Basic energy characteristics and isotherm of methanol adsorption on Cu 2+ ZSM-5 zeolite

. This paper presents isotherm results and basic (  H ,  F and  S ) thermodynamic characteristics of methanol adsorption in Cu2+ZSM-5 zeolite. The trinomial equation of adsorption isotherm by the bulk micropore filling theory (VMOT) is also described. A correlation between adsorption-energy characteristics was found and molecular mechanisms of methanol adsorption in Cu2+ZSM-5 zeolite were revealed in the whole filling region. It was determined that Cu2+ cations are located in shielded positions of the crystal lattice of zeolite ZSM-5. Adsorption of methanol molecule leads to the migration of Cu2+ cations from the zeolite lattice to the crossings formed by the intersection of straight and zigzag channels and to the formation of ion/molecular complexes of different multiplicity in them. It was found that small polar methanol molecules form high-energy hepta complexes with Cu2+ cation and are located in the first coordination sphere in Cu2+ZSM-5 zeolite. It was determined that the average molar entropy (-65 kJ/mol) of methanol adsorption indicates that in the zeolite the mobility of methanol molecules is lower than the liquid phase and close to the mobility of the solid phase.


Introduction
The study of adsorption properties of adsorbents provides useful information on structural characteristics and possibilities of practical applications.In the world, adsorbents derived from natural raw materials and synthetically derived adsorbents are widely used in various fields of industry, construction, agriculture and other areas [1][2][3][4][5][6][7][8][9].Adsorbents of selective action and wide range of application are nano-porous molecular sieves -zeolites.The most common area of their application is the oil and gas industry.ZSM-5 type catalysts are highly effective catalysts for various processes in petrochemical and oil refining industry.Part of the reason for the wide popularity was the unique ability of these zeolites to catalyse the process of obtaining high-octane gasoline with high octane number [OH>95] from nonpetroleum feedstock, such as from methanol, as well as directly convert methanol or ethanol into a mixture of hydrocarbons [1,10].Therefore, all side study of physicochemical and especially energetic characteristics of zeolites of ZSM-5 type is of great theoretical and practical importance.
Suggested new applications of zeolites described in the literature include: molecular electronics, quantum dots/chains, zeolite electrodes, batteries, non-linear optical materials and chemical sensors.Recently, research has been reported on the use of zeolites as low dielectric constant materials for microprocessors [11].
Knowledge of the nature of active centres, and estimation of their concentration is a prerequisite for any attempt to classify adsorbents.Adsorption and catalytic properties of zeolites depend to a large extent on the structure of their porous structure, the number, strength and nature of the active centres they contain.Therefore, a comprehensive study of physicochemical and energetic characteristics of zeolites of ZSM-5 type is of great theoretical and practical importance.In this connection, the accumulation and systematisation of the most important thermodynamic characteristics of adsorption systems, one of the components of which is a zeolite, is of great importance.
The crystal structures of zeolites ZSM-5 and ZSM-11 are the most studied, and the sorption capacity of these zeolites owes much to the cavities that are formed as a result of the intersection of channels, which are different for ZSM-5 and ZSM-11 and apparently determine their different adsorption capacity.The lattice parameters and unit cell volume of ZSM-5 zeolite synthesised with different templates (amines and alcohols) were studied using X-ray diffraction analysis in [12].It is shown that the replacement of the templat molecule strongly affects the volume and parameter a of the unit cell, while parameters b and c change weakly.The authors conclude that the ZSM-5 crystal grows along the a direction.
In [13], the change in the unit cell parameters of zeolite ZSM-5 during adsorption of various organic dyes was studied by X-ray diffraction analysis.It is shown that the change in the zeolite lattice dimensions depends on both the nature of adsorbed molecules and their sizes.Adsorption of paraffins (C6, C8, C14) leads to an increase in all sizes of the zeolite unit cell, with the largest changes observed for tetradecane.The various physicochemical properties of ZSM-5 have been investigated using various characterisation techniques including X-ray diffraction, BET (Brunauer-Emmett-Teller) surface area, Fourier transform infrared and scanning electron microscopy [14][15][16][17].
The adsorption characteristics of pentasil towards molecules O2, N2, CO2, H2, CH3OH and many others have been studied in [18][19][20][21][22][23].Using adsorption microcalorimetry and vibrational spectroscopy techniques, they studied the room temperature interactions of CO2 with NaZSM-5 zeolite [18].They confirmed that these interactions consist of reversible two-step adsorption on approximately energetically equal and non-interacting adsorption centres, which constitute an ideal ensemble in the thermodynamic sense.From adsorption isotherms obtained by bulk adsorption and calorimetric methods, the standard enthalpy and Gibbs free energy changes for 1:1 and 1:2 adducts formed by CO2 molecules and Na + cations have been calculated.Standard entropy changes are also determined in this work.All vibrational modes were measured and the entropy of the adsorption phase was calculated by statistical mechanics method.The adsorption of carbon dioxide and para-xylene on ZSM-5 zeolites at 303 K was also investigated by calorimetric methods [24][25].It was shown that the measured adsorption isotherms are well described by the Dubinin-Radushkevich equation.This indicates that the adsorption of CO2 and p-xylene does not take place on the surface of the zeolite, but inside, in accordance with the mechanism of volume filling of micropores.The volume of micropores and the characteristic energy of adsorption were determined.
There is a large number of data on the adsorption of organic substances in pentasil-type zeolites, which were obtained by various physicochemical methods of investigation.However, the data obtained by adsorption-calorimetric method are few, which puts on the agenda the task of further detailed study of adsorption properties of ZSM-5 type zeolites towards hydrocarbon molecules as well as polar molecules and obtaining the main thermodynamic characteristics of these systems.In addition, the adsorption-calorimetric method applied in this work allows to reveal the mechanism of adsorption processes occurring on adsorbents and catalysts.Adsorption measurements can be used to obtain data on various factors (e.g.channel sizes, pore volume, cation localisation, etc.) related to the structure of a particular zeolite.Of particular interest is the study of adsorption of various substances in zeolites of the ZSM-5 type.Adsorption on these zeolites strongly depends on the cations present in the structure.Because of the large space between adsorption centres, the zeolite is ideal for model studies of adsorption behaviour.

