Adsorption of Small Alkanes on ZSM-5 Zeolites: Influence of Brønsted

Article pubs.acs.org/JPCC

Adsorption of Small Alkanes on ZSM‑5 Zeolites: Influence of Brønsted Sites Yu-Hao Yeh,† Raymond J. Gorte,*,† Srinivas Rangarajan,‡ and Manos Mavrikakis‡ †

Department of Chemical & Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States Department of Chemical and Biological Engineering, University of WisconsinMadison, Madison, Wisconsin 53706, United States

‡

S Supporting Information *

ABSTRACT: The adsorption of a series of small alkanes was studied experimentally on H-ZSM-5 zeolites using calorimetric measurements in order to determine their interactions with the Brønsted sites. Differential heats measured on four ZSM-5 samples with different Si/Al2 ratio and with different defect concentrations were found to depend strongly on the Brønsted-site density but not on the presence of defects. The interactions for CH4 with the Brønsted sites were minimal but the effect was significant (up to 11 ± 2 kJ/mol extra heats) for larger alkanes, such as n-C6H14. The affinity of the alkanes with the Brønsted sites increased with the gas-phase proton affinity of the alkanes and the calculated affinity of the alkanes for the strong acid, fluorosulfonic acid. The extra heats of adsorption in H-ZSM-5 over its siliceous counterparts can therefore be associated with the strength of hydrogen bonding between the adsorbed alkane and the Brønsted sites, which in turn increases with molecular size. Specifically, extra heats were found to vary linearly with acid affinity corrected for dispersion interactions. The comparison of the experimental and computational results, therefore, indicates that the hydrogen bonded interaction theory describes the effect of Brønsted sites for alkane adsorption on zeolites.

1. INTRODUCTION The hydrogen forms of zeolites are of great commercial importance as solid, Brønsted-acid catalysts. Although acidic zeolites catalyze many of the same reactions as homogeneous acids, there are important differences, in large part due to different solvation effects, so that there is a continuing need to characterize and model how molecules interact with the acid sites. The high-silica zeolite, H-ZSM-5 is one of the most widely studied materials, both because of its commercial importance and because of its relative simplicity. Rates for a number of important reactions increase linearly with the Al content of this zeolite.1,2 Furthermore, adsorption studies have demonstrated the formation of stoichiometric adsorption complexes, one molecule per Al site, for alcohols,3 amines,4 nitriles,5 thiols,6 and other classes of molecules.7−9 The reactivities of these adsorption complexes in TemperatureProgrammed Desorption (TPD) do not depend on the Si/Al2 ratio of the sample.10 This, together with the fact that differential heats of adsorption for simple bases like NH3 and pyridine are constant with coverage up to one molecule per site,11 suggests that the Brønsted sites in H-ZSM-5 are essentially equivalent. For bases with proton affinities higher than that of ammonia, heats of formation of the stoichiometric adsorption complexes have been shown to scale with the gas-phase proton affinities;12 however, for weaker bases, the situation is less clear.5 An © XXXX American Chemical Society

important example is that of simple alkanes wherein the adsorption thermochemistry depends on the dispersive interactions with the zeolite wall and the noncovalent interactions with the acid site. In our previous study,13 we showed that interactions of CH4 with the Brønsted sites in HZSM-5 do not contribute significantly to the heats of adsorption; differential heats were found to be independent of coverage or zeolite Si/Al2 ratio. However, there are both experimental14,15 and theoretical16,17 reports that the contributions to the heats of adsorption from interactions with Brønsted sites are significant for larger alkanes. For example, Eder and Lercher14 reported that differential heats for n-hexane to noctane are 10 kJ/mol higher on H-ZSM-5 than on a siliceous ZSM-5. Arik et al. measured the temperature dependences for Henry’s Law constants and also reported that heats of adsorption for normal alkanes increased with Al site density.15 There are several factors which make it difficult to determine how strongly alkanes interact with the Brønsted sites in HZSM-5. First, at realistic measurement temperatures, the alkane/Brønsted-site interaction is relatively small compared to kBT, where kB is the Boltzmann’s constant. On the basis of thermodynamic arguments, this implies that a significant Received: April 15, 2016 Revised: May 20, 2016

