Bergaya F. Handbook of Clay Science

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The thermal decomposition sequence of Mg-based LDH, as typiﬁed by

Mg–Al–CO

LDH, is well documented. The MgO phase is observed between ca.

400 and 850 1C, and has a smaller lattice constant a than pure MgO due to the

incorporation of Al

ions. Above 900 1C, Al ions are released and the spinel

(MgAl

) is formed. Pure MgO and MgAl

are obtained at 1000 1C(Fig. 13.1.8)

(Miyata, 1980; Hibino et al., 1995). Mg–Ga–CO

LDH behave in a similar way

(Lopez-Salinas et al., 1997c). In the case of Mg–Fe–CO

LDH, MgFe

spinel

forms together with a MgO phase at 350 1C(Rouxhet and Taylor, 1969; Ferna

ndez

et al., 1998; Hibino and Tsunashima, 2000).

Mg-based LDH-containing oxidiz able trivalent metal ions yield oxides where the

metals occur in varied states of oxidation (Ferna

ndez et al., 1994 ; Labajos et al.,

1996; Labajos and Rives, 1996; Bahranowski et al., 1999). For LDH with tetravalent

metal ions, all the constituent metal ions are homogeneou sly dispersed in the oxides

that form after calci nation (Velu et al., 1997a, 1999a).

Zinc-based LDH are generally less thermally stable than their Mg-based coun-

terparts (Bertoldi et al., 1988; Fuda et al., 1993a, 1993b; Hussein et al., 1995;

Fig. 13.1.8. Changes in the XRD pattern when Mg–Al–CO

LDH with Mg/Al ¼ 2:1 is

heated at various temperatures. The calcinations temperatures and Miller indices are indicated

in the ﬁgure.

13.1.7. Fundamental Properties 1051

del Arco et al., 1996; Kooli et al., 1997a; Velu et al., 1997b; Pre

vot et al., 1998; Ennadi

et al., 2000; Crespo et al., 2001). Nickel-based LDH have thermal stabilities inter-

mediate between those of Mg- and Zn-based LDH (Clause, 1991, 1992; Triﬁro

et al.,

1994; Barriga et al., 1996; Kannan et al., 1996; Labajos and Rives, 1996; del Arco et

al., 1999; Jitianu et al., 2000b). Cobalt-based LDH decompose at lower temperatures

than other LDH (Kannan and Swamy, 1992; Kannan et al., 1995; Xu and Zeng,

1998, 2001a; del Arco et al., 1998; Zeng and Lim, 2000; Pe

rez-Ramı

rez et al., 2001a,

2001b, 2001c, 2001d).

Other forms of LDH such as calcium-based LDH (Messersmith and Stupp, 1995),

Li–Al LDH (Herna

ndez et al., 1985; Hou and Kirkpatrick, 2001), and multicom-

ponent LDH (Morpurgo et al., 1996; Velu and Swamy, 1996; Alejandre et al., 1999;

Castiglioni et al., 2000; Tichit et al., 2001) show their own distingu ishing phases

during thermal decomposition.

Effect of Interlayer Anions

The thermal decomposition of LDH into the corresponding oxides is moderately

affected by the nature of the interlayer anions. For example, DTA indicates that the

nitrate forms of Mg–Al LDH are thermally more stable when the layer charge x

( ¼ Al/(Mg+Al)) increases (Xu and Zeng, 2001b). By contrast, the carbonate forms

show a decrease in decomposition temperature when x increases. The thermal be-

haviour of the nitrate forms is attributed to the arrange ment of the interlayer nitrate

ions which changes from ‘ﬂat-lying’ to ‘stick-lying’ as the number of nitrate anions

increases (Xu and Zeng, 2001c). Dehydroxylation is apparently retarded when the

nitrate group adopts the ‘stick-lying’ conformation, being alternately attached to top

and bottom hydroxyl sheets of contiguous layers.

