Zeolite Y Synthesis Modification, And purification

8/17/2019 Zeolite Y Synthesis Modification, And purification

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Review ArticleZeolite Y: Synthesis, Modification, andProperties—A Case Revisited

Wolfgang Lutz

Brandenburgische echnische Universit ̈at Cottbus, Volmerstraße , Berlin, Germany

Correspondence should be addressed to Wolgang Lutz; [email protected]

Received October ; Revised January ; Accepted January ; Published May

Academic Editor: Louis-Philippe Leebvre

Copyright © Wolgang Lutz. Tis is an openaccessarticle distributed underthe CreativeCommons AttributionLicense, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Y zeolites dealuminated by steaming were introduced as uid-cracking catalysts in the year . Extensive research has been doneto develop suitable dealumination techniques, to investigate crystal structure, and to characterize catalytic behaviour. However, theorigin o the secondary pore system ormed in the zeolite structure during dealumination process remained completely obscureover a period o our decades. Open questions concerned also the existence o extraramework siliceous admixture in additionto extraramework aluminium species which can dramatically change the catalytic properties o these zeolites. Tis paper gives areview on the synthesis o DAY materials and provides some answers to several open questions.

1. Introduction

Commercial synthesis o Y zeolite was claimed by Breck in [] afer the rst industrial manuacturing o A and Xtypes by Milton [, ]. Zeolite NaY appears to be topologi-cally analogous with the type X aluminosilicate ramework.Te cubic unit cell o all these alumosilicates contains (Si,Al)O4 tetrahedrons. Breck suggested that the change o Xto Y modication occurs at a silicon to aluminium ratio o . []. Zeolites with Si/Al values lower and higher than thiscritical point characterize X and Y composition, respectively.Because o the instability o Al-rich samples in acids or water

at elevated temperatures [], R ̈uscher et al. [] proposed anew denition based on a typical chemical behaviour o thesamples—the dealumination by steam treatment []. AboveSi/Al ratio o ., Y samples may be dealuminated in steamwithout collapse o its ramework. Te reason or that arisesrom long silicate chains with remaining silicate unitswhile aluminate units can be removed.

Te new X/Y denition explains the reason why only high-silica modications o zeolite Y have beenused as crack-ing catalysts [, ]. Since zeolite Y can be directly synthesizedwith maximal Si/Al values o about [], all catalytically relevantmaterials mustbe prepared by postsynthetic removalo ramework aluminium. Zhdanov et al. [] provided an

overview about different techniques o zeolite dealuminationsuch as acid extraction [], isomorphous Al/Si substitution[], and thermochemical treatment o NH4Y []. Heating o NH4Y in dry or wet air brings different dealumination effectsand structural eatures to dealuminated Y samples (DAY)[]. In each case, ramework aluminium will be transormedthereby into extraramework aluminous species (EFAl) [].

Ambs and Flank [] explained the thermal stability o DAY over K as a result o the removal o sodium ionswhich destabilize the ramework due to their mineralizingeffect. Peri [] gave preerence to the ormation o novel Si–O–Si bonds as a result o dehydroxylation o intermediately

ormed hydroxyl nests. Kerr et al. [] explained risingstability o DAY by effect o healing o the Al deects withmigrating H4SiO4 molecules.

Increasing thermal stability o DAY was one o thereasons why the thermochemical process has been com-mercially established in a high extent. Afer discovery o structural undamentals o steamed zeolites, researchers putmore attention on their catalytic behaviors. Tereby, theormation o a secondary pore systemin DAY hasbeen ound.According to [], this process was assumed to enhance themolecular transport o starting materials and nal productsduring catalytic conversion. But, in spite o the ramework repairing, a signicant decrease o the sorption capacity

Hindawi Publishing CorporationAdvances in Materials Science and EngineeringVolume 2014, Article ID 724248, 20 pageshttp://dx.doi.org/10.1155/2014/724248


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Advances in Materials Science and Engineering

was observed []. Te nature o this phenomenon is stillunclear and problems o mesopores ormation and types o mesopores, as well as partial amorphization o the zeoliteramework remain open.

Since a direct relationship between the structure o NaY and the specic dealumination behavior o NH4Y

undoubtedly exists, revisiting the problems o synthesis, ion-exchange, and dealumination behaviors o zeolite Y seems tobe advisable.

2. Results and Discussion

.. Synthesis o Zeolite NaY. Fundamentals o the synthesiso aluminum-rich zeolites o A, X, and Y types were reportedby Kerr [, ], Breck and Flanigan [], Zhdanov [],and McNicol []. Tese and several other authors discussedmainly the crystallization process o zeolites rom theirprimary aluminosilicate gels. Using Molybdate method [],Fahlke and colleagues [, ] investigated in details themechanism o gel ormation and ollowing rearrangement.Tis method determines the length o silicate chains betweenaluminate units in dependence on gel maturation. Indeed,prolonged silicate units dene the process o gel aging.For measurement, the ≡Al–O–SiO()–Al≡ with = 1–∞zeolite ramework is hydrolyzed in mineral acid and addedto molybdic acid. Te degradation rate (the measure o thelength o silicate chains) is optically ollowed by ormationo the yellowish silica Molybdate complex. Tis techniquecharacterizes perectly the Al-rich A and X zeolites, but itails in the case o Y crystals because the innite long silicatechains o latters are practically insoluble in acid.

Properties o DAY samples—crystallinity, silicon to alu-minium ratio, secondary pore volume, and extraramework species—depend directly on the state o the parent NaYcrystals and even on the starting gel. ypical composition o synthesis batches o aluminosilicate zeolites are

2.0 SiO2 : 1 Al2O3 : 3.4 Na2O : 170 H2O NaA

4.5 SiO2 : 1 Al2O3 : 6.3 Na2O : 280 H2O NaX

9.0 SiO2 : 1 Al2O3 : 3.0 Na2O : 120 H2O NaY

()

with nal Si/Al ratioso , –., and.–., respectively [].Basic materials are aqueous solutions o sodium aluminateand sodium silicate or NaA and NaX, silica gel or silicasol or NaY, and, as expected, sodium hydroxide and water.

Te reaction composition diagram according to Breck [] orKostinko [] shown in Figure demonstrates that the eldso gel compositions rom those A, X, or Y zeolites are ormed.

As can be seen rom Figure , the variation o gel compo-sition o X and Y zeolites is signicantly smaller than that o zeolite A.

By mixing o raw materials, aluminosilicate gels occurin all zeolite synthesis batches. For example, when puresolutions o aluminate and silicate are mixed, spontaneously silicon-rich hydrogels precipitate. Some o the monomericaluminate ions are linked to the oligomeric silica molecules.Te residual aluminate ions remain at rst in the solution.Butthe gel reorganizes its structure by dissolution and renewed

Na2O Al2O3

SiO2

10

50

50

90

90

50

10

10

90

A

A

X

X

Y

Y

F : Reaction composition diagram o aluminosilicate zeolitesA, X, and Y (elds-possible gel composition; points-nal zeolitecomposition) [].

precipitation up to the equilibrium state. Over an agingperiod o ten to twenty hours, the gel consumes more andmore aluminate in such a way that nally the solids containsilicon and aluminium in equivalent amounts in zeolitesA and X batches. Te three-dimensional gel structure isstable at ambient temperature in its mother liquid becauseall alkaline soluble ≡Si–O–Si≡ bonds were changed already into chemically stable ≡Si–O–(Al(−))≡ bonds. Te negativecharges o the ourold coordinated Al atoms protect thegel against urther attacks o the likewise negatively chargedhydroxyl ions. On the other hand, the negative chargespromote the attack o protons or spontaneous degradationo the ramework as in the case o Molybdate measurements.

Wieker and Fahlke [] detected % o monomeric≡(Al(−))–O–Si–O–(Al(−))≡ silicate building units, in additionto polymeric silicate, in NaY synthesis batches. Te NaY gelcomposition differs thus rom that o A and X. Due to thesemonomeric species, the crystallization o NaY starts withragments o NaX composition (Si/Al ratio o about ) at – K. Te crystallization is avored i the batch is seededby NaX nanocrystals, exactly by the way exploited in indus-trial zeolite manuacturing. Actually so-called seed-solutionswhich contains silicate ions in the orm o oligomeric chains(length o – silicon atoms) terminated by aluminate

units are used in order to prevent, as much as possible, theormation o Al-rich areas inside the NaY crystals.

With the increasing o time, the crystallization turnsgradually into the actual NaY zeolite composition with aSi/Al ratio over .. Ten, due to the overproportionalconsumption o aluminate at the beginning o the process,the crystallization drops off owing to the total consume o aluminate ions being primary building units in the aujasitestructure. Tis is the reason why the crystal surace becomesnally rich in silicon and no secondary crystal growth above m is observed [, ]. Frequently, the NaY zeolites arecrystallized in polycrystalline particles with smaller actualsize [].


