ZSM-5 zeolite chemical composition (mass %, EDS).
The nanoporosity in zeolite ZSM-5 was analyzed as a function of SiO2/Al2O3 molar ratio (MR). The internal pore structure was studied by high-resolution adsorption. Surface areas, microporous volume, characteristic energy of sorption, and pore-size distributions were calculated from N2 sorption isotherms by the BET, Langmuir, t-method of de Boer, αS-plot of Sing, direct comparative plots of Lee, Newnham, Dubinin-Astakhov, differential adsorption curves, and nonlocal density functional theory methods. The results indicated that MR dependence in these zeolites caused structural defects through micropore opening and widening as well as the emergence of further slit-like mesopores.
- ZSM-5 zeolite
- nanopore measurements
The nanoporous, ordered, and three-dimensional structure of zeolites makes them materials of great practical importance in the hierarchy. The broad use of microporous zeolites (pore diameter
ZSM-5 and, its purely siliceous analog, silicalite (both have a structural code “MFI” in accordance with the IZA database) are among the most widely studied zeolites. MFI is one of the most versatile and commercially significant zeolites; it is widely used in the petroleum industry to convert methanol into complex hydrocarbons in methanol-to-gasoline processes, as well as in the alkylation of aromatic compounds and their subsequent separation . The microporous network of this zeolite consists of intersecting straight and sinusoidal channels. The straight channels have pore openings defined by a cross-section of 10-member rings of 0.54–0.57 nm and sinusoidal channels by elliptic pores of 0.51–0.54 nm in cross-section. The intersections are cavities of 0.8 nm in diameter  (see Figure 1).
A detailed study on the different types of adsorption sites that constitute the structural skeleton of this zeolite was carried out by Cho et al. . They classified the sorption sites into three types: (1) the SS sites located in straight channels; (2) the SZ sites located in zigzag (sinusoidal) channels; and, finally, (3) the SI sites located at the intersections (Figure 2). One of the most important catalytic properties of ZSM-5 is its shape selectivity. This is a consequence of its primary microporous structure and is the basis for most of its successful applications .
Another important parameter that allows to adjust the zeolite properties is their chemical composition, that is, their SiO2/Al2O3 molar ratio (MR). The amount of Al in the framework is proportional to the number of exchangeable cations, H+ among others, which affects both Lewis and Brønsted acidity. The main interactions of the sorbate molecules in the pores of the zeolite are realized through the oxygen atoms of the lattice and extra-framework cations. Microporosity and secondary porosity in zeolites and similar materials can be determined from the low- and medium-pressure regions of the sorption isotherm using various approaches . The shape-selective activity of MFI can be attributed to the presence of active sites in micropores. It was shown that the shape-selective properties of zeolites may be greatly reduced due to the presence of active sites in the secondary porosity with a wide distribution of the pore diameter and on the external surface of their crystallites, so zeolites with a large outer surface area are less selective than those with fewer imperfections.
The presence of molecules that blocks the pores of the zeolite or a partial destruction of its structure can drastically decrease its activity by reducing the microporous volume accessible for the reactants. The effect that the external surface area of ZSM-5 zeolite crystals used for shape-selective reactions causes, was reported previously . Some authors in reported works have used the αS-plot of Sing as alternative method to evaluate the external surface area and the true intra-crystalline capacity .
The aim of this study was to accurately describe the dependence of all the different types of ZSM-5 porosity on MR and to show which methods are best suited for measuring them in each range. This will allow us to develop an approach to the application of various existing methods of texture characterization for samples of zeolite with mixed porosity.
A set of ZSM-5 zeolites in their sodium form (Na-ZSM-5) with a SiO2/Al2O3 molar ratio (MR) varying from 30 to 120 was synthesized using a template of tetrapropylammonium bromide (TPABr) following the methodology reported by Ghiaci et al. . Through the text and figures, these samples are called Z, followed by the MR value (30, 70, 95, or 120), for example, Z30 means an Na-ZSM-5 sample with an MR equal to 30. For comparison, a set of Na-ZSM-5 samples supplied by TOSOH Co., Japan, with MR 20, 23.3, and 30 were also studied. These TOSOH samples are called ZT, followed by MR value. A reference macroporous solid material required to estimate micropore volumes was obtained from the Tehuacan area in the state of Puebla, Mexico. This reference substrate was identified by X-ray powder diffraction (XRD) as α-SiO2. X-ray powder diffraction of ZSM-5 samples was obtained in the 2θ ranges of 5–50 degrees using diffractometer Bruker D8, using nickel-filtered Cu Kα (λ = 0.154 nm) radiation. Scanning Electron Microscopy images were collected from a JEOL JSM-6610LV electron microscope with tungsten filament and an electron detector operated at 20 kV. N2 adsorption isotherms were measured at the boiling point of liquid N2 (76.4 K at the 2200 m altitude of Puebla City, México) in the interval of relative pressures,
3. Results and discussion
3.1. X-ray analysis
The XRD patterns of all samples (Figure 3) are typical of ZSM-5 zeolites . In general, all the samples showed reasonably sharp diffraction patterns, indicating good crystallinity. Please note that commercial TOSOH samples and those prepared in the laboratory are nearly identical. The main peaks appear at the following 2θ angles: 8.0°, 8.9°, 9.8°, 14.0°, 14.8°, 20.9°, 23.2°, 23.9°, 24.5°, 29.4°, and 30.0° (Figure 3). Most of these peaks are not resolved; usually, one peak is a superposition of several closely located reflections. For example, the [−101], , and  reflections positioned at 2θ = 7.92°, 7.93°, and 8.01°, respectively, gave rise to a total peak at ~8.0. The most important difference between the standard XRD pattern and the observed for both sets of samples is the relative intensity of the various peaks, but a detailed discussion of the changes in the structure of ZSM-5 due to MR variations and synthesis conditions is beyond the scope of the present work and will be discussed elsewhere. Three peaks that appear at 2θ = 16.0°, 26.4°, and 30.9° in the ZT23.3 sample (marked with asterisks) are most probably associated with an unidentified impurity.
