Al ratios: A combined experimental and computational study

Accepted Manuscript Paraquat adsorption on NaY zeolite at various Si/Al ratios: A combined experimental and computational study Chalermpan Keawkumay, Wina Rongchapo, Narongrit Sosa, Suwit Suthirakun, Iskra Koleva, Hristiyan A. Aleksandrov, Georgi N. Vayssilov, Jatuporn Wittayakun PII:

S0254-0584(19)30621-2

DOI:

https://doi.org/10.1016/j.matchemphys.2019.121824

Article Number: 121824 Reference:

MAC 121824

To appear in:

Materials Chemistry and Physics

Received Date: 5 October 2018 Revised Date:

29 April 2019

Accepted Date: 7 July 2019

Please cite this article as: C. Keawkumay, W. Rongchapo, N. Sosa, S. Suthirakun, I. Koleva, H.A. Aleksandrov, G.N. Vayssilov, J. Wittayakun, Paraquat adsorption on NaY zeolite at various Si/Al ratios: A combined experimental and computational study, Materials Chemistry and Physics (2019), doi: https:// doi.org/10.1016/j.matchemphys.2019.121824. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Paraquat adsorption on NaY zeolite at various Si/Al ratios: a combined experimental and computational study

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Chalermpan Keawkumaya, Wina Rongchapoa,b, Narongrit Sosaa, Suwit Suthirakuna, Iskra Kolevac, Hristiyan A. Aleksandrovc*, Georgi N. Vayssilovc, Jatuporn Wittayakuna*

School of Chemistry, Institute of Science, Suranaree University of Technology, Nakhon

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a

Ratchasima 30000, Thailand.

School of Environmental Health, Institute of Public Health, Suranaree University of

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Technology, Nakhon Ratchasima 30000, Thailand. Faculty of Chemistry and Pharmacy, University of Sofia, Blvd. J. Bauchier 1, 1126 Sofia,

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Bulgaria.

*Corresponding author: [email protected],

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*Co-corresponding author: [email protected]

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ABSTRACT NaY samples in the crystalline form with Si/Al ratios from 2.15 to 2.40 were synthesized as adsorbents for paraquat. Their surface areas, particle size, concentration of

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basic sites decreased with an increase of Al content in the synthesis gel. The interaction between paraquat molecule and NaY zeolite was investigated by periodic DFT calculations to understand the adsorption behavior. Paraquat adsorption by all samples fit with Langmuir

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model. The capacity of the adsorbents was in the range of 210-240 mg/g-adsorbent, higher than NaY in the previous reports. After paraquat adsorption, the surface areas of the samples

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decreased from around 800 to 30 m2/g indicating that paraquat adsorbed inside the zeolite pores. FAU samples with the highest Si/Al ratio (2.4) had the highest adsorption capacity in line with the results from the DFT calculations. The paraquat molecules located close to the

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negatively charged fragments of the zeolite framework, due to the electrostatic interaction with the positively charged ammonia groups of the organic molecule.

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Keywords: NaY zeolite; Si/Al ratio; paraquat adsorption; DFT calculations

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1. Introduction A herbicide paraquat has been widely used in Thailand including Nakhon Ratchasima province which has the largest agricultural area in the country [1]. Paraquat is toxic [2] and

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could easily contaminate water due to its high solubility. Thus, it is necessary to investigate paraquat removal from aqueous solution as a proposed method for environmental management.

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Various sorbents have been studied for paraquat adsorption including clay, activated carbon, ordered mesoporous materials and zeolites [3-10]. Those materials are different in pore

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structure, surface area and Si/Al ratio. From our previous works on silica-based adsorbents, the adsorption capacity depends on Al content to some extent. The capacity is ranked in the following order: NaY > NaY/SBA-15 composite > NaBEA > NaX > Al-MCM-41 > MCM-41

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> rice husk silica [5,7,8,10]. The presence of Al in the mesoporous Al-MCM-41 leads to increase of the adsorption capacity compared to the silicate MCM-41 sample [5,7]. NaY has larger Al content thus larger adsorption capacity than NaBEA [5]. In contrast, NaX which has

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the same faujasite structure and more Al content than NaY has the smaller adsorption

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capacity. The interaction of sodium ions with the AlO4¯ tetrahedra in the cavity of NaX is stronger than that in NaY, thus, making them less exchangeable with paraquat [10]. From the previous works mentioned above, one can conclude that NaY is the best

adsorbent for paraquat among the studied ones. Thus, our main goal in the current study is to improve its adsorption capacity. Two strategies are employed: changing the Si/Al ratio and modifying the synthesis method to produce NaY with a smaller crystal size. Periodic calculations based on Density Functional Theory (DFT) were also performed to achieve a

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4 better understanding on the interaction between NaY zeolite and the paraquat molecule. Hence, we modeled various structures with different Si/Al ratios loaded with up to four paraquat molecules in one unit cell.

