Disclosed is a process of making mesoporous silico-aluminates from
calcium bentonite by controlled extraction of octahedral aluminum
under mild condition, by acid, preferably phosphoric acid. The mesoporous
silicoaluminates contain only tetrahedral aluminum as the residual
aluminum. As the result of the selective removal of the octahedral
aluminum, the mesoporous silico-aluminates have several unprecedented
properties compared to products produced by conventional processes
either by mild acid-activation or by a removal of all the aluminum.
What is claimed:
1. A method for producing an amorphous mesoporous silicoaluminate
containing at least 0.5 weight percent aluminum from a calcium bentonite
clay mineral containing octahedral and tetrahedral aluminum in the
framework of the mineral, which comprises mixing said calcium bentonite
clay with sufficient acid to leach substantially all of said octahedral
aluminum without removing tetrahedral aluminum as determined by
using .sup.27 Al NMR to determine that tetrahedral aluminum has
not been removed.
2. The method of claim 1, wherein the acid is a mineral or organic
3. The method of claim 1, wherein the acid is phosphoric.
4. The method of claim 3, wherein the concentration of said phosphoric
acid is in the range of 1 to 6M.
5. The method of claim 1, wherein aluminum is leached at a temperature
of 20.degree. to 100.degree. C.
6. The method of claim 3, wherein the residue of phosphoric acid
after leaching is washed with sufficient water to reduce the level
of phosphorous to below 1.0 wt %, expressed as P205.
7. The method of claim 1, wherein the calcium bentonite prior to
leaching contains less than 2.0 wt % of iron, expressed as Fe.sub.2
O.sub.3, and after leaching has a BET surface area in the range
of 600-730 m.sup.2 /g, a pore volume in the range 0.4 to 0.8 cc/g,
and pore diameter in the range of 30-60 .ANG. unit.
8. The method of claim 1, wherein the calcium bentonite prior to
leaching contains more than 5.0 wt % of iron, expressed as Fe.sub.2
O.sub.3 , and the leached clay has a BET surface area in the range
of 300 to 500 m.sup.2 /g, a pore volume in the range of 0.4 to 0.8
cc/g, and pore diameter in the range of 50 to 80 .ANG..
9. A mesoporous silicoaluminate residue of calcium bentonite clay
having a BET surface area in the range of 600 to 730 m.sup.2 /g
and a pore volume in the range of 0.4 to 0.8 cc/g, a pore diameter
in the range of 30-60 .ANG. units, said residue containing tetrahedral
aluminum but no octahedral aluminum.
10. The composition of claim 9 obtained from a Cheto bentonite
11. A mesoporous silicoaluminate residue of calcium bentonite clay
having a BET surface area in the range of 300 to 500 m.sup.2 /g,
a pore volume in the range of 0.4 to 0.8 cc/g and a pore diameter
in the range of 50 to 80 .ANG. units, said residue containing tetrahedral
aluminum but no octahedral aluminum.
12. The composition of claim 11 obtained from Fowlkes clay.
FIELD OF THE INVENTION
This invention relates to novel mesoporous silico-aluminate products
and to a process for making such products by controlled extraction
of octahedral aluminum from a calcium bentonite clay by an acid,
preferably phosphoric acid, leaving mesoporous silicoaluminate containing
tetrahedral aluminum in the solid residue.
BACKGROUND OF THE INVENTION
Calcium bentonite clays, i.e., clay in which the principal exchangeable
cation is a calcium ion, are also referred to as sub-bentonites,
calcium montomorillonites or calcium smectites. Generally, these
are hydrated aluminosilicate crystalline minerals. Usually, magnesium
proxies for some of the aluminum in the crystals of the clay. Iron
content varies with clays from different deposits.
For many years, selected bentonite source clays have been treated
on a commercial scale with acids to leach aluminum from the structure.
The acid leaching has been practiced to produce bleaching earths,
cracking catalysts and reactive pigments for carbonless copying
paper, among other commercial applications. See for example, patents
and publications cited in U.S. Pat. Nos. 5,008,226 and 5,008,227.
