Alkylsilane-Functionalized Microporous and Mesoporous Materials: Molecular Simulation and Experimental Analysis of Gas Adsorption

Alkylsilane-Functionalized Microporous and Mesoporous Materials:Molecular Simulation and Experimental Analysis of Gas AdsorptionS. Builes,*,† P. Lopez-Aranguren,†,‡ J. Fraile,‡ L.F. Vega,†,§ and C. Domingo*,‡

†MATGAS Research Center and ‡Instituto de Ciencia de Materiales de Barcelona (CSIC), Campus de la UAB s/n, Bellaterra, 08193,Spain§Carburos Metalicos, Air Products Group, C/Aragon 300, Barcelona, 08009, Spain

*S Supporting Information

ABSTRACT: Solid sorbents are considered as a potentiallyless-energy-intensive alternative to the use of liquids for theremoval and separation of liquid and gaseous fluids. Thecontrol of the surface characteristics of porous inorganicmaterials via the deposition of an organic layer is of greatinterest for tailoring the properties of the sorbent. Forinstance, organic functionalization of traditional solid sorbents(micro- and mesoporous silica and silicates) allows tuningtheir surface properties, such as hydrophilicity or hydro-phobicity and surface reactivity. However, the underlyingmechanism of the sorption process in highly complex organicfunctionalized materials is not yet fully understood. This incomplete understanding limits the possibilities of designing optimaladsorbents for different applications increasing the interest in performing complementary experimental-simulation studies. In thiswork, the adsorption of N2 in alkylsilane-modified disordered mesoporous silica (silica gel 40) and crystalline aluminosilicate(zeolite Y) is analyzed by a combination of experiments and simulations. The goal of the adsorption simulation study was two-fold: first, to assess the ability of using grand canonical Monte Carlo to obtain quantitative predictions of the adsorptioncharacteristics of gases on alkylsilane postfunctionalized products and, second, to provide new insights into the adsorptionmechanism. A supercritical silanization experimental method was used for the postmodification of the internal surface of thestudied porous substrates. This work demonstrates that even though the models of amorphous hybrid materials requiresimplifications related to the cell size and silane polymerization modes, it is possible to use these models to obtain an adequateinsight of what happens in the macroscopic systems. These models allow us to acquire information on the mechanisms of silanefunctionalization and the interactions of the support and silane chains with the adsorbed gases.

1. INTRODUCTIONSorption on micro- and mesoporous solid materials has beenfound to be one of the most effective, potentially less-energy-intensive, sorption technologies.1 Their main advantages arerelated to their accessibility, easy regenerability, convenientapplication in batch and continuous processes, and highsorption capacity. Traditionally, zeolites,2 porous siliceousminerals,3−6 and superabsorbent hydrogel polymers7 havebeen widely used as sorbents for pollutants in aqueous systems.However, industrial applications of these materials are limitedby the low sorption capacity and selectivity for nonpolar orhydrophobic matter. Currently, a new class of solid poroussorbents based on organic−inorganic hybrids is beingintensively prospected for new uses in organic matterseparation, with applications ranging from sorption of oil andhydrocarbon contaminants to CO2 capture.8−11 The organicfunctionalization of solid materials allows tuning surfaceproperties, such as hydrophilicity or hydrophobicity andreactivity.12−15 Therefore, porous sorbents can be tailored (orfunctionalized) based on the molecular structural features oftarget sorbate molecules.

With the advent of periodic amorphous mesoporous silica(SiO2) materials, organo-modified silica based materials haveattracted more and more attention as sorbents. Furthermore,the family of organo-modified disordered mesoporous silicasupports (silica gels) also deserves attention, mainly because oftheir low cost, large number of silanol groups on the internalsurface, and high surface area and pore volume. Theseproperties make them suitable for a large amount of bulkapplications in the area of sorption of organic compounds.Herein, a disordered mesoporous silica is used as the host forthe organic modifying moiety. Results on sorption capacity forthis material are compared with those of an organo-modifiedcrystalline aluminosilicate (faujasite). We used this lattermaterial for comparison because in commercial applicationsof adsorption the microporous zeolite family is the most widelyused adsorbent.

Received: February 28, 2012Revised: April 12, 2012Published: April 16, 2012

Article

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© 2012 American Chemical Society 10150 dx.doi.org/10.1021/jp301947v | J. Phys. Chem. C 2012, 116, 10150−10161

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One of the most successful surface functionalizationapproaches is the impregnation and chemical grafting of longhydrocarbon chains via silanization.16 In this work, a super-critical silanization method was used for the postmodification ofthe internal surface of porous substrates.8,17 Monoalkyltrialkoxy(R-Si(OH)3) and trialkylmonoalkoxy (R3-SiOH) silanes wereused for the study.The final objective of this study is to identify the adaptation

of hybrid structures for particular applications related tosorption. To utilize these materials for maximum economicaland environmental benefits, we must elucidate the relationshipbetween physical properties (e.g., specific surface area and porestructure) and surface chemistry (e.g., surface charge, hydro-phobicity). From an experimental perspective, both micro- andmesoporous structures are difficult to characterize; therefore, itis not easy to understand the sorbent structure at a molecularlevel. Hence, molecular simulations are used here to comple-ment adequately the experimental efforts.The structures of crystalline solids, such as zeolites, are

