Effect of transition metal cations on the commensurate freezing of n-hexane confined in micropores of ZSM-5 J. Serb. Chem. Soc. 80 (10) 1297–1309 (2015) UDC 547.216+541.183+549.67: JSCS–4798 536.51:536.75 Original scientific paper 1297 Effect of transition metal cations on the commensurate freezing of n-hexane confined in micropores of ZSM-5 RADMILA HERCIGONJA1*, VLADISLAV RAC2, VESNA RAKIĆ2 and ALINE AUROUX3 1Faculty of Physical Chemistry, University of Belgrade, 11000 Belgrade, Studentski trg 12, Serbia, 2Faculty of Agriculture, Nemanjina 6, 11080 Belgrade-Zemun, Nemanjina 6, Serbia and 3Université Lyon 1, CNRS, UMR 5256, IRCELYON, Institut de recherches sur la catalyse et l’environnement de Lyon, 2 avenue Albert Einstein, F-69626 Villeurbanne, France (Received 3 February, accepted 21 April 2015) Abstract: Besides its importance concerning fundamental studies on gas ads- orption in narrow pores, investigation of the commensurate freezing of a fluid within a zeolite is of practical importance in the application of zeolites in the processes of adsorption, separation and catalysis. In this work, the adsorption of n-hexane on HZSM-5 and its transition metal ion-exchanged modified forms was studied at 303 K by means of microcalorimetry. The thermal molar entro- pies changes of adsorption were calculated and thereby, the freezing-like behaviour of n-hexane inside the structure of the zeolite as a confinement media was noticed. This effect is governed by the attractive interactions between n-hexane molecules and the pore walls, and is influenced by the length of the pores and the nature of the charge-balancing cations. Among the inves- tigated zeolites, a solid-like phase of n-hexane in the pores of zeolites with Fe(II) ions was the most similar to solid bulk n-hexane, while Cu(II) ions con- tributed to the lowest ordering of the obtained solid-like n-hexane phase. Keywords: confinement media; adsorption; entropy; microcalorimetry; ZSM-5. INTRODUCTION The confinement of fluids in limited spaces, such as narrow pores, is a very interesting phenomenon. The behaviour of fluid confined in a pore is influenced by many factors, such as pore size and geometry, and the atomic structure of the pore surface. Consequently, its properties are distinctly different from those of bulk phase. For example, a confined fluid can have a higher density or can be in a different aggregation state from its analogue under normal conditions. There are many experimental and theoretical studies reporting that the phase behaviour of * Corresponding author. E-mail: radah@ffh.bg.ac.rs doi: 10.2298/JSC150203032H _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ 1298 HERCIGONJA et al. different fluids under extreme confinement is qualitatively different from that of the bulk.1–13 In the confinement medium, the presence of fluid–pore wall and fluid–fluid forces can lead to interesting surface-driven phase changes. These include new types of phase transitions not found in the bulk phase and shifts in transitions. In a bulk system, freezing is considered a first-order phase transition accompanied by an infinitely sharp change in a suitable order parameter, usually density or composition. In a confinement medium, the freezing phase transition is possible but with evident effects of confinement. The freezing temperature of a fluid confined in pores is determined by the bulk freezing temperature, pore wall–solid and pore wall–fluid surface tensions, the molar volume of the liquid, the latent heat of melting in the bulk and pore width.14 A decrease or increase in the freezing temperature due to confinement is strongly affected by the strength of the attractive forces between the fluid molecules and the pore walls.14,15 For repulsive or weakly attractive pore walls, the shift in the freezing temperature is negative.16,17 For highly attractive adsorbents where the adsorbate–pore wall interactions are strong compared to the adsorbate–adsorbate interactions, an inc- rease in freezing temperature over the bulk value is observed.15,18,19–22 Zeolites are regular crystalline solid microporous materials with peculiar structural characteristics (a three-dimensional lattice with well defined pores, high internal surface area and curvature, high ion-exchange capacity and remark- able thermal stability). Zeolites are extensively used in the chemical industry as catalysts and for the separation of gases, particularly hydrocarbons.23–25 There- fore, the adsorption of hydrocarbons, especially n-alkanes, has been widely stu- died.4,6,26–29 According to the molecular dimensions of their pores, zeolites may be considered as confinement media and the effect