Sorption of CO2 and CH4 on Raw and Calcined Halloysite—Structural and Pore Characterization Study

The article presents comparative characteristics of the pore structure and sorption properties of raw halloysite (R-HAL) and after calcination (C-HAL) at the temperature of 873 K. Structural parameters were determined by optical scanning and transmission electron microscopy methods as well as by mercury porosimetry (MIP, Hg) and low-pressure nitrogen adsorption (LPNA, N2, 77 K). The surface area parameter (LPNA) of halloysite mesopores before calcination was 54–61 m2/g. Calcining caused the pore surface to develop to 70–73 m2/g. The porosity (MIP) of halloysite after calcination increased from 29% to 46%, while the surface area within macropores increased from 43 m2/g to 54 m2/g. The total pore volume within mesopores and macropores increased almost twice after calcination. The course of CH4 and CO2 sorption on the halloysite was examined and sorption isotherms (0–1.5 MPa, 313 K) were determined by gravimetric method. The values of equilibrium sorption capacities increased at higher pressures. The sorption capacity of CH4 in R-HAL was 0.18 mmol/g, while in C-HAL 0.21 mmol/g. CO2 sorption capacities were 0.54 mmol/g and 0.63 mmol/g, respectively. Halloysite had a very high rate of sorption equilibrium. The values of the effective diffusion coefficient for methane on the tested halloysite were higher than De > 4.2 × 10−7 cm2/s while for carbon dioxide De > 3.1 × 10−7 cm2/s.


Introduction
Coal has been one of the dominant fuels of the world and European economy since the industrial revolution in the XIXth century. In 1990, coal made up almost 41% of gross energy consumption in current EU-28 Member States and 39% of energy production in these countries. Despite the 'climate package' and decarbonisation policy, in 2015 16% of the EU's gross energy consumption came from coal and 24% came from electricity production [1]. According to global policy, the use of coal as a fuel is to cease so solutions are sought to use this raw material and its deposits in an alternative way, e.g., as reservoirs for carbon dioxide carbon capture and storage (CCS) technology. In CCS and carbon  Clay minerals-amino acid modified montmorillonite 298 0.1 0.5 - [36] The article presents a study of CO 2 /CH 4 high pressure sorption calcined and raw halloysite. An in-depth analysis of pore structure was performed to evaluate halloysite for further structural modifications.

Methods
The microscopic analyses of the halloysite surface topography were carried out using an Apollo XP scanning electron microscope (SEM-EDS, EDAX, Weiterstadt, Germany). The microscope consists of an EDAX Apollo X energy-dispersive X-ray spectrometer with a secondary electron and back scattered electron detectors and an EDS X-ray dispersion energy spectrometer, on which, in the micro-areas of the sample, the chemical composition of the rocks has been identified. The measurements were carried out in a low-vacuum mode (10 −2 Pa) with an accelerating voltage of 20 kV electron beam. The tests were carried out for halloysite samples before and after calcining. Before measuring, the samples were sprayed with a thin layer of silver. Images were recorded at magnifications of 100-2500×. The Apollo XP scanning electron microscope is owned by the Academic Centre for Materials and Nanotechnology, AGH University of Science and Technology in Poland.
For further characterization of the synthesised halloysite specimens, transmission electron microscope (TEM) studies were performed. Before observations of the halloysite nanoplates and nanotubes, research material was prepared. A small amount of halloysite sample in ethanol was dispersed in ethanol and a drop of suspension was placed on a microscopic carbon-coated copper mesh. TEM tests were carried out using a Titan 80-300 TEM/STEM emission electron microscope (FEI, Hillsboro, OR, USA) with emission transmission equipped with a twin-lens operated at 300 kV and an annular darkfield detector.
The structural analyses of the surface area and the micropore and mesopore size distribution were carried out by low pressure nitrogen adsorption (LPNA) on a ASAP 2020 analyser (Micromeritics, Norcross, GA, USA). For the measurements nitrogen was used as the adsorbate. The analyses were carried out under isothermal conditions at 77 K and in the absolute pressure range of 0-0.1 MPa. Such a pressure range corresponds to the relative pressure that is equal to the ratio of the absolute pressure and the critical pressure of N 2 in the gas phase, in the range of 0 < p/p 0 < 0.996. Prior to the analyses the samples were degassed at 343 K for 24 h in pressure close to vacuum. The measurement consisted in the adsorption of the gas molecules on the surface of the material that occurs due to the interactions at the gas-solid interface. The analyzer registered the volume of gas adsorbed in the pore space of the rock. Knowing the sorption equilibrium points under a given pressure, it was possible to determine the structural parameters of the material.
