Clay minerals as selective and shape-selective sorbents by slappypappy113

VIEWS: 33 PAGES: 10

									Pure & Appl. Chern., Vol. 61, No. 11, pp. 1903-1912, 1989.
Printed in Great Britain.
@ 1989 IUPAC




             Clay minerals as selective and shape-selective
             sorbents

              R.M.   Barrer
                                                                      W
              Chemistry Department, I m p e r i a l College, London, S 7 ZAZ, England




             Abstract - An account has been given o f four modes o f s o r p t i o n based on
             c l a y minerals and t h e i r c a t i o n exchanged and p i l l a r e d forms. S o r p t i o n o f
             non-polar molecules on outgassed l a y e r s t r u c t u r e s c o n t a i n i n g only
             i n o r g a n i c i n t e r l a y e r c a t i o n s such as Na+ or Ca2+ i s confined t o e x t e r n a l
             surfaces only. This i s a l s o t h e case with t h e f i b r o u s c l a y s or
             p a l y g o r s k i t e s . Polar molecules penetrate t h e i n t e r l a y e r r e g i o n o f t h e
             l a y e r s t r u c t u r e s , b u t o n l y a f t e r a t h r e s h o l d pressure i s exceeded. The
             thermodynamic b a s i s o f t h i s behaviour has been given. When t h e i n t e r l a y e r
             space i s completely f i l l e d by l o n g c h a i n organic cations, as i n
             dimethyldioctadecylammonium m o n t m o r i l l o n i t e , i m b i b i t i o n w i t h s w e l l i n g can
             occur t o an e x t e n t governed by the cohesive energy density o f t h e sorbate
             r e l a t i v e t o t h a t o f t h e i n t e r l a y e r region. S e l e c t i v i t y can be s p e c i f i c
             f o r aromatics and heterocycles. F i n a l l y , a f t e r exchange w i t h organic
             c a t i o n s or w i t h i n o r g a n i c oxy-cations which permanently expand t h e i n t e r -
             l a y e r r e g i o n b u t do n o t f i l l a l l t h e space, a d i v e r s e range o f microporous
             sorbents can be produced which possess many z e o l i t e p r o p e r t i e s such as
             shape-selective s o r p t i o n . Some aspects o f these sorbents are discussed.


             1. INTRODUCTION

Clay minerals i n c r e a s i n g l y a t t r a c t a t t e n t i o n as sorbents and c a t a l y s t s . The f i b r o u s c l a y s
or p a l y g o r s k i t e s are channel s t r u c t u r e s i n which s i l i c e o u s s t r i p s occur. A given s t r i p i s
l i n k e d through shared edges t o each o f f o u r other i d e n t i c a l s t r i p s , two above and two below.
( r e f . 1 ) . The cross-section normal t o t h e s t r i p l e n g t h s ressembles a b r i c k w a l l M i t h each
a l t e r n a t e b r i c k removed and the r e s u l t a n t channels r u n p a r a l l e l w i t h t h e s t r i p s . Other c l a y
minerals are l a y e r s t r u c t u r e s . Where these l a y e r s c a r r y a negative charge t h e r e are i n t e r -
l a y e r c a t i o n s e l e c t r o c h e m i c a l l y equivalent t o t h e a n i o n i c charge (smectites, v e r m i c u l i t e s ,
micas).

The bonding between s i l i c e o u s l a y e r s i s n o t covalent, b u t i n v o l v e s e l e c t r o s t a t i c forces,
d i s p e r s i o n forces and sometimes hydrogen bonding. Because o f t h i s l a y e r s i l i c a t e s can
egpand t o i n t e r c a l a t e guest molecules t o an e x t e n t not observed among z e o l i t e sorbents.
Weiss ( r e f . 2) has recorded t h e s w e l l i n g i n d i s t i l l e d water o f a number o f l a y e r s i l i c a t e s
having the t r i p l e l a y e r s found i n mica, w i t h r e s u l t s shown f o r Na- and Ca- forms i n Table 1.
M a r g a r i t e mica w i t h the highest l a y e r charge and p y r o p h y l l i t e and t a l c w i t h zero l a y e r
charge d i d n o t s w e l l , and thus d i d n o t i n t e r c a l a t e water. For a l l the o t h e r s t h e e x t e n t o f
s w e l l i n g was s e n s i t i v e b o t h t o t h e a n i o n i c l a y e r charge and t o the type o f i n t e r l a y e r c a t i o n .

I n t h i s account I s h a l l r e p o r t f o u r k i n d s o f sorbent behaviour w have observed f o r smectites
                                                                                                e
and v e r m i c u l i t e s which can sometimes r e s u l t i n h i g h s e l e c t i v i t i e s i n c l u d i n g molecule s i e v i n g .