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(a) (b)

F : Zeolite A (a) and aujasite-type zeolites X and Y (b) ormed by sodalite cages.

Te synthesis o large aujasite type crystals with diam-eters

>

m was reported by Ferchiche et al. [, ] and

Berger et al. [] only or samples with Si/Al ratios below ..It seems very likely that such samples consist o the mixtureo both X and Y types [].

Te Si to Al gradients in NaY starting rom inside thenuclei and increasing to ∞ at the surace o the zeolite crys-tals are responsible or several urther specic phenomenaobserved in steamed DAY zeolites. Te NaY crystals withSi/Al ratios over . are unortunately too small or a char-acterization o their gradient by physicochemical methodssuch as ESCA or EDX. Additionally, they tend to collapseduring measurements. Tereore, a complex characterizationo DAY samples includingtests o hydrothermal stability, state

o crystallinity, characterization o extraramework species,and pore structure is necessary to describe the actual state o NaY and consecutive products.

.. Sodium/Ammonium Ion Exchange o Zeolite NaY. Teunit cell o zeolite NaA contains atoms and that o theaujasite-type zeolites NaX and NaY contains SiO4 andAlO4 tetrahedrons []. Tey orm sodalite cages which arelinked to oxygen double rings and rings in NaA, NaX,and NaY, respectively, as seen rom Figure . InNaA and low-silica NaX rameworks with Si/Al = , the SiO4 and AlO4tetrahedrons are arranged in rigorous alternation accordingto the Loevenstein rule [].

Cavities with porediameters o. nm in NaA and .nmin NaX, NaY arise. Te diameters o pore entrance are equalto . nm and . nm, respectively [].

Due to the negatively charged AlO4(−) tetrahedrons,

zeolites contain Na(+) ions in different amounts in theirsynthesis orm − in NaA, – in NaX, – in NaY(values o X and Y based on the denition o R ̈uscher etal. in []). Different cations may be exchanged or sodiumions, totally or to a high degree, to adjust the entrance porediameter and specic sorption behaviour o zeolites. Barrerand colleagues published undamental aspects about thismethod o zeolite modication, or example, in [, ].

Because zeolite NaY is used preerentially in dealu-minated state or catalytic processes, the introduction o ammonium ions into this ramework is o specic interest.Ammonium ions are decomposed by heating in H+ andoutgoing NH3 gases thus orming HY modication which isactive in the ramework dealumination by steaming.

At the laboratory scale, the ammonium ions can beexchanged or sodium ions in . M solution o ammoniumnitrate or sulphate at liquid/solid ratio higher than . Foran effective and careul exchange, the procedure should berepeated. In this way, % o sodium ions are removedrom the ramework o zeolite Y. Tus, one-fh o theramework remains inactive in dealumination, or example,by steaming as it will be seen below. Te use o a higherconcentrated salt solution or/and treatment o the zeolite

at K was not successul []. But there is a possibility to increase the active ramework part i a stepwise way is used: rstly the partially ammonium exchanged sampleis heated at – K then the ion exchange procedureis repeated once more. Intermediately produced protonsdisplace the residual sodium ions rom their position insidethe sodalite cages and double-six rings which are removedin the second ammonium exchange rom the large cavities.Partial dealumination o the ramework already takes placeat each treatment step.

.. Dealumination. Dealumination is a method o chemicaland structural modication o zeolites. But not each synthetic

zeolite ramework preserves structural arrangement andstability afer removing some Al atoms. Tus, NaA and NaXzeolites are completely collapsed afer any dealuminationprocedure. NaY zeolite with Si/Al < . also loses, at leastpartially, its crystallinity []. But only with this treatment, Yalumosilicates with denite Si to Al ratios that guarantee theirspecic properties such as thermal stability, hydrophobicity,or catalytic activity can be directly prepared.

Te simplest dealumination method consists o the treat-ment o samples in inorganic or organic acid [] where deca-tionation takes place. Under action o the introduced pro-

tons, ≡Si–O–(Al(−))≡ bonds became hydrolyzed and changedrstly into more stable≡Si–O–H units plus H–O–Al≡ splitted


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structure. At the second step, all the other bindings with Alions are broken off. Utilizing the natural clinoptilolite, rstly,Barrer and Makki [] were successul in the treatment o thezeolite with M hydrochloric acid at K. Te aluminiumcontent decreased down to % without a signicant loss o crystallinity o samples. Aluminium atoms removed rom the

ramework were ound as EFAl species in the solution. Tedisadvantage o this method or zeolite NaY lies in remainingstructural deects involving ormation o hydroxyl groupsinside the silicon ramework [].

A more sensitive extraction process includes a treatmento the zeolite with complex agent o ethylene-diamine-tetra-acetic acid (EDA). By use o this method Kerr [] removedthe hal o the ramework aluminium atoms in NaY zeolitewithout a greater amorphization o the crystals but with ahigh density o structural deects. One can get more intactstructures i zeolite NaY is treated in a vapor o silicon-tetra-chloride as it was shown by Beyer and coworkers [,, ]. In the resulting structure, Al atoms are replaced by silicon atoms at elevated temperature. But rigid control overthis exothermic process and technological demand on theresistance against the aggressive medium appears to be toocomplex so that industrial production was restricted only to the semi-industrial scale. Tereore, the calcination o NH4Y zeolites in steam [] has been established worldwideas technical dealumination process.

... Termochemical reatment o NH 4NaY in Steam.Generally, dealumination o NH4NaY in steam has notbeen changed since the introduction o this procedure by McDaniel and Maher in []. Samples are treated in anopen urnace or rotary kiln through which steam o barwater pressure is going. Steam is generated by evaporationo liquid water with one bar o water pressure or saturationo gas with water vapor o lower partial pressure. Te actualconcentration o water molecules inside the oven reactionzone depends on heating and is reduced with increasing tem-perature o sample treatment. Te extent o dealumination islimited by the degree o ammonium exchange o the startingmaterial and depends on the temperature andpartial pressureo steam [, ].

Salman and colleagues [] have investigated the dealu-mination o zeolite (NH4)39 (Na)10[(AlO2)49(SiO2)143] witha Si/Al ratio o . over a wide eld o variable parameters.Figure shows increasing removal o ramework Al withrising the temperature and period o treatment.

Small and slightly higher dealumination with Si/Al valueso . and . was ound at K and K, respectively.Te evolution o Si/Al at K and K indicates a typicalsaturation behaviour towards values o . and .. Satura-tion effects have also been reported earlier by Wang et al.[], Engelhardt et al. [], and Lohse et al. []. Due toa limitation o sodium/ammonium exchange in the usedone-step strategy, % o the Na+/AlO4

− charge complexesremained stable under steam. Te equilibrium Si/Al ratiosobtained in [] correspond to % ( K), % ( K),and % ( K) o the total content o Al atoms removedrom the ramework. Further removal could be increased by arenewed ammonium exchangeollowed by a second steaming

0 5 10 15 202

4

6

8

10

12

S i / A l r a t i o

Time (h)

573 K673 K 773 K

873 K

973 K

F : Si/Al ratio o DAY zeolites in dependence on steamingtemperature and time [].

10 15 20 25 30 35 40

1073 K

773 K

673 K

473 K

873 K

NaY

NH4Y

573 K

973 K

2 (∘)

F : Selected XRD patterns o DAY zeolites steamed hoursin dependence on temperature [].

process. Following this two-step strategy, the Si/Al ratio o DAY steamed at K or hours changed rom . to .,with a twoold increase o the dealumination effect without

signicant sample amorphization [].Signicantly higher values o Si/Al ratio (up to ) have

been obtained directly or steaming at K. However, thisresult was accompanied by a partial or strong amorphizationo the nal product. Te sample became completely amor-phous at K because o a thermal collapse atthe beginningo steaming. At the lowest temperature investigated ( K),drastic ramework damage has occurred, too. In this case,a sour hydrolysis o the ramework owing to the highconcentration o water molecules in the reaction zone took place.

X-ray diffraction (XRD) patterns in Figure indicate amoderate decrease in signal intensity or NH4NaY and DAY


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400 600 800 1000 1200

HY, 1073 K

HY, 873 K

HY, 773 K

HY, 673 K

HY, 573 K

HY, 973 K

HY, 473 K

NH4Y

NaY

Wave number (cm−1)

F : IR spectra o parent NaY zeolite and DAY samplessteamed or hours in dependence o temperature.

samples. Tis observation will be discussed below, along withthe results on the ormation o extraramework species.

