Molecular sieve abstract
Ferrisilicate molecular sieves of the ZSM-5 type, having SiO.sub.2
/Fe.sub.2 O.sub.3 mole ratios ranging from 20 to 400 are prepared
by adding a silica source and a quaternary ammonium salt in that
order to an acedified solution of an iron (III) compound, crystallizing
the resulting gel to form a ferrisilicate molecular sieve, and thermally
treating the molecular sieve with nitrogen, air and/or steam at
300.degree. to 700.degree. C. Preferred thermal treatment comprises
treating with nitrogen first, then with air or steam. Thermally
treated molecular sieves contain iron both in and outside the crystal
framework; most of the non-framework iron is dispersed as very finely
divided iron oxides on internal surfaces. Molecular sieves are useful
as catalysts in Fischer-Tropsch and other iron oxide-catalyzied
reactions.
Molecular sieve claims
What is claimed is:
1. A process for preparing a ferrisilicate molecular sieve which
comprises:
(a) adding a silica source and one or more compounds selected from
the group consisting of primary, secondary and tertiary amines,
and quaternary ammonium compounds to an acidic aqueous solution
of an iron (III) compound, and maintaining said solution in the
acidic state until the addition of said silica source is complete;
(b) heating the mixture obtained in step (a) at a temperature of
about 100.degree. to about 250.degree. C. until molecular sieve
crystals are formed; and
(c) thermally treating the molecular sieve crystals formed in step
(b) at a temperature from about 250.degree. C. to about 1000.degree.
C. in an inert atmosphere at about 450.degree. to about 800.degree.
C. for about 6 to about 16 hours, then in air at about 400.degree.
to about 1000.degree. C. for about 3 to about 8 hours, and then
with steam at a temperature from about 300.degree. to about 700.degree.
C. for about 1 to about 4 hours.
2. A process according to claim 1 in which said silica source is
added first and said amine or quaternary ammonium compound is added
after the addition of said silica source is complete.
3. A process according to claim 1 in which said silica source
is an aqueous solution of sodium silicate.
4. A process according to claim 1 to which said one or more compounds
added in step (a) are one or more quaternary ammonium compounds.
5. A process according to claim 1 in which said inert atmosphere
is a nitrogen atmosphere.
6. A process according to claim 1 in which iron in the as synthesized
molecular sieve is predominantly in the crystal framework and in
the thermally treated molecular sieve is about 15 to about 40 percent
in the framework and about 60 to about 85 percent outside the framework.
Molecular sieve description
TECHNICAL FIELD
This invention relates to processes for making crystalline ferrisilicate
molecular sieves. More particularly, this invention relates to processes
for making crystalline ferrisilicate molecular sieves of the ZSM-5
type.
BACKGROUND ART
Molecular sieves are ordered, porous crystalline materials having
a definite three-dimensional crystal structure, within which there
are a large number of small cavities which are interconnected by
a number of still smaller channels or pores. These cavities and
pores in any specific molecular sieve material are of precisely
uniform size. Since the pores are of such size as to accept for
adsorption molecules which are small enough to pass through the
pores, while rejecting molecules of larger size, the materials have
come to be known as "molecular sieves" and are utilized
in various ways which take advantage of this property. Molecular
sieves may be used, for example, as catalysts, selective adsorbents,
drying agents, ion exchange materials, and for other purposes. Aluminosilicate
molecular sieves are frequently referred to as zeolites.
The synthetic crystalline aluminosilicate zeolites are the best
known molecular sieves. These materials are characterized by a rigid
three-dimensional network of SiO.sub.4.sup.- and AlO.sub.4.sup.-
tetrahedra, which are cross-linked through shared oxygen atoms.
The electronegativity of the aluminum-containing tetrahedra is balanced
by the inclusion in the crystal of a cation, typically monovalent
or divalent, such as an alkali metal (e.g. sodium) or an alkaline
earth metal (e.g. calcium). The monovalent or divalent ion is typically
at least partially exchangeable by conventional ion exchange techniques.
The aluminum and silicon are not exchangeable. Various alumino-silicate
molecular sieves are known. One of these is ZSM-5 which is described,
for example, in U.S. Pat. No. 3702886 to Argauer et al.
Less well known are the ferrisilicate molecular sieves. One of
these, ZSM-12 is described in published European Patent Application
(EPA) No. 0013630. Another is the crystalline silicate described
in U.S. Pat. No. 4208305 to Kouwenhoven et al. This latter material
is of the ZSM-5 type and, according to the patent, consists structurally
of a three-dimensional network of SiO.sub.4 FeO.sub.4 and optionally
AlO.sub.4 GaO.sub.4 and GeO.sub.4 tetrahedra which are interlinked
by oxygen atoms. The patent discloses a number of catalytic processes
in which the molecular sieves may be used. However, direct conversion
of a carbon monoxide-hydrogen mixture to a hydrocarbon mixture (the
Fischer-Tropsch synthesis) is not among these reactions.
Iron-containing zeolites are also known. These may be prepared
by (a) physical admixture of a zeolite and an iron component, (b)
ion exchange of Fe (III) into a zeolite, (c) adsorption of a volatile
metal compound in the zeolite cavities followed by thermal decomposition,
and (d) impregnation of a zeolite with a solution of a ferric compound
followed by thermal decomposition.
In catalysts where the iron component is physically mixed with
the zeolite ZSM-5 an intimate mixture between the two components
is very difficult to obtain. Thus, all of the iron component is
likely to be on the outside of the pores of the molecular sieve
making these catalysts least selective for the Fischer-Tropsch reaction.
Further, the formation of large metal oxide particles decreases
the amount of surface available for reactions to take place.
Loss of crystallinity and thermal stability is reported for synthetic
zeolites which are ion exchanged with ferric ions. The ion exchange
of Fe (III) cations into the zeolite can give rise to a high dispersion
of the iron component. However, ion exchanging of Fe (III) ions
into the zeolite ZSM-5 has not been completely successful, due to
the size of the hydrated iron complex and the high dispersion of
monovalent exchange sites within the zeolites.
