natural and sodium clinoptilolites submitted to acid treatments: experimental and theoretical...

Natural and Sodium Clinoptilolites Submitted to Acid Treatments:Experimental and Theoretical StudiesAramis Rivera,*,† Tania Farías,† Louis Charles de Menorval,‡ Miguel Autie-Perez,§,∥ and Anabel Lam*,†

†Zeolites Engineering Laboratory, Institute of Materials Science and Technology (IMRE), University of Havana, 10400 Havana, Cuba‡Institut Charles Gerhardt Montpellier, UMR-5253 CNRS-UM2, Equipe Agregats, Interface, et Materiaux pour l’Energie (AIME),Universite Montpellier 2, C.C. 1502 Place Eugene Bataillon, 34095 Montpellier Cedex 5, France§Molecular Engineering Laboratory, Institute of Materials Science and Technology (IMRE), University of Havana, 10400 Havana,Cuba∥Departamento de Fundamentos Químicos Biologicos (FQB), Facultad de Ingeniería Química, Instituto Superior Politecnico Jose Antonio Echeverría, 10400 Havana, Cuba

*S Supporting Information

ABSTRACT: In the present work, the effect of acidtreatments on the structure of a natural and sodium exchangedclinoptilolite was evaluated using experimental and theoreticalmethods. The results demonstrated the good stability of thesamples submitted to HCl treatments, although it was proventhat aluminum was extracted from the framework. It wasverified that the sodium clinoptilolite (AZ) is more resistantthan its natural form (NZ) to the acid treatment since thealuminum extraction is smaller and the percent of estimatedcrystallinity is higher in AZ. An increase in the microporevolume, as well as the creation of new narrow micropores, wasalso verified. The simulation results indicated that thealuminum at T2 position is the easiest to remove during the dealumination process, and it was also noted that, duringdealumination, different slabs are formed in the structure, creating a framework like a clay. Calculations suggested that thestability of the dealuminated frameworks was related to attractive and repulsive interactions, which take place between the speciesinvolved in the dealumination process. Our work demonstrates that sodium modification is an essential step to obtain astructurally stable acidic natural clinoptilolite.

1. INTRODUCTION

Clinoptilolite is one of the most abundant and economicallyimportant natural zeolites. It belongs to HEU-type zeolites andits s impl ified empir ica l formula is (Na,K,Ca,M-g)6[Al6Si30O72]·nH2O.

1 The crystalline structure containsthree types of channels confined by tetrahedral ring systems.The channels A and B run parallel to the c-axis and are confinedby ten- and eight-membered rings, respectively, while the Cchannels, confined by another set of eight-membered rings, runparallel to the a-axis and the [100] direction.2,3 The channelsare occupied by cations partially or totally coordinated by watermolecules, which play an important role in the adsorption andthermal properties of the zeolite.4,5

The modifications of natural zeolites, as ion exchange,thermal treatment, and dealumination by acid treatment, areapplied to change their properties, which have a great influencein practical applications.1,6−9 Acid treatments of zeolites havebeen widely used to modify their adsorptive characteristics.10,11

It is known that, in acid medium, zeolites may interact withhydronium (H3O

+) ions taking place the exchange by theirnatural cations and the hydrolysis of the Al−O−Si bonds,

which lead to the dealumination of the framework. Deal-umination of a zeolite was first reported by Barrer and Makki,12

who progressively removed aluminum from natural clinoptilo-lite by washing it with hydrochloric acid. Although the acidtreatment is one of the most common methods of obtaining H-forms of zeolites, its application is rather limited as a result ofthe instability of the framework of most natural zeolites.Therefore, this simple method can be used only to obtain theH-forms of silica rich and relatively acid-stable zeolites asclinoptilolite, mordenite, and erionite.H-forms of zeolites differ greatly from nonmodified zeolites

in terms of their adsorption properties. Clinoptilolite zeolitesdealuminated via acid treatment have been investigated in orderto prepare efficient catalyst, molecular sieves, and adsorb-ents.1,13−15 Acid forms of Cuban natural clinoptilolite (NZ)were prepared by two different methods in order to obtain Li-enriched forms for their potential use as ions release matrix.16