of confinement on fluids adsorbed in zeolites may be expected. For example, investigations of hexane and heptane adsorption at room and at slightly higher temperatures on silicalite-1 and ZSM zeolite revealed anomalous behaviour of these two hydrocarbons compared to other alkanes.2–6 An explanation was given for the first time by Smit and Maesen.30 Their interpretation is that the adsorption of straight chain hydrocar- bons on silicalite leads to a phase transition of the hydrocarbons inside the pores of the silicalite. Bearing in mind that silicalite-1 and ZSM have two types of channels, straight and zigzag (sinusoidal) connected via intersections, they showed that phase transition occurred when the lengths of the adsorbed mole- cules were similar to the length of the channels. Indeed, the length of n-hexane and heptane molecules are 1.03 and 1.16 nm, respectively, while the length between the centres of channel intersections of the silicalite structure is 1.2 nm, i.e., they are comparable. Under the above conditions, fluid can freeze in a con- figuration that is commensurate with the pore structure.30–33 Phase transitions of hexane and heptane between the gas, liquid and solid phases in the pores of silic- alite-1 and ZSM are generally accepted and were the subject of many studies.34–41 _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ COMMENSURATE FREEZING OF N-HEXANE IN MICROPORES OF ZSM-5 1299 Commensurate adsorption of hydrocarbons (e.g., p-xylene, n-hexane, n-heptane, benzene, etc.) was found in several different types of zeolites, for instance, MFI, ITW, ERI, CHA, LTA, AFX and silicalites.42–46 Evidence for freezing of n-hexane molecules inside the channels of ZSM can be seen from the adsorption isotherms, which can show a step or kink when half of the maximum loading (about 4 molecules per unit) is achieved; such isotherms were obtained for adsorption measurements at temperatures above 338 K.4,6,30,47–49 Lohse et al.50 did not provide evidence for a kink at half of the loading because the temperature of adsorption (298 K) was too low. Addition- ally, Meansen30 showed in a simulation that the kink becomes more pronounced with increasing temperature. The adsorption isotherms measured by Zhu et al.51 showed a kink in the temperature region 338–373 K, but not at 303 K. The volu- metric adsorption isotherms of n-hexane on HZSM-5 zeolite and its transition metal modified forms were reported,52 but they did not show a kink since a low temperature of adsorption (303 K) was applied. In addition to isotherms, temperature programmed desorption (TPD) profiles also gave evidence of commensurate freezing of n-hexane in the zigzag channels of zeolite. The TPD studies showed that among the linear alkanes, hexane and heptane behave distinctly differently.28,32,44,45,48,53–57 While other linear alk- anes showed a single desorption step, n-hexane and especially n-heptane exhi- bited two-step desorption profiles. The first desorption peak of n-hexane and n- heptane occurred at lower temperatures than expected based on the chain length of these two n-alkanes. This low temperature peak corresponds to desorption from the zigzag channels and should be caused by a relatively high gain in entropy upon desorption compared to the other n-alkanes. The relatively high gain in entropy upon desorption can only be the result of a low entropy value in the constrained position of the adsorbed n-hexane and n-heptane molecules at high loadings. Partial desorption then allows a rearrangement of the adsorbed n-hexane or n-heptane molecules, resulting in an ordering similar to the ordering of the other n-alkanes with normal entropy values. The high temperature peak corresponds to desorption from the straight channels and it occurs at tempera- tures that are in accordance with the chain length of the n-alkane. It was reported58 that the TPD profiles of n-hexane sorbed on ZSM-5 were composed of two well-defined peaks in the temperature region from 300 to 550 K, consistent with the results of other researchers.28,32,44,54,56 Accordingly, this indicated that the phase transition of freezing occurred when n-hexane was adsorbed on the samples of ZSM used in the present study. In the last decade, among the transition metal ion-containing zeolites, the Fe-, Cu- and Mn-MFI zeolites have received much attention because they are catal- ytically active in some important reactions, such as N2O and NO decomposition, reduction of NOx to N2 in the reaction with various hydrocarbons59–61 and catal- _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ 1300 HERCIGONJA et al. ytic cracking of hydrocarbons.62–64 Many experimental results