Sorption of Benzene on AlPO4-5 Containing Various Heteroatoms and

Langmuir 1999, 15, 5857-5862

5857

Sorption of Benzene on AlPO4-5 Containing Various Heteroatoms and Analytical Description of the System† M. Rozwadowski,*,‡ J. Kornatowski,‡,§ R. Golembiewski,‡ and K. Erdmann‡ Faculty of Chemistry, Nicholas Copernicus University, 7 Gagarin St., 87-100 Torun, Poland, and Lehrstuhl II fuer Technische Chemie, Technische Universitaet Muenchen, Lichtenbergstrasse 4, 85747 Garching bei Muenchen, Germany Received October 9, 1998. In Final Form: January 14, 1999 Sorption of benzene on nonsubstituted AlPO4-5 occurs on adsorption centers originating from surface heterogeneity and created by structure defects. The metallic heteroatoms present in the framework of substituted AFI-type materials form additional centers able to bind the benzene molecules. This is equal to an increase of heterogeneity in comparison to nonsubstituted AlPO4-5. Sorption capacity of the MeAPO-5 materials depends on both the content of metal and its distribution throughout the crystals. The distribution parameter plays an important role and may even dominate the effect of the number of metallic heterocenters. Some of the equations based on the Polanyi-Dubinin potential theory can be accepted as suitable for characterizing the distribution of the adsorption potential and adsorption energy of the AFI-type molecular sieves. The dependence between the adsorption potential and the metal content may likely be disturbed by inhomogeneous distribution of the heteroatom centers.

Introduction Adsorption measurements are a source of information about adsorbate-adsorbent interactions which may be characterized by the values of either adsorption energy or adsorption potential.1-4 The adsorption energy distribution is the function commonly accepted for quantitative characterization of the global energetic heterogeneity of solids.3-5 The distribution function describes the energetic heterogeneity of a solid with respect to a defined adsorbate.3 An effective way for evaluating the energy distribution function from the total adsorption isotherm is the condensation approximation method.1,5,6 This method offers a simple equation2,7 for calculating the total adsorption potential distribution, X(A):

X(A) ) -dθt(A)/dA

(1)

Here the adsorption potential A is defined as4

A ) RT ln(ps/p)

(2)

and θt is the total adsorption capacity. † Presented at the Third International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, Torun, Poland, Aug 9-16, 1998. ‡ Nicholas Copernicus University. § Technische Universitaet Muenchen.

(1) Steele, W. A. The Interaction of Gases with Solid Surfaces; Pergamon Press: Oxford, U.K., 1974. (2) Jaroniec, M. Langmuir 1987, 3, 795. (3) Jaroniec, M.; Maday, R. Physical Adsorption on Heterogeneous Solids; Elsevier: Amsterdam, 1988; p 320. (4) Jaroniec, M.; Choma, J. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1989; Vol. 22, p 208. (5) Rozwadowski, M.; Wloch, J.; Erdmann, K.; Kornatowski, J. Langmuir 1993, 9, 2661. (6) Cerofolini, G. F. Thin Solids Films 1974, 23, 129. (7) Rozwadowski, M.; Wojsz, R.; Wisniewski, K. E.; Kornatowski, J. Zeolites 1989, 9, 503. (8) Ross, S.; Olivier, J. P. On Physical Adsorption; Wiley-Interscience: New York, 1964. (9) Jaroniec, M. Adv. Coll. Interface Sci. 1983, 18, 149. (10) Jaroniec, M., Braeuer, P. Surf. Sci. Rep. 1986, 6, 65. (11) Dubinin, M. M., Astakhov, V. A. Izv. Akad. Nauk SSSR, Ser. Khim. 1971, 5. (12) Dubinin, M. M. Prog. Surf. Membr. Sci. 1975, 9, 1.