entirely known, and atomistic models are already available.18

However, for noncrystalline materials, such as silica gel, the linkbetween the real and the simulated material is merely statistical.Therefore, the first step to reproduce the functionalized silicagel surface was to build a realistic model to represent thepristine porous matrix. In this study, the method for thegeneration of atomistic models of silica gel of MacElroy andRaghavan19,20 was used.The methodology commonly found in the literature for

simulating functionalization of silica materials consists of usingan algorithm to replace a number of surface silanol groups byorganic moieties. A common procedure consists of adding tothe surface only the organic group (R-) in the silane moleculeand not the whole hydrolyzed compound (R-SiO).21−23 In that

conventional approach, the additional silica group (−SiO) fromthe silane molecule is taken into account in the pure silicamodel as an extra SiO2 layer. Consequently, this model givesbetter results for in situ modified materials than for thepostsynthesis functionalized ones.21 For this reason, theprocedure of grafting the complete surface group (R-SiO) onthe surface of the pure silica model material is taken in thiswork.24 In the last section, information about the porosity, thepore size distribution, the surface area, and the surfacechemistry of pristine and silanized materials is obtained fromthe adsorption data of the GCMC simulations. The equilibriumtheory of simple inhomogeneous fluids is used to obtain areliable prediction of fluid properties in crystalline microporouszeolites.The simulated results are discussed and compared with

experimental data available from our laboratory. Because themain objective of this research was to obtain models for use inthe design of sorbent materials, the performance of thegenerated structure in the simulation of adsorption comparedwith experimental data was the main criterion in the evaluationof the modeling method.

2. EXPERIMENTAL SECTION

2.1. Materials. Commercial supports, pertaining to twogroups of materials, were studied (see Figure 1): (i) acrystalline microporous Zeolite Y (ZY) of the Faujasite class,containing Si and Al atoms (Si/Al ≥1, Strem Chemicals), and(ii) an amorphous mesoporous silica gel 40 (SG40, Aldrich).The zeolite substrate was activated by calcination in a tubularoven (Carbolite 3216) at 520 °C during 48 h under N2. Thematrices were functionalized with either an alkyltrialkoxysilane(octyltriethoxysilane: CH3(CH2)7Si(OCH2CH3)3) or a trialky-l a l k o x y s i l a n e ( o c t y l d i m e t h y l m e t h o x y s i l a n e :

Figure 1. Used matrixes and silane reagents (raw and hydrolyzed). (A) octyltriethoxysilane, (B) octyldimethylmethoxysilane, (C) octylsilanetriol,and (D) octyldimethylsilanol.

The Journal of Physical Chemistry C Article

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(CH3(CH2)7(CH3)2SiOCH3)) both from Fluka, represented asmolecules A or B, respectively, in Figure 1. CO2 (CarburosMetalicos S.A., 99.995 wt %) was used as the solvent forsilanization.2.2. Synthesis of Silane Postfunctionalized Matrices. A

series of experiments was performed to prepare silanized hybridmaterials with the characteristics required for comparison withthe generated models. The supercritical silanization of thesubstrates was performed in batch mode in a high-pressureequipment described elsewhere.8 The alkylsilanes used have arelatively high solubility in scCO2 under the experimentalconditions employed. Hence, experiments were performed bycharging the reactor with the solid substrate, the liquid silane,and scCO2 at the desired pressure (P) and temperature (T)during a reaction time (t) (Table 1). Recovered samples afterdepressurization were washed with a continuous flow of scCO2at 10 MPa and 45 °C during 30 min to remove the excess ofdeposited silane.2.3. Characterization. The thermal behavior of raw and

silane-treated samples as well as the quantification of theimpregnated amount of silane were studied by thermogravi-metric analysis (TGA). Measurements were performed in a N2atmosphere at a heating rate of 10 °C min−1 by using a TGATA Instruments Q5000 IR. Textural characteristics andadsorption behavior of pristine and silanized substrates werestudied by low-temperature N2 adsorption at 77 K using aMicromeritics ASAP 2000 system. Prior to the measurements,mesoporous SG40 samples were dried under reduced pressureat 120 °C for 24 h, whereas ZY samples containing microporeswere dried at 150 °C during 48 h. The surface area (SBET) wasdetermined using the BET (Brunauer−Emmett−Teller)equation.25 The classical pore size model developed byBarret−Joyner−Halenda (BJH) was used for the calculationof the pore volume (VBJH) over the mesopore, using theadsorption branch of the isotherm. The displayed porediameter (Dp) in Table 1 corresponds to the average porediameter calculated from the equation: 4 × VBJH/SBET. Themicropore volume (Vmp) was estimated by the t plot (Harkins−Jura equation).

3. COMPUTATIONAL SECTIONThe models for the surface-modified substrates were generatedin two steps. First, the geometrical structure of the porous silicaskeleton was generated together with their silanol groups.

Second, silane chains were added considering two possiblekinds of interactions with the substrate, physical (impregna-tion) and chemical (grafting). Finally, molecular simulations ofthe generated models were used to understand the N2adsorption phenomena in pristine and silanized materials.

