Microtextural properties of layered double hydroxides: a theoretical and structural model

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Microporous and Mesoporous Materials 67 (2004) 1–17

Microtextural properties of layered double hydroxides:a theoretical and structural model

Juan J. Bravo-Su�aarez a,b, Edgar A. P�aaez-Mozo c, S. Ted Oyama b,*

a Centro de Investigaciones en Cat�aalisis, Escuela de Ingenier�ııa Qu�ıımica, Universidad Industrial de Santander, A.A. 678, Bucaramanga, Colombiab Environmental Catalysis and Nanomaterials Laboratory, Department of Chemical Engineering, Virginia Polytechnic Institute and State University,

Blacksburg, VA 24061-0211, USAc Centro de Investigaciones en Cat�aalisis, Escuela de Qu�ıımica, Universidad Industrial de Santander, A.A. 678 Bucaramanga, Colombia

Received 11 July 2003; received in revised form 14 October 2003; accepted 14 October 2003

Abstract

In this paper, a theoretical method to estimate the textural properties of layered double hydroxides (LDHs) of the type

[M2þ1�xM

3þx (OH)2][A

n�x=n] is presented. The theoretical calculations are based on the structure and composition of the LDH, using

geometrical models of an LDH crystallite and the intercalating anion. Several examples of the application of this model to LDHs

and pillared LDHs are shown. The estimated properties include the interpillar distances, the interlamellar and external areas, the

interlamellar free volume, the fraction of external anions and the apparent and true density of the LDH. For well crystallized LDH

samples, agreement between the estimated and the experimental results is found, for poorly crystalline samples, a correlation be-

tween the degree of crystallite agglomeration and the experimental areas is proposed.

� 2003 Elsevier Inc. All rights reserved.

Keywords: Layered double hydroxides; Hydrotalcite; Textural properties; Theoretical model; Agglomeration

1. Introduction

Among layered compounds, layered double hydrox-

ides (LDHs) represent one of the most technologicallypromising materials because of their relative ease of

preparation and broad uses as adsorbents, anion

exchangers, catalysts and catalyst supports [1]. The

LDHs can be described as layered compounds of bru-

cite-like structure (Mg(OH)2), with positively charged

layers due to a partial substitution of divalent M2þ

metals by trivalent M3þ metals. Between the layers there

are counter anions to balance the electrical charge andcoordinating water molecules. LDHs are commonly

represented by the general formula: [M2þ1�xM

3þx -

(OH)2]xþ(An�

x=n) ÆmH2O, where M2þ and M3þ can be any

divalent and trivalent metal ions (with ionic radius

similar to Mg2þ), x is the metal ratio M3þ/(M2þ +M3þ)

*Corresponding author.

E-mail addresses: [email protected] (J.J. Bravo-Su�aarez), oyama@

vt.edu (S. Ted Oyama).

1387-1811/$ - see front matter � 2003 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2003.10.003

and An� is the interlamellar charge-compensating anion.

The flexibility of the LDHs to incorporate in their

structure a variety of metals and anions has increased

the interest in these materials. Special attention is beingpaid to LDHs with bulky and stable anions, i.e. poly-

oxometalates (POMs), since they can give rise to a wide

range of microporous materials [2].

The specific surface area (SSA) of LDH compounds

can range from a few m2/g up to about 100 m2/g. The

synthesis method can have a great influence on the final

properties of the SSAs. Several factors may play a major

role in determining the textural properties, such as theaging and hydrothermal treatments of LDH precipitates

as well as the degasification regimes used before mea-

suring the adsorption–desorption isotherms [3]. Some

examples of how the SSA changes with these parameters

are given in Table 1 [3–6]. In general, in the case of small

inorganic anions (carbonates, nitrates, chlorides, etc.)

the chemical composition of the LDHs does not have a

significant effect on the obtained SSAs. Although, insome cases, the use of mixtures of organic solvents and

mail to: [email protected]

Table 1

SSAs of several Mg1�xAlx–CO3 LDHs [3–6]

x Synthesis T

(K)

Synthesis t

(h)

SBET

(m2/g)

Comments

0.33 373 36 24 Homogeneous precipitation by urea hydrolysis

0.33 338 18 80 ±6 m2/g after degasification at 343, 378 or 453 K or calcination at 513 K

0.25 r.t. 30 75 Heating at 373 K in air for 16 h. Degasification at 353 K

0.25 r.t. 30 88 Synthesized in H2O/ethanol. Degasification at 353 K

0.25 r.t. 30 136 Heating at 373 K in air for 16 h, synthesized in H2O/ethylenglycol.

Degasification at 353 K

0.23 338 0.5 92 Drying overnight at 343 K and degasification at 303 K for 10 h

0.19 338 0.5 88 Drying overnight at 343 K and degasification at 303 K for 10 h

r.t.¼ room temperature.

2 J.J. Bravo-Su�aarez et al. / Microporous and Mesoporous Materials 67 (2004) 1–17

water can modify the superficial texture of the final

LDH due to an aggregation effect of the crystallites [3,6].

Layered compounds can be classified into two types:

intercalation compounds and pillared compounds.

According to IUPAC [7] rules, an intercalation com-

pound results from the insertion of guest species in the

interlayer region of a layered solid, giving rise to an

increase in the basal spacing. When a layered compoundis transformed into a thermally and chemically stable

micro or mesoporous layered material, then it is

denominated a pillared compound. The thermal and

chemical stability and the porosity are the features that

distinguish a pillared compound from an intercalation

compound. Intercalation compounds do not necessarily

possess stability and porosity, while pillared compounds

must. The interlayer spacings in pillared compoundsshould be at least one N2 diameter and the interlayer

region must be accessible to molecules at least as large as

N2. The porosity in the pillared LDHs can be influenced

by several factors among which the most important are

the dimensions of the anionic pillars, and their number

and distribution in the LDH layers. These last two

characteristics depend on the pillar charge, the charge of

the LDH layers and the distribution of charges in theLDH [8].

Very few examples can be found in the literature of

pillared LDHs that possess relatively high micropore

volumes, especially when compared to the pillared

compounds obtained from cationic clays. High SSAs

and microporous volumes have been attainable thus far

only with hexacyanometal anions [9,10]. Some LDHs

pillared with POMs also show high SSAs [11,12].However, even in these cases, the experimental micro-

porous surface areas are smaller than the theoretical

values expected from calculations using the layer spac-

ing and the charge density of the anions [13]. Cavalcanti

et al. [14] studied the accessibility to the interlayer space

of LDHs intercalated with ferri- and ferrocyanate an-

ions using a simple structural model based on the LDH

unit cell. Other authors also related the experimentalSSA results of some POM pillared LDHs with the areas

estimated from the unit cell of the pillared compounds.

Recently, Nijs et al. [8] made a theoretical evaluation

of the microporosity present in LDHs pillared with

the ferricyanate anion and Keggin type POMs. In this

latter study the SSA was calculated with a geometrical

model based on the LDH unit cell, the micropore vol-

ume was calculated assuming pillars of cylindrical

geometry, and the free interpillar distance (IPD) was

estimated using data for the anionic exchange capacityof the LDHs and the experimental values of the

micropore volumes.

Although there exist in the literature several models

to explain experimental findings such as the above, none

of them can be efficiently used to predict the textural

properties of LDHs compounds by only taking into

account structural and geometrical considerations. Also,

the models are sometimes dependent on experimentalresults, are restricted to a limited range of compositions

or, in some cases, assume spherical particles to estimate

the SSAs of the LDHs [3,15,16]. To overcome these

limitations, the present work describes a procedure for

predicting several microtextural characteristics of LDH

compounds based on only structural and geometrical

considerations of the crystallites. These estimated theo-

retical properties include the specific external area, thespecific interlamellar area, the specific interlamellar free

volume, the apparent and true density, and the inter-

pillar free distance. Contributions due to the edges of

the brucite-like layers and due to the anions in the

borders of these layers have been taken into account.

The main advantage of this procedure is the ability to

relate textural properties of samples with measurable

properties of the LDH crystallites such as their averagethickness (distance in the basal plane 0 0 l), their average

particle diameter and the LDH composition. In this

manner, some new structural and textural relationships

can be readily obtained which are not possible with the

models already published. Some examples of calcula-

tions for several pillared LDHs are given. Among the

anions used as examples are the ferricyanate, deca-

vanadate, Keggin type POMs and the dicobaltatedeca-molybdate anion. Most of these pillared LDHs have

been reported in the literature [12].

J.J. Bravo-Su�aarez et al. / Microporous and Mesoporous Materials 67 (2004) 1–17 3

2. Theoretical calculations

All theoretical calculations are based on the general

LDH formula: [M2þ1�xM

3þx (OH)2]

xþ[An�x=n]. LDHs com-

monly form hexagonal-shaped crystals as seen in Fig. 1.Therefore, for the calculations of the textural properties,

a hexagonal geometry for the brucite-like layers is used.

Figs. 2 and 3 show respectively, the schematic top and

side view of an LDH crystallite layer of mean diameter

(dc).The location of the metallic octahedra in an LDH

layer can also be seen in Figs. 2 and 3. The top view area

of each octahedron in an LDH layer (AEO), as shown inFig. 2, can be calculated by the formula: AEO ¼ a2

p3=2,

Fig. 1. Scanning electron micrograph of a [Mg0:66Al0:33][OH] LDH.

Fig. 2. Schematic top view of a brucite-like layer, a � 0:31 nm.

Fig. 3. Schematic side view of a brucite-like layer, thickness of layer

�0.48 nm.

where a represents the mean distance between OH

groups on the same side of the layer. Many authors have

reported values of a for several LDHs [17–19], for

example: [Mg0:75Fe0:25][CO3]¼ 0.311 nm, [Zn0:75Al0:25]-

[Cl]¼ 0.308 nm, [Zn0:66Cr0:33][Cl]¼ 0.312 nm, [Mg0:66-Al0:33][CO3]¼ 0.304 nm, [Mg0:75Al0:25][CO3]¼ 0.306 nm

and [Mg0:80Al0:20][CO3]¼ 0.308 nm, among others. In

the present work an average value of 0.310 nm is used for

all the calculations. Therefore, the layer area of an

octahedral unit (AEO) is 0.083 nm2.

2.1. Interpillar free distance

The variables for an LDH crystallite include the

mean diameter (dc), the thickness (lc), the number of

brucite-like layers (m), the thickness of a brucite-like

layer (lb) and the interlayer spacing (h). A model for an

LDH crystallite is shown in Fig. 4.