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Changing aluminum content in the framework of zeolite Y provides a way of controlling the properties and the sorption capacity [11]. Zeolite Y can be synthesized with lower and higher critical point of Si/Al value [12]. Ferchiche et al. [13] synthesized zeolite Y

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with Si/Al ratios from 1.7 to 2.1 using different amounts of silica. The lattice parameter decreased with the increasing of silica content indicating the less incorporation of Al atoms in

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the faujasite framework due to the longer Al-O bond length in tetrahedral units than the Si-O [13]. Qiang et al. [14] also studied the effect of Si/Al ratio in the range of 2.6 to 3.1 on the crystallinity of NaY zeolite. The crystallinity remained constant at the Si/Al ratio of 2.6 - 2.8

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but decreased significantly with increasing of the water content in the reaction mixture at Si/Al ratio of 2.8 - 3.0. In this work, NaY samples will be synthesized with Si/Al ratio between 1.8 and 2.4.

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Another approach to improve the adsorption capacity is changing the zeolite synthesis

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gel composition. Variation of water content in the synthesis gel could lead to zeolites with different crystal size, surface charge, hydrophilicity and external surface activity [15, 16]. In this work NaY samples will be synthesized with a smaller amount of water than that reported by Rongchapo et al. [5,10].

Besides the main goal to improve the adsorption capacity, the interactions between paraquat and zeolite Y with different Si/Al ratios are studied to understand the adsorption behavior. DFT method is an effective and reliable tool to complement experimental results.

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5 For instance, there are several examples showing that DFT can be used for modeling of zeolite systems and clarifying various experimental data related to the interaction of zeolites

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with organic molecules [17-20].

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2. Experimental 2.1 Materials

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Chemicals for NaY synthesis were rice husk silica, sodium aluminate Riedel-de Haën®, 41.383% Na2O, 58.604% Al2O3), sodium hydroxide (97%wt NaOH, Carlo-Erba). Commercial grade paraquat solution (27.6 %w/v, Masda) was employed in the adsorption study

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and the solid chemical grade (99.9 %w/w, Fluka) was used to verify the actual concentration.

2.2 Synthesis and characterization of NaY zeolites

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NaY zeolite samples with Si/Al ratio from 1.8 to 2.4 were synthesized with a

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procedure modified from that by Wittayakun [21]. Sodium silicate solution was prepared by dissolving 57.40 g of rice husk silica in NaOH solution prepared from dissolving 23.00 g of NaOH in 119.60 g of deionized (DI) water. A seed gel with a molar ratio of 10.67Na2O:1Al2O3:10SiO2:180H2O was prepared by dissolving 4.09 g of NaOH in 20.00 g of DI water in a polyethylene (PE) bottle followed by and addition of a desired amount of anhydrous NaAlO2 (Riedel-de Haën®, 41.383% Na2O, 58.604% Al2O3) to the NaOH solution. The mixture was stirred for 10 min until the solution became clear. After that 22.72 g of the

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6 Na2SiO3 solution was added and the mixture was stirred for 10 min, capped, and aged at room temperature for 24 h. A feedstock gel with molar ratio 4.3Na2O:1Al2O3:10SiO2:180H2O was prepared with

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the procedure similar to that of the seed gel except that it was used immediately without aging. In brief, 0.14 g of NaOH was dissolved in 131.97 g of DI water in a PE bottle, then, a desired amount of sodium aluminate was added, stirred for 10 min and at the end 143.43 g of

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sodium silicate solution was added. The overall gel compositions and names of the synthesized NaY samples from various gel ratios are summarized in Table S1 (Supplementary

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Materials). Note that the gel composition used in this work was different from the one in the work of Rongchapo et al. [5,10]. The main difference is that their gel composition contains a larger amount of water.