             2. NON-POLAR SORBATES AND INORGANIC INTERLAYER CATIONS
When t h e i n t e r l a y e r c a t i o n s were small i n o r g a n i c i o n s l i k e Na+ or Ca2+ w e l l outgassed
smectites and v e r m i c u l i t e s d i d n o t i n t e r c a l a t e non-polar molecules such as n-, i s o - , neo-
and c y c l o - p a r a f f i n s or benzene or toluene ( r e f . 3 ) . Likewise these non-polar molecules d i d
n o t enter t h e channels present i n t h e p a l y g o r s k i t e s ( r e f . 4). The s o r p t i o n isotherms on
t h e e x t e r n a l surfaces were o f type 2 i n Brunauer's c l a s s i f i c a t i o n ( r e f . 9 ) . A t r e l a t i v e
pressures where m u l t i l a y e r s and c a p i l l a r y condensate occur t h e r e are h y s t e r e s i s loops which
close a t r e l a t i v e pressures i n t h e range 0.27 t o 0.47, according t o t h e sorbate and t h e
temperature. These f e a t u r e s a r e displayed i n F i g . 1. ( r e f . 3 ) . From t h e isotherms t h e
e x t e r n a l monolayer e q u i v a l e n t areas can be determined. As shown i n Table 2 , f o r the c l a y
minerals i n v e s t i g a t e d i n my l a b o r a t o r y , t h e e x t e r n a l areas range from very l i m i t e d t o l a r g e ,
according t o s i z e and morphology o f t h e c r y s t a l s . The f i b r o u s clays, a t t a p u l g i t e and
s e p i o l i t e , are already h i g h area sorbents, without i n v o l v i n g i n t r a c r y s t a l l i n e s o r p t i o n .

                                                                    1903
1904                                       R. M. BARRER


TABLE 1 . The relation between swelling in distilled
    water and charge density of silicates having the
    triple layers found in micas (ref. 2 ) .              TABLE 2.   External surface areas, Ae,
                                                               of some clay minerals
                    Interlayer
                    surfaceper     Extent of swelling                               c.e.c.
                    unit charge          in R :                                       in
Silicate               (I2)        Na- form Ca- form Clay Mineral                 meq/l Dog* Ae/m2g-’

Margarite               12            0           0       Fluorhectorite0             150    8
Muscovite               2k           1.9         2.8      Fluorhectorite 0             90   11
Biotite                 24           1.9         2.8      Bentone 34                   -    44.8
Lepidolite              24           1.9         2.8      (dimethyldioctadecylammo-
Zinnwaldite             24           1.9         2.8      niummontmorillonite)
Seladonite              27           2.4         2.8      Montmorillonite (Na-rich)    91    21.7
Glauconite              31           3.8         2.8      Hectorite                    90    81.4
Dioctahedral illite     32           4.2         2.8      (Ben-a-Gel)
Trioctahedral illite    36           5.1         4.3      Montmorillonite (Ca-rich)   102    85.4
Vermiculite             36           5.1         4.3      Attagulgite                  -    195 (max.)
Vermiculite             36           5.0         4.2      Sepiolite                    -    235 (max.)
Vermiculite             37           5.1         4.2
Batavite                36           5.1         4.3      *   c.e.c. = cation exchange capacity;
Beidellite I            41           5.4         4.9          meq = milliequivalents
Saponite                42           4.9         4.9          0 = Synthetic
Nontronite              46            m          9.2
Beidellite I 1          57            W          9.2
Montmorillonite         60            m          9.2
Montmorillonite         75            m          9.6
Hectorite              100            m         10.6
Pyrophyllite            m             0           0
Talc                    W             0           0

For n-, iso- and neo-pentane, outgassing the attapulgite at each of a series of progressive+
ly increasing temperatures produced the correlated changes between rate of water l o s s ,
surface area, BET affinity constants and uptakes at relative pressure of 0 . 1 which are shown
in Fig. 2 (ref. 4 ) . For attapulgite outgassed below 100°C there was a selectivity sequence
                              n-pentane> iso-pentane> neo-pentane
but after outgassing above 100°C, as the probable result of structural changes in the sorb-
ent (ref. 7 ) , this selectivity almost disappeared.
Because adsorption on external surfaces will always occur one must allow for this when
evaluating amounts intercalated and available interlayer areas.



           3. POLAR SORBATES AND INORGANIC INTERLAYER CATIONS

In well outgassed smectites and vermiculites, in which the interlayer cations were again
small ions such as Na+ and Ca2+, sorption was initially limited to external surfaces.
However, at approximate critical pressures which varied with sorbate, cation and temperat-
ure, intercalation of polar molecules began and resulted in one or two steps in the
isotherms (ref. 3 ) . These steps are illustrated in Fig. 3 (ref. 8 ) for pyridine and water
in the Na-rich montmorillonite of Table 2 . A second feature was hysteresis between sorption
and desorption branches of the isotherms which persisted to very low pressures. Both these
features require explanation.



           4. THE CONDITION FOR INTERCALATION THRESHOLDS

An interpretation of and requirement for intercalation threshold pressures has been given
(ref. 8 ) . The unexpanded, outgassed parent clay mineral will be termed the cx-phase and the
expanded phase after intercalation the p-phase. For uptake of n moles of guest, M, we may
write the equation
            a-phase + nM AG-p-    phase nM                              (1)
the free energy change for which is AG under the experimental conditions of temperature and
pressure. AG has two components. The first is a positive term, AGap, which is the Free
                                    Clay minerals as selective and shape-selective sorbents                                        1905




                                                                               e
                                                                               e
                                                                                    20
                                                                                     0 '
                                                                                          '




