Density Functional Theory Study of Organosulfur Selective Adsorption

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Density Functional Theory Study of Organosulfur Selective Adsorption on Ag-TiO Adsorbents 2

A H M Shahadat Hussain, Michael L McKee, John M Heinzel, Xueni Sun, and Bruce J. Tatarchuk J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 19 Jun 2014 Downloaded from http://pubs.acs.org on June 20, 2014

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The Journal of Physical Chemistry

Density Functional Theory Study of Organosulfur Selective Adsorption on Ag–TiO2 Adsorbents

A. H. M. Shahadat Hussain1, Michael L. McKee2, John M. Heinzel3, Xueni Sun1, and Bruce J. Tatarchuk1* 1

Center for Microfibrous Materials Manufacturing (CM3), Department of Chemical Engineering, Auburn University, Auburn, AL 36849, USA. 2

Department of Chemistry and Biochemistry, Auburn University, Auburn, AL 36849, USA 3

Energy Conversion Section, NAVSEA, Philadelphia, PA 19112, USA

Keywords: Desulfurization, density functional theory, adsorbent, adsorption energy

ABSTRACT: Ag–TiO2 adsorbents have pronounced capacity for selective removal of organosulfur compounds from complex fuel mixtures. Computational calculations were performed to investigate the nature of this pronounced selectivity as well as to study the adsorbent structure. A cluster model was developed for this study. Geometry optimization, frequency analysis and single-point energy calculations

were

carried

out

using

31G(d)(ECP=SDD(Ag,Ti))//B3LYP/LANL2DZ).

density

functional

The computed

theory

(B3LYP/6-

adsorption energies included

dispersion terms (GD3) and were corrected for basis set superposition errors (BSSE). Silver incorporated onto anatase-TiO2 clusters, and with greater preference in the presence of –OH groups. Adsorption energies were calculated for sulfur containing species (thiophene, benzothiophene, dibenzothiophene, 4,6-dimethyldibenzopthiophene) and non-sulfur aromatics (quinoline, benzofuran,

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naphthalene, benzene) typically present in fuel mixtures. Calculated adsorption energies are consistent with the selective binding of organosulfur compounds through the Ag atom of the AgTi6O8(OH)8 cluster rather than the –OH groups of the titania analog. The adsorption orientation of organosulfur compounds was π-preferred. Heterocycles with more aromatic rings were adsorbed more strongly. Organonitrogen compounds (i.e., quinoline) showed the strongest adsorption. Results from equilibrium saturation adsorption experiments were also compared with DFT calculations and the trends and selective separation factors were shown to be in good agreement.

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1.


Introduction Desulfurization of logistic and commercial hydrocarbon fuels is an essential step in fuel

processing for both environmental concerns1,2 and fuel cell applications.3,4 To produce ultra-low sulfur fuels, adsorptive desulfurization is being investigated as a viable supplement to the conventional hydrodesulfurization (HDS) process.3,5,6 Adsorptive desulfurization is promising for its low energy requirements and scalability to small size.1 The adsorbents used in the process must have high selectivity toward refractory sulfur heterocycles.5 Supported and unsupported silver–titania adsorbents have demonstrated regenerable organosulfur adsorption capacities from hydrocarbon fuels such as jet fuels and diesels.6-8 Supported Ag–TiO2 adsorbents were able to selectively remove sulfur down to parts per billion (ppbw) levels7 and were effective even when the ratio of non-sulfur to sulfur aromatics was more than 25000:1. These adsorbents have shown efficacy in removing organosulfur compounds, nonetheless very little is known about the role and structure of Ag in the adsorbent and/or the selective nature of the adsorption mechanism. The affinity of Ag toward organosulfur compounds depends on the position of Ag in a support matrix and on Ag particle sizes. Therefore, it is important to understand how Ag is incorporated into the surfaces of high surface area TiO2 supports and the significance of defect sites and surface –OH groups. Sulfur selectivity is a vital criterion for an effective adsorbent. It is known that petroleum-based fuels are a mixture of various organic compounds ranging from simple straight chain hydrocarbons to complex aromatic molecules with various functional groups. In addition to organosulfur compounds, typical fuel mixtures have much higher concentrations of other aromatic hydrocarbons and organonitrogen and organooxygen compounds. These compounds will compete with organosulfur compounds for active sites on adsorbent surfaces. Therefore an effective adsorbent should have high organosulfur affinity and selectivity. In order to develop a fundamental understanding of sulfur selectivity, a comparison of experimental adsorptive desulfurization performance and computational investigation is warranted. 3 Environment ACS Paragon Plus


