Surface Structure and Acidity Properties of Mesoporous Silica SBA-15

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Surface Structure and Acidity Properties of Mesoporous Silica SBA-15 Modified with Aluminum and Titanium: First Principles Calculations Saul Perez-Beltran, Perla B. Balbuena, and Gustavo E Ramirez-Caballero J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05630 • Publication Date (Web): 21 Jul 2016 Downloaded from http://pubs.acs.org on July 30, 2016

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The Journal of Physical Chemistry

Surface Structure and Acidity Properties of Mesoporous Silica SBA-15 Modified with Aluminum and Titanium: First Principles Calculations Saul Perez-Beltran1,2, Perla B. Balbuena1*, and Gustavo E. Ramírez-Caballero2 1 Department of Chemical Engineering, Texas A&M University, College Station, TX 77843 2 Centro de Investigaciones en Catalysis (CICAT), Departamento de Ingeniería Química, Universidad Industrial de Santander, Bucaramanga, Colombia

Abstract Ordered mesoporous silica materials are considered promising supports for the development of novel hydroprocessing catalysts. Specifically, the mesoporous silica SBA-15 exceeds because of high specific surface area, wall thickness and pore size distribution, features that make this material more stable at enviroments at which hydroprocessing reactions take place. However, the SBA-15 lacks strong Brønsted acid sites and this fact still hinders its widespread commercial use. In this work we report density functional theory analyses of the structure and acidity properties of the SBA-15 surface. Periodic boundary models are used and the temperature dependence of the silanol surface density is taken into account. Surface modification by isomorphic substitutions with aluminum and titanium is investigated. It is found that aluminum substitution favors creation of bridging hydroxyl groups. However, surface modifications with aluminum and titanium also create local structural distortions inducing H-bond interactions which improve the acidity properties of hydroxyl groups on the surface. Calculation of vibrational frequencies of O—H bonds are used to quantify the surface acidity properties.

Introduction Recent reports indicate that the quality of crude oil supply is continuously diminishing without feasible expectations of finding new reserves of light crude oil.1-3 Moreover, the new environmental regulations require of transportation fuels with contents of sulfur and aromatics lower than 10 ppm.1,2,4 Fundamental research is required to overcome these issues and achieve a better balance of the various hydroprocessing catalytic (HPC) reactions taking place in the oil processing industry.2,5 Achieving a better balance of the HPC reactions requires the design of 1 ACS Paragon Plus Environment


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better HPC catalysts. Current design strategies for HPC catalysts are oriented to the search of more robust and more selective active phases, and also to the search of better materials for supports of these active phases.6,7 The mesoporous silica SBA-15 (Santa Barbara amorphous No. 15) has been postulated as a promising catalytic support due to its specific surface area ranging between 690 and 1040 m2/g, its narrow pore size distribution with average pore diameter tunable between 5 and 30 nm, and its high hydrothermal stability due to wall thicknesses between 3 and 5 nm.8,9 Several reports remark the potential of SBA-15-based HPC catalysts.10-12 SBA-15-supported NiW and NiMo catalysts were proposed as catalysts with larger pore diameter and higher surface area than molybdenum-based hydroprocessing catalysts supported on γ-Al2O3.12 Moreover, NiMo catalysts supported on pure siliceous SBA-15, and aluminum modified SBA-15 (Al-SBA-15), were tested and compared with commercial catalysts for hydrogenation of heavy oil derived from coal liquefaction process.

13

From these reports, it was observed that SBA-15-supported

catalysts have equal or better performance than commercial catalysts. Moreover, the Al-SBA15-suported catalysts have higher hydrodesulfuration (HDS) and hydrodenitrogenation (HDN) activities than SBA-15-supported catalysts. The SBA-15 modified through isomorphic substitutions with aluminum or titanium, Al-SBA-15 and Ti-SBA-15, respectively, has been proposed as catalytic support for the HDS of dibenzothiophene and hydrogenation of biphenyl.12 It was found that modified materials shown better acidity properties at the surface, and better dispersion of supported active phases.4,9,12,14,15 The lower catalytic activity of catalysts supported on SBA-15 instead of modified SBA-15 is attributed to the lack of Brønsted acid sites.4 The Brønsted acid sites are the primary seat of catalytic activity for processing reactions of hydrocarbons.16 Bridging hydroxyl groups with strong Brønsted acid properties have been measured in Al-SBA-15 materials,17 whereas better adsorption properties have been associated with Ti-SBA-15 materials because of the presence of 3d electronic orbitals.18 The acidic properties on the SBA-15's surface are strongly influenced by the density and nature of silanol groups, which in turn depends on the temperature.19-21 Regardless of the synthesis 2 ACS Paragon Plus Environment

