-

erization at low temperature. The faster this phenomenon occurs, the stronger the

Br ø nsted acid and the more electron - rich is the oleﬁ nic double bond. The experi-

ment must be performed in a medium – low temperature range (e.g. room tem-

perature) with oleﬁ n pressures of the order of 20 – 200 Torr in order to favor the

oligomerization from the point of view of thermodynamics. The observed polym-

erization rate in our experimental conditions follows the order: 1,3 - butadiene >

isobutene > propene > ethylene [140 – 142] . Actually, ethylene polymerization is (in

the conditions used to evaluate the surface acidity of oxide catalysts) only observed

with very strong Br ø nsted acids, while some weak Br ø nsted acids only allow the

polymerization of butadiene. Additionally, it can be observed that very strong

Br ø nsted acids cause the formation of branched polymeric chains from linear

oleﬁ ns, for example polyisobutene formation from both isobutene and from 1 -

butene and likely also from ethylene [143] . Alumina OH groups, although unable

to cause polymerization of the four butene isomers, cause the polymerization of

1,3 - butadiene.

3.6.3.2 The Br ø nsted Acidity of Simple Metal Oxides

It is evident that the ability to protonate a base depends upon the strength of the

base. The ability to protonate pyridine can be taken as the discriminatory behavior

to deﬁ ne an oxide catalyst as a Br ø nsted acid. Typical ionic oxides, such as alkaline

earth oxides, cupric oxide, zinc oxide, aluminas, gallias, ferric oxides, chromic

oxide (Cr

), titanias, zirconias, ceria and thoria, even when highly hydroxylated,

are not able to protonate pyridine. Also, low - valency typical covalent oxides, such

as silicas (including silicated oxides), germania and boria - containing catalysts, do

not protonate pyridine. Consequently, such oxides are either weakly or non -

Br ø nsted acidic. On the contrary, pyridine is protonated on higher oxidation state

oxides such as vanadia, niobia, molybdena and tungsta, either in bulk form or

when supported. Also, materials containing surface chromic anhydride (CrO

bulk or supported phosphates and sulfated oxides protonate pyridine. This is

associated with the presence of M

O double bonds and the possibility of delocal-

izing the anionic charge after dissociation [101] . Oleﬁ n oligomerization tests agree

with studies performed using protonable bases, showing that surfaces that are able

to protonate pyridine also cause propene oligomerization, while surfaces that

protonate piperidine only, such as alumina, are not able to oligomerize propene

but may cause 1,3 - butadiene oligomerization (Table 3.9 ).

3.6.3.3 The Br ø nsted Acidity of Protonic Zeolites

The strong Br ø nsted acid strength of the bridging OH groups of zeolites is

conﬁ rmed by adsorption of basic probes followed by different techniques. Quite

strong bases such as pyridine are easily protonated, as shown in the spectra on

the left hand side of Figure 3.23 , where the bands of pyridinium ions (1636,

1625, 1547, 1491 cm

− 1

) are strongly predominant after pyridine adsorption on

H - MFI. Weak bases such as nitriles and CO hydrogen bond with these OH

154 3 The Use of Infrared Spectroscopic Methods

groups with a strong shift down of the ν OH mode. In Figure 3.21 , the spectra

of H - MFI before and after low - temperature adsorption of CO are reported. The

band at 3615 cm

− 1

, due to bridging OH groups, shifts down to near 3300 cm

− 1

with a ∆ ν ∼ 315 cm

− 1

, evidencing the strong acidity of these groups. In parallel,

the CO triple - bond stretching shifts up from 2138 cm

− 1

for the liquid like species

to > 2175 cm

− 1

Most data agree that when the Al content is relatively low, the amount of

Br ø nsted sites in zeolites actually strictly depends on Al concentration, according

to theory. The ratio between catalytically active sites and Al ions ranges apparently

from 80 to 100% for highly siliceous extraframework - species - free zeolites.

