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Investigation on the porosity of zeolite NU-88 by means of positron annihilation lifetime spectroscopy
Nuclear Instruments and Methods in Physics Research B 267 (2009) 2550–2553
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Investigation on the porosity of zeolite NU-88 by means of positron annihilation lifetime spectroscopy G. Consolati a,*, M. Mariani b, R. Millini c, F. Quasso a a
Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci, 32, Milano 20133, Italy Dipartimento di Energia, Politecnico di Milano, Piazza Leonardo da Vinci, 32, Milano 20133, Italy c Eni S.p.A. R&M Division, Research and Technological Development, San Donato Research Center Via F. Maritano 26, I-20097 San Donato Milanese, Milano, Italy b
a r t i c l e
i n f o
Article history: Received 17 March 2009 Received in revised form 13 May 2009 Available online 28 May 2009 PACS: 36.10.Dr 71.60.+z 78.70.Bj 81.05.Rm
a b s t r a c t Seven well characterized zeolites were investigated by positron annihilation lifetime spectroscopy. The lifetime spectra were analysed in four discrete components. The third one was associated with ortho-positronium annihilation in the channels, framed in terms of infinite cylinders. Differences between the radii determined from the positron annihilation technique and X-ray diffraction data were found and explained in terms of the physical structure of the channel. An analogous study on a high-silica NU-88 zeolite gave a value of 0.33 nm for the corresponding radius, in agreement with Ar and N2 adsorption data as well as with the catalytic behaviour of this zeolite in several acid catalyzed reactions. The longest lifetime component in NU-88 reveals the existence of mesopores, with average radius of about 1.8 nm, which could explain the importance of hydrogen transfer reactions in this zeolite. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Positron annihilation Zeolites
1. Introduction Zeolites form an important class of materials which have attracted considerable interest because of their ion-exchange, molecular sieving and, particularly, unique shape selective catalytic properties. These properties in turn arise from their peculiar structural characteristics, in primis the presence of voids (channels and eventually, cages) with regular dimensions determined by the arrangement of the four-connected [SiO4] and [AlO4] tetrahedra in a three-dimensional framework. The continuous efforts in zeolite synthesis has produced a large number of different topologies corresponding, today, to 186 zeolite framework types, each of them characterized by a peculiar pore system [1]. The recognition of the porous characteristics of zeolites is strongly related to the determination of their crystal structure performed with the classical single-crystal X-ray diffraction methods in the relatively few cases in which specimen of suitable dimensions are available or, more commonly, through the elaboration of X-ray powder diffraction (XRD) data. In some cases, however, the nanocrystalline nature of the products and/or of the presence of structural disorder render the quality of the XRD pattern too
* Corresponding author. Tel.: +39 2 23996158; fax: +39 2 23996126. E-mail address: [email protected] (G. Consolati). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.05.057
poor for allowing the determination of the structure even with the most advanced structural tools. This is the case for NU-88, a high-silica zeolite synthesized in the presence of a dicationic structure directing agent (SDA) such as 1,5bis(N-methylpyrrolidinium)pentane or 1,6-bis(N-methylpyrrolidinium)hexane [2]. The XRD pattern of this zeolite is characterized by the presence of few sharp and several broad reflections (Fig. 1), indicating the existence of severe disorder phenomena, which hamper the determination of the crystal structure [2]. On the other hand, NU-88 exhibits interesting catalytic properties in the isomerization of n-hexane [3], cracking of n-octane [4], hydroisomerization of n-heptane [5] and in the synthesis of methylenedianiline (MDA) and bis-phenol-A (BPA) [6], attracting the interest of the researchers. Besides the evaluation of its catalytic properties, attempts to define the characteristics of the pore structures has been performed, leading to different conclusions. Examining the results of the hydroisomerization of n-heptane, Lacombe et al. concluded that NU-88 belongs to the family of the medium pore zeolites, its pore system being constituted by 10-membered ring channels with large voids (cages or channel intersections) [4]; on the other hand, Lee et al. included NU-88 in the family of the large pore zeolites with a multidimensional channel system [3]. These contradictory conclusions prompted us to investigate the pore structure of NU-88 by means of positron annihilation lifetime spectroscopy (PALS), a nuclear method useful to probe in a
G. Consolati et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2550–2553
s3 ¼ 1=k3 ¼ 1=ðkp þ ki Þ:
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ð2Þ
3. Experimental 3.1. Materials
Fig. 1. Typical X-ray powder diffraction pattern of NU-88.
non-destructive way the voids present in materials [7]. Many investigations on zeolites were carried out with this method [8– 14]. PALS is based on the fact that a fraction of the positrons injected into a sample may form, in the final stage of their ionization track, an unstable electron–positron complex, termed positronium (Ps). This becomes trapped in low electron density regions of the host matrix, such as voids in porous materials, free volume holes in polymers and channels in zeolites. In the ground state Ps exists in two sublevels, ortho-Ps (o-Ps) and para-Ps (p-Ps), reflecting the different spin states of the particles (parallel and antiparallel, respectively). In a vacuum, o-Ps decays into three quanta, with a lifetime of 142 ns. On the other hand, p-Ps decays into two quanta and has a lifetime of 0.125 ns. In condensed matter o-Ps interacts with the electrons belonging to the walls of the void in which it becomes trapped, and its lifetime is mainly determined by the annihilations of the positron with one of these external electrons in a relative singlet state – ‘pickoff’ mechanism [15]. This process reduces the o-Ps lifetime with respect to that in a vacuum. A correlation exists between o-Ps lifetime and the sizes of the cavity, which can be cast in a quantitative form by suitably modeling the trapping site. Different models were proposed to analyze the behaviour of o-Ps in a void, according to the various materials; in the present work we treated the zeolitic channels as cylinders with infinite length, for which a mathematical treatment is available [16,17].