3.1. Structural Models of the Porous Media.3.1.1. Zeolite Model. Zeolite Y is a microporous crystallinesolid with a well-defined structure consisting of interconnectedchannels that forms a 3-D ordered network. Zeolite Y, with afaujasite framework, is formed by connected TO4 tetrahedral,where the T atoms are silicon or aluminum cations. This zeoliteconsists of almost spherical cavities of 1.3 nm in diameteraccessible through tetrahedral windows of 0.74 nm. Incrystalline zeolites, pore structure follows directly from thecrystallographic data. Hence, only the coordinates of the zeoliteY framework atoms, obtained by X-ray diffraction, are requiredto simulate the structures. In this work, the faujasite structurewas extracted from the zeolite database available in theMaterials Studio software package.26 A ratio Si/Al = 1 wasconsidered in the model, with no lattice defects and thus nosilanol groups on the pore surface. The presence of aluminumintroduces charge defects in the zeolite framework, which arecompensated with Na+ nonframework counterions. The pointcharge values were taken from the works of Maurin et al.27

They previously reported the adsorption of different gasmolecules on Faujasites at room temperature (Si: +2.4e, Al:+1.7e, O: −1.2, and Na: +0.7). The Lennard-Jones (LJ)parameters were taken from the consistent valence force field(CVFF). These parameters have been successfully used tomodel CO2/N2 adsorption on Faujasite.28 The model of theZeolite Y network is shown in Figure 1.

3.1.2. Amorphous Silica Gel Model. Experimentally,mesoporous amorphous silica gels are obtained by randomaggregation of primary dense silica nanospheres dispersed in acolloidal suspension (Figure 2a). The result is an amorphousporous material with a random pore size distribution andextremely high pore connectivity. The method used in thiswork to generate the model for the amorphous silica gel is anatomistic procedure that follows the concepts of theexperimental synthetic procedure.19,20,29 Figure 2b shows themain stages involved in the simulation of the porous silicastructure. Concisely, using the hard-sphere model, a cubic boxwith a preset void volume is filled with a chosen number ofspheres and a predetermined surface area. The spheres are

Table 1. Supercritical Operating Conditions and Some Obtained Results: Textural Properties and Silane Grafting Density(ρgrafting)

sample silanea T [K] P [bar] t [min] SBET [m2 g−1] VBJH [m3 g−1] Vmp [m3 g−1] Dp [nm] ρgrafting [mmol g−1]

1-SG40 556 0.47 3.42-SG40 C 348 100 90 460 0.41 3.6 0.413-SG40 C 348 200 90 410 0.38 3.7 0.584-SG40 D 348 100 45 424 0.35 3.3 0.591-SGT 475 0.44 3.72-SGT C 490 0.37 3.0 0.43-SGT C 532 0.33 2.2 0.64-SGT0.2 C 540 0.29 2.0 0.45-SGT0.2 C 536 0.26 1.9 0.61-ZY 750 0.282-ZY A 348 200 90 360 0.17 0.601-ZYT 740 na2-ZYT A 360 na 0.59

aImpregnated silane form (Figure 1) that was also used to calculate the molecular weight and thus the grafting density (ρdensity).



placed in the box at random positions until they fit in thesimulation cell, being both the dimension of the simulation celland the radius of the sphere found by iteration. The spheres arelinked using LJ interactions, allowing them to move andminimizing the energy. Because of computational constraints,the specific model in this work was constructed using only twointerconnected spheres. Although this restriction may limit thevalidity of some quantitative aspects of the adsorption results,the model is suitable for the thoughtful analysis of obtaineddata, assessed by comparison with experimental observations.The hard spheres in the box were subsequently replaced by a

realistic model of pregenerated amorphous silica spheres. The

primary silica nanospheres in the model were considered asconstituted by nonporous vitreous silica; the initial amorphoussilica blocks were taken from the Materials Studio structuresdatabase.26 The primary silica spheres were built by carving outfrom the bulk vitreous silica model using a random centralpoint and the previously calculated radius. The surface of thesilica spheres obtained in this manner is composed of oxygenatoms bonded to two silicon atoms and oxygen bonded to asingle silicon atom. Single-bonded nonbridging oxygens wereconnected to hydrogen atoms and represent the hydroxylsilanol groups, which normally exist on the surface ofamorphous silica. Figure 2c shows a snapshot of the generated

Figure 2. Silica gel: (a) experimental production process, (b) schematic outline of the main stages that are involved in model development, and (c)generated model (color key: Si: yellow; bridging O: red; nonbridging O: green; H: white).



silica structure. The textural parameters of silica gel 40 (sample1-SG40 in Table 1) were used for computation of the model.Values of cell and sphere dimensions were found by iterationresulting in 9.07 nm length and 3.5 nm radius, respectively. Thecalculated silanol density in the model was 4.5 OHnm−2.The LJ parameters for the solid silica atoms were obtained

from the works of MacElroy et al.19,30 in amorphous silica.These parameters have been previously used for the calculationof diffusion of different gases on models of silica gel.30 Thepoint charges for the silica were calculated by Brodka et al.31

from semiempirical calculations for silica clusters. Moreover,these parameters have been previously used to simulate theadsorption of gases on amorphous silica.21 The parameters forthe intermolecular interaction parameters of the silica gel aregiven in Table 2.

