Differential heats, isotherm and entropies of n-pentane adsorption on LiZSM-5 and CsZSM-5 zeolites

. Artificially synthesised zeolites are widely used in water purification devices as adsorbents, ion exchangers, molecular sieves; they are used as electron donors and acceptors. Also, zeolites are currently the most important catalysts for the processing of various hydrocarbon raw materials. Synthetic zeolites ZSM-5 are highly efficient catalysts of these processes. For o characterisation of zeolite channels and estimation of sorption volume n-pentane are the most suitable. n-Pentane fills all sorption space and adsorbed with higher energy. This paper presents the results of the basic thermodynamic characteristics and isotherms of n-pentane adsorption in Cs 3,17 ZSM-5 and Li 3,37 ZSM-5 zeolites at 303 K. A system consisting of a universal high-vacuum adsorption unit and a Tian-Calvet type differential modified microcalorimeter, DAC-1-1A, coupled to it was used to measure isotherms and differential heats of adsorption. The correlation between adsorption-energy characteristics was found and the molecular mechanism of n-pentane adsorption in CsZSM-5 and LiZSM-5 zeolites in the whole filling region was revealed.

At present, the problem of reducing the release of nitrogen oxides into the atmosphere is the most important one for environmental protection.Therefore, this problem is currently being intensively investigated by numerous groups in both academic and industrial laboratories.
Zeolite catalysts show such high activity for a wide range of chemical reactions that their activity is compared with that of enzymes.However, due to their diversity and the multiplicity of their active centres, the mechanism of action of these catalysts is still a matter of debate.
A special place is occupied by zeolites of ZSM-5 type with different cations due to its ability to decompose nitrogen oxides to elemental N2.There are several ways of NO decomposition: 1.Selective catalytic reduction of NO with ammonia, typical for chemical plants and stationary power plants; 2.Catalytic reduction of NO in the presence of CO, typical for control of car exhaust gases; 3.Selective catalytic reduction of NO in the presence of hydrocarbons, a method that has not yet found industrial application, but can be used both for control of car exhaust gases and emissions from various industrial plants; 4.Direct decomposition of NO.
All of these directions are being intensively researched.
The crystal structures of ZSM-5 and ZSM-11 zeolites are the most studied, and the sorption capacity of these zeolites owes much to the cavities that are formed by the intersection of channels, which are different for ZSM-5 and ZSM-11 and apparently determine their different adsorption capacity [11][12][13][14][15][16].
The lattice parameters and unit cell volume of zeolite ZSM-5 synthesised with different templates (amines and alcohols) were studied by X-ray diffraction analysis in [17][18][19][20].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 [21], 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.
It has been shown by optical microscopy and X-ray diffraction analysis that silicalite-1 crystals expand upon adsorption of alkanes C4-C8 and isobutane at room temperature [22].The linear expansion of 200 μm size crystals in the c-axis direction is 0.20-0.45%.For adsorption of n-pentane, n-hexane and n-heptane, the maximum linear expansion in the b direction is ≈0.54%.When hexane and n-heptane are adsorbed, the maximum volumetric expansion is 1.2%.At the same time at adsorption of benzene the unit cell volume of zeolite almost does not change.The expansion in the a direction for all molecules, except isobutane, is the smallest.During adsorption of n-hexane, the expansion at 180 K is larger than at room temperature.It was observed that the expansion of ZSM-5 zeolite crystals upon adsorption of n-alkanes reduces the size of defects in the polycrystalline zeolite membrane and changes the flux of n-octane through the defects.The adsorption-induced expansion of zeolite crystals is reversible.
In the work of Tamm H. and Stach H. [23] on the basis of calorimetric studies and data [24] it was proposed to divide 1/4 of the zeolite unit cell into three adsorption sites with approximately equal volume: 1st -pores between neighbouring junctions in straight pores; 2nd -pores between neighbouring junctions in zigzag pores; 3rd -pore junction.According to [25] the unit cell of silicalite consists of two straight and four zigzag channels.The lengths of straight and zigzag channels were also determined, which are 2.006 and 0.67 nm, respectively.Close values were obtained by the authors [26] -1.98 and 0.67 nm.
The authors [27][28][29][30][31] studied adsorption of alkanes and alkenes in zeolites by comparing adsorption characteristics for three types of ultrasilica: ferrierite, ZSM-5, and mordenite.The activation energy for the diffusion of propane and n-butane on ferrierite and the heat of adsorption of alkanes C2-C4 and alkenes in zeolites and silica were calculated on the basis of IR-Fourier spectroscopy data; the diffusion processes in micropores were evaluated by comparing the results with previously published activation energies for the diffusion of nbutene.
There is a large number of data on the adsorption of hydrocarbons 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 zeolites of ZSM-5 type with respect to hydrocarbon molecules as well as polar molecules and obtaining the main thermodynamic characteristics of these systems.
We set out to investigate the differential heats of adsorption of n-pentane in zeolites Cs3,17ZSM-5 and Li3,37ZSM-5.These data could provide information on the state of the Cs + and Li + cations, adsorption mechanism, conformation and localisation of the adsorbate/cation complex.In turn, this information could facilitate the understanding of the mechanism of catalytic processes.