Removal o Al atoms results in the shrinking o the totalzeolite ramework because o the different –Al–O– and –Si–O– bond lengths o pm and pm [], respectively.Te lattice constant 0 decreases rom . nm in NaYto . nm in DAY treated at K or h. Tis sampleshows the highest dealumination state, without a signicantstructural amorphization.

IR spectra o DAY samples obtained by Salman et al. []are given in Figure and support the data o XRD measure-ments. Te ramework sensitive double-ring vibration band

DR [] shifs rom cm−1 to cm−1 with increasingsteaming temperature up to K. Te signal completely disappeared in the sample dealuminated at K.

Apart rom DR signal, systematic changes in the shapeo spectra, particularly bending, symmetrical and asym-metrical stretching vibration o the O4 building units

at – cm−1, – cm−1, and – cm−1, havebeen additionally observed. Optimal conditions o steamingappear to be at K as evident rom the shape and intensitieso characteristic peaks in the dealuminated ramework seen

in Figure . Te ramework became more regular in respectto the ormer Si/Al gradient resulting rom NaY synthesis.

At lower temperature, the shape o peaks becomesbroader indicating a higher nonuniorm distribution o theSi/Al ratio in the ramework and a relative stronger contribu-tion o the extraramework species ormed. But, as we willsee below, lower steaming temperatures lead to a reducedmesopore ormation.

One can suggest that all spectra in Figure are super-imposed by extraramework species o aluminosilicate andsilica gel [] ormed by some corrosion o the ramework.Te aluminosilicate may be extracted together with theextraramework aluminium by acid treatment []. It is

known that highly dealuminated and acid leached DAY,requently called US-EX (ultrastable extracted zeolite Y),shows very sharp vibration bands because o homogenisationo the pure SiO2 ramework [].

In samples treated at K, DR signal does not shif romits characteristic place. In this case, due to the low thermal

activation no dealumination occurs inside the ramework butthe dissolution o the zeolite takes place.

... Determination o the Si/Al Ratio o DAY Samples. Tetuning o ramework properties o DAY zeolites requires anexact determination o the Si/Al ratio. Dealumination effectcan be proved by chemical analysis o the acid solutionand washing water in the samples prepared through acidleaching [] and Al/Si substitution [], respectively. How-ever, such approach requires certainty that all extrarame-work aluminium is completely washed out in the samples.Tereore, investigation o the solid state by physicochemicalmethods like XRD, IR, and NMR seems to be the better way.

Tese methods are even indispensable or characterization o steamed DAY samples because these zeolites still contain thewhole amount o EFAl and chemical analysis data appear ascontradictory.

Breck [] has presented the relationship between theunit cell constant, 0, and Al atom density per unit cell orhydrated NaX and NaY zeolites on basis o synthesizedsamples. R ̈uscher and colleagues [] extended this modeupon series o DAY samples prepared by steaming as wellas Al/Si substitution. Tey additionally proposed a thoroughstudy o evolution o the IR double-ring vibration DR ordetermination o Si/Al ratios inside the zeolite ramework and compared these calculation methods [] with empiricalequations o Fichtner-Schmittler et al. [] using the inrared

asymmetrical O valence vibration ( = Si,Al) and 29SiMAS NMR data obtained by Engelhardt and Michel [].

XRDR üscher

= 5.3480 − 12.898 ()

(with Si/Al = (1 − )/ and = Al molar raction by use o the cell parameter 0).

IR R üscher

= 3.857 − 0.00619 DR cm−1 ()

(with Si/Al = (1 − )/ and = Al molar raction by use o DR ).

IR Fichtner

= 4.766 − 0.00439O cm−1 ()

(with Si/Al = (1 − )/ and = Al molar raction by use o O).

NMR Engelhardt

Si

Al =

∑ 4 + 0.753 + 0.5 2 + 0.251

()

( = intensity o Q4/nAl signals in 29Si MAS NMR spectra, = 0–4, without consideration o the signals o the nonze-olitic admixtures).


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: Si/Al values o zeolites NaY, HY, and DAY dealuminated or h in steam.

Sample emperature K Si/Al ratio

0 () DR () O () NMR ()NaY — . . . .

HY . . . .

DAY . . . .DAY . . . .

DAY . . . .

In analogy to the results o Breck [], Fichtner andcoworker [] have shown the correlation between unit cellparameters, skeletal stretching vibrations, and molar ractiono aluminium ( = (1 + Si/Al)−1) or a series o X and Yzeolites and ound an ideal solid solution behaviour or thesematerials.Te lattice constant0 as well as the inrared modesO and DR vary linearly on molar aluminium ractionwhich provides a rather easy method or calculating the

Si/Al ratio o DAY samples independently on their synthetictype. However, differences in the level o Si/Al values havebeen reported in dependence on applied relations o () to(). Signicantly higher Si/Al values seen in able werecalculated according to O and the NMR [, ].

Te Si/Al ratios obtained with XRD data should charac-terize the actual state o the ramework. Tese values wereassumed to be not affected by extraramework species inthe samples. Within each series o steaming temperature,the inrared mode DR closely coincides with the 0 valuebecause the double-ring vibration that characterizes only thepure aujasite ramework []. In contrast, higher values havebeen calculated using NMR as well as O data. Resultsrom both methods seemed to be inuenced by admixtures o DAY samples. o clariy this phenomenon, a detailed phaseanalysis o ramework and extraramework products in DAYspecimens should be perormed.

... Extraramework Species in Steamed Y Zeolites. Firstobservation o extraramework aluminium was published ornatural zeolites afer their acid treatment [, ]. Dealumi-nated samples obtained thereby [] or by Al/Si substitutionin SiCl4 vapor [] are usually ree o EFAl i samples havebeen intensively washed out. Residues may be analyzed by 27Al MAS NMR measurements [] since the signal o Alatoms tetrahedrally coordinated in the zeolite ramework and

the signal characteristics o Al(3+) cations and AlCl3 salt differrom each other by their spectral positions.Te chemical shifo the ormer is situated at ppm, whereas that o the latterlies at about ppm. Extraramework silicon (EFSi) which canbe detected by 29Si MAS NMR measurements appears only in the case o partial amorphization o samples afer theirchemical treatment.

More complicated situation appears during calcinationand steaming o NH4NaY zeolites since all EFAl remainsin the solid state [, ]. Baran et al. [] consideredthat tetrahedrally coordinated Al is not transormed into

exchangeable Al(3+) cations. Breck and Skeels [, ] sug-gested the ormation o precipitated Al(OH)3 which can be

extracted by leaching with solutions o sodium chloride orpotassium uoride. Te remaining deects (hydroxyl nests)o the zeolite ramework can be identied by measurement o the inrared stretching vibration o isolated ≡Si–OH hydroxylgroups at cm−1 [].

Different studies on EFAl have reported their possiblelocation on the surace o crystal and/or mesopores [, ],revealed their role in the catalytic behavior [] and, in

particular, analyzed their nature by means o 27Al MAS NMR spectroscopic measurements []. EFAl has been character-ized as closely related to aluminium-oxo-hydroxo cations o polymeric arrangement described rst by Bertram et al. []with the help o Ferron method []. Teirchemical behaviorin organic and mineral acids was intensively investigated by Scherzer []. An increase o the catalytic selectivity o DAYafer extraction o the extraramework aluminium has beenreported by Rhodes and Rudham []. Disadvantages afertreatment in acid consist in the risk o urther dealuminationo the zeolite with remaining Al(3+) cations which could actas Lewis active sites in catalysis [].

In some papers, EFAl has been described as not isolated

but incorporated with EFSi and called thereore as silica-alumina species [–]. Strong interaction between bothtypes o extraramework species was quantitatively analyzedwith X-ray photoelectron spectroscopy []. Using specicMolybdate measurements [], the silica-alumina was latercharacterized in samples o low-temperature steaming asAl aluminosilicate []. Tereore, Siantar et al. [] andLutz et al. [] have perormed detailed analysis o EFSispecies. Te dealumination behavior o NH4NaY samples inthe steam was demonstrated using two commercial and three

laboratoryNaY zeolite samples.Figures and show 27Al and29Si MAS NMR spectra o initial and dealuminated zeolites.

Independent o the origin o zeolite samples, extrarame-

work species appeared to be o the same type. Te aluminiumspectra in Figure show a signal or tetrahedrally coordi-nated ramework Al at ppm [] (NaY). Afer dealumi-nation, additional two broader signals or extraramework species near ppm and ppm have been ascribed to octa-hedrally and pentahedrally coordinated Al, respectively. Tesignal at ppm characterizes the polymeric oxo-hydroxo-aluminium cations [] whereas the broad shoulder centeredat ppmis assigned to the distortedextraramework octahe-dra and/or aluminium tetrahedra o the ramework [, ].