Received: November 22, 2012Revised: January 31, 2013Published: January 31, 2013

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© 2013 American Chemical Society 4079 dx.doi.org/10.1021/jp3115447 | J. Phys. Chem. C 2013, 117, 4079−4088

pubs.acs.org/JPCC

The atomic details involved in dealumination of zeolites arenot well-known due to the complexity of the process and thelimitations of the experimental techniques. For applicationssuch as catalysis and adsorption, it is very useful to know whichAl sites are more easily removed in the dealumination process,and the changes produced in the framework after this removal.Simulation is a useful tool to know these details. However, onlyfew theoretical papers have been published on this topic andnone about clinoptilolite, as far as we know. For example,Vlught et al.17 studied the dealumination of mordenite using acombination of experiments and simulations based on kineticMonte Carlo technique. Most recently, Swang et al.18 proposedthe reaction paths for the dealumination of chabazite usingperiodic density functional calculations.In the present work, experimental and theoretical studies

have been used to evaluate the impact of the dealuminationprocess, by acid treatments, on the Al framework distribution ofnatural- and sodium-exchanged clinoptilolites, as well as ontheir stability and porosity. Our motivation is to develop newstable materials based on natural zeolites with potentialapplications and to know in detail their chemical and physicalcharacteristics. A further motivation is that, very recently,promising results on the adsorption and separation of n-paraffins on an acid modified sodium clinoptilolite have beenreported by us,9 which has attracted a rapid interest in thecommunity.

2. METHODS2.1. Preparation of the Zeolitic Materials. The raw

zeolitic material used in this study was the purified naturalclinoptilolite, NZ, from Tasajeras deposit (Cuba), which wasobtained by means of a mineral benefit method reportedelsewhere.19,20 NZ is a powder of 37−90 μm particle size,mainly consisting in a mixture of clinoptilolite (70%),mordenite (5%), anorthite (15%), and quartz (10%). Amodified form of NZ enriched with sodium was obtained byion exchange with a 1 mol L−1 NaCl solution.21 The resultingsample was called AZ. The oxide form chemical compositionsof NZ and AZ are presented in Table 1.

The acid forms of NZ and AZ were prepared treating thesolids with HCl solutions at different concentrations (0.001,0.05, 0.1, 0.3, 0.5, and 1 mol L−1). The solid liquid ratioemployed was 1 g/250 mL. The treatments took place during 1h at room temperature (303 K). After that, the systems werecentrifuged, and the solids were dried at 353 K. The obtainedsamples were labeled NZH-0.001, NZH-0.05, NZH-0.1, NZH-0.3, NZH-0.5, and NZH-1, and AZH-0.001, AZH-0.05, AZH-0.1, AZH-0.3, AZH-0.5, and AZH-1, respectively. The numberin the sample names denotes the employed acid concentration.For selected samples (NZH-0.1, NZH-0.5, and NZH-1), aportion of material was washed several times with distilledwater until neutral pH was reached to remove the acid excessand the extra framework aluminum species. The washedsamples were named NZH-0.1w, NZH-0.5w, and NZH-1w.

2.2. Characterization of the Samples. Atomic emissionspectroscopy with inductively coupled plasma (ICP-AES) wasused to determine the Si and Al contents in selected samples.An ARL-3580 ICP-OE spectrometer was employed. For eachanalysis, 0.2 g of material was digested using a mixture ofHClO4, HF, and HCl. For the analysis of Si, the samples weredigested by alkaline fusion.Qualitative X-ray diffraction (XRD) analysis was used to

determine if the transformations applied to NZ and AZproduces variations in the zeolite lattice parameters. Thepossible appearance of new phases and changes in the relativediffraction intensities of the different mineral phases was alsoevaluated. A Philips Xpert diffractometer was employed, usingCu Kα radiation (λ = 1.541838 Å). The 2θ range from 5 to 40°was swept at 2° min−1 in a continuous way.

