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Surface and Pore Structure Assessment of Hierarchical MFI Zeolites by Advanced Water and Argon Sorption Studies Matthias Thommes, Sharon Mitchell, and Javier Perez-Ramirez J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp3051214 • Publication Date (Web): 07 Aug 2012 Downloaded from http://pubs.acs.org on August 8, 2012

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The Journal of Physical Chemistry

Surface and Pore Structure Assessment of Hierarchical MFI Zeolites by Advanced Water and Argon Sorption Studies

Matthias Thommes,*,† Sharon Mitchell,‡ and Javier Pérez-Ramírez‡

†

Quantachrome Instruments, 1900 Corporate Drive, Boynton Beach, FL, USA.

‡

Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Wolfgang-Pauli-Strasse 10, CH-8093, Zurich, Switzerland.

* Corresponding author. Tel: (561) 731-4999, Fax: (561) 732-9888; E-mail: [email protected].

ACS Paragon Plus Environment

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Abstract Advanced physico-chemical characterization of the pore structure of hierarchical zeolites, including the precise knowledge of pore size, interconnectivity, and surface properties, is crucial in order to interpret their superior performance in catalytic and adsorption applications. Post-synthetic mesopore formation by alkaline treatment of zeolites leads to simultaneous compositional changes due to the selective dissolution of silicon. By careful tuning of these effects through subsequent acid treatment, the Si/Al ratio can be restored to that of the parent zeolite. We evaluate the application of argon (87.3 K) and water (298.5 K) adsorption to assess the porous properties of mesoporous ZSM-5 zeolites with equivalent porosity, but differing composition. An accurate and combined micro-mesopore size analysis is obtained by applying NLDFT (non-local density functional theory) analysis on the argon isotherms. The argon adsorption data clearly reveal the two different relative pressure regions of micro- and mesopore filling of hierarchical zeolites. In contrast, the two filling regions overlap upon adsorption of water due to the hydrophobic nature of the ZSM-5 micropores and the much more hydrophilic nature of the mesopores. This indicates that water adsorption is sensitive to the Si/Al ratio, the distribution of aluminum species in the zeolite, and to the presence of polar groups on the mesopore surface. Thus, further insights into the surface and structural properties of the pores in hierarchical zeolites prepared by desilication can be gained. Based on our results, we put forward a hydrophilicity index capable of identifying differences in surface chemistry between distinct porous materials, and also between the micro- and mesopores present within hierarchically-structured nanoporous materials. Our findings are not only important for a comprehensive surface and pore structural characterization of hierarchical zeolites, in particular with regard to optimizing their application in catalytic and sorption processes, but also in general for further advancing the characterization of complex porous materials

Keywords: Hierarchical zeolite; Desilication; Pore size distribution; Argon adsorption; NLDFT; Water adsorption; Surface characterization; Hydrophilicity index


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1. Introduction In recent years, significant advancement has been made in the synthesis and characterization of micro- and mesoporous materials.1-4 Interest in hierarchical (mesoporous) zeolites stemmed from the concept of improving the hindered access and molecular transport of their commercially important microporous counterparts (for recent reviews see refs.5,6). The network of interconnected micro- and mesopores facilitates efficient transfer of fluids to and from active sites located predominantly within the micropores, leading to benefits in numerous catalytic applications.7-11 Mesoporous zeolites can be prepared by direct synthesis or by post-synthetic modification of conventional zeolites. Among the different available routes, desilication is a cost-effective, versatile, and scalable method.12-15 The generation of intra-crystalline mesopores by the base-mediated leaching of silicon results in simultaneous variation in composition. NaOH-treated zeolites exhibit a decreased bulk Si/Al ratio, crystallinity, and micropore volume, and an increased presence of Lewis acidic sites.16-18 Subsequent acid treatment under mild conditions is increasingly applied to restore the bulk Si/Al ratio and reduce the Lewis acidity.16,18 An accurate knowledge of the porous characteristics constitutes primary information for assessment of the synthesis-property-function relations that govern the zeolite performance in a given application. The most popular method to obtain this information is physical adsorption, in particular the adsorption of nitrogen and argon at their boiling temperatures (i.e. 77.4 K, and 87.3 K, respectively). During recent years, major progress has been achieved in understanding the adsorption and phase behavior in micro- and mesoporous materials with simple pore geometries, leading to improved structural characterization (for recent reviews see refs.4,19). This has been further supported by the development and availability of advanced theoretical approaches based on statistical mechanics. For example, the application of non-local density functional theory (NLDFT) based methods not only permits description of the fluids in pores at a molecular level, but also the obtainment of precise pore size distributions over the entire micro- and mesopore range.19,20


