New Approach to the Acidity Characterization of Pristine Zeolite

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New Approach to the Acidity Characterization of Pristine Zeolite Crystals by Ethylene Using Reversed-flow Inverse Gas Chromatography (RF-IGC). Mohammed Amine Benghalem, Amir Astafan, Ludovic Pinard, Toufic Jean Daou, and Thomas Belin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11031 • Publication Date (Web): 16 Jan 2017 Downloaded from http://pubs.acs.org on January 21, 2017

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The Journal of Physical Chemistry

New Approach to the Acidity Characterization of Pristine Zeolite Crystals by Ethylene Using Reversedflow Inverse Gas Chromatography (RF-IGC). M. Amine. Benghalem1, Amir. Astafan1,2, Ludovic. Pinard1, T. Jean. Daou2, Thomas. Belin*,1 1

Institut de Chimie des Milieux et Matériaux de Poitiers, UMR 7285 CNRS, 4 Rue Michel Brunet,

Bâtiment B27, 86073 Poitiers Cedex 9 France. 2

Université de Strasbourg, Université de Haute Alsace, Equipe Matériaux à Porosité Contrôlée (MPC),

Institut de Science des Matériaux de Mulhouse (IS2M), UMR CNRS 7361, ENSCMu, 3 bis rue Alfred Werner, 68093 Mulhouse Cedex, France.

ABSTRACT: The adsorption and diffusion are key parameters for the catalytic conversions of ethylene to hydrocarbons. Using microcrystals of beta zeolites and the reversed-flow inverse gas chromatography technique, a new approach is developed for the catalytic characterization of pristine crystals. Based on the monitoring of the dynamic concentration in a time resolved way, this method allows to follow the surface coverage and adsorption energies among others physicochemical criteria. Pristine crystals were synthesized to rule out the effect of surface defects. The impact of porosity and acidity was studied by ethylene adsorption over porous, non-porous and acid materials. The acidity of the mother zeolite (acid and porous) was decreased by retroexchanges and partial cokage. The modification of the ethylene sorption due to the Brønsted acidity was then highlighted. The local surface coverage was not affected by the presence of ACS Paragon Plus Environment


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porosity while a second adsorption phase of the ethylene was induced by the modification of the acidity. Moreover, a relation between the amount of Brønsted acidity and the ethylene lateral interactions measured by RF-IGC was evidenced.

INTRODUCTION The diffusion and adsorption of hydrocarbons in zeolites are affected by their porosity and their complex acidities.1–4 Perfectly characterized materials as well as the knowledge of the reagent adsorption in real time are then needed. However, it remains difficult with the currently available methods to estimate the diffusional path length of the reactants and the products. This diffusion can be shortened by the modification of the porosity while the number of reaction steps promoting desorption will be limited by decreasing the acidity.5 Thus the molecular condensation which is primarily responsible for the catalytic deactivation could be mitigated.6–8 Determination of the global acidity of zeolite framework could be done in two ways: (i) Intra and extra-framework aluminum are analyzed by aluminium-27 solid-state NMR spectroscopy9, elemental analysis or XPS.5 (ii) The amount of hydroxyl groups is resolved by following the stretching band of these groups by infrared10 or by proton solid-state NMR11 spectroscopies. These analysis allowed the quantification of the overall acidity of the zeolite. The adsorption of molecular probes such as pyridine on zeolite provides both the discrimination of the Brønsted and Lewis sites10 and the quantification of the silanol groups.12 The probe desorption temperature gives an estimation of the strength of acid sites. Other methods are also used to characterize the acidity of zeolites such as the thermal desorption of the NH313,14 or

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the CO and C2H4 adsorption at low temperature.15,16 The combination of these methods with ab initio quantum-mechanical17–19 is necessary to probe proton transfer in zeolite.20,21 It’s already possible to determine the acidity by using model catalytic reactions sensitive to the chemical properties of zeolite such as alcohol dehydration22 and alkane cracking.23 Indeed, the initial activities of the catalysts is representative of the accessible acidity under the operating conditions. These methods are not completely independent of the aforementioned characterization tools. Each method provides useful information about the complex acidity of zeolites. The reversed flow inverse chromatography (RF-IGC) is also considered as a characterization method and a catalytic reactor.24–27 Developed by Katsanos et al.28, it allows to follow the surface coverage in way resolved and to distinguish the nature and strength of the adsorbate/adsorbent interactions.29,30 This method can also provide an additional view of the interactions involved as function of the coverage of the mono and multilayer surface. Besides of acidity properties, the crystallite size is an important factor affecting the zeolite porosity. Our laboratory has already shown that the decrease of the crystallite size tends to seriously affect the acidity of the zeolites and the behavior of the reactants. 31 This is due to the creation of surface defects as extra-lattice aluminum and/or silanols. As an example, the characterization of the acidity by pyridine adsorption followed by infrared does not always represent the real acidity of the zeolite except in the case of the large pristine crystallites.5 The absence of defects ruled out any change on the Brønsted acidity which makes them a good candidate for the fundamental study of acidity. Moreover, the acidity seems to have a real impact on the adsorption and diffusion of reactants. A recent study by our research team on the ethanol transformation into hydrocarbons on