indicate that the physical adsorption of n-alkane reactants contributes to the kinetics of catalytic reactions.65–67 In this work, the phase transition of commensurate freezing of n-hexane confined in microporous of HZSM-5 and in its forms modified by different charge balancing cations: Cu(II), Mn(II) and Fe(II), was investigated. It is known that attractive adsorbate–pore wall interactions in zeolite partly originate from the charge balance cation–adsorbate interactions, and hence, an influence of tran- sition metal cations on the adsorption of n-hexane and on its commensurate freezing could be expected. Recently, molecular dynamics (MD) simulation stu- dies31 clearly showed that the behaviour of n-hexane at a loading of 4 mol per unit cell (u.c.) in silicalite-1 should not be ascribed to an enthalpy effect but to an entropy change of the system approaching 4 mol (u.c.)–1, which was in agree- ment with the results of Smit et al.1 Therefore, in order to investigate the influ- ence of the nature of the charge balancing cation on the commensurate freezing of n-hexane inside the zeolite structure, one physical property, thermal molar entropy, was studied, which was chosen for two reasons: first, the appearance of a low temperature peak in TPD profile, as confirmation that commensurate freez- ing is associated with the entropy change, and secondly, the property in relation to the molar entropy of liquid and solid n-hexane may cast insight into the free- dom of the n-hexane molecules within the zeolite and thus, could provide evi- dence for the occurrence of commensurate freezing. EXPERIMENTAL The parent self-produced NaZSM-5 (Si/Al = 20) was synthesized hydrothermally. HZSM-5 and under-exchanged forms of HZSM-5 containing Cu(II), Fe(II) or Mn(II) cations (mono- or bi-metallic) were obtained by appropriate common wet ion-exchange procedures, fully described previously.22 The crystallinity of the parent NaZSM-5, HZSM-5 and ion- -exchanged forms was proved by X-ray diffraction. The measurements were performed on a Bruker (Siemens) D5005 diffractometer at room temperature using CuKα radiation (0.154 nm), 2θ from 3 to 80°, in 0.02° steps with 1 s per step, and the results showed that the structure of the ZSM-5 zeolite remained unchanged during the ion-exchange process. XRD analysis proved that the structure was also not changed by adsorption of n-hexane. The differential heats and the isotherms of n-hexane adsorption were collected using a coupled microcalorimetric–volumetric line, using the procedure fully described elsewhere.23 Briefly, the heats of adsorption were measured in a heat-flow microcalorimeter (C80, Setaram) linked to a glass volumetric line that permitted the admission of successive known doses of adsorbed gas, until a final equilibrium pressure of 66 Pa. Subsequently, the sample was pumped, the desorption peak was recorded and re-adsorption was performed at the same temperature. Before the adsorption, the samples were pre-treated in vacuum (10-3 Pa) overnight at 673 K, while the adsorption temperature was maintained at 303 K. In order to clarify the states of cation species in the investigated samples, UV–Vis diffuse reflectance spectra were recorded, in 190–1000 nm spectral region, using a Perkin Elmer Lambda 35 UV–Vis spectrometer equipped with a diffuse reflectance accessory. The powder samples were placed in the sample cup and BaSO4 was used as a reference. _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ COMMENSURATE FREEZING OF N-HEXANE IN MICROPORES OF ZSM-5 1301 The thermal entropy of sorbed n-hexane was calculated from the values of the entropy change of adsorption, ΔS0, obtained from calorimetric measurements.52 The obtained values of entropy change of adsorption are in good agreement with the values from the litera- ture.44,56,68,69 The following expression was used to calculate ΔS0 from the microcalorimetry data: 0 0 0 ΔΔ ln p H S R p T = + (1) where T is the adsorption temperature, p0 is the standard pressure, p is the equilibrium pres- sure at temperature T and ΔH0 is the standard enthalpy change in the adsorption process. These values were obtained from microcalorimetry as differential heats of adsorption. The entropy change accompanying the adsorption process can be expressed as: 0 0 *g sΔS S S= − (2) where 0gS is the molar entropy of n-hexane vapour at standard pressure p0 and temperature T, while *sS is the differential molar entropy of adsorbed n-hexane. The differential molar entropy, is a finite, positive quantity that may be separated into thermal and configurational entropy components.70 In this case, *sS can be represented by: *s c thS S S= + (3) where Sc and Sth are