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method, as temperature increases the surface density of silanol groups decreases;20 above 463 K all physisorbed water is removed and silanol groups begin to react between them through condensation reactions. Moreover, at approximately 1373 K the surface becomes completely dehydroxylated. Motivated by the need of rational design of novel supports for HPC catalysts,22 here we report DFT calculations to characterize the SBA-15 surface, together with the study of isomorphic metallic substitutions on surface acidic properties. In earlier works the DFT method has been used for studying the adsorption of chromium oxide on hydroxylated amorphous silica surfaces.23 The chromium oxide - silica system (Cr/SiO2) is an useful catalyst for polymerization of ethylene at low pressure. In this work, flat film models with periodic boundary conditions (PBC) are used to model the SBA-15 pore surface.23-25 Only the pore surface is modeled because this work focuses on studying the atomistic level structural properties at the surface, rather than on modeling of diffusion or confinement effects inside the pore structure.18,26 The use of PBC conditions allowed better representation of the density and type of silanol groups on the surface,25,27,28 while facilitated an easier convergence of computational calculations.29 Despite the translational symmetry of PBC models, an adequate representation of the amorphous nature of the SBA-15's framework was warranted thanks to the use of unit cell dimensions far larger than the interatomic distances.12,30-32 A statistical thermodynamic-based approach19 is used for modeling the surface density of silanol groups as a function of temperature. Theoretical quantification of acidity strength for silanol groups and bridging hydroxyl groups is performed by O-H vibrational frequency calculations. The O-H vibrational frequency calculations are particularly suitable with PBC models;25,33,34 comparison between calculated OH vibrational frequencies and experimental data is direct and straightforward; lower O-H vibrational frequencies are associated with stronger Brønsted acidity.17,25,29 Other theoretical descriptors for this quantification are the deprotonation energy (DPE) calculations,35-37 and the adsorption of basic molecules.37,38 However, neither the DPE calculations nor the adsorption of basic molecules give correct absolute values under PBC conditions.39

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System and computational details A periodic film of hydroxylated amorphous silica is used for modeling the inner pore surface of the SBA-15.23,25 Details regarding its structural characterization are published in an earlier work from where the structure was taken.24 The unit cell dimensions are 17.6 Å x 12.5 Å x 25.2 Å, which ensures an accurate reproduction of amorphous structure of the SBA-15 framework.40 The total number of atoms is 120 (Si27O67H26), the film thickness is approximately 9 Å and the average density is 1.9 g/cm3, which is in qualitative agreement with reported values for bulk silica (2.14 g/cm3). The surface silanol number is 6.31 OH/nm2 and the dangling bonds on the bottom surface are saturated with H atoms. Moreover, the angular Si-O-Si and O-Si-O distributions are in agreement with other works.41-43 Two and three membered Si ring structures are not included as they are expected to be open after surface hydration,25 whereas non-bridging oxygen atoms are saturated with H atoms. This model is qualitatively representative in terms of isolated, geminal, and vicinal silanol groups. A single OH group attached to a Si atom is an isolated silanol. A geminal silanol refers to one of two OH groups bound to the same Si atom, whereas vicinal silanols are those OH groups attached to silicon atoms and involved in H-bond interactions (either single or geminal).19,25 Classical Molecular Dynamics (MD) and Density Functional Theory (DFT) have been synergistically used for the structural relaxations and the dehydroxylation simulations. The MD calculations are performed as an initial relaxation step, whereas the DFT calculations are intended for more refined structural optimizations. The MD calculations are performed using the Universal Force Field (UFF) potential as is implemented in the AVOGADRO code,44 and the DFT calculations are performed using the Vienna Ab-initio Simulation Code (VASP).26,45,46 Movement of all atoms is enabled at any stage of MD and DFTcalculations. For the MD simulations the use of the UFF potential ensures accurate modeling of angular distortions close to linearity, as it is the case for amorphous silica where it is known that the SiO-Si angle has a bending barrier as low as 1.80 kJ/mol.35 The UFF potential has been already applied to structural relaxations for several types of atomic systems.43,47