Testing methods
To solve the problem the adsorption-calorimetric method of investigation was applied, giving directly quantitative and qualitative characterisation of the nature and forces of ad-sorption interaction.For measurements of isotherms and differential heat of adsorption, a sys-tem consisting of a universal high-vacuum adsorption unit and a Tian-Calvet-type, DAC-1-1A thermally conductive differential microcalorimeter connected to it was used, which has high accuracy and stability.The instrument's calorimeter sensitivity is extremely high and its reliability is high (it can measure about 0.2 μW thermal power).The calorimeter makes it possible to obtain the thermokinetics of the process of the adsorption systems under study, which is very important for elucidating the mechanism of adsorption [12][13][14][15][16].
Most of the heat (about 99%) released into the calorimeter chamber is dissipated into the calorimeter block immediately after release.Only about 1% of the heat released remains in the calorimeter chamber, raising 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.
The adsorption-calorimetric method used in this work provides highly accurate mole thermodynamic characteristics and reveals 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 volume-liquid methods.We used a BARATRON B 627 membrane pressure gauge to measure the equilibrium pressures.

Results and discussion
Differential heats of adsorption of n-pentane on LiZSM-5 zeolite at low fillings (up to 0.29 mmol/g) decrease linearly from 84 kJ/mol to 70 kJ/mol (Fig. 1) and almost do not change up to adsorption of 0.55 mmol/g.The content of lithium cations, according to the chemical composition of EJ, is 0.577 mmol/g, i.e., the amount of adsorbed n-pentane follows the pattern C5H12:Li + .The reason for the overestimated heats at low fillings seems to be lithium cations, with which n-pentane can interact due to the induction effect.At high fillings (starting at 0.55 mmol/g), there is first an increase in heat to 73 kJ/mol, followed by a slight decrease to 71.5 kJ/mol and forms 2C5H12:Li + ion-molecular complex at an adsorption of 1.16 mmol/g, although it is known that in the case of silicalite the heats increase strongly with occupancy from 64 kJ/mol to 73 kJ/mol, the heats in NaZSM-5 and HZSM-5 zeolites are almost unchanged (Fig. 1) [33][34][35].Subsequent molecules of n-pentane are adsorbed already without the participation of cations and their heat level is as in the case of silicalite, i.e. cationless structure (Fig. 1) [33].The total adsorption is 3 molecules of n-pentane per cation.The adsorption isotherm of n-pentane on zeolite LiZSM-5 in semi-logarithmic coordinates is presented in Fig. 2. The equilibrium pressures at low fillings reach relative pressures P/Ps=410 -7 , which indicates the strong adsorption of n-pentane on zeolite LiZSM-5.The adsorption isotherm was brought to 1.72 mmol/g at relative pressures P/Ps=0.57(or up to 353 torr).
The adsorption isotherm of n-pentane on LiZSM-5 zeolite corresponds to differential heats of adsorption (Fig. 2).The isotherm concave at low pressures (P/Ps=610 -6 and 0.3 mmol/g), indicating the existence of strongly interacting adsorption centres.Starting from P/Ps=0.00004 (from 0.66 mmol/g) rises up to relative pressures P/Ps =0.000065 at adsorption of 1 mmol/g and forms a step.The adsorption isotherm corresponds to the differential heats of adsorption and confirms the formation of ion-molecular complex 2C5H12:Li + at adsorption of 1.16 mmol/g.Further, the relative pressures increase rapidly to P/Ps=0.57at adsorption of 1.72 mmol/g.The adsorption isotherm of n-pentane on LiZSM-5 zeolite is described by the two-term VMOT equation [36][37]: (1) Fig. 2 shows that the calculated data are in good agreement with the experimental data.
The molar differential entropy of n-pentane adsorption on zeolite LiZSM-5 is presented in Fig. 3, where the entropy of liquid n-pentane is taken as zero.Fig. 3 shows that the curves Sa in the whole region of zeolite channels filling with n-pentane are in the negative region of liquid n-pentane entropy values, which indicates a denser packing of adsorbed molecules in the channels of zeolite LiZSM-5 than on zeolite NaZSM-5 [35].The entropy diagram shows strong dispersion adsorbate-adsorbent interaction.At adsorption of 1.3 mmol/g Sa decreases to -91 J/K mol, which indicates a strong limitation of mobility of n-pentane molecules in the saturation region.The adsorption entropy also confirms the formation