NU-88 was synthesised following the recipe reported in [6], using N,N-pentamethylen-bis-[N-methyl-3-hydroxo-piperidinium] dihydroxide as a SDA. ERS-7 (ESV) was prepared according the recipe reported in [20] while EU-1 (EUO), ZSM-35 (FER), ZSM-5 (MFI), Mordenite (MOR), ZSM-12 (MTW) and Offretite (OFF) were synthesized according the standard recipes reported in the Atlas of Verified Synthesis of Zeolitic Materials [21]. All the zeolite samples were calcined at 550 °C for 5 h in air to remove the SDA trapped in the pores and exchanged in acidic form by repeated treatments with an aqueous solution of ammonium acetate, followed by a further calcination at 550 °C for 5 h in air. The nature of the samples was confirmed by XRD analysis, performed with a Philips X’PERT vertical diffractometer equipped with a pulse-height analyzer and a secondary monochromator. Data were collected in the 3 6 2h 6 55° angular region, with 0.05° 2h step and 5 s/step accumulation time; the Cu Ka radiation (k = 1.54178 Å) was used. 3.2. Positron annihilation measurements The positron source consisted of a droplet of 22Na from a carrier-free neutral solution (activity: 0.3 MBq), dried between two identical KaptonÒ foils (thickness 1.08 mg cm2), which were afterwards glued together. The source was inserted within two identical layers of the specimen in a typical ‘sandwich’ configuration. All the investigated zeolites, in form of powder, were pressed in metallic cylindrical boxes, then were outgassed at 383 K for 4 h to remove water present in the channels. The thickness of the samples (about 1 mm) was sufficient to stop all the injected positrons. PALS spectra were collected through a conventional coincidence set-up, having a resolution of about 240 ps. All the measurements were performed at room temperature and in a vacuum granted by a rotary pump (<10 Pa). Each spectrum contained about 3 106 counts. Three spectra for each sample were collected. Analyses were carried out through the computer code LT [22], with a suitable correction for the positrons annihilated in the KaptonÒ. 4. Results and discussion
2. The pore model The zeolitic pore is assumed to be an infinite cylinder with effective radius R. Such a Ps trap has a potential well with a finite depth; however, for convenience of calculation, the depth is assumed to be infinite, but the radius is increased to R + DR, DR being an empirical parameter which describes the penetration of the Ps wave function into the bulk. The electron density is assumed to be zero for r < R and constant for r > R. The relationship between o-Ps pickoff decay rate kp (ns1) and R is the following [16,17]:
kp ¼ 2:56
Z
a1 RþRDR
a1
J 20 ðrÞr dr;
ð1Þ
where J0(x) is the zero order Bessel function of the first kind and a1 = 2.4048 its first zero. DR may change on passing from solid to liquid structures [18], as well as by using different geometries for holes trapping Ps. A value DR = 0.166 nm [19] is generally reported in the literature. The measured o-Ps lifetime s3 is the reciprocal of the total decay rate k3, which is the sum of the pickoff decay rate and the intrinsic decay rate ki = 1/142 ns1:
Time annihilation spectra of the investigated zeolites were analysed into four discrete components: satisfactory fits were obtained (v2 test in the range 0.95–1.1) in our analyses. The results are shown in Table 1. Concerning the interpretation of the various components, in a complex structure like a zeolite it is hard to unambiguously attribute each component to a single decay process: it is reasonable to consider that some of the components may be average lifetimes from different annihilation channels. The first component can be considered as a mixture of two states, one due to annihilations from p-Ps, the other one coming from free positrons, that is, not forming Ps and annihilated in the bulk. Indeed, due to the finite resolution of the apparatus and to the rather faint value of the p-Ps intensity, it was not possible to extract the corresponding component from a constraint-free analysis. Sometimes the p-Ps lifetime is obtained from the time annihilation spectra by fixing sp-Ps to the value in a vacuum, 0.125 ns. We did not follow such a procedure, since in our opinion it can introduce a bias in the spectrum, with the result to distort the other components. Indeed, it is known that in matter p-Ps lifetime can be different with respect to that in a vacuum, due to the combined
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G. Consolati et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2550–2553