LDH, containing certain organic anions, can change their basal spacings on mild

heating. Expansion or contraction may occur during loss of interlayer water. Thus,

the basal spacing of Li–Al LDH with intercalated lon g-chain fatty acids

(CH

)

COOH, n ¼ 10, 12, and 14) expands on heating to 100 1C, whereas

that of the analogous Mg–Al forms contracts (Borja and Dutta, 1992). Terephtha-

late and benzoate intercalates of Mg–Al LDH also show changes in basal spacing

due to a change in orientation of the anions from vertical to horizontal or to in-

terstratiﬁcation (Vucelic e t al., 1995; Kooli et al., 1996).

Contraction of basal spacings also occurs when anions are grafted to the hy-

droxide layer s. Grafting can enhance thermal stability (Pre

vot et al., 1998). The

higher thermal stability of LDH sulphate relative to the carbonate and hydroxide

forms may be explained in terms of the grafting of sulphate anions to the hydroxide

layers (Constantino and Pinnavaia, 1995).

Calcination/Hydration Cycles and Reconstruction

The thermal behaviour of Mg–Al–CO

LDH is characterized by three major steps

(Fig. 13.1.9)(Miyata, 1980; Sato et al., 1986a, Labajos et al., 1992; Pesic et al., 1 992;

Rey et al., 1992; MacKenzie et al., 1993; Hibino et al., 1995; Hudson et al., 1995;

Chapter 13.1: Layered Double Hydroxides1052

Tichit et al., 1998). Up to 250 1C, interlayer water is lost (step 1). At 250–500 1C, the

layer structure collapses, and the LDH converts into a mixed-oxide MgO-like phase

(step 2). The mixed oxide can reconstruct into the (original) LDH by exposure to an

aqueous solution (see Section 13.1.4.D). At ca. 900 1C the mixed oxide decomposes

to MgO and MgAl

spinel (step 3). The foregoing is a simpliﬁed outline of the

thermal behaviour of LDH.

In reality, the processes involved in steps 1–3 are more complicated than described.

Loss of interlayer water in step 1 is concomitant with the appearance of tetrahedrally

coordinated Al (Bellotto et al., 1996b). The metaphases that form are characterized

by a slight contraction of basal spacing, the absence of (0 0 6) reﬂection (see

Fig. 13.1.8), and the splitting of the carbonate u

band (at 1360 cm

1

) into two bands

at 1330 and 1540 cm

1

(see Fig. 13.1.10)(Rey et al., 1992; Hibino et al., 1995; Bellotto

et al., 1996b; Kanezaki, 1998a, 1998b, 1998c; Tichit et al., 1998; Rocha et al., 1999;

Millange et al., 2000; Pe

rez-Ramı

rez et al., 2001d), due probably to grafting of car-

bonate anions to the hydroxide layers (Stanimirova et al., 1999). Formation of tet-

rahedrally coordinated Al was also observed during step 2 (MacKenzie et al., 1993;

Fig. 13.1.9. Decomposition of Mg–Al–CO

LDHs by calcinations and reconstruction from

calcined LDHs in aqueous solutions.

13.1.7. Fundamental Properties 1053

res et al., 1997; Tichit et al., 1998; Rocha et al., 1999). However, the local location

of Al(IV) is uncertain (Rebours et al., 1994; Shen et al., 1998), and this Al(IV)

disappears after reconstruction (Rey et al., 1992; Parker et al., 1995). A variety of

anions can be intercalated following calcination at ca. 500 1C (step 2) and subsequent

reconstruction (Fig. 13.1.9), (Chibwe and Jones, 1989a, 1989b; Dimotakis and Pin-

navaia, 1990; Cavani et al., 1991; Narita et al., 1991; Newman et al., 1998, 2001)

although some carbonate anions remain in the calcined materials (e.g., Clause, 1991;

Hibino et al., 1995; Kloprogge et al., 2001). A decrease in anion sorption capacity

(Parker et al., 1995) or a change in crystal symmetry (Stanimirova et al., 2001) may

also occur in the ﬁrst calcination/reconstruction cycle, while a MgAl

spinel phase

may appear in further cycles (see Fig. 13.1.11)(Hibino and Tsunashima, 1998).