the differential molar configurational and differential molar thermal entropies, respectively. When the density of the molecules adsorbed in the channels of the zeolite is high, it is generally assumed that these molecules attempt to pack closely together and take up the walls of the anion framework. With such a localized sorption process, Sc, may be calculated with the equation: c ln 1 x S R x = − (4) where 0/x W W= (W is the amount adsorbed at equilibrium pressure p, and W0 is the total adsorption capacity at 303 K).71 Subtraction of Eq. (4) from *sS gives Sth: *th s ln 1 x S S R x = − − (5) RESULTS AND DISCUSSION The values of differential molar entropy, differential molar configurational and differential molar thermal entropies were calculated for the adsorption of n-hexane with different loadings (expressed as N, number of molecules adsorbed per unit cell), on both parent HZSM-5 and its cation modified forms using Eqs. (2)–(5). The profiles of differential molar thermal entropies plotted vs. the amount adsorbed are shown in Fig. 1. The profiles of the Sth values vs. loading gave evidence that the differential molar thermal entropies of the n-hexane adsorbed on zeolite was dependent on the nature of the environment of the adsorbate molecules, i.e., the porous zeolite lattice with its charge balance cations. It can be seen from Fig. 1 that the Sth of parent HZSM-5 was altered by the incorporation of transition metal cations in its _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ 1302 HERCIGONJA et al. lattice. Furthermore, the presence of transition metal cations increased the ads- orption capability of HZSM-5 zeolite from 4 (obtained in the case of HZSM-5) to 6 molecules of n-hexane per unit cell (found for zeolites with transition metal cations). Different numbers (from 2 to 8) of sorbed n-hexane molecules per unit cell were reported in the literature,4,32,72 while the theoretical loading limit for ZSM-5 was 8 hexane molecules per unit cell.73 It could be inferred that the charge balance cations influence the values of Sth since they result from n-hexane attractive interactions with the zeolite pore wall, originating from the macro- anion framework and extra-framework charge-balancing cations. Fig. 1. Differential molar thermal entropies (Sth) as a function of loading N (molecules per unit cell) of n-hexane. In a study of n-hexane adsorption on ZSM, several facts must be taken into account. First, the length of the n-hexane molecule is almost the same as the distance between channel intersections and hence, these molecules cannot be wholly located in the channel segments and the possibility of adsorbate–adsor- bate interaction at the channel intersections always exists at high surface load- ings. In fact, adsorbate–adsorbate interaction are enhanced for a loading of 4 n-hexane molecules per unit cell; while for even higher values (4–8 molecules per unit cell), the adsorbed molecules arrange in such way that additional side-on interactions can occur; while with further increase in the N values, closer packing of n-hexane molecules occurs and thus, repulsion interactions become signi- ficant.4,47,74,75 For all the investigated zeolites, the Sth vs. loading profiles can be divided into three regions of loading. It could be seen from Fig. 1 that these regions are: _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ COMMENSURATE FREEZING OF N-HEXANE IN MICROPORES OF ZSM-5 1303 N < 2, 2 < N < 6 and N > 6. The Sth values found for n-hexane adsorption on HZSM-5, CuZSM-5 and CuMnZSM-5 decreased sharply in the initial, micro- -pore-filling region (N < 2). This is probably due to the fact that the first mole- cules entering the micropores occupy the most favourable sites, which results in their fast ordering and fast loss in Sth. In the case of adsorption on FeZSM-5, the Sth values decreased very slowly up to N < 2. However, it is important to notice that in this low-surface coverage region, the Sth values measured for n-hexane adsorption on MnZSM-5, FeMnZSM-5 and FeCuZSM-5 showed increased values with a more or less marked maxima. Barrer et al.70,76 reported that strong energetic heterogeneity is reflected in a maximum in the entropy curves against coverage for low adsorbate uptakes. Indeed, it seems that on these samples, the ordering of adsorbed molecules occurred as a result of interaction with surface active sites. After the maximum values of Sth were reached on MnZSM-5 and on the samples containing Fe(II) cations (FeZSM-5, FeMnZSM-5 and FeCuZSM-5), the Sth values remained almost constant in the medium surface coverage region (2 < N < 6), thus indicating highly localized adsorption. In