In order to characterize the pores that get filled in with multiple layers the BET model was used [52] in the relative pressure range of 0.05 < p/p 0 ≤ 0.30. It is based on the sorption isotherm that was plotted based on adsorption points according to Equation (1): where: a BET (P) (mmol/g) is the sorption equilibrium point, a mBET (mmol/g) is the multilayer capacity, C (−) is the adsorption equilibrium constant that depends on the difference between the adsorption heat for the first layer and the condensation heat, p (Pa) is the absolute pressure, p 0 (Pa) is the saturation pressure. This model assumes that the surface of the material is covered with multilayer of the gas molecules, the specific surface area of the adsorbed gas is determined by Equation (2): where: SSA (m 2 /g) is the specific surface area, ω (nm 2 ) is the surface occupied by one adsorbate molecule in the mono-molecular layer, N A (1/mol) is the Avogadro's number. Barrett's, Joyner's and Halenda's (BJH) theory [53] in the relative pressure range 0.01 < p/p 0 was used to characterize the mesopores area of the material in which capillary condensation occurs. The BJH model is based on the Kelvin Equation (3) [54]. Based on sorption and desorption points, the total pore volume available for adsorbed gas particles, specific surface area and pore size distribution as a function of their diameter and mean pore diameter were determined: where: σ (m 3 /mol) is surface tension of liquid nitrogen, V (mol) is liquid molar volume of nitrogen, R (J/mol·K) is a gas constant, T (K) is temperature, r k (m) is capillary radius.
To characterize the volume and surface area of micropores in halloysites, t-plot and NLDFT models were used. The calculations of the t-plot method were made based on the results of the BET model and Harkins and Jura thickness equation in the thickness range of 0.35-0.5 nm. NLDFT analyzes were performed for pillared clays.
The macropores and a part of mesopores of the halloysites were analysed by mercury intrusion porosimetry (MIP). Measurements of the volume of mercury intruded into the open pores of the material were carried out on an AutoPore IV 9500 analyser (Micromeritics). Prior to the measurement the sample was heated at 378 K for 25 h and degassed under 6.5 Pa for 1h. The calculations are based on the Washburn theory [55]. The size of the critical capillary into which mercury can intrude under a given pressure is expressed by Equation (4). The surface area was determined from Equation (6) and the pore size distribution from Equation (7). As usual, in simplified calculations pressure p 0 was omitted and constant value of mercury surface tension (0.48 N/m) and a constant mercury contact angle (141.3 • ) were adopted: assuming that: where: D (µm) is the critical pore diameter, γ (N/m) is the surface tension of mercury, θ ( • ) is the contact angle of mercury, ∆p (Pa) is the difference of mercury pressure and gas pressure in the pores, p 1 (Pa) is the hydrostatic pressure of mercury, p 2 (Pa) is the gas pressure in pores of the samples, S (m/g 2 ) is the surface area, V (cm 3 /g) is the pore volume. The analyses of methane and carbon dioxide on halloysites were carried out using an IGA 001 gravimetric gas sorption analyser (Hiden Isochema, Warrington, UK), in the pressure range of 0-2.0 MPa ( Figure 1).  The gravimetric measurement method records the changes of the sorbent weight during sorption under isobaric and isothermal conditions. During diffusion the gas sorbate molecules permeate the pore structure of the sorbent. If the molecules get in the range of intermolecular interactions with the sorbent molecules the number of the degrees of freedom of the sorbate drops. This process is registered as an increase of the weight of the sorbate-sorbent system. Prior to the tests the samples were sieved. The 0.5-1.0 mm grain fracture was selected for the analysis and averaged using multiple cone splitting method. A 0.5 g sample was placed on the scale pan of the analyser in the reactor chamber. The measurement started with the sample preparation stage and then the sorption capacity of sample was determined to a selected gas at a given temperature and under a given pressure. Each time the procedure included: -Sample preparation stage i.e., sieving a halloysite sample to the 0.5-1 mm grain fracture and degassing it in deep vacuum at 353 K for 12 h, The gravimetric measurement method records the changes of the sorbent weight during sorption under isobaric and isothermal conditions. During diffusion the gas sorbate molecules permeate the pore structure of the sorbent. If the molecules get in the range of intermolecular interactions with the sorbent molecules the number of the degrees of freedom of the sorbate drops. This process is registered as an increase of the