The fundamental equation for the total adsorption isotherm may be written as follows:5,8-10

θt )

∫Ωθl(p,Q)f(Q) dQ

(3)

Here θl(p,Q) is the isotherm equation describing the local adsorption, f(Q) is the differential distribution of adsorption energy, and Ω is the range of variance of energy Q. For the description of the isotherms, we applied the Polanyi-Dubinin (PD) potential theory of volume filling of micropores. The Dubinin-Astakhov (DA) equation11 proposed in 1971 occupies a central position in this theory:

[ ( )]

W ) W0 exp -

A βE0

n

[ ( )]

) W0 exp -k

A β

n

(4)

Here W is the volume of the liquidlike adsorbate filling micropores under pressure p and at temperature T, W0 is the total volume of the micropores, A ) RT ln(ps/p) is the differential molar work of adsorption, β is the similarity coefficient that reflects the adsorbate properties,12 E0 is the characteristic energy of adsorption, n is a parameter characterizing the shape of the distribution of the adsorption potential, and k is a structural parameter correlated with micropore dimensions. When the exponent n is equal to 2, eq 4 transforms into the Dubinin-Radushkevich (DR) equation.13 In 1980, Stoeckli and Dubinin proposed the Dubinin-Radushkevich-Stoeckli (DRS) equation14 valid for heterogeneous microporous adsorbents. This equation was later modified by Rozwadowski and Wojsz, who obtained the following relation (DRSI):15 (13) Dubinin, M. M. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1966; Vol. 2, p 51. (14) Dubinin, M. M.; Stoeckli, H. F. Coll. Interface Sci. 1980, 75, 34. (15) Rozwadowski, M., Wojsz, R. Carbon 1984, 22, 363. (16) Jaroniec, M.; Choma, J. J. Mater. Chem. Phys. 1986, 15, 521. (17) Jaroniec, M.; Madey, R.; Rothstein, D. Pol. J. Chem. 1990, 64, 225. (18) Kornatowski, J.; Sychev, M.; Finger, G.; Baur, W. H.; Rozwadowski, M.; Zibrowius, B. In Proceedings of the Polish-German Zeolite Colloquium; Rozwadowski, M., Ed.; Nicholas Copernicus University Press: Torun, Poland, 1992; p 20.

10.1021/la981422d CCC: $18.00 © 1999 American Chemical Society Published on Web 04/09/1999

5858 Langmuir, Vol. 15, No. 18, 1999

W(A) )

[ () 2

W0 exp

∆ A 2 β

2n

[ ( )]

- k0

A β

n

erfc

Rozwadowski et al.

k0

]

∆ A 21/2 β ∆21/2 (5) k0 erfc ∆21/2 n

()

[

]

Here k0 represents the value of k at the maximum of the distribution function f(k), ∆ is the half-width of the distribution, and erfc is the error function complement. Next, Jaroniec and Choma proposed the JaroniecChoma (JC) equation16 as an alternative to eq 5. The authors used the Γ-type distribution and obtained the isotherm equation in the following form:

Θ)

[

]

q q + (A/β)2

n+1

(6)

Here n and q are the parameters of the Γ distribution (n > -1 and q > 0), Θ ) a/a0, where a is the amount of adsorbate in the micropores under the equilibrium pressure p and at temperature T, and a0 is the maximum amount of adsorbate in the micropores. A and β denote the same as in eq 4. Recognition of an analogy between the DR equation and the Langmuir-Freundlich (LF) equation was an important step in the theoretical studies of adsorption on microporous solids. The LF equation is of the following form:3,4,17

{

W ) W0 1 + exp

[RTγ (A - A )]} 0

-1

(7)

Here A0 ) RT ln(psK) and K and γ are constants. AFI-type molecular sieves have unidimensional pore systems composed of parallel channels of 7.3 Å diameter. The other pores, created by six-membered ring channels of diameter equal to 3 Å, are too narrow to allow adsorption of the benzene molecules and are not considered in the present work. This paper is dealing with sorption of benzene on AFI type molecular sieves containing various heteroatoms which are mostly incorporated into the framework positions.18-21 Such heteroatoms are expected to be a source of energetic heterogeneity in the zeolitic materials. We look for an explanation for the observed differences in sorption behavior between the particular MeAPO-5 derivatives in relation to benzene. We try to perform an analytical description of the system by checking (a) the applicability of equations derived from the PolanyiDubinin potential theory to the description of adsorption isotherms of benzene on AFI type molecular sieves and (b) which of those equations could be accepted as suitable for characterizing the distribution of adsorption potential and, thus, of adsorption energy in the studied microporous solids. Experimental Section The samples of AlPO4-5 and its derivatives substituted with Si, Mg, Co, V, Zr, Mo, Fe, Cd, Cu, Ni, and V + Mo as well as a series of samples with various contents of Mg, Zr, V, and Si were (19) Kornatowski, J., Rozwadowski, M., Baur, W. H., Golembiewski, R. In Proc. Symp. “Characterization and Properties of Zeolitic Materials”; Rozwadowski, M., Ed.; Nicholas Copernicus University Press: Torun, Poland, 1994; p 9. (20) Kornatowski, J.; Rozwadowski, M.; Lutz, W.; Lezanska, M. In Proceedings of the 3rd Polish-German Zeolite Colloquium; Rozwadowski, M., Ed.; Nicholas Copernicus University Press: Torun, Poland, 1998; p 51. (21) Kornatowski, J.; Rozwadowski, M. Proc. Int. Zeolite Conf., 12th 1998, in press.

