Adsorption of C6 hydrocarbon rings on mesoporous catalyst supports

Chemical Physics Letters 482 (2009) 296–301

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Adsorption of C6 hydrocarbon rings on mesoporous catalyst supports Róbert Rémiás, András Sápi, Róbert Puskás, Ákos Kukovecz *, Zoltán Kónya, Imre Kiricsi Department of Applied and Environmental Chemistry, University of Szeged, H-6720 Szeged, Rerrich Bela ter 1, Hungary

a r t i c l e

i n f o

Article history: Received 3 September 2009 In final form 7 October 2009 Available online 12 October 2009

a b s t r a c t The adsorption of cyclohexane, cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene and benzene was studied on pristine and shortened multi-wall carbon nanotubes, SBA-15 and a novel high surface area mesoporous carbon (CMH). Data were fitted with the Freundlich adsorption equation and the correlation between the fitted parameters and quantitative structure–activity relationships (QSAR) descriptors of the adsorbates was analyzed. Adsorption on carbon nanotubes is more sensitive to the partial pressure of unsaturated adsorbates, whereas SBA-15 is more sensitive to saturated partners. CMH is a neutral material that appears to be particularly useful for studying catalyst particle efficiency without the influence of the support itself. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Considerable effort is devoted today to the research of noble metal nanoparticles for heterogeneous catalytic applications. Mono- and bimetallic [1,2] nanoparticles of platinum, palladium, ruthenium and gold were shown to be active in various hydrogenation reactions [3]. The main motivation behind these studies is the possibility of controlling the structure of the active sites down to the atomic level which in turn is expected to promote rational catalyst design [4,5]. Metallic nanoparticles are seldom used as standalone catalysts, rather, they are usually mounted on suitable support materials [6]. The pore structure of the support, the nature of the metal-support interaction, catalyst particle morphology and dispersion, etc. are all frequent subjects of high quality research reports. In comparison, the number of reports dealing with the adsorption of the reactants and products on the support surface is considerably smaller. However, a heterogeneous catalytic reaction is a complex process. Even the simple hydrogenation of cyclohexene to cyclohexane consists of several steps as described by the Langmuir–Hinshelwood mechanism (Fig. 1) [7]. Since the available surface area of the support is many orders of magnitude larger than that of the catalytically active particles, reactant molecules are adsorbed on the support (step 1) and reach the hydrogenation center by surface migration (step 2). After the reaction the product migrates away from the active center (step 5) and desorbs from the support (step 6). Therefore, the affinity of the support to adsorb reactants and products directly influences the speed of the hydrogenation reaction. Moreover, in a macroscopic hydrogenation/dehydrogenation catalyst bed there is * Corresponding author. E-mail address: [email protected] (Á. Kukovecz). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.10.016

always a chance for the primary products to re-adsorb again and participate in secondary reactions which of course affects the performance (e.g. selectivity) of the catalyst system. Quantitative structure–activity relationships (QSAR) have been gaining in popularity in the last decades because they provide the basis for the rational planning of molecular interactions [8]. Even though the most important field of QSAR application is pharmaceutical drug design [9], other areas like catalysis [10] and analytical chemistry [11] can also benefit from this technique. In this contribution, we investigate the correlation between QSAR descriptors of C6 hydrocarbon rings as model compounds and their adsorption properties on multi-wall carbon nanotubes, SBA-15 and a novel high surface area mesoporous carbon material (CMH) [12]. These adsorbents were chosen because they are in the focus of contemporary catalysis research as supports in e.g. fuel cell applications [13], CO oxidation [14] carbon–carbon coupling [15] and hydrogenation reactions [16,17]. 2. Experimental 2.1. Adsorbent preparation The studied adsorbents were prepared in our laboratory using well-established and published methods. Multi-wall carbon nanotubes (MWCNTs) were synthesized by catalytic chemical vapor deposition from a C2H4/N2 mixture over a (Fe, Co)/MgO catalyst at 923 K and purified using oxidative and acidic washing steps [18]. This method is estimated to give a MWCNT sample over 95% purity [19]. Some MWCNTs were shortened by milling for 10 h in a vibrating stainless steel ball mill. We have shown earlier that 10 h of this low energy milling does not increase the amount of amorphous carbon in the sample significantly [20]. A new type