The starting clays which are used to produce known forms of acid
leached bentonites typically contain approximately 20% alumina (based
on the dry weight). The aluminum in bentonites are in octahedral
and tetrahedral bonding structures. Acid dosages of about 40-50
gm of 96% H.sub.2 SO.sub.4 /100 gm clay are typically used. Alkaline
earth and alkali metals are removed. The clays are usually leached
to a residual aluminum content in the range of about 10-15 wt. %.
The extent of leaching varies inter alia with the intended use of
the leached clay. However, in general practice, both octahedral
and tetrahedral aluminum remain in the solid residue which, when
studied by XRD, exhibits lines characteristic of the clay crystals.
The acid treated clay is invariably washed to remove soluble salts
and entrained acid. While sulfuric acid is usually the acid of choice,
other acids such as phosphoric and citric acids have been proposed.
It is known that repeated sulfuric acid leaches, resulting in extractions
in excess of those used in the typical commercial prior art practice,
can produce siliceous residues with essentially no aluminum. The
porosity (surface area and pore volume) can be severely destroyed
by such practice. This may explain why exhaustive leaching to remove
virtually all aluminum has not been practiced commercially.
Acid-activated bentonites have been used as reactive pigments for
several decades for paper products, in particular for use as a porous
pigment in carbonless copy paper manufacture. The acid-leached bentonite
was used with normally colorless leuco dyes to develop colored images.
In the case of U.S. Pat. No. 4,405,371, Sugahara et al., proposed
to use a relatively highly leached bentonite. The bentonite was
leached by H.sub.2 SO.sub.4 or HCl to such degree that SiO.sub.2
content was about 82-96.5 wt %, preferably 85-95 wt %. The acid-leached
bentonite was characterized by its loss of X-ray crystallinity,
regardless of its aluminum content or structure. However, it was
noted that the acid-leached bentonite had a relatively low BET surface
area, about 180 m.sup.2 /g.
The use of acid-activated bentonite as petroleum cracking catalysts
was proposed in the 1930s. The mild acid-leaching, usually using
H.sub.2 SO.sub.4 or HCl, generated porosity and acidity which were
required for the catalysis. After the acid-leaching, part of the
clay crystallinity was maintained, and most aluminum still remained
in the structure, about 10-15 wt. % expressed as Al.sub.2 O.sub.3.
It was found that the catalytic activity was reduced after more
aluminum was removed.
The following references are nonlimiting examples of references
that relate to the preparation of acid-leached bentonites and to
their use in catalytic cracking applications:
R. E. Grim, "Applied Clay Mineralogy", McGraw-Hill, New
York, 1962, p. 307-332.
G. R. Bond, "Acid-treated clay catalyst for cracking hydrocarbons",
U.S. Pat. No. 2,551,580 (1951).
A. Grenall, "Montomorillonite cracking catalyst, X-ray diffraction",
Ind. Eng. Chem., 40 (1948) 2148-2151.
U.S. Pat. No. 3,944,482, Bruce R. Mitchell et al., "Process
for the Cracking of High Metals Content Feedstocks".
Since the discovery of zeolites in the 1960s, the role of acid-activated
bentonite in refinery cracking has substantially decreased except
for some uses such as a matrix constituent. For example, in Mitchell
et al patent (U.S. Pat. No. 3,944,482) acid-leached bentonite was
used as matrix for high metals tolerant resid catalysts. Mitchell
et al found that the acid-activated bentonite matrix must meet two
requirements: (1) it must have a high aluminum content above 20
wt %, and (2) its average pore diameter must be larger than 100
SUMMARY OF THE INVENTION
One aspect of the present invention comprises a method for manufacturing
mesoporous particulate silico-aluminate from calcium bentonite clay
minerals containing octahedral and tetrahedral aluminum in the framework
of the mineral. The process comprises mixing the calcium bentonite
mineral with sufficient acid, preferably phosphoric acid, to leach
substantially all of the octahedral aluminum while leaving at least
a predominating amount, preferably all, of the tetrahedral aluminum.
The term "mesoporous" as used herein refers to a pore
diameter (volume average) of about 20-100 .ANG., measured by N.sub.2
The concentration of the acid is in the range of 0.5 to 8M, preferably
at least 1.0M and less than 6.0M, and, most preferably in the range
of 2.0 to 4.0M. Expressed in wt %, the acid concentration is in
the range of 5 to 86 wt %, preferably at least 10 and less than
60 and, most preferably in the range of 20 to 40 wt %. Temperature
is preferably in the range of 20.degree. to 200.degree. C., most
preferably 70.degree. to 100.degree. C.