of 2C5H12:Li + ion-molecular complex at adsorption of 1.25 mmol/g.Fig. 4 shows the thermokinetics of n-pentane adsorption on LiZSM-5 zeolite.At low fillings the process is established in 10 hours.At higher fills, the adsorption rate gradually accelerates to 2 hours at 0.66 mmol/g adsorption.The thermokinetics curve then passes E3S Web of Conferences 458, 02008 (2023) EMMFT-2023 https://doi.org/10.1051/e3sconf/202345802008through a maximum (3 hours) at an adsorption of 0.77 mmol/g.Then the adsorption process accelerates again and at adsorption of 1.5 mmol/g it stabilises and equilibrium is established in 50 minutes.The entropy and adsorption kinetics are consistent with the differential heats of adsorption and also confirm the formation of ion-molecular complex 2C5H12:Li + at adsorption of 1.25 mmol/g.Differential heats of adsorption of n-pentane on zeolite CsZSM-5 are presented in Fig. 1.The heats of adsorption vary little (1 kJ/mol) from low fillings to high fillings (1.2 mmol/g).The content of cesium cations, according to the chemical composition of EY, is 0.54 mmol/g, i.e. it forms 2C5H12:Cs + ion-molecular complex at adsorption of 1.2 mmol/g.The reason for the higher heats on CsZSM-5 than on adsorption on silicalite up to 1.2 mmol/g fills seems to be cesium cations, with which n-pentane can interact due to the induction effect.Starting from the fillings of 1.2 mmol/g the heat of adsorption drops linearly from 70 kJ/mol to the level of the heat of condensation of 26.2 kJ/mol.When comparing the lines of curves of heat of adsorption of n-pentane on zeolites ZSM-5 and on silicalite (Fig. 1), it is seen that they are located at the same level [33][34][35].The adsorption isotherm of n-pentane on zeolite CsZSM-5 in semi-logarithmic coordinates is presented in Fig. 2. The equilibrium pressures at low fillings reach relative pressures P/Ps=5 10 -6 , which indicates a stronger adsorption of n-pentane on zeolite CsZSM-5 than on silicalite [33].The adsorption isotherm was brought to 1.5 mmol/g at relative pressures P/Ps=0.38 (or up to 235 torr).In general, the adsorption isotherm curve on CsZSM-5 zeolite is located below the isotherms on LiZSM-5 and NaZSM-5 zeolites, indicating weaker adsorption on it [35].The adsorption isotherm of n-pentane on CsZSM-5 zeolite is described by the trinomial VMOT equation from low fillings up to 1.35 mmol/g [36][37]: Fig. 3 shows that the calculated data are in good agreement with the experimental data.The molar differential entropy of adsorption of n-pentane on zeolite CsZSM-5 is presented in Fig. 4. In general, the entropy is located below the entropy of liquid n-pentane and the entropy on LiZSM-5 and NaZSM-5 zeolites, but above the entropy of adsorption on silicalite [33][34].The entropy diagram shows a strong adsorbate-adsorbent dispersion interaction.At adsorption of 1.1 mmol/g, the entropy decreases to -97 J/Kmol, which indicates a strong limitation of mobility of n-pentane molecules in the saturation region.Fig. 3 shows that the entropy minimum on ZSM-5 zeolites is almost at the same adsorption amounts (1.2 mmol/g).
The thermokinetics of n-pentane adsorption on CsZSM-5 zeolite is presented in Fig. 4. Adsorption equilibrium depending on the value of n-pentane adsorption on zeolite CsZSM-5 is established faster than on zeolite LiZSM-5.Up to 0.6 mmol/g the adsorption process is established on average in 1 hour.Further it is established in 30 minutes.

Conclusions
Adsorption-calorimetric studies of adsorption of n-pentane molecule on zeolites Cs3,17 ZSM-5 and Li3,37ZSM-5 have been carried out.Complete thermodynamic characteristics of the adsorption of n-pentane on zeolites Cs3,17ZSM-5 and Li3,37ZSM-5 have been obtained.The adsorption isotherms are described by the equations of the bulk micropore filling theory (VMOT).The stepwise character of the heats of adsorption of n-pentane on zeolite Li3,37ZSM-5 was revealed.The extent of the region of high heats correlates with the number of lithium and caesium cations in the zeolite structures.It is shown that the adsorption properties of ZSM-5 zeolites depend on the type of cation as well as on the structure of fragments of the ZSM-5 zeolite structure.n-pentane adsorbed in LiZSM-5 and CsZSM-5 zeolites is located in the first coordination sphere of Li + and Cs + cations, forming diammonium complexes.These complexes are located at the intersections of straight and zigzag channels.The n-pentane molecules form a monocomplex with Li cation + and a bicomplex with Cs cation.