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Adsorption energy is an ideal indicator in the sulfur selectivity study.9,10 It can differentiate the strength of various adsorption sites. For estimating adsorption energy, density functional theory (DFT) is a very popular method. DFT is a quantum mechanical technique for calculating optimized geometries and energies. It is an efficient tool, especially for transition metals, and can be used with an Effective Core Potentials (ECP) representation of the core electrons.11 Researchers have previously reported DFT studies of adsorption on anatase-TiO210,12 and rutile-TiO2.13 DFT calculations were also used to investigate optical properties14 and photocatalytic activities15 of Ag–TiO2 materials. Other researchers have employed DFT techniques in investigating thiophene adsorption on π-complexation adsorbents11,16,17 and thiophene incorporation during HDS reactions.18 A similar adsorption modeling of Ag–TiO2 adsorbents with DFT would be beneficial for a better design of selective organosulfur adsorbents. Here we present a DFT study to investigate the sulfur selectivity of Ag–TiO2 to provide an accurate description of the adsorption phenomena. In addition, we also investigate the adsorption energies of organosulfur compounds to the surface hydroxyl groups on the TiO2 cluster and compare these with experimental equilibrium saturation capacities. Separation factor values of Ag–TiO2 are also presented in order to test its ability as an effective commercial adsorbent.

2.

Experimental and Computational Methodologies:

2.1.

Adsorption Experiments: Equilibrium saturation experiments were carried out for calculating saturation capacities. Neat

and Ag-supported anatase-TiO2 (particle size: 850-1400 µm) were used for this study. Neat TiO2 (source: Saint Gobain Norpro) was used in saturation experiments after drying (at 110 °C for at least 6 h in air) and calcination (at 450 °C for 2 h in air). Ag–TiO2 was prepared by impregnating silver nitrate (source: Alfa Aesar) onto TiO2 support followed by drying and calcination at aforesaid conditions. Experimental steps regarding adsorbent preparation can be found elsewhere.19 In the saturation 4 Environment ACS Paragon Plus

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experiments, the adsorbent was treated with model fuel consisting of various organosulfur (T, BT, DBT, 4,6-DMDBT) compounds in n-octane (C8). These were purchased from Alfa Aesar and Acros Organics. The organosulfur compounds were dissolved into C8 individually where the sulfur concentration was 1000 parts per million by weight (ppmw) in each case. In every saturation test, the adsorbent was mixed with a model fuel and was mixed in a mechanical shaker for 48 h after which the equilibrated fuel was measured for adsorbate concentration. In each case, the fuel to adsorbent ratio was 20 ml/g. For analyzing sulfur concentration, the equilibrated fuels were measured in an Antek 9000S total sulfur analyzer. Each experiment was carried out multiple times and the error was within 10%. Experimental details on saturation tests and the analytical equipment configuration have been reported previously.7 2.2.

DFT Calculations: Geometry optimizations, frequency analysis, and single-point energy calculations were

undertaken using DFT methods. All the calculations were carried out using the Gaussian 09 software package on computers located at the Alabama Supercomputer (ASC) Facility. The models were constructed using Gaussview (version 5.0) and Molden at Auburn University. Geometry optimizations were performed with a double-ζ basis set with ECP20 for Ti and Ag and the B3LYP combination of Becke gradient corrected exchange functional and Lee-Yang-Parr correlation functional.21,22 Effective core potentials (ECP) are used to represent the core electrons for post third row atoms and can substantially reduce computation time with almost no effect on the results since the core electrons are assumed to have a minor effect on adsorption. After optimization, the geometries were subjected to frequency analysis using B3LYP/LANL2DZ in order to verify whether the geometries were at true minima on the potential energy surface. This was followed by single-point energy calculations using B3LYP with the 6-31G(d) basis set on all non-metal atoms and the Stuttgart effective core potential (SDD) on all metal atoms (Ti and Ag). In summary, the single-point energies were calculated at the B3LYP/[6-31G(d)+SDD] level on structures optimized at the B3LYP/LANL2DZ level. An empirical 5 Environment ACS Paragon Plus


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dispersion term “GD3” was added to the single-point energy calculations in order to include dispersion effects. The dispersion term employs structure (coordination number) dependent dispersion coefficients that are based on first principles calculations. It provides more reliable binding energies and is particularly useful for systems with metals.23 The natural population analysis (NPA) charges were also calculated and discussed.9 2.3.

Binding and Adsorption Energy: Binding energies were calculated to study the Ag incorporation and –OH generation

mechanism on the TiO2 clusters. Adsorption energies were calculated for various sulfur and non-sulfur aromatics.

Thiophene

(T),

benzothiophene

(BT),

dibenzothiophene

(DBT),

and

4,6-

dimethyldibenzothiophene (4,6-DMDBT) were used as sulfur aromatics whereas benzene (C6) and naphthalene (C10) were used as non-sulfur aromatics. To compare the organosulfur compounds with the organonitrogen and organooxygen compounds, quinoline and benzofuran were employed. The following equation was employed for calculating the adsorption energy (Eads): Eadsorption = Eadsorbent-adsorbate - (Eadsorbent + Eadsorbate) Where, Eadsorbent-adsorbate, Eadsorbent and Eadsorbate are the energies of adsorbent-adsorbate system, free adsorbent, and free adsorbate, respectively. A more negative value of Eads corresponds to a stronger adsorption. Solvent effects were not taken into account since the adsorption usually takes place in a non-aqueous environment where the effect of polarity and polarizability is negligible. Since these were non-covalent interactions, the Eads values were corrected for Basis Set Superposition Errors (BSSE). The counterpoise correction (CP)24 method was employed to calculate the BSSE for adsorbentadsorbate interactions.25 In our work, the range of BSSE was between 12–26 kJ/mol. Since the CP method is known to overestimate the BSSE, the final energies were calculated by taking an average of the BSSE corrected and uncorrected adsorption energies.26 Details regarding BSSE corrections can be found elsewhere.26 The BSSE correction was not included in the Ag and H2O binding energy


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calculations. Zero-point corrections of the energies were not calculated since our objective was to qualitatively investigate the adsorbent selectivity through the adsorption energies. 2.4.