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SBA-15 was prepared using a slightly modified version of the original Stucky method [21]. Two grams P-10400 copolymer was dissolved in 63 ml distilled water and 10 ml cc. HCl solution. Tetraethoxy-silane (TEOS) (3.1 g) was added to this solution and stirred for 24 h at room temperature, followed by heat treatment at 353 K for 24 h in a Teflon lined stainless steel autoclave. The resulting white material was heated to 773 K at a rate of 5 K min1 in N2 flow, then the gas flow was changed to O2 for 2 h. Finally, the material was allowed to cool down to room temperature in N2 flow. 2.2. Adsorbent characterization

Fig. 1. Schematic illustration of cyclohexene hydrogenation over a carbon nanotube supported noble metal nanoparticle. Step 1: cyclohexene adsorption on the nanotube, step 2: reactant migration to catalyst nanoparticle, step 3: H2 chemisorption on the catalyst, step 4: hydrogenation reaction, step 5: product migration away from the catalyst, step 6: desorption of the product cyclohexane. The rate of steps 1, 2, 5 and 6 is influenced by the hydrocarbon-support interaction.

of mesoporous carbon (CMH) developed by our group was chosen as the third carbonaceous adsorbent to be tested. CMH was prepared by the controlled pyrolysis of acetylene over LudoxÒ SM30 colloidal silica template particles at 1170 K [12].

The specific surface area of the samples was determined from N2 adsorption isotherms recorded at 77 K on a Quantachrome NOVA 2200 instrument. Prior to the adsorption measurement the samples were degassed at 473 K in vacuum for 2 h to remove adsorbed contaminants. XRD profiles were recorded on a Rigaku Miniflex II instrument using Cu Ka radiation. A Hitachi S-4700 and a Philips CM10 microscope were used for scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. Samples for TEM were drop-coated onto carbon mounted holey carbon grids and dried under ambient conditions before measurement. Nanotube dimensions were determined by taking at least 100 independent measurements on the TEM images. 2.3. Adsorption measurements Adsorption isotherms were recorded in a manually operated volumetric apparatus utilizing a capacitive pressure sensor. Fifty

Fig. 2. Characteristic TEM images of the mesoporous supports discussed in this work. (A) Shortened MWCNT, (B) untreated MWCNT, (C) amorphous mesoporous carbon CMH and (D) SBA-15.

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milligrams sample was measured into a glass sample holder, degassed and decontaminated in vacuum at 773 K for 1 h, then the dead volume was determined by He expansion and the adsorption isotherm was measured at 273 K. The equilibrium amount of hydrocarbon adsorbed was measured against the pressure of the hydrocarbon (pHC) relative to its saturated vapor pressure. In order to calculate the prel = pHC  p1 0 relative pressure of the hydrocarbon vapors from the absolute pressure we estimated their saturated vapor pressure (p0) at 273 K using the Antoine equation [22].

2.4. Theoretical calculations The geometry of the hydrocarbons was optimized using the HyperDFT density functional with the 6-31G** orbital basis set as built into the HyperChem 7.0 software package. The SCF convergence limit was set to 1  105, integration was done using the standard Pople grid #1, geometry optimization was performed using the conjugate gradient method. Quantitative structure– activity relationship descriptors were calculated for the optimal

Fig. 3. Adsorption isotherms of the discussed systems measured at 273 K. The abscissa gives the pressure of the hydrocarbon relative to the pressure of the saturated hydrocarbon vapor at 273 K.

Table 1 Freundlich isotherm parameters derived by fitting the measured adsorption isotherms with Eq. (1) in the 0 < prel < 0.3 regime. Adsorbent

ABET (m2 g1)