3.2. Functionalization with Silane Groups. The drivingforce for functionalization was selected between impregnationof nonhydrolyzed species A or B and the surface reaction withhydrolyzed forms C or D (Figure 1). The particular drivingforce selected for generating the models drastically dependedon the concentration of surface silanols on the support.3.2.1. Impregnation in Microporous Zeolite Y. In a perfect

zeolite crystal, the internal pore surface is electrically neutral,and no silanols would be present. Hence, silane addition in thissubstrate would be considered as an impregnation process andemulated by performing GCMC simulations of the adsorptionand desorption of nonhydrolyzed species.3.2.2. Grafting on Mesoporous Silica Gel. Silica gel

materials are characterized by having a large number of reactivesilanol groups on the internal pore surface. Therefore, for thesematerials, it was assumed that silane functionalization wasprimarily driven by surface reaction. The silane chains areexpected to react with the silanols tethering to the supportinstead of simply impregnating the surface. This hybrid materialwas modeled by introducing silane groups to the pregeneratedmodel of silica gel and linking them to a certain number ofsilanols existing on the surface. For comparison withexperimental results, the values of grafting density used tomodel the hybrid (0.4 and 0.6 mmol g−1 for 2-SGT and 3-SGTsamples, respectively) were similar to those obtainedexperimentally under two different working conditions (Table1). Each silane chain was built on the silica surface segment bysegment. The oxygen atom bonded to the silica gel surface wasconsidered as the first atom or segment in the chain (covalentgrafting) after eliminating the H accompanying the silanolgroup, whereas the second atom was the silicon bonded to theorganic chain. The silica material can be modeled as a rigid

structure, but the adsorbed alkylsilane molecules should havecertain mobility in regard to the torsion and bending of thechains. The torsion and bending angles in the surface groupswere handled using a coupled-decoupled configurational bias(CDCB) algorithm.32 Additionally, a pregenerated Gaussiandistribution for the probabilities of generating the bending andtorsion angles for the grafted molecules was used.33 The fulldetails of the functionalization method can be found else-where.24

3.3. N2 Adsorption Simulations. N2 adsorption wasstudied in the pregenerated model materials, both pristine andhybridized with surface groups. Simulations were carried outusing the Monte Carlo method, equilibrating the adsorbentwith a reservoir of N2 and exchanging and moving molecules inthe grand canonical ensemble (GCMC). The adsorptionisotherms were calculated by simulating the average numberof adsorbed N2 molecules at different sets of bulk pressure. Thesoft-SAFT equation of state was used to relate the pressure ofthe bulk fluid to the chemical potential of the adsorbate.34,35

Once the equilibrium was reached, the amount of adsorbedmolecules at this bulk pressure was averaged over a number ofGCMC trials. For each value of pressure, 2.0 × 107 trials wereused for equilibration and 1.4 × 107 steps for data collection.The adsorption simulations took into account that in the

simulation cell, periodic in three dimensions, the only rigidparts of the modeled hybrid material were the silica spheres.Conversely, the adsorbed surface chains had one endpositionally fixed to the silica surface, but bending and torsionmovements during the N2 adsorption were allowed. Thepositions of the different atoms of the silane chain wererecalculated during the adsorbate equilibration using the CDCBalgorithm. Finally, the adsorbate molecules were allowed todisplace to new positions as well as to enter and leave the cell;this was simulated by the insertion and deletion of fluidmolecules.To avoid N2 molecules being adsorbed inside the dense silica

skeleton, the insertion and deletion of fluid molecules wasrestricted to the open pore space using a cavity bias.36 Inaddition to the atomic and geometrical considerations for theadsorption model, the intermolecular interactions amongadsorbate molecules, silane chains, and silica surface wereinput parameters in the simulation. The intermolecularinteractions were calculated through pair-wise-additive 12−6LJ potentials for the repulsive and dispersive terms andCoulombic potentials for the first-order electrostatic contribu-tion.37 The interaction parameters between LJ sites werecomputed according to the Lorentz−Berthelot combinationrules.The interactions of the N2 molecules were modeled using the

TraPPE potential.38 N2 molecules were taken to be rigid, withan N−N bond length of 0.11 nm. The parameters for theintermolecular interaction of the fluid are listed in Table 2. Theparameters for the siloxane part of the organic chain are takenfrom the MM2 force field for silane compounds.39 Theparameters for the rest of the CHn organic groups are takenfrom the TraPPE force field. The charges in the surface chainswere adjusted to maintain electrical neutrality in the simulationcell. The parameters for the intramolecular energy of theimpregnated zeolite Y with octyltriethoxysilane (Figure S1) arelisted in Tables S1−S4, while the parameters for theintramolecular energy of the grafted hydrolyzed silane (FigureS2) in silica gel are listed in Tables S5−S8 (all presented in theSupporting Information).

