Pulse Gas Chromatographic Study of Adsorption of Substituted

Sep 7, 2012 - Aromatics and Heterocyclic Molecules on MIL-47 at Zero Coverage. Tim Duerinck,. † ... of molecular functionality and size on Henry ads...
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Pulse Gas Chromatographic Study of Adsorption of Substituted Aromatics and Heterocyclic Molecules on MIL-47 at Zero Coverage Tim Duerinck,† Sarah Couck,† Frederik Vermoortele,‡ Dirk E. De Vos,‡ Gino V. Baron,† and Joeri F. M. Denayer*,† †

Department of Chemical Engineering, Vrije Universiteit Brussel, Belgium Centre for Surface Chemistry and Catalysis, Katholieke Universiteit Leuven, Belgium

‡

S Supporting Information *

ABSTRACT: The low coverage adsorptive properties of the MIL-47 metal organic framework toward aromatic and heterocyclic molecules are reported in this paper. The effect of molecular functionality and size on Henry adsorption constants and adsorption enthalpies of alkyl and heteroatom functionalized benzene derivates and heterocyclic molecules was studied using pulse gas chromatography. By means of statistical analysis, experimental data was analyzed and modeled using principal component analysis and partial least-squares regression. Structure−property relationships were established, revealing and confirming several trends. Among the molecular properties governing the adsorption process, vapor pressure, mean polarizability, and dipole moment play a determining role.

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INTRODUCTION Metal−organic framework materials have received considerable attention in the past years. A unique feature of these highly structured porous materials is a seemingly unimaginable number of combinations of metal (oxide) clusters and organic linkers. The synthesis of a large variety of these materials has been reported, and their number is ever increasing. It is therefore of utmost importance to gain an insight in the structural trends that govern the adsorption properties. The experimental screening of the seeming endless possible structures is not only time but also cost ineffective and economically not viable. A straightforward and systematic approach in screening for an applications fitting adsorbent at low experimental or calculation cost would be of great value. Accurate methods that allow one to predict adsorption properties for a first preliminary screening toward applications are still lacking. Several attempts, recently reviewed by Smit and Maesen,1 and Gelb,2 to obtain such a prediction based on structural modeling are reported for specific materials. In this paper, we report on the adsorption properties of MIL47. MIL-47 is a rigid metal−organic framework consisting of vanadium cations associated to six oxygen atoms, linked together by terephtalic molecules. The porous structure of the material is that of 1D diamond-shaped straight channels, with pore opening of 7.9 Å × 12.0 Å. The synthesis of MIL-47 was first reported by Barthelet and co-workers in 2002.3 Most adsorption studies on this material focused on CO2, alkanes, and xylenes. The adsorption and diffusion of hydrogen, methane, and carbon dioxide on MIL-47 has been extensively studied throughout the years.4,5 Bourelly et al. described CH4 © 2012 American Chemical Society

and CO2 adsorption at 304 K using direct microcalorimetry at different pressures.6 Ramsahye et al. were able to reproduce experimental adsorption isotherms by Grand Canonical Monte Carlo simulation for CO2 on MIL-47 and MIL-53.7 Using density functional theory and molecular dynamics, it was revealed that MIL-47 has no preferential sites and only a small number of weak CO2 adsorption geometries.8 The resulting adsorption enthalpy could be related to the experimental microcalorimetry data.9 Liu and Smit showed in an extensive comparative molecular simulation study on three zeolites (MFI, LTA and DDR) and seven MOFs (Cu-BTC, MIL-47, IRMOF1, IRMOF-11, IRMOF-12, IRMOF-13, and IRMOF-14) that separation performance for CO2/N2 and CH4/N2 is comparable for all materials, with the exception of somewhat elevated performance in the presence of larger quadrupoles.10 Hydrocarbon adsorption of linear C1−C9 alkanes in MIL-47 has been studied extensively with respect to pore (in)flexibility in comparison with MIL-53 using isotherm measurements and X-ray diffraction (XRD) structure characterization, confirming the rigid nature of the MIL-47 framework.11 Low-coverage adsorption properties of linear and branched C5−C8 alkanes, cyclohexane, and benzene were measured using pulse gas chromatography by Finsy et al.12 For linear alkanes, a 7.6 kJ/ mol increase for each additional methylene group was reported. Henry constants for iso-alkanes were slightly lower but no steric constraints of the side branches were observed. Received: July 10, 2012 Revised: September 5, 2012 Published: September 7, 2012 13883

