Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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ZEOLITE ACTIVATION PROCESS
Filed of the Invention
This invention relates to a method for increasing
the catalytic activity of crystalline zeolites by treatment
with water.
The Prior Art
Zeolite catalysts have become widely used in the processing of
petroleum and in the production of various petrochemicals. Reactions
such as cracking, hydrocracking, alkylation, dealkylation,
transalkylation, isomerization, polymerization, addition,
disproportionation and other acid catalyzed reactions may be performed
with the aid of these catalysts~ ~oth natural and synthetic zeolites are
known to be active for reactions of these kinds.
Recently, synthetic zeolites containing hish proportions of
silica relative to alumina have been developed and zeolites of this kind
have shown themselves to be useful. U.S. Patent No. 3,702,886 to
Argauer et al discloses a class of crystalline aluminosilicates
designated ZSM-5 which have highly advantageous properties. U.5. Patent
No. 3,941,871 and its Reissue No. ~9,948 to Dwyer et al disclose
crystalline organosilicates which exhibit a structure, as evide~ced by
X~ray diffraction pattern, similar to that of Z5M-5, but with high ratios
of silica relative to alumina. Materials of this kind are stated to
exhibit low aging rates and to have low coke making properties when used
in hydrocarbon processing.
Various treatments have been proposed in the past for modifying
the activity of the zeolites, either by reducing it when too active or by
increasing it when insufficient. One such treatment has been steaming
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and in the past .it has generally been used to decrease the activity of
the zeolite, as reported in "Fluid Catalytic Cracking with Zeolite
Catalysts", Venuto and Habib7 Marcel Dekker Inc., N.Y., N.Y. 1979.
The reduction of activity is not, ho~ever~ necessarily
undesirable because it may in certain circumstances be accompanied by an
improvement in other characteristics of the zeolite, for example,
resistance to aging. This fact has been exploited in certain processes,
for example, in the alkylation process described in U.S. Patent NQ.
4,016,218, which employs a zeolite catalyst which has been subjected to a
lo prior thermal treatment either in an iner~ atmosphere or by steaming, to
reduce its activity. The deactivation caused by the steam becomes more
pronounced at higher temperatures and with longer reaction times.
It has also been found that steaming may in certain instances
have beneficial effects upon the catalyst. U.S. Patent No. 3,257,310,
for example, describes a method for preparing a cracking catalyst of high
activity and selectivity by steaming a zeolite for at least two hours at `
a speci~ied temperature. The zeolites described in this patent include
natural zeolites such as mordenite and ~aujasite and synthetic zeolites
such as zeolites X, Y and L.
U.S. Patents 4,149,960 and ~,150,062 describe the use of water
in the feedstock during operation to reduce coking and aging rates. U.S.
Patent No. 3,546,1ûO describes a method for maintaining the selectivity
o~ a hydrocracking catalyst by restricting the partial pressure of water
during the hydrocracking operation~
U.S. Patent No. 3,493,519 describes a method of producing
hydrothermally stable cracking catalysts by calcining zeolite-Y in the
presence of steam, a process which was theorized to cause lattice
aluminum de~ects which, after 5llhse~upnt treatment by base exchange with
ammonium salts, chelation and calcination in air produced the desired
3~ highly active product.
U.S. Patent No. 3,493,49û describes a method for restoring the
activity to used catalyst by controlled treatment with anionic reagents
including water at high temperatures, even with catalysts which had
initially been steamed to reduce their level of cracking activity, such
as zeolites X and Y.
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U.S. Patent No. 3,758,403 describes a method for cracking
hydrocarbon feedstocks using a mixture of zeolites including a ZSM~5 type
zeolite and a large pore zPolite such as zeolites X, Y or L or
~aujasite. The selectivity of the catalyst is said to be improved by
treatment with steam ~hich if desired, may be carried out in the cracking
unit itself.