Table 1. Heteroatom Contents, Crystal Dimensions, and Benzene Sorption Capacities for Nonground MeAPO-5 Samples (298.2 K) C6H6 sorption capacity sample MgAPO-5 SAPO-5 CoAPO-5 ZrAPO-5 VAPO-5 FeAPO-5 CuAPO-5 CdAPO-5 NiAPO-5 VMoAPO-5 MoAPO-5 AlPO4-5

Me content (ICP) (atom %)

dimens of cryst (µm)

4.4 3.3 2.1 1.8 1.3 0.9 0.8 0.3 0.1 0.4; traces traces

450 × 75 100 × 100 300 × 75 360 × 60 50 × 45 525 × 60 530 × 75 265 × 60 520 × 55 170 × 75 300 × 60 200 × 85

p/ps = 0.1

p/ps = 0.9

mol/kg dm3/kg mol/kg dm3/kg 1.320 0.891 1.073 0.343 0.994 0.060 0.106 0.395 0.085 0.754 0.400 0.642

0.117 0.079 0.095 0.031 0.088 0.005 0.009 0.035 0.008 0.067 0.036 0.057

1.482 1.092 1.190 0.683 1.199 0.087 0.155 0.483 0.126 0.877 0.472 0.747

0.133 0.098 0.106 0.061 0.107 0.008 0.014 0.043 0.011 0.078 0.042 0.067

synthesized hydrothermally by following our procedure20,22-24 for growing large crystals of AFI type materials. The formal mole ratios (given as oxides) in the reaction gels were: a:b:c:d:e Al2O3: P2O5:Me2On:R:H2O, where R ) triethylamine, a ) 1 for Si, V, Zr, and Mo and 0.9-0.915 for the other metals, b ) 1 or 0.9-0.915 as opposed to a (always a + c/2 ) b or b + c/2 ) a), c ) 0.17-0.2, d ) 1.35-1.55, and e ) 270-300. The details of the syntheses were published elsewhere.18-20,24 The resulting crystalline materials were decanted, filtered out, washed, dried at 378 K, and sieved if necessary. Then, the products were calcined under air and oxygen at 763 and 793 K, respectively, for at least 48 h each. The crystals were examined by XRD, SEM and light microscopy, sorption measurements for water, benzene, and nitrogen, and TGA and ICP analysis. The results of those measurements were published elsewhere.18-21,25-27 The contents of metals and the upper limits of the crystal dimensions for the investigated samples are listed in Table 1. The adsorption isotherms for benzene were determined at 298.2 K. Prior to the adsorption measurements, the samples of ca. 0.1 g were activated in situ under stationary vacuum of ca. 10-3 Pa at 673 K until constant mass (∆m e 10-5 g/12 h at least). Benzene was carefully degassed by repeatedly freezing at temperature of liquid nitrogen and evacuating. Amounts of benzene adsorbed at various relative pressures (p/ps) were measured using a vacuum device equipped with a McBain quartz spring balance and an MKS Baratron gauge. The sorption equilibrium was assumed to be attained when the observed mass change of ∆m was lower than 10-5 g for at least 12 h under a constant pressure of the adsorbate.

