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

Подождите немного. Документ загружается.

426 9 Thermal Analysis and Calorimetric Methods

The effect of dealumination can be observed in a series of commercial

Y - faujasites with Si : Al ratios of 2.6, 5.8, 12.8 and 24. The modiﬁ cation of the

acidity resulting from the dealumination is illustrated by the curves of differential

heats of ammonia adsorption (Figure 9.16 ).

The initial heats, in the 180 – 230 kJ mol

− 1

range, correspond to particularly strong

Lewis sites. With the exception of the most dealuminated zeolite, the curves

present a quasi plateau of heats around 140 kJ mol

− 1

(attributed to Br ø nsted sites),

then decrease to about 70 kJ mol

− 1

, a value often considered as the chemisorp-

tion – physisorption limit for ammonia. These curves indicate the existence of three

populations of sites: strong sites (Q above 150 kJ mol

− 1

), medium sites (120 < Q <

150 kJ mol

− 1

) and weak acid sites (70 < Q < 120 kJ mol

− 1

). In terms of relative site

populations, the two most dealuminated zeolites present a higher number of very

strong sites, owing to the presence of EFAL species.

Flow adsorption microcalorimetry has been used to measure the heats of adsorp-

tion of ammonia in a nitrogen carrier on the H

and Na

forms of a Y zeolite [21] .

The calorimeter was linked to a thermal conductivity detector in which the rates

of adsorption and desorption and the associated rates of heat evolution or absorp-

tion were measured simultaneously at atmospheric pressure. The authors found

that, as surface coverage increased, the sites covered ﬁ rst were not necessarily

those with the highest molar heats of adsorption.

Calorimetric investigations of the adsorption of water, ammonia, methanol and

other small polar molecules on the Na forms of synthetic zeolites A, X and Y have

demonstrated the heterogeneous nature of the Q

diff

versus n

relation, which can

be explained by the successive interactions of the exchange cations at the various

crystallographic positions [14] .

The energy of interaction with ammonia and therefore the acid strength of

mordenite at low coverages are higher than the corresponding data for HY molecu-

lar sieves. The overall initial acid strengths of the H - forms of mordenite, ZSM - 5

and faujasite may be arranged in the following order: HM ( ≈ 170 kJ mol

− 1

) > H -

ZSM - 5 ( ≈ 155 kJ mol

− 1

) > H - Y ( ≈ 140 kJ mol

− 1

Figure 9.16 Differential heats of ammonia adsorption versus

coverage for dealuminated HY zeolites.

When using pyridine as a probe molecule, the heats evolved are higher than

with ammonia [36] . However, unlike ammonia, pyridine, which is a larger mole-

cule, can titrate only OH groups located in large channels. Heats of adsorption of

pyridine were measured at 573 K on a series of Na - exchanged mordenites [113] .

The curves of differential heats of adsorption versus pyridine coverage were ana-

lyzed, and Figure 9.17 shows the number of sites of a given strength as a function

of the exchange level. Three populations of sites, above 190 kJ mol

− 1

, within 190 –

150 kJ mol

− 1

and within 150 – 90 kJ mol

− 1

, are apparent (Figure 9.17 ). A moderate

increase of the population of very strong sites (heat above 190 kJ mol

− 1

) is apparent

in this ﬁ gure, whereas the number of sites with Q

diff

values lying between 190 and

150 kJ mol

− 1

increases considerably with the exchange degree up to 47% exchange

and then remained constant above this value. These results suggest that, as long

as new hydroxyls are formed in large channels, their average acidic strength

increases. Above 47% exchange, the acidic strength of hydroxyls in the large chan-

nels remains constant, which indicates that the formation of hydroxyls taking place

in side pockets (above 50% of cation exchange) does not inﬂ uence the properties

of hydroxyls in large channels [14] . Concomitantly, the number of sites that display

a moderate strength (between 150 and 90 kJ mol

− 1

) decreases and then remains

constant.