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Figure 1. Structures of the molecules studied by pulse gas chromatography.

adsorption C8−C10 alkyl aromatics was observed. The strongest interactions were found between methyl groups of the adsorbates and carboxylate groups in the pores.25 In regard to stability and the scope of fuel desulfurization, MIL-47 and MIL-53 (except the Fe form of the latter material) proved to be stable upon H2S adsorption.26 The preferential adsorption sites are the μ2-O sites in the initial stage; at higher pore filling, the equilibrium shifts to the formation of dimers with higher orientational disorder.27 Khan et al. reported desulfurization by benzothiophene adsorption in liquid phase, attributed to the high acidity of the highly oxidized vanadium ion.28 A more recent study revealed that CuCl2 loaded MIL-47 adsorbed even more benzothiophene compared to the original MIL-47.29 In the present work, the focus lies on the adsorption of a set of benzene derivatives with alkyl, halogen, and other functional groups and heterocyclic C5−C6 molecules on MIL-47. Adsorption properties of a set of 22 molecules at very low degree of coverage (“zero coverage”) were determined by pulse gas chromatography. Quantitative structure−property relationships between adsorption properties and molecular properties were established using principal component analysis and partial least-squares regression. By gathering a limited amount of experimental adsorption data, we previously showed that it is possible to apply data mining techniques in order to obtain predictive relationships between the physicochemical properties of the adsorbing compounds and the adsorbent on the one hand and the adsorption behavior on the other hand. Such quantitative structure−property relationship (QSPR) modeling could also contribute to a better understanding of the adsorption phenomenon.30

Rosenbach et al. studied the adsorption of light hydrocarbons (C1−C4) in the flexible MIL-53 and rigid MIL-47 framework using a combination of molecular simulations and microcalorimetry measurements.13 They established a relationship between alkyl chain length, mean polarizability, and adsorption enthalpy. A one-dimensional diffusion mechanism for alkanes in the pores of MIL-47 was found by neutron scattering spectroscopic experiments and confirmed in molecular dynamic calculations by Jobic et al.14,15 The adsorption of n-hexane and n-nonane in MIL-47 was studied through gravimetric measurements in a comparative study with MIL-53 on structure breathability.16 Experimental adsorption isotherms of C5−C9 n-alkanes were compared to those obtained by configurational bias Grand Canonical Monte Carlo simulations.17 A fair agreement was found and the trans configurational orientation along the pore axis was predominantly observed for short n-alkanes. The parallel direction was more favored by longer n-alkanes.17 Characteristics of adsorption and diffusion for a number of guest molecules (Ar, CO2, C5−C9 linear and branched alkanes) in zeolites (FAU, NaY) and metal−organic frameworks (IRMOF-1, CuBTC, MIL-47, MIL-53(Cr)-lp, PCN-6′) were studied below the critical temperature of the guest species.18 Linear isomers show a higher degree of clustering than branched species. The adsorption selectivity in mixtures is significantly enhanced toward components with a higher degree of clustering. The selective adsorption and separation of xylene isomers and benzene on MIL-47, CuBTC and MIL-53 was reported by Alaerts et al.19 Preference of para-xylene over meta-xylene and para-xylene over ethylbenzene was found. In a later study, they reported on the influence of temperature, length, bed thickness, and oven geometry during regeneration of MIL-47 regarding its performance.20 In general, it was found that the adsorption selectivity increases with partial pressure or the degree of pore filling.21 Separation at high pore filling results from differences in packing modes of the C8 alkylaromatic components in the pores of MIL-47.22 Similarly, the separation of styrene and ethylbenzene was studied.23 Selectivity and adsorption properties of xylenes were confirmed by Catillo et al. in a molecular simulation study.24 A preference for ortho-isomers in the

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MATERIALS AND METHODS

The adsorption properties of substituted benzene and heterocyclic molecules on MIL-47 were determined by pulse gas chromatography. Henry constants were obtained directly from the chromatographic response, while adsorption enthalpies and pre-exponential factors were calculated from the responses at different temperatures. The experimental data was analyzed and modeled by statistical means. The methodology is based on two main techniques: principal component analysis and bootstrap partial least-squares regression.29 13884