weight of the sorbate-sorbent system. Prior to the tests the samples were sieved. The 0.5-1.0 mm grain fracture was selected for the analysis and averaged using multiple cone splitting method. A 0.5 g sample was placed on the scale pan of the analyser in the reactor chamber. The measurement started with the sample preparation stage and then the sorption capacity of sample was determined to a selected gas at a given temperature and under a given pressure. Each time the procedure included: -Sample preparation stage i.e., sieving a halloysite sample to the 0.5-1 mm grain fracture and degassing it in deep vacuum at 353 K for 12 h, - The measurement stage i.e., the quasi-step change of the pressure of the gas sorbate in the reactor at the rate of 333 mbar/min and recording the changes of the weight of the sorbate-sorbent system for the period of time that it needs to equilibrate.
The measurements were carried out at 313 K and they were repeated for 0.1 MPa, 0.3 MPa, 0.6 MPa, 1.0 MPa and 1.5 MPa. Each time after changing the sorbate (CO 2 , CH 4 ) a new sample was used to avoid changes in the sorption capacities resulting from previous tests. On the basis of the sorption points and by minimizing the sum of squared deviations Langmuir sorption isotherms were plotted according to Equation (8): where: a L (P) (mmol/g) is sorption equilibrium point, a mL (mmol/g) is the total monolayer capacity, b (1/MPa) is the adsorption equilibrium constant equal to the inverse of the half pressure, p (Pa) is the equilibrium pressure of the adsorbed nitrogen, c (mmol/g) is the coefficient representing the hysteresis loop.
In the range of the tested pressures, the Langmuir isotherms extrapolated the measurement points correctly in the case of both CO 2 and CH 4 . From the point of view of the metrology of the phenomenon, the kinetics of sorption processes, understood as a decrease in the degrees of freedom of the sorbate molecules near the sorbent molecules is instantaneous. The slow achievement of sorption equilibrium results from the processes of molecule transport in the sorbent pore structure. The parameter that describes the sorption kinetics quantitatively is the effective diffusion coefficient. For the sphere with the radius R, Fick's second law is: but: takes the following form: where: r is the distance from the sphere centre, C(r, t) is the distribution of the sorbate concentration within it. The solution to this equation has the form of a series expansion: which describes how the mass M(t) of the substance accumulated in the spherical grain tends to a limit value M ∞ . In order to determine the effective diffusion coefficient using a gravimetric method, it is necessary to record the changes of the weight of the sorbent-sorbate system as a function of time, after the step pressure change in the reactor. The maximum rate of the pressure changes in the case of IGA-001 is 333 mbar/min. One of the assumptions leading the analytical solution of the diffusion equation is the linear shape of the sorption isotherm. For the measurement uncertainties due to the isotherm being non-linear to be as small as possible, the test is usually made for the pressure change from vacuum to 0.1 MPa.

Materials
The halloysite for testing came from the halloysite open pit mine in Dunino, Poland (51 • 08 44.0" N 16 • 04 31.3" E). Halloysite is an aluminosilicate belonging to the kaolinite-serpentine group with Al 2 Si 2 O 5 (OH) 4 chemical formula. It occurs naturally in three variants of particle shapes: halloysite nanotubes, halloysite nanoplates and spheres. It consists of one octahedral aluminate sheet and one tetrahedral silicate sheet hence is classified as 1:1 phyllosilicate. Halloysite, like other phyllosilicates, is characterized by two-dimensional sheets consisting of SiO 2 (tetrahedra) and AlO 3 (octahedra). These two layers are separated by interlayer H 2 O molecules. Halloysite from polish deposit "Dunino" is a product of basalt weathering and a raw mineral in the deposit consists mostly of nanotubes and nanoplates. A detailed description of halloysite from Dunino deposit, its chemical and mineralogical characterization are presented in [56]. Table 2 shows the chemical composition and selected trace elements of raw halloysite determined by means of X-ray fluorescence method. It occurs in two different hydration states; the dominant one is hydrated halloysite (10 Å) in which the water in the interlayer increases its thickness. It can also be found in dehydrated forms halloysite (7 Å). Occurs in two different hydration states, the dominant is the hydrated halloysite (10 Å), he is also found in dehydrated forms of halloysite (7 Å). In this work authors focused on the analysis of structural and sorption properties of dehydrated meta-halloysite and calcined halloysite.