Separation Factor: Separation factors (α21) were calculated from adsorption energies.16 The following formula was

used to determine α21: α =

X /Y X /Y

where X and Y are mole fractions in the adsorbed and solution phases, respectively. In the current work, subscripts 1 and 2 corresponded to non-sulfur aromatics (benzene) and heterocycles (i.e. organosulfur, organonitrogen, organooxygen) compounds, respectively. The heterocycles typically have low concentrations in fuels and they follow the Henry’s law. Therefore, the group X2/Y2 yields the Henry constant (H2) for the heterocycles. Non-sulfur aromatics typically have much higher concentration than heterocycles’. In commercial ultra low sulfur diesel, the ratio of non-sulfur aromatics to organosulfur compounds can be 25000:1.7 As a result X1 is much smaller than that estimated from Henry’s law.16 In other words, the Henry constant (H1) for non-sulfur aromatics is no less than X1/Y1. Therefore the following equation can be reached: α ≤

H H

The Henry constant is approximately proportional to e–E/RT, where E, R, and T are the adsorption bond energy, universal gas constant, and temperature, respectively. For low X1 cases where the non-sulfur aromatics follow Henry’s law, the previous equation becomes: α = e

( )

The adsorption energies calculated from DFT methods were employed in separation factor calculations. More information regarding the procedure of separation factor calculation can be found elsewhere.16



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3.

Results and Discussion:

3.1.

(Experimental) Effect on Various Sulfur Heterocycles on Equilibrium Saturation Capacity:

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Desulfurization experiments were carried out to investigate the effect of various sulfur heterocycles on sulfur adsorption capacity and to compare with the computational results. Figure 1 shows the saturation capacities of neat and Ag supported TiO2 adsorbents for model fuels with T, BT, DBT, and 4,6-DMDBT. In terms of saturation capacity, the order from high to low was DBT>4,6DMDBT>BT>T. The addition of benzene rings resulted in higher adsorption capacity. This has been attributed to the π-electron cloud of aromatic rings which can increase dispersion interactions as well as donor-acceptor interactions. The selectivity order of Ag–TiO2 adsorbent makes it ideal as a polishing agent since the thiophenic molecules are more refractory to HDS process. However, the adsorption capacity for 4,6-DMDBT was lower than DBT, possibly due to increased steric hindrance from the attached methyl groups.27 The experimental trend in the adsorption capacities is consistent with the continuous adsorption experiments, however the steric hindrance effect of 4,6-DMDBT is greater because of diffusion.28 We have also carried out competitive adsorption experiments where the Ag–TiO2 adsorbent showed remarkable selectivity toward sulfur heterocycles.7,37


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Figure 1. Equilibrium saturation capacities acquired from saturation experiments (adsorbents: TiO2 and 4 wt% Ag–TiO2, duration: 48 hours, fuel to adsorbent ratio: 20 ml/g, initial sulfur concentration in model fuel: 1000 ppmw S). 3.2.

(Computational) Model Construction for DFT Calculations: We constructed different cluster models in order to simulate supported and unsupported TiO2

and Ag–TiO2 based on the adsorbent characterization results reported previously.29,30 In both unsupported and supported Ag–TiO2, the anatase phase of TiO2 was shown to have higher capacity and the ability to effectively disperse Ag.29,30 The oxidation state of Ag was measured to be +1, as seen from electronic spin resonance (ESR) and x-ray absorption spectroscopy (EXAFS) studies.30,31 EXAFS studies further revealed that the Ag–O coordination number was 4 and the Ag–O bond distance was 2.32 Å.30 Ag–Ag or Ag–Ti interactions were not seen within 3 Å. The supported Ag–TiO2 adsorbent (10 wt% Ag/TiO2–Al2O3; Ti:Al = 1:4.4 by weight) was analyzed via X-ray diffraction (XRD), where the diffractograms did not show any Ag or Ag oxide peaks. It indicated that silver was present as nanosized or even smaller sized particles.29 The diffraction invisible nature of supported Ag (up to 10% by weight) pointed that TiO2 can facilitate high dispersion of Ag. We also carried out infrared (IR) studies on the neat and Ag supported adsorbents and found –OH signatures in both.29 Changes in –OH bands were also observed in the IR spectra of adsorbents after Ag impregnation.29 Therefore, the –OH groups are closely associated with Ag. Based on these observations, the neat and silver-supported titania clusters were constructed. The overall size of each cluster was below 10 Å. The clusters were relaxed and were assumed to be free of contaminants. The Al2O3 and SiO2 supports used in the supported Ag–TiO2 adsorbents7 would have some effect on organosulfur adsorption. However, the contribution is minor as compared to that of Ag–TiO2. Therefore, these were not included in the DFT calculations because of simplicity. For representing anatase TiO2 without any defect site, a small cluster consisting of six Ti atoms was constructed (Ti6O9(OH)6). The cluster model was chosen to model the TiO2 cluster where the number



