Porous clay heterostructures (PCH) as acid catalysts

Article abstract of DOI:10.1039/a703101g

Porous clay heterostructures formed by surfactant assembly of open framework silica in the galleries of a smectite clay represent a new family of solid acid catalysts, as evidenced by the selective dehydration of 2-methylbut-3-yn-2-ol to 2-methylbut-3-yn-1-ene.

Full text of DOI:10.1039/a703101g

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Porous clay heterostructures (PCH) as acid catalysts
Anne Galarneau, Anis Barodawalla and Thomas J. Pinnavaia*
Department of Chemistry and Center for Fundamental Materials Research, Michigan State University, East Lansing, MI 48824,
USA
Porous clay heterostructures formed by surfactant assembly
of open framework silica in the galleries of a smectite clay
represent a new family of solid acid catalysts, as evidenced
by the selective dehydration of 2-methylbut-3-yn-2-ol to
reaction product was recovered by centrifugation, air-dried and
calcined at the desired temperature for 4 h using a temperature
ramp rate of 2 °C min . The calcined products were denoted
FH-PCH-X where X is the calcination temperature in °C.
Chemical analyses indicated the presence of 10.4 moles of
2
1
2-methylbut-3-yn-1-ene.
4
gallery Si per fluorohectorite O20F unit cell.
Pillared interlayered clays have been extensively studied as
promising solid-state alternatives to traditional acid catalysts,
such as aluminium trichloride, sulfuric acid and hydrofluoric
acid.1 We recently reported a new type of clay intercalate with
high surface area and a structural porosity substantially larger
than pillared clays. These new derivatives, which we call porous
clay heterostructures (PCH) are formed by surfactant-directed
assembly of open-framework silica in the galleries of smectite
Table 1 provides the basal spacings, BET surface areas, and
Horvath–Kowazoe pore sizes for FH-PCH samples calcined in
the range 300–550 °C. Three orders of 00l X-ray reflection are
observed in accord with a lamellar structure. Although the basal
spacing decreases from 38.1 to 33.5 Å with increasing
temperature, there is a general increase in the BET surface areas
to a maximum value of 790 m g at 500 °C where the removal
of gallery surfactant ( < 1 mass% C) and the dehydroxylation of
the gallery silica become more complete. The lamellar structure
is retained even at 700 °C, but the basal spacing begins to
decrease more rapidly at this temperature. At 800 °C the PCH
structure degrades substantially with the formation of quartz as
a decomposition product. Clearly, the thermal stability of the
PCH structure degrades substantially with the formation of
quartz as a decomposition product. Clearly, the thermal stability
of the PCH structure is sufficient to conduct a wide range of acid
catalyzed conversions.
,2
2
21
3
clays with high charge density. In contrast to microporous
pillared clays with pore sizes typically below 10 Å, PCH
materials exhibit regular porosity in the supermicropore to small
mesopore range (15–25 Å).
PCH synthesis makes use of the micellar ordering of silicate
species and surfactants in a manner not unlike the supramole-
cular ordering processes for mesoporous MCM-41 molecular
sieves.4 However, as shown in Fig. 1, PCH assembly differs
from MCM-41 chemistry insofar as framework organization
occurs in the restricted two-dimensional gallery region of the
clay rather than in the three-dimensional bulk phase. Because
PCH design combines the open framework structure of the
gallery silica with the chemistry of the clay layer, new