                                                                               + 140.
                                                                               ra
                                                                                    100




           relative pressure                    relative pressure
                                                                                    10 I
F i g . 1. Isotherms t y p i c a l o f l a y e r s i l i c a t e                      0       50  100 150 200 250            300   350
    (here a Na-rich m o n t m o r i l l o n i t e ) behaving as                                Outgassing temperature, 'C.
    a non-permeable sorbent ( r e f . 3 ) . S i n d i c a t e s
    the c h a r a c t e r i s t i c shoulder a t t h e c l o s u r e o f       F i g . 2. C o r r e l a t i o n s between various
    t h e h y s t e r e s i s loop. 0 = s o r p t i o n p o i n t s ;              aspects o f s o r p t i o n o f isomeric
    x I desorption p o i n t s .                                                   pentanes on a t t a p u l g i t e and t h e
                                                                                   outgassing temperatures ( r e f . 6 ) :

                                                                                    1. Rate o f water l o s s from t h e


                                         1
                                       240                                 J
                                                                                        attapulgite.
                                                                                    2. The c o e f f i c i e n t c i n t h e BE1
                                                                                        isotherm.
                                                                                    3. The areas a v a i l a b l e t o the
                                                                                        isomers.
                                                                                    4 . The uptakes a t a r e l a t i v e pressure
                                                                                        o f 0.1.
                                                                                    0, n-C5HI2;A,          iso-C 5 H12;      '
                                                                                    neo-C H
                                                                                         5 12'

                                 relative pressure
  F i g . 3. S o r p t i o n of p y r i d i n e and water i n
      Ns-rich m o n t m o r i l l o n i t e , showing t h r e s h o l d
      pressures, isotherm steps and h y s t e r e s i s per-
      s i s t i n g t o low r e l a t i v e pressures ( r e f . 3 and 8).



energy change on expanding t h e a-phase t o the sorbate f r e e p-phase.     The second, which i s
negative, i s t h e f r e e energy change, AGi, when t h e n moles o f guest enter the p-phase.   Thus

               AG    =     A,
                           Gp       +    AGi                                                              (2)

and whether i n t e r c a l a t i o n can proceed or n o t w i l l depend on t h e r e l a t i v e magnitudes o f AGap
and AGi.   For AG p o s i t i v e , i n t e r c a l a t i o n i s n o t expected; f o r AG negative, i n t e r c a l a t i o n
can occur.

                                                                                                           e
The p-phase i n t e r c a l a t e s are a type o f s o l i d s o l u t i o n o f guest i n host t o which w now apply
t h e Gibbs-Duhem r e l a t i o n a t constant temperature:

               npdpp      +     ndpM           Vdp                                                        (3)

Here ng denotes t h e number o f moles o f t h e host comprising t h e p-phase.                   A convenient
measure o f t h i s mole c o u l d be t h e guest-free gpamme u n i t c e l l content. pp and pM are t h e
chemical p o t e n t i a l s o f h o s t and guest i n t h e s o l u t i o n o f volume V a t pressure p (normally t h e
vapour pressure o f t h e guest a t e q u i l i b r i u m with t h e s o l u t i o n ) . Also d p M = R T d l n a where a
denotes the a c t i v i t y o f t h e guest. W can re-arrange eqn. 3 and i n t e g r a t e between a I 0 and
                                                  e
1906                                             R. M. BARRER

a, the result being
                                              k
              kp   = k
                     ;   +                                                   (4)
                                             '
                                             0
in this expression :
                   p         = Gf is the free energy of a mole of pure (i.e. guest-free) p-phase.
V = V/n, is the molar volume of the &phase, which is a constant because the 8-phase is
already Fully expanded and so does not expand further when accommodating the guest. In eqn.
4 n has been replaced by nsat 8 where n     is the saturation uptake of guest inside the
                                        sat
p-phase and 8 is the degree of saturation for an uptake of n moles of guest.
As 8 in eqn.,4 increases sokamust decrease. The free energy of reaction 1 is the differ-
ence in free energy of the host-guest solution and of the pure components, 1.e.


where I=: is the free energy per mole of pure a-phase and kg the chemical potential of
        .GZ
guest molecules in the gas phase. At intercalation equilibriumMp; = kLM that eqns. 4 and
                                                                       so
5 give




The critical condition for intercalation to commence arises when AG, = 0, i.e. as AG,




The integral can in principle be found from graphical integration of the isotherm plotted as
e/p against p, since usually one can take p as a measure of activity. However, the isotherm
for intercalation can be measured only in the pressure range above the critical or threshold
pressure so that an extrapolation Qo zeroepressure is required and uncertainty is thereby
introduced in evaluating AGap =      - k p ) from eqn. 7 . Vpp is usually negligible.

The extent of swelling required for intercalation varies from one guest molecule to another
(according to molecular size, shape and orientation of the guest between layers) as does fihe
isotherm of the guest in the p-phase. Therefore there will be a different temperature
dependent threshold pressure for each guest molecule in a given clay mineral. Also there
will be a different threshold pressure for the same guest in each different clay mineral,
because the work in separating the layers will differ according to lqyer charge and
composition, and to the type of interlayer cation.
The foregoing analysis has wide generality: it is applicable to zeolite formation from other
minerals in which the usual guest is water, and to clathration, both of which represent
solid solutions of host plus guest. It is also applicable to intercalation in layer
structures other than clay minerals, for example graphite and certain layer sulphides.
Fig. 4 . shows as an illustration, the isotherms we have obtained for clathration of K r in
phenol, above the threshold pressures (ref. 9 ) .
When there is more than one step in the intercalation isotherm (e.g., HzO in Fig. 31, there
is a second transitibn from p-phase intercalate to Y-phase intercalate. The foregoing
treatment then applies to this step with a and p replaced throughout by p and?
respectively.

