Benzene, Toluene, m-Xylene Adsorption on Silica-Based Adsorbents

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Benzene, Toluene, m Xylene adsorption on Silica based adsorbents Thamsanqa Ncube, K. Suresh Kumar Reddy, Ahmed Sultan Al Shoaibi, and Chandrasekar Srinivasakannan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03192 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 21, 2017

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Benzene, Toluene, m Xylene adsorption on Silica based adsorbents T.Ncube, K. Suresh Kumar Reddy, Ahmed Al Shoaibi*, C. Srinivasakannan Chemical Engineering Department, The Petroleum Institute, P.O. Box 2533, Abu Dhabi, UAE ABSTRACT It was proposed that incorporation of nitrogen groups on mesoporous silica pores would enhance its surface polarity and improve adsorption of gaseous BTX (Benzene, Toluene, and m Xylene). Among the popular list of mesoporous silica adsorbents, KIT-6 and SBA-15 were selected as a result of their superior textural properties, as well as they are being widely reported in literature. After preliminary screening using toluene test, KIT-6 was chosen to have better affinity and therefore selected for Aptes (3-Aminopropyl triethoxysilane) surface modification. Aptes modification was performed through grafting method utilizing factorial design of experiments technique to identify the effective process variable covering the parameters; Aptes concentration, Reaction temperature and Reaction duration. The results of a statistical design of experiments using Minitab-15 showed that Aptes concentration was the only significant factor which affects the silica adsorbent BET surface area. Benzene adsorption was found to be highest on KIT-6 adsorbent, whilst m xylene and toluene had their highest values on 0.006% Aptes adsorbent (KIT-6 modified with 0.006% v/v Aptes) relative to other adsorbents. m Xylene had highest adsorption relative to toluene and benzene on all modified silica adsorbents (0.006% Aptes, 0.33% Aptes, and 0.66% Aptes) except for 8% Aptes adsorbent. 0.006% Aptes adsorbent was selected as the best adsorbent for BTX adsorption. Key Words: 3-Aminopropyl triethoxysilane (Aptes), Silica, Benzene, Toluene, m Xylene, Adsorption.

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1. Introduction Third of the world's natural gas reserves contain high concentration of H2S, CO2 and in some cases other sulphur compounds such as carbonyl sulphide (COS). In the gas amine sweetening absorption column not only H2S and CO2 are absorbed but also other volatile organic and aromatic compounds such as benzene, toluene and m xylene (BTX). Many gas processing operations claim to have encountered claus catalyst poisoning in their SRUs (sulfur recovery units). This phenomenon has affected sulfur yield and claus process operation time for many years, mainly attributed to the presence of BTX [1]. It is well established that presence of BTX in the incoming gas lowers the lifetime of the catalyst in the sulphur recovery reactors as a result of coke formation on its surface. Hence in order to overcome this issue, the whole gas flow is currently directed to the furnace to eliminate the BTX through combustion in many operations. Since all the gas need to pass through the furnace, it results in lowering the temperature of the gas exiting the furnace. To solve this problem different methods such as preheating of the combustion air, oxygen enrichment of the feed gas and increasing the concentration of H2S in the feed stream are being explored. Low H2S content in the incoming gas may further lower the furnace flame temperature, resulting in flame blow off and eventually leading to plant shut downs. The present work envisages removing the BTX through adsorption process, prior entry to the furnace, facilitating the conventional 2/3rd bypass. This problem initiated an intense research on BTX removal, especially

by use of adsorbents such as porous carbon, MOFs, silica etc., [1]. Most of the literature on BTX removal refers to use of porous carbon based sorbents as they are the well-researched, commercially available and cheap [1, 2]. Additionally they possess different pore structure, pore size distribution and functional groups, all which render them different from each other, although all are referred as porous carbons [2]. Wang et al [3] reported benzene, toluene and m xylene adsorption on porous carbon as 18.02 mg/g, 16.03 mg/g and 31.09 mg/g