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and lengths of Ti–O nearest neighbors were very similar to experiments on the anatase surface. The overall +4 oxidation states of titanium atoms were preserved by adding OH groups to the edges. These hydroxyl groups were assumed to have minimum effect on the adsorption energies. Smaller nano clusters of TiO2 such as Ti2O2(OH)4 have been used in the past40 to study molecular and electronic effects. Larger clusters (up to Ti12O24) have also been considered41 to determine the most stable cluster structure. The cluster presented here is large enough to bind an Ag atom in a realististic fashion yet small enough to be computationally tractable. The most probable consequence of our truncated cluster model is that the HOMO-LUMO gap was too large.42 A larger TiO2 cluster (and smaller HOMOLUMO gap) would have probably increased donor-acceptor interactions and thereby would have increased the binding energies of the organosulfur compounds considered in this manuscript. To construct the Ag supported cluster, Ti6O9(OH)6 was incorporated with a neutral Ag atom. One Ag atom was placed between four oxygen atoms (AgTi6O9(OH)6). The geometrically optimized neat and Ag supported TiO2 clusters (without any defects) are shown in Figure 2. All the structures had neutral charge with singlet multiplicity, except for the Ag supported clusters which had neutral charge with doublet multiplicity.30 The Ti coordination number in these clusters was 4. Four-coordinated Ti are more active than five- or six-coordinated Ti,32 and are usually more available at the surface than in the bulk and Ag incorporation is a surface phenomenon in our case. The effect of Ag impregnation on the titania cluster was evaluated by calculating the binding energy. The following mechanism was assumed for Ag incorporation:

Ti6O9(OH)6 + Ag AgTi6O9(OH)6 where, E = E

(!) -

(E

(!) +E )

Here E represents the binding energy of Ag incorporation. The structure energies were acquired by means of calculating the single-point energies of the geometrically optimized structures, as described


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in section 2.2. The value of E" was calculated to be -24.6 kJ/mol. The negative value indicated that the incorporation reaction was exothermic.

Figure 2. Geometrically optimized structures of (a) Ti6O9(OH)6 and (b) AgTi6O9(OH)6 (with no extra –OH groups). The clusters discussed earlier contained TiO2 with no defect site or –OH group. Here, –OH groups were introduced to the Ti6O9(OH)6 cluster to investigate the consequent effect in Ag incorporation and organosulfur adsorption. This was carried out by interacting the cluster with H2O. It is well known that anatase TiO2 can dissociate H2O to form –OH groups upon adsorption.33 After H2O adsorption, new –OH groups were formed (Ti6O8(OH)8). The optimized clusters with bridged –OH groups are shown in Figure 3. Some of the bond distances are also shown in Figure 3. Silver was impregnated on the hydroxylated cluster through a similar procedure as before. The overall water/silver incorporation steps can be shown as follows:

Ti6O9(OH)6 + H2O Ti6O8(OH)8 (Two bridged –OH)

Ti6O8(OH)8 + Ag AgTi6O8(OH)8 where, E = E # (!)# - (E

(!) +E! )



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E = E # (!)# - (E # (!)# + E ) Here E and E represent the H2O and Ag binding energies, respectively. The E and E values were calculated to be -72.6 and -69.9 kJ/mol, respectively. The values indicated that the incorporation mechanism was likely to be spontaneous. In addition, Ag incorporation with a hydroxylated titania cluster resulted in an almost three times increase in negative binding energy as compared to a nonhydroxylated cluster. Therefore, –OH groups greatly facilitated Ag incorporation. It should be noted that the present results did not take into account solvent effects. Nevertheless, the results qualitatively indicated that Ag impregnation results in the formation of a highly active adsorbent. The Ti–O calculated distances of the bridged hydroxyls were 2.02 Å, somewhat longer than typical Ti–O bonds in anatase (ca. 1.93-1.97 Å34), while the other calculated Ti-O distances were about 1.81 Å which were shorter than observed in anatase. The position of Ag in AgTi6O8(OH)8 was similar to that in AgTi6O9(OH)6. The Ag–O bond distances were between 2.33–3.01 Å, which were close to the experimental bond distance of 2.32 Å.30 In AgTi6O8(OH)8, the Ti–O bond distances of bridged hydroxyl groups increased to 2.19 and 2.13 Å.