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beta zeolites of different crystallite size and morphology revealed large differences in stability related to obviously diffusional problems.5 The conversions of ethylene to hydrocarbons is a reaction known to be sensitive to the chemical and textural structures. Thus, the aim of this work is to explain the differences in the ethylene sorption and diffusion based on the textural and acidic properties of microcrystalline zeolites with similar sizes. The interaction of ethylene with the zeolite structure at low temperature (340 K) will be studied using the reversed flow inverse gas chromatography (RF-IGC). Various microcrystalline beta zeolites (12 MR, 6.6 × 5.6** ↔ 5.6 × 5.6) are carefully synthesized for the sake of this study. Textural and acidic properties are also well characterized. The impact of acidity and porosity on the ethylene surface coverage will be investigated and then, the Brønsted acidity influence on the lateral interactions will be revealed. EXPERIMENTAL SECTION Synthesis of Beta zeolites Five microcrystalline beta zeolites with different acidities are studied. The first, named (H– BEA) is hydrothermally synthesized according to the protocol of Camblor et al.32 The synthesis mixture is prepared by hydrolyzing tetraethylorthosilicate (TEOS) (98 wt%, Aldrich) in an aqueous solution of tetraethylammonium hydroxide (TEAOH) (35 wt% in aqueous solution, Aldrich). Then, a solution with dissolved metal aluminum (99.95%, Aldrich) in aqueous TEAOH is added and the mixture is kept under stirring until the complete evaporation of ethanol formed upon hydrolysis of TEOS. Finally, hydrofluoric acid (40 wt% in H 2O) is added. The gel with the following molecular composition : 0.573 HF – 0.226 (TEA)2O - 0.016 Al2O3 - 1 SiO2 7.03 H2O is hydrothermally treated at 443 K for 14 days in a Teflon lined stainless steel autoclave.


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After the required crystallization time, the autoclave is cooled down to room temperature. The pH of the mother liquor is in the range 8 – 9.5. The product is filtered and washed extensively with distilled water. The addition of the aluminum to the mixture is intended to obtain protonic properties. The second sample used as reference (Si-BEA) is purely silicic zeolite and is synthesized according to the following protocol without aluminum source: 1.5 g of HF (hydrofluoric acid 40 wt% in H2O) were dissolved in 10 g of distilled water in plastic beaker, 12.6 g of TEAOH (3.5 wt% in H2O) and 3 g of Aerosil (200) were then added, while stirring continuously. After homogenization, the gel is lyophilized for 5 days. 0.06 g of H-BEA and 4.5 g of distilled water were added to the lyophilized gel under stirring. The obtained gel has the following molecular composition: 1 SiO2 – 0.6 HF – 0.6 TEAOH – 5 H2O. After synthesis, the product is filtered and washed with distilled water and dried overnight at 353 K. Finally, all the samples are calcined under air at 823 K during 5 h with a temperature ramp of 274 K per minute in order to remove the surfactants. The three other zeolites are derived from the protonic zeolite H-BEA to obtain various Brønsted acidities. For this purpose, two retro-exchanges with NaNO3 were realized (Na1-BEA and Na2-BEA). To obtain Na1-BEA, 150 mL of NaNO3 (2.27 × 10-3 mol L-1) solution is added to 1.5 g of H-BEA zeolite at room temperature under strong agitation (600 rpm) for 4 h. The same protocol is applied to Na1-BEA with a NaNO3 concentration of 0.2 mol L-1 to obtain the second retro-exchanged zeolite Na2-BEA. Samples are washed three times then dried in a muffle furnace at 363 K for one night. The last sample (C-BEA) is obtained by partial cocking. The goal is to poison some of acidic sites without affecting the porosity.


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Characterizations The crystallinity of these materials is characterized by XRD in a PANalytical X'Pert Pro MPD diffractometer using Cu Kα radiation (λ = 0.15418 nm). The diffraction patterns are recorded between 3-50° with a 2 Ɵ angle step of 0.017° and a time step of 220 s. Small angle powder X-ray diffraction patterns in the 0.5-10° region were recorded on a glass plate with a 0.02° step size (time step = 1 s) and a variable slit mode. Analysis of the textural properties is performed by nitrogen adsorption on a Micromeritic 2420 ASAP. The samples are degassed for 1 h at 363 K then 4 h at 623 K. Nitrogen (99 % pure) doses are then adsorbed across a wide range of relative pressures at constant temperature (typically liquid N2, 77 K). The total volume of each sample is taken at a relative pressure P/P0 of 0.99. The micropore volumes and the external surfaces areas are calculated by the t-plot method. The surface areas were measured by applying the Brunauer–Emmett–Teller (BET) theory.33 The morphology and particle sizes of the reference zeolites (H-BEA and Si-BEA) are analyzed by scanning electron microscopy SEM (Philips XL30 FEG) under an acceleration voltage of 200 kV, with a point-to-point resolution of 0.3 nm. The dispersion of Na+ cation in the crystallites after retro-exchanges is observed by SEM in a JEOL 2100 LaB6 equipped with an EDX detector JED Analysis Program. The amounts are quantified by ICP-OES in a Perkin Elumer Optima 2000 DV analyzer. For the acidity measurement, the catalysts are probed by pyridine at 423 K followed by Fourier transform infrared spectroscopy (FTIR). The samples are pretreated by heating at 723 K under air flow for 8 h. The pyridine is then adsorbed at 373 K and left under vacuum for 1 h before