Fig. 5 shows the schematic arrangement of equally

spaced pillars on the surface of a brucite-like layer,

assuming a hexagonal geometry, where IPD is the meaninterpillar distance and dp is the mean pillar diameter.

Although the lattice of pillars is likely to be commen-

surate with the underlying hexagonal array of LDH

octahedra, the derivations provided below do not make

this assumption.

Based on Fig. 2, the number of metallic octahedra

present in a brucite-like layer (NMb) is given by the ratio

Fig. 4. Expanded schematic view of the layers of an LDH crystallite,

lb � 0:48 nm.

dp

IPD

Fig. 5. Schematic view of equally spaced pillars on a brucite-like layer.

Fig. 6. Cubic pillar model.

Fig. 7. Cylindrical pillar model.


between the area of a brucite-like layer (d2cp3=2) and the

layer area of an octahedral unit (AEO ¼ a2p3=2):

NMb ¼ ðdc=aÞ2 ð1ÞAssuming electroneutrality in the LDH, the number

of pillars per LDH crystallite layer (Yb) is:

Yb ¼ x �NMb=n ð2Þwhere x is the metal ratio M3þ/(M2þ +M3þ) and n is the

charge of the interlayer anion. In order to calculate the

IPD, a relationship between the IPD and the mean

diameter of the LDH layer (dc), the number of pillars

per LDH layer (Yb), and the pillar diameter (dp) needs tobe obtained. For this purpose the ratio between the areaof a brucite-like layer and one-third the area of the

hexagonal unit cell as given in Fig. 5 is used, since the

result is the total number of pillars per brucite-like layer.

Yb ¼area of brucite layer

area accommodating a single pillar

¼ ðd2c

p3=2Þ=½ð1=3Þðp3ðdp þ IPDÞÞ2p3=2� ð3Þ

The factor of one-third appears because the hexago-

nal cell in Fig. 5 is a conventional cell, and the actual

unit cell is a rhombohedron of one-third the hexagon

area containing a single pillar. Therefore, the IPD can be

calculated if the geometry, dimensions (dp) and chargeof the pillars (n), and the composition (x) and dimen-

sions of the LDH crystallite (dc) are known. Using Eq.

(3), the IPD is calculated by:

IPD ¼ ðdc=pYbÞ � dp ð4Þ

In the present study cubic and cylindrical geometries

for the pillars are used. These two models can be seen in

Figs. 6 and 7, respectively. Here, dp is the equivalent

pillar diameter and h is the pillar height.

2.2. Specific areas

The two pillar geometries of cubic and cylindrical

shape are also used in the calculations of the specific

external and interlamellar areas. The areas of the pillars

are easily calculated as 2d2p þ 4dph for a cubic pillar and

2p½ðdp=2Þhþ ðdp=2Þ2� for a cylindrical pillar.

Table 2

Contributions of layers and pillars to the total interlamellar area in an LDH

Contribution Cubic pillar

+LDH layersp3ðm� 1Þd2

c

)Interlamellar pillars 2ðm� 1ÞYbd2p

+Interlamellar pillars 4ðm� 1ÞYbdph)Interlamellar pillars in the crystallite bordersa ðm� 1Þdphð2

p

m¼ number of brucite-like layers in a crystallite, dc ¼mean diameter of a cry

height, and IPD¼ interpillar distance.a It has been assumed that the contribution to the external surface area due

for cubic and cylindrical pillars, respectively. It can be easily demonstrated th

by ð2p3dcÞ=ðdp þ IPDÞ.

2.2.1. Total specific interlamellar area

All of the properties calculated per unit weight need

the molecular weight of the LDH crystallite (MWC),

which is calculated by the following formula:

MWC ¼ ½ð1� xÞAWM2þ þ ðxÞAWM3þ þ ð2ÞAWOH�

þ ðx=nÞAWAn� �NMbm=NAV ð5Þ

where AWM2þ , AWM3þ , AWOH� and AWAn� are the

atomic weights of the M2þ, M3þ, OH� and An�,

respectively. NAV is Avogadro’s number, 6.023 · 1023,and m is the number of brucite-like layers in the LDH

crystallite. With respect to Fig. 4, m can be calculated

by:

m ¼ lc=ðlb þ hÞ ð6Þwhere lc and lb are the crystallite and brucite-like layer

thickness (�0.48 nm), respectively. All of the different

contributions to the total interlamellar area (TIA), as

calculated for the cubic and cylindrical models, are given

in Table 2. Therefore, the total specific interlamellararea (TIA) based on cubic pillars is given by:

TIAcub ¼ ðm� 1Þ p3d2

c

h� 2Ybd2

p þ 4Ybdph

� dphð2p3dcÞ=ðdp þ IPDÞ

i.MWC ð7Þ

crystallite

Cylindrical pillarp3ðm� 1Þd2

c

2pðm� 1ÞYbðdp=2Þ2

2pðm� 1ÞYbðdp=2Þh3dcÞ=ðdp þ IPDÞ ðm� 1Þpdpðh=4Þð2

p3dcÞ=ðdp þ IPDÞ

stallite, Yb ¼number of pillars per layer, dp ¼ pillar diameter, h¼ pillar

to the interlamellar pillars in the crystallite borders is dph and pðdp=4Þhat the number of interlamellar pillars in the crystallite borders is given

Table 4

Contributions of layers and pillars to the total interlamellar free vol-

ume in an LDH crystallite

Contribution Cubic pillar Cylindrical pillar

+LDH layers ðm� 1Þhd2c

p3=2 ðm� 1Þhd2

c

p3=2

)Pillar Ybðm� 1Þhd2p pYbðm� 1Þhðdp=2Þ2

m¼ number of brucite-like layers in a crystallite, h¼ pillar height,


And the total specific interlamellar area for cylindrical

pillars is:

TIAcyl ¼ ðm� 1Þ p3d2

c

h� 2pYbðdp=2Þ2 þ 2pYbðdp=2Þh

� pðdp=4Þhð2p3dcÞ=ðdp þ IPDÞ

i.MWC

ð8Þ
dc ¼mean diameter of a crystallite, Yb ¼ number of pillars per layer,
and dp ¼pillar diameter.

2.2.2. Total specific external area

The calculations are similar to the ones in Section

2.2.1. All of the contributions to the total external area,

as calculated for the cubic and cylindrical models, are

given in Table 3.Finally, the total specific external area (TEA) mini-

mum and maximum based on cubic pillars is:

TEAcub;max ¼p3d2

c

hþ 4Ybdphþ 2

p3mdclb

þðm� 1Þdphð2p3dcÞ=ðdpþ IPDÞ

i=MWC

ð9Þ

TEAcub;min ¼p3d2

c

hþ 2

p3mdclb þ ðm� 1Þdphð2

p3dcÞ

=ðdp þ IPDÞi=MWC ð10Þ

And the total specific external area for cylindrical pillars

is:

TEAcyl;max ¼p3d2

c

�þ 2pYbðdp=2Þhþ 2

p3mdclb

þ ðm� 1Þpðdp=4Þhð2p3dcÞ

=ðdp þ IPDÞ�=MWC ð11Þ

TEAcyl;min ¼p3d2

c

�þ 2

p3mdclb

þ ðm� 1Þpðdp=4Þhð2p3dcÞ

=ðdp þ IPDÞ�=MWC ð12Þ

2.3. Total specific interlamellar free volume

The layer and pillar contributions to the interlamellar

free volume for cubic and cylindrical pillar geometries

are given in Table 4.

The total interlamellar free volume (TIV) based on

cubic pillars is calculated by the following equation:

Table 3

Contributions of layers and pillars to the total external area in an LDH cry

Contribution Cubic pillar

+LDH layersp3d2

c

)External pillar Ybd2p

+External pillar Ybd2p þ 4Ybdph

+Layer edges 2p3mdclb

+Interlamellar pillars in the crystallite borders ðm� 1Þdphð2p

dc ¼mean diameter of a crystallite, Yb ¼number of pillars per layer, dp ¼pi

crystallite, lb ¼ brucite-like layer thickness, and IPD¼ interpillar distance.

TIVcub ¼ ðm� 1Þhðd2c

p3=2� Ybd2

pÞ=MWC ð13Þ

And the total interlamellar free volume for cylindrical

pillars is:

TIVcyl ¼ ðm� 1Þhðd2c

p3=2� pYbðdp=2Þ2Þ=MWC ð14Þ

Other variables of interest that can be calculated are the

molar percent of anions in the external surface of the

LDH crystallite (XAE), the apparent density (ADc) and

the true density of the LDH crystallite (TDc). The

apparent density is defined as the mass of the crystallitedivided by its volume including open pores, and the true

density is the mass of the crystallite divided by its vol-

ume excluding pores.

XAE ¼ ½ð2p3dcÞ=ðdp þ IPDÞðm� 1Þ þ Yb�100=ðYbmÞð15Þ

ADc ¼ MWC=ðlcd2c

p3=2Þ ð16Þ

TDc ¼ MWC=ðlcd2c

p3=2� TIV �MWCÞ ð17Þ

3. Estimated microtextural properties of several interca-

lated LDH materials

In this section some examples of the estimated mi-

crotextural properties of several LDHs intercalated with

various large, complex anions are shown. The structures

of these anions are shown in Table 5 [20–23]. In columns

A and B two different arrangements of the anion in the

interlamellar region are illustrated. Column C shows the

top view of the structure shown in column B with acalculated area.

stallite

Cylindrical pillarp3d2

c

pYbðdp=2Þ2

pYbðdp=2Þ2 þ 2pYbðdp=2Þh2p3mdclb

3dcÞ=ðdp þ IPDÞ ðm� 1Þpdpðh=4Þð2p3dcÞ=ðdp þ IPDÞ

llar diameter, h¼ pillar height, m¼number of brucite-like layers in a

Table 5

Structures of different anions that can be pillared in LDHs [20–23]

Anion A B Ca

Phosphotungstate

i.e. [PW12O40]3�

h ¼ 1:11 nm h ¼ 1:03 nm A � 1:014 nm2

Decamolybdodi-

cobaltate

[H4Co2Mo10O38]6�

h ¼ 1:07 nm h ¼ 0:80 nm A � 1:052 nm2

Decavanadate

[V10O28]6�

h ¼ 1:03 nm h ¼ 0:71 nm A � 0:761 nm2

Ferricyanate

[Fe(CN)6]3�

h ¼ 0:72 nm h ¼ 0:64 nm A � 0:417 nm2

aContributions due to the ionic radius of the terminal atoms and also of the atoms present in the borders of the anion structure have been taken

into account in the calculations of the top view area.