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For the DFT calculations, the generalized gradient approximation proposed by Perdew-BurkeErnzerhof (GGA-PBE) is used for the exchange-correlation functional approximation.48 The GGAPBE functional is one of the most widely used in plane-wave DFT calculations because of its reliability for predicting structural and thermodynamic properties.49 The core-electron interactions are described using the Projector Augmented Wave (PAW) pseudopotential.50,51 The surface Brillouin-zone integration is calculated using the gamma point Monkhorst-Pack mesh.52 The ionic relaxation loop is performed using a conjugated-gradient algorithm until the total energy difference is below to 10-3 eV, whereas the electron self-consistent iteration is set to 10-4 eV. Spin polarized calculations are carried out within the framework of DFT.53A Gaussian smearing with a width of 0.05 eV is also utilized. In all cases an initial DFT optimization is performed using a plane-wave expansion of 230 eV, followed by a final complete relaxation at 400 eV. As mentioned above the silanol density at the surface decreases as temperature increases. To follow this temperature effects a statistical thermodynamic-based approach, outlined in an earlier work is used for modeling this dehydroxylation process by following the most energetically favorable path.19 The condensation reactions are modeled between pairs of silanol groups with Si-Si and O-O distances lower than 5.5 Å and 4.5 Å, respectively. The dehydroxylation energy (ΔEDFT) is calculated using the Eq. 1. ∆ = − −

Eq. 1

where and terms are the total DFT energies before and after the silanol pair condensation, respectively, and the term is the total DFT energy of an isolated water molecule. Full DFT relaxations were applied after each condensation reaction. Effects for migration of hydroxyl groups are not included in our calculations following the suggestion from earlier works that inclusion of such effects is not required to explain dehydroxylation processes at temperatures below of 873 K.19 Several surfaces with decreasing density of silanol groups are generated. The approximated temperature associated to each surface is given by the Eq. 2, where is the temperature, is 5 ACS Paragon Plus Environment


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the Boltzmann constant, is the partial pressure of water, is the rotational partition function of water, and Λ is the thermal de Broglie wavelength of water in the ideal gas state. This equation comes from a Gibbs energy balance, which at equilibrium enables the equality between the dehydroxylation energy and the entropic contribution associated with the release of a water molecule. Details of this derivation are given in references.19,54 ∆ = ln

Eq. 2

The finite difference method within the harmonic approximation as implemented in VASP is used for calculations of OH-stretching modes. In this method the forces acting on single atoms that are displaced from their equilibrium positions at a conserved framework structure are calculated via the Hellmann–Feynman theorem and the numerical derivatives of these forces are used to construct the partial dynamic matrix.26 More accurate prediction of vibrational frequencies have been reported by inclusion of anharmonic contributions.33,55

However,

harmonic calculations still provide reasonable estimates for Brønsted acid properties at the surface.17,25 The Bader charge method was used to quantify the electronic charge associated with specific atomic species. This method is based solely on the calculation of charge density what makes it rather insensitive to the basis set used for the electron wave-function calculation. Complete details about the algorithm for the Bader charge method are found in references.56

Results Surface density of silanol groups vs. temperature Figure 1 shows the surfaces (top view) obtained after modeling of condensation reactions. The frame i at the top left shows the film of hydroxylated amorphous silica with a surface density of silanol groups of 6.31 OH/nm2, whereas frames ii to iv shown the films obtained after condensation reactions. The frame v shows the dehydroxylation energy ∆EDFT for each condensation reaction, calculated as stated in Eq. 1. As can be seen, the dehydroxylation energy of condensation reactions increases as the surface density of silanol groups decreases. The 6 ACS Paragon Plus Environment

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dehydroxylation energy for condensation reactions between two silanol groups on the film with a surface density of 6.31 OH/nm2 is 0.53 eV, whereas the film with a surface density of 5.45 OH/nm2 has a dehydroxylation energy of 1.83 eV. For the surface with a surface density of silanol groups of 3.18 OH/nm2 no silanol pairs available for further condensation reactions. . This sustained increase in the dehydroxylation energy suggests higher surface reconstruction at lower surface density of silanol groups, which is in agreement with previous reports.19 (ii)

(ii)

(i)

2

2

Initial surf, OH/nm = 6.31

2

OH/nm = 5.91

OH/nm = 5.45

(v)

(iv)

1.9 1.4 0.9

ΔE [eV]

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0.4 2

OH/nm = 3.18

6

5

4

3

OH/mn2

Figure 1: Dehydroxylation process outputs: (a-d) Films with varying silanol number, (e) Dehydroxylation energy calculated at each dehydroxylation step using Equation 1. Si atoms are yellow, O atoms are red, and H atoms are white.