Many investigations, also using IR spectroscopic methods, have been performed

with the aim of revealing the degree of acid strength of different zeolites. Experi-

mental as well as theoretical data [77] show that besides the interactions of the

functional groups of the probe molecules with the zeolite ’ s Br ø nsted sites, the van

der Waals interactions of other unreactive groups of atoms with the zeolite cavity

walls may be very relevant, and participate in the stabilization of the adsorbed

species [100] . These interactions may vary signiﬁ cantly as a function of the type

of zeolite and the dimension and shape of the cavities as well as the Al and proton

content and the presence of extra - framework species. Also, they depend on the

size and shape of the probe molecule. These “ conﬁ nement effects ” make the cavi-

ties of the single zeolite structures unique solvation and reactivity environments

and play an important role in catalysis by zeolites. They also account for the dis-

crepancies among the acid strengths measured using different probes and differ-

ent techniques.

In the case of zeolites, studies have also been performed to distinguish sites

located at the external surface from those present in the cavities, and also to dis-

tinguish the position in different cavities for zeolites with a complex pore struc-

ture. This can be performed using molecules having different molecular sizes

but similar chemical behavior. Some of the useful molecules are shown in

Scheme 3.6 .

Internal versus External and Extraframework Sites in Zeolite Acid Catalysis: the Use

of Hindered Basic Probes

Catalytically active sites also exist on the external surface

and at the pore mouth of zeolite crystals. These sites are considered to be respon-

sible for unwanted non - selective catalysis. On the other hand, H - zeolites also cata-

lyze reactions of molecules that do not enter the cavities because of their larger

size. So, the external surface of zeolites is certainly active in acid catalysis. Addi-

tionally, the bulk and surface Si/Al compositions of a zeolite could be different

and different preparation procedures can be chosen to modify this ratio.

The external surfaces of H - FER [143 – 145] , H - MFI [146, 147] and H - MOR [147,

148] have been studied by IR spectroscopy of adsorbed hindered aliphatic nitriles,

pyridine [145, 146] , lutidine and aromatic hydrocarbons [149] . Terminal silanols

and Lewis acid sites exist at the external surface of H - FER, H - MFI and H - MOR.

Interestingly, the acidity of the external silanol OH groups of zeolites can be

enhanced with respect to those of silica, appearing similar to those of SA. The very

3.6 The IR Spectroscopy of Adsorbed Probe Molecules for Surface Chemistry Characterization 155

strong bridging Br ø nsted acid sites, however, do not apparently exist at the external

surface, being totally conﬁ ned to the internal surface.

Zeolite catalysts are frequently applied after treatments that tend to increase

their stability and also to further enhance surface acidity and shape selectivity

effects. These treatments, such as steam dealumination, can cause a decrease in

the framework Al content and the release of aluminum - containing species from

the framework. This can contribute to the stability of the framework, but extra -

framework species can also contain additional catalytically active acid sites. These

particles can also narrow the size of the zeolite channels or of their mouths, so

improving the shape selectivity effects. Extra - framework material ( EF ) can also

Scheme 3.6 Probe molecules for the study of

localization of active sites in microporous

materials: AN = acetonitrile; PrN =

propionitrile; IBN = isobutyronitrile; PN =

pivalonitrile (2,2 - dimethyl - propionitrile);

BN = benzonitrile; oTN = ortho - toluonitrile;

DPPN = 2,2 - diphenylpropionitrile; py =

pyridine; 2,6 - lut = 2,6 - dimethylpyridine, 2,6 -

lutidine; 2,6,ditbpy = 2,6, di - tert - butyl - pyridine;

MA = methylamine; DMA = dimethyl amine;

TMA = trimethylamine; pX, mX, oX = para - ,

meta - and ortho - xylene.

156 3 The Use of Infrared Spectroscopic Methods

arise from the preparation or the activation procedure or by addition of other

components by impregnation or ion exchange. The presence of EF material gives

rise to the presence of strong additional bands in the IR OH - stretching spectrum.

In general, IR bands above 3750 cm

− 1

, and in the region 3730 – 3650 cm

− 1

in pro-

tonic zeolites, is attributed to OH groups on EF materials.