Adsorption enthalpies of N

, CO, CH

CN and NH

on H - BEA and H - MFI zeo-

lites have been measured calorimetrically at 303 K in order to assess the energetic

features of the various interactions occurring within the zeolite nanocavities,

namely: (i) speciﬁ c adsorption on Lewis and Br ø nsted acid sites; (ii) H - bonding

interaction with hydroxyl nests; (iii) conﬁ nement effects. Whereas CO and N

single out contributions from Lewis and Br ø nsted acid sites, CH

CN and NH

are

not preferentially adsorbed on Lewis sites, suggesting that adsorption on Br ø nsted

sites is competitive with that on Lewis sites. For CO adsorbed on a Lewis - rich

H - β zeolite, the initial Q

diff

value is ≈ 70 kJ mol

− 1

, compared with 60 kJ mol

− 1

for

H - ZSM - 5 [114] .

9.3 Surface Properties of Oxides 427

Figure 9.17 Acid site strength distribution of Na,H -

mordenites as a function of the exchange level. Pyridine

adsorption at 573 K, activation at 773 K.

428 9 Thermal Analysis and Calorimetric Methods

Alkali - exchanged zeolites are basic solids and are recognized as effective cata-

lysts for a wide range of organic transformations, such as aldol condensation,

alkylation and etheriﬁ cation. The basic strength and catalytic activity of alkali -

exchanged zeolites can be increased by the occlusion of alkali metal oxide clusters

via impregnation and decomposition of alkali metal compounds such as acetates

or hydroxides. Oxides of cesium and potassium have been synthesized in the

supercages of zeolite X and the alkali - loaded zeolites have been characterized by

both CO

adsorption microcalorimetry and stepwise TPD ( STPD ) of CO

[115] . For

a loading of about two alkali metal atoms per zeolite supercage, the majority of

adsorption sites at 373 K were characterized by a ∆ H

ads

of about 80 – 100 kJ mol

− 1

The CO

adsorption capacity of CsO

/CsX increased linearly with the number of

excess cesium atoms, but the strength of most adsorption sites seemed to be unaf-

fected by the amount of excess cesium species. The basic strengths were found to

vary in the order CsO

/CsX > CsO

/KX > K O

/KX. However the differential heats

at low CO

coverage were about the same for all samples. The values of CO

adsorp-

tion capacities from STPD were slightly higher than those from adsorption micro-

calorimetry [115] .

Silicalite can be regarded as a new polymorph of silica, presenting adsorption

characteristics close to those of a ZSM - 5 zeolite with an extremely high Si : Al ratio.

The heats of adsorption of both quadrupole (N

and CO

) and non - polar (Ar, O

, C

and SF

) gases were determined on a silicalite in order to study the

effect of adsorbate size and polarity on the energetics of adsorption in zeolites

[116] . Silicalite was classiﬁ ed as a relatively homogeneous adsorbent compared to

X - type zeolite. The accuracy of the calculated isosteric heats of adsorption was

estimated to be ± 2% for heats larger than 20 kJ mol

− 1

and ± 5% for heats smaller

than 20 kJ mol

− 1

. Examples of calorimetrically measured heats for adsorption of

pure SF

and CO

on a silicalite sample bonded with alumina have also been

reported and compared with calculated isosteric heats [117] .

ETS - 10 is a titanosilicate with a three - dimensional 12 - ring pore system and a

very high ion - exchange capacity. The integral heats of adsorption of monoalkyl-

amines on ETS - 10 have been measured by isothermal calorimetry as a function

of the n - alkylamine concentration [118] . The heats vary in the order methyl < ethyl

< propyl < butyl amine.

Copper - exchanged ETS - 10 catalysts with low and high degrees of exchange have

been prepared and studied using calorimetry of adsorption of NO, CO, C

and

probe molecules [119] . Moreover, DSC curves of reduction by hydrogen as a

function of temperature yielded useful information concerning the identiﬁ cation

of supported species and the determination of the intermediate oxidation states of

copper during reduction (Figure 9.18 ).

The low - temperature shoulder for the under - exchanged Cu - ETS - 04 and Cu - ETS -

1 samples can be assigned to the reduction of Cu

to Cu

and the main peak to

the reduction of Cu(I) to Cu(0), which conﬁ rms that Cu(I) species are predominant

in these samples, while in the over - exchanged sample Cu - ETS - 5, most of the Cu

species are in close contact and not inserted in the support.

A fairly large number of papers have been published on the use of calorimetry

to study the acidity of mesoporous materials [14] .

