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Insight into the acid sites over modified NaY zeolite and their adsorption mechanisms for thiophene and benzene Junjie Liao, Yunfei Zhang, Lijun Fan, Liping Chang, and Weiren Bao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05046 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on March 1, 2019

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Industrial & Engineering Chemistry Research

Insight into the acid sites over modified NaY zeolite and their adsorption mechanisms for thiophene and benzene Junjie Liao,† Yunfei Zhang,‡ Lijun Fan,† Liping Chang,† Weiren Bao*,† †Key

Laboratory of Coal Science and Technology, Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, PR China

‡Henan

CEP Environmental Protection Technological Service Limited Corporation, Nanyang, PR China

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Series of HY sorbents with different amounts and strength of Bronsted and Lewis acidic sites were prepared. Their adsorption performance in thiophene-benzene solution, pure benzene and pure thiophene were evaluated, and the desulfurization mechanisms were studied. The results show that both thiophene and benzene could be effectively adsorbed through the interaction between Lewis acid sites and the conjugated π bond in these two molecules, and thus thiophene adsorption could be obviously competed by benzene on Lewis acidic sites. By contrast, on Bronsted acidic sites, thiophene can be efficiently adsorbed via H-S bond, which is stronger than the former interaction. Moreover, the middle strong Bronsted acid could cause thiophene oligomerization. Hence, for the desulfurization purpose by NaY zeolite in aromatic free system, its weak and middle strong Lewis acid should be increased, and for that in aromatic existing system, the Bronsted acid site with relative low strength should be promoted. Keywords: NaY zeolite; Lewis and Bronsted acid; desulfurization; thiophene; benzene

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1 Introduction The organic S-containing compounds contained in chemical feedstocks and liquid fuels are the potentially important source of sulfurous pollutants. So much attention has been directed to deep desulfurization processes.1-5 However, most of these focused on the desulfurization of liquid fuels,6-10 and much less work has been done on desulfurization of other hydrocarbons, especially aromatics. As one of the important industrial chemical feedstock, pure benzene is mainly derived from petroleum benzene and coking benzene.11 Coking benzene with relatively rich resources (about 4 million tons last year) has received increasing emphasis in the situation of insufficient petroleum resource and increasing benzene demand.12 The existence of thiophene, which is physically and chemically similar to benzene, severely limited the utilization range of benzene,13 therefore, deep removal of thiophene are quite necessary. The current hydrodesulfurization (HDS) process could remove most of the thiophene14-16 but it is not feasible to produce ultra-clean benzene for the synthesis of cyclohexanone-oxime, maleic anhydride, and aniline

17.

Accomplished at ambient

temperature and pressure, selective adsorption is one of the most promising ultra-deep desulfurization methods18,19. The key challenge of this approach is the development of sorbents with high sulfur capacities and selectivity for sulfur compounds over other aromatic and olefinic compounds. Some literatures20-23 have reported that the modified NaY zeolite shown a very high adsorption desulfurization activity for thiophene due to its high ion-exchange and size-selective capacities as well as thermal and mechanical stabilities. Many scholars

24-29

insisted that surface acidity of zeolites is an important

factor that influences the desulfurization performance of sorbents, and aromatics could cause the competitive effect on thiophene adsorption.26, 30-32 However, the interaction mechanisms for two kinds of acidic sites on the surface of NaY zeolite, the Bronsted 3



acidic site and Lewis acidic site, have not been distinctively discussed and the different strength of them are also not distinguished. The questions needed to be made clear are mainly: (1) how are the thiophene and benzene adsorbed on the acidic sites with different forms and different strength? (2) why does the benzene cause competitive adsorption during desulfurization? In this paper, a series of HY zeolites with different amounts of Bronsted and Lewis acidic sites were prepared by ion exchanging in NH4NO3 solution and calcinating at different temperatures. The forms and strength of the acidic sites were determined. Based on this, the adsorption behaviors for thiophene and benzene and their interaction mechanisms were studied. 2