A

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volumes were determined from gravimetric uptakes of n-hexane at room temperature and 10 Torr, assuming the pore volume becomes filled with liquid-like n-hexane. Key properties of these four samples are given in Table 1. 2.2. Calorimetry. The Tian-Calvet calorimeter was homebuilt and has been described in detail elsewhere.20 The instrument was constructed from five, 2.54 cm square, thermal-flux meters (International Thermal Instrument Company, Del Mar, CA) placed between a cubic, pyrex sample cell and a large Al block. The thermopiles had previously been calibrated by passing current through a Pt wire placed between the sample cell and the thermopiles. During the measurement, 1-g zeolite samples, pressed into wafers and placed at the bottom of the pyrex cube, were covered with quartz chips in order to prevent heats losses due to radiation out the top. The samples were evacuated using a mechanical pump and heated to 573 K overnight in vacuum to remove water. The samples were then exposed to calibrated volumes of gas from a GC sample loop. A series of small alkanes were used as adsorbates in this study, includes CH4, C2H6, C3H8, n-C5H12, iso-C5H12, and n-C6H14. The data from calorimetric measurements are most reliable when the measurements are performed at the lowest temperatures for which adsorption is reversible.18 When there is a distribution of site strengths, low temperatures favor having molecules adsorb preferentially in the strongest sites. This is important when the difference between strong and weak sites approaches kBT. However, adsorption reversibility is critical in order to avoid chromatographic adsorption in the sample and to ensure that adsorption comes to equilibrium. Therefore, different temperatures were used for the different alkanes in this study. The reversibility of the measurements was verified by comparing isotherms obtained by adsorption and desorption of the sample at a certain temperature. Experiments with CH4 were performed at 195 K by placing the Al-block, heat sink of the calorimeter in a styrofoam container with dry ice. Adsorption of C2H6 and C3H8 were performed at room temperature; for n-C5H12, iso-C5H12, and n-C6H14, adsorption measurements were performed between 353 and 373 K. 2.3. Calculations. The binding of alkanes to a gas phase Brønsted acid (and other cationic proton donors) can be viewed as a hydrogen-bonded interaction;21−23 analogously, the additional heats of adsorption of alkanes on H-ZSM-5 can be related to the strength of hydrogen bonding between the alkane and the Brønsted proton of the zeolite. In the complete absence of a solvent, the strength of the hydrogen bond is the proton affinity, but this interaction is moderated in the condensed phase by solvent interactions. Further, while proton affinity intrinsically captures the stability of a three-center-two-electron complex that a proton can form with two neighboring carbon atoms of the alkane and the included bond,24,25 the interactions of an alkane with a Brønsted proton does not lead to such a strong covalently bonded complex.16,17,26−28 Therefore, we explored an alternative descriptor of alkane-Brønsted site interaction by calculating acid affinities for the various alkanes, Eacid−affinity, using a gas-phase superacid, fluorosulfonic acid (FSO3H). The acid affinity is the energy of complexation for an acid-alkane complex, as given in eq 1.

fraction of adsorbed molecules will not be present at the Brønsted sites at any given coverage.18 Measured differential heats will be an averaged value between that of molecules adsorbed at the acid sites and molecules not at acid sites. Second, theoretical determination of the interactions between alkanes and Brønsted sites is difficult because the standard DFT methods may not accurately capture hydrogen-bonding effects with alkanes. For example, in our previous studies of CH4 adsorption, calculations performed using various functionals indicated that CH4 should bind to the Brønsted sites with an additional 1 to 7 kJ/mol,13 but this was not observed experimentally. Finally, most real zeolites have significant concentrations of vacancy defects, observable as nested silanols; and these have a profound effect on the adsorption of polar molecules, like water.5 Although vacancy sites do not appear to affect CH4 adsorption,13 CH4 adsorption was also not affected by the presence of Brønsted sites. In the present study, we set out to examine the interaction between simple alkanes and the Brønsted sites in H-ZSM-5 calorimetrically and theoretically, as a function of alkane size. In the calorimetric measurements, the additional interactions with the Brønsted sites were extracted by measuring the differential heats on materials with varying Si/Al2 ratio; and the possible effects of defect sites were determined by comparison with results on a defect-free ZSM-5 synthesized in a fluoride medium. To understand the effect of alkane size on the additional interactions with the Brønsted sites from a theoretical side, gas-phase acid-affinities were calculated for the strong acid, fluorosulfonic acid (FSO3H), as a possible descriptor. Both experimental and theoretical considerations indicate that the size of the alkane can significantly affect interactions with the Brønsted sites.