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Table 1. Correlation Matrix of the Definitive Descriptor Set vapor pressure density polarizability dipole moment heat of formation χ3 cluster

vapor pressure 1.00 −0.51 −0.72 −0.58 −0.28 −0.61

density

polarizability

dipole moment

heat of formation

χ3 cluster

1.00 0.21 0.31 0.33 0.04

1.00 0.00 0.37 0.75

1.00 −0.07 0.12

1.00 0.22

1.00

Table 2. Experimental and Modeled Adsorption Properties of MIL-47 at Zero Coveragea experimental compd benzaldehyde* benzene* bromobenzene chlorobenzene* dimethylsulfide dioxane* ethylbenzene fluorobenzene* iodobenzene* mesitylene methylbenzoate m-xylene* napthalene nitrobenzene* n-pentane* o-xylene propylbenzene* p-xylene pyridine tertrahydrofurane thiophene toluene

enthalpy −54.8 −43.1 −55.3 −52.0 −34.6 −39.5 −54.5 −44.2 −59.6 −62.6 −67.2 −55.6 −67.2 −58.3 −43.0 −54.8 −62.5 −53.9 −46.5 −36.3 −52.3 −51.0

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1.4 0.8 1.0 1.2 0.7 0.9 1.0 0.8 1.0 2.1 1.1 1.0 2.1 1.4 1.5 1.0 1.4 1.0 1.0 0.8 1.5 0.9

modeling

pre-exponential factor −21.2 −2.03 −21.4 −21.1 −19.7 −19.5 −21.0 −20.4 −21.7 −23.0 −23.1 −21.4 −22.3 −21.5 −20.0 −20.9 −22.0 −21.0 −20.8 −19.3 −19.9 −21.2

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.3 0.2 0.3 0.3 0.2 0.2 0.3 0.2 0.3 0.5 0.3 0.3 0.5 0.3 0.4 0.3 0.4 0.3 0.3 0.2 0.4 0.2

enthalpy

pre-exponential factors

−53.5 −43.5 −55.2 −51.7

± ± ± ±

1.4 1.1 1.5 1.4

−21.2 −21.3 −21.9 −21.1

± ± ± ±

0.6 0.6 0.6 0.6

−39.1 −56.0 −45.7 −61.1 −61.4 −64.2 −56.2

± ± ± ± ± ± ±

1.0 1.5 1.2 1.6 1.6 1.7 1.5

−21.4 −21.3 −20.2 −19.7 −19.9 −20.8 −21.1

± ± ± ± ± ± ±

0.6 0.6 0.6 0.5 0.5 0.6 0.6

−58.6 −42.8 −56.7 −61.5 −55.5 −44.2 −39.9 −41.7 −50.2

± ± ± ± ± ± ± ± ±

1.5 1.1 1.5 1.6 1.5 1.2 1.0 1.1 1.3

−20.1 −20.9 −19.7 −20.3 −21.8 −21.4 −21.5 −20.0 −21.9

± ± ± ± ± ± ± ± ±

0.5 0.5 0.5 0.5 0.6 0.6 0.6 0.5 0.6

a

Adsorption enthalpies are given in kJ/mol and pre-exponential factors (transformed by normal logarithm) in mol/[kg·Pa]. The training set for the modeling is marked by an asterisk (*).

Synthesis and Characterization. MIL-47 was synthesized according to a recipe from literature, by loading a mixture of typically 1.22 g of VCl3, 0.32 g of terephthalic acid, and 14 mL of H2O in a Teflon-lined steel autoclave and placing it in an oven at 473 K for 96 h.3 After cooling, the mixture was washed with water and activated by calcination under air for 21.5 h at 573 K.20 After synthesis, the white powder was treated with N,N-dimethylformamide under ultrasonic treatment at 70 °C, according to the method of Haque et al, so as to evacuate the pores.31 XRD analysis of the samples revealed that the material is phase-pure MIL-47 (Supporting Information Figure S1). Pulse Chromatographic Experiments. Using the pulse gas chromatography method, the adsorption properties of benzene derivates (alkyl, halogen, and heteroatom functionalized) and heterocyclic molecules with 5 or 6 membered rings (Figure 1) were measured in the zero coverage limit, that is, at very low surface coverage, where molecules are not interacting with each other. Adsorption enthalpy ΔH0, Henry constants K′, and pre-exponential factors K′0 were determined for each of the components.32−35 Adsorbent powder was compacted into disks by applying pressure of ca. 600 bar; the disks were broken into fragments and sieved. The 530−600 μm fraction was used to fill an 1/8 in. diameter stainless steel column with a length of 15 cm. In situ activation of the adsorbents was performed by raising the temperature to 300 °C (2 °C/min) and keeping the temperature stable for 6 h. The gas flow was set to 10 cm3/min. The GC pulse measurements were carried out using an Agilent 7820A gas chromatograph with a thermal conductivity detector