27Al MAS NMR measurements were carried out to know theevolution of this element on clinoptilolite after the treatments.The spectra of the solid samples were obtained at roomtemperature using an ASX-300 Bruker spectrometer at 78.2MHz, with a pulse length of 3.00 μs at 30°, a recycle time of 1 s,and a spinning rate of 10 kHz. A total of 2048 scans wereaccumulated and referenced to Al(NO3)3. Considering thatdifferent materials were analyzed, similar amounts of sampleswere used to fill the rotor for NMR measurements, in order tocompare the spectra under the same experimental conditions.The nitrogen adsorption experiments were preformed to

study the variations in the textural parameters of the zeoliticsamples, after the applied treatments. For isotherms measure-ments, the samples were evacuated during 10 h at 573 K and1.3 × 10−2 Pa. The experiments were performed at 77 K using aMicromeritics 2010 system. The experimental data up to P/P0= 0.7 were fitted by the Dubinin’s two terms equation (DTT),which can be expressed as22

β β= − + −

⎛⎝⎜

⎞⎠⎟

⎛⎝⎜

⎞⎠⎟W W

AE

WAE

exp exp0101

2

0202

2

(1)

where W is the adsorbate volume retained in the adsorbent at agiven relative vapor pressure P/P0,W01,W02 and E01, E02 are themicropore volumes and the characteristic adsorption energiesof the narrow micropores and of the broad ones, respectively, βis an affinity coefficient that permits the comparison of theadsorption potential between the test adsorbate and a referenceadsorbate, and A is the thermodynamic adsorption potential,defined as A = RT ln(P0/P), being R the universal gas constantand T the adsorption temperature. The DTT equation, basedon the theory of volume filling of micropores (TVFM), is veryuseful for the description of gas adsorption in adsorbents withtwo micropore distributions (as carbons), but it is notfrequently used in zeolites.23−25

2.3. Simulation Details. Periodic density functional theorycalculations were performed in order to obtain the most stablegeometry of the Na- and H-clinoptilolite and the differentdealuminated samples. The starting structure for the theoreticalstudies was the sodium clinoptilolite unit cell with sixaluminums located in T2 and T3 sites as was suggested byRuiz-Salvador et al.26 Aluminum atoms located at positions 11,12, 15, and 16 are in T2 sites, while those located at positions17 and 21 are in T3 sites (see Figure 1). Inside the cell thereare 20 water molecules and the corresponding compensatingcations. In the starting structure, the six sodium cations arelocated at the center of channels A and B of the clinoptilolite(positions M1 and M2, respectively), and they are allowed to

Table 1. Oxide-Form Chemical Composition of NZ and AZin wt % (With the Balance As H2O)

sample SiO2 Al2O3 Na2O K2O CaO MgO Fe2O3

NZ 63.5 11.1 2.0 0.6 4.3 0.5 1.1AZ 64.5 11.3 4.3 0.6 1.6 0.5 1.0

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move freely during the simulation. Four of the sodium cationscan be found in A channels (located at M1 position), wheretwo of them are sharing the same channel. The other two Na+

cations are in B channels (situated at M2 position), one perchannel. This sodic structure was called AZ-sim. To obtain theacid structure, the sodium cations were substituted by hydrogenprotons. Each proton is bonded to one oxygen, which isbonded to aluminum following the criterion to minimize thesteric hindrance with the framework.In the acid structure (HZ-6Al), gradual aluminum removal

was achieved by taking out one aluminum and its compensatingcation (H+) each time. The valence of the four oxygen atomsbonding to the removed aluminum was completed withhydrogen forming silanol groups (≡SiOH). The energies ofthe different dealuminated structures generated in each removalstep were calculated. The most stable dealuminated structurewas used for the next Al extraction step. The dealuminatedstructures were labeled HZ-nAl, where n corresponds to thenumber of Al atoms that remain in the structure.

The geometries, the cell parameters, and the total energies ofall samples were optimized and calculated using densityfunctional theory (DFT) as implemented in SIESTAcode.27,28 The Perdew−Burke−Ernzerhof generalized gradientfunctional was used to treat electron exchange andcorrelation.29 The Troullier−Martins pseudo potentials30 thathave been used previously31,32 were applied. Localized basissets with double-ζ polarization orbital were used for all valencestates. The positions of all atoms and the lattice were relaxedusing a conjugate gradients algorithm.33