Figure 3. Geometrically optimized structures of (a) Ti6O8(OH)8 and (b) AgTi6O8(OH)8; each with two bridged –OH groups. 12 Environment ACS Paragon Plus

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In the structures discussed earlier, two bridged –OH groups were formed after H2O incorporation. Calculations were carried out for the structures where two single –OH groups were formed after H2O incorporation. The Ti6O8(OH)8 (with two single –OH groups) and the corresponding AgTi6O8(OH)8 structures are shown in Figure 4. The binding energies were calculated for the following mechanisms. $

Ti6O9(OH)6 + H2O Ti6O8(OH)8 (Two single –OH) $

Ti6O8(OH)8 + Ag AgTi6O8(OH)8 where, %&' = E #(!)# - (E

(!) +E! )

%&' = E #(!)# - (E #(!)# + E ) The values of %&′ and %&′ were calculated to be -38.1 and -66.1 kJ/mol, respectively. Although both binding energies were negative, these were less negative than those with bridged –OH groups. Therefore, the Ag–TiO2 structures with bridged –OH groups were observed to be more stable. Optimization was also carried out for a structure consisting of one bridged and one single –OH groups. However, the bridged –OH group was transformed to single –OH group after optimization.

Figure 4. Geometrically optimized structures of (a) Ti6O8(OH)8 and (b) AgTi6O8(OH)8; each with two single –OH groups.



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3.3.

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(Computational) Adsorption Energy Calculation and Selectivity Comparison:

3.3.1. Difference between Neat and Silver Supported Clusters: The Eads of T and BT on neat and silver-supported clusters were calculated and are summarized in Table 1. Both uncorrected and average BSSE corrected Eads values are included. The optimized structures of T and BT adsorbed on both Ti6O8(OH)8 and AgTi6O8(OH)8 (each with bridged –OH groups) are shown in Figure 5 and Figure 6. The orientation of each was such that the organosulfur molecules would adsorb through the S atoms (with a –OH group from Ti6O8(OH)8 and through Ag for AgTi6O8(OH)8). For both T and BT, we can see that AgTi6O8(OH)8 had more negative adsorption energies than Ti6O8(OH)8, further confirming the sulfur affinity of silver. This was also in accord with the sulfur adsorption capacities of neat and silver-supported TiO2 and TiO2–Al2O3.29 However, the hydroxyl groups can act as secondary adsorption sites for sulfur heterocycles. In a desulfurization process, organosulfur compounds can be adsorbed on the –OH groups once all active Ag sites are occupied. This can also be supported by the curved breakthrough characteristics of the adsorbent for commercial and logistic fuels.8 The Ag–O bond distances changed after sulfur adsorption, as shown in the figures.


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Figure 5. Geometrically optimized structures of (a) thiophene adsorbed on Ti6O8(OH)8 (with bridged OH groups and (b) benzothiophene adsorbed on Ti6O8(OH)8 (with bridged OH groups). Avg. BSSE corrected energy: (a) -34.1 kJ/mol; (b) -54.4 kJ/mol.



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Figure 6. Geometrically optimized structures of (a) thiophene adsorbed on AgTi6O8(OH)8 (S–Ag interaction) and (b) benzothiophene adsorbed on AgTi6O8(OH)8 (S–Ag interaction); the clusters consisted of bridged –OH groups. Avg. BSSE corrected energy: (a) -98.0 kJ/mol; (b) -104.7 kJ/mol. The Eads of T adsorption on the Ti6O8(OH)8 and AgTi6O8(OH)8 clusters with single –OH groups were also calculated using DFT methods. The adsorption orientations were S–H for T–Ti6O8(OH)8 and S–Ag for T–AgTi6O8(OH)8. The optimized structures are shown in Figure 7 and the calculated adsorption energies are summarized in Table 1. T adsorption on clusters with single –OH groups were weak, as compared to that on clusters with bridged –OH groups. Therefore, the clusters with bridged – OH groups were more stable as well as had stronger affinity for organosulfur compounds. These clusters were therefore used in further studies.


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Figure 7. Geometrically optimized structures of (a) thiophene adsorbed on Ti6O8(OH)8 (S–H interaction) and (b) thiophene adsorbed on AgTi6O8(OH)8 (S–Ag interaction); the clusters consisted of single –OH groups. Avg. BSSE corrected energy: (a) -27.1 kJ/mol; (b) -77.4 kJ/mol. 3.3.2. Difference between π–Ag and S–Ag Bonding: To distinguish between the π–Ag and S–Ag interactions, T and BT were each adsorbed on AgTi6O8(OH)8 through two initial orientations: one with the S atom close to Ag, and the other one with the ring close to Ag. Multiple trials were carried out to determine the π–Ag and S–Ag orientations which would result in the strongest adsorptions. The corresponding energies are summarized in Table 1 and the optimized structures are shown in Figure 6 and Figure 8. The results indicated that the π–Ag interactions were stronger than S–Ag. This result supported our previous observations in infrared studies demonstrating π-bond between Ag and T.29 The bond distances between Ag and organosulfur molecules were also smaller for π–Ag than those for S–Ag. We also attempted to interact T and BT with Ti6O8(OH)8 through π–H. However, the organosulfur molecules reoriented to S–H after geometry optimization.