effects of pickoff (which decreases the lifetime) and of the electron density at the positron (‘contact density’ [23]), which is generally smaller than in vacuo. This last effect increases p-Ps lifetime in an unpredictable way. The values of the lifetime of the second component are in the range 580-870 ps. These are unusually long lifetimes for an annihilation process involving only free positrons. Certainly most of the annihilations associated to the second component are due to positrons trapped in zones (channels and cavities) having lower electron density than the bulk; as a consequence, the lifetime is longer than that corresponding to the first component. However, since a value longer than 500 ps is usually assumed as an indication of the presence of Ps, part of annihilations corresponding to the second component can be attributed to Ps trapped in small ‘blind’ voids, different from the channels, like vacancies lattice, always present in a real crystalline structure and not revealed by the classical XRD technique. We tried to split the second component by analysing the time annihilation spectra into five components; actually, in some cases we succeeded to obtain a lifetime s2 around 0.4 ns and a fifth component with lifetime of about 1 ns, which would correspond to o-Ps trapped in ‘blind’ holes with sizes smaller than the channels. However, the statistical uncertainty associated to the various components increased, in particular for those components with smaller intensities, which are the most interesting ones for the present work. Furthermore, the v2 test did not improve significantly. We concluded that the analysis in four discrete components is most satisfactory as far as the error bars of the various components are concerned. Analyses in five components, or with some distributed components, would be really better from the point of view of the interpretation, but they would require a statistics of counts higher than those accumulated in our spectra. The third component has a lifetime of the order of few ns and may be related to o-Ps decay by pickoff into the channels of zeolites. Knowledge of s3 allows one to estimate the radius of the zeolitic channel, framed as an infinite cylinder, by using Eqs. (1) and (2), once the parameter DR is known. We used for it the value 0.166 nm [19] and we found the radii RPALS shown in Table 2. Uncertainties on RPALS are estimated around 5%. For comparison, the radii RPORE as resulting from the determination of the crystal structure of the different zeolites [1] are also reported: since none of the zeolites here considered are characterized by perfectly circular pore openings, the values of RPORE correspond to the radii of the equivalent cylindrical channel calculated by averaging the various sizes displayed in [1]. Furthermore, with the exception of MTW,
Table 1 Lifetime parameters of the investigated zeolites. Each value is the average of three measurements. Uncertainties are shown in parenthesis and derived not only from the scattering of the experimental results but also from the conservative estimates adopted. Zeolite
s1 (ns)
I1 (%)
s2 (ns)
I2 (%)
s3 (ns)
I3 (%)
s4 (ns)
I4 (%)
ESV
0.25 (2) 0.33 (3) 0.22 (2) 0.34 (3) 0.25 (2) 0.39 (4) 0.43 (4) 0.39 (4)
18 (2) 34 (3) 25 (3) 43 (4) 24 (2) 47 (5) 56 (6) 43 (4)
0.62 (3) 0.76 (4) 0.58 (3) 0.63 (3) 0.65 (4) 0.75 (4) 0.87 (4) 0.79 (4)
69 (3) 51 (2) 50 (2) 49 (2) 59 (3) 42 (2) 32 (2) 46 (2)
2.9 (1)
6.7 (1)
9.1 (5)
2.8 (1)
12.5 (2) 24.2 (5) 2.6 (1)
11.4 (6) 13.9 (7) 32 (2)
4.1(1)
13.7 (2) 5.9 (1)
11.2 (6) 64 (4)
6.9 (1)
6.5 (1)
100 (5)
3.8 (1)
4.2 (1)
58 (4)
6.3 (3) 2.5 (1) 0.8 (1) 5.4 (3) 3.3 (2) 5.1 (3) 5.5 (3) 6.8 (3)
EUO FER MFI MOR MTW OFF NU-88
1.9 (1) 3.6 (1) 3.3 (1)
Table 2 Size (in nm) of the cylindrical channel as detected by PALS (RPALS) and equivalent radius (RPORE) of the channels calculated from [1]. For each zeolite structure, the characteristics of the porous structure are also reported. Zeolite
RPALS
RPORE
Pore characteristicsa
ESV
0.28
0.20
EUO
0.27
0.24
FER
0.21
0.24
MFI
0.32
0.27
MOR
0.30
0.34
MTW OFF
0.34 0.45
0.29 0.34
NU-88
0.33
–
1D system of cages connected by 8MR windows 3.5 4.7 Å 1D linear 10MR channels 4.1 5.4 Å, with large side pockets 1D linear 10MR channels 4.2 5.4 Å, interconnected with 1D 8MR channels 3.5 4.8 Å 3D pore structure composed by linear 10MR channels 5.3 5.6 Å, interconnected with sinusoidal 10MR channels 5.1 5.5 Å 1D linear 12MR channels 6.5 7.0 Å interconnected with linear 8MR channels 2.6 5.7 Å and linear 8MR channels 3.4 4.8 Å 1D linear 12MR channels 5.6 6.0 Å 1D linear 12MR channels 6.7 6.8 Å interconnected with 2D 8MR channels 3.6 4.9 Å Unknown
a
1D, 2D, 3D define the dimensionality of the pore system (mono-, bi- or tridimensional); 8MR, 10MR and 12MR define the number of tetrahedra defining the opening of the channels (e.g. 8MR stands for eight-membered ring).