For the purpose of the study, two samples were selected. Sample R-HAL was a raw halloysite which was initially dried at 378 K, which removed the interlayer water making it a meta-halloysite, the sample was not subjected to any further treatment except comminution for sorption experiments. The other sample (referred to as C-HAL sample) was a modified halloysite, calcinated at the temperature of 873 K.
The SEM images (Figures 2 and 3) show the microstructure of the raw Dunino halloysite and their phase diversity. Apart from the halloysite phase, the EDS analyses revealed a kaolinite phase and other mineral phases, which are irregular and differ in the surface microstructure. Those phases contained iron, magnesium and titanium compound, which may be indicative of the presence of magnetite, magnesium ferrite, ilmenite and geikielite [57].
temperature of 873 K.
The SEM images (Figures 2 and 3) show the microstructure of the raw Dunino halloysite and their phase diversity. Apart from the halloysite phase, the EDS analyses revealed a kaolinite phase and other mineral phases, which are irregular and differ in the surface microstructure. Those phases contained iron, magnesium and titanium compound, which may be indicative of the presence of magnetite, magnesium ferrite, ilmenite and geikielite [57].   temperature of 873 K.
The SEM images (Figures 2 and 3) show the microstructure of the raw Dunino halloysite and their phase diversity. Apart from the halloysite phase, the EDS analyses revealed a kaolinite phase and other mineral phases, which are irregular and differ in the surface microstructure. Those phases contained iron, magnesium and titanium compound, which may be indicative of the presence of magnetite, magnesium ferrite, ilmenite and geikielite [57].

Structural Analyses
This LPNA and MIP methods were used to analyse the surface structures of the halloysite samples. Within LPNA analysis, nitrogen adsorption and desorption isotherms were plotted (77 K) ( Figure 5) and micro and mesopore areas of the halloysites were described. In both samples type III isotherms were identified according to International Union of Pure and Applied Chemistry (IUPAC) [58]. Such an isotherm shape is typical of mesoporous and macroporous materials. In both samples capillary condensation of nitrogen took place in the multi-layer area. The desorption curve differed in shape from the adsorption curve creating in both cases a hysteresis loop of H3 type, characteristic of materials containing slit pores [59].

Structural Analyses
This LPNA and MIP methods were used to analyse the surface structures of the halloysite samples. Within LPNA analysis, nitrogen adsorption and desorption isotherms were plotted (77 K) ( Figure 5) and micro and mesopore areas of the halloysites were described. In both samples type III isotherms were identified according to International Union of Pure and Applied Chemistry (IUPAC) [58]. Such an isotherm shape is typical of mesoporous and macroporous materials. In both samples capillary condensation of nitrogen took place in the multi-layer area. The desorption curve differed in shape from the adsorption curve creating in both cases a hysteresis loop of H3 type, characteristic of materials containing slit pores [59].
TEM images of halloysite samples (Figure 4) show the morphology of a raw Dunino halloysite. Grains consist of mixed halloysite nanoplates and halloysite nanotubes. Nanotubes are approximately 200-400 nm in length and have usually a thickness of 30-50 nm whereas nanoplates are characterized by an irregular shape and have usually a size of 80 nm in width and 400 nm in length.