Results and Discussion Application of large crystals of zeolites seems to be favorable in order to avoid undesirable side effects such as intercrystalline adsorption, residual amorphous admixture within the crystalline phase, influence of increased external surface area of small crystals, etc., which may occur in powder preparations. Additionally, a slower growth of large crystals usually gives better chances for an appropriate incorporation of heteroatoms into the framework of molecular sieves. The prepared samples were composed of large hexagonal prisms typical for the AFI structure type, and their dimensions (SEM) were mostly within the range of 100-530 × 40-100 µm except one VAPO-5 sample (Table 1). The XRD patterns of these (22) Finger, G.; Kornatowski, J. Zeolites 1990, 10, 615. (23) Finger, G.; Richter-Mendau, J.; Buelow, M.; Kornatowski, J. Zeolites 1991, 11, 443. (24) Kornatowski, J.; Rozwadowski, M.; Finger, G. Pol. Patents PL 166147B1, 166149B1, 166162B1, 166505B1, 167598B1, all 1995. (25) Kornatowski, J.; Sychev, M.; Baur, W. H.; Finger, G. Collect. Czech. Chem. Commun. 1992, 57, 767. (26) Kornatowski, J.; Zadrozna, G.; Wloch, J.; Rozwadowski, M. Langmuir, submitted for publication. (27) Kornatowski, J.; Rozwadowski, M.; Lutz, W.; Sychev, M. Manuscript in preparation.

Sorption of Benzene on AlPO4-5

Langmuir, Vol. 15, No. 18, 1999 5859 Table 2. Heteroatom Contents, Crystal Dimensions, and Benzene Sorption Capacities for Ground MeAPO-5 Samples (298.2 K)

sample MgAPO-5 (4.4)a MgAPO-5 (2.8) MgAPO-5 (1.6) MgAPO-5 (0.6) ZrAPO-5 (1.8) VAPO-5 (0.1) AlPO4-5

C6H6 sorption capacity Me content dimens p/ps = 0.1 p/ps = 0.9 b (ICP) of cryst (atom %) (µm) mol/kg dm3/kg mol/kg dm3/kg 4.4 2.8 1.6 0.6 1.8 0.1

450 × 75 200 × 50 220 × 60 180 × 60 360 × 60 270 × 80 200 × 85

1.375 1.316 1.161 0.460 0.544 0.520 0.656

0.122 0.117 0.103 0.041 0.048 0.046 0.058

1.614 1.615 1.337 0.556 0.657 0.869 0.845

0.143 0.143 0.119 0.050 0.058 0.077 0.075

a Numbers in parentheses: Me contents in atom %. b Crystals prior to grinding.

Figure 1. Isotherms of benzene adsorption on nonground samples of MeAPO-5 and AlPO4-5 at 298.2 K: Me ) Mg (4), Si (×), Co (0), Zr (1), V (2), Fe (b), Cu (3), Cd (]), Ni (+), VMo ([), Mo (9); (O) AlPO4-5.

crystalline products were characteristic of the AFI structure type.18 The contents of the particular metals found with the ICP analysis were 0.1-4.4 atom %, i.e. about 0.025-1.1 atom/uc (Table 1), except Mo (traces). The sorption capacities of all the examined samples for benzene at p/ps = 0.1 and p/ps = 0.9 have been listed in Table 1 as well. Studies on the mechanism of sorption require one to take into account a possible influence of the crystal morphology on sorption. One should differentiate two aspects: (i) dimensions of the crystals which define the length of intracrystalline diffusion pathways; (ii) internal structure of the crystals with possible overgrowing or twinning which might significantly hinder the sorption processes. These two aspects are especially important in the case of our large crystals which are known to commonly form twins.19,20 For those reasons, we determined also the sorption isotherms for a selected group of samples (see below) which had been thoroughly ground. The grinding treatment should open for sorption the crystal regions that could be inaccessible for the benzene molecules in the result of an internal twinning or another type of hindrances for diffusion. The isotherms of benzene adsorption determined for the series of the nonground MeAPO-5 samples are shown in Figure 1. With exception of two samples, SAPO-5 and ZrAPO-5, the sorption capacity for benzene increases significantly with the metal content (Figure 1 and Table 1) and varies from ca. 0.5 (Mo) to ca. 1.5 mmol/g (Mg 4.4). This increase suggests that the sorption mechanism of benzene does not follow the volume filling of micropores as, in such a case, no increase of the capacity above that of parent AlPO4-5 could be observed. On the other hand, that increase occurs within the initial stage of sorption, i.e. at p/ps e 0.15 (Figure 1). At higher values of p/ps, almost no increase of sorption is observed and the isotherms are approximately parallel to each other. This indicates that the samples become saturated with benzene already at p/ps < 0.15. The changes of the sorption behavior within the investigated group of the samples suggest that the sorption potential relative to benzene increases considerably with the metal content. As the representative examples for the measurements on the ground samples, we have chosen parent AlPO4-5, then MgAPO-5 with the highest metal content and sorption capacity, three further MgAPO-5 samples of lower metal contents and sorption capacities as the series with the same metal, and finally two other derivatives, ZrAPO-5 and VAPO-5, which should reveal another sorption