Afer deconvolution (II), the 29Si MAS NMR spectra o NaY samples (I) in Figure show the ve signals typical orthe aujasite ramework with Si(nAl) building units ( = 0–4)


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050100

(ppm)

050100

(ppm)

050100

(ppm)

050100

(ppm)

050100

(ppm)

CKBZeosorb Fahlke Ginter Ferchiche

DAY

NaY

F : 27Al MAS NMR spectra o commercially and laboratory synthesized zeolites beore (NaY) and afer dealumination or hours at K (DAY) (recipes o laboratory synthesized samples according to Fahlke et al. [], Ginter et al. [], and Ferchiche et al. []) [].

at chemical shifs between − ppm and − ppm [].Simulation o the 29Si MAS NMR spectra was perormedby the use o a set o single lines with Gaussian line shape.Tis calculation reveals the lack o siliceous extraramework species (III) in the spectra o parent zeolites. But steamingresults in a drastic change in shape and intensity o theNMR signals (I). All peak positions are shifed to higher

ppm values. Te decrease o the Si(-Al) and increaseo the Si(-Al) structure units conrm the removal o ramework aluminium. Te signicant shoulders between− and − as well as ppm indicate an internal changeo the ramework structure and the presence o siliceousextraramework species (III). Contributions o aluminosil-icate and silica gel are marked with the symbols and .In earlier investigations, these additional signals have beenattributed to in-homogeneously dealuminated high-aluminaand high-silica regions inside a disturbed zeolite ramework [, ].

Content o extraramework species in steamed DAYsamples [] amounted to .–.% or the aluminosilicate

() and .–,% or silica gel (). It has been shown [, ]that the intensity o the peaks assigned to and at low-temperature steaming could be reduced by successive chemi-cal dissolution in hydrochloric acid andsolution o potassiumhydroxide. Acid treatment caused the decomposition o theAl aluminosilicate with ormation o silica gel. Alkalinetreatment revealed a light loss o the silica gel admixture.In high-temperature samples, or example, at K, the Alaluminosilicate decomposes into silica gel and clay. In such acase not the aluminosilicate but the silica gel is determinedin a higher portion.

Te occurrence o extraramework silica-gel is thereason why Si/Al ratios calculated in DAY samples with O

mode () and NMR data () (see italics in able ) appear tobe principally too high. Both IR andNMR signals are strongly superimposed by contributions o those characteristic o Si(Al) and Si(Al) species [–].

Te strong deviation o Si/Al ratios determined withasymmetrical O inrared vibration band results rom thegreat difference between the wave numbers o the zeolite and

the silica gel. Wave numbers between and cm−1

have been observed or silica gels in dependence on their

ramework state []. Te higher the portion o Q4 buildingSi(OSi)4 units, the higher the energy or the activation o their ramework. For “pure” DAY samples, O values rom to cm−1 should be measured. But, owing to thesuperposition o the nonresolved signals o both zeolite andsilica gel, the actually measured values o DAY vary between

and cm−1 [].

Te Si/Al ratios calculated with 29Si MAS NMR aretoo high since the Q3(Si(OSi)3OH) and Q

2(Si(OSi)2(OH)2)signals o the extraramework silica gel superimpose with

those assigned to Si(Al) and Si(Al) in the zeolite [, ]. Inorder to get real values, deconvolution o the spectra becomesthe essential step o the zeolite analysis with NMR technique[]. Tereore, more realistic Si/Al ratios should be obtainedusing the XRD lattice constant 0 or the inrared DR modeaccording to () and (), respectively.

... Origin o Siliceous Extraramework Species. ZeoliteNaY is stable in dry or wet air up to K. Afer thistemperature, the crystalline ramework transorms into theamorphous state. Clay (Al2O3) and silica (SiO2) are ormed.Ten, with the rising temperature, carnegiiete and nephelineoccur above K [, ]. In water or steam o autogenous


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(I)

(II)

(III)

(IV)

(I)

(II)

(III)

(IV)

−80 −100 −120 −80 −100 −120 −80 −100 −120 −80 −100 −120 −80 −100 −120

−80 −100 −120 −80 −100 −120 −80 −100 −120 −80 −100 −120 −80 −100 −120

s s s s s

a a a a

(ppm)(ppm)(ppm) (ppm) (ppm)

(ppm)(ppm)(ppm) (ppm) (ppm)



DAY

NaY

F : 29Si MAS NMR spectra o commercially and laboratory synthesized zeolites beore (NaY) and afer dealumination or hours at K (DAY) [].

pressure, NaY is stable up to K []. It will be decomposedinto an amorphous phase ollowed by transormation into

hydroxysodalite or zeolite P [, ]. Te containing sodiumions act as mineralizer. Buhl et al. [] demonstrated theirinuence on the hydrothermal stability o aujasite typezeolites X (Si/Al = .) and LSX (Si/Al = .).

Te proton modication o zeolite Y is o interestconcerning the application o aujasites as DAY catalyst.According to Rüscher et al. [] zeolite HY (Si/Al > .)becomes dealuminated above K under the ormation o extraramework species. Innite ≡Si–O–Si≡ chains inside theramework prevent the zeolite decomposition and only asmall part o the structure is transormed into aluminiumhydroxide/-oxide, aluminium aluminosilicate, and silica gel.But in the case o lower Si/Al ratios and nite ≡Si–O–Si≡

chains, the structure is hydrolyzed partially or completely. Inthis sequence, zeolite HX with Si/Al = .−. is particularly instable. In contrary, silicon rich aujasites with Si/Al over manuactured by Al/Si substitution [] or steamingand acid leaching [] are instable during steam treatment[, ].

wo chemical processes run simultaneously in the samecrystal o Y zeolite since the Al–Si gradients betweenaluminium-rich nuclei and silicon-rich surace o the zeolite

appear in this material [, ], independent o the origin o

NaY or steaming conditions. Consequently, the center o the

crystal is transormed into X-ray amorphous aluminosilicateand then, with rising temperature, to clay and silica while the

pure silica gel tends to orm on the surace.


9/21


(II)

(I)

(III)

SiO2+ Al(OH)3

+HOH

Si

Si

Si Si Si

Si

SiSiO

O

O

O

O O

O O

O

O

O

O

−

+

Na

Al

Si Si

Si

Si Si

O

O

O

O O

OO

−

Si

Si

Si

O

O

O

O

O

H+

Al

Si

Si

Si Si Si

Si

SiSiO

O

O

O

O

OO

O

O

OO

OH

Al+3HOH

− 2 H

O H

−NaOH

Si

Si

Si Si Si

Si

SiSi

O

O

O

O O

O

OO

O

OOH

H

H

HO

A l ( O

H ) 3

F : Stepwise decomposition o a low-silica zeolite ramework by water [].

Water molecules attack the zeolite ramework accordingto the chemical reactions

H(+) + OH(−) + Na(+) ≡ Al(−)–O–Si ≡

←→ OH(−) + Na(+)+ ≡ Al +HO–Si ≡()

H

(+)

+OH

(−)

+ ≡ Si–O–Si ≡←→≡ Si–OH +HO

–Si ≡ ()

with the rising temperatures. Tereby, the protons o the

water molecules attack the ≡Si–O–(Al(−))≡ bonds over thewhole ramework whereas the hydroxide ions attack espe-cially the terminal ≡Si–OH groups o the surace. Figures and give a schematic impression o both processes []where water acts like a catalyst [].

In addition to the steaming procedure, a treatment inliquid water is helpul orthe understanding o the hydrother-mal chemistry o aujasite type zeolites. Dimitrijevic et al.[] investigated the behavior o NaY and HY with Si/Al. in water o autogenous pressure between K and

K. Figure shows the sorption capacity o both samplesafer the hydrothermal process. Te uptake o water in NaYremains unchanged, but that o HY decreases signicantly above K. Te proton orm is drastically damaged undertreatment. Te reason or this different behavior consists inthe alkaline reaction o NaY in aqueous medium suppressingthus the attack o protons ormed by water dissociation.

In addition to extraramework silica and alumina, traceso kaolinite occurred. Te ormation o a new crystallinephase below K was detected or the rst time. It is dueto the incomplete sodium/ammonium ion exchange o theprogenitor o the HNaY modication which was obtained by careul heating at K.

Surprisingly, the silicon-rich DAY samples obtained by Al/Si substitution are ound to be stronger attacked by liquidwater as compared with the low-silica samples generatedby steaming. Figure shows the corresponding behavior o hydrothermally treated DAY in dependence on the module(SiO2/Al2O3 ratio).