Mesoporous molecular sieves are close to microporous zeolites in their methods

of preparation and principal regions of application. These materials are usually

synthesized by using supermolecular templates and, in particular, the micelles of

cationic surfactants [17] .

According to the data from calorimetric investigations of the adsorption of

ammonia, unlike silica gel, the pure - silica mesoporous sieve MCM - 41 possesses

speciﬁ c although weak acidity. The introduction of titanium or zirconium, and

particularly aluminum, ions into the structure of this sieve substantially increases

the acid strength of the sorbent [120, 121] .

Using ammonia as probe, the strength of the acid centers of the investigated

samples can be arranged in the following order: silica gel < Si - MCM - 41 < Si,Zr -

MCM - 41 < Si,Ti - MCM - 41 < Si,Al - MCM - 41. The heats of NH

adsorption on the

latter sample with molar ratio SiO

: Al

= 32 : 8 are 130 – 150 kJ mol

− 1

, which is

close to the heats of interaction of ammonia with a ZSM - 5 zeolite.

Spectroscopic and calorimetric investigations of the adsorption of acetonitrile

[122] have shown that the sorbent Si,Al - MCM - 41 has strong Lewis (aprotic) acidity

and poorly deﬁ ned Br ø nsted (protic) acidity [17] . The presence of strong Lewis

centers on Si,Al - MCM - 41 was conﬁ rmed by ﬂ ow microcalorimetry for the adsorp-

tion of 1 - butanol from a solution of n - hexane [123, 124] . Depending on the alumi-

num content of the structure, the number of strong Lewis centers on the surface

of this sample was found to vary in the range of 43 to 70% of the total number of

active centers.

9.4

A Case Study: Vanadia Catalysts

It is known that catalytic activity in selective oxidation of hydrocarbons can

be related to the co - operative action of an oxidizing function and an acidic

function. From this perspective, the determination of both the redox and the acidic

Figure 9.18 Heat ﬂ ow proﬁ les for reduction under H

ﬂ ow as

a function of temperature for various Cu - ETS samples.

9.4 A Case Study: Vanadia Catalysts 429

430 9 Thermal Analysis and Calorimetric Methods

properties of the catalytic centers, as carried out in the present study using pulsed

adsorption and ﬂ ow calorimetry, becomes a topic of renewed importance.

Vanadium oxide is known to catalyze oxidation of hydrocarbons, and the selec-

tive ODH of ethane over V

catalysts has been extensively studied. The acid – base

and redox characters of vanadium pentoxide, V

/SiO

, V

/ γ - Al

, V

/TiO

/SiO

, V

/CeO

, V

/Nb

and V

/MgO catalysts have been

investigated by adsorption microcalorimetry of probe molecules and/or TPR/TPO

calorimetric experiments respectively.

adsorption calorimetry experiments performed on bulk V

indicated a

weak acidity, with an initial heat of adsorption of 70 kJ mol

− 1

and a large heteroge-

neity [125] .

TPR/TPO experiments were performed in a TG - DSC apparatus on a bulk V

[125] using a gas ﬂ ow of C

, C

or H

(diluted in helium) as the reducing

agents, at a heating rate of 5 K min

− 1

. The reduction temperatures and heat ﬂ ows

depended greatly on the reducing agent, with reduction heats increasing in the

order C

< C

< H

. According to the weight loss, a similar reduction extent

occurred whether hydrogen, ethane or ethene was used as reducing agent, corre-

sponding to the formation of V

in all cases. In the case of ethene, various

intermediate suboxides could be deduced by examining the derivative of the

thermogravimetric curve. Following each reduction, re - oxidation studies were

performed under oxygen ﬂ ow in the same equipment, clearly showing different

steps in the re - xidation process.

Regular pulses of pure ethane on bulk V

maintained at 823 K in a microcalo-

rimeter linked to a gas chromatograph provided kinetic data of theoretical signiﬁ -

cance, as well as an insight into the mechanism of the reduction process. The

results of this work carried out using mainly calorimetric techniques led to the

conclusion that diffusion of oxygen from the bulk is predominant in the selective

oxidation of ethane and that the redox process plays a more important role than

the acidic sites in the case of unsupported vanadium pentoxide.