characteristic due to different valence and, most likely, coordination symmetry of the metal ions.20,25-28 The grinding treatment was performed by hand in an agate mortar. The maximum particle dimensions of the ground crystals were about 3 µm. The XRD patterns of the ground samples did not indicate any loss of crystallinity. The contents of heteroatoms in these samples and the upper limits of the crystal dimensions prior to the grinding as well as the sorption capacities of the ground materials are listed in Table 2. The sorption of benzene on the ground crystals appeared to be slightly higher than that on the large crystals. This slight increase could apparently be explained as the result of an increase of specific area due to grinding. However, a simple mathematical calculation can show that such an increase of the crystal surface area after grinding is lower than 0.1% in relation to the surface area of the pores. For this reason, we relate the effect of the slight increase of sorption to an inhomogeneous distribution of heteroatoms in our large crystals. In the smaller particles resulting from the grinding treatment, the differences (gradients) in the distribution of the heteroatoms become lower than those in the parent (large) crystals due to a simple mechanical reduction of dimensions. Thus, each separate particle becomes more homogeneous than a parent crystal, and eventually the whole ground sample becomes apparently more homogeneous as well. As the heterocenters interact with the benzene molecules (see below), they control diffusion of benzene in the pores. A more homogeneous distribution of the heteroatoms results this way in a less hindered diffusion. Moreover, the pores become more accessible due to shorter diffusion pathways along the pores in all the smaller particles. An especially good support for that explanation is given by the ZrAPO-5 sample which shows a drastic increase of sorption at low relative pressure although the final sorption capacity is almost the same (Tables 1 and 2).20,27 A separate consideration has to be devoted to the sorption capacity of parent AlPO4-5. The sorption isotherm for that sample locates in the middle of the group of the isotherms (Figure 1); i.e. some metals cause an increase and the others a decrease of the capacity in relation to the nonsubstituted AlPO4-5. Three derivatives containing Ni, Fe, and Cu show almost no adsorption of benzene and should rather be considered as samples with extraframework metal species which clog the pores. This is supported by the similar effects observed for the nitrogen sorption on these samples.20,25-28 Nevertheless, each decrease of the sorption capacity below that of AlPO4-5 cannot be simply interpreted as an effect of clogging the pores by (28) Kornatowski, J.; Zadrozna, G. Proc. Int. Zeolite Conf., 12th 1998, in press.

5860 Langmuir, Vol. 15, No. 18, 1999

Rozwadowski et al.

Figure 2. Isotherms of benzene adsorption on selected ground MeAPO-5 samples at 298.2 K: (]) MgAPO-5 (4.4), (3) MgAPO-5 (2.8), (1) MgAPO-5 (1.6), (b) MgAPO-5 (0.6), (4) ZrAPO-5 (1.8), (×) VAPO-5 (0.1); (0) AlPO4-5. Table 3. Parameters of the Particular Isotherm Equations for Benzene Adsorption on Ground MeAPO-5 Samples (p/ps ) 0.1, T ) 298.2 K) sample

sorption capacity (dm3/kg)

MgAPO-5 (4.4)

0.122

MgAPO-5 (2.8)

0.117

MgAPO-5 (1.6)

0.103

MgAPO-5 (0.6)

0.041

ZrAPO-5 (1.8)

0.048

VAPO-5 (0.1)

AlPO4-5

0.046

0.058

eq

W0 (dm3/kg)