2. EXPERIMENTAL SECTION 2.1. Materials. Four different H-ZSM-5 samples with widely varying Brønsted-site concentrations were used in this study. The commercial ZSM-5 samples (CBV 5524G, Si/Al2 ratio of 50; CBV 28014, Si/Al2 ratio of 280) were obtained in their ammonium-ion form from Zeolyst and were converted into the acidic, H-ZSM-5 form by calcination in flowing air at 773 K for 4 h. The samples are referred to here as H-ZSM5(50) and H-ZSM-5(280). Two low-defect samples with different Si/Al2 ratios were prepared by using fluoridecontaining media at the University of Delaware. The synthesis and characteristics of these samples (ZSM-5(F,Si) and H-ZSM5(F,Al) have been described in detail in a previous publication.13The Si/Al2 ratio for these two samples were determined by ICP-OES and listed in Table 1. Brønsted-site concentrations were determined using simultaneous Temperature-Programmed-Desorption (TPD) and Thermogravimetric-Analysis (TGA) measurements of 2-propanamine.10,19 Pore Table 1. Zeolite Samples Used in This Study zeolite

Si/Al2

Brønsted-acid site densities (μmol/g)b

pore volume with n-hexane (cm3/g)

ZSM-5(F,Si) H-ZSM-5(F,Al) H-ZSM-5(50) H-ZSM-5(280)

>1100a 75a 50 280

10 260 470 90

0.17 0.17 0.19 0.19

Eacid − affinity = Ealkane − acid − Eacid − Ealkane

(1)

Here, Ealkane−acid is the energy of the hydrogen-bonded, alkaneacid complex, Eacid is the energy of the acid in the gas phase, and Ealkane is the energy of the alkane in the gas phase. Because

a

Determined by ICP-OES analysis. bMeasured by TPD-TGA of 2propanamine. B

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The Journal of Physical Chemistry C the acid affinity of an alkane is expected to depend on the strength of the acid, the optimal choice of acid to explore hydrogen bonding would be one with a deprotonation energy (DPE) equal to that of H-ZSM-5 (1200 kJ/mol as computed by Sauer et al.).29 It should be noted that the particular choice of acid is, in principle, not critical as any acid could be used as reference if (i) it is not so weak that the corresponding acid affinity of most alkanes is negligibly small (200 kJ/mol). This is because, while proton affinity captures strong covalent interactions (as discussed in Calculations), the extent of interactions (and electron transfer) for alkanes with Brønsted sites are significantly weaker. The extent of the electron transfer (or the extent of overlap of the occupied orbitals of the alkane and the unoccupied orbitals of the acid, from classical molecular orbital theory) can also depend on the intrinsic electronic properties of the alkane and the acid, such as the energy of the lowest unoccupied molecular orbital of the acid or the highest occupied molecular orbital of the alkane. To explore this further, we tried comparing the energy of the highest occupied molecular D

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estimated by calculating the energy of the hydrogen-bonded complex and that with the OH bond of the fluorosulfonic acid rotated so that the hydrogen atom points away from the alkane, thereby breaking the hydrogen bond. The coordinates of the alkane and the rest of the atoms of the acid are fixed (at the positions of the hydrogen-bonded complex) while calculating the latter energy so that the dispersion contributions can be subtracted from the energy of the hydrogen-bonded complex. It should be noted that the final structure of the acid is the mirror image of Figure 4(i). Mathematically, ΔEacid − affinity = Ealkane − acid − with − HB − Ealkane − acid − without − HB (2)

where Ealkane−acid−with−HB is the energy of the hydrogen bonded complex and Ealkane−acid−without−HB is the energy of the complex where the hydrogen bond was broken. Figure 5 shows a clear

Figure 3. Extra differential heats of adsorption between ZSM-5(F,Si) and H-ZSM-5(F,Al), from Table 2, plotted as a function of the proton affinity of the various alkanes examined.

orbital (EHOMO) of the alkane with the experimentally measured additional heats of adsorption. Figure S1 clearly shows a monotonic variation across alkanes; EHOMO of methane is significantly lower than that of other alkanes (1−2 eV) and this can explain the negligible additional heat observed experimentally. As in the case of proton affinity (Figure 3), however, the variation in EHOMO is much larger than that of the extra heats. To overcome the limitations of these two descriptors, we considered acid affinity, as described in eq 1, as an alternative descriptor. The individual acid−alkane complexes are shown in Figure 4. All energies and structures were calculated using the

Figure 5. A comparison of experimental extra differential heat between H-form and siliceous ZSM-5 and computed extra acid affinity (−ΔEacid−affinity). Experimental and computational errors are included for each data point; the latter errors are based on reported mean absolute error in the prediction of noncovalent interactions (∼1.8 kJ/ mol)41 using ωB97X-D functional with the 6-311++G(3df,3dp) basis set and are shown only to provide a qualitative guidance on the magnitude of the error. The trend line (dashed red line) fit is given by y = 2.1217x − 25.01 kJ/mol with an R2 > 0.97.