Table 2. Lennard-Jones Interaction Parameters for the Non-Bonded Interaction of Silica Gel and Nitrogen in the GCMCSimulation of Adsorption

site σ (nm) ε/kB (K) q (e) ref

Silica GelSi 0 0 1.283 30,31Obridging 0.2708 228.4 −0.629 30,31Ononbridging 0.3000 228.4 −0.533 30,31H 0 0 0.206 30,31

N2

N 0.331 36.0 −0.482 38COMa 0.0 0.0 +0.964 38

aCOM: center of mass of the nitrogen molecule.



4. RESULTS AND DISCUSSION

The main objective of this work is to study the influence ofsilane functionalization on gas adsorption for different poroussupports. The basic process of study is the functionalizationwith an alkyltrialkoxysilanes (A in Figure 1), which afterhydrolysis of the alkoxy groups forms species of the type C. Forcomparison, the adsorption of trialkylmonoalkoxysilane (B inFigure 1), with a single hydrolyzable group that forms D, wasalso studied. For the systems analyzed here, the characteristicsof silane interaction with the substrate, physisorption byimpregnation or chemisorption by surface reaction, dependeddrastically on the concentration of surface silanols on thesupport.4.1. Adsorptive Behavior of Zeolite Y. Silanol groups in

crystalline zeolite Y are mostly present in the external surface asterminal groups. Experimentally, some internal silanols havebeen noticed and are taken as an indication of lattice defects.However, all together, the amount of silanols in 1-ZY sample

could be considered very small in comparison with amorphous1-SG40, as established by TGA curves (Figure 3a). In therecorded thermograph of raw 1-ZY, the only important weightloss occurred below 200 °C, and it was attributed to desorptionof physically adsorbed water. The weight loss occurring in therange 350−650 °C, assigned to silanol condensation, wasinsignificant.For zeolite Y due to the absence of surface silanols in the

substrate, the impregnation of monomeric nonhydrolyzedspecies was expected to be the primary driving force forfunctionalization opposed to surface reaction. For the super-critically octyltriethoxysilane impregnated zeolite, TGA showedthat the weight loss corresponding to silane desorptionoccurred mainly at temperatures below 350 °C (Figure 3bfor 2-ZY sample). The loss of surface groups at this relativelylow-temperature range is associated with the vaporization ofphysisorbed silane molecules. The TGA estimated impregnatedamount was ca. 0.60 mmol g−1 (Table 1).

Figure 3. TGA profiles of: (a) pristine substrates and (b) samples prepared by supercritical silanization with octyltriethoxysilane.

Figure 4. Results for zeolite Y: (a) octyltriethoxysilane adsorption/desorption isotherm, (b) image of the silane chains adsorbed in the crystallineframework, and N2 adsorption isotherms for the (c) pristine and (d) impregnated samples for the experimental (blue) and simulated materials (red).



The adsorptive impregnation of zeolite Y with silane A(Figure 1) was simulated by GCMC. Figure 4a shows thesimulated adsorption/desorption isotherm, while Figure 4brepresents an image of the chains impregnated in the crystallinezeolite. The maximum calculated loading was 0.59 mmol g−1

(sample 2-ZYT), which is a value equivalent to the one obtainedexperimentally (sample 2-ZY). The maximum impregnationvalue was limited by sterical constrains related to the length andshape of the silane molecule and the geometry of the pores andchannels. The calculated adsorption/desorption isotherm(Figure 4a) indicates that the modeled 2-ZYT hybrid materialwas saturated at relatively low pressures, but desorbing thesilane required high vacuum. Because the modeled 1-ZYTmaterial did not contain any surface silanol group, the organicgroups added were not able to form chemical bonds with thesurface and were simply impregnated. However, the analysis ofthe desorption curve indicates that they were bonded to thesurface via a strong physical interaction. Therefore, the silaneimpregnated inside of the zeolite would require for its removalthe use of a temperature swing process.The simulated and experimental N2 adsorption isotherms for

the pristine and impregnated zeolite samples (Figure 4c,d,respectively) showed an excellent agreement. The main effectof silane functionalization was the reduction of the pore volumeand thus the zeolite adsorption capacity.4.2. Adsorptive Behavior of Silica Gel. 4.2.1. Pristine

Silica Gel. The model of the pristine silica gel was constructedwith dimensions consistent with the structural properties of themesoporous silica gel 40 (sample 1-SG40 in Table 1). Owing tothe broad range of parameters involved when modeling thepristine amorphous silica gel, it was necessary to imposerestrictions on the solid phase by limiting the number ofparticles in the simulation cell to two. The low number ofspheres used could affect the correct reproduction of therandomness of the void space in the silica gel. Nevertheless, thiswas required as a compromise between the CPU requirementsfor the simulation and a reliable estimation of the generatedpore space in the unit cell. The capability of the method topredict accurately textural and adsorption properties wasevaluated by comparing N2 adsorption isotherms (at 77 K)for pristine and impregnated materials obtained by eitherGCMC simulation or BET measurements. The validity of thesilica gel model was first verified by comparing the silanoldensities in the experimental and simulated materials. From theTGA thermograph, a silanol density of 4.6 OHnm−2 wasestimated for sample 1-SG40 (Figure 3a), which matches thecalculated value in the model of 4.5 OHnm−2 for sample 1-SGT.