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The Si/Al ratios of the products were determined by X-ray fluorescence spectroscopy (ED-XRF, Horiba 5200). Morphologies of the NaY were studied by scanning electron microscopy (SEM, CARL ZEISS-AURIGA). The zeolite basicity was determined by

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temperature-programmed desorption of carbon dioxide (CO2-TPD) in a Belcat-B equipped

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with a thermal conductivity detector. Bare and paraquat containing NaY zeolite samples were characterized by X-ray

diffraction (XRD, Bruker D8 ADVANCE) with a Cu Kα radiation. N2 adsorption-desorption isotherms were obtained at a liquid nitrogen temperature from a Bel Sorp mini II. The surface area was calculated using a Brunauer–Emmett–Teller (BET) method. Functional groups of all adsorbents before and after adsorption of paraquat (500 mg/L) were determined by a Fourier

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7 transform infrared spectroscopy (FTIR, Perkin Elmer Spectrum GX) using KBr pellet technique with 2 cm-1 resolution.

2.2 Adsorption of paraquat

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The adsorption procedure was done as described in the literature [5]. Each adsorbent (0.05 g) was added into 20 mL of paraquat solution with a concentration ranging from 100 to

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1500 mg/L in a 125-mL polypropylene bottle. The mixture was stirred for 60 min with mixing speed of 400 rpm at room temperature (25 ºC). As shown in Figure S1 (Supplementary

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Materials), paraquat adsorbed rapidly on NaY samples and equilibrium was reached in 5 min. The adsorption time of 60 min was used in the study to ensure that the system was in equilibrium. The solution was collected using a 0.45 µm syringe filter. The remaining paraquat concentration was determined using a UV-Vis spectrophotometer (Varian CARY 300) at 257

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nm. The effect of pH was investigated with the same procedure using 500 mg/L of paraquat solution at various pH (1, 3, 5 (as-prepared), 7, 9). The amount of paraquat adsorbed at

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equilibrium (qe) was calculated by the equation below [22,23].

qe =

(C 0 - C e ) × V w

C0 and Ce are the initial and equilibrium concentration of paraquat (mg/L), respectively. V is the volume of paraquat solution (L) and w is the amount of adsorbent (g). Maximum adsorption capacity of paraquat on NaY samples was determined based on Langmuir isotherm shown below [22-23].

Ce C 1 = + e qe K L qm q m

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KL is the Langmuir constant related to the affinity of binding site (L/mg) and qm is the maximum adsorption capacity (mg/g). Both KL and qm can be determined from the linear plot

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of Ce/qe versus Ce.

2.3 Computational model

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The interaction between paraquat molecule and NaY zeolite structure was performed by periodic DFT calculations with the PW91 exchange-correlation functional [24] using

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Vienna ab initio simulation package, VASP [25,26]. Ultrasoft pseudopotentials [27,28] were used as implemented in the VASP package. Due to the large unit cell (see below) the Brillouin zone was sampled using only the Γ point [29]. The valence wave functions were expanded in a

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plane-wave basis with a cutoff energy of 400 eV.

The rhombic unit cell of the FAU-type zeolite framework [30] was optimized for the pure silicate structure containing 48 crystallographically equivalent T atoms with dimensions

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a = b = c = 17.34 Å and α = β = γ = 60°.

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3. Results and discussion

3.1 Si/Al ratio, morphology and basicity of NaY zeolites The Si/Al ratios of the synthesized NaY samples are shown in Table 1, in the range of

FAU-type zeolite NaY [12]. The morphologies of NaY zeolite samples obtained by SEM are shown in Figure 1. All samples composed of small particles with uniform size lower than 1 µm. The size of individual crystals, of which each particle is composed, is approximately 100-

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9 200 nm. However, the particle size of NaY samples with a higher Al content becomes smaller, due to the higher alkalinity of the synthesis gel. Similarly, there are several reports for the synthesis of nanozeolites with small crystal sizes (10-100 nm) [15,31,32]. These results

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are compared with NaY with the Si/Al ratio of 2.2 synthesized with a larger amount of water (256H2O) in the gel [10]. In this study the gel with less amount of water (180H2O) produces the zeolite with smaller crystals which might improve the zeolite textural properties leading to

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increased adsorption capacity.