Some authors believe that EF material is released at the external surface of zeo-

lites. The use of hindered nitriles, however, demonstrated that the EF material

produced by thermal treatment in H - MOR is in the interior of the side pockets

[150] . Similarly in a sample of H - MFI, EF material was found to be located in the

interior of the channels [150] .

IR Determination of the Location of Acid Sites and of the Diffusivity of Large Molecules

in Protonic Zeolites with Complex Pore Structure

The use of hindered nitriles as

probe molecules can be used to to distinguish Br ø nsted sites located in different

cavities in complex pore zeolites. The framework of the FER zeolite gives rise to

two kinds of channels, one of which is a 10 - membered ring, and the other a smaller

8 - membered ring. H - FER sample shows a single band due to bridging OH groups

centered at 3595 cm

− 1

. Acetonitrile ( AN ) perturbs this band strongly, while propio-

nitrile ( PN ) does not perturb it at all, being unable to enter either type of cavity.

Consequently, we attempted to distinguish the sites in the two channels by using

isobutyronitrile ( IBN ) as a probe (Figure 3.24 ). Upon IBN adsorption, the band of

Figure 3.24 FTIR spectra of H - FER zeolite (Zeolyst, Si/Al =

27.5) (a) after activation; (b) after contact with IBN vapour.

b − a: subtraction spectrum.

3.6 The IR Spectroscopy of Adsorbed Probe Molecules for Surface Chemistry Characterization 157

the external terminal silanols is fully perturbed and shifted to 3420 cm

− 1

with a

component near 3370 cm

− 1

. IBN interacts also with part of the bridging OH groups

of H - FER. Upon adsorption of IBN in fact, the band of bridging OH groups

decreases only slightly in intensity. Part of it is unperturbed and does not change

its shape and breadth. Accordingly, a weak A,B,C pattern appears, due to the strong

interaction of part of the H - FER bridging OH groups with IBN. The maximum of

the unperturbed band of the bridging OH groups is at 3598 cm

− 1

, while the

maximum of the band of the OH groups which are perturbed by IBN (read as a

minimum in the subtraction spectrum) is at 3591 cm

− 1

. We consequently assign

the band at 3591 cm

− 1

to the OH groups located in the 10 - membered ring channel

(accessible to IBN) and that at 3598 cm

− 1

(inaccessible to IBN) to the OH groups

located in the 8 - membered ring channels. Analysis of the intensities of the two

bands indicates that they are not far from the ratio 1 : 1.

Similar studies have been performed on H - BEA [151] , allowing the distinction

of the sites in the two channels of this structure, and on H - MOR [149, 150] where

three different types of OH group have been distinguished, located in the main

channels, in the intersection between main channels and side pockets and in the

side pockets. In the case of H - Y faujasite, the sites located in the supercages and

in the sodalite cages are easily distinguished. Using trimethylamine as a probe, a

third component, which is not perturbed at all by the probe, at 3501 cm

− 1

, assigned

to OH groups in the hexagonal prisms, is also observed [152] .

These studies also allowed the determination of the diffusivity of molecules

having different steric hindrance in the cavities of these zeolites, as summarized

in Table 3.10 .

3.6.4

Spectroscopic Detection and Characterization of the Surface Lewis Acid Sites

As already mentioned, coordinatively unsaturated cations exposed at the surface

of ionic oxides give rise to surface Lewis acid sites. Basic molecules can, conse-

quently, interact with these sites by forming a new coordination bond, so complet-

ing or increasing the overall coordination at the surface cation, as shown in

structure I of Scheme 3.5 . Upon this interaction, electrons transfer from the basic

molecules towards the catalyst surface. This electronic perturbation (and some-

times the molecular symmetry lowering) arising from this contact are the causes

of a vibrational perturbation of the adsorbate. In most cases, the vibrational per-

turbation only consists in shifts of some vibrational frequencies, the more pro-

nounced the stronger the interaction, that is, the greater the Lewis strength of the

surface site. Consequently, the shift of the position of some very sensitive bands

of the adsorbate upon adsorption can be taken as a measure of the Lewis acid

strength of the surface sites.