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In contrast to the breakthroughs achieved in the analysis of pore structure by physical adsorption, far less progress has been made in the assessment of surface chemistry. The effects of geometrical and chemical heterogeneities of the pore walls on the sorption, phase, and wetting behaviors of fluids in porous particles, are actively researched. Within this context, the use of water to probe the surface chemistry and pore structure has been applied for both organic (e.g. carbons21-26) and inorganic (e.g. zeolites and molecular sieves27-38) nanoporous materials. Water adsorption is attractive because measurements can be performed at room temperature, with favorable kinetics. Furthermore, the small kinetic diameter of water (0.28 nm) permits entry into pores that are not accessible to carbon dioxide or nitrogen.39 More relevantly, water adsorption is strongly affected by the nature of the surface. However, interpretation of the experimental isotherms is not always straightforward since the underlying adsorption mechanism in nanoporous materials has not been unequivocally described. Although water adsorption is known to be sensitive to the surface chemistry of purely microporous ZSM-5 zeolites (e.g. adsorption on strong/weak Brønsted/Lewis sites, internal defects and external surfaces),28-37 it has not been applied widely for the characterization of hierarchical zeolites. Mesoporous LTA zeolites prepared by carbon templating were recently reported to be more hydrophobic than solely microporous LTA.38 This was explained in terms of the presence of carbon residues formed upon removal of the template used for mesopore introduction by this route, and emphasized the fact that the properties of hierarchical zeolites are highly dependent on how they are prepared. Herein, we examine the adsorption and phase behavior of simple fluids (nitrogen, argon) and water vapor in conventional and hierarchical MFI-type zeolites with different Si/Al ratios. Extensive intracrystalline mesoporosity is introduced by alkaline treatment. Sequential acid washing is applied to tune the composition of the desilicated zeolites. Comparison of acid-washed hierarchical zeolites is particularly interesting, since they can have very similar pore architectures to the as-desilicated, but differing surface properties. Accurate pore size/structure determination over the whole micro- and


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mesopore size range is achieved by NLDFT analysis of the argon adsorption data. A hydrophilicity index is introduced to characterize the surface chemistry of different zeolites studied.

2. Experimental 2.1. Materials preparation ZSM-5 with two different nominal Si/Al ratios of 25 (Z25, CBV 5524G) and 40 (Z40, CBV 8014) were obtained in the ammonium form from Zeolyst International. These two zeolites served also as a kind of reference material to validate the effect of different surface chemistry (here different Si/Al ratios) on water adsorption. Hierarchical ZSM-5 (Z40-H) was prepared by alkaline treatment of Z40 (1 g) in aqueous sodium hydroxide (0.2 M, 30 ml) for 30 min at 378 K. Acid-washed hierarchical zeolite (Z40-HW) was obtained by treatment of Z40-H (1 g) with dilute HCl (0.1 M, 30 ml) for 6 h at 378 K. Both hierarchical zeolites were subjected to multiple step ion-exchange in aqueous ammonium nitrate solution (0.1 M). All zeolites were characterized in their acidic form following calcination in static air at 823 K for 5 h. 2.2. Adsorption studies Argon sorption experiments were performed with manometric techniques (Quantachrome’s Autosorb IQ and Autosorb I MP) at 87 K. Nitrogen isotherms were measured in a Quantachrome Quadrasorb-SI gas adsorption analyzer at 77 K. Water sorption experiments were performed at 298 K with gravimetric and manometric sorption analyzers (Quantachrome’s Aquadyne and Hydrosorb). Prior to the sorption experiments, the samples were outgassed for 12 h under turbomolecular pump vacuum at 623 K. Pore size distributions were determined from the argon isotherms by application of an advanced NLDFT method, assuming argon adsorption at 87 K in cylindrical siliceous zeolite pores in the micropore range and an amorphous silica pore model for the mesopore range.17 The total pore volume of micro- and mesopores was obtained by applying the Gurvich rule on the argon adsorption data. The Gurvich rule allows conversion of the adsorbed amount (here at a rel. pressure of 0.92) into a pore