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respectively. However, research on utilization of other adsorbents for BTX removal are very sparingly reported in literature. In this connection the present work would assess mesoporous silica based sorbents as a potential BTX adsorbent. Mesoporous silica continues to be one of the most promising adsorbents since its discovery by Mobil Corporation, as a result of its favorable textural properties, ability to tune desirable properties and ease of chemical surface modification [2]. Sporadic literature reported surface properties of functionalized and virgin silica adsorbents and their influence on BTX adsorption [4-5]. Silica adsorbent-aromatic interaction is attributed to the hydrogen bond between the hydrogen atom bonded to the Si-OH and the aromatic delocalized pi electrons, and the adsorbed aromatic is reported to lie flat on the interface [6]. These interactions are said to be weak hence impacting negatively on BTX adsorption capacity and kinetics. As a result attempts to investigate the performance of surface modified silica adsorbents on BTX adsorption is reported in the literature [4, 5]. Benzyl group based modified periodic mesoporous organosilica (PMO) was reported to have adsorption efficiency in aqueous solution higher than 60% for toluene, p-xylene and o-xylene and 44% for benzene [4]. Studies on the effect of hydrophobicity on BTX adsorption on silica aerogels through silica gel doped with methyl groups showed that benzene adsorption is higher for hydrophilic adsorbents than hydrophobic ones [5]. Amine functionalized silica has been widely reported to enhance carbon dioxide and hydrogen sulfide gas adsorption [7-9]. BTX adsorption on amine modified silica adsorbents has not been reported in open literature. The literature with some relevance is the adsorption of polycyclic aromatics (naphthalene) using Aptes modified SBA-15 in aqueous environment. The results were favorable showing 79.3% adsorption at highly acidic conditions (pH of 2). The authors attributed


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this result to stronger interactions between the positively charged amine functional groups (NH3+) of the NH2-SBA-15 adsorbent and the delocalized pi electron system of the aromatic ring of naphthalene [10]. Authors have reported distinct amine structures on the modified Aptes-silica adsorbents surface, which includes hydrogen bonded, free, protonated amino groups (R-NH3+,

-

O-Si) and others [9-13]. The protonated amine group (R-NH3+) was observed to be dominant at low Aptes loading [14]. It was observed that Aptes modified SBA-15 exhibited an FT-IR spectra peak at 1520 cm-1 and a wide band at 2700-3400 cm-1 which is assigned to the NH3+ bending and stretching vibrations respectively. The NH3+ group was seen to be more visible as the Aptes concentration increase in the modified SBA-15 adsorbent [13]. It is possible that the NH3+ species at lower Aptes concentration could be more exposed on the surface than at higher Aptes concentration as a result of formation of oligomers. Aromatic pi orbitals overlap to create a cloud of delocalized electrons, creating a dense negative region which is expected to be a point of attraction to the positively charged amine group (NH3+) [15]. It is also important to note that on the modified adsorbent surface, the availability of organic groups associated with amino silane structure on the modified silica surface contribute to its hydrophobicity. This may provoke van der waals interactions with the aromatic molecules. Therefore balance need to be struck between the surface polarity as a result of NH3+ presence and the hydrophobic organic groups which are in the amino silane structure [16]. The present work focuses on amine functionalization of silica based adsorbent to impart positive effects on BTX adsorption capacity. Synthesis of modified silica sorbents with grafting functionalization method using Aptes amino silane has been reported to improve surface polarity [17]. This process is known to cause physical damage to the adsorbent due to pore blockage and hence it is important to functionalize the adsorbent with minimal pore damage. Towards this


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objective the statistical design of experiments was utilized to establish the relationship between the dependent and independent variables. The independent variables being Reaction temperature, Aptes concentration and Reaction duration. The dependent variable being nitrogen adsorption BET surface area. Pure vapors of benzene, toluene and m xylene adsorption was carried out on KIT-6 and Aptes modified adsorbents.

