Al Ratio and Al Distribution

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Atomistic Investigations of the Effects of Si/Al Ratio and Al Distribution on the Adsorption Selectivity of n-Alkanes in Brønsted-Acid Zeolites Chi-Ta Yang, Amber Janda, Alexis T. Bell, and Li-Chiang Lin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11190 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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The Journal of Physical Chemistry

Atomistic Investigations of the Effects of Si/Al Ratio and Al Distribution on the Adsorption Selectivity of n-Alkanes in Brønsted-acid Zeolites Chi-Ta Yang1, Amber Janda2†, Alexis T. Bell2*, and Li-Chiang Lin1*

1

William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus, OH 43210, USA

2

Department of Chemical and Biomolecular Engineering, University of California, Berkeley, Berkeley, CA 94720, USA †

Present address: Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA

*To whom correspondence should be addressed: [email protected]; [email protected]

Abstract The adsorption of n-alkanes onto Brønsted-acid sites is a key step in the catalytic cracking of alkanes. Employing configurational-bias Monte Carlo simulations, we have investigated how the ratio of equilibrium adsorption constants for central C-C bonds relative to terminal bonds of nalkanes (i.e., the adsorption selectivity ratio) in Brønsted-acid zeolites is influenced by the Si/Al ratio and the Al distribution. A new computational approach was implemented and the developed force field was validated by a comprehensive comparison between simulation results and experimental data for a number of Brønsted-acid zeolites. While the adsorption selectivity seems to be relatively insensitive to the Si/Al ratio, our results reveal that the Al distribution plays a crucial role in determining the adsorption selectivity. Changes in the Al distribution result in a change of as much as two fold in the adsorption selectivity ratio for nhexane. The selectivity generally shows larger variations with respect to Al distribution in zeolites with a larger Si/Al ratio. The two factors identified by this work that substantially influence the selectivity ratio are the siting of Al atoms among T-sites and their spatial proximity, and an atomic-level understanding of each of these effects was achieved. The siting of Al atoms at more or less selective T-sites significantly influences the overall selectivity ratio, and Al atoms

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in close proximity can synergistically enhance the adsorption of central C-C bonds, leading to a higher selectivity ratio relative to isolated Al atoms. We anticipate that these results will have important implications for future large-scale computational screenings and the development of advanced synthesis approaches to target certain Al distributions in zeolites.

1. Introduction In the past several decades, zeolites have been widely employed in many fields such as catalysis,1 gas separation,2 and ion exchange3 due to a variety of desirable properties such as excellent thermal and chemical stability, large surface areas, and geometrically diverse topologies. Zeolites are crystalline, microporous materials consisting of corner-sharing [AlO4]and [SiO4] tetrahedra that form pores, cages, and/or channels of molecular dimensions. Brønsted-acid sites result when the negatively charged oxygen of [AlO4]− is compensated by a proton. Such acidic zeolites are vital in petroleum refining,4 petrochemicals production,5 and pollution control,6 and also catalyze the formation of 60-65% of the world’s propylene via steam cracking and 30% through fluid catalytic cracking.7 Cracking of a C-C bond in an alkane at a zeolite Brønsted-acid site results in the formation of a smaller alkane and an alkene. This process can occur through both mono- and bimolecular mechanisms.8,9 The monomolecular mechanism, involving the interaction of an alkane C-C or C-H bond with a Brønsted proton, dominates at conditions of low surface coverage (low pressure and conversion),10 while the bimolecular mechanism, involving hydride transfer as well as oligomerization and beta scission of alkene intermediates,11,12 primarily occurs under industrial conditions (high pressure, conversion, and surface coverage). Because of its simplicity and well-defined kinetics, the monomolecular alkane cracking, and also dehydrogenation, have been the subject of a number of studies aimed at elucidating the influence of zeolite structure on cracking kinetics.13–21 Adsorption thermodynamics play a crucial role in monomolecular cracking kinetics because the apparent rate coefficient is proportional to the equilibrium constant for alkane adsorption onto Brønsted protons and the intrinsic rate coefficient.13,14,22–24 Measured activation parameters are consequently equal to the sum of the adsorption enthalpy and