contrast, the Sth values of HZSM-5 and the zeolites containing Cu(II) cations (CuZSM-5 and CuMnZSM- 5) slightly decreased from the constant values, thus indicating that there were some deviations of the localized sorption. For high surface coverages (N ˃ 4 for HMnZSM-5 and N ˃ 6 for all other samples), the Sth values increased sharply as a result of repulsion interactions among the closely packed n-hexane molecules. At low surface coverage, the adsorbate molecules can move freely in the zigzag channels and thus fill one part of the intersections for some time. As a consequence, further adsorption would become restricted, since the intersections are blocked while the straight channels are too short to accommodate n-hexane molecules. To fill the zeolite completely, the molecules adsorbed in a zigzag channel have to be confined in their position, which leads to a loss in entropy.8 It is accepted that when a coverage of half a loading per unit cell is achieved, the intersections are blocked. Then, the adsorbed molecules in the zigzag zeolite channels undergo phase transition, which is known as commensurate freezing. The Sth values of n-hexane adsorbed that are achieved at half-loading (≈3 mole- cules per unit cell) may be compared with the entropy of liquid n-hexane at 303 K (300 J mol–1 K–1) and the sum of the vaporization entropy change (94 J mol–1 K–1) and the fusion entropy change of n-hexane (72 J mol–1 K–1) at 178 K.77 According to Eq. (3), the values of the differential molar entropy at half-loading enable the derivation the Sth part of the respective differential molar entropy since c 0S = . These values of Sth are listed in Table I. The Sth part of the respective *sS obtained for the configurational entropy c 0S = of n-hexane adsorbed within the channels of FeZSM-5, FeMnZSM-5 and FeCuZSM-5 were between 55 and 62 J mol–1 K–1; values sufficiently lower than the sum of the vaporization entropy change and the fusion entropy change of n-hex- _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ 1304 HERCIGONJA et al. TABLE I. The Sth part of respective *sS (for c 0S = ) Zeolite *sS (= thS ) / J mol-1 K-1 ( c 0S = ) CuMnZSM-5 134 CuZSM-5 136 MnZSM-5 98 FeMnZSM-5 60 FeZSM-5 55 FeCuZSM-5 62 ane at 178 K to suggest that degree of ordering of the molecules adsorbed in the pores of these zeolites was similar to that in solid n-hexane. Similar results were found for n-hexane adsorbed on MnZSM-5. In this case, the Sth part of the respective *sS , for c 0S = equalled 98 J mol–1 K–1, which is more than the Sth found in the case of the zeolites containing Fe(II) cations, but still less than the sum of the vaporization entropy change and the fusion entropy change of n-hexane at 178 K. The Sth parts of the respective *sS obtained for c 0S = within the channels of CuMnZSM-5 and CuZSM-5 were practically equal (134 and 136 J mol–1 K–1) and also less than the sum of the vaporization entropy change and the fusion entropy change of n-hexane at 178 K. Obviously, the presence of Mn(II), Cu(II) and especially Fe(II) cations changes the attractive interactions between n-hexane molecules and the pore walls in such a way to enable the adsorbed n-hexane molecules to be arranged as in a solid-like state of n-hexane. It is noteworthy that the Sth values obtained for the samples containing Fe(II) cations were about 100 J mol–1 K–1 lower than the sum of the vaporization entropy change and the fusion entropy change of n-hexane at 178 K, while the Sth values for the samples containing Cu(II) ions were only about 20 J mol–1 K–1 lower than the above mentioned sum, indicating a higher ordering of the hexane mole- cules in the zeolites containing Fe(II) cations than inside those containing Cu(II) cations. The general conclusion, based on the values of Sth, is that a phase transition from gaseous n-hexane to a solid-like structure of n-hexane, i.e., com- mensurate freezing of n-hexane, occurred in the pores of ZSM-5 containing tran- sition metal ions. The commensurate freezing occurred at 303 K, well above the temperature of the bulk freezing of n-hexane (178 K). Increasing of the freezing temperature is a characteristic of fluids confined in a confinement medium with highly attracting pore-walls, as is the case with the investigated zeolites. In order to better illustrate the influence of the extra framework cations on decreasing Sth, the difference between Sth and 0gS was calculated and plotted as 0 th g( )S S− vs. loading (Fig. 2). At the very beginning of the sorption, the entropy loss followed the order: FeCuZSM-5 > FeMnZSM-5 > FeZSM-5 > MnZSM-5 > HZSM-5 > CuZSM-5 > CuMnZSM-5. At half of the maximum loading, the entropy loss followed