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ELSEVIER
Acid dealumination of EM-5P.J. Kooyman, P. van der Waal, and H.
van BekkumLaboratory of Organic Chemistry and Catalysis, Delji
University of Technology,Julianalaan 136, 2628 BL Del& The
NetherlandsThe acid dealumination of various samples of ZSM-5 is
studied using mineral acids as thedealuminating agent. Apart from
the synthesis method of the ZSM-5, the acid concentration,
thetemperature, and the duration of the acid treatment are varied.
The results are evaluated usingelemental analysis and 27AI MAS
n.m.r. A comparison with literature data is presented. It seemsthat
the dealumination of MFI is not as straightforward as is often
assumed. 0 Elsevier ScienceInc. 1997Keywords: MFI zeolite;
dealumination; aluminum; mineral acid treatment
INTRODUCTIONGenerally, the less framework aluminum acidic sites
aunit cell of a zeolite possesses, the stronger the acidityper site
is. Thus, by controlling the Si/Al ratio of thezeolite framework,
the acidic catalytic properties of thezeolite can be influenced.
However, many types of zeo-lites (e.g., MOR and zeolite Y (FAU))
cannot be syn-thesized directly in a high-silica form. These
zeoliteshave to be dealuminated using a postsynthesis treat-ment.
Although it is well known that most zeolites canbe dealuminated
without seriously affecting the crystalstructure of the materials,
not many types of zeoliteshave been studied systematically for
their behavior un-der acid dealumination conditions. The most
com-monly used dealumination methods are the so-calledsteaming
treatment (in which the zeolite is heated tohigh temperatures,
e.g., 5OOC, in a flow of steam) andtreatment with gaseous SiCl,,
also at elevated tempera-tures. Many papers have been published on
the behav-ior of zeolite Y (FAU) under these treatments,
mainlybecause the resulting material, USY, shows improvedcatalytic
cracking properties, which is of great interestto the petroleum
industry. Using either of the afore-mentioned treatments, zeolites
are obtained that con-tain less framework Al species than the
parent materi-als, but few framework vacancies (silanol nests).
This isdue to direct substitution of Si for Al in the SiCl,
treat-ment, and to the T-jump mechanism in steaming.Because of the
latter mechanism, mesopores are alsocreated during the migration of
SiO, under the actionof heat and steam. Less frequently used
dealumina-tion techniques include reaction with chelating
agentslike EDTA and oxalic acid, reaction with aqueous(NH,) iF,,
and reaction with F, gas.
The creation of framework vacancies upon dealumi-nation can be
desirable when another transition metal(e.g., Ti, Fe, B, Ga, V) has
to be incorporated into theAddress reprint requests to Dr. P.J.
Kooyman.Received 10 January 1994; accepted 4 August 1996Zeolites
18:50-53, 19970 Elsevier Science Inc. 1997655 Avenue of the
Americas, New York, NY 10010
zeolite framework, leading to suitable catalysts for vari-ous
processes. However, as the direct incorporation ofsome transition
metals (e.g., I?) into the MFI frame-work is not straightforward,3
it can be beneficial to fol-low a secondary synthesis route, in
which Al-containingMFI is first dealuminated via acid treatment to
createsilanol nests. Subsequently the desired transition metalis
inserted into or attached to the silanol nests,
creatingtransition-metal-containing MFI.
Literature data on the acid dealumination of Al-containing MFI
show a great variety of results. Althoughhydrochloric acid
treatment is often used to dealumi-nate MFI, several authors use
hydrochloric acid ion ex-change as a means to convert MFI to the
H-form.4 Al-though treatment conditions are similar, it is then
as-sumed that only ion exchange occurs; the possibility
ofdealumination is not even taken into account.From a comparison of
literature data concerning hy-drochloric acid treatment of
ZSM-5,5-12 the questionarises whether the synthesis method of ZSM5
is impor-tant in the resistance of the material to acid
dealumi-nation. Thus, we have synthesized various samples ofZSM-5
according to different synthesis methods, and wehave studied the
results of treating these samples ofZSM-5 with varying
concentrations of different mineralacids and of changing the
temperature of the acid treat-ment.