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volume by assuming that that the pores are filled with the liquid adsorptive (see reference texts for a detailed overview).40-42 2.3. Additional characterization X-ray diffraction (XRD) was measured in a PANalytical X’Pert Pro MPD diffractometer (Ni-filtered Cu Kα radiation, 0.05° step size, 8 s per step). Transmission electron microscopy (TEM) imaging was performed with a Phillips CM12 instrument operated at 100 kV. The bulk Si/Al molar ratio in the solids was obtained by atomic absorption spectroscopy (AAS) using a Varian SpectrAA 220 FS spectrometer. X-ray photoelectron spectroscopy (XPS) was undertaken using a Quantera SXM spectrometer equipped with an Al Kα monochromatic source. Tableted samples were measured under ultrahigh vacuum (1×10-10 kPa). A 250 eV flood gun was used to minimize sample charging.

3. Results and discussion 3.1. Materials and primary characterization Two conventional zeolites of distinct composition (Z25 and Z40) and two hierarchical zeolites, the first prepared by NaOH treatment of Z40 (Z40-H), and the second by sequential HCl treatment of Z40-H (Z40-HW), formed the basis of this study. The long-range crystalline MFI structure in both the conventional and hierarchical zeolites was confirmed by XRD. TEM micrographs of the samples evidenced the introduction of an intracrystalline mesopore network upon desilication of Z40 and the similar mesoporous structures of Z40-H and Z40-HW (Figure 1). A similar average particle size was observed for all zeolite studied (0.3-1 µm). Compositional changes induced by the alkaline and acid treatments were assessed by analysis of the molar Si/Al ratio in the bulk (AAS) and surface (XPS) of the zeolites (Table 1). The bulk Si/Al ratio decreased upon alkaline treatment from 39 in Z40 to 28 in Z40-H, due to the selective extraction of silicon. Conversely, the removal of aluminum species by sequential acid treatment restored the bulk Si/Al ratio of the hierarchical Z40-HW sample to 38. The surface molar Si/Al ratios followed a similar trend. No compositional gradients were evidenced in the


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conventional Z40 zeolite. A larger deviation between the bulk and surface composition observed for Z40-H (Si/Al surface = 20), was suggestive of aluminum enrichment of the external surface in this sample. This is in agreement with previous findings based on the study of adsorbed probe molecules of varying sizes by IR spectroscopy.43,44 Although this difference was reduced following acid washing, Z40-HW retained a slightly higher surface concentration of aluminum than that of Z40. Thus, based on the results of primary characterization alone, differences in the surface properties of Z40 and Z40-H can immediately be envisaged due to their distinct compositions. The similarity in the composition of hierarchical Z40-HW and Z40, from which it was derived, is important for subsequent evaluation of the adsorption properties. 3.2. Adsorption studies Pore structure determination by Ar and N2 sorption. Figure 2 displays the argon and nitrogen sorption isotherms of the conventional (Z25 and Z40) and of the hierarchical (Z40-H and Z40-HW) ZSM-5 zeolites. The low-pressure adsorption range associated with argon uptake in the zeolite micropores is highlighted in the semi-logarithmic plot (Figure 2a). Despite significant differences in the Si/Al ratios of these zeolites (ranging from 25 to 40), the isotherms coincide in the low-pressure region of pore filling. This confirms (i) the relative insensitivity of argon sorption (87.3 K) to eventual differences in surface properties, and (ii) its suitability as an adsorptive for pore size characterization of micro-mesoporous zeolites. Expected differences between the conventional and hierarchical zeolites are clearly apparent in the shape of the argon adsorption isotherms at relative pressures > 0.15 when displayed in a linear scale (Figure 2b), in close correspondence with the nitrogen (77.4 K) adsorption isotherms, which were obtained in the relative pressure range from 10-2 to 1 (Figure 2c). Cumulative pore volume and pore size distribution curves obtained from the argon and nitrogen isotherms are shown in Figures 3a-b, respectively. Combined micro-mesopore size distributions of the hierarchical zeolites were obtained by applying a NLDFT method dedicated to the adsorption of argon adsorption at 87.3 K in cylindrical, siliceous zeolite pores.19,20 Similarly the nitrogen isotherms were


