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

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224 5 Solid-State NMR of Oxidation Catalysts

trum of the same VO

/Al

catalyst shown in Figure 5.22 . The advantages of

MQMAS in identifying different sites are evident with Al in tetra - , penta - and

octahedral environments being clearly distinguished.

Quartararo has employed

Al MQMAS NMR to the study of phosphorus - modi-

ﬁ ed aluminas [160 – 162] . 3Q - MAS NMR spectra reveal that the inﬂ uence of the

phosphorus modiﬁ er on Al

is critically dependent on the synthesis method

employed, with two new aluminum phosphate phases forming if P is added at a

later stage in the synthesis [160] . MQMAS NMR has also been applied to alumi-

nophosphate materials by Antonijevic and coworkers [28] .

Al 3Q - MAS NMR

spectra were used in conjunction SATRAS NMR under MAS conditions in order

to probe molecular - scale dynamics. Figure 5.24 shows

Al MQMAS and SATRAS

spectra for both a calcined and dehydrated aluminophosphate (AlPO - 14) and for

an isopropylamine - templated aluminophosphate (ipa - AlPO - 14). The broadening

Figure 5.24 (a) – (d)

Al MQMAS (triple - quantum) and

SATRAS NMR spectra of calcined - dehydrated AlPO - 14 and

ipa - AlPO - 14. The MAS rate was 20 kHz. In the STMAS spectra,

the central - transition autocorrelation peak and its contribution

to the isotropic projection is indicated by * , while an outer

satellite - transition correlation peak is indicated by * * .

Adapted from ref. [28] ; reprinted with permission from the

American Chemical Society.

observed in the SATRA spectrum of ipa - AlPO - 14 relative to the AlPO - 14 spectrum

can be assigned to dynamics - driven modulation of the

Al quadrupolar interaction

on the microsecond timescale.

In addition to MAS and MQMAS NMR, other techniques have also been applied

to the study of aluminum oxides to provide further information. Off - resonance

nutation NMR can assist in the interpretation on MAS and MQMAS spectra by

providing information on the average quadrupolar interaction in a sample. Kraus

and co-workers have applied this technique to γ - Al

supports and to γ - Al

supported hydrotreating catalysts [24] . Using this method quadrupolar coupling

constants were calculated for Al atoms in tetrahedral ( > 5.0 MHz) and octahedral

sites (4.5 MHz). These were then used in conjunction with results from MAS and

MQMAS experiments to develop a model of both the bulk and the surface struc-

ture of the alumina under study.

5.3.3.2 Other Oxide Supports

It is not only alumina supports which may be investigated through NMR spectros-

copy. Any support material containing NMR - active nuclei is suitable for study.

MAS has been applied to silica [164] , aluminum phosphate [131, 165] and SnO

supports [166, 167] . The nuclei of interest in studies of SnO

119

Sn.

119

Sn NMR

studies have also been conducted on pyrochlores (Ln

where Ln is a lantha-

nide) which are catalysts for the oxidative coupling of methane [168 – 170] . In such

investigations, analyses of the NMR lineshape, in addition to information gained

from the chemical shift value, have proved informative in extracting structural

information, notably on the ionic radius of Sn atoms in such compounds.

Gallium oxide (Ga

) is also used as a support material for metal catalysts in

addition to being a photocatalyst in its own right [94, 171 – 175] . As with Al

different polymorphs of Ga

exist, with the distribution of gallium coordination

environments differing between polymorphs. For instance, in β - Ga

50% of Ga

atoms are located in tetrahedral sites and 50% in octahedral sites. α - Ga

contains

only octahedral Ga sites.

There are two NMR active isotopes of gallium, both of which, like

Al, are

quadrupolar. These are

Ga (40% natural abundance, spin = 3/2) and

Ga (60%

natural abundance, spin = 3/2). Solid - state NMR studies of Ga

, however,

present a much greater technical challenge than the study of Al

. As discussed

above, in

Al NMR spectra the second - order quadrupolar broadening can be

reduced through the application of high magnetic ﬁ elds and, in MAS experiments,

high spinning rates. This allows for the resolution of different sites without inter-

ference from overlapping spinning side - bands. For

Ga, however, the second -

order quadrupolar broadening is so large that even at high magnetic ﬁ elds and

MAS spinning rates spectra can remain unresolved. As a result the resolution of

static spectra is greater than that of those acquired under MAS conditions. The

quadrupolar moment of

Ga is 70% larger than that of

Ga, accentuating the

difﬁ culties encountered with

Ga. As a result the majority of studies have focused

Ga NMR, despite the lower natural abundance of this nucleus. These prob-

lems have been discussed in detail by a number of workers [28, 136, 176, 177] .