Summary of the Invention
It has now been found that the degree to which the activity of
the zeolites can be enhanced by steaming is increased if the zeolite is
lo steamed in the presence of a binder for the zeolite. The binder
preferably used is alumina, either on its own or in the presence of other
porous matrix materials. It is believed that the steaming produces
additional stable active sites in the zeolite and that these additional
sites are responsible for the observed increase in activity.
Description of Preferred Fmhodi~ents
The zeolites which may be used in the present process have a
silica to alumina ratio of at least 12 and preferably much higher. It
has been found that the degree of enhancement in the activity o~ the
zeolite becomes greater as the silica to alumina ratio of the zeolite
increases. ~ccoxdingly, the higher silica to alumina ratios are
preferred and generally ratios of above 100:1 are preferred. I~
possible, the ratio should exceed 500:1 and we have found that marked
enhancement of activity is obtained at ratios over 1200:1, for example,
1600:1. The use of ratios even higher than this is contemplated, going
as high as 3200:1 or even higher. The silica to alumina ratio may be
determined by conventional analysis. The ratio represents, as closely as
possible, the ratio in the rigid anionic framework of the zeolite crystal
i.e. the structural or frame..^oIk silica:alumina ratio and excludes
materials such as aluminum in binder or in another form within the
channels of the zeolite. The ratio may be determined by conventional
methods such as ammonia desorption/rGA.
The zeolites are also characterized by their Constraint Index,
which is to be within the approximate range of 1 to 12. The Constraint
Index is a measure of the constraint imposed by the crystal structure of
the zeolite on the access by molecules of dif~ering sizes to the internal
structure of the crystal. A measure o~ such constraint is desired in
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order to obtain the desired conversions. It is sometimes possible to
judge from a known crystal structure whether constrained access of this
kind~exists. For example, if the only pore windows in a crystal are
~ormed by 8-membered rings of silicon and aluminum atoms, molecules with
S a cross-section larger than normal hexane will be excluded and the
ze~lite is not of the desired type. Windows of 10-mcmbered rings are
preferred, alth~ugh, in some instances, excessive puckering or pore
blockage may render these zeolites ineffective. Twelve-membered rings do
not generally appear to o~fer sufficient constraint to produce the
advantageous conversions, although puckered structures exist such as TMA
offretite which is a known ef~ective zeolite. Also, structures can be
Conceived, due to pore blockage or ot~er cause, that may be suitable.
The Constraint Index provides a convenien~ indication of the
extent to which a zeolite provides this restrained access. A method for
detenmining Constralnt Index, together with values of the Index for
exemplary zeolites, is described in U.S. Patent No. 4,016,218 and J.
Catalysis 67, 218-222 (1981) to which reference is made for details of
the me~hod. ~ec~llse Constraint Index is a characteristic which is
dependent ~pon the structure of the zeolite but is measured by means of a
test which is dependent upon the cracking or acid activity of the
zeolite, the test candidate should be representative of the zeolite in
structure and have adequate cracking activity. Cracking activity may be
varied by known artifices such as steaming, base exchange or variation of
the silica:alumina ratio.
Zeolites which may be treated by the present activation process
include ZSM-5, ZSM-ll, ZSM-12, ZSM-23, ZSM-35, ZSM-3~ and other similar
materials having the appropriate characteristics. ZSM-5 is described in
U.S. Patent 3,702,886; ZSM-ll in U.S. Patent 3,709,979; ZS~-12 in U.S.
Patent 3,832,449; ZSM-23 in U.S. Patent 4,076,842; ZSM-35 in U.S. Patent
4!016,~45 and ZSM-38 in U.S. Patent 4,046,859. T~ese are the preferred
zeolites and of these ZSM-5 is particularly preferred.
Highly siliceous forms of ZSM~5 are described in
U.S. Patent Re. 29~948, highly siliceous forms of ZSM-11 in
Canadian Patents 1,139,733 and 1,139,732 and highly siliceous
forms of ZSM-12 in Canadian Patent 1,139,731.