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entropy and the intrinsic activation enthalpy and entropy, respectively.22,23 To elucidate the influence of the adsorption equilibrium on apparent kinetics, and to extract intrinsic kinetic parameters, it is therefore necessary to obtain adsorption thermodynamics of alkanes at protons (e.g., specific adsorption enthalpy and entropy, denoted respectively as ∆Hads-H+ and ∆Sads-H+; see Computational Method). We note that the adsorption of alkanes directly onto protons cannot be measured at reaction temperatures (> 673 K), due to the tendency of the alkane to react and to locate increasingly at siliceous parts of the framework.25–27,24 As a result, computational studies have been employed to access this information.28,29 Recently, we have developed an approach to calculate adsorption of alkanes at protons in zeolites13 in which an efficient domain decomposition method was developed for use together with the Widom particle insertion method (described below). We also developed improved force field parameters,13 fit to describe the interactions between the alkane and the proton, which moves rapidly among the O atoms of the negatively charged AlO4− groups at the temperature of cracking.30,31 This methodology enabled us to systematically investigate the effects of zeolite pore and cage topology on the adsorption thermodynamics of n-alkanes adsorbed at Brønsted acid sites.14 While a compensating effect of correlated changes in adsorption entropy and enthalpy on the free energy generally exists, it was found in our previous work that the adsorption free energy can be tuned by manipulating a characteristic dimension (e.g., changing the pore size) and topology (e.g., adding cages) simultaneously.14 It is anticipated that this development can also facilitate the discovery of novel Brønsted-acid zeolites, serving as a powerful approach to discover zeolite candidates possessing desirable adsorption properties for alkane cracking. For example, by examining adsorption data reported in our recent theoretical work14 together with selectivity data from our recent experimental work,13 it can be seen that zeolites with higher selectivity to central C-C adsorption are also generally more intrinsically selective to central C-C cracking. Thus, an improved understanding of what zeolite properties influence the selectivity to adsorption of different C-C bonds of an alkane can serve as guidelines for the rational design of structures that are most likely to crack an alkane at a desired location.


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Although significant progress has been made in understanding the influence of zeolite structure on alkane adsorption,13,14 it is noted that these studies were performed for zeolites containing only isolated Brønsted protons. An atomic level understanding of the effects of the distributions of multiple Al atoms among different T-sites and channel environments, as well as of their proximity, is lacking. In this study, the adsorption properties of C4-C6 n-alkanes in Brønsted-acidic FAU, MOR, MFI, TON, FER, KFI, and MWW were investigated, using CBMC simulations in the Henry region,18 which corresponds to the low coverages relevant to monomolecular cracking, as functions of the Si/Al ratio and Al distributions. Attention is focused on the selectivity ratio for adsorption via a central vs. terminal C-C bond, which is expected to influence the selectivity for monomolecular alkane cracking. As noted above, it is expected that zeolites that are more selective to central C-C adsorption will generally also be more intrinsically selective to central C-C cracking.13 Cracking at this location would be preferred in industrial applications such as the cracking of naphtha range alkanes to liquefied petroleum gas (LPG). In the present study, the accuracy and transferability of the force field was first evaluated by comparing theoretical predictions with experimentally available measurements of ∆Hads-H+ and ∆Sads-H+ at room temperature in several Brønsted-acid zeolite frameworks having a range of Si/Al ratios. Next, the effects of the Si/Al ratio (ranging from 2 to 71) for adsorption of C4-C6 n-alkanes in a given zeolite framework was investigated for sets of 10 zeolite samples of the framework with random distributions of Al atoms. The role of the Al distribution in influencing the adsorption selectivity for a given framework at a constant Si/Al ratio was also studied. Notably, our results show that the selectivity toward adsorption via central C-C bonds can vary by as much as a factor of two as a function of the Al distribution among different Tsites. High selectivity to central C-C adsorption in 10 zeolite samples with randomly generated Al distributions was found to correlate strongly with the number of Al atoms located at T-site locations that exhibit high selectivity, while the range of the overall selectivity observed among samples correlated with the variation in selectivity among the individual T-sites. T-sites located in a more confined space appear to promote the adsorption of central C-C bonds. However, further study is needed in order to achieve a quantitative understanding of the effects of