Figure 2 shows the surface density of silanol groups vs. temperature as calculated with Eq. 2, and this is compared with experimental data average between 16 different samples with different specific surface areas.20 Calculations are performed at 0.01 bar of partial water pressure, which is typical for HDS processes.57 Although no experimental data is available below 473 K, our calculations indicate a flattening in the slope which is due to the lower dehydroxylation energy for the condensation reactions. Lower dehydroxylation energy suggests lower surface reconstruction upon reaction between silanol groups, which in turn is due to higher availability of silanol groups at the surface.19 There is a deviation between our calculations and the experimental data at temperatures above 600 K. This is because at higher 7 ACS Paragon Plus Environment


temperatures the condensation reactions are promoted by migration of protons on the surface,20 but this effect is not included in the statistical thermodynamic-based approach used here.19 However, our surfaces are still representative for HPC processes, because most of the HPC reactions are performed at or below these temperatures.57 7 6 5 OH/nm2

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4 3 2 1 0 200

400

600

800

1000

1200

1400

Temp [K]

Figure 2: OH/nm2 vs. temperature:

, average experimental values;

, computed values @ 0.01 bar.

Analysis of the type of bond that silicon atoms form facilitates comparison between our films and experimental data. Table 1 shows the bond type distribution of the silicon atoms as a function the surface density of silanol groups. Silicon atoms bonded to four siloxane bridges are called Q4, whereas those bonded to three siloxane bridges plus one hydroxyl group are Q3, and those that bond to two siloxane bridges and two hydroxyl groups are referred as Q2. Both the as-synthesized SBA-15 and the film i show similar percentage of Q3 atoms, but the percentages of Q4 and Q2 show a significant deviation. Again, both the calcined SBA-15 and the film iv have similar percentage of Q3 atoms, but now both surfaces also share similar percentages of Q3 and Q4 atoms. From these observations it is inferred that physical accuracy of our films increases as the surface density of silanol groups decreases. The type-bond distribution for silicon atoms fits better the experimental data as the dehydroxylation procedure is applied. Hereafter, only the films ii, iii and iv are used for calculations.


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Table 1: Silicon’s type-bond distribution vs. surface density of silanol groups. Film

Surface Density of Silanol Groups

Unit Cell % Q2

% Q3

% Q4

Si27O67H26

28.6

42.8

28.6

5.91

Si27O66H24

12.5

56.2

31.3

iii

5.45

Si27O65H22

11.8

47.0

41.2

iv

3.18

Si27O64H20

7.1

35.7

57.2

58

-

8

41

51

58

-

4

29

66

2

(OH/nm )

Formula

i

6.31

ii

-

As-synth. SBA-15 (300 K) Calcined SBA-15 (773 K)

Structural models for modified SBA-15 Al-SBA-15 The difference in valence electrons between Si and Al atoms imply protonation of one oxygen atom after each substitution. Figure 3 outlines how the isomorphic substitutions of aluminum atoms are performed in our model. An H+ proton is bonded to aluminum, oxygen, and silicon atoms, whereas the H-O distance is set at 0.915 Å, and the Al-O-H angle is set to be equal to the Si-O-H angle. The H, O, Al, and the Si atoms are all included in the same plane. For each Si/Al substitution, all the Al-O-Si bridges are tested as possible sites for protonation. The Loewenstein rule has been taken into account for the calculation of isomorphic substitutions, in such a manner that existence of -Al-O-Al- bridges is avoided. The Loewenstein rule states that whenever two tetrahedra are linked by one O bridge, the center of only one of them can be occupied by Al; the other center must be occupied by Si or another small ion of electrovalence four or more. 59,60 DFT optimizations are performed in all cases.

H 0.915 Å

O Si

Al

Figure 3: Oxygen protonation after Al incorporation, structure before relaxation. Al atom in purple.



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In order to study the early stages of SBA-15’s modification by aluminum, the film ii (5.91 OH/nm2) is subject to substitution of one Si atom by one Al atom (Al/Si ratio of 1/26). The silicon atoms type Q4, Q3, and Q2 are subject to substitution, and the stability of each substitution is quantified by substitution energy as stated in equation 3, where the index n indicates the number of Al atoms, and the first two terms on the right side are the DFT energies for the structures with n and n-1 Al atoms, respectively. Finally, the terms ESi, EAl and EH are the energies of isolated Si, Al and H atoms, respectively. More negative values indicate more stability for substitution. ΔE = E(AlnSi27-nO65H22+n) - E(Aln-1Si27-n+1O65H22) + ESi – EAl – EH

Eq. 3.