(TCD). An inert gas carrier (helium) was passed through the column filled with pellets. The flow rate was regulated by a mass flow controller and set to 25 Ncm3/min. Pressure drop over the column was measured with a pressure transducer placed at the outlet of the mass flow controller. After injection, a baseline correction of the response curve was performed. First order moments were calculated by integration. The adsorption measurements were performed between 100 and 300 °C; 1 μL of sample was injected during each experiment. A minimum of five data points, covering a minimal range of 50 °C was recorded. The absolute column inlet pressure varied between 1.21 and 1.49 bar. The adsorption enthalpies at zero coverage were calculated from the temperature dependence of the Henry constants as given by the van’t Hoff equation.36 The adsorption enthalpy at zero coverage is proportional to the slope of ln K′ versus 1/T plot. Correlations (r2) of 0.997 and above were obtained in all cases. Statistical Modeling. The pre-exponential factors and adsorption enthalpies at zero coverage from the previously described chromatographic experiments were analyzed using principal component analysis (PCA) and partial least-squares regression. By establishing statistical models relying on few empirical or calculated descriptors, direct interpretation of these descriptors is possible. A correct and well considered interpretation of the adsorption phenomena regarding the adsorbate−adsorbent properties could lead to a more profound understanding or the predictive power of new material’s properties. In the analysis phase, the statistical program SPAD37 was used for principal component and cluster analysis (analysis phase), while partial 13885

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Figure 2. Chromatographic response for the monohalogen aromatics series at 220 °C. Baseline correction was performed, and signals were rescaled by a factor 10−3 for fluorobenzene and 5 × 10−3 for chloro-, bromo-, and iodobenzene.

Figure 3. van't Hoff plots of the experimental Henry constants for tetrahydrofurane, dioxane, thiophene, pyridine, benzaldehyde, nitrobenzene, and methylbenzoate in the 200−300 °C range. least-squares regression was performed in R38 using a bootstrap-PLS methodology in the modeling phase.39 An initial set of molecular descriptors was taken from literature40 or calculated using Materials Studio41 to describe the diversity of the adsorbate set. Redundant descriptors were removed from the structural and physicochemical parameter set. A cut off point of r2 = 0.75 was used. The correlation matrix of the definitive parameter set can be found in Table 1. By applying “tree classification” with the “maximal difference between the adsorbate molecules” criterion (described by descriptor set) on the principal components, a classification in 12 classes was done. The number of principal components taken was equal to the number of descriptors minus one. From the total of 22 molecules (Table 2), a set of 10 training molecules was randomly determined from 10 classes; 2 classes of 1 molecule, being “mesitylene” and “methyl benzoate”, were directly placed into the external validation set. The remainder of the data set was used as an external validation set (prediction set). Molecules that make part of the training set are

marked with an asterisk (*) in Table 2. Napthalene was left out of the modeling set, as not all descriptors were available. Projection to latent structures or partial least-squares (PLS) regression was employed to establish absorbent specific predictive models for adsorption enthalpies and pre-exponential factors at low coverage. Direct modeling of the entropy had the benefit that these values require no logarithmic transformation and allow for simple error calculation without further transformation afterward. Models were trained with the earlier established training sets using a combined bootstrap-PLS methodology.39 The number of bootstraps was set to 500. Models were internally validated using goodness of fit criteria (RMSEP, Q2) on the out-of-bag data. Model coefficients were determined as the median values of the bootstrap coefficient distributions. The tolerance level (alpha, α) was varied between 0.01 and 0.25 in steps of 0.01 to find the optimal model. Redundant descriptors were iteratively removed (p < α). The final model’s coefficients with error estimates are obtained by taking the median for each description over the 500 bootstraps at 13886

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Figure 4. Relationship between mean molecular polarizability and adsorption enthalpy at zero coverage.

Figure 5. Experimental relationship between adsorption enthalpy and pre-exponential factor at zero coverage. Molecules used for the trendline are depicted by full symbols. optimized alpha and iteration. The predicted value for the adsorption property of each adsorbate is calculated by applying this final model. Error estimation is done by error progression from the coefficient’s standard errors. The obtained models are evaluated using the diagnostic tools: correlation (r2), average (|Δxav|), and maximal (|Δxmax|) deviation between experimental and calculated values.