EXPERIMENTALMaterials: synthesis of ZSM-5
Samples denoted as ARGN were synthesized accord-ing to the
original patent method, described by Ar-gaucr and Landolt,s
Nindicating the Si/Al ratio of theresulting material. Typical
features of this synthesismethod are the high TPA/Si (TPA =
tetrapropylammo-nium cation) and H,O/Si ratios employed. Thus, a
so-lution A was prepared by dissolving 40.1 g of Aerosil200
(Degussa) in 209.5 g of 40% TPAOH in water (CFZZaltbommel). To
obtain a solution, it is necessary to
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MFI dealumination: P. J. Kooyman et al.glass vessel; at
temperatures higher than 100C a Tef-lon-lined stainless steel
autoclave (not stirred) was used.heat the mixture to about 100C
under stirring. Solu-tion B was obtained by dissolving the
appropriateamount of NaAlO, (ICN Pharmaceuticals) in 72.3 g ofwater
and was stirred into solution A. The resultinggel was autoclaved at
150C for 6 days.Sample SSG40 (Si/Al = 40) was synthesized
accord-ing to a method described by Sand and Ghamami.i4105 g of
Ludox AS-40 (Du Pont) , 127 g of 25% NH, inwater, 16 g TPABr (CFZ
Zaltbommel), and 2.86 g of
aluminum isopropoxide (Janssen Chimica) were mixedto a
homogeneous gel, which was autoclaved at 180Cfor 2 weeks.Sample
(HA-15 (Si/Al = 15) was synthesized accord-ing to a method
described by Change et all5 by makinga solution A of 150 g of Ludox
HS-40 and 100 g of1.3% TPABr in water, and a solution B by
dissolving8.65 g of NaOH in 11 g of H,O and subsequently
slowlyadding 6 g of sodium aluminate. Then solution Bwas stirred
into solution A, and the resulting gel wasautoclaved for 3 days at
175C.Sample RR-38 (Si/Al = 38) was synthesized accordingto the
method used by Rraushaar and van Hooffs byfirst making three
solutions: solution A by dissolving12 g of NaOH and 30 g of
Cab-0-Sil (the Cabot Com-pany) in 90 g of water; solution B by
dissolving 3.4 gof Al,(SO,), . 18H,O and 24.0 g of TPABr in 74.3 g
of4 N H,SO,; solution C by dissolving 14.7 g of NaCl in45.3 g of
water. Then, starting with solution C, solu-tion B and about 3/4 of
the total amount of solutionA were added simultaneously under
vigorous stir-ring. The pH was set at 3 by adding a little bit more
ofsolution A, and the mixture was stirred for 2 h. ThepH was then
set at 9 by adding the remaining part ofsolution A and a small
amount of NaOH while st ir-ring, and the mixture was autoclaved at
160C for 3days.All MFI samples synthesized were identified as
pureMFI by X-ray diffraction and i.r. spectroscopy. Beforethe
dealumination experiments, all samples were cal-cined in air in a
muffle furnace at at least 500C over-night, to remove occluded TPA.
