Overview of sorption analysis capabilities for meso- and microporous zeolites nanomaterials Chimica Techno Acta ARTICLE published by Ural Federal University 2021, vol. 8(3), № 20218302 eISSN 2411-1414; chimicatechnoacta.ru DOI: 10.15826/chimtech.2021.8.3.02 1 of 4 Overview of sorption analysis capabilities for meso- and microporous zeolites nanomaterials S. Tokmeilova * , E.V. Maraeva Saint Petersburg Electrotechnical University «LETI», Prof. Popova St., 5, Saint Petersburg 197376, Russia * Corresponding author: ak_saya@mail.ru This article belongs to the regular issue. © 2021, The Authors. This article is published in open access form under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Abstract In this paper we consider the main application features of the ther- mal desorption method of inert gases, implemented on the Sorbi MS (Meta, Russia) device, for the analysis of meso- and microporous ma- terials. Recommendations on the choice of measurement modes for stable operation of the Sorbi MS device are offered (including rec- ommendations on mass, sample preparation mode). The article pre- sents the results of the micropores analysis by the t-plot and Sing method. Keywords micropores mesopores capillary condensation sorption hysteresis Received: 27.04.2021 Revised: 12.06.2021 Accepted: 30.06.2021 Available online: 07.07.2021 1. Introduction Gas sorption methods are commonly used to characterize porous materials. Porous materials are widely used in mi- cro- and nanoelectronics. According to the classification of pores by size, micropores are less than 2 nm in size, mes- opores are in the range from 2 to 50 nm [1]. At present, nitrogen sorption at 77 K has become a standard tool for the analysis of materials with pores in a range of 0.5– 50 nm. The mechanism of nitrogen sorption at 77 K occurs in the following way. At a low relative pressure (0.02– 0.1), the filling of micropores with adsorbate begins. The monolayer adsorption occurs after the completion of ad- sorption in micropores. Capillary condensation is observed in relatively small mesopores when the relative pressure and pore width correspond to the Kelvin equation. The desorption isotherm is obtained by reversing the adsorp- tion process, releasing the liquid adsorbate and decreasing the equilibrium relative pressure [2]. The evaporation process takes place from the meniscus of the condensed liquid. In the case of polymolecular adsorption, the adsorption potential interaction does not change with the distance from the surface of micropores, as a result of dispersion potentials overlapping of closely spaced pore walls [3]. As a result, adsorption in the entire volume of micropores becomes equiprobable to adsorption on their surface (mi- cropores volumetric filling). The adsorption potential in- teraction of adsorptive molecules with microporous mate- rials is much higher than with the surface of mesoscale materials. All this together determines some peculiarities and imposes certain restrictions on the applicability of the Brunauer-Emmett-Teller (BET) method. Therefore, other procedures are used for the micropore adsorption iso- therms analysis, the so-called comparative methods of analysis, which make it possible to isolate the contribution of adsorption in the volume of micropores and to calculate the size of the mesopores and macropores surface. Cur- rently, a large number of comparative methods are known that are used to determine the specific surface area of a microporous material and to estimate the volume of mi- cropores in the presence of mesopores: t-method, Sing’s αs-method and others. The works [4-7] illustrating the current state of the question of the specific surface area analysis of mesopo- rous materials are presented. Mesoporous materials are widely used in practice (bioengineering, sensorics of gase- ous media and environmental monitoring, power engi- neering, lithium-ion batteries), are used as adsorbents and catalysts for oil refining processes. In all cases, the porous structure parameters play a key role. The paper [4] presents and discusses the kinetic re- sults of nitrogen adsorption and desorption experiments performed at 77 K on one aluminum-based and three ma- terials based on carbon, differing in their microporous and mesoporous nature. The paper is devoted to the study of transport phenomena during adsorption / desorption ex- periments in pores of various sizes. The authors of the article noted that the rates of transport phenomena http://chimicatechnoacta.ru/ https://doi.org/10.15826/chimtech.2021.8.3.02 http://creativecommons.org/licenses/by/4.0/ https://orcid.org/0000-0001-6180-7359 Chimica Techno Acta 2021, vol. 8(3), № 20218302 ARTICLE 2 of 4 change significantly with the gradual filling / emptying of the pores. The authors of [5] synthesized and investigated CoO- 3D ordered mesoporous carbon nitride (CoO@mpgCN) catalyst. The authors managed to find out that the signifi- cant catalytic performance of CoO@mpgCN was due to its uniformly