the order: FeZSM-5 > FeCuZSM-5 = FeMnZSM-5 > HZSM-5 > MnZSM-5 > _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ COMMENSURATE FREEZING OF N-HEXANE IN MICROPORES OF ZSM-5 1305 CuZSM-5 = CuMnZSM-5, while at the maximum loading (N = 6), the entropy loss followed the order: FeZSM-5 > FeCuZSM-5 > FeMnZSM-5 > MnZSM-5 > CuMnZSM-5 > CuZSM-5. Fig. 2. Dependence of the entropy loss 0th g( )S S− of adsorbed n-hexane on loading N. The maximum entropy loss was achieved for n-hexane adsorption in the zeolites with Fe(II) cations and the minimum entropy loss was found for n-hex- ane adsorption in zeolites with Cu(II) cations. The rigid zeolite structure is the same in all these samples, which means that the presence of the charge-balancing cations influences the distribution and ordering of n-hexane molecules inside the zeolite channels, and thus the Sth values. It is important to note that27 Al MAS NMR experiments showed that the HZSM-5 investigated in this work does not contain extra framework Al, which means that some steric hindrance for n-hex- ane adsorption was not to be expected.78 The different values of the Sth part of respective *sS (for c 0S = ) of the samples with different charge balancing cations indicate that the created solid-like phase was not always the same and depended on the type of the extra framework cations. Cu(II), Mn(II) and Fe(II) have the same charge, but different ion radii (Cu: 0.73 nm, Mn: 0.82 nm and Fe: 0.78 nm) and hence, they create different electrostatic interactions with n-hexane mole- cules. In addition, Cu(II), Mn(II) and Fe(II) have different electron configur- ations, i.e., (Mn(II) possesses five and Fe(II) four incomplete d-orbitals, while Cu(II) has only one unpaired electron in the 4s1 orbital). The electrons of uncom- pleted orbitals can form bonds with oxygen atoms from the lattice, as well with _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ 1306 HERCIGONJA et al. adsorbed n-hexane molecules. If these electrons “react” with the oxygen of the lattice, the reaction of n-hexane with oxygen atoms is reduced, while the cations partly lose the possibility of interaction with n-hexane. Both effects cause an inc- rease in Sth. However, the interaction of spin unpaired electrons with n-hexane molecules produce the opposite effect, i.e., a decrease in Sth. The low Sth obtained for samples with Mn(II) and Fe(II) cations shows that Mn(II) and Fe(II) present in zeolite lattice coordinated the electrons of adsorbed n-hexane easier than the electrons of the oxygen atoms from the lattice. Therefore, the Sth of the samples containing Mn(II) and Fe(II) ions are influenced both by ion-induced dipole interactions, while the additional interactions originate from the possibility of Mn(II) and Fe(II) to behave as electron acceptors. On the contrary, considering that Cu(II) ions have only one unpaired electron in the 4s1 orbital, the Sth value of samples possessing Cu(II) ions is mostly determined by ion-induced dipole interactions through electrostatic and dispersive forces. If all the interactions achieved between n-hexane molecules and the samples possessing Fe(II) and Mn(II) ions are compared, it could be concluded that Fe(II) ions can achieve stronger electrostatic (its ionic radius is smaller), but weaker additional interact- ions (Fe(II) possesses four uncompleted d-orbitals, Mn(II) five). Based on the values of Sth, it seems that both these interactions are stronger in the samples containing Fe(II) cations, which led to better ordering of the adsorbate and finally to the formation of a solid-like phase in the course of a phase transition known as commensurate freezing. CONCLUSIONS The results obtained in this work show that the phase transition of commen- surate freezing of n-hexane occurred during the adsorption of n-hexane into the parent HZSM-5 and its transition metal modified forms: CuZSM-5, MnZSM-5, FeZSM-5, CuMnZSM-5, FeMnZSM-5 and FeCuZSM-5. Freezing of n-hexane, the molecules of which “fit” the zigzag channels of ZSM-5, occurred at 303 K, which is well above the freezing temperature of bulk n-hexane (178 K). The positive shift in the freezing temperature was affected by the strong attractive forces between the n-hexane molecules and pore walls of the zeolite, originating from the presence of charge balancing cations. The results showed that above the circumstances of reduced dimensionality, such is in the case of zeolite lattice, the type (size and charge) and nature (electron configuration) of the charge balancing cation determine the ordering of the solid like phase of n-hexane obtained in the phase transition of commensurate freezing. Among the investigated