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being evacuated at 423 K. The areas under the peaks at 1545 and 1454 cm-1 are used to quantify Brønsted [PyH+] and Lewis [PyL] sites, respectively. Reversed-flow inverse gas chromatography (RF-IGC) This specific experimental device is used as a reactor and a tool for characterizing physicochemical parameters at once. This method is based on the temporal study of the adsorption/desorption and diffusion of a very small amount of adsorbate as already stated by Katsanos et al.34,35 Materials The sample gas ethylene is injected into a solid bed column placed at the bottom of an empty diffusion column of stainless steel (L) of 36 cm length and a sectional area of 0.145 cm 2. This column is interconnected by the middle with a chromatographic column of stainless steel of 50 cm length (2l) of the same diameter. One of these side (x = 0) leads to a flame ionization detector (FID) and the other side (x = 2l) to a reversing valve (Valco Vici 1/16 '' fitting). Both columns were placed in the furnace of a conventional chromatography Varian GC3400 (Figure 1). Conditions 0.1 g of each zeolite are sieved to have grain sizes between (0.2 - 0.4 mm) and pre-treated under nitrogen flux (99.99 % pure filtered with silica gel) at 523 K for 6 hours. Identical quantities of ethylene (99 % pure from Air Liquide) are then injected (48 µmol) using another automatic 6way valve (Valco Vici 1/16'' fitting) at 343 K. Once arrived to the chromatographic column (sampling column), the ethylene diffusing in the 1 cm catalyst bed and in the diffusion column, is carried to FID by flux of nitrogen with a rate of 40 mL min-1. Every 120 s the flow direction is


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reversed for 5 s by the mean of the reversing valve. This inversion aims to correct the FID signal drift. The corresponding signal represents a concentration gradient comprising extrachromatographic peaks due to flux reversals in the chromatography column versus time (Figure 2). The reaction times vary from sample to another depending on their capacity of retention and transformations, it ranges between 4 and 7 hours. Subtracted from the baseline, the heights of peaks H (V) are fitted with a sum of four exponentials. The pre-exponential Ai and exponential Bi parameters are used to calculate all the physicochemical properties described hereafter.35,36 Calculations The main equation relies on the heights of the experimental peaks as a function of time and is obtained using Laplace’s space and time transforms35,37: 3

𝐻 = 𝑔𝑐(𝑙, 𝑡) = ∑ 𝐴𝑖 exp(𝐵𝑖 𝑡)

(1)

𝑖=1

where H (V) is defined as the peak heights, g (V cm3 mol−1) the calibration factor of the detector and c(l, t) (mol cm−3) the measured concentration of the injected gas at x = l and time t (s). Ai and Bi were respectively the pre-exponential factors (V) and the exponential coefficients of time (s−1). The program has been written in the Scilab script language38 and the variable projection method is used to determine the values of Ai and Bi parameters.39 The concentration of the gaseous analyte cy (mol cm−3) is derived from the previous relation:

𝑐𝑦 (0, 𝑡) =

𝜐𝐿 𝑐(𝑙, 𝑡) 𝐷𝑧

(2)


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where 𝜐 (cm s−1) is the linear velocity of the carrier gas in the sampling column, L (cm) the length of the diffusion column and Dz (cm2 s−1) the diffusion coefficient of the injected adsorbate in the carrier gas. This last value was estimated from the Fuller, Schettler and Giddings (FSG) relation (0.2002 cm2 s−1 for ethylene/nitrogen at 343 K).40,41 The value of the equilibrium adsorbed concentration 𝑐𝑠∗ (mol g−1) is obtained by assuming a nonlinear adsorption isotherm: 𝑡

𝑎𝑦 𝑐𝑠∗ = 𝑘 ∫ 𝑐𝑦 (0, 𝜏)𝑑𝜏 𝑎𝑠 1

(3)

0

where ay (cm2) is the cross-sectional area of the void space in the bed column, as (g cm−1) the amount of *BEA sample per unit length, k1 (s−1) the dynamic adsorption rate constant describing the local experimental isotherm of the adsorbate on the solid surface and τ is a dummy variable for time. A system of only three partial differential equations is needed to describe the whole analytical technique: (i)

in the bed column: 𝜕𝑦 𝜕 2𝑦 𝑎𝑠 = 𝐷𝑒𝑓𝑓 2 − 𝑘𝑟 (𝑐𝑠∗ − 𝑐𝑠 ) 𝜕𝑥 𝜕𝑦 𝑎𝑦

(4)

Fick diffusion and adsorption phenomena are reported in this relation. 𝐷𝑒𝑓𝑓 (cm2 s−1) is the effective diffusion coefficient of the adsorbate in the void space of the solid bed, k r (s−1) the rate constant for desorption from the bulk solid and cs (mol g−1) the concentration of analyte adsorbed on the solid at time t.


