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

194 4 Resonance Raman Spectroscopy – Θ-Al

2

O

3

-Supported Vanadium Oxide Catalysts

terization of V species in doped - V/Ti/O

catalysts by means of TPR and TPO

measurements: a study of the effect of

promoters in the oxidation of o - xylene .

Studies in Surface Science and Catalysis ,

155 , 153 – 65 .

55 Magg , N. , Immaraporn , B. , Giorgi , J.B. ,

Schroeder , T. , Baumer , M. , Dobler , J. ,

Wu , Z. , Kondratenko , E. , Cherian , M. ,

Baerns , M. , Stair , P.C. , Sauer , J. and

Freund , H. - J. ( 2004 ) Vibrational spectra

of alumina - and silica - supported vanadia

revisited: An experimental and

theoretical model catalyst study . Journal

of Catalysis , 226 ( 1 ), 88 – 100 .

56 Weckhuysen , B.M. , Verberckmoes , A.A. ,

Debaere , J. , Ooms , K. , Langhans , I. and

Schoonheydt , R.A. ( 2000 ) In situ UV - Vis

diffuse reﬂ ectance spectroscopy – on line

activity measurements of supported

chromium oxide catalysts: relating

isobutane dehydrogenation activity with

Cr - speciation via experimental design .

Journal of Molecular Catalysis A: Chemical ,

151 ( 1 – 2 ), 115 – 31 .

57 Wu , Z. and Stair , P.C. ( 2006 ) UV Raman

spectroscopic studies of V/ θ - Al

2

O

3

catalysts

in butane dehydrogenation . Journal of

Catalysis , 237 ( 2 ), 220 – 9 .

Solid - State NMR of Oxidation Catalysts

James McGregor

195

5

5.1

Introduction

5.1.1

Oxides in Catalysis

Metal oxide materials are employed extensively both as catalysts and catalyst

supports for a wide variety of reactions [1 – 4] . Industrially relevant processes in

which bulk oxides act as the catalyst include the production of sulfuric acid from

SO

2

over mixed potassium/vanadium oxides, the oxidation of butane to maleic

anhydride over vanadium - phosphorus oxides and the production of synthesis gas

from methane and steam over rare earth pyrochlores. Such reactions exploit the

redox properties of oxides, while their acid – base character is employed in trans-

formations such as alkane isomerization and catalytic cracking. Aluminum oxide

(alumina) is an effective catalyst for double - bond isomerization, although it is

perhaps more widely employed as a catalyst support. A wide variety of metal

oxides, including Al

2

O

3

, SiO

2

, TiO

2

, Ga

2

O

3

, SnO

2

and ZrO

2

are employed to support

and disperse the active metal phase in a huge number of catalysts. Such materials

have applications in many areas including hydrogenation, dehydrogenation and

oxidation reactions. Frequently, the beneﬁ ts provided by supported catalysts are

combined with the activity of metal oxides. For instance, vanadium oxide sup-

ported on alumina is widely employed as a catalyst in the oxidative dehydrogena-

tion of light alkanes [5 – 7] .

5.1.2

Studying Metal Oxide Catalysts

Owing to the importance of metal oxides in catalysis their structure is widely

studied. The aim of such work is to establish structure – activity relationships,

whereby the structural parameters of the materials responsible for their activity,

and their mode of action, are identiﬁ ed. Industrial catalysts, however, are often

highly complex materials and identifying such relationships is not a simple task.

Metal Oxide Catalysis. Edited by S. David Jackson and Justin S. J. Hargreaves

ISBN: 978-3-527-31815-5

196 5 Solid-State NMR of Oxidation Catalysts

Furthermore, both bulk and surface properties of the materials can play important

roles. For instance the oxidation of alkanes over vanadium - based oxides is believed

to proceed via a Mars – van Krevelen mechanism [8, 9] with lattice (bulk) oxygen

being the active oxygen species. In metal oxide - based cracking catalysts, however,

activity is directed by the surface acidity of the material.

There are, therefore, a number of distinct structural characteristics which must

be identiﬁ ed in order to fully understand the action of oxide catalysts. Bulk proper-

ties of interest include the identiﬁ cation of distinct crystallographic phases present

in the catalyst; the local environment of the nuclei in either crystalline phases or

amorphous materials; and the redox properties of the catalyst. Surface properties

impacting on catalytic activity include the local environment of nuclei at the

surface; acid – base behavior; the number and concentration of acid sites, including

hydroxyl groups and the nature of these acid sites.

Solid - state NMR is perhaps unique in its ability to probe each of these charac-

teristics. In particular, its sensitivity to local geometry and coordination environ-

ments provides advantages over many more traditional characterization methods.