3. RESULTS AND DISCUSSION

Figure 2 shows the X-ray diffraction patterns of NZ and AZbefore and after the acid treatments at selected concentrations.According to the most intense diffraction peaks, clinoptiloliteand mordenite are the main zeolitic phases in the samples. Thesharpness of the peaks and the negligible variation of theirpositions before and after the transformations suggest a goodstructural stability of the materials. Therefore, the appliedtreatments do not seem to provoke collapse or significantdestruction of the zeolitic structure, at least detectable by thistechnique. The relative intensities of some diffraction peakscorresponding to the zeolitic phases varied as a result of theacid treatment, in particular, the reflections corresponding tothe (020) and (200) planes. These results are coherent withthose reported by Petrov et al.34 In their work, the authorsattributed the intensity variations to the ionic exchange. Inaddition, our simulation results suggest that the decrease in theintensity is also due to the removal of aluminum atoms locatedat these planes. It produces a restructuration of the frameworkin such way that a lower number of atoms are located in theseplanes. This also generates a decrease in the interplanardistances (d) during the dealumination process (see Table 2).In the present work, the crystallinity is defined as the sum of

the intensities of the [020], [200], [131 400], [4 21], [151], and[530] diffraction planes, normalized by assuming that this sumfor NZ, AZ, and HZ-6Al corresponds to 100% of crystallinity,following previous works.17,35,36 Figure 3 shows the estimated

Figure 1. Acid clinoptilolite unit cell. The colors of aluminum, oxygen,silicon, and hydrogen atoms are blue, red, green, and white,respectively.

Figure 2. X-ray diffraction patterns of NZ before and after HCl treatments at different concentrations (A) and AZ before and after HCl treatment aswell as the patterns obtained from simulations (B). The Miller indices of the diffraction planes of the clinoptilolite and mordenite are pointed out forthe NZ sample.



crystallinity percent of the experimental samples (NZ and AZ)and simulated structures (HZ-nAl) submitted to dealumination,as a function of the framework silicon/aluminum ratio (Si/Al).The framework Si/Al ratio of the experimental data wasestimated from the 27Al MAS NMR and chemical analysis, aswill be seen later.

Simulation results suggest that even after extreme deal-umination (sample HZ-1Al) about a 67% of crystallinity isretained. This fact can be explained taking into account that, inthe simulations, the aluminum atoms are removed in a perfectand infinite crystal, and the surface defects produced by the acidattack are not taken into account. In addition, the frameworkcan be reordered after each removal step.Regarding the experimental samples, AZ has better resistance

to the acid attack than NZ. It is evidenced in the highercrystallinity and smaller aluminum removal of AZ, under thesame experimental conditions. This result confirms the role ofthe exchangeable cation in the acid resistance of theclinoptilolite. It is accepted that during the acid treatmentdifferent reactions could occur, which generally take place bysteps.37 At the beginning of the treatment, the native cations areexchanged by H+. In the next step occurs the dealumination ofthe framework without visible structural changes. In the laststage, the disintegration of the framework may be observed,resulting in an amorphous phase. The extent of these reactionsdepends on the acid concentration, the temperature and thecontact time. Rozic et al.38 studied the clinoptilolite cationexchange with H+ ions for a zeolite from Croatia. The authorsconcluded that the exchange order of the clinoptilolite nativecations is Na+ > Mg2+ > Ca2+ > K+. Since the ion exchangereaction Ca2+/H+ is less favored than Na+/H+,38 it is expectedthat in NZ the attack of the H+ to the Al−O bonds could beginat a lower acid concentration. Note in Table 1 that NZ ismainly a calcium clinoptilolite and AZ is basically a sodiumclinoptilolite.The unit cell parameters and unit cell volume of the

experimental samples, determined with the celref software (unitcell refinement software), and those obtained from the resultsof the simulations are presented in Table 3. For bothexperimental and simulated samples, when the Na+ cationsare exchanged by H+, no significant changes in the cellparameters are observed. Such parameters do not changenoticeably when the raw materials, NZ and AZ, are treated with0.1 and 0.5 M HCl solutions. However, with the increment ofthe acid concentration to 1 M, a slight decrease in the unit cellvolume and in the unit cell parameters takes place. In thesimulated samples, the cell volume decreases almost linearly

Table 2. X-ray Parameters for Simulated Samples: Acid andDealuminated Clinoptilolite, HZ-6Al and HZ-4Al,Respectively

HZ-6Al HZ-4Al

hkl 2θ (deg) d (Å) 2θ (deg) d (Å)

020 9.87 9.063 9.98 8.856200 11.35 7.834 11.34 7.799131 22.68 3.972 22.85 3.889400151 30.37 2.982 30.70 2.910530 32.73 2.749 32.90 2.720

Figure 3. Crystallinity percent as a function of the framework silicon/aluminum ratio (Si/Al) for the experimental samples and simulatedstructures, before and after dealumination.