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When the zeolites are prepared in the presence of organic
cations they are initially catalytically inactive, possibly because the
intracrystalline free space is occupied by organic cations from the
forming solution. They may be activated by heating in an inert
atmosphere at 1000F (538C) for one hour, for exar~le, followed by base
exchange with arnmoni~m salts followed by calcination at 1000f (538C) in
air. The preser~e of organic cation in the forr~ng solution may not be
absolutely essential to the ~orrnation of the zeolite but these cations do
~opear to favor the formation o~ the desircd crystal structure.
Natural zeolites may sometimes be converted to the desired
zeollte by ~arious activation prrxedures and other treatments such as
base exchange, stearnir~, alumina extraction and calcination. Natural
minerals which may be so treated include ferrierite, brewsterite,
stilbite, dachiardite, epistilbi~e, heulandite, and clir~tilolite.
According to a preferred aspect of the present invention, the
preferred zeolites have a crystal framework density, in the dry hydrogen
form, not substantially below about 1.6 g.cm~3. The dry density for
known structures may be calculated frotn the number of silicon plus
aluminum atoms per 100 cubic Angstrams, as given, e.g., on page 19 of the
article on Zeolite Structure by W. M. Meier, includir~ in "Proceedings of
the Conference on Molecular Sieves, London, April 1967~, published by the
Society of Chemical Ir~ustry, London; 1968. When the crystal structure
is unknown, the crystal fr.~~wolk density may be deterrnined by classical
pyknometer techniques. For exar~le, it may be determined by irnmersing
~5 the dry hydrogen form of the zeolite in an organic solvent which is not
sorbed by the crystal.
Crystal framework densities of some typical zeol ites
are disclosed in European Pa~ent Application No. 34,444,
published August 26, 1981.
When it has been synthesized in the alkali rnetal form, the
zeolite may be converted to the hydrogen form, generally by intermediate
formation of the ammonium form by ammonium ion exchange and calcination
of ammonium form to yield the hydrogen form. In addition to the hydrogen
form, other forms of the zeolite wherein the original alkali metal has
3S been reduced to less than about 1.5 percent by weight may be used. Thus,
the original alkali metal of the zeolite or introduced hydrogen cations
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may be replaced by ion exchange with other suitable ions of Groups IB to
VIII of the Periodic Table, including, by way of example, nickel,
cadmium, copper, zinc, palladium, calcium or rare earth metals.
It is normally preferred to use zeolites of large crystal size,
that is, of about O.l microns or larger as opposed to small crystal
zeolites of about 0.02 to O.û5 micron crystal size because the large
crystal zeolites respond better to steaming.
The zeolite is composited with an activating metal oxide which
is capable of activating the zeolite by the creation of ad~itional active
lo sites when the zeolite/oxide composite is steamed. The oxide will
normally act as a binder for the zeolite. The preferred binder is
alumina, preferably in the form of alpha-alumina or alpha alumina
monohydrate but other binders may also be used either on their own or in
combination with alumina, for example, silica-alumina~ silica-zirconia,
LS silica-thoria, silica-berylia, silica-titania as well as ternary
compositions, such as silica-alumina-thoria, silica- alumina~zirconia,
silica-alumina-magnesia or silica-magnesia-ziIconia. Other metal oxides
which may be employed include titania, zirconia and chromia. Simple
experiment may be employed to determine other useful materials.
The relative proportions of zeolite and binder will generally be
adjusted in accordance with the silica to alumina ratio o~ the zeolite~
with the zeolites of higher silica to alumina ratio being able to bene~it
more from a larger proportion of binder than those with a lower ratio.
In general, the amount of binder will be from 10 to 90 percent by weight
of the combined zeolite and binder, preferably 20 to 80 percent by
weight. A zeolite with a silica to alumina ratio of about 1600:1 can
usefully be composited with 25 to 50 percent by weight of alumina binder.
