Basic thermodynamic characteristics of toluene adsorption in Cu2+ZSM-5 Zeolite

. This paper presents the results of isotherms and basic (  H,  F and  S) thermodynamic characteristics of toluene adsorption in CuZSM-5 zeolite. For measurements of isotherms and differential heat of adsorption, a system consisting of a universal high-vacuum adsorption unit and an attached differential modified Tian-Calve microcalorimeter DAC-1-1A was used to provide direct quantitative and qualitative characteristics of the nature and forces of adsorption interaction. The correlation between the adsorption-energy characteristics was found and the molecular mechanism of toluene adsorption in CuZSM-5 zeolite in the whole filling region was revealed. Toluene adsorbed in CuZSM-5 zeolite is located in the first coordination sphere with Cu 2+ cation, forming two-dimensional complexes. It was found that the charge density significantly affects the mechanism, the energy of adsorption and the number of adsorbed molecules. It was determined that the average molar entropy of toluene adsorption in CuZSM-5 zeolite indicates that the mobility of toluene molecules in zeolite is below the liquid phase and close to that of the solid phase, indicating that the mobility of toluene molecules on cations is strongly inhibited. This work shows how calorimetric data can be used to complement crystal structure results and detect subtle adsorbent/adsorbate interactions at the molecular level.


Introduction
Volatile Organic Compounds (VOC's) visibly pollute the atmosphere and harm human health. These compounds destroy the ozone layer (1 molecule of chlorine compound destroys 100,000 molecules of ozone). The products of human industrial activity include aromatic hydrocarbons, chloralkenes, alcohols, carbon dioxide, which are strategic gases in many industrial processes. Large quantities of carbon dioxide are emitted into the atmosphere, causing global environmental problems. One way of recovering carbon dioxide is by using an adsorption process. Recently, the adsorption method has been increasingly used to reduce and control emissions as the most efficient and economical. The potential material for selective adsorption and separation of carbon dioxide into toxic chemicals is adsorbents. Therefore, a comprehensive study of the physico-chemical and especially the energy characteristics of adsorbents is of great theoretical and practical importance.
The phenomenon of adsorption of gases and vapours underlies many chemical and biological processes and the interest of researchers has long been driven by the need to solve purely practical problems. The study of adsorption energy of gases and vapours is of paramount importance not only for understanding interesting phenomena and practically valuable processes occurring on various synthetic zeolites, but also for accumulation, systematization and standardization of the most important thermodynamic characteristics of heterogeneous systems, one of whose components is zeolite.
Natural and synthetic zeolites have found wide application as adsorbents. Synthetic zeolites are aluminosilicate, microporous adsorbents with not only high selective adsorption but also the ability to separate substances, with different size, shapes of adsorbate molecules. They are distinguished by their strictly crystalline structure and large specific surface area. The zeolite pores are spherical cavities connected by channels. Nowadays industrial companies manufacture different grades of zeolites, which have different cations and sizes of pores. The most practical application is for zeolites, type A, X and ZSM-5.
Zeolites are most widely used in gas and petrochemicals, ion exchange (water purification and softening), adsorption and separation of vapours and gases and removal of impurities (particularly environmentally and biologically harmful) from gases and solutions. In addition, zeolites are increasingly being used in ecology, agriculture, animal husbandry, paper industry and construction [10].
Anticipated 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][12].
Nanoporous molecular sieves -zeolites -are currently one of the most sought-after materials for selective adsorption and separation in the world. They are of great interest due to the possibility to adjust their texture (pore size and architecture), chemical properties (Si/Al ratio) and the nature of off-lattice cations, which influence the adsorption and catalytic properties of crystalline materials regulated by the structure, number and nature of the active centres they contain. Today, vapour and gas adsorption processes are major chemical processes in the world. Therefore, a certain (clear) chemical composition and structure of zeolites enables their production and theoretical and practical application.
A comprehensive study of physical-chemical and especially energetic characteristics of zeolites of ZSM-5 type is of great theoretical and practical importance. Energy characteristics, due to strict definiteness of chemical composition and crystal structure of zeolites, should be well reproduced and can be applied in practical calculations or theoretical discussions. However, a number of questions on the specificity of zeolite structure, especially those related to the problem of spreading of active centres, mechanism of adsorption of polar and non-polar molecules, nature and participation of defects in adsorption, are still unclear. The solution of these questions is decisive for the successful application of adsorbents and the directional regulation of their properties, and also makes it possible to study and establish general regularities of adsorption and catalysis.
Among aromatic hydrocarbons, benzene, toluene and p-xylene are the easiest to adsorb on zeolites of ZSM-5 type (all of them have kinetic diameter of 0.58 nm). The adsorption of benzene, toluene and p-xylene on a silicate has been studied in [27][28][29][30][31][32]. The differential heats of adsorption of benzene and toluene on silica have a complex dependence on the degree of filling [28][29][30][31]. The adsorption heat curves for benzene and toluene showed minima and maxima. The authors attributed them to redistribution and reorientation of the adsorbed molecules at fills above 0.5.
Toluene is the only aromatic hydrocarbon whose adsorption does not cause a change in zeolite lattice size. In the adsorption of aromatic hydrocarbons, the influence of molecular size is more important than their nature and the adsorption of all aromatic hydrocarbons, larger than toluene, causing a decrease of lattice constants a and c and an increase of constant b [32].
A large number of data on the adsorption of organic substances in pentasil-type zeolites are available, which have been obtained by various physico-chemical methods of investigation. The adsorption-calorimetric method applied in this work reveals the mechanism of adsorption processes occurring on adsorbents and catalysts.
Differential heat of adsorption together with other differential adsorption energetic characteristics (enthalpy, free energy and entropy) most fully characterise the physical, chemical, crystallochemical and geometrical nature of the adsorbent surface and allows the study of adsorption phenomena on a molecular structural level.