5.3 Structure of Bulk Oxides 225

226 5 Solid-State NMR of Oxidation Catalysts

Signiﬁ cant progress in overcoming these challenges was made through the work

of Massiot and coworkers on the Quadrupolar Phase Adjusted Spinning Sideband

( QPASS ) technique [136, 176 – 180] . QPASS is a two - dimensional NMR technique

that separates spinning sidebands ( ssbs ) before summing them to reconstruct an

inﬁ nite spinning rate spectrum. This removes the problem of overlapping ssbs;

however, it comes at the expense of signiﬁ cantly greater acquisition times. Figure

5.25 shows a comparison of static, MAS and the projected QPASS spectra for β -

. Two different sites, corresponding to tetrahedral and octahedrally coordi-

nated Ga nuclei, are observed. As for Al

chemical shifts of Ga are indicative of

the coordination environment. Tetrahedrally coordinated Ga typically has a chemi-

cal shift around 200 ppm, 5 - coordinate Ga is ∼ 75 ppm and 6 - coordinate Ga ∼ 0 ppm.

As with Al NMR, peak asymmetry, with an upﬁ eld tail, indicates a distortion from

ideal geometry.

Ash and coworkers have studied a wide variety of model compounds containing

Ga in different coordination environments [29] . Employing a range of solid - state

NMR techniques including Variable Offset Cumulative Spectra ( VOCS ), Rotor -

Assisted Population Transfer ( RAPT ), SATRAS and MQMAS NMR to overcome

the difﬁ culties associated with gallium nuclei, and probing both

Ga and

Ga,

the dependence of chemical shift on coordination environment has been clearly

shown. Furthermore, through consideration of the magnitude of the electric ﬁ eld

gradient, the inﬂ uence of the second coordination sphere can be observed, and

charged environments (e.g.

GaO

−

) can be easily distinguished from neutral ones

(e.g. GaO

). Solid - state NMR is therefore, in the case of Ga, more sensitive to such

properties than solution NMR.

The relationship of gallium oxide structure, as revealed by NMR, to catalytic

activity in the conversion of methanol to diethyl ether has been investigated by

Figure 5.25 Static, MAS and QPASS spectra of β - Ga

Adapted from ref. [177] ; reprinted with permission from EDP

Sciences.

Lavalley [181] , revealing that both α - and β - Ga

showed the same apparent rate

despite the differences in their bulk structure. This surprising result was explained

through the proposed similarity of the surface properties of the materials under

reaction conditions. Other Ga - containing catalyst systems have been investigated

through the efforts of a number of research groups [136, 138 – 140] .

5.4

Surface Properties of Metal Oxides

Section 5.3 considered NMR spectroscopic approaches to the bulk characterization

of oxides and oxidation catalysts. Catalytic activity is, however, intrinsically linked

with the nature of the catalyst surface and hence a number of techniques have

been developed in order to probe this. As discussed in Section 5.1 , two of the most

signiﬁ cant parameters impacting on catalyst activity are the acid – base character-

istics of a surface and the redox properties of the material, and NMR techniques

exist to probe both of these characteristics. One of the most common techniques

to probe surface structure is CP - MAS NMR, in particular CP from hydrogen to

the nucleus of interest – either the metal or the oxygen of the metal oxide. Histori-

cally, the source of surface

H species has often been those naturally present on

the catalyst surface, as chemisorbed hydroxyls or physisorbed water. As such,

much of the work in this area involves the study of supports such as SiO

. Applica-

tions of CP - MAS and other spectroscopic approaches to the study of oxide surfaces

are outlined in the following sections.

5.4.1

Surface Structure

The most widely employed tool to probe the surface structure of oxide catalysts is

CP - MAS NMR. Sindorf and Maciel ﬁ rst demonstrated

H →

Si CP in 1980 [182] ,

in a study of the surface of silica gels. A discussion of the development of CP and

other surface characterization techniques is provided by Maciel and Ellis [183] .

More recently Aramendia and coworkers have applied CP - MAS NMR to the study

of the silica support of a Pd/SiO

catalyst [184] . Figure 5.26 shows the CP - MAS

NMR spectrum of a silica support showing three distinct signals at − 110, − 101

and − 92 ppm, which are assigned to Si atoms in Si

−

O tetrahedra of the SiO

structure, silanol groups and silanodiol groups respectively.