         5. HYSTERESIS ASSOCIATED WITH INTERCALATION
W have also to account for the persistent hysteresis between the sorption and desorption
 e
branches shown in Fig. 3 . The change of the crystal from its a-phase to the guest-bearing
 p-phase involves nucleation at the periphery of the a-phase. Nucleation involves two
 Positive free energy terms: a strain free energy A g s and an interface free energy, A g a .
 For a nucleus with j included guest molecules its f r e e energy of formation is thus
          Agj       = AGz(j/NA)   + Ag, + Ags
rather than
          Agj       = AG,(j/NA)                                              (9)

In these expressions AGz is the free energy of formation of the amount of p-phase intercal-
ate containing one mole of the guest at the value of 8 corresponding with pressure p of the
intercalation isotherm and NA is Avogadro's number. At the true threshold pressure AG2 is
                                    Clay minerals as selective and shape-selective sorbents                                            1907




                                                                           - 0.30
                                                                           I


                                                                            'M
                                                                           v
                                                                            M
                                                                           f      .0
                                                                                 02
                                                                           8
                                                                           B
                                                                            I    OJO


                                                                                   0          0.4       08 0
                                                                                                         .            0.4        0.8
                                                                                                relative vapour pressure
                pressure in cm of mercury                               F i g . 5. Isotherms f o r toluene and ethylbenzene
F i g . 4. S o r p t i o n isotherms of K r i n -phenol                     obtained w i t h Bentone 34 (dimethyldioctadecyl-
    above t h e t h r e s h o l d pressures ( r e f . 9 . ) .               ammonium m o n t m o r i l l o n i t e ) ( r e f s . 10 and 8 ) .
    ( 1 ) 195K; ( 2 ) 212K; ( 3 ) 222.2K; and ( 4 ) 228K.                  0,    sorption;      ., e s o r p t i o n
                                                                                                  d


zero, as considered i n t h e p r e v i o u s section, b u t Ag. i n eqn. 8 i s s t i l l p o s i t i v e . Accord-
i n g l y n u c l e a t i o n o f P-phase i n t e r c a l a t e i s delayed $0 pressures above t h e t r u e thermodyna-
mic t h r e s h o l d pressure. For t h e same reason, on desorption, n u c l e a t i o n o f the a-phase w i l l
a l s o be delayed t o pressures below the t r u e thermodynamic threshold.                   The r e s u l t i s t h e
i n t e r c a l a t i o n h y s t e r e s i s which was seen i n F i g . 3.

             6. SORBATES IMBIBED BY ORGANO-CLAYS
The t h i r d k i n d o f behaviour o f sorbents obtained from c l a y m i n e r a l s i s e x e m p l i f i e d by
Bentone 34, (Table 2 ) which i s a m o n t m o r i l l o n i t e exchanged w i t h dimethyldioctadecylammonium
ions. The l a r e organic c a t i o n s f i l l a l l the space between s i l i c e o u s l a y e r s and expand
d(001) t o 23.2w.         This sorbent imbibed some types o f sorbate, n o t a b l v aromatic hydrocarbons
and heterocycles, b u t n o t o t h e r s ( r e f . 10). Where i m b i b i t i o n was s u b s t a n t i a l t h e d(001)
spacing expanded Further, and t h e isotherms tended t o be o f type 3 i n Brunauer's c l a s s i t i c -
a t i o n , as seen i n F i g . 5 ( r e f . 8 ) f o r toluene and ethylbenzene. These isotherm contours
r e c a l l the contours found when benzene i s sorbed by c r o s s - l i n k e d elastomers, and t h e r e f o r e
suggest t h a t i m b i b i t i o n may be c o n t r o l l e d by the cohesive energy d e n s i t i e s o f t h e sorbates
(C.E.D. = molar energy o f v a p o u r i s a t i o n o f l i q u i d sorbate d i v i d e d by t h e molar volume o f t h e
l i q u i d ) . This view i s supported by the r e s u l t s i n Table 3.

For iso-octane and cyclo-hexane d(001) i s t h e same, w i t h i n experimental u n c e r t a i n t y , as
t h a t f o r t h e parent Bentone 34 and the small observed uptakes are t h e r e f o r e n e a r l y a l l on
t h e e x t e r n a l surface. For toluene and benzene d(001) increased s u b s t a n t i a l l y and t h e uptake
i s l a r g e l y by i m b i b i t i o n . The s e l e c t i v i t y of i m b i b i t i o n f o r aromatics and h e t e r o c y c l e s over
p a r a f f i n s and c y c l o - p a r a f f i n s i s notable. The maximum i m b i b i t i o n should occur when t h e
C.E.D.        o f the sorbate most c l o s & l y matches t h a t o f t h e organic i n t e r l a y e r as m o d i f i e d by t h e
presence o f adjacent s i l i c e o u s l a y e r s .