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Figure 8. Geometrically optimized structures of (a) thiophene adsorbed on AgTi6O8(OH)8 (π–Ag) and (b) benzothiophene adsorbed on AgTi6O8(OH)8 (π–Ag); the clusters consisted of bridged –OH groups. Avg. BSSE corrected energy: (a) -111.1 kJ/mol; (b) -117.9 kJ/mol. 3.3.3. Effect of Benzene Rings and Methyl Groups We compared the Eads of various sulfur heterocycles adsorbed on AgTi6O8(OH)8 in order to study the effect of benzene rings and methyl groups. Figure 8, Figure 9, and Table 1 illustrate the corresponding structures and the Eads values of T, BT, DBT, and 4,6-DMDBT adsorbed on AgTi6O8(OH)8. Both π–Ag and S–Ag orientations were studied and the corresponding Eads values are included in Table 1. The S–Ag oriented 4,6-DMDBT could not be calculated because the steric effect was significant. For DBT adsorbed onto AgTi6O8(OH)8, π-interaction was the preferred mode of adsorption as well. From Table 1, it can be emphasized that the adsorbent had stronger adsorption toward sulfur heterocycles with more benzene rings. It is interesting to note that adding two methyl


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groups to DBT to form 4,6-DMDBT resulted in a slightly less negative adsorption energy (-123.8 versus -126.2 kJ/mol). From more negative to less negative, the Eads values followed the order: DBT>4,6-DMDBT>BT>T. This indicated that the adsorbent had the strongest affinity for DBT among these molecules. The order was in accord with that from the saturation experimental results done here. However, the Eads of 4,6-DMDBT adsorption was close to that of DBT adsorption in the calculations, whereas the adsorbent capacity for 4,6-DMDBT was close to that for BT in the experiments (see Section 3.1). During saturation tests, 4,6-DMDBT might have more diffusion resistance into the adsorbent mesopores because of its structure and thus the adsorbent might have a reduced capacity.

Figure 9. Geometrically optimized structures of (a) dibenzothiophene adsorbed on AgTi6O8(OH)8 (π– Ag) and (b) 4,6-dimethyldibenzothiophene adsorbed on AgTi6O8(OH)8 (π–Ag); the clusters consisted of bridged –OH groups. Avg. BSSE corrected energy: (a) -126.2 kJ/mol; (b) -123.8 kJ/mol. 3.3.4. Effect of Non-Sulfur Aromatics To investigate the effect of non-sulfur aromatics on sulfur adsorption, we performed DFT calculations regarding benzene and naphthalene adsorption on AgTi6O8(OH)8. The optimized clusters



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and the corresponding Eads values are included in Figure 10 and Table 1, respectively. Since the molecules have no functional groups other than C, interactions though π electrons were predominant. Benzene and naphthalene adsorption on AgTi6O8(OH)8 were weaker than T and BT adsorption. The presence of S increases the electron density, thereby enhancing the adsorption affinity. It also indicated that silver has very high selectivity toward sulfur aromatics over non-sulfur aromatics. Therefore, the observation that sulfur heterocycles are preferentially adsorbed onto the silver surface over aromatic hydrocarbons is supported by DFT calculations. Because of this, AgTi6O8(OH)8 has high selectivity even when the concentration ratio of non-sulfur aromatics to sulfur aromatics in hydrocarbon fuels is more than 25000:1.7 Naphthalene had more negative adsorption energy than benzene due to the added benzene rings. This was also reported by other researchers.35 The adsorption energies were consistent with the experimental heats of adsorption data.36

Figure 10. Geometrically optimized structures of (a) benzene adsorbed on AgTi6O8(OH)8 (π–Ag) and (b) naphthalene adsorbed on AgTi6O8(OH)8 (π–Ag); the clusters consisted of bridged –OH groups. Avg. BSSE corrected energy: (a) -101.4 kJ/mol; (b) -115.5 kJ/mol. 20 Environment ACS Paragon Plus

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3.3.5. Effect of Different Hetero Atoms In the previous section, we observed that the adsorption energy was less negative for aromatic hydrocarbons whereas it was more negative for the molecules containing sulfur. Therefore, the adsorbent had higher selectivity toward organosulfur compounds. To study its selectivity toward organonitrogen and organooxygen compounds, Eads were calculated for quinoline and benzofuran adsorbed on AgTi6O8(OH)8 and were compared with that for BT adsorption (Figure 6, Figure 8, Figure 11, and Table 1). Quinoline and benzofuran had N and O as the functional groups, respectively. For Eads calculation, S–Ag, N–Ag, and O–Ag interactions were considered for BT, quinoline, and benzofuran, respectively. For each molecule, π–Ag interaction was considered as well. Figure 11 shows quinoline and benzofuran adsorbed on AgTi6O8(OH)8 though orientations that resulted in the stronger adsorptions. We can see from the figure that quinoline was adsorbed perpendicular to the adsorbent surface, indicating σ-bonding. Similar bonding was observed in the case of benzofuran adsorbed through O–Ag orientation. From the DFT studies, the order in terms of Eads values from more negative to less negative was: quinoline (N–Ag)>benzothiophene (S–Ag)>benzofuran (O–Ag) or N>S>O. This was similar to the basicity order of the functional groups. These results supported the conclusions from the desulfurization experiments reported previously.37 The π-interacted adsorbate molecules were also studied and the corresponding Eads values are included in Table 1. π-bonding was preferred for BT and benzofuran but not for quinoline. The highest occupied molecular orbital (HOMO) is π-type for both quinoline and benzofuran (8.6238 and 8.839 eV, respectively). However, the first σ orbital is much higher for quinoline compared to benzofuran which means quinoline is a much better donor species.