Iron (O) (i.e., metallic iron) species can be introduced into the
pores of a zeolite by adsorption and subsequent decomposition of
the iron complexes. The most common volatile metal compound that
is used to prepare iron containing zeolites is iron pentacarbonyl,
Fe(CO).sub.5. The size of the iron pentacarbonyl is just about ideal
to be adsorbed by zeolite Y. The iron pentacarbonyl is first adsorbed
by the zeolite and then the carbon monoxide is driven off from the
iron pentacarbonyl by thermolysis. This process of making iron containing
zeolites has the disadvantage that during the process of thermolysis,
The adsorbed metal compound tends to come out of the pores of the
zeolite. Moreover, the iron pentacarbonyl is too large in size to
enter the zeolite ZSM-5 and hence when used over zeolite ZSM-5 will
have all of the iron present outside the pores of the zeolite ZSM-5.
DISCLOSURE OF THE INVENTION
This invention according to a first aspect provides novel crystalline
ferrisilicate molecular sieves of the ZSM-5 type. These molecular
sieves have an overall silica to ferric oxide (SiO.sub.2 /Fe.sub.2
O.sub.3) mole ratio in the range of about 20 to 400. A first portion
of the iron content is in the crystal framework or lattice, and
the remaining portion of the iron is outside the crystal framework.
This remaining portions constitutes from about 0 to about 80 percent
by weight of the total iron content and is dispersed in the form
of finely divided particles on the internal and external surfaces
of the molecular sieve. At least about 30 percent, preferably at
least about 50 percent, most preferably at least about 80 percent,
of the non-framework iron is dispersed on the internal surfaces.
Nearly all of the non-framework iron on the internal surfaces is
in the form of iron oxide particles having a particle size less
than about 5 Angstrom units, while iron on the external surfaces
is in the form of iron oxide particles predominantly from about
5 to about 15 Angstrom units.
This invention according to a second aspect provides a process
for preparing ferrisilicate molecular sieves of the ZSM-5 type.
This process comprises: (a) adding a silica source and one or more
compounds selected from the group consisting of primary, secondary
and tertiary amines and quaternary ammonium compounds to an acidic
aqueous solution of an iron compound, and maintaining said solution
in the acidic state until the addition of said silica source is
complete; (b) heating the mixture obtained in step (a) at a temperature
of about 100.degree. to about 250.degree. C. until molecular sieve
crystals are obtained; and (c) thermally treating the molecular
sieve crystals formed in step (b) at a temperature from about 300.degree.
to about 1000.degree. C. The preferred thermal treatment comprises
treating the molecular sieve of step (b) in an inert atmosphere
at about 450.degree. to about 800.degree. C. for about 6 to about
16 hours, then in air at about 400.degree. to about 1000.degree.
C. for about 3 to about 8 hours, and then optionally with steam
at about 250.degree. to about 700.degree. C. for about 0.5 to about
36 hours.
This invention according to a third aspect provides a process for
carrying out chemical reactions using a ferrisilicate molecular
sieve of the ZSM-5 type as described above. According to this process,
a gaseous reactant or mixture thereof is contacted with the ferrisilicate
molecular sieve under reaction conditions. In particular, this process
may be a catalytic process in which a mixture of carbon monoxide
and hydrogen is contacted with the molecular sieve at a temperature
from about 250.degree. to about 400.degree. C. and at a pressure
from about one to about 20 atmospheres, whereby a reaction produce
comprising a hydrocarbon or mixture thereof is obtained. This hydrocarbon
or mixture thereof comprises a major portion of gasoline range (C.sub.6
to C.sub.10) hydrocarbons when the temperature is maintained from
about 250.degree. to about 350.degree. C. and the pressure is from
about 10 to about 20 atmospheres.
BEST MODE FOR CARRYING OUT THE INVENTION
The ferrisilicate molecular sieves of the present invention have
an overall SiO.sub.2 /Fe.sub.2 O.sub.3 mole ratio in the range of
about 20 to about 400 preferably from about 30 to about 200 and
exhibit ZSM-5 structure. These molecular sieves consist structurally
of a three-dimensional framework of SiO.sub.4.sup.- and FeO.sub.2.sup.-
tetrahedra which are interlinked by common oxygen atoms.
Only a portion of the iron content of final product (i.e., thermally
treated) molecular sieves of this invention is in the framework.
Framework iron is in the form of tetrahedra. Framework iron may
constitute as little as 20 percent (by weight) of total iron; typically
however, framework iron is from about 50 to 100 percent of the total
iron content. The remainder of the iron is outside of the framework
in the form of octahedra, and consists essentially of finely divided
particles of iron oxides dispersed on the internal and external
surfaces of the molecular sieve. Nearly all of the particles on
the internal surfaces are smaller than about 5 Angstroms (A) in
size. Particles on the external surfaces are predominantly from
5 to 15 .ANG.. Most of the non-framework iron is dispersed on the
internal surfaces, i.e. surfaces of the pores and the cavities (which
for convenience will simply be referred to as the pore surfaces)
of the sieve. The thermally treated molecular sieves may range from
off white to brown in color. Distribution of the iron content of
the molecular sieve between the framework and non-framework sites
may be shown by Mossbauer spectra. Thermally treated molecular sieves
of this invention have a high degree of thermal stability.
The electronegativity of framework iron is balanced by exchangeable
cations, e.g. hydrogen, ammonium, alkali metal or alkaline earth
metal, in the crystal structure. The ion exchange capacity of a
product molecular sieve furnishes a quantitative measure of the
amount of framework (tetrahedral) iron present. Thermally treated
molecular sieves as produced are in the hydrogen form; other exchangeable
cations may be introduced by conventional ion exchange techniques.
The overall SiO.sub.2 /Fe.sub.2 O.sub.3 mole ratio of a thermally
treated molecular sieve herein is based on the total quantity (framework
plus non-framework) iron present.
The ferrisilicate molecular sieves of this invention exhibit ZSM-5
structure and may be regarded as analogs of the known crystalline
aluminosilicate zeolite molecular sieves. Such molecular sieves
are described, for example, in U.S. Pat. No. 3702886 cited above.
One indication of ZSM-5 structure is the presence of pores of a
uniform diameter of about 5.5 Angstroms. Another indication is an
x-ray diffraction pattern which is similar to that of known ZSM-5
molecular sieves. The x-ray diffraction pattern of the molecular
sieves of this invention is shown in Table I below.