Table 2 summarizes the substitution energy of Si4+ atoms by Al3+ atoms. The sites type Q2, Q3, and Q4 are favorable for Al substitution with substitution energies ranging from -5.144 eV to 4.233 eV. No clear trend for Al substitution into sites Q2, Q3 or Q4 was observed. However, certain preference is observed for those substitutions where the protonated Al-O-Si Bridge involves a Q3 Silicon atom. The Silicon atoms Q3 are bonded to one hydroxyl group and three siloxane bridges, and the presence of hydroxyl bonds is favored at low temperatures. Table 2: Substitution energy of Si4+ atoms by Al3+ atoms at a Li/Si ratio of 1/26 Site

ΔE (eV)

Site

ΔE (eV)

AlQ3-OH-SiQ3

-5.144

AlQ3-OH-SiQ4

-4.421

AlQ4-OH-SiQ3

-5.030

AlQ2-OH-SiQ4

-4.408

AlQ2-OH-SiQ3

-4.609

AlQ4-OH-SiQ4

-4.296

AlQ4-OH-SiQ3

-4.522

AlQ3-OH-SiQ3

-4.223

Structural distortions are also observed after isomorphic substitution of Si4+ atoms by Al3+ atoms. In case of non-protonated oxygen atoms, the Al-O bond length is on average 0.089 Å longer than the equivalent Si-O bond length. Whereas for protonated O atoms, this distance is on average 0.338 Å longer than the corresponding Si-O bond length. This is in quantitative agreement with structural distortions reported for (001) surfaces of Mordenite, where the Al10 ACS Paragon Plus Environment

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bond length is 0.08 - 0.11 Å and 0.27- 0.29 Å larger for non-protonated and protonated oxygen, respectively.17 The substitution energy at higher Al/Si ratios is shown in Figure 4. The aluminum substitution becomes increasingly less favorable as the Al/Si ratio increases. Similar behavior was measured in Al-SBA-15 materials synthesized by post-synthesis procedures,61 where tetrahedral substitution of silicon atoms was favored at low Al/Si ratios, however, at higher Al content, the Al substitution in octahedral sites became predominant.

-4 -4.2 ∆E (eV)

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-4.4 -4.6 -4.8 -5 -5.2 0

0.05

0.1 0.15 Al/Si molar ratio

0.2

0.25

Figure 4: Formation energy as a function of Al/Si ratio. The blue line is for guiding the eye.

Ti-SBA-15 Figure 5 shows the film iii (5.45 OH/nm2) subject to isomorphic substitutions with titanium atoms at sites Q4, Q3, and Q2. The substitution's stability is evaluated by equation 4. The n index refers to the number of Ti atoms, whereas the first two terms on the right side correspond to the DFT energies for the structures with n and n-1 titanium atoms. In the same manner, the ESi and ETi are the DFT energies for the isolated Si and Ti atoms, respectively. For a constant Li/Si ratio of 1/26, Figure 5 shows that the site Q4 is the most stable for substitution, followed by sites Q3 and Q2. At low Ti content, the preferred sites for substitutions with Ti atoms are those where the Ti4+ is bonded to four Ti-O-Si bridges. ΔE = E(TinSi27-nO65H22) + ESi – E(Tin-1Si27-n+1O65H22) + ESi – ETi

Eq. 4



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After substitution, structural distortions arise regardless where the Ti4+ cation is located; the average Ti-O bond length is on average 0.197 Å larger than the equivalent Si-O bond length. These structural distortions are attributed to differences in atomic radius between Ti and Si atoms (Ti atomic radius = 140 pm, Si atomic radius = 110 pm).62

O1

Si1

O1

H1

H2 O2

Si2 O2 O4

Si2

Q4

O4

Si4 O3 Si3 a) Q4; ΔE = -3.530 eV

Si4

Q3

H1 O1

O2

Si3 Q2

O3

O4

O3 Si4 Si3 b) Q3; ΔE = -2.923 eV

c) Q2; ΔE = -2.367 eV

Figure 5: Isomorphic substitution of Si atoms by Ti atoms in positions Q4, Q3, and Q2 at a Ti/Si ratio of 1/26.

Modification with titanium was evaluated up to a Ti/Si ratio of 5/22. As shown in Figure 6, the substitution becomes less favorable as the titanium content increases. This behavior is in agreement with UV-vis diffused reflectance (DRS) measurements of SBA-15 materials modified with Ti through post-synthesis procedures with titanium butoxide.