2. Materials and Methods All reagents used for the experiments were analytical grade, Aptes (99.9%, 0.946g/ml), benzene (99.9%), toluene, m xylene were purchased from Aldrich. KIT-6 and SBA-15 were synthesized as per reported literature in our research laboratory [18, 19]. X ray diffraction (XRD) was used to identify the KIT-6 structure and modified samples with Pan Analytical X Pert Pro, using CuK-α radiation. The BET surface area and pore size distribution was determined by use of nitrogen adsorption isotherm from an Autosorb 1-C Adsorption Apparatus (Quanta Chrome Instruments, USA). The samples were initially dried in an oven at 110⁰C for 12 hrs and quickly transferred into the sample tube. The sample was then elevated to 170⁰C and evacuated for 4 hrs until the pressure less than 10-4 Torr. The BET surface area was determined using multi point Brunauer– Emmett–Teller (BET) method. FEG-250 SEM instrument (FEI, Holland) was used at an accumulation voltage of 30 KV with a magnification of 2.5 K to determine the surface morphology of sorbents. Fourier Transform Infrared (FT-IR) spectra of porous adsorbents, prior and post modification, were determined by a Bunker Tensor 27 and Opus data collection program. CHN analysis was also carried out both for modified and silica based sorbents using Euro vector EA 3000 (Euro Vector Instruments, Italy). BTX adsorption was carried out using a gravimetric sorption analyzer “IsoSORP STATIC 3xV-MP” (Rubotherm, Germany).


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3. Experimental 3.1 Sorbent preparation and screening The sorbents utilized in this research were KIT-6 and SBA-15, which were prepared at our research laboratory as per the reported literature [18, 19]. During SBA-15 synthesis only Pluronic P123 was used as structure directing agent, a blend of Pluronic P123 and n-butanol was employed in the preparation of KIT-6 and methods both used tetraethylorthosilicate (TEOS) as a silica precursor [18]. To estimate the sorption capacity of these materials and to screen the sorbent adsorption studies were carried out with toluene at 45oC in the gravimetric sorption analyzer. 3.2 Optimization of surface functionalization of KIT-6 with Aptes KIT-6 (50 mg) powder was suspended in 15 ml anhydrous toluene and Aptes (0.006 v/v% or 8v/v%) solution was refluxed at 110 oC under nitrogen atmosphere for 24 hrs. The product was filtered and washed with excess 2-propanol to remove adhering Aptes and then dried at 105oC under a vacuum and these materials were used for sorption studies. To optimize surface functionalization of KIT-6 with Aptes, statistical design of experiments “Response Surface Methodology” (RSM) was used with process variables being Reaction temperature (X1), Aptes concentration (X2) and Reaction duration (X3). The minimum and maximum ranges of these variables were established based on literature and preliminary experimental studies. The complete experimental plan with respect to the range of these variables values is listed in Table 1. The three independent variables with the responses are shown in Table 2.. The response variable was BET surface area of sorbent (Y). The response was correlated with process variables using a second order polynomial equation as seen below;


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Where ‘Y’ is the predicted response (BET Surface area), bo is the value for the fixed response; and bi, bii, bij are the linear, quadratic and cross product coefficients respectively. The significance of the model parameters were tested using “hypothesis testing” while the appropriateness of the model was validated through analyses of variance (ANOVA) using the Minitab-15 Software. Table 1. High and low levels of factors Factor Reaction temperature (X1) Aptes concentration (X2) Reaction Duration (X3)

Low Level (-1) 80⁰C 0.006% 10 hrs

High Level (+1) 110⁰C 8% 20 hrs

Table 2. Experimental Data Reaction

Aptes

Reaction

BET

temperature,

concentration

duration,

Surface area,

C

%

hrs

m2/g

1

80

8

10

378

2

80

0.006

20

653

3

110

0.006

10

669

4

80

8

20

334

5

110

8

10

450

6

110

8

20

331

Experiment

o


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7

110

0.006

20

629

8

80

0.006

10

779

4. Results and discussion The physical properties and toluene adsorption capacity of SBA-15 and KIT-6 in the prescreening test were shown in Table 3.