TABLE I ______________________________________ Inten- Number 2
Theta sity I/Io ______________________________________ 1 7.78 689
28 2 8.72 520 21 3 11.68 153 6 4 13.66 156 6 5 13.84 226 9 6 15.78
192 7 7 17.6 116 4 8 19.14 162 6 9 20.22 252 9 10 22.06 173 7 11
22.96 2144 88 12 23.14 2431 100 13 23.58 676 27 14 23.82 1105 45
15 24.28 780 32 16 25.64 130 5 17 26.5 180 7 18 26.84 240 9 19 29.14
212 8 20 29.78 236 9 ______________________________________
As synthesized ferrisilicate molecular sieves of this invention
are crystals having a white to pale lemon yellow color, indicating
that all or most (e.g., at least 90 percent) of the iron content
is in the framework, and having the same mole ratio of silica to
ferric oxide (i.e. from about 20 to about 400) that characterizes
the final product. The percentage of iron in the framework is lower
at SiO.sub.2 /Fe.sub.2 O.sub.3 mole ratios below about 50. The as
synthesized ferrisilicate molecular sieves may be represented on
the water free basis by the following formula:
where R is alkylammonium, dialkylammonium, trialkyl-ammonium or
tetraalkylammonium; a is from about 1 to about 6; and b is from
about 20 to about 400. R is preferably tetraalkylammonium, and the
alkyl groups are lower alkyl groups, i.e., alkyl groups containing
from one to about 8 carbon atoms. A minor amount of R may be accounted
for by an alkali metal ion, e.g. sodium.
The x-ray diffraction pattern of the as synthesized molecular sieve
is substantially the same as that of the final product molecular
sieve, i.e., as shown in Table I.
Preparation of the product ferrisilicate molecular sieves of this
invention requires two operations, i.e. (1) preparation of the as
synthesized ferrisilicate, and (2) thermal treatment of the as synthesized
ferrisilicate i order to form the product ferrisilicate molecular
sieve.
The as synthesized ferrisilicate is preferably formed by adding
a silica source to an acidic solution of an iron (III) (i.e., ferric
compound, adding to the resulting gel a primary amine, a secondary
amine, and tertiary amine, or a quaternary ammonium salt and heating
the resulting mixture (which is a gel), preferably in an autoclave
under autogenous conditions at about 100.degree. to about 250.degree.
C., until crystallization takes place.
The acidified solution of a ferric [i.e., ions (III)] compound
is obtained by dissolving an iron (III) compound, such as ferric
nitrate, ferric chloride or ferric sulfate, in water and acidifying
the resulting solution with a strong mineral acid such as hydrochloric
or sulfuric acid to pH not higher than about 5.
The silica source (or precursor) may be either an aqueous solution
of an alkaline metal silicate or an aqueous silica sol. Alkali metal
silicate solutions are ordinarily preferred, because these result
in better incorporation of the iron into the framework, while use
of a silica sol results in a substantial amount of non-framework
(octahedral) iron in the as synthesized ferrisilicate. Representative
alkaline metal silicates are N-Brand silicate (PQ Corporation),
which has the formula Na.sub.2 SiO.sub.3.5H.sub.2 O. Sodium metasilicate
from other vendors can also be used. Other sodium silicates having
different SiO.sub.2 /Na.sub.2 O mole ratios may also be used. Representative
silica sols (less suitable as previously indicated) include "LUDOX"
(E. I. Dupont Company) and "Cab-O-Sil" (Cabot Corp.),
both of which contain particles of high molecular weight polymeric
silica beads.
It is important to add the silica source to the iron (III) solution,
rather than to add the iron solution to the silica source or to
charge both simultaneously to a reaction vessel, because it is important
to maintain an acidic pH, preferably below about 5 throughout the
addition of the silica source. If this is not done, iron (III) hydroxide
will precipitate and the desired incorporation of substantially
all of the iron into the framework of the as synthesized ferrisilicate
gel, and the desired distribution and particle size characteristics
of the iron in the final product molecular sieve, will not be obtained.
The amine or quaternary ammonium salt is preferably added after
the addition of the silica source is complete. The amines are primary,
secondary or tertiary alkyl amines in which the alkyl group contains
from 1 to about 8 carbon atoms. Tertiary amines are preferable to
the primary or secondary amines. A representative tertiary amine
is tripropylamine. Preferred, however, are the quaternary ammonium
salts, which are tetraalkyl ammonium salts of strong acids, the
alkyl group containing from about 1 to about 8 carbon atoms. A representative
quaternary ammonium salt is tetrapropylammonium (TPA) bromide.
A minor amount of alkali metal (e.g., sodium) salt may be used
in addition to the amine or quaternary ammonium salt, but the latter
must constitute the major source of exchangeable ions in the molecular
sieve as synthesized.
The amine or quaternary ammonium salt and the silica source may
be added simultaneously to the acidified iron (III) solution is
desired, provided that the pH of the solution is maintained in the
acidic state and preferably at a pH not over about 5 until addition
of most of the silica source is complete. (When simultaneous addition
is used, addition of the silica source may be completed before addition
of the amine or quaternary ammonium salt is completed). However,
it is ordinarily preferred to add the amine or quaternary ammonium
salt after all of the silica source has been added.
The mole ratios of quaternary ammonium compound, iron compound
and silica source expressed as R.sub.2 O, Fe.sub.2 O.sub.3 and SiO.sub.2
respectively, in the reactants are substantially the same as the
ratio in the as synthesized ferrisilicate gel.
Ferrisilicate gel is placed in an autoclave and heated under autogenous
pressure at about 100.degree. to about 250.degree. C. (preferably
about 170.degree. C.) for 2 to 5 days. The resulting white solid
may be separated from the mother liquor, e.g., by filtration or
centrifugation, then washed with water and dried at about 100.degree.
C. The resulting material is an as synthesized highly crystalline
ferrisilicate molecular sieve. X-ray powder diffraction confirms
the formation of the ZSM-5 structure.
The as synthesized molecular sieve is thermally treated. This generally
causes a portion of the iron to migrate from the framework to the
internal surfaces (and to a slight degree to the external surfaces
as well). Thermal treatment comprises treatment with nitrogen, air
and/or steam at a temperature from about 250.degree. C. to about
1000.degree. C. Preferred thermal treatment according to this invention
includes treatment in an inert atmosphere, preferably a flowing
stream of nitrogen, at a temperature from about 450.degree. to about
800.degree. C. for about 6 to about 16 hours, followed by calcining
in air at a temperature from about 400.degree. to about 1000.degree.