Based on the anion structural information given inTable 5, the calculations of the textural properties of the

intercalated materials are easily obtained by applying

the already described equations: the IPDwith Eq. (4), the

total interlamellar area (TIA) with Eqs. (7) and (8), the

total external area (TEA) with Eqs. (9)–(12), the total

interlamellar free volume (TIV) with Eqs. (13) and (14),

the molar percent of anions in the external surface of the

LDH crystallite (XAE) with Eq. (15), the apparentdensity (AD) and true density (TD) with Eqs. (16) and

(17), respectively, and the absolute total interlamellar

free volume (TIV) of the crystallite with Eqs. (5), (13)

and (14), using the product TIV ÆMWC. All of these

properties are shown in Tables 6–11. For the results in

Tables 6–9, the LDH crystallite dimension (dc � lc) hasbeen estimated as 200 nm · 20 nm. In the case of the

meixnerite-like LDH (LDH-OH) an ionic radius of0.135 nm is used for the OH� group [24]. In general, the

anion height based on the arrangement in column B of

Table 5 has been used. In the determination of the anion

base for a given model, the area of the anion viewed

normal to the basal plane (top view) shown in column C

of Table 5 is utilized. For instance, for a cylindrical

model the ferricyanate anion has the following dimen-

sions: dp ¼ 0:729 nm and h ¼ 0:644 nm, and for a cubicmodel the dimensions are: dp ¼ 0:646 nm and h ¼ 0:644nm. In order to convert the values of the textural

properties expressed per unit weight, in units of charge

of the LDH, the following conversion factor is used:

MWC=ðNMb � mÞ. Some examples of the texturalproperties per unit charge of the LDH are given in Table

6. The experimental textural properties of some LDHs

and pillared LDHs are shown in Tables 12 and 13

[12,25,26].

4. Discussion

4.1. Interpillar distances

An expression that relates the number of pillars per

brucite-like layer (Y ), the mean diameter of the LDH

layer (dc) and the pillar diameter (dp) with the IPD has

been obtained by relating the area of a hexagonal-

shaped brucite-like layer with a hexagonal conventional

cell area. This relation is given by Eq. (4). A simplerequation is attained by substituting Eqs. (1) and (2) into

Eq. (4):

IPD ¼ apðn=xÞ � dp ð18Þ

For a ¼ 0:310 nm, Eq. (19) reduces to:

IPD ¼ 0:310pðn=xÞ � dp ð19Þ

Eq. (19) is a simple relationship that shows the depen-

dence of the IPD with variables such as the pillar charge

(n), the LDH composition (x) and the equivalent pillar

diameter (dp). Therefore, for pillars with similar geo-

metry and dimensions but different charges, it is expected

Table 6

Estimated textural properties of a Mg1�xAlx LDH intercalated with different anions using a cubic pillar model and crystallite dimensions of

200 nm· 20 nm

Anion x IPD

(nm)

TIA

(m2/g)

TEAmin

(m2/g)

TEAmax

(m2/g)

TIV

(cm3/g)

TIVc · 1016(cm3)

MWC· 1016(g)

[PW11O39]7� 0.15 1.12 1012.3 74.4 110.0 0.304 3.173 10.428

0.25 0.63 868.8 56.7 101.3 0.175 2.426 13.888

0.35 0.38 782.7 46.0 95.9 0.097 1.678 17.348

[PV2W10O40]5� 0.15 0.78 930.9 63.6 105.8 0.221 2.725 12.312

0.25 0.38 797.4 46.8 97.7 0.099 1.678 17.028

0.35 0.16 721.9 37.2 93.0 0.029 0.632 21.744

[PW11Fe(H2O)O39]4� 0.15 0.59 842.7 54.3 98.9 0.160 2.332 14.542

0.25 0.23 718.3 38.8 91.0 0.049 1.024 20.745

0.35 0.04 651.3 30.4 86.6 )0.011 )0.284 26.948

[PW12O40]3� 0.15 0.38 746.1 43.8 91.4 0.092 1.678 18.198

0.25 0.06 637.5 30.4 84.2 )0.002 )0.066 26.838

[H4Co2Mo10O38]6� 0.15 0.94 1090.6 74.8 107.7 0.255 2.673 10.463

0.25 0.49 937.2 59.2 102.0 0.138 1.849 13.409

0.35 0.25 839.1 49.2 98.3 0.063 1.025 16.355

[V10O28]6� 0.15 1.10 1298.8 85.1 113.4 0.313 2.859 9.137

0.25 0.65 1176.7 72.0 111.5 0.210 2.294 10.931

0.35 0.41 1089.2 62.5 109.9 0.136 1.730 12.725

[Fe(CN)6]3� 0.15 0.74 1696.8 96.0 138.5 0.329 2.675 8.141

0.25 0.43 1737.9 87.9 152.0 0.231 2.078 9.003

0.35 0.26 1772.0 81.0 162.9 0.150 1.481 9.864

[CO3]2� 0.15 0.72 1596.2 67.7 79.7 0.174 2.049 11.793

0.25 0.46 1561.8 64.8 83.7 0.145 1.794 12.403

0.35 0.32 1530.7 62.1 87.4 0.118 1.538 13.012

[OH]� 0.15 0.57 1779.2 69.6 83.7 0.191 2.181 11.429

0.25 0.38 1855.5 67.8 90.5 0.171 2.015 11.796

0.35 0.29 1927.2 66.0 96.9 0.152 1.848 12.163

x ¼ M3þ=ðM2þ þM3þ), IPD¼ interpillar distance, TIA¼ total interlamellar area, TEAmin ¼ total external area minimum, TEAmax ¼ total external

area maximum, TIV¼ total interlamellar volume, TIVc ¼ total interlamellar volume of crystallite, and MWC¼molecular weight of crystallite.

Table 7

Estimated textural properties per unit charge (eþ) of a Mg0:75Al0:25 LDH intercalated with different anions using a cubic pillar model and crystallite

dimensions of 200 nm· 20 nm

Anion IPD

(nm)

TIA

(nm2/eþ)

TEAmin

(nm2/eþ)

TEAmax

(nm2/eþ)

TIV

(nm3/eþ)

TIV

(cm3/g)

TIVc · 1016(cm3)

[PW11O39]7� 0.63 0.892 0.058 0.104 0.180 0.175 2.426

[PV2W10O40]5� 0.38 1.004 0.059 0.123 0.125 0.099 1.678

[PW11Fe(H2O)O39]4� 0.23 1.102 0.059 0.140 0.075 0.049 1.024

[PW12O40]3� 0.06 1.265 0.060 0.167 )0.004 )0.002 )0.066

[H4Co2Mo10O38]6� 0.49 0.805 0.051 0.088 0.119 0.138 1.849

[V10O28]6� 0.65 0.773 0.047 0.073 0.138 0.210 2.294

[Fe(CN)6]3� 0.43 0.884 0.045 0.077 0.118 0.231 2.078

[CO3]2� 0.46 0.689 0.029 0.037 0.064 0.145 1.794

[OH]� 0.38 0.779 0.028 0.038 0.072 0.171 2.015

Heading descriptions as in Table 6.


that those with the highest anion charges will show the

largest IPD values. This trend can be observed in Table

6 for the intercalated anions with Keggin geome-

try, where the predicted order of IPDs is the fol-

lowing: [PW11O39]7� > [PV2W10O40]

5� > [PW12O40]4� >

[PW12O40]3�. Another variable that affects the IPDs is

the equivalent pillar diameter. In this case, pillars with

the same charge will show an inverse IPD trend, the

larger the diameter the smaller the IPD. For example,

from Table 6, the IPD of the [Mg0:75Al0:25][V10O28]6�

(0.65 nm) is larger than the IPD of the [Mg0:75Al0:25]-

[H4Co2Mo10O38]6� (0.49 nm), duly following an inverse

Table 8

Estimated textural properties of a Mg0:75Al0:25 LDH intercalated with different anions using a cylindrical pillar model and crystallite dimensions of

200 nm · 20 nm

Anion IPD

(nm)

TIA

(m2/g)

TEAmin

(m2/g)

TEAmax

(m2/g)

TIV

(cm3/g)

TIVc · 1016(cm3)

XAE

(%)

AD

(g/cm3)

TD

(g/cm3)

[PW11O39]7� 0.50 808.4 56.3 95.8 0.175 2.426 10.32 2.01 3.09

[PV2W10O40]5� 0.24 728.4 46.4 91.5 0.099 1.679 9.91 2.46 3.24

[PW11Fe(H2O)O39]4� 0.10 647.5 38.4 84.7 0.049 1.026 9.68 2.99 3.51

[PW12O40]3� )0.07 564.5 30.1 77.7 )0.002 )0.064 9.42 3.87 3.84

[H4Co2Mo10O38]6� 0.35 869.6 58.8 96.7 0.138 1.851 9.13 1.94 2.64

[V10O28]6� 0.54 1109.8 71.6 106.6 0.210 2.294 8.71 1.58 2.36

[Fe(CN)6]3� 0.35 1621.7 87.3 144.2 0.231 2.078 7.63 1.30 1.86

[CO3]2� 0.41 1505.7 64.6 81.4 0.145 1.793 5.16 1.79 2.42

[OH]� 0.35 1788.3 67.6 87.8 0.171 2.014 4.74 1.70 2.40

IPD¼ interpillar distance, TIA¼ total interlamellar area, TEAmin ¼ total external area minimum, TEAmax ¼ total external area maximum,

TIV¼ total interlamellar volume, TIVc ¼ total interlamellar volume of crystallite, XAE ¼molar fraction of external anions, AD¼ apparent density,

and TD¼ true density.

Table 9

Estimated textural properties of a [M2þ1�xAlx][PV2W10O40]

5� LDH using a cubic pillar model and crystallite dimensions of 200 nm· 20 nm

M2þ x IPD

(nm)

TIA

(m2/g)

TEAmin

(m2/g)

TEAmax

(m2/g)

TIV

(cm3/g)

TIVc · 1016(cm3)

XAE

(%)

AD

(g/cm3)

TD

(g/cm3)

Mg 0.15 0.78 930.9 63.6 105.8 0.221 2.725 10.55 1.78 2.93

0.25 0.38 797.4 46.8 97.7 0.099 1.678 9.91 2.46 3.24

0.35 0.16 721.9 37.2 93.0 0.029 0.632 9.57 3.14 3.45

0.45 0.02 673.4 31.0 89.9 )0.016 )0.415 9.35 3.82 3.60

Zn 0.15 0.78 741.9 50.7 84.3 0.176 2.725 10.55 2.23 3.68

0.25 0.38 685.9 40.3 84.0 0.085 1.678 9.91 2.86 3.77

0.35 0.16 650.1 33.5 83.7 0.026 0.632 9.57 3.49 3.83

0.45 0.02 625.4 28.8 83.5 )0.015 )0.415 9.35 4.11 3.88

Heading descriptions as in Tables 6 and 8.