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analyzed using an NLDFT method dedicated to sorption of N2 in cylindrical silica pores19,20 to extract related pore size and volume data. DFT and computer simulation methods (such as Molecular Dynamics and Monte Carlo simulation) have become powerful methods to describe the sorption and phase behavior of inhomogeneous fluids confined in nanoporous materials and are considered by now the state-of-the art for the pore size analysis of micro-mesoporous materials. These approaches describe the distribution of adsorbed molecules in pores, providing detailed information about the local fluid structure near curved surfaces in comparison to the structure of the bulk fluid. They are the most advanced methods currently available for pore size analysis of micro- and mesoporous materials. The calculation of the pore size distribution is based on a solution of the Generalized Adsorption Equation (GAE) which correlates the kernel of theoretical adsorption (or desorption) isotherms for a given adsorptive/adsorbent system (of given pore geometry) with the measured adsorption (desorption) isotherm.45-49 The NLDFT cumulative pore volume plots enable clear differentiation of the micro- and mesopore volumes (Table 2). Similar micropore volumes are derived for both the conventional and the mesoporous zeolites. The total pore volumes are determined at a relative pressure of 0.92 (by applying the Gurvich method40-42), which reflects the upper closure point of the hysteresis loop indicating the completion of mesopore filling. The NLDFT argon pore size distributions (Figure 3b) clearly identify the distinct groups of pores present in the hierarchical zeolites, i.e. the ZSM-5 micropores (pore diameter 0.52 nm) and significant mesoporosity in the pore diameter range from 2-10 nm. In the mesopore range, the curves derived from the N2 isotherms agree well with those of argon. Furthermore, the pore size and volume distributions of the hierarchical Z40-H and Z40-HW samples are essentially identical. It is important to note that the additional peak, appearing in the differential pore size distribution curves at ca. 1 nm, does not reflect real pores within the zeolite structure. In fact, it is most likely associated with the reminiscence of a phase transition in connection with nitrogen and argon adsorption


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in highly siliceous MFI zeolites.50-52 The appearance of unusual steps which may be accompanied by hysteresis loops are characteristic of this transition. N2 adsorption isotherms (77 K) in silicalite-1 and highly siliceous ZSM-5 exhibit a step in the pre-capillary condensation range (p/p0 = 0.1-0.2). For Ar (87.3 K or 77.4 K), this phase transition is observed at much lower relative pressures (p/p0 = 10-4-10-2).52 Although still not completely understood, these interesting phenomena are thought to originate either from a zeolite phase transition (orthorhombic - monoclinic) or from an adsorbate phase transition i.e., from a disordered (liquid-like) to a more ordered (solid-like) state.50-52 As the aluminum content of the zeolite framework increases, the magnitude of the phase transition can be significantly reduced if not entirely absent. At intermediate Si/Al ratios, the phase transition may still occur in the absence of hysteresis, and the associated step in the adsorption isotherm becomes less steep.53 In the context of the present study, the fact that this transition is not associated with the filling of empty pore space needed to be accounted for in subsequent pore volume and surface area analysis. On comparison of the cumulative pore volume and differential pore size distribution curves, one notices the reminiscence of such a transition and corresponding artifacts in the pore size distribution for the parent and mesoporous ZSM-5 zeolites, and the associated change in adsorbed volume is most pronounced in Z40, which has the highest Si/Al ratio. Sensitivity of water adsorption to zeolite framework composition. Figure 4 compares the adsorption of argon and water by the conventional zeolites. An almost perfect coincidence between the argon isotherms of Z25 and Z40 is observed up to a relative pressure of 0.2 due to essentially identical microporosity of the MFI framework. Minor deviation at higher relative pressures is attributed to small differences in secondary mesoporosity correlated with the external surface (the external surface areas of Z25 and Z40 derived by application of the t-plot method on the N2 data are 68 and 95 m2 g-1, respectively). In contrast to argon adsorption, the water isotherms deviate appreciably in the relative pressure range up to 0.6, i.e. Z25 exhibits a higher uptake of H2O in comparison with Z40 (Figure 4b). Above a relative pressure of 0.6 the water isotherms converge, in