At onehand,appearance o EFAl bringsa stabilizing effect

to the zeolite ramework [] and a more regular structurein the steamed DAY samples. Some observations indicatedthat the structure o these zeolites heals duringdealumination[, ]. Orthosilicic acid (H4SiO4) migrates rom thecrystal surace into ree ramework vacancies generated dueto removal o aluminium atoms. Migration o H4SiO4 takesplace too, i silica gel obtained by precipitation is hydrother-mally treated in analogy to the preparation procedure o DAY zeolites []. All deects—sorption centers or gases orliquids—are repaired by reorganization o the ramework.Because the ramework becomes more regular with rising o time andtemperature the silica gel loses its sorption behavior.Te asymmetrical O ( = Si) valence vibration in inrared

spectra shifs into the region o wave numbers characteristico pyrogenic silica at – cm−1.

An important observation gives another explanation orthe different behavior o low-silica and high-silica DAY sam-ples. Te degradation o silicon-rich suraces according to ()proceeds ast. But i the surace o such a sample is aluminatedagain [], stabilization o the structure takes place []. Testabilizing effect o the surace layer consisting o sodiumaluminosilicate may be associated with the elimination o theterminal silanol ≡Si–OH groupsand a blocking o the energy-rich ≡Si–O–Si≡ bonds near the crystal surace, where watermolecules attack the ramework. Contrary to the polymericsilica in the dealuminated ramework, this aluminium-rich


10/21


Si Si

Si

Si Si Si

Si

SiSi

SiSi

Si

Si

Si

Si Si

Si

Si Si

Si

Si

Si

Si

Si Si Si

SiSi

Si Si

Si

Si

Si Si

Si

Si SiO O

O

O OO

O

O

O

O

O

O

O O O

O

OOO

O

O O

O

OO

O

O

O

OO

O

O

O

O O

O

O

O

O O

O

O O O

OO

O

O

O

O

O

O

O

OO

O

OH

OH

OH

OH

OH

H

H

H

H

H

H

HOH

HOH

HO

HO

(II)

(I)

(III)

+2HOH

+ 2 H

O H

−

−

−

−

−−

−

−

+

+

+2OH

OH + 2OH

F : Stepwise decomposition o a high-silica zeolite ramework by water [].

280 320 360 400 440 480

150

200

250

300

350

NaY

HNaY

Temperature (K)

W a t e r u p t a k e (

m g g −

1 )

F : Water sorption uptake o zeolites NaY and HNaY aferhydrothermal treatment in water o autogenous pressure or hours in dependence on temperature [].

layer contains only monomeric and dimeric silicate unitswhich are detected by Molybdate measurement. Using suchan approach, high-silica zeolites may be stabilized against ahydrothermal or alkaline attack [].Lutzetal.[] reportedin detail on the external introduction o extraramework sodium aluminosilicate species through an alkaline and com-bined acid/alkaline pathways using sodium aluminate as wellas aluminium-oxo-hydroxo cations []or(Al,SiO) speciesinto aujasite type zeolite []. Alumination in solutions o sodium or potassium aluminate was reported also or other

0 50 100 150 200 250 300

0

20

40

60

80

100

473 K

443 K

423 K

403 K

R e s i d u e o

f D A Y z e

o l i t e ( % )

SiO2/Al2O3 ratio

F : Residue o DAY zeolites afer hydrothermal treatment inliquid water in dependence on module and temperature [].

high-silica zeolites such as silicalite [], MCM- [], andzeolite Beta [].

Te sodium aluminosilicate surace layer can be unc-tionalized as waer or catalytically active ingredient. Teobtained adsorber/catalyst composites were successully tested in the conversion o several organics and applied orwaste water purication.

... Adsorber/Catalyst Composites. Active ingredients inadsorber/catalyst composites on the basis o high-silica DAYzeolites are exclusively localized on the crystal surace while


11/21


0 20 40 60 80 100 1200

20

40

60

80

100

Without catalyst

Pt-DAY

Time (min)

p - c

h l o r o p

h e n o

l ( % )

Fentons reagent (Fe++ salt)

F : Degradation o g per L p-chloro-phenol in water by adding o g H2O2 on g o Fenton’s reagent, respectively, zeolitePt-DAY loaded with mg/g platinum [].

the bulk remains ree or adsorption. Tus, high conversiono acetone on an H-DAY composite with Si/Al = or suchone o -butane on a Ni-DAY or Pt-DAY analogues has beenound []. But the specic application o the compositesremains in the purication o waste waters. Pollutants suchas halogenated hydrocarbons must be removed up to aew nanogram. At the rst step o cleaning, the pollutionwill be separated rom liquid phase by adsorption and atthe second step it is decomposed by chemical reaction, orexample, by oxidation with hydrogen peroxide. Because o the degradation o the toxic agent any regeneration o the

composite is not provided.Te decomposition o p-chloro-phenol in water on Pt-

DAY was tested comparing to Fenton’s reagent (FeSO4) [].Te zeolite was loaded by mg, mg, mg, and mg o Pt per g composite. Te best effect was obtained with mg/gbecause o an optimized activation o the admixed hydrogenperoxide. Figure shows the signicant advantage o thenew composite compared with the classical FeSO4 using aone-step strategy. In a semitechnical plant, pureness degreelower than ppm has been achieved. Details o preparationand application o adsorber/catalyst composites are discussedin a series o patents [–].

... Reinsertion o Extraramework Aluminium into Frame-work Positions. Te catalyticactivity o zeolites is determinedmainly by their ramework aluminium. Te aluminium con-tent may be varied directly in the synthetic process (zeoliteZSM-) or, as has been reported here, by subsequent rame-work dealumination (zeolite Y). Te subsequent insertiono external aluminium into zeolites rameworks has beenhighly discussed rom the beginning o eighties. In this case,aluminium chloride [], clay [], and alkali aluminate[] have been suggested as aluminium sources.

For steamed DAY samples, Breck and Skeels [] haveproposed a reinsertion o the extraramework aluminiumback into the aujasite ramework by treatment o samples

in alkaline solution. Liu and colleagues [] conrmed this

approach by interpretation o 29Si MAS NMR spectra andtermed this process as “ne tuning o perormance o zeoliticcatalysts.” Hamdan et al. [] saw in the “secondary synthe-sis,” a method o achieving an optimal number o Brønstedacid sites responsible or the crack activity or hydrocarbons

within the zeolite ramework. But reexamination o theprocess by Engelhardt and Lohse [] did not conrm thishypothesis. Teir doubts concerned especially the interpre-

tation o measured 29Si MAS NMR spectra. Nevertheless, aseries o NMR studies supported the reinsertion concept o Klinowski et al. [–]. Te signals o Si(nAl) buildingunits shif to a higher values by ramework dealumination.While the peaks o the Si(Al) and Si(Al) units in Figure decrease in intensity, and the signals assigned to Si(Al) andSi(Al) increase.

Lutz et al. reexamined the concept proposed by Liu et al.[] using steamed DAY samples with varying Si/Al ratio[], different conditions o alkaline leaching [], and

acid leached samples beore the alkaline treatment in KOHsolution []. Te authors reported the drastic decreaseo the Si(Al) and the increase o the Si(Al–Al) peakswhich indicates actually an Al enrichment inside the zeolitesramework. However, this phenomenon was also observedwhen the extraramework aluminium was eliminated by acidextraction [, ] (see Figure ). Since in alkaline treatedDAY samples, the newly ormed alkali aluminosilicates con-tribute especially to the Si(Al) and Si(Al) signals; thedecrease o the Si(Al) signal should be ascribed to a yetunknown phenomenon.

Bezmann [], using the Liu approach, studied the massbalance afer KOH treated Y samples and ound % o

the silicon and % o the aluminium in the ltrate o thebatch. Furthermore, the crystallinity o sample decreased to%. Lutz et al. [] provided a detailed mass balance witha similar tendency. Te treatment o aluminosilicates and,particularly, metastable zeolites in alkaline solution gives riseto the change in their crystal structure []. Te effect o increasing solubility o zeolites ramework with increasingSi/Al ratio and the ormation o Al-rich gels during alkalinetreatment o zeolites have already been reported by Zhdanov and Egorova [] and Stach et al. [].

Aouali et al. [] observed desilication and partialdecomposition o highly dealuminated zeolite Y ramework resulting in Al enrichment. Tis is the reason why the amor-

phization o DAY samples increases by treatment in alkalinesolution with the rising o Si/Al ratio seen in badly resolved29Si MAS NMR spectra []. Tereby, % silicon o theramework with Si/Al = was migrated into newly ormedextraramework aluminosilicate or into alkaline solution.