characterized by a mono-dimensional linear channel system, all the zeolite here considered possess cages (as ESV and EUO) and/ or voids in correspondence to the intersections among different channel systems (MFI, OFF) (Table 2). In such a complex situation, it is difficult if not impossible to approximate the pore systems to an equivalent cylinder; therefore, we preferred to consider the values of RPORE as the average radii of the larger channel present in each zeolite structure. As a consequence, it is observed that RPALS is systematically slightly larger than RPORE, which, in our approximation, does not take into account the presence of cages and channels intersections. Therefore, the Ps trapped into the different pore systems probes the presence of such large voids and the radius deduced in cylindrical approximation is larger than that defined by the simple value of RPORE. Only in two cases (FER and MOR) the value of RPALS is smaller than RPORE and that finds a justification with the presence of extended channel systems with eight-membered ring openings (Table 2). In these cases, Ps is expected to probe an average value of the different channels, which is necessarily smaller than the value of RPORE determined for the sole largest channel. Concerning NU-88, its channel radius as deduced from PALS results is 0.33 ± 0.01 nm. In this case we cannot make a comparison with RPORE because of the lack of structural data; however, it should be pointed out that our determination is nicely in agreement with the pore size distribution curve calculated from Ar adsorption isotherms [3]. When compared with the RPALS determined for the other zeolites, the value of 0.33 nm deduced for NU-88 cannot solve alone the problem of the classification of its pore system. Surely it does not consist of mono-dimensional linear channels with 10MR openings, even with large voids as in EUO, because a smaller average radius is expected. The possibility remains of a bi- or tri-dimensional system of intersecting channels with 10MR openings, as in the case of MFI, or of 12MR openings as in the case of Beta zeolite (here not considered because of the complexity of its disordered structure). The N2 adsorption data reported in [3] seem to be in favour of the second hypothesis, since a micropore volume of 0.24 cm3 g1 is consistent with a structure belonging to the family of the large pore zeolites with a multidimensional 12MR channel system such as Beta (micropore volume 0.22 cm3 g1) instead of the medium pore zeolites even with multi-dimensional 10MR channel system such as MFI (micropore volume 0.19 cm3 g1). Moreover, the results of the catalytic tests
G. Consolati et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2550–2553
are consistent with the presence of a multi-dimensional 12MR channel system. It is not only the conclusion reached by Lee et al. [3], even in view of the similarity of the catalytic performances of NU-88 and zeolite Beta in the hydroconversion of n-heptane and in the cracking of methylcyclohexane [4]. In a further examination of the PALS data of NU-88, we considered the longest lifetime component s4, which lies in a range between 10 and 100 ns and can be ascribed to annihilations from o-Ps trapped in pores larger than the channels. Such component was thoroughly investigated [10,11,24]. Concerning NU-88, we transformed again s4 into the radius of a spherical cavity [25,26], since we cannot guess the shape of the void. In this case we used an extension of the Tao–Eldrup model [27–29], necessary when Ps lifetime overcomes 30 ns. We found for the radius the value of 1.8 ± 0.2 nm, a value too large to be admitted as a crystallographic void present in the structure of a zeolite. It is likely that such pores originate from the peculiar morphology of the NU-88 powder, which usually crystallizes in form of blackberry-like aggregates of nanocrystals with dimensions of the order of 20–30 nm [3]. Though these agglomerates contain mesopores with a wide size distribution (diameter up to ca. 30 nm), it is likely that what is revealed by PALS is the fraction of intercrystalline mesoporosity with lower dimensions. Under a practical point of view, the presence of mesoporosity in the agglomerates has little influence on the catalytic reaction itself, which usually occurs within the zeolite channels, but is important for enhancing the mass transport efficiency of reactant and product molecules, which, in turn, may influence the entire process. In the present work discussion was centred on o-Ps lifetimes; corresponding intensities (Table 1) were considered as secondary parameters. In fact, their value depends on the energy window on the stop discriminator: a window in an energy region below 450 keV increases the registration of decays into three quanta with respect to pickoff annihilations, by increasing the fraction of the components with very long lifetimes [24]. Also the different efficiencies of the detector for the 3c/2c counting ratio should be taken into account, in order to get corrected values of the intensities [30]. However, this has a negligible influence on the values of the shortest lifetimes [11]. In our case we set the stop energy window around 511 keV, in order to efficiently investigate the o-Ps annihilations into the channel. 5. Conclusions PALS is able to estimate an equivalent radius of the zeolitic channel, in reasonable correlation with the value as determined by XRD, by taking into account various approximations. In PALS the channels are framed as infinite cylinders with a circular section, although real channels generally deviate from this shape and, depending on the zeolite structure, connect large voids represented by cages and/or intersections with other channels. We evaluated the average size of the channels in zeolite NU-88 (0.33 nm), which resulted in good agreement with the estimate given by the Ar and N2 adsorption data, as well as with the results of the catalytic tests performed on several reactions. All these data are consistent with a porous structure of NU-88 consisting in a multi-
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dimensional system of intersecting 12MR opening channels, allowing to classify this material in the family of the large pore zeolites. PALS spectra showed the presence of another o-Ps component with a longer lifetime, ascribed to annihilations in mesopores, which, in the case of NU-88, have an average radius (using the spherical approximation) of 1.8 nm. Such kind of pores are too large for being considered as part of the crystallographic structure of NU-88. Most likely, they represent the smaller fraction of the intercrystalline mesoporosity formed by the assembly of the nanocrystals (dimensions 20–30 nm) of NU-88 in blackberry-like agglomerates.