The zeolite is composited with the binder by intimately grinding
the two materials together, in the presence of water, after which the
mixture is ~ormed .into suitable partlcles and dried. It has been found
that the desired enhancement of activity does not occur if the zeolite
and binder are simply mixed together instead of being intimately ground
as described above. The finely ground mixture of zeolite, binder and
water may conveniently be formed into particles by extrusion using an
extrusion press or, alternatively, other shaping methods may be used such
as pelletizing or pressing. The amount of water is chosen as to give a
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mixture which has a satisfactory consistency for the forming step. The
zeolite may contain sufficient occlu~ed water or sufficient water may be
present in the binder.
The zeolite may be treated to convert it to the desired form
either before or after it is composited with the binder. Thls, if it is
synthesized in the alkali metal form it may be converted to the hydrogen
or another catlonic form e.gO the alkal~ metal, alkaline earth metal or
ammoniu~ form before or after comoositing with the binder. If conversion
entails more than one step the requisite steps may, if desired, be
carried out at different stages of the process, some before compositing
and some a~ter. Generally, however, the zeoli~e should be at least
partly in the hydrogen form during the steaming or, alternatively, in a
form which will be wholly or partly converted to the hydrogen form under
the conditions employe~ during the steaming e.g. the ammonium form or the
alkylammonium form.
After the zeolite/binder composite has been formed it is
subjected to steaming. During this step, the cr~osite is held in an
atmosphere entirely or partly of steam at an elevated temperature.
Generally, it is preferred to operate with an atmosphere of 100% steam
although partial steam atmosp~eres may also be used with some 1QSS of
effectiveness. If a gas other than steam is present it should be an
inert gas such as nitregen. The steaming is generally carried out at a
temperature from 200 to 500C, pre~erably from 300 to 450C. Good
results have been obtained at about 400 to 425C. ThR pressure during
steaming will nonmally be carried out at atmospheric or under
s~eratmospheric pressure, generally in the range of 100 to 500 kPa,
pre~erably from 100 to 200 kPa. The steaming should generally be
continued for at least one hour and usually durations of 12 to 48 hours
will be preferred.
3~1 The steam may be produced in-situ, for example, by the
dehydration of alcohols such as methanol, ethanol, propanol, n-butanol or
pentanol to produce the steam, with olefins as a by-product or by the
combustion of hydrocarbons to produce carbon oxides and steam.
The steaming may be carried out under conditions
such as those described in European Application 34,444,
published August 26, 1981.
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The activity of the catalyst is measured in terms of its alpha
value. The alpha value reflects the relative activity of the catalyst
with respect to a high activity silica-alumina cracking catalyst. To
determine the alpha value, n-hexane conversion is determined at a
suitable temperature between about 550F to lOOO~F (288 to 538U~),
preferably at 1000F (538C). Conversion is varied by variation in space
velocity such that a conversion level of up to about 60 percent of
n-hexane is obtained and converted to a rate constant per unit volume of
zeolite and compared with that of silica-alumina catalyst which is
nor~ali?ed to a reference activity of 1000F (538C). The catalytic
activity of the catalyst is then expressed as multiple of this standard,
i.e. the silica-alumina standard. The silica-alumina reference catalyst
contains about 10 weight percent A1203 and the remainder SiO2.
This method o~ determining alpha, modified as described above, is
L5 described in the Journal of Catalysis, Vol. VI, pages 278-287, 1966, to which reference is made for further details of the method.
The extent of the activation produced by the present method is
notable. Increases of over lO0 percent in the alpha value may be
obtained with zeolites having a silica to alumina ratio of 1200:1 or
more. C~ml~ns~rate results may be obtained with other zeolites of
differing silica to alumina ratio. The enhancement in activity is
believed to be caused by the creation of additional, stable active
internal sites in the zeolite because after the steaming treatment is
complete, the Constraint Index remains consistent with that of the
original zeolite structure although the alpha value has increased
significantly. The catalyst therefore retains its original selectivity
but with an improved acid activity.