Experimental

2.1 Reagents used in experiments Thiophene with analytical pure grade was purchased from Alfa Aesar Company. Benzene with chromatographic pure grade was purchased from Tianjin Institute of Chemical Reagents. NaY zeolite was commercially purchased from Nanjingheyi Chemical Company Limited. 2.2 Acid modification of NaY zeolite using NH4NO3 Six copies of 10 g NaY zeolite were respectively added to 100 mL NH4NO3 aqueous solution with concentrations of 0.05, 0.10, 0.30, 0.50, 0.70, and 1.00 mol/L. After holding at room temperature for 12 h, the zeolite was filtrated, washed, and then dried at 100 °C for 2 h in an oven and calcined at 500 °C for 3 h in a muffle furnace. The zeolites obtained by modifying in 0.05, 0.10, 0.30, 0.50, 0.70, and 1.00 mol/L NH4NO3 aqueous solution are labeled as HY0.05, HY0.1, HY0.3, HY0.5, HY0.7, and HY1.0 respectively.

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2.3 Acid modification of NaY zeolite using calcination During calcination, some Bronsted acidic sites would be transformed into Lewis acidic sites. According to this, the NH4Y sample was firstly prepared by NaY ion exchanging in 1.00 mol/L NH4NO3 aqueous solution using above preparation condition. Then the NH4Y was calcined at 300 °C, 500 °C, 620 °C, or 750 °C for 3 h in a muffle furnace to prepare the HY sorbents with different ratio of Bronsted acid to Lewis acid, which are marked with HY1.0C300, HY1.0C500 (this’s also the HY1.0 sorbent), HY1.0C620, and HY1.0C750 respectively. 2.4 Adsorption desulfurization evaluation The adsorption desulfurization performances of sorbents were evaluated by static adsorption experiment. Thiophene-benzene solution with the thiophene concentration of 500.00 mg/L was used. In the adsorption experiment, 0.25000 ± 0.00002 g sorbent was weighted and immerged into 3.50 mL thiophene-benzene solution. After sustained at room temperature for 24 h, it was considered that the adsorption quasi-equilibrium had been reached. The concentration of thiophene remained in the thiophene-benzene solution before (c0) and after (c) adsorption experimental was measured by gas chromatography coupled with flame photometric detector (GC-FPD) according to our previous study.12,17 The desulfurization efficiency (η, %) was calculated by equation (1).



Co  C 100% Co

(1)

where, co is the initial concentration of thiophene in the solution (500 mg/L); and c is the concentration of thiophene in the solution (mg/L) after adsorption desulfurization. The chemical adsorption capacity of sorbent for thiophene or benzene was 5



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measured as follows: about 2 g HY0.05, HY0.1A, HY0.3A, HY0.5A, HY0.7A, or HY10A sorbents was soaked in pure benzene or pure thiophene and hold for 24 h at room temperature. Then the sorbent was separated from liquid by filtration. The sorbent was treated in a vacuum oven at 50 °C for 12 h to remove the physically adsorbed thiophene/benzene. The roles of Lewis and Bronsted acid sites on thiophene/benzene adsorption mainly present by chemical adsorption, which is closely related to the surface of sorbent, so the physical adsorption should be eliminated. The chemical adsorption capacity for thiophene (Qthiophene, mmol/g) and benzene (Qbenzene, mmol/g) of these sorbents was calculated using equation (2).

Qthiophene (or Qbenzene ) 

1000  (maa  mba ) M  mba

(2)

where, mba and maa are the weight of sorbent before and after adsorption (g), and M is the molar mass of adsorbate (g/mol). 2.5 Characterization of sorbents 2.5.1 XRD X-ray powder diffraction (XRD) patterns were collected in a 2θ range of 5 ° to 35 ° using a D/MAXRB XRD instrument (Rigaku, Japan) equipped with CuKα radiation (λ = 0.154 nm), 40 kV tube voltage and 100 mA tube current. 2.5.2 NH3-TPD NH3-temperature programmed desorption (NH3-TPD) was carried out using CHEMBET 3000 chemical adsorption instrument (Quantachrome, USA). At the preprocessing stage, 0.20 g sorbent was placed into a quartz reactor and heated in helium flow at 500 °C for 30 min. After the sample temperature decreased to 100 °C, NH3 was introduced into the reactor for 30 min. After NH3 adsorption, the sample was flushed 6