Experimental and computed adsorption isotherms of thepristine material are both of type IV (Figure 5a), exhibiting awell-pronounced stepwise character related to a two-stagemesoporous behavior.25,40,41 In these systems, the convex part,representing the region from capillary condensation to porefilling (point D in Figure 5a), is preceded by a concave wettingtransition going from an empty state to intermediate conditionscharacterized by layer-by-layer growth on the pore walls. Thesimulation yielded the absolute adsorbed amount of N2straightforwardly correlated to the mesoporous volume insideof the cell because the model did not contemplate largemesopores or the macroporous interparticle volume. Hence,measured VBJH values for the supercritically prepared sampleswere used for comparison with the model. The SBET (Table 1)of model materials was calculated from the simulatedadsorption isotherm applying the BET equation.The point at which pore filling occurs is controlled by the

accessible porosity, the pore geometry, the pore networking,and the tortuosity. In the studied systems, the total adsorbedamount at P/P0 = 1 was higher for the synthesized materialthan for the simulated one (Figure 5a). Moreover, the positionof total pore filling (point D in Figure 5a) occurred at lowerpressure in the simulation (P/P0 ca. 0.6) than in the experiment(P/P0 ca. 0.8). Both facts indicate smaller pore size and totalpore volume for the model (sample 1-SGT) with respect to thepristine material (sample 1-SG40), as reflected in the texturaldata (Table 1). This finding is on account of the practical limitsof the simulation cell size, which did not allow the accuratereplication of the complete pore size distribution (large pores).However, at P/P0 = 0.6, similar adsorbed amounts were foundfor the experimental and simulated isotherms. Therefore, tofacilitate comparison, the adsorption at relative pressures higherthan 0.6 will not be considered in further discussion, since theextra-adsorption at P/P0 > 0.6 observed for the experimentalmaterial and related to that occurring in large mesopore andmacropore voids could not be represented by the modelmaterial.The microscopic details of the silica should be reflected

mostly in the low-pressure range (P/P0 < 0.1) becauseadsorption in this region is determined by preferred adsorptionsites. In mesoporous materials, the initial concave part of theadsorption isotherm is attributed to monolayer formation.Point B in Figure 5a is taken to indicate the stage at whichmonolayer coverage is completed and multilayer adsorption isabout to begin. Figure 5b shows in more detail the low-pressurerange of the isotherms. In this pressure region, the agreementbetween simulation and experimental data for adsorption on

Figure 5. Experimental and simulated adsorption isotherms of N2 on pristine silica gel at 77 K for: (a) whole and (b) enlarged low pressure range.



the pristine material was excellent. Still, the amount of adsorbedN2 was slightly larger in the experimental sample than in thesimulated one, which can be corrected by applying to thesimulation isotherm a scaling factor (ϕ)21,42 based on theestimated porosity values for the experiment and the model (ϕ= 0.47/0.44 in Table 1). Thereafter, the validated model ofsimulated silica gel can be expanded to study functionalizationby considering the grafting of the silane chains inside thematerial.4.2.2. Functionalized Silica Gel. In the second section of this

work, the silica gel matrix was functionalized with alkylsilanechains. A supercritical CO2 silanization procedure was followedto synthesize the experimental materials. For a trialkoxysilane ofthe type RnSi(OR′)3 (A in Figure 1), the surface silanizationreaction starts with the hydrolysis of the alkoxy OR′ groups toform silanols (RnSi(OR′)3−x(OH)x, C in Figure 1), which canthen interact by hydrogen bonding with the OH groups on thesurface of the porous silica material (#Si−OH···HO−Si) orwith other silane molecules (Si−OH···HO−Si). Dehydration ofthe formed hydrogen bonds produces siloxane linkagesbetween the silane and the surface (#Si−O−Si) and self-assembled monolayers by condensation of neighboring silanemolecules (Si−O−Si). Hence, for the hydrolyzed species C,covalent grafting with the surface can be coupled to horizontalself-assembly. In the TGA plot of the supercritically silanizedsamples (Figure 3b), the loss of weight occurred within 350−600 °C was related to the decomposition of strongly bondedspecies (highly cross-linked and/or chemisorbed silanes) bycleavage of C−C and Si−C bonds. The calculated amounts ofoctyltrisilanol that reacted with the substrate at two differentexperimental pressures were 0.42 and 0.57 mmol g−1 for thesamples 2-SG40 and 3-SG40, respectively (Table 1).Experimental observations indicate that the highly reactive

surface of amorphous silica gel might assist the formation ofcovalently bonded silanes, as surface reaction is the primarydriving force for absorption versus impregnation. Moreover, themesoporous space allows for the self-condensation reactionbetween adjacent silanols. Even though the steps of theexperimental silanization process are well known, the complex-ity of the reaction makes it necessary to simplify the chemistryemployed to model the functionalization. The most usedgeneralization consists of not taking into account the formationof siloxane bridges between neighboring silane chains.21−23,43