Table 3. Textural properties of adsorbents with toluene sorption capacity Pore Adsorbent

Surface area

volume Pore diameter (Å)

Toluene adsorption capacity (mg/g)a

(m²/g)

(cc/g)

SBA-15

1221

1.40

28.1

156

KIT-6

882

1.37

27.8

190

a: Toluene adsorption at p/po=0.6, and temperature at 318 K

It was observed that despite KIT-6 having low surface area compared to SBA-15, had much higher toluene sorption capacity. This may be due to its cubic pore structure with unique 3-D channel network which offers an easy access to the adsorbate in comparison to the 2-D hexagonal pore structure of SBA-15 [15]. Hence KIT-6 was chosen as the base sorbent for surface modification in the present work. 4.1 Surface functionalization of KIT-6 and optimization

As a result KIT-6 was used as a base adsorbent to carry out surface modification with Aptes and to assess the effect of functionalization process parameters on pore blockage, BET surface area


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and adsorption capacity. The conditions which minimize and maximize surface area reduction were identified through a design of experiments with the aid of Minitab-15 software and the results are shown in Table 4 and 5.

Table 4. Estimated Effects and Coefficients for modified KIT-6 BET surface area. Term Constant Reaction temperature Aptes concentration Reaction time Aptes concentration* Reaction temperature Reaction temperature* Reaction time R2 = 98.5%; R2 (adj) = 94.7%

Coefficient 527.9 -8.1 -154.6 -41.1 25.4

T 37.09 -0.57 -10.86 -2.89 1.78

P 0.001 0.626 0.008 0.102 0.217

1.4

0.10

0.932

Table 5. Analysis of variance (ANOVA) for the RSM model for Aptes- KIT-6 BET Surface area Source

Main Effects 2-Way Interactions Residual Error Total

Degrees of freedom (DF) 3 2 2 7

Sum of squares (SS) 205329 5166 3241 213737

Mean squares (MS) 68443 2583 1621

F

P

42.23 1.59

0.023 0.386

From Table 4, the ‘p’ values show that Aptes concentration is the only significant parameter which affects the BET surface area during KIT-6 surface modification process and all other parameters seems to be insignificant at 95% confidence level. Reported literature confirms that 50-60% of the total Aptes bound to KIT-6 at equilibrium happens in the first 60 seconds, which additionally confirms the insignificance of reaction duration [20]. So the relationship between the BET surface area and the Aptes concentration can be shown as below: BET Surface area (m2/g) = 527.9 - 154.6 Aptes concentration (%)


(1)

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An increase in Aptes concentration decreases adsorbent BET surface area [9, 16] and also this can be confirmed from equation (1). KIT-6 modified with 0.006% Aptes concentration at reaction temperature of 80oC and reaction time of 10 hrs was chosen as the best while the one modified with 8% Aptes was chosen to be the worst sorbents. The virgin and modified KIT-6 adsorbents were subjected to detailed characterization and BTX adsorption assessment. Table 5 shows the ANOVA results of the model utilized to relate the independent and dependent variables. The low ‘F’ and the high ‘p’ shows the absence of 2 way interaction. Only the main effects are seen to be significant.


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4.2 Sorbent Characterization

6000 KIT-6

5000

0.006% Aptes 8% Aptes

4000 Intensity

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3000 2000 1000

0 0

10

20

30

40

50

60

70

80

2-Theta

Figure 1. X-Ray diffraction of KIT-6, 0.006% Aptes, 8% Aptes silica adsorbents

The KIT-6 X-ray diffraction pattern indicate a sharp diffraction peak at around 2θ =15-30o which can be indexed as the (211) reflection of the bicontinous cubic Ia3d symmetry [2, 21]. Modified silica samples also exhibit the same pattern therefore confirming that KIT-6 structure was maintained after Aptes incorporation [15]. However small shifts of peaks to lower 2θ angles were observed from Figure.1 especially for 8% Aptes adsorbent. This indicates a slight effect on the cubic structure and may be due to slight pore blockage of Aptes modified sorbent [22].