Testing methods
To solve the problem the adsorption-calorimetric method of investigation was applied, giving directly quantitative and qualitative characterization of the nature and forces of adsorption interaction. For measurements of isotherms and differential heat of adsorption, a system 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.
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.
In this work isotherms, differential heat, entropy and kinetics of toluene adsorption Cu 2+ ZSM-5 (Si/Al=27,5) at 303 K were studied. Before starting the experiment, the adsorbent was evacuated at 723 K for 10 hours to high vacuum (10 -5 torr).

Results and discussion
Organic substances whose molecular size in cross-section does not exceed 5.5A 0 , are well adsorbed on all pentasils, but in different amounts (volumes). For example, while n-paraffins entirely fill the entire sorption volume of a silicate or ZSM-5, densely arranged in all channels by the end-to-end mechanism, benzene adsorbs in smaller amounts (60%) and appears to fill straight and zigzag channels [33]. The authors [33,34] believe that aromatic hydrocarbons are adsorbed only in the straight channels and in the intersections of the ZSM-5 zeolite. However, what is the molecular mechanism of adsorption of benzene and toluene on pentasils is not yet clear. For example, it was shown in [35] that adsorption of benzene on a silicate is accompanied by complex changes, both in the shape of the isotherm and, especially, in the shape of the differential heat dependence curve on the adsorption values. These complex changes were attributed by the authors to various reorientations and redistributions of the adsorbate in the silicate channels, but the specific mechanisms of these changes were not considered.
The adsorption isotherm of toluene in Cu 2+ ZSM-5 zeolite in semi-logarithmic coordinates is shown in Figure 1. of toluene in zeolite Cu 2+ ZSM-5. The adsorption isotherm is brought to 1.12 mmol/g at relative pressures of P/Ps=0.64 (or up to 24 torr). If the density of toluene in zeolite is assumed to be the same as that of a normal liquid at temperature of experiment and the volume occupied by toluene molecule at saturation is calculated, it turns out that toluene occupies 0.102 cm 3 /g sorption volume of zeolite Cu 2+ ZSM-5, which is 62 %. Figure 1 shows that the adsorption isotherm of toluene on Cu 2+ ZSM-5 is S-shaped with an almost vertical section in the middle. At small P/Ps5,2 10 -5 fills it rises rapidly upwards, indicating strong sorption of toluene. At medium fillings (0,6 mmole/g and P/Ps0,07) the isotherm forms a step. The content of Cu 2+ cations, according to the chemical composition of EС (elementary cell), is 0.3 mmole/g, i.e., each cation interacts with two toluene molecules.
Apparently, a sandwich-like -complex is formed with Cu cation 2+ in the middle. The only question is where this complex is located -either completely in the channel intersections (obvious for tetrahedral complexes, e.g., with alcohols [36]) or in the straight channels with the toluene molecules strongly protruding into the channel intersections (less likely model). Apparently, the benzene rings form a dihedral angle equal to the angle between the zigzag. The ring planes are oriented perpendicular to the axes of the zig-zag channels.
With this conformation of the adsorption complex copper cations can simultaneously interact with -electrons of benzene rings and with oxygen atoms of the lattice, and hydrogen atoms of toluene molecules -with oxygen atoms of zigzag channels.
At P/Ps=0.4 the fill degree of 0.76 (0.843 mmole/g) is achieved. Thus, the isotherm fits almost entirely within the narrow range P/Ps from 5.2 10 -5 to 0.4. Such a shape of the isotherm is characteristic of phase transformations of adsorbed substance [37] and is one of the main phenomenological features of homogeneity of the sorption system as a whole [38]. Comparing with the adsorption isotherm of benzene on silicate [39] one could expect even more complicated character of isotherm changes with Cu 2+ ZSM-5 filling (due to additional appearance of Al and Cu ions in silicate structure) but it didn't happen. Even more significant changes took place in differential heat curve when comparing it for silicate shown in Figure 2 [40,41]. Numerous minima and maxima have disappeared. The heat of adsorption of molecules on ZSM-5 zeolites is characterized by a stepwise decrease indicating stoichiometric interaction of these molecules with lattice negative charge compensating Me + cations [42,43].  