The leached silicoaluminate residue is washed, preferably with
water, until the residual acid is below 1.0% weight, expressed as
P.sub.2 O.sub.5, (based on the anhydrous weight of the solid). After
leaching, the resulting mesoporous residues contain 4.0 to 0.5 weight
% Al.sub.2 O.sub.3 (based on the anhydrous weight) and have surface
areas from 300 to 730 m.sup.2 /g, depending on the starting clays.
Preferably, the characteristic XRD lines of bentonite are absent
in the silicoaluminate product. Only two broad peaks at about 2.theta.=2.2
and 23.degree. are observable, which are due to the amorphous mesoporous
SUMMARY OF FIGURES
FIG. 1 displays small-angle XRD powder patterns of fresh and H.sub.3
PO.sub.4 -leached Cheto clay. Notice the disappearance of clay peaks
and the formation of the mesoporous silicoaluminate (MEPSA) peaks
as leaching time increases.
FIG. 2 shows .sup.29 Si MAS NMR of fresh and H.sub.3 PO.sub.4 -leached
Cheto clay. Notice the disappearance of the clay Si--O--Al bond
and the formation of MEPSA Si--O--Si and Si--O--OH bonds as leaching
FIG. 3 shows water adsorption isotherms of MEPSAs at 25.degree.
C. All products have very high water adsorption capacity.
FIG. 4 displays .sup.27 Al MAS NMR of Fowlke clay and the fresh
and steamed MEPSA-3 obtained therefrom. For the starting clay, most
aluminum is octahedral. H.sub.3 PO.sub.4 leaching removes almost
all the octahedral Al and keeps the tetrahedral Al intact. The tetrahedral
Al has very high hydrothermal stability.
DESCRIPTION OF PREFERRED EMBODIMENTS
In one embodiment of this invention novel silico-aluminate products
are produced. These products have a remarkably high hydrothermal
stability and a high BET surface area, typically about 300-500 m.sup.2
/g, a pore volume in the range of 0.4 to 0.8 cc/g, and a pore diameter
in the range of 30-80 .ANG. units. Suitable source clays for these
ultra highly stable products are commercially available as Mississippi
or Fowlkes clay. These stable mesoporous silicoaluminates material
can be used both as a cracking catalyst and a matrix for a zeolitic
resid cracking catalyst.
Such porous materials are used to improve the gasoline yield and
increase the bottoms upgrading, but at no significant penalty of
hydrogen and coke. They also tolerate high metal content in the
feedstock. This is achieved by the extensive, but controlled acid-leaching,
which generates much higher porosity and removes all the octahedral
aluminum, but keep all the tetrahedral aluminum.
In other preferred embodiments of the invention, the products are
mesoporous silicoaluminates with ultra high surface area (up to
730 m.sup.2 /g) and brightness (typically about 90% using the TAPPI
procedure). The mesoporous silicoaluminates may find many applications
such as reactive pigments for carbonless copy paper and ink-jet
printing as well as desiccants and adsorbents. Mesoporous silicoaluminates
derived from bentonite, such as the calcium bentonites known as
Cheto (Arizona) clays, can be processed to provide materials that
are not only ultra highly porous and bright, but also have favorable
water adsorption isotherms for use as desiccants. Surface areas
of such products are typically about 600 to 730 m.sup.2 /g; pore
volume in the range of 0.4 to 0.8 cc/g and pore diameter in the
range of 30-60 Angstrom units.
In practice of this invention, the range of phosphoric acid concentration
is preferably 1-6M when producing high BET surface area products
at reasonable leaching time at 95.degree. C. Especially preferred
is the use of 2-4M H.sub.3 PO.sub.4 when producing high surface
area products using 2-10 hours leaching time at 95.degree. C.
Generally the minimum acid concentration is about 1.0 M; using
lower concentration leaching time may be excessive even at 100.degree.
C. The maximum concentration is about 6.0M. Use of stronger acid
can result in a reaction that is too vigorous to control.
Optimum temperature varies with the concentration of the acid.
Preferred leaching temperature is in the range 70.degree.-100.degree.