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The CO2-TPD profiles of NaY samples are shown in Figure S2 in Supplementary Materials. All NaY samples showed one peak between 190 and 370 °C contributing to a weak basicity [33]. As shown in Table 1, the amount of total basic sites decreased as the Al content increased. Typically, oxygen atoms in the framework of zeolites are intrinsic Lewis basic sites

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which form acid-base pairs with the charge compensating cations [34].

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Figure 1. SEM images of NaY zeolite samples with various Si/Al ratios, i.e., 2.4 (NaY1), 2.31 (NaY2), 2.25 (NaY3), 2.19 (NaY4) and 2.15 (NaY5).

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3.2 Properties of NaY zeolite before and after paraquat adsorption XRD patterns of all NaY zeolite samples before and after paraquat adsorption (Figure

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2a and 2b, respectively) were similar to those of the standard NaY zeolite [35]. Sharp peaks

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indicated that all samples were in a crystalline form. Other zeolite phases were not observed. After paraquat adsorption, all peak positions remained the same. However, intensity ratio of the plane (220) and (311), I(220)/I(311) decreased significantly indicating an incorporation of paraquat in the zeolite supercages which changes the distribution of sodium ions (Na+) [36,37]. Such change is an evidence of ion exchange. Similar results are reported on NaY and NaX zeolites [5, 10]. As shown in Table 1, the crystal size and the crystallinity of NaY zeolite

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11 decreased with increasing Al content because amorphous phase occurred. This effect led to

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less adsorption sites in the zeolite sample.

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Figure 2. XRD pattern of (a) NaY zeolite samples with various Si/Al ratios, i.e., 2.4 (NaY1), 2.31 (NaY2), 2.25 (NaY3), 2.19 (NaY4) and 2.15 (NaY5) and (b) NaY zeolite samples after paraquat adsorption.

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13 Nitrogen adsorption-desorption isotherms of all NaY samples are shown in Figures 3ae. All samples possessed a type I isotherm. A high nitrogen uptake at low relative pressure (P/P0 = 0.0-0.1) indicated that a monolayer adsorption was achieved at low relative pressure.

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After a nearly constant uptake, a steep adsorption step appears at P/P0 = 0.9-1.0, related to the narrow pore size distributions [38]. The BET surface area (SBET) of NaY zeolite samples increased with increasing of the Al content (Table 1). The external surface area (SEXT)

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samples have similarly micropore volume (VMicro).

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increased with Al content probably due to the decrease in crystallinity and crystal size. All

The isotherm of all paraquat containing NaY samples are shown in Figure 3a-e. The nitrogen uptake at low P/P0, BET surface area and micropore volume decreased significantly (Table 1) confirming that paraquat occupied micropores of NaY. These results are consistent

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with the previous report [10] but the percent decrease of surface area in this study is higher. Thus, NaY zeolite synthesized with the less amount of water had the higher paraquat

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adsorption capacity.

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Figure 3. Nitrogen sorption isotherms of pristine NaY zeolite samples with various Si/Al ratios, i.e., 2.4 (NaY1), 2.31 (NaY2), 2.25 (NaY3), 2.19 (NaY4) and 2.15 (NaY5) and NaY zeolite

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samples with adsorbed paraquat: (a) NaY1 and NaY1-PQ, (b) NaY2 and NaY2-PQ, (c) NaY3

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and NaY3-PQ, (d) NaY4 and NaY4-PQ, and (e) NaY5 and NaY5-PQ.

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16 Table 1 Properties of pristine zeolite samples including Si/Al ratio, crystal size, relative crystallinity, BET surface area (SBET), micropore volume

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(VMicro), external surface area (SExt); porosity of NaY samples with adsorbed paraquat; Langmuir parameters of paraquat adsorption on NaY

Si/Al ratioa

Total basicity (mmol/g)

Crystal size (nm)b

% Relative crystallinityc

NaY1

2.40

0.274

42.16

NaY2

2.31

0.252

NaY3

2.25

NaY4

Sample

Paraquat-adsorbed NaY

NaY

Langmuir parameters

VMicro (cm3/g)

Sext (m2/g)

SBET (m2/g)

VMicro (cm3/g)

Sext (m2/g)

qm (mg/g)

KL (L/mg)