Table 3. Unit Cell Parameters and Cell Volume for NZ, AZ, Their Acid Modified Forms at Different Concentrations, and theSimulated Structures

sample a (Å) b (Å) c (Å) β (deg) volume (Å3)

NZ 17.657 ± 0.014 17.913 ± 0.013 7.410 ± 0.006 116.33 ± 0.01 2101 ± 3NZH-0.1 17.649 ± 0.015 17.914 ± 0.016 7.395 ± 0.006 116.38 ± 0.01 2094 ± 3NZH-0.5 17.611 ± 0.020 17.930 ± 0.016 7.397 ± 0.007 116.25 ± 0.01 2095 ± 4NZH-1 17.625 ± 0.016 17.898 ± 0.016 7.372 ± 0.007 116.26 ± 0.01 2085 ± 3AZ 17.631 ± 0.018 17.940 ± 0.018 7.407 ± 0.006 116.24 ± 0.01 2101 ± 3AZH-0.1 17.651 ± 0.015 17.963 ± 0.016 7.417 ± 0.007 116.55 ± 0.01 2104 ± 3AZH-0.5 17.637 ± 0.018 17.968 ± 0.016 7.399 ± 0.007 116.26 ± 0.01 2103 ± 4AZH-1 17.654 ± 0.017 17.953 ± 0.017 7.363 ± 0.007 116.10 ± 0.01 2096 ± 3AZ-sim 17.716 18.227 7.541 115.71 2193HZ-6Al 17.613 18.136 7.474 117.14 2123HZ-5Al 17.628 17.918 7.461 117.85 2083HZ-4Al 17.603 17.723 7.452 117.54 2060HZ-3Al 17.624 17.538 7.473 117.88 2040HZ-2Al 17.587 17.322 7.499 118.03 2015HZ-1Al 17.518 17.185 7.528 117.93 2000HZ-0Al 17.558 17.242 7.542 118.16 2009



with the number of aluminum atoms removed. The unit cellcontraction during dealumination is probably related to theshorter length of Si−O bonds with respect to those of Al−O, asit has been reported in previous work.39 Behaviors similar toour experimental and theoretical results have been reported byothers for zeolite samples submitted to similar treatments.39−41

The 27Al MAS NMR spectra of the raw zeolites NZ and AZand the acid modified samples are shown in Figure 4. Thespectra of NZ and AZ show only one line at around 55 ppm,associated to Al atoms located in framework tetrahedralpositions. However, the spectra corresponding to the treatedsamples show an additional line at 0 ppm, characteristic of Alatoms with octahedral coordination, which indicates thedealumination of the framework. As can be noticed, thedealumination process begins to occur at relatively low valuesof acid concentration (0.05 mol L−1), and the dealuminationdegree increases with the acid concentration being evidencedby an increment in the intensity of the octahedral peak.Figure 4 shows also the spectra of the washed samples

(NZH-0.1w, NZH-0.5w, and NZH-1w). It is evident that bymeans of the washing processes it is possible to remove most ofthe octahedral aluminum species that are present in thesamples. As it is expected, the aluminum atoms near the surfacewill be more susceptible to the acid attack,42 and that is why theresulting species will be easily removed during the washingprocesses. It is interesting to note that the intensities of the 0ppm signals for these samples are very similar, so the extra-framework aluminum amount is almost the same in all thecases. The remaining octahedral species could be due tohydrolysis of the complex [Al(H2O)6]

3+ that takes place whenincreasing the pH successively with the washed steps. Thesespecies are poorly soluble in water, thus they are retained in thematerials. In addition, it could be also possible that somealuminum is partially connected to the framework, so itscomplete elimination is not possible.