The samples are ex-pected to be mainly in the protonic form. The
Si/Alratios were determined by ICP analysis of the samplesdissolved
in diluted HF/H,SO,/H,BO,.Acid treatmentThe samples of ZSM-5 were
treated with differentmineral acids at different temperatures to
study alumi-num removal from the framework. Threefold treat-ment
with 100 ml of 1 N aqueous acid per gram ofzeolite, for at least
half an hour, was applied. The zeo-lite was placed in the reaction
vessel with the acid so-lution and was heated to the desired
temperature,which was maintained for the amount of time indi-cated,
after which the source of heat was removed andthe suspension was
left to settle. The acid solution wasdecanted, fresh acid solution
was added, and the wholeprocedure was repeated. After the third
heating period,the suspension was filtrated, and the zeolite was
washedthoroughly with water. Samples were dried overnight at120C in
air. Up to 100C the dealumination experi-ments were performed in a
stirred round-bottomed
Al MAS n.m.r.Solid-state *Al n.m.r. spect a were recorded on
aVarian VXR-400 S spectrometer, using magic anglespinning and a
Doty probe. I M aqueous Al(NOs)s wasused as an external chemical
shift reference (8 = 0).Acquisition time was 0.3 s with an
acquisition delay of
0.3 s. The number of datapoints was 16,000, and thepulse width
was 1.5 ps, to correspond to a flip an-gle
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MFI dealumination: P. J. Kooyman et a/.Figure I shows the *Al
MAS n.m.r. spectra of thesample ARL22 before and after 3fold
treatment with 1N HCl for 1 and 24 h at 80C. From these spectra it
isclear that the starting material contains only tetrahe-drally
coordinated aluminum (signal at 54 ppm),whereas a small part of the
aluminum is coordinatedoctahedrally in the acid-treated samples
(signal at 0ppm) . Both acid-treated samples contain approxi-mately
the same ratio of tetrahedrally to octahedrally
coordinated aluminum. From these n.m.r. spectra wecan conclude
that although some aluminum is ex-tracted from the framework upon
acid treatment, de-alumination is not very efficient, and
dealuminationefficiency does not increase upon prolonging the
acidtreatment from 3 * 1 h to 3 * 24 h. We can also con-clude that
the removal of octahedral aluminum is in-complete.As none of the
samples obtained using the AFU syn-thesis method showed significant
dealumination,samples of ZSM-5 obtained using different
synthesismethods were treated with 1 N aqueous HCl. However,sample
CHA-15 is not easily dealuminated.The Al MAS n.m.r. spectra of
sample SSG-40 after3-fold 2-h treatment with 1 N HCl shows only a
verysmall signal at 0 ppm, indicating that practically noaluminum
is removed from the framework.Kraushaar and van Hooffs are the only
authors toreport almost 100% dealumination upon treatingZSM5 with 1
N HCl. Our sample KR-38 was preparedaccording to the synthesis
method these authors use.6The *Al MAS n.m.r. spectra of this
material before andafter 1 N HCI treatment (3 * 1 h at 80C) show
that ourmaterial KR-38 is very resistant to acid treatment
(thesignal for octahedrally coordinated aluminum is very
40 0 -40wm
Figure 1 AI MAS n.m.r. spectra of ARL-22: (a) starting
mate-rial; (b) after 3 * l-h acid treatment; and (c) after 3 * 24-h
acidtreatment.
small after acid treatment). After completion of thisstudy,
Prof. van Hooff informed us that their dealumi-nation results are
difficult to reproduce.6 Subtle differ-ences in the synthesis
conditions might play an impor-tant role in determining the
stability of the materialobtained.Kornatowski et al.,* reported
reasonable dealumina-tion using Mostowicz synthesis method and
subject-ing this sample of ZSM-5 to boiling 1.25 N HCl. More-over,
in that case the degree of dealumination increaseswith increasing
acid treatment period.** However, themajority of the literature
data as well as the data of thepresent work show that the acid
dealumination ofZSM-5 is not as feasible as one would be inclined
tothink. It seems that most ZSM-5 synthesis methods pro-vide
samples that are very resistant to acid dealumina-tion. This
stability might be because of the virtual al,sence of structural
defects. One would expect thatsamples of ZSM-5 obtained via a very
fast synthesismethod will possibly contain intrinsic structural
defectsand may be more susceptible to acid dealumination.This fast
crystallization might be achieved by addingseeds to the synthesis
gel or by applying microwaveheating s* during the
synthesis.CONCLUSIONSThe acid dealumination of zeolite ZSM-5 is not
asstraightforward as it seems to be. Apparently no dealu-mination
is found when using aqueous HBr or H,SO,,and hardly any
dealumination is found when usingaqueous HCl. For the samples
studied, the temperatureor the period of time of the acid treatment
do notinfluence the extent of dealumination, nor do the Si/Al ratio
of the samples or the synthesis method accord-ing to which the
samples were obtained.ACKNOWLEDGMENTSThanks are due to Dr. J.A.
Peters and Mr. A. van Estrik,who recorded the Al MAS n.m.r.
spectra. Mr. J. Padmosperformed the elemental analyses. Dr. A.
Sinnema isgrateful1garding r7 acknowledged for valuable discussions
re-Al MAS n.m.r.
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