R2

100.00

789

0.29

47

21

0.01

10

234.40±1.36

0.0462

0.9771

41.43

98.93

804

0.30

51

24

0.01

11

228.30±1.35

0.0445

0.9760

0.222

38.81

97.53

844

0.31

56

27

0.01

12

222.83±1.68

0.0435

0.9721

2.19

0.212

37.83

93.30

849

0.31

61

29

0.02

14

214.35±1.46

0.0438

0.9724

NaY5

2.15

0.192

36.24

89.50

856

0.31

63

39

0.02

15

208.23±1.46

0.0423

0.9696

NaYd

2.20

N/Ae

N/Ae

N/Ae

N/Ae

N/Ae

92

N/Ae

N/Ae

185

N/Ae

0.9997

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SBET (m2/g)

870

from XRF; bFrom XRD using Scherrer equation; cfrom XRD; dfrom Rongchapo et al. [5,10]; eNot Reported

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a

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samples with various Si/Al ratios compared to NaYa sample obtained from Rongchapo et al. [5,10].

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17 Figures 4a and 4b show FTIR spectra of the pristine NaY samples and the samples with adsorbed paraquat. The assignments are summarized in Table S2 in the Supplementary Materials. All samples showed typical peaks of NaY zeolite. After adsorption, the

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measurement by KBr method at an ambient condition did not show clear evidence of the interaction between paraquat and NaY zeolite samples. The paraquat peaks around 2990-2850 cm-1 were also observed in some samples (NaY1-PQ and NaY3-PQ) and seemed to shift to

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higher wavenumber. The strongest peak of paraquat at 1645 cm-1 overlapped with the hydroxyl of water. The peaks at 1639, 1504, and 1459 cm-1 were observed on some samples

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and did not shift [39]. The paraquat peak at 815 cm-1 was not observed from all paraquat-

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containing NaY samples.

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Figure 4. FTIR spectra of (a) the pristine NaY zeolite samples with various Si/Al ratios, i.e., 2.4 (NaY1), 2.31 (NaY2), 2.25 (NaY3), 2.19 (NaY4) and 2.15 (NaY5) and (b) NaY samples with adsorbed paraquat.

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3.3 Adsorption capacity of paraquat The amount of paraquat adsorbed on all NaY samples at various equilibrium concentrations is shown in Figure 5. The adsorbed amount on all NaY adsorbents increased

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rapidly at low equilibrium concentrations indicating that the interaction was chemisorption. The isotherms of paraquat adsorbed on NaY samples based on Giles classification were class H (high affinity) sub-group 2 which is a strong adsorption [40]. The adsorption curve tended to

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be a plateau, which for adsorption on surface indicates formation of a monolayer, while in paraquat sorption in the zeolite pores indicates occupations of sorption positions with similar

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binding affinity. The similar adsorption behavior is reported on NaY and NaX zeolites [5,7].

Figure 5. Adsorption isotherms of paraquat in NaY zeolite samples with various Si/Al ratios i.e., 2.4 (NaY1), 2.31 (NaY2), 2.25 (NaY3), 2.19 (NaY4) and 2.15 (NaY5).

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We noticed that the amount of paraquat adsorbed in the various samples is different at high paraquat concentrations (> 100 mg/L). A possible explanation is that paraquat adsorbs on

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NaY by ion exchange with Na+ ions, according to the XRD results [10]. Osakoo et al. [9] proposed that C and N atoms in the paraquat molecules interacted with oxygen centers from NaY-SBA-15 composite.

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Paraquat adsorption on NaY samples followed Langmuir isotherms indicating a sorption at sites with uniform binding affinity. The Langmuir parameters are summarized in

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Table 1. The sample with a higher Si/Al ratio has a higher maximum adsorption capacity (qm) and KL value indicating a higher affinity of paraquat to the binding sites [40]. This trend is opposite to the trend of surface area indicating that changing Si/Al ratio is the main key to improve the adsorption capacity.