             TABLE 3.       S o r p t i o n by Bentone 34 a t p/po              0.2 r e l a t e d t o C.E.D.    and
             d(001) spacings ( r e f . 1 0 )


                                        r,,,            4
                                                   -1
                                                                                 mmole sorbed per g
             Sorbate                    I L.L.U.                  T'C                                                  d(001)
                                       -I

                                        Lca)/cm3
                                                                                 a t T & p/po         0.2               ;1
             isa-butane                      6.25                 -30                      0.062                         -
             n-butane                        6.7                  -30                      0.077                         -
             iso-octane                      6.85                  45                      0.11                        23.2
             n-heptane                       7.45                  45                      0.11                        23.2
             cyclo-hexane                    8.20                  45                      0.16                        23.2
             ethylbenzene                    8.8                   60                      0.52                          -
             toluene                         8.90                  45                      0.65                        27.2
             benzene                         9.15                  45                      0.81                        27.2
             dioxane                        10.0                   60                      0.92                          -
             pyridine                       10.7                   60                      1.79                          -
             nitromethane                   12.6                   45                      1.38                          -
1908                                                         R. M. BARRER


Bentone 34 is just one member of a class of expanded but non-porous layer silicates in which
all interlayer space is filled by large organic cations. Their selectivities desenve more
investigation.

                7. SORPTION IN PERMANENTLY MICROPOROUS LAYER SILICATES
The fourth type of behaviour is exhibited by layer silicates which have been made
permanently microporous. About 1950 it seemed to me that if the small inorganic cations
between the layers were replaced by larger ions such as Me4Nf (Me CH3) the siliceous
layers would be held apart permanently, thus creating interlamellar micropores accessible
to any molecules of the right shape and size to fit between the cations and the eiliceous
layers. This reasoning is shown schematically in Fig. 6 (ref. 11).   It is the dimensions d1
and d3 which control access to the interlamellar pore spaces.
                                                  11/1111111111/11/1/1
                 0.      Na*,Ca2*ctc                                             Out g a s s e d p o r c n t
                                                  111//11/1////11///11



                                                  11111111/111111///11Il                    Per mane n l Y
                                                                                            expanded and
                         i   orgonic
                                                                                            porous c or
                             c a t I on


                Fia. 6 . Representation of the permanent expansion of a layer
                  silicate by exchanging small interlayer cations for large ones
                  (ref. 11).

The correctness of this surmise was shown in a paper of 1955 (ref. 12) in which the exohange
ions were Me4Nf and Et4Nf(Et    C2Hg). The expanded clay minerals functioned as molecular
sieves and freely intercalated many permanent gases and non-polar hydrocarbons which were
npt intercalated by the parent Na-montmorillonite. The sorption capacity was thereby
greatly increased. This work was developed further over subsequent years using seyzral
different types of layer silicate and a variety of organic cations and also Co(en) , where
‘len’l denotes ethylenediamine. It revealed many interesting selective and shape-selective
separation capabilities.
Inclusion isotherms (derived from the total uptakes corrected for sorption upon external
surfaces) are of type 1 in Brunauer’s classification (ref. 5) and so recall those normally
observed in zeolite sorbents. This is seen in Fig. 7 (ref. 13) for N2 and A r at 78K in a
series of alkylammonium and alkyldiammonium montmorillonites and hectorites. The



       40                           5

       30                            8


                                                                            Fig. 7 . Interlamellar parts o f the
                                                                              sorption isotherms for N2 and A r at 78K
                                                                              (refs. 13 and 8 )
                                                                              (a) N2 in alkyldiammonium
 7
 U



 .-
       60
                                                                                  montmorillonites.
 n.
 8     50
                                                                              (b) N 2 in alkylammonium montmorillon-
                                                                  3
                                                                  5               ites.
       40                                                         9
                                                                              (c) N 2 in alkyldiammonium hectorites.
       30
                                                                              (d) A r in alkyldiammonium hectorites.

       2ot                                    t                               The numbers by the curves are the

       lot  1   I    I          I        I
                                              t
                                              I   1    I      I       I
                                                                              carbon numbers of the organic cations.


       0            0.4                 0.8   0       0.4             0.8
                         PIP0                         PIP,
                             Clay minerals as selective and shape-selective sorbents              1909


saturation interlayer uptakes decline with increasing chain lengfih. Saturation capacities
are greater, and the decline with carbon number of the cation is less marked, for
alkyldiammonium than for alkylammonium ions. This is expected because the total number of
divalent exchange ions is only half the number of monovalent ones.
Permanent intracrystalline pore volumes were estimated for the sorbate-free layer silicates
from the total interlayer areas less the areas covered by the cations, multiplied by the
measured free distances, d l in Fig. 6 , between adjacent siliceous layers. Table 4 (ref. 13)
shows the considerable total micropore volumes.
The free distance, d,,, between the sheets of the expandld clay mineral is not necessarily
fixed, but may expand further to intercalate appropriate guest molecules (see below). Total
miffypore volumes of the guest-bearing layer silicates were therefore also evaluated for two
Co   (en) -fluorhectorites with c.e.c. ' s of 90 and 150 meq per 1009, rgspectively termed
FH90 and ?HISO. The pore volumes in Table 5 ( r e f . $ 5 ) are given as c m of liquid sorbate
intercalated per g, of sorbent at saturation of the interlayer pore volume.
The behaviour of these permanently porous layer silicates can be considered in terms of eqn.
2 ( AG I AGae + A G . ). If d l and d, are sufficient for the guest to be interclated
without further inirease in d 1 then AG.p will be zero, so that AG = AG. and therefore is
always negative. No threshold pressure is then required for intercalatih and so the
isotherm is continuous, as is normal for a zeolite sorbent.