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by helium flow to remove the physically adsorbed NH3. Finally, NH3-TPD curve was recorded under helium atmosphere from 100 °C to 900 °C with a heating rate of 10 °C/min. 2.5.3 FTIR FTIR spectra were obtained on a TESOR27 FT-IR spectrometer (Bruker, Germany) in the range 400-4000 cm-1, with a resolution of 4 cm-1. For each run, 2 mg sample with 100 mg KBr was ground into powder and pressed to make tablet. Sixteen FTIR spectra were averaged and compared with the spectrum of the blank, a 102 mg KBr tablet. 2.5.4 Py-FTIR The Bronsted acid and Lewis sites were characterized by Py-FTIR using TESOR27 FT-IR spectrometer (Bruker, Germany). The Py-FTIR spectra of samples were obtained at a resolution of 4 cm-1 for 16 scans. To be detailed, about 15 mg sample was pressed into a 13 mm wafer and placed in a glass cell with CaF2 windows. Then, the wafer was pretreated at 380 °C under vacuum condition for 2 h and then cooled down to room temperature for pyridine adsorption. After adsorption, the sample was purged by vacuum for 0.5 h and subjected to the thermal desorption of the adsorbed pyridine in vacuum at different temperatures (150, 250 and 350 °C) and monitored by FTIR spectroscopy. The amounts for Bronsted acid (QB, mol/g) and Lewis acid (QL, mol/g) were calculated by equations (3) and (4) respectively according to the reference.33 QB  1.88S1542 R 2 / W

(3)

QL  1.42 S1452 R 2 / W

(4)

where, S1542 and S1452 (cm-1) are the band area of 1540 cm-1 and 1450 cm-1 respectively, R (cm) is the radius of the wafer, W (mg) is the mass of the wafer. 2.5.5 N2 adsorption The specific surface area and pore volume of the sorbents were measured by nitrogen 7



adsorption/desorption isotherms at -196 °C obtained through a SORPTOMATIC 1900SERIES (Micromeritics, USA) using BET and HK equations. 2.5.6 TGA Thermogravimetric analysis (TGA) of samples was carried out using STA-490C equipment (Netzsch, Germany). The temperature rose from 30 °C to 800 °C with a heating rate of 10 °C/min, and the carrier gas was air with a flow rate of 100 mL/min. 3 Results and discussion 3.1 Surface acidity of sorbents The Py-FTIR spectra of samples were shown in Figure 1. The band at 1452 cm-1 is assigned to Lewis bonded pyridine, 1542 cm-1 is attributed to Py-Bronsted bounding, and 1490 cm-1 can be assigned to pyridine associated with both Bronsted sites and Lewis sites.34, 35 The bands at 1442 cm-1 and 1593 cm-1 in Py-IR can be assigned as the hydrogen-bonded pyridine.36 For the NaY zeolite, there is a shoulder peak around 1452 cm-1, but no obvious band could be found around 1542 cm-1, which indicates that on the NaY surface, the vast majority of acidic sites are Lewis acidic sites provided by the unsaturated coordinative aluminum. Moreover, for the series of HY sorbents, a new peak around 1542 cm-1 apparently appears, and the density of this peak and that of the peak around 1452 cm-1 all increase with NH4NO3 concentration. This suggests that the treatment in NH4NO3 solution could promote both the formation of Bronsted acid and Lewis acid.