/SiO

catalysts prepared by grafting or wet impregnation methods have also

been studied in the ethane ODH reaction. The surface characteristics and the

reactivity of these catalysts have been investigated using microcalorimetry linked

to other techniques such as volumetric studies and thermogravimetry. The vanadia

amounts ranged from 1 to 20 wt%. The numbers of acid sites of the supported

catalysts were found to increase with vanadium loading, leading to materials with

a moderately acidic character [126, 127] .

Compared to bulk vanadia, higher values of the initial heats of NH

adsorption

have been measured for the V

/SiO

catalysts (78 to 124 kJ mol

− 1

). Considering

the overall value of chemisorbed ammonia versus the vanadium content, the

population of titrated acid sites was slightly higher for the grafted V

/SiO

cata-

lysts than for the samples prepared by wet aqueous impregnation for similar V

loadings. A linear relation was also observed between the integral enthalpy of

ammonia adsorption and the vanadium content of the catalysts.

Interaction with the support was enhanced after catalytic reaction, as indicated

by the detection of new strong Lewis acid sites by DRIFT spectroscopy measure-

ments and higher heats of ammonia adsorption [128] . The catalytic activity

increased with the vanadium content and the ammonia uptake of the V

/SiO

catalysts, that is with their acidity. However, selectivity to ethane appeared to be

more closely related to the coordination of the vanadium atoms than to the acidic

character of the surface of the V

/SiO

catalysts. The activity and selectivity to

ethane were enhanced on V

/SiO

especially for very low vanadium contents,

presumably corresponding to isolated vanadium cations, which are more easily

reduced in ethene as shown by DSC. As stated above, both the number and the

strength of the acid sites of the silica - supported catalysts increased as the vanadia

loading increased, independently of the preparation method. Comparing catalysts

at similar V

loadings prepared by impregnation and by atomic layer deposition

( ALD ) it appears that the ALD preparation yields a V

phase characterized by a

higher number of acid sites and a stronger surface acidity than the preparation by

impregnation (Figure 9.19 ) [129] . ALD is a gas - phase layer - by - layer processing

method based on surface - saturating precursor adsorption on the support, thus

allowing a high dispersion of the deposited species.

The acid – base properties of V

/ γ - Al

catalysts prepared by the impregnation

method have been characterized by ammonia, pyridine and sulfur dioxide adsorp-

tion microcalorimetry. Sulfur dioxide adsorption made it possible to differentiate

a vanadate layer from free alumina.

Ammonia and sulfur dioxide adsorption experiments also showed that,

at low vanadium coverage, a large part of the vanadate layer is bound to acid – base

pairs of alumina. At low vanadium coverage ( < 3 wt% V

), the acidic character

of the V

/ γ - Al

catalyst can be ascribed to vanadium - free alumina, whereas

at higher vanadium coverage it is largely attributed to vanadate compounds

[130] .

9.4 A Case Study: Vanadia Catalysts 431

Figure 9.19 Differential heats of ammonia adsorption over a

silica support and silica - supported vanadia catalysts prepared

by ALD (ﬁ lled symbols) and impregnation (open symbols).

VS - A6 and VS - I6 on one hand, and VS - A12 and VS - I10 on the

other hand, have comparable vanadia contents.

432 9 Thermal Analysis and Calorimetric Methods

Figure 9.20 displays the differential enthalpy of SO

adsorption at 353 K as a

function of the probe uptake on samples with various vanadium contents and on

pure γ - Al

The adsorption of sulfur dioxide of the catalysts rapidly decreased with the

vanadium content of the samples. As sulfur dioxide is not chemisorbed on bulk

vanadium pentoxide and also appears not to chemisorb on vanadate species, on

/ γ - Al

catalysts this probe can be considered to be selective for the titration

of the basic sites of vanadium - free alumina. Thus, by selectively probing uncovered

alumina, SO

further allows the distinction between the acid – base features of the

vanadium oxide layer and those of uncovered alumina.