14

The DRS spectra reveals

that at low Ti/Si ratios the Ti4+ cations are tetrahedrally incorporated into the SBA-1. However, these measurements also indicate that at Ti/Si ratios above 1/20, the tetrahedral incorporation becomes unfavorable and extra-framework TiO2 is formed. Although having the same trend, comparison between Figure 4 and Figure 6 shows that in case of Ti the substitution energy has a slightly less smooth behavior. This behavior may be attributed to a higher difference in atomic size between Ti and Si than between Al and Si. Further studies are needed to corroborate this hypothesis. The titanium species are also preferentially located at Q4 sites at high Ti/Si ratios, however, the availability of Q4 sites also diminishes as the titanium content increases, which could explain the Ti incorporation in octahedral sites as indicated in experiments.


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0 ΔE (eV)

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-0.5 -1 -1.5 0

0.05

0.1 0.15 0.2 Ti/Si molar ratio

0.25

Figure 6: Formation energy plot as a function of Ti/Si ratio. The blue line is for guiding the eye.

Brønsted Acid properties Pure siliceous SBA-15 Figure 7 shows the calculated OH-stretching frequency vs. the O-H bond length for films iii and iv before modification, 5.45 OH/nm2 and 3.18 OH/nm2, respectively. The OH-stretching frequencies range from 3817 cm-1 to 3601 cm-1,

in qualitative agreement with FT-IR

measurements of SBA-15, which display broad band between 3759 cm-1 to 3743 cm-1.63 Linear regression coefficients of 0.9839 and 0.9876 are calculated for the films iii and iv, respectively, which is in agreement with previous works.17,25,64 Moreover, nearly the same slope is observed for both surfaces, which indicates that type-bond distribution does not significantly influence the OH-stretching frequency.25 Table 1 already showed that the type-bond distribution for silicon atoms changes as the surface density of silanol number decreases. The OH-stretching frequency seems to be grouped in two domains: A first domain of high frequencies spanning between 3821 cm-1 and 3740 cm-1, which includes isolated silanols and Hbond acceptors silanols, and a second domain of low frequencies varying between 3724 cm-1 and 3601 cm-1, which corresponds to H-bond donor silanols in agreement with experimental evidence.65



3850

3750 f [cm-1]

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3650

3550 0.968

0.973 O-H distance [Å]

Figure 7: OH stretching frequencies:

0.978

, OH/nm2 = 5.45;

, OH/nm2 = 3.18

As the surface density of silanol groups diminishes the lowest vibrational frequency becomes higher. The film iii displays a lowest vibrational frequency of 3601 cm-1, whereas the film iv has a lowest vibrational frequency of 3677 cm-1. This correlation is explained in terms of H-bond interactions. Figure 8 shows the structures corresponding to the two lowest vibrational frequencies from film iii. Both hydroxyl groups are involved in H-bond interactions (O--H) with surrounding oxygen atoms, and the OH-stretching frequency decreases as the O-H distances becomes shorter. Compared to the vibrational frequency of 3749 cm-1 assigned to terminal silanols,17 displacements of 118 cm-1 and 148 cm-1 are observed for the hydroxyl groups with O-H distances of 2.07 Å and 2.04 Å, respectively. These vibrational frequencies are comparable to vibrational frequencies assigned to Brønsted acid sites in bulk Mordenite (3608 cm-1).17


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-1

νOH = 3631 cm H

H O dO--H = 2.07 Å

O

H dO--H = 2.04 Å O

H O -1

νOH = 3601 cm

a)

b)

Figure 8: H-bond interactions between hydroxyl groups on the surface: a) Hydroxyl groups connected through a siloxane bridge; b) Hydroxyl groups connected at least through a -Si-O-Si-O-Si- bridge

Figure 9 shows a linear correlation (linear regression coefficient, 0.982) between the vibrational

frequency vs. O--H distance for several hydroxyl groups from films iii and iv. The lower is the O-H distance, the lower the vibrational frequency. From these results, we infer that the H-bond interactions can enhance the surface’s acidity strength becoming comparable to that of Brønsted acid sites created upon Al modification of H-Mordernite zeolites.66



3760 3740 3720 3700 f [cm-1]

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3680 OH/nm2 = 3.18

3660 3640

OH/nm2 = 5.45

3620 3600 3580 2

2.2

2.4

2.6

dO--H [Å]

Figure 9: OH-stretching frequency as a function of O-O distance: 3.18.

, OH/nm2 = 5.45;

, OH/nm2 =

Al-SBA-15 Figure 10 (left) shows the vibrational frequency for several surface silanol groups before and after Al3+ modification (Al/Si = 5/22). Besides the silanol group labeled as c, no relevant changes in O-H stretching frequency were observed. Figure 10 (right) shows the silanol group labeled as c, before and after modification with Al. After modification, the O--H distance is 0.22 Å shorter and the vibrational frequency is 83 cm-1 lower. However, only this silanol group displayed this behavior. We inferred that modifications with aluminum do not induce significant changes in the acidic properties of silanol groups already present on the surface before modification, only small changes in vibrational frequencies are observed.