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The pH influences on the adsorption of paraquat (Figure S3 in supplementary materials). We found that the lowest pH of 3 correspond to the lowest adsorption capasicty

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due to the competition between H+ cations and paraquat to the adsorption sites. Increasing of pH from 3 to 9 leads to increasing of the adsorption capacity of NaY samples towards

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paraquat due to the loss of Brønsted acidic sites in the zeolite samples. A similar effect is reported on adsorption of paraquat on silica [39]. In comparison with the previous work [5] that the NaY with the Si/Al ratio of 2.20

shows the qm value of 185 mg/g-adsorbent. In this work, the NaY with the Si/Al ratio of 2.19 gave the qm value of 214 mg/g-adsorbent. Both samples were synthesized with different

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21 amount of water in gel composition [5] and had different crystal size. Hence, we can conclude that the modification of zeolite synthesis is another way to increase the adsorption capacity. According to the number of supercages per gram in NaY zeolite and the

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computational result (see below) that two paraquat molecules can be accommodated in one supercage, we may estimate that 7.26 x 1020 paraquat molecules can be sorbed per gram zeolite. For the sample with the highest sorption the amount of paraquat is 5.45 x 1020

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molecules, i.e. by about 25% lower than the maximal amount. As discussed above, the maximal sorption capacity of the five samples decreases in the order NaY1 to NaY5, which is also in

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line with decreasing percentage of crystallinity, as shown in Table 1.

3.4 Computational results of paraquat adsorption in supercage of NaY zeolite FAU structure was modeled by insertion 8, 12, 14, 16, and 22 Al centers in the unit

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cell, which corresponds to Si/Al ratio of 5.0, 3.0, 2.4, 2.0, and 1.2, respectively. The charge compensation of the negative charge around Al centers in the framework is accomplished by

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sodium cations. For all structures ion exchange with 1, 2, and 4 paraquat dications was considered as in each case one paraquat dication replaced two Na+ cations located in the

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corresponding region. Note that one may not expect quantitative agreement between experiment and the modeling due to various possible factors that may influence the experimental measurements, which cannot be adequately accounted by the computational modeling. To estimate the preference for the process of exchange of Na+ cations in NaY structures with different Si/Al ratios by one or more paraquat dications we used the structure

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22 with the Si/Al of 5 (with eight aluminum ions per unit cell, denoted as NaY-5.0-6Na-PQ) as a reference. For the equilibria with one paraquat, the energies are calculated as follow: +

∆E1 = E(NaY-5.0-6Na-PQ) + 2×E(Na ) – E(NaY-5.0-8Na) – E(PQ) +

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∆E2 = E(NaY-3.0-10Na-PQ) + 2×E(Na ) – E(NaY-3.0-12Na) – E(PQ) Erel(1PQ) = ∆E2 – ∆E1

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In this way the energy contributions of the solvated sodium and paraquat ions are canceled and Erel is an estimate for the preference for exchange of sodium ions by paraquat compared

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to NaY zeolite with the Si/Al ratio of 5.0. A negative value suggests higher preference for incorporation of paraquat. Negative energy values were obtained for the zeolites with Si/Al ratio from 1.2 to 3 (Table 3). These results suggested that the structure with higher Al content

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will have a higher preference to exchange sodium by paraquat as the maximal preference, 0.64 eV, corresponding to the model with the Si/Al ratio of 2.0. This is consistent with the experimental results from this work and previous works [5, 7, 10] that adsorption occurs

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quickly at a low paraquat concentration.

way:

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For the equilibria with two paraquat molecules, the energies are calculated in a similar

∆E3 = E(NaY-5.0-4Na-2PQ) + 4×E(Na+) – E(NaY-5.0-8Na) – 2×E(PQ) ∆E4 = E(NaY-3.0-8Na-2PQ) + 4×E(Na+) – E(NaY-3.0-12Na) – 2×E(PQ) Erel(2PQ) = ∆E4 – ∆E3

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In all cases positive relative energy (Erel(2PQ)) is obtained (Table 2), as it is particularly high for the zeolite structures with lowest Si/Al ratio, 2.0 and 1.2. These results agree with the previous experimental results [7, 10] that the adsorption capacity on NaY is higher than that

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on NaX, as well as with the results reported here. In the modeled complexes with four paraquat ions per unit cell additional interactions between the sorbed molecules occur and the analysis of the energy trends becomes very

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complicated and is not discussed. Such paraquat loading, however, correspond to the results

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reported by Rongchapo et al. [10] that the number of paraquat cations per supercage of NaX and NaY were 1 and 2 ions, respectively, meaning that the number of cations per unit cell were 2 and 4, respectively.