Table 10

Estimated textural properties of a [Mg0:75Al0:25][H4Co2Mo10O38]6� LDH with variable crystallite thickness (lc) using a cubic pillar model and

crystallite dimensions of 200 nm· lclc(nm)

IPD

(nm)

TIA

(m2/g)

TEAmin

(m2/g)

TEAmax

(m2/g)

TIV

(cm3/g)

TIVc · 1016(cm3)

XAE

(%)

AD

(g/cm3)

TD

(g/cm3)

5 0.49 669.4 264.8 478.7 0.098 0.264 35.09 1.55 1.83

10 0.49 860.7 118.0 209.7 0.127 0.792 16.54 1.81 2.34

15 0.49 912.8 77.9 136.3 0.134 1.321 11.48 1.89 2.54

20 0.49 937.2 59.2 102.0 0.138 1.849 9.12 1.94 2.64

25 0.49 951.3 48.4 82.2 0.140 2.377 7.75 1.96 2.70

30 0.49 960.4 41.4 69.3 0.141 2.905 6.86 1.98 2.75

35 0.49 966.9 36.4 60.2 0.142 3.434 6.24 1.99 2.78

40 0.49 971.7 32.7 53.4 0.143 3.962 5.77 2.00 2.80

50 0.49 978.4 27.6 44.1 0.144 5.018 5.13 2.01 2.83

100 0.49 991.2 17.7 26.0 0.146 10.169 3.88 2.01 2.85



relation with the occupied anion areas (0.761

nm2 < 1.052 nm2). Similarly, the effect of the LDH metal

ratio x ¼ M3þ=ðM2þ þM3þÞ on the IPDs can be ana-

lyzed. It is easily deduced from Eq. (19) that for a givenanion the smaller the value of LDH charge (x) the largerthe IPD, as it is seen in the examples of Table 6.

The results of IPDs for the [Mg1�xAlx][Fe(CN)6]

LDH obtained in the present study are similar to those

reported by several authors. For instance, Cavalcanti et

al. [14], using an ideal structural model of the unit cell,

estimated minimum and maximum distances of 0.35 and

0.45 nm among anions in the [Mg0:75Al0:25][Fe(CN)6]4�

LDH. The estimated IPDs using Eq. (19) are 0.51 and

0.59 nm for the cylindrical and cubic pillar model,

respectively. Nijs et al. [8] reported some IPD results for

[Mg1�xAlx][Fe(CN)6]3�. For example, the IPDs for

Table 11

Estimated textural properties of a [Mg0:75 Al0:25][H4Co2Mo10O38]6� LDH with variable crystallite mean diameter ðdcÞ using a cubic pillar model and

crystallite dimensions of dc · 20 nm

dc(nm)

IPD

(nm)

TIA

(m2/g)

TEAmin

(m2/g)

TEAmax

(m2/g)

TIV

(cm3/g)

TIVc · 1016(cm3)

XAE

(%)

AD

(g/cm3)

TD

(g/cm3)

50 0.51 925.5 81.7 124.5 0.138 0.116 16.32 1.94 2.64

75 0.50 930.7 71.8 114.5 0.138 0.260 13.14 1.94 2.64

100 0.50 933.2 66.8 109.6 0.138 0.462 11.55 1.94 2.64

150 0.49 935.9 61.8 104.5 0.138 1.040 9.93 1.94 2.64

200 0.49 937.2 59.2 102.0 0.138 1.849 9.12 1.94 2.64

300 0.49 938.5 56.7 99.5 0.138 4.160 8.31 1.94 2.64

400 0.49 939.1 55.5 98.2 0.138 7.395 7.89 1.94 2.64

500 0.49 939.5 54.7 97.5 0.138 11.555 7.65 1.94 2.64

1000 0.49 940.3 53.2 96.0 0.138 46.220 7.16 1.94 2.64


Table 12

Experimental textural properties of some LDHs [25,26]

LDH-A Method Treatment time (min) lc (nm) SBETa (m2/g) TEAmin

b (m2/g) TEAmin/SBET

Mg0:75Al0:25–CO3 MW 0.5 13.3 80 100.6 1.3

Mg0:75Al0:25–CO3 MW 10 18.9 85 72.3 0.9

Mg0:80Al0:20–CO3 MW 0.5 9.5 50 140.7 2.8

Mg0:80Al0:20–CO3 MW 10 15.4 90 89.2 1.0

Mg0:86)Al0:14–CO3 MW 0.5 9.0 15 144.6 9.6

Mg0:86)Al0:14–CO3 MW 10 11.4 95 118.2 1.2

Mg0:75Al0:25–NO3 HT 10 7.2 2.2 180.7 82.1

Mg0:75Al0:25–NO3 HT 60 7.9 8.0 162.1 20.3

Mg0:75Al0:25–NO3 HT 600 9.6 65.5 134.9 2.1

Mg0:75Al0:25–NO3 US 5 7.8 48.4 162.1 3.3

Mg0:75Al0:25–NO3 US 30 10.3 69.9 124.8 1.8

Mg0:75Al0:25–NO3 US 60 15.5 71.7 87.5 1.2

lc ¼ crystallite thickness, SBET ¼ area BET, TEAmin ¼ total external area minimum, MW¼microwave, HT¼ hydrothermal at 363 K, and US¼ultrasound.

a Samples of MgAl–NO3 were dried at 423 K for 5 h before BET analysis. For samples of MgAl–CO3 no data on the treatment before BET

analysis were reported.bAssumed crystallite mean diameter (dc) of 150 nm and cubic anion model.


x ¼ 0:25 and 0.35 are about 0.40 and 0.20 nm, respec-

tively (read from a graphic in the original reference).

The IPD values estimated in this work and shown in

Table 6, for the cubic pillar model, are 0.43 and 0.26 nm,respectively. The IPD values for the cylindrical model

are 0.35 and 0.18 nm, respectively. Using idealized

models of the unit cell for a [Mg0:66Al0:33][V10O28]6�

LDH, Drezdzon [15] estimated that the IPD is large

enough to accommodate water molecules (�0.29 nm,

measured from a figure in the original reference). From

Table 6 or using Eq. (19), the estimated IPD is 0.44 nm

for the cubic pillar model and 0.33 nm for the cylindricalpillar model, in agreement with Drezdzon’s results.

In some cases the 100% intercalation of pillars is not

possible. For this, the IPD can be estimated by intro-

ducing a new term in Eq. (2), the intercalated fraction

(IF): Yb ¼ IF � x �NMb=n. With this change, the IPD is

given by:

IPD ¼ 0:310pðn=ðIF � xÞÞ � dp ð20Þ

Eq. (20) indicates that if the pillars are uniformly dis-

tributed along the layers of the LDH crystallite layers,

the IPDs will be larger when the intercalated fraction is

smaller. However, it should not be forgotten that in thecases where IF< 1 for the anion of interest, there must

be other compensating anions to reach electroneutrality

in the LDH, although the presence of different anions in

the interlamellar region may not be thermodynamically

favored [27]. The analysis of IPDs of mixed interlayer

anions is complex. One way to facilitate this is by

introducing in Eq. (4) the concepts of anions average

charge (nave) and diameter (dpave). If yi is the molar ratioof the ith anion with charge ni, the anions average

charge can be calculated by nave ¼ ½Rðyi=niÞ��1and dpave

is lower or equal to Rðyi � dpiÞ. These mixed interlayer

anions also affect the textural properties by modifying

the molecular weight of the crystallite.

In practical terms, the prediction of IPDs in the

LDHs can be used to know in advance if the synthesis of

a pillared material is feasible or not, and if the obtained

Table 13

Experimental textural properties of some pillared LDHs [12]

LDH-A xia xf b SBET

(m2/g)

lPVc

(cm3/g)

IPDb

(nm)

TIA

(m2/g)

TEAmin

(m2/g)

TEAmax

(m2/g)

TIV

(cm3/g)

[Mg1�xAlx][V10O28]6� d ;e ; f 0.20 0.31 73 0.011 0.50 1045.1 126.1 215.2 0.152

0.28 0.38 58 0.008 0.36 995.2 114.6 213.5 0.110

[Mg1�xAlx][Fe(CN)6]3� e ;f ;g ;h 0.20 0.20 294 0.110 0.56 1596.1 185.9 300.3 0.258

0.23 0.23 438 0.159 0.48 1607.2 180.8 308.6 0.231

0.25 0.25 367 0.140 0.43 1614.2 177.6 313.9 0.215

0.33 0.33 254 0.090 0.29 1640.0 165.8 332.9 0.154

0.40 0.40 173 0.045 0.20 1659.7 156.7 347.3 0.107

[Mg1�xAlx][PV2W10O40]5� d ;e ; f 0.20 0.34 47 0.003 0.18 656.1 77.3 197.3 0.031

0.25 0.39 45 0.004 0.10 631.1 69.9 193.7 0.008

0.34 0.45 63 0.003 0.03 607.0 62.7 190.3 )0.014

[Zn1�xAlx][PV2W10O40]5� f ; i ; j ;k 0.21 0.19 166 0.048 0.59 645.5 94.4 177.3 0.121

0.25 0.25 136 0.032 0.39 618.3 82.5 177.2 0.077

0.29 0.30 123 0.028 0.26 600.5 74.6 177.1 0.047

0.33 0.35 98 0.016 0.17 586.0 68.2 177.0 0.024

Heading description as in Table 6.a x before intercalation.b x after intercalation.c Experimental micropore volume.d Synthesis by anion exchange with a slurry of LDH–NO3 recently synthesized.eDegasification at 423 K for 16 h under vacuum.f Assumed crystallite dimensions of 150 nm· 10 nm.g Synthesis by anion exchange with an LDH–NO3 pre-swelled in water.hm varies between 7 and 11 (lc ¼ 7:8� 12:3 nm).i Synthesis by anion exchange with an LDH–terephthalate (prepared by anion exchange with an LDH–NO3).j Degasification at 393 K for 12 h under vacuum.km varies between 5 and 8 (lc ¼ 7:6� 12:1 nm).


material could present microporous properties. Re-

cently, Hui et al. [28] reported the synthesis of the

[Zn0:68Al0:32][PW12O40]3�0:11 LDH. In spite of satisfactory

characterizations by FTIR spectroscopy, elemental

analysis and powder XRD, the IPD of this materialresults in a negative value ()0.06 nm). This result indi-

cates that the synthesis of the totally exchanged and

pure [PW12O40]3� pillared LDH compound is not the-

oretically feasible. In this particular case, the presence

of a mixture of [PW11O39]7� and [PW12O40]

3� can

be a plausible explanation of the results, since the

[PW12O40]3� can decompose into [PW11O39]

7� at the pH

of the synthesis (4.5–5.0) and the anion [PW11O39]7�

shows higher positive IPD values, as shown in Table 6.