Mixed oxides obtained from LDH often serve as catalysts. Reconstruction leads

to a redu ction in catalyti c activity. In addition, undesirable oxide phases can appear

upon repeated cycles of calcination and reconstruction (Alcaraz et al., 1998; Hibino

and Tsunashima, 1998; Marchi and Apesteguı

a, 1998; Pe

rez-Ramı

rez et al., 2001a).

The ease of reconstruction depends on the nature of the constituent metal cations

Fig. 13.1.10. Changes in the IR spectrum when Mg–Al–CO

LDH with Mg/Al ¼ 2:1 is

heated at various temperatures. The calcinations temperatures are indicated on the spectra.

Chapter 13.1: Layered Double Hydroxides1054

(Sato et al., 1988; Kooli et al., 1997a; Tichit et al., 2002a). For example, mixed oxides

from Ni-based LDH are much more difﬁcult to reconstruct than those from Mg- or

Zn-based LDHs. Mixed oxides from Mg-based LDH can be reconstructed in an

aqueous solution at room temperature. With Zn-based LDH, some ZnO-like phase

remains under the same mild conditions. Ni-based LDH require hy drothermal

treatment for reconstruction, whereas thermal treatment gives rise to crystalline ZnO

in the case of Zn-based LDH.

An excellent example of a metal cation-dependent reconstruction was reported by

Alcaraz et al. (1998), who were able to solve the problem of deactivation (induced by

rehydration of the oxide catalysts back to LDH) by using Ni

instead of Mg

for

the synthesis of the LDH precursor.

Layered Double Oxides and Pre-spinel Phases

Oxides obtained by decomposition of LDH form a single-phase rock salt-type solid

solution before segregating into M(II)O and spinel phases when the temperature is

Fig. 13.1.11. Changes in the XRD pattern when Mg–Al–CO

LDH with Mg/Al ¼ 2:1 is

subjected to repeated calcinations/reconstruction. Diffraction peaks corresponding to spinel

are labeled with a solid circle.

13.1.7. Fundamental Properties 1055

further increased. There is no doubt that the mixed oxides retain some structural

homogeneity. Models for local ordering or local segregation in the calcined materials

were proposed. One such model involves formation of a layered double oxide (LDO)

(Hudson et al., 1995; Carlino et al., 1996; Carlino, 1997). Like LDH, LDO can

intercalate guest anions.

Another model involves formation of a pre-spinel or quasi-amorphou s spinel-type

phase. Calcination of Ni–Al LDH between 350 and 800 1C gives rise to the following

local phases: (i) a NiO phase; (ii) a spinel-type phase; and (iii) an alumina-type phase

(Clause et al., 1992; Triﬁro

et al., 1994; Jitianu et al., 2000b; Pe

rez-Ramı

rez et al.,

2001d).

When Ni–Fe LDH are calcined at 450 1C, an amorphous phase forms with a

structure related to inverse spinels such as NiFe

or g-Fe

(del Arco et al., 1999).

C. Surface and Colloidal Properties

Surface Characterization

The surface properties of LDH inﬂuence catalytic reactivity, colloidal stability, ad-

sorption, and intercalation. The speciﬁc surface area and AEC of LDH were de-

scribed in Sections 13.1.6.B and 13.1.4.G, respectively.

Zeta potentials were measured for Mg–Al–CO

/NO

LDH as a function of pH or

electrolyte concentration (Table 13.1.7). At a ﬁxed salt concentration the isoelectric

point (IEP) decreases sharply for divalent anion (Chaˆ telet et al., 1996). Divalent

anions also reduce particle mobility (Lagaly et al., 2001a). These results show that

divalent anions are more strongly adsorbed by LDH than monovalent anions.

The zero point of charge (z.p.c.) (see Chapter 5) ranges from 12.0 to 12.3 for

Mg–Al LDH and from 10.3 to 10.8 for Mg–Fe LDH (Han et al., 1998; Hou et al.,

2001). The amount of charge may be calculated from potentiometric titration.