The differential heat of adsorption of toluene in Cu 2+ ZSM-5 zeolite is shown in Figure  2. The adsorption heats for toluene are rather complex. Figure 2 shows that the adsorption heats of toluene in Cu 2+ ZSM-5 zeolite changes stepwise with increasing filling. The initial heat of adsorption (without taking into account adsorption on the impurity centre) is 116 kJ/mole. Depending on the adsorption filling, the enthalpy forms a step and decreases from 116 kJ/mole to 84 kJ/mole with an adsorption of 0.3 mmole/g.
The content of copper cations, according to the chemical composition of EС, is 0.3 mmole/g, zeolite i.e., the heat drop from 116 kJ/mole to 84 kJ/mole for the adsorption of 0.3 mmole/g associated with the adsorption of one toluene molecule on one Cu 2+ cation. The amount of high-energy adsorbed toluene corresponds to the scheme 1С6H5CH3:Cu 2+ . With increasing adsorption there is the formation of Cu 2+ cation complexes with two toluene molecules with a decrease in heat from 84 kJ/mol to 70 kJ/mole at 0.6 mmole/g adsorption, i.e., forms 2С6H5CH3:Cu 2+ . Further adsorption proceeds in zigzag channels (second stage) and in straight channels (third stage) of zeolite, i.e., the part where no Cu 2+ cations, because they correspond to the heat of adsorption of toluene in these channels (numerous data on the heat of adsorption of benzene on different cationic forms of ZSM-5 zeolite and silicalite). Considering that the dispersion potential in zig-zag channels are higher, it can be assumed that adsorption first takes place in the zig-zag channels. After zig-zagging toluene is adsorbed in straight channels. The reason for the inflated heat at high fills seems to be the lithium cations, with which toluene can interact due to the induction effect.
The corresponding entropy diagram is shown in Figure 3. As can be seen, Sd is significantly lower than the entropy of liquid toluene. Consequently, the adsorption thermodynamics definitely indicate a strongly localized nature of the adsorbed toluene on Cu 2+ ZSM-5. Sd gradually increases from -185 J/K mole to a maximum value of -20 J/K mole at 0.3 mmole/g adsorption.
The entropy also confirms the strong interaction of toluene with the Cu 2+ cation up to filling 0.3 mmole/g, as well as in the heat and adsorption isotherm, i.e., strong interaction of toluene with Cu 2+ cation in a 1:1 ratio. Further S d changes in a wave-like manner and rises sharply to the entropy of liquid toluene at 1.11 mmole/g. The time of establishment of adsorption equilibrium of toluene on Cu 2+ zeolite ZSM-5 ( Figure 4) at small fillings up to 0.1 mmole/g varies from 6.7 hours to 8.2 hours and further decreases until filling 0.6 mmole/g (until complete molecules/cation complex formation in the ratio С6H5CH3:Cu 2+ -2:1)). Starting from 0.6 mmole/g filling the adsorption process slows down dramatically, reaching a maximum (3.5 hours) at 0.72 mmole/g filling. Then the adsorption rate gradually increases and an equilibrium is reached at 1.11 in 1 hour.

Conclusion
Adsorption of toluene in zeolite Cu 2+ ZSM-5 has been calorimetrically studied. Isotherms and full thermodynamic (H, F and S) characteristics of toluene adsorption in zeolite Cu 2+ ZSM-5 were obtained. The correlation between the adsorption-energy characteristics was found and the molecular mechanism of toluene adsorption in Cu 2+ ZSM-5 zeolite in the whole filling region was revealed. Toluene adsorbed in Cu 2+ zeolite ZSM-5 is located in the first coordination sphere with Cu 2+ cation, forming dimeric complexes. The heat of adsorption correlates with the number of Cu cations in the zeolite structures. It was shown that adsorption properties of ZSM-5 zeolite depend on the type of cation as well as on the structure of fragments of ZSM-5 zeolite structure. It was found that the charge density has a significant effect on the mechanism, the adsorption energy, and the number of adsorbed molecules. The average molar entropy of toluene adsorption in Cu 2+ ZSM-5 zeolite was determined, indicating that the mobility of benzene molecules in the zeolite is below the liquid phase and close to the mobility of the solid phase, indicating a strong inhibition of the mobility of adsorbed toluene on cations.