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agreement with the equivalent textural properties of the conventional zeolites. A strong correlation is known to exist between water uptake and the content of framework aluminum29,54,55 in MFI zeolites and the differences in adsorption observed between Z25 and Z40 are consistent with the increased hydrophilicity expected based on the greater aluminum content in the former. The fact that water adsorbs at much higher relative pressure than Ar at 87 K (where micropore filling is essentially completed at relative pressures < 10-3 (Figure 1a)) reflects the prominent hydrophobic nature of these ZSM-5 zeolites. In a very recent publication37 highly accurate water adsorption measurements in various MFI zeolites with different Si/Al including the extremely hydrophobic fluoride-mediated silicalite-1 and ZSM-5 samples with various Si/Al ratios have been reported. These data reveal a relationship between the Al content and the hydrophilicity of the MFI material which is clearly reflected in the shape of the water adsorption isotherms. The neutral framework of pure silica MFI-type silicalite-1, which has no cation exchange capacity, is known to be extremely hydrophobic.36 Due to the lack of attractive interaction of water with the surface (i.e. water-water attraction is stronger than the affinity of water for the zeolite framework), water adsorption into the silicalite channels is very limited, and in fact for fluoride-mediated silicalite the water adsorption is essentially zero37. In principal structural silanol defects can allow also for some water adsorption37, however, in the presence of aluminum, as in the case of ZSM-5, it is thought that water initially strongl y adsorbs at specific sites throughout the z e o l i t e crystal, but contributions to the water uptake comes also from adsorption and formation of water clusters at the external surface.51 Our results clearly indicate that water adsorption occurs to some appreciable extent within the micropores of Z25 and Z40, and that the varying uptake at low relative pressures is indicative of differences in their respective surface chemistry. These conclusions are also supported by the fact that Z25 and Z40 samples exhibit very similar textural properties; even more, despite displaying a slightly lower external surface area and total BET surface area (445 m2 g-1 compared with 465 m2 g-1, for Z25 and Z40 respectively; linear BET range was found to be below p/p0 = 0.1), the adsorption of water on


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Z25 was appreciably higher than on Z40 at low relative pressures. Furthermore, based on the textural parameters derived from the Ar isotherms (Table 2) and by taking into account that; i) the mesopore volume (ca. 0.01 cm3 g-1 for Z25) is negligible and ii) the external surface comprises less than ca. 13% of the total area for both Z25 and Z40, it follows that approximately 60% of the micropore volumes are indeed filled with water at a relative pressure of 0.92 for both zeolites. The shift to higher relative pressures of the filling of pores of specific diameter with water, due to the hydrophobic nature of micropore surfaces (in comparison with the filling of adsorbates such as nitrogen at 77 K or argon at 87 K which completely wet the pore walls), was also previously observed in the water isotherms of microporous activated carbons.21-26 In the case of the latter, where the micropore and external surface properties are equivalent, water adsorption and pore filling are associated with cluster formation. This leads to adsorption isotherms characterized by very low water uptake over a wide range of relative pressures (up to p/p0 = 0.4) followed by a step-like filling extending over a narrow relative pressure range.21,25,26 In the case of ZSM-5, however such a step like pore filling is not observed. More work is needed to investigate the origin of the different water adsorption behavior observed in hydrophobic carbon and hydrophobic ZSM-5, however the fact that in ZSM-5 adsorption occurs simultaneously on the more hydrophilic external surface and in the hydrophobic zeolite framework may contribute to this as well the effect of different pore geometry and size (in contrast to the MFI channels, the pores in activated carbon are considered to be of slit-like geometry with a wide distribution of pore widths) on water cluster formation. Water adsorption in hierarchical zeolites. A comparison of the water adsorption of conventional and mesoporous zeolites is shown in Figure 5a. The additional mesoporosity of Z40-H and Z40-HW zeolites is seen by the significantly increased measured uptakes. Differences in the wetting behavior of argon (adsorbate film completely wets surfaces) and water (only partial wetting) are clearly visible in the shape of the corresponding isotherms, particularly in the low-pressure range (Figures 5b-d). In case of argon, adsorption and filling of micropores is indicated by a strong increase in