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Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Investigation on the porosity of zeolite NU-88 by means of positron annihilation lifetime spectroscopy G. Consolati a,*, M. Mariani b, R. Millini c, F. Quasso a a
Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci, 32, Milano 20133, Italy Dipartimento di Energia, Politecnico di Milano, Piazza Leonardo da Vinci, 32, Milano 20133, Italy c Eni S.p.A. R&M Division, Research and Technological Development, San Donato Research Center Via F. Maritano 26, I-20097 San Donato Milanese, Milano, Italy b
a r t i c l e
i n f o
Article history: Received 17 March 2009 Received in revised form 13 May 2009 Available online 28 May 2009 PACS: 36.10.Dr 71.60.+z 78.70.Bj 81.05.Rm
a b s t r a c t Seven well characterized zeolites were investigated by positron annihilation lifetime spectroscopy. The lifetime spectra were analysed in four discrete components. The third one was associated with ortho-positronium annihilation in the channels, framed in terms of infinite cylinders. Differences between the radii determined from the positron annihilation technique and X-ray diffraction data were found and explained in terms of the physical structure of the channel. An analogous study on a high-silica NU-88 zeolite gave a value of 0.33 nm for the corresponding radius, in agreement with Ar and N2 adsorption data as well as with the catalytic behaviour of this zeolite in several acid catalyzed reactions. The longest lifetime component in NU-88 reveals the existence of mesopores, with average radius of about 1.8 nm, which could explain the importance of hydrogen transfer reactions in this zeolite. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Positron annihilation Zeolites
1. Introduction Zeolites form an important class of materials which have attracted considerable interest because of their ion-exchange, molecular sieving and, particularly, unique shape selective catalytic properties. These properties in turn arise from their peculiar structural characteristics, in primis the presence of voids (channels and eventually, cages) with regular dimensions determined by the arrangement of the four-connected [SiO4] and [AlO4] tetrahedra in a three-dimensional framework. The continuous efforts in zeolite synthesis has produced a large number of different topologies corresponding, today, to 186 zeolite framework types, each of them characterized by a peculiar pore system [1]. The recognition of the porous characteristics of zeolites is strongly related to the determination of their crystal structure performed with the classical single-crystal X-ray diffraction methods in the relatively few cases in which specimen of suitable dimensions are available or, more commonly, through the elaboration of X-ray powder diffraction (XRD) data. In some cases, however, the nanocrystalline nature of the products and/or of the presence of structural disorder render the quality of the XRD pattern too
* Corresponding author. Tel.: +39 2 23996158; fax: +39 2 23996126. E-mail address: [email protected] (G. Consolati). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.05.057
poor for allowing the determination of the structure even with the most advanced structural tools. This is the case for NU-88, a high-silica zeolite synthesized in the presence of a dicationic structure directing agent (SDA) such as 1,5bis(N-methylpyrrolidinium)pentane or 1,6-bis(N-methylpyrrolidinium)hexane [2]. The XRD pattern of this zeolite is characterized by the presence of few sharp and several broad reflections (Fig. 1), indicating the existence of severe disorder phenomena, which hamper the determination of the crystal structure [2]. On the other hand, NU-88 exhibits interesting catalytic properties in the isomerization of n-hexane [3], cracking of n-octane [4], hydroisomerization of n-heptane [5] and in the synthesis of methylenedianiline (MDA) and bis-phenol-A (BPA) [6], attracting the interest of the researchers. Besides the evaluation of its catalytic properties, attempts to define the characteristics of the pore structures has been performed, leading to different conclusions. Examining the results of the hydroisomerization of n-heptane, Lacombe et al. concluded that NU-88 belongs to the family of the medium pore zeolites, its pore system being constituted by 10-membered ring channels with large voids (cages or channel intersections) [4]; on the other hand, Lee et al. included NU-88 in the family of the large pore zeolites with a multidimensional channel system [3]. These contradictory conclusions prompted us to investigate the pore structure of NU-88 by means of positron annihilation lifetime spectroscopy (PALS), a nuclear method useful to probe in a
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s3 ¼ 1=k3 ¼ 1=ðkp þ ki Þ:
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ð2Þ
3. Experimental 3.1. Materials
Fig. 1. Typical X-ray powder diffraction pattern of NU-88.
non-destructive way the voids present in materials [7]. Many investigations on zeolites were carried out with this method [8– 14]. PALS is based on the fact that a fraction of the positrons injected into a sample may form, in the final stage of their ionization track, an unstable electron–positron complex, termed positronium (Ps). This becomes trapped in low electron density regions of the host matrix, such as voids in porous materials, free volume holes in polymers and channels in zeolites. In the ground state Ps exists in two sublevels, ortho-Ps (o-Ps) and para-Ps (p-Ps), reflecting the different spin states of the particles (parallel and antiparallel, respectively). In a vacuum, o-Ps decays into three quanta, with a lifetime of 142 ns. On the other hand, p-Ps decays into two quanta and has a lifetime of 0.125 ns. In condensed matter o-Ps interacts with the electrons belonging to the walls of the void in which it becomes trapped, and its lifetime is mainly determined by the annihilations of the positron with one of these external electrons in a relative singlet state – ‘pickoff’ mechanism [15]. This process reduces the o-Ps lifetime with respect to that in a vacuum. A correlation exists between o-Ps lifetime and the sizes of the cavity, which can be cast in a quantitative form by suitably modeling the trapping site. Different models were proposed to analyze the behaviour of o-Ps in a void, according to the various materials; in the present work we treated the zeolitic channels as cylinders with infinite length, for which a mathematical treatment is available [16,17].
