Langmuir 1995,11, 1726-1730
1726
Adsorption of Gaseous p - or m-Xylene in BaX Zeolite: Correlation between Thermodynamic and Crystallographic Studies Caroline Mellot, Marie-H61&neSimonot-Grange,*$tEmmanuelle Pilverdier,? Jean-Pierre Bellat,? and Didier Espinat Institut Franqais du Pdtrole, 1 et 4 Avenue de Bois Prdau, B P 311, 92506 Rueil-Malmaison Cedex, France, and Universitd de Bourgogne, Laboratoire de Recherches sur la Rkactivitd des Solides, CNRS URA 23, BP138, 21004 Dijon Cedex, France Received November 7, 1994. In Final Form: February 8, 1995@ This paper presents a synthesis of macroscopic and microscopic measurements about adsorption of xylenes in faujasite type zeolite. Adsorption of pure gaseous p-xylene or m-xylene in BaX zeolite was investigated by thermogravimetry, isothermal differential calorimetry, and neutron diffraction. A very good correlation was obtained between adsorption heats of both isomers in BaX zeolite and the crystallographicpositions of the molecules in the zeolitic structure as a function of filling. This correlation between phenomenologic and microscopic aspectshas shown important differences in the adsorption process between both xylenes when high fillings are considered. As for m-xylene, all molecules are adsorbed near Ba2+ions in the a-cage in the whole adsorptionrange, i.e. up to the total filling, which is about 3 molecules per a-cage. Consequently, the n-type interaction between each aromatic ring and a Ba2+cation gives rise to rather high adsorption heats which decrease slightly as filling increases. As forp-xylene, the first two molecules adsorbed in each a-cage are located near Ba2+ions, giving rise to similar adsorption heats as for m-xylene. Strong steric hindrance prevents the third molecule ofp-xylene to adsorb near a Ba2+cation. The molecule is constrainedto adopt a less enqgetic position in the a-cage. A strong drop in the adsorption heats is therefore observed when the filling with p-xylene is increased above 2 molecules per a-cage.
1. Introduction The separation of p-xylene from CS(p-,0-,m-xylenes and ethylbenzene) aromatics is performed industrially by sele,ctiveadsorption processes on molecular sieves. The BaX zeolite is of great interest because of its high selectivity forp-xylene when competitive adsorption ofpand m-xylene is performed. The present work aims to investigate the adsorption process of both isomers in the BaX zeolite through an original synthesis of microscopic and macroscopic results. Many studies have been reported in the literature about the adsorption of aromatics in Y and X zeolites, giving either macroscopic information such as isotherms and differential adsorption heats' or structural descriptions such as the crystallographic positions of the adsorbed m o l e c ~ l e s .Yet, ~ ~ ~microscopic and macroscopic data are rarely correlated to one another with the aim of precisely understanding the adsorption process a t a molecular level. In this work, the results obtained from macroscopic measurements such as adsorption heats are interpreted in the light of structural information such as the crystallographic arrangement of the adsorbate in the zeolite. With the aim to elucidate the energetic and structural properties which mark the differencebetween both xylenes in the separation process, the thermodynamic and structural aspects of the adsorption of each isomer in the BaX zeolite are compared. For that purpose, results from thermogravimetry, isothermal differential calorimetry, and neutron diffraction are correlated. Starting from structural refinement^,^^^ the adsorption heats of both isomers are correlated to the crystallographic positions of
* Author to whom correspondance should be sent.
' Universit6 de Bourgogne.
Abstract published in Advance A C S Abstracts, April 15,1995. (1)Ruthven, D.M.; Goddard, M. Zeolites 1986,6, 275. (2) Goyal, R.; Fitch, A. N.; Jobic, H. J. Chem. SOC.Chem. Commun. 1990,1152. (3) Czjzek, M.; Vogt, T.; Fuess H.Zeolites 1991,1 1 , 5255. (4)Mellot, C.; Espinat, D.; Rebours, B.; Baerlocher, C.; Fischer, P. Catal. Lett. 1994,27, 159. @
0743-7463/95/2411-1726$09.00/0
p - or m-xylene molecules. The results show that the evolution of adsorption heats as a function of filling is directly influenced by the different positions that xylene molecules adopt as filling increases.