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the adsorption at relative pressures p/p0 < 0.15. In contrast, the adsorption of water in this range is small and adsorption increases linearly with p/p0 up to 0.5. The water and argon isotherms intersect at p/p0 = 0.7, above which the water uptake exceeds that of argon. Unlike the behavior observed with N2 and Ar, where micro- and mesopore filling occur in separated relative pressure ranges (Figure 1), it is characteristic that the uptake of water in hierarchical zeolites occurs gradually across the whole relative pressure range. Whereas for wetting fluids (e.g. Ar) there is a direct relationship between pore size and the pressure of pore filling, the lack of clear distinction between filling regimes in the water isotherms indicates non-uniform wetting of the pore system by water. The later clearly demonstrates differences in the surface chemistry of the micro- and mesopores in the desilicated zeolites. Well-defined hysteresis is observed in the isotherms of both mesoporous zeolites above a relative pressure of 0.3 due to the capillary condensation of water into mesopores. Hysteresis of type H4/H3 is also seen in the argon isotherms, resulting from the evaporation of some of the confined liquid via cavitation, and indicating that some large mesopores are only accessible through narrow meso- or micropores.4 Both the water and argon hysteresis loops exhibit an upper closure point at a relative pressure of 0.92, implying that capillary condensation in mesopores is essentially complete at this point. The isotherms are reversible at higher relative pressures, where adsorption is attributed to the external or macroporous surfaces. Wider hysteresis is observed in the H2O isotherms as the thermodynamic state of water confined in narrow mesopores at 298 K is far more subcritical than that of argon at 87.3 K. It is well known that argon adsorption in mesopores of 2-4 nm diameter is entirely reversible.19,20 At a given temperature, the width of the hysteresis loop in a mesoporous solid becomes smaller with decreasing pore size, and disappears below a certain critical pore size.56,57 For argon adsorption at 87.3 K (T/Tc,bulk = 0.58 where Tc,bulk is the critical temperature of the bulk fluid) in cylindrical silica pores this critical pore size is located between 3.6 and 3.9 nm.56 On the other hand it has been shown that pore condensation and water adsorption hysteresis can still be observed in ca. 3 nm pores at 298 K


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(T/Tc,bulk = 0.46) indicating that here water is still below the hysteresis critical temperature for this pore size.30,57 Hence, the difference in hysteresis behavior for water and argon in these mesoporous zeolites is due to the difference in the thermodynamic state of the pore fluid. 3.3. Hydrophilicity index In order to assess the surface chemistry of zeolites Weitkamp et al. proposed a hydrophobicity index (HI) which is determined by studying the competitive adsorption of water/toluene or water/methylcyclohexane vapor mixtures.58,59 While the concept introduced through the hydrophobicity index was indeed useful, it followed an experimentally quite demanding approach, requiring the measurement of breakthrough curves. Here, we suggest a more general, and simpler strategy to compare the surface chemistry (e.g. hydrophilicity/hydrophobicity) of nanoporous materials, which is demonstrated in relation to the characterization of hierarchical zeolites. Our methodology involves the comparison of the adsorption isotherms of an adsorptive which is sensitive to surface chemistry but does not necessarily completely wet the adsorbent surface/adsorption sites (here water), with the adsorption isotherm of an adsorptive which leads to an adsorbate film that completely wets the adsorbent surface (i.e. having a contact angle of zero degrees). It is well known that fluids such as nitrogen and argon at their boiling temperature (77.35 K and 87.27 K, respectively) form complete wetting adsorbates on surfaces. Hence, by comparing water adsorption with argon adsorption at 87 K we are able to define a so-called hydrophilicity index, or in case the adsorptive chosen which is sensitive to surface chemistry, is not water, then by comparing with argon adsorption we determine in more general terms a wettability index. In order to further evaluate the differences in H2O adsorption between the zeolites studied, we calculated the degree of pore filling with water at relative pressures of 0.15 and 0.92 (Table 2). In the case of argon at 87.3 K (a wetting fluid), micropore filling is complete at p/p0 = 0.15. At p/p0 = 0.92 both the micro- and mesopores of the hierarchically-structured materials are completely filled as indicated by the fact that at p/p0 = 0.92 the hysteresis loop closes (upper closure point of the hysteresis loop). Hence,