          TABLE 4. Calculated interlayer free areas and porosities of some organo-
          montmorillonites and hectorites (ref. 13)


          Cation                9.u.c.      Ion co-area        dl
                                                               -      Free area        Porosity
                                wt.                                      2 -1            3 -1
                                                  i2           i        m g            cm 9

          (a) Alkylammonium montmorillonites (c.e.c. 85; meq/IOOg)
          CH~NH;                 746            21.6          2.2          294          0.065
          C~H~NH;                755            29.8          3.4         214           0.073
          C~H~NH;                765            35.6          3.7         184           0.068
          n-C4H9NH;              774            41.4          3.9         155           0.060
          n-C5H1 N ;
                   H             783            47.1          4.0         126           0.050
          n-C6H1 3NH;            792            52.9          4.0          98           0.039

          (b) a, o-alkyldiammonium montmorillonites (c.e.c. 85 meq/100g)
                      ) 2
          + N H ~ ( C H ~~ ~ f 746              34.8          2.8         278           0.078
                (
          + N H ~ c H ~ ) ~ ~ ; 750
                       3                        40.6          3.5         262           0.092
          +N H ~ ( C H ~ ; ~ N H ; 755          46.4          4.0         246           0.098
                        ~
          + N H ~ ( C H5 ~ ~ ; 759              52.2          3.9         231           0.090
          +NH,(cH,),NH;            764          58.0          3.8         216           0.082
          +NH,(CH~)~NH; 769                     63.7          3.9         201           0.078
                        H~
          + N H ~ ( c 8 ~ )~ ; 773              69.5          3.9         187           0.073
                (
          + N H ~ c H ~ ) ~ N H ~ 778           75.3          3.9         172           0.067

          (c) a, w-alkyldiammonium hectorites (c.e.c. 91 meq/100g)
          +NH~(cH,),NH;           769           34.8          2.8         240           0.067
          + N H ~ C H ~ )~ ~ ; 774
                (       3                       40.6          3.6         224           0.080
          + N H ~ c H ~ ) ~ N H ; 779
                (                               46.4         L3.91        208          [0.0811
          + N H ~ ( c s ~ ~ ' ; 783
                        H~)                     52.2          3.9         192           0.075
          + N H ( C H ) 9 ~ ~ ;802
                ~       ~                       75.3          3.9         128           0.050
1910                                                      R.   M. BARRER

             TABLE 5. Pore volumes i n C0"~(en)~-FH90 and C 0 ~ I ~ ( e n ) ~ - F H 1 estimated
                                                                                      50
             from monolayer s a t u r a t i o n uptakes, v ( r e f . 15)
                                                          m

             Sorbate        T
                            -                      m
                                                   '            Mol.Vo1.          at T       i(
                                                                                         cm3 q)
                                                                                              l
                                         3                                    3          per g. o f sorbent
                             K          m
                                       d a t s t p g-"                   cm

             (a) C O I I I ( ~ ~ ) ~ - F H ~ O

                             77.3                 67.8                  26.5                      0. oeo
                    :
                    2        90.2                 66.5                  27.9                      0. OE3
                    N2       77.3                 51 .3                 34.7                      0.079
                    Ni       90.2                 45.7                  37.4                      0.076
                    Ar       77.3                 65.6                  27.4                      o.oeo
                    Ar       90.2                 63.7                  29.1                      O.OE3
                   CH4       90.2                 50.0                  38.7                      0.OE6
             iso-C4HI0      273.2                 19.2                 100.1                      0.OE6
             neo-C HI2      273.2                 23.7                 117.4                      0.1z4
                   ca,      273.2                 31 .4                 47.4                      0.066

             (b) C 0 I ~ ~ ( e n ) ~ - F H 1 5 0

                             77.3                 51 .7                26.5                       U:061
                   :
                   2         90.2                 49.3                 27.9                       0.061
                  CO;       273.2                 21 .&                47.4                       0. 045


Sometimes i n t e r c a l a t i o n f u r t h e r increases d    .
                                                              I n t h i s case AG,p i s n o t zero. As examples
one may consider methylammonium and t e t r a m e t h y l ammonium m o n t m o r i l l o n i t e s i n s o r p t i o n
e q u i l i b r i u m w i t h n-heptane and benzene :

             Exchange form                   dl   ;
                                                  /             dl (
                                                                   ;
                                                                   /     n-heptane)                dl/;(   benzene)

                   MeNHf                      2.2                         3.6                               5.6
                   MellN*                     4.1                         4.1                               5.2

