Selective dealumination of zeolites

This invention relates to a process for selective dealumination, particularly surface dealumination of zeolites by sequential ion-exchange and calcination. The process is selective in the sense that the aluminium atoms within the pore structure of the zeolite remain virtually intact. The surface dealuminated zeolites can be used, after loading with a gallium compound, as catalysts in hydrocarbon conversion reactions.

The present invention relates to a process for selective dealumination, 
particularly surface dealumination of zeolites by sequential ion-exchange 
and calcination. 
Calcination of zeolites to achieve a degree of dealumination thereby 
modifying the activity thereof is well known. Such methods are described 
for instance in EP No. 95305 and in the artices by Kerr, G. T. , in J. 
Phys. Chem. 71, 4155 (1967) and by Scherzer, J. in J. Catalysis, 54, 
285-288 (1978). 
One of the major drawbacks of the prior art dealumination techniques is 
that they remove aluminium atoms from the entire framework of the zeolite 
i.e. both the external surface and the internal pores within the zeolite. 
Whilst removal of the aluminium atoms from the external surface is 
desirable to moderate non-shape-selective activity of the zeolite, the 
removal of aluminium atoms from within the pore structure is undesirable 
because it reduces the internal acid sites and hence a reduction in 
catalytic activity in the shape-selective environment of the zeolite 
pores. Removal of internal aluminium atoms from within the pores can also 
cause partial destruction of the zeolite pore structure which is 
detrimental to the zeolite. 
It is therefore an object of the present invention to selectively achieve 
surface dealumination of the zeolite without adversely affecting the 
internal acid sites or pore structure of the zeolite. 
Accordingly the present invention is a process for surface dealumination of 
a zeolite to improve the catalyst selectivity thereof for hydrocarbon 
conversion reactions, said zeolite containing in the framework a first set 
of ion-exchangeable cations and having a pore size which is incapable of 
allowing to pass therethrough a second set of cations said process 
comprising: 
(a) refluxing the zeolite with an aqueous solution of metal cations capable 
of (i) entering the zeolite pores and (ii) exchanging with the first set 
of cations in the framework until substantially all the first set of 
cations within the pores have been replaced by the metal cations, 
(b) refluxing the metal cation exchanged zeolite from step (a) with an 
aqueous solution of the second set of cations which are capable of thermal 
decomposition into vapourisable components until substantially all of the 
metal cations on the external surface of the zeolite have been replaced by 
the second set of cations, 
(c) calcining the second set cation exchanged zeolite from step (b) at 
elevated temperature so as to thermally decompose the second set cations, 
and 
(d) contacting the calcined zeolite from step (c) with an aqueous solution 
of a third set of cations capable of exchanging with the internal metal 
cations introduced therein by step (a) until substantially all of the 
metal cations have been replaced with protons or ions capable of giving 
rise to protons upon subsequent thermal decomposition. 
The first set of cations referred to herein are those present in the 
framework of the zeolite e.g. organic cations, inorganic cations and/or 
protons and these may be present in the internal pores, on the external 
surface or both. For instance these may be protons, alkali metal cations 
and/or template cations which are present in the as-synthesised zeolite or 
any cations which may have been accidentally or by design introduced into 
the zeolite framework prior to the commencement of the actual process of 
surface dealumination. In order to ensure that cations on the zeolite are 
of a uniform type prior to exchange with the second set of cations and in 
order to protect the internal acid sites of the zeolite during subsequent 
dealumination by calcination it is necessary to exchange the first set of 
cations on the zeolite with a metal cation. These metal cations exchange 
with all the framework cations, both external and internal, in the zeolite 
by virtue of their ability to enter readily the pores within the zeolite. 
Examples of metal cations which may be exchanged with the first set of 
cations include alkali metal cations, cations of Groups II and III of the 
Periodic Table and lower valent cations of the transition metals. Of these 
the alkali metal cations, especially sodium ions are preferred. 
The ion-exchange with the metal cations is carried out by refluxing the 
parent zeolite with an aqueous solution of the metal cation until 
substantially all the first set cations within the pores of the zeolite 
have been exchanged for the metal cations. The completion of this 
ion-exchange step can be detected by atomic absorption spectroscopy of the 
refluxing solution. 
After the metal ion-exchange step is completed, the exchanged product may, 
if desired, be washed with an aqueous solvent to remove any surplus 
unexchanged metal ions therefrom and then dried. 
The metal ion-exchanged product, with or without the washing and drying 
step is then ready for exchange with the second set of cations referred to 
above. 