NU-88 was synthesised following the recipe reported in [6], using N,N-pentamethylen-bis-[N-methyl-3-hydroxo-piperidinium] dihydroxide as a SDA. ERS-7 (ESV) was prepared according the recipe reported in [20] while EU-1 (EUO), ZSM-35 (FER), ZSM-5 (MFI), Mordenite (MOR), ZSM-12 (MTW) and Offretite (OFF) were synthesized according the standard recipes reported in the Atlas of Verified Synthesis of Zeolitic Materials [21]. All the zeolite samples were calcined at 550 °C for 5 h in air to remove the SDA trapped in the pores and exchanged in acidic form by repeated treatments with an aqueous solution of ammonium acetate, followed by a further calcination at 550 °C for 5 h in air. The nature of the samples was confirmed by XRD analysis, performed with a Philips X’PERT vertical diffractometer equipped with a pulse-height analyzer and a secondary monochromator. Data were collected in the 3 6 2h 6 55° angular region, with 0.05° 2h step and 5 s/step accumulation time; the Cu Ka radiation (k = 1.54178 Å) was used. 3.2. Positron annihilation measurements The positron source consisted of a droplet of 22Na from a carrier-free neutral solution (activity: 0.3 MBq), dried between two identical KaptonÒ foils (thickness 1.08 mg cm2), which were afterwards glued together. The source was inserted within two identical layers of the specimen in a typical ‘sandwich’ configuration. All the investigated zeolites, in form of powder, were pressed in metallic cylindrical boxes, then were outgassed at 383 K for 4 h to remove water present in the channels. The thickness of the samples (about 1 mm) was sufficient to stop all the injected positrons. PALS spectra were collected through a conventional coincidence set-up, having a resolution of about 240 ps. All the measurements were performed at room temperature and in a vacuum granted by a rotary pump (<10 Pa). Each spectrum contained about 3 106 counts. Three spectra for each sample were collected. Analyses were carried out through the computer code LT [22], with a suitable correction for the positrons annihilated in the KaptonÒ. 4. Results and discussion
2. The pore model The zeolitic pore is assumed to be an infinite cylinder with effective radius R. Such a Ps trap has a potential well with a finite depth; however, for convenience of calculation, the depth is assumed to be infinite, but the radius is increased to R + DR, DR being an empirical parameter which describes the penetration of the Ps wave function into the bulk. The electron density is assumed to be zero for r < R and constant for r > R. The relationship between o-Ps pickoff decay rate kp (ns1) and R is the following [16,17]:
kp ¼ 2:56
Z
a1 RþRDR
a1
J 20 ðrÞr dr;
ð1Þ
where J0(x) is the zero order Bessel function of the first kind and a1 = 2.4048 its first zero. DR may change on passing from solid to liquid structures [18], as well as by using different geometries for holes trapping Ps. A value DR = 0.166 nm [19] is generally reported in the literature. The measured o-Ps lifetime s3 is the reciprocal of the total decay rate k3, which is the sum of the pickoff decay rate and the intrinsic decay rate ki = 1/142 ns1:
Time annihilation spectra of the investigated zeolites were analysed into four discrete components: satisfactory fits were obtained (v2 test in the range 0.95–1.1) in our analyses. The results are shown in Table 1. Concerning the interpretation of the various components, in a complex structure like a zeolite it is hard to unambiguously attribute each component to a single decay process: it is reasonable to consider that some of the components may be average lifetimes from different annihilation channels. The first component can be considered as a mixture of two states, one due to annihilations from p-Ps, the other one coming from free positrons, that is, not forming Ps and annihilated in the bulk. Indeed, due to the finite resolution of the apparatus and to the rather faint value of the p-Ps intensity, it was not possible to extract the corresponding component from a constraint-free analysis. Sometimes the p-Ps lifetime is obtained from the time annihilation spectra by fixing sp-Ps to the value in a vacuum, 0.125 ns. We did not follow such a procedure, since in our opinion it can introduce a bias in the spectrum, with the result to distort the other components. Indeed, it is known that in matter p-Ps lifetime can be different with respect to that in a vacuum, due to the combined