Adsorbate

Short MWCNT

146

Cyclohexane Cyclohexene 1,4-Cyclohexadiene 1,3-Cyclohexadiene Benzene

1.20 ± 0.05 1.25 ± 0.09 1.48 ± 0.09 1.46 ± 0.04 1.70 ± 0.08

0.309 ± 0.022 0.347 ± 0.060 0.351 ± 0.023 0.356 ± 0.016 0.391 ± 0.025

MWCNT

193

Cyclohexane Cyclohexene 1,4-Cyclohexadiene 1,3-Cyclohexadiene Benzene

1.61 ± 0.15 1.68 ± 0.16 1.93 ± 0.15 2.40 ± 0.16 2.41 ± 0.14

0.332 ± 0.051 0.385 ± 0.076 0.423 ± 0.056 0.461 ± 0.057 0.474 ± 0.050

CMH

1630

Cyclohexane Cyclohexene 1,4-Cyclohexadiene 1,3-Cyclohexadiene Benzene

11.12 ± 0.23 11.47 ± 0.33 12.29 ± 0.83 11.70 ± 0.22 15.55 ± 0.48

0.305 ± 0.009 0.310 ± 0.012 0.375 ± 0.017 0.3391 ± 0.007 0.456 ± 0.015

SBA-15

860

Cyclohexane Cyclohexene 1,4-Cyclohexadiene 1,3-Cyclohexadiene Benzene

5.13 ± 0.19 5.28 ± 0.16 5.86 ± 0.26 5.60 ± 0.40 6.71 ± 0.46

0.421 ± 0.019 0.424 ± 0.014 0.311 ± 0.014 0.295 ± 0.008 0.289 ± 0.026

Kf

n

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geometries by the same software. Statistical calculations were done in Minitab 14.

3. Results and discussion The morphology and crystal structure of the adsorbents was checked by SEM and XRD (not shown here) and TEM (Fig. 2). The mean diameter of the multi-wall nanotubes was 14.7 nm and their mean length was >2 lm and 320 nm for the original and the shortened tubes, respectively. Although the nanotubes were not purposefully opened by e.g. oxidation [23,24], they can be considered as mesoporous supports because (i) their mean inner channel diameter (3.2 nm) falls into the mesoporous range, and (ii) it is well known that a random MWCNT assembly exhibits mesoporous behavior due to the size of the intertube openings [25]. The CMH mesoporous carbon (Fig. 2C) possessed an amorphous structure with small spherical features determined by the LudoxÒ SM30 template particles and a mean pore diameter of 9.8 nm [12]. The channel structure of the SBA-15 was regular with a characteristic diameter of 6.6 nm (Fig. 2D). The specific surface area as determined from N2 adsorption isotherms (not shown) of shortened MWCNTs, pristine MWCNTs, SBA-15 and CMH carbon was 146, 193, 860 and 1630 m2 g1, respectively. The specific surface area difference between pristine and shortened MWCNTs is due to the milling induced flattening of tube ends and reduced nanotube entanglement as discussed in detail previously [20]. Summarizing, the studied adsorbents can be considered as typical examples of their species and therefore, we may anticipate that hydrocarbon adsorption characteristics measured on them will provide general insight into the behavior of such materials as catalyst supports. In Fig. 3, we present the adsorption isotherms of cyclohexane, cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene and benzene over the studied mesoporous catalyst supports. Let us first observe that all five molecules feature the same isotherm type on the same adsorbent, indicating that the fundamental mechanism of the adsorption does not depend on the saturation level of the C6 ring even though these molecules are known to adsorb in different positions even on a simple Si(1 0 0)-2  1 surface [26]. The isotherms measured over carbonaceous materials can be classified as Type II (strong adsorbate–adsorbent interaction on meso- or macroporous adsorbent), whereas those recorded on SBA-15 classify as Type IV according to the IUPAC recommendation [27]. Adsorption capacities extrapolated to prel = 0 are reported in the Supporting information. All isotherms diverge to infinity as prel approaches 1 because of the condensation of the hydrocarbon vapor [28]. In a typical gas–solid heterogeneous catalytic reaction reactants and products are heavily diluted by other reactants/products and/ or inert gases. Therefore, only the lower section of the adsorption isotherm will be considered now. In the 0 < prel < 0.3 region both Type II and Type IV isotherms can be fitted adequately using the empirical Freundlich isotherm (Eq. (1)):