On the other hand, the IPDs can be directly related to

the microporous properties of pillared LDHs. Hence,

those materials that show interlayer spacings and IPDs

larger than the diameter of a N2 molecule (�0.37 nm),

will have a higher probability of having microporosity.

Such is the case for [Mg0:75Al0:25][Fe(CN)6]3� LDH

(h ¼ 0:64 nm and IPD¼ 0.43 nm) [10]. Based on merelygeometrical and structural arguments, to obtain micro-

porous pillared LDHs the ideal pillar should possess a

large relative ratio of charge to equivalent diameter

(high charge, small diameter) and a molecular height

larger than the diameter of a N2 molecule. With all these

conditions fulfilled, the best microporosity results

should be obtained at small values of x.

4.2. External areas

It was seen in Section 2.2 that several variables

can influence the calculations of the external and inter-

lamellar specific areas of LDH materials such as

the charge and geometrical model of the pillar, and the

composition, dimensions and charge of the LDH. In the

case of the cubic pillar model some results are shown in

Table 6. A typical dimension of an LDH crystallite de-

scribed by several authors is 200 nm · 20 nm [12]. Thesedimensions can vary and are mainly functions of the

synthesis method and the hydrothermal treatments gi-

ven to the LDH. In this work, the external and inter-

lamellar areas of several intercalated anions (POMs,

ferricyanate, carbonate and hydroxyl) in LDHs have

been estimated. As seen in Table 6 for hexagonal-shaped

crystallites of dimensions 200 nm · 20 nm, the interla-

mellar areas of the intercalated materials can potentiallyreach maximum values of up to 1700 m2/g and external

areas of up to 150 m2/g. However, up to now only a

maximum experimental value of BET area of only 438

m2/g has been obtained [11]. This result suggests that the

textural possibilities offered by pillared LDHs have not


been totally exploited. According to Nijs et al. [8],

interlamellar pore blocking and collapse of brucite-like

layers can occur during the intercalation process, which

will reduce the predicted values. The predictions for

minimum and maximum SSAs are also shown in Table6. The maximum TEA includes all possible contribu-

tions, such as the external surface and edges of the LDH

layers, the anions present in the borders and in the

exterior of the top and bottom layers. The minimum

TEA does not take into account the contribution due to

the anions in the external surfaces. In cases where the

LDH materials have a uniform distribution of anions on

the external surface and the values of h and IPD aremuch larger than 0.37 nm, the TEA may be close to the

maximum value. On the other hand, when the h or IPDs

are close to or smaller than 0.37 nm, the TEA will be

near the minimum value. The LDH charge can influ-

ence, in many different ways, the total specific areas of

the LDH. For the interlamellar areas, the larger the

LDH charge the higher the concentration of the anions

in the interlayer region, so that the interlamellar areaof the crystallite (TIA ÆMWC) should be larger. For

example, [Mg1�xAlx][PW11O39]7� LDH (x ¼ 0:15, 0.25

and 0.35) has crystallite TIA values of 1.056 · 10�12,

1.207 · 10�12 and 1.358 · 10�12 m2, respectively. How-

ever, when the TIA is expressed per unit charge of the

LDH, as proposed by Yun and Pinnavaia [29], a com-

pletely opposite behavior is obtained, the higher the

charge of the LDH the smaller the TIA, expressed innm2/eþ. When the TIAs are expressed in m2/g, it is

difficult to establish a relationship with the LDH charge

since in this case the TIA is a function not only of the

geometry and charge of the anion and the brucite-like

layers, but also of the molecular weight of the interca-

lated LDH. The difference in molecular weight for dif-

ferent LDHs can be seen in the values of MWCs for

different LDH charges, either for the same anion oranions with the same geometry (i.e. Keggin type). These

results are shown in Table 6.

When considering the effect of the charge of similar

anion geometries on the specific areas, the results are

very similar to those observed for the charge of the

LDH. The smaller the anion charge the higher the

number of anions necessary to reach electroneutrality,

giving larger crystallite TIAs and TEAs (expressed inm2). For instance, the crystallite TIA for the LDHs:

[Mg0:75Al0:25][PW11O39]7�, [Mg0:75Al0:25][PV2W10O40]

5�

and [Mg0:75Al0:25][PW11Fe(H2O)O39]4� are 1.207 ·

10�12, 1.358 · 10�12 and 1.490 · 10�12 m2, respectively.

The results in Table 7 show that the same trend is ob-

tained when comparing the TIAs expressed in nm2/eþ

and this is because the pillar dimensions and the LDH

charge have been kept constant. If the TIAs expressed inm2/g are compared, then the smaller the anion charge

the smaller the TIA values due to a higher anion con-

centration and therefore a higher molecular weight of

the LDH. From the results in Tables 6 and 8, the specific

areas are similar for both cubic and cylindrical anion

models, although the estimated IPDs are slightly smaller

when using the cylindrical anion model. Similarly, the

effect of the LDH molecular weight on the texturalproperties for different metals in the brucite-like layers

can be observed in Table 9. The textural properties ex-

pressed per unit weight of the LDH will be higher for the

[Zn1�xAlx][PV2W10O40]5� than for the [Mg1�xAlx][PV2-

W10O40]5� because of the greater molecular weight of

the Zn in comparison to the Mg. If the same results are

expressed in nm2/eþ, then the areas will be the same for

both materials as long as LDH charges are the same.For example, the TEAmax for the LDHs [Zn0:75-

Al0:25][PV2W10O40]5� and [Mg0:75Al0:25][PV2W10O40]

5�

are both equal to 0.123 nm2/eþ. This result is expectedsince the textural properties expressed per unit charge do

not depend on the LDH molecular weight. The crys-

tallite TIA and TEA results for an LDH crystallite

of [Zn1�xAlx][PV2W10O40]5� and [Mg1�xAlx][PV2W10-

O40]5� in m2 are also the same.

The estimated textural properties of the LDH

[Mg0:75Al0:25][H4Co2Mo10O38]6� for crystallites with

different thickness and mean diameter dimensions are

given in Tables 10 and 11, respectively. From the results

in Table 10, it is observed that the crystallite thickness

(lc) has a considerable effect on the specific areas,

especially on the TEA. Thus, the larger the values of lcthe smaller the values of TIA and TEA. It is worthwhileto note this inverse relationship of the lc’s with the TEAs

(in m2/g). For example, a reduction of 50% in the

crystallite thickness results in an increase of almost

100% in the TEAs. On the other hand, the crystallite

diameter (dc) also has some effect on the specific areas,

although not as pronounced as in the case of the crys-

tallite thickness. From the results in Table 11, it is noted

that the TEA and the TIA reach an almost constantvalue when dc is over 150 nm.

Reichle [30,31] reported BET areas for the LDH

Mg0:75Al0:25–CO3 of 120 m2/g for a sample crystallized

at 338 K for 18 h and 14 m2/g for a sample crystallized

at 473 K for 18 h. These BET area results were

explained using crystallite dimensions of 200 nm · 10 nm(TEAmin ¼ 125 m2/g) and 1000 nm · 100 nm (TEAmin ¼13 m2/g), respectively. However, these dimensions aresomewhat large, and from the published TEM results of

these materials [30,31] the dimensions 100 nm · 10 nm

(TEAmin ¼ 133.8 m2/g) and 500 nm · 50 nm (TEAmin ¼26.2 m2/g) may be more appropriate. In addition, only a

fraction of the TEAmin values can be experimentally

measured by a probe molecule such as N2. With regards

to Fig. 2, the fraction of a hexagon covered by a

spherical probe molecule is p=ð2p3Þ. Therefore the mea-surable TEAmin(mTEAmin) will be given by pTEAmin=ð2p3Þ. Because the height of the carbonate anion

(h ¼ 0:27 nm) is smaller than 0.37 nm, a contribution to


the total area due to external and interlayer anions is

excluded. The estimated mTEAmin for the MgAl–CO3

LDHs crystallized at 338 and 473 K are 121.3 and

23.8 m2/g, respectively. These results are in agree-

ment with the experimental findings. Many otherexamples of the synthesis and characterization of LDH

compounds, including their morphological characteris-

tics, have been published [3]. However, very few of these

comprise a full textural characterization with crystallite

thickness and mean diameter, specific areas and micro-

pore volumes. In most of the cases, the values of the

average crystallite thickness (distance in the basal plane

0 0 l), shown in the present work to have an appreciableinfluence in the specific areas of LDHs, have not been

reported, even though they can be easily calculated from

powder XRD results by means of Scherrer’s equation

[32].

The BET area of LDHs can show substantial differ-

ences even for the same material, as seen in Table 1.

According to Rives [3], the observed differences are

mostly a result of the aging and hydrothermal treat-ments given during the synthesis, or the drying process

before BET analysis. From a structural point of view,

these discrepancies in BET areas can be explained by the

differences in the average dimensions of the LDH crys-

tallite. For instance, in the synthesis of the LDH

[Mg0:75Al0:25][CO3] by the urea method, Constantino

et al. [4] obtained a BET area of 24 m2/g. The measured

crystallite mean diameter by the SEM method variedbetween 0.5 and 10 lm. Using similar synthetic condi-

tions, Oh et al. [33] observed average diameters of 2 lm.