This assumption was also employed in this work. The hybridsimulated material was generated starting from the O atomcovalently bonded to the silica surface, whereas the secondatom was the Si bonded to the organic chain, which was addedsegment by segment. Figure 6 shows an image of the resultingoctylsilane chains grafted on the amorphous silica gel. Themolecules were always required to react in a monodentatecovalent configuration with the surface and never withneighboring silane molecules. The designed algorithm isexpected to allow analyzing and understanding simultaneouslythe surface of silica functionalized with both tri- andmonoalkoxysilanes. Actually, monodentate bonding is theexpected natural experimental behavior for trialkylmonoalkox-ysilane molecules with a single hydrolyzable group (B in Figure1). After hydrolysis, octyldimethylmethoxysilane generates theform D, which reacts with the hydroxylated surface (#SiR2−O−Si). For ease of comparison, a silanized 4-SG40 sample wasprepared in the laboratory with a monoalkoxysilane, containinga grafting density of 0.59 estimated by TGA (Table 1).

The adsorption isotherms of silica gel samples functionalizedwith silanes, measured for the synthesized products (C or D)and calculated with GCMC for the simulated materials (C),reflected many properties of the porous hybrid materials. Theavailable space for N2 adsorption, measured as the VBJH,diminished with functionalization for all studied materials(Table 1). The reduction of mesopore volume was similar forthe experimental (Figure 7a) and model (Figure 7b) materials.The VBJH was reduced with respect to pristine silica gel in ca.16% by functionalizing with ca. 0.4 mmol g−1 of silane, whereasa decrease of ca. 25% was estimated for deposited silaneconcentrations of ca. 0.6 mmol g−1. Hence, the model was ableto simulate accurately the reduction in the mesoporeadsorption capacity due to the increasing void space occupiedby hydrolyzed silane as the concentration increased. Con-versely, the simulation slightly overpredicts the adsorption inthe low-pressure region (Figure 7c,d for 0.4 and 0.6 mmol g−1

functionalization, respectively), which is the opposite behaviorto that observed for the pristine materials (Figure 5b). Fortrialkoxysilanes, the overprediction likely reflects the absence ofhorizontal self-assembly in the simulated product, theimportance of which increases with the functionalizationdegree. Whereas in the simulation the microvoids createdamong the randomly distributed chains can act as strongadsorption sites (Figure 8a2), the chains in the experimentalmaterial are horizontally self-assembled, leading to a morecompact packing (Figure 8a1). Significant differences betweenthe chemisorbed monoalkoxyde and the model were alsoobserved (Figure 7d). The structural monodentate absorptionin D (Figure 1) could be considered to be similar to thatadopted in the model (Figure 8b and a2, respectively).However, D exposes lateral methyl groups (CH3) amongchains, which shield the polar surface of the metal oxide.Therefore, the methyl groups act as a structural barrier thatprevents N2 molecules interaction with the surface (Figure 8b),resulting in low adsorption values in the low pressure range.15,44

The lateral silanol moieties present in the model likely acted asstronger adsorption sites than the CH3 in the monoalkoxide.

Figure 6. Snapshot of hydrolyzed octyltriethoxysilane functionalizedsilica gel (similar color key of Figure 2).



Figure 7. Experimental and simulated adsorption isotherms of N2 on functionalized silica gel at 77 K for: (a) experimental materials, (b) modeledmaterials, and experimental and simulated materials loaded with (c) 0.4 mmol g−1 and (d) 0.6 mmol g−1 in the enlarged low-pressure range.

Figure 8. Schematic representation of the different functionalization possibilities for species: (a) A and (b) B in Figure 1.



The macroscopic consequences found from this analysiswere similar values of pore volume for both experimental andsimulated materials but significant differences in the surfacearea (Table 1). The BET surface area decreased after silaneadsorption for laboratory obtained samples and slightlyincreased for modeled compounds, indicating a largeproportion of micropores in the simulated model. Geometri-cally the smallest pores have little contribution to the porevolume but a large contribution to the surface area. The extramicropores could be generated in the simulated material afterfunctionalization of the mesopores. The reduction in the meanpore size was significant only for the simulated materials.In a different approach, and following the work of

Schumacher et al.,21,42 the order in the −Si−O-Si-O- horizontalpolymerization observed experimentally for trialkoxysilanes wassimulated by adding an extra layer of SiO2 to the silica primaryparticles model, then building on the silica surface only theorganic chain of the silane molecule. To compare both models,we calculated a new set of simulations, generating an additionalamorphous silica gel model with the configuration of 1-SGT butwith a radius 0.2 nm higher (the van der Waals radius of siliconand oxygen is 0.038 and 0.152 nm, respectively) than the

original one. The alkyl chains were grafted in concentrations ofeither 0.4 or 0.6 mmol g−1 (samples 4-SGT0.2 and 5-SGT0.2,respectively) to the generated spheres of 3.7 nm radius.Schematic representations of the experimental and 0.2 nmmodel used are shown in Figure 9a,b, respectively. For thesesamples, the reduction in adsorption capacity with respect tosample 1-SGT was on the order of 33 and 38% forconcentrations of silane chains of 0.4 and 0.6 mmol g−1,respectively (Figure 9c,d, respectively). These percentages arenot comparable to the reductions observed in the experimentalmaterials, which were on the order of 15 and 25% for samples2-SG40 and 3-SG40, respectively. Hence, the macroscopicadsorption behavior at different functionalization degrees wasnot accurately described by the silica gel model with a largerradius of the spheres. Using the approach of increasing the silicaradius sphere in 0.2 nm, it was considered that thefunctionalized hybrid was constituted by a continuous film of−Si−O-Si-O- around all primary spheres (Figure 9b). There-fore, the accuracy of the results obtained using this approachwould be higher for systems with high degrees offunctionalization, that is, near a monolayer. Note that atgrafting densities of 0.4 mmol g−1, the deviation between