DC

DR DAR DRSI JC LF DR DAR DRSI JC LF DR DAR DRSI JC LF DR DAR DRSI JC LF DR DAR DRSI JC LF DR DAR DRSI JC LF DR DAR DRSI JC LF

0.155 0.117 0.119 0.155 0.118 0.162 0.113 0.117 0.162 0.112 0.140 0.104 0.102 0.140 0.105 0.046 0.046 0.046 0.046 0.054 0.053 0.052 0.047 0.053 0.058 0.054 0.050 0.048 0.054 0.056 0.074 0.058 0.057 0.074 0.058

0.9312 0.9968 0.9961 0.9312 0.9976 0.9186 0.9970 0.9950 0.9186 0.9993 0.9650 0.9971 0.9977 0.9650 0.9994 0.9928 0.9928 0.9928 0.9928 0.9847 0.9962 0.9963 0.9959 0.9962 0.9944 0.9821 0.9847 0.9875 0.9821 0.9735 0.9455 0.9919 0.9920 0.9455 0.9955

extraframework species in all the cases. The phenomena of decreasing the sorption capacity are also observed for the groups of samples containing the same metal in various amounts. An example for such a group of the MgAPO-5 samples is presented in Figure 2 and Table 2. The sorption capacities of the samples with higher Mg contents are significantly higher than that of the parent AlPO4-5 while the sample with the lowest Mg content shows a clearly decreased capacity. Moreover, the increase of the capacity is approximately proportional to the Mg content. Thus, it would not be logical to accept that the pores in the lowest substituted samples are strongly clogged and those in the highly substituted ones are free for sorption. At this point,

one might also exclude an imaginable effect of inhomogeneous distribution of the metal centers in the framework as the isotherms shown in Figure 2 have been determined for the ground samples, and the grinding should average a possible inhomogeneity of the distribution (compare below). Then, the discussed observations suggest that energetic relations rather than simple steric hindrances are the reasons for the variations in sorption capacities. The calculations presented below support that suggestion. For the reasons indicated above, the analytical considerations deal with the sorption isotherms determined for the group of the ground samples. In routine calculations, W0 is assumed as one of the adjustable parameters that should correspond to the total volume of micropores in the examined samples. The values of W0 for benzene calculated from eq 4 with n equal to both 2 and noninteger numbers as well as from eqs 5-7 are listed in Table 3. In addition to W0, also the determination coefficients (DC) illustrating the fit of the calculated isotherms to the measured ones were examined in order to verify the applicability of the isotherm equations to the description of the experimental data. The DC values relating to eqs 4-7 are listed in Table 3 as well. Considering both the W0 and DC values for benzene (Table 3), one can state that the results obtained with use of eq 4 with n being noninteger numbers (DAR), eq 5 (DRSI), and eq 7 (LF) are generally better than those from the DR and JC equations. It is also evident that the W0 values which characterize the volume of micropore space agree with the micropore volumes obtained from the benzene adsorption at p/ps ) 0.1 (Table 3). Similar observation was already reported by us for the MFI type zeolites.29,30 Moreover, that agreement is much better for the DAR, DRSI, and LF than DR and JC equations. For microporous solids, the total heterogeneity of an adsorbent is the sum of the structural and surface heterogeneities and is characterized by the distribution function of the adsorption potential (eq 1, compare Introduction).3,4 Using eqs 4-7, the distribution can be expressed by eqs 8-12.7,17

X(A) ) -dW/dA ) -W0(dΘ/dA) ) (2W0kA/β2) exp[-k(A/β)2] (8) X(A) ) (nW0kAn-1/βn) exp[-k(A/β)n] X(A) ) nW0An-1(k0βn - ∆2An)/β2n +

() π 2

( )

1/2

nW0∆An-1 exp -

[

( )]

βn 1 + erf

(9)

k02

2∆2

k0

(10)

∆21/2

X(A) ) 2(n + 1)qn+1(A/β2)[q + (A/β2)]-(n+2) (11)

]{

γ γ exp (A - A0) 1 + X(A) ) W0 RT RT γ (A - A0) exp RT

[

[

]}

-2

(12)

The above functions (eqs 8-12) are shown for several representative samples in Figure 3. Parameters characterizing the distribution function, Xmax, A(Xmax), and the (29) Kornatowski, J.; Rozwadowski, M.; Lutz, W.; Baur, W. H. Stud. Surf. Sci. Catal. 1995, 97, 259. (30) Kornatowski, J.; Sychev, M.; Rozwadowski, M.; Lutz, W. Stud. Surf. Sci. Catal. 1997, 105, 1795.