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(ii)

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in the diffusion column: as no solid was present in this region, only the Fick’s law is needed, then: 𝜕𝑐𝑧 𝜕 2 𝑐𝑧 = 𝐷𝑧 𝜕𝑡 𝜕𝑧 2

(5)

where cz (mol cm−3) is the concentration of the analyte in the diffusion column28,42. (iii)

the rate of change of the adsorbed concentration is described by the relation: 𝜕𝑐𝑠 = 𝑘𝑟 (𝑐𝑠∗ − 𝑐𝑠 ) − 𝑘2 𝑐𝑠 𝜕𝑡

(6)

where k2 (s−1) is the rate constant of a possible first order surface reaction of the adsorbate. All these equations are solved using Laplace transforms. With the assumption of the perfect gas law, the Jovanovic isotherm model is used to determine the values of adsorption parameters43:

𝜃(𝑐𝑦 , 𝑇, 𝜀) =

𝑐𝑠∗ ∗ 𝑐𝑠𝑚𝑎𝑥

= 1 − exp(−𝐾𝑅𝑇𝑐𝑦 )

(7)

∗ where θ is the local surface coverage (–); 𝑐𝑠𝑚𝑎𝑥 is the local monolayer capacity (mol g−1) and K =

K0(T) exp(𝜀 / RT) is defined as the Langmuir constant (MPa−1). K0 is derived from the statistical thermodynamic. KRT values as a function of time are obtained from Ai and Bi parameters as following: 𝑔𝐷𝑧 ∑3𝑖=1 𝐴𝑖 𝐵𝑖2 𝑒𝑥𝑝(𝐵𝑖 𝑡) 1 𝐾𝑅𝑇 = { 3 − 3 } (8) 2 𝑣𝐿 [∑𝑖=1 𝐴𝑖 𝐵𝑖 𝑒𝑥𝑝(𝐵𝑖 𝑡)] ∑𝑖=1 𝐴𝑖 𝑒𝑥𝑝(𝐵𝑖 𝑡) where R and T are respectively the perfect gas law constant (J mol−1 K−1) and the experimental temperature (K). This value is needed to determine the adsorption energy 𝜀 (J mol−1):


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𝐾 𝜀 = 𝑅𝑇 ln ( ) 𝐾0

(9)

and the probability density function f(𝜀, t) from the experimental data.44,45 This function is defined ∗ as the derivative of the number of adsorption sites 𝑐𝑠𝑚𝑎𝑥 with respect to the adsorption energy 𝜀.

The following relation is then obtained: ∗ 𝜕𝑐𝑠𝑚𝑎𝑥 1 𝐾𝑅𝑇((𝜕𝑐𝑠∗ )⁄(𝜕𝑡)) + ((𝜕 2 𝑐𝑠∗ )⁄(𝜕𝑐𝑦 𝜕𝑡)) 1 𝜕𝑐𝑠∗ 𝑓(𝜀, 𝑡) = = [ − ] 𝜕𝜀 𝑅𝑇 ((𝜕(𝐾𝑅𝑇))⁄(𝜕𝑡)) 𝐾𝑅𝑇 𝜕𝑐𝑦

(10)

However, it is demonstrated that the use of a modified probability density function 𝜑 (mol J−1) is a better choice for the true energy distribution function.44

𝜑(𝜀, 𝑡) =

𝜃𝑓(𝜀, 𝑡) ∗ 𝑐𝑠𝑚𝑎𝑥

(11)

Various groups of active sites are clearly resolved by plotting the modified probability density function as a function of time. Moreover, the relative proportions to the overall explored sites are obtained by measuring the areas under the time curves of the separate peaks46. In addition to the adsorption energy parameter 𝜀, lateral interactions are estimated using the following parameter 𝛽 44,47:

𝛽=

𝑧𝜔 𝑅𝑇

(12)

ω being the lateral interaction energy and z the number of neighboring atoms for each adsorption site. Thus, the θzω is the differential energy of adsorption added to 𝜀 due to lateral interactions.


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RESULTS AND DISCUSSIONS The diffractograms of the reference materials (H-BEA, Si-BEA) are shown in Figure 3A. They are very similar and the single crystalline phase corresponding to beta zeolite (P4122) is also confirmed. The SEM images of H-BEA and Si-BEA zeolite show bipyramidal square base crystals (Figure 3B). The average particle sizes for the samples Si-BEA and H-BEA are 5.1 and 7.6 µm respectively. Their large sizes excludes the appearance of interparticular mesoporosity. The absence of surface defects was an important criterion in the choice of this material. In fact the amount of silanols and extra-framework aluminum responsible for the Lewis acidity is strongly reduced. The EDX snapshots in Figure 4 show the dispersion of Na, Si and Al amount on retroexchanged zeolite Na1-BEA. The exchange of protons H+ by Na+ cation is done homogeneously inside the crystallites thereby excluding any impact on the textural property or any dispersion gradient which could affect the diffusion pathway of ethylene molecular probe. The EDX snapshots of the second retro-exchaged zeolite is similar. The textural and acid properties of fresh zeolites as well as sodium Na+ and carbon C percentages in zeolites after experiments are shown in Table 1. All the samples have negligible external surfaces due to the big crystallite sizes. The microporous volumes are between 0.22 and 0.24 cm3 g-1 which is characteristic of beta zeolites. The mesoporous volumes are close to zero and this excludes the contribution of an intermediate porosity due to inhomogeneous sizes of particles. The specific surface areas (SBET) of the fresh zeolites are similar (≈ 590 m2 g-1). Only a very slight decrease is observed for the zeolites with post synthesis treatment (< 6 % of average