Structural Analyses
This LPNA and MIP methods were used to analyse the surface structures of the halloysite samples. Within LPNA analysis, nitrogen adsorption and desorption isotherms were plotted (77 K) ( Figure 5) and micro and mesopore areas of the halloysites were described. In both samples type III isotherms were identified according to International Union of Pure and Applied Chemistry (IUPAC) [58]. Such an isotherm shape is typical of mesoporous and macroporous materials. In both samples capillary condensation of nitrogen took place in the multi-layer area. The desorption curve differed in shape from the adsorption curve creating in both cases a hysteresis loop of H3 type, characteristic of materials containing slit pores [59].  The isotherms were fitted based on the adsorption points as the function of pressure. The values of the structural parameters are presented in Table 3. Assuming that the adsorbate molecules cover the surfaces of the tested materials completely, the BET multilayer surface area (SSA BET ) as well as the BJH surface area (SSA BJH ) were calculated. In the calculations it was assumed that the area covered by N 2 molecules was ω = 0.162 nm 2 . In the raw halloysite the BET surface area was 62.7 m 2 /g, the BJH surface area was in the range of 53.9 m 2 /g and the total pores volume was 0.10 cm 3 /g. After calcination the values of the structural parameters increased. The value of SSA BET increased by nearly 70 m 2 /g, the value of SSA BJH was 72.5 m 2 /g and the V BJH increased to 0.18 cm 3 /g. To separate the area of micropores and mesopores, the t-plot method was used. According to this method, R-HAL had a negligible surface area of micropores (4.1 m 2 /g). After calcining, the surface of the micropores in C-HAL slightly developed to 16.3 m 2 /g. The obtained values of the structural parameters are at the level typical of macroporous carbonate rocks used in the energy industry [24,60,61]. Figure 6 presents the curves of the cumulative and incremental pore volume of halloysites as a function of their diameter, which was determined according to the BJH theory. This model assumes that the pores fill with gas on the principles of capillary condensation under the influence of increasing pressure. The C-HAL calcination sample had almost twice as large the cumulative volume in the mesopores area as the R-HAL non-calcined material.       To characterize the area of macropores in halloysite, they were subjected to mercury porosimetry (MIP) analyzes. The test results are summarized in Table 3. Halloysite showed diversification in the value of structural parameters but the halloysite after calcination had a much more extensive macroporous structure. The surface area in the C-HAL increased from 43 m 2 /g to 54 m 2 /g, while the porosity from 29% to 46%. Figure 8 shows the volume distribution of halloysite pores as a function of their diameter. In the R-HAL sample the pore distribution curve was unimodal, and the largest pore volume was in pores with a diameter of about 0.005-0.02 cm 3 /g. In the C-HAL sample, two ranges of pore diameters with the largest volume were distinguished. Fine macropores of 0.005-0.5 µm in diameter prevailed and a second pore peak with diameters of several µm was distinguished. To characterize the area of macropores in halloysite, they were subjected to mercury porosimetry (MIP) analyzes. The test results are summarized in Table 3. Halloysite showed diversification in the value of structural parameters but the halloysite after calcination had a much more extensive macroporous structure. The surface area in the C-HAL increased from 43 m 2 /g to 54 m 2 /g, while the porosity from 29% to 46%. Figure 8 shows the volume distribution of halloysite pores as a function of their diameter. In the R-HAL sample the pore distribution curve was unimodal, and the largest pore volume was in pores with a diameter of about 0.005-0.02 cm 3 /g. In the C-HAL sample, two ranges of pore diameters with the largest volume were distinguished. Fine macropores of 0.005-0.5 µm in diameter prevailed and a second pore peak with diameters of several µm was distinguished.

CH4 and CO2 Sorption Analysis
On the basis of sorption capacities, registered under specified measurement conditions, CH4 and CO2 sorption isotherms were determined for halloysite samples. Table 4 presents the values of

CH 4 and CO 2 Sorption Analysis
On the basis of sorption capacities, registered under specified measurement conditions, CH 4 and CO 2 sorption isotherms were determined for halloysite samples. Table 4 presents the values of equilibrium sorption points at pressures 0.1, 0.3, 0.6, 1.0 and 1.5 MPa and the total sorption capacity relative to CH 4 and CO 2 , approximated by Equation (8). where: a L (p) is the point of sorption equilibrium in pressure p. a mL is the total capacity of the monolayer, b is the Langmuir adsorption equilibrium constant equal to the inverse of half pressure.