zeolites, the solid-like phases obtained in zeolites possessing Fe(II) cations (FeZSM-5, FeMnZSM-5 and FeCuZSM-5) were the most similar to the solid n-hexane phase. Obviously, the presence of Fe(II) as the charge balancing cation facilitated the ordering of the adsorbed n-hexane molecules inside the zeolite. The smallest _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ COMMENSURATE FREEZING OF N-HEXANE IN MICROPORES OF ZSM-5 1307 ordering of the solid like phase of n-hexane obtained in the process of com- mensurate freezing was in the zeolites with Cu(II) as the charge balancing cations (CuMnZSM-5 and CuZSM-5). Acknowledgement. The authors acknowledge the support from the Ministry of Education, Science and Technological Development of the Republic of Serbia (Project No. 172018). И З В О Д УТИЦАЈ КАТЈОНА ПРЕЛАЗНИХ МЕТАЛА НА “МРЖЊЕЊЕ” ХЕКСАНА У ОГРАНИЧЕНОМ ПРОСТОРУ МИКРОПОРА ZSM-5 ЗЕОЛИТА РАДМИЛА ХЕРЦИГОЊА1, ВЛАДИСЛАВ РАЦ2, ВЕСНА РАКИЋ2 и ALINE AUROUX3 1Факултет за физичку хемију, Универзитет у Београду, Студентски трг 12, 11 000 Београд, 2Пољопривредни факултет, Универзитет у Београду, Немањина 6, 11080 Београд-Земун и 3Université Lyon 1, CNRS, UMR 5256, IRCELYON, Institut de recherches sur la catalyse et l’environnement de Lyon, 2 avenue Albert Einstein, F-69626 Villeurbanne, Francе Commensurate freezing је назив за појаву фазног прелаза првог реда (мржњења) гасова на температурама које су знатно изнад температуре сублимације слободног гаса, у условима када се гас адсорбује у уским порама чије димензије одговарају димензијама молекула гаса. Поред значаја у основним испитивањима адсорпције гасова, испитивање “мржњења” флуида унутар зеолита као микропорозног материјала је од практичног зна- чаја у примени зеолита у процесима адсорпције, раздвајања компонената и катализи. У овом раду је проучавана адсорпција n-хексана из гасне фазе на ZSM-5 зеолиту, као и његовим облицима модификованим јонима прелазних метала, применом микрокалори- метрије на температури од 303 K. ZSM-5 зеолит има два типа канала, праве и синусо- идалне (цик-цак) који се међусобно секу. Дужина молекула n-хексана (1,03 nm) одго- вара растојању између центара пресека правих и цик-цак канала зеолита (1,2 nm), тако да под овим условима n-хексан може да се адсорбује на такав начин да је његово кре- тање толико ограничено да може да се схвати као замрзавање у каналима зеолита. Ова појава је у литаратури названа commensurate freezing и повезана је са променом ентро- пије система, па је у раду на основу израчунатих вредности промене термалне моларне ентропије адсорпције потврђено да је дошло до формирања структуре n-хексана која личи на чврсти n-хексан. Такође је показано да наведена фазна промена n-хексана зависи од природе катјона прелазних метала (електронске конфигурације, димензија и наелектрисања) који компензују негативно наелектрисање решетке. Међу испитиваним зеолитима, у оном са Fe(II) јонима је постигнут највећи степен уређености молекула n- хексана тако да је у том случају он у стању које је најсличније чврстом док је у зеолиту са Cu(II) јонима постигнута његова најмања уређеност. (Примљено 3. фебруара, прихваћено 21. апила 2015) REFERENCES 1. B. Smit, T. Maesen, Chem. Rev. 108 (2008) 4125 2. G. Rakhmatkariev, K. Zhalalov, K. Akhmedov, Uzb. Khim. Zh. 3 (1998) 68 3. M. Dubinin, G. Rakhmatkariev, A. Isirikyan, Izv. Akad. Nauk. SSSR Ser. Khim. 10 (1989) 2333 4. F. Eder, J. Lercher, Zeolites 8 (1997) 75 5. F. Eder, J. Lercher, J. Phys. Chem., B 101 (1997) 1273 _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ 1308 HERCIGONJA et al. 6. H. Zhang, S. Peng, L. Mao, X. Zhou, J. Liang, C. Wan, J. Zheng, X. Ju, Phys. Rev., E 89 (2014) 89 7. C. Malheiro, B. Mendiboure, J. Miguez, M. Pineiro, C. Miqueu, J. Phys. Chem., C 118 (2014) 24905 8. C. Bilgic, A. Askin, J. Chromatogr., A 1006 (2003) 281 9. A. Chica, A. Corma, P. Miguel, Catal. Today 65 (2001) 101 10. J. Delgado, T. Nijhuis, F. Kapteijn, J. Moulijn, Chem. Eng. Sci. 59 (2004) 2477 11. J. Denayer, G. Baron, J. Martens, P. Jacobs, J. Phys. Chem., B 102 (1998)3077 12. L. Domokos, L. Lefferts, K. Seshan, J. Lercher, J. Catal. 203 (2001) 351 13. J. Ndjaka, G. Zwanenburg, B. Smit, M. Schenk, Micropor. Mesopor. Mat. 68 (2004)37 14. M. Pera-Titus, J. Phys. Chem., C 115 (2011) 3346 15. M. Miyahara, K. Gubbins, J. Chem. Phys. 106 (1997) 2865 16. K. Morishige, K. Kawano, J. Chem. Phys. 110 (1999) 4867 17. M. Sliwinska-Bartkowiak, J. Gras, R. Sikorski, R. Radhakrishnan, L. Gelb, K. Gubbins, Langmuir 15 (1999) 6060 18. J. Klein, E. Kumacheva, Science 269 (1995) 816 19. J. Klein, E. Kumacheva, J. Chem. Phys. 108 (1998) 6996 20. M. Miyahara, M. Sakamoto, H. Kanda, K. Higashitani, Stud. Surf. Sci. Catal. 