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SBET). Moreover, the microporous volumes remain unaffected. The textural properties of the zeolites after post-synthesis treatment by partial cokage (C1-BEA) or retro-exchanges (Na1-BEA, Na2-BEA) are then preserved except for the totally coked zeolite (C2-BEA) where the porosity is totally blocked. Only the grain surface is accessible. The Brønsted acidity corresponding to the bridged OH (Si-OH-Al) are measured at 423 K by pyridine desorption. The concentrations shown in Table 1 are calculated by integrating the band at 1545 cm-1 of the recorded FTIR spectras. The purely silicic zeolite (Si-BEA) has no Brønsted acidity due to the absence of aluminum. Because of pore blocking and acid sites poisoning with 18 % of carbonaceous deposits, the totally coked zeolite (C2-BEA) has no porosity or acidity. The other four samples have different acidities ranging from 758 µmol g-1 for H-BEA to 398 µmol g-1 for Na2-BEA. Even so, these materials have the same theoretical Si/Al ratio of 23. The reference zeolite (H-BEA) is considered as parent sample with 100 % of acidity. A partial cokage by poisoning of 20 % of the acid sites yielded to the first derived zeolite (C1-BEA), with a carbon content of 2.6 % which does not affect the microporosity. The two samples obtained by retroexchanged have acidities of 65 % (Na-BEA) and 50 % (Na2-BEA) of the parent zeolite (H-BEA), and their sodium contents are of 0.36 wt% and 0.55 wt% respectively. Reversed-flow inverse gas chromatography (RF-IGC) 1- Impact of porosity and acidity Three materials with different textural and acidic properties are chosen to study the impact of the presence of porosity and acidity. The adsorption of ethylene is carried out on three samples. The first one (H-BEA) is purely microporous with high acidity (Table 1). On the contrary, the pure silica zeolite sample (Si-BEA) is free of acidity but with preserved microporosity. In order to compare these materials to a porosity and acidity free zeolite, the (H-BEA) sample is totally


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deactivated to become C2-BEA sample via the catalytic reaction with ethanol at high temperature and pressure.5 The local surface coverage θ is the area occupied relative to the surface explored at time t and the values calculated using equation 7 are shown in Figure 5. In the presence of acidity (H-BEA), two covering phases are distinguished. The first is characteristic of the monolayer and involves two steps: covering of the strongest adsorption sites (θ ≤ 1) then on the moderate adsorption sites (0 ≤ θ ≤ 1).31,48 The second phase corresponds to the multilayers. In the absence of acidity (Si-BEA and C2-BEA), phase 1 is retained with its two steps: strong adsorption sites in the first and a mixed contribution of moderate adsorption sites with the physisorption in the second step.31,48 The second phase disappears somehow and the slope differences are related to the way in which the probe molecule interacts with the adsorption sites of the sample as a function of their quantity. The presence or the lack of porosity is shown by a simple time lag between Si-BEA and C2BEA. It means that the difference of covering is due to the nature and strengths of adsorption sites. The normalized energy density probability function 𝜑 (𝜀,t) is a parameter defined as the ∗ variation of the number of adsorption sites (𝑐𝑠𝑚𝑎𝑥 ) that depends on the adsorption energy (𝜀) as

already stated by the equation 11. This parameter is represented as a function of time in Figure 6. For the (H-BEA), three regions are observed with similar intensities: "A" corresponding to the strong sites previously identified. The end of this region corresponds to that of the first step of local surface coverage (θ ≤ 1) (Figure 5). The second region "B" corresponds to moderate sites and occurs in the same time interval as second step of local surface coverage (0 ≤ θ ≤ 1).The last region "C" corresponds to weak sites due to the multilayers. For both non-acidic zeolites (Si-BEA