distributed mesopores, large specific surface area, and high electron transferability at the active CoO sites. The textural parameter and the mesoporous extent of materials originated from parent template were exam- ined by nitrogen adsorption-desorption isotherm along with corresponding BET surface area and pore distribution analyses. The authors of [6] aimed to explore a facile route to synthesize mesoporous zinc silicate composites. The spe- cific surface area and the pore properties of the samples were tested by the Brunauer-Emmett-Teller (BET) method based on N2 adsorption / desorption measurements on Quantachrome Autosorb-IQ2. The authors provided the N2 adsorption / desorption isotherms and pore size distribu- tions of ZS-3 and ZS-5 that are rather different in struc- ture. The fact that the structure is mesoporous is indicated by the increase of adsorbed nitrogen at relative pressure P > 0.45 and related to the multilayer adsorption and ca- pillary condensation of N2. The excellent adsorption prop- erty endows the composite with potential application in the field of dye wastewater treatment. The authors of [7] attempted to enhance nitrogen ad- sorption capacity modified X zeolite adsorbent. For this, procedures including both NH4 + treatment and Ca 2+ ion- exchange were carried out. These modifications lead to a hierarchical mesopore-micropore structure with a multi- layer N2 adsorption capacity. The properties of adsorbents are characterized by N2 adsorption–desorption in 77 K, XRD, and XRF analysis. In paper [7] adsorption isotherms are modeled by Langmuir, Dual-Site Langmuir (DSL), Freundlich, and Langmuir-Freundlich (Sips) models, and the corresponding parameters are determined. These ad- sorbents can be used effectively in the helium purification by the pressure swing adsorption (PSA) process, which is based on nitrogen adsorption. It is known that the zeolites considered in this work are microporous materials. However, catalytic processes require materials containing both meso- and macropores. At the same time, the advanced system of zeolite mi- cropores has a significant effect on the rate of the reac- tions. In this regard, it is necessary to develop analysis techniques that provide simultaneous control of the micro- and mesoporous structure of materials. The aim of this paper is to consider the application pe- culiarities of the thermal desorption method of inert gas- es, implemented on the Sorbi MS (Meta, Russia) device, for the analysis of meso- and microporous zeolite materi- als. 2. Experimental In this work, zeolites of the type ZSM-5 (samples 1,3) and BETA (samples 2,4–6) were investigated. Aluminum hy- droxide (AH), including silica addition, was selected as the binder. The sample synthesis features and the selection of the peptizers are shown in Table 5. Zeolites were synthesized at the Irkutsk National Re- search Technical University. In the previous paper [8], the results of the studying the properties of compositions in the mesopore range are presented; here we will empha- size the analysis of micropores. Adsorption isotherms were built on the basis of the sorption study data obtained. The samples were studied by nitrogen sorption at 77 K on the Sorbi MS device. The de- vice works by comparing the volumes of adsorbate gas sorbed by the test sample and the standard sample. The specific surface area is measured using the 4-point BET method. Micropores volume was determined on the basis of analysis of inert gas adsorption isotherms. 3. Results and Discussion 3.1. Investigation of capillary condensation pro- cesses, sorption hysteresis A characteristic feature of adsorption in mesopores is as- sociated with capillary condensation, which leads to filling the mesopores volume with a liquid adsorbate phase at a relative pressure, for example, nitrogen vapor at 77 K, 0.4 < P/P0 < 1. As a rule, the process of capillary conden- sation is irreversible. This means that the magnitude of sorption with increasing pressure of the sorbent does not coincide with the magnitude of sorption with decreasing pressure. In this case, a characteristic hysteresis is formed on the sorption isotherm, formed by mismatched branches of adsorption and desorption (Fig. 1) [3]. 3.2. Recommendations on the choice of measure- ment modes of the Sorbi MS device During researching the parameters of the nanomaterials porous structure by the sorption method, it is important to correctly estimate the mass of the adsorbent material re- quired for the study, select the sample preparation mode, and set the range of the relative partial pressure variation of the adsorbate gas at which the measurement will be carried out. Fig. 1 Characteristic types of capillary-condensation hysteresis loops according to the IUPAC classification. Adapted from [3] Chimica Techno Acta 2021, vol. 8(3), № 20218302 ARTICLE 3 of 4 3.2.1. Determination of the analyzed material mass and sample preparation The features of the thermal desorption method realized in Sorbi MS are discussed in [9,10]. When studying composi- tions by the thermal desorption method of nitrogen, the choice of the analysed material mass is determined by two factors: the possibility of obtaining a stable desorption signal, which is used to calculate the volume of desorbed gas, and the range of the total measured surface S = Ssp∙m. For example, the selected masses for various series of zeo- lite compositions that provide a stable desorption signal are shown in Table 1. 