The integrated area intensities of the signals at 55 and 0 ppmand the total and framework Si/Al ratio for each sample arepresented in Table 4. The chemical analysis provides the total

amount of aluminum (framework and extra-framework) in thesample and, consequently, the total Si/Al ratio, (Si/Al)t. Thedata from 27Al MAS NMR allows to calculate the frameworkSi/Al ratio (Si/Al)fr, through the expression

=+I I

I(Si/Al) (Si/Al)

( )fr t

55 0

55 (2)

where I55 and I0 are the integrated areas of the signals at 55 and0 ppm, respectively, in the 27Al MAS NMR spectra.As can be observed, the aluminum content diminishes with

the increase in the HCl concentration employed in thetreatment, increasing the total Si/Al ratio. In the acid samples(Si/Al)fr > (Si/Al)t, which indicates that only a small part of thealuminum extracted from the framework abandons the material,while extra-framework aluminum species remain in the sampleafter the treatment. It should be also noted that the extent of

Figure 4. 27Al MAS NMR spectra of NZ (A) and AZ (B) before and after the treatments with HCl solutions at different concentrations. The dottedlines correspond to the spectra of the washed samples NZH-0.1w, NZH-0.5w, and NZH-1w.

Table 4. Total and Framework Si/Al Ratio Calculated byAAS and 27Al MAS NMR for NZ, AZ, and Their AcidModified Forms

sample Si (wt %)Al (wt%) (Si/Al)t

a I55 (%) I0 (%) (Si/Al)frb

NZ 29.7 5.9 4.8 100.0 0.0 4.8NZH-0.1 31.4 5.3 5.7 94.4 5.6 6.0NZH-0.5 30.8 4.7 6.3 82.9 17.1 7.6NZH-1 30.2 4.2 6.9 76.2 23.8 9.0AZ 30.2 6.0 4.8 100.0 0.0 4.8AZH-0.1 31.4 5.4 5.6 88.4 11.6 6.3AZH-0.5 30.9 5.0 5.9 82.9 17.1 7.1AZH-1 30.2 4.6 6.3 74.9 25.1 8.4

aTotal Si/Al ratio. bFramework Si/Al ratio.



the dealumination is greater for acid samples of NZ, which iscoherent with the percent of crystallinity of the samples thatwas previously reported.The nitrogen adsorption isotherms of the samples are shown

in Figure 5. A simple inspection suggests that the experimentalisotherms are of Type I, according to the IUPAC classification.However, a more detailed observation of the first region revealsthat it is a combination of Type I and Type II isotherms.Therefore, the isotherms can be suitably described by theDubinin’s two terms equation, which indicates that the sampleshave two micropore distributions. The different parametersobtained from the fit of the N2 adsorption isotherms accordingto this model are presented in Table 5. The NZH-0.1 isothermfitted according to the DTT equation is show in Figure 6, as arepresentative plot.The increment in the micropore volume of different natural

clinoptilolites after acid treatments have been reported by otherauthors.9−11,16,43,44 In the present work, an increase in themicropore volume is observed when the raw samples, NZ andAZ, are submitted to acid treatments (see Table 5), inparticular for the AZ acid samples. Such increase probablyarises from the liberation of supplementary space on thesamples by the dealumination process and the cation exchangewith the proton from the acid medium. This space will behigher in AZ because it is easier to exchange the Na+ than thenative cations of NZ by the H+ ions from solution, as it wasdiscussed previously. The increment of W0, which depends of

the concentration of HCl employed, takes place mainly inregion 1, i.e., for narrow micropores. Note that the W02practically does not change, indicating that the broadmicropores are less affected by the acid treatments. Both thecharacteristic energy E01 and W01 increase from NZ and AZ toNZH-0.1 and AZH-0.1, respectively. This fact indicates thecreation of new micropores with smaller diameters in the

Figure 5. Nitrogen adsorption isotherms of the raw materials, NZ (A) and AZ (B), and their acid forms.