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and C2-BEA), only two regions are observed named "D" and "E". The first one is identified to the strong sites (θ ≤ 1) already described with H-BEA sample. The second region “E” covers the whole moderate and weak sites distribution. The relative proportion of the adsorption sites is calculated by integration of each regions. The results are shown in Table 2. For the non-acidic samples, The relative proportion of sites “D” implicated in strong repulsive ethylene interactions (β < 0 and θ ≤ 1) is about 21.09 % for purely silicic and 12.89 % for totally coked samples. This moderate variation is obviously due to the textural properties especially the lack of the micropores on C2-BEA. The ethylene interactions within micropores are then impossible. These zeolites contain predominantly sites “E” responsible of attractive lateral and van der Walls interactions (respectively 78.90 and 87.11 %). The acidic sample H-BEA contains of about 12.41 % of strong sites “A” (β < 0 and θ ≤ 1) which could be associated with the direct interactions of ethylene with empty Brønsted acid sites. As expected by the presence of acid properties, the second region “B” represents 39.34 %. This corresponds to the average sites and is probably involved in more heterogeneous attractive lateral interactions. The third region “C” (48.24 %) corresponds mainly to the van der Walls interactions. The adsorption energy 𝜀 decreases as a function of lateral attractive interactions β for the three zeolites with a little higher energies for porous zeolites (Si-BEA and H-BEA) (figure 7). A discontinuity is observed in the case of the acidic zeolite H-BEA close to β = 2.31 which could be logically related to the presence of acid sites. The first step of attractive lateral interactions (0 ≤ β ≤ 2.31) corresponds to the region “B” of adsorption sites while the second one (β > 2.31) is related to the region “C”.


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In a first assumption, four lateral interactions are possible (ethylene-acidic sites, ethylenemicropores walls, ethylene-grain surface and ethylene-ethylene). In the absence of acidity (Si-BEA), only three types of lateral interactions are possible and the minimum of energy are not observed. This confirms that the amount of acid sites is directly involved in the presence of an energy minimum as the size and porosity are identical to the sample H-BEA. Concerning the totally coked sample (C2-BEA), only the interactions of ethylene with the grain surface and ethylene-ethylene are possible. The shape of the curve is similar to that of Si-BEA sample but with lower energies. The adsorption sites “B” and “C” (observed on H-BEA sample) are not distinguished for non-acidic samples (Si-BEA and C2-BEA). The Figure 7 shows that the lateral interactions involved in the ethylene adsorption on zeolite samples depends on the textural properties and predominantly on the acidic properties. 2- Effect of Brønsted acidity on the lateral interactions The Brønsted acidity is involved in the majority of reactions involving zeolites and hydrocarbons.49,50 Ethylene is converted to several hydrocarbon molecules over zeolitic acid sites.51 It has been shown that the conversion of olefins with the acidic sites requires passing through three types of reaction intermediates (Figure 8): π-complex, carbocation and alkoxide called σcomplex.18,19 The formation of the π-complex between the double bonds of the ethylene and the bridged OH (Si-OH-Al) of the zeolite is the primary step of adsorption. The stability of this complex depends on the distance H-π and which varies with the amount of Brønsted sites of the material.17 Indeed, a strong acid sites involves a strong attraction and thus the reduction of the


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distance H-π. This is what probably happens with sites “A” as the ethylene molecules are difficult to desorb. This state can give way to a second step of interactions: free molecules of ethylene interact with the previously adsorbed ethylene molecules.52 It represents the attractive lateral interactions. The formation of π-complex can also probably take place between the free ethylene molecules in the absence of Brønsted acidity (Figure 8). However, its transformation remains limited because the transition state is difficult to be achieved because of the very high activation energy.53 However, it is difficult to predict whether the adsorbed ethylene is in the form of π-complex surface or alkoxide surface. Indeed the stability of these reactive intermediates is dependent on the temperature and acidity.17 In the presence of acidity at low temperature, energy differences between the intermediaries are low. Indeed, the occupation of the strongest sites is energetically promoted.53 Figure 9 shows the adsorption energies as a function of the attractive lateral interaction coefficients (𝛽 > 0) of the microcrystals with a similar porosities but different Brønsted acidities (Table 1). The repulsive surface interactions (𝛽 < 0) are not represented. A correlation between strong and average/weak interactions is identified (Figure 9). Indeed, the adsorption energies (𝜀) measures the adsorbent-adsorbate interactions while the lateral interactions (𝛽) involve the adsorbate-adsorbate interactions. In the presence of acidity, the two phases of attractive lateral interactions (𝛽) are observed dues to the previously adsorbed molecules which are considered acting like a secondary adsorption sites. The average adsorption energies of ethylene are similar for the four samples as the curves overlap each other. However the lateral interactions corresponding to the energy minimum


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previously seen in reference zeolite H-BEA (Figure 6 at 𝛽 = 2.31) decreases for the zeolites of lower acidities. The minimum 𝜀 position on the 𝛽 axis is plotted versus Brønsted acidity in Figure 10. A correlation between the amount of bridged OH (Si-OH-Al) of the zeolites and the end of the first phase of attractive lateral interactions is observed. Indeed, this passage by a minimum of energy is observed only on the zeolite having a Brønsted acidity (Figure 7). Brønsted acid sites are responsible for the formation of the π-complex while it stability depends on the quantity, strength, and the vicinity of the acidic sites. The adsorbed ethylene attracts the free molecules in the role of weakest secondary adsorption sites. Their total coverage involves an energetic discontinuity hence the passage by an energy minimum. It characterize a state of low probability such as the end of monolayer. The adding of multilayers ethylene do not lead to an increase the adsorption energies. It is similar for all samples. CONCLUSIONS The microcrystalline zeolites synthesized for this study are perfectly crystallized with a bipyramidal particle sizes of 5 – 10 µm. Three samples are obtained by tailoring acidity and porosity properties. The impact of these physicochemical parameters on the ethylene sorption are investigated. The surface coverage of porous acidic material involves the interactions with grain surface, the microporosity and adsorption sites. The mono and multilayers are already distinguished with two adsorption phases characterized by a passage by a minimum local surface coverage. This minimum is not observed without acidity.