C. for about 3 to about 8 hours. The extent for iron migration depends
on the treating agent or agents used (steam causing the greatest
migration), and the temperature and time of treatment. Thermal treatment
causes decomposition of the organic material (amine or quaternary
ammonium salt). At least a portion of the thermal treatment should
be with air in order to assure complete decomposition.
After calcination with nitrogen and air, or with air alone, the
ferrisilicate molecular sieve is ammonium ion exchanged, in order
to remove any sodium ion present. This may be done with an aqueous
solution of an ammonium salt of strong mineral acid, such as ammonium
nitrate.
After ammonium ion exchange, the molecular sieve is again thermally
treated, either by calcination in air or by hydrothermal treatment
with steam. According to one mode of treatment, the molecular sieve
may be air dried at about 100.degree.-120.degree. C., then heated
at a somewhat higher temperature, (e.g. about 250.degree. to about
350.degree. C.) for about 2 to 6 hours, and then calcined at high
temperature, (e.g. about 550.degree. to about 650.degree. C.) for
a longer time, (e.g. 6 to 24 hours). Finally, the calcined material
may be ion exchanged, for example with potassium ion (as dilute
KOH to a pH of 8.0), washed with water, filtered and air dried at
about 100.degree.-120.degree. C. The produce molecular sieve formed
in this manner typically contains about 50-100 percent of the iron
in the framework, the remainder (about 0 to 50 percent) being finely
dispersed throughout the molecular sieve, including the pore surfaces.
Only a small amount of the non-framework iron is on the outside
surfaces, which is desirable because iron on the outside surfaces
is less reactive for catalytic reaction purposes. The non-framework
iron is in the form of particles of iron oxides; nearly all the
particles on internal surfaces are smaller than 0.5 .ANG. while
those on external surfaces are predominantly from 5 .ANG. to 15
.ANG..
The ammonium exchanged molecular sieve described above may be hydrothermally
treated with steam at about 300.degree. to about 700.degree. C.
for about 1 to 4 hours, washed with water, filtered and air dried
at about 100.degree. to 120.degree. C. Hydrothermal treatment with
steam causes a much larger portion of the framework iron to migrate
outside the framework and to become dispersed as finely divided
iron oxide particles on the pore surfaces. Hydrothermal treatment
with steam also causes a greater percentage of the non-framework
iron to migrate to the external surfaces than is the case when a
molecular sieve is thermally treated with nitrogen and air only,
or with air alone. For example, a molecular sieve treated with steam
at 550.degree. to 650.degree. C. for 1 to 4 hours may contain about
15 to 40 percent of the iron in the framework, and conversely about
60 to about 85 percent of the iron outside the framework, principally
in the form of finely divided iron oxide particles not larger than
about 5 Angstroms dispersed mainly on the internal surfaces. Typically
about 95-97 percent of total non-framework iron in thermally treated
molecular sieves (less in those having SiO.sub.2 /Fe.sub.2 O.sub.3
mole ratio less than 50) is on the internal surfaces.
The ion exchange capacity of final product molecular sieves may
be determined by ion exchange with dilute KOH to pH 8.0 prior to
final washing and drying if desired.
The unit cell diameter of molecular sieves of this invention ranges
from about 5330 .ANG. (at a SiO.sub.2 /Fe.sub.2 O.sub.3 mole ratio
of 100) to about 5410 .ANG. (at a SiO.sub.2 /Fe.sub.2 O.sub.3 mole
ratio of 20). Little further change in the unit cell diameter takes
place as the SiO.sub.2 /Fe.sub.2 O.sub.3 ratio is increased above
100.
When molecular sieves according to this invention are used for
catalytic purposes, the materials should generally be available
in the form of particles with a diameter of about 0.5 to about 5
millimeters. Typically the final product molecular sieve have a
particle diameter in the range of about 0.5 to about 8 microns.
To achieve larger size, and in increase thermal stability, the molecular
sieve may be composited with an inorganic matrix or binder material
if desired. Examples of suitable matrix or binder materials are
naturally occurring clays, such as kaolin and bentonite. Other suitable
matrix or binder materials are synthetic inorganic oxide, such as
alumina, silica, zirconia or combinations thereof, as for example
silica-alumina and silica-zirconia. The ratio of molecular sieve
to matrix material may be as desired, and typically molecular sieve
constitutes from about 10 to about 100 percent by weight of a composite.
Molecular sieves according to this invention may be used as catalysts
in various reactions, but are particularly suitable as Fischer-Tropsch
catalysts for the direct conversion of mixtures of carbon monoxide
and hydrogen to hydrocarbons, without forming and recovering methanol
as an intermediate. The carbon monoxide-hydrogen mixture may be
derived by conventional means, as for example steaming of coal.
The mole ratio of CO to H.sub.2 in the reactant mixture may range
from about 1:1 to about 3:1. Such a reactant mixture is contacted
with a molecular sieve of this invention under reaction conditions,
e.g. a pressure ranging from about atmospheric to about 20 atmospheres,
a temperature ranging from about 250.degree. to about 400.degree.
C., and at a weight hourly space velocity from about 0.1 to about
100 reciprocal hours (h.sup.-1). The reaction product is a hydrocarbon
mixture. The reaction mixture formed includes both the reaction
product and unreacted carbon monoxide and hydrogen. The term "reaction
produce", in this specification is used to denote only those
materials produced in the chemical reaction, while "reaction
mixture" denotes the mixture of reaction product and unreacted
starting materials obtained).
Ferrisilicate molecular sieves of this invention may also be used
for other catalytic reactions, particularly iron oxide-catalyzed
catalytic reactions. In particular, the molecular sieves of this
invention may be used as dehydrogenation and oxidation catalysts,
e.g. in the oxidation of butene to butadiene, oxidation of olefins
to alkene acetate esters, dehydrogenation of ethylbenzene to styrene,
and oxydehydrogenation of isobutyric acid to methyacrylic acid or
a lower alkyl ester thereof. Other reactions include decomposition
of 2-butanol. Catalysts of this invention can also be used by hydrogenation
catalysts for liquification of coal.
A major advantage of the molecular sieves of this invention is
their ability to catalyze direct formation of hydrocarbons, particularly
gasoline hydrocarbons from carbon monoxide-hydrogen mixtures without
the necessity of producing and recovering methanol as an intermediate.