However, neither of the two studies indicated values of

the average crystallite thickness. The estimated TEAmin

for a crystallite with dimensions of 2000 nm · 40 nm is

28.8 m2/g and the mTEAmin is 26.1 m2/g. This result

indicates that the crystallite thickness may have a value

close to 40 nm, which may be in agreement with the highcrystallinity of these materials as observed in the pub-

lished powder XRD patterns. For the rest of the LDHs

in Table 1, with the exception of the LDH [Mg0:75Al0:25]-

[CO3] synthesized in a water/ethylene glycol medium,

the observed BET areas are around 80–90 m2/g. These

BET area results can be estimated using a value of

crystallite diameter (dc) close to 150 nm, and a crystallite

thickness (lc) of about 15.0 nm. The differences observedwith the sample synthesized in the water/ethylene glycol

media can be explained by a variation in the mean

diameter and thickness of the crystallite. A more de-

tailed discussion of this difference is not possible since

the values of dc and lc were not described in the original

reference. Velu et al. [5] reported the values of crystallite

thickness for the LDHs [Mg0:77Al0:23][CO3] (17.6 nm)

and [Mg0:81Al0:19][CO3] (12.9 nm); the BET areasof these materials are shown in Table 1. If an aver-

age crystallite diameter of 150 nm is assumed, then

the estimated mTEAmin for these LDHs are 70.9

and 93.7 m2/g, respectively. These estimated values

are comparable with the experimental results, 92 and

88 m2/g.

Recently, Hussein et al. [25] and Seida et al. [26]

studied the effect of microwave radiation and ultrasoundon the synthesis of MgAl LDHs and the effect that these

treatments have on the textural properties of the

resulting materials. The first authors presented the

evolution of the crystallite thickness (lc) together with

the BET area of the resulting LDHs, when the LDH was

synthesized with hydrothermal treatments under a

microwave field using exposure times between 0.5 and

10 min. The experimental results are given in Table 12.The observed crystallite thickness for the LDHs

[Mg0:75Al0:25][CO3], [Mg0:80Al0:20][CO3] and [Mg0:86-

Al0:14][CO3] varied between 13.3–18.9, 9.5–15.4 and 9.0–

11.4 nm for microwave exposure times of 0.5 and 10

min, respectively, and the BET areas were 80–85, 50–90

and 15–95 m2/g, respectively. Assuming a particle

diameter (dc) of 150 nm, the estimated TEAmin for these

materials are 100.6–72.3, 140.7–89.2 and 144.6–118.2m2/g, respectively. The larger deviations presented by

the BET areas at the shortest exposure time can be ex-

plained by the poor crystallinity of the samples and due

to a crystallite aggregation of the type face-to-face or

card-house [19], resulting in a reduction in the total

external area. The resulting face-to-face type aggregate

from two crystallites would form a ‘‘new crystallite’’ of

greater thickness. If the interfacial space between thesetwo crystallites is not accessible or very limited to N2

molecules, then the total external surface area can be

decreased up to 50%. An analysis of the ratio TEAmin/

SBET shows that when large values are obtained there is

a high tendency to form face-to-face aggregates with a

higher number of crystallites. In this way, from the re-

sults in Table 12, there are more aggregates at shorter

microwave exposure times for an LDH with the samecharge (½Mg0:86Al0:14�½CO3� � 0:5min ¼ 9:6 > ½Mg0:86-

Al0:14�½CO3� � 10min ¼ 1:2) and at smaller values of

x for different LDH charges (½Mg0:86Al0:14�½CO3� �0:5min ¼ 9:6 > ½Mg0:80Al0:20�½CO3� � 0:5min ¼ 2:8 >½Mg0:75Al0:25�½CO3� � 0:5min ¼ 1:3). Similar behavior is

noted from the results of the synthesis of the LDH

Mg0:75Al0:25–NO3 subjected to hydrothermal (363 K)

and ultrasound treatments. The results of TEAmin/SBETfor the hydrothermally treated samples (t ¼ 10, 60 and

600 min) are 82.1, 20.3 and 2.1, respectively, and for the

samples treated under ultrasound (t ¼ 5, 30 and 60 min)

the TEAmin/SBET are 3.3, 1.8 and 1.2, respectively. At

elevated treatment times (HT or US), the crystallinity of

the sample is higher and the values of lc are much larger,

resulting in TEAmin/SBET values close to 1.0. Therefore,

there is a lower probability of forming face-to-faceaggregates, which means that at these conditions the

experimental values of the BET area agree with the

mTEAmin.


[Mg0:75Al0:25][NO3] LDHs have been also synthesized

by Fetter et al. [34] using high concentrations of metals

(2.5 M), relatively short precipitation times (10 min) and

a subsequent treatment of the LDH slurry with micro-

wave radiation for 5 min. After a degasification of theLDH samples at 373 K for 2 h, the resulting areas are

quite small (�3 m2/g). The BET area differences of the

MgAl–NO3 with the MgAl–CO3 are explained by these

authors in terms of the higher interlayer density of the

NO�3 anions in the LDH, which prevents N2 molecules

from accessing the interlayer space. For the MgAl–CO3,

with a lower anion density, the N2 can supposedly dif-

fuse more easily into the interlayer region. From theresults in the present study and from the literature, these

explanations are not completely satisfactory. It is true

that in an LDH the NO�3 density is higher than for the

CO2�3 , since both anions possess a similar geometry and

a different charge, as given by the estimated IPDs for

these materials. For the [Mg1�xAlx][NO3] (x ¼ 0:15,0.25, 0.35) the IPDs are 0.38, 0.20 and 0.11 nm,

respectively, while for the [Mg1�xAlx][CO3] (x ¼ 0:15,0.25, 0.35) the IPDs are 0.72, 0.46 and 0.32 nm,

respectively. Although the IPDs for the MgAl–CO3 are

larger than a N2 molecule, it would be erroneous to

think that these N2 molecules can reach the interlamellar

region since the interlayer distance for the CO2�3 is about

0.27 nm. Also, if we compare the equivalent molecular

weights of the crystallites of MgAl–NO3 and MgAl–

CO3 with the same dimensions, the obtained values aresimilar. A more satisfactory explanation of Fetter et al.’s

results would have to do with the poor crystallinity of

the synthesized materials together with a high agglom-

eration of the crystallites. The textural properties of

others LDHs of NO�3 and CO2�

3 have been also pub-

lished in the literature. For instance, Velu et al. [35]

synthesized the LDHs of [Zn0:64Al0:36][CO3] and

[Zn0:63Al0:37][NO3]. The crystallite thickness (lc) and theBET areas were 25.2 and 18.0 nm, and 45 and 32 m2/g

(read from a graphic in the original reference), respec-

tively. If a crystallite diameter of 150 nm is assumed,

then the measurable TEAmin (mTEAmin) are 35.6 and

48.6 m2/g, respectively. These values are comparable

with the experimental results.

Although the results of the model presented in this

study are applicable to LDHs of the type[M2þ

1�xM3þx (OH)2][A

n�x=n], similar trends for the textural

properties have been found in LDHs of the type

[Mþ1�xM

3þx (OH)2]

ð2x�1Þþ[An�ð2x�1Þ=n] [36]. Ulibarri et al.

[37] studied the textural properties of the LDH

[Liþ0:33Al3þ0:67(OH)2]0:33þ[An�

0:33=n] as a function of the time

and the temperature of the hydrothermal treatment. The

experimental results indicated that the longer the time of

the hydrothermal treatment at 403 K the larger thecrystallite thickness. The values of the crystallite thick-

ness varied from 2.4 nm for a sample not treated

hydrothermally, up to 31.9 nm for a sample treated for

48 h. The BET areas of these samples ranged from 55.5

to 5.5 m2/g, respectively. These results follow the

same behavior observed for the LDH of the type

[M2þ1�xM

3þx (OH)2][A

n�x=n], where the larger the crystallite

thickness the smaller the TEAmin. Regarding the tem-perature of the hydrothermal treatment, a similar

behavior is also noted. As the temperature of the

hydrothermal treatment at 48 h increased, the LDH

crystallite thickness becomes greater giving as a result a

reduction of the BET area.

The intercalation of POMs in LDHs has been well

studied owing to the potential that these materials

present as catalysts and adsorbents with shape selectiv-ity [2]. Several methods of intercalation of POMs in

LDHs have been described in the literature, including

direct synthesis [29], direct anion exchange [38,39], anion

exchange by acid elimination [15] and reconstruction of

the LDH structure [40,41]. Recently, Nijs et al. [12]

studied the microporous properties of some of these

intercalated compounds. Some of the results of this

study are shown in Table 13. In the case of the LDHs[Mg1�xAlx][V10O28] and [Mg1�xAlx][PV2W10O40] the

values of the TEAs and the crystallite diameter (dc) andthickness (lc) were not described. For calculation pur-

poses, LDH crystallites with dimensions of 150 nm · 10nm have been assumed. From the results in Table 13, the

values of TEAmin are almost twice the BET area for the

LDH [Mg1�xAlx][V10O28]. This result may indicate that

the BET area is mainly due to the external area of theLDH crystallite. The explanation for why the BET area

is smaller than the TEAmin may be that face-to-face

aggregates are formed or simply a crystallite thickness

larger than 10 nm is present. For the LDH [Mg1�xAlx]-

[PV2W10O40], the values of TEAmin estimated with the

model are much closer to the BET areas, but still larger.

Again, a difference in the crystallite thickness could

explain these variations. For these two intercalatedcompounds, there is no formation of interlamellar

microporosity, even for the [Mg1�xAlx][V10O28] that

shows an IPD of 0.50 nm, since it is very probable that

the synthesized material contains other isopolyvana-

dates species of lower charge that would make access

more difficult to the interlayer region. In the LDH

[Mg1�xAlx][PV2W10O40] (xf ¼ 0:34, 0.39, 0.45), small

and even close to zero values of IPDs are observed (0.18,0.10 and 0.03 nm). This may be because the initial xvalues of the original LDH (x ¼ 0:20, 0.25, 0.34) are

much smaller than those of the final intercalated mate-

rial, due to a partial dissolution of the Mg from the

brucite-like layers. These faults in the lamellar structure

could be used by the POMs as exchange sites, and

according to Nijs et al. [9], this would explain why in

some cases the obtained interlayer distances are smallerthan expected. Consequently, the IPDs of this LDH

[Mg1�xAlx][PV2W10O40] should be calculated for x val-

ues between xi and xf . The obtained IPDs are 0.55–0.18,

Fig. 8. Model of a brucite like layer with equally spaced cubic anions.


0.39–0.10, and 0.18–0.03 nm, respectively. Additionally,

the decomposition of the [PV2W10O40]5� in other Keg-

gin type POMs cannot be excluded, which could be also

intercalated in the LDH.