2. Literature Survey6 The BaX zeolite belongs to the faujasite family. In this structure, the stacking of TO4 tetrahedrons (T = Si, Al) creates sodalite cages (P-cages, aperture diameter: 0.22 nm), linked together by means ofhexagonal prisms (Figure 1). The tetrahedral arrangement of the ,&cages creates a supercage (a-cage), the diameter of which is 1.25 nm. Each a-cage leads to four P-cages through a hexagonal window and to four other a-cages through a dodecagonal window, the aperture of which is 0.75 nm. The unit cell is cubic and contains eight a-cages and eight P-cages.Small molecules, such as H2O and NH3, may be adsorbed into both kinds of cages. Large organic molecules like xylenes enter only the a-cages. The cations, which neutralize the negative charge of the framework, are mainly located in three crystallographic sites: SI, inside the hexagonal prism; SI., inside the P-cage; and ,311, inside the a-cage toward the 6-ring windows. A detailed structural study of the framework and of the cationic distribution in the BaX zeolite is reported e l ~ e w h e r e .The ~ Ba2+cations of the a-cage, distributed in two of four SIIsites, are possible adsorption sites for the xylene molecules. It is noteworthy that the adsorption ofxylene can lead to a partial migration of cations from the SI.and SIsites to the SIIsites in such a way that, at total filling, the four SIIsites of the a-cage are occupied with Ba2+cations. As for p- and m-xylene, both molecules have similar properties. The main differences between these two xylenes are the relative position of their methyl groups and consequently their dimensions and the dipolar mo(5) Mellot, C. Thesis,Universit4 Pierre et Mane Curie, Pans, France,
1993.
(6) Barrer,R. M. Zeolites and Clay Minerals as Sorbent and Molecular Sieues; Academic Press: New York, 1978.
0 1995 American Chemical Society
Langmuir, Vol. 11, No. 5, 1995 1727
Adsorption of Xylenes in BaX Zeolite
Table 2. Definition of Thermodynamic Values Site I1
Um,ads,f(T)= molar internal energy of adsorbate at final equilibrium state (defined by adsorbed amount nf under pressure Pr a t temperature T). Um,ad,i(T)= molar internal energy of adsorbate a t initial equilibrium state (defined by adsorbed amount ni under pressure Pi a t temperature T).
Anads= nf - ni = adsorbed amount between both equilibrium states (defined under pressures Pf and Pi a t temperature 79.
Site I'
Site I
Table 3. Chemical Composition and Unit Cell Parameter of BaX Zeolite Samples sample composition per unit cell
filling (moleda)
unit cell parameter (nm)
Figure 1. Cationic sites in the faujasite type zeolite. Table 1. Physical Features of Xylenes parameter critical diameterhm lengthhm breadthhm thicknesshm Pm
p-xylene 0.67 0.98 0.67 0.4 0
m-xylene 0.71-0.74 0.86 0.79 0.4 0.36
expression of this heat quantity is
According to Letoquard e t a1.,8 the molar heat measured at constant temperature is given by the expression T7
ment of the m-xylene molecule (Table 1).The dimensions of both molecules are close to the aperture diameter of the 12-ring windows of the a-cages. Thereafter, a weak difference of size and geometry may cause steric bulk and modify the adsorption process.