8


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1593

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1442

B acid 1542

L acid 1490 1452 NaY HY0.05 HY0.1 HY0.3 HY0.5 HY0.7

1650

1600

1550

1500

1450

1400

-1

Wavenumber (cm )

Figure 1 Py-FTIR spectra of NaY, and HY sorbents obtained in NH4NO3 solution with different concentrations In order to obtain the amounts of the Bronsted acid and Lewis acid, the deconvolution was carried out on the Py-FTIR spectra. As displayed in Figure S1, from the areas of the peak around 1542 cm-1 (S1542) and 1452 cm-1 (S1450), the amount of acids could be calculated according to equations (3) and (4), and the results were illustrated in Table 1. With the increase of NH4NO3 concentration, the amounts of these two acid all increase. For the Bronsted acid, its amount increases more rapidly than that of Lewis acid, and its amount is higher (Figure 2). Table 1 Deconvolution results of Py-FTIR spectra and the acid amount of NaY and HY sorbents L acid B acid QL QB S1542 (cm-1) (mol/g) (mol/g) (mol/g) (mol/g)

Sorbent

Wafer mass (mg)

S1452 (cm-1)

NaY

14.0

0.43

0.00

18.41

0.00

0.00

0.00

HY0.05

14.8

0.74

0.68

29.70

11.29

27.47

27.47

HY0.1

12.5

1.11

1.23

53.15

34.74

58.90

58.90

HY0.3

13.2

1.91

2.03

86.86

68.45

92.19

92.19

HY0.5

13.9

2.30

3.42

99.28

80.87

147.48

147.48

HY0.7

12.3

2.32

3.51

113.12

94.71

171.30

171.30

9



180 Lewis acid Bronsted acid

Acid amount (umol/g)

150

HY0.7 HY0.5

120 90 HY0.3

60

HY0.1

30

HY0.05 NaY

0 0.0

0.2

0.4

0.6

0.8

NH4NO3 concentration (mol/L)

Figure 2 Effect of NH4NO3 concentration on the amounts of Bronsted and Lewis acid of NaY and HY sorbents For the Py-FTIR spectra of HY0.5 and HY0.7 sorbents at different desorption temperatures, as present in Figure 3, the band around 1542 cm-1 diminishes slightly with the desorption temperature increasing, whereas the band around 1452 cm-1 firstly decreases obviously and then tend to increase slightly with the temperature increasing from 150 °C to 250 °C and to 350 °C, indicating that almost all the Bronsted acid sites and some of the Lewis acid sites are relatively strong, while most of the Lewis acid sites are relatively weak. HY0.5

HY0.7 L acid

B acid

Absorbance (a.u.)

L acid

B acid 1542

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1452

o

150 C o

250 C

1452

1542

o

150 C o

250 C

o

350 C

1575

1550

1525

1500

1475

1450

1425

o

350 C

1575

1400

1550

1525

-1

1500

1475

1450

1425

1400

-1

Wavenumber (cm )

Wavenumber (cm )

Figure 3 Py-FTIR spectra of HY0.5 and HY0.7 sorbents obtained at 150, 250 and 350 °C To further verify this, the NH3-TPD was carried out to characterize the strength of 10


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acidic sites on sorbents. As displayed in Figure 4, for NaY zeolite, there are two NH3 desorption peaks, one large peak is at 250 °C and one shoulder peak is around 347 °C, which are attributed to the weak and middle strong Lewis acids respectively. Compared with NaY zeolite, a new NH3 desorption peak appears at 375 °C caused by the middle strong Bronsted acids on NH3-TPD curves of HY sorbents, and its density also increases with NH4NO3 concentration, which is consistent with the Py-FTIR results shown in Figure 3.

HY1.0

o

375 C

HY0.7 HY0.5 HY0.3

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


HY0.05 NaY

o

250 C

o

347 C

100

200

300

400

500

600

700

o

Temperature ( C)

Figure 4 NH3-TPD curves of NaY, and HY sorbents obtained in NH4NO3 solution with different concentrations During the modification process of NaY zeolite in NH4NO3 aqueous solution, NH4+ ions would exchange with Na+ ion in zeolite, and most of NaY zeolite was transformed into NH4Y zeolite.37 And during the calcination process, NH4+ decomposed and NH4Y zeolite was finally converted into HY sorbent.38 During the whole modification process, although the crystallinity of HY sorbents decreases as the NH4NO3 concentration increases (Figure S2), the similarity in the XRD patterns for HY sorbents and NaY zeolite indicates that the structure of zeolite Y is retained after NH4+ modification 11