Figure 9. Schematic models used in samples: (a) 2-SGT and 3-SGT and (b) 4-SGT0.2 and 5-SGT0.2. Experimental and simulated adsorption isothermsof N2 on silica gel at 77 K for samples functionalized with (c) 0.4 mmol g−1 and (d) 0.6 mmol g−1 of silane (species C in Figure 1).

Figure 10. Degree of functionalization as a function of the number of computational cycles required for the grafting simulation for: (a)octyltrialkoxysilane and (b) trialkoxysilanes with different tail lengths (ethyl 2C, butyl 4C, hexyl 6C, and octyl 8C).



macroscopic adsorption values of the experimental and thesimulated material was on the order of 55% (compare samples2-SG40 with 4-SGT0.2), whereas this value was reduced to 35%by increasing the functionalization degree to 0.6 mmol g−1

(compare samples 3-SG40 with 5-SGT0.2). In contrast, thedeviation <10% for samples where each silane molecule wasgrafted entirely to the silane surface (compare samples 2-SG40with 2-SGT and 3-SG40 with 3-SGT). The model with the largerspherical particle radius had less void volume and a smaller porediameter, entering the region of micropore (Dp in Table 1).From the simulated material, the maximum grafting densities

of C was calculated. Figure 10a depicts a plot of the number ofMC cycles needed to obtain a given grafting density; each MCcycle is an attempt to change the grafting point of one of themolecules and moving the previously grafted chains. The pointsrepresent the molecules as they are tethered during thesimulation, and the continuous line is the total number ofmolecules grafted. From this plot, it is possible to obtain theupper limit (geometrically and energetically) of functionaliza-tion of the silica gel model, which corresponds to 0.9 mmol g−1

for octyltriethoxysilane.To study the influence of the grafted chain length in the

maximum amount of molecules that can be grafted on the silicagel surface, we functionalized the model silica gel with severalalkyltriethoxysilanes of different alkyl lengths (ethyl, butyl,hexyl, and octyl). Because the grafted part ((OH)2SiO−) of thechain was kept constant, only the esteric impediment due to thedifferent tails has an effect on the maximum grafting densityLarge differences in grafting densities were reached by reducingthe number of carbons from 8 (8C, octyl) to 6 (6C, hexyl) andto 4 (4C, butyl) (Figure 10b). However, a further reduction totwo carbons (2C, ethyl) in the chain did not produce asignificant reduction in the grafted amount with respect to fourcarbons.

5. CONCLUSIONSResults concerning the adsorption of N2 in alkylsilane-functionalized silica by Monte Carlo simulations werepresented and compared with available experimental datafrom our laboratories. The validity of the generated silica gelmodel was first verified by stating similar silanol densities forexperimental and model samples (ca. 4.5 OHnm−2). Analyzingthe textural properties, it can be concluded that the simulatedmaterial is an appropriate model for silica gel 40 for pore sizesbelow the intermediate mesoporous range. Next, experimentaland simulated N2 adsorption isotherms of postsynthesisfunctionalized materials were compared to obtain insight intothe behavior of the functionalized chains. The simulationsaccurately predicted the experimental adsorption isotherms fora zeolite matrix in which silane physisorption was expected. Forcovalently bonded silane-functionalized silica gel, the alkylsilanemolecule was grafted entirely to the silica surface. This modelwas able to simulate accurately the reduction in the adsorbentcapacity due to the increasing pore space occupied by thehydrolyzed silane as the concentration increased, with adeviation of <10% with respect to experimental values. Incontrast, for a model where only the alkyl chain was grafted tothe silica substrate, deviations as high as 55% were found.Moreover, it was shown that the simulation method forcovalent functionalization of silica gel described in this work ismore realistic in regards to the prediction of the adsorptioncapacity at different grafting densities than the models thatconsider only the organic tail of the silane chain as the grafting

moiety, at least for concentrations lower than thosecorresponding to monolayer formation.

■ ASSOCIATED CONTENT*S Supporting InformationSimulation parameters for the functionalized alkylsilane chains.This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] (C.D.); [email protected] (S.B.).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe financial support of the Spanish Government underprojects CENIT SOST-CO2 CEN2008-1027 (MEC, CDTI)CTQ2008-05370 and MAT2010-1855 is gratefully acknowl-edged. Additional support for this work has been provided bythe Generalitat of Catalonia under project 2009SGR-666 andby Carburos Metalicos (Air Products Group). The computa-tional time provided by CESCA, the supercomputer Center ofCatalonia is deeply appreciated. S.B. acknowledges a TALENTgrant from the Commission for Universities and Research ofthe Generalitat de Catalunya.

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