with (Table 2)
3. Experimental Section The BaX zeolite was prepared starting with pure synthetic
NaX faujasite (Union Carbide) having a SUM ratio of 1.25, with the chemical formula N@6Silo&&84.n&O written in terms of the unit cell. The BaX zeolite was obtained by ion exchange in successivebatches with a 0.01 M aqueous solution of Ba(NO& a t 50 "C. The degree of exchange is within the 82-94% range.5 3.1. Thermodynamic Measurements. Phenomenologic aspects were studied by means of thermogravimetric (TGA) and differential calorimetric (DCA) analysis. TGA was performed in a McBain-type balance well-suited to impose pure xylene pressure controlledby a "cold point''.' Prior to use, the xylene was dehydrated in situ by means of activated 4A zeolite. About 15 mg of zeolite was used. Before each experiment the zeolite was activated in situ at 400 "C under lov4 mbar (1mbar = 101.3 Pa) during 12 h. DCA was carried out in a heat flow microcalorimeter ((280Setaram), coupled with a volumetrictechnique to detect accurate thermal effects and adsorbed amounts. The mass of the sample was about 700 mg. The conditionsof gas purification and zeolite activation were the same as previously defined. Adsorption-desorption isotherms were drawn in graduated steps by increasing or decreasing pressure in small successive increments under pressure ranging from 10-l to 10 mbar. For adsorption measurements, the initial state was the activated state and the final state was reached under 10 mbar of pressure. For desorptionmeasurements, the previouslydefined initial and final states were reversed. Adsorption enthalpies were measured by successive adsorptions of xylene amounts, Anade,of mol (0.3 molecule per a-cage or moleda), with the same initial and final states as those previously defined. Desorption enthalpies were not measured. Adsorption isotherms inferred from the volumetric technique are the same as those drawn in TGA. The calorimetric method was an isothermal adsorption involving an open system. The calorimeter was at 150 "C and the reserve of gas a t 20 "C. Under these conditions, the measured adsorption heat is the result of the average integral enthalpy on the adsorbedamount range at 150"C, hade, and the gas warming up from 20 "C to 150 "C. For the molar adsorbed amount, the (7) SimonobGrange,M. H. Clays Clay Minerals 1979,27,423.
Experimental
conditions
show
that
the
term
(pf- Pi) VadmrbendAnade is less than 1%. Therefore, it will be neglected. The molar heat then becomes an average molar integral enthalpy which is similar to a derivative enthalpy:
On the other hand, it is difficult to estimate the term for gas warming. In this work, it will be omitted. Indeed, experiments are performed under the same conditions for both xylenes, and these gases have similar thermal capacities. The obtainedvalues are then minimized, but the comparative argument is good. 3.2. Dif€raction Experiments. For the diffraction analysis, the samples were dehydrated a t 100 "C under vacuum mbar) during4 h and then a t 300 "C during48 h, before admitting a controlled pressure of deuterated xylene vapor to the cell. An equilibrium between the zeolite sample and the vapor was maintained during 4 h. The amounts of adsorbed xylene were estimated by weighting the samples before and after adsorption. Two fillings corresponding to approximately 1 and 3 moleda were prepared (Table 3). The samples were then stored in airtight vanadium sample holders prior the diffraction experiments. Further details about the neutron data sampling, the refinement procedure, and the structural parameters of the different samples had been previously r e p ~ r t e d . ~ The . ~ conditionsof data collection were exactly the same for all the samples. The diffraction patterns were analyzed using the Rietveld refinement method9 and the XRS-82 package of programs.1° The crystallographicpositionsof the adsorbedp- or m-xylene molecules were determined for both fillings, i.e. for 1 and for 3 moleda.
4. Results 4.1. Phenomenologic Study. 4.1.1. AdsorptionDesorption Isotherms at 150 "C. Adsorption-desorption (8) Letoquard, C.; Rouquerol, F.; Rouquerol, J.;J. Chim. Phys. 1973, 3,559. (9)Rietveld, H.M. J. Appl. Crystullogr. 1969,2,65. (10)Baerlocher, C. The X-rayRietveld System; Institut fur Kristal-
lographie und Petrographie, ETH: Zurich, 1982.
1728 Langmuir, Vol. 11, No. 5, 1995
Mellot et al.
m-xylene
I
log[n/(molec/a)]
10’6(Tlogpo/p)2/KZ 0
2
4
6
8
10
Figure 2. Adsorption-desorption isotherms ofp- and m-xylene in BaX zeolite, at 150 “C.