3.2.2. Modes selection and sample preparation of the analyzed material Sample preparation of the analysed material, as a rule, consists in controlled heating of the sample in a flow of inert gas (helium). Preparation is necessary, first of all, to remove moisture and surface contamination. Variable sample preparation modes are heating temperature and time. Table 2 shows, for example, the results of measuring the parameters of the porous structure of some samples depending on the conditions of preliminary degassing. Also all samples were subjected to a single annealing procedure in a muffle furnace for 1 hour. This procedure is required in case of long-term storage or transportation of samples in high humidity conditions. The results of meas- uring the specific surface area for a sample of the BEA type are presented in Table 3. As can be seen from Table 3, in the case of long-term storage of zeolite samples in high humidity conditions, the measured specific surface area of the samples decreases by 2–3 times. Zeolites annealing at a temperature of 500 °C restores the specific surface area by removing moisture from the micro- and mesoporous system. In the study of zeolites, such heat treatment is similar to the an- nealing at the final stage of their preparation does not lead to structure destruction and is recommended. 3.2.3. Measurement in the given range of the relative partial pressures of the adsorbate gas P/Po The range selection of P/Po values is determined by the investigated porous structure parameter. The measure- ment of the specific surface area by the BET method, the external surface and the construction of the size distribu- tion of mesopores suggest the choice of different study modes. For example, the P/Po parameters that are selected on the Sorbi MS device used in this work are listed in Table 4. 3.3. The micropores volume determination of a series of zeolite compositions by the Sing and t-plot methods For the successful and high-quality application of porous materials, the control of their parameters and properties is an important criterion. Analysis of microporous struc- ture parameters in zeolite compositions was carried out in Table 1 Selected masses for various series of zeolite compositions Series BEA+2MNaOH BEA Mass, mg 83 150 49 211 Ssp value at standard conditions of sample preparation, m 2 / g 296.6 Exceeding the maximum measurable desorption signal 303.4 Exceeding the maximum measurable desorption signal Table 2 Specific area values for different sample preparation modes T Prep, °С t Prep, min Ssp, m 2 /g BEA+2MNaOH - - 99.79 150 20 296.6 150 45 399.2 300 15 434.4 Table 3 Specific area values for different sample preparation modes before and after annealing at 500 °C T Prep, °С t Prep, min Ssp, m 2 /g BEA before annealing - - 170 150 60 366.1 BEA after annealing - - 453 BEA after annealing, 3 days later - - 300 150 60 470 Table 4 The value range of P/P0 Investigated parameter Method Range of relative partial pressures of adsorbate gas P/P0 Specific surface area BET 6% to 20% Micropore indication t-plot method 15% to 40% External surface of mesopores, size distribution of mesopores Capillary condensation of an inert gas 6% to 97% this paper. Zeolites of the type ZSM-5 (samples 1,3 in Ta- ble 5) and BETA (samples 2,4–6 in Table 5) were used. Studies of the zeolites porous structure were carried out by low-temperature nitrogen adsorption on a Sorbi MS device (t-plot method). As an alternative method for estimating the micropores volume, a comparative Sing method is proposed. It is based on the use of the relative adsorption value αs on the reference sample. The value αs is calculated from the ratio of the current adsorption value V to adsorption at the relative pressure P/P0 = 0.4. The reference is a non-porous zeolite (without heating) of the same chemical nature as the sample under study. Using normalized value we rebuild previously con- structed adsorption isotherms into dependence of type V = f(V/V0.4), replacing relative pressure by value αs. The segment, cut off on the ordinate axis, is the value by which the micropores volume is then calculated. The obtained t-plot and Sing micropore volume values are shown in Table 5. Chimica Techno Acta 2021, vol. 8(3), № 20218302 ARTICLE 4 of 4 Table 5 Micropore volume values by the t-plot and Sing method Sample № Sample composition zeolite/AH, wt% AH producer Peptizer Micropores volume (t-plot method), cm 3 /g Micropores volume (Sing method), cm 3 /g 1 ZSM-5 - - 0.07 0.07 2 BEA - - 0.066 