Molecular sieve abstract
Noncrystalline mesoporous molecular sieve substances are prepared
using alkyltrimethylammonium halide as a surfactant in combination
with water-soluble organic or inorganic salts capable of binding
to a monovalent cation. The sieve substances are superior to the
M41S series in thermal stability and hydrothermal stability. The
sieve substances are structurized in such a manner that channels
uniform in diameter are randomly arranged interconnecting with one
another in a three-dimensional way so as to allow reactants to readily
diffuse therein.
Molecular sieve claims
What is claimed is:
1. A method for preparing a disordered mesoporous molecular sieve
substance exhibiting a three-dimensionally disordered channel network
and a narrow pore-size distribution having a line width less than
0.5 nm at the medium height in a channel size distribution curve
obtained from an adsorption isotherm of nitrogen, comprising the
steps of:
(A) mixing silicate salt and alklytrimethylammonium halide as a
surfactant, represented by the following formula I:
wherein n is an integer of 12 to 18 and X is Cl or Br, to prepare
a mixed aqueous solution;
(B) adding in the mixed aqueous solution one or more water-soluble
organic salts selected from the group consisting of ethylenediaminetetraacetate
tetrasodium salt, adipic acid disodium salt, sodium acetate, 13-benzenedisulfonic
acid disodium salt and nitrilotriacetic acid sodium salt;
(C) subjecting the resulting solution to a hydrothermal reaction
in such a way that suitable pH and temperature are maintained for
a period so as to give precipitates of the molecular sieve substance;
(D) filtering and drying precipitates of the molecular sieve substances;
and
(E) calcining the filtered and dried precipitates.
2. The method in accordance with claim 1 wherein said surfactant
is selected from the group consisting of hexadecyltrimethylammonium
chloride, dodecyltrimethyl ammonium bromide, tetradecyltrimethylammonium
bromide and octadecyltrimethylammonium bromide.
3. The method in accordance with claim 1 wherein said silicate
salt is sodium silicate.
4. The method in accordance with claim 1 wherein said mixed aqueous
solution in said step (A) further comprises an ammonia aqueous solution.
5. The method in accordance with claim 1 wherein said step (E)
for calcining is carried out at a temperature of 500 to 600.degree.
C.
6. A method for preparing a disordered mesoporous molecular sieve
substance exhibiting a three-dimensionally disordered channel network
and a narrow pore-size distribution having a line width less than
0.5 nm at the medium height in a channel size distribution curve
obtained from an adsorption isotherm of nitrogen, comprising the
steps of:
(a) mixing silicate salt, one or more salts selected from the group
consisting of aluminates, borates and acid salts of 3d transition
metals on the periodic table, and alkyltrimethylammonium halide
as a surfactant, represented by the following formula I:
wherein n is an integer of 12 to 18 and X is Cl or Br, to prepare
a mixed aqueous solution;
(b) adding in the mixed aqueous solution one or more water-soluble
organic salts selected from the group consisting of ethylenediaminetetraacetate
tetrasodium salt, adipic acid disodium salt, sodium acetate, 13-benzenedisulfonic
acid disodium salt and nitrilotriacetic acid sodium salt;
(c) subjecting the resulting solution to a hydrothermal reaction
in such a way that suitable pH and temperature are maintained for
a period so as to give precipitates of the molecular sieve substance;
(d) filtering and drying precipitates of the molecular sieve substances;
and
(e) calcining the filtered and dried precipitates.
7. The method in accordance with claim 6 wherein said aluminate
is sodium aluminate (NaAlO.sub.2 ).
8. The method in accordance with claim 6 wherein said mixed aqueous
solution contains, 1.0 to 15.0 moles of said silicate salt, and
0.0025 to 1.5 moles of said salts selected from the group consisting
of aluminates, borates and acid salts of 3d transition metals on
the periodic table per mole of the alkyltrimethylammonium halide
of Formula I.
9. The method in accordance with claim 6 wherein said surfactant
is selected from the group consisting of hexadecyltrimethylammonium
chloride, dodecyltrimethyl ammonium bromide, tetradecyltrimethylammonium
bromide and octadecyltrimethylammonium bromide.
10. The method in accordance with claim 6 wherein said silicate
salt is sodium silicate.
11. The method in accordance with claim 6 wherein said mixed aqueous
solution in step (a) further comprises an ammonia aqueous solution.
12. The method in accordance with claim 6 wherein said step (e)
for calcining is carried out at a temperature of 500 to 600.degree.
C.
Molecular sieve description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, in general, to a mesoporous molecular
sieve substance and methods for preparing the same. More particularly,
this invention relates to a noncrystalline mesoporous molecular
sieve substance, superior over the M41S series of Mobil Co. in thermal
stability, hydrothermal stability and structured in such a manner
that channels of uniform diameter are randomly arranged in a three-dimensional
way so as to allow the ready diffusion of reactants therein. Also,
the present invention is concerned with methods for preparing the
noncrystalline mesoporous molecular sieve substance.