Relative to the calculated HOMO energy (B3LYP/6-31G(d)), the first σ

molecular orbital of quinoline was only 0.6 eV lower, while for benzofuran, the first σ molecular orbital was 3.2 eV below the HOMO. Thus, it can be rationalized that the π-interactions are very similar for quinoline and benzofuran, while the σ-interaction is much stronger for quinoline. The π-



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interaction was 8.9 kJ/mol stronger for quinoline compared to benzofuran (-109.4 vs. -100.5 kJ/mol), and was consistent with a π ionization potential (IP) higher by 0.2 eV (better donor), while the σinteraction was 63.1 kJ/mol stronger (-141.4 vs. -78.3 kJ/mol) consistent with a much higher σ IP (better donor). The computational calculations indicated that organonitrogen compounds are highly detrimental to desulfurization through this route. These compounds compete for the active sites with organosulfur species.11 Alternatively, the Ag–TiO2 adsorbent can also act as a good denitrogenation adsorbent. In the optimized structures, the Ag–N bond distance was the shortest.

Figure 11. Geometrically optimized structures of (a) quinoline adsorbed on AgTi6O8(OH)8 (N–Ag orientation) and (b) benzofuran adsorbed on AgTi6O8(OH)8 (π–Ag orientation); the clusters consisted of bridged –OH groups. Avg. BSSE corrected energy: (a) -141.4 kJ/mol; (b) -100.5 kJ/mol.


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Table 1. Adsorption energies (uncorrected and average BSSE corrected) of sulfur and non-sulfur aromatics adsorption on Ti6O8(OH)8 and AgTi6O8(OH)8 Structure Ti6O8(OH)8 (With single –OH groups) AgTi6O8(OH)8 (With single –OH groups) Ti6O8(OH)8 (with bridged –OH groups)

Adsorbate

Eads (kJ/mol)

a

Orientation

Uncorrected

Avg. BSSE Corrected

S–H

-30.8

-27.1

S–Ag

-85.4

-77.4

Thiophene

S–H

-40.1

-34.1

Benzothiophene

S–H

-62.8

-54.4

S–Ag

-105.9

-98.0

π–Ag

-121.0

-111.1

S–Ag

-113.8

-104.7

π–Ag

-127.7

-117.9

S–Ag

-129.0

-117.3

π–Ag

-139.2

-126.2

4,6-dimethyl dibenzothiophene

π–Ag

-136.7

-123.8

Benzene

π–Ag

-111.2

-101.4

Naphthalene

π–Ag

-125.8

-115.5

N–Ag

-153.3

-141.4

π–Ag

-120.1

-109.4

O–Ag

-89.2

-78.3

π–Ag

-113.1

-100.5

Thiophene

Thiophene

Benzothiophene

Dibenzothiophene AgTi6O8(OH)8 (with bridged –OH groups)

Quinoline

Benzofuran a) “S–H” indicates the adsorbate hydrogen bonds to –OH groups of the cluster; “X–Ag” indicates that the adsorbate interacts through a σ-interaction with the silver atom (X = S, N, or O); and “π–Ag” indicates that π-system of the adsorbate interacts with the silver atoms. 3.4.

(Computational) NPA Charges: The NPA charges of Ag and S in the adsorbate-adsorbent clusters were calculated and are

presented in Table 2. The charges of Ag before adsorption were consistent with the experimental data, where the +1 oxidation state of Ag were observed to be the active form in Ag–TiO2 adsorbent.31 Silver charges were decreased after adsorption; whereas S charges were increased for π-interacted molecules 23 Environment ACS Paragon Plus


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and decreased for S-interacted molecules. The S charges were seen to be positive in all adsorbateadsorbent clusters whereas the C atoms had negative charges. Apart from S, both N and O functional groups in quinoline and benzofuran, respectively, had negative charges. After adsorption, the charges of N and O were reduced during σ-interaction, and were increased during π-interaction. Table 2. The NPA charges (calculated via the B3LYP/6-31G(d)+SDD method) of Ag and S/N/O from optimized adsorbate, adsorbent, and adsorbate-adsorbent structures Charge (e-) Cluster

Adsorbate

Ti6O8(OH)8 (with single –OH groups) AgTi6O8(OH)8 (with single –OH groups)

Thiophene

Thiophene Ti6O8(OH)8 (with bridged –OH groups) Benzothiophene Thiophene

Benzothiophene

Dibenzothiophene AgTi6O8(OH)8 (with 4,6-dimethyl bridged –OH groups) dibenzothiophene

Orientation

Ag (free)

Ag (adsorbed)