0.3
0
1
2
3
4
Figure 3. Transformed characteristiccurves of the DubininRadushkevich model of isotherms of p-xyleneBaX and mxyleneBaX systems at 150 “C.
isotherms of gaseousp- or m-xylene in BaX zeolite a t 150 150 “C are reversible and of type I, characteristic of miIApdSH(/(kJh”) croporous adsorption (IUPAC classification,ref ll)(Figure 2). For very low pressures, animportant adsorbed amount of xylene is observed. At the lowest relative pressure (p/po= each a-cage contains statistically about 2.5 molecules in the case of m-xylene, and 2.25 molecules in the case of p-xylene. The complementary adsorbed amount is only about 0.5 molec/a when the relative pressure is increased to the maximum value of 7 x A previous study investigating BaY and NaY z e o l i t e ~ l ~ - ~ ~ under the same conditions of pressure, showed that, in n/(molecla) 0 the case of BaY zeolite, at the lowest pressures, Henry’s 0 1 2 3 law is not yet obeyed a t 400 “C, while, in the case of NaY Figure 4. Adsorption heats ofp-xylene and m-xylene adsorbed zeolite, this law is followed as soon as 250 “C. This results in BaX zeolite as a function of filling, at 150 “C. in the BaY zeolite adsorbing xylenes more strongly than the NaY zeolite. As the X and Y zeolites are quite similar, adsorbate and increasing adsorbate-adsorbate interacit appears that the strong affinity of both xylenes for the tions. BaX zeolite may be attributed to the Ba2+cation. In the 0-2 moleda range, i.e. for the first two aromatic At 150 “C, more m-xylene thanp-xylene is adsorbed in molecules adsorbed in each a-cage, the adsorption derivathe BaX zeolite. The difference between both isomers is tive enthalpies are of the same order of magnitude for about 0.25 molec/a. For example, the BaX zeolite is filled both isomers. with 3 moleda under 10 mbar of m-xylene, but only with In the 2-3 molec/arange, a clear difference betweenp2.75 molec/a under the same pressure ofp-xylene. This and m-xylene appears. Indeed, a strong decrease in the result could be explained from m-xyleneladsorbent specific adsorption derivative enthalpies of p-xylene is observed: interactions owing to the dipolar moment of the m-xylene AadaPx(2 moleda) is about 105 kJmol-l as h,d$l,,(3 moled molecules. The Dubinin-Radushkevich model15applied a)is less than 50 kJ-mol-’. Such behavior is not observed to each isotherm yields a theoretical maximum adsorption in the case of m-xylene: Aadam(2molec/a) is about 103 of 3.28 moledafor m-xylene and only 3 moledaforp-xylene kJ-mol-1 as A,damx(3 molec/a) is about 90 kJ-mol-l. (Figure 3). Adsorption enthalpies of m-xylene are much higher than 4.1.2. Isothermal Derivative Enthalpies. Adsorption those ofp-xylene in the 2-3 molec/a range. Adsorption derivative enthalpies of p-xylene and m-xylene in BaX enthalpies of m-xylene begin to decrease strongly only zeolite at 150 “C as a function of filling show several above a filling of 3 molec/a. features (Figure 4). A few conclusionsmay be drawn. In the case ofp-xylene, At filling “zero”,the adsorption derivative enthalpies of these results clearly show that the increase of filling leads p- and m-xylene are similar ( A , d a at filling “zero”% 115 to a great decrease in the adsorbent-adsorbate interackJ-mol-l). It may be concluded that, at low filling, tions in the 2-3 molec/a range. From the main features interactions between the aromatic molecule and the of figure 4, it may be suggested that p-xylene molecules adsorbent are of the same order of magnitude for both occupy two different adsorption sites corresponding to isomers. This result especially shows that the dipolar strongly different adsorbate-adsorbent interactions. The moment of m-xylene has no significant energetic contribution in the adsorbent-adsorbate interactions. last molecules adsorbed in the 2-3 molec/a range would be much less energetically adsorbed than the first For both xylenes, adsorption derivative enthalpies molecules adsorbed in the 0-2 molec/a range. In the case decrease as filling increases, showing a weakening of total of m-xylene, adsorption is energetically homogeneous in interactions which results from decreasing adsorbentthe whole 0-3 moleda range. All adsorption sites have similar characteristics, giving rise to rather high adsorp(11)Sing,K. S. W.;Everett,D.H.;Haul,R.A. W.;Moscou,L.;Pierotti, R. A.; Rouquerol, J.; Siemieniewska,T. Pure Appl. Chem. 1985,54 (4), tion enthalpies which decrease slightly as filling increases 603. from 0 to 3 molec/a. (12)Bellat, J. P.; Simonot-Grange,M. H.; Jullian, S. C. R.Acad. Sci. 4.2. Microscopic Study. 4.2.1. Adsorption of pParis 1992,314, sene 11, 777. (13)Bellat, J. P.; Simonot-Grange, M. H.; Jullian, S. C. R.Acad. Sci. Xylene in BaX Zeolite. The neutron diffraction patterns Paris 1993, 316, s4rie 11, 1363. of BaX zeolite with deuteratedp-xylene a t low filling ( ~ 1 (14) Bellat, J. P.; Simonot-Grange,M. H.; Jullian S. Zeolites in press. molec/a) and at total filling ( ~ molec/a) 3 were analyzed (15)Dubinin, M. M.; Zaverina. E. D.: Radushkevich. L. V. Zh. Fiz. Khim.Acta 1947,21, 1351 by the Rietveld method and allowed us to establish a model
Langmuir, Vol. 11, No. 5, 1995 1729
Adsorption of Xylenes in BaX Zeolite an-
Position I3'
Position A
Deuterated p-xylene
2.5 +I
Position I S
J
.I*W
2.0-
c, u
g 1.5w c.