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followed by heat treatment. Figure S3 provides the nitrogen adsorption and desorption isotherms of sorbents. In adsorption isotherms, under low pressure, with the relative pressure (P/P0) increasing, the curves rise up sharply. The adsorption isotherms of all the sorbents belong to the type I isotherms, and all sorbents have microporous adsorption behavior. When the relative pressure increases from 0.4 to 0.9, the N2 volume adsorbed increases again, which indicates that there are some mesopores in the sorbents. The BET surface area (SBET), micropore surface area (Smicro), micropore surface volume (Vmicro), and pore diameter of different sorbents are listed in Table 2. For these pore structure diameters of all HY sorbents, they are all higher than those of NaY zeolite and increase with the NH4NO3 concentration because the higher the NH4NO3 concentration is, the more Na+ ions could be normally exchanged which results in the more H+ ions in HY sorbent. The size of H+ ion is smaller than that of Na+ ion, which would increases the pore volume and surface area. Figure 5 shows the relationship between NH4NO3 concentration and BET specific surface area of sorbents, it can be seen that they have a good linear relationship. Table 2 Pore structure parameters of NaY and HY sorbents obtained in NH4NO3 solution with different concentrations Sorbent

SBET (m2/g)

Smicro (m2/g)

Vmicro (cm3/g)

Pore diameter (nm)

NaY

642

592

0.25

0.78

HY0.05

659

607

0.26

0.79

HY0.1

669

616

0.27

0.80

HY0.3

691

635

0.28

0.80

HY0.5

720

684

0.29

0.82

HY0.7

746

693

0.29

0.83

HY1.0

743

706

0.31

0.85

12


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760 HY0.7

Y=122X+653 R=0.999

740

HY0.5

2

SBET (m /g)

720 700 HY0.3

680

HY0.1

660 HY0.05

640 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

NH4NO3 concentration (mol/L)

Figure 5 BET specific surface area of NaY and HY sorbents vs NH4NO3 concentration 3.2 Desulfurization performance of sorbents and its relationship with surface acidity Thiophene-benzene solution with a concentration of 500 mg/L was used to investigate the desulfurization efficiency of sorbents. The results were shown in Figure 6(a), it can be found that the desulfurization efficiency changes in the following sequence: HY1.0>HY0.7>HY0.5>HY0.3>HY0.1>HY0.05>NaY, which have a linear relationship with the amount of Bronsted acid, as illustrated in Figure 6(b). It indicates that the middle strong Bronsted acid could enhance the desulfurization performance of HY sorbents. 34 32 29.8

29.4

30 28.2

28

30.2

28.4

26.9

26 24 22

NaY

30.5

33.3

a

Desulfurization efficiency (%)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


HY0.05

HY0.1

HY0.3

HY0.5

HY0.7

HY0.7

30.0 HY0.5

29.5

HY0.3

29.0 28.5 28.0

HY1.0

b

HY0.1 HY0.05

20

40

60

80

Y=0.0145X+27.78 R=0.96

100 120 140 160 180

Bronsted acid (mol/g)

Sorbent

Figure 6 Desulfurization efficiency of NaY and HY sorbents (a) and its relationship with Bronsted acid amount (b) 13



In order to investigate how the thiophene and benzene were adsorbed on Bronsted and Lewis acidic sites on Y zeolite, pure benzene and pure thiophene were used to evaluate the sorbents. For the thiophene molecule, its sulfur atom has six electrons, two of them bond with two carbon atoms to form  bonds; two of them are contributed to form conjugated π bond with the four electrons provided by four carbon atoms; another two of them are lone pair electrons which could act as the electron donor. Whereas, for benzene molecule, it only has the similar conjugated π bond with thiophene but no sulfur. As listed in Table 3, the adsorption capacities for benzene and thiophene are closed to each other, which probably caused by the adsorption on Lewis acidic sites in NaY zeolite because most the acidic sites are Lewis type (shown in Figure 1), and both benzene and thiophene all have conjugated π electrons which could interact with Lewis acidic sites, the electron acceptor. Moreover, the adsorption capacities of HY sorbents for benzene and thiophene are all higher than those of NaY zeolite and increase in the same order with the desulfurization efficiency in thiophene-benzene solution shown in Figure 6(a). The adsorption capacity for benzene increases slightly, whereas that for thiophene rises obviously, e.g. the adsorption capacity for thiophene over HY1.0 are l.63 times as much as that over NaY zeolite. This indicates that the Bronsted acid could obviously favor the thiophene adsorption but not for benzene. The main reason is that the sulfur atom in thiophene has one lone pair electrons (electron donor), which could interact with H+ (electron acceptor) of Bronsted acidic site to form H-S bond.