properties for selective heterogeneous catalysis may be antici-
pated. Using the dehydration of 2-methylbut-3-yn-2-ol
,5
Table 1 also includes the results for the catalytic conversion
of MBOH over calcined FH-PCH samples. This probe reaction
is sensitive to the presence of acid-base sites on the catalyst
6
surface. Acidic sites result in the dehydration of MBOH to
2-methylbut-3-yn-1-ene (MByne), whereas basic sites cause
cleavage to acetone and acetylene. As indicated by the high
selectivity to MByne, particularly for FH-PCH-350, the sur-
faces of FH-PCH contain mainly acidic sites. In contrast,
(
MBOH) as a probe reaction, we demonstrate in the present
work the acid catalytic activity of PCH materials relative to
alumina-pillared clays and related catalysts.
+
pristine Li fluorohectorite is highly basic, affording primarily
Cetyltrimethylammonium (CTMA) exchange cations and
decylamine were used as co-surfactants to form a PCH with
fluorohectorite as the clay host. Synthetic lithium fluorohec-
acetone and acetylene as reaction products. Thus, the inter-
calation of surfactant-assembled silica and the subsequent
calcination of the heterostructure imparts acidic surface func-
tionality.
torite (Li-FH) with the unit cell composition Li1.12
Li1.12Mg4.88](Si 20)F ·xH O (Corning, Inc.) was allowed to
react at 50 °C with 0.3 m aqueous [(C16 33)NMe ]Br in twofold
-
[
8
O
4
2
The results given in Table 1 further show that FH-PCH-350
is a much more active acid catalyst than either mesoporous
MCM-41 silica or microporous alumina pillared fluorohectorite
(APF-350). The latter intercalate is an especially active
H
3
excess of the clay cation exchange capacity. After a reaction
time of 24 h, the product was washed with ethanol and water to
remove the excess surfactant and air dried. The clay then was
added to the decylamine in the molar ratio CTMA-FH:decyla-
mine = 1:20, and the resulting suspension was stirred for 30
min. Sufficient tetraethylorthosilicate (TEOS) was added to
achieve a final molar ratio of CTMA-FH:decylamine:TEOS =
7
alkylation catalyst among pillared clays. The reactivity of FH-
PCH-350 even approaches that of K-10 montmorillonite, a
commercially available acid-restructured clay. This latter
material, however, lacks framework porosity and is incapable of
shape selective catalysis.
1
:20:150. After a reaction time of 4 h at room temp., the
The FH-PCH materials reported here are hybrid structures of
the smectite clay fluorohectorite, which is basic, and weakly
acidic MCM-41 silica (cf. Table 1). Some Brønsted acidity is
expected for PCH materials, however, because the initial gallery
TEOS
calcination
+
cations (i.e. Li ) are replaced first by surfactant cations and then
by protons upon destruction of the surfactant through calcina-
8
tion. Also, fluorohectorite is known to undergo hydrolysis of
+
Amine-solvated Q -clay
Templated heterostructure
Porous clay heterostructure
lattice fluorine at temperatures above 250 °C. Proton dissocia-
tion from the resulting hydroxyl groups also can contribute to
Brønsted acidity. In addition, the silicate layers of fluorohector-
ite can undergo local restructuring upon calcination at elevated
temperatures, giving rise to Lewis-acid sites in the gallery
region.8
Fig. 1 Schematic illustration of porous clay heterostructure (PCH)
formation through surfactant-directed assembly of open framework silica in
the galleries of a layered silicate co-intercalated by a quaternary ammonium
ion surfactant (filled head groups) and a neutral amine co-surfactant (open
head groups)
Chem. Commun., 1997
1661