Sorption of Benzene on AlPO4-5

Langmuir, Vol. 15, No. 18, 1999 5861 Table 4. Parameters of the Distribution Function of the Adsorption Potential for Benzene on Ground MeAPO-5 Samples (298.2 K) sample

eq

MgAPO-5 (4.4) DR DAR DRSI JC LF MgAPO-5 (2.8) DR DAR DRSI JC LF MgAPO-5 (1.6) DR DAR DRSI JC LF MgAPO-5 (0.6) DR DAR DRSI JC LF ZrAPO-5 (1.8) DR DAR DRSI JC LF VAPO-5 (0.1) DR DAR DRSI JC LF AlPO4-5 DR DAR DRSI JC LF

103Xmax A(Xmax) ∆ (mol cm3 kJ-1 kg-1) (kJ mol -1) (kJ mol-1) 7.101 8.887 8.581 7.101 9.252 8.168 9.372 8.902 8.167 10.083 7.303 7.109 7.500 7.303 7.549 2.255 2.265 2.257 2.257 2.295 2.667 2.644 2.821 2.668 2.648 2.374 2.369 2.513 2.374 2.342 3.511 3.859 3.983 3.511 4.035

13.261 19.965 19.664 13.251 19.302 12.003 19.267 18.678 12.010 18.755 11.594 17.376 17.181 11.603 17.235 12.262 12.099 12.240 12.237 13.024 12.067 12.374 13.626 12.064 13.306 13.747 16.000 15.632 13.747 16.448 12.819 18.672 18.323 12.819 18.310

21.250 12.563 13.250 21.250 11.375 19.188 11.500 12.563 19.188 9.875 18.563 14.125 12.438 18.563 12.250 19.625 19.750 19.625 19.625 20.125 19.375 19.250 14.625 19.375 19.375 22.125 20.375 16.750 22.125 20.250 20.500 14.375 13.125 20.500 12.875

Figure 3. Functions of the adsorption potential distribution calculated for benzene adsorbed on the samples of MgAPO-5 (4.4) (a), MgAPO-5 (0.6) (b), and parent AlPO4-5 (c) with use of the DR, DAR, DRSI, JC, and LF equations.

half-width of the distribution ∆ are listed for all the examined samples in Table 4. The shape of the distribution functions (Figure 3), which characterizes heterogeneity of microporous solids, and the relevant distribution parameters (Table 4) depend generally on the used isotherm equation. As can be seen from Figure 3 and Tables 3 and 4, the DR equation gives results similar to those derived from the JC equation while another group of the results similar to each other are obtained with use of the DAR, DRSI, and LF equations. Thus, the above considerations seem to indicate that, out of all the presented equations based on the Polanyi-Dubinin potential theory, eqs 4, 5, and 7 are the most useful for the description of benzene adsorption on AFI type molecular sieves as they yield higher DC values and better correlations for W0 than the other equations. Generally, the results show that the metal heteroatoms play the role of adsorption centers which affect the adsorption potential (Figure 3) and influence considerably the sorption capacity for benzene (Figures 1 and 2 and Tables 1 and 2). The sorption capacity is dependent on both the amounts of metals and their distribution (Figures 1 and 2 and Tables 1 and 2). These facts indicate that the

Figure 4. Dependence of A(Xmax) calculated from the DRSI equation on heteroatom content for some MeAPO-5 and AlPO4-5 samples.

metal ions create centers capable of bonding the benzene molecules; i.e. they exert a significant effect on the heterogeneity of the examined solids. In the case of the ground MgAPO-5 samples, the increase of the Mg content causes a systematical shift of the maximum of the distribution curve X(A) to higher values of A (Figure 4). Such a simple and clear dependence should theoretically be expected for all the investigated metal derivatives.