Furthermore, the catalysts of this invention have good selectivity
for this reaction, which is believed due to the dispersion of iron
oxides on the internal surfaces (i.e. the pores) of the molecular
sieve with only a comparatively small amount of iron oxides on the
external surface. (Iron oxides dispersed on external surfaces tend
to catalyze reactions non-selectively, while iron oxides dispersed
on internal surfaces promote selective reactions). Molecular sieves
of this invention are also selective catalysts for other iron oxide-catalyzed
reactions.
Molecular sieves of this invention are also useful as isomerization
and cracking catalysts. For example, they may be used for isomerization
of straight chain alkanes to branch chain alkanes, e.g. n-hexane
to isohexane. They are also useful as cracking catalysts for cracking
heavy hydrocarbon fractions to produce gasoline range hydrocarbons.
The preferred molecular sieves for isomerization and cracking are
those in which a major portion, e.g. from about 50 to about 100
percent of total iron, remains in the framework after thermal treatment.
This invention will be further described with reference to the
examples which follow. The SiO.sub.2 /Fe.sub.2 O.sub.3 ratio given
in each example refers to overall mole ratio.
Samples of both as synthesized and thermally treated molecular
sieves were analyzed according to the procedures indicated below.
The x-ray powder defraction data were obtained using a Phillips
X-Ray Difractometer (Ni filtered Cu K-alpha, 2-theta range 5.degree.-40.degree.).
For comparison, known samples of the aluminosilicate ZSM-5 were
used. Chemical analysis of the samples was done by atomic absorption
spectroscopy. SEM analysis was conducted using a Cambridge Scanning
Electron Microscope with Trace Northern X-ray detector.
Mossbauer spectra were measured using a conventional constant acceleration
spectrometer, using a source of .sup.57 CoinRh. Spectra were recorded
in 0 field at room temperature (RT) or liquid nitrogen temperature
(LNT; 77.degree. K.) and at 4.2.degree. K. with either a low magnetic
field (0.05 T) or a high field (8 T) applied parallel to the gamma
ray direction. All isometer shifts are quoted relative to an absorber
of metallic iron at room temperature. Fits at low field were performed
using a standard lease square fitting routine. When fitting quadruple
doublets, both peaks were constrained to have the same line with
and intensity. Hyperfine split sextats were fit to 3 doublets and
the hyperfine field estimated splitting of the outermost line. In
all cases, it was found that the quadropole splitting (magnetic)
was negative 0 mms/second. Average isomer shifts were calculated
directly from the raw data by summation or from the fits. High field
fits of paramagnetic spectra were obtained using a spin Hamiltonian
simulation program.
Mossbauer spectral analysis disclose: the distribution between
framework and non-framework iron, and the approximate particle size
of the latter. Framework iron is in the form of tetrahedra, which
in the Mossbauer spectra are indicated by a singlet or single peak
(when absorption is measured against velocity in mm/sec) regardless
of measurement temperature, with an average isomer shift (IS) no
more than 0.3 mm/sec at room temperature. A doublet in the Mossbauer
spectrum indicates a mixture of tetrahedral and octahedral iron,
with the latter dispersed in a fine state of subdivision (no larger
than 0.6 nanometers). A sexlet or six-line spectrum indicates the
presence of large agglomerates (larger than 0.6 nanometers) of iron
oxides. The Mossbauer spectrum indicates the presence of all iron
present, regardless of its location.
Color of all samples was observed. All as synthesized samples were
white, indicating that all or nearly all of the iron is in the framework
at this stage. Thermally treated samples range from off-white to
brown in color. Any discoloration indicates that at least part of
the iron is present outside the framework as iron oxides. Color
furnishes a qualitative indication as to the presence or absence
of non-framework iron.
All thermally treated samples were base exchanged with the dilute
potassium hydroxide to pH 8.0 in order to obtain base exchange capacity.
Both the acidity and the base exchange capacity diminish as the
amount of framework iron decreases. Therefore, observed base exchange
capacity furnishes a confirmation of the amount of framework iron
as determined by atomic absorption spectroscopy.
EXAMPLE 1
As-synthesized ferrisilicate molecular sieve. SiO.sub.2 Fe.sub.2
O.sub.3 =98
100 g N-brand Silica (PQ Corp., Valley Forge, PA, Na.sub.2 SiO.sub.3.5H.sub.2
O) in 100 g H.sub.2 O, is added to a solution containing 4.16 g
Fe(NO.sub.3).sub.3.9H.sub.2 O dissolved in 50 g H.sub.2 O. The pH
is adjusted to be strongly acidic with 7.5 g H.sub.2 SO.sub.4 (96
percent). To the resulting pale lemon colored gel is added 12 g
tetrapropylammonium bromide (Aldrich Chemical) in 50 g H.sub.2 O.
After vigorous agitation the mixture is placed in a stainless steel
autoclave, sealed and heated under its own pressure at 170.degree.
C. for 2 to 5 days, without stirring. The resulting white solid
is filtered, washed with water and dried at 100.degree. C. X-ray
powder diffraction confirms the formation of the ZSM-5 structure.
The material contains almost all of its iron in the framework of
the molecular sieve, as shown by Mossbauer spectroscopy.
EXAMPLE 2
Thermally modified ferrisilicate molecular sieve, SiO.sub.2 /Fe.sub.
2 O.sub.3 =98
The sample from Example 1 is heated in air at 110.degree. C. for
three hours. It is then carefully calcined in dry air in two stages.
In the first stage it is heated in dry air at 300.degree. C. for
2-3 hours followed by a second stage, where it is calcined in dry
air at 600.degree. C. for 18 hours. The sample obtained was then
ammonium exchanged using excess 1M ammonium nitrate solution at
65.degree. C. for two hours. The resulting solid sample is named
as sample A for future reference herein. A portion of the sample
A is heated at 110.degree. C. for two hours. It is then heated in
dry air at 300.degree. C. for three hours and calcined in dry air
at 600.degree. C. for 18 hours. The sample is then potassium exchanged
by titrating it with dilute KOH to a pH of 8.0. The sample is finally
washed with water, filtered and air dried at 110.degree. C. This
material contains 71 percent of the iron in the framework of the
molecular sieve. The remaining 29 percent of the iron is very finely
dispersed throughout the molecular sieve, including the inside of
the pores.