As an example, in Figure 3.14 the spectra of pyridine molecularly adsorbed on

β - Ga

are shown. The spectra are due to molecular Lewis - bonded pyridine,

without any absorption assignable to pyridinium ions. It is evident that the 8a and

19b pyridine bands (1580 and 1438 cm

− 1

in the liquid) are shifted signiﬁ cantly

158 3 The Use of Infrared Spectroscopic Methods

Table 3.10 Summary of the characteristics of protonic zeolites and on the diffusion of molecular probes in their cavities

as measured by IR spectroscopy at room temperature.

Code Channels/cages Channel size

( Å )

IR ν OH

(cm

− 1

)

FER 8 - ring channel [010]

3.5 × 4.8

3598 E E D D D D

10 - ring channel [001]

4.2 × 5.4

3591 E E E D D D

MFI 10 - ring channel [100]

(sinusoidal)

5.1 × 5.5 ∼ 3610

channel

E E E E D D

10 - ring channel [010]

(straight)

5.3 × 5.6 ∼ 3620

intersect

E E E E D D

BEA 12 - ring channels

[001]

5.6 × 5.6

3609 E E E E D D

12 - ring channels

[100]

6.6 × 6.7

3628, 3608,

3590

E E E E E D

MOR 8 - ring compressed

channels [001]

2.6 × 5.7

3588

side - pocket

E D D D D D

8 - ring side pockets

[010]

3.4 × 4.8

3609

intersect

E D D D D D

12 - ring main

channels [001]

6.5 × 7.0

3605 main

chann

E E E E E D

FAU Hexagonal prisms

accessed through

6 - ring channels

ca 2.7 × 2.7

3501 D D D D D D

Sodalite cages

accessed through

6 - ring channels

ca 2.7 × 2.7

3553 D D D D D D

Supercages accessed

through 12 - ring main

channels [111]

7.4 × 7.4

3625 E E E E E E

E = easy; D = difﬁ cult

3.6 The IR Spectroscopy of Adsorbed Probe Molecules for Surface Chemistry Characterization 159

upwards upon adsorption. The 8a mode is clearly multiple (three components can

be distinguished, at 1622, 1615 – 1605 and ∼ 1590 cm

− 1

, due to multiple adsorption

sites. The behavior observed with β - Ga

is very similar to that observed on

transitional aluminas (see below).

Similar experiments can be performed using nitriles as probe molecules. These

molecules are weaker as bases and this sometimes allows a better resolution in

the detection of the Lewis sites. Analysis is based on the study of the C

≡

N stretch-

ing region. In this region liquid acetonitrile shows a strong doublet at 2294,

2254 cm

− 1

, where the latter band is deﬁ nitely stronger than the former. This

doublet is due to the Fermi resonance between the C

≡

N stretching and a δ CH

ν C

–

C combination. The virtual position of the CN stretching fundamental shifts

upwards by increasing electron withdrawal from the N lone pair upon adsorption,

and this causes the experimental position of both components of the doublet to

shift up. Simultaneously, however, the relative intensities of the two components

progressively inverts. Deuteroacetonitrile CD

CN and other nitriles have single CN

stretchings. They can also be used to determine the location of Lewis sites in pores

and cavities.

Low - temperature adsorption of CO is widely used as an acid site probe. In Figure

3.3 the spectra of CO over different solids are compared. The CO stretching band

may shift strongly up when it bonds as terminal carbonyl species on highly acidic

cationic centers, and can shift down signiﬁ cantly when it bridges over two or three

metal atoms over an extended metal surface. In Table 3.11 the position of some

sensitive bands over oxide surfaces is reported, allowing the measurement of their

Lewis acidity.

3.6.4.1 The Lewis Acid Sites of Aluminas and SA s

The catalytic activity of transitional aluminas ( γ − , η − , δ − , θ − Al

) are undoubtedly

mostly related to the Lewis acidity of a small number of low coordination surface

aluminum ions, as well as to the high ionicity of the surface Al

–

O bond [101] .

Alumina ’ s Lewis sites have been well characterized by adsorption of several probes.

They are the strongest among metal oxides. The number of such very strong Lewis

sites present on transitional alumina surfaces depend on the dehydroxylation

degree (depending on the activation temperature) and on the particular phase and

preparation.