I n t h e MeNH -form t h e increase i n d was p a r t i c u l a r l y l a r g e f o r benzene i n t e r c a l a t i o n and
                                                    1.
made AC,p g i g enough f o r i n t e r c a l a t i o n t o r e q u i r e a low t h r e s h o l d pressure ( r e f . 16). This
meant t h a t when benzene and n-heptane were sorbed from t h e i r m i x t u r e s n-heptane was i n i t a l l y
preferred.     I n c o n t r a s t i n t h e s o r p t i o n o f n-heptane/benzene m i x t u r e s by t h e M N-form, t h e
                                                                                                                e
                                                                                                                 4
benzene was always p r e f e r r e d . F i g . 8 ( r e f . 17) then g i v e s t h e f r a c t i o n a t i o n f a c t o r s f o r




    70
                                         ?                F i g . 8 . Enrichment f a c t o r s o f benzene, ?, as
                                                              f u n c t i o n s o f temperature and t o t a l e q u i l i b r i u m
                                                              pressure f o r a benzene-heptane mixture. The
                                                              sorbent was (CH3)4N-montmorillonite and t h e feed
                                                              m i x t u r e contained 0.67 mole f r a c t i o n o f benzene.

                                                               Q, 80°C; 0, 85OC; x, 90°C;A,                95OC.




            total rguihprtrrure (ern.)



benzene a t s e v e r a l temperatures as a f u n c t i o n o f t o t a l pressure.               T h i s f a c t o r , 189 is
d e f i n e d as




The n denote moles and t h e p are p a r t i a l pressures. Subscripbs B and H denote benzene and
n-heptane and s u p e r s c r i p t s s and g denote sorbed and gaseous, r e s p e c t i v e l y .
                                        Clay minerals as selective and shape-selective sorbents                                     1911


When one component was n o t i n t e r c a l a t e d and t h e other was (eg. cyclohexane and benzene i n
MeNH3- m o n t m o r i l l o n i t e ) t h e s e p a r a t i o n f a c t o r was even greater. Many mixtures were w e l l
separated using shape-selective microporous cA8y m i n e r a l sorbents, as e x e m p l i f i e d f o r t h e
f o l l o w i n g p a i r s , u s i n g Me4N-montmorillonite a t 77OC ( r e f . 18) :

             benzene/cyclohexane                                                 cyclohexane/cyclohexanol
             toluene/cyclohexane                                                 n-heptane/met hanpl
             benzene/n-heptane                                                   n-hexane/cyclohexane
             benzene/carbon t e t r a c h h o r i d e                            n-heptandiso-octane
             benzene/toluene                                                     n-heptane/cyclohexane
             methanolfcarbon t e t r a c h l o r i d e                           thiophene/benzene



             8. TAILORING THE INTERLAYER MICROPORES

The microporous c l a y m i n e r a l s can be m o d i f i e d i n t h r e e main ways :

              ( a ) By a l t e r i n g t h e a n i o n i c l a y e r charge and hence t h e c o n c e n t r a t i o n o f
                    i n t e r l a y e r cations.

              ( b ) By a l t e r i n g t h e charge on t h e i n t e r l a y e r c a t i o n s and t h e r e f o r e t h e i r
                    number.

              ( c ) By v a r y i n g t h e s i z e and shape o f the c a t i o n s .

An example of yfyh o f these t h r e e               hods follows.          The e f f e c t O f l a y e r charge i s seen i n
Table 5 f o r Co        (WI)~-FHSO and Coqefi(en)3-FH150, and i s due t o changes i n t h e f r e e d i s t a n c e
d between adjacent p a i r s o f c a t i o n s . d3 was estimated as 6.7 and 3 . 6 A r e s p e c t i v e l y ,
wzereas the v e r f i f y a l free d i s t a n c e between s i l i c e o u s l a y e r s was 4.2 A f o r each sorbent. As
a f y y u l t t h e Co    (er1)~-FH90i n t e r c a l a t e d molecules as l a r g e as neo-pentane whereas i n
Co      (er1)~-FH150a t 77.3K O2 was i n t e r c a l a t e d b u t N2 was not.

The e f f e c t of charge on t h e c a t i o n i s s en among t h e elkylammonium and alkyldiammonium
c a t i o n s of Table 4. The i o n s CH3CH2CH$HJ                and NH3CH2CH2NH3 have about t h e same l e n g t h and
volume, b u t o n l y h a l f t h e number o f the d i v a l e n t i o n s i s r e q u i r e d as compared with t h e mono-
v a l e n t ones. This increased t h e d i s t a n c e d and gave mgrelopen space between c a t i o n s , as
r e f l e c t e d i n the i n t e r l a y e r f r e e areas o f 278 and 184 i g
                                                                            n             .
The e f f e c t o f shape and s i z e of c a t i o n s i s i l l u s t g a t e d by comparing CH NH              -
                                                                                                          and (CH ) N-
m o n t m o r i l l o n i t e s , having d equal t o 2.2 and 4.1 A r e s p e c t i v e l y . A t 3j3K3khe CH2NHz-4form
d i d n o t i n t e r c a l a t e iso-C 4          $so-C H   or cyclohexane appreciably, whereas a l l these
m l e c u l e s were i n t e r c a l a t z d ’ a b the (pH\54N-montmorillonite.        A l a r g e increase i n d 1 would
be needed bo i n t e r c a l a t e these hydrocarbons i n CH NH3-montmorillonite, w i t h t h r e s h o l d
pressures f o r i n t e r c a l a t i o n which were n o t reached i n our c o n d i t i o n s . For (CH )4N-montmor-
i l l o n i t e the increase needed i n dt i s much less and i n t e r c a l a t i o n proceeded                     (dG
                                                                                                                i n eqn. 2
i s negative).