Table 5. Parameters Obtained by Fitting the N2 Adsorption Isotherms of NZ, AZ, and Their Acid Modified Forms According toDubinin’s Two Terms Equationa

region 1 region 2

sample Nm1 E01 W01 Nm2 E02 W02 W0 E01 × W01

NZb 1.090 7608 0.038 0.141 585 0.005 0.043 289NZH-0.1 1.739 9541 0.060 0.256 778 0.009 0.069 572NZH-0.5 2.043 8939 0.071 0.239 664 0.008 0.079 635NZH-1 2.133 7107 0.074 0.251 604 0.009 0.083 526AZc 0.195 3945 0.007 0.184 590 0.006 0.013 28AZH-0.1 2.968 7917 0.103 0.269 614 0.009 0.112 815AZH-0.5 3.139 7800 0.109 0.308 534 0.011 0.120 850AZH-1 2.891 7450 0.100 0.290 573 0.010 0.110 745

aNm1 and Nm2, maximal amount adsorbed (mmol g−1); E01 and E02, characteristic energy (J mol−1); and W01 and W02, micropore volume (cm

3 g−1),for regions 1 and 2, respectively. W0, specific total micropore volume (cm

3 g−1). bFrom ref 9 (values from region 2 have been recalculated). cfrom ref9.

Figure 6. N2 adsorption isotherm of NZH-0.1 sample, fitted accordingto the DTT equation.



material. The decrease in E0, when increasing the acidconcentration, for the other acid samples, indicates anincrement of the micropore diameter. The increase in W01,with a lesser E0, points toward the creation of new microporesof higher diameter.Figure 7 shows the nitrogen amount adsorbed as a function

of time, which depends largely of the adsorptive characteristics,the pore diameter, and the pore volume of the adsorbent. Forthe selected samples, the order followed was NZH-1 > NZH-0.1 and AZH-1 > AZH-0.1 (although in the micropore region,the behavior is very similar). The replacement of the nativecations from NZ and AZ and the removal of aluminum fromthe framework in the modified acid samples make the nitrogenmolecules diffuse through more open channels, so they can beadsorbed in sites that are not accessible in the case of rawmaterials. It is important to note that the time for themicropore volume filling and micropore volume follow arelatively similar order. It indicates that a smaller time to fill thebigger micropore volumes is needed, when increasing the acidconcentration, in particular for NZ.Connolly surface analysis was used to determine the

occupied and free volume of different simulated structures(HZ-6Al, HZ-4Al, and HZ-0Al) in order to compare them withthe nitrogen adsorption results. With the aim of evaluating thefree volume, the water molecules were removed from theframework. The calculated values are shown in Table 6. As it

was expected, the hydrated frameworks in the differentdealumination stages have lower free volumes than theanalogous without water. The free volume of the frameworksfor the samples HZ-6Al and HZ-4Al without water are of thesame order as those microporous volumes (W0) obtainedexperimentally for AZ acid samples (see Table 5). When all theAl atoms are removed from the simulated structure (HZ-0Al)the free volume decreases due to the contraction of the unit cell

during the dealumination process as it was previously discussed(see Table 3).Another simulation result that it is worthwhile to mention is

that, in all the structures, strong hydrogen bond interactionsbetween the zeolitic framework and the 20 water molecules areestablished. In many cases, the zeolite acid protons pass to thewater forming hydronium ions, which interact with theframework oxygens by hydrogen bonds. Although in theliterature the formation of these species have been reported,45

in our case for the HZ-6Al structure, four out of the six protonsare forming hydronium ions. In this sense, Armbruster et al.40

expected that the H3O+ species within the structural channels

should be considered instead of H, for the case of highlyhydrated heulandite.On the other hand, theoretical results indicate that the order

of Al extraction according to its positions is the following: 12,16, 11, 21, 15, and 17 (see Figure 1). The first three and thefifth are at T2 position, the fourth and the sixth at T3, whichsuggests that Al atoms located at T2 (the most populated site)are the easiest to extract from the clinoptilolite framework. It isimportant to note that we are not taking into account thekinetics of the reaction and activation energies involved. Onlythe energy of one of the possible products is being considered.The energy differences between the frameworks of twoconsecutive removal steps are shown in Table 7. These energydifferences increase, while the Al content decreases in theframework.The majority of the 10 ring window channels of the

clinoptilolite structure modeled has four Al atoms. However,one out of every five has only two Al atoms located in 12 and16 positions (see Figure 8a). As was mentioned previously,

Figure 7. Amount of nitrogen adsorbed at 77 K as a function of time for the NZH-0.1, NZH-1 (A) and AZH-0.1, AZH-1 (B) samples.