Table 13.1.7. Isoelectric point for LDH in various electrolytes

Electrolyte IEP

pH at 0.01 M salt concentration

Salt concentration (mmol/L) at pH 7

Water 11.1 —

NaCl >11.1 >100

NaSCN — 87

7.6 3

7.8 3

CrO

6.7 —

HPO

— 0.7

Values from Chaˆ telet et al. (1996).

Values from Lagaly et al. (2001a).

Chapter 13.1: Layered Double Hydroxides1056

However, the separate contribution of surface adsorption and ion exchange to z.p.c.

values is difﬁcult to assess.

Surface Modiﬁcation

Hydrophobization of LDH by exchange with organic anions (surfactants) is an

important procedure for obtaining new types of adsorbents. Thus, intercalation of

dodecyl sulphate or dode cyl benzenesulphonate into Ca–Al LDH yields hydropho-

bic LDH capable of adsorbing organic solvents, such as n-heptane, benzene, toluene,

and n-propanol. Thes e surfactant-modiﬁed LDH show interlayer swelling in n-hep-

tane (De

ny et al., 1997). Hydrophobic LDH can also be obtained by intercalation

of myristic acid, or esteriﬁcat ion of sebacoyl chloride with surface hydroxyl groups

on the interlayer surface (Jakupca and Dutta, 1995; Morioka et al., 1995; Esumi and

Yamamoto, 1998). In studying the adsorption of surfactants, Pavan et al.

(1998–2000) found that the interlayer carbonate anions of LDH are not replaced

by surfactants through simp le ion exchange. However, some carbonate anions can be

exchanged by organic acids from the melt (Carlino and Hudson, 1994, 1995; Carlino

et al., 1996; Carlino, 1997).

Extraction of interlayer dodecyl sulphate by a cationic surfactant ( N-cetyl-N,N,

N-trimethylammonium bromide) leads to fast anion exchange, the amount of do-

decyl sulphate exchanged being proportional to the amount of cationic surfactant

added (Crepaldi et al., 1999, 2000b). New hybrid ﬁlms of LDH and surfactants were

prepared by the Langmuir–Blodgett (LB) method to obtain electrodes for sensors

(He et al., 2001). Finally, modiﬁcation of the interlayer surface with polymerized

silicate was reported, offering a way of synthesizing new catalysts (Yun et al., 1995;

Depe

ge et al., 1996).

Colloidal Dispersions and Delamination

The term ‘colloidal solution of LDH’ is somewhat ambiguous since it can be in-

terpreted in two ways: a suspension of LDH particles, or a dispersion of exfoliated

single layers of LDH (i.e., delaminated LDH). For example, the turbid supernatant

after intense washing with water, or treatment with ultrasound, consists of a sus-

pension of LDH particles. Such suspensions were used to measure electrophoretic

mobility (Lagaly et al., 2001a) and rheological properties (Albiston et al., 1996).

Colloidal suspensions of LDH were also used as solid emulsion stabilizers (Abend

et al., 1998; Neuha

usler et al., 1999; Thieme et al., 1999; Lagaly et al., 2001b).

Oil–water emulsions, stabilized by heterocoagulates of bentonite and LDH, are

useful for cosmetic or pharmaceutical formulations.

A dispersion of delaminated LDH is totally different from a suspension of par-

ticles. Delamination, which can be thought of as inﬁnite swelling, results in colloidal

solutions that are very stable. Several methods for delaminating LDH were reported:

treatment of Zn–Al–dodecyl sulphate LDH in alcohols under reﬂux at 120 1C

(Adachi-Pagano et al., 2000), simple mixin g of Mg–Al–glycine LDH and formamide

(Hibino and Jones, 2001), mixing water and Mg–Al –alkoxide LDH prepared in

13.1.7. Fundamental Properties 1057

alcohols (Gardner et al., 2001), and mixing Mg–Al–dodecyl sulphate LDH and

acrylate monomers at 70 1C(O’Leary et al., 2002). When colloidal dispersions of

delaminated LDH are gently dried, well-ordered LDH are recovered (Hibino and

Jones, 2001; Leroux et al., 2001a). On the other hand, when such dispersions are

freeze-dried, the recovered materials are X-ray amorphous (Leroux et al., 2001a).