The second set of cations referred to herein are those cations which are of 
a size/diameter greater than the pore size of the zeolite to be 
selectively dealuminated and which are readily decomposed thermally into 
vaporisable components. The second set cations therefore upon ion-exchange 
with the zeolite to be selectively dealuminated only exchange with the 
cations on the external surface of the zeolite. Specific examples of the 
second set of cations include those derived from organic bases such as 
trialkylamines, alkanolamines, and the like. When such cations are 
calcined after exchange with the cations on the external surface of the 
zeolite, they cause surface dealumination by known mechanisms. The 
second-set cation is preferably a tetra-alkyl ammonium ion, preferably a 
tetrapropyl ammonium ion. The second set cation exchange of the metal 
cation exchanged zeolite is carried out by refluxing the latter in an 
aqueous solution of the second set-cation. Since the second-set cation 
cannot enter the pores of the zeolite, the metal cations therein remain in 
tact during this stage and only the metal cations on the external surface 
undergo ion-exchange. The completion of the surface ion-exchange can be 
determined by atomic absorption spectroscopy of the refluxing solution. 
Upon completion of the surface ion-exchange, the resultant exchanged 
product may be optionally washed with an aqueous solvent to remove any 
surplus unexchanged second-set cations from the zeolite surface. 
Thereafter the washed product may be dried. The product from the 
second-set cation exchange step is then calcined at an elevated 
temperature so as to thermally decompose the second-set cations into 
vaporous components leaving a zeolite which has protons on the surface and 
metal cations within the pores. The calcination step is preferably carried 
out at a temperature from 450.degree. to 900.degree. C. The calcination 
may be carried out at reduced, atmospheric or elevated pressures. The 
calcination is preferably carried out in an oxidising atmosphere. Examples 
of the oxidising atmosphere that may be used include air, steam, mixtures 
thereof and these may be optionally diluted with a carrier gas such as 
nitrogen. This step results in dealumination of the zeolite surface but 
the acid sites within the pores remain unaffected. 
When the calcination step is completed, the product zeolite from that step 
is then subjected to a final treatment by contacting with a third set of 
cations capable of converting the internal metal cations, e.g. those 
within the pores of the zeolite, into protons or into ions capable of 
giving rise to protons upon subsequent thermal treatment. This may be 
achieved either by bringing the calcined product into contact with an 
aqueous solution of either an acid, preferably a mineral acid, or a salt 
readily decomposable thermally into vaporous components thereby giving 
rise to protons. An example of such a salt is ammonium nitrate. If an 
aqueous solution of a salt is used in the final ion-exchange to remove the 
internal metal cations, e.g. within the pores of the zeolite, the zeolite 
may have to be subjected to calcination to convert the cations from the 
salt within the pores into protons. 
The resultant surface dealuminated zeolite with the internal acid sites and 
pore structure intact may be used as a catalyst or a catalyst support in 
the usual manner. For instance the surface deactivated zeolite may be 
loaded e.g. with a gallium oxide catalyst by known methods and may in 
addition be bound in an inert binder e.g. a silica matrix to improve the 
physical characteristics thereof. 
The zeolites which have ion-exchangeable cations and pores which are 
incapable of allowing the second set of cations, e.g. a tetraalkyl 
ammonium ion, to pass through suitably have a silica to alumina molar 
ratio greater than 1.5:1, preferably greater than 20:1. Such zeolites are 
suitably selected from mordenites, ferrierites and zeolites of the 
MFI-type including the ZSM variety of zeolites designated by nos. 2, 3, 4, 
5, 11, 12, 35, 38 and 48. Mordenites, ZSM-5, ZSM-11, ZSM-12, ZSM-35, 
ZSM-38 and ZSM-48 are preferred, and ZSM-5 is most preferred. 
The zeolites selectively dealuminated as above are useful as catalyst or 
catalyst support for various hydrocarbon conversion reactions. 
In particular, surface dealuminated zeolites of the present invention when 
loaded with a gallium catalyst are effective in converting C.sub.2 to 
C.sub.5 hydrocarbons especially ethane selectively to aromatic 
hydrocarbons. Typically, the conversion of ethane to aromatics may be 
carried out 600.degree.-700.degree. C., 0.1-10 bar pressure and 0.1-5 
WHSV. 
The dealuminated zeolites produced by the process of the present invention 
reduce significantly the proportion of heavy polycyclic aromatics formed 
during such a reaction.

The present invention is further illustrated with reference to the 
following Examples. 