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Table 1 Physical and acid catalytic properties of calcined FH-PCH derivatives and related solids
MBOH product selectivitya
Basicb
Surface
area/
m g
HK
pore
size/Å
MBOH
conv.
(%)
Acidic
2
21
Sample
d001/Å
Acetylene
Acetone
MByne
FH-PCH-300
FH-PCH-350
FH-PCH-450
FH-PCH-500
FH-PCH-550
Li-FH
38.1
38.7
37.1
35.3
33.5
10.5
—
575
555
630
790
750
50
250
214
1100
18.9
19.6
22.3
22.6
21.7
0
47.0
52.7
32.9
23.6
19.0
5.0
67.9
16.4
11.0
0
0
0
100
0.1
0.2
0.8
3.4
46.5
0.1
3.6
7.0
99.9
99.6
98.9
93.8
17.3
99.9
92.9
85.0
0.2
0.4
2.9
36.2
0
c
K-10
APF-350
0
d
18.9
—
< 10
22.0
3.6
8.0
e
MCM-41
a
Reactions of MBOH were carried out over 150 mg quantities of catalyst at 180 °C in a packed-bed reactor linked to an on-line Perkin-Elmer 8500 gas
chromatograph equipped with FID detection and a packed column of 3% SP-1500 on Carbopack B. The catalysts were activated at 200 °C for 2 h prior to
testing under a helium flow of 20 ml min2 . MBOH in the gas phase (0.35 mmol h ) was diluted to 2.5% in helium. b Basic active sites afford acetone and
1
21
c
d
actylene as reaction products, whereas acid sites afford exclusively MByne. A commercial acid-treated clay (Aldrich) calcined at 350 °C. Alumina-pillared
fluorohectorite, prepared according to ref. 8 and calcined at 350 °C. Mesoporous silica molecular sieve assembled from cetyltrimethylammonium bromide
e
according to ref. 4 and calcined at 350 °C.
analogous to APF-350. Upon layer restructuring at calcination
temperatures up to 350 °C, the Q3 resonance of the clay
broadens and shifts progressively from d 293.3 to 296.6
signifying an increase in the Si–O–Si bond angle of the
tetrahedral sheet. The Q (d 2103) and Q (d 2110) resonances
of the assembled gallery silica, however, are not affected by
calcination over this temperature range. Inversion and grafting
of the clay tetrahedral sheet to the gallery silica, as evidenced by
a new resonance at d 299.7, does not occur until the FH-PCH
is calcined in the range 450–550 °C. Such local restructuring of
the silicate layer also occurs for APF over the same temperature
range.
3
50 °C
3
4
600 °C
On the basis of the above FTIR and MAS NMR results we
conclude that FH-PCH-350 and APF-350 share the same type of
acid functionality and layered structural features. The important
difference, of course, is that FH-PCH-350 is a surfactant-
assembled supermicroporous to mesoporous intercalate,
whereas APF-350 is exclusively a microporous pillared deriva-
tive. The enhanced access to the framework gallery sites in FH-
PCH-350 most likely contributes greatly to the superior
catalytic activity of this intercalate.
1
800 1700 1600 1500 1400
–1
ν / cm
Fig. 2 IR spectra of pyridine chemisorbed at 150 °C on FH-PCH samples
calcined at 350 and 600 °C (solid lines). The dashed lines are spectra for the
pristine FH-PCH samples prior to pyridine adsorption.
The partial support of this research by the National Science
Foundation through CRG grants CHE-9224102 and -9633798 is
gratefully acknowledged.
FTIR spectra for pyridine chemisorbed at 150 °C on FH-
PCH-350 and -600 provide insights into the nature of the acid
sites in PCH materials (Fig. 2). The C–C ring stretching
frequencies near 1445 and 1547 cm21 are diagnostic of the
Footnote and References
*
E-mail: pinnavaia@cem.msu.edu
9
bonding of pyridine to Lewis and Brønsted sites, respectively.
The relative intensities of these bands are very similar to those
observed for APF. This similarity in acid functionality is
somewhat surprising, because the Lewis acidity of alumina
pillared clays normally is associated with the presence of
coordinatively unsaturated sites on the alumina pillars. FH-
PCH, however, contains no aluminium.
1 F. Figueras, Catal. Rev.-Sci. Eng., 1988, 30, 457.
2 Y. Izumi, K. Urabe and M. Onaka, Zeolite, Clay, and Heteropoly Acid in
Organic Reactions, VCH Publishers, 1992, pp. 49–98.
3
A. Galarneau, A. Barodawalla and T. J. Pinnavaia, Nature, 1995, 374,
29.
5
4
J. S.Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge,
K. D. Schmitt, C. T.-W. Chu, D. H. Olson, E. W. Sheppard, S. B.
McCullen, J. B. Higgins and J. L. Schlenker, J. Am. Chem. Soc., 1992,
To better understand the origin of the observed acidity for
FH-PCH materials and the relationship of the acidity to APF,
we carried out 19F and Si MAS NMR studies for materials
1
14, 10 834.
29
5 C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck,
Nature, 1992, 359, 710.
6 H. Laron-Pernot, F. Luck and J. M. Popa, Appl. Catal., 1991, 78, 213.
1
9
calcined to 550 °C. F NMR showed that the fluorine sites
associated with the negatively charged octahedral triads of the
silicate layers (d 216) diminish to ca. 25% of the initial
intensity when calcined at 350 °C. This latter result is in accord
with the previously observed hydrolysis chemistry of fluo-
7
8
J.-R.Butruille and T. J. Pinnavaia, Catal. Today, 1992, 14, 141.
J.-R. Butruille, L. J. Michot and T. J. Pinnavaia, J. Catal., 1993, 139,
6
64.
9
E. P. Parry, J. Catal., 1963, 2, 371.
8
29
rohectorite (APF-350). Also, Si MAS NMR showed that the
silicate layers in FH-PCH are locally restructured in a manner
Received in Columbia, MO, USA, 6th May 1997; 7/03101G
1662
Chem. Commun., 1997