C. Especially preferred are temperatures in the range 90.degree.
to 100.degree. C.
Mesoporous silicoaluminate products of the invention may be prepared
by batch or continuous operations, preferably employing continuous
agitation either by mechanical stirring or by bubbling steam into
After reaction is complete, the mesoporous silico-aluminate is
separated from the bulk of the liquid by known means such as filtration
or centrifugation. The residues are washed, preferably with deionized
water, to reduce H.sub.3 PO.sub.4 residual to below 1.0 wt %, expressed
by P.sub.2 O.sub.5.
Before acid-leaching, clays can be processed by drying and crushing
into powder. Typically particle size of the powders is about 10-40
micrometer in diameter.
Suitable but nonlimiting sources of clay useful in practice the
invention have the following composition (on a dry weight basis):
______________________________________ Source Clay (wt %): Cheto
Fowlke (Arizona) (Mississippi) ______________________________________
SiO2 66.7 66.4 Al2O3 19.9 19.4 CaO 3.4 3.6 MgO 6.1 3.6 Fe2O3 1.9
5.7 TiO.sub.2 0.3 0.9 K.sub.2 O 0.2 0.8 Na.sub.2 O 0.1 0.2 ______________________________________
Clay mineralogy: XRD powder diffraction patterns indicate that
Fowlkes and Cheto clays are of typical layered montmorillonite smectite
structure except that Fowlke has a higher impurity content. The
main smectite peaks of as-received clays are located at
______________________________________ 2 Theta (degree) Intensity
______________________________________ 5.8 vs 17.4 w 20.0 s 29.5
w 35.2 s ______________________________________
In one especially preferred embodiment of the invention a mesoporous
silicoaluminate material, dubbed MEPSA-1, with high surface areas
about 730 m.sup.2 /g, is synthesized. The high surface area can
be obtained only when H.sub.3 PO.sub.4 acid and a special smectite
clays are used. Such bentonite clays are mined in the Cheto deposit
and are supplied under the trade name F2 by Engelhard Corporation.
To our knowledge, products obtained by practice of this invention
using such clays have the highest surface areas of acid-leached
bentonites ever achieved. Both XRD and .sup.29 Si NMR provide evidence
that the acid-leaching has fundamentally transformed the layered
bentonite into an amorphous silica-like structure. The resulting
products, designated MEPSA-1 in the illustration examples is no
longer a bentonite. The high surface area is related to the removal
of aluminum in the clay. However, a maximum surface area is obtained
only when some or all the tetrahedra aluminum (Al.sub.2 O.sub.3
0.5-2.0 wt. %) remains in the structure. A complete removal of aluminum
leads to a destruction of some of the porosity.
MEPSA-1 can be made by stirring as-mined (dried and pulverized)
F2 bentonite in an aqueous phosphoric acid solution. We found that
the same surface area can be obtained by different combinations
of the three reaction variables: acid concentration, reaction time,
and temperature. After the acid-treatment, the solid is filtered,
washed, and dried without further pretreatment. In a typical laboratory
procedure a slurry with a F2 clay/3M H.sub.3 PO.sub.4 acid ratio
of 1g/10 ml is made, stirred at 95.degree. C. for 2.5 hours, filtered,
washed three times with deionized water, and dried at 105.degree.
FIG. 1 shows XRD powder patterns of H.sub.3 PO.sub.4 -leached F2
as a function of reaction time. As the acid-treatment time increases,
the signature peaks of bentonite (at about 2.theta.=5.8, 20 and
35.degree.) gradually disappear, and a new peak is formed at 2.18.degree.
which corresponds to a very large d-spacing of about 40.5 .ANG..
N.sub.2 adsorption BET surface area (727 m.sup.2 /g) and average
pore size (42 .ANG. in diameter) of MEPSA-1 is entirely consistent
with the large XRD d-spacing value. Furthermore, as shown in FIG.
2, .sup.29 Si MAS (magic-angle spinning) NMR indicates that the
acid-treatment has almost completely changed the local structure
of bentonite. Fresh bentonite is characterized by its single silicon
peak at about -93 ppm due to silicon sites that are connected to
three other Si and one Al (or Mg) atoms through oxygen bridges (FIG.