It seems that, although the different alumina spinel - type phases react a little

differently to outgassing, the density of the strongest Lewis acid sites tends to

decrease a little as the calcination temperature of the alumina increases (i.e. upon

the sequence γ → δ → θ , which is also a sequence of decreasing surface area). As

a result of this the number of strongest acid sites per gram signiﬁ cantly decreases

in this sequence, although catalyst stability increases.

Although it is clear that surface Lewis acid sites on alumina are due to coordi-

natively unsaturated Al

ions, it is not fully clear what is the coordination of such

surface ions. Most authors agree that at least three different types of Lewis acid

sites (weak, medium, strong) exist on transitional aluminas, arising in some way

from the two or three coordinations of the ions in the bulk spinel - type structure,

160 3 The Use of Infrared Spectroscopic Methods

namely octahedral and tetrahedral (normal spinel positions) and trigonal. Pyridine

adsorption produces three components for the 8a vibrations at 1624, 1618 and

1597 cm

− 1

, attributed to three diffent Lewis bonded species. Liu and Truitt [126]

emphasized the close proximity of Lewis acid sites to surface OH groups while in

their study Lundie and coworkers [153] identiﬁ ed four different Lewis acid sites

arising from coordinatively unsaturated octahedral (the weakest) and tetrahedral

sites (the three strongest), three of which are considered to be associated with three

different types of hydroxyl groups.

Very strong Lewis acid sites can be detected on the surface of SA, characterized

by ν CO of adsorbed carbon monoxide at 2230 cm

− 1

(Figure 3.21 ), as well as by the

8a mode of adsorbed pyridine at 1625 cm

− 1

(Figure 3.23 ) They are certainly due to

highly uncoordinated Al ions and correspond to the strongest Lewis sites of tran-

sitional alumina or perhaps are even stronger, owing to the induction effect of the

covalent silica matrix. This makes SA also a very strong catalyst for Lewis acid

catalyzed reactions.

3.6.4.2 Lewis Acidity of Other Ionic Oxides

The strongest Lewis acidic oxides in normal circumstances are alumina and gallia,

that is, oxides of elements at the limit of the metallic character. The same elements

also give rise to halides characterized by even stronger Lewis acidity. Medium to

Table 3.11 Position (cm

− 1

) of the sensitive IR bands of adsorbed basic probe molecules on

different catalyst surfaces. The Lewis acid strength roughtly decreases from top to bottom.

Adsorbate CO Pivalonitrile Pyridine Ammonia Adsorbing

site type

IR mode

ν C O ν CN

sym

AlF

2309, 2305 1627

Zeolites (external surface) 2230 2300 1625 Masked

Silica - alumina 2235 2296 1625 Masked

γ - Al

2235 2296 1625 1295

2210 – 2190 1615 1265

1595 1220

2170

Alumina - pillared

montmorillonite

2290 1625 Masked

Acid - treated montmorillonite 2295 1625 Masked

SiO

–

TiO

2226 w 2308 w

1610

Masked Ti

2208 2285

, unsupported = 2290 1613 1275, 1222

ZrO

2195 1606 1210, 1160 Zr

2170

Sulfated zirconia 2160 1606 1210, 1150 Zr

TiO

anatase 2208 2285 1610 1225

2182 2260 1185

Liquids 2143 2236 1583 1054

3.6 The IR Spectroscopy of Adsorbed Probe Molecules for Surface Chemistry Characterization 161

medium - strong Lewis acidity is found for other ionic oxides such as zirconias,

titanias, iron oxides, and so on.

3.6.4.3 Lewis Acidity of Highly Covalent Oxides

Lewis acidity is not usually observed for the covalent oxides of non - metal elements.