In the range of tested methane pressures, the sorption isotherms determined had a small course, well described by the Langmuir isotherm (Figures 9 and 10). After methane sorption, the C-HAL calcination sample obtained higher values of sorption capacities at each pressure. The analysis of sorption points indicated that as the measurement pressure increased, so did the difference between CH 4 sorption efficiency by R-HAL and C-HAL. The value of the maximum sorption capacity approximated by Equation (8) was in the range of 0.39-0.49 mmol/g. The obtained level of methane sorption capacity differed from the level typical for microporous materials, e.g., coal [12]. At the pressure of 0-1.5 MPa Langmuir sorption isotherms correctly extrapolated the measuring points also in the case of CO2. The carbon dioxide sorption value for both samples was over twice as high as for methane sorption. The value of the maximum sorption capacity determined by the Langmuir Equation (8) in the halloysite before calcining was 0.66 mmol/g. After the calcining process, At the pressure of 0-1.5 MPa Langmuir sorption isotherms correctly extrapolated the measuring points also in the case of CO2. The carbon dioxide sorption value for both samples was over twice as high as for methane sorption. The value of the maximum sorption capacity determined by the Langmuir Equation (8) in the halloysite before calcining was 0.66 mmol/g. After the calcining process, it increased slightly to 0.76 mmol/g. At the pressure of 0-1.5 MPa Langmuir sorption isotherms correctly extrapolated the measuring points also in the case of CO 2 . The carbon dioxide sorption value for both samples was over twice as high as for methane sorption. The value of the maximum sorption capacity determined by the Langmuir Equation (8) in the halloysite before calcining was 0.66 mmol/g. After the calcining process, it increased slightly to 0.76 mmol/g.

Diffusion Kinetics
The kinetics of halloysite diffusion is so high that it is impossible to accurately determine the value of the effective diffusion coefficient by the gravimetric method. Analysis of the kinetics plots (Figures 11 and 12) shows that changes in the mass of the sorbent-sorbate system almost immediately map the changes in sorbate pressure in the reactor. In this case, only a bottom estimate of the diffusion coefficient is possible [62]. Timofejew proposed some simplification of the solution to the analytical diffusion equation. After solving the Equation (12) looking for a moment of time for which the gas mass is 50% of the initial mass: we will obtain a solution described in the literature as Timofejew's formula (14): where: (s) is the half time, (cm) is the so-called equilibrium radius of grain, equal: Timofejew proposed some simplification of the solution to the analytical diffusion equation. After solving the Equation (12) looking for a moment of time for which the gas mass is 50% of the initial mass: we will obtain a solution described in the literature as Timofejew's formula (14): where: (s) is the half time, (cm) is the so-called equilibrium radius of grain, equal: Timofejew proposed some simplification of the solution to the analytical diffusion equation. After solving the Equation (12) looking for a moment of time for which the gas mass is 50% of the initial mass: we will obtain a solution described in the literature as Timofejew's formula (14): where: t 1 2 (s) is the half time, R (cm) is the so-called equilibrium radius of grain, equal: The study used a grain size of 0.5-1mm, resulting in a radius R = 0.35 mm. In the case of CH 4 , both raw and calcined halves t 1 2 can be estimated at about 90 s. This allows us to state that the value of the effective diffusion coefficient for CH 4 on raw and calcined halloysite is higher than De > 4.2 × 10 −7 cm 2 /s. In the case of CO 2 , the process is similar and the half time t 1⁄2 can be estimated at about 120 s. With the same grain size, this means that the effective CO 2 diffusion coefficient value for raw and calcined halloysite is higher than De > 3.1 × 10 −7 cm 2 /s. The values of effective diffusion coefficients for halloysite are to be considered high. The comparison of these values with one of the materials most often tested for diffusivity such as coal, indicates that in the case of halloysite, the diffusion faster by 2 to 3 orders of magnitude.