2-a). After the acid-treatment, shown in FIG. 2-b and -c, the Si--O--Al
bond is almost completely replaced either by a Si--O--OH (-103 ppm)
bond or a Si--O--Si bond (-113 ppm). Only about 6% of original signal
is left. Thus, both XRD and NMR data are definitive that MEPSA-1
is no longer a bentonite. Chemical analysis data also indicate that
the acid-treatment has significantly reduced alumina content from
20 to 1.3 wt. % and increased the Si content from 67 to 95 wt. %.
X-ray diffraction evaluations reported herein were performed on
a Philips APD 3720 diffractometer. The instrument settings are:
Voltage: 45 K.check mark.
Current: 40 MA
Radiation: Cuk.alpha. 1.5406 .ANG.
Divergency slit: automatic compensator
Receiving slit: 0.2 mm
Scan range (2.theta.): 1.degree.-40.degree.
Step size 0.04.degree.
Count time: 2 sec/step
Solid-state nuclear magnetic resonance (NMR) was used to determine
the local structure of the starting bentonite clays and the end
silica products. This method is particularly useful in determining
local atomic bonding structures in the materials. All the high resolution
NMR spectra were taken from a Varian Unity-400 MHZ spectrometer
at room temperature under a so-called magic angle spinning (MAS)
The aluminum spectra were taken using a Doty 5mm probe with MAS
at about 11 kHz spinning speed. The spectra were referenced to a
1.0M aluminum nitrate aqueous solution. In order to further eliminate
the quadrupolar broadening associated with .sup.27 Al nucleus, a
short RF excitation pulse, 0.5 .mu.s, was used and the samples were
moisturized at 80% humidity for at least 24 hours before the analysis.
These conditions had been established in literature. Reference is
made to the following papers and the references therein:
X. Yang, Structure identification of intermediate aluminum species
in USY zeolite using high resolution and spin-lattice relaxation
.sup.27 Al NMR, J. Phys. Chem., 99 (1995)1275.
X. Yang and R. Truitt, Observation and study of new tetrahedral
Al sites in NH3-treated, steamed zeolites using MAS .sup.27 Al and
.sup.15 N NMR, Zeolites, 17 (1996) 249.
The silicon spectra were acquired using a Chemagnetics 7.5 mm probe
with MAS at about 5 kHz. A 7.0 .mu.s excitation pulse and 40 s recycle
time were used. The spectra were referenced to tetramethyl silane
(TMS). The detailed acquisition conditions can be found in the following
X. Yang and P. Blosser, Location and bonding of cations in ETS-10
titanosilicate molecular sieve: A multinuclear NMR investigation,
Zeolites, 17 (1996) 237.
The chemical composition analysis was performed with a standard
X-ray florescence technique. The elemental composition was based
on a volatile free weight basis (1000.degree. C.). For all the analyzed
elements of the clays and the silicon in MEPSA, the accuracy is
within .+-.0.1 wt %. For the low aluminum residual in MEPSA, the
accuracy is within .+-.0.5 wt %.
The water adsorption isotherms were analyzed using a TGA (thermal
gravametric analysis) method. The samples were dried at 120.degree.
C. before the analysis. Each measurement point represents the amount
of water gained after an equilibrium is reached under the specific
The BET surface area, pore volume, and pore size were determined
by nitrogen gas adsorption at liquid nitrogen temperature, using
either of two automated instruments: Quantachrome.RTM. Autosorb-6
or Micrometrics.RTM. ASAP2400. The samples were heated at 250.degree.
C. under vacuum for at least 6 hours before the analysis. The sample
weight was obtained on a dried sample. The surface area was obtained
by B.E.T. method with 39 relative pressure points. The pore volume
represents the total pore volume with pore radius less than 1000
TAPPI brightness and yellowness were measured using a Technidyne-S4M
and Technidyne-MicroTB1C instrument, respectively. The TAPPI brightness
is also referred to as GE or Germ brightness. The samples were ground
to 325 mesh for the analysis. The instruments were calibrated against
the manufacturer's master instruments. A sample of fully calcined
kaolin sample supplied by Engelhard Corporation was used as a reference.
The abrasion was measured using an Einlehner abrasion apparatus.
An aqueous slurry containing 15 wt % solids and 87,000 revolution
of abrasion (equivalent of 40 minutes) were used.