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the total pore volumes and the micropore volumes of parent and hierarchical zeolites can be reliably determined from the argon (87 K) adsorption data. Using pore volumes determined in this way allows the calculation of the degree of pore filling with water from the amounts of adsorbed water (assuming that the average density of water confined in the pores is equivalent to the bulk liquid density, i.e. 0.997 g cm-3, which can be expected provided that water shows complete wetting behavior on the zeolite surfaces). If water would completely wet the pore and external surfaces of the adsorbent like argon, micropore filling would be expected to be close to 100% at a relative pressure of 0.15, with liquid-like adsorbate and complete micro-/mesopore filling observed at a relative pressure of 0.92. Deviation from complete pore filling evidences the incomplete wetting behavior of water, i.e. the degrees of micro- and total pore filling by H2O (X0.15 or X0.92, respectively) depend on the surface properties of the pore network. Alternatively, the average density of confined water could be appreciably lower than that of the bulk liquid state, but the average density of confined water also depends on the wetting characteristics of the adsorbed phase. The smaller the deviation from 100%, the more hydrophilic the surface is, while a larger divergence from 100% would reflect a greater surface hydrophobicity. Furthermore, differences in the surface properties of the micro- and mesopores would give rise to variation in the relative pore filling at different p/p0. Hence, the degree of pore filling determined as described in the previous section corresponds to a ‘hydrophilicity index’. For the conventional zeolites Z25 and Z40, X0.15 was 25 and 21%, respectively (hydrophilicity index 0.2-0.25). The greater hydrophilicity of Z25 is consistent with the higher framework charge density expected for this zeolite. However, a relatively low degree of pore filling is observed for both samples, even at p/p0 of 0.92 (X0.92 = 62 and 61%, respectively), confirming that the MFI micropores are fairly hydrophobic in nature. In comparison, hierarchical Z40-H exhibits degrees of pore filling of X0.15 = 24% and X0.92 = 79%. Both values are higher than those of the conventional Z40 zeolite from which it was derived. Based on the assumption that X0.15 corresponds to mainly micropore filling, the micropores of Z40-H seem to be more hydrophilic than those of Z40. This could be expected in line


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with the lower Si/Al ratio of Z40-H (28), which is comparable to that of Z25. Indeed, the hydrophilicity indices of Z25 and Z40-H are very similar (0.24 and 0.25, respectively). However, the difference for X0.15 (24% for Z40-H and 21% for Z-40) is also affected by a small contribution coming from the amount of water adsorption (at rel. pressure p/p0 = 0.15) on the much larger and more hydrophilic mesopore/external surfaces of Z40-H (mesopore/external areas: 285 m2 g-1 for Z40-H compared with 95 m2 g-1 for Z40, derived from t-plot analysis of nitrogen adsorption data). Since the water only partially wets the micropores, they continue to fill at relative pressures larger than 0.15. Therefore, at the relative pressure of 0.92, a much higher fraction of the available micro- and mesopore volume has been filled in all zeolites (ca. 60% for the conventional and ca. 80% for the mesoporous zeolites). However, the significantly increased percentage of overall pore filling in hierarchical zeolites indicates that mesopore surfaces are appreciably more hydrophilic than those of the micropores. If the mesopore surfaces possessed the same surface properties as the micropore surfaces, then the degree of pore filling at a relative pressure of 0.92 should be identical for both parent and mesoporous zeolites. This is in line with the lack of clear distinction between micro- and mesopore filling ranges in the water sorption isotherms. The increased hydrophilicity of the hierarchical zeolites with respect to the parent zeolite can be explained by the existence of a higher number of polar surface groups related to the introduction of mesoporosity by base leaching. For example, interactions of water with surface silanol groups, which are known to be more prevalent in hierarchical zeolites,16 are more favorable than with the Si-O-Si groups which terminate the surface within the micropores. In addition to the increased mesopore surface area in hierarchical zeolites, an increased hydrophilicity could also be favored by the re-distribution of aluminum during desilication. As affirmed by N2 and Ar sorption (Figure 2) and by TEM (Figure 1), the structures, pore size distributions, and pore volumes of zeolites Z40-H and Z40-HW are essentially identical (Figure 3). Hence, the textural comparability of these pore systems, enables the use of water adsorption to detect potential differences in their surface chemistry (e.g. arising from the distinct Si/Al ratios of these two


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zeolites). Figure 6 displays the water and argon sorption in these samples Z40-H and Z40-HW. In contrast to argon, we observe appreciable differences in the water adsorption between the Z40-H and Z40-HW zeolites (Figure 6b) indicating that the surface of the acid-washed zeolite has slightly increased hydrophilicity, which is not expected from analysis of the bulk Si/Al composition alone. In fact, the hydrophilicity index (X0.15) estimated for Z40-HW (0.33) is higher than that of Z40-H (0.23), despite having a higher bulk molar silicon to aluminum ratio (Si/Albulk = 38 and 28, respectively). The fact that the higher aluminum content of Z40-H does not result in a more favorable interaction with water compared to Z40HW, indicates that the water adsorption is not only sensitive to the overall Si/Al ratio in the zeolite, but also to the metal distribution. The increased water uptake even at lower relative pressures for Z40-HW could indicate a higher content of framework aluminum in the ZSM-5 micropores. Alternatively, acid washing could lead to an increased population of silanol groups in the zeolite micropores of Z40-HW, as compared to the mesoporous zeolite Z40-H. However, no differences in the absorbance of terminal silanol groups (3740 cm-1) were evidenced by DRIFT spectroscopy. In either case the surface properties of hierarchical zeolites could not be derived purely based on the results of bulk chemical analysis. Furthermore, no conclusive explanation of the differences between the surface properties could be drawn from the results of XPS analysis alone; proper analysis of the water/argon adsorption data offers not only complimentary information but an alternative way of detecting differences in the surface properties of hierarchical zeolites.