             9. MICROPOROUS PILLARED CLAY MINERALS

;$ntion           i s i n c r e a s i n g l y being d i r e c t e d towards replacements o f i n t e r l a y e r c a t i o n s f(Na+,
         etc) ( i n montmorillonites, b e i d e l l i t e , nontronite, hectorites, fluorhectorites,
r e c t o r i t e and t e t r a s i l i c i c micas) by oligomeric c a t i o n s o f elements such as A l , Zr, T i , C r
and Fe. An example o f such a c a t i o n i s the Keggin i o n

               1A l l   304( OH) 24( H20)     17+
Found i n aluminium c h l o r h y d r o l s o l u t i o n s . The o b j e c t i v e s have been t o increase t h e i n t e r -
l a y e r f r e e distances, d      ,
                                  and, by r e p l a c i n g organic i o n s by i n o r g a n i c ones, t o improve the
thermal and hydrothermal s t a b i l i t i e s o f the products. I n t h i s way i t i s hoped t o o b t a i n
good microporous c a t a l y s t s more open than z e o l i t e s and o f h i g h surface area, f o r dewaxing,
hydrocracking, and c r a c k i n g o f heavy o i l f r a c t i o n s and biomass o i l s .

Table 6 g i v e s examples o f r e p o r t e d r e s u l t s f o r i n t e r l a y e r f r e e distances and f o r monolayer
equivalent areas i n some s t u d i e s on p i l l a r e d days (PILCS).

The distances d have been s u c c e s s f u l l y increased, as were t h e thermal and hydrothermal
s t a b i l i t i e s , wh4n compared w i t h those o f t h e microporous organoclays.               The BET monolayer
e q u i v a l e n t areas a l s o show increases r e l a t i v e t o t h e organoclays i n some b u t n o t a l l the
products. The value o f d a f t e r c a l c i n a t i o n i s a f u n c t i o n o f the method o f p r e p a r a t i o n ,
                                     1
the c a l o i n a t i o n temperatures and t h e type o f l a y e r s i l i c a t e and o l i g o m e r i c oxycation.
1912                                                      R. M. BARRER


             TABLE 6.  I n t e r l a y e r free distances, d l =2dLy01)-9.4, i n 8 , and
             monolayer e q u i v a l e n t areas (MEA) i n     m g   i n some PJLCS.


              P i l l a r based         Pillaring              After Calcination:               Reference
                       on:                 via:                           MEA
                                                                  dl

                    A1        Oligomeric c a t i o n s         8.4-91      N 400                   19,20
                    A1              II          II
                                                               6.8           300                   21
                 (Si, A l )                    I1
                                                               6.0-10.2     85-430                 22
                 (Si, A l )         (I
                                               11
                                                               6.6-8.8      340-500                22
                    Si      [ S i (acetylacetonate)      31+   3.2 (max. )  40-1 90                23
                    Si      Silsesquioxanes                    6.8-10.4     140-400                24
                    Zr      Oligomeric c a t i o n s           8.6          260                    19
                                                  I1
                     Zr            I1
                                                               12.4         290                    19
                     Zr            II             I1
                                                               12.4 ti 16.2 250                    19
                     T i           11             I1
                                                               15.6         300                    25
                     T i           If             I1
                                                               19.4         330                    25
                     Cr            11             II
                                                               11.6         430                    26
                     Fe     [ Fe3( OCOCH3)70H]+                 7.3         280                    27



The free distances, d          ,
                           between adjacent p i l l a r s are, however, n o t w e l l defined. One i d e a l
model assumes t h e pi1;fars t o be e q u a l l y spaced, b u t t h e r e i s n o t y e t much supporting
evidence. I t seems more probable t h a t i n c a l c i n a t i o n soma p i l l a r s may fuse together, g i v i n g
a range i n values o f d3. I f t h i s i s the case the micropores w i l l be more i r r e g u l a r than
those i n the organoclay molecular sieves and i n z e o l i t e s . Isotherms a r e o f types 1 or 2
and some g i v e evidence o f mesoporosity, as estimated from N2 desorption.


             10. CONCLUDING REMARKS
I t i s hoped t h a t enough has been s a i d t o c l a r i f y t h e d i v e r s i t y o f behaviour found among
sorbents based on l a y e r s i l i c a t e s . Much remains t o be explored i n a l l areas, and e s p e c i a l l y
i n t h e f a s t expanding chemistry o f PILCI's. I n t h i s l a t t e r area more knowledge i s needed
about the e x t e n t and o r i g i n o f mesoporosity and t h e degree o f r e g u l a r i t y i n micropore
dimensions, and a l s o about t h e n a t u r e o f the p i l l a r s a f t e r and b e f o r e c a l c i n a t i o n , t h e ways
i n which they are attached t o t h e s i l i c e o u s l a y e r s and how they may be chemically m o d i f i e d
t o improve c a t a l y t i c p r o p e r t i e s . More knowlege i s a l s o r e q u i r e d about t h e chemical species
present i n p i l l a r i n g s o l u t i o n s .


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 3.    R.M. Barrer and D.M. MacLeod, Trans. Faraday Soc., 1954, 50, 980.
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