Dessau et al. [] observed a strong corrosion o thelarge crystals o silicalite by a treatment in alkaline solution.Groen et al. [] and Verbroekend and Pérez-Ramirez []generated thus controlled mesoporosity in zeolites. Lutz et al.[] have treated this zeolite on a rit, separating the ormedaluminosilicate in the ltrate. Mirodatos and Barthemeu [] assumed the participation o extraramework silicon inthe ormation o the aluminosilicate.


12/21


(ppm)

(a)

(b)

(d)

(e)

(c)

(f)

−120−110−100−90−80

F : 29Si MAS NMR spectra o zeolites NaY (a),steamed DAY(b), acid leached DAY (c), alkaline leached DAY (d), and alkalineleached DAY afer acid extraction o the extrarameworkaluminiumwith moderate (e) and intensive stirring () [].

In a detailed study on the alkaline reactivity o extra-ramework species Lutz et al. [] have shown that EFSi—silica gel at the crystal surace and, in dependence on thesteaming conditions, aluminosilicate or clay and silica inthe nuclei—contributes to the sample transormation only at a low extent. Te aluminosilicate admixture is protectedagainst the attack o sodium or potassium hydroxide by its negative charge and silica gel ormed at steaming tem-perature o K or more is lowly soluble. Te observednewly ormed sodium or potassium aluminosilicate is gen-

erated exclusively rom Al(3+) cations and silicon removed

0

50

100

150

200

250

P/P0

0.0 0.2 0.4 0.6 0.8 1.0

1 h

5 h

10 h A d s o r p t i o n ( c m

3 ( s t p ) / g )

F : Nitrogen isotherms measured at K on DAY steamedat K and bar water pressure in dependence on time [].

rom the dealuminated ramework. Tis means that the “re-insertion” o extraramework aluminium consists in a desili-cationo the zeolites rameworkand a ormation o additionalX-ray amorphous alkali aluminosilicate.

Because o the exchange o protons against alkali metalcations, the Brønsted activity, or example, in conversiono -pentane [], breaks totally down by use o DAYsamples prepared according to “re-insertion” concept givenby Liu et al. [].

... Secondary Pore Volume in Steamed Y Zeolites. Muchwork has been done to characterize the transition o bulk micropores o DAY zeolites into a secondary pore volume[–]. Formed mesopores were attributed to reorganiza-tion o the ramework afer removal o aluminium ollowedby generation o structural deects. Te healing o the rame-work by migrating o orthosilicic acid (H4SiO4) into deectpositions was discussed too [, , ]. Formation andhealing can be ollowed by inrared analysis o the ormedinternal hydroxyl groups [].

First investigations o the interaction o DAY sampleswith gases and vapors related to their catalytic behaviors havebeen perormed [–] and ollowed later by systematicstudies o their sorption capacity []. Afer the characteri-zation o sorption centers by hydrocarbons, the investigation

o the pore structure using nitrogen at K came into theocus o interest. A typical sign o the mesopores in DAYis the hysteresis between the adsorption and desorptionloop o the nitrogen isotherm above a relative pressureat / over .. Nitrogen liquees in the secondary pore

volume and needs, thereore, more energy or evaporationthan adsorbed nitrogen molecules. Figure shows typicalnitrogen isotherms with such a hysteresis loop.

Using nitrogen uptake techniques, Lutz and colleagues[] detected a ormation o various bulk mesopores independence on the conditions o steaming. Framework deects tend to be occupied by migrating H4SiO4, especially at temperatures o K and K. Corresponding diffusion


13/21


o orthosilicic acid was described in detail by Iler [ , ].Insertion goes down at temperatures above K since thedehydroxylation o the hydroxyl nests accompanied by a rear-rangement o the ramework and the reduced concentrationo offered water takes place.

Mesopores were unquestionably assumed to be the result

o pure dealumination o the microporousbulk. Using textureanalysis and D-EM measurements, Jansen et al. [] visualized large mesopores o .–. nm not only within thenuclei but also near to the surace o the DAY crystals. Similarmorphology was observed also by Lynch et al., who reportedon a secondary pore system without direct connection to theexterior o the crystals [].

Te origin o the closed nuclei and open suracemesopores must be attributed to the inhomogeneous sili-con/aluminium contribution over the Y zeolite crystallites[, , ]. Te semiempirical approaches, which describethe Si/Al ratio o aujasite-type zeolites by use o 29Si MASNMR [] or X-ray data [], did not take into accountpossible effect o such Si/Al gradients [, ]. Due to thene size o the crystallites, a direct proo o the gradientpresence is difficult, or example, using EDX analysis. Buta hydrothermal instability o the aluminum-rich nuclei andthe silicon-rich surace o DAY crystals can be deducedrom investigation o aujasite–type zeolites with analogouscomposition [].

Kortunov et al. [] reported on molecular transportin DAY samples. Te diffusion measurements have beenperormed by use o PFG NMR technique with ,,-triisopropylbenzene and n-octane as probe molecules. Tecritical diameters are, respectively, larger and smaller thanthe inlet o the zeolite micropores. It was ound that -octane molecules entered the whole crystal whereas ,,-triisopropylbenzene could not enter the microporous bulk and thus the nuclei mesopores. Te idea that the secondary pore system improves the molecular transport o startingmaterials and products in catalytic conversion processes [,] must be corrected.

One should urthermore assume that DAY samples usedby Kortunov contained no or only small amounts o bulk mesopores because o healing o the microporous bulk at K. It seems likely that stronger dealuminated DAYsamples steamed, or example, at K, contain such amounto bulk mesopores that traffic o large molecules through themesoporous network becomes possible [–].

Based on current results, one can propose, complemen-

tary to the review on mesoporosity in [, ], a modelcontaining three types o mesopores with closed mesoporesin the crystal nuclei, open mesopores at the crystal surace,and bulk mesopores within microporous bulk. Tis model isshown schematically in Figure .

Generally speaking, even a ourth type o mesoporesmay occur in the steamed zeolite as a result o nonuniormpacking o crystallites with different size in the polycrystallineDAY material. Te variation in average crystallite size hasbeen ound or a series afer dealumination at K, K,and K []. It was shown here and in [] that theaverage diameter determined by XRD measurements usingthe Scherrer equation does not vary signicantly at K

Nuclei mesopore

Crystallite

Macropore

Surface mesopore

Bulk mesopores

F : ypes o mesopores in steamed DAY crystals [].

0 2 4 6 8 100.0

0.2

0.4

0.6

0.8

Time (h)

973 K

873 K

673 K 573 K

V m e s o

/ V t o t a l

F : Relative mesopore volume (Vmeso/Vtotal) o DAY samples

in dependence on temperature and steaming time [].

and K but decreases rom nm to nm afer hourso steaming at K in sample with zeolite particles o nm in size. I crystallites become smaller in this way,their package density increases. Tereore, macropores o the intracrystalline space may shrink into larger mesoporeswhich can be detectable by nitrogen measurements at K.However, this assumption cannot be proved actually.

Formation o the secondary pore system leads to a pro-nounced loss o crystallinity, sorption capacity, and catalyticactivity o the zeolite materials. It would be helpul to reduce

the mesoporous volume. Te rst possibility lies in themoderate steaming o samples to guarantee the healing o theramework deects in the microporous bulk (hydroxyl nests)[]. Perorming a gentle and homogeneous dealuminationwith KY zeolite o higher ramework cell which possessesrather equilibrium distribution o ammonium ions inside thestructure could provide another solution o this problem.And a third way is associated with minimizing the Si/Algradient by seeding o NaY synthesis batches with seedsolution [, ] instead o an insertion o nanoseeds o Xzeolite [].

For an investigation o moderate steaming, commercialNaY zeolite with a Si/Al ratio o . was exchanged with


14/21


0

50

100

150

200

250

300

P/P0

0.0 0.2 0.4 0.6 0.8 1.0

V ( c m

3

g − 1 )

USY, 723 K

USY, 873 K

NaYseedsolution

(a)

0

50

100

150

200

250

P/P0

0.0 0.2 0.4 0.6 0.8 1.0

NaYX-seedsUSY, 723 K

USY, 873 K

V ( c m

3

g − 1 )

(b)

F : Nitrogen isotherms o the initial NaYseed−solution (a) and NaYX−seeds (b) zeolites and the corresponding DAY samples steamed or hours at K and K, respectively [].

ammonium ions up to %. Te ormation and healing o thebulk mesopores in dependence on temperature were detectedby the presentation o the relative change o the pore volumeVmeso/Vtotal during the steaming time []. Figure showsthat the contribution o mesopores to the total volume isconstant at K and K. Only the mesopore surace S

changes slightly with the time. It seems likely that Vmeso isdominated by the volume o the closed nuclei mesoporeswhile the mesopore surace Smeso characterizes mainly theopen surace mesopores.