However, no single NMR experiment can furnish the investigator with data in

each and all of these areas. A number of distinct, innovative, approaches have been

developed to probe different structural features. A number of these are outlined

below.

In addition to providing information on the structural characteristics of catalysts,

NMR is also widely employed in the study of catalytic processes. Through studying

the interaction of reactant and product molecules with the catalyst surface, insights

can be gained into the mechanism of the reactions occurring. Both in situ and

ex situ methods have been used to great effect in this area. Such studies are

outside the scope of this chapter. However, excellent reviews in this area have

been prepared by a number of authors [10 – 15] , while the application of NMR

imaging to catalysts and catalytic processes is discussed by Gladden and coworkers

[16] .

This chapter focuses on the application of solid - state NMR techniques for the

characterization of oxidation catalysts. Initially, a brief introduction to these tech-

niques is provided (Section 5.2 ), within which methods suitable for the study of

both bulk structure (Section 5.2.1 ) and surface characteristics (Section 5.2.2 ), are

described. Examples of the application of these techniques are then provided in

Section 5.3 , for bulk oxides, and Section 5.4 , for surface properties. Finally, Section

5.5 provides an outlook as to future directions in this area.

5.2

NMR Techniques

Solid - state NMR has become a fundamental part of the toolkit to characterize cata-

lysts and other solid materials. It is now routine to employ one of the many NMR

techniques available to extract information about both the surface properties and

the bulk properties of catalysts. Additional information on redox properties and

5.2 NMR Techniques 197

metal oxidation states – key properties for metal oxides and oxidation catalysts – can

also be elucidated.

The principal difference between the NMR spectra of liquid samples and those

acquired in the solid state is that linewidths of solid samples are very much broader

than those for liquids. This difference arises because, in liquids, molecular

tumbling averages anisotropic interactions and therefore removes them from the

spectrum. To be in a position to overcome this problem it is necessary to under-

stand the properties that give rise to an NMR spectrum.

The key properties of nuclei that allow them to be probed by NMR are nuclear

magnetism and nuclear spin. Quantum mechanics dictates that the angular

momentum of any system is quantized and can take only the values:

h

SS

2

1

π

+

()

(5.1)

As the spin quantum number, S , can take only integer or half - integer

values, nuclear spin can only take either integer, or half - integer values. Only

when nuclear spin is greater than zero does a nucleus have a magnetic moment

( µ ). It is the magnetic moment which is manipulated in NMR spectroscopy,

and hence only nuclei with a spin > 0 can be investigated by NMR. Table 5.1 shows

the nuclear spin of a number of nuclei commonly probed in solid - state NMR

spectroscopy.

The relationship between the magnetic moment and angular momentum, P, is

given by:

µγ=×P (5.2)

where γ is the gyromagnetic ratio.

1

H has a nuclear spin I = 1/2. Quadrupolar

nuclei, that is, those with I > 1/2, such as

51

V ( I = 7/2) or

27

Al ( I = 5/2), possess a

non - spherical charge distribution that can couple to electric - ﬁ eld gradients present

in most solid - state materials. This can have a large effect on the observed spectrum

and leads to, for example, quadrupolar broadening of peaks, hindering spectral

interpretation.

In the presence of a magnetic ﬁ eld a nucleus with I > 0 experiences a number

of interactions. These interactions may each be expressed as a Hamiltonian with

the overall effect being expressed as:

HH H H H H

ZDCSJQ

=++ ++

(5.3)

where H

Z

, H

D

, H

CS

, H

J

and H

Q

correspond to the Zeeman, dipolar, chemical shift,

indirect (J - ) coupled and quadrupolar interactions.

The Zeeman interaction is the lifting of the degeneracy of the energy levels in

a nucleus by the application of a magnetic ﬁ eld, B

0

. This results in the development

of 2 I + 1 energy levels as shown in Figure 5.1 . Nuclear spins exhibit a dipole

moment which interacts with that of other nuclei, known as dipolar coupling, D.

198 5 Solid-State NMR of Oxidation Catalysts

The extent of this is dependent upon factors including the species involved and

the internuclear distance between them. In a strong magnetic ﬁ eld, the dipolar

coupling is dependent upon the orientation of the internuclear vector with the

external magnetic ﬁ eld such that:

D ∝−31

2

cos θ

(5.4)

The chemical shift interaction results in a shift in the basic resonance frequency

of the nucleus being examined and arises from the concurrent interaction of

Table 5.1 Nuclear spin and other properties of NMR active

nuclei commonly found in oxidation catalysts.