S–H

-

-

S–Ag

0.73

0.61

(S–H)

-

-

(S–H)

-

-

(S–Ag)

0.61

(π–Ag)

0.66

(S–Ag)

0.60

(π–Ag)

0.67

(S–Ag)

0.59

(π–Ag)

0.67

(π–Ag)

0.72

S/N/O (free)

S/N/O (adsorbed) 0.34

0.38

0.35 0.35

0.36 0.38

0.36

0.35

0.34 0.34 0.43 0.34 0.40 0.34 0.38

0.66

0.33

0.34

Benzene

(π–Ag)

0.67

-

-

Naphthalene

(π–Ag)

0.68

-

-

(N–Ag)

0.64

(π–Ag)

0.68

(O–Ag)

0.69

(π–Ag)

0.65

Quinoline

Benzofuran


-0.45

-0.48

-0.54 -0.43 -0.53 -0.46

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3.5.


(Computational) Separation Factor: Separation factors are employed in adsorption/absorption processes to determine the quality of

separating a particular component from a mixture.16 It is essential in hydrocarbon fuel desulfurization since every fuel contains numerous compounds which can interfere with the process. This factor can adequately describe the adsorbent ability to selectively remove sulfur derivatives from the fuels. The α values of various sulfur, nitrogen, and oxygen aromatics over benzene are given in Table 3. The Eads values of species adsorbed through preferred orientations (N–Ag for quinoline, π–Ag for the rest) were employed for separation factor calculation. Benzene represented the non-sulfur aromatic content of hydrocarbon fuels. As seen from the table, all α values except αBenzofuran/C6 were greater than 2, indicating that the adsorbent can be a good separation agent commercially.16 Therefore, Ag–TiO2 is a feasible adsorbent for removing all the aforesaid sulfur heterocycles. The value of αThiophene/Benzene was 50, indicating high selectivity toward T over C6. The values were consistent with experimental separation factors reported by other researchers.36

Table 3. Separation factors of all sulfur heterocycles as compared to benzene Separation Factor

Adsorption on AgTi6O8(OH)8

αThiophene/Benzene (π–Ag)

50

αBenzothiophene/Benzene (π–Ag)

794

αDibenzothiophene/Benzene (π–Ag)

22466

α4,6-dimethyldibenzothiophene/Benzene (π–Ag)

8648

αQuinoline/Benzene (N–Ag)

1.0 × 107

αBenzofuran/Benzene (π–Ag)

0.7

4.

Conclusions: In this work, a computational study was carried out using DFT methods to investigate the sulfur

selectivity of Ag–TiO2. We also studied the adsorbent activation steps and found out that the Ag–TiO2 25 Environment ACS Paragon Plus


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structures (both hydroxylated and non-hydroxylated) were stable. Silver incorporation with TiO2 was likely to be spontaneous and the presence of –OH facilitated the process. The –OH groups contributed not only in Ag incorporation, but also acted as secondary adsorption sites. Bridged –OH groups were more stable and demonstrated stronger affinity toward organosulfur compounds than single –OH groups. Ag–TiO2 clusters had much stronger adsorption affinities than neat TiO2 clusters. For all sulfur and most of the non-sulfur heterocycles, π-interaction was the preferred mode of adsorption. For AgTi6O8(OH)8 cluster with bridged –OH groups, the order of adsorbate molecules in terms of the adsorption energies from more negative to less negative was: quinoline (N–Ag)>DBT (π–Ag)>4,6DMDBT (π–Ag)>BT (π–Ag)>C10 (π–Ag)>T (π–Ag)>C6 (π–Ag)>benzofuran (π–Ag). This was also in agreement with the results from saturation experiments. The effect of aromaticity was found significant as organosulfur species with more aromatic rings tended to adsorb more strongly onto silver. Functional groups in heterocycles had considerable effect on adsorption energy, and the presence of functional groups such as S and N significantly enhanced heterocycle adsorption on the adsorbent. The separation factors of Ag–TiO2 reflected its ability to adsorb sulfur aromatics even in the presence of 25000 times higher concentration of non-sulfur aromatics. The adsorption energy of silvertitania calculated in this work can be utilized in the adsorbent formulation for maximizing adsorbent activity and in the design of other adsorption processes.


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ASSOCIATED CONTENT Supporting Information: The self consistent field (SCF) energies with added GD3 dispersion corrections (calculated via B3LYP/6-31G(d)+SDD) of the adsorbent-adsorbate clusters are available in Table S1 and the geometrically optimized structures (via B3LYP/LANL2DZ) of all the adsorbent, substrate, adsorbate, and adsorbent-adsorbate clusters are available in Table S2. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Telephone: +1 3348442023. Fax: +1 3348442065. E-mail address: [email protected] (B.J. Tatarchuk).

ACKNOWLEDGMENT This project has been funded by the U.S. Army Tank Automotive Research, Development and Engineering Center (TARDEC) under contract no. W56HZV-07-C-0577. Computer time was made available on the Alabama Supercomputer Facility.



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Density Functional Theory Study of Organosulfur Selective Adsorption

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