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Silanol group labeled as c Al-SBA-15 SBA-15

3900 3800 f [cm-1]

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


3700 3600

H

H

3500

O

O

dO--H = 2.22 Å

dO--H = 2.00 Å

a

1

2 b

3 c

4 d

5 e

Silanol group

6 f

Figure 10: O-H stretching frequency in selected hydroxyl groups on the surface:

, SBA-15;

, Al-SBA-

15 (Al/Si = 5/22).

Modification with Al creates bridging hydroxyl groups on the surface. Figure 11 shows the OHstretching frequency of a bridging hydroxyl group as the Al/Si content increases. At frame a the OH-stretching frequency is 55 cm-1 higher than the OH-stretching frequency of a strong Brønsted acid site for a Mordenite zeolite (3608 cm-1).66 However, at frame b the vibrational frequency is 177 cm-1 lower than 3608 cm-1. This shift to lower frequencies is due to the increase in aluminum content. At frame b, the Al3+ is incorporated into the same ring containing the Al3+ already incorporated in frame a. This induces structural distortions which reinforce the H-bond interaction between the bridging hydroxyl group and the framework oxygen atom. The plot in Figure 11 helps to show this behavior. However, at higher Al/Si ratios we also observe that further aluminum incorporation does not change significantly the vibrational frequency. This is because for frames c, d and e, the Al3+ incorporation occurs in tetrahedral sites not included in the same ring having the bridging hydroxyl group under study. From this, we inferred that Al incorporation only induces local distortions affecting nearby hydroxyl groups. Creation of Brønsted acid sites with milder acidity on amorphous silica-alumina due to interaction of hydroxyl groups with Al atoms has been also reported in earlier works.67



H

H

H O

O a

O

b 3700

c 2.6

a

2.5 3600

O

d

2.4 2.3

3500

2.2

b

d

e

c

2.1

dH--O [Å]

H f [cm-1]

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2

3400

1.9 H

1.8

3300 O

0

0.05

0.1

0.15

0.2

0.25

Al/Si ratio e

Figure 11: Calculated OH-stretching frequency as a function of Al content: Al/Si = 1/26, b) Al/Si = 2/25, c) Al/Si = 3/24, d) Al/Si = 4/23, and e) Al/Si = 5/22.

We conclude that Al3+ incorporation not only creates bridging hydroxyl groups with stronger acidity properties than silanol groups, but also induces local distortions that may reinforce Hbond interactions between hydroxyl groups and framework oxygen atoms. Not only the electron valence of Al3+ atoms is important for modifying the surface acidity properties; the differences in atomic radius between Si and Al are also relevant. The vibrational frequency of bridging hydroxyl groups created upon aluminum modification (Al/Si ratio 5/22) is shown in Figure 12. The vibrational frequency oscillates between 3700 cm-1 and 3080 cm-1, which are 49 cm-1 and 669 cm-1 lower than the vibrational frequency of isolated silanol groups (3749 cm-1), respectively. Lower vibrational frequencies imply a stronger acid site, which indicates that bridging hydroxyl groups have better acidity properties than isolated silanol groups. This behavior has been also reported experimentally.17,68


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a

b

c

3800 3700

o

3600

o f [cm-1]

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


d

3500 3400 3300 3200 3100

o o

3000 a1

b2

c3

d4

e5

Site e

Figure 12: Calculated OH-stretching frequency of hydroxyl groups on protonated O (Al/Si ratio of 5/22)

Ti-SBA-15 Modification with titanium does not create additional hydroxyl groups on the surfaces. Changes in the vibrational frequency of a silanol group occurs when Ti4+ is incorporated in the first coordination sphere of that silanol group. Table 3 compares the vibrational frequencies before and after titanium modification. The column Δf quantifies changes in OH-frequency after Ti4+ incorporation. After modification most of hydroxyl groups suffer a decrease in their vibrational frequency. In most cases, this is possibly due to local distortions induced by substitution of nearby tetrahedral sites. This generalized decrease in frequency could explain the observed increment on acidity strength in supported SBA-15 catalyst after Ti4+ incorporation.12,14 Figure 13 shows two hydroxyl groups before and after modification. The vibrational frequency is

lowered by H-bond interaction. After modification, the vibrational frequencies of hydroxyl groups a and b are 3515 cm-1 and 3680 cm-1, respectively. For a, the vibrational frequency is only 72 cm-1 above the OH-stretching frequency of Brønsted acid sites in Mordenite, whereas 19 ACS Paragon Plus Environment