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Table 3 Adsorption capacities of NaY and HY sorbents for pure benzene and pure thiophene and its increase of HY sorbents compared with NaY Sorbent

Qbenzene (mmol/g)

Qbenzene (mmol/g)

Qthiophene (mmol/g)

Qthiophene (mmol/g)

NaY

2.35

0.00

2.64

0.00

HY0.05

2.55

0.20

2.85

0.21

HY0.1

2.61

0.26

3.04

0.40

HY0.3

2.67

0.32

3.67

1.03

HY0.5

2.71

0.36

4.04

1.40

HY0.7

2.72

0.37

4.16

1.52

HY1.0

2.74

0.39

4.32

1.68

If we subtract the adsorption capacities for benzene and thiophens of NaY zeolite from those of HY sorbents (Table 3), the growth of adsorption capacities for benzene (Qbenzene) and thiophens (Qthiophene) of HY sorbents could be obtained. In the same way, the growth of Bronsted (QB) and Lewis (QL) acid amount (Table 1) of HY sorbents would also be obtained. As displayed in Figure 7, the Qbenzene and Qthiophene have good relationship with QL and QB respectively, whereas the linear relationship between Qbenzene and QB is not good as illustrated Figure 7(c). This could provide a proof for the above statement that the Lewis acid is beneficial to the adsorption of the compounds containing conjugated π bond (benzene and thiophene), whereas the Bronsted acid is beneficial to that of thiophene.

15



1.4

0.33 0.30 0.27 0.24 0.21

0.39

Y=0.0096X-0.042 R=0.98

0.36

1.2


0.36

1.6

Y=0.0021X+0.18 R=0.99

Qthiophene (mmol/g)

0.39


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.0 0.8 0.6 0.4

a

Y=0.0018X+0.099 R=0.89

0.33 0.30 0.27 0.24 0.21

0.2

0.18

Page 16 of 26

b

0.0

c

0.18

0 20 40 60 80 100

40 80 120 160

40 80 120 160

QL (umol/g)

QB (umol/g)

QB (umol/g)

Figure 7 Relationships of Qbenzene with QL (a), Qthiophene with QB (b) and Qbenzene with QB (c) In order to further verify above statement, HY1.0C300, HY1.0C500, HY1.0C620, and HY1.0C750 are used for the desulfurization in benzene. As present in Figure 8, as the calcination temperature increases from 300 °C to 750 °C, the desulfurization efficiency decreases from 36.31% to 22.29%. For the HY zeolite, it was reported that there is an equilibrium between the Bronsted and Lewis acidic sites. During the calcination process, the Bronsted acidic sites would be transformed into Lewis acidic sites because the OH group which is the provider of Bronsted acidic sites,39 would react with each other to release H2O.40 This transformation equilibrium would move to the production of Lewis acid direction when the calcination temperature increases,38,41 which could be proved by the in situ FT-IR results illustrated in Figure 9 (the data were obtained by using a stainless steel reactor) that with the calcination temperature rising from 350 °C to 680 °C with the heating rate of 5 °C/min, the brand intensity of hydroxyl (the provider of Bronsted acidic sites) appears at 3750 cm-1-3000 cm-1 decreases obviously. This means that the decrease of Bronsted acidic sites in sorbents would obviously reduce the thiophene adsorption capacity (shown in Table 4) with calcination temperature. 16


36 HY1.0C300 HY1.0C500

32

HY1.0C620

28

24 HY1.0C750

20

300

400

500

600

700

800

o

Calcination temperature ( C)