z 1.0c! 0 4
Filling of the zcolitc indicntcd in numbcr of molcculcs pcr a-cagc
0.5-
0
2 niolwiiles in position B
-n e ! 1
I
I
I
I
I
I
0
20
40
60
80
100
120
I
@ n a o n .c Figure 7. Adsorption of m-xylene. Filling model of the BaX zeolite from low filling to total filling. The different crystallographic positions of m-xylene molecules are indicated according to the filling range. 0
2-THETA
;:;{I
Deuterated m-xylene
+, 2.5-
w
2.0-
b
z 1.51
z
1.00.5
a
-
0.01 0
20
40
60
80
100
2-THETA
Figure 5. Neutron powder diffraction patterns of BaX zeolite with deuteratedp- and m-xylene at 11 K (a) at low filling (% 1 moleda) and (b) at total filling (a 3 moleda). Filling of the zeolite imlicated in number of molecules pr acage I
2
POSI'I'ION A
I
0
3
POSITION F (no located)
- K-IYpC Colllpk% - van der Waals inter;ictionsbctwecn
no x-type complex
methyl groups imd l'nincwork oxygciis
+ I
i
high adsorption dcrivativcenthalpy 00
@ nil
OD
/
and
i I molcculc in psilion A
a c
5
3
3-
i 2 tnolealles in psition B'
0.0 -V.J
1
J.
low actsorption
dcnvativccnthalpy
0 (1
Figure 6. Adsorption of p-xylene. Filling model of the BaX zeolite from low filling to total filling.
for the filling of the a-cages (Figures 5a,b). The results show the following main features. In the 0-2 moleda range, each p-xylene molecule is adsorbed in a high symmetry position in the a-cage, close to a Ba2+ion, according to the orientation A (Figure 6).5 The molecule is stabilized by a n-type interaction between the aromatic ring and the Ba2+ion and by van der Waals interactions between the methyl groups and the framework oxygen atoms. In the 2-3 moleda range, a third molecule ofp-xylene is introduced in each a-cage. The experimental crystallographic analysis has shown that this third molecule has
0
no direct coordination to a barium cation via its n-system and is stabilized by different interactions than those of molecules in position A. The adsorption of this third molecule within the space of the a-cage has no significant effect on the crystallographic location of the two other p-xylene molecules adsorbed in position A. It may be concluded that, with increasing filling, a new adsorption site (denoted as site F), less energetic because no n-type interaction is involved, appears. This site has not been located. 4.2.2. Adsorption of m-Xylene in BaX Zeolite. A detailed crystallographicstudy4 leads to a different model than that previously described (Figure 7). In the 0-2 moleda range, all m-xylene molecules are adsorbed in position B, close to a Ba2+cation. During adsorption a migration of Ba2+cations from SIand SI. to SIIsites inside the a-cages is observed. In the 2-3 moledarange, a third molecule is introduced in each a-cage. The two previously B positioned m-xylene molecules have to reorient in position B' to allow the adsorption of the third molecule and to minimize repulsive intermolecular interactions. This third m-xylene molecule is adsorbed close to a Ba2+ cation in position A. At total filling, it is noteworthy that the four SIIsites of a-cages are occupied by a Ba2+ion and consequently each m-xylene molecule is in interaction with a Ba2+cation. 5. Discussion As forp-xylene,adsorption derivative enthalpieschange slightly in the 0-2 moleda range (Figure4). For the filling of 2 moleda, the structural analysis shows that allp-xylene molecules are adsorbed close to a Ba2+cation. The n-type interaction between the aromatic ring and the cation is then responsible for the rather high measured adsorption enthalpies. The important decrease observed in the enthalpy above the filling of 2 moleda is to be attributed to the appearance of the less energetic adsorption site F. Crystallographicanalysis states that, in the 2-3 moleda range, the third p-xylene molecule introduced in each a-cage and adsorbed a t site F has no specific interaction with the Ba2+ cations of the a-cage: lower derivative adsorption enthalpies are consequently observed up to total filling. As for m-xylene, the structural analysis shows that increasing filling leads to important steric hindrance, which determines the crystallographic location of the aromatic molecules. Yet, the adsorption of m-xylene shows .some constant features in the phenomenologic study. In