2. Description of the Prior Art
Among solid substances known thus far, those having uniform channels,
such as zeolites of porous crystalline aluminum silicate and of
porous crystalline aluminum phosphates (AlPO.sub.4) are defined
as molecular sieves, because they selectively adsorb molecules smaller
than the size of the channel entrance or they allow molecules to
pass through the channel. In view of crystallography, zeolite and
AlPO.sub.4 are fully crystalline substances, in which the atoms
and channels are arranged in complete regularity. These fully crystalline
molecular sieves are obtained naturally or synthesized through hydrothermal
reactions. The number of fully crystalline molecular sieves obtained
or synthesized thus far amount to several hundred species. They
play an important role as catalysts or supports in modern chemical
industries by virtue of their characteristics including selective
adsorption, acidity and ion exchangeability. Examples of the current
catalyst processes utilizing the characteristics of zeolite include
the petroleum cracking reaction using ZSM-5 and the aromatic conversion
reaction of paraffin using KL-zeolite impregnated with platinum.
A significant problem of the fully crystalline molecular sieve is
that it cannot be used in reactions of molecules larger than 1.3
nm in size.
Recently, a group of researchers at Mobil Co. reported a series
of mesoporous molecular sieves, named the M41S series, including
MCM-41 and MCM-48. See U.S. Pat. Nos. 5057296 and 5102643. These
molecular sieves show a structure in which mesopores uniform in
size are arranged regularly. Existing molecular sieves have been
produced by using inorganic or organic cations as templates, whereas
those mesoporous molecular sieves are synthesized through a liquid
crystal template pathway by using surfactants as templates. These
mesoporous molecular sieves have the advantage that their pore sizes
can be adjusted in a range of 1.6 to 10 nm by controlling the kinds
of surfactants or synthetic conditions employed during the production
process.
Stuky, a professor at the University of California, U.S.A. and
his colleagues reported mesoporous molecular sieves, designated
as SBA-1 2 3 in Science, 268 1324 (1995). The channels of the
mesoporous molecular sieves are regularly arranged, while the constituent
atoms show an arrangement similar to that of amorphous silica.
Mesoporous molecular sieves have regularly arranged channels larger
than those of existing zeolites, thus enabling their application
to adsorption, isolation or catalyst conversion reactions of relatively
large molecules. Of the aforementioned mesoporous molecular sieves,
the most widely researched is MCM41 which has a uniform structure
exhibiting hexagonal arrangement of straight mesopores, such as
honeycomb, and has a specific surface area of about 1000 m.sup.2
g.sup.-1 as measured by ordinary BET. After being subjected to calcination
to remove template materials, MCM-41 samples disclosed by early
researchers undergo, although there is a little difference depending
upon synthetic conditions, structure contraction ranging from 20
to 25% relative to pre-calcination. This contraction is attributed
to the fact that silanol groups are condensed by the calcination.
However, the research data from the present inventors show that
if the equilibrium of the silicate condensation reaction is shifted
toward the product by controlling the pH of the reactants during
the hydrothermal synthesis of MCM-41 the condensation of the silanol
group is completed in advance, so that the weak thermal stability
can be overcome. This achieves a considerable improvement in structural
uniformity as disclosed in Journal of Chemical Society, Chem. common.,
1995 p711. The MCM-41 thus obtained did not undergo structural
decomposition even at 500.degree. C. under 1 atm inthe presence
of 100% water vapor. It contracted only slightly in structure even
upon heating up to 900.degree. C. under an oxygen atmosphere. A
modified MCM-41 in which aluminum is substituted for silica has
an acidity and ion exchangeability similar to that of existing zeolites.
The modified MCM-41 can be used for various reactions requiring
such properties.
Superior in thermal stability as it is, MCM-41 begins to lose its
structure in water heated at 65.degree. C. or higher, which is ascribed
to the hydrolysis of the silicate constituents. Twelve hours after
being heated in boiling water at 100.degree. C., MCM-41 completely
loses its structural characteristics. This poor hydrothermal stability
serves as a serious limiting factor at temperatures 60.degree. C.
or higher. Three examples include: (1) the case of using a titanium-substituted
molecular sieve in a partial oxidation reaction; (2) the case of
a catalyst conversion reaction under a hydrothermal condition in
which hydrogen peroxide is used as an oxidizing agent; or (3) the
case of impregnating in the molecular sieve a transition metal,
such as platinum, as a catalytically active ingredient. Further,
when MCM-41 is subjected to calcination to remove templates, it
is decomposed to form coke which is, in turn, likely to close the
mesopores because MCM-41 has straight channels. Even when catalytically
active ingredients, such as platinum or palladium, are impregnated,
the passages are readily closed, which prevents molecules from diffusing.
Thus, the metal molecules present on the inner side of the closed
passages cannot be in contact with reactants and only those present
at the opposite ends show catalytic activity. Therefore, active
research and development efforts have been and continue to be directed
to an improvement in the hydrothermal stability of mesoporous molecular
sieves and in overcoming the molecular diffusion problem attributable
to closed channels. |