14 (2002) 411 21. U. Raviv, P. Laurat, P. Klein, Nature 413 (2001) 51 22. R. Radhakrishnan, K. Gubbins, A. Watanabe, K. Kaneko, J. Chem. Phys. 111 (1999) 9058 23. K. Sirkar, Chem. Eng. Commun. 157 (1997) 145 24. S. Bates, R. Van Santen, Adv. Catal. 42 (1998) 1 25. B. Newalkar, N. Choudary, U. Turaga, R. Vijayalakshmi, P. Kumar, S. Komarneni, T. Bhat, Chem. Mater. 15 (2003) 1474 26. R. Kolvenbach, L. Gonzalez-Pena, F. Luis, A. Jentys, J. Lercher, Catal. Lett. 143 (2013) 1116 27. T. Vlugt, M. Schenk, J. Phys. Chem., B 106 (2002) 12757 28. S. Bates, M. Gillan, G. Kresse, J. Phys. Chem., B 102 (1998) 2017 29. P. Jacobs, H. Beyer, J. Valyon, Zeolites 1 (1981) 161 30. B. Smit, T. Maesen, Nature 337 (1995) 42 31. N. Floquet, J. Simon, J. Coulomb, J. Bellat, G. Weber, G. Andre, Micropor. Mesopor. Mater. 122 (2009) 61 32. D. Olson, P. Reischman, Zeolites 17 (1996) 434 33. W. Well, J. Wolthuizen, B. Smit, J. Hooff, R. Santen, Stud. Surf. Sci. Catal. 105 (1997) 2347 34. M. Henri, H. Loven, Phys. Rev. Lett. 85 (2000) 366 8 35. D. Majda, W. Makowski, J. Therm. Anal. Calorim. 101 (2010) 519 36. H. Morell, K. Angermund, A. Lewis, D. Brouwer, C. Fyfe, H. Gies, Chem. Mater. 14 (2002) 2192 37. J. Coulomb, P. Llewellyn, Y. Grillet, J. Rouquerol, Stud. Surf. Sci. Catal. 87 (1994) 535 38. U. Müller, H. Reichert, E. Robens, K. Unger, Y. Grillet, Anal. Chem. 333 (1989) 433 39. P. Llewellyn, J. Coulomb, Y. Grillet, J. Patarin, H. Lauter, H. Reichert, J. Rouquerol, Langmuir 9 (1993) 1846 40. D. Dubbeldam, S. Calero, T. Maesen, B. Smit, Phys. Rev. Lett. 90 (2003) 245901 41. G. Manos, L. Dunne, M. Chaplin, Z. Du, Chem. Phys. Lett. 335 (2001) 77 42. R. Krishna, S. Calero, B. Smit, Chem. Eng. 88 (2002) 81 _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ COMMENSURATE FREEZING OF N-HEXANE IN MICROPORES OF ZSM-5 1309 43. W. Makowski, D. Majda, Thermochim. Acta 412 (2004) 31 44. W. Makowski, D. Majda, Appl. Surf. Sci. 252 (2005) 707 45. W. Haohan, Q. Gong, D. Olson, J. Li, Chem. Rev. 112 (2012) 836 46. D. Olson, A. Lan, J. Seidel, K. Li, J. Li, Adsorption 16 (2010) 559 47. R. Richards, L. Rees, Langmuir 3 (1987) 35 48. R. Marguta, S. Khatib, J. Guil, E. Lomba, E. Noya, J. Perdigon-Melon, S. Valencia, Micropor. Mesopor. Mater. 142 (2011) 258 49. T. Vlugt, R. Krishna, B. Smit, J. Phys. Chem., B 103 (1999) 1102 50. U. Lohse, H. Thamm, M. Noack, B. Fahlke, J. Incl. Phenom. 5 (1987) 307 51. W. Zhu, F. Kapteijn, B. Linden, A. Moulijn, Phys. Chem. Chem. Phys. 3 (2001) 1755 52. R. Hercigonja, V. Rac, V. Rakic, A. Auroux, J. Chem. Thermodyn. 48 (2012) 112 53. S. Ashtekar, A. McLeod, M. Mantle, P. Barrie, L. Gladden, J. Hastings, J. Phys. Chem., B 104 (2000) 5281 54. Y. Yang, C. Rees, Micropor. Matter. 12 (1997) 117 55. B. Millot, A. Methivier, H. Jobic, J .Phys. Chem., B 102 (1998) 3210 56. D. Majda, W. Makowski, J. Therm. Anal. Calorim. 101 (2010) 519 57. W. Makowski, L. Ogorzale, Thermochim. Acta 465 (2007) 30 58. V. Rac, V. Rakic, S. Gajinov, V. Dondur, A. Auroux, J. Therm. Anal. Calorim. 84 (2006) 239 59. J. Pieterse, S. Booneveld, R. van den Brink, Appl. Catal., B-Environ. 51 (2004) 215 60. R. Burch, P. Millington, Appl. Catal., B-Environ. 2 (1993) 101 61. S. Yashnik, Z. Ismagilov, V. Anufrienko, Catal. Today 110 (2005) 310 62. X. Li, B. Shen, C. Xu, Appl. Catal., A-Gen. 375 (2010) 222 63. O. Bortnovsky, P. Sazama, B. Wichterlova, Appl. Catal., A-Gen. 287 (2005) 203 64. K. Kohei, I. Hajime, N. Seitaro, I. Akira, Catal. Commun. 29 (2012) 162 65. W. Haag, R. Dessau, R. Lago, Stud. Surf. Sci. Catal. 60 (1991) 255 66. T. Narbeshuber, H. Vinek, J. Lercher, J. Catal. 157 (1995) 388 67. W. Haag, Stud. Surf. Sci. Catal. 84 (1994) 1375 68. W. Makowski, B. Gil, D. Majda, Catal. Lett. 120 (2008) 154 69. J. Janchen, H. Stach, L. Uytterhoven , W. Mortier, J. Phys. Chem. 100 (1996) 12489 70. R. Barrer, J. Suterland, Proc. R. Soc. A-Math. Phys. 237 (1956) 439 71. W.J. Mortier, Compilation of Extra-Framework Sites in Zeolites, Butterworth Scientific Limited, Guildford, 1982 72. P. Jacobs, H. Beyer, J. Valyon, Zeolites 1 (1981) 161 73. K. De Meyer, S. Chempath, J. Denayer, J. Martens, R. Snurr, G. Baron, J. Phys. Chem., B 107 (2003) 10760 74. P. Titus, J. Phys. Chem., C 115 (2011) 3346 75. K. M. A. De Meyer, M. Kurt, S. Chempath, J. Denayer, J. Martens, A. Johan, R. Snurr, G. Baron, J. Phys. Chem., B 107 (2003) 10760 76. R. Barrer, R. Gibbons, Trans. Faraday Soc. 59 (1963) 2875 77. NIST Chemistry WebBook, http://www.webbook.nist.gov/chemistry/ 78. H. Zou, M. Li, J. Shen, A. Auroux, J. Therm. Anal. Calorim. 72 (2003) 209. _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. 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