Q ¼ K f  pnrel

ð1Þ

Here Q (mol g1) denotes the amount of hydrocarbon adsorbed, Kf (mol g1) is a proportionality constant, prel is the relative pressure of the hydrocarbon vapor and n is the exponent of the adsorption. The Freundlich isotherm is widely used to describe the adsorption of volatile organic carbons on carbon adsorbents [29] and has been applied with success to the adsorption of light hydrocarbons on SBA-15 as well [30]. Fitted Freundlich parameters are summarized in Table 1. The Kf constant is determined by the amount of adsorption sites available on the adsorbent and therefore, it is roughly proportional to the specific surface area of the samples. A secondary trend also exists for each adsorbent: the lower the saturation of the hydrocarbon ring, the larger Kf is. This is an intriguing finding as it suggests that the number of active sites could depend on the adsorbate molecule itself. The n parameter of the Freundlich isotherm is correlated with the interaction between the adsorbate and the adsorbent: it measures the sensitivity of the adsorption to a minor increase in the prel pressure of the adsorbate. The larger n is, the larger the relative change in the Q amount adsorbed will be for the same perturbation of the adsorption process by prel variations. Data in Table 1 indicate that on carbonaceous supports, n increases with the increasing aromaticity of the C6 ring. This is in agreement with the known affinity of carbonaceous materials towards planar and aromatic adsorbates [31]. A plausible explanation for this phenomenon could be the increased stacking ability of the less saturated molecules. On the other hand, there are also strong arguments against this hypothesis: (i) Freundlich isotherms were fitted only to data in the prel < 0.3 section, whereas stacking interactions are more pronounced at higher hydrocarbon loadings, and (ii) on SBA-15 the trend is actually reversed. Here the largest effect appears to be between the saturated cyclohexane and the support, with n values decreasing progressively as aromaticity increases. Therefore, the simple stacking interaction is not likely to be the real force determining the behavior of the Freundlich exponent. Table 2 summarizes the most important QSAR parameters of the studied C6 hydrocarbon rings as calculated from their 631G** density functional optimized geometries. Four of these parameters (surface, volume, polarizability and mass) are size related while the other three (HOMO, LUMO, gap) are determined by the electronic structure. Distance based cluster analysis was performed on Table 2 data (see Supporting information) and revealed that the two most similar molecules are the two dienes. Cyclohexene and cyclohexane are closely related to each other and to the dienes, and the whole non-aromatic group is significantly different from benzene. Thus, the calculated QSAR parameters appear to be suitable descriptors of the studied hydrocarbons. This hypothesis was also confirmed by extrapolating the monolayer adsorption section of the adsorption isotherms to prel = 0 and fitting the extrapolated adsorption capacity results to the QSAR descriptors by Partial Least Squares (PLS) regression (see Supporting information for quantitative data). The following conclusions could be made: (i) two factorized QSAR parameters are enough to roughly describe the adsorption phenomenon in the studied systems, since the measured and the PLS-fitted adsorption

Table 2 QSAR parameters of the studied adsorbates calculated using the 6-31G** basis set.

Cyclohexane Cyclohexene 1,4-Cyclohexadiene 1,3-Cyclohexadiene Benzene

Surface (Å2)

Volume (Å3)

Polarizability (Å3)

Mass (amu)

HOMO (eV)

LUMO (eV)

Gap (eV)

360.52 345.84 333.28 331.67 306.83

545.56 514.46 480.87 481.11 437.62

11.01 10.82 10.63 10.63 10.43

84.16 82.15 80.13 80.13 78.11

1.846 2.734 2.781 3.119 2.648

6.149 5.858 5.811 5.232 5.602

4.303 3.124 3.030 2.113 2.954

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Table 3 Pearson correlation coefficients (Pr) between the fitted Freundlich isotherm parameters and the calculated QSAR molecule descriptors. QSAR factor