The acid – base properties of a 5.2 wt% V

/ γ - Al

catalyst have also been

studied after performing measurements of catalytic activity for ethane conversion

to ethene by microcalorimetry of adsorption of NH

and SO

at 353 K. Figure 9.21

displays the acidic and basic features of this catalyst before and after the catalytic

run. The acid – base features of γ - Al

are also reported for reference. This ﬁ gure

conﬁ rms that V

is preferentially anchored on the basic sites of the alumina,

resulting in a large decrease in basicity. It was also observed that some additional

acid sites (0.1 µ mol m

− 2

) have appeared after the catalytic run. This amount corre-

sponds to 5% of the overall acidic sites of the catalyst. It is interesting to note the

same increase in basic sites. These features are consistent with the restoration of

some acid – base pairs of γ - Al

and show the overall stability of the acid – base

character of the vanadium oxide layer [131] .

A 20 wt% V

/ γ - Al

catalyst has been subjected to a TPR in ethane and re -

oxidation in air using TG - DSC equipment, and compared to bulk vanadium pent-

oxide. Reduction in ethane occurs at a lower temperature on the V

/ γ - Al

catalyst (717 compared with 794 K for bulk vanadium pentoxide). The exothermic

Figure 9.20 Differential heat of sulfur dioxide adsorption at

353 K as a function of the probe uptake on V

/ γ - Al

catalysts with different wt% of V

as indicated in the ﬁ gure.

enthalpy evolved in the reduction is much higher on the alumina - supported cata-

lyst. A reduction step by itself is an endothermic process. The observed exothermic

enthalpies involve two terms: an enthalpy of reaction (exothermic) and an enthalpy

due to lattice oxygen migration (endothermic). It was deduced from the calorimet-

ric data that lattice oxygen migration to react with ethane requires less energy on

the V

/ γ - Al

catalyst, that is lattice oxygen mobility is improved on that

catalyst.

Nano - sized V

/SiO

, V

/Al

, V

/TiO

and V

/TiO

/SiO

can be pre-

pared by ALD. Compared to liquid - phase preparations, the technique gives greater

beneﬁ ts in terms of species dispersion and uniformity.

Alumina - , silica - and titania - supported vanadium oxide systems with V

load-

ings ranging from 3 to 12 wt%, corresponding to 0.02 to 0.09 V/(Al, Si, Ti) atomic

ratios, were prepared by ALD and compared with the corresponding impregnated

catalysts [129] . The surface acidic properties of the supports and catalysts were

investigated using ammonia adsorption microcalorimetry to determine the number

and strength of the surface acid sites. Deposition of V

on alumina and titania

supports gave rise to catalysts with lower numbers of acid sites than the respective

supports, while for the samples prepared on silica, an increase of the number of

acid sites was observed after V

deposition, thus conﬁ rming the study performed

by Le Bars and coworkers [131] . As a common trend, the surface acid strength was

greater for the ALD catalysts than for the impregnated ones, suggesting a stronger

interaction of the VO species with the support centers, which act as electron attrac-

tor centers creating Lewis - like vanadium species.

Synergistic interactions of highly dispersed vanadia and titania species on silica

have been shown to control the acidity, reducibility and thus the overall activity of

the V

/TiO

/SiO

catalysts in the selective oxidation reaction of o - xylene to

phthalic anhydride [132] . Using ammonia adsorption calorimetry and pyridine

adsorption IR spectroscopy it was shown in this study that the strong Lewis acidity

of a pure titania support was masked by the deposition of vanadia, the decrease

in the number of acid sites being especially pronounced in the case of the highly

9.4 A Case Study: Vanadia Catalysts 433

Figure 9.21 Differential enthalpies of adsorption of ammonia

and sulfur dioxide at 353 K on 5.2 wt% V

/ γ - Al

434 9 Thermal Analysis and Calorimetric Methods

dispersed ALD samples. On V

/TiO

/SiO

samples, the better dispersion of

active species in the ALD samples was veriﬁ ed by the acidity measurements. The

number of strong, mainly Lewis, acid sites increased in the ALD catalysts with

sub - monolayer titania coverage. In the catalysts with high titania content, the ALD

vanadia species masked the acidity better than the species prepared by impregna-

tion. It was also shown using ammonia and o - xylene adsorption microcalorimetry

that the presence of strong surface acidic and o - xylene adsorption sites decreased

the activity of the catalysts in the test reaction of o - xylene oxidation. Medium -

strength and strong o - xylene adsorption sites, with evolved heats between 120 and

150 kJ mol

− 1

, seemed to be the most detrimental for the catalytic activity.