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0.9

KIT-6 0.006% Aptes 8% Aptes

0.8 0.7 % Transmittance (a.u)

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0.6 0.5 0.4 0.3 0.2 0.1 0 500

1000

1500

2000

2500

3000

3500

4000

Wavelength (cm-1)

Figure 2. Fourier transform infrared transmittance spectra for silica based adsorbents

FT-IR spectra of KIT-6, 0.006% Aptes and 8% Aptes adsorbents are shown in Figure 2. KIT-6 shows a peak at 960 cm-1 which is associated with the Si-OH group [13], and is also be seen for 0.006% Aptes adsorbent but disappears for 8% Aptes adsorbent. This shows that the 0.006% Aptes adsorbent is partially modified and they are free unreacted silanol groups (Si-OH) on its surface whilst for 8% Aptes adsorbent the silica surface have been fully covered with Aptes.

The presence of a C-H group peak at 2800-3000 cm-1 in 8% Aptes sorbent was observed, and for 0.006% Aptes sorbent overall spectra resembles that of the base material. Also the N-H group is not visible for 0.006% Aptes adsorbent due to fewer amounts of bound Aptes molecules, but for 8% Aptes adsorbent this peak is visible at 687 cm-1 [15]. A broad band at 2700-3400 cm-1 and a


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peak at 1510 cm-1 attributed to NH3+ [13, 15], are clearly observed for 8% Aptes adsorbent. The spectra observed at 1630 cm-1 is attributed to adsorbed water [13, 15]. These FT-IR results confirm the incorporation of Aptes in the KIT-6 structure.

a)

b)

c)

Figure 3. SEM Analysis: a) 8% Aptes, b) 0.006% Aptes, c) KIT-6 Figure. 3 shows SEM images of KIT-6, 0.006% Aptes and 8% Aptes adsorbents. SEM images analysis clearly show that KIT-6 and 0.006% Aptes sorbent are more porous than the 8% Aptes,


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indicating the presence of excess Aptes on the external surface, contributing to the loss of porosity as evidenced through the N2 adsorption isotherms [6].

Table 6. Physical properties of adsorbents Nitrogen

Carbon

Surface

Pore diameter

content

content

area

Å

%

%

m2/g

KIT-6

-

-

882

27.9

0.006 % Aptes

0.87

4.91

779

27.8

0.33% Aptes

2.50

8.52

425

27.8

0.66% Aptes

2.55

9.85

388

24.2

8% Aptes

4.28

11.67

311

24.5

Adsorbent

The physical properties of KIT-6, 0.006% Aptes, 0.33% Aptes, 0.66% Aptes and 8% Aptes adsorbents are shown in Table 6. An increase in the Aptes concentration was found to increase nitrogen and carbon content in the adsorbent, whilst the surface area and average pore diameter was found to decrease [9, 16]. The surface area reduction for 0.006% Aptes adsorbent was minimal and highest for 8% Aptes adsorbent.


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4.3 BTX adsorption

200 KIT-6 Adsorption Capacity (mg/g)

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0.006% Aptes 150

0.33% Aptes 0.66% Aptes 8% Aptes

100

50

0 0

20

40

60

80

Pressure (mbar)

Figure 4. Effect of Aptes concentration on benzene adsorption Figure 4. shows an increase in adsorption capacity with an increase in pressure, this behavior is as a result of an increase in adsorbate concentration. KIT-6 has the highest benzene adsorption at all pressures and it decreases with an increase in amount of incorporated Aptes. KIT-6 is highly hydrophilic due to the presence of Si-OH groups on its surface and benzene is reported to have affinity to hydrophilic surfaces [5, 23]. This affinity is as a result of hydrogen bonds between Si– OH and the delocalized pi electron system in the aromatic ring [24].