4. Conclusions Our study combining argon and water adsorption provides further insights into the pore structure and surface properties (e.g. hydrophobicity/hydrophilicity) of hierarchical ZSM-5 zeolites prepared by post-synthetic base and acid treatments. NLDFT (non-local density functional theory) analysis of the Ar isotherms provided an accurate understanding of the pore size and volume distributions. The two different relative pressure regions of micro- and mesopore filling of hierarchical zeolites are clearly


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revealed by argon, which at 87.3 K completely wets all pore surfaces. In contrast, upon adsorption of H2O at 298.5 K, which only partially wets pore surfaces of ZSM-5, the two filling regions overlapped. It follows that water adsorption is sensitive to the Si/Al ratio, the distribution of aluminum species in the zeolite, and to the presence of polar groups at the mesopore surface. A hydrophilicity index capable of identifying surface chemistry differences between the micro- and mesopores within a hierarchical pore structure and between distinct zeolites, was defined by comparing the argon

and water sorption

behaviors. These findings are not only important for a comprehensive surface and pore structure characterization of hierarchical zeolites, in particular with regard to optimizing their application in catalytic and sorption processes, but also in general for further advancing the characterization of complex porous materials.

Acknowledgement. The Swiss National Science Foundation (Project Number 200021-134572) is acknowledged for financial support.

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Tables

Table 1. Bulk and surface composition of the zeolites studied.

a

Molar (Si/Al)bulk [-] b

Molar (Si/Al)surface [-]

Z40

Z40-H

Z40-HW

39

28

38

41

20

35

a

Atomic absorption spectroscopy.

b

X-ray photoelectron spectroscopy.


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Table 2. Textural parameters derived from analysis of the H2O and Ar adsorption isotherms.

Sample

Vmicro(Ar)a [cm3 g-1]

Vpore(Ar)b [cm3 g-1]

V0.15(H2O)c [cm3 g-1]

V0.92(H2O)c [cm3 g-1]

X0.15(H2O)d [%]

X0.92(H2O)d [%]

Z25 Z40 Z40-H Z40-HW

0.20 0.19 0.17 0.15

0.21 0.23 0.47 0.49

0.05 0.04 0.04 0.05

0.13 0.14 0.37 0.41

25 21 24 33

62 61 79 84

a

Micropore volume estimated by NLDFT analysis.

b

Total pore volume estimated assuming bulk liquid density of Ar = 1.42 g cm-3 (Gurvitch rule).

c

Volume of H2O adsorbed at different p/p0 (0.15 and 0.92) assuming a bulk liquid density at 298 K (0.997 g cm-3).

d

Degree of pore filling by water adsorption at different p/p0 (0.15 and 0.92)


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Figure captions

Figure 1. TEM micrographs of the zeolites studied.

Figure 2. Semi-logarithmic Ar (a), linear Ar (b) and linear N2 (c) isotherms of Z25, Z40, Z40-H, and Z40-HW.

Figure 3. NLDFT cumulative pore volumes (a) and pore size distributions (b) of Z40, Z40-H, and Z40-HW derived from the Ar (solid symbols) and N2 (open symbols) isotherms presented in Fig. 2.

Figure 4. Ar (a) and H2O (b) isotherms of Z25 and Z40.

Figure 5. H2O adsorption (a, open symbols) in Z40, Z40-H, and Z40-HW and individual comparison with Ar isotherms (solid symbols, b, c, and d, respectively).

Figure 6. Comparison of the Ar (a) and H2O (b) adsorption of Z40-H and Z40-HW.


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Z40

Z40-H

50 nm

Z40-HW

100 nm


100 nm

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Figure 1


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Figure 2 ACS Paragon Plus Environment

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Figure 3


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Figure 4 ACS Paragon Plus Environment

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Figure 5


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Figure 6


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