In another study [133] , microcalorimetric adsorption of reactants and products

on supported vanadia catalysts for the selective oxidation of propene to acetone

was used to provide information about the surface bonding strengths that may be

used for the microkinetic analysis of the corresponding surface reactions. The

initial heat for water adsorption on the studied V

/TiO

catalyst is lower than

90 kJ mol

− 1

, indicating that water is reversibly adsorbed at temperatures higher

than 360 K. The heat of adsorption of acetone on the reduced V

/TiO

catalyst

is about 100 kJ mol

− 1

, which can be used to estimate the activation energy for

desorption of acetone from the catalyst [133] .

The acid – base properties of V

/CeO

catalysts prepared by the wetness impreg-

nation technique have been determined by NH

and CO

adsorption calorimetry

at 423 K [134] . Variations in V

loading and calcination temperature brought

about changes in the surface structure of the dispersed vanadium species, and

hence in the surface acidic and redox properties. CeO

possessed fairly strong

surface acidity and basicity. Addition of V

enhanced the surface acidity as well

as the redox properties, but decreased the surface basicity. Calcination of the

10 wt% V

/CeO

catalyst at 873 K resulted mainly in the formation of CeVO

the surface, which showed low surface acidity and redox ability.

Microcalorimetry was also used to measure the heats of adsorption of ethanol

and 2,2,2 - triﬂ uoroethanol as a function of coverage at 300 K for a 6 wt% V

/CeO

sample. The calorimetric data showed a constant heat of adsorption of 65 kJ mol

− 1

[135] .

catalysts supported on niobyl phosphate (NbOPO

) with V

loadings

from 5 to 25 wt% were prepared by incipient wetness impregnation, and the

surface acidity was determined by microcalorimetry using NH

as a probe mole-

cule. The results suggested that the loaded V

weakened the surface acidity of

NbOPO

while increasing the proportion of weak acid sites (Figure 9.22 ). The

surface of NbOPO

presented a purely acidic character and weakened the redox

properties of supported V

[136] .

Magnesium vanadates, also known as VMgO catalysts, are known to be efﬁ cient

catalysts for the ODH of light alkanes. Corma and coworkers [137] suggested that

the incorporation of small amounts of V on a MgO surface notably modiﬁ ed not

only the redox properties of the system but also the acid – base character of the

oxygen species present on the catalyst surface. These acid – base properties were

related to the nucleophilic character of the lattice oxygen anions.

A series of VMgO catalysts with various compositions and surface areas were

characterized at 353 K, using adsorption calorimetry of NH

and SO

to probe their

acid – base properties. The acid strength of the VMgO samples increased up to

14 wt% of V loading. For higher loadings, the strong acidity decreased, while

weaker acid sites developed above 25 wt%. The basicity of the VMgO samples

remained nearly unchanged for loadings up to 25 wt% V. At higher loadings, the

basicity decreased, and vanished for the 45 wt% V loading sample [138] .

All these studies have demonstrated the usefulness of calorimetry for studying

the surface characteristics of vanadia - based catalysts. Among the surface proper-

ties of vanadia catalysts of relevance to their catalytic activity in selective oxidation

reactions, acidity is one of the most signiﬁ cant.

Protonic acidity of the support is not desirable as it leads to side - reactions such

as polymerization or cracking of organic molecules at high temperatures and

coking. Complete coverage of the support by a V

phase might give rise to Lewis -

like vanadium centers, which are characterized by a lower acid strength than the

acid centers of the oxide supports.

For vanadia catalysts supported on alumina, titania or niobyl phosphate, the

main trend evidenced by ammonia adsorption microcalorimetry is a remarkable

decrease of the number of acid sites, together with a decrease of the total acid

strength compared with the acidic properties of the bare support. By contrast,

owing to the neutral behavior of the silica surface, the acidity of V

phases

deposited on silica becomes much more apparent. A similar trend was observed

on the basic magnesia and amphoteric ceria. A decrease in basicity was observed

on all amphoteric supports ( γ - Al

,TiO

/SiO

, CeO

), allowing the distinc-

tion between acid – base features of the vanadium oxide layer and those of the

uncovered support.

9.4 A Case Study: Vanadia Catalysts 435

Figure 9.22 Differential heats of ammonia adsorption versus

coverage for various niobyl phosphate supported vanadia

samples.