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effects of pickoff (which decreases the lifetime) and of the electron density at the positron (‘contact density’ [23]), which is generally smaller than in vacuo. This last effect increases p-Ps lifetime in an unpredictable way. The values of the lifetime of the second component are in the range 580-870 ps. These are unusually long lifetimes for an annihilation process involving only free positrons. Certainly most of the annihilations associated to the second component are due to positrons trapped in zones (channels and cavities) having lower electron density than the bulk; as a consequence, the lifetime is longer than that corresponding to the first component. However, since a value longer than 500 ps is usually assumed as an indication of the presence of Ps, part of annihilations corresponding to the second component can be attributed to Ps trapped in small ‘blind’ voids, different from the channels, like vacancies lattice, always present in a real crystalline structure and not revealed by the classical XRD technique. We tried to split the second component by analysing the time annihilation spectra into five components; actually, in some cases we succeeded to obtain a lifetime s2 around 0.4 ns and a fifth component with lifetime of about 1 ns, which would correspond to o-Ps trapped in ‘blind’ holes with sizes smaller than the channels. However, the statistical uncertainty associated to the various components increased, in particular for those components with smaller intensities, which are the most interesting ones for the present work. Furthermore, the v2 test did not improve significantly. We concluded that the analysis in four discrete components is most satisfactory as far as the error bars of the various components are concerned. Analyses in five components, or with some distributed components, would be really better from the point of view of the interpretation, but they would require a statistics of counts higher than those accumulated in our spectra. The third component has a lifetime of the order of few ns and may be related to o-Ps decay by pickoff into the channels of zeolites. Knowledge of s3 allows one to estimate the radius of the zeolitic channel, framed as an infinite cylinder, by using Eqs. (1) and (2), once the parameter DR is known. We used for it the value 0.166 nm [19] and we found the radii RPALS shown in Table 2. Uncertainties on RPALS are estimated around 5%. For comparison, the radii RPORE as resulting from the determination of the crystal structure of the different zeolites [1] are also reported: since none of the zeolites here considered are characterized by perfectly circular pore openings, the values of RPORE correspond to the radii of the equivalent cylindrical channel calculated by averaging the various sizes displayed in [1]. Furthermore, with the exception of MTW,
Table 1 Lifetime parameters of the investigated zeolites. Each value is the average of three measurements. Uncertainties are shown in parenthesis and derived not only from the scattering of the experimental results but also from the conservative estimates adopted. Zeolite
s1 (ns)
I1 (%)
s2 (ns)
I2 (%)
s3 (ns)
I3 (%)
s4 (ns)
I4 (%)
ESV
0.25 (2) 0.33 (3) 0.22 (2) 0.34 (3) 0.25 (2) 0.39 (4) 0.43 (4) 0.39 (4)
18 (2) 34 (3) 25 (3) 43 (4) 24 (2) 47 (5) 56 (6) 43 (4)
0.62 (3) 0.76 (4) 0.58 (3) 0.63 (3) 0.65 (4) 0.75 (4) 0.87 (4) 0.79 (4)
69 (3) 51 (2) 50 (2) 49 (2) 59 (3) 42 (2) 32 (2) 46 (2)
2.9 (1)
6.7 (1)
9.1 (5)
2.8 (1)
12.5 (2) 24.2 (5) 2.6 (1)
11.4 (6) 13.9 (7) 32 (2)
4.1(1)
13.7 (2) 5.9 (1)
11.2 (6) 64 (4)
6.9 (1)
6.5 (1)
100 (5)
3.8 (1)
4.2 (1)
58 (4)
6.3 (3) 2.5 (1) 0.8 (1) 5.4 (3) 3.3 (2) 5.1 (3) 5.5 (3) 6.8 (3)
EUO FER MFI MOR MTW OFF NU-88
1.9 (1) 3.6 (1) 3.3 (1)
Table 2 Size (in nm) of the cylindrical channel as detected by PALS (RPALS) and equivalent radius (RPORE) of the channels calculated from [1]. For each zeolite structure, the characteristics of the porous structure are also reported. Zeolite
RPALS
RPORE
Pore characteristicsa
ESV
0.28
0.20
EUO
0.27
0.24
FER
0.21
0.24
MFI
0.32
0.27
MOR
0.30
0.34
MTW OFF
0.34 0.45
0.29 0.34
NU-88
0.33
–
1D system of cages connected by 8MR windows 3.5 4.7 Å 1D linear 10MR channels 4.1 5.4 Å, with large side pockets 1D linear 10MR channels 4.2 5.4 Å, interconnected with 1D 8MR channels 3.5 4.8 Å 3D pore structure composed by linear 10MR channels 5.3 5.6 Å, interconnected with sinusoidal 10MR channels 5.1 5.5 Å 1D linear 12MR channels 6.5 7.0 Å interconnected with linear 8MR channels 2.6 5.7 Å and linear 8MR channels 3.4 4.8 Å 1D linear 12MR channels 5.6 6.0 Å 1D linear 12MR channels 6.7 6.8 Å interconnected with 2D 8MR channels 3.6 4.9 Å Unknown
a
1D, 2D, 3D define the dimensionality of the pore system (mono-, bi- or tridimensional); 8MR, 10MR and 12MR define the number of tetrahedra defining the opening of the channels (e.g. 8MR stands for eight-membered ring).