EXAMPLE 3
Hydrothermally modified ferrisilicate molecular sieve under mild
conditions, SiO.sub.2 /Fe.sub.2 O.sub.3 =98
A portion of the sample A (of Example 2) is hydrothermally treated
using steam at 650.degree. C. for one hour. The sample is then potassium
exchanged by titrating it with dilute KOH to a pH of 8.0. The resulting
sample is washed with water, filtered and air dried at 110.degree.
C. This material contains 35 percent of the iron in the framework
of the molecular sieve. The remaining 60 percent of the iron is
outside the framework and is present inside and outside the pores
of the molecular sieve.
EXAMPLE 4
Hydrothermally modified ferrisilicate molecular sieve under severe
conditions. SiO.sub.2 /Fe.sub.2 O.sub.3 =98
A portion of the sample A (of Example 2) is hydrothermally treated
using steam at 600.degree. C. for 4 hours. The sample is then potassium
exchanged by titrating it with dilute KOH to a pH of 8.0. The resulting
sample is washed with water, filtered and air dried at 110.degree.
C. This material contains only 20 percent of the total iron in the
framework of the molecular sieve. The remaining 80 percent of the
iron is outside the framework and is present inside and outside
the pores of the molecular sieves.
EXAMPLE 5
As synthesized molecular sieves; SiO.sub.2 /Fe.sub.2 O.sub.3 ranging
from 20 to 200 (General Procedure)
The iron containing reagent (Fe(NO.sub.3).sub.3. 9H.sub.2 O, or
FeCl.sub.3.H.sub.2 O) Fisher Reagent Grade) was dissolved in 100
g H.sub.2 O. The solution, was acidified with 16 g H.sub.2 SO.sub.4
(96 percent) and 200 g N-brand silica (PQ Corp. Na.sub.2 SiO.sub.3.5H.sub.2
O) in 200 g H.sub.2 O was added to this fresh solution. Immediate
formation of a pale yellow gel was observed. To the gel was added
24 g tetrapropylammonium bromide (TPABr Aldrich Reagent Grade) in
40 g H.sub.2 O. The gel was heated in a stirred autoclave (Autoclave
Engineers 1 dm.sup.3 capacity) at 170.degree. C. for 3 days under
autogeneous pressure. The resulting white solid was filtered, washed
and dried at 100.degree. C. X-ray powder diffraction confirmed the
presence of highly crystalline ferrisilicate with the zeolite ZSM-5
structure. Atomic absorption confirms S10.sub.2 /Fe.sub.2 O.sub.3
ratio.
FeZSM-5 (20): To 3.4 g, FeCl.sub.3.H.sub.2 O in 50 g H.sub.2 O
with 4.5 g H.sub.2 SO.sub.4 is added 50 g N-brand silica in 50 g
H.sub.2 O. to the resulting gel was added 6.3 g TPABr in 10 H.sub.2
O.
FeZSM-5 (50): To 15 g Fe(NO.sub.3).sub.3.9H.sub.2 O in 100 g H.sub.2
O and 16 g H.sub.2 SO.sub.4 was added 200 g N-brank silica in 200
g H.sub.2 O. After precipitates of the gel, 24 g TPABr in 40 g H.sub.2
O was added.
FeZSM-5 (90): To 4.16 g Fe (NO.sub.3).sub.3.9H.sub.2 O dissolved
in 50 g H.sub.2 O and acidified with 7.5 g H.sub.2 SO.sub.4 was
added 100 g N-brank silicate in 100 g H.sub.2 O. After formation
of the milky gel, 12 g TPABr in 25 g H.sub.2 O was added.
FeZSM-5 (171): to 3.5 g Fe(NO.sub.3).sub.3.9H.sub.2 O dissolved
in 100 g H.sub.2 O and 16 g H.sub.2 O.sub.4 was added 200 g N-brand
silica in 200 g H.sub.2 O. After the white milly gel formed, 24
g TPABr in 50 g H.sub.2 O was added.
Silicalite: To a 15 g H.sub.2 SO.sub.4 in 75 g H.sub.2 O solution
was added 150 g N-brank silicate in 150 g H.sub.2 O to this was
added 18 g TPA Br in 100 g H.sub.2 O. (This is included for comparison).
EXAMPLE 6
Thermally modified ferrisilicate molecular sieve; SiO.sub.2 /Fe.sub.2
O.sub.3 =20
The as-synthesized sample of ferrisilicate molecular sieve with
SiO.sub.2 /Fe.sub.2 O.sub.3 of 20 is dried in air at 110.degree.
C. for three hours. It is then heated in dry nitrogen for two hours
at 145.degree. C., followed by heating at 500.degree. C. for 8-10
hours. It is cooled to 145.degree. C. in dry nitrogen and then switched
to dry air at 145.degree. C. for 2 hours. Finally, the sample is
calcined in dry air at 500.degree. C. for 4 to 5 hours.
An ammonium exchanged form of the sample was obtained by ammonium
exchange using excess 1M ammonium nitrate solution at 65.degree.
C. for two hours.
A portion of this sample is heated at 110.degree. C. for two hours,
then heated in dry air at 145.degree. C. for two hours and finally
calcined in dry air at 500.degree. C. for 5 hours. The sample is
then potassium exchanged by titrating it with dilute KOH to a pH
of 8.0.
EXAMPLE 7
Hydrothermally modified ferrisilicate molecular sieve under very
mold conditions SiO.sub.2 /Fe.sub.2 O.sub.3 =20
A portion of the ammonium exchanged sample (of Example 6) is hydrothermally
treated using steam at 300.degree. C. for one to four hours. The
sample is then potassium exchanged by titrating it with dilute KOH
to a pH of 8.0. The resulting sample is washed with water, filtered
and air dried at 110.degree. C.
EXAMPLE 8
Hydrothermally modified ferrisilicate molecular sieve under mild
conditions. SiO.sub.2 /Fe.sub.2 O.sub.3 =20
A portion of the ammonium exchanged sample (of Example 6) is hydrothermally
treated using steam at 550.degree. C. for one hour. The sample is
then potassium exchanged by titrating it with dilute KOH to a pH
of 8.0. The resulting sample is washed with water, filtered and
air dried at 110.degree. C.