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for alkyl aromatics or minor tailing when hetero atoms are present. The chromatographic response obtained after pulse injection for the monohalogen aromatic series is shown in Figure 2. The adsorbates elute sequentially in order of molecular weight, with no observable peak tailing. The peak first order moments correlate logarithmically with the polarizability of the molecules (Figure S2). The changing strength of dipole moment has no observable quantifying effect. Similarly, no peak tailing was observed for alkyl derivatives and only minor tailing for heterocyclic molecules. The van't Hoff plots of some heterocyclic molecules and hetero functionalized aromatics used for enthalpy and pre-exponential factor calculation are shown in Figure 3. Regression correlations of 0.999 and above were obtained.

RESULTS

Low Coverage Adsorption Properties. The adsorption enthalpies and pre-exponential factors at zero coverage of all studied components on MIL-47 can be found in Table 2. The temperature dependency of the Henry adsorption constants was in accordance with the van't Hoff plots (r2 = 0.997 and above). All recorded chromatograms show symmetrical peaks 13887

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Figure 6. Correlation plot of experimental (x-axis) and calculated (y-axis) adsorption enthalpies at zero coverage for training (hollow symbols), prediction (full symbols), and outliers (asterisk).

substituents in combination with a coordinated π-system for the current data set. The experimental values of mesitylene (ΔH0 = −62.7 kJ/ mol) and methyl benzoate (ΔH0 = −67.2 kJ/mol) clearly fall out of the general trend. Although no significant difference in adsorption enthalpy between mesitylene and propylbenzene is observed, mesitylene clearly lies off the general ΔH0−ln K0 trend (Figure 5). Both molecules have an identical chemical composition, but compared to propylbenzene the steric radius of mesitylene is significantly larger due to a different isomeric structure, resulting in a different overall “closeness” to the pore wall. The entropy factor of mesitylene is much larger and cannot be explained by loss of rotational freedom upon adsorption at the pore surface alone. This points toward a limited translational freedom of mesitylene through the structure. An estimate of the molecular size is given by de molecular shadow indices.46 In comparison to other alkyl aromatics, its size in the Lx·Ly plane is much larger (Figure S3). One dimension can be orientated along the pore axis, and thus, Ly is the dimension of importance. Given the diamond shaped pore cross section of 7.9 Å × 12.0 Å, the smallest dimension is 7.9 Å. Mesitylene, having a Ly value of 8.3 Å, is thus not able to freely rotate perpendicular to the pore axis. As a result, a difference in adsorption entropy of 8.45 J/[mol·K] between mesitylene and propylbenzene is obtained (Figure S3), which translates into in an adsorption selectivity, defined as the ratio of Henry constants, of 2.73 at 200 °C, in favor of propylbenzene. The lower pre-exponential value of methyl benzoate can be explained by a combination of factors. The presence of the ester function strengthens the interaction between the functional group and the metal−organic framework, thus lowering the adsorption enthalpy value. The ester function is much larger than any other substituent in the data set. Similar to mesitylene, steric hindrance in the pores plays a significant role. The size of methylbenzoate in the Lz dimension (Table S1) is approximately 2.6 Å larger than that of benzaldehyde and nitrobenzene, its structurally most similar adsorbates.

A closer look at the experimental data reveals several trends in adsorption properties (Table 2). With regard to the hetero functionalized adsorbates, adsorption properties seem to correlate with polarizability and/or electronegative strength of the substituent. An influence of the resulting dipole moment from substituent positioning can also be observed. These effects will be discussed in the following paragraphs. The experimental data correlates well with the mean molecular polarizability (Figure 4) for most components, making it a suitable descriptor in structure−property relationship modeling. The halogen series nicely falls in the alkyl aromatics series, indicating that the exhibited properties are likely the result of increased polarizabilty and not due to higher electronegative properties of the halogen atoms. The Ofunctional groups carbonyl, nitro, and methyl ester are somewhat off this trend. In all three cases, an extended πsystem is present thus allowing further delocalization of πelectrons from substituent and the aromatic ring. Due to the electronegative character, electrons will be inductively drawn from the benzene ring toward the substituent. The latter lowers the interaction potential of the benzene molecule. The electron rich substituent on the other hand will bind more strongly to the metal−organic framework than alkyl fragments. A clear example is pyridine, that is almost isostructural to benzene (6membered ring, flat, delocalized π-system): a difference of −3.5 kJ/mol in adsorption enthalpy is measured albeit a loss of one 1 hydrogen atom. In the study of adsorption properties of hydrocarbons, a linear relationship between adsorption entropy or logarithm of the pre-exponential factor and adsorption enthalpies is often observed, due to the increasing loss of freedom when interaction becomes larger.42−45 Figure 5 depicts this relationship between adsorption enthalpy (x-axis) and logarithmic preexponential factor (ln K0). A linear relationship can be observed, although the correlation for this data set as a whole is of lesser degree (r2 = 0.89) than for linear and branched alkanes (0.94).12 This can be attributed to the much larger diversity in physicochemical properties resulting from, for example, the presence of electronegative or highly polarizable 13888

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Figure 7. Correlation plot of logarithmic (left) and nontransformed (right) experimental (x-axis) and calculated (y-axis) pre-exponential factors for training set (hollow symbols), prediction set (full symbols), and outliers (asterisk).