During amine functionalization Aptes propyl groups were introduced in the KIT-6 structure resulting in the silica surface being hydrophobic [25]. Aptes impacts hydrophobic behavior not only by eliminating the hydroxyls, but also by providing anchor points for the nonpolar organic


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substitution of the silane which shields the NH2 group from the surface [25, 26]. This hydrophobicity of modified silica is the reason for poor benzene adsorption [5, 27]. The extent of hydrophobicity depends on the Aptes concentration used in the modification process [26] resulting in a decrease in benzene adsorption as the Aptes concentration increases.

Toluene and m xylene have highest adsorption on 0.006% Aptes adsorbent as seen in Figure 5. It was observed that Aptes concentrations higher than 0.006% reduced toluene and m xylene adsorption.

250

Adsorption Capacity (mg/g)

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KIT-6

200

0.006% Aptes 0.33% Aptes

150 100 50 0 0

10

20

30

40

Pressure (mbar)

(a)


50

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200 Adsorption Capacity (mg/g)

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KIT-6 0.006% Aptes 0.33% Aptes 0.66% Aptes 8% Aptes

150

100

50

0 0

2

4

6

8

10

12

14

Pressure (mbar)

(b) Figure 5. (a) Effect of Aptes concentration on toluene adsorption (b) Effect of Aptes concentration on m xylene adsorption

m-Xylene and toluene have higher molecular weights and attached methyl groups, as a result they exhibit stronger hydrophobic interactions with the Aptes propyl groups on the modified silica surface [27, 28]. Similar observations were reported [23] and claim that the presence of alkyl groups in m xylene and toluene result in their stronger attraction to hydrophobic surfaces (organic groups) and their adsorption is preferred to the aromatic ring.

The introduction of minimal amount of Aptes propyl groups (0.006% Aptes) on the silica surface resulted in less reduction in BET surface area, pore volume and pore diameter damage hence an improved toluene and m-xylene performance. As the Aptes concentration increases the


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hydrocarbon chains on the silica surface increase contributing to a reduction in the BET surface area, pore volume and pore diameter, offsetting the beneficial stronger hydrophobic interactions with surface Aptes propyl groups [9, 16].

200 Adsor[tion Capacity (mg/g)

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(a) 150

100 Benzene Toluene Xylene

50

0 0

0.1

0.2

0.3 P/Po

0.4

0.5


0.6

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400 300

(b)

200 100 0 0

0.1

0.2

0.3 0.4 P/Po

0.5

0.6

200


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(c)

150

100

50

0 0

0.1

0.2

0.3 P/Po

0.4

0.5


0.6

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Adsorptiom Capacity (mg/g)

150 (d)

100

50

0 0

0.1

0.2

0.3 0.4 P/Po

0.5

0.6


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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100 (e)

80 60 40 20 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

P/Po

Figure 6. BTX adsorption on a) KIT-6 adsorbent b) 0.006% Aptes adsorbent c) 0.33% Aptes adsorbent d) 0.66% Aptes adsorbent e) 8% Aptes adsorbent The effect of various Aptes modified adsorbents on BTX adsorption is shown in Figure 6. The reported interaction between silica adsorbents and aromatics as a result of hydrogen bonding depend on the polarity index of the aromatic species. The higher the index the better and stronger the interaction [24, 27]. It was observed from Figure 6 that benzene had the highest adsorption