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Three adsorption sites with different strength are identified in the presence of porosity and acidity while only two types for the non-acidic and non-porous zeolites. Indeed, in the absence of acidic sites, mostly the moderate-weak interactions are then developed as expected the adsorption over stronger acidic sites give rise to the strongest interactions. The lateral interactions are observed at lower adsorption energies. In the absence of acidity and porosity, a continuous decrease in the adsorption energies is observed. The microporosity increase the whole adsorption energies of about 10 kJ mol-1 compared to the non-porous material. The presence of acidity involves a passage by an energy minimum at a known value of the lateral interaction coefficients. The Brønsted acid sites could be indirectly responsible of the presence of this energy minimum by producing secondary attractive sites where the ethylene molecules are gathered as pseudo-clusters. The amount of secondary sites then depends on the amount of the Brønsted sites. As shown by the linear relation between the FTIR determined Brønsted acidity and the value of lateral interaction coefficients.


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AUTHOR INFORMATION Corresponding Author * Email: [email protected] Tel: +33 (0) 549 454 819, Fax: +33 (0) 549 453 779. ** Email: [email protected] Tel: +33 (0) 549 453 914, Fax: +33 (0) 549 453 779.

ABBREVIATIONS RF-IGC Reversed flow inverse gas chromatography; NMR Nuclear magnetic resonance; XPS XRay photoelectron spectrometry; FTIR Fourier Transform Infrared spectroscopy; FID flame ionization detector; SEM Scanning Electron Microscopy; EDX energy dispersive X-ray spectroscopy.

REFERENCES (1) (2) (3) (4) (5) (6)

Song, L.; Sun, Z.; Duan, L.; Gui, J.; McDougall, G. S. Adsorption and Diffusion Properties of Hydrocarbons in Zeolites. Microporous Mesoporous Mater. 2007, 104 (1–3), 115–128. Limtrakul, J.; Nanok, T.; Jungsuttiwong, S.; Khongpracha, P.; Truong, T. N. Adsorption of Unsaturated Hydrocarbons on Zeolites: The Effects of the Zeolite Framework on Adsorption Properties of Ethylene. Chem. Phys. Lett. 2001, 349 (1–2), 161–166. Dixit, L.; Rao, T. S. R. P. New Approach to Acid Catalysis and Hydrocarbon - Zeolite Interactions. In Studies in Surface Science and Catalysis; T.S.R. Prasada Rao and G. Murali Dhar, Ed.; Elsevier, 1998; Vol. Volume 113, pp 313–319. Jentys, A.; Lercher, A. Techniques of Zeolite Characterization. In Studies in Surface Science and Catalysis; 2001; Vol. 137, p 345. Astafan, A.; Benghalem, M. A.; Pouilloux, Y.; Patarin, J.; Bats, N.; Bouchy, C.; Daou, T. J.; Pinard, L. Particular Properties of the Coke Formed on Nano-Sponge *BEA Zeolite during Ethanol-to-Hydrocarbons Transformation. J. Catal. 2016, 336, 1–10. Lakiss, L.; Ngoye, F.; Canaff, C.; Laforge, S.; Pouilloux, Y.; Qin, Z.; Tarighi, M.; Thomas, K.; Valtchev, V.; Vicente, A.; et al. On the Remarkable Resistance to Coke Formation of Nanometer-Sized and Hierarchical MFI Zeolites during Ethanol to Hydrocarbons Transformation. J. Catal. 2015, 328, 165–172.


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(7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24)