Nucleus Spin Resonant

frequency (MHz)

Natural

abundance (%)

Sensitivity

wrt

1

H

Quadrupole moment

(10

− 28

m

2

A)

1

H

1

2

200.106 73 100 1 0

2

H 1 30.7176

< 0.1 < 0.000 01

0.0028

11

B

3

2

64.2020 80.3 0.133 0.04

13

C

1

2

50.331 538 1.1 0.000 18 0

14

N 1 14.455 32 99.6 0.001 0.01

15

N

1

2

20.276 74 0.3

< 0.000 01

0

17

O

5

2

27.1284

< 0.1

0.000 01

− 0.026

19

F

1

2

188.2543 100 0.0834 0

23

Na

3

2

52.9314 100 0.092 0.14

27

Al

5

2

52.1418 100 0.210 0.15

29

Si

1

2

39.7342 4.7 0.0004 0

31

P

1

2

81.0045 100 0.066 0

43

Ca

7

2

13.463 62 0.15 0.000 01

− 0.049

50

V 6 19.951 50 0.3 0.017 0.209

51

V

7

2

52.593 99.7 0.381

− 0.05

69

G a

3

2

48.034 60.4 0.042 0.19

71

Ga

3

2

61.0248 39.6 0.056 0.12

85

Rb

5

2

19.320 76 72.2 0.0073 0.26

87

R b

3

2

65.4757 27.9 0.049 0.12

89

Y

1

2

9.803 73 100 0.000 12 0

95

Mo

5

2

13.0378 15.7 0.0005 0.011

97

Mo

5

2

1.3104 9.46 0.0003 0.013

115

Sn

1

2

65.7624 0.4 0.000 12 0

117

S n

1

2

71.2896 7.6 0.0034 0

129

X e

1

2

55.3519 26.4 0.0056 0

131

X e

3

2

16.408 17 21.2 0.0006

− 0.12

133

Cs

7

2

26.248 04 100 0.047

− 0.003

138

La 5 26.400 37

< 0.1 < 0.000 01

0.51

139

La

7

2

28.267 68 99.9 0.059 0.22

5.2 NMR Techniques 199

Figure 5.1 The effect of quadrupole interactions on an I = 3/2

nucleus in a magnetic ﬁ eld. The Zeeman interaction splits the

levels by an equal amount, ω

0

(the Larmor frequency in

frequency units). The central +1/2 to − 1/2 transition is

unaffected by ﬁ rst order coupling ω

Q

1

()

; however, second order

coupling,

ω

Q

2

()

, affects all transitions.

the nucleus with the orbital moment of the surrounding electrons, and of these

electrons with B

0

, the static ﬁ eld. Nuclei rarely possess spherically symmetric

electron distributions and as a result there is a dependence of the chemical shift

on molecular orientation, referred to as chemical shift anisotropy ( CSA ). Powder

samples exhibit a broad (static) NMR signal, the boundaries of which represent

the molecular orientations with the smallest and largest effect on chemical shift.

Indirect (or J - ) coupling is the coupling of spins to each other through chemical

bonds. For a pair of spins, i and j , the Hamiltonian of the spin - spin coupling is:

HJ

J

=×× ⋅π 2II

ij

(5.5)

where J is the coupling constant.

The ﬁ nal nuclear spin interaction is quadrupolar coupling. Quadrupolar nuclei

are those with I > 1/2. For I > 1 only

10

B ( I = 3),

50

V ( I = 6) and

177

Lu ( I = 7) have

integral spin. All remaining nuclei, including catalytically relevant nuclei such as

51

V ( I = 7/2) and

27

Al ( I = 5/2), have half - integer spins. As stated above, the non -

spherical charge distribution of quadrupolar nuclei leads to coupling with electric

ﬁ eld gradients in solid - state materials. This interaction is described by:

H

Q

=⋅⋅IQI

ii

(5.6)

where Q is the quadrupole moment of spin i. Figure 5.1 shows the effect of quad-

rupolar interactions on a quadrupolar nucleus. For simplicity a nucleus of spin =

3/2 has been shown although this can be extended to higher spin nuclei.

The resonance frequency of the central transition (+1/2 → − 1/2) is unchanged

by the ﬁ rst - order quadrupolar Hamiltonian. Other (satellite) transitions are,

however, shifted by an amount proportional to the ﬁ rst - order quadrupolar cou-

pling constant ω

Q

1

()

. The result of this is to substantially broaden resonances from

200 5 Solid-State NMR of Oxidation Catalysts

the satellite transitions. Second - order quadrupolar coupling does have an effect on

the central transition, broadening it and thus causing the linewidth to increase.