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on hydrophilic KIT-6 [5] followed by m xylene and toluene, and this trend was according to the increase in polarity index (Benzene (2.7)> m Xylene (2.5)> Toluene (2.4)) [27]. From Figure 6, it was observed that m xylene has the highest adsorption followed by toluene and benzene, on all modified KIT-6 except for 8% Aptes modified adsorbent, where toluene adsorption was highest followed by m xylene and benzene. This behavior is attributed to the fact that m xylene has the highest molecular weight and two methyl groups in its structure, therefore has stronger hydrophobic interactions with surface Aptes propyl groups [23, 25]. Hence m xylene performs better on Aptes modified silica adsorbents, followed by toluene then benzene. This trend is according to the increase in molecular weight and number of methyl groups present (m xylene (106.6 g/mol) >toluene (92.1 g/mol) > benzene (78.1 g/mol)) [25].

Low m xylene adsorption on 8% Aptes adsorbent was as a result of pore blockage during functionalization of KIT-6 with high Aptes concentration, restricting m xylene access to the adsorbent pore structure [9, 16]. m Xylene sorption is the most affected since it has a larger molecular size. At high adsorption pressures there is a sudden jump in adsorption capacity for all adsorbents which could be due to multilayer adsorption and capillary condensation.

The results show that adsorption on the Aptes modified silica adsorbents were mainly influenced by the presence of Aptes propyl group than the nitrogen species. The NH3+ groups which were expected to improve surface polarity were overshadowed by the presence of the carbon structure which causes modified KIT-6 to be more hydrophobic [25]. It was observed that even at very low Aptes concentrations (0.006% Aptes adsorbent) hydrophobic behavior is significant, which caused benzene despite having highest polarity index to perform poorly relative to KIT-6 [5, 23,


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25]. Nevertheless 0.006% Aptes adsorbent was selected as the best adsorbent for BTX due to its high adsorption capacity of m xylene, toluene and acceptable performance towards benzene.


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318 K 298 K

300

200

100

0 0

0.1

0.2

0.3

0.4

0.5

P/Po

Figure 7. m Xylene adsorption on 0.006% Aptes adsorbent at 318 K and 298 K Figure 7 shows the response of m xylene sorption with temperature. An increase in the temperature was found to decrease adsorption. A decrease in adsorption with increase in temperature indicate the adsorption to be exothermic and predominance of physisorption over chemisorption. Physisorption can also be validated by the formation of multilayers at high adsorbate concentration/pressures for all adsorbents [29]. 0.006% Aptes adsorbent recovered its activity after each m xylene adsorption (45oC) /desorption (150oC) cycle. Ten regeneration cycles were carried out with helium purging after each cycle.

5. Conclusion The effect of presence of nitrogen groups as against the pore structural damages in the process of Aptes incorporation in the surface of silica based adsorbents was assessed. Its effect on the adsorption of the aromatics namely BTX has resulted in the following key conclusions;


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The process of nitrogen group incorporation in the KIT-6 using Aptes at different concentration on one hand increased the nitrogen and carbon content while on the other hand significantly reduced the sorbents BET surface area. The BET surface area reduction was minimal for the lowest of Aptes concentration sorbent and its incorporation had no significant structural change on KIT-6. Benzene adsorption was highest on KIT-6 compared to all modified silica adsorbents. m Xylene and toluene had highest adsorption on 0.006% Aptes adsorbent relative to all other adsorbents utilized in the present work. 0.006% Aptes adsorbent was selected as the best adsorbent for BTX removal due to its slight hydrophobicity. This enhances m xylene and toluene sorption even though slightly decreased benzene sorption. Slight Aptes modification of KIT-6 proves to be the best sorbent for removal of toluene and m xylene. m Xylene had the highest adsorption as compared to benzene and toluene on all the Aptes modified silica adsorbents except for 8 % Aptes adsorbent. On all adsorbents the adsorption phenomenon was found to be predominantly physisorption in nature.

Funding: This work was supported by the Gas Research Center, Petroleum Institute, Abu Dhabi, [11003, 2013-2016].

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Benzene, Toluene, m-Xylene Adsorption on Silica-Based Adsorbents

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