Table 6. Free Volumes Determined for Selected SimulatedStructures with and without Water Molecules

free volume (cm3 g−1)

structure with water without water

HZ-6Al 0.045 0.240HZ-4Al 0.046 0.213HZ-0Al 0.034 0.188

Table 7. Energy Difference (ΔE) between Two ConsecutiveStructures Involved in the Dealumination Procces

structure Al removeda ΔE (kJ mol−1)

HZ-6AlHZ-5Al 12 990.82HZ-4Al 16 1152.32HZ-3Al 11 1177.75HZ-2Al 21 1142.28HZ-1Al 15 1202.31HZ-0Al 17 1222.13

aThe number corresponds to the aluminum atom position in theframework, as was assigned in Figure 1.



these aluminums are the easiest to remove from the modeledclinoptilolite. When these Al are removed, a slab is created inthe framework, forming a structure similar to a clay, keeping theperiodicity without losing the porous structure (see Figure 8b).In addition, a complete clinoptilolite dealumination generatesextra slabs in the framework. This result confirms thosesuggested by Armbruster et al.40 In their work, the authorsstudied acid-treated zeolites (heulandites) where dealuminationis just beginning, and they speculated that the resulting materialcan be considered a metastable product toward clay formation.However, in the simulations, the formation of these slabs doesnot increase considerably the free volume of the cell. This isdue to the fact that the removed Al is replaced by fourhydrogen atoms, which complete the oxygen valences, formingthe same number of silanol groups in the framework. Thepresence of these terminal groups decreases the free volume ofthe cell.The simulation results suggest that different factors

determine the stabilization of the dealuminated structures:the hydrogen bonds between the zeolitic framework and thewater molecules, the interaction (repulsion) between thehydrogen of formed silanol groups, and the interaction betweenthe silanol groups and water molecules. The two last ones willbe more significant while the Al content decreases and thestructure becomes more hydrophobic (i.e., the system will beless stable). At high Al content, the structures with higherhydrogen bonds between the framework and the water are themost stable. On the contrary, at low Al amount (i.e., high Si/Alratio), those structures with lowest interactions between −OHof silanol groups and lowest hydrogen bond interactionsbetween silanol groups and water molecules will be the moststable.

4. CONCLUSIONS

The structural stability of clinoptolite samples, in their naturaland sodium forms, was not significantly affected by acidtreatments. The estimated crystallinity percent indicated thatthe sodic sample (AZ) has better resistance to the acid attackthan the natural one (NZ). Simulation results suggested that,for the modeled structures, a high crystallinity is retained, evenafter extreme dealumination. For both experimental samples

and simulated structures, variations in the unit cell volume(contraction) and in the unit cell parameters took place afterthe acid transformations. The simulations indicated that the Alin T2 positions are the easiest to remove.The results of the nitrogen adsorption isotherms indicated an

increase in the micropore volume of the raw materials, due toacid treatments, as well as the creation of new narrowmicropores. It is coherent with the calculations results, wheresmall additional slabs were created in the framework when theAl atoms were removed.Simulation studies suggested that the stability of the

dealuminated frameworks is related to different factors, whichchange depending on the Si/Al ratio. For those structures withlow Si/Al ratio, the most stable will be those where a largernumber of hydrogen bonds between the framework and waterare established. While for the frameworks with high Si/Al ratio,the repulsion between the formed silanol groups, as well as theinteraction between the silanol groups and the water moleculesdetermine the stability of the final product.In general, the obtained results suggested that to obtain a

structurally stable acid-natural clinoptilolite, the raw materialshould be first modified to its sodium form and then acidtreated.

■ ASSOCIATED CONTENT*S Supporting InformationOutput files of all simulated structures. This material is availablefree of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Tel: +53 7 8788959, ext. 221. E-mail: [email protected](A.R.); [email protected] (A.L.).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank The Academy of Sciences for the Developing World(TWAS) for the research grants no. 00-360 RG/CHE/LA, no.04-033 RG/CHE/LA, and no. 07-016 RG/CHE/LA. Partialfinancial support from the Pole Universitaire Europeen de

Figure 8. Acid clinoptilolite with the six Al (a) and the acid structure without Al in 12 and 16 positions (b).



http://pubs.acs.org

mailto:[email protected]

mailto:[email protected]

Montpellier is also acknowledged. A.L. thanks ProfessorClaudio M. Zicovich-Wilson for the helpful discussion. Wewould like to thank E. Altshuler for the revision of themanuscript.

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