Figure 8 Effect of calcination temperature on the desulfurization efficiency of different HY sorbents

o

350 C o 400 C o 450 C o 500 C o 550 C o 600 C o 650 C o 680 C

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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4000

3750

3500

3250

3000

2750

-1

Wavenumber (cm )

Figure 9 In situ FT-IR spectra of HY1.0 sorbent calcinated from 350 °C to 680 °C with the heating rate of 5 °C/min Table 4 Adsorption capacity for pure benzene and pure thiophene over HY sorbents obtained at different calcination temperatures Sorbent

HY1.0C300

HY1.0C620

HY1.0C750

Qbenzene (mmol/g)

2.13

2.08

1.97

Qthiophene (mmol/g)

3.14

2.10

1.43

For the HY1.0C300, HY1.0C620, and HY1.0C750 sorbents adsorbed pure benzene or pure 17



thiophene, their TG and DTG curves are shown in Figure 10. In the DTG curves, whatever the pure benzene or pure thiophene adsorbed, the curves all have a weight loss peak around 100 °C, but for the sorbents absorbed thiophene, another weight loss peak appears around 550 °C. In order to analyze what the substances are released during heating process of HY1.0C300 sorbent adsorbed pure thiophene and pure benzene, a fixed bed reactor coupled with MS were used. The data are shown in Figure S4. It can be seen that weight loss peak around 100 °C is attributed to the benzene or thiophene adsorbed on Lewis acid sites, and water adsorbed on the surface of sorbent. The peak appears around 550 °C, is assigned to the combustion of the thiophene adsorbed on Bronsted acid sites. The benzene adsorbed on HY1.0C300 sorbent could almost all release in its parent form at about 100 °C, whereas thiophene sulfur is not. The thiophene sulfur release mainly releases in form of thiophene at about 100 °C, and then in form of SO2 at about 500 °C. 0.0 HY1.0C300 after benzene adsorption HY1.0C620 after benzene adsorption

Weight (%)

95

HY1.0C750 after benzene adsorption

90

85

80 100

200

300

400

500

600

700

Weight change rate(%/min)

100

-0.5

-1.0

HY1.0C750 after benzene adsorption

-2.0

800

HY1.0C300 after benzene adsorption HY1.0C620 after benzene adsorption

-1.5

100

200

o

Temperature ( C)

400

500

600

700

800

o

HY1.0C300 after thiophene adsorption HY1.0C620 after thiophene adsorption HY1.0C750 after thiophene adsorption

90 85 80

0.0

Weight change rate (%/min)

95

300

Temperature ( C)

100

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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75

-0.4

-0.8

-1.2

HY1.0C300 after thiophene adsorption HY1.0C620 after thiophene adsorption HY1.0C750 after thiophene adsorption

-1.6

100 200 300 400 500 600 700 800

100

200

o

300

400

500

600

700

800

o

Temperature ( C)

Temperature ( C)

Figure 10 TG and DTG curves of HY sorbents adsorbed pure benzene and pure thiophene 18


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Moreover, as displayed in Figure 11(a), the percentage of the first weight loss step in TG curves of different sorbents are closed to each other (about 20%). As shown in Figure 11(b), the weight loss percentage of the second step is 6.57%, 5.84%, and 1.55% for HY1.0C300, HY1.0C620, and HY1.0C750 after thiophene adsorption, and it decreases significantly with Bronsted acid sites, which also indicates that the thiophene could be adsorbed on Bronsted acid sites by H-S interaction, and this interaction is stronger than the former.