Short MWCNT

MWCNT

Kf

n

Kf

n

Kf

n

Kf

n

Surface Volume Polarizability Mass HOMO LUMO Gap

0.98 0.98 0.97 0.97 0.50 0.60 0.56

0.98 0.97 0.96 0.96 0.65 0.65 0.67

0.86 0.87 0.87 0.87 0.64 0.89 0.77

0.93 0.95 0.96 0.96 0.79 0.87 0.86

0.90 0.86 0.82 0.82 0.18 0.29 0.24

0.93 0.92 0.89 0.88 0.27 0.36 0.32

0.96 0.95 0.92 0.92 0.34 0.42 0.39

0.84 0.88 0.90 0.90 0.63 0.76 0.71

capacities were in good qualitative agreement for each adsorbent, but (ii) two factorized QSAR parameters are not enough to take the fine details of the adsorption process into account. The PLS fit was not able to reproduce the adsorption capacity differences between the hydrocarbons on the same adsorbent. Therefore, all seven QSAR parameters should be used instead of their factorized versions regardless of their obvious correlation with each other. The Pearson correlation coefficient (Pr) is an established indicator of the strength of the linear correlation between datasets. The nearer |Pr| is to one, the greater the probability that a linear relationship exists between the datasets. In order to uncover the links between adsorbate properties and Freundlich isotherm parameters the Pearson correlation coefficient was calculated and is reported in Table 3. The strongest observable trend is the negative correlation between the size of the adsorbent and the Freundlich proportionality constant Kf. On this basis it is possible to suggest a physically sound explanation for the apparent adsorbate dependence of Kf. Regardless of the adsorbent type, saturated C6 rings are larger and it takes less of them to cover the adsorption sites active at a given relative pressure and therefore, Kf decreases with increasing adsorbate size. A more detailed analysis of Table 3 reveals that the studied adsorbents can be classified into three groups on the basis of the Freundlich exponent n. Both carbon nanotube samples exhibit a similar behavior characterized by negative correlations between the Freundlich parameters and all QSAR descriptors except the HOMO for which a medium strength positive correlation exists. The reason behind the anomalous behavior of HOMO correlations is not clear at the moment; it is probably a spurious effect related to the theoretical approximation used for the calculations. The negative correlation between the Freundlich exponent n and electronic descriptors LUMO and gap can be interpreted as a clear preference of carbon nanotubes towards more aromatic adsorbates. This effect has been observed earlier by Crespo et al. for benzene and cyclohexene on single-wall carbon nanotubes (SWCNTs) [32] and exploited by Marquis et al. in the chiral separation of SWCNTs [33]. CMH isotherms are dominated by size related effects; the correlation between adsorbate electronic structure and isotherm parameters is weak. This means that CMH is a ‘neutral’ adsorbent that does not differentiate between adsorbates on the basis of their saturation level. On the other hand, on SBA-15 the positive correlation between the Freundlich exponent n and the QSAR parameters (except HOMO) indicates that SBA-15 – adsorbate interaction is more sensitive to pressure changes for (i) larger and (ii) more saturated molecules. This interesting phenomenon is dissimilar to the behavior of carbon nanotubes or CMH. The absolute amount of hydrocarbon adsorbed on SBA-15 is significantly larger for e.g. aromatics than for saturated molecules (Fig. 3 and Supporting information) which agrees with literature wisdom well [30,34,35], thus SBA-15 adsorbs more aromatic C6 rings than saturated ones at the same prel pressure. However, increasing or decreasing this prel value will result in a larger relative change in the amount adsorbed if the adsorbate is saturated.

CMH

SBA-15

It appears possible to exploit this feature to the fine-tuning of SBA-15 based catalytic hydrogenation process parameters. 4. Conclusion We compared the adsorption of C6 hydrocarbon rings on four different mesoporous catalyst support materials in order to facilitate the rational design of supported hydrogenation/dehydrogenation catalysts. Pearson correlation analysis performed on QSAR adsorbate descriptors and experimentally derived Freundlich adsorption isotherm parameters revealed that the adsorbentadsorbate interaction is more sensitive to pressure changes for aromatic molecules on carbon nanotubes and for saturated molecules on SBA-15. This effect makes rational process variable planning possible and should be taken into account when optimizing a catalyst system for a certain task provided that all other major effects, e.g. metal-support interaction, temperature, catalyst particle morphology, etc. are under control. For example, a MWCNT supported catalyst could be useful when hydrogenating a minor aromatic target component in a more saturated feed. On the other hand, SBA15 based systems could be preferred for the selective hydrogenation of a mono- or diene in a feed containing mostly aromatics. The novel high surface area mesoporous carbon CMH exhibits a neutral behavior and could be useful for studying the effects of e.g. catalytic metal nanoparticle variations without the distortions introduced by preferential reactant or product adsorption sensitivity on the support. Acknowledgements The financial support of the Hungarian Scientific Research Fund (OTKA) through projects NNF-78920 and 73676 is acknowledged. The authors thank Zoltán Fodor for contributing to the experimental work and two anonymous reviewers for their helpful remarks that have helped in improving the manuscript significantly. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2009.10.016. References [1] M.V. Seregina et al., Chem. Mater. 9 (1997) 923. [2] J.M. Thomas, B.F.G. Johnson, R. Raja, G. Sankar, P.A. Midgley, Acc. Chem. Res. 36 (2003) 20. [3] H.U. Blaser, C. Malan, B. Pugin, F. Spindler, H. Steiner, M. Studer, Adv. Synth. Catal. 345 (2003) 103. [4] G.A. Somorjai, J.Y. Park, Surf. Sci. 603 (2009) 1293. [5] H. Tada, T. Kiyonaga, S. Naya, Chem. Soc. Rev. 38 (2009) 1849. [6] R.J. White, R. Luque, V.L. Budarin, J.H. Clark, D.J. Macquarrie, Chem. Soc. Rev. 38 (2009) 481. [7] G. Rothenberg, Catalysis: Concepts and Green Applications, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2008. [8] M. Karelson, V.S. Lobanov, A.R. Katritzky, Chem. Rev. 96 (1996) 1027. [9] R. Perkins, H. Fang, W.D. Tong, W.J. Welsh, Environ. Toxicol. Chem. 22 (2003) 1666.

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