Testing methods
The adsorption-calorimetric method used in this work makes it possible to obtain highly accurate molar thermodynamic and to reveal detailed mechanisms of adsorption processes occurring on adsorbents and catalysts.Adsorption measurements and adsorbate dosing were carried out using a universal high-vacuum adsorption unit.The unit allows adsorbate dosing by both gas-volume and volumetric-liquid methods.Methanol was dosed from a pre-calibrated microcapillary with a cross-section of 0.095 mm 2 , the liquid level in which was measured using a cathetometer B-630 with an accuracy of 0.01 mm.To prevent vapour condensation on the walls of the capillary, its temperature was maintained by an electric heater slightly higher than the temperature of the liquid in the capillary.To measure equilibrium pressures, we used a BARATRON B 627 membrane manometer.A modified DAC-1-1A thermally conductive microcalorimeter with high accuracy and stability was used as a calorimeter.Despite its external insulating shells, it is non-adiabatic because the heat released in it is introduced from the calorimeter chamber as it is released and dissipated in a large metal block.Although the temperature of the calorimetric chamber changes only slightly, the apparatus cannot be called strictly isothermal; it exhibits small changes in temperature, which are inevitable and form the basis of measurement.
Most of the heat (about 99%) released in the calorimeter chamber is dissipated in its block immediately after release.Only about 1% of the released heat remains in the calorimeter chamber, increasing its temperature very slightly.The measurement is mainly concerned with the heat flux that passes through the surface of the calorimeter chamber and the calorimeter block.