Discussion
The use of porous materials in sorption processes depends on a number of properties. One of the decisive factors is a high sorption capacity. It is directly correlated with the high surface area (SSA) of the sorbent, the diameter of its pores and the chemical nature of its surface. An extremely important factor determining the use of sorbent is favorable adsorption kinetics. Phenomena associated with the accumulation of gas adsorbate in the pores of the material include filtration, diffusion and specific sorption processes. Specific sorption is an almost instantaneous process. Filtration is slower, but it is much faster than diffusion, which is the longest process. Therefore, the analysis of the gas transport process in the pores is mainly based on the analysis of the phenomenon of diffusion kinetics [63]. Some of the adsorbents, especially heterogeneous, are characterized by high selectivity, which depends on the size of the pores, their shape, tortuosity and distribution. Selectivity is a particularly important factor in the case of adsorption from multi-component solutions. Important factors in the applicability of adsorbents are also their good mechanical properties and good stability and strength as well as low production costs.

Structural Analyses
The surface area parameter is directly related to the amount and size of pores in the material. The pores may have different dimensions, shapes and volumes, and may form different channel systems. Due to the size and the adsorption phenomena occurring in them, according to IUPAC [58], pores are divided into macropores with a diameter above 50 nm, mesopores with a diameter of 2-50 nm and micropores with a diameter below 2 nm. Macropores act as transport channels in the processes of gas diffusion and filtration. Mesopores, also called transient pores, play a significant part in the adsorption mechanism., micropores on the other hand, make a decisive contribution to the size of the surface area of the material.
The pore structure of the examined halloysites mainly consists of mesopores and macropores. This is showed by the shapes of type III adsorption isotherms (LPNA method) and the relatively low BET surface value, which was in the range of 62.7-69.8 m 2 /g. T-plot analysis revealed that in R-HAL the share of micropores is negligible (6.5%), while in C-HAL it is 23.4%. The studies on the pore volume distribution determined by the LPNA method (NLDFT model) showed that the value of the total pore volume in the halloysite after calcination increased from 0.046 cm 3 /g to 0.054 cm 3 /g. In both halloysites, the largest volume was found in pores 10-33 nm wide, which according to IUPAC are classified as mesopores. According to the BJH model, which does not assume the presence of micropores, but only mesopores in which capillary condensation occurs, the total pore volume almost doubled its value, and the largest volume contained mesopores and fine macropores.
Halloysite belongs to the group of layered silicates. Other minerals of this group, such as: saponite, montmorillonite or vermicult, usually have a surface of several dozen m 2 /g [64]. The values of structural parameters of halloysite and other layered silicates are therefore comparable. The calcination used in the examined halloysite allowed for the increase the share of micropores in the mineral and increased SSA by 12%. The micropores do not have a large volume, but they affect the development of the surface area. In the case of adsorption processes, this is a very important factor.
Comparing halloysite to other natural meso-and macroporous materials such as carbonate rocks (limestones and dolomites), halloysite has a much larger surface area and pore volume [24,60,61,65]. The SSA BET value of these rocks often do not exceed about 20 m 2 /g, hence they are not considered as reservoirs for CO 2 or CH 4 . In contrast to mesoporous silica gels, halloysites have significantly lower SSA BET values. Typical silica gels have the surface area in the range 500-800 m 2 /g [66].
In comparison with microporous rocks, e.g., hard coal, the SSA BET value of halloysites is also much higher. The SSA BET of coal is only a few m 2 /g. However, coal is a highly selective rock in relation to gases. Nitrogen is a gas with which carbon interacts so weakly that the particles of this gas are not able to penetrate into its smallest pores. The heterogeneity of coal and its properties, such as swelling, for example, make comparing its structural parameters with other rocks very difficult [50].
Synthetic homogeneous materials such as carbon nanotubes or graphene compounds have a much more developed surface area than halloysite, reaching even several thousand m 2 /g. They have a well-developed and ordered structure but their production is rather expensive [48,67].

Sorption Analysis and Diffusion Kinetics
Analyses of the examined halloysite consisted in determining the sorption capacity for CO 2 and CH 4 gases. Due to the mainly meso-and macroporous structure of halloysite, the sorption processes described in the article were considered as physical adsorption processes.
In the high pressure range the tested halloysites were characterized by sorption capacities. Individual sorption points made it possible to plot Langmuir sorption isotherm. The CO 2 sorption capacities of the raw halloysite were in the range of 0.18-0.54 mmol/g. Calcination increased this parameter to a value in the range of 0.20-0.63 mmol/g.