As we know, a silicon-rich surace acts as source o orthosilicic acid [, ] in silica gel aswell as in the steamedDAY samples. Incorporation o H4SiO4 into the deects insidebulk ramework runs better at low temperature o K– K because water is available under these conditions ina sufficient amount or healing procedure. With temper-ature rise, the portion o bulk mesopores grows slightly at K and signicantly at K. At these temperaturesnonhealed bulk mesopores contribute mainly to the total

mesopore/macropore volume. But healing cannot compen-sate the ramework collapse because o the strong thermalactivation o the ramework and the drastically reducedconcentration o water in the reaction zone.

Synthesis o the ammonium orm o zeolite Y rom aKY modication ollowed the idea to get a homogeneousdealumination with a reduced mesopore ormation. Owingto a bigger size o potassium ions, the ramework o Y zeoliteis expanded rom . nm (NaY) to . nm (KY) and theexchange process should run easier. However, exchange o ammonium or potassium ions succeeded surprisingly only to %, similar to NaY zeolite. But steaming o KY zeolite at K where a certain healing o bulk mesopores still takes

place demonstrated an unexpected result []. Te microp-ore volume o K-DAY was reduced stronger than that o Na-

DAY. It appears that residual Na(+) ions, localized mainly inthe aluminum-rich center o crystals, stabilize the sensitive

≡Si–O–(Al(−))≡ bonds stronger than K(+) ions. Te latter wasmore homogeneously distributed over the ramework whileprotons resulting rom ammonium complex show a tendency to occupy positions within the nuclei. Due to a repeated ionexchange and steaming procedure, the mesopore volume didnot change signicantly or both samples but the Si/Al ratioincreased gradually rom . (parent NaY andKY) to . (rststeaming) and to . or both (second steaming).

Modication o the seeding procedure in NaY synthesislooks more attractive in order to decrease the amountmesopores inside the zeolite nuclei []. NaYseed−solutionand NaYX−seeds samples o same chemical composition weredealuminated in their ammonium orm at K and K.Dealumination changed the ramework ratios to . and .,respectively.

Te mesoporous area Ameso o both series determinedby nitrogen adsorption at K differs signicantly. Values o DAYseed−solution were smaller than those o DAYX−seeds. Tisobservation gives the hint to the absence o nuclei mesoporesin DAYseed−solution. Te corresponding nitrogen isotherms inFigure show no hysteresis loop at both temperatures inves-tigated. Te weak hysteresis or DAYseed−solution prepared at K is only responsible or emptying the surace mesoporeswhich inevitably appear during steaming.

Te occurrence o nuclei mesopores can be reduced by starting the crystallization o NaY with oligomeric alumi-nosilicate that hinders the appearance o a strong Si/Al gra-dient typical o seeding procedures with NaX nanocrystals.


15/21


3. Conclusion

Te ne-tuning o the Si/Al ratio in zeolite Y ramework isachieved by a subsequent steam treatment o NH4NaY sam-ples at temperatures between K and K in dependenceon time. At lower and higher temperatures, decompositionoccurs due to acid hydrolysis in steam or the thermalcollapse o the ramework, respectively. But a certain degreeo the decomposition is observed in all other steamed DAYproducts too. Tis effect results rom a corrosion o theAl-rich nuclei and the Si-rich surace o crystallites whichare both hydrothermally instable. Te Si/Al gradient insidethe ramework resulting rom the specic synthesis o theparent NaY zeolite can be diminished i the synthesis batchis seeded by a so-called seed solution rather than with NaXnanocrystals.

Due to the ramework corrosion, inner and outer meso-pores occur in addition to bulk mesopores which appearafer removal o ramework aluminium. Concentration o inner and outer mesopores can only be reduced decreasing

the Si/Al ramework gradient. In contrast, bulk mesoporesare healed by the migration o orthosilicic acid (H4SiO4)into vacancies o the ramework at steaming temperatures o K and K, especially. All observed nuclei mesoporesare closed thereore they cannot contribute to the transporto greater molecules.

In addition to extraramework aluminium, extrarame-work siliceous species occur as Al aluminosilicate and sil-ica gel. With increasing steaming temperature, the alumi-nosilicate is decomposed into clay and silica gel. With therising temperature, all extraramework components becomeinactive in alkaline treatment. Tereore, the process o Alenrichment o the zeolite ramework during alkaline leaching

is associated actually not with migrating o extra ramework Al species back into the lattice but it results rom the act thatthe DAY ramework becomes desilicated afer such alkalinetreatment.

Conflict of Interests

Te author declares that there is no conict o interestsregarding the publication o this paper.

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[] N. Salman, C. H. R ̈uscher, J.-C. Buhl, W. Lutz, H. ouar, andM.Stöcker, “Effect o temperatureand time in the hydrothermaltreatment o HY zeolite,” Microporous and Mesoporous Materi-als, vol. , no. –, pp. –, .

[] U. Lohse, E. Alsdor, and H. Stach, “Dealuminierte Moleku-larsiebe vom yp Y. Herstellung und Charakterisierung durchIR-Spektren, DA/DG-Messungen und Adsorptionsdaten,”Zeitschrif ̈ur Anorganische und Allgemeine Chemie, vol. , no., pp. –, .

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[] W. Lutz, C. H. R ̈uscher, and D. Heidemann, “Determination o the ramework and non-ramework [SiO2] and [AlO2] specieso steamed and leached aujasite type zeolites: calibration o IR,NMR, and XRD data by chemical methods,” Microporous and Mesoporous Materials, vol. , no. , pp. –, .

[] R. Bertram, U. Lohse, and W. Gessner, “Zur Charakterisierungdes Extragitter-Aluminiums in Y-zeolithen mittels der Ferron-methode,” Zeitschrif ̈ur Anorganische und Allgemeine Chemie, vol. , no. , pp. –, .

[] H. Fichtner-Schmittler, U. Lohse, H. Mießner, and H. E.Maneck, “Correlation between unit-cell parameter, skeletalstretching vibrations and molar raction o aluminium o aujasite type zeolites or Si/Al = .–,” Zeitschrif ̈ur Physikalische Chemie, vol. , pp. –, .

[] C. H. R ̈uscher, J.-C. Buhl, and W. Lutz, “-P--determinationo the Si/Al ratio o aujasite-type zeolites,” in Zeolites and Meso- porous Materials at the Dawn o the st Century: Proceedingso the th International Zeolite Conerence, A. Galarneau, F.di Renzo, F. Fajula, and J. Vedrine, Eds., vol. o Studies inSurace Science and Catalysis, p. , Elsevier, Amsterdam, TeNetherlands, .

[] G. Engelhardt and D. Michel, High Resolution Solid State NMR

o Silicates and Zeolites, John Wiley & Sons, NewYork, NY, USA,.

[] W. Lutz, C. H. R ̈uscher, . M. Gesing et al., “Investigations o the mechanism o dealumination o zeolite Y by steam: tunedmesopore ormation versus the Si/Al ratio,” in Recent Advancesin the Science and echnology o Zeolites and Related MaterialsPart B: Proceedings o the th International Zeolite Conerence,E. van Steen, L. H. Callanan, and M. Claeys, Eds., vol. o Studies in Surace Science and Catalysis, pp. –, Elsevier,London, UK, .

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[] R. K. Iler, Te Chemistry o Silica, John Wiley & Sons, NewYork,NY, USA, .

[] W. Lutz, E. Löffler, and B. Zibrowius, “Non-ramework alu-minium in highly dealuminated Y zeolites generated by steam-ing or substitution,” in Progress in Zeolite and Microporous Mate-rials: Preceedings o the th International Zeolite Conerence, H.Chon, S.K. Ihm, and Y. S.Uh, Eds., vol.o Studies in SuraceScience and Catalysis, pp. –, Elsevier, Amsterdam, TeNetherlands, .

[] D. P. Siantar, W. S. Millman, and J. J. Fripiat, “Structural deectsand cation exchange capacity in dealuminated Y zeolites,”Zeolites, vol. , no. , pp. –, .

[] W. Lutz, H. ouar, D. Heidemann et al., “Siliceous extra-ramework species in dealuminated Y zeolites generated by steaming,” Microporous and Mesoporous Materials, vol. , no.–, pp. –, .

[] K. U. Gore, A. Abraham, S. G. Hegde, R. Kumar, J. Amoureux,and S. Ganapathy, “29Si and 27Al MAS/Q-MAS NMR studieso high silica USY zeolites,” Journal o Physical Chemistry B, vol., no. , pp. –, .