Nanocomposites of LDH and polymers were obtained from colloidal dispersions of

delaminated LDH and polymers (Leroux and Besse, 2001).

Surface Properties of Calcined Materials

Thermal decomposition of the LDH structure increases the speciﬁc surface area

(Table 13.1.8)(Kannan and Swamy, 1992; Labajos et al., 1992; Barriga et al., 1996;

Kannan et al., 1996; Marchi and Apesteguı

a, 1998; Shen et al., 1998; Hussein et al.,

2001; Tichit et al., 2001; Olsbye et al., 2002). Formation of oxide phases and sin-

tering of particles at high temperature cause a marked de crease in speciﬁc surface

area. Nitrogen adsorption–desorption isotherms of the calcined materials are clas-

siﬁed as type IIb rather than type IV. Type IIb isotherms are characteristic of ag-

gregates of plate-like particles, classically found with clay minerals (Tichit et al.,

2001).

The acidity and basicity of calcined Mg–Al–CO

LDH were evaluat ed by micro-

calorimetric adsorption of NH

and CO

. Acidity decreases in the order

g-Al

>LDH calcined at 400 1C>LDH calcined at 600 1 CELDH calcined at

800 1C, while basicity follows the order MgO>LDH calcined at 600 1C>LDH cal-

cined at 800 1C>LDH calcined at 400 1C. IR spectroscopy indicates that the acid/

base sites on the surface of calcined LDH are mainly of the Lewis type (Shen et al.,

1998). Weak Lewis acid sites appear to be associated with Al

ions, while basicity is

related to isolated surface hydroxyl groups (del Arco et al., 1993). Calcined Mg–Al

LDH contain surface sites of low (OH



group), medium (Mg–O pairs), and strong

2

anions) basicity. The relative contribution of low- and medium-strength basic

sites to the total basicity increases with increasing Al

content (Di Cosimo et al.,

1998).

13.1.8. INDUSTRIAL AND ENVIRONMENTAL APPLICATIONS:

CONVENTIONAL AND NOVEL

A. Catalytic Applications

Mixed oxides obtained by calcination of LDH can serve as solid-base catalysts for (i)

polymerization of alkene oxides; (ii) aldol condensation of aldehydes and ketones;

(iii) methane or hydrocarbon steam reforming; (iv) methanation; (v) synthesis of

methanol and higher alcohols; (vi) hydrocarbon (Fischer–Tropsch) synthesis; and

(vii) hydrolysis of nitriles (Cavani et al., 1991; Vaccari, 1998, 1999b). The thermal

decomposition of LDH, containing transition metal ions, was intensively studied

Chapter 13.1: Layered Double Hydroxides1058

Table 13.1.8. Speciﬁc surface area of calcined LDH

Composition

Treatment for

original LDH

Degassing

temperature

(1C)

Calcination

temperature

(1C)

Speciﬁc

surface area

/g)

References

Co/Al ¼ 2/1 Aging, 65 1C,

24 h

120 Not calcined 47 Kannan and

Swamy (1992)200 270

Mg/Al ¼ 2/1 None Not described 750 ca. 200 Labajos et al.

(1992)1100 ca. 50

Ni/Mn ¼ 2/1 None Not described Not calcined 70 Barriga et al.

(1996)125 80

450 58

700 15

1000 o1

Ni/Mn ¼ 3/1 Not calcined 70

125 72

450 56

700 17

1000 4

Ni/Al ¼ 2/1 Aging, 65 1C,

24 h

Not described Not calcined 92 Kannan et al.

(1996)450 170

Ni/Al ¼ 2.5/1 Not calcined 124

450 146

Ni/Al ¼ 3/1 Not calcined 83

450 137

Ni/Al ¼ 3.5/1 Not calcined 121

450 92

(Cu+Co+Zn)/

Al ¼ 3/1

60 1C during

coprecipitation

400 400 203 Marchi and

Apesteguı

(1998)

500 163

600 79

Mg/Al ¼ 3/1 None Not described 400 195 Shen et al.