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(43) Sircar, S. Effect of Local Isotherm on Adsorbent Heterogeneity. J. Colloid Interface Sci. 1984, 101 (2), 452–461. (44) Margariti, S.; Siokos, V.; Roubani-Kalantzopoulou, F. Experimental Determination of Adsorption Energies, Adsorption Isotherms, Probability Density Functions, and Lateral Molecular Interactions on CXHY/CaO Systems. J. Chromatogr. A 2003, 1018 (2), 213– 223. (45) Katsanos, N. A.; Arvanitopoulou, E.; Roubani-Kalantzopoulou, F.; Kalantzopoulos, A. Time Distribution of Adsorption Energies, Local Monolayer Capacities, and Local Isotherms on Heterogeneous Surfaces by Inverse Gas Chromatography. J. Phys. Chem. B 1999, 103 (7), 1152–1157. (46) Katsanos, N. A.; Gavril, D.; Kapolos, J.; Karaiskakis, G. Surface Energy of Solid Catalysts Measured by Inverse Gas Chromatography. J. Colloid Interface Sci. 2004, 270 (2), 455– 461. (47) Katsanos, N. A.; Roubani-Kalantzopoulou, F.; Iliopoulou, E.; Bassiotis, I.; Siokos, V.; Vrahatis, M. N.; Plagianakos, V. P. Lateral Molecular Interaction on Heterogeneous Surfaces Experimentally Measured. Colloids Surf. Physicochem. Eng. Asp. 2002, 201 (1– 3), 173–180. (48) Metaxa, E.; Kolliopoulos, A.; Agelakopoulou, T.; Roubani-Kalantzopoulou, F. The Role of Surface Heterogeneity and Lateral Interactions in the Adsorption of Volatile Organic Compounds on Rutile Surface. Appl. Surf. Sci. 2009, 255 (13–14), 6468–6478. (49) Martens, J. A.; Jacobs, P. A. Chapter 14 Introduction to Acid Catalysis with Zeolites in Hydrocarbon Reactions. In Studies in Surface Science and Catalysis; van Bekkum, H., Flanigen, E. M., Jacobs, P. A., Jansen, J. C., Eds.; Elsevier, 2001; Vol. Volume 137, pp 633–671. (50) Guisnet, M.; Ribeiro, F. R. Les Zéolithes: Un Nanomonde Au Service De La Catalyse; EDP Sciences, 2006. (51) Lin, B.; Zhang, Q.; Wang, Y. Catalytic Conversion of Ethylene to Propylene and Butenes over H−ZSM-5. Ind. Eng. Chem. Res. 2009, 48 (24), 10788–10795. (52) Allotta, P. M.; Stair, P. C. Time-Resolved Studies of Ethylene and Propylene Reactions in Zeolite H-MFI by In-Situ Fast IR Heating and UV Raman Spectroscopy. ACS Catal. 2012, 2 (11), 2424–2432. (53) L. Yu. Doigikh; N. I. II’chenko; N. V. Pavlenko. Factors Determining Selectivity of Ethylene Conversion to Butadiene Over Aluminosilicate Catalyst. Theor. Exp. Chem. 1997, 33 (2), 67–70.


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FIGURES

Figure 1. Experimental assembly of RF-IGC.


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Figure 2. Signal of the flame ionization detector versus time during RF-IGC experiment. The narrow peaks due to the flow reversal are easily shown superimposed to the diffusion band.


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Figure 3. (A) Wide angle powder XRD pattern of calcined micron-sized H-BEA and SiBEA samples and (B) respective scanning electron microscopy images.

Figure 4. EDX snapshots of retro-exchanged zeolites Na1-BEA.


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Figure 5. Plot of local surface coverage θ as a function of time for Si-BEA, C2-BEA and H-BEA samples.

Figure 6. Plot of modified energy density probability 𝝋 as a function of time. Three levels of C2H4 interactions with fresh zeolite H-BEA (“A”, “B”, “C”) and two (“D”, “E”) with coked C2BEA and purely silicic zeolite Si-BEA are identified.


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Figure 7. Plot of adsorption energy 𝜺 as a function of lateral interactions coefficient 𝜷 for Si-BEA, C2-BEA and H-BEA samples.


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Figure 8. Calculated structure for adsorbed ethylene (a), transition state of ethoxylation reaction (b) and final ethoxide (c).

Figure 9. Plot of adsorption energy as a function of lateral interactions coefficient 𝜷 of fresh H-BEA, coked C1-BEA and exchanged Na1-BEA Na2-BEA zeolites.


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Figure 10. Plot of Brønsted acidity [PyH+] as a function of laterals interactions 𝜷 of fresh H-BEA, coked C1-BEA and exchanged Na1-BEA, Na2-BEA zeolites. Dotted line is given as guideline.


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TABLES. Table 1. Selected characteristics of zeolites samples: reference H-BEA, exchanged Na1BEA, Na2-BEA, coked C1-BEA C2-BEA and purely micron-sized crystal Si-BEA. SBET

Sext

Vmeso

Vmicro

(m2 g-1)

(m2 g-1)

(cm3 g-1)

(cm3 g-1)

CH+ (µmol.g-1) % Na

Catalyst

(± 3 m2 g-1)

(± 0.01 cm3 g-1)

%C 423 K

(± 0.01 %)

(± 1 %)

(± 5 %)

Si-BEA

594

8

0.02

0.24

0

0

0

C2-BEA

12

6

0.01

0.02

0

18

0

H-BEA

588

13

0.02

0.22

0

0

758

C1-BEA

582

12

0

0.22

0

2.6

596

Na1-BEA

552

16

0.01

0.22

0.36

0

492

Na2-BEA

549

7

0.01

0.22

0.55

0

398

Table 2. Relative proportions and relative errors of identified adsorption sites for C2H4 observed on micron-sized crystal *BEA Samples. Relative proportions (%) Catalyst H-BEA

Sites (A)

Sites (B)

Sites (C)

12.41 (±0.01)

39.34 (±0.01)

48.24 (±0.02)

Sites (D)

Sites (E)

Si-BEA

21.09 (±0.02)

78.90 (±0.02)

C2-BEA

12.89 (±0.01)

87.11 (±0.01)


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TOC Graphic


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New Approach to the Acidity Characterization of Pristine Zeolite

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