0.09 3 70 ZSM -5/30 AH Sasol aqueous solution of nitric acid 0.053 0.065 4 70 BEA /30 AH-1 Sasol aqueous solution of nitric acid 0.039 0.074 5 70 BEA /30 AH-2 OAO AZK and OS aqueous solution of nitric acid 0.1 0.1 6 70 BEA /30 AH-3 OAO AZK and OS mixture of aqueous solutions of nitric acid and ammonia (1:1) 0.072 0.097 As it can be seen, the micropore volume values calcu- lated by Sing do not differ significantly compared to the values taken from the Sorbi MS device. Thus, it is possible to estimate the volume of micropores by both the t-plot method and the comparative Sing method. The analysis showed that the largest micropores volume (0.1 ml/g) is typical for samples of series 5, where in the synthesis pro- cess an aqueous solution of nitric acid was used as a pep- tizer, and aluminum hydroxide from the manufacturer OAO AZK and OS was used as a binder. Replacement of the peptizer (sample 6 in Table 5) led to a slight decrease in the micropores volume in the composition. However, such a replacement results in an increase in the total specific surface area [8] and the total pore volume. 4. Conclusions The paper considers the method application features of nitrogen thermal desorption for the study of micro- mesoporous materials on the example of zeolite composi- tion. The recommended mass values for obtaining a stable desorption signal and the recommended modes of sample preparation of zeolite compositions were selected. As a rule, in the study of materials by the thermal desorption method of inert gases, an insufficient mass of the sample can act as a significant limitation for the analysis. In the case of studying zeolite compositions, on the contrary, too large an adsorbent mass can lead to an incorrect sensor signal for thermal conductivity and peak truncation. Nitrogen adsorption at 77 K on a series of zeolite com- positions was investigated. It was shown that using analy- sis sorption methods, it is possible to estimate the volume micropores by the t-plot and Sing method. References 1. Everett D. Manual of Symbols and Terminology for Physico- chemical Quantities and Units, Appendix II: Definitions, Ter- minology and Symbols in Colloid and Surface Chemistry. Pure and Applied Chemistry. 1972;31(4):577-638. doi:10.1351/pac197231040577 2. Wang G, Wang K, Ren T. Improved analytic methods for coal surface area and pore size distribution determination using 77K nitrogen adsorption experiment. Int J Min Sci Technol. 2014;24(3):329-334. doi:10.1016/j.ijmst.2014.03.007 3. Gavrilov VY. Fiziko-himicheskie osnovy adsorbcionnogo ana- liza dispersnyh i poristyh materialov [Physicochemical bases of adsorption analysis of dispersed and porous materials]. Novosibirsk: NCTC; 2007. 67 p. Russian. 4. Zelenka T. Adsorption and desorption of nitrogen at 77 K on micro- and mesoporous materials: Study of transport kinet- ics. Journal of Microporous and Mesoporous Materials. 2016;227:202-209. doi:10.1016/j.micromeso.2016.03.009 5. Nguyen TB, Huang CP, Doong RA, Chen CW, Dong CD. CoO-3D ordered mesoporous carbon nitride (CoO@mpgCN) compo- site as peroxymonosulfate activator for the degradation of sulfamethoxazole in water. Journal of Hazardous Materials. 2021;401:123320. doi:10.1016/j.jhazmat.2020.123326 6. Dong GY, Tian GY ,Gong LL, Tang QG. Mesoporous zinc sili- cate composites derived from iron ore tailings for highly effi- cient dye removal: Structure and morphology evolution. Journal of Microporous and Mesoporous Materials. 2020;305:110352. doi:10.1016/j.micromeso.2020.110352 7. Hadis M, Jafar T, Babak M. Enhanced nitrogen adsorption capacity on Ca 2+ ion-exchanged hierarchical X zeolite. Separation and Purification Technology. 2021;264:118442. doi:10.1016/j.seppur.2021.118442 8. Kononova IE, Maraeva EV, Skornikova SA, Moshnikov VA. Influence of binder on porous structure of zeolite composi- tions and their catalytic activity. Glass Physics and Chemis- try. 2020;46(2):162-169. doi:10.1134/S1087659620020066 9. Gracheva IE, Moshnikov VA, Maraeva EV, Karpova SS, Alexsandrova OA, Alekseyev NI, Kuznetsov VV, Olchowikc G, Semenov KN, Startseva AV, Sitnikov AV, Olchowikc JM. Nanostructured materials obtained under conditions of hier- archical self-assembly and modified by derivative forms of fullerenes. Journal of non-crystalline solids. 2012;358(2):433-439. doi:10.1016/j.jnoncrysol.2011.10.020 10. Maraeva EV, Permiakov NV, Kedruk YY, Gritsenko LV, Ab- dullin KhA. Creating a virtual device for processing the re- sults of sorption measurements in the study of zinc oxide na- norods. Chimica Techno Acta. 2020;7(4):154-158. doi:10.15826/chimtech.2020.7.4.03 https://doi.org/10.1351/pac197231040577 https://doi.org/10.1016/j.ijmst.2014.03.007 https://doi.org/10.1016/j.micromeso.2016.03.009 https://doi.org/10.1016/j.jhazmat.2020.123326 https://doi.org/10.1016/j.micromeso.2020.110352 https://doi.org/10.1016/j.seppur.2021.118442 https://doi.org/10.1134/S1087659620020066 https://doi.org/10.1016/j.jnoncrysol.2011.10.020 https://doi.org/10.15826/chimtech.2020.7.4.03