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the vibrational frequency of hydroxyl b is 93 cm-1 lower than the vibrational frequency of Brønsted acid sites in Mordenite.17 This is other indicative that the Ti modification effectively enhances the acidity properties of the surface, as has been reported in other works.12,14 Table 3: Calculated changes in OH-stretching frequency of silanol groups after Ti incorporation (Ti/Al = 5/22). -1

f [cm ] Hydroxyl group

Ti-SBA-15

Δf =

(Ti/Si = 5/22)

fTi-SBA-15 - fSBA-15

Pure SBA-15

a

3817.50

3801.72

-15.78

Decrease

b

3807.98

3515.28

-292.70

Decrease

c

3797.03

3680.85

-116.18

Decrease

d

3740.34

3694.60

-45.74

Decrease

e

3724.52

3771.99

47.47

Increase

f

3631.88

3688.12

56.24

Increase

g

3601.86

3593.92

-7.94

Decrease


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Before substitution

After substitution Ti/Si = 5/22

a

H H

O

O dO-H = 3.52 Å

dO-H = 1.94 Å

b

H H

O

O

dO-H = 3.37 Å

dO-H = 2.17 Å

Figure 13: Structure of two hydroxyl groups before and after modification with Titanium

Conclusions No modeling of the entire pore channel of SBA-15 was required for obtaining information about the surface’s structural features, validating the use of PBC models for modeling the amorphous framework of mesoporous silica materials. The advantage of this approach is evidenced in an adequate modeling of the type of bonding of the silicon atoms, which determines the surface reactivity of amorphous silica. Representative films have been obtained for different temperatures, which will be useful for future elucidation of temperature effects on HPC reaction mechanisms. Regardless the silanol number and the surface modification, all studied cases showed a linear relationship between the O-H vibrational frequency and the O-H bond length. It is observed 21 ACS Paragon Plus Environment


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that H-bond interactions between hydroxyl groups and oxygen atoms can lower the O-H vibrational frequencies of hydroxyl groups at the surface, which in turn increase their acidity strength. Moreover, this local effect is favored on surfaces with higher silanol number. It was determined that the SBA-15 modifications with Al and Ti are energetically favorable. However, we observe that both the isomorphic substitutions with Al and Ti become increasingly less favorable as the content of these species increases. This behavior could explain the appearance of these species in octahedral sites, as has been reported in earlier works, which does not contribute to enhance the Brønsted acid surface properties. Preferential sites for isomorphic substitution are identified for both Al and Ti. In the case of Al, the most favorable places for substitution are those where the protonated Al-O-Si bridge involves a Si atom type Q3, which suggests that Al modification through post synthesis procedures could be facilitated at low temperatures because the Si atoms type Q3 are more abundant in surfaces with a higher silanol number. In the case of Ti, the Q4 site is identified as the preferred site for substitution. This result indicates that at low Ti loadings the isomorphic substitutions are preferred in sites where the titanium cation is bonded to four Ti-O-Si bridges. We noticed that differences in atomic size between Si atoms and Al or Ti atoms contribute to changes in surface acidity. The modification with Al not only creates stronger Brønsted acid sites, but also generates local distortions inducing additional H-bond interactions between surface hydroxyl groups and surrounding oxygen atoms. In this way, not only the Al electronic valence is important for changing the surface acidity properties; differences in the atomic radius between Si and Al also play a relevant role. Moreover, we observe a generalized decrease in O-H vibrational frequencies after titanium modification. In most cases this was attributed to local distortions induced by substitution of tetrahedral sites near to surface hydroxyl groups.

This generalized decrease in wavenumbers could explain the observed

increment of acidity strength in catalysts supported on Ti-SBA-15. Future work should be focused on determining changes of adsorption energies after isomorphic substitutions. The enhancements in population of hydroxyl groups might be relevant for tuning bifunctional HPC catalysts. 22 ACS Paragon Plus Environment

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Acknowledgments Computational resources from Texas A&M Supercomputing Center, Brazos Supercomputing Cluster at Texas A&M University and from Texas Advanced Computing Center at UT Austin and computational resources from Supercomputación y Cálculo Científico (SC3) at Universidad Industrial de Santander are gratefully acknowledged.

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Table of Contents Graphic


Surface Structure and Acidity Properties of Mesoporous Silica SBA-15

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