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geometry on the selectivity ratio. The presence of nearby Al atoms (i.e., proximate Al atoms) was also found to affect the selectivity at a given T-site and increase the selectivity to central CC adsorption. In order to gain atomistic understandings, insights into the enthalpy and entropy of adsorption are also discussed in the context of density maps that illustrate the distribution of configurations of the alkane near the proton. Overall, the selectivity of alkanes for adsorbing via central vs. terminal C-C bonds at Brønsted protons in zeolites is not generally constant for a given framework type, and exhibits more sample-to-sample variation for high Si/Al ratios and for more heterogeneous frameworks having a large range of the adsorption selectivity among individual T-sites. The results of this study provide a systematic understanding of the effects of the Si/Al ratio and the Al distribution on adsorption selectivity in zeolite materials, and facilitate the rational design of better zeolite catalysts to promote the cracking of central C-C bonds.

2. Computational Method Configurational-bias Monte Carlo (CBMC) simulations were used to compute the adsorption properties of linear alkanes (propane through n-hexane) in Brønsted-acid zeolites. Seven zeolites including MFI, TON, FER, MWW, MOR, KFI, and FAU were studied, chosen mainly on the basis of available experimental data.32–35 The atomic structure of these zeolites was taken from the database of the International Zeolite Association (IZA).36 In these calculations, to describe intermolecular and intramolecular interactions of alkanes, the TraPPE model,37 an united atom approach, was adopted for representing methyl (−CH3) and methylene (−CH2−) groups of the alkanes. Non-bonded intermolecular interactions were described by LennardJones 6-12 potentials, while appropriate potential functions were adopted to describe intramolecular interactions of bond stretching, bending, and torsion. To account for the interactions between linear alkane molecules and zeolite framework oxygen atoms, parameters developed by Dubbeldam et al.38,39 for all-silica zeolites were used. The interaction between an alkane molecule and a Brønsted-acidic proton associated with the framework Al atom was described in our previous work;13 an effective potential was developed for the interaction between the alkane and the oxygen atoms attached to Al atoms to account for the rapid relocation of the proton occurring at temperatures of cracking (> 673 K).30,31 This parameter set


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was determined by fitting experimental data for n-alkane adsorption in Brønsted-acidic FAU (a structure which has 12-MR pores and large cages), using the experimental values of ΔHads (by contrast to the specific adsorption at protons, ΔHads corresponds to the enthalpy change for overall adsorption anywhere within the zeolite, including at protons).40 The Si/Al ratio of the experimental sample was matched to that used in the simulation. This potential was further validated for alkanes adsorbed in CHA (8-MR pores and smaller cages), and ΔHads determined by CBMC was found to differ from experimental values by only 0.1−0.3 kJ mol−1.41 Thus, the force field developed is likely to be transferable to zeolites with a wide range of pore sizes; however, a more detailed evaluation of the potential is still needed. Although the previously developed potential has been shown to reproduce overall adsorption properties well in the aforementioned two structures,13 its accuracy remains unknown for its predictions of specific adsorption properties in zeolites with a wide range of structural features and varying Si/Al ratios. As noted previously and discussed below, we have therefore carried out a comprehensive comparison between CBMC-predicted values of ∆Hads-H+ and ∆Sads-H+ and experimental values for alkanes in seven different zeolites. The Widom test particle insertion method13,42 was used to probe the energy surface for alkane adsorption in zeolites (i.e., , interaction energies between zeolites and alkanes) at infinite dilution (i.e., the Henry region).13,43,22,23,44 Several million test particle insertions were carried out to ensure statistically accurate averages. Using the Widom particle method, the enthalpy of adsorption (∆ ) can be computed by23,44 ∆ = ∆ − = 〈 〉 − 〈 〉 − 〈 〉 − ,

(1)

where 〈 〉, 〈 〉, and 〈 〉 are the Boltzmann-weighted averages of the interaction energies of the zeolite-adsorbate, the zeolite, and the gas, respectively. In these calculations, the zeolite framework is treated as rigid and, therefore, the value of Uh is essentially zero. The Henry’s law coefficient (KH) can also be derived from the inserted configurations using the ratio of the average Rosenbluth weights of the adsorbate in the zeolite framework (i.e., 〈〉) and in the gas phase (i.e., 〈 〉), respectively; =