correlation between the extra heat and extra acid affinity (−ΔEacid−affinity). The variation of the calculated difference in acid affinity across alkanes is a factor of 2 smaller than that of the experimentally measured additional heats. This can be attributed to the higher deprotonation energy of fluorosulfonic acid. The ΔEacid−affinity value for methane is about 12 kJ/mol while our experimental data shows negligible effect of the Brønsted site. This difference can arise from (1) not including temperature corrections which was calculated to be about 3.5 kJ/mol for methane, (2) Ealkane−acid−without−HB slightly overestimating the energy of non-hydrogen bonded complex because the alkane and acid molecules are fixed, although correctly canceling dispersion effects from the first term of eq 2, and (3) intrinsic errors in the functional which only included relatively more strong hydrogen-bonded complexes in parametrization and testing. The calculated affinity differences values are, however, consistent with similar values reported for weaker acids (HCl and HF) with higher levels of theory, including MP2 and CCSD(T) with aug-cc-pVTZ basis sets.22

Figure 4. Hydrogen bonded alkane-acid complexes. The acid is fluorosulfonic acid (i), and the alkanes are (ii) methane, (iii) ethane, (iv) propane, (v) pentane, (vi) iso-pentane, and (vii) hexane. The energy of the complex without hydrogen bonded interactions is obtained by rotating OH bond so that the hydrogen points away from the alkane; the structure of the acid in the resulting complex relaxes to a structure that is the mirror image of (i) owing to the symmetry. Carbon, hydrogen, oxygen, sulfur, and fluorine atoms are colored gray, white, red, yellow, and aqua, respectively.

ωB97X-D density functional41 with the 6-311++/G(d) basis set implemented in Gaussian 09.42 Clearly, the OH bond of the fluorosulfonic acid points toward the alkane indicating the formation of a hydrogen bond. We believe that extra acid affinity (ΔEacid−affinity) is a more relevant descriptor than proton affinities for understanding hydrogen-bonding of weak bases in zeolites. This can be E

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Specifically, the difference in acid affinity for methane with HF and HCl was about 6 kJ/mol at MP2/aug-cc-pVTZ level of theory; super acids like FSO3H is expected to show a larger difference in acid affinity at this level of theory. Nevertheless, the high degree of correlation between ΔEacid−affinity and experimental heats reinforces our proposition of the hydrogen-bonded nature of the alkane-H-ZSM-5 interaction.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b03855. A comparison of the experimental extra heats with EHOMO of alkanes and complete author list for Gaussian 09 (PDF)

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REFERENCES

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4. CONCLUSIONS The differential heats of adsorption of simple alkanes on HZSM-5 zeolite strongly depends on the presence of Brønstedacid sites. The extra heats of adsorption in H-form zeolites over its siliceous counterparts can be associated with the strength of hydrogen bonding between the adsorbed alkane and the Brønsted sites, as can be inferred from the correlation between experimental extra heats and computed extra acid affinity which captures noncovalent interactions between a weak base and strong acid. The comparison of the experimental and computational results demonstrated that the hydrogen bonded interaction theory accurately describes the effect of Brønsted sites for alkane adsorption on zeolites.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 215-898-8351; fax: 215-573-2093; e-mail: gorte@seas. upenn.edu (R.J.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.-H.Y. and R.J.G. acknowledge support from the Air Force Office of Scientific Research under AFOSR Award No. FA9550-14-1-0302. S.R. and M.M. acknowledge support from the Air Force Office of Scientific Research under a Basic Research Initiative grant (AFSOR FA9550-12-1-0481). The computational work was performed partly using supercomputing resources at EMSL, a National scientific user facility at Pacific Northwest National Lab (PNNL), the Center for Nanoscale Materials (CNM) at Argonne National Lab (ANL), and the National Energy Research Scientific Computing Center (NERSC). EMSL is sponsored by the Department of Energy’s Office of Biological and Environmental Research located at PNNL. CNM and NERSC are supported by the U.S. Department of Energy, Office of Science, under contracts DE-AC02-06CH11357 and DE-AC02-05CH11231, respectively. Additional CPU resources at the DoD High Performance Computing Modernization Program (U.S. Air Force Research Laboratory DoD Supercomputing Resource Center, the U.S. Army Engineer Research and Development Center (ERDC), and the Navy DoD Supercomputing Resource Center (Navy DSRC)) supported by the Department of Defense, were used to conduct this work. The assistance of Prof. Raul Lobo and Dr. Trong Pham in the preparation of zeolite samples is gratefully acknowledged. F

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DOI: 10.1021/acs.jpcc.6b03855 J. Phys. Chem. C XXXX, XXX, XXX−XXX