Results and discussion
To characterise the adsorption properties of zeolite, the differential heats and isotherms of methanol adsorption were measured and the entropy and free energy of methanol adsorption were calculated.The solvation of cations located on the surface and in the volume of microporous materials such as zeolites is a process of a more complex nature, since here the interaction of cations and adsorbate molecules with the anionic solid matrix must be taken into account, which may hinder or even prevent complexation in the cation/molecule system.Pentasil zeolites Cu 2+ ZSM-5 and NaZSM-5 possess absolutely identical alumina-silica frameworks and differ almost twofold in the amounts of exchangeable Cu 2+ and Na cations (0.3 and 0.5 mmol/g).One might have expected, since the charge of copper cations is twice that of sodium cations, that their adsorption properties towards the same substance (CH3OH) should also be essentially the same.Indeed, in fact, when investigating the differential heats of adsorption of methanol vapour at 303 K on these zeolites, signs of both similarity and substantial similarity of their adsorption properties were found [26][27][28][29].
Differential heats of adsorption of methanol vapour on zeolite Cu 2+ ZSM-5 are presented in Fig. 1.The of differential heats of adsorption of methanol on zeolite Cu 2+ ZSM-5 can be divided into 2 sections corresponding to the region of high heats of adsorption.These are adsorption of an average of 7 alcohol molecules per cation, when the heat varies from 115 kJ/mol to 52 kJ/mol; and a region of relatively low (48 kJ/mol) adsorption of another 6 alcohol molecules.The initial heat of adsorption (excluding adsorption on the impurity centre) is 115 kJ/mol.At higher fillings they pass the maximum and at adsorption of 0.3 mmol/g they fall again to the level of 110 kJ/mol, i.e. the curve of differential heats of adsorption forms a high-energy step at the level of 110 kJ/mol.The high-energy adsorption of methanol with Cu 2+ cation proceeds in a 1:1 ratio.This result also confirms our established phenomenon of diffusion of Cu 2+ cation from the side channels of the zeolite, as in the case of Li + cation [24][25].At a ratio of 1:1 all cations are located in the interstices, where the formation of S-dimensional complexes methanol/ Cu 2+ .
Then with increasing adsorption there is a consecutive formation of complexes of Cu 2+ cation with two, three, four, five, six and seven alcohol molecules with decreasing heat from 110 kJ/mol to 52 kJ/mol.The minimum on the curve corresponds exactly to the hepta complex Cu 2+ with seven methanol molecules.This complex due to its size can fit only in the interstices of the straight and zigzag channels of the zeolite.Adsorption of the next six molecules of methanol goes with a wave-like change of heat, then again decreases to the heat of condensation and goes already in the "silicate" part of the zeolite, i.e. in the part where there are no Cu 2+ cations.The total adsorption of methanol on zeolite Cu 2+ ZSM-5 is 13 molecules ( 4 mmol/g) of methanol per cation.
In general, the heat of adsorption of methanol (Fig. 1) on zeolite Cu 2+ ZSM-5 issignificantly higher than on NaZSM-5, LiZSM-5, CsZSM-5 and silicalite [26][27][28][29].The increased heat of adsorption of methanol on zeolite Cu 2+ ZSM-5 is characterised by high surface charge density of divalent copper cations.the zeolite to be the same as that of a normal liquid at the temperature of the experiment and calculate the volume occupied by a methanol molecule at saturation, it turns out that methanol occupies 0.17 cm 3 /g of the sorption volume of the Cu 2+ ZSM-5 zeolite, which is 91%.
The isotherm in semi-logarithmic coordinates is shown in Figure 2 and it confirms the energetic data.The equilibrium pressures at low fillings reach P/Ps=1.67 10 -6 , indicating strong sorption of methanol on zeolite Cu 2+ ZSM-5, but in general it is located below the curves of isotherms on zeolite NaZSM-5 and above the curves on silicalite [26][27][28].delayed from the entropy of liquid methanol, and is all below zero (Fig. 3).It confirms the strong 1:1 interaction of methanol with Cu 2+ cation.Sd initially decreases from -210 J/Kmol to -260 J/Kmol forming CH3OH/Cu 2+ monocomplex at 0.3 N/M adsorption.Further Sd gradually increases to -20 J/mol at adsorption of 2.1 mmol/g.Further adsorption takes place in the "silicate" part of the zeolite.Methanol molecules interact much more strongly in the "silicate" part than on Na cation and silicate.The average molar (-65 kJ/mol) entropy of adsorption indicates that the mobility of alcohol in zeolite Cu 2+ ZSM-5 is lower than the mobility of methanol in the liquid phase and close to its mobility in the solid phase.

Conclusions
Full thermodynamic characteristics of methanol adsorption on zeolite Cu 2+ ZSM-5 have been obtained.The step character of heat and entropy of methanol adsorption was revealed.The correlation between adsorption-energy characteristics was found and the molecular mechanism of methanol adsorption in zeolite Cu 2+ ZSM-5 in the whole filling region was revealed.It was found that Cu 2+ cations are located in shielded positions of the crystal lattice of zeolite ZSM-5.Methanol adsorption leads to the migration of Cu 2+ cations from the zeolite lattice into the intersections formed by the intersection of straight and zigzag channels and the formation of ion/molecular complexes of different multiplicity in them.Polar methanol molecules form high-energy heptocomplexes with Cu 2+ cation and are located in the first coordination sphere in Cu 2+ ZSM-5 zeolite.The average molar entropy of methanol adsorption indicates that in the zeolite the mobility of methanol molecules is lower than the liquid phase and close to the mobility of the solid phase.

Fig. 1 .
Fig.1.Differential heats of methanol adsorption on Cu 2+ ZSM-5 zeolite.The horizontal dashed line is the heat of condensation.The adsorption isotherm of methanol on zeolite Cu 2+ ZSM-5 was brought to 4.1 mmol/g at relative pressures P/Ps =0.76 (or to P=115 torr).If we assume the density of methanol in