Interestingly in the case of silica, Lewis acidity is generally not found but it can

appear after very harsh pretreatment under outgassing [154] . The polarizing power

of the “ cations ” in the covalent oxide, is higher than 8. This shows that the reason

for the absence of Lewis acidity on the surface of these oxides is the difﬁ culty of

breaking the M

–

(OH) bonds, which are so covalent owing to the strong polarizing

power of the cation that would result by dissociation, with water desorption. Addi-

tionally, when Si(OH) groups actually break, couples of them tend to give rise to

“ strained ” Si

–

Si bridges, becuase of the strength of Si

–

O bonds, neutralizing

Lewis acidic coordinatively unsaturated Si ions. On the contrary, dehydroxylation

of ionic oxides is easier, and results in the generation of Lewis acid sites that are

sufﬁ ciently stable to stay as such under relatively mild conditions.

On germania, Lewis acidity is generally not found. Oxides of transition metals

in very high oxidation states, such as vanadia, niobia, tungsta and molybdena,

show strong Lewis acidity, attributed to the coordinative unsaturation of vanadyl

[82] , niobyl [83] , tungstyl [84] and molybdenyl ions at the surface. The supported

oxides of pentavalent vanadium and niobium and of hexavalent tungsten and

molybdenum also display strong Lewis acidity [99] .

3.6.4.4 Lewis Acidity of Protonic Zeolites

Studies of adsorbed hindered molecules demonstrate that in non - defective zeolites

quite strong Lewis acidity is usually present at the external surface [149 – 152] .

Additional Lewis acidity occurs due to extra - framework aluminum oxide species

[100, 153] .

3.6.5

Determination of the Oxidation State of Cationic Centers

Carbon monoxide is a very weak base, widely used for the surface characterization

of cationic centers on metal oxide surfaces [155] . The electronic structure of CO

implies a triple bond between C and O, according to the stretching frequency

measured at 2143 cm

− 1

for the free molecule in the gas. In principle, the 14 elec-

trons are distributed symmetrically between C and O atoms, so that the lower

positive charge of the C nucleus with respect to O implies the formation of a dipole

with the negative charge at the C atom, in spite of the lower electronegativity of C

with respect to O. For this reason, the CO molecule tends to interact through the

C end with cationic centers. This interaction is rather weak, usually completely

reversible by outgassing at ambient temperature and should be studied at room

or lower temperature (e.g. at liquid nitrogen temperature, 77 K). According to

theoretical calculations, this interaction is a simple polarization, with no formation

of a true coordinative σ bond with the cationic center. This interaction tends to

162 3 The Use of Infrared Spectroscopic Methods

increase the CO bond order, so that the CO stretching frequency tends to increase.

Accordingly, the experimental measure of the CO stretching frequency for CO

interacting with surface cations can be taken as a measure of the polarizing power

of the cation or, in other terms, of its Lewis acidity (see Table 3.11 ).

However, when the cation or the metal atom contains, besides empty orbitals,

full or partly ﬁ lled d - type orbitals, they can interact with the empty π

- type orbitals

of CO, via a π - type electron backdonation from the metal to CO. This implies that

these antibonding orbitals become partly ﬁ lled, so that the bond order and the CO

stretching frequency is decreased by this last interaction. In this case, the interac-

tion can become very strong and very stable metal – carbonyl complexes can be

formed. The experimental CO stretching frequency in this case is a complex func-

tion of the electron accepting power of the cation (Lewis acidity) and of its π - type

electron donating power. Accordingly, the CO stretching frequency of CO adsorbed

on several transition metal cations is very informative concerning the oxidation

state of the adsorbing ion.

In Figure 3.25 the IR spectra of CO adsorbed at low temperature over a Pd/ γ -

catalyst after oxidizing (upper spectra) and reducing pre - treatments (lower

spectra) are reported. In the oxidized catalysts a strong band composed of a main

maximum at 2149 cm

− 1

and a pronounced shoulder at 2160 cm

− 1

is evident. This

Figure 3.25 FTIR spectra of carbon monoxide adsorbed on

PdO

/ γ - Al

catalyst, after previous calcination in air at 673 K

(upper spectra) and after reduction in hydrogen (1 atm) at

673 K (lower spectra). The ﬁ rst spectra in each series has been

obtained in contact with 10 Torr CO at 130 K for 5 min. The

other spectra were recorded upon outgassing (10

− 3

Torr) on

warming to 270 K.