Sasikala and coworkers have employed

H -

Si CP - MAS NMR in the study of

/SiO

catalysts [97] . They have acquired spectra of SiO

, modiﬁ ed silica samples

and vanadia - impregnated samples and have compared them to

Si single - pulse

and CP - MAS NMR spectra. Surface hydroxyl groups provide the

H source in these

experiments. The CP - MAS technique is inherently sensitive to the structure of the

surface silica units. This allows a number of distinct silica units on the surface to

be identiﬁ ed, something that is impossible through conventional single - pulse

MAS NMR. The effect of vanadium impregnation is to reduce the intensity of the

5.4 Surface Properties of Metal Oxides 227

228 5 Solid-State NMR of Oxidation Catalysts

peak corresponding to silica units comprising two SiO

tetrahedra attached to Si

(manifested as the shoulder on the downﬁ eld side of the peak at ∼ − 100 ppm). This

is due to the breakage of Si

−

Si units in larger structural units.

In addition to studies of silica supports work has also been carried out focusing

on aluminas. However, the application of

H →

Al CP - MAS NMR is complicated

as compared to

H →

Si CP, as this involves transfer between a half - integer

nucleus (

H) and a quadrupolar nucleus (

Al). The issues surrounding this have,

however, been discussed in depth by a number of workers [183, 185 – 190] . Despite

these challenges, the application of

H →

Al CP - MAS NMR to catalytic systems

has been successfully demonstrated. The ﬁ rst such study was carried out by Morris

and Ellis in 1989 [186] for hydrated γ - Al

. However, this was only partially suc-

cessful as it failed to observe any tetrahedrally coordinated surface aluminum

as result of processes which are unfavorable to CP, including fast spin – lattice

relaxation [186] .

Some of the problems with CP from adsorbed hydroxyls or water molecules can

be overcome through adsorbing a hydrogen - containing base molecule, such as

pyridine, and using this as the source of

H species [186] . This allows the surface

to be dehydroxylated, eliminating the possibility of dynamic surface processes

providing highly efﬁ cient quadrupolar relaxation pathways broadening signals

beyond detection. Furthermore, as demonstrated by Coster and coworkers [191] ,

chemisorbed water induces surface reconstruction of aluminas. Therefore, CP -

MAS NMR spectra acquired from hydrated alumina surfaces do not represent the

true nature of the surface during catalytic operation. Ammonia may used in the

same manner as pyridine to provide a

H source for

H →

Al CP - MAS NMR

experiments. This has been exploited by Alvarez and coworkers [192] , who com-

pared the distribution of 4 - , 5 - and 6 - coordinate Al atoms at the surface, as indi-

cated by CP - MAS, with molecular dynamics simulations. Good agreement was

achieved between theory and experiment.

In addition to CP to the metal nuclei in metal oxides,

H →

O or M →

(where M is a metal nucleus) CP - MAS NMR is also possible. The pioneering work

in this area was conducted by Oldﬁ eld and coworkers [38, 124, 149, 193] . In a study

of SiO

, a previously unobserved resonance for amorphous silica, assigned to

silanol (Si

−

H) sites present on the surface, was identiﬁ ed through the use of

Figure 5.26

Si CP - MAS NMR spectrum of an SiO

catalyst

support. Adapted from ref. [184] ; reprinted with permission

from Elsevier.

O →

H CP. Ashbrook and Smith have reviewed the application of

O NMR

techniques, including

H →

O CP - MAS NMR, to oxygen - containing materials

[194] . Ashbrook and coworkers had previously performed

H →

O CP - MAS NMR

to boehmite, an aluminum hydroxide precursor to catalytic aluminas [53] . This

allowed the observation of the hydroxyl oxygen species at natural abundance levels,

providing signiﬁ cant sensitivity gains over conventional NMR methods. Spectra

were also acquired of

O - enriched boehmite which, as Figure 5.27 shows, provides

a signiﬁ cant signal enhancement.

5.4.2

Surface Acidity

In addition to understanding the bulk structure of catalysts and the distribution

of bulk nuclei at the surface, an appreciation of the number and nature of acid

sites is also of key importance. A number of NMR techniques have been developed

in order to probe both Lewis and Br ø nsted acidity at surfaces, and extensive

reviews on many of them have been produced by various authors [183, 195,

196] .