As a result, linewidths from quadrupolar nuclei are greater than those from I =

1/2 nuclei such as

1

H. One of the effects of second - order quadrupolar coupling is

to produce an irremovable isotropic quadrupolar shift. In liquids, rapid molecular

orientation eliminates the effects of this shift.

There are therefore a wide number of interactions that inﬂ uence NMR - active

nuclei, giving rise to potentially very complex spectra from which it can be difﬁ cult

to extract the parameters of interest, particularly in the case of complex materials

such as catalysts. The ever increasing development of instrumentation, however,

making, for instance, higher magnetic ﬁ elds or higher spinning speeds (where

appropriate) accessible, has made possible the development of many advanced

NMR techniques. These techniques, applied correctly, allow a wealth of informa-

tion to be extracted from NMR spectra. The approach taken can be to remove

interactions which complicate spectra, such as line broadening in the case of

magic angle spinning ( MAS ) (Section 5.2.1.1 ). Alternatively, it may be desirable

to selectively probe otherwise unwanted interactions. Off - resonance nutation

NMR (Section 5.2.1.4 ), for instance, reveals the structural information present in

the quadrupole interaction, while double resonance techniques (Section 5.2.2.1 )

re - introduce dipolar couplings, removed under magic angle spinning. Through

such approaches it is possible to extract a wide variety of information, even from

structurally complex oxidation catalysts. The following sections outline techniques

that are relevant in this area.

5.2.1

Bulk Structure of Catalysts

Spectroscopic approaches to the study of oxidation catalysts focus on two main

areas – the characterization of the bulk structure of the material and the identiﬁ ca-

tion of distinct surface environments. The bulk structure and, where appropriate,

phase of catalytic materials is of key importance in dictating their activity, particu-

larly in processes involving redox mechanisms. Solid - state NMR techniques can

prove particularly advantageous in this area. For instance, unlike X - ray diffraction,

NMR can probe both crystalline and amorphous structures. However, as discussed

above, achieving high - resolution spectra in the solid - state is complicated by the

numerous interactions giving rise to line broadening effects, complicating spectral

interpretation. The following sections outline experimental approaches to over-

come these challenges, with a focus on those techniques that can be applied to

metal oxide catalysts.

5.2.1.1 Magic Angle Spinning

MAS is the most common technique employed in solid - state NMR to remove line

broadening. It involves fast mechanical sample rotation about an axis inclined at

54 ° 44 ′ (the value where 3 cos

2

θ − 1 = 0, Equation 5.4 ) to the direction of the external

magnetic ﬁ eld. It can remove line broadening from dipolar interactions (averaged

5.2 NMR Techniques 201

to zero), chemical shift anisotropy (averaged to a non - zero value) and quadrupolar

interaction to ﬁ rst order, and reduce quadrupolar interaction to second order by a

factor of four.

MAS NMR mimics the rapid time - averaged tumbling molecular motion that

occurs in liquid samples. The overall result is that the signal becomes much

narrower and the isotropic peaks can be individually observed. In practice, MAS

is achieved by loading the sample into a rotor that is axially symmetric about its

axis of rotation. The sample is then spun, via an air turbine mechanism, at the

magic angle to the applied magnetic ﬁ eld. MAS NMR is discussed in detail in a

number of references, for example [17, 18] .

Applications of solid - state MAS NMR, discussed in the forthcoming sections,

include the measurement of

1

H chemical shifts, allowing distinct Br ø nsted acid

sites to be identiﬁ ed and differentiated; the study of different Al coordination

environments in aluminas by

27

Al MAS NMR; and the characterization of vana-

dium sites in vanadium oxide catalysts by

51

V MAS NMR. An example of the effect

of MAS, as compared to static NMR measurements, is shown in Figure 5.2 for

V

2

O

5

. Both

51

V and

27

Al are quadrupolar nuclei, hence anisotropic effects are not

removed through MAS alone. Other techniques may therefore be necessary to

achieve high - resolution spectra, particularly for non - crystalline samples.

5.2.1.2 Multiple Quantum Magic Angle Spinning

In order to remove the anisotropic effects of quadrupolar interactions which can

obscure MAS spectra other techniques such as Double Rotation ( DOR ) [18, 19] ,

Dynamic Angle Spinning ( DAS ) [18] or Multiple Quantum Magic Angle Spinning

( MQMAS ) have to be employed. Of these the most simple to apply practically is

MQMAS, which can be conducted on a standard MAS - equipped NMR spectrom-

eter. A number of texts and review papers discuss MQMAS in signiﬁ cant technical

detail [20, 21] .