The sorption capacities of halloysite to CO 2 were much higher than the sorption capacity to CH 4 , particularly in the lower pressures area. Raw halloysite in the pressure range of 0.1-1.5 MPa showed a CH 4 sorption capacity from 0.02 mmol/g to 0.18 mmol/g, and after calcination it increased to 0.03-0.21 mmol/g. Langmuir's total sorption capacity relative to CH 4 was about 3 times lower than relative to CO 2 . Calcination increased the total CO 2 sorption capacity from 0.66 mmol/g to 0.76 mmol/g, while in the case of CH 4 sorption a decrease from 0.49 mmol/g (R-HAL) to 0.39 mmol/g (C-HAL) was noted.
The process of calcining led to an increase of the sorption capacity by several percent for individual pressures relative to raw halloysite. It is worth noting that in the case of CH 4 sorption, calcination significantly affected the value of the b coefficient of sorption isotherms. Thanks to that change the Langmuir isotherm describing CH 4 sorption on the calcined halloysite tends to an asymptotic value at lower pressures relative to the CH 4 isotherm on the raw halloysite. As a result, despite the fact that sorption capacities relative to CH 4 at the sorption points tested were higher in calcined halloysite than in the raw one, the Langmuir total sorption capacity based on extrapolation of measurement points was lower. Table 1 presents sorption capacities of various materials of natural and synthetic origin. The CO 2 and CH 4 sorption capacities of halloysite are comparable in value to high-carbonated coal.
The tested material displayed a very interesting sorption kinetics course. The diffusion process was so fast that it was impossible to precisely specify the diffusion coefficient value. This is due to the fact that the maximum rate of change of gas pressure in the reactor is comparable to the time to reach the value of asymptotic sorption. It is only possible to estimate that the De value is not lower than De > 4.2 × 10 −7 cm 2 /s for CH 4 and not lower than De > 3.1 × 10 −7 cm 2 /s for CO 2 . Such high values of diffusion coefficients are very beneficial in terms of potential sorbent applications for the uptake of tested gases.
Sorption kinetics plots for halloysite compared to synthetic nanomaterials were instant. The surface available for gas molecules was sorbed very quickly. According to [47], stabilization of mass change for carbon nanotubes and graphene-based materials as a result of sorption of both CO 2 and CH 4 took place after a few minutes. For these materials, however, it was not possible to quantify the process kinetics.
Compared to natural coals, the values of diffusion coefficients in halloysite are significantly higher, even by one or two orders of magnitude. Phenomena resulting from the lengthening of sorption processes are mainly associated with the transport of gas particles inside the network of cracks and sorbent pores. This factor is marginal in the case of mesoporous materials but it is of great importance in the case of microporous materials. The duration of the diffusion of gas particles towards the pore volume depends on the pores the particles penetrate. The smaller the pores, the longer the diffusion time.

Conclusions
The article compares the structure and sorption properties of calcined halloysite to CO 2 and CH 4 with the properties of raw halloysite. The research aimed to characterize halloysite for further structural modifications. The calcination process applied had an effect on the development of the pore space of the studied halloysite. Calcining at the temperature of 873 K led to opening of the smallest mesopores (up to 3.5 nm), but the values of both their volume and the surface area after calcination were very low. Calcining opened up the previously inaccessible macropores and increased the porosity of this clay rock by 17%.
The CH 4 and CO 2 sorption properties of the measured halloysites was very interesting for technologies that require the use of significant amounts of sorbent. Compared to synthetic nanomaterials such as activated carbon, zeolites, modified graphene or nanotubes the sorption properties of halloysite are lower even by an order of magnitude. However, the cost of mining halloysite is much lower compared to the synthesis of nanomaterials. This is particularly true of geographical areas where this rock occurs in large quantities. It is due to that factor that the use of halloysite is beneficial.
Sorption capacities of halloysite are comparable to those of clay minerals and hard coal. Due to the equally high availability in nature, hard coal can also be a natural and cheap sorbent. However, low values of diffusion coefficients and low coal permeability pose a huge limitation in gas sorption on hard coal. These restrictions are a significant obstacle to the concept of CO 2 storage in off-balance coal seams. Halloysites do not have this disadvantage because they have higher values of diffusion coefficients than typical hard coals by several orders of magnitude.