5.4.2.1 Br ø nsted Acid Sites

Br ø nsted acid sites can be directly probed through solid - state

H NMR spectros-

copy, as chemical shifts can be correlated with acid strength [195, 197, 198] . The

precise chemical shift observed for any given Br ø nsted acid site is dependent on

the material upon which it is located. For instance, on silica values of ∼ 1.6 ppm

are typically observed; zirconia has two distinct OH sites, at 2.4 and 4.8 ppm; while

on alumina a typical range may be − 0.2 to 4.3 ppm. Early studies employing

NMR to study Br ø nsted acid sites focused on the characterization of the surface

of amorphous silica – alumina materials [165, 199 – 201] . Extensive work, however,

Figure 5.27

O cross - polarized MAS NMR spectra of (a) 35%

O - enriched and (b) natural abundance boehmite. In both

cases, the MAS rate was 7 kHz. Adapted from ref. [53] ;

reprinted with permission from Elsevier.

5.4 Surface Properties of Metal Oxides 229

230 5 Solid-State NMR of Oxidation Catalysts

has also been conducted on supported catalysts and catalyst supports, including

alumina - supported catalysts.

The possible coordination environments of OH groups on Al

is dictated by

the surface termination of the alumina; thus a wider variety of OH sites is possible

on γ - Al

, which contains both octahedrally and tetrahedrally coordinated Al

nuclei, than on α - Al

in which only octahedral sites are present. The interaction

between OH groups and the surface of a transition alumina, and alumina -

supported vanadia and molybdenum catalysts, has been probed by

H MAS NMR

spectroscopy [202] . In all cases a number of distinct OH sites are clearly identiﬁ ed.

The effect of vanadia deposition on Al

is to titrate the surface hydroxyl groups,

with the most basic groups being titrated preferentially (Figure 5.28 ).

H MAS

NMR also reveals unreacted alumina hydroxyls present in the catalysts, which are

located in closed pores of the alumina support. These were not directly detectable

by IR spectroscopy. In the same work, deuteration experiments indicate the pres-

ence of some accessible

H nuclei present only at high vanadia loadings. These

H species, also not revealed by IR studies, are suspected to be Br ø nsted sites.

These sites are not removed, even at monolayer coverages of vanadium oxide.

In addition to alumina - supported catalysts, Mastikhin and coworkers have also,

through

H MAS NMR, investigated Br ø nsted acid sites on V/SnO

, V/Sb

[75] ,

V/TiO

- ZrO

, [76] , V/TiO

[81] , V/AlPO

[203] , V/TiO

- SiO

[83] and (VO

/WO

(TiO

- Al

) [85] catalysts.

Wang and coworkers have studied the strength and concentration of acid sites

on a MoO

/SnO

catalyst [167] . As shown in Figure 5.29 , at least two

H signals

Figure 5.28

H MAS NMR spectra of VO

/Al

catalysts as a

function of vanadium oxide content: (a) 0%, (b) 5%, (c) 17%,

and (d) 19%. Asterisks denote spinning sidebands. Adapted

from ref. [202] ; reprinted with permission from Elsevier.

(0.9 and 4.2 ppm) corresponding to OH groups are observed over the pure support.

Deposition of molybdenum species onto the support results in a shift of the down-

ﬁ eld peak to 4.5 ppm, and the intensity of the signal at 0.9 ppm is reduced, most

probably indicating the interaction of Mo with the surface Sn

−

OH groups. Else-

where

H MAS NMR has been applied to the study of the coordination of OH

groups on MgO [204] and to SiO

, AlPO

and Mg

(PO

)

, which are used as sup-

ports for palladium hydrogen - transfer hydrogenation catalysts [184] . Figure 5.30

Figure 5.29

H MAS spectra of SnO

support (a) and MoO

SnO

catalyst (b). The asterisk denotes spinning sidebands.

Adapted from ref. [167] ; reprinted with permission from the

Royal Society of Chemistry.

Figure 5.30

H MAS NMR spectra for (a) SiO

, (b), AlPO

and

(PO

)

supports. Sidebands are marked with an asterisk.

Adapted from ref. [184] ; reprinted with permission from Elsevier.

5.4 Surface Properties of Metal Oxides 231

232 5 Solid-State NMR of Oxidation Catalysts

compares the

H MAS NMR spectra for these three materials. The SiO

spectrum

contains only one peak, at 1.8 ppm, corresponding to isolated Si

−

OH groups;

AlPO

contains two distinct OH groups as shown by signals at − 0.1 and 3.3 ppm;

while the spectrum of Mg

(PO

)

contains three distinct hydroxyl peaks (3.1, 1.2

and − 0.1 ppm) that have been assigned to P

−

OH, Mg

−

OH and basic Mg

−

groups, respectively.

In addition to conventional

H MAS NMR measurements, more sophisticated

techniques such as CRAMPS [201] and spin - echo double resonance [205] can be

applied to the study of surface hydroxyl groups. Deng and coworkers [205] have

applied

Na} double resonance to the study of Na

– γ - Al

catalysts.