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Table 2. Relative energy, Erel (in eV), calculated as described in the text for exchange of Na+ by paraquat in zeolite structures with various Si/Al ratios containing different amount of paraquat in supercage. Si/Al

Erel 2PQ

-0.02

0.43

2.4

-0.41

0.10

2.0

-0.64

2.25

1.2

-0.52

2.57

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Erel 1PQ

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3.0

As expected, in the optimized structures of the paraquat in the zeolite pores the positively charged parts of the paraquat are located close to the negatively charged fragments of the zeolite framework as in this way the electrostatic interaction is maximized (Figure 6).

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24 The calculated distances between nitrogen centers of the paraquat and the aluminum centers of the framework for most of the structures with one sorbed paraquat molecule are in the range 450 – 520 pm. The shortest distances between the carbon centers of the methyl groups

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and the aluminum centers of the framework are in the range 390 – 450 pm. In all structures weak hydrogen bonds between the paraquat hydrogen atoms and zeolite oxygen centers around Al in the framework are formed (see Figures 6b-d).

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The shape of the paraquat molecules in the zeolite pores depends on the loading. For models with one or two sorbed molecules per unit cell both carbon atoms connecting the

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pyridine rings and the nitrogen centers in each ring are located essentially on one line. In the structures with higher loading, four molecules per unit cell, this line is distorted and the

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molecules are bent due to too close location of the neighboring molecules.

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Figure 6. Structures of pristine NaY zeolite (a) and zeolite with one (b), two (c) and four (d) sorbed paraquat molecules per unit cell. Color coding of different centers is shown

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in panel b: Al – green, Na – yellow, C – black, N – blue, H – light pink; Si and O

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centers are not shown.

4. Conclusions

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Zeolites NaY with Si/Al ratios in the range of 2.1-2.4 were synthesized by hydrothermal method. All samples were in crystalline form with particle sizes in the range of 100-300 nm. Zeolite samples had smaller crystal sizes than those in previous works due to the lower amount of water in the synthesis gel. The surface areas of NaY zeolite samples

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prepared in the current study were in the range 789 - 856 m2/g. The surface area increased with increasing of the Al content. The sample with the highest Si/Al ratio (lowest Al content) had

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the highest basicity (0.274 mmol/g). The adsorption of paraquat on NaY samples followed the

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Langmuir model. The capacity of the adsorbents was in the range of 210-240 mg/g-adsorbent. NaY sample with the highest investigated Si/Al ratio 2.4 had the highest adsorption capacity. When paraquat was introduced in the zeolite cavities, the surface area of the samples decreased significantly from about 800 to about 30 m2/g. In addition, the interaction between paraquat molecule and NaY zeolite structure was studied by periodic DFT calculations to understand the adsorption behavior. Our results showed that in case of one paraquat dication, the structures with higher Al contents are more prone to exchange sodium. For instance, NaY

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26 with Si/Al ratio 2.4 is the most favorable to exchange sodium by paraquat. Our finding that the paraquat molecules prefer to be located close to the negatively charged fragments of the zeolite framework is consistent to the electrostatic interaction between the organic molecule

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and the zeolite framework.

Acknowledgement

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Chalermpan Keawkumay is supported by Suranaree University of Technology (SUTPhD Scholarship and Full-Time Doctoral Researcher Grants). Narongrit Sosa is supported by

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the Science Achievement Scholarship of Thailand (SAST). Iskra Z. Koleva, Hristiyan A. Aleksandrov and Georgi N. Vayssilov acknowledge the support by European Union’s Horizon 2020 programme (“Materials Networking” project, grant agreement No 692146, and COST Action MP1306). This work was also supported by the European Regional Development Fund

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within the Operational Programme “Science and Education for Smart Growth 2014 - 2020” under the Project CoE “National center of mechatronics and clean technologies“

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BG05M2OP001-1.001-0008-С01.

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Highlights o NaY zeolites with Si/Al ratios from 2.15 to 2.40 were synthesized.

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o The zeolites had paraquat adsorption capacity from 210 to 240 mg/g-adsorbent. o NaY zeolite with the highest crystallinity had the highest adsorption capacity.

o Interaction between paraquat and the zeolite is investigated by DFT calculations.

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o Paraquat located close to the negatively charged of the zeolite framework.