MQMAS was ﬁ rst proposed in 1995 by Harwood and Frydman [22, 23] . The aim

of the technique is to refocus the second - order quadrupole effects, which otherwise

Figure 5.2 (a) Static and (b) MAS NMR spectra of V

2

O

5

.

Spinning rate (for MAS) = 14 kHz. B

0

= 9.40 T.

202 5 Solid-State NMR of Oxidation Catalysts

broaden the observed resonance [22] . This involves the correlation of single and

odd - integer multiple quantum coherences in a two - dimensional NMR experiment

while spinning at the magic angle. To observe this, it is necessary to convert the

multiple quantum coherence to a single quantum coherence by inserting pulses

or delays into the applied pulse sequence.

The result of the MQMAS experiment is to generate a two - dimensional spec-

trum consisting of a number of distinct features, as illustrated in Figure 5.3 . Along

one axis the isotropic chemical shifts, and hence the crystallographic inequivalent

nuclei, are mapped out, while along the other quadrupole - induced shifts are

mapped out. Figure 5.3 shows a triple quantum spectrum of γ - Al

2

O

3

acquired by

Kraus and coworkers [24] . Aluminum in tetrahedral coordination environments

is denoted Al(IV), while those nuclei that are octahedrally coordinated are denoted

Al(VI). That the signals curve away from the isotropic chemical shift axis at low

parts per million ( ppm ) values indicates that the resonances experience a distribu-

tion in both quadrupolar and chemical shift interactions.

5.2.1.3 Satellite Transition Magic Angle Spinning

Satellite Transition MAS ( SATRAS or STMAS ), developed by Gan in 2000 [25] , is

an alternative approach to MQMAS for the acquisition of high - resolution NMR

spectra of quadrupolar nuclei. The principal advantage of SATRAS over MQMAS

is that it is not dependent upon an efﬁ cient transfer of multiple - quantum coher-

ences. Like MQMAS, SATRAS is a 2D experiment performed under MAS condi-

tions. The technique involves exciting the satellite transitions in the spin manifold

of quadrupolar nuclei using short radio frequency ( rf ) pulses. The second - order

Figure 5.3

27

Al 3Q - MAS spectrum of γ - Al

2

O

3

. Both tetrahedral,

Al(IV), and octahedral, Al(VI), environments are detected.

Spinning sidebands are marked with an asterisk. Adapted

from ref. [24] ; reprinted with permission from the American

Chemical Society.

5.2 NMR Techniques 203

quadrupolar broadenings of these satellite and the central transitions differ only

by a scaling factor. As a result, a high - resolution spectrum can be obtained through

a correlation of the single - quantum satellite and central transitions. This is achieved

through a two - pulse sequence where the ﬁ rst pulse excites satellite transitions and

the second pulse transfers coherence to the central transition. Since its develop-

ment, SATRAS has been applied to a range of catalytically relevant systems [26 –

29] . Figure 5.4 shows a SATRAS spectrum of γ - Al

2

O

3

acquired by Ashbrook and

Wimperis [26] . Signals at ∼ 8 and 65 ppm correspond to aluminum in octahedral

and tetrahedral coordination environments respectively. The broadening of the

resonances is due to both quadrupolar and isotropic parameters, indicating the

amorphous nature of the material.

5.2.1.4 Off - Resonance Nutation NMR

Techniques such as MQMAS aim to enhance spectral resolution through averag-

ing quadrupolar contributions to spectra. However, the quadrupole interaction

itself can be a source of valuable information. This information is, however, often

difﬁ cult to extract from static or MAS spectra, owing to the poor spectral resolution

resulting from a distribution of chemical shifts, residual dipolar broadening or

quadrupolar interactions. Off - resonance nutation NMR, a technique developed by

Samosan and Lippmaa [30, 31] , aims to overcome these problems. Nutation NMR

describes the behavior of the spin system in the rotating frame during rf irradia-

tion whereby the inﬂ uence of the chemical shifts is dramatically reduced. Off -

resonance nutation is a variant on this where the behavior of the spin system is

instead monitored in the effective ﬁ eld, thereby increasing the upper limit of acces-

sible quadrupolar coupling values. A comprehensive review of both theoretical and

practical aspects of off - resonance nutation NMR is provided by Kentgens [32] .

Figure 5.4

27

Al 2D - SATRAS NMR of γ - Al

2

O

3

. The MAS rate

was 30 kHz. Contour levels are shown at 8, 16, 32, and 64%

of the maximum intensity. Adapted from ref. [26] ; reprinted

with permission from Elsevier.

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

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