〈〉

〈 〉

,

(2)


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where is the mass density of the zeolite framework. With eqns. 1 and 2, the entropy of adsorption (∆

)

∆

can then be calculated from

=

∆!"#

+ [1 + ln) *]

(3)

It is important to note that ΔUads, ΔHads,, ΔSads and KH (denoted as overall adsorption properties) correspond respectively to the adsorption energy, enthalpy, entropy and Henry’s coefficient for alkanes located anywhere within the zeolite, which includes adsorbates in a reactant state near Brønsted acidic protons (i.e., specific adsorption) and in siliceous parts of the framework.13,23 For alkane cracking, as pointed out in our previous work and in the introduction,13,43 specific adsorption properties are of particular interest given that they are directly related to the overall catalytic activity for monomolecular cracking. To obtain the properties corresponding to specific adsorption, a domain decomposition approach13,42 was developed. This method extracts the configurations at reactant states from all inserted trial configurations, allowing one to effectively and directly compute specific adsorption properties at Brønsted acidic protons. The resulting specific adsorption properties for internal energies, enthalpies, the Henry’s coefficients, and entropies at protons are denoted as ΔUads-H+, ΔHads-H+, KH-H+, and ΔSadsH+.

An inserted alkane configuration is assigned to the reactant state when a C-C bond j is

located within a cutoff radius (rc) of an Al atom positioned at T-site i.22 A value of j=1 indicates the terminal bond, while values of 2, 3, etc. indicate nonterminal bonds with larger values corresponding to bonds further away from the terminal bond. Using n-hexane as an example, j=1 (j1) specifies its terminal bond, while j=3 (j3) refers to its centermost bond and j=2 (j2) to the bond in between. From the domain decomposition, specific adsorption properties of bond j of an alkane at a particular T-site i, (i.e., ΔUads-H+(i,j) and KH-H+(i,j), the Henry’s law constant for bond j of a guest alkane adsorbed at T-site i), can be derived from CBMC simulations. By using the aforementioned equations (1), (2), and (3) with becoming 1/- . / . (where / . is the moles of protons per kilogram of zeolite and - . is the volume contained within one mole of spheres of cutoff radius, rc), ΔHads-H+(i,j) and ΔSads-H+(i,j) can then be calculated.22 To obtain average values of ΔHads-H+(i,j) over all bonds j of an alkane at each T-Site i, ΔHads-H+(i) at a T-Site i is determined from the Boltzmann-weighted average of ΔHads-H+(i,j) over all C-C bonds j. The


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value of ΔSads-H+(i) is then determined from the value of ΔHads-H+(i) as described in refs. 13 and 22. Although the aforementioned approach has been shown to predict specific adsorption properties for isolated T-sites in Brønsted-acid zeolites, it lacks the ability to compute specific adsorption properties in zeolites at varying Si/Al ratios (i.e., with multiple Al atoms).13,22 We have extended the capability of the domain decomposition approach to calculate the specific alkane adsorption in zeolites at varying Si/Al ratios. We utilized Zeo++45 to construct zeolite structures for a given Si/Al ratio by randomly distributing Al atoms following Lowenstein’s rule.46 For each studied structure at a Si/Al ratio of interest, ten random samples s were generated. The specific adsorption properties of n-alkanes onto all protons in each of the ten generated samples were sampled directly via the Widom particle insertion method with the domain decomposition approach. The obtained specific internal energy and Henry’s coefficients of the bond j of a guest alkane in sample s are represented as ΔUads-H+(s,j) and KH-H+(s,j), respectively. The same notation is used for the specific adsorption enthalpy ΔHads-H+(s,j) and entropy ΔSads-H+(s,j). To obtain averaged adsorption properties of the sample s over j bonds (e.g., ΔHads-H+(s)), the same approach as described above was used. Assuming that each of the 10 sampled Al distributions for a given Si/Al ratio is equally probable, the specific adsorption properties for each Si/Al ratio (averaged over the 10 different randomly generated distributions) correspond to the ensemble averages given by: < ∆12 >=

Al Ratio and Al Distribution

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