3.2. Scanning electronic microscopy
In Figure 4, it can be seen that the effect of the templates used during the synthesis process affects the morphology of the zeolite crystals obtained. Thus, for example, in Figure 4(a) and (b) corresponding to zeolites ZT-20 and ZT-23.3, it can be seen that the crystals obtained have lath-like shapes. In the case of the ZT-30 and ZT-23.3 zeolites, clusters of spheroidal crystals are observed where the crystals of the zeolites coexist, as seen in Figure 4(c) and (d). Finally, the SEM images of the zeolites ZT-30 and ZT-23.3 do not exhibit a predominant or defined geometry, as seen in Figure 4(e) and (f) .
3.3. High-resolution adsorption
N2 sorption isotherms at 77 K for both sets of samples are shown in Figure 5 as sorbed volume at standard temperature and pressure (STP) in cm3 per gram of zeolite versus
3.3.1. External surface area
To calculate the volume of the micropores from the sorption data, De Boer
The total micropore volumes in cm3 g−1 for all the samples are given in Table 2. These values were calculated from: (1) αS-plots, (2)
22.214.171.124. High-resolution αS-plots
The filling of macro, meso, and micropores can be proved by analyzing high-resolution αS-plots starting at low relative pressures, that is, 10−5; see Figure 7. There are some significant differences in the form of αS-plots as a function of MR, mainly for Z120. A pronounced distortion of the isotherm shape is observed at a very low
3.3.3. Pore-size distributions calculated by the DAC, D-A, and NLDFT approaches
126.96.36.199. DAC method
Calculation of pore-size distributions from desorption branches of N2 isotherms using the differential adsorption curves (DAC)  method yields bimodal distributions (Figure 10), with the thickness of the pore size of ca. 0.36 and 0.55 nm for all samples. The plots are unimodal with the pore ca. 0.36–0.40 nm. This approach correctly describes the essential qualitative features of N2 sorption in the microporous zeolites, such as ZSM-5, that is, pores in the range of 0.3–0.6 nm. The results of these estimates are shown in Table 3.
188.8.131.52. D-A method
The pore-size distributions obtained by the D-A method  are shown in Figure 11. The average pore diameter, seen as a maxima on the curves by this method, varies according to MR. Table 3 lists the optimized
184.108.40.206. Nonlocal density functional theory method
Nonlocal density functional theory (NLDFT) was developed to take into account pore sizes in voids of well-defined geometry . With this approach, the molecules adsorbed in the pores tend to be packaged in accordance with the adhesion forces established with the substrate (i.e. attractive forces between adsorptive and adsorbent molecules) and interactions with the remaining fluid molecules. The molar density of the adsorbed phase varies as a function of pore size. The adsorption isotherm is calculated from a given pore shape (spherical, cylindrical, slit-like, etc.), and the experimental isotherm is given as the sum of a series of individual single-pore isotherms multiplied by their relative abundance over a range of pore sizes. In the present case, the microporous structure of ZSM-5 zeolite can be approximated as a bundle of parallel cylindrical pores and the nature of the adsorbent can be assumed as that of the silica. In this way, the distribution of supermicroporous zeolitic adsorbents can be calculated from high-resolution adsorption isotherms. The results of the analysis of the size of the supermicropores using the NLDFT method are shown in Figure 12 and are listed in Table 4. The pore-size distributions obtained from the N2 isotherms using the NLDFT cylindrical pore model yield bimodal distributions with pore size characteristics of 1.8 and 5.0 nm. It is observed from this figure that the intensity of the distribution at 5.0 nm is poorly developed; however, it is represented in all distributions. A possible explanation is that the structures are not homogeneous and that it contains a significant amount of slit-like pores and pores of other irregular shapes. Based on the NLDFT method, one can get an idea about the actual widths and pore sizes of the supermicropore voids existing in ZSM-5 zeolites for which high-resolution N2 isotherms are available.
αS is Sing’s αS method, DAC is direct comparison plots,
The obtained samples exhibit reasonable diffraction patterns, indicative of good crystallinity. The most important difference between the standard XRD pattern and those observed for both sets of samples is the relative intensity of the various peaks. The ZSM-5 samples synthesized are composed of crystals with different geometry in a range of sizes 5–10 μm. N2 isotherms have been measured, starting at a relative pressure of 10−5 and up to 1. To evaluate the texture properties of ZSM-5 zeolites, BET, Langmuir, Ast, surface areas, and external surface area were used. A significant amount of micropores was found in all ZSM-5 zeolites. Such methods as αS,
This work was partially supported by DGAPA-UNAM IN107817 Grant, VIEP, and the Academic Body “Investigación en zeolitas,” CA-95 (PROMEP-SEP).
Conflicts of interest
The authors declare no conflict of interest.