EXAMPLE 9
Hydrothermally modified ferrisilicate molecular sieve under moderate
conditions; SiO.sub.2 /F.sub.2 O.sub.3 =20
Two portions of the ammonium exchanged samples (of Example 6) hydrothermally
treated using steam at 550.degree. C. for two and four hours respectively.
The two portions are then potassium exchanged separately by titrating
them with dilute KOH to a pH of 8.0. The resulting samples are washed
with water, filtered and air dried at 110.degree. C.
EXAMPLE 10
Hydrothermally modified ferrisilicate molecular sieves under moderate
conditions for long periods of time; SiO.sub.2 /Fe.sub.2 O.sub.3
=20
Portions of the ammonium exchanged samples of Example 6 are hydrothermally
treated using steam at 550.degree. C. for 8 to 72 hours. The samples
are then potassium exchanged by titrating them separately with dilute
KOH to a pH of 8.0. The resulting samples are washed with water,
filtered and air dried at 110.degree. C.
EXAMPLE 11
Hydrothermally modified ferrisilicate molecular sieves under severe
conditions; SiO.sub.2 /Fe.sub.2 O.sub.3 =20
Portions of the ammonium exchanged samples of Example 6 are hydrothermally
treated using steam at 600.degree. to 700.degree. C. for one to
8 hours. The samples are then potassium exchanged by titrating them
separately with dilute KOH to a pH of 8.0. The resulting samples
are washed with water, filtered and air dried at 110.degree. C.
EXAMPLE 12
Thermally modified ferrisilicate molecular sieve; SiO.sub.2 /Fe.sub.2
O.sub.3 =50
The as synthesized sample of ferrisilicate molecular sieve with
SiO.sub.2 /Fe.sub.2 O.sub.3 of 50 is heated in air at 110.degree.
C. for three hours. It is then heated in dry nitrogen for two hours
at 145.degree. C. followed by heating at 500.degree. C. in dry nitrogen
for 8-10 hours. It is cooled to 145.degree. C. in dry nitrogen and
then switched to dry air at 145.degree. C. for 2 hours. Finally,
the sample is calcined in dry air at 500.degree. C. for 4 to 5 hours.
An ammonium exchanged form of the sample was obtained by ammonium
exchange using excess 1M ammonium nitrate solution at 65.degree.
C. for two hours.
For hydrothermal modification of the material, an ammonium exchanged
form of the sample is obtained by ammonium exchanged using excess
1M ammonium nitrate solution at 65.degree. C. for two hours.
A portion of this sample is heated at 110.degree. C. for two hours,
then heated in dry air at 145.degree. C. for two hours and finally
calcined in dry air at 500.degree. C. for 5 hours. The sample is
then potassium exchanged by titrating it with dilute KOH to a pH
of 8.0.
EXAMPLE 13
Hydrothermally modified ferrisilicate molecular sieve under very
mild conditions; SiO.sub.2 /Fe.sub.2 O.sub.3 =50
A portion of the ammonium exchanged sample of Example 12 is hydrothermally
treated using steam at 300.degree. C. for one to four hours. The
sample is then potassium exchanged by titrating it with dilute KOH
to a pH of 8.0. The resulting sample is washed with water, filtered
and air dried at 110.degree. C.
EXAMPLE 14
Hydrothermally modified ferrisilicate molecular sieve under mild
conditions; SiO.sub.2 /Fe.sub.2 O.sub.3 =50
A portion of the ammonium exchanged sample of Example 12 is hydrothermally
treated using steam at 550.degree. C. for one hour. The sample is
then potassium exchanged by titrating it with dilute KOH to a pH
of 8.0. The resulting sample is washed with water, filtered and
air dried at 110.degree. C.
EXAMPLE 15
Hydrothermally modified ferrisilicate molecular sieve under moderate
conditions; SiO.sub.2 /Fe.sub.2 O.sub.3 =50
Two portions of the ammonium exchanged sample of Example 12 is
hydrothermally treated using steam at 550.degree. C. for two and
four hours respectively. The two portions are potassium exchanged
separately by titrating them with dilute KOH to a pH of 8.0. The
resulting samples are washed with water, filtered and air dried
at 110.degree. C.
EXAMPLE 16
Hydrothermally modified ferrisilicate molecular sieves under moderate
conditions for long periods of time; SiO.sub.2 /Fe.sub.2 O.sub.3
=50
Portions of the ammonium exchanged samples of Example 12 are hydrothermally
treated using steam at 550.degree. C. for 8 to 72 hours. The samples
are then potassium exchanged by titrating them separately with dilute
KOH to a pH of 8.0. The resulting samples are washed with water,
filtered and air dried at 110.degree. C.
EXAMPLE 17
Hydrothermally modified ferrisilicate molecular sieves under severe
conditions; SiO.sub.2 /Fe.sub.2 O.sub.3 =50
Portions of the ammonium exchanged samples of Example 12 are hydrothermally
treated using steam at 600.degree. to 700.degree. C. for one to
8 hours. The samples are then potassium exchanged by titrating them
separately with dilute KOH to a pH of 8.0. The resulting samples
are washed with water, filtered and air dried at 110.degree. C.
EXAMPLE 18
Thermally modified ferrisilicate molecular sieve; SiO.sub.2 /Fe.sub.2
O.sub.3 =90
The as-synthesized sample of ferrisilicate molecular sieve with
SiO.sub.2 /Fe.sub.2 O.sub.3 of 90 is heated in air at 110.degree.
C. for three hours. It is then heated in dry nitrogen for two hours
at 145.degree. C. followed by heating it at 500.degree. C. in dry
nitrogen for 8-10 hours. It is cooled to 145.degree. C. in dry nitrogen
and then switched to dry air at 145.degree. C. for 2 hours. Finally,
the sample is calcined in dry air at 500.degree. C. for 4 to 5 hours.
For hydrothermal modification of the material, an ammonium exchanged
form of the sample is obtained by ammonium exchange using excess
1M ammonium nitrate solution at 65.degree. C. for two hours.
A portion of this sample is heated at 110.degree. C. for two hours,
then heated in dry air at 145.degree. C. for two hours and finally
calcined in dry air at 500.degree. C. for 5 hours. The sample is
then potassium exchanged by titrating it with dilute KOH to a pH
of 8.0.