5862 Langmuir, Vol. 15, No. 18, 1999

However, it has been recorded as an exception among all the studied materials, and no dependence between A and the content of heterocenters has been found for the metals other than Mg. Moreover, the value of the adsorption potential for the parent AlPO4-5 has appeared to be unexpectedly high which seems to be unjustified (Figure 4). In other words, that adsorption potential is much higher than those for the samples substituted with lower amounts of Mg or several other metals (these values are reflected in lower sorption capacities of those samples; compare Figures 1 and 2 and Tables 1, 2, and 4). This observation suggests that the decrease of sorption capacity in relation to the parent AlPO4-5, observed for all materials substituted with lower amounts of various metals, is a general tendency. This fact as well as the high value of A for the parent AlPO4-5 supports the conclusion that the relation between the sorption capacity and modification of the sample heterogeneity by isomorphous substitution of heterometals is of an energetic origin rather than a simple steric effect hindering the sorption process. One might accept a picture that the interactions between the heterocenters and the benzene molecules (bonding strength) apparently decrease with the observed increase of the sorption capacities of the MeAPO-5 samples. This seems to hold so for the samples with various metals (Figure 1) as for those with one metal in various amounts (Figure 2). Taking into consideration the fact that distances between the particular heterocenters increase with a decrease of the substitution level, the observations seem to be justified. Interactions of the sorbate molecules with the isolated heterocenters should be stronger than in the case of the heterocenters located close to each other. Thus, sorption on the isolated heterocenters has a more localized character which results in a decrease of sorption capacity caused by hindering, or even braking, the diffusion between the adsorption centers of any type. That picture requires one to accept that, in nonsubstituted AlPO4-5, there is a relatively high number of the adsorption centers created exclusively by a surface heterogeneity, i.e. mainly by various structure defects. Interaction of such centers with the sorbate molecules can obviously be relatively weak. The occurrence of any “stronger” heterocenters should dominate the surface effects, and just such a behavior has actually been observed. The lack of a relationship between the adsorption potential and the content of metal with one exception of the ground MgAPO-5 samples suggests an extraordinarily important role of the inhomogeneous distribution of the metallic heterocenters. A strong support for this conclusion is given by the fact that the calculations for the isotherms determined for the nonground MgAPO-5 samples could also yield no dependence between the adsorption potential and the Mg content. On the other hand, it is clear that the role of the distribution parameter increases with a

Rozwadowski et al.

decrease of the metal content. That is why the dependence has been found only for the series of the highest substituted and ground MgAPO-5 samples. Verification of that picture can be possible by measurements on the samples either much higher substituted or with almost ideal homogeneous distribution of the metals in the framework. A very thorough grinding of the samples seems to offer an alternative way for those hardly possible synthesis tasks. Further investigations are in course. Conclusions Sorption of benzene (and likely also other nonpolar adsorbates) on nonsubstituted AlPO4-5 occurs on the adsorption centers created by surface heterogeneity (mostly structure defects). The metallic heteroatoms present in the framework of the substituted AFI type materials create centers capable of bonding the benzene molecules. This is equal to an increase of heterogeneity in comparison to nonsubstituted AlPO4-5. The sorption capacity of the MeAPO-5 samples for benzene is dependent on both the content of the metal and its distribution throughout the crystals. Substitution of low amounts of metal can lead to a decrease of the sorption capacity below that characteristic of nonsubstituted AlPO4-5 due to hindering of diffusion of the sorbate molecules at the isolated metal centers. Increase of the metal content results in the increase of the sorption capacity. The distribution of the metallic heterocenters plays a decisive role and may even dominate the effect of the metal content. The importance of the distribution parameter increases with a decrease of the metal content. The isotherm equations DAR, DRSI, and LF, based on the Polanyi-Dubinin potential theory, seem to be useful for the description of benzene adsorption on AFI-type molecular sieves. These equations can be accepted as suitable for characterizing the distribution of the adsorption potential and, thus, of the adsorption energy of such solids. An expected dependence of increase of the adsorption potential with the metal content has been found only for the series of the ground MgAPO-5 samples. The lack of such dependence in the case of the other metals is most likely due to inhomogeneous distribution of the metallic heterocenters in the framework and/or a too low substitution. Acknowledgment. The work was partially supported by the Polish Committee for Scientific Research within the Grant No. 3TO9A04714. The authors gratefully thank Dr. J. Wloch for the computer calculations. LA981422D