(1998)600 132

800 114

Mg/Al ¼ 4/1 Aging, 70 1C,

18 h

Not described Not calcined 68.0 Hussein et al.

(2001)500 201.9

Co/Al ¼ 3/1 None 200 Not calcined — Tichit et al.

(2001)300 166

500 145

Co/Mg/Al ¼ 2/1/1 Not calcined 174

300 230

500 149

Co/Ni/Mg/Al ¼ 1.5/

0.5/1/1

Not calcined 17

300 209

500 163

Co/Ni/Mg/Al ¼ 1/1/

1/1

Not calcined 20

300 235

500 199

Co/Ni/Mg/Al ¼ 0.5/

1.5/1/1

Not calcined 30

300 254

500 204

Ni/Mg/Al ¼ 2/1/1 None 200 Not calcined 20 Tichit et al.

(2001)300 251

(continued on next page)

13.1.8. Industrial and Environmental Applications: Conventional and Novel 1059

because the products are potentially useful as catalyst precursors. The transition

metals include nickel (Clause , 1991, 1992; Triﬁro

et al., 1994; Alcaraz et al., 1998; del

Arco et al., 1999; Jitianu et al., 2000b), cobalt (del Arco et al., 1998; Pe

rez-Ramı

rez

et al., 2001a, 2001d; Tichit et al., 2001), iron (Raja and Santhanalakshmi, 1996; Ge

et al., 2001), an d copper (Morpurgo et al., 1996; Velu and Swamy, 1996; Marchi and

Apesteguı

a, 1998 ; Castiglioni et al., 2000). The reactivity of LDH itself (as a solid-

base catalyst) was investigated using 2-methyl-3-butyn-2-ol as a probe (Constantino

and Pinnavaia, 1995).

An emerging trend is the application of LDH-derived catalysts in green chemistry.

Solid-base catalysts are environment friendly, as compared with their liquid counter-

parts, because their waste products are easier to dispose than the huge volumes of

liquid alkali waste (Ono and Baba, 1997). Solid bases from LDH were used in mer-

captan oxidation for sweetening (Alcaraz et al., 1998), formation of C–C bonds

(Choudary et al., 2000, 2001b), and condensation of citral and ketones (Roelofs et al.,

2001). Environment friendly technologies using LDH were also developed for the

production of hydrogen from vegetable oils (Marquevich et al., 2001), the simulta-

neous production of hydrogen and nanocarbon by decomposition of methane

(Li et al., 2000), and the high-rate synthesis of amine N-oxides (Choudary et al.,

2001a). Velu et al. (1999b, 2000b) investigated Cu–Zn–Al–Zr LDH as precursors of

catalysts for steam reforming of methanol to produce CO-free hydrogen for fuel cells.

Another environmental application of LDH-derived catalysts involves decom-

posing gaseous air pollut ants and greenhouse gases. The performance of Cu–Mg–Al

mixed oxides (obtained by calcination of LDH) in the selective catalytic reduction of

NO by NH

is comparable to that of a commercial V

–WO

/TiO

catalyst

Table 13.1.8 (Continued )

Composition

Treatment for

original LDH

Degassing

temperature

(1C)

Calcination

temperature

(1C)

Speciﬁc

surface area

/g)

References

500 192

Co/Ni/Al ¼ 1.5/

1.5/1

Not calcined 13

300 201

500 144

Co/Ni/Mg/Al ¼ 1.2/

1.2/0.6/1

Not calcined 15

300 234

500 127

Co/Ni/Mg/Al ¼ 0.8/

10.8/1.4/1

Not calcined 96

300 296

500 270

Mg/Al ¼ 3/1 60 1C during

coprecipitation

Not described 800 162 Olsbye et al.

(2002)

Compensating anions are carbonate or nitrate.

Chapter 13.1: Layered Double Hydroxides1060