characterized by a mono-dimensional linear channel system, all the zeolite here considered possess cages (as ESV and EUO) and/ or voids in correspondence to the intersections among different channel systems (MFI, OFF) (Table 2). In such a complex situation, it is difficult if not impossible to approximate the pore systems to an equivalent cylinder; therefore, we preferred to consider the values of RPORE as the average radii of the larger channel present in each zeolite structure. As a consequence, it is observed that RPALS is systematically slightly larger than RPORE, which, in our approximation, does not take into account the presence of cages and channels intersections. Therefore, the Ps trapped into the different pore systems probes the presence of such large voids and the radius deduced in cylindrical approximation is larger than that defined by the simple value of RPORE. Only in two cases (FER and MOR) the value of RPALS is smaller than RPORE and that finds a justification with the presence of extended channel systems with eight-membered ring openings (Table 2). In these cases, Ps is expected to probe an average value of the different channels, which is necessarily smaller than the value of RPORE determined for the sole largest channel. Concerning NU-88, its channel radius as deduced from PALS results is 0.33 ± 0.01 nm. In this case we cannot make a comparison with RPORE because of the lack of structural data; however, it should be pointed out that our determination is nicely in agreement with the pore size distribution curve calculated from Ar adsorption isotherms [3]. When compared with the RPALS determined for the other zeolites, the value of 0.33 nm deduced for NU-88 cannot solve alone the problem of the classification of its pore system. Surely it does not consist of mono-dimensional linear channels with 10MR openings, even with large voids as in EUO, because a smaller average radius is expected. The possibility remains of a bi- or tri-dimensional system of intersecting channels with 10MR openings, as in the case of MFI, or of 12MR openings as in the case of Beta zeolite (here not considered because of the complexity of its disordered structure). The N2 adsorption data reported in [3] seem to be in favour of the second hypothesis, since a micropore volume of 0.24 cm3 g1 is consistent with a structure belonging to the family of the large pore zeolites with a multidimensional 12MR channel system such as Beta (micropore volume 0.22 cm3 g1) instead of the medium pore zeolites even with multi-dimensional 10MR channel system such as MFI (micropore volume 0.19 cm3 g1). Moreover, the results of the catalytic tests
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are consistent with the presence of a multi-dimensional 12MR channel system. It is not only the conclusion reached by Lee et al. [3], even in view of the similarity of the catalytic performances of NU-88 and zeolite Beta in the hydroconversion of n-heptane and in the cracking of methylcyclohexane [4]. In a further examination of the PALS data of NU-88, we considered the longest lifetime component s4, which lies in a range between 10 and 100 ns and can be ascribed to annihilations from o-Ps trapped in pores larger than the channels. Such component was thoroughly investigated [10,11,24]. Concerning NU-88, we transformed again s4 into the radius of a spherical cavity [25,26], since we cannot guess the shape of the void. In this case we used an extension of the Tao–Eldrup model [27–29], necessary when Ps lifetime overcomes 30 ns. We found for the radius the value of 1.8 ± 0.2 nm, a value too large to be admitted as a crystallographic void present in the structure of a zeolite. It is likely that such pores originate from the peculiar morphology of the NU-88 powder, which usually crystallizes in form of blackberry-like aggregates of nanocrystals with dimensions of the order of 20–30 nm [3]. Though these agglomerates contain mesopores with a wide size distribution (diameter up to ca. 30 nm), it is likely that what is revealed by PALS is the fraction of intercrystalline mesoporosity with lower dimensions. Under a practical point of view, the presence of mesoporosity in the agglomerates has little influence on the catalytic reaction itself, which usually occurs within the zeolite channels, but is important for enhancing the mass transport efficiency of reactant and product molecules, which, in turn, may influence the entire process. In the present work discussion was centred on o-Ps lifetimes; corresponding intensities (Table 1) were considered as secondary parameters. In fact, their value depends on the energy window on the stop discriminator: a window in an energy region below 450 keV increases the registration of decays into three quanta with respect to pickoff annihilations, by increasing the fraction of the components with very long lifetimes [24]. Also the different efficiencies of the detector for the 3c/2c counting ratio should be taken into account, in order to get corrected values of the intensities [30]. However, this has a negligible influence on the values of the shortest lifetimes [11]. In our case we set the stop energy window around 511 keV, in order to efficiently investigate the o-Ps annihilations into the channel. 5. Conclusions PALS is able to estimate an equivalent radius of the zeolitic channel, in reasonable correlation with the value as determined by XRD, by taking into account various approximations. In PALS the channels are framed as infinite cylinders with a circular section, although real channels generally deviate from this shape and, depending on the zeolite structure, connect large voids represented by cages and/or intersections with other channels. We evaluated the average size of the channels in zeolite NU-88 (0.33 nm), which resulted in good agreement with the estimate given by the Ar and N2 adsorption data, as well as with the results of the catalytic tests performed on several reactions. All these data are consistent with a porous structure of NU-88 consisting in a multi-
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dimensional system of intersecting 12MR opening channels, allowing to classify this material in the family of the large pore zeolites. PALS spectra showed the presence of another o-Ps component with a longer lifetime, ascribed to annihilations in mesopores, which, in the case of NU-88, have an average radius (using the spherical approximation) of 1.8 nm. Such kind of pores are too large for being considered as part of the crystallographic structure of NU-88. Most likely, they represent the smaller fraction of the intercrystalline mesoporosity formed by the assembly of the nanocrystals (dimensions 20–30 nm) of NU-88 in blackberry-like agglomerates.
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