EXAMPLE 19
Hydrothermally modified ferrisilicate molecular sieve under very
mild conditions; SiO.sub.2 /Fe.sub.2 O.sub.3 =90
A portion of the ammonium exchanged sample of Example 18 is hydrothermally
treated using steam at 300.degree. C. for one to four hours. The
sample is then potassium exchanged by titrating it with dilute KOH
to a pH of 8.0. The resulting sample is washed with water, filtered
and air dried at 110.degree. C.
EXAMPLE 20
Hydrothermally modified ferrisilicate molecular sieve under mild
conditions. SiO.sub.2 /Fe.sub.2 O.sub.3
A portion of the ammonium exchanged sample of Example 18 is hydrothermally
treated using steam at 550.degree. C. for one hour. The sample is
then potassium exchanged by titrating it with dilute KOH to a pH
of 8.0. The resulting sample is washed with water, filtered and
air dried at 110.degree. C.
EXAMPLE 21
Hydrothermally modified ferrisilicate molecular sieve under moderate
conditions; SiO.sub.2 /Fe.sub.2 O.sub.3 =90
Two portions of the ammonium exchanged sample of Example 18 are
hydrothermally treated using steam at 550.degree. C. for two and
four respectively. The two portions are potassium exchanged separately
by titrating them with dilute KOH to a pH of 8.0. The resulting
samples are washed with water, filtered and air dried at 110.degree.
C.
EXAMPLE 22
Hydrothermally modified ferrisilicate molecular sieves under moderate
conditions for long periods of time; SiO.sub.2 /Fe.sub.2 O.sub.3
=90
Portions of the ammonium exchanges samples of Example 18 are hydrothermally
treated using steam at 550.degree. C. for 8 to 72 hours. The samples
are then potassium exchanged by titrating them separately with dilute
KOH to a pH of 8.0. The resulting samples are washed with water,
filtered and air dried at 110.degree. C.
EXAMPLE 23
Hydrothermally modified ferrisilicate molecular sieves under severe
conditions SiO.sub.2 /Fe.sub.2 O.sub.3 =90
Portions of the ammonium exchanged samples of Example 18 are hydrothermally
treated using steam at 600.degree. to 700.degree. C. of done to
8 hours. The samples are then potassium exchanged by titrating them
sieve dilute KOH to a pH of 8.0. The resulting samples are washed
with water, filtered and air dried at 110.degree. C.
EXAMPLE 24
Thermally modified ferrisilicate molecular sieve; SiO.sub.2 /Fe.sub.2
O.sub.3 =200
The as-synthesized sample of ferrisilicate molecular sieve with
SiO.sub.2 /Fe.sub.2 O.sub.3 of 200 is heated in air at 110.degree.
C. for three hours. It is then heated in dry nitrogen for two hours
at 145.degree. C. followed by heating it at 500.degree. C. in dry
nitrogen for 8 to 10 hours. It is cooled to 145.degree. C. in dry
nitrogen and then switched to dry air at 145.degree. C. for 2 hours.
Finally, the sample is calcined in dry air at 500.degree. C. for
4 to 5 hours.
For hydrothermal modification of the material, an ammonium exchanged
form of the sample was obtained by ammonium exchange using excess
1M ammonium nitrate solution at 65.degree. C. for two hours.
A portion of this sample is heated at 110.degree. C. for two hours,
then heated in dry air at 145.degree. C. for two hours and finally
calcined in dry air at 500.degree. C for 5 hours. The sample is
then potassium exchanged by titrating it with dilute KOH to a pH
of 8.0.
EXAMPLE 25
Hydrothermally modified ferrisilicate molecular sieve under very
mild conditions; SiO/Fe.sub.2 O.sub.3 =200
A portion of the ammonium exchanged sample of Example 24 is hydrothermally
treated using steam at 300.degree. C. for one to four hours. The
sample is then potassium exchanged by titrating it with dilute KOH
to a pH of 8.0. The resulting sample is washed with water, filtered
and air dried at 110.degree. C.
EXAMPLE 26
Hydrothermally modified ferrisilicate molecular sieve under mild
conditions; SiO.sub.2 /Fe.sub.2 O.sub.3 =200
A portion of the ammonium exchanged sample of Example 24 is hydrothermally
treated using steam at 550.degree. C. for one hour. The sample is
then potassium exchanged by titrating it with dilute KOH to a pH
of 8.0. The resulting sample is washed with water, filtered and
air dried at 110.degree. C.
EXAMPLE 27
Hydrothermally modified ferrisilicate molecular sieve under moderate
conditions SiO.sub.2 /Fe.sub.2 O.sub.3 =200
Two portions of the ammonium exchanged sample of Example 24 are
hydrothermally treated using steam at 550.degree. C. for two and
four hours respectively. The two portions are then potassium exchanged,
separately by titrating them with dilute KOH to a pH of 8.0. The
resulting samples are washed with water, filtered and air dried
at 110.degree. C.
EXAMPLE 28
Hydrothermally modified ferrisilicate molecular sieves under moderate
conditions for long periods of time; SiO.sub.2 /Fe.sub.2 O.sub.3
=200
Portions of the ammonium exchanged samples of Example 24 are hydrothermally
treated using steam at 550.degree. C. for 8 to 72 hours. The samples
are then potassium exchanged by titrating them separately with dilute
KOH to a pH of 8.0. The resulting samples are washed with water,
filtered and air dried at 110.degree. C.
EXAMPLE 29
Hydrothermally modified ferrisilicate molecular sieves under severe
conditions; SiO.sub.2 /Fe.sub.2 O.sub.3 =200
Portions of the ammonium exchanged samples of Example 24 are hydrothermally
treated using steam at 600.degree. to 700.degree. C. for one to
8 hours. The samples are then potassium exchanged by titrating them
separately with dilute KOH to a pH of 8.0. The resulting samples
are washed with water, filtered and air dried at 110.degree. C.
EXAMPLE 30
Preparation of gasoline range hydrocarbons from synthesis gas
Mixture of carbon monoxide and hydrogen (synthesis gas) are passed
over the thermally treated ferrisilicate molecular sieve of Example
12 under the following process conditions: H.sub.2 /CO ratio of
1.0 to 3.0 pressure of 15 to 30 atmospheres, temperature of 300.degree.
C. to 400.degree., flow rates of 8 to 60 cc/min. The weight of catalyst
is approximately 0.4 grams. The products include gasoline range
hydrocarbons. |