Through this it can be deduced with which surface nuclei different OH groups

are associated. In particular, under the strong irradiation of sodium during the

spin - echo experiment the

H signals corresponding to hydroxyl groups strongly

associated with sodium atoms will be suppressed, while those associated with

other surface species will remain unaffected. In this example three distinct surface

OH groups were identiﬁ ed, manifested in signals at 0 ppm (more basic Al

−

OH),

2 ppm (more acidic Al

−

OH) and 4.5 ppm (OH associated with Na).

Taking a similar approach, Crepeau and coworkers have conducted

Al}

TRAPDOR experiments on silica – alumina catalysts in order to distinguish

nuclei on sites adjacent to Al nuclei from other OH groups [206] . Two types of

H species are observed in conventional

H MAS spectra – at 1.8 and 2.8 ppm.

During the TRAPDOR experiment the use of

Al irradiation during

H signal

acquisition suppresses the signal from hydrogen nuclei in close proximity to

aluminum atoms. As Figure 5.31 shows, this results in the suppression of the

H signal at 2.8 ppm. These nuclei, representing a relatively small fraction of the

Figure 5.31

H {

Al} TRAPDOR experiment of Si

OAl

(Spinning rate = 10 kHz. B

= 9.4 T). The full and the dotted lines

correspond, respectively, to

H MAS spectrum acquired without

and with

Al irradiation. Adapted from ref. [206] ; reprinted with

permission from the American Chemical Society.

total

H density, are therefore located close to Al sites. The majority of hydrogen

nuclei are therefore located further from Al sites and are therefore likely to exist

as silanol groups.

In addition to directly probing Br ø nsted acid sites through

H NMR, character-

ization can also be carried out through observing the resonance of an appropriate

probe molecule. The

P - resonance of organic phosphine and phosphine oxides

bound to Br ø nsted acid sites is distinct from that of the same materials bound to

Lewis acid sites [207 – 209] . Again the chemical shift can be correlated with acid

strength, with higher chemical shifts corresponding to more acidic sites. Such a

method has been employed by Wang and coworkers to characterize acid sites on

MoO

/SnO

catalysts using trimethylphosphine as the probe molecule [167] .

Chen and coworkers employed trimethylphosphine oxide ( TMPO ) as the probe

molecule to characterize pure and sulfated zirconia both with and without a metal

promoter [210] . On pure ZrO

resonances were detected at 62, 53, 41 and 34 ppm.

The peaks at 53 and 62 ppm correspond to physisorbed TMPO while the peaks at

62 and 53 ppm can be assigned to TMPO adsorbed on Br ø nsted (B) and/or Lewis

(L) acid sites. Upon hydration the intensity of both these signals diminished,

indicating that they in fact correspond to Lewis acid sites. Had they been Br ø nsted

sites the interaction between TMPO and the catalyst would have been too strong

to be inﬂ uenced by hydration. Upon addition of a metal promoter additional reso-

nances are observed at 87, 76, 68 and 65 ppm. Hydration experiments conﬁ rm that

these new sites are Br ø nsted acid sites. Furthermore, quantiﬁ cation of the distinct

acid sites identiﬁ ed on the catalyst has been carried out, based on the assumption

that each TMPO molecule can only be adsorbed on one acid site.

In addition to organic phosphines and phosphine oxides, other probe molecules

which have been successfully employed in the study of Br ø nsted sites include

alkanenitriles [211 – 213] (

C and

N NMR), acetone [167] and CO [214] (

NMR).

5.4.2.2 Lewis Acid Sites

As alluded to above, probe molecules such as organic phosphates can also be used

to probe Lewis acid sites. However, the most common probe molecules employed

are nitrogen - containing species, in particular ammonia, alkylamines and pyridine.

For the latter two either

C or

N NMR techniques may be applied; however,

NMR is generally preferred as the resultant spectra appear less complex and, as

adsorption to the surface occurs via the nitrogen atom, the observed resonance

shifts are larger than they would be for

C spectra. The development of this

method to investigate Lewis acidity of aluminas and silica – aluminas has previously

been discussed in detail by Eckert [196] and by Maciel and Ellis [183] and will only

brieﬂ y be recapped here.

The initial studies demonstrating the feasibility of using probe molecules to

study acid sites were conducted by Gay and coworkers from 1977 onwards [215,

216] . This approach was further developed by Dawson and coworkers who employed

C CP - MAS NMR to investigate the number and nature of acid sites on γ - Al

through the adsorption of n - butylamine and pyridine [217, 218] . In the case of

5.4 Surface Properties of Metal Oxides 233