EXAMPLE 1 
A. A sample of an MFI-type zeolite containing ammonium cations (zeolite 
prepared using NH.sub.3 template according to the general process of our 
published EP 0030811-A) was exchanged with Na+ions by refluxing for a 
period of 4 hours with a 1M solution of NaCl in water (pH of the solution 
ca. 10). The process was repeated to ensure complete exchange of the 
NH.sub.4.sup.+ ions with Na.sup.+ ions. The exchanged zeolite was then 
washed with distilled water to remove any unexchanged cations and dried in 
air at 100.degree. C. 
The Na.sup.+ cations on the external surface of the zeolite were exchanged 
with tetrapropylammonium cations by refluxing with an aqueous solution of 
0.5M tetrapropylammonium (TPA) bromide for a period of 4 hours. The 
Na.sup.+ ions within the pores remained intact. 
The exchanged zeolite was then washed with distilled water to remove any 
unexchanged cations and dried in air at 100.degree. C. 
The zeolite was then calcined for 16 hours at a temperature of 600.degree. 
C. in a still air atmosphere at atmospheric pressure. The temperature was 
raised from 100.degree. C. to 600.degree. C. at a rate of 100.degree. 
C./hour. This caused vaporisation of the TPA.sup.+ ions and dealumination 
of the zeolite surface. The Na.sup.+ ions remaining within the pores of 
the surface-dealuminated zeolite were exchanged with ammonium ions by 
refluxing with 0.81M aqueous ammonium nitrate for 4 hours. The process was 
then repeated to ensure that all the Na.sup.+ ions had been exchanged. 
(The NH.sub.4.sup.+ ions were converted to protons by thermal 
decomposition during the activation procedure of the finished catalyst). 
The NH.sub.4.sup.+ exchanged, surface-dealuminated zeolite was contacted 
with 20 ml of an 0.04M aqueous solution of gallium nitrate. This solution 
was just sufficient so as to wet the zeolite. The aqueous solvent was then 
removed by drying at 130.degree. C. at sub-atmospheric pressure. 
The gallium impregnated zeolite was then mixed with an equal weight of 
LUDOX AS 40 (Registered Trade Mark, 40% colloidal silica in water) to 
obtain a slurry which was dried for 16 hours at 100.degree. C. to give a 
cake of zeolite in the inert silica binder. This was broken up and sieved 
to give catalyst particles which passed through a standard 12 mesh sieve 
but were retained by a 30 mesh sieve. 
This procedure gave a final catalyst containing 0.72 wt % Ga. 
6 ml of this catalyst (3.5g) was loaded into a vertical fixed bed reactor. 
The catalyst was contacted with nitrogen and the temperature of the 
reactor raised to 550.degree. C. The catalyst was maintained under these 
conditions for 16 hours. 
The catalyst was then contacted with H.sub.2 at 600.degree. C. for 0.5 
hours prior to testing for ethane aromatisation by contacting with ethane 
at 625.degree. C., 1 WHSV, 3 bar pressure. 
The following results were obtained. 
__________________________________________________________________________ 
Selectivity 
Contact 
Ethane 
to Selectivity 
Selectivity 
Temp. 
Pressure 
WHSV 
Time Conv. 
aromatics 
to C.sub.6 -C.sub.8 
to C.sub.9.sup.+ 
.degree.C. 
Bar hr.sup.-1 
S wt % 
wt % wt % wt % 
__________________________________________________________________________ 
625 3 1 7.3 36.3 
58.0 47.0 11.0 
__________________________________________________________________________ 
Comparative Test 1--Unmodified MFI Catalyst 
A sample of an MFI-type zeolite containing ammonium cations (zeolite 
prepared using ammonia as template according to the general process of our 
published EP 0030811-A) was impregnated with 20 ml of an 0.04M aqueous 
solution of Ga (NO.sub.3).sub.3. This solution was just sufficient so as 
to wet the zeolite. The aqueous solvent was then removed by drying at 
130.degree. C., at subatmospheric pressure. 
The gallium impregnated zeolite was then bound in an inert silica matrix as 
in Example 1 above and sieved to give catalyst particles which passed 
through a standard 12 mesh sieve but were retained by a 30 mesh sieve. 
This procedure gave a final catalyst containing 0.8% Ga. 
The catalyst was loaded into a vertical fixed bed reactor and was contacted 
with nitrogen while the temperature was raised to 550.degree. C. This was 
maintained for 16 hours. The catalyst was then contacted with hydrogen at 
600.degree. C. for 0.5 hours prior to testing for the aromatisation of 
ethane at 625.degree. C., 1 WHSV, and 3 bar pressure. The following 
results were obtained. 
__________________________________________________________________________ 
Selectivity 
Contact 
Ethane 
to Selectivity 
Selectivity 
Temp. 