20

after benzene adsorption after thiophene adsorption 20.85

18.14

19.05 18.49

15 10 5 0

5.84

5 4 3 2 1.36

1

HY300

b

6.57

6

20.22 18.71

7

a Weight loss (%)

25

Weight loss (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


HY620

0

HY750

HY300

1.55 1.1

1.09

HY620

HY750

Sample

Sample

Figure 11 Weight loss percentage calculated from TG curves of samples in the temperature range of 30-300 °C (a) and 300-700 °C (b) In order to study the interaction between thiophene and sorbent, the FTIR was carried out to characterized the HY1.0C300, HY1.0C620, and HY1.0C750 sorbents after thiophene adsorption. As illustrated in Figure 12, the band at 1396 cm-1 is attributed to the C=C bond in thiophene ring. The bands around 1436 cm-1, 1454 cm-1, and 1505 cm-1 were reported that the thiophene was transferred into oligomerization under the catalyzing on middle strong Bronsted acidic sites.42 On acidic zeolite, unsaturated compound could be easily induced by proton, and π electron that existing on thiophene ring is susceptible to the electrophilic attack as shown in Figure 13. As a result of protonation effects, capacity of thiophene increased in large range. After protonation, there may occur oligomerization between thiophene and protonated thiophene (Figure 13), and this 19



oligomerization was also observed on HFAU zeolites.43 This means that thiophene could be adsorbed on middle strong Bronsted acidic sites and then catalyzed to form oligomer.

HY1.0C300 1436

Transmittance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1396

HY1.0C620 HY1.0C750 1505

1454

1600

1550

1500

1450

1400

1350

1300

-1

Wavenumber (cm )

Figure 12 FT-IR spectra for HY sorbents obtained at different calcination temperatures after thiophene adsorption

Figure 13 Polymerization process of thiophene 3.3 Thiophene adsorption mechanism over HY sorbent From above discussion, it can be concluded that thiophene could be adsorbed over HY sorbents by two interactions: one is formed by the conjugated π bond in thiophene ring providing electrons and Lewis acid sites accepting the electrons, and the other one is H-S bond between the sulfur atom in thiophene molecule and Bronsted acidic sites (Figure 14). For these two interactions, the former is weaker to let the thiophene 20


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Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


adsorbed could be desorbed at 100 °C. The former interaction is more suitable for the desulfurization for thiophene from aromatic hydrocarbon free environment because the benzene derivatives also have conjugated π bond, which would bring the competitive adsorption effect, whereas the latter interaction would enhance the desulfurization performance in aromatics. Moreover, thiophene could be catalyzed into oligomer over middle strong Bronsted acidic sites in NaY zeolite. From this point of view, it is suggested that, for the desulfurization purpose by NaY zeolite in aromatic free system, its weak and middle strong Lewis acid should be increased, and for that in aromatic existing system, the increase of Bronsted acid site with relative low strength (to avoid the oligomer production) should be thought about.

Figure 14 Schematic diagram of adsorption of thiophene and benzene on Bronsted and Lewis acidic sites in zeolite 4 Conclusions During the preparation of HY sorbent from the modification of NaY in NH4NO3 solution, when the NH4NO3 concentration increases from 0.05 mol/L to 1.0 mol/L, the Bronsted and Lewis acidic sites, surface area all increase, which result in the promotion of the desulfurization performance of HY sorbent in thiophene-benzene. On Lewis acidic sites, both thiophene and benzene could be effectively adsorbed through the interaction between conjugated π bond in thiophene or benzene and Lewis acid sites. 21



And thus on Lewis acidic sites, thiophene adsorption could be obviously competed by benzene. On Bronsted acidic sites thiophene can be efficiently adsorbed via the H-S bond between the sulfur atom in thiophene molecule and Bronsted acidic sites, and this H-S bond is stronger than the former interaction. Moreover, the middle strong Bronsted acid could cause the oligomerization of thiophene. Associated Content Supporting Information Deconvolution of Py-FTIR spectra, XRD patterns, Nitrogen adsorption- desorption isotherms, and fixed bed-MS results (PDF) Author Information Corresponding Author *Tel: +86 035 6010482. Fax: +86 035 6010482. E-mail: [email protected]. ORCID Weiren Bao: 0000-0003-3822-0239 Notes The authors declare no competing financial interest. Acknowledgments This work was supported by the National Natural Science Foundation of China [grant number 21406151]; Research Project Supported by Shanxi Scholarship Council of China [grant number 2015-040]; Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi; and the Natural Science Foundation of Shanxi Province [grant number 201601D021129].

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Insight into the Acid Sites over Modified NaY ... - ACS Publications

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