1730 Langmuir, Vol. 11, No. 5, 1995
Mellot et al.
Table 4. Relevant Bond Lengths for m-Xylene Molecules Adsorbed in the BaX Zeolite, According to Their Crystallographic Position in the a-Cages m-xylene position position B position B' position A
d(Ba2+*.aromatic ring) (nm) 0.244(1) 0.275(1) 0.290(3)
d(&thyl'
* 'Okamework)
(nm) 0.238(2) 0.258(1) 0.267(3)
the whole adsorption range, m-xylene molecules are adsorbed close to the Ba2+cations of the a-cages, being forced to reorient and even to weaken their n-type interaction with the cations in order to allow another molecule to adsorb and to minimize methyl-methyl repulsion^.^ At high filling, a displacement of the aromatic ring 0.03 nm toward the a-cage center along the 3-fold axis is observed. This migration is probably not to be assigned to clustering or mutual attraction (the ring centers of adjacent molecules are at a distance of more than 0.46 nm) but is more significant of steric hindrance. When considering the thermodynamic results, a slow linear decrease in the adsorption derivative enthalpies in the whole filling range is shown (Figure 4). According to crystallographic results, this evolution is to be attributed to a continous weakening ofthe strength of the adsorbateadsorbent interactions. This was clearly observed in the structural analysis as longer aromatic ring to cation and methyl groups to framework distances were obtained as increasing filling was studied (Table 4). The adsorption of m-xylene differs strongly from the adsorption ofp-xylene: n-type interaction between each m-xylene molecule and a Ba2+cation is observed in the whole filling range, giving rise to rather high adsorption
enthalpies up to total filling. Consequently, no important decrease in the enthalpies is observed. These results show that the n-type interactions have the energetic largest contribution in the adsorption derivative enthalpies. Furthermore, analysis shows that the differencesobserved between m- and p-xylene adsorption are not to be attributed to the different dipolar moments of both isomers but to specific steric hindrance occurring when the filling of the a-cage is increased. The different crystallographic positionsthat m- andp-xylene molecules adopt when filling increases have to be assigned to specific steric constraints for each isomer, owing to the relative position of their methyl groups. Particular adsorbent - adsorbate interactions and adsorption derivative enthalpies are therefore observed.
Conclusion The originality of this work consists in the complementarity of phenomenologic and microscopic studies on the adsorption process of purep-xylene or m-xylene in the BaX zeolite. A precise correlation between structural data and thermodynamic measurements has been established. This correlation has shown that p-xylene and m-xylene adsorption processes are similar in the 0-2 moleda filling range: either the crystallographic positions of p- and m-xylene molecules or the respective derivative adsorption enthalpies of both isomers are comparable. In the 2-3 moleda range, strong differences appear between both xylenes. The strong drop in the derivative adsorption enthalpies ofp-xylene is correlated to the appearance of a less energetic adsorption site. LA940875P