Modeling. The modeling of the adsorption properties using partial least-squares regression (PLS) was performed in two steps. Separate models for adsorption enthalpy and preexponential factors at zero coverage were established. The initial descriptor set consists of vapor pressure, density, polarizability, dipole moment, heat of formation, and χ3 cluster. The modeling procedure was identical in both cases. An overview and discussion of the obtained results is presented in the following paragraphs. Table 2 contains the numerical experimental and modeling results. Modeling of adsorption enthalpies was achieved to a very high degree. The correlations between experimental and calculated values for training and prediction set are, respectively, 0.99 and 0.97. The resulting average (|Δxav|) and maximal (|Δxmax|) deviation between experimental and calculated values is 1.0 and 2.3 kJ/mol. The average standard deviation for the complete set is 2.1 kJ/mol. This value is larger than the determined average (1.2 kJ/mol) and equal to the maximum (2.1 kJ/mol) error of the experimental set. The descriptor set was further reduced by removal of nonsignificant descriptors on the basis of nonsignificantly different from zero within a confidence interval of 95%. The resulting descriptor set consists of mean polarizability and dipole moment. If one focuses onto the performance of the visually identified outliers mesitylene and methyl benzoate in the pair plot of experimental and modeled data (Figure 6), it should be noted that also these molecules are well represented by the model. The full line with slope 1 represents the points having equal values on both the x- and y-axis. An almost perfect prediction is obtained for mesitylene, while methyl benzoate still lies somewhat of the general trend. The latter is not unexpected, as the descriptor “mean polarizability” describes the nature of methyl benzoate to a lesser degree. The third descriptor “dipole moment” compensates partially for this. The modeling of the pre-exponential factors was achieved to a lesser degree (Figure 7). The non-normal distributed experimental values require preprocessing as partial leastsquares regression inherently assumes normal distribution of the data. The data was transformed to a logarithmic scale to satisfy this underlying assumption. This transformation leads to a loss in accuracy when recalculating the original values. A prediction accuracy of within 3% of the original value becomes magnified to 1−47% error after transformation at lower or

upper extremities of the modeled data. This effect is visually shown in Figure 7. Therefore, only logarithmic values will be compared and used for model quality assessment. In the left graph, logarithmic values are shown whereas, the right graph depicts the recalculated values. The previously magnifying or minimizing effect in course of the recalculations is very well illustrated by mesitylene and benzyl benzoate, indicated by asterisk symbols in Figure 7. As above, the descriptor set was further reduced to mean polarizability and dipole using the same elimination scheme. The combination of both descriptors allows one to describe the basic features of the molecules in this set. Correlations between experimental and calculated values for training and prediction set are, respectively, 0.88 and 0.98. The resulting average (|Δxav|) and maximal (|Δxmax|) deviation between experimental and calculated values equal 0.6 and 0.6 in the logarithmic scale. As in the case of adsorption enthalpy, the properties of mesitylene are approximated to a higher degree than those of methyl benzoate, although both components on average have significantly larger differences between experimental and calculated values.

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CONCLUSION The adsorption of a diverse set of functionalities and molecular sizes in the C5−C9 range was studied experimentally. Remarkably, the largest fraction of the obtained data obeys the same compensation effect between adsorption enthalpy and entropy. The experimental data was analyzed and modeled using principal component analysis and partial least-squares regression. The modeling of these adsorptive properties could be done on the basis of a small number of data points, yet within the accuracy of experimental uncertainties. Structure−property relationship revealed several trends. Among the molecular properties governing the adsorption process, mean polarizabilitiy, and dipole moment play a determining role in the case of hetero cyclic molecules and nonalkyl functionalized benzene rings.

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures. This material is available free of charge via the Internet at http://pubs.acs.org. 13889

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J. Denayer and D. De Vos are grateful to FWO Vlaanderen for financial support (G.0453.09 N).

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