The LDHs [Mg1�xAlx][Fe(CN)6]3� and [Zn1�xAlx]-

[PV2W10O40] have crystallite thicknesses between 7.8–

12.3 and 7.6–12.1 nm, respectively [9]. Thus, an average

value of 10 nm has been taken for both materials and a

value of 150 nm has been estimated for the crystallite

diameter (dc). With these dimensions the estimated tex-

tural properties are given in Table 13. The

[Mg1�xAlx][Fe(CN)6]3� LDH presents values of TEAmin

smaller than the BET area. This BET area increases asthe value of x decreases, in agreement with the increas-

ing IPDs. These results together with the micropore

volume are an indicative of the presence of micropo-

rosity. The BET areas register a maximum value at

x ¼ 0:23, and below it the BET areas start decreasing.

This reduction is not in agreement with the theoretical

predictions. According to Nijs [12] this reduction in the

BET areas can be attributed to the loss of LDH crys-tallinity at the degasification conditions of the sample,

due to a lower thermal stability of the pillared materials

at low x values. On the other hand, aggregation of these

low crystalline materials can contribute to the decrease

of the total external area. If the external area of these

intercalated materials is assumed to have a value close to

the TEAmin, then the micropore areas can be calculated

based on experimental data. The interlamellar areasreach a maximum of up to 16% of the theoretical value.

On the one hand, this result can be understood if the

IPD is viewed as an average value. For example, the

LDH [Mg0:75Al0:25][Fe(CN)6]3� that has a BET area of

367 m2/g and a relatively high thermal stability at the

degasification conditions has an IPD of about 0.43 nm.

Therefore, if this material is assumed to have a normal

distribution of IPDs with an average value of 0.43 nmand a standard deviation of 0.05 nm, then only 88% of

the total IPDs will show values larger than 0.37 nm.

Consequently, not all of the pores formed in the inter-

lamellar region will be accessible to N2 molecules. On

the other hand, the low interlamellar areas can be

understood if we take into account that the IPDs and

the interlayer spacing (h) for the ferricyanate anion are

smaller than 0.74 nm (twice the diameter of a N2 mole-cule). For this reason, only a fraction of the total

interlamellar area (TIA) estimated theoretically can be

measured experimentally.

4.3. Interlamellar areas

To estimate the fraction of TIA measured experi-

mentally, an equilateral parallelogram geometry for abrucite-like layer with equally spaced cubic pillars is

assumed. This model is schematically shown in Fig. 8.

For IPD values between 0.37 and 0.74 nm, an equation

to estimate the fraction of the TIA that can be experi-

mentally measured (FIAmea) is obtained in the following

manner. Two different types of ‘‘empty spaces’’ can beidentified: the first type lies between pillars in the same

row and the second type lies between two rows of pillars.

The total number of available empty spaces for the first

type is given by NPsðNPs � 1Þ and for the second type

by ð2NPs � 1ÞðNPs � 1Þ, respectively. NPs is the number

of pillars on one of the sides of the structure in Fig. 8.

The fraction of TIA that can be experimentally mea-

sured is given by the ratio between the area measured byN2 molecules and the theoretical area. This ratio ex-

pressed as a function of NPs is: ½AN2NPsðNPs � 1Þþ

AN2ð2NPs � 1ÞðNPs � 1Þ�=½IPD2NPsðNPs � 1Þp3þ 2dp-

hNPsðNPs � 1Þ þ IPD2ð2NPs � 1ÞðNPs � 1Þp3 þ dp-hð2NPs � 1ÞðNPs � 1Þ�. Simplifying common terms and

also for values of NPs � 1 the following equation is

obtained:

FIAmea ¼ 3AN2=ð3p3IPD2 þ 4dphÞ ð21Þ

where AN2is the molecular cross sectional area of a

nitrogen molecule (0.163 nm2). Using Eq. (21), the

fraction of TIA that can be experimentally measured for

the LDH [Mg0:77Al0:23][Fe(CN)6]3� corresponds

approximately to 17%, that is, 273.2 m2/g. By comparing

this result with the experimental 257 m2/g, the model

explains adequately the experimental results for the

[Mg0:77Al0:23][Fe(CN)6]3� LDH. In the case of the

[Zn1�xAlx][PV2W10O40], the BET areas are also larger

than the values of TEAmin; together with the micropore

volumes it can be said that this material has micropo-

rosity. Similarly, the BET areas show an increasing

trend as the x values decrease. If the micropore areas are

calculated by SBET)TEAmin, then only up to 11% of the

TIA theoretical value is obtained. These differences can

be explained with similar arguments used for the[Mg1�xAlx][Fe(CN)6]

3� materials.

4.4. Interlamellar free volumes

Some results of the estimated TIVs for several inter-

calated LDHs with crystallites dimensions (dc � lc) 200nm · 20 nm are shown in Table 6. In all cases, as ex-

pected, the TIV decreases when the LDH charge (x)


increases, as a result of a higher concentration of anions

in the interlayer region. If two anions of similar charge

are compared, with the same LDH charge, the TIV is

lower for the bulkier anion. For example, the LDH

[Mg0:75Al0:25][H4Co2Mo10O38]6� (0.138 cm3/g) has a

lower TIV than the LDH [Mg0:66Al0:33][V10O28]6� (0.210

cm3/g). On the other hand, a comparison of anions of

similar volume within an LDH of the same charge

shows that the TIVs are higher for the anions of higher

charge. Therefore, for the Keggin anion series shown in

Table 6, the estimated TIVs follow the trend:

[PW11O39]7� > [PV2W10O40]

5� > [PW11Fe(H2O)O39]4� >

[PW12O40]3�. For a given LDH compound, independent

of the anion model utilized, the estimated TIVs are the

same since, in the calculations, the area occupied by the

anion remains the same. When the effect of the metals in

the brucite-like layers is studied, the lower TIVs are for

the LDHs that contain the heavier metals due to the

molecular weight effect on the TIVs expressed in cm3/g.

In absolute terms, the TIVc (in cm3) is the same as long

as the LDH charge and crystallite dimensions are thesame. For instance, the [Zn0:75Al0:25][PV2W10O40]

5� and

the [Mg0:75Al0:25][PV2W10O40]5� have the same TIVc ¼

1:678� 10�16 cm3. A similar result is obtained when the

TIVs are expressed per unit charge of the LDH (nm3/eþ)because the LDH charge is the same for both materials.

The effect of the crystallite dimensions can be seen in

Tables 10 and 11. In the case of the LDH

[Mg0:75Al0:25][H4Co2Mo10O38] with a constant crystal-lite diameter (dc), when the crystallite thickness (lc) in-creases the TIV also increases, reaching an almost

constant value over a lc of 25 nm. The same result is

obtained when the units of the TIVs are expressed in

nm3/eþ since the conversion factor g/eþ, MWC/

(NMc � x �m), is only function of the LDH charge and

molecular weight. For this same material, if the crys-

tallite thickness is kept constant and the crystallite di-ameter is modified, then the estimated TIV remains

constant. Similarly to the TIA, the crystallite thickness

presents a more pronounced effect on the TIVs than the

crystallite diameter. However, in the case of the TIVc’s,

as shown in Tables 10 and 11, a constant increase of the

crystallite interlamellar free volume is observed as the

values of dc or lc are incremented.

Three different strategies can be utilized to compareTIVs of intercalated LDHs with similar crystallite di-

mensions: using TIVs expressed in cm3/g, in nm3/eþ or

in cm3. Therefore, for the LDHs Mg0:75Al0:25�An� ob-

served in Table 7, the anion order of the five highest

TIVs expressed in cm3/g are: [Fe(CN)6]3� (0.231) > [V10-

O28]6� (0.210) > [PW11O39]

7� (0.175) > [OH]� (0.171) >

[CO3]2� (0.145); in nm3/eþ: [PW11 O39]

7� (0.180) > [V10-

O28]6� (0.138) > [PV2W10O40]

5� (0.125) > [H4Co2Mo10-O38]

6� (0.119) > [Fe(CN)6]3� (0.118) and in cm3 · 1016:

[PW11 O39]7� (2.426) > [V10O28]

6� (2.294) > [Fe(CN)6]3�

(2.078) > [OH]� (2.015) > [H4Co2Mo10O38]6� (1.849).

The experimental results of the micropore volumes of

several pillared LDHs are shown in Table 13. The lPV/TIV ratio of these materials varies between 7% and 69%.

The highest value is presented by the LDH [Mg0:77-

Al0:23][Fe(CN)6]3�. This result can be explained if the

method used to measure the micropore volume using N2

is taken into account, because when the IPDs and the

interlayer spacings fall between 0.37 and 0.74 nm, it is

not possible to experimentally measure all of the theo-

retical TIV. In this case, an expression similar to Eq.

(21) is derived to determine the fraction of TIV that can

be experimentally measured (FIVmea): ½VN2NPsðNPs �

1ÞþVN2ð2NPs� 1ÞðNPs� 1Þ�=½ðp3=2ÞIPD2hNPsðNPs�

1Þ þ ðp3=2ÞIPD2hð2NPs � 1Þ ðNPs � 1Þ�. Simplifying

terms and for NPs � 1 the following expression is ob-

tained:

FIVmea ¼ 2VN2=ðp3IPD2hÞ ð22Þ

VN2is the volume of a N2 molecule, calculated from the

liquid density of N2 at the boiling point (0.808 g/cm3).

Eq. (22) is valid for values of h and IPDs between 0.37

and 0.74 nm. For [Mg0:77Al0:23][Fe(CN)6]3� the maxi-

mum fraction of the TIV that can be measured is about45%. However, the experimental value is 69%. This re-

sult could be explained by the presence of IPDs > 0:74nm and by a contribution to the micropore volume from

the external area, the LDH mesoporosity or both [42].

4.5. Other properties

Other LDH properties that can be estimated in the

present study include the molar fraction of the anions

present in the external surface of the crystallite and the

LDH apparent and true densities. The molar fraction of

the LDH external anions (XAE) is an important variablein the pillared LDH compounds, especially when they

are used as catalysts. For example, in the POM pillared

LDHs, the polyoxometalate usually plays the role of

active site in the catalytic process. In catalysis, the cat-

alytic activity can be expressed as turnover rates (TOR),

molecules reacting per active site per unit time. The

TORs in reactions catalyzed by LDH-POMs are typi-

cally calculated on the basis of the total POMs present inthe LDH [13]. However, in many cases the reacting

molecules are too big to access the interlamellar region.

Thus, it is more likely that the reactions will be carried

out in the external surface of the LDH. In this work, Eq.