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[] A. Corma, F. V. Melo, and D. J. Rawlence, “Effect o thenonuniorm dealumination on the acidity and catalytic activity

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[] K. Ehrhardt, M. Suckow, and W. Lutz, “Hydrothermal decom-position o aluminosilicate zeolites and prediction o theirlong-term stability,” in Catalysis by Microporous Materials:Proceedings o ZEOCA ’, H. K. Beyer, H. G. Karge, I. Kiricsi,and J. B. Nagy, Eds., vol. o Studies in Surace Science and Catalysis, pp. –, Elsevier, .

[] W. Lutz, E. Löffler, M. Fechtelkord, E. Schreier, and R. Bertram,“Non-ramework aluminium in highly dealuminated Y zeolites

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[] W. Lutz, D. Heidemann, C. Hübert, and W. Wieker, “Contribu-tion o silica gels to superimposed 29Si MAS NMR spectra o Yzeolites dealuminated by steaming,” Zeitschrif ̈ur Anorganischeund Allgemeine Chemie, vol. , no. , pp. –, .

[] W. Lutz, W. Wieker, D. Müller, M. Schneider, C. H. R ̈uscher,and J.-C. Buhl, “Phase transormations in alkaline and acidleached Y zeolites dealuminated by steaming,” Zeitschrif ̈ur Anorganische und Allgemeine Chemie, vol. , no. , pp. –, .


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[] W. Lutz,R. Bertram, D.Heidemann, R. Kurzhals, C. H. Rüscher,and G. Kryukova, “Reactivity o extra-ramework species o USY zeolites in alkaline medium,” Zeitschrif ̈ur Anorganischeund Allgemeine Chemie, vol. , no. , pp. –, .

[] W. Lutz, D. äschner, R. Kurzhals, D. Heidemann, and C.Hübert, “Characterization o silica gels by 29 Si MAS NMR andIR spectroscopic measurements,” Zeitschrif ̈ur Anorganischeund Allgemeine Chemie, vol. , no. -, pp. –, .

[] J. Felsche and S. Luger, “Phases and thermal decomposi-tion characteristics o hydro-sodalites Na+,[AlSiO],(OH) ⋅HO,” Termochimica Acta, vol. , pp. –, .

[] G. Engelhardt, J. Felsche, and P. Sieger, “Te hydrosodalitesystem Na+,[SiAlO],(OH) ⋅ HO: ormation, phase com-position, and de- and rehydration studied by 1H, 23Na, and 29SiMAS-NMR spectroscopy in tandem with thermal analysis, X-ray diffraction, and IR spectroscopy,” Journal o the AmericanChemical Society , vol. , no. , pp. –, .

[] J.-C. Buhl, M. Gerstmann, W. Lutz, and A. Ritzmann,“Hydrothermal stability o the novel zeolite type LSX in com-parison to the traditional X modication,” Zeitschrif ̈ur Anorganische und Allgemeine Chemie, vol. , no. , pp. –, .

[] U. Lohse, Zur Struktur und zu den Adsorptionseigenschafenvon siliciumreichen Molekularsieben [Ph.D. thesis], Academy o Sciences o GDR, Berlin, Germany, .

[] W. Lutz, B. Zibrowius, and E. Löffler, “Hydrothermal andalkaline stability o high-silica Y-type zeolites in dependenceon the dealumination procedure,” in Zeolites and Related Microporous Materials: State o the Art —Proceedings o theth International Zeolite Conerence, Garmisch-Partenkirchen,Germany, – July , J. Weitkamp, H. G. Karge, H. Peier,and W. Hölderich, Eds., vol. o Studies in Surace Science and Catalysis, pp. –, Elsevier, .

[] W. Lutz, B. Zibrowius, and E. Löffler, “Hydrothermal and

alkaline stability o high-silica Y zeolites generated by combin-ing substitution and steaming,” in Zeolites: A Rened ool or Designing Catalytic Sites: Proceedings o the International ZeoliteSymposium, L. Bonneviot and S. Kaliaguine, Eds., vol. o Studies in Surace Science and Catalysis, pp. –, Elsevier,.

[] W. Lutz, D. Heidemann, R. Kurzhals, and G. Kryukova, “Char-acterisation o siliceous extra-ramework species in day zeo-lites by 29Si MAS NMR and IR spectroscopic measurements,”Zeitschrif ̈ur Anorganische und Allgemeine Chemie, vol. ,no. , pp. –, .

[] W. Lutz, H. ouar, R. Kurzhals, and M. Suckow, “Investigationand modeling o the hydrothermal stability o technically relevant zeolites,” Adsorption, vol. , no. -, pp. –, .

[] R. Dimitrijevic, W. Lutz, and A. Ritzmann, “Hydrothermalstability o zeolites: determination o extra-ramework specieso H-Y aujasite-type steamed zeolite,” Journal o Physics and Chemistry o Solids, vol. , no. , pp. –, .

[] W. Lutz, “Stabilizing effect o non-ramework AL on the struc-tureo dealuminated y zeolites underhydrothermal conditions,”Crystal Research and echnology , vol. , no. , pp. –,.

[] A. . Steeland E. Dooryhee, “ime dependence o thestructuralchanges occurring in NH-Y zeolite on dealumination: apreliminary study using energy-dispersive X-ray diffraction,”Zeolites, vol. , no. , pp. –, .

[] U. Lohse and M. Mildebrath, “Dealuminierte Molekularsiebe vom yp Y zur Porosität dealuminierter Molekularsiebe,”

Zeitschrif ̈ur Anorganische und Allgemeine Chemie, vol. ,no. , pp. –, .

[] W. Lutz, E. Löffler, and B. Zibrowius, “Increased hydrothermalstability o highly dealuminated Y zeolites by alumination,”Zeolites, vol. , no. , pp. –, .

[] W. Lutz, W. Gessner, R. Bertram, I. Pitsch, and R. Fricke,“Hydrothermally resistant high-silica Y zeolites stabilized by covering with non-ramework aluminum species,” Microporous Materials, vol. , no. –, pp. –, .

[] S. Schönherr, H. Görz, W. Gessner, and R. Bertram,“Protolysevorgänge in w ̈aßrigen Aluminiumchloridlösungen,”Zeitschrif ̈ur Chemie, vol. , no. , pp. –, .

[] I. Pitsch, U. Kürschner, D. Müller et al., “Synthesis, charac-terization and catalytic activity o amorphous Al,SiO gelsrom weakly acidic aqueous solutions,” Journal o MaterialsChemistry , vol. , pp. –, .

[] M. W. Anderson, J. Klinowski, and X. Liu, “Aluminationo highly siliceous zeolites,” Journal o the Chemical Society,Chemical Communications, no. , pp. –, .

[] H. Hamdan, S. Endud,H. He, M. N. M. Muhid, and J.Klinowski,

“Alumination o the purely siliceous mesoporous molecularsieve MCM-and its hydrothermal conversion intozeolite Na-A,” Journal o the Chemical Society, Faraday ransactions,vol.,no. , pp. –, .

[] C. Yang and Q. Xu, “Aluminated zeolites and their propertiespart .-Alumination o zeolites ,” Journal o the Chemical Society, Faraday ransactions, vol. , pp. –, .

[] W. Lutz, D. L. Hoang, G. Lischke, and B. Parlitz, “Extra-ramework aluminium in DAY zeolites as carrier or catalyticingredients,” in Proceedings o the rd Polish-German ZeoliteColloquium, M. Rozwadowski,Ed., pp. –, Nicolas Coper-nicus University Press, .

[] W. Lutz, R. Bertram, W. Wieker, and M. Jank, “Adsorber-Katalysator-Komposite ̈ur Umweltprozesse,” in Neue Entwick-

lungen zur adsorptiven Gas- und Wasserreinigung , W. Henschel,Ed., vol. o Freiberger Forschungshefe A: Verahrenstech-nik/Umwelttechnik, p. , Bergakademie Freiberg, Freiberg,Germany, .

[] W. Lutz, P. Kleinschmit, and E. Roland, “Removing organicsubstances rom aq. soln. or sepn. o harmul or useul organicsubstances,” DE Patent , .

[] W. Lutz, “Verahren zum Enternen von Stoffen aus w ̈aßrigenLösungen und Adsorptionsmittel,” DE Patent , .

[] W. Lutz, D. Hoang, R. Fricke, and H. Lieske, “Asorber/catalystcomposite,” DE Patent , .

[] W. Lutz, W. Gessner, and R. Bertram, “Hydrothable catalytically active composites or separating organics rom uid phase,” DEPatent , .

[] B. Schlicht, H. J. Redlich, G. Höhne, and W. Lutz, “Ver-ahren zum

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Zeolite Y Synthesis Modification, And purification