The zeolite/binder composites produced by the present method may
be used as catalysts in acid catalyzed conversion reactions of the kind
cat~lyzed by khe type of zeolite used in the method. Hydrocarbon
conversion reactions such as cracking, hydrocracking, alkylation,
dealkylation, transalkylation, isomerization, polymerization,
disproportionation and aromatization are particularly important but other
reactions such as the conversion of oxygenates such as methanol or
dimethyl ether to hydrocarbons are also of interestO The conditions
employed in these reactions will be those appropriate to the particular
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catalyst being used, having due regard to its enhanced activity. The
method is of particular utility for restoring activity to catalysts whioh
have become inactivated by exposure to water during use. For example, in
processes such as the conversion of methanol to hydrocarbons, substantial
quantities of wa~er in the form of steam are produced and they may tend
to deactivate the ca~alyst. I~ this happens, the present prccess may be
used to reac~ivate it.
The following Examples illustrate the improvement wrought by the
present invention.
Example 1
A sample of zeolite ZSM-5 in the hydrogen form and having a
structural silica:alumina ratio of 1600:1 was mulled by ball milling with
35 percent by weight ot` alpha-alumina monohydrate, adding sufficient
deionized water to form a mixture which could be conveniently mulled.
The mull was extruded into pellets (small cylinders of 1.6 mm diameter)
and the pellets air dried at 110C, precalcined in nitrogen at about
54û~C after which the zeolite was converted to the hydrogen form by
ammonium cation exchange, air drying at about 110C and calcination in
air at about 540C. The alpha value of this catalyst was 7.7.
A sample of the catalyst was contacted with 100 percent steam at
atmospheric pressure and at a temperature of 425C for 18 hours. The
steam treated product had a Constraint Index of 1.6 at 450C, consistent
with the ZSM-5 structure, and an alpha value of 17.6.
After the measurement of the alpha value had been made, the catalyst was
~5 regenerated by being heated in air to 540C. The alpha value of the
regenerated catalyst was 17.4, indicating that the activation ~as stable.
Example 2
A sample of the 1600:1 zeolite ZSM-5 of Example 1 was obtalned
in the hydrogen form by ammonium exchange of the as-synthesized zeolite,
followed by air calcination of the ammonium ZSM-5 at about 5~0C. The
bin~er-free zeolite was then treated in 100% steam at atmospheric
pressure at 425C for varying periods of time, after which the activity
of the catalyst was determined. The results are shown in the Table below.
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TABLE
Steaming Time
(Hours) Alpha-activity
6.1
6 7 2
18 7.6
7.2
44 7.0
These results, in comparison with those of Example 1, show that the
presence of the binder is necessary
for the activation to occur.
Example 3
Two parts o~ the 1600:1 zeolite ZSM-5 of Example 1 in the
hydrogen form produced by the air calcination of the ammonia form zeolite
were mixed well with one part of alpha-alumina monohydrate and the
mixture was then pelletized and steamed for 1~ hours in 100% steam at
atmospheric pressure and 425C. The alpha value of the steamed catalyst
was 6.7, showing that mere admixture of the zeolite and the binder is
insufficient for activation.
Example 4
The 1600:1 zeolite ZSM-5 of the preceding Examples in the
ammonium form was mulled with ~5% alpha alumina monohydrate by ball
milling after which the mixture was extruded into 1.6 mm cylindrical
pellets. The extruded catalyst was then dried in air at 120C,
precalcined in nitrogen at 540~C, followed by an ammonium exchange, alr
drying at 120C, air calcination and steaming for 18 hours at 425C under
atmospheric pressure. The alpha value of the steamed catalyst was 12.3,
a substantial increase over the origlnal alpha value of 6.1.
The catalyst was then steamed for an additional hour at 540C
and under atmospheric pressure, after which the alpha value was found to
be 12.1, consistent with a theoretical prediction of 12Ø