(15) is used for the calculation of the fraction of external

anions. This equation takes into account contributions

due to the top and bottom external crystallite layers and

also the anions in the borders of the LDH crystallitelayers. Eq. (15) is based on the assumption that there are

enough interlamellar anions in each layer (Y ) to equili-

brate the LDH charge (x). Therefore, the total anions

present in the top and bottom external crystallite layers

are Y . As noted from Eq. (15), the XAE is a function of


the dimensions and charge of the LDH and the interca-

lated anion. The results of XAE for several intercalated

Mg0:75Al0:25LDHs, with crystallites of dimensions 200

nm · 20 nm, are shown in Table 8. The values of XAE for

the POMs can vary between 8.7% and 10.3%, and for theferricyanate anion it is 7.6%. The effect of the LDH

charge on the XAE can be observed in Table 9. A small

reduction in the values of XAE is noted when increasing

the LDH charge. Additionally, the results of XAE for the

LDH [Mg0:75Al0:25][H4Co2Mo10O38], with a crystallite of

constant diameter (dc) and variable thickness (lc), arepresented in Table 10. Note that XAE values decrease as

the crystallite thickness increases. A similar trend is ob-served when the crystallite diameter is increased and the

crystallite thickness is kept constant, although at values

of dc over 300 nm the variation of XAE is slight.

Density is one of the properties of the LDHs that has

been little studied. Typically, density values of 2.00 g/

cm3 are assumed. However, as shown in this study,

the LDH density can vary appreciably depending on the

intercalated anion. As seen from Eqs. (16) and (17), theapparent density is a function of the dimensions and

molecular weight of the LDH crystallite. The true den-

sity is also a function of the interlamellar free volume.

Density calculations shown in Table 8, for the LDHs

Mg0:75Al0:25-A with crystallites of dimensions 200

nm · 20 nm, indicate that the apparent density values

vary from 1.30 g/cm3 for the [Mg0:75Al0:25][Fe(CN)6] up

to 3.00 g/cm3 for the [Mg0:75Al0:25][PW11Fe(H2O)O39].The true density values vary from 1.86 g/cm3 for the

[Mg0:75Al0:25][Fe(CN)6] up to 3.51 g/cm3 for the

[Mg0:75Al0:25][PW11Fe(H2O)O39]. The density calcula-

tions for the [M2þ1�xAlx][PV2W10O40](M

2þ ¼Mg,Zn)

LDHs with crystallite dimensions of 200 nm · 20 nm are

given in Table 9. For both materials the apparent and

true densities are larger at higher LDH charges. On the

other hand, the densities of the [Zn1�xAlx][PV2W10O40]are greater than for the [Mg1�xAlx][PV2W10O40] due to

the heavier molecular weight of the Zn with respect to

the Mg. The results of densities for the LDH [Mg0:75-

Al0:25][H4Co2Mo10O38] as a function of the crystallite

diameter and thickness are given in Tables 10 and 11.

Although the variation of the crystallite diameter (dc)does not affect the obtained density values, the apparent

density is larger when the crystallite thickness (lc) isincreased; showing little variation at values of lc over 30nm and reaching an almost constant value over 50 nm.

The true density values are also larger as the crystallite

thickness increases, but the variation is greater than that

observed for the apparent density.

5. Conclusions

Several simple equations have been presented for the

calculation of the textural properties of LDH com-

pounds. These estimates were made by modeling the

properties of an LDH crystallite and the intercalating

anion. One of the main advantages of this approach is

the correlation of the LDH textural properties with

LDH characteristics that can be experimentally mea-sured, such as the average crystallite diameter and

thickness (spacing in the basal plane 0 0 l). The results

obtained with this model are comparable with experi-

mental results for BET areas and micropore volumes. In

cases of poor crystallinity, the results of the model do

not agree with the BET values. However, these diver-

gences are explained in terms of the formation of

agglomerates of a face-to-face type. Taking into accountthe results predicted by the model and the method for

the experimental determination of the LDH textural

properties, we were able to explain satisfactorily the

textural results of the LDH [Mg0:77Al0:23][Fe(CN)6]3�,

which has thus far exhibited the highest BET area. It

was also demonstrated that properties such as the areas

and interlamellar free volumes expressed per unit charge

of the LDH give better results than when expressed perunit weight of the LDH, and also that they can be useful

when comparing different LDHs. The results of pre-

dicted LDH densities indicate that it may be possible to

obtain textural information of the LDH crystallite from

the experimental measurement of the LDH apparent

and true densities. In general, the results of this study

contribute to a better understanding of the textural

properties of the LDHs, giving some correlations fortheir prediction. The results also potentially assist in the

synthesis of LDHs for specific applications.

Acknowledgements

We acknowledge financial support for this work by

the Director, Division of Chemical and Thermal Sys-tems of the National Science Foundation under grant

CTS-0321979, and from Universidad Industrial de

Santander and COLCIENCIAS, in the frame of the

project ‘‘Synthesis, characterization and testing of bi-

omimetic catalysts for selective oxidation’’, code 1102-

05665-95.

References

[1] F. Cavani, F. Trifiro, A. Vaccari, Catal. Today 11 (1991) 173.

[2] V. Rives, M.A. Ulibarri, Coord. Chem. Rev. 181 (1999) 61.

[3] V. Rives, in: V. Rives (Ed.), Layered Double Hydroxides: Present

and Future, Nova Science Publishers, Inc, New York, 2001,

p. 229.

[4] U. Costantino, F. Marmottini, M. Nocchetti, R. Vivani, Eur. J.

Inorg. Chem. (1998) 1439.

[5] S. Velu, K. Suzuki, M. Okasaki, T. Osaki, S. Tomura, F. Ohashi,

Chem. Mater. 11 (1999) 2163.


[6] F. Malherbe, C. Forano, J.P. Besse, Micropor. Mesopor. Mater.

10 (1997) 67.

[7] R.A. Schoonheydt, T. Pinnavaia, G. Lagaly, N. Gangas, Pure

Appl. Chem. 71 (1999) 2367.

[8] H. Nijs, M. de Bock, N. Maes, E.F. Vansant, J. Porous Mater. 6

(1999) 307.

[9] H. Nijs, P. Cool, E.F. Vansant, Interface Sci. 5 (1997) 83.

[10] H. Nijs, M. de Bock, E.F. Vansant, Micropor. Mesopor. Mater.

30 (1999) 243.

[11] H. Nijs, M. de Bock, E.F. Vansant, J. Porous Mater. 6 (1999) 101.

[12] H. Nijs, Ph.D. thesis, Universiteit Antwerpen, Belgium, 1999.

[13] E.A. Gardner, S.K. Yun, T. Kwon, T.J. Pinnavaia, Appl. Clay

Sci. 13 (1998) 479.

[14] F.A.P. Cavalcanti, A. Schutz, P. Biloen, in: P. Delmon, P.A.

Jacobs, G. Poncelet (Eds.), Preparation of Catalysts IV, Studies in

Surface Science and Catalysis, vol. 31, Elsevier, Amsterdam, 1987,

p. 165.

[15] M.A. Drezdzon, Inorg. Chem. 27 (1988) 4628.

[16] J.H. De Boer, in: D.H. Everett, R.H. Ottewill (Eds.), Surface Area

Determination, Butterworths, London, 1970, p. 7.

[17] R. Allman, Acta Crystallogr. B 24 (1968) 972.

[18] A. De Roy, C. Forano, J.P. Besse, in: V. Rives (Ed.), Layered

Double Hydroxides: Present and Future, Nova Science publishers,

Inc, New York, 2001, p. 1.

[19] S.K. Yun, T.J. Pinnavaia, Chem. Mater. 7 (1995) 348.

[20] G.M. Brown, M.R. Noe-Spirlet, W.R. Busing, H.A. Levy, Acta

Crystallogr. B 33 (1977) 1038.

[21] A.L. Nolan, C.C. Allen, R.C. Burns, D.C. Craig, G.A. Lawrance,

Aust. J. Chem. 51 (1998) 825.

[22] H.T. Evans Jr., Inorg. Chem. 5 (1966) 967.

[23] B. Morosin, Acta Crystallogr. B 34 (1978) 3730.

[24] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751.

[25] M.Z.B. Hussein, Z. Zainal, O.W.S. Tat, R. Ibrahim, Oriental J.

Chem. 15 (1999) 23.

[26] Y. Seida, Y. Nakano, Y. Nakamura, Clays Clay Miner. 50 (2002)

525.

[27] T. Kwon, T.J. Pinnavaia, Chem. Mater. 1 (1989) 381.

[28] J.B. Hui, Q.F. Liu, Y.X. Ma, H.Z. Liu, L.S. Li, R.R. Xu, Chin.

J. Process Eng. 1 (2001) 152.

[29] S.K. Yun, T.J. Pinnavaia, Inorg. Chem. 35 (1996) 6853.

[30] W.T. Reichle, Solids State Ion. 22 (1986) 135.

[31] W.T. Reichle, S.Y. Kang, D.S. Everhardt, J. Catal. 101 (1986) 352.

[32] C. Suryanarayana, M. Grant Norton, X-ray Diffraction: A

Practical Approach, Plenum Press, New York, 1998, p. 207.

[33] J.M. Oh, S.H. Hwang, J.H. Choy, Solids State Ion. 151 (2002)

285.

[34] G. Fetter, M.T. Olguin, P. Bosch, S. Bulbulian, J. Porous Mater. 7

(2000) 469.

[35] S. Velu, V. Ramkumar, A. Narayanan, C.S. Swamy, J. Mater. Sci.

32 (1997) 957.

[36] M.A. Ulibarri, M.J. Hernandez, Mater. Chem. Phys. 14 (1986)

569.

[37] M.A. Ulibarri, J. Cornejo, M.J. Hernandez, J. Mater. Sci. 22

(1987) 1168.

[38] T. Kwon, G.A. Tsigdinos, T.J. Pinnavaia, J. Am. Chem. Soc. 110

(1988) 3653.

[39] E.D. Dimotakis, T.J. Pinnavaia, Inorg. Chem. 29 (1990) 2393.

[40] K. Chibwe, W. Jones, Chem. Mater. 1 (1989) 489.

[41] E. Narita, P. Kaviratna, T.J. Pinnavaia, Chem. Lett. (1991)

805.

[42] K.S.W. Sing, in: D.H. Everett, R.H. Ottewill (Eds.), Surface Area

Determination, Butterworths, London, 1970, p. 25.

Microtextural properties of layered double hydroxides: a theoretical and structural model

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