Pressure 
WHSV 
Time Conv. 
aromatics 
to C.sub.6 -C.sub.8 
to C.sub.9.sup.+ 
.degree.C. 
Bar hr.sup.-1 
S wt % 
wt % wt % wt % 
__________________________________________________________________________ 
625 3 1 7.2 27.5 
41.7 24.4 17.3 
__________________________________________________________________________ 
EXAMPLE 2 
A catalyst was prepared from an MFI-type zeolite containing ammonium 
cations by the procedure described in Example 1 above. The gallium loading 
was 0.95 wt %. This catalyst was placed in a vertical fixed bed reactor 
and was contacted with nitrogen as the temperature was raised to 
550.degree. C. The catalyst was maintained under these conditions for 16 
hours. The catalyst was then contacted with hydrogen at 600.degree. C. for 
0.5 hours prior to testing for the aromatisation of ethane at 650.degree. 
C., 0.5 WHSV and atmospheric pressure. The following results were 
obtained. 
__________________________________________________________________________ 
Selectivity 
Contact 
Ethane 
to Selectivity 
Selectivity 
Temp. 
Pressure 
WHSV 
Time Conv. 
aromatics 
to C.sub.6 -C.sub.8 
to C.sub.9.sup.+ 
.degree.C. 
Bar hr.sup.-1 
S wt % 
wt % wt % wt % 
__________________________________________________________________________ 
650 1 0.5 5 62.8 
59.2 33.0 26.2 
__________________________________________________________________________ 
Comparative Test 2--Unmodified MFI Catalyst A catalyst was prepared from an 
MFI-type zeolite containing ammonium cations by the procedure described in 
comparative Test 1 above. The gallium loading was 0.9 wt %. 
The catalyst was loaded into a vertical fixed bed reactor and was contacted 
with nitrogen as the temperature was raised to 550.degree. C. These 
conditions were maintained for 16 hours. The catalyst was then contacted 
with hydrogen at 600.degree. C. for 0.5 hours prior to testing for the 
aromatisation of ethane at 650.degree. C., 0.5 WHSV and atmospheric 
pressure. 
__________________________________________________________________________ 
Selectivity 
Contact 
Ethane 
to Selectivity 
Selectivity 
Temp. 
Pressure 
WHSV 
Time Conv. 
aromatics 
to C.sub.6 -C.sub.8 
to C.sub.9.sup.+ 
.degree.C. 
Bar hr.sup.-1 
S wt % 
wt % wt % wt % 
__________________________________________________________________________ 
650 1 0.5 5 60.8 
60.7 27.0 33.6 
__________________________________________________________________________ 
Comparative Test 3--Unselective Dealumination 
A sample of an MFI-type zeolite as described in Example 1 above was 
exchanged with NH.sub.4.sup.+ ions by refluxing for a period of 4 hours 
with a 0.81M aqueous ammonium nitrate solution. 
The zeolite was then placed in a horizontal tubular furnace the temperature 
of which was raised to 550.degree. C. while contacting the zeolite with 
air flowing at 40 ml/min. The airflow was diverted through a water bath at 
60.degree. C. which gave a 20% v/v steam in air mixture on entering the 
furnace. The zeolite was contacted with this mixture for 2 hours with the 
temperature at 550.degree. C. This caused dealumination within zeolite 
pores as well as the surface. The zeolite was then refluxed with 0.81M 
ammonium nitrate as previously. 
The zeolite was impregnated with gallium nitrate solution and bound in an 
inert silica matrix as in Example 1. 
The final catalyst contained 0.8 wt % Ga. 
The bound catalyst was tested for ethane aromatisation as in Example 1. 
The following result was obtained. 
__________________________________________________________________________ 
Selectivity 
Contact 
Ethane 
to Selectivity 
Selectivity 
Temp. 
Pressure 
WHSV* 
Time Conv. 
aromatics 
to C.sub.6 -C.sub.8 
to C.sub.9.sup.+ 
.degree.C. 
Bar hr.sup.-1 
S wt % 
wt % wt % wt % 
__________________________________________________________________________ 
625 3 1 8.5 38.0 
46.6 25.6 21.0 
__________________________________________________________________________ 
These results demonstrate the higher selectivity to C.sub.6 -C.sub.8 
aromatics (shape selective products) and lower selectivity to 
C.sub.9.sup.+ aromatics (products of non shape selective zeolite surfaces) 
obtained with the selectively dealuminated MFI catalysts (Examples 1 and 
2) compared with the unmodified MFI catalysts (Comparative Tests 1 and 2) 
and the conventionally dealuminated MFI catalyst (Comparative Test 3).